ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS
VOLUME 57
CONTRIBUTORS TO
THISVOLUME
R. Stephen Berry P. Braun D.-J. David Delon C. Hanson Robert J. Kelly Sydney Leach Henry W. Redlien F. Riidenauer F. P. Viehbock
Advances in
Electronics and Electron Physics EDITEDBY CLAJRE MARTON Smithsonian Inst itut ion Washington, D.C.
VOLUME 57
198 1
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81 82 83 84
9 8 7 6 5 4 3 2 1
CONTENTS CONTRIBUTORS TO VOLUME 57 . . . . . . . . . . . . . . . . . . . . . FOREWORD. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vii ix
.
Elementary Attachment and Detachment Processes I1 R . STEPHEN BERRY AND SYDNEY LEACH
I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .
1
. . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 125
11. Specific Processes
Fiber Optics in Local Area Network Applications DELONC . HANSON I . System Requirements and Trends . . . . I1. The Optical Communication Medium . . 111. Terminal Device and System Performance . References . . . . . . . . . . . . . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . .
145 162 189 225
Surface Analysis Using Charged-Particle Beams P . BRAUN.F. RUDENAUER. AND F. P. VIEHBOCK I . Introduction . . . . . . . . . . . . . . . . . . I1 . Classification of Methods . . . . . . . . . . . . . 111. Quantitative Elemental Analysis . . . . . . . . . IV . Depth Profiling . . . . . . . . . . . . . . . . . V . Elemental Mapping . . . . . . . . . . . . . . . VI . Three-Dimensional Isometric Elemental Analysis . . VII . Sensitivity and Resolution Limits . . . . . . . . References . . . . . . . . . . . . . . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . .
231 233 242 259 215 292 298 306
Microwave Landing System : The New International Standard HENRYW . REDLIEN AND ROBERT J . KELLY
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . I1. The Operational and Functional Requirement for MLS . . . . . . . .
111. Description of the Microwave Landing System .
. . . . . . . . . . . IV. System Design Considerations . . . . . . . . . . . . . . . . . . . V . Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . V
311 320 327 358 405 406
vi
CONTENTS
Microprocessor Systems D.-J. DAVID I . The Microprocessor Revolution
. . . . . . . . . . . . . . . . . . 411
I1. Components of a Microprocessor System . . . . . . . . . . . . . . 424
111. How to Deal with a Microprocessor-Based Application . . . . . . . . IV . System Development . . . . . . . . . . . . . . . . . . . . . . . V . The Choices in the Designofa Microprocessor System . . . . . . . . VI . Conclusion : A Glimpse into the Future . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . .
448 454 462 469 470
AUTHORINDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SUBJECT INDEX
473 489
CONTRIBUTORS TO VOLUME 57 Numbers in parentheses indicate the pages on which the authors’ contributions begin
R. STEPHEN BERRY,Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637 (1) P. BRAUN,Institut fur Allgemeine Physik, Technische Universitat Wien, Karlsplatz 13, 1040-Vienna, Austria (231)
D.-J. DAVID,University of Paris and ENSAM, Paris Cedex 05, France (41 1) DELON C. HANSON,Hewlett-Packard Company, Optoelectronic Division, Palo Alto, California 94304 (145) ROBERTJ. KELLY,Communications Division, The Bendix Corporation, Towson, Maryland 21204 (31 1) SYDNEY LEACH,Laboratoire de Photophysique Moleculaire du C.N.R.S., Universitt Paris-Sud, 91405 Orsay, France (1) HENRY W. REDLIEN, Communications Division, The Bendix Corporation, Towson, Maryland 21204 (311)
F. RUDENAUER, Institut fur Allgemeine Physik, Technische Universitat Wien, Karlsplatz 13, 1040-Vienna, Austria (23 1)
F. P. VIEHBOCK, Institut fur Allgemeine Physik, Technische Universitat Wien, Karlsplatz 13, 1040-Vienna, Austria (23 1)
vii
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FOREWORD The five articles that make up this volume survey modern research in two areas of fundamental physics and three areas of engineering. The article by R. Stephen Berry and Sydney Leach completes an extensive review of elementary collision processes and free charged particles at thermal energies in gases. We expect this article to be widely cited. With the extraordinarily rapid growth of fiber optic communications, in-depth reviews of current work such as that by Delon C. Hanson are vital for the development of the field. His article on local area network applications focuses on a particularly important aspect of the subject. One of the strong relations between the principles of electron physics and of materials science is discussed in the article by P. Braun, F. Rudenauer, and F. P. Viehbock in their review of surface analysis, a subject of great interest. Thecontribution by Henry W. Redlien and Robert J. Kelly on microwave landing systems has value not only to those working in the subject, but to a full complement of researchers concerned with ranging and guidance. We are particularly pleased to have this article because it helps us to expand the perspective of the advances. Little need be said about the role of microprocessors in research. D.-J. David’s article deals with information important to anyone working with microprocessors or considering their value to his or her work. As is our custom we present a list of articles to appear in future volumes of Advances in Electronics and Electron Physics. Critical Reviews: Atomic Frequency Standards Electron Scattering and Nuclear Structure Large Molecules in Space The Impact of Integrated Electronics in Medicine Electron Storage Rings Radiation Damage in Semiconductors Visualization of Single Heavy Atoms with the Electron Microscope Light Valve Technology Electrical Structure of the Middle Atmosphere Microwave Superconducting Electronics Diagnosis and Therapy Using Microwaves Computer Microscopy Image Analysis of Biological Tissues Seen in the Light Microscope Low-Energy Atomic Beam Spectroscopy History of Photoemission Power Switching Transistors ix
C. Audouin G. A. Peterson M. and G . Winnewisser J. D. Meindl D. Trines N. D. Wilsey and J. W. Corbett J. S. Wall J. Grinberg L. C. Hale R. Adde M.Gautherie and A. Priou E. M. Glaser E. M. Horl and E. Semerad W. E. Spicer P. L. Hower
X
FOREWORD
Radiation Technology Diffraction of Neutral Atoms and Molecules from Crystalline Surfaces Auger Spectroscopy High Field Effects in Semiconductor Devices Digital Image Processing and Analysis Infrared Detector Arrays Energy Levels in Gallium Arsenide Polarized Electrons in Solid-state Physics The Technical Development of the Shortwave Radio Chemical Trends of Deep Traps in Semiconductors Potential Calculation in Hall Plates Gamma-Ray Internal Conversion CW Beam Annealing Process and Application for Superconducting Alloy Fabrication Polarized Ion Sources Ultrasensitive Detection The Interactions of Measurement Principles, Interfaces and Microcomputers in Intelligent Instruments Fine-Line Pattern Definition and Etching for VLSI Recent Trends in Photomultipliers for Nuclear Physics
L. S. Birks G. Boato and P. Cantini M. Cailler, J. P. Ganachaud, and D. Roptin K. Hess B. R. Hunt D. Long and W. Scott A. G. Milnes H. C. Siegmann, M. Erbudak, M. Landolt, and F. Meier E. Sivowitch P. Vogl G. DeMey 0. Dragoun
J. F. Gibbons H. F. Glavish K. H. Purser W. G. Wolber Roy A. Colclaser J . P. Boutet, J. Nussli, and D. Vallat
Waveguide and Coaxial Probes for Nondestructive Testing of Materials Holography in Electron Microscopy The Measurement of Core Electron Energy Levels Millimeter Radar Recent Advances in the Theory of Surface Electronic Structure Rydberg States Long-Life High-Current-Density Cathodes
Henry Krakauer R. F. Stebbings Robert T. Longo
Supplementary Volumes: Microwave Field-Effect Transistors
J . Frey
Volume 58 : Modeling of Irradiation-Induced Changes in the Electrical Properties of Metal-Oxide Semiconductor Structures
Point Defects in Gap, GaAs, and InP The Collisional Detachment of Negative Ions Implementation for Very Large-Scale Integration Stimulated Cerenkov Radiation Materials Consideration for Advances in Submicron Very Large-Scale Integration
F. E. Gardiol K. J. Hanssen R. N. Lee and C. Anderson Robert D. Hayes
J. N. Churchill, P. E. Hollmstrom, and T. W. Collins U. Kaufmann and J. Schneider R. L. Champion Heiner Ryssel John E. Walsh D. K. Ferry
Our sincere thanks to all of the authors for such splendid and valuable reviews. C. MARTON
ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS
VOLUME 57
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ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS. VOL. $7
Elementary Attachment and Detachment Processes. I1 R. STEPHEN BERRY Departmeni of Chemistry and The James Franck Institute The University of Chicago Chicago, Illinois
SYDNEY LEACH Laboratoire de Photophysique Moldculaire du C.N.R.S.* Vniversitd Paris-Sud Orsay, France
I. Introduction ..... ....................................... 11. Specific Processes . .......................... A. Dissociative Recombination and Attachment . . . . . . B. Collisional Detachment and Ionization. . . . . . . . . . . . C. Ion-Pair Formation . . . . . .......................................... D . Ion-Ion Neutralization ......................... E. Photoionization ............................................
.......................................... G . Multiphoton Ionization.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H. Photodetachment . . . . . . . . . . . . . . . .................................. References ..................................................
1
42 60 93
112
I. INTRODUCTION This review continues the task begun in the article by Berry (1980a),which is referred to here as Part I.’ The goal of the review is a description of the elementary collision processes associated with the production and capture of free charged particles in gases at thermal energies, up to a few electron volts, apart from photoionization, which only commences with photons of several volts. We address the scientist who is not a specialist in atomic and molecular colIisions. It is our intent to convey the currently held physical pictures of the most important processes, to give a sense of the orders of magnitude of their rates and cross sections, and to provide an entry into the enormous literature
* Laboratoire associe a I’UniversiteParis-Sud.
’ See Advances in Electronics and Electron Physics, Vol. 51, pp. 137-182 (1980). Copyright 8 1981 by Academic Press, Inc. All rights of reproductionin any form reserved.
ISBN 0-12-014651-6
TABLE I CLASSIFICATION OF ELEMENTARY ATOMIC/MOLECULAR PROCESSES Initial state Process number
Nuclear
Final state
Electronic
Nuclear
Electronic
Special conditions
Name and process Radiationless transition; internal conversion (intersystem crossing if forbidden with respect to change of electron spin) AB +AB* Autoionization, preionization AB or AB* + AB' + e ; autodetachment AB- 4AB e
1
Bound
Bound, excited
Bound, excited
Bound
2
Bound
Bound
Free
3
or Bound, excited Bound or
Bound Bound, excited Bound, excited
Free
Bound
Bound Free
Predissociation ABorAB*+A
4
Bound, excited Bound
Bound
Bound, excited
Inverse autoionization or preionization ; dielectronic recombination AB+ + e -+ AB* ; (radiationless, nondissociative) attachment AB + e + A B Inverse predissociation A + B+AB*orAB
N
5
Free
Bound
6
Free
Bound
or Bound, excited Bound
or Bound, excited Bound
+
Bound Bound, excited Bound Bound
Photon released
+B
Radiative recombination A f B + A B + hv, AB+ radiative attachment AB + e + A B - + hv
+ e + A B + hv,
W
7
Free
Bound
Bound
Free
8
Free, excited
Bound
Bound
Free
9
Free
Bound, excited
Bound
10
Bound
Free
Free, rearranged Free
Bound
11
Free
Bound
Free
Free
12
Free
Bound
Free
Bound, ionic
13
Free
Bound, ionic
Free
Bound
14
Bound
Bound, excited
Free
Bound, ionic
15
Bound
Bound
Bound
Free
Photon absorbed
16
Bound
Bound
Free
Bound
Photon absorbed
17
Bound
Bound
Free
Bound
Associative ionization chemionization, Hornbeck-Molnar process A or A* + B-t AB + e; associative detachment A + B- +AB + e Penning ionization A* + B -t A t B+ + e; Penning detachment A* + B- - t A + B + e Rearrangement ionization, Kuprianov process A* + B C - + A B + + C + e Dissociative recombination AB+ + e+A + B; dissociative attachment AB + e+-A + BColIisional ionization A + B - + A + B+ + e ; collisional detachment A + B-+A + B + e Ion-pair formation A+B-+A++BIon-ion neutralization (occasionally recornbination) A+ + B - - t A + B ( o r A B ) (Dissociative) ion-pair formation AB* + A + + BPhotoionization AB + hv + AB+ + e ; photodetachment AB- + hv -t AB + e Photodissociation AB + hv + A + B or AB + hv+A+ + B Dissociative ionization A B * - t A + B+ + e
4
R. STEPHEN BERRY AND SYDNEY LEACH
in this field. We have also taken this opportunity to point out several unresolved problems, inconsistencies, and discrepancies in the current literature. We do not attempt to be exhaustive in our coverage. As in Part I (Berry, 1980)a), we apologize in advance to our colleagues for omission of material that they may feel we should have included. The organization follows the pattern set in Part I, with minor variations. The processes of interest were given in Table I of Part I ; that table is reproduced here (Table I).Just as the main sections of Part I corresponded approximately to the first 9 categories of this table, the sections of this discussion begin with the 10th category and continue through the 17th. However, we have grouped some closely related processes, notably ion-pair formation (process 12) with dissociative ion-pair production (process 14). Photodissociation is treated only as a competitor for photoionization or photodetachment, on the basis that we are concerned with charged systems, principally. Photodetachment is given its own section, separate from photoionization. Because of the great increase in activity in the subject, we have also given a separate section to multiphoton ionization.
11. SPECIFIC PROCESSES A . Dissociative Recombination and Attachment
1. General Description Dissociative recombination of electrons e
+ AB++A + B
(1)
and dissociative attachment of electrons,
+ AB+A- + B
(2) have so much in common that we treat them together. The close relation of these processes was recognized long ago by Bates (1950b). Note that A or B may be a polyatomic fragment, and that either A or B might be left in an excited state-electronically, or, if the species is molecular, then vibrationally or rotationally-following reaction (1) or (2). The essential physics of both processes is the same: a free electron becomes a bound electron and the reason the electron remains bound is that some or all of the energy released in the attachment process is carried off in the kinetic energy of the dissociating heavy products. Typically, both processes carry the system “downhill” in the following sense. Translational energy of an electron is given up for translational energy of relative motion of heavy particles. For a given amount of translational e
5
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
energy, the density of states and, therefore, the partition function and the entropy are higher if the effective mass is large rather than small. While one can find cases in which dissociative recombination or attachment is endoergic enough to bind fast, hot electrons and generate cold, slowly moving heavy particles, we can expect that when these processes are iso- or exoergic, then they are entropy increasing. Dissociative recombination was reviewed by Bardsley and Biondi (1970) and earlier by Danilov and Ivanov-Kholodnyi (1965).The topic was included in broader reviews by Flannery (1972, 1976) and Bates (1975, 1979). An older review by Biondi (1964) concentrated on the processes relevant to atmospheric recombination. Dissociative attachment was reviewed by FiquetFayard (1974a) and was included both in a review of resonant scattering of electrons (Bardsley and Mandl, 1968) and, peripherally, in the review of the reverse process of associative detachment written by Fehsenfeld (1975). The book edited by Bates (1962) and that by Massey et al. (1974), of course, discuss both processes. Dissociative recombination has been studied extensively for the very simplest systems such as e + H,+ and somewhat for clusters of hydrogen and helium; the process has been pursued in depth by a few laboratories for the heavier rare gas molecule ions: recombination of CH+ + e and H 3 + + e have generated considerable interest for their astrophysical implications: N O + + e has been and continues to be the most popular system for the study of dissociative recombination. A few other systems, often related to these, such as HCO+ + e, have been examined, and recent studies have now e, H 3 0 + e, and been carried out for polyatomic ions such as NH,+ clusters of H,O+.(H,O), .The subsequent discussion is organized along these lines, approximately, following a description of how the process is supposed to occur. Dissociative attachment has been studied almost exclusively with molecules containing halogens or with other highly electronegative “electron catchers” such as 0 atoms or OH groups. There are exceptions; the most notable is H, + e giving HH (Schulz and Asundi, 1965). The physical processes of recombination and attachment are similar in that the electron is drawn to the molecule by a long-range attractive potential, whereupon the energy of attachment must be carried off by the breaking of a chemical bond faster than the electron can regain enough energy to go free again. The essential problem for theorists has been finding out what makes the electron stick long enough to allow a bond to break, and identifying the mechanism of energy transfer between the incoming electron and the other degrees of freedom of the system. The indices available for evaluating the various interpretations have been the temperature dependence of recombination and attachment coefficients-a very difficult and meager index for
+
+
+
6
R. STEPHEN BERRY AND SYDNEY LEACH
testing theory in this particular situation-and, more recently, the dependence of the corresponding cross sections on electron energy (Peart and Dolder, 1973b, 1974a,c; McGowan et al., 1976, 1979; Auerbach et al., 1977; D. Mathur et al., 1978; Mitchell and McGowan, 1978),and the cross sections for production of specific states of the products (Phaneuf et al., 1975; Vogler and Dunn, 1975). Whatever the details of the mechanism at a microscopic level, the most significant gross features of the two processes are these. In dissociative recombination, the cross sections and rate coefficients, in the large, are decreasing functions of electron energy over the range from zero to tens of electron volts. The smallest, simplest molecules, such as H,+, HD’, and D,+, have maximum cross sections between and cm’, for zero-energy electrons (Auerbach et al., 1977); cross sections for larger, more complex molecules are somewhat larger: NH4+ has a recombination cross section of 3 x 10- l 3 at an electron energy of about 0.065 eV (Dubois et al., 1978): that of H,O+ is about 1.5 x 10- 14, based on a thermal rate coefficient of (4.1 f 1.0) x lo-’ cm3/sec at temperatures between 2000 and 2450 K (Hayhurst and Telford, 1974). Clusters such as H5+(Leu et al., 1973b) and H30+.(H,0), (Leu et al., 1973a) have recombination rate coefficients as much as ten times larger than those of the simple H3+ and H 3 0 + ions. An illustration of the behavior of a typical rate coefficient with temperature of the electrons is in Fig. 1. This is a representation of a large number of data assembled by Heppner et al. (1976) for e + H 3 0 + . The cross sections and rate coefficients for dissociative attachment do not have as universal a pattern as their counterparts for dissociative recombination, nor are the parameters for attachment as large. In many cases, the cross sections for attachment of electrons to neutrals are very small for low-energy electrons and rise with electron energy to a maximum somewhere in the range of a few tenths of an electron volt to a few electron volts; moreover, these cross sections frequently exhibit peaks and dips comparable in amplitude with the cross sections themselves. The process e + F, + F + F- is a simple illustration: The rate coefficient for this process appears to have a single sharp peak at or very near the zero ofelectron energy E , and to decrease sharply as E , goes up to about 6 eV (Schneider and Brau, 1978; Tam and Wong, 1978). This coefficient falls as Ep3/’ from just under lo-* to about 6 x lo-’ cm3/sec between about 0.2-0.4 and 6 eV. By contrast, the cross section for formation of 0 - from e + 0, has a sharp onset at E, of 4.4 eV (Schulz, 1962). Carbon monoxide, CO,, SO,, and H,O were also shown by Schulz to have sharp thresholds for the onset of dissociative attachment, with cross sections that rise and then fall with electron energy. Moreover, the cross sections for such processes are comparable to or smaller than gas-kinetic cross sections; the peak of the cross
7
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
I
W v)
‘I
m
z W
I
TEMPERATURE
( O K )
+
+
FIG.I . Rate coefficients measured for the recombination process e HaO++ H H,O. The curves were calculated by Heppner et a/. (1976), with various uncertainties; the solid curve is, in essence, their “base case.” All the points shown represent data considered by Heppner ef a/., with the exception of the two solid circles, which were taken from the recent work of Ogram er a/. (1980). This work was done at high temperatures in a shock tube, and this could account for the high rate at 3000 K.Taken, with permission, from Heppner et al. (1976).
section for e + O2 -+ 0 + 0 - ,just below 7 eV of electron energy, is 1.3 x, lo-’* cm2 (Schulz, 1962), and the peaks for 0-production from CO and CO, are 2.2 x and 4.6 x cm2, respectively. Christophorou and Stockdale (1968) examined cross sections for dissociative attachment, nDA,in some 30 molecules. They found it useful to classify these molecules into three groups according to the energy and magnitude of the peak of cDAas a function of electron energy. For the first group, the peak falls at an energy below the first electronically excited state of the molecule and is 10- l 7 cm2 or more. The second group has peak cross sections between cm2, and the peak occurs above the energy of the first and electronically excited data of the molecule. The third group has cross sections of less than 2 x lo-,’ cm2. The first group includes halogen-containing molecules and one peak of N 2 0 ; the second includes H 2 0 , NO, CD,, one
8
R. STEPHEN BERRY AND SYDNEY LEACH
peak of CO, and N,O and two peaks of H,. The last group includes the lowest peak for H,, one peak of CO,, and the C O molecule. In simplest terms, the reasons for these differences are, first, the difference between the Coulomb attraction of an ion for an electron and the much weaker dipolar or polarization field that a neutral molecule exerts on an electron ; and second, the electron with thermal energy approaching a molecular positive ion always finds bound molecular states available to it, whereas an electron approaching a neutral will, in most cases, have to supply some energy to the molecule to make a bound state available for itself. This second distinction has meant that theorists have been able far more frequently to assign labels to the states responsible for electron capture leading to dissociative attachment than to assign the labels to states responsible for dissociative recombination. 2. Dissociative Recombination: The Rare Gases The demonstration and interpretation of dissociative recombination began with the dilemma of explaining observed ionospheric recombination rates several orders of magnitude greater than those of the mechanisms then considered radiative recombination and ion-ion neutralization. Bates and Massey (1946b) suggested such a dissociative process. High rates of recombination were demonstrated in the laboratory from microwave studies of the decay of plasmas in helium (Biondi and Brown, 1949), whereupon Bates (1950a,b) attributed these rates to the specific process e
+ He:
(3)
42He
Biondi then took up a systematic investigation to establish the mechanism of electron-ion recombination in gases, an investigation that has continued to the present time. The first product of Biondi’s work was the demonstration (1963) that in afterglows following microwave discharges, for a wide range of conditions, electron-ion recombination is a volume process, not a wall process, and that the volume process has kinetics consistent only with the above type of reaction. Notably, he used argon and helium-argon mixtures to show that the recombination of electrons with argon ions occurs via e
+ Ar:
-+Ar
+ Ar*,
Ar* + A r
+ hv
(4)
The mechanism was taken to be the one suggested by Bates, a direct transition from e Ar,+ to a state with a repulsive potential curve dissociating to Ar + Ar*. The presence of excited atoms in rare-gas afterglows was previously established and correlated with the decay of the free electron concentration (Holt et al., 1950; Johnson et al., 1950). Biondi (1963) pointed out in that first paper of his series how important it would be t o measure the
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
9
kinetic energy distribution of the emitting atoms, in addition to the temporal dependence of the electron concentration and the temporal, spatial, and spectral distribution of the radiation. [The importance of the spatial distribution of the electron distribution, especially for microwave experiments, was demonstrated by Kasner and Biondi (1965). J The first effort by Biondi and his collaborators (Rogers and Biondi, 1964) to find the high kinetic energies in atoms produced by process (1) was inconclusive. This work, of recombination in helium, was particularly influential in stimulating a series of theoretical and experimental studies that eventually established the special nature of helium; we shall discuss recombination in helium shortly. Connor and Biondi (1965) then investigated recombination in neon, by monitoring the temporal dependence of electron concentration and, by interferometry, the shapes of atomic emission lines in neon afterglows. The allowed singlet-singlet p + s transition at 585.2 nm showed a width appropriate to two neon atoms separating with a relative kinetic energy of 1.25 f 0.07 eV. In fact, the line shapes in the afterglow were interpreted as having narrow central components rising out of the middle of broader lines. The latter parts were attributed to neon atoms that emitted radiation after recombination and dissociation occurred, but before the excited atoms suffered collisions. The narrow central parts were attributed to neon atoms with thermal velocity distributions that had become excited by transfer of electronic energy from a fast excited atom colliding with a slow atom after dissociation. The recombination coefficient at 300 K was found to be about 2 x lo-’ cm3/sec and the cross section for energy exchange, (8 & 2) x cm2 at the velocity of 2.5 x lo5 cm/sec, of the dissociating atoms. Later, Frommhold and Biondi (1969) used the same methods to reinvestigate recombination in neon and to examine the process in argon. They confirmed the basic finding of Connor and Biondi, but pointed out that if the dissociating ions have several possible exit channels, each channel should contribute its own broad component to any Doppler-broadened emission line. All the 22 neon lines and the 5 argon lines studied by Frommhold and Biondi showed broadening indicating dissociative recombination. Some lines, such as the 585.2 nm line of Ne, displayed one dominant broad component, but virtually all of them had at least indications of other components. Frommhold and Biondi were led to suppose that some molecular ions were in an excited state, and that recombination could lead to several possible excited states of the product atoms. Biondi and his group continued to study recombination in the heavier rare gases (Shiu and Biondi, 1977, 1978; Shiu et al., 1977) with emphasis on the dependence of the rate coefficient on electron temperature T, and on the relationship between the electron temperature and which excited states
10
R . STEPHEN BERRY AND SYDNEY LEACH
occur in the product atoms. The trend seems to be that the recombination coefficients a increase slightly with the size of the atoms: (9.1 i- 0.9) x at 300 K for Ar,' + e, (1.6 0.2) x for Kr,' e, and (2.3 i-0.2) x cm3/sec for Xe,' + e. The temperature dependencies of these coefficients are not so systematic: a(Ar) T-o.61, a(Kr) T - 0 . 5 5 and , a(Xe) T - u 3 for 300 < T, < 700 K merging smoothly into T-'.', dependence for higher T . The three substances showed the same behavior regarding the atomic excited states that appeared: When T, was 300 K, the only atomic lines seen were attributable to electrons recombining with the diatomic molecular ions in their ground electronic and vibrational states. When T, was increased to make the mean electron energy of order 0.6-0.7 eV, higher atomic lines were detected, corresponding to electrons recombining with excited ion-molecules. The processes specific to the recombination of hot electrons are presumably much like those that occur in dissociative attachment, involving collisional excitation accompanied by electron capture in the excited state. Hence, these processes are analogous to dielectronic recombination (cf. Berry, 1980a, p. 155; Bates, 1975).In principle, electronic, vibrational, or rotational degrees of freedom of the ion-molecule could be excited by the incoming electron and act as the energy sink to stabilize the newly formed neutral. The evidence suggests that electronic and vibrational excitation may play significant roles in dissociative recombination, but that rotational excitation would only play a role in very cold systems (Bardsley, 1968b). However, the experimental results for neon show significant differences in the behavior of the recombination rate coefficients, when the electrons are heated, roughly as T,- '/', as against T,,'.5 when the electrons and ions are heated in a shock wave (Cunningham and Hobson, 1969). O'Malley (1969) has attributed the rapid falloff in the recombination rate coefficients in the latter case to excitation of vibrations of the ion-molecule. O'Malley's interpretation begins with the supposition that the overlap of the wave function of e Ne, is large for the vibrational-electronic ground state of the ion and the dissociating state responsible for capture, for states of relative translational motion accessible near Re of the ion by collision with thermal electrons. This is generally supposed to be the situation for many diatomics including Ne,', Ar,', Kr,', Xe,', NO', and others. (We shall return to this point.) If the conditions supposed by O'Malley are fulfilled, as in Fig. 2a, then he has the overlap of the initial and final nuclear wave functions-that is, the FranckCondon factor-falling off as u-l/', where u is the vibrational quantum number: but, more important, the factor governing the probability that the molecule dissociates before it autoionizes back to e + ion falls exponentially as 1 - exp(hv/kT,,,). In other words, for the diatomic systems that exhibit very rapid dissociative recombination, the only state of the molecule-ion
+
N
-
+
-
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
I
I
I
(el
(d1
11
(f)
FIG.2. Schematic representations of potential curves involved in various cases of dissociative recombination : (a) crossing curves in the most favorable situation for dissociative recombination, yielding A + B in a strongly Fepulsive state from a strongly bound AB' and an electron; (b) another direct case, moderately favorable for dissociative recombination but without a true crossing of the potential curves; (c) an indirect process in which bound, neutral AB is formed in a highly excited state (AB)**, perhaps a Rydberg state, from AB' + e; (AB)** then undergoes a transition to the repulsive state that yields A t B; (d) an indirect process like the preceding, except that the bound Rydberg state (AB)** has a potential that does not cross the curve of AB' + e. In (d), the curves of (AB)** and the repulsive state (AB)* do cross, but they need not, yielding the case shown as (e). In (f), there is a neutral state (AB)* that plays exactly the same role as (AB)* in (b), the difference being that in (f) the state of the neutral has bound vibrational levels, but the transition of interest occurs above the dissociation energy of (AB)** so the excited molecule dissociates as in (b). With the attractive well, some (AB)* molecules could have lifetimes considerably longer than those of (b). Case (a) is the basis of the model of OMalley (1969); (f) was studied by Nielsen and Berry (1971). Note that (f) may also involve a crossing of the two potential curves.
that plays a role is supposedly its ground vibronic state, and the only state of the neutral that enters is the one that crosses through the ionic potential as in Fig. 2a. Now let us return to recombination in helium. The results of Rogers and Biondi (1964) raised questions about the importance of dissociative recombination, just at the time that this mechanism was becoming widely accepted. Shortly before, Hinnov and Hirschberg (1962) had made a strong case that a process He"
+ 2e
-V
He*
+e
(5)
would account much better for their observations: They noted that spectral lines from highly excited states of He became more intense after their discharge was turned off, whereas lines from lower states died away. Then Ferguson et a/. (1965) studied the dependence of several He lines on T, and
12
R. STEPHEN BERRY AND SYDNEY LEACH
summarized the existing data from many sources, concluding that the dissociative recombination coefficient for He,' could be no greater than about 3 x 10- l o cm3/sec. They emphasized the observation of Niles and Robertson (1964) that the emission from the afterglow of a helium discharge is largely from molecular helium very soon after the discharge is cut off. This picture is certainly contrary to the expectations of a system undergoing fast dissociative recombination to excited atoms. The history of the unraveling of this problem has been given in the review by Bates (1979). The set of coupled processes, together called collisional-radiative recombination, including the three-body process (9, its inverse of ionization by electron collision, excitation and deexcitation of atoms by electrons, and radiative processes, appears to account well for what happens in helium except at very low temperatures, and presumably plays an important role in other dense plasmas hot enough that molecules are not present in quantity. First suggested by D'Angelo (1961), Bates and Kingston (1961), and McWhirter (1961), the process was the subject of extensive computations (Bates et al., 1962a,b), followed by many others, culminating in those of Stevefelt et al. (1975). The expression obtained by the last-named group for the collisional-radiative recombination coefficient aCRof electrons in helium is = 1.55 x 10-10T;0.63 aCR(cm3/sec)
+ 6.0 x + 3.8 x
10-9Te-2.18N,0.37
10-9T,-4.5N, (6) The first term, independent of the electron concentration N , , is clearly due purely to radiative processes; the third term, linear in N,,is purely collisional and corresponds to process (5). The middle term is a numerical fit to the complex results of many microscopic processes occurring simultaneously ;in effect, it is the result of the interaction between the collisional and radiative processes. The analysis by Stevefelt et al. seems to give agreement within about a factor of two with experimental data including their own (Boulmer et al., 1973,1974). Other analyses have included the possibilities of molecular contributions, either by dissociative recombination or by the analog of process (l),but with molecular ions instead of atomic (Collins, 1965).Collins referred to the process 2e
+ He,+ +2He + e
(7) as collisional-dissociative recombination. At low pressures, Boulmer et al. (1977) found that Eq. (6) nearly accounted for all their experimental results; they replaced the first term of (6)with u1 = 3.5 x
10-5T;1.9 cm3/sec
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
13
amounting to a small correction, which they attributed to the process He+
+ c + He + 2 H e
(8)
that is, to helium rather than an electron acting as a third body. Typically, the effective recombination coefficient for electrons in helium at pressures of order 25 torr, temperatures between 300 and 500 K,and N , about 10' cm3is about 6 x (Boulmer et al., 1973). It is ironic indeed that the helium plasma, the system that stimulated the entire investigation of dissociative recombination, turns out to be one of the few clear counterexamples. However, it turns out that at cryogenic temperatures, about 10 K, the effective recombination rate coefficient in helium cm3/sec, proportional to T,-10.2 vapor is very large, (4 f 0.6) x (Delpech and Gauthier, 1972) and independent of N , . In agreement with this is the value (3.4 k 1.4) x lo-' at 80 K (Gerard0 and Gusinow, 1971). The reason for the large cross section and behavior more like "traditional" dissociative recombination is that in cold helium plasmas, many of the ions exist as He,+ and He,', not just as He2+.It is the larger ions that are responsible for the large cross sections at low temperatures. Why is helium so different from neon and the other rare gases? Why does dissociative recombination of He,' have a cross section small enough that collisional-radiative recombination is the dominant mechanism for its neutralization? The answer, as best we know, now comes from Mulliken's discussion (1964) of the potential curves of He, and He,'. There are simply no dissociating states of He, whose potential curves stand in relation to the curve of the ground vibronic state of He,' as do the curves of Fig. 2a or b. Only vibrationally excited states of He2+ appear to be available for direct coupling to the dissociating states that produce He He*. The hydrogen molecule also has a paucity of excited states, like helium because of the small number of electrons. In H, one again finds no crossings of repulsive curves with the ground vibronic state of H,', from which H H* can be formed by direct dissociative recombination.
+
+
3. Theories Before turning to the recent experimental work that has dealt largely with very simple systems, we review the state of theoretical discussions of the subject. We shall not attempt to describe the many formalisms and computational procedures; rather, we restrict ourselves to describing the pictures that have emerged. The theoretical treatments range from models intended to be just elaborate enough to rationalize the observations, such as the very early treatment by Bauer and Wu (19561, to full quanta1 calculations of specific kinds and channels of dissociative recombination (Nielsen and Berry, 197 1 ) and formal general theoretical statements (Chen and Mittleman,
14
R. STEPHEN BERRY AND SYDNEY LEACH
1968). For H,’ + e, the most “successful,” i.e., the most recent, theoretical treatments still do not manage to represent the experimental results quantitatively (Bottcher, 1976, for example), but for this system theorists have tried to work with nonempirical treatments and, by and large, the theoretical representation accords reasonably with experiment (Fig. 3). For larger systems, theorists have been more successful by using semiempirical or phenomenological formulations that can be based either on spectroscopic data or ab initio calculations. Probably the most influential formulation has been that of Bardsley (1968a,b). It clarified the vocabulary by comparing previous formulations with Bardsley’s own in common terms; it laid out explicitly the distinctions among several physical processes and their mathematical counterparts; it criticized weaknesses in some of the earlier formulations ; and it showed how to make quantitative estimates from empirical data and thus to estimate the temperature dependence of recombination rate coefficients. More recent theoretical work carried out in a similar spirit includes the application of multichannel quantum defect theory, first by Lee (1977) to N O + + e, and most recently, in a n elegant manner that more properly couples different continua-the electron continuum of e AB’ with the atomic continuum of A + B-by Giusti (1980), with application to CH’ + e. Current dogma divides dissociative recombination into several kinds of processes. These are (1) direct processes in which the transition is made from a state of e + ABf in which AB’ is vibrationally bound to a compound state (AB)* of the neutral that has a repulsive potential leading to A + B, either of which may be excited; (2) indirect processes in which the transition is made first from e AB* to a neutral level (AB)** that is bound both with respect to the electrons and the nuclei, and then from (AB)** to the repulsive state (AB)*, which dissociates; (3) processes-not always distinguished as clearly from the other two in the literature as the physics demands-in which e AB’ makes a transition into a state of the neutral that we shall label (AB)*, whose electronic potential curve supports bound vibrational states, but whose internal energy of nuclear motion is greater than the dissociation energy of the electronic state. The greater part of dissociative recombination appears to go through one or both of the first two processes. However, in hydrogen and perhaps helium, the inverse process of associative ionization seems to contain a significant component from the third mechanism and the dissociative recombination of these species at high temperatures, where vibrationally excited H 2 + is present, probably involves some of the third process as well (Nielsen and Dahler, 1965; Nielsen and Berry, 1971). The direct processes are represented by couplings between curves of the kinds shown in Fig. 2a,b. The indirect processes can occur through processes of the types shown in Fig. 2c,d,e. A fourth type of indirect process through a
+
+
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
I5
single intermediate is possible, but is not shown, in which the curve of the e, but the dissociating state (AB)* bound state (AB)** crosses that of AB’ does not cross the curve of (AB)**. Processes of the third kind are illustrated by Fig. 2f; again, the curve of (AB)* could cross that of A B + . Much of the discussion of dissociative recombination by direct processes has the aspect of calling for a deus ex machina in the form of a crossing of the potential curves of (AB)* and (AB)’ in a very specific, rather narrow range of the A-B distance R . Within the conventions of the Franck-Condon picture, electronic transitions occur (1) with no change of internuclear distance, and therefore “vertically” in diagrams of the types of Figs. 2 and 3; and (2) with no change in relative momentum of nuclei, and overwhelmingly at classical turning points, and therefore, “from potential curve to potential curve.” Suppose the free electron approaching AB’ has energy 6 ; this much energy must, within the Franck-Condon picture, go entirely into electronic excitation. Hence, the only internuclear distance at which the transition may occur is R , , as shown in Fig. 4, where the separation between the two potential curves is precisely E. This classical model for the direct mechanism requires that the curves of (AB)* and of (AB)’ have a separation comparable to the kinetic energies of the electrons within the range of R available to the AB+
+
Center-of-moss energy ieV1
FIG.3. Dissociative recombination cross section for Hi + e, with H; in a distribution of vibrational states. Points are measuredby Auerbach et al. (1977); x ’s are from the experiments of Peart and Dolder (1974a). The dashed curve was calculated by Bottcher (1976). Taken, with permission, from Auerbach et al. (1977).
16
R. STEPHEN BERRY AND SYDNEY LEACH
ion-molecules. This means that in a typical afterglow, the separation between the curves of (AB)* and AB' would have to be comparable to the thermal energy of the free electrons in the range of R between the classical turning points of the ground vibrational state $o(R)of AB'. In Fig. 4,the condition is met for electrons having energy 6 at the distance R, and a thermal distribution P(6). The distribution P(R,) of distances at which this thermal distribution could cause capture would be simply the reflection of P ( E )in the difference E,,(R) - EAB+(R).This distribution is not quite yet the probability distribution PDR(R)for dissociative recombination is a function of the distance R. To obtain PDR(R) from P(R,),we must multiply by the probability distribution for finding AB' at distance R ; suppose AB' is in its ground the square of the ground-state vibrational state; this distribution is II+$~(R)(', vibrational wave function for AB'. The construction of P D R ( R ) is shown in Fig. 5, which is based on the curves of Fig. 4 for the states (AB)* and AB'. In the center top is the difference in energy of these two curves, for the region in which the difference is positive and classical dissociative recombination is physically possible. At left is a hypothetical distribution P(E)of electron energies. The curve of P(R,) is constructed by reflection as shown. Below the curve of P(R,) is the probability distribution for the internuclear distance with (AB)* in its ground vibrational state. At the bottom is the product P(R,)[$,,(R)~'; in this case it is only slightly distorted from P(R,), but this is not necessarily always the case.
Et I AB'te
E
I
I
:
I
R-
RCR, R,
+
FIG.4. Closer specification of the potential curves for AB' e ( k = 0 ) and a crossing, repulsive state (AB)+ for dissociative recombination [case (a) of Fig. 21. The internuclear distances indicated by It&), Rotand R, are, respectively, the capture distance for an electron with energy t, the equilibrium distance for AB' and the crossing distance beyond which it is increasingly improbable that A B go into a stable AB' e. The energy of the electron is shown at left from E o , the minimum of the potential curve of AB' ; it is shown again at the center of the diagram starting from the zero-point level of AB' and terminating on the curve of (AB)* ; the point at which this energy separation equals t is precisely the capture distance of R&).
+
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
17
If this nearly classical model of direct dissociative recombination gave the full picture, then intersections of the curve of (AB)* with that of AB+ would have to occur at values of R somewhat larger than the left-hand (small R ) classical turning point of $,,(R), and not too far to the right of its right-hand classical turning point. This restricts the crossing region to a ~, p is the reduced mass of the A-B oscillator range of order ( h / p c ~ ) ”where and w is the AB’ vibrational frequency in rad/sec. This range typically is about 0.04 A or less, and the corresponding energy range in which the crossing must occur is of the order of the zero-point energy of AB’. Curves of (AB)*, such as those in Fig. 6, would correspond to states rather ineffective for dissociative recombination ;for the curve labeled (AB):, the electron energy would have to be less than c1 and for the curve labeled (AB)f, the energy of the electron would have to be greater than c 2 for dissociative recombination to occur with a high probability, at least according to the classic model. It is not surprising that H, and He,, with relatively few excited potential curves, seem not to fulfill the required condition. Rather, it is at least a little surprising that
R
FIG.5. Construction from a semiclassical model of the probability distribution P,,(R) of direct dissociative recombination as a function of internuclear distance. The curve P(6) is the exogenous distribution ofelectron energies; the curve of E,,,(R) - E,,+(R) is the separation of the two relevant potentials for the direct recombination; $,(R) is the vibrational wave function of the AB’ molecule. One can apply this model to any of the direct cases of Fig. 2a, b, or f, including the unillustrated case as in Fig. 2f but with a crossing. If the AB+ molecules may have vibrational excitation and if vibrational relaxation is slow compared with recombination, the probability P(R,) should be multiplied by the vibrational distribution ~ p j l $ j ( R ) lofz the AB’ molecules; p j is the probability of thejth vibrational state and I , ~ ~ (isRits ) wave function.
18
R. STEPHEN BERRY AND SYDNEY LEACH
I
FIG.6. Details of the potential curves involved in dissociation recombination. For recombination to be effective in the direct, curve-crossing model, the potential of (AB)* must fall in the range between the dashed curves (AB): and (AB):.
more than one or two diatomic systems can be interpreted successfully in terms of this model. Why, one is forced to ask, d o neon, argon, krypton, and xenon, as well as nitric oxide, oxygen, and nitrogen all seem to conform to a picture based on such a restricted crossing? For polyatomics, there is no problem because of the high density of vibrational levels; the problem lies with diatomics and perhaps triatomics. One part ofthe answer is easily found in the simplification from quantummechanical behavior to classical behavior. As soon as one allows the (AB)* molecule to be described by a translational wave function, the requirement of a curve crossing disappears. The less restrictive condition is only that the curve of the state (AB)* be close enough to that of AB’ for the two translational wave functions to exhibit significant overlap. Second, to meet the conditions of the Franck-Condon approximation, we need only require that the transition from AB’ e to (AB)* be vertical and that the nuclear momentum not change during the transition. This means that the transition need not be “from potential curve to potential curve,” but that if it occurs at an A-B distance R , , the kinetic energy of the nuclei at R , in the upper state (AB*) be the same as that at R , in the lower state. As in the classical model, the energy c of the free electron goes entirely into electronic excitation in this approximation, but the system can make transitions even when the nuclei are moving. This situation is precisely the same as that invoked for Penning ionization, as discussed in Part I (Berry, 1980a). Finding the relative importance of direct and indirect pathways has been a popular way to describe many recent studies of dissociative recombination. The temperature dependence of the recombination rate coefficient has been considered one guide because theoretical arguments have indicated that the rate coefficient for the direct process oftype 1 (Fig. 2a-c) and oftype 3 (Fig. 2f) should vary as Te-1’2at low temperatures (Bardsley, 1968b; Nielsen and
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
19
Berry, 1971),but may fall off faster with increasing T at “high” temperatures, and that the rates for indirect processes should fall faster than Te-’”, perhaps as fast as 7‘-3/2.The corresponding behavior of the cross sections would be like Ee- for the low-temperature direct processes and like a higher inverse power of electron energy in other cases. 4. Fitting Experiments and Theories: e
+ Hzi
Experiments with merged electron and ion beams, which give very high energy resolution, have been done to determine the energy dependence of the cross sections for dissociative recombination. These show (Mitchell and McGowan, 1978; McGowan et al., 1979; Mu1 and McGowan, 1979)that the cross sections are smooth, decreasing functions of electron energy, going as Ee- within experimental uncertainty, up to electron energies of about 0.1 eV. Above this energy range, the cross sections fall faster with electron energy and seem to display maxima and minima suggestive of resonances-long-lived intermediate quasibound states-and therefore of indirect processes. Hence, the present state of interpretation would have direct processes playing an important role for many, perhaps all recombination processes at low energies, with indirect processes progressively more important with increasing electron energy. As theoretical work has become more sophisticated, the discussion has begun to turn more toward identifying specific intermediate electronic states and specific product states. For example, in the analysis of e + H2’, the dissociative Rydberg states of(H2)*built on one electron in the antibonding core orbital la, and one in a Rydberg orbital, e.g., the ( laU)(3s) configuration, were suggested by Zhdanov and Chibisov (1978). This proposal was criticized by Derkits et al. (1979) who claimed that Zhdanov and Chibisov overestimated the role of (la,)(nl) repulsive Rydberg states by neglecting their autonionization. Derkits et al. (1979) estimated that such states c;ould only contribute at most about 8 x lo-’’ cm2 to the cross section and then only for electrons with energies well above 3 eV. However, Zhdanov (1980) recalculated the contribution of such states to the limit H(1s) H(n = 4)and compared the results with those of Phaneuf et d.(1975). The excellent agreement implies that repulsive Rydberg states must be included in future calculations of the dissociative recombination of hydrogen. The computed contribution of capture into dissociating regions of bound e. Here, based largely on states, as in Fig. 2f, is a bit larger, at least for Hz the capture of a free s electron, if R is smaller than Re(H2’) or a freed electron if R is larger, Nielsen and Berry (1971) found a cross section having a maximum for E, = 0, and strongly increasing with the vibrational state of H,’.
’
+
+
+
20
R. STEPHEN BERRY AND SYDNEY LEACH
Its magnitude is 2 x lo-'* cm2 for v = 2 when E , is eV, but yields a total cross section of considerably less than 10- l 9 cm2 for an electron energy of 0.1 eV. This picture is based on capture into the (log)(4sa)Rydberg state of (H,), which dissociates to H(1s) H(n = 3). Bottcher's (1976) model has the ion catching the electron in a d wave and going into the (10,)~lC,+ state of (H2)*as the dissociating state, by way of lC,+ intermediate Rydberg states built on one electron in the la, orbital of H,'. The cross sections derived this way are of order cm2 for electrons with about 1 eV of energy, about a factor of 2 higher at this energy than the experimental value of Peart and Dolder (1974a) and considerably higher still than the values of Auerbach et al. (1977) for H,' with vibrational quantum numbers 0, 1, and 2. The experimental and theoretical values are essentially the same for energies of about 0.5 and 2.1 eV and above; below 0.5 eV, Bottcher's theoretical values are too low. Putting this observation together with the results just cited from the group of McGowan, one is led to suspect that a direct low-energy process supplementing the indirect processes called upon by Bottcher would explain this simplest of systems. It is quite unknown whether the direct process would be one of type 1, AB' e + (AB)*-+ A + B, or type 3, AB' e + (AB)* + A + B, or of some more complex form. For example, in H, low-energy recombination might involve transitions through several electronic states, with small amounts of energy being exchanged between electrons and molecular rotation, and larger amounts between electrons and vibrations, in a succession of promotions of the nuclear vibrational state until the molecule reaches a state whose potential is in a favorable relation to a dissociating curve, perhaps but not necessarily that of the (la,)' 'C,' state. The absolute cross sections for e H 2 + and e D 2 + were first determined unambiguously by the inclined beam measurements of Peart and Dolder (1973a,b, 1974a). The cross sections were an order of magnitude less than what had been expected and, in effect, rationalized by Bauer and Wu (1956). The observed cross sections were 17.3 x cm2 at 0.5 eV and and 8.2 x for 6.4 x at 1.01 eV for H2' + e, and 15.3 x D2+ + e at the same energies. These values were confirmed by the measurements of D. Mathur et al: (1978) by an ion-trap method. The results of Auerbach et al. (1977) are also consistent with the others, apart from showing irregular oscillations at electron energies above about 0.1 eV, with amplitudes increasing with E , . Figure 3 shows the results obtained by Auerbach et al. (1977) for H 2 + + e. The oscillations are of order 0.1 eV and their amplitudes contribute roughly 15% of the total cross section. Moreover, the oscillations are dependent on the vibrational state of the H2'ion (McGowan et al., 1976).
+
+
+
+
+
21
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
McGowan and his collaborators attribute the resonance structures to Rydberg states, which we would designate as (H,)** or (H2)* here. One further set of experiments with D 2 + + e may be very important for the interpretation of the mechanism of this recombination. Dunn and coworkers (Phaneuf et al., 1975; Vogler and Dunn, 1975)have followed emission by excited hydrogen atoms to determine the contributions of specific final hydrogenic states to the total cross section for dissociation recombination. About 10% of the total cross section comes from states yielding D(n = 4), over the range 0.6 I E, I 7 eV, from the results of Phaneuf et al. A slightly smaller fraction of the dissociation products are D(n = 2) atoms, according to Vogler and Dunn. It is possible, but seems very unlikely, that fast groundstate atoms could be the principal products. Whether the major excited products are atoms with n = 3 as suggested by Nielsen and Dahler (1965)and used in the model of Nielsen and Berry (1971) remains to be tested. 5. Larger Ions The dissociative recombination cross section of H3+ + e has been measured by Auerbach et al. (1977)for H, in an undertermined assembly of states, and by Peart and Dolder (1974~)using a source of vibrationally cold H 3 + (Peart and Dolder, 1974b). The cross sections are all larger than for H2+ + e, 5 x 10-'6-10-'5 cmz at 1 eV, for example, and slightly larger, perhaps a factor of 2, with the vibrationally deexcited H,+ than with the distribution of Auerbach et al. As with H 2 + + e, the high-energy resolution of Auerbach et al. showed resonance-like structure in the cross section for H,' + e at energies above about 0.2 eV. The process H 3 + e has been treated theoretically only very recently by Kulander and Guest (1979). For electron energies a little above 1 eV, they expect the products to be H(n = 2) H, in its vibrationless ground state. At higher energies, they expect a variety of products; H 2 + + H - should appear above 5.41 eV. At the very lowest energies, they predict the products to be three hydrogen atoms. Now that the vibrational levels are known for H,+ (Carney and Porter, 1980; Oka, 1980), D,' (Carney and Porter, 1980; Shy ef al., 1980),and Rydberg states of H 3 and D, (Herzberg, 1979),we can expect a still more refined theoretical analysis of this recombination process. Cross sections for larger clusters of hydrogen around a proton have not been studied, but thermal rate coefficients have been studied for H,+ e. Leu et al. (1973b) found a value for the rate coefficient of this recombination of 3.6 x cm3/sec at 205 K, by working at pressures up to 0.6 torr in their discharges. Trainor (1978) carried out measurements at higher electric +
+
+
+
22
R. STEPHEN BERRY AND SYDNEY LEACH
fields, and therefore at higher mean electron energies, that were consistent with the results of Leu et al. and indicated that the rate coefficient falls offwith increasing electron energy, as one expects. Diatomic molecules larger than H 2 + and He2+ have been studied, but, apart from NO" + e, not extensively. The recombination of CH+ e plays an important role in certain models of the chemistry of the interstellar medium (Solomon and Klemperer, 1972), and has consequently been studied both theoretically (Bardsley and Junker, 1973; Krauss and Julienne, 1973; Giusti-Suzor and Lefebvre-Brion, 1977; Raseev er al., 1978; Giusti, 1980) and experimentally (Mitchell and McGowan, 1978). The experimental results give a large cross section, (5.1 & 0.5) x cmz for electrons with energies of 0.01 eV, and a rate coefficient of (3.1 f 0.3) x lo-' cm3/sec. This value is large enough-by two orders of magnitude-to be somewhat upsetting to the astrophysical model. Earlier calculated values were lower, of order cm2/sec for the rate coefficient. The large experimental cross section could, however, reflect the presence of vibrationally excited states of the ion: This rationalization could make the laboratory results consistent with the theoretical values and the values used in the astronomical model. e was realized to be a key process for The recombination of NO' removal of electrons in the ionosphere (Biondi, 1964; Danilov and IvanovKholodny, 19 5). The rate coefficient for this process was measured most recently by H iang et al. (1975). It is large, especially for a diatomic, 3 x lo-' cm3/sec at an electron temperature of 500 K. Cross sections were measured by Walls and Dunn (1974) and by Mu1 and McGowan (1979). Theoretical interpretations of the process have been given by Bardsley (1968a,b, 1970), Michels (1975), and Lee (1977). The cross-section measurements appear to agree well for electron energies from below 0.1-1 eV, the region where the two sets overlap: about 2 x cm2 at an energy of about 0.05 eV to about 3 x at an electron energy of 1 eV. They also agree well with the theoretical predictions of Michels. The results of Huang et al. are about 1.7 times the rate coefficients inferred from the cross sections measured by Mu1 and McGowan and by Walls and Dunn. Mu1 and McGowan suggest that the NO' in Huang, Biondi, and Johnsen's experiment could be vibrationally hotter than that in either the Walls-Dunn trapped-ion experiment or the Mul-McGowan merged-beam experiment. Michels' theoretical cross sections do increase with the vibrational quantum number of the N O f . A little commentary is appropriate concerning the states of NO that might be formed in the recombination process. Bardsley invoked two states, the Bf2Aand B2n,after considering several others. Michels derives smaller cross sections using five states; in addition to the B"A and B2n, he considers the 'Zf, the 'll 111 and the states, all of which dissociate to N(2D) + O(3P),and whose potential curves cross that of the X'C' state of NO' in the
+
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
23
vicinity of the ground or low vibrational states of the ion. This is an example par excellence of the situation where either Fig. 2a or c could apply. Michels' analysis is based on the former, that is, upon a direct mechanism. However, since all the states he invokes have bound vibrational levels, the situation as Michels supposes it is really a type 3 process, direct but via the part of an attractive potential on the left turning-point side above its dissociation limit-a type 3 process, accomplished by a curve crossing rather than by the proximity of two states as it is portrayed in Fig. 2f. The multichannel quantum-defect treatment of Lee (1977) gives cross sections larger than those of Michels or of the Walls-Dunn and MulMcGowan experiments. When the improved theory of Giusti (1980)is applied to this problem, we may hope to see values from this presumably very powerful formulation that represent the experiments accurately. The same theory can be used, of course, to describe autoionization of NO, a subject discussed at some length in Part I (Berry, 1980a). Like the studies of products from D,, there is one experimental investigation of the products of the dissociative recombination of e + NOt (Kley er al., 1977). Bardsley (1968b) predicts that about half the NO dissociates to give N('D) atoms; Michels has all the N O going to this limit, but with a qualification allowing for a curve crossing that would give some ground-state nitrogen atoms. Kley et al. find that 76 6% of the dissociating N O molecules formed from e + NO" give N('D). This is consistent with the rather crude theories now available and can be used as a test for more elaborate calculations. More important, it provides a basis for a supposed source of excited metastable nitrogen atoms in the upper atmosphere. The studies of recombination of electrons with clusters such as H 3 0 + . (H,O), and NH,+(NH,), were mentioned earlier. A theoretical analysis rationalizing their large cross sections was presented by Bottcher (1978). In his treatment, the vibrational levels of the cluster act as energy acceptors. The number of available vibrational levels is large, so large that there are resonance levels available everywhere to permit indirect recombination. The theory gives a linear dependence of the cross section on the numbers of monomer units in the cluster. Dissociative recombination of clusters was included in a general review of ionic clusters by Smirnov (1977). There is one important problem that arises with such systems and that has been overlooked by almost everyone concerned with such species. The exception is the analysis by Herbst (1978) of the question of what products are formed when a polyatomic molecule dissociates. Herbst uses the statistical model of Light and Pechukas (Pechukas and Light, 1965; Light, 1967), with an orbiting model-a critical radius within which an electron has unit probability of recombining, which may be adequate for polyatomics-and makes specific predictions of (the products. For example, Herbst predicts
R. STEPHEN BERRY AND SYDNEY LEACH
24
+
+
that H,O+ e will give as a primary product H, O H in its ground state. Even a “most extreme case” example gives only 50% of the (H30)*molecules going to HzO + H. Neutralization of CH3+ is expected to give largely CH Hz. However, the product distribution from NH4’ e is probably divided relatively evenly among NH, + H, NH2(XZB,)+ H,, NH2(AzA,) + H, and, NH(X3X-) H, H. The method is too statistical in nature to be valid for H 3 + e. The products from this simplest polyatomic system are H? Both are possible. Kulander and still not known: are they 3H or H, H(n = 2) for Guest (1979) predict 3H for thermal electrons and H, electron energies above 1 eV. It would be worthwhile to consider treating the fragmentation problem by the methods of Quack and Troe (1974) and of Silberstein and Levine (1980).
+
+
+
+
+
+
+
6. Dissociative Attachment: The Simplest Paradigms Most dissociative attachment processes have been interpreted as direct resonance processes: capture of a n electron by a neutral molecule, putting the compound system into a repulsive state or above some dissociation limit of a state with bound vibrational levels, so that the compound state splits into a neutral fragment and a negative fragment. Analogous to dissociative recombination, the rate k,, of this process or its cross section cDAhas been described quantitatively by the crude representation of a product of a cross section (7, for capture of an electron into a resonant state, and a survival probability ps that the electron stays together to preserve the compound negative ion long enough for the nuclei to reach a point of no return, beyond which the electron is truly bound and the molecule separates irrevocably into its fragments (Holstein, 1951; Bardsley et al., 1964; Bardsley, 1968a). The formulation of the rate in this manner was refined to take proper account of the angular momentum of the restrictions on the incoming electron and the capturing molecule (Chen and Peacher, 1967). Chen and Peacher also argue that the kinetic energy of the nuclei at their point of no return must be large relative to the inverse of the survival probability-that is, dissociation must be rather more probable than auto detachment of the electron-or the rate of dissociative attachment may vary significantly with the rotational state of the capturing molecule. The symmetry restrictions arise through the relation between the angular momentum of the incoming electron, the compound state, and the molecules. The most primitive example of dissociative attachment, e H, + H + Hat the lowest possible energies, illustrates this phenomenon. The lowest energy resonance of H, + e is a transient state of H,- with a very short lifetime, of order sec. when the nuclei are separated by about the equilibrium distance for the neutral H,. The incoming electron is captured
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
25
into the lowest empty molecular orbital, the la,, which becomes a hydrogenic 1s orbital at the separated-atom limit, but a 2pa orbital in the united atom limit. The latter is the important one for dissociative attachment because the direct resonance model has the incoming electron going from its free state into a transient bound state of the same symmetry type as i t $ initial free $tare. The model supposes that no angular momentum is exchanged between the incoming electron and the nuclei in the capture process. This restricts to p waves the incoming electrons approaching H, to form H + H - via (lo,)’( lo,) 2Zu+state. The p-wave electron encounters a centrifugal barrier which, according to Chen and Peacher, is strongly dependent on the molecular rotational state if the electron is slowly moving. The inference they draw is that for a case in which s-wave electrons can be captured, the simple picture of survival probability suffices, but when electrons must be captured from free states of higher angular momentum, the simple expression of aDA= n,p, should be replaced by a sum of terms, one for each angular momentum state of the molecule. The resonant capture and dissociation process is strongly reminiscent of the picture of chemical reactions of the form A B C + A B + C in which the entire reaction takes place on a single potential surface. For dissociative attachment, the process is usually represented in terms of what appear at first sight to be different potentials, as, for example, those of Fig. 2 or 6. However, these curves can be thought of as sections of a hypersurface drawn for two different placements of the incoming electron. The curves corresponding to e A B or e AB’ have the electron infinitely far away. The curves with the electron attached, e.g., as AB- for dissociative attachment, correspond to the energy of the compound system as a function of R(AB) averaged over the distribution of the electron when it is in its bound or quasi-bound state. Because this limit involves averaging the energy over the position of the electron, it is not exactly the same as the counterpart in the system of three heavy particles, ABC, for which the potential of the exit channel is drawn as E(RAB)when C is very far away. However, the entrance channels are precisely equivalent with e playing the role of A. Moreover, the selection rule restrictions invoked by Chen and Peacher are, in this sense, very much like the role of rotational and nuclear spin constraints invoked by Quack (1977)and Quack and Troe (1975a,b) to describe the state distributions in products of photodissociation and, by implication, of three-body rearrangements. It is interesting to note the parallel between the survival factor p , and the transmission coefficient that appears in mechanistic models of reactive collisions, models such as “absolute rate theory.” The direct resonance model so widely accepted now for dissociative attachment (Bardsley et at., 1964; O’Malley, 1966; Schneider et al., 1979)was not the first explanation for this process. The first explanation, which was
+
+
+
26
R. STEPHEN BERRY AND SYDNEY LEACH
accepted for many years, was given by Bloch and Bradbury (1935). Their picture was the counterpart of the indirect process described previously for dissociative recombination, and shown in Fig. 2f: the potential curves of the neutral and the negative ion (the authors treated only diatomics) were supposed to be identical but displaced on an energy scale. The mechanism of capture was presumed to be the exchange of electronic energy for vibrational energy by the mechanism of breakdown of the Born-Oppenheimer approximation. That is to say that the mathematical representation of Bloch and Bradbury has the e + AB channel coupled to the A - + B channel through the action of the nuclear kinetic energy operator on the electronic wave function. The physical counterpart is a slight tardiness of the electronic wave function in reaching its adiabatic stationary state as the nuclei move, so that the system finds itself in a state that is a mixture of the entry channel and the exit channel, and possibly other states. Bloch and Bradbury illustrated their calculations with the attachment of electrons to oxygen; the contrast between their formulation and the resonance picture was made particularly clear by Herzenberg (1969) when he treated the same molecule. There were three points on which the Bloch-Bradbury model seemed to contradict experiments carried out during the 1960s: the electron-scattering experiments showed several peaks in the electron energy-loss spectrum indicating that vibrational excitation occurs to several final states of 0, when slow electrons scatter inelastically from 0,. This implies, in turn, that the Franck-Condon envelopes and potential curves of the neutral molecule and the compoundstate negative ion molecule do not overlap closely. In fact Herzenberg’s interpretation puts the potentials for 0,- and 0, in about the same relative positions as those of AB’ e and (AB)** in Fig. 2c. Second, Bloch and Bradbury inferred that the electron affinity of O,, which is everywhere the distance from the potential of 0, to that of 0,- in their model, could be no greater than 0.19 eV, but the thermodynamic electron affinity of 0, is now known to be 0.4 eV (Hotop and Lineberger, 1975). Third, the rate of attachment of electrons to 0, goes up as the square of the pressure, not linearly. This implies that the lifetime of a compound state is short and that most compound 0, states lose electrons, rather than become 0 - + 0, contrary to the Bloch-Bradbury picture. It would not be surprising if some examples are found that accomplish dissociative attachment by unambiguous vibronic coupling, but thus far the common and better understood examples seem to be adequately explained by the simpler resonance model. The consistency of the direct resonance model with observations must be taken cautiously, especially in view of the need for both kinds of processes to explain dissociative recombination. One recent suggestion has been made (Tronc et al., 1979) to explain structure in a cross section for e H, + H - + H above 14 eV, in
+
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
27
FIG.7. Schematic diagram of the potential curves for H2 (solid curve) and H,- (solid at large R, dissolving at R < R J . The width of the distribution of the diffuse portion of the H2“curve” corresponds to the inverse Iiibtime, or to the imaginary part of the potential if the total potential is represented as a complex function.
which breakdown of the Born-Oppenheimer approximation has been invoked, but calculations of even this simple case have yet to be carried out. This brings us back to the example of hydrogen. The process that occurs at lowest electron energies has a threshold at 3.75 eV and a cross section that rises nearly vertically there to a peak of about 1.7 x cm2, which is rather small, due to the short lifetime of the H,- compound state. The event can be visualized in terms of Fig. 7. To the right of R , , 3 bohr, the state of H, is more stable than that of H, + e ; to the left, where R is less than R , , the state of H,- lies in the continuum of the H, + e system so some process, however slow, couples the quasi-bound electron with the continuum. In a simple Born-Oppenheimer model, the lifetime of the compound electronic state toward autodetachment is a simple function of R ;its inverse, the width, grows from zero at R , to about 3 eV at R = 1 bohr (Bardsley and Mandl, 1968). If the incoming electron has enough energy to raise the molecule to the diffuse energy range of H,- above the Franck-Condon region for H, + e (Fig. 6), then the electron is temporarily trapped and the system stays in its ( 1 0 ~ ) ~ ( 1 0 ’Xuf ~ ) resonant state for a time of order sec. This corresponds to a broad, diffuse peak in the elastic scattering cross section for electrons from H, (Bardsley and Mandl, 1968). If the incoming electron has enough energy to generate the compound state above the dissociation limit of H + H-, 3.75 eV, there is a finite probability that the nuclei can separate to R , or beyond or during the lifetime of the resonance, and dissociative attachment occurs. Under such circumstances, the cross section for dissociative attachment is nonzero at the threshold energy for the process; this is precisely what was found by Schulz and Asundi (1965, 1967). In this case, where the lifetime ofthe complex is very short and the survival factor is small,
28
R. STEPHEN BERRY AND SYDNEY LEACH
the effect of isotopic substitution is the consequence of its effect on the to survival factor. Replacement of H, by D, reduces aDAfrom 1.6 x cm2, nearer to that of H2 cm2, whereas cDA for H D is 1 x 3 x than of D,. If the electron impinging on H, has an energy of about 10 eV, another process may occur. A new resonant channel opens, corresponding to the ( l ~ ~ ) ( l n ”2C,+ ) ~ , state. The potential curves for this state are of course entirely repulsive because of the antibonding nature of the la, orbital. This process was observed by Khvostenko and Dukel’skii (1958)and Schulz (1959) and then studied by Rapp et al. (1964) and interpreted soon thereafter by Bardsley et al. (1966).The cross section is much larger than for the process at about 4 eV; the peak for H2 is 1.3 x cm2. Moreover, the peak in aDAis about 4 eV wide (full width at half height) in contrast with the width of only about half a volt for the low-energy process. Again, the heavier the isotope, the smaller is the likelihood of dissociative attachment. A third “peak” in the cross section for e H, + H H occurs for electron energies between 14 and 15.5 eV (Rapp et al., 1964). In this region, the cross section rises above 2 x lo-,’ cm2 and has a number of small, sharp maxima on the high-energy side of the principal maximum (Tronc et al., 1979). This peak rises just at the threshold energy required to generate H - + H(n = 2) as the dissociation products. Much of the interpretation of these peaks in the cross section for dissociative attachment has been devoted to assigning the angular momentum and spin of the dissociative transient state of the H 2 - . This exercise is much like trying to make assignments of electronic band spectra in the absence of rotational structure; often one can go far by combining calculations and chemical intuition, and with a little luck, make predictions that allow the assignments to be tested. For example, the 14-eV peak with its subsidiary maxima was assigned by Tronc et al. (1979) as due to a transient state ,A, state of H,- built on a lagnun, configuration that correlates with H - + H(n = 3), and that undergoes rotational coupling with a repulsive lagluunu, ,lIs state that dissociates to give H- + H(n = 2). This is a testable suggestion: one could look for the emission of the Lyman-a line from the dissociation products, and even correlate its polarization with the angular distribution of products. High resonant states of H2- and N,- have been identified from differential inelastic electron scattering cross sections by Comer and Read (1971a,b),but the precise states invoked by Tronc et al. have not been seen this way. The theory of the angular distribution of the products of dissociative attachment has been studied by O’Malley and Taylor (1968) and developed further by Klar and Morgner (1979), but as yet there seems to have been no attempt to use the relation between these distributions and
+
+
29
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
other properties of the products-in the case here, the polarization of emitted atomic radiation-to assign quantum numbers to the intermediate state. The angular distributions themselves, however, have been measured and interpreted (Van Brunt and Kieffer, 1970, 1974; Hall et al., 1977; Tronc ef al., 1977; Azria et al., 1979, 1980). A recent and provocative advance in the interpretation of dissociative attachment is the suggestion by Bottcher and Buckley (1979) that the cross [H,-, 'nu]+ H- + H(2p) may be greater than section fore H,(c311,) 10- ' cm2. For hydrogenic plasmas, this result raises the prospect that molecules in the metastable c 3 n Ustate of H, might be as important as molecules in their ground state for capturing thermal electrons, under circumstances in which the metastables are generated. More broadly, the suggestion leads one to question whether electronically excited molecules might be important generally for dissociative attachment in decaying plasmas and plasma sheaths, where electrons are available to generate the excited species. The one other available datum is the comparison of gDRfor O,(a'A,) (Burrow, 1973) with that ofthe ground state of 0, (Henderson et al., 1969); the cross sections for the metastable and the ground state are, at their maxima (4.6 f 1.3) x lo-'' and 1.3 x lo-'' cm2, respectively. Admittedly, the alAg state of 0, has the same configuration as the ground state and might be expected to resemble that state more than the c state of H, resembles the H, ground state. The important hint is merely that attachment cross sections may be considerably larger for excited states than for ground states. It may be useful at this point to remind the reader unfamiliar with the jargon of molecular collision physics about the designations of transient compound states. In many cases, such as the 10-eV peak of uDAfor e H,, the compound system gets its long life because some energy is transferred from the relative motion of the collision partners into a specific, clearly identifiable degree of freedom, leaving the collision partners in a well-defined (but nonstationary) quantum level of the compound system having a welldefined excitation of the target. In our example of e + H,, one electron is promoted from the lo, orbital to the lo, excited orbital, which it then shares with the incoming electron to give H , - ( 1 ~ 7 ~ ) ( 1 0 , ),ng+. ~ When such an internal excitation can be assigned, the resonance is called a Feshbach resonance. In other cases, the collision partners go into a compound state that is not readily identified as having energy from the collision stored in a ,) of e H, at about specific internal excitation. The ( l ~ , ) ~ ( l o , resonance 3 eV that gives rise to dissociative attachment above 3.75 eV is of this type. Such a resonance owes its lifetime to the kinematics of motion in the effective potential, including the centrifugal and polarization contributions to the potential. In fact, in this example, it is generally believed that the centrifugal
'
+
--f
+
+
R. STEPHEN BERRY AND SYDNEY LEACH
30
barrier experienced by an electron in a p-wave about the H, acts as the trap that keeps the electron temporarily bound to the H,. Such a resonance is called a shape resonance. The classical counterpart of a shape resonance is orbiting as it is induced by potential scattering. Thus, both polarization potentials and permanent dipoles can also give rise to resonances in electron scattering that one would call shape resonances. 7. Dissociative Attachment: Halogen and Hydrogen Halide Molecules
The halogens, the hydrogen halides, and larger halogenated molecules play special roles in electron capture because of their large capture cross sections for electrons of low energy. Dissociation of X, e to X- + X is exothermic for all the halogens, as Tam and Wong point out (1978). In the cases of halogens, much effort has gone toward determining thermal rate coefficients for dissociative attachment, particularly because these coefficients are important parameters for the description of halogen lasers. The experimental results are not easy to reconcile, particularly when the electrons have an effective temperature much higher than the halogen molecules. For example, although the rate coefficients for equilibrated e and I, e appear to rise with temperature (Birtwistle and Modinos, Br, 1978; Schneider and Brau, 1978; Trainor and Boness, 1978), their reported magnitudes seem more different than one might expect. Birtwistle and Modinos found rate coefficients of about lo-'' cm3/sec at 250 K and 2 x lo-'' at about 340 K for I, + e. They predict a rapid falloff in the rate coefficient if the molecules are at 300 K but the electron energies rise to more than about 500 K. Truby (1969) finds a rate coefficient at least twice that predicted from Birtwistle and Modinos' data at lower temperatures and, more important, a coefficient that increases with electron energy. The rate e a t about 300 K is not at all well coefficient for thermally equilibrated Br, established: Values have been reported of 0.82 x lo-', cm3/sec (Truby, 1971) and (1.0 f 0.9) x lo-" (Sides et al., 1976), both far less than the value for e I, at the same temperature. Moreover, the rate coefficient fore + Br, (300 K) only reaches 10- cm3/sec when the average electron energy is above about 0.5 eV (Trainor and Boness, 1978). One might expect the halogens to be more similar to one another. The fluorine case has been examined by several groups; the results are in qualitative agreement but certainly differ by factors of 2-4. Schneider and Brau (1978) obtain a rate coefficient of 7-8 x lo-' cm3/sec for e F, with electrons of 1 eV, whereas Chen et a/. (1977) obtain only 2 x lo-' under conditions supposedly about the same. The rate coefficient for e + F, does appear to be somewhat larger than those for the heavier halogens at electron energies of order 1 eV or less. This last observation is itself peculiar in light of the relative cross-section
+
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31
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
measurements made by Tam and Wong (1978). These experiments showed only one peak in oDAnear two electron energy for fluorine and three peaks for the other halogens, in the range of electron energies up to about 8 eV. Three peaks, at zero, 2.51, and 5.76 eV, were reported for C1, just previously (Kurepa and Belic, 1977). From arguments of symmetry, Tam and Wong assign the fluorine peak to a somewhat forbidden capture of an electron into the o,, orbital of a shape resonance, which should occur in the other halogens as well. However, this 2C,,+ resonance would, according to Tam and Wong, be masked by the next resonance, a 'lie core-excited or Feshbach resonance with configuration (o,np)2(n,np)4(n,np)3(a,np)2.This assignment attributes the next two peaks to core excited resonances built on excitation of an electron from the nu or IT,orbital. The puzzlement here is, of course, that it is hard to reconcile the assignments of Tam and Wong with the reported rate for thermal e F, + F- + F, 10-fold larger than those for the heavy halogens. The explanations remain to be found; one suggestion from the discussion of Tam and Wong is that the effects of impurities can be very large in systems containing halogens, considerably larger than is suspected by many investigators. Tam and Wong, by the way, report peaks near zero electron energy for all four halogens; Frost and McDowell(l960) had reported a peak for C1, with a maximum at about 2.5-2.6 eV, which probably corresponds to Tam and Wong's second peak. But, again, the results are puzzling because Tam and Wong say that the peak at zero energy for C1, is much stronger than those at higher energy. However, Frost and McDowell do report peaks at or near zero electron energy for bromine and iodine and find no peaks at higher energies. For iodine, Tam and Wong find the peak just above 2 eV stronger than that at zero ! One can only say that dissociative attachment by halogens is not well understood. The hydrogen halides are somewhat better understood, but the interpretation of some details of their dissociative attachment cross sections are controversial. The process was recognized and accepted at least from the work of Fox (1957);theearly work demonstrated that e HX gives H X-. C1 can also be produced from e HCl Later it was shown that H(Buchel'nikova, 1959). The cross sections for halide formation commence at electron energies of about 0.7 eV for HCl and about 0.2 eV for HBr (Ziesel et al., 1975),essentially at their thermodynamic thresholds. Hydride ions are only produced at somewhat higher electron energies, 6 eV and above (Azria et al., 1973).The process generating C1- has a cross section with one maximum and has generally been attributed to a single 'E+ compound state of HCI- (Fiquet-Fayard, 1974b).Steplike structure appears in the falloff of this cross section at energies above its maximum at about 0.85 eV (Abouaf and Teillet-Billy, 1977). In H F and HBr, the same sort of structure-one peak with steps on the high-energy side-is observed (Abouaf and Teillet-Billy,
+
+
+
+ +
32
R. STEPHEN BERRY AND SYDNEY LEACH
1980a,b).The cross section for formation of H- has two peaks, at about 7.0 and 9.5 eV (Azria et al., 1980), with structure on the high-energy side of the higher energy peak, between 9.5 and 11 eV. These peaks have been identified C1. Taylor er al. with 2X' and 211states of HCI- that dissociate to H(1977) invoked the three states just mentioned and three more states that dissociate to H + C1 + e, in order to rationalize the peaks and steps. FiquetFayard (1974b) supposed in a somewhat ad hoc but intuitively appealing argument that the cross section for dissociative attachment falls to a lower value above each point where the incoming electron has the energy to excite another vibrational state of the HC1 molecule. Nesbet (1977)invoked virtual states for each step, agreeing with Fiquet-Fayard's picture that the steps correspond to successivelyhigher channels of e + HCI (vibrationally excited). The interpretation as vibrational excitation is strongly supported by the study of Azria et al. (1980) of the intensity of zero-energy scattered electrons as a function of the energy of incident electrons. It is clear from this work that at every energy threshold for excitation of a new state of vibrationally excited HC1, one finds some scattered electrons that have given up essentially all their energy, presumably to vibrational excitation, and also a drop in the cross section for dissociative attachment. In short, vibrational excitation and dissociative attachment to H X- are competitive modes of decay of the 'Z' shape resonance of HX- available for very low-energy electrons. At higher energies the resonances are Feshbach resonances derived from configurations with two electrons in Rydberg orbitals around 217 ground states of the HX' ions (Spence and Noguchi, 1975). These states may decay to H + F- in the case of e H F (Abouaf and Teillet-Billy, 1980a), to HC1 with e + HCl (Azria et al., 1980), and presumably to H- + Br and to H- + I with HBr and HI, respectively. Figure 8 shows both the C1- current and the intensity of very low-energy scattered electrons, as functions of the energy of the incident electrons, as measured by Azria et al. It still remains to be seen why the cross sections have step shapes on their high-energy sides; Fiquet-Fayard showed that one assumption would account for them, and both Nesbet and Taylor et al. gave rational bases for this assumption but it still lacks real justification. The absolute cross sections and thermal rate coefficients for these processes show enormous variation from halogen to halogen. The peak cross section fore HCl -P H C1- is just under 0.2 x 10- cm2,at an electron energy of 0.78 eV, as shown in Fig. 8; the peak for HBr is 2.7 x 10- l 6 cm2 and occurs at 0.28 eV; the peak for HI is 2.3 x cm2 and appears at threshold (Christophorou et al., 1968). These cross sections are consistent with the rate coefficients for dissociative attachment of HC1, from 2 x to 1.2 x lo-'' cm3/sec, for flames with temperatures from 1725 to 2475 K
+
+
+
+
+
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
33
Electron energy IeV) FIG.8. The spectra of C1- ions (a) and very-low-energy ( 5 8 0 meV) electrons (b) formed by HCI + e +H + CI-. Both the electrons and the ions are those at a 90% scattering angle, and both are restricted to correspond only to C1- ions formed with kinetic energy between 0 and 20 meV. Taken, with permission, from &ria et al. (1980).
(Miller and Gould, 1978) and with a coefficient of approximately 8 x cm3/sec for HBr at 300 K and electrons with 0.6 eV of energy (Trainor and Boness, 1978). However, these data and the cross sections of Buchel'nikova (1959), Azria et af. (1973), and Christophorou et al. (1968) are not consistent with the 1000-fold larger cross sections for HCl implied by the rates of associative detachment found from flowing afterglow studies by Howard et al. (1974) and from flame studies by Burdett and Hayhurst (1977a,b). The discrepancies are smaller for HBr but are nevercheless still well over an order of magnitude. (The various results for HI seem consistent.) In terms of kinetic parameters of an Arrhenius form, with k = A exp( - A E / k q , the discrepancies lie in the preexponential; the activation energies AE are accepted to be equal to the endothermicities of the reactions. Burdett and Hayhurst ascribe the discrepancy to a rapid increase in oDAwith the vibrational state of HCI and HBr. In view of the difference between the findings of Miller and Gould and of Burdett and Hayhurst, it would seem worthwhile to measure oDAfor HCI carrying vibrational excitation, using a beam or swarm experiment. Three orders of magnitude ought not to go unquestioned.
34
R. STEPHEN BERRY A N D SYDNEY LEACH
Alkyl halides and nitrites appear to behave much like the hydrogen halides at low electron energies. One finds dissociative attachment cross sections with peaks in cDAas a function of electron energy corresponding presumably to resonant states (Stockdale et al., 1974). Some production of His observed with e + CH,CN at about 4 eV, but the peaks at lowest electron energies invariably correspond to halide or CN - production. The cross section at its peak is probably about 5 x lo-'' cmz for CH,Br. (Stockdale et al. also find that argon in a high Rydberg state can transfer an electron to CH,I to give CH, + I - with high efficiency.)The behavior of CH,N02 + e is not the same as that of the other methyl compounds; three-body processes play an important role for nitromethane, in contrast to the halomethanes or hydrogen halides.
8. Other Molecules
+
We have commented already on dissociative attachment of e 02.This molecule played an important role in the understanding and interpretation of apparent cross sections, and especially in the importance of the kinetic energy of the ions (Chantry and Schulz, 1967). Until this work, it had seemed that the energy dependence of the cross section for dissociative attachment was inconsistent with the electron affinity of the oxygen atom. Including the effect of the ion kinetic energy reconciled the observations very well. Dissociative attachment to N2 cannot, of course, give rise to N- + N unless the N- is in a metastable state. However, collisional dissociation of N2 by low-energy electrons has been interpreted as going through a transient N(4S) + N-(,P) channel, which in turn gives 2N(4S) + e (Mazeau et al., 1978; Spence and Burrow, 1979). Total cross sections for the process were measured by Spence and Burrow; the peak of 2.5 x 10- cm2 occurs just above the threshold energy of 9.8 eV, leaving electrons with energy of 0.07 eV. At higher electron energies, N - is formed in a higher state, either the 'D or 'S (Hiraoka et al., 1977). The dissociative attachment for triatomic molecules-H2S (Azria et al., 1979),H,O (Compton and Christophorou, 1967), and CO, (Schulz, 1962), for example-have been interpreted with some success by semiclassical statistical theories (Fiquet-Fayard et al., 1972; Goursaud et al., 1976, 1978). This brings the dissociative attachment process to about the same level of understanding as dissociativerecombination of triatomics. Dissociative attachment has also been studied for much larger molecules: SF, (Compton and Cooper, 1973; Astruc et al., 1979) and cyclic anhydrides (Cooper and Compton, 1972, 1973). The latter were shown to produce COz- in a metastable state. The interpretation of the decomposition of such molecules has been given in
'*
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
35
terms of a vibrational predissociation (Klots, 1976a,b)and related to similar processes for neutrals (Quack and Troe, 1974). Electrons attach to acetylene, C2H2,to give dissociative attachment if the electron energy is above 2.3 eV; the product ion in this case is C,H-, and the onset is a sharp function of electron energy. At higher electron energies, one sees H-and C,- ions (ca. 7 eV) and a broad C,- peak between 11 and 15 eV (Azria and Fiquet-Fayard, 1972). B. Collisional Detachment and Ionization
Here we shall look at those processes of the forms A A
+ B + A + B+ + e
+ 0- - + A + B + e
that occur at collision energies of a few electron volts or less, the conditions that may be met in a discharge, arc, or simple plasma. A very recent review of this subject has been given by Lacmann (1980). We shall not discuss ionization processes that rely on direct transfer of momentum from a heavy particle to a bound electron, the kind of ionization that occurs with collisions at energies of tens of kilovolts or more. Such collisions are studied, for example, in beam-foil collisions, but their interpretation is best given in terms of the Born or Bethe-Born approximation, which have been discussed elsewhere (Inokuti, 1971; Inokuti et al., 1978).However, this subject, long studied as it has been, has some important areas that were investigated only relatively recently. An example is the ionization of alkali atoms by fast neutral atoms (Kikiani et al., 1966). 1. Simple Collisional Ionization
Collisional ionization in low-energy collisions is closely related to associative ionization, a process discussed in Part I (Berry, 1980a). If the collision partners A + B are in their ground electronic states or in states that go adiabatically into the compound state AB' + e, then the only apparent difference between collisional ionization and this sort of associative ionization is in the final states of the nuclei: free in the former case, bound in the latter. The same relation holds for collisional detachment and associative detachment; in the former case, A + B- gives e and A + B in a continuum state, and in the latter, e and AB in a bound state. We shall make some comments later about the cases in which one or the other process is dominant or even exclusive.
36
R. STEPHEN BERRY A N D SYDNEY LEACH
Collisional ionization at relatively low energies is especiallyimportant for alkali and alkaline earth atoms, in flames, for example. Although such processes have apparently not been studied extensively by beam collisions, there is information available directly from flame studies. Rate coefficients for these reactions have been reported for all the alkalis with such collision partners as Ar, H,, N,, CO, CO,, and H20(Jensen and Padley, 1966; Hayhurst and Telford, 1972;Kelly and Padley, 1972;Hayhurst and Kittelson, 1974). The kinetics are generally expressed in terms of an Arrhenius form
k
= A exp(EA/kT)
(11)
where, in flames, EA, the activation energy, is approximately the ionization potential; in shocks, the activation energy seems to be the first excitation energy (Johnston and Kornegay, 1963). The crucial quantity to measure for these processes in flames is the preexponential factor A. Kelly and Padley x [T(K)/2000] found, for example, that with Na, A is about 5 x cm3/sec. Generally, the preexponentials are about 5 x lo-' cm3/sec at 2000 K. The corresponding cross sections are about lo-', cm2, ranging from about 0.5 x lo-', for Na + Ar to 3.5 x lo-', for Na + H20,according to Kelly and Padley. These are as much as a 1000-fold greater than the cross sectionsfor gas-kinetic or excitation collisionswith the same species. Bates (1976) criticized Kelly and Padley's method of estimating rates and cross sections for individual species from data taken for the complex mixtures found in flames. In the cases examined by Kelly and Padley, the numerical values of their preexponential factors are fortuitously close to the values estimated by Bates from his more complex expressions for data reduction. However, there are other cases cited by Bates for which the rate coefficients based on an overly simplified model are much too large. No theoretical analysis has yet been put forth to explain these very large preexponential factors, at least not at a microscopic level. 2. Collisional Ionization of Rydberg States and Other Special Cases Collisional ionization of atoms and molecules in high Rydberg states is rather akin to collisional ionization of alkali atoms in that the perturbation of the collision knocks a single, loosely bound electron into a continuum state from a bound orbital or even to an inelastic collision of an atom with a free electron (Fermi, 1936). The process also has much in common with the transfer or capture of an electron from a Rydberg orbital into a bound negative ion level, as was recognized by Stockdale et al. (1974)in their study of dissociativeattachment of methyl halides and other alkyl compounds. The collisional ionization of atoms in Rydberg states has obvious applications to the chemistry of the interstellar medium, where this process is very possibly a
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
37
competitor with radiation and collisional relaxation for the fate of neutrals freshly formed by collisional-radiative recombination. Collisional ionization of atoms and especially of molecules in Rydberg states became especially popular when isotope separation schemes were proposed that were based on Rydberg states (Ambartsumyan et al., 1975, for example).A very natural and intuitively useful way of studying the effects of collisions on molecules in Rydberg states was the determination by Person et al. (1976)of the ratio q of the number of ionizing collisions to the total number of all collisions. This study was done for acetone, methyl bromide, acetaldehyde-both CH,CHO and CD,CDO-and ethylene-d,, with several collision partners including helium, argon, and nitrogen. The values of q are higher for high Rydberg levels than for low levels, of course. In thermal collisions, with Rydberg states of only about 0.04 eV or less from the ionization limit, the values of q for C,H,O, C,D,O, and CH,Br were found to be about 0.9 or more. However, both the acetaldehydes gave less than 10% ions when they were excited to Rydberg levels about 0.07-0.08 eV below the ionization limit. It is puzzling that the fraction of collisions yielding ions from CH,Br was only 0.65 when the Rydberg state or states were only 0.03 eV from the limit, whereas q was almost unity for states 0.04 eV below the limit. The fractional yields of ions from acetone and especially from C2D4 were distinctly lower than those for CH,Br or acetaldehyde, except for the latter at 0.08 eV below the ionization potential. Collisional ionization cross sections of rare gas atoms in Rydberg states were measured for groups of states by Hotop and Niehaus (1968), in the energy range from 0.034 eV below the ionization limit up to only 0.001 eV below that limit. The cross sections were reported to rise from below lo-', cm2 for quantum numbers n c 25, to well over cm2 for quantum numbers of about 40. Cross sections for specific Rydberg states of xenon in collision with SF, were measured more recently by West et al. (1976); these are all about 1.2 x lo-" cm2, for f states with n from 25 to about 40. The comparison has been made of Rydberg state collisions of rare gas atoms with polyatomics and with atoms and diatomic molecules. It appears that the cross sections are larger for collisions with polyatomic neutrals than with diatomics or atoms (Sugiura and Arakawa, 1970; Latimer, 1977). A simple and elegant process that falls between Penning and collisional ionization was demonstrated by Arrathoon et al. (1973)and recently studied in detail by Hultzsch et al. (1979).Consider the collision of He+ with Ba. The energy released by capture of a free electron to form He in its ground state is greater than the sum of the first two ionization potentials of Ba. Hence, the process He' + Ba -+ He + Ba2+ + e can occur in arbitrarily slow collisions. Hultzsch et al. found that He+ + Ba goes through a step in which (Ba+)*is formed by electron transfer, probably of a 5p electron, from Ba to
R. STEPHEN BERRY AND SYDNEY LEACH
38
Hef, and then through an autoionizing transition. The cross sections for both He+ and Na' are decomposable according to the energies of the released electrons. These electrons exhibit sharp energy spectra like photoelectron spectra; Hultzsch et al. were able to measure some cross sections for production of electrons of specific energies, as functions of collision energy. The first prominent electron group, with 8.27 eV of energy, corresponds to a cm2 at a collision process with a peak cross section of about 17 x energy of about zero. Hultzsch et al. interpret this as corresponding to a compound state of (HeBa)' that dips just as low as the energy of the He Ba' * asymptote from which Bat * autoionizes to give the 8.27 eV electrons. The next prominent electron group has an energy of 9.50 eV; its peak cross section of 2.8 x 10- cm2 occurs at a collision energy of 20.5 eV. With He+ on Ca, the system undergoes its autoionization during the lifetime of the (HeCa)' compound state. No estimate was made of the absolute cross Ba. section for this system; it is not as large by any means as that for Hef
+
'
+
3. Collisional and Associative Detachment
Turning now to collisional detachment, A-+ M+A+ M + e
we note the similarity of this process in low-energy collisions to associative detachment, A
+ M +AM + e
(13) which was introduced in Part I (Berry, 1980a) in parallel with associative ionization. The topic was recently reviewed by Fehsenfeld (1975). In the present context we are particularly interested in associative detachment when the colliding species are in their ground states. These processes are the inverses of dissociative attachment and the three-body attachment process M,the colliding that begins with A and M free. To form A M and e from Apair of heavy particles must reach a spatial region and an energy where A M is stable, and the electron must escape before A- and M can separate. The rates for such processes are, in some instances, quite large: about 1.6 x cm3/sec for F- H and 9.6 x lo-'' for C1H, but the nonobservation H, at 300 K of HI e implied a coefficient less than 6 x lo-" for I (Fehsenfeld, 1975).A major reason for such large rate coefficients is the high escape velocity of the electron; Christophorou et a!. (1968) give a total lifesec for HCI-, roughly equally divided between time of about 1.8 x the two decay channels of H C1- and HCl e. The rate coefficients for H + H - +H2 e is about cm3/sec for collision energies up to about 1 eV, and rises possibly almost an order of magnitude over the next decade of energies (Fehsenfeld, 1975), according to
+
+
+
+
+
+
+
+
39
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
calculations from theory. Bieniek and Dalgarno (1979) have elaborated earlier theoretical models to estimate the cross sections for specific vibrational and rotational states of H,. The very important reason for such an elaboration is the possibility that associative detachment of H- + H may be responsible for production of the vibrationally excited hydrogen molecules observed in space (for example, Gautier et al., 1976; Beckwith et al., 1978). Indeed, the cross sections for production of H, with u = 3-6 and J between 7 and 15 are almost as high as 5 x cm2 for collisions at 0.0129 eV in some instances. At an energy 10-fold larger, the cross sections are only about 0.5 x 10- l 6 cm2 at maximum. The total computed cross sections at the two energies are 85.5 x 10- l6 cm2 and 22.0 x 10- l 6 cm2 at the two energies. The computed thermal rate coefficientof 1.89 x cm3/sec is consonant with the experimental value from flowing afterglow studies (Schmeltekopf et al., 1967). While vibrationally excited H, from associative detachment has not been observed in the laboratory, the distribution of vibrational states of HCl from H C1- has been determined very recently (Zwier et al., 1980). From the infrared chemiluminescence of a flowing afterglow, Zwier et al. found that the ratio of HCI (u = 2) to HCI ( u = 1) is 0.60 _+ 0.03; the temperature of 296 K is too low for production of significant amounts of the states with u 2 3. The authors infer by comparison with the displacement reaction C1- + HI that very little HCl (u = 0) is generated by associative detachment. These results are very much in accord with results of Allan and Wong (1981) that the cross section for the reverse process increases 10-fold with each additional vibrational quantum of the HCI. Comer and Schulz (1974), in their study of detachment cross sections from 0 - and S - on a variety of molecular gases, found it useful to distinB, the guish three cases, as follows. In the first class of reactions, with Across section is zero from a collision energy of zero up to an energy equal to the electron affinity of A, above which the products are A B e. In the second class, the cross sections are large for low-energy collisions because AB + e may be formed exothermically. The third class contains those molecules having small but nonzero cross sections at energies below that of the onset of direct collisional detachment. The process 0- 0, falls in the first class; the threshold for this process is 1.465 eV and the cross sections rise from zero at this energy to 10- l 6 cm2 at 5 eV. By contrast, 0 - + CO and 0- H, are in Comer and Schulz’ class 2, with cross sections above 8 x 10- l6 cm2 at zero energy, but dropping to about 10- cm2 at 3-4 eV. The N, molecule with 0 - is examined carefully by these authors; the cross section does rise at collision energies below 2 eV but only from about 0.1 x 10- l6 cm2 to about 0.62 x 10- l 6 at 0.32 eV. The implication is that this system falls in class 3. The O--N, system is compli-
+
+
+ +
+
+
40
R. STEPHEN BERRY AND SYDNEY LEACH
+
cated by the existence of the stable N,O molecule; the potential of NZ(’X) 0- is estimated to cross that of N 2 0 + e over a range of energies ranging from about half a volt above the minimum for the N - 0 stretch for linear N 2 0 to close to that minimum for a bent molecule. In other words, the probability of reaction is very sensitive to the 0 - N z orientation. The results for 0-+ NO at very low energies are controversial; Comer and Schulz find a cross section below 10- l 6 eV for collision energies less than 3 eV, but they cite much higher cross sections reported by others. Hydrocarbons such as C,H2 and CzH4 seem to be in their class 2; CH4 is put in the same class, but the data they give suggest that it may be as appropriately put in class 3. The results of Comer and Schulz are neatly consistent with Risley’s (1977) results for “collisional” detachment of H- on N, . Here, one sees oscillations in the energy spectra of the ejected electron, which Risley interprets as due to Franck-Condon oscillations associated with the formation of a transient N,- molecule. The Nz- then decays into N, + e. The transient N z - is presumably the same as the resonant state seen in a variety of electronscattering experiments,at electron energies between 1 and 4 eV. This process of electron transfer followed by electron loss is almost, but not quite, simple, direct collisional detachment.
4. “Simple” Dissociative Attachment Simple, direct collisional detachment occurs for negative ions colliding with rare-gas atoms. Such processes do have significant cross sections at low collision energies; both the cross sections and rate coefficients have been studied by a number of techniques. Remarkably, there is a very significant unresolved discrepancy regarding the collisional detachment of the halogen atoms; we shall discuss this problem below. Collisional detachment from H- by He at low energies was measured by Bailey e f al. (1957), for H - on 0,by Bailey and Mahadevan (1970), and for H- and D- in beam collisions with He, Ne, Ar, and N, (Champion et al., 1976).The process was modeled theoretically by Lam et al. (1974) who used the formalism of a complex potential to account for passage of the system out of the initial (H- + atom) channel. Collisional detachment of H- by He in the range 0-3 keV was studied theoretically by Herzenberg and Ojha (1979); readers interested in that higher energy range can use this work as a recent entry into the literature. Born-Oppenheimer breakdown dominates the process at low energies. The cross sections for H- and D- on He have flat maxima of approximately 3.4 x cm2 for collision energies of about 15 eV and above (Champion et al., 1976). The detachment cross sections are about half that
41
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
large at 3 eV, and the data are consistent with a threshold at the electron affinity of hydrogen, 0.74 eV. The cross section is slightly larger for D- than for H-, as one would expect from the relative velocities. The detachment cross sections of H- and D- with neon are somewhat smaller than-roughly half of-those with helium (Champion et al., 1976),which may be associated with the small elastic scattering cross section for electrons on neon at low energies. Champion et al. point out the puzzling observation that at each energy, the low-energy detachment cross section for D - on neon is smaller, not larger, than that for H - on neon, despite the longer interaction time for D - . This order holds also for D- and H - on argon, but the cross sections are larger than for neon, more like those with helium. With N,, H- and D- have large cross sections, about 6 x 10- cm2 for collisions of 10-50 eV, and the cross sections of the two isotopes are about the same. Collisional detachment cross sections from 0 - ,OH-, and 0, - have been measured with 0, (Bailey and Mahadevan, 1970) and with rare-gas atoms (Wynn et al., 1970). In the latter case, it was possible to study some cross sections in the very important threshold region-important, because the rates of such processes under thermal conditions depend on the product of the fast-rising cross section and the rapidly falling Boltzmann distribution of relative speeds of the collision partners. Like H - , the cross sections for these cm2 at collision energies of 15 eV or other species are of order 4 x more. The threshold behavior, especially for 0- and He, shows clearly that the cross section ud is directly proportional to the square of the relative energy, and that the detachment process first occurs at the thermodynamic electron affinity of the oxygen atom or the OH radical; the threshold behavior for 0,- has the same dependence, but the threshold seems to be at a collision energy of zero. The collisional detachment cross sections of halide ions with rare gases were studied by Bydin and Dukel’skii (1957) and by Fayeton et al. (1978)at high energies, and at thermal energies by Champion and Doverspike and their collaborators (Champion and Doverspike, 1976a,b; B. T. Smith et al., 1978). The thermal rate coefficients for these processes have been determined by measurements in shock waves (Mandl et al., 1970; Mandl, 1971, 1973, 1976a,b, 1978; Luther et al., 1972; Milstein, 1972; Berry, 1980b). The shock-tube studies are consistent with one another, giving rate coefficients of order 3 x cm3/sec (even for I - ) at temperatures of 5000-5500 K down to about cm3/sec for temperatures near 3500 K. The beam experiments, on the other hand, have yielded cross sections having thresholds at energies roughly double the electron affinities of the halogen atoms, and staying in the range of 10- l 6 cm3 for energies up to 20 eV. The upper limits to the rate coefficients implied by the measurements of B. T. Smith et al. (1978)are, for example, 0.14 x cm3/sec for C1Ar at
’
+
42
R. STEPHEN BERRY AND SYDNEY LEACH
+
4000 K and 1.1 x cm3/sec at 5000 K. For BrAr, the rate coeffiand 0.23 x In cients at the same temperatures are 0.027 x short, the beam measurements yield values far too low to be consistent with the shock-tube studies. Moreover, the low cross sections and the energy thresholds at about twice the electron affinity of the halogens are observed with C1- and Br- on H,, D,, O,, N,, CO, CO,, and CH4 (Doverspike et al., 1980). It is tempting to suppose that the shock-tube systems have additional processes contributing to the detachment rate, possibly X- + e-+ X + 2e or X- M' X M (Berry, 1980b), but there is no positive evidence yet for such a process. On the other hand, it is puzzling that the thresholds for collisional detachment from halide ions are so much higher than for H - , 0 - , or OH-. This remains one of the striking unresolved anomalies of elementary attachment and detachment processes.
+
-+
+
C . Ion-Pair Formation Charge formation and removal processes need not involve free electrons. The negative charge carriers may be heavy particles, particularly if they originate with very electronegative neutrals, such as halogen atoms. The ion-pair formation processes A B A + B- and AB M A+ BM are known to occur and in some circumstances, even dominate the charge formation process. In the latter process, we would usually understand M to mean a heavy particle that produces collisional dissociation of AB to ions. However M may also be a photon or an electron. And the state formed by absorption of the quantum of excitation may be either a state that dissociates directly to A + B-, or a state with a measurable lifetime as an AB* complex; this situation approaches a half-collision parallel to the A B+A+ B- process. Photoproduction of ion pairs is discussed later in the context of photoionization. High-energy collision processes producing ion pairs were the titular topic of a review by Baede (1975).This discussion also summarizes a very large part of the theoretical work on the subject through about 1972. More recently, Los and Kleyn (1978) reviewed the literature on ion-pair formation emphasizing processes involving alkali atoms with halogen atoms and molecules. Berry (1980b) has discussed alkali-halogen and related ion-ion neutralization processes with particular attention to the chemical kinetic phenomena in gases in which these neutralizations are important. The theory of ion-pair production has been essentially a theory of the crossing of potential curves or surfaces. In its simplest form, exemplified by an alkali atom and a halogen atom, the system is somehow produced in an initial state-by excitation of a neutral molecule, by collision of neutral atoms, typically-energetic enough that the atom pair may become an ion
+
+
+
+
+
-+
+
+
-+
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
43
pair and dissociate. Figure 9 shows the lowest relevant potential curves of Na I. This system illustrates several of the elementary processes of ion-pair production. If these two neutral atoms collide with kinetic energy greater than Q (the difference of the ionization potential of Na and the electron affinity of I), it is possible, in principle, for an electron to jump from the Na atom to the I atom to form Na’ and I-.
+
1. Collision-Induced Dissociation to Ion Pairs
If a molecule of NaI is struck by one or more argon atoms and is thereby given enough energy to dissociate to atoms in their ground states, no ionpair production can occur. However, it may be that no dissociation occurs either. This would happen if the ionic NaI molecule could not successfully transform itself by electron transfer into two neutral atoms when the distance R between the Na nucleus and the I nucleus is large. The molecule would persist as two ions at all the internuclear distances accessible to it and would stay bound by the attractive Coulomb force at energies as high, perhaps, as I - . However, if the system were to receive the limit of dissociation of Na’
+
Na’(’SltI-(’S)
.-0
t
-
-
Re-1
Re
%+1
R,+2
Re+3 Re+4 R,t5
Internuclear Distance ( A )
FIG.9. The lowest potential curves for the NaI molecule. The region around R = R, is taken from Davidovits and Broadhead (1967); for NaI, Re = 2.71 I A. The region around the crossing distance of 6.9296 is based on an estimated splitting of 0.11 eV at the crossing distance (Grice and Herschbach, 1974). While only one potential curve is drawn from each dissociation limit, there are, of course, several; for &le there are two states with total axial angular momentum R = 1, one with R = 2 (all doubly degenerate), and two with R = 0, all from the limit Na(2S) + l(2P3,2).
44
R . STEPHEN BERRY AND SYDNEY LEACH
energy exceeding the dissociation limit of two ions, then one would expect to see such ion pairs. Experiments with shock-heated alkali halide molecules (Berry et al., 1968; for reviews see Mandl, 1978; Berry, 1980b)and with beams of rare gases colliding with alkali halide molecules (Tully et a!., 1971) give direct evidence for production of alkali-halogen ion pairs by dissociative collisions with heavy particles: MX+A-cMC
+ X- + A
(14)
The thallium halides exhibit similar behavior (Parks et al., 1973a,b, 1977). The process may occur by a succession of collisions, as in the heating behind a shock, or by a single collision with sufficient energy. In these cases, dissociation occurs to ions rather than to atoms because the molecules, which are ionic in their ground electronic states, cannot make the electron transfer that would be required if the electrons behaved entirely adiabatically. Adiabatic behavior would require the dissociating atoms to change from being an ion pair, as they are at distances near the molecular equilibrium point Re, to being an atom pair, as they are at very long distances, if the system is in its electronic ground state, The NaI molecule whose potential curves are shown in Fig. 9 is actually a borderline case; a molecule such as CsCl dissociates overwhelmingly to ions (Sheen et al., 1978) and a molecule with a light alkali, especially Li, dissociates primarily to atoms, following the adiabatic course (Ewing et al., 1971; Berry, 1978, 1980b). The reason, in simple terms, is that the region in which the transition between ionic and atomic charge distributions must occur, namely the region of internuclear separation around R , , where the ionic curve crosses the curve for atoms in their ground states, is at relatively small internuclear distances for Li salts and is a relatively broad region. For salts of Cs and Rb, this transition region occurs at very large internuclear distances and is very narrow indeed. The consequence is that the salts of Cs and Rb could only exhibit adiabatic behavior if the nuclei were moving exceedingly slowly as they pass through the transition region: for the lithium salts, the nuclei may have quite high kinetic energy, even several eV, and the electrons still behave adiabatically. It is important to note that Kr or Xe CsCl yield the alkali-rare gas molecule ion as the principal positive species for the 2-3 eV range from threshold upward (Sheen et al., 1978). Ion-pair production by collision with a third body, M AB M A + + B- has also been studied in flames (Burdett and Hayhurst, 1977a,b, 1979).The rate coefficientsderived this way are in reasonably good agreement with those from shock-tube studies. However, the values one obtains from the Arrhenius parameters inferred by Burdett and Hayhurst correspond to considerably larger variations of the rate coefficients with temperature than
+
+
--+
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
+
45
are found in the shock-tube studies. For example, for RbBr M, the flame results give a coefficientof 1.44 x 10- l 7 cm3/sec at 2000 K and 3.83 x 10- l 4 at 3000 K ;the shock-tube results are 1.25 x 10- at 2860 K and 3.6 x 10at 4570 K. The implication is that the effective activation energy is not constant, but depends somewhat on the temperature, presumably the vibrational temperature of the diatomic AB. To achieve ion-pair production by collisional dissociation, the molecule to be dissociated must receive adequate energy. In the processes discussed thus far, this energy comes from the kinetic energy of relative motion of the colliding pair, and, if any is available, from vibrational energy in the diatomic. The experiments of Tully et al. (1971)indicate that the cross sections for such processes may be greater than 10 A2, within about 1 eV above theshold, for a single impact. Hence, direct dissociation to ions by collision is not an unimportant process in gases containing alkali halides, at temperatures in the range lo4 K. Other quantitative information regarding the absolute rates of reactions AB M + A + B- + M comes from the shock-tube experiments of Milstein, Weber, and Berry (Mandl, 1978; Berry, 1980b), and the flame studies of Burdett and Hayhurst (1977b, 1979). The latter give rates for the chlorides, bromides, and iodides of the alkalis and Ga, In, and TI. Their data have been reduced to Arrhenius parameters [ k = A T - 3 . 5exp( - A H / R T ) , in terms of A and AH] (Burdett and Hayhurst, 1979).The rate coefficients for the temperature range of their experiments are typically in the range of cm3/sec for RbCl and cm3/sec for NaCl at 2000 to 2 x about CrCl at 3000. Even if one neglects any decrease in the effective activation energy H with temperature, the rates predicted for 8000 K are of order 10- lo, consistent with the estimates of Tully et al. One might expect rates of these processes to increase faster with temperature than a simple exponential form would predict, because of vibrational excitation of the target molecule. The experiments of Tully et al. (1971) show that the process near threshold is very much more probable if the molecular target contains vibrational energy. The probability of dissociation was unfolded from the dependence of the relative cross section on the vibrational energy of the alkali halide; the result is a steeply rising function, as shown in Fig. 10. From this behavior, we know that vibrational as well as translational energy can contribute to collisional dissociation to ion pairs. If we generalize from this example, we are led to suppose that vibrational energy is the more effective, by a considerable margin. Energy to produce ion pairs by dissociative collisions need not come only from translation or vibration. Bush et a!. (1972) showed that this energy could also be supplied by electronic excitation in a collision partner: helium metastables striking 0, molecules at thermal energies have energy
+
+
46
R. STEPHEN BERRY AND SYDNEY LEACH
Vibration01 Quantum Number
CsBr Internal Energy ( kcol/mole) FIG. 10. The probability of dissociation of CsBr in collision with Ar, as a function d ( n ) vibrational quantum number (with effects of rotation disregarded), and as a function a&,) of internal energy. The dashed curve uHsis the cross section for a hard-sphere endothermic process.
above the 17.3-eV threshold and produce 0' + 0-ion pairs. However, this process is far less likely than the competitive Penning ionization that generates O,+ + e. Electrons can also induce ion-pair formation. The process operates by electronic excitation of the target molecule to a state dissociating to ions, with more energy in the relative motion of the nuclei than the dissociation energy D,*of the excited molecule. The simplest example, H, + e + H f
+ H- + e
(15) was probably observed first by Lozier (1930),but was shown definitively by Khvostenko and Dukel'skii (1958). Its threshold is at 17.3 eV, and the cross section for the process is very small, about cm2 at its maximum. This cross section is displayed in the subsequent discussion of photoionization. The excited state is difficult to identify; in H,, the ionic character passes from one adiabatic excited state to another as the H-H internuclear distance varies (Davidson, 1960; Kolos and Wolneiwicz, 1969; Glover and Weinhold, 1977). Other molecules show the same sort of behavior, especially in the narrow energy band bound between the threshold for ion-pair production A + B- and the threshold for the production of A + B + e. For example, besides H,, CO, NO, and O2 were studied by Locht and Momigny (1971),
+
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
47
who were able to interpret structure in the energy dependence of ion-pair production curves in terms of thresholds for specific processes and molecular states. In general, these processes all seem to have small cross sections.
