Adsorbed Layers on surfaces

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Landolt-Börnstein Numerical Data and Functional Relationships in Science and Technology New Series / Editor in Chief: W. Martienssen

Group III: Condensed Matter Volume 42

Physics of Covered Solid Surfaces Subvolume A Adsorbed Layers on Surfaces Part 4 Adsorbed Species on Surfaces and Adsorbate-Induced Surface Core Level Shifts Editor H.P. Bonzel Authors H.P. Bonzel, R. Denecke, W. Eck, A. Föhlisch, G. Held, W. Jaegermann, N. Mårtensson, T. Mayer, H. Over, H.P. Steinrück

ISSN 1615-1925 (Condensed Matter) ISBN 3-540-20281-1 Springer Berlin Heidelberg New York Library of Congress Cataloging in Publication Data Zahlenwerte und Funktionen aus Naturwissenschaften und Technik, Neue Serie Editor in Chief: W. Martienssen Vol. III/42A4: Editor: H.P. Bonzel At head of title: Landolt-Börnstein. Added t.p.: Numerical data and functional relationships in science and technology. Tables chiefly in English. Intended to supersede the Physikalisch-chemische Tabellen by H. Landolt and R. Börnstein of which the 6th ed. began publication in 1950 under title: Zahlenwerte und Funktionen aus Physik, Chemie, Astronomie, Geophysik und Technik. Vols. published after v. 1 of group I have imprint: Berlin, New York, Springer-Verlag Includes bibliographies. 1. Physics--Tables. 2. Chemistry--Tables. 3. Engineering--Tables. I. Börnstein, R. (Richard), 1852-1913. II. Landolt, H. (Hans), 1831-1910. III. Physikalisch-chemische Tabellen. IV. Title: Numerical data and functional relationships in science and technology. QC61.23 502'.12 62-53136 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in other ways, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution act under German Copyright Law. Springer is a part of Springer Science+Business Media springeronline.com © Springer-Verlag Berlin Heidelberg 2005 Printed in Germany The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Product Liability: The data and other information in this handbook have been carefully extracted and evaluated by experts from the original literature. Furthermore, they have been checked for correctness by authors and the editorial staff before printing. Nevertheless, the publisher can give no guarantee for the correctness of the data and information provided. In any individual case of application, the respective user must check the correctness by consulting other relevant sources of information. Cover layout: Erich Kirchner, Heidelberg Typesetting: Authors and Redaktion Landolt-Börnstein, Darmstadt Printing and Binding: AZ Druck, Kempten SPIN: 10932216

63/3020 - 5 4 3 2 1 0 – Printed on acid-free paper

Preface Surface Science is understood as a relatively young scientific discipline, concerned with the physical and chemical properties of and phenomena on clean and covered solid surfaces, studied under a variety of conditions. The adsorption of atoms and molecules on solid surfaces is, for example, such a condition, connected with more or less drastic changes of all surface properties. An adsorption event is frequently observed in nature and found to be of technical importance in many industrial processes. For this reason, Surface Science is interdisciplinary by its very nature, and as such an important intermediary between fundamental and applied research. Intense world-wide research in this field over the last 50 years has lead to a considerable degree of maturity, such that a documentation of quantitative results in a single source seems desirable. Tribute is being paid to this effect by the renowned Series of LANDOLT-BÖRNSTEIN whose editor-in-chief Werner Martienssen, Frankfurt/ Main, has initiated several volumes of collected scientific data in the field of Surface Science. The beginning has been made with LANDOLT-BÖRNSTEIN volume III/24, entitled Physics of Solid Surfaces. This volume, consisting of four subvolumes, appeared in 1993-96 and covers the properties of clean solid surfaces. The current volume III/42 is devoted to Physics of Covered Solid Surfaces and, in particular, to Adsorbed Layers on Surfaces. It is as such a collection of data obtained for adsorbates on well-defined crystalline surfaces. "Well-defined" means surfaces of known crystallographic structure and chemical composition. It was almost clear at the beginning, that the amount of general information and quantitative data on Adsorbed Layers on Surfaces is enormous, too large to fit into a single book. Hence several subvolumes had to be planned. Unfortunately, the chapters anticipated for each of the subvolumes did not arrive synchronously with the production schedule, such that the sequence of chapters actually printed in the subvolumes deviates from that in the original outline of the whole volume. We apoligize for this inconvenience, but in the age of electronic information distribution this problem will be solved, once all volumes are available electronically. Search routines will guide the reader to the data of his desire. Until that time, the index of each subvolume will have to do. Three subvolumes A1 to A3 of volume III/42 have already appeared in the years 2001-2003. The present subvolume A4 entitled Adsorbed Species on Surfaces and Adsorbate-Induced Surface Core Level Shifts is the fourth in this sequence. Another final subvolume is currently in preparation. Finally, it is again my pleasure to thank all authors of this volume for their excellent contributions, and the editing and production offices of the Landolt-Börnstein Office of the Springer-Verlag for efficient cooperation and excellent support. Jülich, June 2004

Hans P. Bonzel

VI

Contributors

Editor H.P. Bonzel Forschungszentrum Jülich Institut für Schichten und Grenzflächen (ISG 3) 52425 Jülich Germany

Authors E.I. Altman Department of Chemical Engineering Yale University New Haven, CT 06520 USA 3.4.3 Halogens on metals and semiconductors M. Bienfait CRMC2/CNRS Faculté de Luminy Physique - Case 910 F-13288 Marseille Cedex 9 FRANCE 3.1.2 Noble gases on graphite, lamellar halides, MgO, NaCl H.P. Bonzel Forschungszentrum Jülich Institut für Schichten und Grenzflächen (ISG 3) 52425 Jülich Germany 1 Introduction to physical and chemical properties of adlayer/substrate systems 3.7.1 CO and N2 on metals W.A. Brown Department of Chemistry University College London London WC1H 0AJ U.K. 3.7.2 NO, CN, O2 on metals H. Brune Institut de Physique Expérimentale (IPE) École Polytechnique Fédérale de Lausanne (EPFL) PHB-Ecublens CH-1015 Lausanne 3.3.1 Metals on metals

Contributors K. Christmann Institut für Physikalische und Theoretische Chemie Freie Universität Berlin 14195 Berlin Germany 3.4.1 Chemisorbed hydrogen on metals and semiconductors R. Denecke Universität Erlangen-Nürnberg Lehrstuhl für Physikalische Chemie II Egerlandstraße 3 91058 Erlangen Germany 4.3 Adsorbate induced surface core level shifts of metals R.D. Diehl Department of Physics Pennsylvania State University University Park, PA 16802 USA 3.2.1 Alkali metals on metals W. Eck Universität Heidelberg Angewandte Physikalische Chemie Abteilung Materialchemie Im Neuenheimer Feld 253 69120 Heidelberg Germany M. Enachescu Candescent Technologies 6320 San Ignacio Ave. San José, CA 95119 USA 3.4.4 P, S, As, Sb on metals and semiconductors N. Esser Institut für Festkörperphysik Technische Universität Berlin D-10623 Berlin Germany 4.6 Surface optical properties J.E. Fieberg Department of Chemistry Georgetown College Georgetown, KY 40324 USA 3.8.9 Halogen-substituted hydrocarbons on metals and semiconductors

VII

VIII

Contributors

A. Föhlisch Institut für Experimentalphysik Universität Hamburg Luruper Chaussee 149 D-22761 Hamburg Germany 3.7.1 CO and N2 on metals H.-J. Freund Fritz-Haber-Institut der Max Planck Gesellschaft (MPG) D-14195 Berlin Germany 3.9 Adsorption on oxides H.J. Grabke Max-Planck Institut (MPI) für Eisenforschung GmbH D-40074 Düsseldorf Germany 3.5 Surface segregation of atomic species (non-metal on metal) E. Hasselbrink Institut für Physikalische und Theoretische Chemie Universität Essen D-45117 Essen Germany 3.8.3 NH3 and PF3 on metals and semiconductors G. Held University of Cambridge Department of Chemistry Lensfield Road Cambridge CB2 1EW United Kingdom 3.8.7 Cyclic hydrocarbons on metals and semiconductors K. Hermann Fritz-Haber-Institut der Max-Planck Gesellschaft (MPG) Abteilung Theorie D-14195 Berlin Germany 4.1 Surface structure on metals and semiconductors H. Ibach Institut für Schichten und Grenzflächen (ISG 3) Forschungszentrum Jülich D-52425 Jülich Germany 4.4 Surface free energy and surface stress

Contributors K. Jacobi Fritz-Haber-Institut der Max-Planck Gesellschaft (MPG) D-14195 Berlin Germany 4.2 Electron work function of metals and semiconductors W. Jaegermann Fachbereich Materialwissenschaft Fachgebiet Oberflächenforschung Technische Universität Darmstadt D-64287 Darmstadt Germany 3.8.2 H2O and OH on semiconductors M.Y.L. Jung Deppartment of Chemical Engineering University of Illinois Urbana, IL 61801 USA 3.11 Surface diffusion on metals, semiconductors and insulators B.E. Koel Department of Chemistry, SSC 606 University of Southern California Los Angeles, CA 90089-0482 USA 3.8.4 CO2, NO2, SO2, OCS, N2O, O3 on metals and semiconductors H. Kuhlenbeck Fritz-Haber-Institut der Max-Planck Gesellschaft (MPG) Abteilung Chemische Physik D-14195 Berlin Germany 3.9 Adsorption on oxides V.G. Lifshits Institute of Automation and Control Processes 690041 Vladivostok Russia 3.3.2 Metals on semiconductors N. Mårtensson Department of Physics Uppsala University S-751 21 Uppsala Sweden 4.3 Adsorbate induced surface core level shifts of metals

IX

X

Contributors

T. Mayer Fachbereich Materialwissenschaft Fachgebiet Oberflächenforschung Technische Universität Darmstadt D-64287 Darmstadt Germany 3.8.2 H2O and OH on semiconductors R. McGrath Surface Science Research Centre and Department of Physics The University of Liverpool Liverpool L69 3BX U.K. 3.2.1 Alkali metals on metals E.G. Michel Departimento Fisica de la Materia Condensada C-III Instituto Universitario de Ciencia de Materiales "Nicolas Cabrera" Universidad Autonoma de Madrid 28049 Madrid Spain 3.2.2 Alkali metals on semiconductors R. Miranda Departimento Fisica de la Materia Condensada C-III Instituto Universitario de Ciencia de Materiales "Nicolas Cabrera" Universidad Autonoma de Madrid 28049 Madrid Spain 3.2.2 Alkali metals on semiconductors D.R. Mullins Oak Ridge National Laboratory Oak Ridge, TN 37831-6201 USA 3.8.5 Substituted hydrocarbons on metals B.E. Nieuwenhuys Gorlaeus Laboratory Leiden University NL 2300 Ra Leiden The Netherlands 3.7.3 Diatomic molecules on alloys K. Oura Department of Electronic Engineering Faculty of Engineering Osaka University Osaka 565-0871 Japan 3.3.2 Metals on semiconductors

Contributors H. Over Physikalisch-Chemisches Institut Justus Liebig Universität Gießen Heinrich-Buff Ring 58 D-35392 Gießen Germany 3.4.2 C, N, O on metals G. Pirug Institut für Schichten und Grenzflächen (ISG 3) Forschungszentrum Jülich D-52425 Jülich Germany 3.8.1 H2O and OH on metals W. Richter Institut für Festkörperphysik Technische Universität Berlin D-10623 Berlin Germany 4.6 Surface optical properties M.A. Rocca Centro di Fisica delle Superfici e Basse Temperature del CNR Istituto Nazionale di Fisica della Materia I-16146 Genova Italy 4.5 Surface phonon dispersion G. Rupprechter Fritz-Haber-Institut der Max-Planck Gesellschaft (MPG) Abteilung Chemische Physik D-14195 Berlin Germany 3.8.6 Linear hydrocarbons and CH4 on metals and semiconductors M. Salmeron Lawrence Berkeley Laboratory Materials Science Bldg. 66/208 Berkeley, CA 94720 USA 3.4.4 P, S, As, Sb on metals and semiconductors D. Sander Max-Planck Institut (MPI) für Strukturphysik D-06120 Halle Germany 4.4 Surface free energy and surface stress

XI

XII

Contributors

A.A. Saranin Institute of Automation and Control Processes 690041 Vladivostok Faculty of Physics and Engineering Far Eastern State University 690000 Vladivostok Russia 3.3.2 Metals on semiconductors E.G. Seebauer Deppartment of Chemical Engineering University of Illinois Urbana, IL 61801 USA 3.11 Surface diffusion on metals, semiconductors and insulators G.A. Somorjai Department of Chemistry University of California Berkeley, CA 94720 USA 3.8.6 Linear hydrocarbons and CH4 on metals and semiconductors H.-P. Steinrück Lehrstuhl für Physikalische Chemie II Universität Erlangen-Nürnberg D-91058 Erlangen Germany 3.8.7 Cyclic hydrocarbons on metals and semiconductors J. Suzanne Departement de Physique CRMC2 - Centre National de la Recherche Scientifique (CNRS) Faculte des Sciences de Luminy F-13288 Marseille, Cedex 9 France 3.6 Molecules on graphite, BN, MgO (except noble gases) W.T. Tysoe Department of Chemistry and Laboratory for Surface Studies University of Wisconsin - Milwaukee Milwaukee, WI 53211 USA 3.8.5 Substituted hydrocarbons on metals Ch. Uebing Department of Physics and Astronomy Rutgers, The State University of New Jersey Piscataway, NJ 08854-8019 USA 3.5 Surface segregation of atomic species (non-metal on metal)

Contributors H. Viefhaus Max-Planck Institut (MPI) für Eisenforschung GmbH D-40074 Düsseldorf Germany 3.5 Surface segregation of atomic species (non-metal on metal) J.M. Vohs Department of Chemical Engineering University of Pennsylvania Philadelphia, PA 19104-6315 USA 3.8.8 Oxygenated hydrocarbons on metals and semiconductors M.A. Van Hove Lawrence Berkeley National Laboratory Materials Science 66 Berkeley, CA 94720 and Department of Physics University of California-Davis Davis, CA 95616 USA 4.1 Surface structure on metals and semiconductors P.R. Watson Department of Chemistry Oregon State University Corvallis, OR 97331 USA 4.1 Surface structure on metals and semiconductors J.M. White Department of Chemistry and Biochemistry University of Texas at Austin Austin, TX 78712 USA 3.8.9 Halogen-substituted hydrocarbons on metals and semiconductors H. Wiechert Institut für Physik der Johann Gutenberg-Universität D-55099 Mainz Germany Molecules on graphite, BN, MgO (except noble gases) Ch. Wöll Lehrstuhl für Physikalische Chemie I Ruhr-Universität Bochum D-44801 Bochum Germany 2 Characterization of adsorbate overlayers: Measuring techniques

XIII

XIV

Contributors

P. Zeppenfeld Institut für Experimentalphysik Atom- und Oberflächenphysik Johannes-Kepler-Universität Linz A-4040 Linz, Austria 3.1.1 Noble gases on metals and semiconductors A.V. Zotov Faculty of Electronics Vladivostok State University of Economics and Service 690600 Vladivostok, Russia Institute of Automation and Control Processes 690041 Vladivostok , Russia 3.3.2 Metals on semiconductors

Landolt-Börnstein Editorial Office Gagernstr. 8, D-64283 Darmstadt, Germany fax: +49 (6151) 171760 e-mail: [email protected]

Internet http://www.landolt-boernstein.com

Contents

XV

Contents III/42 Physics of Covered Solid Surfaces A: Adsorbed Layers on Surfaces

Part 4: Adsorbed species on surfaces and adsorbateinduced surface core level shifts 1

Introduction to physical and chemical properties of adlayer/substrate systems (H.P. BONZEL) ............................................................................................................................................ see subvolume III/42A1

2

Characterization of adsorbate overlayers: measuring techniques (CH. WÖLL).................................................................................................................................................... see subvolume III/42A2

3 3.1 3.1.1 3.1.2

Data: Adsorbate properties Adsorption of noble gases Noble gases on metals and semiconductors (P. ZEPPENFELD)................... see subvolume III/42A1 Noble gases on graphite, lamellar halides, MgO and NaCl (M. BIENFAIT).............................................................................................................................................. see subvolume III/42A1 Adsorption of alkali metals Alkali metals on metals (R.D. DIEHL, R. McGRATH) ......................................... see subvolume III/42A1 Alkali metals on semiconductors (E.G. MICHEL, R. MIRANDA) ..... see subvolume III/42A1 Adsorption of metals Metals on metals (H. BRUNE)...................................................................................................... see subvolume III/42A1 Metals on semiconductors (V.G. LIFSHITS, K.OURA, A.A. SARANIN, A.V. ZOTOV) .................................. see subvolume III/42A1 Non-metallic atomic adsorbates on metals and semiconductors Chemisorbed hydrogen on metals and semiconductors (K. CHRISTMANN).................................................................................................................................... see subvolume III/42A5 Adsorption of C, N, and O on metal surfaces (H. OVER) ............................................................................................ 2 Introduction ................................................................................................................................................................................................................ 2 General remarks..................................................................................................................................................................................................... 2 List of acronyms .................................................................................................................................................................................................... 2 Oxygen adsorption on metal surfaces ............................................................................................................................................. 4 The dissociative sticking coefficient of oxygen on metal surfaces and its dependence on the impact energy of the incident O2 molecule ............................................................................................................................. 13 The heat of adsorption of chemisorbed oxygen overlayers on metal surfaces ................................... 18 Oxygen-metal bond strength (ab initio calculations).................................................................................................... 20 Vibrational properties of chemisorbed oxygen................................................................................................................... 30 Local atomic oxygen-metal geometry .......................................................................................................................................... 33 Ordered overlayers of chemisorbed oxygen and surface oxides on metal surfaces ...................... 37 Phase diagrams and phase transitions in the O-metal surface system ......................................................... 40 Nitrogen adsorption on metal surfaces ........................................................................................................................................ 41 The dissociative sticking coefficient of nitrogen on metal surfaces.............................................................. 43 The heat of adsorption of chemisorbed nitrogen overlayers on metal surfaces................................. 44 Nitrogen-metal bond strength (ab initio calculations) ................................................................................................. 45 Electronic properties of chemisorbed nitrogen on metal surfaces................................................................... 46 Vibrational properties of chemisorbed nitrogen atoms .............................................................................................. 47 Local atomic nitrogen-metal geometry ....................................................................................................................................... 48 Ordered overlayers of chemisorbed nitrogen atoms on metal surfaces ..................................................... 49

3.2 3.2.1 3.2.2 3.3 3.3.1 3.3.2 3.4 3.4.1 3.4.2 3.4.2.1 3.4.2.1.1 3.4.2.1.2 3.4.2.2 3.4.2.2.1 3.4.2.2.2 3.4.2.2.3 3.4.2.2.5 3.4.2.2.6 3.4.2.2.7 3.4.2.2.8 3.4.2.3 3.4.2.3.1 3.4.2.3.2 3.4.2.3.3 3.4.2.3.4 3.4.2.3.5 3.4.2.3.6 3.4.2.3.7

XVI

Contents

3.4.2.4 3.4.2.4.1 3.4.2.4.2 3.4.2.4.3 3.4.2.4.4 3.4.2.4.5 3.4.2.5

Carbon adsorption on metal surfaces ............................................................................................................................................ 50 Carbon-metal bond strength ................................................................................................................................................................... 51 Electronic properties of chemisorbed carbon on metal surfaces....................................................................... 51 Vibrational properties of chemisorbed carbon atoms................................................................................................... 52 Local atomic carbon-metal geometry ........................................................................................................................................... 53 Ordered overlayers of chemisorbed carbon atoms on metal surfaces.......................................................... 54 References for 3.4.2........................................................................................................................................................................................ 55

3.4.3 3.4.4

Halogens on metals and semiconductors (E.I. ALTMAN) ............................... see subvolume III/42A1 Adsorption of S, P, As, Sb and Se on metals, alloys and semiconductors (M. ENACHESCU, M. SALMERON) ............................................................................................ see subvolume III/42A3

3.5

Surface segregation of atomic species (H. VIEFHAUS, H.-J. GRABKE, CH. UEBING) ................................................................. see subvolume III/42A3

3.6 3.6.1 3.6.2

Molecules on graphite, BN, MgO (except noble gases) Adsorption of molecules on MgO (J. SUZANNE) .................................................... see subvolume III/42A3 Adsorption of molecular hydrogen isotopes on graphite and BN (H. WIECHERT) ........................................................................................................................................... see subvolume III/42A3

3.7 3.7.1 3.7.1.1 3.7.1.1.1 3.7.1.1.2 3.7.1.1.3 3.7.1.1.4 3.7.1.1.5 3.7.1.2 3.7.1.3 3.7.1.4 3.7.1.5 3.7.1.6 3.7.1.7 3.7.1.8 3.7.1.9

Molecular diatomic adsorbates on metals and semiconductors CO and N2 adsorption on metal surfaces (A. FÖHLISCH, H.P. BONZEL)....................................... 74 Introduction ............................................................................................................................................................................................................. 75 Thermodynamic properties...................................................................................................................................................................... 76 Vibrational properties ................................................................................................................................................................................... 77 Geometric structure ......................................................................................................................................................................................... 78 Electronic structure and adsorption models ........................................................................................................................... 80 Atom specific electronic structure and the allylic model of CO adsorption ......................................... 85 CO adsorption on fcc metal surfaces............................................................................................................................................. 87 CO adsorption on bcc metal surfaces ........................................................................................................................................102 CO adsorption on hcp metal surfaces ........................................................................................................................................110 CO adsorption on simple cubic metal surfaces ................................................................................................................120 CO adsorbed on relevant binary systems, modelled by ultra-thin metal overlayers..................121 Adsorption of N2 on metals .................................................................................................................................................................124 Organization of the tables......................................................................................................................................................................135 References .............................................................................................................................................................................................................202

3.7.2 3.7.3

NO, CN, O2 on metals (W.A. BROWN) ............................................................................. see subvolume III/42A3 Adsorption of diatomic molecules on alloy surfaces (B. E. NIEUWENHUYS) ......................................................................................................................... see subvolume III/42A3 Molecular polyatomic adsorbates on metals and semiconductors H2O and OH on metals (G. PIRUG) ....................................................................................... see subvolume III/42A5 H2O and OH on semiconductors (W. JAEGERMANN, T. MAYER) ................................................................226 Introduction ..........................................................................................................................................................................................................226 Surface preparation ......................................................................................................................................................................................229 Surface structure: relaxation and reconstruction ............................................................................................................229 Surface electronic structure and surface potentials .....................................................................................................230 Methods of investigation ........................................................................................................................................................................231 Adsorption mode ............................................................................................................................................................................................232 Thermodynamic data of adsorption .............................................................................................................................................234 Kinetic data of adsorption/desorption, surface diffusion and surface reactions ............................234 Local structure ...................................................................................................................................................................................................236

3.8 3.8.1 3.8.2 3.8.2.1 3.8.2.2 3.8.2.3 3.8.2.4 3.8.2.5 3.8.2.6 3.8.2.7 3.8.2.8 3.8.2.9

Contents 3.8.2.10 3.8.2.11 3.8.2.12 3.8.2.13 3.8.2.14 3.8.2.15 3.8.3 3.8.4 3.8.5 3.8.6 3.8.7 3.8.7.1 3.8.7.2 3.8.7.3 3.8.7.4 3.8.7.4.1 3.8.7.4.2 3.8.7.4.3 3.8.7.4.4 3.8.7.5 3.8.7.5.1 3.8.7.5.2 3.8.7.5.3 3.8.7.5.4 3.8.7.5.5 3.8.7.6 3.8.7.7 3.8.7.8 3.8.7.9 3.8.7.10 3.8.7.11 3.8.8 3.8.9 3.8.10 3.8.10.1 3.8.10.2 3.8.10.2.1 3.8.10.2.2 3.8.10.2.3 3.8.10.2.4 3.8.10.3 3.8.10.4 3.8.10.5 3.9 3.10

XVII

Long range order ............................................................................................................................................................................................237 Electronic properties ...................................................................................................................................................................................238 Core level lines .................................................................................................................................................................................................240 Vibrational properties ................................................................................................................................................................................241 Figures for 3.8.2 ..............................................................................................................................................................................................275 References for 3.8.2.....................................................................................................................................................................................296 Adsorbate properties of NH3 and PF3 on metals and semiconductors (E. HASSELBRINK) ................................................................................................................................... see subvolume III/42A3 CO2, NO2, SO2, OCS, N2O, O3 (B.E. KOEL) ............................................................. see subvolume III/42A5 Substituted hydrocarbons on metal surfaces (W.T. TYSOE, D.R. MULLINS) ........................................................................... see subvolume III/42A3 Linear hydrocarbons and CH4 on metals and semiconductors (G. SOMORJAI, G. RUPPRECHTER) ........................................................................................... see subvolume III/42A5 Cyclic hydrocarbons (G. HELD, H.P. STEINRÜCK) ........................................................................................................300 List of symbols and abbreviations ................................................................................................................................................300 Benzene (C6H6) ................................................................................................................................................................................................301 Cyclohexane (c-C6H12) .............................................................................................................................................................................303 Other saturated cyclic hydrocarbon molecules (cycloalkanes)........................................................................304 Cyclopropane (c-C3H6).............................................................................................................................................................................304 Cyclobutane (c-C4H8) ................................................................................................................................................................................304 Cyclopentane (c-C5H10) ...........................................................................................................................................................................305 Cyclooctane (c-C8H16)...............................................................................................................................................................................305 Non-saturated cyclic hydrocarbon molecules (other than benzene)...........................................................305 Cyclopentene (c-C5H8) .............................................................................................................................................................................305 Cyclopentadiene (c-C5H6) .....................................................................................................................................................................305 Cyclohexene (c-C6H10) .............................................................................................................................................................................306 Cyclohexadiene (c-C6H8) .......................................................................................................................................................................306 Cyclooctadiene (c-C8H12) and Cyclooctatetraene (c-C8H8) ................................................................................306 Ethylene Oxide (C2H4O).........................................................................................................................................................................306 Pyridine (C6H5N) ............................................................................................................................................................................................307 List of Tables ......................................................................................................................................................................................................308 Tables for 3.8.7.................................................................................................................................................................................................309 Figures for 3.8.7 ..............................................................................................................................................................................................354 References for 3.8.7.....................................................................................................................................................................................362 Oxygenated hydrocarbons on metals and semiconductors (J. VOHS) ... see subvolume III/42A3 Halogen-substituted hydrocarbons on metals and semiconductors (J. FIEBERG, J.W. WHITE) ................................................................................... see subvolume III/42A3 Polyatomic chain-like hydrocarbons on metals and semiconductors (W. ECK) .............................371 Introduction ..........................................................................................................................................................................................................371 Physical and Chemical Properties .................................................................................................................................................371 Structural data: Tilt and twist angles, packing and lattice structures ........................................................371 Heat of formation and thermal stability...................................................................................................................................373 Wettability .............................................................................................................................................................................................................374 Anchor groups for SAMs on inorganic substrates .......................................................................................................374 List of abbreviations....................................................................................................................................................................................374 Tables .........................................................................................................................................................................................................................375 References for 3.8.10 .................................................................................................................................................................................380 Adsorption on oxides (H. KUHLENBECK, H.J. FREUND) .................................. see subvolume III/42A5 Surface diffusion on metals, semiconductors, and insulators (E.G. SEEBAUER, M.Y.L. JUNG) ............................................................................................... see subvolume III/42A1

XVIII 4 4.1 4.2 4.3 4.3.1 4.3.2 4.3.2.1 4.3.2.2 4.3.2.3 4.3.2.4 4.3.2.5 4.3.2.6 4.3.2.7 4.3.2.8 4.3.2.9 4.3.2.10 4.3.2.11 4.3.2.12 4.3.2.13 4.3.2.14 4.3.2.15 4.3.2.16 4.3.2.17 4.3.2.18 4.3.2.19 4.3.2.20 4.3.2.21 4.3.2.22 4.3.2.23 4.3.2.24 4.3.2.25 4.3.2.26 4.3.2.27 4.3.2.28 4.3.2.29 4.3.2.30 4.3.2.31 4.3.2.32 4.3.2.33 4.3.3 4.4 4.5 4.6

Contents Data: Adsorbate-induced changes of substrate properties Surface structure on metals and semiconductors (M.A. VAN HOVE, K. HERMANN, P.R. WATSON) ................................................. see subvolume III/42A2 Electron work function of metals and semiconductors (K. JAKOBI) ... see subvolume III/42A2 Adsorbate induced surface core level shifts of metals (R. DENECKE, N. MǺRTENSSON) ....................................................................................................................................................388 Introduction ..........................................................................................................................................................................................................388 Data section ..........................................................................................................................................................................................................396 Al(001) ......................................................................................................................................................................................................................397 Al(111) ......................................................................................................................................................................................................................398 Ni(100) ......................................................................................................................................................................................................................398 Mo(110) ...................................................................................................................................................................................................................399 Ru(0001) .................................................................................................................................................................................................................399 Ru (10 1 0) ..............................................................................................................................................................................................................400 Rh(100).....................................................................................................................................................................................................................401 Rh(110).....................................................................................................................................................................................................................401 Rh(111).....................................................................................................................................................................................................................401 Stepped Rh surfaces ....................................................................................................................................................................................403 Pd(100) .....................................................................................................................................................................................................................403 Pd(110) .....................................................................................................................................................................................................................404 Pd(111) .....................................................................................................................................................................................................................405 Ta(100) .....................................................................................................................................................................................................................406 Ta(110) .....................................................................................................................................................................................................................406 Ta(111) .....................................................................................................................................................................................................................407 Ta (poly) ..................................................................................................................................................................................................................407 W(100) ......................................................................................................................................................................................................................407 W(110) ......................................................................................................................................................................................................................409 W(111) ......................................................................................................................................................................................................................411 W(320) and other stepped W .............................................................................................................................................................411 W (poly)...................................................................................................................................................................................................................412 Os(0001)..................................................................................................................................................................................................................413 Ir(100) ........................................................................................................................................................................................................................413 Ir(110) ........................................................................................................................................................................................................................413 Ir(332) ........................................................................................................................................................................................................................413 Pt(110) .......................................................................................................................................................................................................................414 Pt(111) .......................................................................................................................................................................................................................414 Stepped Pt surfaces.......................................................................................................................................................................................416 Au(100) ....................................................................................................................................................................................................................416 Au(110) ....................................................................................................................................................................................................................416 Au(111) ....................................................................................................................................................................................................................417 Au (poly) .................................................................................................................................................................................................................417 References .............................................................................................................................................................................................................418 Surface free energy and surface stress (D. SANDER, H. IBACH) ............... see subvolume III/42A2 Surface phonon dispersion (M.A. ROCCA) ........................ see subvolume III/42A2 Surface optical properties (N. ESSER, W. RICHTER) ................ see subvolume III/42A5

Erratum ...........................................................................................................................................................................................................................................................422

2

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

3.4.2 Adsorption of C, N, and O on metal surfaces H. OVER 3.4.2.1 Introduction 3.4.2.1.1 General remarks I would like to start with some general remarks about the completeness of the data presented in this chapter. The amount of data reported in the literature about the properties of adsorbed O, N, and C layers on metal surfaces is hardly tractable on a reasonable time scale, so that the present chapter is inevitably incomplete. This is particularly the case for the electronic properties for which a last comprehensive compilation of data goes back to 1982 [82W1]. The presented tables should rather serve as a first introduction into the wealth of literature about this topic from which the reader may start a more exhaustive literature research. General trends and properties of the adsorbates O, N, and C among the metal surfaces precede each subsection. 3.4.2.1.2 List of acronyms Acronym AES APS ARPES ARSIMS ARUPS b BE c.t. 1O CEM Cluster Cluster DFT DFT-GGA disp. DLEED E(E)LS Eact EELFS EELS Ei ELS EMT ESS EXAFS EXELFS expos. FES FFAK FLAPW FP-LAPW HeD HEIS

Explanation Auger-electron spectroscopy appearance potential spectroscopy angle-resolved photoemission spectroscopy. angle resolved secondary ion mass spectrometry angle-resolved ultraviolet photoemission spectroscopy bulk binding energy coordinated to one O atom corrected effective medium calculations cluster calculations cluster calculations (in contrast to slab calculations) density functional theory calculations DFT-generalized gradient approximation dispersion diffuse low energy electron diffraction electron (energy) loss spectroscopy. activation energy electron energy loss fine structure electron energy loss spectroscopy impact energy of the incident molecule beam electron loss spectroscopy. effective medium theory equilibrium segregation study extended X-Ray absorption fine structure extended electron energy loss fine structure exposure forward-electron scattering. forward focusing of Auger and Kikuchi electrons full potential linearized augmented plane wave method full potential linear augmented plane wave method He diffraction high energy ion scattering Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces

HRCLS HREELS ICISS ID IHA IPE IPES ISS KRIPES KW L LEED LEIS LT phase MCS MC MDS MEIS ML Mol. Beam: MR MS NDRS NRA

high resolution core level shifts high resolution energy electron loss spectroscopy impact-collision ion scattering spectroscopy isothermal desorption isosteric heat of adsorption inverse photoemission: energies given in eV above EF inverse photoemission spectroscopy ion scattering spectroscopy k-resolved inverse photoemission spectroscopy King Wells method [74K1] gas exposure in Langmuir (1 L = 1.33 10-6 mbar s) low energy electron diffraction low energy ion scattering low temperature phase Monte-Carlo simulations micro calorimetry metastable deexcitation spectroscopy. middle energy Ion scattering monolayer molecular beam according to the King-Wells method missing row reconstruction meta-stable phase negative direct recoil spectroscopy nuclear resonance analysis frequency factor photoemission spectroscopy photoelectron diffraction Raman spectroscopy Rutherford backscattering spectroscopy raster electron microscope reflected high energy electron diffraction room temperature surface core level self consistent linearized augmented plane-wave method surface core level shift surface enhanced Raman scattering surface extended X-ray-absorption fine structure secondary ion mass spectroscopy spot profile analyzing LEED surface soft-X-ray absorption scanning tunneling microscopy STM-light emission spectroscopy substitutional subsurface soft-X-ray emission spectroscopy surface X-ray diffraction standing X-ray wave critical temperature (order-disorder transition) thermal desorption spectroscopy thermal energy atom scattering time-of-flight scattering and recoiling spectrometry. ultra high vacuum

νD

PES PhD Raman RBS REM RHEED RT SCL SC-LAPW SCLS SERS SEXAFS SIMS SPALEED SSXA STM STM-LES sub subs. SXES SXRD SXW Tc TDS TEAS TOF-SARS UHV

Landolt-Börnstein New Series III/42A4

3

4

3.4.2 Adsorption of C, N, and O on metal surfaces

UPS XAS XPD XPS ∆Φ

ultraviolet photoemission spectroscopy X-ray absorption spectroscopy X-ray photoelectron diffraction X-ray photoemission spectroscopy workfunction change

[Ref. p. 55

3.4.2.2 Oxygen adsorption on metal surfaces Sources of oxygen used in UHV experiments are molecular oxygen O2 (the most frequently used source), nitrogen oxides N2O and NO2, atomic O (produced by glow discharge plasma) and ozone O3. These latter three sources are used to produce high-O-coverages even under UHV conditions. Care has to be taken since NO2 and O3 are strongly oxidizing agents which attack even the gaskets of the UHV chamber and the oil in the back pumping system. The interplay of chemisorption, subsurface diffusion and oxidation governs the transformation from a metallic to an oxide material and can be monitored with surface sensitive methods. The most weakly bound oxygen species on metal surfaces is the physisorbed oxygen. This kind of oxygen reveals structural, vibrational and electronic properties that are very close to those of gaseous oxygen. On the other hand, chemisorbed molecular oxygen (superoxo- and peroxo species) is bound by about 0.7-1.0 eV, as observed for instance on Pt(111) [98N3] (and references therein) and Ag(110) [96G1] (and references therein). In this section we concentrate on the properties of chemisorbed (atomic) oxygen on metal surfaces. Chemisorbed (atomic) oxygen needs dissociation of molecular oxygen prior to the adsorption (for dissociative sticking coefficient see Table 1), establishing a strong bond between the atomic oxygen species and the metal surface (see Tables 2 and 3). Details about the adsorption of molecular oxygen on metal surfaces can be found in section 3.7.2. In general the bond strength of chemisorbed atomic oxygen on the metal surface is substantially higher than the binding energy of oxygen in corresponding metal oxides; typical values for chemisorbed oxygen are 5 - 10 eV. Typical experiments in surface science (UHV conditions) are far from thermal equilibrium with the surrounding gas phase. The following scenario is therefore typical for UHV experiments. Beyond a critical coverage of on-surface oxygen, the binding energy of oxygen on the surface is lower than of oxygen accommodated in the selvedge region of the metal surface. Consequently O penetrates into the subsurface region or even dissolves into the bulk for energy reasons. Finally, oxide formation takes place on the metal surface. Oxygen adsorption on metal surfaces plays a crucial role in the oxidation reaction of molecules over metal catalysts (such as the CO oxidation reaction and the partial oxidation of organic molecules) whose efficiency varies widely with the oxygen coverage on the surface. This variation in catalytic activity is attributed to the dependence of the oxygen adsorption energy which determines predominantly the activation barrier for simple oxidation reactions. In turn, the binding energy of oxygen to the underlying catalyst surface is a function of the mutual interaction among the adsorbed O atoms and depends on the actual configuration of the surface [79E2, 77B7, 81S2, 98O1]. For instance, beyond a critical coverage several metal surfaces allow O penetration and diffusion into the bulk region which eventually may result in the formation of a metal oxide. There are some (rare) exceptions from this general tendency: For Zr, Ti and Ta subsurface O is more stable than on-surface oxygen. These variations in the binding energy of oxygen affect directly the catalytic activity. Catalysis by transition-metal surfaces exhibits characteristic trends across the periodic table whereby metals that form chemical bonds of intermediate strength have the highest activity. The strength of the Ometal bonding is frequently related to its propensity to dissociate molecular oxygen on metal surface. For instance the O-Ag bonding is much weaker than on Ru or Ti, and also the dissociative sticking coefficient is much smaller on Ag than on Ru or Ti (see Table 1). An exception to this general rule is aluminum: Although the O-Al bonding is strong, dissociative sticking is very low (see Table 1). The reason is that the missing d-electron density of Al does not allow for high dissociation probability while the s-electron density causes a strong bonding. For a more thorough discussion of this effect, the reader is referred to [95J1]. Norskov et al explain the binding energy of oxygen among the transition metals to be related to Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces

5

the position of the d-band center (see Fig. 1). To reach high activity on metal surfaces, a low O-metal bonding has to be balanced against the simultaneous reduction in the dissociative sticking probability. This is accomplished with transition metals that bind atomic oxygen moderately strong (so called Sabatier Principle). 0

O chemisorption potential energy rel.to ½ O 2 [eV ]

-2

-4

-6

-8

Zr

Nb

Mo

Tc

Ru

Rh

Pd

Ag

0 -2

Fig. 1. Calculated and experimental values of the binding energy of adsorbed O atoms on various transition metals are indicated along the 5th row in the Periodic Table. Also shown are the O binding energies as a function of the energetic position of the d-band center; [00H1].

-4

Simple model DFT - GGA Exp. (polycryst.)

-6

-8

-4

0 -2 d - band center ed [ eV ]

2

Molecular beam techniques have become a powerful tool to study the dynamics of dissociative chemisorption of molecules, such as oxygen, on well-characterized single crystalline surfaces in UHV (see Table 1). The advantage of using molecular beams is that kinetic and vibrational energy of the impinging gas molecules are well-defined and can be controlled. Many of these investigations have indicated that dissociative chemisorption occurs mainly through two different mechanisms: a direct dissociative mechanism and a precursor-mediated mechanism [97D2]. If the translational energy of the incident O2 molecule promotes the dissociative chemisorption of a molecule at a surface then such a mechanism has been termed direct dissociation. However, it is also possible that kinetic energy may assist in surmounting barriers to molecularly chemisorbed surface states as well (a direct molecular chemisorption mechanism) which then serve as precursors to dissociation. According to [97D2] the general trend is as follows: Systems demonstrating molecularly chemisorbed states which are stable at low temperatures and coverages appear most consistent with dissociation mechanisms involving direct molecular chemisorption for incident energies up to few electron volts. These molecular states have been identified by HREELS and NEXAFS (see for example 98N2). Specific values for the dissociative sticking coefficient and other details about the dissociation process are compiled in Table 1. In general, the dissociative sticking coefficient depends strongly on the impact energy of the incident O2 molecules. This property is illustrated in Fig. 2 for various close-packed metal surfaces. In Fig. 3 we show an example for the dissociative sticking coefficient of O2 on various orientations of Ni as a function of the adsorbed O coverage [97S1].

Landolt-Börnstein New Series III/42A4

6

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

1.0

Initial adsorption probability S 0

O 2 / Ru (001) 0.8 O 2 / Ir (111)

0.6

0.4 O 2 / Pt (111) 0.2 N 2 / W (100) 0

0.2

0.4 0.8 0.6 1.0 Kinetic energy E i [eV]

1.2

1.4

Fig. 2. Measurements of the initial adsorption probability S0 versus kinetic energy Ei for O2 on Ru(001) (filled squares: 77 K, open squares: 500 K); for O2 on Ir(111) (filled discs: 77 K, open discs: 425 K); for O2 on Pt(111) (filled triangles: 200 K, open triangles: 350 K); and for N2 on W(100) at 300 K; [97D2].

0.8 Ni {110} Ni {100} Ni {111}

Sticking probability r0

0.6

0.4

0.2

0

0.1

0.2 0.3 0.4 0.5 0.6 Oxygen atom coverage [ Ni ML ]

0.7

Fig. 3. Dissociative oxygen sticking probability on Ni{100}, Ni{110} and Ni{111} at 300 K in the low Ocoverage region; [97S1]. A thermal molecular beam at room temperature was used.

Other experiments, which do not use molecular beam techniques, introduce the O2 gas through a leak valve. The introduced oxygen gas is at room temperature with a Maxwellian energy distribution for the kinetic energy. Thus the experimental value for the dissociative sticking probability provides an energy averaged value. In general, these values are closer to the reality in catalytic reactions than those obtain by molecular beam experiments. The binding of O atoms to metal surfaces is prevalently covalent. It encompasses two contributions, one is coming from the coupling of O(2p) to the metal s states, and the other is due to the extra coupling to the d-states. Since the contribution from the metal s states to the O-metal bonding is approximately the same for all late transition metals, the main trends in the chemisorption energy is given by the interaction with d electrons (see Fig. 1). The coupling of the localized d states gives rise to a bonding and an antibonding state (tight binding argument). As we move from Cu, Ag, or Au to the left in the Periodic Table, the d-bands move up in energy, and progressively more anti-bonding adsorbate-metal d states become empty. For Cu, Ag, and Au the anti-bonding states are completely filled because the d-bands are well below the Fermi level. The variation in the adsorption strength from Cu, Ag, and Au is determined by the Pauli repulsion between O(2p) and the completely filled d-states. Au has the most extended d states, and therefore the strongest repulsion. This explains why Au is the noblest metal among Cu, Ag, and Au [95H2]. With oxygen in metal oxides the contribution of ionic bonding becomes more important than in the chemisorbed phase of oxygen on metal surfaces. Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces

7

The chemisorbed O species is mostly covalently bound to metal surfaces, as characterized by typical XPS values of O(1s) appearing at 531.5 eV. In the valence band region, the peaks characteristic for atomic oxygen are located at about 6.5 eV below the Fermi level (EF) (see Table 4). This energy position for the O(2p) derived emission from adsorbed atomic oxygen is typical for most transition metals [78K1]. The emission of molecular surface oxygen is centered at 8 eV below EF and has a remarkable width (FWHM) of 4.5 eV.

Cu (110)

I I

II

Ag (110)

I II

I I Intensity

Ni (111)

I

I II I

Pd (111)

I II Pt (100)

Ir (111)

Fig. 4. Angle-integrated UPS Spectra (hν = 40.8 eV) from various oxygen covered transition metal surfaces. The spectrum for Ag(110) originates from the work by Bradshaw et al. [74B1], while all other spectra are taken from the work by Küppers and Ertl [78K1]. The bars under the spectra denote one-electron multiplet energies as calculated by Doyen and Ertl [78D1]. The presentation is taken from [82W1]. O(2p) derived emission appears at about 6 eV.

12 8 EF 4 F Binding energy rel.to Femi level E B [eV ]

In Fig. 4, we show angle-integrated UPS spectra from various oxygen covered transition metal surfaces. ARUPS data have established the energetic splitting of both levels into O(2pz) derived σ, σ* and O(2px,y) derived π, π* states. On transition metals with high density of d-states at EF only the bonding molecular orbitals (MO) are filled and show up in UPS, whereas the anti-bonding MO's are located above EF and are empty. As shown by Hammer and Norskov [00H1] this MO scheme with bonding and antibonding orbitals is of general validity for the oxygen-metal chemisorption bond. Subsequent oxygen incorporation and incipient oxidation is accompanied by a substantial redistribution of the valence band emission. The O(1s) emission changes from 531.5 eV to 529.5 eV, which is largely independent of the substrate metal [82W1]. In the chemisorbed oxygen phases on metal surface, the surface core level shifts of the metal atoms are linearly related to the number of O atoms coordinated to it, see for example the systems O-W(110) [98R1] and O-Ru(0001) [01O1]. Typically, the core levels of surface metal atoms shift to lower values with increasing O-coordination (see Table 4 and compare also section 4.3). The oxygen against metal vibrations are in the energy range of 40 meV to 90 meV, a typically value is 60 meV (see Table 5). The local adsorption geometry of oxygen on metal surface (see Table 6) can roughly be divided into non-reconstructive adsorption, which frequently occurs on low-index surfaces, and reconstructive adsorption. The adsorption site found for oxygen is in good agreement with the general tendency that adsorption of chalcogen atoms on high density transition metal surfaces takes place at those adsorption sites with the highest coordination number and also on which an additional metal layer would have grown. Marcus et al. [75M2] advocated first this empirical law.

Landolt-Börnstein New Series III/42A4

8

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

The general trend of atomic adsorbates to chemisorb in high-coordinated sites was later corroborated by Effective Medium Theory (EMT) considerations. According to this theory, which has been put forward by Nørskov and coworkers [90N2], the actual bonding configuration of the adsorbed oxygen atom is the result of a delicate balance of reaching the optimum electron density offered by the metal substrate surface and minimizing the Pauli repulsion between the metal and the adatom charge density. If the adatom is coordinated to many metal atoms the optimum electron density is provided by a larger bond-lengths that minimizes the Pauli repulsion, and thus making high-coordination adsorption sites favorable. Some early transition metals behave quite differently. With the O-Zr(0001) system we are faced with the interesting situation where only after a critical O coverage is accommodated below the surface, onsurface oxygen becomes stabilized, while for other metal surfaces, a critical on-surface O coverage is needed for the commencement of oxygen penetration into the sub-surface region [95W1, 95W2, 97W1]. In general the heat of adsorption does not vary very much from one surface plane to another [79E3] so that values found for polycrystalline samples serve (at least) as a guideline (see Fig. 5).

Heat of adsorption [kcal / mol]

260

O2

240 220 200 180 160 140 120 100 80 60 40 20 0

a Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn La Hf Ta W Re Os Ir Pt Au Hg Tl Pb

220

O2

Heat of adsorption [kcal / mol]

200 180 100

160

110 100 100

140 120 100

100 110

80 60 40 20

b

110 110 111 111 100 110 100 110 111

111

0

Fig. 5. (a) Heats of adsorption of O2 on polycrystalline transition metal surfaces. (b) Heats of adsorption of oxygen on various single crystal surfaces of transition metals; [94S4]. These are TDS data.

Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn La Hf Ta W Re Os Ir Pt Au Hg Tl Pb

Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces

9

In a naive picture one would assume that the strength of the chemisorption bond increases as the number of “unsaturated” valencies of surface atoms increases, i.e. if the coordination number of the metal surface atoms decreases. This would imply a lower adsorption energy at the most densely packed planes compared to planes with higher Miller indices. A more elaborated view was put forward by Hammer and Nørskov [00H1]. From thermal desorption experiments it is known that with increasing O coverage the activation energy for desorption (see Table 2) and therefore the O-metal bonding (see Table 3) weakens as the result of a net repulsion among the O atoms. This observation is exemplified with oxygen adsorption on various orientations of Ni surfaces (see Fig. 6). 600

Ni {110} Ni {111} Ni {100}

Adsrption heat [kJ / mol ]

500 400 300 200 100

0

1 2 3 Apparent oxygen atom coverage [ Ni ML ]

4

Fig. 6. Caloric heats of adsorption of O2 as a function of coverage in the oxide film formation region for all three low index Ni surfaces; [98B2].

Yet the O-metal bond-lengths do not follow this general trend [98O1]. In most of the cases, the Ometal bond-length remains constant or even decreases slightly with increasing O coverage as demonstrated with Ru(0001) and Ni(111). The effective O radii, which are derived from the value for the O-metal bond-length, slightly exceed the covalent radius by 0.1 Å. This is consistent with a comparatively small net charge transfer from the substrate to the oxygen adatom and a prevalently covalent bonding. The effective O radius increases also with the coordination number. Varying the coordination from threefold to fourfold results in an increased O radius of about 0.1 Å (cf. Ni, Pd, and Rh, Table 6). The adsorption energy of oxygen increases only slightly by a few tenth of an eV (see Table 3), when increasing the coordination number, e.g. from three to four, such as with fcc(111) and fcc(100). It is interesting to note that TD data (as collected in Table 2) are quite difficult to find by a literature research and most of the derived values of the heat of adsorption are not very reliable. The reason is that during the heating ramp of a typical TD experiment not only desorption takes place but also bulk dissolution, phase transition, oxide formation, reaction with other adsorbed species etc.. For instance for aluminum, oxygen desorption is not possible to measure since the O-Al bonding is so strong that Al will melt before O2 desorption can take place. For hexagonal cobalt oxygen desorption takes place at temperatures where Co transforms from hcp to fcc lattice. The interaction between O atoms has partly electrostatic origin in that the O induced dipoles interact with each other. The chemisorption-induced dipoles of on-surface oxygen cause in general an increase of the workfunction (see section 4.2), while subsurface O decreases the work function. If O atoms come to close to each other direct orbital overlap may occur, which could lead to O2 formation with subsequent desorption. Similar to many other chalcogens, oxygen atoms try to prevent a situation where O atoms have to share a common metal atom [98S1]. A very important type of interaction among the O atoms is indirect through their bonds with the metal surface. This interaction exhibits an oscillatory character, i.e. it may be attractive or repulsive depending on the mutual separation. It decays within distances of two to three lattice constants to values below kT. The indirect interaction is considered to be crucial for the development of ordered oxygen layers on metal surfaces (see Table 7). In order to form ordered overlayers the mobility of the O atoms has to be high Landolt-Börnstein New Series III/42A4

10

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

enough to reach the thermodynamical stable configuration. The diffusion barrier determines the mobility of O atoms across the surface. Their values are about one order of magnitude smaller than the strength of the chemisorption bond itself. The interaction among the O atoms in an ordered phase manifests itself in the formation of 2D-band structures, which can be identified for instance with ARUPS even in the case of a (1×1) overlayer. On more open surfaces, such as the fcc(100), disordered O-overlayers are more frequently observed than on densely-packed surfaces, e.g fcc(111). This is presumably due to the higher activation barrier for O diffusion on fcc(100) compared to fcc(111). Evidently the energy of the system depends on the mutual configuration of the adsorbed particles and therefore no longer the configuration with maximum entropy (equal to random distribution) will characterize the equilibrium. As a result long-range order may occur, depending on the interaction between the O atoms and the thermal energy kT. The adsorbate system may be treated with the methods of statistical thermodynamics. At finite temperatures the statistical properties of adsorbate systems may be described by two-dimensional models [76B1], such as the Ising model or the 3-state, 4-state Potts models, leading to an order-disorder transition at a critical temperature (see Table 8). A comprehensive collection of experimental phase diagrams are indicated in Figs. 7 - 10. 700

500

c O / Mo (110)

2.order

p (2×2) antiphase domains + liquid

500

Temperature T [K ]

Temperature T [K ]

gas

p (2×2)

1.order

gas

200

400 complex structures

0

0.1

0.2 Coverage q [ML]

(Ö3×Ö3) R 30°

p (2×2) + gas

p (2×2) + gas 300

p (2×2)

300

p (2×2) antiphase domains

400

600

0.3

0.4

Fig. 7. Phase diagram for O-Mo(110); [86W2]. The dashed lines are extrapolations from LEED measurements.

100 0.10

0.15

0.20 0.25 Coverage q [ML]

0.30

0.35

Fig. 8. Coverage versus temperature phase diagram for O-Ni(111); [81K1].

Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces LG+ p (2×2)

LG+ p(1×1)

11

Desorption states α none

800 LG+ p (2×2)

α+β

α+β+γ

Desorption onset

700 p (2×2)

p (1×1)

1

LG 600 p (2×1)

p (2×1) +p (2×2)

p (2×2) +p (1×1)

Temperature T [K ]

Temperature T [K ]

700

600

Disorder and dissolution

3

5 c (2×2)

p (2×2) + c (2×2)

500

500 2

400

0

0.4 0.6 Coverage q [ML]

0.2

p (2×2)

400

p (2×1)+LG

0.8

1.0

Fig. 9. Phase diagram for O-W(110). LG denotes “lattice gas”; [89W2].

0

0.1

6

p (5×5) + c (2×2)

4

7

p (5×5) + p (2×2) + c (2×2)

p (5×5)

0.3 0.4 0.2 Coverage q [ML]

0.5

0.6

Fig. 10. Phase diagram fro the system oxygen/Pd(100). Solid lines are used to connect data points; dashed lines are assumed or possible boundaries; [88C1].

Since the chemisorption strength of O on metal surfaces is quite high, the adsorption is accompanied by substantial reconstructions of the metal surface (see section 4.1), either locally [94S3] or with mass transport involved [93B1, 96T1, 98C1]. Prominent examples of the latter class are O-induced added row reconstructions on the fcc(110) surface of Ni, Pt, Rh, Pd, Cu, and Ag. The reconstruction is driven by the prospect to form stronger O-metal bonds; this tendency is facilitated by soft metals, which exhibit relatively weak metal-metal bonding. The added row reconstruction on Cu(110) has been considered as the first step towards oxidation [95L2]. The preference of oxygen to bind to low-coordinated metal atoms was explained in the framework of EMT [90N2]. Thus, oxygen chemisorption in long-bridge sites on a (2×1) added row reconstructed Cu(110) surface [90C2] (see Fig. 11) becomes energetically more favorable than adsorption in the first Cu layer of the unreconstructed surface, which overcompensates even the cost of breaking metal-metal bonds.

Fig. 11. The added row (or missing row) structure induced by oxygen adsorption on Cu(110), Ni(110) and Ag(110); [96O2]. The small black balls represent the oxygen atoms. The big hatched and white balls represent the substrate atoms of the first and second substrate layer, respectively. The arrows indicate the growth direction of the metal-O rows.

[001]

[110]

Note that the coordination of the outermost Cu atoms on the (2×1) missing row reconstructed surface is reduced from six (ideal (110) surface) to four and oxygen in long-bridge sites allows oxygen to bind to two under-coordinated metal atoms.

Landolt-Börnstein New Series III/42A4

12

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

Rh(110)-(2×2)p2mg-2O, q = 0.5, 1×2 missing row

Rh(110)-c(2×6)-8O, q = 0.67, 1×3 missing row

Rh(110)-c(2×8)-12O, q = 0.75, 1×4 missing row

Rh(110)-(2×1)p2mg-2O, q = 1.0

Fig. 12. The stable O-phases on Rh(110) as they develop with increasing O-coverage. Except the (2×1)p2mg-2O structure, all other phases are characterized by a missing row type reconstruction of the underlying Rh(110) surface; [98O1]. The small black balls represent the oxygen atoms. The big white, grey and dark grey balls represent the substrate atoms of the first, second and third substrate layer, respectively.

On Rh(110) oxygen adsorption induces the (1×n) missing row reconstruction (see Fig. 12), where densely packed Rh-rows are completely missing. Oxygen adsorption proceeds then on the (1×2) troughs in quasi-threefold sites instead of (1×1) troughs. Oxygen atoms are attached to two Rh atoms in the topmost layer and one Rh atom in the second. This site preference is explained by the propensity of oxygen atoms to bind to lower coordinated metal atoms. For steric reasons the oxygen does not form a (n×1) added row reconstruction on Rh(110) because the topmost Rh-Rh separation along the [001] direction is too small to allow the O atoms to be incorporated [98C1].

Landolt-Börnstein New Series III/42A4

Table 1. Sticking coefficient Substrate

Coverage

Ag(110)

initial initial initial

Ag(111)

initial initial

Ag(100)

0-0.41 ML initial initial

Impact energy Ei [eV] 0.8

0.1 >0.4 0.1 0.9 1.8

4.4×10−3

<0.8

~1 >0.3

0.8 >0.5

8×10−4 Al(110) Al(111)

Al(100) Be(0001)

initial initial initial initial initial initial initial initial initial initial

0.04 0.0045 <0.004 <0.01 0.9a) 0.04 0.0053

0.030 0.6 -2.0

Remarks

Method

Ref.

direct molecular chemisorption O2(ad) → O(ad) molecular adsorption 100 K: O2(gas) → O(ad) 500 K: O2(gas) → O(ad) 477 K RT RT

Mol. beam Mol. beam STM, backfilling

94V2 94V3 98Z1

TDS, XPS, LEED

84C2

Mol. beam Mol. beam

96R2 96R1, 95B3, 97R2, 97R3

TDS Theory Mol. beam

85C1 97Z1 96B3

RT: dissociative sticking sticking probability strongly Mol. beam decreases for Ei <0.5 eV Mol. beam Ekin = 88 meV

96R2

1.7 × higher than on Al(111)

79M2, 77M1 84B2 93B2 99Z1 97Ö1 97Ö1 84B2

direct and precursor mediated steps are important 490 K explain mol. beam of [96R1] no direct dissociation channel, molecular precursor O2(ad) → O(ad)

on defect free surfaces RT RTg)

1.4 × smaller than on Al(110) 0.008 0.1

XPS, AES XPS, AES STM, backfilling HREELS Mol. beam Mol. beam Mol. beam Ar bombardment XPS XPS TDS

90R3

3.4.2 Adsorption of C, N, and O on metal surfaces

initial

Sticking coefficient 0.63 ~1 1.7×10−3 1.9×10−3 2×10−4 10−3 - 10−2 0.1 - 0.4 10−7 10−6 10−3 10−6

Ref. p. 55]

Landolt-Börnstein New Series III/42A4

3.4.2.2.1 The dissociative sticking coefficient of oxygen on metal surfaces and its dependence on the impact energy of the incident O2 molecule, Ei

79M2 84B2 84F1

13

Coverage

Co (10 1 0)

initial

Sticking coefficient ~0.25

Co(0001)

initial initial initial 1/8 ML initial

0.3 0.6 0.8 0.08 0.65

fcc-Co(100) Cr(110) Cu(110)

0.02

initial

0.13/0.20

Ir(110)

1 ML O 0.15 ML initial initial

Ir(111)

initial

0.1 0.32 0.6 0.8 0.9 0.2 0.1 0.7

initial initial Fe(110)

Fe(111) Fe(100)

120 K, 300 K

0.050 0.050 0.5

averaged value; sticking coeff. increases with O-cov. 100 K 330 K 330 K d); direct dissociation

300 K, constant 0.050 0.365

0.020 0.800 0.050

direct and activated

Method

Ref.

∆Φ

90S1

∆Φ UPS, AES UPS, AES AES Mol. beam, trapping-mediated

79B1 82C1 77R1 86S3 93H1

Mol. beam, 93H1, 97D2 Quantum dynamics 96G2 AES Mol. beam

79H3, 82S4 93H2

Mol. beam 99M1 XPS, TDS 79H1 Quantum dynamics 96G2 RT; directly activated for Ei >150 meV, precursortrapping-dissociation for low surface temperature and low Ei.

Mol. beam

94H2

backfilling the chamber backfilling the chamber AES TDS + LEED

80P1 80P1, 76D2 00A1 84S3

extrapolated

Landolt-Börnstein New Series III/42A4

0.050 >0.5

85 - 1000 K 77 K 425 K

0.8

Mol. beam, trapping mediated Mol. beam, trans. energy 0.10 eV; trapping mediated

95K1 97D1

[Ref. p. 55

c(2×2)O

Cu(111) Cu(001)

0.3 0.8 0 - 0.3 ML initial

Remarks

3.4.2 Adsorption of C, N, and O on metal surfaces

initial

0.25 0.8 a), b) 0.050 0.8 0.001 0.04 0.8 a), f) 0.03 0.02 0.6 0.3

Impact energy Ei [eV]

14

Substrate

initial initial initial initial initial 0.33 ML initial initial 0 - 0.33 ML 0.33 - 0.5 ML 0.5 - 0.8 ML 0.8 - 1.0 ML initial initial initial initial initial 0 - 0.3 ML >0.5 ML initial

0.24 0.25 0.50 0.10 ~0.70 ~1.0 1.0 0.87 0.65 0.50 0.23 0.78 0.63 1.0 0.95 ~0.8 →0 0.65 → 0.4

initial 0.05 ML 0 - 0.25 ML initial initial

0.1 0.87 0 0.3 0.75 0.50

Ir(100)-(1×1) Mo(110) stepped (110) Mo(111) Mo(100)

Ni(110) Ni(111) Ni(100) Pd(110)

Pd(111)

Impact energy Ei [eV]

Remarks

Method

Ref.

425 K

Mol. beam TDS

98A1 79K1

Mol. beam TDS AES, TDS, LEED

98A1 79K1 83B2

XPS AES, ∆Φ

85M1 79B2

Mol. beam Mol. beam Mol. beam TDS, LEED TDS

97S1 97S1 97S1 74H1 99Y2

Mol. Beam

98S2, 98N1

TPD, AES

89G1

TPD, LEED, UPS Mol. beam, STM

77C2 01K1

425 K

step distance 25 Å sticking coeff. is constant

473 K sticking coeff. is fairly constant 100 K → 650 K; Direct molecular chemisorption; physisorbed state is precursor of the chemisorbed molecules and these again are precursors for the dissociated O species; precursor conversion from peroxide to atomic O: Energy barrier 0.32 eV. 300 K

323 K 623 K

15

Ir(100)-(1×5)

Sticking coefficient 0.10 0.05

3.4.2 Adsorption of C, N, and O on metal surfaces

Coverage

Ref. p. 55]

Landolt-Börnstein New Series III/42A4

Substrate

Coverage

Pd(100)

0.25 ML 0.5 ML

Pt(110)-(2×1)

initial initial initial

>0.25 ML initial initial

0.3 0.4 0.55 0.42 0.24 0.03 0.3

Pt(100)-(1×1)

initial

Re (10 1 0)

0 - 0.5 ML

0.53

Rh(110)

Ru (10 1 0)

initial initial initial initial initial >0.5 ML <0.8 ML <0.5 ML

0.7 0.95 ~0.5 0.6 0.74 0.04 0.8 0.4

Ru(0001)

>0.5 ML initial

0.24 0.3 0.95 a), b), c), f)

Rh(111) Rh(100)

Method

Ref.

300 K, p(2×2): 2 L O2 300 K; c(2×2): 180 L O2

TDS/LEED

93K2

300 K 300 K 170 K 300 K 600 K

TDS, AES TDS, AES TDS, AES

86F1 76D1 98W2

0.05

200 K

91R1, 88L1

0.05

350 K e); direct molecular chemisorption

Mol. beam, precursor mediated Mol. beam, precursor mediated TDS, XPS TDS, XPS Mol. beam Mol. beam Mol. beam, precursor mediated

300 K mechanism ambiguous

stick. coeff. constant and decreases then rapidly (T = 353 K) 125 K sample temperature 258 K, 573 K sample temp. 100 K; assuming saturation at 0.5 ML 110 K - 150 K 300 K 100 K; stick. coeff. constant; partially molecular

Landolt-Börnstein New Series III/42A4

0.050 >0.2

77 - 500 K d); direct dissociation

91R1 78N1 73B2, 77H2, 81C1 84N1, 94G2 97D2 84N1, 96B2 79P1

AES, LEED TDS, LEED TPD, AES Mol. beam Mol. beam

92C4 90S2 85M2 97B3 98K1

XPS, LEED, TDS LEED, TDS

83F2 77K1, 96R1

Mol. beam, trapping mediated

96W3

[Ref. p. 55

Pt(100)-hex

initial initial initial

0.1 a), c) 0.2 0.02 - 0.08 <0.003 a), b), c), f) >0.20

Remarks

Impact energy Ei [eV]

3.4.2 Adsorption of C, N, and O on metal surfaces

Pt(111)

Sticking coefficient ~0.1 ~0.003

16

Substrate

Coverage

V(111)

initial

V(100)

0 - 0.3 ML (1×5)O

W(110)

initial 0.33 ML 0.5 ML 0 - 0.5 ML 0 - 0.25 ML 0.25 - 0.5 ML 0.5 - 0.75 ML 0.75 - 1.0 ML

Zr (10 1 0)

0.0 - 1.0 ML <0.5 ML

Impact energy Ei [eV] 0.045 0.160 0.085

Remarks

473 K 873 K

300 K 1000 K 300 - 1350 K

d); typical values; direct dissociation 300 K 1050 K 300 K 1050 K 300 K 1050 K 300 K 1050 K linearly for T = 90, 293, 473 K up to 0.5 ML; stick. coeff. remains even high during oxidation

Ref.

Mol. beam, direct chemisorption

00B2

AES KW AES, TDS, LEED AES, TDS, LEED SCLS, UPS O(2p)

01S1 77B4 80M1 98R1, 79W2

TDS AES, TDS, LEED SCS, UPS O(2p) Mol. beam TDS, AES

75E1 77B4, 80M1 98R1 86R3 76B3, 79W2

TDS, AES LEED, NRA, AES

72M1 94Z1

17

a) =Initial adsorption probability increases with increasing kinetic energy a') =Initial adsorption probability decreases with increasing kinetic energy b) =Initial adsorption probability scales with normal kinetic energy c) = Initial adsorption probability is surface temperature dependent d) = Saturation coverage is kinetic energy dependent. e) = Molecular intermediates detected after high kinetic energy exposure f) = Initial adsorption probability increases with increasing surface temperature f') = Initial adsorption probability decreases with increasing surface temperature g) = No surface temperature dependence observed between 300 and 1000 K

Method

3.4.2 Adsorption of C, N, and O on metal surfaces

W(100)

initial initial initial

Sticking coefficient 0.95 0.58 0.75 0.40 0.60 0.28 0.38 0.55 0.35 0.43 0.19 0.05 0.2 - 1.0a), b) 1.0 1.0 1.0 0.73 0.39 0.44 0.24 0.24 0.98 - 0.05 1

Ref. p. 55]

Landolt-Börnstein New Series III/42A4

Substrate

18

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

3.4.2.2.2 The heat of adsorption of chemisorbed oxygen overlayers on metal surfaces Table 2. Heat of adsorption. substrate

surface

heat of adsorption/ activation barrier for desorption

Ref./method

Ag(110)

initial initial (8×1)O, (6×1)O, (4×1)O (2×1)O (4×4)4O initial

76E1 80B1, 96R2/TDS 95C1/TDS 99C1/TDS 96R1/TDS 60B1 01S2

0.25 ML >1.2 ML initial initial θ = 0.5 - 0.8 initial initial

335 kJ/mol 173±5 kJ/mol 163 kJ/mol w.r.t. O2 180 kJ/mol 259±16 kJ/mol 820 - 880 kJ/mol No O2 TDS available, due to phase transition of hcp-Co to fcc-Co 420 kJ/mol 727 kJ/mol Annealing to 800 K: removes 0.3 ML O 459 kJ/mol No desorption of O2 up to 1050 K 490 kJ/mol (272 − 41.8θ) kJ/mol 635 kJ/mol 802 kJ/mol 869 kJ/mol 498±5 kJ/mol 605 kJ/mol 440 - 470±15 kJ/mol 532±5 kJ/mol, that decreases rapidly with coverage 300 - 481 kJ/mol 188 kJ/mol (activation energy) 230 kJ/mol 222 kJ/mol 230 kJ/mol 800 K TD peak 750 K TD peak 160 kJ/mol 130 kJ/mol (173+34θ) kJ/mol, 280 kJ/mol 360 kJ/mol

>0.6 ML desorption

170 kJ/mol 860 K → 775 K

Ag(111) poly-Al Co(10 1 0) Co(0001) poly-Co poly-Cr Cu(110) poly-Cu Fe(110) poly-Fe Ir(111) poly-Mn poly-Mo poly-Nb Ni(110)

Ni(111) Ni(100) poly-Ni Pd(110) Pd(111)

Pd(100)

poly-Pd Pt(110) (2×1)

initial initial c(2×6): 0.8ML initial Variable O cov. initial initial initial initial initial initial initial initial initial initial c(2×4) initial

66B1 60B1 87M1 69M1 90S3/TDS 92G2 76I1/TDS 60B1/TDS 66B1/TDS 60B1/TDS 93A2/MC 91B3/MC 93A2/MC 93A2/MC 60W1, 60B1 99Y2/TDS 99Y2/TDS 89G1/TDS 77C2/TDS 90B1/TDS 90B1/TDS 88C1/TDS 93K2 93K2 60B1 96W2/MC 96W2/MC 77W2/TDS

Landolt-Börnstein New Series III/42A4

Ref. p. 55]

substrate

3.4.2 Adsorption of C, N, and O on metal surfaces

surface

Pt(111) 0.75 ML 0→0.75 ML

Pt(100) poly-Pt Rh(110)

0 → 0.25 ML 0.04 → 0.25 ML 0 → saturation (3×1), >0.3 ML 0.13 - 0.27 ML initial

(2×2)O, 0.5 ML c(2×8)O, 0.95 ML Rh(111)

Rh(001)

poly-Rh Ru(0001) poly-Ta poly-Ti W(110) W(111) W(100)

poly-W poly-Y

initial β1 (1250 K) β2 (920 K) β3 (820 K) >0.6 ML initial (2×2)O (2×1)O initial initial 0.1 ML 0 - 1 ML 0 - 1 ML 0.25 L O2 2.5 L O2 0 - 0.3 ML initial initial

19

heat of adsorption/ activation barrier for desorption

Ref./method

213 - 176 kJ/mol 4 desorption states at 800, 720, 690, 570 K 184 kJ/mol → 110 kJ/mol BE = 3.26 → 2.5 eV 192 kJ/mol → 154 kJ/mol 232±36 kJ/mol 208 kJ/mol − 13.5 kJ/mol.θ/θsat 260 kJ/mol 160 kJ/mol, (3×1) → hex 288 - 301 kJ/mol Several desorption states: β1 – β5 β5 (1150 K) 294±35 kJ/mol, β4 (1095 K) 280±10 kJ/mol, β3 (909 K) 234±10 kJ/mol, β2 (835 K) 215±10 kJ/mol, β1 (797 K) 205±10 kJ/mol 300 kJ/mol, Rh-O binding: 395 kJ/mol 205 kJ/mol, Rh-O binding: 348 kJ/mol Desorption state β β (1200 K) 235±10 kJ/mol β (700 K, 900 K) 235±10 kJ/mol 355 kJ/mol 386 kJ/mol 360 kJ/mol 260 kJ/mol 210 kJ/mol 110 kJ/mol 486 kJ/mol 400 kJ/mol 315 kJ/mol 886 kJ/mol 986 kJ/mol 965 kJ/mol 656 kJ/mol - 482 kJ/mol 627 kJ/mol - 473 kJ/mol 550 kJ/mol 492 kJ/mol 489 kJ/mol - 579 kJ/mol 878 kJ/mol 1107 kJ/mol

81C1 89P1/TDS 89P1/TDS 81C1 84D1/IHA 99K1/TDS 84G3/ID 84N1/ID 60B1 90S1/ TDS (10 K/s)

92C4/TDS 92C4/TDS 79T1/TDS (24 K/s) 95P1/TDS (27 K/s) 83R1 98K1/ MC 83F2/TDS (8.5 K/s) 98K1/MC 60B1 85S2 60B1 60B1 75E1/TDS 75B3/ID 75B3/ID 76B3/TDS 76B3/TDS 75B3/ID 66B1 88C1

Remark: Adsorption micro calorimetry in surface science studies: The reader can find calorimetrically measured molar heats of adsorption of gaseous adsorbates on wires and ribbons, which are not included in this table [96C2].

Landolt-Börnstein New Series III/42A4

20

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

3.4.2.2.3 Oxygen-metal bond strength (ab initio calculations) Table 3. The O-metal bond strength as computed by ab-initio calculations.

substrate Ag(110) Ag(111) Ag(100) Al(111)

surface

O-metal bond strength

Ref./method

3.25 eV w.r.t. O overlayer: 1.0 eV 3.32 eV w.r.t. O 8.0 eV 8.5 eV, (1×1)O island growth; strong bonding due to O2 px,y 7.16 eV (2×2)O, 7.32 eV (2×1)O, 7.44 eV (2×2)3O, 7.63 eV (1×1)O 7.0 eV overlayer: −1.0 eV; not stable w.r.t. O2

94R1/cluster 00H1/DFT 94R2/cluster 95J1 95J1 01K2/DFT-GGA

5.20 eV 2.08 eV 2.84 eV, 3-fold hollow overlayer: 2.0 eV 4.56 eV w.r.t. atomic oxygen

(1×1)O (2×2)O (2×2)O (2×2)O low cov. single O (2×2)O low cov. single O (2×2)mgO

5.0 eV, Fe-O: 2.57 Å 4.35 eV O-overlayer: 5.1 eV 5.2 eV 4.77 eV 4.98 eV 5.9 eV 5.03 eV 5.60 eV 6.7 eV 4.36 eV (1.26 eV w.r.t O2)

94R1/cluster 98F2/DFT 97L1/cluster 00H1/DFT 01Z1/DFT 94R2/cluster 93B4/cluster 97B2/DFT 00H1/theory 00H1/theory 99H1/DFT 92S2/cluster 97L1/cluster 99H1/DFT 92S2/cluster 93G2/cluster 02H1/DFT

(2×2)-O (√3×√3)-O c(2×2)-O (√3×√3)2O (1×1)O single O (2×2)-O c(2×2)O (1×1)O (2×2)O (2×2)O (2×2)O single O single O (2×2)O

4.15 eV/3.53 eV 4.15 eV 3.8 eV 3.6 eV 3.1 eV 3.99 eV 4.2 eV/3.53 eV 3.8 eV 2.2 eV 2.75 eV 4.26 eV 5.51 eV w.r.t. O atom; energy diff. hcp-fcc: 0.5 eV 3.14 eV, 2.61 eV 2.5 - 2.7 eV 3.97 eV w.r.t. atomic oxygen

98L1/99H1/DFT 98L1 98L1 98L1 98L1 97L1/cluster 98L1/99H1/DFT 98L1 98L1 97M1 00L2/DFT 97F2,97F3 97L1, 97C3/cluster 96I1/cluster 01Z1/DFT-slab

(2×2)O initial (2×2)O (1×1)O 0.25 - 1ML

Al(100) Au(111)

initial (2×2)O

Cu(110)

initial (2×1)O single O (2×2)O (2×2)O initial

Cu(111)

Cu(100) Fe(100) Mg(0001) Mo(110) Nb(110) Ni(111)

Ni(100)

Pd(110)(1×2) Pd(111)

Pd(100)

Pt(111)

98D3/cluster 00H1/DFT

Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces

21

substrate

surface

O-metal bond strength

Ref./method

Pt(100)(1×1)

0.25 ML 0.50 ML 1.0 ML (2×2)pg (2×2)O: (2×1)O: (√3×√3)2O (1×1)O single O (2×2)-O c(2×2)O (1×1)O c(2×4)-2O: (2×1)-2O (2×2)O: (2×1)O: (2×2)3O: (1×1)O (2×2)O (1×5)O

3.80 eV 3.0 eV 2.52 eV 2.75eV 5.03 eV, 2.5 eV 4.85 eV (4.93 eV) 4.6 eV 4.24 eV (4.38 eV) 4.61 eV 5.20 eV, 4.77 eV 4.75 eV 3.8 eV 5.36 eV 5.14 eV 5.55 eV, 2.8 eV 5.28 eV 5.06 eV 4.84 eV overlayer: 3.8 eV 5.26 eV (0.6 ML) - 5.14 eV (0.8 ML) depending on the O coverage 5.18 eV 10.01 eV/9.0 eV 9.13 eV/8.5 eV on-surface ads.: 6.5 eV

97G1/DFT-GGA

Rh(110) Rh(111)

Rh(100)

Ru(10 1 0) Ru(0001)

Tc(110) V(100)

Zr(0001)

c(2×2)O (2×1)O (1×1)O (2×2)O

97S6/DFT 98L1, 99G1, 0H1/DFT 98C1/cluster 98L1/99H1 (DFT)

98S1/DFT 96S2, 00H1/DFT

00H1/theory 01K1/DFT 01K1/DFT 96Y1/01J1/DFT 96Y1/01J1/DFT 00H1/DFT

Remarks: a) Cluster Calculations produce less reliable values for O-metal binding energies than slab calculations. b) Some oxygen binding energies are given w.r.t. atomic O in the gas phase, others are given w.r.t. half of the binding energy of O2. Both values differ by about 2.5. eV.

Landolt-Börnstein New Series III/42A4

22

3.4.2.2.4 Electronic properties of chemisorbed atomic oxygen Table 4. Electronic properties of chemisorbed atomic oxygen overlayers on metal surfaces. Substrate Ag(110)

Surface (2×1)O

c(6×2)O c(2×2)

Ag(111)

Al(110) Al(111)

chem. O oxidic O (4×4)O

25 L -100 L 100 L 0.5 L O2 (1×1)O (islands)

Remarks strong dispersion (2 eV) along the Ag-O chains

Method ARUPS

O(2p) anti-bonding O(1s) O(2p) anti-bonding

1.4 528.1 1.5

490 K

theory XPS

O(2pz) O(2p) bonding O(2p) anti-bonding O(1s) O(2p) bonding O(2p) anti-bonding Ag(3d5/2) Ag(3d5/2) O(1s) O(1s) bonding O(1s) anti-bonding TD states: O(1s) O(β) Ag-3d5/2 O(β) O(1s) O(γ) Ag-3d5/2 O(γ) O(2p) O(γ) O(1s) (4×4)O O(2p) O-induced Al(3p3/2) peaks

3.1 6-8 1.5 - 4 528.5 9.7 3.1 368.2 367.7 530.4 3.8 8.2

weak dispersion of 0.3 eV along Ag chains

chemisorbed

O(2pz)

530.3 368.0 529.0 367.3 2.8, 1.8 528.2 7-8 73.9, 75.1 74 - 81 6.7

O(2px,y)

7.7

Al(3p3/2)

72.5 73 73.5

Ref. 86P1, 90T1, 97C1 76R1, 90T1 84C2 97C1

ARUPS MDS

98O3 92C3

XPS UPS

95B5 88S2

XPS

00B1 77G2, 90R2

UPS

oxidic similar to Al(100) chemisorbed O strong dispersion of O(2p) band structure clean c.t. 1O c.t. 2O

XPS XPS XPS XPS UPS XPS PES PES

95B4 96B5 95B4 96B5 96B5 85C1, 95B5 76M1, 78E1 78E1

ARPES

79H2, 79E1, 86M1

theory PES

83B4, 82B1 87M2, 91B2, 93B7

[Ref. p. 55

Landolt-Börnstein New Series III/42A4

(1×1)O (1×1)O (islands)

Binding energy [eV] 6

3.4.2 Adsorption of C, N, and O on metal surfaces

0.1-0.6 ML (3×1)O, (4×1)O (2×1)O (n×1)O

State O(2p)

Surface

State

oxide (1×1)O (islands)

Al(3p3/2) O(1s)

10 L O2 (1×1)O

Al(100)

Co( 10 1 0 )

600 L O2

O(KLL) Al(L1)O(L22L33) Al(2p)

533.5 74 - 81 6.7 7.5 5-9 532.1, 535.5 73

perimeter anti-bonding Al-O complex (unoccupied state)

1500 L O2

O(1s) O(KLL)

3 - 100 L O2

O(2px,y)

505 55 75.3 74.3 531 469.8, 484.4, 490.6, 505 2.5, 7.5

100 L O2

Al(2p)

75.3, 74.3

(1×1)O 10 L O2 10 L O2 25 L O2 (2×1)O

O(2p)

8 - 10 74 - 81 75.8 7.0, 9.5 6

(2×1)pg-2O

Co(0001)

O(2p) O(1s) Al(2p)

2pz 2px, 2py

Remarks c.t. 3O

c(2×4)2O (2×1)pg-2O 0.5 L, 120 K 2500 L, 120 K

Al(2p) O(2p) O(2px), O(2py) O(2pz) O(2px) O(2py) O(2pz) 2 states O(2p) O(2p)

Ref.

PES XPS/ theory

87M2 91B2

SSXA ARUPS

79B4 79E1, 79H2

SC-FAPW

81W1 91B2

AES

87H2

XPS AES

87H2 87H2

with 0.7 eV dispersion

clean Al(111): 72.6 eV; small (1×1)O islands: internal and perimeter O atoms

PES interpretation: O in-plane with topmost Al plane theory theory PES theory anti bonding Al-O complex (unoccupied state) SSXA XPS UPS disperse by less than 0.5 eV

76Y1, 78E1 77B6 84B2 78E1 81K3 79B4 77F1 76M1 90S1

disperses by 1 eV 90S1

disperse by 0.5 eV disperses by 0.2 eV

O(on-surface) O(bulk)

IPES IPES UPS

96R3 96R3 82C1

23

7 5 5.8 5.8 −1.8, −2.1 −2 - −4 2.7, 5.3 0.9, 2.8, 5.0, 6.6, 9.3, 11.2

interior

Method

3.4.2 Adsorption of C, N, and O on metal surfaces

(1×1)O 0.3 ML

O(1s)

Binding energy [eV] 74 75.1 532.1

Ref. p. 55]

Landolt-Börnstein New Series III/42A4

Substrate Al(111) cont.

Surface -

Cr(110)

c(4×2)O 100 L O2 <1 L O2

Cr(111) Cr(100)

Cu(111)

(2×1)O 500 L O2, 100 K

(2√2×√2)R45°

Fe(110)

(1×1)O (1×1)O

O(2p) anti-bonding states Covalent bond: O2p-metal-sp

1.5

O(1s) O(2p) O(1s) O(1s) O-Cu: 2p-3d bonding anti-bonding O-K emission O(2p) derived states O(2p) O(2pz), O(2px,y) well separated O(1s) O(1s) O-px,y, pz-derived levels O(2p) O(2p) O(2p) O(2p) O(px, py) O(pz) O(2p)

−0.2, −0.3 530.5 6.2, 10.1 529.8 531.3 ~6 1-2 525.4 1.4, 6 6-7

Remarks

weak dispersion

oxide dispersion 0.3 eV disperse by more than 1 eV

XPS ARUPS

Ref. 95G1 88K3 85F1 78G1 82G1

theory XAS STM-LES XPS UPS XPS

85F1 84D2, 87C2, 96O1, 97C1 92W1 89P3 01U1 90R2 82S4, 90R2 95D3

PES

77T1, 80L1

UPS ARUPS SXES SXES, XAS XPS XPS ARUPS

92W2 77L2, 77T1 80L1 93T1 98W3 80P1 93S6, 79B3 85S3

ARUPS

85S3

minority spin polarized

PES IPE ARPES

90C1 92H2, 95D2 85P1

vertical and horizontal bonding

theory

85H1

strong dispersion(2eV) along the Cu-O chains.

unoccupied single O states on-surface sub-surface FWHM ≈ 4.2 eV: wide distribution of O(2p) dispersion 1 eV Oxygen K-emission spectra: filled 2p-3d state

530.1 530.3 5.5, 7.0 dispersion ≈0.1 eV dispersion ≈1.6 eV 7.1, 5.8, 5.0, 4.55 −1.7 4.9, 2.2 disp. 6.9, 1.9 disp. 4-8

Method UPS XPS ARUPS XPS XPS UPS XPS ARUPS

[Ref. p. 55

Landolt-Börnstein New Series III/42A4

Fe(100)

(2√2×√2)R45° (2√2×√2)R45° (2√2×√2)R45° (2√2×√2)R45° low O coverage c(2×2)O c(3×1)O c(2×2)O c(3×1)O c(2×2)O c(3×1)O (1×1)O

Binding energy [eV] 5.5 529.5 6.2, 7.4 574 - 584 529.6 5.5 530.6 6.6 4.9, 6.8 574 - 584 5 - 7.8

3.4.2 Adsorption of C, N, and O on metal surfaces

Cu(110)

>1 L O2 c(2×2)O (1×1)O 100 L O2 (2×1)O

State O(2p) O(1s) O(2p) Cr(2p) O(1s) O(2p) O(1s) O(2p) O(2p) Cr(2p) O(2p) bonding states

24

Substrate Co(1120)

State O(2p) O(2p) O(2p) O(2p) Gd(4f)

Ir(110)

oxidic adsorbed O

O(1s) O(1s) Ir(4f7/2) O(2p) O(1s) O(1s) O(2p) Mg(2p)

Ir(111)

Mg(10 1 0)

expos. <630 K expos. >650 K 1 - 10 L O2 1 L O2

Mg(0001)

9 L O2 O(1s)

Mo(110)

0.5 L O2 (2×2)-1O

Mo(111)

low cov.

O(2p)

Ir(100)

100 L O2 0.8 ML Mo(100) Ni(110)

Ni(111)

2 - 10 L O2 (2×1)O

O(2p) O(2px,y)

Binding energy [eV] 5.5, 5.3 ~6 5.4, 6.5 4.8, 5.4, 6.5, 7.6 8.75 → 8.85 8.35 → 8.60 530.6 528.9 60.7 6-7 529.8±0.2 531.5 5 50.79 51.44 2.9

O(1s)

(2×2)O

O(2p)

chemisorbed O oxidic-O surface oxidic-O bulk

on-surface O subsurface O MgO O chem. transition from O-derived 2p to levels of an MgO species. (1×1)O underlayer coexists with MgO

530.6, 533

5.5 8 9.5, 7 530.6 → 530.2 227.2 → 227.4 4-6 6 −3.2 −2.0

(2×1)O (3×1)O (3×1)O 30 L, 100 K

Remarks

530 531.5 −1.1 → −2.8

Method UPS EELS ARPES ARPES PES

Ref. 76B1, 77R3 84S3 95Z1 95Z1 96M1

XPS

97L2

UPS XPS

77C3, 78K1 76Z1

UPS PES

79K1 89T1

ELS

81H1, 81N1

EELS XPS

82F1 81G1 94D1

Fermisurface mapping: global Peierls distortion induces the (2×2) ordering. UPS HeI HeII HeII O(1s)-shift XPS Mo(3d)-shift UPS strong dispersion of 2 eV along the Ni-O ARUPS chains. theory

77W3 91P1, 97C1, 96S4 90N1 85D1, 85D2

ARUPS XPS

96S4 95R2

IPE XAS

85A1 89P3

25

similar to (2×1)O O2− O1− dispersion Covalent bond: O(2p)-metal-sp

85M1

3.4.2 Adsorption of C, N, and O on metal surfaces

Surface c(2×2)O <3 L O2 0 - 1 L O2 1 - 3 L O2 0.25 L O2

Ref. p. 55]

Landolt-Börnstein New Series III/42A4

Substrate Fe(100) cont. Gd(0001)

c(2×2)O

5.5

(√3×√3)R30°O >0.7 ML

c(2×2)O c(2×2)O (2×2)O (2×2)O 30 L O2, 100 K

O(2pz) O(2px,y) Ni(3p3/2)

O(1s) O(1s) O(2pz) O(2px,y) O(1s)

529.9 524 529.75 8 6 530 531.5

(2×2)O Pd(110)

Pd(3d5/2)

c(2×4)O 100 L O2, 300 K 100 L O2, 1000 K 0.4 ML

335.58 336.3 6 21.2, 22.2 24.3 335.0 532.3 336.6 529.6 335.54

529.9 529.9 6.5

Pd(100)

c(2×2)

O(2pz) O(2s) O(2s) Pd(3d5/2) O(1s) Pd(3d5/2) O(1s) Pd(3d5/2)

Pt(110)

(1×2)MR+O2 (1×2)MS+O2 (2×2)O

O(1s) O(1s) O(2p)

Pd(111)

>3 ML Landolt-Börnstein New Series III/42A4

Pt(111)

more pronounced at grazing incidence O2− O1−

Method XPS

Ref. 76K1

XPS

95R2

PES

79R1, 79C2

XPS

01D1

O(2p) hybridized with Ni(s,p), strong dispersion IPE, KRIPES of up to 3 eV O(2p) derived state (hybrid of O(2p) and Nitheory 4s4p) UPS SCLS O-K emission SXES SCLS UPS O2− O1− Oxygen K-emission spectra: 2p-3d antibonding state partly occupied. clean surface component 334.96 eV surface oxide 1 eV dispersion along densely packed rows. O chemisorbed O subsurface

94H1 85G2, 71E1 89N1 92W2 89N1 64G1, 77J1

XPS

95R2, 00K1

SXES

93T1

SCLS SCLS ARUPS UPS

91C1 96B1 93Y1 83W1

XPS

90B1

clean surface 335.40 eV; angular dependence of PES the intensity: O in 4-fold hollow 0.92 Å above the Pd layer. XPS

94G1, 96P1

86F1 89P1, 80G1

[Ref. p. 55

c(2×4)O

Remarks

3.4.2 Adsorption of C, N, and O on metal surfaces

c(2×2)O

30 L, 100 K

Ni(100)

State Ni(3p3/2) clean O(1s) O(1s)

Binding energy [eV] 861.5 852 531.6 530 531.5 8 6 853.7 - 553.9, 855.5 855.9 −1 - −4

Surface (2×2)O

26

Substrate Ni(111) cont.

Re(0001)

Rh(110)

Binding energy [eV] 530.8 529.8 6

0.2 ML 0.8 ML -

O(1s) O(1s) O(1s) O(2p)

530.9 529.8 530 6

(1×5)2O (1×3)2O c(2×4)O disordered <0.3 ML >0.8 ML (2×2)p2mg

Rh(100)

Ru(10 1 0)

(2×2)4pg c(2×4)2O c(2×4)2O, (2×1)p2mg-2O

O(2p)

5.5, 6.6, 7.8

O(1s) O(1s)

XPS UPS ARUPS

80D3

ARUPS

92L3 94C1

2 O-induced states (2pz, 2px) disperse by 1.2 eV ARUPS and 0.8 eV CLS

98C1

O(2px) and O(2pz) disperse by about 1.0 eV along the close-packed Re rows. No dispersion perpendicular to the Re rows. 3 weakly dispersing O(2p) bands

O(1s) O(1s) O(1s) O(1s) Rh(3d5/2) Rh(3d5/2) O(2pz) O(2s) Rh(3d5/2) O(1s)

Ru(3d5/2)

530.25 529.75 529.4 529.4 −0.12 0.3 5.9 21.0 −0.25 529.8±0.2 4.0, 4.7, 5.2, 5.8, 6.2, 6.8 0.395 (Ru-O) 0.695 (Ru-2O) 2.8, 4.2, 4.6, 5.0, 6.3

O(1s) O(2p) induced states O(1s)

92L2

529.8±0.2 6.4 530.24

01V1

PES

99J1

Rh c.t 1O w.r.t. bulk Rh c.t 2O w.r.t. bulk

SCLS

00A2

derived states disperse by about 1.4 eV

PES

96Z1

w.r.t. bulk

SCLS XPS

6 O-induced bands; all of them are weakly dispersing (<0.1 eV) SCLS shift by 75 meV 5 O(2p) induced bands for various polarizations of the light

83F2 93R1 00B1 95R1 75F1 01O1

27

≤1.0 ML

UPS theory XPS

Ref. 84D1 80G2 76H2, 84N2 92B1 84D1

529.65 530.25

O(1s) <0.50 ML

Method XPS

low temperature adsorption

(2×1)p2mg-2O Ru(0001)

Remarks

strong interaction between O(2p) and Pt-5d

(2×2)p2mg (10×2) (2×2)O (2×1)O (2×2)O (2×1)O (2×2)4pg

Rh(111)

State O(1s) O(1s) O(2p)

3.4.2 Adsorption of C, N, and O on metal surfaces

Re(10 1 0)

Surface 0.2 - 0.8 ML

Ref. p. 55]

Landolt-Börnstein New Series III/42A4

Substrate Pt(111) cont. Pt(100)(1×1)

Ta(110)

State

1 L O2 1 L O2 5 L O2

Ta(4f7/2) Ta(4f7/2) Ta(4f7/2)

Ta(4f7/2)

Binding energy [eV] 280.1 280.60 281.04, 280.66 281.04 23 24.1 24.9 27.5 22.76 22.94, 24.04, 22.14

Ti(0001)

Surface state of Ti(0001) just above EF disappears upon O-exposure.

V(110) V(111)

c(2×6) <10 L O2

V(100)

(1×5)O

W(110)

(4×1)O (2×1)-O (1×1)-1O (1×1)-1O 0.50 - 1.0 ML (1×1)O

W(111)

0.1 L O2 0.7 L O2 initial (1×1), <0.5 ML (1×1) 1 ML (2×1) (4×1)

O(2p) V(2p1/2), V(2p3/2), O(1s) V(2p1/2) V(2p3/2) O(1s) O(2p) V(2p1/2) V(2p3/2) O(1s) O(2p) W(4f7/2)

5.9

W(4f7/2) clean W(4f7/2) W(4f7/2) O(2p) O(2p) O(1s) W(4f7/2) W(4f7/2) O(1s) W(4f7/2) W(4f7/2) W(4f7/2) O(2p)

32.0 31.1

Method

PES

Ref. 00L1 01O1, 00L1 01O1, 00L1 01O1 95R3

PES

82V1

APS

81J1

UPS XPS, UPS

81H2 00G2 91S3

XPS

10S1, 83F4

ARUPS SCLS

88Z1 94P1, 98R1

XPS SCL FLAPW UPS, 100 eV PES XPS PES

98D1, 98D2, 98R1 99O1 94P1 99F1 81O1 82V1

XPS SCLS

74Y1 89A1

KRIPES

88K2

only kinetic energies are given 524 516 534 5-6 519.7 512.2 531 5-7 31.7

O segregation

shift 0.60 eV 6.3 5.1, 6.1, 7.7 530.35 31.83 31.73 530.35 31.13 → 31.9 31.13 → 33.2 31.13 → 32.8 −2.3

WO2 like phase reconstructed 2D “WO2”

[Ref. p. 55

Landolt-Börnstein New Series III/42A4

W(100)

Remarks surface surface surface surface clean (2×1)O coplanar Ta+O layer Ta2O5

3.4.2 Adsorption of C, N, and O on metal surfaces

Ta(111)

Surface (2×2)O (2×1)O (2×2)3O (1×1)O 0.5 L O2

28

Substrate Ru(0001) cont.

Zr(0001)

Surface 100 L cluster 1 - 1000 L (1×1)O (2×1)O

State O(2p) O(2p) and Zn(4s, 4p) O(1s)

Binding energy [eV] 4.3 - 5

Remarks strong bonding character

530 6, 7.6

O-derived bands

Method UPS DV-Xα-MO XPS theory

Ref. 75B1, 78A1 83M3 85S4 96Y1

Ref. p. 55]

Landolt-Börnstein New Series III/42A4

Substrate Zn(0001)

3.4.2 Adsorption of C, N, and O on metal surfaces 29

30

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

3.4.2.2.5 Vibrational properties of chemisorbed oxygen Table 5. Vibrational properties of (atomic) oxygen overlayers on metal surfaces and the beginning of surface oxide formation. Suggested adsorption sites are indicated. If not otherwise indicated the technique of HREELS or IRAS was used.

substrate

structure

Me-O stretch vibration [meV]

Ag(110)

(2×1)

39, 41 surface phonon: 17 anharmonicity 40.9, 40.2, 39.8 38.5 78.5 104 37.2, 42.4 30 78.5 80.8 104 104 37 30 50 - 54 65 - 81

Ag(111)

Ag(100) Al(111)

(2×1), (3×1), (4×1) low cov. O(β) O(γ) c(6×2)O disordered O O(β) O(β) O(γ) O(γ) c(2×2) (1×1) isolated O (1×1)O (1×1)O (1×1)O (1×1)O islands

Co(10 1 0)

c(2×4), p(2×1) (2×1)pg-2O

Cr(110)

initial p(4×2) 0.75L O2 at RT 5L O2 at RT 0.6 - 1.5 L O2 at RT (2×1)

Cr(100) Cr(111) Cu(110)

c(6×2) Cu(111)

disordered

65 - 81 105 105 - 120 re-interpretation: 72, 60, 105 Motion of the Al3O coordination cluster: perpendicular, umbrella, lateral. 50 - 65

76.9 75.6 68 65 44, 72.5 29.6, parallel [1 1 0] 48.5, perpendicular [1 1 0] 48.5 42.1, 62 29.4

adsorption site

Ref. 81B3, 80S1 95S1 94S2

subsurface on-surface dissolved O embedded O fourfold hollow

94S2 94P3/SERS 94P3/SERS 95B5 83B3, 95B3 94P3/SERS 95B7/Raman 94P3/SERS 95B7/Raman 90A1

fcc hollow on- and subsurface-O; O-islands subsurface-O oxidic O fcc-hollow

86C1 87A1

C1 symmetry Cs symmetry no C2v symm. long-bridge long-bridge -

90S1 90S1

long-bridge long-bridge O-3Cu threefold hollow

87A1 82E3 87A1 96F1, 97H1

86S3, 86S4 86S3, 86S4 86B4 86B4 86B5 90V1 87M1 87M1 82D1

Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces

31

substrate

structure

Me-O stretch vibration [meV]

adsorption site

Ref.

Cu(100) Fe(110)

(2√2×√2)R45° p(2×2) c(3×1) p(1×1)O LT phase 0 - 0.52 ML subs. O <10 L O2

36, 55.2, 84.9 62.5 69 54, 35 56, 81 62 - 66 coverage dependent 68 100 57, 77

fourfold hollow long-bridge long-bridge 4-fold 4,1-fold threefold

89W1 81E1 81E1 90L2 89L2 86B2 87M1 87M1 89T1

(√5×√5)R26° (√5×√5)R26° (2×1) (1×1)O (2×1) (2×2) (√3×√3)R30° (2×2)O p(2×2) c(2×2) disordered c(2×2)

66 (adsorbed O) 47, 75 (adsorbed O) 92.5 (oxide layer) 51, 77 121.6 70 55, 69, (91) 62, 74, 94 67.5, 51, 67, 82 24, 46 72 72 surface phonon 53 39 49 (0.11 ML), 46 (0.56 ML) 77, 88, 98, 105, 121

Fe(100) Fe(111) Ir(111)

Mg(0001) Mg(10 1 0) Mo(110) p(2×2)O LT >0.25 ML LT 1000 K dose Mo(111) 0.8 ML Mo(100)

Ni(110) Ni(111)

Ni(100)

Ni(113)

0 - 0.5ML

Pd(110)

Pd(111)

Pd(100)

MgO FK , interstitial O long-bridge 3-fold threefold on-top 3-fold 3-fold (on-top) MoO2 oxidized surface long-bridge threefold hollow fourfold hollow 5 types of subsurface oxygen 3-fold 4-fold O between rows long-bridge long-bridge

(2×3)-1D c(2×4), (2×3)-1D (2×2)O (2×2)O >1.4 ML (2×2)-O

55 - 65, 149 - 155 35 - 40, 85 - 90 100, 124 32, 59 56 56 59.4 O-induced Raleigh-Ph.-gap 34, 53, 84, 94 44

surface oxide 4-fold hollow

c(2×2)-O

44

4-fold hollow

p(5×5) (√5×√5)

44, 50 44, 54 60, 41 perpendicular and lateral Pt-O vibration

reconstruction reconstruction fcc site

Pt(110)2×1

Landolt-Börnstein New Series III/42A4

3-fold hollow

92C2, 98K4 92C2 92C2 88S1 88S1 83M2 00K1 00K1 83M2 90V1 80I1 80I1 94T1 83A1, 84R1 83A1, 84R1 85R1 93O1 96S6 96S6 96S6 87K1, 86N1 86N1 86N1 86I1, 90B1 96K1 90B1 91S1, 82N1, 84S1 91S1, 82N1 84S1 91S1 91S1 93S1

32

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

substrate

structure

Me-O stretch vibration [meV]

adsorption site

Ref.

Pt(111)

(2×2)1O (2×2)3O “oxide” low cov.