2. Photoproduction of Ion Pairs Intermediate between collisional dissociation to ion pairs and ion-pair production by two-body collisions, A B + A + B-, is photodissociation to ions. This phenomenon occurs frequently as one of two or more competitive decay channels available to a photoexcited molecule. This aspect is discussed later (Section II,E,4,a). One might expect light alkali halides such as LiI to dissociate to ions if they absorb photons of sufficient energy. However, the evidence for ion-pair production by photoabsorption comes primarily from other species more easily studied. Klewer et al. (1977) used two-photon absorption to provide Cs-. The experiments were enough energy for Cs, to dissociate to Cs' carried out with photon energies below the threshold for formation of Cs+ + Cs e. However, formation of Cs2+ is a process competitive with ion-pair formation. In this particular case, ion-pair production is typically two orders of magnitude less probable between the threshold photon energy of 1.95 eV and about 2 eV; above about 2.1 eV, ion-pair production has the higher probability. (The energy absorbed as light is of course twice the energy per photon.) At higher energies, 17-30 eV, photodissociation of O,, NO, and CO to ion pairs has been observed by Oertel et al. (1980; see also Section II,E,4).
+
+
+
+
3. Ion-Pair Production by Charge-Exchange Collisions The simplest two-body processes yielding ion pairs by charge exchange are probably those of hydrogen atoms with rare-gas atoms. Although such processes are too endothermic to be very important in the range of conditions of our primary concern, they nevertheless so basic that some new, definite information about them deserves mention here. Total (i.e., integral cross sections have been measured by Aberle et al. (1980) for collisions of H and D on Ar, Kr, and Xe, to produce H - or D- and the positive rare-gas ion. The cross sections rise slowly from the threshold energies for about 1 eV and then more rapidly. They show large-amplitude variations; that for H + Ar has cm2 at about maxima of about 0.5 x 10- cm2 at about 21 eV, 2 x 20 eV, and 4 x cm2 at about 70 eV. The other combinations have different oscillatory patterns, but their cross sections have maxima of the same order. Aberle et al. interpreted the variations in cross sections with energy as due to interference between the ionic channel and two neutral states, the initial state and one or more Rydberg states whose potential curves lie close to and parallel with that of the ionic state.
48
R . STEPHEN BERRY AND SYDNEY LEACH
At thermal energies, ion-pair production by collision of an electron donor and an electron acceptor is probably of greater general interest than the processes requiring collisions with external sources of energy, such as fast Ar, He* metastables, electrons, or photons. It is useful to distinguish simple electron transfers, as in K + I + K + + I-, from rearrangements, such as N, + CO + NO' + CN-. Again, we refer to Baede (1975) for an extensive review of both theoretical and experimental work on these topics, particularly involving alkali atoms as donors. Experimental studies of these processes began with the simpler systems of alkali atoms colliding with molecules, even in the 1930s and 1940s (Polanyi, 1932; Magee, 1940), but the more elementary atom-atom processes were only studied definitively by the use of colliding beams, beginning in about 1970 with the study of Cs + 0 - Cs' + 0- by Woodward (1970). The cross section for production of Cs' in the energy range 200-1800 eV was found to be about cm2, about an order of magnitude less than that given by the Landau-Zener-Stueckelberg calculations of van den Bos (1970, 1972). The alkali-halogen atom process is simpler still. The energy dependence of the relative cross sections for Li, Na, and K on I to give I - were obtained by Moutinho et al. (1971a, 1974)in the kinetic energy range from threshold to a few eV above threshold. Delvigne and Los succeeded in measuring absolute differential cross sections for Na + I + Na' + I- in the energy range from 13 to 55 eV. Experimental values for the total cross sections for these processes have not been reported, but Los and Kleyn (1978),comparing experimental and theretical results, put them in the range of 5-10% of the geometric cross section based on the internuclear distance of the crossing point of ionic and ground-state atomic potential curves, in the range a few electron volts above threshold. The differential cross section offers a rich subject for study because it shows oscillations due both to the rainbow and Stueckelberg contributions. The rainbow effect arises from the interference of different deflection processes from a single potential that contains both attractive and repulsive parts. The Stueckelberg oscillations are due to interferences between two different potentials inside the crossing radius. The results of Delvigne and Los (1973) were not in good agreement with the theoretical values for large impact parameters, but were quite satisfactory for close collisions. More recent calculations of Li and Na on I, particularly by Faist et al. (1975)and by Faist and Levine (1976)based on close coupling, are more consistent with the experimental results both for differential and total cross sections (Los and Kleyn, 1978).The results support the idea that the Landau-Zener-Stueckelberg picture-the transition probability is dominated by what occurs at a curve crossing, but is modified a bit by what happens elsewhere-is satisfactory for these systems. The Stueckelberg
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
49
semiclassical form, which retains phase information lost in the LandauZener expression, seems to be quite adequate to represent the stringent tests of the total and especially the differential cross sections for K + I, down almost to the threshold energy (Andresen and Kuppermann, 1978; Andresen et al., 1978).We shall see that this is not the case for all A + B = A + + Bprocesses. The flame studies of Burdett and Hayhurst (1977a,b, 1979)provide quantitative values of rates of ion-pair formation. Burdett and Hayhurst measured values of these rates for the alkali, gallium, indium, and thallium salts of C1, Br, and I, at temperatures from 1820 to 2590 K and fit the rates to Arrhenius and parameters. Typically, these rates fall between about cm3/sec. They are not in very close agreement with the values one would naively infer from the shock-tube studies of Milstein, Weber, and Berry (Mandl, 1978; Berry, 1980b) if one assumes that the rates can be computed from the relation (forward rate)/(backward rate) = equilibrium constant. However, the flame results are intended to be the values of the true two-body process, whereas the shock-tube studies give only the effective two-body rate coefficients t h e n the pressure of the third body is effectively constant. By combining Burdett and Hayhurst’s two- and three-body contributions, one obtains results reasonably compatible with the shock-tube results. Collisions of alkali atoms with molecules comprise by far the largest part of the data for ion-pair production processes. The alkali atom-halogen molecule pairs were among the first to be studied by neutral-neutral beam collision processes. The cross sections are of order 100 A’; the processes occurring at lowest energies include both electron transfer and M + X, -+ MX + X. Los and Kleyn (1978) point out how, up to relative velocities of order 3 x lo3 m/sec, K + Br, goes to KBr + Br, but almost entirely to ion-pair formation for the next decade of velocities. The negative ions formed in this process are about 50% Br- at the lower velocities and about 30% Br- at higher velocities (Hubers, 1976; Hubers et d.,1976). The interpretation of the process is summarized by Los and Kleyn, based largely on the interpretations given by the Amsterdam group. The probability of electron transfer from alkali to halogen molecule is large when the initial collision occurs, because the crossing of the ionic M + + X,- and neutral M + X2curves occurs at a small interparticle distance. This in turn is due to the rather small electron affinity of X2 when the X-X distance is near its equilibrium value. However, X2-formed at such a distance is produced at a point high on the inner repulsive part of the potential curve for Xz-,so the two X’s move apart rapidly in the newly born XI- molecule. If the collision occurs in a time longer than the X2- vibration, or at an energy too low to form M + + X,- or M + + X- + X, then MX + X are produced, presumably with MX in a high vibrational state. If the collision happens in a
50
R. STEPHEN BERRY A N D SYDNEY LEACH
time comparable to or somewhat shorter than an X2- vibration, the M + ion leaves before X- can tag along, and the residue appears as X,- or X- + X, depending on the distribution of energy between relative motion of the halogens and the other degrees of freedom. At still higher energies, the process becomes sudden with respect to nuclear motions, and is dominated by the Landau-Zener picture with Franck-Condon factors determining the branching between X,- and X- X fragments. The alkali atom-oxygen molecule system is one of those studied most extensively with regard to ion-pair formation. One of the most recent efforts (Kleyn et al., 1978), which summarizes most of the previous work (see also Baede, 1975) verifies that 0 2 -is the dominant product and that a LandauZener picture suffices to describe the system approximately. A minor product is 0 - ,above the threshold for formation of 0 + 0 - , typically about 9 eV (Moutinho et al., 1971b). The cross sections for 0 2 -production rise from their thresholds of about 1 eV (Cs + 0,) about 5 A2 at 5-10 eV above threshold and then to as much as about 8 A2 at their maxima, in the range of 100 eV. The cross sections should show oscillations with collision duration, according to the analysis of these authors. Ion-pair production in collisions of still more complex species is becoming a subject of quite broad interest. Some simple examples are: the collisions of Li, Na, or K with halomethanes to give alkali positive ions and CH,X- ions (Moutinho er al., 1974); collisions of the same alkalis with SF, to give alkali positives and either SF,- (near threshold) or (SF,- + F) and (SF, + F-) at slightly higher energies (Hubers and Los, 1975);collisions of K and Cs with UF, and WF, to give UF,- and WF,- (Stockdale et al., 1979);and collisions of K and Cs with H,O (Warmack et a/., 1978).These processes all presumably involve a first step of simple electron transfer which, in the last two examples, is followed by dissociation. In the case of SF,-, this dissociation may occur only for collision energies of about 0.5 eV or more, depending on the alkali. Values of0.43 eV (Fehsenfeld,1970)and 0.51 eV (Hubers and Los, 1975)have been reported for K + SF,, for example. Formation of F- occurs only above a second, much higher threshold, 8.2 eV for Na + SF, and 7.30 eV for K SF,, according to Hubers and Los. Like the M AB process (Tully et al., 1971) and the M + I2 process (Aten et al., 1977),the M + SF, process has been examined to determine the sensitivity of the reaction probability both to variation in translational energy and to internal vibrational energy. The M-SF, system shows a complete equipartition of vibrational and translational energy for collisions with very low kinetic energy. However, for SF,- formed above threshold and for SF,- (or F - ) formed more than 2 eV above their thresholds, vibrational energy in the SF, is more effective in enhancing the reaction rate than is energy in translation. This is easily reconciled in terms of “vertical” or rapid transitions from SF, to SF,-.
+
+
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
51
The experiments with K or Cs and H 2 0 showed only OH- and, with D20, OD- negative ions. The H 2 0 - molecule is unstable. The ground 'A, state dissociates to H + OH-, whereas higher states dissociate to 0 - + H, and to H- + OH. The results, based on angular distributions of both positive and negative ions, show that electron transfer to form the ground state of H 2 0 - is the only important process for ion-pair formation. The lowest energy of the collisions in these studies was 10 eV; one can be reasonably sure that only H,O- in the 'A, ground state would be produced in collisions at lower energies. The most complex ion-pair formation processes involving alkali atoms are probably those studied by the Oak Ridge group (Compton et al., 1974; Stockdale et al., 1974; Cooper et al., 1975). Collisions of cesium atoms with maleic anhydride
succinic anhydride,
and parabenzoquinone,
yield ion pairs. With maleic anhydride, the principal negative ion from collisionswith energies between 2.5 and about 7 eV is the primary C4H203ion. At higher collision energies, both C2H2C02-and metastable C 0 2 - are as important in the negative ion mass spectrum. No parent negative ion was found from succinic anhydride, but CdH203- and C4H303- (unresolved) were formed at energies of collision from the threshold of 3 eV upward; at 5 eV, C,H,CO,- appeared and dominated the spectrum above about 5.5 eV. The negative ion spectrum of Cs + p-benzoquinone gave a parent ion at energies from 2 eV upward and very small quantities of fragment ions from about 3 eV and beyond. Even species as unstable as anhydrides (which give up C 0 2 under many conditions) can, it appears, become stable negative ions under gentle electron transfer conditions.
52
R. STEPHEN BERRY AND SYDNEY LEACH
4. Charge Exchange with Excited Species Collisions of excited donors with acceptors can provide energy to make otherwise highly improbable ion-pair formation take place. Rare-gas atoms in high Rydberg states react with I, to form 1,- (Gillen et al., 1978), with SF, to form SF,- (Hotop and Niehaus, 1967), and with CH,CN to form CH,CN- (Sugiura and Arakawa, 1970; Stockdale et al., 1974). In these cm2, examples, the cross section for ion-pair formation is about 1.7 x a figure presumably due more to the size of the average orbit of the Rydberg state than to any characteristic of the acceptor. More recently, Dimicoli and Botter (1980, 1981a,b) have carried out further studies of ion-pair formation with argon and xenon atoms in high Rydberg states and with acceptors CCI,, CCl,F, CH31, SF,, and C6F6 from hypersonic nozzle beam sources, so that the acceptors have low internal temperatures. Detection was done by time-of-flight coincidence mass spectrometry. Both CCl, and CC1,F give dissociative attachment, forming C1- and CCl, and CCl,F, respectively. Hence, these are examples of reactive ion-pair formation processes. However, they probably occur in two steps, an elementary electron transfer step to form Ar' + CC1,- or CCl,F, followed by dissociation of the negative ion molecule. With SF,, in slow collision one finds SF,- as the sole product, but at higher energies one obtains SF,- + F, as with alkali atoms as donors. Such processes as these may provide significant channels for the capture of electrons at the boundaries of hot plasmas, in which rare-gas ions are present in high concentration within a hot zone, but electron acceptors are available from cooler regions. With argon (in a high Rydberg level) as a collision partner, the cross sections for ion-pair formation are in the range cm2, one to two orders of magnitude larger than for detachment collisions as with Ar* + CO or CF,. Matsuzawa (1972) and Flannery (1973) have given theoretical descriptions of this process in which the electron in a Rydberg state is considered to be slow moving and nearly free. While the model predicts the correct magnitude of the cross section, it fails to predict the observed rise in cross section with relative velocity exhibited at velocities below about 7 x lo4 cm/sec and quantum numbers n s 28. The theories do not include any allowance for near-resonant behavior that is known from ion-ion neutralization studies (see later) to heighten transition probabilities considerably. A more refined theory will presumably show some kind of maximum for just this reason.
5 . Ion-Pair Formation in Reactive (Rearrangement) Collisions The alkali-oxygen system mentioned previously (Kleyn et al., 1978)is one of the simplest that may bring us to examine truly reactive ion-pair formation. That is, with Na + O,, for example, a rearrangement might, in principle, In fact, such a process was occur of the type Na O2-+NaO+ 0-.
+
+
53
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
observed by Rol and Entemann (1968) and by Neynaber et al. (1969). The cm2 cross section for this particular process is not large, about 4 x according to Neynaber et al., at a collision energy of 7 eV, 1.5 eV above the threshold energy. In this case, the reaction is probably the result of interactions in a compound state NaO,, and not of final-state interactions as CCl, from CCl,-. was the formation of C1Fite and Irving (1972)and Cohen et al. (1973) have looked in some detail at the competitive processes that occur when a metal atom M produces charged species from collision with oxygen. These include associative e , electron transfer M 0, M' + 0,-, ionization M 0, M 0 2 ' and the rearrangement process M + 0, + MO' + 0 - . This work was mentioned in Part I (Berry, 1980a) in connection with associative ionization, which is the dominant process. More recently, Patterson et al. (1978) showed that thorium colliding with ozone gives 0 - and 0,- with Tho,' and T h o + , respectively, as relatively minor products among processes that lead largely to associative ionization and Kuprianov processes; i.e., to the production of free electrons. In the reactions studied by Cohen, Young, and Wexler, the 02-, predominant process gave electron transfer and formation of M' with barium, titanium, and aluminum. Barium and titanium gave about an order of magnitude less MOe + 0 - , and still smaller amounts of pure associative ionization, MO,' -t e. With aluminum, no oxide ions, either A10' or AlO,', were observed, which seems a little surprising in view of the chemical lore that aluminum has a great affinity for oxygen. A still more complicated reaction that generates ion pairs is one of the reactions that takes place between alkali diatomic molecules and halogen molecules. Lin el al. (1974) showed that K, reacted with Br,, I,, IBr, and ICI X-,and (3) KX,- K', with a to give (1) K' + KX + X-,(2) K2X' cm'. This is an order of magtotal reactive cross section of 3-10 x nitude less than the cross section for production of neutrals (Grice, 1975). Rothe el al. (1976), Reck et al. (1977), and Dispert and Lacmann (1977) studied the process in more detail. When the process (1) is energetically allowed, it clearly predominates among the ion-forming channels; when process (1) is not energetically possible but the other two are, the products MX,- and M,X+ are found. In addition, Dispert and Lacmann observed the process K + C1, -+K' + C1,- at energies between 0.5 and 5 eV, and K' C1C1 from 3 eV upward; the latter is about 4 times as likely at its peak (6 eV) as the peak at 3.5 eV of the process giving C1,-. Similar results were found for K -+ Br, . Extensive cross section measurements for K, and Cs, with a number of halides were reported by Wells et al. (1980). The most complex process leading to ion-pair formation from collisions of simple species is the rearrangement reported by Utterback and van Zyl (1978).The process occurs at collision energies above its threshold of 12.1 eV, and therefore falls a bit outside the range we would normally include in this
+
+
+
+
+
+
+
+
+
+
N
+
54
R. STEPHEN BERRY AND SYDNEY LEACH
discussion. However, one can imagine similar processes with lower thresholds that might be quite relevant in flames or plasmas, so we mention the rearrangement ionization reaction here. The remarkable process is N, + CO --t NO+ + CN-. The molecule NO has a relatively low ionization potential and CN has a large electron affinity, so it is not surprising that the process is observed. At high energies, no doubt one gets CN + e, rather than CN-. However, near threshold where the cross section is of order 2 x 10- l 9 cm', one observes CN- in the mass spectrometer detector. In fact, one observes CN- at collision energies up to and a bit beyond a local maximum (- 2 x lo-'' cm') at an energy of 10 eV. D. Ion-Ion Neutralization The inverse of the simplest of the ion-pair formation processes is ion-ion mutual neutralization, A+
+ B- + A + B
(16)
We shall discuss this topic somewhat more briefly than ion-pair formation because its literature is much sparser and because it has been relatively unchanged since its most recent reviews (Moseley et al., 1975;Flannery, 1976; Berry, 1980b). However, we shall use this opportunity to summarize the theoretical picture for both the neutralization process and its inverse. Modern work on ion-ion neutralization begins with the work of Mahan and his collaborators (Carlton and Mahan, 1964; Mahan and Person, 1964a,b; Mahan, 1973), because these were the first studies to extend to pressures low enough to distinguish two-body neutralization from three-body processes. Previously, the studies were restricted to relatively dense gases and were interpreted by models appropriate to those conditions (Langevin, 1903; Thomson, 1924; Loeb, 1960; Natanson, 1960; Mahan and Person, 1964b). Interest in ion-ion recombination in dense gases has been sustained recently by the importance of the rare gas-halide lasers. Consequently, the most elaborate of the earlier theories of three-body neutralization, that of Natanson, has been extended (Mahan and Person, 1964b; Flannery, 1972;Flannery and Yang, 1978a,b; Wadhera and Bardsley, 1978). This theory is based on the notion of a critical radius for the ion pair; if the pair comes closer than this distance, they orbit and are sure to suffer a deactivating collision with a third body. Unfortunately, the work of Mahan and Person seems to have been overlooked in some of the recent literature. This is a precise sorting out of the collisional mechanics of A + + B- + M within the notion that two ions moving in a hyperbolic orbit may be deactivated if one of them suffers a collision with a third body.
55
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
Ion-ion recombination rates can be described by pressure-dependent rate coefficients whose minimum values are, in principle, independent of any carrier gas. In practice, this is not quite true (Carlton and Mahan, 1964), which suggests the formation of some clusters around the ions. However, it seems to be approximately so and the rate coefficients increase linearly with pressure. The limiting low-pressure values are effectively reached when the total pressure is below about 5 or 10 torr with NO' and NO,-, with NO+ and SF,-, and C6H,+ and SF,-. The iodine system, where iodine provides both positive and negative ions of uncertain structure, may have to be carried to slightly lower pressures (Carlton and Mahan, 1964). For all these species, the two-body neutralization rate coefficient at 300 K is between 1 and 2 x lo-' cm3/sec, corresponding to a geometric cross section whose radius is about 200 A, that is, to about lo-" cm2. The rate coefficients for mutual neutralization of alkali positives and halide negatives as measured by kinetic studies behind shock waves are in the range 2-5 x lo-'' cm3/sec for temperatures of 3000 K (Berry, 1980b). The rate coefficients include both the two-body and three-body processes; no pressure dependence was studied in this shock-tube work. The largest value reported for an alkali-halogen pair is 9.3 x lo-' cm3/sec for Na' + C1-, calculated by Olson (1977), who found a value of 1.8 x lo-'' cm3/seg for Kf C1-. The reason for the 100-fold larger value for Na' Cl- is the near-resonant character of mutual neutralization, a point discussed in more detail below. Other atom-atom systems were studied in a flowing afterglow (Church and Smith, 1977),but only upper limits to the rate coefficients could F-, and Kr+ + F-, all in He carrier gas, the be set. For Xe+ + C1-, Xe' Frate coefficients at 300 K were all below 5 x lo-' cm3/sec; for Ar' in He, the upper limit was 1 x lo-' cm3/sec. The values are notably lower than the rate coefficients reported by Mahan and his group, despite the much higher temperatures. The interpretation, as we shall see, lies in the fact that the alkali halide studies all involve electron donors whose parent neutrals have large electron affinities and acceptors whose available levels are sparse and relatively high-lying. Rate coefficients for mutual neutralization of halide ions with alkalis, with Ga, with In, and with T1 have been measured in flames by Burdett and Hayhurst (1977b, 1979).Their values, taken at temperatures between 1900 and 2600 K, are generally considerably lower, sometimes by two orders of magnitude, than the shock-tube values. However, Burdett and Hayhurst distinguished two-body and three-body processes. If one uses the results of Burdett and Hayhurst to distinguish two-body from three-body contributions in the shock-tube data, then thearesults from the flame studies and those from the shock-tube studies appear to be fairly consistent, but then do not fit with Olson's calculated coefficients. Examples of the two-body rate
+
+
+
+
56
R. STEPHEN BERRY AND SYDNEY LEACH
coefficients from Burdett and Hayhurst are: 4.8 x lo-'' cm3/sec for Na', CI-; 1.0 x lo-" cm3/sec for K', Br-; 1.5 x lo-" cm3/sec for Rb', Br-; and 1.5 x cm3/sec for Cs', I-. The three-body contributions, with argon at about 0.1-1 atm and 3000 K, bring the total effective two-body rate coefficients up to 2.5 x lo-", 2.3 x lo-", and 2.5 x 10- l o cm3/sec for the last three of these salts. The combination Na', C1- was not studied in the shock experiments. Species whose ion-ion neutralization has been studied in molecular beams extend from the very simplest H' + H- and He' + H - pairs through some diatomic-triatomic examples such as NO' + NO2- and 02+ + NO2-. These were all reviewed by Moseley et al. (1975), but some more recent work deserves to be singled out. Peart et al. (1976a)improved the resolution of the Hf H- system to show convincing oscillatory structure in the cross section for collisions with center-of-mass energies between 20 and 100 eV. At about 118 eV, the cross section has a very sharp peak at 2.1 x cm2; between about 2 and 1000 eV, its value is between 1 and 2x cm2. At energies below 20 eV, the cross section is a decreasing function of energy; typical of exoergic ion-ion neutralization, its cross section varies as ur;,ttive at very low energies (Aberth et al., 1968). The cross cm2 for section for the He+ H- process is also in the range 1-2 x collision energies between 30 and 10oO eV. The corresponding rate coefficients at the temperatures where these cross sections would be appropriate are of order lo-' cm3/sec. At the lowest energies studied, about 0.2 eV, the cross section for Hf + H - neutralization is about 2.5 x lo-', cm3, corresponding to a rate coefficient again slightly above lo-' cm3/sec. With more complex ions, the cross sections are larger still. For example, 02++ 0,has a cross section for mutual neutralization of about 2 x lo-', cm2 at a relative energy of 0.2 eV, and this is typical of such systems (Moseley et al., 1975). The beam results are in general quite consistent with the bulk gas discharge results of Mahan, Person, and Carlton. The flowing-afterglowstudies have given neutralization coefficientsfor a NO2-, N O + + NO,-, number of molecular species, including NO' CC13+ C1- (Smith and Church, 1976), C1,+ + C1- (Church and Smith, 1977),and NH4+ + C1- (D. Smith et al., 1978a).The rate coefficients for all these processes at 300 K are apparently 4-6 x 10- cm3/sec,indicating both that the detailed nature of the acceptor is not important and, more surprising, that the strength of binding of the last electron to the negative ion is also nearly irrelevant to the rate coefficient or the effective cross section. Even clustered ions such as H,0f-(H20), neutralizing with C1- or N 0 3 - . H N 0 , have about the same cross sections of loTi2 cm2 and rate coefficients of about 5 x lo-* cm3/sec @. Smith et al., 1978a). The flowing-afterglow studies of NO' + NO,- give a value of the rate coefficient of (6.4 & 0.7) x
+
+
+
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
57
cm3/sec at 300 K, a large rate on the scale of atomic processes. It was pointed out by Smith and Church (1976) that this is nevertheless almost an order of magnitude lower than the value of 5.1 x lo-' cm3/sec from merged-beam studies (Person et af., 1976), and is significantly smaller than the value of 1.75 x cm3/sec reported for NO' NO2- in a stationary afterglow (Eisner and Hirsh, 1971). It is just consistent with a value based on a (Olson, 1972); Landau-Zener (LZ) curve-crossing model, 1.2 k 0.3 x the crossing, in this case, may be at a close enough internuclear distance to make the LZ model applicable. While the low value seems plausible in light of the many other values for ion-ion neutralization of molecular species, the origin of the discrepancy remains unknown. It would be valuable if flowingafterglow studies were done with some systems involving weakly bound negative ions, such as H- or 0-. Flame studies of molecular positive ions neutralizing with halides (Burdett and Hayhurst, 1976, 1978) also gave rate coefficients of about 3 x lo-' cm3/sec.These investigations were done with both H 3 0 +and N O + and with C1-, Br-, and I-. Again, no measurements were done for negative ions with more weakly bound electrons, ions such as S-, 0 - ,or H-. Information regarding the states involved in ion-ion neutralization comes from the merged-beam experiments of Weiner et al. (1971). While most ion neutralization experiments in beams have been monitored by the amount of total neutral beam formed, or the decrease in charged-particle beam intensity, these experiments followed the light emission from Na' 0 - .The results indicate that the greatest part of the cross section, which is of order 8 x cm2 in the range of 0.5-1.5 eV, is due to electron transfer from the negative ion to the empty 3d level of Na'. This level is the one most nearly resonant with the level of the outermost electron in 0 - ,i.e., with the detachment energy for 0 - + 0 e. A similar phenomenon occurs with Li' + 0 - (B. Blaney, unpublished) and with Na' + I-, where the 3p level of Na is nearly resonant with I - (Gait and Berry, 1977). One other study of product states was that of the light emission from NO+ + NO2- (D. Smith ef al., 1978b). The only observed emission from their flowing afterglow was a set of bands of the 1 system of NO, from the ground vibrational state of NO(A2C') to ground 211 state. The FranckCondon constraints of conservation of nuclear position and momentum would favor the formation of higher electronic states of NO, notably the C211,D2C+,and possibly the B211 in a high vibrational level. The C and D states would be the ones favored by near-resonant conditions for charge exchange. We suspect that the emission observed in the afterglow experiments is the result of collisional relaxation of both electronic and vibrational excitation; Smith et al. noted that this could be occurring at the relatively high pressures (ca. 1 torr) of carrier gas in their afterglow.
+
+
+
58
R. STEPHEN BERRY AND SYDNEY LEACH
It would be valuable to have more information about the products formed in mutual neutralization. We do not know, for example, how energy is partitioned among the degrees of freedom or what dissociation may occur, or even how well the Franck-Condon approximation holds in such processes. What, for example, are the products of neutralization of NH4+ and C1- or H,O+ + C1- ? Clementi's calculations (Clement], 1967; Clementi and Gayles, 1967) indicate NH3 HCl would give NH4Cl, but those analyses did not consider the neutralization reaction with possible subsequent dissociation. The latter reaction was discussed by Burdett and Hayhurst (1976), and the possibilities were considered that H 2 0 H C1 or H,O HCl' is formed, but the real products are unknown. The very large cross sections for ion-ion neutralization were something of a puzzle from the viewpoint of microscopic interpretation, despite their prediction from phenomenological theory (Aberth et a[., 1968). LandauZener theory, based on the supposition that the electron transfer occurs almost exclusively as the nuclei pass through the crossing distance, could account well for processes in which this crossing occurs at distances where atomic orbitals from the two ions overlap significantly, say for R < 30A. However, this model implies that the cross sections for neutralization must become very small if the crossing distance is large (Bates and Lewis, 1955). Until the acceptor states could be identified, this conception was unchallenged. However, the crossing distance for Na(3d) 0 with Na+ + 0 - is 288 A, a distance at which the LZ crossing probability is infinitesimal, in contrast to the measured cross section of almost 10- l 2 cm2.This corresponds to a radius of 50 A, or to a probability of neutralization of about 1 in 33 for collisions within the 288-A radius. While the LZ model and its extensions have continued to be used for many ion neutralization systems (Demkov, 1964; Bandrauk and Child, 1970; Bandrauk, 1972; Janev, 1976; Janev and RaduloviC, 1978), the cross sections obtained this way are sometimes considerably smaller than those observed (Burdett and Hayhurst, 1979, for example). A correct theoretical description presumably could be obtained from the use of the full theory of Delos and Thorson (1972). However, even without this machinery, and without a general theory that can account for the longrange processes quantitatively, we nevertheless have considerable insight into what occurs and into what kind of theory is required. Ion-ion neutralizations with long-range crossings are the worst cases for the LZ model. For one thing, the exponential governing the probability of a curve crossing in LZ theory contains the difference of the slopes of the two potentials in its denominator. When two curves have nearly the same slope, this denominator becomes extremely large and the computed probability, correspondingly unstable. Second, the two curves in these systems are almost degenerate for long ranges of internuclear distance, whereas the LZ model
+
+ +
+
+
59
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
supposes that they cross sharply enough that the two states interact only near the crossing. In mathematical terms, the LZ theory supposes that the stationary phase approximation suffices to give the transition amplitude in a semiclassical action integral representation of the phase of the wave function. The point of stationary phase is, essentially, the crossing point. One might suppose, by contrast, that when two potential curves are nearly parallel the stationary-phase approximation is not adequate and that a significant part of the transition amplitude accumulates over a range of internuclear distances, much of it well away from the crossing distance. This situation is well known in a context only slightly different from ionion neutralization, namely in near-resonant ion-neutral charge transfer. The experimental work (Baker et al., 1962; Fite et a/., 1962)fits very well with the theory (Rapp and Francis, 1962), which accounts for nonresonant, resonant, and near-resonant processes. In the resonant and near-resonant cases, very large contributions to the total transition amplitude come from oscillations of the charge between the two colliding centers. We can expect similar behavior for ion-ion neutralization, especially in systems having long-range crossings. In these instances, the ionic potential curve is nearly parallel to and degenerate with one atomic curve for a long interval. In cases where the only crossings are at shorter distances-cases in which the ionic curve has a low-lying limit for R co,crosses only one or two neutral curves and has these intersections at distances of order 30 A or less, here we expect LZ theory to hold very well. Moreover, the cross sections for such processes are expected to be only slightly larger than geometric; this is just what we saw for the ion-ion neutralization cross sections of most of the alkali-halogen combinations. Sodium iodide is the most extreme exception; this combination has a near-resonant crossing of a neutral curve with its ionic curve. In systems other than the alkali halides, one generally finds that the ionic limit is high enough in energy to cross Rydberg states or other high-lying states, so that a near-resonance is almost always possible. The firmest confirmation of this interpretation comes from a calculation CIby Olson (1977) of the ion-ion neutralization cross sections of Na' and K + + Cl- by the LZ method and by a close-coupling formalism that includes the contributions of the entire collision trajectory to the transition probability. While the curve crossings of these systems are much closer than for Na' + I - with Na(3p) + I, they are large enough, it seems, to show the main point. The cross section for the K-Cl system calculated by the LZ method is about 40% below the close-coupling value. In the direction Na + C1(2P,,z)+ Na' + C1-, the close-coupling cross section is approximately 1.5 x cm2 between 1.5 and 4 eV, and the crossing distance is 9.5 A and the close-coupling cross section is approximately 6 x 10- * cmz between 0.7 and 0.85 eV; the LZ value is several orders of magnitude smaller.
-
+
60
R. STEPHEN BERRY AND SYDNEY LEACH
We conclude, as Weiner et al. suggested (1971),that the LZ picture must be augmented for near-resonant processes. The challenge for theorists now is to formulate a representation, perhaps a reduction of the Delos-Thorson theoretical description, that gives a criterion for when the near-resonant picture and the LZ curve-crossing picture apply, tells us whether these two regions overlap or are separate, and gives us a reliable, convenient algorithm for computing the transition probability in almost all instances.
E . Photoionization We turn now to two processes that involve electron ejection via photon absorption : photoionization of neutral species and photodetachment of electrons from negative ions. These are examples of photoelectric effects, which have been of interest since Hertz’s original discovery in studying the impact of ultraviolet light on solids nearly 100 years ago (Hertz, 1887a,b), and the observation of similar effects in the gas phase (Hughes, 1910).They play an important role in chemical physics, astrophysics, planetary atmospheres, and ionosphere studies, as well as as in various branches of plasma physics and chemistry. Light absorption as an initiator or modifier of electrical discharges was known from the early days of Hertz, and this is now known to be due to photoionization, photodetachment, and, in some cases, to so-called optogalvanic effects in which the atoms and molecules in excited neutral states of atoms and molecules created by light absorption, modify the electrical resistance of the gas, positively or negatively (i.e., changes its propensity to ionization). We point out also that photoionization, photodetachment, and, perhaps, optogalvanic processes are of importance not only in laboratory discharges but also in lightning discharges. Experimental results and theoretical treatments of the followingaspects of photoionization will be discussed : photoionization sources, the physics of the photoionization process; photoelectron ejection (energies, angular distributions, and spin polarization) ;photoionization cross sections and their shape, particularly near thresholds, autoionization features; photoionization efficiencies and their variation with photon energy; dissociative ionization, including the role of autoionization and theoretical and experimental approaches to ion fragmentation; multiphoton ionization (spectroscopy and fragmentation). General results are discussed in these fields and some specific atoms and molecules are treated in depth. Among subjects not discussed are the photoionization of atoms and molecules in external electric and magnetic fields, the photoionization of positive ions (on which relatively little work has been done), and postcollision
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
61
interaction effects, studied recently both in electron scattering experiments and inner-shell photoionization. Recent reviews in some of the areas treated are those of Marr (1967,1976), Eland (1974, 1979), Leach (1974), Samson (1976), Rosenstock (1976), Berkowitz (1979), Johnson (1980a,b), van der Wiel (1980), and Krause (1980), as well as the series of reviews on electron spectroscopy edited by Brundle and Baker (1977, 1978, 1979). Theoretical methods for calculation of photoionization cross sections and photoelectron angular distributions have been expounded in the proceedings of three recent colloquia (Kleinpoppen and McDowell, 1976; Wuilleumier, 1976; Rescigno et al., 1979). Discussion of photodetachment in atoms and molecules is limited to negative ions having positive electron binding energies. Photodetachment cross sections near threshold and dissociative processes are treated. The appropriate review articles are mentioned in that section. 1 . Some General Remarks
a. Photoionization sources. Much of the earlier work on photoionization processes was carried out using line-emission sources of the gas discharge or vacuum spark varieties. Detailed structure in cross-section curves is not easily revealed with such sources. Continuum sources, particularly the highpressure rare-gas continua, the Lyman continuum generated by a highcurrent density discharge through a low-pressure gas, such as in the Garton (1953,1959) source, and the Ballofet-Romand-Vodar (1961) (BRV) vacuum spark source (which has a low inductance, is triggered by an auxiliary sliding spark, and uses refractory high-atomic-weight anode material) have been the principal VUV sources used for quantitative studies. Descriptions and references to these line and continuum sources are given by Samson (1967) and by Zaidel’ and Shreider (1970). Recent developments of the very useful BRV plasma source are described by Damany et al. (1966), Fox and Wheaton (1973), Boursey and Damany (1974), and Lucatorto et al. (1979). In more recent years, the quasi-ideal synchrotron radiation (SR) source has provided more reliable data on photoionization cross sections and other processes induced by photon absorption, The SR sources can provide continuum radiation from the far IR to the X-ray region and, with adapted monochromators, give tunable radiation over this entire range. A comparison of various VUV sources is made in Table 11, taken from Jortner and Leach (1980). It should be noted that one difficulty in the use of continuum sources is to account for scattered light and, especially with synchrotron radiation, eliminating higher spectral-order radiation.
62
R. STEPHEN BERRY A N D SYDNEY LEACH
TABLE 11 COMPARISON BETWEEN SR SOURCES AND VACUUM UV DISCHARGE LAMPS ~
Source Conventional rare-gas continua He continuum (50 torr) Ne continuum (100 torr) Ar continuum (200 torr) Kr continuum (200 torr) Xe continuum Pulsed discharge in rare-gas continua Ar (2000 torr) Kr (2000 torr) Hydrogen discharge Hinterreger lamp Resonance lamps He(1) resonance He(I1) resonance Ne(1) resonance Ne(I1) resonance Plasma sources BRV source
Soft X-ray lines YM5 ZrMr NbMr RhMt MgKu AlKcl CuKu Synchrotron radiation
~
~
~
Useful energy range (eV)
Linewidth (eV)
12-21
-
~~~
~
Pulse length (sec)
Intensity at monochromator exit slit photons/(sec A)
-
lo8
- ID8
12.4- 16.8 8.0-11.8
-
-
6.9-9.9
-
- lo8
6.2-8.4
-
8.0- I 1.8 6.9-9.9
-
-
4-12 21.2 40.8 16.8 26.9 4-250
132.3 151.4 171.4 260.4 1254 1487 8055 10- 1- 1000
--
lo8
lo8
109-1010 109-1010 107
---
10-3
< 10-3
10'O 109
1O'O 109
3 x 1012(at5Hz) [6 x 101'photons/ (pulse A,]
0.5 0.8 1.2 4.0 0.7 -0.8 2.5
lo9 (photons/sec) 10' (photons/sec)
-
-
2 0 . 4 x lo-"
lo1'
63
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
Laser-generated plasmas can also be used to obtain continuum radiation over the 50-2000 A region. Results have been obtained that show promise as sources for quantitative absorption spectroscopy in the far UV (Carroll ef al., 1978). Work on suitable target materials is being actively pursued (Mahajan er al., 1979; Carroll et al., 1980). The rapid development of VUV lasers also shows promise for photoselective studies over a narrow tunability range in the 5-10 eV spectral region for one-photon excitation and 15-30 eV for multiphoton excitation (Jortner and Leach, 1980). The use of visible and near-UV laser sources, especially tunable ones, has given great impetus to the study of multiphoton ionization. This field is of interest not only as a tool for spectroscopy, where easily detected ions monitor intermediate real or virtual states which play a role in their formation, but also for studies of fragmentation processes (Johnson, 1980a).
b. The photoionization process. The (single-photon, single-ionization) phoroionization process corresponds to the removal of a bound electron, nominally to infinity, following absorption of a photon of adequate energy. The probability that an atomic or molecular species in an initial (i) bound state will undergo a transition to a final (f) electron continuum state is usefully defined in terms of a cross section at the particular photon energy absorbed. The differential cross section da( is proportional to the ratio of the boundcontinuum transition rate Pif(v)to the exciting light intensity Z(v): 11)
do(v) =
Ef(v)hr/Z(19)
(17)
The transition rates can be calculated using first-order time-dependent perturbation theory within the context of the semiclassical theory of radiation-matter interaction; i.e., where the radiation field is treated classically as a plane wave with vector potential A, so that I ( v ) = (2nv2/c)(AI2.The transition rate is given by: S A v ) = (2n/h)((f /Hint I i> I ' ~ r (18) where H,,,is the interaction Hamiltonian and pr is the density of continuum final states. Hintcan be written a6 Hint= H,JEl) H,,,(M 1) Hi,,(E2) . . . where the three terms on the right-hand side are the electric dipole, magnetic dipole, and electric quadrupole interactions, respectively. We will be concerned mainly with electric dipole transitions. A key problem in calculations of the transition probability is the description of the wave functions used, especially the adequate form of the continuum wave function. We will not go into detail here, but note that different approaches will be mentioned dulring the course of this part of the review. A
+
+
+
64
R. STEPHEN BERRY AND SYDNEY LEACH
convenient, but not unique, approach to the description of atomic and molecular electronic states that take part in the photoionization process, is the independent-particle approximation. An energy state will be an eigenstate of an approximate Hamiltonian; it can be described by its electronic configuration, i.e., by a “listing” of the single-particle states that are occupied and the values of angular momentum quantum numbers (or approximation to them). On photoionization, the removal of a bound electron is accompanied by the quasi-simultaneous reorganization of the orbitals and energies of the remaining electrons. The diabatic or adiabatic nature of this process has been discussed by Berry (1963,1969a). The amount ofelectron reorganization is a function of the rapidity of electron ejection, the process being adiabatic when the kinetic energy of the outgoing electron is relatively small. This can be considered in terms of the orbital relaxation time (Berry, 1963; Meldner and Perez, 1971). For example, with photons in the 10-eV range, the interaction time is of the order of 2 x lo-’ sec, assuming 500 cycles of the radiation field. Since the orbital reorganization time is of the order sec, e.g., in saturated hydrocarbons, photoionization will correspond to an electronic adiabatic process. This is in contrast to electron impact ionization where electrons with kinetic energy of 10 eV will have interaction times of the order of 10- l 6 sec, making ionization a partially diabatic process. Some of the discrete bound states, for example, as described on the independent-particle approximation, will have the same energy as one or more of the continuum states and can thus decay to the continuum if an adequate value of the matrix element of the interaction operator exists on this level of approximation. These superexcited (i.e., above the lowest ionization energy) states are Rydberg levels converging to limits above the lowest ionization potential. They are known as autoionizing (or preionizing) states and the decay process as autoionization (or preionization) (see Berry, 1980a, Part I). Autoionization can occur only if the atomic or molecular ion core is excited with an energy at least equal to the binding energy of the detachable electron in the superexcited state. Some useful theories of this autoionization process, e.g., the multichannel quantum defect theory (MQDT, see later) are “collisionist” in approach, in which one conceives of an inelastic collision occurring between excited electron and ion core, leading to exchange of energy and angular momentum. Although for atoms the energy exchange can only be electronic in nature, for molecules there also exists the possibility of conversion of vibrational and/or rotational energy of the ion core into kinetic energy of the outgoing electron. Thus, for molecules, there exist rotational, vibrational, and electronic autoionization mechanisms. As we
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
65
will see later, these processes can be competitive and can even interfere with each other. Ion or electron currents are generally used to determine total (integrated) photoionization cross sectionso,.Absolute values of ci are not easy to obtain; most literature values are relative to standard calibrations. Electron measurements can be used to faithfully represent ionization processes except when ion pairs are formed (no electrons ejected)or when multiple ionization occurs (multiple-electron ejection). Much useful information has indeed been obtained from photoelectron measurements. Before examining photoelectron ejection we wish to mention that although we are concerned here with ionization through photon absorption, it is possible to quantitatively study a wide range of energy-dependent “simulated” photoionization phenomena by the detection of fast scattered electrons in coincidence with ejected electrons (e, 2e process) and ions (e, e + ion process). The fast-electron differential scattering cross sections are determined at negligibly small momentum transfers (Tan et al., 1978; Brion and Hamnett, 1979). Photoabsorption cross sections can be obtained by kinematic correction of the forward-scattering electron energy loss spectrum according to the Bethe-Born theory. Partial photoionization cross sections are then available via the ion branching ratios. Ion yields from ionization and dissociative ionization have been measured with this technique for a number of molecular species, e.g., NH, (Wight et at., 1977), H,O (Tan et a!., 1978), CH, (van der Wiel et al., 19761, C 0 2 and N,O (Brion and Tan, 1978, 1979), and 0, (Brion et al., 1979). Whenever comparisons can be made there is very satisfactory agreement with the results of studies using photon excitation. 2. Photoelectron Studies a. Photoelectron spectroscopy. Since one electron is emitted for every singly charged ion (excluding ion pairs) formed by photoionization with photons of energy hv, the ion appearance potentials 1 can be determined by measurement of the kinetic energy of the photoejected electrons, according to the relation: I = hv - mo2/2, where m is the mass and u the velocity of the electron. For a molecular species, the ion will be formed not only in a particular electronic state, but also in well-defined vibrational and rotational levels. The energies of these ion levels are displayed in photoelectron spectrum (PES) bands (Fig. 11). The energy resolution in PES depends on the linewidth of the exciting radiation, the translational motion of the absorbing species, and the resolving power of the electron-energy analyzer. With present-day techniques (Leach, 1974; Wanneberg et al., 1974; Briggs, 1977; Eland, 1978; Berkowitz, 1979), resolutions down to about 5 meV (-40 cm- ) can be achieved with
’
66
R. STEPHEN BERRY AND SYDNEY LEACH
PHOTOELECTRONIC SPECTROSCOPY
~\\\\\\\\~ He 58L A
L
+
i AB+ 4 -Jl
4
Ethv
4
AB
PE S A fixed coincidence a t f i x e d E(e-)
PES h variable coincidence a t fixed h
1. e--ION 2 . e--hVF 3 hVF-ION
1. e - - I O N 2. e-- h V F 3. hVF -ION FIG. 11. Principles of photoelectron spectroscopy (PES). On the left, “normal” PES at fixed incident photon energy. On the right, threshold photoelectron spectroscopy using variable incident photon energy. Three types of coincidence measurement possibilities are indicated.
care and to about 2 meV (- 16 cm- ’) with extreme care. Resolved (and that partially) rotational structure has only been achieved for the hydrogen molecule. Another form of photoelectron spectroscopy is of interest in photoionization studies, that of threshold photoelectron spectroscopy (TPES). In TPES, the incident photon energy is varied continuously and electrons of fixed kinetic energy (usually approximately zero energy) are detected (Fig. 11). This method can pick up not only ion levels but also autoionizing resonances. The most useful photon source for such studies is monochromatized synchrotron radiation (Guyon and Nenner, 1980). b. The asymmetry parameter p : Some generalities. Besides the ion energy values, useful structural and dynamic information stems from measurement of the angular distribution of the photoejected electrons. With the relatively low photon energies used in most molecular photoionization
67
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
studies, the velocity of the photoejected electron is nonrelativistic, so that the electric dipole approximation holds (Tseng et al., 1978), and the angular distribution F ( 8 )is given for linearly polarized light (Yang, 1948;Cooper and Zare, 1968; Tully et al., 1968) by:
m)cc (oi/4n)[i + (fi/2)(3cosz 8 - 113
(19)
where 8 is the angle between the direction of the ejected electron and the electric vector of the polarized incident radiation. The expression on the right-hand side of Eq. (19) is the differential cross section do/dR for linearly polarized light. The asymmetry parameter /? is very informative since it can be a function of the energy of the ejected electron and also depends on the nature of the outgoing partial waves-hence, on the initial (neutral) and final (ion continuum) electron configurations, and so can also throw light on multichannel interactions or, in other words, on electron configuration interactions. Indeed, because of its role in a differential cross section, fi is very sensitive to small perturbations. In the electric dipole approximation, the selection rules A1 = f 1 apply whenever 1 is a good quantum number. The value of fi can vary from +2, which corresponds to photoejection from s orbitals, the ejection favoring the electric vector direction, to - 1 where ejection tends to follow the direction perpendicular to the electric vector. The value fi = 0 corresponds to an isotropic distribution. For p electrons, for example, where there will be two outgoing waves, 1 1 and 1 - 1, there will be interference effects between the continuum d and s waves; fi then varies with photon energy, due in large part to phase shifts of the radial matrix elements (Kennedy and Manson, 1972). Furthermore, even for s electrons, anisotropicelectron-ion interaction (Dill et a/., 1974; Starace et al., 1977) and, for heavier atoms, relativisticeffects (Walker and Waber, 1973,1974;Dehmer and Dill, 1976b; Torop et al., 1976), can cause fi to be less than 2. Recent years have seen an increasing effort in the measurement of /? values of atoms and molecules. This has been reviewed by West and Marr (1976) and by Marr (1978) who described experimental techniques and discussed results for the rare gases. The polarization properties of synchrotron radiation have been put to good use for /? measurements (Mitchell and Codling, 1972), mainly on atomic species: Ne (Lynch et al., 1972; Codling et al., 1976), Ar (Watson and Stewart, 1974; Houlgate et al., 1974, 1976), Kr (Lynch et al., 1973; Watson and Stewart, 1974; Miller et al., 1977), Xe (Lynch et al., 1973; Torop et al., 1976). Studies on rare gases have also been made using other excitation sources, e.g., by Wuilleumier and Krause (1974)and by Dehmer et al. (1975). As an example we show in Fig. 12 experimental and theoretical values ofg for the Sp electrons in Xe as a function of photon energy. The experimental
+
+
R. STEPHEN BERRY AND SYDNEY LEACH
68 h
& . a
2
4
6
8
10
I2
14
PHOTON ENERGY o ( R y ) FIG. 12. Photoelectron angular distribution parameter p for the 5p electrons of Xe as a function of photon energy (1 Ry = 13.6 eV). Experimental data by Dehmer et al. (1975) (m) and Torop ef al. (1976) ( 0 ) .Calculated results by Amusia et al. (1972) (---) using the RPAE method but neglecting interchannel correlations between 5p and 4d electrons. These correlations are included in an RPAE calculation by Amusia and Ivanov (1976) (-). [By permission from Samson (1980).]
values of Torop et al. (1976) and Dehmer el al. (1975) are not matched by some of the various calculated values (Amusia et al., 1972; Kennedy and Manson, 1972) above 50 eV (- 3.7 Ry). That this disagreement is due to total or partial neglect of electron correlation effects is indicated by the much better agreement with a later random phase approximation with exchange (RPAE) calculation by Amusia and Ivanov (1976). c. The angular momentum transfer theory. Although we will not enter into details of the many calculations of fl that have been made, particular mention must be made of the angular momentum transfer theory of Fano and Dill (1972), which makes it possible to separate p into geometric and dynamic factors. Within the LS coupling approximation in which spin-orbit interaction is negligible compared to the residual Coulomb interaction, photoionization of an atom is described by the following process: The angular momentum of the photon jhvcouples to La,the orbital angular momentum of the atom, forming an invariant angular momentum L that characterizes an intermediate state, which itself decays into an electron of orbital momentum I and an ion whose orbital angular momentum is Li . In this approximation there is no spin change. The angular momentum transfer], obeys the relation : j , = 11 - 1 I = ILi - La1, where the allowed values of j , are defined by rules of conservation of total angular momentum and parity.
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
69
The asymmetry parameter is given as a weighted average in j, (Dill, 1972, 1973; Fano and Dill, 1972),
where the photoionization amplitudes, characterized by particular values of j , , superimpose incoherently. There is, therefore, a clear separation of the
which depends, for example, on symmetry properties, geometric factor )(it), and the dynamic factor which relates, for example, to electron correlations and interaction between the excited electron and the ion core (Dill, 1976). Such a separation takes advantage of the fact that the orbital from which photoejection takes place is quantized on the internuclear axis, and so described within a molecular frame, whereas the final-state continuum function is conveniently considered within the laboratory frame (spacefixed axes). A further important concept is that of parity transfer (Dill and Fano, 1972),i.e., change in parity of the target species, which enables one to distinguish between parity-favored and parity-unfavored (Dill, 1973)transitions. In parity-favored cases, the perimeter rule j , 1 + j h , = even integer, is observed. Parity-unfavored transitions, in which the LS coupling approximation breaks down, and involves a change of spin orientation, have indeed been observed by Samson and Gardner (1973b) in the autoionization features of Xe just above the threshold for ionization. The considerable variations in B in sweeping through autoionizing resonances (states) is shown in Fig. 13, along with the theoretical values calculated by Dill (1973). Similar concepts, adapted to the presence of cylindrical symmetry, are valid for diatomic molecules (Chang and Fano, 1972; Chang, 1978b). The existence of a reflection symmetry in the potential field results in C+-C+and C--C- transitions being parity favored, whereas (forbidden) Z+-Z- transitions are parity unfavored. On the other hand, Z-ll transitions contain both parity-favored and -unfavored contributions to p. It is therefore of interest to separate these contributions in calculations. For both atoms and molecules, the factorization of fi is complete for parity-unfavored transitions and its value is always - 1. For parity-favored transitions, the mean value of p gives an indication of the relative importance of the two ionization continua 1 = ( j , f 1). Extension of the Fano-Dill theory beyond the diatomic molecule case has been made by Druger (1976). He was able to generate angular momentum transfer selection rules applied to direct ionization, in an orbital approximation, for 40 different symmetry types as well as for the general case of an nfold rotational symmetry.
+
70
R. STEPHEN BERRY AND SYDNEY LEACH I I
I
7d
I
8d
'
' 9s w
8s
-
-
Q
-1.0-
1000
'
I
980
WAVELENGTH
i '
(A)
i
1
960
FIG. 13. Variation of the photoelectron angular distributisn parameter /3 through two of the autoionizationresonances between the 'P,,, and *P,,,states of Xe'. Experimental data ( 0 ) obtained by Samson and Gardner (1973b) are compared with the results of a MQDT calculation (-) by Dill (1973). [By permission from Samson and Gardner (1973b).]
d. The p of H,. Measurements of fl have been made on a number of molecular systems, but mostly at one fixed wavelength, usually the He1 584 A or NeI 736/744 A lines (Berkowitz, 1979).Synchrotron radiation sources are now being used for photon-energy dependent studies and work has been done on H, (Marr et a/., 1980), N,, O,, and C O (McCoy et al., 1978; Marr et al., 1979; Hoimes and Marr, 1980). As an example of measurements on diatomic molecules we will first consider H,. This species is light enough for rotational effects to be observable. The theoretical principles have been worked out by Buckingham er al. (1970)and by Sichel(l970) for both the unresolved and the resolved rotation case; Tully et al. (1968) had limited their analysis to the fixed-molecule approximation. The microscopic aspects of these treatments are based in part on approximations which are not always valid, ejection of an electron from a definite orbital, without subsequent electron reorganization, i.e., Koopmans theorem (Koopmans, 1934), and the Born-Oppenheimer approximation, which leads to the postulation that the vibrational branching ratios for photoionization are proportional to the Franck-Condon factors. These assumptions are invalid, for example, in the vicinity of autoionizing states. Outside autoionizing regions, fl has been evaluated by use of correlated
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
71
pseudopotentials (Shaw and Berry, 1972).The p parameters of H, associated with vibrational-rotational autoionization at various vibrational and rotational ionization thresholds have been calculated by Raoult and Jungen (1980) using multichannel quantum defect theory (MQDT, see later) and the formulation of Dill (1972). For ionization from the initial J ” = 0 level, the angular distribution is completely determined, independently of dynamics, by the rotational level N = 0 or N = 2 of the residual ion. When J ” # 0, the angular distribution depends also on the electron-ion core rotational coupling. Raoult and Jungen (1980) show that in some cases the influence of vibrational autoionization extends far beyond the energy region of the autoionizing resonances in modifying the value of p. Measurements of partial angular distributions from H z are required to confirm the conclusions of Raoult and Jungen (198 1). Several measurementsofp for H, have been made using the NeI 736/744A lines in which the transitions are not resolved rotationally, so that the values represent the sum of final and average of initial rotational states (McGowan et al., 1969; Carlson and Jonas, 1971; Niehaus and Ruf, 1971; Kibel et al., 1979a). Experimental artifacts that can account for the dispersion of the results are discussed by Kibel et al. (1979a), in particular electron-scattering effects. A “best value” appears to be = 1.95 k 0.05. Some theoretical calculations, of various degrees of refinement, give results in reasonable agreement with the experimental value (Shaw and Berry, 1972; Ritchie, 1975; Chandra, 1977). Niehaus and Ruf (1971) also report p values for partially resolved rotational transitions. For the AN = 0 transition they obtain 6 = 1.95 k 0.03; for the AN = + 2 transition the value given is p = 0.85 i-0.14, but the data have been reanalyzed by Chang (1978a) to yield j? = 0.45 k 0.87. The calculations of Dill (1972) are in good agreement with the P(AN = 0) result, but he predicts a value of p = 0.2 for the AN = f 2 transition. Raoult et al. (1980), using the powerful multichannel quantum defect theory have calculated the following values: B(AN = 0) = 1.954, p(AN = +2) = 0.2. Synchrotron radiation studies on H, have been carried out by Marr ef af. (1980) in which they determined p over the photon energy range 18-30 eV. Significant departures from p = + 2 were observed; differences between measured and theoretically calculated values over this energy range zre attributed to the presence of unresolved A N = 2 rotational branches, the effects of which were not included in the calculations of Hirota (1976) and Dutta et al. (1977). The minimum, p 1.5 near 25 eV, may reflect the influence of electronically inelastic processes related to the existence of doubly excited singlet states of H, converging to the 2pa, state of H,’ (Raoult et al., 1980).The difficulties inherent in extending the MQDT approach to the case of electronic autoionization have been discussed by Raoult (1980).
-
72
R. STEPHEN BERRY A N D SYDNEY LEACH
e. p Measurements on other diotomic mi on polycitomic systems. Several studies have revealed that fl can vary as a function of vibrational level of the same electronic state of the ion formed. This is particularly clear not only in H2 (Raoult and Jungen, 1981; Raoult et al., 1980), but also in N,, O,, and C O (e.g., Holmes and Marr, 1980). These effects can be due to unresolved autoionization structures (cf. Raoult and Jungen, 1980),to differing outgoing partial waves for the direct and autoionizing processes (Berry, 1980a, Part I) and/or to the presence of shape resonances (Cole et al., 1980). Shape resonances in the molecular electronic continuum are due to centrifugal barrier effects in the high 1 components of the final-state wave function. The barrier creates a temporary trap of the photoelectron, thus enhancing electronvibrational coupling and leading to non-Franck-Condon intensities, as well as to vibrationally dependent p values over a more-or-less broad spectral range in the vicinity of the resonance (Dehmer et al., 1979). Calculations by Wallace et al. (1979)have been made using the multiple-scattering method. In comparing with experimental values of Marr et al. (1979)for N, and CO, it is evident that the states for which the 0 , wave resonance is accessible exhibit oscillations in /I over a small energy range (see also Raseev et a!., 1980). Measurements of p values have been made on several other diatomic molecules, as well as on many triatomic and polyatomic systems by a number of groups. We mention in particular Carlson and co-workers (Carlson, 1971; Carlson and Anderson, 1971; Carlson and Jonas, 1971; Carlson and McGuire, 1972; Carlson et al., 1972: Jonas et al., 1972; White et af., 1974), Berkowitz and co-workers (see appropriate references in Berkowitz, 1979), and Kuppermann and co-workers (see references 9a-9h given in Sell and Kuppermann, 1979). Many of the reported studies, made at single excitation wavelengths, seek to investigate trends in p values, correlating values for particular orbital ionizations in structurally related chemical series, e.g., substituted ethylenes, linear dienes, heterocyclic molecules, or benzene derivatives. Photon energy-dependent studies, albeit at three fixed wavelengths have also been carried out on a series of unsaturated hydrocarbons (Kibel et al., 1979b). Kibel and Nyborg have also studied NO (1979). In Fig. 14 is given the He1 584 A (21.22 eV) photoelectron spectrum of C6H6, and the angular distribution parameter /I measured for the different PES bands (Carlson and Anderson, 1971). It is clear that overlapping PES bands are clearly revealed through the p measurements. Calculations of p for molecules other than diatomics have been rarely attempted and have generally not been very fruitful. Recently, Grimm et al. (1980) have used the multiple-scattering method, with a tangent-spheres muffin-tin potential, to calculate p as a function of photon energy for the series of triatomic species CO,, COS, CS,. A number of resonances were
73
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
t.2
0.8
m 0.4
0
g
60
240
z
$20 n
"20
(9
(8
I7
16
IS (4 BlNMMG ENERGY (&'I
13
12
11
1
0
9
FIG. 14. He1 photoelectron spectrum of CsH, (bottom) and the photoelectron angular distribution parameter measured at selected energies. (By permission from Carlson and Anderson, 1971b.)
predicted in each case. The experimental data is sparse, being restricted to the NeI (16.85eV), He1 (21.22eV), and He11 (40.8eV) line excitations. Reasonable agreement is obtained for CO, and COS (sign of 8, qualitative order of /? values), but calculated values are less satisfactory for CS,, although the generally higher values of the sulfur compound are correctly predicted. The calculations are very sensitive to the number of partial waves so that general application of this method to large molecules may not be feasible.
f. Spin polarization of photoelectrons. The spin polarization of photoelectrons ejected from CO, and N,O has been observed (Heinzmann et al., 1980), using as excitation the circularly polarized light from a synchrotron radiation source to form the Q = and $ sublevels of the R2II ground state of the ion. This Fano effect (Fano, 1969; Kessler, 1976) can be used to probe the partial cross sections for photoionization transitions to different degenerate continua (Heinzmann, 1978) and has previously been studied in some atoms (Lube11 and Raith, 1969; Baum et al., 1972; Heinzmann et al., 1970, 1975, 1976). (See also the discussion in Section II,G on multiphoton ionization in atoms.) The ejection of polarized photoelectrons from unpolarized
74
R. STEPHEN BERRY AND SYDNEY LEACH
atoms by nonpolarized radiation, prediced by Lee (1974), has been observed by Heinzmann et al. (1979) for Xe (see also Huang et al., 1979, for a theoretical study of this case for a series of rare gases).
3. Photoionization Cross Sections a. General characteristics. The partial photoionization cross section a,(k) for producing a singly charged ion in the kth level is ideally a step function of the photon energy, corresponding to a constant branching ratio (Wigner, 1948; Geltman, 1956; see also Section 11,H). Two kinds of general behavior are exhibited for atoms. In Fig. 15 is shown the photoionization cross section for helium (Lowry et al., 1965).The sharp rise ofa,(k)at threshold is followed by a slow decrease to higher energies. The second type of behavior is illustrated by the results for neon (Ederer and Tomboulian, 1964), which exhibits a maximum in the cross-section function (Fig. 16).The threshold for the production of the 2P3i2and 2P,i, states of the 2s' 2p5 NeII ion occurs at about 575 A with a total cross section ui 6.3 x l o - ' * cm2. After passing through a maximum of lo-'' cm2 at 375 A, ci decreases, falling to 7.8 x lo-'' cm2 at about 260 A, the onset for formation of the 2s 2p6 2S,,, state of NeII. The two types of functions are related to the presence or absence of a node in the initial bound-state radial wave function of the photoejected electron. Minima can also occur when at certain energy regions, the radial wave functions in the initial state and the continuum state tend to cancel in the matrix elements (Cooper, 1962). A new threshold step might be expected in crosssection curves as each new ionization level k' is reached with increasing photon energy. Above this threshold there will be a superposition of the partial photoionization cross sections to form the total cross-section cri . This is seen in Fig. 16 at the 'S,,, level threshold where the jump is of the order of 0.7 x lo-'* cm'. N
FIG.15. Photoionization cross-section function for He. (By permission from Lowry el a/., 1965.) 1 Mb = LO-'' cm2.
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
01 I 600
1
500
I
LOO
I
I
I
300
200
100
75
-A@)FIG. 16. Photoionization cross-section function for Ne in the 100-600 A region. (By permission from Ederer and Tomboulian, 1964.) The discontinuity at the L, threshold at 260 A was not observed by Samson (1966); 1 Mb = 10-’* cmz.
6. Autoionization features for atoms and molecules. Discontinuities at the photoionization thresholds above the lowest are not always observed. Indeed, Samson (1966) has found no evidence for the discontinuity at 260 A shown in Fig. 16 for neon. Fano and Cooper (1965) have shown that autoionization processes can give rise to interference effects, which can cause oi to increase, decrease, or remain relatively unmodified at ionization thresholds. For molecules, of course, these thresholds can correspond to rotational or vibrational, as well as electronic-state formation. Resonance peaks and dips (antiresonance minima) of various symmetrical or asymmetrical shapes can occur due to interferences between the probability amplitudes for direct ionization and indirect ionization (autoionization from superexcited levels). The shape factor has been parameterized by Fano (1961)in a treatment based on isolated autoionization resonances. The “Fano profile” function f (c, q ) is given by the following expression:
fk4) = (6 + d2/k2+ 1)
(21)
where c is the energy of the incident photon in units of peak width and q is related to the ratio of electric dipole coupling strength from the ground (neutral) state t,bo to the compound state YEand to the continuum state i,bE (where YEis a linear combination between the continuum state t,bE and the discrete excited state t,bs), as well as to the interaction between discrete and continuum states. Fano profiles in autoionizing features are illustrated in Fig. 17 by the photoionization cross-section function for atomic nitrogen in the 610-850 A region obtained by Dehmer et al. (1974). The photoionization threshold for production of the 2s2 2pz 3P state of NII is at 852.2 A. The first excited state
76
R. STEPHEN BERRY AND SYDNEY LEACH
I
620
.
.
.
l , 640
.
.
I
.
660
.
.
I
.
.
680
.
PHOTON WAVELENGTH
I
.
. 700
A
l , , L I
" 8 4 0
860
fh
FIG.17. Photoionization cross-section function for atomic nitrogen in the 610-850A region. (By permission from Dehmer et a/., 1974.) Wavelength resolution FWHM = 0.83 A.
of the ion to which an optical dipole transition is allowed from the 2s' 2p3 4S ground state of the neutral NI atom is the 2s 2p3 'S state of NII at 609.3 A. The asymmetrical peaks and antiresonance minima in Fig. 17 correspond essentially to autoionization into the continuum associated with the 3P ion state from the n = 3, 4, . . . Rydberg series levels leading to the 2s2p3 5P state of the ion. The photoionization cross-section functions of molecules are very rich in autoionization features for several electron volts in energy above the first ionization threshold. This subject was discussed in Part I (Berry, 1980a)and will be examined and brought up to date in the context of competing decay channels (Section II,E,4). Mies (1968; see also Smith, 1970) extended Fano's (1961) original treatment to several continua and also to the general case of several closely spaced autoionizing states and any number of continua. Jungen (1980) has stressed that due to overlap of resonances, photoionization cross sections can have forms that are much more complex than the relatively simple analytical forms that derive from Fano's original treatment. This is clear from the results of the theoretical calculation carried out for H 2 in the threshold region by Jungen and Dill (1980), based on Seaton (1966) and Fano's (1975) quantum defect theory. The observed (Dehmer and Chupka, 1976) and calculated cross-section curves are in excellent agreement and well demonstrate the existence of structures far more complex than a single Fano profile. In general, molecular high-resolution photoionization spectra will require analyses of the same order of refinement as developed by Jungen and Dill (1980) for the case of hydrogen.
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
77
c. Photoionization cross sections in atoms: Experimental results. The development of photoelectron spectroscopy techniques has given great impetus to the study of partial cross sections for photoionization, i.e., for formation of specific states of the ion. Ionization from the valence-shell orbitals leads in atoms to formation of singly charged ions and also, in some energy regions, to multiple ionization. The latter process can be related to electron correlation effects. In Fig. 18 are plotted the partial cross sections for formation of ionic states in neon (Wuilleumier, 1973; Wuilleumier and Krause, 1974) over an extensive photon energy range. The ground state of Ne has the configuration ls2 2s2 2p6. Ionization threshold is at 21.565 eV and this process involves ejection of a 2p electron (cf. Fig. 16);further thresholds are at 48.476 eV for ejection of a 2s electron and 870.2 eV for Is electron ionization. Although the cross section for (2p)- is about three times that of (2s)- at their respective maxima, the decline to higher energies is much faster for (2p)-’, the two partial cross sections being equal at about 650 eV. Two other types of process are also observed. These are the two-electron “shake-up” and “shake-off’ processes. The shake-off process leads to multiple ionization and gives rise to broad continua of photoelectron energies
hv (eV)
FIG.18. Partial cross sections for formation of various ionic states of Ne by photoabsorption (Wuillemier, 1973; Wuilleumier and Krause, 1974; by permission from Berkowitz, 1979). 1 Mb = 10-’8cmZ.
78
R. STEPHEN BERRY AND SYDNEY LEACH
in the PES spectrum since, e.g., in double ionization, the two electrons ejectec can share the excess kinetic energy. The use of a mass spectrometer can hell to sort out multiple from single ionization (Schmidt et al., 1976; Holland et al 1979). The shake-up process involves single ionization with concurren excitation and gives rise to discrete PES peaks. Figure 18 also shows th partial cross sections for double ionization, as well as the shake-up 2p and 2 processes. The shake-up processes have cross sections that are approximate1 constant and one-tenth of the corresponding (2s)-’ and (2p)- cross section over the whole energy region. Further shake-up and shake-off processe occur in conjunction with formation of a vacancy in the K shell (Carlso et al., 1971; Krause, 1971). It should be noted that the (2s)-’, (2p)-’, an1 their corresponding shake-off and shake-up partial cross sections do no exhibit discontinuities at the (1s)- threshold at 870.2 eV (Wuilleumier, 1973 Multiple ionization is relatively important in the heavier rare gases. Fo example, for Xe, 43% of the ions formed near the 4d threshold are doubl charged, values of 28% and 20% are found for Kr and Ar, respectivelj whereas for He the ratio does not rise above 5% (Carlson, 1967 ;Samson ant Haddad, 1974; Schmidt et al., 1976). In our discussion of the experimental results on atoms we have concen trated largely on the rare gases, whose stability and monatomicity mak them relatively easy to study. We stress however, the importance of obtainin results on other “less easy” species for testing theoretical methods. Earl measurements on atomic photoionization concerned the alkali metals, whos ionization potentials are very low, the threshold falling in the 2000-3000 1 region (Samson, 1976).The use of furnaces makes it difficult to maintain well defined conditions over the absorption path ;furthermore, the coexistence c atomic and molecular species in the absorption cell creates problems. Studie on the alkaline earth metals, Cd, Zn, Hg, etc., have been reviewed by Mar (1967) and, more recently, by Samson (1980) who also reviewed results on th alkali metals. Photographic spectra have now been taken for many atomi species, and a number of measurements have been made of photoionizatio cross sections. We mention work on the following atoms (photoabsorption o photoionization): Mg (Mitchell, 1975) ;0 (Cairns and Samson, 1965; Kok ef al., 1978); Mn (Connerade et al., 1976); Ba (Connerade et al., 1979); ant Hg (Brehm, 1966; Berkowitz and Lifshitz, 1968; Cairns et al., 1970; Con nerade and Mansfield, 1973; Mansfield, 1973). Sources of data on atom! including work on photoionization of electronic excited states, and on som atomic ions, are given in the literature (Marr, 1967; Kieffer, 1968, 1976 Gallagher et al., 1978). Recent information is reported in reviews by Wuilleu mier (1980), Samson (1980), and Starace (1980a,b).Cross sections for ioniza tion of excited states have been studied via two (or more) photon processe (see Section 11,G).
’
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
79
d. Photoionization cross sections in atoms: Theoretical methods. Much theoretical work has been done on developing methods for calculating photoionization cross sections of atoms (Manson, 1976, 1977). The simplest approximation used is the central potential model (e.g., Cooper, 1962; McGuire, 1967; Manson and Cooper, 1968; Combet-Farnoux, 1969). The results show the general features of photoionization over a large range of incident photon energies, including manifestations of the “Cooper minimum” in the cross section. This method does not provide understanding of features due to several electrons since it involves the consideration of only one active electron. Models using the Hartree-Fock approximation to describe the initial and final wave functions are more satisfactory, since all electrons and their exchange are now considered. Many calculations have used this approximation (e.g., Kennedy and Manson, 1972).Use of these approximate wave functions leads to photoionization cross-section values, which differ accordingly as the dipole matrix element is expressed in the dipole length or the dipole velocity forms (see, e.g., Stewart and Webb, 1963; Grant and Starace, 1975). The Hartree-Fock approximation is unsatisfactory when a strong interaction exists between outgoing channels coupled to ground and excited states of the residual ion. This is largely overcome with the RPAE (Amusia and Cherepkov, 1975; Wendin, 1976) in which matrix elements are neglected which, in the case of a large number of electrons, cancel each other through their random phases. Relativistic effects can also be incorporated (Johnson and Lin, 1979). Calculations on closed-shell atoms have been done by Amusia and his collaborators (Amusia, 1974; Amusia et al., 1974; Amusia and Cherepkov, 1975),by Wendin (1976)and Wendin and Starace (1978),and by Johnson and Cheng (1979).The method has also begun to be applied to openshell systems (Cherepkov, 1976; Starace and Armstrong, 1976; Starace and Shahabi, 1980). Another approach is that of many-body perturbation theory (MBPT) (Brueckner, 1955a,b),which has been applied to atomic systems, particularly by Kelly (1964, 1968, 1969; Kelly and Ron, 1972; Kelly and Simons, 1973). Although in practice, if not in principle, this is a higher order calculation than RPAE, the number of terms (or Feynmann diagrams) to be included in determining the dipole matrix element rapidly becomes very large. The method has, however, been applied to open-shell atoms such as sodium (Chang and Kelly, 1975)and carbon (Carter and Kelly, 1975),as well as to the problem ofdouble ionization (Carter and Kelly, 1976; Changand Poe, 1975). The configuration interaction (CI) method (Burke and Seaton, 1971) can also be used to calculate quite accurate cross sections. Two techniques are currently used to determine the important radial functions and “final-state’’ coefficients that determine the initial bound state and final free-electron plus
80
R. STEPHEN BERRY AND SYDNEY LEACH
ion state wave functions. These are the R-matrix method (Burke et al., 1971 which, like MQDT, treats correlations within a partitioned space, and th linear algebraic equations approach (Eissner and Seaton, 1972; Seaton, 1974 Both closed- and open-shell atoms, as well as some positive ions, have beel studied by these techniques (LeDourneuf et al., 1975,1976,1979; Taylor ant Burke, 1976; Pradhan and Saraph, 1977; Burke and Taylor, 1979; Combet Farnoux and Ben Amar, 1980). Some work has also been done in applying quantum defect theor, (Dubau, 1978; Seaton, 1978) to the calculation of atomic photoionizatioi cross sections (Starace, 1976), e.g., for beryllium (Dubau and Wells, 1973). e. Photoionization cross sections in molecules. As mentioned earlier, thl quantum defect method is proving extremely useful, in its multichannel forn (Raoult et al., 1980; Jungen and Dill, 1980) for calculating photoionizatioi cross sections and p values for molecules, particularly in the threshoh regions exhibiting autoionizing resonances. The advantage of MQDT is it validity for any strength of interaction between discrete and continuun levels. It enables complex photoionization spectra to be interpreted with i small number of parameters and, as stated earlier, has a “collisionist approach in that an important concept is the exchange of energy and angula momentum between the excited electron and the ion core in a defined limitec volume of radius ro, outside of which the photoelectron is subjected to i long-range potential, e.g., a pure Coulomb field. MQDT can indeed deal wit1 the whole range of electron-core nuclei interactions, between the valid an( invalid limits of the Born-Oppenheimer approximation (Atabek an( Jungen, 1976). The final-state wave function outside the interaction region can bl calculated numerically or analytically in terms of a number of scatterin] parameters. These parameters are often obtained semiempirically, but i i some cases they can be calculated ab initio. A similar situation pertains ii R-matrix theory, which is used to calculate photoionization cross section over a large energy range, but where the theory stresses boundary con ditions at the limiting interacting volume edger = rot whereas in MQDT thi boundary conditions emphasized are r = 00. One of the principal difficulties in calculation of molecular photo ionization cross sections and angular distributions of photoelectrons is in thl adequate representation of the wave function of the outgoing continuun electron. Direct ab initio calculation of the continuum wave function wa made by Duzy and Berry (1976) in the case of N,, but their results did no reveal the existence, and effects, of the nu shape resonance in N, photo ionization (see later). A particular difficulty in molecular photoionizatioi calculations is in the nonseparable nature of the multicenter wave equation One of the most useful representations of the continuum involves thl
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
81
multiple-scattering method (Dill and Dehmer, 1974; Dehmer and Dill, 1979), which uses an approximate static and exchange-correlation potential. Other methods, such as the R-matrix approach and the single-center expansion (in which the molecular wave functions are expanded about a single center, thus reducing the problem to purely radial integrals, as for atoms), are described in a recent volume (Rescigno et al., 1979). A recent calculation on N, by Raseev et al. (1980) uses a one-center static-exchange approximation for the electronic continuum wave function (Raseev er al., 1978). A promising method for cross sections, but of little use for fl calculations, is that introduced by Langhoff (Langhoff et al., 1976,1980; Langhoff, 1979), in which Stieltjes and Tchebycheff imaging techniques (moment theory methods) are used for smoothing discrete L2 eigenfunctions in representing the continuum photoionization cross sections. One advantage is that with the use of L2 basis functions, atomic and molecular structure computational procedures can be employed (Langhoff et al., 1980). Let us consider in some detail the experimental and theoretical results on partial cross sections for vibrational and electronic levels of nitrogen. A detailed experimental investigation up to about 40 eV was carried out by Woodruff and Marr (1977) using a synchrotron radiation source and photoelectron detection. For the latter, the “magic angle” of 54”44’was used so as to make the measurements independent of the asymmetry parameter variations with photon energy (Gardner and Samson, 1975). Similar studies, on electronic state branching ratios, were carried out by Plummer et al. (1977). We will first consider electronic state partial cross sections. The ground state X’C,’ electron configuration of neutral N, is lag2lau, 20,’ 20,’ lnu43ag2. Successive removal of one 3a,, ln,, or 20, electron produces the N2+ ion in the X2C,+, A’II,, and B’C,’ states, respectively, at the following thresholds: 15.6, 17.0, and 18.8 eV. The partial cross section for the X2Cgt state of N,’ is seen in Fig. 19 (Woodruff and Marr, 1977). The data represent the vibrationally unresolved case. A striking observation is the rise in the cross section between 27 and 30 eV. Cohen and Fano (1966) attributed this feature to a transition between the 30, electron and an f-continuum wave. A resonance of this type was found for la, photoionization (Dehmer and Dill, 1975, 1976a). Calculations by Davenport (1976) confirmed that in the 3a, case, it is principally a finalstate resonance involving an f wave with a, symmetry, although the shape resonance was predicted to have a maximum about 31 eV, somewhat higher than the observed value of 28 eV. The results of more recent 3a, calculations of Dehmer er a/. (1979) and of Raseev et al. (1980), using methods briefly described earlier, are also given in Fig. 19. The photoionization cross sections were calculated as a function of the internuclear distance and an average taken on the vibrational motion, assuming that the initial state has u” = 0 and that the ionization potential is the same for all vibrational levels u’ of the
82
R. STEPHEN BERRY AND SYDNEY LEACH
1c
-I n C
.-c0. U
Y
ul ul
ul
2
V
E
C
I
20
I
1
30
I
I
40 Photon energy (eV)
FIG.19. Partial photoionization cross section for forming the X2Zs+ state of N 2 + .The experimental data).( of Woodruff and Marr (1977) are compared with the results ofcalculations by Dehmer et al. (1979) (---) and of Raseev el al. (1980) (-) (see text); i Mb = lo-'' cm2. (By permission from Raseev el al., 1980.)
ionic state. A peak is predicted around 30 eV by these two calculations, as well as by one, which did not average over vibrations, carried out by Rescigno et al. (1978) who used the Stieltjes-Tchebycheff imaging technique in representing the bound-continuum transition moments by bound-virtualbound ones (Langhoff et al., 1980). These calculations do not take into account autoionization, which is mainly responsible for the strong peaks observed at energies less than 25 eV. Since the f partial wave has CT, symmetry, it cannot be coupled to ungerade states, so that shape resonance effects are not expected or found (Woodruff and Marr, 1977; Plummer et al., 1977) for the (lq,)-' and (2oU)-' partial cross sections. It is interesting to note than in CO, isoelectronic with N, but
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
83
',
having no inversion symmetry, shape resonances are predicted in the (50)(40)- and (3a)-' photoionizations but not in the (ln)-' (Davenport, 1976). This has been confirmed experimentally by Plummer et al. (1977). Another important experimental observation on N, photoionization is the marked dependence of the partial cross section on the vibrational level in the residual ion in its X2Cg+ state. The relative intensities of the partial cross sections for the different vibrational states are markedly at variance with the Franck-Condon factors (Woodruff and Marr, 1977; West et al., 1980). Similar vibrationally dependent behavior is observed in the (50)- photoionization to form the X2X,+ ground state of CO' (Cole et al., 1980).These results for N, and CO, as well as the accompanying vibrationally dependent photoelectron angular distributions can be accounted for by autoionizing features and by the shape resonances discussed above, both of which processes can create a strong coupling between electronic and vibrational motion (see the discussion on vibrationally dependent fi values earlier in Section E,2,e).The relevant shape resonance calculations are those of Dehmer et al. (1979) and Raseev et al. (1980). Photoionization cross-section data are known for only a few molecules (Berkowitz, 1979). The rapidly expanding use of synchrotron radiation sources will indubitably improve this situation. As intimated earlier, such information will be of importance in studies of atmospheric pollution, upper-atmosphere chemical physics, the ionosphere, and planetary atmospheres in general. As an example of a polyatomic species, we will discuss the partial ionization cross sections in the linear species carbon dioxide. The klC,+ground state of neutral CO, has the electron configuration .. . 4ag23aU2lnU4lng4. Two methods, photoelectron spectroscopy and ion fluorescence, have been used to obtain the partial cross sections from state branching ratios in conjunction with calibration techniques. Photoelectron spectroscopy has been used mainly at a number of fixed wavelengths over the range 400-900 A (Bahr et al., 1969, 1972; Samson and Gardner, 1972; Samson et al., 1972; Gardner and Samson, 1973; J. A. R. Samson, private communication, 1977), but synchrotron radiation sources have also been used (Gustafsson et al., 1978). The first four ionic states, k211,, &,nu,B2C,+ and, e2C,+, are formed by successively removing an electron from each of the four outermost orbitals (Fig. 20). Energy resolution was insufficient to separate theA211, and P2Cu+states in the synchrotron radiation study of Gustafsson et al. The data given in Fig. 21 are those of J. A. R. Samson (private communication, 1977) using a line source. Marked variations in partial cross sections are observed in autoionization regions. Partial ionization cross sections have also been inferred for the B2Zu+ and&,& states from measurements of the ion-emission transitions B2C,' -+
',
eV
19.3E
18.OE
17.3'
!I
28005000A
~ bAo
I I 13.77
0 FIG.20. Electronic states and configurations of CO,' relative to the ground state of CO,
I
\
I
15
c
t
FIG. 21. Partial photoionization crass sections for production of CO,+ x2n,(o), A21 (O), D2Z,' ( A ) , and C2Z,' ( 0 )states over the incident photon-energy range 17.59-23.88 e using line sources (J. A. R. Samson, private communication, 1977). Data at discrete energi
have been connected by straight lines (see text).
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
85
X2n,and A2n,-+ X2n,over the photon excitation range 185-716 A (Lee and Judge, 1972; Carlson et a/., 1973; Samson and Gardner, 1973a; Gentieu and Mentall, 1976; Lee et al., 1976). The relative values of the b- and A-state partial cross sections are very different than those determined from PES measurements. At 584 A, for example, the fluorescence measurements indicate a branching ratio A/B = 2.2 0.3 (Leach et al., 1978a), whereas the PES data give A/B = 0.63 f 0.04 (J. A. R. Samson, private communication, 1977; Leach et al., 1978a). This discrepancy has been shown to be due to interelectronic coupling between the P2Xu and A2n,states of the molecular ion, which gives rise to a significant shift of photon-emission intensity from the spectral region usually considered to be '8-w (2800-2900 A), to that considered to be A-% (3000-5000 A); in contrast, there is only a very limited spectral redistribution in the photoelectron spectrum (Leach et al., 1978a,b). This exampfe shows that extreme care must be taken in interpreting fluorescence data to infer partial cross sections whenever there exist radiationless transitions involving theemitting states ofa molecular ion (Leach et al., 1980). +
4. Photoionization Eficiencies and the Decay
of Superexcited States
a. Generalities. An important parameter in photoionization is its efficiency or quantum yield. The photoionization efficiency yI is defined as the number of ions produced per photon initially absorbed. For atoms, yI is generally unity above the first threshold, but it can be less than unity in regions where superexcited states occur if autoionization is a slower process than other decay processes such as fluorescence emission. Such behavior has been noted for atomic oxygen (Dehmer et a!., 1973; Samson and Petrosky, 1974).Nevertheless, in most atomic cases, photoionization cross sections can be considered as identical to those of photoabsorption. The situation is markedly different for molecules, where there is a greater diversity of decay channels for superexcited states and a greater density of the latter since these now include rovibronic levels of electronic states. The primary act of photoabsorption can lead to direct ionization and also to the formation of superexcited states whose decay processes include not only autoionization and fluorescence, as is possible for atoms, but also other nonionic processes besides fluorescence such as neutral dissociation, predissociation, and intramolecular coupling to stable states. To a first approximation each process can be considered as independent and competitive, and thus have a distinct partial cross section, but strong interactions can also occur between these channels, leading to interference effects, just as we have seen in the case of direct and indirect ionization.
b. Competing decay channels of superexcited H , . Some of these competitive effects have been demonstrated for the hydrogen molecule. Figure 22
86
R. STEPHEN BERRY A N D SYDNEY LEACH
19-
18-
->,
17-
I
> 0 a
16-
W
2
w
15-
-I
4
F 14w z
I3-
12
-
II-
%
I
I
I
I
0.5
1.0
1.5
I
1
I
I
2.0 2.5 3.0 3.5 INTERNUCLEAR DISTANCE
I
I
I
4.0
4.5
5.0
(A)
.
’*
FIG.22. Selected potential energy curves for H2and H 2 + .(By permission from Sharp, 1971.)
reproduces some of the potential energy curves for H, and H2+given by Sharp (1971).The first ionization potential is at 15.34 eV (804 8).The dissociation limit H,+ H(1s) H(21) is at 14.67 eV (845 A), whereas the limit for ion-pair formation H, + H + H - is at 17.32 eV (716 8). We look first at autoionization processes over a much larger spectral region than in our earlier discussion on the threshold region calculations of Jungen and Dill (1980).Figure 23 gives, for the 762-807 8 region, the relative photoabsorption and photoionization cross sections for para-HZ observed at 77 K by Chupka and Berkowitz (1969) at a resolution of 0.04 A. The ions were analyzed by mass spectrometry for detection and mass analysis. The relative ionization ai and absorption a, cross sections were measured. The extensive structure is due to autoionization features. Detailed analysis is rendered difficult because of considerable configurational mixing in the superexcited states of H,. The multichannel quantum defect theory is proving most useful in this respect (Herzberg and Jungen, 1972;Atabek et a!., 1974; Jungen and Atabek, 1977). The superexcited-state assignments to
+
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
87
features in Fig. 23, as well as to the higher-resolution photoionization spectrum of Dehmer and Chupka (1976) given in Fig. 1 of Part I (Berry, 1980a), are therefore to be considered as only zero order in sense. It is clear from Fig. 23 that the ratio oi/o,, which measures kau,/ktOt,the autoionization rate relative to total decay rate, varies markedly among autoionization peaks, providing a direct demonstration of the existence of
FIG.23. Relative photoionization (uJ and photoabsorption (us)cross sections for para-H, at 78 K measured between 807 and 762 A with a spectral resolution half-width of 0.04 A. The numbers above many of the peaks in the u, data give the ui/uaratio. (By permission from Chupka and Berkowitz, 1969.)
88
R . STEPHEN BERRY AND SYDNEY LEACH
I
competitive decay channels. Autoionization rates can be obtained from the bandwidths only if the latter are greater than the energy resolution of the monochromator, which is not the case in Chupka and Berkowitz's (1969) spectra, nor, in general, in the better-resolved photoionization spectra of Dehmer and Chupka (1976). However, by deconvoluting the measured bandwidths to correct for instrumental resolution width, Dehmer and Chupka (1976) obtained experimental widths, which they compared with calculated values based on the two-channel quantum defect approach. Earlier calculations of H2 autoionization rates have been made by Berry and
89
ELEMENTARY ATTACHMENT A N D DETACHMENT PROCESSES
2;
0;
I0
-n
E
npo v.3
-0.9
770
771
772
773
PHOTON
WAVELENGTH
(A
774
775
776
FIG.23c
Nielsen (1970) and Shaw and Berry (1972),as well as by Herzberg and Jungen (1972), who compared the corresponding calculated linewidths with those they measured in low-temperature optical absorption spectra of H,. Autoionization propensity rules have been derived concerning principal quantum number (Bardsley, 1967) (rate K 3 )and vibrational level values (Berry, 1966) [probability p(Au = - 1) >> p(Au = -2) etc.]. The vibrational branching ratios are predicted to oscillate and differ considerably from the FranckCondon values as the photon energy passes through autoionizing resonances (Raoult and Jungen, 1981). The corresponding behavior for the photoelectron asymmetry parameter has been discussed earlier. Autoionization in H, has also been studied by threshold photoelectron spectroscopy at low (Villarejo, 1968; Stockbauer, 1979) and at much higher resolution (Peatman, 1976a,b). Propensity and selection rules have been
-
90
R. STEPHEN BERRY AND SYDNEY LEACH
tested, and the detailed routes examined of several degenerate autoionization processes (i.e., where the superexcited state is quasi-degenerate with the ion state to which it decays). Molecular fluorescence from superexcited states of H,, as well as from states above the dissociation limit at 14.67 eV, has been observed by Roncin et al. (1974) (see also Dieke, 1958) and the states identified. All lines are Q ( J ) , whereas R and P branch lines are absent. This indicates that the emitting levels are 'nu-states. These states, unlike the others known from optical spectra, have no available channels to which decay is much faster than photon emission. Predissociation of superexcited states has also been identified as an important decay channel in H,. The experimental observations of Guyon and his collaborators, using a synchrotron radiation source, are of H atom Lyman-cr, indicating predissociation to H(2p) + H(ls), and H-atom Balmer a, /?,and y lines, corresponding to predissociation to H(n = 3, 4, and 5) H(1s) (Borrell et al., 1977). [The H(2s)/H(2p) ratio has also been measured by Mentall and Guyon (1977), but only for predissociation below the ionization threshold.] Identification ofthe lsa, npo, I&,+ and lsa, npn, 'nustates involved has been made; some of the features are common to the molecular fluorescence excitation spectrum. A particular study has been made of predissociation of npn 'nulevels (n = 3-9) and the predissociation yields deduced for H, rovibronic levels (Guyon et al., 1979). Predissociation was found to be an important decay channel even when in competition with autoionization for the lower npn 'nu+states, whereas the npn 'nu-components have much smaller predissociation yields and decay mainly by a molecular fluorescence transition to the E, F'C,+ state ofH,, thus confirming the observations ofRoncin et al. (1974) mentioned above. Guyon et al. (1979) argue that the 'nu+components predissociate via coupling to 'Xu+ states, perhaps involving accidental predissociation (cf. Glass-Maujean et al., 1978; Glass-Maujean, 1979). Ion-pair formation has been observed by McCulloh and Walker (1974) and, at high resolution, by Chupka et al. (1975). This process is of relatively of the H2+ yield at little importance, having an intensity of about 4 x 714 A. The H - excitation spectra for para-H, are shown in Fig. 24 taken with a resolution of 0.07 A (middle curve) and of 0.035 A (bottom curve). The peaks at 710.2 and 714.3 A correspond to major peaks in the H Balmer-P excitation spectrum. There is no underlying continuum, which indicates that ion-pair formation is entirely by predissociation. The threshold of 17.32 eV ( - 716 A ) lies between the energies of dissociation to H(41) and (51). The features in the H- excitation spectrum can be correlated with Rydberg states converging to H2+ X2Cg+ (u 2 9). Work has also been done on D, and on HD. In the
+
91
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
8t
1
L
7
-
J
,
702
m c .c
3
10-
.
.
.
,
'
'
.
I
.
.
.
,
.
.
.
I
.
704
706
708
710
704
706
708 710 WAVELENGTH
.
.
,
.
.
.
,
.
.
.
,
.
712
714
716
712
714
716
.
(b)
-
.-
-e 4
-
50
-
z Y
-
u
-
o
IA
W
702
PHOTON
(a)
FIG.24. Photoionization efficiency curves for H2+(a) and H - (b,c) from para-H, taken at 78 K. The curves (a) and (b) were taken at wavelength resolution FWHM = 0.07 A, the curve (c) at FWHM = 0.035 A. Same identifications of high-n Rydberg levels converging to H,' X2Z,+, u = 9-11, are indicated. (By permission from Chupka el al., 1975.)
92
R . STEPHEN BERRY AND SYDNEY LEACH
latter case it has been possible to independently observe the channels leading D +. A theoretical discussion has been given by to H + D- and H Durup (1978).[It is perhaps appropriate to mention at this point that ion-pair , and CO has been studied over the photon energy range formation in 0 2NO, 17-30eV using synchrotron radiation (Oertel et al., 1980);see Section II,C,2.] We see that the whole range of decay channels of superexcited states has been observed for the hydrogen molecule. Insufficient information exists for a complete quantitative study of the rates and yields of the various processes over a large spectral range.
+
+
c. Molecular photoionization eficiencies: Larger molecules. In general, it is found that the ionization efficiency for molecules does not reach unity until several electron volts above the ionization threshold. This implies that nonionic channels must have decay rates that are comparable, to within a factor of 100, with autoionization rates over this energy region. There is little detailed work on the competitive processes for molecular species other than hydrogen. When yi reaches unity, autoionization becomes the overwhelming decay process of the superexcited states. In this region, the photoionization crosssection curves show little or no structure as is consistent with the linewidths associated with ultrafast autoionization decay rates ( - 10l6 sec- ') and the high densities of superexcited states at these energies. Illustrative of this behavior is Fig. 25, which shows the absorption and photoionization cross wave number 1.3 1.4
1.2
-n
I'
[lo' cm-l)
I
I
1.5
1.6
1.7
I
I
I
1.8 1.9 2 l
l
600
I
v I I'A1
I
photoionization
-
n 0 1
900
800
.
700 _ I . _ _ . L
w a v e ienyrn
/I\
I
I
600
500
e
51
n 0
\HI
FIG. 25. Absorption cross-section and photoionization cross-section functions for H2in the 900-500 A region; 1 Mb = 10- '*cmz.(Adapted, with permission, from Cook and Metzger, 1964.)
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
93
TABLE 111
DOMAIN OF COMPETITIVE NONIONIC DECAYPROCESSES OF SUPEREXCITED STATES IN SOME MOLECULAR SPECIES
Molecule H2 N2 0 2
co
NO H2 0
co,
NH, CH4 CZH, C2H4 C2H6
C6H6
Adiabatic first ionization potential (eV)
Lower energy limit” for q5i = 1 (eV1
Energy difference AEh (eV)
15.43 15.58 12.07 14.01 9.26 12.62 13.77 10.15 12.62 11.40 10.51 1 1.56 9.26
18.23 20.66 18.09 19.37 17.71 20.66 19.37 17.71 15.49 17.71 16.75 I8 16.53
2.80 4.08 4.02 5.36 8.45 4.04 5.60 7.56 2.87 6.31 6.24 ,.. 6.4 7.27
-
These limits have an accuracy of the order of 0.2-0.5 eV. Difference between third and second columns equals the domain of competitive nonionic decay. a
sections for H, from the work of Cook and Metzger (1964). Photoionization onset is at 804 A, but the corresponding cross section does not attain that of photoabsorption until about 680 A. Table I11 presents for a number of di-, tri-, and polyatomic molecules: column 2 shows the adiabatic first ionization potential (Berkowitz, 1979; Huber and Herzberg, 1979);column 3 gives the photon energy corresponding to ion formation with unit efficiency, estimated mainly from compilations given by Berkowitz (1979); and in column 4, AE, the energy difference between ( 3 ) and (2).AE therefore corresponds to the energy range above the first ionization limit in which superexcited states have nonnegligible probabilities for nonionic decay. It is seen that for this series of molecules, AE has values from -3 to -9 eV, but no systematic trends are obvious. F . Dissociative Ionization 1. Dissociative Ionization in H2
States reached by photoabsorption can give rise not only to stable ionic states by direct or indirect ionization but also to dissociative ionization. In
94
R. STEPHEN BERRY AND SYDNEY LEACH
H,, the dissociative ionization threshold is at 18.076 eV (Fig. 26). At this state that is reached by absorption within the threshold it is the lsog X2Z:,+ Franck-Condon zone (hatched area, Fig. 26).To reach the 2p0, ,Xu+and the still higher 2pn, repulsive states, higher photon energies are required. Dissociative ionization of molecular hydrogen is a particularly important process in interstellar molecule chemistry (Watson, 1975). Experimental studies have been carried out by Browning and Fryar (1973),Fryar and Browning (1973),and Strathdee and Browning (1976)using mass spectrometric detection; Glass-Maujean et al. (1979) studied the emission from excited H atoms formed in the dissociative processes, including dissociative ionization at E > -40 eV where the 2pn, state dissociates to H H + . The H + / H 2 + ratio is a constant 2% over most of the photon range up to 30.5 eV, after which it rises to 5% with the opening up of the 2pa, channel and to 11% at 40.8 eV when the 2pn, channel becomes accessible.
+
Potential mqy (eV)
Photon energy (eW
FIG.26. The ratio HC/H2+ from photodissociative ionization of H, ( 0 )and D+/D2+ from D, between 18 and 30 eV. Curves (a)-(d) are based on calculations using wave functions of varying sophistication for the ground states of H2and H,+ ; (e) shows to scale the potential energy curves for H, and H 2 + necessary to interpret the experimental results. (By permission from Browning and Fryar, 1973.)
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
95
The results of Strathdee and Browning (1976)argue in favor of autoionization effects in the 27 eV region; those of Glass-Maujean et al. (1979) support the contention that above 30 elf, dissociative ionization is a more probable process than (pre)dissociation of neutral superexcited states. Theoretical studies have been carried out by Ford et al. (1975).Agreement with the experimental H + / H 2 + ratios is good up to about 26 eV, beyond which the theory predicts too few H + ions. The “missing” protons could arise from opening up of the Iso, channel or, more probably (Bottcher and Docken, 1974; Hazi, 1974), from absorption into resonance states of H2 formed by adding an electron to the H 2 + 2po, orbital. These resonance processes deserve further experimental and theoretical study. 2. Role of Autoionization in Dissociative Ionization The optical resolution available in recent photoionization mass spectrometric studies has made it possible to study the profiles of autoionization features not only for the parent ion, as was discussed earlier, for example, in the case of H,, but also in the various fragment ion channels of small molecules. The results of systematic studies carried out by Berkowitz and his co-workers on diatomic and triatomic molecules, as well as on neopentane, have been brought together in a paper by Eland et al. (1980),which gives an empirical analysis of the autoionizing resonances observed in the total and partial photoionization cross sections. This analysis is based on the Fano single-resonance formulation extended also to the case of a single resonance interacting with several continua. Although it is clear that in reality there can be many overlapping resonances, as discussed for H, (Jungen, 1980; Jungen and Dill, 1980), nevertheless, in the absence of a full multichannel quantum defect theory treatment, which would require much unavailable data on energy levels, the analysis was carried out, faure de mieux, on a singleresonance basis. The approach can be considered as a simple parameterization in which the observed profile data is expressed in effective Fano q-shape parameter, resonance width r, and the resonance and continuum coupling strengths. The parameterization for the series of species studied gives consistent resonance energies and widths independent of the particular channel. Peak shapes are found to vary systematically with the relative intensity of the resonance and the continuum in the channel examined. Triatomic species of related electronic structure are found to have similar series of low 14) resonances. Further work on this parameterization approach is discussed by Eland (1980). Many results show that thresholds for dissociative photoionization are often associated with autoionization processes rather than direct photoionization. This can be seen, for example, in comparing the ionization cross-
96
R. STEPHEN BERRY AND SYDNEY LEACH
section curve with photoelectron spectra taken at a nonautoionizing photon energy. Such comparisons often show that ionization events, including dissociative processes, can take place in a "Franck-Condon gap." The onset of many dissociative ionization processes occurs at the thermodynamic threshold, which lies within the Franck-Condon gap where the probability of ion formation is extremely low as determined by He1 PES. This is the case for the formation of 0' from N,O (Dibeler e f al., 1967; Berkowitz and Eland, 1977; Nenner et al., 1980); 0 ' from CO, (Dibeler and Walker, 1967; McCulloh, 1973; Eland and Berkowitz, 1977); S+ from COS (Dibeler and Walker, 1968; Eland and Berkowitz, 1979); S' and CS' from CS, (Coppens et al., 1979; Eland and Berkowitz, 1979); SO' from SO, (Dibeler and Liston, 1968; Weiss et al., 1979); C,' from C,N, (Eland, 1979); HCO' from H,CO (Guyon et al., 1976; Vaz Pires et al., 1978); C3H3+from CH3C ECH (Parr (Parr et al., 1978). Some and Elder, 1968); and C3H3+from H,C=C=CH, further examples are given by Murray and Baer (1979). 3. Dissociative Ionization in CO,
The example we will discuss is CO,. The electronic energy levels of the CO,' ion are given in Fig. 20. Figure 27 shows the photoionization efficiency
co; z
0
LW
0'
In In
0
a
0
z
0
l-
a N z 9 w
? la W J
a
I
600 I
20.66 eV
.
I
610
#
I
620
.
I
630
.
'
640
I
650
660
I
I
I
19.99eV
19.37eV
18.78 eV
,
I
670
,
I
680 I
18.22 eV
WAVELENGTH (8) FIG.27. Relative photoion yield curves of C 0 2 + ,O + ,and CO+ in the vicinity of the dissociative ionization thresholds. Each curve is in different arbitrary units. The vertical arrow indicates the C 0 2 + C2Z,+-state threshold energy. (By permission from Eland and Berkowitz, 1977.)
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
97
curves for CO,', CO+, and 0' in the neighborhood of the dissociative ionization thresholds in the 19 eV region taken from the work of Eland and Berkowitz (1977).Each curve is in different arbitrary efficiency units so that their relative intensities are not directly comparable. The He1 PES spectrum of CO, (Fig. 28) (Eland, 1978) shows that this region is not accessible by direct ionization. The threshold for 0' is at 19.07eV (650A) [CO, O'(4S) CO X'X' (u = O)], about 0.32 eV below the e2Z,+-state threshold. The structure in the 0' curve between 19.07 and 19.39 eV indicates clearly that the dissociation processes occur here via superexcited states-the Rydberg levels of the Tanaka, Jursa, LeBlanc series converging to the C2E,+ state of C 0 2 +-which are coupled to dissociative ionization continua corresponding to lower lying CO,' states. The 0' current increases sharply at state threshold. This state has been shown to be completely prethe C'Z,' dissociative (Eland, 1972),thus accounting for the large increase in the partial cross section. However, spin conservation rules are violated here, since the dissociation products O'(4S) CO('Z ') can only form quartet states. At 19.337 eV lies the threshold for forming 0' and the CO molecule in its u = 1 state. Eland (1972), in a study of the kinetic energy distribution of the 0' ion, has concluded that at the e2Z.,' state onset, predissociation to 0' + CO creates about 85% of the CO X'Z,' molecules in the u = 1 state, the remainder in u = 0. The CO' ion is formed at its thermodynamic threshold at 19.446 eV, the process being CO, CO' X2Z,+(u = 0) + O(3P). At this energy the observed CO'/O' ratio from the two dissociative channels is 0.3 at 300 K. Rotationally excited levels of the c2Z,+(u = 0, 0,O) state with J > 40 are above the CO' dissociation limit and predissociation is principally to CO+ + 0. Rather large impact parameters are involved in con--+
+
+
--+
I
20
I
I
I8
I
I
I
16 ionization energy ( e V 1
I
1L
FIG.28. He1 photoelectron spectrum of C 0 2 .(By permission from Eland, 1978.)
98
R. STEPHEN BERRY A N D SYDNEY LEACH
serving angular momentum, which makes it somewhat surprising that the CO+ dissociation channel dominates over the 0' channel. However, this probably reflects the fact that CO+(2C,+) O(3P)correlates with doublet states of C 0 2 +(Leach, 1970) and could thus favor predissociation of c2C,' with respect to the quartet state formed by CO 0'. The structure in the CO+ partial photoionization yield curve indicates that vibrationally excited states open up new channels for CO' production. An experimental difficulty in determining the exact branching ratios for forming parent and fragment ions is the discrimination of ions formed with, or given, different kinetic energies. Most mass spectrometric methods used tend to discriminate against energetic ions so that carefully designed instrumentation is required (Masuoka and Samson, 1980). Measurements of the C 0 2 + / C O + / O + / C +ratios have been made at a number of wavelengths between 304 and 740 A (Berkowitz, 1979).This has been extended down to 90 A with measurements also including doubleionization processes forming C 0 , 2 and C2 (Masuoka and Samson, 1980). Cross-section data can be obtained by two methods: (1) the branching-ratio method in which the mass spectrum is accumulated at a given photon energy and the ratio is determined for the number of ions collected for a particular species to the total number of ions produced; and (2) the ions per photon method in which the wavelength is scanned for a fixed mass, the photon intensity being monitored simultaneously with the ion intensity. The relative cross sections determined by either method are put on an absolute basis by normalization at some calibration point. Double-ionization cross sections are given in Fig. 29. Maximum-peak partial cross sections are as follows: co2+(-37x 10-1Rcm2at-6OOA);CO+(-3.2 x 10-"cm2at -450A); 0 ' (-3.8 x lo-'' cm2, at -310 A); C + (-2.8 x lo-'* cmz at -340 A); C02,+ (-0.35 x lo-'' cm2 at -230 A); and C 2 + (-0.8 x cm2 at 130 8).Cross-section accuracy is estimated to be 10% for C 0 2 +;15% for CO', C + , and 0'; and 50% for C 0 2 2 + and C 2 + .Less-extensive data by other workers are given by Berkowitz (1979)and, for comparison, in the paper by Masuoka and Samson (1980).Recently, the (e, e ion) technique has been used to study ion fragmentation in CO, up to 80 eV (Hitchcock et a/., 1980) with results in good agreement with the photon data. Structure in all the fragmentation curves obtained by the photon and (e, e ion) techniques can be correlated with the multielectron transitions observed in the pseudophotoelectron spectra obtained with the (e,2e) technique by Brion and Tan (1978,1979).The total fragmentation cross section has three principal peaks: (1) -8 x lo-'' cm2 at -420 A ; (2) -9.5 x lo-'' cm2 at -330 A; and (3) 7.8 x 10- cm2 at 230 A (Masuoka and Samson, 1980).The excitation spectrum of vacuum ultraviolet fluorescence from CO, has been measured by
+
+
+
+
-
+
+
-
-
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
0.4,
,
1
1
(a).
- 0.3-Iz
-t
‘CC
0
0
-
0
0
-
.
. 0
-
0
0.2-
w m
$ ‘ *
-
0.
VI
,
g 0.1 E
0
0 0 0 0
u A
U
z o (I:
,
,
,
1
99
:
:
:
I
!
-
0
-
I
’
Lee et al. (1975). The spectrum is similar in shape to the total fragmentation curve determined by Masuoka and Samson ( 1 980). The absolute fluorescence cross sections appear to be about 5-10% of the total fragmentation values. The fluorescence is probably due to excited fragments produced by dissociative photoionization. 4. Theoretical and Experimental Approaches to Ion Fragmentation
Fragmentation of small ions can be treated theoretically in terms of potential energy hypersurfaces and determination of configurational point trajectories. Symmetry and quasi-symmetry properties of initial and final product states can be useful. Unimolecular fragmentation involves interelectronic-state coupling and, in many cases, intramolecular vibrational redistribution. In a recent review (Lorquet et a/., 1980),a discussion is given of nonadiabatic interactions and radiationless transitions in molecular ions. The various topologies of patential energy surfaces presenting nonadiabatic
100
R. STEPHEN BERRY AND SYDNEY LEACH
couplings are exemplified and their kinetic and dynamic implications considered, with applications to a number of small polyatomic ions. A more phenomenological discussion of predissociation processes in molecular ions is given in a review by Momigny (1980) who emphasizes the role of rotational predissociation. Fragmentation of molecular ions has mainly been discussed in terms of the quasi-equilibrium theory (QET) (Rosenstock et al., 1952, 1980; Rosenstock, 1968) or the essentially equivalent RRKM theory (Rice and Ramsperger, 1927; Kassel, 1928; Marcus and Rice, 1951)of mass spectra in which a statistical distribution or redistribution of vibronic energy in excess of a dissociation limit is assumed. A systematic experimental study of the dissociative photoionization of all alkanes from ethane through hexane, as well as n-heptane and n-octane, was carried out by Steiner et al. (1961) in the photon energy range up to 11.9 eV. The results tend to disagree with the statistical theory in the restricted energy region where there is a low density of ion electronic states. Following this valuable and enterprising early work, much has been published in this field, mainly using electron-impact ionization, but more recently, increasingly involving photoionization as well as photodissociation of ground-state molecular ions (Dunbar, 1979). New experimental techniques have been developed (Lifshitz, 1978), in particular coincidence techniques (Baer, 1979). Radiative and nonradiative yields (Leach et al., 1980)and fragmentation yields (Eland, 1979) can be determined by complementary photoion-fluorescence photon and photoion-photoelectron coincidence techniques. The photoion-photoelectron coincidence (PIPECO)techniques are reviewed in detail by Baer (1979). In the latter, the energy-analyzed electron selects the ion internal energy and provides a starting time for ion time-of-flight measurements. This gives much information on ion dynamics, in particular, the kinetic energy released in dissociation and its distribution (KERDs) and the parent ion lifetime with respect to dissociation. In earlier experiments, single wavelength sources, mainly He1 lamps, were used but more recently monochromatized continuum sources, in particular synchrotron radiation, have provided a means of ion-state selection by detection in coincidence with threshold electrons (Fig. 10). We will not enumerate the vast number of experimental results using the large variety of techniques mentioned above. In general (Lifshitz, 1978), the results show that vibrational randomization within a particular electronic state of the ion is complete prior to dissociation, but that internal conversion between electronic states is not always complete, giving rise to the notion of “isolated electronic states,” sometimes involving isomerization. Ion lifetimes have been determined in the lop3-10- sec range. A continuous distribution of lifetimes is observed under photon (and electron) impact ionization. The QET-RRKM theory is found to give reasonable rate constants, whereas the
’’
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
101
phase-space version of QET (Klots, 1964, 1972, 1976a; Pechukas and Light, 1965; Light, 1967), in which emphasis is on product states rather than the activated complex, can be used for energy disposal calculations. However, many questions are still open for particular cases; the general field of molecular ion dissociation dynamics is bound to flourish with the opening up of a variety of new experimental techniques and theoretical approaches. Among the theoretical methods we mention the possible application to the unimolecular decomposition of ions of the phase-space formulation (statistical adiabatic channel model) of Quack and Troe (1974, 1975a,b) and the maximum entropy formulation of the statistical theory of Levine (see, e g , Silberstein and Levine, 1980). G . Multiphoton Ionization 1. Multiphoton Ionization of Atoms
The advent of lasers has made it possible to study multiphoton ionization ( M P I ) processes in atoms and molecules via either real or virtual intermediate states. The ionization probability is a function of the laser frequency, coherence, and polarization, as well as the energy-level structure of the species. A considerable amount of work has been done on the physics of MPI in atoms. This has been reviewed by Lambropoulos (1976), van der Wiel and Granneman (1977), Letokhov (1978), Mainfray and Manus (1978), and Mainfray (1980). On the theoretical level, it is found that time-dependent perturbation theory gives a reasonably good description of MPI (Bebb and Gold, 1965).The ionization probability as a function of laser intensity is given by the lowest-order term in the perturbation series as long as one is far from a real state resonance region. The probability is modified very markedly in the resonance regions. A treatment analogous to that of Fano (1961) for autoionization has been developed for MPI in the resonance region (Beers and Armstrong, 1975; Feneuille and Armstrong, 1975). Laser bandwidth effects on MPI have also been studied in model calculations (Armstrong and Eberly, 1979). High fields (2lo9 W cm-') can induce level shifts and broadening and thus modify the resonance profiles. The variation of the effective order of nonlinearity K in a nominal 4-photon MPI of cesium, is shown in Fig. 30 as a function of resonance detuning AE = E(6F) - E(6S) - 3E(hv) where E(6F) and E(6S) are cesium atom energy levels, E(hv) is the laser photon energy (Morellec et al., 1976). The resonance involved is the 3-photon 6S-6F transition. The fourth power of the intensity law is valid in the off-resonance ( A E 2 10 c m - ' ) region. The corresponding resonance profile in terms of relative number of ions formed is given in Fig. 31 (Mainfray and Manus, 1978).
-30
20 Resonance
0
10
10
-
20
-
detunng B E = EBF E~~ 3 Ep
FIG.30. Variation of the effective order of nonlinearity K as a function of the resonance detuning AE in the four-photon ionization of Cs. (By permission from Morellec el al., 1976.) K = 6 log NJ6 log I . N; (arb1t r a r y u n i t s 1
lo!
10'
10:
102
10
I I
I
10588
10589
ARes. I
4
10590
c
0.3A FIG. 31. Resonance profile due to the 6S+6F three-photon transition in four-photon ionization of Cs. (By permission from Mainfray and Manus, 1978.) I = 4 x 10' W/cmZ.
103
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
Coherence effects become relatively important at high fields (210l2 W cm-2). The characteristic time for off-resonance MPI is -0.1 psec for AE 300 cm- '. For a single-mode laser pulse the coherence time t , is of the order of 10 nsec, whereas for an incoherent laser pulse of bandwidth 10 A, t c 1-10 psec, with the photons now arriving in bunches during the pulse duration, in contrast to the uniform sequential arrival in the single-mode case. The quasi-instantaneous laser intensity is therefore much enhanced in the incoherent laser-pulse case (bunched photons).For example, an enhancement of lo7 in the number of ions was observed in the 11-photon ionization of Xe with a Nd-glass laser pulse when the coherence time was changed from 10 nsec to 10 psec (Lecompte et al., 1975). The off-resonant N-photon ionization rate W is given by the following expression :
-
-
-
where oN is the generalized N-photon ionization cross section, I is the average laser intensity, and fN is the nth-order autocorrelation function. fN = 1 for a single-mode or bandwidth-limited laser pulse, whereas fN = N ! for an incoherent laser pulse whose statistical properties are related to thermal radiation. It follows that off-resonance MPI processes are excellent probes for the statistical properties of laser pulses. It should be noted that oN is a function of the polarization properties of the laser radiation, since selection rules for electric dipole transitions will apply for each stage of the N-photon absorption process. Electric quadrupole transitions have also been observed in a 3-photon MPI process in Na (Lambropoulos et al., 1975). The laser polarization properties were used to study the angular distribution of photoelectrons from the resonant 2-photon MPI process + Na(3p 2P,i2)+ Na+ e - as excited sequentially by two Na(3s 2S1/2) linearly polarized pulsed laser beams (Duncanson et al., 1976; Strand et al., 1978; Leuchs et al., 1979; Hansen et a/., 1980). A theoretical treatment predicted that the photoelectron angular distribution depends on the degree of coherence among the hyperfine levels and on the time interval between the exciting pulse and the ionizing pulse (Strand et al., 1978; Strand, 1979). The experimental results, including the observation of incipient quantum beats (Strand et a/., 1978; Leuchs et a/., 1979; Hansen et al., 1980), confirmed these predictions and also showed that an essentially completely coherent superposition of Na(3p 2P,,2) intermediate hyperfine states is produced. The theory of this process was developed for resonant two-photon ionization of diatomic and linear molecules by Hansen (1979), but no such experiments have been reported at the time of this writing. Laser polarization properties can also be used to produce spin-polarized photoelectrons via MPI. The total cross section for this process gives an MPI
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R . STEPHEN BERRY A N D SYDNEY LEACH
version of the Fano effect (Fano, 1969). However, the angular dependence of spin polarization can be considerable. The coupling between the angular momentum of an excited electron and that of the core, of course, influences the angular distribution of photoelectrons. Moreover, this distribution depends on the value of the Hund’s case coupling of the core (Hansen, 1979). The study by Strand et a!. (1978) included a theoretical prediction of the effects of hyperfine interactions on the angular dependence and the total electron spin polarization produced by two collinear, circularly polarized laser beams via the process Na(’S,,,) -t Na(’P,,,) --* Na+(’S,) e - . It was concluded that laser pulses shorter than 1 nsec arriving almost simultaneously would be necessary to produce completely polarized electrons by resonant two-photon ionization of Na. It is, of course, necessary for the laser bandwidth to be much smaller than t h e j = i,2 separation in the intermediate ’P state, a condition very easily met. Recently, electrons produced by MPI have been observed to undergo free-free transitions as they continue to interact with the laser radiation while in the ion field (Agostini et a/., 1979). It should also be mentioned that multiphoton ionization transitions can be used to obtain coherent emission in the vacuum ultraviolet in Ne 532 A and He 380 (Reintjes et al., 1976, 1977). We mention here the possibility of MPI where the penultimate state is a very highly excited Rydberg state, so that the last stage of ionization would require very little energy. Beiting et a / . (1979) have carried out a two-laser experiment in Na, in which the first laser saturated a D transition, populating the 3p 2P3,2state, while the second excited selected ns or nd Rydberg levels. They observed Na+ ion signals which were consistent with the photoionization of the highly excited atoms by 300 K blackbody radiation. These results illustrate the necessity of avoiding unwanted thermal radiation effects in studying processes involving highly excited atoms and molecules. They also demonstrate the potential of these species as infrared detectors. Finally, for atoms, we stress that MPI studies at high light intensities can be severely perturbed by the existence of even a small fraction of molecular dimers of an atomic gas. This is particularly important in the case of alkali atoms (Held et a/., 1972; Granneman et a/., 1976; Hermann et a/., 1977a,b, 1978; Klewer et a/., 1977).
+
-
-
a
2. Multiphoton Ionization of Molecules: Spectroscopy; Cross Sections As for any other kind of ionization process, multiphoton ionization of molecules is more complex than for atoms because of the greater number of decay channels. Furthermore, the possibility of dissociation of either ion or
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
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neutral into (at least some) neutral fragments means that MPI of the fragment(s) can also occur. The effects much studied for atoms-saturation, level shift and broadening, laser-pulse profile-have thus far been little investigated in the case of molecules. Molecular spectroscopy is beginning to benefit considerably from MPI. The one-photon selection rules of traditional absorption spectroscopy are no longer limiting factors; new states can be reached and identified. Assignments of molecular excited states can be determined or verified using polarization properties (McClain, 1974; Berg et al., 1978; Lehmann et al., 1978; McClain and Harris, 1977; Parker and Avouris, 1979; Heath et al., 1980). Photoionization thresholds can also be measured, as is illustrated in Fig. 32 for pyrrole (Williamson et al., 1979). Furthermore, the lifetimes of electronic excited states can also be determined using the MPI technique with a variable delay between pulses from the two lasers used to produce ionization (Parker and El-Sayed, 1979).An example is the H,CO molecule for which a nitrogen and a hydrogen laser were used (Andreyev et al., 1977; Antonov et al., 1977). Spectroscopic studies have been done both using a single wavelength and also a tunable laser. Much more information is obtained when tunable radiation brings the molecule to a resonant intermediate state with a second laser to provide the ionization step. A notable simplificatiofi of the spectrum can be obtained, as has been demonstrated in the case of I , (Williamson and Compton, 1979) in which “competing” 1- or 3-photon resonances can be greatly reduced in importance by this two-laser technique. Further details on, and references to, MPI techniques in molecules are given in the review papers of Johnson (1980a,b). The electronic states revealed by MPI spectroscopy are usually the Rydberg states. This results from the fact that highly excited valence states are relatively strongly coupled to decay channels so that they largely disappear before ionization can occur. However, the valence-excited states of benzene and its derivatives have been studied in detail by MPI (KroghJespersen et al., 1979; Johnson, 1980b). In many cases, Rydberg levels previously masked by valence absorptions in traditional spectroscopy have been revealed via the MPI technique. For example, 3s Rydberg levels, which are parity forbidden in one-photon transitions in molecules having a center of symmetry, have been observed in a number of polyatomic species (Johnson, 1975, 1976; Nieman and Colson, 1978; Turner et al., 1978). As a further example, we present the recent work of Johnson (1980b) on methylbenzenes. Figure 33 shows the one-photon absorption curve of toluene and the MPI spectrum for which there is obviously much more structure and which clearly shows the presence of two Rydberg states in this region. Similar behavior is observed for p-xylene where the Rydberg transition, although
106
R. STEPHEN BERRY AND SYDNEY LEACH
'
"6 "5
ij'...;.11
Lb' ?
I
'
l
IP
4
1 1
l (0
*\ i \
t 0-
I
I
I
I
c
1
I
4480 (500 VUV WAVELENGTH
I >
4520
(A)
2
0
0 4400
4500 LASER WAVELENGTH
4600
(d)
FIG. 32. (a) Single-photon ionization efficiency of pyrrole. Arrows indicate ionization energies of the first band in the photoelectron spectrum of pyrrole. (b) Three-photon ionization signal of pyrrole at a pressure 60 mtorr. The data are not corrected for variation of laser power with wavelength. The background ion signal below threshold energy is due to four-photon ionization. (c) Three-photon ionization signal of pyrrole at its room-temperature vapor pressure of about 8 torr. (By permission from Williamson et al., 1979.)
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
107
I I
I
I
I
I
180
185 190 WAVELENGTH i nm) FIG. 33. Single-photon absorption spectrum (upper curve) and multiphoton ionization spectrum (lower curve) for toluene in the 180-190 nm region. (By permission from Johnson, 1980b.)
parity forbidden, is induced vibronically. Polarization properties were used to assign the Rydberg state symmetry and the 3p, nature of the atomic orbital involved. Multiphoton ionization cross-section calculations are rare for molecules. We mention in particular the case of four-photon ionization of NO, which Cremaschi et al. (1978) have treated using a quantum defect method to calculate the matrix elements. The cross sections obtained enable one to describe the behavior of the system in the short time limit. Rate equations were developed for the long time limit. This kinetic approach to multiphoton processes (Bradley et al., 1972; Zakheim and Johnson, 1980) is often not particularly suitable in practice because of uncertainties in laser intensity, but it can be used to account for most of the spectral feature intensities in NO (Zakheim and Johnson, 1978). The calculated cross sections are in accord with experiment in predicting a greater intensity for a 2 2 ionization (resonance on absorption of second photon) than in a 3 1 ionization process. Order of magnitude values for 2-, 3-, and 4-photon MPI are IO-" cm'sec' and 10-'Oo cm8 sec2. cm4 sec, In much of the work reported, MPI takes place in a static gas cell. Recently, Johnson has introduced the use of supersonic jet (expanded nozzle) beams in MPI (Zakheim and Johnson, 1978).The target molecules are thus cooled to rotational temperatures of the order of 1 K and vibrational temperatures of a few tens of degrees Kelvin (Smalley et al., 1975, 1977). This technique has been applied to the MPI of N O (Zakheim and Johnson, 1978), where the low temperature simplified MPI spectrum aids in analysis, and to aniline (Dietz ef al., 19801, metal carbonyls (Duncan et a/., 1979)and benzene,
+ +
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R. STEPHEN BERRY A N D SYDNEY LEACH
fluorobenzene and chlorobenzene (Murakami et al., 1980). In this last case, it was concluded that the 4-photon MPI spectrum correctly reflects the two-photon absorption cross sections to the excited S , state. Some work has been done on MPI in liquids, in particular by Vaida et al. (1978)and by Scott et al. (1979).The photon-energy onset of conduction in the liquid phase can, in principle, be used to determine ionization potential stabilization energies with respect to the gas phase.
3. Multiphoton Ionization of Molecules: Dissociative Processes In most of the molecular MPI experiments mentioned above, where the principal aim is spectroscopic in nature, the total ion current is detected. Much more detailed information is available with concurrent mass analysis. Since pulsed lasers are often excitation sources in MPI, mass analysis can be achieved by time-of-flight measurements, although magnetic spectrometers have been used in some cases. With the mass analysis capability comes the possibility of studying dissociative ionization following multiphoton absorption. Fragmentation can ensue by several possible processes: (1) multiphoton transition to a repulsive or predissociative ion state, (2) transition to a stable ion state followed by photon absorption to a dissociative region, and (3) transition to a neutral dissociative or predissociative state that leads to fragments which can undergo ionization via further photon absorption. Mass analysis in MPI was introduced by Klewer et al. (1977) who observed the two-photon ionization of Cs, and also showed the formation of Cs- at photon energies below the Cs2+ threshold. the ion pair Cs' Instruments have been built that give a time-of-flight analysis of the ions or neutrals, a high resolution of the kinetic energy, and the possibility of studying angular distributions (de Vries et al., 1980; van der Wiel, 1980). Angular distributions, as well as polarization ratios of ion current, are of course important in the analysis of the symmetry of intermediate states in MPI. Molecular beams of the effusive kind are also used in MPI. Feldman et al. (1977)studied the MPI (without mass analysis) of a Na, beam as well as of the BaCl product of the Ba HCI reaction, thus demonstrating the utility of MPI as a detection technique in reactive collision experiments. Herrmann et al. (1977a,b, 1978) used a two-laser system and mass analysis to study the MPI of Na,. The isotopically selective formation of Liz+ has been studied by two-photon ionization of Li, beams (B. P. Mathur et al., 1978; Rothe et al., 1978). Two-laser, two-photon ionization of molecular beams, with mass analysis, has been discussed by Letokhov (1977)and results presented on the kinetics of stepwise photoionization and fragmentation of NO,, benzaldehyde, benzophenone, nitrobenzene, and nitrotoluene by Antonov et al.
+
+
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
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(1978). Polyatomic species were first studied by MPI of a molecular beam with mass analysis by Boesl er al. (1978) whose work on benzene confirms that MPI is a very selective and versatile ionization source for mass spectrometry; in particular it should be stressed that the distinct structure of the intermediate state, reflected in the MPI spectrum, is highly species specific. Indeed, this latter quality has led to consideration of two-photon ionization spectroscopy as a method for teal-time monitoring of atmospheric polluants (Brophy and Rettner, 1979). Ionization of molecular beams, involving the absorption of more than two photons, and with mass analysis, was first carried out by Zandee et al. (1978)on 1,. Extensive studies have been carried out by Zandee and Bernstein (1979a,b) on NO, I,, benzene, and butadiene using a tunable, nitrogenpumped, dye laser. At each resonance corresponding to the m-photon ionization of an n-photon intermediate state, the fragmentation pattern of the ions is measured. The minimum number of photons absorbed per molecule to form a fragment can be deduced if the appearance potential of the fragment is known (which is not always the case). MPI and the ensuing fragmentation remain extremely wavelength selective, even at the highest laser-peak power densities used. The nonresonant contribution to ionization is only a small fraction of the resonance-enhanced ion yields. About of the beam molecules within the focal region are ionized by the laser pulse. Several studies have been carried out on benzene. Using a low-power peak laser, Boesl et a/. (1978)observed formation only of the parent ion in a two-photon ionization process. Rockwood et al. (1979) and Reilly and Kompa (1979), using KrF or ArF lasers, observed fragmentation of benzene, the fragmentation patterns and relative intensities being dependent on the laser-power density. This agrees with the results of Zandee and Bernstein (1979a,b)who found that increasing the laser power favored the formation of smaller fragments. Indeed, it is possible to achieve almost exclusive formation of C + ions, as found also by Cooper et a/. (1980). The fragmentation patterns are also vibronic state dependent as depicted in Fig. 34, which gives a two-dimensional optical-mass spectrum of benzene (Zandee and Bernstein, 1979a,b)for relatively low laser power densities. Zandee and Bernstein’s (1979b)analysis of the results lead them to suggest that the resonance multiphoton dissociative ionization process involves pumping neutral C,H, up its vibronic energy levels until an autoionizing level is reached to yield the fragment ions. Alternatively, the up-pumping to dissociation proceeds via the ionic ladder. In similar molecular beam MPI studies on azulene and naphthalene, but using both single laser and two-laser excitation (Lubman et al., 1980),the data obtained support the suggestion ofZandee and Bernstein that autoionizing states play an important role in the MPI dissociative ionization process, at least in these aromatic systems.
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R. STEPHEN BERRY AND SYDNEY LEACH
I-
2 Y
a 0:
FIG.34. Two-dimensional optical-mass spectra o f benzene for relatively low laser power intensities. Wavelength spectrum of total ions (top) is broken down into its constituent contributions from each of the Ci fragment ions. (By permission from Zandee and Bernstein, 1979a.b.)
It is clear that dissociative ionization in MPI brings in interesting new theoretical problems to add to those concerning QET-RRKM theory and its validity that we mentioned earlier in the case of one-photon ionization. In a statistical interpretation of the MPI fragmentation in benzene, Silberstein and Levine (1980) argue that differences in fragmentation patterns at different power levels (or also in comparison with electron-impact ion fragmentation studies) do not necessarily reflect dynamical effects. They suggest instead that these differences are due to the phase space available to the different fragments at increasing levels of energy deposition in the molecule. In their view, the initial absorption provides selectivity in MPI, but the subsequent energy uptake and fragmentation will be dominated by the available phase space, unless there are special dynamic constraints such as could exist in molecules having bonds of very different strengths. An example of the latter is to be found in the MPI study ofmetal carbonyls (Duncan et al., 1979).For the case of benzene, Silberstein and Levine (1980)show that on varying the mean energy deposited per parent molecule, a simple statistical theory based on the maximum-entropy formalism was able to predict,
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
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in reasonable agreement with experiment, the variation of the fragmentation pattern from almost exclusively parent ions at low energies, to fairly large, stable fragment ions at medium energies and small energy-expensive ions at high energies. Fisanick et al. (1980) have obtained experimental results on the multiphoton dissociative ionization of acetaldehyde, which, in general, parallel those mentioned above for the aromatic species, in particular a change of fragmentation pattern with laser power. They used a rate equation approach to explain their results and developed an expression for the individual fragment ion intensities as a function of laser power. All ionization processes were assumed to be direct, i.e., no long-lived predissociating or autoionizing states, with all fragmentation occurring during the laser pulse. Eventual saturation and spatial effects are considered as well as ion loss through dissociation and the further photofragmentation of ions formed. Application to the case of acetaldehyde does account reasonably for the power dependencies found for the parent and fragment ions. 4. Infrared Multiphoton Ionization of Molecules
Besides excitation with visible and ultraviolet lasers, multiphoton ionization of molecules can also be achieved using infrared radiation: BCI, (Akulin e f al., 1975; Karlov et al., 1976), SiF, (Karlov, 1978), H,O and D,O (Chin, 1977; Chin and Faubert, 1978), C H 3 N 0 2(Avouris et a!., 1979), but the exact mechanisms in each case require further investigation. The initiation of CO, laser-induced plasmas has been thought to involve the acceleration and multiplication of preexisting free electrons (Kroll and Watson, 1972; Yablonovitch, 1973),but it is possible that the initiating process is IR-MPI. Further, the production of free electrons by IR-MPI could play an important part in the chemistry of CO, laser-irradiated polyatomic molecules (Avouris et al., 1979). Finally, we mention that previously produced gas-phase ions trapped in ion traps can be dissociated by multiphoton absorption of visible or infrared radiation; most interestingly, the IR intensities used are very much lower than in the usual IR multiphoton dissociation experiments. This work has recently been reviewed by Woodin et al. (1979).Recent work by Coggiola et al. (1980) has investigated the final step in IR multiphoton dissociation of polyatomic ions. They formed highly vibrationally excited CF31+,CF,Br+, and CF,Cl+ ions under collision-free conditions and studied the fragmentaX on absorption of a single lop IR photon. The dissociation tion into CF,' yields peaked sharply at absorption frequencies in the molecular ion, but it is considered that absorption takes place in the vibrational quasi-continuum.
+
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R. STEPHEN BERRY AND SYDNEY LEACH
H . Photodetachment
1. General Remarks The photoionization processes we have discussed involve ionization thresholds in the UV and vacuum UV regions. An important class of processes is photodetachment, which corresponds to ionization of a negative ion. The outermost electron is weakly bound ( 55 eV) so that photodetachment thresholds are in the IR, visible or near-UV domains. In the present section we will be concerned with photodetachment from negative ions which have positive binding energies (electron affinities); we thus exclude temporary attachment processes that are observed as electron-scattering resonances (Schultz, 1973a,b). Determination of binding energies will not be discussed. Our concern is with processes reviewed in the section on photoionization, cross sections in particular. Furthermore, since theoretical methods of calculating photodetachment cross sections are similar to those used for neutrals, this topic will be little discussed except for a few particular cases. Much of the work in this area stems from activity at the National Bureau of Standards, from the first measurements of the A- + hv -+ A + e - process, on Rb and Cs by Mohler and Boeckner (1929),the extensive pioneering work of Branscomb and his co-workers (Branscomb, 1962)and the development of laser photodetachment studies by Lineberger (1974) and his co-workers. An early, influential, review was a book by Massey in 1936, which has been updated in 1950 and 1976 (1976). Other reviews of interest are those of Berry (1969b), Steiner (1972), Franklin and Harland (1974), Lineberger (1974), Hotop and Lineberger (1975), Lineberger et al. (1976), and Janousek and Brauman (1979). Bibliographies of interest are included in NBS Special Supplement No. 426 (Kieffer, 1976) and its supplements (Gallagher et al., 1978). A review of radiation processes of atomic negative ions, in the context of plasma physics, has been written by Popp (1975). Important driving forces in this field include astrophysics and planetary atmosphere and ionosphere research. In 1939, Wildt suggested that opacity of the sun’s atmosphere in the red and infrared regions could be due to photon absorption by H-. Theoretical calculations of the photodetachment rate supported this idea (Bates and Massey, 1940; Chandrasekhar, 1945; Chandrasekhar and Elbert, 1958). Experimental confirmation in the laboratory was achieved by Branscomb and Smith (1955) and Smith and Burch (1959). The use of lasers has made very high-energy resolution possible in the crossed-beam configuration developed early by Branscomb, but whose resolution was of the order of 100A. Furthermore, PES techniques have also been used on negative ions, using fixed-frequency lasers, in which energy analysis, and sometimes values of the photoelectrons are studied (Celotta et al., 1972, 1974; Hotop et al., 1973a; Siege1 et al., 1972).
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Other techniques have also been used in which high concentrations of negative ions are produced in ion-pair production processes by shock-tube techniques. For example, halogen negative-ion photodetachment has been studied by Berry et al. (1961), as well as the reverse process of radiative attachment (Berry and David, 1964; Muck and Popp, 1968), and the corresponding electron affinities determined with precision (Milstein and Berry, 1971). However, this is a generally more limited technique than that using crossed beams. Another useful technique involves ion storage, in particular ion cyclotron resonance (ICR), which can be used to study photodetachment (Smyth and Brauman, 1972a,b,c; Reed and Brauman, 1974; Janousek and Brauman, 1979). This has been done mainly for molecular negative ions. 2. Photodetachment Cross Sections Near Threshold: Theoretical Remarks Understanding cross-section behavior near threshold is important for photodetachment of negative ions, in particular for determination of accurate threshold energies to give electron affinity values. Furthermore, this behavior provides information on the orbital momentum of the bound detachable electron. Wigner’s (1948) general treatment of the threshold behavior of reactions involving the collision of two particles and having two final products can be applied to this situation. The cross section will be proportional to the product of the squared matrix element between the discrete initial and continuum final states, and the density of states in the continuum. is the photoThe latter is proportional to E’’’ where E = (hv - Ethreshold) electron energy. In determining the behavior of the matrix element with photon energy, it must be recognized that there is an important difference between photoionization and photodetachment. This resides in the nature of the potential field. The radial wave equation contains a 1(1 l)/r2 contribution to the potential. In photoionization, this term falls off much more rapidly than the l/r Coulomb term. The threshold dependence of the photoionization cross section then tends to be a step function. However, in photodetachment, the centrifugalbarrier term dominates over other contributions at long distances, the interaction between photodetached electron and neutral-core final products being short-ranged. This means that Wigner’s result for two neutral products is applicable.There results a variation ofthe photodetachment cross section with energy at threshold, dependent on the angular momentum of the electron in the final free state. Dipole selection rules make it possible to correlate this dependence with the angular momenta of the electron in the initial state (Massey, 1976). With appropriate representation of the continuum wave function, the photodetachment cross section near threshold can be expressed as a power
+
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R. STEPHEN BERRY A N D SYDNEY LEACH
series in k, the linear momentum of the freed electron, raised to a power that is a function of I, which is essentially the angular momentum component of the continuum state in which the photoejected electron moves under the influence of the residual atom field (Wigner, 1948; Branscomb et al., 1958; Massey, 1976): r ~ ”cc
vk2‘+‘(ao
+ a , k 2 + bk21nk + a2k4 + . . .)
(23)
where the a, and b are constant coefficients whose values depend on the bound-state wave function and on details of the potential (O’Malley, 1965; Hotop et al., 1973b). Since k will be proportional to the square root of the kinetic energy of the photoejected electron, it can be replaced in Eq. (23) by E”’. In general, the electron will be ejected preferentially into the lowest angular momentum state allowed by conservation of angular momentum and parity. For diatomic molecules, the appropriate power law will depend upon the angular momentum, projected along the internuclear axis, associated with the orbital from which the electron is photoejected (Geltman, 1958).It should be noted that Geltman’s theory may not apply very close to threshold since the magnitude of the velocity of the outgoing electron there will be less than or comparable to nuclear velocities, leading to a breakdown of the BornOppenheimer approximation. However, at a few hundred cm- above threshold, this is no longer a problem. A further problem, mentioned later, is that Geltman’s theory takes no account of the cross-section dependence on molecular rotation. Extension of Wigner’s analysis to the case of a species of any molecular symmetry has been achieved by Reed et al. (1 976), using group theory considerations. The limitations of this approach are most evident for systems of low symmetry where there are, of course, few restrictions on the types of allowed transitions. Another shortcoming is that the symmetry rules do not inform us on the relative intensities and post-threshold behavior of allowed transitions. A one-electron formalism has been used very successfully by Reed et al. (1976) to calculate the relative photodetachment cross sections in the threshold region for a number of diatomic and polyatomic negative ions, with results that agree well with experiment in spite of neglect of electrondipole interactions, as in Geltman’s work. Examples will be given later.
’
3. Atomic Negative Ions Photodetachment threshold studies have been made for a number of atomic negative ions (Lineberger et al., 1976). Let us consider the case of photodetachment from Se-, whose electron configuration is . . . 4s2 4p5.
115
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
1
i -*--f 1
14
I
15
16
I
I
17
1 18
I
I
I
19
PHOTON ENERGY ( 1 0 3cm-1) FIG. 35. Se- photodetachment cross section in the energy range (14.000-19,000 cm-': Se-(2P,,,,,,,1 ~ V - + S ~ ( ~ P , . , ,+, , e) - . Fine structure transition thresholds are indicated by vertical arrows. (By permission from Hotop et ol., 1973a.)
+
Detachment of a p electron leads to an outgoing s or d wave, since photon absorption leads to a unit change of the I quantum number. Close to threshold, the d-wave contribution is negligible, so that the cross section at threshold should be proportional to k . Experimentally, several fine-structure transitions are observed by conventional spectroscopy (Berry et al., 1965) and by laser photodetachment spectroscopy (Hotop et al., 1973a), as shown in Fig. 35. The Se- negative ion can exist in the 2P,i2and 2P3/2states, whereas there are three possible final Because of the large 2P,1z-2P312splitting, states in the residual atom, 3P0,L,2. most ions in the beam are in the lowest state, 'P312, which helps in the identification of the various thresholds. It is found that the individual thresholds have shapes predicted by the Wigner threshold law, but only over a restricted photon-energy range (Fig. 36), e.g., as can be seen in the partial photodetachment cross section for Se-(2P,/,) + Se('P,). The relative strengths of the fine-structure transitions measured for Sehave been compared with the results of a number of model calculations (Lineberger et al., 1976). Reasonable agreement is obtained with a model in which the final state is considered as a (Se e - ) complex within the LS
+
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R. STEPHEN BERRY A N D SYDNEY LEACH
500
I
1
I
1
1
1
I
I
ELECTRON MOMENTUM k [l/a,] FIG. 36. Se-('P3,,)+Se(3P,) + e- partial photodetachment cross section plotted as a function of electron momentum. The straight line represents the Wigner threshold law. (By permission from Hotop et al., 1973a.)
coupling approximation and following the dissociation of the complex into the various fine-structure exit channels (Lineberger and Woodward, 1970; Rau and Fano, 1971). This is in the spirit of the original approach by Wigner (1948)who, in his general discussion of two-particle collision reactions, proposed that the probability of a particular reaction near threshold will depend solely on the propensity to dissociation of the collision complex. The important factor is thus the long-range interaction of the dissociating particles and not the nature of the transition involved. Rau (1976)has pointed out that the behavior of the cross section away from threshold is relatively little influenced by the dynamics of the photodetachment process and that the branching ratios for the individual exit channels are given by an easily evaluated geometrical factor. This approach accounts well for the experimentally observed departures of the branching ratios from the statistical ratios expected at a lower level of approximation. Interesting results are observed in photodetachment to an excited state of the neutral, because of interference effects between ground- and excitedstate channels. Cusps can arise in the ground-state channel cross section as discussed by Wigner ( 1948) and, for photodetachment processes, by Moores and Norcross (1974).Experimental observations in light alkali negative ions, e.g., Na- (Lineberger, 1974) show cross-section shapes corresponding to Wigner cusps near thresholds at opening of 'P channels. For heavier alkali
c
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negative ions, such as Rb- and Cs-, photodetachment in energy regions where the lower np final-state channel opens up is dominated by the doubly excited states of the negative ion, which lie close to this threshold (Patterson et al., 1974; Slater et al., 1978). This gives rise to Fano lineshapes corresponding to window resonances such as is shown in Fig. 37 for Cs- (Slater et al., 1978). The Cs- resonances are consistent with the close-coupling calculations of Moores and Norcross (1974), but the latter d o not take into account the effects of the 'P fine structure. The semiempirical multichannel photodetachment model of Lee (1975) includes the fine structure, and is based on MQDT adapted to the case of an electron interacting with a neutral-atom core. The multichannel wave functions are defined in terms of an R-matrix, whose elements are determined empirically. However, the constraints of the model are such that Lee's theory does not adequately account for the long-range polarization interaction between the electron undergoing photoejection and the neutral core. Furthermore, two-electron correlation becomes more and more important as higher and higher Rydberg levels are involved in the final neutral state. The limiting case is the threshold for photodetachment of two electrons. This is a field of great interest (Lineberger el al., 1976), both from the theoretical and from the experimental
PHOTON E N E R G Y , cm-1 FIG.37. Cs- photodetachment ground-state partial cross section near the 'P threshold: Cs- + hv-+Cs(ZS) + e - . The depth of the minimum near 14,962 c m - ' is limited by the laser linewidth of 3 cm-'. The error bars reflect the uncertainty in the normalization of the 'P partial cross section to the total cross section. (By permission from Slater P I a/., 1978.)
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R. STEPHEN BERRY AND SYDNEY LEACH
viewpoint, but only preliminary experiments have been reported thus far (Slater et al., 1978). It should also be noted that energetics has been the main concern of photodetachment studies. Relatively little work has been done on photoelectron asymmetry behavior in this case, although there is certainly some theoretical interest (e.g., Moores and Norcross, 1974)and some experimental work has also been done (e.g., Celotta et al., 1974). Before turning to negative molecular ions, we mention a number of other results on atomic negative ions. Following the suggestion of Herzberg (1955) that diffuse interstellar absorption lines at a number of wavelengths in the visible spectral region might be due to autoionizing states of H-, C - , or 0 - , an experimental study of the photodetachment spectrum under high resolution (Herbst et ai., 1974a) shows that these are not viable candidates for the carrier. Multiphoton (two-photon) detachment via a virtual state has been observed and studied in detail for I - by Hall et a/. (1965). Finally, we stress also that our discussion is limited to a few representative ions and that data on photodetachment of other atomic negative ions can be found via the bibliographic review material cited at the beginning of this section, especially the NBS Special Report 426 and its supplements. 4. Diatomic Negative Ions
Photodetachment studies have been made on a large range of diatomic negative ions. The form of the photodetachment threshold law has been predicted for a nonrotating diatomic molecular ion having no permanent dipole moment (Geltman, 1958). This is of little use for most practical cases where the threshold law should depend on molecular rotation and on the permanent electric dipole moment (if any) of the species. Furthermore, from the experimental viewpoint, the negative molecular ions will usually be in a conglomerate of rovibronic states each of which will have its own threshold energy and with various possible behaviors. The O H - / O D - photodetachment cross sections in the 4000-7000 A region are shown in Fig. 38 for the process OH-/OD-(X'C+) hv+ OH/OD(X211i) e - as studied at low resolution ( - 100 A) by Branscomb (1966).The OH-(X'C+) and OH(X211i)potential energy curves are similar in form and parallel to each other [re(OH-) 2 r,(OH)]. The corresponding Franck-Condon transition in the photodetachment process accounts for the existence of a clear-cut threshold. Quasi-identical onsets were found for O H - and OD-. The maximum in the photodetachment function at about 6300 A requires interpretation. High-resolution, laser photodetachment
+
+
ELEMENTARY ATTACHMENT A N D DETACHMENT PROCESSES
h
I2
”EV
11
2
10
0
-
119
9
. -g
8
Y
u
$ $ 0, U -C E c * 0 +
r
a 2
7 6 5 4
3 2 I t -
01
4000
I
4500
1
5-
1
1
5500 6OOO Wavelength (A)
I
6500
hJ 8 7-
7500
FIG.38. Photodetachment cross sections for OH- ( 0 )and OD- (0) using a monochromator with 100-8, resolution (Branscomb, 1966); ( 0 )using bandpass filters (Smith and Branscomb, 1955). (By permission from Branscomb, 1966.)
studies have been made in the threshold region by Hotop et al. (1974) (Fig. 39), and over the whole 3500-7000 8, region by Lee and Smith (1979).There is an unexplained disagreement between the cross-section values of Lee and Smith and those of Branscomb, which are 40% higher. On theoretical grounds, the threshold law for OH-/OD- is expected to have behavior intermediate between the two limiting cases o a Eo (step function) and o a El” (no permanent dipole moment); the true threshold law should also be J dependent, since effects of dipole field rotation will differ with rotation velocity (Hotop et a!., 1974). Further theoretical aspects of rotational effects are discussed by Walker (1973). In his data analysis, Branscomb (1966) assumed a step-function behavior at threshold. This was found not to hold for significantly populated J states in the high-resolution work of Hotop et al. (1974). In the latter, analysis was attempted with both o a E”’ and o a for all J ’ . Best agreement with experiment was found with o a Ell4 (Fig. 39). Another interesting diatomic negative ion is Cz-, which may play an important role in the absorption continua observed in stellar spectroscopy. This ion has the distinction of having a stable, excited electronic state at -2
120
R. STEPHEN BERRY AND SYDNEY LEACH
PHOTON ENERGY
(lo3cm-'1
FIG. 39. OH- photodetachment cross section in the energy range 14,300-15,400 cm-' (700-650 nm): OH-('C+) ~ V + O H ( ~ ~ I e, )- . The dots represent the experimental data. The sharp onset near 14,700 cm-' corresponds to the opening of Q branch channels in the OH 2n3,2 final state. The solid line is a fit to the data using an threshold law and a negative ion temperature of 1200 K. (By permission from Hotop et a]., 1974.)
+
+
eV, i.e., below the photodetachment threshold at -3.5 eV. This also makes C2- the only gas-phase negative molecular ion for which an electronic transition between stable states is known by optical spectroscopy (Herzberg and Lagerqvist, 1968). The photodetachment cross section has been studied near threshold at low resolution (Feldmann, 1970). Lineberger and Patterson (1972) have used a tunable dye laser to study photodetachment in C2- by absorption of two photons, the first of which is used to excite the real 'ELI+ excited state, which is a resonant intermediate state in this process. The structure observed in the apparent photodetachment cross section correlates with levels to be expected from the optical work on the A2X,'-X2Xgf transition of C2-. The absolute oscillator strength of this transition of C,has been measured using shock-tube techniques (Cathro and Mackie, 1973). The 0,-ion, which is of great importance in aeronomy, as well as in plasma physics and also in some biological processes, has been extensively studied by laser PES techniques in efforts to determine the electron binding energy (Celotta et al., 1972). The cross sections for photodetachment have been measured over the photon-energy range 1.93-2.71 eV using a drift-tube
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
121
mass spectrometer and a tunable dye laser (Cosby et al., 1975, 1976). Some structure was suspected in the earlier measurements (1975) but is less evident in the later (1976). In particular, no structure is observed at 2.18 eV, which is the theoretical vertical threshold energy for forming the b'E:,+ final state of 0 2This . result, which is consistent with the threshold dependence predictable for this state, is confirmed by Lee and Smith (1979),whose measurements over the 1.48-3.5 eV photon energy range give cross sections of about 0.9 x lo-'' cm2 at 1.48 eV, about 2.5 x lo-'' cm2 at 3 eV and 3.7 x lo-'* cm2 at 3.5eV. Over the common energy range, these values are in good agreement with earlier measurements, not only of Cosby ef al. (1975,1976) but also those, at lower resolution, of Burch et al. (1958), as well as the fixed-frequency laser-excitation data of Beyer and Vanderhoff (1976). Strong resonances, interpreted as autoionizing features, were first observed in the photodetachment cross section of molecular negative ions near threshold by Zimmerman and Brauman (1977b) in a study of the enolate anion of acetophenone using ICR and laser techniques. A systematic study was carried out by Novick ef al. (1979a) on resonances in the photodetachment cross sections of NaCl-, NaBr-, and NaI- at over 1 eV above the photodetachment threshold. These resonances are interpreted as autodetachment of a 'll state of the NaX- ion embedded in the photodetachment continuum and give rise to a highly vibrationally excited ground state of the neutral NaX molecule. The autoionizing state is estimated to have a lifetime of the order of lo-'' sec on the oversimplified Fano formalism basis of a single discrete state interacting with a single continuum. We give here a partial list of other diatomic negative ions studied by one or other of the experimental techniques mentioned earlier: SH- and SD(Steiner, 1968; Eyler and Atkinson, 1974); SeH- (Smyth and Brauman, I972c); CH- (Feldman, 1970; Kasdan et al., 1975a). and SiH- (Kasdan et a!., 1975b),which also have low-lying bound excited states whose energies were determined, as well as excited states of the neutral, hydrides; NH- (Celotta et al., 1974; Engelking and Lineberger, 1976),for which the PES was used to determine the a'A-X3Z- intercombination energy separation in the neutral NH; NO- (Siege1 et al., 1972); S2- (Celotta et al., 1974); SO- (Feldmann, 1970; Bennett, 1972), for which the energy of the a'A state above the X3Eground state of SO was measured; C1,- (Sullivan et al., 1977; Lee et al., 1979), which undergoes photodissociation rather than photodetachment; C10(Lee et al., 1979), which exhibits both processes; PO- and P H - (Zittel and Lineberger, 1976) for which the a'A-X3E- intercombination energy separation was determined for the neutral hydride; LiCI- (Carlsten et a/., 1976); FeO- (Engelking and Linebttrger, 1977a); F2-,As,- (Feldmann et al., 1977); BeH-, MgH-, CaH-, ZnH-, PH-, ASH- (Rackwitz et al., 1977).
122
R. STEPHEN BERRY AND SYDNEY LEACH
5 . Triatomic and Polyatomic Negative Ions The ICR technique was used by Smyth and Brauman (1972b,c) to study the photodetachment cross-section behavior of NH,-, PH,-, and ASH,for which sharply defined thresholds were observed. The PES spectrum of NH,- was observed by Celotta et al. (1974) and other work on NH,- has been done by Feldmann (1970) and by Reed et al. (1976) who carried out theoretical calculations to compare with the experimental data of Smyth and Brauman (Fig. 40), and on PH,- by Zittel and Lineberger (1976). SO,- was the first triatomic negative ion studied by the PES technique (Celotta et al., 1974).The analysis enabled a determination to be made of the symmetrical valence-mode frequency v of the negative ion. The vibrational structure in the PES is very similar to that calculated by Cederbaum et af. (1977) for the (radiative) electron attachment spectrum. There is as yet no experimental determination of the latter. The NO2- photodetachment cross section has been measured at low resolution by Warneck (1969).The threshold is not well defined in this work. Laser detachment studies were carried out by Herbst et al. (1974b) who determined absolute cross sections and discussed the problem of their energy dependence. The NO, - photodetachment cross section is of importance in understanding the chemistry of the D-region of the ionosphere (Thomas, 1974). The observation of two thresholds in the photodetachment studies of Richardson et al. (1974) and of Herbst er al. (1974b) has led to the suggestion that two NO,- isomers are involved, the ONO-(Czy)species and a higher peroxy NOO- form. Calculations of Pearson et al. (1974)
0.76
0.80
0.84 0.00 0.92
0.96
1.00
104
PHOTON ENERGY ( e V )
FIG.40. Relative experimental ( 0 )and theoretical (-) photodetachment cross sections for NH,- as a function of incident photon energy. (By permission from Reed et a/., 1976.)
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
f 23
support the existence of these two distinct isomers. However, further work on NO2- photodetachment by Huber et al. (1977), using a drift-tube mass spectrometer and a tunable dye laser, has cast some doubt on the existence of the peroxy form of NO2- in these experiments. A peroxy form is indeed known in the case of NO3-, from ion-molecule reaction studies. Photodissociation of NO,- at photon energies less than 2 eV points to this being the peroxy form, since the threshold for normal NO,- should lie at 3.7 t 0.4 eV (Smith el al., 1979b). Photodetachment studies of a number of other molecular negative ions of importance in aeronomy have been carried out using a variety of techniques. For example, photodetachment from CO,-, C 0 3 -.H,O,and 0,ions in a drift tube has been studied by Burt (1972a,b),and 0,by Sinnott and Beaty (1971)and Smith and Lee (1979).Much work on 0, - and 04-has been carried out by Cosby et a!. (1975, 1976, 1978a,b); the results on 0,- indicate that this species has an excited electronic state, which leads to dissociation along the symmetric stretch coordinate. Recent work by Novick et al. (1979b) indicates that the photodissociation cross section is at least one order of magnitude greater than for photodetachment. For 04-,photodissociation is observed, but it is not clear whether or not photodetachment occurs (Cosby et al., 1975, 1976). A detailed study on C 0 3 - has been made by Moseley et al. (1976)using a drift-tube mass spectrometer and various lasers. These authors monitored the destruction of C 0 3 - and the formation of the 0 - ion resulting from the process C 0 3 - + hv -+ C O , + 0-.The results have been extended to shorter wavelengths (3500 A) by Smith et al. (1979a). Structure in the photodissociation cross section of C 0 , - in the photon energy range 1.8-2.7 eV was assigned to transitions from the ground R2B, state to vibrational levels of the first electronic excited state of CO, - which is predissociative. The excited state is thought to have ,A2 electronic symmetry, and to lie at 1.52 eV above W2B2,well below the photodissociation threshold at around 1.8 eV. Frequencies of three vibrational modes of the excited state were determined. CO, - is therefore a case where negative-ion photodissociation dominates over photodetachment in the energy region studied, as confirmed by the work of Novick et al. (1979b), who detected no photoelectrons with laser excitation of CO,-. Very recently, Hiller and Vestal (1980) in a study of CO, - photodissociation have measured cross sections at variance with those of Mosely et al. (1976) and Smith et al. (1979a); they also observed a twophoton dissociation process below 2.2 eV. Work on C 0 3 - (and on 0 2 - ) photodissociation has also been done by Beyer and Vanderhoff (1976) at a number of selected laser excitation wavelengths and using a drift-tube mass spectrometer.
124
R. STEPHEN BERRY AND SYDNEY LEACH
Very recently, photodissociation and photodetachment cross-section measurements for a number of the negative ions discussed above have been made at 2484 a by Hodges et al. (1980) using a rare-gas-halogen excimer laser. The hydride CH, is an important chemical and astrophysical species. The structure of the ground state and the singlet-triplet intercombination energy have long been the subject of controversial determinations. The PES of CH,- was used by Zittel et al. (1976) to make such determinations for the neutral. In particular, a value of 19.5 L 0.7 kcal/rnol was obtained for the g1A,-R3B, splitting, but this is in serious disagreement with a number of values obtained by various techniques and which converge to a value of about 8 k 1 kcal/mol (Lengel and Zare, 1978). Photodetachment has been studied for a considerable number of large polyatomic negative ions. The appropriate references are given by Janousek and Brauman (1979). Most of the work is directed to the determination of electron affinities and on interpreting the effects of chemical substitution in chemically related series. As mentioned earlier, a systematic study comparing theoretical and experimental cross-section behavior near threshold for polyatomic negative ions has been made by Brauman and his co-workers using ICR techniques (Reed et al., 1976; Zimmerman and Brauman, 1977a,b; Janousek and Brauman, 1979). We mention only a few cases. The cyclopentadienide ion C J - (Richardson et a!., 1973; Reed et al., 1976) has a slowly rising cross section over a large photon energy region, well reproduced by calculations (Fig. 41). Jahn-Teller characteristics of the final, neutral
08 m ul
'
'
O
6
0.0 1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
PHOTON ENERGY ( e V )
FIG.41. Relative experimental ( 0 )and theoretical (-) photodetachment cross sections for C,H,- plotted as a function of incident photon energy. (By permission from Reed et al., 1976.)
ELEMENTARY ATTACHMENT AND DETACHMENT PROCESSES
125
state (cyclopentadienyl radical C,H, ) have been explored in a PES spectrum of the negative ion by Engelking and Lineberger (1977b). In the case of the pyrrolate ion C4H4N-, Richardson et al. (1975) have shown that there is an orbital reordering when an electron is photodetached, producing a CT radical in spite of the 71 electrons being the most weakly bound. This highlights one of the pitfalls in PES interpretation, i.e., the failure of Koopmans theorem, which postulates no electron reorganization on ionization.
ACKNOWLEDGEMENTS R.S.B. would like to express his thanks to the Laboratoire de Photophysique Moleculaire, Universite de Paris-Sud, Orsay, for hospitality and support during much of the writing of this article. The final preparation of the manuscript was supported in part by a Grant from the National Science Foundation. We would like to thank Drs. Christian Jungen and Helen Lefebvre-Brion for their helpful comments on the manuscript.
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ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS. VOL. 57
Fiber Optics in Local Area Network Applications DELON C. HANSON Hewletr-Packard Company Optoelecfronic Division Palo Alto, California
I. System Requirements and Trends A. Introduction . . . . . . . . . . . . . . ............................ B. Transmission-MediumConsiderations . . . . . . . C. Local Area Network Topologies and Trends .............................. D. Standards and Performance Limits of the Transmission Medium . . . . . . . . . . . . . ................
145 149 154 157
.......... C. Connector Design.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 111. Terminal Device and System Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 189 A. Optical Source and Transmitter Circuit Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Optical Detector and Receiver Considerations. . . . . . . . 203 213 C. System Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
I. SYSTEM REQUIREMENTS AND TRENDS
A . Introduction In the past few years, major progress has been made in the design of fiber optic data-link components and subsystems. Much of the early effort was applied to the development of long-distance, point-to-point, digital telecommunications (DT) hardware. This has resulted in several major planned installations on routes where Aber optics is now cost competitive. In contrast to the relatively controlled environment and structured communication hierachy of DT systems, local data communication (LDC) links encompass a very diverse set of potential applications. This results from a broad spectrum of data rates, equipment interface requirements, link lengths, and a wide range of packaging and environmental conditions. With this diversity, as actual systems take shape, many of the fundamental issues have shifted from strict technological considerations to such practical issues as the physical size of compohents, availability of power-supply voltages, 145 Copyright Q 1981 hy Academic Press. Inc. All rights of reproduction tn any form reserved. ISBN 0-12-014657-6
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compatibility of optical connector options, and the feasibility of nonspecialists assemblying connectors in the field. In addition to individual data-link considerations, local area networks (LANs) are rapidly evolving to meet the interconnect requirements of lower cost digital terminal hardware which is distributed within a local site. Hence, in this article LDC will refer to performance, cost, and technology issues for individual links. This will provide a direct comparison with pointto-point DT links; LAN discussions will relate to the interconnection of links to form a network configuration. The purpose of this article is to bridge the gap between system requirements and technology in order to arrive at a reasonable understanding of the cost-performance trade-offs for fiber optics in LDC and LANs. It may yet be too early to determine the ultimate impact of fiber optics on evolving communication equipment standards; however, trends are becoming apparent.
B. Transmission-Medium Considerations Quite often potential users of fiber optic data links do not have a clear understanding of the relative merits of fiber optic links as compared to other transmission media, e.g., twisted-wire pair or coaxial cable. This situation is particularly true when optocouplers (which contain photon-coupled optical sources and detectors in a single package) are used to provide some degree of the isolation properties which are inherent with optical fiber links. Figure 1 shows a schematic drawing ( 1 ) of the world’s shortest optically coupled link, i.e., an optocoupler built in a dual in-line package having a direct optically coupled path between the source and the detector. The major reasons for using optocouplers in conjunction with metallic links is to suppress common mode voltages which are induced into the cable by the ambient environment and to provide a coupling device between parts of a system which are at different electrical potentials. As Fig. 1 indicates, internal capacitance C&,, induces a parasitic common mode path between the source and the detector and C , , C2 induce parasitic input-output capacitance. Utilizing an integrated detector-amplifier in an optocoupler (discussed in Section III,B,3,a) significantly reduces this common mode capacitance. In addition, if a short fiber stub is used to separate and optically couple the source to the detector in the same package, the input-output capacitance approaches that of a fiber optic link. Optical fiber and wire have inherently different transmission-bandwidth limitation mechanisms. In the case of optical fiber, there is great latitude in controlling the fiber refractive index profile (see Fig. 9) so that multimodepath-propagation delay distortion can be effectively eliminated. The product of the optical fiber 3-dB transmission bandwidth and its effective length [see
FIBER OPTICS IN LOCAL AREA NETWORKS
147
---_L f_ _ _ _ _
Film Shield
Dual In-Line Package
Y
Four-Pin Emitter Lead Frame
I
fv-
GaAsP LED Emitter (Underneath Lead Frame)
Sili det Am Four-Pin Detector Lead Frame
FIG.1. Schematic drawing and cross section of dual in-line optocoupler showing the internal shielding designed to reduce parasitic capacitive coupling ( I ) .
Eq. (7)] can be in the > 1 GHz-km range (2).Since fiber is a dielectric medium having a dielectric constant similar to that of the dielectric between the inner and outer conductors in metallic cable, the propagation delay of the initial propagating wavefront is essentially identical in both optical fiber and coaxial cable. However, for metallic links, skin effect loss increases as the square root of frequency (3).Hence, the step response of a skin-effect-limited cable is non-Gaussian, i.e., it rises slowly for a short time after the incidence of the initial propagating wavefront to a small amplitude, then increases rapidly to a much larger amplitude, and then very slowly to its final value. As a result, the 10-90x response time of a skin-effect-limited cable is about 30 times longer than the 0-50% response time (4). Due to this effect, metallic cable 3-dB bandwidth is inverSely proportional to the square of the link length, rather than being inversely proportional to the effective link length, as in the optical fiber case.
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Figure 2 shows a comparison of attenuation versus modulation bandwidth for coaxial cable having various diameters and a relatively low-loss optical fiber cable having the same diameter as the smallest coaxial cable. Skin-effect loss governs the slope of the coaxial cable attenuation curves. As noted in Section II,B,l, the typical outer diameter D of LDC fiber optic cable is 2.5 mm. This is equivalent to the diameter of the RG-l79B/U cable, but there is an order of magnitude difference in attenuation even for a moderate bandwidth of 10 MHz. The coaxial cable diameter must be increased to 23 mm (RG-219/U) to achieve an equivalent lO-dB/km attenuation at 10-MHz modulation bandwidth.
149
FIBER OPTICS IN LOCAL AREA NETWORKS
WIRE
FIBER OPTIC
BANDWIDTH-LENGTH I
l
l
1
I
l
l
l
l
ISOLATION ( EM1 )
DURABILITY
FIG.3. Summary of relative merits of wire versus fiber optic cable (5).
As a result, even though optocouplers may be used with metallic cable to reduce effects of ground loops, common-mode coupling, and voltage differences between nodes, the inherent pulse distortion (or reduced bandwidth) caused by skin-effect loss in metallic cables and their susceptibility to electromagnetic noise pickup and emission are not reduced. Since it becomes more costly to compensate for both of these effects with increasing data rate, optical fiber transmission becomes increasingly more attractive with the trend toward higher speed and distributed data transmission. Figure 3 shows a summary of the relative merits of wire versus fiber optic cable (5).The rating system is a subjective assessment of the importance of the various performance factors. Cost is particularly subjective and is variable with application since there are many contributions beyond the raw cable cost which enter in. These contributions may include: special cable shielding, duct installation cost (which may be $10-20/m), and the complex terminal electronics required to compensate for cable pulse distortion and temperature variation if performance is pushed to the limit. As a result, in many cases, fiber optics is cost effective today and will become even more attractive as the data rate and link length of distributed systems increase.
C. Local Area Network Topologies and Trends
Local area networks (LANs) refer to communication capability within a building or between adjacent 'buildings on a common site. The LAN is designed to interconnect a broad spectrum of data nodes, e.g., computers,
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terminals, mass storage devices, plotters, printers, and gateways to other networks within a restricted area which is typically less than 2 km in diameter. Until now, LANs have primarily used proprietary interfaces with protocols designed to solve a specific problem, or have interconnected only a local cluster of nodes via a standard interface, e.g., IEEE 488 (see Fig. 5). With the rapid expansion of lower cost digital hardware, many more nodes will exist in a local area and will require interconnection for efficient operation. The 1980s will witness a major thrust toward LAN standardization with the IEEE Computer Society spearheading this effort. Individual companies are beginning to specify multiple-user, distributed data communication equipment (6) and may create de fucto standards in the interim. Fiber optic network components must either be compatible with or compete against this movement. The objective of LAN standardization (7), is to separate the required end-to-end protocols into a prescribed communication hierarchy and to specify the interface between the respective levels. The International Standards Organization (ISO) levels of primary importance to LANs are shown in Table I. In order to successfully complete a message transfer between network nodes, there must be compatibility up through level 4. The primary emphasis of this article is on fiber optic physical hardware at level 1. The separation of the protocol levels is particularly important for incorporating a new technology, e.g., fiber optics, since if the level definitions are properly thought out, they avoid the need for changes at all levels prior to implementation. The number of LANs which have been developed or proposed is as large as the number of organizations who have considered the problem. A recent summary (8) includes more than 70 alternatives which use a spectrum of TABLE I
I S 0 DATACOMMUNICATION HIERARCHY Level designation
Level functions
4
Network transport protocol Network control protocol Link control protocol Physical (electrical, mechanical and functional description along with procedures for establishing, maintaining and disconnecting the link)
3 2 1
FIBER OPTICS IN LOCAL AREA NETWORKS
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network topologies, transmission media, and communication protocols. These topologies can be summarized by four basic generic types, as shown in Fig.4 and summarized in Table 11. For the fully connected network, shown in Fig. 4a, any two nodes which must communicate have a dedjcated channel between them. In the case where all N nodes communicate, (N)x (N - 1)/2 duplex links must exist. Each link may operate over a different medium, at different data rates and with different control protocols. Respective nodes are addressed through polling. Since each link has permanently dedicated hardware and there is no reallocation of transmission paths, considerable total communication capability and flexibility is wasted. It is difficult to expand the network since N extra duplex links are required for each new fully connected node. The perceived reliability of this network is high since breaking a link only affects one communication path. If alternate routing is incorporated, this topology offers the highest reliability. The star topology, shown in Fig. 4b, has low connectivity, since it requires only N duplex links. Nodes are interconnected through a central control
Loop Control Node
4 (C)
(d)
FIG.4. Four generic network topc/logiesfor local data communication: (a) fully connected; (b) star; (c) ring/loop; (d) bus.
TABLE I1 LOCAL AREA NETWORK P
~
R ANDSFEATIJRFS
Parameters
Network topology
Number of network links
Network protocol
Features
Link data rate
% O ( N - 1)
Polling
Node rate
Star
N
Polling
Node rate
Ring or loop (active repeaters) Distributed data bus
N
Multiplexing or contention Contention
> N.(node rate)
Fully connected
1
>(node rate)
Ease of expanding or tapping
Perceived network Ease of reliability synchronization
Difficult (many extra lines) Moderate
High
-
High
Moderate (shut down network) Easy (with coax)
Low
Easy (nodes independent) Easy
High
Difficult
FIBER OPTICS IN M C A L AREA NETWORKS
153
node. The performance features of this topology are similar to the fully connected case, but it has no redundancy and has potentially low reliability due to dependence on the central node. This network is preferred for local cluster communication. The ring or loop, in Fig. 4c, has minimum connectivity, since it requires only N simplex links. In an active repeatered ring, each node is connected to two other nodes with a desigpated direction of data flow. The distinction between a ring and loop is that a loop has a single control node (shown dashed), whereas a ring distributes the control among the various nodes. Extensive analysis and implementation of rings has been reported (9, 10). They have many desirable features for widely distributed network nodes. Rings and loops utilize various forms of time division multiplexing (TDM) in which particular time slots of an information field (which is passing around the loop) are either allocated to each node permanently or messages are inserted into time slots on a dynamic contention basis. Since all nodes are interconnected by a common data path, TDM requires that the link data rate be more than N times the node rate, in order to include the framing and error control required to operate the network. With active repeaters at each node, it is relatively straightforward to extend a ring network. However, the need to shut down the ring while adding a node is a distinct disadvantage. Due to the fact that all information flows through each node, the perceived network reliability is low since a “down” node or broken cable may shut down the entire network. Methods (11) of building in redundancy or of achieving a “soft”failure procedure are of major importance. The ring has a major advantage for high data rate TDM systems because network throughput is not limited by propagation delay between the network’s most remote nodes, since the protocol is not based on dynamic optical contention on the network between nodes competing for control. The distributed bus network, shown in Fig. 4d, is the topology traditionally thought of when wire or coaxial LANs are used, since low-perturbation electrical taps onto the network are relatively straightforward. Ethernet (12)is a well-documented version of this topology using CATV-type coaxial cable and taps. The network protocol incorporates dynamic contention by the individual nodes to gain control of the network. Certain rules of behavior (13), e.g., “listen before talking” (by sensing the carrier) and “listen while talking” (by comparing injected data with network data) are incorporated to increase network utilization efficiency. From a coaxial hardware standpoint, it is relatively simple to “tap on” additional nodes without disturbing the network. The perceived network reliability is high due to the totally passive transmission medium. Extensive work has been done to mathematically model (13) and measure (14) the multiple-access Ethernet. Operating at 2.94 Mb/sec with 120 nodes distrib-
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uted over a 600-m coaxial cable, the Xerox results (14) demonstrated up to 90% network utilization efficiencyeven under high offered load conditions. In contrast to the baseband Ethernet coaxial contention network, broadband coaxial networks are also being developed using CATV coaxial hardware (15).This approach utilizes distributed control and packet network protocols, derived from public packet networks (CCITT X.28), to combine voice, video, and data transmission on a single coaxial cable network. From a bandwidth standpoint, thousands of low-speed terminals can be supported simultaneously or can be intermixed with other communication requirements. This article is primarily concerned with the physical link design trade-offs at level 1 (Table I) to provide a perspective for fiber optic network development. McQuillan (16) has stated that networks have traditionally been designed backwards, i.e., starting from technology and working toward user needs. The higher level protocols (to allow equipment manufactured by different vendors to communicate) have come last and require one to two orders of magnitude more man-years of work to resolve than the hardware itself. Why then consider fiber optics for LANs when the transmission capability of coaxial cable has barely been exploited for even moderate bandwidth LANs? The answer to this question is dependent on the application. Even though it is possible, in principle, to solve any problem caused by transmission distortion, induced or radiated noise, common-mode or system differential voltages when using a metallic cable, the cost of the terminal hardware and required special engineering may outweigh the cost of an equivalent fiber optic alternative which provides an assured solution. The challenge for the system designer is to determine the preferred solution, based on true costs, ahead of time so that the installation does not require a rework cycle at even greater cost. D. Standards and Performance Limits of’ the Transmission Medium Figure 5 shows a comparison of projected data rate versus distance limits of various level 1 communications standards (17).The lower portion of this figure shows four widely used wire-based standards. Of these, RS-232, RS-422, and RS-423 are serial data links, whereas IEEE 488 incorporates eight parallel data lines and eight control lines in a byte serial data bus protocol. The oldest and most widely used standard (RS-232) couples the communication level specifications and has low performance, but is very cost effective when it satisfies the transmission requirements.
FIBER OPTICS IN LOCAL AREA NETWORKS LIN ME
LOCAL DATA COMMUNICATIONS
1
2nd GENERATION
155
DIGITAL TELECOMMUN -
1' LEVEL 1 DATA TRANSMISSION OPTIONS
0.1
1.0
10
LINK
100
1K
10K
lOOK
LENGTH ( m )
FIG.5. Level 1 data transmission standards and trends with respect to data rate versus link length.
Transmission performance is improved by one or two orders of magnitude by using integrated electronic interface circuits, as in the case of RS-422 and RS-423 standards. The order of magnitude performance improvement of RS-422 over RS-423 is the result of using balanced drive and detection circuits to compensate for skin-effect-induced pulse distortion in the cable. The IEEE-488 (or HP-IB) standard was originally developed for interconnecting clustered instrumentation systems but has been considerably expanded in application over the past 10 years. The maximum data rate is 1 Mbaud (MBd) over each of B data lines. Due to reflections and impedance mismatches, the allowed link length is 2 m per node or a maximum of 20 m. This data bus has recently been extended to provide a 1-km separation between instrument clusters by serializing the 16 data and control lines and utilizing a 1-km full duplex fiber optic link (18). In the region above 1 MBd per channel, the main-frame parallel data bus is typically less than 1 m long and is used to transmit very high-speed data along the back plane of a cohputer. Above 20 MBd, parasitics, impedance mismatching, and cross-talk become increasingly more serious even with
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this short link length. Fiber optics provides an excellent solution for these problems but introduces an additional, potentially serious, fixed propagation delay due to the required optoelectronic transducer devices at each end of the fiber links. Digital telecommunications (DT) transmission relates to the region greater than 1 MBd- 1 km. The designation baud (one symbol per second) is often used to distinguish between the transmission rate of information (in bits/second) and the transmission rate of pulse transitions (in baud). For the designer of transmission media and terminal hardware, the baud has more meaning because it directly relates to the response time of the physical link. Thus, a transmission medium can be specified independent of how the information is coded on the line. For comparison, Table I11 shows a summary of United States-based, T-carrier digital telecommunications options which fit into this region. The repeater spacings for the various conventional transmission technology options are shown plotted in Fig. 5. Since central offices are typically spaced 4-10 km apart, it is clear that repeaters are required for the wire/coaxialbased links. It is expected that DT fiber optic links will “prove in” at greater than 20 Mb/sec due to the fact that fiber costs are dropping relative to copper and intermediate repeaters can be eliminated (19). In the local data communications (LDC) region, applications are very diverse and the established communication hierarchy of DT does not currently exist. Data rates are often limited by higher level computer software protocols rather than the transmission medium itself (19). First-generation LDC links encompass data rates from 100 Bd to 10 MBd and link lengths from 1 to 2000 m. This region includes the existing wire-medium standards described earlier and has a segment not covered by existing wire standards. With the ground swell of activity (6) among major LSI microprocessor TABLE I11 UNITED STATESDIGITAL TELECOMMUNICATIONS T-CARRIER SUMMARY
System
Data rate (Mbisec)
Voice channels
TI
1.5
24
T2
6.3
96
T3 T4
44.7 214
672 4032
Transmission-medium technology
Repeater spacing (km)
“Screened” twisted wire Digital radio (2 GHz) “Lo-cap” twisted wire Digital radio (2-6 GHz) Digital radio (6- 1 1 GHz) Air dielectric coax Digital radio (18 GHz)
1.7 221 4.5 221 2-21 1.6 4.8
FIBER OPTICS IN LOCAL AREA NETWORKS
I57
companies to dominate the coaxial-based, LAN standards in this region (<10 MBd and c2 km), the initial LAN protocol may be Ethernet (12), a similar contention protocol or distributed control by means of passing a control “token.” If fiber optics is to have a significant impact on LANs before 1985, it is imperative to have interfaces compatible with coaxial networks in this first-generation region, particularly for extending the link length and dealing with noisy environments. Otherwise, fiber optics for LANs will not have a significant impact until higher speed protocols are developed which require fiber optics, e.g., in the second-generation region having transmission data rates up to 100 MBd. This represents a severe technological challenge for fiber optics since a direct implementation of the multiply tapped bus, shown in Fig. 4d,is very difficult at best. This is the case because of the following: (1) Optical taps having negligible perturbation on the main network do not exist. As a result, the optical loss per tap severely restricts the system length and the number of tap$ (see Section III,C,3,b). (2) A dynamic contention protocol, e.g., Ethernet (12), places severe contraints on optical receiver design since it requires both wide dynamic range and fast response time.
These issues will be discussed more extensively in Section II1,B. The third-generation LDC region (c1 GBd) will not have significant impact until the late 1980s. By this time, advances in single-mode fiber technology, lasers, and integrated optics will provide a major thrust of activity. This article, however, will concentrate primarily on fiber optics for LANs, which will come into place by the mid-1980s and operate at c 100 MBd.
E . Fiber Optic Component Design Trends With the above challenges in LANs, it is appropriate to consider the status and trends in fiber opzic components needed to implement these networks. Key concerns are hardware standardization and the comparison of differences between telecommunication and local data communication systems. Many of these differences will merge in the second- and thirdgeneration regions shown in Fig. 5. 1. Standardization Issues
There is considerable fiber optic standardization activity in every industrial nation as well as internationally through the International Electrotechnical Commission (IEC) Subcommittee 46E and its associated
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DELON C . HANSON
working groups. It is clear that committees will not decide fiber optic standards in a vacuum. Hence, suitable component availability, cost-performance trade-offs, and user acceptance are of primary importance before final standards are set. In general, standardization is a very difficult task in a new technology in which many competitive approaches attempt to achieve similar end results. A proper balance must exist between the designer's desire to advance the state of the art and the user's desire to have multiple vendors of low-cost components immediately. If standards are set too soon, performance will not be as good and costs will not be as low as would ultimately be achievable. It is important to achieve physical compatibility and interchangeability of devices without forcing all electrical and optical parameters into a narrow standard specification range. To proceed with standards relevant to LDC, it is necessary to define the functional interface between the transmission fiber and the associated node communication hardware. This transducer interface is defined by a terminal device. Figure 6 shows a summary classification of fiber optic terminal devices which provide this interface for a broad variety of applications (17). The individual function of these devices can be separated into four distinct categories, i.e., electrical signal-processing circuitry unique to fiber optics; optoelectronic transducer device(s); optical signal-processing element(s) ; and connector(s) or equivalentfs). Below each functional category are examples of individual elements which may be included in that particular category. The optical interface to the
PROCESSING 'IGNAL CIRCUITRY NOT UNIQUE
TO FIBER OPTICS
I 1
I DPTOELECTRONIC 1
ELECTRICAL c ~ PROCESSING
I
UNIQUE TO FIBER OPTICS
I
TRANSDUCER ~ ~ DEVICEISI
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1
OPTICAL ~SIGNAL y PROCESSING ELEMENTIS)
CONNECTDRISI 1
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EQUIVALENTISI
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EXAMPLES
DrivingiAmplifuation LED/LD Stabiliation p-i-nIAPD 8 Monitoring IDrviaS on
8
Stabilization Monitorinn Modulation ~ ~ $ l i n l l 8 Amplification 8 Iral.tion 8
8
F i b Pitail Conneetor
8
FIG.6. Fiber optic terminal device interfaces and examples of individual elements: P, power; C. control; M, monitor.
FIBER OPTICS IN LOCAL AREA NETWORKS
159
network fiber is accomplished by means of a fiber pigtail, a connector mounted directly on the terminal device or an equivalent, e.g., a permanent splice. Power (P), control (C), and monitor (M) ports may exist for the individual functions and require definition and standardization if terminal devices from different vendors are to be interchangeable. With this definition of a fiber optic terminal device, one medium, e.g., coaxial cable and its necessary terminal hardware, can be replaced by an optical fiber and its associated terminal devices and achieve equivalent transmission performance. This capability defines a transparent interchange. The electrical signal(s) interfacing the terminal device’s signal-processing circuitry may have either serial or parallel transmission paths. This article explores the probable direction of fiber, connector, and terminal-device standardization and the reasons for the choices which will be made.
2. Contrasts in Digital Telecommunications and Local Area Networks Digital telecommunicatiops (DT) is concerned with transmitting voice circuits from point to point. Transmission economics has required the development of electronic circuits which combine many voice circuits into a composite waveform before transmission. Table I11 summarized the T-carrier hierarchy in the United States with its corresponding number of voice channels. In other countries, different hierarchies occur, but in all cases fixed-channel relationships exist. Time-division multiplexing (TDM)is used in conjunction with pulse-code modulation (PCM) to transmit the digital equivalent of analog signals. The PCM terminals are used to combine the voice channels to form a composite waveform (20). This results in a well-defined signal format which is easily converted to binary-logic levels. At the repeater spacings for metallic links shown in Table 111, the composite signal is reshaped, retimed, and regenerated by the repeater in order to compensate for induced pulse distortion and attenuation in the cable. Since the typical cost for repeaters is $1000-2000, it is highly desirable to eliminate repeaters and only require terminal electronics in central offices (21).This is a significant advantage and can be achieved with optical fiber. Since costs are spread on a channel-km basis, fiber optics for DT will likely “prove in” economically in the region greater than 20 Mb/sec (19). In a central office, ambient temperature control is very good and a full range of power supply voltages can be made available through dc-to-dc conversion. Since the digital switching hardware already exits, only a restricted set of design parameters must be cdnsidered in determining whether optical fiber systems are cost competitive.
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DELON C. HANSON
In LANs, as a result of the present speed limitations of higher level protocols, the full fiber bandwidth is not typically utilized. Thus, link costs are calculated on a cost-per-channel basis rather than the channel-km basis used in DT (19). This is a more severe test in general and, thus, practical issues such as terminal component size, cost and, power supply compatibility become primary selection considerations. Although computer and peripheral manufacturers have typically provided f 12 V on printed circuit boards, the trend is toward providing only a single +5-V supply (19). This has a particular impact for optoelectronic receiver design where the requirement for high sensitivity, wide dynamic range, good power supply ripple and noise rejection become more difficult with the constraint that a single polarity, relatively low-voltage supply imposes. A summary of certain design contrasts for DT and LAN applications is shown in Table IV, based on application requirements summarized in earlier sections of this article. The DT requirements for long link lengths at relatively high data rates imposes a demanding requirement for low fiber attenuation (<7 dB/km) and relatively high bandwidth (> 200 MHz-km). To achieve this with low cost, the numerical aperture (NA) is specified to be relatively low, e.g., 0.22. Because the digital terminal multiplexing hardware and protocol is complex and relatively expensive, higher cost can be allocated to the optoelectronic transducer devices. Thus, laser diodes (LDs) operating at lo = 850 nm and silicon avalanche photodiodes (APDs) have been selected for first-generation DT application due to their availability and high optical power output and sensitivity, respectively. It is likely that second-generation DT systems will operate near &, = 1.3 pm and utilize GaInAsP light-emitting diode (LED) sources and GaInAs p-i-n detectors because of improved reliability and reduced complexity (22). For LANs, shorter link lengths at relatively lower data rates places a lower premium on low fiber attenuation but emphasizes the importance of using larger fiber core diameter and higher NA to minimize total coupling loss from the optical source into the fiber. The larger fiber diameter reduces the requirements for connector precision. However, the need for small terminal hardware which will mount directly on tightly spaced printed circuit boards, along with low cost and the ability to withstand repeated disconnection, imposes demanding requirements on the optical connectors. Due to the confined space, snap-on connector designs are often desired so that insertion and removal can occur without the need to insert fingers into the narrow spacing between circuit boards. Light-emitting diodes (LEDs) are chosen in preference to laser diodes (LDs) for LANs operating up to 100 MBd because of lower cost, lower temperature variation, easier electrical interface, and simpler associated package design. The lower output power, slower optical response time, and
TABLE IV CONTRASTS IN DT AND LAN PARAMETERS ~
Parameters
~~~~
Local area networks (LANs)
Long (> 1.51 Moderate- high Complex
Low-moderate ( <2) Low-high Simple High High
Application summary Link length (km) Data rate Digital terminal hardware Diversity of applications Disconnections and changes Design choices Optical source Optical detector Multimode fiber design Attenuation (dB/km) Bandwidth-distance (MHz-km) Numerical aperture (NA) Core diameter (pm) Optical connector loss (dB)
~
~
Digital telecommunications(DT)
LOW
Low First generation: LD at 1, = 850 pm Silicon APD
Second generation : LED at 1, = 1.3 pm GaInAs p-i-n
LOW (t7)
Low (<2)
High ( z200) Moderate (0.2) Small ( I50) Low (0.5-1.0)
Light-emitting diode (LED) Siliconp-n or p-i-n diode (PD) Moderate-high (5-300) Moderate (10-200) Moderate-large (0.2-0.5) Moderate-large (80-400) Moderate (1 -3)
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DELON C . HANSON
broader spectral linewidth of LEDs are acceptable for the vast majority of applications. p-i-n Photodiodes (PDs) are selected in preference to APDs in order to avoid the requirement for a high-voltage supply (e.g., 50-300 V) and due to their reduced temperature sensitivity. The specifics of these component design trade-off issues are explored in more detail in subsequent sections of this article. Primary emphasis is on the status and trends in technology for LANs.
11. THEOPTICAL COMMUNICATION MEDIUM
A. Optical Fiber The key to the application benefits of fiber optics is the fiber transmission medium itself, even though, for those applications which do not push the fiber transmission performance limits, this is often overlooked. Following the proposal by Kao and Hockman (23)in 1966 that optical fiber could serve as a 1000
;100 &
850 nm
\
a
w
v
z 0
10
CI
c. d
3
z w c. Ed
.6 1.0
0.47
MINIMUM ATTENUATION BEYOND 1.Opm 0.1
1
I68
1
1
1970
1
1
1972
1
1
1974
1
1
1976
1
1
1978
1
191
YEAR FIG.7. Progress in reduction of optical fiber attenuation (27)
FIBER OPTICS IN LOCAL AREA NETWORKS
163
WAVELENGTH (pm) 0.8
0.9
. = E
A
30-3 01
1.0 1.1
1.3
1.5 1.7 2.0
Infrared Absorption
m
lo--
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
PHOTON E N E R G Y ( e V ) FIG.8. Assignment of loss mechanisms in ultra-low-loss silica fiber (24).
suitable transmission medium, major emphasis has been directed toward reducing attenuation. This progress, illustrated in Fig. 7, resulted in the achievement of 0.2 dB/km attenuation at lo= 1.6 pm by Miya er a/. in 1979 (24). This is near the fundamental attenuation limits shown in Fig. 8. The Raleigh scattering limit, which has a dependence, is caused by scattering which arises from phase and compositional fluctuations that are frozen in during the glass cooling process (25). The magnitude of this disturbance depends on the particular glass system. The infrared absorption limit is determined by fundamental vibration modes of the Si04 molecule in the silica fiber core. The absorption peaks are caused by the hydroxyl (OH) ion content in the glass system. There have been many papers which have developed and reviewed the fundamental fiber performance parameters, e.g., (26-28). This article emphasizes only parameter modeling which is pertinent to characterizing the system trade-offs in LDC applications. 1. Fiber Performance Parameters
For LDC applications, the primary interest is in relatively large core fibers in order to minimize the cost of optical sources and connectors. As a consequence of the larger fiber core cross section, a large number of optical modes are trapped and gui4ed by the refractive index profiles, shown schematically in Fig. 9. In order to produce total internal reflection for propagating modes, the refradtive index must be reduced in going from the fiber core axis to the claddin4 interface.
164
DELON C. HANSON
CLADDING
INDEX
I (a)
(b)
FIG.9. Fiber index profiles and ray trajectories versus fiber radius and profile parameters: (a) refractive index profile 01 versus fiber radius; (b) ray trajectory for step index (a = m) and graded index (a = 2).
The lower refractive index, outer cladding layer is important since it confines the optical modes to the fiber core and thus reduces optical absorption loss caused by scattering at the core-cladding interface. It also protects the core region from contaminants and is a major factor in determining fiber strength, as a consequence of its outer surface quality and material bonding strength. a. Index profiles and mode parameters. The refractive index profiles versus radius are characterized by the parameter o! through the relationship:
n(r) = n,[l
-
2A(r/a)"]1'2,
n(r) = n2 = n l [ l - 2A]'I2,
0I rI a
r >a
(1)
Graded index fibers have a parabolic profile (a = 2) in order to achieve maximum bandwidth. As the optical ray trajectories for a = 2 in Fig. 9 indicate, this profile provides continuous ray focusing in the core medium. Since a nonaxial ray travels a greater distance in a region which has a progressively lower index, its travel time is equivalent to axial rays and, thus, all rays arrive at the output nearly simultaneously. This is indicated schematically by the common axial crossover points. On the contrary, in step index fibers (a = 00) rays travel in straight lines. Thus, no self-compensation occurs and, consequently, substantially greater pulse spreading results. The ray concept is only a partial description of the character of optical transmission in fibers. It describes the direction a plane wave component takes in a dielectric medium, such as an o2tical fiber core, but it ignores the interference phenomenon which restricts rays to certain discrete characteristics or modes in order for them to match electromagnetic boundary
FIBER OPTICS IN LOCAL AREA NETWORKS
165
conditions required for propagation in a cylindrical optical waveguide. Gloge and Marcatili (28)have shown that this quantization limits the number of guided optical modes to a number N which is given by the relationship:
N
=
(knl)’[a/a
+ 2)] Au2
(2)
where n, = n(at r = 0), k = 2n/A is the free-space propagation constant, and A is the free-space optical wavelength. Hence, a step index fiber (a = co) will sustain twice as many modes as a graded index fiber (a = 2) having the same radius and index difference. This 3-dB coupling-loss penalty for graded index fibers is a significant factor in optimizing fiber parameters for LDC applications. Note also that the number of modes is proportional to the core area. The ability of an optical fiber to couple efficiently to an LED source is determined primarily by the maximum acceptance angle Om,, shown in Fig. 9. Rays with 8 < Omax are trapped by total internal reflection. Applying Snell’s law at the fiber front face and the core-cladding interface, this angle is specified in terms of the numerical aperture (NA) for the uniform index profile case (it applies to any profile) by the relationship NA = sin8,,,
= (n: -
ni)’/2 = n,(2A)1’2
(3)
Substituting A from Eq. (3) into Eq. (2) yields the following expression for the number of fiber modes
N = %( (NA)’(na2) L) A a+2
(4)
Thus, the ability of an optical fiber to couple efficiently to a Lambertian LED source [i.e., a source which has optical intensity l ( 8 ) = locos 8, where 8 is the angle measured from the normal to the LED surface] is determined by N and is proportional to (NA)’ and the fiber core area nu2.As indicated previously, the fiber-transmission bandwidth is limited by pulse spreading or dispersion. One component, illustrated in Fig. 10 versus index-profile parameter a, is modal pulse dispersion. The second component, material dispersion (caused by the nonlinear dependence of refractive index on wavelength A) results in dispersion proportional to the spectral linewidth 6A of the source. b. Pulse dispersion. As shown in Section II,C, modal dispersion is the primary bandwidth determinant in step index and partially graded (a > 3) fibers for LDC applications. The maximum modal group delay spread T,, determined by ray tracing, between a ray at 8, and an axial ray, is given by
166
DELON C. HANSON
40 -
I
I
I
NA -
z 0
30--
20--
a I I
d
n 0
z
2
6
4
PROFILE
10
8
PARAMETER,
(Y
FIG. 10. Modal dispersion versus profile parameter tl for various numerical aperatures. All modes are assumed equally excited and there are no cladding modes and no mode coupling.
where L is the link length and c is the velocity of light in free space, 3 x lo5 km/sec. As the refractive index profile is adjusted to 01 = 2, the continuousfocusing property of the fiber reduces the modal dispersion by the factor A/2. Hence, z,(a
=
2) = (~5/8cn:)(NA)~
(6)
where NA is NA (at r = 0). At N A = 0.25, this results in a factor of 16 times reduction in modal pulse dispersion. Modal dispersion is not uniform with link length since diffusion of optical power between fiber modes only reaches steady state after coupling length L,. The resultant effective fiber length, as given by Marcatili (29),is Leff = [l
+
L (L/L,)]”Z
(7)
For L << L,, modal dispersion is approximately proportional to link length L, whereas for L >> L,, modal dispersion is proportional to L1/2.Since fiber
FIBER OPTICS IN LOCAL AREA NETWORKS
167
bandwidth is inversely proportional to Lee, the bandwidth-distance product is not a constant with respect to the physical link length L. Figure 10 shows the modal dispersion versus profile parameter a for various values of A (or, correspondingly, NA). It is clear from this figure that both a and NA play a major role in determining modal dispersion. This data will be used later in relation to fiber bandwidth specification in Fig. 21. Material dispersion results when the group velocity (u,) is a function of wavelength. This has been described by Gloge (27) through the relationship
where s A / A is the source fractional spectral linewidth and A2 (d2n,/dA2)is the material dispersion of the waveguide. Material dispersion is a function of wavelength for silica fibers. It is shown later, in relation to Fig. 21, that there is real merit from a dispersion standpoint for operating at 1.3 ym with LED sources for high-speed systems since this dispersion component goes to zero at this wavelength (30). c. Microbending loss. The fiber parameters dealt with thus far have assumed that the fiber is uniform and straight and, consequently, all rays maintain the same scattering angles throughout their propagation journey. In actual applications, fibers are subject to bends having various radii of curvature. Figure 11 illustrates that with a curved, step index fiber, the reflection angle will be changed by
where I,, the reflection length, is 4a/tan(BaXi,,)and R is the bend radius. If Ooutside > 8, (critical angle, shown in Fig. 9), the ray will no longer be trapped in the core. The fractional loss F at this bend is given by Marcatili (29) as a F--=RA
2an: R(NA)2
According to Eq. (4), a fiber with A = 0.02 (NA = 0.29), a = 50 pm, a = 10, and l o = 820 nm will trap N = 5400 modes. Olshansky (31)has shown that these modes can be divided into M = N’” groups. All modes in a given group have an axial propagatian constant
where 0 < m < M and the Mth mode group contains m modes.
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DELON C . HANSON
,-e
outside
i
(b)
FIG. 11. Schematic drawing of ray paths for step index fibers with (a) straight and (b) curved configuration.
)”’(tg-””””
The propagation constant spacing betweet adjacent mode groups is
sp - (2A)a /’
(
2u u+2
(12)
Gloge (27) has shown that mode coupling, which causes microbending loss, occurs almost exclusively between modes in adjacent groups. As a consequence, significant mode coupling will occur only if the fiber perturbation has Fourier components with spacing equal to the mode spacing given in Eq. 12. For an LDC fiber having a = 50 pm, A = 0.02, and d = 2, Sp = (2A)’l2/a = 40cm-’
FIBER OPTICS IN LOCAL AREA NETWORKS
169
for all modes. This corresponds to a spatial period A = 2n/@ = 1.6mm
For step-index fibers having the same Q and A, the highest order modes (which have the greatest scattering loss) have A = 1.56 mm. Hence, fiber perturbations having a spacing or 1-2 mm will create the greatest microbending loss. Cable design will be considered in some detail in Section I1,B. Extensive analyses by Olshansky (31), Marcuse (32),and Gloge (33)have been summarized and expanded by Hanson (34). The fiber attenuation increase due to microbending is related by B
=
- 10 log, e-yL = 4.34yL
(13)
where L is the fiber length (cm) and y is the propagation loss coefficient. Olshansky has pointed out the direct connection which exists between B and the coupling length L,, given in Eq. (7). Neither term can be accurately predicted separately from first principles, but theory by Olshansky (31) predicts that their product is related by
BL, = LQ(a,p)
(14)
where p is the power law exponent in the Fourier spectrum of the perturbations. Figure 12 shows the dependence of microbending loss Q (dB) per unitmode coupling length L, versus wfor values of p. Hanson (34) points out that p = 2 for isolated perturbations with a hard jacket in direct contact with the fiber, and p = 4 for a soft buffer filling the space between the fiber and the jacket. From Eqs. (13) and (14), y = Q(a, p)/4.34LC. From the trend in Fig. 12, the advantage of using a soft buffer in contact with the fiber is clear, i.e., it reduces microbending loss B because of the reduction of Q(a, p) with increasing p. Olshansky (35)has calculated that 100 perturbations of 3-pm amplitude randomly distributed over a l-km length of fiber results in B = 24 dB. The importance of smooth, soft fiber buffering is evident. It is also evident from Fig. 12 that a more nearly step-index fiber is less susceptible to microbending loss. Minimizing microbending loss is a major consideration in cable design, discussed in Section I1,B. Just as in the case of other fiber parameters, it is useful to establish the dependence of microbending loss B on fiber NA and core radius a. Hanson (34) has shown that B can vary from a*/(NA)" to ao/(NA)* depending on fiber buffering and perturbatibn conditions. This is a drastic spread of performance and is an indicatiqn why individual results have been reported
170
DELON C . HANSON
2.0,
I
I
I
6
8
1.5
m W
0 W,
cn
0 1.0 -3
cn cn Y U
X
w 0.5
0
4 PROFILE
D
PARAMETER, a
FIG. 12. Microbending loss Q (dB) (per unit mode coupling length L c ) versus profile parameter a for various values of power law exponent p (31).
(36, 37) in the middle of this range without noting the full range of the dependence which is possible. d. Fiber strength. In addition to the minute bends which result in mode scattering and microbending-induced attenuation, fiber stress caused by bends of much smaller bend radius are important in specifying the necessary fiber strength and reliability. The term “fiber strength” can mean many things depending upon the application. During cable installation it refers to the ability of fiber to withstand short-term tensile and bending stresses. For long-term reliability, it
FIBER OPTICS IN LOCAL AREA NETWORKS
171
refers to the ability of fiber to resist fracture caused by a much smaller stress in the presence of moisture, which may cause minute cracks to grow in size. In either case, fiber-strength characterization is a statistical process since the nature and location of imperfections on the surface of good-quality glass fiber is random. The flaws may result from the cladding-material parameters, from abrasion occurring at high temperature, from pulling/coating operations, or from room-temperature handling operations. Since glass is a brittle material, a fracture is caused by a crack which propagates at velocity V (38),given by V = AK” (15) where A is a material constant and n is the stress-corrosion susceptibility constant. The stress-intensityparameter K, which derives from early work by Griffith (38),is
K = Ycr,a”2
(16)
where Y is a geometrical factor, cr, is the applied stress, and a is the crack depth. At a critical value K , of K , the crack velocity V disobeys Eq. (15) and increases rapidly and catastrophically. Neither crack depth a nor applied stress 8, alone are sufficient to specify K , . It is generally accepted that the statistical strength of fibers is described by a Weibull distribution (39).For system installation, there is interest in the strength of fibers which are greater than 1 km in length. For practical reasons, measurements are typically made on lengths <20 m. Weibull statistics relate to the survival probability S at stress level cr for sample length L The statistics are characterized in terms of corresponding reference values So, stress level no,and length Lo through the relationship In h[(1 - F ) - ‘ ] = m In(cr/cr,)
+ In&/,!,,)
(17)
where F, the failure probability, is 1 - S. A Weibull plot of In ln[(l - F ) - ’ ] versus In cr yields, according to Love (do), a straight line of slope rn when there is a single-flaw type. The stress at which In ln[(1 - F ) - ’ ] = 0, corresponding to the 63.2% probability of failwe point, is defined by go. Figure 13 shows a Weibull plot of the strength distribution of a fiber measured with a 10-m gauge length. For this fiber, two distinct types of flaws are evident, as represented by straight-line slopes with m = 3.5 and rn = 15, respectively. Today’s strong fibers have a steeper slope and are shifted to the right in Fig. 13. Ektensive test results have been presented for strong optical fibers (41). For LDC system applicatioas, the strength of 1-km fibers is of primary interest even though measure nts are made on 10-m gauge lengths. Assuming the parameters in Eq. are constants, the 1-km fracture stress can
172
DELON C. HANSON
I
I
I
I
I
I
I
I
I
6
I
-- 4 w
m
o!
= 15+'
--2 I
I 0.1
I
I
I
I
LL
-- -6
I
I
-- ao( 1000m)= 0.9
--
ao(lOm) = 3.5 I I
1
I
I I
I
I
I
I I
I I
I
I
I
-8
be derived from Fig. 13 by shifting the horizontal dashed line down by -ln(lO/1000) = 4.6 units. Hence, at 1 km the fracture stress parameter o0 is reduced to 0.9 GN/mZ. The Weibull slope parameter m is derived from the ratio
m =
4.6
1n(3.5/0.9)
= 3.5
With such a distribution of fractures versus tensile force, fiber manufacturers and users are concerned with the question of whether a test can be performed which will ensure that during installation and for a prescribed operating
FIBER OPTIC6 IN LOCAL AREA NETWORKS
173
period, the probability of failure is low. It is important that the test not significantly degrade the fiber strength. Proof testing of fiber has become an accepted method for accomplishing these objectives. In this process, an optical fiber is subjected to a tensile force for a few seconds which is larger than the load expected in subsequent handling. The fiber sections which pass the proof test have a maximum possible size flaw and a minimum guaranteed failure time because of static fatigue at a particular stress level. The key concern is to establish a proper proof test stress level which will minimize the increase in fiber cost incurred due to breaking the fiber into unacceptably short sections before cabling, while maintaining a sufficiently stringent proof test to ensure long-term reliability. Kalish and Tariyal(42)have characterized the time to failure t , caused by static fatigue in terms of a stress power law expression t, = W(a;-2/a;)
(18)
where oP is the proof-test stress and ofis the applied stress in service which leads to static fatigue,
w
=
2/[AYZ(n - 2)K;,z]
and A, n are material constants in the crack velocity expression [Eq. (15)] ;K, Y are stress intensity factors [in Eq. (16)] at the proof-test level. Equation (18) is very significant in its strong dependence on the stress corrosion susceptibility constant n. The upper portion of Fig. 14 is a plot of Eq. (18) for three proof-test stress levels, as presented by Tariyal et al. (42). The parameters are for n = 32, room-temperature operation, and 45% relative humidity. Significantly, the dashed lines show that to project a 20year life before static fatigue, the applied stress in service must not be more than 30% of the proof-test stress. The lower half of Fig. 14 shows that if the material constant n can be increased well above 30, the applied stress on the fiber for a given life can be significantly increased. Hiskes (43) has recently shown that through the application of a first coat of vitreous silicon nitride via chemical vapor deposition, the stress corrosion parameter can be increased from the range 20-26 to the range n = 40 to infinity. If this proves to be a viable production process it will have major significance in both fiber and cable construction. It is generally recognized that moisture will penetrate any organic coating. Therefore, while organic coatirigs, e.g., silicone, provide excellent abrasion resistance, they do not protect tbe fiber against static fatigue. Charles (44) has shown that stress corrosion in the presence of ambient moisture will eventually result in fracture at a service stress far below the initial fiber strength. If n
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DELON C . HANSON
I
I
I
I
100 yr 20 yr 1 Yr
1 month
\
1 hr
I
10
0
I
I
70
I
140
210
280
1
I
I
I
N
-
P
0 v1
v1
-
-
I
7
-
1
FIBER OPTICS IN LOCAL AREA NETWORKS
175
can be made to approach infinity, it will not be necessary to require that the service stress be less than 30% of the proof-test stress. Consequently, a smaller fiber bend radius can be tolerated and lighter-weight cable construction can be used.
2. Fiber Selection a. Attenuation versus bandwidth-distance. Having summarized the basic parameters of optical fibers which are relevant to LDC, independent of technology, it is now appropriate to consider the choices which are available. The outer LDC boundaries in Fig. 5 establish the minimum bandwidthdistance products which the transmission link must possess to meet system requirements. For LDC applications, this is summarized in Table V (17). These bandwidth-distance requirements are a factor of 30 lower than those corresponding to DT systems. The outer boundaries in Fig. 15 show the range of fiber parameters in the attenuation versus bandwidth-distance plane which meet these system requirements. For the LDC sector, the highest bandwidth-distance is 200 MHz-km. This is a factor of two larger than is dictated by the above system applications list because it is necessary to account for additional speed limitations in the system, e.g., the source and detector-amplifier bandwidths, The precise bandwidth margin which is required depends on the particular application. The slope of the upper LDC boundary in Fig. 15 accounts for the fact that as the attenuation exceeds 20 dB/km, link length will be limited by the fiber attenuation and, consequqntly, there is a corresponding reduction in the required fiber bandwidth-distance. The lower sloping boundary in Fig. 15 is a recognition that optical fibers have a minimum dispersion-limited bandwidth, independent of how high the attenuation may become (45). The upper and lower slopes in the DT region (above 200 MHz-km) imply that attenuation must decrease with increased bandwidth-distance to allow using the increased fiber bandwidth for the DT system requirements shown in Fig. 5. TABLE V
LDC generation
System bandwidth-distance (MHz-km)
First Second Third
20 100 100
176
DELON C. HANSON
Local Data Communications
BANDWIDTH-DISTANCE (MHz-km) FIG.15. Optical transmission parameters (at 1, = 825 nm) versus technology choices for local data communications and digital telecommunicationsfibers.
b. Technology choices. Shown in Fig. 15 are the suggested performance boundaries for the four primary technologies which can be used for the respective segments in order to meet LDC application requirements. The vertical boundary at 10 MHz-km is determined by pulse dispersion. The allplastic and plastic-clad silica (PCS) fibers are step index. According to Eq. (5), if the steady-state NA = 0.25, the modal pulse dispersion is
z,/L = (NA)2/(2cn:)
=
50 nsec/km
Since a 10-MHz signal has a 50-nsec baud period, this level of distortion significantly limits performance for a l-km link. Boundaries in Fig. 15 should be viewed somewhat broadly since there have been reported results beyond those in the d2signated areas. Beales and Day (25) have extensively reviewed fiber technologies with particular emphasis on the liquid-phase (double-crucible) compound glasses. The advantages of compound glasses are (1) lower cost, in principle, because fibers can be drawn into continuous lengths (since the melt can be replenished) rather than being limited by the fixed preform lengths, and (2) a wide range of available numerical apertures exist, e.g., 0.2-0.5, depending on the glasses which are chosen for the core and cladding. The disadvantages are (1) difficulty in getting attenuation below 10 dB/km at 825 nm due to the enhanced
FIBER OPTICS
IN LOCAL AREA NETWORKS
177
scattering loss of compound gbsses, (2) fiber strength tends to be somewhat lower than in silica-clad fibers, and (3) the ultra-low-attenuation window in the 1.0-1.6-pm range does not exist. Partially graded (3 u < 10) and graded-index (M = 2) silica fibers are most extensively fabricated by vapor-phase techniques. There are currently three leading process alternatives:
-=
(1) Flame hydrolysis (outside tube). This method was used to produce the first fiber (23) with attenuation less than 20 dB/km. The procedure involves depositing “soot” on the outside of a rotating “bait” rod. The rod, usually made of A120, in order to match the thermal expansion coefficient of the soot, is carefully removed before collapsing the preform to draw the fiber. This is a relatively high-volume process, but it is difficult to achieve the ultimate low loss since the hydroxyl ion is not as easily eliminated from the deposition. (2) Thermal oxidation (inside tube or MCVD). This process is the most common method for making low-loss optical fiber. It results in “soot” deposition in layers on the inside of a carefully heated rotating quartz tube as a result of the appropriate gas flow and many passes of a torch along the rotating tube. The dopant concentrations can be changed on each pass to control the doping profile by forming minute step changes in the index profile. After collapsing the preform, the original quartz tube becomes the fiber outer cladding. (3) Vapor axial deposition (VAD). In this process both the core and cladding of the preform are deposited simultaneously on the end of a rotating fused silica starter rod using special torches. This preform is placed inside a quartz tube and collapsed before fiber drawing. Excellent attenuation and bandwidth results have been reported (24). Recent results have yielded 150 km of 150-pm-0.d. fiber from a single preform. There is some difficulty in achieving large NA, e.g., 0.3, with this process. In addition, control of the refractive index profile is somewhat more difficult and thus the resultant bandwidth is more variable. However, contrary to the two other vapor-phase processes, VAD does not have gn axial discontinuity in the refractive index profile caused by the out diffusion of germanium from the interior deposited layers during the high-temperature, preform collapse process. With much additional process engineering, VAD has the potential of making arbitrarily long-length preforms of the highest quality. was the first fiber seriously considered for use in Plastic-clad silica (PCS) LDC. With reference to Fig. 9, the core is made from a homogeneous quartz rod having index n, = 1.458 a d the cladding is a polymer with index n sufficiently lower, e.g., A = .02 ith n2 = 1.429. The attractiveness of PCS is that the starting core material is a relatively low-cost homogeneous rod.
L
178
DELON C. HANSON
Thus, it can have a relatively large diameter in order to ease source and connector coupling. The polymer cladding, which is added during the pulling process, serves the dual role of trapping the core optical modes and protecting the core from mechanical abrasion. Figure 16, due to Blyler and Hart (46), shows the effect of polymer cladding-material attenuation on overall PCS fiber attenuation. Cladding materials typically have attenuation greater than 1000 dB/km. It is readily evident from Fig. 16 that cladding loss seriously increases overall fiber attenuation. Only silicone cladding exhibits overall fiber attenuation < 10 dB/km with the given set of conditions. In contrast to the PCS advantages listed above, there are also some significant disadvantages, as noted by Hanson et al. (47,48). These include the following: (1) Much greater susceptibility to attenuation increase with moisture and temperature cycling since water can penetrate directly to the optical guiding surface. (2) Much greater difficulty in designing and installing low-loss connectors which will reliably hold the optical fiber in a fixed position. Securing the fiber is necessary in order to prevent damage to associated optical coupling devices. Claddings are typically soft and may soften unacceptably I
f
Assumed 1.5--
---
"SiOP
1
I
acORE = SdB/kni
p*]
= 0.75
--
-
FIBER OPTICS IN LOCAL AREA NETWORKS
179
in the desired operating-temperature range. Thus, the glass core of the fiber cannot be secured by gripping the cladding material. Epoxy materials, which will rigidly secure to the core after the cladding is removed in the connector tip, typically have a higher index of refraction than the core and will extract optical power from the fiber core in the areas of contact and may increase connector attenuation by 1-3 dB. This implies that the cladding should be left on the fiber. This adds the burden of maintaining precise cladding concentricity in order to minimize connector radial alignment loss. A recent design improvement by Raychem for PCS fiber termination replaces the soft cladding on a short-fiber length near the connector tip with a low-index “Fiber Sleeve” which adheres directly to the glass core, is relatively hard, and has a broad temperature range. This may yield practical PCS connector termination results for rigidly locking the fiber in the connector tip. All plastic fiber, having attenuation typically > 300 dB/km, is attractive for 1-20-m link lengths in which the fiber core diameter is sufficiently large, e.g., lo00 pm, so that conventional large-area sources can be used without the need for precision alignmept. Due to the ductile nature of plastic, the fiber can be tied in knots without fear of breakage. Figure 17 summarizes these fiber parameter options in the numerical aperture versus core diameter plane. It is clear from this figure that certain areas of overlap occur between technologies. Also shown are the parameter ranges for graded-index and single-mode silica fibers used in DT applications. The pros and cons of selecting fibers in the four corners of this plane are listed. It is evident from this comparison that compromises must be made, particulary since the corresponding attenuation versus bandwidth-distance parameters in Fig. 15 must be included in the design trade-off process. As a result of these trade-offs, certain fiber standards have been proposed or are in the process of being proposed. “Standard” parameter choices are shown by circles in Fig. 17. In the case of DT, the standard fiber has a 50-pm core diameter, a 125-pm cladding o.d., and NA = 0.22. This represents the IEC and CCITT International !Standards consensus as the best choice for the variety of technologies which csn be used to make this fiber. Higher-cost, greater-precision connectors are acceptable with this relatively small core fiber since fiber cost for long cable lengths between repeaters is reduced. As noted in Section I, a broad spectrum of applications exists in LDC. Because of the shorter link lengths involved in LDC, there is a desire to utilize lower cost, less-precision connectors, and larger active area sources. As a result, the evolving standards have core-area stepping by factors of four, i.e., the core diameters incremeat from 50 to 100 and 200 pm, respectively. There is also an 80-85-pm core/ 125-pm-cladding 0.d. fiber standard evolving
180
DELON C. HANSON
*Reduced Fiber Cost *Improved Flexibility 0.5 2 I-
cc
r-----1 L i
7
Compound Classes
W
&
a
I
is,Ic
0.4
a
Partially Graded Silica
V
m
,
Reduced Connector Cost Easier Source Alignment
IMPROVED: Input Coupling Microbending Loss
L-
p!
’ w
2
0.3
z
REDUCED: *Fiber Cost Attenuation
W I-
a
c. rA
*n
0.2
I
a w
c
rA
Single-Mode Silica 0.1
I
I I
I
200
300
400
I
100
CORE
DIAMETER
t
(pm)
FIG. 17. Steady-state numerical aperature versus core diameter for various optical fiber technology and application choices.
for LDC applications in order to use the same connector hardware as in DT applications. From Fig. 15, it is evident that LDC fiber becomes “telecommunication-like” when bandwidth-distance becomes > 100 MHz-km. Thus, it is probable that 80-85/125-pm and 100/140-pm fibers, having the same NA and bandwidth, will be standardized depending on whether the LDC equipment evolves from the telecommunications or local data communications sectors. Because of the moderate bandwidth-distance products shown in Fig. 15, improved input coupling efficiencies and microbending loss result from increasing the NA. However, there is a slight penalty of increased fiber cost and attenuation with increased NA. Table VI summarizes the projected fiber standards for LDC applications. As noted by Hanson (22),the 140-pm 0.d. was chosen for the 100-pm core fiber to reduce fiber cost, since a 20-pm glass cladding thickness is the thinnest which gives negligible increase in attenuation due to optical mode interaction with the surrounding fiber jacketing, and is consistent with established
181
FIBER OPTICS IN LOCAL AREA NETWORKS
TABLE VI DATACOMMUNICATIONS FIBER PARAMETERS Optical parameters Fiber diameter
Fiber type All plastic Plastic-clad silica Glass/glass (LDC) Glass/glass (DT)
Core (pm)
Clad 0.d. (Fm)
lo00 370 200 200 100 50
1030 400 300-600 230-280 I40 125
Numerical 3-dB Attenuation aperture bandwidth (850 pm) (90%power) (MHz-km) (dB/km) 0.45
25
0.2-0.3 0.3 0.3 0.2
2 10
z 10 20-200 2 200
1600 @ 660 nm 2 30 2 I5 I10 17
fiber fabrication methods (47, 49). The proposed 80-85/125-pm fiber supports this same conclusion concerning cladding thickness. The cladding outside diameters for the 200-pm core fiber is very much undecided since a variety of all glass fiber technologies can be used which yield 0.d.s in the range 230-280 pm. In addition, for the PCS fiber there is an even broader spectrum depending on whether the polymer cladding is relatively soft, e.g., silicone, and must be removed and reclad in the connector installation process or is relatively hard with the intention of leaving it intact to form the cladding in the connector alignment mechanism.
B. Cable Requirements for LDC Considerable attention has been given in the literature to multichannel cable design for use with telec~mmunicationssystems (50). Emphasis is on long runs of relatively heavy cable construction which can be pulled through existing underground ducts or plowed underground behind heavy earthmoving equipment. In cities, runs may be shorter and splicing may be required at intermediate manhole separations since it may not be feasible to shut down an entire section of a city during the cable-pulling operation. For local data communications (LDC), the emphasis is primarily on relatively lightweight, single- or dual-channel cable which can be laid within a building or between adjacent buildings. Although the cable is usually laid in floor or ceiling cable trays, in many cases the preference is to have the cable attached directly to walls or Supporting structures or to be pulled into existing ducts. Because of the need to make many repeated interconnections
182
DELON C. HANSON
during normal equipment operation and the need for equipment to be moved many times, the cable must be flexible and yet durable enough to withstand being run over by carts and being pulled around relatively sharp corners.
I. Cable Design Because of these requirements, single- and dual-channel LDC cables have the basic structure shown in Fig. 18. This construction consists of an optical fiber surrounded by a soft buffer coating (typically silicone) and an inner buffer jacket having outside diameter d. Aramid strength members, typically Kevlar, surround the inner cable jacket and provide load-bearing and impact protection. The outer jacket, typically made of polyurethane or polyvinyl chloride (PVC), provides abrasion resistance and structural rigidity, as well as controlling the surface friction properties of the cable. The webbing between the two cable channels yields separation qualities much like that in an electrical lamp power cord. The outer diameter D is typicalIy 2.5-3.0 mm. Aramid Outer Jacket
Buffer Coating (Silicone)
'Optical Fiber kdd
SINGLE CHANNEL
I Separation Web
DUAL CHANNEL
FIG.18. Single- and dual-channel fiber optic cable construction for local data communications applications.