58 (perp. vibration) 59.4 95 70

89P1, 82S3 89P1 80G1 97W2, 98F1

low cov. (2×1)p2mg (2×2)p2mg (2×2), (2×1) (2×2) c(2×2) (2×2)4p4g c(2×4)-2O (2×1)pg-2O initial (2×2)O (2×1)O (2×2)3O (1×1)O

72 45, 63 47, 65 62 - 68 48 - 54

3-fold hollow 3-fold hollow “oxide” O adsorbed in edge bridge of A-type steps long-bridge

threefold hollow fourfold hollow

86R1 82S1, 90F1, 87G1, 82D3

3-fold 3-fold 3-fold 3-fold 3-fold 3-fold 3-fold on-surface sub-surface long-bridge quasi-3-fold quasi-3-fold quasi-3-fold 3-fold

oxide layer

98S1 98S1 97H1 79T3, 95M1 79T3, 97K1 97K1 97H1 95K2 95K2 84D1 84D1, 97E1 97E1 97E1 76F1 76F1 76F1 76F1 76F1 90G1, 85S1 90G1 85 S1 85S4

octahedral

96Y1

Pt(322)

Rh(110)

Rh(111) Rh(100)

Ru(10 1 0) Ru(0001)

V(110) W(110)

W(100)

Ti(0001)

Zn(0001) Zr(0001)

initial (2×1)O (2×2)3O (1×1)O 0.17 ML 0.27 ML 0.45 ML 1.0 ML (2×2)O 25 L (1×1)O 1 - 30 L O2 100-1000 L O2 (1×1)O

67 64 54 64 73 (perp.), 53 (lateral) 80 (perp.), 67 (lateral) 81 (perp.) 77 131 69.5 72, 48 79, 50 82, 53 75 50, 78 50, 78, 90 65, 90 127.5 65 65, 87 40 - 45, 90 - 92 70.5 70.5, 49 60 (theory)

oxide on-top octahedral

93A1

Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces

33

3.4.2.2.6 Local atomic oxygen-metal geometry Table 6. The O-metal bond length and other structural characteristics.

substrate Ag(110)

surface (2×1)-O (2×1)-O c(6×2)-4O

Ag(111)

Al(111)

Al(100)

Au(111) Co(10 1 0) fccCo(100) Cr(100)

(1×2)-1O 1 bar, 400 h, 800 K (4×4)-4O

O-metal [Ǻ] Adsorption site/Remarks 2.04 MR reconstruction O in long-bridge 2.13 MR, O in long-bridge 2.05, 2.07 MR reconstruction analog to Cu(110)-c(6×2) every 2nd row missing subsurface induced reconstruction.

(4×4)-4O (1×1)O

1.8

Trilayer of Ag2O(111) on Ag(111) rotated by 30° Ag-deficient trilayer fcc, (1×1)O islands

(1×1)O (1×1)O (1×1)O

1.92 1.76 1.80

fcc assumed 3-fold coordinated. O fcc

(1×1)O (1×1)O (1×1)O >0.1 ML chemisorbed O oxidic O (1×1)O low O cov. low O cov <0.30 ML

1.80 1.79±0.05 1.88±0.03 1.79 2.14 1.75 -

O fcc O-Al distance: 0.58 Å subsurface O ruled out O(1×1) islands + oxide O-O distance 2.90±0.05 Ǻ

>0.30 ML (1×1)O (√3×√3)R30° (2×1)pg c(2×2)O

1.98 1.83/1.99 1.94

(√5×√5)R26°-5O 2.05

Landolt-Börnstein New Series III/42A4

Ref./Method 84P1/SEXAFS 93C2/ICISS 93S1/DFT 93D1/LEIS 98O3/STM 93B6/STM, RHEED, REM 74R1/LEED

oxide phase starts to grow 4-fold “gold oxide” O hcp like sites O 4-fold hollow

00C1/STM&DFT 92K1, 95J1, 01K2/DFT 81B1/SEXAFS 81N2/SEXAFS 81S1, 82N2, 83M1/ LEED 92K1/SXW 93W1/MEIS 98M1/NDRS 93B2, 98T1/STM 80S3/SEXAFS 80S3/SEXAFS 81W1/SC-LAPW 86M1/cluster calc. 98D3/cluster calc. 90L1, 91L2/ ARSIMS 80D2/LEED 91L2/ARSIMS 80D2/EXAFS 96H1/STM 97G2/LEED 78M3/LEED

Cr vacancy structure

99S1/LEED-STM

4-fold 4-fold 4-fold, O almost within the Al surface plane, randomly.

34 substrate Cu(110)

3.4.2 Adsorption of C, N, and O on metal surfaces surface (2×1)-O

c(6×2)-4O

Cu(111)

√73R5.8°× √21R10.9° single O (2√2×√2)R45°

O-metal [Ǻ] Adsorption site/Remarks 1.84 O: long bridge, MR reconstruction 1.81, 2.00 1.84, 1.85 1.81, 2.01 1.81, 1.77 1.89, 2.27 1.83, 2.09 1.83, 2.08 MR, O: long-bridge; super Cu atom links two Cu/O chains; In-plane structure only. 1.78, 1.89 1.90 averaged Cu-O bond length Added CuO2 oxide layer

Ir(110)

disordered c(2×2)O c(3×1) >0.4 ML (2×1)O (1×1)O (1×1)O (1×1)O (1×1)O (1×1)O c(2×2)-O

Ir(111) Ir(100)

(2×2)-O (1×2)-O

Mg(0001)

8 L O2

1.93 1.82, 2.14 1.86, 2.07 1.88, 2.07 2.05, 1.95 2.07, 2.00 2.02 2.10 2.06, 1.97 2.06 1.93 1.80 2.04 1.95 1.93 -

Mo(100)

(1×1)O (√5×√5)-4O

2.70 1.97, 2.08 2.12, 1.6

(2×1)-2O low O-cov. (2×1)O (2×1)O (1×2)O initially

1.78 1.77, 1.86 -

Cu(100)

Fe(110)

Fe(211) Fe(100)

Ni(110)

3-fold hollow assumed O: 4-fold, MR reconstruction O: 4-fold, MR reconstruction O: 4-fold, unreconstructed O-overlayer, O: long-bridge MR reconstruction FeO islands grow O: long-bridge, MR reconstruction O: deep in 4-fold hollow sites

Fe-Fe layer distance: +23 % O: short bridge O: fcc O: bridge site O: bridge site 90 % on-surface, 10 % subsurface, Mg-O layer distance: 0.7 Å; note the work function decreased by 1 eV. O octahedral O-tetrahedral, islanding 4Mo+4O per unit cell with Mo-Mo bond length: 2.85 Å MR reconstruction O 4-fold without reconstruction O:long-bridge, MR O:long-bridge, MR every 2nd Ni row is missing

[Ref. p. 55 Ref./Method 86B1/SEXAFS 91D1/ICISS 90F2/SXRD 90P2/LEED 93D1/LEIS 93S4/DFT 98F2/DFT 98L2/DFT 90F3/SXRD 93D2/ICISS 95L1/LEED 01M1/STM 01G1/cluster 90Z1/LEED 93L1/SEXAFS 93L1/SEXAFS 95W1/STM 95W1/STM 95W1/STM 86S1/LEED 77L2, 87J1/LEED 81R1/Theory 87V1/LEIS 89H3/MEIS 87C3/FLAPW 78C2/LEED 95B6/TOF-SARS 79C1/LEED 00J1/DFT 00J1/LEED 98M2/NDRS 89C1/FFAK 97B2/DFT 96X1/STM 92R1/SXRD 96X1/STM 82O1/LEIS 85N1/ICISS 90K1/LEED 94E1/STM

Landolt-Börnstein New Series III/42A4

Ref. p. 55] substrate Ni(111)

Ni(100)

Pd(110)

Pd(111)

Pd(100)

Pt(210) Pt(100)

Rh(110)

3.4.2 Adsorption of C, N, and O on metal surfaces surface (2×2)O (2×2)O (2×2)O (2×2)O (√3×√3)O (√3×√3)O (√3×√3) 0.3 ML

O-metal [Ǻ] 1.83 1.85±0.03 1.85±0.05 1.80±0.04 1.80±0.02 1.85 1.87 -

Adsorption site/Remarks fcc fcc fcc fcc fcc fcc fcc fcc, hcp (domain boundary)

>0.7 ML disord., <0.1 ML (2×2)O c(2×2)O c(2×2)O p(2×2), c(2×2) oxide form. c(2×2)O c(2×2)O p(2×2)O, c(2×2)O (2×2)O (2×2)O single O (2×3) c(2×4)O (2×2)mgO c(2×4), (2×3)-1D (√3×√3)O (2×2)O (√3×√3)O c(2×2)O p(2×2)O (√5×√5)R27°O (5×2)rect-O c(2×2)-1O p(2×2)-1O

1.94±0.03 1.93±0.03 1.92±0.03 1.96±0.03 1.92 1.92 1.96±0.05

oxide formation 4-fold 4-fold 4-fold 4-fold 4-fold, d(O-Ni)=0.9 Å >80L visible in NEXAFS 4-fold 4-fold 4-fold

1.95±0.05 1.96±0.05 1.93 2.07, 1.80 1.86, 1.88 2.02 -

4-fold 4-fold 4-fold fcc, (1×3) MR fcc, (1×2) MR fcc, (1×2) MR reconstructed surface

2.03 1.98±0.08 2.02±0.08 2.18 2.11 1.73 1.79±0.05 1.94 1.94

(2×2)p2mg-2O c(2×8)-12O (5×10)np (2×1)pmg-2O (2×2)

2.0±0.1 1.97, 2.02 2.00

fcc (assumed) fcc fcc 4-fold 4-fold single PdO(001) on Pd(100) 3-fold on both (110) and (310) facets O: bridge O: bridge and 4-fold site are energetically degenerated fcc, (1×2) MR fcc, (1×4) MR fcc fcc (1×2)MR, O fcc

Landolt-Börnstein New Series III/42A4

35 Ref./Method 90V1, 94S1/LEED 92H1/SEXAFS 90P1/NEXAFS 96D1/PhD 91M1/LEED 92H1/SEXAFS 81N3/HEIS 96S5, 97S5/ DLEED 01D1/XPS 91O1/LEED 91O1/LEED 91O1/LEED 82S2/SEXAFS 83N1/NEXAFS 83N1/NEXAFS 93S2/PhD 99A1/DFT 94X1/SIMS 83F3/RBS 94X1/SIMS 93G2/cluster theo. 89H2/RBS 97B1/LEED 97B1/LEED 01H1/DFT 98L1/DFT 98O1/LEED 98O1/LEED 98L1/DFT 96K2/LEED 94V3/LEED 94L1/SEXAFS 97G1/DFT 97G1/DFT 93G1/LEED 94O1/LEED 95O1/LEED 93G1, 95B1/LEED 97S5/DFT

36 substrate Rh(111)

Rh(100)

Ru(10 1 0)

Ru(0001)

stepped Ru(0001) Ta(100) W(110)

W(100) W(100) W(211) W(100)2×1

3.4.2 Adsorption of C, N, and O on metal surfaces surface (√3×√3)O (2×2)O (2×1)O (2×2)O (2×1)O (1×1)O O subsurface Single O c(2×2)O (2×2)O (2×2)4pg-2O

O-metal [Ǻ] 2.0 2.00±0.08 2.02±0.08 2.00 1.99 1.95 1.8 2.02 2.17 2.12±0.06 2.00, 2.06

c(2×4)-2O (2×1)p2mg-2O c(2×4)-2O (2×1)p2mg-2O (1×1)2O artificial (2×2)O (2×1)O (2×2)3O (1×1)O (2×2)O (2×1)O (1×1)O (2×2)O

2.08, 2.03 2.03, 2.03 2.09, 2.10 2.11, 2.11 1.98, 2.08

(3×1)-O (2×1)O (2×1)O (2×1)O ≈(1×1)O (1×1)O (1×1)O O disordered (4×1) (2×1)-3O O disordered

2.03±0.06 2.02±0.06 11.98±0.0 6 2.00±0.04 22.10 2.08 2.03 2.0±0.1 2.08 2.05, 2.11 1.73 2.10 1.65 2.0, 2.24 -

[Ref. p. 55

Adsorption site/Remarks fcc (assumed) fcc fcc fcc fcc fcc octahedral fcc, assumed 4-fold 4-fold quasi-3-fold, clock reconstruction type 2a), 0.2 Ǻ displacements of Rh atoms hcp along the Ru flanks hcp along the Ru flanks hcp hcp fcc, hcp

Ref./Method 98L1/DFT 97S3, 86W1/LEED 97S3,96W1/LEED 99G1/DFT 99G1/DFT 99G1/DFT 98W1/PhD 97C3/cluster 98L1/ DFT 88O1/ LEED 99A1/DFT 98S7, 99B1/LEED 99N1/SXRD 98S1/LEED 98S1/LEED 98S1/DFT 98S1/DFT 98S1/DFT

hcp hcp hcp hcp hcp hcp hcp hcp (terrace), four-fold (step)

89L1/LEED 89P1/LEED 98K2/LEED 96S3/LEED 96S2/DFT 96S2/DFT 96S2/DFT 95H1/LEED

4-fold interstitial 3-fold 3-fold 3-fold 3-fold two types of 3-fold O W-O layer distance: 1.18 Ǻ 4-fold ----3-fold trough

85T1/LEED 75V1/LEED 01Y1/LEED 93J1/STM 98D1, 01T1/PhD 93J1/STM 99O1/XPD 86R1/DLEED 98O2/EELFS 89B1, 89R2/TOFSARS 89M1/LEIS 91M2/STM

O disordered in 2nd layer, MR 3-fold or 4-fold hollow, MR

Landolt-Börnstein New Series III/42A4

Ref. p. 55] substrate Zr(0001)

3.4.2 Adsorption of C, N, and O on metal surfaces surface (2×2)2O (2×1)O (1×1)1O

O-metal [Ǻ] 2.28±0.05 2.11 2.28

(1×1)1O (1×1)2O

2.05 2.21, 2.07

Adsorption site/Remarks O-octahedral below 1st and 2nd layer O-octahedral below 1st and 2nd layer 0.5 ML below 1st and 0.5 ML below 2nd layer (octahedral) O-octahedral tetrahedral/on-surface similar to ZrO2

37 Ref./Method 95W1/LEED 01J1/DFT 95W2/LEED 01J1/DFT 97W1/LEED

Remarks: a) Clock reconstruction type 1 (black) and type 2 (white) according to 99A1 b) In general, DFT slab calculations are more reliable than Cluster calculations concerning the atomic geometries. 3.4.2.2.7 Ordered overlayers of chemisorbed oxygen and surface oxides on metal surfaces Table 7. Ordered overlayers of chemisorbed oxygen and surface oxides on metal surfaces.

Substrate fcc- Ag

Orientation (110) (110) (331) (111)

fcc-Al

fcc-Au hcp-Be

(100) (110) (111) (100) (111) (0001)

fcc-Co

(10 1 0) (0001) (100)

bcc-Cr

(110)

hcp-Co

fcc-Cu

Landolt-Börnstein New Series III/42A4

(111) (100) (110) (210) (111) (332) (100) (h11) h=5,...,15

O-surface structure (2×1), (3×1), (4×1), (5×1), (6×1), (7×1), c(6×2)-6O c(2×2) high pressure phase (2×1) (2×2), (√3×√3)R30°, (4×4)-4O disordered, c(2×2) (331) facets, (111) facets (4×4), (1×1)O, oxide-like (1×1)O islands disordered, amorphous oxide (√3×√3)R30° (1×1)O, BeO(0001)-(1×1), BeO(0001)-(2×2) (2×1), c(2×4), (2×1)pg disordered (2×2), c(2×2) and then nucleation of CoO crystallites. (3×1), (100)-facets p(4×2) (√3×√3)R30° c(2×2), c(2×4), (1×1) (2×1), c(6×2) (2×1), (3×1) disordered, (4,3,−3,5) disordered, (9,1,1,5) (2√2×√2)R45°, c(2x2) c(2×2)

Ref. 73E1, 76E1, 84C1 88S2 78M1 65M1, 74R1 85C1, 95B5 76E1, 85G1 71B1, 72V1 71B1, 78F1, 83M1, 93B2, 98T1 88W2/STM 67J1, 77F1 96C1 84F1 90S1 82C1 77R1 73M1 88K3 78J1 77H1, 82G1 67E1, 83F1 78M2 67E1 82M1 79W1, 98T2 92S1

38 Substrate bcc-Fe

3.4.2 Adsorption of C, N, and O on metal surfaces Orientation (110) (111)

hcp-Gd fcc-Ir

(100) (0001) (110)

hcp-Mg

(111) (766) (100) (0001)

bcc-Mo

(110) (111) (211) (100)

bcc-Nb

fcc-Pd

(110) (111) (100) (110) (771) (111) (100) (110)

fcc-Pt

(331) (111) (100) (110)

fcc-Ni

(111)

(654) (766) (12,9,8) (62,62,60)

O-surface structure c(2×2), (3×1), (2×2) beyond 0.4ML O: FeO(111) (6×6), (5×5), (4×4), (2√7×2√7)R19.1°, (2√3×2√3)R30° c(2×2), (1×1)O (1×1)-disordered (2×2), c(2×2), (3×2), (1×1), (1×4)oxide (2×2) or/and (2×1) (2×1) (2×1), (5×1), (1×1) (1×1) disordered, MgO(111)(1×1)R30°, MgO(111)(√7×√7/2)R19° (2×2)-1O, (2×1)-O, (1×1) 1-dim. ordered structures (211) facets, (110) facets, (4×2), (4×4), (1×3), (112)-(1×2) facets, (112)-(1×3) facets (2×1), (1×2), (1×3), c(4×2) c(4×4), (2×1), (√5×√5)R26°, (4×1) c(2×2), (6×2), (3×1), (5×5), (110) and (112)facets (1×1)O (3×1) (2×2), (1×1) c(2×2), (1×1), (3×10)-oxide c(2×4), (2×1), (3×1), (9×4) (2×1) (2×2), (√3×√3)R30° (2×2), c(2×2) (1×3), (1×2), pseudo-(2×1), c(2×6), c(2×4)-4O (1,2,2,0) (2×2), (√3×√3)R30°, (2×2)-oxide (2×2), c(2×2), (√5×√5), (5×5) (2×1), (4×2), c(2×2), (1×3), (1×5), (1×7) (2×2)-O, (√3×√3)R30°, (4√3×4√3)R30°, PtO2(0001), (3×15) (2×2)-3O (√3×√3)R30° (2×2), (√3×√3)R30° (√3×√3)R30° (2×2)

[Ref. p. 55

Ref. 62G1, 84K1 95W4 77N1 76B1 86W3, 95Z1 79T1 71G1, 79C1 76H1 69G1, 76R1, 81H1 81N1, 82F1

68H1, 89G2, 91D2, 86W2, 89G2 96K1/RHEED 77C1, 75K1, 85Z1 70D1 68H1, 69K1, 77C1, 75R1, 85Z1, 79B2 83M2 67H1 77P1 77P1, 73F1 85B1, 64M1, 68M1, 93B1 91H1 64M1, 81K1 64M1, 83D1 69E1, 89H2 81D1 77C2 82O1, 87C1, 88C1, 91S1 64T1, 76D1, 80S1 64T2, 77W1, 77L1 89P2 80D1 77L1 80D1 80D1

Landolt-Börnstein New Series III/42A4

Ref. p. 55] Substrate fcc-Pt (cont.) hcp-Re

fcc-Rh

hcp-Ru Ru stepped fcc-Ta

hcp-Ti fcc-Th bcc-V bcc-W

hcp-Zr

3.4.2 Adsorption of C, N, and O on metal surfaces Orientation (100)

O-surface structure (2√2×2√2)R45°, (5×1), (2√2×√2)R45°, (2×1), (3×1) (2√2×√2)R45° (611) (10 1 0) (2×3)O, c(2×4)2O, (1×5)2O, (1×4)2O, (1×3)2O (0001) (2×2) (110) (2×2)p2mg-2O, (2×3)p2mg, c(2×2n)3nO, n=3,4,5, ... (2×1)p2mg-2O (10×2) (2×1), (1×3) (331) (111) (2×2), (2×1), (8×8)-oxide (111) (1×1) (755), (331) (2×2) (100) (2×2), (2×2)gg, (10 1 0) c(4×2), (2×1)p2mg, c(2×6), (7×1) (101) (1,1,3,0), (2,1,5,0), (4,1,9,0) (0001) (2×2), (2×1), (2×2)-3O, (1×1)O (2×1)O (110) (3×1) (100) (2×8/9), c(3×1), (4×1), (3×3), (1×2), (1×3) (3×1) (211) (0001) (1×1), (2×2)O (111) disordered (100) disordered (110) (3×1), c(6×2) (100) (5×1) (110) (2×1)-O, (2×2)-3O, (1×1)-1O, c(14×7), c(2×2), c(21×7), c(48×16) (2×1) (10,1,1) (111) disordered, (211) facets (100) disordered, (4×1), (2×2), (2×1), (3×3), c(2×2), c(8×2), (3×1), (8×1), (4×4) (211), (221) (2×1), (1×n), n = 1, 2,..., 7 (0001) (2×2)2O, (1×1)1O, (1×1)2O (10 1 0) (2×4)

39

Ref. 77L1, 79M1, 77P2, 84B1 79M1, 83L1 91L1, 72Z1 69F1, 70D2 66T1, 67T1, 90S2, 91B1 01V1 93V1, 95F1 79T2, 80C1 99G1 79C1 78C1, 88O1 77O1 77R2 70G1, 75M1, 82D1, 97K1, 96S1 79P1 67H1 74C1, 85T1 67H1 58F1, 81J1, 85S1, 90G1 77B1 76T1 67H1, 00G2 82J3, 84G1 75E1, 78C3, 93J1, 67T2 73B1, 66G1, 78B2 77E1 79N1 73P1, 76B1, 81K2 67C1, 83W2, 85B1, 85W2 95W1, 95W2, 97W1 94Z1

Remarks: a) (1×1) can mean ordered (1×1)O phase or disordered phase, while (1×1)O is used for an ordered (1×1) overlayer of oxygen. b) Disorder occurs more frequently on (100) presumably due to higher diffusion barriers.

Landolt-Börnstein New Series III/42A4

40

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

3.4.2.2.8 Phase diagrams and phase transitions in the O-metal surface system Table 8. Phase diagrams and phase transitions of O-metal systems.

substrate

surface

fcc-Ag

c(2×2)O → (1×1)O phase transition at 180 K
Cu(110) Mo(110)

Ni(110) Ni(111)

Ni(100)

(100)

Rh(110)

(2×2) c(2×2) (2×2)pmg

Rh(111) Rh(100)

(1×n) (2×2)O (2×2)p4g

Pd(100)

Ru(0001) W(110)

W(100) W(112)

phase diagram/remarks/kind of order-disorder transition/critical temperature Tc

(2×2)O (2×1)O (2×1)O (2×2)3O (2×2)3O (2×1)→(2×2) (2×1)+(2×2) (2×1)+(2×2) (2×1)O→(1×1) (2×1)O (2×1)O (2×1)O

Tc = 550 K Tc = 590 K, T-θ phase diagram (2×2)pmg-(2×1) transition: 2D Ising model, Tc = 750 K Missing Row Reconstructions 4-states Potts model; Tc = 280±5 K transition to c(2×2): 2-state Ising model; Tc = 450±5 K. Tc = 754 K, 4-state Pott Tc = 555 K, 3-state Pott Tc = 709 K, 2-D Ising model 3-particle interaction: 15 - 40 meV order-disorder: trans. may be 1st-order. universal class: XY model disordering kinetics phase separation Deconstruction (order-order) phase transition, Tc = 903 K Tc = 900 K Self-similar growth Tc = 899 K, 2D-Ising

Ref./method 90A1/HREELS 90C2/STM 91D2/MCS 86W2/LEED 90G2/LEED 82K2/STM 80S1/RBS 95L2 94S1 87P1/LEED 81K1/LEED 85R1/HREELS 83R3/HeD 87C4/LEED 93B8/HeD 97K2/MC 97O1/HeD, LEED 96B4/SPALEED 87P1/LEED 90P3/LEED 93B8, 89W2/LEED 78C3/MCS 84R2 82K1 93J1 90T2/LEED 92G1, 86R4/LEED 83W2/LEED 89Z1/LEED+MCS 85W1/LEED

Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces

41

3.4.2.3 Nitrogen adsorption on metal surfaces

The interaction of atomic nitrogen with metal surfaces has been less intensely studied than that of oxygen. One reason might be that the high activation energy for dissociative adsorption of molecular nitrogen prevents easy production of atomic nitrogen adlayers on metal surfaces. The adsorption of atomic nitrogen and that of atomic oxygen is quite similar so that many general trends observed with O adsorption are equally found with atomic nitrogen. The most common methods to deposit atomic nitrogen on transition metal surfaces are: 1) Atomization of N2 using high frequency discharge [77S1], a hot filament [66M1] or an ion gun [82R1]. 2) Dissociative adsorption of hydrides of N (NH3 or N2H4) followed by removal of the hydrogen by thermal desorption or by oxidation with pre adsorbed oxygen [81K4, 86B3, 89M2, 91W1]. This method is not as “clean” as method 1. 3) Dissociative adsorption of oxides of N (e.g. NO) followed by removal of the oxygen by reduction, and then thermal desorption of the reducing agent [91T1, 92C1]. 4) Dissociation of NO by electron bombardment accompanied by electron stimulated oxygen desorption and following reduction of the residual oxygen by CO [93B3]. This method is not as “clean” as method 1. 5) Segregation of nitrogen (existing as impurity) from the bulk of the metal towards the surface. This effect was used with Cr(100) [97S4]. Other impurities in the bulk may segregate as well. A major motivation in surface chemistry to study nitrogen adsorption on metal surfaces is coming from the ammonia synthesis, i.e. the production of NH3 from N2 and H2 (Haber-Bosch synthesis) [50M1, 94T2]. A good catalyst for ammonia should be able to dissociate molecular nitrogen without forming too strong bonds to the metal surface or without forming nitrides [80E1]. This interplay of trends renders Fe, Ru and Os good catalysts for ammonia synthesis (see Fig. 13). Investigations of nitrogen adsorption are also interesting for the formation of metal-nitrides. The development of metal nitrides poisons the activity of a metal surface towards ammonia synthesis; this is particularly true with Fe [87S1].

10

2

NH 3 activity [arb.units]

Ru 10

1 10

-1

Os Mo Re

Ir

Rh Co Ni

10 -2 10 -3 0.4

Fe

Fig. 13. The activity of various transition metals for ammonia synthesis is shown as a function of the degree of filling the d-band [81O2].

Pt 0.5

0.6 0.7 0.8 d-band occupancy [%]

0.9

1.0

In general, the bonding of atomic nitrogen on metal surfaces is stronger than that of atomic oxygen (see Table 11). The main reason is that the valence states of N are so high that the coupling to the d states is strong enough to push anti-bonding states above the Fermi level. Although the N-metal bonding is strong, desorption temperatures are usually still quite low due to the strong internal bond of N2 (see Table 10).

Landolt-Börnstein New Series III/42A4

42

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

The strong N-metal bond causes frequently local and global reconstructions of the metal surface. The hard sphere radius of adsorbed nitrogen is between 0.55 and 0.6 Ǻ (see Table 14) which is close to the covalent radius of nitrogen (0.55 Ǻ). Like oxygen, the chemisorbed N species is mostly covalently bound to the metal surfaces, as seen by typical XPS values of N(1s) appearing at about 397.6 eV. In the valence band region, the peaks characteristic for atomic nitrogen are located at about 5 eV below the Fermi level (EF). This energy position for the N(2p) derived emission from adsorbed atomic nitrogen is typical for most transition metals (see Table 12). Typical N against metal vibration are in the range 30 - 60 meV (see Table 13). Again similar to O the adsorbed N atoms are able to form ordered overlayers on the metal surfaces (see Table 15). However, since the binding energy and therefore the diffusion barrier of adsorbed nitrogen is higher than that of adsorbed oxygen, the ordering process of N is frequently kinetically hindered. Due to the strong intra-molecular binding of N2, the dissociative sticking of N2 is also very low, typical values of 10−6 on Fe surfaces and 10−12 on Ru surfaces (see Table 9). Both metals are particular active catalysts for ammonia synthesis [94T2].

Landolt-Börnstein New Series III/42A4

Ref. p. 55]

Landolt-Börnstein New Series III/42A4

3.4.2.3.1 The dissociative sticking coefficient of nitrogen on metal surfaces Table 9. Sticking coefficient Surface

Sticking coefficient

Fe(110)

initial initial

1×10−7 0 6×10−6

Fe(100)

initial

10−6→ 0.01a),c) 1.2×10−5 1 - 4×10−7

Re(0001)

initial

Fe(111)

Re(1120)

initial

Ru( 10 1 0 )

initial

Ru( 112 1 ) Ru(0001)

initial initial

W(110)

initial

W(111) W(100)

initial

a) a') c) e) f')

Impact energy Ei [eV] 0.18 - 0.65

0.25 → 1

(9±2)×10−7 ~10−5 (4±1)×10−4 −12

<10

−12

<10

−12

<10 <10−3 5×10−7→10−2

1.25 0.15→4.0

0.003 <3×10−3→0.35 0.31→1 0.08 0.14→0.4a’),f’) 0.45→5 0.63

Remarks

Method

Ref.

583 K - 733 K zero sticking for entropy reasons 430 K; essentially no activation barrier, but a high entropy barrier e), direct molecular chemisorption (a-N2) 400 K - 580 K 380 K - 510 K

TDS Mol. beam TDS/DFT

77B2 83B1 82E2, 77B3/99M1

Mol. beam TDS

87R1 97A1 79E2, 82E1, 77B3, 76E1

218 K, Eact = 14±2 kJ/mol 218 K, Eact = 6±1 kJ/mol RT; little N in subsurface

TDS theory TDS

87H1 90B3 87H1

TDS

96D1

RT; little N in subsurface

TDS

96D1

RT sample 600 K activation barrier: ~2 eV, dissociative sticking is achieved by >1.3 eV 300 K

TDS Mol. beam Mol. beam/DFT

96D1 96S4 97R1/97M3

TDS Mol. beam (RT) KW Mol. beam (300 K) TDS, KW

79S1 86P3, 84L1 72K1 89R1 81B1, 74K1

300 K 340 K

3.4.2 Adsorption of C, N, and O on metal surfaces

Substrate

Initial adsorption probability increases with increasing kinetic energy Initial adsorption probability decreases with increasing kinetic energy Initial adsorption probability is surface temperature dependent Molecular intermediates detected after high kinetic energy exposure Initial adsorption probability decreases with increasing surface temperature

43

44

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

3.4.2.3.2 The heat of adsorption of chemisorbed nitrogen overlayers on metal surfaces Table 10. Heat of Adsorption. substrate

Ag(111) Cu(111)

surface

heat of adsorption [kJ/mol]

<0.42 ML

activation barrier for desorption [kJ/mol] 106 and 130 143

>0.42 ML

88

Remarks/Method

Ref.

Recombinative desorption

97C2

recombinative desorption via the Cu(100)-c(2×2)N phase

98M3

Fe(110)

212

Fe(111)

203

TDS: 860 K, first order

98S4 77B3, 76E1 77B2

Fe(100) poly-Fe

222

TDS: 980K, first order

77B3

initial initial

167 293 531

51B1 55B1 79E3

initial

136

96V1

Ir(111) Ni(100)

TDS: 920 K, first order

Re(0001)

601

Rh(110)

(2×1)N

83

Rh(111) Rh(100) Rh(410) Rh(533)

disordered 88 (2×1)N 107 - 120 (2×1)N 180 disordered 149 165 100 70

recombinative desorption desorption energy: (260 − 80θ) kJ/mol, 0 ≤ θ ≤ 1.0 TDS, desorption at 580 K, isothermal desorption: νd = 2×107 s-1 νd = 2×108 s-1 νd = 2×108 - 2×1010 s-1 TDS, νd =5×1011 s-1 TDS, νd = 1011 s-1 TDS, νd = 107 s-1 TDS, νd = 107 s-1

87H1 87G1 92C1 93K1 92L1 98S6 93B3 97J1 97J1 97J1

Re(0001)

487

Ru(10 1 0)

120

TDS

Ru(0001)

TDS TDS TDS

initial

184 190 112-120 575

poly-W

initial

397

51B1

W(110)

(2×2)N

648

71T1

poly-Ta

90H1 97D2 93S4 93R1 50B1

Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces

45

3.4.2.3.3 Nitrogen-metal bond strength (ab initio calculations) Table 11. The N-metal bond strength as computed by ab-initio calculations.

substrate

surface

N-metal bond strength

Ref./method

Ag(100) Cu(100) Fe(110)

initial initial 1/3 ML (2×1)N (√3×√3)N (1×1)N c(2×2)N c(2×2)N (1×1)N (2×2)-N (√3×√3)-N c(2×2)-N (√3×√3)-2N (1×1)N (2×2)-N c(2×2)N (1×1)N (2×2)N (2×2)N (√3×√3)N c(2×2)-N (√3×√3)-2N (1×1)N (2×2)-N c(2×2)N (1×1)N (2×2)N (2×2)N (2×1)N (2×2)3N (1×1)N (2×2)N (1×1)N

−14.5 kJ/mol (endotherm) 230 kJ/mol 135 kJ/mol (probably wrong structure) 550 kJ/mol w.r.t. atomic N 135 kJ/mol (probably wrong structure) 111 kJ/mol 234 kJ/mol 572 kJ/mol w.r.t. atomic N (assumes the atomic geometry) 116 kJ/mol 383 kJ/mol 383 kJ/mol 314 kJ/mol 276 kJ/mol 246 kJ/mol 407 kJ/mol 362 kJ/mol 169 kJ/mol 415 kJ/mol 441 kJ/mol 446 kJ/mol 408 kJ/mol 371 kJ/mol 303 kJ/mol 490 kJ/mol 462 kJ/mol 383 kJ/mol 561 kJ/mol 539 kJ/mol 513 kJ/mol 475 kJ/mol 436 kJ/mol 617 kJ/mol w.r.t. atomic N 409 kJ/mol, N-octahedral between 2. and 3. Zr layer

97R4/Cluster 97R4/Cluster 99M1/DFT 90R4/CEM 99M1/DFT

Fe(111) Fe(100)

Pd(111)

Pd(100)

Pt(111) Rh(111)

Rh(100)

Ru(0001)

W(110) Zr(0001)

99M1/DFT 90R4/CEM 99M1/DFT 98L1/DFT

98L1/DFT

01M2/DFT 98L1/DFT

98L1/DFT

97S2/DFT

90R4/CEM 97Y1/DFT

Remark: Cluster calculations produce less reliable values for N-metal binding energies than slab calculations.

Landolt-Börnstein New Series III/42A4

46

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

3.4.2.3.4 Electronic properties of chemisorbed nitrogen on metal surfaces Table 12. Electronic properties of chemisorbed atomic nitrogen overlayers on metal surfaces.

substrate

surface

Ag(111) Cr(110)

10 L N2

Cr(111)

Binding energies of core and valence states

Ref.

N(1s): 397.9 eV, UPS: 3.4 eV, 8.2 eV

77G2/XPS, UPS

At room temp.: surface nitride; shift of the peak at 41.5 eV

84M1/EELS

N(1s): 397.8 eV

88F1/XPS

Cr(100)

(1×1)N

N(2p): 2.5, 4.7eV disperse strongly by 2eV, 1eV

80G3/ARUPS

Cu(110)

(2×3)-3N >0.5 ML

N(1s) 396.5eV N(1s): 396.5, 397.2 eV a second N species on 4fold site

90B2/XPS

Cu(100)

c(2×2)N

N(2p): 1.2 eV, 5.6 eV below EF N(1s): 396.3 eV N(2pz), N(2px,y) separated Calculated XAS, XES N(1s) spectra N K emission: 391.4, 396 eV and 390.3, 393.2, 395.2, 397 eV: strong 2p-4sp bonding state, strong 2p-3d antibonding state N(1s): 396.4 eV N(2p): 1 eV, 6 eV below EF

77T1/UPS 77T1/XPS 98W3/SXES, XAS 98T3/cluster 94W1/SXES

Fe(111)

N(2p): 5 eV, 1.8 eV N(1s): 397.5, 397.0 eV on- and sub-surface nitrogen

77B3 87A3/PES

Gd(0001)

N(2pz): 5.6 eV, N(2px,y): 3.4 eV

95W3/PES

Ir(110)

NO dissociation; atomic N: N(1s): 398.14eV

00D1/XPS 94H1/ IPE

(2×2)p4g-N (2×2)p4g-N

−1.0, −3.5 eV above EF, probably due to hybridization of N(2p) with Ni(d), KRIPES: flat dispersion. N K emission: 391.6 eV, 397 eV: 2p-4sp bonding state; weak 2p-3d antibonding state N(1s): 397.3 eV N(2p): 1.5, 5 - 6 eV below EF

Re(0001)

(2×2)N

5 eV

89H1/ELS

Rh(110)

(2×1)N

dispersion of N(2py) along Rh-N chains (y95D1/ ARUPS direction) from 5.4 to 6.3 eV, no dispersion perp. to Rh-N rows.

Ti(0001)

1 - 1000 L

300 K: N(1s): 397.3 eV (octahedral), 395.8 eV (on-surface) 200 K: N(1s): 398.2 eV (on-surface), 397.3 eV (octahedral) 473 K: N(1s): 397.3 eV (octahedral), 397.8 eV (dissolved), 395.8 eV (on-surface)

97F1/XPS

N(2p) 6 eV below EF

79S1/UPS

low N coverage low N coverage c(2×2)N c(2×2)N c(2×2)N

Ni(100)

(2×2)N (2×2)p4g-N

1 - 1000 L 1 - 1000L W(110)

(2×2)N

90N1/XPS 94W1/UPS

94W1/SXES 90N1/XPS 89K1/UPS

97F1/XPS 97F1/XPS

Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces

47

3.4.2.3.5 Vibrational properties of chemisorbed nitrogen atoms Table 13. Vibrational properties and suggested adsorption sites of (atomic) nitrogen overlayers on metal surfaces. If not otherwise indicated the technique of HREELS or IRAS was used.

substrate

structure

Cr(111)

Me-N stretch vibration [meV]

adsorption site

Ref.

67

3-fold

88F1

Cr(100)

initial

70

2-fold

85B3

Cu(110)

(2×3)-N (2×3)N

50 10.5, 45.5, 54.5, 68.0, 45.5, 68.0, 82.0, 82.0, 86.5, 71.5, 48.5, 84.5

quasi-5-fold N-modes confirmed by theory

88H1 96H2

Cu(111)

34.5, 50.5 distorted Cu(100)-c(2×2)

4-fold

86H1

Cu(100)

c(2×2)N

40.5, 94 perpendicular Cu-N

4-fold

87M4

Fe(111)

-

56

-

84G1, 85T2

Fe(100)

c(2×2)N

61

4-fold

90L2

Ni(110)

(2×3)-N

24, 46

long-bridge

88K1

Ni(100)

(2×2)p4g-N

89.0 - 92.5 (parallel mode) 34 - 54 (perpendicular) Rayleigh mode temperature dep.

reconstruction

86D1

Pd(110)

(2×3)-N

32 55 90 130

long-bridge long-bridge short-bridge on-top

87K1

Rh(110)

(2×1), (3×1)

Re(0001)

56

long-bridge

92C1

46

3-fold

90H1

Ru(10 1 0)

0.6 ML (−1,1,2,1)-N

60 (perp.), 41 (lateral)

hcp-sites

97D2

Ru(0001)

(2×2)N (√3×√3)N (2×2)N

72 (perpendicular)

hcp site

93S1

67 (perp.), 63 (lateral)

hcp site

97M3/DFT

W(110)

(4×1)N

90 (β1-TD peak) 60 (β2-TD peak)

-

89S2

W(100)

c(2×2)N

60

4-fold hollow

80H1

Landolt-Börnstein New Series III/42A4

48

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

3.4.2.3.6 Local atomic nitrogen-metal geometry Table 14. The N-metal bond length and other structural characteristics.

substrate Ag(100)

surface initial

N-metal [Ǻ] 2.12

Adsorption site/Remarks 4-fold

Cr(100)2Co

(1×1)N

2.04, 1.94

deep in hollow site of bcc Co bilayer 93S3/LEED

Cr(100)

(1×1)N

2.04, 2.02

deep in hollow site

Cr(110)

c(2×2)N (√6×√6)R35°

2.07, 1.97 -

Cu(110)

(2×3)-4N

deep in hollow site N-induced herringbone reconstruction quasi-5-fold, square reconstruction

1.83, 2.12 1.87, 1.88 1.81 Cu(111) Cu(100)

LT 500 K c(2×2)N c(2×2)N c(2×2)N c(2×2)N initial

1.81, 1.95 1.85, 2.29 1.77 1.87 1.84

disordered N Cu(100)-c(2×2)N N: deep in 4-fold hollow site N-Cu co-planar, lower symmetry

Ref./Method 97T1/Cluster

89J1, 92J1, 97R5/LEED 98S5/LEED, STM 97S4/STM 90A2/LEIS 90R1/PhD 91S2/NICISS 93B5/SXRD 94V1, 97M1/LEED 96W1/SEXAFS 91D3/EXELFS 98S4/STM 87Z1/LEED 93L2/SEXAFS 01H1/PhD 01D3/PhD 97T1/Cluster 91D3/EXELFS 99M1/DFT

Fe(110)

0.5 ML

Vertical rumpling of 0.34 Ǻ Fe(100)-c(2×2)N like overlayer

Fe(111)

0.5 ML

Fe(100)-c(2×2)N like overlayer

99M1/DFT

Fe(100)

Mo(100)

c(2×2)N c(2×2)N (2×2)N (1×1)N c(2×2)N

1.81 1.90 1.96 1.85 2.45, 2.60

4-fold 4-fold 4-fold, no experiment 4-fold, no experiment 4-fold

82I1/LEED 99M1/DFT 99M1/DFT 99M1/DFT 75I1/LEED

Ni(110)

(2×3)-3N

1.86±0.03

N hollow, square reconstruction

94W1/SEXAFS

-

N-hollow at room temperature Ni(100)-c(2×2)N like structure after annealing at 670 K deep in 4-fold site, clock reconstruction type 1

93O2/NEXFAS

Ni(111)

(2×2)p4g-N

1.89

(2×2)p4g-N (2×2)p4g-N

1.85 1.85 -

Pd(111)

(√3×√3)N

Pd(100)

c(2×2)N

Ni(100)

87W1/SEXAFS 91K1/PhD 99A1/DFT 99D2/SXRD

1.94

lateral displacements of topmost Ni atoms by 0.3 Ǻ fcc (assumed)

98L1/DFT

2.07

4-fold

98L1/DFT Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces

substrate Rh(110)

surface (2×1)N

Rh(111)

(√3×√3)N

Rh(100)

c(2×2)N

Ru(0001)

49

N-metal [Ǻ] 1.87±0.08 1.94±0.08 1.95

Adsorption site/Remarks long-bridge, missing row

Ref./Method 95G2, 95D1/LEED

fcc (assumed)

98L1/DFT

W(110)

(2×2)N (√3×√3)N (2×2)N (√3×√3)N (1×1)N 1-1000L at RT (2×2)

2.07 2.03 1.93±0.05 1.93±0.06 2.00, 1.98 1.97 2.10 -

4-fold 4-fold hcp hcp hcp hcp N underlayer, octahedral N-octahedral and on-surface N-underlayer

98L1/DFT 99A1/DFT 97S2/LEED 97S2/LEED 97S2, 97M2/DFT 97S2/DFT 76S1/LEED 97F1/XPD, XPS 79S1/ISS

W(100)

<0.5 ML

1.67, 1.87

-

98O4/EELFS

W(100)

c(2×2)N

2.28, 2.13

deep in 4-fold site 0.27 Å buckling in the 2. W-layer

95B1, 82G2/LEED

Zr(0001)

(1×1)N

2.27 2.27

N underlayer N-octahedral

87W1/LEED 97Y1/DFT

Ti(0001)

3.4.2.3.7 Ordered overlayers of chemisorbed nitrogen atoms on metal surfaces Table 15. Ordered overlayers of chemisorbed nitrogen on metal surfaces. If not otherwise indicated the technique of LEED was used. No structure determination.

Substrate Orientation

N-surface structure

Ref./Method

fcc-Cu

(2×3) (2×3) Pseudo (100) reconstruction c(2×2) c(2×2) 2 phases, reconstruction (2×5) 4 phases c(2×2), (3×3) reconstruction (3√3×3√3)R30°, (5×5) c(2×2) initial adsorption of nitrogen (2×3) (2×3) c(5√3×9)rect., “(2×6)” (2×2)p4g (2×3) c(2×2)N (2×1), (3×1) disordered c(2×2)

94V1 98T4/STM 99D1/STM 76B1 01D2, 01E1/STM 77B2 97H2 77B3 97A1 82I1 00P1/STM 82R1 96T2, 93V2/STM 93G3 93L3/STM 88K1 90Y1, 91Y1 92C1 92B1 91T1

(110) (111) (100)

bcc-Fe

(110) (211) (111) (100)

fcc-Ni

fcc-Pd fcc-Rh

(110) (111) (100) (110) (100) (110) (111) (100)

Landolt-Börnstein New Series III/42A4

50

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

Substrate Orientation

N-surface structure

Ref./Method

hcp-Re hcp-Ru hcp-Ti bcc-W bcc-W

(2×2) (2×2), (√3×√3)R30° (1×1)N underlayer c(2×2) (2×2), N underlayer

89H1 94B1 76S1 68O1 79S1

(0001) (0001) (0001) (100) (110)

3.4.2.4 Carbon adsorption on metal surfaces

The interaction of atomic carbon with metal surfaces has been even less intensely studied than oxygen and nitrogen, mainly because the preparation of pure carbon layers is more involved. The preparation of carbon overlayers requires the dissociation of carbon containing molecules, such as C2H2, CO, C2H4 etc., leading to coadsorption. In order to end up with a pure carbon overlayer, the other constituents, such as oxygen and hydrogen, have to be removed from the surface. Another popular procedure to produce Coverlayers on metal surfaces is segregation of carbon from the bulk. Carbon is able to form carbidic carbon (isolated adsorbed C atoms) and graphite overlayer on metal surfaces. Only the carbidic carbon is desired in catalytic reaction. Graphite layers are considered to poison the catalytic activity of metal surfaces. CO and H2 can be catalyzed to form CH4 and higher alkanes together with water [82B1]. This process requires as the rate determining step the dissociation of CO on the catalyst’s surface. If the C-O bond is preserved, the reaction with H2 results in products containing oxygen, such as alcohols. For the catalytic CO hydrogenation reaction Sabatier was awarded with the Nobel Prize in chemistry 1912 [12S1]. The binding mechanism of carbidic carbon to metal surfaces is similar to that of oxygen and nitrogen. This is reflected in the electronic properties (see Table 17) where about 5 eV below the Fermi energy C(2p) derived states are observed. Typical C against metal vibrations reveal energy losses of about 50 60 meV (see Table 18). The bond strength determined by ab-initio calculations is about 6 eV (see Table 16). The resulting hard sphere radius of carbon on metal surfaces (determined by the carbon-metal bond length minus the half of the metal-metal bond length) is about 0.55 Å which is substantially smaller than typical O and N hard sphere radii of about 0.70 Å and 0.65 Å, respectively; for C-metal bond length the reader is referred to Table 19. However, due to the smaller size of carbon in comparison to oxygen and nitrogen, carbon atoms can easily penetrate the topmost metal layer. Quite in contrast, a graphite overlayer is characterized by a very strong internal C-C binding energy and a weak interaction to the metal surface. Due to the strong C-C bonding, graphite forms always a hexagonal overlayer regardless of the symmetry of the metal surface. Carbidic carbon, on the other hand, is able to form ordered structures as manifested in Table 20. Many physical-chemical quantities cannot be measured. For instance, the sticking coefficient of carbon does not make sense, since the formation of carbon on a metal surface requires several reaction steps on the surface such as with the dissociation of C2H2 or CO and other C containing products. The same problem is encountered with the heat of adsorption. Therefore sticking coefficients and experimentally determined heats of adsorption are not presented for carbon. The interaction with C is also important for studying the formation of metal carbides, diamond films, nano-tubes and C60 molecule.

Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces

51

3.4.2.4.1 Carbon-metal bond strength Table 16. The C-metal bond strength as computed by ab-initio calculations.

substrate

surface

C-metal bond strength [kJ/mol]

Me-C bond length [Å], Ref./method adsorption site

Co(0001)

Ni(100) Pd(111) Pt(111)

(1×1)C (2×2)C single C (2×2)C single C

680 646 530 655 296 656 743 589 644 708 480 589 608 728 690, 713 614 526 651

1.79, hcp 1.85, subs. 1.84, hcp 1.90, subs.

Cu(111) Fe(100) Ni(111)

(2×2)C (2×2)C 1 ML C 1 ML C single C single C single C (2×2)C (2×2)C (2×2)C 1 ML C 1 ML C

2.17 3-fold hollow fcc 1.89, fcc 1.86, subs. 1.89, fcc 1.91, subs.

3-fold hollow

98K3/DFT 98K3/DFT 98K3/DFT 97L1/cluster 93B4/cluster 97L1/cluster 00W2/DFT 98K3/FP-LAPW

98B1/DFT 74S1/ESS 75I1/ESS 97L1/cluster* 97M1/DFT 97L1/cluster

Remarks a) *be careful: for oxygen on the same surface the authors favored on-top adsorption! b) Cluster Calculations produce less reliable values for C-metal binding energies than corresponding slab calculations. 3.4.2.4.2 Electronic properties of chemisorbed carbon on metal surfaces Table 17. Electronic properties of chemisorbed atomic carbon overlayers on metal surfaces. Substrate

Surface

Co(100) Cu(110)

(2×2)-C

Cu(100) Fe(100) Ir(110) Mo(110) Mo(001)

State

Binding energy [eV]

273 272 288, 295 272 281, 288, 295 c(2×2)-C C(2p) 3.0 c(2×2)-C C(2p), Fe(3d) −2.5, −3.9 contamination C(1s) 284.5 (4×4) C(1s) 273 (1×1)C C(1s) 283 Mo 225.5, 229.5

Landolt-Börnstein New Series III/42A4

Remarks

Ref./Method

fcc-Co(100): carbidic C Carbidic C C K-edge Carbidic C C K-edge

78M4/AES 87S2/AES 87S2/EELS 87S2/AES 87S2/EELS 77R3/UPS 92H2/IPE 97L2/XPS 92H3/AES 76G1/XPS

antibonding states

Carbidic C

52

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

Substrate

Surface

State

Binding energy [eV]

Remarks

Ref./Method

Ni(110)

(4×5) (2×1)-C

C(2p) C(2p)

~4

85P1/UPS 91P3/PES

(2×2)C 0.2 ML (2×2)C

C(2p)

Carbidic C disperses from 6 eV to 6.5 eV, from 4.2 eV to 5.2 eV disperses from 4 eV to 6 eV

N(111) Ni(100)

1.0, 4.2, 13.0 −1.5 - −2

(2×2)C (2×2)-C

Re(0001) Rh(111) Ru(0001) W(100)

(2×2)-C (2×2)p4g-C (2×2)p4g-C 7×√19 C+O coadsorption Carbon 0.5-1.0 ML 0.1-1.0 ML

C(2p) C(2s) C(1s) Ni(2p3/2) C(1s)

C(s,pz) derived C(2p) W(4f7/2)

4.0, 6.0 11 - 13 282.9 282.9 853.2 273 5.2, 9.4, 11.5

probably hybridization of O(2p) with Ni(d), KRIPES: flat dispersion. C(2p) derived state disperses from 3.4eV to 4.6eV

86M1/ARUPS 83R2/PES 94H1/IPE 86P2/ARUPS 82K2/UPS

C KVV

91H2, 91H3/AES 91N1/XPS 94Z2/XPS 72Z2/AES 89K1/UPS

9.8

82H1/UPS

4.8 31 - 32

84J1/UPS 95L3/HRCLS

several peaks

3.4.2.4.3 Vibrational properties of chemisorbed carbon atoms Table 18. Vibrational properties of (atomic) carbon overlayers on metal surfaces. If not otherwise indicated the technique of HREELS or IRAS was used.

Substrate

Structure

Me-C stretch Remarks vibration [meV]

Cr(111) Cr(110) Cr(100) Fe(111) Fe(100) Ni(100)

55, 71 53, 70 low coverage 65 51 c(2×2)C 54 50 disordered (2×2)p4g-C 42.5 87.5 - 90.6

dissociation of CO, 0.5 L CO dissociation of CO, 0.2 L CO dissociation of CO

(<0.3 ML) perpendicular perpendicular mode parallel mode

Adsorption site

Ref.

3-fold, 2-fold 3-fold, 2-fold 4-fold 3-fold 4-fold

86B5 86B5 85B3, 86B5 86B2 90L2 87R2, 89S3

clock-wise reconstruction

Landolt-Börnstein New Series III/42A4

Ref. p. 55]

3.4.2 Adsorption of C, N, and O on metal surfaces

53

3.4.2.4.4 Local atomic carbon-metal geometry Table 19. The C-metal bond length and other structural characteristics.

Substrate

Surface

C-metal [Ǻ] Adsorption site/Remarks

Ref./Method

Al(111)

C disordered low C cov.

-

hcp!

90B3, 92B2, 95F2

2.1 1.75 1.79 1.85 1.84 1.90 2.09, 1.92 2.28, 2.17 -

4-fold hcp hcp substitutional hcp substitutional C-underlayer 4-fold 4-fold, no reconstruction deep in 4-fold site upon annealing C penetrates the region (5 Å below the surface) 3-fold in troughs fcc fcc substitutional fcc substitutional deep in 4-fold site, clock reconstruction type 1

86M1/cluster 87A1/LEED 98K3/FP-LAPW

Al(100) Co(0001)

Fe(111) Fe(100) Mo(100) Mo(111) Ni(110) Ni(111)

Ni(100)

(2×2)C (2×2)C 1 ML C 1 ML C cluster c(2×2)C+O low C cov. c(2×2)C small amounts of C (2×1)C (2×2)C (2×2)C (2×2)C 1 ML C 1 ML C (2×2)p4g-2C (2×2)p4g-2C (2×2)p4g-2C (2×2)p4g-C 0.15 ML

Rh(100) W(100)

c(2×2)-2C (1×5)4C

Zr(0001)

(√2×√2)R45° (1×1)C

Landolt-Börnstein New Series III/42A4

1.85 1.89 1.76 1.86 1.89 1.91 1.82, 1.95 1.85, 1.99 1.85 1.85 1.87, 1.99 1.79, 1.94 2.01 C-W layer distance: 0.5 Å 2.29

undistorted 4-fold site no clock reconstruction 4-fold hollow O-coadsorption drives C into the subsurface region surface reconstruction C-underlayer, octahedral

91A1/cluster calc. 78J2/LEED 83O1/LEIS 95J2/LEED 87O1/LEIS 91C2/EELFS 98K3/FP-LAPW 00W2/DFT 98K3/FP-LAPW 98K3/FP-LAPW 98K3/FP-LAPW 91G1, 79O1/LEED 91K1/PhD 87B1/SEXAFS 89A2/SEELFS 99A1/DFT 99T1/PhD 99A1 88M1/LEIS 89M1/AES, LEIS 95L3/HRCLS 88W1/LEED

54

3.4.2 Adsorption of C, N, and O on metal surfaces

[Ref. p. 55

3.4.2.4.5 Ordered overlayers of chemisorbed carbon atoms on metal surfaces Table 20. Ordered overlayers of chemisorbed carbon on metal surfaces.

Substrate Orientation

N-surface structure

Ref.

fcc-Co hcp-Co bcc-Fe bcc-Mo

(100) (1120) (100) (100) (100) (110)

fcc-Ni

(110) (100)

p(2×2)C (2×5) c(2×2) (1×1)C (4×4)C; α-Mo2C(0001) (2×1), (1×2) (4×4) (4×5)pmg (2×2)p4g, splitted c(2×2)

78M4 98V1/STM 90L2 80K1 91Y1 76G1 92H3 85P1 79O1, 87B1, 87R2

clock reconstruction clock reconstruction (1×1)2C, graphitic C c(5√3×9)rect (√39 × √39)R16.1° 7×√19 c(2×2)-2C, c(3×2)-4C, (5×1)-4C R(15×12) R(15×3) (1×1)C underlayer

94H3/STM 00N1/STM 83R2 93G3 89N2 72Z2 73L1, 84J1, 89M1 68B1/LEED 95B8, 96B6/STM 88W1

(100) (997) (111)

hcp-Re bcc-W

(0001) (100) (110)

hcp-Zr

(0001)

Landolt-Börnstein New Series III/42A4

3.4.2 Adsorption of C, N, and O on metal surfaces 3.4.2.5 References for 3.4.2

12S1 50B1 50M1 51B1 55B1 58F1 60B1 60W1 62G1 64G1 64M1 64T1 64T2 65M1 66B1 66G1 66M1 66T1 67C1 67E1 67J1 67H1 67T1 67T2 68B1 68H1 68M1 68O1 69E1 69F1 69G1 69K1 69M1 70D1 70D2 70G1 71B1 71E1 71G1 71T1 72B1 72K1 72M1 72V1 72Z1 72Z2 73B1 73B2 73E1 73F1 73F2 73L1

Sabatier, P., Senderens, J.B.: C.R. Acad. Sci. Paris 134 (1902) 514. Beeck, O, Cole, W.A., Wheeler, A.: Disc. Faraday Soc. 8 (1950) 314. Mittasch, A.: Adv. Catal. Relat. Subj. 2 (1950) 81. Beeck, O.: Adv. Catal. 2 (1951) 151. Bagg, J., Tompkins, F.C.: Trans. Faraday Soc. 51 (1955) 1071. Farnsworth,H.E., Schlier, R.E., George, T.H., Buerger, R.M.: J. Appl. Phys. 29 (1958) 1150. Brennan, D., Hayward, D.O., Trapnell, B.M.W.: Proc. R. Soc. (London) Ser. A 256 (1960) 81. Wedler, G.: Z. Phys. Chem. (Frankfurt) 24 (1960) 73. Germer, L.H., MacEae, A.U.: J. Appl. Phys. 33 (1962) 2923. Gobeli, G.W., Allen, F.G., Kane, E.O.: Phys. Rev. Lett. 12 (1964) 94. MacRae, A.U.: Surf. Sci. 1 (1964) 319. Tucker jr., C.W.: J. Appl. Phys. 35 (1964) 1897. Tucker jr., C.W.: Surf. Sci. 2 (1964) 516. Müller, K.: Z. Naturforsch. 20A (1965) 153. Brennan, D., Graham, M.J.: Discuss. Faraday Soc. 41 (1966) 95. Germer, L.H., May, J.W.: Surf. Sci. 4 (1966) 452. Mimeault, V.J., Hansen, R.S.: J. Phys. Chem. 70 (1966) 3001. Tucker, C.W.: J. Appl. Phys. 37 (1966) 4147. Chang, C.C., Germer, L.H.: Surf. Sci. 8 (1967) 115. Ertl, G.: Surf. Sci. 6 (1967) 208. Jona, F.: J. Phys. Chem. Solids 28 (1967) 2155. Haas, T.W., Jackson, A.G., Hooker, M.P.: J. Chem. Phys. 46 (1967) 3025. Tucker, C.W.: J. Appl. Phys. 38 (1967) 124. Tucker, C.W.: Surf. Sci. 6 (1967) 2696. Baudoing, R, Stern, RM: Surf. Sci. 10 (1968) 392. Haydek, K., Farnsworth, H.E.: Surf. Sci. 10 (1968) 429. May, J.W., Germer, L.H.: Surf. Sci. 11 (1968) 443. Onchi, M, Farnsworth, H.E.: Surf. Sci. 11 (1968) 203. Ertl, G., Rau, P.: Surf. Sci. 15 (1969) 443. Farnsworth, H.E., Zehnder, D.M.: Surf. Sci. 17 (1969) 7. Grant, J.T.: Surf. Sci. 18 (1969) 228. Kann, H.K.A., Feuerstein, S.: J. Chem. Phys. 50 (1969) 3618. Murgulescu, I.G., Vass, M.I.: Rev. Roum. Chim. 14 (1969) 1201. Dooley, G.J., Haas, T.W.: J. Vac. Sci. Technol. 7 (1970) 49. Dooley, G.L., Haas, T.W.: Surf. Sci. 19 (1970) 1. Grant, J.T., Haas, T.W.: Surf. Sci. 21 (1970) 76. Bedair, S.M., Smith jr., H.P.: J. Appl. Phys. 42 (1971) 3616. Eastman, E., Cashion, J.K.: Phys. Rev. Lett. 27 (1971) 1520. Grant, J.T.: Surf. Sci. 25 (1971) 460. Tamm, P.W., Schmidt, L.D.: Surf. Sci. 61 (1971) 317. Becker, G.E., Hagstrum, H.D.: Surf. Sci. 30 (1972) 505. King, D.A., Wells, M.G.: Surf. Sci. 29 (1972) 454. Madey, T.E.: Surf. Sci. 33 (1972) 355. Van Hove , H., Leysen, R.: Phys. Status Solidi A9 (1972) 361. Zehner, D.M., Farnsworth, H.E.: Surf. Sci. 30 (1972) 335. Zimmer, R.S., Robertson, W.D.: Surf. Sci. 29 (1972) 230. Baker, J.M., Eastman, D.E.: J. Vac. Sci. Technol. 10 (1973) 223. Bonzel, H.-P., Ku, R.: Surf. Sci. 40 (1973) 85. Engelhardt, H.A., Bradshaw, A.M., Menzel, D.: Surf. Sci. 40 (1973) 410. Farrell, H.H., Strongin, M.: Surf. Sci. 38 (1973) 18, 31. Farrell, H.H., Isaacs, H.S., Strongin, M.: Surf. Sci. 38 (1973) 31. Levy, R.B., Boudart, M.: Science magazin (AAAS) 181 (1973) 547.

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56 73M1 73P1 74B1 74C1 74H1 74K1 74M1 74R1 74S1 74Y1 75B1 75B2 75B3 75B4 75C1 75E1 75F1 75I1 75I2 75K1 75M1 75M2 75R1 75V1 76B1 76B2 76B3 76B4 76C1 76D1 76D2 76E1 76E2 76F1 76G1 76H1 76H2 76I1 76K1 76M1 76R1 76R2 76S1 76S1 76T1 76Y1 76Z1 77B1 77B2

3.4.2 Adsorption of C, N, and O on metal surfaces Michel, P., Jardin, Ch.: Surf. Sci. 36 (1973) 478. Papageorgopoulous, C.A., Chen, J.M.: Surf. Sci. 39 (1973) 313. Bradshaw, A.M., Menzel, D., Steinkilberg, M.: Discuss. Faraday Soc. 58 (1974) 46. Chesters, M.A., Hopkins, B.J., Leggett, M.R.: Surf. Sci. 43 (1974) 1. Holloway, P.J., Hudson, J.B.: Surf. Sci. 43 (1974) 123. King, D.A., Wells, M.G.: Proc. R. Soc. London Ser. A339 (1974) 245. Messmer, R.P., Tucker, C.W., Johnson, K.H.: Surf. Sci. 42 (1974) 341. Rovida, R., Pratesi, F., Maglietta, M., Ferroni, E.: Surf. Sci. 43 (1974) 230. Shelton, J.C., Patil, H.R., Blakey, J.M.: Surf. Sci. 43 (1974) 493. Yates, J.T., Erickson, N.E.: Surf. Sci. 44 (1974) 489. Briggs, D.: Faraday Discuss. Chem. Soc. 60 (1975) 81. Brundle, C.R., Carley, A.F.: Chem. Phys. Lett. 33 (1975) 41. Baenninger, U., Bas, E.B.: Surf. Sci. 50 (1975) 279. Batra, I.P., Robaux, O.: Surf. Sci. 49 (1975) 653. Conrad, H., Ertl, G., Küppers, J., Latta, E.E.: Surf. Sci. 50 (1975) 296. Engel, T., Niehus, H., Bauer, E.: Surf. Sci. 52 (1975) 237. Fuggle, J.C., Madey, T.E., Steinkilberg, M., Menzel, D.: Surf. Sci. 52 (1975) 521. Ignatiev, A., Jona, F., Jepsen, D.W., Marcus, P.M.: Surf. Sci. 49 (1075) 189. Isett, L.C., Balekley, J.M.: Surf. Sci. 47 (1975) 645. Kennett, H.M., Lee, A.E.: Surf. Sci. 62 (1975) 606. Madey, T.E., Engelhardt, H.A., Menzel, D.: Surf. Sci. 48 (1975) 304. Marcus, P.M., Demuth, J.E., Jepesen, D.W.: Surf. Sci. 33 (1975) 501. Riwan, R., Guillot, C., Paigne, J.: Surf. Sci. 47 (1975) 183. Van Hove, M.A., Tong, S.Y.: Phys. Rev. Lett. 35 (1975) 1092. Bruker,C.F., Rhodin, T.N.: Surf. Sci. 57 (1976) 523. Burkstrand, J.M., Kleiman, G.G., Tibbets, C.G., Trancy, C.J.: J. Vac. Sci. Technol. 13 (1976) 291. Bauer, E., Poppa, H., Vishanath, Y.: Surf. Sci. 58 (1976) 517. Binder, K, Landau, D.P.: Surf. Sci. 61 (1976) 577. Comrie, C.M., Weinberg, W.H., Lambert, R.M.: Surf. Sci. 57 (1976) 619. Ducros, R., Merrill, R.P.: Surf. Sci. 55 (1976) 227. Dorfeld, W.G., Hudson, J.B., Zuhr, R.: Surf. Sci. 57 (1976) 460. Engelhardt, A.M., Menzel, D.: Surf. Sci. 57 (1976) 591. Ertl, G., Grunze, M., Weiss, M.: J. Vac. Sci. Technol. 13 (1976) 314. Froitzheim, H., Ibach, H., Lehwald, S.: Phys. Rev. B 14 (1976) 1362. Gillet, E., Chiarena, J.C., Gillet, M.: Surf. Sci. 55 (1976) 126. Hagen, D.I., Nieuwenhuys, B.E., Rovida, G., Somorjai, G.A.: Surf. Sci. 57 (1976) 632. Helms, C.R., Bonzel, H.P., Kelemen, S.: J. Chem. Phys. 65 (1976) 1773. Ivanov, V.P., Boreskov, G.K., Savchenko, V.I., Egelhoff, W.F., Weinberg, W.H.: Surf. Sci. 61 (1976) 207. Krishnan, N.G., Delgass, W.N., Robertson, W.D.: Surf. Sci. 57 (1976) 1. Martinsson, C.W.B., Petersson, L.-G., Flodström, S.A., Hagström, S.B., in: Proc. Int. Study Conf. on Photoemission from Surfaces (Willis, R.F., Feuerbacher, B., Filton, B., Backx, C. eds.), Nordwijk, Holland, 1976, p. 177. Rhodin, T.N., Brodén, G.: Surf. Sci. 60 (1976) 466. Rösch, N., Menzel, D.: Chem. Phys. 13 (1976) 243. Shih, H.D., Jona, F.: Surf. Sci. 60 (1976) 445. Shih, H.D., Jona, F., Jepsen, D.W., Marcus, P.M.: Surf. Sci. 60 (1976) 445. Taylor, T.N., Colmenares, C.A., Smith, R.L., Somorjai, G.A.: Surf. Sci. 54 (1976) 317. Yu, K.Y., Miller, J.N., Chye, P., Spicer, W.E., Land, N.D., Williams, A.R.: Phys. Rev. B 14 (1976) 1446. Zhdan, P.A., Boreskov, G.K., Bronin, A.I., Egelhoff jr., W.F., Weinberg, W.H.: Surf. Sci. 61 (1976) 25. Bastasz, R., Colmenares, C.A., Smith, R.L., Somorjai, G.A.: Surf. Sci. 67 (1977) 45. Bozso, F., Ertl, G., Weiss, M.: J. Catal. 50 (1977) 519. Landolt-Börnstein New Series III/42A4

3.4.2 Adsorption of C, N, and O on metal surfaces 77B3 77B4 77B5 77B6 77B7 77C1 77C2 77C3 77E1 77F1 77G1 77G2 77H1 77H2 77J1 77K1 77L1 77L2 77L2 77N1 77O1 77P1 77P2 77P2 77R1 77R2 77R3 77S1 77T1 77W1 77W2 77W3 78A1 78B1 78B2 78B3 78C1 78C2 78C3 78D1 78E1 78F1 78G1 78H3 78J1 78J2

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Bozso, F., Ertl, G., Grunze, M., Weiss, M.: J. Catal. 49 (1977) 18. Besocke, K., Berger, S., in: Proc. 7th Intern. Vacuum Congress and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, p. 893. Brucker, C., Rhodin, T.N.: J. Catal. 47 (1977) 214. Batra, I.P., Ciraci, S.: Phys. Rev. Lett. 39 (1977) 774. Bonzel, H.P.: Surf. Sci. 68 (1977) 236. Clark, L.J.: Proc. 7th Intern. Vacuum Congress and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, p. A2725. Conrad, H., Ertl, G., Küppers, J., Latta, E.E.: Surf. Sci. 65 (1977) 235, 245. Conrad, H., Küppers, J., Nitschke, F., Plagge, A.: Surf. Sci. 69 (1977) 668. Engel, T., Von dem Hagen, T., Bauer, E.: Surf. Sci. 62 (1977) 361. Flodström, S.A., Bachrach, R.Z., Bauer, R.S., Hagström, S.B.M.: Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, p. 869. Gartland, P.O.: Surf. Sci. 62 (1977) 183. Gary, G., Burkstrand, J.M.: Phys. Rev. B 16 (1977) 1536. Horn, H., Hussain, M., Pritchard, J.: Surf. Sci. 63 (1977) 244. Hopster, H., Ibach, H., Comsa, G.: J. Catal. 46 (1977) 37. Jacobi, K., Scheffler, M., Kambe, K., Forstmann, F.: Solid State Commun. 22 (1977) 17. Ku, R., Gjostein, N.A., Bonzel, H.P.: Surf. Sci. 64 (1977) 465. Légaré, P., Maire, G., Carière, Deville, J.P.: Surf. Sci. 68 (1977) 348. Lloyd, D.R., Quinn, C.M., Richardson, N.V.: Surf. Sci. 68 (1977) 419. Legg, K.O., Jona, F., Jepsen, D.W., Marcus, P.M.: Phys. Rev. B 16 (1977) 5271. Nakanishi, S., Horiguchi, T.: Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, p. A2727. Orent, T.W., Hansen, R.S.: Surf. Sci. 67 (1977) 325. Pantel, R., Bujor, M., Bardolle, J.: Surf. Sci. 62 (1977) 739. Pirug, G., Brodén, G., Bonzel, H.P.: Proc. 7th Intern. Vacuum Congr. and 3rd Int. Conf. on Solid Surfaces, Vienna, 1977, p. 907. Pirug, G., Brodén, G., Bonzel, H.P.: Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, p. 907. Rovida, R., Maglietta, M.: Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces, Vienna, 1977, p. 963. Reed, P.D., Comrie, C.M., Lambert, R.M.: Surf. Sci. 64 (1977) 603. Rhodin, T.N., Brucker, C.W.: Solid State Commun. 23 (1977) 275. Schwaha, K., Bechtold, E.: Surf. Sci. 66 (1977) 383. Tibbets, G.G., Burkstrand, J.M., Tracy, J.C.: Phys. Rev. B 15 (1977) 3652. Weinberg. H.W., Monroe, S.R., Lampton, V., Merrill, R.P.: J. Vac. Sci. Technol. 14 (1977) 444. Wilf, M., Dawson, P.T.: Surf. Sci. 65 (1977) 399. Weng, S.-L., Plummer, E.W.: Solid State Commun. 23 (1977) 515. Abbati, I., Braicovich, L., Bertoni, C.M., Calandra, C., Manghi, F.: Phys. Rev. Lett. 40 (1978) 469. Brundle, C.R.: IBM J. Res. Rev. 22 (1978) 235. Bauer, E., Engel, T.: Surf. Sci. 71 (1978) 695. Bauer, E., Engel, T.: Surf. Sci. 71 (1978) 695. Castner, D.G., Sexton, B.A., Sonorjai, G.A.: Surf. Sci. 71 (1978) 519. Chan, C.-M., Luke, K.L., Van Hove, M.A., Weinberg, W.H., Withrow, S.P.: Surf. Sci. 78 (1978) 386. Ching, W.Y., Huber, D.L., Lagally, M.G., Wang, G.-C.: Surf. Sci. 77 (1978) 550. Doyen, G., Ertl, G.: J. Chem. Phys. 68 (1978) 5417. Eberhardt, W., Kunz, G.: Surf. Sci. 75 (1978) 709. Floström, S.A., Martinson, C.W.B., Bockrach, R.Z., Hagström, S.B.M., Bauer, S.M.: Phys. Rev. Lett. 40 (1978) 907. Gewinner, G., Peruchetti, J.C., Jaegle, A., Kalt, A.: Surf. Sci. 78 (1978) 439. Hofmann, P., Urwin, R., Wyrobisch, W., Bradshaw, A.M.: Surf. Sci. 72 (1978) 635. Jardin, J., Michel, P.: Surf. Sci. 71 (1978) 312. Jona, F., Legg, K.O., Shih, H.D., Jepsen, D.W., Marcus, P.M.: Phys. Rev. Lett. 40 (1978) 1466.

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3.4.2 Adsorption of C, N, and O on metal surfaces Küppers, J., Ertl, G.: Surf. Sci. 77 (1978) L657. Marbrow, R.A., Lambert, R.M.: Surf. Sci. 71 (1978) 107. McKee, C.S., Remny, L.V., Roberts, M.W.: Surf. Sci. 75 (1978) 92. Maglietta, M., Zanazzi, E., Bardi, U., Jona, F.: Surf. Sci. 77 (1978) 101. Maglietta, M., Rovida, G.: Surf. Sci. 71 (1978) 495. Netzer, F.P., Wille, R.A.: Surf. Sci. 74 (1978) 547. Bridge, M.E., Lambert, R.M.: Surf. Sci. 82 (1979) 413. Bauer, E., Poppe, H.: Surf. Sci. 88 (1979) 31. Brundle, C.R.: Surf. Sci. 66 (1979) 581. Bianconi, A., Bachrach, R.Z., Flodström, S.A.: Phys. Rev. B 19 (1979) 3879. Chan,C.M., Weinberg, W.H.: J. Chem. Phys. 71 (1979) 2788. Capehart, T.W., Rhodin, T.N.: J. Vac. Sci. Technol. 16 (1979) 594. Eberhardt, W., Himpsel, F.: Phys. Rev. Lett. 42 (1979) 1375. Ertl, G., Weiss, M., Lee, S.B.: Chem. Phys. Lett. 60 (1979) 391. Ertl, G., in: The Nature of Surface Chemical Bond, Rhodin, T.N., Ertl, G. (eds.), Amsterdam: North Holland, 1979, p. 314. Hofmann, P., Wyrobisch, W., Bradshaw, A.M.: Surf. Sci. 80 (1979) 344. Hofmann, P., von Muschwitz, C., Horn, K., Bradshaw, A.M., Kambe, K., Scheffler, M.: Surf. Sci. 89 (1979) 327. Habraken, F.H.P., Kiefer, E. Ph., Bootsma, G.A.: Surf. Sci. 83 (1979) 45. Küppers, J., Michel, H.: Appl. Surf. Sci. 3 (1979) 179. Maire, G., Légaré, P., Lindauer, Vorname: Surf. Sci. 80 (1979) 283. Martinson, C.W.B., Flodström, S.A.: Surf. Sci. 80 (1979) 306. Niehus, H.: Surf. Sci. 87 (1979) 561. Onuferko, J.H., Woodruff, D.P., Holland, B.W.: Surf. Sci. 87 (1979) 357. Pantel, R., Bujor, M., Bardolle, J.: Surf. Sci. 83 (1979) 228. Rhodin, T.N., Capehart, T.W.: Surf. Sci. 89 (1979) 337. Somerton, C., King, D.A.: Surf. Sci. 89 (1979) 391. Taylor, J.L., Ibbotson, D.E., Weinberg, W.H.: Surf. Sci. 79 (1979) 349. Thiel, P.A., Yates, J.T., Weinberg, H.W.: J. Vac. Sci. Technol. 16 (1979) 438: Surf. Sci. 82 (1979) 22. Thomas, G.E., Weinberg, W.H.: J. Chem. Phys. 70 (1979) 954. Wuttig, M., Franchy, R., Ibach, H.: Surf. Sci. 213 (1979) 193. Wang, C., Gomer, R.: Surf. Sci. 84 (1979) 329. Bowker, M., Barteau, M.A., Madix, R.J.: Surf. Sci. 92 (1980) 528. Castner, D.G., Somorjai, G.A.: Appl. Surf. Sci. 6 (1980) 29. Castro, G.R., Küppers, J.: Surf. Sci. 123 (1982) 456. Davis, S.M., Somorjai, G.A.: Surf. Sci. 91 (1980) 73. Den Boer, M.L., Einstein, T.L., Elam, W.T., Park, R.L., Roelofs, L.D., Laramore, G.E.: Phys. Rev. Lett. 44 (1980) 496. Ducros, R., Alnot, M., Eberhard, J.J., Hoosley, M., Piquard, G., Cassuto, A.: Surf. Sci. 94 (1980) 154. Ertl, G.: Catal. Rev. Sci. Eng. 21 (1980) 201. Gland, J.L., Sexton, B.A., Fisher, G.B.: Surf. Sci. 95 (1980) 587. Gland, J.L.: Surf. Sci. 93 (1980) 487. Gewinner, G., Peruchetti, J.C., Riedinger, R., Jaegle, A.: Solid State Commun. 36 (1980) 785. Ho, W, Willis, R.F., Plummer, E.W.: Surf. Sci. 95 (1980) 171. Ibach, H., Bruchmann, D.: Phys. Rev. Lett. 44 (1980) 36. Ko, E.I., Madix, R.J.: Surf. Sci. 109 (1980) 221. Ling, D.T., Miller, J.N., Weissman, D.L., Oianetta, P., Stefan, P.M., Lindau, I., Spicer, W.E.: Surf. Sci. 96 (1980) 89. Madey, T.E.: Surf. Sci. 94 (1980) 483. Pirug, G., Broden, G., Bonzel, H.P.: Surf. Sci. 94 (1980) 323. Park, R.L., Einstein, T.L., Kortan, A.R., Roelfs, L.D., in: Ordering in Two Dimensions, Sinha, S.K. (ed.), Amsterdam: Elsevier, 1980, p. 17. Landolt-Börnstein New Series III/42A4

3.4.2 Adsorption of C, N, and O on metal surfaces 80S1 80S2 80S3 80S4 81B1 81B2 81B3 81C1 81D1 81E1 81G1 81H1 81H2 81J1 81K1 81K2 81K3 81K4 81N1 81N2 81N3 81O1 81O2 81R1 81S1 81S2 81W1 82B1 82B2 82C1 82D1 82D2 82D3 82E1 82E2 82E3 82F1 82G1 82G2 82H1 82I1 82J1 82J3 82K1 82K2 82M1 82N1 82N2 82O1 82R1 82S1

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Salmeron, M., Somorjai, G.A.: Surf. Sci. 91 (1980) 199. Sexton, B.A., Madix, R.J.: Chem. Phys. Lett. 76 (1980) 294. Stöhr, J., Johansson, L.I., Brennan, S., Hecht, M., Miller, J.N.: Phys. Rev. B. 22 (1980) 4052. Smeenk, R.G., Tromp, R.G., Van der Veen, J.F., Saris, F.W.: Surf. Sci. 95 (1980) 156. Bickly, H., Arlow, J.S., Morris, M.A., King, D.A.: Vacuum 31 (1981) 509. Bachrach, R.Z., Hansson, G.V., Bauer, R.S.: Surf. Sci. 109 (1981) L560. Backx, C., de Groot, C.P.M., Biloen, P.: Surf. Sci. 104 (1981) 300. Campbell, C.T., Ertl, G., Kuipers, H., Segner, J.: Surf. Sci. 107 (1981) 220. Davies, P.W., Lambert, R.M.: Surf. Sci. 110 (1981) 227. Erley, W., Ibach, H.: Solid State Commun. 37 (1981) 937. Ghijsen, J., Namba, H., Thriy, P.A., Pireaux, J.J., Caudano, P.: Appl. Surf. Sci. 8 (1981) 397. Hayden, B.E., Schweizer, E., Kötz, R., Bradshaw, A.M.: Surf. Sci. 111 (1981) 26. Hanson, D.M., Stockbauer, R., Madey, T.E.: Phys. Rev. B 24 (1981) 5513. Jonker, B.T., Morar, J.F., Park, R.L.: Phys. Rev. B 24 (1981) 2951. Kortan, A.R., Park, R.L: Phys. Rev. B 23 (1981) 6340. Ko, E.I., Madix, R.J.: J. Phys. Chem. 85 (1981) 4019. Krakauer, H., Psernak, M., Freeman, A.J., Koelling, D.D.: Phys. Rev. B 23 (1981) 3859. Kiskinova, M., Goodman, D.W.: Surf. Sci. 109 (1981) 113. Namba, H., Denille, J., Gilles, J.M.: Surf. Sci. 108 (1981) 446. Norman, D., Brennan, S., Jaeger, R., Stohr, J.: Surf. Sci. 105 (1981) L297. Narusawa, T., Gibson, W.M., Tornquist, E.: Surf. Sci. 114 (1981) 331. Opila, R., Gomer, R.: Surf. Sci. 105 (1081) 41. Ozaki, A., Aika, K., in: Catalysis: Science and Technology, Vol. 1, Anderson, J.R., Boudart, M. (ed.), Berlin: Springer, 1981, p 143. Ribarsky, M.W.: Solid State Commun. 38 (1981) 935. Soria, F., Martinez, V., Munoz, M.C., Sacedon, J.L.: Phys. Rev. B 24 (1981) 6926. Somorjai, G.A.: Chemistry in Two Dimensions: Surfaces, Ithaca, NY: Cornell University Press, 1981. Wang, D.-S., Freeman, A.J., Krakauer, H.: Phys. Rev. B 24 (1981) 3104. Bylander, D.M., Kleinman, L., Mednick, K.: Phys. Rev. Lett. 48 (1982) 1544. Bonzel, H.P., Krebs, H.J.: Surf. Sci. 117 (1982) 639. Castro, G.R., Küppers, J.: Surf. Sci. 123 (1982) 456. Doering, D.L., Madey, T.E.: Surf. Sci. 123 (1982) 305. Dubois, L.H.: Surf. Sci. 119 (1982) 399. Dubois, L.H.: J. Chem. Phys. 77 (1982) 5228. Ertl, G., Lee, S.B., Weiss, M.: Surf. Sci. 114 (1982) 527. Ertl, G., Lee, S.B., Weiss, M.: Surf. Sci. 114 (1982) 515. Erskine, J.L., Strong, R.L.: Phys. Rev. B 25 (1982) 5547. Flodström, S.A., Martinsson, C.W.B.: Surf. Sci. 118 (1982) 513. Gewinner, G., Peruchetti, J.C., Jaéglé, A.: Surf. Sci. 122 (1982) 383. Griffiths, K., King, D.A., Aers, G.C., Pendry, J.B.: J. Phys. C15 (1982) 4921. Himpsel, F.J., Christmann, K., Heimann, P., Eastman, D.E., Feibelman, P.J.: Surf. Sci. 115 (1982) L159. Imbihl, R., Behm, R.J., Ertl, G., Moritz, W.: Surf. Sci. 123 (1982) 129. Jonker, B.T., Morar, J.F., Park, R.L.: Phys. Rev. B 24 (1981) 2951. Jensen, V., Andersen, J.N., Hielsen, H.B., Adams, D.L.: Surf. Sci. 116 (1982) 66. Kaski, K., Kinzel, W., Gunton, J.D.: Phys. Rev. B 27 (1982) 6777. Koel, B.E., White, J.M., Goodman, D.W.: Chem. Phys. Lett. 88 (1982) 236. Milne, R.H.: Surf. Sci. 121 (1982) 347. Nyberg, C., Tengstal, C.G.: Surf. Sci. 126 (1982) 163. Neve, J., Rundgren, J., Westrin, P.: J. Phys. C 15 (1982) 4391. Orent, T.W., Bader, S.D.: Surf. Sci. 115 (1982) 323. Roman, E., Riwan, R.: Surf. Sci. 188 (1982) 682. Semancik, S., Haller, G.L., Yates jr., J.T.: Appl. Surf. Sci. 10 (1982) 546.

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3.4.2 Adsorption of C, N, and O on metal surfaces Stöhr, J., Jaeger, R., Kendelewicz, T.: Phys. Rev. Lett. 49 (1982) 142. Steininger, H., Lehwald, S., Ibach, H.: Surf. Sci. 123 (1982) 1. Spitzer, A., Lüth, H.: Surf. Sci. 118 (1982) 136. Van der Veen, J.F., Himpsel, F.J., Eastman, D.E.: Phys. Rev. B 25 (1982) 7388. Wandelt, K.: Surf. Sci. Rep. 2 (1982) 1. Andersson, S., Karlsson, P.-A., Persson, M.: Phys. Rev. Lett. 51 (1983) 2378. Böheim, J., Brenig, W., Engel, T., Leuthäuser, U.: Surf. Sci. 131 (1983) 298. Bauer, E., Poppa, H.: Surf. Sci. 127 (1983) 243. Benndorf, C., Franck, M., Thieme, F.: Surf. Sci. 128 (1983) 417. Batra, I.P.: J. Electron Spectrosc. Relat. Phenom. 29 (1983) 83. Demuth, J.E., DiNardo, N.J., Cargill, G.S.: Phys. Rev. Lett. 50 (1983) 1373. Feidenhansl, R., Stensgaard, I.: Surf. Sci. 133 (1983) 453. Fisher, G.B., Schmieg, S.J.: J. Vac. Sci. Technol. A 1 (1983) 1064. Frenken, J.W., Van der Veen, J.F., Allen, G.: Surf. Sci. 135 (1983) 147. Foord, J.S., Reed, A.P.C., Lambert, R.M.: Surf. Sci. 129 (1983) 79. Lindauer, G., Légaré, P., Maire, G.: Surf. Sci. 126 (1983) 301. Martines, V., Soria, F., Munoz, M.C., Sacedon, J.L.: Surf. Sci. 128 (1983) 424. Miles, S.L., Bernasek, S.L., Gland, J.L.: J. Phys. Chem. 87 (1983) 1626. Miyazaki, E., Tsukada, M., Adachi, H.: Surf. Sci. 131 (1983) L390. Norman, D., Stöhr. J., Jaeger, R., Durham, P.J., Pendry, J.B.: Phys. Rev. Lett. 51 (1983) 2052. Overbury, S.H., Stair, P.C.: J. Vac. Sci. Technol. A 1 (1983) 1055. Root, T.W., Schmidt, L.D.: Surf. Sci. 134 (1983) 30. Rosei, R., Modesti, S., Sette, F., Quaresima, C., Savoia, A., Perfetti, P.: Solid State Commun. 46 (1983) 871. Rieder, K.-H.: Surf. Sci. 128 (1983) 325. Weissman-Wenocur, D.L., Shek, M.L., Stefan, P.M., Lindau, I., Spicer, W.E.: Surf. Sci. 127 (1983) 513. Wang, G.C., Lu, T.M.: Phys. Rev. B 28 (1983) 6795. Behm, R.J., Thiel, P.A., Norton, P.R., Bindner, P.E.: Surf. Sci. 147 (1984) 143. Batra, I.P., Kleinman, L.: J. Electron Spectrosc. Relat. Phenom. 33 (1984) 175. Campbell, C.T., Paffett, M.T.: Surf. Sci. 144 (1984) 469. Campbell, C.T., Paffett, M.T.: Surf. Sci. 143 (1984) 517. Derry, G.N., Ross, P.N.: Surf. Sci. 140 (1984) 165. Didio, R.A., Zehner, D.M., Plummer, E.W.: J. Vac. Sci. Technol. A 2 (1984) 852. DiNardo, N.J., Blanchet, G.B., Plummer, E.W.: Surf. Sci. 140 (1984) L229. Fowler, D.E., Blakely, J.M.: Surf. Sci. 148 (1984) 265, 283. Gutmann, A., Zwicker, G., Schmeisser, D., Jacobi, K.: Surf. Sci. 137 (1984) 211. Grunze, M., Golze, M., Hirschwald, W., Freund, H.-J., Pulm, H., Seip, U., Küppers, J., Ertl, G.: Phys. Rev. Lett. 53 (1984) 850. Griffiths, K., Jackman, T.E., Davies, J.A., Norton, P.R.: Surf. Sci. 138 (1984) 113. Grant, R.B., Lambert, R.M.: Surf. Sci. 146 (1984) 256. Jupiter, P.J., Viescas, A.J., Carbone, C., Lindau, I., Spicer, W.E.: J. Vac. Sci. Technol. A 3 (1984) 1517. Kirschner, J.: Surf. Sci. 138 (1984) 191. Lee, J., Madix, R.J., Schlaegel, H.E., Auerbach, D.J.: Surf. Sci. 143 (1984) 626. Miyano, T., Kamei, K., Sakisaka, Y., Onchi, M.: Surf. Sci. 148 (1984) L645. Norton, P.R., Griffiths, K., Binder, K.E.: Surf. Sci. 138 (1984) 125. Norton, P.R., Binder, K.E., Griffiths, K.: J. Vac. Sci. Technol. A2 (1984) 1028. Puschmann, A., Haase, J.: Surf. Sci. 144 (1084) 559. Rahman, T.S., Mills, D.L., Black, J.E., Szeftel, J.M., Lehwald, S., Ibach, H.: Phys. Rev. B 30 (1984) 589. Rikvold, P.A., Kaski, K., Gunton, J.D., Yalabik, M.C.: Phys. Rev. B 29 (1984) 6285. Stuve, E.M., Madix, R.J., Brundle, C.R.: Surf. Sci. 146 (1984) 144. Szeftel, J., Lehwald, S.: Surf. Sci. 143 (1984) 11. Landolt-Börnstein New Series III/42A4

3.4.2 Adsorption of C, N, and O on metal surfaces 84S3 85A1 85B1 85B2 85B3 85C1 85D1 85F1 85G1 85G2 85G3 85H1 85M1 85M2 85N1 85P1 85P2 85R1 85S1 85S2 85S3 85S4 85T1 85T2 85W1 85W2 85Z1 86B1 86B2 86B3 86B4 86B5 86C1 86D1 86F1 86H1 86I1 86K1 86M1 86N1 86P1 86P2 86P3 86R1 86R2 86R3 86R4 86S1 86S3

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Sakisaka, Y., Miyano, T., Onchi, M.: Phys. Rev. B 30 (1984) 6849. Altmann, W., Desinger, K., Donath, M., Dose, V., Goldmann, A., Scheidt, H.: Surf. Sci. 151 (1985) L185. Behm, R.J., Ertl, G., Penka, V., Schwanker, R.: J. Vac. Sci. Technol. A 3 (1985) 1595. Benziger, J.B., Preston, R.E.: Surf. Sci. 151 (1985) 183. Baca, A.G., Schulz, M.A., Shirley, D.A.: J. Chem. Phys. 83 (1985) 6001. Campbell, C.T.: Surf. Sci. 157 (1985) 43. Dose, V.: Surf. Sci. Rep. 5 (1985) 337. Foord, J.S., Lambert, R.M.: Surf. Sci. 161 (1985) 513. Garfunkel, E.L., Ding, X., Dong, G., Yang, S., Hou, X., Wang, X.: Surf. Sci. 164 (1985) 511. Godby, R.W., Benesh, G.A., Haydock, R., Heine, V.: Phys. Rev. B 32 (1985) 655. Godby, R.W.: Phys. Rev. B 32 (1985) 5432. Huang, H., Hermanson: J., Phys. Rev. B 32 (1985) 6312. Marinova, T.S., Stefanov, P.K., Neshev, N.: Surf. Sci. 164 (1985) 196. Matsushima, T.: Surf. Sci. 157 (1985) 297. Niehus, H., Comsa, G.: Surf. Sci. 151 (1985) L171. Paolucci, G., Rosei, R., Prince, K.C., Bradshaw, A.M.: Appl. Surf. Sci. 22/23 (1985) 582. Panzner, G., Mueller, D.R, Rhodin, T.N.: Phys. Rev. B 32 (1985) 3472. Rocca, M., Lehwald, S., Ibach, H.: Surf. Sci. 163 (1985) L738. Strong, R.L., Erskine, J.L.: J. Vac. Sci. Technol. A 3 (1985) 1428. Surnev, L., Rangelov, G., Bliznakov, G.: Surf. Sci. 159 (1985) 299. Sakisaka, Y., Komeda, T., Miyano, T., Onchi, M., Masuda, S., Harada, Y., Yagi, K., Kato, H.: Surf. Sci. 164 (1985) 220. Sen, P., Rao, C.N.R.: Solid State Commun. 54 (1985) 309. Titov, A.V., Jagodzinski, H: Surf. Sci. 152-153 (1985) 409. Tsai, M.C., Seip, U., Bassignana, I.C., Küppers, J., Ertl, G.: Surf. Sci. 155 (1985) 387. Wang, G.C., Lu, T.M.: Phys. Rev. B 31 (1985) 5918. Wang, G.C., Pimbley, J.M., Lu, T.M.: Phys. Rev. B 31 (1985) 1950. Zhang, C., Van Hove, M.A., Somorjai, G.A.: Surf. Sci. 149 (1985) 326. Bader, M., Puschmann, A., Ocal, C., Haase, J.: Phys. Rev. Lett. 57 (1986) 3273. Bartosch, C.E., Whiteman, L.J., Ho, W.: J. Chem. Phys. 85 (1986) 1052. Bassignana, I.C., Wagemann, K., Küppers, J., Ertl, G.: Surf. Sci. 175 (1986) 22. Baca, A.G., Klebanoff, L.E., Schulz, M.A., Paparazzo, E., Shirley, D.A.: Surf. Sci. 171 (1986) 255. Baca, A.G., Klebanoff, L.E., Schulz, M.A., Paparazzo, E., Shirley, D.A.: Surf. Sci. 173 (1986) 215. Crowell, J.E., Chen, J.G., Yates, J.T.: Surf. Sci. 165 (1986) 37. Daum, W., Lehwald, S., Ibach, H.: Surf. Sci. 178 (1986) 528. Freyer, N., Kiskinova, M., Pirug, G., Bonzel, H.P.: Surf. Sci. 166 (1986) 206. Higgs, V., Hollins, P., Pemble, M.E., Pritchard, J.: J. Electron Spectrosc. Relat. Phenom. 39 (1986) 137. Imbihl, R., Demuth, J.E.: Surf. Sci. 173 (1986) 395. Kern, K., David, R., Palmer, R.L., Comsa, G., He, J., Rahman, T.S.: Phys. Rev. Lett. 56 (1986) 2064. McConville, F.C., Woodruff, D.P., Kevan, S.D., Weinert, M., Davenport, J.W.: Phys. Rev. B 34 (1986) 2199. Nishijima, M., Jo, M., Kuwahara, Y., Onchi, M.: Solid State Commun. 60 (1986) 257. Prince, K.C., Paolucci, G., Bradshaw, A.M.: Surf. Sci. 236 (1986) 101. Prince, K.C., Surman, M., Lindner, Th., Bradshaw, A.M.: Solid State Commun. 59 (1986) 71. Pfnür, H., Rettner, C.T., Lee, J., Madix, R.J., Auerbach, D.J.: J. Chem. Phys. 85 (1986) 7452. Root, T.W., Fisher, G.B., DiMaggio, C.L.: J. Chem. Phys. 85 (1986) 4679, 4687. Rous, P.J., Pendry, J.B., Saldin, D.K., Heinz, K., Mueller, K., Bickel, N.: Phys. Rev. Lett. 57 (1986) 2951. Rettner, C.T., DeLouise, L.A., Auerbach, D.J.: J. Chem. Phys. 85 (1986) 1131. Roelofs, L.D., Chung, J.W., Ying, S.C., Estrup, P.J.: Phys. Rev. B 33 (1986) 6537. Sokolov, J., Jona, F., Marcus, P.M.: Europhys. Lett. 1 (1986) 401. Shinn, N.D., Madey, T.E.: Surf. Sci. 173 (1986) 379.

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3.4.2 Adsorption of C, N, and O on metal surfaces Shinn, N.D., Madey, T.E.: Surf. Sci. 176 (1986) 635. Wong, K.C., Hui, K.C., Zhou, M.Y., Mitchell, K.A.R.: Surf. Sci. 165 (1986) L21. Witt, W., Bauer, E.: Ber. Bunsenges. Phys. Chem. 90 (1986) 248. Weller, D., Sarama, D.: Surf. Sci. 171 (1986) L425. Astaldi, C., Geng, P., Kacobi, K.: J. Electron Spectrosc. Relat. Phenom. 44 (1987) 175. Atrei, A., Bardi, U., Rovida, G., Torrini, M., Zanazzi, E., Maglietta, M.: J. Vac. Sci. Technol. A 5 (1987) 1006. Arabczyk, W., Mussig, H.-J.: Vaccum 37 (1987) 137. Bader, W., Ocal, C., Hillert, B., Haase, J., Bradshaw, A.M.: Phys. Rev. B 35 (1987) 5900. Bertram, M.E., Windham, R.G., Koel, B.: Surf. Sci. 184 (1987) 57. Chang, S.-L., Thiel, P.A.: Phys. Rev. Lett. 59 (1987) 296. Courths, R., Cord, B., Wern, H., Saalfeld, H., Hüfner, S.: Solid State Commun. 63 (1987) 619. Chubb, S.R., Pickett, W.E.: Phys. Rev. Lett. 58 (1987) 1248. Chang, S.-L., Thiel, P.A.: J. Chem. Phys. 88 (1987) 2071. Jona, F., Marcus, P.M.: Solid State Commun. 64 (1987) 667. Gurney, B.A., Ho, W.: J. Chem. Phys. 87 (1987) 1376. Haase, G., Asscher, M.: Surf. Sci. 191 (1987) 75. Hoffman, A., Maniv, Ts., Folman, M.: Surf. Sci. 182 (1987) 56. Kuwahara, Y., Fujisawa, M., Jo, M., Onchi, M., Nishijima, M.: Surf. Sci. 188 (1987) 490. Mundenar, J.M., Baddorf, A.P., Plummer, E.W., Sneddon, L.G., Di Dio, R.A., Zehner, D.M.: Surf. Sci. 188 (1987) 15. McConville, C.F., Seymour, D.L., Woodruff, D.P., Bao, S.: Surf. Sci. 188 (1987) 1. Marinova, T.S., Kostov, K.L.: Surf. Sci. 185 (1987) 203. Mohamed, M.H., Kesmodel, L.L.: Surf. Sci. 185 (1987) L467. Overbury, S.H.: Surf. Sci. 184 (1987) 319. Piercy, P., Pfnür, H.: Phys. Rev. Lett. 59 (1987) 248. Rettner, C.T., Stein, H.: Phys. Rev. Lett. 59 (1987) 2768. Rocca, M., Lehwald, S., Ibach, H., Rahman, T.S.: Phys. Rev. B 32 (1987) 9510. Stoltze, P.: Phys. Scr. 36 (1987) 824. Santoni, A., Urban, J.: Surf. Sci. 186 (1987) 376. Van Zoest, J.M., Fluit, J.M., Vink, T.J., Van Hassel, B.A.: Surf. Sci. 182 (1987) 179. Wong, Y.M., Mitchell, K.A.R.: Surf. Sci. 187 (1987) L599. Zheng, H.C., Sodhi, R.N.S., Mitchell, K.A.R.: Surf. Sci. 188 (1987) 599. Chang, S.-L., Thiel, P.A.: J. Chem. Phys. 88 (1998) 2071. Cerny, S., Kovar, M.: Coll. Czech. Chem. Commun. 53 (1988) 2412. Fukuda, Y., Nagoshi, M.: Surf. Sci. 203 (1988) L651. Heskett, D., Baddorf, A., Plummer, E.W.: Surf. Sci. 195 (1988) 94. Kuwahara, Y., Fujisawa, M., Onchi, M., Nishijima, M.: Surf. Sci. 207 (1988) 17. Krainsky, I.L.: J. Vac. Sci. Technol. A 6 (1988) 780. Komeda, T., Sakisaka, Y., Onchi, M., Kato, H., Suzuki, S., Edamoto, K., Aiura, Y.: Phys. Rev. B 38 (1988) 7345. Luntz, A.C., Williams, M.D., Bethune, D.S.: J. Chem. Phys. 89 (1988) 4381. Mullins, D.R., Overbury, S.H.: Surf. Sci. 193(1989) 455. Oed, W., Dötsch, B., Hammer, L., Heinz, K., Müller, K.: Surf. Sci. 207 (1988) 55. Stefanov, P.K., Marinova, T.S.: Surf. Sci. 200 (1988) 26. Segeth, W., Wijngaard, J.H., Sawatzky, G.A.: Surf. Sci. 194 (1988) 615. Wong, P.C., Lou, J.R., Mitchell, K.A.R.: Surf. Sci. 206 (1988) L913. Wintterlin, J., Brunse, H., Hoger, H., Behm, R.J.: Appl. Phys. A 47 (1988) 99. Zhang-Long, X., Gu, L., Zhen-Guo, J., Xiao-Xia, Z.: Acta Phys. Sin. 37 (1988) 311. Alnot, P., Auerbach, D.J., Behm, J., Brundle, C.R., Viescas, A.: Surf. Sci. 213 (1989) 1. Atrei, A., Bardi, U., Maglietta, M., Rovida, G., Torrini, M., Zanazzi: E., Surf. Sci. 211/212 (1989) 93. Bu, H., Grizzi, O., Shi, M., Rabalais, J.W.: Phys. Rev. B 40 (1989) 10147. Cronzher, H., Heinz, K., Müller, K., Xu, M.L., Van Hove, M.A.: Surf. Sci. 209 (1989) 387. Landolt-Börnstein New Series III/42A4

3.4.2 Adsorption of C, N, and O on metal surfaces 89G1 89G2 89H1 89H2 89H3 89J1 89K1 89K2 89L1 89L2 89M1 89M2 89N1 89N2 89P1 89P2 89P3 89R1 89R2 88S1 89S2 89S3 89T1 89W1 89W2 89Z1 90A1 90A2 90B1 90B2 90B3 90C1 90C2 90F1 90F2 90F3 90G1 90G2 90H1 90K1 90L1 90L2 90N1 90N2 90P1 90P2 90P3 90P4

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Guo, X., Hanley, L., Yates jr., J.T.: J. Chem. Phys. 90 (1989) 5787. Grzelakowski, K., Lyuksyutov, I., Bauer, E.: Surf. Sci. 216 (1989) 472. He, W.J., Goodman, D.W.: Surf. Sci. 218 (1989) 211. He, J-W., Memmert, U., Griffiths, K., Norton, P.R.: J. Chem. Phys. 90 (1989) 5082, 5088. Headrick, R.L., Konarski, P., Yalisove, S.M., Graham, W.R.: Phys. Rev. B 39 (1989) 5713. Joy, Y., Gauthier, Y., Baudoing, R.: Phys. Rev. B 40 (1989) 10119. Kilcoyne, A.L.D., Woodruff, D.P, Rowe, J.E., Gaylord, R.H.: Phys. Rev. B 39 (1989) 12604. Kiss, J., Klivenyi, G., Revesz, K., Solymosi, F.: Surf. Sci. 223 (1989) 551. Lindroos, M., Pfnür, H., Held, G., Menzel, D.: Surf. Sci. 222 (1989) 451. Lu, J.-P., Albert, M.R., Bernasek, S.L., Dwyer, D.J.: Surf. Sci. 215 (1990) 348. Mullins, D.R., Overbury, S.H.: Surf. Sci. 210 (1989) 481. Matsuo, L, Nakamura, J., Hirano, H., Yamada, T., Tanaka, K., Tamaru, K.: J. Phys. Chem. 93 (1989) 7747. Nilsson, A., Martensson, N.: Phys. Rev. Lett. 63 (1989) 1483. Nakamura, J., Hirano, H, Xie, M., Matsuo, I., Yamada, T., Tanaka, K.: Surf. Sci. 222 (1989) L809. Pfnür, H., Held, G., Lindroos, M., Menzel, D.: Surf. Sci. 220 (1989) 43. Parker, D.H., Bartram, M.E., Koel, B.E.: Surf. Sci. 217 (1989) 489. Pedio, M., Fuggle, J.C., Somers, J., Umbach, E., Haase, J, Lindner, Th., Höfer, U., Hillert, B., Becker, L., Robinson, A.: Phys. Rev. B 40 (1989) 7924. Rettner, C.T., Schweizer, E.K., Stein, H., Auerbach, D.J.: J. Vac. Sci. Technol. A 7 (1989) 1863. Rabalais, J.W., Grizzi, O., Shi, M., Bu, H.: Phys. Rev. Lett. 63 (1989) 51. Stefanov, P.K., Marinova, T.S.: Surf. Sci. 200 (1988) 26. Sellidj, A., Erskine, J.L.: Surf. Sci. 220 (1989) 253. Szeftel, J., Mila, F., Khater, A.: Surf. Sci. 216 (1989) 125. Thiry, P.A., Ghijsen, J., Sporken, R., Pireaux, J.J., Johnson, R.L., Caudano, R.: Phys. Rev. B 39 (1989) 3620. Wuttig, M., Franchy, R., Ibach, H.: Surf. Sci. 213 (1989) 103. Wu, P.K., Tringides, M.C., Lagally, M.G.: Phys. Rev. B 39 (1989) 7595. Zuo, J.K., Wang, G.C., Lu, T.M.: Phys. Rev. B 39 (1989) 9432. Ares Fang, C.S.: Surf. Sci. 235 (1990) L291. Ashwin, M.J., Woodurff, D.P.: Surf. Sci. 237 (1990) 108. Banse, B.A., Koel, B.E.: Surf. Sci. 232 (1990) 275. Baddorf, A.P., Zehner, D.M.: Surf. Sci. 238 (1990) 255. Billing, G.D., Guldberg, A., Henriksen, N.E., Hansen, F.Y.: Chem. Phys. 147 (1990) 1. Clarke, S., Brookes, N.B., Johnson, P.D., Weinert, M., Sinkovic, B., Smith, N.V.: Phys. Rev. B 41 (1990) 9659. Coulman, D.J., Wintterlin, J., Behm, R.J., Ertl, G.: Phys. Rev. Lett. 64 (1990) 1761. Fisher, G.B., DiMaggio, C.L.: Surf. Sci. 233 (1990) 12. Feidenhans'l, R., Grey, F., Johnson, R.L., Mochrie, S.G.J., Bohr, J., Nielsen, M.: Phys. Rev. B 41 (1990) 5420. Feidenhans'l, R., Grey, F., Nielsen, M., Besenbacher, F., Jensen, F., Lagsgaard, E., Stensgaard, I., Jacobson, K.W., Norskov, J.K., Johnson, R.L.: Phys. Rev. Lett. 65 (1990) 2027. Garrett, S.J., Egdell, R.G., Rivière, J.C.: J. Electron Spectrosc. Relat. Phenom. 54/55 (1990) 1065. Grzelakowski, K., Lyuksyutov, I., Bauer, E.: Phys. Rev. Lett. 64 (1990) 32. Hendriksen, N.E., Billing, G.D., Hansen, F.Y.: Surf. Sci. 227 (1990) 224. Kleinle, G., Wintterlin, J., Ertl, G., Behm, R.J., Jona, F., Moritz, W.: Surf. Sci. 225 (1990) 171. Lauderback, L.L., Larson, S.A.: Surf. Sci. 234 (1990) 135. Lu, J.-P., Albert, M.R., Chang, C.C., Bernasek, S.L.: Surf. Sci. 227 (1990) 317. Noffke, J., Weimert, B., Fritsche, L.: Vacuum 41 (1990) 163. Norskov, J.: Rep. Prog. Phys. 53 (1990) 1253. Pedio, M., Becker, L., Hillert, B., D'Adddato, S., Haase, J.: Phys. Rev. B 41 (1990) 7462. Parkin, S.R., Zeng, H.C., Zhou, M.Y., Mitchell, K.A.R.: Phys. Rev. B 41 (1990) 5432. Pfnür, H., Piercy, P.: Phys. Rev. B 41 (1990) 582. Parker, D.H., Koel, B.: J. Vac. Sci. Technol. A 8 (1990) 2585.