FIBER OPTICS IN LOCAL AREA NETWORKS
183
The tension T applied to ti cable along its axis can be expressed in terms of the allowed fiber elongation S, by the Hooke’s law relationship T
S,(E,A,
+ ErA,)
(19) where E is the Young’s modulus and A is the cross-sectional area of the strength members (c) and fiber (f), respectively. If Sf is set at the fiber maximum S,,,, as determined by the fiber initial proof test and long-term derating (Fig. 14), the allowed cable tension can be approximated by =
(20) From Eq. (20) the required cable strength-member design is prescribed. Clearly, if the fiber can withstand a greater strain Sf,,,, the cable construction can be lighter weight and Iess costly. In addition to providing the required cable tensile strength, the strength-member configuration is very important in determining the impact resistance of the cable and may contribute to the temperature dependence of attenuation because of inducing microbending in the optical fiber. T
=
Sfme,EcA,
2. Fiber Buffering As previously discussed in Section II,A,l,c, the soft buffer coating on the fiber is crucial for reducing the microbending loss caused by small fiber perturbations. In addition, this buffer coating provides the first level of protection against abrasion immediately after the fiber is drawn, in order to reduce the probability of static fatigue. Ishihara et al. (51) have characterized the optimum dimensions for the coated fiber in order to minimize the fiber stress caused by compressive loads and attenuation increase due to microbending as a result of temperature cycling and cable loading. Their results show that the optimum inner-cable buffer-jacket diameter d = 0.9 mm and the optimum buffer-coating thickness is 0.09d. Thus, the optimum soft buffer-layer thickness is 80 pm. For a 140-pm-0.d. fiber, the optimum buffer-coating outer diameter is 300 pm. These were the dimensions specified for the LDC cable described by Hanson et al. (47).If the buffer coating is made thicker, the fiber production process is slowed (and cost is increased) because of the need to allow additional time for the buffer coating to cure before the fiber is wound on a reel for storage. If the buffer coating is thinner, less protection results. Use of a two-layer buffer coating (with a higher modulus over a lower modulus) has yielded improved attenuation performance at low temperature because of reduced microbending. There are various inner buffer jacket constructions, particularly for plastic-clad silica (PCS) fiber. For PCS, the buffer coating or the inner buffer
184
DELON C. HANSON
jacket serve the dual purpose of providing the optical cladding interface as well as mechanical buffering. As discussed in Section II,A,2,b, certain vendors have chosen to use a relatively hard combined cladding and jacket, and to incorporate this within the tight cable assembly shown in Fig. 18. This hard inner jacket must be concentric with the fiber since it cannot be easily removed for direct fiber-contact alignment in the connector. Selection of the material for the outer jacket of LDC cables is important for several reasons, e.g., scuff resistance, flame-retardant qualities, temperature dependence, and friction qualities during duct installation. The primary choices are PVC, polyurethane, and Hytrel. Of these, PVC has the lowest cost and has experienced the greatest use in electrical cables. There are serious concerns, however, about its outgassing qualities if it is burned, particularly when laid in air ducts. Hytrel has a broad temperature range and is very durable, but is difficult to make flame retardant and is more expensive than PVC or polyurethane. Polyurethane is tough and scuff resistant, has a broad temperature range, and can be made sufficiently flame retardant for most applications. For these reasons, the trend seems to be to use polyurethane for LDC cable. Clearly, one cable design will not meet the requirements of all applications without making it excessively expensive for the lowest performance requirements. The most difficult environmental conditions will occur between buildings on a local site where temperature and moisture extremes may occur. Setting mechanical specification standards for LDC cable is a significant challenge since the right balance must exist between performance and cost. As shown in Fig. 14, to achieve a 20-year life for optical fiber having stress corrosion susceptibility parameter n = 32 and low humidity, requires that the applied stress be less than 30% of the proof-test stress np. If the fiber is hermetically sealed so that n approaches infinity, it is possible that cable cost can be reduced by allowing the fiber to experience an applied stress approaching the proof-test stress and also be free from static fatigue caused by the presence of both moisture and applied stress. C . Connector Design
Much has been written supporting the virtues of various types of optical connectors. The fact that there are as many connector designs on the market as there are different connector vendors is ample evidence that no single design approach has won general acceptance. It remains a distant goal for an untrained installer to apply fiber optic connectors which will precisely align two hair-size fibers in less than 10 min at a remote site with no auxiliary power, epoxy, or polishing.
FIBER OPTICS IN LOCAL AREA NETWORKS
185
The objective of this article is not to list and chronicle all of the connector design and vendor options which exist, but rather to summarize the performance requirements, primary design options, and trends. Quoted results for connector loss are usually stated for identical fibers in each half of a connector. Whereas this may be a satisfactory approach for specifying precision reference connectors and for quoting results for permanently splicing a fiber break, it is not satisfactory for system specification in which the full range of fiber parameters will be experienced. The contributions to optical insertion loss of fiber optic connectors may be divided into two classes (52): (1) intrinsic loss due only to fiber parameter differences for perfectly aligned fibers, and (2) extrinsic losses due to fiber misalignment tolerances caused by the particular fiber alignment technique. 1. lntrinsic Loss As outlined in Section II,A,l,a, Gloge and Marcatili (28)have shown that the total number of optical modes N in a fiber is given by
N
=
g1’( La) + ( N2 A ) ’ n a ’
(4)
When all modes of a source fiber (S) are equally excited and there are no leaky modes, the intrinsic coupling loss L,, from source fiber (S) into receiving fiber (R) is given by the ratio of the number of steady-state trapped modes. For NR < Ns, Li, = - 10 log, o(NE/NS)
(21)
Substituting Eq. (4) into Eq. (21) yields Li, = La
+ L, + L,,
(22)
where
L = -20 log, o(aR/~s),
L=
- 20 log, o(NAR/NAs)
All optical parameters are referred to the local region of the connection. Other contributions not taken into account by these expressions result from (1) noncircularity of the fiber core, (2) concentricity error between the core and cladding reference surfaces, and (3) noncircularity of the outer reference surface. It is very difficult to separqtely account for the individual contributions listed above. Even if individualkontributions add on an rms basis, connector
186
DELON C. HANSON
loss is reduced in actual fibers because higher order modes are more highly attenuated. The remaining steady-state optical modes tend to concentrate near the fiber-core axis. Thus, the resultant mismatch loss is less than that calculated from assuming that all modes are equally excited. 2. Extrinsic Loss
Loss contributions in this category are associated with the alignment tolerences of the physical connector hardware which supports the fiber. The four categories of orthogonal contributions are (a) radial misalignment of the fiber cores, (b) axial separation of fiber end surfaces, (c) angular misalignment of fiber axes, and (d) fiber end surface finish and Fresnel reflections. The analytical characterization of the extrinsic misalignment loss is dependent on the model used for the distribution of optical modes at the exit of the source fiber. If the model assumes that the entire source fiber-core end surface consists of uniform point sources which emit into a solid angle specified by the fiber NA (53,54),a representation of optical fiber misalignment loss can be obtained using simple geometrical optics. Calculated loss contributions based on this model are shown in Fig. 19. It is evident that the radial displacement is the most sensitive parameter and that the NA has opposite sensitivity variation for the axial and mutual angle contributions. In actual fibers the equilibrium optical mode distribution will differ from the assumptions of this model. Individual contributions, added on an rms basis, reasonably account for the overall extrinsic loss of the connector. Because of different perceived needs for alignment precision and cost, a broad spectrum of incompatible connector alignment hardware options have been introduced into the market. Table VII includes representative alignment methods and certain vendors who use the alignment approach. In addition to this set of fiber alignment options, there is an even broader set of choices for the physical coupling means for mating the connector halves. These range from SMA-type screw threads to plastic snap-on shells. Suffice it to say that user selection of preferred options and resultant standardization has not yet occured. As a result, the “let the buyer beware” label certainly applies at this time. In addition, many connector vendors have not adequately characterized the effects of temperature cycling of terminated cables over their prescribed storage limits. Particularly with dissimilar expansion coefficients, e.g., glass fibers in plastic ferrules, a distinct possibility of fiber movement relative to the ferrule end surface can exist as a result of temperature cycling and may cause damage due to physical contact of fiber end surfaces.
187
FIBER OPTICS IN LOCAL AREA NETWORKS
3.0
I Pa = CORE OIAMETER
1.0-
R A D I A L DISPLACEMENT RATIO, 6 / 2 a
W
0
0.1
0.2
0.3
A X I A L SEPARATION RATIO, t / 2 a
I
0"
4"
8"
12"
M U T U A L ANGLE, 0
FIG. 19. Calculated extrinsic connector loss components versus mechanical offset parameters for various numerical apertures (NAs). All models are assumed to be equally filled over the source fiber surface within the angle defined by the NA.
As noted in Section II,A,2, a key design trade-off for LDC fiber optic links is the relative cost of fib& versus the cost of the associated connectors required to achieve a prescribed insertion loss for a given link length. Since the fiber cost increases about ib proportion to fiber cross-sectional area, it is desirable to use smaller diambter fibers with the resultant requirement for
188
DELON C . HANSON
TABLE VII Fiber-alignment means Tapered plastic ferrule Metallic ferrule Precision-drilled fiber hole Precision tube insert which fits fiber Diamond insert which fits fiber Rod-lens interface Biconical plastic ferrule, transfer molded directly onto fiber Four bent glass rods forming V-groove for fiber Internal plastic lens
Vendors AMP (55) Amphenol, H. P. (47) NTT (56) (NEC, SEIKO) ITT (57) British Post Office (58) Bell Labs (59) TRW(60) Deutsch (61)
greater connector alignment precision as the required link length increases. Figure 20 shows a projection of connector piece part costs versus core diameter to achieve the required precision. As a rule, for long lengths, e.g., >300 m, it is less expensive to use smaller diameter fibers and allow the additional connector cost.
0
100
200
300
400
CORE DIAMETER ( p m ) FIG.20. Relative connector piece part cost versus fiber core diameter.
500
FIBER OPTICS IN LOCAL AREA NETWORKS
I89
111. TERMINAL DEVICE AND SYSTEM PERFORMANCE A . Optical Source and Transmitter Circuit Design
In previous sections of this article, parameters of the optical medium which are relevant to LDC have been explored in some detail. Of particular importance to overall system performance is the optical source and the interface between the optical qource and the fiber. This includes such considerations as operating wavelength, optical linewidth, response time, and coupling efficiency. The significance of each of these factors will be examined in the following sections. 1. Optical Wavelength and System Bandwidth It is clear from Fig. 8 that from a fiber attenuation standpoint, it is preferable to operate between 1.0 and 1.6 pm. However, operation in this region introduces severe technology constraints, particularly for the optical detector. Consequently, for LDC attention is primarily directed toward the 650-850 nm wavelength region in which 111-V optical source and silicon optical detector options are readily available; 111-V devices refer to semiconductors listed in Columns I11 and V of the periodic table. Table V!II summarizes the material and device parameters of optical 111-V sources for use in fiber optics. The choice of an optimal source has many alternatives depending upon the desired operating wavelength, optical output power, response time, and packaging constraints. Short links using all-plastic fiber prefer conventional, surface-emitting GaAsP LEDs in the 650-750 nm region for ultimate low cost and for system bandwidths < 10 MBd. The primary emphasis of this section will be GaAlAs source alternatives in the 750-850 nm region in which >l-km link lengths can be achieved. Based on the communication system requirements shown in Fig. 5, the starting point for parameter donsiderations is the projected bandwidthdistance product of the link. Source-fiber coupling efficiency is considered in the next section as it relates to allowed link length. The link bandwidth is limited by the pulse dispersion of the fiber and the response time of the source and detector. One of the key distinctions between laser diodes (LDs) and light emitting diodes (LEDs) is their spectral linewidth. As shown in Table VIII, the linewidth of LEDs is typically 30-100 nm. The corresponding linewidth of LDs is less than 2 nm. In Section II,A,l,a, the modal and material pulse dispersion properties of optical fibers were summarized. It was shown that for LDC applications, where it is desirable to improve the source coupling efficiency as a trade-off
TABLE VIII
FIBEROpnc SOURCE COMPARISON GaAlAs
Parameter Material Wavelength (nm) Spectral linewidth (nm) Ext. quantum efficiency 9 (%) Response time (nsec)
GaAsP surface LED
650-750 30 0.1-0.2
<40
GaAs (Zn doped) surface LED
GaInAsP
Surface LED
Etched-well LED
stripe LED
Stripe laser
750-850 40 1.0
750-850
750-850 40
750-850 <2 50
890-910
40
< 10
< 10
L Easy Moderate
L Easy Higher
C Most difficult Highest
< I5
3.5 i15
2.0
< 15
40 1
.o
Surface LED
Stripe laser
1100-1600 100
1100-1600 2 20
1 .o
Device Optical flux configuration" Ease of use Relative cost a
L Easy Low
L, Lambertian; C, collimated.
L C L C Easy More difficult More difficult Most difficult Moderate Higher Higher Highest
FIBER OPTICS IN LOCAL AREA NETWORKS
191
for reduced fiber bandwidth, the index profile parameter u is chosen in the range 3-10. As shown in Fig. 10, this yields substantially improved modal pulse dispersion as compared to step-index fibers. Figure 21 summarizes the total pulse dispersion and resultant transmission bandwidth versus source spectral linewidth (17). The analysis of modal pulse dispersion versus a, developed by Olshansky and Keck (36)and shown in Fig. 10, neglects mode coupling and thus overestimates pulse dispersion. Experimental results with fibers of known u indicate that the calculated results in Fig. 10 should be reduced by a factor of two to better approximate actual fiber performance. This has been done in developing the curves in Fig. 21 from the data shown in Fig. 10. The material dispersion contribution at 820 nm is approximately 0.1 SL/km, where 61 (nm) is the source spectral linewidth. Modal and material pulse dispersion contributions have been combined on an rms basis in Fig. 21. The data in Fig. 21 are plotted for three values of NA and index profile parameter a. The lower curve is for standard graded-index telecommunication fiber, as presented by Li (62). The dashed lower curve illustrates the reduced material pulse dispersion contribution at Lo = 1.25 pm. From Table VIII, it is noted that the GaInAsP LED spectral linewidth is 100 nm. Figure 21 shows that even with this relatively broad linewidth, the overall system pulse dispersion is nearly four times less at Lo = 1.25 pm than with a narrower linewidth source at A. = 820 nm. The summary of LDC data communication requirements in Fig. 5 shows that first-generation links require a bandwidth-distance product > 20 MBd-km. Second-generation link requirements extend to > 100 MBd-km. The upper two curves in Figure 21 are fiber types intended for firstgeneration links utilizing relatively high NA (0.29) for A = 0.02. The stepindex case shows a calculated pulse dispersion of 50 nsec/km. In practice, the dispersion is less due to the more rapid attenuation of higher-order modes in the cable. The plot for NA = 0.29, a = 8, is the fiber type proposed by Hanson et al. (47) for use in moderate-bandwidth LDC applications. This 0.d. and is listed for comparison fiber has a 100-pm-core/l40-ym-cladding in Table VI. With an LED linewidth of 40 nm centered at Lo = 820 nm, the composite pulse dispersion is 17.5 nsec/km for this fiber. This corresponds to a 20-MHz-km fiber bandwidth-distance product. The two intermediate curves are fiber parameters intended for use with second-generation LDC systems having data rates < 100 MBd. The NA has been reduced from 0.29 to 0.25 and u from 8 to 3 in order to achieve greater than 100 MBd-km performance with a 40-nm spectral linewidth LED at 820 nm. Note that there is not a significant pulse dispersion advantage in using a LD source with this fiber at this wavelength. As will be noted later in
192
DELON C. HANSON
100 1
LASER-4
80+60--
+LED-
10
STEP INDEX ( 01 = 00)
40--
-E
( A = 0.02, NA = 0.29)
E 20--
. 24
(y
Y
--
=8
I U
a
0
0
.o
2
-
E 100
x
+
u
z w
bl
Y
2 I
X
Lal b
1000
*
e c e I(A= 0.01, N A
o*2 0.1
t :
0.1
n
= 0.21)
I
II
1.o
10
SOURCE SPECTRAL LINE WIDTH
1
(nm)
FIG.21. Pulse spreading versus source spectral linewidth for various fiber profile parameters I,.
a, NAs, and operating wavelengths
FIBER OPTICS IN LOCAL AREA NETWORKS
193
this section, this reduction in NA and c( introduces more than 2.5-dB additional coupling loss in order to achieve the improved bandwidth. 2. LED Response Time If 100-MBd-km system performance is desired, there is not much point to specifying an LED wavelength which minimizes transmission pulse distortion unless the LED optical power can be modulated fast enough to avoid its being the bandwidth-limiting factor for the link. Figure 22 shows an equivalent circuit for LEDs used in fiber optic applications; R, is the source series resistance, c d the diffusion or recombination capacitance, C,the space charge junction capacitance, and id = Io[exp(vd/nkT)-']
(23)
where vd is the junction voltage and n w 2 for heavily doped GaAs devices. In an ideal LED, the rise time would be governed solely by the spontaneous recombination time of the carriers. In practical LEDs, however, the junction and stray capacitance delay the arrival time of carriers and thus both factors contribute to the overall response time. As a result, both materiallimited and circuit-limited design contributions enter in. The LED effective response time 7 is given by 'l= (cs
+ cd)/gd
(24)
where g d = aid/&),= qZo/nkT [from Eq. (23)]. The relationship between the optical output power from an LED with constant current drive and the modulation frequency (63) is given by
P(d =po/p
+ (04 3 112 2
(25)
where Po is the optical power at zero modulation frequency. To increase the modulation rate, 7 can be reduced by increasing the active-layer doping
FIG.22, LED equivalent circuit.
194
DELON C. HANSON
lo3
N
X
-
E X
P
102
I
5
n
z
d m
DOPING DENSITY ( G e ) , c ~ n - ~ FIG.23. DH Burrus-type LED bandwidth and effective carrier lifetime versus active-layer doping density (63).
level or by decreasing the thickness of the active layer at lower doping levels, as discussed by Lee and Dentai (63)and shown in Fig. 23. Unfortunately, for system designers interested in 100-MBd performance, reducing T results in a proportional reduction (6.3)in output power Po, as shown in Fig. 24. Thus, in addition to the more than 2.5-dB coupling-loss penalty described in the previous section (due to increasing the fiber bandwidth from 20 to 100 MHz-km), reducing the LED response time from 15 to 5 nsec for operation at 100 MBd reduces the optical output power by an additional 4.8 dB. 3. LED Design and Fiber-Coupling Eficiency
The objective of LED device design is to maximize the conversion of LED drive current into optical power coupled into steady-state fiber modes. The starting point for this is the LED internal quantum conversion efficiency. Another objective is to create an exit radiation pattern which couples effectively into the acceptance cone of fibers of interest. Device design also has significant implications on junction temperature, heat-sinking capability, and the ability to easily package the device with a well-designed optical port.
195
FIBER OPTICS IN LOCAL AREA NETWORKS
-- 400
30
- __ -E
3
w
3
, \
DIAMETER = 50pm
\
--200 --loo
0 0 d I-1 * a l o5: :
I
a
2.-
‘\t.
0
1-
- -50
!
1
1
I
--20
I
of band-
The source comparison summary in Table VIII lists three basic LED types, i.e., surface emitters (64),etched well (Burrus) surface emitters (65) and edge or stripe emitters (66). The considerations described in Sections III,A,l and III,A,2 apply to eqch of these LED types, independent of device configuration. Devices of each type, designed for operation in the 800-850 nm range, are shown in Figs. 25,26, and 27, respectively. The spherical lens in Fig. 25 could, in general, be aspherical or cylindrical in shape. These structures can be described as an “epitaxial layer cake.” It is evident from these figures that this is an apt description. Multiple layers of dissimilar semiconductors having different thicknesses and doping levels are needed for efficient operation in order to define the operating wavelength, confine injected electrons and holes in an active layer, and confine the optical power generated by the recombination of these carriers. The active region is confined to a small physical area (preferably smaller than the fiber-core area) by the contact area restriction and the lateral resistance of the epitaxial layers. Junctions between dissimibr semiconductors with different energy gaps are called heterojunctions. With respect to Figs. 25 and 27, the relatively
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DELON C. HANSON
SPHERE LENS Zn DIFFUSED LAYER
1
I
FIG.25. Surface-emitting GaAlAs LED (641.
highly doped aluminum layers, e.g., Alo.3and Alo.4 for 30 or 40% aluminum concentration, provide optical confinement since the added aluminum content reduces the refractive index and traps the optical radiation, just as in the case of an optical fiber illustrated in Fig. 9. This heterostructure also provides carrier confinement because of the energy barriers which are formed as a result of the relative energy gaps. Chin et al. (67) have theoretically and experimentally compared the performance of single-heterostructure (SH) and double-heterostructure (DH) LEDs for use in optical data links. When interfacing with high numerical aperture (NA = 0.36) fibers desired for LDC applications, they concluded that the D H LEDs launch at least eight times more optical power into a fiber than a similarly designed SH LED. This is due to the fact that for SH LEDs the active layer must be a compromise between the need for current confinement and the resultant undesired increase in nonradiative recombination at the contact. The active layer in a SH device (67)must be doped 100times more heavily than a DH device, with a resultant factor of two reduction in efficiency. D H LEDs of interest for fiber optic applications have internal quantum efficiency > 50% (63). Both types of surface emitters have emission patterns which are essentially Lambertian [Z(O) = I, cos O ] with about 120" beamwidth at the halfpower points. On the contrary, the edge-emitting DH structure, shown in Fig. 27, uses partial internal reflection of the spontaneous radiation to funnel
197
FIBER OPTICS Xh’ LOCAL AREA NETWORKS
LIGHT
ETCHED “WELL”
GLASS FIBER
METAL TAB
EPOXY RESIN n-TYPE GaAs
PRIMARY LIGHT EMITTING AREA FIG.26. Burrus-type surface-emitting GaAlAs LED (65).
CONTACT-
FIG.27. High-radiance edge-emitting GaAlAs LED (66).
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DELON C. HANSON
the emitted optical power into a beam exiting from the junction edge. As a result of the very thin active layer, e.g., 0.05 pm, which is required for efficient edge emitter operation and of a junction width of 50-80 pm, the radiation pattern is very asymmetrical, i.e., about 30" half-power angle in the vertical plane and 120" in the horizontial plane (68). The number of steady-state optical modes N which a fiber can trap was developed in Section II,A,l,a and is given by
For a perfectly aligned LED-fiber interface to an LED junction area which is smaller than the fiber-core area nu2,the power coupling efficiency qc into an optical fiber (69) is proportional to N and is given by
A plot of the dependence of coupling efficiency on the index profile parameter c( for various NA values is shown in Fig. 28. It is noted that there is 3-dB penalty in coupling efficiency for a graded index (a = 2) fiber relative to the step index (a = co)in each case. In addition, for a graded index fiber, optimal direct butt coupling between an LED source and fiber requires that the source diameter be approximately one-half the core diameter (69). A great deal of controversy exists about the relative merits of surface emitters and edge emitters for use in fiber optic systems. Based on idealized assumptions, Marcuse (70) has shown that an edge-emitting LED with an internal guiding region and the same intrinsic radiance and active layer thickness, e.g., 2.5 pm, as a surface emitter should be capable of coupling 3.5 times more power into a fiber. Botez and Ettenberg (71)and Gloge (72) state that for NA < 0.2, edge emitters have a factor of four coupling efficiency advantage over surface emitters. In addition, Botez and Ettenberg conclude that edge emitters are superior to surface-emitting LEDs for optical data rates > 20 Mb/sec and fibers with NA < 0.3. Although the collimation of the exit radiation pattern improves coupling efficiency, it creates the challenge of growing very thin, e.g., 0.05-pm, active layers. Also, there is some difficulty in packaging edge-emitting LEDs because of the need to heat sink the device effectively and yet mount it near the heat-sink edge for efficient optical fiber butt coupling to the emitting junction. The emitting junction has the advantage, however, of being located near the heat-sink surface (68). The above arguments are based on direct fiber butt coupling to the collimated beam of an edge-emitting junction in order that the fiber can
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199
16.
14-
FIG.28. Minimum direct-sourcefiber-coupling loss versus profile parameter a and NA.
extend directly up to the junction region. The coupling efficiency advantage for butt coupling into low-NA fibers is, of course, due to the narrow, e.g., 30",emission beam in the vertical junction plane. Surface emitters on the other hand are much less affected by reabsorption and interfacial recombination (65). In the case of the etched well (Burrus)
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DELON C . HANSON
emitter, the contact on the junction side of the heat sink may have a relatively high reflection coefficient.This allows photons emitted toward the contact to be reflected back and coupled into the fiber. This advantage also brings the disadvantage of handling a very thin, e.g., 15-pm, junction membrane during the final device process and dieattach to the heatsink. It is generally conceded that etched well emitters have the advantage of radiating about 3 times more optical power into air. The problem is that the 60" half-power emission angle of surface-emitting LEDs greatly exceeds the fiber acceptance angle (Om,, = 17.50' for NA = 0.3, Fig. 9). Considerable progress has been made recently in matching the half-power emission angle of surface emitters to the acceptance angle of fibers through the use of relatively high-index microlenses which are mounted directly on or near the emitting surface (73, 74). It is claimed (73) that improvements in coupling efficiency of up to 30 times over butt coupling can be achieved by this method. This exceeds the claimed advantage for edge emitters. Usually, engineers trained in electronics and communication systems do not have a good feel for the limiting performance factors of optical lenses. In summary, the best that can be achieved for incoherent sources, regardless of how complex the lens system may be, is that radiance be conserved (75) by the lens in translating from one surface to another. This means that the product of source diameter d and emission angle 8, can at best be transformed by the lens into the product of fiber-core diameter 2a and fiber acceptance angle Om,, without loss of optical power. A lens cannot increase the power coupled from a larger area incoherent source into a smaller area fiber. Thus, a microlens which theoretically could couple all the optical power from a surface emitter (8, = 60') into a telecommunications fiber (2a = 50 pm, NA = 0.21, Om, = 12.1') would require that the source diameter be less than 10 pm. For an LDC fiber having core diameter 2a = 100 pm, NA = 0.30, Om,, = 17.50", the source must have d < 29 pm in order to avoid exceeding the optical pattern transforming capability of even a perfect lens. These are not totally impractical junction dimensions, but they place a severe burden on the quality of the semiconductor junction material and contact since they must reliably sustain current densities in the range lo5 A/cm2. Speer and Hawkins (74) have taken the approach of magnifying the emitting junction to achieve a prescribed exit NA through the use of a microlens. This relaxes the lateral position sensitivity caused by dimensional tolerances in the optical connector and allows the possibility that larger fibers may be coupled to the optical port with increased coupling efficiency. However, this also results in a corresponding reduction in coupling efficiency
FIBER OPTICS IN LOCAL AREA NETWORKS
20 1
for smaller fibers. The small physical size of this optical component is a significant factor for achieving practical fiber-optic link hardware. Yamaoka and Masayuki (76)have taken an alternate approach to achieve improved coupling by forming a spherical lens on the end of the coupling fiber. While this does improve coupling efficiency, it requires more skill during fabrication and final assembly than using microspheres, and is not likely to be used extensively. 4. Transmitter Circuit and Package Design
With reference to Fig. 22, the circuit required to drive an LED effectively for LDC applications is relatively simple when compared to receiver circuit considerations which are discussed in Section II1,B. The primary functions of the transmitter circuit are the following: Providing the desired current level with adequate compensation to minimize optical power variation due to changes in temperature and supply voltage. Reducing the overall response time t of the LED by providing a current spike at the leading and trailing edges of the drive pulse and providing a small hold-on bias current to reduce the diffusion time constant. Possible incorporation of special circuitry to transform the time dependence of the input data stream so as to produce a data-independent average optical power level which will lead to zero dc component in the electrical spectrum at the receiver. At data rates under 10 MBd, discretely packaged devices may satisfy the above functions without serious parasitic problems by utilizing separate resistor and capacitor components mounted on a printed circuit board. However, if board space is critical, it is much more space efficient to integrate these functions onto a single custom integrated circuit, thereby achieving better parameter control as a bonus. For high-volume applications, cost may also favor transmitter circuit integration. In the range 10-100 MBd it is even more desirable to develop custom integrated circuits, since it beaomes important to reduce circuit parasitics and control the LED switching time. Above 100 MBd, laser sources are recommended although they bring with them the additional associated complexity of optical detection and feedback in order to control the drive current over temperature and with aging. A complete transmitter module containing a custom integrated circuit which incorporates all of the ~ b o v efeatures has been developed (47), as shown in Fig. 29. The integrated circuit functional diagram is shown in Fig.
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DELON C. HANSON
30. The current drive for the LED is supplied by three current sources: I , is normally OFF, I , is normally ON, I , is a low-level hold-on current used to reduce the LED switching time. With this arrangement, the LED can be driven in either of two modes: (1) Mode select high. The LED drive current and output optical power are direct replicas of the TTL level electrical input signal, as shown in Fig. 30. (2) Mode select low. The drive current and output optical power have the time dependence shown on the middle plot in Fig. 30. This optical power waveform has a midlevel from which equal area positive and negative pulses occur at the positive and negative input data transitions, respectively. Refresh pulses having the same pulse area are generated with the same polarity as a previous data pulse if the input waveform remains in a particular state for more than = 3 p e c . If a refresh pulse occurs simultaneously with a data pulse, the data pulse overrides (as shown in Fig. 30) so that pulse jitter does not occur. This waveform has the purpose of producing zero dc and negligible low-frequency components in the electrical spectrum at the receiver so that the receiver can be effectively ac-coupled while still functioning from zero to the maximum specified data rate (47).This results from operating on the differential pulses defined at the edges of the input pulse waveform.
I
I
I
i5cm
DIELECTRIC STANDOFF
CAPACITOR
SOURCE OR DETECTOR>,
cm
L 4.34cm _ 1 FIG.29. Hewlett-Packard fiber optic module (47).
203
FIBER OPTICS IN LOCAL AREA NETWORKS
With Mode Select
’ c
OUT MODE SELECT LOW
OUT MODE SELECT
HIGH
U
n n n
U
s
n
n
-
FIG.30. Transmitter equivalent circuit and related electrical and optical waveforms ( 5 ) .
The hybrid package shown in Fig. 29 incorporates butt coupling between the optical fiber and surface or etched-well (Burrus) emitters. The optical interface from the hybrid module to the optical connector uses a precision ferrule which is an integral part of the module. The precisely positioned fiber stub is sealed in the hybrid package feedthrough port. This terminal device is a good physical example of a particular case for the general model shown in Fig. 6. B. Optical Detector and Receiver Considerations
The optical detector in a fiber optic link serves the vital function of converting the received optical power into the electrical current for the receiver preamplifer input. As such, it has several key requirements: High responsivity R (A/W) at the optical wavelengths of interest, e.g., R > 0.5 A/W; low leakage and dark current, e.g., < 1 nA; fast response time, e.g., < 3 nsec; low-voltage operation, e.g., < 5 V ; low cost; small size; and small variation with temperature.
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DELON C . HANSON
1. Optical Detector Design
As noted in Section I,E,2, the requirement for low-voltage operation in LDC practically eliminates avalanche photodiodes (APDs) from serious consideration for all but the most stringent applications which can afford the extra physical size and cost of dc-to-dc converters and temperature stabilization circuitry. Similar practical considerations argue against using the lower fiber attenuation which is achievable in the region 1.0-1.6 pm (as shown in Fig. 8), because of the less mature technological development status and higher cost of both sources and detectors in this longer-wavelength region. This conclusion also results from the fact that the tandem insertion loss contributions of connectors, couplers and switches in distributed networks may far exceed the link-length-dependent insertion loss in most LANs. Thus, the wavelength range 800-900 nm is of primary interest for present- and next-generation LDC applications. Silicon p-i-n and p-n junction detectors provide efficient, mature, and low-cost solutions to the optical detection problem in this range. Figure 31 shows a cross-sectional drawing of p-n and p-i-n junction detectors. For the p-i-n detector, the lightly doped n region is depleted of carriers by a relatively high electric field. The electron-hole pairs formed by photorecombination are swept across the reverse-biased junction. For efficient operation, the depletion width must be sufficiently thick relative to the recombination length (l/cto)so that a large fraction of the incident optical power is absorbed and thus creates electron-hole pairs. The photodiode responsivity R (A/W) is defined by
R = ip/Po = [q(l
- r)/hv](l - edaoL)
(27)
where i, is the detected photocurrent; Po, incident optical power; q, electron charge; 1 - r, surface transmission coefficient; hv, photon energy; cia, absorption coefficient (cm- ) : L, width of depletion layer. R is a function of
'
FIG.31. Cross section of (a) p-n and (b) p-i-n photodiode detectors.
FIBER OPTICS IN LOCAL AREA NETWORKS
205
optical wavelength since the photon energy must exceed the junction energy gap in order for photorecombination to occur, and thus Q, decreases with increasing wavelength. This wavelength dependence of R is shown for a p-i-n photodetector in Fig. 32, as developed by Melchior (77). It is evident that at A0 = 850 nm, it is necessary to have L > 10 pm in order for R > 0.5 A/W. As wavelength is increased, a correspondingly greater depletion-layer width is required for a given responsivity. The speed of response of a photodiode is primarily determined by the drift velocity in the depletion region, if the photodiode is sufficiently reversebiased. When the depletion-layer electric field in silicon is above 2 x lo4 V/cm, the carrier velocity is saturated at u > 5 x lo6 cm/sec. As a result, the top scale in Fig. 32 indicates that the carrier transit time is well under 1 nsec. A significant potential advantage of p-n junction photodetectors for LDC is that they can be integrated directly with the associated amplifier and digital output circuits on a single silicon chip. In the region less than 20 MBd, where dc-coupled amplifiers can be used effectively, this represents a significant cost advantage. It may also be a significant EM1 advantage since
CARRIER TRANSIT TIME ( n s e c )
D E P L E T I O N L A Y E R W I D T H (pm) FIG.32. Absorption efficiency versus depletion layer width or carrier transit time for silicon p-i-n photodiode near the absorption band edge (771.
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DELON C. HANSON
+j&-; tis(t)
FIG.33. Photodiode equivalent circuit.
long bonding leads between the detector and preamplifier are eliminated and thus the package does not require electromagnetic shielding. There are several significant limitations, however, for p-n junction photodetectors: To be compatible with the process for the associated silicon integrated circuits, the epitaxial thickness is typically less than 7.5 pm. Since this places an upper limit on the depletion width L, Fig. 32 shows that this results in a substantial, e.g., 3-dB, reduction in optical conversion efficiency. Since the junction capacitance is inversely proportional to depletion width L, the p-n diode junction capacitance is proportionally larger than for a p-i-n detector. This ultimately limits the upper frequency of operation. The equivalent circuit of a photodetector is shown in Fig. 33. The fact that the detected current i, is represented by a high-impedance current source is significant for preamplifier circuit design. 2. Receiver Preamplijier Design Comparison Before the detected signal from the photodetector can reestablish the input data-pulse sequence, it must be amplified by many orders of magnitude and threshold detected to distinguish the signal from noise. The total noise at the preamplifer input includes the noise associated with the signal and the photodetector. Pulse distortion associated with the originating signal or the transmission medium has the potential of creating intersymbol interference and thus increasing the probability that a signal pulse will be incorrectly identified by the threshold detector. Thermal and shot-noise sources associated with the receiver can be treated through the use of Gaussian statistics (20).If this same assumption is made for other noise sources and if intersymbol interference is neglected [it is later included in Eq. (29)], then the probability of error (P,) can bt: directly related to the signal-to-noise ratio Q.For a two-level signal, P, is very closely approximated (78) by
FIBER OPTICS IN LOCAL AREA NETWORKS 10-5,
I
207
1
SIGNAL-TO-NOISE R A T I O , Q FIG.34. Probability of error versus signal-to-noise ratio for two-level digital signals (78).
Equation (28) is shown plotted in Fig. 34. Note that P, = for Q = 6 and that P, is a very rapidly decreasing function of Q,i.e., increasing Q by 6:( reduces P by an order of magnitude. This is in sharp contrast with analog transmission systems which require signal-to-noise ratios of 40 dB and thus severely strain the capability of fiber optic subsystems (19). There have been many reviews of receiver circuit designs for accomplishing these objectives, in particular by Personick et al. (19,7840).The primary emphasis of the reviews has been on digital telecommunications (DT) applications. A typical DT receiver/regenerator (19) is shown in Fig. 35. At the minimum optical input signal level, the avalanche photodiode (APD) back bias is automatically adjusted for an avalanche gain of about 50 and the following variable gain amplifier provides about 66 dB of gain in order to achieve a 1-V output signal level. At the maximum optical input signal, the APD gain is reduced to less than 10 (a 7-dB reduction) so that, along with a 33-dB automatic reduction in gain of the variable gain amplifier, a 40-dB dynamic range is achieved with a constant output signal level. a. Preamplijer design alternatives. The preamplifier design for a DT receiver, shown in Fig. 35, and for LDC receivers, described later in Section III,B is the primary factor for acheving a prescribed receiver SNR. Figure 36 shows the two primary candidate designs for fiber optic preamplifier circuits
208
DELON C . HANSON
TIMING RECOVERY
t LOW NOISE PREAMPLIFIER
t
VARIABLE GAIN AMP1 IF IER
OPTICAL SIGNAL
VARIABLE H V
FIG.35. Typical receiver/regenerator block diagram for telecommunicationsapplications (18).
which interface between the photodetector and the postamplifier. The highimpedance (HZ) approach has been utilized extensively in early D T applications, whereas the transimpedance (TZ) design has been used extensively in LDC applications. b. Preamplifier modeling. For modeling, the photodiode equivalent circuit in Fig. 33 has been simplified to a single current source i,(t) in parallel with capacitance C, . In both cases the preamplifier is characterized by the input parameters R , and C, and noise sources in@) and en@).The preamplifier
I
I lbl
FIG.36. Comparison of equivalent circuits for (a) transimpedance (TZ) and (b) highimpedance (HZ)receiver preamplifiers (f8).
FIBER OPTICS IN LOCAL AREA NETWORKS
209
gain stage is assumed to be noiseless. In the TZ case, it is represented by open-loop frequency dependence A , ( o ) and by constant A in the HZ case. The noise contribution from TZ feedback resistor R, is represented by if@), while the noise contribution from the large value, parallel bias resistor R , for the HZ preamplifier is represented by ib(t).The noise of the additional amplification stage A at the output of the TZ preamplifier is accounted for e,,(t). The transfer function Iffi&) at the output of the HZ preamp is needed to compensate for the integration characteristic of the HZ input. The output filter in the TZ case is much less critical since the much lower input impedance does not cause integration of the input signal. The required average optical input signal power Pavefor a specified SNR Q is given by where q is the electronic charge; R , photodetector responsivity (A/W) ; Tb, baud period; 2, dimensionless noise parameter (80) for the receiver. Equation (29) is a particularly useful relationship for the system designer since it explicitly contains all of the parameters needed to specify a digital receiver’s performance. The receiver’s noise performance is included in the parameter Z which was described originally by Personick (80) as Z”’. In order to compare HZ and TZ preamplifiers directly, Z can be represented in a common form:
where R , = R , for TZ; R , = R , for HZ; SI is the single-sided current spectral density (AZ/Hz);SE, single-sided voltage spectral density (V2/Hz); S,, = SE S,, for TZ, SE. = S , for HZ; R , = R , for TZ, R , = R , for HZ; R , = [ l / R , 1 / R b ] - ’ ;k, Boltzmann’s constant; T, absolute temperature; C, = C, C,; I,, I , , parameters relating the input and equalized output pulse shapes (80). For bipolar preamplifiers, SI = 2qIb,,, and S, = 2kT/g,. It is clear from Eq. (30)that to minimize the noise contribution it is necessary to minimize C, and maximize the resistance parameters. The 2 parameter is very similar for TZ and HZ amplifiers if R , = R,, respectively. However, the reason HZ amplifiers are so named is that, typically, an FET input device i s used and R, must be made very large to minimize noise. As a result, the input admittance is dominated by C, and thus signal integration results. The equalizer H,,&) plays a vital role for the HZ amplifier since it is necessary to restore the pulse shape of the integrated input signal. If a simple differentiator is used for this purpose, the low-frequency signal components
+ + +
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DELON C. HANSON
are heavily attenuated. The problem is that to minimize the noise contribution, the amplification must occur before equalization. As a result, high-level, low-frequency signal components occur ahead of the equalizer. The dynamic range of the HZ preamplifier is limited because the input voltage continues to build up on CTif the signal stays in one state for multiple baud periods. In addition, to utilize the noise advantage of the HZ preamplifier it may be necessary to individually adjust the equalization circuitry and compensate it over temperature. The TZ preamplifier has found widespread acceptance because of its wide bandwidth capability, wider dynamic range, and ease of integration on silicon. When the amplifier gain is very large, the TZ amplifier is a current to voltage converter (hence the name) as defined by the feedback resistor R , . In practice, the amplifier gain is finite and thus there is a limit to how large R, can be. As a result, from Eq. (30), there is a lower limit on the noise contribution of R , . Even with this limitation, practical system dynamic range requirements of 20-40 dB, along with reasonable freedom from input signal pulse distortion, is achieved. 3. LDC Receiver Design Alternatives
The TZ preamplifier, which is primarily used in LDC applications for the reasons sited in Section III,B,2 is incorporated into one of the three following basic receiver circuits, depending on the operating frequency range and required receiver dynamic response time. Performance trade-offs for each circuit design will be summarized in the following sections with emphasis on functional performance differences, ease of integration, and overall complexity. a. dc-Coupled receivers. The dc-coupled receiver is shown in Fig. 31. It is the simplest to realize in integrated form. The dc current I,, can be set to
-
RF
FIG.37. Generalized block diagram of dc-coupled receiver.
FIBER OPTICS IN LOCAL AREA NETWORKS
21 1
balance the nominal level of the detected signal current. Particularly when a p-n photodetector is incorporated as part of the same silicon chip, it can be packaged on a simple header without the use of any external hybrid components. Because of dc coupling and the absence of energy storage elements, it responds rapidly to changes in pulse amplitude, which makes it suitable for passive distributed data bus networks in which dynamic contention occurs between signals received from different nodes in the network. A major disadvantage of this design is the pulse distortion which occurs as a result of using a fixed-threshold detection level for all input signal levels. Since the signals have nonzero rise and fall times, this causes substantial pulse distortion and ultimately limits the maximum data rate and allowed dynamic range. Since dc levels are coupled through this circuit, the design is relatively sensitive to component mismatch. As a rule, the integrated dccoupled receiver for fiber optic applications is limited to less than 20 MBd due to these difficulties. b. ac-Coupled receivers. This design approach, shown in Fig. 38, uses series-coupling capacitors to eliminate the component mismatch problem. In addition, pulse distortion is minimized since the detection threshold is maintained in the middle of the waveform over the entire signal amplitude range, if the average signal duty factor is 50%. If a TZ preamp is utilized, a relatively wide dynamic range results. If the AGC, shown dotted in Fig. 38, is not incorporated and the coupling capacitors are small, a relatively fast response time occurs. As noted above, this circuit is limited to use with 50% duty factor signals, e.g., with Manchester encoding. As a consequence, even with large interstage coupling capacitors it does not accommodate asynchronous operation without transmitter encoding. Particularly with integrated circuit design, the requirement to use relatively large, off-chip, interstage capacitors increases
I T
r-+-k&- 1
@@+qp&-p I
- -1 -
4
RF FIG.38. Generalized block diagram of ac-coupled receiver.
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DELON C . HANSON
T RF
DATA
T
--
A FIG.39. Generalized block diagram of dc-feedback receiver.
the packaging difficulty. This integrated circuit design approach has been taken by Biard (81). c. dc Feedback receivers. A generalized block diagram of a dc feedback (DCFB) receiver is shown in Fig. 39. This is a hybrid of the first two design approaches since it is basically a dc-coupled design in which effective ac coupling is achieved through DCFB, which supresses the dc response and compensates for component drift. With this design, the difficult to package interstage coupling capacitors are replaced by two capacitors which are connected to ground for DCFB and AGC signal averaging, respectively (82). The DCFB signal is obtained from the average value of the three-level middle waveform in Fig. 30 or from a 50% duty factor, two-level signal. The AGC signal is derived from the peak value of the amplified signal. The DCFB resistor provides an additional source of noise relative to the ac-coupled receiver for a TZ preamplifier. This circuit is primarily intended for point-to-point applications in which constant-amplitude signals are received, since the acquisition time is very large due to the DCFB loop. A schematic of a fully integrated example of this receiver (82)is shown in Fig. 40. Because of the two sets of threshold detectors, this circuit recovers either the two-level or three-level signals shown in Fig. 30. In addition, a link monitor output signal is provided to indicate link continuity. This continuity signal occurs even in the absence of data for the three-level waveform in Fig. 30. The link monitor is a useful function for many applications where system status is required.
213
FIBER OPTICS IN LOCAL AREA NETWORKS BIAS
GAIN CONTROL ? PREAMP STAGE
DIFFERENTIAL AMPLIFIER STAGE
LOGIC HIGH
ALC AMP
LlNK MONITOR
MONITOR OUTPUT
FIG.40. Schematic drawings of Hewlett-Packard integrated dc feedback receiver circuit (5).
C . System Design Considerations
Having summarized the design considerations for the individual components of a fiber optic link, it is appropriate to discuss how the individual device specifications interrelate so that when they are interconnected a viable link with adequate performance margin results. From a SNR or bit error rate (BER) standpoint, the key consideration is the sensitivity specification of the receiver relative to the available received optical signal power. 1. Link-Reliability Margin
Figure 41 shows schematically the distribution of received optical power. There may be a reduction (movement toward the left) as time increases, due to LED or connector degradation. For high production yield, the receiver optical power specification limit for a given BER must be well above the actual receiver sensitivity distribution, when temperature and supply voltage margins are accounted for. As a result of these distributions and the absolute minimum-maximum specification limits which are assigned to individual devices, it is very likely that the total link will function satisfactorily even if one component is outside of its specification limits. Thus, there is some controversy about how to
214
DELON C . HANSON OPTICAL SIGNAL POWER AVAILABLE A T RECEIVER PORT __c
A~RANSMITTER TYPICAL
TYPICAL RECEIVER DISTRIBUTION RECEIVER SENSITIVITY SPECIFICATION
FIG.41. Sketch of possible reduction of receiver optical signal power with time versus receiver specification and sensitivity distribution.
establish device specification margins in order to achieve realistic system reliability projections. The reliability R of a device is the probability that it will perform its intended function for a specified period of time under specified conditions. Assuming it is in a constant failure rate region R can be expressed by R
=
exp(-r/MTBF)
(311
where t is the designated mission time, MTBF is the mean time between failures, and R is the probability of survival for time t . The failure rate F is the number of failures relative to stated conditions in a given total number of operating hours. The MTBF is related to F by the inverse relationship, MTBF
=
1/F
(32)
Generally, users are concerned with the reliability of a fiber optic component for a given mission time t. If the component has an MTBF = 100,000 hr and the mission time is 5 yr with 70% utilization, then t = 5 yr x (8760 hr/yr) x 0.7 = 30,660 hr. As a consequence, the component reliability R = exp(-30,660/100,000) = 0.74, even though the MTBF is more than three times longer than the mission time. Note that if the mission time equals the MTBF, R = 0.37. Thus, 63% of the components will have failed by the end of the mission. A data link network consists of several components in tandem such that
FIBER OPTICS IN LOCAL AREA NETWORKS
215
catastrophic failure of any one component will cause network failure, Thus, the system MTBF, is given by MTBF, =
[ ']' i =I
MTBF,
(33)
where MTBF, is the mean time between individual component catastrophic failures. Except for catastrophic failures, individual devices may drift out of their data sheet specification limits and the link will continue to function satisfactorily. With reference to Table I, the optical output power of GaAsP LEDs degrades slowly and continuously with time but tends not to instantly fail because of dark-line defects. On the other hand, GaAlAs LEDs tend to maintain nearly constant optical output power but may experience a rapid degradation mechanism. Clearly, the link optical power margins should be set differently in these two cases. As discussed in relation to the generic network topologies shown in Fig. 4, fiber optic local area networks (LANs) are configured using either point-topoint (PP) links with active repeaters or passive data bus (PDB) hardware. The PP case includes simplex and duplex links, which are configured into either fully connected, star or ring/loop networks having active nodes. The PDB case includes linear and cluster interconnections in which dynamic optical contention occurs on the network in order to gain control before transmitting a block of data. System considerations for these two cases will be summarized in the following sections. 2. Distributed Point-to-Point Local Area Networks Figures 42 and 43 show point-to-point (PP) links with active repeaters configured in simplex loop and full duplex linear configurations. In both
I'
4
HYI'ASSEI)
PORT
FIG.42. Loop data bus using point-to-point links and optical bypassing of disabled node.
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DELON C . HANSON
I.
OPTICAL BYPASS
SWITCH
FIG.43. Dual-direction linear network using point-to-point links and optical bypassing of disabled nodes.
cases, local bypass switching (discussed later) is shown to prevent an inactive repeater from disabling the network. a. Sysrem power budget. Figure 44 shows a typical example of a system optical power budget. It accounts for an operating temperature range of 0-70°C, a worst-case BER = at the highest specified data rate of 10 MBd. For this particular example, the total worst-case-system optical power budget is 18 dB. This is the difference between the transmitter output power at worst-case temperature and the required receiver input optical signal 10-9BER AT 10 Mbaud
O°C-70°C v1 v1
I
0
20
Z 0
16
a
* e Z
1
12
8
TOTAL INSERTION LOSS
4 I
E
a
0 100
300
500
700
900
1100 1300
P - C A B L E LENGTH ( m ) FIG.44. Typical point-to-point link optical power budget showing total insertion loss versus cable length and associated power margin in triangular region.
FIBER OPTICS IN LOCAL AREA NETWORKS
217
power for the specified BER and for worst-case conditions. The total link insertion loss for worst-case temperature is offset upward by the connector loss. The optical power budget margin is represented by the triangular region. In particular, at 1 km a worst-case margin of 3.5 dB exists. The typical power margin at 1 km is over 10.0 dB. For a first-generation L D C link operating at 10 MBd, the optical budget is the primary link design consideration. As the data rate is increased to 100 MBd, the link pulse distortion limits, shown in Fig. 21, must also be accounted for to ensure satisfactory system operation. If the assumption is made that from a system reliability standpoint, it is only necessary to allow for a single inactive node, the optical power budget of the network must only accommodate the attenuation of two cable lengths (one on each side of the “down” mode) and the bypass switch loss. An additional cable and switch insertion-loss contribution must be added for each additional inactive node allowed in tandem. Thus, the major advantage of P P active repeater networks is that localized, well-defined optical power budgets can be specified on the network independent of the number of tandem nodes. The received power level is relatively constant (except for the instant when an inactive repeater is bypassed), since it only originates from one source. b. Optical bypass switch design. Several design approaches have been proposed for node bypass switches. These include solid-state optoelectronic designs which require no moving parts, as well as optomechanical designs. The practicality and ultimate performance specifications of either approach remains to be proven, but several comparisons can be made. The optoelectronic switches use liquid-cystal (83), magneto-optic, or acousto-optic materials and applied voltage to shift a divergent optical beam, which is leaving one fiber, between at least two other fibers with reasonable efficiency.Since there are no moving parts, switching can occur in the submillisecond range and the components have small physical size. For the liquid-crystal switch, key concerns are the lack of adequate isolation between channels, the relatively high minimum insertion loss, restricted temperature range, and relatively high switching voltages. A variety of optomechanical bypass switches have been proposed. These include rotating spherical mirrors which reflect an optical beam between selected channels (84), moving optical-fiber pigtail switches (85), and rigid single-motion fiber switch assemblies (86). The latter approach (86),which is potentially attractive for LAN applications, is shown in Fig. 45. Using 100-pm core/140-pm-o.d. fibers, this design has achieved greater than 50-dB isolation between channels with less than 2-dB insertion loss. Switching speed is in the 10 msec range. Prototypes have been operated 25,000 cycles before failure.
218
DELON C. HANSON DATA OUT
t
, T O REPEATER
SPACER FIBER
0,
ym
I
& ’
FROM REPEATER (a)
(b)
FIG.45. Schematic drawing of prototype optical fiber bypass switch (86): (a) top view; (b) side view.
c. Loop network implementation. Casto (87)has described Harris Corporation’s decision to implement a fiber optic loop LAN using PP links. A second redundant loop was included to provide a reliability backup. The network, designed to distribute high-resolution satellite image data between 35 nodes in a meteorological center, operates at 50 Mb/sec. TDM time slots are allocated dynamically, based on traffic requirements. Twisted wire cable was eliminated due to bandwidth and ground loop problems. CATV coax provided the necessary bandwidth performance and ground loop immunity for this application but had insufficient EM1 and radiation protection. Okuda et al. (88) summarized Toshiba’s Ring Century Bus, which uses PP links for separate data and control rings. Projected data rate using TDM is 100 Mbfsec with up to 300-m separation between nodes. Hence, P P links allow the implementation of real-world distributed loop networks with performance exceeding that achievable with conventional coaxial-based network hardware. The development of practical automatic bypass switching and “soft failure” network redundancy schemes will considerably enhance the acceptance of this network technology. 3. Distributed Data Bus As discussed in relation to the generic network topologies in Section I,C, the distributed data bus in Fig. 4d is the topology usually chosen when coaxial cable is the transmission medium. The perceived advantages of this network were outlined previously in Section 1,C. They are primarily the totally passive nature of the transmission medium and the ability to easily install low-perturbation taps on the transmission line without disrupting the operating network.
FIBER OPTICS IN LOCAL AREA NETWORKS
219
For a fiber-optic-based network, the configuration in Fig. 4d is the most difficult to implement since an optical equivalent of essentially zero perturbation, bidirectional electrical taps does not exist to couple the optical data signals efficiently into and out of the main network fiber. In addition, it is necessary to sever the main network fiber (i.e., shut down the network) in order to introduce the coupler. a. Passive optical fiber couplers. Considerable research has been undertaken to develop optical fiber couplers and power splitters because of the key role these devices will play in distributed data bus networks. Coupler designs include micro-optic Selfoc lenses with mirrors (89), bifurcation of quartz rods (90),offsetting of fibers in a plastic waveguide housing (91)and biconically tapered multimode fibers (92-94). For LANs, the most practical design approach appears to be the biconically tapered, multimode coupler sketched in elementary form in Fig. 46. The same technology is used both for two fiber couplers [in which the reported directivity is > 55 dB and excess loss < 0.5 dB (92,93)]and for 4-100 fiber star couplers (94-96). Because the couplers are fabricated from the same type of fiber which is used in the network itself, minimal discontinuities occur from inserting the coupler into the network. The operation of biconically tapered couplers is based on the principle that by reducing the modal volume of the fiber core through tapering, the higher-order optical modes are forced to the outer tapered cladding surface which has an air interface. In the expanding tapered region of the fused coupler, the source fiber optical modes tend to become equally distributed among the exit optical fibers. Through scattering, these modes become trapped in the cores of the exit fibers. For most efficient and uniform performance, care must be given to achieve the proper degree of twisting and tapering and to avoid bubbles and discontinuities during the fusion process. Lightstone(94) has discussed the fact that in order to avoid the irreversible loss of optical modes from the biconical taper into air, a fiber having core
DIRECTIVITY
= -10 log
(z)
FIG.46. Schematic drawing of two-fiber biconically tapered multimode coupler.
220
DELON C. HANSON
radius a with a given NA must have its tapered-core diameter aT limited so that aT/a > NA/(n: - 1)1/2
(34)
For an F fiber-channel coupler, the coupling fraction C,, from the source fiber to an auxiliary output is limited by the area ratio (aT/a)2through the relationship
(35)
CFA < (1/F){1 - [(NA)’/(n: - I)])
The intrinsic nonuniformity of the optical power coupling to the exit end of the source fiber C,, (due to lower order modes which propagate directly through the coupler), relative to the other auxiliary fibers is given by (94) CFS
=1
- (F - 1)CFA
(36)
Thus, for NA = 0.3 and n, = 1.458, the taper must satisfy (aT/a)> 0.28. Table IX shows the limiting coupler asymmetry for various numbers of fiber channels F. It is clear from Table IX that as the number of fiber channels increases, the imbalance in optical power between the output end of the source fiber and the other auxiliary fibers becomes quite drastic, e.g., 5.4 dB for F = 32. Since this imbalance adds directly to the required receiver dynamic range, the designer is challenged to circumvent this phenomenon, e.g., through local mode mixing. b. Passivejber optic loop and linear data buses. The fiber optic implementation of the LAN shown in Fig. 4d is more readily achieved in the loop configuration sketched in Fig. 4c and shown in more detail in Fig. 47. When the network is closed into a loop, transmitter To and receiver R , are located at the same loop control node. The loop active repeater nodes, discussed in Sec. III,C,2, are thus replaced by passive, optical fiber couplers which provide efficient coupling into and out of the main network fiber, and their associated terminal devices. TABLE IX TRANSMISSIVE STARCOUPLING PARAMETERS Number of fiber channels 4
8 16 32
Auxilary fiber coupling fraction (CF1
Source fiber Coupling fraction (CFsl
0.230 0.115
0.195
1.30 2.32
0.13 0.10
3.51 5.45
0.058
0.029
0.31
Ratio 10 b g , , ( c F , / o (dB)
FIBER OPTICS IN LOCAL AREA NETWORKS
22 1
FIG.47. Passive loop data bus insertion-loss components (see text for explanation of symbols).
The worst-case optical power budget between the nodes in Fig. 47 must be examined with several cases in mind since the same terminal device specifications apply at any location. Hudson and Thiel(97) and Barnowski (98)have analyzed this network for fiber bundles. Single-fiber systems are considered here since they are more practical for LANs. Depending on the choice of parameters, the highest attenuation loss between nodes could be: u0.N- 1 u0.N
= P(N- i)RIPoT, = PNRIPoT,
u1.N- 1 ul,N
=
P(N-1 ) R I P i - r
= PNR/PIT
where transmitter and receiver power parameters Pi are identified in Fig. 47. The relationships for these optical power ratios are (for N > 3):
+ LN + Lc + ( N - 2)Lu (37) = Lc, + LcT - L, + ( N - 2)L" (38) = 2LN + 2Lc + LT + ( N - 2)Lu (39) = Lc, + LN + Lc + ( N - 2)Lu (40) where Lu = LN + 2Lc + LT, network unit attenuation per added node and a0,N-I
=
L,,
a1,N-l
UO,N a1.N
associated cable; LCT,transmitter coupling loss; LCR,receiver tap loss; L,, coupler transmission loss; Lc, connector loss; LN,fiber transmission loss. In the node coupler configuration in Fig. 47, the receiver coupler precedes the transmitter coupler in the direction of optical power transmission. Because of the relatively high isolation this configuration provides between the local transmitter and receiver (due to the opposite coupler fiber orientation into the network), it is possible to "listen while talking" without serious receiver dynamic range problems. This is a key feature of the coaxial Ethernet data bus network described in Section 1,C. A disadvantage of this configuration is that the dual coupler, redrawn in Fig. 48a, is substantially more
222
DELON C . HANSON
FIG.48. Two-fiber node coupler alternatives for passive loop data bus: (a) dual coupler; (b) single coupler.
difficult to fabricate than the coupler configuration shown in Fig. 48b. Although easy to fabricate in the latter case, the local transmitter is directly coupled to the local receiver, thus increasing the required receiver dynamic range. Electronic switching may be used to disable the local receiver when its transmitter is “talking” but this overrides the “listen while talking” feature for the network. Selection of parameters for LAN performance evaluation depends on the technology used for the passive couplers. If biconically tapered couplers of the type shown in Fig. 46 are used, the typical excess loss equals -10 log(B1) = 0.8 dB or p1 = 0.83. In order to minimize the receiver dynamic range requirement, it is desirable that all of the attenuation coefficients in Eqs. (37)-(40) be equal. In addition, it is desirable to minimize the network unit attenuation L, in order to increase the number of allowed tandem nodes. Assuming that aO,Ncan be adjusted (as a special case if necessary), this implies that in designing network coupler parameters the boundary condition should be imposed that LcR
+ L, + Lc = LcR +
LCT
- LT
(41)
If the network is designed to accommodate 300 m of lO-dB/km cable between nodes, then L, = 3.0 dB. It is reasonable to assign a second-generation connector loss Lc = 1.0 dB. Thus, Eq. (41) reduces to Bi(1
-
Copt)= 2.581COpt
(42)
so that Copt= 0.29
or
- 10 logl0(C,,,) = 5.4 dB
In summary, the network parameters are:
(43)
223
FIBER OPTICS IN LOCAL AREA NETWORKS
Thus, for N > 3, Eqs. (37)-(40) reduce to
+ ( N - 2)7.3] dB M O ~ N= [10.3 + (N - 2)7.3] dB
a O , N - l= a l , N - l = a I , N= [10.2
(44) (45)
It is interesting to observe that at each node there is nearly an equal balance between the connector and coupler transmission loss, i.e., LT = 2Lc. Adding an individual node introduces 2Lc + LT = 4.3-dB loss plus the associated cable attenuation. With this parameter optimization, it is noted from Eqs. (44) and (45) that the worst-case insertion loss between all nodes is essentially equal. In addition to characterizing the maximum insertion loss between nodes, it is also necessary to establish the minimum insertion loss in order to determine the required receiver dynamic range. With reference to Fig. 47, the minimum insertion loss occurs at the end nodes when there is zero cable attenuation LN and is given by a,
=
ag, 1
= aN- 1,N = Lc
+ L,,
= Lc
+ Lc-
(46)
For the parameters selected above, i.e., Lc = l.OdB,
L,,
=
LCT= 6.2dB,
then amin= 7.2dB.
c. Transmissive star data bus. Figure 49 shows a schematic drawing of a transmissive star data bus configured with F = 3 fiber channels. The parameter designation is the same as in Fig. 47. From a functional standpoint, the network can be configured with dual channel cables between the individual T/R modules and the star coupler at the central node. Because all links are effectively in parallel, the optical power budget is very simple, i.e.,
aij = PjR/PIT= 4Lc
+ 2LN + bS,
FIG.49. Passive transmissive star data bus for F = 3 fiber channels with L, L, = 3.0 dB, and L, = 1.0 dB; hsis coupler transmission loss.
(47)
=
1.0 dB,
224
DELON C. HANSON
where L , = 10 log,,(F) + LE,F is the number of optical star channels, and L, is the excess loss per channel. Assuming the same parameter values as previously, i.e.,
L, = 1.0 dB,
L, = 3.0 dB,
LE = 1.0dB,
Equation (47) is plotted in Fig. 50 versus the number of nodes N (or star channels F). Also plotted is the maximum and minimum insertion loss between nodes for the loop data bus. It is useful to note that a 32-channel transmissive star data bus having a 300-m radial arm length has no more node-to-node insertion loss than a loop data bus with 4 hops of 300-m length between nodes. Also, a loop bus with 8 hops requires over 50dB of worst-case optical power budget. This is a major challenge for terminal device design and severely restricts the general implementation of the passive loop fiber optic data bus networks. lo(1
m
m
80
0
-1
z
LOOP OR LINEAR DATA BUS
0ILL Y
-
in
z
60
Y
n
0
z
I
0
7 40 Y
a 0
z Y v1
d
u c
20
in
LL
0
3 0 8
I I
I
16
24
I
NUMBER OF N O D E S , N
FIG.50. Worst-case node-to-node insertion loss for loop and transmissive star passive data buses versus number of nodes. Star coupler parameters: Lc = 1.0 dB, L, = 3.0 dB, and L, = 1.0 dB. Loop data bus parameters: Lc = 1.0 dB, L, = 3.0 dB, LT = 2.3 dB, L,, = L C T = 6.2 dB.
FIBER OPTICS IN LOCAL AREA NETWORKS
225
ACKNOWLEDGMENT The author would like to acknowledge his interaction with many individuals in the HewlettPackard Company who have contributed to his understanding of design trade-offs for fiber optics in local-area network applications. A partial list includes : Tom Hornak, Bob Burmeister, George Kaposhilin, Bill Brown, Eric Hanson, and Ron Hiskes of the Hewlett-Packard Corporate Research Laboratory, who have been instrumental in exploring technology and design alternatives; Roland Haitz, Lee Rhodes, Joe Tajnai, Bob Weissman, and Steve Garvey of the Optoelectronic Division, who have contributed extensively to discussions and projects directed toward the development of commercial products for fiber optic data link and network applications. He would also like to express appreciation for the diligent efforts of Terry Lincoln in typing the manuscript and Betty Downs in drawing the figures.
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68. Kressel, H., el al. (1980). Laser diodes and LEDs for-fiber optical communication, Top. Appl. Phys. 39, p. 43. 69. Marcuse, D., Gloge, D., and Marcatili, E. A. J. (1979). Guiding properties of fibers, in “Optical Fiber Telecommunications” (Miller, S. E., and Chynoweth, A. G., eds.), p. 68. Academic Press, New York. 70. Marcus, D. (1977). LED fundamentals: comparison of front- and edge-emitting diodes, IEEE J. Quantum Electron. QE-13, p. 819. 71. Botez, D., and Ettenberg, M. (1979). Comparison of surface and edge-emitting LEDs for use in fiber-optical communications, IEEE Trans. Electron Devices ED-26, p. 1230. 72, Gloge, D. (1977). LED design for fiber system, Electron. Lett. 13, No. 4, 399. 73. Carter, A. C., Goodfellow, R. C., and Davis, R. (1980). 1.3-1.6 pm GaInAsP LEDs and their application to long haul, high data rate fiber optic systems, Digest of Int. ConJ on Communications, Seattle, Washington, p. 28. I . 74. Speer, R., and Hawkins, B. M. (1980). Planar DH GaAlAs LED packaged for fiber optics, Proc. of Elect. Comp. Con$, Sun Francisco. 75. Hudson, M. C. (1974). Calculation of the maximum optical coupling efficiency into multimode optical waveguides, Appl. Opt. 13, No. 5, 1029. 76. Yamaoka, T., and Masayuki, A. (1978). GaAlAs LEDs for fiber optical communications systems, Fujitsu Sci. Tech. J. 14, No. 1, p. 133. 77. Melchior, H. (1973). Semiconductor detectors for optical communications, Comp. Laser Eng. Abstract, IEEE J. Quantum Elecrron. QE-9, p. 659. 78. Smith, R. G., and Personick, S.D. (1980). Receiver design for optical fiber communication systems, Top. Appl. Phys. 89, p. 135. 79. Personick, S . D. (1977). Receiver design for optical fiber systems, Proc. IEEE 65, No. 12, 1670. 80. Personick, S . D. (1973). Receiver design for digital fiber optic communications systems, Bell Syst. Tech. J. 52, No. 6, 843. 81. Elmer, B. R., and Biard, J. R. (1978). “Fiber Optics Receiver Integrated Circuit Development,” AFAL-TR-78-185, Final Report, p. 5. 82. Brown, W. W., et al. (1978). System and circuit considerations for integrated industrial fiber optic data links, lEEE Trans. Commun. COM-26, No. 7,976. 83. Soref, R. A. (Aug., 1978). Multimode switching components for multi-terminal links, Proc. Soc. Photo-Opt. Instrum. Eng., 150, Laser and Fiber Opt. Commun. San Diego, p. 142. 84. Spillman, Jr., W. B. (1979). Mechanical one-to-many fiber optic switch, Appl. Opt. 18, No. 12,2068. 85. Yamamoto, H., and Ogiwara, H. (1978). Moving optical fiber switch experiment, Appl. Opt. 17, No. 22, 3675. 86. Rawson, E. G . , and Bailey, M. D. (1980). A fiber optical relay for bypassing computer network repeaters, Opt. Eng. 19, No. 4, 628. 87. Casto, T. L. (1980). Dynamic bandwidth allocation of a fiber optic data bus, Proc. of Electro Program, p. 2812. 88. Okuda, N., Kunikyo, T., and Kaji, T. (1978). Ring century bus-An experimental high speed channel for computer communications, Proc. of 4th ICCC, Kyoto, p. 161. 89. Kobayashi, K. et al. (1977). Micro-optics devices for branching, coupling, multiplexing and demultiplexing, Proc. ConJ on Int. Optics, Opt. Comm., Tokyo, p.367. 90. Milton, A. F., and Lee, A. B. (1976). Optical access couplers and a comparison of multiterminal fiber communication systems, Appl. Opt. 15, p. 244. 91. Auracher, F., and Witte, H. H. (1977). New planar optical coupler for a data bus system with multimode fibers, Appl. Opt. 15, p. 2195.
FIBER OPTICS IN LOCAL AREA NETWORKS
229
92. Ozeki, T., and Kawasaki, B. S. (1976). Optical directional coupler using tapered sections in multimode fibers, Appl. Phys. Leii. 28, p. 528. 93. Kawasaki, B. S., and Hill, K. 0. (1977). Low-loss access couplers for multimode optical fiber distribution networks, Appl. Opt. 16, p. 1794. 94. Lightstone, A. W.(1980). Characteristics of multiport biconical duplex and star couplers for military and commercial applications, Proc. of Fiber Opt. and Commun. (FOO-Sun Francisco, p. 261. 95. Rawson, E. G., and Nafarrate, A. B. (1978). Star couplers using fused biconically tapered multimode fibers Electron. Lett. 14, p. 274. 96. Rawson, E. G. (1979). Optical fibers for local computer networks, Digest of Opt. Fiber Corn. Topical M f g . , Washington, D.C., p. 60. 97. Hudson, M. C., and Thiel, F. L. (1974). The star coupler: A unique interconnection component for multimode optical waveguide communications systems. Appl. Opt. 13, p. 2540. 98. Barnowski, M. K. (1975). Data distribution using fiber optics, Appl. Opi. 14, No. 1 I , 2571.
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ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS, VOL. 51
Surface Analysis Using Charged-Particle Beams P. BRAUN, F. RUDENAUER,*
AND
F. P. VIEHBOCK
Institut f u r Allgemeine Physik Technische Universitat Wien Vienna, Austria
I. introduction . . . . . . . . . . . . . . . . . . . . . . , . . .
.. . . . , , .. ... .. , .. .. ... . , .
11. Classification of Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . , . ,
A. Analysis of Electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Analysis of Ions.. C. Short Comparison of Selecte 111. Quantitative Elemental Analysis A. Quantitation of AES . . . . . , . . . . , . , . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . , , B. Quantitation of SIMS C. Quantitation ofISS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Quantitation of RBS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1V. Depth Profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Nondestructive Methods . . . . . . . . . . . . . . . . . . . . . . . , . . . . . , . . . . . . . . . . . . . . . . B. Destructive Methods. . . . . . . . . . . . . . . . . . . . . V. Elemental Mapping.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Auger Mapping . . . ... B. SIMS Mapping ...................................................... C. Elemental Mapping Using Other Ion Method VI. Three-Dimensional Isometric Elemental Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Terminology Used in Multidimensional Digital Imaging . . . . . . . . . . . . . . . . . . . B. Practical Examples of Three-Dimensional SIMS Analysis . . . . . . . . . . . . . . . . . VII. Sensitivity and Resolution Limits . . . . . . . . . . . . . . . . . . . . A. Sensitivity and Resolution in AES . . . . . . . . . . . . . . . . B. Resolution and Sensitivity Limits in SIMS . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . ...
23 I 233 233 237 24 1 242 242 245 255 258 259 259 26 1 275 275 219 290 292 29 3 296 298 298 300 306
I. INTRODUCTION Surface analysis, i.e., the investigation of the compositional, structural, and electronic properties of the solid-vacuum interface is a relatively new field of investigation. Neverthekss, many of the surface analytical techniques have found wide application in such diverging areas as microelectronics, corrosion research, biology, geology, and cosmology. Also, the relative merits and capabilities of these techniques have been discussed in many * Also affiliated with 6sterreichiscbes Forschungszentrum Seibersdorf 1082-Wien, Lenaugasse 10, Austria. 23 I Copyright 01981 by Academic Press, Inc. All rights of reproduction in any form rewrved. ISBN 0-12-014657-6
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P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
excellent review papers so that, at first glance, a further review appears to be somewhat inappropriate. We have therefore intentionally limited the scope of this article to a selection of techniques and to a few important subjects which either have received increased attention or which have been newly developed during the last few years. As far as the selection of techniques is concerned, we will describe only surface analytical methods in which charged-particle beams are used for signal stimulation (primary beams) and also constitute the informationcarrying signal radiation itself (secondary beam) ; AES, ISS, RBS, SIMS, among others, belong to this category. One of the most important capabilities of a surface analytical technique is quantitative elemental analysis, i.e., the determination of fractional atomic concentrations of the elements present in the sample surface ; generally, the element-specific signal supplied directly by the detection arrangement is not proportional to the element concentration. Mathematical algorithms have to be developed to transform relative signal intensities to elemental concentration values. A survey of these algorithms is presented. However, it will become clear that practical quantitation is still performed on an empirical basis (using experimentally determined elemental sensitivity factors) ; quantitation algorithms based on physical models of the particular signal stimulation and emission process, in almost any case, only allow analysis with considerably reduced accuracy. The reduced accuracy, however, is somewhat balanced by the less stringent requirements on a priori knowledge on the analytical sample. A second subject which is treated in some depth in this article is elemental imaging; i.e., the determination of the spatial distribution of elements in a solid sample. Conventional imaging techniques which have been extensively used in the past have the disadvantage that they primarily determine the two-dimensional distribution of an element-specific signal strength rather than the distribution of true elemental concentrations across the solid surface. With the increased use of digital computers controlling both, data acquisition and data evaluation in surface analytical instruments, imageprocessing techniques, applying elemental quantitation routines in each pixel of the uncorrected image are receiving increased attention. In a recent development, the techniques of computerized image analysis have been extended to perform complete three-dimensional elemental characterization of the surface near volume of a solid sample. This review therefore begins in Section I1 with a description of the fundamentals of selected particle-beam techniques (divided into electron methods and ion methods), continues with a survey of quantitation methods (Section III), and finally (Section IV-VI) deals with different analytical modes (point analysis, depth profiling, and image and volume analysis). In the concluding
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
233
section (Section VII), the analytical limitations of the two most frequently applied techniques, AES and SIMS, are discussed. It appears, that surface analytical techniques eventually will be able to perform three-dimensional quantitative analysis of solids with spatial resolution of 10 nm (and much lower than that in the depth-profiling mode) for the major elements of a sample.
11. CLASSIFICATION OF METHODS Analytical methods using charged particles are based on electron and ion interactions with surfaces. A . Analysis of Electrons
In Fig. 1 an overview of the most important methods discussed in this section is given. Methods using electrons as probe and secondaries are Auger electron spectrometry (Harris, 1968; Chang, 1971; Taylor, 1971; Palmberg, 1970; Weber, 1970; Connell and Gupta, 1971), electron energy-loss spectrometry (Raether, 1965; Boersch et al., 1962), threshold spectrometries, ionization loss spectrometry (Gerlach et al., 1970; Gerlach, 1971), and ion neutralization spectrometry (Hagstrum, 19721. 1. Auger Electron Spectrometry ( A E S )
The technique is based on the Auger effect whereby an incoming electron with sufficient energy ionizes an atom by removing a core electron. The relaxation to the ground state follows several possible paths. One possibility is the emission of a photon with characteristic energy, but here the interesting case is the emission of a kinetic electron, the Auger electron. The difficulty in this technique is to measure the small number of Auger electrons in the secondary electron distribution. Energy analysis of these electrons yields elemental identification of surface atoms with an information depth depending on the Auger electron energy. The primary electrons will penetrate the material analyzed to a significant depth. However, Auger electrons generated deeper in the substrate will also undergo ineleastic collisions and contribute to the secondary electron distribution (Feibelman, 1973; Brundle, 1974). Only Auger electrons from atoms at the very surface will not change their kinetic energy and are detected in the spectrum. The detection depth will depend slightly on the material analyzed, but will in general follow the square root of kinetic energy for energies > 80 eV. In practical application, the information depth is roughly 0.5-2 nm. The contribution of the various layers decreases exponentially and hence the signal depends mostly on the
234
P. BRAUN, F. RUDENAUER,AND F. P. VIEHBOCK
A*
(C)
(d 1
FIG.1. Overview ofelectron methods discussed in Section II,A. (a) AES; (b) ELS; (c) ILS; (d) INS.
surface layer; AES thus became a very popular technique for surface analysis. Using electronic differentiation, about 0.1% of a monolayer can be detected. A second reason for the popularity of AES is that it is a simple way of obtaining atomic and chemical information from the energy spectrum. On the other hand, quantitative analysis needs improved operation of the system by careful calibration as well as sufficient knowledge about the influencing parameters like current density, ionization cross section, Auger transition probability, analyzer transmission, contribution of backscattered electrons, and surface topography. High-energy resolution is necessary if chemical information and type of bonding of surface atoms is demanded. Surface atoms involved in a chemical bonding change their electron binding energy corresponding to the charge transfer caused by the electronegativity of the
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
235
atoms. For a certain Auger transition, the kinetic electron energy is determined by the sum of the shifts of the energy levels involved. The strongest change both in energetic position and peak shape will take place if the valence band is involved in the Auger transition (Fahlman et al., 1966).
2. Electron Energy-Loss Spectrometrv ( E L 8 In ELS the inelastic electrons are collected either in the vicinity of a diffracted beam or by a wide angular aperture. With the use of ELS, surface electronic transitions and surface vibrations of clean and gas-covered surfaces have been investigated. ELS provides information about the spectrum of empty electron states near the surface in combination with valence- and corelevel spectroscopies. Localized vibrations of adsorbates contain collective and structural information about the degree of dissociation, binding energies, and lateral interactions. These studies may therefore contribute significantly to catalytic researeh. The information depth of ELS is not simply determined by the electron mean free path but by the wavelength of the charge density fluctuation near the surface. Sensitivities of lo-’ and l o p 2 monolayers have been reported for electronic and vibrational excitations, respectively. The electrons in the conduction band of a metal closely approximate a degenerate gas or plasma of free electrons in a positive potential. Excitations of the plasma can arise in electron density oscillations of discrete energy values, which depend on the free-electron density and dielectric properties of the metal (Ferrel, 1956; Pines, 1963). The experiment can be carried out in an electron spectrometer if sufficient energy resolution is provided. The information and results obtained by this method are closely related to those of optical methods, but ELS is in many cases an simpler method and gives more direct information. Another advantage of this method arises from the fact that theories are more easily applicable. ELS has been demonstrated not only for vibrational spectra of adsorbed gases, but also for loss spectra arising from electronic transitions (Froitzheim, 1977). For single electronic transitions and collective electronic transitions (plasmon excitation), a resolution of about 0.5 eV is sufficient, whereas for vibrational spectra of adsorbates a resolution of a few milli-electron volts is necessary. 3. Threshold Spectrometries In threshold spectrometries the excitation or ionization of an atom in the inner shells corresponds to an interband transition between a core level and the unoccupied states of the valence band. Excitation can only occur if the excitation energy is sufficient to raise an electron from deep-lying levels into
236
P. BRAUN, F. RUDENAUER,AND F. P. VIEHBOCK
the unoccupied states above E F ;then the number of electrons per energy interval, given by N(E), is the density of states. As in free atoms, excitation is not necessarily accompanied by the loss of an electron. The loss of an electron will occur only if it has enough energy to pass the work function barrier. a. Observation of secondaryparticles. Secondary particles emitted during deexcitation, e.g., Auger electrons or characteristic X rays, leads to Auger electron appearance potential spectroscopy (AEAPS) and soft X-ray appearance potential spectroscopy (SXAPS). sec, accomA core-hole state decays after a time of the order of panied by the emission of Auger electrons or X rays. In general, the decay is not a single-electron transition, but a transition cascade including CosterKronig transitions, creating various kinetic secondary electrons. A series of secondaries is emitted and leaves the atom multiple-charged (Plesonton and Snell, 1957). The measuring principle is simple: by detecting the target current, which is the difference between the primary and secondary currents, AEAPS results as the most sensitive threshold spectrometry (Kirschner and Staib, 1975). b. Analysis of the exciting beam. The observation of the exciting beam and its variation near threshold leads to disappearance potential spectrometry (DAPS). The results of the former group depend on the deexcitation mechanism, whereas the latter does not by analogy to the FranckHertz experiment. When the primary energy is sufficient to produce a new excitation, those electrons which created a core hole lose energy and disappear from the measured reflected beam (Kirschner and Staib, 1973). The reflection coefficient decreases at excitation thresholds and is a measure of the excitation probability. All secondary decay processes are neglected by this method, because the excitation is observed directly. Electronic differentiation improves the rather poor signal-to-noise ratio. The experimental setup is simple: the energy of the electron gun is varied over the energy range of interest and the pass energy of the analyzer is varied simultaneously to measure the quasi-elastic part of the electron energy spectrum (Kirschner, 19771. 4. Ionization Loss Spectrometry (ILS)
Ionization loss spectrometry is related to threshold spectrometries insofar as characteristic losses are observed, but in ILS the energy spectrum is analyzed. The applicability of ILS to elemental analysis is evident. The core-level excitation process is the same as in threshold spectrometries; a sensitivity to chemical effects can be expected by modification of the density of states or by shifts of the core levels. ILS resembles DAPS in that it samples the excitation itself, independent of the following decay processes. Again,
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
237
Auger- or X-ray yields are unimportant in this method, but the mean free path for primary and secondary electrons depends on their different energy compared with DAPS. The main advantage of ILS for elemental analysis lies in the simplicity of the spectra. As in threshold spectrometries, in ILS the number of lines is relatively small, the lines are sharp, and overlap is rare (Kirschner, 1977).
5 . Zon Neutralization Spectrometry (INS) Secondary electrons for surface analysis can also be generated by bombarding ions. INS is a method that uses ion-electron interaction whereby slow primary ions at the surface are neutralized by a resonance process or an Auger transition. The potential of this method is, of course, not elemental analysis but investigation of adsorption and desorption phenomena on solid surfaces. The results are comparable with UV-photoelectron spectroscopy studies, especially if single-electron processes are involved (Hagstrum, 1972).
B. Analysis of Ions I . Zon Scattering Spectrometry (ZSS) Elastic binary collisions of low-energy (< 10 keV) primary ions with target atoms provide information on the elemental composition of the sample surface via the energy spectrum of backscattered primary ions. Figure 2 PRIMARY ION
SCATTERED ION
SAMPLE
000000 000000 FIG.2. Experimental geometry of ISS (schematic); formula for energy of scattered ions for scattering angle of 90" is shown. [From Honig and Harrington (1973).]
238
P. BRAUN, F. RUDENAUER,AND F. P. VIEHBOCK
shows a schematic of the scattering geometry. An incoming ions of mass number M1 and energy EO is elastically scattered by a surface atom mass number of M , through a laboratory scattering angle 8. The scattered primary ion retains an energy E l , given by (Smith, 1967), E1/Eo = ((1
+ M2)-1/M1)Z[cos8 + (M2/M1)' sin2 01
(1)
Energy analysis of primary ions backscattered through an angle 8 can therefore, in principle at least, provide unambiguous information on the mass number M , of elements present at the sample surface. In many practical ISS arrangements (Goff, 1973), a scattering angle 0 = 90" is chosen; the range of detectable elements in these cases is limited to those with M , > M , . The backscattered ion current Is=+can be written as (Honig and Harrington, 1973; Baun, 1978),
where Zp is the primary current, k is a constant G ( E o ) ,a factor taking into account the geometric arrangement of atoms at the sample surface [masking effect (Werner, 1977a)], c the differential scattering cross section, Pn(vl)is the velocity-dependent neutralization probability for the emerging scattered particle, and N o is the number of target atoms with mass number M , per square centimeter of surface. Scattering cross-section data have been calculated on the basis of a screened Coulomb potential and, for a given primary ion, typically vary within one order of magnitude through the period system, the lighter elements being the less sensitive ones. The factor 1- Pn(ul)depends on the primary ion/target combination and has been measured for a limited number of systems only (Verbeek and Eckstein, 1980). Quantitative determination of elemental concentrations by means of ISS does not yet have a firm theoretical basis; practical applications, therefore, frequently resort to an empirical sensitivity factor approach (see Section 111,C). The advantages of ISS are its surface sensitivity (the origin at the ions scattered into the elastic peak is limited to the topmost monolayer of the sample), and its capability of practically nondestructive analysis, particularly when light primary ions (H', He+) are used. With suitable instrumental additions the method can be applied for depth profiling and low-resolution lateral imaging (Minnesota, Mining and Manufacturing Co., 1979). 2. Rutherford Backscattering Spectroscopy (RBS) Similar to ISS, in RBS the elemental composition of a sample is also deduced from the energy losses of an incoming primary ion beam in a single
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
239
collision with a target atom. In RBS, however, light ions ( H + , He+) are used almost exclusively, together with much higher incident ion energies (several 100 keV to several MeV). As a consequence, not only the topmost atomic layer but also subsurface layers are accessible to analysis. Figure 3 schematically shows the scattering geometry used in RBS. A primary ion with energy E l is incident at the target surface; due to its high energy it penetrates into the solid to a depth x where it suffers a single wideangle collision with a target atom. The scattered ion is deflected back to the surface where it emerges with an energy E , . The incoming and outgoing ion-path sections are straight lines; here the ion loses energy in collisions with electrons and plasmons but is not deflected. The differential electronic energy loss dE per unit path length s can be written in the form of a power law,
d E / d s = -AYE'
(3)
where E is the ion energy and v an exponent depending on the energy range ( - 1 < v < 1/2) (Behrisch and Scherzer, 1973); A , is a constant, depending on v. When, therefore, the ion has penetrated into the solid to a depth x below the surface (see Fig. ll), it has slowed down to an energy E; (Behrisch and Scherzer, 1973),
E;
=
[Ei-' -(1
- v)A,(x/cosM)]~'('-')
(4)
The energy loss which this ion now suffers in a collision with a target atom
\ FIG.3. Experimental geometry of RBS (schematic); for further explanation see text., [From Behrisch and Scherzer (1973).]
240
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
of mass number M 2 in a depth x is characterized by the “nuclear energy-loss factor” k ,
M, cose M, (5) M , M, M , M2 where E ; is the ion energy immediately after the collision. The outgoing ion again suffers electronic energy loss [analogous to Eq. (4)]so that the final ion energy E , can be calculated from Eqs. (4) and ( 5 ) as
+
+
+
Obviously, measurement of the scattered-ion energy yields information on the mass number M , of the target atom (via k ) as well as on the depth x where the collision has taken place. RBS can therefore provide compositional as well as depth-profiling information in a practically nondestructive analysis mode. Note, however, that a simultaneous determination of these two parameters is not always unambiguous since a light surface element may appear at the same energy E , as a heavy element in a greater depth x. Figure 4 shows a typical application of RBS for the analysis of a multilayer thin-film sample. The thickness of the individual layers can be calculated from the width of the elemental scattering peaks. Note that the stoichiometry of the SiO, film can be obtained from the step in the Si continuum.
-
, a]4
ENERGY OF BACKSCATTEREO 4He (keV) 5yO 500 I 1000 lop0 1500 2000 II 15pO I I I T I I NI on
69000-
rz
=I
0
40,000-
2 ; , 0 0
-
Surfoce
*ev
u
AU
LT--l
EXPANDED SCALE FIG. 44. Examples for depth profiling with RBS; thickness of Au layer ca. 600 A. [From Ziegler (1975).]
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
24 1
Quantitative determination of the composition of the oxide film can be performed by integration of the peak areas for Si and 0 (see Section 111,D).
3. Secondary Ion Mass Spectrometry (SIMS) Bombardment of a solid with an energetic ion beam (1-20 keV) gives rise to the development of collision cascades between knock-on target particles (Sigmund, 1969), transfering energy and momentum from the bombarding ion to target atoms. Some “branches” of the collision cascade return to the surface where they may cause ejection of surface particles when the transferred energy exceeds the surface binding energy of the solid. A generally small fraction of the ejected atomic and molecular particles are (positively or negatively) ionized ; mass analysis of this ionized fraction yields information on the elemental and (with limitations) molecular composition of the surface. The range of the primary ion in a solid is of the order of 10 nm, the developing collision cascade having approximately the same dimensions ; a secondary ion may therefore be emitted in lateral distances of the order of 5 nm from the impact point of the primary ion. By far the largest contribution to the secondary ion-emission originates from the momentary topmost atomic layer of the solid, the contribution of deeper layers being not quite certain (“information depth” generally is less than three atomic monolayers). Continued primary beam bombardment causes erosion of the sample and the macroscopic surface recedes; continued analysis of this receding surface is the basis of the depth-profiling capability of SIMS. The lateral distribution of elements may be determined using one of three different experimental techniques (see Section V,B, l), and a combination of lateral “imaging” with continued surface erosion permits three-dimensional isometric compositional analysis of a solid (see Section V1,B). Further advantages of SIMS are the extreme sensitivity for trace element detection (ppb to ppm); the capability of analyzing all elements in the periodic system including hydrogen (down to the 10-ppma level) and the ability of in situ determination of isotopic abundances. The quantitation of SIMS, i.e., the determination of relative or absolute elemental concentrations from the SIMS spectrum has not yet been put on a satisfactory theoretical basis; phenomenological methods have, however, been developed, range permitting semiquantitative analysis with accuracy in the 10 rel. (see Section 111,B). C . Short Comparison of Selected Particle-Beam Methods
Table I gives order of magnitude information on the essential analytical features of the surface analytical particle-beam methods described in the
242
P. BRAUN, F. RUDENAUER, AND F. P. VIEHROCK TABLE 1 GENERAL ANALYTICAL FEATURES OF PARTICLE BEAMTECHNIQUES Technique ~
a
Feature
AES
ISS
RBS
SIMS
Elemental range Elemental resolution Yield variation Limit of detection (atomic fraction) Lateral resolution Depth resolution (atomic layers) Atomic location Chemical information
2 Li AZ= I 10
2 Li
2 Li
>ti
10
100 10- 4- 10-
10-3-10-~
0.1-0.5 pm 2-20
No Some
10- 3- 10- *
'
1000
5 Pm 8-80
< I pm
1-2 Yes No
Yes No
No Some
100 pm
1-3
Poor for high-Z isotopes.
previous sections (Morgan and Werner, 1978a). The values listed in the table are those actually obtained in existing instruments; for some of the parameters, particularly for the lateral resolution capability, the intrinsic limits of the respective methods have not yet been experimentally obtained. 111. QUANTITATIVE ELEMENTAL ANALYSIS
In this section a description of quantitation methods is given, insofar as they exist for elemental analysis in electron and ion methods. A . Quantitation of AES
Quantitation means the determination of elemental concentration in solid surfaces by evaluating the rough data. One has to distinguish between two methods : first, determination of the elemental concentration with no specimen standards, directly from the measured signal, and second, comparison with standard measurements (Joshi et al., 1975; Palmberg, 1973 ; Powell, 1978; Carlson, 1975; Bauer, 1975; Sevier, 1972). 1. Basic Mechanism
For quantitative analysis a relation between the Auger current from a certain element and the density of that element in the surface is a necessity. The source volume in which Auger electrons are generated by the primary
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
243
electron beam depends on beam diameter d and energy E , related to escape depth. The emitted Auger current by a WXY Auger transition in element i is given by (Palmberg, 1973),
li(wxy)=
J J’”J: R
Ew
lp(E, z)ai(E,Ew)ni(z)yi(WXY)
x exp( - z/A) dQ d E dz
(7)
where lp(E,z ) is the total exciting electron current including the contribution of backscattered electrons; ai(E,E,) is the ionization cross section ofthe core level W ;ni(z)is the atomic density of element i at a depth z from the surface; exp( - z / A ) is the Auger electron probability for escape; and yi(WXY)is the probability for a WXY Auger transition. Two-dimensional homogeneity of the elemental composition normal to the z direction has been assumed. Three-dimensional homogeneity of the elemental composition simplifies Eq. (7) (Joshi et a!., 1975; Palmberg, 1973): li(WXY)= lpToi(E,,E,)niy,(WXY)A(l
+ r)
(8)
where T is the transmission of the analyzer and r is the contribution of backscattered electrons. To approach quantitative analysis from first principles, the ionization cross section, the Auger transition probabilities, and the backscattering factor must be known precisely. Furthermore, surface topography will influence the Auger current as well. In general, these factors are not known precisely enough for analytical purposes, thus use of first principles is not practical at the present time. The second approach by standard measurements will offer more reliable methods for quantitative Auger analysis. 2. Comparison with Standard Measurements The measured Auger signal of a specimen with unknown composition is compared with the Auger signal of a standard with known elemental concentration. The density of element i in the specimen ni can be related to that in the standard ni by using Eq. (8): assuming specimen and standard are excited by the same primary electron current. If the composition of specimen and standard are almost the same, the influence of the informatipn depth and the backscattering factor will also be almost the same and Eq. (9) reduces to:
244
P.
BRAUN, F. RUDENAUER, AND F. P.
VIEHBOCK
A remaining effect is the influence of surface topography ;specimen and standard should thus have equal surface roughness. In this method the initial ionization cross section and Auger transition probability are not required and the determination of the Auger currents is reduced to a relative measurement. In Fig. 5 the backscattering factor r is plotted versus reduced energy E , / E , for several elements (Smith and Gallon, 1974).The values of I increase with increasing atomic number, varying from about 1.1 for carbon to about 1.8 for gold at E J E , = 3. From this, operation at the ionization threshold minimizes the backscattering correction, but the ionization cross section decreases very fast too, at E , / E , near 1, and minimizes the Auger current.
3. U s e of Elemental Sensitivity Factors
A very useful method for quantitative analysis is the application of elemental sensitivity factors, If all Auger signals I i of the elements representing the specimen are known, the active concentration of element A can be given by (Joshi et al., 1975; Davis et al., 1976):
where K i is the relative sensitivity factor of element i . The important advantage of this method is the elimination of standard measurements. Neglected are the matrix dependence of the sensitivity factor and influence of surface topography. Despite the uncertainties of this method and the use of relative sensitivity factors of pure elemental standards, quantitative analysis can be carried out with an accuracy of 10-30%.
2.0
L
1.8 r
1.6
-
1.4 1.2 1.0
0
1
2
3
I
I
I
I
I
I
L
4
5
6
7
8
9
10
UX
FIG.5. Backscattering correction factor r for several elements as functions of U, = E,,/E,. [From Smith and Gallon (1974).]
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
245
B. Quantitation of S l M S
It is the aim of quantitative elemental SIMS analysis to infer relative elemental concentration values of the elements contained in the sample from secondary ion peak intensities. A number of phenomenological and theoretical models have been proposed for that purpose and have been reviewed in the literature (Riidenauer, 1977; Blaise and Nourtier, 1979). Here we want to confine ourselves to the description of the most frequently used algorithms for which sufficient experimental data have been collected to allow an assessment of analytical accuracy. 1. Theoretical Basis for Empirical Models
It is an experimentally observed fact that the total emitted secondary ion current Zq(A) of a molecular species containing A atoms in charge state q generally also depends, aside from the atomic concentration c(A) of element A, on the concentrations c(X) of other elements present in the sample (matrix effect!) as well as on sample and experimental parameters (e.g., surface coverage of active species, crystallographic orientation, energy, and type of primary particles). The dependence on these experimental parameters is denoted symbolically by the single variable c. It is further accepted that, other parameters being constant, Zq(A) is proportional to primary current I , . When we choose a set of mutually independent experimental variables 6 and concentration variables c [note that concentrations are interdependent via Cc(X) = 11, we can formulate this dependence of the secondary ion current as an “equation of state” in the thermodynamic sense 14(A) = g[c(A),
C(X)Y
€11,
(12)
where g is a function of elemental concentrations and experimental parameters. Equation (12) is simply the expression of the fact that under the same experimental conditions, a sample of elemental composition c(X) will always emit the same secondary ion current Iq(A)when bombarded by a primary ion current I , . It is impossible at the present time to calculate the “state function” g from first principles. It can, however, be shown that a few general relationships (see Sections III,B,l,a-c) between ion current I q ( X ) and elemental concentrations c(X) can be derived without detailed knowledge of the function g. In the same way, empirical quantitation models which are in use for other surface and thin-film analytical techniques (e.g., AES, ISS) can be put on a theoretical basis using the concept of a state equation. a. External standards (calibration-curve method). The fundamental state equation (12) is a statement of the most general relationship between total emitted secondary ion current Jq(A) of element A (not necessarily a trace
246
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
element) and the parameters describing target properties [primarily elemental concentrations c(X)] and experimental parameters. When all parameters except the concentration of element A, c(A), are kept constant (because of the interdependence of relative elemental concentration this is, strictly speaking, impossible), the detected ion current P ( A ) of an A-containing molecule (cluster, atom) in charge state q can be considered a function g of c(A) and of the primary current only
The exact form of the function g cannot be derived from the state equation (12), but is an individual property of the particular target under investigation. Equation (13) can be considered a “calibration curve” in the sense that if 1’4(A) can be measured in calibration samples containing element A in various concentrations (keeping the other components in the samples approximately constant), the concentration of an unknown sample of approximately the same composition can be obtained by interpolating the ion current from the unknown sample between the values measured. b. Working-curve approach. It can be shown (Benninghoven et al., 1981) that the ratio of detected ion currents ofelements A and B, P(A)/P(B), is only a function of the concentration ratio c(A)/c(B) of these elements 1’4(A)/1’4(B)= K[c(A)/c(B)]
(14)
when A and B are trace elements. Equation (14)can be considered a “working curve” for quantitation of SIMS intensities. The working curve can be determined from a measurement of a series of calibration samples containing A and B in different relative concentrations. The concentration ratio of an unknown can be determined by interpolation from the working curve under the following conditions: (i) Experimental conditions during measurement of the working curve and the unknown are identical. (ii) The compositions of calibration samples and unknown, aside from the trace elements A and B, are approximately identical (“same matrix type”). Compared to the external standard approach the primary ion current need not be accurately specified, because of the rationing of secondary ion signals. Note that the absolute concentration of an element A can only be determined when: (i) The absolute concentration c(R) of a reference element (R) is known
Here, [c(A)/c(R)Iwc is the concentration ratio of elements A and R obtained
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
247
from the working curve [Eq. (14)] using the experimentally determined current ratio I''(A)/I'q(B). (ii) The working curves of all elements present in the sample (with the same reference element R) have been previously determined, and secondary ion currents Z''(X) of all sample constituents have been measured ;in this case :
c. Relative sensitiuityfactors (RSF). The RSF method is a special case of the working-curve approach (see Section III,B,l,b). As has been shown in Section III,B, the working curve degenerates into a straight line when A is a trace element and R is a matrix element with an atomic concentration close to 1. This behavior has also been experimentally demonstrated (Ishizuka, 1974; Okano, 1979; Tsuruoka et al., 1974; Werner, 1969) and in many cases appears to apply even up to rather high elemental concentrations (< 10%) and for samples where the reference element concentration is distinctly smaller than c(R) = 1. In the case of the linear working curves equation the concentration ratio of elements A and R can be written as
c(A) SRZ"(A) __---
I"(A) c(R) - SAI"(R) - SA,RI"(R)
where S,, SAare absolute sensitivity factors SA
=
I"(A)/c(A)
(18)
and SA,Ris the "relative sensitivity factor" (RSF) of element A (referred to R) (Ganjei and Morrison, 1978; Smith and Christie, 1978), SA,R
=
SA/S,
(19)
Again, relative sensitivity factors can be empirically determined from measurements of calibration standards; principally, one standard only with a trace amount of A in a matrix of B is sufficient to determine the RSF of element A
The determination of the absolute concentration of an element A is possible: (i) When the absolute concentration of the reference element (internal standard) is known:
248
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
in this case the ion currents of trace and reference element only have to be measured. (ii) When the RFSs of all elements contained in the sample are known and ion currents of all sample constituents have been measured; in this case no internal standard is necessary:
Note that according to the state function concept, the following requirements generally have to be fulfilled for application of the relative sensitivity factor concept : (i) The test element is a trace element. (ii) The reference element is a matrix element with c(R) close to unity. (iii) The composition of the unknown sample has to be similar to the calibration sample. The exact definition of “similar” is somewhat arbitrary; within the state function concept, however, “similar” can be defined as Ac(X) << 1 or [ag/dc(X)] << 1, i.e., either closely matching composition of unknown and standard or practical independence of ion current P ( A ) from other elemental concentrations. (iv) Experimental conditions in the measurement of unknown and sample have to be “similar.” Deviations from these requirements introduce systematic (nonlinear regions in working curve) or random errors (not carefully specified experimental conditions) into the determination of elemental concentrations. d. Znnternal indicators. As has been mentioned before, the relative sensitivity factor for a particular element may vary when matrix composition or experimental conditions are changing. Such experimental conditions influencing the RSF may be local angle of incidence of primary beam, local surface charge, and surface coverage of reactive gases, etc. The RSF therefore is a multidimensional function of all these parameters. It may be hoped, however, that there exists a single variable reflecting the “state” of the ionbombarded surface so that the RSF of a particular element may be expressed as a function of that single variable (“internal indicator,” 6 ) (McHugh, 1975) only. A variety of readily observable ratios of ionic species, usually involving atoms or molecules containing atoms of the matrix element M [matrix ion species ratios (MISR) (Larsson et al., 1977)], have been used as internal indicators (McHugh, 1975; Ganjei et al., 1978; Larsson et al., 1977),
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
249
For a complete analysis of a sample, the dependence of the relative sensitivity factors from the internal indicator t has to be known for each elemental constituent (series of curves SX,R(t), see Fig. 6). These SXpR(c) curves may be obtained from calibration samples measured under different experimental conditions (e.g., oxygen partial pressure, sample inclination). Note that any set of indexed RFSs applies to a particular class of matrices only ; the conditions for “similarity” of matrices and experimental environment, however, may be considerably relaxed compared to the constant RSF method (see Section III,B, 1,b). Nevertheless, a considerable amount of a priori information is necessary before the method can be actually applied for quantitation of an “unknown” (in parentheses because at least the matrix type has to be known before analysis in order to select the appropriate set of indexed RFSs) ; this is, however, compensated by increased analytical accuracy of the method. In an actual analysis the internal indicator t (ion species ratio) has to be measured in addition to the ion currents of the elements to be quantitated. With the measured value of the internal indicator, the RSFs applying to the sample environment prevailing in the course of that particular analysis are taken from graphs similar to Fig. 3. The calculation of elemental concentrations then proceeds exactly as in Eqs. (21) and (22). e. Standard addition. The present state of quantitative elemental SIMS analysis is not quite satisfying. The empirical quantitation models described I
t
I
1
ES 6-
FIG. 6 . Internal indicator curves (schematic); Sx,R, relative sensitivity factor; 6 , internal indicator (e.g., matrix ion species ratio, MISR). [From Benninghoven et al. (1981).]
250
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
in the previous sections require a great amount of apriori knowledge (calibration curves, indexed RSF, etc.) and suffer from the somewhat arbitrary definition of the requirements for the applicability of a specific set of calibration curves (RSFs, indexed RSFs, etc.) to a particular sample. In addition, RSFs determined for one individual SIMS instrument may not be transferred to another one, even of the same type (Newbury, 1979), when an accurate analysis is required. The quantitation procedures based on physical models still suffer from the lack of understanding of the detailed processes leading to secondary ion emission and of the accurate knowledge of atomic and molecular parameters entering into their mathematical description ; their accuracy can be considered to be semiquantitative at best. A conceptually simple way of determining the number of atoms of a certain element A present in a limited sample volume from the intensity of a characteristic secondary ion mass peak is to add an accurately known additional number na of A atoms to the originally present A-atom no (“standard addition”) and determine the increase of secondary ion emission of element A, Z, , caused by the added atoms. If the physicochemical state of the “added” standard A atoms is exactly the same as that of the originally present A atoms and if the number of added atoms is approximately the same or smaller than n o ,proportionality between volume concentration and secondary ion emission can be assumed so that ndna = l o l z a (23) where I , is the secondary ion emission of the sample previous to standard addition. Ion implantation is a suitable means to incorporate atoms of almost any kind into the host lattice of the sample. Figure 7 schematically shows how an unknown volume concentration c b (atoms/cm3) of an element can be determined in a sample implanted with a known dose F (atoms/cm2) of the same element (Leta and Morrison, 1980b). The original atom concentration c b is assumed to be independent of depth; the added standard atoms show a characteristic implant profile (approximately a Gaussian peak centered at the mean range of the implant). This characteristic depth distribution allows us to discriminate between the original uniform concentration level cb (observable at larger depths) and the peaked implant superimposed on the uniform “background.” The measurement is performed by sputtering the sample to a depth D beyond the tails of the implant peak and by measuring the integrated detected ion current Z of the test element (area under the curve, measured in counts) and the signal level S b corresponding to the original “background” concentration level c b ; c b then can be determined through the relation (Leta and Morrison, 1980a), Cb
= SbtF/ID
(at0mS/Cm3)
(24)
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
25 1
D
0
Depth FIG.7. Simulated depth profile of an ion-implanted sample; see text. [From Leta and Morrison (198Oa).]
where t is the total time measured in seconds. The sputter depth D (cm) can be determined from Talystep measurement. Although up to now this method has only been tested with a limited number of samples (Leta and Morrison, 1980a,b), it appears promising because it requires no a priori knowledge at all, instrumental parameters (transmission etc.) do not enter, experimental conditions are uncritical, and because it is based on a minimum set of not unreasonable assumptions: (i) Implanted and original atoms are present in the same physicochemical state in the implanted region. (ii) The yield of the test element from the implanted region is identical to that from the unimplanted zone at greater depth. (iii) The secondary ion emission is proportional to the concentration up to the maximum implant cancentration.
2. Theoretical and Semitheoretical Models When SIMS first was being developed as analytical tool it was originally hoped that a theoretical understanding of the secondary ion emission process would be the basis for any reliable quantitation method of SIMS spectra. This stage, however, has not yet been reached and the analytical accuracy of physical models is markedly inferior to that obtainable with empirical models such as described in Section III,B,l. We will therefore devote much less space to theoretical model$ and refer the reader to reviews which have
252
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
appeared in the literature (Werner, 1980b; Riidenauer, 1977; Blake and Nourtier, 1979). Still the most frequently applied group of theoretically based algorithms are the various types of LTE models. The original version [CARISMA (Andersen and Hinthorne, 1973)] relied on the existence of a plasma in local thermodynamic equilibrium (LTE) at the ion-bombarded surface. In such a plasma the volume concentrations of neutral ionized atoms species (no, n +) and of electrons (nJ, respectively, would obey the Saha-Eggert operation equation (n+n,)/no
=
2[Z+(T)/Zo(T)](2nm,kT)3/2h-3 x exp[-(Ei
- AEi)/kT]
(25)
where Z + and Zo are the partition functions of ionized and neutral species respectively, me is the electron mass, and Ei is the ionization energy of the neutral species; k is Boltzmann’s constant, h is Planck’s constant, T is the plasma temperature, and Ei is the depression of ionization energy due to plasma effects (Drawin and Felenbok, 1965). The connection between the variables on the left side of Eq. (25), the observable (positive) secondary ion current Z+(A) of an element A, and the desired atomic concentration c(A) is made through the postulates I+(A) x n+,
c(A) x ntot = no
+ n+ +
*
.
*
(26)
where not only neutral atoms and positive atomic ions but also negative ions and A-containing neutral and ionized molecules add up to the total number ntotof atoms of type A in a unit plasma volume. Obviously, elemental concentrations can be expressed in terms of measured secondary ion currents, known atomic parameters, and electron concentration n, , and temperature Tof the plasma. The latter two variables cannot be determined from first principles; in this case it can be shown (Rudenauer, 1977) that the concentrations of at least two elements (“internal standards”) have to be known in order to solve the system of simultaneous equations such as Eq. (25) and to calculate the concentrations of all elements present in the sample. In the original CARISMA algorithm of Andersen and Hinthorne (1973) oxide and dioxide species are taken into account in Eq. (26). Other authors have simplified the algorithm by considering neutral and singly ionized atomic species only [SLTE (Simons et al., 1976); QUASIE (Riidenauer and Steiger, 1976)l. Also, the interpretation of the model in terms of LTE became questionable and was formulated somewhat differently (Morgan and Werner, 1978b); it became a frequently expressed opinion that Eq. (25) should rather be considered a practical formula without resorting to any theoretical interpretation, T and n, acting as fitting parameters only. This
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
253
encouraged further simplification of the algorithm, particularly because such simplifications apparently did not influence analytical accuracy. At present, models containing only one fitting parameter ( T ) are in use [QUASIMS (Morgan and Werner, 1978a,b); SIQUAS (Riidenauer and Steiger, 1976; Steiger and Rudenauer, 1979)], requiring one internal standard element only. When the value of the fitting parameter T is known from previous measurements on samples of the same class, even quasi- “standardfree” LTE analysis can be performed (Morgan and Werner, 1978a). The LTE results often show considerable deviations from actual elemental concentrations (Rudat and Morrison, 1979; Newbury, 1979). Two normalization procedures have been used to force elemental concentrations to obey reasonable boundary conditions : (a) Concentration normalization. An additional equation of condition is introduced stating that the sum of calculated concentrations is unity. This implies that ion currents of all elements present in the sample can be accurately measured and are free from mass interferences. (b) Sputter normalization. This method has been used for quantitation of secondary ion images (G. H. Morrison, private communication). It uses a one-parameter, single internal standard LTE version, which is sometimes called the “collision cascade model” (Werner, 1980 ). Starting from a modified Saha-Eggert equation, an ionization fraction for the internal standard element S is calculated, using an arbitrary T value:
( N + / N 0 ) ,= 2(Z+/Zo)s(2zrn,kT)3/2/h3 exp[ -
- AEi)/kT]
f ( T ,Z ) (27) where N s + and Nso are the number of ionized and neutral standard atoms, =
respectively, sputtered from the target per unit time. From Eq. (27) the total (r neutral) number of standard atoms per second is expressed as a function
of standard ions detected per second Nsi’
’
fis” = [ f (T, Z ) ]- (Ns
+
’its)
(28)
where rS is the total instrument transmission for standard ions. The atomic concentration ( c ~of) the~ standard ~ ~ ~ can now be calculated as (CS)calc
= fis0mot
(29)
where N,,,, is the total number af sample atoms sputtered per second; Nto, can be experimentally determined, e.g., from TALYSTEP measurements. This procedure is repeated for different values of T until the difference between calculated and specified standard concentration is less than a tolerated limit. Let T* be the T value for this optimum fit; then the concentra-
254
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
tion of any element X can be calculated from the known standard concentration and the ratio of detected ion currents Z(X)/Z(S): c(X)/c(S) = [~(X)/~(S)](z+/z,)X(zo/z+), exp[ -(Eix - Eis)/kT] (30) Only those elements X enter into the calculations, for which the concentrations have to be calculated. Therefore, only the peak height of the elements to be quantified have to be measured. 3. Comparison of Models
A comparison of the usefulness of different models for the quantitation of SIMS spectra should take into account the following parameters: (a) Analytical accuracy ; often defined as relative percentage deviation 6 of calculated versus certified elemental concentration, averaged over a great number of analyses on a variety of sample types
6 = 100(cca,c -
Ccert)/Ccert
(%I
(31)
Eq. (31) is a reasonable definition for methods with small quantification errors (6 << 100%). When errors are large an “error factor” F is a more reasonable measure, F = ~cad~cert, for
Ccalc
for
Ccalc
>
Ccert
(32) < ccert (b) The amount of apriori knowledge required (sample data, calibration curves, internal standard, etc.) other than extractable from the SIMS spectrum of the unknown sample. (c) The amount of SIMS data to be collected from the unknown sample. (d) Computation time for the algorithm; this is of particular importance in the quantification of SIMS images or three-dimensional data (see Section VI). F=
Ccert/Ccalc>
Table I1 gives a compilation of relevant data for different quantitation models; data on computational speed can be taken from Table IV. The definition of analytical accuracy in particular is not unambiguous since some of the models were tested against a limited choice of standard sample types only and were implemented on different types of SIMS instruments, not all allowing comparable experimental conditions to be realized. The analytical accuracy in the last line of Table 11(defined as the percentage of analyses with F < 1.5), therefore should be considered as a rough guideline only. More detailed evaluations of analytical accuracy of SIMS models have appeared in the literature (Rudat and Morrison, 1979; Werner, 1980b; Smith and
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
255
Christie, 1978; Newbury, 1979). It can be seen that the empirical models in general are still superior with respect to accuracy and computation time, but they require a great deal of apriori knowledge. In addition, relative sensitivity factors (calibration curves, working curves) obtained on one individual instrument are not transferable to other instruments so that each analyst has to do extensive time-consuming calibration work on his own machine. The physical models, by contrast, are less accurate and can be considered as semiquantitative at best. However, they do require a minimum amount of a priori knowledge, which may however be difficult to obtain (internal standard elements ), and are generally transferable between different instruments (Rudenauer et al., 19761, provided the dependence of instrument transmission on mass number is known. For double-focusing instruments it is general practice not to consider mass discrimination effects (aside from, perhaps, detector discrimination). In instruments employing quadrupole mass filters the transmission/mass number relationship is complicated and depends on the adjustment of many individual parameters. In these types of instruments, therefore, physical quantitation models (which, in their original versions, all rely on a mass-independent transmission) are hardly used. C . Quantitation of I S S
A description of the physical processes playing a role in scattering of low-energy ions from a multielemental solid surface would require knowledge of surface potential and structural data which is generally not available from an unknown sample. A quantitative determination of the elemental composition of a sample surface will therefore have to resort to empirical “relative sensitivity factor” methods. The applicability of these methods in a limited range of concentrations (relative surface coverage in ISS, because of the single monolayer response), can be theoretically derived from the existence of a “state function” (see Section III,B,l). In the particular case of ISS the relative sensitivity factor SA,Rof a test element A, referenced to the reference element R, can be derived from Eq. (2) in Section II,B,l as
where I ’ ( A ) and I’(R) are the intensities of the elastic scattering peaks of elements A and R, respectively, and N,(A), N,(B) are the corresponding surface-atom densities, measured in cm-*. The RSFs can be determined from calibration measurements on pure elements. In this particular case the
TABLE I1 COMPARtSON OF
QUANTITATION MOD=
FOR
SIMS
Methodasb ~
EST
WC
RSF
X
X
X
X
X
X
IIN
SAD
CAR
SLTE
1P
NS
SNLTE
2
2
1
0
1
X
X
X
VI N
m
A priori information needed to quantitate one element (A): Matching sample composition Sample type only Identical experimental condition Primary current measurement Calibration curve of element A of all elements RSF of element A of all elements No. of internal standards RSF ( 6 ) of element A for all elements Parameter T Atomic and molecular parameters Implanted dose of element A
X
X
X
X
X
Secondary ion data needed to determine c(A) : I(A) I*(A)/I*(R) t = I*(M,)/I*(MJ I*(X)/I*(R); X = 1,. . ., N" I*(X)/I*(S); X = I , . . ., M < N' Sputtered volume Depth profile of A
X
X
X
X X
X
X
4J
Percent of total analyses with F < 1.5
4
X
41
809
>90h
F
1.5'
358*J
45'
5Oj
6 x 20%"'
F zz l.SL"
EST, external standard; WC, working curve; RSF, relative sensitivity factors; IIN, internal indicators (MISR); SAD, standard addition; CAR, CARISMA; SLTE, simplified LTE; 1 P, 1 parameter LTE; NS, standard, free LTE; SNLTE, sputter-normalized LTE. PweRtkeses -ate d t m m t e evafuationmethods. M , , M , are matrix ion species. N is the number of elements present in the sample. S is the internal standard element. 1 Estimated average accuracy of analyses 20% with careful measurement Smith and Christie (1978). Werner (1980b). Leta and Morrison (1980a). Rudat and Morrison (1979). Limited experience. Smith and Christie (1977). " Morgan and Werner (1978a). " Drummer and Morrison (1980).
-
'
258
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
ratio of elemental area densities No(A)/No(B)may be substituted by the ratio of the squares of the atomic radii a. (Baun, 1978),
The concentration (relative surface coverage) B(A) of element A is then calculated from the standard formula (see Section III,B,l,c),
where the I(X) are the measured elastic peak currents of the elements X present in the sample. The general limitations to the application of a simple sensitivity factor formula such as Eq. (34) are discussed in Section III,B,l. In particular, the definition of similarity of calibration sample and unknown may be complicated in ISS by effects such as crystal structure, multiple scattering, selective sputtering, etc. (Honig and Harrington, 1973 ; Baun, 1978).
D. Quantitation of RBS The probability R for a high-energy primary ion to be backscattered into the solid-angle interval dsZ and energy internal dE, is given by Behrisch and Scherzer (1973) as d2N/N,
=
n,do(E;) dxfcos
= R(E1, E z , CI,
p, ~ p dE, ) dR
(36)
(see also Fig. 4), where dn is the Rutherford differential cross section do
=
ZlZ2e2 cos e - (1 - [ ( M , / M , ) ~ i n e ] ’ ) ~ / ~ 4E;’ sin4B{1 - [(M1/MZ)sin B]2)1/2
~
(37)
d2N is the number of ions scattered into the interval (dE,,dR), N , is the number of primary ions incident onto the target during the registration time of a spectrum, and nT is the number of target atoms per cubic centimeter. For a given primary ion and fixed scattering angle, the cross section is therefore a function of mass number M , and atomic number of the target atom only. Using Eqs. (4)-(6) from Section II,B,2, Eq. (36) can be written in the form d 2 N ( X )d& dR
= N,n(X)f(Zz,
M , , a, 0, E l )
N,n(X)f(X) (38) Here, d2N(X)is the number of ions scattered from target atoms of element X into the interval (dE2,dR), n(X) is the number of X atoms per cubic centimeter =
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
259
of the target, and f is a function depending on the indicated variables (for fixed primary energy and geometry, f is a property of the target element only). Integrating d 2 N ( X )with respect to dE, in effect means determination of the peak area A ( X )corresponding to scattering from a particular element X (e.g., the Ni peak in Fig. 4): A ( X ) = d N ( X )d o = N,n(X)
s
f ( X )dE, = N , n ( X ) f ( X )
(39)
where, again, f(x) = I f ( X ) dE2 is a function of element X . Obviously, one can define elemental sensitivity factors S ( X ) (see Section III,B), S(X) = 4 X ) / n ( X )= N , f ( X )
(40)
which, in principle at least, can be analytically calculated from the formulas given in this section and Section II,B,2. For a particular experimental arrangement, it is better to determine relative sensitivity factors SR(X)with respect to a reference element R from measurements on calibration samples (e.g., thin films of pure elements and known thickness),
The absolute number n(X) of X atoms per cubic centimeter of sample then can be calculated with the familiar formula
A set of sensitivity factors determined from pure elements is valid, in principle, for a particular primary energy and mean target atomic number only. Taking the appropriate experimental precautions, a quantitative determination of absolute elemental volume concentration can be performed with accuracies of a few rel. %.
IV. DEPTH PROFILING A . Nondestructive Methods
1. Methods Using Electron Spectroscopy An obvious possibility is to tqke advantage of the dependence of the mean free path of electrons on the primary energy (see Section 111,A). In AES the simplest way is to use different Auger electron transitions for the same
260
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
element if different transitions are available. In this way information about the depth distribution of elements in a substrate can be found. The Auger signal I , for a given transition depends on (Section 111,A): I,
N
1;
cA(z)
exp( - z / l , cos a) dz
(43)
cA(z) is the concentration of element A with depth z, I , is the mean free path of the electrons of the Auger transition considered, and a is the angle between surface normal and analyzer entrance slit. Because of Eq. (43), depth distribution can also be obtained by variation of angle a. We therefore define an effective escape depth according to Fig. 8 (Hofmann, 1980):
leff = L A cos a
(44)
This method can be used directly if the solid angle of acceptance of the analyzer is small enough to select the angle a. This is the case for analyzers that do not have a symmetric axis, which includes source, entrance, and exit, e.g., spherical analyzer. In the case of a great acceptance angle, e.g., a cylindrical mirror analyzer (CMA), the following relation for the take-off angle can be given (Zeller, 1980): cos a = C O S ~ ~coscp ,
+ sinrp, sinrp cos 9
(45)
wherecp, is the analyzer entrance angle (for the CMAqA equals 42.3"),rp the sample normal, and 9 the azimuthal angle (see Fig. 9 for explanation). Using a CMA, the depth resolution is rather complicated but can be simplified by selecting the azimuthal angle 9. This can be done by using a drum device (Hofmann, 1980). In this CMA a diaphragm decreases the azimuthal angle from 2n to about 6" and is rotatable by a feed through.
FIG.8. Definition of effective electron escape depth.
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
26 1
FIG.9. Variation of take-off angle for a CMA.
Moving 9 from 0" to 180" the take-off angle a ranges forq =qAfrom 0" to 84.6".The effective escape depth then becomes
Aetr = A,
for a = 0";
AeR
=
0.1AA
for a = 84.6"
Changing the take-off angle from surface normal to about 5" off the surface plane, the information depth changes by a factor of 10.
2. Depth Profiling Using RBS The method of depth profiling with RBS has been briefly described in Section II,B,2. With limitations, RBS is suitable for quasi-nondestructive determination of depth profiles in surface layers of thicknesses not greater than about 10 pm and for elements present in concentrations of the order of 100 ppm or above. The interaction of light ions in the 100-keV range is intrinsically accompanied by radiation damage in the sample lattice. When the degree of damage is tolerable with respect to the postanalysis use of the sample (e.g., for analysis with a different method or, in the case of microelectronic devices, to the postanalysis functioning of the device), RBS analysis may be considered nondestructive. A theoretical and practical survey of the method has been given in the literature, e.g., by Ziegler (1975). B. Destructive Methods
Depth profiling can be done in an destructive way by removing surface layers by ion bombardment simultaneously with many analytical techniques (Wehner, 1975), or by methods using tapered sectioning (Werner, 1980a). 1. Sputtering of Composite Materials
The ejection of material from solid surfaces under energetic ion bombardment is well known as sputtering. Although sputtering of elements has
262
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
been extensively investigated, sputtering of composite materials has been studied more extensively only recently. One of the reasons is that within the last 10 years sputtering has become a widely used method for depth profiling in combination with different surface analytical techniques such as AES, I S , XPS, and SIMS (Wehner, 1975). It has been found that sputtering of composite materials will often change the very surface composition of the sample that is analyzed by this method (Andersen, 1974; Oechsner, 1976). Whenever a multicomponent system is ion bombarded, selective sputtering generally occurs. Due to different partial sputtering yields of the individual components, an altered surface layer will build up. If steady-state conditions are reached, the target will be sputtered stoichiometrically, simply from conservation of matter. However, in general the surface will have a composition other than the bulk. Also, total sputtering yields of multicomponent systems have been found to be different from a superposition of the yields of the components (Betz et al., 1977b; Dahlgren and McClanahan, 1972; Ogar et al., 1969; Szymonski et al., 1978; Poate et al., 1976). Considering the sputtering process of composite materials, one has to distinguish between solid solutions or compounds and multiphase systems. For the first group, steady-state conditions will be reached quickly after removing a layer comparable to the penetration depth of the primary ions. For the second group, grains of different phases are present at the surface and it is obvious that selective sputtering will occur, if the individual phases have different sputter rates. Steady-state conditions will be reached after removing a layer with a thickness of several grain diameters (Henrich and Fan, 1974), which will generally be in the pm range. Surface roughness will develop and due to “back and forth” sputtering between the phases, the very surface composition of a phase will be also determined by the surface topography and the other phases present (Wehner and Hajicek, 1971). Cone formation and accumulation of “difficult-to-sputter” species result in a very complicated sputtering behavior, which depends on the evolution of the topography with time and also on impurity concentrations. Although sputtering of elements is quite well understood and different theoretical approaches (Sigmund, 1981 ; Betz et al., 1971 ; Ishitani and Shimizu, 1974) have yielded satisfactory agreement between experiment and theory, no comprehensive theory of alloy or compound sputtering has yet been established. Semiquantitative calculations by various authors (Haff, 1977; Haff and Switkowski, 1976; Kelly, 1978)predict surface enrichment of the heavier component. Andersen and Sigmund (1974) have extended their theory of random collision cascades to the problem of energy sharing among the components of a multicomponent system. They predict for systems with a large mass ratio of the constituents, that the energy
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
263
spectrum of the heavier component goes considerably faster to zero than for the light constituent, as compared to the components. This should also result in a higher ejection probability for the lighter constituent and therefore produce surface enrichment of the heavier component. However, these predictions are complicated by the surface binding energies. Two components having nearly the same mass may have very different surface binding energies and hence very different sputter yields, although recoil energy and slowing down densities are the same. The second complication appears as soon as an altered surface layer develops, due to different partial sputtering yields, and the condition of homogeneity for applying the theory is no longer fulfilled. a. Selective sputtering. Surface enrichment of one component due to selective sputtering has been shown for different binary and ternary alloy systems (Betz, 1980; Opitz e i al., 1980). To observe changes in the surface composition of alloys under ion bombardment it is necessary to start from a “clean” surface with a composition equal to the bulk composition. Different methods can be used to create such a surface for solid solutions (Braun and Farber, 1975): Fracturing the samples in the ultra-high-vacuum (UHV) system prior to analysis. If the fracture is transgranular, e.g., through the grains, the surface created should have a composition equal to the bulk. For an intergranular fracture, segregation to the grain boundaries can give rise to a surface different in composition to the bulk. Scribing the samples with a stainless steel or diamond tip in UHV also results in a surface with a composition equal to the bulk by a nonthermal melting process in a microscopic region (Meyer, 1977). Provided the same composition is found for these quite different methods to create a new surface, it can be assumed that it is also equal to the bulk composition. However, a different surface composition can be found for heterogeneous alloy systems, e.g., Ag-Cu (Farber et al., 1976). Using AES for determining the surface composition, the ratio of the Auger signals of the alloy components from a sputtered surface can be compared with the Auger signal ratio of scribed and fractured samples to determine changes in surface composition due to ion bombardment. This procedure is more advantageous than the use of pure standards for two reasons. First, different surface topography of the standards and the sputtered alloys can give rise to different results and, second, the contribution of backscattered primary electrons can be different for the standards and the alloys (Joshi et al., 1975). When sputtered and scribed or fractured alloys are compared, however, we find that the matrix will be the same except for the altered layer thickness. Under steady-state conditions one can derive the following relation between the component sputtering yields S(A), S(B),and the Auger intensi-
264
P. BRAUN, F. RUDENAUER,AND F. P. VIEHBOCK
ties Z, ZB for the bulk and rA , 4 for the surface composition after sputtering (Shimizu and Saeki, 1977): S(A)/S(B) =
(zA/zB)/(TA/rB)
(46)
This equation for the equilibrium case remains also valid when diffusion processes are assumed to take place, possibly enhanced due to sputter damage (Ho, 1978). Therefore, the ratio of the component sputtering yields can be seen directly as the difference between the sputtered surface and the scribed or fractured bulk composition ratios. For example, analysis of the Ag-Au system using AES is shown in Fig. 10 ;S(Ag)/S(Au) was found to be 1.8 according to Eq. (46). To study the sputter behavior of composite materials, a first approximation is needed to compare the component sputtering yields in the alloy with the sputtering yields of the pure elements. According to Sigmund’s theory (Sigmund, 1981) the sputtering yield of the elements is given as: S(E) = 0.042LSn(E)/U0
(47)
where S,(E) is the nuclear stopping power, E the energy of the incoming ion, U, the surface binding energy taken as the heat of sublimation, and L a function of M , / M , . The expression S(E)U, is a quantity proportional to the recoil energy density deposited at the surface from the collision cascade created by the impinging ion. This function is plotted in Fig. 11 for 2-keV Ar ions and different target materials (dashed line). It has been shown that Sigmund’s L function is high by at least a factor of 2 for large mass ratios (Sigmund, 1977; Andersen and Bay, 1973), which is probably caused by a lack of surface corrections in the theory. Using the experimentally derived
FIG.10. Measured surface concentration ratio of Ag (351-355 eV) and Au (2024 eV) versus ratio of bulk concentrations for Ag-Au alloy; [from Betz (1980)l:0 , fractured; A, scribed; 0, sputtered.
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
lo-
A Al
;"w
Ag
Cr
N
Au
265
U
Pt
FIG.11. Recoil energy density SU,,for 2-keV Ar ions on different target materials according to the sputtering theory of Sigmund (1981) (dashed line) and with values for L ( M , / M , ) according to Andersen and Bay (1973) (solid line).
L function from Andersen and Bay (1973), the fully drawn line in Fig. 11 is found for S(E)U,. As can be seen, the recoil energy density increases strongly only for light target materials and is constant for target atomic weights above 100, indicating that differences in the individual sputtering yields for the heavier elements are only determined by the surface binding energies. b. Consequences of selective sputtering. The observations for all binary, one-phase systems analyzed thus far, show that the sputtering yields of the components are more or less preserved in the alloys, and enrichment occurs for the component that has the lower yield as a pure element. In addition, the results for the ternary systems Ag-Au-Cu (Betz et al., 1980)and Fe-Cr-Ni (Opitz et al., 1980) exhibit the same behavior as the corresponding binary systems. Contrary to these observations, it has been proposed from RBS measurements (Liau et al., 1977) that, as a rule, enrichment of the heavier component occurs under ion bombardment. For Ag-Au enrichment in Au, Cu-Pd in Pd, Cu-Pt in Pt, and Ni-Pt in Pt, it is observed that the enrichment component is the heavier one as well as the component with the lower yield of the pure constituents. For Cu-Au, with a large mass ratio of 3: 1, no Au enrichment exists at 1 and 2 keV and only a minor enrichment exists for higher ion energies, in agreement with equal sputtering yields for Au and Cu. For Cu-Ni and Pd-Ni the lighter component Ni is enriched; and this is also true for Au-Pd wherein the lighter component is Pd. For Ag-Pd, with a mass ratio of 1, strong Pd enrichment is also observed in accordance with the much smaller yield of Pd compared to Ag. On the other hand, for metallic compound phases AI-Au,, A1,-Au, AI-Cu, and intermetallic silicides Si-Ni, Si-Pt, and Si-Pt, enrichment of
266
P. BRAUN, F. RUDENAUER,AND F. P. VIEHBOCK
the heavier component was found using RBS (Liau et al., 1977; Chu et al., 1976), although the sputtering-yield ratios of the pure components would indicate in all these cases enrichment of the lighter component. A study of Al-Au, Al-Fe, A1-Ni, and Al-Cr compounds (Opitz, 1979) under 2-keV argon ion bombardment shows depletion in Al, which is always the lighter and low-sputtering-yield component. Partly for these alloys the mass ratio is exceptionally large. It is however interesting to note that enrichment of the heavier component is only found if the recoil energy densities of the pure components are quite different, as can be seen from Fig. 11. For all the alloys with surface enrichment in agreement with the yields of the pure elements, the recoil energy density is either the same, because both components have the same mass, e.g., as for Cu-Ni and Ag-Pd, or their atomic weights are large enough so that the recoil energy density no longer varies with mass. If, on the other hand, the recoil energy densities of the pure components are quite different, enrichment of the heavier component, which is also the one with the higher recoil energy density, exists. All these experimental results are in agreement with the following assumption: For composite materials with components of nearly the same recoil energy density UoS, surface enrichment and component sputtering yields are only determined by the surface binding energies of the alloy constituents, which seem to be in qualitative agreement with those of the pure components. Therefore, the ratio of the component sputtering yields agrees qualitatively with that of the pure elements, and enrichment of the low-yield constituent is observed. This indicates that the recoil energy densities are preserved in the alloy. For alloys with quite different recoil energy densities of the constituents, collision cascade effects generally play the dominant role, and enrichment of the heavier component, as predicted by theoretical models (Haff, 1977; Haff and Switkowski, 1976; Kelly, 1978; Andersen and Sigmund, 1974) is observed.
2. Limits of Depth Profiling Using the Sputtering Method A universally applicable method for removing thin layers from a solid is bombardment by energetic ions of energies between a few hundred to a few thousand electron volts. Simultaneous detection of sputtered ions by SIMS makes this method very powerful for depth analysis, although any surface analysis method may be used for depth profiling in combination with sputtering (e.g., AES). The vital result of any depth-profile measurement is the concentration cA of element A as a function of depth z from the surface. The measured
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
267
distribution should be transformed to the actual depth profile if cA is a function of the measured signal (Auger electron current or secondary ion current; see Section 111) and z is a function of the sputtering time. The constant sputtering rate with time is only a rough assumption in depth profiling where the sputtering time is taken as a measure for the removed layers. However, the sputtering yields of the various elements are all within one order of magnitude; therefore, a rough proportionality between sputtering time and depth is given. More accuracy is necessary for the sputter yields, not only of the pure elements but also of composite materials which are present in the specimen. The sample consists of a two- or multiphase system, with no solid solubility of one constituent in the other. It was then found that the sputter rate remains close to that of the Eow-yield constituent (Braun, 1979), if at least 30 at.% of this component are present in the alloy; for lower amounts, the total sputtering yield increases to that of the high-yield component. For a nonmixable two-phase system, the number of atoms NAand N , , sputtered by an ion dose It from the two phases, should be NA =
SAAAIt
(48)
NB = SBABIt
(49)
where SA, SBare the sputtering yields of the pure elements, and A,, A B are the fractional surface areas of each component. For steady-state conditions, NJNB = = S A A A / S B A B if cA, cBare the bulk concentrations of the two elements. Therefore, one obtains for the total sputtering yield st
*
~/(cA/SA
+ cB/SB>
(50)
The sputtering yields for the nonmixable systems Ag-Cu (Betz, 1980) and Ag-Ni and Ag-Co (Dahlgreq and McClanahan, 1972) are shown in Fig. 12. The disagreement between the calculated and the measured yield values, and the constant sputtering yield over a wide concentration range, indicates that the low rates were caused by coating the Ag phase with the low-yield phase (Cu, Ni, Co) after changes in surface topography, due to the different erosion rates of the crystallites. Wehner and Hajicek (1971) observed similar reductions in the sputter yield due to coating the high-sputter yield material with a low-yield one. They explain the yield reduction on basis of cone formation and back and forth sputtering between the cones. Furthermore, for Ag-Cu thin films it was found that after ion bombardment is stopped, the surface becomes enriched in Ag within a few minutes. This effect was explained by surface diffusion of Ag covering the Cu crystallites by approximately one monolayer of Ag (Betz et al., 1977a).This indicates the presence
268
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
8 C
._ \
g 6 c
0
L
2
0
20
LO
M)
80
100
at% Ag
FIG. 12. Sputtering yields (2-keV A r t ) for Ag-Cu alloy films on glass (0) and on Ta,O, ( 0 )substrates, and for (1.5-keV Kr') Ag-Ni (m) and Ag-Co ( 0 )alloys. [From Dahlgren and McClanahan (1972).] The dashed lines are the yield curves according to Eq. (50).
of two controversial processes for nonmixable systems : Topography changes and contamination of the high-yield phase (Ag) with low-yield atoms resulting in low-sputter rate, and after sputtering the surface becomes covered with one overlayer (Ag) due to surface diffusion. For a solid solution of two components with nearly equal mass, the recoil energy density SV, (Section IV,B,l) in the alloy is the same as in either pure constituent because different, but equal, mass atoms are not distinguished in the development of the collision cascade. Different sputtering yields of the components and selective sputtering effects in the alloy can therefore only arise due to different surface binding energies of the components. For Cu-Ni and also for Au-Cu and Ag-Au, a linear increase of the sputtering yield with the concentration from the value of the low-yield component to the value of the high-yield component is observed (Betz, 1980); Fig. 13 shows this behavior for Au-Cu alloys. I ._: 5 \
5
5 L3-
I
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
269
3. Methods Using Tapered Sectioning Tapered areas of samples can be established by mechanical angle lapping or by sputter-crater etching and profiling with the probing beam by a line scan over the cross section. Obviously, the preparation method uses destructive processes, but the depth information obtained is nondestructive. Inherent to the preparation method are variations of the sample composition as a result of mechanical lapping or sputtering. By lapping composite materials, one has to distinguish between single- and multiphase alloys or compounds. In the latter, a smearing of constituents with tensile strength lower than the other ones can take place in the surface layers. Generally, sputtering also will vary the surface composition of composite materials, by selective sputtering, and leads to a new equilibrium surface concentration; this is treated in detail in Section IV,B,l. By mechanically preparing a tapered section at an typically of 0.1" and using a probing beam of diameter I pm, a depth angle /?, resolution of about 2 nm should be obtainable. After a surface is sputtered for a short period of time, a crater will develop with a nearly Gaussian profile (Hofmann, 1976): z(u) = zo exp( - u 2 / 2 0 2 )
(51)
Az(u)/Au = tan /? = uz(u)/02
(52)
Figure 14 illustrates the crater profile. In the deepest part of the crater the slope will be tan /? = for typical values of crater radius o = 1 mm and of depth z,, = 200 nm, corresponding to an angle /? = 20".If the electron beam (AES) is scanned from the bottom of the crater to the edge, the depth analyzed changes gradually. The important difference to the conventional
4 FIG.14. Development of 8 sputtering crater with a Gaussian profile.
270
P. BRAUN, F. RUDENAUER,AND F. P. VIEHBOCK
angle-lapping method often used in electron microprobe analysis is that the angle can be made much smaller in crater sputtering. The advantages of tapered sectioning over conventional sputter depth profiling (see Section IV,B,2) are that the depth profile can be obtained after sputtering with no limitation in data recording over the depth of interest and that lateral inhomogeneities of elements in depth distribution can be studied quickly in more detail.
4. Considerations in Auger Depth Projiling When an element has to be depth profiled in a homogeneous matrix, the sputtering yield yIoIis generally constant in that the depth scale is proportional to sputtering time. When, e.g., an element has to be profiled through a multilayer system, variations in sputtering yield may cause distortions in the measured profile. Recently, optical methods have been developed allowing in situ measurement of sputtering speed with a depth resolution down to the nanometer range (Kempf, 1979). The quality of a depth profile measurement may be assessed by two parameters: depth resolution Az and dynamic range R D . These parameters may be defined with respect to a step function (100-0%) concentration change: Az is the depth which has to be sputtered before the elemental signal at the interface changes from 95 to 5% of its maximum level; and R D is the ratio of maximum to minimum signal when sputtering through the interface into a region originally containing no X atoms. The true depth profile may be smeared and distorted in the measurement due to several instrumental and physical factors: ion beam homogeneity ; material transport from crater rim to crater center ; sputter redeposition of material on surrounding electrodes ; mass interference in low-resolution SIMS instruments or line interference in AES ; statistical nature of layer removal; atomic mixing in collision cascade; finite information depth of sputtered ions in SIMS and of characteristic electrons in AES; surface roughness and topography ; distortion of profiles by field-induced migration caused by ion beam charging of insulators. If the sputter rate dz/dt varies within the area analyzed, but is constant with time, this leads to a constant relative depth resolution Az/z(Werner, 1974). In AES a further limitation to depth resolution is set up by the statistical
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
27 1
nature of sputtering [factor (5)]. A quantitative estimate can be obtained from the successive-sputtering model proposed by Benninghoven ( 1970). The basic assumptions of this model are that sputtering takes place only in the actual surface layer and that the sputtering yield is the same for all layers independent of any different layer composition. As result, the relation between depth and relative depth resolution is given by
Az/z
=
2(d/~)”~
(53)
with the monolayer thickness d. For example, with a value d = 0.4 nm, the relative depth resolution Az/z at a 4-nm sputtered depth is 63%, at 40 nm it is 20%, and at 400 nm, 6%. Evidence of the validity of this relation could be shown in the work of Hofmann (1976) and Evans (1972). For example, an Auger depth profile of a multilayer interference coating of a selective solar absorber for 2-keV Ar+ ion sputtering is given in Fig. 15. The Auger signal versus time is in most cases sufficient for comparing of data but not if the absolute depth distribution is desired. In Fig. 15 the layers consist of A1203,MOO,, AI2O, on Mo with their individual sputtering yields, which differ strongly. The ratio S(Mo03)/S(A120,) for 10-keV
10
2
sputtering time FIG. 15. Auger depth profile of AlZ0,-MoO,A1,0, with 2-keV Ar’ ions.
coating on Mo substrate; sputtered
272
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
Kr’ ion bombardment (Kelly and Lam, 1973) equals about 6. The absolute depth distribution will then show the A1,0, layers to be 6 times as thin as given in Fig. 15. For the time-to-depth transformation, the sputter rates of composite materials and compounds are important. Data from different authors for the same element and energy can vary greatly and a careful data selection should be done in any case study (Wehner, 1975; Betz, 1980). 5. Practical Examples for SIMS Depth Projling In SIMS depth profiling, the sample to be analyzed is bombarded with a primary beam current Z,(t), constant in time. The secondary ion current Zx(t)of a selected element X is recorded as a function of time. The time scale t can be transformed into a depth scale z through apriori knowledge or in situ measurement of the sputtering speed so that the desired depth profile of an element cx(z) can be obtained, CX(4 =
klx(t)
(54)
where k is a constant and cx(z) is the elemental concentration of element X as a function of depth (Werner, 1978). Experimentally obtained values for the relative depth resolution Az/z are of the order of 0.4% remaining constant at great sputtering depths (Werner, 1974,1977b). The effects distorting a true profile have already been listed in Section IV,B,4. Dynamic ranges in excess of lo4: 1 (measured in the tail of implantation profiles) have been obtained in well-adjusted instruments (Werner, 1974). Concerning the fidelity of the shape of a measured profile, the distorting effects (1)-(3) in Section IV,B,4 can be minimized through experimental refinements (see Fig. 16). Here the profile of a “B” implant in Si is severely smeared because an unfocused primary beam is used; rastering a focused beam only slightly increases the dynamic range due to remaining crater-edge effects. Rastering plus electronic gating the central crater section brings about a dramatic increase in dynamic range [better than with rastering and mechanical gating by extracting lens and diaphragm (Magee et al., 1977)l; finally, four orders of magnitude in the dynamic range can be obtained by combined use of rastering a focused beam, electronic gating, and mechanical aperture (extraction lens). The residual signal level may then affect only the traced instrumental effect (3) or any of the physical limitations (4)-(9) (see p. 270). The statistical nature of sputtering sets a limit to depth resolution mainly in extremely shallow gradients (Az = 5-10 atomic layers); atomic mixing may cause an asymmetric profile broadening at the interior side (McHugh, 1975). Serious modifications of profiles in insulators may be expected for ions with high mobility when using positive primary ions. In
273
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
static beam
\
rasteronly
\-r
raster +lens
Si
I
5 keV4'Ar+
raster +. electronic gate
raster electronic gate
+ +
lens
0
0.5
1.0
1.5
depth ( p m )
FIG. 16. Optimization of dynamic range in SIMS depth profiling; see test. [From Magee et al. (1977).]
these cases (e.g., Na, K in glass matrices), the use of negative primary ions and means for external charge suppression are achievable (Werner and Morgan, 1976; Gossink et al., 1978). Mass interference of isobaric molecules can be avoided in high-resolution instruments when the required resolution does not exceed MjAM = 15,000 (Magee, 1979). A different type of profile distortion may be caused by variations in the degree of ionization of the sputtered particles with depth (e.g., sputtering through an interface between different matrices). Efficient means to correct for this effect have not yet been developed; fortunately, in depth profiling of ion implants (one of the most important fields requiring high-sensitivity,
274
P. BRAUN, F. RUDENAUER,AND F. P. VIEHBOCK I5KeV PROTON IMPLANT INTOSILICON
t
o
y
-
r
B AND
-
ldsO
0.1
0.2
0.3
0.4
P DOUBLE DIFFUSION
p
0.5
0.5
0.6
1.0
1.5
2.0
DEPTH ( p m )
(b)
FIG.17. (a) Depth profiles of H - and 30SiH- using Cs+ bombardment; no background subtraction. (b) Simultaneous profiling of B and P diffusion layer to determine junction depth. Both figures from Magee (1979).
TABLE 111 DEPTH-PROFILING DETECTION LIMITS' IMPLANTS I N Si USINGAES AND POSITIVE (0,') OR NEGATIVE (CS') SIMSb
FOR [ON
SlMS Element
AES
0 2+
cs
1019
10i7
1OI6
1017
3 x 10'6
H C 0
+
1017
B
1019 1019
1014-1015
As
loi9
P
I O ~ ~ - I O *3~
1019
S Se Te Au In atoms/cm3. After Werner (1978b).
1017
1oi5
3 x 1015 2 x 10'6 3 x 10'6
1OI6 1015
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
275
high-resolution profiles), yield variations may be neglected in virtually all practical cases. When all of these artifact effects are properly dealt with, SIMS is the most sensitive and precise method for depth-profiling elements throughout the periodic system (hydrogen included). Examples of measured ion implant profiles (Perkin-Elmer, 1979) are shown in Fig. 17. A compilation of practically obtained detection limits in depth profiling is given in Table 111.
V. ELEMENTAL MAPPING The development of surface analytical instrumentation with spatial resolution capabilities makes possible two-dimensional and, by utilization of the sputtering method for removal of the surface layer, even three-dimensional (steric) elemental analysis of surfaces and the surface near volume of a solid sample. The primary information obtained by these instruments is always the two-dimensional distribution of an element-characteristic signal (e.g., Auger electron or secondary ion signal). These two-dimensional distributions have generally been termed Auger or secondary ion “maps,” or, less accurately, “elemental maps” of the particular element to which the instrument has been tuned. In many cases these “micrographs,” as they should correctly be called, give valuable information concerning the distribution of an element across the sample surface. Extreme care should be taken, however, when considering micrographs as directly representative of the local distribution of elemental concentrations. Ideally, the signal intensity across such micrographs should be directly proportional to the local elemental concentration (“concentration contrast”). In general, however, the physical processes involved in the stimulation, emission, and detection of the electron and ion signals from a solid surface lead to a nonlinear relationship between local element concentration and local signal intensity, thus introducing artifacts into the contrast distribution of electron and ion micrographs. Analog and digital methods have been devised, capable of removing, in many cases, “artifact contrast” from electron and ion micrographs, thereby transforming “micrographs” into “concentration maps” of the respective element. A . Auger Mapping
For a comprehensive, surface analysis determination of the surface composition of a point, line, or area with high, uniform sensitivity, high spatial resolution, designed for practical problem solving is important. A scanning Auger microprobe provides an almost nondestructive elemental analysis of
276
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
a specimen surface with atom-layer-depth resolution and lateral resolution in the submicron range. It employs a scanning electron beam (energy, 1-30 keV) as a probe for AES analysis. The diameter of the impinging electron beam can be preselected (see Section VILA) and determines the minimum size of the area analyzed. 1. Instrumentation
An Auger image, or “elemental map,” shows the spatial distribution of a single element over the surface analyzed. It is obtained by setting the spectrometer to a specific Auger peak and scanning the electron beam over the selected area of the sample. The electron beam can be stepped point by point in unit beam diameter steps as done in commercial instruments (Perkin-Elmer, 1979; MacDonald and Waldrop, 1971). The Auger peak intensity above the adjacent background level for each spatial point is measured and stored in a memory. Images are composed of up to 250 lines with 250 points per line and have a number of gray levels discernable in the picture, determined by the signal-to-noise ratio (Browning and Prutton, 1979).If the electron gun is coaxial in the CMA a minimization of misleading effects due to topographic distortions is given. Topography effects can be also minimized by dividing each data-point signal N ( E J by the background signal N(E,). Figure 18 shows an example of computer-generated Auger maps and line scans of a sulfide inclusion in iron (Perkin-Elmer, 1979). One line scan gives a quantitative indication of the elemental distribution across the sample. Selection of horizontal or vertical line location and number of elements per line depend on the analytical problem and uses routine programs in computerized system control.
2. Quantitation of Auger Maps By elemental mapping, the relative concentrations of the various constituents in a sample can be determined with high lateral resolution. Use of quantitation routines for the Auger signal of each data point lead to lateral distribution of the elements over the selected sample area. Generally, the local signal and elemental concentration will not be proportional. The topography of the surface especially determines the local dependency of the Auger electron yield. For different elements the influence of surface topography will partly level off and a first-order approximation can be given. The contribution of backscattered electrons to the Auger signal also depends on topography and can be minimized only if they are not significant or essentially different for the elements of interest (El Gomati et al., 1979).
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
277
I.tm FIG.18. Computer-generated Auget maps of S, Fe, and Mn (left) and line scans (right) of a sulfide inclusion in iron. [After Perkin Elmer (1979).]
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P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
FIG.19. SEM image of a steel fracture surface (top) and Fe Auger images before (center) and after (bottom) topographical correction. [After Perkin-Elmer (1980).]
279
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Application of topographical correction routines using background subtraction results in a correct picture of elemental distribution. Figure 19 shows a scanning electron micrograph (SEM) of a steel fracture surface and Auger images of iron before and after application of this topographical correction routine (Perkin-Elmer, 1980). Additional effects in elemental mapping are vibrations of the sample against the focused electron beam and magnetic stray fields, which should be Minimized also. B. SIMS Mapping 1. Instrumentation
Three basic types of SIMS instrumentation are available for obtaining information on the lateral distribution of elements on a solid surface (see Fig. 20): a. The ion microscope (direct imaging instrument). Here, the sample is bombarded by a large primary ion beam (of the order of 500-pm diam.); secondary ions are emitted from the bombarded area with a local current density distribution related to the local concentrations of the different elements present in the bombarded area. The sample plane is “imaged” in a rigid ion optical sense onto the surface of an ion/electron converter. The optical system performing this imaging consists of an extraction and
U
\I/
(a)
(b)
(C)
FIG.20. Three basic modes of SIM$ Imaging (schematic). The direct-imaging scanning ion microprobe (b) combines the secondary ion extraction system of the direct-imaging ion microscope (c) with the mass filter and detecoon systems of the scanning ion microprobe (a). [From McHugh et a/. (1977).]
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P. BRAUN, F. RUDENAUER,AND F. P. VIMBOCK
immersion lens, a transfer lens system, a (single or double focusing) mass analyzer system, and a projection lens system (Castaing and Slodzian, 1962; Rouberol et al., 1968; Gourgout, 1967). The essential action of the mass spectrometer is to separate out an individual mass-analyzed elemental image from the global image produced by the immersion lens. The magnified elemental ion image at the converter surface is transferred into an electron image with the same current distribution; this electron image may be visually observed or photographed from a scintillator screen. Total lateral magnification is of the order of !OO. Different versions of the ion microscope have been built. The original instrument, designed by Castaing and Slodzian (1962) used a single focusing magnet for mass analysis; owing to the large energy spread of secondary ions, mass resolution was therefore limited to about M / A M = 250. A second-generation instrument, produced by CAMECA S.A. of France, used a double-magnetic prism/electrostatic mirror system to limit the energy bandpass of secondary ions, thereby reducing peak tailing and increasing mass resolution. With the same instrument high-resolution mass spectra (MIAM 5 5000) were obtained using an optional electrostatic sector (Morrison and Slodzian, 1975). Lateral resolution in the imaging mode was of the order of 1 pm or better. The third-generation instrument of CAMECA (the IMS-3F) incorporates a truly double-focusing stigmatic mass spectrometer allowing high mass resolution (MIAM I7500) to be obtained in both the imaging and the mass spectrometric mode; in the latter mode, mass resolution may be increased to about 12,000 (Gourgout, 1977). The primary beam can be focused to small diameters (2 pm) and can be scanned to obtain a flat-bottomed crater; this feature is useful in depth profiling (see Section IV,B,4). b. The scanning ion microprobe. Here the lateral resolution is obtained by scanning a microfocus primary ion beam (of the order 2-pm diam.) across the sample surface. The secondary ions emitted from the sample are collected by an electrostatic extraction optics and mass analyzed by a mass spectrometer. The mass-analyzed secondary ions are usually registered by a high-gain multiplier detector. The detector current may be used to modulate the beam intensity of an oscilloscope, which is scanned in synchronism with the primary ion beam. Thus, an image of the secondary ion current density distribution at the sample surface is produced at the oscilloscope screen, the lateral resolution corresponding to the diameter of the scanning primary ion beam. Since in this type of instrument the mass analyzer does not have to transport image information it can be designed for high transmission (Slodzian, 1979) and high mass resolution (Williams and Evans, 1975); also, nonfocusing types of mass analyzers, such as the quadrupole mass filter, can be used, thus considerably reducing the cost of the total instrument
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
28 1
(Wittmaak, 1978; Riidenauer et al., 1978). The original version of the IMMA scanning ion microprobe of ARL (Liebl, 1967) was the predecessor of various types of commercial (Banner and Stimpson, 1974; Tamura et al., 1970; Wittmaak, 1978) and laboratory instruments (Riidenauer and Steiger, 1974; Riidenauer et af., 1978). c. The image-dissecting ion microprobe. This type of instrument combines features of the ion microscope and the scanning ion microprobe (McHugh et al., 1977). As in the ion microscope a large primary beam and an emission lens system is used which produces a global (non-mass-analyzed) image of the secondary ion current density distribution at the bombarded sample surface. This global image is raster-scanned across an aperture; thus, only ions corresponding to a small surface area are allowed to pass into a mass spectrometer, which, as in the scanning ion microprobe, does not have image-conserving properties and can be designed for high transmission and abundance sensitivity. An elemental image can be obtained by modulating the beam intensity of an oscilloscope, scanned in synchronism with the global image, with the output signal of the mass spectrometer detector. 2. Quantitation of SIMS Maps a. Contrast mechanisms, “Contrast” in secondary ion micrographs, i.e., spatial variations in signal intansity, may be due to several sample- and instrument-related phenomena. On flat surfaces the most prominent contrast effects are the following: ( i ) Concentration contrast; the secondary ion signal only depends on local elemental concentration. When, in addition, this dependence is linear, the ion micrograph directly reflects the distribution of the respective element across the surface. (ii) Matrix contrast; i.e., local variations in absolute and relative ion yields due to local variation in matrix composition or locally variable reactive gas adsorption. (iii) Crystallographic contrqst ; i.e., variations in secondary ion yield due to locally variable orientation of microcrystallites with respect to the sample surface (Prager, 1975; Scilla and Morrison, 1977).
On rough or nonplanar surfaces additional artifacts may be introduced by the following : (iu) Topographic contrast; i.e., local variations in primary beam incidence angle (Kobayashi et al., 1977) and secondary ion collection efficiency due to topographic microstructure (shading effect).
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(u) Chromatic contrast; i.e., element-dependent ion collection efficiency due to element-dependent ion emission energies (Steiger and Rudenauer, 1979). In the presence of a strong ion extraction field the electrostatic potential distribution near microstructures at a sample surface acts to some degree as an energy preselector for the secondary ions; generally, the signal extraction from deep crevices or from behind sharp ridges is easier for slow ions than for fast ions.
In addition to these sample-related effects [(g-(u)], the ion detector itself may cause distortions of the contrast in secondary ion images : (ui) Detector contrast appears to be particularly important in ion microscopes due to the various steps involved in image recording with instruments of this type. Fasset et al. (1977) have taken into account nonhomogeneous detector response and detector nonlinearity due to blackening of photographic film when the ion micrograph is recorded by direct film exposure in the ion microscope and digitized in a scanning microdensitometer. Analog and digital methods have been devised to remove one or more of these “artifact” contrast effects [(i)-(ui)]from an ion micrograph and obtain “corrected maps” more representative of the actual elemental distribution than the original ion micrographs. b. Analog processing of ion micrographs. Kobayashi et al. (1 977) have developed a fast on-line hardware current-division method capable of removing predominantly topographic contrast from ion micrographs produced by a scanning ion microprobe. The experimental arrangement is shown in Fig. 21. The essential feature is a “total ion monitor” situated between the electrostatic and magnetic sectors of the double-focusing TOTAL ION MONITOR
Etec1ron Multiplier I
+
FIG.21. Scanning ion microprobewith total ion monitoring (schematic). [From Kobayashi et af. (1977).]
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secondary ion mass analyzer. Part of the energy-analyzed secondary ion beam impinges on an ion/electron converter plate; the ion-induced secondary electrons are accelerated to a scintillatorfphotomultiplier combination, thus producing a reference current It', which is modulated as the primary ion beam is scanned across the sample; I,' is proportional to the locally emitted total secondary ion current. The central portion of the energy-analyzed secondary ion beam, correspanding to the energy bandpass of the instrument, is passed to the magnetic sector of the spectrometer through a diaphragm in the converter plate. When it is assumed that topographic features alone are causing the modulation in the total emitted secondary ion current, the mass-analyzed element-specific ion current I + , which is detected at the final multiplier detector, can be expected to carry the same local modulation as the reference current Zt+. In the arrangement of Kobayashi et al., the mass-analyzed current I + is referenced to the total ion monitor current I,+ in a fast analog reference circuit. Therefore, topographic contrast will be removed in the I+/Z,' signal. The remaining signal modulation can thus be interpreted as being due to either concentration contrast or a superposition of the artifact contrast effects (other than topographic) described in V,B,2,a. Figure 22 illustrates the effect of on-line total ion referencing on line-scan signals from a fracture surface of steel. Obviously, the modulation in the Fe+ signal and the total ion monitor signal are similar so that only a small modulation remains in the referenced signal ZFe+/It+,leaving open only the possibility for a considerably smaller local variation in Fe concentration than the uncorrected ion signal IFe+ appears to indicate. A similar reduction of topographic contrast has been demonstrated by applying the same technique to correct full secondary ion micrographs ("ion images") (Kobayashi
I
I
0
1
I
100 200 300 ANALYZED DISTANCE ( p n )
I, 400
FIG.22. Line scans across steel fracture surface showing effect of total ion monitor correction; I , + , total ion monitor current; IF,+, Fe current. [From Kobayashi et al. (1977).]
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P. BRAUN, F.
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P. VIEHBOCK
et al., 1977). Since fast electronic circuitry is used for analog division, the time required to record a fully referenced or unreferenced ion micrograph (characteristically, 2 10 msec) only depends on amplifier response speed (generally limited by the I+ signal) ; moreover, both maps are simultaneously available (see Fig. 21) immediately after completion of a scanning frame. c. Digital processing of ion micrographs. Quantitative correction of a set of corresponding locally registered ion micrographs is equivalent to performing a complete quantitative elemental point analysis in each image point of a “scene” (as defined in Section V1,A). Often, it is desired that the quantitation be carried out not only at selected characteristic points or features of the ion micrographs, but that the calculated concentration values be arranged in a “concentration map” of sufficient spatial detail. In this case the huge number of analytical points contained in a scene and the computation time required for calculating absolute concentration values in a single image point (see Section V,B,2,c,v) practically necessitates the extensive use of digital computers for instrument control as well as for data recording and evaluation. Digital image handling and image-processing techniques therefore will have to be widely applied. (i) Global sensitiuityfuctors. Buger et al. (1977) and Schilling and Biiger (1978) were the first to utilize the advantages of digitally recorded spaceresolved secondary ion intensity data for display and quantitation of secondary ion micrographs. Their experimental setup consisted of an ARL IMMA controlled by a PDP-11 computer. The computer, among other functions, controlled the scanning of the primary ion beam (digital two-dimensional step scan) and the secondary ion counting electronics. The ion intensity data were punched on paper tape, which was further evaluated on a larger computing facility in an off-line mode. The quantitation was carried out in each pixel of the input scene (see Section VI,A) and relied essentially on a set of pixel-independent “correction factors” ai applied to the elemental ion currents Zi(x, y ) in each pixel (x, y). The correction factors ai essentially were reciprocal relative sensitivity factors and were defined as
Here, fi,fR and t i ,tR are the secondary ion currents (count rates) and relative atomic concentrations, respectively, of element i and a reference element R, averaged across the full images. The t i were obtained by applying a twointernal-standard global LTE-correction routine to the spatially averaged secondary ion currents ii.The image correction then proceeds as a pixelwise image application of the RSF method. The computer was also employed to manipulate and record micrographs and corrected elemental maps.
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Hard copy-image data were obtained in the form of y-modulated twodimensional images or selected elemental line scans outputted on a digital plotter. Figure 23 shows “quantitative line scans” (note that scale is calibrated in atomic percent) obtained by plotting the results of the correction routine for different elements across a selected line of the image. The sample in this case was modular cast iron ;a single carbon precipitate of 30-pm diam. may be identified in the center of the line scan. Since the correction factors mi are pixel independent, this method obviously cannot account for local variations in the matrix contrast, which can be expected when phases of widely different composition are present in the scanned sample area. Note, however, that topographic contrast will be removed (although, in the particular case of Fig. 23 the sample was polished flat), and that the concentrations (averaged across the field of view) will be correctly returned with an acauracy characteristic of LTE correction.
FIG.23. Line scans across polished&eel surface, quantitated by local application of global sensitivity factors. [From Biiger er a/. (1977).]
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P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
(ii) Internal indicators (MZSR). Internal indicators, or MISR, quantitation requires the ion micrographs of the elements to be quantified, plus micrographs of two matrix ion species as input data. Additional a priori information required is the knowledge of the MISR curves (RSF versus internal indicator or MISR, respectively) for all elements of interest (see Section III,B,l,d). The first step in a MISR-image quantitation is the calculation of a “ratio map” (ratio of matrix ion species intensities in each pixel); for the second step, local concentration values are calculated by referring to the RSF versus MISR curves for the respective elements. For ease of computation the RSF curves are stored in lookup table format containing RSFs for the unknown elements at suitable intervals of the MISR. Between these points RSFs are linearly interpolated. Drummer and Morrison (198 I) applied this technique to obtain elemental concentration maps of minor elements in low-alloy steel samples and demonstrated an analytical accuracy of the order of 30% relative error, which is of the same order as the sampling error which can be expected in the particular microsampling situation. Note that the applicability of a certain set of MISR curves in a particular analytical point is ensured only when the local sample matrix is similar to the matrix in which the particular MISR curves have been determined. This is additional a priori information which may not always be available in an unknown sample. The application of the same set of MISR curves in all different phases (inclusions etc.) of a multiphase sample might considerably reduce analytical accuracy. ( i i i ) Local LTE correction. Quantitative elemental point analysis by means of simplified two- or one-parameter LTE (Andersen and Hinthorne, 1973; Simons et al., 1976; Riidenauer et al., 1969; Morgan and Werner, 1978c) requires as input data the monoatomic elemental ion currents of all elements present in the analytical sample point, as well as absolute atomic concentration values for one or more internal standard elements (see Section III,B,3). Extending this to LTE analysis of a two-dimensional sample area, the following input data are needed (see Fig. 24) :
(a) Registered ion micrographs of all elements present in the imaged sample area (“input scene”). These should be already available in digitized form, i.e., a digitized ion current (ion count) value per element in each of the N ( x ) , N ( y ) pixels and should be corrected for detector distortion. (b) Absolute concentration values for one or more internal standard elements in each pixel; these values may be thought of being arranged in two-dimensional “internal standard maps.” These maps should already be registered with respect to the ion micrographs.
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
ion micrographs
..........
I L T E correction
287
IMi ( N ( x ) , N(y))
1 .....
internal standard map( 6)
SMj IN(x), N(y))
concentration maps
CMj (N(x), N(y)) FIG.24. Local LTE correction of ion micrographs. [From Steiger and Riidenauer (1979).]
Now, in each of the N ( x ) , N ( y ) pixels, corresponding ion currents and concentration values from the set of input micrographs and internal standard maps are sent through a LTE correction routine, yielding absolute concentration values for all elements for which ion micrographs are available. Thus, computed elemental “concentration maps” are obtained showing the “true” two-dimensional distribution of elements across the imaged sample area. Obviously, correction of secondary ion micrographs generally requires “local standards” on a microscale as compared to the “global standards” used in conventional LTE analysis of flat, compositionally homogeneous samples. There, internal standard concentrations may be derived from a bulk analysis of the sample by other standardized analytical techniques. Such an approach may be taken in image correction only if it can be assumed that the element selected as global internal standard is homogeneously distributed within the imaged sample area so that contrast in the ion micrograph of that standard element is “artifact contrast” in the sense discussed above. In all the other cases, local standards have to be defined in each pixel.
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New techniques have to be devised for this purpose, e.g., application of multiple space-resolved analytical techniques on the same sample (electron microprobe, Auger microprobe, etc.), or the use of ion-implanted local or global standards. Riidenauer et al. (1979), Riidenauer and Steiger (1977), and Steiger and Riidenauer (1979) have used two- and one-parameter LTE algorithms with internal concentration normalization (Cci = 1 ; see Section III,B,2) to correct elemental line scans and full ion micrographs, taken from fractured glass surfaces. In these samples it could be ascertained by optical techniques that the elemental oxygen of the glass matrix was homogeneously distributed and therefore was a suitable “global” internal standard element. Results of the one-parameter, one internal standard correction procedure are shown in Fig. 25. Note the following important facts : (a) Contrast distributions of corresponding ion micrographs and concentration maps are often grossly different so that interpretation of a single ion micrograph in terms of quantitative element distribution may lead to entirely wrong conclusions. (b) Since the ratio of two local elemental ion currents depends exponentially on the “temperature” parameter T, the strong contrast in the T-map indicates a strong nonproportionality of local elemental ion currents across the analytical area and, therefore, the presence of other than only topographic contrary effects ;a global sensitivity-factoralgorithm would therefore give inaccurate results on this particular sample. In the computer-generated images of Fig. 25 the local brightness is proportional to the registered number of ion counts/pixel (ion micrographs) or to absolute atomic concentration, respectively (concentration maps). The images were generated on a bistable storage oscilloscope from image data stored on a magnetic disk by suitably quantizing the pixel intensities and point-density modulation of the oscilloscope screen. The image-processing software package also allows for continuous “zooming” and “roaming” of a selected subimage, image interpolation to a finer image raster (for smoother display appearance), display of isointensity lines (ion count or concentration), contrast modification by arbitrary selection of quantitation levels, etc. (Steiger and Riidenauer, 1979). (iu) Sputter normalized L TE (SNLTE).Drummer and Morrison (1981) used the simplified LTE algorithm described in Section III,B,2 for pointwise correction of ion micrographs. This method requires in each image point, the apriori knowledge of the concentration of at least one internal standard element and the a posteriori measurement of the total amount of sample atoms sputtered during one frame (exposure) time (local data). The local
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P. BRAUN, F. RUDENAUER,AND F. P. VIEHBOCK
internal standard was generated by implanting fluorine (as BF3) in a maskgenerated pattern into the sample (NBS-SRM-664, low-alloy steel). At the low doses used (<9 x 1015 atoms/cm2), the implanted fluorine ions are practically quantitatively retained in the sample (Gries, 1979a,b). The number of sample atoms sputtered was determined by a global measurement of the crater depth. Consequently, the LTE results thus obtained are resting on the assumption of a locally constant sputtering rate. This assumption may not be rigorously justified in a multiphase sample. In this particular sample the absolute concentration values of the alloy components were calculated correctly (within accuracy limits characteristic for the LTE analysis) using the local fluorine implant standard ; alternatively, the locally variable concentration of the implanted fluorine was correctly returned using the certified concentration of iron as internal standard. In many cases, however, the effects of the variations in local sputtering speed on the analytical accuracy will be outweighed by the inherent inaccuracies of the LTE algorithm (see Section III,I3,3). ( 0 ) Computational considerations. In most of the laboratories engaged in digital correction of ion micrographs the correction routines are run on dedicated microcomputers which also are used for instrumental control and data acquisition. Consequently, image correction requires considerably longer computation times than might be possible on larger mainframes. The next generation of minicomputer-based SIMS image quantitation facilities probably will be fitted with larger main memories and mass storage devices (disk packs, tape stations, etc.). High-speed data networks, coupling the dedicated mini- or microcomputer running the ion probe to a fast mainframe central computing facility will be an alternate setup to perform fast quantitation of ion micrographs. In Table IV effective average computation times for quantitation of a 128 x 128 pixel scene (ca. 5 elements) are listed for different correction methods and hardware configurations. C . Elemental Mapping Using Other Ion Methods
Elemental mapping techniques in SIMS and Auger analysis have been developed to a very advanced stage and are widely applied in a practical analytical environment. The imaging capabilities of the other ion methods described in Section II,B are based on the fact that any primary ion beam, be it the low-energy beam in ISS or the mega-electron volt beam in RBS, can be focused to small diameters and raster scanned across the sample; the respective analytical signal can be recorded as a function of primary beam position thus building up an “image” of the element under investigation. Even with a focused stationary beam the sample can be raster scanned under the beam to yield information on elemental distribution. The practical
TABLE IV COMPUTATION TIMES FOR Computer
Algorithm
Timeiscene
MICROGRAPH OF TYPICAL SIZE" Remarks
Reference
PDPll/34; 48k words
CARISMA
90 hr
44 hr 40 min
PDP11/20; 24k words
Simplified two-parameter LTE Fast one-parameter LTE Int. arithm. Flt. pt. arithm. Arr. proc. SNLTE
D. Newbury (private communication, 1978) Steiger and Riidenauer (1979) Steiger and Riidenauer (1 979)
8 min 32 secb 4.8 hr (4.0 hr) (10 min) 20 min (5 min) (3 min) > I 2 sec (>2.5 sec)
W. Steiger (unpublished data, 1979) Steiger and Riidenauer (1979) Drummer and Morrison (1980) Drummer and Morrison (1980) Drummer and Morrison (1980) Drummer and Morrison (1980) Drummer and Momson (1980) Drummer and Morrison (1980) Kobayashi et 41.(1977) Kobayashi er nl. (1977)
MISR
Analog circuit
~
CORRECTION OF ION
Analog processing (tot. ion monit.)
~
' 128 x 128 pixels; ca. five elements.
* Estimate.
Total time/scene T map Each element Total timeiscene Ratio map Each element Total timeiscene Each element ( 20 msec/line)
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P. VIEHBOCK
problems with these techniques, however, are in the difficulties of focusing sufficient ion current in small beam spots and of collecting sufficient signal counts to obtain images with tolerable noise levels. Imaging in ISS has been achieved in one case by raster scanning a 100-pm primary beam and synchronously detecting the scattered ion signal in a fixedenergy window corresponding to an elemental scattering peak. A field of view of the order of 1 mm is obtainable (Minnesota Mining and Manufacturing Co., 1979; Rush et al., 1974). With a secondary count rate of the order of lo3 counts/sec, it can be estimated that an image consisting of 100 x 100 pixels with an average signal-to-noise ratio of 10 is built up in a measurement time of the order of 20 min. RBS imaging has been performed by step scanning the target under a stationary beam of dimensions 2 x 2 pm2; the small beam is obtained by suitably aperturing a 10-4A primary He beam so that the target current is of the order of lo-’’ A. Main-element imaging with 2-pm resolution and a 64 x 64 pixel data matrix (field of view, ca. 320 pm2) has been performed in a measurement time of 15 min (Bayerl and Eichinger, 1978). The “proton microprobe” at Heidelberg (Bosch et al., 1978) achieves beam dimension of 2 x 2 pm’ and ion currents of up to 100 pA (for H’) by ion optical focusing of an apertured beam in a small magnetic quadrupole doublet lens. Protons of heavier atoms (e.g., I6O4+) from a 6-MeV Van de Graaf accelerator can also be focused. The system is mainly used for local analysis of small sample5 by means of proton-induced X rays (PIXE). Detection limits of the order of 10 ppm for elements with 2 > 12 have been obtained: A similar PIXE microprobe has been described by Wilde et al. (1978). Suter et al. (1979) have described a 3-MeV small beam facility used for PIXE imagiag. For protons, beam sizes of the order of 30 pm2 have been obtained using magnetic quadrupole doublets for focusing, and electrical deflection for digital raster scanning; field of view is 1.5 x 1.5 mm; images are stored in digital form in an MCA; instrument control is via a PDP-15 microcomputer.
VI. THREE-DIMENSIONAL ISOMETRIC ELEMENTAL ANALYSIS Any surface-sensitive imaging (or quasi-imaging) analytical method can be extended to perform three-dimensional analysis of a solid by combining it with sputter removal of surface layers. When elemental mapping is performed at certain well-defined time intervals during the sputtering process, the two-dimensional elemental maps can be “stacked” to yield a threedimensional distribution of a certain element(s) in a surface near volume of the solid. The main problems with the implementation of this analytical
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mode are of a technical rather than a principal nature and are related to the large amount of image data to be handled and to the difficulties of efficiently displaying three-dimensional information. Although three-dimensional mapping data can and have been presented as a series of two-dimensional analog elemental maps (photographs), each corresponding to a certain depth below the surface, this procedure is cumbersome, time consuming, and can be considered inefficient in the sense that the time required for presentation and display of data may be orders of magnitude larger than the instrument time required for actual data acquisition. It therefore appears that significant progress in three-dimensional Characterization and analysis of solids is only possible when data acquisition and display are fully computerized and ion or electron intensity data can be converted to digital form directly in the dataacquisition process. A . Terminology Used in Multidimensional Digital Imaging
Space-resolved solids characterization and analysis can be performed in four “primary” modes (Fig. 25): point, line, image, and volume analyses. All of these primary modes require knowledge of the lateral position and extent of the sampled analytical area. Knowledge of the depth scale (with respect to the original sample surface) is then available through measurement of local primary current dose and calculation of local sputtering rate, but it may not be of primary interest in any mode other than volume analysis. Other “operating modes” as defined in the literature (Benninghoven et a / ., 1981) (e.g., static SIMS, bulk analysis, depth profiling) do not, in principle, require lateral definition of the analytical volume and are therefore not included in this section. Information equivalent to that obtained in bulk analysis or depth profiling can also be obtained from the volume analysis mode by integrating over individual “planes” or the total cuberille (see below), respectively. 1. Point Analysis Mode (Zero Dimensional)
In the analytical point under consideration, element-specific signals (e.g., peak heights, peak/peak heights, peak areas) have to be collected for all elements ( A , . . ., N ) of interest. This data collection is termed the “spectrum” (mass or Auger spectrum) in the analytical point. 2. Line Analysis Mode (One Dimensional)
This mode can be considered as a series of point analyses performed along a straight line in the plane of the momentary sample surface (e.g., the x coordinate). The spatial exteot of this “analytical line” is called the line
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POINT ANAl Y
I
u
1 A<-
1 “point”
+/
.... +
element *& \
“
LINE ANALYSIS 1 ”channel” 1”linescan”
element~~N*~ element.A* /
”spect rum ”
“
element -Nt@
"line spectrum’’ I
2-DIMFNSIONAI A N A l m Y
Y
..... element aNiv
X
”scene”
3-DIMENSIONAI ANALYSE V
,I ”cuberille”
Y
.I “voxel“
.....
1 “plane” (.image) element*Ai* \
Z
.,
element uN* /
”volume” 63D scene)
FIG.26. Four “primary modes” of space-resolved multidimensionalsolids analysis.
amplitude, each individual spatial quantization element along the line is a “channel,” the element-specific signal is the “channel count,” all channel counts for one element constitute a “line scan,” and the collection of line scans for all elements constitutes a “line spectrum.”
3 . Image Analysis Mode (Two Dimensional) This mode is equivalent to a series of point analyses performed in the points of a two-dimensional grid raster at the momentary sample surface. Practical digital beam scanning or image digitizing systems (Fassett et al., 1977) almost exclusively use rectangular grids, not necessarily with equal numbers of linear spatial quantization elements in both coordinate axes.
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29 5
In the case of such a rectangular “data grid,” the spatial extent of the scanned area in the direction of the x and y axes at the sample surface is called the “x amplitude” and “y-amplitude,” respectively, the areal quantization element a “pixel” (picture element), and the array of all pixels for one element an “image”; the collection of images for different elements, in the same surface area is a “scene.” When the data in all elemental images correspond to exactly the same spatial location at the momentary sample surface the images or scene are said to be “registered.” 4. Volume Analysis Mode (Three Dimensional)
This mode is equivalent to a series of point analyses performed in the grid points of a three-dimensional grid. One set of planes in this grid (x,y planes) is usually parallel to the original sample surface (in the case of flat samples), the third coordinate axis ( z axis) is perpendicular to the sample surface so that a parallelepipedic section of the near-surface volume is covered by the analytical points. The smallest spatial quantization elements are the “voxels” (volume elements). The spatial extent of the sputtered volume in the direction of the x, y , z axes is called the ‘‘x amplitude,” “ y amplitude,” and “sputter depth,” respectively. The array of all voxels for a certain element is called a “cuberille,” the set of cuberilles for all elements analyzed is a “volume.” Obviously, the measurement of a (three-dimensional) cuberille is the only inherently destructive mode of the four operational modes described above. When one desires to measure three-dimensional element distributions using the quasi-nondestructive SAM-method, the sample has to be homogeneously eroded by ion beam sputtering of an area exceeding the area scanned by the SAM electron beam. A sample with topographically structured original surface will generally develop changing surface topography owing to a locally variable sputtering yield (locally variable composition). The analytical points in such a case will therefore be arranged irregularly in the sputtered volume. It is therefore obvious that spatial registration of raw data may be essential in threedimensional analysis of inhomogeneous nonisotropic and topographically structured solids. Such a registration, however, has, to the author’s knowledge, not yet been applied in an actual analysis, and it has been common practice to display the analytical information as it is contained in the “data grid,” i.e., in equidistant planes, respresenting successive scanning frames, with the same metric as the scanning pattern of the microprobe or digitizing equipment, respectively (Schilling and Biiger, 1978; Steiger and Riidenauer, 1979; Riidenauer and Steiger, 1977; Werner, 1976). Examples of applications where three-dimensional registration of data would be of great use
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are particulate analysis, analysis of fracture surfaces, and three-dimensional analysis of microelectronic circuits. B. Practical Examples of Three-Dimensional SIMS Analysis It is within the capability of the SIMS method to follow the distribution of an element simultaneously in the “lateral” (parallel to the surface) and in the “depth” dimension. This type of “three-dimensional isometric solids analysis” can be principally performed in any ion microprobe instrument (imaging or scanning). Information on the lateral distribution (at a given depth) is obtained by the “imaging” capability (see Section V,B,1) ; the depth information is supplied automatically when recording successive ion micrographs from the same sample area, because sample surface layers are removed during recording of an individual ion micrograph. This method of “stacked images” actually has been repeatedly applied in the past (Berkey, 1973; Werner, 1976); in this way, for example, the distribution of inclusions in tungsten metal could be followed with depth (Berkey, 1973).The visualization of a three-dimensional distribution of an element, however, is cumbersome when applying standard image-recording techniques [photographing from an oscilloscope screen, possibly with subsequent digitization using a microdensitometer (Fassett et al., 1977)l. The registration of subsequent photographically recorded images is difficult and the relative location of corresponding features in images from different sample depths is not easily quantifiable. When an ion microprobe is directly connected to a computer which is programmed to record and store a great number of successive ion images of different elements, a new practical dimension in elemental analysis of solids may be realized. Rudenauer and Steiger (Suter et al., 1979) have coupled a scanning ion microprobe to a PDP-11/34 minicomputer via a CAMAC interface; among other instrument parameters, this computer controls the primary beam scan and records elemental images on-line by sequentially counting single ion pulses during each step-scan interval. Thus, digital ion micrographs of different elements can be stored as consecutive sample layers are sputtered away. The amount of information that can be stored is limited only by storage capacity; in the arrangement of Steiger and Rudenauer, up to 17 images of 256 x 256 pixels each can be stored on a magnetic disk. Note that such a three-dimensional “volume” contains all information on the total sputtered volume, obtainable by SIMS. Thus, depth profiles can be obtained from these primary data by integrating voxel counts across individual depth planes ;distribution of elementsin any desired sample cross section can be obtained by considering only voxels intersecting this particular cross-sectional plane ; “line scans” between two arbitrary points
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
297
within the sputtered volume can be obtained by selecting only the appropriate voxel data. Figure 27 shows an example for this three-dimensional analytic capability. Digital ion images for the elements carbon and indium at subsequent sample depths are shown in Fig. 27a and b. The imaged area is a 800 x 800-pm section of a contact pad on a printed circuit card. The last image recorded corresponds to a depth of approximately 1200 A from the original surface. Obviously, a thin inhomogeneous layer of carbon and indium is
FIG.27. Three-dimensional SIMS analysis of a thin surface layer of a PCB circuit board: (a) and (b), coaxial projections of C and In at correspondingdepths (increasing clockwise from upper left); (c) and (d), transaxial projetion of C and In; total sputtered depth ca. 1200 A. All images are uncorrected ion micrographs. Intensity levels for In images: 5, 30, 55, 75, 100, 120, 220 counts/pixel. For C images: 3, 20,40, 60, 80, 90, 150 counts/pixel
29 8
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
covering the contacts; the two elements tend to accumulate in different regions of the sputtered volume. An exact description of the relative local relationship between these elements, however, is very difficult using only from these images (looking at the sample in the direction of the surface normal). When we rearrange each cuberille in such a way as to permit to look sideways “through” the sample onto a plane perpendicular to the surface (“transaxial projection,” Fig. 27c), one easily recognizes an individual carbon precipitate on top of the superficial indium layer. It must be noted that the data displayed in Fig. 27c are still arranged in the “data grid” rather than the “isometric analytical grid” (Suter et al., 1979). Nevertheless, this information, easily deduced from the transaxial projection, could be derived only with great difficulty from the conventional ion images of Figs. 27a and b alone. This new three-dimensional analysis mode appears to be particularly suited to the analysis of integrated circuits that exhibit threedimensional structure with spatial dimensions (order of 100 in the lateral dimension with local definition in the 1-pm range and depth structure down to a few microns with definition in the 100-A range) accessible to modern ion microprobe hardware.
VII. SENSITIVITY AND RESOLUTION LIMITS A . Sensitivity and Resolution in AES
The sensitivity in AES is determined by the signal-to-noise ratio that can be obtained with the analysis instrumentation. The limit of elemental concentration that can be detected is confined by electron shot noise, which causes the background with the Auger peaks to be superimposed. The background consists mainly of primary electrons, which are scattered back through the surface after loss of energy several times by single and collective electronic excitation (Bauer, 1975). The total backscattered electrons with energy greater than 50 eV are about one third the primary electrons. Assuming the background to be uniform over the energy range from zero to E,, the background current transmitted through a CMA is given by Joshi et af., (1975): IB= O.3TIpAEIE,
where I, is the primary current, T and AE are the transmission and energy resolution of the analyzer, respectively.
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
299
The transmitted Auger current is given by (Section III,A, I):
I,
=
yIpTAE/w
for AE << w
(574
I,
=
yIpT
for AE >> w
(57b)
where w is the Auger peak width. The shot-noise current is given by (Spangenberg, 1948): fN =
(58)
(2eIBB)”’
where B is the instrument bandwidth. From Eqs. (56)-(58) follows l,/IN = ( ~ / W ) ( I ~ T A E E ~ / O . ~ for ~ B )AE ’ ~ ~<< w
(59a)
i S / I N = (y/w)(lpTE,,/0.6 AEeB)’I2
(59b)
for AE >> w
The maximum &/INof about lo4 can be reached under typical operating conditions by choosing AE equal to w . Normally, I,/& will be lowered by a factor of 10, especially by a mixmatch of AE and w. The bandwidth B is related to the time constant for recording (depending on the electronic circuitry) by B
- I/T
(60)
The recording time is determined by the stability of the surface analyzed, which depends strongly on the vacuum conditions. Figure 28 shows the relation between the time needed for build-up of a monolayer of contamination versus the pressure in the test chamber. By this fact the time T limits the
TEST CHAMBER PRESSURE (rnbar)
FIG.28. Relation between pressureand time needed for buildup of one monolayer of oxygen on a clean surface.
300
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
04'*
1
10-10
I
10-9 beam current
1
10-8
I
10-7
1-6
(A1
FIG.29. Dependence of electron beam diameter on the beam current for a lanthanum hexaboride cathode. [From Perkin-Elmer ( 1980).]
signal-to-noise ratio. To compensate for a higher signal-to-noise ratio a higher primary beam current is necessary; but for a certain primary electron current density, 9 = 41,,/nd2 (61) depending on the stability of the target material under the electron beam (Braun et al., 1977), a decrease in spatial resolution will be the result. Figure 29 demonstrates the dependence of the beam diameter from the beam current for a commercial instrument (Perkin-Elmer, 1980). For any surface analyzed the parameters mentioned should be well tuned. A prerequisite are the vacuum conditions which determine the recording time. The primary beam current determines the spot size and therefore the spatial resolution. Both recording time and beam current are responsible for the signal-to-noise ratio, which defines the elemental detection limit. B. Resolution and Sensitivity Limits in SIMS 1. La feral Resolution
As has been pointed out in Section V,B,l, information on the lateral distribution of elements can be obtained by three types of instrumentation : the scanning ion microprobe, the ion microscope, and the image-dissecting ion microscope. The lateral resolution in a mass-separated image produced by any of these techniques is subject to three types of limits : a. Intrinsic resolution limit. This limit is imposed by the fact that the point of impact of a primary ion, owing to the formation of collision cascades near the sample surface, is not identical with the point of emission of the
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
30 1
secondary ion. A value of the order of 150 will be representative for the diameter of the cascades and, therefore, the intrinsic resolution limit (Benninghoven et al., 1981). b. Useful resolution limit. This limit is imposed by the fact that owing to the destructive nature of the SIMS technique, a certain number of atoms of the target material have to leave the surface before enough secondary ions have been accumulated on the detector so that the local secondary ion emission of element i can be determined with a precision p (rel. %). These sputtered atoms naturally occupy a finite volume V of the solid (Castaing and Slodzian, 1962)
v = 104/upZa+(X)c(~)~, (cm3)
(62)
where n is the total number of target atoms/cm3, a+(X) is the degree of ionization, c(X) is the fractional atomic concentration of element X, and I; is the total transmission of the secondary ion mass analyzer (defined as number of ions X being detected per total number of emitted ions X). This limiting volume may have different dimensions in all three coordinate directions. When, however, in three-dimensional analysis it is desired to have equal lateral resolution in all coordinate axes, the “useful resolution” follows from Eq. (62) as 6, =
P I 3 =
104i3(p2na+(X)c(X)1;)-”3(cm)
(63)
c. Ion optical resolution limit. This limit is set up by the imperfections of ion optical lenses and depends on the mode of image formation. For scanning ion microprobes the ion optical limit is imposed by the finite primary beam spot size which can be focused onto the target by the primary ion gun. Figure 30 shows a graph of spot diameters achievable when the conversion angle of the primary beam at the target is limited to the value given at the abscissa. Four sets of lines representing different contributions to spot size (Benninghoven el al., 1981) are shown in the figure: the brightness limit (I/b), determined by the brightness p of the ion gun only; the spherical aberration limit (ds),determined by the spherical aberration of the last primary focusing lens; the chromatic aberration limit (dch),determined by both the energy spread of the primary ion beam and the chromatic aberration of the last primary lens; dzffraction limit (dJ, set up by the wave properties of a particle beam.
It can be seen from Fig. 30 that spherical aberration is dominating at spot sizes of 1 pm and above; begow 1 pm chromatic aberration is the limiting factor for duoplasmatron sources (Benninghoven et al., 198l), whereas for surface ionization sources chramatic aberration becomes dominating only below 100 A. The graph also shows that for a minimum required probe current of lo-’’ A, the minimum obtainable spot size is of the order of
302
P. BRAUN, F. RUDENAUER, AND F. P. VIEHFIOCK
FIG.30. Dependence of spot diameter on angle of convergence at target for microfocus ion beam; focal length of last focusing lens is 2 cm. Chromatic aberration limits shown for ion sources with different energy spread SV/V (duoplasmatron, SVjV = 5 x l W 4 ; surface ionization source, SV/V = 2 x lO-’-EHD source, SV/V = 1.5 x
0.1 pm when duoplasmatron sources are used (Liebl, 1978) (source brightness fi I100 A/cm2.sr) and of the order of 200 A with a cesium surface ionization source (p I1000 A/cm* * sr). A major improvement in spot sizes and focusable ion currents appears to be possible with field ionization or electrohydrodynamic ion sources (Levi-Setti and Fox, 1980; Krohn and Ringo, 1976; Seliger, 1972). In ion microscopes and image-dissecting ion microscopes, the spatial resolution is determined by the properties of the emission lens only. This lens accelerates the secondary ions from the target and forms a virtual image of the illuminated surface area (Slodzian, 1964); the following focusing and dispersing lenses mainly magnify the aberrations of this lens, together with the image, introducing only a small additional aberration. The resolution
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
303
limit minimum set up by the combined aperture and chromatic aberration of the emission lens is given by
hmi, = Z/Fo (cm) (64) where 1 (in electron volts) is the maximum lateral energy of the secondary ion beam, and F, (in electron volts) the electrostatic field strength in the acceleration gap. In practical instruments, F, is of the order of lo4 V/cm and 5 of the order of 1 V, so that lateral resolution limits of the order of 1 pm are obtained. Further reduction of the lateral ion energy (by suitable aperturing in the lens crossover) improves lateral resolution at the cost of transmitted intensity. 2. Sensit iv itv Limits The factors influencing the sensitivity of a SIMS instrument with respect to the capability of trace-element detection can be understood by rearranging Eq. (62)
cmi,(x) = 104/npza+(xiT~v
(65)
essentially connecting the minimum detectable fractional concentration cmin of an element to the sample volume V consumed during an analysis. The larger this volume, the lower is the detection limit of a certain element (at a given precision of measurement); a’(X) cannot be calculated from first principles (see also Section 111,B,2); the instrument transmission can be theoretically calculated, requiring the knowledge of ion optical, geometric, and target parameters (e.g., energy distribution of selected element). Theoretical transmission figures, however, are hardly ever achieved due to unavoidable misalignments and the dependence of energy distributions on the different experimental parameters. Nevertheless, the capabilities of an individual instrument with respect to trace-element detection, depth profiling, and local microspot analysis can be assessed when the practical sensitivities Sp(X) of this individual instrument are known for an element X. Sp(X) is a global measure of secondary ion emission and instrument transmission (see also Section III,B,l) SP(X) = Po<)PPC(X) = T0P+(X)T
(66)
and can be easily determined experimentally by measuring the detected ion current Z’(X) for element X [present with concentration c@)] at a given is the total sputter ion yield primary ion current Z, on bulk standards; of the sample. Inserting Eq. (66) into Eq. (65) gives the trace-element detection limit in bulk analysis
xot
c(X)min
v
= 1O4 Kot/np2Sp(X)
(67)
304
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
The sensitivity in the depth-profiling and spot-analysis modes can be obtained from Eq. (67) by substituting Ad and h 3 , respectively, for the consumed sample volume V. In depth profiling, therefore, c(x),, = 1041;,,/np2s,(x)~d
(68)
where A is the surface area from which ions are admitted into the mass spectrometer ( A may be smaller than the bombarded surface area, owing to electronic or mechanical aperturing; see also Section IV,B,4), and d is the differential layer thickness necessary to detect an element of concentration c(X) with a precision p . In microspot analysis, = 104
C(X),,,
I;,,/~~~s,(x)s~
(69)
Sn
iSh le
10
1
20
1
1 40
50 60 ATOMIC NUMBER It\
AU
70
eo
I 90
too
FIG. 31. Practical sensitivity (relative secondary ion yield) (a) for positive (M') and (b) negative (M-) secondary ions; bombarding ions, 13.5-keV I6O- and 16.5-keV Cs+,respectively (in units of counts/sec/nA). [From Storms el al. (1977).]
305
SURFACE ANALYSIS USING CHARGED-PARTICLE BEAMS
where 6 is the diameter of the (spherical) volume element sputtered away during the measurement. Values for the practical sensitivity of elements vary from instrument to instrument. Figure 31 shows values for positive and negative secondary ions determined by Storms et al. (1977) on an ARL IMMA instrument. These values are representative of modern microprobe and microscope equipment. Choosing an element with SJX) = lo6 counts/sec/nA ( x detected 108
I
1
1
I
1
5
Si
Au
Hg
I
11
9
Mn
90 ATOMIC NUMBER ( 2 )
FIG.3 I b.
306
P. BRAUN, F. RUDENAUER, AND F. P. VIEHBOCK
secondaries per primary ion) and a signal-to-noise ratio of 3, a 10-ppb impurity can be profiled with a 1000-A depth resolution when the apertured area is of the order 300 x 300 pm’; alternatively, a 100-ppm impurity can be detected in a volume of 1 pm3. REFERENCES Andersen, C. A., and Hinthorne, J. R. (1973). Anal. Chem. 45, 1421. Andersen, H. H. (1974). In “Physics of Ionized Gases 1974” (V. Vujnovic, ed.), p. 361. Inst. Phys. Univ. Zagreb, Zagreb, Yugoslavia. Andersen, H. H., and Bay, H. L. (1973). Radiut. Ef. 19, 63. Andersen, N., and Sigmund, P. (1974). Mar.-Fys. Medd.-K. Dan. Vidensk. Selsk. 39, No. 3. Banner, A. E., and Stimpson, B. P. (1974). Vacuum. 24,511. Bauer, E. (1975). In “Interaction on Metal Surfaces” (R. Gomer, ed.), p. 225. Springer-Verlag, Berlin and New York. Baun, W. L. (1978). ASTM Spec. Tech. Publ. STP 643, p. 150. Bayerl, P., and Eichinger, P. (1978). Nucl. Instrum. Methods 149,663. Behrisch, R., and Scherzer, B. M. U. (1973). Thin Solid Films 19, 247. Benninghoven, A. (1970). 2. Phys. 230,403. Benninghoven, A., Riidenauer, F. G., and Werner, H. W. (1981). “Secondary Ion Mass Spectrometry” (in press). Berkey, E. (1973). In “Microstructural Analysis; Tools and Techniques” (J. L. McCall and W. M. Mueller, eds.), p. 287. Plenum, New York. Betz, G. (1980). Surf. Sci. 92,283. Betz, G . , Dobrozemsky, R., and Viehbock, F. P. (1971). Int. J . Muss. Spectrom. Ion Phys. 6.45 I. Betz, G., Braun, P., and Farber, W. (1977a). J . Appl. Phys. 48, 1404. Betz, G., Farber, W., and Braun, P. (1977b). Int. Con$ At. Coflisions Solids, 7th, 4977 p. 50, Vol. 11. Betz, G., Arias, M., and Braun, P. (1980). Nucl. Instrum. Methods 170, 347. Blaise, G.,and Nourtier, A. (1979). Surf. Sci. 90,495. Boersch, H., Miessner, H., and Raith, W. (1962). 2. Phys. 168,404. Bosch, F., et ul. (1978). Nucl. Instrum. Methods 149,665. Braun, P. (1979). Vuk.-Tech.28, 76. Braun, P., and Farber, W. (1975). Surf. Sci. 47, 57. Braun, P., Farber, W., Betz, G., and Viehbock, F. P. (1977). Vacuum 27, 103. Browning, R., and Prutton, M. (1979). Phys. Technol. 10,259. Brundle, C. R. (1 974). J . Vuc. Sci. Technol. 11, 2 12. Biiger, P. A., Schilling, J. H., and Fidos, H. (1977). Pup. Int. Secondary Ion Mass Spectrosc. Conf., Ist, 1977, unpublished. Carlson, T. A. (1975). “Photoelectron and Auger Spectroscopy.” Plenum, New York. Castaing, R., and Slodzian, G. (1962). J . Microsc. (Paris) 1, 395. Chang, C. C. (1971). Sui-6 Sci. 25, 53. Chu, W. K., Howard, J. K., and Lever, R. F. (1976). J . Appl. Phys. 47,4500. Connell, G . L., and Gupta, Y. P. (1971). Muter. Res. Stand. 11,8. Dahlgren, S . D., and McClanahan, E. D. (1972). J . Appl. Phys. 43, 1514. Davis, L. E., MacDonald, N. C., Palmberg, P. W., Riach, G . E., and Weber, R. E. (1976). “Handbook of Auger Electron Spectroscopy.” Physical Electronics Ind., Minnesota. Drawin, H. W., and Felenbok, P. (1965). “Data for Plasmas in Local Thermodynamic Equilibrium.” Gauthier-Villars, Paris.
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Rush, T. W., McKinney, J. T., and Leys, J. A. (1975). J . Vac. Sci. Technol. 12,400. Schilling, J . H., and Biiger, P. A. (1978). Int. J . Mass Spectrom. Ion Phys. 27, 283. Scilla, G. J., and Morrison, G. H. (1977). Anal. Chem. 49, 1529. Seliger, R. L. (1972). J . Appl. Phys. 43, 2352. Sevier, K. D. (1972). “Low Energy Electron Spectroscopy.” Wiley (Interscience), New York. Shimizu, R., and Saeki, N. (1977). Surf Sci. 62, 751. Sigmund, P. (1969). Phys. Rev. 184,383. Sigmund, P. (1977). In “Inelastic Ion Surface Collisions” (N. H. Tolk, J . C. Tully, W. Heiland, and C. W. White, eds.) p. 121. Academic Press, New York. Sigmund, P. (I98 I). In “Sputtering by Ion Bombardment-Physics and Applications” (R. Behrisch, ed.), Vol. I, Chapter 2. Springer-Verlag, Berlin and New York. Simons, D. S., Baker, J. E., and Evans, C. A. (1976). Anal. Chem. 48, 1341. Slodzian, G. (1964). Ann. Phys. (Paris) [I41 9, 591. Slodzian, G. (1979). In “Secondary Ion Mass Spectrometry, SIMS 11” (A. Benninghoven et al., eds.), p. 81. Springer-Verlag, Berlin and New York. Smith, D. H., and Christie, W. H. (1977). Oak Ridge Naif. Lab. [Rep.] ORNL-TM (US.) ORNL/TM-5728. Smith, D. H., and Christie, W. H. (1978). Int. J . Mass Spectrom. Ion Phys. 26, 61. Smith, D. M., and Gallon, T. E. (1974). J . Phys. D 7, 151. Smith, D. P. (1967). J . Appl. Phys. 38, 340. Spangenberg, K. K. (1948). “Vacuum Tubes.” McGraw-Hill, New York. Steiger, W., and Riidenauer, F. G. (1979). Anal. Chem. 51,2107. Storms, H. A,, Brown, K. F., and Stein, J. D. (1977). Anal. Chem. 49,2023. Suter, M., Bonani, G . , Jug, H., Stoller, C., and Wolfli, W. (1979). IEEE Trans. Nucl. Sci. NS-26, 1373. Szymonski, M., Bhattacharya, R. S., Overeijnder, H., and de Vries, A. E. (1978). J . Phys. D 11, 751. Tamura, H., Kondo, T., Doi, H., Omura, I., and Taya, S. (1970). In “Recent Developments in Mass Spectroscopy” (K. Ogala and T. Hayakawa, eds.), p. 205. Univ. of Tokyo Press, Tokyo. Taylor, N. J. (1971). Bull. Acad. Phys. SOC. 16, 1353. Tsuruoka, K.,Tsunoyama, K., Ohashi, Y., and Suzuki, T. (1974). Jpn. J . Appl. Phys., Suppl. 2, Pt. 1, 391. Verbeek, H., and Eckstein, W. (1980). Proc. Symp. Suyf. At. Phys., Znd, 1980, p. 14. Weber, R. E. (1970). Solid State Technol. 13, 49. Wehner, G. K.(1975). In “Methods of Surface Analysis” (A. W. Czanderna, ed.), p. 5. Elsevier, Amsterdam. Wehner, G. K., and Hajicek, D. J. (1971). J . Appl. Phys. 42, 1145. Werner, H. W. (1969). Dev. Appl. Spectrosc. 7A, 239. Werner, H. W. (1974). Vacuum 24,493. Werner, H. W. (1976). Acta Electron. 19, 56. Werner, H. W. (1977a). Proc. Int. Vac. Congr., 7th, 1977 p. 2135. Werner, H. W. (1977b). Pap. Secondary Ion Mass Spectrom. Int. Conf., Ist, 1977, unpublished. Werner, H. W. (1978). Pittsburgh Meet. Anal. Chem. Appl. Spectrosc. Paper No. 341. Werner, H . W. (1980a). Muter. Sci. Eng. 42, 1. Werner, H. W. (1980b). Surf. Interface Anal. 2, 56. Werner, H. W., and Morgan, A. E. (1976). J . AppC Phys. 47, 1232. Wilde, H. R., Roth, M., Uhlhorn, C. D., and Gousier, B. (1978). Nucl. Instrum. M e f h o h 149, 675.
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Williams, P., and Evans, C . A. (1975). NBS Spec. ( U S . ) Publ. 427. Wittamaak, K. (1978). Adu. Muss Spectrom. 7, 758. Zeller, M. V. (1980). “Seminar on Sputtering and Plasma Etching and Surface Analysis.” Perkin-Elmer, Vaterstetten, FRG. Ziegler, J. F., ed. (1975). “New Uses of Ion Accelerators.” Plenum, New York.
ADVANCES IN ELECTRONICS A N D ELECTRON PHYSICS, VOL. 57
Microwave Landing System : The New International Standard HENRY W. REDLIEN
AND
ROBERT J. KELLY
Communications Division The Benciix Corporation Towson, Maryland
I. Introduction. . . . . . . .
311 312 312 314 316 317 318 319 319 320 320 320 32 1 322 322 327 327 328 C. Distance-Measuring Equipment . . . . . , 357 358 358 B. Error Analysis.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372 C. Power-Budget Considerations, . . , . , . , . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . 393 D. Siting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394 F. Examples of MLS Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399 F. Monitoring . . 403 V. Definitions . . . . . . . . . 405 References . , , . . . . . . 406
A. Instrument Landing System (ILS B. The Need for MLS as a Replacement for ILS . . . . . . C. The Landing Operation with ML D. The Operational Application . . . E. The Development Process . . . . . . . . . . . . . . . . . . . . . . , . . . . . , , . . . . . , . . . . . . . . . . F. International Acceptance . . . . . . . G . The Current Process of Standardization . . . . . . . . . , . . . . . . . . . . . . , . . . . . . . . , , . H. The Single Accuracy Standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I. MLS Transition and Implementation . J . The Emphasis of This Article . . . . . . . . 11. The Operational and Functional Requirement for MLS . . . A. MLS Configurations.. . . . . . . . . . . . , , , . . . . . , . , . . . . . . . . . . , , . , , , . . . . , , . . . . . B. Angle and Range Coverage. . . . . . . , , , . . . . . . . . . , . , . . . . . . . . . . . . . . . . . . . . , . . C . Accuracy at Threshold and over the Coverage Volume . . . . . . . . . . . . . . . . . . , , . 111. Description of the Microwave Landing System . . . . . . . . . . . . . . . . , . . , . . . . . . A. Overall System
.
I. INTRODUCTION
In April, 1978, a worldwide meeting of the All Weather Operations Division (AWOD) of the International Civil Aviation Organization (ICAO) selected the time reference scanning-beam (TRSB) technique from several alternatives to be the new international standard microwave landing system 31 I Copyright Q 1981 by Academic Press, Inc All rights of reproduction in any form reserved ISBN 0-12-014657-6
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(MLS). MLS has been configured to overcome certain limitations and is to replace the current ICAO standard instrument landing system (ILS), which has provided approach guidance for aircraft for more than 40 years. ILS is currently located at 623 runways in the United States and at 750 other runways throughout the world. The selection of the new MLS system resulted from over 10 years of activity in the United States and over six years in ICAO. ICAO is currently preparing definitive specifications for MLS called SARPS (standards and recommended practices), which will assure a uniform quality of landing guidance at the facilities of the 172 member nations. A . Instrument Landing System (ILS)
ILS is an “air-derived” system where ground transmitters radiate signals which are then received in the landing aircraft to derive a lateral displacement from the runway centerline and a vertical displacement from a given glideslope profile (11,53, 66). ILS operates at 110 MHz for its lateral guidance (localizer) and 330 MHz for its vertical guidance (glideslope). The localizer antennas, ranging from 50 to 150 ft, provide relatively wide beams of 6-10” along a single approach path. These wide localizer beams are subject to interference from structures on and near the airport. The glideslope antenna, which consists of two or more elements mounted on a mast, require reflection from the airport surface to form the guidance signal. A flat surface, free of obstacles, at least 1500 ft in front of the antenna is needed for proper operation. Although obstacle-free environment and flat surfaces are available at many airports and good service is supported, there are many airports which cannot provide these conditions. Beam bends occur at some airports which limit operations. Some sites have such irregular terrain that ILS cannot be installed. Also, ILS does not always provide accurate guidance signals to the very low altitudes required for automatic “hands-off’ landings. B. The Need for M L S as a Replacement for ILS MLS has been configured to overcome these limitations by the use of high frequencies or “microwaves” (5000 MHz), which can develop very narrow beams of 0.5-3” from antennas of reasonable sizes, 4-12 ft in standard applications and to a maximum of a 24-ft flare antenna for one form of autolanding guidance. The narrow beams can avoid most airport structures and they do not require the flat surface in front of the vertical guidance elements. In particular, the azimuth antenna, which has a “fan” beam narrow in the horizontal direction and a wide beam in the vertical direction, can be designed to have a very sharp “cutoff’ on its lower side. This cutoff reduces
MICROWAVE LANDING SYSTEM
313
radiation toward the airport surface to avoid reflections and distortion of the guidance beam. MLS antennas can be installed in a wider variety of airport environments than ILS and provide a better signal quality to lower altitudes on the approach path. In particular, the ICAO SARPS has specified a single high-quality accuracy and coverage standard at every runway end for equipment installed by ICAO states. Accuracy will be sufficient to perform autolanding on a regular basis. In addition, MLS has been designed to provide guidance over a wide range of azimuth angles up to f60" from the runway centerline and up to 30" in elevation from the airport surface. Distance measurement equipment (DME) is included as an essential element of the system to provide range from the runway. The necessary information is available to determine the three-coordinate position from the runway over a wide-coverage volume. Figure 1 shows the single-path guidance of ILS and Fig. 2 shows the volume coverage of MLS (57, 72-74).
FIG.1. The conventional instrument landing system (ILS) has a single approach for landing guidance.
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HENRY W. REDLIEN A N D ROBERT J. KELLY
FIG.2. The microwave landingsystem(MLS) is designed to provide wide-angle coverage for elevation, front azimuth, back azimuth, and flare in the expanded application.
C . The Landing Operation with MLS
The landing process consists of curved paths for noise abatement, vertical curved or segmented guidance, and the transition to the final centerline approach (Fig. 3a). The decision heights where the pilot must be able to “see to land” are 200- and 100-ft ceilings-categories I and 11, respectively. The flare maneuver, touchdown, and rollout complete the landing. MLS must support each of these maneuvers under “instrument flight rule” conditions. The elevation element, located 1000 ft from threshold and offset from the centerline by 400 ft, provides elevation guidance to the decision window in category I and I1 operations and to threshold in category I11 (zero ceiling). The approach-azimuth element supports lateral guidance to the decision window in category I and I1 operations and to touchdown and rollout in
MICROWAVE LANDING SYSTEM
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category 111. The flare maneuver is typically performed manually for categories I and I1 by visual reference, but, for category 111, positive azimuth and vertical control are required to touchdown. The autoland maneuver is facilitated with MLS by its increased accuracy which is extended to lower elevations further down the landing path. Flare to touchdown requires the height above the runway and in MLS, it may be provided by a combination of the flare elevation angle and DME. Current ILS practice obtains height from a radar altimeter after a transition from the glideslope antenna typically occurring prior to runway threshold. Because of the limited low-altitude coverage of the ILS, flat terrain is required over an extended area for accurate radar altimetry. The extended lowaltitude coverage of MLS permits this transition to be made at lower altitudes closer to threshold without the need for extensive flat terrain.
/-/-
- -------_
FIG.3a. The landing operation with MLS.
MISSEO APPROACH A2
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HENRY W. REDLIEN AND ROBERT J. KELLY
4-
0 - 4 0 30
,
1
1
,
I
40
50
60
70
80
90
100
1
1
1
I
I
110
120
130
140
150
AIRCRAFT VELOCITY (KTS)
FIG.3b. Approach angle capability for range of pilot acceptable sink rates. Each aircraft shown at minimal velocity. H i s sink rate.
D . The Operational Application
MLS is intended for universal international application serving both civil and military users. It is to provide landing guidance for all classes of airports and all classes of aircraft including air carriers, vertical and short takeoff and landing aircraft, business and light aircraft of general aviation as well as the wide variety of military aircraft. Figure 3b shows the maximum
MICROWAVE LANDING SYSTEM
317
and preferred sink rates for a large number of different aircraft types, requiring the selectable glideslope angles which can be provided by MLS. MLS will support service at the large commercial airports with categories of service which permit all-weather landings as well as curved or segmented paths to increase the number of aircraft landing in high-traffic situations. For simpler applications, simplified equipment will permit inexpensive installation at small airports, which serve the private avaiation sector and the “commuter” airlines. The key to the universal application is the standardized format of the “signal-in-space” which is being specified by ICAO. Every MLS ground transmitter must provide a minimum set of signals in a well-defined format which can be decoded by every airborne MLS receiver. Every MLS receiver will be able to operate with every MLS ground station with at least some minimum level of service. The airborne and ground equipment are to be completely “interoperable.” E . The Development Process
Following the introduction of ILS in the 1940s, there had been continued development of new and improved systems, many operating at microwave frequencies. Developers included the Department of Defense (DOD), the Federal Aviation Administration (FAA), and private industry in the United States, and this activity was matched by international developments. The large number of different systems being proposed led to the formation of Special Committee No. 117 of the Radio Technical Commission for Aeronautics (RTCA) in late 1967 (4). This committee, with a membership covering a wide spectrum of government, military, airline industry, manufacturing industry, and international representatives developed an operational requirement for the new MLS and selected two possible formats from the many proposed : the “scanning-beam” MLS and the “Doppler scan” MLS. The Doppler system had been introduced by the representatives of the United Kingdom. In the United States the recommendations of RTCA SC-117 led to the preparation of a five-year development plan (15,38). The FAA was the lead Government Agency, but the plan was approved by the DOD and the National Aeronautics and Space Administration (NASA), who were to participate in the development program. More than six major industrial contractors participated including ITT Gilfillan, Hazeltine Corporation, Raytheon Company, Airborne Instruments Laboratory, Texas Instruments, and The Bendix Corporation. After study and feasibility demonstrations of different approaches, the FAA led a government-sponsored selection process which chose the time reference scanning-beam technique (TRSB)
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HENRY W. REDLIEN AND ROBERT I. KELLY
in December, 1974. The process was later ratified by an executive review with representatives of FAA, DOD, NASA, and DOT in February, 1975 (22, 24). The Bendix Corporation and Texas Instruments then designed and supplied prototype equipment in 1976 which formed the basis for the United States proposal to ICAO. F. International Acceptance
In 1972, the Seventh Air Navigation Conference of ICAO, meeting in Montreal (17),had decided to proceed with MLS. They adopted an operational requirement for MLS and charged the All Weather Operations Panel (AWOP), which had been established in 1964, to request proposals from member states and make a selection of the most suitable technique. Five major proposals were submitted : Australia Federal Republic of Germany France United Kingdom United States
Time reference scanning beam (TRSB) DME-based landing system (DLS) Air-ground data link system (AGDLS) (withdrawn before final selection) Doppler MLS (DMLS) Time reference scanning beam (TRSB)
At the sixth meeting of AWOP in March, 1977, the time reference scanningbeam technique was selected to be recommended to a worldwide meeting to be held the following year (20). In 1978 over 80 ICAO states confirmed the panel selection in a meeting of the All Weather Operations Division (AWOD) (19). This had followed a year of intensive competition, where the United Kingdom and the United States conducted field demonstrations in Europe, Asia, South America, and the United States and Canada to demonstrate the capabilities of their respective approaches (33). The MLS program was characterized by intense competition both in the United States selection process as well as in the international selection in ICAO. The debate, often acrimonious, led to much publicity in the technical as well as the popular press. It included investigation by a U S . Congressional Committee. These aspects of the program are another story which will not be treated here. However, following the final selection of TRSB in 1978, all sides supported the decision and embarked enthusiastically on the preparation of technical standards for the new system. G . The Current Process of Standardization
Following the selection, the AWOP panel was charged with preparing “Standards and Recommended Practices” (SAWS) and other supporting
MICROWAVE LANDING SYSTEM
3 19
material for both the angle and DME system. The SARPS (white pages) and the “guidance material” (green pages) form the primary MLS system specification and will be included in the ICAO “Annex 10,” which is the basis of international agreement on navigation aids and landing systems. In March, 1980, a draft SARPS was adopted by the AWOP Panel (21) and in April 1981, the SARPS was adopted by the worldwide Communications Divisional Meeting (13,23). Additional activities directed toward standardization include the RTCA SC-139 (75) in the United States and the EUROCAE WG-19 in Europe, which are preparing airborne receiver standards. These standards will provide guidance to the aircraft certifying authorities in the Unites States and in Europe. The preparation of the “SARPS” for MLS by ICAO represents a unique achievement in the development of a new, highly technical system for aviation in an international forum. Previously, systems had been fielded extensively before ICAO standards had been prepared. MLS required a long and exacting process of “consultative’’ participation by a large number of government organizations, airline business, and general aviation groups, as well as the electronics industries from many nations. The existence of the SARPS will permit all nations to adopt an international standard MLS assuring a worldwide standard of safety. H . The Single Accuracy Standard
ICAO compatible MLS calls for a single accuracy standard to be implemented worldwide. In ILS there are three accuracy standards, one for each weather minimum (i.e., categories I, 11, and 111). For the ICAO MLS, the accuracy guidance is guaranteed to be of autoland quality, which is the accuracy performance specified for category 111 ILS (see 1and 3). This means that the guidance system, because it has the highest quality defined for ILS is not the limiting factor in establishing weather minimums. In addition, the structural and electronic components which make up each “single-thread” ground equipment module will satisfy ILS category 111 reliability requirements. The redundancy and monitoring requirements necessary to satisfy category I1 or 111 weather minimums are but modular additions to the basic single-thread system. There are some exceptions to this when the standard accuracy cannot be achieved. For example, at some “humped” runways, the guidance signals cannot be generated with sufficient coverage at low angles to permit automatic landings. In these cases the SARPS only requires that guidance accuracy be provided to 0.9” above the humped surface. Also, for those “nonfederal” facilities in the United States which are maintained by local
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HENRY W. REDLIEN A N D ROBERT J. KELLY
or state governments, the U.S. Government permits installations of a reduced-coverage capability MLS whose accuracy is equivalent to the SARPS accuracy specification. These nonfederal specifications are defined in FAR171 (16). I . MLS Transition and Implementation The transition from ILS to MLS is not a simple task and requires very careful planning. Any plan requires a clearly presented economic justification. The Department of Transportation released a draft transition plan and a cost benefit study for MLS in November 1980 and is currently soliciting public comment (7,8). The transition plan lists a number of different strategies, but calls for the phasing out of ILS and the introduction of MLS over a 20-year period. ILS is protected in ICAO to 1995, which means that it will still have a recognized status in ICAO until that time. There will be a period where there will be duplicated facilities with both ILS and MLS equipment installed on the ground and carried in the aircraft. Current implementation includes the FAA Service Test and Evaluation Program (STEP), which in an initial phase will install four prototype ground equipments. Two will be located at Washington National, one at Philadelphia, and one at Clarksburg, West Virginia. Additional equipments will be procured and installed to demonstrate the performance capabilities of MLS.
J. The Emphasis of This Article The overall objective of this article is to provide a definitive description of the MLS and the ICAO signal format. The international standards were accepted by an ICAO worldwide meeting in April 198 1. However, the technical rational which permits the system to achieve its objectives will be presented in some detail to show that MLS design is completely integrated to meet a wide variety of requirements and is fundamentally sound. 11. THEOPERATIONAL
AND
FUNCTIONAL REQUIREMENT
FOR
MLS
The original operational requirement for MLS was described in ICAO by the Air Navigation meeting of 1972, which has provided the guidance for system development, and now these requirements are indirectly reflected in the current SARPS document. Although the primary emphasis was placed on accurate guidance to be provided over wide sectors in azimuth, elevation
MICROWAVE LANDING SYSTEM
32 1
(to very low altitudes over the runway), and other technical features, considerable attention was devoted to safety features and to its operational application. Specific requirements were placed on the integrity and reliability of the new system. The requirement assured that MLS would be applicable to all types of aircraft, with a wide range of speeds and altitudes. The system would be compatible with Air Traffic Control procedures and permit improved operations in the terminal area. It would be operable in the severe meteorological conditions of very reduced visibility and heavy rain. The configuration of MLS, and their coverage and accuracy will be summarized below. Individual requirements placed on the ground equipment, airborne equipment, and those allocated to the propagation path will be described under system design considerations. An abstract and portions of the draft SARPS (based on AWOP meeting, September, 1980) form a concise description of a basic and an expanded MLS configuration. A . MLS Configurations
“Basic M L S . The basic configurations of the MLS shall be composed of the following: (a) Approach azimuth’ equipment, associated monitor, remote control, and indicator equipment. (b) Approach elevation equipment, associated monitor, remote control, and indicator equipment. (c) A means for the transmission of basic data words, associated monitor, remote control, and indicator equipment. (d) DME, associated monitor, remote control, and indicator equipment.
Expunded MLS configurations. It shall be permissible to derive expanded configurations from the basic MLS, by addition of one or more of the following function or characteristic improvements : (a) Back azimuth equipment, associated monitor, remote control, and indicator equipment. (b) Flare elevation equipment, associated monitor, remote control, and indicator equipment. (c) DME, associated monitor, remote control, and indicator equipment.
’
In addition to the nominal azimuth approach function a “high-rate’’ azimuth has been defined. This permits more efficient us0 of the time available in the signal format, when only approach azimuth, back azimuth, and elevation functions are implemented. There is also a growth to 360” azimuth accommodated for navigation in the terminal area.
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HENRY W. REDLIEN AND ROBERT J. KELLY
(d) A means for the transmission of auxiliary data words, associated monitor, remote control, and indicator equipment. (e) A wider proportional guidance sector exceeding the minimum specified in Section B.”
B. Angle and Range Coverage
The coverage requirements for each of the four guidance functions are shown schematically in Figs. 4-7. In the coverage sectors accurate indications of angle from the runway centerline or a selected angular reference are provided. This is referenced to as “proportional guidance.” In some cases proportional guidance need only be supplied over a limited sector less than the specified coverage, but a “clearance” signal indicating a simple fly-left or fly-right must be provided to the edges of the specified coverage. The $40” laterally for azimuth is a firm requirement for proportional guidance, but f60” is recommended for some applications. Azimuth guidance is to be provided down to 8 ft above the runway and to 20” elevation angle with a recommended 30”. Range is 20 nautical miles (NM). Missed-approach coverage is less in all dimensions except that it is still required down to 8 ft above the runway. In elevation coverage is required to a minimum angle of 0.9” which is typically 16 ft at the end of the runway. Flare coverage is also down to 8 ft in the touchdown zone. The wide-angle coverage in azimuth is an obvious significant difference from ILS. Also, the low-altitude coverage (not always available in ILS) for both azimuth and elevation and flare permits reliable landings in very lowceiling conditions. C . Accuracy at Threshold and over the Coverage Volume
The key accuracy specifications are defined at the “MLS reference datum” which, except for missed-approach azimuth, is 50 ft above the runway at its threshold (start of concrete at approach end). It is also intended that these accuracies be met for runways up to 15,000-ft long. This uniform accuracy specification means that suitable guidance signal quality to permit autolandings is provided at every runway end (threshold). It does not necessarily mean that autolandings can be performed in low-visibility conditions for this requires many other criteria to be met with respect to monitoring, redundancy, etc., but it does mean that autolandings can be performed routinely in most conditions.
323
MICROWAVE LANDING SYSTEM
The uniform accuracy standard is a key feature of MLS, not achievable with ILS. A pilot can expect the same guidance performance at every runway threshold, further assuring safe and reliable operation. There will be some airports, because of such factors as “humped” runways, for which the standards cannot be achieved, but these will be treated as well-defined and
APPF EL€\ ANTENNA
/
\
20NM
BMX)m(20,000FTl
/
\
/ THRESHOLD
-
-----
6Wml2000FT)
I
M
NM+
APPROACH AZIMUTH ANTENNA
FIG.
4. Approach azimuth coverage: (a) lateral; (b) vertical
~~~$~~~~THRESHOLD ANTENNA MLS DATUM POINT
e----f APPROACH DIRECTION
ADDITIONAL E E E N D E D
-_-_
6OWm (20,000 FT I APPROACH ELEVATION ANTENNA
MLS DATUM
I
I
2.6m
--Ib)
FIG.5. Approach elevation coverage: (a) lateral; (b) vertical. THRESHOLD
FLARE ANTENNA
e----f APPROACH DIRECTION
la)
0
46m 1150 FT I
J
0
0
0
:.:. -1
(2 500 FT I
I
"."
................................................. ................................. +760m
\
0 '
'(b)
FIG.6 . Flare coverage: (a) lateral; (b) vertical.
\
.
325
MICROWAVE LANDING SYSTEM
\
BACK _. .-...AZlMllT ... - .