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3.4.2 Adsorption of C, N, and O on metal surfaces Robinson, A.W., Woodruff, D.P., Somers, J.S., Jilcoyne, A.L.D., Ricken, D.E., Bradshaw, A.M.: Surf. Sci. 237 (1990) 99. Rao, C.N.R., Prabhakaran, K., Rajumon, M.K.: Rev. Solid State Sci. 4 (1990) 843. Rocca, M., Traversaro, P., Valbusa, U.: J. Electron Spectrosc. Relat. Phenom. 54/55 (1990) 131. Raekter, T.J., DePristo, A.E.: Surf. Sci. 235 (1990) 84. Schwarz, E., Ernst, K.H., Goner-Buntrock, C., Neubert, M., Christmann, K.: Vacuum 41 (1990) 180. Schwarz, E., Lenz, J., Wohlgemuth, H., Christmann, K.: Vacuum 41 (1990) 167. Smentkowski, V.S., Yates jr., J.T.: Surf. Sci. 232 (1990) 113. Tjeng, L.H., Meinders, M.B.J., Sawatzky, G.A.: Surf. Sci. 236 (1990) 341. Tringides, M.C.: Phys. Rev. Lett. 65 (1990) 1372. Voigländer, B., Lehwald, S., Ibach, H.: Surf. Sci. 225 (1990) 162. Vu Grimsby, D.T., Wu, Y.K., Mitchell, K.A.R.: Surf. Sci. 282 (1990) 51. Yamada, T., Matsuo, I., Nakamura, J., Xie, M., Hirano, H., Matsumoto, Y., Tanaka, K.: Surf. Sci. 231 (1990) 304. Zeng, H.C., Mitchell, K.A.R.: Surf. Sci. 239 (1990) L571. Arabczyk, W., Rausche, E., Storbeck, F.: Surf. Sci. 247 (1991) 264. Bowker, M., Guo, Q., Joyner, R.: Surf. Sci. 257 (1991) 33. Bagus, P.S., Brundle, C.R., Illas, F., Parmigiani, F., Polzonetti: Phys. Rev. B 44 (1991) 9025. Borroni-Bird, C.E., Al-Sarraf, N., Andersson, S., King, D.A.: Chem. Phys. Lett. 183 (1991) 516. Comelli, G., Sastry, M., Paolucci, G., Prince, K.C., Olivi, L.: Phys. Rev. B 43 (1991) 14385. Caputi, L.S., Amoddeo, A., Tucci, R., Papagno, L.: Phys. Rev. B 44 (1991) 1357. Duerr, H., Fauster, Th., Schneider, R.: Surf. Sci. 244 (1991) 237. Dünweg, B., Milchev, A., Rikvold, P.A.: J. Chem. Phys. 94 (1991) 3958. Dai, Q., Gellman, A.J.: Surf. Sci. 248 (1991) 86. Gauthier, Y., Baudoing-Savois, R., Heinz, K., Landskron, H.: Surf. Sci. 251/252 (1991) 493. Haase, O., Koch, R., Borbonus, M., Rieder, K.H.: Phys. Rev. Lett. 66 (1991) 1725. Hutson, F.L., Ramaker, D.E., Koel, B.E.: Surf. Sci. 248 (1991) 104. Hutson, F.L., Ramaker, D.E., Koel, B.E., Gebhard, S.: Surf. Sci. 248 (1991) 119. Jensen, F., Besenbacher, F., Lagsgaard, E., Stensgaard, I.: Surf. Sci. 259 (1991) L774. Kilcoyne, A.L.D., Woodruff, D.P., Robinson, A.W., Lindner, Th., Somers, J.S., Bradshaw, A.M.: Surf. Sci. 253 (1991) 107. Lenz, J., Rech, P., Christmann, K., Neuber, M., Zubragel, C., Schwarz, E.: Surf. Sci. 269-270 (1991) 410. Lauderback, L.L., Lynn, A.J., Waltman, C.J., Larson, S.A.: Surf. Sci. 243 (1991) 323. Mendez, M.A., Oed, W., Fricke, A., Hammer, L., Heinz, K., Müller, K.: Surf. Sci. 253 (1991) 99. Meyer, J.A., Kuk, Y., Estrup, P.J., Silverman, P.J.: Phys. Rev. B 44 (1991) 9104. Nilsson, A., Martensson, N.: Chem. Phys. Lett. 182 (1991) 147. Oed, W., Starke, U., Heinz, K., Müller, K., Pendry, J.B.: Surf. Sci. 251/252 (1991) 488. Pollak, P., Courths, R., Witzel, St.: Surf. Sci. 255 (1991) L523. Por, E., Haase, G., Citri, O., Kosloff, R., Asscher, M.: Chem. Phys. Lett. 186 (1991) 553. Papagno, L., Conti, M., Caputi, L.S., Anderson, J., Lapeyre, G.J.: Phys. Rev. B 44 (1991) 1904. Rettner, C.T., Mullins, C.B.: J. Chem. Phys. 94 (1991) 1627. Simmons, G.W., Wang, Y.-N., Klier, K.: J. Phys. Chem. 95 (1991) 4522. Spitzl, R., Niehus, H., Comsa, G.: Phys. Rev. B 43 (1991) 12619. Siller, L., Pervan, P., Milun, M.: Fizika (Moscow) 23 (1991) 221. Tanaka, K., Yamada, T., Nieuwenhuys, B.E.: Surf. Sci. 242 (1991) 503. Wagner, M.L., Schmidt, L.D.: Surf. Sci. 257 (1991) 113. Yamada, T., Tanaka, K.-I.: J. Am. Chem. Soc. 113 (1991) 1173. Young, M.B., Slavini, A.J.: Surf. Sci. 245 (1991) 56. Zhang, C.-S., Flinn, B.J., Mitchell, I.V., Norton, P.R.: Surf. Sci. 245 (1991) 373. Berko, A., Solymosi, F.: Appl. Surf. Sci. 55 (1992) 193. Brune, H.: PhD Thesis 1992, FU Berlin. Benesh, G.A., Liyanage, L.S.G.: Surf. Sci. 261 (1992) 207.

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Comelli, C., Lizzit, S., Hofmann, Ph., Paolucci, G., Kiskinova, M., Rosei, R.: Surf. Sci. 277 (1992) 31. Colaianni, M.L., Chen, J.G., Weinberg, W.H., Yates jr., J.T.: Surf. Sci. 279 (1992) 211. Canepa, M., Cantini, P., Materra, L., Terrini, S., Valdenazzi, F.: Phys. Scr. T 41 (1992) 226. Comelli, G., Dhanak, V.R., Kiskinova, M., Pangher, N., Paolucci, G., Prince, K.C., Rosei, R.: Surf. Sci. 260 (1992) 7-13. Grzelakowski, K., Lyuksyutov, I., Bauer. E.: Phys. Rev. B 45 (1992) 6877. Ganzmann, I., Borgmann, D., Wedler, G.: Mol. Phys. 76 (1992) 335. Goldmann, A., in: Angle-Resolved Photoemission: Theory and Current Applications, Kevan, S.D (ed.), Amsterdam: Elsevier, 1992, p. 291. Haase, J., Hillert, B., Becker, L., Pedio, M.: Surf. Sci. 262 (1992) 8. Himpsel, F.J., Ortega, J.E.: Surf. Sci. 268 (1992) L279. He, J.-W., Kuhn, W.K., Goodman, D.W.: Surf. Sci. 262 (1992) 351. Joly, Y.: Phys. Rev. Lett. 68 (1992) 951. Kerkar, M., Fisher, D., Woodruff, D.P., Cowie, B.: Surf. Sci. 271 (1992) 45. Koch, R., Borbonus, M., Haase, O., Rieder, K.-H.: Appl. Phys. A 55 (1992) 417. Lizzit, S., Comelli, C., Hofmann, Ph., Paolucci, G., Kiskinova, M., Rosei, R.: Surf. Sci. 276 (1992) 144. Lenz, J., Rech, P., Christmann, K., Neuber, M., Zugräbel, C., Schwarz, E.: Surf. Sci. 269/270 (1992) 410. Lenz, J., Rech, P., Zubrägel, C., Christmann, K., Schwarz, E.: BESSY Jahresbericht 1992, 231. Robinson, I.K., Smilgies, D.-M., Eng, P.J.: J. Phys. Condens. Matter 4 (1992) 5845. Sotto, M.: Surf. Sci. 260 (1992) 235. Siegbahn, P.E.M., Wahlgren, U.: Int. Quantum Chem. 42 (1992) 1149. Weimert, B., Noffke, J., Fritsche, L.: Surf. Sci. 264 (1992) 365. Wassdahl, N., Nilsson, A., Wiell, T., Tillborg, H., Duda, L.-C., Guo, J.H., Mårtensson, N., Nordgren, J., Andersen, J.N., Nyholm, R.: Phys. Rev. Lett. 69 (1992) 812. Alfé, D., Rudolf, P., Kiskinova, M., Rosei, R.: Chem. Phys. Lett. 211 (1993) 220. Al-Sarraf, N., King, D.A.: Surf. Sci. 307-309 (1993) 1. Besenbacher, F., Norskov, J.K.: Prog. Surf. Sci. 44 (1993) 5. Brune, H., Wintterlin, J., Trost, J., Ertl, G., Wiechers, J., Behm, R.J.: J. Chem. Phys. 99 (1993) 2128. Belton, D.N., DiMaggio, G.L., Ng, K.Y.S.: J. Catal. 144 (1993) 273. Blyholder, G., Lawless, M.: Surf. Sci. 290 (1993) 155. Baddorf, A.P., Zehner, D.M., Helgesen, G., Gibbs, D., Sandy, A.R., Mochrie, S.G.J.: Phys. Rev. B 48 (1993) 9013. Bao, X., Barth, J.V., Lehmpfuhl, G., Schuster, R., Uchida, Y., Schlögl, R., Ertl, G.: Surf. Sci. 284 (1993) 14. Berg, C., Raasen, S., Borg, A., Andersen, J.N., Lundgren E., Nyholm, R.: Phys. Rev. B 47 (1993) 13063. Baek, D.H., Chung, J.W., Han, W.K.: Phys. Rev. B 47 (1993) 8461. Comicioli, C., Dhanak, V.R., Comelli, G., Astaldi, C., Prince, K.C., Rosei, R., Atrei, A., Zanazzi, E.: Chem. Phys. Lett. 214 (1993) 438. Canepa, M., Cantini, P., Fossa, F., Mattera, L., Terreni, S.: Phys. Rev. B 47 (1993) 15823. Dorenbos, G., Boerma, D.O.: Surf. Sci. 287/288 (1993) 443. Dorenbos, G., Breeman, M., Boerma, D.O.: Phys. Rev. B 47 (1993) 1580. Gierer, M., Over, H., Ertl, G., Wohlgemuth, H., Schwarz, E., Christmann, K.: Surf. Sci. 297 (1993) L73. Goursot, A., Mele, F., Russo, N., Salahub, D.R., Toscano, M.: Int. Quantum Chem. 48 (1993) 277. Gardin, D.E., Batteas, J.D., Van Hove, M.A., Somorjai, G.A.: Surf. Sci. 296 (1993) 25. Hodgson, A., Lewin, A.K., Nesbitt, A.: Surf. Sci. 293 (1993) 211. Hall, J., Saksager, O., Chorkendorff, I.: Chem. Phys. Lett. 216 (1993) 413. Johnson, K.E., Wilson, R.J., Chiang, S.: Phys. Rev. Lett. 71 (1993) 1055. Jesina, A., Tringides, M.C.: Phys. Rev. B 48 (1993) 2694. Kiskinova, M., Lizzit, S., Comelli, G., Paolucci, G., Rosei, R.: Appl. Surf. Sci. 64 (1993) 185.

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3.4.2 Adsorption of C, N, and O on metal surfaces Klier, K., Wang, Y.-N., Simmons, G.W.: J. Phys. Chem. 97 (1993) 633. Lederer T., Arvanitis, D., Comelli, G., Troeger, L., Baberschke, K.: Phys. Rev. B 48 (1993) 15390. Lederer, T., Arvanitis, D.,Tischer, M., Comelli, G., Troeger, L., Baberschke, K.: Phys. Rev. B 48 (1993) 11277. Leibsle, F.M.: Surf. Sci. 297 (1993) 98. Oehlers, M., Vichon, Th., Szeftel, J., Souikiassian, P.: J. Electron Spectrosc. Relat. Phenom. 64/65 (1993) 269. Okawa, Y., Namba, H., Kuroda, H., Yimagawa, M., Fujikawa, T.: Jpn. J. Appl. Phys. 32 (1993) 362. Rech, P., Muschiol, U., Schneider, F., Schwarz, E., Christmann, E.: BESSY Jahresbericht 1993, 256. Schmidt, J., Stuhlmann, Ch., Ibach, H.: Surf. Sci. 284 (1993) 121. Saiki, R.S., Kaduwela, A.P., Sagurton, M., Osterwalder, J., Friedman, D.J., Fadley, C.S., Brundle, C.R.: Surf. Sci. 282 (1993) 33. Scheurer, F., Ohresser, P., Carriere, B., Deville, J.P., Baudoing/Savois, R., Gauthier, Y.: Surf. Sci. 298 (1993) 107. Schimizu, T., Tsukada, M.: Surf. Sci. 295 (1993) L1017. Shi, H., Jacobi, K., Ertl, G.: J. Chem. Phys. 99 (1993) 9248. Stambouli, V., Palacio, C., Mathieu, H.J., Landolt, D.: Appl. Surf. Sci. 70/71 (1993) 240. Tillborg, H., Nilsson, A., Wiell, T., Wassdahl, N., Martensson, N., Nordgren, J.: Phys. Rev. B 47 (1993) 16464. Voss, C., Gaussmann, A., Kruse, N.: Appl. Surf. Sci. 67 (1993) 142. Voetz, M., Niehus, H., O’Connor, J., Comsa, G.: Surf. Sci. 292 (1993) 211. Wouters, E.R., O'Connor, D.J., Van der Veen, J.F., Zagwijn, P.M., Vrijmoeth, J., Slijkerman, W., Frenken, J.W.M.: Surf. Sci. 296 (1993) 141. Yagi, K., Higashiyama, K., Fukutani, H.: Surf. Sci. 295 (1993) 230. Bludau, H., Gierer, M., Over, H., Ertl, G.: Chem. Phys. Lett. 219 (1994) 452. Comelli, G., Baraldi, A., Lizzit, S., Cocco, D., Paolucci, G., Rosei, R., Kiskinova, M.: Chem. Phys. Lett. 261 (1996) 253. Dhar, S., Smith, K.E., Kevan, S.D.: Phys. Rev. Lett. 73 (1994) 1448. Eierdal, L., Besenbacher, F., Lagsgaard, E., Stensgaard, I.: Surf. Sci. 312 (1994) 31. Gürer, E., Klier, K., Simmons, G.W.: Phys. Rev. B 49 (1994) 14657. Guo, X.C., Bradley, J.M., Hopkinson, A., King, D.S.: Surf. Sci. 310 (1994) 163. Hayden, A.B., Pervan, P., Woodruff, D.P.: Surf. Sci. 306 (1994) 99. Hodgson, A., Wight, A., Worthy, G.: Surf. Sci. 319 (1994) 119. Hoermandinger, G., Pendry, J.B., Leibsle, F.M., Murry, P.W., Joyner, R.W., Thornton, G.: Phys. Rev. B 50 (1994) 8807. Ladas, S., Siokou, A., Kennou, S., Fink, T., Imbihl, R., Mertens, F., Haase, J.: Surf. Sci. 319 (1994) 337. Over, H., Gierer, M., Bludau, H., Ertl, G., Tong, S.Y.: Surf. Sci. 314 (1994) 243. Peterlinz, K.A., Sibener, S.J.: J. Phys. Chem. 99 (1995) 2817. Peden, C.H.F., Shinn, N.D.: Surf. Sci. 312 (1994) 151. Pettinger, B., Bao, X., Wolcock, I.C., Muhler, M., Ertl, G.: Phys. Rev. Lett. 72 (1994) 1561. Ricart, J.M., Torras, J., Clotet, A., Sueiras, J.E.: Surf. Sci. 301 (1994) 89. Ricart, J.M., Torras, J., Illas, F., Rubio, J.: Surf. Sci. 307-309 (1994) 107. Schmidtke, E., Schwennicke, C., Pfnuer, H.: Surf. Sci. 312 (1994) 301. Stietz, F., Pantförder, A., Schaefer, J.A., Meister, G., Goldmann, A.: Surf. Sci. 318 (1994) L1201. Starke, U., Van Hove, M.A., Somorjai, G.A.: Prog. Surf. Sci. 46 (1994) 305. Somorjai, G.A.: Introduction to Surface Chemsitry and Catalysis, John Wiley and Sons, Inc., 1994. Tisdale, G., Sibener, S.J.: Surf. Sci. 311 (1994) 360. Topsoe, H., Boudart, M., Norskov, J.K. (eds.): Frontiers in catalysis: Ammonia synthesis and beyond. Top. Catal. 1 (1994) 185. Vu Grimsby, D.T., Mitchell, K.A.R.: Phys. Rev. B 49 (1994) 11515.

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Vattuone, L., Boragno, C., Restelli, P., Pupo, M., Rocca, M., Valbusa, U.: Phys. Rev. Lett. 72 (1994) 510. Vattuone, L., Boragno, C., Restelli, P., Pupo, M., Rocca, M., Valbusa, U.: Phys. Rev. B 49 (1994) 5. Wiell, T., Tillborg, H., Nilsson, A., Wassdahl, N., Martensson, N., Nordgren J.: Surf. Sci. 304 (1994) L451. Xu, C., Burnham, J.S., Goss, S.H., Caffey, K., Winograd, N.: Phys. Rev. B 49 (1994) 4882. Zhang, C.-S., Bing Li, Norton, P.R.: Surf. Sci. 313 (1994) 308. Zdansky, E.O.F., Nillson, A., Martensson, N.: Surf. Sci. 310 (1994) L583. Batteas, J.D., Barbieri, A., Starkey, E.K., Van Hove, M.A., Somorjai, G.A.: Surf. Sci. 339 (1995) 142. Bessent, M.P., Hu, P., Wander, A., King, D.A.: Surf. Sci. 325 (1995) 272. Buatier de Mongeot, F., Valbusa, U., Rocca, M.: Surf. Sci. 339 (1995) 291. Bao, X., Muhler, M., Pettinger, B., Schlögl, R., Ertl, G.: Catal. Lett. 32 (1995) 185. Bare, S.R., Griffiths, K., Lennard, W.N., Tang, H.T.: Surf. Sci. 342 (1995) 185. Bu, H., Shi, M., Boyd, K., Rabalais, J.W.: J. Chem. Phys. 95 (1991) 2882. Bao, X., Muhler, M., Pettinger, B., Uchida, Y., Lehmpfuhl, G., Schlögl, R., Ertl, G.: Catal. Lett. 32 (1995) 171. Bode, M., Pascal, R., Wiesendanger, R: Surf. Sci. 344 (1995) 185. Canepa, M., Salvetti, M., Traverso, M., Mattera, L.: Surf. Sci. 331 (1995) 183. Dhanak, V.R., Baraldi, A., Comelli, G., Prince, K.C., Rosei, R., Atrei, A., Zanazzi, A.: Phys. Rev. B 51 (1995) 1965. De Rossi, S., Duo, L., Ciccacci, F.: Eur. Phys. Lett. 32 (1995) 687. Davies, P.R., Roberts, M.W., Shukla, N., Vincent, D.J.: Surf. Sci. 325 (1995) 50. Farias, D., Tröger, H., Rieder, K.H.: Surf. Sci. 333 (1995) 150. Furthmüller, J., Kresse, G., Hafner, J., Stumpf, R., Scheffler, M.: Phys. Rev. Lett. 74 (1995) 5084. Grellner, F., Klingenberg, B., Borgmann, D., Wedler, G.: J. Electron Spectrosc. Relat. Phenom. 71 (1995) 107. Gierer, M., Mertens, F., Over, H., Ertl, G., Imbihl, R.: Surf. Sci. 339 (1995) L903. Held, G., Uremovic, S., Menzel, D.: Surf. Sci. 331-333 (1995) 1122. Hammer, B., Norskov, J.K.: Nature (London) 376 (1995) 238. Jacobson, J., Hammer, B., Jacobson, K.W., Norskov, J.K.: Phys. Rev. B 52 (1995) 14954. Jentz, D., Rizzi, S., Barbieri, A., Kelly, D., Van Hove, M.A., Somorjai, G.A.: Surf. Sci. 329 (1995) 14. Kelly, D., Verhoef, R.W., Weinberg, W.H.: J. Chem. Phys. 102 (1995) 3440. Kim, C.M., DeVries, B.D., Fruehberger, B., Chen, J.G.: Surf. Sci. 327 (1995) 81. Liu, W., Wong, K.C., Mitchell, K.A.R.: Surf. Sci. 339 (1995) 151. Liu, W., Wong, K.C., Zeng, H.C., Mitchell, K.A.R.: Prog. Surf. Sci. 50 (1995) 247. Lyman, P.F., Mullins, D.R.: Phys. Rev. B 51 (1995) 13623. Materer, N., Starke, U., Barbieri, A., Doell, R., Heinz, K., Van Hove, M.A., Somorjai, G.A.: Surf. Sci. 325 (1995) 207. Over, H., Mertens, F., Gierer, G., Bludau, H., Ertl, G.: Surf. Sci. 334 (1995) 344. Peterlinz, K.A., Siebener, S.J.: J. Phys. Chem. 99 (1995) 2817. Rech, P., Vollmer, A., Schmidt, K., Muschiol, U., Schwarz, E., Schneider, F., Christmann, K.: BESSY Jahresbericht 1995, p. 342. Rao, C.N.R., Vijayakrishnan, V., Kulkarni, G.U., Rajumon, M.K.: Appl. Surf. Sci. 84 (1995) 285. Ruckman, M.W., Qiu, S.-L., Strongin, M.: Appl. Surf. Sci. 89 (1995) 401. Stietz, F., Meister, G., Goldmann, A., Schaefer, J.A.: Surf. Sci. 339 (1995) 1. Wang, Y.M., Li, Y.S., Mitchell, K.A.R.: Surf. Sci. 342 (1995) 272. Wang, Y.M., Li, Y.S., Mitchell, K.A.R.: Surf. Sci. 343 (1995) L1167. Waldfried, C., McIlroy, D.N., Li, D., Pearson, J., Bader, S.D., Dowben, P.A.: Surf. Sci. 341 (1995) L1072. Wight, A., Condon, N.G., Leibsle, F.M., Worthy, G., Hodgson, A.: Surf. Sci. 331-333 (1995) 133. Zhang, J., Dowben, P.A., Li, D., Onellion, M.: Surf. Sci. 329 (1995) 177.

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3.4.2 Adsorption of C, N, and O on metal surfaces Bondzie, V.A., Kleban, P., Dwyer, D.J.: Surf. Sci. 347 (1996) 319. Bradley, J.M., Guo, X.-C., Hopkinson, A., King, D.A.: J. Chem. Phys. 104 (1996) 4283. Buatier de Mongeot, F., Rocca, M., Valbusa, U.: Surf. Sci. 363 (1996) 68. Baraldi, A., Dhanak, V.R., Comelli, G., Prince, K.C., Rosei, R.: Phys. Rev. B 53 (1996) 4073. Bao, X., Muhler, M., Schedel-Niedrig, Th., Schlögl, R.: Phys. Rev. B 54 (1996) 2249. Bode, M., Pascal, R., Wiesendanger, R.: Z. Phys. B 101 (1996) 103. Chevrier, J., Huang, L., Zeppenfeld, P., Comsa, G.: Surf. Sci. 355 (1996) 1. Cerny, S.: Surf. Sci. Rep. 26 (1996) 1. Davila, M.E., Asensio, M.C., Woodruff, D.P., Schindler, K.-M., Hofmann, Ph., Bao, S., Fritzsche, V., Bradshaw, A.M.: Surf. Sci. 359 (1996) 185. Frederick, B.G., Lee, M.B., Richardson, N.V.: Surf. Sci. 348 (1996) L71. Gravil, P.A., Bird, D.M., White, J.A.: Phys. Rev. Lett. 77 (1996) 3933. Ge, J.-Y., Dai, Z., Job, Z.H.: J. Phys. Chem. 100 (1996) 11432. Huang, L., Zeppenfeld, P., Chevrier, J., Comsa, G.: Surf. Sci. 352 (1996) 285. Hannon, J.B., Dürr, H.A., Plummer, E.W.: Phys. Rev. B 54 (1996) 16444. Illas, F., Rubio, J., Ricart, J.M., Pacchioni, G.: J. Chem. Phys. 105 (1996) 7192. Kamei, M., Obayashi, T., Tsunematsu, H., Tanaka, Y., Gotoh, Y.: Surf. Sci. 356 (1996) 137. Kolthoff, D., Jürgens, D., Schwennicke, C., Pfnür, H.: Surf. Sci. 365 (1996) 374. McIlroy, D.N., Waldfried, C., Li, D., Pearson, J., Bader, S.D., Huang, D.-J., Johnson, P.D., Sabiryanov, R.F., Jaswal, S.S., Dowben, P.A.: Phys. Rev. Lett. 76 (1996) 2802. Ozawa, R., Yamane, A., Morikawa, K., Okwada, M., Suzuki, K., Fukutani, H.: Surf. Sci. 346 (1996) 237. Over, H., Tong, S.Y., in: Handbook of Surface Science, Vol. 1, Unertl, W.N. (ed.), Elsevier 1996, p 425. Park, K.T., Simmons, G.W., Klier, K.: Surf. Sci. 267 (1996) 307. Raukema, A., Butler, D.A., Box, F.M.A., Kleyn, A.W.: Surf. Sci. 347 (1996) 151. Raukema, A., Butler, D.A., Kleyn, A.W.: J. Phys. Condens. Matter 8 (1996) 2247. Rech, P.: PhD Thesis 1996, FU Berlin. Stampfl, C., Schwegmann, S., Over, H., Scheffler, M., Ertl, G.: Phys. Rev. Lett. 77 (1996) 3371. Stampfl, C., Scheffler, M.: Phys. Rev. B 54 (1996) 2868. Stampfl, C., Schwegmann, S., Over, H., Scheffler, M., Ertl, G.: Phys. Rev. Lett. 77 (1996) 3371. Suzuki, K., Yamane, A., Ozawa, R., Gunji, Y., Higashiyama, K., Fukutani, H.: Surf. Sci. 365 (1996) 248. Schwennicke, C., Pfnür, H.: Surf. Sci. 369 (1996) 248. Schilbe, P., Siebentritt, S., Pues, R., Rieder, K.-H.: Surf. Sci. 360 (1996) 157. Tismuss, S., Wander, A., King, D.A.: Chem. Rev. 96 (1996) 1291. Takehiro, N., Matsumoto, Y., Okawa, Y., Tanaka, K.-I.: Phys. Rev. B 53 (1996) 4094. Vattuone, L., Yeo, Y.Y., King, D.A.: J. Chem. Phys. 104 (1996) 8096. Wong, K.C., Liu, W., Mitchell, K.A.R.: Surf. Sci. 360 (1996) 137. Wartnaby, C.E., Stuck, A., Yeo, Y.Y., King, D.A.: J. Phys. Chem. 100 (1996) 12483. Wheeler, M.C., Seets, D.C., Mullins, C.B.: J. Chem. Phys. 105 (1996) 1572. Xu, H., Ng, K.Y.S.: Surf. Sci. 356 (1996) 19. Yamamoto, M., Chan, C.T., Ho, K.M., Naito, S.: Phys. Rev. B 54 (1996) 14111. Zacchigna, M., Astaldi, C., Prince, K.C., Sastry, M., Comicioli, C., Rosei, R., Quaresima, C., Ottaviani, C., Crotti, C., Antonini, A., Matteucci, M., Perfetti, P.: Surf. Sci. 347 (1996) 53. Alstrup, I., Chorckendorff, I., Ullmann, S.: J. Catal. 168 (1997) 217. Brena, B., Comelli, G., Ursella, L., Paolucci, G.: Surf. Sci. 375 (1997) 150. Bungaro, C., Noguera, C., Ballone, P., Kress, W.: Phys. Rev. Lett. 79 (1997) 4433. Brault, P., Range, H., Toennies, J.P.: J. Chem. Phys. 106 (1997) 8876. Courths, R., Hüfner, S., Kemkes, P., Wiesen, G.: Surf. Sci. 376 (1997) 43. Carter, R.N., Murphy, M.J., Hodgson, A.: Surf. Sci. 387 (1997) 102. Chen, M., Bates, S.P., van Santen, R.A., Friend, C.M.: J. Phys. Chem. B 101 (1997) 10051. Davis, J.E., Nolan, P.D., Karseboom, S.G., Mullins, C.B.: J. Chem. Phys. 107 (1997) 943. Davis, J.E., Mullins, C.B.: Surf. Sci. 380 (1997) L513. Landolt-Börnstein New Series III/42A4

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Elbe, A., Meister, G., Goldmann, A.: Surf. Sci. 371 (1997) 438. Frickel, D.P., Kuznetsov, M.V., Shalaeva, E.V.: Surf. Rev. Lett. 4 (1997) 1309. Feibelman, P.J.: Phys. Rev. B 56 (1997) 2175. Feibelman, P.J.: Phys. Rev. B 56 (1997) 10532. Ge, Q., Hu, P., King, D.A., Lee, M.-H., White, J.A., Payne. M.C.: J. Chem. Phys. 106 (1997) 1210. Gierer, M., Over, H., Rech, P., Schwarz, E., Christmann, K.: Surf. Sci. 370 (1997) L201. Head, J.D.: Surf. Sci. 384 (1997) 224. Haneman, D.: Surf. Sci. 375 (1997) 71. Janssen, N.M.H., Chlach, A.R., Ikai, M., Tanaka, K., Nieuwenhuys, B.E.: Surf. Sci. 382 (1997) 201. Kostov, K.L., Gsell, M., Jakob, P., Moritz, T., Widdra, W., Menzel, D.: Surf. Sci. 377-379 (1997) L138. Kundrotas, P.J., Lapinskas, S., Rosengren, A.: Surf. Sci. 377-379 (1997) 7. Liao, M.-S., Au, C.-T., Ng, C.-F.: Chem Phys. Lett. 272 (1997) 445. Ladas, S., Kennou, S., Hartmann, N., Imbihl, R.: Surf. Sci. 382 (1997) 49. Morikawa, Y., Mortensen, J.J., Hammer, B., Norskov, J.K.: Surf. Sci. 386 (1997) 67. Moré, S., Berndt, W., Stampfl, C., Bradshaw, A.M.: Surf. Sci. 381 (1997) L589. Mortensen, J.J., Morikawa, Y., Hammer, B., Norskov, J.K.: J. Catal. 169 (1997) 85. Österlund, L., Zoric, I., Kasemo, B.: Phys. Rev. B 55 (1997) 15452. Over, H., Schwegmann, S., Cvetko, D., De Renzi, V., Floreano, L., Gotter, R., Morgante, A., Peloi, M., Tommasini, F., Tennaro, S.: Phys. Rev. B 55 (1997) 4717. Romm, L., Katz, G., Kosloff, R., Asscher, M.: J. Phys. Chem B 101 (1997) 2213. Rocca, M., Cemic, F., Buatier de Mongeot, F., Valbusa, U., Lacombe, S., Jacobi, K.: Surf. Sci. 373 (1997) 125. Raukema, A., Butler, D.A., Kleyn, A.W.: Surf. Sci. 373 (1997) 127. Ricart, J.M., Torras, J., Rubio, J., Illas, F.: Surf. Sci. 374 (1997) 31. Ri, C.-S., Cho, Y.-P., Park, J.-B., Kang, J.-S., Kim, S.-H., Lee, K.-H. : J. Vac. Sci. Technol. B 15 (1997) 1623. Stuckless, J.T., Wartnaby, C.E., Al-Sarraf, N., Dixon-Warren, St.J.B., Kovar, M., King, D.A.: J. Chem. Phys. 106 (1997) 2012. Schwegmann, S., Seitsonen, A.P., Dietrich, H., Bludau, H., Over, H., Jacobi, K., Ertl, G.: Chem. Phys. Lett. 264 (1997) 680. Schwegmann, S., Over, H., De Renzi, V., Ertl, G.: Surf. Sci. 375 (1997) 91. Schmid, M., Pinczolits, M., Hebenstreit, W., Varga, P.: Surf. Sci. 389 (1997) L1140. Schwennicke, C., Pfnür, H.: Phys. Rev. B 56 (1997) 10558. Stokbro, K., Baroni, S.: Surf. Sci. 370 (1997) 166. Torras, J., Toscano, M., Ricart, J.M., Russo, N.: J. Mol. Catal. A: 119 (1997) 387. Wang, Y.M., Li, Y.S., Mitchell, K.A.R.: Surf. Sci. 380 (1997) 540. Wang, H, Tobin, R.G.,Lambert, D.K., DiMaggio, C.L., Fisher, G.B.: Surf. Sci. 372 (1997) 267. Wintterlin, J., Trost, J., Renisch, S., Schuster, R., Zambelli, T., Ertl, G.: Surf. Sci. 394 (1997) 159. Yamamoto, M., Kurahashi, M., Chan, C.T., Ho, K.M., Naito, S.: Surf. Sci. 387 (1997) 300. Zhdanov, V.P.: Phys. Rev. B 55 (1997) 6770. Ali, T., Klötzer, B., Walker, A.V., Ge, Q., King, D.A.: J, Chem. Phys. 109 (1998) 9967. Besenbacher, F., Chorkendorff, I., Clausen, B.S., Hammer, B., Molenbroek, AM., Norskov, J.K., Stensgaard, I.: Science magazin (AAAS) 279 (1998) 1913. Brown, W.A., Kose, R., King, D.A.: Chem. Rev. 98 (1998) 797. Comelli, G., Dhanak, V.R., Kiskinova, M., Prince, K.C., Rosei, R.: Surf. Sci. Rep. 32 (1998) 165. Daimon, H., Ynzunza, R., Palomares, J., Takabi, H., Fadley, C.S.: Surf. Sci. 408 (1998) 260. Daimon, H., Ynzunza, R., Palomares, J., Tober, E.D., Wang, Z.X., Kaduwela, A.P., Van Hove, M.A., Fadley, C.S.: Phys. Rev. B. 58 (1998) 9662. Duarte, H.A., Salahub, D.R.: J. Chem. Phys. 108 (1998) 743. Feibelman, P.J., Hafner, J., Kresse, G.: Phys. Rev. B 58 (1998) 2179. Frechard, F., Van Santen, R.A.: Surf. Sci. 407 (1998) 200. Kose, R., Brown, W.A., King, D.A.: Surf. Sci. 402-404 (1998) 856. Kim, Y.D., Wendt, S., Schwegmann, S., Over, H., Ertl, G.: Surf. Sci. 418 (1998) 267.

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3.4.2 Adsorption of C, N, and O on metal surfaces Klinke, D.J., Wilke, S., Linda, J., Broadbelt, J.: J. Catal. 178 (1998) 540. Kröger, J., Lehwald, S., Ibach, H.: Phys. Rev. B 58 (1998) 1578. Loffreda, D., Simon, D., Sautet, P.: J. Chem. Phys. 108 (1998) 6447. Liem, S.Y., Kresse, G., Clarke, J.H.R.: Surf. Sci. 415 (1998) 194. Mitrovic, B.C., O'Connor, D.J.: Surf. Sci. 405 (1998) 261. Mitrovic, B.C., O'Connor, D.J., Shen, Y.G.: Surf. Rev. Lett. 5 (1998) 599. Murphy, M.J., Skelly, J.F., Hodgsson, A.: J. Chem. Phys. 109 (1998) 3619. Nolan, P.D., Lutz, B.R., Tanaka, P.L., Mullins, C.B.: Surf. Sci. 419 (1998) L107. Nolan, P.D., Wheeler, M.C., Davis, J.E., Mullins, C.B.: Acc. Chem. Res. 31 (1998) 798. Nolan, P.D., Lutz, B.R., Tanaka, P.L., Davis, J.E., Mullins, C.B.: Phys. Rev. Lett. 81 (1998) 3179. Over, H.: Prog. Surf. Sci. 58 (1998) 249. Ota, K., Tanaka, M., Usami, S.: Surf. Sci. 402 (1998) 813. Ozawa, R., Gunji, Y., Sekiba, D., Nakamizo, H., Fukutani, H.: J. Electron Spectrosc. Relat. Phenom. 88-91 (1998) 717. Ota, K., Tanaka, M., Usami, S.: J. Electron Spectrosc. Relat. Phenom. 88-91 (1998) 571. Riffe, D.M., Wertheim, G.K.: Surf. Sci. 399 (1988) 248. Schwegmann, S., Seitsonen, A.P., De Renzi, V., Dietrich, H., Bludau, H., Gierer, M., Over, H., Jacobi, K., Scheffler, M., Ertl, G.: Phys. Rev. B 57 (1998) 15487. Sjövall, P., Uvdal, P.: Chem. Phys. Lett. 282 (1998) 355. Skelly, J.F., Bertrams, T., Munz, A.W., Murphy, M.J., Hodgson, A.: Surf. Sci. 415 (1998) 48. Sporn, M., Platzgummer, E., Pinczolits, M., Hebenstreit, W., Schmid, M., Hofer, W., Varga, P.: Surf. Sci. 396 (1998) 78. Siokou, A., Van Hardeveld, R.M., Niemantsverdriet, J.W.: Surf. Sci. 402-404 (1998) 110. Shen, Y.G., Qayyum, A., O’Connor, D.J., King, B.V.: Phys. Rev. B 58 (1998) 10025. Trost, J., Brune, H., Wintterlin, J., Behm, R.J., Ertl, G.: J. Chem. Phys. 108 (1998) 1740. Tanaka, K., Fujita, T., Okawa, Y.: Surf. Sci. 401 (1998) L407. Triguero, L., Petterson, L.G.M.: Surf. Sci. 398 (1998) 70. Takehiro, N., Besenbacher, F., Laegsgaard, E. Tanaka, K., Stensgaard, I.: Surf. Sci. 397 (1998) 145. Venvik, H.J., Borg, A.: Appl. Phys. A 66 (1998) 491. Wider, J., Greber T., Wetli, E., Kreutz, T.J., Schwaller, P., Osterwalder, J.: Surf. Sci. 417 (1998) 301. Walker, A.V., Klötzer, B., King, D.A.: J. Chem. Phys. 109 (1998) 6879. Wiell, T., Klepeis, J.E., Bennich, P., Björneholm, O., Wassdahl, N., Nilsson, A.: Phys. Rev. B 58 (1998) 1655. Zambelli, T., Barth, J.V., Wintterlin, J.: Phys. Rev. B 58 (1998) 12663. Alfè, D., de Gironcoli, S., Baroni, S.: Surf. Sci. 437 (1999) 18. Baraldi, A., Cerdá, J., Martin Gago, J.A., Comelli, G., Lizzit, S., Paolucci, G., Rosei, R.: Phys. Rev. Lett. 82 (1999) 4874. Böttcher, A., Niehus, H.: Phys. Rev. B 60 (1999) 14396. Canepa, M., Terreni, S., Narducci, E., Mattera, L.: J. Chem. Phys. 110 (1999) 2257. Driver, S.M., Woodruff, D.P.: Surf. Sci. 442 (1999) 1. Dudzik, E., Norris, A.G., McGrath, R., Charlton, G., Thornton, G., Murphy, B., Turner, T.S., Norman, D.: Surf. Sci. 433-435 (1999) 317. Feydt, J, Elbe, A, Engelhard, H, Meister, G, Goldmann, A: Surf. Sci. 440 (1999) 213. Gunduglia-Pirovano, M.V., Scheffler, M.: Phys. Rev. B 59 (1999) 15533. Jaworowski, A.J., Beutler, A., Strisland, F., Nyholm, R., Setlik, B., Heskett, D., Andersen, J.N.: Surf. Sci. 431 (1999) 33. Gibson, K.D., Viste, M., Sanchez, E.C., Sibener, S.J.: J. Chem. Phys. 110 (1999) 2757. Hammer, B., Hansen, L.B., Nørskov, J.K.: Phys. Rev. B 59 (1999) 7413. Kreuzer, H.J., Payne, S.H., Drozdowski, A., Menzel, D.: J. Chem. Phys. 110 (1999) 6982. Mortensen, J.J., Hansen, L.B., Hammer, B., Norskov, J.K.: J. Catal. 182 (1999) 479. Norris, A.G., Schedin, F., Thornton, G., Dhanak, V.R., Turner, T.S., McGrath, R.: Phys. Rev. B 62 (1999) 2113. Oguchi, T.: Surf. Sci. 438 (1999) 37.

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Schmid, M., Leonardelli, G., Sporn, M., Platzgummer, E., Pinczolits, M., Varga, P.: Phys. Rev. Lett. 82 (1999) 355. Stampfl, C., Kreuzer, H.J., Payne, S.H., Pfnür, H., Scheffler, M.: Phys. Rev. Lett. 83 (1999) 2993. Terborg, R., Hoeft, J.T., Polcik, M., Lindsay, R., Schaff, O., Bradshaw, A.M., Toomes, R., Booth, N.A., Woodruff, D.P., Rotenberg, E., Denlinger, J.: Phys. Rev. B 60 (1999) 10715. Yata, M., Rouch, H.: Appl. Phys. Lett. 75 (1999) 1021. Yagi, K., Sekiba, D., Futani, H.: Surf. Sci. 442 (1999) 307. Zhukov, V., Popova, I., Yates, J.T.: Surf. Sci. 441 (1999) 251. Arabczyk, W., Narkiewicz, U.: Surf. Sci. 454-456 (2000) 227. Andersen, J.N.: Private communication, June 2000. Bukhtiyarov, V.I.: Private communication, June 2000. Beutl, M., Lesnik, J., Lundgren, E., Konvicka, C., Varga, P., Rendulic, K.: Surf. Sci. 447 (2000) 245. Carlisle, C.I., King, D.A., Bocquet, M.-L., Cerda´, J., Sautet, P.: Phys. Rev. Lett. 84 (2000) 3899. De Wolf, C.A., Bakker, J.W., Wouda, P.T., Nieuwenhuys, B.E., Baraldi, A., Lizzit, S., Kiskinova, M.: J. Chem. Phys. 113 (2000) 10717. Gomes, J.R.B., Gomes, J.A.N.F., Illas, F.: J. Mol. Catal. A 170 (2001) 187. Gourieux, T., Frechard, S., Dulot, F., Eugene, J., Kierren, B., Malterre, D.: Phys. Rev. B 62 (2000) 7502. Hammer, B., Nørskov, J.K.: Adv. Catal. 45 (2000). Johnson, K., Ge, Q., Tismuss, S., King, D.A.: J. Chem. Phys. 112 (2000) 10460. Kim, C.M., Jeong, H.S., Kim, E.H.: Surf. Sci. 459 (2000) L457. Lizzit, S., Baraldi, A., Groso, A., Reuter, K., Ganduglia-Pirovano, M.V., Stampfl, C., Scheffler, M., Stichler, M., Keller, C., Wurth, W., Menzel, D.: Phys. Rev. B 63 (2001) 205419. Lynch, M., Hu, P.: Surf. Sci. 458 (2000) 1. Nakano, H., Kawakami, S., Fujitani, T., Nakamura, J.: Surf. Sci. 454-456 (2000) 295. Wiklund, M., Borg, M., Nyholm, R., Andersen, J.N.: unpublished. Watwe, R.M., Bengaard, H.S., Rostrup-Nielsen, J.R., Dumesic, J.A., Norskov, J.K.: J. Catal. 189 (2000) 16. Domnick, R., Held, G., Witte, P., Steinrück, H.-P.: J. Chem. Phys. 115 (2001) 1902. Driver, S.M., Woodruff, D.P.: Surf. Sci. 492 (2001) 11. Driver, S.M., Hoeft, J.T., Polcik, M., Kittel, M., Terborg, R., Toomes, R.L., Kang, J.-H., Woodruff, D.P.: J. Phys. Condens. Matter 13 (2001) L601. Ellmer, H., Repain, V., Rousset, S., Croset, B., Sotto, M., Zeppenfeld, P.: Surf. Sci. 476 (2001) 95. Gomes, J.R.B., Gomes, J.A.N.F.: Surf. Sci. 471 (2001) 59. Hoeft, J.T., Polcik, M., Kittel, M., Terborg, R., Toomes, R.L., Kang, J.-H., Woodruff, D.P.: Surf. Sci. 492 (2001) 1. Jomard, G., Pasturel, A.: Appl. Surf. Sci. 177 (2001) 230. Koller, R., Bergermayer, W., Kresse, G., Hebenstreit, E.L.D., Konvicka, C., Schmid, M., Podloucky, R., Varga, P.: Surf. Sci. 480 (2001) 11. Kiejna, A, Lundqvist, B.I: Phys. Rev. B 63 (2001) 085405. Lee, C.S., Lin, T.M.: Surf. Sci. 471 (2001) 219. Matsumoto, T., Bennet, R.A., Stone, P., Yamada, T., Domen, K., Bowker, M.: Surf. Sci. 471 (2001) 225. Michaelides, A., Hu. P.: J. Chem. Phys. 114 (2001) 5792. Over, H., Lundgren. E., Wiklund, M., Andersen, J.N.: Chem. Phys. Lett. 342 (2001) 467. Pedersen, M.O., Osterlund, L., Mortensen, J.J., Mavrikakis, M., Hansen, L.B., Stensgaard, I., Laegsgaard, E., Norskov, J.K., Besenbacher, F.: Phys. Rev. Lett. 84 (2000) 4898. Schiechl, H., Winkler, A., Fresenius, J.: Anal. Chem. 371 (2001) 342. K.-H. Ernst: Private communication. Takagi, H, Daimon, H,. Palomares, F.J., Fadley, C.S.: Surf. Sci. 470 (2001) 189. Uehara, Y., Matsumoto, T., Ushioda, S.: Solid State Commun. 119 (2001) 671. Vesselli, E., Alfrich, C., Baraldi, A., Comelli, G., Esch, F., Rosei, R.: J. Chem. Phys. 114 (2001) 4221. Yamazaki, H., Kamisawa, T., Kobubun, T., Haga, T., Kamimizu, S., Sakamoto, K.: Surf. Sci. 477 (2001) 174.

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3.4.2 Adsorption of C, N, and O on metal surfaces Zhang, C.J., Baxter, R.J., Hu, P., Alavi, A., Lee, M.-H.: J. Chem. Phys. 115 (2001) 5272. Hansen, K.H., Sljivancanin, Z., Hammer, B., Lagsgaard, E., Besenbacher, F, Stensgaard, I.: Surf. Sci. 496 (2002) 1. Norskov, J.N., K. Nørskov, T. Bligaard, A. Logadottir, S. Bahn, L. B. Hansen, M. Bollinger, H. Bengaard, B. Hammer, Z. Sljivancanin, Y. Xu, S. Dahl, and C. J. H. Jacobsen: J. Catal. 209 (2002) 275.

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3.7 Molecular diatomic adsorbates on metals and semiconductors 3.7.1 CO and N2 adsorption on metal surfaces A. FÖHLISCH, H.P. BONZEL List of symbols and abbreviations ARUPS ARPEFS ass CDAD CSOV DLEED DOS ESD ESDIAD EELS fr. transl. FWHM H HR-EELS IETS IPS IRAS LEED LITD MB MBS ML NEXAFS PED PES PFDMS PJ-EELS PM-IRAS RAIRS RBS REMPI S0 SXD SFG SHG T TDS TEAS T g, T s Tp TPD TP-EELS

angle resolved UPS angle resolved photoemission fine structure assumed circular dichroism in angular distribution of photoelectrons constrained space orbital variation diffuse (intensity) low energy electron diffraction density of states electron stimulated desorption electron stimulated desorption ion angular distribution electron energy loss spectroscopy frustrated translational mode full width at half maximum hollow adsorption site high resolution EELS inelastic electron tunneling spectroscopy inverse photoemission spectroscopy infrared reflection absorption spectroscopy low energy electron diffraction laser induced thermal desorption molecular beam (sticking coefficient) molecular beam scattering monolayer near-edge X-ray absorption fine structure photoelectron diffraction (energy scanned mode; also: PhD [82Woo]) potential energy surface pulsed field desorption mass spectrometry [86Kru] pressure jump EELS [89Whi] polarization modulation infrared reflection absorption spectroscopy reflection absorption infrared spectroscopy (IRAS) Rutherford backscattering Resonance enhanced multi-phonon ionization [97Sch2] initial sticking coefficient (at zero coverage) soft X-ray diffraction sum frequency generation second harmonic generation atop adsorption site thermal desorption spectroscopy thermal energy atom scattering gas temperature / surface temperature temperature of maximum desorption rate (peak) thermally programmed desorption temperature programmed EELS Landolt-Börnstein New Series III/42A4

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time resolved EELS ultra-violet photoelectron spectroscopy work function change X-ray absorption spectroscopy X-ray emission spectroscopy X-ray photoelectron spectroscopy X-ray photoelectron diffraction (angle-scanned mode) XPS using Y Mζ radiation at 132.3 eV [89Düc]

3.7.1.1 Introduction In this chapter the properties of adsorbed CO and N2 on fcc, bcc and hcp metal surfaces of well-defined structure will be presented. All properties except surface diffusion coefficients [01See], work function changes [01Jac1] and surface core level shifts [01Den] due to adsorbed CO (or N2) are listed in tables. The metals for which data have been collected are highlighted in the partial periodic table below. The larger portion of tabulated results concerns adsorbed CO. Data for adsorbed molecular N2 are much less numerous. No relevant data have been found for elements in gray. Al

Si

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Ga

Ge

Zr

Nb

Mo

Tc

Ru

Rh

Pd

Ag

Cd

In

Sn

Hf

Ta

W

Re

Os

Ir

Pt

Au

Hg

Tl

Pb

Bold character: single crystal data reviewed in this section Plain black character: thin film data reviewed (Ti, Mn, Zn, Sn) Grey character: no data available (Si, V, Ga, Ge, Tc, Cd, In, Hf, Hg, Tl, Pb) Single component elemental crystals of a single orientation have not been available in all cases. To provide some element-specific information regarding the adsorptive properties of rarely studied crystals, such as Ti, Zn, Mn, and Sn, we have also considered binary systems to a limited extent in section 3.7.1.6. The substrates consist of a small coverage (submonolayer to several monolayers) of the element of interest on a carrier single crystal of a different element. The modification of CO and N2 adsorption behavior due to the added component may serve as a rough indicator of the chemical activity of that component. A list of such systems is given in section 3.7.1.6. There have been a very large number of studies of CO adsorption on metal surfaces during the past 40 years. Many different aspects of CO adsorption have been discovered and described. It is impossible to cite all of those studies in this chapter and nearly equally impossible to present all published quantitative data in the respective tables. Although we have tried to cover all important and especially the recent publications, it is unavoidable that we have missed some. Therefore we admit at this point that our data collection is likely to be incomplete. We apologize to those whose work slipped through the search grid. What is true for the citations and data, is even more the case for the figures included in this chapter. Here the choice was rather subjective and arbitrary, motivated by our desire to present some outstanding examples. Most refractory metals (bcc) surveyed in this chapter exhibit a high activity for CO adsorption and for breaking the C-O bond. fcc metals, on the other hand, are less reactive in general although most of them adsorb CO. The dissociation of CO, if it occurs, is in general structure sensitive. Several fcc metals, such as Ag, Au, Al, Sn and Pb, are fairly inactive or even inert to CO adsorption. It is therefore an important Landolt-Börnstein New Series III/42A4

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issue in the context of surface reactivity to distinguish molecular and dissociated CO, once CO is adsorbed on the surface. Thermal desorption spectra (pressure versus temperature) have been used as an early and simple means to classify the adsorbed states. A convention evolved to label thermal desorption peaks, beginning at low temperature, with Greek letters, e.g. α1, α2, . . , β1, β2, etc. which are often meant to indicate molecular (sometimes physisorbed) species and dissociated species, respectively. This designation of peaks has been adopted here as well, but the meaning of α and β has changed and does not necessarily stand for a molecular or dissociated adsorbed species. The nomenclature of adsorbed nitrogen is different than for adsorbed CO. At least four different states have been found, e.g. on Fe(111) [85Tsa, 86Whi1, 86Whi2, 87Fre, 87Gru1, 87Gru2], which are designated as follows (in order of increasing adsorption energy): δ-state: physisorbed N2, γstate: chemisorbed σ-bonded N2 (oriented vertical to the surface), α-state: chemisorbed π-bonded N2 (oriented nearly parallel to the surface), β-state: atomic nitrogen N. Depending on the availability of adsorption sites, several sub-states (e.g. of β and γ) may be distinguished. In the case of molecular N2 these different states can be determined by thermal desorption or by vibrational and electron spectroscopies which have been summarized for transition metal surfaces in 1991 [91Rao]. Historically, the inability to make a clear distinction of adsorbed chemical states has led to quite a bit of confusion in the years prior to about 1960. Once electron and vibrational spectroscopies came into use, this was no longer an issue. Hence the data characterizing the adsorbed state in situ are primarily spectroscopic data, such as molecular vibrational frequencies and molecular valence orbital as well as atom- and chemistry-specific core level binding energies. Further important characteristics are the rates of adsorption and desorption, i.e. sticking coefficients and temperatures of desorption, but also measurements of energies of adsorption and desorption. Finally, structural data concerning the adsorption site on the surface and the bond parameters of the substrate atom − molecule configuration are desirable. Additional physical and chemical data, e.g. two-dimensional ordered CO structures, two-dimensional dispersion of CO molecular orbital states, coverage dependencies of various adsorption related quantities, or activation energies for CO dissociation, have been measured in some cases and will be reported in this section. The adsorption of CO and the isoelectronic homonuclear N2 on transition and noble metals are prototypes for molecular adsorption, mediated through the CO orbitals of σ and π symmetry, for example. The adsorption of CO on transition metal surfaces can also be dissociative, especially at elevated temperature. The dissociation to atomic carbon and oxygen, or alternatively the disproportionation of CO into adsorbed carbon and CO2 is of considerable technological relevance, because this step is important for the catalytic hydrogenation of CO to methane (and higher hydrocarbons) which takes place on metals, notably Ni, Fe, Co, Rh and Ru [74Dal, 75Dal, 76Van, 77Pal, 78Dwy, 79Kre, 80Goo, 80Kre, 82Bon, 94Som]. 3.7.1.1.1 Thermodynamic properties For the molecular adsorption, the heat of adsorption determined via thermal desorption spectroscopy (TDS) and/or measurements of isosteres or isobars, gives us access to the total energy difference between the clean metal surface and the CO gas which is an important characteristic of CO molecules adsorbed to the metal surface. However, this quantity does not specify the adsorption path, eventually involving precursor states and special geometric effects, e.g. site selectivity and site blocking. The information on the dynamics of the adsorption process is contained in quantities, such as the coverage dependent sticking coefficient and the pre-exponential factors of adsorption. The heat of adsorption on fcc transition metals is relatively low but increases for bcc metals, such as seen in Fig. 1 [94Som]. Since no distinction between molecular and dissociative adsorption of CO is made, the latter increase may be related to a predominance of CO dissociation on these metals.

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Heat of adsorption [k cal/mol ]

200

CO

Ti

150

W Nb

100 50

Pt Pd

0

Mo

Mn

Rh

Co Ni Ir Fe Cu

Zr Ta

Re Cr Ge

150 250 50 300 100 200 Heat of formation of oxide per metal atom [k cal/mol]

Fig. 1. Energy of adsorption of CO on transition metals as a function of the heat of formation of corresponding oxides [79Toy].

The heat of adsorption of CO and N2 to metal surfaces is determined not only by the substrate material but also by the crystallographic orientation of the surface. It is highly sensitive to the coverage which is due to the interplay between the local interaction between the adsorbed species and the metal adsorbent, and the adsorbate-adsorbate interaction, both being direct and substrate-mediated. Although the molecules N2 and CO are isoelectronic, there are important differences in their adsorption behavior on metals. In comparison, the heat of adsorption or the activation energy of desorption, respectively, of the isoelectronic N2 is for the same metal surface always significantly lower than for CO. So is the activation energy of desorption for c(2×2)CO/Ni(100) 138.6 kJ/mol [83Koe1] and for the geometrically equivalent c(2×2)N2/Ni(100) only 25 kJ/mol [84Gru3]. Thus N2 is adsorbed in a much narrower range of temperatures (under vacuum conditions) than CO. One important aspect of CO adsorption is that its heat of adsorption is about one order of magnitude smaller than its gas phase dissociation energy (11.23 eV). The fraction is even less for the N2 molecule. This observation has led to the longstanding assumption of a weak molecule surface interaction, where the chemisorption process causes only a small modification of the molecular orbital structure of the adsorbed relative to the free molecule. Consequently, the adsorbate electronic structure has been approximated by that of the free molecule, treated as a nearly unperturbed species weakly interacting with the substrate. Based on recent X-ray emission spectroscopic observations, a different scenario has emerged in order to explain the relatively weak heat of adsorption: Here a a strong covalent interaction between the molecule and the substrate has been found, characterized by significant orbital mixing: This implies a modification of the internal molecular bond, which costs energy, and the formation of the surface chemical bond, where energy is gained. In this case, a low heat of adsorption is also expected, but it is the result of these relatively large, but almost equal opposing contributions. 3.7.1.1.2 Vibrational properties Further properties of the adsorbate are accessible through vibrational spectroscopies, such as electron energy loss spectroscopy (EELS), infrared reflection absorption spectroscopy (IRAS), sum frequency spectroscopy (SFG) [94She, 96Klü, 01Ric], or atom and molecular beam scattering (TEAS, MBS). With these techniques the local bond strengths within the adsorbate-metal complex are determined through the excitation and observation of the moiety’s vibrational modes [54Eis, 58Eis, 59Eis, 72Eis]. CO in the gas phase is characterized by a single vibrational mode, the C-O stretch at 2169.8 cm−1. Upon adsorption the frequency of the C-O stretch decreases to values between 1700 cm−1 and 2050 cm−1. This characteristic red shift of the C-O stretch indicates that in the adsorption process the internal C-O bond is weakened. The C-O stretch has also been instrumentalized to assign adsorption sites and substrate coordination [54Eis, 58Eis, 79Ric]. These assignments have been successful in many cases, but caution is advised, as several counter examples have surfaced [93Sch, 94Dav, 96Dav]. CO adsorption on well-defined stepped single crystal surfaces [78Bra1, 79Erl, 79Hop, 83Hof1, 85Hay1, 90Ben, 92Luo, 95Ram, 96Sve, 02Unt] as well as artifically roughened low-index surfaces [82Ort, 83Hof1, 90Tüs2, 02Unt] have been analyzed to correlate vibrational CO stretch frequencies with defect sites. Knowledge of this kind is important for Landolt-Börnstein New Series III/42A4

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elucidating the adsorption of CO on small supported metal clusters, as they are used in heterogeneous catalysis [92Bad, 96Eva, 96Goy, 96Rai, 98Wol, 2002Unt, 02Yud]. These clusters may exhibit welldefined low-index facets and/or a multitude of defect sites, depending on their size and thermal history. Vibrational spectroscopies, especially SFG, have been used successfully to characterize defect sites on these small metallic clusters even for a wide range of CO pressures (10−7 mbar to 103 mbar). There are also low energy frustrated translational and rotational modes of adsorbed CO. In particular, the frustrated translation normal to the surface (equivalent to a metal-carbon stretch) provides a qualitative indication of the CO-metal bond strength. Vibrational low energy modes of single adsorbed CO molecules have been detected by inelastic tunneling spectroscopy (IETS), either in a thin film diode geometry [81Kro] or by utilizing the scanning electron microscope in vacuum [99Lau, 99Lee, 00Lee, 01Hah, 03Wal]. Thermal energy He atom scattering (TEAS, HAS) of adsorbed CO on metals has also provided frequencies of the frustrated translational mode [96Hof, 03Gra]. The dispersion of low energy modes has been studied by this technique for the c(2×2)-CO layer on Cu(001) [95Ell, 96Hof] and the (4×2) structure of CO on Pt(111) [86Lah, 98Gra1]. The interaction within a molecularly adsorbed CO layer at higher coverage is substantial. This leads to a continuous shift in vibrational frequencies with coverage, observed for many systems, e.g. for CO on Ru(0001) [80Pfn]. Using high resolution XPS, the vibrational progressions of the core-ionized adsorbate species have been observed [98Föh, 99Föh1, 99Föh2]. The latter vary significantly with adsorption site [98Föh]. The intra-molecular bond of gas phase N2 is stronger than that of CO with a N-N stretch at 2358 cm−1 [91Rao]. An adsorption induced weakening of this N-N bond is found to be small as long as the molecule is σ-bonded through one of its nitrogen atoms and more or less perpendicularly adsorbed at the surface. On the basis of the adsorption energy and the vibrational properties a chemisorbed N2 species is not easily distinguished from a physisorbed species. 3.7.1.1.3 Geometric structure To characterize the geometric structure of adsorbed CO molecules, i.e. bond distances and angles, techniques such as X-ray photoelectron diffraction (XPD, PED or ARPEFS) and, in case of long range order, low energy electron diffraction (LEED) have been utilized. Photoelectron diffraction is performed either in an angle-scanned mode at constant energy (XPD) or in an energy-scanned mode with variable photon energy (PED, ARPEFS). Whereas the first mode yields primarily bond directions (angles), the second yields bond distances and angles. CO adsorbs in general in an upright adsorption geometry, with the carbon atom pointing towards the metal surface. In the case of Ag surfaces there is some controversy whether the weakly chemisorbed CO is oriented parallel to the surface or in a random orientation [84Kra] [94San1, 94San2]. Upon adsorption the internal bond length of the gas phase CO molecule of 1.128 Å is typically extended towards 1.13-1.15 Å at a metal carbon distance between 1.26 Å and 1.9 Å. The adsorbed CO molecule can occupy both low coordination and high coordination sites, with both selective adsorption into a single site or simultaneous adsorption into differently coordinated sites. The adsorption of CO to most fcc metal single crystal surfaces leads to the formation of ordered superstructures, as determined by LEED. A particular metal surface of low-index orientation can support several different superstructures at certain coverages. Tilting of the molecular axis can be induced by the repulsive lateral interaction between the dipole moments of the adsorbed CO molecules. The adsorption of N2 takes place only in on top sites, where the N2 molecular axis is oriented normal or nearly normal to the surface. This alignment causes the two N-atoms of the N2 molecule to become chemically inequivalent. The determination of the adsorbate orientation in the chemisorbed state was helped significantly by XPS, where the chemisorbed N2 phase exhibits a very distinct N 1s photoelectron spectrum, such as seen in Fig. 2 for N2 adsorbed on Ru(001), W(110) [82Umb, 83Umb, 84Umb] and Ni(100) [78Fug, 84Gru4, 84Umb]. The spectra are quite similar and exhibit three peaks, aside from the fact that spectrum (b) for Ni(110) is less well resolved than the others [84Gru4, 84Umb]. The two N 1s peaks (1,2) at binding energies below 400 eV are so-called screened states while the broad peak (4) near 406 eV is due to shake up processes, originally called the unscreened state [78Sch, 82Umb, 83Men, 83Umb, 84Umb, 85Fre, 86Bre, 86Gol, 91Nil]. Weakly chemisorbed molecules generally exhibit shake-up photoemission features of considerable intensity [93Til]. Landolt-Börnstein New Series III/42A4

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N2

N1s

on

Intensity

d

Ru (001)

c W (110)

b Ni (100)

a

4

410

2 1

Fig. 2. N 1s core level spectra of chemisorbed molecular N2 on three different single crystal surfaces as indicated. Spectrum (a) for Ni(100) was recorded at higher resolution than spectrum (b). The maximum near 400 eV is a double peak on all metals; [84Umb], [84Gru4].

Ni (100)

405 400 Binding energy [eV ]

In particular the investigations of the ordered c(2×2) layer of N2 on Ni(100) have clarified the adsorbate configuration as well as the physics of photoemission from adsorbed N2. It was shown that chemisorbed N2 on this surface is oriented perpendicularly and that the two low binding energy peaks are representing the two chemically inequivalent N-atoms of N2, one involved in bonding to the metal surface, the other pointing towards the vacuum [74Ege, 91Nil]. A representative set of high resolution N 1s spectra is given in Fig. 3 for several polar angles of emission [91Nil]. The screened state binding energies are 399.4 and 400.7 eV for the outer and inner N-atom, respectively. The assignment of peaks to individual N-atoms of N2 is based on a study of the polar angle variation of N 1s intensity [74Ege]. The intensity ratio of the 400.7 eV peak to the 399.4 eV peak, shown in the inset of Fig. 3, reveals a maximum near normal emission, due to forward elastic scattering, which proves the perpendicular orientation of N2 relative to the Ni surface, and furthermore, that the 400.7 eV peak corresponds to emission from the inner N-atom [91Nil].

N 2 /Ni (100)

q = 0°

e-

80°

Intensity

40°

N q N

q = 5° q = 35° diff.

420

415

Landolt-Börnstein New Series III/42A4

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395

Fig. 3. N 1s core level photoemission spectra of N2 adsorbed on Ni(100) in a c(2×2) structure. Spectra are recorded at two polar angles of 5 and 35° relative to the normal. The difference spectrum shows a large peak at 400.7 eV typical of electron forward scattering at the second (outer) nitrogen atom of a perpendicularly adsorbed N2 molecule [91Nil]. The inset shows the polar angle dependence of the relative intensity at 400.7 eV to that at 399.4 eV.

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The very intense N 1s shake-up peak was found to be different for the inner and outer N-atom. It consists of three contributions for the outer N-atom and is as such quite similar to the C 1s shake-up peak of adsorbed CO on Ni(100). The fully screened final states of adsorbed N2 and CO are -NN* and -C*O which are in both cases equivalent to ground state -NO (* indicates core hole). Hence the similarity of the shake-up peaks is expected. The three contributions stem from excitations of bonding-to-antibonding states, excitations into Rydberg states, and intra-molecular excitations which are also known from the free molecule. For the inner N-atom, on the other hand, the fully screened final state of -N*N is equivalent to -ON which is a very weakly adsorbed species [74Ege]. Hence there is practically no split bondingantibonding 2π orbital and no shake-up process involving related excitations [91Nil, 93Til]. It is important in this context that an ab initio calculation of the N 1s spectrum for a NiN2 cluster, shown in Fig. 4, reproduces the experimental result of N2 adsorbed on Ni(100) fairly well [85Fre]. Experiment N2 / Ni (100) N1s

Theory Ni _ N 2 _ N r N1s

1.3 eV 0.92 eV

q i =35°

~ 4 eV

4.8 eV

1.14 eV ~1.1 eV

angle integrated 410

a

405 400 Binding energy EB ( EF ) [eV ]

12

b

10

N2 Nr

8 6 4 2 0 Relative energy E rel [eV]

-2

-4

Fig. 4. Comparison of experimental [84Umb] (a) and theoretical N 1s spectrum (b) for N2 adsorbed on Ni(100) and a NiN2 cluster [85Fre]. The double peak near 400 eV is due to the two inequivalent N-atoms (initial state effect) while the broad peak near 405 eV is due to shake-up processes (final state effect).

3.7.1.1.4 Electronic structure and adsorption models To understand why a particular structural arrangement of the adsorbed CO (or N2) molecules on a metal surface yields the largest heat of adsorption, the valence electronic structure of the adsorbate needs to be studied in detail. The gas phase CO molecule has C∞v-symmetry and the ground state electron configuration (1σ)2(2σ)2(3σ)2(4σ)2(1π)4(5σ)2. The highest occupied molecular orbital (HOMO) is the 5σ, and the lowest unoccupied molecular orbital (LUMO) the 2π*. The gas phase N2 molecule has D∞hsymmetry with the ground state valence electron configuration (1σg)2(1σu*)2(2σg)2(2σu*)2(1πu)4(3σg)2, where the HOMO is the 3σg and the LUMO is the 1πg*. Upon adsorption, the symmetry of the gas phase molecule is reduced to the lower symmetry of the surface. However, traditionally, the electronic states are still described in the framework of the gas phase symmetries. There is an abundance of theoretical papers dealing with various aspects of CO and N2 adsorption on metal surfaces. Different theoretical approaches make an assessment of accuracy difficult. Whenever theoretical data were reported that could be compared to experiment, they were included in the table section. However, quite often it was difficult or even impossible to locate theoretical data which are directly amenable to a comparison with experimental results. For this reason only a limited effort has been made to prepare a complete survey of such data. Instead we present results of more recent theoretical calculations for adsorbed CO on well defined metal surfaces. Hopefully these will serve as a starting point for locating additional studies. Landolt-Börnstein New Series III/42A4

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Early work on CO adsorption on metals started with a molecular orbital approach. The starting point were infra-red spectra of CO adsorbed on supported metals (V, Cr, Mn, and Co) which showed a variation in the CO stretching region [64Bly, 69Bly, 87Bly]. Two main bands of CO for each metal were found, located at 1940 and 1890 cm−1 for V, at 1940 and 1880 cm−1 for Cr, at 1950 and 1890 cm−1 for Mn and at 2000 and 1880 cm−1 for Co. The two bands were suggested to correspond to two-fold and one-fold coordinated CO molecules [54Eis, 58Eis, 59Eis]. These data together with additional data for Fe, Ni, and Cu allowed a comparison across the first transition series from V to Cu. A molecular orbital model for the π-electron system of CO adsorbed on a cluster of metal atoms successfully accounted for the frequency shifts of the two principal bands, as the metal is varied across the series [64Bly, 69Bly, 87Bly]. Experimental observations like these were the basis of the well known Blyholder model of molecular CO adsorption [64Bly, 69Bly, 87Bly] which essentially calls for donation of electron charge from the carbon σ-orbital to the metal surface atom and backdonation of charge into the π bonding orbital of the C-O bond [64Bly, 69Bly, 87Bly]. Other theoretical studies of CO adsorption have been carried out in semi-empirical and ab initio frameworks of increasing complexity, see for example [77Doy, 80Ros, 83Bag, 84Bae, 84Bau, 84Her, 85Mes, 96Ham, 97Gro, 97Mor, 98Mor1, 00Kop]. Trends in chemisorption energy of CO for a number of fcc transition metal surfaces and metallic overlayer surfaces [92Rod] can be well described by a rather simple model taking into account the metal d states and the CO 2π* and 5σ states [96Ham]. Fig. 5 shows the correlation between the chemisorption energy and the d-band contribution, representative of the substrate. The local site structure on a surface can have a significant influence on the adsorption energy for the same metal. Calculations by DFT for a reconstructed Pt(110) (1×2) surface showed a variation of the CO adsorption energy from the weakest bonding site, a bridge site inside a one-dimensional trough, to the strongest bonding site, an atop site on an isolated Pt adatom, of about 1 eV [01Tho]. Thus CO adsorption on Pt adatoms is energetically preferred and may lead to considerable surface roughness or step formation.

CO/Ir (100)

Au(111)

c Cu:Cu8Pt(111)

− 0.5

Cu:Ni@Cu(111) Pd/Ru(0001) − 1.0

− 1.5

− 1.5

Pd(111) Pt(111) Pt:Cu9Pt(111)

Cu(111)

Cu/Pt(111)

Intensity

DFT _ GGA chemisorption energy, Echem [eV]

Ag(111) 0

Ni(111) Ni/Ru(00001) Ni:Ni@Cu(111)

− 1.0 − 0.5 0 Model of the d contribution, Ed-hyb [eV]

Ir 4 (CO) 12 b

0.5

Fig. 5. Calculated energy of CO chemisorption versus a model dependent metal d-band contribution to the metal-CO bond. For a number of transition metal systems [96Ham]. Symbols: experimental values.

Ir (5d) CO a

4σ 16

1π

5σ

8 4 12 EF Binding energy [eV ] for CO/Ir (100)

Fig. 6. UV photoelectron spectra of (a) gas phase CO, (b) Ir4(CO)12 carbonyl and (c) CO adsorbed on Ir(100) [78Plu].

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3.7.1 CO and N2 adsorption on metal surfaces

[Ref. p. 202

Hückel molecular orbital theory was used to elucidate the π-orbital structure. Within this framework, the allylic configuration of the Ni-C-O trimer was derived [64Bly, 69Bly, 87Bly]. In analogy to bonding in transition metal carbonyls which have been described previously [51Dew] (historic overview by[01Mic]), a σ-bond between the carbon 2p and metal dσ orbitals is formed, leading to additional charge on the central metal atom which is removed by a back-donation bond into the CO 2π* [64Bly, 69Bly, 87Bly]. The π-interaction was thought to be attractive while the σ orbitals were assumed unchanged for the free and adsorbed CO [64Bly, 69Bly, 87Bly]. With UPS the similarity between the adsorption bonds in a carbonyl and CO bonded to a metal surface was experimentally demonstrated for CO on Ir(100) and Ir4(CO)12 [78Plu] (Fig. 6). With the gas phase spectra included as a guideline, one can see that the 5σ level of adsorbed CO is shifted to higher binding energy in both cases (disregarding intensity changes). Details of carbonyl formation were also analyzed by IETS and STM for single Fe and Cu atoms deposited on a Ag(110) surface [99Lee, 00Lee]. Vibrational frequencies were shown to be quite similar to those for CO adsorbed on Fe and Cu surfaces [99Lau]. Angle-resolved ultra-violet photoelectron spectroscopy (ARUPS) as a band mapping technique has been particularly useful to determine the occupied valence states of adsorbed CO (or N2) and their dispersion. By analogy, the unoccupied states have been determined by inverse photoemission spectroscopy (IPS). A thoroughly characterized system is the well ordered (2×1)p2mg CO layer on Ni(110) [89Kuh, 89Mem, 91Ran] which was analyzed by ARUPS and IPS. The resulting 2D band structure is summarized in Fig. 7. In general, the importance of the CO nearest neighbor distance as a parameter determining the band width is illustrated in Fig. 8 which shows a plot of the measured 4σ dispersion bandwidth versus CO nearest neighbor distance for hexagonally close-packed ordered structures. It can be seen that a logarithmic dependence is obtained, independent of the different substrate material [83Gre, 85Hof]. Many models concentrated solely on the frontier orbitals of the CO molecule, the 2π* and 5σ, where the 2π* was thought to interact with the metal dπ-states, forming a bonding 2π*b and antibonding 2π*a combination [81Fre, 82Mes, 83Fre]. The occupation of the bonding 2π*b was in this model equivalent to back bonding into the 2π*. Even though the two level orbital structure in this severely simplified model is not the three level structure derived by Blyholder [64Bly, 69Bly, 87Bly] this model has been and still is commonly called the “Blyholder model” of CO chemisorption. Another approach for the π interaction was taken by Gumhalter and coworkers, called the resonance model which is based on the NewnsAnderson model for a discrete electronic level interacting with a broad band. Here, the discrete CO 2π* level is broadened resonantly by the substrate interaction, leading to a partial occupation of the resulting broad band. This model implies a large occupied density of states near the Fermi level which could not be verified experimentally [89Kuh, 89Nil, 00Föh1]. In these models the molecule surface interaction is implied to be weak in comparison to the intra-molecular bond. The chemisorption process is expected to cause a small modification of the frontier orbitals HOMO and LUMO, leaving the electronic structure of the CO unit largely intact. A dative bond between the CO 5σ and metal states of σ symmetry is formed, leading to charge donation into the metal which is compensated by a back-donation into the CO 2π*. In this frontier orbital picture, a synergetic π and σ bond is achieved, where the internal C-O bond is weakened due to the increased population of the antibonding CO 2π* orbital in the back-donation. An increase of CO π population through charge transfer into the CO 2π* has been repeatedly interpreted in support of the frontier orbital model and the resonance model. However, this is not conclusive. The calculations in [00Föh1] give a population of the 2π* by 22 % and a depopulation of the 1π by 12 % upon adsorption, thus yielding an increase of CO π population. However, this increase is not the result of a simple charge-transfer into the CO 2π*, leaving the 1π unaffected, but the result of orbital mixing within the CO π-system and with the metal band. The importance of orbital mixing for the σ orbitals, notably between the CO 5σ and 4σ, has been pointed out both from theory and experiment [81Bru, 83Bag, 83Gre, 84Bag2, 85Sun, 86Bag, 87Her, 90Wur, 91Won, 95Hu]. In some studies, the σ interaction was considered attractive, [83Gre, 85Sun, 90Li, 90Wur, 91Won, 95Hu] based upon the finding that the 5σ state measured in UPS is shifted towards larger binding energies, relative to the 4σ. Bagus and coworkers found the σ interaction to be repulsive [81Bru, Landolt-Börnstein New Series III/42A4

Ref. p. 202]

3.7.1 CO and N2 adsorption on metal surfaces

83

83Bag, 84Bag2, 86Bag, 87Her]. They propose Pauli repulsion between the CO 5σ and the metal s-band which is partially compensated by a dative 5σ-dσ bond. This mechanism explains the UPS binding energy shift of the 5σ, where the CO 5σ lone-pair penetrates the metal and its electrons are attracted to the not fully screened metal nucleus. Although 4σ-5σ orbital mixing is considered unimportant for adsorbed CO, [81Bru, 83Bag, 84Bag2, 86Bag, 87Her] they find for isoelectronic N2 a substantial 4σ-5σ mixing [81Bru, 83Bag, 84Bag2, 86Bag, 87Her]. CO (2 ×1)p 2 mg/Ni(110) LEED

CO − 4 s bandwith [eV ]

-7 -6

2π y+ Γ4

-5 -4

2π x+

-3

2π x− Γ3

-1

Binding energy [eV]

Γ2

2π y− Γ1

-2

0.5 0.4 0.3

Ö3 CO

0.2

C (4×2)

0.1 0.08 0.06 0.04 3

Ef

Γ4 Γ4 Γ2 Γ1

1 2

4 5 CO nearest neighbour distance [Å]

Fig. 8. Plot of 4σ bandwidth versus CO nearest neighbor distance in hexagonally close-packed ordered layers on Co(0001), [83Gre] Ir(111), [80Sea, 81Sea] Pd(111) [84Mir] and Ru(0001) [85Hof].

Σ2 Σ1

3

Co Ir Pd Ru

2 Ö3 CO

T = 120 K

4 5

1π x+ Γ4

6

+ 1π y− 1π y

Σ1

X1

Γ2 Γ1 Γ2 5σ − Γ3 1π x−

Σ2 Σ1 Σ2 Σ2

9

5σ + Γ1

Σ1

10

4σ − Γ2

Σ2

4σ + Γ1

Σ1

X1

Σ [110]

X

7 8

11 12 Y

Landolt-Börnstein New Series III/42A4

∆ [100]

Γ

X1 X1 ← Fig. 7. Valence band dispersion of the (2×1)p2mg-CO structure on Ni(110) from ARUPS [86Kuh, 87Kuh, 89Kuh] and IPS [89Mem, 91Ran].

84

3.7.1 CO and N2 adsorption on metal surfaces

[Ref. p. 202

3.7.1.1.5 Atom specific electronic structure and the allylic model of CO adsorption Resonant inelastic X-ray scattering or resonantly excited X-ray emission spectroscopy (XES) has the capability to resolve the valence electronic structure of an adsorbate in a local atom-specific and orbital symmetry selective way [92Was, 93Til3, 94Wie, 95Nil, 97Nil1, 97Nil2, 98Wei, 98Wie, 99Has, 99Sta2, 99Tri, 00Föh1, 00Föh2, 00Föh3, 00Kar, 00Nil, 00Nyb, 00Sti, 00Tri, 02Oga, 03Öst, 04Föh1, 04Föh2, 04Öst]. These properties make XES the experimental equivalent to a population analysis of the valence electronic structure. Thus, for adsorbed CO the valence contributions at the carbon atom and the oxygen can be distinguished and for adsorbed N2 the inner and outer nitrogen atoms can be distinguished [98Ben]. Furthermore, XES allows to seperate and identify the adsorbate-specific valence states from the overlapping bulk metal valence band often dominating the valence region in UPS. In addition, Near edge x-ray absorption fine structure spectroscopy (NEXAFS) has been used to study the unoccupied adsorbate density of states [92Stö]. Using XES as an atom specific probe of the valence electronic structure, [00Föh1, 00Föh2, 00Föh3, 04Föh2] the relative distribution of the valence orbitals and their orbital symmetry at the carbon and oxygen atoms becomes accessible, shown in Fig. 9 for c(2×2)CO on Ni(100) and c(2×2)CO on Cu(100) in direct comparison to gas phase CO. In particular, the XES spectral distributions clearly discern adsorbate states which are energetically overlapping with the metal d-band, a result which was not visible in previous UPS spectra due to the strong photoemission signal from the metal substrate. Theoretical spectral distributions from ab initio DFT calculations of CO/Ni13 and CO/Cu26 clusters (Fig. 10) are in full agreement with the experimental results [00Föh1, 00Föh3]. We can thus visualize in Figs. 11 and 12 the adsorbate electronic structure with orbital contour plots from these calculations. XES experiment C_K O_K

π states ~ 1π

~ dπ

π states ~ 1π

s states ~ 5s

CO/Ni (100) ~ 4s ×3

XES calculation C_K O_K

~ dσ

~ dσ

CO/Cu (100)

Intensity

Intensity

~ 4s

×3

~ 5s

~ 4s

~ dσ

×3

×3

~ 1π

CO/Cu (100)

~ 5s

CO/Ni (100) ~ dπ

~ 1π ~ dπ

s states

~ 4s

~ dπ

~ 5s ~ dσ

×3

×3

×3

×3

1π

15

10

4s

CO gas

1π

4s

5

15 10 0 Binding energy [eV ]

CO gas

5s

5s

5

0

Fig. 9. X-ray emission spectra of CO gas [97Sky] adsorbed in on-top adsorption sites for c(2×2)CO/Cu(100) and c(2×2)CO/Ni(100) [00Föh1, 00Föh2, 00Föh3]. Note the atom specific and symmetry selective projection of the valence electronic structure.

15

10

5

15 10 0 Binding energy [eV ]

5

0

Fig. 10. Computed X-ray emission spectra of CO, CO/Cu26 and CO/Ni13 [00Föh1, 00Föh2, 00Föh3]. The experimental spectra in Fig. 9 are fully reproduced.

Landolt-Börnstein New Series III/42A4

Ref. p. 202]

3.7.1 CO and N2 adsorption on metal surfaces

85

The π-system: Allylic configuration In the free CO molecule there is only one occupied orbital of π-symmetry, the 1π. This is strongly polarized towards the oxygen atom of the molecule as seen in the experimental spectra of Fig. 9 and the calculated spectra in Fig. 10. Upon adsorption we note two important effects. In comparison to the 1π orbital of gas phase CO, the adsorbate 1π is less polarized towards the oxygen atom and new electronic states are found in the energy region between 5 eV and the Fermi-level. We denote this broad distribution of states as the dπ-band, since these states, as will be seen later, are largely derived from the metal d-band. Within this dπ-band an oxygen lone-pair state is observed with an orbital amplitude at the oxygen atom alone and none at the carbon atom. This finding gave rise to the allylic bonding model based on the covalent interaction in the CO-metal trimer moiety [00Föh1, 00Föh3].

CO / Cu

CO gas 2π*

CO / Ni

~ 2π *

~ 2π *

~ dπ − t

~ d π −t

~ dπ−b

~ − dπ b

~ 1π

~ 1π

O C

1π

Fig. 11. Orbital contour plots of CO, CO/Cu26 and CO/Ni13 cluster corresponding to spectral features of π orbital symmetry in Figs. 9, 10 [00Föh1, 00Föh2, 00Föh3].

From the ground state orbital contour-plots in Fig. 11 which have been obtained from the same calculations that led to good agreement (Fig. 10) with the experimental XES in Fig. 9 the allylic configuration is directly accessible. In the lower panel of Fig. 11, the gas phase 1π and the adsorbate 1π are shown. The 1π is polarized upon adsorption towards the carbon atom mixing with the dπ -orbital of the interacting metal atom. The amplitude of the 1π has the same phase between the O, C, and metal, constituting a bonding orbital between all atomic centers. In the two middle panels of Fig. 11 orbital plots from two different energy regions within the dπ-band are shown. We have labeled the orbital representing the high-energy region at the ‘bottom of the band’ as the dπ-b and the orbital close to the Fermi-level at the ‘top of the band’ as dπ-t. The orbital plots of the dπb and dπ-t are characterized by one nodal plane at the central atom (carbon), opposite orbital amplitudes at the oxygen atom and the metal atoms. It is also evident that these represent a non-bonding oxygen lonepair with strong metal d-character. In the top panel of Fig. 11 the evolution of the 2π* orbital is shown. It has two nodal planes and is antibonding between all three atomic centers (metal, carbon and oxygen). In the free molecule the 2π* orbital is polarized towards the carbon atom, opposite to the polarization in the 1π orbital. Upon adsorption the orbital is polarized towards oxygen in comparison to the free molecule and shows some metal d contribution antibonding with respect to the molecule. Landolt-Börnstein New Series III/42A4

86

3.7.1 CO and N2 adsorption on metal surfaces

[Ref. p. 202

The observed π-electronic structure can be summarized into the conceptually simple allylic configuration of a covalent triatomic molecule modified by the varying electronegativity of the involved atoms. Thus, in a first step the adsorbate–substrate complex is reduced to the treatment of a covalent triatomic molecule consisting of the coordinating metal atoms (depending on on-top, bridge or hollow adsorption sites), the carbon atom and the oxygen atom [00Föh1, 00Föh3]. Such a molecular orbital view of CO chemisorption was first proposed by Blyholder [64Bly, 69Bly, 87Bly] using Hückel molecular orbital theory where independent of the chosen basis, the allylic configuration of the adsorbate π system is found, with the three characteristic hybrid orbitals. In a second step modifications from the threeatomic case are added due to the energetic width of the metal d-band and the delocalized sp-band [00Föh1, 00Föh3, 04Föh2]. However, an experimental verification of this theoretical model has not been possible prior to the application of XES to the adsorbate-electronic structure [00Föh1, 00Föh2, 00Föh3, 04Föh2] which allows an atom and symmetry projected analysis of the adsorbate valence states. To deduce the various contributions to the adsorption energy and the local bond-properties, e.g. the CO stretch frequency, the Constrained Space Orbital Variation (CSOV) method [84Bag1, 84Bag2] has been applied to the CO/Ni13 and CO/Cu26 clusters [00Föh1, 00Föh2, 00Föh3] and a weakening of the internal CO π-bond was found upon adsorption leading to a decreased C-O vibrational frequency. The π-interaction stabilizes the adsorbate complex, since only the bonding 1π-orbital and the nonbonding dπ-band are occupied, whereas the antibonding 2π* remains unoccupied. Upon adsorption the internal C-O π-bond is partially broken which is compensated by the formation of a bonding metal-carbon interaction. For isoelectronic N2 on Ni(100) the equivalent behavior was found [98Ben]. The σ-system In the free CO molecule there are two occupied outer valence orbitals of σ-symmetry; the 4σ and the 5σ where the latter is the HOMO of the molecule. As seen experimentally in the lower part of Fig. 9 and in the calculations in Fig. 10 the 5σ orbital has a larger contribution on the carbon atom than on the oxygen atom while the 4σ orbital shows the reverse situation. Upon adsorption of the CO molecule, strong orbital mixing takes place in the σ-system, visible in the polarization of the 5σ orbital towards the oxygen atom and the 4σ towards the carbon atom (Figs. 9, 10). The 5σ and 4σ states show significant changes both in peak positions and in relative intensities. Additional states with low intensity in the XES spectra are found between the 5σ -state and the Fermi-level and are denoted dσ band. The computed adsorbate 5σ and 4σ orbital plots in Fig. 12, reveal in-phase (bonding) orbitals between the adsorbate and the metal, whereas the dσ orbitals are antibonding between the metal and carbon atoms. In general, the σ orbital structure is more complex than the π-orbital structure due to the involvement of more initial CO orbitals in contrast to the π-system with only the CO 1π and 2π∗ orbitals. In the σsystem, the occupied CO valence orbitals (3σ, 4σ and 5σ) and the unoccupied CO 6σ, with a large energy separation, mix with the metal states. This is seen in the XES spectra of adsorbed CO on the late transition metals Ni and Cu through the polarization of the 4σ and 5σ and the occurance of the dσ band. As in the late transition metals the metal bands are highly occupied, anti-bonding adsorbate orbitals are occupied, making the adsorbate-substrate σ interaction repulsive. These qualitative arguments of a repulsive σ interaction have been investigated systematically with CSOV [84Bag1, 84Bag2] calculations on the CO/Ni13 and CO/Cu14cluster model [00Föh1, 00Föh3], where the repulsive σ-interaction between the CO and the metal surface is accompanied by a strengthening of the internal C-O σ bond. In summary, the adsorption of CO and N2 is governed by a strong covalent adsorbate-substrate bond, characterized by significant orbital mixing of the initial molecular orbitals and the metal bands within each orbital symmetry. The orbital structure of the π system can be summarized as an allylic configuration and is, for the example of CO, the result of orbital mixing between the CO 1π, 2π* and the metal dπ-band. Experimentally, this is manifested in the observation of a characteristic oxygen lone-pair state. The π-interaction stabilizes the adsorbate-substrate complex, weakening the internal C-O π-bond upon adsorption. In the σ system equally strong orbital mixing takes place, leading to a strong polarization of the 5σ and the 4σ orbitals within the molecular unit. Depending on the filling of the metal Landolt-Börnstein New Series III/42A4

Ref. p. 202]

3.7.1 CO and N2 adsorption on metal surfaces

87

bands, antibonding σ orbitals become occupied and the σ-interaction for the late transition metals destabilizes the adsorbate-substrate complex, strengthening the internal C-O σ bond. In a valence bond model, the adsorption process can be described as the modification of the internal bonds in the CO gas molecule which is then compensated by the formation of the surface chemical bond to the metal substrate. The equilibrium properties of adsorbed CO are therefore the direct result of the balance between the σ and π-interaction; both in terms of the total energy and the local bond properties, such as bond-distance and vibrational frequencies. This behaviour has been found for CO adsorbed in on-top sites and in higher coordinated sites, such as bridge and hollow sites [00Föh1, 00Föh3]. With increasing density of the adsorbate layers, direct and substrate mediated adsorbate-adsorbate interactions need to be taken into account in addition to the local contributions summarized in the allylic model of the metal-CO moiety [04Föh2].

CO / Cu

CO gas

CO / Ni

~ dσ

~ dσ

~ 5σ

~ 5σ

~ 5σ

~ 4σ

~ 4σ

~ 4σ

O C

Fig. 12. Orbital contour plots of CO, CO/Cu26 and CO/Ni13 cluster corresponding to spectral features of σ orbital symmetry in Figs. 9, 10 [00Föh1, 00Föh2, 00Föh3].

Landolt-Börnstein New Series III/42A4

References for this document 51Dew 54Eis 58Eis 59Eis 64Bly 69Bly 72Eis 74Dal 74Ege 75Dal 76Van 77Doy 77Pal 78Bra1 78Dwy 78Fug 78Plu 78Sch 79Erl 79Hop 79Kre 79Ric 79Toy 80Goo 80Kre 80Pfn 80Ros 80Sea 81Bru 81Fre 81Kro 81Sea 82Bon 82Mes 82Ort 82Umb 82Woo 83Bag 83Fre 83Gre 83Hof1 83Koe1 83Men 83Umb 84Bae 84Bag1 84Bag2 84Bau 84Gru3 84Gru4 84Her 84Kra 84Mir 84Umb

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85Fre 85Hay1 85Hof 85Mes 85Sun 85Tsa 86Bag 86Bre 86Gol 86Kru 86Kuh 86Lah 86Whi1 86Whi2 87Bly 87Fre 87Gru1 87Gru2 87Her 87Kuh 89Düc 89Kuh 89Mem 89Nil 89Whi 90Ben 90Li 90Tüs2 90Wur 91Nil 91Ran 91Rao 91Won 92Bad 92Luo 92Rod 92Stö 92Was 93Sch 93Til 93Til3 94Dav 94San1

94San2

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Shen, Y.R.: Surf. Sci. 299-300 (1994) 551. Somorjai, G.: Introduction to surface chemistry and catalysis; New York: John Wiley & Sons, 1994. T. Wiell, H. Tillborg, A. Nilsson, N. Wassdahl, N. Martensson, J. Nordgren, Surf. Sci. 304, L451 (1994) Ellis, J., Witte, G., Toennies, J.P.: J. Chem. Phys. 102 (1995) 5059. Hu, P., King, D.A., Lee, M.-H., Payne, M.C.: Chem. Phys. Lett. 246 (1995) 73. A. Nilsson, P. Bennich, T. Wiell, N. Wassdahl, N. Martensson, J. Nordgren, O. Björneholm, J. Stöhr. Phys. Rev. B. 51, 10244 (1995). Ramsier, R.D., Lee, K.-W., Yates jr., J.T.: Surf. Sci. 322 (1995) 243. Davis, R., Woodruff, D.P., Hofmann, P., Schaff, O., Fernandez, V., Schindler, K.M., Fritzsche, V., Bradshaw, A.M.: J. Phys. Condens. Matter 8 (1996) 1367. Evans, J., Hayden, B.E., Lu, G.: Surf. Sci. 360 (1996) 61. Goyhenex, C., Croci, M., Claeys, C., Henry, C.R.: Surf. Sci. 352-354 (1996) 475. Hammer, B., Morikawa, Y., Norskov, J.K.: Phys. Rev. Lett. 76 (1996) 2141. Hofmann, F., Toennies, J.P.: Chem. Rev. 96 (1996) 1307. Klünker, C., Balden, M., Lehwald, S., Daum, W.: Surf. Sci. 360 (1996) 104. Rainer, D.R., Wu, M.C., Mahon, D.I., Goodman, D.W.: J. Vac. Sci. Technol. A 14 (1996) 1184. Svensson, K., Rickardsson, I., Nyberg, C., Andersson, S.: Surf. Sci. 366 (1996) 140. Groenbeck, H., Rosen, A., Andreoni, W.: Z. Phys. D 40 (1997) 206. Mortensen, J.J., Morikawa, Y., Hammer, B., Norskov, J.K.: Z. Phys. Chem. 198 (1997) 113. A. Nilsson, M. Weinelt, T. Wiell, P. Bennich, O. Karis, N. Wassdahl, J. Stöhr, M. G. Samant, Phys. Rev. Lett. 78, 2847 (1997). A. Nilsson, N. Wassdahl, M. Weinelt, O. Karis, T. Wiell, P. Bennich, J. Hasselström, A. Föhlisch, J. Stöhr, M. G. Samant, Appl. Phys. A. Mat. Sci. Proc. 65, 147 (1997). Scheuer, M., Menzel, D., Feulner, P.: Surf. Sci. 390 (1997) 23. Skytt, P., Glans, P., Gunnelin, K., Guo, J.-H., Nordgren, J.: Phys. Rev. A 55 (1997) 134. Bennich, P., Wiell, T., Karis, O., Weinelt, M., Wassdahl, N., Nilsson, A., Nyberg, M., Pettersson, L.G.M., Stohr, J., Samant, M.: Phys. Rev. B 57 (1998) 9274. Föhlisch, A., Wassdahl, N., Hasselström, J., Karis, O., Menzel, D., Martensson, N., Nilsson, A.: Phys. Rev. Lett. 81 (1998) 1730. Graham, A.P.: J. Chem. Phys. 109 (1998) 9583. Mortensen, J.J., Hammer, B., Norskov, J.K.: Surf. Sci. 414 (1998) 315-29. M. Weinelt, N. Wassdahl, T. Wiell, O. Karis, J. Hasselström, P. Bennich, A. Nilsson, J. Stöhr, M. G. Samant, Phys. Rev. B. 58 7351 (1998). T. Wiell, J. Klepeis, P. Bennich, O. Björneholm, N. Wassdahl, A. Nilsson, Phys. Rev. B. 58, 1655 (1998). Wolter, K., Seiferth, O., Kuhlenbeck, H., Bäumer, M., Freund, H.J.: Surf. Sci. 399 (1998) 190. Föhlisch, A., Hasselström, J., Karis, O., Menzel, D., Martensson, N., Nilsson, A.: J. Electron Spectrosc. Relat. Phenom. 101 (1999) 303. Föhlisch, A., Hasselström, J., Karis, O., Mårtensson, N., Nilsson, A., Heske, C., Väterlein, P., Stichler, M., Keller, C., Wurth, W., Menzel, D.: Chem. Phys. Lett. 315 (1999) 194. J. Hasselström, A. Föhlisch, O. Karis, N. Wassdahl, M. Weinelt, A. Nilsson, M. Nyberg, L. G. M. Pettersson, J. Stöhr, J. Chem. Phys. 110, 4880 (1999). Lauhon, L.J., Ho, W.: Phys. Rev. B. 60 (1999) R8525. Lee, H.J., Ho, W.: Science 286 (1999) 1719. M. Staufer, U. Birkenheuer, T. Belling, F. Nörtemann, N. Rösch, M. Stichler, C. Keller, W. Wurth, D. Menzel, L. G. M. Pettersson, A. Föhlisch, A. Nilsson, J. Chem. Phys. 111, 4704 (1999). L. Triguero, Y. Luo, L. G. M. Pettersson, H. Agren, P. Väterlein, M. Weinelt, A. Föhlisch, J. Hasselström, O. Karis, A. Nilsson, Phys. Rev. B 59, 5189 (1999). Föhlisch, A., Nyberg, M., Bennich, P., Triguero, L., Hasselström, J., Karis, O., Pettersson, L.G.M., Nilsson, A.: J. Chem. Phys. 112 (2000) 1946. Föhlisch, A., Hasselström, J., Bennich, P., Wassdahl, N., Karis, O., Nilsson, A., Triguero, L., Nyberg, M., Pettersson, L.G.M.: Phys. Rev. B 61 (2000) 16229. Föhlisch, A., Nyberg, M., Hasselström, J., Karis, O., Pettersson, L.G.M., Nilsson, A.: Phys. Rev. Lett. 85 (2000) 3309.

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O. Karis, J. Hasselström, N. Wassdahl, M. Weinelt, A. Nilsson, M. Nyberg, L. G. M. Pettersson, J. Stöhr, M. G. Samant, J. Chem. Phys. 112, 8146 (2000). Koper, M.T.M., Santen, R.A. v., Wasileski, S.A., Weaver, M.J.: J. Chem. Phys. 113 (2000) 4392. Lee, H.J., Ho, W.: Phys. Rev. B 61 (2000) 16347. A. Nilsson, J. Hasselström, A. Föhlisch, O. Karis, L. G. M. Pettersson, M. Nyberg, L. Triguero, J. El. Spec. Relat. Phenom. 110, 15 (2000). M. Nyberg, O. Karis, N. Wassdahl, M. Weinelt, A. Nilsson, L. G. M. Pettersson, J. Chem. Phys. 112, 5429 (2000). M. Stichler, C. Keller, C. Heske, M. Staufer, U. Birkenheuer, N. Rösch, W. Wurth, D. Menzel., Surf. Sci. 448, 164 (2000). L. Triguero, A. Föhlisch, P. Väterlein, J. Hasselström, M. Weinelt, L. G. M. Pettersson, Y. Luo, H. Ågren, and A. Nilsson, J. Am. Chem. Soc. 122, 12310 (2000). Denecke, R., Martensson, N.: Surface core level shifts of metals, in: Physics of Covered Solid Surfaces. Landolt-Börnstein III/42 A4. Bonzel, H.P. (ed.), Berlin: Springer-Verlag, 2004. Hahn, J.R., Ho, W.: Phys. Rev. Lett. 87 (2001) 166102. Jacobi, K.: Electron work function of metals and semiconductors, in: Physics of Covered Solid Surfaces. Landolt-Börnstein III/42 A1. Bonzel, H.P. (ed.), Berlin: Springer-Verlag, 2001. D. Michael, P. Mingos, J. Organomet. Chem. 635, 1 (2001). Richmond, G.L.: Annu. Rev. Phys. Chem. 52 (2001) 357. Seebauer, E.G., Jung, M.Y.L.: Surface diffusion on metals, semiconductors and insulators, in: Physics of Covered Solid Surfaces. Landolt-Börnstein III/42 A1. Bonzel, H.P. (ed.), Berlin: Springer-Verlag, 2001, . Thostrup, P., Christoffersen, E., Lorensen, H.T., Jacobsen, K.W., Besenbacher, F., Norskov, J.K.: Phys. Rev. Lett. 87 (2001) 126102. H. Ogasawara, B. Brena, D. Nordlund, M. Nyberg, A. Pelmenschikov, L. G. M. Pettersson, A. Nilsson, Phys. Rev. Lett, 89, 276102 (2002). Unterhalt, H., Rupprechter, G., Freund, H.-J.: J. Phys. Chem. B 106 (2002) 356. Yudanov, I.V., Sahnoun, R., Neyman, K.M., and Rosch, N.: J. Chem. Phys. 117 (2002) 9887. Graham, A.P.: Surf. Sci. Rep. 49 (2003) 115. H. Öström, L. Triguero, M. Nyberg, H. Ogasawara, L. G. M. Pettersson, A. Nilsson, Phys. Rev. Lett. 91, 46102 (2003) Wallis, T.M., Nilius, N., Ho, W.: J. Chem. Phys. 119 (2003) 2296. A. Föhlisch, F. Hennies, W. Wurth, N. Witkowski, M. Nagasono, M. N. Piancastelli, L. Moskaleva, K. Neyman, N. Rösch, Phys. Rev. B 69, 153408 (2004). A. Föhlisch, W. Wurth, M. Stichler, C. Keller, A. Nilsson, Journal of Chemical Physics, 121, 9606 (2004) H. Öström, A. Föhlisch, M. Nyberg, M. Weinelt, C. Heske, L. G. M. Pettersson, A. Nilsson, Surf. Sci. 559 85 (2004).

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3.7.1 CO and N2 adsorption on metal surfaces

3.7.1.2 CO adsorption on fcc metal surfaces Al The adsorption of CO on single crystal Al surfaces, possible only at low temperature of <40 K, has been studied under various aspects [79Pir, 80Chi, 82Flo, 83Sch, 86Pau1, 86Pau3, 88Ryb, 89Jac, 90Jac]. The C-O stretch vibration of adsorbed CO on Al(111) and Al(100) is with 2137 and 2135 cm−1 [88Ryb, 89Jac] nearly identical and close to the gas phase value, suggestive of rather weak physisorption. The adsorption energy is estimated as 20 kJ/mol [80Chi, 89Jac] and 33 kJ/mol [86Pau3]. Another study of CO/Al(100) reports 2060 cm−1 for the C-O stretch [86Pau3]. Also the influence of coadsorbed K was studied, showing a substantially modified adsorbed CO species with CO stretch frequencies as low as 1250 cm−1 and dissociation at elevated temperature [86Pau2, 87Pau]. UPS illustrates weak adsorption of CO on clean Al(111) by well separated 5σ and 1π molecular orbital peaks, in addition to the 4σ peak [80Chi, 89Jac]. Compared to the experimental heat of adsorption of CO on Al(111), theoretical values for atop bonded CO of 22.1 kJ/mol [83Bag] and 47.3 kJ/mol [96Ham] have been reported. A study of CO adsorption on a Cu-promoted Al(111) surface, where the coverage of Cu is less than half a monolayer, is interesting because EELS detects a C-O stretch at 1260 cm−1 and dissociation of CO at 348 K [93Col]. Although neither pure Al nor Cu are able to dissociate CO under such conditions, the Cu-modified Al surface is. Electron irradiation also leads to CO dissociation on Al(111) [82Flo]. Ni The adsorption of CO on oriented Ni single crystal surfaces is one of the most intensely studied systems in the context of this chapter. Many examples of results were presented in the general introduction to this volume [01Bon]. Overall, CO adsorption is predominantly molecular on Ni surfaces. On the Ni(100) surface three ordered CO overlayers are formed: The c(2×2) at 0.5 coverage [72Tra2, 78All, 78Hor1, 79And2, 79Hei2, 79Pet, 80And, 80Ton, 81Kev, 82Bib, 88Uvd], the c(5√2×√2)R45° structure at 0.6 coverage [72Tra2, 82Bib, 88Uvd] and the (3√2×√2)R45° structure at 0.67 coverage [72Tra2, 82Bib, 88Uvd]. Both the thermodynamics of adsorption and desorption have been studied [72Tra2, 78Mad, 79Bor, 80Yat, 81Joh, 83Koe1, 93Stu, 93Tak, 95Vas]. The coverage dependent surface stress [94Gro], the thermodynamics of CO diffusion on the Ni(100) surface [87Roo] and the role of defects on dissociative adsorption [86Ste] have been determined. The vibrational spectra have been measured with EELS and IRAS [82Bib, 87Ber, 88Uvd, 93Sin, 95Vas]. Based on the CO stretch frequencies, pure on top adsorption is assigned to the c(2×2) structure, whereas mixed on top and bridge adsorption takes place for the (3√2×√2)R45° and c(5√2×√2)R45° structures [82Bib, 87Ber, 88Uvd, 93Sin, 95Vas]. The valence electronic structure has been analyzed with photoemission spectroscopy [71Eas, 77All1, 78All, 78Hor1, 80Smi, 81Bru, 83Koe1, 94San3] and the atom specific and orbital symmetry resolved valence electronic structure has been determined with XES [00Föh1, 00Föh2, 00Föh3] (cf. previous section). The C1s and O1s core level binding energies of adsorbed CO have been determined with XPS [81Bru, 83Koe2, 88Uvd, 92Bjö, 98Föh, 99Föh1], where even the vibrational fine structure of the core-ionized species has been resolved [98Föh, 99Föh1]. The unoccupied density of the Ni-CO valence states has been investigated as well [87Joh, 88Zae, 92Bjö, 92Nil, 92Stö]. On the Ni(111) surface the following CO overlayer structures have been observed: (2×2) at 0.25 ML coverage [81Kev, 96Dav], (√3×√3)R30° between 0.30 and 0.42 ML coverage [74Chr, 76Con, 81Kev], c(2×4) at nominal 0.5 ML coverage [82Net, 93Bec, 93Sch, 94Dav, 94Map, 96Dav, 98Hel], (√7×√7)R19° at 0.56 ML coverage [76Con, 82Net, 98Hel] and c(2√3×4)rect at 0.62 ML coverage [98Hel]. The thermodynamic properties of the different phases have been studied by different techniques [74Chr, 76Con, 79Rub, 80Iba, 81Cam1, 82Net, 84Gij, 87Fro, 88Sur, 89Zhu, 93Stu, 98Hel]. Also the adsorbate induced surface stress [94Gro] and the surface diffusion activation energy of 29 kJ/mol with a preexponential factor of 1.2×10−5 cm2s−1 [88Zhu] have been determined. Lan dolt-Börn stein New Series III/4 2A4

3.7.1 CO and N2 adsorption on metal surfaces

89

The assignment of local adsorption sites has evolved considerably. On the Ni(111) surface, the traditionally used assignment of local adsorption sites through the C-O stretch frequency has turned out to be questionable [93Sch, 94Dav]. Vibrational spectroscopy has been widely used for this purpose [77Ber, 79Cam, 80Erl, 85Per, 85Ryb, 87Fro, 88Sur, 91Ha, 94Dav, 97Smi]. CO bridge adsorption has been initially reported for the (√3×√3)R30° structure, based on PED data [81Kev] which later was reassigned to three-fold hollow sites [94Dav, 97Smi, 98Hel]. For the c(4×2) phase, the traditional assignment to bridge sites by vibrational spectroscopy [77Ber, 79Cam, 80Erl, 87Fro, 88Sur] has been replaced by threefold hcp and fcc hollow sites based on LEED [94Map], XPS [98Hel], SEXAFS [93Bec], IRAS [97Smi] and PED [93Sch, 94Dav]. The same assignment was found for the (2×2) structure [96Dav]. Towards higher coverage in the (√7×√7)R19° phase, both on top and bridge sites are occupied [93Sch, 94Dav, 98Hel]. Using UPS the valence electronic structure has been determined [76Con, 76Wil, 88Gum, 90Sch]. Also the unoccupied states have been investigated [83Boz, 88Fra, 88Gum]. A detailed investigation of the CO core level binding energies has been used to support the assignment of local adsorption sites [84Jug, 98Hel]. Three ordered CO structures exist on the Ni(110) surface. The most frequently studied is the (2×1)p2mg at 1 ML coverage with the CO molecules occupying short-bridge sites and being tilted by about 20° relative to normal [73Mad2, 85Beh, 85Rie, 86Kuh, 88Han, 89Kuh, 92Kna, 93Hua, 93Pan, 94Zha, 95Rob, 95Spr, 98Emu, 01Pet]. At 0.75 ML a c(4×2) and at 0.62 ML a c(8×2) overlayer has been observed [85Beh]. In early work also other LEED patterns, e.g. a c(2×2) [73Tay, 80Mah], have been reported that could not be reproduced in later work. The thermodynamic adsorption and desorption properties have been investigated [73Mad2, 73Mad3, 73Tay, 75Fal, 81Ber, 85Beh, 86Fro, 87Bau, 90Fei, 92DeA, 93Stu]. Also the CO surface diffusion parameters have been reported [91Xia, 92Xia, 96Ber]. The vibrational properties of CO on the Ni(110) surface have been analyzed with EELS [81Ber, 81Nis, 87Bau, 90Voi] and IRAS [92Lov]. Based on these investigations, a tilted short-bridge adsorption site for the (2×1)p2mg structure was supported. The valence electronic structure and dispersion has been been studied with ARUPS [82Hor, 84Rie, 86Kuh, 89Kuh] and the unoccupied states with inverse photoemission [86Fre2]. Vibrational studies of CO adsorption on stepped Ni(100) [93Sin, 96Sve] and Ni(111) [79Erl, 90Ben] surfaces have also been reported. The adsorption energy at step sites of Ni(510) is higher than on the (100) terrace sites although the energy difference is small. Ordering on the terrace is hindered by step adsorption [96Sve]. Cu On the Cu(100) surface a disordered overlayer is found at 77 K below a coverage of 0.45 ML, but a well ordered c(2×2) structure exists at 0.5 ML coverage [72Tra1, 79Pri, 80And, 80Ton, 82Bib, 83Stö, 86McC, 88Uvd]. Further adsorption leads to a uniaxial compression and a c(7√2×√2)R45° structure at 0.57 ML [72Tra1, 79Pri, 82Bib, 88Uvd, 96McC, 97McC]. Furthermore it has been suggested that at even higher coverage of 0.6 ML even a c(5√2×√2)R45° structure exists [72Tra1, 82Bib]. On the Cu(100) surface the on-top adsorption site is preferred [72Tra1, 79Pri, 80And, 80Ton, 82Bib, 83Stö, 86McC, 88Uvd, 96McC, 97McC]. The thermodynamic properties have been investigated [72Tra1, 83Bur, 86Dub, 90Pet, 91Pet2, 92Tru]. Both the heat of adsorption and the activation energy of desorption follow the same trend of a rapid decrease from the zero coverage limit to a plateau between 0.1-0.5 ML coverage [72Tra1, 83Bur, 86Dub, 90Pet, 91Pet2, 92Tru], declining further with higher coverage. The pre-exponential sticking factor in the zero-coverage limit is found at 10−15.3±0.4 [91Pet2]. The sticking behaviour is described by the precursormediated adsorption model [81Cas], allowing for direct sticking into unoccupied chemisorption sites [96Dvo, 00Dvo]. The initial sticking coefficient at 110 K is 0.85, slowly decreasing until a rapid drop sets in reaching the compression phase beyond 0.5 ML coverage [96Dvo, 00Dvo]. The vibrational properties have been investigated with EELS [79And1, 88Uvd], IRAS [85Ryb, 90Hir, 92Tru, 94Hir, 95Hir, 96McC, 97McC, 98Gra2], SFG [90Har], Helium atom scattering [94Ber, 95Ell, 96Bra, 96Gra, 98Gra2, 03Gra] and STM-IETS [99Lau, 00Lee, 02Hei]. On the Cu(100) surface lateral adsorbate-adsorbate interactions La ndolt-Bö rnstein New Series III/4 2A4

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and dephasing has been studied [90Har, 90Hir, 91Pet2, 92Bjö, 92Bor, 92Mor, 94Kam, 95Hir, 95Wei, 96Gra, 97McC, 98Gra2]. For example, the energy of lateral repulsion for the c(2×2) structure is at 4.2-8.3 kJ/mol [90Pet, 91Pet2]. CO adsorption at step and defect sites was characterized by low-energy vibrational modes [96Bra]. The binding energies of the valence states have been determined by UPS [75Bru, 76Dem, 77All2, 85Hes, 94San2, 94San3] and XES [00Föh1, 00Föh2]. The unoccupied states were studied by inverse photoemission [92Tsu] and NEXAFS [84McC, 86McC, 92Bjö]. The core-level binding energies have been determined with XPS [75Bru, 78Gun, 79Pri, 90Ant, 92Bjö] and resonant Auger investigations [87Wur, 94San3] and PSD [91Tre] were performed. On the Cu(111) surface, adsorption above 90 K yields a (√3×√3)R30° structure at 0.33 coverage [77Kes, 79Pri, 80Hol, 96Mol, 99Bar], followed by a (1.5×1.5)R18° structure at 0.44 coverage [80Hol, 96Mol] and a (1.39×1.39) hexagonal overlayer at 0.52 saturation coverage [80Hol]. Tip-functionalized STM indicates that the saturation coverage exhibits a (4×4) supercell [99Bar]. Previously, the (1.5×1.5)R18° structure has also been assigned as a c(4×2) at 0.5 coverage [72Pri, 77Hor1] or as a (√7/3×√7/3)R49.1° [77Kes]. Likewise, the (1.39×1.39) hex. structure has been discussed as a (1.4×1.4) hex, (√2×√2) [79Pri] or (7×7) [79Pri]. For all phases, besides the saturation phase, CO occupies on-top adsorption sites [85Hay2]. In the saturation phase on-top and bridge sites are populated [80Hol, 85Hay2]. The thermodynamic properties of CO adsorption on Cu(111) have been studied extensively [75Con, 77Kes, 79Hol, 86Kir, 90Hin, 99Kne2]. The heat of adsorption and activation energy of desorption below 0.33 coverage are about 50 kJ/mol and above 0.33 coverage 38 kJ/mol [75Con, 77Kes, 79Hol, 86Kir, 90Hin]. The sticking coefficient remains more or less constant close to saturation coverage, indicative of precursor adsorption kinetics but also exhibits characteristic minima at the formation of the ordered overlayers [86Kir, 99Kne2]. The vibrational properties [79Hol, 79Pri, 85Hay2, 90Hir, 93Hir, 94Hir, 95Hir] and the valence electronic structure have both been determined [75Con, 86Kir]. Also two-photon photoemission has been performed of the CO covered Cu(111) surface [94Her, 95Her, 95Kno, 99Vel, 99Wol]. Two ordered CO overlayer structures have been observed on the Cu(110) surface below 215 K: a (2×1) structure between 0.25 and 0.5 coverage [77Hor2, 82Woo, 85Har, 86Hol, 95Hof, 96Ahn] and a compression structure at a CO coverage of 0.8, leading to a streaky (4/5×2) structure [96Ahn], also denoted as c(4/5×2) [85Har] or c(1.3×2) [77Hor2, 82Woo]. It is under debate, whether annealing to 170 K improves [85Har] the long range order in these two phases [96Ahn]. Next to them exist disordered phases both towards lower and higher coverage [77Hor2, 82Woo, 85Har, 95Hof, 96Ahn]. The thermodynamic properties of CO adsorption on Cu(110) have been studied with various techniques [73Wac, 77Hor2, 85Har, 91Pet2, 92Chr, 96Jin, 01Kun]. In particular the sticking coefficient [85Har, 91Pet2, 96Jin, 01Kun] is well described by a Kisliuk model with K=0.07±0.02 and S0=0.95±0.05 [01Kun]. The vibrational modes of CO/Cu(110) have been determined, indicating on-top adsorption [79Wen, 82Woo, 84Hol, 92Mor, 94Hir, 98Bra, 99Lau]. In particular a clear anisotropy of the frustrated translation along and perpendicular to the atomic rows of the Cu(110) surface has been observed [97Ahn, 98Bra]. The valence electronic structure has been determined [78Kan, 82Mar, 85Che2] as well as the corelevel binding energies [86Hol, 92Chr]. Also the unoccupied adsorbate states have been investigated [82Stö, 84Rog, 85Che2, 91Dav]. Rh LEED studies have found three ordered overlayer structures on the Rh(100) surface. A c(2×2) structure is seen below 370 K for coverages between 0.2 and 0.6 ML, with an optimum coverage of 0.5 ML [78Cas, 82Kim, 87Gur, 90Leu, 94deJ, 96Bar, 97Wei, 98Str]. Above 0.5 ML CO coverage at 300 K, the structure coexists with the (4√2×√2)R45° structure, which saturates the surface at 300 K at 0.75 ML coverage [66Tuc, 82Kim, 87Gur, 90Leu, 94deJ, 96Bar, 97Wei, 98Str]. Heating to 350 K leads to the reapearance of the c(2×2) structure [98Str]. High CO exposure below 280 K results in a third structure, first denoted ‡split (2×1)— [78Cas] but later designated as a c(6×2) with 0.83 ML saturation coverage [94deJ, 98Str]. Lan dolt-Börn stein New Series III/4 2A4

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The activation energy of desorption for CO/Rh(100) in the zero coverage limit has been determined with TPD as 131±4 kJ/mol, with a preexponential factor of (4±3)×1016s−1 [94deJ], or 134 kJ/mol at 8.4×1012s−1 [82Kim]. Molecular beam studies yielded more reliable values of 149±10 kJ/mol and 135±8 kJ/mol at 1016.3±1.1s−1 and 1014.5±0.9s−1 [97Wei]. The coverage dependent sticking coefficient obeys the Kisliuk model with a zero coverage value of 0.75 and a Kisliuk parameter of K=0.7 [94deJ]. The CO molecules occupy up to 0.5 ML coverage (c(2×2) structure) on top sites, based on vibrational and XPS data [87Gur, 96Bar, 98Str]. However, with increasing coverage both on-top and bridge sites are occupied. The observation of a c(2×2) structure at 100 K with CO coverage as low as 0.2 ML implies island formation [87Gur]. The (4√2×√2)R45° and c(6×2) structures at coverages of 0.75 ML and 0.83 ML, respectively, exhibit both on-top and bridge site occupation in a coverage dependent way. For the (4√2×√2)R45° structure bridge/top occupation ratios of 2 [96Bar] and 2.3±0.5 [98Str] have been found. For the c(6×2) structure ratios of 1.2 [96Bar] and 0.8±0.2 [98Str] were reported. For the (4√2×√2)R45° superstructure IRAS indicates two on-top bound species and one bridge bound species, [87Gur] but another study claims one on-top bound and one bridge bound species [94deJ]. Bridge sites are occupied at elevated temperature for 0.4 ML coverage, whereas at 90 K CO favors on-top sites [87Gur]. Also surface contamination leads to increased bridge occupation [87Ric, 94deJ]. The numerical values of the difference in binding energy between on-top and bridge sites varies significantly: Gurney et al [87Gur] report for the (4√2×√2)R45° structure at 0.5 ML an energy difference of 4.60±0.25kJ/mol. Much lower values of 0.4 to 1.7 kJ/mol between 0.2 and 0.5 ML coverage were also reported [90Leu] but found to be influenced by contamination [94deJ]. The adsorption of CO on Rh(100) has also been investigated by UPS [83Koe1], XPS of the Rh surface core level shift [96Zac] and ion scattering spectroscopy [84Mol] and ESD [90Cra]. On the Rh(111) surface CO adsorbs in several different overlayer structures depending on sample temperature, CO coverage and the partial pressure of CO. At temperatures below 120 K a (2×2) CO structure at 0.25 ML coverage has been observed by LEED [79Thi2, 98Beu], followed at 0.33 ML by a (√3×√3)R30°-CO structure [97Gie, 97Ove, 98Beu] and a (4×4)-8CO structure at 0.5 ML [98Beu]. At a CO coverage above 0.5 ML, the LEED pattern exhibits continuously changing structures which early on have been termed ‡split (2×2)— [78Cas, 84deL] because the LEED spots coalesce with increasing coverage into the (2×2)-3CO pattern at the saturation coverage at 0.75 ML [97Gie, 97Ove, 98Beu]. At temperatures above 120 K, the low coverage (2×2)-CO and (4×4)-8CO patterns do not form, but a (1×1) LEED patterm has been found around 0.25 ML [98Beu]. At room temperature the (√3×√3)R30° CO structure at 0.33 ML and the (2×2)-3CO structure at 0.75 ML is found [78Cas, 79Thi2, 81Koe, 83vHo2, 84deL, 97Gie, 97Ove, 98Beu]. An investigation of the temperature dependence of the (√3×√3)R30°-CO and the (2×2)-3CO structures reported for the (√3×√3)R30°-CO structure an order-disorder transition at a temperature of 330±5 K [91Pet1, 97Ove]. Early LEED measurements, all performed above 120 K, have observed the (√3×√3)R30°-CO and (2×2)-3CO LEED patterns [78Cas, 79Thi2, 81Koe, 83vHo2, 84deL] but could not observe well the (2×2)-CO and (4×4)-8CO patterns. The earliest indication of a (2×2) pattern at 0.25 ML coverage was given by [79Thi1]. Structural analysis with LEED of the (√3×√3)R30°-CO [81Koe] and the (2×2)-3CO structures [97Gie] as well as surface X-ray diffraction of the (2×2)-3CO structure [99Lun] have led in combination with XPS work [97Beu, 98Beu, 01Sme] and He diffraction [97Ove] to a consistent determination of adsorbate geometry. In the (2×2)-3CO structure, one molecule sits in an on-top site while the other two molecules occupy three-fold fcc and hcp sites [97Beu, 97Gie, 97Ove, 98Beu, 99Lun, 01Sme]. Earlier work proposed erroneously a bridge site model, where two CO molecules were thought to occupy near-on-top sites and one CO molecule a bridge site [80Dub, 83vHo1, 83vHo2, 84deL]. XPS has been used extensively to determine the adsorption site [84deL, 97Beu, 98Beu, 01Sme]. It has been found, [97Beu, 98Beu, 01Sme] that up to 1/3 ML CO coverage, all molecules occupy on-top sites. At intermediate coverage up to 0.5 ML, a small number of threefold hollow sites becomes occupied, leading at saturation coverage to 1/3 in on-top and 2/3 in hollow sites. At coverage above 0.4 ML, the relative top/hollow occupation depends on the sample temperature. Up to 0.54 ML the relative amount of CO molecules in hollow sites increases with increasing temperature, wheras at coverages above 0.54 ML increasing temperature leads to a decreasing occupation of hollow sites [97Beu, 98Beu, 01Sme].

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CO adlayers at variable CO background pressure have been studied at room temperature using scanning tunneling microscopy [00Cer]. Exposure of the Rh(111) surface to 5×10−8 torr leads to a (2×2) LEED pattern with three domains of (2×1) symmetry at 0.5 ML coverage [00Cer]. At 1×10−6 torr a STM image with (√7×√7)R19° periodicity has been observed with 3/7 ML coverage [00Cer]. For a CO pressure between 5 torr and 700 torr a STM image with (2×2) periodicity has been reported which has been proposed to represent the (2×2)-3CO structure [00Cer]. The thermodynamic properties of CO adsorption on the Rh(111) surface have been studied with TPD [78Cas, 79Thi2], laser desorption spectroscopy [88See] and molecular beam scattering [91Pet1, 97Wei, 99Beu]. TPD determined a first order adsorption via a mobile precursor. In particular, the coverage dependence of CO desorption [88See, 91Pet1] and the dependence of the initial sticking coefficient on the kinetic enery of impinging CO molecules [99Beu] have been investigated. Peterlinz et al. measured essentially the isothermal and isosteric rates of desorption by using CO molecular beams and in addition time-resolved specular Helium scattering from the adsorbed CO layer as a kinetic response amplifier [91Pet1]. The desorption rates of CO at coverages between 0 and 0.22 ML and at temperatures of 440555 K, shown in Fig. 13, obey an Arrhenius behavior. The coverage dependence can be be fitted by either a constant desorption energy of 135 kJ/mol or a constant pre-exponential factor of 1.33×1014 s−1. Neither can be quite correct because the repulsive interaction between adsorbed CO molecules calls for a decreasing adsorption energy with increasing coverage. However, the effect is small for this investigation (less than 10 %) because of the limited range of coverage. 1.5

1.5

q< 0.003

q< 0.003 q = 0.026

q = 0.026

1.0

q = 0.165 q = 0.072

q = 0.19 q = 0.219

0

q = 0.219

0

14

n d = 1.33 × 10 s -

E a = 32.3 kcal/mol

1

− 1.0

− 1.0

a

q = 0.19

− 0.5

− 0.5

1.8

q = 0.165 q = 0.072

0.5

log (k m )

0.5

log (k m )

1.0

2.0 2.1 2.2 1.9 Reciprocal temperature 1000/ T [K −1]

1.8

2.3

b

2.0 2.1 2.2 1.9 Reciprocal temperature 1000/ T [K −1]

2.3

Fig. 13. Semi-log plots of isosteric CO desorption rates versus 1/T from Rh(111) at coverages of 0 to 0.22 ML and temperatures between 440 and 555 K [91Pet1]. Solid lines are best fits to the data assuming (a) a constant desorption energy and variable pre-exponential factor, and (b) a constant pre-exponential factor and a coverage dependent desorption energy of Ed(θ) = 135 œ 6.77 θ − 160 θ2 [kJ/mol].

The binding enthalpy difference for hollow and on-top bound CO was determined as ~14.7 kJ/mol at an assumed pre-exponential factor of 1013 s−1 [80Dub]. The activation energy of the CO exchange reaction has been measured with TPD and isotope switch techniques as 35.6±2 kJ/mol [94Son], using supportive ESD work [94Cam]. Vibrational spectroscopy has played an important role in the determination of the registry of the CO on Rh(111) [80Dub, 81Koe, 84Koe, 84Tom, 93Wit, 01Wit]. Dubois et al. found in coverage dependent EELS investigations on-top adsorbed CO at a C-O stretch of 2070 cm−1 and bridge bonded CO at 1870 cm−1 with a coverage and temperature dependent ratio [80Dub]. The CO-metal frustrated translation was found at 420 cm−1 [80Dub]. However, based mostly on XPS, the correct assignment into fcc and hcp hollow sites in favor of the bridge sites was accomplished. In addition vibrationally resolved XPS, known to be highly sensitive to different adsorption states [98Föh], indicates Lan dolt-Börn stein New Series III/4 2A4

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next to on-top and hollow site adsorption also at 3L CO exposure a small population of bridge sites [01Sme]. The adsorption of CO on Rh(111) was also studied with UPS [78Bra2] and EDS [92Cra, 94Cam]. On the CO covered Rh(110) surface early LEED investigations showed at 300 K a c(2×2) pattern at 0.5 ML coverage and a (2×1)p1g1 pattern at 1 ML coverage [77Mar]. Subsequent structural work has uncovered a very complex adsorption behaviour illustrated in Fig. 14 [94Wei]. For all temperatures a disordered lattice gas has been found at CO coverages below about 0.08 ML. A highly intense pattern characterized by split c(2×2) spots appears at a CO coverage between 0.3 and 0.6 ML below 180 K. The —split“ c(2×2) pattern converts to a pattern with diffuse, broad c(2×2) spots above 180 K. Overlapping diffraction features from —(3×2)“, —(4×2)“, and —(5×2)“ symmetries are observed for CO coverage above about 0.62 ML and below 240 K. The —(5×2)“ predominates at a coverage of 0.8 ML. With further increase in coverage, (2×1)p2mg LEED spots appear with the —(5×2)“ features. A pure (2×1)p2mg pattern appears at 1 ML saturation coverage [94Wei]. desorption region

CO/Rh (110) 400

Temperature T [K ]

incommensurate fluids

300

compressed

fluid “(5 × 2)”

“(4 × 2)” “(3 × 2)”

lattice gas “c (2× 2)”

200 “split” + gas

100 0

0.1

0.2

“split” c (2 × 2) solid 0.3

0.4

0.5 0.6 0.7 CO coverage q CO

(2 × 1) solid

“(5 × 2)” + (2 × 1)

0.8

0.9

1.0

1.1

Fig. 14. Phase diagram of CO/Rh(110) according to [94Wei].

Next to the unreconstructed Rh(110)-(1×1) surface, also a clean (1×2) missing row reconstruction can be prepared [92Dha, 93Bel, 93Mur]. With increasing coverage the following LEED patterns are found: (1×2) at 0.24 ML, p(2×2) at 0.46 ML, diffuse c(2×4) at 0.52 ML, c(2×4) at 0.75 ML and (2×2)p2mg at 1 ML [92Dha]. On the unreconstructed Rh(110) surface, the zero coverage limit Arrhenius parameters for CO desorption have been determined [77Mar, 80Bai, 91Bow, 97Wei] with TPD to 130 kJ/mol [77Mar] and 132 kJ/mol [91Bow] at an assumed pre-exponential factor of 1013 s−1. Modulated beam and single beam studies [97Wei] yielded 173.8±4 kJ/mol and 164.1±6 kJ/mol and pre-exponential factors of 1017.8±0.4 s−1 and 1016.8±0.6.s−1, respectively. The coverage dependent sticking coefficient follows well the Kisliuk model, with an initial sticking coefficent of 0.68±0.01 and a Kisliuk parameter of K=0.3 [91Bow]. While the ordered superstructures are well recognized in the system, the registry of the CO molecules is debated. As TPD gives a single desorption peak at all coverages, a single adsorption site for any of the structures could be suggested. LEED suggests a short bridge adsorption site for the (2×1)p2mg structure at 1 ML [94Bat] and for the c(2×2) at 0.5 ML coverage [77Mar, 93Dha, 94Wei]. However, coverage dependent XPS [93Dha] shows a significant O1s binding energy shift with coverage and it was suggested, that below 0.54 ML on-top sites are occupied, whereas above 0.54 ML only bridge sites are occupied. Vibrational spectroscopy (EELS) yields CO stretch frequencies at 2008 cm−1 below 0.5 ML and at 1968 cm−1 above 0.5 ML [97Wei] which represents a rather small shift. To reconcile the experimenal findings, two different structural models, off-top and tilt models have been proposed for ordering in the overlayers, based on the symmetries of features in the LEED patterns. In off-top models, CO adsorbs perpendicular in short bridge sites along [110]. In tilt models, CO molecules occupy sites mainly on-top of Rh atoms, but tilted in [001] towards the troughs (long bridge). Based on the LEED investigation [94Wei] off-top models are favored for all overlayers except the (2×1)p2mg. The (2×1)p2mg phase is thought to consist of tilted CO molecules in on-top sites or to be due to a zig-zag reconstruction of the substrate [94Wei]. La ndolt-Bö rnstein New Series III/4 2A4

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Pd The adsorption of CO on the Pd(100) surface has been investigated with LEED [69Tra1, 78Bra1, 82Bib, 82Ort, 88Uvd, 91And, 92Ber2]. At half monolayer coverage a (2√2×√2)R45° is formed, followed at 0.67 ML coverage by a (3√2×√2)R45° phase and at 0.75 ML coverage by a (4√2×√2)R45° LEED pattern. The evolution of the LEED features above 0.5 ML coverage has attracted considerable attention, as they evolve in a rapid series of phases, or quasi-continuous [69Tra1, 79Bat]. In this coverage regime also a c(5√2×√2)R45° and a c(7√2×√2)R45° have been reported [82Bib]. This behaviour has been explained by the formation of domain superlattices above 0.5 ML coverage, where antiphase domains of the (2√2×√2)R45° structure, separated by domain walls with a higher density of CO molecules, are formed leading to a continuous splitting of LEED spots including the (3√2×√2)R45° and (4√2×√2)R45° phases [92Ber2, 96Sch]. CO occupies bridge positions in an upright adsorption geometry in the (2√2×√2)R45° phase [78Bra1, 80Beh, 82Ort] which are also occupied at higher coverages where eventually some tilting occures [88Uvd]. The thermodynamics of adsorption have been investigated by several groups [69Tra1, 78Bra1, 80Beh, 97Yeo1] yielding the isosteric heat of adsorption at 161 kJ/mol from zero up to 0.45 ML coverage, decreasing rapidly to higher coverage, and a pre-exponental factor of 3×1016s−1, plus a sticking factor of 0.6 between 0 ML and 0.2 ML coverage, dropping rapidly thereafter indicating a precursor state [80Beh]. The vibrational properties have been investigated with EELS and IRAS [78Bra1, 79Beh, 82Ort, 83Hof1, 85Bro, 88Uvd] where the C-O stretch in all cases can be attributed to bridge bound species. The latter is even supported by IRAS on single crystal electrodes in aquaeous solution [90Yos]. Further studies of CO adsorption on Pd(100) and (111) were carried out by static SIMS [83Bro, 85Bro] where dominant Pdx(CO)y ions were correlated with the coverage dependent site specificity of adsorption. The valence electronic states have been determined by UPS [79Hor2, 80Beh, 94San1] and Auger resonant Raman scattering [94San1, 94San2, 94San3]. The core level binding energies of the C1s and O1s have been determined with XPS, [92Bjö, 94San1, 94San2, 94San3, 95Ped] as well as the adsorbate induced Pd core level shifts [91And]. On the Pd(111) surface CO forms a large number of ordered LEED structures [70Ert, 78Con, 83Hof1, 84Mir, 87Oht, 90Tüs1, 98Gie, 00Zas2]: θ=0.33 coverage (√3×√3)R30°, θ=0.5 c(4×2) or (√3×2) rect, θ=0.514 (√3×35) rect., θ=0.529 c(√3×17) rect., θ=0.545 (√3×11) rect., θ=0.556 c(√3×9) rect., θ=0.571 (√3×7) rect., θ=0.6 c(√3×5) rect., θ=0.63 (4√3×8) rect. and at θ=0.75 (2×2). It can not be excluded that a full sequence of (√3×n)rect. and c(√3×n)rect. (n odd) with coverage θ=0.5(n+1)/n exist [90Tüs1]. The adsorbate structures have also been investigated with STM [00Sau, 02Ros]. The preferred adsorption of CO into 3-fold hollow sites was experimentally found, supported by total energy calculations, whereby STM showed a height difference of 0.1 Å for CO in fcc and hcp hollow sites [00Sau]. A high-resolution two-photon UPS study of CO on Pd(111) illustrated significant changes in the image states of that surface [96Wal]. The thermodynamic properties, in particular as a function of coverage and temperature have been determined [70Ert, 74Con1, 89Guo, 93Sza2, 99Car, 01Sta, 02Bou], in addition to surface diffusion coefficients characterized by an activation barrier of 16.9 kJ/mol and a pre-exponential factor of 2.2×10−3 cm2s−1 [97Sna]. Vibrational spectroscopy has established the initial adsorption in hollow sites up to 0.33 ML coverage, giving at intermediate coverages ~0.5 ML way to mixed bridge and hollow adsorption and towards saturation coverage mixed on-top and hollow adsorption [83Hof1, 90Tüs1, 92Kuh1, 93Sza2, 96Rai, 98Bou, 99Car, 00Sur, 02Bou, 02Unt]. The assignment of adsorption sites as a function of coverage via XPS has been reported [00Sur] and the valence electronic structure has been studied with photoemission [76Llo, 84Mir, 95Ban] und inverse photoemission [88Jen]. The adsorption of CO on stepped Pd surfaces was also studied by vibrational spectroscopies and ESDIAD which led to the identification of CO adsorbed at step and localized defect sites [95Ram, 96Sve]. Noteworthy is the rather different behavior on Pd(510) and Ni(510) surfaces: step-adsorbed CO is more weakly bound than CO on (100) terraces for Pd while the opposite is observed for Ni [96Sve]. It seems that CO on Pd(510) is adsorbed at the upper step edges while for Ni(510) it occupies initially sites at the bottom of the step edge. Lan dolt-Börn stein New Series III/4 2A4

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The coverage and temperature dependent adsorption of CO on the Pd(110) surface has been investigated with LEED [74Con1, 75Lam, 86Gos, 88He, 90Rav2, 91Hu, 93Wan, 94Loc, 96Loc, 97Ram, 99Kat2, 00Yag], EELS [85Che1, 99Kat2 , 99Kat1], IRAS [85Che1, 89Rav, 90Rav1, 90Rav2], TPD [86Gos, 87Gos, 88He, 89Ehs, 99Jon, 00Yag], molecular beam scattering [98Jon, 99Jon, 01Hir], XPS [94Loc, 96Loc, 97Ram, 98Jon, 99Jon], UPS [78Weh], work funktion changes [88He] and field ion microscopy [92Gau1, 92Gau2]. The clean Pd(110) surface shows bulk truncation [85Die]. With increasing CO exposure the following LEED patterns have been observed below 140 K: a weak c(2×4) below 0.25 ML, a sharp (2×1) at around 0.4 ML, coexisting diffuse (3×1) and (2×1)p2mg around 0.6 ML and a sharp (2×1)p2mg pattern above 0.7 ML up to a full monolayer [99Kat2]. Below 0.4 ML CO adsorbs in an upright geometry, whereas nearest-neighbor repulsion tends to tilt the CO molecules alternatingly, [99Kat2] where the tilt angle for the (2×1)p2mg has been determinded as 24±3° [94Loc, 96Loc] and for the (2×1) as 11±4° [93Wan]. The previously named studies conclude bridge adsorption sites except for one [93Wan]. Adsorption of CO in the coverage range between 0.3 ML and 0.75 ML leads at elevated temperature above 250 K to an adsorbate induced missing row Pd(110)-(1×2) surface reconstruction [90Rav2, 91Hu]. Saturation coverage at 300 K or annealing of the low temperature (2×1)p2mg phase between 330 K and 250 K leads to a (4×2) LEED pattern at 0.75 ML coverage. Annealing to higher temperature leads to a (1×1) pattern [88He, 90Rav2, 91Hu]. Also the adsorbate induced surface core-level shifts have been determined [91Com]. TPD measurements after CO exposure at 130 K report five desorption peaks α1, α2, α3, β1, β2 , where the α3, β2 peaks at 318 K and 393 K have been associated with the surface structure reconstruction of (2×1)p2mg→(4×2) and (4×2)→(1×1), respectively, at an activation energy of 55.5 kJ/mol for T >393 K, 24.3 kJ/mol for 393 K >T >318 K and 10.1 kJ/mol for 318 K>T [00Yag]. The zero coverage desorption activation energy lies in the range of 126-149 kJ/mol [99Jon, 00Yag, 01Hir], [74Con1]. The overall sticking probability has been described by a Kisliuk model with S0≈1 [87Gos, 90Rav1, 90Rav2, 00Yag] and a low K parameter [00Yag]. Only one investigation reported a significant lower S0≈0.5 [98Jon, 99Jon]. Vibrational spectroscopy finds at 135 K the C-O internal stretching mode above 1895 cm−1 to blueshift continuously with increasing coverage, whereas the external Pd-CO vibrational modes, i.e. the stretching mode at 363 cm−1 change drastically and simultaneously with the LEED patterns, a feature that has been attributed to a switching of the tilting direction of the adsorbed CO molecules during the phase change [99Kat2]. For the (2×1)p2mg structure six vibrational modes have been found [99Kat1, 99Kat2, 02Kat]. It has been pointed out, that the CO stretch frequency at 2003 cm−1 makes it difficult to directly assign the bonding site solely from vibrational data [93Wan]. At 300 K the CO coverage dependent adsorbate induced surface reconstruction has been monitored with IRAS [85Che1, 89Rav, 90Rav1, 90Rav2]. Ag On Ag(111) reversible CO adsorption above 77 K was found by measuring the surface potential changes between 77-123 K and 10−1-10−8 torr where the isosteric heat of adsorption varies from 26.7 kJ/mol to 16.7 kJ/mol linear with the surface potential [76McE1]. This was interpreted as weak chemisorption or rather physisorption without long range order (no LEED pattern). Schmeisser et al. find for physisorbed CO on Ag(111) an orientationally ordered phase with two molecules per unit cell and with the CO oriented parallel to the surface, in analogy to the previously reported ordered herringbone structure of N2 on graphite [85Sch]. The bonding of CO to the Ag(110) surface has been discussed controversially as physisorption [84Kra] or chemisorption [94San2]. Physisorption has been concluded from photoelectron spectra of the core and valence region of CO adsorbed on Ag(110) at low temperature [84Kra]. The valence spectra show that the large bonding shift of the 5σ level observed for CO adsorbed on transition metals as well as the large relaxation shifts are absent for CO/Ag(110). This proves that the CO molecule is physisorbed on Ag(110). Polarisation-dependent spectra indicate that CO is not bound to the surface in an upright La ndolt-Bö rnstein New Series III/4 2A4

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geometry but rather with its molecular axis parallel to the surface, or in a random orientation. The C 1s and O 1s core levels exhibit pronounced satellite structure. By comparison with the spectra from the CO/Ni and CO/Cu it is shown that the satellite structure and the differences in extra-atomic relaxation shifts can be well understood by invoking a model in which both core and valence photoelectron spectra are governed by the degree of screening of the photon-induced hole through metal-adsorbate charge transfer. On the other hand, a chemisorptive bond is concluded from studies of CO adsorption on Cu, Ag and Au using core and valence photoemission, X-ray absorption and autoionization of core excited states [94San2, 94San3]. The purpose was to investigate the nature of the adsorption bond of CO on Ag(110) and Au(110) by starting out from the well-established chemisorption system CO/Cu(100)-c(2×2). The photoemission spectra of CO on Ag(110) and Au(110) showed strong shake-up satellites both for the valence and core levels. The separation of the satellite closest to the main line is observed to follow the position of the substrate d-band relative to the Fermi level. The CO adsorption strength for the noble metals is deduced to decrease in the order Cu-Au-Ag. This estimate is based on the widths of the XAS resonances, which are related to the adsorbate-substrate interaction strength of the core excited states, and the relative shake-up intensities, which are expected to increase with a decreasing adsorption strength in the ground state [94San2, 94San3]. The same trends regarding the shake-up intensities are observed both for the valence and core levels. Further support for the formation of a chemisorption bond comes from the study of single Ag-CO and Au-CO complexes on a NiAl(110) surface because IETS indicates a hindered frustrated rotational mode at 209.7 cm−1 and a C-O stretch of 2145 cm−1 for the AgCO complex [03Wal]. EELS measurements have been performed [93Hom] on a physisorbed disordered bilayer of CO on Ag(110) as a function of electron incidence angle and scattering angle as well as primary electron energy. It has been found that in the submonolayer regime the a3π and a1π electronic excitations are quenched because of adsorbate-substrate interactions, whereas strong losses with a clearly resolved vibrational splitting are observed at coverages above a monolayer. The intensities of these excitations behave differently as a function of the experimental geometry which can be explained by the different characters of excitations, i.e. one is optically allowed whereas the other is optically forbidden. Also the electron trajectories play an important role for the ratio of the cross-sections of these two excitations due to electron-image charge interaction and diffraction effects. Ir Clean Ir(100) and Ir(110) surfaces show stable surface reconstructions [73Chr, 76Hag, 78Nie, 91Koc, 92Avr] whereas Ir(111) is unreconstructed [78Tay2]. Stable and clean surfaces of both Ir(100)-(1×1) and Ir(100)-(5×1) can be obtained. Adsorption of CO on the Ir(100)-(1×1) surface leads to the formation of a c(2×2) [69Gra, 91Kis] and, depending on the speed of dosing, an unstable (1×1) LEED pattern [91Kis]. On the Ir(100)-(5×1) surface the exposure to CO has led to the formation of a (2×2) LEED pattern [69Gra] which has not been verified in later investigations, reporting (1×1) LEED patterns [76Bro, 91Kis]. It was further found, that there is no difference in reactivity towards CO between the Ir(100)(1×1) and Ir(100)-(5×1) surfaces [76Rho]. To elucidate the subtle differences for the adsorption of CO on both the (1×1) and the (5×1) reconstructed Ir(100) surface at room temperature, EELS and LEED experiments have been conducted [91Kis]. CO is adsorbed on both surfaces at all coverages in on-top sites. All four vibrational modes of the adsorbate have been detected. Adsorption of CO on the (5×1) surface lifts the reconstruction locally giving rise to a (1×1) LEED pattern. The vibrational frequencies of the CO-molecules on both surfaces differ only slightly. At saturation the iridium-CO and the C-O stretching frequencies are 485 and 2075 cm−1 on the (5×1) and 497 and 2068 cm−1 on the (1×1) surface, respectively. The frequency of the rotational mode of the CO molecule is found to be at 425 cm−1 and the frustrated translation at 53 cm−1, both showing no dispersion along the ΓM direction. The C-O stretching vibration shows dispersion due to dipole-dipole interaction, even when the overlayer is not ordered. An IRAS and LEED investigation [93Mar] of CO adsorbed on the reconstructed (5×1) and unreconstructed (1×1) surfaces of Ir(100) at Lan dolt-Börn stein New Series III/4 2A4

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300 K report a single C-O stretch initially at 2026 cm−1 on the (1×1) surface and a more complex adsorption behavior on the (5×1) surface. For the latter at least five distinct C-O stretching bands have been observed sequentially between 2025 cm−1 and 2097 cm−1 as a function of exposure. In contrast to previous studies, LEED results suggest that the (5×1) reconstruction is not fully lifted, even after exposure to relatively high CO pressures [93Mar]. The adsorption of CO on the Ir(111) surface with three-fold rotational symmetry leads to the typical (√3×√3)R30° [69Edm, 71Gra, 73Wei, 74Doy, 76Com1, 76Com2, 76Hag, 76Küp, 77Iva, 78Com, 81Sea, 89Mar, 96Lau] and (2√3×2√3)R30° [76Com1, 76Com2, 76Hag, 76Küp, 77Iva, 78Com, 81Sea, 89Mar, 96Lau] superstructures with 1/3 and 7/12 coverage, respectively. Analogous to Ru(100), the question has been raised whether the CO binding state at coverages exceeding 1/3 for the (√3×√3)R30° superstructure differs from the low coverage adsorbed CO [74Mad] or are merely governed by the repulsive adsorbateadsorbate interaction, increasing with the continuing compression of the original (√3×√3)R30° layer to a (2√3×2√3)R30° layer [76Com2]. In earlier LEED investigations highly coordinated bridge and hollow adsorption sites were favored [74Doy, 76Com1, 76Com2, 76Hag, 76Küp, 77Iva, 78Com]. However, more recent investigations based on EELS and IRAS have clearly established bonding in on-top sites for the (√3×√3)R30° and for the (2√3×2√3)R30° structures. Similar to the geometrically equivalent Ru(001) surface [80Pfn], all the CO molecules feel a similar substrate potential, leading to the (2√3×2√3)R30° structure with equivalent terminal adsorption sites. TPD indicates the existence of three sequentially populated CO bonding states [96Lau]. The first peak at ~550 K fills in until the maximum intensity of the (√3×√3)R30° LEED pattern is observed. At this point a second state, desorbing at lower temperatures appears leading to the (2√3×2√3)R30° structure. At even higher coverage a third low temperature shoulder appears around 300 K, indicating that some mobility of the CO molecules is required for its formation [97Sus1]. The adsorption and desorption kinetics of CO on Ir(111) has been extensively studied [76Com1, 76Com2, 76Küp, 76Zhd, 78Tay2, 78Zhd]. [96Lau, 97Sus1, 97Sus2]. The vibrational properties have been investigated as a function of coverage with EELS [89Mar] where with increasing CO coverage an Ir-CO frequency between 475 and 490 cm−1 and a C-O stretch between 2025 and 2050 cm−1 has been found. Using Fourier transform IRAS, TPD and LEED at sample temperatures between 90 and 350 K, only a single absorption band, between 2030-2090 cm−1 has been observed with increasing CO coverage up to 0.7 ML [96Lau]. The coveragedependent frequency shift of the IR band can be described quantitatively using an improved dipole coupling model. The contribution of the dipole shift and the chemical shift to the total frequency shift were separated using isotopic mixtures of CO. The chemical shift is positive with a constant value of approximately 12 cm−1 for all coverages, whereas the dipole shift increases with coverage up to a value of 36 cm−1 at a coverage of 0.5 ML [96Lau]. The Ir(110) surface shows a clean surface reconstruction, which has been initially interpreted as a missing row (1×2) surface reconstruction but subsequent investigations STM [91Koc], ion-scattering [92Avr] and LEED [95Lyo] have determined a grooved, microfaceted (331) reonstruction, consisting of (111) terraces two atoms wide, which has a LEED pattern similar to a (1×2) missing row surface reconstruction [91Koc]. Early work on CO adsorption led to the observation of a (2×2) structure [73Chr, 78Nie, 78Tay1, 78Tay2, 78Tay3]. This is to be understood as relative to the (1×1) structure of the unreconstructed surface, equivalent to a (2×1) with respect to the (1×2) reconstructed surface. In addition a (4×2) structure was found [78Tay1, 78Tay2, 78Tay3]. In the early literature a (2×1)p1g1 was proposed, which turned out to be due to coadsorbed oxygen [78Tay1, 78Tay2, 78Tay3]. Also the observation of c(2×2) surface structures appears to be due to dissociation of CO and the subsequent CO adsorption onto carbon and oxygen contaminated Ir(110) surfaces [78Nie]. A combined LEED, IRAS and TPD study at 300 K [95Lyo] has come to the conclusion, that CO adsorbs on the reconstructed Ir(110) surface in on-top sites at all coverages, presumably oriented near the (110) surface normal. IRAS shows a single band for the C-O stretch that shifts from a low-coverage value of 2001 cm−1 to a saturation-coverage limit of 2086 cm−1. This frequency shift was separated into two contributions by using isotopic substitution, one being due to dipole-dipole interactions and a smaller one due to chemical effects (d-electron competition). TPD spectra yield in agreement with previous work three adsorption states at 380 K, 490 K and 610 K [73Chr, 78Tay1, 78Tay2, 78Tay3, 94Kan, 94Men]. To La ndolt-Bö rnstein New Series III/4 2A4

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reconcile the three adsorption states observed in TPD with the single C-O stretch frequency, continuously changing with coverage from a low-coverage value of 2001 cm−1 to a saturation-coverage limit of 2086 cm−1, indicative of a single adsorption site, a model of discontinuous binding energy decrease as a function of CO-CO next neighbor coordination has been proposed. Starting from the clean surface, the first state consists of CO molecules of approximately the same binding energy in an ordered structure with short range or poor long range order up to a coverage of 0.33. Increasing the coverage further leads to the filling of the second TPD state up to a coverage of 0.8, where the CO molecules are having up to two direct CO neighbours causing a IRAS dipole shift. At 0.8 coverage also some ordering was observed. The ordered structures at 0.33 and 0.8 coverage seem to correspond to the previously observed (2×2) amd (4×2) structures [78Tay1, 78Tay2, 78Tay3]. Towards even higher coverage, up to maximum coverage 1 ML, the number of next neighbours increases even more [94Kan, 94Men]. For the (2×2) structure a desorption energy of 154.9 kJ/mol [73Chr] was found. Coverage dependent measurements yielded (146.5 œ 67 × θ) kJ/mol [78Tay1, 78Tay2, 78Tay3], with a pre exponential factor falling from 3×1012 s−1 to 1×109 s−1 at saturation coverage. Initial sticking coefficients of 0.8 [87Ste], 0.9 [78Tay1] and 1.0 [78Nie] have been reported which are initially independent of coverage but decreasing to small values at high coverages. A detailed investigation of the sticking coefficient and the formation of an extrinsic precursor state (weakly bound state above occupied sites) has been performed using a supersonic molecular beam [87Ste]. The sticking coefficient drops with increasing molecular beam energy (0.8 at 8.4 kJ/mol to 0.35 at 142.3 kJ/mol beam energy) because inelastic scattering occurs with increasing beam energy, in contrast to a trapping/desorption scattering into an extrinsic precursor state at low beam enegies [87Ste]. These findings were later reproduced [97Bur]. Pt The clean Pt(100) surface exhibits a (5×20) reconstruction at room temperature, with the outermost layer showing a quasi-hexagonal arrangement, normally referred to as the hex-phase [79Hei1, 86Beh, 92Guo]. Depending on the annealing temperature, the hexagonal layer undergoes for above 1070 K a rotation of 0.7° denoted as hex-R phase [78Bon, 95Mar]. Next to the stable reconstructed surfaces, there exists also a metastable (1×1) bulk terminated surface [78Bon, 78Bro] which transforms above 400 K [81Dav, 81Nor] through an intermediate (1×5) structure [82Hei] to the stable hex-phase. A conversion of the hex-phases to the metastable (1×1) surface can be carried out by adsorption, lifting the hex reconstruction and subsequent removal of the adsorbate by a low temperature catalytic reaction [75Bon2, 78Bon, 81Bar2]. The surface free energy difference between the clean hex and (1×1) phases has been derived as 20 kJ/(mol Pts) [95Yeo]. It has been suggested that the adsorbate induced hex to (1×1) transition plays an important role for oscillatory reactions on Pt(100) [86Imb, 88Sch, 93Imb]. The calorimetric heat of CO adsorption on Pt(100), measured by a supersonic molecular beam in normal incidence, has been determined as a function of coverage at 300 K [96Yeo]. On the (1×1) surface, the differential heat is 225 kJ/mol near zero coverage, stays constant at 215 kJ/mol between 0.1 and 0.25 ML, then decreases to a second plateau at 179 kJ/mol between 0.25 and 0.5 ML coverage, finally decreasing abruptly to a value of 85 kJ/mol. The sticking coefficient on the (1×1) surface decreases linearly between 0 and 0.5 ML coverage from S=0.6 to S=0.26, then falling to a steady state value S=0.04. Earlier results quoted a constant sticking coefficient of 0.6 [78Bro] and 0.75 [83Beh] below 0.4 ML coverage followed by a rapid decrease in the range up to 0.5 ML. On the hex surface, the differential heat is initially 180 kJ/mol and decreases to a plateau at 170 kJ/mol between 0.15 and 0.5 ML coverage. Towards higher coverage an abrupt decrease to the steady state value of the (1×1) surface is observed [96Yeo]. The sticking coefficient on the hex surface is S=0.73 at low coverage, decreasing almost linearly [83Beh, 93Hop, 96Yeo] to S=0.26 at coverage 0.5 ML, then approaching the steady state value S=0.04. For coverage larger than 0.5 ML, the sticking coefficient is the same for the hex and (1×1) surfaces [96Yeo]. Some earlier measurements [77McC, 83Beh] show the same trend, whereas others show an almost constant value up to 0.3 ML coverage and a subsequent decrease [80Cros].

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The mechanism of the CO induced hex to (1×1) phase transition has been proposed to involve a lifting of the hex reconstruction by the initial adsorption, followed by migration and trapping of CO into (1×1)CO islands with a local coverage of 0.5 ML, starting at a critical coverage of 0.05 ML [82Thi, 83Beh, 83Thi] or 0.08 (± 0.05) ML at 300 K [83Jac2]. The resulting island formation has been investigated by various methods, especially STM [83Beh, 87Rit, 94Bor]. With increasing coverage, the area of the surface covered by the (1×1) islands increases but the islands themselves remain constant in size [94Bor]. On the (1×1) surface, combined IRAS and LEED investigations have reported at both 90 and 300 K a single linear band of an on-top species, shifting from 2065 to 2090 cm−1 with increasing coverage, and at least three different bands in the bridge region at frequencies between 1867 and 1910 cm−1, depending on coverage [95Mar]. The ratio of bridge to on-top varies with coverage and temperature. On the hex surface and at low exposure the CO stretch is observed at 2083 cm−1 and assigned to an on-top adsorption site on this surface [91Gar, 95Mar]. At a coverage exceeding 0.13 ML the hex surface reconstruction appears to start by converting into the (1×1) structure, indicated by the observation of an on-top band at 2089 cm−1 and a band at 1873 cm−1 due to a bridge species [95Mar]. However, the assignment of the feature at 2083 cm−1 as an on-top species on the hex-reconstructed Pt(100) surface at 0.13 ML coverage is challenged by the observation that the hex to (1×1) conversion starts at 0.01 to 0.03 ML at 400 K and independent of temperature above 200 K [93Hop]. Then, at low coverages, the small frequency shifts on the hex and (1×1) surface are likely to be due to a small change in the local coverage within the (1×1) islands [95Yeo]. These investigations clarify in particular for the c(2×2) overlayer at 0.5 ML coverage the previously controversial discussion of bonding sites, bridge [82Bib] or on-top [83Beh, 84Ban]. The adsorption of CO to the Pt(100) surface has also been studied with UPS [82Bro]. The adsorption of CO on Pt(111) leads to a series of ordered superstructures identified by LEED [77Ert, 82Ste, 87Ogl, 87Tüs, 88Bla, 90Ryb, 91Won, 94Vil, 98Jen, 98Ma, 00Zas1]. Up to 0.33 ML, a (√3×√3)R30° superstructure is formed with CO occupying on-top sites. At 0.5 ML, CO forms a c(4×2) structure containing 0.25 ML on-top and 0.25 ML bridge-bonded CO. At higher CO coverages, the c(4×2) structure is compressed along the [ 1 1 0 ] direction, leading to the observation of a (√3/2×√3/2)R15° at 0.58 ML [82Ste] and a 2/3(√3×3)rect at 0.67 ML [98Ma]. Additional phases have been reported, such as (4×4) at 0.18 - 0.19 ML [90Ryb] and (8×8) at 0.3 ML [87Tüs, 90Ryb]. The use of high pressure STM has led to the observation of ordered structures of CO at pressures of 200 Torr and above [94Vil, 98Jen]. The adsorption and desorption energies have been studied in great detail. A very sensitive measurement of CO adsorption at low coverage was developed on the basis of thermal energy He atom scattering [82Poe1, 82Poe3, 82Poe4, 84Poe, 87Ver]. The high scattering cross section of 125 Å2 of adsorbed CO, measured for He atoms of 63 meV energy at 40° incidence, is responsible for this sensitivity [82Poe1, 82Poe2, 83Poe2, 84Poe, 88Yin]. The coverage dependent desorption energy of CO has been found to decrease from 134 kJ/mol to 105 - 84 kJ/mol between zero and 0.5 ML, then a further decrease to around 41 kJ/mol at 0.67 ML coverage [77Ert, 84Poe, 86See]. Calorimetric measurements [97Yeo2] find a similar trend, but higher values, where the heat of adsorption at 300 K is initially 183±8 kJ/mol, declining to 118±19 kJ/mol at 0.5 ML coverage. At higher coverage up to 0.75 ML a constant value of 65±3 kJ/mol is found. CO adsorption on vicinal Pt(111) surfaces is characterized by higher binding energy sites, such as step and kink sites [82Poe4]. The coverage dependent sticking probability of CO on Pt(111) shows a precursor mediated behavior [82Poe] which is up to 0.5 ML described by the Kisliuk expression [77Kis] with K = 0.55 and an initial sticking coefficient of 0.8 [97Yeo2]. Above 0.5 ML, the sticking probability falls to the steady state value ~0.05. Similar low temperature adsorption kinetics have been found [93Cud]. This general trend is reported in a large number of studies [74Lam1, 76Chr, 77Col, 77Ert, 77McC, 79Hor1, 79Nor, 81Cam2, 81Lin, 82Ste, 86See, 93Cud, 96Zae, 97Yeo2]. Since CO adsorption on step edges with an adsorption energy of 146 kJ/mol and a pre-exponential factor of 1.25×1015 s−1 plays an important role [81Cam2], there is quite a variation of the numerical values of the adsorption energy in the literature due to non-flat (111) surfaces. The diffusion parameters of CO on Pt(111) have been investigated in great detail by various methods [67Lew, 82Poe2, 88Reu, 92 Kwa, 94Fro, 98Ma] and depend significantly on the presence of step edges. Using flat and stepped Pt(111) surfaces and a linear optical diffraction method [98Ma] CO diffusion on La ndolt-Bö rnstein New Series III/4 2A4

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Pt(111) between 133 and 313 K and 0.1 to 0.67 ML coverage has been found to follow an Arrhenius law with a diffusion activation energy of 12.5 - 19.7 kJ/mol and 30.6 - 33.0 kJ/mol for flat and stepped surfaces, respectively [98Ma]. The desorption process has also been investigated with REMPI [97Sch2] and ESD [93Sza1, 95Sza]. The vibrational spectra of CO on Pt(111) have indicated bridge and on-top bound species. In particular the c(4×2) phase at 0.5 ML coverage, with half the molecules in on-top and bridge sites, has been studied extensively [77Iba, 79Hop, 82Ste, 86Lah, 89Mal, 89Per, 90Per, 90Ryb, 96Klü, 97Eng, 99Eng]. The CO stretch mode for on-top and bridge adsorbed CO is established at 2081 - 2104 and 1850 1855 cm−1, respectively, and the CO metal stretch for on-top and bridge at 468 - 480 and 363 - 380 cm−1, respectively. Also the low energy vibrational modes have been determined by He scattering [86Lah, 98Gra1, 03Gra] and EELS [82Ste]. The vibrational dynamics have been studied as well [86Lah, 89Mal, 89Per, 90Bec, 91Bec, 97Eng]. For low coverage only on-top occupation has been found, whereas at higher coverage the ratio of bridge to on-top varies [82Ste, 88Ols, 89Mal]. As a function of coverage, the evolution of the core level binding energies [95Bar] and resonant core level excitation and decay [89Mur, 92Wur, 94Bjö] have been studied. Integrated O1s and C1s intensities yield relative coverages (which may be calibrated) while binding energy shifts indicate relative site occupancies, such as seen in Fig. 15 [94Bjö]. The use of SFG has allowed to study CO adsorption up to 700 Torr pressure. Here the on-top and bridge modes gradually disappear and a broad band at 2045 cm−1 prevails [96Su]. Vibrational spectra of CO adsorbed on stepped Pt surfaces show stretch frequencies in the range 2065 - 2081 cm−1 which are associated with CO bonded to step sites [85Gre, 85Hay1]. CO/Pt (111) C1s

O1s

c (5 × Ö 3) q = 0.6 on top 0.4 bridge 0.2

Intensity

c (4 × 2) q = 0.5 on top 0.25 bridge 0.25 (4 × 4) q = 0.2 on top 0.2

534

532

530 288 286 Binding energy [eV]

284

Fig. 15. C1s and O1s core level spectra of three ordered CO layers on Pt(111) at coverages of 0.2, 0.5 and 0.6 ML. Note changes in ratio of on-top to bridge adsorbed CO. Data were obtained with monochromatized Al Kα radiation at 1487 eV [94Bjö].

The clean Pt(110) surface exhibits a (1×2) reconstruction [72Bon2] where every alternate atomic row in the [100] direction is missing [82Jac1, 88Fer]. The exposure to CO at temperatures above 250 K lifts the reconstruction, and a (1×1) LEED pattern of a surface with residual disorder is observed for coverages above 0.5 [76Com3, 82Fer, 82Hof1, 82Jac1]. Annealing of the Pt(110) surface at 500 K and exposure to CO leads to an ordered (2×1) superstructure with coverage 1 on an unreconstructed substrate [76Com3]. Models with (2×1)p1g1 [74Lam2, 76Com3, 81Bar1, 82Bar, 82Hof1, 82Hof2, 84Bar, 87Wes] and (2×1)p2mg [84Rie, 92Win, 01Now] symmetry have been proposed. Angle-resolved XPD results seem to provide solid evidence for the (2×1)p2mg-CO structure [01Now]. The CO-induced conversion of the Pt(110) reconstruction to a non-reconstructed surface proceeds via defect formation [89Gri]. The reason is most likely the high adsorption energy of CO on Pt adatoms compared to regular surface sites, Landolt-Börnstein New Series III/42A4

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101

illustrated in Fig. 16 [01Tho]. The lower the coordination of a Pt atom, the higher the adsorption energy. The energy scales also with the center of the local d-band which is related to band narrowing [01Tho]. Below 250 K a metastable c(8×4) phase has been observed [82Fer, 82Jac2, 86Fre1, 01Now] where during adsorption the equivalent of an entire monolayer of Pt atoms is displaced [82Jac1]. Here CO effectively adsorbs well ordered on a (1×2) reconstructed Pt(110) surface [82Fer]. A structure model was proposed for this layer [01Now]. The clean non-reconstructed Pt(110) surface was also prepared [82Fer]. The structural transformation has been studied with STM [89Gri], RHEED [95Sch] and surface core level shifts [87Düc]. trough bridge -1.1

-1.2

trough bridge

facet on top

-1.7

facet hcp

CO binding energy [eV ]

CO binding energy [eV ]

facet on top facet next to step on top ridge on top ridge bridge -2.2

facet hcp

-1.6

ridge on top ridge bridge

-2.1

ridge step on top adatom on top

adatom -2.6

-2.7

4

5

a

6

7 8 9 Coordination number

10

-3.0

11

b

-2.8

-2.6 -2.2 -2.4 Value of d-band center [eV]

-2.0

-1.8

Fig. 16. Chemisorption energy of CO on different sites of a Pt(110) surface versus (a) local coordination of the bonding Pt, (b) the value of the d-band center [01Tho].

On the Pt(110)-(1×2) surface at 300 K, using the temperature modulation technique, the activation energy of CO desorption has been determined in the zero coverage limit to 150.7 ± 6 kJ/mol at a preexponential factor of 3×1014 s−1, decreasing linearly up to a coverage of 0.15 ML to 134.0 ± 6 kJ/mol at a pre-exponential factor of 3×1014 s−1 [88Eng]. Below 0.5 ML coverage a constant sticking coefficient 0.8 has been observed, indicating adsorption via a precursor state [76Com3, 82Hof1]. Towards higher coverage a molecular beam study reports at 0.1 ML and 0.6 ML coverage 148 and 140 ± 15 kJ/mol [80Fai]. Isosteric heats of adsorption have been reported up to saturation coverage [76Com3, 77McC, 80Fai, 82Hof1, 82Jac1]. Vibrational spectroscopy (IRAS, EELS) has shown, that adsorption of CO on Pt(110) at 300 K is exclusively in on-top sites at all coverages [82Hof1, 84Bar, 87Hay, 96Klü, 98Sha2]. The C-O stretch frequency shift with coverage of 50 cm−1 has been found to be due to dipole coupling. Bridge bonded species, which do not occur at 300 K, have been reported after partial desorption [84Hof, 86Fre1] and for adsorption at lower temperatures [83Hof2, 86Fre1, 01Now] but an IRAS investigation between 90 and 300 K could not observe any bridge species at any coverage [98Sha1, 98Sha2]. No C-O frequency change has been observed when the (1×2) reconstructed surface phase converts to the (1×1) surface. At 90 K, the observed saturation coverage on the frozen (1×2) surface is the same [98Sha2] or about 10 % higher [01Now] than at 300 K. In the latter case nearly 20 % bridge adsorbed CO is detected by a binding energy shift of the O1s XPS core level. Hence on-top and bridge sites on second layer Pt atoms must also be occupied. Heating the saturated adlayer formed at 90 K to 140 - 160 K leads to a marked intensity increase in the CO band which is assigned to the creation of the c(8×4) surface phase [98Sha2]. Further heating above 250 K completely converts the Pt surface to a disordered (1×1) phase with adsorbed CO. The driving force for the restructuring process has been attributed to a small increase in the heat of adsorption for CO, located in on-top sites on Pt atoms along the (1×2) ridges compared with CO in on-top La ndolt-Bö rnstein New Series III/4 2A4

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sites on Pt atoms in the (1×2) troughs [98Sha2]. The situation is different again below 30 K, where the clean surface reconstruction is not lifted and hence CO adsorption on the missing row reconstructed Pt(110)-(1×2) has been observed [98Sha1]. Here, two C-O stretch bands are observed. Warming to 40 K starts surface diffusion, leading to chain condensation, limited towards higher temperature at 60 K by further thermodynamic transition. A complex structure model for the c(8×4)-CO has been proposed [01Now]. Further studies of adsorption of CO on the Pt(110)-(1×2) surface have been carried out with UPS [77Shi1, 81Bar1, 82Bar, 82Hof2, 89Düc], IPE [91Ber] and XPS [86Fre1, 87Düc, 01Now]. Au For the adsorption of CO on Au surfaces, the formation of a chemisorptive bond has been difficult to establish experimentally. Whereas for the Au(111) and Au(100) surface only physisorption occures, the most reactive Au(110) surface shows indications of weak chemisorption. Based on the similarity between UPS data on CO/Cu(100) and CO/Au(110), the formation of a chemisorptive bond on the Au(110) surface is derived in analogy to the weak chemisorption on the Cu(100) surface [89Düc]. The chemisorption of CO on Au(110) is further supported by the analysis of XPS shake-up intensities, XAS line shapes and the comparison of autoionization spectra for the weak CO chemisorption on Cu(100), Ag(110) and Au(110) [94San2, 94San3]. In He scattering studies of the CO/Au(111) system the angular dependence of the cross sections has been found to be very sensitive to the adatom potential. It was found that a hard bump representation of the adatom is inadequate for the differential scattering [85Ell]. Reflection-absorption infrared spectra of CO adsorbed on Au(332) show a single band which first appears above 2120 cm−1 [97Rug] and shifts to lower wavenumber with increasing coverage. The band is broad and consists of at least three poorly resolved components. The study of single Au-CO complexes on a NiAl(110) surface is of interest in this context because IETS indicates a hindered frustrated rotational mode at 282 cm−1 [03Wal]. 3.7.1.3 CO adsorption on bcc metal surfaces Cr The study of CO adsorption on Cr(110) by a number of different techniques, e.g. LEED, EELS, UPS, XPS, ESDIAD and Auger electron spectroscopy, has shown this metal surface to be highly reactive towards CO adsorption and dissociation [84Shi, 85Shi2, 85Shi3, 86Meh, 86Shi]. Vibrational spectra provided clear evidence for the sequential adsorption of CO into two different molecular states at 120 K. The first, called α1-state, is characterized by very low C-O stretch frequencies at 1150 and 1330 cm−1 and a complete lack of ion emission in ESDIAD. This α1-state was atributed to a π-bonded CO molecule oriented nearly parallel to the Cr(110) surface. An ordered c(4×2) structure was formed at 120 K and a coverage of about 0.25. The second CO species, called α2-state, observed at higher coverage and at the same temperature was characterized by three vibrational frequencies at 495, 1865 and 1975 cm−1. ESDIAD showed O+ ion emission in nearly perpendicular direction. Hence this state was identified as CO bonded through the carbon atom in atop and bridge sites, respectively, both oriented perpendicular to the surface. The α1-state was also identified as a precursor to CO dissociation [85Shi2]. Pre-adsorbed oxygen inhibits the formation of the α1-state CO but not the α2-state CO. Even the c(4×2)-CO layer is disordered and converted to α2-state CO by adding adsorbed oxygen [85Shi3]. Cr(110) exposed to CO at 300 K leads to only dissociated C and O. Overall there must be a low barrier for the dissociation of adsorbed CO on Cr. A related theoretical molecular orbital study of adsorbed CO on Cr(110) shows that a high coordinate lying down configuration is favored at low coverage [86Meh]. This is mainly due to a destabilization and emptying of the antibonding counterparts to σ and π donation bonds. Backbonding to the CO 2π* orbital is enhanced which stabilizes a nearly horizontal orientation of CO.

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Of interest is also the effect of Cr thin films deposited on the Ru(001) surface on CO adsorption [02Eng]. Adsorption and dissociation of CO is considerably enhanced compared to clean Ru(001). Fe The combined use of TDS and vibrational spectroscopy (EELS) has been demonstrated to great advantage in the study of CO/Fe(100) [87Moo1, 87Moo2]. Figure 17 illustrates the procedure and some of the results. A TDS trace on the right shows the different desorption states due to molecular and dissociated CO, α- and β-states, respectively. Heating a saturated CO layer from 120 K to temperatures, where one or several states have desorbed, and taking EELS spectra of the residual layer, yields information on the various states still adsorbed on the surface. It is clear from the data in Fig. 17 that α1 and α2 are molecular CO states, because of the C-O stretch frequencies at 2020 and 2070 cm−1, respectively. The most interesting case is the α3 species, characterized by a very low frequency of 1210 cm−1 which is attributed to a tilted CO molecule. This particular α3 CO partly desorbs and partly dissociates in the temperature range 343-483 K leaving C and O on the surface which give rise to the β desorption peak at 830 K. The tilt angle of the adsorbed α3 CO has been studied by three different techniques, NEXAFS [87Moo3], XPD [89Sai] and CDAD [94Wes]. It was found to be about 55° relative to normal [89Sai]. The most recent result of the angular dependence of the circular dichroism of the CO 4σ orbital is displayed in Fig. 18 and compared to calculated angular dependencies assuming a range of tilt angles. From the comparison of experiment and theory a tilt angle of 40°±10° is concluded [94Wes]. The conversion of adsorbed CO among different adsorption states on an Fe(100) surface was investigated through the coadsorption of labelled 13C16O and unlabelled 12C16O [88Lu]. In these experiments selected adsorption states were initially populated by one type of CO and the surface was then exposed to the other type. While no conversion occurred between the β-CO states and the α-states, or between α2 and α1 states, significant conversion was observed between the α2 and the (tilted) α3 states. A relative energy profile of the adsorption state conversions was proposed, based on these observations [88Lu].

TPD

HREELS × 400

395 530

483 K

343 K

× 400

Intensity

483 K 1260

1210

× 200

963 K

530

2020

2070 223 K

× 200

× 200

343 K α2

223 K

α3

α1

× 400

470

CO partial pressure p CO

× 400

963 K

β

Fig. 17. CO on Fe(100), correlation between vibrational spectra (left) and thermal desorption data (right) for conditions where several α and βstates have been separated [87Moo2].

125 K

70 × 400 × 200

0

1000 2000 3000 Energy loss [cm -1 ]

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500 700 Temperature [K]

900

104

3.7.1 CO and N2 adsorption on metal surfaces

Thermal energy atom scattering (TEAS) has been used to study CO adsorption on Fe(111) [92Ber1]. The He specular intensity drops rapidly and smoothly with CO exposure on the Fe(111) surface. In contrast to CO/Pt(111) [83Poe1] the diffuse scattering of He from CO on Fe(111) is not a negligible fraction of the intensity. The surface exhibits a finite reflectivity even at high CO coverages. An effective scattering cross section for molecular adsorbed CO of 56 Å2 was derived from a fit of the reflectivity data to a model taking this diffuse scattering into account. The He atom scattering cross section of dissociated C,O was determined to be 93 Å2 [92Ber1].

CDAD asymmetry A CDAD [%]

60

free CO 4 σ orbital h n = 30.7 eV

a = 0° 5°

20

15°

40

CDAD asymmetry A CDAD [%]

80

25°

20 35°

a

0 O

-20

CO/Fe (100) h n = 30.7 eV

55° 53° 0

51° 49°

-20

45°

45°

C -40 -15

a

0

45 15 30 Tilt angle of CO-axis Q [deg]

-15

60

b

a = 35°

0

47°

45 15 30 Tilt angle of CO-axis Q [deg]

60

Fig. 18. Measured CDAD asymmetry of CO 4σ orbital and calculated values for several tilt angles Θ of the CO axis relative to the surface. (a) free CO molecule at hν = 30.7 eV. (b) For CO adsorbed on Fe(100) [94Wes].

Nb An early study of CO adsorption on a Nb field emitter (FEM) tip showed evidence of desorption of CO at 150 K [64Kle]. Changes in the FEM emission pattern at about 600 K were interpreted to indicate CO dissociation. No direct evidence of adsorbed molecular or dissociated CO on Nb surfaces has been presented. In general, the behavior of CO on Nb is thought to be similar to CO on Ta, Mo and W surfaces [70For]. Although clean surfaces of Nb(110) crystals have been prepared [66Haa], no studies of CO adsorption on (110) oriented single crystals of Nb are known. The sticking probability of CO on Nb(111) at 300 K was determined as 0.06 and a maximum work function change of 0.25 eV has been reported [64Oma]. Mo We briefly discuss the adsorption of CO on Mo(110). Molecular and dissociative adsorption of CO occur [81Kel]. The rate of dissociative CO adsorption was analyzed in detail for clean and pre-covered Mo(110) surfaces by utilizing XPS and UPS as an indicator of molecular and dissociated CO [81Sem, 86Eri]. The thermally induced conversion of adsorbed CO to carbon and oxygen was measured at several coverages and temperatures. The rate of CO dissociation in the adsorbed state is described by an Arrhenius expression, according to rdis (T ) = Aexp(−Edis /kT ) , with Edis as the activation energy of dissociation and A as the pre-exponential factor. Isothermal as well as thermal conversion spectra (applying a linear temperature ramp) were used to extract A and Edis separately [86Eri]. Results were obtained for clean Mo(110) and surfaces modified with coadsorbed S, C, O and K. The results are listed in Table 3. PreLan dolt-Börn stein New Series III/4 2A4

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105

exponential factors and activation energies show a pronounced compensation effect, as seen in Fig. 19, which means that high activation energies are associated with high pre-exponential factors, and vice versa.

Pre - exponential factor A [s -1 ]

S 10

12

unmodified

K

unmodified 38 %

unmodified 13 %

10 8

O 10 4

Fig. 19. Plot of pre-exponential factor versus activation energy for CO dissociation on modified Mo(110) surfaces. The rate parameters for all surfaces fall on a single line in this plot of log(A) versus Edis. At 300 K dissociation rates are nearly the same, however, for all surfaces. [86Eri].

C

0

0.5 Activation energy E dis [eV ]

1.0

Vibrational spectra (IRAS) of CO adsorbed on Mo(110) at 95 K are shown in Fig. 20 [91He1]. At low exposure (2 L = 2.6 mbar⋅s of CO) no evidence of a C-O stretching frequency is seen in the range of 1800-2100 cm−1. Three separate peaks at 1885 - 2010 cm−1 are observed at higher CO exposures, with the one near 2040 cm−1 rising to high intensity at 13 mbar⋅s. Although CO sticking is expected to be high at this low temperature, no evidence of molecular CO is seen at low coverage by IRAS. A plot of the total amount of carbon, measured by Auger electron spectroscopy, versus CO exposure confirms the presence of CO at low exposures but the integrated IRAS intensity of all CO peaks in Fig. 21 versus CO exposure reveals a range of zero intensity up to 3 L = 4 mbar⋅s. The solution to this apparent problem is given by Fig. 22 which shows vibrational spectra (EELS) of CO/Mo(110) at 120 K over a larger range of frequencies [91Che]. At low to intermediate coverages a C-O stretch frequency at 1345 cm−1 is detected and interpreted as a molecular tilted CO species whose intramolecular bond is weakened due to its adsorption geometry. This species is easily dissociated at higher temperature via an intermediate species characterized by an even lower frequency at 1130 cm−1 [91Che]. He et al. could not see the lowfrequency CO species by IRAS but they produced indirect evidence for its presence by a titration experiment with vapor deposited Cu [91He1]. A small amount of Cu was added (about 0.9 ML) to a low coverage of CO adsorbed at 95 K. A peak at 2101 cm−1 was then detected. Heating this surface to 145 225 K and recording IRAS spectra showed this peak to be stable. The peak was attributed to CO that had migrated from Mo surface sites to Cu sites where it assumes an upright configuration. The disappearance of the tilted CO species with high CO coverage [91Che], Fig. 22, is most likely due to crowding on the surface, leaving no room for a tilted molecule. The orientation and bonding of adsorbed CO on Mo(100) was studied by EELS, angle-resolved UPS and near-edge X-ray absorption fine structure (NEXAFS) [87Ful]. Two different CO species can be clearly distinguished by their C-O stretch frequencies measured by EELS. At low coverage CO exhibits a very low stretching frequency of about 1200 cm−1 [85Zae]. Both NEXAFS and ARUPS unequivocally indicate that at this coverage CO is tilted at approximately 40° to the surface normal. This CO tilting at low coverages cannot be ascribed to adatom-adatom interactions. Measurements of the positions of the photoelectron peaks of the tilted molecule indicate that both the 1π and the 5σ orbitals participate in surface bonding. Based on these observations, a bonding model is proposed in which the tilted CO molecule is chemisorbed in a fourfold hollow site [87Ful]. At high coverage CO exhibits a stretch frequency near 2100 cm−1 which is compatible with an on-top bonded species. ARUPS indicates that this CO chemisorbs with its axis perpendicular to the surface in an analogous manner to that observed on other transition metal surfaces. Overall, a similarity to CO adsorbed on Fe(100) is obvious [85Ben, 89Dwy, 89Sai]. La ndolt-Bö rnstein New Series III/4 2A4

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3.7.1 CO and N2 adsorption on metal surfaces 2040

CO/Mo (110)

CO/Mo (110)

2055

Tads = 120 K

2029

400

0.1 %

× 100

Absorbance

CO exposure : (molecules / cm 2)

CO exposure 10 L

× 120

Intensity

6L 1896

2010 1993

1927 1885

1345 1500

1970

1935

4L

× 333

3L

× 600

2L 2100

2000 1900 Wavenumber [cm -1 ]

f 2.6 ×10 15

1500

1920

2200

g 6.3 ×10 15

575

1800

e 1.7 ×10 15 d 8.6 ×10 14

× 1000

c 4.2 ×10 14 × 1000

Fig. 20. Vibrational (IRAS) spectra of CO on Mo(110) at 95 K; [91He1].

× 1000

0

b 2.1 ×10 14 a clean

1000 2000 Energy loss [cm -1]

Fig. 22. EELS spectra of CO on Mo(110) at 120 K taken for increasing coverage; [91Che]. 0.30

1.00

CO/Mo (110) 0.25

AES ratio C (273 eV) / Mo (186 eV)

Integtated intensity [a.u.]

0.08

0.06

0.04

0.02

0.15 0.10 0.05

0

0 0

a

0.20

2

4

6 8 CO exposure [L]

10

12

0

b

2

4

6 8 CO exposure [L]

10

12

Fig. 21. (a). Integrated intensity of IRAS peaks versus CO exposure at 90 K. (b). AES peak ratio C(273eV)/ Mo(196eV) versus CO exposure at 95 K; [91He1].

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107

Ta A study of CO adsorption on a (110) oriented FEM tip was analyzed by changes in the emission pattern [63Kle]. Annealing to 125 K produced some CO desorption but no direct diagnostic of the adsorbed layer was possible in this work. In another study of adsorbed CO on a single crystal of Ta(100) at 300 K showed evidence of dissociative adsorption and dissolution of C into the bulk at elevated temperatures [74Che]. Ordered surface structures appear being due to atomic oxygen. No desorption takes place at temperatures below about 2000 K. Further information on reactions of CO with Ta are given by Horz [78Hor2]. Several studies of CO adsorption on Ta(110) surfaces modified by added Cu or Pd have appeared more recently [93Kuh1, 93Sel, 95Pic1]. W For CO adsorption on W surfaces the initial rate of adsorption is high for a range of surface (Ts) as well as gas (Tg) temperatures, such as shown in Fig. 23 and Fig. 24 for the coverage dependent sticking coefficient of CO on W(110) and W(100) surfaces [71Koh, 79Wan]. Thermal desorption spectra from W(110) exhibit multiple states, seen in Fig. 25, which may be grouped into two regions, at 200 - 400 K and at 900 - 1200 K. These states were designated α- and β-states, respectively, with the special notion that the α-state of a primary CO layer (—virgin“), prepared on the clean W(110) surface, produced a single peak at 375 K. This peak was attributed to the —virgin“ state of CO. Adsorbing CO on surfaces that had been flashed to 500 - 600 K after primary CO adsorption, exhibited a broad range of several α-states, distinctly different from virgin CO, Fig. 25. These α-states seemed to be more weakly bound than —virgin“ CO [71Koh]. Is CO initially adsorbed molecularly or dissociatively, i.e. breaking into separately adsorbed C and O species? A clue to answering this question comes from Fig. 26 which is a plot of the amount of CO desorbed at T <500 K versus the total amount adsorbed at 100 K [71Koh]. No CO desorbs below 500 K for coverages up to 0.25 which means that all of the initially adsorbed CO must either be strongly bound to the surface or be converted into such a species during heating, so that all surface species desorb at T >900 K. The initially adsorbed CO may be all molecular or a mixture of molecular and dissociatively adsorbed CO - situations which can not be resolved by thermal desorption spectra alone. Most likely only dissociated CO is present after heating to 500 K.

CO/W (100)

ì

Ts = 90 K

Tg í

1.0 0.8 0.6 0.4 0.2

Ts í

î

110 K 201 K 295 K 367 K 435 K

1.0 0.8 0.6 0.4 0.2

0

0 0

a

ì

Tg = 110 K

Absolute sticking coefficient S

Absolute sticking coefficient S

î

CO/W (100)

110 K 200 K 300 K 400 K 540 K

2

4

6

12

10

8 2

14

CO coverage [molecules / cm × 10 ]

0

b

2

4

6

8

10

12

CO coverage [molecules / cm2 × 1014 ]

Fig. 23. Coverage dependence of absolute sticking coefficients for CO on W(100) at (a) 90 K and (b) 110 K [79Wan].

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3.7.1 CO and N2 adsorption on metal surfaces

1.0

Relative sticking coefficient

0.8

0.6

0.4 400 K

600 K

300 K

100 K

0.2 500 K

2

0

200 K

4 6 Number of doses adsorbed

8

Fig. 24. Coverage dependence of relative sticking coefficient of CO on W(110) at 30 K [73Koh].

10

12

v

Amount desorbed [a. u.]

10 8 6

α2

4

β2

α1

2

Fig. 25. (Ο) Thermal desorption spectrum of saturated CO (—virgin“) layer on clean W(110) adsorbed at 77 K. (∆) Desorption of layer saturated at 77 K after heating virgin layer to 600 K. Dotted line: desorption from polycrystalline W surface [71Koh].

β1

0 100 200 300 400

800 900 1000 Temperature [K]

1400

1200

0.7

Amount desorbed at T < 500 K Total amount at 100 K

0.6 0.5 0.4 0.3 0.2 0.1 0

0.1

0.2

0.3 0.4 0.5 0.6 Initial coverage qi ( at 100 K )

0.7

0.8

0.9

1.0

Fig. 26. Amount of CO desorbed at T <500 K relative to total amount of CO (adsorbed at 100 K) versus initial CO coverage (adsorbed at 100 K) [71Koh].

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109

Early spectroscopic work by UPS of adsorbed CO on W(100) and W(110) concentrated on detecting CO molecular orbital related emission peaks for various conditions where α and β desorption states may be predominant [76Plu]. The β3 state on W(100) was identified as dissociated CO, giving rise to a c(2×2) LEED pattern, and tentatively attributed to C and O in 4-fold hollow sites. Low coverage adsorption of CO into this state even at room temperature seemed likely. Other β states consist also of C and O strongly bound to the surface, with the CO bond considerably weakened or fully dissociated. Virgin and α CO are molecular and bonded through the C atom to the surface (—carbonyl-type bond“), with some remaining uncertainty, whether CO is linearly or bridge bonded. On W(110) the virgin state is molecular, it both desorbs and converts to β-CO on heating to 500 K. Readsorption on a saturated layer heated to 700 K forms α-CO which is not identical to the initial virgin state CO [76Plu]. A further clarification of CO adsorption on W came from various sources, quantitative X-ray photoemission spectra being one of them [83Umb]. Molecular and dissociated CO in the adsorbed state can be well distinguished by their O 1s and C 1s core level binding energies (Table 7), and the intensities of the respective lines can be taken as a reliable measure of relative CO coverages [73Mad1, 74Yat3]. Hence the amounts of molecular and dissociated CO, here virgin and β-states, respectively, were measured for a CO layer, initially adsorbed up to saturation at 100 K, and then heated successively to a temperature T [83Umb]. It is concluded from Fig. 27 that only molecular CO is present up to about 270 K where it begins to partially desorb and mostly convert into dissociated C and O, the latter being equivalent to the sum of β-states. Note that at >400 K all of the adsorbed CO is dissociated. 1.0 v - CO

Relative coverage

0.8

0.8

β - CO

0.6

0.6

0.4

0.4

0.2

0.2

0

0 100

200

300

400 500 900 Temperature T [K ]

1000

1100

1200

Absolute coverage [×10 15 ]

1.0

Fig. 27. Evolution of adsorbed CO layer on W(110) separated into virgin (v) and β CO at 100 K, based on quantitative O 1s photoemission spectra [83Umb].

Further insight into the nature of CO/W(110) was gained by vibrational spectra of virgin CO adsorbed at 90 K [91Hou]. The spectra were taken at several CO coverages in the range 0.23 - 0.67 and show two peaks in the C-O stretching region, Fig. 28. The growth of the two peaks with coverage suggests that the species at 1360 cm−1 is adsorbed first, followed by the second species at 2020 cm−1. A maximum coverage of the low frequency state is found near 0.4 total CO coverage. As the total CO coverage increases, the amount of the 1360 cm−1 state decreases and becomes undetectable at >0.7 . Separate annealing experiments demonstrated that the 1360 cm−1 CO state converts to dissociated CO below 250 K. All of this behavior, together with measured work function changes due to adsorbed CO, are consistent with a molecular CO species whose axis is strongly inclined, estimated at about 70° relative to normal [91Hou]. Although CO adsorption characteristics seem to be similar for W(110) and W(100), a low frequency CO state has so far not been verified for the W(100) surface.

La ndolt-Bö rnstein New Series III/4 2A4

110

3.7.1 CO and N2 adsorption on metal surfaces

2020 cm

×200

−1

1360 cm -1

Relative intensity

qCO 0.67 0.60

0.46 0.33

Fig. 28. Coverage dependent EELS spectra of CO adsorbed at 90 K on W(110). Note transient appearance of 1360 cm−1 frequency indicative of a tilted CO species [91Hou].

0.23 - 4000

- 3000

- 2000 -1000 0 Electron energy loss [cm-1]

1000

3.7.1.4 CO adsorption on hcp metal surfaces Ti Several studies of CO adsorption on Ti( 10 1 1 ) and Ti(0001) are known [78Fuk, 80Fuk3, 98Kuz]. UPS and XPS were used to study the valence band DOS and the effect of adsorbates on individual photoionization cross sections [80Fuk3]. Dissociation of CO leads to the formation of oxides and carbides at the surface, characterized by O1s and C1s spectra at 529.8 ± 0.2 eV and 281.8 ± 0.2 eV, respectively [78Fuk]. The kinetics and mechanism of dissociative chemisorption of CO on Ti(0001) at T = 300 K and exposures of 0 - 300 Langmuir were investigated by X-ray photoelectron spectroscopy and X-ray photoelectron diffraction [98Kuz]. The results were used to characterize the initial elementary stages of chemisorption, to determine the positions of adsorbed atoms and to distinguish between the different types of kinetics of filling in the non-equivalent adsorption centers with carbon or oxygen atoms. A series of possible variants of the CO chemisorption on the Ti(0001) surface was calculated by the first-principle linear method of muffin-tin orbitals in the full-potential version (LMTO-FP) [98Kuz]. Co CO adsorption on the basal plane of Co(0001) has been studied in fair detail by several groups [77Bri, 79Fre, 83Gre, 83Pap, 96Bei, 00Cab, 00Lah]. Three ordered CO structures at coverages between 0.33 and 0.58 were detected by LEED. The coverage dependent energy of desorption was determined as 96 - 128 kJ/mol [83Pap]. The dispersion of the electronic valence orbital derived states at 0.33 and 0.58 coverage, illustrated in Fig. 29 and Fig. 30, was measured by ARUPS [83Gre]. The local structure of adsorbed CO at 160 L was determined by LEED, yielding a C-O bond length of 1.17 ± 0.06 Å and a Co-C bond length of 1.78 ± 0.06 Å [00Lah]. Beitel et al. studied adsorbed CO at pressures between 10−10 and 600 mbar at 300 - 550 K by polarisation modulated RAIRS [96Bei]. Linearly and bridge-bonded CO could be distinguished. Annealing in high pressure CO at 100 mbar and 450 - 490 K produces defect sites at the Co(0001) surface which are possibly related to the formation of Co carbide due to dissociation of CO Lan dolt-Börn stein New Series III/4 2A4

3.7.1 CO and N2 adsorption on metal surfaces

111

[96Bei]. Cabeza et al. compared CO adsorption on clean Co(0001) and Pt modified Co(0001) using LEED and photoemission spectroscopy [00Cab]. CO adsorbs molecularly on both Pt and Co surfaces. The coverage of CO on Co(0001) is higher than on the modified Pt/Co surface. Pt causes also a lowering of the adsorption energy which is in agreement with theory. Γ

7.2 7.4

Γ

M

M K

K

CO/ Co (0001) RT dispersion

Binding energy E B [eV]

7.6 7.8 M

5 s -1 π

8.0

Γ

Γ

8.2

Γ

M Γ

KM K Γ

10.2 10.4

Fig. 29. Dispersion of CO molecular orbital states for the ordered (√3×√3)R30° CO layer on Co(0001) at room temperature. Inserts show LEED pattern and measured directions in reciprocal space [83Gre].

4s

10.6 10.8

2.0 2.0 1.0 0 1.0 Parallel component of wavevector k II [Å - 1]

b

7.2 7.4

Γ

a

CO/ Co (0001) low T dispersion

Binding energy E B [eV]

7.6 7.8

5 s -1 π

M b

8.0

Γ

Γ

10.89° a

8.2

Γ

10.2

KM K

M Γ

Γ

10.4

4s

10.6 10.8 2.0 2.0 1.0 0 1.0 Parallel component of wavevector k II [Å - 1]

La ndolt-Bö rnstein New Series III/4 2A4

Fig. 30. Dispersion of CO molecular orbital states for the ordered (2√3×2√3)R30° CO layer on Co(0001) at room temperature. Inserts show LEED pattern and measured directions in reciprocal space [83Gre].

112

3.7.1 CO and N2 adsorption on metal surfaces

The behavior of CO adsorption on higher index surfaces of Co, such as Co( 10 1 2 ) and Co( 11 20 ) was also investigated [78Pri, 82Pap, 84Hab, 85Pap, 96Too]. CO is molecularly adsorbed on Co( 112 0 ) below 300 K, with a coverage independent isosteric heat of adsorption of 143 kJ/mol [85Pap]. Molecular and dissociative adsorption occur above 300 K. Molecular adsorption of CO is found at 100 - 420 K on Co( 10 1 0 ) with an isosteric heat of adsorption of 143 kJ/mol, dropping to 120 kJ/mol with increasing coverage [82Pap]. Two ordered structures, p(2×1) and c(2×1), are observed at 300 K by LEED. CO adsorption on Co( 10 1 2 ) is molecular at 300 K and leads to a (3×1) structure [78Pri]. Heating the adlayer causes both desorption and dissociation of CO. Recently, the formation of the CO-induced (3×1) structure on Co( 11 2 0 ) has been investigated by scanning tunnelling microscopy [98Ven]. The molecular adsorption of CO is found to create a trough-and-ridge structure running along the (0001) direction and comprising a (3×1) periodicity in well-ordered regions. STM measurements conducted during CO exposure show that the adsorption induces migration of Co atoms along the surface. A —missing rowadded row“ model for the formation of the (3×1) structure is proposed, in which every third zig-zag row of atoms in the clean Co( 11 20 ) surface is absent [98Ven]. Zn Studies of CO adsorption on Zn crystals are not known. We briefly consider CO adsorption on thin films of deposited Zn. CO was reported to not adsorb on thick Zn films at 80 K under ultra-high vacuum conditions [90Car, 93Rod]. In contrast, adsorption is observed on Zn atoms bonded to Ru(0001). Bimetallic bonding induces a reduction of approximately 0.5 eV in the binding energy of the core and valence levels of Zn. This shift probably increases the strength of the Zn-CO bond. CO molecules adsorbed on Zn adatoms show an O(1s) binding energy close to 535.2 eV and a desorption temperature of approximately 125 K. Both properties indicate a very low energy of adsorption and a very weak contribution of π-backbonding to the ZnC-O bond. By comparison, CO adsorption on polar ZnO surfaces, either Zn or oxygen terminated, is relatively weak, with adsorption energies between 29 [00Bec]. and 52 kJ/mol [80Gay] for both surfaces. Zr CO readily dissociates on adsorption on a ploycrystalline Zr surface [80Foo]. Quantitative studies of CO behavior are complicated because of dissolution of C and O into the bulk of the material. Studies of CO adsorption on single crystal surfaces are not known. Ru An extensively studied example of molecular adsorption is CO on Ru(0001). The dependence of the sticking coefficient of CO on coverage at surface temperatures between 100 and 400 K is shown in Fig. 31 [83Pfn1]. There is a slow nearly temperature independent decrease with coverage up to about 0.3 where a well ordered (√3×√3)R30°-CO structure forms. This result, corroberated by a measurement of the intial sticking coefficient as a function of translational kinetic energy of incident CO in Fig. 32, [00Rie] indicates non-activated adsorption. Thermal desorption spectra of CO in Fig. 33, measured by the change of electron work function, reveal two adsorption states and first order desorption kinetics [83Pfn2]. When the heating rate for thermal desorption, βH=dT/dt, is varied over about three decades, the energy of desorption and the pre-exponential factor for a certain coverage can be determined from a plot of βH/Tp2 versus 1/Tp as shown in Fig. 34 [83Pfn2]. Alternatively, desorption isotherms can be constructed from the desorption data of Fig. 33. A set of isotherms for the temperature range 432 - 513 K is given in Fig. 35 for the same system [83Pfn2]. Evaluation of this large amount of data leads to the coverage dependent isosteric adsorption energy of CO on Ru(0001) and the associated pre-exponential factor in Fig. 36 [83Pfn2]. The discontinuity in both quantities at the CO coverage of 0.33 is related to a structural transition from the (√3×√3)R30° to a more compressed CO layer.