OR DEPARTUR
45m (150 FT
J
DIRECTION ----1-5NM
T
I
/
/
I
I
t\
--- E
la)
1500rn (5 000 FT I
L I (bl
BACK AZIMUTH ANTENNA
FIG.7. Back azimuth coverage: (a) lateral; (b) vertical
understood exceptions so that suitable procedures can be adopted. For the azimuth site which is at the far end of the runway, the accuracy in angular units must increase (smaller errors) for the long runways. Thus, more accurate and larger azimuth antennas are usually required for the longest runways. This is not the case for elevation, since it is located at a fixed distance typically 750- 1000 ft from the threshold reference. The size requirements of the elevation antenna are determined by the nature of the approach terrain (sloping, flat, etc.).
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HENRY W. REDLIEN AND ROBERT J. KELLY
The specification of accuracy is divided into several components each having a specific effect on aircraft guidance and control. The “path-following error” represents those error components whose variations are sufficiently slow (less than 0.5 rad/sec) so that most aircraft will follow the guidance error. It is divided into a bias term (PFE) and a noise term (PFN). The “control motion noise” (CMN) represents error components which vary sufficiently rapidly such that the aircraft cannot follow the errors. However, when the MLS receiver is coupled to an automatic flight control system, these errors can cause vibrations in the control surfaces or at the pilot control column, which are disconcerting to the pilot. Table I summarizes the guidance accuracy requirements as specified by the current ICAO SARPS. It contains the requirements for the four functions, the three error components at the runway threshold reference datum in terms of a displacement in feet and meters. These displacements have been converted to degrees for elevation assuming a nominal 954 ft from
TABLE I GUIDANCE ACCURACY AT RUNWAY THRESHOLD Guidance accuracy at reference datum MLS function accuracy component Approach azimuth PFE PFN CMN RECOM PFE Approach elevation PFE PFN CMN RECOM PFE Back azimuth PFE PFN CMN Flare PFE PFN CMN
meters
feet
f 6 3.5 f 3.2 *4
+ 20
k0.6 k0.4 k0.23
+2 f 2 k0.75 -
*kk 63.43.2
f 20
k0.6 f0.4 k0.23
+2 f 1.3 f0.75
*
Angular units (degrees) Azimuth (approx. runway length) Elevation
k11.5 f 10.5 f13.5
5000 ft
10,000 ft
15,000 ft
0.229 0.132 0.120 0.155
0.115 0.066 0.060 0.077
0.076 0.044 0.040 0.032
0.458 0.264 0.241
0.229 0.132 0.120
0.153 0.088 0.080
*0.120 f 0.078 f0.045
+11.5 f 10.5
0.046 0.030 0.017
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MICROWAVE LANDING SYSTEM
TABLE I1 GUIDANCE ACCURACY OVER COVERAGE VOLUME AZIMUTH PATHFOLLOWING ERROR
5000 ft AZ 0 A 2 40" 10,000 ft AZ 0" A 2 40" 15,000 ft A 2 0" A 2 40" Elevation A 2 0" AZ 40"
At NILS reference datum (deg)
At 20-NM distance (deg)
At IS" elevation angle (deg)
0.23
0.46 0.69
0.46
0.12
0.24 0.36
0.24
0.076
0.152 0.228
0.076
0.06
0.2 0.26
0.30
threshold and for azimuth assuming runway lengths of 5000, 10,000, and 15,000 ft. It will be shown in Section IV that the guidance accuracies are sufficient to permit automatic landings as well as category I and I1 approaches. Table I1 shows that reduced accuracy is permitted at other positions in the coverage volume away from the reference datum.
OF THE MICROWAVE LANDING SYSTEM 111. DESCRIPTION
A . Overull System The basic component elements for a full-capability MLS are shown in Fig. 3. These include angle guidance elements and distance-measuring equipment to provide the three dimensions of aircraft position relative to the instrumented runway. Like ILS, it is an "air-derived'' system, where signals are radiated from ground antennas in a standard format and then processed in an airborne MLS receiver. In the case of DME, a signal interrogation is originated at the aircraft which is received at a ground transponder located near the approach azimuth antenna. The DME transponder returns a reply pulse with a calibrated delay for two-way ranging processing. The DME for MLS operates in a similar manner to current DME, and it is compatible with the current system,
328
HENRY W. REDLIEN A N D ROBERT J. KELLY
but it can provide greater precision (less than 100-ft error) to support the autolanding and other maneuvers. The angle-guidance output information can be in the form of a course deviation indicator (CDI) from a fixed azimuth and elevation path or it may be displayed as angular coordinates relative to the runway. Alternatively, it may be in the form of a high-precision digital or analog data stream fed directly to an automatic flight control system. MLS is a highly modular system and it may be implemented in simpler versions. The highest capability ground systems may contain approach azimuth ; approach elevation ; flare elevation ; back azimuth ; and distancemeasuring equipment. A 360” coverage in azimuth is also available. Simpler versions will have only approach azimuth and elevation with or without DME. There is also a range of capability in the airborne systems. Highest capability will provide very precise digital guidance data as well as the decoding and display of “basic” and “auxiliary” data. Less capable systems will simply provide azimuth and elevation guidance without data display. MLS is designed with a high degree of “integrity,” which means that the equipment is highly reliable and it is self-checked or monitored to assume that only correct and accurate information will be derived. If any of a number of self-tests either on the ground or in the air are not passed, warnings will be displayed to avoid potential unsafe conditions.
B. The Angle System The angle-guidance system has ground and airborne equipments and is tied together by the standard “signal-in-space” format (71). The format contains certain data and guidance information organized for each MLS function.
“TO‘ SCAN BEAM
“ F R O SCAN BEAM - --
-- - -
.. +e*
1
+em
RECEIVED SIQNALS (dB1
t
FIG.8. MLS angle measurement technique.
1
I
.
MICROWAVE LANDING SYSTEM
329
TABLE I11
MLS ANGLEAND DATACHANNELING Channel number
Frequency WHz)
500 501 502 503 504
5031.O 5031.3 503 1.6 503 I .9 503 1.2
Channel number
Frequency (MHz)
695 696 697 698 699
5089.5 5089.8 5090.1 5090.4 5090.7
The basic principle to derive the angle information is based on the time reference scanning-beam (TRSB) technique. The TRSB technique for the azimuth function is illustrated in Fig. 8. A beam narrow in azimuth and wide in elevation scans horizontally with carefully controlled timing (65).The time between successive TO and FRO passes of the beam, as determined in the MLS receivers is calculated to provide the azimuth angle of the aircraft in a “proportional guidance” region. These signals are referred to as the ‘‘scanning beams.” There may also be provided a “fly-left or a fly-right” indication in regions beyond proportional coverage, but with the coverage limits. Such indications are derived from “clearance beams” as will be described below. 1. The Channel Plan The four basic angle-guidance functions are time multiplexed on each of the 200 frequency channels available for MLS in the microwave C-band. This large number of channels overcomes the limitations of the 40 channels of ILS and permits more than one installation at each airport and other installations at nearby airports in such dense traffic areas as the New York Metropolitan area and Chicago. The channel allocations are shown in Table 111. ‘The200 channels are spaced 300 kHz apart to occupy 60 MHz of the spectrum in the microwave C-band from 5031.0 to 5090.7 MHz. 2. The Signal Format
a. Sequences. The four basic MLS functions are radiated on the same frequency in several specified sequences to accommodate the variety of guidance functions and data to be provided (Fig. 9). The MLS receiver does not require a particular sequence, since it identifies each function each time
330
HENRY W. REDLIEN AND ROBERT J. KELLY
7 5 p n c SEQUENCE
VARIABLE
ENVELOPE DETECT
PROCESSING 0
MEASURED ANGLE
DATA DEMO0 SCAN FUNCTION IDENTITY AUXILIARY DATA
lbl
FIG.9. The signal format with ground and airborne elements: (a) expanded MLS siting and signal format; (b) receiver.
it is received independent of any others which may be present on the same channel. Figure 10 shows the sequences as recommended in the ICAO SARPS. The sequences have been arranged to provide the average function rates shown in Table IV and to provide a “jittered” format. The jitter does not allow the functions to repeat at regular intervals to avoid being synchronized with possible periodic errors. Such errors might be introduced by the regular blocking of the received guidance signals by a rotating
33 1
MICROWAVE LANDING SYSTEM ALL ANGLE GUIDANCE FUNCTIONS
SEQUENCE WITH HIGH RATE AZIMUTH
TIME
(msecl
SEOUENCE NO. 1
SEOUENCt NO. 2
SEQUENCE NO. 1
TIME {msec)
SEOUENCE NO. 2
0
APPROACH ELEVATION
ELEVATION FLAPE
FLARE 10
APPROACH AZIMUTH
APPROACH AZIMUTH
2o
BASIC OATA WOROS(N0TE 11 AZIMUTH
FLARE
30
APPRO*eH ELEVATION (NOTE 1)
HIGH RATE APPROACH AZIMUTH
~o
M
.-
I
BACK AZIMUTH
NOTE 2
5o
(NOTE 21
FLARE
FLARE
68.7 8
6
.
8
I
APPROACH AZIMUTH
ELEVATION
APPROACH ELEVATION
d
67.5
I
(NOTE 3)
NOTES 1. WHEN BACK AZIMUTH I S PROVIOEO, BASIC OATA WORO NO. 1 MUST BE TRANSMITTEO ONLY IN THIS POSITION 2. BASIC OATA WOROS MAY BE TRANSMITTEO I N ANY OPEN TIME PERIODS
NOTES 1. BASIC OATAWOROS MAY BE TRANSMITTEO IN ANY OPEN TIME PERIODS 2. WHEN BACK AZIMUTH IS PROVIOEO BASIC OATA W O R O NO. ZMUST BE TRANSMITTE~ ONLY IN THIS POSITION 3. THE TOTALTIME OURATION OF SEOUENCE # 1 PLUSSEOUENCE I 2 MUST NOT EXCEEO 134 msnc
3. THE TOTAL TIME DURATION OF SEOUENCE # 1 PLUS SEOUENCE I 2 MUST NOT EXCEBO 134 mm:
NO WOROS 2WOROS
13
HIGH RATE APPROACH AZIMUTH
50
(NOTE 3)
1
I
ELEVATION
HIGH RATE
I))RLUCH) ELEVAflON
APPROACH ELEVATION
HIGH RATE APPROACH AZIMUTH
I
ELEVATION
-1
I
3W OROS
19
NO WOROS 3 WOROS 1 WORO
2
20
6
FULLCVCLE=616 msec (MAXIMUMI
FIG. 10. The signal format function sequences.
NO WORDS 3 WOROS
0
-I
18 msec
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HENRY W. REDLIEN AND ROBERT J. KELLY
TABLE IV THEAVERAGE TRANSMISSION RATESFOR Function Approach azimuth guidance High-rate approach azimuth guidance Back azimuth guidance Approach elevation guidance Flare elevation guidance Basic data” a
THE
MLS FUNCTION
Average function rate (Hz) 13 39 6.5 39 39
f 0.5
f 1.5 0.25 f 1.5 f 1.5
See Table VII.
propeller. There are 15 different and independent functions which are separately identified. Six are angle-guidance functions, eight are basic data, and one is reserved for “auxiliary data” (Table V). b. Angle function organization. Each angle function occupies its own time slot, which must be suitably synchronized so as not to overlap the other functions on the same frequency channel. The information for each function is organized into a standard format at precisely specified times as shown in Fig. 11. A “preamble” and “sector” signals are radiated from an antenna which covers the entire angular coverage volume with a single wide beam. The proportional guidance is radiated from a ground antenna which provides a beam narrow in the scan (or guidance) dimension and which extends to the edge of coverage in the other dimension. Clearance beams are radiated into a sector bounded by the limit of proportional coverage and the edge of coverage. c. Preamble. The preamble is radiated throughout the coverage sector plus or minus 40” in azimuth to permit (1) the acquisition of the radio frequency carrier, (2) the detection of a time synchronization code, and (3) the identification of the guidance function. Detailed timing for a preamble is shown in Table VA. ( i ) Carrier acquisition for phase synchronization. There is first a pure unmodulated carrier, which is followed by differential phase-shift keying of a carrier. A digital “zero” is represented by 0 phase shift between successive pulses occurring every 64 psec. A “one” is represented by 180” of phase shift. The signaling rate is 15,625 bits/sec. (ii) Synchronization code. The synchronization or “Barker” code follows the carrier with the bit sequence identified in Table VA, which permits a reference time to be established in the receiver. The remaining elements of the individual function format then occur at precisely established times as
TABLE VA. PREAMBLE TIMING^ Event time slot begins at :
Event Carrier acquisition : (CW transmission) Receiver reference time code : I, = I I, = I I, = 1
I4 = 0 I, = I Function identification: Is I, I8
19
I10 1,I I1 2 End preamble a
15.625 kHz clock pulse (number)
0
Time (msec)
0
13 14 15 16 17
0.832 0.896 0.960 1.024 1.088
18 19 20 21 22 23 24 25
1.152 1.216 1.280 1.344 1.408 1.472 1.536 I .600
See Table VC. TABLE VB. APPROACH AZIMUTH FUNCTION TIMING" Event time slot begins at:
Event Preamble Morse code Antenna select Rear OC1 Left OCI Right OCI TO test TO scan Pause Midscan point FRO scan FRO test End function (airborne) End guard time; end function (ground)
' See Table VC.
15.625 kHz stock pulse (number) 0 25 26 32 34 36 38 40
Time (msec) 0 1.600 1.664 2.048 2.176 2.304 2.432 2.560 8.760 9.060 9.360 15.560 15.688 15.900
334
HENRY W. REDLIEN A N D ROBERT J. KELLY
TABLE VC The following 15 functions have preamble identification codes Approach azimuth High rate azimuth Approach elevation Flare elevation Back azimuth 360" azimuth
Basic data word 1 Basic data word 2 Basic data word 3 Basic data word 4 Basic data word 5 Basic data word 6 Basic data word 7 Basic data word 8 Auxiliary data
Time allocated for each function (msec) Approach azimuth High rate azimuth Approach elevation Flare elevation Back azimuth 360" azimuth Basic data words Auxiliary data
15.9 11.9 5.6 5.3 11.9 3.1 -
shown for the preamble and for the approach azimuth function. This timing specification is a basic feature of the receiver design and it permits certain types of spurious multipath signals to be identified and rejected. (iii) Function identijlcation. The function identification code follows to indicate which function is being received. There are codes for all the functions previously indicated as well as function codes for a 360" azimuth, eight
Y v)
Y 3 0
3 0
GROUND RAOIATEOTEST
~WLE
SEG~%O~
'70" SCAN TIME SLOT
"FR0"SCAN TIME SLOT
FIG. I I . The angle function organization. (I) Azimuth functions only.
335
MICROWAVE LANDING SYSTEM
“basic” data words, and auxiliary data (Table VC). These codes permit any and each function or data to be radiated, received, and decoded in any sequence and timing. The function identification is decoded and validated assuring a correct interpretation of the guidance informance received. Five bits are needed for identification and two for parity. d . The remaining sector siqnal. Four additional signals are radiated on the sector antenna to provide : ground equipment identification (on azimuth only); airborne antenna selection ; out-of-coverage (OCI) indication (separate sector antennas) ; and receiver processor test with special pulses. ( i ) Theground equipment Identijication. A single bit in the DPSK stream from the azimuth antenna forms the basis for generating a four-character Morse code for ground system identification. It must be radiated at least six times per minute and requires a number of azimuth function scans to generate the code. A DPSK “one” initiates a Morse code dot or dash and a zero terminates the symbol. Figure 12 shows the timing specification and the number of azimuth functions required for one example. ( i i ) Airborne antenna selection. For a 6-bit period following the Morse code bit, a “zero” is transmitted providing a constant amplitude signal for 0.384 msec. This signal is available for comparing the signal strengths from different airborne antennas to make a selection of the strongest. (iii) Azimuth out-of-coverage indication (OCZ). One feature of MLS is the provision of a positive indication that an aircraft is not in the specified coverage region of the MLS ground station. There are three separate pulses radiated by three different sector-type antennas to provide OCI to the left, right, and to the rear of the azimuth antenna to fill the entire azimuth region not in coverage. These antennas are described in Section II1,B. Two 64-psec time slots for each of the three pulses are available in each azimuth-site transmission. The left, rear, and right sector patterns must meet the following specifications:
,
,
The pulse amplitude must be greater than any guidance signal in the out-of-coverage sector.
H
rt
o.itas SECONDS
;
DASH
I AZIMUTH SCANS
I I I I I I 1 I I 1 I 11.1 I I I I I I I I I I I I I I I I I TIME
FIG.12. The Morse code identification timing.
-
336
HENRY W. REDLIEN AND ROBERT J. KELLY
The pulse amplitudes must be 5 dB less than the appropriate clearance signals in the clearance regions. The pulses shall be at least 5 dB less than the scanning beam level within the proportional coverage region. With these specifications a comparison of the level of the OCI pulses with either the proportional or clearance guidance signals provides a positive indication of being out of coverage. (iv) Receiverprocessor test signals. Two time slots of 128 psec each just before and after the guidance signals are reserved for two pulses whose fixed separation in time corresponds to a fixed-azimuth guidance angle greater than 60". These pulses are available for checking the calibration of the receiver. e. Angle-guidance signals. There are two basic types of signals for angle guidance. These are the scanning beams which provide proportional guidance and the clearance beams which provide fly-left or fly-right signals. (i) Scanning beams for proportional guidance. The basic specifications for the five basic scanning-beam functions (including high-rate azimuth) are summarized in Table VI and Fig. 13. These specifications define a unique proportional relationship between the time difference between the successive passages of the beam and the guidance angle to be derived. Sufficient time is allocated in the format to provide guidance up to 62" in azimuth and 29" in elevation. (ii) Clearance signals. Should the proportional coverage provided by the ground station be less than the coverage limits, fly-left and fly-right signals may be generated in the airborne receiver by the use of clearance pulses. The
PREAMBLE
SECTOR SIGNALS
I .--L
"TO" PULSE
PULSE
I
I
"FRO
" F R O SCAN TIME SLOT
TIMESLOT
-------
l-
MIDSCAN POINT
FIG. 13. Scanning beam timing.
337
MICROWAVE LANDING SYSTEM
TABLE VI VALUS OF THE ANGLE-GUIDANCE PARAMETERS’
Parameter Approach azimuth High-rate approach azimuth Back azimuth Approach elevation Flare elevation
Maximum scan angle (degrees) -62 to -42 to -42 to - 1.5 to - 2 to
Value of i for max scan angle (psec)
f62 +42 f42 i-29.5” 10
+
13,000 9000
9000 3500 3200
T, (psec)
Degreesper microsecond
6800 4800 4800 3350 2800
0.020 0.020 0.020 0.020 0.010
See Eq. (1) for coding and definition of parameters.
clearance pulse timing is shown in Fig. 14. These pulses will be radiated into the clearance region by a separate antenna with the following specifications : The right clearance pulse shall exceed the left clearance pulse by at least 15 dB. It shall exceed sidelobes of the scanning beam by 5 dB. It shall be 5 dB below the scanning-beam signal at the edge of coverage. In order to identify the clearance signals and not interpret them as scanning-beam signals at an angle slightly greater than the maximum intended, some additional information is needed. This is provided in the “basic data” transmitted from the ground station to the airborne receiver. f : Basic and auxiliary data. In addition to the information provided with each guidance function, certain other information is needed to perform the landing operation, In the case of the clearance indication, it is actually necessary to receive the limits of proportional azimuth coverage as data to properly interpret the received signals. (i) Basic data. Basic data is required to be provided by every ground station. It is organized into eight 32-bit words, each with a 12-bit preamble identical in format to the guidance function preambles. Parity bits are included with each word and some spare bits are designated. The eight basic data words are organized as shown in Table VII. The data may be used in several ways. Some represents the dimensions of the physical airport layout to permit manipulation of the guidance data for particular purposes. For example, the azimuth-to-threshold distance permits a conversion of azimuth data as an angle to a lateral displacement in feet at the runway threshold. By standardizing on this lateral displacement, deviation indicators will have the same deflection sensitivity at threshold.
338
HENRY W. REDLIEN AND ROBERT J . KELLY TIME SEPARATION OF FLY-LEFT CLEARANCE PULSES -TIME
SEPARATION OF SCANNING-BEAMPULSES AT NEGATIVE LIMITS OF SCAN
\
WIDTH OF PROPORTIONAL GUIDANCE ECTOR
"FRO" SCAN
"TO" SCAN TIME SLOT
TIME SLOT
I
oo LEGEND:
MI DSCAN POINT
FLY-LEFT CLEARAWE PULSES
FLY-RIGHT CLEARANCE PULSES
SCANNING BEAM PULSES
FIG. 14. Clearance pulse timing for azimuth functions.
Other dimensions permit the coordinate conversion of the guidance data to any type and reference origin. Some data transmits the status of the equipment to determine if certain categories of landing may be executed. Operational parameters such as minimum glideslope assure that selected glideslopes are free of obstacles. Other data such as the value of the beamwidths radiated may assist the MLS angle receiver in the processing of the guidance data. (ii) Auxiliary data. There are also provisions made in the format for additional or auxiliary data. It will be organized into 64-bit words. Transmission time is available between specific sequences as shown in Fig. 10. As of this writing, the content of the auxiliary data has not yet been defined by ICAO. g. Power dertsity. In order for the MLS receiver to accurately decode the signal format with sufficient guidance accuracy and data-bit integrity, sufficient signal power is required at the aircraft. A minimum power density,
339
MICROWAVE LANDING SYSTEM
TABLE VII
BASICDATAWORDS
Word number
Maximum time between transmissions (see)
Data content ~
~
Approach azimuth to threshold distance Approach azimuth proportional Coverage limit (in two sectors)
0.4
Ground equipment performance level Minimum glide path Back azimuth next function DME status
0.16
Approach azimuth beamwidth Approach elevation beamwid th Flare elevation beamwidth Approach azimuth sector guidance
10
DME distance DME offset
10
Approach azimuth antenna offset DME (standard or precision) DME channel
10
6
MLS ground system identification
10
7
Ground equipment performance level Back azimuth antenna distance Back azimuth proportional coverage limit Back azimuth beamwidth
8
Elevation antenna height Elevation antenna offset MLS datum point to threshold distance
3
I
10
which must be supplied at the maximum range of 20 nautical miles, has been specified as well as increased power density at certain other regions of the coverage volume in Table VIII. It is to be noted that higher power densities are required for the wider beamwidth ground antennas to obtain guidance accuracy. The reception and processing of the wider beams is inherently less accurate under low signal-to-noise conditions, so more power is needed. The exact relationship between beamwidth and accuracy is discussed later in Section IV.
340
HENRY W. REDLIEN A N D ROBERT J . KELLY
TABLE VIII MINIMUM POWER DENSITY REQUIRED AT AIRCRAFT‘
Function
DPSK signals (dBW/m2)
Approach azimuth guidance High-rate approach azimuth guidance Back azimuth guidance Approach elevation guidance
-89.5 -89.5 -81.0 - 89.5
Angle signals (dBW/mZ) (antenna beamwidth) 1”
2”
3”
Clearance signal (dBW/m*)
-88.0 -88.0 -79.5 -88.0
-85.5 -88.3 -77.0 -88.0
-82.0 -86.8 -73.5 N/A
-88.0 -88.0 -88.0 N/A
At the approach reference datum the densities will be 15 dB greater. At 2.5 m (8 ft) above the runway at the MLS datum the densities will be 5 dB greater.
3. The MLS Ground Equipment The MLS ground station generates the “signal-in-space” to provide angle guidance and data. There are separate ground stations and locations for each of the guidance functions, although the azimuth and elevation equipments can be “collocated” under certain circumstances. The azimuth station, in addition to providing azimuth guidance, generates the key timing, control, and switching for the other guidance functions located on the runway. Several types of antennas are needed to implement the sector coverage, the scanning beams, the “OCI” function, and clearance. Internal control is provided for the transmitter, the switching among the various antennas and for the steering of the scanning beams. There is also provided a high degree of equipment monitoring for both “executive” actions and maintenance alarms. a. Control, timing, and switching. A block diagram of typical azimuth ground equipment (Fig. 15) shows how the equipment is interconnected under a control unit which initiates the synchronization and timing to implement the various aspects of the signal format. A clearance antenna may be included, but this is only required if the full coverage is not provided by the scanning-beam antennas. The elevation ground system is synchronized from the azimuth system. All the basic data words are radiated from the azimuth station. The remaining guidance elements are organized in a similar manner without the need to provide basic data or synchronization. 6. Transmitlers. The transmitters provide the signal in the microwave C-band with a frequency stability of 10 kHz at power levels of 10-20 W. Traveling-wave tube amplifiers with low-level exciters have been generally used in the developmental equipment, but solid-state amplifiers are under
I
@
I
-I v)
u r
a L LI
v)
-
a 0 k z
0
z
I I I I
I
. .
I
4
&--
I -
--I
FIG. 15 . Typical system block diagram.
342
HENRY W. REDLIEN A N D ROBERT J. KELLY
development and now are coming into common use in MLS. Power budgets have been designed to anticipate solid-state transmitters. c. Antennas. There are a number of different antenna types needed for each function. In particular, the azimuth scanning-beam antennas must generate beams narrow in azimuth but which fill the elevation coverage. Elevation beams are narrow in elevation and fill the azimuth coverage. The antenna patterns of all the antennas for two typical azimuth stations are shown in Fig. 16. One has the full proportional coverage of 80" provided by the scanning beams and the second has only a limited proportional guidance of 20". In this case clearance beams are added to give a fly-left and a fly-right display as appropriate. The remaining 360" is filled in with OCI beams to indicate out of coverage as described previously. ( i ) Scanning-beam antennas. These antennas are the heart of MLS. They provide the angle guidance signal for proportional guidance. Narrow beams which scan at precisely controlled rates are needed. There are a number of antenna techniques available for this purpose, but two general types have been implemented during the development process. These are the "phasedarray" and the "beam port" lens or reflector antennas. Conceptual diagrams for each are shown in Fig. 17. The phased array has a central feed point, a power distribution network, a bank of phase shifters, and a number of individual radiating elements. By incrementing the phases at discrete time intervals, the beam is steered. Fourbit phase control (22.5" phase steps) has proved adequate to obtain smooth beam steering. Ferrite phase shifters have been implemented, but pin-diode types are currently being developed and will find application in the Joint Tactical MLS being developed under a tri-service program by the US.Army. A typical beam port type is the "Rotman lens." A microwave-focusing structure feeds a series of radiating elements in much the same manner as in the phased array. One such structure is a "pill box" (a pair of metal parallel plates) in which a series of input and output probes are placed. The geometry of the probe placement has been selected so that a signal injected at an input port forms a directive beam at a direction associated with the input probe location. By switching from probe to probe, the beam scans. "Blended" switching in which more than one probe is excited permits a very smooth and accurately controlled scanning beam. The placement of the input and output probes permits perfect focusing (or beam formation) for three different beam angles to obtain wide angle performance. Antenna beamwidths from 0.5" to 3.0" are available. In azimuth they are typically 1" for 15,000-ft runways, 2" for 8000 ft, and 3" for 5000 ft or less. The selection is made to assure the same accuracy at the runway threshold. In elevation they range from 2" to 0.5" with 0.5" being used for the flare function.
343
MICROWAVE LANDING SYSTEM
7 FORWARD IDENT
SCANN,NG-,
+goo
RIGHT REAR
ocI
1800 A2 BEAM SCAN COVERAGE
LEFT
FIG.
344
(a'
HENRY W. REDLIEN A N D ROBERT J. KELLY
lN%T
PHASE
POWER DISTRIBUTION NETWORK
PHASE SHIFTERS
ANTENNA ELEMENTS
~PLANE !'$~~~$~
(b)
LENS INPUT
\ 0 4 4 ANTENNA ELEMENTS
AMPLITUDE APERTURE PLANE
FIG. 17. Phased array (a) and Rotman lens concept (b).
(ii) Clearance-beam antennas. In the example shown for azimuth, (Fig. 16b) clearance beams are needed for to the left and right of the runway centerline from 10" to 40".They are fixed in space and have certain requirements placed on their gain relative to the scanning beam antenna gain as specified in Section III,B,2. There are a number of ways to generate the clearance beams. They may be formed in separate antennas, by specialized excitations of the phasedarray antennas, or rather conveniently in the Rotman lens antenna. In this case the probes which correspond to the clearance directions are connected as a group and fed with the clearance signal at the appropriate time in the signal format. (iii) Sector coverage and OCI antennas. The preamble and data are required to be available simultaneously over the complete guidance coverage sector, over 80" in our example. A single antenna element is one convenient way to provide the coverage, but it may also be obtained from special excitations of many elements of a phased-array antenna. Two or three OCI antennas are needed to fill the angles behind the antenna as shown in Fig. 16 in these cases the OCI antennas are typically single elements oriented appropriately. (iv) Vertical coverage for azimuth antennas. Perhaps one of the most significant features of MLS is the ability to control the directivity of the antenna patterns, not only in the guidance direction, but also in the nonscanning direction. In the case of the azimuth antennas the shape of the
MICROWAVE LANDING SYSTEM
345
elevation or vertical pattern is controlled to reduce the effect of ground reflections on the accuracy of the guidance signals. In particular, the pattern has full amplitude down to the lower limits of coverage and then "cuts off' very rapidly towards lower angles. This underside cutoff prevents any significant amount of radiation from reaching the airport surface and avoids reflections which could otherwise cause errors in guidance. The sharp cutoff patterns are obtained by the use of vertical aperture and specially designed aperture excitations. An example pattern is shown in Fig. 18 for 20" of elevation coverage. ( v ) Typical antenna implementations. An azimuth and an elevation system have been installed at Washington National Airport by the FAA. They were designed and fabticated by Bendix Corporation under contract (6). The azimuth scanning beamwidth is 2" and the elevation 1.5". The azimuth antenna has an array of 64 vertical column radiators fed by a Rotman lens. Full 40" scanning-beam coverage is provided for proportional guidance. The sector coverage antenna is one vertical column, and the three OCI antennas are mounted on an equipment shelter external to
-10
+10
+20 DEGREES
FIG.18. Vertical pattern.
+30
W
m P
FIG.19. Azimuth system at NASA Wallops Station, Virginia.
341
348
HENRY W. REDLIEN AND ROBERT J. KELLY
the array, also single vertical columns but enclosed within cylindrical dielectric radomes. The elevation antenna has a single row of 64 dipole antenna elements fed by a Rotman lens. The sector and OCI antennas are each single-column radiators mounted to the side of the dipoles one on top of the other. (vi) Alternate approaches. Other antennas have been implemented by the FAA in the United States, and the Australian Government has fielded and tested a number of beam port reflector types. There have been two sets of 1” beamwidth azimuth phased-array antennas developed. The first was a test bed for demonstrating feasibility of MLS and was evaluated extensively at the FAA test center in Atlantic City, New Jersey. This set was also airlifted to Brussels, Belgium, as part of a worldwide MLS demonstration program. There it provided the guidance to perform automatic landings as well as curved approaches. A second set is installed at the National Aviation and Space Administration (NASA) test facility in Wallops Station, Virginia (18) (Figs. 19 and 20). Rotman lens antennas have also been applied to a reduced capability “small-community” system with clearance beams. In other cases where limited proportional coverage is permissible an antenna developed by Hazeltine Corporation and called COMPACT has been developed. It is a technique applied to phased arrays and permits the reduction of the number of active phase shifters. The elevation antenna installed at Wallops Station is one example. d. Monitoring system. One of the features of a landing system is the need to continuously monitor performance so that any potential unsafe operation can be detected and suitable warnings issued to the pilot. In some cases equipment failures are detected so that standby equipment can be switched in to take its place. This latter feature is especially important for category I1 and I11 landings should any failure occur during the final touchdown phase. Monitoring is also useful in performing routine maintenance. “Executive” monitoring measures those parameters essential to the safe operation of the system. Examples include the beam accuracy (mean course error), the radiated power level for the various subsystems, system timing, DPSK integrity, and the synchronization among the functions to provide the proper time multiplexing. Failure of any of these parameters to meet certain specifications will result in a system shutdown and a flag is raised in the MLS receiver. There is permitted, however, a time of 1 sec to switch to standby equipment and retain operation. If this period is exceeded the system must be shut down. Each function has its monitoring and shutdown capability, but if the azimuth system shuts down, the approach elevation,
MICROWAVE LANDING SYSTEM
349
flare, and back azimuth also are shut down, and the appropriate warnings issued. Many quantities are measured for maintenance. These include system voltage levels and the operational status of components and subsystems. One example is unique to phased-array antennas. An off-line maintenance monitor exercises every bit of every phase shifter in sequence and detects any failures. The phase shifter can be replaced at the next convenient opportunity. To achieve high reliability the executive and maintenance functions are configured independently.
4. The Airborne Receiver and Processor The MLS receivers will be implemented with a wide variety ofapproaches and with a variety of capabilities depending on the application. An RTCA Committee (SC- 139) is currently defining the “minimum operational performance standards” (MOPS) for receivers to be manufactured and certified by the FAA “TSO” process. A corresponding activity is taking place under the European Organization for Civil Aviation Electronics (EUROCAE). However, there are a number of functions which every receiver must perform and these will be described. In order to derive the guidance information from the MLS signal in space, there are a number of equipments installed in the aircraft. These include one or more antennas, connecting transmission lines, a control unit, a combined receiver/processor, and some form of display for the guidance information and data (Fig. 21a and b). The guidance information may be in the form of an analog output or a digital data stream fed directly to automatic flight control equipment. There may also be provided raw auxiliary data to be processed external to the MLS receiver. Within the range of the MLS signal, the receiver processes the format and provides accurate proportional guidance, clearance guidance, or it indicates that it is not within the MLS coverage by the display of a suitable flag or warning. It decodes, displays, or stores for later use certain basic data words as appropriate. The receiver must operate with integrity and accuracy in the presence of “multipath” reflections of the radiated signal format, signal interference of various types, and with internal and external electrical noise. A positive indication of any equipment failure must be provided. Some of the requirements placed on the receiver to meet the more critical landing operations are the need to “acquire” the MLS signal and output data within 1 sec. It must also be able to accommodate short interruptions of less than 1 sec and continue to output the last valid data or “coast.” These
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HENRY W. REDLIEN A N D ROBERT J. KELLY la)
CHANNEL SELECT EL SELECT
FIG.21a. Basic units of airborne angle receiver.
last two requirements apply to categories I1 and I11 where long interruptions on the last part of the final approach would force the operation to be abandoned. The need to operate in strong multipath environments, which at least momentarily can exceed the direct signal amplitude, requires processing to assure that the correct guidance information is provided. For example, should the receiver momentarily lock up on a strong and spurious multipath signal, it must examine all incident signals so that as the direct signal becomes stronger, as it invariably does, the receiver must be able to shift to the stronger direct signal. On the other hand, after the direct signal has been acquired and established some history, the receiver should not lock on short bursts of multipath. Up to about 10 sec of strong multipath have been found to be a tolerable amount. a . Antenna and transmission lines. One or more antennas may be positioned on the aircraft surface to provide the coverage required for the landing operation. For a minimum operation of approaching a selected course followed by a straight-in approach, a coverage of 140" in azimuth and 35" in elevation is considered adequate. For other operations and flight procedures 360" coverage may be needed. For the minimum coverage a small monopole antenna, backed up by a reflector, placed forward of the windscreen or on top of the pilot's cabin will give the forward coverage. A monopole antenna, extending about one-half inch from the aircraft, placed at some location either on top or bottom of the aircraft, depending on location of engines, tail assembly, and landing gear can provide the 360" coverage. A combination of two antennas, one for forward and one for aft coverage, can also be implemented. b. The angle-control unit. This unit provides for selection of one of 200 MLS channels and in some installations permits the selection of an "azimuth radial" or a particular glidepath angle. In ILS there is only a centerline approach path available at a nominal elevation or glidepath angles from 2.5" to about 3.5" depending on terrain and obstacles about the airport. MLS on the other hand can provide straight paths at angles away
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HENRY W. REDLIEN AND ROBERT 3. KELLY
from the runway centerline and at higher and lower glidepath angles. These angles can be inserted into the receiver, which then forms the reference for course-deviation indications. There are limits to the selection, especially for low-glidepath angles. If too low an angle is selected, data from the ground system is available to warn the pilot of this condition (see Section III,B,2,f. c. Receiuerlprocessor. The basic functions of the receiverlprocessor may be divided into two groups. The first is concerned with those operations preliminary to the processing of the guidance data and the second is the actual processing. An equipment block diagram of a possible typical approach is shown in Fig. 22. Key to this implementation is the “microprocessor,” which is a very small computer programmed to control all the MLS receiver processing (61). (i) Preliminary functions. The first step during reception of each “frame” of the format is -tb establish phase synchronization on the unmodulated C-BAND ANTENNAAND FRONT END
LOG AMP AND VIDEO DETECT ZBkHz FILTER
I ID DECODE
ENVELOPE MICROPROCESSOR ACDUISITION/ VALIDATION
ANGLE PROCESSING 10 RAD/SEC FILTER
ANGLE OUTPUT
FIG.22. Receiver processing block diagram.
MICROWAVE LANDING SYSTEM
353
carrier for the subsequent DPSK modulation. The next is the extraction of a timing reference for subsequent data decoding from the 5-bit “Barker code.” This is followed by the identification of the MLS function out of the 15 possible which may be received. Once the identification is made certain, data stored in the receiver is then recalled for the processing of the guidance data. The remaining functions include the Morse code ground system identification, the airborne antenna selection process, and the measurement of the amplitude of the OCI. (ii) The basic processing of the angle-guidance information. The basic concept of processing to obtain angle guidance from the receiver scanning beams is very straightforward, but before any guidance data is made available, it is subject to a series of tests to verify its correctness. In the simplest terms, the receiver TO-FRO scanning beams generate a video waveform which has two peaks. The time that each is received is determined from the two individual times which occur at a - 3 dB threshold level for each beam (see Fig. 13 and Table VI). The difference between the average of each pair is a time directly related to the guidance angle, which may then be calculated. This process is repeated for each scan of the signal format and provides an output stream of raw data, which may then be further processed or filtered. However, there are criteria which the data must meet before it is presented to the pilot. (iii) Acquisition and validation of guidance information. To understand the criteria to which the angle data must be subjected, we need to identify the types of distortion which might occur in the MLS signal. At long ranges, the signal level may be at a low amplitude and noise generated in the receiver can distort it. There may be interference from any number of outside sources or there may be reflections of the MLS signal from objects in and surrounding the airport. These reflections or “multipath” can introduce extra scanning beams to be received which could cause erroneous data to be decoded, unless the appropriate provisions are made. Since it is sometimes possible that the multipath will exceed the direct and desired guidance signal in amplitude, we need a method to select the direct signal. In addition, it is possible for the guidance signal to be absent momentarily because of ground power or equipment failure or by the blockage of the signal from the ground antenna by a large aircraft or some ground vehicle. Operational considerations have set criteria for the receiver operation : (1) The MLS receiver must acquire the largest guidance signal within a period of 1 sec. (2) The receiver must continually evaluate the signals received so that the largest and most persistent signal will be processed.
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HENRY W. REDLIEN AND ROBERT J. KELLY
(3) Once having acquired the largest signal and developed a history verifying that it is the largest, short bursts of very large amplitude multipath must be rejected. (4) The receiver must not acquire signals where detection statistics fall below about 50%. (5) Should there be a total loss of signal, a flag must be raised warning the pilot within 1 sec. (6) For a loss of signal shorter than 1 sec, the receiver should “coast” through this interruption. Several of these criteria are required by the autolanding operation, which must have uninterrupted guidance in the final approach phases to touchdown and rollout on the runway. It also requires that the guidance not be disturbed by short bursts of strong multipath which could be introduced by large hangars.
1
MEASURE BEAM
I
MEASUREMENT STORE PEAKS AND THRESHOLD CROSSING TIMES
-------PROCESS SIGNAL PROCESSING CYCLE
MEASUREMENT
I INCREM:NT~
JDE,CREM;NT
I
‘IN> ‘OUT INCREMENT PIN
1
,
LIFT “C“ FLAG
LIFT “F“ FLAG
“AND
4 LIFT SYSTEM FLAG
FIG. 23. Frame/confidence counter logic and definition of the measurement cycle and the signal processing cycle.
MICROWAVE LANDING SYSTEM
355
( i v ) Example implementation of acquisition and validation. Figure 23 diagrams the essential elements of an acquisition and validation scheme implemented in protypical equipment developed for the FAA by Bendix in 1976.The signal is subjected to: (1) frame validation; (2) slew rate validation ; and (3) comparative signal amplitude validation. The basic approach is to receive each signal format frame (13 times per second for azimuth and 39 times for elevation) and conduct checks to determine if certain criteria are met. If the signal meets the criteria, a counter is incremented and if it does not meet the criteria, the counter is decremented. The rates of incrementing and decrementing are adjusted such that all valid data being received will provide a sufficient count to remove the warning flags from the receiver in 1 sec. The frame validation checks preamble data and certain beam characteristics. The preamble function identification is compared with stored identification codes and checked for parity. The width of the dwell gate, the presence of two gates, and the symmetry of their location with respect to format timing references are evaluated. A successful check increments a “frame” counter, which is automatically decremented at one-half the scan rate. This approach requires that at least 50% of the received frames be validated to remove the warhing flag. It precludes spurious interference from being processed and assures that valid MLS signals are being received. A “slew-rate” limiter is an additional check which decrements the frame counter if angle-guidance information changes at a large rate which could not result from possible movement of the aircraft. It precludes erroneous data which might have passed other frame checks. The check of comparative signal amplitude provides protection against strong multipath signals. If the scanning beam first illuminates the aircraft receiving guidance and then, at a later time in the scan, illuminates a large reflector, there will be a set of beams for the direct signal and a set for the multipath. Under normal circumstances, the multipath is the weaker, so a “tracking” gate is set about both beams of the stronger signal. The weaker multipath is not processed. For each frame the signal within the gate is compared in amplitude with that outside the gate, and as long as the in-gate signal is stronger, a separate “confidence” counter is incremented. One second of consistent counts permits the flag to be removed, but confidence continues to accumulate for about 20 sec. The confidence counter is decremented each time the out-of-gate signal is the stronger. When the counter is reduced to a count corresponding to 1 sec, by a persistent out-of-gate signal, a flag is raised and the now stronger signal is acquired within 1 sec and confidence is then accumulated for that signal. This process has several strong implications of how multipath is handled in MLS and is based on experience gained during the MLS development
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HENRY W. REDLIEN AND ROBERT J. KELLY
program. Multipath can be strong for short periods, but not persistently stronger than direct signals. So when a receiver is first acquiring the strongest signal, it usually locks on the direct signal and builds confidence for 20 sec. Short bursts of very strong multipath for times up to 20 sec will not break the track on the direct signal. This is important in final approaches where the presence of some large aircraft or structure near the runway might produce a momentarily large multipath signal. It may be possible that on initial acquisition the multipath could be the stronger signal for several seconds. The receiver in this case would lock on the multipath, raise the flag, and momentarily output incorrect guidance. However, as soon as the direct signal becomes larger, it becomes the signal being tracked in a short time because it would be unlikely that the multipath, being transitory, would be able to build any substantial confidence time. This approach provides a means of tracking the more persistent and larger (on the average) direct signal, precludes short bursts of multipath from causing erroneous guidance data, and also permits a shift to the direct signal if multipath has been initially acquired. ( u ) ReceiuerJ7ltering. There are two key frequency filters in the MLS receiver. One is a beam envelope filter and the other is the output filter. Both are specified in the ICAO SARPS since they impact overall system performance. There is a low-pass filter of 26-kHz bandwidth which follows the video detector. This filter is adequate to pass the desired beam envelope undistorted, but is narrow enough to reject some rapid steps and modulation which may be introduced on the scanning beams by the ground antenna. There are a number of different ways the beams from the ground antenna may be scanned. Just about all the techniques so far identified involve some form of digital switching and phase shifting in small, but rapid steps. The ICAO SARPS accuracy specification assumes the presence of this filter so that these very high-frequency steps can be eliminated from the beams being processed. There is also a filter on the output data. The data can be rather heavily averaged since pilots and aircraft cannot respond to very rapidly changing guidance signals. A single pole filter with a cutoff frequency of 10 rad/sec (1.6 Hz) has been found adequate to pass all the guidance signals necessary to properly control an aircraft. This subject is discussed more fully in Section IV. To assure proper autopilot interface, the SARPS guidance material states that the receiver output filter, for sinusoidal input frequencies, should not induce phase lags which exceed (1) 4" from 0.0 to 0.5 rad/sec for the azimuth function; and
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357
( 2 ) 6.5" from 0.0 to 1 .O rad/sec and 10" at 1.5 rad/sec for the elevation function. Amplitude variations relative to the zero frequency response should be limited to f0.5 dB.
C . Distance-Measuring Equipment Integral to the MLS concept is the distance-measuring equipment (DME), which measures range to touchdown. It must satisfy to the maximum extent possible approach and landing operational requirements for all user aircraft (CTOL, STOL, and VTOL). These requirements dictate aircraft range and range rate measurements with an accuracy at least an order of magnitude more precise than those needed for conventional terminal and enroute DME applications. This precision DME system must (1) satisfy the CTOL and STOL category I1 decision height and category 111 flare maneuver accuracy requirements (100 ft, 2 sigma) when combined with the appropriate elevation data; and (2) for VTOL applications provide accurate range (20-40 ft, 2 sigma) and range rate information (k2 knots, 2 sigma) to permit IFR decelerated approaches to hover. These specifications must be satisfied in the presence of large signal attenuation caused by ground multipath interference and signal time-of-arrival errors induced by lateral multipath (hangars, etc.). The conventional DME is designated DME-N in Ref. 10 and the precision DME has been designated as the DME-P in Ref. 23. Because there are user airborne-equipment space and cost restrictions, the DME-P must be interoperable with conventional (e.g., VORTAC) DME ground transponders and must be capable of being combined with DME-N in the same package. The precision distance-measuring equipment is based upon an evolution of the DME-N principle as described in Ref. 53.That system is a radio aid to navigation which provides distance information by measuring total round-trip time of transmissions from an airborne interrogator to a ground transponder and return.The DME-P is a multimode range-measurement system completely compatible with DME-N, which obtains increased accuracy by pulse-shape modification. It is to operate at L-band frequencies between 960 MHz and on the same rf channels as conventional DME. The DME-P concept and specific channel plan are evolving from the MLS/DME standardization activity presently underway within ICAO. The DME-P technique, its history, and the technical problems associated with
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HENRY W. REDLIEN AND ROBERT J. KELLY
DME in the approach and landing application are treated in detail in Refs. 47,48, and 56. In April of 1981, ICAO adopted the precision DME Operational Requirements (23). In addition, modifications were proposed to the existing DME-N ANNEX-10 which would permit the conventional DME to be paired with the angle portion of the MLS. It is anticipated that the conventional DME with 0.2-NM system accuracy will be used to replace the marker beacons now used for category I approaches during the early stages of MLS implementation and may continue to serve many MLS applications in the future. The radar altimeter currently used by air carriers would, during this period, provide the highly accurate vertical guidance necessary for category I1 approaches and automatic landings. In addition, the availability of DME at MLS sites will permit the automatic gain scheduling required in autopilot operation. The DME-P ICAO standardization process is expected to be completed in 1982.
IV. SYSTEM DESIGNCONSIDERATIONS The design of the MLS system is based on thorough analysis and test which have provided a complete and fundamentally sound system specification in equipments which can be implemented. The requirements of the specification are traceable to the basic physics of the landing operation. The design of MLS resulted from the efforts of the many government agencies and laboratories involved as well as the many participating industrial contractors both in the United States and abroad. This section presents (1) the rationale for the accuracy requirements; (2) the sources of error in MLS; (3) the factors which affect the power received at the aircraft; (4) the considerations in siting an MLS system in existing airport configurations; (5) examples of MLS performance; and (6) the monitoring of the signal to provide system integrity.
A . Accuracy The ICAO accuracy specification provides a signal-in-space of uniform guidance quality at all SARPS compatible MLS-equipped runways. The signal quality will be equivalent to present-day category 111 accuracy and automatic landing specifications ( I , 3). Further, the basic specification will lead to economic and simple flight inspection standards and measurement procedures by defining only two error components : the path-following error and the control motion noise.
MICROWAVE LANDING SYSTEM
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The following material will provide the operational basis for the accuracy and its measurement, catalog the error contributions, and finally develop the accuracy budgets, including experimental justification and rationale. 1, The Accuracy Specification
Guidance accuracy needs to be considered in all regions of the terminal area, but as would be expected, the most critical region is in the vicinity of runway threshold. As mentioned earlier, the accuracy standard is measured at the single MLS reference datum (50 ft above runway threshold). This is in distinction with ILS, wherein the accuracy is measured at the different minimum guidance height (MGH) associated with the appropriate decision height.2 The permissible path deviations are expressed as a linear displacement from the nominal runway centerline and glide slope. The lateral guidance error is specified at f.20 ft, 2 sigma, and the elevation error is + 2 ft. The accuracy will be maintained regardless of runway length, ground terrain, multipath, or weather conditions. The point here is one of signal quality and not runway weather minimum certification. The ground system could be commissioned for any one of the category I, 11, or 111 service categories ( 2 , 3 ) .
2. The Operational Basis,for Accuracy The obvious purpose of a landing system is to assist a pilot and his aircraft in the effort necessary to have a successful touchdown and rollout. The importance of the aircraft landing phase and landing guidance accuracy is highly evident in the statistics presented in Ref. 44, where it states that 55% of commercial jet accidents occur during the landing phase, which is only 14% of the total flight exposure. A successful touchdown is defined as one in which all pertinent aircraft performance variables are within their respective ranges of acceptable values. The purpose is first to place the aircraft in proper position and attitude to execute either manual or autolanding with an extremely high probability of success. Ifone or more variables exceeds its range ofacceptable values, an accident does not necessarily result because there is a “cushion” built into each of these ranges. The “decision height” is a height above the runway below which a pilot must not descend unless adequate visual references have been obtained. He must land by visual means or execute a missed approach. The decision hei@ for category 1 operations is 200 ft with a minimum guidance height (MGH) of 150 ft; category I1 is 100 ft with MGH of 50 ft. ILS at best does not provide guidance below 50 ft, but a radio altimeter can provide height information to touchdown for autolandings.
TABLE IX RANGEOF ACCEPTABLE VALUBOF AIRPLANEVARIABLES AT TOUCHDOWN (CTOL)” Acceptable value Variable
X,:
Min 800 ft
longitudinal position from runway threshold -hTD: sink rate
0
~ T :D
0
pitch attitude
uTD
:
airspeed
C, : lift coefficient
A D:
lateral runway
J~TD: lateral drift velocity
&D
~
’
misalignment angle between inertial velocity and fuselage centerline T :D bank angle
Max
Reason for limiting values
2300 ft To ensure a touchdown on the runway and within the lighted touchdown zone 5 ft/sec Too “hard” a touchdown will damage the landing gear
Comments The FAA requires a 2a total dispersion of less than 1500 ft about some nominal point 5 ft/sec is considered to be limiting value for passenger comfort
To keep from landing on the nose wheel or hitting the tail on the runway 125 kt 125 and 145 kt corre145 kt A lower limit is required to avoid spond to u,,, k 10 kt,which gives 1.1 stalling or losing control of the airand 1.311.1aII plane. An upper limit is required to avoid overrunning the runway after touchdown 0.83 CLmaxcorreAn upper limit is 0.83cLm.x required to avoid sponds to 1.1pc,,,,, in lg flight; and to losing maneuver capability 1.5pSlal, during 1.lg flight -27 ft 27 ft To ensure a touchThe FAA requires a 2 sigma dispersion down and rollout of 27 ft from the without running off runway centerline the side of the runway - 8 ft/sec 8 ft/sec To avoid running off 8 ft/sec corresponds to about a 2” track the side of the runerror way after touchdown Passenger comfort is -4” 4” To avoid excess side also a factor loads on the landing gear
- 5”
5”
5”
To avoid having a wing tip or engine pod hit runway
Annex-10 specifies k2O
The guidance errors must be structured so that they also satisfy the attitude and maneuverability criteria listed in this table. CTOL, conventional takeoff and landing.
MICROWAVE LANDING SYSTEM
36 1
A list of the pertinent parameters to be met at touchdown and their corresponding ranges of acceptable values is given in Table IX (52). They were obtained as a consensus of FAA and industry judgment with FAA limits taking precedence when they were available. As shown in Table IX, many factors contribute to the lateral and longitudinal touchdown dispersion for typical landings. These include the various wind components, flight control errors, and the MLS guidance error, which is the difference between the indicated angle provided by the MLS and the actual angular position. The MLS guidance error specification is based on a number of considerations including the longitudinal dispersion of the touchdown location, the permitted lateral dispersion, the implications of the category I1 decision window of +75 ft lateral dimension and _+ 12 ft vertical, and the requirement to be able to control the vertical deviation from the intended glide path within specified limits. The guidance error is budgeted with other sources of touchdown dispersion to fall within the tolerances indicated in Table IX. It is reasonable to allow the accuracy to degrade away from the MLS reference datum in both the horizontal and vertical dimensions of the service coverage in such a manner that it is consistent with all the operational requirements in the approach maneuver zones. These zones are defined in Fig. 3. The MLS accuracy degrades at the limits of MLS coverage so as to be within the NAVAID error cells which interface or supplement the MLS guidance functions. These degradation factors, which are specified in the MLS SARPS, allow the MLS azimuth function path-following error to increase to 1200 ft at the extreme lateral MLS coverage limits. This is well within (10 sigma), the proposed (1985) 2-NM airplane separation criteria (27), which, in fact, is determined by “pilot blunder” distributions (50) rather than 10-sigma guidance errors. Similarly, the MLS accuracy on final approach (10 NM) is 300 ft, and again is well within the 5000-ft (2500-ft, future) runway separation criteria, which, in turn, is also a strong function of pilot blunders rather than the guidance errors. 3. The Components of the MLS Error To further understand the effect of the MLS error on the landing operation, the actual dynamics of the landing process need to be taken into account. It is instructive to view the operation as a sampled data control system, as shown in Fig. 24a. The azimuth or glidepath which is selected is the input to the system and the output is the actual path. The guidance loop contains
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HENRY W. REDLIEN AND ROBERT J. KELLY
r---1 FLIGHT CONTROL SYSTEM
,
SELECTED
LINEAR AIRCRAFT DYNAMICS
-POSlTlON
r T
SCANNING BEAM
GEOMETRY
(ANGLE) (a)
9AOIANSISEC LUNLllUUlNAL CliANNtL [ k L t V A I I U N I
RECEIVER OUTPUT
0.1 SEC (10 RAOBEC 1
FIG.24. (a) The landing system block diagram. (b) Error spectrum. (c) Lateral Autoland, closed loop response-radio noise to localizer deviation. (d) Lateral Autoland, closed loop response-radio noise to aileron deflection.
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MICROWAVE LANDING SYSTEM
250
200
ILu n 150
E0 u '
100
$ P
\
2%
0 10-3
50
0 100
10.2
(d)
101
F R E W E N C Y IRADIANS/SEC)
FIG.24c and d
the flight control system, the aircraft-response dynamics, a geometry factor to account for distance, the ground-scanning beam, and a difference device which represents the MLS receiver. For most aircraft the guidance loop bandwidth is less than 0.5 rad/sec for azimuth and 1.5 rad/sec for elevation. Several conclusions can be drawn considering the case for azimuth. The first is that it is only the guidance error components below 0.5 rad/sec which can cause the aircraft to deviate from an intended path. A second is that the
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HENRY W. REDLIEN A N D ROBERT J. KELLY
output bandwidth of the receiver must be greater than the loop bandwidth so that guidance signals can be provided at the maximum frequency of the loop without significantly eroding the amplitude and phase margins of an automatic flight control system (AFCS) necessary to assure aircraft system stability. A 10-rad/sec single-pole filter has been found adequate to do this. However, the 10-rad/sec filter permits error components between 0.5 and 10 rad/sec to be input to the AFCS. Although they do not contribute to a path-following error, they can cause vibrations in the control surfaces or the pilot control column and therefore affect pilot confidence in the guidance system. The path-following error (PFE) is equal to those components of the MLS guidance error which have a frequency of 0.5 rad/sec or less for azimuth and 1.5 rad/sec or less for elevation. The control motion noise (CMN) is equal to those components which lie between 0.3 and 10 rad/sec for azimuth and between 0.5 and 10 rad/sec for elevation. These parameters represent a general government/industry consensus. The concepts discussed above are illustrated in Fig. 24b and shown to correspond to flight control practice. Figure 24c and d are the control loop and guidance transfer junctions for localizer deviation and aileron deflection for the Lockheed, LlOll flight control system. These concepts also apply to short takeoff and landing aircraft (STOL) (34). 4. Basis for the Path-Following Error The azimuth path-following error specification of 6 m (20 ft) is intended to confine the lateral touchdown dispersion for automatic landings in the presence of turbulance so as to achieve the equivalent landing performance obtained by manual landing under VFR conditions. The 3.4-m (1 1.5-ft) path-following noise component is intended to restrict lateral displacements to less than 3 m (15 ft) in the region from 1000 m (3300 ft) to the reference datum3 [(9), Attachment C to Part I, Para. 2.1.41. The following simple calculation demonstrates that the 20-ft PFE is consistent with the lateral dispersion permitted the FAA’s automatic landing specification (FAA-AC-20-57A; 1) of +27.5 ft (Table IX). For the Boeing 707 with 22-ft wheel span as an example, the probability that the outer wheel will exceed the edge of the runway apron is less than one in a million landings for this dispersion. This corresponds to the performance achieved by jet
’
The azimuth PFE and PFN specifications are intended to be similar to the ILS category I11 standard. The experimental basis for this standard is reported in (36) and (37).
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airliners in VFR conditions. The assumed breakdown of the error components which contribute to the 27.5-ft lateral dispersion is as follows: Path-following error Wind shear (12 knots/100 ft) Aircraft flight control system errors Total error (RSS)
20 ft 18 ft I ft f27.5 ft
The approach elevation path-following error specdications of 0.6 m (2.0 ft) is intended to restrict the aircraft's vertical displacements to less than 1.2 m (4 ft) [ ( I J ) , Attachment C to Part I. Para. 2.1.51. 5. Basis for Control Motion Noise It is intended that the CMN specifications satisfy the criteria for automatic landing and be acceptable to the pilot with respect to aircraft attitude, wheel and column motion, and control surface movement. Table X lists the azimuth control activity criteria which will provide pilot/passenger acceptance. These levels are subjective in nature, but represent a consensus of opinion in the AFCS and airframe industries. Table XI
TABLE X AZIMUTH CONTROL ACTIVITY CRITERIA' Cooper rating: Lateral acceleration: Roll attitude variation: Control wheel variation: Aileron deflection: Condition:
Cooper 11, pleasant to fly 0.04 g 1'-2" 2"-8" =2 15,000-ft runway, at threshold
'2 sigma. TABLE XI AZIMUTH CMN STUDYRESULTS Research group
CMN recommendations
Sperry Flight Systems 0.059" (10-Ht data rate), 0.048" (13.5-Hz data rate), and 10 rad/sec bandwidth McDonnell/Douglas 0.039" at 5 Hz and 10,000-ft runway; 0.036"at 13.5 Hz and 15,000-ft runway
Reference Hazeltine MLS Phase I Final Report (References 9, 69) ITT/Gilfillan MLS Phase I Final Report (Reference 12)
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HENRY W. REDLIEN A N D ROBERT 1. KELLY
lists study results by various researchers; it indicates the level of CMN which will keep the control activity within the acceptability limits listed in Table X. Computer studies reported in Ref. 76 support these results. From these studies, a consensus value (0.04", 2 sigma) was selected as the azimuth-angle CMN specification limit. Similar studies were performed on the longitudinal channel, and a range of values between 0.05 and 0.07 for elevation CMN was deemed acceptable. The azimuth CMN error is therefore specified as 3.2 m (10.5 ft), which corresponds to 0.04" for a 4600-m (15,000-ft) antenna-to-threshold distance runway plus antenna offset). This increases to 0.08" for 8000-ft runways and 0.12" for 5000-ft runways, assuming that there is an automatic desensitization of the autopilot gain to accommodate for the shorter runway. The approach elevation CMN error is specified as 0.03 m (1.O ft) at the approach reference datum, which, for 3" glideslope-siting geometry, including a 121-m (400-ft) antenna offset and a 10-ft antenna phase-center height, corresponds to a 0.067" angular error. For the region beyond threshold out to 20 NM the CMN error needs to be specified within tight limits to support autopilot coupled approaches. The current specification, which can be as high as 0.2", is considered marginal and is currently under review by the FAA and others. It is anticipated that operational experience will limit the specification of CMN to be no greater than 0.1". 6. System Error Budgets For the purpose of facility planning and equipment procurement specifications, the system PFE and CMN can be partitioned into three error components : ground-equipment error, airborne error, and propagation error (multipath). It is intended that the error budgets be satisfied with 95% probability; i.e., on the average the budgets will be exceeded 5 times out of every 100 approaches and landings to a given runway. MLS error budgets must be configured to satisfy the system PFE, system PFN, the groundequipment alignment error, and the airborne centering error as specified in the MLS SARPS. These specifications are used as the basis for the example error budgets which are presented next. a. System path-following error budgets. The ground, airborne, and propagation error components are assumed to be independent, random variables for each aircraft approach and landing, and the combined error for most multipath error sources can be estimated by root-sum-square (RSS) of the three components. The ground-equipment PFE is composed of the alignment error and a scanning-beam-pointing error that results in a PFN component. The course
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MICROWAVE LANDING SYSTEM
alignment error (azimuth) and the minimum glide-path alignment error (approach elevation) include both the actual physical misalignment of the antenna and the drift components arising from temperature variations, aging, etc. These errors vary slowly with time and are considered to be constant over the duration of a single approach and landing (that is, constant for a single flight inspection record). The ground equipment beam-pointing PFN component arises from the imperfections that may OCCUF in the antenna beam-steering mechanism which generate beam-pointing errors that fluctuate as a function of the beampointing angle. These spatial variations will generate the PFN component as the approaching aircraft deviates about the selected azimuth course or glidepath angle. Since the airborne receiver’s PFN is negligible, the only remaining system path-following noise component are the errors induced by multipathparticularly those due to antenna sidelobe reflections and diffuse reflections. The multipath error component may also contain a residual bias component. The PFE components are tabulated in Tables XI1 and XI11 to form example azimuth and elevation error budgets. The ground component is the sum of the “maintain and adjust” limit and the “monitor alarm” margin, TABLE XI1 EXAMPLE AziMum PATH-FOLLOWING ERRORBUDGETS“
Error components (95% probability) Executive monitor alarm limit Maintain and adjust limit Monitor alarm margin Total Path following noise Ground antenna Multipath Total (RSS) Airborne centering System PFE (RSS)
Angular error (degrees) for antenna-to-threshold distances (ft)
Linear error (ft)
15,000
8000
5000
10.0 6.4 16.4
0.062
0.1 16
0.186
0.044 0.017 0.076
0.083 0.017 0.143
0.132 0.017 0.221
5.2 10.3
11.5 4.5 20.0
The system PFE is not allowed to exceed t 2 0 ft for a duration greater than 1 sec. The monitor alarm margin may have to be reduced or additional multipath control techniques employed for those multipath errors which cannot be summed on an RSS basis with the maintain and adjust limit and still satisfy the 20-ft PFE. The system PFE (20 ft), system PFN ( I 1.5 ft), the maintain and adjust limit (10ft), add the airborne centering error (4.5 ft) are ICAO SARPS specifications. It is recommended that an integral monitor be used as the sensor for the executive alarm.
368
HENRY W. REDLIEN AND ROBERT J. KELLY
TABLE XI11 EXAMPLE ELEVATION PATH-FOLLOWING ERRORBUDGETS” Error components (95% probability) Executive monitor alarm limit Maintain and adjust limit Monitor alarm margin Total Path-following noise Ground antenna Multipath Total (RSS) Airborne centering System PFE (RSS)
Linear errorb (ft)
Angular error (degrees)
1.o 0.52
1.52 0.3 1.26 1.30 0.25
2.00
0.10
0.087 0.017 0.133
a The system PFE is not allowed to exceed k 2 ft for a duration greater than 1 sec. The monitor alarm margin may have to be reduced or additional multipath control techniques employed for those multipath errors which cannot be summed on an RSS basis with the maintain and adjust limit and still satisfy the 2-ft PFE. The system PFE (2 ft), system PFN (1.3 ft), the maintain and adjust limit (1 ft), and the airborne centering error (0.25 ft) are ICAO SARPS specifications. It is recommended that an integral monitor be used as the sensor for the executive alarm. * Antenna-to-reference datum 861 ft.
and is called the “executive monitor alarm limit.” (Ground equipment errors which exceed the alarm limit for 1 sec or more shut the site down.) The airborne component is the receiver-centering error, whereas the propagation error is the RSS combination of the multipath error component and the beam-pointing PFN. Example field monitor budgets are tabulated in Tables XVI and XVII. Back azimuth and flare guidance error budgets are not presented. In conclusion it should be emphasized that the executive alarm limit should be adjusted such that the system PFE is not exceeded with 95% probability for a duration of more than 1 sec. This means that the executive alarm margin presented in Tables XI1 and XI11 may in practice be more or less than those margins at a number of sites. 6 . System control motion noise error budget. Example CMN budgets are listed in Tables XIV and XV. The ground and airborne components of the system CMN are the temporal noise errors generated by the ground antenna and airborne receiver. These errors are statistically independent and can be combined on an RSS basis with the propagation (multipath) CMN components.
369
MICROWAVE LANDING SYSTEM
TABLE XIV EXAMPLE APPROACH AZIMUTH95 PERCENT PROBABILKY CMN ERRORBUDGETS
Beamwidth
Runway4 (ft)
System
Ground
Air
Propagation
I" 2" 3"
15,000 8000 5000
0.04" 0.075" 0.1"
0.01" 0.016" 0.024"
0.005" 0.009" 0.015"
0.037" 0.072" 0.096"
a Runway distances include antenna offset distance from stop end of runway.
TABLE XV EXAMPLE APPROACH ELEVATION 95 PERCENTPROBABILITY CMN ERRORBUDGETS'
Beamwidth
Antenna-tothreshold distance (ft)
System
Ground
Air
I" 2"
861 86 1
0.067" 0.067"
0.009"
0.005" 0.010"
0.018"
Propagation 0.066" 0.064"
a At 50-ft threshold crossing on 3" glide path and 10-ft antenna phase center. Antenna offset 400 ft from runway centerline.
7. MLS Measurement Methodology The path-following error (PFE) and control motion noise (CMN) error are evaluated by passing the measured error time history or recording through the standardized filters defined in Fig. 25. Although the term "path-following error" suggests the difference between a desired flight path and the actual flight path taken by an aircraft following the guidance signal, in practice this error is evaluated by instructing the flight inspection pilot to fly a straight course and recording the difference between the airborne equipment output guidance indication from the standard PFE filter and the corresponding aircraft position as measured by a suitable optical or radar tracking system. A similar technique determines the control motion noise error by making measurements with the CMN standard filter. The errors as measured at the output of the filter cannot exceed the specification for more than 5% of the time over any 40-sec portion of a flight record from the limits of coverage to the MLS reference datum (corresponding to the 95% probability; see Fig. 26). This method for determining
G U I D ~ ~ C E CORNER FREWENCIES (RAO/SECl FUNCTION wo *l w2
RECEIVER OUTPUT FILTER:
APPROACH AZIMUTH
0.5
942 PATH FOLLOWING s2+2{wRs+wR2 FILTER-
APPROACH ELEVATION FLARE
1.5 2.0
0.3
10
0.5
10
0.5
lo
- CONTROL MOTION NOISE FILTER:
FIG. 25. Filter configurations and corner frequencies.
FIG. 26. MLS measurement methodology. c is the error specification; T, region to be evaluated; TI, T,, T,, . . . , time intervals that error exceeds specifications. For the ground equipment to be acceptable in this region, the following inequality should be true: ( T, + T, + T3 + . . .)/T 5 0.05.
37 1
MICROWAVE LANDING SYSTEM
the 95% probability was originally proposed in Ref. 36 and was subsequently adopted by ICAO to determine conformance to the ILS beam-bends standards. As an alternative to the use of filters, ground and airborne instrumentation errors can be determined using the following procedures. The average error over a 10-sec measurement interval is equivalent to the path-following error, whereas the square root of its corresponding variance is equivalent to be the instrumental control motion noise error. The errors associated with the various parts of the system are separated by a series of measurements made on the “bench,” in a reflection-free environment, and then in the actual airport environment. First, the instrumentation errors associated with the standard airborne receiver are measured using a bench test instrument and the centering error is adjusted to zero. Second, with this standard receiver the total system instrumentation error is measured by operating the ground equipment 011an antenna range or in some other reflection-free environment. Since the standard receiver centering error is negligible, the measured path-following error can then be attributed to the ground equipment. The ground equipment AZIMUTH TOTAL ERROR (3’ BEAMWIDTH)
0.20
-0.20
CMN COMPONENT
l
-1.0
1
1
1
0
1
1.0
1
1
2.0
1
1
3.0
.
1
4.0
1
1
5.0
1
1
8.0
GROUND RANGE FROM THRESH010 (NM)
FIG.27. Components of MLS error data (Bendix SMCS).
372
HENRY W. REDLIEN A N D ROBERT J. KELLY
CMN is obtained by subtracting the known standard receiver CMN variance from the CMN variance of the measurement. To assist in visualizing these error components, examples of MLS flight test data taken with the Bendix “Small Community System” on Runway 4/22 at FAATC, New Jersey, are shown in Fig. 27. The total guidance error (raw data) is shown in the upper trace. In the remaining traces, the total error has been resolved into the critical specification components, the PFE and CMN. Note that both the PFE and CMN are well within their respective system error budgets, which is to be expected because in this case the runway is relatively free of specular multipath sources, and there is a remaining budget for multipath. This approach to MLS measurements with PFE and CMN filters was suggested originally by one of the authors (R. J. K.) in Ref. 60. It was intended to provide data more directly related to the aircraft flight dynamics than that provided by ILS data. It would also provide further assurance that flight inspection would not reject good installations nor accept poor installations. Several authors (36, 49) have suggested that the appropriate filters applied to ILS would improve and simplify ILS flight inspection. There is currently consideration being given to having ICAO ILS standards modified.
B. Error Analysis As is true of all radio position-fixing devices, errors caused by equipment imperfections and propagation effects may be introduced in the course of generating and detecting the MLS guidance information. An appreciation for these error mechanisms is helpful for a more complete understanding of the system and is essential for those involved in the design of MLS equipment. 1. Error Sources
Landing guidance systems will have, in one form or another, the following potential error sources : (1) (2) (3) (4)
ground guidance encoding errors (clock, beam steering, and antenna) ; transmitter noise-induced errors (negligible); absolute ground reference errors (i.e., antenna alignment) ; propagation errors (multipath); ( 5 ) receiver noise-induced errors; ( 6 ) analog/digital quantizing errors ; (7) airborne guidance decoding errors ; (8) guidance readout truncation errors. The following sections summarize the relations which estimate the
MICROWAVE LANDING SYSTEM
3 73
error magnitudes of the MLS sources in terms of its fundamental system parameters. 2. Fundamental MLS Error Mechanism The effect of the error sources is the distortion of the symmetry of the scanning-beam envelope. These effects can be quantified by beginning with the MLS coding equation
e = (T,, - t y / 2
(1)
where 6 is the azimuth or elevation guidance angle in degrees; t is the time separation in microseconds between TO and FRO mainlobe centers; To is the time separation at 0" scan angle; V is the scan velocity constant in degrees per microsecond. Taking the differential of this expression yields SO = -3V6t = +(OBW/T)
St
(2)
where V, the beam-scan velocity is equal to the 3-dB antenna beamwidth Oaw divided by the 3-dB beam dwell time. The differential time error between the TO-FRO scans is 6t. Equation ( 2 ) is the starting point for all MLS error analysis wherein the problem is to determine the actual form 6t assumes for each error source. The differential error 6t depends on the exact form of the interfering or distorting signal, including its relative amplitude, rf phase, and its distribution or shape in time. Using Eq. (2), a first-order approximation for most error sources (62) is
68 = ) p 6 B W
(3)
where 68 is the peak error in degrees and p is the ratio of the undesired perturbing signal amplitude to the desired signal. The ratio p has very general application. In the case of propagation effects, it is the ratio of the multipathto-direct signal ratio, whereas for receiver noise effects, it represents the inverse of the voltage signal-to-noise ratio (SNR). The ipBBwapproximation represents an upper bound on the errors when p is less than - 3 dB (0.707) because neither the differential rf phase between the direct and unwanted signal nor the disturbing signal shape have been considered. However, it is very useful in estimating errors from receiver noise, diffraction multipath, and antenna sidelobe errors. There will also be error reduction factors resulting from random processes and aircraft motion averaging, which is possible because the guidance data rates are higher than the aircraft-guidance loop bandwidth. The thermal noise error component for receivers with dwell-gate processors can be estimated using Eq. (3). This equation applies when even signal perturbations which effect the leading and trailing edge of the scanning beam
374
HENRY W. REDLIEN AND ROBERT J. KELLY
are uncorrelated. Reference 54 shows that the single TO-FRO scan noise error is he = +(eBw)/(s/N)1/2 (1 sigma)
(4)
where 8 is the 1-sigma error, p = (S/N)-'12,and S / N is the peak video signal-to-noise power ratio. Since by virtue of the noise process each TO-FRO scan is an independent sample, the samples can be averaged by the single-pole 10-rad/sec receiver output filter. This smoothing can be estimated by a reduction factor g defined as g = (data rate/2)(2n/filter noise bandwidth)
(5)
where the filter-noise bandwidth for the single-pole 10-rad/sec filter is 15 rad/sec. Thus, at the output of the receiver the SNR-dependent errors are
68,
=
fe,,[(sj~)~l112
(6)
The video SNR (SjN) is related to the carrier SNR (CjN) at the I F output by (S/N) = (BW,,/BWvidco)(C/n?
(7)
A typical receiver I F bandwidth is 150 kHz. 3 . Instrumentation Errors Simply stated, the ground equipment consists of a CW source and an antenna which shapes the radiation into a narrow beam. A clock in conjunction with a beam-steering mechanism causes the narrow beam to scan TO and FRO at a rate prescribed by the angle-coding calibration in degrees per second. In the airborne receiver, an inverse process, beginning with beam-envelope detection, uses a clock to decode the scanning beams by measuring the elapsed time between the reception of the TO and FRO beams. The instrumentation errors are considered to be those associated with the electronics of timing and the beam formation by the antenna independent of propagation effects. a. Ground equipment errors. The ground error sources associated with the angle encoding process are the time-code generator and the beamsteering mechanism, which, together with the antenna imperfections, establish the accuracy of the angle code [see Eq. (I)]. Noise introduced by the transmitter has been found to be negligible. Misalignment of the antenna from its absolute reference course (centerline or reference glide path) due to temperature and weather effects can introduce measurable errors. Figure 28 itemized the ground error sources. In most ground equipment implementations, only the beam-pointing
375
MICROWAVE LANDING SYSTEM BEAM WlNTlNG ERRORS , GENERATED BY THE ANTENNA IMPERFECTIONS AND THE BEAM
SIGNAL-TO-NOISE INEGLIGIBLEJ
STEERING MECHANISM
STEERING COMMANDS
ANGLE GENERATOR
FIG. 28. Ground error sources (phased array implementations). implementations)
errors and antenna drift error due to temperature and environmental effects are significant. The permitted drift errors are listed as line items 2 and 3 in Tables XVI and XVII. Tables XI1 and XI11 list the permitted beam pointing errors. Rationale for classifying the antenna beam-pointing error as a PFE is as follows. In some antennas the stepping mechanism which scans the beam TABLE XVI AZIMUTH FIELDMONITOR EXAMPLE MAINTAIN AND ADJUST BUDGETERROR
I"
3'
2"
1. Ground antenna alignment (max) 2. Antenna drift (max, without radome effects) 3. Radome drift effects (rain) 4. Ground antenna temporal noise" and monitor receiver noise (5.2 sigma) 5. Monitor receiver drift
0.015" 0.070"
0.010" 0.040"
0.00s
0.023" 0.002"
0.0157" 0.0013"
0.0013" 0.00077"
0.005"
0.005"
0.005"
6. Maintain and adjust alertb (add 1 , 2 , 3 , 4 ,and 5)
0.1 15"
0.072"
0.038'
Antenna beamwidths ~~
0.02"
a Line 4 is a noise margin which allows, on the average, only one false alarm in 100 days. Time constant for the maintenance alert is 10 sec as measured by the field monitor. The longtime constant permits wind-loading effects to be averaged. The executive monitor warning limit is 1 sec, as measured by the integral monitor. The maintain and adjust alert (in degrees) corresponds to 10-ft linear error (SARPS specification) as measured with respect to a given antenna-to-threshold distance. This limit is a maximum value which will be exceeded only in the event of a component failure. The executive monitor alarm limit will, in general, exceed the maintain and adjust alert; see Table XIII.
HENRY W. REDLIEN AND ROBERT J. KELLY
316
TABLE XVII ELEVATION FIELD MONITOREXAMPLE MAINTAIN AND ADJUSTBUDGETERROR Antenna beamwidth
I"
2"
Ground antenna alignment (max) Antenna drift (without radome effects) Radome drift effects (rain) Ground antenna temporal noise" and monitor receiver noise (5.2 sigma) 5 . Monitor receiver drift 6 . Maintain and adjust alertb (add 1, 2, 3,4, and 5)
0.005" 0.02" 0.005" 0.002"
0.01" 0.04" 0.01" 0.002"
0.005" 0.037"
0.005"
1. 2. 3. 4.
__
0.067"
Line 4 is a noise margin which allows, on the average, only one false alarm in 100 days. Time constant for the maintenance alert is 10 sec as measured by the field monitor. The iong-time constant permits windloading effects to be averaged. The executive monitor warning limit is 1 sec, as measured by the integral monitor. The maintain and adjust alert (in degrees) corresponds to I-ft linear error (SARPS specification). Limit is a maximum value which will be exceeded only in the event of a component failure. The executive monitor alarm limit will, in general, exceed the maintain and adjust alert. Antenna with 10-ft phase-center height is placed 861 ft from reference datum (3" glide path) and 400 ft from runway centerline.
develops a beam-pointing characteristic with spatially distributed errors which are constant in time. In other words, each time the beam repeats a scan, the same angular error pattern would be generated at each point throughout the coverage sector. They are converted to apparent time-varying errors by the path deviations of the aircraft. They vary too slowly to contribute to the control motion noise, Fig. 29.4 The antenna CMN error components are small. The values listed in Tables XIV and XV are the recommended values for the monitor maintenance limits. Because of the many different antenna implementations a single relation using Eq. (2) cannot be developed to provide an upper bound on the antenna beam-pointing errors. It is recommended that these errors be determined Some phase-array antenna designs employ a technique called "phase cycling." In this technique, a constant phase increment is added to each antenna-radiating element on each scan of the beam. Its effect is to disturb the beam-pointing spatial pattern such that a time-varying error is introduced. This effect can be beneficial because the beam-pointing error can be reduced by transforming the error into a CMN component, which in turn can be filtered. Another purpose of "phase cycling" is that all the antenna phase-shifter states can be observed by a monitor placed at a fixed point.
MICROWAVE LANDING SYSTEM
377
AIC TRACK
ERROR SOURCE ANTENNA PATTERN
TIME
FIG.29. Relation of antenna beam spatial variations with the path following error
using a complete computer simulation of a proposed antenna implementation to derive the exact value of t . (Reference 58 derives such a relation for the line phased array.) b. Airborne receiver instrumentation errors. As mentioned earlier, the “dwell-gate processor” has been field operational since late 1974. Since then it has become the reference receiver for determining MLS system performance. The receiver detects the envelopes of both the TO and FRO beams, determines the times at each - 3 dB threshold level to establish two dwell gates. The time differential between the midpoints of each dwell gate is directly related to the beam-pointing angle. Although there are alternatives to the dwell-gate processor, it will be used for the presentation because it is generally representative of performance which can be obtained. For undistorted beams, the beam peak and the midpoint lying between the - 3 dB points of the beam are identical, and an accurate indication of the aircraft’s angular position is obtained. Any technique can be used to determine the beam-pointing angle; the only requirement is that the estimates correspond to the true guidance angles. In many applications, guidance systems use algorithms which determine the angular direction corresponding to the peak of a symmetric unimodal beam. Several such techniques as the split-gate tracker and single-edgeprocessor are described in Refs. 30 and 54. These systems have become practical with the advent of the microprocessor, which is integral to all modem MLS angle receivers.
378
HENRY W. REDLIEN AND ROBERT J. KELLY AID OUANTIZATION ERRORS (REMOVED BY 26kH2 DIGITAL FILTER) MDRMALIZE TO OPE
LOG IF
VIDEO DETECT
ENVELOPE FILTER 35 kHz
AID
A AID CONVERTER BIAS DRIFT AND AMPLITUDE NONLlNEARlTlES ARE SMALL
PEAK DETECT
PRECISION CLOCK
LONG-TERM DRIFT @ ERRORS (NEGLIGIBLE)
DEVICE
TRUNCATION ERROR
PROCESSOR
-3 dB THRESHOLD
DWELL GATE MEASUREMENT
ANGLE CLOCK OUANTIZATION ERROR
FILTER
SNR DEPENDENT ERRORS
FIG.30. Angle measurement error sources (phase 111 receiver implementation).
The receiver error sources may be described with reference to Fig. 30 and develop as follows. First, the guidance signal is corrupted by the addition of receiver front-end noise. This converts to an “input” error at the digital angle signal processor generated by the conversion of the analog data to a digital format. The time between the TO-FRO beams is decoded using the precision angle clock whose stability and quantization errors determine the accuracy of the angle code sensitivity (degrees per second). The final airborne error source is the digital truncation error associated with the angle receiver readout. 4. Propagation Efects Any radiating system operating in airports with many hangars, buildings, control towers, etc., will be subject to unwanted reflections (or multipath) from these obstacles. Measures must be taken to maintain system accuracy in even the most severe environments. The analysis and control of MLS propagation errors form a voluminous literature. The Lincoln Laboratory reports collectively document the principal research in this area (32, 35,40-43,67, 68). At the C-band MLS frequency with wavelengths about 2 inches, almost any metal or concrete surface will reflect, scatter, diffract, or shadow the transmitted MLS signal. Relatively small flat surfaces can introduce specular (mirror-like) reflections of high intensity. However, the
MICROWAVE LANDING SYSTEM
379
short wavelengths and the basically irregular distribution of reflecting surfaces cause the reflections to be highly variable in amplitude, phase, and duration to produce a diffuse-like reflection. These vary more rapidly than comparable reflections in ILS which result from the longer wavelengths (2.7 and 0.9 m). The MLS system needs to be designed to operate with short burst of very strong reflected signals as well as a lower level which may be more persistent. a . Operation with strong bursts of’multipath. It was described in Section I11 that the MLS receiver is designed with acquisition and validation circuits to acquire the strongest and most persistent signal and build a confidence that it remains the strongest signal. Should other strong signals dominate, the receiver must reject the originally acquired signal and reacquire the dominant signal. This characteristic permits the MLS to operate in strong multipath environments. b. Accuracy in the presence of multipath. Having been assured that the receiver will decode the direct and strongest and most persistent signal, the question arises of how accurate will the guidance information be in the presence of multipath? As discussed previously, it is recognized that the receiver MLS scanning beams may be distorted, if interfering signals are present at the same time as the direct signal beams are received. The interfering signal will add or subtract to the direct beam, distorting the beam envelope at the - 3 dB thresholds, and cause timing and, subsequently, guidance errors as indicated by Eq. (2). On the other hand, multipath signals arriving at the receiver at other times will not distort the beam envelope nor cause guidance errors. The error disturbance magnitude is a function of p (the multipath-to-direct signal ratio), the separation angle & A , differential rf phase, and the ground antenna beam width. Figure 31 defines the separation angle concept using ground multipath as an example reflection source. Differential rf phase and &A involve straightforward calculations, whereas the estimation of p may involve analysis and careful field e~perimentation.~ Both the multipath duration and error fluctuation rate are readily determined using the multipath geametry. The basic criteria to making MLS immune to multipath effects is to follow the following general guidelines. (1) Take steps to assure that the multipath reflections do not occur at the same time as the scanning beam in the receiver. (2) If multipath does occur at the same time (a) reduce multipath magnitude to a negligible value; (b) average rapidly varying components. References 4 2 , 6 7 , and 68 provide the field data and experimental validation for the computer simulations and theoretical analyses which have been applied to estimate the magnitude, frequency, and duration of the MLS propagation effects.
380
HENRY W. REDLIEN AND ROBERT J. KELLY SEPARATION ANGLE
MULTIPATH
FIG.31. Definition of separation angle.
In the next section multipath control techniques in the control of specular reflections are described. These techniques, however, will be shown to apply also to diffuse multipath and diffraction effects, discussed in Section IV,B,4,d. c. Multipath control techniques, specular rejection. The criteria for eliminating or minimizing multipath effects are achieved by careful design and selection of the antenna patterns and by taking advantage of the high data rate of the TRSB MLS system. Figure 32 illustrates the basic antenna pattern design approaches for four multipath cases. (i) Lateral multipath in the scan direction. Figure 32a shows a plan view of an aircraft on a final approach path and two obstacles, a hangar at a wide angle with respect to the approach path and another obstacle at a small angle. In the case of the hangar, the main beam is received directly as it intercepts the aircraft. At a later time the beam scans toward the hangar and the resulting reflection is received at the aircraft. The fact that the multipath reflection of the main beam arrives at a different time than the direct signal means that it will cause no guidance error. The hangar is considered to be an “out-of-beam’’ signal in the receiver. Reflections are out of beam when the “separation angle” of coding between the direct approach path and the obstacle are greater than about 1.7 antenna beamwidths. Figure 31e illustrates the “in-beam’’ and “out-of-beam” multipath definitions. (ii) Motion averaging. As will be described below, when direct and multipath signals arrive at the same time in the receiver, guidance errors can result. However, depending on the basic geometry and aircraft velocity the errors may vary rapidly so as to be averaged by the receiver filter and reduced. Averaging is permitted because the data rate is higher than the sample rate necessary to provide an accurate reconstructed guidance signal. The excess samples are averaged to reduce both the sinusoidal-like and noiselike errors induced by the aircraft motion as it flies through the multipath interference region. The rate of change of the rf carrier phase of the direct and reflected signal with time may be expressed as a “scalloping” frequency,
f, = d+,,/dt
= ( V/A)(COS LY
- cos /?)
(7a)
MICROWAVE LANDlNG SYSTEM
VERTICAL FAR PATTERN
B LOW HORIZON PATTERN F h OFF AT BdB1DEGREE
DIRECT SIGNAL (d)
FIG.32a-d. (See p. 382 for legend.)
38 1
382
HENRY W. REDLIEN AND ROBERT J. KELLY Kl
01
DIRECT SIGNAL A
p
!
I
REFLECTED SIGNAL
21
A
-
,
OUT-0F-BEAM MULTIPATH
A
( eSEP>1.7 I
I
I
I
t
2 K
U
Ma
'1
t
I
I
ELEVATION ANGLE
s
dew)
\
-
\ \
L
I AIRCRAFT VELOCITY
= 200
FT/SEC
25
O ! 0
I
I
2
4
6
6
10
DISTANCE FROM THRESHOLD (FT
12
1
lo3)
(f)
FIG. 32. Specular rnultipath error source configuration: (a) azimuthal lateral multipath (scan direction); (b) low-angle azimuth vertical path (nonscan direction);(c) low-angle elevation vertical multipath (scan direction); (d) elevation lateral multipath (nonscan direction). ( e ) Illustration of in-beam and out-of-beam multipath. (f) Scalloping frequency geometry.
383
MICROWAVE LANDING SYSTEM
where a is angle from the aircraft to the guidance antenna relative to the direction of the aircraft motion and is the angle between the aircraft motion and the reflected signals. If fi is large, there is a large scalloping frequency (see Fig. 32f). The reduction factors for the azimuth, high-rate azimuth, and the elevation system for a receiver with a 10-rad/sec filter are given in Fig. 33 for various scalloping frequencies. The nonuniform sampling resulting from the jittered function sequence (see Fig. 10) of the signal format reduces the aliased spectrum peaks of the uniform sampled transfer function (bottom right). Noiselike error components are reduced by the noise-improvement factor I/& as indicated by the dotted lines in Fig. 33. A second mechanism is the reflection of the azimuth antenna pattern sidelobes at the time the scanning beam is pointing at the aircraft. These 1.oI' W
I
w
0
1
1.01
1 AZIMUTH (13Hz DATA RATE)
a
v)
J
2 ul
W
Y
HIGH RATE AZIMUTH (39Hz DATA RATE)
2
k
as 0.5
W
I c
0.5
2
I-
4
w
I
0
20
K
.
1
40
60 FREQUENCY (Hz)
80
I
- I
0
100
io
JITTERED FORMAT
40 $0 FREQUENCY (Hz)
rio
1bo
80
100
JITTERED FORMAT
1.o
ELEVATION (40Hz DATA RATE)
Y
n
Lu ul
z
3
P
v)
Y
a 0.5 Lu
Ec
4
Y
K
~~
0
0
0
20
40 60 FREQUENCY (Hzl
80
100
20
40 60 FREQUENCY (Hz)
JITTERED FORMAT
FIG. 33. Average scan function motion averaging improvement factor obtained using a single-pole 10-rad/sec filter.
384
HENRY W. REDLIEN A N D ROBERT J. KELLY
can cause guidance errors because the reflections arrive at the same time as the direct guidance beam. This source of error is controlled by (1) designing the pattern with low sidelobes; and (2) the suitable averaging of the data stream permitted by the high data rate of the system. For example, for complete reflection of sidelobes which are suppressed 25 dB (Fig. 34) for a 3” beamwidth antenna and an assumed averaging factor of 0.38 = l / & (Fig. 35b), the error is
66 = (l/JZ)p6,,(l/Jg>
= 0.045
This error is well within the CMN system budget for azimuth (Table XIV). The I / @ factor arises because the sidelobes independently disturb the leading and trailing edge of the - 3-dB threshold crossings of the beam. Obstacles more in line with the direct beam, at less than the 1.7 beamwidth criteria, are considered to be “in-beam’’ and they arrive at the same time as the direct beam and could cause guidance errors if the distortion is sufficiently large. In this case, the errors are avoided by selecting a sufficiently small beamwidth to meet the 1.7 beamwidth separation angle criteria. (iii) Vertical multipath in nonscan direction. In-beam multipath cannot be avoided in the nonscan plane as shown in Fig. 32b because of reflections from the ever-present airport surface. Although azimuth errors would not be introduced unless there were some physical asymmetry in the reflecting surface, there is usually some asymmetry, and there could be errors if the reflection were strong. In this case the vertical antenna pattern for the azimuth beam is designed to have a very rapid reduction in amplitude at directions toward the airport surface. To obtain this specially shaped pattern requires some vertical aperture for the azimuth antenna and a special
0
0.5 1.0 TIME (msecl
1.5
2.0
FIG.34. Typical azimuth antenna dynamic sidelobe levels.
MICROWAVE LANDING SYSTEM
385
0
10
20
0 4 8 12 16 20 24 ELEVATION ANGLE (DEGREESI
-72
-48 -24 0 24 40 72 AZIMUTH ANGLE (DEGREES)
FIG.35. (a) Azimuth angle and DPSK antenna vertical pattern control (4-ft vertical aperture). (b) Elevation pattern control.
antenna design, as has been described in Section 111. A typical "sharp cutoff' pattern obtained from a 4-ft vertical aperture is shown in Fig. 35a. (iv) Vertical multipath in the scan direction. This type of multipath occurs in the elevation guidance case and it follows the same general principles as developed for azimuth. Figure 32c shows the main beam and the reflection from the sidelobes. The main beam is chosen sufficiently narrow to resolve its specular reflections at the lowest angle of guidance.The separation angles are kept greater than 1.7 beamwidths, which, because of the reflection geometry permits accurate guidance down to about one half that amount. For a 1" beam, accurate guidance is obtained to 0.85" elevation angle. This is substantiated by the detailed error curves of Fig. 36 for in-beam multipath, where a complete computer evaluation has been made for reflections from - 1 to - 12 dB, for beamwidths from 0.5" to 3", and for two values of relative phase between the direct and reflected signal. These curves permit an evaluation of the minimum separation angle permitted, and by calculation, the minimum elevation angle for accurate guidance. Antenna sidelobes need to
386
HENRY W. REDLIEN A N D ROBERT J . KELLY
R F PHASE = 180
1
-'45::
W I
2
3
5
6
Id)
0.5
RF PHASE = 180
4
SEPARATION ANGLE I O s ~ p IN l DEGREES
R F PHASE = 180
0.3j 0.1 ..
X -0.5 *
-0.2
..
1
2
3
4
5
SEPARATION ANGLE ( O s ~ p IN l DEGREES
6
-"'-: -0.3
1
2
3
4
5
SEPARATION ANGLE I esEpjIN DEGREES
be suppressed to an even greater degree than in azimuth since there is always a very strong reflection from the flat airport surface, and because of the geometry at low elevation angles, essentially no data averaging takes place. (u) Lateral multipath in nonscan direction. The elevation antenna has a wide lateral pattern filling the system coverage angle, typically f40",Fig. 32d. Reflections from laterally displaced obstacles may be in-beam and are potential sources of error. The two factors which limit the error are the control of the lateral pattern amplitude and the rather favorable geometry for motion averaging. At an angle 40" from centerline, the pattern amplitude may be reduced as much as 8 dB (0.4 reduction factor) as shown in Fig. 35, which is reduced further by an averaging factor of about 0.4 for a total reduction factor of 0.16. Even an unlikely complete reflection does not cause an error outside the budget of Table XV. Propagation errors, although less tractable than instrumentation errors, are kept well within specified limits by the use of design techniques described above. d. Additional multipath considerations (i) Difuse multipath. In the literature of MLS, diffuse multipath is loosely defined to be the combination of all the other reflection sources in
6
MICROWAVE LANDING SYSTEM
387
the environment which do not scatter energy in a definable direction. Signal perturbations from these effects appear noiselike on flight error traces, and in a benign specular multipath environment, their presence can be detected as an apparent increase in the measured system instrumentation errors. Diffuse reflection from rough surfaces alone is usually very small and has no impact on MLS accuracy. Diffuse multipath as defined here is a spatially distributed interference field which is formed by multipath arising from many (10-20) small reflecting surfaces. As the aircraft moves through the interference field, the relative phases of the components which make up the interference field change with time so as to generate a random (noiselike) time waveform as viewed by the airborne receiver. Diffuse multipath is important because it establishes a noise “floor” independent of receiver noise, which limits the ultimate accuracy of the airborne receiver signal processing. This may be observed as increased noise at low-elevation angles as shown in Figs. 45d and f. Diffuse multipath is minimized by using the control techniques discussed previously and summarized in Figs. 33-36. Diffuse multipath as it applies to MLS or equivalent radar systems is treated in Refs. 28 and 29. (ii) Dijaction multiputh. Scanning beams which impinge upon physical discontinuities, such as the vertical edges of hangars, may be distorted by the phenomina of diffraction. In the beam scan direction, and when the line of sight to the aircraft is not blocked, the diffracted multipath is a replica of the direct signal and can be analyzed using the same concepts which characterize specular multipath (i.e., separation angle reflection coefficient). In addition, it conforms to all the in-beam and out-of-beam relations that are valid for specular multipath where, for example, the separation angle is defined as the angle between the line-of-sight to the aircraft and the ray to the diffraction edge as viewed from the antenna. Figure 37 summarizes these concepts for an elevation signal diffraction geometry where the equivalent of Eq. (3) has been used to approximate the errors in diffraction region and the shadow region. Guidance errors generated by diffraction can be large and must be treated with the same concern as specular multipath. This includes the placement of the MLS elevation antenna with respect to the ILS glideslope near field monitor as well as the placement of the MLS azimuth and elevation field monitors. Reference 70 and the Lincoln Laboratory reports treat the effects of diffraction multipath. To demonstrate the ability of MLS to cope with a diffracted signal, the Belgium Government arranged a test in January, 1978, for the MLS in which two C-130 Hercules aircraft were parked about 1000 ft forward of the Bendix 1” phased-array elevation antenna (5). (See Fig. 38 for the obstacle
388
HENRY W. REDLIEN A N D ROBERT J . KELLY
‘ Rm
=
Rq R?
%d >A
Rf =
dhR,
1
; PDIFFRACTION =
2,
Rf 7
rn AIRCRAFT IS I N THE FAR FIELD OF THE ELEVATION ANTENNA
s pDIFFRACTION IS ALSO THE SIGNAL AMPLITUDE IN THE SHADOW REGION
ELEVATION ANTENNA
n
\
FIG.37. Diffraction multipath plus shadowing.
geometry.) The effects of diffraction multipath were most apparent on the 2” glide-path approach where the pilot’s line of sight to the phase center of the elevation antenna just grazed the tip of the C-130 tail. For these tests, the accuracy performance results were highly dependent upon the aircraft flight path relative to the tip of the C-130 tail. As shown in this extremely stressful multipath situation, the system met the elevation autoland specifications. (iii) Shadowing. Objects which partially or totally obscure the line of site between the airborne receiver antenna and the ground antenna result in a propagation effect commonly known as “shadowing.” In this case there is no unblocked direct signal and it is only the highly attenuated diffracted signal into the shadow region which reaches the aircraft. The preamble function and the scanning-beam signal levels have been designed with sufficient SNR margins in the critical approach region to overcome signal attenuation due to shadowing effects. The magnitude of the signal attenuation can be calculated using Fresnel diffraction theory. Experimental data is limited, but attenuation in the range of - 10 to - 15 dB, as reported in Ref. 31 and the Lincoln Laboratory reports, are indicative of the magnitude to be expected. Significant blockage is observed when a substantial portion of the first Fresnel zone is blocked. The performance of MLS in the shadow region is dependent on the degree of shadowing, the geometry of the situation, and the possible presence
389
MICROWAVE LANDING SYSTEM
BENDIX TEST BED 1' ELEVATION ANTENNA
-1000 FEET
CAFU 740
1
0
1.o
I
2.0
i
3.0
4.0
5.0
GROUND RANGE FROM THRESHOLD (NM)
FIG.38. Brussels C-130 diffraction test?
of multipath from other obstacles on the airport. For blockage alone without multipath, there is an attenuated signal which the MLS receiver may acquire, but there is an attendant guidance error when the primary diffracting edge is normal to the scanning-beam direction. This error is proportional to the angular distance from the antenna to obstacle ray and the ray from the antenna to the aircraft as the aircraft descends behind the obstacle. The physical explanation is that the signal which is present at the line-of-sight position (top of obstacle) is diffracted down to the aircraft which is at a lower angle; hence, the angular difference is the error. When the obstacle discontinuity is along the nonscan beam direction, there is signal attenuation, but no errors are induced; e.g., an azimuth beam which is partially blocked by,the top of an aircraft tailplane.
390
HENRY W. REDLIEN AND ROBERT J. KELLY
For cases of blockage in the presence of multipath, the error depends on the prior history of the guidance. If the receiver has acquired the attenuated blocked signal, then the introduction of strong multipath will have no effect for a least 10-20 sec. If there is no acquisition history and the multipath is strong, the receiver can acquire the multipath and introduces a significant error. However, studies and experiments (.35,40) to date indicate that shadowing phenomena, which are common to all microwave landing systems, can generally be avoided and do not affect MLS operations. Figure 39 illustrates qualitatively the sources of specular reflection, diffraction multipath, and shadowing. ( i u ) Sidelobe considerations. The antenna sidelobe6 design should satisfy two conditions: (1) the dynamic sidelobe level should not prevent the airAREA OF
DIFFRACTION
AREA OF DIFFRACTION
AZ
FIG.39. Hangar-caused multipath and signal blockage areas. There are three classifications of sidelobes depending on how they affect the processing of the guidance signals by the MLS receiver. They are defined in Section V.
MICROWAVE LANDING SYSTEM
39 1
borne receiver from acquiring and tracking the main beam; (2) the effective sidelobe level should be compatible with the system error budget. Lateral multipath reflections from the azimuth antenna sidelobes and ground multipath reflections from elevation antenna sidelobes can perturb the main beam and induce angular errors. To ensure that the error tl generated by these sidelobes is within the propagation error budgets recommended in Tables XI11 and XIV, the required “effective sidelobe level” (pESL)can be defined to be
60/bw (8) This effective level is the ratio of an observed peak error ( h e ) to the beamwidth (OBw) for a situation where the obstacle reflection coefficient and motion-averaging factor are unity. If these two factors are present, the observed error is P E ~ L=
69 = P ~ PRL P M A ~ B w where pR is the multipath reflection coefficient, 8Bw is the ground antenna beamwidth, and PMA is the motion-averaging factor.’ Since the effective sidelobe level is related to dynamic sidelobe level, it is important that the antenna design specification also include the dynamic level. The effective sidelobe level is related to the dynamic sidelobe level PDYM by KPDYM where K is a reduction factor which depends upon the antenna implementation. Example sidelobe reduction factors are : PESL
=
( 1 ) a factor related to the antenna element pattern which reduces the multipath signal level relative to the coverage volume ; (2) a factor related to the degree of randomness in the dynamic sidelobe; for the azimuth antenna this factor equals l / $. The motion-averaging factor depends on the specific multipath geometry, the aircraft velocity, and the function data rate and the output filter bandwidth. For combinations of multipath geometry and aircraft velocity such that the motion frequency is greater than 1.6 Hz, the motion factor is given by Eq. (5). In general, this factor only applies to the azimuth antenna. Sidelobe errors for the elevation antenna typically have very low scalloping frequencies.
’
The factor does not apply because the sidelobes are typically half the width of the main beam. Therefore, the lobes are of-opposite s i p ; thus one threshold crossing is raised while the other is lowered. The error is therefore twice the value predicted by Eq. (3).
TABLE XVIII AIRBORNE POWERBUDGET Elevation beamwidth
Azimuth beamwidth Power budget item
IF SNR required for: 72% decode rate (dB) 0.2% CMN (dB) Acquisition (dB) Noise figure (dB) Noise power in 150-kHz IF bandwidth (dBm) Cable loss (dB) Airborne antenna gain (dBi)b Margin (dB) Signal required at aircraft (dBm) High-rate azimuth function. dB above an isotropic antenna.
DPSK
Clearance
11 - 122 5 0
6 - 95
2"
-
-
-
-
9
6.5 11 - 122 5 0 6 -93.5
6.5 I1 -122 5
-
5 -
1"
0
6 -93.5
11 -122 5 0
6 -91
3"
3"a
-
7.7 11 - 122 5 0 6 -92.3
12.5 -
11 - 122 5 0 6 - 87.5
1"
2"
-
-
6.5 11 - 122 5 0
6.5 11 - 122 5 0 6 -93.5
6
-93.5
393
MICROWAVE LANDING SYSTEM
This factor can be further reduced at high "scalloping" frequencies where the multipath-induced errors in the TO and FRO beams are uncorrelated. In order to satisfy the azimuth CMN error budget (Table XV) and the elevation guidance PFE it is recommended that the effective sidelobe level not exceed -23 dB fot a 3" beamwidth azimuth beam and -25 dB for a 2" beamwidth elevation beam.
C. Power-Budget Considerations Power budgets serve a purpose similar to that discussed previously for error budgets. It is necessary to identify both the ground and airborne equipment losses and antenna gains to ensure that the signal-in-space can be adequately detected and measured by the airborne receiver. Tables XVIII and XIX are example airborne and system power budgets. TABLE XIX SYSTEM POWERBUDGEP Angle beamwidth Azimuth Power budget itemsb
DPSK
Clearance
2"
3"
Elevation 2"
Signal required at aircraft (dBm) Propagation loss' (dB)
- 95 138.7
-93.5 138.7
-91 138.7
-87.5 138.7
-93.5 138.1
0.5 2.2 0.3 3.0 2.0 4.3
0.5 2.2 0.3 3.0 2.0 4.3
0.5 2.2 0.3 0.5 2.0 3.1
0.5 2.2 0.3 0.5 2.0 3.1
0.5 2.2 0.3
I .o
2.0
2.0
2.0
I .5
1.5 - 16.3
1.5 -14.4
I .5 - 14.1
43.4'
40.5
Probabilistic losses (dB) (a) Polarization (b) Rain (c) Atmosphere (d) Horizontal multipath (e) Vertical multipath Total (a)-(e) (RSS) (dB) Horizontal and vertical pattern loss (dB) Monitor loss (dB) Antenna gaind (dB) Net power gain at coverage extremes (dB) Required transmitter power (dBm)
-
1.5 -
-7.3 42.1
- 13.3 -
38.6
-
I .o 2.5
-
38.0
k40" azimuth coverage; 0-20" vertical coverage; 20-NM range. Losses and antenna gains are representative values. ' Distance to azimuth antenna taken as 22.5 NM. The required transmitter power can be reduced by using higher efficiency antennas. High data rate for 3" azimuth beamwidth will reduce required transmitter power by 4.8 dB.
*
394
HENRY W. REDLIEN A N D ROBERT J. KELLY
Three criteria established the angle power budgets for typical receiver implementation : (1) DPSK transmissions must have at least a 72% detection probability; (2) angle CMN must be maintained within 0.2" (2 sigma) at the limits of range coverage (this CMN may be reduced; see Section IV,A,6). (3) angle single-scan acquisition requires at least a 14-dB video SNR as measured at the beam envelope filter. The system power budget is presented in Table XIX. The power density (dBw/m2) specified in Table VIII is related to the signal power specified in Table XVIII at the aircraft antenna by the relation power into isotropic antenna (dBm) = power density - 5.5
(9)
At the limits of range coverage (20 NM) the principal source of CMN is internal receiver noise. This noise-induced error can be estimated using Eq. (6), which reflects the CMN dependence upon ground antenna beamwidth and sample rate. The power-budget tables highlight these dependencies. The angle function measurement assumes a 26-kHz beam envelope filter bandwidth. The video SNR given in Table XVII is related to the IF SNR by Eq. (7). The DPSK preamble function analysis assumes (1) a carrier reconstruction phase lock loop airborne receiver implementation, and (2) that the receiver preamble decoder rejects all incorrect preambles which do not satisfy the Barker code or are identified by the preamble parity bit. Items (a)-(e) in Table XIX are functions of the aircraft position and weather, which have been assumed to be random events. That is, they will only on rare occasions simultaneously reach their worst-case values. Therefore, these losses are viewed as random variables and are root-sum-squared to obtain the RSS loss component. Table XVIII provides an example of an airborne power budget that contains the assumptions which were used in developing the power density standards. Criteria for CMN and acquisition use Eqs. (6) and (7) and assume a 26-kHz beam envelope filter to determine the SNR in the 150-kHz I F filter. The cable loss is user dependent and therefore can be traded with other parameters depending upon aircraft type. Finally, the adjacent channel interference standard has been structured such that there is at least a 5-dB margin to account for variations in the effective radiated power above the minimum power density specified in Table VIII.
D . Siting MLS siting is concerned with minimizing propagation effects caused by airport structures, ground terrain, ILS facilities, and interference by MLS
MICROWAVE LANDING SYSTEM
395
A2
8BW
500
la* ) DEGREES 7
FIG.40. Azimuth function maximum beamwidth criteria.
on existing ILS installations. These effects were identified and discussed in Section IV.
1. Azimuth Antenna Figure 40 defines the geometry and gives the equation for determining the ground antenna beamwidth so that lateral main beam multipath is out-of-beam relative to the runway centerline. References 40 and 43 discuss the airport critical areas which are related to the beamwidth formula. In the nonscan direction vertical pattern control prevents ground multipath interference from attenuating the desired signal both over the runway in the touchdown region and at long range. Because the separation angle is small, azimuth site ground multipath is primarily a SNR or power budget problem. Figure 41 shows the expected signal amplitude response for a 4-ft vertical aperture over flat terrain as a function of elevation angle. The free-space pattern for this design is shown in Fig. 35a, where the underside slope of the vertical pattern at the horizontal can be designed to at least 7.5-dB/degree and so that the first sidelobe should not exceed - 13 dB. For this design small errors may be generated for terrain in front of the antenna which slopes significantly or is highly inhomogenous or has obstructions such as approach lights. For further details, see Refs. 32, 35, 39,63, and 64. 2. Elevation Antenna
As mentioned in Section IV, the antenna beamwidth is selected such that ground reflections for the minimum elevation angle (glide path) and the maximum slope of the front course terrain are out of beam. Figure 42 defines the geometry and presents the equation for determining the beam-
H = 7.28 FT
40 361
RUNWAY
k
0'
16 16..
I a 12.-
8t--
LI
8 -=
4
1
I
0.333 0.667
1
I
I
I
1.00 1.33 1.66 2.00 2.33 2.66 3.00 ELEVATION ANGLE (DEGREES)
FIG.41. Azimuth antenna vertical pattern, flat terrain with H = 7.28 ft and A
2 ELEVATION
'BW
'SA 1 . 7
FIG.42. Elevation beamwidth criteria for front course terrain.
4
3.33 =4
ft.