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113

0.4

230 K

0.2

0

Absolute sticking coefficient S

Absolute sticking coefficient S

120 K 0.6

400 K 380 K 360 K

0

260 K 295 K 320 K 340 K

0.4 0.2 Coverage q [M L]

0.6

100 K

150 K 0.4

175 K 200 K

0.2

0

0.6

0

0.4 0.2 Coverage q [M L]

0.6

Fig. 31. Coverage dependence of absolute sticking coefficients S for CO on Ru(0001) at adsorption temperatures between 100 and 400 K; [83Pfn1].

Fig. 33, see next page 1.00

q i = 60° q i = 0°

10 - 4

2

0.90

10 - 5

bH / T P

Initial sticking coefficient S o

0.95

0.85

10 - 6

0.80 0.75 0

0.2

0.4 1.0 0.6 0.8 Translational energy E t [eV]

1.2

Fig. 32. Dependence of the initial sticking coefficient S0 on translational (kinetic) energy for CO on Ru(0001) at 0° and 60° incidence angle at Ts = 273 K; [00Rie].

Landolt-Börnstein New Series III/42A4

10 - 7 1.9×10 - 3

2.0

2.1 1 / TP

2.2

2.3

Fig. 34. Evaluation of peak temperatures from thermal desorption data of CO/Ru(0001), according to Falconer and Madix [75Fal], to obtain the effective desortion energy; [83Pfn2].

114

3.7.1 CO and N2 adsorption on metal surfaces

Differentiated work function _d D f dt

D fo [ meV ] 795 741 716 662 634 608 555 497 472 434 404 357 334 290 241 200 141 100

300

500 400 Temperature T [K ]

Fig. 33. Differentiated work function versus temperature curves for CO on Ru(0001), illustrating the coverage dependent thermal desorption of CO; [83Pfn2].

600

T [K ]

6

432 435 438 441 444 447 449 452 455 458 460 463

ln N / N

5 4 3

2 1

499 496 507 502 513 510 505

475 484 481 478 493 490 487

300 100 200 Work function change D f [meV ]

472

466 469

Fig. 35. Desorption isotherms obtained from the data of Fig. 33 for CO on Ru(0001) according to the procedure by Bauer et al. [75Bau] [83Pfn1]. Temperatures in K are given for each isotherm. 400

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1020

Pre - exponential factor k o [s -1]

Isosteric heat of adsorption E B [ k J/mol ] Effective desorption energy Eeff [ k J/mol ]

3.7.1 CO and N2 adsorption on metal surfaces

150

100

0.1

a

0.2

0.3 0.4 0.5 Coverage q [ ML]

0.6

1018

1016

1014

0.1

b

0.2

0.4 0.5 0.3 Coverage q [ ML]

0.6

Fig. 36. Coverage dependence of (a) isosteric heat of adsorption as well as effective desorption energy and (b) preexponential factor of desorption for CO on Ru(0001); [83Pfn2].

The dynamics of CO scattering from a Ru(0001) surface has also been investigated. Fig. 37 shows the angle-resolved flux distribution of scattered CO molecules at surface temperatures of 273 and 550 K from the clean and for 273 K also from the CO-covered surface, at an incident kinetic energy of 0.8 eV [00Rie]. The maximum of the distribution at 68° is shifted relative to the angle of incidence (60°), indicative of parallel momentum increase or refraction due to the deep molecular adsorption well for CO. Surface phonon - molecule interaction is responsible for broadening and asymmetry of the distribution in Fig. 37. 0.25

Intensity [a. u. ]

0.20

CO covered, Tg = 273 K clean, Tg = 273 K clean, Tg = 550 K

Ei = 0.8 eV

0.15

0.10 ×10

0.05

0 30

40

50 60 70 Scattering angle qf [deg]

80

90

Fig. 37. Angle-resolved flux distribution of CO molecules scattered from Ru(0001) at 60° incidence angle. Surface temperature as indicated; [00Rie].

A comparison of CO adsorption on differently structured surfaces of the same material is of interest from the point of view of structure sensitivity of adsorption. Thermal desorption spectra for CO from Ru( 10 1 0 ) are seen in Fig. 38 [75Bon1, 89Lau]. Peaks at about 370 and 500 K are representing molecular CO adsorption and the similarity to the spectra in Fig. 33 suggest that structure sensitivity is not very significant. This observation is supported by the coverage dependent activation energy of desorption evaluated from a line shape analysis of the desorption traces [89Lau]. The energy data in Fig. 39 for Ru( 10 1 0 ) are in the same range as those for the Ru(0001) surface in Fig. 36a. La ndolt-Bö rnstein New Series III/4 2A4

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3.7.1 CO and N2 adsorption on metal surfaces 200

b2

CO partial pressure

b1

Activation energy of desorption E des [k J / mol ]

b3

150

100

50 Exposure [L] 5.0 2.5 2.0 1.2 0.8 0.5 0.4

300

400 500 Temperature T [K ]

600

0

0.25

0.75 0.50 CO coverage q [ML]

1.00

1.25

Fig. 39. Activation energy of desorption versus CO coverage, derived from line shape analysis of TDS traces; [89Lau].

Fig. 38. Thermal desorption spectra of CO from Ru( 10 1 0 ) showing three desorption states; [89Lau].

The interaction within a molecularly adsorbed CO layer at higher coverage is substantial. This leads to a continuous shift in vibrational frequencies with coverage, such as seen for CO on Ru(0001) in Fig. 40 [80Pfn]. The C-O stretch vibration frequency increases with increasing CO coverage from 1984 to about 2060 cm−1 at saturation of 0.67. The actual saturation density is here 1.11 × 1015 CO/cm2. The density for the well ordered (√3×√3)R30°-CO structure is 5.45 × 1014 CO/cm2. By comparison, measured frequencies of CO on the more open Ru( 10 1 0 ) surface also show a similar shift in stretch frequency with coverage, Fig. 41 [89Lau]. There are at least four well ordered structures of CO in this case, and if we calculate the absolute CO coverage for the (3×1) structure, for example, it is 5.2 × 1014 CO/cm2 and the C-O stretch frequency is 2019 cm−1. This compares very well with the absolute CO density of the (√3×√3)R30° structure on Ru(0001) and its frequency of 2021 cm−1. Similarly at saturation: the density on Ru( 10 1 0 ) is 1.06 × 1015 CO/cm2 at a frequency of 2062 cm−1 and on Ru(0001) it is 1.11 × 1015 CO/cm2 at a frequency of 2060 cm−1. Hence the registry of CO with the substrate surface seems to be only of secondary importance for CO-CO interactions.

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260

245 1984

T = 200 K

117

2021.5

q ~ 0.003 0.02 0.05 0.09 0.13

443

Ru (1010) / CO 100 K

2062

× 40 433

(41 12)

2048

1810

q 1.22

1%

2043

0.26 0.33 0.37

Intensity

IR absorption

0.18 433 440

0.39 0.48 0.56 0.61 0.65 0.67

2100

2050 2000 Wavenumber [cm -1]

1950

Fig. 40. Coverage dependence of the C-O stretch vibrational frequency of CO adsorbed on Ru(0001) at 200 K, measured by IRAS; [80Pfn].

2019

1.0

2010

443

(” 4×1”)

0.43 2060.5

(2×1) p2mg

453

(3×1)-2 CO

2000

0.75 0.6

70

(” 2×1”) p (3×1) 0

400

800 1200 1600 Energy loss [cm -1 ]

0.5 0.33 0 2000 2400

Fig. 41. Correlation of vibrational spectra (EELS) and CO overlayer structure on Ru( 10 1 0 ) at 100 K. Note coverages of CO on the right; [89Lau].

CO on Ru(0001) at the highest coverage forms the ordered structure designated (2√3×2√3)R30° [83Pfn1]. Orientation of CO and lateral intermolecular interactions were studied for this structure by ARUPS [85Hof]. Using p-polarized light at 35 eV energy and measuring the polar angle dependence of the intensity of the 4σ molecular orbital, not involved in the adsorption bond, yields the distribution shown in Fig. 42. Compared to theory, the experimental data are consistent with perpendicularly adsorbed CO. Recording the variation of molecular orbital peak positions with emission angle, using both p- and spolarized light, the two-dimensional band structure of the (2√3×2√3)R30° CO layer could be determined [85Hof]. The result is presented in Fig. 43 as a plot of binding energy versus parallel component of the electron wave vector (which is conserved in photoemission). There is a substantial dispersion in all orbitals but strongest in the 5σ orbital of CO which is involved in Ru-CO bonding. The dispersion gap is about 0.7 eV at the Γ -point. Corresponding energies for the 4σ and 1π orbitals are 0.4 eV and 0.5 eV, respectively. The dispersion indicates a large lateral interaction between CO molecules in this compact layer. Similar results for ordered CO layers of high density have been obtained for both the (√3×√3)R30° and the (2√3×2√3)R30° structures on Co(0001) [83Gre], shown in Fig. 29 and Fig. 30, respectively, and for CO/Ir(111) [80Sea, 81Sea], CO/Pd(111) [84Mir] and CO/Ni(110) [86Kuh]. The comparison of these data shows that the amount of dispersion is lower for the lower coverage structure. A more general correlation is given in Fig. 8 which shows a plot of the measured 4σ dispersion bandwidth versus CO nearest neighbor distance for all of these hexagonally close-packed ordered structures. It can be seen that a logarithmic dependence is obtained regardless of the substrate material [83Gre, 85Hof]. Hence the dispersion is greatly governed by CO-CO repulsive lateral interactions.

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3.7.1 CO and N2 adsorption on metal surfaces ~ 1π

7.2

~ 1π

4 s - peak intensity

Initial energy [ eV ]

7.4

tilt 20° 10° 0°

10°

0°

10° 20° 30° 40° 50° ~ Out of plane polar angle J

7.6

~ 5s

7.8

~ 5s

8.0

10.3

~ 4s

10.5

60°

Fig. 42. Intensity of CO 4σ peak as a function of polar angle for the (2√3×2√3)R30° structure on Ru(0001). The angle of incidence is 45°, p-polarized light at hν = 35 eV [85Hof]. The full and dashed lines are theoretical dependencies according to Davenport theory taken from [82Bar].

~ 4s

1.6 GM

0.8 H (H’)

0

0.8

1.6

H (H’)

2.4 GK

Parallel component of wavevector k II [Å - 1]

Fig. 43. Two-dimensional band structure of CO-related molecular orbitals along the substrate Γ Κ and Γ Μ directions of the (2√3×2√3)R30° structure on Ru(0001); [85Hof].

The interaction of CO with Ru field emitter surfaces has been studied by pulsed field desorption mass spectrometry (PFDMS) at a pressure of 1.3×10−6 mbar CO and temperatures of 328 K and 458 K. Probing the stepped region in the vicinity of the (0001) pole of the Ru field emitter, various ionic species have been detected, including singly and doubly charged subcarbonyls Ru(CO) n+ x (x=1 - 4). The intensities of these species as well as their relative abundances depended on the field strength, the repetition rate of the field pulses (field free reaction time) and the surface temperature. A consecutive reaction is considered involving chemisorbed Ru subcarbonyl molecules. These Ru(CO)x species reach steady surface concentrations at different relaxation times which depend on temperature. Ru(CO)2 is formed by an activated process. A reaction model is presented which describes the removal of Ru lattice atoms and their diffusion into the adsorbed layer via Ru(CO)2 formation as part of a process inducing morphological changes of the Ru emitter apex [86Kru]. Irradiation of a Ru(0001) surface covered with CO using intense femtosecond laser pulses (800 nm, 130 fs) leads to desorption of CO with a nonlinear dependence of the yield on the absorbed fluence (100 380 J/m2). Two-pulse correlation measurements reveal a response time of 20 ps (FWHM). The lack of an isotope effect together with the strong rise of the phonon temperature (2500 K) and the specific electronic structure of the adsorbate-substrate system strongly indicate that coupling to phonons is dominant. The experimental findings can be well reproduced with a friction-coupled heat bath model [00Fun]. Broadband IR sum frequency generated radiation (SFG) was also used to excite the fundamental and higher order stretch vibration of low-coverage adsorbed CO on Ru(0001) [00Hes1]. Due to the high intensity of the incident IR radiation the vibrational transitions υ = 0→1 at 1989 cm−1, 1→2 at 1962 cm−1 and even the 2→3 at 1936 cm−1 were excited at a CO coverage of 0.004 ML. The anharmonicity constant was determined to be 13.6±0.6 cm−1 from fitting the experimental spectra, in good agreemnet with the analysis of overtone spectra [98Jak1]. The dissociation energy of the adsorbed C-O bond was evaluated in the framework of a Morse potential as 9.18±0.03 eV (compared to 11.09 eV for gaseous CO) [00Hes1]. The excitation of the υ = 1→2 hot band depends on the CO coverage. As the coverage increases to 0.33, this band is no longer observed and the fundamental 0→1 transition shifts to lower frequencies. A single broadened band is detected at coverages above 0.025. This phenomenon is interpreted in terms of a local oscillator to delocalized phonon transition [00Hes2]. High-resolution infrared reflection absorption spectroscopy of CO on Ru(001) utilizing a continuously tunable infrared laser has been carried out at 85 300 K [86Hof]. A small shift in the CO stretch of 2.5 cm−1 was reported with increasing temperature. Lan dolt-Börn stein New Series III/4 2A4

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119

Investigations of CO adsorption on structurally more diverse surfaces, such as Ru( 10 1 0 ) [89Lau], Ru(1120) [80Car, 01Wan1, 01Wan2], Ru(1121) [03Fan] and stepped Ru(1,1,10) [85Shi1] showed a larger variety of adsorbed CO states but also CO dissociation at 300 K and elevated temperatures. CO adsorbed on Ru(1120) gives rise to two ordered structures, a p(1×2) and a (1×2)p2mg with increasing coverage [01Wan1]. The latter is characterized by a glide plane related to the zig-zag rows of Ru surface atoms and on-top bonded CO molecules on them. Whether the CO coverage of the (1×2)p2mg is 0.5 or 1.0 is not quite clear [01Wan1, 03Fan]. The C-O stretch of >1994 cm−1 supports an upright configuration of CO. A tilted CO species exhibiting a low stretch frequency of 1552 cm−1 and a bending mode at 690 cm−1 is observed at low coverage and temperatures up to about 240 K [01Wan2]. Dissociation of this adsorbed CO species is observed around 300 K by EELS [01Wan2]. A four-fold hollow adsorption site is suggested for this tilted CO species. The Ru(1121) with its multi-site surface is particularly interesting [03Fan]. Three molecular species, α1-CO, α2-CO and β-CO can be distinguished by EELS as well as TDS. β-CO is observable only at low coverage and characterized by a low stretch frequency of 1335 cm−1, thus typical for a molecule inclined towards the surface. This state dissociates at about 300 K. The weakly bound α-states have stretch frequencies in the range 1770 - 2050 cm−1 and are perpendicularly adsorbed. The α1-state converts to βCO at moderate total coverage and T >360 K, increasing the amount of dissociated CO [03Fan]. On the other hand, β-CO is destabilized at low temperature and increasing CO coverage by converting to α1-CO. Self-consistent density functional calculations have been carried out for the adsorption of O and CO as well as the dissociation of CO on Ru(001) [98Mav]. The effect of straining the surface on its chemical properties have been studied in particular. It was found that the surface reactivity increases with lattice expansion (up to 3 %), coupled to an upshift of the metal d-states. As a consequence, the energy of adsorption and the activation barrier for dissociation of CO decreases, thus increasing the amount of total dissociated CO [98Mav]. The underlying reason for the strain-induced effect is the shift in the center of the relatively narrow d-band. A more general relationship between the interaction strength of adsorbed species and the position of the center of the d-band is demonstrated for several transition metals and simple adsorbed species [98Mav]. Re CO adsorption on Re surfaces has been much less studied than on W or Mo surfaces [77Hou, 80Duc, 80Fuk2, 83Tat, 85Tat, 88Kel]. The adsorption on Re(0001) and a stepped basal plane by TPD, LEED and AES showed two α-states and one β-state. The coverage of the latter is dependent on surface structure and presumably dissociated [77Hou]. Molecular and dissociated CO states are also observed in TDS, as seen in Fig. 44 for a clean Re( 10 1 0 ) surface [88Kel]. This figure also demonstrates how adsorbed sulfur blocks efficiently adsorption sites for CO. The ability of Re to dissociate CO is particularly perturbed since β-states are eliminated faster than α-states, Fig. 44b. Spectroscopic data are not available in this case. CO adsorption on stepped Re surfaces has also been studied, with emphasis on the question of stepinduced dissociation of CO [81Duc]. The results turned out to be complicated and difficult to reproduce. The study of CO adsorption on the thin film system Re/Pt(111) is also of interest in this context because it demonstrates an increase in adsorption energy relative to clean Pt(111) and Re(0001) [99Ram]. Furthermore, the modification of a Re(0001) surface by metallic adsorbates and ist effect on CO adsorption has been studied [90He].

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3.7.1 CO and N2 adsorption on metal surfaces

CO/S/Re (1010)

qs [ML ] 0.95 0.78

0.10

CO TDS peak area [a. u.]

CO mass intensity

0.31

CO/S/Re (1010)

40

0.53

30 total area 20

10

b area only

0

200

a

600 400 Temperature [K ]

0

800

b

0.2

0.6 0.8 0.4 Sulfur coverage qs [ML]

1.0

1.2

Fig. 44 (a). Thermal desorption spectra of CO for clean and sulfur covered Re (1010) surfaces. Initial CO

exposure was constant for all traces. (b). Relative CO coverage versus sulfur coverage on Re (1010) illustrating a —poisoning“ effect of S; [88Kel].

Os A single study of CO adsorption on polycrystalline Os is known [80Fuk1]. Photoemission and desorption studies indicate that CO is molecularly adsorbed on clean Os. A desorption peak at 550 K is recorded. Adsorption of CO on a carbonized Os surface produces a desorption peak at 440 K and different CO photoemission peaks. Spectral data for the latter structure are consistent with molecular adsorption in which the CO axis is nearly parallel to the surface and both C and O are involved in bonding. 3.7.1.5 CO adsorption on simple cubic metal surfaces Mn No studies of CO adsorption on well defined poly- or single crystalline Mn surfaces are known. The reactivity of Mn for adsorbed CO can only be estimated from a few studies of CO adsorption on thin films of Mn deposited on carrier crystal surface, such as Fe(110) [93Sie], Ru(001) [87Hrb1, 87Hrb2, 90Hrb, 96Sha], Pd(100) [95Wu], and Cu(100) [99Grü]. At room temperature, Mn grows layer by layer on Ru(001). The first three layers are pseudomorphic with respect to the Ru substrate. At higher coverages, manganese layers adopt a commensurate structure with a (√3×√3)R30° LEED pattern. At coverages of more than 10 layers, the growing layer of manganese becomes disordered. These structures are thermally unstable, and annealing to 700 K transforms them to a 1×1 pseudomorphic structure. A new, higher temperature CO desorption state at 650 - 800 K appears at submonolayer coverages. The indications are that added Mn increases significantly the propensity of CO to dissociate [90Hrb]. Details are briefly reported in the next section on binary systems. Lan dolt-Börn stein New Series III/4 2A4

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3.7.1.6 CO adsorbed on relevant binary systems, modelled by ultra-thin metal overlayers The properties of adsorbed CO on some of the binary systems listed in Table 1 are included in the data tables section. Much of the experimental work has been reviewed in several papers [91Rod3, 91Rod4, 92Cam, 92Rod, 93Kuh2]. CO adsorption on compound or alloy systems has also been investigated, such as on surfaces of NbC(111) [92Eda], Pd3Mn [99Del] and Pt3Me where Me stands for Ti, Co or Sn [86Pau1, 92Ros]. Because the investigations on thin film bimetallic surfaces are of interest in connection with CO adsorbed on clean metal surfaces, additional results will be briefly presented below. HREELS and Auger electron spectroscopy (AES) have been utilized to study CO adsorption and dissociation on copper deposited on an Al(111) single crystal [93Col]. Exposing Cu/Al(111), possessing approximately one-half monolayer of copper, to CO produces a species which exhibits a weak vibrational mode with an unusually low CO stretching frequency of 1260 cm−1. The low-frequency CO mode indicates that this species possesses a severely weakened C-O bond. This is verified by thermal CO dissociation upon heating to 348 K indicated by the formation of adsorbed carbon and oxygen on the surface. The thermal instability and the vibrational spectra of the low frequency CO species suggest that the CO is bound in a di-sigma structure, where both the C and O atoms are chemically bonded to metal atoms [93Col]. The vibrational properties of CO adsorbed on fcc Fe(100) thin films, grown at 300 K on Cu(100), were studied using IR reflection absorption spectroscopy [99Tan]. At low exposure only a single C-O stretch band appears at 1920 cm−1 which shifts to 1998 cm−1 with increasing coverage. At higher exposure a second C-O stretch band is observed at 2020 cm−1 which increases in intensity and shifts to 2048 cm−1 at high coverage. The initially observed band can be ascribed to the bridging CO and the second band to atop CO. Although the bridging CO was stable up to about 380 K, the atop CO band was less stable and disappeared above 313 K [99Tan]. The adsorption of CO on Fe monolayers on W(110) was studied by thermal desorption, XPS, UPS, work-function and electron-impact measurements [97Nah]. The clean Fe layers deposited/annealed at 90 K and 600 K show very different work functions, but they behave similarly in CO adsorption. The latter is qualitatively similar to clean W(110). CO is molecularly adsorbed at 90 K to a coverage of CO/W = 0.8 - 0.85. Heating leads to desorption of part of the CO with peaks at 320 K for the 600 K Fe layer and 250 and 340 K for the 90 K layer. Also an amount of CO/W = 0.27 converted to dissociated CO, indicated by a shift in the O 1s binding energy from 531.8 eV for virgin CO to 530.1 eV for dissociated CO. UPS spectra of CO for the two Fe layers are very similar and do not differ appreciably from those of CO on W(110). All results indicate that the properties of adsorbed CO on these Fe films are quite similar to CO on W(110) [97Nah]. Adsorption of carbon monoxide on the c(8×2) phase of Mn/Cu(100) at 100 K was studied by means of UPS, work function change measurements, and LEED. Two stages of adsorption can be separated. The first type of molecularly adsorbed CO does not change the substrate structure. No adsorbate-induced superstructure can be observed, and the work function is increased by 0.9 eV. Further adsorption leads to a loss of long range order of the Mn film, and the CO-metal interaction as revealed by UPS is changed, accompanied by a decrease of the work function by 0.16 eV. It is not possible to re-establish the ordered film structure by annealing. No dissociation of CO occurs below 200 K [99Grü]. The interaction of vapor-deposited Mn with the Ru(0001) surface and its effect on CO chemisorption were studied by Auger electron spectroscopy, thermal desorption spectroscopy, isotopic exchange, and LEED [87Hrb2, 90Hrb]. Layer by layer growth of Mn is observed at room temperature. The first three layers are pseudomorphic with respect to the Ru substrate [96Sha]. At higher coverages, Mn layers adopt a commensurate structure with a (√3×√3)R30° LEED pattern. At coverages of more than 10 ML, the growing Mn layer becomes disordered. Disordered and (√3×√3)R30° structures are thermally unstable, since annealing to 700 K transforms them to a pseudomorphic (1×1) structure, together with a large decrease in the Mn/Ru AES ratio. A new, higher temperature CO desorption state at 650 - 800 K appears at Mn coverages of 0.3 - 1 ML. Coadsorbed CO isotopes show random mixing on Mn/Ru which together with the high temperature desorption state is indicative of CO dissociation [90Hrb].

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3.7.1 CO and N2 adsorption on metal surfaces

Table 1. CO adsorption studied on model binary metal systems, prepared as a thin film of metal A on an oriented single crystal of metal B. Metal A or B belong to the category of metals reviewed here in context of CO adsorption properties. Binary system overlayer/ substrate Co/Mo(110) Cr/Ru(001) Cu/Al(111) Cu/Mo(110) Cu/Re(0001) Cu/Ru(0001) Cu/Ta(110) Fe/Cu(100) Fe/Mo(110) Fe/Ru(0001) Fe/W(110) Fe/W(110) Fe/W(100) Fe/Re(0001) K/Fe(110) Mn/Cu(100) Mn/Fe(110) Mn/Pd(100) Mn/Ru(0001) Ni/Mo(110) Ni/Ru(0001) Ni/W(110) Pd/Co(0001) Pd/Re(0001) Pd/Ru(0001) Pd/Ta(110) Pd/W(110) Pt/Co(0001) Pt/Ti(111) Pt/W(110) Re/Pt(111) Sn/Ni(111) Sn/Pt(111) Ti/Nb (poly) Zn/Ru(0001)

Maximum coverage of overlayer [ML] 1.0 - 5 0.1 - 0.5 0.5 ∼5 ∼ 40 8 1.5 2 1 1 1 <0.4 0.9 <2 3.5 1.5-6 Theory Theory 3.1 ∼8 Theory 3 Theory 0.6 Theory Theory 3 1.85 ∼1 0.4 alloy 1.0

Data in tables (T) References or detailed (D) below 91He2, 91He3 02Eng D, T 93Col 92Kuh2 91Rod1 T 91Rod2, 99Kne1 93Kuh1 D, T 99Tan 90He T 88Ega T 90Ber D, T 97Nah 90Ber 90He T 79Bro, 92Zhu D, T 99Grü T 93Sie 95Wu T, D 90Hrb 91Cam, 91He3, 93He 96Ham 90Rod 98Pic 92Cam 92Cam 96Ham D 90Koe, 93Sel, 95Pic1 90Kuh, 92Kuh2 95Pic2 T 00Cab 97Pic 95Pic2 T 88God T 99Ram D 95Xu D 94Xu T 98Yos D 93Rod

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3.7.1 CO and N2 adsorption on metal surfaces

123

A study of CO adsorption on Mn/Fe(110) using XPS as a means to distinguish molecular and dissociated CO on the surface provided evidence for the promotion of CO dissociation by Mn [93Sie]. This effect was seen at several Mn coverages. The onset of dissociation could be located at 200 - 300 K, depending on Mn coverage, and was considerably lower than about 380 K observed for clean Fe(110) [79Bro]. The total amount of dissociated CO at 523 K was 14 and 80 % of the initial coverage for clean Fe(110) and Mn/Fe(110), respectively [93Sie]. CO chemisorption properties of Pd monolayers and ultrathin Pd films of various thicknesses on Ta(110) have been investigated using TDS, HREELS and LEED [93Sel]. Using Arrhenius plots constructed from CO desorption data, the activation energies of CO desorption on these films were determined. On the pseudomorphic Pd monolayers with coverages of 0.6 - 1.0, CO desorbs with an activation energy of approximately 63 kJ/mol. This value is in contrast to 150 kJ/mol measured on the bulk Pd(111) surface and indicates a strong alteration in the CO chemisorption properties at low Pd coverage. For Pd films annealed to 550 K, the CO desorption energy increases with the size of the Pd clusters, which form on top of the Pd monolayer. HREELS, used to probe the CO adsorption site on these Pd films, shows that CO adsorbs predominantly in atop sites on the pseudomorphic and the fcc(111) Pd monolayers. A small amount of bridge adsorbed CO is detected which corresponds to CO molecules adsorbed on small Pd clusters. This is consistent with LEED observations which indicate that the fcc(111) structure is reconstructed to the pseudomorphic structure upon exposure to CO [93Sel]. The chemisorption of CO on a Sn-covered Ni(111) surface (Sn coverage 0.33) has been studied with reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption. Formation of the (√3×√3)R30°-Sn/Ni(111) surface strongly suppresses CO adsorption. Only 0.04 ML CO can be chemisorbed on this surface at 110 K, exclusively at atop sites. The binding energy of this adsorbed CO is reduced to only about 43 kJ/mol. Additionally, no significant effect of subsurface Sn on CO chemisorption was observed. These results are in sharp contrast to a previous study of CO chemisorption on the (√3×√3)R30°-Sn/Pt(111) surface [94Xu], where the chemisorbed CO saturation coverage, adsorption site distribution and desorption temperature were quite similar to those of the Pt(111) surface. The kinetics is strongly influenced by Sn, indicative of a modified (physisorbed) precursor state for chemisorbed CO. Whereas the sticking coefficient of CO on clean Pt(111) is about 0.9 at 100 - 300 K, it decreases to 0.2 at 250 K with 0.33 Sn coverage [94Xu]. The influence of Sn on CO chemisorption is ascribed to repulsive Sn-CO interactions. The differences in the chemisorption properties of Pt-Sn and NiSn surface alloys are rationalized by considering the different sizes of the surface unit cells and the location of Sn with respect to the surface plane [95Xu]. The adsorption of CO on Zn/Ru(0001) surfaces has been studied using thermal desorption mass spectroscopy, core and valence level photoemission and Auger spectroscopy. At 80 K, CO does not adsorb on thick Zn films under ultra-high vacuum conditions [90Car, 93Rod]. In contrast, adsorption is observed on Zn atoms bonded to Ru(0001). Bimetallic bonding induces a reduction of approximately 0.5 eV in the binding energy of the core and valence levels of Zn. This shift probably increases the strength of the Zn-CO bond. CO molecules adsorbed on Zn adatoms show an O(1s) binding energy close to 535.2 eV and a desorption temperature of approximately 125 K. Both properties indicate a very low energy of adsorption and a very weak contribution of π-backbonding to the ZnC-O bond. For CO chemisorption on Zn/Ru(0001) surfaces, Zn blocks Ru sites approximately on a one-to-one basis and induces a weakening of the Ru-CO bond [93Rod]. By comparison, CO adsorption on polar ZnO surfaces, either Zn or oxygen terminated, is relatively weak, with adsorption energies between 29 [00Bec] and 52 kJ/mol [80Gay] for both surfaces.

La ndolt-Bö rnstein New Series III/4 2A4

References for this document 63Kle 64Kle 64Oma 66Haa 66Tuc 67Lew 69Edm 69Gra 69Tra1 70Ert 70For 71Eas 71Gra 71Koh 72Bon2 72Pri 72Tra1 72Tra2 73Chr 73Koh 73Mad1 73Mad2 73Mad3 73Tay 73Wac 73Wei 74Che 74Chr 74Con1 74Doy 74Lam1 74Lam2 74Mad 74Yat3 75Bon1 75Bon2 75Bru 75Con 75Fal 75Lam 76Bro 76Chr 76Com1 76Com2 76Com3 76Con 76Dem 76Hag 76Küp 76Llo 76McE1 76Plu 76Rho 76Wil

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98Gra2 98Hel 98Jak1 98Jen 98Jon 98Kuz 98Mav 98Pic 98Sha1 98Sha2 98Str 98Ven 98Yos 99Bar 99Beu 99Car 99Del 99Eng 99Föh1 99Grü 99Jon 99Kat1 99Kat2 99Kne1 99Kne2 99Lau 99Lun 99Ram 99Tan 99Vel 99Wol 00Bec 00Cab 00Cer 00Dvo 00Föh1 00Föh2 00Föh3 00Fun 00Hes1 00Hes2 00Lah 00Lee 00Rie 00Sau 00Sur 00Yag

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00Zas1 00Zas2 01Bon 01Hir 01Kun 01Now 01Pet 01Sme 01Sta 01Tho 01Wan1 01Wan2 01Wit 02Bou 02Eng 02Hei 02Kat 02Ros 02Unt 03Fan 03Gra 03Wal

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124

3.7.1 CO and N2 adsorption on metal surfaces

3.7.1.7 Adsorption of N2 on metals The nomenclature of adsorbed nitrogen states is traditionally different from that of adsorbed CO. At least four different states have been found, e.g. on Fe(111) [85Tsa, 86Whi1, 86Whi2, 87Fre, 87Gru1, 87Gru2], which are designated as follows (in order of increasing adsorption energy): δ-state: physisorbed N2 γ-state: chemisorbed σ-bonded N2 (oriented vertical to the surface) α-state: chemisorbed π-bonded N2 (oriented nearly parallel to the surface) β-state: atomic nitrogen N. Depending on the availability of adsorption sites, several sub-states (e.g. of β and γ) may be distinguished. The PES should therefore exhibit at least four different minima, one for each state, with corresponding barriers between them [87Gru2]. Ni On the Ni(100) surface N2 adsorbs with the molecular axis normal to the surface (γ state), which leads to two inequivalent N atoms, which have been separated by XPS [90Mar]. Earlier XPS studies mostly focussed on the significant shake-up intensity accompanying the XPS main lines [81Bru, 84Umb, 90Rao]. The valence electronic structure of chemisorbed N2 has been determined with UPS [81Bru], in particular the c(2×2) structure with UPS [84Dow], and XES [98Ben]. The c(2×2) structure at 0.5 ML coverage has been established by LEED [84Dow, 84Gru3] and the thermodynamic properties as a function of coverage have been determined [84Gru3]. Desorption spectra of N2 from Ni(111) are shown in Fig. 45, starting at about 22 K, with increasing coverage [86Bre]. At low coverage a chemisorbed N2 state is occupied first, recognized by the desorption peak at 88 K. This peak shifts to lower temperature with increasing coverage and a second peak at T = 45 K indicates a physisorbed N2 state. Further exposure leads to even another group of peaks at 28 - 30 K which are attributed to a second and third layer of physisorbed N2 molecules (condensed solid phase) [86Bre]. The peak temperatures of 45 and 85 K correspond to adsorption energies of about 10 and 20 kJ/mol, respectively, based on a first order desorption process and a pre-exponential factor of 1 × 1013 s−1. Heating the N2 covered surface to 60 - 70 K allows to form a pure chemisorbed phase and thus a separate spectroscopic characterization of both phases. Vibrational spectra of adsorbed N2 support the general assignment of physisorbed and chemisorbed attributes to these adsorbed molecular states because the N-N stretch vibrational frequency is almost identical to that of gaseous N2 for physisorbed N2 while it is about 130 cm−1 lower for the chemisorbed species [92Shi]. On the Ni(111) surface two LEED patterns have been observed. At 0.33 ML coverage a (√3×√3)R30° structure [91Yos] and at 0.5 ML coverage a (2×2) structure [89Qui]. The thermodynamic properties have been determined as a function of coverage [86Bre, 91Yos] as well as the vibrational evolution [89Qui, 91Yos]. The valence electronic structure has been determined with UPS [86Bre, 88Umb, 90Rao] and the N1s core levels with XPS [86Bre, 88Umb]. N2 adsorption on the Ni(110) surface forms a (1×1) structure between 0 - 0.3 ML coverage above 140 K [83Gru1, 84Gru3], a (2×1) structure at 0.5 ML coverage, a fluid phase between 0.4 and 0.72 ML coverage and a c(1.4×2) at 0.72 ML saturation coverage [80Gol, 83Gru1, 83Jac1, 84Gru3, 88Kuw]. Their thermodynamic properties [80Gol, 83Gru2, 83Jac1, 84Gru3, 88Kuw] and their vibrational behaviour [82Ban, 82Hor, 86Bru, 86Str, 87Bru, 88Kuw, 89Sch] have been determined. The XPS data of adsorbed N2 on Ni(100) have already been discussed in section 3.7.1.1.3 in context with Figs. 2 - 4 which show N1s core level spectra in detail. These spectra support the perpendicular orientation of N2 on Ni as well as its relative weak chemisorption. Furthermore, the system at saturation coverage has been investigated with UPS [82Hor, 90Rao] and XPS [80Gol, 90Rao].

Landolt-Börnstein New Series III/42A4

3.7.1 CO and N2 adsorption on metal surfaces ps 2

125

N 2 - TPD

ps 3 CS ps 1

N 2 mass intensity

Exposure [L] 31

23

14 12.5 11 9.4 4.7 1.5 20

40

60

Fig. 45. Thermal desorption spectra of N2 adsorbed on Ni(111) for different exposures (equivalent to coverages) at 20 K. All peaks shown are due to molecular N2 labelled ps1ps3 = physisorbed N2 , cs = chemisorbed N2 . Exposures are given in 1 × 1014 molecules/cm2; [86Bre].

80 100 Temperature [K ]

Rh On Rh surfaces TDS has identified three desorption features [83Hen]. Field emission microscopy indicates structure sensitive adsorption and the activation energies of adsorption have been determined [83Hen]. Pd Adsorption of N2 on Pd films [68Kin] and on the Pd(110) surface [87Kuw] has been reported and characterized with TDS. On the Pd(110) surface a (2×1) LEED structure has been observed and vibrational spectroscopy indicates a perpendicular (γ state) orientation [87Kuw]. On the Pd(111) surface, photoemission spectroscopy indicates only physisorption [82Hor]. Ir Early work using field emission microscopy showed indications of crystal-face specific adsorption of N2 [83Hen]. On the Ir(100)-(1×1) and the Ir(100)-(5×1) surfaces N2 adsorption has been studied with IRAS, TDS and LEED [93Gar]. N2 appears to adsorb in on-top sites without lifting the clean surface reconstruction [93Gar]. On the Ir(111) surface reversible adsorption has been observed with EELS and TDS [90Cor]. The Ir(110)-(1×2) surface has been investigated by XPS, UPS, AES, LEED and TDS [81Ibb]. In particular a p1g1(2×2) LEED pattern has been observed [81Ibb].

Landolt-Börnstein New Series III/42A4

126

3.7.1 CO and N2 adsorption on metal surfaces

Pt The adsorption of N2 on the Pt(111) surface originally reported [77Shi2, 79Ris] based on EELS and TDS was in subsequent investigations found to be due to defect sites [96Aru]. Nevertheless, condensed layers of N2 form ordered structures [00Zep]. Fe

Adsorbed N 2 molecules [×10 14 cm -2 ]

The adsorption of N2 on Fe surfaces has been studied intensely because of its relevance to the technical catalytic production of NH3 [80Ert, 81Ert, 85Sto]. The rate of dissociative chemisorption on low-index Fe surfaces is very slow, with the fastest rate reported for the Fe(111) plane [77Boz1, 77Boz2]. This surface behaves differently from other well studied metal surfaces, such as Ru(0001), Re(0001), Ni(100), Ni(111) and W(110). The bcc Fe(111) surface has three differently located surface atoms (A, B and C with 4, 6 and 7 nearest neighbors) and hence there is a variety of adsorption sites. Two different types of chemisorbed N2 have been detected on this surface, the γ-state being an upright species and the α-state being a π-bonded or at least severely tilted species [82Ert2, 84Gru1, 85Tsa, 86Whi2, 87Fre]. Their desorption temperatures are 110 and 145 - 160 K, respectively, and signify rather different energies of adsorption. The γ-state tends to convert to the α-state of N2 at 110 K while the latter is known to dissociate to atomic N at T >150 K. Fig. 46 shows a set of adsorption isotherms for the equilibrium between α-state and gaseous N2 which is evaluated to yield the isosteric heat of adsorption of 31.4 kJ/mol [82Ert2].

0.3 140 K 145 K 0.2

152 K 156 K

0.1

0 3 4

6 8 10 6

2 3 4 6 8 10 5 2 3 N 2 partial pressure PN 2 [Torr ]

4

6 8 10 4

Fig. 46. Adsorption isotherms (N2 coverage versus pressure) for the equilibrium N2,gas ⇔ N2,ad (α-state) on Fe(111) [82Ert2]. The isosteric heat of adsorption evaluated from this data is 31.4 kJ/mol.

A theoretical 2D potential energy diagram for N2 on Fe(111) has been calculated using a LCAO formalism [85Tom]. An interesting result is the region illustrating the transition between γ- and α-states, Fig. 47, with an activation energy of about 20 kJ/mol [84Gru2]. The same figure gives also the N-N stretch vibrational frequencies [84Gru1, 85Tsa, 86Whi2] and heats of adsorption of the two states [87Fre]. Detailed descriptions of molecular and dissociative N2 adsorption on Fe(111) and the conversion from δ to γ and from γ to α to β state are given in [87Gru1, 87Gru2]. The well depths (heats of adsorption) of the molecular N2 states are 20.7, 24.5 and 31.4 kJ/mol for δ, γ and α states, respectively. The activation energies for the conversion from δ to γ, from γ to α and from α to β are about 16.6, 18.5 and 28 kJ/mol, respectively [87Gru2]. These are approximate numbers since the actual PES is quite complex because of the additional degrees of freedom of the N2 molecule.

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127

90° Tilt angle with respect to surface normal [ deg ]

a

nNN :1490 cm-1 E ad :31 kJ mol

Eact :~20

g 0

C

N N

kJ mol nNN :2100 cm-1 E ad :24 kJ mol

0°

A B

N2

B A C

Fig. 47. Schematic two-dimensional potential energy surface for N2 on Fe(111) [85Tom] illustrating the transition between molecular γ and α states. The insets show the adsorption energies [84Gru2] and vibrational stretch frequencies [85Tsa, 86Whi2] as well as the activation energy of conversion [84Gru2, 87Fre].

Position on Fe (111) surface

Despite the existence of two well defined molecular N2 chemisorbed states (γ and α), the initial sticking coefficients of dissociative adsorption on Fe(111), Fe(110) and Fe(100) are lower than 10−6 at 300 K and above [77Boz1, 77Boz2]. However, the initial rates of adsorption are not equal and were reported as 60:3:1 in the sequence of (111), (100) and (110) [77Boz2]. Increasing the crystal temperature leads to a decrease of the sticking probability, Fig. 48 [87Ret2]. Such a behavior suggests the existence of a molecular precursor state and an activation barrier for the dissociative adsorption of N2. Increasing the kinetic energy of the incident N2 molecules greatly enhances sticking, as seen in Fig. 49 for Fe(111) at a surface temperature of 520 K [87Ret1, 87Ret2]. At 100 kJ/mol and above the sticking coefficient is about 0.1, an increase of five orders of magnitude. There is evidence that dissociative adsorption of N2 on this surface goes via the α-state as precursor since population of this state increases also with increasing kinetic energy of N2. This behavior is in contrast to N2 adsorption on W(110) where no precursor seems to be involved [87Ret2]. Additional studies with the adsorption of vibrationally excited N2 molecules (nozzle heated to 2000 K, 15 % υ = 1 and 3 % υ = 2) at higher kinetic energy showed that only a small increase in sticking was observed due to the additional vibrational energy [87Ret1]. The interaction between adsorbed N atoms on Fe(100) has been studied by high resolution STM at 298 K [99Öst]. There is a tendency to form small islands of c(2×2)-N order due to nearest neighbor repulsion and next-nearest neighbor attraction. These interactions are quantitatively assessed by the technique of configuration distribution analysis [99Öst]. The effect of coadsorbed alkali metals on N2 adsorption on Fe surfaces has also been studied [82Ert1, 85Tsa, 86Whi2, 88Rao]. Altered adsorption sites and enhanced dissociative adsorption rates, coupled with changes of the adsorbed molecular precursor state, have been observed. In this context, the rate of ammonia synthesis by promoted Fe, for which the dissociative adsorption of N2 was found to be most important, is also increased considerably [86Bar1, 88Str]. In this context it is of interest to draw attention to a study of N2 adsorption on Cr-Fe alloys of (110) orientation [88Dow]. The initial sticking coefficient of N2 on Cr(110) is very high compared to that on Fe(110). On the Fe72Cr28(110) alloy N2 sticking is considerably impeded whereas surface Cr forms some nitride phase.

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3.7.1 CO and N2 adsorption on metal surfaces

-2.0

0 -1

-2.5

-2

Ln ( So )

Log10 ( So )

-3.0

-3.5

-4.0

-4 -5

-4.5 -5.0

-3

-6

0

8 2 4 6 Reciprocal surface temperature 1/ T [×10 3 K -1 ]

10

Fig. 48. Log-plot of initial sticking coefficient S0 of dissociative N2 adsorption on Fe(111) versus reciprocal surface temperature, at normal incidence and a kinetic energy of N2 of 1.05 eV; [87Ret2].

-7

0

1

2 3 Kinetic energy [eV ]

4

5

Fig. 49. Initial sticking coefficient S0 of dissociative N2 chemisorption on Fe(111) versus kinetic energy of incident molecules at normal incidence and a surface temperature of 520 K; [87Ret2].

W Historically, one of the first extensive investigations of N2 adsorption on well defined surfaces was carried out on (110), (100) and (111) planes of tungsten [61Ehr1, 61Ehr2, 62Ehr1, 65Del, 88Ehr]. This work was also the first clear demonstration of structure sensitivity in adsorption. Adsorption at 300 K was mostly dissociative on W(100) and W(111) (β-state, heat of adsorption about 315 kJ/mol) [65Del] but no sticking was observed on W(110) [79Bes, 79Pol, 82Liu]. A unique quantitative demonstration of the structure sensitivity of N2 adsorption in the vicinity of the W(110) orientation was carried out by Besocke et al. who studied a W(110) single crystal which exhibited a central region of ideal (110) and four vicinal regions, Fig. 50, two tilted toward the (100) face having steps running in [001] direction, and two tilted towards the (112) face having steps running in [ 1 10] direction [79Bes]. Tilt angles were 10° and 5° in each case. The step densities of the vicinal orientations varied between 8×106 cm−1 and 4×106 cm−1. Exposing this crystal to N2 at 300 K showed that no adsorption could be detected on the (110) surface (step density <2×105 cm−1) but significant amounts on the vicinal surfaces. On the latter the amount was proportional to the step density (or the number of step sites), see Fig. 50 [79Bes]. Once the step sites were filled, adsorption came to a halt. At elevated temperature of 920 K, filling of the terrace sites occurred by diffusion of atomic N from the step sites. A weakly bound γ-state on W(110) and W(100) was found at lower temperatures of 100 - 130 K, with an estimated heat of adsorption of 38 kJ/mol [65Del]. This γ-state was thought to be atomic. It was later identified as a molecular state [79Fug]. The situation for the W(111) plane was more complicated because adsorption/desorption at 110 - 220 K provided evidence for three nitrogen states: γ, α and β (in the sequence of increasing desorption temperatures). The α-state was suggested to be molecular N2 based on its dipole moment [65Del]. The heats of desorption were 38, 67 and 314 kJ/mol for γ, α and β-states from W(111), respectively. The α-state, typical for the (111) orientation, has also been found on polycrystalline tungsten [62Ehr2]. This early work proved to be pace-setting for all later studies of N2 adsorption on metal surfaces. Spectroscopic investigations of N2 adsorption on tungsten single crystal surfaces clarified to a large extent the nature of the various adsorbed states (see Tables).

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129

0.5

Coverage qN [ML ]

0.4

0.3

Fig. 50. Plot of nitrogen coverage versus exposure for dissociative adsorption of N2 on W(110) (Ο) and four vicinal W surfaces at room temperature [79Bes]. Symbols for the vicinal surfaces: ! steps parallel to [001] , 10° miscut; ■ steps parallel to [001] , 5° miscut; ∆ steps

0.2 0.1

parallel to [ 1 10] , 10° miscut; ▲ steps parallel to [ 1 10] , 5° miscut. 0

2

4

6 8 N2 exposure [ L]

10

12

Ru A comprehensive study of the coverage dependence of the heat of adsorption/desorption, pre-exponential factor and sticking coefficient of N2 on Ru(0001), analogous to CO on the same surface [83Pfn1, 83Pfn2], has been carried out by Menzel et al. [83Men]. The results are summarized in Fig. 51. There are essentially three coverage regimes to be noted: in the first regime up to θ = 0.16, the heat of adsorption is 38 - 42 kJ/mol, with a pre-exponential factor of 2 × 1014 s−1, and the sticking coefficient at 0.38; the second regime is characterized by an increase in the adsorption energy, pre-exponential factor and sticking coefficient; in the third regime all quantities are observed to drop to much lower values. Regime 1 is a pure phase of chemisorbed N2 (γ-state), regime 2 a mixture of chemisorbed and physisorbed N2 (δstate), and the adorption/desorption data in regime 3 are dominated by the physisorbed N2 which has a significantly lower heat of adsorption. The sticking coefficient data are separated into γ and δ parts. A pure physisorbed phase can not be obtained. The dissociative adsorption of N2 on Ru(0001) has been widely studied [00Jac]. Initial reports of low sticking at 300 K were confirmed by more recent investigations [96Die, 99Dah]. The presence of an activation barrier to dissociative adsorption, suspected on the basis of low sticking coefficients, was confirmed by checking the kinetic energy dependence of sticking [97Rom]. An increase in kinetic energy up to 4 eV caused an increase in the sticking coefficient of more than four orders of magnitude. Dahl et al. measured the sticking coefficient of N2 dissociative adsorption on Ru(0001) as a function of substrate temperature, Fig. 52, and found an increase with rising temperature, equivalent to an activation energy of 39 kJ/mol [99Dah]. The sticking coefficient at room temperature was about 2 × 10−12, in good agreement with Dietrich et al. [96Die]. To find out whether residual steps on the surface were more active than the perfect terrace, they deposited a small amount of Au onto this crystal. Au is known to diffuse to and bind at the step sites, making these sites inactive for N2 dissociation. As a result, the measured sticking coefficient of N2 dropped by more than seven orders of magnitude (at 500 K), suggesting the clean Ru(0001) terraces to be even more inert towards N2 dissociation than hitherto believed [99Dah]. A density functional calculation of the activation barriers for N2 dissociation at terrace and step sites, shown in Fig. 53, supports the experimental results. The dynamics of collision-induced desorption (CID) of molecular N2 from Ru(0001), exposed to hyperthermal rare gas colliders generated in a supersonic atomic beam source, have been studied [98Rom]. As a non-thermal desorption process, however, it will not be discussed here.

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3.7.1 CO and N2 adsorption on metal surfaces

Isosteric heat of adsorption energy of desorption E [kJ/mol ]

130

( ) 42 38 34 30 26

a

Pre -exponential factor k o [S -1 ]

10 16 1014

10 12

Sticking coefficient S

1010

b

0.6

st

0.4 0.2 sd

0 0

0.1

sg

0.2 0.3 Coverage q [ML ]

c

Fig. 51. Low temperature adsorption of N2 on Ru(0001): Coverage dependence of (a) isosteric heat of adsorption (x) and energy of desorption (•), (b) pre-exponential factor of desorption, (c) sticking coefficients into physisorbed δ and chemisorbed γ molecular N2 states. st is the total sticking coefficient; [83Men].

0.4

Ru (0001) E a = 0.4 ± 0.1 eV

log S 0

-11

-12

-13

1-2% Au on Ru (0001) E a = 1.3 ± 0.2 eV

Potential energy change DE [eV ]

-10

TS

2.0 1.0 0

N 2,ad

2N ad

-1.0

-14

0.0020 0.0030 0.0035 0.0015 0.0025 Reciprocal substrate temperature 1/ T [1/K ]

Fig. 52. Initial sticking coefficient of dissociative adsorption of N2 on clean and Au-covered Ru(0001) as a function of reciprocal substrate temperature [99Dah]. The Au coverage of 1 - 2 % is so low that only residual step sites are believed to be occupied. Activation energies of adsorption are obtained from the slopes. The open cicle is a separately measured point at room temperature; [96Die].

Fig. 53. Results of a density functional calculation of potential energies of adsorbed N2 on Ru(0001) in its molecular state, transition state (TS) and dissociated state, 2N. The upper curve represents the dissociation on a flat terrace, while the lower curve is the result of a similar calculation for a step site. Configurations are shown in each case. Note the large differences in activation barrier of dissociation; [99Dah].

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131

Re Similar to iron, rhenium is a very active catalyst for ammonia synthesis [82Spe1, 82Spe2, 84Ass, 87Haa1, 87Haa2]. Adsorption of N2 on various Re surfaces was shown to be structure sensitive [87Haa1] and a corresponding striking structure sensitivity was demonstrated for ammonia synthesis at 870 K where a ratio of 1:50:1700 was found for the (0001), ( 10 1 0 ) and ( 11 2 0 ) orientations of Re, respectively [84Ass]. The small activity of Re(0001) was even attributed to the edges of the crystal. Indeed, a very low sticking coefficient of about 10−5 for N2 has been reported at 300 K on Re(0001) [76Liu, 91Por]. A strong increase in the dissociation probability was found with increasing kinetic energy of incident N2 molecules, such as shown in Fig. 54 [91Por]. This behavior could be well reproduced by a quantum mechanical calculation assuming an activation barrier for N2 dissociation [90Hen, 91Por].

Dissociation probalility

10 - 1

10-3

10-5

10-7 0

0.4

0.8 Kinetic energy [eV ]

1.2

1.6

Fig. 54. Plot of dissociation probability (sticking coefficient) versus translational kinetic energy of incident N2 on Re(0001) at a surface temperature of 300 K [91Por]. The points are experimental data and the solid line is obtained theoretically for the υ=0 state. The dashed line represents a calculation for the υ=1 state, at a fixed total energy.

Activated dissociative adsorption Many investigations show that there is substantial evidence for activated dissociative chemisorption of N2 on close-packed surfaces of metals, e.g. on Fe(110) [77Boz1, 77Boz2], Ru(0001), [96Die, 97Rom, 99Dah], Re(0001) [76Liu, 82Liu, 90Hen, 91Por], W(110) [65Del, 71Tam, 75Sin, 76Yat2, 81Cos, 84Lee, 86Pfn] and possibly Mo(110) [72Mah, 88Ehr]. These systems are characterized by low initial sticking coefficients, such as shown in Table 2, but also by a strong dependence of the adsorption rate on incident kinetic energy and even incident angle of N2 molecules. Low sticking coefficients can only be measured reliably, if the presence of pre-dissociated N2 in the gas phase, e.g. due to interaction of N2 with hot tungsten filaments in a vaccum chamber [93Shi, 96Die], can be ruled out. In the particular case of Ru(0001), this effect caused a difference in the initial sticking coefficient of six orders of magnitude [96Die]. Experiments on Ru(0001) have demonstrated [99Dah] that a relatively higher rate of adsorption at step sites can be sufficient to cause an overall measurable rate of adsorption, even if the ideal step-free surface may possibly not be able to dissociate any N2. Similar effects had been noted on a Re field emitter tip of (0001) orientation where no adsorption of N2 took place on the perfect (0001) terrace as long as the temperature stayed between 300 and 550 K [68vOs, 76Liu]. At higher temperature the terrace was filled with atomic N via surface diffusion from the vicinal range.

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3.7.1 CO and N2 adsorption on metal surfaces

Table 2. Activated dissociative adsorption of N2 on densely packed surfaces. Substrate

Fe(110) Mo(110) Re(0001)

Crystal structure bcc bcc hcp

Initial sticking coefficient −7

<1 × 10 0.09 <1 × 10−5 9 × 10−6 4 × 10−5

Re(0001) Re(0001)

Temperature [K] 508 300 300 218 830 theory theory

Ru(0001) hcp Ru(0001)/Au Ru(0001) hcp

1 × 10−12 1 × 10−17

300 500 theory

W(110)

0.004 0.003

300 800

bcc

Activation energy of dissociation [kJ/mol] ∼ 29

14±2 58±14 73 55-167 site dependent 39 126 135-183 212 41-92

References

77Boz2 72Mah, 82Liu 76Liu, 82Liu 87Haa1 91Por 88Ass 90Hen 96Die, 99Dah 97Jac, 99Dah 98Mor2, 99Dah 97Rom 71Tam 84Lee, 86Pfn

Angle-resolved desorption of N2 from W(110) showed a non-cosine distribution, Fig. 55, indicative of an activation barrier for dissociative adsorption [81Cos]. Molecular beam studies with variable translational energy of impinging N2 molecules on W(110) have shown that the coverage and hence the sticking coefficient for dissociative adsorption rises with increasing kinetic energy [84Lee, 86Pfn]. A series of uptake curves of nitrogen taken for several kinetic energies up to about 100 kJ/mol are presented in Fig. 56 [84Lee]. Different binding states of nitrogen are being populated as the incident kinetic energy rises. This is best seen by the TDS traces of nitrogen with increasing coverage, Fig. 57, one set at low (4.2 kJ/mol) and one at high kinetic energy (104 kJ/mol). The corresponding dependence of the initial sticking coefficient versus the incident N2 energy is given in Fig. 58 [84Lee]. A second investigation of this kind also measured the adsorption rate at various incident angles relative to normal; the results are summarized in Fig. 59 [86Pfn]. In all of these studies the initial sticking coefficient of N2 for nearly thermal beams is 0.003. A rapid increase in sticking coefficient (note logarithmic scales) sets in at kinetic energies above 40 kJ/mol. The maximum activation barrier for dissociative adsorption was derived from the data at normal incidence, Fig. 60, and is found to be 92 kJ/mol [86Pfn]. Realistically, one expects a distribution of activation barriers of a width of about 50 kJ/mol. Hence the maximum activation energy is higher than the values given before [81Cos, 84Lee]. The finding of activated N2 adsorption on the close-packed W(110) surface is an example for the structure sensitivity of adsorption, because the variation of the sticking coefficient with coverage for various other tungsten single crystal surfaces, summarized in Fig. 61, illustrates the rather unique behavior of the (110) plane [75Sin].

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3.7.1 CO and N2 adsorption on metal surfaces 0°

q = 45° -7.5°

15°

0.6 N atom fractional coverage [ML]

7.5°

133

-15°

22.5°

-22.5°

30°

E K [kcal/mol ] 24.9

0.5

22.4

b1

12.6 9.5 7.0

0.4 0.3

2.04

b2

0.2

-30°

0.1 0 37.5°

0

-37.5°

45° 52.5°

200 400 600 N 2 exposure [Equivalent Langmuir]

800

Fig. 56. Uptake curves of N coverage versus N2 exposure on W(110) for various kinetic energies of the incident N2 beam, with the surface temperature at 820 K. The sticking coefficient and total amount of adsorbed N increase with higher kinetic energy; [84Lee].

-45° -52.5°

Fig. 55. Angle-resolved intensities of desorbing N2 molecules from a N covered W(110) surface, where the peak temperature of desorption is 1560 K. The non-cosine distribution is consistent with an activation energy of adsorption of 17.4 kJ/mol. The full line is the calculated desorption flux distribution based on this activation energy; [81Cos].

N 2 / W (110)

b2 E K = 2.04 [kcal /mol] q i = 45°

E K = 24.9 [kcal /mol] q i = 45°

b

N 2 mass intensity

N 2 mass intensity

b3

Exposure 600 L 300 L 150 L 175 L 50 L 25 L 1000 1200 1400 1600 1800 Surface temperature [K ] Landolt-Börnstein New Series III/42A4

b1 Exposure 180 L 60 L 30 L 10 L 4L 2L 0.5 L 1000 1200 1400 1600 1800 Surface temperature [K ]

Fig. 57. Thermal desorption spectra of N2 from a N covered W(110) surface as a function of exposure (coverage) for two different kinetic energies of the incident N2 molecular beam, angle of incidence 45°. (a) Ek = 8.54 kJ/mol, (b) Ek = 104 kJ/mol; [84Lee]. Note the occupation of several β-states for the higher incident kinetic energy.

134

3.7.1 CO and N2 adsorption on metal surfaces

0.20

1

N 2 beam energy [kcal /gmol ] 10 15 25 20

5

30

35

Initial sticking probability

0.18 Initial sticking probability

0.16 0.14 0.12 0.10 0.08

qi = 0° = 30° = 45° = 55° = 60°

10-1

10-2

0.06 0.04

10-3

0.02 0

0.2

0.4 0.6 0.8 1.0 N 2 beam energy [eV ]

1.2

0

50

1.4

Fig. 58. Initial sticking probability of dissociative N2 adsorption on W(110) as a function of N2 kinetic energy at 45° incidence; [84Lee]. The sticking coefficient at thermal energies was 0.0035. 0.4

100 150 Beam energy [kJ mol -1]

200

Fig. 59. Log-plot of initial sticking coefficient of dissociative N2 adsorption on W(110) versus kinetic energy of incident molecules for angles of incidence between 0 and 60°, with the surface temperature at 800 K. The dashed line indicates the predicted behavior for 60° incidence, based on normal energy scaling of the 0° data; [86Pfn].

0.8

0.3

qi = 0° Ts = 800 K

0.2

0.1

Sticking probability S

Initial sticking probability

0.9 Ts = Tg = 300 K

0.7 {310} 0.6 0.5

{100}

0.4 {320}

0.3 {411}

0.2 0.1 0

50

150 100 200 Beam energy [kJ mol 1]

250

Fig. 60. Initial sticking coefficient of dissociative N2 adsorption on W(110) versus kinetic energy of incident molecules for normal incidence and a surface temperature of 800 K. The dashed curve represents a Gaussian barrier height distribution corresponding to the probability of molecules dissociating at a given incident energy; [86Pfn].

{110}

0

{111}

4 6 2 8 Surface coverage [×1014 atoms /cm 2 ]

10

Fig. 61. Structure sensitivity of N2 adsorption: Comparison of sticking probabilities for dissociative N2 adsorption on various tungsten single crystal surfaces at room temperature. The lowest sticking coefficient is found for the densely packed W(110) surface while the highest is observed for the open (310) and (320) faces; [75Sin].

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Adsorption of N2 on W(100) is apparently also activated because the initial sticking coefficient decreases strongly with increasing surface temperature above 300 K [83Aln]. The experimental data could be modelled by assuming a molecular precursor state which traps N2 molecules prior to their dissociation. The latter rate is competing with desorption from the precursor state. A difference in activation energies for desorption and adsorption into the dissociated state was found to be 18 kJ/mol [83Aln]. A study of the angular velocity distribution of N2 molecules scattered from the W(100) surface has shown that the trapping probability into the precursor state is nearly independent of surface and beam temperatures, and thus confirmed the earlier interpretation of measured dissociative sticking coefficients [88Ret]. The situation for N2 adsorption on Cr(110) seems to be unresolved. Although molecular N2 states exist at low temperature [90Shi, 91Dow], one of them a π-bonded species serving as a possible precursor state to dissociation [90Shi], there are insufficient data on the rate of dissociative adsorption as a function of temperature or kinetic energy of incident molecules [84Miy], to decide on the existence of an activation barrier. The high propensity for forming a stable chromium nitride seems to enhance sticking. No detailed study of the influence of steps on the rate of dissociative adsorption is known. Finally it is worth noting that the gas phase reaction of N2 molecules with metal nano-clusters of varying sizes has been studied for for Mo, Nb, V and W [95Mit, 96Mit, 98Ber, 98Hol, 99Hol]. Molecular as well as dissociative adsorption on these clusters has been found.

Landolt-Börnstein New Series III/42A4

References for this document 61Ehr1 61Ehr2 62Ehr1 62Ehr2 65Del 68Kin 68vOs 71Tam 72Mah 75Sin 76Liu 76Yat2 77Boz1 77Boz2 77Shi2 79Bes 79Fug 79Pol 79Ris 80Ert 80Gol 81Bru 81Cos 81Ert 81Ibb 82Ban 82Ert1 82Ert2 82Hor 82Liu 82Spe1 82Spe2 83Aln 83Gru1 83Gru2 83Hen 83Jac1 83Men 83Pfn1 83Pfn2 84Ass 84Dow 84Gru1 84Gru2 84Gru3 84Lee 84Miy 84Umb 85Sto 85Tom 85Tsa 86Bar1

Ehrlich, G.: J. Chem. Phys. 34 (1961) 29. Ehrlich, G., Hudda, F.G.: J. Chem. Phys. 35 (1961) 1421. Ehrlich, G.: J. Chem. Phys. 36 (1962) 1171. Ehrlich, G., Hudda, F.G.: J. Chem. Phys. 36 (1962) 3233. Delchar, T.A., Ehrlich, G.: J. Chem. Phys. 42 (1965) 2686. King, D.A.: Surf. Sci. 9 (1968) 375. van Oostrom, A., in: Proc. 8th Intern. Conf. on Phen. in Ionized Gases, Vienna, 1968, p. 291. Tamm, P.W., Schmidt, L.D.: Surf. Sci. 26 (1971) 286. Mahnig, M., Schmidt, L.D.: Z. Phys. Chem. N.F. 80 (1972) 71. Singh-Boparai, S.P., Bowker, M., King, D.A.: Surf. Sci. 53 (1975) 55. Liu, R., Ehrlich, G.: J. Vac. Sci. Technol. 13 (1976) 310. Yates jr., J.T., Klein, R., Madey, T.E.: Surf. Sci. 58 (1976) 469. Bozso, F., Ertl, G., Grunze, M., Weiss, M.: J. Catal. 49 (1977) 18. Bozso, F., Ertl, G., Grunze, M., Weiss, M.: J. Catal. 50 (1977) 519. Shigeishi, R.A., King, D.A.: Surf. Sci. 62 (1977) 379. Besocke, K., Wagner, H.: Surf. Sci. 87 (1979) 457. Fuggle, J.C., Menzel, D.: Surf. Sci. 79 (1979) 1. Polizzotti, R.S., Ehrlich, G.: J. Chem. Phys. 71 (1979) 259. Rissmann, E.F., Parry, J.M.: J. Phys. Chem. 79 (1975) 1975. Ertl, G.: Catal. Rev. Sci. Eng. 21 (1980) 201. Golze, M., Grunze, M., Driscoll, R.K.: Appl. Surf. Sci. 6 (1980) 464. Brundle, C., Bagus, P.S., Menzel, D., Hermann, K.: Phys. Rev. B 24 (1981) 7041. Cosser, R.C., Bare, S.R., Francis, S.M., King, D.A.: Vacuum 31 (1981) 503. Ertl, G., Huber, M., Lee, S.B., Paál, Z., Weiss, M.: Appl. Surf. Sci. 8 (1981) 373-86. Ibbotson, D.E., Wittrig, T.S., Weinberg, W.H.: Surf. Sci. 110 (1981) 313. Bandy, B.J., Canning, N.D.S., Hollins, P., Pritchard, J.: J. Chem. Soc. Chem. Commun. (1982) 58. Ertl, G., Lee, S.B., Weiss, M.: Surf. Sci. 114 (1982) 527. Ertl, G., Lee, S.B., Weiss, M.: Surf. Sci. 114 (1982) 515. Horn, K., DiNardo, J., Eberhardt, W., Freund, H.-J., Plummer, E.W.: Surf. Sci. 118 (1982) 465. Liu, R., Ehrlich, G.: Surf. Sci. 119 (1982) 207. Spencer, N.D., Somorjai, G.A.: J. Phys. Chem. 86 (1982) 3493. Spencer, N.D., Shoonmaker, R.C., Somorjai, G.A.: J. Catal. 74 (1982) 129. Alnot, P., King, D.A.: Surf. Sci. 126 (1983) 359. Grunze, M., Kleban, P.H., Unertl, W.N., Rys, F.S.: Phys. Rev. Lett. 51 (1983) 582. Grunze, M., Unertl, W.N.: J. Vac. Sci. Technol. A 2 (1983) 896. Hendrickx, H.A.C.M., Hoek, A., Nieuwenhuys, B.E.: Surf. Sci. 135 (1983) 81. Jacobi, K., Rotermund, H.H.: Surf. Sci. 133 (1983) 401. Menzel, D., Pfnür, H., Feulner, P.: Surf. Sci. 126 (1983) 374. Pfnür, H., Menzel, D.: J. Chem. Phys. 79 (1983) 2400. Pfnür, H., Feulner, P., Menzel, D.: J. Chem. Phys. 79 (1983) 4613. Asscher, M., Somorjai, G.A.: Surf. Sci. 143 (1984) L389. Dowben, P.A., Sakisaka, Y., Rhodin, T.N.: Surf. Sci. 147 (1984) 89. Grunze, M., Golze, M., Hirschwald, W., Freund, H.J., Pulm, H., Seip, U., Tsai, M.C., Ertl, G., Küppers, J.: Phys. Rev. Lett. 53 (1984) 850. Grunze, M., Golze, M., Fuhler, J., Neumann, M., Schwarz, E., in: The intermediates in N2 dissociation on Re and Fe; 8th International Congress on Catalysis, Berlin, 1984, p. 133-143. Grunze, M., Dowben, P.A., Jones, R.G.: Surf. Sci. 141 (1984) 455. Lee, J., Madix, R.J., Schlaegel, J.E., Auerbach, D.J.: Surf. Sci. 143 (1984) 626. Miyano, T., Kamei, K., Sakisaka, Y., Onchi, M.: Surf. Sci. 148 (1984) L645. Umbach, E.: Solid State Comm. 51 (1984) 365. Stoltze, P., Norskov, J.K.: Phys. Rev. Lett. 55 (1985) 2502. Tomanek, D., Bennemann, K.H.: Phys. Rev. B 31 (1985) 2488. Tsai, M.C., Seip, U., Bassignana, I.C., Küppers, J., Ertl, G.: Surf. Sci. 155 (1985) 387. Bare, S.R., Strongin, D.R., Somorjai, G.A.: J. Phys. Chem. 90 (1986) 4726.

86Bre 86Bru 86Pfn 86Str 86Whi1 86Whi2 87Bru 87Fre 87Gru1 87Gru2 87Haa1 87Haa2 87Kuw 87Ret1 87Ret2 88Ass 88Dow 88Ehr 88Kuw 88Rao 88Ret 88Str 88Umb 89Qui 89Sch 90Cor 90Hen 90Mar 90Rao 90Shi 91Dow 91Por 91Yos 92Shi 93Gar 93Shi 95Mit 96Aru 96Die 96Mit 97Jac 97Rom 98Ben 98Ber 98Hol 98Mor2 98Rom 99Dah 99Hol 99Öst

Breitschafter, M.J., Umbach, E., Menzel, D.: Surf. Sci. 178 (1986) 725. Brubaker, M.E., Trenary, M.: J. Chem. Phys. 85 (1986) 6100. Pfnür, H., Rettner, C.T., Lee, J., Madix, R.J., Auerbach, D.J.: J. Chem. Phys. 85 (1986) 7452. Stroscio, J.A., Persson, M., Ho, W.: Phys. Rev. B 33 (1986) 6758. Whitman, L.J., Bartosch, C.E., Ho, W., Strasser, G., Grunze, M.: Phys. Rev. Lett. 56 (1986) 1984. Whitman, L.J., Bartosch, C.E., Ho, W.: J. Chem. Phys. 85 (1986) 3688. Brubaker, M.E., Malik, I.J., Trenary, M.: J. Vac. Sci. Technol. A 5 (1987) 427. Freund, H.J., Bartos, B., Messmer, R.P., Grunze, M., Kuhlenbeck, H., Neumann, M.: Surf. Sci. 185 (1987) 187. Grunze, M., Strasser, G., Golze, M.: Appl. Phys. A 44 (1987) 19. Grunze, M., Strasser, G., Golze, M., Hirschwald, W.: J. Vac. Sci. Technol. A 5 (1987) 527. Haase, G., Asscher, M.: Surf. Sci. 191 (1987) 75. Haase, G., Asscher, M.: Chem. Phys. Lett. 142 (1987) 241. Kuwahara, Y., Jo, M., Tsuda, H., Onchi, M., Nishijima, M.: Surf. Sci. 180 (1987) 421. Rettner, C.T., Stein, H.: J. Chem. Phys. 87 (1987) 770. Rettner, C.T., Stein, H.: Phys. Rev. Lett. 59 (1987) 2768. Asscher, M., Becker, O.M., Haase, G., Kosloff, R.: Surf. Sci. 206 (1988) L880. Dowben, P.A., Miller, A., Ruppender, H.J., Grunze, M.: Surf. Sci. 193 (1988) 336. Ehrlich, G.: Activated Chemisorption, in: Chemistry and Physics of Solid Surfaces VII. Springer Series in Surface Science 10; Vanselow, R., Howe, R. (eds.), Berlin: SpringerVerlag, 1988, p. 1-106. Kuwahara, Y., Fujisawa, M., Onchi, M., Nishijima, M.: Surf. Sci. 207 (1988) 17. Rao, C.N.R., Rao, G.R.: Chem. Phys. Lett. 146 (1988) 557. Rettner, C.T., Schweizer, E.K., Stein, H., Auerbach, D.J.: Phys. Rev. Lett. 61 (1988) 986. Strongin, D.R., Somorjai, G.A.: J. Catal. 109 (1988) 51. Umbach, E.: Appl. Phys. A 47 (1988) 25. Quick, A., Browne, V.M., Fox, S.G., Hollins, P.: Surf. Sci. 221 (1989) 48. Schenk, A., Hock, M., Küppers, J.: Surf. Sci. 217 (1989) L367. Cornish, J.C.L., Avery, N.R.: Surf. Sci. 135 (1990) 209. Henriksen, N.E., Billing, G.D., Hansen, F.Y.: Surf. Sci. 227 (1990) 224. Martensson, N., Nilsson, A.: J. Electron Spectrosc. Relat. Phenom. 52 (1990) 1. Rao, G.R., Rao, C.N.R.: Appl. Surf. Sci. 45 (1990) 65. Shinn, N.D.: Phys. Rev. B 41 (1990) 9771. Dowben, P.A., Ruppender, H.-J., Grunze, M.: Surf. Sci. 254 (1991) L482. Por, E., Haase, G., Citri, O., Kosloff, R., Asscher, M.: Chem. Phys. Lett. 186 (1991) 553. Yoshinobu, J., Zenobi, R., Xu, J., Xu, Z., Yates jr., J.T.: J. Chem. Phys. 95 (1991) 9393. Shi, H., Jacobi, K.: Surf. Sci. 278 (1992) 281. Gardner, P., Martin, R., Tüshaus, M., Shamir, J., Bradshaw, A.M.: Surf. Sci. 287 (1993) 135. Shi, H., Jacobi, K., Ertl, G.: J. Chem. Phys. 99 (1993) 9248. Mitchell, S.A., Lian, L., Rayner, D.M., Hackett, P.A.: J. Chem. Phys. 103 (1995) 5539. Arumainayagam, C.R., Tripa, C.E., Xu, J., Yates, J.T.: Surf. Sci. 360 (1996) 121. Dietrich, H., Geng, P., Jacobi, K., Ertl, G.: J. Chem. Phys. 104 (1996) 375. Mitchell, S.A., Rayner, D.M., Bartlett, T., Hackett, P.A.: J. Chem. Phys. 104 (1996) 4012. Jacobi, K., Dietrich, H., Ertl, G.: Appl. Surf. Sci. 121 (1997) 558. Romm, L., Katz, G., Kosloff, R., Asscher, M.: J. Phys. Chem. B 101 (1997) 2213. Bennich, P., Wiell, T., Karis, O., Weinelt, M., Wassdahl, N., Nilsson, A., Nyberg, M., Pettersson, L.G.M., Stohr, J., Samant, M.: Phys. Rev. B 57 (1998) 9274. Berces, A., Hackett, P.A., Lan, L., Mitchell, S.A., Rayner, D.M.: J. Chem. Phys. 108 (1998) 5476. Holmgrem, L., Andersson, M., Rosen, A.: J. Chem. Phys. 109 (1998) 3232. Mortensen, J.J., Hammer, B., Norskov, J.K.: Phys. Rev. Lett. 80 (1998) 4333. Romm, L., Zeiri, Y., Asscher, M.: J. Chem. Phys. 108 (1998) 8605. Dahl, S., Logadottir, A., Egeberg, R.C., Larsen, J.H., Chorkendorff, I., Törnqvist, E., Norskov, J.N.: Phys. Rev. Lett. 83 (1999) 1814. Holmgren, L., Rosen, A.: J. Chem. Phys. 110 (1999) 2629. Österlund, L., Pedersen, M.O., Stensgaard, I., Laegsgaard, E., Besenbacher, F.: Phys. Rev. Lett. 83 (1999) 4812.

00Jac 00Zep

Jacobi, K.: Phys. Status Solidi (a) 177 (2000) 37. Zeppenfeld, P., David, R., Ramseyer, C., Hoang, P.N.M., Girardet, C.: Surf. Sci. 444 (2000) 163.

136

3.7.1 CO and N2 adsorption on metal surfaces

3.7.1.8 Organization of the tables The collected data related to physical and chemical aspects of adsorbed CO and N2 on metal surfaces are presented in a total of 13 tables. The content of these tables is as follows: Table 3: CO adsorption, thermodynamic properties (2D ordered structures; coverage; heat of adsorption; activation energy and pre-exponential factor of desorption; initial sticking coefficient) Table 4: Adsorbed CO dissociation parameters (adsorbed state, coverage; temperature, pre-exponentail factor and activation energy of dissociation; desorption temperature of state) Table 5: Crystallographic and vibrational data of adsorbed CO (frequencies; adsorption site) Table 6: Adsorbed CO (low frequency vibrational data by EELS, TEAS [2003Gra] and IETS [2002Ho]) Table 7: Additional structural data of adsorbed CO (2D ordered structure; coverage; adsorption site; MeC and C-O bond lengths; tilt angle of CO) Table 8: Electronic structure of adsorbed CO (MO valence levels in eV) Table 9: Core level spectroscopic data of adsorbed CO (C1s and O1s binding energies; chemical state of CO) Table 10: Thermodynamic properties of adsorbed N2 (2D ordered structures; coverage; heat of adsorption; activation energy and pre-exponential factor of desorption; initial sticking coefficient) Table 11: Adsorbed N2 dissociation parameters (adsorbed state, coverage; temperature, pre-exponentail factor and activation energy of dissociation; desorption temperature of state) Table 12: Crystallographic and vibrational data of adsorbed N2 (frequencies; adsorption site) Table 13: Additional structural data of adsorbed N2 (2D ordered structure; coverage; adsorption site; MeN and N-N bond lengths) Table 14: Electronic structure of adsorbed N2 (MO valence levels in eV) Table 15: Core level spectroscopic data of adsorbed N2 (N1s binding energies)

Lan dolt-Börn stein New Series III/4 2A4

Landolt-Börnstein New Series III/42A4

Table 3. CO adsorption (thermodynamics) Substrate

Coverage of adsorbate

Technique Heat of adsorption Ead [kJ/mol] 27.2 27.2

Disordered Disordered ∼1

22.1 47.3 Al(100) Au(110) Co(0001)

Disordered (√3×√3)R30° (√7×√7)R19.2° (2√3×2√3)R30°

Co(10 1 0)

p(2×1) c(2×1)

below 33.5 128 → 96 1/3 0.59 7/12 0.5 1.0 ≤ 0.5 1.0 >1.0

Co(101 2)

p(2×1) (2×1)p2mg c(2×6) (3×1)

Co(102 0)

(3×1)

2/3

Cr(110)

c(4×2)

0.25 α1 ≥ 0.3 α2

Co(10 1 0)

145 decr. to 120

TDS TDS Desorpt. Rate vs. 1/T; Theory Theory TDS Isotherms TDS

Activation energy of desorption Ed [kJ/mol]

∼20

Preexponential factor νd [s−1] 1×1013 ass. 1×1013 ass. 1×1013 ass.

33

1×1014 ass. 1×1013 ass.

103±8

1×1013 ass.

Sticking coefficient s(θ)

Temperature of desorption [K]

<40

76McE1 76McE1 80Chi, 89Jac 83Bag 96Ham 86Pau3 87Out 83Pap 77Bri 83Gre 82Pap

Isotherms TDS TDS

References

∼92 110.7 105

2×1013 3.1×1013 2×1013

0.89

∼400 α2 ∼340 α1 600 β

84Hab 96Too

3.7.1 CO and N2 adsorption on metal surfaces

Ag(111) Ag(110) Al(111)

Structure

78Pri 143

85Pap

Isotherms

84Shi, 85Shi3, 86Shi

137

≈0.9-0.4 coverage dependent

Structure

Coverage of adsorbate

Heat of Technique adsorption Ead [kJ/mol]

Cu(100)

c(2×2) c(7√2×√2) R45°

0 0-0.15 0.15-0.35 0 0-0.57 Cu(111)

(√3×√3)R30°

69 56 53

69.9-60.8 60.8

1/3 50 44.2

(1.5×1.5) R18°

0.44

50 35.1

Landolt-Börnstein New Series III/42A4

(1.39×1.39) hex 0.52 0-0.4 0-0.25

53.6

TDS, MB TDS IRAS AES, LEED, EELS, TDS LEED, UPS AES, LEED, EELS TDS TDS MB TDS

Sticking coefficient s(θ)

65-40 55.27 ± 1.26

Temperature of desorption

300

1016-1013 ± 10−15 1 ± 10−15.3 0.4

148/168 148/168

45 127

34 0.87 3 × 10 15 1× 10 15

72Tra1

83Bur 86Dub 90Pet, 91Pet2 92Tru 0.85 at 110 K 0.85-0.1 0.70

51

References

[K]

1×1013 ass

Isosteric heat LEED, ∆φ Laser, TDS TREELS TDS, EELS Isosteric heat IRAS Opt. Refl., TDS

Preexponential factor νd [s−1]

0.9±0.1

127 112

96Dvo, 00Dvo 99Kne2 86Kir 79Hol 77Kes 99Kne2 75Con 77Kes 86Kir 99Kne2 90Hin

3.7.1 CO and N2 adsorption on metal surfaces

0.5 0.57 0-0.1 0.1-0.5 0.5-0.6 0-0.6

Activation energy of desorption Ed [kJ/mol]

138

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Coverage of adsorbate

Heat of Technique adsorption Ead [kJ/mol]

Cont'd.

Fe(110)

Fe(100)

Fe(100)

Fe(111)

50 38 54.8-58.6 57-38

55

Sticking coefficient s(θ)

Temperature of desorption [K]

86Kir

40 26

50.24 ± 4.18 53-42 52 75-50

79Hol

10−13.6 ± 1.0 1013 - 2×109 1×1013 ass. 1018 - 1012

13

96 134-201

1×10 ass. 1×10−2

60-105 220 53.6 75.4 109.7

1×1013 ass.

0.46 0.95±0.05 1-0.25 0.8 0.85 0.08

215 205-215 210 205-220 215 215 205-220 400-405 α 800 β

Isosteric heat TDS

TDS

TDS

88 105

References

1×1013 ass.

1×1013 ass

73Wac 92Chr 91Pet2 85Har 01Kun 96Jin 77Hor2 79Bro, 81Yos 82Gon, 83Wed 85Vin 80Ben

220 306 440 820 340 420

87Moo1, 87Moo2, 89Lu 84Sei, 85Sei

139

77±4 0.5 ML C 0.25 ML C,O 100±5 α1-states β α1 α2 α3 β α1 α2

TDS TDS IRAS IRAS TDS TDS TDS, EELS TDS, MB Opt. Refl.,TDS TDS TDS

Preexponential factor νd [s−1] 2 × 10 14

3.7.1 CO and N2 adsorption on metal surfaces

Cu(110)

0-0.3 >0.35 0-0.33 0.33-0.52 0 0.1-0.5 0 0-0.5 0-1 0.2-0.6 0-0.77 0.38 α β

Activation energy of desorption Ed [kJ/mol]

Structure

Coverage of adsorbate

Heat of Technique adsorption Ead [kJ/mol]

Cont'd. Fe(111)

(√3×√3)R30°

(2√3×2√3)R30°

Ir(110)

(2×2)

Landolt-Börnstein New Series III/42A4

(4×2)

0.25 (0.9-1)*1/4 coverage θ 0<θ<0.75 θ=0 0.75 variable θ

Preexponential factor νd [s−1]

Sticking coefficient s(θ)

TDS

146.5±0.4 113.0-132.7 142.4 141.5±2.1 125.6±0.4

17

91

∼10

137

∼1013 2.4×1014 ass. ~1 1×1013 ass. 1-0.7 2.4×1014 ass. (1−θ )2 2.4×1014 ass. 2.4×1014 ass.

Isosteric heat Isosteric heat LEED/UPS TDS Isosteric heat

Temperature of desorption [K] (700) 820 ∼250 ∼340 ∼395 ∼760 150-300 450-525 533

2.4×1014 ass. 133.9 139.0±2.1 142.8±2.1 154.5±2.1

154.9

TDS Isosteric heat Isosteric heat Isosteric heat TDS Isosteric heat TDS/AES/ LEED Isosteric heat

TDS

175-110

2.4×1014 ass. 0.065 2.4×1014 ass. 0.5 2.4×1014 ass. 0.87 2×1015

113.0 123.5

1×10 ass

<1

146.5 œ 67 θ 154.9 83.7 146.5±8

1×10131×109 1×1013 1×109

1-0.8 1.04±0.05 0.8

13

423 465 520 380-450

References

86Bar2 89Whi

76Com1 76Com2 78Zhd 76Küp 76Zhd 78Tay2 76Küp 76Zhd

97Sus1

448 73Chr 433, 503, 613 78Nie 78Tay1

600

89Mar

3.7.1 CO and N2 adsorption on metal surfaces

Ir(111)

β α0 α1 α2 β 1/3 1/3 1/3 1/3 = 5.2×1014 cm−2 5.2×1014 cm−2 7/12 = 9.1×1014 cm−2 7.1×1014 cm−2 4.0×1014 cm−2 2.1×1014 cm−2 from θ=0 to θ = 0.7

Activation energy of desorption Ed [kJ/mol] 209

140

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Coverage of adsorbate

Heat of Technique adsorption Ead [kJ/mol]

Cont'd. Mo(100)

Mo(100)

Mo(100)

Mo(110) Ni(100)

c(2×2) p(3√2×√2)R45° c(5√2×√2)R45°

α1 α2 β1 β2 β3 β1 β2 0.5 0.5 0.67 0.67 0.6 0-0.6

Preexponential factor νd [s−1]

MB, variable beam energy TDS

TDS

TDS

0.28 0.21 125.6 121.4

126-112

Isosteric heat TDS Isosteric heat TDS, IRAS TDS, IRAS SIMS, isosteric heat TDS TDS

Temperature of desorption [K]

87Ste

0.8 0.35 260 328 357 280-357 coverage dep. 54 96 264 293 389 209 414 138.6 22 70

220-300

73Lec, 73Vis, 80Ko2

490 1030 1290 150 290 840 940 1250

78Fel

1×1013 ass.

1×1013 ass.

1×1015 ass. 1×10−2 and 2nd ord. ass. 1×1010 1×1019 1×1013 ass. 1×1013 ass. 1×1013 ass. 105 1013 1×1013 ass. 1×1013 ass. 1×1013 ass.

References

85Zae

77Gil

450

72Tra2 83Koe1 72Tra2 95Vas 79Bor

170 α0 240 α0

80Yat

141

40.6 58.2

TDS

Sticking coefficient s(θ)

3.7.1 CO and N2 adsorption on metal surfaces

8 kJ/mol to 142.4 kJ/mol α β1 β2 β3 β-states

Activation energy of desorption Ed [kJ/mol]

Structure

Coverage of adsorbate

[kJ/mol] 70.8 86.2 110.5

Cont'd.

c(4×2) c(2√3×4)rect (√3×√3)R30°

(√7×√7)R19°

0 0 0-sat. <0.1 0 0.5 0.5 0.62 1/3 1/3 1/3 0.35 0.57 0.57 0-sat. 0 0-0.3 >0.3-0.6

123-99

98

130-95

Landolt-Börnstein New Series III/42A4

134 0 0-0.4 0-0.5

149.5 111-84

TDS TDS TDS TDS TDS Calorimetry TDS TDS, IRAS TDS TDS, XPS TDS, XPS TDS Isosteric heat TDS, XPS TDS TDS TDS, XPS Calorimetry TDS TDS TDS TDS TDS TDS Isosteric heat

Activation energy of desorption Ed [kJ/mol]

109.3 116.4 ± 2 127 134 -70 55

108.8

Preexponential factor νd [s−1] 1×1013 ass. 1×1013 ass. 1×1013 ass. 1×1013 ass. 6×1015 1×1015 1.6×1014 1017.5 8×109

Sticking coefficient s(θ)

Temperature of desorption [K] 290 α0 350 β1 440 β2

0.74 0.91 445 320 100 440

1×1013 1×1013

370 110 58

13

1×10 2×109

220 3×1014 125 125-58

0.72-0.28 0.9

8×1014 1×109

1017-1021 1×1013

References

0.91 1.05-0.2 1-0.54

440

81Joh 78Mad 93Stu 93Tak 95Vas 87Fro 98Hel 76Con 74Chr 98Hel 87Fro 87Fro 98Hel 93Stu 88Sur 87Fro 84Gij 81Cam1 80Iba 74Chr

3.7.1 CO and N2 adsorption on metal surfaces

Ni(111)

Heat of Technique adsorption Ead

142

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Cont'd.

0.1-0.9 sat. (2×1) p2mg c(4×2) c(8×2) c(2×2)

Os(0001)

(√3×√3)R30°

Os (poly)

(2√3×2√3)R30° saturation struct.

Heat of Technique adsorption Ead [kJ/mol] 113-88

1.0 1.0 0.78±0.01 0.68±0.02 ² 0-1 0-1

101 129.7±4 131.9±2.5 130.2±1.6 125.6 133-101

0 0 10−3 - 0.2

bridge 133.1 top 136.1

0.7-1.0 <0.7 0.33 0.42 0.63

121 121 104.7 125

Sticking coefficient s(θ)

LITD PES Isotherms

Preexponential factor νd [s−1] 1×1013 1×1013

Calorimetry Isotherms,TDS Isotherms,TDS Isotherms,TDS isosteric heat Calorimetry TDS

3×1015 6×1016±1.4 9.2×1017±0.5 7.6×1014±0.2 1×1013 ass. 3×1015 1014.7

0.26

Activation energy of desorption Ed [kJ/mol]

121.4±8.4 143.6±3.7 130.2±1.6 117.2 127-109

Temperature of desorption [K]

89Zhu 79Rub 93Stu 92DeA

0.75-0.26 355 α1 425 α2

EELS, isobars EELS, TDS TDS TDS TDS TDS Isosteric heat Isosteric heat

73Tay 93Stu 90Fei 87Bau

108 83.7-96.3 138.2

3×1012 1013 - 1015 8.5×1015

0.9 330 α1 420 α2 360 α1 455 α2

1×1013 ass. 1×1013 ass. 450 α1 505 α2 526 α3

TDS

References

131

1×1013 ass.

86Fro 75Fal 85Beh 81Ber

3.7.1 CO and N2 adsorption on metal surfaces

Ni(110)

Coverage of adsorbate

73Mad2 73Mad3 83Ven

80Fuk1

143

Structure

Pd(100)

(√3×√3)R30° c(4×2)

Pd(110)

Landolt-Börnstein New Series III/42A4

(2×1)p2mg (4×2) c(2×2) Pt(100)(hex)

Low 0.45 low-0.5 low-0.45 0.15 low 0.45 0.8 0.5 0 0 0-1/3 1/3 0.5 0.55 0-0.4 Low

1.0 0.75 0.5 0-1 0

Heat of Technique adsorption Ead [kJ/mol] 150.7 128 150-85 161 168 115 76 167.5 125.6 142.4 142.4 134.0 23.4±0.8

180-85

Isosteric heat Isosteric heat Isosteric heat Isosteric heat TDS Calorimetry Calorimetry Calorimetry Calorimetry TDS TDS Isosteric heat TDS TDS MB SHG MB Isothermic MB Isosteric heat Isosteric heat Isosteric heat MB LEED, EELS

Activation energy of desorption Ed [kJ/mol]

154.1

148.6

Preexponential factor νd [s−1] 1×1013 ass. 1×1013 ass.

Sticking coefficient s(θ)

3×1016 2×1016 1×1013 ass. 1×1013 ass. 1×1013 ass. 1×1013 ass. 1×1015.3 1×1013 ass 1×1013 ass 1×1013 ass 1×1013 ass

0.6

130 126-149 113.0 150.7 167.5

1×1013 ass 1×1013 ass 1×1013 ass 1×1013 ass

References

[K] 69Tra1

0.83 0.65 0.35 0.65

0.96

1-0.1 0.93 0.95 0.5

0.74-0.04 115.1

Temperature of desorption

0.7

78Bra1 80Beh 97Yeo1

89Guo 93Sza2 70Ert 74Con1 01Sta 02Bou 01Hir 00Yag 99Jon 74Con1

93Hop, 96Yeo 83Beh

3.7.1 CO and N2 adsorption on metal surfaces

(2√2×√2)R45° Pd(111)

Coverage of adsorbate

144

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Coverage of adsorbate

Heat of Technique adsorption Ead [kJ/mol]

Cont'd.

0-0.4

Pt(100)(1×1)

Pt(111)

c(2×2) c(5√2×√2)R45° c(3√2×√2)R45° c(4×2) (√3×√3)R30° c(4×2) c(4×2)

LEED, EELS Isosteric heat Isosteric heat

0.75 0.5

TDS

138.2 225-85

138.2±8 130±5 100±5 133−68.5 θ 134 118±19 183±8 118±19-65±3

134.0 108.9

1×1013 ass 1×1013 ass

98.8 111.0 121.8 133.5

1×1013 ass 1×1013 ass 1×1013 ass 1×1013 ass

Temperature of desorption

References

[K] 0.70-0.1

XPS, uptake LEED,EELS LEED,EELS MB

Isosteric heat TDS TDS, EELS TDS, EELS TEAS TEAS Calorimetry Calorimetry Calorimetry

Sticking coefficient s(θ)

117.2 100.5

75Kne

0.24 0.24 0.24 0.24 0.6 0.7 0.6 0.61-0.04

1×1013 ass 1×1013 ass 5×1014±50% Function 1×1013±50% of T and B 1.4×1014 4.3×1014 0.7-0.1 0.8 0.05

400 450 525 550

77McC

78Bro 83Beh 96Yeo

74Lam1 76Chr 93Cud 400 430

84Poe 87Ver 97Yeo2

145

0 0-0.4 0.5 0-1 0.5 0.6 0.67 0.75 1/3 0.5 0.25 top 0.25 bridge 0-0.13 <0.01 0.1-0.5 0 0.5-1

Preexponential factor νd [s−1]

3.7.1 CO and N2 adsorption on metal surfaces

p(1×1) c(4×2) c(2×2) β1 β2 β3 β4

Activation energy of desorption Ed [kJ/mol]

Structure

Coverage of adsorbate

Heat of Technique adsorption Ead [kJ/mol]

Cont'd.

0-0.5-0.67 0.1-0.5

Pt(110)(1×2)

(1×1) (2×1)p1g1 (2×1)p2mg

c(8×4) β1 β2 α2 α3 β1 Landolt-Börnstein New Series III/42A4

Re(0001)

0.1-0.5 0 0.5-1 1 1 1 0.2-0.5 0.1 0.6 0.78 ± 0.05 1

148 140 ± 15

α β

145±15 150.7 ± 6 129.4 133 133 ± 4 104 160 ± 15

Isosteric heat

135 ± 10

TPD

Sticking coefficient s(θ)

Temperature of desorption

0.8-0.04

77Ert

82.9 108.9 105.1 120.2 131.5 105-113

0.8 3×1014 1×1013 ass 1×1013 ass 1×1013 ass 1×1013 ass

0.8 0.8 0.8

425 505

1×10 ass 1×1013 ass 1×1013 ass 1×1013 ass 1×1013 ass 1×1013 ass.

1 0.64 0.64

86See 96Zae 82Ste 88Eng 76Com3 82Hof1 76Com3 76McC 82Jac1 80Fai

1

13

References

[K]

134-67

Isosteric heat MB

TDS

Preexponential factor νd [s−1] 1×1015 ass

380 350 460 412 511 511 450 810

82Jac1 77McC 73Bon

77Hou, 80Duc 90Ros

3.7.1 CO and N2 adsorption on metal surfaces

TDS,LEED, ∆Φ LI-TDS MB Isosteric heat T-modulation Isosteric heat Isosteric heat Isosteric heat Isosteric heat

Activation energy of desorption Ed [kJ/mol] 134-41

146

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Coverage of adsorbate

Heat of Technique adsorption Ead [kJ/mol]

Rh(100)

c(2×2) (4√2×√2)R45° c(6×2) Rh(111) (2×2) 3CO

0 for 100 500 K T<170 K 0.5 0.75 0.83 0 to 0.75 0 0 variable θ

122 33 94

134±8 to 59

135 − 6.8 θ − 160 θ 2

MBS mod MBS single TPD TPD TPD TDS Reverse flash Reverse flash Reverse flash TPD

LDS LDS MBS mod. MBS single Isosteric heat by MBS He TPD TPD MBS

1016.3±1.1 1014.5±0.9 (4±3) × 1016 8.4×1012 1×1013 ass 1×1013 ass

Sticking coefficient s(θ)

Temperature of desorption [K]

88Kel

97Wei 94deJ 82Kim

0.75

78Cas 97Med

0.86-0.58 0.04 0.7 129.8

161.2±6 164.9±3

132 130

References

1×1013 ass

78Cas

1014±0.5to 0.8±0.05 13 ~0 10 1016.5±0.6 1016.9±0.2 1.33×1014 × exp(0.344θ+ 48.8 θ 2) 0.76±0.04 1013.6±0.3 1×1013 ass 0.95-0.62

88See 97Wei 91Pet1

273

79Thi2 78Cas 99Beu

147

0 5L 0

TPD

Preexponential factor νd [s−1] 1×1013 ass.

3.7.1 CO and N2 adsorption on metal surfaces

α1 α2 β-states 0 0 0 0 0

Re (1010)

Activation energy of desorption Ed [kJ/mol] 80 105 176-218 149.5±10 135.2±8 131.0±4 134.4 127.7±4 121.4

Structure

Rh(110)(1×1)

Ru(0001)

(√3×√3)R30°

Rh(110)(1×2)

0 0-1 low low low low 0.2<θ<0.33 0.33<θ<0.55 >0.55 0.2<θ<0.4 0.4<θ<0.63 >0.63

Heat of Technique adsorption Ead [kJ/mol] 132 132-108

150-120 120-70 70-50 100-60 60-45 45-20

MBS, TPD MBS, TPD TPD TPD MBS mod. MBS single TPD TPD TPD TPD TPD TPD TDS

MBS at 85 K

Landolt-Börnstein New Series III/42A4

MBS at 273 K

Activation energy of desorption Ed [kJ/mol]

124 130 174 ± 4 164 ± 6

∼98 ∼117

Preexponential factor νd [s−1] 1×1013 ass. 1×1013 ass. 1×1013 ass. 1×1017.8 ± 0.4 1×1016.8 ± 0.6 1014 - 1012 1012 - 107 107 - 104 109 - 105 105 - 103 103 - 100 1×1013 ass.

Sticking coefficient s(θ)

Temperature of desorption

References

[K] 91Bow

0.68±0.01 485, 425, 390 495 485 485 485 490 490, 440, 390 490, 440, 390 485 485, 400 485, 400 0.85

0.92-0.58 with variable Ekin 0.96-0.84 with variable Ekin

80Bai 77Mar 97Wei 93Bar

93Bar

70Gra, 74Mad, 75Fug1, 75Fug3, 77Fug, 79Tho, 93Ove 99Kne1

2000Rie

3.7.1 CO and N2 adsorption on metal surfaces

Ru(0001)

(1×1) c(2×2) (2×1)p2mg (1×2) c(2×4) (2×2)p2mg (√3×√3)R30°

Coverage of adsorbate

148

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Ru(0001)

Structure

(√3×√3)R30°

Coverage of adsorbate

0.05-0.25 0.33 >0.35 atop site Hollow site α1 α2 β

Ru(0001)

[kJ/mol] 160±6 175 120-110 160.3 114 91

TDS

Ru (1010)

Ru (1011)

(3×1) ("2×1") (3×1) (2×1)p2mg ⎛ 4 1⎞ ⎟ ⎜ ⎝1 2⎠

0.3 0.47 0.67 1.0 1.22

(3×1) (4×1) ⎛ 4 1⎞ ⎟ ⎜ ⎝1 2⎠

<0.3 ∼1.0 >1

⎛1 1⎞ ⎜ ⎟ ⎝ 3 0⎠

α β

157 at θ <0.5

Preexponential factor νd [s−1] ∼2×1016 ∼1×1019 ∼1×1014

∼127 ∼109 ∼146 102 126 150 ∼125 100 100

1×1013 ass.

Sticking coefficient s(θ)

Temperature of desorption

Isosteric heat and TDS

TDS

TDS

References

[K] 0.7-0.55 ≈0.3 <0.25

78Pfn, 83Pfn1, 83Pfn2 96Ham 00Kop

Theory Theory TDS

Ru (1010) Ru (1010)

Isotherms and TDS

Activation energy of desorption Ed [kJ/mol] 160±6 175 120-110

440 520-460 580 403 513

1×1013 ass. 1.0 at θ <0.85

85Shi1

77Ku 89Lau

1×1015

140 75

∼1×1014 ∼1×109

∼50

∼1×106

∼105 ∼118

1×1013 ass.

3.7.1 CO and N2 adsorption on metal surfaces

Ru(0001)

Heat of Technique adsorption Ead

96Rot

0.6

76Ree

149

Structure

Coverage of adsorbate

Heat of Technique adsorption Ead [kJ/mol]

Ru(1120)

p(1×2) (1×2)-p2mg

W(100)

c(2×2)

W(110)

W(110)

p(2×7) c(4×1) p(3×1) p(4×1) p(5×1)

TDS

Preexponential factor νd [s−1]

Sticking coefficient s(θ)

Temperature of desorption [K] 440-460 400 500 540 380 430-450 540

TDS

1.0 Isothermal desorption 40 230 291±13 LEED, XPS

∼104 2.8×1012 1.2×1012

0.72-0.95, temp. depend.