MICROWAVE LANDING SYSTEM
397
width. In the figure, multipath reflection “2” effects are computed with = 0. Lateral in-beam multipath in the nonscan direction can be minimized by use of the pattern control shown in Fig. 35b. The pattern should be controlled to - 3 dB at f20” and -7 dB at +40”. More details concerning elevation low-angle performance can be found in Refs. 28, 42, 45, 46. Vertical multipath problems which are associated with the flare maneuver are discussed and resolved in Refs. 54 and 59. &OPE
3. Split Siting Figure 9 depicts ground facility locations for a typical expanded split-site installation with respect to the runway. The azimuth antenna, DME antenna, and field monitor are normally located on the extended runway centerline, whereas the shelter housing the azimuth angle coding/transmitter electronics and DME transponder might be offset approximately 200 ft to provide personnel safety during the perforinance of maintenance on system components other than the angle antenna. The approach azimuth and missed-approach azimuth antennas are placed sufficiently far beyond the end of the runway to meet obstruction limitation restrictions. The elevation and flare antennas are normally located approximately 860 and 3700 ft, respectively, from the threshold and beside the runway, sited so as to illuminate their respective areas of coverage during approach, flare, and touchdown. Offsets of these stations from runway centerline could range from 250 to 600 ft (76 to 183 m), depending upon the category of service desired and the ILS glideslope facility/taxiway geometry. Missed-approach requirements dictate that the missed-approach azimuth facility should be located at the threshold end of the runway.
4. Collocated Siting Systems may be sited with the azimuth, elevation, and DME located together, in substantially the same location as the split-site EL elevation location. It is expected that the centerline guidance plane will pass through the runway centerline extended at minimum guidance height (MGA) for the nominal glideslope and would therefore be rotated slightly toward the runway. 5. MLS/ILS Collocation The MLS must operate satisfactorily and without degradation when collocated with existing vhf/uhf instrument-landing systems to serve the same or other runways. At the same time, the performance of the ILS should not be degraded by the presence of the MLS.
398
HENRY W. REDLIEN AND ROBERT J. KELLY
Various types of ILS localizers currently in use are listed below. Each of these will require different standards for collocating the MLS azimuth antenna. a. Eight-loop antenna. The MLS azimuth antenna may be located 80 m (350 ft) in front or in back of the 8-loop localizer antenna. The MLS antenna should not affect the ILS signal appreciably. b. ANIMRM-7. This is an array of dipole elements which is opaque and somewhat shorter than the waveguide localizer. The MLS needs to be located in front of the dipole and waveguide localizer. c. Alford traveling-wave and log periodic antennas. These antennas occur in two lengths: 14 m (45 ft) and 26 m (85 ft). The shorter antennas may be treated like an 8-loop antenna and the longer antennas like the V-ring array, where the MLS may be mounted in front without causing undue interference. d. Parabola. Because of its greater height, this localizer antenna is located back from the runway threshold to provide obstacle clearance. The MLS may be located in front of it in its near-field region. Siting of the MLS elevation facility will depend upon the presence or absence of an ILS glideslope facility. When collocating MLS with an existing ILS glideslope, their threshold crossing heights should be identical for that MLS glide path which is equal to the commissioned ILS glide path. Since the phase center height of the MLS elevation antenna may be 2.74 m (9 ft) (flat terrain assumed), the MLS antenna must then be positioned forward of the JLS antenna. The position inboard to the ILS antenna (nearer to runway centerline) is also preferred. For example, a 3" glideslope will place a 2.74-m (9-ft) phase center height MLS antenna 52 m (172 ft) in front of the ILS antenna. The selection of the MLS elevation antenna location should take into account the position of the ILS monitor antenna which can be a source of diffraction multipath. For full-capability systems, the flare antenna is located 820-900 m (2600-2950 ft) directly behind or slightly inboard of the elevation antenna. In cases of conflict with the ILS glideslope, the flare antenna may have to be located on the opposite side of the runway. When an MLS provides guidance to a runway which is served by an approach lighting system (ALS), a potential conflict exists between the azimuth antenna and/or back azimuth antenna and the ALS. The MLS antenna(s) should not be elevated so high as to violate the ALS blockage requirements of ICAO Annex 14, Supplement 8, and, in this siting, should provide the required guidance accuracy. The MLS approach azimuth antenna and its monitors may have to be located within the approach light lane of a runway instrumented for two-way service. The missed-approach azimuth antenna and its monitor (when split approach azimuth siting is used) will have to be located in the approach light lane even for one-way instrumented runways.
399
MICROWAVE LANDING SYSTEM
E. Examples of MLS Performance
In this section, test data results are discussed which illustrate the various classes of error components. The data was collected with the Bendix "Small Community System" (SMCS), and the Phase I11 prototype airborne receiver (6). The Bendix SMCS antenna beamwidths are 3" and 2" for the AZ and EL antennas, respectively, developed by the use of Rotman lenses. Performance details of this system are summarized in Ref. 51. Table XX summarizes the data taken on the Phase 111 receiver and the multimode receiver assembled at CALSPAN, Inc. (see 30). Note that the PFE (bias) of the error receivers is negligible (< 0.005*). Ground-equipment noise errors measured on the azimuth beam was also within the tables and specifications for control motion noise. 1. Beam-Pointing Errors
Figure 43 is an example of beam-pointing errors measured on the Bendix antenna range in Towson, Maryland. It is the error specification listed in Table XI1 which addresses these antenna errors in the vicinity about runway centerline (i.e., k2"). 2 . Field-Test System Errors Unlike the antenna range and airborne receiver bench measurements, the field tests measure the entire system including ground and airborne equipments with respect to a given site. TABLE XX COMPARISON OF AIRBORNE PROCESSOR TECHNIQUES' Airborne-receiver accuracy bench-test measurements (2a)
Receiver Bendix
Calspan
Measured by CMN (BW-26 kHz)b 40-Hz data rate CMN (BW-26 kHz) 13.5-Hz data rate PFE centering error CMN (BW-26 kHz) 40-Hz data rate PFE centering error
Dwell gate
Split-gate processor
0.003"
-
0.004"
~
Single-edge processor
Signal input
0.010" ~
- 60 dBm
<0.005" 0.0048"
<0.005" 0.0046'
<0.005" 0.014"
-85 dBm - 85 dBm
10.01
<0.0 1
<0.01
- 85 dBm
If the receiver design matches the transmitter beamwidth, the numbers listed in the table are beamwidth independent. BW. bandwidth.
400
HENRY W. REDLIEN A N D ROBERT J. KELLY (a)
ANTENNARANGE ERRORS
I ** 0
-0.1
*,* ' *a' ,' * , * *** ** * * **
1 ,
I
,
1 .
T
L t
:
:
:
-2
;
r
:
:
:
:
0
1 2
AZIMUTH ANGLE (DEGREES)
2 -
W
; 0.10.-
:
1
B K
K W
w
- MEAN AND 5/D Y
A 2 ANTENNA
2.5 AND 97.5 PERCENTILES
0.05::
0.00-0.05.-
x
Y,rw**dtrt
$ ; ; $ ; : ) * * y** ** 10
*
20
30
*** 40
50
60
FIG.43. (a) Antenna range test, Bendix SMCS AZ antenna. (b) Bendix SMCS static test.
The small community MLS (Bendix) was initially installed for acceptance testing to serve runway 08 at Federal Aviation Administration Test Facility (FAATC). The site has little potential for specular lateral multipath reflections in either the azimuth or elevation antenna lateral coverages. Thus, diffuse multipath and specular ground reflection effects would be the only expected contributors to distortion of the radiated signals other than the instrumental errors discussed earlier. The terrain in the vicinity of Runway 08 is generally flat, with slightly rising terrain in the final approach region of
MICROWAVE LANDING SYSTEM
40 1
the runway. Both static tests (stationary probes of the signal-in-space) and flight tests were performed. Summary results are presented in the following sections. a. Static-test results. Static tests are conducted at a large number of surveyed points within the azimuth and elevation coverage volumes using a mobile test van with an adjustable receiving antenna mast. The accuracy performance should not be substantially larger than the RSS of the antenna range data with the airborne receiver bench test data. Consequently, any increase in the flight error trace PFE or CMN can then be attributed to multipath related effects. The bias errors indicated in the static data are of particular interest in that they @rethe sum of the instrumental beam-pointing errors and any multipath errors caused by signal reflections from the airport environment. Figure 43a presents the total system static error for the 3" beamwidth azimuth, almost 4000 ft from the antenna site, as a function of the receiver antenna height. Note that the total system CMN for the static error is less than 0.013", which would correspond to the combined ground and airborne measurements described in Table XIV. Figure 44 is a static test plot example of a 2" beamwidth elevation antenna measured at various heights above the runway. It is noteworthy that the accuracy is achieved down to 28 ft corresponds to about 2" elevation angle and predicted by the separation angle relation specified in Fig. 42. b. Flight-test results. The system was flight tested extensively on Runway 08 at FAATC where the precision instrumentation required to determine absolute error was available. Flight data traces are presented in Fig. 45, which interpret the total errors in terms of the PFE and CMN components. The ICAO SARPS accuracy requirements are indicated on these figures.
9a
0.16.0.12::
AUTOLAND PATH FOLLOWING ERROR SPECIFICATION
NOTE: 95% NOISE MEASUREMENTS ARE EQUAL TO THE CONTROL MOTION NOISE: SPEClFlCATlON IS? 0.05'
U
0
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68
POLE HEIGHT (FEET)
FIG.44. Bendix SMCS elevation antenna static test runway centerline
ELEVATION PFE
(4 2.5' APPROACH CMN SPECIFICATION LIMIT
(dr
AZIMUTH PFE f
0.20
(4
0
P P w
f
-P
-0.20 0.20
7' APPROACH
---- _ _ _ CMN _ _SPECIFICATION _ _ _ _ LIMIT
0
0
r
AZIMUTH CMN FILTER
W
O
If)
------_-_____-____
P P
-0.20
(9)
7'APPROACH CMN SPECIFICATION LIMIT (h)
a
w -0.20. 3
-1.00
0.00
1.00
I
I
I
1
1
2.00
3.00
4.00
5.00
6.00
GROUND RANGE FROM THRESHOLD (NM)
FIG.45. Bendix SMCS flight tests (NAFEC, New Jersey).
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The data is presented for azimuth and elevation, along centerline for glideslopes of 2.5” and 7”. The azimuth data shows no significant change in either PFE or CMN at the different glide paths. This result is entirely consistent with the static data shown earlier. There is a slight increase due to the diffuse multipath scattering arising from the azimuth main beam and the antenna sidelobes. Also, data is available which demonstrates the stabiiity of the system. Some flights were flown nearly six months apart, and there was essentially no change in the system characteristics. Similar results can be observed in the elevation data, except that, at the lower glide path angle, the noise components of the error data increased due to the expected influence of the ground reflection. Again, this result was forecast by the fact that the front course terrain has a positive slope in front. However, even at the lowest glide path (2.5”, Fig. 45d), the elevation CMN is within the f0.06”specification, and the PFE is well within the requirements. Note that in Figs. 45b, f, and h that the CMN decreases as the glide slope is increased. It is reasonable to assume that at a high enough glide path, the site ground effects on the scanning beam should be negligible and the CMN error will be equal to the error observed in the elevation static tests. In summary, accuracy performance is evaluated on the bench and the antenna range to derive the equipment errors. Upon installation at a designated site, the equipment instrumentation errors, as a complete system, are measured again, using a mobile van to perform static probe tests. The errors are compared with the bench and range data. If all discrepancies are identified, the equipment is then available for field deployment.
F. Monitoring In the design of radio aids to approach and landing of aircraft, it is not sufficient simply to provide precise guidance information, but assurances must be given that this precision is being maintained during landing and that it will be available for long periods. This assurance is provided in MLS by a monitoring approach which is an integral part of the system design and error budgets as defined in Ref. 26’ and specified in Tables XII,XIII,XVI,
* It is important to distinguish between the concepts of integrity and reliability. Integrity is the trust which can be placed in the correctness of the information supplied by the facility. The MLS monitor function provides this assurance. Reliability, on the other hand, relates to system availability and the system hazard probability. For example, category I1 approach and landing availability requirements dictate redundant ground and airborne equipment because the reliability of one system (“single-thread”) is not high enough. Similarly, the lack of sufficient reliability also requires that ground equipment certified for category 111 landings be configured with redundant “hot” standby equipment. The reliability of a “single thread” system cannot satisfy the overall lo-’ system hazard probability required for the 10-sec critical flare maneuver.
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and XVII. An “executive” monitor makes the appropriate measurements continuously to ensure that the signal quality at the ground station never exceeds the alarm limit. In addition, it is desirable to “adjust and maintain” the course alignment error such that it is well within the executive alarm limit. Both the alarm limit and the “adjust and maintain” limit are specified error components in the PFE budgets. A “maintenance” monitor continuously checks the operation of all equipment subassemblies at the “line-replaceable-unit’’ (LRU) level and provides warning when performance starts to degrade, but before it would result in sufficiently poor quality to take executive action. In this way, degraded LRUs can be replaced at noncritical periods thereby maintaining continuous system service. The monitoring function is performed by field, antenna aperture (integral), and internal sensors which may be coordinated by the use of modern, inexpensive “microprocessors.”9 These processors can be designed for the simpler category I systems and then upgraded for category I1 and I11 systems by modular additions to the software programs. In its simplest configuration, the monitor design requires only one field monitor location (or integral angle position) to provide complete executive control and be assured that the proper signals are radiated at all angles. The monitor processor examines the radiated signal-in-space for accuracy throughout the service volume. Specifically, the executive monitoring action are the following: (1) A shift in the mean courseline from its nominal position at the appropriate reference datum beyond the limits indicated in the error budgets Tables XVI and XVII ; (2) an error in the preamble DPSK transmissions for a period of more than 1 sec; (3) a reduction in transmitted power of each signal below the minimum specified power defined in Table VIII for a period of more than 1 sec; (4)an error in the signal format timing; ( 5 ) an error in the TDM synchronization. Because of the inherent simplicity of the MLS design concept, the airborne monitoring and redundancy design has been able to be patterned along proven approaches. The monitoring in the airborne units provides a sufficient amount of self-test to isolate all failure modes which affect flight control. For category I1 and I11 operations only a single configuration, using An approach being considered recommendsthat the 1-sec executive alarm be sensed by an integral monitor. Wind-loading effects on the field monitor antenna may make the I-sec monitor time constant difficult to achieve under the condition that the executive false alarms be less than 1 every 100 days.
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two identical self-monitored receivers, is necessary to provide the failoperational category I11 integrity and the required category I1 availability. V. DEFINITIONS Angle-measurement reference (beam center). The reference for the MLS scanning-beam measurement process is the beam center as estimated by averaging the times corresponding to the - 3-dB thresholds on the leading and trailing edges of the received scanning-beam pulse. Standard receiver for dejning overall system accuracy. An airborne receiver with (1) signal processing based upon the angle measurement reference; (2) negligible centering error; (3) control motion noise error less than or equal to the values contained in Tables XIV and XV; and (4)26-kHz bandwidth beam envelope filter, 10 rad/sec bandwidth angle output data filter, and 1 1-dB noise figure. Separation angle. The difference in coding between the multipath and the direct signals. Out-ofibeam multipath. The condition where the separation angle between the main beam direct signal and the multipath signal is more than 1.7 times the ground equipment antenna beamwidth. In-beam multipath. The condition wherein the separation angle between the main beam direct signal and the multipath signal is less than 1.7 times the ground equipment antenna beamwid t h. Static antenna sidelobe level. The peak of the nonscanning (or static) far-field radiation pattern of the antenna as measured in the scan direction exclusive of the main beam. Dynamic antenna sidelobe level. The level that is measured in a receiver of fixed position which exceeded 5% of the time by the scanning antenna far-field radiation pattern as measured at the function scan rate in the scan direction exclusive of the main beam using a 26-kHz beam envelope video filter. Efective sidelobe level. That level of scanning-beam sidelobe which generates main beam multipath interference corresponding to a given angular error with 95% probability. Antenna beam shape. The shape of the main beam static far-field pattern is measured at 3 dB and 10 dB below the beam peak. The ratio of the 3-dB and 10-dB beamwidths for a wide class of antenna aperture functions (uniform, Taylor, cosine, etc.) is to be from 0.55 to 0.71. Motion averaging. A method of time-averaging the time-varying guidance signal perturbation in the airborne receiver introduced by the aircraft motion through a multipath interference field.
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Pattern control. The technique to shape the beam pattern in the nonscan direction such that potential multipath reflections are attenuated. Scalloping rate. The rate of change of phase between the direct and reflected signal as the aircraft moves through the multipath interference field. It is expressed as a frequency f; and is a strong function of the site geometry and the aircraft velocity vector. Specular reflection coeficient. Relates to the reflected (mirrorlike) radiation that conforms to the geometric optical laws of reflection. The coefficient is a function of reflector size, surface roughness, surface contour, and polarization. Surface roughness tends to diffuse the reflected energy and therefore lowers the specular reflection coefficient. Similarly, the reflector surface contour can modify the magnitude of the reflected radiation whenever the reflector departs from a plane surface. Znstrumentation error. The error component induced solely by the ground and airborne hardware independent of propagation or signal-to-noise ratio effects. Propagation error. The total error induced on the signal-in-space by signal reflections or blockage from hangars, buildings, aircraft, and ground terrain in the surrounding environment. Signal reflections include specular and diffuse multipath, whereas signal blockage effects include diffraction multipath and shadowing. System error. The total system performance as specified in the accuracy budget is determined by adding the individual error components in a rootsum-square fashion to obtain the “system error.” Time averaging. A method of averaging each valid data sample with preceding data to minimize the effects of time-varying signal perturbations. Time gating. A method of using a fixed or variable time gate in an MLS airborne receiver to provide for out-of-beam multipath protection. This term is synonymous with tracking gate.
REFERENCES ANONYMOUS 1. “Automatic Landing System,” Advisory Circular 20-57A, FAA, January 1971. 2. “Criteria for Approving Category I and Category I1 Landing Minima for FAR121 Operations,” FAA Advisory Circular 120-29, September 1970. 3. “Criteria for Approval of Category 111 A Landing Weather Minima,” FAA Advisory Circular 120-28A, January 1973. 4. “A New Guidance System for Approach and Landing,” Special Committee- 117 Radio Technical Commission for Aeronautics, Document No. DO- 148, December 18, 1970. 5. “Bendix-Bell MLS Phase I1 Contract DPT, FA72WA,” Final Report, July 1974.
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6 “Bendix Microwave Landing System Phase 111 (Basic Narrow and Small Community Configurations),” Final Report, FAA Report No. MLS-BCD-R-2801-1, June 1978. 7. “An Analysis of the Requirements for, and the Benefits and Costs of the National Microwave Landing System (MLS),” FAA Report No. FAA-EEM-80-7 (Executive Summary, Vols. I, II), June 1980. 8. “Microwave Landing System Transition Plan-Draft,” Prepared by the U S . Department of Transportation, October 20, 1980. Y. Hazeltine Five-Year MLS Development Program Plan, Hazeltine Report No. 110326, September 1972. ZO. “UHF Distance Measuring Equipment,” Section 35, Chapter 3, International Standards and Recommended Practice, Aeronautical Telecommunications Annex, Vol. 10, ICAO, May 23, 1974. 11. “Instrument Landing System,” Section 3.1, Chapter 3, International Standards and Recommended Practice, Aeronautical Telecommunications Annex, Vol. 10, ICAO, May 23, 1974. 12. “ITT-Gilfillan MLS Development Plan Phase I Final Report,” FAA Report No. FAARD-74-118, September 1972. 13. “Microwave Landing System (MLS) Characteristics,” Section 3.11, Chapter 3, International Standards and Recommended Practices, Aeronautical Telecommunications Annex, Vol. 10, ICAO, 1981. 14. “Microwave Landing System Signal Format and System Level Functional Requirements.” FAA Report No. FAA-ER-700-08C, April 1979. 15. “National Plan for Development of the Microwave Landing System,” FAA/DOT, July 1971. 16. “Non-Federal Navigation Facilities; Proposed Microwave Landing System Requirements,” DOT/FAA 14 CFR Part 171, September 8, 1980. 17. “Operational Requirements for a New Non-Visual Precision Approach and Landing Guidance System for International Civil Aviation,” Report of 7th Air Navigation Conference, WP-90, Agenda Item 3, Appendix A, Montreal, 1974. 18. “Bendix Microwave Landing System Phase I11 (Basic Wide Configuration), Final Report, FAA Report No. MLS-BCD-R-4110-1, December 1980. 19. “Report of the All Weather Operations Divisional Meeting,” ICAO DOC 9242, AWO 78, Montreal, April 4-21, 1978. 20. “Report of All Weather Operations Panel, Sixth Meeting,” ICAO DOC 9200, AWOP/6, March 18, 1977. 21. “Report of the All Weather Operations Panel Eighth Meeting,” ICAO, Montreal, March 11-21, 1980. 22. ”Report of the MLS Executive Committee Concerning Technique Selection,” issued by FAA, DOD, NASA, DOT, February 21, 1975. 23. Report of the ICAO Communications Divisional Meeting, Montreal, March 30 to April 16, 1981. 24. “Scanning-beam MLS chosen by U.S.,” Auiat. Week Space Technol. 102, No. 9,26 (1975). 25. “Time Reference Scanning Beam Microwave Landing System,” US.submission to ICAO, FAA/DOT, December 1975. 26. “TRSB Monitor Philosophy,” Working Paper 35, ICAO All Weather Operations Divisional Meeting, Montreal, March 1978.
AUTHORS 27. Alexander, B., “Report of DOT Air Traffic Control Committee,” FAA/DOT, December 1969.
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28. Barton, D., “Multipath Fluctuation Effects in Trackwhile Scan Radar,” 23rd Annual
Tri-Service Symposium, 1977 (unclassified copy). 29. Beckmann, P., and Spizzichino, A., “The Scattering of E & M Waves from Rough Sur-
faces.” MacMillan, New York, 1963. 30. Beneke, et al., “TRSB Multimode Digital Processor,” CALSPAN Final Report FAARD-78-84, April 1978. 31. Benjamin, J., and Reich, P., “A Survey of Microwave Guidance Problems Related to the Operational Requirements,” Proceedings of the ION National Aerospace, 1972. 32. Berger, H., and Evans, J., “Diversity Techniques for Airborne Communications in the Presence of Ground Reflection Multipath,” Lincoln Laboratory Technical Note 1972-78, September 8, 1972. 33. Boyle, D., “MLS Cleared for Take-Off.’’ Interavia, 10/1978. 34. Brown, S., ef al., Microwave landing system requirements for STOL operations. J . Aircr. 3, No. 2 (1976). 35. Capon, J., “Multipath Parameter Computations for the MLS Simulation Computer Program,” Project Report, ATC 68, Lincoln Laboratory, April 8, 1976. 36. Doniger, J., “Analytical Study of ILS Beam Characteristics for the Systems Research and Development Service, Federal Aviation Agency,” The Bendix Corp., Eclipse-Pioneer Div., Final Report, FAA Project No. 114-1312D,August 31, 1962. 37. Doniger, J., “Analog Computer Study of Category 111 ILS Airborne and Ground Equipment Standards,” The Bendix Corp., Eclipse-Pioneer Div., Defense Documentation Center No. AD616005, September 1964. 38. Edwards, J. W., MLS: On the beam for ’76. Astronaut. Aeronaut., March, 24-30 (1973). 39. Evans, J. E., ef al., “MLS Multipath Studies: Application of Multipath Model to Key MLS Performance Issues,” Project Report ATC-63, Vol. 11, Lincoln Laboratory, February 25, 1976. 40. Evans, J. E., et al., “MLS Multipath Studies: Mathematical Models and Validations,” Project Report ATC-63, Vol. I , Lincoln Laboratory, February 25, 1976. 41. Evans, J. E., et al., “MLS Multipath Studies: Phase 3 Final Report, Development and Validation of Model for MLS Techniques,” Project Report ATC-88, Vol. 11, Lincoln Laboratory, February 7, 1980. 42. Evans, J. E., ef al., “MLS Multipath Studies: Phase 3, Final Report, Overview and Propagation Model Validation/Refinement Studies,” Project Report ATC-88, Vol. 1. Lincoln Laboratory, April 25, 1979. 43. Evans, J. E., et al., “TRSB Critical Area Studies-Part I: Reflection Effects,” ATC Paper 44 wp-5028. Lincoln Laboratory, November 5, 1975. 44. Fries, J., “Commercial Aviation Benefits to Be Derived from the MLS,” DOT-FA72WA3010, December 1974. 45. Frisbie, F., “Low Elevation Capabilities of the TRSB Technique,” London BIP No. I I, AWOP-6 Working Group A, November 1976. 46. Frisbie, F. L., “MLS Performance in Rising Terrain,” London BIP No. 12, AWOP Working Group A, November 1976. 47. Hirsch, C., “L-Band DME for the Microwave Landing System,” FAA Contract WI-713086-1, Final Report, February 1972. 48. Hirsch, C., “L-Band MLS/DME Compatible with ICAO Annex 10,” prepared for Automation Industries, Inc., Vitro Laboratories Division, Final Report, October 26, 1975. 49. Hofman, F., et al., “ILS Glide Slope Standards: A Review of Flight Inspection Standards Affecting Landing Performance and Comparison with Limits Evolved from a System Analysis,” FAA Final Report FAA-RD-74-119, August 1974. 50. Holt, J., Safe separation in controlled flight. J . Inst. Navigation 21, No. 1 (1974).
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51. Jensen, G., and Vickers, D., “The Performance of a Simple Microwave Landing System Configuration,” System Research and Development Service Progress Report, DOT, August 8-9, 1978. 5.7. Johnson, W., and Roger, H., Determination of ILS Category I1 decision height window requirements. NASA [Contract Rep.] CR NASA-CR-2024, May ( 1972). 53. Kayton, M., and Fried, W., “Avionics Navigation Systems,” Chapter 5. Wiley, New York, 1969. 54. Kelly, R. J., and LaBerge, C., “Comparison Study of Airborne Signal Processing Techniques.” NAECON “78.” Dayton, Ohio, May 16, 1978. 55. Kelly, R. J., Guidance accuracy considerations for the microwave landing system. J. Inst. Navigalion 24, No. 3 (1977). 56. Kelly, R. J., and LaBerge, C., Guidance accuracy considerations for the microwave landing system precision DME. J. Inst. Navigation 27, No. I (1980). 57. Kelly, R., Redlien, H., and Shagena, J., Landing aircraft under poor conditions. IEEE Spectrum 15, No. 9, 52 (1978). 58. Kelly, R. J., “MLS Error Sources,” Bendix Technical Memo, MLS-ICAO-58, Baltimore, Maryland, August 8, 1978. 59. Kelly, R., and LaBerge, C., “ M U Flare Low Elevation Angle Guidance Considerations,” Proceedings of the National Telecommunications Conference ’80, Houston, Texas, December 1, 1980. 60. Kelly, R. J., “System Design and Flight Test Results of the Bendix/Bell MLS Category 1/11] Elevation Approach Guidance Function,” Proceedings of the AIAA Mechanics and Control of Flight Conference, AIAA Pup. 74-909. Anaheim. California, August 5-9 1974. 61. Kelly, R., and LaBerge, C., “TRSB Angle Processor Description and Computers Model,” Bendix Memorandum MLS-ICAO-62, Rev. A, December 13, 1977. 62. Kelly, R. J., TRSB multipath control techniques. J. Inst. Nuuig. 23, No. 1 (1976). 63. Lopez, A. R., Scanning beam microwave landing system-multipath errors and antennadesign philosophy. IEEE Trans. Antennas Propag. AP-25, No. 3 (1977). 64. Lopez, A. R., Sharp cutoff radiation patterns. IEEE Trans. Antennas Propag. AP-27, No. I I (1979). 65. Meyer, M. A. “Stepped Beam Instrument Landing System.” U.S. Patent Number 3,735,407, May 22, 1973. 66. Sandretto, P. G., “Electronic Aviation Engineering.” International Telephone and Telegraph, New York, 1958. 67. Shnidman, D. A., “Airport Survpy for MLS Multipath Issues,’’ Project Report, ATC-58. Lincoln Laboratory, December 15, 1975. 68. Shnidman, D. A., “The Logan MLS Multipath Experiment,” Project Report, ATC-55, Lincoln Laboratory, September 25, 1975. 69. Sperry Rand Report on MLS Data Format Analog Computer Studies. Prepared for Hazeltine Corp., August 1972. 70. Wheeler, H., “Multipath Effects in Doppler MLS,” Appendix Q of Hazeltine MLS Phase I1 Final Report, FAA/DOT, Oct. I, 1974. 71. Zeltser, M., “MLS Signal Format Rationale,” 35th Annual Institute of Navigation Meeting, St. Louis, Missouri, June 1979. 72. Cox, R. M., and Kolb, E. F., “Microwave Landing System (MLS)-A New Approach and Landing System,” Aviation Forum, Jan. 1979. 73. Cox, R. M.,and Sebring, J. R., “MLS-A Practical Application of MicrowaveTechniques,” IEEE Trans. Microwaue Theory Tech. Symposium Issue, December 1976. 74. Cox, R. M., and Shirey, J. M., “MLS-A New Generation Landing Guidance Is Here.” IEEE Symposium on Terrestrial Based Navigation, Atlantic City, N.J., December 1980.
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75. “Minimum Operational Performance Standards for Microwave Landing System (MLS) Airborne Receiving Equipment,” Special Committee 139, Radio Technical Commission for Aeronautics, Document No. DO-I77 (approved by RTCA Executive Committee, 17 July 1981). 76. Lanrnan 111, M . H., “An Investigation of Microwave Landing Guidance System Signal Requirements for Conventionally Equipped Civilian Aircraft” Report DOT-TSC-FAA71-24, June 1971.
ADVANCES IN ELECTRONICS A N D ELECTRON PHYSICS, VOL. 57
Microprocessor Systems D.-J. DAVID Universily of Paris and ENSAM Paris. France
I . The Microprocessor Revolution . . . . . . . . . . , . , , . . . , . , . . . . . . . . . . . . . . . . . . . , , . . . A. Introduction . . . , .................. B. Place of Micropro rket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Microprocessor Applications . . . . . . . . . . . . , . . ..................... D. Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. From Logic Gate to Micropcocessor . . . . . . . . . . . , . . , , . . . . . . . . . . . . . . . . 11. Components of a Microprocessor System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. The Memories.. . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. The Microprocessor . . . . . . . C . The Interface Chips . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Assembling a Microprocessor System . . . . , . . , . . . . . . , . 111. How to Deal with a Microproce$sor-Based Application . . . . . . . . . . . . . . . . . . . . . . . . A. Specifying an Application . . B. Example.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Standard Interface Buses.. . IV. System Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Programming Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Development Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. The Choices in the Design of a Microprocessor System . . . . . . . . . . . . . . . . . . . . . . . . ................. A. Microprocessor or Not? . . .
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C. Which Microprocessor?
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I. THEMICROPROCESSOR REVOLUTION A . introduction
We are living the microcomputer revolution. The advent of microprocessors has made data processing facilities available to new areas of application at the same time that it has upset classical application areas by the spectacular cost reduction that results. The purpose of this study is to examine the causes of this revolution, its future direction, and what can be expected from microprocessors. We shall take the user’s point of view rather than the technician’s or the manufacturer’s. Our list of references will be 41 I Copynght 0 1981 by Academic Press, Inc All nghrs of reproduction in any form reserved ISBN 0-1 2-014657-6
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somewhat short since, in this field, more accomplishmentshave been achieved by putting products on the market than by writing theoretical articles. B. Place of Microprocessors on the Market The microcomputer revolution, circa 1975, succeeded the computer revolution of the 1960s. The advent of computers upset problem-solving methods in many fields, but because of their price and size, and attendant expenses, computers were limited to distribution among large companies or administrations having computation or management needs that justified the corresponding investment. In 1968- 1970, new kinds of computers appeared, called minicomputers. They were of lower performance than mainframe computers, but their utilization cost was considerably lower. This has widened their distribution, making them affordable for small businesses, laboratories, town halls, etc. The field of application was getting wider, too, because of the possibility of dedicating a minicomputer to smaller tasks such as industrial process control. This evolution continued until the advent of microcomputers. Microcomputers are real computers with regard to global structure, functions, applications, and, to some extent, performance. However, their price is extremely low: A microcomputer costs roughly 25 times less than a miniand 500 times less than a mainframe computer. In addition, microcomputer size is decreasing more and more. A full computer is now contained in a single integrated circuit chip. Both diminutions have led to a phenomenal spread of microcomputers and data processing because lower prices open new markets. A microcomputer may be devoted to a single experiment in a laboratory or to a single function in a robot machine; home appliances contain microcomputers (e.g., washing machines, microwave ovens, or cameras). Smaller size also allows new applications, especially in the medical field (microcomputerized pacemaker worn by the patient), the automobile industry (ignition systems), and in military domains. There is in fact an avalanche effect: Mass distribution lowers prices, hence still larger distribution. Another effect may be observed, and it was also true in large and minicomputer domains. Progress always leads to two results for a given computer: a product of equal price but better performance and a product of equal performance but much lower price (see Fig. 1). This explains why in a given price category, the best performing product has a strongly increasing power, whereas low-range products have constant performance but decreasing prices. This yields a “bundle” as shown in Fig. 2. It is interesting to examine the relative placement of the three bundles corresponding to mainframe, mini-, and microcomputers. Figure 3 shows
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f year n 4
Price FIG. 1 . The two directions of progress.
that after a certain time, the highest range elements of a family reach the lower range of the family above. Note that the highest performing micros will achieve parity with low-range mainframe computers. We feel that if large mainfkame computers survive (there will always be a race for increasing performance at any cost), minis are doomed at some point. Minis which were not used at their full power have been already replaced by micros, and computers are now sold as minis which were in fact built as micros. On the other hand, microcomputers supersede wired-logic devices in almost all situations. In effect, except for very special cases, or for problems which demand a very fast device (and, in such a case, the cost does not matter), microcomputers have the decisive advantage of programmability.
Ti me FIG.2. PerformOnce evolution of a computer family.
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Time (years) FIG.3. Performance evolution of three computer families.
The same microcomputer is able to deal with different applications by means of a simple program change. Thus, a standard product sold in large quantities (hence, a low price) may be used, which eliminates the need to study a new system for each application. This increases reliability as well. The most important advantage, however, concerns product modification : With a microcomputer, it is sufficient to modify the programs, whereas in wired logic, the whole circuit has to be restudied and rebuilt, and this is much more expensive. The last characteristic of microcomputers which contributes efficiently to the ease of access to data processing facilities offered by microprocessors is the simplicity of building a microprocessor system. A microcomputer system is very easy to understand and to adapt to user needs, even for a nonspecialist. In effect, the technological progress of recent years has allowed the integration in a single chip of an increasing number of functions of increasing complexity. Therefore, building a microcomputer now requires only the assembly of a very few chips [it is now possible to achieve a complete microcomputer with a single integrated circuit (IC)]. Without going to this extreme, a typical minimal system may be built with as few as three ICs (see
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1
I Interface
1
FIG.4. Minimal microcomputer system
Fig. 4).The main chip is the microprocessor, which plays the role of central processing unit (CPU), interpreting and executing instructions. A second chip will fulfill the memory function, preserving instructions and data. A third IC will implement interfaces with the external world. It is perhaps worthwhile to take this opportunity to clarify the terminology with regard to the difference between a microcomputer and a microprocessor. A microcomputer is a computer built around a microprocessor; an end user “sees” only microcomputers, since he is interested only in complete working systems. A microprocessor is only one of the components of a microcomputer: the chip which plays the role of a CPU. The microprocessor extracts instructions from the memory, executes these instructions, and controls all other companents in the system. [Microprocessor market considerations can be found in Faggin (1979) and Zaks (1980b) and advantages of microprocessors are well summarized in Zaks (1977).]
C . Micvoprocessor Applications Microprocessor applications are innumerable, albeit largely unexplored. They are limited more by a lack of imagination on the part of engineers than by microprocessor potentialities. There is a considerable need for education, and the potential profits are enormous. Here, we review the prinoipal, present applications of microprocessors and classify them by: Size (i.e., number of involved chips). One chip (i.e., single-chip microcomputer), or 2 or 3 main chips constitute the low range; 5-10 chips form the average range; 20 or more chips constitute the higher range. Bit-slice microcomputer systems need roughly 50 chips. Complexity (i.e., size of the pertinent program). It has been observed that 80% of applications occupy less than 4K, and even 70% occupy less than 1K bytes of program (K = 2’’ = 1024).
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Domain. We distinguish data processing applications, industrial applications, appliances, and special domains.
1. Data Processing Applications The first microprocessor applications were developed in this domain, since, in the beginning, only data processing specialists were able to understand what use could be made of microprocessors, and they were the only people accustomed to programming. Applications in this domain are divided into two main categories : Central processing unit ( C P U ) construction for lower range minis replacement. A whole generation of microcomputers used in small business applications have appeared, and their capabilities are increasing. On the other hand, some well-known minis have a CPU built around bit-slice microprocessors. Intelligent peripheral control. Here, “intelligent” means that the presence of a microprocessor relieves the CPU of some tasks (e.g., data coherence checking, syntactical analysis, data smoothing). This unburdens the central computer all the more with regard to several similar peripherals. At present, all terminals (especially screen terminals) have a microprocessor. Plotters are equipped with an interpolator, too. Optical character-recognition readers use a built-in microprocessor to perform pattern recognition.
2 . Industrial Applications
Here, again, two main categories may be distinguished: Minicomputer replacement. When a process-control minicomputer is not used at its full capacity, it represents a considerable savings to replace it by a microcomputer. Besides, it is probably worth replacing a minicomputer which controls, for instance, a whole workshop or a whole laboratory by a set of microcomputers, each controlling a single machine or a single experiment. It favors modularity and versatility. It is especially convenient in case of failure because it avoids hanging up the whole system. Classical automaton replacement. Microcomputers are particularly suited for relay or wired-logic automaton replacement. Computational capabilities allow the setup of more sophisticated devices. More complex regulation algorithms involving more parameters may be used. A set of target values, some of which may be conditional, may be stored in the memory. This is said to constitute an “intelligent” device. A typical example of “intelligence” is provided by validity tests: Suppose we have several sensors of the same parameter, e.g., a liquid flow in a pipe. A microprocessor assigns a weight to the value given by each sensor. If one of the sensors goes wrong, it can be momentarily excluded by assigning it a null weight. How-
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ever, its measures are still examined and if they return correct, the sensor is used again. Only programmed logic allows this. Programmed logic allows interpolation between values, and it allows a log to be kept of different events or measures in view of a later statistical treatment. More security constraints may be taken into account and they can be more sophisticated. Finally, use of a microprocessor allows a more convenient and pleasant-touse device to be built : Target values can be entered by means of a keyboard and data can be displayed more clearly. All above considerations eoncern the following applications [Capocaccia (1979)l: fluid distribution system in a plant, tint titration system for paints (microprocessors offer the only system which allows color reproducibility in the automobile industry), silo-filling system, waggonet-filling system, traffic regulation, robots, especially in the automobile industry, heating regulation, office-lights control (the simple system which switches off the lights clerks have forgotten results in considerable savings), and energy-saving systems. 3 . Appliances
We list in this category all applications which are accessible to the man in the street. Applications involving usual devices and improving the quality of life : pollution controllers; heating regulation (energy saving) ; traffic-lights regulation; point-of-sales terminals ; bank-desk terminals (lowered costs due to microprocessors allow multiplication of branch offices linked to the central office and the user may perform all operations on his account, even very far from his home). Applications involving home devices making data processing available eventual(y without user’s knowledge: automatic phone dialers; photocopiers ; washing machines; automatic microwave ovens [here, fiction is becoming reality-you type on your keyboard : “ckiicken, 3 lbs, 8,” and at 8 o’clock, everything
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is ready. The program includes validity tests-if you type “chicken, 30 lbs” (instead of 3 lbs) an error will be signalled.] robbery alarm systems, even capable of telephoning the police; automobile ignition systems with antipollution and energy-saving devices ; in the near future, automatically guided motor-cars ; special applications such as video games and home or hobby computers. Such systems, are real computers, offered at a surprising price. They are used for computer-science self-education, personal accounting or, simply, for entertainment. They have undergone a boom in the United States and the tide is now reaching Europe. It can be estimated that around 300,000 such machines have been sold throughout the world. [Applications in this field are described in a pleasant manner in Zaks (1980a).]
4. Special Domains Home applications have been developed mainly because of the low cost of microprocessors which has allowed the introduction of data processing facilities in fields where it was formerly impossible, and, also, for some applications, because of their small size which opens domains once forbidden to classical computers. In the applications of this section, the price does not matter, rather it is the small size which is taken advantage of. This concerns the military domain where miniaturization is important, as in avionics (on-board computers where weight is fundamental, bus signals are even transmitted through optical fibers because glass is lighter than copper, and this suppresses glitches), and medical applications. For instance, microprocessor pacemakers are now used. Such pacemakers measure breathing speed or other parameters implying that the patient is making an effort and they adapt the stimulated cardiac rythm to such an effort, whereas classical pacemakers did not allow any effort because they could only maintain a constant rhythm. Another application is that of an artificial pancreas for diabetic people, where a microprocessor measures glycemia and controls injection of the needed quantity of insulin. A sophisticated algorithm is necessary because of reaction delays to the injections. Many studies are still necessary to obtain a portable device. [A very good classfication of microprocessor applications according to many criteria is given in Mori et al. (1979). It applies to Japan but we feel it should be representative of any country.]
D. Technology This section is not intended to be a full account of technological details but only a reminder, mainly of terminology. [Two good references are De Man (1976) and Salama (1978).]
MICROPROCESSOR SYSTEMS
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A technology is characterized by (1) the way each transistor is realized and ( 2 ) how different transistors are grouped to form a basic gate. Groups of technologies are defined by (l), the main distinction being between bipolar technologies and MOS technologies. Bipolar technologies are faster but less suitable for large-scale integration. MOS technologies are slower but their ability for large-scale integration has been at the forefront of the microprocessor revolution. Moreover, recent advances in MOS technologies have greatly increased their speed and, especially with regard to memories, MOS circuits have reached “bipolar speeds.” Besides speed and ease of integration, other parameters which are important for the choice of a technology are power consumption, noise immunity, and manufacturing-process complexity. These different characteristics are summed up for the principal actual technologies in Table I. 1. Technology Compatibility
Each technology is characterized by its own logical levels, current source and sink capabilities, current input requirements, and pulse rise time and propagation delays. A given technology is, of course, self-compatible, i.e., an output of a given family may drive at least one input of the same family. In fact, it can drive several inputs of the same family, usually 10, sometimes 20 or 50. This allows the designer to focus his attention on the logical function performed by a chip and not to worry at all about currents and voltages, provided he assembles chips of the same family. In order to assemble chips of different families, sets of level translators and buffers were used. Now, however, progress has achieved that whatever their technology, recent chips are TTL compatible. This allows all support logic used with microprocessors to be TTL or LS TTL standard. A word of caution : A TTL compatible MOS output is not equivalent to a TTL output able to drive 10 TTL inputs-it can drive only 1 TTL input or 5 LS TTL inputs
2. Microprocessor Technologies Because of the lower integration capability of bipolar technologies, they are used only in bit-slice microprocessors. All other microprocessors are in MOS and especially in NMOS. Some are in CMOS for applications which need the particular advantages of this technology. In fact, the advent of microprocessors is an accident of technology. Manufacturers had become able to produce higher and higher density integrated chips but they Wondered how to use this capability to have 10,000-20,000 transistors on a chip, i.e., what LSI chips to make. The first
TABLE I IC TECHNOLOGIES
Familf TTL s TTL LP TTL LS TTL
tpd bsec)
Standby consumption (mW/gate)
Easeof integration
Noise immunity
Low Low Low Average
Good Average Average Good
1: 5 v 1: 5V
10
10 20 2 2
2: -2and -5.2V 2: 5 V and current injection Now 1 : 5 V, formerly like PMOS 2 o r 3 : +12, +5, - 5 , or 12 V : 5-20 V
10 3 30
ECL I ~ LI, ~ L
1
50
10-50
2
Low Good
Low Average
NMOS
20-50
0.5
Excellent
PMOS
100
0.5
Excellent
Good enough Average
CMOS
60
0.001
Excellent
Excellent
CMOS-SOS
20
0.001
Excellent
Excellent
0.5
Still better than NMOS
Good
HMOS, VMOS, DMOS
Supply voltages
= 30
1: 5V 1: 5 v
: 5-20 V
I:5V
Comments The most widespread family Faster, but more expensive than TTL Slower than TTL Consumption of LPTTL, speed of standard TTL, is becoming the standard Fastest technology New bipolar technology Used in most LSI devices and microprocessors Oldest MOS technology (used in pocket calculators) Very low consumption used for battery-powered memories or military applications Cumulative advantages of NMOS and CMOS, but costly manufacturing process New technologies derived from NMOS to achieve VLSI (very large-scale integration)
"TTL, transistor-transistor; S 'ITL, Schottky TTL; LP TTL, low-power TTL; LS TTL, low-power Schottky TTL; ECL, emitter coupled; CMOS, complementary MOS;CMOS-SOS, silicon on sapphire.
MICROPROCESSOR SYSTEMS
42 1
idea was to make memories, which were sold to minicomputer manufacturers. Then, the idea was to produce UART chips, because only a very standard product could exist in LSI due to the yield problem (see below). The idea to manufacture a single chip having all the functions of a computer CPU emerged from a contract between Intel (at this moment a memory manufacturer) and Datapoint, Inc., who wanted a single-chip display terminal controller. This was the first microprocessor. To tell the truth, Datapoint did not use the chip because it was too slow, but Intel put it on market, expecting it would enhance memory sales. The success went beyond expectation and it started it all. This explains why in the beginning, no one knew what characteristics to give to their product, since they were not data processing specialists. The result is that old microprocesgors had strange features and odd instruction sets. But now, they have reached adulthood and such oddities no longer exist.
3. The Yield The yield is a major cost factor for a chip, but the more you manufacture a given chip, the better yield you obtain. This is known as the “learningcurve” phenomenon. As you manufacture more chips, you control the process better and the yield rises. This is the reason why an LSI chip cannot exist if it cannot expect some minimum distribution. Therefore, LSI chips can only be standard products.
E. From Logic Gate to Microprocessor
The purpose of this section is to show that the possibility of producing microprocessors is only an extrapolation of the current trends in the progress of semiconductor fabrication. There is no qualitative change. The simplest logic circuits are gates where the output results from the permanent inputs. The first advance leads to synchronized logic where some of the outputs are used to provide a timing base for elaboration of results. This avoids the “racing” problems which occur by the addition of unbalanced propagation delays. The D-type flip-flop is an example of synchronized logic (e.g., 7474): Q output reproduces D input but only after a rising edge on CP input. A second advance leads to controlled logic. For instance, consider the quadruple selector 74157 of Fig. 5a. Pins may be reordered as in Fig. 5b. Doing this represents two advances : First, we have defined a data width that we shall manipulate ;second, we have isolated the select input which will control IC function-either copy A inputs or copy B inputs. Finally, the
422
D.-J.
DAVID
3
* B A 3 B
74157
Eiz
Fi
l
74157
3Y
y
4
b
a FIG.5. Pin reordering of 74157 selector.
74157 has another control input (enable), which controls when the outputs will be valid: It is therefore a synchronization input. As a second example, let us examine the 74169 counter. Here, again, there are data inputs and outputs (4 bits each) and control/synchronization pins: the clock input (starts counting), the load input (initializes count to value shown on data input pins), kinput (decides if count will be incrementing or decrementing). Thus, we arrive to the concept of a universal logic circuit, equipped with data inputs and outputs with a fixed number of bits, control pins where the shown binary pattern decides which operation will be performed on the data, and synchronization pins which decide when the operation will be performed (Fig. 6). Such circuits exist and are called arithmetic and logic units (ALUs), e.g., 74481. We are then not far from a microprocessor. What is missing now is what will ensure successive operation sequencing. In effect, any treatment can be seen as a sequence of elementary operations which follow each other in time. It must be ensured that at time t , , the binary pattern which will govern the
Control
Synchro.
FIG.6. Universal logic circuit.
MICROPROCESSOR SYSTEMS
423
first operation is presented on control pins, then at time t 2 , the pattern for the second operation, and so on . . . . This can be obtained by adding very few classical logic circuits to the circuit of Fig. 6 , thus obtaining Fig. 7. Successive control signal groups are kept in successive locations of a memory where they constitute a program. A register [instruction address register, or program counter (PC)], incremented at every clock pulse will provide the memory address which must be read to obtain the signals corresponding to the current operation. The concerned memory location is copied into instruction register I. For correct operation, output data must be entered back as input data for the next manipulation. This results in the necessity for a buffer register which will be accumulator A. Now data may have to be stored in the memory if several data items are to be handled, hence a path between A and memory. The ALU generally has two data inputs, the first being linked to the accumulator and the second linked to memory. The result of the ALU is generally sent directly to the accumulator. The last indispensable register is the program status register P which holds states resulting from some events which may occur in the ALU and must be kept to modify the behavior of subsequent operations (e.g., carry or overflow events). Thus, we arrive at Fig. 8 which represents the minimum livable microprocessor structure. A real microprocessor will be equipped with buffers and some additional registers (for instruction-set enhancement), which do not appear in Fig. 8. Thus, the microprocessor appears as a device which spends most of its time sending addresses to the memory to obtain information from it or to store information in it. When the obtained information are instructions, they go into the I register. When they are data, they go to or come from the accumulator.
Universl logic circuit
FIG.7, Skeleton of a computer,
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D.-J. DAVID
-.-.-.-.
.- -.Memory
Address bi1s
FIG.8. Microprocessor structure.
We see now that what is new when going from wired logic to microprocessor use is that instead of considering operations spacewise, they must be considered timewise. Instead of being performed side by side in different logic circuits, the successive actions of a process will be performed after one another in successive instructions. The difficulty is precisely to order the different operations along time. This order of operations which leads to the solution of a given problem constitutes the algorithm to be devised, and-in the form of an instruction sequence-the program which has to be written. Another point to keep in mind is that the time-programmed form is often longer than wired logic, and this may be important in some applications. 11. COMPONENTS OF A MICROPROCESSOR SYSTEM
The general architecture of a microprocessor system is very simple (see Fig. 9). A few packages linked to one another by buses are sufficient. The
Address bus
MPU ,.
Memory
Chips
Interface
Chips
1
Data bus
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D.-J. DAVID
address bus transmits the addresses which allow the selection of the wanted information, the data bus transmits precisely this information, and the control bus transmits the signals which govern system operation. The main system chips are: the microprocessor (MPU or pP) which creates system buses and performs operations on information ; the memory packages which store handling instructions as well as data to be handled; and the interface chips which connect the microcomputer to the external world. Progress in chip integration has been such that each of the above functional units (which in mainframe computers occupy entire desks) is contained in a single integrated circuit package. Now, ICs are produced which hold the whole microcomputer in a single chip. [General principles of computer systems are recalled in Doty (1979),Zaks (1977),and Osborne (1976, Volume 2 details specific products).] A . The Memories
There are two main categories of semiconductor memories : RAMs (random access memories) which can be read or written. They are used to store current data or programs which are likely to be modified. ROMs (read only memories) which can only be read. They are used to store permanent data and, mainly, programs when they are fixed up. 1. RAMs
According to the constitution of a 1-bit cell, a RAM is said to be static or dynamic. Static RAMs use a flip-flop to store a bit. Dynamic RAMs use a (parasitic) capacitor. In fact, the static cell needs four transistors, whereas the dynamic cell needs only one. Thus, dynamic memories are denser (larger capacity within a chip), faster, and they have a lower consumption. In fact, at present, current static chips are 4K bits (e.g., 2114) and the limit is 16K bits (e.g. 4016), whereas, in the dynamic case, current chips have 16 K bits, and 64K-bit chips are being sampled (4164). Integration doubles every year, but it takes two years to produce a new chip. Thus, every second year, a new chip with fo'ur times the preceding capacity appears on the market. The drawback of dynamic memories is that capacitors lose their charge because of current leakage. The memory must therefore be refreshed and this demands auxiliary circuits more complicated than those for static memories. A disadvantage of semiconductor memories is that they lose
MICROPROCESSOR SYSTEMS
427
information as soon as they are switched off. The solution is to use batteries, which is more feasible with CMOS chips which have a very low consumption. Memories able to hold several months can be achieved. 2. ROMs ROMs offer a radical solution to the volatility problem, but they can be used only for fixed information. There are four distinct types:
a. Proper ROMs. The information is written (we could even say “engraved”) permanently during manufacturing, since it is the metallization mask which “programs” the memory. Manufacturers charge $1000-$2000 to prepare a mask and they only accept minimum orders of roughly 1000 chips. Thus, you must be sure of your program when you order a mask ! b. PROMs (programmable ROMs). The program is written by fuse burning within the chip, where zeros are wanted. This operation may be done in the field, which is more convenient than mask ordering, but here, again, you can program only once. However, errors are less costly (and zeros can always be added). c. EPROMs or REPROMs (erasable or reprogrammable ROMs). In NMOS EPROMs, each bit is stored as an electric charge trapped in an isolated transistor gate [floating-gateavalanche MOS technology (FAMOS)]. These charges can escape under ultraviolet light, and then the memory may be (electrically) reprogrammed. This cycle may be repeated many times. This gives a good versality for debugging programs because a UV lamp and an EPROM programmer are sufficient. Those ROMs (e.g., 2716: 2K x 8 bits; 2532: 4K x 8 bits; this is the present maximum, whereas in simple ROMs, 8K x 8 bits exist) are easily recognized because their package is covered by a quartz window. Generally, a pin-compatible simple ROM corresponds to each EPROM type. It is used in mass production when the program is fixed up, because it is cheaper. But EPROMs have achieved recent progress in reliability, speed, and density so that they can be used in small series production, thus, making PROMs and, eventually, ROMs obsolete. d. Electricall-v alterable ROMs (EAROMs). Here, reprogramming is simpler because all operations are electric, but these memories, which use the MNOS technology, are still too slow; however, advances are expected. 3. Functional Parameters of Memories
An important choice criterion for a memory is its organization. For instance, a 1K-bit chip may be 128 x 8 bits (e.g., 6810) or 1K x 1 bit (e.g., 2102). For large memory sizes, the total number of chips is the same in both cases, but N x 1-bit chips are more valuable (fewer pins).
t cycle
4
R/W
cs
b
I
I
I
I
I I
I
Write pulse width
I I I
I I I
I
I
I I
taccess
t DSU I
*--tH
Data stable time Data hold time
MICROPROCESSOR SYSTEMS
429
Another parameter is speed, defined by the access time (time elapsed between selection and data availability) and the cycle time (time between start of an operation and the moment when the next operation may be started). These times are of the order of 60 nsec (2147) and 200-450 nsec (2114) for present MOS chips. They appear in Fig. 10 which shows also which signals are to be found on a normal memory package. Of course, the least significant bits of the address bus are present (for 1 K: 10 lines AO-A9); for the data bus, there are p lines if the memory is a N x p bits. If p is small, there can be 2p lines if, for eaoh bit, there is an input and an output line, but the trend is to have a single bidirectional line. 2102s and 41 16s have unidirectional lines. Some chips have inverted input and output: DIN and DOUT. The R/W line tells if the operation is a read (R/W = 1) or a write (R/W = 0). Finally, the chip-select lines result from decoding the most significant address bits. The 21 14 has only one such pin, active low: CS. In the case of a ROM, there is no R/W line. However, there are programming control lines, if applicable. [The above problems are detailed in Nicoud (1975a).J B. The Microprocessor
1. Dixerent Types of Microprocessors The central processing unit of a microcomputer system is formed of one or several microprocessor chips and a few auxiliary circuits. The market is at present shared by six types of microprocessors. a. Four-bit microprocessors (e.g., 4004). These chips perform all functions of a CPU, but for a word length of 4 bits. They are now obsolete since they are as expensive as 8-bit microprocessors. b. Eight-bit microprocessors. They are at present the standard type since they are well suited to many applications. c. Sixteen-bit microprocessors. Their use is becoming more widespread and they are tending to replace some minis with which they are compatible (e.g., the Nova-compatible 9440) ; but they settle the dilemma between a large number of pins (the 9900 and 68000 have 64 pins) or multiplexing address and data buses. d. Bit-sfice microprocessors. Bipolar technologies are faster than MOS, but they do not allow such a large-scale integration as MOS. In a simple bipolar chip, it is possible only to group the logic which performs all arithmetic functions on 2 or 4 bits. A 16-bit CPU will be made, for instance, of 4 slices of 4 bits with other chipg performing the sequencing functions. Thus, the assembly is more complex than with a single-chip microprocessor, but it is adaptable to user needs and performance is higher. The most widely
430
D.-J. DAVID
spread bit-slice family is the 2900 family. A very fast ECL family exists, the 10,800. c. Single-chip, four-bit microcontrollers. Pocket calculators, for instance, use chips which perform all functions: CPU, storage, display control, etc., in a somewhat simplified form. There is no assembly problem and cost is minimal (e.g., TMS 1000 series). d. Single-chip, eight-bit microcomputers. In 1977, the first complete single-chip microcomputers appeared. For instance, the single-chip version of the F8, the 3870 has 2K bytes of ROM, 64 bytes of RAM, the CPU, and 16 1 / 0 pins. The 8748 has a UV REPROM of 1K bytes. Other families have their own single-chip version (6801, 6500/1, 28). From now on, we shall devote ourselves to the second category (monolithic 8-bit microprocessors). 2. Microprocessor Operation Viewed from the exterior, the essential of microprocessor operation appears as a succession of memory cycles. For each cycle, the microprocessor deposits an address on the address bus and, in case of a read, it expects to find the wanted data a little later on the data bus. If it is a write cycle, the microprocessor deposits the concerned data on the data bus. The R/W pin indicates the sense of the exchange which is going to occur. The sequence of cycles needed to execute a given instruction is specific to the instruction and it also depends on the microprocessor internal architecture. In any case, the first cycle (called fetch cycle) reads the first byte of the instruction, which is the operation code. According to the concerned operation code, additional read cycles will occur to read the next bytes of the instruction, if any, and then read or write cycles for the operands. Some cycles may occur, without using the read data, in order to leave time for the microprocessor to perform internal operations. Manufacturers’ documents describe all the needed cycles for each instruction of their products. We refer the reader to these documents. However, we describe such an instruction in Section II,B,4 below for illustration purposes. This sequence is not important: what matters is to know the phenomenological effect of the instruction. Conversely, the number of needed cycles is important to preview the execution time of a program.
3 . Tvpical Microprocessor Pins We shall take the example of the 6502. This implies no endorsement, but the 6502 offers a simpler example than others. Any microprocessor presents : at least two power supply pins (0 and
+ 5 V) ;
MICROPROCESSOR SYSTEMS
43 1
clock pins-the 6502 has an input ‘po where it is sufficient to provide a square signal, and two outputs ‘pl and ‘ p 2 ; the address-bus pins-most micros have 16 of these, hence an addressable space of 216 = 64 K bytes; data pins-bidirectional; there are 8 of them for an 8-bit micro; control pins; the 6502 is quite simple in this respect:
R / W (read/write) indicates the sense of current memory cycle; W T is pulled to 0 to restart the processor (reset) ; SYNC is set to 1 during a fetch cycle; RDY (ready); if this input is set to 0, the processor goes into a wait state to adapt to a slow memory or to allow DMA (direct memory ; access) IRQ (interrupt request) set to 0 if an interrupt is requested, NMI (Nonmaskable interrupt request), and so on. Other microprocessors have different halt-request signals and also signals allowing a n exterior device to take control of buses (e.g., for DMA). Timing relationships between some of these signals are illustrated by Fig. 11, where: TCYC = 1 psec, versions exist with TCYC up to 750 nsec; TA is the maximum access time allowed to a memory to remain compatible with the microprocessor; TB is the maximum time within which the microprocessor is guaranteed to settle data on the data bus for a write operation. Used memories must accept it. It is seen that the 6500 series timing is simple. Each cycle is divided in two phases: “ql” where an address is prepared and sent to the address bus and ‘cp,” where data are sent or expected on the data bus. This discipline is exactly the same as the one followed by the 6800 series.
4. Example of Instruction Execution The only goal of this section is to show a simple example of the events which occur along a typical instruction execution. We choose one of the simplest instructions of the 6502, since we have taken this micro as an example: an immediate accumulator load supposed to reside at memory address 1040.In 1040 is A9 (hexadecimal),which is the corresponding operation code; in 1041 is the data item 25 which is to be transferred into the accumulator as a result of the instruction. At the beginning, the program counter PC contains the instruction address, i.e., 1040 (follow the events in Fig. 8). First cycle ‘pk.PC is sent to the address bus; thus 1040 appears on the address pins. R / W is set to 1 (read) as well as SYNC (since it is a fetch cycle).
Read cycle
Write cycle
t cyc
I
I I
I
I
I
I
4
,
I FIG. 1 I . 6500 Series timing.
43 3
MICROPROCESSOR SYSTEMS
First cycle ‘pz. PC is incremented (+1041). Contents of address 1040 (i.e., A9) appears on the data bus. It is transferred into current instruction register I. Second cycle cpl. PC is sent to address pins. R/W remains equal to I while SYNC is reset to 0. Second cycle, q2. PC is incremented (-1042). The data byte which appears on the data bus (i.e., 25) is stored in an intermediate register which does not appear in Fig. 8, the input data latch. Third cycle ‘pl. We have arrived at the fetch cycle of the instruction following the one we are considering-actually, contents of location 1042 will be read while SYNC will be 1. However, at the same time, under the influence of the operation code A9 which is still in the I register, the contents (25) of the input data latch will be transferred into the accumulator, while pertinent flags in status register P will be positioned. It is seen that the end of the effective execution of an instruction overlaps the fetch cycle of the next instruction. This method, which serves to save time, is called “pipe lining.” Here, the apparent duration of the instruction is two cycles.
C. The Interface Chips The essential roles of the chips designed to interface the microprocessor with the external world are latching and multiplexing. It is effectively necessary to create signals which remain at a given level during a time longer than must be allowed to execute some instructions to take the decision to maintain or to change the signal, according to a pertinent algorithm and to external information which may be involved (Fig. 12). A line of the data bus cannot constitute such a signal. In effect it remains constant only during, at most, one clock period. Conversely, it is from a
in the instr.
Processing ... stop decision
Reset instr.
t FIG. 12. Controlling an external signal.
434
D.-J. DAVID
1
ID 92
R/W
->
1/2 7474
Q-
CP
data bus line that the external signal will be generated. During a write cycle the line will be set to the wanted level and, at the same time, a device will memorize it. A simple D flip-flop may be used (Fig. 13). The data present on Do line is stored at the (p2 descending edge, i.e., at end of a write cycle (R/W = 0), provided the address is correct. The Q signal will be changed only by another write instruction at the allocated address. Actual interface chips are a mere extrapolation, but very sophisticated, of the Fig. 13 circuit. In our opinion, the ease of building a microprocessor system and using it for different applications is mainly due to these interface chips, rather than to the microprocessor itself. These chips have multiple functions and some are very elaborate. They are split in two general categories : serial interfaces and parallel interfaces. All these chips contain a number of internal registers which are accessed in the simplest way if the method of memory-mapped 1 / 0 is used. In this method, each register is seen from the processor as if it were a particular memory cell. The address is decided by the choice of the address lines to which the chip is connected. A read at this address transfers the register contents to the data bus allowing the microprocessor to know the information (which may come from the external world). A write allows the processor to impose data to this register. These data may then be transferred to the external world. These interfaces are programmable in the sense that writing some information in the so-called chip-control registers allows the modification of the chip behavior. 1. Serial Interface
A serial interface presents an external pin on which the different informati6n is transmitted successively along time. The chips which implement such interfaces are called UARTs (universal asynchronous receiver-transmitter), USARTs (universal synchronous/asynchronousRT, e.g., 825 l), or ACIAs
MICROPROCESSOR SYSTEMS
435
(asynchronous communications interface adaptor, e.g., 6850 or 655 1). They are programmable in the sense that the user may decide the transmission speed, the transmission form (synchronous or asynchronous), the transmission protocol (which parity checks will be adopted, number of bits per character). New chips in this category are more and more sophisticated. They are adapted to the new protocols which are developed in view of the spreading transmission nets, such as SDLC or XDLC. Teletypes” operate in the asynchronous mode. The term is actually a little incorrect : “asynchronous” does not mean that the transmitter and the receiver do not have to be synchronized, it means that they need to be synchronized only during one character time. Between two characters, the line is held in logic state 1. To recover synchronism, each character is preceded by a start bit (0 state) and followed by one or two stop bits according to the terminal type (1 state). The Teletype uses two stop bits. It transmits in 7-bit ASCII code, with a parity bit generally fixed to zero (see Fig. 14). This contrasts with synchronous transmission where synchronism must be ensured during the whole message, which demands much more precise agreement between the clocks at both transmission ends and, hence, a much higher cost. In order not to lose synchronism when there is no character to transmit, a special character called synchronization or rest character is transmitted repetitively. The receiver should sample each bit as closely as possible to the middle of the bit period. For doing this, it receives a clock signal whose frequency may be equal to the bit frequency (isosynchronous transmission), or to 16 or 64 times the bit frequency. Sampling is more precise in the last two cases. Let us now take a closer look at the 6850 ACIA, for example. It is seen from the programmer as a set of four 8-bit registers selected by an RS pin connected to address bit AO. There will therefore be two consecutive addresses, a and a 1, where a is the base address determined by the chipselect pins connection. The first register is the Data Transmit Register (address a, write cycle) where the character to be transmitted is written. It will be automatically serialized by the ACIA. Then, there is the Data Receive Register where the currently received character is being assembled. When it is assembled, it is obtained by a read at address a. Thus two registers share the same address: They are discriminated by the read or write nature of the operation. The other two registers share address a + 1. In a write operation, the control register is accessed, where one specifies the clock division rate (1, 16. or 64), the number of data bits (7 or 8, in general), the number of stop bits, and the parity. In a read, access is to the status register which reports events that have occured in the chip: The most important bits are the seventh position (an interruption has occured), the zeroth position
+
Start bit
Actual character
1
0
1
0
Parity
1
1
0
0
1
2 stop bits I
Intercharacter gap
I
t
t FIG.14. Teletype’ transmission of digit “5”: hex 35: 00110101.
MICROPROCESSOR SYSTEMS
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(receive register full, i.e., a receive character has just been assembled, it must now be recuperated by reading at address a), and the first (transmit register empty, i.e., a character transmission is just finished, the next must now be written at address a). The most important ACIA pins are: Microprocessor interface p h s . Data bus (8 pins), RS (register select), IRQ (interrupt request), chip selects, R/W (read/write), and E (clock, linked to q 2 ) . Serial transmission pins. RxD (data receive), TxD (data transmit), and associated clocks RxC and TxC. Clock signals must be provided apart. For instance, to receive at 110 bits/sec, with divide rate 16, a 110 x 16 = 1760-Hz signal must be provided on RxC pin. Other chips such as the 5501 or the 655 1 have a built-in programmable transmission-rate generator. Elementary modem controls. Useful if a modem is connected.
Two main hardware interface types are used in asynchronous serial transmissions. In the RS 232 C (CCITT V24) norm, voltage control is used : Logic 1 corresponds to a positive voltage (generally +12 V), whereas 0 corresponds to a negative voltage (usually, - 12 V). Most frequently, Teletypes'Duse a 20-mA current loop interface (two transmission wires, two reception wires). It is a way to prevent noise: A glitch has a greater effect on the voltage of a wire than on a current which circulates in two wires because of compensation between the two wires. Logic 1 state corresponds to the presence of current, state 0 to its absence. It is very easy to switch from one interface to the other or to TTL levels by means of optocouplers. 2. Parallel Interfaces These chips are the most fundamental for general interfacing of all kinds of peripherals (CRTs, printers, punched-tape readers, etc.). They are called PIAs (peripheral interface adaptor), PIOs (parallel input/output), or PPIs (programmable parallel interface), which is, in fact, the most precise designation since it insists on two essential attributes of these chips : Parallel. Exchanges occur 8 bits by 8 bits, hence they are faster than serial communications. Programmable. The chip behavior may be modified by the program; for instance, it can be decided if a line will be an input or an output. a. Notion of elementary I / O port. We take the example of the 6820 PIA, since it was the first device to implement such techniques, departing from using standard logic as in Fig. 13. All chips of this category have several (usually two) 8-bit 1/0ports.
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A port corresponds to eight external pins of the chip. Each pin is directly linked to a bit in an 8-bit internal register of the chip called the data register (of the considered port). This corresponds to having eight times Fig. 13, one for each data-bus line. The data register is selected from among other internal registers by means of register-select pins connected to least-significant address bits. Thus it has some address a. If the port behaves as an output and if some binary pattern is written into the register, then the external pins which correspond to ones in the pattern will maintain a potential higher than 2.4 V, whereas those corresponding to zeros will maintain a potential lower than 0.4 V (TTL convention). Thus, a potential can be imposed on the external world, allowing switching of lamps, relays, motors, etc. If the port behaves as an input, it is the external world which imposes a potential on the PIA’s pins. In a read of the register at address a, a 1 will be obtained on bits linked to pins where the imposed potential is 2 2 V, whereas a 0 will be obtained where the imposed potential is 10.8 V. The originality of PIAs is that each line may be programmed individually to act as an input or an output (all lines of a port do not have to be programmed in the same way). On the other hand, the behavior of a line may be changed at any moment in the program. This is very surprising for specialists in wired logic where the input or output character of every line is decided once and for all. Now, how are lines programmed as inputs or outputs? The data register (address a) is associated to another 8-bit register, called a direction register (generally at address a + 1).Each bit of the direction register corresponds to a bit of same position in the data register. To program a line as an output, it is sufficient to write a 1 in the corresponding bit of the direction register, whereas a 0 makes the corresponding line act as an input. Since the RESET writes zeros in all chip registers, all ports are in input state just after reset. Our 6820 example has two such ports, A and B. b. Bidirectional behavior. When you write a 0 or a 1 on an input-programmed bit, the potential of the corresponding pin is not affected (only an external circuit can do this). When you write on an output-programmed bit, the corresponding pin takes the corresponding TTL potential, except if the pin is connected to a circuit which overloads it. For instance, if you write a 1 on a bit linked to ground through a small resistor (not an exactly null resistance which could damage the chip), the potential will remain close to 0. Now if you read the data register under those circumstances, port A and B of the 6820 will not have the same behavior. On port B you will read a 1 because there is a buffer which ensures that you always read what you have written. On the contrary, on port A, you will read a zero: A read always reflects the electric potential of the pin at reading time. Thus, port A, programmed as an output has in fact, a bidirectional behavior. It is to be noted that this bidirectional behavior makes obsolete the above-
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MICROPROCESSOR SYSTEMS
described principles. If we have a bidirectional behavior we d o not need a direction register. It is precisely what occurs in the recent single-chip microcomputers which incorporate the CPU and I/O ports in the same chip, and it is recommended to save registers in such complex chips. Therefore, there are no direction registers because the ports are always in output mode. If you want to impose voltages, you write the appropriate pattern. If you want to read, you write an all-ones pattern and you read back to see on which pins the external world imposes zeros. c. Handshaking. In addition to the eight lines which allow the transmission of, for instance, a character, other signals may be exchanged. For example, an input peripheral may generate a “data ready” signal. Receiving this signal, the processor will read the input port and then it will generate a “data taken” signal indicating to the peripheral that it can now prepare the next data. Such procedures are called “handshaking.” Some chips like the 6820/6520, the 6522, or the ZSO-PI0 have, for each port, a set of additional lines used for handshaking. The exchange is automatic: The mere reading of the port data register positions the “data taken” line. On some simplified 1 / 0 chips, or, rather, on combination chips which have other functions in addition to I/O (e.g., 6530 or 6532), IjO functions may be kept as simple as possible and, therefore, handshaking features may be omitted. It is, however, possible to perform handshaking with those chips, but this needs to involve lines of another port, and the programmer has the responsibility of completely controlling these lines (see Fig. 15). It is obvious that with suchchips, it is very easy for the microprocessor to “know” what happens in the external world. It is sufficient to read the data register of a port to obtain the voltages on the corresponding pins. It may be necessary to use an analog-to-digital converter if the problem is to measure a continuous parameter. In most cases too, a sensor is needed to transform the physical parameter into an electric voltage.
Data ready
I Port isread Peripheral data
Data to be read I
Data taken (CA21
CA2 is set
Peripheral prepares new data t
-
FIG. IS. Handshaking (CA2 applies to 6820 or 6522 chips; 1 and 2 are simultaneous on a 6820/6520/6522. )
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In output, a pulse of determined duration is generated by writing twice on an output-programmed bit :
L-rJ Write 1
b Wait for wanted delay
The output pin can be connected to transistors, relays, or thyristors if power control is needed. The step of delay generation is made much easier by the use of a new circuit type-the timers.
3. Timers A timer consists essentially of an addressable register which is continuously decremented at system clock rate or at one of its submultiples. To start the timer, an initial value n is written into the timer register. Then, it is decremented every Tunits of time (T = kt where t is the system clock period and k is the frequency division rate). After delay nT, the register reaches value 0. This positions a flag (exhausted delay) or generates an interrupt request which allows the program to know that the delay is over. If the register is read before delay exhaustion, the obtained value allows the time which remains to be known. Delays that can be generated in this way are T, 2T, . . ., 256T if the register is %bit wide. In principle, two parameters can be determined by program: the initial value of the counting-down register and (in some cases) the predivision factor k . The obtained delay is proportional to these parameters. Let us take two examples. They are chosen for illustrative purposes and although they are interesting chips, this implies no endorsement. First we examine the timer of the 6530. The 6530 is a multifunction chip which combines ROM, RAM, two IjO ports, and a timer. Its timer is made of an 8-bit register, a clock predivisor, and a 1-bit status register. The predivisor may divide the clock rate by 1, 8, 64, or 1024. This allows delays to generate from I psec to 262.144 msec if the system clock is at 1 MHz. Besides the divisor rate, another program option is whether delay end will cause an interrupt request. We have here a very interesting philosophy. In any case, when the delay is over, the status bit is set to 1. Now, if specified, an interrupt may be requested in addition. This allows working in either the polling mode or the interrupt mode (these terms are explained in Section 111).
MICROPROCESSOR SYSTEMS
44 1
A very particular feature of the 6530 is that the operating mode is chosen in the following way: The timer register has a set of eight addresses in the system; writing to any of these eight addresses writes the initial count in the same register, but, according to the particular address used, the divisor rate and the interrupt option are different. The 6522 has two 1 / 0 ports and two timers. Each of these timers has a 16-bit counter and no predivisor. Starting a delay by writing an initial count is performed in two steps since the microprocessor can write only eight bits at a time. The same philosophy as above is applied to sense the end of a delay : Each timer corresponds to a bit in a status register and this bit is set in any case at end of delay; now an interrupt option exists. Both timers may operate in the same way as the 6530 to generate single delays. Possible delays are 1 p e c to 65535 msec. However, additional modes exist : Timer 1 may operate in “free-running” mode, i.e., as soon as a delay is finished, timer 1 is restarted for a new delay. Moreover, at each end of delay an external pin of the 6522 may optionally be inverted, which allows the generation of pulses or waveforms. Timer 1 decrements only at system clock rate. Timer 2 may decrement at this system clock rate or, optionally, according to a clock signal entered on an external 6522 pin. This allows using it as an event counter. At variance with the 6530, the 6522 timers operation modes are decided by writing pertinent binary patterns into certain control registers in the chip. We have no room here to give more details, rather we refer the reader to the manufacturers’ data sheets or to Osborne (1976). 4. Other Chips We have described above the general-purpose interface chips that can be used. Two other kinds of interface chips exist. a . The combination chips. These combine several general functions among RAM, ROM, I/O, and timers. We have already quoted the 6530 (1 K ROM, 64-bytes RAM, 2 1/0 ports) and the 6522 (2 1/0 ports, 1 serial register, 2 16-bit timers). Such chips exist now in almost every family, since use of such combination chips is very economical in a system. The ultimate combination chip is the chip which combines RAM, ROM, I/O, and timers plus the CPI: It is the single-chip microcomputer. b. The special interface chips. These are assigned to control a particular category of peripherals. Most frequently used are : CRT terminal controllers (e.g., 96364, 6845), and floppy-disk controllers (e.g., 1771-1795 series, 6843, pPD 472). In addition, there are chips specialized for interrupt priority control (6844, Z80-DMA). [The general reference for this section is Zaks-Lesea (1977). Repko (1978) gives interesting elements about serial I/O. 1/0 chips are described in
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Osborne (1976),Vol. 11. Nicoud (1975b)details handshaking problems, while David (1980) describes a specific use of PIAs, together with interrupts.] D . Assembling a Microprocessor System
With the components we have just seen, the assembly of a microprocessor system is quite a simple task. No special electronic skills are required. In most cases you have to link pins bearing the same name on different chips. 1. Support Logic around the Microprocessor
Aside from some details, the problems to be dealt with here concern clock signal generation, reset control, and bus-buffering devices. With an 8080, these problems make it mandatory to add two auxiliary chips: the 8224 and the 8228. With a 6800, a special clock circuit is necessary. With more recent microprocessors such as the 6500 series or the Z-80, the clock problem is solved much more easily. It is sufficient to provide a TTL square wave on the clock input pin (cpo on a 6500). A crystal is necessary only if the application requires a precise and stable time reference. a. Reset control. Here, again, the problems are simple. Use of a Schmidt trigger as shown in Fig. 16 ensures a clean rising edge at power-up. Then the system starts the reset routine automatically. b. Bus buflering. Bus buffering is not necessary as long as the system remains small enough : recent microprocessor pins are TTL compatible, i.e., they can drive one TTL load (a true TTL output can drive 10 TTL loads). This means that they can drive quite a large number of MOS chips such as memories. Only some control lines may have to be buffered if they are connected to TTL ICs such as decoders. Use of LS components should in most cases avoid any problem. If the system is quite large, all lines must be buffered. Bidirectional buffers are necessary for the data bus. The 74 LS 245 is very useful for this, since it may deal with eight lines at a time. This chip was so successful1that last year there was a shortage of it for several months throughout the world ! For the address bus, unidirectional buffers are sufficient, such as the 8 1
FIG. 16. Typical reset circuit. Reset
Push button
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LS 95 or the 74 LS 244 (8 bits). These buffers have three-state outputs. Most microprocessors have a three-state address bus in order to allow DMA. This is not the case with the 6500 series, but this is not penalizing since DMA applies only to large systems where buffers are necessary anyway. In a 6500 system, the buffers bring the DMA possibility as a by-product. When memory chips which have separate data pins (e.g., 2102) are used, it is necessary to put three-state buffers between the DOUT pins and system data lines (e.g., 74125). The buffer will be enabled only when the memory is selected for a read : The read data will be transmitted to the bus. In the 2102 case, since thischip has a iK x 1 bit organization, eight chips are needed, each being connected to one line of the data bus : A chip will have all bits of position i for all 1024bytes. With 21 14s,which are 1K x 4-bit chips. two chips are sufficient to form 1 K bytes: one for DO-D3, the other for D4-D7. c. Particular problems: Interrupt pins. Interrupt-request pins are designed to be connected to several sources which will be open collector to allow wired or (active low). Therefore, a pull-up resistor to + 5 V is needed. Even if the pin is not used, it is recommended to mount a pull-up resistor of, say, 3 kR to avoid spurious interrupts. Write pulse signal. In the 6500 or 6800 series, a single control line, R/ W indicates if the cycle is a read (l), or a write (0). This state is known early within the cycle (see Fig. 11). However, some memories require receiving a write-enable pulse a little before the microprocessor puts the data to be written on the bus. In the 6500 series, the data appear on the bus early enough after the q2 rising edge to allow correct operation of the circuit in Fig. 17a. Some chips, instead of the couple (R/W, CS) use the couple RE (read E (write enable). Here, again, Fig. 17b gives a possible solution. enable) and W It now remains to connect the address bus of our system. Least-significant lines will connect to the address lines of the memory chips. This ensures that inside a chip, the addresses will be consecutive. If the chip has 2" locations, n lines will be connected in this manner. For instance, for a 1024-cell chip, 10 lines AO-A9 will be connected to the lines of the same name of the address bus. We now have to form the chip-select signals using the most significant lines of the address bus; this is the addressing problem. 2. The Addressing Problem a. Nondecoded logic. This problem is solved in two steps :
(1) Assign to each chip in the system (memory or I/O) an address range (of size equal to the chip-memory size, in principle). Thus, you have to decide a partition of the addressable space. Some particular constraints may have
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R/W
7404
7400
WP
1
I I
Address logic
R/m
b FIG.17. Write pulse, write enable, and read enable generation.
to be taken into account. For instance, with a 6500 microprocessor, it must be ensured that adresses FFFC and FFFD (thus, actually, highest addresses), be in ROM (it is the RESET vector), and that there is some RAM between 0100 and OlFF (for the stack) and between 0000 and OOFF (for zero page). This implies that lowest addresses be in RAM. The same holds for the 6800, whereas it is exactly the reverse for an 8080 or Z-80. (2) Build the chip-select signals to implement the above partition. Of course, you try to use as little logic as possible. A helpful feature is that very often, a system does not use the whole addressable space. For instance, only 4 or 8K out of the possible 64K may be used. In that case, you can decide to leave some of the highest address lines unconnected, thus making them indifferent. It is to be noted that as soon as a line becomes indifferent, the addressable space is replicated, i.e., each cell takes two addresses. Example 1. If the most significant four lines become indifferent, the address space is replicated 16 times (= Z4). The byte of address OXXX also has addresses IXXX, 2XXX, . . ., FXXX. Thus, to satisfy the above constraint (high ROM, low RAM), it is sufficient to put the ROM on top of the 4K subspace, and the RAM at the bottom. If there is ROM in FFF, it will be also in FFFF; if there is RAM in 000 it will also be in 0000. Another way to save addressing logic when the addressable space is not completely filled up, is to use linear selection where a given chip is selected by a single address bus line.
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Example 2. Assume two 1K-byte ROM chips, A and B (Fig. 18a). Each chip is connected to AO-A9 for internal byte select. Moreover, A is selected if A15 = 1, B is selected if A14 = 1. Thus A is selected for addresses of the form 8000 . . . hex; B is selected for addresses of the form 4000 . . . hex. Actually, all addresses 28000 must be assigned to A. Thus we see that A (size 1K)occupies, in fact, a “space” of 32K. Whenever an address line is used in linear selection, the available address space is divided by two. Likewise, selection of B by A14 divides by two the remaining 32K. This is not awkward as long as the system contains little memory (this is very often the case), and this is very cost effective because there is no logic to implement. However, there is a very important drawback: Suppose that in the preceding example, we try to read at an address such as COOO.Both A14 and A1 5 are set to I ; therefore, A and B are selected at the same time. This should neuer occur. The software must therefore ensure that this will never occur, and this is very risky. It is better to add a little hardware to obtain the selection of B if A14 = 1 andA15 = 0 (Fig. 18b). Some chips have several chip-select pins of opposite
FvFiL A1 5
a r
A14
cs
cs -Cs
B
B
*I5=
cs
-cs S
A Frq. 18. Linear selection.
A
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D.-J. DAVID
polarity. They avoid the logic of Fig. 18b, or, rather, they have it inside. They are therefore easy to use (Fig. 18c) in small or average systems. c. Example of a small system. Suppose we want to build a small system with a 6503 (microprocessor of the 6500 family which has only 12 address lines, thus 4K of addressable space, A12-Al5, may be considered as indifferent), a 2708 EPROM (1K bytes), and a 6532 (128-byte RAM, 2 IjO ports, and a timer). Such a system is already sufficient to deal with interesting applications. The 6532 has two The 2708 has an active low chip-select pin, chip selects of both polarities CS1 and .C Moreover, RS pin (RAM select) = 0) and other functions = 1 discriminates between RAM activates the ports or the timer). The whole set holds within 2K; thus we will leave A1 1 unconnected, too. A10 will discriminate between the ROM and the 6532. Since the ROM must be on higher addresses, it will be selected if A10 = 1. The 6532 will be selected for A10 = 0. To do this we link CSZ to A10 and CS1 to the +5-V source. AO-A9 select a byte out of 1024 within the 2708. AO-A6 will select a byte out of 128 within the 6532 RAM. The simplest is to connect A7 to RS: The RAM will have addresses 0000-007F, the 1-0, addresses 0080 and so on. The A8 line is indifferent to the 6532. The result is that addresses 100-17F will also be RAM and this allows implementing the stack. The stack pointer is initialized to 7F and if the stack has 20 hex of depth (this is seldom exceeded), addresses 0-60 will correspond to variables, while addresses 6 1 -7F (under the name 161-17F) will correspond to the stack. The ROM has addresses 400-7FF, but also COO-FFF, or FCOOFFFF as well. The complete schematics are given on Fig. 19. d . Use of decoders. Going beyond the preceding example, it is probably not profitable to use discrete logic because things soon become too complex. It is simpler and more cost effective to use decoders. A decoder is a circuit which has essentially n inputs and 2" outputs, numbered 0, 1, 2, . . . , 2" - 1. At any moment, only one of these outputs is active. We shall consider that active equals 0, inactive equals 1 (for wired-or reasons). This explains that most chip selects are active low. When the binary pattern made by then inputs is number k,the active output is output number k . The decoder has additionally one or two enable inputs which allow twostage decoding. Mainly used decoders are 3-8 (74 LS 138) or 4-16 (74 LS .154 or open collector 74159). The 74 LS 145 is used too because it is an open colleWor. It to :Fyf), is in fact a BCD 4- 10 decoder, but if only outputs 0-7 are used it is a 3-8 decoder whose input D plays the role of an enable. You can always use a higher order decoder of which only half of the outputs are connected. To partition the memory space, you have to choose the size of the
m.
(a
(m
(v;,";
PAO-PA7 PBO-PB7
FIG. 19. Small system
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segments. The most cost effective (but not always possible) is to use only chips of a given size, and this will be the segment size. According to the needed number of segments, you will see how many highest address lines may be indifferent if any. In a given segment, you may have several chips of smaller size: You will then use a decoder inside the segment (two-stage decoding) ; or you may have a chip which covers two segments, and its will then be obtained by wired-oring two decoder outputs. The most widely used segment sizes are 1K and 4K; 8K-byte ROMs and 16K-byte RAMS exist, hence sometimes larger segment sizes. Finally, for small systems, smaller segments (128, 256, or 512 bytes) may be imagined. If you choose 4K segments and a 4-16 decoder, you obtain a fully decoded system with 16 4K segments. The decoder inputs are connected to A19-Al5. This is, for instance the case of PETjCBM computers. If all segments are not occupied, the corresponding decoding lines may be used for expansion ! At this point, two concepts are concurrent. In the first one, only one decoder is used. Additional boards will be connected to the decoder outputs which must therefore be added to the mother board lines. In the second one, decoding is made on each additional board. You can even implement switches which allow the board base address to vary. The first concept has two slight drawbacks: (1) There are additional bus lines. (3) The segment size chosen for the decoder may be different from that which would be appropriate for the additional board, hence some additional logic may be needed. The second concept is more expensive since the decoder must be repeated on each board. However, this solution is the most widely used in industrial systems since it offers more modularity and versatility. The board base address is fixed by positioning a set of miniature switches. If you know that no more than eight 4K segments will be occupied, you can leave A1 5 unconnected and connect A13-Al4 to the inputs of a 74LS138. Thus you have a combination of complete and partial decoding, and each byte has two addresses (e.g., 0000 and 8000 hex). [The above problems are dealt with in the first chapters of Zaks and Lesea (1977) and in Sawin (1975).]
111. H O W
TO
DEALWITH
A
MICROPROCESSOR-BASED APPLICATION
A . Specifying an Applicutian
Microprocessor applications are extremely diverse. The list given in Section I is only an outline. However, when the moment arrives to build the
MICROPROCESSOR SYSTEMS
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microprocessor system able to deal with the application and to write the corresponding program, the process to follow will be exactly the same whatever the application. All applications are in fact similar. For any application, you have to measure a certain number of physical quantities, take a number of decisions according to the obtained values, and express these decisions as actions on some parameters. The system loops on itself in the sense that measures are done again to see if the taken actions have resulted in the desired effects. The scheme of Fig. 20 is valid for any process control. The process to follow to deal with an application is then obvious:
( I ) Inputs. List the quantities which are needed and design the sensors able to obtain them. (2) Outputs. List the parameters which must be acted upon and design the pertinent actuators. ( 3 ) Algorithm. Prepare the decisions to be taken to generate the signals which will control the actuators according to obtained information. Note that it is sometimes required to return to (1) after having performed (2) and (3), since the need for additional input information may have been pointed out. Order (2), (3), (I) may also be convenient. The advantage of microprocessor systems is that all the logic (i.e., the microprocessor and its program, the 1/0 ports, the analog-to-digital and digital-to-analog converters) is now almost completely standardized, thus simple. This allows the designer to devote most of his time to the real problems which are effectively specific to the application, i.e., to find the sensors able to obtain the needed parameters and the actuators able to control the considered process. 1. Sensors
Sensors are devices which transform the quantities you wish to measure into electrical quantities. It is then sufficient to transform them into numeric value with an A / D converter, There are many difficulties: (a) It is necessary to know a physical phenomenon which ensures the transformation into electricity. This is not obvious. (b) Measurement precision and fidelity are important. (c) The duration of the measurement is an essential parameter: The sensor must be able to follow the evolution of the measured quantity. Except for measurement of electrical quantities, measurements of temperature, pressure, stress, and speed are the easiest to obtain at present. More delicate are, for instance, the measurement of spatial position and
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N D Converter
c c
Input port
Microprocessor
.I output port
.I DIA Converter
FIG.20. Loop control of any application.
orientation. Depending on their nature and environment, measurements of chemical concentrations may be easy or difficult. 2 . Actuators Actuators are devices able to act upon the process. Among actuators are display devices (indirect action), relays, electric magnets, motors, valves, and so on. The problem is to devise actuators which are powerful enough and have the appropriate reaction time to control the concerned process. Generally, between an actuator and the microprocessor system, there is a digital-to-analog converter and/or an appropriate amplifier. Security devices. In any correctly designed process control, a security imperative is that every actuator be associated to a device (and a program module) designed to check that a command has actually been sent to the actuator. The check circuit must contain an independent sensor: It is not sufficient to check that you have written a 1 in a PIA register. What must be verified is, for instance, that the flow of some liquid has effectively been stopped. These sensors allow the execution, at the beginning, of a program called an “exercizer” which quickly sends all possible commands of the system and checks that they have been effectively obeyed. This allows immediate detection of defective devices and to signal them before catastrophic breakdown. Note that it is checked that the command is executed, and only this command. For instance, in a traffic controller, it would be catastrophic to switch on green lights in both directions. In the United States, the law imposes that if this occurs, the microprocessor disconnect itself, the flashing yellow mode be installed, and the central computer warned. In any case, the microprocessor allows refinement of security devices. Another problem with actuators is that of initialization. The successive states through which the actuator passes before being explicitly controlled by the application program must be known and correct. Think of what
MICROPROCESSOR SYSTEMS
45 1
would happen if the actuator were the magnet supporting a several ton press ! The states to consider are : before power-up of the microprocessor; during power-up and RESET sequence ; and during the I j0 port initialization routine. Wiring must be such that appropriate states are maintained until the microprocessor takes control. Afterward, it is the responsibility of the program. At power-down, problems are normally simpler since the system is under program control. The case of power failure must be dealt with in some applications. Generally, a safeguard routine started by a nonmaskable interrupt is executed. We have no room, here, to do more than quote the problems of noise and isolation in the industrial context. They are outlined in Zaks and Lesea (1977) and in Harrison (1978).
3. The Timing Problem One of the most important characteristics of the control signals that the microcomputer must provide to the controlled process concerns time : When must a signal be set up? After which delay must it be put down? When should a measurement be made? With what periodicity should data be obtained? The time which concerns a process is determined by the process itself and it is called real time. The time which concerns the microprocessor is characteristic of its operation speed, especially the time taken to execute an elementary instruction, and we call it proper time (this denomination is proposed by the author-it is not a widely accepted concept as is real time). The timing problem consists of synchronizing proper time and real time. Effectively, the microprocessor can work in any case, but according to its proper time; on the other hand, however, it must, apparently, provide commands adapted to the process run, and therefore it must operate in real time. If real time is faster than proper time, no synchronism is possible. This means that the considered microprocessor is not fast enough for the application. For instance, it is evident that a microprocessor which has a 2-pec minimum instruction time will be able to get data every 6-10 psec, roughly. It will not be able to deal with a signal-processing application requiring a 1-MHz sampling rate. If real time is slower than proper time, the application is feasible. I t should be pointed out that sensor measurement times, actuator reaction times, A / D or D / A conversion times slow down the apparent microprocessor operating time. In addition, two main synchronizing methods may be used to allow the microprocessor b “follow” the process. a. Watchdog loop (or “piling"). The program loops over a series of
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read operations of the different quantities to be obtained. Some delays are possibly implemented to ensure that different readings of the same quantity have the appropriate frequency to observe significant evolution. In some cases, the loop takes too long a time so that the second method is used. b. Event-drioen system. Some events (such as contact closure, key hit, quantity going beyond an alarm value) create an interrupt. An interrupt is a signal which causes the microprocessor to terminate the current instruction and then jump to a special routine to handle the interrupt. When this routine is completed the microprocessor returns to the interrupted program. The interrupt handling routine has two tasks : (1) recognize the interrupt cause; this may involve some polling but much shorter than a whole polling loop; and (3) take the appropriate actions for the considered interrupt, e.g., ring a bell, close a valve or a relay. This ensures the best possible response time for urgent problems. It is often recommended to use a mix of both methods. There is a cruising operation mode with a watchdog loop for certain parameters, whereas most crucial events are able to create interrupts.
B. Example
We now give the analysis scheme of a typical example-an elevator. The reader is encouraged to think of other examples such as a photocopier, traffice lights, temperature regulation, alarm central. [A lot of examples are described in the literature, such as: Basafiez et al. (1976), Dash and Mathur (1976), De Monte et al. (1980), Doyle (1976), El Shirbeeny and Zakzouk (1979), Houle and Lavoie (1978), Henschen and Yau (1976), Lataire et al. (1979), Leffen (1976), Radhakrishnan et al. (1976), and Tully et al. (1976) (chosen for diversity). Small business application treatment is described in Warren and Miller (1979. Robotics problems are detailed in Gupton (1980) and Loofbourrow (1978)J For our elevator, we follow exactly the scheme outlined above. 1. Input Data
Destination story. In elaborate versions, several requests have to be kept in the memory. Requests may be distinguished according to their source: corridor call button or inside the elevator keyboard. sensors: keys and contacts. Story where the elevator is going to arrive.
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sensors: contacts or photocells. Securities: door openings, overload sensors: contacts or photocells. 3. outputs
Door closure (if any resistance is sensed, the door must be reopened). actuator: small motor. Main motor control (up and down). actuator: relay Brake control actuator: relay or electromagnet. Display lights to indicate where the elevator is, which stories are requested, sense of motion, alarm conditions.
3 . Algorithms and Timing According to sophistication level, you may have an elaborate algorithm to optimize the run in function of requested stories. A simple method is the following: When the elevator is going up and an upper story is requested, the elevator continues to go up. When it is going down and a lower story is requested, it continues to go down. In other words, the direction changes are minimized. Timing. Story requests are not urgent. A watchdog loop is used to poll each story successively to see if it is requested. All securities must create an interruption to actuate the break as soon as possible. Story arrivals may be dealt with by either method. When it is known that the elevator is reaching a story, the program tests whether it is requested and, if yes, the brake is actuated.
C . Standard Interface Buses A recent development very much simplifies the task of sensor and actuator development in an important application field, i.e., instrumentation. In this field a whole set of devices is available on the market, such as sophisticated voltmeters, spectrometers, Fourier analyzers, and signal generators. According to the application a number of different devices must be connected to the system at the same time. This implies some particular hardware rules and handshaking protocols. Two standards have emerged : the CAMAC standard which is mainly used in laboratory experiments and
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the IEEE 488 which is now very widely spread. The CAMAC standard is very much hardware oriented and it even imposes the shapes of the devices-a special card-cage has been designed called the “crate.” On the contrary, the IEEE 488 is more protocol oriented. Its concepts are described in Loughry (1976). The only hardware specifications are the bus pinning, the connector shape and pin-out, the load and current specifications, and the maximum total and interdevice distances. The IEEE 488 protocols allow connection of different speed devices. Three fundamental functions exist: controller, listener, and talker. A device may have one or several of these functions, at different sophistication levels. Although implemental in wired logic in their simplest form, these protocols are especially well suited to be implemented with the help of a microprocessor. This corresponds to the present trend to use “intelligent” devices, i.e., devices equipped with a microprocessor. It is very easy to equip a microprocessor system with an IEEE 488 interface. Besides the software to implement the protocols, you just have to interface PIA ports with 3446s, or else you can use special IEEE 488 interface chips which are now appearing on the market, such as the 68488. Moreover, use of the hundreds of products made by many manufacturers is now very easy and inexpensive : Following the PET/CBM (Fisher and Jensen, I980), new very low-cost microcomputers offer a built-in IEEE 488 bus.
IV. SYSTEM DEVELOPMENT A . Programming Steps After you have listed the inputs and the outputs of your system and decided on an algorithm according to Section III,A, and have built your system according to Section II,D, the longest step is writing your application program. We have room here to list only a few concepts. Whole volumes have been written on programming, among which are Barden (1977), David (1981), Morton (1977), and Zaks (1978, 1979). An important point to note is that all programming concepts and languages developed for large computers are perfectly applicable to microcomputers as well. The programming step is itself split in three steps : flowcharting, program coding, and debugging. The last step is generally the most difficult. It will be described in Section IV,B, below. 1. Flowcharting and Documenting the Program Let us insist thoroughly on the fact that this step is absolutely necessary prior to proper coding. The flowchart is a drawing which allows logic flow
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to be visualized: It is a very useful design aid and too many programmers who believe that they can do without it are wrong and lose time in the end. Moreover, the flowchart is an excellent documentation tool for the program. It is an absolute necessity to provide complete and accurate documentation. This makes debugging much easier, but here, again, programmers’ psychology acts against it. The attempts to use structured programming (e.g., Wirth, 1973) are perhaps a way toward the solution because it favors topdown design. 2. Program Coding This is the proper programming phase. If the application has been carefully designed and a detailed flowchart has been prepared, the coding should be quite easy and short. The choice of the programming language is important, and it will be examined in the next section. Here, we review the instruction types offered by machine language, since this is the language representative of the real possibilities of microprocessors. a. Instruction types. Many classifications are possible. Here is ours. (i) Transfer of information without modification. This covers : memory-register transfers, for instance transfer from memory to the accumulator and transfer of the accumulatory into memory; inter-register transfers; stack instructions.-push and pull of a register contents. ( i i ) Arithmetic and logic instructions. This includes : two-operand instructions, which are of the form
A (accumulator) + A op M (memory cell) op may be one of the classical logic operations AND, OR, and exclusive OR, or addition and subtraction. Direct multiplication and division exist only on the most recent 16-bit microprocessors. one-operand instructions, which operate on a register or a memory cell. The operations may be: shifts and rotations; incrementationdecrementation (add or subtract 1);others, such as complementation. ( i i i ) Jump instructions. The unconditional jump transfers control to another part of the program. Branch instructions allow us to test a
bit in the flag register. If a condition is satisfied, a jump occurs, whereas the program continues in sequence if the condition is not satisfied. Other special jumps are the call to a subroutine and the return instruction.
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( i u ) Operation on the j a g s . Nearly all instructions influence some flags,
but some instructions do nothing but force a flag to a value. Other instructions in this group, called comparisons, perform virtual arithmetic operations. They perform a register-minusmemory subtraction, but the result is only to position the status flag in view of a branch instruction; the register remains unchanged. ( u ) Input-output. In the 6800 and 6500 series, this category is absent. The trend is to use memory-mapped IjO where the interface chip registers are considered normal memory cells, manipulated by ordinary instructions. A typical sequence could be a load of a port register into the accumulator, then some arithmetic operations, and a store of the accumulator into another port to act upon the external world. b. Addressing modes. Another component of the power of a microprocessor is the set of its addressing modes. An addressing mode is the way the concerned address is specified in an instruction involving memory. Classical modes, present in almost all microprocessors are : inherent, where the instruction performs an internal operation, and thus no address is specified; immediate, where the data itself and not its address is provided within the instruction; absolute, where the complete 16-bit address is specified ; short, where the address is specified only in 8 bits. being within the first 256 bytes of memory (zero page). relatiue, where the address is given as an 8-bit displacement relative to the program counter. indexed, where the contents of an index register are automatically added to the provided address-this makes it easy to deal with tables.
Other modes, which are common on large computers or minis, are quite rare on microcomputers. They are, for instance: autoincrement, which is a variant of indexed mode, where the index register is automatically incremented (or decremented) ; indirect, where it is not the operand address which is provided but the address of the address; indirect indexed, indexing may be combined with indirection in two ways-indexing after the indirection or before. The 6502 possesses both and we refer the reader to its data sheets for details.
We acknowledge that this presentation of software is too sketchy, however, its purpose was only to give a general view of microprocessor possibilities.
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Barden (1977) details machine-language programming for three microprocessors : the 8080,6800, and 6502. Specific references for microprocessors are : for the Z-80, Zaks (1979) ; for the 6800, Leventhal(l978) ;for the 6502, Zaks (1978), Leventhal(1978), De Jong (1980), and Foster (1978).
B. Development Systems 1. General Presentation
The need for a development system to help in the building of a microprocessor system results in the intermediate position of the microprocessor world between wired-logic and classical computers. There is no need for a development system to build up a wired-logic system because probes and oscilloscopes are generally sufficient. On the contrary, in a microprocessor system, signal inspection is not sufficient. It is necessary, first, to enter the program which will govern the system and then check that it leads to the wanted behavior. If not, it is necessary to understand what is wrong and correct the program. These program-debugging steps are the same as in classical computers. However, they have no development system, simply because they dispose of all that is needed as peripherals and debugging-aid software. However, an industrial microprocessor system has none of this. In its final-usage phase, its only peripherals are the sensors and actuators useful for the controlled process, and its sole software is the corresponding program. Obviously, no superfluous ROM chips will be incorporated. The result is-and it is specific to the microprocessor world-that a microprocessor system will exist successively with two configurations. Before the final configuration, where the system has only what is necessary for the controlled process, there is a development phase in which peripherals allowing data introduction, display, and debugging-aid programs are added to the system. All this hardware and software which is temporarily “grafted” to the system is called the “development system.” In other words, the development system “disguises” the system as a classical computer. However, there are some specific features that development systems have with regard to classical peripherals and high-level programming languages, aspects similar to the usual minicomputers. All microprocessor manufacturers propose development systems for their products at different sophistication (hence, cost) levels, but the cost of a development system is to be distributed: (1) over the number of items of the developed product and if mass production is expected, the most sophisticated development system is affordable :
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(2) over the set of different products designed with help of this system since, of course, when a product is developed, the next product is being designed. The only condition is to use the same microprocessor, although polyvalent development systems begin to appear.
We now see how the main two functions of a development system are fulfilled. 2 . Program Entry and Modijication With regard to hardware, the development system provides the microprocessor system with the usual computer peripherals. Input peripherals most frequently used in the microprocessor world are keyboards (complete or only hexadecimal) and paper-tape readers. The minimal input device is the hexadecimal keyboard available on single-board microcomputers such as the KIM-]. This implies use ofmachine language, whereas complete keyboards allow character-string manipulation and high-level languages. The minimal display peripheral consists of a row of seven-segment LEDs. Much more comfortable are CRT displays which allow reading of a full page: Different combinations exist from 10 lines of 16 characters to 25 lines of 80 characters. A cheap solution is to use a domestic television set. If hard copy is needed, a printer is necessary. Prices are decreasing considerably now. A microprinter which prints 20-40 characters a line is a possible solution. For program storage, the ideal solution is the disk. Microprocessor development systems very often offer floppy disks which store from lOOK bytes to 1M byte, with direct access. Magnetic tapes allow only sequential access. A minimal solution, but one which suffers very drastic limitations, is to use an audio cassette deck. The development system comprises the software able to operate the above peripherals. The minimum system is a program which gets information from the keyboard, enters them in memory, displays, and corrects them. It is the operating system of a single-board microcomputer. If a complete keyboard is available, this system becomes a text editor which stores texts in a file and allows any modification with simple commands such as “search a line,” “change a word,” “save a file,” etc. Languages. Edited texts are often programs, written either in symbolic assembly language or in a high-level language. Let us be reminded that the main disadvantages of machine language are suppressed if symbolic assembly is used. The nature of the operation is specified by a mnemonic which avoids remembering the binary operation code, and operands are specified by names which avoid handling addresses explicitly.
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High-level languages are available for ease of programming. They are more synthetic (fewer instructions to do the same treatment), closer to usual mathematical functions, and, as far as possible, machine independant. This allows “portable” programs. Most frequently used high-level languages in the microprocessor world are : PL/ M-type languages (name varies according to the manufacturerMPL, PL/Z, etc.), similar to ALGOL or PL/l and allowing access to machine resources; BASIC, the language easiest to learn; FORTRAN, the most widespread among minis; APL; and PASCAL. Use of a high-level language leads to a considerable diminution of time and effort devoted to program setup (Rooney, 1980). However, the obtained program is less efficient in execution speed and occupies more memory than the same program written in machine language. Use of a high-level language is therefore recommended if the problem under consideration leads to a very complex program, or if, to take place on the market, a very fast setup is mandatory; this supposes that the need of one or two additional ROM chips is acceptable, thus the product is not a mass product. Whatever the language used, in order to be executed, the program must be translated into machine langpage. This is done by an assembler (for assembly language) or a compiler (for high-level languages). In the beginning these programs were available only on large mainframe computers accessed, for instance, through a time-sharing net. This was called a cross-assembler or a cross-compiler because the translation occured on a machine other than the target machine. Development systems now have enough memory and disks to do the operation locally. Assemblers and compilers are then the main part of the development-system software. In some cases, the high-level language program is executed instruction by instruction as each instruction is translated: This is called an interpreter and it is the case with most BASIC implementations. [Among the immense number of books devoted to highlevel language programming, we quote Morton (1 977) for BASIC and Wilson-Addyman (1978) for PASCAL. See also Zaks (1980a. Chapter 6).] A possible way of using the development system is the “ROM simulation” mode. The system under development does not yet have its ROMs, since the program is not yet in its definitive state. In order to try the current version, it is introduced in a system-development RAM by means of its peripherals. Then the system under development is connected, the addresses of the just-filled RAM are defined 8s ROM, and the system is tried out. If its behavior is satisfactory, the program is (nearly) correct. To continue experiments on a full scale, a REPROM will be programmed and installed in
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place of the ROMs on the underdevelopment board. This programming can be done by a simple copy of the above RAM if the development system is equipped with a REPROM programmer, a very convenient feature. We have now seen how a program can be entered from typing to REPROM programming, through its translation into machine language. The development system allows also the detection of errors during execution. 3 . Execution Analysis
Wrong operation of the system can be due to either a hardware or a software cause (e.g., program error). a. Hardware. Hardware errors are detected by examining the important signals in a CRT oscilloscope equipped with sufficient frequency range and sweep delay device to examine initial edges of signals. It will often be necessary to use a storage oscilloscope to display transient phenomena. One of the most frequent errors is to overload an output (e.g., to try to drive more than 10 TTL inputs by a TTL output). In that case, it is seen clearly on the oscilloscope that the voltage is not low enough in the zero state (>0.8 V) and that it is not high enough in the 1 state ( <2 V). Other errors are forgotten connections or short circuits. The problem is that many signals must be displayed at the same time. For instance, it may be necessary to see what addresses the microprocessor sends to the bus: This needs 16 lines. The problem is solved by logic analyzers, which are actually storage oscilloscopes but with 8, 16, or 32 channels. Some features make use easier, and three forms of display are available: square waves, binary, or hexadecimal. Sweep can be started upon recognition of a specified bit pattern. These devices are very useful but quite expensive [see Zaks and Lesea (1977), Chapter 81. 6. Software. Programming errors are detected in classical computers by means of a well-proven method: The program is executed step by step and between instructions, the different internal registers and some memory locations are examined. As soon as a discrepancy is found with respect to the correct data, one is close to understanding the error. Application of this method to microprocessor systems settles two problems : Step-by-step. Most microprocessors do not allow the clock to stop. Addition of a microprocessor-dependent circuit is mandatory. It uses most frequently the interrupt feature to stop execution. Register display. Unless you remove the cover and dispose microprobes where they should be, any access to the internal registers is forbidden. The registers can only be displayed by a program.