∼150 ∼300 ∼375 ∼975 ∼1125

References

01Wan1, 01Wan2

03Fan

67And, 79Wan 71Koh, 73Koh 77Leu, 83Umb 77Ste1, 77Ste2

3.7.1 CO and N2 adsorption on metal surfaces

Ru(1121)

α1 α2 β1 β2 α1 α2 β 0.5 α, β α1 α2 "virgin" β1 β2 ∼0.2 >0.3 ∼0.5 ∼0.65 ∼0.8

Activation energy of desorption Ed [kJ/mol] 103 85 89

150

Substrate

Landolt-Börnstein New Series III/42A4

3.7.1 CO and N2 adsorption on metal surfaces

151

Table 4: Adsorbed CO (dissociation parameters) Substrate

Cr(110) Mn/Fe(110)

α1 α β α β

Mo(110)

Mo(100)

α

Ni(100) defects Ni[5(111)× (1 1 0) ]

Fe(110) Ir(111) Re(0001) W(110)

Zr polycrystal

La ndolt-Bö rnstein New Series III/4 2A4

Temperatur e of dissociation [K] ∼300 0.25 ∼200 Mn/Fe=0.0 ∼220 6 ∼270 Mn/Fe=0.1 "clean" C O C,O S K 0.54-0.63 ∼220

Adsorbed Coverage State θ

α β "virgin" α1 β1 β2 β

low 0.36 >10−6 torr 0.31 0.026

500 300

Activation Pre-expo- References energy of nential dissociation factor [kJ/mol] [s−1] 82Kat 84Shi 93Sie

71 36 51 79 79 74 58-65

1010.6 103.9 106.6 1011.6 1012.3 1010.9 1013 ass.

83.7

<300 >650 >400 >300 275-400

<300

46-63

1013

81Kel, 81Sem, 86Eri

73Lec, 73Vis, 80Ko2, 81Sem 86Ste 78Erl

79Bro 76Com2 77Hou, 83Tat, 85Tat 83Umb

80Foo

152

Table 5. Adsorbed CO (crystallographic and vibrational data) Substrate

Structure

Al(111)

Coverage/ adsorbed state saturated

Vibrational frequency [cm−1] Me-C

440 ∼ 1 ML K

Ag(110) Au(332) Co(0001)

Cr(110) Landolt-Börnstein New Series III/42A4

Cu/Al(111)

References

CO perpend. to surface

EELS

89Jac

IRAS T <40 K EELS

88Ryb 86Pau3, 87Pau

IRAS PMIRAS

01Hah 97Rug 96Bei

atop 2-fold bridge, tilted CO

IRAS

96Too

"lying down" in 2-fold hollow; perpendicular in atop, bridge di-sigma bonded

EELS

84Shi, 85Shi2, 85Shi3

2135

Al(100) K/Al(100)

Co (1010)

Technique

at 13 K

2060 1060 1250 1750 1910 2153 2120 2012-2048 1850-1900 2055-2080

p(2×1) (2×1)p2mg

<0.56 0.5-1.0

1972-2020 1900-1967

c(2×6) c(4×2)

>1.0 0.25; α1

450-475

(1×1)

α2

495

1984 1150-1200 1330 1865 1975 1260

0.5 ML Cu

atop bridge defect sites

93Col

3.7.1 CO and N2 adsorption on metal surfaces

Al(100)

C-O 2137

Adsorption site / configuration

Landolt-Börnstein New Series III/42A4

Substrate

Cu(100)

Structure

c(2×2)

Cu(111)

Cu(110)

Vibrational frequency [cm−1] Me-C 342.8 344.5 345.2 344.56 ± 0.16 338.8

(√3×√3)R30°

0.57 0.1 - 0.57 0 0 1/3

334.0

(1.4×1.4)hex, (7×7)

0 0 0.52 at 7 K 0.52 at 77 K

(2×1)

0.52 0.5

334.3 338.5

685 345

2084 2093.0 2076-2086 2076 2065 2076 2076 2076 2068 2078 2078 1837, 2068 1812, 1830, 2068 2070 2094 2082 2078 2073 2089 2090, 2106

Technique

References

atop atop atop atop atop atop, bent atop atop atop atop atop atop atop atop atop bridge, atop bridge, atop

EELS IRAS IRAS IRAS IRAS EELS IRAS IRAS STM-IETS IRAS IRAS IRAS IRAS IRAS IRAS IRAS

88Uvd 95Hir

IRAS

79Pri

IRAS EELS IRAS SFG STM-IETS IRAS IRAS

82Woo 79Wen 94Hir 92Mor 99Lau 82Woo 84Hol

T T T T T T T, step edges

98Gra2 85Ryb 88Uvd 97McC 95Hir 99Lau 95Hir,94Hir, 93Hir,90Hir 79Pri 85Hay2 95Hir 79Hol 85Hay2

153

0 0 0-sat.

C-O 2089.0 2086 2079.3

Adsorption site / configuration

3.7.1 CO and N2 adsorption on metal surfaces

c(7√2×√2) R45°

Coverage/ adsorbed state 0.5

Structure

Fe(100) Fe(100)

Fe(111)

A B C a1 a2 b c 8 ML Fe

Fe(111)

Fe/Cu(100) Ir(100)(1×1)

c(2×2)

Ir(100)(5×1)

(1×1) (5×1) not lifted

Vibrational frequency [cm−1] Me-C 470 530 395 375 400 510-520 515 550

550

Landolt-Börnstein New Series III/42A4

0.5 low cov. 0.5 variable θ

497

Ir(111)

Variable θ

475-490

Ir(110)

Variable θ Low θ

475-490

485

C-O 1180-1245 2070 2020 1210 1155-1170

1530 1805-1850 2000 1325-1485 1520-1575 1735-1860 1945-2015 1929-1998 2020-2048 2068-1998 2026 2075-2005 5 bands 20252097 2025-2050 2030-2090 2012-2070 2001

Adsorption site / configuration

Technique

References

"lying down" T 2-fold 4-fold

IRAS EELS

85Ben 87Moo2

CO O (>400 K) C Deep hollow Shall. Hollow atop Deep hollow Deep hollow Shall. Hollow atop bridge atop atop

EELS

89Lu

EELS

84Sei, 85Sei

EELS

86Bar2 89Whi

IRAS

99Tan

EELS IRAS EELS IRAS

91Kis 93Mar 91Kis 93Mar

EELS IRAS EELS IRAS

89Mar 96Lau 89Mar 95Lyo

atop

atop atop atop

3.7.1 CO and N2 adsorption on metal surfaces

Fe(100)

Coverage/ adsorbed state Low θ, α3 α1 α2 α3 β α3 β

154

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Cont'd. Mo(100)

Vibrational frequency [cm−1]

Coverage/ adsorbed state Saturation θ <0.2

400

θ >0.4

400 565-575 400

Mo(110)

<0.3

>0.3

Ni(100)

C-O 2086 1065 1235 1085 2100

c(2×2)

0.5

(3√2×√2) R45°

0.67

484

c(5√2×√2) R45° 0.6

References

atop "lying down" or inclined

IRAS EELS

85Zae

EELS

91Che

inclined CO, titrated with coadsorbed Cu; upright CO in different sites

IRAS

91He1

atop atop bridge atop atop, bridge atop, bridge 2/3 atop 1/3 bridge atop

EELS EELS EELS EELS EELS IRAS EELS EELS IRAS

88Uvd 82Bib 88Uvd

atop O C CO "lying down" or inclined, upright CO

82Bib 95Vas 88Uvd 93Sin

155

0 - 0.5

468

1130 1345 1500 1920-2055 low-frequency comp. not seen by IRAS; 1885-1896 1927 1993-2040 2065 2068 2008 2049 2068,1931 2056, 1976 2041 1920 2045 - 2076

Technique

3.7.1 CO and N2 adsorption on metal surfaces

Mo(110)

Me-C

Adsorption site / configuration

Ni(111)

Structure

(√3×√3)R30°

0.5

(√7/2×√7/2)R19.1° 0.57

Vibrational frequency [cm−1] Me-C 400

400

400

0 - 0.2

Landolt-Börnstein New Series III/42A4

0.05 0 saturat. 0 - 0.57 0 - 0.57 0 - 0.2

380 380

C-O 1910 1878 1878 1900 1910, 2050 1898 1910, 2055 1910 2055 1910 1903- 1912 2055 1903- 1912 2055 2045, 1910 1817-1823 1817-1823 1831-1857 1831-1857 1817 1815 1912, 2033 1920, 2050 1830-1912, 2050 1820

Adsorption site / configuration

bridge fcc, hcp hollow bridge bridge, atop bridge bridge, atop fcc, hcp hollow atop bridge bridge atop fcc, hcp hollow atop atop, bridge 3 fold hollow fcc, hcp hollow bridge fcc, hcp hollow 3 fold hollow bridge atop, bridge atop, bridge fcc hollow, atop hcp hollow

Technique

References

EELS IRAS IRAS, XPD IRAS EELS IRAS IRAS IRAS, XPD IRAS, XPD IRAS IRAS TRAS IRAS, XPD IRAS, XPD IRAS IRAS IRAS, XPD IRAS IRAS, XPD IRAS EELS EELS EELS IRAS IRAS

79Cam, 80Erl 88Sur 94Dav 79Cam 80Erl 85Ryb 88Sur 94Dav 79Cam 88Sur 94Dav 79Cam 88Sur 94Dav 88Sur 94Dav 79Cam 80Erl 77Ber 87Fro 97Smi

3.7.1 CO and N2 adsorption on metal surfaces

c(4×2)

Coverage/ adsorbed state 0.33

156

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Ni(110)

Structure

(2×1) p2mg

Pd(100)

(2√2×√2) R45° (3√2×√2) R45° (4√2×√2) R45°

Pd(111)

C-O 1900 1960 2016, 1904

<0.85

430

1840, 1960

0.05 0.2 low ~0.5 increasing exposure

436 444

378

1855, 1960 1879, 1976 1880, 1990 1935, 2015 1594 1876-1900

402 459

2004-2012 2052

347 339 315 338 282

1952 1904 1960 1984 1984 1895 - 1949 1975 1890 - 1940 1849 1918

0.5 0.5 0.67 0.75 0.75 low - 0.5 0.6 low-0.75 1/3 0.5

450, 345

360

Technique

References

atop, short br. atop atop, tilted

EELS EELS EELS

90Voi 87Bau 81Nis

atop, bridge, ratio 2.5:1 atop, bridge atop, bridge atop, bridge atop step (bottom) bridge (step, terrace) atop (terrace) defect sites (step, terrace) bridge bridge bridge bridge bridge, tilt bridge bridge bridge hollow bridge

EELS

87Bau

EELS EELS EELS EELS EELS, LEED

81Nis

EELS EELS EELS EELS EELS IRAS IRAS EELS, SIMS IRAS IRAS

88Uvd 79Beh 88Uvd

81Ber 96Sve

78Bra1 82Ort 85Bro 83Hof1

157

(√3×√3)R30° c(4×2)

Me-C 410 430 444

Adsorption site / configuration

3.7.1 CO and N2 adsorption on metal surfaces

Ni(510)

Vibrational frequency [cm−1]

Coverage/ adsorbed state 1.0 1.0 >0.9

Cont'd.

(2×1)p2mg (1×1) (4×2) (4×2)

0.5-0.6

(√3×2)rect (√3×35)rect (√3×11)rect (√3×7)rect c(√3×5)rect c(4√3×8)rect (2×2)

(4×2) (2×1)p1g1 c(2×2) Landolt-Börnstein New Series III/42A4

p(4×2) (2×1)p2mg

Vibrational frequency [cm−1] Me-C

383

0.6-0.7 1 ~370 0-15 L at 110 K ~370 0.9-15 L at 110 K 0-15 L, 300K 347-379

Adsorption site / configuration

Technique

References

bridge bridge bridge bridge bridge atop, hollow atop, hollow atop, bridge atop, hollow atop, bridge hollow atop, hollow short bridge bridge bridge

EELS EELS EELS EELS IRAS IRAS IRAS SFG SFG SFG SFG EELS EELS IRAS IRAS

90Tüs1

bridge

IRAS

bridge bridge bridge atop

IRAS IRAS IRAS

1883-1912

bridge

IRAS

1972-2000

bridge

IRAS

C-O 1920 1910, 1957 1901, 1962 1962 1951 1951, 2097 1951, 2097 2083, 1955 2103, 1890 2093, 1940 1890 1823 - 1936 2003.5 1882-1912 1925, 1982, 1943 2068, 1992, 1951,1902, 1864 1959-1976 1903-1909 2003 1890-2000 ~2110

83Hof1

98Bou

00Sur 99Kat1 89Rav, 90Rav1 90Rav2

85Che1

3.7.1 CO and N2 adsorption on metal surfaces

Pd(110)

Coverage/ adsorbed state 0.5 0.514 0.545 0.571 0.6 0.63 0.75 <10−6 mbar >10−6 mbar domain boundaries 0.3 L - 2.7 L 1.0 0.05-0.3 0.3-0.5

Structure

158

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Pd(510)

Structure

p(2√2×√2)R45° c(5√2×√2)R45°

c(4×2)

0.5

c(4×2)

0.5

c(4×2)

0.5

c(4×2)

0.5

p(1×1) c(4×2) c(2×2) Pt(100)-(1×1)

Pt(111)

Me-C 354 346

480 380 480 380 468 363 451 369

Adsorption site / configuration

Technique

References

C-O 1948 1956 1996 2125 2083 2086,2089,1873 2030, 1950

bridge (terrace) bridge bridge (terrace) atop (step) atop atop, bridge atop, bridge

EELS, LEED

96Sve

IRAS IRAS EELS

90Hol, 95Mar 95Mar 83Beh

2065-2090 1867, 1910 2030, 1950

atop bridge atop, bridge

IRAS IRAS EELS LEED, EELS

90Hol, 95Mar 95Mar 80Cros, 83Beh 64Tuc, 78Bro, 80Cros, 83Beh

2065 2100 1850 2100 1850 2081 1855

atop 50% atop 50% bridge 50% atop 50% bridge 50% atop 50% bridge 50% atop 50% bridge 50% atop

EELS EELS EELS EELS EELS EELS EELS IRAS IRAS RAIRS

83Hay 86Lah

2104

82Ste 77Iba 97Eng, 99Eng 90Per

159

c(2×2) c(5√2×√2)R45° c(3√2×√2)R45° c(4×2) (√3×√3)R30° c(4×2)

0 >0 0.1-0.6 0.75 0.5 0-1 0-1 0.1-0.6 0.5 0.6 0.67 0.75 1/3 0.5

Vibrational frequency [cm−1]

3.7.1 CO and N2 adsorption on metal surfaces

Pt(100)-(hex)

Coverage/ adsorbed state 0.5 >0.5 saturated

Structure

Coverage/ adsorbed state

Vibrational frequency [cm−1] Me-C

Cont'd.

(1×1) (2×1)p1g1

Rh(100)

473-462 478-466 470 and 380 379

0.5-1 1 1 0-1 at 300 K low at 160 K 1 at 160 K low at 300 K 1 at 300 K 1 at 300 K

476 476 475

Landolt-Börnstein New Series III/42A4

c(2×2) (4√2×√2)R45°

0 0.5 0.75

c(6×2)

0.83

465 465 and 380

472

Technique

50% bridge

RAIRS IRAS IRAS EELS EELS SFG SFG IRAS SFG SFG SFG EELS EELS EELS IRAS EELS EELS SFG SFG EELS

2100 and 1850 1855 2082.7 ± 1.8 2093.3 ± 1.6 2084.0-2094.7 2095 1845 2045 2097 2097 2105 2080-2130 2105 2105 and 1855 2064.9 ± 1.6 2093.7 ± 1.6 2093.6

atop atop atop atop atop atop, bridge atop atop atop

1995 2052 2054 2031, 1944 2074

atop atop atop atop, bridge atop

atop, bridge bridge atop atop atop atop bridge

IRAS

IRAS

References

89Per, 90Ryb 89Mal 82Ste 79Hop 96Klü 88Ols 96Su

82Hof1 84Bar 87Hay 83Hof2 96Klü

94deJ

3.7.1 CO and N2 adsorption on metal surfaces

Pt(110)

0-0.5 0-0.6 0.17-0.58 0.5 0.5 1 0-0.6 10−7 up to 700 torr

C-O 1854

Adsorption site / configuration

160

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Coverage/ adsorbed state

Vibrational frequency [cm−1] Me-C

Rh(111)

(√3×√3)R30° (2×2) (4×4) (2×2) 3CO

0.08, 300 K 0.32, 300 K 0.48, 300 K 0.60, 300 K 0.10, 90 K 0.30, 90 K 0.50, 90 K 0.75, 90 K 0 - 0.83 at 90-300 K 1/3 0.25 0.5 0.75

2000 1990

420

2070 1870 2065 1855 2008 2008 - 1968 1968 2017 2017 2033 1984 2021

0.75 Rh(110)

Ru(0001)

Ru(0001) (√3×√3)R30°

0.05 - 0.50 0.50 - 0.71 0.71 - 1 0.1 0.3 0.6 ∼ 0.003 0.33

436 444 428

bridge atop, bridge atop, bridge atop, bridge atop, bridge atop, bridge atop, bridge atop, bridge atop, bridge atop, bridge atop atop atop 67% atop, 33% bridge atop bridge short bridge short bridge short bridge atop

References

IRAS IRAS IRAS IRAS IRAS IRAS IRAS IRAS EELS

90Leu

87Gur 81Koe 80Dub

84Koe EELS EELS EELS EELS

97Wei

CO stretch, 200 K IRAS

80Pfn

79Thi1

161

460 480

Cont'd.

Technique

3.7.1 CO and N2 adsorption on metal surfaces

436

C-O 1880-1970 2005, 1881 2026, 1908 2049, 1936 2062, 1942 2013, 1896 2034, 2023, 1898 2045, 1910 2086, 1964 2000, 1895

Adsorption site / configuration

(√3×√3)R30°

0.33

452.8 458.9

Cont'd.

Me-C

Ru(0001) Ru (1010)

(3×1) (2×1)p2mg ⎛ 4 1⎞ ⎟ ⎜ ⎝1 2⎠

0.33 1.0 >1

453 433 443

Ru(1120)

p(1×2) (1×2)-p2mg

0.25 0.5 Low

691 402-482 442

Intermediate

498

W(100)

α β

W(100)

α1 "virgin" α+β

363 548 (C) 605 (O) 360 545 625

Ru(1121)

C-O 2061 2030.8 1941 3940

Adsorption site / configuration

Technique

References

96Jak

at 30 K (IRAS) 13 16 C O overtone (2ν)

98Jak2 1901 1648 2000 2048 1810 2062 1552 1930-2050 1335 1946 1487 1769 2065

Landolt-Börnstein New Series III/42A4

2100 2065

Fermi resonance atop hollow atop

Tilted CO atop 4-fold hollow atop defect site bridge CO in atop, dissociated into C and O at 100 K; β state is dissociated CO

Theory

00Kop

EELS

89Lau

EELS EELS

01Wan1, 01Wan2 03Fan

EELS

77Fro

EELS

85Fra

3.7.1 CO and N2 adsorption on metal surfaces

Vibrational frequency [cm−1]

(√3×√3)R30°

Coverage/ adsorbed state 0.67 0.33

Structure

162

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Coverage/ adsorbed state

Vibrational frequency [cm−1] Me-C

References

CO stretch at 90 K at 295 K

IRAS

93Rif

2023-2099 2082-2102 2061-2070 1360 1960-2040

tilted CO upright CO

82Hou, 91Hou

3.7.1 CO and N2 adsorption on metal surfaces

W(110)

Technique

C-O

W(100) α α1 α2 0.23-0.5 0.23-0.77

Adsorption site / configuration

163

164

3.7.1 CO and N2 adsorption on metal surfaces

Table 6. Adsorbed CO (low frequency vibrational data by EELS, TEAS [03Gra] and IETS [02Ho])

Substrate

Structure

Coverage

θ Al(111) Ag/NiAl(110) Single atom Au/NiAl(110) Single atom c(2×2) Cu(100)

Cu(111)

low

0.5

c(7√2×√2)R45° 0.57 0 0 (√3×√3)R30° 1/3

Cu(110) (2×1)

Cu(211)

Adsorption site

atop atop atop atop atop atop atop atop, bent atop atop atop

0 0 0 0.5

atop atop atop atop atop 0.07, 40K atop atop 0.07, 110K atop atop atop single CO atop

Cu(511) Cu(211)

atop

Vibrational frequency [cm−1] 34.7 209.7 282.2 284.7 287.8 32.2-45.1 287.2±0.16 31.69±0.16 290.3 284 292.8 282.3 294.2 32.83±0.4 293±2.5 39.5±5 25±4 29±0.8 26±0.8 30±0.8 27±0.8 288 46.7 289 24.6±0.8 24.2±0.8 25.8±0.4 20.2±0.4 37 425 53 425 53

Fe(110) Ir(100)(1×1)

Step site Defect site p(1×2) c(2×2)

0.5

atop atop

Ir(100)(5×1)

(1×1)

0.5

atop

Ni(100)

c(2×2) (3√2×√2)R45°

0.5 0.67 0.07 0.5

atop bridge 258 atop, bridge 28.2 atop 30.6

c(5√2×√2)R45° 0.6

33% atop, 67% bridge

363

Mode assignment References

ν4 fr. Transl. ν3 fr. Rotation ν3 fr. Rotation ν3 fr. Rotation ν3 fr. Rotation ν4 fr. Transl. ν3 fr. Rotation ν4 fr. Transl. ν3 fr. Rotation ν3 fr. Rotation ν3 fr. Rotation ν3 fr. Rotation

03Gra 03Wal 03Wal 90Hir 95Hir 95Ell 98Gra2

ν3 fr. Rotation ν4 fr. Transl. ν4 fr. Transl.

88Uvd 87Ber

ν3 fr. Rotation

88Uvd

88Uvd 95Hir 99Lau 95Hir,94Hir, 93Hir, 90Hir ν3 fr. Rotation 95Hir ν4 fr. Transl. 96Bra, 02Hei ν3 fr. Rotation 99Lau ν4 fr. Transl [110] 97Ahn ν4 fr. Transl [100] ν4 fr. Transl [110] 98Bra ν4 fr. Transl [100] ν4 fr. Transl [110] ν4 fr. Transl [100] ν3 fr. Rotation 94Hir 99Mor ν4 fr. Transl. ν3 fr. Rotation 96Bra ν4 fr. Transl. ν4 fr. Transl. ν4 fr. Transl. ν4 fr. Transl. ν4 fr. transl. 92Toe ν3 fr. Rotation 91Kis ν4 fr. Transl. ν3 fr. Rotation 91Kis ν4 fr. Transl.

Lan dolt-Börn stein New Series III/4 2A4

3.7.1 CO and N2 adsorption on metal surfaces Substrate

Structure

Ni(111)

c(4×2)

Ni(110)

(2×1)p2mg c(4×2) c(8×2)

Pd(100) Pd(110)

(3√2×√2)R45° (4√2×√2)R45° (2×1)p2mg

Pt(110)-(1×2) (1×1) (2×1)p1g1

Pt(111)

Rh(111)

Ru(0001) Ru(0001)

W(110)

La ndolt-Bö rnstein New Series III/4 2A4

c(4×2)

Low θ (√3×√3)R30° (2×2)-3CO (√3×√3)R30° (√3×√3)R30°

Vibrational frequency θ [cm−1] 0.5 bridge 184 76 0.5 bridge 302 95 atop, bridge 384.7 1.0 100 40 30 31.4 atop 0.1-0.3 60.4 bridge 1.0 0.67 bridge 403 0.75 bridge 411 1.0 short bridge 201.6 427.5 338.7 69.4 bridge 69.2 1 atop 420 1 atop 420 475 1 at 300 K atop 404 411 atop, 50% 0.5 48.5 bridge, 50% 425 144 360 60 47.8 0.03-0.05 atop 64.4 bridge 0.25 55.9 atop atop 46.4 1/3 atop 45.2 3/4 atop 46.0 hollow 92.8 atop 45 40±8 0.33 atop atop 46.3 <0.09 atop 47.5 low 64 Coverage

Adsorption site

165

Mode assignment References

ν3 fr. Rotation ν4 fr. Transl. ν3 fr. Rotation ν4 fr. Transl. ν5 fr. Rotation ν6 fr. Transl. ν4 fr. Transl. ν4 fr. Transl. ν4 fr. Transl. ν4 fr. Transl. ν3 fr. Rotation ν3 fr. Rotation. ν3 fr. Rotation ν4 fr. Rotation ν5 fr. Rotation ν6 fr. Transl. ν4 fr. Transl. ν3 fr. Rotation ν3 fr. Rotation ν2 fr. Rotation ν3 fr. Rotation ν3 fr. Rotation ν4 fr. Transl. ν3 fr. Rotation ν4 fr. Rotation ν5 fr. Rotation ν6 fr. Transl. ν4 fr. Transl. ν4 fr. Transl. ν4 fr. Transl. fr. Transl. fr. Transl. fr. Transl. fr. Transl. fr. Transl. ν4 fr. transl. ν4 fr. Transl. ν4 fr. Transl. ν4 fr. Transl.

85Per 91Ha 90Voi

97Ber 88Uvd 99Kat1

00Kaw 84Bar

96Klü 82Ste, 86Lah, 93Sza1

98Gra3 98Gra1 93Wit 01Wit 79Wil 96Gie 97Bra 03Gra

166

Table 7. Adsorbed CO (additional structural data, bond lengths and bond angles) Substrate Ag(100) Ag(111)

Structure c(2×2) c(4×2) p(√3×√3)R30° p(2×2)

Cover./ Ad-state

Adsorption site

Bond length [Å] Me-C C-O

1/3

n-n 4.09 5.78 5.01 5.78

disordered

Co(0001)

(√3×√3)R30°

0.33

atop

1.78±0.06 1.17±0.06

Cu(100)

c(2×2)

0.5 0.5 0.5 0.5 0.5 0.5 0.58 0.57 0.57 0.57 0.6 0.6 1/3

atop atop atop atop atop atop atop atop atop atop atop atop T

1.90±0.10 1.13±0.10 3.61 1.92±0.05 1.13 3.62

c(7√2×√2)R45°

c(5√2×√2) R45° Cu(111)

(√3×√3)R30°

Landolt-Börnstein New Series III/42A4

(1.5×1.5)R18° 0.44

T T T

Tilt angle [deg]

References 76McE1

2.89

LEED

85Sch

2.88

LEED LEED

94San2 94San3 00Lah

LEED NEXAFS, PED LEED EELS, LEED NEXAFS LEED LEED LEED LEED EELS, LEED LEED LEED STM ARPEFS LEED, IRAS LEED ARPEFS

80And 86McC 82Bib 88Uvd 83Stö 80Ton 72Tra1 79Pri 82Bib 88Uvd 72Tra1 82Bib 99Bar 96Mol 85Hay2 80Hol 96Mol

2.04 in 1st layer 2.56 2.56

no tilt

3.6 1.15±0.10 1.90±0.10 1.15±0.10 3.61

Technique

2.56

3.6-3.0

2.56

2.56

2.56

1.91±0.01

2.56

1.91±0.02

2.56

6-15°

3.7.1 CO and N2 adsorption on metal surfaces

Au(110)

Me-Me 2.89

Landolt-Börnstein New Series III/42A4

Substrate Cont'd.

Cu(110)

Structure

Cover./ Ad-state

(1.39×1.39) hex (1.4×1.4)hex, (7×7)

0.52 0.52

(2×1)

0.52 0.5

Cu(210) Fe(100)

c(2×2)

α3 β α3

Fe(100) Fe(100)

c(2×2)

dissoc. CO 4-fold hollow 4-fold holl. dissociated 4-fold hollow

2.083 2.083

0.5 atop

n-n

Me-Me

Technique

References

LEED LEED LEED LEED, IRAS STM LEED,ESDIAD ARUPS XPD LEED, TDS LEED, IRAS LEED, IRAS LEED,ESDIAD LEED, TDS LEED, IRAS LEED, IRAS PED

80Hol 80Hol 79Pri 85Hay2 99Bar 96Ahn 95Hof 86Hol 85Har 82Woo 77Hor2 96Ahn 85Har 82Woo 77Hor2 01Ter

LEED

78Jon

54.7°

NEXAFS

89Dwy

55°±2° 45°±10° >45°

XPD NEXAFS CDAD LEED LEED

89Sai 87Moo3 94Wes 69Gra 78Nie

Tilt angle [deg]

2.556

no tilt no tilt no tilt ±10°

2.556

±2.5°

1.87±0.02 1.15±0.05

3.61

1.87±0.02 1.16±0.03

1.43

2.54±0.75 θCO=18±6 θCuC=6±5

167

Ir(100)(1×1)

0.25 C 0.25 O

Bond length [Å] Me-C C-O

3.7.1 CO and N2 adsorption on metal surfaces

(5/4×2) c(5/4×2) c(1.3×2)

0.25-0.5 0.5 0.5 0.8 0.8 0.77

Adsorption site T T, B B:T, 12:13 B:T, 12:13 B, T T T T T T (H) T T T (H) atop

Structure

Cover./ Ad-state

Cont'd. (1×1) (2×2) (1×1)

Ir(111)

(5×1) not lifted (√3×√3)R30°

Ir(111)

(2√3×2√3)R30°

0.25 <0.94 0.5

Bond length [Å] Me-C C-O

n-n

Me-Me

Tilt angle [deg]

atop

1/3

7/12

Landolt-Börnstein New Series III/42A4

3-fold H 3-fold H 3-fold H 3-fold H 3-fold H 3-fold H 2-fold Br 3-fold H 2-fold Br atop atop 2 fold br. 2 and 3 fold 2 and 3 fold 2 and 3 fold 2 and 3 fold 2 and 3 fold 2 and 3 fold

4.69

2.72

4.7

2.72

4.7

2.72

4.7

3.56 3.55 3.55 3.55 3.55

2.72 2.72 2.72 2.72 2.72

0 ±15°

Technique

References

LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED EELS IRAS/LEED LEED LEED LEED LEED LEED LEED LEED

91Kis 93Mar 91Kis 69Gra 76Bro 91Kis 93Mar 69Edm 71Gra 77Iva 81Sea 78Com 76Com1 76Com2 73Wei 74Doy 76Hag 76Küp 89Mar 96Lau 77Iva 76Hag 81Sea 78Com 76Com1 76Com2 76Küp

3.7.1 CO and N2 adsorption on metal surfaces

Ir(100)(5×1)

Adsorption site atop atop atop atop

168

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

4-fold H

EELS IRAS/LEED LEED LEED

3/4

4 and 2 fold

LEED LEED

0.33 0.8

top top

89Mar 96Lau 73Chr 78Tay1 78Tay2 78Tay3 78Nie 78Tay1 78Tay2 78Tay3 95Lyo

(2×2)

(4×2)

Ir(110) (331)+(111) microfacets Mo(100) Mo(100) Ni(100)

1 1/4

Adsorption site atop terminal

n-n

Me-Me

IRAS/LEED

α

θ <0.5 c(2×2)

T T T T T T T T

1.80±0.04 1.13±0.05 3.52

2.489

3.52 3.52 3.52 3.52 3.52

2.489 2.489 2.489 2.489 2.489

1.80±0.10 1.71±0.10 1.70±0.10 1.80±0.10 1.80

1.15±0.10 1.15±0.10 1.13±0.10 1.15±0.05 1.15

no tilt 40°±10° 40°±5° no tilt no tilt

no tilt no tilt

ESDIAD NEXAFS ARUPS ARPEFS LEED LEED LEED LEED LEED, EELS LEED LEED LEED LEED XPD

80Nie 87Ful 81Kev 82Bib 72Tra2 78Hor1 78All 88Uvd 79And2 80And 80Ton 79Hei2 79Pet

169

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

4-fold hollow T T

Tilt angle [deg]

3.7.1 CO and N2 adsorption on metal surfaces

References

Cont'd. Ir(110)(1×2)

Bond length [Å] Me-C C-O

Technique

Cover./ Ad-state

Cont'd.

Structure (3√2×√2)R45°

c(5√2×√2)R45°

c(4×2)

(2×2)

(√3×√3)R30°

Landolt-Börnstein New Series III/42A4

(√7×√7)R19°

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.25 0.25 0.25 1/3 0.33

Adsorption site

Bond length [Å] Me-C C-O

n-n

Me-Me

B B

tilted

B, T 1/3 B, 2/3 T

H fcc, hcp H fcc-H 64% hcp-H fcc-H 64% hcp-H fcc-H 65% hcp-H fcc-H 2/3 hcp-H fcc-H 64% hcp-H atop bridge 24% atop, B,76% H

0.33 1/3 0.57

bridge no tilt, atop tilt 1.78±0.10 1.19±0.23 2.88 2.88 2.88 1.32±0.10 1.29±0.05 1.27±0.07 2.88 1.29±0.07 1.32±0.07 2.88 1.28±0.04 1.29±0.08 1.18±0.07 2.88 1.34±0.07 1.15±0.07 1.30±0.08 4.89 1.26±0.04 1.8±0.04 1.13 4.89 1.27±0.05 1.13 4.33

4.33

T, B

Tilt angle [deg]

3.30 3.30

2.03 ass. 2.10±0.15 2.10±0.15 2.03 ass. 2.03 ass. 2.04+0.12 2.04-0.04 2.07±0.11 7+10° 2.07±0.11 6+10° 2.10±0.06 2.10±0.06 2.10 2.10

Technique

References

LEED LEED LEED, EELS LEED LEED LEED, EELS

72Tra2 82Bib 88Uvd 72Tra2 82Bib 88Uvd

LEED SEXAFS XPS PED PED PED

82Net 93Bec 98Hel 94Dav 93Sch 94Dav

PED

96Dav

PED

94Map

PED

96Dav

ARPEFS

81Kev

LEED

76Con

XPS LEED LEED XPS

98Hel 74Chr 76Con 98Hel

3.7.1 CO and N2 adsorption on metal surfaces

Ni(111)

Cover./ Ad-state 0.69 0.67 0.67 0.6 0.6 0.6

170

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Cont'd. Ni(110)

c(2√3×4)rec (2×1) p2mg

Bond length [Å] Me-C C-O

Adsorption site

1.0

B

1.0

B, T

2.49

1.0

B

2.49

1.0 1.0

B B

Intermed. B B B

1.83±0.02 1.117±0.1 1.21±0.03 1.84±0.02 1.19±0.03 1.15±0.07 1.85±0.04 1.12±0.05 1.95±0.05

n-n 2.77 2.49

Me-Me

2.490 2.492

2.49 2.492 2.49 2.49 2.49 2.49 2.49 2.49

1.94±0.02 2.49 2.49

1.13±0.05 1.92±0.05

1.0

B

2.49

1.20±0.04

2.49

1.87±0.03

c(4×2) c(8×2)

1.0 1.0 1.0 1.0 1.0 0.75 0.62 0.3 - 0.9

B B B B B B, T B, T

2.49 2.49

2.49 2.49 2.49 2.49 2.49 3.3 4.0

2.49 2.49 2.49 2.49 2.49 2.49

Tilt angle [deg]

θCO=23.9±0.7° θNiC=21.3±0.5° θCO=23.5±0.7° θNiC=21.7±0.5° θCO=20±4° θNiC=20±4° θCO=17±3° θNiC=27±3° θCO=19° θNiC=19° θCO=19° θNiC=16±2° θCO=17±2° θCO=21±3° θNiC=27±2° θCO=26±4° θNiC=26±3° θCO=20±5° θCO=17±2° θCO=17±2°

References

LEED XPS X-ray diff.

82Net 98Hel 01Pet

LEED

94Zha

LEED

88Han

XPD

92Kna

ARPES

93Hua

STM X-ray diff.

95Spr 95Rob

SEXAFS LEED ESD LEED LEED LEED

93Pan 86Kuh 85Rie 85Beh 73Mad3 85Beh

XPD

89Wes

171

Tilted Tilted no tilt no tilt θCO=(0-21)±1°

Technique

3.7.1 CO and N2 adsorption on metal surfaces

Cover./ Ad-state 0.57 0.62 1.0 at 2.3 bar 1.0 at 10−10 bar 1.0

Pd(100)

Structure (2√2×√2)R45°

Cover./ Ad-state 0.5

Adsorption site B

Bond length [Å] Me-C C-O n-n 1.93±0.07 1.15±0.10 2.745

Me-Me 2.74

Tilt angle [deg]

2.74

2.74 2.8

2.74 2.74

14-30°

2.8 2.8

2.74 2.74

29°

1.3±0.2 (3√2×√2)R45°

(4√2×√2)R45°

Pd(111)

c(4×2)R45° c(5√2×√2)R45° c(7√2×√2)R45° (√3×√3)R30°

Landolt-Börnstein New Series III/42A4

c(4×2)

0.67

0.75

0.5

B

B

B

3.9 2.74 2.74

1/3 1/3 1/3 1/3 1/3 1/3 1/3 0.5 0.5 0.5 0.5

Fcc H Fcc H Fcc H Fcc H Fcc H Fcc, hcp H Fcc H B B B B

4.71 4.71 4.71 1.29±0.05 1.15±0.05 4.71 1.25±0.04 1.11±0.04 4.71 4.71 1.27±0.04 1.25±0.14 4.71 3.6 3.6 3.6 3.6

2.748 2.2462 2.25

0±23°

References

LEED LEED LEED LEED LEED, EELS LEED X-ray diffr. LEED LEED, EELS LEED LEED LEED LEED, EELS LEED LEED LEED LEED LEED LEED LEED LEED LEED STM PED LEED LEED LEED STM

80Beh 78Bra1 82Ort 92Ber2 88Uvd 91And 96Sch 92Ber2 88Uvd 91And 82Bib 92Ber2 88Uvd 91And 69Tra1 82Bib 70Ert 83Hof1 78Con 87Oht 00Zas2 02Ros 98Gie 83Hof1 78Con 84Mir 02Ros

3.7.1 CO and N2 adsorption on metal surfaces

3.9

Technique

172

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

1

T

2.11±0.06 1.16±0.04 2.74

3.28

(2×1)p1g1 or p2mg (2×1)p2mg (2×1)p2mg +

1

B

1.97±0.03 1.15

2.74

3.28

0.7-1 0.4-0.7

B B

2.75

3.28

(√3×2)rect (√3×35)rect c(√3×17)rect (√3×11)rect c(√3×9)rect (√3×7)rect c(√3×5)rect

Pd(110)

0.5 0.514 0.529 0.545 0.556 0.571 0.6 0.6

c(4√3×8)rect

0.63

split (2×2)

2/3

(2×2)

3/4

(2×1)p2mg

Me-Me 2.25 2.25

1.84±0.14 1.15

Tilt angle [deg] 0±25° 0±25°

Alt. tilt. 6±6, 24±3 Alt. tilt. 2±5, 11±4 22±5 Alt. tilt Alt. tilt

Technique

References

PED

98Gie

LEED

90Tüs1

LEED LEED LEED LEED LEED LEED STM

83Hof1 78Con 83Hof1 78Con 83Hof1 78Con 02Ros

LEED LEED LEED, XPS XPD

83Hof1 74Con1 97Ram 94Loc

TLEED

93Wan

PED

02Kit

LEED,EELS

99Kat2

173

(2×1)

Cont'd.

Cover./ Ad-state 0.5

3.7.1 CO and N2 adsorption on metal surfaces

Bond length [Å] Me-C C-O n-n 1.31±0.06 1.14±0.12 3.6 1.37±0.06 1.14±0.14 3.6

3/4 0.7-1.0 0.75 1

Adsorption site Fcc H Hcp H B B B B B B B B B B, H, T B, H, T T, H T, H T, fcc H, hcp H T, H Short B B Short B

Structure

Cont'd.

Structure (3×1) (2×1) c(2×4) (2×1)p2mg (4×2)

Adsorption site

0.3-0.4 0.1-0.3

B B B B B B

0.3-0.75

Bond length [Å] Me-C C-O

n-n

Me-Me

Tilt angle [deg]

Pt(100)(hex)

0.05-0.3

(1×1)

B and T c(4×2)

0.75

B and T

Landolt-Börnstein New Series III/42A4

Pt(100)(1×1)

c(2×2) c(2×2) or (√2×√2)R45°

c(5√2×√2)R45°

0.5 0.2-0.5 0.5 0.5 0.5 0.6

T, islands T, islands 1/3 B, 2/3 T B B 2/3 B. 1/3 T

References

LEED LEED LEED,IRAS LEED LEED,XPS LEED LEED LEED, IRAS LEED LEED LEED LEED,EELS LEED LEED LEED LEED LEED LEED,EELS LEED LEED,EELS LEED,EELS LEED,IRAS LEED,EELS LEED LEED,IRAS

00Yag 91Hu 90Rav2 00Yag 97Ram 88He 74Con1 90Rav1 81Nor 75Kne 81Bar2 83Beh 68Mor 64Tuc 81Nor 75Kne 81Bar2 83Beh 68Mor 83Beh 83Beh 95Mar 78Pir 82Bib 95Mar

upright upright

0.75 (2×1)p2mg (4×2) (1×1)

Technique

normal

3.7.1 CO and N2 adsorption on metal surfaces

Cover./ Ad-state

174

Substrate

Landolt-Börnstein New Series III/42A4

Substrate Cont'd.

Adsorption Bond length [Å] Me-C site C-O 1/2 B, 1/2 T

c(4×2)

0.75

2/3 B, 1/3 T 2/3 B, 1/3 T

(√3×√3)R30°

1/3

c(4×2)

0.5

T T T T 1/2 T 1/2 B 1/2 T 1/2 B 1/2 T, 1/2 B ~² T, ² B ³ T, ½ B 0.88 T 0.12 B T T T

0.5

Pt(110)(1×2)

0.5 (√3/2×√3/2)R15° 0.58 0.67 2/3(√3×3)rect 1/3 (2×2) disordered 0.18 (4×4) 0.19 0.3 (8×8) 200 Torr hexagonal (√19×√19)R23.4° 0.67 0.5-1 (1×1)

n-n

4.80

1.85±0.025 1.85±0.025 1.92±0.04 1.40±0.04

2.78

1.15±0.05 T:5.4;4.8 2.769 1.15±0.05 T-B:3.7 1.12±0.04 2.77 1.19±0.04

Tilt angle [deg]

Normal

Normal Normal Normal

Tilt? 1.85±0.10 1.15±0.10 2.08±0.10 1.15±0.10

2.77

3.7 3.7

2.77 2.77 2.77 25°

Technique

References

LEED,IRAS LEED,EELS LEED,IRAS LEED ARUPS LEED,EELS LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED LEED

95Mar 83Beh 95Mar 78Bro 82Bro 83Beh 77Ert 91Won 82Ste 90Ryb 87Ogl 88Bla 00Zas1 82Ste 90Ryb 82Ste 98Ma 87Ogl

LEED LEED

87Tüs 90Ryb

STM STM LEED LEED, ARUPS LEED RBS

98Jen 94Vil 76Com3 82Hof1 82Fer 82Jac1

175

T T T T

Me-Me

3.7.1 CO and N2 adsorption on metal surfaces

Pt(111)

c(3√2×√2)R45°

Cover./ Ad-state 0.67

Structure

Structure

Cover./ Ad-state

Cont'd.

(2×1)p1g1

Rh(100)

c(2×2)

Landolt-Börnstein New Series III/42A4

(4√2×√2)R45°

1 1 1 >0.2 at 160 K 2L 0.5

0.75

1.9 ± 0.1 T T T T T T 4 fold H 0.7 B, 0.3 T 2/3 B, 1/3 T 0.5 B, 0.5 T 2/3 B, 1/3 T

n-n

Me-Me

Tilt angle [deg]

Zig-Zag 26 ± 2°

2.77 2.77

normal

20° normal

2.77 2.77

15 ± 5°

1.3 ± 0.1

10° - 24° 3.80 3.80 3.80 3.80 3.80 3.80

2.69 2.69 2.69 2.69 2.69 2.69

3.59 2.69

2.69 2.69

normal

tilt normal

Technique

References

LEED LEED LEED LEED LEED ARUPS

72Bon1 74Lam1 74Lam2 76Com3 82Hof1 82Bar 84Bar 86Fre1 84Rie

EELS,ARUPS LEED, XPS UPS RBS LEED,ARUPS UPS ARUPS, LEED RHEED XPS, LEED EELS, LEED XPS, LEED LEED, IRAS LEED, EELS LEED LEED XPS, LEED XPS, LEED LEED, IRAS LEED, EELS LEED

82Jac1 82Hof1 82Fer 83Hof2 95Sch 98Str 97Wei 96Bar 94deJ 87Gur 82Kim 78Cas 98Str 96Bar 94deJ 87Gur 82Kim

3.7.1 CO and N2 adsorption on metal surfaces

(2×1)p2mg c(8×4)

1

Adsorption Bond length [Å] Me-C site C-O T T T T T T T 1/4 B, 3/4 T 0.16 B, 0.84 T T T T 1/3 B, 2/3 T

176

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Cover./ Ad-state

Cont'd. c(6×2)

"split (2×1)" (2×2) at <120 K

0.83 0.25

(1×1) at >120 K

0.25

(√3×√3)R30°

1/3 1/3 1/3

(4×4)-8CO at <120 K split (2×2)

0.5 >0.5

T, B, H

(2×2)-3CO

0.75 0.75 0.75

T, H

0.75

1.94±0.1 1.15±0.1 2.03±0.07 1.15±0.1

1.86±0.05 1.17

3.23 2.69 5.38 5.38

Me-Me

2.69 2.69 2.69 2.69 2.69 2.69 2.69 4.7 2.69 4.7 2.69 4.7 2.69 4.7 2.69 4.7 2.69 2.69 2.69 3.6 2.69 2.69 3.2 2.68 3.11 2.85±0.1 2.68 3.23±0.1 2.69 2.69 3.1 2.69 3.1 2.69 3.1 2.69 3.1

Tilt angle [deg]

tilt

Normal

<15° normal

normal

Technique

References

LEED XPS, LEED

66Tuc 98Str

XPS, LEED

96Bar

LEED, IRAS LEED LEED, XPS LEED, XPS

94deJ 78Cas 97Beu 98Beu

XPS LEED LEED LEED LEED, XPS LEED, XPS LEED, XPS LEED, XPS LEED EELS LEED LEED LEED LEED LEED, XPS LEED, XPS LEED, XPS SXD

01Sme 78Cas 79Thi2 81Koe 97Beu 98Beu 97Beu 98Beu 78Cas 80Dub 78Cas 79Thi2 83vHo1 97Beu 98Beu 01Sme 99Lun

177

0.75

2/3 T 1/3 B 1/3 T 1/3 H hcp 1/3 H fcc 1/3 T

n-n 3.17

3.7.1 CO and N2 adsorption on metal surfaces

Rh(111)

0.83

Adsorption Bond length [Å] Me-C site C-O T, B, H 0.45 B, 0.55 T 0.55 B, 0.45 T 1/5 B, 4/5 T B, H T T T T H T 1.95±0.1 1.07±0.1 T T T

Structure

Cover./ Ad-state

Cont'd. (2×1) (√7×√7)R19° (2×2)-3CO

(2×1)p1g1 c(2×2) c(2×2) split c(2×2) (3×2) (4×2) (5×2) (2×1)p2mg (1×1) c(2×2) streaky c(2×2) (2×1)p2mg (2×1)p2mg

Landolt-Börnstein New Series III/42A4

Rh(110)(1×2)

(1×2)

(2×2) poor

0.5 3/7 5-700torr 0 to 0.75 T, B 0 to 0.75 T, H hcp, H fcc Short bridge 1 0.5 Short bridge 0.3-0.5 Short B 0.5 Short B 2/3 Short B 0.75 Short B 4/5 T 1 T 0.18 at 120 K T 0.37 T 0.54 0.57 B, 1.0 at 0.43 T 120 K 0.80 B, 1.0 at 0.2 T 270 K 0.45 B, 0.24 at 120 K 0.55 T 0.46 at

0.54 B,

Bond length [Å] Me-C C-O 1.49±0.05 1.16 1.49±0.05 1.16

n-n 3.1 3.1

Me-Me 2.69 2.69

Tilt angle [deg] normal normal

Technique SXD SXD STM STM STM LEED,XPS XPS

References

00Cer

84deL 01Sme

LEED

77Mar

LEED

94Wei

LEED, XPS

93Dha

LEED, XPS

93Dha

13°

3.7.1 CO and N2 adsorption on metal surfaces

Rh(110)(1×1)

Adsorption site 1/3 H hcp 1/3 H fcc

178

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Cont'd. c(2×4) diffuse c(4×2) (2×2)p2mg (√3×√3)R30° (√3×√3)R30°

Adsorption site 0.46 T 0.46 B, 0.54 T 0.57 B, 0.43 T 0.53 B, 0.47 T atop atop

Bond length [Å] Me-C C-O

2.0±0.1

1.1±0.1

1.93±0.04 1.10±0.05 Ru(0001)

(2√3×2√3)R30° disordered

0.05-0.2

Ru(0001) W(110) W(111)

0.23-0.5 α (v)

atop atop hollow

2.0 ± 0.15 (1.1) 1.92 1.165 1.58 1.199

n-n

Me-Me

Tilt angle [deg]

Technique

References

no tilt 12°-16°

LEED ESDIAD

dynamic tilt, temp. depend. ≤ 10° tilt upright

LEED

83Mic 79Mad, 93Ove 96Gie

ARUPS DLEED Theory

85Hof 89Pie 00Kop

ESDIAD ESDIAD

82Hou 76Mad

∼ 70° no tilt at 100K

3.7.1 CO and N2 adsorption on metal surfaces

Ru(0001) Ru(0001)

Cover./ Ad-state 120 K 0.52 at 270 K 0.75 at 270 K 1.0 at 270 K 0.33 0.33

179

Substrate

Ag(110) Al(111)

Structure

Coverage/ adsorbed state

disordered disordered

Au(110)

disordered

Co(0001) (√3×√3)R30° (2√3×2√3)R30° Co(1010)

(2×1)

Cr(110)

c(4×2)

Cu(100)

c(2×2)

Cu(111)

(√3×√3)R30°

Cu(110)

(2×1)

1/3 7/12

α1 α2 0.5

0.33 0.33 0.5

Landolt-Börnstein New Series III/42A4

0.3 Fe(110) (1×2)

0.5

5σ 9.1 7.9 12.35 12.99 9.8±0.6 8.2 7.9±0.1

1π 11.9 11.0 15.33 15.99 10.0 9.8±0.6 6.8 7.25±0.3

4σ 14.8 13.7 18.04 18.74 12.5 12.7 10.8 10.75±0.05

∼7.5

∼7.5

11

∼7.5 ∼7.5 8.5 8.5 8.5 8.8 8.6 8.5 8.4

∼7.5 ∼7.5 8.5 8.5 8.5 7.5 8.6 8.5 8.4

11.6 10.8 11.8 11.5 11.8 12.0 11.7 11.6 11.8

8.5 7.6 8.1

8.5 7.6 6.9

12.0 10.6

Technique

Reference

UPS UPS UPS

84Kra 80Chi 89Jac

UPS UPS UPS ARUPS normal emission UPS

89Düc, 94San2

82Pap

UPS

86Shi

UPS UPS UPS XES UPS UPS RPES UPS ARUPS UPS

85Hes 77All2 94San3 00Föh1, 00Föh2 86Kir 75Con 85Che2 82Mar 78Kan 79Bro, 83Jen 94Mar

79Fre, 83Pap 83Gre

3.7.1 CO and N2 adsorption on metal surfaces

monolayer multilayer

MO valence levels [eV]

180

Table 8. Adsorbed CO (valence electronic structure). Electron binding energies are referenced to the Fermi-level.

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Cont'd. Fe(100) Fe(100)

(2√3×2√3)R30°

Ir(110) Mn/Cu(100) Ni(100)

c(8×2)-Mn c(2×2)

5σ 7.5-8.3

8.54 7.37 9.2 7.2 top 1/3 9.1 7.3 7/12 8.5 9.2 7.8 8.6 0.83 ML Mn 7.4 0.5 7.2 0.5 8.3 0.5 8.0 0.5 7.5 0.5 8 0.5 8 0.5 7.5

Technique 4σ 10.5-10.7

Reference

1π 7.5-8.3 cov. dep. 7.1

10.5

UPS UPS

77Rho 88Cam

8.93-9.21 8.72-9.65 8.6 8.5 8.8 8.7 9.0 8.6 8.6 7.2 7.4 5.5 7.8 7.5 7.5 8 8 7.5

11.45 10.81 11.7 11.3 11.7 11.2 11.9 11.7 11.3 11.2 10.7 10.3 10.8 10.6 10.7 11 11 10.7

Theory

89Ron

ARUPS UPS ARUPS UPS/XPS ARUPS ARUPS UPS UPS UPS XES UPS ARUPS PES UPS ARUPS UPS

80Sea 76Bro 81Sea 78Zhd 81Sea 80Sea 76Zhd 89Düc 99Grü 00Föh1, 00Föh3 77All1 78All 94San3 80Smi 78Hor1 71Eas

ARUPS

3.7.1 CO and N2 adsorption on metal surfaces

α1 α2 α3 β atop bridge

(1×1) (√3×√3)R30°

MO valence levels [eV]

7.1

c(2×2)

Fe(111) Ir(100) Ir(100)(5×1) Ir(111)

Coverage/ adsorbed state ∼ 0.1-0.5

181

Cont'd.

Structure

(3√2×√2)R45°

Coverage/ adsorbed state 0.66 0.69

Ni(111)

Ni(110)

(2×1)p2mg

Os (poly) Pd(100)

(2√2×√2) R45°

Pd(111)

Pd(110) Pt(100) Pt(111)

(√3×√3) R30° c(4×2)

c(4×2) (4×4)

1.0 1.0 1.0 ∼0.4 0.5 0.5 L-20 L 1/3 0.5 1 0.75

Landolt-Börnstein New Series III/42A4

10 L Pt(110)

0.6

5σ 8.2 6.5 8.7 8.4 8.5 8 7.8 8.5 8.4 8.4 8.0 8.3 7.6 7.9 8.2 7.8 7.8 7.9 8.8 9.2 9.2 9.6 8.6 8.1 9.2 9.2

1π 7.2 7.9 7.1 7.3, 6.1 8 7.8 8.5 6.7 6.5 8.0 8.3 7.3 7.9 7.5 7.3 7.8 7.9 7.6 8.3 8.4 8.6 8.6 9.1 7.9 8.2

4σ 10.8 10.9 11.7 11.2 11.2 10.9 10.6 11.6 10.5 - 11.4 11.7 11.0 11.0 10.5 10.8 11.2 10.7 11 10.8 11.4 11.7 11.9 12.0 11.7 11.8 11.6 11.7

Technique

Reference

UPS UPS UPS ARUPS ARUPS UPS UPS UPS ARUPS UPS UPS UPS UPS UPS UPS UPS UPS UPS ARUPS UPS UPS UPS UPS UPS YPES UPS

83Koe1 81Bru 76Wil 90Sch 88Gum 76Con 86Wur 86Kuh, 89Kuh 82Hor 84Rie 80Fuk1 94San1 80Beh 76Llo 84Mir 95Ban 78Weh 82Bro 94San1 79Nor 89Mur 81Mil 89Düc 81Bar1

3.7.1 CO and N2 adsorption on metal surfaces

0.5 c(4×2) (√7/2×√7/2) R19.1° 0.57 0.57 (√7/2×√7/2) R19°

MO valence levels [eV]

182

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Cont'd. (2×1)p1g1 c(4×2) Re(0001)

(2×1)p2mg

(√3×√3)R30° (2√3×2√3)R30°

5σ 8.7 9.25 9.2 7.9 α ∼7 7.3-8.15 7.6 Sat. at 300 K 8.3 1 7.6

W(100)

W(100) W(110)

7.8 8.7

4σ 11.8 11.7 11.7 11.0 10.8 11.1 10.8 11.2 10.6

1π 8.7 8.2 8.3 7.9 8.15 7.3-8.15

Technique

Reference

UPS UPS UPS UPS

77Shi1 82Bar 82Hof2 80Fuk2 85Tat 80Duc 83Koe1 78Bra2 81Bai 80Bai 75Fug1 85Shi1

UPS UPS UPS

α

7.7 7.7

7.7 7.7

10.5 10.8

UPS

0.33 0.56

7.55 7.9 7.4

7.4 7.3 7.4

10.5 10.6 10.6

ARUPS

8.9 5.9 (C) 7.6 8.7 6.5 8.7 8.3 ∼7.3 7.8

8.9

Ru (1010) W(100)

MO valence levels [eV]

7.6 8.7 6.5 8.70 8.3 ∼7.3 7.8

11.4

75Bon1 UPS

73Bak

UPS

76Vor

ARUPS

76Plu

11.4 10.4 10.8

183

α β "virgin" α β α1 α2 "virgin" α

85Hof

3.7.1 CO and N2 adsorption on metal surfaces

Rh(100) Rh(111) Rh(110)(1×1) Ru(0001) Ru(0001) Ru(1,1,10) Ru(0001)

Coverage/ adsorbed state 5L 1

184

Table 9. Adsorbed CO (core level binding energies; adsorption states; molecular/atomic species). Electron binding energies are referenced to the Fermi-level. Substrate

Coverage/ adsorbed state

(√3×√3)R30°

Co(1012)

Cu(100)

c(2×2) c(7√2×√2)R45°

Cu(110) (2×1)

0.5 0.57 Sat. 0.5

Fe(110)

Fe(100) Fe(100)

α1 α2 α3 β

Fe(111) Landolt-Börnstein New Series III/42A4

Fe/W(110) Ir(111)

α (v), <300 K β, >300 K (2√3×2√3)R30° 7/12

Chemical state

Core level energies [eV]

References

CO CO CO CO

C 1s 286.4 286.8 285.8±0.4 285.3

O 1s 533.9 533.7 531.9±0.4

94San2 94San2 83Gre, 00Cab, 00Fre 81Pri

atop atop

286.3 286.3

atop CO Dissociated CO CO at 123-150 K CO CO CO tilted Dissociated CO Dissociated CO Dissociated CO

286

533 533 533.2 533 531.9 530.3 531.6 532.1 532.4 532.2 531.4 530.6 529.9 532.0 530.0 531.8 530.1 530

285.5 285.6 284.8

282.0 285.4 282.8

92Bjö 90Ant 92Chr 86Hol 79Bro 98Sey 78Bru 80Ben 87Moo1

77Tex 97Nah 76Zhd

3.7.1 CO and N2 adsorption on metal surfaces

Ag(110) Au(110) Co(0001)

Structure

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Coverage/ adsorbed state

Chemical state

Core level energies [eV] C 1s

c(4×2)

7L 0.5

(√3×√3)R30° (√7×√7)R19°

1/3 0.57

c(2√3×4)rect

0.62

Mo(100) Ni(100)

c(2×2) (3√2×√2)R45°

Ni(111)

Os (poly) Pd(100)

(2√2×√2)R45°

∼0.4 0.5

CO Dissociated CO Dissociated atop atop bridge 33% atop, 67% bridge

fcc-hollow hcp-hollow fcc-hollow bridge atop interm. bridge CO bridge

286.0 284.5 285.8 285.8 285.7 285.6 285.8 285.5 285.5 285.24 285.24 285.24 285.32 285.96 285.32 287 286.3 285.9

(3√2×√2)R45°

0.67

bridge

286.0

531.2 531.6

(4√2×√2)R45°

0.75

bridge

286.1

531.7

93Sie 85Zae 92Bjö 98Föh, 99Föh1 88Uvd 81Bru 88Uvd 83Koe2 98Hel

185

84Jug 80Fuk1 92Bjö, 94San1, 94San2, 94San3 95Ped 92Bjö, 94San1, 94San2, 94San3 92Bjö, 94San1, 94San2, 94San3

3.7.1 CO and N2 adsorption on metal surfaces

c(5√2×√2)R45°

α β α-states β-states 0.5 0.5 0.67 0.69 0.6

Mn/Fe(110)

O 1s 531.8 530.2 532.5 530.8 532.2 531.4 531.5 531.5 532.2 531.3 531.2 530.86 530.86 530.86 531.01 532.15 531.01 532.3 532.4 531.4

References

Structure

Pd(111)

(4×2)

c(8×4) (2×1)p2mg c(8×4) (4×4) c(4×2)

low 0.75 at 135 K 0-1 at 300 K 1 at 120 K 1.0 at 80-600 K 1.09 at 80-240 K 0.18 0.5

c(5×√3)

0.6

(1×1) (2×1)p2mg Pt(110)-(1×2)

Pt(111)

α, at 100 K

Re(0001) Landolt-Börnstein New Series III/42A4

(2×2) Re(0001) Re/Pt(111)

0.03 0.1 0.21 0.32 0.41 0.48 0.50 0.75, 300 K

β, at 420 K α β 0.55 ML Re

Chemical state

100% hollow 94% H, 6% bridge 89% H, 11% bridge 76% H, 24% bridge 63% H, 37% bridge 45% H, 55% bridge 44% H, 56% bridge bridge on ridge bridge in grooves bridge on ridge atop 75% atop, 25% bridge atop 80% atop, 20% bridge atop 50% atop, 50% bridge 33% atop, 67% bridge CO CO Dissociated CO Dissociated CO at Pt-site

Core level energies [eV] C 1s 285.53 285.53, 285.73 285.53, 285.73 285.54, 285.74 285.55, 285.74 285.60, 285.78 285.59, 285.76 286.3 286.1 284.8 286.3

287.1 287.1 286.7 286.7 286.0 286.7 286.0

286.0

References

O 1s 00Sur

97Ram

532.7 532.8, 531.0 533.2 533.1, 531.2 532.6 532.7 531.0 532.7 531.0 533.6 532.1 530.5 532 530 531.1

86Fre1 01Now 94Bjö 95Bar

85Tat

80Duc, 80Fuk2 99Ram

3.7.1 CO and N2 adsorption on metal surfaces

Pd(110)

Coverage/ adsorbed state

186

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Coverage/ adsorbed state

Chemical state

Cont'd. Rh(100)

0.5 0.75 0.83

(√3×√3)R30° (2×2) (4×4) (2×2) 3CO

low 1/3 0.25 0.5 0.75

Rh(111)

0.75 3L

1 L-20 L Rh(110)(1×1)

1 (10 L dose)

(1×1) c(2×2) streaky c(2×2) (2×1)p2mg

0.18 at 120 K 0.37 0.54 1.0 at 120 K

O 1s 532.8 98Str 532.4 531.5

284.04 286.0 286.0 286.0 285.35 286.0 285.99 285.29 286.03 285.42 285.25 286.1 285.3 285.4

532.1 532.1 532.1 530.6 532.1

96Bar 01Sme 97Beu

01Sme

532.0 530.8 531.3 530.8 531.9

84deL 80Bai, 81Bai 93Dha

187

(2×1)p2mg

CO at Re-site atop bridge atop bridge atop atop atop atop 67% fcc hollow 33% atop 33% atop 33% fcc H, 33% hcp H atop bridge hollow atop (bridge) atop bridge atop atop atop atop 57% bridge, 43% atop

C 1s 286.6 285.7 285.85 285.45

References

3.7.1 CO and N2 adsorption on metal surfaces

c(2×2) (4√2×√2)R45° c(6×2)

Core level energies [eV]

Structure

Coverage/ adsorbed state

Chemical state

Core level energies [eV] C 1s

Rh(110)(1×2) 0.24 at 120 K 0.75 at 270 K 1.0 at 270 K α β α β Ru (1010)

θ ≤0.6 θ >0.6 θ >1

Ti(0001) Ti(0001)

W(poly)

Landolt-Börnstein New Series III/42A4

W(100)

atop CO atop bridge dissociated dissociated CO, bulk C and O phases

α1 α2 "virgin" β α1

CO CO CO dissociated CO

α "virgin", β1

CO CO and dissoc.

281.8±0.2 281.9 283.2 284.4 ≈287.3 ≈287.3 ≈285.4 ≈283.1 286.2

531.7±0.2 531.85±0.15 530.1±0.2 532 ∼530.5 531.7 531.9 531.9 530.9 529.8±0.2 530.4 531.8 534.2 532.8 531.5 530.5 533.0 533 531.7

93Dha

75Fug2, 75Fug3, 77Fug 85Shi1

96Rot

78Fuk, 80Fuk3 98Kuz

74Yat1

74Yat2, 74Yat3 76Yat1

3.7.1 CO and N2 adsorption on metal surfaces

Ru(0001)

(1×2) c(4×2) (2×1)p2mg (√3×√3)R30°

bridge atop 45% bridge, 55% atop 57% bridge, 43% atop 53% bridge, 47% atop CO

O 1s 530.8 531.9

References

188

Substrate

Landolt-Börnstein New Series III/42A4

Substrate

Structure

Coverage/ adsorbed state

Zn/Ru(0001) O/Zn(0001)

dissociated CO CO dissociated CO physisorbed CO

References

Core level energies [eV] C 1s 283.2 285.8±0.5 285.5±0.3 283.1±0.3

291.5

O 1s 530.1-530.6 532.0±0.4 531.6±0.3 530.4±0.2 535.2

93Rod

537.7

90Car

77Umb, 83Umb 77Ste1

Table 10. Molecular N2 adsorption (thermodynamics) Substrate

Structure

Cr(110)

Fe(110) Fe(100) Fe(111)

Coverage/ adsorbed state

Heat of adsorption Ead [kJ/mol]

0.36 γ state

c(2×2)

β β α γ

209 222 218 31.4 ∼24

Technique

Activation energy of desorption Ed [kJ/mol]

TDS

14 28 234 243 214

TDS TDS Isotherms TDS TDS

Sticking Pre-exponential factor coefficient s(θ) νd [s−1]

1×1013 ass. 1×1013 ass. 1×1013 ass.

∼2×10

References

0.09

84Miy 91Dow

10−7 - 10−6 10−7 - 10−6 10−7 - 10−6 0.003 at 120 K

77Boz2 77Boz1, 77Boz2

3.7.1 CO and N2 adsorption on metal surfaces

β2, β3 α "virgin" β 0.16-0.57 ML Zn ∼ 1 ML O

Cont'd. W(110)

Chemical state

82Ert2

10

84Gru1, 84Gru2, 85Str

189

Structure

Coverage/ adsorbed state

Heat of adsorption Ead [kJ/mol]

β,

Fe(111)

Ir(110)-(1×2) p1g1(2×2) Ir(100)-(1×1) Ir(100)-(5×1)

Mo(110) Mo(100) Mo(111)

Ni(100)

clean c(2×2)-C c(3×2)-N, facets to (433) at 850 K c(2×2)

Landolt-Börnstein New Series III/42A4

c(2×2) Ni(111)

γ (on α) 0-1 1 γ, 90 K 97 K 130 K 160 K β at 300 K

Sticking Pre-exponential factor coefficient s(θ) νd [s−1] 1×10−6 at 300 K, ∼0.1 at high incident kinetic energy

31 38

30 25

Isobars TDS Isobars TDS

20.7 24.5 25-28

2.1×1013 1×1013

35.5-25.1

108 - 1011

TDS TDS

β at 300 K θmax = 0.35

AES

0.5, γ, 120 K 0.1 - 0.5 0.5 35 0 at 90 K

TDS, LEED TDS LEED TDS TDS

339±13

25 44 - 25

0.01

1×1013 ass. 1×1013 ass. 13

20

1×10 ass. 1×1013 ass.

87Ret1, 87Ret2

87Gru2

1

1×1013 ass. 1×1013 ass.

TDS TDS

References

81Ibb 93Gar 93Gar

0.09 0.6±0.1 at 200 K <1×10−3 0.05

72Mah 80Ko1

1 1- 0

84Gru3

83Ega

84Dow 91Yos 86Bre

3.7.1 CO and N2 adsorption on metal surfaces

δ γ

Activation energy of desorption Ed [kJ/mol]

MB, variable incidence energy

θmax = 0.55

Fe(111)

Technique

190

Substrate

Landolt-Börnstein New Series III/42A4

Structure

Coverage/ adsorbed state

Ni(110)

(2×1) fluid c(1.4×2)

0 - 0.65 0.65 - 0.75 0.75 - 1.0 0-1 0.5 0.5 - 0.67 0.72 0.5 0.72 0 - 0.3, >140K 0.5 0.5 - 0.72 0.72 γ at 78 K γ at 120 K 120 K γ β α β α β

(2×1) fluid c(1.4×2) (2×1) (2/3×1/3) (1×1) gas (2×1) fluid c(1.4×2) Pd poly Pd(110) Pt(111) Re(0001) Re(0001) Re (1120)

36.6-42-20

Technique

TDS TDS TDS TDS LEED LEED LEED, TDS LEED LEED LEED LEED LEED LEED TDS TDS TDS TDS, FIM TDS TDS

TDS, FEM

Activation energy of desorption Ed [kJ/mol] 42 42 - 21 21

Sticking Pre-exponential factor coefficient s(θ) νd [s−1]

References

1×1013 ass. 1×1013 ass. 1×1013 ass. 8×1012

84Gru3 83Gru2 84Gru3 80Gol 88Kuw

1 - 0.45 0.45 - 0.5 0.5 - 0

13 83Jac1 83Gru1

25 - 41 25 40 ∼40

1×1013 ass. 1×1013 ass. 1×1013 ass. 1×1013

24±1 260±20 21±2 29±2 240±40 38

1×1011 1×10−2 1×109 1×1010 1×10−2 1×1013 ass.

0.67 0.15 adsorbs at 80 K <10−5 at 300 K 0.88 9×10−6 0.92

68Kin 87Kuw 77Shi2 76Liu 87Haa1 87Haa1

4×10−4 83Hen

191

Rh(100)

(2×1)

Heat of adsorption Ead [kJ/mol]

3.7.1 CO and N2 adsorption on metal surfaces

Substrate

Structure

Rh(111) Rh(110) Rh (poly)

Ru(0001)

Ru(0001) Ru(0001)

(√3×√3)R30° (√3×√3)R30°

clean

γ1 at 110 K γ2 at 160 K γ3 at 225 K γ 75 K 95 K δ γ γ β β

Heat of adsorption Ead [kJ/mol]

Technique

TDS, FEM TDS, FEM TDS, FEM TDS, FEM TDS, FEM TDS 28 42

Isotherms TDS

59

Theory

Activation energy of desorption Ed [kJ/mol] 35 43 30 45 60 24.4 31.4 38-40

Sticking Pre-exponential factor coefficient s(θ) νd [s−1]

References

1×1013 ass. 1×1013 ass. 1×1013 ass. 1×1013 ass. 1×1013 ass. 1×1013 ass.

83Hen 83Hen 83Hen

∼1×1012 ∼1×1016

TPD

Au decorated steps,θ Au= 0.01 Ru (1010)

β

TDS

γ β

TDS (WFC) TDS

∼38 331±13

0.05

TDS isothermal desorption

27 43

1×1011 1×1015

Ru (1121) W(110) W(110) Landolt-Börnstein New Series III/42A4

(2×2) W(110)

0.25 γ-state (0.7)

83Ant, 86Ant

0.4 (increase to 0.65 at θ = 0.17)

82Feu, 83Men 97Mor

1×10−12 at 300 K ∼2×10−12 at 300 K ∼7×10−10 at 500 K ∼2×10−17 at 500 K

96Die, 97Jac 99Dah

1×10−12 at 300 K 1×10−12 at 300 K

96Die, 97Jac

0.004 ∼0.01 at 300 K ∼0.7 at 100 K

65Del 71Tam, 75Sin 79Som 76Yat2 90Lin

3.7.1 CO and N2 adsorption on metal surfaces

Ru(0001)

Coverage/ adsorbed state

192

Substrate

Landolt-Börnstein New Series III/42A4

Technique

Heat of adsorption Ead [kJ/mol]

W(110)

β3

θmax = 0.2

W(110)

β

W(100)

γ (γ+ ,γ− )

TDS (WFC)

W(100)

β γ (γ+ ,γ− )

42-46 314

TDS

39-44 308

Structure

c(2×2)

W(111)

W(111)

δ (precursor) β γ α β

Sticking Pre-exponential factor coefficient s(θ) νd [s−1] 0.003 at 820 K Ei <20 kJ/mol 0.003 at 800 K Ei <30 kJ/mol

β β

W(100)

Activation energy of desorption Ed [kJ/mol]

TDS TDS (WFC)

≥ 18 335±13 38 67 ∼314

References

84Aue, 84Lee 86Pfn 65Del

1×1013 0.23

0.05

70Cla 0.4 at 300 K 0.59±0.01 at 300 K 0.60 at 90 K 0.50-0.58 at 300405 K

74Kin 83Aln 66Est, 71Tam 65Del

72Kin, 74Kin 0.08±0.01 0.09 Sticking coefficients of N2 adsorption at 300 K on various surfaces of W, such as (100), (310), (320), (411), (111) and (110), were published by [75Sin] (see Fig. 61). The heat of adsorption in the β-state is given as ∼320 kJ/mol.

3.7.1 CO and N2 adsorption on metal surfaces

Coverage/ adsorbed state

Substrate

193

194

Table 11. Adsorbed N2 (desorption temperatures and dissociation parameters) Substrate

Re(0001) Re(0001) Re (112 0) Ru(0001) Ru(0001)

Coverage

θ

β β β γ

Landolt-Börnstein New Series III/42A4

W(110)

γ

W(110)

β

Activation energy Pre-exponential factor of dissociation [s−1] [kJ/mol] ∼29 ∼21-46 ∼0

110 (conversion to α-state) 145 - 160

α β α β α, 4×1 (N2) β, 2×2 (N) α1 α2 δ γ β

Temperature of dissociation [K]

Temperature of desorption [K] 920 920-1000 880 ∼90 160 850 ∼120 ∼1100 160 900-1300 129 155

14±2 1700.17 0 - 0.4

88-100 113-124

128-148 conversion γ to β-state 300 K

39±10 126±20 183 212 ∼40

(steps) (terrace) (theory) (theory)

17.4 (single activation barrier)

References

77Boz2 76Ert, 77Boz1 82Ert2, 85Tsa, 86Whi1

70Doo, 87Haa1 89He 87Haa1, 87Haa2 82Feu 99Dah 97Rom

150

76Yat2, 90Lin

1560

81Cos

3.7.1 CO and N2 adsorption on metal surfaces

Fe(110) Fe(100) Fe(111) Fe(111)

Adsorbed State

Landolt-Börnstein New Series III/42A4

Substrate

W(110)

W(110)

Adsorbed State

Coverage

θ

Temperature of dissociation [K]

β1 β2 β3 β

0.5 (max.)

820 K

low cov.

800 K

* populated at higher kinetic energy of incident N2 [84Lee].

92 (maximum of activation barrier distribution) no barrier

Temperature of References desorption [K] 84Lee 1180* 1245* 1310-1430 86Pfn

1390-1420

81Cos

3.7.1 CO and N2 adsorption on metal surfaces

W(310)

Activation energy Pre-exponential factor of dissociation [s−1] [kJ/mol] ∼41 (estimated activation barrier)

195

196

Table 12. Adsorbed N2 (crystallographic and vibrational data) Substrate

Structure

Coverage/ adsorbed state

δ at 20 K

Cr(111)

α at 100 K

Fe(111)

γ α1

K/Fe(111) Fe(111)

T <80 K T >110 K

K/Fe(111) Ir(111) Ir(100)-(1×1) (√2×√2)R45° Ir(100)-(5×1) Ni(111) Ni(110)

(√3×√3)R30° (2×2)

Landolt-Börnstein New Series III/42A4

α2 (with K) β γ α1 α2 (with K) 84 K θ=0-1 θ = 0.5 θ=0-1 θ = 0 - 0.33 0.5 at 83 K 100 K 125 K 90 K 80 K 115 K

435 460 470 435 460 330

335 322 339 346 339

1490 (15N2) 1555 (14N2) 1370 (15N2) 2100 1415-1490 1170 2210 2211-2190 2201 2208 2218 - 2194 2186 2195 2185 2194 2194 2194

Adsorption site / configuration

Technique

References

physisorbed N2 vertical π-bonded N2 atomic N

EELS

89Jac, 90Jac

EELS

88Fuk

EELS

84Gru1, 85Tsa

EELS

85Tom, 86Whi1, 86Whi2

EELS IRAS IRAS IRAS IRAS IRAS EELS EELS EELS EELS EELS

90Cor 93Gar

side-on bonded side-on bonded atomic N terminal bonded side-on bonded K-influenced vertical vertical vertical vertical vertical vertical chemisorbed chemisorbed chemisorbed chemisorbed chemisorbed

93Gar 91Yos 89Qui 89Sch 82Hor 88Kuw 86Str 82Ban

3.7.1 CO and N2 adsorption on metal surfaces

Al(111)

Vibrational frequency [cm−1] Me−N Me−N2 N−N 2339 + satellites 1170 530

Landolt-Börnstein New Series III/42A4

Structure

Coverage/ adsorbed state

Cont'd.

c(1.4×2)

θ = 0.72

Pd(110) Pt(111) Ru(0001)

(2×1)

Ru(0001)

(√3×√3)R30° (√3×√3)R30°

0.1 - 0.72 γ sat. 120 K 120 K γ 75 K 95 K

θK = 0.05 Ru(0001)

Ru(0001) Ru (10 10)

W(100)

(√3×√3)R30° (2×3), θ=0.5

γ saturated at 40 K δ β γ at 0.33 δ γ β

2329, 4610 568

282 330 484

α at 105 K β γ−

W(100)

Vibrational frequency [cm−1] Me−N Me−N2 N−N 2194 2200 - 2194 242 2242 2240 2247-2198 278-291 2252-2212 280-301 2209-2189 2195 300 2150 305 2192 282

2239 2329 2192

1048 1209 520

γ+

274

α β

604

2136 1451

Technique

References

chemisorbed chemisorbed chemisorbed

IRAS IRAS EELS IRAS EELS

87Bru 86Bru 87Kuw 77Shi2 83Ant, 87deP, 92Shi 87deP

chemisorbed N2 vertical

IRAS EELS chemisorbed N2, atop, vertical; physisorbed N2 atomic N chemisorbed N2 physisorbed N2 chemisorbed N2 νII atomic N ν ⊥ atomic N

EELS

92Shi

(off specular) Theory EELS

93Shi 97Mor 95Gru 97Die

chemisorbed N2 π-bonded atomic N chemisorbed N2

EELS

89Sel

chemisorbed N2 bridge-bonded N2 atomic N

EELS

80Ho

197

967

1500 2200

Adsorption site / configuration

3.7.1 CO and N2 adsorption on metal surfaces

Substrate

198

Table 13. Adsorbed N2 (additional structural data; bond lengths) Substrate

Ni(100)

Structure

(√3×√3)R30°

Ru(0001) Ru(0001) Ru(0001)

c(2×2)

(√3×√3)R30° (√3×√3)R30° (2×2)-N (√3×√3)R30° O/Ru(0001) (2×2)-O + N2 W(100) (√2×√2)R45°

0.5 0.33 (95 K) 0.33 0.25 0.33 0.5

Species

N2 N2 γ-state at 60 K atomic N β-state N2 N2 atomic N N2 atomic N

Adsorption site Bond length [Å]

atop

Me-N 2.25±0.01

N-N 1.10±0.07

>2.5 4-fold hollow

1.83

atop

2.00±0.05 2.00 1.93 1.93

3-fold hcp atop 4-fold hollow

2.29

Tilt angle

Technique

References

no tilt

ARPEFS

96Mol

no tilt

NEXAFS SEXAFS LEED

89Wen, 90Wen

LEED Theory LEED

94Blu 97Mor 97Sch1

LEED

95Ove 82Gri

1.10±0.04 1.13

no tilt

1.12±0.06

no tilt

82Imb

3.7.1 CO and N2 adsorption on metal surfaces

∼0.2

Fe(111) Fe(100)

Coverage θ or adsorbed state 0.33

Landolt-Börnstein New Series III/42A4

Landolt-Börnstein New Series III/42A4

Table 14. Adsorbed N2 (valence electronic structure). Electron binding energies are referenced to the Fermi-level. MO valence levels [eV] 3σg 9.85

1πu 11.0

2σu 12.85

Cr(110)

θ<0.7 α at 90 K

8.4

7.1

12.7

Fe(111)

γ at ∼77 K α at 110 K 1 at 95 K γ at 77 K γ, 0.5 γ, 0.5, 90 K γ at 70 K γ at 80 K γ γ at 90 K γ at 80 K 1 at 80 K At 90 K

8.6 8.4 8.0 7.6 7.6 8.3 9.0 8.6 8.7 8.1 8.6 9.2 8.7 7.9* 7.5 9.2 7.9 7.0 7.5

8.3 7.5 8.0 7.6 7.3 8.1 9.0 8.6 8.2 7.8 8.6 9.2 10.3 7.9 7.5 (10.5) 10.5 7.9 7.0 7.6

11.8-11.2 12.3 11.8 12.4 12.4 12.8 13.0 12.2 12.5 11.8 11.8 12.8 11.9 12.4 12.0 12.2 12.0 11.7 11.9-12.1

Structure

Al(111)

Ir(110)-(1×2) Ni(100)

p1g1(2×2) c(2×2) c(2×2)

Ni(111)

Ni(110) Pd (poly) Pd(111) Ru(0001)

(√3×√3)R30°

Ru(0001) (√3×√3)R30° W(110) W(110)

δ γ at 40 K γ γ at∼120 K

Technique

Reference

physisorbed N2 vertical molecular N2 inclined, π-bonded vertical inclined vertical vertical vertical vertical vertical vertical vertical vertical vertical vertical random near vertical

UPS

89Jac

UPS

90Shi

ARUPS

87Fre

UPS UPS XES UPS UPS UPS UPS UPS UPS UPS ARUPS ARUPS

81Ibb 81Bru 98Ben 84Dow 86Bre 90Rao 88Umb 82Hor 90Rao 91Rao 82Hor 85Hes, 87deP

ARUPS

92Shi

UPS ARUPS

79Fug 80Umb, 82Umb

physisorbed N2 chemisorb. N2 chemisorbed N2 chemisorbed N2 vertical

199

*Dispersion of 3σg level found to be <0.1 eV [85Hes].

Molecule orientation

3.7.1 CO and N2 adsorption on metal surfaces

Coverage/ adsorbed state δ at 20 K

Substrate

200

3.7.1 CO and N2 adsorption on metal surfaces

Table 15: Adsorbed N2 (core level binding energies; adsorption states; molecular/atomic species). Electron binding energies are referenced to the Fermi-level.

Substrate

Coverage/ Core level adsorbed state energies [eV] N 1s Cr(111) α at 100 K 397.8 398.9 Fe(111) γ T <85 K 401.2, 405.9 399 α 397 β Ir(110)-(1×2) p1g1(2×2) 1 at 95 K 399.2 404.2 satellite 403.6 Ni(111) δ at 20 K 400 shoulder γ 401.0 405.5 satellite 401 γ 405.5 sattelite 399.3 Ni(100) γ 400.5 405.2 satellite 399.4 c(2×2) γ, θ = 0.5 400.7 406 satellite 400.0 γ at 80 K 405.2 satellite 400.7 γ at 77 K 405.8 satellite 400.5 Ni(110) γ at 80 K 406.0 satellite 400.1 γ at 130 K 401.5 406.7 satellite Pd (poly) At 80 K 401.3 405.1 satellite 400.0 Re(0001) γ 405.4 396.3 β, T >160 K Ru(0001) 399.3 γ 400.7 405 satellite Ru(0001)

Structure

γ

400.3

Chemical state

References

chemisorbed N2 π-bonded terminal bonded N2 side-on bonded N2 atomic N vertical N2

88Fuk 84Gru1, 84Gru2

81Ibb

physisorbed N2 chemisorbed N2

86Bre

chemisorbed N2

88Umb

chemisorbed N2

84Umb

chemisorbed N2

90Mar

chemisorbed N2

90Rao

chemisorbed N2

81Bru

chemisorbed N2

90Rao

chemisorbed N2

80Gol

chemisorbed

91Rao

molecular N2

84Gru2

atomic N molecular N2

82Umb, 84Umb

molecular N2

95Bir Lan dolt -Börn stein New Series III/ 4 2A4

3.7.1 CO and N2 adsorption on metal surfaces Substrate

W(110)

W(110)

La ndolt-Bö rnstein New Series III/4 2A4

Structure

Coverage/ Core level adsorbed state energies [eV] N 1s 405.3 397.4 β γ (0.5) 399.1±0.2 400.4±0.2 405.5±0.5 ∼397 β 399.3 γ 400.5 405.5 satellite

201

Chemical state

References

atomic N molecular N2

79Fug, 90Zha

N2 shake-up sat. atomic N molecular N2

82Umb, 84Umb

References for this document 64Tuc 65Del 66Est 67And 68Kin 68Mor 69Edm 69Gra 69Tra1 70Cla 70Doo 70Ert 70Gra 71Eas 71Gra 71Koh 71Tam 72Bon1 72Kin 72Mah 72Tra1 72Tra2 73Bak 73Bon 73Chr 73Koh 73Lec 73Mad2 73Mad3 73Tay 73Vis 73Wac 73Wei 74Chr 74Con1 74Doy 74Kin 74Lam1 74Lam2 74Mad 74Yat1 74Yat2 74Yat3

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76Con 76Ert 76Hag 76Küp 76Liu 76Llo 76Mad 76McC 76McE1 76Plu 76Ree 76Vor 76Wil 76Yat1 76Yat2 76Zhd 77All1 77All2 77Ber 77Boz1 77Boz2 77Bri 77Ert 77Fro 77Fug 77Gil 77Hor2 77Hou 77Iba 77Iva 77Kes 77Ku 77Leu 77Mar 77McC 77Rho 77Shi1 77Shi2 77Ste1 77Ste2 77Tex 77Umb 78All 78Bra1 78Bra2 78Bro 78Cas 78Con 78Erl 78Fel 78Fuk 78Hor1 78Jon 78Kan

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78Mad 78Nie 78Pfn 78Pir 78Pri 78Tay1 78Tay2 78Tay3 78Weh 78Zhd 79And2 79Beh 79Bor 79Bro 79Cam 79Fre 79Fug 79Hei2 79Hol 79Hop 79Nor 79Pet 79Pri 79Rub 79Som 79Thi1 79Thi2 79Tho 79Wan 79Wen 79Wil 80And 80Bai 80Beh 80Ben 80Chi 80Dub 80Duc 80Erl 80Fai 80Foo 80Fuk1 80Fuk2 80Fuk3 80Gol 80Hol 80Iba 80Ko1 80Ko2 80Nie 80Pfn 80Sea 80Smi

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80Ton 80Umb 80Yat 81Bai 81Bar1 81Bar2 81Ber 81Bru 81Cam1 81Cos 81Ibb 81Kel 81Kev 81Koe 81Mil 81Nis 81Nor 81Pri 81Sea 81Sem 81Yos 82Ban 82Bar 82Bib 82Bro 82Ert2 82Fer 82Feu 82Gon 82Gri 82Hof1 82Hof2 82Hor 82Hou 82Imb 82Jac1 82Kat 82Kim 82Mar 82Net 82Ort 82Pap 82Ste 82Umb 82Woo 83Aln 83Ant 83Bag 83Beh 83Bur

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01Pet

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3.8.2 H2O and OH on semiconductors W. JAEGERMANN, TH. MAYER 3.8.2.1 Introduction In this data collection the adsorption and adsorbate properties of H2O and OH on well defined semiconductor surfaces in UHV are summarized. The adsorbate induced changes of substrate properties are only considered, if they are intrinsically related to the adsorption process. They are subjects of other parts of this LANDOLT-BÖRNSTEIN III/42A2. Here only those effects are mentioned which are absolutely necessary to understand the basic adsorbate properties. Reactions with the substrate are only covered, if they are immediately connected with dissociative adsorption below or at room temperature. Thermally induced reactions at elevated temperatures, usually an oxidation of the semiconductor substrate and details about the involved mechanisms, are not presented here in any detail, despite the fact that many papers dealing with H2O adsorption are motivated by this process and contain results in this respect. A number of review articles have been published in the past, which present and discuss the interaction of H2O with semiconductor surfaces (see e. g. [87F, 88F, 95W1]). The interaction of H2O with semiconductor surfaces is studied to achieve a fundamental understanding of solid/liquid interfaces involving aqueous solutions. Such interfaces are the essential part of semiconductor based devices as (photo)electrochemical and (photo)catalytic cells and sensors. They are also of central importance in wet chemical etching and processing steps in semiconductor microelectronics as e.g. in passivating oxide formation. Usually the number of reactants is rather large in these applications and the interface interactions may become very complex. As a consequence the electronic properties of the interfaces may vary depending on the processing steps strongly influencing the operation of the devices. In many cases the microscopic origin of the involved changes are not very well understood, as the species interacting with the semiconductor surfaces have not been identified and their electronic interactions remain unclear. As H2O is often the main constituent with high surplus in the solution its interaction with the semiconductor must be understood in very detail. Thus, H2O as an adsorbate on defined semiconductor surfaces is of basic interest and also a key ingredient to a fundamental understanding of wet-chemical processes in many technological applications. In contrast to metals the bonding interactions of semiconductors are governed by localized and directional chemical bonds. Therefore the adsorption process as well as the adsorbate properties are in general strongly influenced by the structure and chemical composition of the substrate surface, which again depends on the preparation procedure used for a specific semiconductor [92B, 95M]. The atomic structure of semiconductor surfaces deviate considerably from bulk truncated surfaces as most semiconductors undergo severe surface relaxations and reconstructions to minimize their specific surface energies. Thermodynamic driving force is the attempt to minimize the number of energetically unfavourable surface dangling bonds produced by the loss of translational symmetry across the surface. Furthermore compound semiconductors exhibit surface atoms of different polarity adding electrostatic potentials and different dangling bond energies to the surface energy. As a consequence the adsorption is strongly dependent on the specific surface orientation and the kind of semiconductor used for investigation. In addition, the procedure of surface preparation and the obtained defect concentration may vary in different investigations which also influences the adsorption process. Therefore the data and conclusions given in literature may differ due to these effects. These factors will be discussed in more detail in section 3.8.2.2 and 3.8.2.3. Different surface terminations imply very different surface electronic structures ([92B, 94H, 94S2, 95M] and LANDOLT-BÖRNSTEIN III/24B, chapter 3.2 by Calandra and Manghi). Already on clean semiconductor surfaces the energetic position and dispersion of surface states and surface resonances strongly influences the charge distribution between semiconductor bulk and surface. Local dipoles involve only the surface atoms, but extended space charge layers lead to extended layers of ionized dopants in the bulk. As the adsorption of H2O as an electron donating adsorbate induces charge transfer from its occupied molecular orbitals to the semiconductor, the electronic structure of the surface changes with adsorption. On the other hand a dissociative adsorption will lead to surface bound OH and H groups Landolt-Börnstein New Series III/42A4

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which may be neutral or partially negatively/positively charged. Depending on their bonding properties and surface molecule energy states the charge transfer between the bulk and the surface may change again. As a result adsorbed H2O or OH and H may strongly change the electronic surface potentials and, vice versa, the given surface potentials of the semiconductor substrate controlled by its doping may influence the adsorption process. A more detailed discussion on these effects is given in section 3.8.2.4. The variation in structural and electronic properties of different semiconductors with their specific surface terminations leads to strongly varying adsorption properties and bonding interactions. The interaction with adsorbates as H2O and its dissociation products will depend on the number, distances and relative orientations of specific surface atoms and related bonding sites. Due to adsorption the substrate structure and electronic structure will change again in most cases. It turns out that depending on the experimental conditions as e.g. temperature and dosage the exposure of most semiconductors to H2O does not only lead to adsorption but the adsorbate species may further react with surface and subsurface atoms. These surface reactions following the adsorption step lead to oxidation and etching, which may already be initiated at low sample temperatures. Therefore, it is hardly possible to prepare identical adsorption stages in different experiments. Many contrary interpretations on H2O adsorption on semiconductors are probably related to this effect that adsorption and subsequent reaction steps occur in parallel. Most of the work on water adsorption on semiconductors has been performed with Si surfaces. This is evidently in part motivated by the fact that Si is the best established semiconductor material, which can rather easily be prepared in different surface orientations and defined reconstructions. In addition, it is the semiconductor most important in technology. Especially the interaction with H2O is of interest for etching processes, the formation of H-terminated surfaces, and oxide growth. A large number of investigations have been performed using different surface science techniques. For the technically important Si(100) surface and to a lesser extent for the Si(111) surface the interaction of H2O (OH) as an adsorbate is well studied and reasonably well understood. In cases where some uncertainties may still exist these are given in the text and added as remarks in the tables. For all other semiconductors the number of investigations is much smaller. In these cases there is no general agreement on H2O adsorption, yet. Even the most simple question on the mode of adsorption – dissociative or molecular – is still not clarified for many semiconductors. For these semiconductor substrates it is attempted to give a survey of the performed experiments and the main conclusions, as given in the original contributions. We do not attempt to draw any final conclusions on the mode of adsorption and the adsorbate properties. In cases, where parts of the results seem to be questionable based on other and more recent investigations, remarks will be added in the text and as a comment in the tables. List of Acronyms and Symbols used in this chapter Acronyms AES ARUPS CNDO DB DAS DFT ELS EHT ESDIAD HREELS IR IRAS LCAO LDA LDFT LDFT/LDA Landolt-Börnstein New Series III/42A4

Auger electron spectroscopy angle resolved UPS complete neglect of differential overlap of atomic orbitals on the same atom dangling bond dimer adatom stacking fault density functional theory energy loss spectroscopy extended Hückel theory electron stimulated desorption ion angular distributions high resolution electron energy loss spectra infrared infrared absorption spectroscopy linear combination of atomic orbitals local density approximation local density functional theory LDFT within the local density approximation

228 LEED LEIS LITD MNDO MO ODA PED PSID PYS SIMS STM SXPS TDS TPD TOF-SARS UHV UPS XPS

3.8.2 H2O and OH on semiconductors low energy electron diffraction low energy ion scattering laser induced thermal desorption modified neglect of diatomic overlap molecular orbital outer dimer atom photo electron diffraction photon stimulated ion desorption photoemission yield spectroscopy secondary ion mass spectrometry scanning tunneling microscopy soft XPS or synchrotron induced XPS thermal desorption spectroscopy thermal or temperature programmed desorption time of flight scattering and recoiling spectrometry ultra high vacuum ultraviolet photoelectron emission spectroscopy x-ray photoelectron emission spectroscopy

Symbols

∆φ χ ∆χ ∆EB

EB EBF EBvac EBVBM EF Evac eVbb EVBM

θ θ/θsat θsat

Hads L ML Oads OHads RT Siad Sirest S S0 S/S0 100 K →300 K

change of workfunction electron affinity change of surface dipol change of binding energy binding energy binding energy referenced to EF binding energy referenced to vacuum level binding energy referenced to valence band maximum Fermi level vacuum level band bending valence band maximum coverage relative coverage saturation coverage adsorbed hydrogen Langmuir (1 × 10−6Torr × 1s = 1.33 × 10−6mbar × 1s) monolayer adsorbed oxigen adsorbed OH room temperature adatom of Si(111)7×7 reconstruction restatom of Si(111)7×7 reconstruction sticking coefficient initial sticking coefficient for θ →0 relative sticking coefficient sample temperature in K adsorption at low temperature, measurement taken after annealing to 300 K

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3.8.2.2 Surface preparation The adsorption of H2O on semiconductor surfaces evidently depends on the type of semiconductor, its surface orientation, and the procedure applied to prepare clean and defined substrate surfaces. For this reason some early experiments on H2O adsorption on powdered/crushed semiconductors published in the literature will not be considered in this data collection (see e.g. [68E]). The different techniques known for the preparation of metal surfaces in UHV, namely thermal cleaning, ion bombardment, ion bombardment and annealing, and in situ deposition by any vacuum deposition process cannot be applied with the same success to most semiconductor surfaces. The different surface orientations of elementary semiconductors like Si and Ge are usually prepared by cleaving in vacuo or by ion bombardment and annealing. Also starting from chemically etched surfaces is possible, which is followed by a thermal cleaning process. In general, there are well established procedures available, which allow to prepare surfaces of comparable quality. For compound semiconductors the surface preparation may be very complex depending on the specific surface orientation under consideration. High quality substrate surfaces are obtained for the cleavage planes, which are the non-polar surfaces of the substrate material. For compound semiconductors with Zinkblende structure as e.g. GaAs these are the (110) planes. All other surface orientations are usually prepared by several ion bombardment and annealing cycles, which may result in many different surface reconstructions or facetting. In principle, it is also possible to obtain high quality surfaces of most surface orientations by in-situ MBE growth or by transferring epitaxial films from any growth chamber using shielding surface layers, which can be heated off at elevated temperatures. To our knowledge such surfaces have hardly been used for the investigation of H2O adsorption. Therefore, adsorption studies on compound semiconductors suffer from different surface pre-treatments and for this reason from differing surface properties. High quality defect free and chemically inert substrate surfaces can be prepared from the (0001) van der Waals cleavage plane of layered chalkogenide semiconductors. The chemically saturated hexagonally closed packed cleavage plane allows physisorption of H2O at low sample temperatures only. Some general information on preparation methods used for the different semiconductor surfaces can be found in the following references [83K1, 92B, 95L, 95M], see also LANDOLT-BÖRNSTEIN III/24A. Details on the preparation of the substrates are usually given in the experimental part of the cited papers. 3.8.2.3 Surface structure: relaxation and reconstruction Clean semiconductor surfaces of a defined crystallographic plane do usually not exist in their bulk truncated structure. Assuming homonuclear cleavage of bonds in surface formation unsaturated directional dangling bonds are formed which are only occupied with a single electron. In order to minimize the surface free energy these semiconductor surfaces exhibit a pronounced tendency to surface relaxation or reconstruction, which may involve a severe structural rearrangement of surface atoms. Thermodynamic driving force is the reduction of the number of energetically unfavourable dangling bonds and/or of their electronic character by charge redistribution. Whereas relaxation processes and also some surface reconstructions can occur spontaneously, some of the thermodynamically more stable surface reconstructions need the thermal activated diffusion of surface atoms. Therefore, the structural rearrangement on the idealized surface is also influenced by the substrate temperature profile used within the preparation sequence. As a consequence different surface terminations are known to exist for elementary and compound semiconductors depending on the preparation sequence used, which are listed in several reviews [83K1, 92B, 95L, 95M] and a previous LANDOLT-BÖRNSTEIN III/24A. The reader should refer to this article for the representation of the structural arrangement of a certain surface, given in the established notation of its superstructure as e.g. found in LEED. The number of dangling bonds, their distance and orientation to each other, as well as their atomic origin deviate considerably on the different semiconductor surfaces. As these dangling bonds act as adsorption sites one expects strong variations in H2O adsorption properties on different semiconductor surfaces.