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To do this, even in the simplest single-board microcomputers, a monitor is available to allow step-by-step processing in the following way: Between any instruction an interrupt is requested provided that (1) a step-by-step mode is enabled, and (2) the current instruction is not part of the monitor itself. When the interrupt occurs, a monitor module is executed to store the registers in a memory area called “register image” and display the stop address. Then a monitor command allows display of the register image and resumption of execution. This is the minimal system which allows program debugging. Therefore, single-board microcomputers allow only limited development. i. Breakpoints. Step-by-step execution of a long sequence is tedious. More sophisticated monitors allow breakpoints to be set in which execution stops upon arrival there. This allows stopping only on some instructions. A repeat count may be associated with a breakpoint: The stop occurs when the breakpoint is reached for the nth time. This is especially useful in loops. A similar device is the stop-on-address possibility which also concerns operands. The stop occurs as soon as a specified address appears on the address bus. This allows detection when the program fetches an address it should not, and it is immediately seen which instruction is wrong. ii. Simulators. Another way is to use a simulator running on a large computer. It interprets each binary instruction and performs the wanted operations on memory images of the registers. Results are printed after each “instruction” as well as expected execution times. iii. Emulators. A drawback of step-by-step monitors and of simulators is that all timing problems escape from them. An error which occurs because the system responds too late to an external event cannot appear in step by step. To see this, the program must run in real time, i.e., at normal speed. A real-time simulator is called an emulator. The debugging system which offers the most extended possibilities is the In-Circuit Emulator (Intel’s ICE, Motorola’s USE, System 65’s USER-all manufacturers have it now), and it operates as explained in Fig. 21. The system under development is stripped from its microprocessor : On its socket a set of 40 lines coming from the development system, the “umbilical cord,” is connected. On each line of the umbilical cord, the development-system device simulates the signal which would have been present on the corresponding microprocessor pin. However, at the same time, the development-system program stores certain information as each instruction is executed. Then, on a breakpoint, you may display the image of all that has occurred during, for instance, the last 44 cycles, and the operation is close to the real, final system operating conditions. This is therefore a very interesting debugging tool.
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"Umbilical" cord
Controlled automaton
Keyboard-display
system
FIG.21. In-circuit emulator.
Thus, we have seen the different levels of development systems from the minimal to most sophisticated functions. An aspect of simple development systems is that they are educational tools for teaching microprocessor systems via case studies [see Schertz and Stewart (1976) and Dewitte et al. (1979)]. V . THECHOICES IN THE DESIGNOF
A
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The design of a microprocessor system is influenced by a series of choices. Let us examine them in sequence. A. Microprocessor or N o t ? This is clearly the first choice to be made. Alternatives are to use a minicomputer or wired logic. 1. Microprocessor versus Minicomputer When the application justifies using a minicomputer, the argument against using a microprocessor is its low performance. This deserves closer examination.
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In many cases, especially process control in laboratories, minicomputers used at a fraction of their capabilities can be found. It is then obvious that they should be replaced by microcomputers. In other cases, a minicomputer controls several experiments simultaneously. It may be profitable to replace it by several microcomputers, each devoted to a single experiment. The total cost will be probably lower and there is better modularity, which is more versatile, especially in case of breakdown (with a minicomputer everything is stopped, whereas with several microcomputers, only one experiment is affected). Besides, microprocessors are not so slow. Some 8-bit microprocessors exist in faster versions (the 2-80 has a 4-MHz version, the 6800 has a 2-MHz version, and the 6500 family has 4-MHz chips). Moreover, auxiliary chips which perform floating-point arithmetic 10 times faster are available (Schenkel, 1976). Sixteen-bit microprocessors, which are more and more available, are precisely aimed at minicomputer applications. Finally, if very high performance is needed, bit-slice microprocessors can be used. They lead to more expensive systems, but they operate five to ten times faster. Moreover, the graphics of Fig. 3 show that performance is still increasing. Business applications have even become feasible for microcomputers now that Winchester technology hard disks of 5-20M bytes have become available at low cost (Zaks, 1980d). 2. Microprocessor versus Wired Logic Wired logic should be used rather than a microprocessor in two cases. (1) If the application is so simple that its implementation in wired logic is easier than with a microprocessor. However, the threshold of microprocessor applicability is very low. Complexity and cost of wired-logic implementation effectively increase much faster according to the functional sophistication than for a microprocessor system where there are only program and storage to add on. The microprocessor also has decisive advantages : (a) Manufacturing costs are lower because there are fewer components to assemble. (b) Set-up costs are lower because the essential is program debugging, and in case of error, it is the program which has to be modified instead of an electronic circuit. (c) If the application is simple enough to be feasible in wired logic, the programmed logic solution may allow the addition of new functions to the system; e.g., the microprocessor washing machine does more than an ordinary one. The company creative enough to add new functions to a classical device by means of a microprocessor may obtain huge profits.
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(d) Programmed logic allows operations to be performed which are impossible in wired logic-correctness tests for sensors or complex regulation algorithms. (e) Maintenance of microprocessor systems is easier because there are fewer components. Another advantage is that diagnostic routines can be incorporated in the ROMs to automatically localize failures. (f) Finally, product evolution is very cheap. It is generally sufficient to modify the program, thus replacing or adding a ROM chip. (2) The second case is that of very high performance. This is often the case of signal processing. Before rejecting the idea of using a microprocessor, however, all possibilities must be examined : fast versions, auxiliary arithmetic chips, 16-bit micros, or bit slices. A clever solution may be to do an information preprocessing in wired logic, where the microprocessor is too slow, and then use a microprocessor for the processing part, which can be handled by the microprocessor. For instance, that it is what we have done in David (1977). An additional advantage of programmed logic in process control is that it allows more time to be devoted to sensor and actuator design. The limit could be to use an identical, standard microcomputer for different processes, changing only sensors and actuators and providing a specific program. B. Which Microprocessor Category ?
The different microprocessor categories have been listed in Section II,B,l, according to the number of bits. Another criterion is the technology which may be imposed to fullfill certain application requirements : the bipolar technology is required for faster processing; the CMOS technology is particularly suitable if the application requires very low power consumption (battery backup) or large noise immunity. Let us recall that the 4 bits are now obsolete, except for pocket calculators or very small applications. For new designs they are supplanted by 8 bits, which are not more expensive. PMOS 8-bit microprocessors are obsolete. More than 50% of all applications are solved by 8-bit microprocessors, mainly NMOS and some CMOS processors (see Mori et al., 1979). This category is really the standard, at present. It is well suited to character handling, decimal computation, analog-to-decimal conversions (%bits offer a sufficient precision in most cases). Eight-bit microprocessors allow implementation of small systems to very large systems with many interface chips. On the low end, complete single-chip microcomputers are suitable for mass production. For higher range applications, we think of 16-bit micros (see Section VI), or if bipolar speeds are required, it is mandatory to use bit slices in Schottky
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TTL technology or in ECL. The obtained systems are more complex (around 50 chips) and more expensive, but an addition takes 100-500 nsec instead of 4-5 psec. C . -Which Microprocessor?
The appropriate microprocessor category is almost directly imposed by the application. Another choice which arises immediately from the nature of the application is the language in which the program will be written. This choice has been described in the Section IV. Three levels of programming languages exist : binary machine language, symbolic assembly language, and high-level language. Binary machine language is used only if a complete keyboard is not available. Otherwise, symbolic assembly language is used, since it is equivalent to machine language except for its more comfortable presentation. Assembly language is used if performance is important. However, if debugging and programming speeds are important, a high-level language is used. The choice of a high-level language among others, depends on availability and personal preference since all such languages have more or less equivalent capabilities. We arrive now at a much more delicate choice: that of a particular microprocessor within its category. In effect, the different products in a category have very similar performances : Each has particular advantages which the others have not. None has all the advantages. Moreover, vendor literature is not designed to make this choice easy! On the contrary, manufacturers’ data sheets insist on the good points of the product but do not mention its weaknesses. They must be studied carefully and with critical sense. For instance, the data sheet insists on the high clock frequency which is accepted but does not say how many cycles are needed to perform an operation. Thus, a 2-80 with a 2.5-MHz clock and a 6502 at 1 MHz have essentially the same performance. A 2-MHz 6502 is faster than a 4-MHz Z-80. We now review the main criteria for choices. Of course, the goal is to optimize the unit cost of the system you are designing, i.e., UC = (F/N) V where N is the number of pieces you expect to produce ; Fare the fixed costs (independent from N , their main components are the development costs) ; V are the variable costs (the same for each copy), for instance, the intrinsic cost of the involved chips. The different criteria act upon V or F. Therefore, their importance changes completely according to whether N is small (small diffusion product such as a laboratory automaton) or large (appliance). We shall first examine criteria which act upon both F (development costs) and V (intrinsic costs). These are performance criteria such as the instruction set-an efficient
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instruction set makes programming easy, thus reducing development costs. On the other hand, it allows better use of the memory which is thus able to save some memory chips and, hence, lowers intrinsic cost. 1. Performance Criteria
a. Clock frequency. This parameter directly determines execution speed, but it must be examined carefully. It is not sufficient to define the frequency. It is necessary to see how many cycles it takes to perform a typical operation. The best way to use this criterion is to have two versions of the same microprocessor working at different speeds. One then chooses the version which is best suited to the application. Programming costs are reduced if the processor has sufficient speed because it is not necessary to try to save execution time for every instruction you write! A word of caution if you are looking for a fast system: Use of a fast version of the microprocessor makes it necessary to use fast versions of all associated chips, especially memories, and this increases cost considerably. b. Instruction set and addressing modes. This is the most subjective of all-everyone has his preferences. In fact, the main processors are roughly equivalent in this domain. In a whole program, what a microprocessor loses at first with respect to others, it regains a little later. Caution should be exercised about gadget instructions which are pointed out by the vendor, but are seldom used. The number of internal registers may be impressive, but real usefulness must be examined. Personally, we think that efficient addressing modes are more important. The instruction set influences development costs very much. Therefore, it has little importance in mass production. Addressing modes influence both the cost of development (good addressing modes make programming easy) and the intrinsic cost (good addressing modes lead to more efficient memory usage, which allows chip-count reduction). The way in which the microprocessor deals with interrupts is part of this criterion, and this may be important for some applications. c. Benchmark programs. The best way to compare several microprocessors is to run on each of them an identical program called benchmark, written either to be general enough to test all possibilities of the machines, or to be representative of the considered application. However, the representativity problem is the source of unending discussions. In fact, manufacturer-written benchmarks should be avoided. In effect, the manufacturer has invested much time to optimize his program in order to take advantage of all the peculiarities of his processor and to carefully avoid its weaknesses. The user should write a benchmark program himself, not too time consuming, and in keeping with his programming habits. He will thus have a result that represents what he can expect to obtain
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from each of the investigated microprocessors. [Interesting benchmark results are given in Flippin (1980a,b)].
2. Intrinsic Cost Criteria These criteria concern, mainly hardware features since they influence the system building cost. a. Price of the microprocessor. We list it only to say it is of absolutely no importance. The price of the microprocessor is negligible compared to total system cost. Moreover, all microprocessors in each category have very similar prices. b. Price of auxiliary chips. A complete system may comprise a large number of auxiliary chips (support logic, interface chips). Thus, they tend to cost more than the microprocessor itself, all the more as manufacturers have a tendency to lower microprocessor prices and compensate their loss with support chips. It is better to prefer a “family” with a comprehensive set of cheap interface chips. One of the most complete on the market is the 6500 family. It is interesting that, among interface chips, multifunction packages be available, associating RAM, ROM, 1/0 ports, and timers. It is very convenient to dispose of several “blends” associating the different functions in variable ways, allowing optimal fit to the application. The price of associated memories must also be taken into account in this connection. c. Hardware facilities. Different criteria may be listed as: number of chips needed to complete a system ; clock signal complexity ; built-in clock generator; power consumption ; and number of power supplies. These criteria are not contestable. Others are more subjective, such as:
D M A ,facility. (Its real necessity in the system under consideration must be carefully examined. Generally, D M A is all the more unnecessary the more efficient the microprocessor.) Different control signals. For our part, we prefer simplicity, provided all possibilities remain available. Particular features. For instance, the 2-80 has signals for dynamic memory refresh. However, careful examination shows that this does not solve all problems. In general, we can say that in this domain, more recent microprocessors which are improvements of the 8080 (Z-80 and 8085) or of the 6800 (6500) are preferable. Two philosophies divide the &bit microprocessor world : (1) the 8080, featuring a machine cycle composed of several clock periods, many internal registers but weak memory manipulation possibilities, and specialized 1/0 instructions.
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(2) the 6800, featuring a machine cycle made of a single clock period less internal registers but extended memory manipulation and addressing features, memory-mapped I/O, and use of interface chips. The 2-80 goes beyond the 8080 in register number and instruction-set extension. It has better hardware facilities (single power supply). The 6500 goes beyond the 6800 in the following directions: better performance (more pipelining and up to 4-MHz clock); still more addressing modes; family of differently tailored microprocessors and more versatile multifunction interface chips. d. Availability (second sources). It is fundamental in mass production not to suffer from very long delivery delays. The existence of second sources (i.e., other manufacturers who produce the same microprocessor) is a guarantee in this area. Besides, the presence of second sources can result in price reductions because of “price wars.” It should also be noted that for the first manufacturer if the existence of second sources results in a loss of some share of the market, it is also an affirmation of the product, giving it credibility. To sum up, let us say that microprocessors should not be chosen only in relation to second sources, but it is comforting to have a least one. 3. Development Cost Criteria
Since the main development operation is program writing and debugging, criteria in this field are mainly software oriented. Let us recall the importance of instruction set and addressing modes. This criterion is fundamental for program design ease. However, it not very important for a mass product where the program is generally simple and the design cost is to be divided by the number of pieces produced. On the contrary, it is crucial for complex program applications. Other criteria in this category are as follows. a. Development system availability. This is absolutely mandatory in order to implement a product. It should be pointed out that some training single-board microcomputers may be used as minimal development systems. Otherwise, complete development systems are expensive. It is advantageous to adopt a microprocessor family able to deal with the whole range of applications, and the development-system investment will have to be made once and for all. b. Software support. This is a key criterion since it determines ease of designing. Software support includes : the monitor; an assembler; a text editor ; the loader, possibly a relocatable loader ; compilers or interpreters for high-level languages; and a subroutine library for, e.g., floating-point arithmetic. The last two criteria are important, but they are not very discriminating since, now, all microprocessors on the market have good development
MICROPROCESSOR SYSTEMS
469
systems and extensive software support. [Hidden costs in a microprocessor system development are analyzed by Jinadasa (1978), whereas different trade-offs and products are evaluated by Osborne (1976).] VI. CONCLUSION: A GLIMPSE INTO THE FUTURE
We have now seen what microprocessor systems are, what they consist of, and how they are developed. Of course, progress never stops. Let us now describe some directions for future advances. A . Sixteen-Bit Microprocessors
This is the natural step forward, and a number of 16-bit microprocessors are now produced. For some of these, mass production is late with respect to announcements, but, anyway, they exist. Main families are 9900, 9440, 8086, 2-8000, and 68000, which is actually in the 32-bit direction. [Comparisons are given in Zaks (1980c) and in Heering (1980).] A 16-bit microprocessor is by no means the obvious choice if an 8-bit is sufficient for the application. We think that 16-bit products will completely supplant 8-bits only when (1) they cost no more than 8 bits, and (2) all support chips (interface chips) are produced. An interesting intermediate step is represented by microprocessors which have 16-bit internal registers but have an 8-bit data bus (e.g., 6809, 8088, and announced 6516). The real domain of 16-bit microprocessors is business or number crunching (a domain which has been much neglected by 8-bits), but we feel that in process control, the 8-bits have still many years of service. B. A Possible Crisis
To go beyond the present MOS state of the art in integration scale and speed, the lines must be thinner and thinner. We are already beyond the limits of optical resolution, which results in using electron or proton beams to prepare the ‘‘photo’’-reduced masks. However, if we go beyond this, another problem appears : Dielectric rupture will occur unless the operating voltages are taken under 5 V, but this kills TTL compatibility. What are the solutions? The advent of a “new TTL” technology, operating at, say, 3 V would settle a very difficult problem of compatibility with prior products. Another solution would be that these future MOS products incorporate built-in level-translation buffers, but this adds complexity. Nevertheless, we are sure that solutions will be found and that further progress will be achieved in this exciting field. [A manufacturer’s view on microprocessor trends is to be found in Faggin (1979).]
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REFERENCES Barden, W. Jr. (1977). “How to Program Microcomputers.” Howard W. Sams, Indianapolis. Indiana. Basanez, L., Fuertes, J. M., Huber, R. M., and Morell, J. (1976). Proc. MIMI’76, Zurich, p. 191. Capocaccia, F. (1979). Euromicro J. 5, 341. Dash, P. K., and Mathur, R. M. (1976). Proc. MIMI ’76, Toronto, p. 199. David, D. J. (1977). Proc. MIMI’77, Zurich, p. 275. David, D. J. (1980). Euromicro J. 6, 175. David, D.-J. (1981). “PET Basics.” dilithium Press, Portland, Oregon. De Jong, M. L. (1980). “Programming and Interfacing the 6502.” Howard W. Sams, Indianapolis, Indiana. De Man, H. (1 976). Proc. MIMI ’76, Zurich, p. 1I. De Monte, V., Fiorini, P., Rotoloni, R., and Scire, G. (1980). Euromicro J . 6, 17. Dewitte, J., Pellegrin, J., and Brie, C. (1979). Euromicro J . 5, 93. Doty, K. L. (1979). “Fundamental Principles of Microcomputer Architecture.” Matrix Publishers, Portland, Oregon. Doyle, J. (1976). Proc. MIMI’76, Toronto, p. 213. E. Shirbeeny, E.-H. T., and Zakzouk, E. E.-D. (1979). Euromicro J . 5,384. Faggin, F. (1979). Euromicro J . 5, 344. Fisher, E., and Jensen, C. W. (1980). “PET and the IEEE Bus.” Osborne-McGraw-Hill, Berkeley, California. Flippin, A. (1980a). Microcomputing 39,26. Flippin, A. (1980b). Microcomputing 46, 182. Foster, C. C. (1978). “Programming a Microcomputer: 6502.” Addison-Wesley, Reading, Massachusetts. Gupton, J. A. (1980). “Microcomputers for External Control Devices.” dilithium Press, Portland, Oregon. Harrison, P. G. (1978). Euromicro J. 4, 18. Heering, J. (1980). Euromicro J . 6, 135. Houle, J. L., and Lavoie, M. (1978). Int. J . Mini Microcomput. 1, 25. Henschen, L. J., and Yau, S. H. (1976). Proc. MIMI ’76, Toronto, p. 206. Jinadasa, N. (1978). Euromicro J . 4, 8. Lataire, P., Meyers, R., Maggetto, G . ,and Van Eck, J. L. (1979). Euromicro J. 5, 380. Leffen, D. J. (1976). Proc. M I M I ’76, Toronlo, p. 196, Leventhal, L. A. (1978). “6800 Assembly Language Programming.” Osborne-McGraw-Hill, Berkeley, California. Loofbourrow, T. (1978). “How to Build a Computer Controlled Robot.” Hayden, Rochelle Park, New Jersey. Loughry, D. C. (1976). “ELECTRO ’76-Professional Program.” IEEE, Boston, Massachusetts. Mori, R., Kita, J., Morishita, J., Tojo, A,, Kokubu, A,, Okkawa, K., Nishikawa, S., Okada, Y., and Uchida, S. (1979). Euromicro J . 5, 155. Morton, J. B. (1977). “Introduction to BASIC.” Matrix Publishers, Portland, Oregon. Nicoud, J. D. (1975a). Euromicro Newsl. 1/3, 3. Nicoud, J. D. (1975b). Proc. MIMI ’75, Zurich, p . 22. Osborne, A. (1976). “An Introduction to Microcomputers,” Vols. 0, I , and 2. OsborneMcGraw-Hill, Berkeley, California. Radhakrishnan, T., Sum, G. Y., and Venkatesh, K. (1976). Proc. MIMI ’76, Toronro, p. 203. Repko, M. (1978). Euromicro J. 4,39.
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47 1
Rooney, M. (1980). Euromicro J . 6,28. Salama, C. A. T. (1978). Int. J. Mini Microcornput. 1, 2. Sawin, D. H. (1975). Euromicro Newsl. 1/4, 26. Schenkel, D. P. G. (1976). Proc. MIMI ‘76, Toronto, p. 24. Schertz, I. D. R . , and Stewart, T. L. (1976). Proc. MIMI ‘76, Toronto, p. 235. Tully, P. D., Moore, A. W., and Dexter, A. L. (1976). Proc. MIMI ’76, Zurich, p. 197. Warren, C., and Miller, M. (1979). “From the Counter to the Bottom Line.” dilithium Press, Portland, Oregon. Wilson, I. R., and Addyman, A. M. (1978). “ A Practical Introduction to Pascal.” SpringerVerlag, Berlin and New York. Wirth, N. (1973). “Systematic Programming: An Introduction.” Prentice-Hall, Englewood Cliffs, New Jersey. Zaks, R . (1977). “Microprocessors: From Chips to Systems.” Sybex, Berkeley. Zaks, R. (1978). “Programming the 6502.” Sybex, Berkeley. Sybex, Berkeley. Zaks, R. (1979). “Programming the 2-80.’’ Zaks, R. (1980a). “Your First Computer.” Sybex, Berkeley. Zaks, R. (1980b). Bus 1.5. Zaks, R. (1980~).Bus 1, 18. Zaks, R. (1980d). Bus 1,22. Zaks, R., and Lesea, A. (1977). ”Microprocessor Interfacing Techniques.” Sybex, Berkeley. ADDENDUM TO REFERENCES The work by De Jong (1980) contains experiments on interfacing. The work by Foster (1978) contains very deep insights. Dr. Leventhal’s book (1978) is very accurate and complete. The books by Osborne (1976) are a must, especially Vol. 2, which is periodically updated and gives an objective evaluation and a thorough description of all microprocessor families. The work by Wilson and Addyman (1978) is a very concise text. And finally. the work by Zaks (1978) is the most accessible text on the 6502.
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Author Index Numbers in parentheses are reference numbers and indicate that an author’s work is referred to although his name is not mentioned in the text. Numbers in italics indicate the pages on which the complete references are given. A
Aberle, W., 47, 56, 58, 125 Aberth, W . H., 139 Abouaf, R., 31,32, 125 Adams, N . G., 57,142 Addyman, A. M . , 459,471,471 Agostini, P., 104, 125 Agron, P., 72,133 Akulin, V. M . , 1I I , 125 Albrecht, A. C., 108, 141 Albritton, D. L., 45, 128 Alexander, B., 361(27),407 Alimpiev, S. S., 1 11, 125 Allan, M . , 125 Ambartsumyan, R. V., 37, I25 Amusia, M . Ya., 68, 79, 125 Andersen, C . A,, 252, 286, 306 Andersen, H. C., 114, 122, 124, 140 Andersen, H. H., 262, 264, 265, 306 Andersen, N . , 266, 306 Anderson, C. P., 72, 128 Anderson, R. J., 159(21),226 Andresen. B., 49, 125 Andrews, R. A., 104, 140 Andreyev. S . V . , 105, 125 Antonov, V. S., 105, I25 Arakawa, K., 37, 52,142 Archer, J . D., 188(57),227 Arias, M . , 265, 306 Armstrong, L., 101, 127, 132 Armstrong, L., Jr., 125 Armstrong, L. A,, Jr., 79, 142 Armstrong, W . T., 7 , 133 Arrathoon, R., 37, 125 Aslam Baig, M . , 78, 130 Astruc, J . P., 34, 125 Asundi, R. K., 5, 27, 141 Atabek, 0.. 80,86,126, 135 Aten, J . A., 48, 50, 126, 138 Atkinson, G. H., 121, 131 Auerbach, D., 6 , 15, 19,20, 21, 126
Augustyniak, W. M., 262, 308 Auracher, F., 219(91),228 Avouris, P., 105, 1 I I , 126, 139 Azria, R., 28, 31, 32, 33, 34, 35, I26
B Baede, A. P. M., 48,138 Baer, T., 96, 100, 126, 138 Bahr, J. L., 83, 126 Bailey, M . D., 217(86),228 Bailey, T. L., 40,41, 126, 144 Baker, A. D., 61,128 Baker, C. E., 59, 126 Baker, E. A. M., 63,137 Baker, J . E., 286, 309 Ballofet, G., 61, 126 Bandrauk, A. D., 58,126 Banner, A. E., 281,306 Barat, M.,41, 132 Barbi, R., 34, 125 Bardsley, J . N., 5 , 10, 14, 18, 19, 21, 22, 23, 24, 25,27,54, 126,130, 143 Barnowski, M . K., 221,229 Barton, D., 387(28), 397(28),408 Basanez, L., 452,470 Bates, D. R., 2, 5 , 8, 10, 12, 36, 58, 112, 126 Bauer, E., 13, 20, 126, 242, 298, 306 Baum, G., 73,127 Baumgartel, H., 47, 92, 139 Baun, W . L., 238,258,306 Bay, H. L., 264, 265,306 Bayed, P., 292,306 Beade, A. P. M., 42,48, 50, 126 Beales, K. J . , 163(25), 176,226 Beaty, E. C., 112, 123, 132, 141 Beauchamp, J. L., I l l , 121, 142, 143 Bebb, H . B., 101, 127 Beckmann, P., 387(29), 408 Beckwith, S., 39, 127 473
474
AUTHOR INDEX
Beerlage, M. J. M., 47, 104, 108, 135 Beers, B. L., 101,127 Behrisch, R., 239,306 Beiting, E. J., 104, 127 Bekov, G. I., 37,125 Belic, D. S., 30, 136 Ben Amar, M., 80, 129 Bender, C. F., 82,140 Benjamin, J., 388(31), 408 Bennett, R. A., 112, 115, 118, 120, 121, 122, 123,127,129, 130,134,141 Benninghoven, A., 246,249,271,293,301,306 Berg, J. 0.. 105, 127,143 Berger, H., 378(32), 395(32), 408 Berkey, E., 296,306 Berkowitz, J., 61, 65, 67, 68, 70, 72, 75, 76, 77, 83, 85, 86, 87, 88, 93, 95, 96, 97, 98, 127, 129. 130, 131, 133 Berkstresser, G. W., 196(67), 227 Bernstein, R. B., 109, 110, 144 Berry, R. S., 1, 2, 10, 11, 12, 13, 14, 18, 19, 21, 23, 35, 38, 41, 42, 44,45, 49, 50, 53, 54, 55, 57, 60, 64,67, 70, 71, 72, 76, 80, 87, 88, 89, 103, 104, 113, 115, 127, 131, 132,133,138, 139, 141, 142,143 Betz, G., 262,263,264,265,267,268,272,300, 306,307,308 Beyer, R. A., 121, 123, 127 Bhattacharya, R. S., 262, 309 Biard, J. R., 212,228 Bieneck, R. J., 39, 127 Bierbaum, V. M., 39,144 Biondi, M.A., 5,6,8,9, I1,22,126, 127, 130, 132, 134, 135, 136, 140, 141 Birtwistle, D. T., 29, 127 Blake, G., 245, 252, 306 Blake, A. J., 83, 126 Blankenship, M. G., 147(2), 225 Bloch, F., 25, 127 Blyler, L. L., 178,227 Boeckner, C., 112, 138 Boersch, H., 233,306 Boesl, U., 109, 127 Boggs, D. R., 153(12), 157(12), 225 Bomse, D. S., I 11, 143 Bonani, G., 292,296,298,309 Boness, M. J. W., 30, 33, 142 Borrell, P., 90, 127 Bortner, T. E., 134
Bosch, F., 292,306 Botez, D., 198, 228 Bottcher, C., 14, 15, 19,23, 28,95, 127 Botter, R., 52, 131 Boulmer, J., 12, 13, 127, 142 Boursey, E., 61, 127 Bowen, T., 188(55), 227 Boyle, D., 318(33), 408 Brackmann, R. T., 29,133 Bradbury, N. E., 25,127 Bradley, D. J., 107, 127 Branscomb, L. M., 112, 114, 118, 119, 127, 128, 133 Branscomb, S. J., 119,142 Brau, C. A., 6,30,141 Brauman, J. I., 112, 113, 121, 122, 124, 134, 139,140, 142, 144 Braun, P., 263,265,267,300,306,307,308 Brehm, B., 128 Breton, J., 90, 133 Brie, C., 462, 470 Briggs, D., 65,128 Briglia, D. D., 27, 28, 140 Brion, C. E., 65,98,128, 134, 142, 143 Brodhead, D. C., 43,130 Brophy, J. H., 98, 109,128 Brown, K. F., 304, 305,309 Brown, S., 364(44), 408 Brown, S. C., 8,127 Brown, W. L., 262,265,266,308 Brown, W. W., 212(82), 228 Browning, R., 94,95,128, 132, 142, 276,306 Brueckner, K. A., 79,128 Brundle, C. R., 61, 128, 233,306 Buchel’nikova, I. S., 31, 33, 128 Buckingham, A. D., 70,128 Buckley, B. D., 28, 127 Biiger, P. A., 284,285,295, 306, 30Y Burch. D. S., 112, 114, 128, 142 Burdett, N. A., 44,45,49, 55, 57, 58, 128 Burgess, D. D., b , 137 Burhop, E. H. S., 5, 138 Burke, P. G., 79,80, 128, 136, 142 Burrow, P. D., 29, 34,128, 142 Burrus, C. A., 195(65), 197(65), 199(65), 227 Burt, J. A., 123, 128 Burton, R., 129 Bush, Y.A., 45,128 Bydin, Iu. F., 41, 128
475
AUTHOR INDEX C Cacak, R., 6, 15, 19,20, 21, 126 Cadez, I., 28, 133 Cairns, R. B., 78, 128 Capocaccia, F., 417, 470 Capon, J., 378(35), 395(35), 408 Carlson, T. A,, 71, 72, 78, 85, 99, 128, 133, 135,136, 143,242,306 Carlsten, J. L., 121, 129 Carlton, T. S., 54, 55, 129 Carney, G. D., 21, 129 Carroll, P. K., 63, 129 Carter, A. C., 200(73), 228 Carter, S. T., 79, 129 Carver, J. H., 83, 126 Castaing, R., 280, 301, 306 Casto, T. L., 218,228 Cathro, W. S., 120,129 Caudano, R., 6,20,126, 129,137 Cederbaum, L. S., 122, I29 Celotta, R. J., 112, 118. 120,121,122,129, 141 Center, R.E., 30, 129 Cernoch, T., 44,I27 Champion, R. L., 40.41, 42, 129, 131, 136 Chan, I. Y., 111,126 Chan, K. H., 156(19), 159(19), 160(19), 207 (19). 226 Chandrasekhar, S., 112, 129 Chang, C. C., 233,306 Chang, E. S., 71,129 Chang, J.-J., 79, 129 Chang, J.-S., 7, 139 Chang, T. N., 79, 129 Chanty, P. J., 34, 129 Chapman, C. J., 129 Chapman, F. M., Jr., 71, 131 Charles, R. J., 173, 226 Chen, H.-L., 30, 129 Chen, J. C. Y., 13, 14,24, 129 Cheng, K. L. 72.128 Cheng, R. T., 74, 79,134, 135 Cherepkov, N. A., 68.79, 125, 129 Chernysheva, L. V., 68, 79, I25 Chibisov, M. I., 19, I44 Chien, R.-L., 103, 104, 133,142 Child, M. S., 58. 126 Chin, A. K., 196, 2-77 Chin, S . L., 11I , 129
Christie, W. H., 247, 254, 255, 257, 309 Christophorou, L. G., 7, 32, 33, 34, 38, 129, I30 Chu, W. K., 266,306 Chupka, W. A., 67, 68, 75, 76, 85, 87, 88, 90, 91, 96, 129, 130, 133 Church, M. I., 55, 56, 57, 129, I41, 142 Clementi, E., 58,129 Codling, K., 67, 70, 71, 72, 78, 83, 129. 134, 137, 138 Coggiola, M. J., 1 1 1, 129 Cohen, H. D., 81,129 Cohen, R. B., 53,129 Cole, B. E., 72, 83, 129, 143 Collins, C. B., 12, 129 Colson, S. D., 105, 139 Combet-Farnoux, F., 79, 80, 129 Comeaux, A. R., 71,137 Comer, J., 28, 39, 130 Compton, R. N., 34, 36, 50, 51, 52, 105, 106, 109, 129,130,142, 143 Connell, G. L., 233,306 Connerade, J. P., 78, I30 Connor, T. R., 9, 130 Cook, C. J., 56, 58, 125 Cook, G. R., 92,93,130 Cooper, C. D., 34,51,130 Cooper, 1. W., 34, 51,67, 74,75, 79, 109, 130, 132 Coplan, M., 44, 127 Coppens, P.,96, 130 Corcoran, C. T., 81, 136 Cosby.P.C., 111,121, 123,129,130, 134,136, 138,142 Cotton, 1. W., 150(7), 225 Cox, R. M., 313(72, 73, 74), 409 Crandall, D. H., 6, 19, 20, 139 Cremaschi, P., 107, 130 Csanak, G., 81,82, 136 Cunningham, A. J., 10,130 Curtis, L., 188(59),227 D Dagnot, J. P . , 280,308 Dahler, 1. S., 14, 21, 139 Dahlgren, S. D., 262, 267, 268,306 Dalgarno, A., 39, 95, 127, I32 Dalton, B. J., 67, 70, 143
476
AUTHOR INDEX
Damany, H., 61,90,127,130, 140 D’Angelo, N., 12,130 D’Angelo, V. S., 6 , 18, 137 Danilov, A. D., 5, 22, 130 Dash, P. K., 452,470 Davenport, J. W., 72,81, 83,130, 133 David, C. W . , 113, 115, 127 David, D. J . , 464,470 Davidovits, P., 43, 130 Davidson, E. R., 130 Davis, F . J., 34, 36, 51, 52, 142 Davis, L. E., 242,243,244,263,298,306,307 Davis, R., 200(73), 228 Davy, P., 12, 13,127 Day, C. R., 163(25), 176,226 Defrance, P., 6, 18, 137 de Grefte, H . A. M., 273, 307 Dehareng, D., 99, 137 Dehmer, J. L., 67, 72, 81, 83, 129, 131, 143 Dehmer, P. M., 67, 68, 72, 75, 76, 81, 82, 83, 85, 87, 88, 90,91, 129, 130 De Jong, M. L., 470,471 Delos, J . B., 40, 58, 130, 136 Delpech, J.-F., 12, 13, 127, 130, 142 Delvigne, G. A. L., 48, 130 De Man, H . , 418, 470 Eemkov, Yu. N., 58,130 De Monte, V., 452,470 Dentai, A. G., 193(63), 194, 195(63),227 Derkits, C., 19, 130 Desal, M., 410 Deschamps, P., 280, 308 Devoret, M., 85, 136 Devos, F . , 12, 127 de Vries, A. C., 49, 125 de Vries, M. S., 108, 130 Dewitte, J., 462,470 Dexter, A. L., 452, 471 Dhez, P., 78, 141 Dhuicq, D., 41, 132 Dibeler, V. N . , 96, 131 Dickson, H . W., 129 Dieke, G. H., 90,131 Dietz, T . G., 107, 110,131 Dill, D., 67, 68, 69,71, 72, 76,80, 81, 86,95, 126,130, 131,132,135,140, 143 Dimicoli, I., 52, 131 Dimoplon, G., 141 Dineson, M. A., 154(15),225 Dispert, H., 53, 131
Dobrozemsky, R., 262,306 Docken, K. K., 95,127,132 Doi, H., 281, 309 Dolder, K. T., 6 , 19, 20, 21, 56, 139 Domke, W . , 122,129 Doniger, J., 364(36,37), 371(36), 372(36), 395 (36),408 Dorman, F. H., 99, 131 Doty, K. L., 426,470 Doverspike, L. D., 40,41,42,129, 131, 136 Dow, J. D., 67,138 Doyle, J., 452,470 Drawin, H. W . , 252,306 Dress, W . B., 72, 133 Drowart, J . , 96,130 Druger, S. D., 69, 131 Drummer, D. M., 257,286,291,307 Dubau, J., 80, 131 DuBois, R. D., 6,131 Dujardin, G., 100, 136 Dukel’skii, V. M., 27, 41, 46, 228, 135 Dunbar, R. C., 100, 131 Duncan, M. A., 107, 110,131 Duncanson, J. A., Jr., 103,131, 133 Dunn, G. H., 6,7,19,20,22,13I,133,139,143 Dunning, F. B., 104,127,143 Durup, J., 92, 131 Dutta, C . M., 71, 131 Duzy, C., 80, 131
E Eastman, D. E.,81, 82, 83,133, 139 Eberly, J. H., 125 Eckhardt, R. C., 104,140 Eckstein, W.. 238,307, 309 Ederer, D. L., 72, 74, 75, 83, 129, 131, 137, 143 Edwards, J. W., 317(38), 408 Eichelberger, T . S., IV., 111, 105, 132, 133 Eichinger, P., 292,306 Eisner, P. N., 57, 131 Eissner, W., 131 Eland, J. H . D., 61,65,85,95,96,97,100, 105, 106, 127, 131, 136,143 Elbert, D. D., 112, 129 Elder, F . A., 96, 139 El Gomati, M . M., 276,307 El-Goresy, A,, 292, 306
477
AUTHOR iNDEX
Ellison, G . B., 39, 124, 144 Elmer, B. R., 212(81), 228 El-Sayed, M. A., 105, 127, 139 Elton, R. C., 104,140 Engelking, P. C., 121, 125, 131 Entenmann, E. A,, 53, 140 E. Shirbeeny, E.-H. T., 452,470 Ettenberg, M., 195(66), 197(66), 198, 227, 228 Evans, C. A., 271,280,286,307,309,310 Evans, D., 146(1), 147(1), 225 Evans, E. W., 41,137 Evans, J., 378(32), 395(32), 408 Evans, J. E., 378(40, 41. 42, 43). 379(42), 395 (39, 40). 408 Ewing, J. J., 44, 127, 131 Eyler, J. R., 121, 131 Eyring, H., 100, 140
F Fabre, F., 104, 125 Faggin, F., 415,469,470 Fahlman, A., 235,307 Faist, M. B.,48, 131,132 Fan, J. C. C., 262,307 Fano, U., 68, 69, 73,75,76,81, 101, 104, 116, 129, 131,132, 140 Farber, W., 263,300,306, 307 Farley, J. W., 21, 141 Fassett, J. D., 282, 294, 296, 307 Faubert, D., 11 I , 129 Fayeton, J., 41, 132 Fehsenfeld, F. C., 5, 11,33,38,39,45,50,128, 132, 134, 141 Feibelmann, P. J., 233, 307 Feldmann, D.. 53,108, 120,121,122, 132, 140 Felenbok, P., 252,306 Feneuille, S., 101, 132 Ferguson, E. E., I1,39,132, 141 Fermi, E., 36, I31 Ferrel, R. A,, 235.307 Fidos, H., 284,285,306 Fink, U., 39, 132 Fiorini, P., 452,470 Fiquet-Fayard, F., 5, 28, 31. 34, 35, 126. 132, 133, 142 Fisanick, G. J., 105, 11 I , 132, 133 Fisbie, F. L., 408 Fisher, E., 454,470
Fite, W. L., 29, 53, 59, 132,133, 139 Flaks, I. P., 35, 135 Flannery, M. R., 5, 52, 54, 132 Flippin, A., 461,470 Folts, H.C., 150(7), 225 Foltz, G. W., 104, 127, 143 Ford, A. L., 95, 132 Foster, C. C., 471,470 Fox, J. N., 61, 132 Fox, R. E., 31, 132 Fox, T. R., 302,307 Francis, W. E., 59, 140 Franklin, J. L., 112, 132 Freiser, B. S., 121, 142 Fried, W., 357(53), 409 Fries, J., 359(44), 408 Frisbie, F., 397(45, 46), 408 Froitzheim, H., 235, 307 Frolich, H.R., 6, 18, 137 Frommhold, L., 9, 132 Frost, D. C., 31,132 Fryar, J., 94, 128,132 Fuertes, J. M., 452, 470 Futrell, J. H.,123, 139 Fyfe, W. I., 30, 129 G Gage, S., 146(1), 147(1), 149(5),203(5), 213(5), 225 Gaily, T. D., 6, 15, 19, 20, 21, 52, 126, 133 Gait, P. D., 57, 132 Gallagher, J. W., 112, 132 Gallon, T. E., 244,309 Galloy, C., 96, 143 Gambling, W. A., 167(30), 226 Ganjei, J. D., 247, 248, 307 Gardner, A. B., 67,137 Gardner, J. L., 81, 83, 126, 132, 141 Gardner, W. B., 170(37), 226 Carton, W. R. S., 61, 132 Gatley, I., 39, 127 Gauthier, J.-C., 12, 13, 127, 130 Gautier, T. N., 39, 132 Gayles, J. N., 58, 129 Gelbart, W. M., 85, 136 Gelius, U., 65, 143 Geltman, S., 74, 114, 118, 128,132 Gentieu, E. P., 85,132 Gerardo, J. B., 13, 132
478
AUTHOR INDEX
Gerlach, R. L., 233,307 Giese, C. F., 100, 142 Gilbody, H. B., 5, 138 Gillen, K. T., 52, 133 Giusti, A., 14, 21, 22, 133 Giusti-Suzor, A,, 21, 81, 133, 140 Glass-Maujean, M., 90, 94, 95, 127, I33 Gloge, D., 160(22), 162(27), 163(27, 28), 165, 167, 168, 169, 181(50), 185, 198,226,227, 228 Glover, R. M., 46, 81,133, 136 Goff, R.F., 238,307 Gold, A., 101, 127 Goldstein, E., 31, 142 Goodfellow, R. C., 200(73), 228 Goodman, L., 105, 136 Gossink, R. G., 273,307 Could, R. K., 33, 138 Gourgout, J. M., 280,307 Goursaud, S.,34, 132, 133 Gousier, B., 309 Covers, T. R.,96, 138 Grady, R. B., 155(18), 208(18), 225 Granneman, E. H., 101,143 Granneman, E. H. A,, 104, 133 Grant, I. P., 79, 133 Gresteau, F., 34, 138 Grey, R., 56,139 Grice, R., 53, 133, 136 Cries, W. H., 290,307 Griffith, A. A,, 171,226 Grimm, F. A., 72,133, 135 Grosser, J., 47, 125 Gudat, W., 81, 82, 83, 133, 139 Guernet, J., 280,308 Guest, J. A., 121, 136 Guest, M. F., 21, 23, 136 Gupta, Y . P., 233, 306 Gupton, J. A,, 452, 470 Gusinow, M., 13,132 Gustafsson, T., 81, 82, 83, 133, 139 Guydon de la Berge, J. M., 280,308 Guyon, P. M., 60,90,96, 127, 133, 138
H Haddad, G. N., 78,141 Haff, P. K., 262,266,307 Hagstrum, H. D., 233,237, 307
Hajicek, D. J., 262, 267,309 Hall, J. L., 112, 118, 120, 121, 122, 129, 133, 141
Hall, R. I., 26, 28, 34, 133, 138, 142 Hamnett, A., 65, 128, 143 Hamrin, K., 235,307 Hanrahan, L. R., 279,281,308 Hanrahan, R. J., 30, 141 Hansen, J., 103, 104, 142 Hansen, J. C., 103, 104, 133 Hansen, N. J., 44, 139 Hanson, D. C., 154(17), 155(18), 156(19), 158 (17), 159(19), 160(19), 175(17), 178, 181 (47), 183, 188(47), 191, 201(47), 202(47), 207(19), 208(18), 225, 226,227 Hanson, Eric., 169, 226 Harland, P., 133 Harland, P. W., 112,132 Harrington, W., 272,273,308 Harrington, W. L., 237, 238, 258,307 Harris, L. A,, 233, 307 Harris, R. A,, 105, 137 Harrison, H., 78, 128 Harrison, P. G . ,45 1,470 Hart, A. C., 178,227 Hasted, J. B., 6,20, 108, 138 Hawk, R. M., 185(52), 227 Hawkins, B. M., 200, 228 Hayes, E. F., 71, I31 Hayes, J. F., 153(10), 225 Hayhurst, A. N., 6, 36,44,45,49, 55, 57, 58, 128, 133 Hazi, A. U., 95, 133 Heath, B. A., 105, 11 1, 132, 133 Hedman, J., 235,307 Heering, J., 469, 470 Heinicke, E., 53, 108, 121, 132, 140 Heinzmann, U., 73,74, 133 Held, B., 104, 133 Henderson, W. R.,29, 133 Henrich, V. E., 262, 307 Henschen, L. J., 452,470 Heppner, R. A., 7,133 Herbst, E., 23, 118, 121,122,124,133,135,144 Herrmann, A,, 104, 108, 133 Herschbach, D. R.,133, 144 Hertz, H., 60, 133 Herzberg, G., 21, 86, 89,93, 120, 134 Herzenberg, A., 24, 25, 27.40, 126, 129, 134 Hess, B. A., 133
AUTHOR INDEX
Heuer, H., 73, 133 Hibbert, A., 80, 128, 136 Hicks, H. S., 129 Hildebrandt, G. F., 104, 127 219(93), 228 Hill, K. 0.. Hiller, J. F., 123, 134 Hinnov, E., 11, 134 Hinthorne, J. R., 252, 286, 306 Hiraoka, H., 34,134 Hirota, F., 71, 134 Hirsch, C., 358(47, 48). 408 Hirschberg, J. G., I I , 134 Hirsh, M. N., 57, 131 Hiskes, R., 173, 226 Hitchcock. A. P., 98. 134 Ho, P. S., 264, 307 Hobson, R. M., 7, 10, 130, 139 Hockhan, G. A,, 162, 177(23). 226 Hodapp, M., 146(1), 147(1), 225 Hodge, M. H., 188(60),227 Hodges, R. V., 124,134 Hofman, F., 372(49), 408 Hofmann, S., 260,269. 271,307,310 Hohla, K., 109, 140 Holland, D. M. P., 78, 134 Holmes, R. M., 70, 71, 72, 134, 138 Holstein, T., 24, 134 Holt, J., 361(50), 408 Holt, R. B., 8, 134, 135 Holzman, M. A,, 188(61),227 Homer, R., 262, 265, 266,308 Honig, R. E., 237,238,258,272,273,307,308 Hotop,H., 26,37,52, 112, 114, 115, 117, 119, 120,134,139 Houle, J. L., 452, 470 Houlgate, H., 67, 129 Houlgate, R. G., 67, 134, 138 Houston, J. E., 233,307 Howard, C. J., 33, 134 Howard, D. L., 37, 57, 139 Howard, J. K., 266,306 Howland, B., 8, 134 Hsieh, T.-C., 96, 143 Huang, C.-M., 22, 134 Huang, K. N., 74, 134 Huber, B. A., 134 Huber, K. P., 93, 134 Huber, R. M., 452,470 Hubers, M. M., 49, 50, 52, 123,134, 135 Hudson, M. C., 200(75), 221,228,229
479
Huetz, A., 34,138 Hughes, A. L., 60,134 Hultzsch, W., 37,134 Hunt, V. B., 153(13),225 Hupp, J. A., 153(14), 154(14), 225 Hurst, G. S., 134 Hutchinson, M. H. R., 107, 127
I Inghram, M. G., 100, 142 Inokuti, M., 35, 134 Irving, P., 53, 132 Ishihara, K., 183,227 Ishitani, T., 262,281,282, 283,284,291,307 Ishizuka, T., 247,307 Itikawa, Y., 35, 134 Ito, K., 94,95, 133 Ito, M., 108, 138 Ivanov, V. K., 68,79, 125 Ivanov-Kholodny, G. S., 5,22, 130
J Janev, R. K., 58, 134 Janousek, B. K., 112, 113, 124, 134 Janssen, A. P.,276, 307 Jason, A. J., 96, 139 Jeffries, J. B., 6, 131 Jensen, C. W., 454,470 Jensen, D. E., 36,134 Jensen, G., 399(51), 409 Jinadasa, N., 469,470 Jivery, W. T., 67, 68, 90, 91, 129, 130 Johnsen, R., 6,22,134, 136 Johnson, B. R., 48, 132 Johnson, P. M., 61, 105, 107,130, 134, 144 Johnson, R. A,, 8, 135 Johnson, W., 361(52), 409 Johnson, W. R., 74, 79, 134, 135 Johnston, H. S., 36, 135 Jonas, A. E., 71, 72, 128, 135 Jones, P. L., 121, 123, 139 Jortner, J., 61, 63. 135 Joshi, A., 242,243,244. 263. 298,307 Judge, D. L., 85.99, 128, 136 Jug, H., 292, 296, 298,309 Julienne, P. S., 21, 136
480
AUTHOR INDEX
Junger, C., 71, 72,76, 80, 86, 89,90,95, 126, 134,135,140 Junker, B. R., 21,126 Justice, B., 171(41), 226
K Kallne, E., 78, 141 Kaiser, H. J., 53, 108, 121, 132, 140 Kaji, T., 218(88), 228 Kalish, D., 173,226 Kao, K. C., 162, 177(23), 226 Kapron, F. P., 175(45),226 Karangelen, N. E., 104, 140 Karpov, N. A., 111, 225, 135 Karlov, N. V., 111, 125, 135 Kasdan, A., 117, 121,235,139 Kasner, W. H., 9, 135 Kassel, L. S., 100, 135 Kawasaki, B. S., 219(92,93), 228 Kaya, K., 108, 138 Kayton, M., 357(53), 409 Keck, D., 170(36), 175(45), 191, 226 Kellert, F. G., 104, 127 Kelly, H. P., 79, 100, 129, 135 Kelly, R., 36,135,262,266,272,313(57), 397 (59), 307,409 Kelly, R. J., 358(56), 372(60), 373(62), 377(54, 58), 397(54), 409 Kempf, R. A,, 181(50), 227 Kemph, J., 270,307 Kennedy, D. J., 67,68,79, 135 Kennedy, E. T., 63,129 Keramides, V. G., 196(67), 227 Kessler, J., 73, 74, 133, 135 Keyser, J., 6, 15, 19,20,21, 126, 137 Khan, S. V.,6,20, 108, 138 Khawaja, E., 103,136 Khvostenko, N. I., 27,46, 135 Kibel, M. H., 71, 235 Kieffer, L.J., 28, 78, 112, 135, 243 Kikiani, B. I., 35, 135 Kingston, A. E., 12,126 Kirschner, J., 236, 237, 307 Kita, J., 418,464,470 Kittleson, D. B., 36,133 Kivel, B., 41, 137 Klar, H., 135 Kleinpoppen, H., 61,135 Klemperer, W., 21, 142 Klewer, M., 47, 104, 108, 133, 135
Kley, D., 23, I35 Kleyn, A. W.,42,48,49,50,52, 123,134,135, 137 Klots, C.E., 8,34, 36,51, 52, 101,135,142 Knyazev, I. N., 105, 108, 109, 125 Kobayashi, H., 281,282,283,284,291,307 Kobayashi, K., 219(89), 228 Kollmann, K., 94,95, 133 Koetser, H., 107, 227 Kohl, J. L., 78, 135 Kokubu, A., 418,464,470 Kolb, E. F., 313(72), 409 Kolos, W., 46, 135 Kompa, K. L., 109,140 Kondo, T., 281,309 Koopmans, T., 70,135 Kornegay, W. M., 36, 135 Kraus, U., 281,308 Krause, M. O., 21,61,67,77,78,128,136,144 Kressel, H., 195(66), 197(66), 198(68), 227 KroghJespersen, K., 105, 136 Krohn, V. E., 302,307 Kroll, N., 111, 236 Kronast, W., 37,134 Kriiger, W., 47,125 Kuebler, N. A., 108,143 Kuhry, J. G., 44,239 Kulander, K. C., 21, 23, 136 Kumar, V., 83,126 Kunikyo, T., 218(88), 228 Kuntzemuller, H., 78, 141 Kupperrnann, A., 49, 72, 225, I42 Kurepa, M. V., 30, 236
L LaBerge, C., 352(61), 358(56), 373(62). 374 (54). 377(54, 55), 397(54, 59), 409 Lacmann, K., 35,53,131, 136 Lafyatis, G. P.,78, 235 Lagerqvist, A,, 120,134 Lam, N. Q., 272,307 Lam, S. K., 40,41, 229, 136 Lamb, W. E., Jr., 21, 241 Lambropoulos, M., 103, 136 Lambropoulos, P., 101,136 Langevin, P., 54, 136 Langhoff, P. W., 81,82,136,140 Lanman 11, M. H., 410 Lanting, G. E. H., 50, 126
48 I
AUTHOR INDEX Larson, H. P., 39, 132 Larsson, S. J., 248,307 Lataire, P., 452, 470 Latimer, C. J., 37, 136 Latimer, C. S., 143 Lavoie, M., 452, 470 Lawrence, G. M., 23, 135 Leach, S., 61,63, 65, 85, 98, 100,135,136 LeCoat, Y.,28, 31, 32, 33, 34, 126 Lecompte, C., 103, 136 LeDourneuf, M., 25, 80, 136, 141 Lee, A. B., 219(90), 228 Lee, C. M., 14,22, 74, 107,136 Lee, L.C., 85,99, 121, 123, 124,134,136, 142 Lee, T. P., 193(63), 194, 195(63), 227 Lee, Y.T., 45,50,143 Lefebvre-Brion, H., 21,72,81,82,83,133,140 Lefevre, G., 28, 34, 126 Leffen, D. J., 452, 470 Lehmann, K. K., 105,136 Leng, F. J., 71, I35 Lengel, R. K., 124, 132, 136 Leone, S. R., 39,144 Le Rouzo, H., 72,81,82,83,140 Lesea, A., 448,451,460,471 Leta, D. P., 248,251,257,307 Letokhov, V. S., 37, 101, 105, 108, 109, 125, 136 Leu, M. T., 6, 136 Leuchs, G., 103,136 Leutwyler, S., 104, 108, 133 Leventhal, L. A., 457, 470,471 Lever, R. F., 266,306 Levine, J., 112, 120, 121, 129, 141 Levine, R. D., 24,48, 101, 110, 131, 132, 141 Levi-Setti, R., 302, 307 Levy, D. H., 107, 141 Lewis, J. T., 58, 126 Leys, J. A., 292,309 Li, T., 191,227 Liau, Z. L., 265,266,308 Lichtin, D. A,, 109, 144 Liebl, H. J., 281, 302,308 Lifshitz, C., 100, 127, 136 Light, J . C., 23, 101,136, 139 Lightman, A. J., 138 Lightstone, A. W., 219, 220(94), 229 Lin, C. D., 79, 135 Lin, S. M., 53, 136 Lindgard, A,, 103, 131
Lineberger, W. C., 26, 103, 112, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 129, 131, 133, 134, 135, 137, 139, 141,144 Ling, J. H., 121, 123, 130 Liston, S. K., 96, 131 Littlewood, I. M., 37, 125 Liverman, M. G., 107, 131 Livett, M. K., 135 Locht, R., 46, 137 Lodding, A., 248, 307 Loeb, L. B., 54,137 Loofbourrow, T., 452,470 Lopez, A. R., 395(63,64), 409 Lorents, D. C., 52, 56,58, 125, I33 Lorenz. J., 73, 133 Lorquet, J. C., 96,99, 137, 143 Los, J., 42, 47, 48, 50, 52, 104, 108, 123, 126, 130,134, 135, 137,138 Loughry, D. C., 454,470 Love, R. E., 171,226 Lowry, J. F., 74,137 Loy, M. M. T., 111,126 Lozier, W. W., 46, 137 Lu, C. C., 72,128 Lubell, M. S.,73, 127, 137 Lubman, D. M., 109, 137 Lucatorto, T. B., 61,137 Luther, K., 41, 137 Lynch, M. J., 67,137
M McClain, W. M., 105, 137 McClanahan, E. D., 262,267,268,306 McClure, B. T., 8, 134, 135 McCoy, D. G., 70,72,137,138 McCulloh, K. E., 90,96, 137 MacDonald, N. C., 276,306,308 McDowell, C. A,, 31,132 McDowell, M. R. C., 61, 135 McFarland, M., 33, 134 McGowan, J. T., 18,22, 138 McGowan, J. W., 6, 15, 18, 19, 20, 21, 71, 126,129, 137, 138 McGuire, E. J., 79, 137 McGuire, G. E., 72,128 McGuire, J. M., 59, 126 McHugh, J., 248,272,308 McHugh, J. A., 279,281,308
482
AUTHOR INDEX
McIlrath, T. J., 61, 137 Mackie, J. C., 115, 120, 127, 129 McKinney, J. T., 292,309 McKoy, B. V., 81.82, 136, 140 McKoy, V., 61,81, 137,140 McQuillan, J. M., 154, 225 McWhirter, R. W. P., 12, 126, 137 Madden, P., 410 Magee, C., 272,273,274,308 Magee, J. L., 48,137 Maggetto, G., 452,470 Mahadevan, P., 40,41,126 Mahajan, C. G., 63,137 Mahan, B. H., 54,55,129,137 Mainfray, G., 101,103,104,125,133,136,137 Mandl, A,, 41,44,45,49, 137 Mandl, F., 5, 24, 25, 27, 126 Mansfield, M. W. D., 78, 130,137 Manson, S. T., 67,68, 79, 131,135,142 Manus, C., 101, 103, 104,133,136, 137 Marcatili, E. A. J., 163(28), 165,166,167,185, 198(69), 226, 227,228 Marcus, D., 198,228 Marcus, R. A., 100,138 Marcuse, D., 169, 198(69), 226,227,228 Maricq, M. M., 39,144 Marr, G. V., 37, 61, 67, 70, 71, 72, 78, 81, 82, 134, 137,138,143 Martin, J. D., 41, 144 Martin, M . A. P., 78, 130 Masayuki, A., 201,228 Mason, S. T., 79,137,138 Massey,H. S. W., 5, 112, 113, 114, 126,138 Masuoka, T., 98,99,138 Mathur, B. P., 53, 108,138,140 Mathur, D., 6, 20, 108, 138 Mathur, R. M., 452,470 Matiuk, V. M., 108, 109, 125 Matsuzawa, M., 52,138 Matthews. K., 39,127 Maurer, R. D., 175(45), 266 May, C. J., 40, 126 Mayer, J. W., 262,308 Mazeau, J., 34,138 Mehlrnan, G., 61, 137 Meisels, G. G., 96, 143 Melchior, H., 205, 228 Meldner, H. W., 64, 138 Mentall, J. E., 83, 85,90, 132, 138, 141 Metcalf, R. M’, I50(8), 153(12), 157(12), 225 Metzger, P. H., 92,93, 130
Meyer, K., 263,308 Meyer, M. A., 329(65), 409 Meyers, R., 452, 470 Michels, H., 22, 138 Mies, F., 76, 138 Miessner, H., 233, 306 Miller, B. I., 195(65), 197(65), 199(65), 227 Miller, D. L., 67, 138 Miller, J. C., 109, 130 Miller, J. F., 138 Miller, M., 452, 471 Miller, T. M., 141 Miller, W. J., 33, 138 Milstein, R., 41, 44, 113, 131, 138 Milton, A. F., 219(90), 228 Mishin, V. I., 37, I25 Mitchell, C. J., 78, 138 Mitchell, J. B. A., 6, 15, 18, 19, 20, 21, 126, 137,138 Mitchell, P., 67, 138 Mittleman, M. H., 13, 14,129, 138 Miya, T., 163(24), 177(24), 226 Miyazaki, K., 186(54), 227 Moddeman, W. E., 78,128 Modinos, A., 29,127 Mohler, F. L., 112, I38 Molini, C. A., 105, 143 Momigny, J., 46, 100, 137, 138 Monahan, J. E.,95,131 Moody, S. E., 103,136 Moore, A. W., 452,471 Moores, D. L., 116, 117, 118, 138 Morell, J., 452, 470 Morellec, J., 101, 102, 104,133,138 Morgan, A. E., 257,273,308,309 Morgner, H., 135 Mori, R., 418,464, 470 Morishita, J., 418,464, 470 Morrison, G. H., 247,248,251,253,254,257, 280,281,282,286,291,294,296,307,308, 309 Morrison, J. D., 99, 131 Morrow, A. J., 181(49), 227 Morrow, T., 107,127 Morshev, V. G., 105, 108, 109, 125 Morton, J., 67, 68, 142 Morton, J. B., 454,459,470 Morton, J. M., 70,72,137,138 Moseley, J. T., 54, 56, 121, 123, 124,130, 134, 136,138, 139,142 Moutinho, A. M. C., 48,50,138
483
AUTHOR INDEX
Muck, G., 113, 138 Mul, P. M., 6, 18, 22, 137, 138 Mulliken, R.S., 13, 138 Mulloney, T. J., 121, 139 Murakami, J., 108, 138 Murray, P. T., 96. 138 Muschlitz, E. G., 40, 59, 126 Myers, B. F., 53, 139
N Naaman, R.,109, 137 Nafarrate, A. B., 219(95), 229 Naff, W. T., 51, 130 Natanson, G . L., 54, 138 Nawata, K., 188(56),227 Nenner, I., 31, 66,96, 133, 138, 144 Nesbet, R. K., 31,34, 134, 138 Neugebauer, G., 39,127 Neusser, H. J., 109, 127 New, G. H. C., 107,127 Newbury, D., 250,253,255,308 Newson, G. H., 78,130 Neynaber, R. H., 53,139 Nicoud, J. D., 429, 470 Niehaus, A., 37,52,71, 134,139 Nielsen, S. E., 11, 13, 14, 18, 19, 21, 88, 89, 127,139 Nieman, G. C., 105, 139 Niles, F. E., 12, 139 Nishikawa, S., 418,464, 470 Noguchi, T., 32, 142 Norcross, D. W., 116, 117, 118, 133, 138, 139 Nordberg, R.,235, 307 Nordling, C., 235,307 Nonnand, D., 101, 102, 138 North, J. C., 188(58),227 Nourtier, A,, 245, 252,306 Novick,S. E., 117. 118, 121, 123, 139, 141 Nyberg, G. L., 71, 135 Nygaard, K. J., 104, 133 0 Odelius, H., 248, 307 Oechsner, H., 262,308 Oertel, H., 47,92, 139 Ogar, W. T., 262,308 Ogawa, M., 85, 128 Ogiwara, H., 217(85), 228
Ogram, G. L., 7, 139 Ogurtsov, G. N., 35, 135 Ohashi, Y.,247,309 Ojha. P., 40, 134 Oka, T.. 21. 139 Okada, Y., 418,464,470 Okano, J., 247,308 O’Kelly, L. B., 134 Okkawa, K., 418,464,470 Okuda, N., 218,228 Olshansky, R.,167, 169, 170(31, 36). 191,226 Olson, N. T., 262,308 Olson, R.E., 54, 55, 56,57, 59, 138, 139 O’Malley, T. F., 10, 11, 25,28, 114, 139 Omura, I., 281,309 O’Neil, S. V., 124, 144 Opitz, M., 263, 265, 266,308 Orr, B. J., 70, 128 Osborne, A,, 426,441,469,470,471 O’Sullivan, G. O., 63, 129 Overeijnder, H., 262, 309 Ozeki, T., 219(92), 228
P Padial, N., 81, 82, 136 Padley, P. J., 36, 134, 135 Paineau, R.,31, 33, 126 Palenius, H. P., 78, 135 Palmberg, P. W., 233,242,243,244,263,298, 306,307,308 Park, R. L., 233,307 Parker, D. H., 105,127,139, 143 Parkinson, W. H.. 78,135 Parks, E. K., 44, 139, 141 Parr, A. C., 72,83,96, 100,129,139,140,143 Patterson, T. A., 53, 114, 117, 118, 119, 120, 122, 133,134, 139 Payne, D. N., 167(30), 226 Peacher, J. L., 24, 129 Pearson, P. K., 122, 139 Peart, B., 6, 19, 20, 21, 56, 139 Peatman, W. 9.. 57,60,89,139, 143 Pechukas, P., 23, 101, 139 Pellegrin, J., 462,470 Perez, J. D., 64, 138 Perkin-Elmer, 275, 276,279, 300, 308 Person, J. C., 37, 54, 57, 137, 139 Personick, S. D., 156(19), 159(19), 160(19), 206(78), 207,209,226,228
484
AUTHOR INDEX
Peterson, J. R., 54, 56, 58, 111, 121, 123, 125, 129, 130,134,138,139 Peterson, L. G., 248,307 Petite, G., 101, 102, 104, 125, 138 Petrosky, V. E., 85, 141 Petrov, Yu.N., 1 1 1, 125, 135 Petty, M. S., 107, 127 Phaneuf, R.A,, 6, 19,20,139 Picazo, J . J ., 154( 15), 225 Pierce, J. R., 153(9), 225 Pines, D., 235,308 Pinnow, D. A., 163(26),226 Plesonton, F., 236,308 Plummer, E. W., 81,82,83,133, 139 Poate, J. M., 262,265, 266,308 Poe, R. T., 79,129 Pogram, K. T., 153(1l), 225 Polanyi, M., 48, 139 Poliakoff, E. D.. 72. 83, 129 Popp, H. P., 112, 113,138,139 Porter, R. N., 21, 129 Potapov, V. K., 108, 109,125 Powell, C. J., 242, 308 Pradham, A. K., 80,140 Prager, M., 281,308 Pratt, R. H., 67, 143 Prokhorov, A. M., 111,125, I35 Prutton, M., 276,306,307 Pullen, B. P., 72, 128
Q Quack, M., 24, 25,35, 101, 140
R Rackwitz, R., 108, 121, 132, 140 Radhakrishnan, T., 452,470 RaduloviE, Z. M., 58, 134 Raether, H., 233,308 Rahman, N., 104, 125 Raith, W., 73, 127,137,233,306 Ramsperger, H. C., 100,140 Raoult, M., 71, 72, 80, 89, 140 Rapp, D., 27,28, 59,140 Raseev, G., 21, 72, 81, 82, 83, 99, 137, 140 Rast, R. H., 67, 142 Rau, A. R. P., 116,140 Rauberol, J. M., 280,308 Rava, R. P., 105,136
Rawson, E. G., 217(86), 219(95,96), 228,229 Read, F. H., 28, 117, 118,130,141 Reck, G. P., 53, 108,138, 140, 143 Redlien, H., 313(57), 409 Reed, K. J., 113, 114, 122, 124, 140 Reich, P., 388(31), 408 Reilly, J., 109,140 Reilly, J. P., 109, 140 Reimann, C. W., 113, 127 Reinhardt, P. W., 51, 130, 142 Reinhardt, W. P., 124, 144 Reintjes, J., 104, 140 Repko, M., 470 Rescigno, T. N., 61, 81, 82, 136, 140 Rettner, C. T., 98, 109,128 Reynaert, J. C., 96, 130 Rhodes, N. L., 156(19), 159(19), 160(19), 207 ( 19), 226 Riach, G. E., 306 Rice, 0. K., 100, 138,140 Richardson, J. H., 122, 124, 125, 139, 140 Richardson, J. M., 8,134 Ringo, G., 302,307 Risley, J. S., 40, 140 Ritchie, B., 71, 140 Robb, W. D., 80,128 Robertson, W. W., 12, 139 Robin, M. B., 105, 108, 111, I32, 133,143 Robinson, E. J., 118, 133 Rockwood, S. D., 109,140 Roger, H., 361(52), 409 Rogers, W. A., 9, 11, 140 Rohr, K., 140 Rol, P. K., 53,140 Romand, J., 61, 126 Ron, A,, 67, 79, 135,143 Roncin, J. Y.,61, 90, 130, 140 Rooney, M., 459,470 Rosenstock, H. M., 61, 100,140 Roth, J. R., 282,294,296,307 Roth, M., 309 Rothe, E. W., 53, 108, 138, 140, 143 Rotoloni, R., 452,470 Roussier, L., 31, 33, 126 Rudat, M. A., 253, 254, 257,308 Riidenauer, F. G., 245,246,249,252,253,255, 281,282,287,288,291,293,295,301,306, 308,309 Ruf, M. W., 37,71,134,139 Rumble, J. R., Jr., 112, 132
AUTHOR INDEX
Runge, P. K., 188(59), 227 Rush, T. W., 292,309
t
S
Saeki, N., 264,309 Sakzouk, E. E.-D., 452,470 Salama, C. A. T., 418,470 Saltzer, J. H., 153(11), 225 Samson, J. A. R., 61,75,78,81,83,85,98,99, 128,132,138,141
Sanchez, F., 103,136 Sandner, N., 78, 141 Sandretto, P. G., 312(66), 409 Sannen, C., 99, 137 Saraph, H. E., 80,140 Sawin, D. H., 448,470 Schaefer, H. F., 122, 139 Schafers, F., 133 Schenk, H., 47,92, 139 Schenkel, D. P. G., 463,471 Schermann, C., 26,28,133,142 Schermann, J. P., 34, 125 Schertz, 1. D. R., 462, 471 Scherzer, B. M.U., 239,306 Schilling, J. H., 284, 285, 295, 306, 309 Schlag, E. W., 109, 127 Schmeltekopf, A. L., 1 I , 39.45, 128, 132, 141 Schmidt, V., 78, 141 Schneider, B., 61, 81, 137, 140 Schneider, B. I., 6, 25, 30, 141 Schoen. R. I., 78,128 Schonhense, G., 74, 133 Schulz, G. J., 5 , 6, 7 , 27, 31, 34, 39, 112, 129, 130. 141,144
Schumacher, E., 104, 108,133 Schwartz, M.I., 181(50), 227 Schweinler, H. C., 142 Schweitzer, G. K., 72, 135 Scilla, G. J., 28 1, 309 Scire, G., 452, 470 Scott, T. W., 108,141 Seaton, M.J., 76, 80, 131, 141 Sebring, J. R., 313(73), 409 Segal, G. A,, 31, 142 Seliger, R. L., 302,309 Sell, J. A,, 72, 141 Sevier, K. D., 242, 309 Shagena, J., 313(57), 409 Shahabi, S., 142
485
Sharp, T. E., 27, 28,86, 140, 141 Shaw, G. B., 71,89,141 She, C. Y., 104,140 Sheen, S. H., 141 Sheffield, J. C., 279,281,308 Shelepin, L. A., 1 1 1,125, 135 Sheridan, J. R., 139 Sherman, D. N., 153(10), 225 Shimizu, R., 262,264,307, 309 Shirahata, K., 195(64), 196(64), 227 Shirey, J. M.,313(74), 409 Shiu, Y.-J., 9, 141 Shnidman, D. A., 378(67,68), 379(67,68), 409 Shoch, J. F., 150(8), 153(14), 154(14), 225 Shreider, E. Ya., 61, 144 Shy, J. T., 21, 141 Sichel, J. M.,70, 128, 141 Sides, G. D., 30, 141 Siegbahn, K.,65,143,235,307 Siegel, M. W., 53, 112, 120, 121, 129, 139, 141 Sigmund, P., 241,262,264,266,306,309 Silberstein, J., 24, 101, 110, 141 Simon, D., 28, 31, 32, 33, 126 Simons, D. S., 279,281,286,308,309 Simons, R. L., 79,135 Simpson, C. S. J. M.,39, 144 Sims, J. S., 81, 136 Sinnott, G., 123, 141 Sipler, D. Y.,9, 141 Sizun, M.,34, 132, 133 Slater, J., 117, 118, 141 Slodzian, G., 280, 301, 302,306, 308,309 Smalley, R. E., 107, 110, 131, 141 Smirnov, B. M.,23, 141 Smith, A. C. H., 59,132 Smith, A. L., 141 Smith, B. T., 42, 131 Smith, D., 5 5 , 56, 57,129, 141,142 Smith, D. H., 247,254,255,257,309 Smith, D. M.,244,309 Smith, D. P., 238,309 Smith, G. P., 121, 123, 130,136, 142 Smith, H. P., Jr., 262,308 Smith, K. A., 104,127 Smith, L. M., 119, 142 Smith, R. G., 206(78), 207(78), 228 Smith, S. J., 103, 112, 114, 127, 128, 136, 142 Smolarek, J., 105, 136 Smyth, K. C., 113, 121, 122,142 Snell, A. H., 236,308
486
AUTHOR INDEX
Solomon, P. M., 21, I42 Soref, R.A,, 217(83), 228 Sorensen, H., 146(1), 147(1), 225 Spangenberg, K. K., 299,309 Spears, D. P., 72, 143 Speer, R.,200,228 Spence, D., 32,34,142 Spillman, W. B., Jr., 217(84), 220(84), 228 Spizzichino, A., 387(29), 408 Spokes, G. N., 113, 127 Staib, P., 236,307 Stannard, P. R.,85,136 Starace, A. F., 61,78,79,80,I31,133,142,143 Stebbings, R.F., 59, 104, 127, 132, 143 Steiger, W., 252, 253, 255, 281, 282, 287, 288, 29 1,295,308,309 Stein, J. D., 304, 305, 309 Steiner, B., 100, 121, 142 Stephenson, L. M., 122, 124, 125,139, 140 Stevefelt, J., 12, 127, 142 Stewart, A. L., 79,142 Stewart, D. T., 67, 143 Stewart, J. H., I88(58), 227 Stewart, T. L.. 462,471 Stimpson, B. P., 281,306 St. John, G:, 144 Stockbauer, R.,72, 83, 89,96, 100, 129, 139, 140, 142,143 Stockdale, J. A. D., 7, 32, 33, 34, 36, 38, 50, 51,52, 129,142,143 Stoll, W., 65, 143 Stoller, C., 292, 296, 298,309 Stone, E. J., 23,135 Storms, H. A., 304, 305,309 Strand, M., 103,131 Strand, M. P., 103, 104,142 Strathdee, S., 94,95, 142 Suen, G. Y., 452,470 Sugiura, T., 37, 52, I42 Sullivan, G. O., 63, 129 Sullivan, S.A., 121,142 Suter, M., 292, 296, 298, 309 Suzuki, K., 281, 282, 283, 284, 291, 307 Suzuki, T., 247,309 Switkowski, Z. E., 262, 266, 307 Szymonski, M., 262,309
T Taieb, G., 100,136 Tam, W.-C., 6,29, 30,142
Tamura, H., 281,282,283,284,291,307,309 Tan, K. H., 65,98,128,142 Tariyal, B. K., 173,226 Taya, S., 281,309 Taylor, H. S., 26, 28, 31,139, 142 Taylor, K. T., 80,136, 142 Taylor, N. J., 233, 309 Teillet-Billy, D., 3 1, 32, 125 Telford, N. R.,6, 36, 133 Thiel, F. L., 185(52), 221,227,229 Thimm, K., 78,130 Thomas, L., 122,142 Thomas, R.J., 147(4), 225 Thomson, J. O., 72, 133 Thomson, J. T., 54,142 Thorson, W. R.,58,130 Thynne, J. C. J., 133 Tierman, T. O., 30, 141 Tobagi, F. A., 153(13), 225 Tojo, A., 418,464,470 Tomboulian, D. H., 74,75,131,137 Torop, L., 67, 68, 142 Tracy, D. H., 78,130 Trainor, D. W., 21, 30,33, 129, 142 Treffers, R. R.,39, 132 Troe, J., 24,55,41, 101,137, 140 Tronc, M., 26, 28, 31, 32, 33, 126, 133, 142 Truby, F. L., 30,142,143 Trujillo, S. M., 53, 139 Tseng, H. K., 67,143 Tsunoyama, K., 247,309 Tsuruoka, K., 247,309 Tu, K. N., 262,308 Tully, F. P., 45, 50, 143 Tully, J. C., 67, 70, 143 Tully, P. D., 452, 471 Turner, J. E., 35, 134 Turner, R.E., 105, 143 Twvarowski, A. J., 108, 141 U
Uchida, S., 418,464, 470 Uhlhorn, C. D., 309 Utterback, N. G., 53,143
V Vaida, V., 105, 108, 143 Van Brunt, R.J., 28, 143 van den Bos, J., 48,143
487
AUTHOR INDEX
Vanderhoff, J. A,, 121, 123,127 van der Leeuw, P. E., 65,128, 142 van der Weg, W. F., 262,308 van der Wiel, M. J., 47, 61, 65, 98, 101, 104, 108, 128,133,134,135,142, 143 Van Eck, J. L., 452,470 van Veen, N. J. A,, 108, I30 van Zyl, B., 53, 143 Vaz Pires, M . , 96, 143 Venables, J. A., 276,307 Venkatesh, K., 452,470 Verbeek, H., 238,307,309 Vestal, M . L., 123, 134,138 Vickers, D., 399(51), 409 Viehbock, F. P., 262,300,306 Villarejo, D., 89,143 Vodar, B., 61,126 Vogler, M . , 6 , 20, 143 Vo Ky Lan, 80,136 von Niessen, W., 122, 129 Vroom. D . A., 71,137
W Wadhera, J. M., 19, 54,130, 143 Wagner, A., 44,139 Wagner, H . G., 41, 137 Wahrhaftig, A. L., 100, 140 Waldrop, J. R., 276,308 Walker, J . A., 90,96, 131, 137 Walker, T . E . H., 67,143 Wallace, S., 72, 81, 82, 83, 130, 143 Wallenstein, M . B., 100, 140 Walls, F . L., 7 , 22, 133, 143 Walther, H., 103, 136 Wang, S. F., 125 Wanneberg, B., 65, 143 Warmack, R.J., 50,142, 143 Warneck, P., 143 Warren, C., 452,471 Watkins, R. L., 37,57,139 Watson, K. M . , 111, 136 Watson, W. D., 94, 143 Watson, W. S., 67, 143 Webb, C. E., 37,125 Webb, T . G., 79,142 Weber, 1. T., 67,143 Weber, R. E., 233,306,309 Wehner, G. K., 261,262,267,309 Weibull, W., 171, 226 Weiner, J., 57, 60, 143
Weinhold, F., 46, 81,133, 136 Weiss, M . J., 96, 143 Wells, J., 80, 131 Wells, G. J., 53, 143 Wells, W. E., 129 Welsh, L. W., Jr., 34, 134 Wendin, G., 79, 143 Werner. H . W., 246, 247, 249, 255. 257, 270, 212, 273, 214, 293, 295, 296, 301, 306, 307,308,309 West, J. B., 37,67,68,72,78,83,129,134,138, 142, 143 West, W. P., 143 Wexler, S., 44, 53, 129, 139, I41 Wharton, L., 107, 141 Wheaton, J . E . G., 61, 132 Wheeler, H . , 387(70), 409 White, R. M., 72, 128, 143 Whitehead, J. C., 53, 136 Whitten, J. L., 107, 130 Wight, G. R.,143 Wigner,E. P., 113, 114, 116, I43 Wilde, H. R.,309 Wildt, R.,143 Wilk, S . F . J., 129 Wilk, S. J., 6 , 15. 19, 20, 21, 126 Williams, P., 280,310 Williamson, A. D., 105, 106, 109, 130, 143 Wilson, I . R.,459, 471,471 Wing, W. H., 21, 141 Winningstad, 147(3),225 Wirth. N., 455, 471 Witte, H. H., 219(91), 228 Wittke, J. P., 195(66), 197(66), 227 Wittmaak, K., 281,310 Wolfli, W., 292, 296, 298, 309 Wolneiwicz, L., 46, I35 Wong, S. F., 6 , 29,30, 142 Waste, L., 104, 108, 133 Woodin, R.L., 1 11,143 Woodruff, P. R.,67,81,82,129,143 Woodward, B. W., 144 Wu, T. Y . , 13,20, 126 Wuilleumier, F., 78, 141 Wuilleumier, F . J., 61, 67, 77, 78, 144 Wynn, M . J., 41, 144
Y Yablonovitch, E., 1 1 1 , 144 Yamamoto, H., 217(85), 228
488
AUTHOR INDEX
Yamaoka, T., 201,228 Yang, C. N., 67, 144 Yang, T. P., 54, 132 Yau, S. H., 452,470 Young, C. E., 53, 129 Young, M., 186(53), 227 Young, R. A., 144 Young, W. C., 188(59), 227 Yu, S., 67, 143 Yukawa, K., 281,282,283,284,291,307
Z Zaidel’, A. N., 61, 144 Zakheim, D. S., 107,144
Zaks, R., 415, 418, 426, 448, 451, 454, 457, 459,460,463,469,471,471 Zandee, L., 109, 110, 144 Zare, R. N., 67, 109, 124,130, 132, 136, 137, I44 Zeippen, C. J., 80, 136 Zellar, M. V., 260, 310 Zeltser, M., 313(71), 328(71), 409 Zhdanov, V. P., 19,144 Ziegler, J. F., 240, 261, 310 Ziesel, J. P., 31, 144 Zimmerman, A. H., 114, 121, 122, 124, 140, I44 Zittel, P. F., 121, 122, 124, 144 Zwier, T. S., 39, 144
Subject Index
A
ac-coupled receiver, 2 1 1-2 12 ACIA (asynchronous communications interface adaptor), 434-435 Adiabatic process, photoionization, 64 AEAPS, see Auger electron appearance potential spectroscopy AES, see Auger electron spectrometry Airborne equipment, microwave landing system, 349-357 errors, 377-378 Aircraft landing systems, see Instrument landing system; Microwave landing system Aircraft touchdown, 359-361 Alkali atom and molecule charge-exchange collisions, 48-5 1 collisional ionization, 36 ion-ion neutralization, 55 ion-pair formation, 42,44 photoionization, 78 reactive collisions, 52-53 Alkanes, dissociative photoionization, 100 Alkyl halide, dissociative attachment, 34 Alloy, sputtering, 262-268 All-plastic fiber, for local data communications, 176, 180-181 ALU, see Arithmetic and logic unit Angle-control unit, airborne, microwave landing system, 350 Angle-guidance system, of microwave landing system, 328-357 Angle-measurement reference (beam center), of microwave landing system, 405 Antenna instrument landing system, 3 12 microwave landing system, 312-313, 325, 340-350, 395-398,405 errors, 374-377, 383-389,399 sidelobe considerations, 390-39 1, 393 APD, see Avalanche photodiode
Ar charge-exchange collisions, 47-48 dissociative recombination, 8-9 multiple ionization, 78 Ar', ion-ion neutralization, 55 Arithmetic and logic unit (ALU), 422-423 Associative detachment, 38-40 Asychronous communications interface adaptor, see ACIA Asynchronous transmission, 435 Atom, multiphoton ionization, 101-104 Auger electron appearance potential spectroscopy (AEAPS), 236 Auger electron spectrometry (AES), in surface analysis, 233-235, 242 depth profiling, 259-260, 263-264, 266, 269-271,274 quantitation, 242-244 sensitivity and resolution, 298-300 Auger mapping, in surface analysis, 275-279 Autoionization, 64,75-76,86-89,92, 109, 121 role in dissociative ionization, 95-96 Avalanche photodiode (APD) in digital telecommunications, 160- 161 in local data communications, 204 Azimuth antenna, microwave landing system, 312-313, 325, 383-384, 395-398, 342345 Azimuth guidance, of microwave landing system, 322-323.325-328 errors, 384-385 Azimuth ground equipment, microwave landing system, 340-341
B Ballofet-Romand-Vodar (BRV)vacuum spark source, 61 Bandwidth, for local data communications, 189-193
489
490
SUBJECT INDEX
Barker code, microwave landing system, 332, 353 Baud, 156 Beam experiments detachment cross sections, 41 -42 ion-ion neutralization, 57 Beam port lens antenna, microwave landing system, 342 Benchmark program, in microprocessor testing, 466-467 BER, see Bit error rate Binary machine language, 458,465 Bipolar technology, in microprocessor system, 419,429-430 Bit error rate (BER), fiber optic system, 213, 2 17-21 8 Bit-slice microprocessor, 429-430, 463-464 Br-, collisional detachment, 42 Br, charge-exchange collisions, 49 dissociative attachment, 30-3 1 reactive collisions, 53 Broadband coaxial network, 154 BRV vacuum spark source, see BallofetRomand-Vodar vacuum spark source Buffer coating, on optical fibers, 169, 183- 184
c C2-, photodetachment, 119-120 Cable, for local data communications, 18 1184 CAMAC standard, for microprocessor system, 453-454 CATV coaxial cable, 153- 154 CCI,F, charge exchange with excited species, 52 CCI,, charge exchange with excited species, 52 CD,, dissociative attachment cross section, 7 CDI, see Course deviation indicator Central potential model, of photoionization cross sections, 79 Central processing unit (CPU), of microprocessor, 415-416 CH+, dissociative recombination, 21 CH, -, photodetachment, 124 CH,', neutralization, 23 CH,CHO, multiphoton ionization, I 1 1 CH,CN, charge exchange with excited species, 52
C2HZ,dissociative attachment, 35 C.&O,, charge-exchange collisions, 51 C,H,O,-, charge-exchange collisions, 5 1 C4H4N-, photodetachment, 125 C,H,N, photoionization, 105-106 C,H,-, photodetachment, 124 C6H6
multiphoton ionization, 105, 107, 109-1 10 photoelectron spectrum, 72-73 C6H6+, ion-ion neutralization, 55 C,H,, multiphoton ionization, 105, 107 Charge-exchange collision, ion-pair production, 47-51 CI collisional detachment, 41 -42 ion-ion neutralization, 55, 59 CI,, dissociative attachment, 30-31 Clearance-beam antenna, microwave landing system, 344 CMN, see Control motion noise
co
dissociative attachment cross section, 8 photoionization cross section, 82-83 reactive collisions, 54 CO, dissociative attachment, 8, 34 dissociative ionization, 96-99 partial ionization cross sections, 83-84 C o t laser-induced plasma, 1 1 I C 0 3 - , photodetachment, 123 Coaxial cable attenuation versus modulation bandwidth, 148 distributed bus network, 153-154, 218 propagation delay, 147 Collisional detachment and ionization, 35-42 and associative detachment, 38-40 Rydberg states, 36-38 simple dissociative detachment, 40-42 simple ionization, 35-36 Collision-induced dissociation to ion pairs, 43-47 COMPACT antenna, 348 Composite material, sputtering of, 261 -272 Compound, sputtering of, 262-266 Compound glass fiber, for local data communications, 176-177, 180-181 Computer, 412-415; see also Microprocessor system Continuum radiation sources, 61-63
49 1
SUBJECT INDEX
Control motion noise (CMN), microwave landing system, 326, 364-366, 368-369, 372,376,401-403 Course deviation indicator (CDI), microwave landing system, 328 CPU, see Central processing unit Cross section, in dissociative recombination and attachment, 6-7,24; see also specific ions photodetachment, 113-1 14 photoionization, 63, 65, 74-85 cs charge-exchange collisions, 48, 50-5 I multiphoton ionization, 101-102 Cs-, photodetachment, I17 Cs2
multiphoton ionization, I08 photodissociation, 47 CsCI, collision-induced dissociation, 44
D D -, collisional detachment, 40-41 D 2 + ,recombination cross section, 6 DAPS, see Disappearance potential spectrometry Data transmission medium, 146- 149 standards and performance limits, 154- 157 dc-coupled receiver, 210-21 I dc-feedback receiver, 212-213 Decision height, in landing systems, 359 Depth profiling, in surface analysis, 259-275 DH LED, see Double-heterostructure lightemitting diode Diabatic process, in photoionization. 64 Diffraction multipath, microwave landing system, 387-388 Diffuse multipath, microwave landing system, 386-387 Digital imaging, in surface analysis, 284,293298 Digital telecommunications (DT), 145, 155156 contrasted with local area networks, 159162 fiber selection, 179-181 receivers, 207 Diode in digital telecommunications, 160- 161 in local area network, 160- I62
for local data communications, 189-201, 204-205 Disappearance potential spectrometry (DAPS), 236 Dissociative ionization, 93-101, 108-1 1 I CO, ,96-99 in H,,93-95 ion fragmentation, theoretical and experimental approaches to, 99-101 role of autoionization in, 95-96 Dissociative recombination and attachment, 4-35 experiments, 18-21 halogen and hydrogen halide molecules, 29-34 large ions, 21 -24 rare gases, 8- I3 simplest paradigms of attachment, 24-29 theories, 13- 18 Distance measurement equipment (DME), microwave landing system, 313,327-328, 357-358, 397 Distributed bus network, 151-154, 218-224 DME, see Distance measurement equipment Doppler scan microwave landing system, 3 17 Double-heterostructure (DH) light-emitting diode, 196-198 DT, see Digital telecommunications Dual-channel cable, for local data communications, 181-182 Dynamic antenna sidelobe level, in microwave landing system, 405 Dynamic memory, 426
E EAROM (electrically alterable read only memory), 427 Edge-emitting diode, 195, 197-198 Effective sidelobe level, in microwave landing system, 405 Eight-bit microprocessor, 429,463-464,467469 Electrically alterable read only memory, see EAROM Electron energy-loss spectrometry (ELS), in surface analysis, 234-235 Electrons, analysis of, in surface analysis, 233-237 Elemental imaging, 232
492
SUBJECT INDEX
Elemental mapping, in surface analysis, 275292 Elementary attachment and detachment processes, 1-144 classification of processes, 2-3 collisional detachment and ionization, 3542 dissociative ionization, 93- 101 dissociative recombination and attachment, 4-35 ion-ion neutralization, 54-60 ion-pair formation, 42-54 multiphoton ionization, 101-1 1 1 photodetachment, 112-125 photoionization, 60-93 Elevation antenna, microwave landing system, 325,348, 395-398 errors, 385-386 Elevation coverage, of microwave guidance system, 322,324, 326, 328 ELS, see Electron energy-loss spectrometry Emulator, microprocessor system, 461-462 EPROM (erasable read only memory), 427 Equivalent circuit, for LEDs in fiber optic applications, 193, 203, 206 Erasable read only memory, see EPROM Etched well light-emitting diode, 190, 195, 197,200 Ethernet, 153-154, 157 Excited species, charge exchange with, 52
F F-, ion-ion neutralization, 55 F,, dissociative attachment, 30 FAMOS, see Floating-gate avalanche MOS technology Fast-electron differential scattering cross section, 65 Fiber optics, local area network applications, 145-229 optical communication medium, 162- 188 standardization issues, 157-1 59 system requirements and trends, 145- 162 terminal device and system performance, 189-224 terminal device interfaces, 158-159 Filter, microwave landing system receiver, 356 Flame hydrolysis, in fabrication of silica fibers, 177
Flame studies charge-exchange collisions, 49 collision-induced dissociation, 44-45 collisional ionization, 36 ion-ion neutralization, 55 ion-ion neutralization, 57 Flare antenna, microwave landing system, 397-398 Flare coverage, in microwave guidance system, 322, 324,326, 328 Floating-gate avalanche MOS technology (FAMOS), 427 Four-bit microprocessor, 429, 464 Frequency channels, microwave landing system, 329 Fully connected local area network, 151-152
G GaAIAs, fiber optic source, 189-190, 192, 196- 197 GaAs (Zn doped), fiber optic source, 190 GaAsP, fiber optic source, 189-190 GaInAs, p-i-n detector, 160-161 GaInAsP, fiber optic source, 160, 190-192 Carton photoionization source, 61 Glidescope antenna, instrument landing system, 3 12 Graded-index silica fiber, for digital telecommunications, 176-177, 179-180 Ground equipment, microwave landing system, 335, 340-349 errors, 374-377
H H charge-exchange collisions, 47 dissociative attachment in hydrogen halides, 29-34 H', ion-ion neutralization, 56
Hcollisional detachment, 40-41 ion-ion neutralization, 56 Hz associative detachment, 39 asymmetry parameter, 70-72 collision-induced dissociation, 46 decay of superexcited states, 85-92
493
SUBJECT INDEX
dissociative ionization, 93-95 dissociative recombination and attachment, 17,24,26-29 cross section, 8 H2+ dissociative recombination and attachment, 14-15, 18-21 cross section, 6 H3 + dissociative products, 23 dissociative recombination, 2 1 H,+ dissociative recombination, 21 recombination rate coefficient, 6 Halide collisional detachment, 41-42 dissociative attachment, 29-34 ion-ion neutralization, 55 Halogen charge-exchange collisions, 48-49 collisional detachment, 40-41 dissociative attachment, 29-34 cross sections, 7 ion-pair formation, 42 reactive collisions, 53 Halomethane, charge-exchange collisions, 50 Handshaking procedure, in microprocessor system, 439-440 Hartree-Fock approximation, in photoionization cross section calculations, 79 HBr, dissociative attachment, 3 1-33 HCI collisional detachment, 39 dissociation attachment, 31-33 HCI-, lifetime, 38 HD', recombination cross section, 6 He dissociative recombination, 9, 11- 13, 17 multiple ionization, 78 photoionization cross section, 74 He' collisional ionization, 37-38 ion-ion neutralization, 56 Heterojunction, 196 HF, dissociative attachment, 31 -32 HI, dissociative attachment, 32-33 High-impedance (HZ) receiver preamplifier, 208-2 10 High-level language, microprocessor system, 458-459,465
H2O charge-exchange collisions, 50-5 1 dissociative attachment, 34 cross sections, 7 H 2 0 + ,recombination cross section, 6 H30+ dissociative recombination, 23 recombination rate coefficient, 6-7 H 3 0 + .(H,O),, recombination rate coefficient, 6 H 3 0 +'(H20), , ion-ion neutralization, 56 H2S, dissociative attachment, 34 Hytrel, 184 HZ preamplifier, see High-impedance receiver preamplifier
I I charge-exchange collisions, 48 dissociative attachment, 31 ion-ion neutralization, 55 IC, see Integrated circuit ICR, see Ion cyclotron resonance IEEE 488 standard, for microprocessor system, 454 ILS, see Instrument landing system Image-dissecting ion microscope, 281, 300, 302 In-beam multipath, microwave landing system, 405 Inelastic collision, autoionization, 64 Infrared multiphoton ionization of molecules, 111 INS, see Ion neutralization spectrometry Instrument landing system (ILS), 312-313, 3 15,350 accuracy, 319, 359 channels, 329 collocation with microwave landing system, 397-398 phasing out, 320 Integrated circuit local data communication applications, 201-203 in microprocessor system, 414-41 5 Interface chips, microprocessor, 433-441 Ion cyclotron resonance (ICR), photodetachment, 113, 121-122, 124 Ion fragmentation, 99-101
494
SUBJECT INDEX
Ion implantation, in surface analysis, 250251,273 Ion-ion neutralization, 54-60 Ionization loss spectrometry (ILS), in surface analysis, 234,236-231 Ion microprobe, 279-283,296, 300-301 Ion microscope, 279-280.282, 300,302 Ion neutralization spectrometry (INS), in surface analysis, 234, 237 Ion-pair formation, 42-54 charge-exchange collisions, 47-5 1 charge exchange with excited species, 52 collision-induced dissociation, 43-47 photoproduction, 47 predissociation of superexcited states, 90 in reactive collisions, 52-54 Ions, analysis of, in surface analysis, 237-241 Ion scattering spectrometry (ISS), in surface analysis, 237-238, 242 elemental mapping, 290, 292 quantitation, 255,258-259
K K, charge-exchange collisions, 49-5 I K', ion-ion neutralization, 55 K,, reactive collisions, 53 Kr, multiple ionization, 78
L Lambertian light-emitting diode source, 165, 196 LAN, see Local area network Landing system, see Instrument landing system; Microwave landing system Laser multiphoton ionization, 101, 103-105, 108109 photodetachment, 112, 120-121 Laser diode (LD), 160-161, 189-191 Laser-generated plasma, 63 LD, see Laser diode LDC, see Local data communication Learning-curve phenomenon, 421 LED, see Light-emitting diode Li charge-exchange collisions, 48 collision-induced dissociation, 44
Light-emitting diode (LED), 160-161, 165, 189-201 Local area network (LAN), 146 contrasted with digital telecommunications, 159- 162 distributed point-to-point links, 215-2 I8 optical fiber coupler, 219 passive fiber optic loop and linear data buses, 220-223 standards and performance limits, 157 topologies and trends, 149-154, 215 transmissive star data bus, 223-224 Local data communications (LDC), 145-146, 156- 157 cable requirements, 181-184 fiber performance parameters, 163- 175 fiber selection, 175-181 optical connectors, 184-188 optical detector and receiver considerations, 203-213 optical source and transmitter circuit design, 189-203 standardization issues, 158 system design considerations, 21 3-224 Localizer antenna, instrument landing system, 312 Local thermodynamic equilibrium (LTE), in secondary ion mass spectrometry, 252253,256, 286-290 Logic circuit, 421 -424 Loop topology, of local area network, see Ring topology Low-energy collision collisional detachment, 38 ionization, 35-36 Low-pass filter, microwave landing system receiver, 356 LTE, see Local thermodynamic equilibrium Lyman continuum, photoionization source, 61
M Many-body perturbation theory (MBPT), 79 Material dispersion, in optical fibers, 167, I89 MBPT, see Many-body perturbation theory Memory, microprocessor, 426-429 Merged-beam experiments, in ion-ion neutralization, 57
495
SUBJECT INDEX
Metal oxide semiconductor, see MOS technology Metals photoionization, 78 reactive collisions, 53 MGH, see Minimum guidance height Microbending loss, in optical fibers, 167- 170 Microcomputer, 412-415; see also Microprocessor system Microlens, 200-201 Microprocessor system, 41 1-471 applications, 415-418,448-454 assembly of, 442-448 components, 424-448 Costs, 467-469 design of system, 462-469 development, 454-462 future of, 469 operation, 430-433 programming, 454-460 technology, 418-421 Microwave landing system (MLS), 31 1-409 airborne receiver and processor, 349-357 angle and range coverage, 322 basic configuration, 321 channel plan, 329 collocation with instrument landing system, 397-398 definitions, 405-406 description of, 327-358 development process, 317-318 distance-measuring equipment, 357-358 error analysis, 372-393, 406 expanded configurations, 321-322 ground equipment, 340-349 guidance accuracy, 322-327, 358-372 international acceptance, 318 landing operation, 314-315 monotoring, 403-405 operational and functional requirement for. 320-327 operational application, 316-317 performance examples, 399-403 power budget, 392-394 propagation effects, 378-393 signal format, 329-340 single accuracy standard, 3 19-320,322-323 siting, 394-398 standardization, 318-319 system design considerations, 358-405
Minicomputer, 412-414,416,462-463 Minimum guidance height, 359 Missed-approach coverage, of microwave landing system, 322, 397 MLS, see Microwave landing system Modal pulse dispersion, in optical fibers, 165-167, 189, 191 Molecular spectroscopy, 105 Molecule autoionization mechanisms, 64 ion fragmentation, 99-101 multiphoton ionization, 104-1 11 photoionization cross sections, 80-85 photoionization efficiencies, 85, 92-93 Monitoring system, microwave landing system, 348-349 Monopole antenna, microwave landing system, 350 MOS technology, in microprocessor system, 419,429 Motion averaging, in microwave landing system, 405 MPI, see Multiphoton ionization Multichannel quantum defect theory (MQDT), &Q,80,86 Multipath effects, and microwave landing system accuracy, 378-388, 390-391, 393 Multiphase system, sputtering, 262, 267 Multiphoton ionization (MPI), 63, 101-1 1 I
N N, photoionization cross sections, 75-76 N2 dissociative attachment, 34 photoionization cross section, 8 1-83 reactive collisions, 54 Na charge-exchange collisions, 48, 50 multiphoton ionization, 103-104 reactive collisions, 52-53 Na', ion-ion neutralization, 55, 57, 59 Na-, photodetachment, 116 Na,, multiphoton ionization, 108 NaI, collision-induced dissociation, 43-44 Ne dissociative recombination, 9- 10 photoionization cross section, 74-75, 77 NH,-,photodetachment, 122
496
SUBJECT INDEX
NH4+ dissociative products, 23 recombination cross section, 6 NO dissociative attachment cross section, 7 four-photon ionization, 107 NO+ dissociative recombination, 22-23 ion-ion neutralization, 55-56 NO,photodetachment, 122- 123 ion-ion neutralization, 55-57 N,O, dissociative attachment cross section, 7-8
0 0collisional detachment, 41 cross section for formation of, 6-7 detachment cross sections, 39-40 ion-ion neutralization, 57 0 2
collision-induced dissociation, 45-46 dissociative attachment, 26, 34 reactive collisions, 52-53 0,collisional detachment, 41 charge-exchange collisions, 50 photodetachment, 120- 121 0 3 -photodetachment, , 123 04-,photodissociation, 123 OCI, see Out-of-coverage indication OD-, photodetachment, 118-1 19 OHcollisional detachment, 41 photodetachment, 118- 120 Optical bypass switch, 217 Optical connector, 184-188 Optical detector, for local data communications applications, 203-206 Optical fiber, 162-181 attenuation, 162-163, 189 versus bandwidth-distance, 175-176 versus modulation bandwidth, 148 cable, 149, 181-184 connector design, 184- 188 in digital telecommunications, 159 performance parameters, in local data communication applications, 163- 175
propagation delay, 147 refractive index, 146, 164 Optical fiber coupler, 219-220 Optical lens, for communications system, 200-201 Optical source, for local data communications, 189-203 Optical taps, 157 Optical wavelcngth, for local data communications, 189-193 Optocoupler, 146-147 Out-of-beam multipath, microwave landing system, 405 Out-of-coverage indication (OCI), microwave landing system, 335-336 antenna, 344-345, 348
P Pacemaker, microprocessor, 418 Pancreas, artificial, 418 Parallel input/output, see P I 0 Partially-graded silica fiber, for local data communications, 176- 177, 180 Partial photoionization cross section, 74, 77, 81 Path-following error (PFE), microwave guidance system, 326,364369,372,401-403 Path-following noise (PFN), microwave landing system, 326, 366-368 Pattern control, in microwave landing system, 406 PCM, see Pulse-code modulation PCS fiber, see Plastic-clad silica fiber Peripheral interface adaptor, see PIA PES, see Photoelectron spectroscopy PFE, see Path-following error PFN, see Path-following noise Phased-array antenna, microwave landing system, 342, 348-349 Photoabsorption cross section, 65 Photodetachment, 60-61, 112-125 atomic negative ions, 114-1 18 cross sections near threshold, 113-1 14 diatomic negative atoms, 118-121 triatomic and polyatomic negative ions, 122-125 ion-pair production, 47
497
SUBJECT INDEX
Photoelectron spectroscopy (PES), 65-66, 112,120-122, 124-125 Photoelectron measurements, 65 Photoelectron studies, 65-74 angular momentum transfer theory, 68-69 asymmetry parameter b, 69-73 spectroscopy, 65-66 spin polarization, 73-74 Photoionization, 60-93 cross sections, 74-85 efficiencies, 85-93 photoelectron studies, 65-74 process, 63-65 sources, 61-63 Photoion-photoelectron coincidence (PIPECO) technique, 100 PIA (peripheral interface adaptor), 437-438 p-i-n detector in digital telecommunications, 160-161 in local data communications, 204-205 p-i-n photodiode (PD), in local area network, I62 P I 0 (parallel input/output), 437 PIPECO, see Photoion-photoelectron coincidence technique PIXE microprobe, 292 Plasma, continuum radiation sources, 60-63 Plastic-clad silica (PCS) fiber for local data communications, 176- 181 buffer coating, 183-184 p-n junction photodiode, for local data communications applications, 204-206 Point-to-point local'area network, 21 5-218 Polarized light, 67 Polyurethane, 182, 184 Polyvinyl chloride (PVC), 182, 184 Power density, microwave landing system, 338-340 PPI (programmable parallel interface), 437 Preamplifier, for local data communications applications, 206-21 0 Preionization, see Autoionization Programmable parallel interface, see PPI Programmable read only memory, see PROM Programming, microprocessor, 454-460 Programming language, microprocessor system, 458-460,465 PROM (programmable read only memory), 427
Propagation error, in microwave landing system, 406 Proportional guidance, of microwave landing system, 322, 329, 336, 342 Proton microprobe, 292 Pulse-code modulation (PCM), I59 PVC, see Polyvinyl chloride
Q QET, see Quasi-equilibrium theory Quantitative elemental surface analysis, 232, 242-259,276-279,281-290 Quasi-equilibrium theory (QET), of ion fragmentation, 100- I01
R Rainbow effect, in charge-exchange collisions, 48 Raleigh scattering, in optical fibers, 163 RAM (random access memory), 426-428,441 Rare gases charge-exchange collisions, 47 charge exchange with excited species, 52 collisional detachment, 41 collisional ionization, 37 continuum radiation sources, 61 -62 dissociative recombination, 8-13 multiple ionization, 78 Rate coefficient, in dissociative recombination and attachment, 6-7; seealso specific ions Rb, collision-induced dissociation, 44 RbBr, collision-induced dissociation, 45 RBS, see Rutherford backscattering spectroscopy Reactive (rearrangement) collisions, ion-pair formation in, 52-54 Read only memory, see ROM Receiver local data communications, 206-21 3 microwave landing system, 329, 349-357, 364,405 errors, 377-379, 390 noise, 373 Repeater, in digital telecommunication, 159 REPROM (reprogrammable read only memory), 427
498
SUBJECT INDEX
Resonance processes, in dissociative attachment, 24-25 Ring (loop) topology, of local area network, 151- 153,220-223 ROM (read only memory), 426-427,429,441 Rotman lens antenna, 342,344-345 Rutherford backscattering spectroscopy (RBS), in surface analysis, 238-242 depth profiling, 261,265-266 elemental mapping, 290, 292 Rydberg states, 64 charge exchange with excited species, 52 collisional ionization, 36-37 MPI spectroscopy, 105 multiphoton ionization, 104
S SARPS, see Standards and recommended practices Scalloping rate, in microwave landing system, 406 Scanning beam, microwave guidance system, 317,329,336-337 Scanning-beam antenna, microwave landing system, 342, 345 Scanning ion microprobe, 279-283,296,300301 Se-, photodetachment, 114-1 16 Secondary ion mass spectrometry (SIMS), in surface analysis, 241 -242 depth profiling, 266,270, 272-275 elemental mapping, 279-290 quantitation, 245-257 resolution and sensitivity limits, 300-306 three-dimensional analysis, 296-298 Sector coverage antenna, microwave landing system, 344-345, 348 Selective sputtering, in surface analysis, 263266 Separation angle, in microwave landing system, 405 SF, charge-exchange collisions, 50 charge exchange with excited species, 52 SF,-, ion-ion neutralization, 55 Shadowing, in microwave landing system, 388-390
SH LED, see Single-heterostructure lightemitting diode Shock-tube studies charge-exchange collisions, 49 collision-induced dissociation, 44-45 ion-ion neutralization, 55 photodetachment, 113 of thermal rate coefficients, 41 -42 Signal format, microwave landing system, 329-339 Signal-in-space, microwave landing system, 317 Silicon avalanche photodiode, in digital telecommunications, 160- 16 1 SIMS, see Secondary ion mass spectrometry Single-channel cable, for local data communications, 181, 183 Single-chip, eight-bit microcomputer, 430 Single-chip, four-bit microcontroller, 430 Single-heterostructure (SH) light-emitting diode, 196 Single-mode silica fiber, for digital telecommunications, 179- 180 Single-photon ionization, 63 Single pole filter, microwave landing system receiver, 356 Sixteen-bit microprocessor, 429,463-464,469 Skin-effect-limited cable, 147 SNLTE, see Sputter-normalized local thermodynamic equilibrium SO,-, photodetachment, 122 Soft X-ray appearance potential spectroscopy ' (SXAPS), 236 Soft X-ray lines, continuum radiation sources, 62 Solid solution, sputtering, 262 Specular reflection coefficient, in microwave landing system, 406 Spin polarization, 73-74, 103-104 Sputter-normalized local thermodynamic equilibrium (SNLTE), 253-254, 256, 288-290 Sputtering, in surface analysis, 261 -273 SR, see Synchrotron radiation Standards and recommended practices (SARPS), for microwave landing system, 312-313, 318-321,326 Star topology, of local area network, 15 I - 153, 220,223-224
I
499
SUBJECT INDEX
Static antenna sidelobe effect, in microwave landing system, 405 Static memory, 426 Stripe laser, 190 Stripe light-emitting diode, 190, 195, 197 Stueckelberg oscillations, in charge-exchange collisions, 48 Superexcited states, decay of, 85-93 Supersonic jet, in multiphoton ionization studies, 107 Surface analysis, using charged-particle beams, 231-310 depth profiling, 259-275 elemental mapping, 275-292 methods, 233-242 quantitative elemental analysis, 242-259 sensitivity and resolution limits, 296-306 three-dimensional isometric elemental analysis, 292-298 Surface light-emitting diode, 190, 195-200 SXAPS, see Soft X-ray appearance potential spectroscopy Symbolic assembly language, 458-459,465 Synchronous transmission, 435 Synchrotron radiation (SR) measurement of asymmetry parameter fl. 67 photoionization cross sections, determination of, 83 photoionization source, 61 -62, 66
Time averaging, in microwave landing system. 406 Time-division multiplexing (TDM), 153, 159 Time gating, in microwave landing system, 406 Time-of-flight analysis, in multiphoton ionization, 108 Timer, in microprocessor system, 440-441 Time reference scanning-beam (TRSB) technique, microwave landing system, 31 1, 317-318, 329 TI halide, collision-induced ion-pair production, 44 TPES, see Threshold photoelectron spectroscopy Transimpedance (TZ) receiver preamplifier, 208-212 Transistor-transistor logic, see TTL Transition probability, in photoionization process, 63 Transmitter, microwave landing system, 340, 342 Transmitter circuit, for local data communications, 189-203 TRSB technique, see Time reference scanningbeam technique TTL (transistor-transistor logic), in microprocessor system, 419 Two-body ion-ion neutralization, 54-56 TZ preamplifier, see Transimpedance receiver preamplifier
T Tapered sectioning, in surface analysis, 269270 T-carrier digital telecommunications, 156 TDM, see Time division multiplexing Teletype, 435-437 Thermal noise error, microwave landing system, 373-374 Thermal oxidation, in fabrication of optical fibers, 177 Three-body ion-ion neutralization, 54-56 Three-dimensional isometric elemental analysis, 292-298 Threshold photoelectron spectroscopy (TPES), 66 Threshold spectrometries, in surface analysis, 235-236
U UART (universal asynchronous receivertransmitter), 435 USART (universal synchronous/asynchronous receiver-transmitter), 434
V Vacuum ultraviolet (VUV) laser, 63 Vacuum ultraviolet (VUV) sources, 61 -62 Vapor axial deposition (VAD), in fabrication of silica fibers. 177
SUBJECT INDEX
W Wire cable distributed bus network, 153 relative merits of, 149 Wired logic, 463-464
X Xe collisional ionization, 37 multiple ionization, 78 Xe+, ion-ion neutralization, 55