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The semiconductor substrates considered in this review are: • the elementary semiconductor surfaces Si(100), Si(100)2×1, Si(100)2×1 vicinal, Si(111)7×7, Si(111)2×1, Si(113)3×1, Si(113)3×2, Ge(100)2×8, Ge(100)2×1, Ge(111)2×8, GexSi1-x(100)2×1; • the 3-5 compound semiconductor surfaces GaAs(100), GaAs(100)2×4, GaAs(100)4×6, GaAs(110), _

InP(110), InP( 1 1 1 ), AlAs(100)1×1; • the chalcogenide semiconductor surfaces GaSe(0001), InSe(0001), MoSe2(0001), MoS2(0001), WSe2(0001); pyrite FeS2(100); • and the ternary compound semiconductor surface CuInSe2(011) Furthermore, the adsorption also is influenced by the ideality (structural quality) of the prepared surface. The number of defects depends on the detailed experimental procedure used for the preparation. The experimentally observed results may be governed by these defects, which may act as adsorption and dissociation sites on otherwise inert surfaces. For more complex surface terminations the „intrinsic“ adsorption properties of a specific surface orientation can hardly be discerned, if the number of defects after preparation is large. But the number of defects, which are actually obtained, are hardly known for most investigations (besides when using techniques like STM) and are usually not quantified. A summary of possibly formed defects on semiconductor surfaces is also given in LANDOLT-BÖRNSTEIN III/24A, chapter 2.3 by Henzler and Ranke. 3.8.2.4 Surface electronic structure and surface potentials The electronic structure of different semiconductor surfaces may also vary depending on the type of semiconductor surface and the existing reconstruction. The energy levels of dangling bonds formed on the surface may be situated in the bulk bandgap forming surface states or may be outside the gap forming surface resonances. As a consequence the semiconductor substrate has different surface electronic properties. In some cases e.g. with Si(111)7×7 the surface is metallic; in other cases e.g. for the perfect GaAs(110) cleavage plane a semiconducting surface is found without electronic states in the bandgap. Also the charge carried by different surface atoms may be different. This is evident for compound semiconductors: on GaAs(110) the surface relaxation is accompanied by a partial electron transfer from Ga to As dangling bonds. But also for elementary semiconductors differently charged surface atoms exist: On the Si(100)2×1 surface the outer dimer atom is negatively charged whereas the inner dimer atom is positively charged. On Si(111)7×7 the adatoms are positively charged whereas the neighboring rest atoms are negatively charged. Polar surfaces as e.g. GaAs (100) or (111) are usually strongly reconstructed with a surface excess of one atomic species, which then dominates the surface electronic structure. Thus, one may expect the adsorption of H2O as nucleophylic species and subsequent dissociation to occur on different surface sites. The reader should refer to [88H, 94H, 94S2, 95M] and LANDOLT-BÖRNSTEIN, III/24B, chapter 3.2, where the electronic surface properties of clean semiconductor surfaces have been summarized in very detail. It should be noted at this stage that similar to the structural properties of semiconductor surfaces also the electronic properties depend very much on the quality of surface preparation. Defects or contaminations may introduce extra surface states and surface resonances, which do not exist on the perfect surface. The energetic positions of the semiconductor band edges in reference to the vacuum level are given by the values of the ionization potential Ip = (Evac – EVB) and electron affinity χ = (Evac – ECB) of a specific semiconductor surface. For different surface orientations these values only depend on changes of the localized surface dipole extending about a few monolayers. This surface dipole is determined by the polarity of the surface bonds. The surface position of the Fermi level in reference to the semiconductor band edges and thus the work function φ = (Evac – EF) additionally depends on the relative position of the bulk Fermi level and the energetic level and occupation of surface states [95M, 96J]. The bulk Fermi level Landolt-Börnstein New Series III/42A4

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is given by the type of doping states (acceptor states: p-type; donor states: n-type) and their concentrations. Flat band conditions, which correspond to the lack of extended space charge layers, can only be expected for surfaces free of surface states. In this case the work function of a p-doped semiconductor is larger than that of an n-doped one by the difference of the bulk Fermi level positions. Active surface states may lead to band bending eVbb in the surface region of the semiconductor typically in the range of 100 to 104 Å (inverse proportional to bulk doping). Vice versa the work function is changed by the shift of the Fermi level given by eVbb versus the band edges. Acceptor (donor) like surface states will accept (donate) electrons from (to) the bulk of the semiconductor, if their energy position is below (above) the bulk Fermi level. For high concentrations of surface states the surface Fermi level is pinned at the energetic level of the surface states; as a consequence p- and n-doped semiconductors show the same work function. The electronic properties of clean semiconductor surfaces are summarized in Ref. [88H, 94H, 94S2, 95M] and LANDOLT-BÖRNSTEIN III/24A, chapter 3.2. With adsorption and depending on the bonding properties the electronic surface states or resonances may change their character (acceptor or donor) and/or energy position in respect to the band edges. As a consequence the band bending and surface dipole will usually be changed. The value of the binding energy EB of electronic states measured e.g. for adsorbed species by photoemission depends on the reference level used. If the Fermi level is used as common reference level (EBF) a shift of binding energy value may occur with adsorption, which is related to a change of semiconductor band bending corresponding to a surface Fermi level shift. This band bending can be related to a charge transfer from the bulk to the adsorbate or may be induced by the adsorbate due to passivation of electronically active surface states originally present in the bulk bandgap of the material. As a consequence the binding energy values EBF may change with coverage without any change in the adsorbate electronic state. The induced band bending will also influence the changes of the work function ∆φ = eVbb + ∆χ; with ∆χ as the change in electron affinity due to changed surface dipole potentials. To avoid changes due to band bending the binding energy values are also referred to the valence band maximum EVBM. In this case only surface dipole potentials showing up between the adsorbate and the substrate may shift the relative binding energy values of adsorbate vs. substrate lines. If the vacuum level is taken as reference level, the binding energy values EBvac are affected by band bending eVbb and surface dipoles [95M, 96J] e.g. by the overall shift of work function ∆φ. The changes of surface potentials are not specifically covered in this part of the LANDOLT-BÖRNSTEIN series on adsorption but in the contribution of Jacoby and Nilsson et al. (LANDOLT-BÖRNSTEIN III/42A2, chapters 4.2 and 4.3). But for presenting the electronic properties of adsorbate states the different reference values and their influence on the experimentally determined binding energy values always have to be kept in mind. In principle, the different binding energy scales can be referred to each other, but, unfortunately, the necessary data on band bending and work functions are not given in all papers presented in this work. One may also expect a strong influence of semiconductor bulk doping on H2O adsorption. If charge transfer from the bulk to the adsorbate is involved, the type of doping and the related band bending defines, which type of charge carrier, electron or hole, is more easily available in the adsorption process. This has been extensively discussed in older work on ionosorption (ionic adsorption) of adsorbates on oxide semiconductors [55H, 63W, 77M], but has not been studied in any detail in more modern work with defined semiconductor surfaces. As in many cases even the doping of the semiconductors are not given in the cited papers, we did not specifically address this point in this data summary. 3.8.2.5 Methods of investigation For the investigation of H2O adsorption on semiconductor surfaces all relevant techniques of surface science have been applied during the last years. Only experiments performed in UHV have been considered thus excluding e.g. scanning probe experiments at air or in solutions. The principles of the most important methods of investigations are described in detail in LANDOLT-BÖRNSTEIN III 42/IA2. chapter 2. General information on experimental techniques can also be found in other LANDOLTBÖRNSTEIN volumes as III/24 and in many monographies on surface science techniques. We use the standard abbreviations without further explanations. They are summarized in section 3.8.2.1. Also theoretical calculations using different methods were applied. For details and limitations the reader may refer to the original literature.

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We also want to note that the application of a certain technique may disturb the adsorption process and the obtained results. In general, the degree of disturbance decreases from ions to electrons to photons, and also from high energies to small energies. Especially with semiconductors the formation of defects may already be introduced by the use of high energy photon beams of high brilliance as e.g. given by synchrotron radiation. These effects may be even more severe, if adsorbates are present. It is usually expected that spectroscopic techniques as UPS and IR do not disturb or modify the surfaces. But in any case changes of spectral features with time may indicate probe induced surface processes. Also the thermal excitation of the adsorbate by hot filaments must be taken into account. For well defined experiments pressure measurements are performed in clear separation from the substrate. This could not be verified for all of the quoted experiments. A discussion on probe induced modification of the adsorption process was part of the controversy on molecular and dissociative H2O adsorption on Si surfaces. 3.8.2.6 Adsorption mode For all semiconductors and their different surface terminations there was a long lasting controversial discussion in literature, if the mode of H2O adsorption is dissociative or molecular. This controversy seems to be settled now at least for Si(100), Si(111), and Ge, for which the experimental results clearly support dissociative adsorption at room temperature saturating the surface dangling bonds (see Table 1). On Si even at low temperatures dissociation occurs. On top of the dissociative adsorption layer molecular H2O may be condensed as H-bonded physisorbed ice layer, at substrate temperatures below 150 K. At elevated temperatures above RT the oxidation of the surface sets in after complete dissociation of H2O into Oads and 2Hads. This process is (at least partly) already observed at RT or even below. The adsorption mode of H2O on 3-5 semiconductors seems to be less clear. Some data suggest an initial dissociative adsorption even at low temperature, whereas others support a dissociative adsorption only at RT and some authors conclude on strongly chemisorbed H2O (Table 1). The reactivity may also depend on the surface orientation and the applied preparation conditions. The orientation dependence of H2O adsorption on 3-5 semiconductors has not been systematically studied, yet. The layered chalcogenides with their chemically inert van der Waals (0001) surfaces seem to be the only system identified so far, for which physisorbed H2O (ice layer) is condensed at low temperatures (<150 K). It is completely desorbed without dissociation above this temperature. We do not include in Table 1 results from theoretical calculation as they are only significant in comparison to experimental results (see Table 2 and 6). Si (100) and its reconstructions All the earlier investigations were mainly addressed to the question whether water adsorption on Si surfaces is either molecular or dissociative. According to ellipsometry data, it was proposed [71M] that H2O dissociates into 2H and O to saturate all the surface dangling bonds. Early UV photoemission spectra were interpreted to be due to molecular adsorption [81F1, 81F2, 83S2] but could not be confirmed later on. UPS measurements performed later give different spectra [84O, 89F], which now are attributed to OH and H groups saturating the Si(100)2×1 dangling bonds. HREELS and IR spectra clearly prove dissociative chemisorption forming Si-OH and Si-H surface molecules [82I2, 84C1]. There is now general agreement that the dominating process below about 400 K is dissociation into OH and H species which saturate two dangling bonds (DBs) of the 2×1 reconstructed surface (Fig. 1, Fig. 12). STM work [93A, 93C2] shows that the reconstructed dimer bonds of the substrate are not broken which was also concluded from the LEED observation of retaining the (2×1) superstructure [82I2, 84S1, 85C, 87L2]. At low temperatures (below 140 K) molecular water may be condensed on top of the dissociated layer. Above 400 K chemisorbed OH species begin to react further forming atomically adsorbed oxygen [96R2, 97W].

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Si (111) and its reconstructions There is now general agreement that on Si(111)7×7 adsorbed H2O initially dissociates into OH and H adsorbate species even at about 80 - 90 K, which saturate two neighboring adatom-restatom pairs in the DAS model of the 7×7 surface reconstruction. This conclusion is now consistently supported by HREELS [82I2, 83K1, 85N, 86N, 86S2], UPS [89F], SXPS [95P] and STM [91A, 97S1] data. Early UPS experiments [79F, 81F1, 81F2] were never reproduced in this way and the observed spectra are probably due to the onset of oxidation. Later UPS results [85R1, 85R2, 86S2] show spectra dominated by OHads due to dissociatively adsorbed H2O, which have been misinterpreted. Depending on coverage and temperature different dangling bond states are involved leading to different stages of adsorption in the range of about 0.12 and 0.19 ML. It is suggested that OH occupies the adatom sites and H the restatom sites [91A, 97S1], but theoretical calculations also propose a reverse occupation of bonding sites [97E]. The energy and angle dependencies of the vibrational modes in HREELS indicate a preferential orientation of the Si-OH and Si-H bond nearly normal to the surface and of the SiO-H bond tilted by 75° from the surface normal [86N]. The 7×7 reconstruction remains unchanged during the initial adsorption process. At room temperatures and high exposures a slow saturation of all surface dangling bonds is found and subsequently a thermally activated reaction with Si back bonds and complete dissociation into Oads and 2Hads is evident. At this stage the surface reconstruction is lost and the oxidation of Si sets in [85N, 85R1, 89K, 95P, 96F, 97S1]. The adsorption of H2O on laser annealed Si(111)1×1 surfaces also leads to a dissociative adsorption in a much higher surface concentration, which is related to the increased number of surface dangling bonds on this surface [95W2]. The adsorption mode on Si(111)2×1 is less clear. Also here a dissociative adsorption is experimentally found, which is accompanied by a loss of the π−bonded chain structure [84S1]. Ge in different surface orientations H2O is suggested to be molecularly adsorbed on Ge(100)2×1 and Ge(111)2×8 surfaces at low temperatures (110 K) or as mixture of dissociative and molecular adsorption. [84C2, 87K2, 87K3, 89L, 91L1, 91L2, 91P1, 92C, 93R1, 93R2]. At RT dissociative adsorption is found for all orientations. The OHads and Hads species saturate the surface dangling bonds available at the surfaces. The surface reconstruction is lost due to adsorption as deduced from the loss of superstructure diffraction spots in LEED. At elevated temperatures the surface is at least partly oxidized [87K3, 91P1]. 3-5 semiconductors in different surface orientations Only a small number of H2O adsorption studies with 3-5 semiconductors have been performed so far and no conclusive results on the mode of adsorption for different compounds and surface orientations can be given up till now. Adsorption studies have been performed either with the unreconstructed but relaxed (110)1×1 cleavage plane or with the reconstructed (100) and (111) planes. Based on SXPS data some authors have suggested an initial dissociative adsorption even at low temperatures on (110) surfaces forming Ga(In)-OH and As(P)-H pairs in correspondence to the related Si and Ge surfaces [96H, 97H, 00H]. Others suggest physisorbed and chemisorbed molecular adsorption but the obtained UPS spectra do not agree to the expected emission pattern observed later on other substrates [79B]. On (100) surfaces molecular adsorption is suggested at low temperatures, which is followed by thermally activated dissociation [97M1, 97M2, 98C]. Dissociation always leads to the formation of group 3-OH and group 5-H bonds as the group 5-OH and group 3-H bonds are thermodynamically unfavorable. In all cases the long range surface structure as given by LEED remains unchanged during initial adsorption.

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Miscellaneous semiconductors The van der Waals (0001) surfaces of layered chalcogenides are composed of a close packed array of chemically saturated chalkogenide surface atoms free of dangling bonds. Thus H2O is only adsorbed molecularly as condensed ice layer at low temperatures (< 150 K) and is completely desorbed at temperatures above. No oxidation is found despite on step edges. For FeS2 (100) and CuInSe2 (011) at low temperatures a preferential adsorption of molecular H2O is suggested on Fe and Cu sites, respectively. The stable molecular adsorption was related to the coordination type bonding to transition metal sites. 3.8.2.7 Thermodynamic data of adsorption There are no recent attempts published to measure thermodynamic data quantitatively e.g. in calorimetric experiments (adsorption isotherms), due to the thermal activated reactivity of most semiconductors with the adsorbate species H2O and OH. Older data as given in [68E] suffer from an insufficient characterization of the surface structure and composition. Informations on adsorption energies are thus only available from kinetic experiments. But also in such studies the determined activation energies on desorption may be affected by the parallel process of thermally activated reactions of the chemisorbed species with the semiconductor substrates (oxidation). At elevated temperatures most semiconductors form oxides before the chemisorbed adsorbates (H2O and OH) desorb. Only for multilayer coverage (ice formation) at low temperatures the physisorbed layers will be desorbed at typical temperatures above 150 K. Data on adsorption energies are also available from theoretical calculations, which give bonding energies as calculated for isolated adsorbate/substrate surface molecules. As an example, on the Si(100)2×1 surface dissociation is observed even at low temperatures (80 - 100 K) [84C2, 85R1] indicating that a possible activation barrier, if existing at all, is quite low. Engler used a theoretical calculation (empirical potential method) to argue that the energy difference between molecular and dissociative adsorption is small, though he predicted a large activation barrier between these states [90E]. In an LDA-DFT calculation the enthalpy change accompanying dissociative chemisorption of one H2O molecule was calculated to be 3.9 eV [95V]. Using other DFT calculations and cluster models of the surface the energy and mechanism of adsorption, the stability of the dissociated products, and the minimum energy reaction path (Fig. 1) were calculated by Konecny et al. [97K]. In agreement with experimental data they found evidence for a weakly bound molecular precursor but no net activation barrier for adsorption. The values given for the adsorption energy from the different applied methods are summarized in Table 2. 3.8.2.8 Kinetic data of adsorption/desorption, surface diffusion and surface reactions The kinetics of adsorption are usually determined by the relative sticking coefficient S/So (So: sticking coefficient for θ → 0) versus relative coverage θ/θsat. (θsat: saturation coverage). These curves are derived from adsorbate coverages versus exposure. For all well studied semiconductors the experimentally observed sticking coefficients and saturation surface coverages depend strongly on the experimental conditions as surface orientation, temperature, and pressure regime. The changes in relative adsorbate coverage for different surface orientations have been clearly demonstrated with cylindrical single crystals of Si [85R3] (Fig. 2a) or Ge [87K3] (Fig. 6) or with lensshaped samples of Si [90S]. The shape of the adsorption curves cannot be explained by Langmuir adsorption, but are typical for an adsorption process with a mobile precursor state [96R2] as treated by Kisliuk [57K, 58K]. In this model impinging molecules may migrate both over occupied and unoccupied regions of the surface and are finally chemisorbed on two adjacent unoccupied dangling bond sites. It is now well accepted that for the most often studied semiconductor surfaces of Si and Ge initially a fast and nearly activation free dissociative adsorption occurs on bonding sites in close neighborhood to each other. Subsequently, substrate surface bonds are attacked in a thermally activated process, which is accompanied by further adsorption of additional H2O

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molecules. This process is related to the disruption of reconstruction bonds and back bonds. Thus, the adsorption kinetics depend on the number and distances of specific dangling bonds as given by the surface reconstruction of the semiconductor surface. There are no data available on the desorption kinetics of adsorbed H2O and OH besides the desorption of condensed ice layers covering the first adsorbate layer. This is due to the fact that after dissociative adsorption of H2O the surface species react in a thermally activated process with semiconductor substrate atoms to form oxides, which involves the complete dissociation of H2O with the intermediate step to OHads and Hads and finally to Oads and 2Hads. Thus, in thermal desorption spectra first the desorption of H2 is observed at elevated temperatures which is followed by the desorption of SOx species (S = surface atom). H2O adsorption studies on semiconductor surfaces and subsequent activated formation of oxides is motivated by the interest in understanding the basic processes involved in the high temperature wet-oxidation of semiconductors used in electronic device processing. Detailed studies on the thermally activated wet oxidation following adsorption of H2O is found for example in the following papers (Si: [85N, 89K, 96F] Ge: [87K3, 91P1] 3-5: [93C1, 96H, 97M1, 98C, 00H]). In this data collection the substrate oxidation is considered only if it occurs in adsorption/desorption experiments of adsorbed H2O or of the dissociation product OH, but not as a subject in itself. The results obtained on kinetic aspects of adsorption and desorption as well as surface diffusion and reaction are summarized in Table 3. Si (100) and its reconstructions The kinetics of adsorption has been investigated in detail in the temperature range from below 140 K, where multilayer adsorption becomes possible, to above 400 K, where OH radicals begin to react further eventually forming atomically adsorbed oxygen [96R2, 97W]. STM work [93A, 93C2] shows that the reconstruction dimers of the substrate are not broken, during dissociative adsorption of H2O with the formation of Si-OH and Si-H surface molecules (Fig. 1 [97K]). Correspondingly, LEED experiments show that the (2×1) periodicity is conserved [82I2, 84S1, 85C, 87L2]. The coverage dependence on dosing as well as the detailed analysis of Si 2p core-level shifts [85R3] indicate a saturation coverage θsat of 1/2 monolayer (ML). However, a small fraction of dangling bonds always remains unoccupied [93A] since water dissociation needs two adjacent empty dangling bonds, which do not always belong to one dimer. When approaching saturation, some isolated single dangling bonds remain unsaturated, which are unable to dissociate water. Thus, the highest coverage achieved is below 0.5 ML (0.48 ML [93A], 0.41 ML [96F]). All studies agree that the initial sticking coefficient S0 is high (near unity). It is reported to be almost constant up to saturation [85R3, 90S] leading to the assumption that a mobile precursor state exists which allows to model the coverage vs exposure time dependence as in Fig. 3 [96R2]. Si (111) and its reconstructions The RT adsorption of H2O on Si(111)7×7 can be divided into three adsorption regimes. The values given for the sticking coefficients and the transition coverage or saturation coverage vary in different reports (see Table 3) depending evidently on sample preparation and experimental parameters (e.g. temperature, dosing, applied experimental technique). The first fast regime is attributed to a fast, precursor mediated, dissociative adsorption probably on rest-atom/adatom pairs forming OH and H radicals, which saturate two neighboring dangling bonds (Figs. 4, 5) [91P3, 96F]. This dissociation is also observed at low temperature (180 K). The second regime corresponds to the complete saturation of all Si(111) surface dangling bonds forming a two-dimensional hydrosilicate phase (Si-OH + Si-H). This process is eventually accompanied by complete dissociation of adsorbed H2O in minor amounts (Si-O-Si + 2SiH). Both processes involve the surface diffusion of Si atoms and of the adsorbed species e.g. OHads. The last very slow regime at coverage above approximately θ = 0.6 is related to Si-oxide formation and reactive attack of back bonds.

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Ge in different surface orientations There are only very few studies on the kinetics of H2O adsorption on Ge surfaces and no final conclusion can be drawn. The sticking coefficient is lower compared to Si surfaces being highest for the Ge (100) ([87K3], Fig. 6). A physisorbed precursor state is assumed [92C]. On Ge(111) adsorption of one H2O molecule induces structural rearrangements of 50 to 80 atoms on the substrate surface [73H, 75S]. Heating to elevated temperatures (T > 600 K) leads to a complete dissociation of H2O with the formation of Ge-O-Ge groups in analogy to Si(100) [91P1]. 3-5 semiconductors in different surface orientations The experimentally found adsorption rate of H2O is very low at RT for all surface orientations [84M, 89S, 96H, 00H]. Desorption spectra are characterized by a molecular desorption peak at low sample temperatures (150 - 180 K) due to desorption of physisorbed H2O including multiple ice layers [93C1, 97M1, 97M2, 98C]. Another well defined desorption peak occurs at 350 K due to desorption of strongly interacting H2O (dissociative or molecularly?) and broad ill defined desorption peaks extend up to 750 K depending on surface orientation and conditions, which are related to desorption from surface hydroxyls. H2O desorption and dissociation is induced by annealing (needed temperatures depend on the system) and can also be induced by irradiation with 50 eV electrons [96S2]. At elevated temperatures complete dissociation and formation of oxides sets in. On GaAs(110) the differential sticking coefficient shows a pronounced discontinuity, which has been related to an adsorbate induced loss of surface relaxation ([84M], Fig. 8). Miscellaneous semiconductors No systematic kinetic studies on the desorption of H2O from layered semiconductors have been performed. Only qualitative results are given on the desorption temperatures of the adsorbed ice layer at substrate temperatures of about 150 K [92M, 93M].

3.8.2.9 Local structure Detailed information on the local structure of the adsorbed species is only available for the Si(100)2×1 surface. Here a number of different experimental techniques have been applied and have been complemented with theoretical calculations. As a result, the electron rich O atom is placed close to the electrophilic dangling bond (downward shifted Si atoms of the dimer) and the hydrogen towards the nucleophilic dangling bond (upward shifted Si atom of the dimer, i.e. the outer dimer atom ODA). The reaction mechanism may be described as simultaneous weakening of an O-H bond and formation of Si-O and Si-H bonds on the electrophilic and nucleophilic sites of the dimer, respectively (Fig. 1). Also information on the orientation, bonding distances and bonding angles of the dissociation products OH and H are given (see Table 4). ESDIAD results clearly indicate that the O-H bond direction is off-normal (Fig. 9a, 9b) [87L2, 94G] and that the Si-OH bond direction is normal to the dimer bond direction. Detailed numbers on bond angles are given from photoelectron diffraction data [98F](Fig. 12) and theoretical calculations [89O, 95V, 97K]. On “clean” Si(100)2×1 surfaces three defects are usually observed in STM images, which were interpreted as missing dimers (A), pairs of missing dimers (B) and pairs of half dimers with two Si atoms missing on the same side of a dimer row (C). Also cluster defects CD composed of the defects A, B, C are found [89H, 92W]. The number of sites appearing like C and A defects increases with exposure to 0.05 L H2O and are termed W and M in Fig. 10 [93C2]. The occupied state image obtained with H2O saturated surfaces shows that the dimers bound to the dissociation products H and OH are atomically resolved. In addition, the density of missing dimers is reduced after water adsorption [93A]. Thus the dark features M Landolt-Börnstein New Series III/42A4

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and W induced by low H2O dosage are attributed to nondissociated H2O molecules. Also the C defects are attributed to two water molecules on the same side of adjacent dimers in a row and A defects to two water molecules on each side of one dimer. The water molecules are assumed to be stabilized by hydrogen bonds. Since single water molecules are not found in STM, they are assumed to be rather mobile. The formation of chains of molecular H2O appears to be an important precursor for dissociative chemisorption [93C2]. With increasing exposure to H2O the growth of islands of dimers saturated with H and OH is observed while the number of islands is constant. At saturation a few single dangling bonds remain uncovered due to the fact that two dangling bonds are necessary for dissociation, which also may belong to different (neighboring) dimers (Fig. 11) [93A]. Both STM studies have noticed that adsorbed water appears identical to features previously assigned to intrinsic defects of the Si surface, suggesting that at least some of those apparent defects are actually due to water adsorbed from the background pressure (see above). For all other semiconductor surfaces there is only rudimentary information on the structural arrangement of adsorbed H2O/OH available in the literature. On Si(111)7×7 OHads is considered to be adsorbed on top sites with an Si-O bond orientation normal to the surface [85N, 86N]. STM data indicate dissociative adsorption of adatom-restatom pairs and adsorbate islands due to H-bond formation (Fig. 13) [91A, 97S1]. On FeS2 and CuInSe2 a preferential occupation of molecularly adsorbed H2O on Fe and Cu sites has been deduced (Fig. 14) [91P2, 92S]. This selective bonding to only one kind of the available surface atoms has been explained by the atomic orbital character of the conduction band state involved in the donor like coordination bonding of the H2O lone pair orbitals. Based on the limited information available it can be concluded that the adsorption of H2O is governed by a coordination type bonding of its lone pair orbitals with the nucleophylic sites (conduction band forming orbitals) of the semiconductor substrate. In addition, interadsorbate interactions due to H-bond formation have to be taken into account. After dissociation the bonding geometry of OHads to the surface is governed by the orientation of the σ bonds formed with the semiconductor dangling bonds. 3.8.2.10 Long range order No detailed quantitative investigations of the surface structure of adsorbed H2O and its dissociation products on semiconductors are given in the literature. LEED has mostly been used as qualitative argument for the discussion of adsorption modes and bonding sites. In addition, a few STM experiments are available, giving information on the long range order of substrate and adsorbates (see Table 5). If the coverage is kept at about one monolayer (for molecular adsorption at low temperatures) the LEED superstructure spots of the substrates remain unchanged indicating that the surface reconstruction is retained. Also in the regime where OHads and Hads are formed by dissociative adsorption the LEED superstructures are usually conserved as e.g. for Si(100)2×1 [87L2, 93L] or Si(111)7×7 [86N, 91P3, 92D, 97Z]. This indicates that the adsorbates only saturate the available danglings bonds on the surface and do not destruct the stabilizing surface back bonds. But there are also cases known, where the superstructure is lost and a (1×1) structure is formed as for Si(111)2×1 [85S] or Ge(111)2×1 [73H, 75S, 79G] indicating a change in substrate surface bonding. The detailed analysis of LEED data shows that the adsorption of one H2O molecule converts the superstructure in an area of 50 - 300 surface atoms around the adsorption site [73H, 75S]. On GaAs(110) a qualitative interpretation of I(V) curves in LEED indicates the loss of surface relaxation with H2O adsorption [84M]. The LEED reflections and thus the long range order are completely lost, when the surface conditions favor surface oxidation by complete dissociation of adsorbed H2O, which already starts at RT for high exposures. STM studies show that adsorbed water is not uniformly distributed over the Si(100)2×1 surface at low coverages. Instead the water fragments form small islands, consistent with adsorption through a mobile precursor state (Fig. 10, 11) [93A, 93C2]. Simultaneous changes in both dangling bonds of a single dimer were observed, so that there is a preference for adsorbed fragments to remain paired. Anderson and Köhler [93A] observed a strong correlation among occupied dimers, with adsorption much more likely to occur at a dimer adjacent to an already occupied dimer. Adsorbate islands grow preferentially in the direction along and perpendicular to dimer rows. Also on Si(111)7×7 the 7×7 superstructure is conserved and adsorbate islands are formed [91A, 97S1] (Fig. 13). Landolt-Börnstein New Series III/42A4

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3.8.2.11 Electronic properties The investigation of the electronic structure of adsorbed H2O by photoelectron valence band spectra (UPS) has mostly contributed to the controversy about dissociative vs. molecular adsorption (Table 1 and 6). Adsorbed H2O in its molecular state should lead to three typical main emission lines in the upper valence band (see Fig. 17, 19, 23, 24, 25, 26) corresponding to the 1b1 molecular orbital (MO) (non bonding, O lone pair, gas phase ionization potential Ip = 12.6 eV), 3a1 MO (partly bonding, partly nonbonding, O lone pair, Ip = 14.7 eV) and the 1b2 MO (OH-bonding, Ip = 18.5 eV). The gas phase spectra and the spectra of condensed molecules are given e.g. in [87T] and in LANDOLT-BÖRNSTEIN III 23A, chapter 2.3. After adsorption these states may broaden, split or shift in energy depending on interadsorbate interaction and on the interaction with the substrate [83S1, 87K3]. The 3a1 state of H2O will split into two levels by the formation of dimers and will lead to a broad feature by ice layer formation due to hydrogen bonding. The characteristic emission pattern of molecular H2O adsorption is given in Fig. 19 for H2O monomers, H2O dimers and ice on Ge(100) [87K3]. Typical ice spectra are found in Fig. 26 for layered chalcogenide substrates [92M, 93M]. When the binding energy values measured for the gas phase spectra are compared to the measurements of the adsorbates different contributions of binding energy shifts must be considered. One important contribution is the unknown decrease of the measured binding energy value due to extraatomic relaxation effects (final state hole screening), which depends on the spatial distance of the adsorbate to the substrate. If the measured binding energy values of adsorbates are referred to the Fermi level of the substrate, a further reduction of the binding energy by the value of the work function must be taken into account which may further depend on band bending effects (see also section 3.8.2.4). For the valence band maximum as reference level the ionisation potential must be taken into account. Its value may change with adsorbate coverage. The experimentally determined values of the adsorbate induced additional emission lines and reference value used in the experiments are given in Table 6. However, as in many cases the position of EF in the bandgap and the value of the work function was not reported, the different binding energy values are not compared to each other. However, a typical value for referring the gas phase ionisation potentials to EF is ∆EB ≈ 7 eV which is composed of a typical value of 5 eV for φ and 2 eV for the extraatomic relaxation term. For dissociated H2O the molecular orbitals of OHads and Hads should be found. For OHads assuming a negative charge, e.g. OH-ads gas phase MOs are often derived from the isoelectronic HF molecule which leads to a degenerate 1π MO (nonbonding, F lone pairs, Ip = 16.0 eV ) and the 3σ MO (FH-bonding, Ip = 19.9 eV) in the upper valence band region [82G]. The ionization potentials of OH- as measured for solid NaOH are at ~ 8.2 eV (1π) and ~ 12.5 eV (3σ) (vs. Evac) [87T]. However, if the orientation of the adsorbed OH will not be exactly perpendicular to the substrate surface the degeneracy of the 1π MO is lifted and a bonding and non-bonding state with respect to substrate-bonding interaction may be formed. The assignment of the OHads emission lines is still done in a controversial way. The experimentalists tend to assign the strong emission and the often observed shoulder line at low binding energy to the 1π MO of adsorbed OH, which has lost its degeneracy due to interaction with the substrate [87K3, 88M, 89L, 91L1, 91L2]. The high binding energy line is due to the 3σ MO OH-bonding state. However, based on theoretical calculations the assignments of the lines are given as (from low to high binding energies): O 2p non-bonding lone pair (deduced from 1π MO), the 3σ MO OH-bonding state energetically shifted to lower binding energies due to reduced OH bond strength, and the Si-O σ bonding state (MO formed by the overlap of the second 1π MO and Si s orbitals) [83C1, 83C2, 87K1, 89F, 94S1]. Which assignment is correct cannot be decided, yet, and therefore both are used in Table 6. Furthermore, the contributions of Hads should lead to additional adsorbate induced states in the upper valence band region, which, however, are generally believed to be of low intensity. Again the binding energy position of the adsorbate states may depend on the given energy scale and interatomic relaxation effects. Thus, depending on the assumptions used for spectral interpretations experimentally found adsorbate induced emissions may be related to different energy states of adsorbate species and substrate bonds. Furthermore, if the binding energy scale is referred to the Fermi level, band bending effects induced by the adsorbate or differences in the doping level, which shift the position of the Fermi level in the bandgap, may lead to additional problems in assigning the UPS valence band emission lines. Finally, for most semiconductors the adsorption of H2O does not lead to a stable adsorbate state, but depending on the conditions, e.g. the substrate temperature, chemical reactions with the semiconductor back bonds may occur in parallel. 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As already discussed before the electronic surface potentials, as surface position of the Fermi level, respectively band bending and work function, may change considerably during the adsorption process. But in many cases published in literature the authors were evidently not specifically interested in these effects and therefore no systematic data are available. For this reason we did not include the scarce data in our review. The reported changes of work function due to adsorption are given by Jacobi in LANDOLT-BÖRNSTEIN III/42A2, chapter 4.2 Si(100)2×1 On Si(100)2×1 after saturation of H2O adsorption at RT the photoemission spectrum shows two strong adsorbate induced lines at about 6.5 and 11.5 eV, the former one indicating a shoulder at about 7.5 eV (see Figs. 15, 16) [84O, 85R1, 85R3, 87L1, 88M, 89F, 90S, 91L1, 94S1, 96R2]. These lines are now assigned to adsorbed OH formed after dissociation; the corresponding Hads does not give prominent structures. The surface states contribution of the Si(100)2×1 surface close to the Fermi edge is quenched during the adsorption process indicating the loss of the corresponding dangling bonds [85R3]. These typical emission lines are already found at lowest coverage even at low sample temperatures (Fig. 16) [85R1], which excludes a stable initial molecular adsorption state. Molecular adsorption of H2O showing the typical emission features is obtained for multilayer condensation at low temperatures (Fig. 16) [85R1]. After annealing such low temperature adsorbate phases to RT first a spectrum typical for OHads is obtained and later the substrate oxidation sets in. The UP spectra, which originally have been interpreted to be due to chemisorbed molecular H2O [81F1], has not been reproduced by other authors besides in difference spectra [85R1]. They are probably due to a mixed adsorption state composed of surface oxides with OH and H-bonded H2O. The given assignments are supported by a number of theoretical calculations performed for different orientations of molecular and dissociatively adsorbed H2O (see Table 6). The energetic order of the OH states and H2O molecular states depends on the assumed orientation of the adsorbates to the surface. Si(111)7×7 Also on Si(111)7×7 after low temperature H2O adsorption and annealing to RT or after quasi-saturation coverage at RT a valence band photoemission spectrum is obtained, which is dominated by two strong lines with the low binding energy feature evidently composed of two overlapping emission lines (see Fig. 17) [85R1, 85R2, 86S2, 89F]. As for Si(100) this spectrum can be assigned to dissociatively adsorbed H2O in the form of OHads and Hads. The adsorbed H leads to very weak emission lines at even lower binding energy and thus is not discriminated in the experimental data. At low sample temperature H2O is adsorbed as ice layer showing the typical three emission line spectrum [85R1, 85R2, 85R3, 86S2]. The three line emission spectrum as obtained by Fujiwara [79F, 81F1, 81F2] assigned to chemisorbed H2O was obtained later on only in difference spectra (RT saturation coverage- RT low coverage regime, Fig. 17) [85R1, 85R2]. It is clear now that the originally given interpretation as due to molecular adsorption does not hold. Based on the given spectral features it also does not fit to coadsorbed H2O stabilized by OHads and Hads. Therefore, in agreement to the vibrational data the best interpretation may be to assume a mixture of OHads, Hads, and a certain concentration of Si-O due to complete dissociation of H2O. The expected electron states of molecularly adsorbed H2O on Si(111) dangling bonds have also been calculated and compared to the experimental data (Table 6). The electron states found depend on the assumptions on binding geometry: for H2O adsorbed normal to the surface (O pointing downwards) the original 3a1 MO state of H2O is strongly shifted in energy close to the 1b2 state due to overlap with the Si 3s states. The 1b2 state is hardly affected. For H2O tilted 90° the original 1b1 state is shifted close to the 1b2 state and the 3a1 state is hardly affected. For an intermediate bonding angle also an intermediate situation with remnants of all H2O based MOs is found [87K1].

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240

3.8.2 H2O and OH on semiconductors

Si in other orientations Also on Si(110) [85R1] and typical stepped surfaces (e.g. Si (113) [97R]) H2O is dissociatively adsorbed at RT showing the typical spectral features of OHads and Hads, which saturate the existing dangling bond states. Ge in different surface orientations On Ge(100) or (111) surfaces at low sample temperature (110 K) molecular adsorption has been observed showing the transition of physisorbed isolated H2O molecules, to H2O dimers, and finally to H2O clusters (ice) in the UP spectra (Fig. 19, 20, 23) [87K3]. The spectral features are very similar to each other besides a splitting of the 3a1 level and finally the broadening of this level as expected for interadsorbate interaction mediated by H-bonding. After annealing to RT dissociatively adsorbed H2O is found with spectral features in close correspondence to the above given data on Si. The dispersion found for these adsorbate states is small (Fig. 21, 22) [91L2]. 3-5 semiconductors in different surface orientations The spectral features as obtained for different surfaces of 3-5 semiconductors after H2O adsorption so far do not allow a final conclusion on the adsorbate species formed and an unambiguous assignment of the valence band features. Only the spectra, which are obtained as condensed ice layers at low temperatures (Fig. 24, 25) can be clearly assigned [97H, 00H]. It seems reasonable to assume dissociative adsorption on the (110) plane for certain conditions, as the surface arrangement of dangling bonds are similar as for Si(100)2×1. The spectral features obtained may be interpreted in this way but the given assignment to the possibly formed group 3-OH and group 5-H surface molecules is not completely clear yet, as the intensity pattern does not agree to the expectations (strong -OH and weak -H emissions) (Fig. 24, 25) [97H, 00H]. Miscellaneous semiconductors The adsorption of H2O on different chalcogenide surfaces (layered chalcogenide (0001), FeS2(100), CuInSe2(011) is only possible at low temperatures with spectral features in UPS which can well be assigned to molecularly adsorbed H2O in the form of ice (Fig. 26) [75Y, 87J, 88J, 91P2, 92M, 92S, 93M, 96M]. 3.8.2.12 Core level lines The core level photoelectron spectra found for H2O adsorption on semiconductors are mostly related to substrate binding energy shifts. We therefore present these data as they are good indications for the mode of adsorption. In some cases it is not clear whether the use of high brilliance light as e.g. synchrotron light enhances the surface oxidation after dissociative adsorption due to electron impact of the secondary electrons (see e.g. [95P]). Alternatively, the data may suggest partial oxidation to occur already in parallel to the adsorption process by thermal activation of surface diffusion. All semiconductor surfaces show additional emissions in their photoelectron core level spectra besides the bulk emission lines (surface core level shifts) which are due to surface reconstructions/relaxation [92B, 94S2, 95L, 95M]. On the clean Si(100)2×1 surface three surface components of the Si 2p core level are discriminated. Their binding energy shift relative to the bulk component and assignment are given as: -0.49 eV (outer dimer atoms ODA), +0.06 eV (inner dimer atoms) and +0.22 eV (second layer atoms) [92L]. The accepted model of the 7×7 reconstruction of Si(111) surface as proposed by Takayanagy et al. [85T] leads to complex Si 2p core level spectra [94K, 94L, 97P] which have been assigned differently by different authors. We follow Landolt-Börnstein New Series III/42A4

3.8.2 H2O and OH on semiconductors

241

here the assignment as suggested by [94K] as it was used for the discussion of H2O adsorption [95P] consisting of surface components at S1: -0.698 eV (rest atom), S3: 0.530 eV (adatom), S4 : -0.183 eV and S5: 0.930 eV with respect to the bulk component. Also Ge(100)2×1 [89L, 91L1, 93R1] and the surfaces of compounds with Zinkblende structure show surface core level shifts [94S2, 95M, 97H] which must be considered in the interpretation of adsorbate induced binding energy shifts. For all investigated semiconductors dissociative adsorption leads to the loss of surface core level shifts in the substrate emissions. These core level shifts indicate a different charging of different surface atoms (in addition to different final state screening), which is immediately lost when the surface dangling bonds become saturated indicating adsorption on the corresponding sites. Nearly no high quality core level photoemission data do exist for the O core lines as the O 1s level was beyond the high resolution limit for second generation synchrotrons and the O 2s line has a very low photoionization cross section. 3.8.2.13 Vibrational properties Vibrational data of adsorbed H2O (OH) species on semiconductor surfaces have been obtained either from electron energy loss spectra (HREELS) or infrared absorption spectroscopy (IRAS). In order to separate adsorption from the residual background in the vacuum chamber in many cases the deuterated analogs D2O (OD) have been studied (in addition). It was suggested to account for different interpretations of adsorption modes from UPS and HREELS data that the formation of OH from the molecular precursor H2O was induced by the electron beam in HREELS. But later studies also with IRAS indicate no strong disturbance of the adsorbate phase by these low energy electrons. Different adsorption modes can be discriminated depending on adsorption conditions (see Table 8): molecular adsorption of H2O (ice) is characterized by the typical broad features due to the hindered rotational and translational modes (around 100 meV or 800 cm-1 and 25 meV or 200 cm-1), the scissor mode (at around 200 meV or 1600 cm-1), which is the most significant mode for molecular adsorption, and the stretching modes (in the range of 410-460 meV or 3300-3700 cm-1). The determined values and the widths of spectral features depend strongly on interadsorbate interactions due to hydrogen bonding and also on the bonding to the substrate. The dissociation of H2O forming OHads and Hads leads to the loss of H2O modes especially the scissor mode and to the occurrence of sharp stretching modes of O-H (around 460 meV or 3700 cm-1) and S-H (S substrate adsorption site, around 260 meV or 2100 cm-1). The S-H stretching mode is more significant as the O-H stretching mode, which may also be broadened by coadsorbed H2O. The S-O stretching modes, and the S-OH and S-H bending modes of dissociatively adsorbed H2O are all found in the spectral range of 80 to 120 meV or 650 to 950 cm-1 and are also not very specific. At elevated temperatures H2O is completely dissociated and the beginning oxidation of the substrate leads to the transformation of spectra. The resulting spectra may now be rather complex as depending on the oxidation mechanism differing bonding geometries, numbers of substrate atoms, and intermediately formed surface molecular species will be involved. Si(100)2×1 On Si(100)2×1 the vibrational data obtained by EELS and IRAS (see Fig. 34, 35, 36) have unambiguously established that dissociation of H2O into Hads and OHads takes place on the clean surface even at low sample temperatures [82I2, 84C1, 84C2, 84S3, 85C, 97S2]. The spectral assignments were confirmed by isotope shifts (H-D and O16-O18). The observed isotope frequency shifts have been confirmed by cluster calculations assuming different coupling constants of the stretching and bending modes [83B, 95V, 96R1, 97K]. Additional information on the local site and orientation of the adsorbates have been deduced from the vibrational spectra, which already have been given in Table 4. Not included in the data collection are detailed information on the technically important initial oxidation which were drawn from vibrational measurements on annealed surfaces and respective cluster calculations [82I1, 82I2, 97W]. The temperature dependence of ESDIAD patterns have been attributed to a low frequency (200 cm-1) Si-OH torsion mode, which is already thermally activated at substrate temperatures below RT (Fig. 9) [94G].

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242

3.8.2 H2O and OH on semiconductors

Si(111) and its reconstructions On Si(111)7×7 vibrational spectra typical for the different adsorption modes (molecular and dissociative) have been obtained depending on the adsorption conditions (Table 8). A comparison of spectra of molecularly adsorbed H2O (D2O) and dissociatively adsorbed H2O clearly show the formation of Si-OH and Si-H in the latter case [82I2, 83K1, 85N, 86N, 86S2]. The spectral changes related to complete dissociation and Si-oxide formation are presented in [85N, 86N]. By changing the angle and primary electron energy in HREELS the variation in vibrational loss intensities were determined and used to estimate the relative contribution of the dipole, impact and resonance mechanism to vibrational excitation [86N]. On Si(111)2×1 the coexistence of molecular and dissociative adsorption has been found and related to interadsorbate stabilization of H2O by Si-OH groups due to H-bonding (Fig. 39) [84S2, 85S, 86S1]. Besides monohydrides also dihydrides are found indicating a reorganisation (diffusion) of adsorbate species. For such surface conditions extremely high resolution in the HREEL spectra were obtained [85S]. Ge in different surface orientations At low sample temperature the adsorption of H2O on Ge(100)2×1 is characterized by spectral features in HREELS, which are attributed to the simultaneous existence of dissociatively and molecularly adsorbed H2O in contrast to Si(100)2×1 [89P]. Annealing to RT leads to the loss of the vibrational features of adsorbed H2O with the formation of OHads and Hads. In mixed GeSi alloys a surface segregation of Ge is discussed, which indicates a pronounced surface mobility of substrate atoms and adsorbate species [86B, 89P]. 3-5 semiconductors in different surface orientations No vibrational spectra have been reported, yet, for H2O adsorption on the well defined (110) cleavage plane of 3-5 semiconductors. Most of the presented studies deal with the (100) surfaces also because of the interest in the subsequent thermally induced oxidation process after adsorption [96S2, 97M1, 98C]. The different dimer reconstructions of the (100) surfaces are not well defined and depend strongly on the preparation method used. As the dissociation of H2O is preferred, when group 3 and group 5 sites exist in close neighborhood to each other, the experimentally observed dissociative adsorption at RT may involve some rearrangement of the initially present surfaces. Furthermore, the starting conditions of the surface for adsorption was not completely free of preadsorbed H2O from the background pressure.

Landolt-Börnstein New Series III/42A4

Landolt-Börnstein New Series III/42A4

Si(100)2×1

Crystal surface Si(100)

dissociative molecular dissociative dissociative

RT, θsat RT, 0.05L 0.1L RT, θsat heating to 400°C 90 K, 1-80L and RT, 1-80L 137 - 412 K, 0.03 ML - θsat

UPS

dissociative

dissociative

molecular dissociative

100 K, 0.25L RT, θsat

UPS, XPS, static SIMS SXPS

dissociative dissociative

80-500 K, ~1L 160, 300, 600 K, θsat

IR polarised LEED, AES, UPS UPS UPS, SXPS, LEED UPS, SXPS STM

Adsorption modeb dissociative molecular dissociative molecular

Conditionsa RT, θsat RT, θsat RT, θsat RT, θsat

Technique Ellipsometry UPS HREELS UPS, SXPS

H-Si-Si-OH formation assumed 94S1 H-Si-O-Si-H formation assumed Si-O-Si and oxide formation assumed even at 90K 95P (oxidation reaction enhanced by SXPS radiation ?) H-Si-Si-OH formation assumed also at low coverage 96R2 and low temperature

91L1 93C2

84S2 87L1

Si...OH2 physisorption of H2O molecules assumed Si-OH and Si-H bond formation assumed Si-OH and Si-H bond formation assumed -Si-Si-OH2 formation assumed for very low coverages on conserved Si-dimers.

84C2 84O

Ref. 71M 81F1 82I2 83S2

Conclusions and remarksc H-Si-O-Si-H formation assumed Si-OH2 molecular chemisorption assumed physisorbed H2O can be excluded Si-OH2 molecular chemisorption assumed (spectra to be assigned to dissociative adsorption?) only Si-H and SiO-H modes detected H-Si-Si-OH bond formation assumed

Table1. Mode of adsorption on different semiconductor surfaces: Molecular or dissociative in dependence on experimental conditions. In this table the mode of adsorption - molecular or dissociative - as suggested by the authors are presented. In cases, where there is now general agreement of the mode of adsorption, we have added a respective comment on deviating conclusions. In other cases, where the mode of adsorption is still not clarified, only the opinion of the authors is presented.

The data are organized in relation to the different adsorbate properties as given in the introductory section of this volume by Bonzel (LANDOLTBÖRNSTEIN III 42A1) whenever possible, which means, if data are available. We do not subdivide the data according to the different semiconductor substrates, as it would impede the direct comparison of typical adsorbate properties. Thus, the data are summarized in the following tables:

3.8.2.14 Data as given in Tables

3.8.2 H2O and OH on semiconductors 243

Si(111)2×1

Si(111)7×7

Crystal surface Si(100)2×1 vicinal

100 K > 2 L RT

RT, > 100 L 100 K, < 0.3 L LEED, HREELS, XPS 100 K 0.3 - 2 L

EELS, Auger, PYS, LEED

SXPS

STM XPS, EELS

UPS UPS

RT, < 100 L

≤ 100 K, 2 L 100 K, 2 L → 300 K RT, 60 - 103 L 80 K, 2.5 L → 300 K RT, 8 - 1200 L RT, θsat ≤ 100 K,< 0.5 L > 0.5 L RT, < 25 L 150 K 150 – 900 K 90 K, 5 - 95 L 300 K, 5 - 95 L

HREELS HREELS

dissociative dissociative and molecular molecular dissociative

dissociative and some oxidation

dissociative

dissociative dissociative

dissociative molecular

molecular

RT, >100 L

UPS

UPS

molecular dissociative

RT, θsat RT RT, < 100 L

disruption of π-bonded chains of 2×1 reconstruction

intermediate coverage region

Si-OH species assumed physisorbed H2O condensed ice layer dissociative adsorption on adatom-restatom pair Si-OH and Si-H bond formation assumed; reaction to oxide at T >175 K initial formation of OH bonds to Si adatoms, further reaction to SiOX x ≤ 4 even at 90K; (oxidation reaction enhanced by SXPS radiation ?) some Si-O-Si formation, no influence of electron beam reported SiOX formation

Si-OH2 molecular adsorption assumed; (spectra to be assigned to dissociative adsorption?)

84S1

97Z

95P

89F 85R1, 85R2 86S2 91A, 97S1 92D

82I2, 86S2 82I2, 83K2, 85N, 86N, 86S2 85R1, 85R2, 86S2

79F, 81F1, 81F2

89F 71M 75F, 77F

dissociative dissociative molecular/ dissociative molecular

RT, θsat

ARUPS, SXPS, LEED ARUPS Ellipsometry EELS Si-OH and H-Si bond formation assumed Si-O-Si and Si-H bond formation assumed molecular adsorption assumed; electron beam induced oxidation with time Si-OH2 molecular adsorption assumed; (spectra probably due to the onset of oxidation?) condensed ice layer Si-OH and Si-H bond formation

Adsorption modeb Conclusions and remarksc Ref. dissociative dissociation takes place at terrace dimers, not at step 85C dimers dissociative 88M Si-OH and Si-H bond formation assumed

Conditionsa RT, θsat

Technique IR polarised

244 3.8.2 H2O and OH on semiconductors

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Landolt-Börnstein New Series III/42A4

GaAs(100)2×4

GexSi1-x (100)2×1 GaAs(100)

Ge(111)2×8

transforms to Ge(100)2×1 Ge(100)2×1

Ge(100)2×8

Crystal surface Si(113)3×1 Si(113)3×2

RT, 7×1012 L

UPS, XPS, SIMS

HREELS, TDS 100 K, 0.2 L > 200 K, 0.2 L

RT, 1L

345 K 110 K, 10 L → 300 K 170 K, 30 ML 110 K, 0.3 - 3 L → 300 K

→ 300K 100 K

110K, 0.4 L 1.1 L 3.7 L →300 K 80 K RT 160 K, 0.5 -102 L

Conditionsa RT, 1 L RT, θsat

HREELS

TDS UPS

UPS, SXPS

HREELS

UPS, SXPS LEED

IR

Technique HREELS HREELS, UPS, SXPS UPS

molecular dissociative

dissociative

dissociative molecular dissociative and molecular dissociative

chemisorbed H2O As-H and Ga-OH bond formation after annealing

no adsorption at RT ice clusters Ge-OH and Ge-H bond formation assumed, some remaining rests of molecular H2O mostly Ge-OH and Ge-H due to Ge surface segregation Ga-OH bond formation assumed

Adsorption modeb Conclusions and remarksc dissociative adsorption at dimer dangling bonds dissociative 3×2 is transformed to 3×1 by a fraction of a monolayer of H2O adsorbed molecular monomeric H2O dimerized, H-bridged H2O ice clusters Ge-OH and Ge-H bond formation assumed dissociative molecular no adsorption at RT for exposure of 10L dissociative molecular ice layer and some dissociative dissociative Ge-OH and Ge-H bond formation assumed dissociative and Ge-OH and Ge-H bond formation assumed molecular dissociative dissociative Ge-OH and Ge-H bond formation assumed

98C

82W

86B

92C 87K2, 87K3

93R1, 93R2

89P, 91P1

89L, 91L1, 91L2

84C2

87K2, 87K3

Ref. 96I 93J, 96S1, 97R

3.8.2 H2O and OH on semiconductors 245

molecular molecular

100 K, 2 L 100 K, 0.5 - 1.5 L

TPD, Auger, HREELS XPS,UPS, LEIS UPS,SXPS UPS,SXPS UPS UPS UPS,SXPS UPS,LEIS,XPS 100 K, 0.5 - 7 L 100 K, 0.3 - 10 L 80 K, 0.5 - 5 ML 100 K 100 K, 1-10.5L 80 K, 0.01 - 10 L

RT, 4×1010 L SXPS 100 K, < 0.5 L 100 K, > 0.5 L UPS, HREELS RT, 2×103 L

molecular molecular molecular molecular molecular molecular

molecular

condensation of ice clusters condensation of ice clusters condensation of ice clusters condensation of ice clusters condensation of ice clusters preferential adsorption on Cu sites

preferential adsorption on Fe sites

Ga-suboxide In-OH and P-H bond formation assumed condensed ice layer partial dissociative adsorption induced by Na codeposition thermal induced desorption and reaction

92M, 93M 92M, 93M 75Y 87J, 88J 92M, 93M, 96M 92S

91P2

97M1, 97M2

88D

00H

97H

81T 96H

79B

96S2

Ref. 93C1

papers; remarks by the authors of this data collection are given in parenthesis

a Experimental conditions as given in the references b Proposed adsorption mode as suggested by the authors of the given papers c Additional information as given by the authors of the given

GaSe(0001) InSe(0001) MoSe2(0001) MoS2(0001) WSe2(0001) CuInSe2

FeS2(100)

AlAs(100)1×1

InP( 1 1 1 )

InP (110)

RT, 400 L 100 K, < 0.5 L > 0.5 L RT, 106 L RT, 2×109 L

PSID SXPS

SXPS

dissociative dissociative molecular dissociative dissociative and molecular oxidation dissociative molecular no adsorption

180K, 0.1 - 600 L 300 K, 102 - 106 L

molecular

UPS

100 K, 3 ML chemisorbed H2O; H2O ice layer on top; dissociative after electron beam irradiation chemisorbed H2O assumed chemisorbed H2O assumed for low coverage physisorbed H2O assumed for high coverage (origin of spectral features unclear) As-OH and Ga-OH bond formation assumed As-H and Ga-OH bond formation assumed ice formation As-H and Ga-OH bond formation assumed mixture of As-H and Ga-OH and H2O

Adsorption modeb Conclusions and remarksc physisorbed and chemisorbed H2O assumed molecular

GaAs(110)

Conditionsa RT, <100 L

Technique Photoreflectance XPS, TPD, HREELS

Crystal surface GaAs(100)4×6

246 3.8.2 H2O and OH on semiconductors

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Table 2. Thermodynamic data on adsorption energies of H2O and its dissociation products OH and H on different semiconductor surfaces Thermodynamic data obtained by experiments approaching thermodynamic equilibrium do not exist for well defined semiconductor surfaces. In this table we will therefore summarize data on adsorption energies as obtained by kinetic experiments and by theoretical calculations. Adsorption Modelb Crystal surface Technique Conclusions and remarks Fig. Ref. Conditionsa energy [eV] 180-300K, 96R2 binding energy of mobile molecular precursor Si(100)2×1 adsorption 0.062 Si...OH2 0-θsat as derived from adsorption kinetics measured kinetics by at different temperatures UPS around 400K, 0.311 adsorption binding energy of mobile molecular precursor 96F, Si...OH2 kinetics by UPS 0-θsat as derived from adsorption kinetics 96R2 Si-OH2 ideal, unreconstructed clusters of 12-16 Si 87B, Si(100) THEORY 1.19 atoms; most stable: OH bridge bonded and H 87R Si-H2O-Si 1.7 MNDO on top; 5.1 Si-OH-Si dissociation barrier of 2.51eV 4.2 Si-OH 2.0 Si-H 2.0 Si-H-Si 2.6 H-Si and Si-OH-Si on one Si cluster difference between molecular and dissociative THEORY 1.07 Si-OH2 90E Si-H2O-Si adsorption is small; Empirical Pair 2.21 dissociation barrier of 2.61eV Potential 1) 2.05 Si-OH-Si and Si-Si-H H-Si-Si-OH 0.20 THEORY dissociative adsorption, calculation for a 95V H-Si-Si-OH 3.9 DFT/LDA2) OH-OH interadsorbate repeatable slab geometry 0.1 hydrogen bond 0.31 - 0.61 Si...OH2 no net activation barrier to dissociation 1 97K THEORY 2.13 - 2.73 H-Si-Si-OH DFT 3) 0.1 OH-OH interadsorbate hydrogen bond 97E Si(111)7×7 THEORY comparison of adsorption energies depending low coverage -1.26 Siad...OH2 4) EHT Siad-OH and Siad-H on adsorption sites: regime 4.38 molecular adsorption endothermic; best sites Siad-OH and Sirest-H 4.27 Si adatoms (Siad); dissociative adsorption on Siad=O and 2Siad-H 11.52 two adatoms exothermic; dissociative adsorption on adatom-restatom pair exothermic; complete dissociation on three adatoms strongly exothermic

3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

247

Technique

0.5 3.5 2.5 23 ML, 100 K 0.49 0.55

2)

Values are given as calculated for the symmetric dimer geometry Density functional theory in the local density approach 3) Density functional theory with different basis sets and functionals 4) Extended Hückel theory; values are given for the best adsorption site 5) Selfconsistent LCAO approach with local density approximation 6) Empirical tight binding method 7) Freshly cleaved surface 8) Annealed surface to 130°C or H2O exposed surface (3x10-6 Torr)

1)

b Proposed adsorption model as suggested by the authors of the given papers

a Experimental conditions as given in the references

GaAs(110) 4×6 Ga rich

THEORY LCAO / LDA5) TPD

0.56

RT

Isothermal desorption TDS 250 K, 0.9ML 0.7

1.4

RT

TDS

Ge(111)1×18) (8×2) GaAs(100) 4×6 GaAs(110)

1.1

As-OH2 Ga-OH2 Ga-OH and As-H ice layer chemisorbed molecular H2O

As-OH2

probably dissociative adsorption molecular

Si-OH2 on different Si sites, Si-OH and Si-H molecular or dissociative adsorption in close neighbourhood

Adsorption Modelb energy [eV] 1.49 Si-OH2 Si-OH and Si-H 1.13

TDS RT

Conditionsa

Ge(111)2×17)

THEORY Empirical Pair Potential 1) Si(110) stepped THEORY ETB6)

Si(111)1×1

Crystal surface

93C1 84M

local surface rearrangement assumed molecular adsorption, formation of As-OH2 bonds assumed preferential molecular adsorption on Ga-sites, dissociation possible after adsorption on Assites. high coverage regime studied.

96S2

96R3

75S

75S

96K

90E

Fig. Ref.

normal adsorption state

total electronic energy calculation leads to dissociative adsorption, core-core repulsion neglected metastable adsorption state

difference between molecular and dissociative adsorption is small

Conclusions and remarks

248 3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

Landolt-Börnstein New Series III/42A4

80 K, 0 - θsat, different pressures 180 - 300 K, 0 - θsat, pressure ranges around 5×10-9mbar RT, 0 - θsat, 1×10-8mbar RT, θsat RT, 0.005 ML ~ 400 K, 0 - θsat, 1×10-8 mbar resp. 5×10-9 mbar 423 K, 0 - θsat, 1×10-8 mbar

HREELS, IR, UPS, Auger, TPD

UPS, IR

TPD

STM

STM

UPS

Si(100)2×1

TPD

UPS

RT and below, 0 - θsat, different pressure ranges around 5×10-9mbar

Technique

Crystal surface

Conditionsa

some single dangling bonds always remain unsaturated the formation of chains of several H2O molecules along the dimer rows is assumed to be a precursor for dissociation first order Langmuir like adsorption isotherm; binding energy of mobile molecular precursor around 0.311 eV/molecule at 400 K adsorption modeled by Langmuir isotherm; 0.291 eV/molecule difference between activation energies for adsorption and desorption

3

85R3, 90S, 96F, 96R2

S(θ) is constant nearly up to saturation → mobile physisorbed molecular precursor on occupied and unoccupied sites dissociation occurs even at 80 K → activation energy is small or zero S/S0 versus θ/θsat modeled by two reaction channels: chemisorption of H and OH on two dangling bonds of one dimer or on two randomly adjacent dangling bonds; binding energy of mobile molecular precursor around 0.062 eV/molecule at RT and below θsat = 0.41

96F

96F, 96R2

93C2

93A

96F

96R2

84C2, 85R3, 96R2

84C2, 85R3, 96F, 96R2

θsat ~ 0.5 → one H2O molecule saturates two dangling bonds

93C2

Fig. Ref.

S0 ~ 1; abrupt saturation;

Results and conclusions

Table 3. Kinetics of adsorption/desorption, surface diffusion, and surface reactions on different semiconductor surfaces In this table the results on saturation coverage and sticking coefficients are summarized. Conclusions on surface diffusion and reactions, which follow from the investigations of adsorption kinetics, are also given. Data on desorption are only available from physisorbed H2O (ice layer) as chemisorbed species tend to react with the substrate forming oxides, when the sample temperature is increased.

3.8.2 H2O and OH on semiconductors 249

Si(111)2×1

Si(111)1×1

UPS

Si(111)7×7

XPS

EELS, Auger, PTS, LEED LITD1), TPD

TPD

LITD1) TPD

STM

TPD

fast adsorption up to θsat ~ 0.1 ML

Results and conclusions

dissociative adsorption followed by ice layer; ice layer desorption at 160 K; coadsorbed H blocks dissociation sites RT dissociative adsorption with 3 regimes; in the 2. regime θsat ~ 0.6 ML and the sticking coefficient S is depending on the pressure: S = 0.08 at 10-11-10-9 torr S = 0.016 at 2×10-8 torr S = 0.009 at 4×10-8 torr initial dissociative adsorption on adatom-restatom RT, < 25 L pairs followed by slow reaction with back bonds 180 - 800 K, 0 - 800 L precursor-mediated dissociative adsorption; S0 = 1.9×10-2 at 180 K S0 = 9.6×10-3 at 300 K constant adsorption rate up to θ = 0.15 - 0.2 ML and θsat ~ 0.35 ML at 300 - 700 K D2O exposure dissociative adsorption on all dangling bonds; 373 K, Si-OD, Si-D formation; initial fast adsorption up to 1×10-6 - 1×10-7 mbar θ ~ 0.12 ML on adatom-restatom pairs S0 = 0.23±0.08 θsat= 0.22±0.02ML dissociative adsorption for exposure < 100 L RT, oxidation reaction for exposure > 100 L 0.6 - 15000 L

RT, 2 L

Conditionsa

180 - 800 K, 0 - 800 L precursor-mediated dissociative adsorption laser heated modified S0 = 6.9×10-1 at 180 K surface 100 K, 0 - 10 L dissociative adsorption followed by ice layer; 3 adsorption regimes

LITD , TPD, 140 K, ~10 - 180 L; AES →700 K

1)

Technique

Crystal surface

5

4

85S

95W2

97Z

96F

95W2

91A, 97S1

91P3, 91P4, 91U

89K

84S1

Fig. Ref.

250 3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

Landolt-Börnstein New Series III/42A4

LEED, Auger TPD

LEED, Auger photoreflectance

Ge(100)2×1

Ge(111)2×1 → 8×2 GaAs(100)4×6

TDS

TDS

GaAs(100)2×4

GaAs(110)

TPD

Technique

Crystal surface dissociative adsorption, θsat= 0.2ML θsat = 0.5 ML physisorbed precursor, θsat = 0.5 ML assumed S0 = 0.28 S0 = 0.02 dissociative adsorption, θsat = 0.25 - 0.3 ML molecular adsorption, S strongly depending on exposure: S ~ 10-2 at 100 L S ~ 5×10-7 at 3×106 L molecular chemisorption via physisorbed precursor, 4 surface sites involved per H2O molecule; estimate of kinetic quantities by model calculation molecular adsorption; D2O ice layer desorption at 170 K; D2O desorption tail up to 600 K of molecular D2O formed by thermally activated association of dissociatively adsorbed D2O chemisorbed molecular H2O; H2O desorption at 180 K; H2O desorption tail up to 750 K of molecular H2O formed by thermally activated association of dissociatively adsorbed H2O chemisorbed H2O, θsat = 1 ML S strongly depending on exposure: S ~ 1.3×10−3 at 6L S ~ 7×10−5 at 200 L S > 6×10−4 at 4800 L: singularity due to loss of relaxation; desorption of chemisorbed H2O at 350 K, desorption tail up to 630 K

RT, 102 - 105 L; 110 K, 1L → 300 K 173 K, 20 L

250 K, 10 - 2×104 L; → 670 K

100 K, < 2.5 L; → 850 K

D2O exposure 100 K, < 23 L; →700 K

180 K - 320 K, 0 - 3000 L

273 K RT 10-9 - 10-1 torr.min 200 K, 100 - 3×106 L

Results and conclusions

Conditionsa

8

7

6

84M

98C

96S2

93C1

89S

73H

92C

87K2

Fig. Ref.

3.8.2 H2O and OH on semiconductors 251

a

Auger M1M15V UPS

TDS

UPS

GaAs(110)

AlAs(100)1×1

InP (110) 100 K, 0.1 - 5 L; → RT

D2O exposure 100 K, 0.1 - 3 L; → 750 K 00H

97M1, 97M2

96H

dissociatively adsorbed H2O, θsat= 0.5 ML; ice layer on top; ice layer desorbs at ~155 K; thermally activated formation of GaOx and desorption of AsH3 at ~250K molecular adsorption of D2O; desorption of D2O and thermally activated dissociation at 150 K - 180 K and recombinative desorption of D2O from Al-OD and As-H at 550 - 750 K dissociatively adsorbed H2O, θsat ~ 0.5 ML; ice layer on top desorbs at ~ 150 K; thermally activated formation of InOx and desorption of PH3 at ~250 K

100 K, 0.1 - 8 L; → RT

84C3

S is 2-3 orders larger than for O2 adsorption

RT, 10 - 106 L

Fig. Ref.

Results and conclusions

Conditionsa

Experimental conditions as given in the references

Technique

Crystal surface

252 3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

3.8.2 H2O and OH on semiconductors

253

Table 4. Local stucture of adsorbed H2O and OH on semiconductor surfaces. Information on the bonding site and the orientation of the adsorbed species is given. Detailed investigations only exist for the Si(100) surface. Crystal Surface Si(100)2×1

Technique

Conditionsa

IR, polarisation RT, θsat dependent ESDIAD 130 - 145 K, H+-desorption 0.5 L, two domain 2×1 LEED RT , 0 - 2 L differential ESDIAD

130 K, θsat, RT, θsat, two domain 2×1 LEED

ESDIAD

160 K, 0.5 L, (100) vicinal, one domain 2×1 LEED RT, θsat one domain 2×1 LEED RT, 0.05 ML

ARUPS polarzation dependent STM

Landolt-Börnstein New Series III/42A4

STM

RT, 0.1 ML - θsat

STM

RT, 0.1 L - θsat, (100) vicinal, one domain 2×1

Results and Conclusions

Fig.

Ref.

bond angle Si-Si-H 130°

84C1

O-H bond direction off-normal; 9a azimuthal orientation of O-H bond in [011], [ 0 1 1 ], [ 01 1 ], [ 0 1 1 ] direction;

87L2 94G

essentially no structure in H+ desorption pattern OH vibration anisotropic, 9b additional information given on thermal activation of vibrational modes.

94G

O-H bond direction nearly perpendicular to dimer bond

87L2

ΟΗ−σ-orbital not perpendicular to the surface and not constrained to lie in the dimer plane non dissociative adsorption assumed; 10a H2O molecule assumed to adsorb on lower atom of buckled dimer; adsorbed molecule assumed to induce static buckling on neighboring dimers of the same row; two H2O molecules on one dimer appear as A defects of “clean” surface, two H2O molecules on the same side of two dimers appear like C defects of “clean” surface; dissociative adsorption; growing 10b,c patches of dissociatively adsorbed H2O indicate adsorbate-adsorbate interaction; single Si atoms of occupied dimers atomically resolved growing patches of dissociatively 11 adsorbed H2O on terraces; chemisorbed H2O molecules appear like A defects of “clean” surface

88M

93C2

93C2

93A

254 Crystal Surface Si(100)2×1

3.8.2 H2O and OH on semiconductors Technique

Conditionsa

Results and Conclusions

TOF-SARS 4 keV Ar+ H, O, Si recoil

RT, θsat, 5×10-9 Torr H2O during experiment

H and OH bound to dangling bonds of 1st-layer Si atoms; Si-H bond shorter than Si-OH; Si-O and Si-H aligned with dimer bond axis bond angle Si-OH: 22±5° from surface normal; bond length Si-OH: 1.7±0.1Å; additional information given for Si bonding geometry of Si-OH bond length [Å] Si-OH 1.93 O-H 1.04 Si-H 1.54 bond length [Å] Si-OH 1.65 O-H 0.98 Si-H 1.53 Si-Si 2.38 bond angle buckling 2° Si-Si-O 111° Si-Si-H 109° Si-O-H 117° O-H perpendicular to dimer bond length [Å] Si-Si 2.458 Si-OH 1.757 O-H 0.974 Si-H 1.499 bond angle buckling 0.0° Si-Si-O 116.7° Si-O-H 116.4° O-H 62.2° to dimer similar values as for single dimer but O-H bond angle 85.3° to dimer

scanned-energy RT, θsat mode PED, O1s emission

THEORY CNDO cluster calculation THEORY LDFT/LDA

geometry according to ESDIAD results

THEORY DFT cluster calculation, single dimer

Si(100)2×1 vicinal

Si(111)7×7

THEORY DFT, two dimers UPS, HeI, cut off 6°, 9° and 12.5°

HREELS

RT, θsat

RT, 2000 L

dissociative adsorption on dimer dangling bond and on adatom like dangling bond since the intensity of OH related features follows the total density of dangling bonds OH and H at on-top sites, normal to surface; O-H tilted away from surface normal by 75°

Fig.

Ref. 94B

12

98F

89O

12

95V

12

97K

97K

90S, 97R

85N, 86N

Landolt-Börnstein New Series III/42A4

3.8.2 H2O and OH on semiconductors

255

Crystal Surface Si(111)7×7

Technique

Conditionsa

Results and Conclusions

STM

RT, ~5 L; RT, ~25 L

Si(113)3×1

THEORY Ext. Hückel HREELS

dissociative adsorption on adatom 13 restatom pairs, center adatoms react twice as often as corner adatom as they have twice the number of neighboring restatoms; island formation of adsorbates due to Hbond interaction; at high coverages a statistical distribution of covered adatom sites is reached dissociative adsorption, bond length Si-OH: 1.74Å dissociative adsorption at dimer dangling bonds only; intensity ratio of saturated (113) to saturated (100) is similar to the ratio of the respective number of dimers molecular adsorption of H2O on top site, bond length Ge-OH2: 2.5Å

Ge(100)2×1 comparison of UPS and THEORY

RT, 1 L

160 K

Fig.

Ref. 91A, 97S1

97E 96I

91L1 , 91S

RT, 0.5 - 100 L

two adsorption modes assumed: OH and H on-top sites bond length Ge-OH: 1.6Å, OH at bridge sites bond length Ge-OH: 1.7Å FeS2(100) LEISS 100 K, 0 - 1L site selective molecular adsorption of H2O on Fe sites CuInSe2(011) LEISS 80 K, site selective molecular adsorption of 0.01 - 10 L H2O on Cu sites a Experimental conditions as given in the references, e.g. Si(100)

Landolt-Börnstein New Series III/42A4

14

91P2 92S

Si(111)7×7

vicinal (100)

Crystal surface Si(100)2×1

150 K, 0 - 5 L;

LEED

LEED

RT, < 25 L

STM

fading of diffraction spots

7×7 conserved

7×7 conserved

dissociative adsorption on unchanged Si adatom-restatom pairs

dimer bond not broken by dissociative chemisorption dimer bond not broken by dissociative chemisorption; no change of step structure dissociative adsorption forming disordered structures dissociative adsorption on dangling bonds involving diffusion of Si adatoms dissociative adsorption on Si adatoms; formation of adsorbate islands

Conclusion and remarksb) dimer bond not broken by dissociative chemisorption dimer bond not broken by dissociative chemisorption dimer bond not broken by dissociative chemisorption, H and OH bound to Si dangling bonds

formation of disordered SiOx surface layer 7×7 conserved, fading of diffraction dissociative adsorption on Si adatoms RT, < 150 L spots RT, 800-1000 L loss of diffraction spots formation of disordered SiOx surface layer

→RT

RT, 0 - 450 L

2×1 reconstruction conserved

RT, θsat

LEED

2×1 reconstruction conserved

RT, θsat

7×7 is conserved but fading of diffraction spots induced 7×7 is transformed to 1×1 at high coverage

2×1 reconstruction conserved

RT, θsat, 5×10-9 mbar

TOF-SARS 4 keV Ar+ H, O, Si ion recoil LEED, one domain 2×1 STM

RT, 2000 L

2×1 reconstruction conserved

RT, θsat

STM

LEED

Results 2×1 reconstruction conserved

Conditionsa RT, θsat

Technique LEED

13

10, 11

Fig.

97Z

92D

91A, 97S

91P2, 91P3, 91U

85N, 86N

93A

87L2

94B

93A, 93C2

Ref. 93L

Table 5. Long-range order of the substrate/adsorbate interface for different semiconductor surfaces. The results obtained by LEED and STM on the long-range order of the adsorbate and/or the substrate/adsorbate complex are given. These data mostly give information on the persistence of substrate reconstructions. No quantitative results on the geometry of the adsorbate layer are available.

256 3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

a

Landolt-Börnstein New Series III/42A4

2×1→ 1×1 or 8×2

loss of diffraction spots 1×1 reconstruction conserved 1×1 reconstruction conserved 1×1 reconstruction conserved, fading of diffraction spots partial loss of 1×1 spots loss of diffraction spots 1×1 reconstruction conserved

RT, < 10 L

> 10 L RT, < 4×103 L RT, < 105 L, θ ≤ 1 ML 100K, < 0.5L → RT RT, 104-105 L RT, >109 L 100 K, < 0.5 L

LEED

LEED LEED

LEED

LEED

Ge(111)2×1

GaAs(100)1×1 GaAs(110)1×1

InP(100)1×1

InP(110)1×1

→ RT Experimental conditions as given in the references

fading of diffraction spots

2×1 weak → 2×1 sharp 2×1 is conserved

→RT 100 K, 1 L

Ge(100)2×1

LEED

LEED

Ge(100) c(4×2)/2×1

c(4×2) → 2×1 weak

LEED

1×1 reconstruction formed transformation 3×2 → 3×1

→ RT RT, 10% of θsat 160 K, 0 - 5 L

Si(113)(3×2)

Results 2×1 lost

Conditionsa LT, 2L

Technique LEED

Crystal surface Si(111)2×1

onset of surface oxidation

oxide formation dissociative adsorption on In-P surface pairs

onset of surface oxidation molecular adsorption

dissociative adsorption on Ga-As surface pairs

I(V) curves indicate loss of relaxation

formation of disordered surface oxide layer

dissociative adsorption on Ge dimers molecular and dissociative adsorption on Ge dimers dissociative adsorption on 2 adsorption sites; area of 50-300 surface atoms involved/per adsorbed water molecule.

transformation of reconstruction induced by dissociative adsorption molecular adsorption at LT

7

Fig. Conclusion and remarksb) π-bonded chains broken by dissociative adsorption

00H

82M

97H

81T 84M

73H, 75S, 79G

91P1

89L, 91L1, 91L2

93J

Ref. 85S

3.8.2 H2O and OH on semiconductors 257

Table 6. Valence band electron states, binding energies, and assignments for different semiconductor surfaces and adsorption conditions. Only for physisorbed H2O the measured electron states are strictly adsorbate molecular orbital states. For chemisorbed species the measured electron states result from adsorbate states which strongly hybridize with substrate states (semiconductor dangling bonds) in covalent bond formation. The states as measured on different semiconductor surfaces and for different adsorption conditions are given together with their assignments. Also results from theoretical calculations for different adsorption geometries are presented here. Refe- Binding Additional information or Remarksd Fig Ref Crystal Molecular Technique Conditionsa rence energies orbital assignment by authors surface of data collection levelb [eV] assignmentc as given in Ref. Si(100)2×1 UPS, HeI 6.1 300 K, 1 L - 100 L EF assignment questionable 1b1 of H2O (spectra unclear, 81F1 3a1 of H2O 8.0 probably due to 1b2 of H2O 10.6 beginning oxidation?) SXPS, EVBM 6.2 molecular adsorption 15 83S2 1b1 of H2O OH-1π 300K, θsat 3a1 of H2O 7.2 assumed hν=51eV OH-1π 1b2 of H2O 11.5 (spectra due to OH-3σ dissociation ?) UPS, HeII 10.9 160 K, 1.5 L Evac O lone pair 84O 11.7 annealed 300 K SiO-H 16.2 Si-OH UPS, HeI 300 K, 6.6 cylindrical Si crystal; assignments EF 85R1 OH-1π 0.7-1000 L 7.6 difference spectra left open OH-1π 11.9 1000 L - 0.7 L are OH-3σ similar to the spectra presented by [81F1] spectra identical to RT 16 85R1 100 K, assignments 6.6 EF OH-1π spectra < 0.3 L left open 7.6 OH-1π 11.9 OH-3σ 100 K, 6.8 1b1 16 physisorbed H2O EF 3a1 0.3 - 0.7 L 9.2 molecular H2O 1b2 12.9 100 K, EF 7.2 1b1 condensed ice layer 16 > 0.7 L 3a1 10.2 molecular H2O 1b2 13.3

258 3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

Landolt-Börnstein New Series III/42A4

EVBM

EF

EVBM

EF

RT, θsat

RT, θsat

300 K, θsat

295 K, θsat

SXPS, hν=36 eV

SXPS, hν=30eV

UPS, HeII

THEORY extended Hückel THEORY tight binding approach Evac

Evac

EVBM

300 K, 10 L - θsat

SXPS, hν=30 eV ARUPS, hν=50 eV p-polarized

6.3 7.4 11.2 5 6.4 7.6 11.9 12.1 14.8 18.5 12.2 16.0 18.8

6.5 7.5 11.9

6.3 11.2 6.1 7.1 11.4

6.2 7.2 11.5 24.5

Refe- Binding rence energies levelb [eV] EVBM

Conditionsa

300 K, 0.06 ML - θsat; 233 K, θsat

Technique

Si(100)2×1 UPS, HeI SXPS, hν=73eV

Crystal surface

OH-1π OH-1π OH-3σ Si-H OH SiO-H Si-OH OH-1π OH-3σ Si-OH OH OH-3σ Si-OH

O-2p SiO-H Si-OH

OH-σ

Si-OH SiO-H OH-1π

Molecular orbital assignmentc as given in Ref. O-2p of OH O-2p of OH O-2p of OH O-2s of OH

Si-sp3 + H-1s O-2p lone pair O-2p + H-1s Si-sp3 + O-2p O-π nonbonding O-p + H-s Si-s + O-p, σ bonding O-p, O-π nonbonding O-p + H-s Si-s + O-p, σ bonding

Oπ nonbonding Si-OH weak bonding

OH-1π OH-1π OH-3σ O-2s of OH

Additional information or assignment by authors of data collection

94S1

83C1, 83C2 87K1

OH adsorbed on dimer dangling bonds OH adsorbed on dimer dangling bonds;

91L1

89F

88M

87L1

85R3, 90S, 96R2

Fig Ref

OH adsorbed on dimer dangling bonds

OH adsorbed on dimer dangling bonds; EVBM is taken at the Fermi edge of the surface state of clean Si(100)2×1 OH adsorbed on dimer dangling bonds; vicinal (100) one domain (2×1); no dispersion observed on changing the emission angle OH adsorbed on dimer dangling bonds; vicinal (100) one domain (2×1) OH adsorbed on dimer dangling bonds

Remarksd

3.8.2 H2O and OH on semiconductors 259

Technique

UPS, HeI

Si(111)7×7 EELS

Si(110)

THEORY fully selfconsistent calculation UPS, HeI

Si(100)2×1 THEORY tight binding approach

Crystal surface

RT, < 135 L; 100 K, 2.5 L →RT

EF

EF

100K, > 0.45L

RT, 10 - 100 L; 150 K, 0.25 - 5 L

EF

100K, 0.45L

6.5 7.5 11

H2O-orbitals

no assignment given

molecular H2O

molecular H2O

assignments left open

14.38 15.58 19.99

Evac

6.6 7.6 11.9 6.8 9.2 12.9 7.2 10.2 13.3 3.7 4.7 8.7

OH OH Si-OH

12.2 18.4 20.0 35.8

Evac

EF

Molecular orbital assignmentc as given in Ref. 1b1 of H2O 1b2 of H2O σ of Si-OH2 2a1 of H2O

Refe- Binding rence energies levelb [eV]

300 K, 2 - 170 L

Conditionsa

OH-1π OH-1π Si-3σ

OH-1π OH-1π OH-3σ 1b1 3a1 1b2 1b1 3a1 1b2 transitions from occupied to unoccupied states

Si-s + O-p

O-p

O-p + Si-s, σ bonding

Additional information or assignment by authors of data collection

16

17

85R1, 85R2, 86S2

75F, 77F, 92D

85R1

89F

87K1

Fig Ref

extrinsic surface states 9b induced by H2O, molecular adsorption assumed molecular adsorption 17 assumed (spectra due to dissociation ?)

condensed ice layer

for low exposures same spectral features as on Si(100) RT, θsat physisorbed H2O

molecular H2O adsorption; values given for 180° orientation; also different orientations of the bond direction calculated OH adsorbed on dimer dangling bonds

Remarksd

260 3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

Landolt-Börnstein New Series III/42A4

Si(111)

Evac

molecular

Evac

Evac

dissociative

THEORY Ext. Hückel

Evac

EF

EF

molecular THEORY tight binding

molecular

< 100 K

UPS, HeI

THEORY Ext. Hückel

RT

UPS SXPS, hν = 36 eV

EF

RT, < 103 L

Si(111)7×7 UPS, HeI

1b1 of H2O 3a1 of Si-OH2 1b2 of H2O 2a1 of H2O Si-H OH-1πx OH-3σ Si-OH OH-2σ 1b1 of H2O 1b2 of H2O 3a1 of Si-OH2 2a1 of H2O 3a1 of H2O 1b2 of H2O 2a1 of H2O

35.8

1b1 of H2O 3a1 of H2O 1b2 of H2O

OH SiO-H Si-OH

Molecular orbital assignmentc as given in Ref. 1b1 of H2O 3a1 of H2O 1b2 of H2O

12.0 18.8 19.4 36.6 9.9 12.1 14.8 18.5 33.8 12.2 18.4 19.2 35.8 14.6 19.2

~7 ~ 9-10 ~ 13

6.2 7.2 11.6

~6 ~8 ~ 11

Refe- Binding rence energies levelb [eV]

Conditionsa

Technique

Crystal surface

and O-py+Si-s, O-σ bonding

O-pz + Si-s, σ bonding

H-1s + Si-sp, σ bonding O-px, O-π nonbonding

O-px, nonbonding O-pz + Si-s, bonding

O-2p lone pair O-2p +H-1s Si-sp3+O-2p

assignment questionable

Additional information or assignment by authors of data collection

H2O on top of Si dangling bond perpendicular to surface H2O on top of Si dangling bond , parallel to surface

Si-OH and Si-H on top of Si dangling bond

dissociative adsorption of H2O; emission dominated by Si-OH groups for low exposures physisorbed H2O, then condensed ice layer; spectra as on Si(110), see Fig. 23 H2O on top of Si dangling bond

molecular adsorption assumed (spectra due to onset of oxidation?)

Remarksd

17

87K1

87K1

83C2

83C1

85R1, 85R2, 85R3, 86S2

79F, 81F1, 81F2, 85R1, 85R2 89F

Fig Ref

3.8.2 H2O and OH on semiconductors 261

UPS, HeII

Ge(100)2×1 UPS, HeI

Si(113), Si(115), Si(5,5,12), Si(112) EF

EVBM EVBM

EVBM

EVBM

110 K, 0.4 L

110K, 1.1 L

110K, 3.7 L

110 K, 3.7 L → 300 K

Evac

5.9 9.8 12.9 6.1 8.5 10.0 12.9 6.3 8.8 9.9 12.9 5.6 7.5 9.2 10.0

16 18.8 33.2 6.2 7.2 11.5

10 12.2

Refe- Binding rence energies levelb [eV]

RT, θsat

THEORY dissociative tight binding

Si(111)

Conditionsa

Technique

Crystal surface

OH-σp

1b1 of 3a1 of 1b2 of 1b1 of 3a1 of 3a1 of 1b2 of 1b1 of 3a1 of 3a1 of 1b2 of OH-π OH-π

H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O H 2O

OH-3σ Si-OH OH-2σ emission shape characteristic for OH +H

Molecular orbital assignmentc as given in Ref. Si-H OH-1π

nonbonding Ge-OH bonding assignment unclear

OH-1π OH-1π Si-3σ

thermally induced dissociation

H2O dimers with Hbonding; 3a1 MO is split by dimer bond H2O ice cluster; 3a1 MO is broadened 20

87K3

97R

87K1

Fig Ref

dissociative adsorption 18 of H2O; orientation dependent saturation coverages derived and related to the number of dimers and adatomlike rebonded atoms at steps 19 H2O monomers

Si-OH and Si-H on top of Si dangling bond

Si-pz + H-1s, σ-bonding O-px lone pair, π non bonding O-pz + Si-s, O-σ bond.

Remarksd

Additional information or assignment by authors of data collection

262 3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

Landolt-Börnstein New Series III/42A4

Ge(111)2×8 UPS

THEORY LCAO xα

EVBM

EVBM

110 K, 3 L →300 K

Evac

Evac

Evac

110 K, 0.3 - 3 L

molecular

dissociative

EF

EVBM

160 K, 0.5 - 100 L

160 K, SXPS, hν=17,20,24 2 L →300 K eV

EVBM

160 K, 0.5 - 100 L → 300 K

Ge(100)2×1 SXPS, hν = 60 eV

12 16.9 12 16.8 15.8 12.2 14.5 19 6.4-6.6 8.9 9.9 13.0 5.2 7.6 10.2 Ge-OH-π Ge-OH-σp Ge-OH-πa Ge-OH-πb Ge-OH-σ 1b1 of H2O 3a1 of H2O 1b2 of H2O 1b1 of H2O 3a1 of H2O 3a1 of H2O 1b2 of H2O OH-π OH-π Ge-OH-σp

Molecular orbital assignmentc as given in Ref. 5.5 OH-π 7.3 OH-π 9.3 Ge-H 10.4 OH-σp 5.9-6.5 1b1 of H2O 8.1-8.4 3a1 of H2O 12.4-12.7 1b2 of H2O ~ 4.8 Ge-H ~ 5.8 OH-π ~ 7.8 Ge-OH-π ~ 9.5 Ge-H ~10.6 OH-σ

Refe- Binding rence energies levelb [eV]

Conditionsa

Technique

Crystal surface

nonbonding Ge-OH bonding

nonbonding weak bonding

nonbonding lifting of degeneracy

Additional information or assignment by authors of data collection

thermally induced dissociation; additional emissions (due to coadsorbed molecular H2O?)

H2O ice cluster, 3a1 splitting due to H-bonding

H2O on top site

OH on bridge site between two Ge

OH on top site

dispersion investigated, only weak dispersion observed

H2O ice layer

thermally induced dissociation

Remarksd

20

23

21, 22

87K3

91S

91L2

89L, 91L1

Fig Ref

3.8.2 H2O and OH on semiconductors 263

UPS, HeI

GaAs (110)

SXPS, hν = 41 eV

UPS, HeI

InP(110)

FeS2(100)

GaSe(0001) SXPS, hν = 21 eV

UPS

GaAs (100)

SXPS, hν = 41eV

Technique

Crystal surface

EVBM

~100 K, > 2L

100 K, 0.5 L - 7 L

EF

EVBM

100 K, > 1.3 L

100 K, 0.5 - 1.5 L

EVBM

100 K, 0.5 L

RT, 7×1012 L

EVBM

EVBM

300 K, 104 L

~100 K, 0.5 L

EVBM

4.2 6 8.4 11.1 5.7 8.1 11.6 ~7 ~10 ~13 6.5-7.3 9.5-10 12.7-13.2

5 7 9.1 12.1 5.5 8 12 6.1 10.9

3.2 4.4 8.4

3.2 5 9.4

Refe- Binding rence energies levelb [eV]

180 K

Conditionsa

In-OH P-H In-OH P-H 1b1 of H2O 3a1 of H2O 1b2 of H2O 1b1 of H2O 3a1 of H2O 1b2 of H2O 1b1 of H2O 3a1 of H2O 1b2 of H2O

Ga-OH As-H Ga-OH As-H 1b1 of H2O 3a1 of H2O 1b2 of H2O OH orbitals

H2O orbitals

H2O orbitals

Molecular orbital assignmentc as given in Ref.

1b1 3a1 1b2

1b1 3a1 1b2

Additional information or assignment by authors of data collection

molecular adsorption preferentially on Fesites condensed ice layer; EBF depending on coverage;

dissociatively adsorbed H2O (origin of emissions unclear?) dissociatively adsorbed H2O (given assignment unclear?) physisorbed H2O, ice layer

physisorbed H2O assumed (origin of spectral features unclear ?) chemisorbed H2O assumed (origin of spectral features unclear ?) dissociatively adsorbed H2O (given assignment unclear?) physisorbed H2O, ice layer

Remarksd

25

24

24

92M, 93M

91P2

00H

82W

96H, 97H

79B

Fig Ref

264 3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

Landolt-Börnstein New Series III/42A4

UPS

CuInSe2 (001)

80 K, 0.01 - 10 L

EF

Evac

~7.3 ~9.9 ~13

~7 ~10 ~13 6.4-8.0 9.2-10.9 12.3-14.3 25.9-27.9 11.0-11.4 ~ 14 16.9-17.5 30.4-31.3

~11.2 ~13.5 ~17

1b1 of H2O 3a1 of H2O 1b2 of H2O

1b1 of H2O 3a1 of H2O 1b2 of H2O 1b1 of H2O 3a1 of H2O 1b2 of H2O 2a1 of H2O 1b1 of H2O 3a1 of H2O 1b2 of H2O 2a1 of H2O

1b1 of H2O 3a1 of H2O 1b2 of H2O

Additional information or assignment by authors of data collection

condensed ice layer; EBF depending on coverage and substrate doping; ionisation energies EBvac nearly independent on coverage; dipole between substrate and adsorbate assumed molecularly adsorbed H 2O

condensed ice layer; EBF strongly depending on coverage and substrate doping; ionisation energies EBvac weakly depending on coverage condensed H2O; ionisation energies EBvac nearly constant; dipole between substrate and adsorbate assumed condensed ice layer

Remarksd

26

92S

92M, 93M, 96M

87J, 88J

75Y

92M, 93M

Fig Ref

b

Experimental conditions as given in the references EF: Fermi level; Evac: vacuum level; EVBM: valence band maximum c For a detailed presentation of the orbital assignment see text of section 3.8.2.11; Si-OH indicates Si-OH bonding state, SiO-H indicates O-H bonding state. d Remarks in parenthesis like e.g. (spectra due to dissociation ?) are given by the authors of this data collection

a

UPS SXPS hν = 21 eV hν = 30 eV

WSe2 (0001)

EF

UPS

MoS2 (0001)

Evac

100 K, 1 - 10.5 L

80 K, 0.5 - 5 ML

UPS

MoSe2 (0001)

Evac

EF

EF

100 K, 0.1 L - 10 L

InSe(0001) SXPS hν = 21 eV hν = 30 eV

Molecular orbital assignmentc as given in Ref. 6.8-7.7 1b1 of H2O 9.5-10.3 3a1 of H2O 13.0-13.9 1b2 of H2O 11.4 1b1 of H2O 3a1 of H2O 14 1b2 of H2O 17.6

Refe- Binding rence energies levelb [eV]

100 K

Conditionsa

Technique

Crystal surface

3.8.2 H2O and OH on semiconductors 265

Table 7. Core level electron binding energies EB of H2O adsorbate species and binding energy shifts ∆EB of substrate emissions obtained for different semiconductor surfaces and adsorption conditions. Core level binding energy shifts measured for the substrate lines as well as the binding energies of the adsorbate lines are presented here, as they have been used for the determination of adsorption mode and adsorbate properties. Fig. Ref. Core level EB Crystal/ Technique Conditionsa Assignmentb,c Remarksc ∆EB surface [eV] [eV] hν [eV] Si(100)2×1 SXPS molecular adsorption assumed 83S2 +0.9 Si-OH2 Si 2p3/2 300 K, θsat (Si-OH?) (shift due to Si-OH?) hν = 120 SXPS 98.74 bulk ODA disappeared after saturation; Si-OH 85R3 350 K, 2 L, θsat Si 2p3/2 –0.46 ODA(clean) intensity equal to original ODA intensity hν =138.5 +0.80 Si-OH after annealing to 640 K Si+and Si2+ +1.0 Si 2p3/2 Si+ → 640 K oxidation states formed +1.8 Si2+ 3+ after annealing to 870 K aditional Si3+ +2.6 Si 2p3/2 Si → 870 K 4+ and Si 4+ emissions and ODA intensity +3.5 Si restored –0.43 ODA(clean) SXPS ODA intensity strongly decreased after Si 2p 87L1 300 K, 10L, θsat 3/2 +0.25 Si-H saturation; hν = 130 +1.00 Si-OH no oxidation observed Si 2p3/2 Si(100) SXPS –0.52 ODA(clean) ODA intensity strongly decreased after 27 88M RT, 2 L, θsat, +0.31 Si-H saturation; hν = 130 (100) vicinal, +1.00 Si-OH no oxidation observed one domain 2×1 LEED –0.43 ODA(clean) ODA intensity strongly decreased after 91L1 300 K, 5 L, θsat Si 2p3/2 +0.24 Si-H saturation; +1.04 Si-OH no oxidation observed 432.8 O 1s Si-OH O1s binding energy monitored in the XPS 94S1 RT, θsat; referred to 432.2 Si-O-Si temperature range 0 - 650 °C → 670 K; 433.2 Si 2p3/2 Si-oxide sub→ 920 K; stoichiometric

266 3.8.2 H2O and OH on semiconductors

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Landolt-Börnstein New Series III/42A4

XPS

SXPS hν =135

Si(113)

SXPS hν=145

XPS

SXPS hν=145

Si-OH Si-H Si-OH

532.6

RT, θsat; RT, 0.01 - 2 L

+0.22 +0.90

H2O ice

534.6

100 K, > 2 L; Si 2p3/2

H 2O Si-OH

100 K, 0.2 - 2 L;

534.0 532.6

O1s

Si-O Siad-OH

530.2

→RT 300 K, ≤ 90 L; 90 K, 40 L; → RT +0.89

Si-OH

531.7

150 K, 0.25 - 5 L; Si 2p3/2

Si-O

530.0

ODA(clean) Si-H Si-OH/Si+ Si2+ Si3+ Si4+ bulk ODA(clean) Si-OH/Si+ Si2+

dissociation at dimers assumed

surface oxide OH bonding to Si adatom; slow 30 attenuation of restatom intensity to 50%; oxidation states up to Si4+at RT and 90 K observed (source induced enhancement of oxidation?) no chemically shifted component in Si 2p 31 line evident

H2O dissociation assumed

(source induced enhancement of oxidation?) preadsorbed state (origin of spectral features unclear?)

96S1

85S

95P

92D

95P

95P

Fig. Ref.

28 ODA intensity slowly decreased with exposure; Si2+ already observed at 5 L; with higher exposures (80L) also Si3+ and Si4+ observed; (source induced enhancement of oxidation?) 29 ODA intensity slowly decreased with exposure;

Assignmentb,c Remarksc

150 K,<0.25 L; O1s

–0.55 +0.85 +1.65

[eV] –0.55 +0.30 +0.85 +1.65 +2.45 +3.35

∆EB

99.6

Core level EB [eV] Si 2p3/2

300 K, 1 - 40 L Si 2p3/2

Technique Conditionsa hν [eV] 90 K, 1 - 80 L SXPS hν = 145

Si(111)2×1

Si(111)7×7

Crystal/ surface Si(100)

3.8.2 H2O and OH on semiconductors 267

SXPS hν = 60 hν = 80

XPS

GaAs(110)

GaAs

120 K, 0.5 - 3.5 L

Ga 3d5/2

2×109 L4×1010 L RT, 7×1012 L Ga 3d Ga 2p O1s O2s

Ga 3d5/2 As 3d5/2

Ga 3d5/2 As 3d5/2

O 1s O 1s

Ge 3d5/2

RT, 106 L

100 K, 3 ML 100 K, 3 ML addtional 50 eV electron beam 100 K, < 0.5 L

110 K, 10 L → 360 K

530.8

533.2 531.0-531.6 529.4-530.4

Core level EB [eV] Ge 3d5/2

conditions as given in the references Assignment as given by the authors of the original paper c Remarks in parenthesis are given by the authors of this data collection

b

a Experimental

GaSe(0001) SXPS InSe(0001) MoS(0001) WSe2(0001)

XPS

GaAs(100) 4×6

SXPS hν = 64

Crystal/ Technique Conditionsa surface hν [eV] Ge(100)2×1 SXPS 160 K, 1 L 300 K, 1 L hν = 70

26.0

+1.6 +1.7

+1.0

+0.60 +0.60 –0.65 +0.6 +0.6

+0.56 0 ±0.1

+0.75 +0.43 –0.45

[eV] –0.43

∆EB

Ga-OH Ga-OH Ga-OH H 2O

Ga-suboxide

Ga-OH As-H As* Ga-OH As-H

Ge dimer (clean) Ge-OH Ge-H Ge dimer (clean) Ge-OH Ge-H H 2O OH Ga-oxide

dissociative adsorption assumed; no binding energy shift of As resolved; (onset of surface oxidation?) molecular adsorption, no chemically shifted substrate components observed; adsorbate binding energy shifts assumed to be due to surface dipols

As* due to Ga-OH formation

ice layer electron induced dissociation of H2O

loss of emission of Ge dimer due to adsorbed H2O

loss of emission of Ge dimer due to adsorbed H2O

Assignmentb,c Remarksc

33 a,b

32

87J, 88J, 92M, 93M, 96M

82W

97H

96S2 96S2

93R1

89L, 91L1

Fig. Ref.

268 3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

Table 8. Vibrational properties of adsorbed H2O or its dissociation products for different adsorption conditions on different semiconductor surfaces Vibrational frequencies of adsorbate/substrate bonding: In this table the vibrational modes and their assignments obtained after adsorption on different semiconductors and for varying adsorption conditions are given. For chemisorbed and dissociatively adsorbed species the measured eigenmodes and frequencies depend on the specific surface molecule formed thus being strongly influenced by substrate atoms and bonding geometry. Crystal Fig. Ref. Vibrational Vibrational Vibrational mode Conclusion and remarks Technique Conditionsa surface frequencyb energyb Adsorbate [cm-1] [meV] 165 - 420 K, Si(100)2×1 HREELS 78.1 34 82I2 630 Si-H bend dissociative adsorption; 0.1 - 1 L H 2O E0 = 14 eV 101.7 820 Si-OH stretch/ Si-OH stretch also observed as Si-OH bend double loss at 1600 cm-1 259.1 2090 Si-H stretch 459.0 3700 SiO-H stretch IRAS p-pol. 35 1670 84C2 80 K, 10 L condensed molecular H2O; H2O scissor 207.1 H2O stretch the H2O stretching mode is a broad ~409.0 ~3300 feature due to H-bonding 257.9 2080 Si-H stretch dissociative adsorption; 35 84C1, IRAS s-pol. 80 K, 0.5 L 258.1 2082 Si-H stretch for the given temperatures and 84C2 p-pol. 300 K, 10 L 453.7 3660 SiO-H stretch exposures identical spectra are p-pol. 80 K,10 L →RT found HREELS RT, 10 L Si-H bend 78 dissociative adsorption 629 84S3 E0 = 1.5 eV 830 103 Si-OH stretch 2097 260 Si-H stretch 3694 458 SiO-H stretch IRAS s-pol. RT, 2 L 257.6 2078 Si-H stretch dissociative adsorption; 85C p-pol. 257.9 2080 Si-H stretch 9° cut off vicinal surface IRAS RT, 2 L 102.3 825 Si-OH stretch dissociative adsorption; 36 97S2 internal 111.3 898 Si-OH bend calculated frequencies show mirror 258.5 2085 Si-H stretch unusual isotope shift Si-OH(D) THEORY 78.9-80.6 dissociative 637-650 Si-H bend calculation with different coupling 83B cluster 102-104 adsorption 823-840 Si-OH stretch constants between different modes calculation 101-102 816-824 Si-OH bend lead to wave numbers in the given 259.0 2089 Si-H stretch range 458.4 3697 SiO-H stretch

3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

269

Si(100)2×1 D 2O

Crystal surface Adsorbate Si(100)2×1 H 2O

dissociative adsorption

dissociative adsorption

THEORY Si9H13OH DFT cluster calculation

THEORY LDA/MD Car-Parrinello THEORY DFT cluster calculation

RT, 10 L

RT, 2 L

HREELS E0 = 1.5 eV

IRAS internal mirror THEORY cluster calculation

dissociative adsorption

165 - 420 K, 0.1 - 1 L

HREELS E0 = 14 eV

dissociative adsorption

Conditionsa

Technique

876 3596 69 700 922 2108 3672 480 650 840 1520 2700 484 645 1532 2702 658 840 1517 501-506 611-634 815-828 1504 2693

445.8 8.6 86.8 114.3 261.4 455.3 59.5 80.6 104.1 188.5 334.8 60 80 190 335 81.6 104.1 188.1 62.1-62.7 75.8-78.6 101-103 186.5 333.9

Vibrational frequencyb [cm-1] 821 909

108.6

Vibrational energyb [meV] 101.7 112.7

Si-OH stretch/ Si-OH bend SiO-H stretch Si-O-H torsion Si-OH stretch Si-OH bend Si-H stretch SiO-H stretch Si-D bend Si-OD bend Si-OD stretch Si-D stretch SiO-D stretch Si-D bend Si-OD stretch Si-D stretch SiO-D stretch Si-OD bend Si-OD stretch Si-D stretch Si-D bend Si-OD bend Si-OD stretch Si-D stretch SiO-D stretch

Si-OH stretch Si-OH bend

also sum frequencies observed dissociative adsorption; intensity of Si-OD stretching mode decreased by 55% versus Si-OH calculations with different coupling constants between different modes lead to wave numbers in the given range; stretching mode intensity Si-OD/Si-OH = 50%

dissociative adsorption; (assignment to Si-OD bend ?)

unconstrained cluster model underestimates Si-OH stretch frequency, calculated frequencies show unusual isotope shift Si-OH(D) dissociative adsorption

67% stretching + 33% bending; 33% stretching + 67% bending; coupling of stretching and bending modes; intensity bend/stretch = 16% only the Si-OH group was allowed to move in the MD calculation

Vibrational mode Conclusion and remarks

36

34

Fig.

83B

97S2

84S3

82I2

97K

95V

96R1

Ref.

270 3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

Landolt-Börnstein New Series III/42A4

Si(111)7×7 H 2O

Si(100)2×1 H218O

Crystal surface Adsorbate Si(100)2×1 D 2O

HREELS

~ 95 ~ 210 ~ 435 95 258 460

300 K, θsat, 60 L; 80 K, condensed H2O →RT 766 2081 3710

765 1610 3510 Si-OH bend Si-H stretch SiO-H stretch

H2O trans/rot H2O scissor H2O stretch

Si-18OH stretch Si-18OH bend

795 904

98.6 102.1

dissociative adsorption

100 K, 2 L

Si-18OH stretch Si-18OH bend

793 855

98.3 106.0

molecular H2O adsorbed; combined translation and rotation; H2O stretching mode is a broad feature due to H-bonding different adsorption procedures used for obtaining dissociative H2O

calculated for H-O-Si/O-Si-Si bend-bend interaction force constant = -0.1 mdyn Å stretching mode intensity Si-18OH/Si-16OH = 90%

Si-18OH stretch Si-18OH bend

805 891

dissociative adsorption

dissociative adsorption; unpublished results by Ibach,Wagner,Bruchman cited by [83B] stretching mode intensity Si-18OH/Si-16OH = 97%

Si-18OH stretch

~800

THEORY cluster calculation THEORY Si9H13OH cluster calculation HREELS

99.8 110.5

~99.2

coupling of stretching and bending modes leads to unusual isotope shift

Vibrational mode Conclusion and remarks Si-OD bend Si-OD stretch

Vibrational frequencyb [cm-1] 666 835

RT, 2 L

dissociative adsorption

THEORY DFT cluster calculation HREELS

Vibrational energyb [meV] 82.6 103.5

IRAS; internal mirror

Conditionsa

Technique

37

Fig.

86S2

82I2, 86S2

96R1

83B

97S2

83B

96R1

Ref.

3.8.2 H2O and OH on semiconductors 271

100K, 2L→RT

HREELS

HREELS RT, θsat, E0 = 2.5 - 11 eV 200 - 1000 L

100 K, < 2 L

100 K, 5 - 100 L

HREELS E0 = 2 - 15 eV

HREELS

Si(111)2×1 H 2O

HREELS RT, θsat, E0 = 2.5- 11 eV 200 - 2000 L

Conditionsa

Technique

Si(111)7×7 D 2O

Crystal surface Adsorbate Si(111)7×7 H 2O

Vibrational frequencyb [cm-1] 630 807 950 2090 3420 3680 766-790 2081-2097 3678-3710 589 839 1516 2678-2694 807-823 2057-2073 3630 1573 3388 202 774-823 1573-1614 3388-3428

Vibrational energyb [meV] 78 100 118 259 424 456 95-98 258-260 456-460 73 104 188 332-334 100-102 255-257 450 195 420 25 96-103 195-205 420-425

H2O scissor H2O stretch H2O frust-trans. H2O rotation H2O scissor H2O stretch

37

Si-H bend Si-OH stretch Si-OH bend Si-H stretch SiO-H stretch/ H bonded SiO-H stretch Si-OH stretch/ SiOH bend Si-H stretch SiO-H stretch Si-OD bend Si-OD stretch Si-D stretch SiO-D stretch Si-OH bend Si-H stretch SiO-H stretch

ice layer at coverage > 2 L; additional overtone losses observed

dissociative adsorption; additional overtone losses observed; angle and energy dependent measurements of scattering intensities dissociative adsorption; additional 38 overtone losses observed; angle and energy dependent measurements of scattering intensities dissociatively and molecularly adsorbed H2O present

dissociative adsorption

Fig.

Vibrational mode Conclusion and remarks

84S1, 85S

84S1, 85S

83K2, 86N

83K2, 85N, 86N

82I2

Ref.

272 3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

Landolt-Börnstein New Series III/42A4

RT, 1 L

GexSi1-x (100)2×1 H 2O

HREELS E0 = 5 - 6.5 eV

100 K, 5 L → 345 K

Ge(100)2×1 HREELS D 2O E0 = 6.5 eV

100 K, 1 L → 345 K

100 K 0.32 - 1 L

RT, 1 L

RT, < 20 L 100 K, 2 L →RT 100 K,100 L→RT

HREELS

HREELS

Conditionsa

Technique

Ge(100)2×1 HREELS H 2O E0 = 6.5 eV

Si(113)3×1

Crystal surface Adsorbate Si(111)2×1 H 2O

Vibrational frequencyb [cm-1] 194 1573-1654 807-830 2057-2073 3630-3654 484 629 895 613 807 2065 670 887-928 1960 3630 524-605 1613 670 928 1992 3630 686 1371 2662 790 1976 2073 3710

Vibrational energyb [meV] 24 195-205 100-103 255-257 450-453 60 78 111 76 100 256 83 110-115 243 450 65-75 200 83 115 247 450 85 170 330 98 245 257 460

H2O libration H2O scissor Ge-OH stretch Ge-OH bend Ge-H stretch GeO-H stretch Ge-OD stretch Ge-D stretch GeO-D stretch Ge-OH and Si-OH stretch Ge-H stretch Si-H stretch Ge-OH and Si-OH stretch

Si-OH bend and stretch Si-H stretch SiO-H stretch Si-H2 rocking Si-H2 wagging Si-H2 scissor Si-H bend Si-OH stretch Si-H stretch Ge-OH stretch Ge-OH bend Ge-H stretch GeO-H stretch

39

H2O frust-trans. H2O scissor

additional overtones observed; surface segregation of substrate atoms and mobility of adsorbed species discussed

dissociative adsorption

dissociative adsorption

dissociatively adsorbed H2O on dangling bonds; additional overtone observed at 200 meV dissociatively and molecularly adsorbed H2O present

molecularly and dissociatively adsorbed H2O present; additional lines due to Si-H2 formation; additional overtone losses observed

Fig.

Vibrational mode Conclusion and remarks

86B, 89P

89P, 91P1

89P, 91P1

91P1

96I

84S1, 85S, 86S1

Ref.

3.8.2 H2O and OH on semiconductors 273

b

a

> 200 K

100 K, 0.5 L D2O

1505 2120 2720 3740

2410

298.8 186.6 262.8 337.2 463.7

2110 3670 1170 2720

Vibrational frequencyb [cm-1] 1188 1446 2664 2885 1630 3670

261.6 455.0 145.1 337.2

Vibrational energyb [meV] 147.3 179.3 330.3 357.7 202.1 455.0

As-D stretch As-H stretch AlO-D stretch AlO-H stretch

D2O stretch

As-H stretch GaO-H stretch D2O scissor D2O stretch

D2O scissor overtone O-D stretch overtone H2O bend H2O stretch

dissociation of D2O; contamination by dissociatively adsorbed background H2O in all spectra

D-bonded D2O (ice)

isolated D2O

dissociative adsorption

Ga stabilized surface; molecular adsorption

D2O ice layer

Vibrational mode Conclusion and remarks

Experimental conditions as given in the references For comparison the data of the original papers are given in vibrational energy [meV] as well as wavenumbers [cm-1] (1 cm-1 = 0.123981 meV)

HREELS

AlAs(100) 1×1 D 2O

> 200 K, 0.2 L

100 K, 0.2 L

100 K, 3 ML D2O

HREELS

HREELS

Conditionsa

Technique

GaAs(100) 4×2 H 2O

Crystal surface Adsorbate GaAs(100) 4×6 H 2O

Fig.

97M1

98C

96S2

Ref.

274 3.8.2 H2O and OH on semiconductors

Landolt-Börnstein New Series III/42A4

References for this document 55H 57K 58K 63W 68E 71M 73H 75F 75S 75Y 77F 77M 79B 79F 79G 81F1 81F2 81T 82G 82I1 82I2 82M 82W 83B 83C1 83C2 83K1 83K2 83S1 83S2 84C1 84C2 84C3 84M 84O 84S1 84S2 84S3 85C 85N 85R1 85R2 85R3 85S 85T 86B 86N 86S1 86S2 87B 87F 87J 87K1 87K2 87K3 87L1 87L2

Hauffe, K.: Advanced Catalysis 7 (1955) 213. Kisliuk, P.J.: J. Phys. Chem. Solids 3 (1957) 95. Kisliuk: J. Phys. Chem. Solids 5 (1958) 78. Wolkenstein, T., The Electron Theory of Catalysis on Semiconductors, (Pergamon Press, Oxford, 1963). Ertl, G.: Ber. Bunsenges. Phys. Chem. 72 (1968) 74. Meyer, F.: Surf. Sci. 27 (1971) 107. Henzler, M.,Töpler, J.: Surf. Sci. 40 (1973) 388. Fujiwara, K.,Nishijima, M.: Phys. Lett. 55A (1975) 211. Sinharoy, S.,Henzler, M.: Surf. Sci. 51 (1975) 75. Yu, K.Y., McMenamin, J.C., Spicer, W.E.: Surf. Sci. 50 (1975) 149. Fujiwara, K.,Ogata, H.: J. Appl. Phys. 48 (1977) 4360. Morrison, S.R., The Chemical Physics of Surfaces, (Plenum Press, New York, 1977). Büchel, M.,Lüth, H.: Surf. Sci. 87 (1979) 285. Fujiwara, K.,Ogata, H.: Surf. Sci. 86 (1979) 700. Galaev, A.A., Gamosov, L.V., Parkhomenko, Y.N.,Shirkov, A.V.: Sov. Phys. Crystallogr. 24 (1979) 72. Fujiwara, K.: Surf. Sci. 108 (1981) 124. Fujiwara, K.: Journal of Chemical Physics 75 (1981) 5172. Thornton, G., Rosenberg, R.A., Rehn, V., Green, A.K.,Parks, C.C.: Solid St. Commun. 40 (1981) 131. Gmelin, Handbook of Inorganic Chemistry, H. Koschel ed. ed., (Springer Verlag, 1982). Ibach, H., Bruchmann, D.,Wagner, H.: Appl. Phys. A 29 (1982) 113. Ibach, H., Wagner, H.,Bruchman, D.: Solid St. Commun. 42 (1982) 457. Montgomery, V.,Williams, R.H.: J. Phys. C 15 (1982) 5887. Webb, C.,Lichtensteiger, M.: J. Vac. Sci. Technol. 21 (1982) 659. Black, J.E., Bopp, P.,Wolfsberg, M.: Surf. Sci. 134 (1983) 257. Ciraci, S., Erkoc, S.,Ellialioglu, S.: Solid St. Commun. 45 (1983) 35. Ciraci, S.,Wagner, H.: Phys. Rev. B 27 (1983) 5180. Kahn, A.: Surf. Sci. Rep. 3 (1983) 193. Kobayashi, H., Kubota, T., Onchi, M.,Nishijima, M.: Phys. Lett. 95A (1983) 345. Schmeisser, D., Himpsel, F.J., Holliger, G., Reihl, B.,Jacobi, K.: Phys. Rev. B 27 (1983) 3279. Schmeisser, D., Himpsel, F.J., Hollinger, B.: Phys. Rev. B 27 (1983) 7813. Chabal, Y.J.: Phys. Rev. B 29 (1984) 3677. Chabal, Y.J.,Christman, S.B.: Phys. Rev. B 29 (1984) 6974. Childs, K.D., Luo, W.-A.,Lagally, M.G.: J. Vac. Sci. Technol. A 2 (1984) 593. Mokwa, W., Kohl, D.,Heiland, G.: Surf. Sci. 139 (1984) 98. Oellig, E.M., Butz, R., Wagner, H.,Ibach, H.: Solid St. Commun. 51 (1984) 7. Schaefer, J.A., Stucki, F., Frankel, D.J., Göpel, W.,Lapeyre, G.J.: J. Vac. Sci. Technol. B 3 (1984) 359. Schmeisser, D.: Surf. Sci. 137 (1984) 197. Stucki, F., Anderson, J., Lapeyre, G.J.,Farrell, H.H.: Surf. Sci. 143 (1984) 84. Chabal, Y.J.: J. Vac. Sci. Technol. A 3 (1985) 1448. Nishijima, M., Edamota, K., Kobayashi, H.,Onchi, M.: Surf. Sci. 158 (1985) 422. Ranke, W.,D.Schmeisser: Surf. Sci. 149 (1985) 485. Ranke, W., Schmeisser, D.,Xing, Y.R.: Surf. Sci. 152/153 (1985) 1103. Ranke, W.,Xing, Y.R.: Surf. Sci. 157 (1985) 339. Schäfer, J.A., Anderson, J.,Lapeyre, G.J.: J. Vac. Sci. Technol. A 3 (1985) 1443. Takayanagi, K., Tonishiro, Y., Takahashi, M.,Takahashi, S.: J. Vac. Sci. Technol. A 3 (1985) 1502. Broughton, J.Q., Schaefer, J.A., Bean, J.C.,Farrel, H.H.: Phys. Rev. B 33 (1986) 6841. Nishijima, M., Edamoto, K., Kubota, Y., Tanaka, S.,Onchi, M.: J. Chem. Phys 84 (1986) 6458. Schaefer, J.A., Anderson, J.,Lapeyre, G.J.: J. Electr. Spectr. Rel. Phen. 38 (1986) 21. Schmeisser, D.,Demuth, J.E.: Phys. Rev. B (1986) 4233. Barone, V., Lelj, F., Russo, N.,Toscano, M.: Journal de chimie physique 84 (1987) 5. Feenstra, R.M., Strodcio, J.A.,Fein, A.P.: Surf. Sci. 181 (1987) 295. Jaegermann, W.,Schmeisser, D.: J. Vac. Sci. Technol. A 5 (1987) 627. Katircioglu, S.: Surf. Sci. 187 (1987) 569. Kuhr, H.J.,Ranke, W.: Surf. Sci. 189/190 (1987) 420. Kuhr, H.J.,Ranke, W.: Surf. Sci. 187 (1987) 98. Larsson, C.U.S., Flodström, A.S., Nyholm, R., Incoccia, L.,Senf, F.: J. Vac. Sci. Technol. A 5 (1987) 3321. Larsson, C.U.S., Johnson, A.L., Flodström, A.,Madey, T.E.: J. Vac. Sci. Technol. A 5 (1987) 842.

87R 87T 88D 88F 88H 88J 88M 89F 89H 89K 89L 89O 89P 89S 90E 90S 91A 91L1 91L2 91P1 91P2 91P3 91P4 91S 91U 92B 92C 92D 92L 92M 92S 92W 93A 93C1 93C2 93J 93L 93M 93R1 93R2 94B 94G 94H 94K 94L 94S1 94S2 95L 95M 95P 95V 95W1 95W2 96F 96H

Russo, N.,Toscano, M.: Surf. Sci. 180 (1987) 599. Thiel, P.A.,Madey, P.E.: Surf. Sci. Rep. 7 (1987) 211. Dong, G., Ding, X., Hou, X.,Wang, X.: Surf. Sci. 201 (1988) 531. Froitzheim, H., in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 5, Eds. D. A. King, D. P. Woodruff, (Elsevier, Amsterdam, 1988), pp. 183. Hansson, J.G.V.,Uhrberg, R.I.G.: Surf. Sci. Rep. 9 (1988) 197. Jaegermann, W.,Tributsch, H., in: Progress in Surface Sciences, Vol. 29, Ed. S. G. Davison, (Pergamon Press, New York, 1988), p. 1. McGrath, R., Cimino, R., Braun, W., Thornton, G.,McGovern, I.T.: Vacuum 38 (1988) 251. Fives, K., McGrath, R., Stephens, C., McGovern, I.T., Cimino, R., Law, D.S.-L., Johnson, A.L.,Thornton, G.: J. Phys.: Condens. Matter 1 (1989) SB105. Hamers, R.J.,Köhler, U.K.: J. Vac. Sci. Technol. A 7 (1989) 2854. Koehler, B.G., Mak, C.H.,George, S.M.: Surf. Sci. 221 (1989) 565. Larsson, C.U.S., Flodström, A.S., Karlsson, U.O.,Yang, Y.: J. Vac. Sci. Technol. A 7 (1989) 2044. Ong, C.K.: Solid St. Commun. 72 (1989) 1141. Papagno, L., Caputi, L.S., Anderson, J.,Lapeyre, C.J.: Phys. Rev. B 40 (1989) 8443. Seebauer, E.G.: J. Vac. Sci. Technol. A 7 (1989) 3279. Engler, C.: phys. stat. sol. (b) 159 (1990) 721. Schröder-Bergen, E.,Ranke, W.: Surf. Sci. 236 (1990) 103. Avouris, P.,Lyo, I.: Surf. Sci. 242 (1991) 1. Larsson, C.U.S.,Flodström, A.S.: Vacuum 42 (1991) 297. Larsson, C.U.S.,Flodström, A.S.: Phys. Rev. B 43 (1991) 9281. Papagno, L., Frankel, D., Chen, Y., Caputi, L.S., Anderson, J.,Lapeyre, G.J.: Surf. Sci. 248 (1991) 343. Pettenkofer, C., Jaegermann, W.,Bronold, M.: Ber. Bunsenges. Phys. Chem. 95 (1991) 560. Podolsky, B.S., Ukraintsev, V.A.,Chernov, A.A.: Surf. Sci. 251/252 (1991) 1033. Podolsky, B.S., Ukraintsev, V.A.,Chernov, A.A.: Sov. Phys. Solid State 33 (1991) 65. Shao, Y.,Paul, J.: Vacuum 42 (1991) 313. Ukraintsev, V.A., Podolsky, B.S.,Chernov, A.A.: Appl. Surf. Sci. 48/49 (1991) 151. Brillson, L.J., Handbook of Semiconductors, Vol. I, North Holland, 1992). Cohen, S.M., Yang, Y.L., Rouchance, E., Jin, T.,D´Evelyn, M.D.: J. Vac. Sci. Technol. A 10 (1992) 2166. Dai, D., Zhu, F., Luo, Y.,Davoli, I.: J. Phys.: Condens. Matter 4 (1992) 5855. Landemark, E., Karlsson, C.J., Chao, Y.-C.,Uhrberg, R.I.G.: Phys. Rev. Lett. 69 (1992) 1588. Mayer, T., Klein, A., Lang, O., Pettenkofer, C.,Jaegermann, W.: Surf. Sci. 269/270 (1992) 909. Sander, M., Jaegermann, W.,Lewerenz, H.J.: J. Phys. Chem. 96 (1992) 782. Wolkow, R.A.: Phys. Rev. Lett. 27 (1992) 2636. Andersohn, L.,Köhler, U.: Surf. Sci. 284 (1993) 77. Carlson, C.R., Buechter, W.F., Che-Ibrahim, F.,Seebauer, E.G.: J. Chem. Phys 99 (1993) 7190. Chander, M., Li, Y.Z., Patrin, J.C.,Weaver, J.H.: Phys. Rev. B 48 (1993) 2493. Jacobi, K., Myler, U.: Surf. Sci. 284 (1993) 223. Lacharme, J.P., C. Sébenne, Chérif, S.M., Safta, N.,Zaibi, M.A.: Appl. Surf. Sci. 65/66 (1993) 598. Mayer, T.: Ph.D. Thesis, Technische Universität Berlin, 1993. Ranke, W.: J. Electr. Spectr. Rel. Phen. 61 (1993) 231. Ranke, W.,Wasserfall, J.: Surf. Sci. 292 (1993) 10. Bu, H.,Rabalais, J.W.: Surf. Sci. 301 (1994) 285. Gao, Q., Z. Dohnalek, Cheng, C.C., W.J. Choyke,J.T. Yates, J.: Surf. Sci. 312 (1994) 261. Himpsel, F.H., in: Handbook of Semiconductors, Vol. II, Eds. T. S. Moss, M. Balkowski, Amsterdam, 1994), pp. 161 . Karlsson, C.J., Landemark, E., Chao, Y.-C.,Uhrberg, R.I.G.: Phys. Rev. B 50 (1994) 5767. Lay, G.l., Göthelid, M., Grekh, T.-M., Björkquist, M., Karlsson, K.O.,Aristov, U.Y.: Phys. Rev. B 50 (1994) 14277. Schulze, R.K.,Evans, J.F.: Appl. Surf. Sci. 81 (1994) 449. Sebenne, C., in: Handbook of Semiconductors, Vol. II, Eds. T. S. Moss, M. Balkowski, Amsterdam, 1994), pp. 33 . Lüth, H., in: Surfaces and Interfaces of Solid Materials 3 Ed. Springer-Verlag, Berlin, 1995). Mönch, W., in: Semiconductor Surfaces and Interfaces 2nd ed., (Springer-Verlag, Berlin, 1995). Poncey, C., Rochet, F., Dufour, G., Roulet, H., Sirotti, F.,Panaccione, G.: Surf. Sci. 338 (1995) 143. Vittadini, A., Selloni, A.,Casarin, M.: Phys. Rev. B 52 (1995) 5885. Waltenburg, H.N.,Yates, J.T.J.: Chemical Reviews 95 (1995) 1589. Wise, M., Okada, L., Sneh, O.,George, S.: J. Vac. Sci. Technol. A 13 (1995) 1853. Flowers, M.C., Jonathan, N.B.H., Morris, A.,Wright, S.: Surf. Sci. 351 (1996) 87. Henrion, O., Löher, T., Klein, A., Pettenkofer, C.,Jaegermann, W.: Surf. Sci. 366 (1996) L685.

96I 96J 96K 96M 96R1 96R2 96R3 96S1 96S2 97E 97H 97K 97M1 97M2 97P 97R 97S1 97S2 97W 97Z 98C 98F 00H

Ikeda, H., Yamada, T., Hotta, K., Zaima, S.,Yasuda, Y.: Appl. Surf. Sci. 100/101 (1996) 431. Jaegermann, W., in: Modern Aspects of Electrochemistry Ed. R. E. White, New York, 1996). Katircioglu, S.,Ercoc, S.: phys. stat. sol. (b) 196 (1996) 77. Mayer, T., Pettenkofer, C.,Jaegermann, W.: J. Phys. Chem. 100 (1996) 16966. Raghavachari, K., Chabal, Y.J.,Struck, L.M.: Chem. Phys. Lett. 252 (1996) 230. Ranke, W.: Surf. Sci. 369 (1996) 137. Rincón, R., García-Vidal, F.J.,Flores, F.: Appl. Surf. Sci. 92 (1996) 216. Scholz, S.M.,Jacobi, K.: Surf. Sci. 369 (1996) 117. Sloan, D.W., Sun, Y.-M.,White, J.M.: J. Vac. Sci. Technol. A 14 (1996) 216. Ezzehar, H., Stauffer, L., Leconte, J.,Minot, C.: Surf. Sci. 388 (1997) 220. Henrion, O.: Ph.D. Thesis, Technische Universität Berlin, 1997. Konecny, R.,Doren, D.J.: Journal of Chemical Physics 106 (1997) 2426. Mitchell, W.J., Chung, C.-H., Yi, S.I., Hu, E.L.,Weinberg, W.H.: Surf. Sci. 384 (1997) 81. Mitchell, W.J., Chung, C.-H., Yi, S.I., Hu, E.L.,Weinberg, W.H.: J. Vac. Sci. Technol. B 15 (1997) 1182. Piancastelli, M.N., Paggel, I.J., Weindel, C., Hasselblatt, M.,Horn, K.: Phys. Rev. B 56 (1997) R 12737. Ranke, W.,Xing, Y.R.: Surf. Sci. 381 (1997) 1. Self, K.W., Yan, C.,Weinberg, W.H.: Surf. Sci. 380 (1997) 408. Struck, L.M., Jr., J.E., Bent, B.E., Flynn, G.W., Chabal, Y.J., Christman, S.B., Chaban, E.E., K.Raghavachari, Williams, G.P., Radermacher, K.,Mantl, S.: Surf. Sci. 380 (1997) 444. Weldon, M., Stefanov, B., Raghavachari, K.,Chabal, Y.J.: Phys. Rev. Lett. 79 (1997) 2851. Zaibi, M.A., Lacharme, J.P.,Sebénne, C.A.: Surf. Sci. 377-379 (1997) 639. Chung, C.-H., Yi, S.I.,Weinberg, W.H.: J. Vac. Sci. Technol. A 16 (1998) 1785. Franco, N., Chrost, J., Avila, J., Müller, C., Dudzik, E., Patchett, A.J., McGovern, I.T., Giebel, T., Lindsay, R., Fritsche, V., Bradshaw, A.M.,Woodruff, D.P.: Appl. Surf. Sci. 123/124 (1998) 219. Henrion, O., Klein, A.,Jaegermann, W.: Surf. Sci. 457 (2000) L337.

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275

3.8.2.14 Figures for 3.8.2

Si(100)2×1

MP

TS

E3

Energy E

E2

E1

P Reaction coordinate

Fig. 1. Energy profile of the reaction of water with the Si(100) surface as calculated by DFT. At a certain level of the calculation (BLYP/TZ94P) the calculated values are: Total adsorption energy E1 = 2.13 eV, molecular precursor adsorption energy E2 = 0.31 eV activation energy for dissociation E3 = 0.23 eV [97K]. 112 113

001

110 331 221 111

H2O/Si(cyl.)

a 350 K

Auger peak height

111 001

1.9 L

110 113

001 6

b 670 K

1/2 ML on 100 L 5L

112 113

4 100 L 2

Coverage q [1014 cm -2 ]

110 331 221 111

1.9 L

1L

0 0

90

Orientation a [deg]

0

90

Fig. 2a. Orientation dependence of the oxygen KLL Auger peak intensity on a cylindrical Si crystal after exposure to the indicated amounts of water: (a) exposure at 350 K, (b) exposure at 670 K. The intensity corresponding to 1/2 ML (oxygen atoms per surface unit cell) for the main orientations is indicated. The calibration of the coverages is given in the right hand scale [85R3].

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Si+H2O 117

111

0.5 183 233 273 391 412

qSAT

-10

0

10

50

30 40 20 Orientation a [deg]

0

60

Fig. 2b. Dependence of the OH + H saturation coverage on orientation of a disk shaped Si crystal as derived from photoelectron spectra for exposures between 300 and 350 K. The respective surface orientations are indicated by Miller Indices. Solid triangles: hν = 50 eV; solid squares: hν = 40.8 eV; solid diamonds: hν = 21.2 eV; open squares: hν = 73 eV; open circles: hν = 21.2 eV . Measurements normalized to 0.5 ML at (001) [97R].

10

3

H2O exposure [ Torr ⋅s] 20 30 −11

1 2×10 Torr ≤ PH2O ≤ 2×10 Torr 2 2×10 −8 Torr

Si (111)7×7 3 4×10 −8 Torr

2

3

1

0

0

0.5

4×10 −8 Torr

2

1

0

4 8 12 16 20 H2O exposure [1016cm −2 ]

2.0 1.0 1.5 H2O exposure [1016cm −2 ]

2.5

500

1500 1000 Exposure time t [s]

2000

Fig. 3. OH+H photoelectron intensity versus exposure time to water for different sample temperatures as indicated. θsat refers to saturation at T > 220 K (corresponding to 0.5 ML)[96R2]. The linear range of the plot (constant adsorption rate) indicates a mobile molecular precursor state.

40

−9

q [10 14 cm −2 ]

Coverage q [10 14 cm −2 ]

T [K] 137

Si(001) + H2O

5.5.12 113 112

Intensity

Saturation coverage QSAT [ML]

001

115

3.0

Fig. 4. Coverage of (dissociated) H2O molecules on the Si(111)7×7 surface as a fun-ction of exposure to water vapor with various pressures P: solid circles, line 1: 2×10-11 Torr < P < 2×10-9 Torr; crosses, line 2: P = 2×10-8 Torr; open triangles, line 3: P = 4×10-8 Torr. The inset presents the same dependence for water vapor pressure P = 4×10-8 Torr at longer exposures. (The same units are used for coordinate axes both in the main figure and in the insert) [91P3]. The graph covers the second and third adsorption regime above θ > 0.2 ML. The transition between the second and third regime occurs at θ ≈ 0.6 ML.

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277

0.25

Si (111)

0.15

D2O coverage q [ ML]

D2O coverage q [ ML]

0.20

0.15

0.10

0

400

800

Coverage q [ML]

Coverage q [1014 cm−2 ]

2

Fig. 5. D2O coverage on Si(111) at 373 K as a function of exposure. Solid squares: ion gauge off. Solid triangles: ion gauge on. Line: spline through data [96F]. The graph covers the first and second adsorption regime.

100 150 200 250 300 350 400 D2 O exposure L [L ]

2400

2000

113

001 0.5 ML

H2O/Ge(cyl.)

Fig. 6. Orientation-dependent coverage as derived from the intensity of the OKLL Auger Signal after water adsorption on a cylindrical Ge crystal. The respective surface orientations are indicated by Miller Indices. The intensity is scaled to the number of oxygen atoms per cm2. (A) 104 L H2O exposed at 300 K. (B) 1 L H2O exposed at 110 K and warmed up to 300 K. Insert: H2O coverage in monolayers (ML) on Ge(001) and Ge(111) at 300 K [87K3].

0.1

B

0 10

2

10 3 10 4 10 5 Exposure [L]

1

0 −120

50

1200 1600 D2 O exposure L [L ]

(001) (111)

0.2

0

111 331 110 331 111

113

001 3

0.05

0

0.05

0

0.10

0.2 ML

A

− 90

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− 60

−30 30 0 Orientation a [deg]

60

90

120

Auger intensity ratio [ I 02 / I Ge ]

278

3.8.2 H2O and OH on semiconductors

Ge(111) 2×1 0.4 0.3 0.2 0.1

LEED intensity I LEED [ I / I clean ]

0 270 eV peak of the (00) beam

2.0

1.0

95 eV peak of the (00) beam extra spot intensity

0 clean

10 −8 10 − 6 10 −2 10 − 4 Water vapor exposure [Torr⋅min]

1

GaAs (110)

6 ×10 − 4 Differential sticking coefficient S

Fig. 7. Upper part: Adsorbed amount of water vapor on Ge(111)2×1 (as ratio of oxygen to germanium Auger signal) versus water vapor exposure to a freshly cleaved germanium surface. Each spot has been used only once to avoid effects of the electron beam. Lower part: LEED intensities of the (00) beam and of a half order beam of the freshly cleaved germanium surface. The two chosen voltages are typical for all voltages of the complete I – V plots [75S].

crystal 18 crystal 22 crystal 24 4×10 − 4

2×10 − 4

0

0

5000

10000 15000 H2 O exposure [L ]

20000

Fig. 8. Differential sticking coefficient of H2O adsorption on GaAs(110) as a function of H2O exposure. The relative sticking coefficient was obtained by dividing the difference between two consecutive values of the integrated H2O desorption flux by the difference of the corresponding H2O exposures. A sticking coefficient of 0.003 is obtained at exposures below 20 L [84M].

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279

〈01 1

〉

1 〈01

〉

〈011〉

Si(100)

a

〈01 1

b

〉

1 〈01

〉

〈011〉

〈011〉

〈011〉

Fig. 9a. Reversible temperature-dependent ESDIAD patterns of OH/Si(100). (a) 130 K Si substrate temperature; (b) 305 K Si substrate temperature. H2O was adsorbed at 305 K to saturation. The figures on the left are perspective plots; they are generated by mapping the counts of H+ ion of the adsorbed OH group in the z direction as a function of the (x, y) coordinates on the detector. The figures on the right are contour plots. Each contour line in each individual plot represents an increment of 1/7 of the maximum count rate at a peak, with a width of ±5% of the median value of that contour [94G]. For a vicinal surface a two fold symmetry is observed (see [87L2]).

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3.8.2 H2O and OH on semiconductors

Si(100)

〈01 1

〈011〉

B

〉

1 〈01

A

〉

〈011〉

a

〈01 1

b

〉

1 〈01

〉

〈011〉

B

A

〈011〉

Fig. 9b. The difference ESDIAD patterns for OH/Si(100). (a) Experimental difference pattern obtained by digital subtraction of the 130 K ESDIAD pattern from the 305 K ESDIAD pattern (see Fig. 9a). (b) Modeled difference ESDIAD pattern with Gaussian distribution peak profiles [94G].

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281

Si(100) 2×1

Fig. 10. (a) Drift-corrected occupied-states STM image of Si(100)2×1 after 0.05 L H2O exposure (sample bias –1.79 V). H2O adsorption produces dark features that appear as isolated M and W features, as well as cluster defects (CD) derived from groups of W´s and M´s. The W´s and some of the M´s are related to adsorbed H2O, with each dark atomic feature representing one molecule bonded to a Si atom. (b) Occupied-state image obtained after 0.2 L H2O exposure (sample bias −2.01 V). Atomic resolution of the dimers is now possible on a substantial portion of the surface but it was not possible on the clean surface. Atoms within the two 2×1 unit cells outlined in the bottom exhibit different intensities of brightness that are attributed to Si-H and Si-OH states. Most of the bright features are unreacted. W and M features are also recognizable. (c) Occupied-state image of the saturated Si(100)2×1 surface after 0.5 L H2O exposure (sample bias –1.61 V). Most of the dimers can be atomically resolved. The different intensities within a dimer reflect Si-H and Si-OH bonding but there is no long-range ordering of these species. Bright atomic protrusions marked O reflect oxidation. The dark M features appear to be dimer vacancies that remain unreacted [93C2].

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References for this document 75S 84M 85R3 87K3 87L2 91P3 93C2 94G 96F 96R2 97K 97R

Sinharoy, S.,Henzler, M.: Surf. Sci. 51 (1975) 75. Mokwa, W., Kohl, D.,Heiland, G.: Surf. Sci. 139 (1984) 98. Ranke, W.,Xing, Y.R.: Surf. Sci. 157 (1985) 339. Kuhr, H.J.,Ranke, W.: Surf. Sci. 187 (1987) 98. Larsson, C.U.S., Johnson, A.L., Flodström, A.,Madey, T.E.: J. Vac. Sci. Technol. A 5 (1987) 842. Podolsky, B.S., Ukraintsev, V.A.,Chernov, A.A.: Surf. Sci. 251/252 (1991) 1033. Chander, M., Li, Y.Z., Patrin, J.C.,Weaver, J.H.: Phys. Rev. B 48 (1993) 2493. Gao, Q., Z. Dohnalek, Cheng, C.C., W.J. Choyke,J.T. Yates, J.: Surf. Sci. 312 (1994) 261. Flowers, M.C., Jonathan, N.B.H., Morris, A.,Wright, S.: Surf. Sci. 351 (1996) 87. Ranke, W.: Surf. Sci. 369 (1996) 137. Konecny, R.,Doren, D.J.: Journal of Chemical Physics 106 (1997) 2426. Ranke, W.,Xing, Y.R.: Surf. Sci. 381 (1997) 1.

282

3.8.2 H2O and OH on semiconductors

Si(100) 2×1 vicinal

Fig. 11. H2O adsorption sequence measured by STM on vicinal Si(100) tilted 4° towards [011]. Exposure is increased from 0 to 1.0 L in steps of 0.25 L. The surface shown in (f) was additionally exposed to 1 L. The images show the same area of 150 x 150 Å2 with only small displacements due to thermal drift. The step structure is not altered during adsorption. In (b) and (c) a growing island is marked [93A].

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3.8.2 H2O and OH on semiconductors H

H

283

Si(100) 2×1

O H O S1 S3

H

S2 S1 S3

S4

S2 S4

[110] Side view Asymetric dimer model

Fig. 12. Schmematic sectional view showing the structural parameter labels for the O atom in the Si(100)2×1-OH system, which have been optimised by a fitting procedure between the O 1s photoelectron diffraction data and multiple scattering simulations. In the plot the configuration of the H2O molecules, OH fragments and Si dimers before and after the surface reaction have been schematically indicated [98F]. The values of distances and angles are given in table 4.

Si(111)7×7

Fig. 13. (A) STM topograph of a clean Si(111)7×7 surface. (B) after exposure to 5 L of H2O and (C) after exposure to 25 L of H2O at RT [91A].

Landolt-Börnstein New Series III/42A4

References for this document 91A 93A 98F

Avouris, P.,Lyo, I.: Surf. Sci. 242 (1991) 1. Andersohn, L.,Köhler, U.: Surf. Sci. 284 (1993) 77. Franco, N., Chrost, J., Avila, J., Müller, C., Dudzik, E., Patchett, A.J., McGovern, I.T., Giebel, T., Lindsay, R., Fritsche, V., Bradshaw, A.M.,Woodruff, D.P.: Appl. Surf. Sci. 123/124 (1998) 219.

284

3.8.2 H2O and OH on semiconductors

FeS2(100)

Si(100)-(2×1)

Fe

h n = 80 eV

I Fe /I S

O

2.2

1.0 L

1.0

0.3 L

1.3

Intensity

Intensity

(300 K)

S

h n = 51 eV clean Si(100)

h n = 21 eV clean 900

2.6 800

700 600 500 He+ Kinetic energy of He+ ions Ekin [eV]

300

400

Fig. 14. He+ LEIS-spectra (1 keV) for different H2O coverages on FeS2 (100). The S/Fe intensity ratio is shown in the spectra, indicating preferential adsorption on Fe-sites [91P2].

Si+H2O

−15

−10

1.1 L- 0.7 L

1.5 L- 0.9 L

0.7 L- 0.3 L

0.9 L- 0.45 L

0.3 L

0.45 L

−5

300 K

Intensity

Si+H2O

2

Fig. 15. Angle-integrated photoelectron valence band spectra for saturation coverage of H2O Si(100)2×1 taken at room temperature with different photon energies. The Fermi level is 0.4 eV above the valence-band maximum [83S2].

b (110)

Intensity

−10

0

100 K

a (001)

−15

−18 −16 −14 −12 −10 −8 −6 −4 −2 Binding energy EBVBM [eV]

−5

0 −15 −10 Binding energy EBF [eV]

−5

0

b (110)

a (111) F

E

E 1200 L - 0.8 L

D 170 L - 2 L

D 1200 L C 135 L B 18 L A 0.8 L

C 170 L B 20 L A 2L

0 −15 −10 Binding energy EBF [eV]

Fig. 16. HeI UP spectra of H2O adsorption at 100 K for the Si (001) surface a) and the (110) surface b). Displayed are the difference spectra between subsequent exposure steps as indicated [85R1].

−5

Fig. 17. HeI UP spectra of H2O adsorption on Si (111) and Si (110) at room temperature. (a) Curves A-D: exposure dependence on (111) (difference spectra); curve E: difference between 0.8 and 1200 L spectra; curve F: same as curve E, but reduced by 35% of the difference curve of 0.7L on Si(100). (b) Curves A-C: exposure dependence on (110) (difference spectra); curve D: difference between 2 and 170 L spectra; curve E: same as curve D, but reduced by 15% of the difference curve of 0.7L on Si(100) [85R1].

0

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3.8.2 H2O and OH on semiconductors

285

Si

Relative intensity

Si (112)

(5, 5, 12)

Fig. 18. Angle integrated valence band photoelectron spectra (hν = 40.8 eV) from five flat Si samples of the indicated orientations on one sample holder, taken in one run before and after exposure. Dots: clean surfaces; lines: after exposure to water equivalent to the number of 2.4 ML of Si (100) leading to saturation with OH + H [97R].

(113) (115)

I2

I1

(001) − 20

− 15

− 10 −5 Binding energy E BF [eV]

0

b2

Ge(001)

b1 a1

×0.4

Intensity

×0.4

D-A

D

C B A

a −15

−10

Landolt-Börnstein New Series III/42A4

C-A B-A b

−15 −5 0 −10 VBM Binding energy EB [eV]

−5

0

Fig. 19. HeI photoelectron spectra on Ge(001) at 110 K. (a) Original spectra of the clean surface (A), exposed to 0.4 L (B), 1.1 L (C) and 3.7 L water (D). The intensities are scaled relative to the intensity of the clean surface spectrum (A). (b) Difference curves (covered minus clean surface spectra) [87K3].

286

3.8.2 H2O and OH on semiconductors

Ge(001)

Ge(111) 450 K

450 K

300 K Intensity

300 K 140 K 140 K

×0.4 ×0.4

3.7 L, 110 K

a −15

−10

3 L, 110 K

b −5

−15 −5 0 −10 Binding energy EBVBM [eV]

Ge(100)

Fig. 20. HeI difference spectra of H2O on Ge(001) (a) and Ge(111) (b). The surface was first covered with a condensed H2O layer at 110 K and then warmed up to the indicated temperatures [87K3].

0

C

F

D

B

A qe = 50° 45° 40°

Intensity

D

35°

E

30° 25°

20° 15° 10° 5° 0° −12

−10

−8 −6 −4 Binding energy EBF [eV]

−2

Fig. 21. Angle-resolved photoelectron spectra of Ge(100) + H2O recorded at hν = 24 eV. The analyzer was rotated in the incident photon ionization plane and the sample was oriented such that the [010] direction was probed. The photon incident angle was θi = 60°. The surface was exposed to 2 L H2O at 160 K and annealed to 300 K. The spectra were recorded with the surface at 300 K [91L2].

0

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3.8.2 H2O and OH on semiconductors

287

0

Ge(100)

Binding energy EBF [eV]

−2

A

−4 B C

−6

D

−8 E −10

F 0

0.5

1.0 Wavevector k [Å−1]

1.5

2.0

Fig. 22. Energy dispersions for the states related with dissociated H2O adsorbed on the Ge(100) surface. The open symbols indicate dispersions in the spectra recorded with an incident photon angle θi = 60° and the solid symbols indicate dispersions in the spectra recorded with an incident photon angle θi = 15° [91L2]. (See also Fig. 21; A: Ge-Ge bond, B: Ge-H bond, C: OH π (nonbonding), D: Ge-OH π (weak bonding), E: Ge-H bond, F: OH σ)

Ge(111) ×0.4

b2 b1 a1

Intensity

×0.4

D D-A C B A

a −15

−10

Landolt-Börnstein New Series III/42A4

C-A B-A b

−15 −10 −5 0 Binding energy EBVBM [eV]

−5

0

Fig. 23. HeI photoelectron spectra of H2O on Ge(111) at 110 K. a: Original spectra of the clean surface (A), exposed to 0.3 L (B), 1 L (C) and 3 L water (D). The intensities are scaled relative to the intensity of the clean surface spectrum (A). (b) Difference curves (covered minus clean surface spectra) [87K3].

288

3.8.2 H2O and OH on semiconductors

GaAs(110)

3a 1

H2O: 1b 2

1b 1

Intensity

1.5 L 1.2 L 1.0 L 0.8 L

0.6 L 0.4 L

Fig. 24. SXP valence band spectra of H2O adsorbed onto GaAs(110) surface at 100 K, excitation energy hν = 41 eV. The marks at 0.4 L H2O indicate As-H and Ga-OH bonds [97H].

0.2 L GaAs(110) cleaved 15

5 10 Binding energy EBVBM [eV]

0

InP(110) H2O: 1b 2

3a 1

1b 1

Intensity

1.3 L 0.6 L

0.4 L 0.3 L 0.2 L 0.1 L

Fig. 25. SXP valence band spectra of cleaved InP(110) with increasing H2O dosage at 100 K. Excitation energy hν = 41 eV. The marks at 0.4 L H2O indicate In-OH and P-H bonds [00H].

0L

12

4 8 Binding energy EBVBM [eV]

0

Landolt-Börnstein New Series III/42A4

References for this document 83S2 85R1 87K3 91L2 91P2 97H 97R 00H

Schmeisser, D., Himpsel, F.J., Hollinger, B.: Phys. Rev. B 27 (1983) 7813. Ranke, W.,D.Schmeisser: Surf. Sci. 149 (1985) 485. Kuhr, H.J.,Ranke, W.: Surf. Sci. 187 (1987) 98. Larsson, C.U.S.,Flodström, A.S.: Phys. Rev. B 43 (1991) 9281. Pettenkofer, C., Jaegermann, W.,Bronold, M.: Ber. Bunsenges. Phys. Chem. 95 (1991) 560. Henrion, O.: Ph.D. Thesis, Technische Universität Berlin, 1997. Ranke, W.,Xing, Y.R.: Surf. Sci. 381 (1997) 1. Henrion, O., Klein, A.,Jaegermann, W.: Surf. Sci. 457 (2000) L337.

3.8.2 H2O and OH on semiconductors

InSe/H2 O

In4d

cleaved

H2O [L] 10 6.0 4.0 2.5 1.5 1.0 0.6 0.3 0.2 0.1 cleaved

1b 1

1b 2

Intensity

289

3a 1

cleaved

Se p z

10 L 10 L

−15

−20

−10 Binding energy EBF [eV]

−5

Fig. 26. SXP valence band spectra of water adsorbed onto InSe(0001) surface at 100 K, excitation energy hν = 30eV [93M]. EF

Vicinal Si(100) B a S3

S1

Intensity

B

S4

b

S3

S2

S1

B

S4

c 3

Landolt-Börnstein New Series III/42A4

S1

H

2 1 0 Binding energy EB [eV]

−1

Fig. 27. (a) Si 2p core level photoemission spectrum recorded at a bulk-sensitive photon energy of hν = 112 eV for the clean Si(100) surface. The energy scale is in binding energy referenced to the bulk Si 2p3/2 component. The solid lines denote a fit with five spin-orbit doublets S1, B, S2, S3 and S4. (b) As in (a), but at a surface-sensitive photon energy hν = 130 eV. (c) Si 2p spectrum as in (b) after exposure to saturation coverage of 2 L of H2O. The new peak labelled H is attributed to emission from Si atoms bonded to H [88M].

290

40

3.8.2 H2O and OH on semiconductors

Si(001)

40

Si(001)

1+ 20

2+

20

3+ 80 L

4+

0

0

40

40

20

1+ 3+

20 L

Si-H

40 L

1+

20

2+

2+

20 L 0

40

40 Intensity

Intensity

0

20 1+

20

Si-H

2+

5L

5L

0

0

40

40

20

20

1+ 0

Si-H

1+ 1L

1L 0

40

40

20 A

0L

O.D.A.

20

O.D.A.

A

0L

0

0

4

3

2 1 0 Binding energy EB [eV]

−1

−2

Fig. 28. Si 2p3/2 spectra (peak maxima normalized to 100) of the Si(001) surface exposed at 90 K to increasing water exposures. Excitation energy hν = 145 eV. The vapor pressure was 2×10-8 Torr for the first two exposures and 5×10-8 Torr for the last three ones. Curve reconstructions are also given, a SiH contribution (dashed line) at about +0.3 eV is introduced for the best resolved spectra [95P]. (1+, 2+ etc. denote the different oxidation states of Si; ODA contribution, dotted line, of outer dimer atoms, A not defined by authors).

4

3

1 2 0 Binding energy EB [eV]

−1

−2

Fig. 29. Si 2p3/2 spectra (peak maxima normalized to 100, EB is given relative to bulk Si 2p3/2) of the Si(001)2×1 surface exposed at 300 K to increasing water exposures (under 2×10-8 Torr). Excitation energy hν = 145 eV. Curve reconstructions are also given [95P]. (1+, 2+ denote the different oxidation states of Si; ODA contribution, dotted curve, of outer dimer atoms, A not defined by authors).

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3.8.2 H2O and OH on semiconductors

40

Si(111)

20

×5

291 534.6

Si(111)

T = 100 K

99.7

80 L

Exposure [L]

×1

0

10

40 1+

×5

60 L

4+

2+

0

5

534

Intensity

20

×1/3

532.6 2

40 ×1

×1

20

1

40 L

0.4

Intensity

0

0.2

40

H2O saturat. 300 K

20

0

20 L 0

102

40

10 L 0 40 20 1+ 5L

40 A1

20 A2

0L

RA

0 4

536 98 Binding energy EB [eV]

533

530

Fig. 31. Si 2p (left) and O 1s (right) Mg Kα photoelectron spectra for water adsorbed on Si(111)2×1 [85S].

20

0

100

3

2 1 0 Binding energy EB [eV]

−1

−2

Fig. 30. Si 2p3/2 spectra (peak maxima normalized to 100, EB is given relative to bulk Si 2p3/2) of the Si(111)7×7 surface exposed at 300 K to increasing water exposures (under 5×10-8 Torr). Excitation energy hν = 145 eV Curve reconstructions are also given [95P]. (RA: rest atom, A1: Si-adatoms, A2: not defined by authors, +1, +2, etc. denote the different oxidation states of Si).

Landolt-Börnstein New Series III/42A4

292

3.8.2 H2O and OH on semiconductors clean 300 K

Ge(100)

a

Intensity

1LH2 O 160 K

b 1LH 2 O 300 K

c 2

−1 1 0 Binding energy EB [eV]

Fig. 32. Ge(100) 3d core-level spectra recorded at 70 eV photon energy. The filled circles show the raw data after background subtraction and the lines show the calculated fits to the data: (a) the spectrum from the clean surface recorded at 300 K, (b) the spectrum from the surface after a 1 L H2O exposure at 160 K, and (c) the spectrum from the surface after a 1 L H2O exposure at 160 K and heating to 300 K. The binding energy is referred to the bulk Ge 3d5/2 components [89L].

WSe 2 162 K

H2O-expose [L] 3.5

150 K

Intensity

Intensity

2a 1

2a 1

1.5 0.5 120 K 0

−30 a

−28 −26 −24 Binding energy EB [eV]

−22

−30 b

−28 −26 −24 Binding energy EB [eV]

−22

Fig. 33. H2O adsorption on WSe2 (0001) at 120 K. SXP spectra of the O 2s level as function of coverage during adsorption (a) and during desorption, after 3.5 L H2O-exposure (b), excitation energy hν = 80 eV [93M].

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3.8.2 H2O and OH on semiconductors

Rel. intensity

Si(100) ×80

820

10−2

293

Si-OH SiH (×10 )

10−3 0.1 1 Exposure L [10 −6 Torr s]

Intensity

2090 3700

1640

630

×2000

×800 ×300

650 840

90 480

1520 ×1000

0

1L H2 O

1000

2090

2700 ×2000

2000 3000 Energy loss Eloss [cm −1 ]

1L D2 O 4000

Fig. 34. Electron energy loss spectra of 1 L of H2O and D2O, respectively, on a Si(100) surface at 300 K. The loss at 1640 cm-1 in the upper spectrum is a double loss of 820 cm-1. The 2090 cm-1 loss on the D2O exposed surface is produced through some H-D-exchange or H2O contamination. The electron impact energy was 14 eV. The insert displays the intensities of the 820 cm-1 loss and the 2090 cm-1 (SiH) loss vs coverage [82I2].

a ×1

b Relative reflectivity DR /R

×1

c ×0.67

Si(100) P- polarization

∆R / R = 4×10 −4

d ×1

1700 2000 2300 2600 2900 3200 3500 Frequency n [cm −1]

Landolt-Börnstein New Series III/42A4

Fig. 35. Water-induced infrared reflection spectra, ∆R/R, upon water exposure on clean Si(100)2×1 at 0.5 L with 80 K: a, 0.5 L with TS = 80 K; b, TS = 80 K and after annealing to 285 K for 1 min with no additional water exposure; c, 10 L with TS = 80 K; and d, 10 L with TS = 80 K and after annealing to 280 K for 1 min [84C2].

294

3.8.2 H2O and OH on semiconductors 840 cm −1 Si-OD stretch

Si(100)

1517 cm −1 Si-D stretch

a D2 O 2085 cm −1 Si-H stretch

825 cm −1 Si-OH stretch Absorbance

Absorbance

658 cm −1 SiOD bend

2×10 −3

5×10 −4

2000 2100 Frequency n [cm −1]

898 cm −1 SiOH bend

b H216 O 600

1200 1000 Frequency n [cm −1]

800

1400

1600

Fig. 36. Infrared absorption spectra of Si(100) surfaces after 2 L exposures to (a) O at ~300 K [97S2]. D2O and (b) H16 2

Si(111)

×18

Intensity

a ×100

90

810 630

b

2090

950

3680 3420

×1000 ×2500

0

1000

2000 Energy loss Eloss [cm −1 ]

Fig. 37. Electron energy loss spectra of (a) 2L of H2O condensed on a Si(111)7×7 surface held at 100 K. (b) The same surface after annealing briefly to 300 K [82I2].

3000

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3.8.2 H2O and OH on semiconductors

1000

0

Energy loss Eloss [cm −1 ] 2000 3000

295

4000

Intensity

D2O/Si(111)7×7

18 mV ×2000

Fig. 38. Electron energy loss spectrum for heavy water adsorbed on Si(111)7×7 surface at saturation coverage (300 K), E0 = 7.98 eV [83K2].

×250

200 300 400 Energy loss Eloss [meV ]

100

0

500

Energy loss Eloss [cm −1 ] 2000

0

4000 453

Si(111)2×1

257

103 ×1000

×50

H2 O - exposure [L] f 20

206 103

364

×100

257

103

453

206 e 10

Intensity

454 ×500

57

258 210

d5

102 453

56

c3

257 207 56

453 b2

102 257 55

a0 15

0

Landolt-Börnstein New Series III/42A4

200 400 Energy loss Eloss [meV ]

Fig. 39. EELS spectra for (a) clean cleaved and (b)-(f) water exposed Si(111)2×1 at room temperature, E0 = 6.5 eV, θi = θs = 65° [84S1].

References for this document 82I2 83K2 84C2 84S1 85S 88M 89L 93M 95P 97S2

Ibach, H., Wagner, H.,Bruchman, D.: Solid St. Commun. 42 (1982) 457. Kobayashi, H., Kubota, T., Onchi, M.,Nishijima, M.: Phys. Lett. 95A (1983) 345. Chabal, Y.J.,Christman, S.B.: Phys. Rev. B 29 (1984) 6974. Schaefer, J.A., Stucki, F., Frankel, D.J., Göpel, W.,Lapeyre, G.J.: J. Vac. Sci. Technol. B 3 (1984) 359. Schäfer, J.A., Anderson, J.,Lapeyre, G.J.: J. Vac. Sci. Technol. A 3 (1985) 1443. McGrath, R., Cimino, R., Braun, W., Thornton, G.,McGovern, I.T.: Vacuum 38 (1988) 251. Larsson, C.U.S., Flodström, A.S., Karlsson, U.O.,Yang, Y.: J. Vac. Sci. Technol. A 7 (1989) 2044. Mayer, T.: Ph.D. Thesis, Technische Universität Berlin, 1993. Poncey, C., Rochet, F., Dufour, G., Roulet, H., Sirotti, F.,Panaccione, G.: Surf. Sci. 338 (1995) 143. Struck, L.M., Jr., J.E., Bent, B.E., Flynn, G.W., Chabal, Y.J., Christman, S.B., Chaban, E.E., K.Raghavachari, Williams, G.P., Radermacher, K.,Mantl, S.: Surf. Sci. 380 (1997) 444.

300

3.8.7 Cyclic hydrocarbons

3.8.7 Cyclic hydrocarbons G. HELD, H.-P. STEINRÜCK 3.8.7.1 List of symbols and abbreviations Geometry, Coverage: σ(d), σ(v) hcp site fcc site bridge pedestal site Θr Θ ML

Edes Eact Ediff Etrans Tmax

orientation with molecular σ(d/v) mirror plane parallel to surface mirror plane (see Fig. 1) 3-fold coordinated hollow site with a substrate atom in the 2nd layer underneath. 3-fold coordinated hollow site with a substrate atom in the 3rd layer underneath. 2-fold coordinated adsorption site. see Fig. 4. relative coverage (fraction of saturation coverage) absolute coverage (adsorbed molecules per substrate surface atom) “monolayer”, unit for absolute coverage. Note: following a common practice, in some cases the word “monolayer” is also used to describe the saturated chemisorbed layer. “ML”, however, always refers to the absolute coverage of molecules, i.e. the ratio of adsorbed molecules to substrate surface atoms. activation energy for desorption activation energy for dissociation (dehydrogenation) activation energy for diffusion translational energy of gas particles desorption temperature (rate maximum)

Experimental techniques 2PPE ∆Φ AES ARPES ARUPS DLEED (E)ELS ESDIAD FTMS HAS HREELS IETS IPES LEED(-IV) LITD MAES MIES NEELFS NEXAFS PED PhD RAIRS Raman SRUPS

2 Photon Photoemission Work Function Change Auger Electron Spectroscopy Angle Resolved Photoelectron Spectroscopy Angle Resolved UV Photoelectron Spectroscopy Diffuse Low Energy Electron Diffraction Electron Energy Loss Spectroscopy Electron Stimulated Desorption Ion Angular Distribution Fourier Transform Mass Spectroscopy Helium Atom Scattering High Resolution Electron Energy Loss Spectroscopy In-Elastic Tunnelling Spectroscopy Inverse Photoemission Spectroscopy Low Energy Electron Diffraction (Intensity vs. Voltage dependence) Laser Induced Thermal Desorption Metastable Atom Electron Spectroscopy Metastable Impact Electron Spectroscopy Near Edge Electron Energy Loss Fine Structure Near Edge X-ray Absorption Fine Structure ( = XANES) Photoelectron Diffraction Photoelectron Diffraction Reflection Absorption Infrared Spectroscopy Raman Spectroscopy Spin Resolved UV Photoelectron Spectroscopy

Landolt-Börnstein New Series III/42A4

3.8.7 Cyclic hydrocarbons SHG STM STS TPD UPS XANES XPD XPS

301

Second Harmonic Generation Scanning Tunnelling Microscopy Scanning Tunnelling Spectroscopy Temperature Programmed Desorption UV Photoelectron Spectroscopy X-ray Absorption Near Edge Spectroscopy ( = NEXAFS) X-ray Photoelectron Diffraction X-ray Photoelectron Spectroscopy

Theoretical techniques ADC(3) CI PM3 DFT (GGA) EHT HF LCGTO-DFT MO calc. MINDO

Algebraic Diagrammatic Construction Green’s function method Configuration Interaction Parameterized Method 3 (semi empirical) Density Functional Theory (General Gradient Approximation) (ab initio) Extended Hückel Theory (semi-empirical) Hartree Fock calculation (ab initio) Linear Combination of Gauss-Type Orbitals– DFT (ab initio) Molecular Orbital calculations (semi-empirical) Modified Intermediate Neglect of Differential Overlap (semi empirical)

3.8.7.2 Benzene (C6H6) Among the cyclic hydrocarbon molecules adsorbed on surfaces, benzene is by far the most intensely studied system. Most surface science techniques have been applied in order to elucidate the geometry and the bond/electronic structure of benzene adsorbed on single crystal surfaces. Due to its high symmetry in the gas phase (D6h), vibrational spectroscopies (HREELS, RAIRS) and photo-electric spectroscopies (ARUPS, NEXAFS) can also be used to retrieve information about the orientation and the symmetry of the adsorption complex besides the actual spectroscopic information. The vast majority of studies concentrate on transition metal surfaces of particular catalytic importance. Recently, there is also growing interest in the adsorption on silicon surfaces. The present article is restricted to the adsorption and coadsorption with atoms / small molecules on low-index (i.e. non-stepped) single crystal surfaces and flat well-defined thin films. The adsorption of benzene on stepped surfaces (Pt, Ir) has been studied to some extent [e.g. 73Gla, 74Bar] because of their potential importance in catalytic processes involving small metal particles. These results are not included in the tables because they are dominated by step effects and do, therefore, not represent low index surface properties. The available information is provided in Tables 1a-1c: After listing typical gas phase or multilayer values for comparison, the adsorbate systems are divided into metals and semiconductors / insulators are listed alphabetically starting with pure benzene layers and then listing coadsorption systems for each substrate. Table 1.a “Temperature Dependence and Adsorption Geometry” compiles the desorption temperatures from temperature programmed desorption spectroscopy (TPD) and the information about long range order (LEED patterns) and local adsorption geometries, bond length, molecular orientation, etc.. The adsorption geometry is characterized by spectroscopic methods (see above), local probes (STM), crystallographic diffraction methods (LEED-IV, PhD), and semi-empirical or ab initio theoretical calculations. Especially for the latter two techniques, benzene provides a particular challenge, since these structures are among the most complex surface structures solved with those methods so far.

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3.8.7 Cyclic hydrocarbons

TPD: Molecular multilayer desorption is observed around 150 K. For the chemisorbed layer a wide range of desorption temperatures has been reported (150 K - 570 K). Hydrogen desorption indicates decomposition of benzene leaving a layer of carbon behind on the surface. Typically, the onset of hydrogen desorption is somewhat above the onset of molecular desorption; the lowest onset is 250 K, reported for Pt(111). As a typical example, TPD spectra of benzene on Ni(111) are shown in Fig. 1. Adsorption Geometry: On metals the surface bond is formed through the interaction between metal dstates and the molecular π-electrons. Therefore, on unreconstructed close-packed surfaces (fcc 111, 100, hcp 0001) intact molecules usually adsorb with the ring parallel to the surface, whereas on more corrugated surfaces (e.g. fcc 110) also tilted geometries are reported with the ring parallel to one of the facets. There is a great variety of adsorption sites (bridge, hollow) and orientations observed, not following any apparent rule. The key geometrical parameters listed in the table are adsorption site, orientation of the molecule (for a description of the nomenclature, see Fig. 2), C-C bond lengths (usually larger than the gas phase value of 1.40 Å) and the substrate-carbon bond lengths (typically around 2.0 Å). On most metal surfaces benzene does not order very easily and in some cases (e.g. Rh(111)) it was argued that all long range ordered structures observed are actually induced by coadsorption with CO from residual gas. In general, pure ordered overlayers tend to be observed at low temperatures near saturation coverage, with the exception of Ru(0001), where ordered phases are observed for various coverages - as an example the corresponding LEED patterns are shown in Fig. 3. Coadsorption with O, NO, CO helps to induce long range order at higher temperatures and lower coverage. For insulator surfaces very weak surface bonds are reported with little information about the adsorption geometry available. On silicon surfaces the situation is different, since benzene is able to form covalent bonds with the surface Si atoms, completely changing the intra-molecular bonding. The main body of structural studies has focussed on Si(100)-(2×1), applying spectroscopic and theoretical techniques. There, a 1,4-cyclohexadiene-like adsorption structure seems to emerge as the most stable adsorption configuration [98Bir, 95Jeo] (see also Fig. 4). Table 1.b “Electronic structure” lists the binding energies of the occupied molecular orbitals and excitation energies for transitions into unoccupied levels. The spatial distribution of the molecular orbitals is shown in Fig. 5. Most data were collected using angle resolved photoelectron spectroscopy (ARUPS). Due to the bond formation through molecular π-electrons on metal surfaces, these orbitals (1e1g, 1a2u) show the largest differential shifts (“bonding” shifts) upon adsorption. The magnitude of the differential shift is also a measure for the surface bond strength (small on group Ib metals, large on group VIII metals). The spectroscopies of occupied and unoccupied molecular levels (ARUPS, EELS, NEXAFS) allow a characterization of the symmetry of the molecule/adsorption complex. As examples, ARUPS spectra of a dilute benzene layer on Ni(110) and NEXAFS spectra of benzene on Au(111), Rh(111), and Pt(111) are shown in Fig. 6 and Fig. 7, respectively. Detailed descriptions of the application of symmetry selection rules in order to retrieve geometry information and discussions of the electronic structure of adsorbed benzene can be found in several recent reviews [92Sto, 94Ste, 95Fre, 96Ste, 97Dow]. The excitation energies for molecular vibrations are listed in Table 1.c “Vibrational properties”. The two main techniques used here are HREELS and RAIRS. The frequencies for adsorption on metal surfaces are very similar to the corresponding values of the molecule in the gas phase, indicating essentially undistorted molecules. On Si surfaces larger differences are observed due to changes in the bond order of the C-C bonds. No assignment is made to individual frequencies. Instead, the molecular vibrations for each adsorption system are grouped into four main energy ranges (400 - 1000 cm-1, 1000 - 1200 cm-1, 1200 - 1600 cm-1, ∼3000 cm-1) with typical modes for each range listed at the top of the table. One band observed for crystalline layers around 1850 cm-1 [96Jak] is listed separately. Two additional columns list the frequencies of molecule-surface vibrations. The frustrated vertical translations (C-M, C-S) can be measured by HREELS (Benzene-Metal typically 250 - 400 cm-1, Benzene-Si, up to 600 cm-1), the energies of the lateral modes (frustrated translations and rotations) are usually not obtained by the above mentioned methods. In some cases such data are, however, available from other techniques (HAS, IETS). A detailed discussion of the assignment of vibrational frequencies from adsorbed benzene can be found in the recent review by Sheppard and De la Cruz [98She]; as a typical example a HREELS spectrum of benzene on (2×1) Si(100) together with the calculated normal frequencies of a C6H6Si13H12 cluster is shown in Fig. 8.

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3.8.7.3 Cyclohexane (c-C6H12) The adsorption of cyclohexane has been studied in some detail on low index and stepped metal surfaces of Cu, Ni, Mo, Ru, Pd, Ag, W, Ir and Pt – the key results of these studies are summarized in Table 2. It contains information on the rate maxima in temperature programmed desorption (Tmax), characteristic vibrational frequencies (soft modes), ordered adsorbate structures, dehydrogenation upon heating as well as the orientation, symmetry and conformation on the surface, along with the corresponding references. The different metals are listed according to their position in the periodic table of elements. For semiconductor or oxide surfaces to the best of our knowledge no detailed studies are available. The majority of studies focus on the nature of the chemical interaction between cyclohexane, in particular the C-H bonds, and the metal surface. From vibrational spectroscopies (HREELS and RAIRS) the signature of the interaction is a softening (red shift) of one C-H mode from ∼2900 cm-1 for the free molecule or in the condensed phase to lower wave numbers by up to 400 cm-1. A comparable mode softening has also been observed for a number of different saturated hydrocarbons on metal surfaces – for a detailed discussion of the vibrational spectra see the reviews by Sheppard [88She, 98She]. There is general agreement that the origin of the softened C-H mode is due to H-atoms perturbed by their proximity to the metal surface in a hydrogen bond-like, C-H···M bonding environment, often also termed as agostic (hydrogen-bond-like) bond. However, its relation to the desorption temperature and bond strength as well as the extent of dehydrogenation is subject of ongoing discussion: For Cu(111), Cu(100), Cu(110), Cu3Pt(111), Ni(111), Ni(100) and Ni(110) surfaces the observed mode softening is relatively small (2690 - 2775 cm-1) and no dehydrogenation is observed upon heating a monolayer of cyclohexane. The desorption temperatures for these systems range from 178 to 206 K. As examples TPD and RAIRS spectra for Cu(111), Cu(100), Cu(110), Cu3Pt, Ni(111) are shown in Fig. 9. (Note that in some cases studies of the same system by different authors yield significant differences in the desorption temperatures, which cannot be attributed only to differences in the heating rate). A special case is cyclohexane adsorption on one single layer of K on Ni(111), where the interaction to the substrate is so weak, that three-dimensional growth occurs and cyclohexane desorbs from the condensed phase at 135-140 K, even at nominal submonolayer coverages. For Mo(110), Ru(0001), Pd(111), Pd(110), Pt(111), Pt(100)-(5×20) a significantly stronger softening of the C-H mode is observed to values between 2550 and 2635 cm-1. For these systems significant dehydrogenation of cyclohexane directly or via stable intermediates to benzene occurs upon heating. In addition, molecular desorption of cyclohexane at temperatures between 182 and 236 K occurs. However, there are several systems that do not fall into these two categories, indicating that there is no general direct correlation between the magnitude of the mode-softening, the desorption temperature and the reactivity towards dehydrogenation. A very clear example is Mo(110) where a significant difference of nearly 100 cm-1 in the vibrational frequency of the soft mode is seen for the clean and the C-modified surface, whereas the desorption temperature remains essentially unchanged. Also for Cu(111) and Cu3Pt(111) a shift of 21 K is seen in the desorption temperature, while the vibrational frequency of the softened mode is identical. At first sight, there seems to be a general trend that dehydrogenation is observed if the soft mode has a wave number smaller than 2650 cm-1. However, there is again an example, which does not fit in that trend: for the non-reconstructed Pt(100)-(1×1) surface no mode softening, but complete dehydrogenation occurs. Also, a strong isotope effect for dehydrogenation is observed for Pt(111) and Mo(110). Summarizing these studies, one has to conclude that no general trends and correlations can be derived at the moment, suggesting that the details of the C-H···metal interaction strongly depend on the substrate electronic structure and that there are several factors that contribute to the bonding interaction. Attractive as well as repulsive contributions to the mode-softening and the bonding interaction are discussed. Furthermore, the role of the geometric matching between three of the H-Atoms and specific adsorption sites on the surfaces has been considered [84Hof, 95Zae, 98Tep, 89Rav2, 85Kan, 88She, 98She]. There are only a few studies on the electronic structure of cyclohexane monolayers by UPS or ARUPS. In all studies, no differential shifts, i.e. bonding shifts, of particular molecular orbitals are observed, indicating only minor changes in the electronic structure due to a weak chemical interaction involving a C-H···metal bond.

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Concerning the geometric structure, for four surfaces the formation of ordered cyclohexane layers has been observed, namely a (√21×√21)R10.9° structure for Pt(111), (√7×√7)R19.1° structures for Cu(111) and Ni(111) and a (9×9) structure for Ag(111). For none of the surfaces a detailed structural analysis has been performed, which partly may be due to the high sensitivity of cyclohexane to beam damage and the large unit cells. The conformation of the molecule, its orientation with respect to the surface plane, and the symmetry of the adsorption complex have been investigated using vibrational spectroscopies (HREELS and RAIRS) or ARUPS in combination with symmetry selection rules. In most cases it is concluded that the molecule is adsorbed in the chair form with the molecular plane parallel or slightly inclined to the surface. The symmetry of the adsorption complex differs for the various substrates, from C3v to C1. 3.8.7.4 Other saturated cyclic hydrocarbon molecules (cycloalkanes) Besides cyclohexane, the adsorption of a number of other cycloalkanes has been studied on metal surfaces – an overview is given in Table 3; for each molecule the metals are listed according to their position in the periodic table of elements. The table compiles the rate maxima in temperature programmed desorption (Tmax), characteristic vibrational frequencies (soft modes), dehydrogenation upon heating and some additional data (column “comments”). For semiconductor or oxide surfaces to the best of our knowledge no detailed studies are available. One central aspect in these studies is again the nature of the chemical interaction with the substrate, in particular, the formation of C-H···metal bonds, which are reflected in mode softening of a C-H vibration. Studies by TPD show that there is a steady increase of monolayer desorption ranges for the different cycloalkanes, from 123 - 150 K for cyclopropane, 175 K for cyclobutane, 161 - 200 K for cyclopentane, 160 - 236 K for cyclohexane to ∼260 K for cyclooctane. 3.8.7.4.1 Cyclopropane (c-C3H6) From a chemical point of view, cyclopropane is interesting, since although saturated, it possesses some “olefinic character”, owing to the strain exerted on the three-membered carbon ring due to the 60° angle between C-C orbitals. Thus cyclopropane lies between a paraffinic and an olefinic hydrocarbon. Cyclopropane adsorption has been studied on Ni(100), Cu(111), Cu(110), Ru(0001), Ir(110) and Ir(111) surfaces. Vibrational studies by HREELS for several of these substrates indicate that, in contrast to cyclohexane (see above) and cyclopentane (see below), no softening of a C-H mode occurs. This is attributed to the fact that the interbond C-C-C angles of 60° for cyclopropane (and 90° for cyclobutane, see below) would not provide convenient geometric requirements for multiple C-H···metal interactions, in contrast to the C6 ring and also the C5 ring in cyclohexane and cyclopentane, respectively [98She]. For cyclopropane it has been suggested that surface bonding occurs via a delocalized σ(CC) orbital [82Hof]. Upon heating of molecularly adsorbed cyclopropane molecular desorption is observed with rate maxima in the temperature range between 123 and 150 K. For some substrates more or less exclusively molecular desorption occurs, for others significant to nearly complete dehydrogenation occurs upon heating. 3.8.7.4.2 Cyclobutane (c-C4H8) Cyclobutane adsorption has only been studied on Ru(0001). No softening of C-H vibrations has been observed.

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3.8.7.4.3 Cyclopentane (c-C5H10) Cyclopentane adsorption has been studied for Cu(111), Cu(110), Rh(111), Ru(0001), Ir(111) and Pt(111) surfaces. Quite general, a softening of C-H vibrations is observed, indicating multiple C-H···metal interactions with the substrate; consequently the C5 ring is assumed to be approximately parallel to the surface. Upon heating of cyclopentane molecularly adsorbed at low temperatures, molecular desorption is observed with rate maxima in the temperature range between 161 and 200 K. For some substrates more or less exclusively molecular desorption occurs, for others significant to nearly complete dehydrogenation is observed upon heating. 3.8.7.4.4 Cyclooctane (c-C8H16) Cyclooctane adsorption has only been studied on Ru(0001). Interestingly, no softening of C-H vibrations are observed, indicating that the majority CH2 groups do not interact with the substrate. Upon heating, molecular desorption of cyclooctane up to temperatures of ∼260 K is observed. 3.8.7.5 Non-saturated cyclic hydrocarbon molecules (other than benzene) The adsorption and dehydrogenation of non-saturated cyclic hydrocarbons has been investigated for various metal surfaces due to the importance of this class of molecules as intermediates in a manifold of surface reactions with great industrial relevance. In contrast to the saturated cycloalkanes, the interaction of the non-saturated cyclic hydrocarbons is significantly stronger, which is attributed to the interaction of the π-electrons with the substrate. A variety of experimental techniques has been applied. Similar to benzene and in contrast to the cycloalkanes there are also a number of studies for group IV semiconductor surfaces with particular emphasis on cycloaddition chemistry. The results are summarized in Table 4, which contains information on various aspects of the adsorbate substrate bonding (column “comments”) and the thermal evolution. For each of the included molecules the data for the different metals are listed according to their position in the periodic table of elements, followed by the data for Si and Ge surfaces. 3.8.7.5.1 Cyclopentene (c-C5H8) Measurements have been performed for cyclopentene adsorbed on Mo(110), Rh(111), Ag(111), Ag(112), Ir(111), Pt(111), Si(100), Si(111) and Ge(100). For Ag(111) and Ag(112) reversible adsorption is observed, indicating a very weak interaction with the substrate. For all other metal surfaces decomposition is observed upon heating. Depending on the substrate and the adsorption temperature, c-C5H6, c-C5H5, and c-C5H3 species have been identified as intermediates. On the Si(100)-(2×1) and Ge(100)-(2×1) surfaces cyclopentene forms di-σ Si/Ge-C bonds, which corresponds to a [2+2] cycloaddition reaction with the surface dimers. For these surfaces no decomposition occurs upon heating. For Si(111)-(7×7) molecular desorption of cyclopentene is observed between 300 and 650 K with some dehydrogenation to c-C6H5. 3.8.7.5.2 Cyclopentadiene (c-C5H6) The adsorption of cyclopentadiene has been studied on Rh(111), Ir(111), Pt(111) and on Si(111)-(7×7). For Ir(111) and Pt(111) the formation of c-C5H5 and c-C5H3 intermediates has been proposed upon heating. On Pt(111) a disproportionation reaction to c-C5H5 and c-C5H8 occurs at 90 K. In contrast to the metal surfaces studied, exclusively molecular desorption of cyclopentadiene is observed for the reconstructed Si(111)-(7×7) surface.

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3.8.7.5.3 Cyclohexene (c-C6H10) Cyclohexene adsorption has been investigated for Ni(111), Cu(100), Ru(0001), W(100), Pt(111), Pt(100)-(1×1), Pt(100)-(5×20), Si(100)-(2×1) and Ge(100)-(2×1). On Cu(100) cyclohexene adsorption is completely reversible. For the Ni and Pt surfaces decomposition to benzene (C6H6) occurs upon heating, partly accompanied by cyclohexene desorption; for Ru(0001) the formation of ethylene (C2H4) is reported. For Si(100)-(2×1) and Ge(100)-(2×1) a di-σ type interaction with the Si dimer dangling bond, i.e. a [2+2] cycloaddition reaction, is proposed. For Si(100)-(2×1) there is evidence for two stable adsorption states, assigned to boat type and twist boat type geometries (Fig. 10). 3.8.7.5.4 Cyclohexadiene (c-C6H8) The adsorption of 1,3-cyclohexadiene and/or 1,4-cyclohexadiene has been studied on Ni(111), W(100), Pt(111), Pt(100)-(1×1), and Si(100)-(2×1) surfaces. For the metal surfaces decomposition to benzene is reported for both conformations. Interestingly, for Pt(111) a partial rearrangement of 1,3- to 1,4- cyclohexadiene is proposed as intermediate reaction step. Upon adsorption of 1,4-cyclohexadiene on Si(100)-(2×1) the formation of di-σ Si-C bonds is reported with the Si dimer structure maintained after adsorption; for 1,3-cyclohexadiene a mixture of [2+2] and [4+2] cycloaddition products is observed. 3.8.7.5.5 Cyclooctadiene (c-C8H12) and Cyclooctatetraene (c-C8H8) These two molecules have been studied on Pt(111) and Si(100)-(1×2). For the latter the interaction of 1,5-Cyclooctadiene indicates the formation of a [2+2] cycloaddition product. 3.8.7.6 Ethylene Oxide (C2H4O) Ethylene oxide (EO) adsorption and reaction on single crystal metal surfaces has been subject to numerous investigations due the industrial importance of ethylene epoxidation – in particular the silvercatalyzed epoxidation represents the largest volume of any catalytic oxidation reaction on an industrial scale. Detailed studies have been performed only for the single crystal surfaces of Fe, Ni, Cu, Mo, Rh, Pd, Ag and Pt. The key results of these studies are summarized in Table 5, listed according to the position of the metal in the periodic table of elements. The table compiles data on the rate maxima in temperature programmed desorption (Tmax), the work function change upon adsorption (∆Φ), ordered adsorbate structures, decomposition upon heating, orientation and symmetry of EO on the surface, bonding shifts of molecular orbitals (2b1, 6a1) upon adsorption and some other information (column “comments”). Overall, the interaction between EO and metal surfaces is weak. At low temperatures EO is reported to adsorb on all surfaces in molecular form. Upon heating the saturated monolayer, molecular desorption in the temperature range between 120 and 255 K is reported. For clean IB metals, Ag and Cu, no decomposition occurs upon heating. For group VIII metals for some substrates ring opening and decomposition occurs; however no general trends can be derived, since e.g. EO decomposition is observed for Pt(111), but not for Pt(110). The situation for Ni(110) is controversial, since reversible and non-reversible adsorption have been reported in different studies. There is general agreement that the molecule bonds to the surface via the O-atom. Upon adsorption, a strong decrease in the work function (∆Φ) is observed, ranging from −1.1 to −2.8 eV (as an example see Fig. 11), which is attributed to an alignment of the large permanent dipole moment of the molecule (6.3 × 10-30 Cm in the gas phase) with a component perpendicular to the surface (O end down). The electronic structure has been mainly investigated by ARUPS. These studies show bonding shifts of up to 0.9 eV to higher binding energies of the topmost molecular levels (2b1, 6a1) due to a weak bonding interaction to substrate. One should note here that in the ARUPS studies until 1993 it was proposed that ordering of these two molecular levels was reversed from that in the free molecules upon Landolt-Börnstein New Series III/42A4

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adsorption. However, theoretical studies performed later on for Ni(110) [95Ulr] and Pd(110) [96She] indicate that the order remains the same as in the gas phase, which appears reasonable, considering the relatively weak interaction of EO with the different surfaces. The vibrational structure of adsorbed EO was studied by HREELS. For molecularly adsorbed EO only small changes in the vibrational frequencies are observed, which indicates a weak bond of EO to metal surfaces. Ordered EO structures have been reported only for the (110) planes of Ni, Cu, Ag and Pt; in each case a c(2×2) structure was observed. One should note that EO is very sensitive to radiation damage by electrons or photons. Information on the orientation of the molecule has been derived from ARUPS and HREELS data based on symmetry selection rules, and from XPD. For Ag(111) and Ag(110) EO is proposed to be oriented perpendicular to the surface, for the other substrates studied, a tilt of the molecular plane is suggested. The symmetry of the adsorption complex is reduced to CS or C1, from C2v for the free molecule. For several surfaces coadsorption of EO with K or O was investigated. Depending on the substrate, O coadsorption leads to a weakening or a strengthening of the EO surface bond; K coadsorption leads to a reaction between EO and K for high K precoverages. 3.8.7.7 Pyridine (C6H5N) Besides benzene, pyridine adsorbed on single crystal metal surfaces represents the second model system for the interaction of aromatic compounds with metals. Whereas the benzene-metal bond is dominated by the interaction with the π-system, pyridine has the additional option of bonding to the metal via the lone pair electrons of the N atom. Due to structural interests, most of the studies have been directed towards the adsorption geometry of pyridine [92Net]. Generally, five different adsorption structures of pyridine on metal surfaces have been proposed (Fig. 12): (a) perpendicular adsorption via the nitrogen lone pair electrons, (b) flat adsorption of the aromatic ring via the π-electrons, (c) tilted adsorption via the nitrogen lone pair orbitals and the π-electrons and (d, e) edge on adsorption through the N and C(2) atoms with the molecular plane more or less perpendicular to the surfaces – i.e. formation of α-pyridile by breaking the C-H bond. In Table 6 the corresponding results are presented as an overview. It compiles the rate maxima in temperature programmed desorption (Tmax), ordered adsorbate structures, the orientation of pyridine on the surface and the thermal evolution. The data are listed beginning with different metals according to their position in the periodic table of elements, followed by data for Si and ZnO surfaces. Structural information has been obtained for the different substrates using predominantly spectroscopies (ARUPS, HREELS, RAIRS, NEXAFS, ESDIAD) and only in very few cases true structural methods (PED) were applied; one should note that no structural analysis by LEED is available, which is most likely due to the small number of ordered pyridine phases and the large unit cells for these few structures. The investigations by one or more different methods show that in most cases the molecules are oriented with the molecular plane parallel to the surface (π-bonded) at low surface coverage, and they undergo a transition to an inclined or perpendicular phase (N-bonded) at higher coverages. The distinction between a perpendicular molecule and a molecule with a small inclination angle with respect to the surface normal is in many cases difficult due to the error bars of between 5 and 20o of the different studies. This could explain the observation that for the same system perpendicular and inclined geometries are reported in different studies. Also, the adsorption geometry (and the nature of the species - see below) sometimes depends on the substrate temperature. The coverage driven (and sometimes also temperature driven) reorientation seems to be a quite general phenomenon of pyridine adsorption on metal surfaces. The only clear exceptions are Pd(110) and Pd(111) were a parallel orientation is maintained from low coverages up to saturation, and Cu(110) where an upright orientation is observed already at low coverages. Interestingly, for these two surfaces a well defined azimuthal orientation is reported.

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For a number of systems the formation of a more or less upright α-pyridile species has been proposed at room temperature – for some systems, e.g. Pt(111) and Ir(111) the situation is controversial, since in studies by different authors either pyridine or α-pyridile have been proposed, both being oriented more or less upright. One should note that the identification of α-pyridile in electronic and vibrational spectroscopies is difficult and not necessarily unique [92Net], and the evolution of H2 from the surface in TPD is often the most direct hint for the formation of α-pyridile.

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References for this document 73Gla 74Bar 82Hof 84Hof 85Kan 88She 89Rav2 92Net 92Sto 94Ste 95Fre 95Jeo 95Zae 96Jak 96She 96Ste 97Dow 98Bir 98She 98Tep

Gland, J., Somorjai, A.: Surf. Sci. 38 (1973) 157. Baron, K., Blakely, D.W., Somorjai, G.A.: Surf. Sci. 41 (1974) 45. Hoffmann, F.M., Felter, T.E., Weinberg, W.H.: J. Chem. Phys. 76 (1982) 3799. Hoffmann, F.M., Upton, T.G.: J. Phys. Chem. 88 (1984) 6209. Kang, D.B., Anderson, A.B.: J. Am. Chem. Soc. 107 (1985) 7858. Sheppard, N.: Annu. Rev. Phys. Chem. 39 (1988) 589. Raval, R., Chesters, M.A.: Surf. Sci. 219 (1989) L505. Netzer, F.P., Ramsey, G.R.: Crit. Rev. Solid State Mater. Sci. 17 (1992) 397. Stöhr, J.: NEXAFS Spectroscopy, Berlin: Springer-Verlag, 1992. Steinrück, H.-P.: Appl. Phys. A 59 (1994) 517. Freund, H.-J., Kuhlenbeck, H.: Applications of synchrotron radiation. High-resolution studies of molecules and molecular adsorbates on surfaces, Berlin: Springer-Verlag, 1995, p. 9. Jeong, H.D., Ryu, S., Lee, Y.S., Kim, S.: Surf. Sci. 344 (1995) L1226. Zaera, F.: Chem. Rev. 95 (1995) 2651. Jakob, P., Menzel, D.: J. Chem. Phys. 105 (1996) 3838. Shekhar, R., Barteau, M.: J. Vac. Sci. Technol. A 14 (1996) 1469. Steinrück, H.-P.: J. Phys. Condens. Matter 8 (1996) 6465. Dowben, P.A.: Z. Phys. Chem. 202 (1997) 227. Birkenheuer, U., Gutdeutsch, U., Rösch, N.: Surf. Sci. 409 (1998) 213. Sheppard, N., De la Cruz, C.: Adv. Catal. 42 (1998) 181. Teplyakov, A.V., Bent, B.E., Eng jr., J., Chen, J.G.: Surf. Sci. 399 (1998) L342.

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3.8.7.8 List of Tables Table 1.a:

Benzene (c-C6H6): Temperature dependence and adsorption geometry

Table 1.b:

Benzene (c-C6H6): Electronic structure

Table 1.c:

Benzene (c-C6H6): Vibrational properties

Table 2:

Cyclohexane (c-C6H12)

Table 3:

Saturated hydrocarbons other than Cyclohexane: Cyclopropane (c-C3H6), Cyclobutane (c-C4H8), Cyclopentane (c-C5H10), Cyclooctane (c-C8H16)

Table 4:

Non-saturated hydrocarbons other than Benzene: Cyclopentene (c-C5H8), Cyclopentadiene (c-C5H6), Cyclohexene (c-C6H10), Cyclohexadiene (c-C6H8), Cyclooctadiene (c-C8H12), Cyclooctatraene (c-C8H8)

Table 5:

Ethylene Oxide (C2H4O)

Table 6:

Pyridine (C5H5N)

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3.8.7.9 Tables for 3.8.7 Table 1a. Temperature dependence and adsorption geometries of adsorbed benzene Metal surfaces surface

superstructure (coverage, ML) exp. temperature

shortest Csubstrate bond length [Å]

comment

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method

Refs.

TPD

95CRC 90Zho

p(3×1) (0.33 ML) 300 K

saturation at 200 K and 300 K

C3v, ring parallel to surface. fcc hollow site, ring parallel to surface. ring parallel to surface asymm. near hollow site

ARUPS

90Dud

1.40

C-H bent upwards by 2°

MO calc.

84And

NEXAFS

98Wei3

model 1: σ(v) parallel to [0001] ring tilted by 20°

1.50 - 1.60

2.20

model 2: σ(d) parallel to [0001] ring tilted by 25° C3v, ring parallel to surface

1.39 - 1.49

2.02

model 1 slightly favoured by LEED-IV, strong ring distortions in model 1.

LEED-IV

01Pus

A/SRUPS

95Get

3.8.7 Cyclic hydrocarbons

cluster

Au(111)

Co / W(110)

C-C bond length [Å]

mol. des. 144→230 K

Ag(111)

Co(10-10)

adsorption site / symmetry/ orientation

1.399

gas phase Ag(111)

Ag(111)

Desorption temperature

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Cu(110)

3 L at 80 K

Cu(110)

low cov.: 0.01 ML ads. at 300 K, expt. at 4 K 0.24 ML, 140 K

Cu(110) Cu(111)

Desorption temperature

adsorption site / symmetry/ orientation

C-C bond length [Å]

4-fold hollow, ring parallel to surface, C-C parallel to close packed rows. long bridge site

1.41 - 1.42

Cu(111)

mol. des. 140→240 K

Cu(111)

mol. des. 140→240 K mol. des. 160→230 K no hydr. des. mol. des. 150→340 K no hydr. des.

sat. chemisorbed layer, 100 K

Cu/ Ru(0001)

sat. chemisorbed layer, 100 K

Fe / W(110)

saturation at 200 K and 300 K

comment

method

Refs.

H-flip structure: ring planar, C-H bonds bent upwards by 17.6° / 5.3°

DFT(GGA) NEXAFS

98Pet 98Wei3

no step decoration

STM

98Doe

ARUPS, HREELS

96Lom

STM

94Str 95Str 98Wei2 94Xi

ring tilted with respect to surface by more than 20°

(few % of 1 ML) 4 K, 77 K

Cu/ Ni(111)

shortest Csubstrate bond length [Å] 2.21

1st layer: ring parallel, 2nd layer: ring tilted

up to 4 rows of ordered molecules along step edges (distance 5.1 Å)

NEXAFS, TPD TPD

98Vel

ring parallel to surface

no azimuthal ordering

ARUPS TPD

99Kos2

ring parallel to surface, σ(d) C3v, ring parallel to surface

azimuthal ordering

ARUPS TPD

00Kos1 99Kos1

A/SRUPS

95Get

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surface

311

Ir(110)

adsorption at 313 K mol. des. ~ 425 K hydr. des. 450→800 K p(3×3) (0.11 ML) mol. des. ~ 400 K at 300 K hydr. des. 370→500 K adsorption at 120 K mol. des. ~ 350 K hydr. des. 350→600 K c(4×4) (0.125)

Ir(111)

Mo(110)

Ni(100) Ni(100)

Ni(110)

Desorption temperature

adsorption at 100 K mol. des. ~ 150 K (multilayer) ~ 480 K hydr. des. 340→510 K c(4×2) (0.25)

adsorption site / symmetry/ orientation

C-C bond length [Å]

shortest Csubstrate bond length [Å]

comment

C3v, ring parallel to surface

4-fold hollow σ(d) or σ(v)

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asymmetric site near 4-fold hollow, σ(d) mirror plane parallel to [110] (troughs), ring tilted by 5°

1.44 / 1.45

1.42 / 1.45

2.02

2.09

method

Refs.

TPD

78Nie

ARUPS, ESDIAD, TPD

85Mac 76Nie

TPD

88Liu

DFT (GGA) C-H bonds bent upwards (0.16 Å) substrate buckling (0.21 Å) coadsorption w. C, O, and AES, TPD CO inhibit dissociation.

C-H bonds bent upwards (19°/35°) .

DFT (GGA)

01Mit 87Bla 87Mye

01Mit

3.8.7 Cyclic hydrocarbons

superstructure (coverage, ML) exp. temperature

312

surface

Landolt-Börnstein New Series III/42A4

superstructure (coverage, ML) exp. temperature

Ni(110)

0.1 ML annealed to 200 K, expt. at 60-65 K

Ni(110)

0.01 ML ads. at 300 K, expt. at 4 K mol. des. c(2×4) (0.2 ML) up to 460 K, 300 K hydr. des. up to 435 K c(2×4) (0.2 ML)

Ni(110)

Ni(110) Ni(110)

Ni(111) Ni(111)

Desorption temperature

diff. coverages, mol. des. adsorption at 100 K 360, 321, 460 K, hydr. des. up to 390 K

adsorption site / symmetry/ orientation

C-C bond length [Å]

4-fold hollow, σ(d) mirror plane parallel to [110] (troughs), ring parallel 4-fold hollow site atop or short bridge site,ring parallel to surface C2v ring parallel to surface C2v in dilute layer C1 in saturated layer

1.45

bridge, σ(v) or fcc, σ(d) bridge, σ(v) or hcp hollow, σ(d)

1.43 / 1.45

2.05

1.44 / 1.45 1.41 / 1.44

2.07 2.02

1.42 / 1.44

2.02

shortest Csubstrate bond length [Å] 1.81

comment

method

Refs.

molecular plane parallel to surface

PhD

00Kan

no step decoration

STM

98Doe

HREELS MO calculations

92Hun

ARUPS NEXAFS

91Ram

ARUPS TPD

91Hub2

DFT (GGA)

01Mit

DFT (GGA)

01Yam3

2.24

C-H bonds bent upwards, substrate buckling (0.18 / 0.22 Å) adsorption on bridge sites slightly preferred over hcp, C-H bonds bent upwards by 19-28°, substrate buckling (0.21 / 0.25 Å)

3.8.7 Cyclic hydrocarbons

surface

313

Ni(111)

(√7×√7)R19.1° (0.14 ML), ads. at 245 K expt. at 160 K (√7×√7)R19.1° (0.14 ML), ads. at 200 K expt. at 300 K disordered phase (0.10 ML), ads. at 300 K expt. at 120 K diff.coverages up to 0.14 ML, disordered phase and (√7×√7)R19.1°

Ni(111)

Ni(111)

Ni(111)

Ni(111)

low coverage

Ni(111) Ni(111) Landolt-Börnstein New Series III/42A4

(2√3×2√3)R30° (0.08 ML) coadsorbed w. NO (0.10 ML), 140 K

Desorption temperature

mol. des. 154, 136, 146 K (multilayer), 270→460 K hydr. des. 380→650 K

adsorption site / symmetry/ orientation

C-C bond length [Å]

hcp hollow site, σ(d)

1.43 / 1.55

hcp hollow site, σ(d)

1.40 / 1.46

bridge, σ(v)

1.40 / 1.44

shortest Csubstrate bond length [Å] 2.02

comment

method

Refs.

buckling in top-most Ni layer (0.14 Å)

LEED-IV

96Hel

1.91 (vert.)

substrate buckling <0.10 Å

PhD

96Sch

1.92 (vert.)

substrate buckling <0.08 Å

PhD

96Sch

reorientation due to lateral interaction

ARUPS TPD, LEED

91Hub1 89Ste 87Mye

σ(v) at low coverage, σ(d) in saturated layer

fcc hollow site, σ(d) hcp hollow site, σ(d) bridge site, C2v, σ(v)

1.43 (2 % expansion) 1.40

2.2 (vert.)

C-H bonds bent upwards by 8.5°

CI

91Jin

2.0 2.02

MO cluster calc. PhD

84And

1.40 / 1.44

C-H bonds bent upwards by 8° NO on hcp and fcc sites. buckling of ring (0.13 Å) and 1st Ni layer (0.08 Å)

01Bao

3.8.7 Cyclic hydrocarbons

superstructure (coverage, ML) exp. temperature

314

surface

Landolt-Börnstein New Series III/42A4

superstructure (coverage, ML) exp. temperature

Ni(111)

(2√3×2√3)R30° (0.08 ML) coads. w. NO, CO, O (0.166 ML) 95 K p(3×3) (0.11 ML) coadsorbed w. NO (0.22 ML) 95 K coadsorbed w. NO 4 ordered structures (0.04 0.125 ML) (0.14 ML) 80 K

Ni(111)

Ni(111)

Ni/ Cu(111) (adlayer) Os(0001)

(√7×√7)R19.1° 200-290 K

Pd(110)

c(4×2)

Pd(110)

diff. coverages at 100 K

Desorption temperature

adsorption site / symmetry/ orientation

C-C bond length [Å]

shortest Csubstrate bond length [Å]

comment

method

Refs.

ARUPS, LEED

91Hub1 91Zeb1

ARUPS, LEED

91Hub1 91Zeb1

TPD

91Zeb2

ARUPS TPD

99Kos1

ARUPS TPD

89Gra

C-H bonds bent upwards (17 - 23°), STM simulations

DFT

00Fav

diffusion above 220 K (barrier 0.57 eV)

STM

96Yos

σ(v)

reorientation due to lateral interaction

σ(d)

mol. des. 325→440 K hydr. des. 400→500 K no mol. des. hydr. des 350→450 K hydr. des 290→830 K

adlayer (Ni/Cu) -tosublayer (Cu/Ni/Cu) transition between 300 and 600 K

σ(v)

σ(d), ring parallel to surface, C3v 4-fold hollow site, σ(v)-plane rotated by 11° w/r to [001], C2 4-fold hollow site

1.42 - 1.44

1.81

3.8.7 Cyclic hydrocarbons

surface

315

superstructure (coverage, ML) exp. temperature

Desorption temperature

adsorption site / symmetry/ orientation

Pd(110)

c(4×2)

mol. des. 410 K hydr. des >390 K

ring tilted by 1020° into troughs, Cs

Pd(111)

p(3×3) coads. w. CO (0.11 ML benz, 0.22 ML CO)

1.40 / 1.46

shortest Csubstrate bond length [Å]

2.39

comment

2 CO per unit cell on fcc sites

mol. des. 350→600 K

Landolt-Börnstein New Series III/42A4

Pt(110)(1x2)

p(4×2) (0.25 ML) anneal to 275 K expt. at 95 K

mol. des. 180→440 K hydr. des 300→700 K

Pt(111)

saturated chemisorbed layer

mol. des. 250→500 K hydr. des 250→750 K

Pt(111)

0.001 ML, 4K

Pt(111)

saturation at 170 K (ca. 0.16 ML)

Pt(111)

saturation at room temp.

molecular plane tilted by 30° in [001], C-H bonds parallel to [1-10]

3 different adsorption sites: hcp, fcc, top σ(d) bridge site, σ(d), C2v σ(d), C3v, ring parallel to surface

1.45 / 1.63

2.02

butterfly-like ring distortion

method

Refs.

ARUPS TPD

88Net4

LEED-IV

88Oht1

TPD

85Wad

ARUPS NEXAFS TPD

98Zeb

TPD

96Lut2

STM STMsimulations

94Sau 96Sau 93Wei3

DLEED

91Wan

ARUPS

87Som1 87Som2

3.8.7 Cyclic hydrocarbons

Pd(111)

fcc hollow site, σ(d)

C-C bond length [Å]

316

surface

Landolt-Börnstein New Series III/42A4

surface

superstructure (coverage, ML) exp. temperature

Pt(111)

Desorption temperature

C-C bond length [Å]

shortest Csubstrate bond length [Å]

comment

mol. des. 370→570 K hydr. des 500→800 K

Pt(111) Pt(111)

adsorption site / symmetry/ orientation

1.36 / 1.44

2.0

bridge site, ring parallel to surface C2v

1.65/ 1.76

2.25

C-H bent upwards, Kekule-type distortion planar ring (within 0.05Å) 2 benzene and 4 CO (on bridge sites) per unit cell

Re(0001) Rh(111)

diff. coverages at 300 K

Rh(111)

(√19×√19)R23° – (0.159 ML) 20 L at 360 K c(2√3×4)rect (0.13 ML) 4 – 5L at 300 K

Rh(111)

Rh(111) Rh(111)

adsorption at 290 K mol. des. 395 K hydr. des 450→700 K

Refs.

TPD

85Abo

MO cluster calculation LEED-IV

84And

LEED, AES, TPD STM

82Tat

no ordering even at saturation, only molecules imaged at the upper step edges LEED high coverage LEED structure σ(d), ring parallel to surface C6v adsorbate symmetry 3-fold hollow site, 1.50 / 1.64 σ(d), ring par-allel to surface

2.06

C-H bonds bent upwards

87Ogl 87Van 88Som

97Yoo 91Neu

ARUPS

87Net2

EHT cluster calc.

86Gar

TPD

86Koe 84Koe 83Lin

3.8.7 Cyclic hydrocarbons

c(2√3×4)rect coads. w. CO (0.13 ML benz, 0.25 ML CO) 140 K

hcp hollow site

method

317

Rh(111)

coads w. CO diff coverages at 300 K, p(3×3) (0.11 ML benz, 0.22 ML CO) c(2√3×4)rect coads w. CO (0.13 ML benz, 0.13 ML CO) p(3×3) coads w. CO (0.11 ML benz, 0.22 ML CO) p(3×3) coads w. CO (0.11 ML benz, 0.22 ML CO)

Rh(111)

Rh(111)

Rh(111)

Desorption temperature

mol. des. 350→450 K hydr. des 450→550 K CO. des 350→500 K

adsorption site / symmetry/ orientation

C-C bond length [Å]

shortest Csubstrate bond length [Å]

comment

method

Refs.

ordering induced through CO

STM

97Yoo 88Oht2

Landolt-Börnstein New Series III/42A4

hcp hollow site, σ(d), ring parallel to surface

1.33 / 1.81

2.35

1 CO (on hcp sites) per unit cell

LEED-IV

86Van 88Som

hcp hollow site, σ(d), ring parallel to surface ring parallel to surface C6v symmetry

1.46 / 1.58

2.30

2 CO (on hcp sites) per unit cell

LEED-IV

87Lin 87Van 88Som

molecules essentially undistorted

ARUPS TPD

86Ber

ring buckling (0.07 Å), substrate buckling (<0.06 Å) C-H bond bent upwards by 8.9 / 23.5° ring buckling (0.06 Å), substrate buckling (<0.06 Å) C-H bond bent upwards by 13.9 / 21.5°

LEED-IV

01Hel

DFT

01Hel

Ru(0001)

(√7×√7)R19.1° (0.143 ML) 200 K

hcp site, σ(v), ring parallel to surface

1.39 / 1.47

2.04

Ru(0001)

(√7×√7)R19.1° (0.143 ML) 200 K

hcp site, σ(v), ring parallel to surface

1.42 / 1.43

2.10

3.8.7 Cyclic hydrocarbons

superstructure (coverage, ML) exp. temperature

318

surface

Landolt-Börnstein New Series III/42A4

superstructure (coverage, ML) exp. temperature

Ru(0001)

(√7×√7)R19.1° (0.143 ML) 200 K

Ru(0001)

c(2√3×4)rect (0.125 ML) 200 K

Ru(0001)

(2√3×2√3)R30° (0.083 ML) 200 K

Ru(0001)

saturation at 230 K

Ru(0001)

adsorption at 120 K mol. des. 164, 142, 152 K (multilayer) 360 K hydr. des 400→720 K multilayer des. 160, 130, 145 K (√13×√13)R13.9° mol. des. 389 K coads w. CO hydr. des (0.08 ML benz, 390→700 K 0.30 ML CO) CO des. ads. at 120 K 380→500 K

Ru(0001)

Ru(0001)

Desorption temperature

adsorption site / symmetry/ orientation

C-C bond length [Å]

hcp site, σ(v), ring parallel to surface hcp site, σ(v), ring parallel to surface hcp site, σ(v), ring parallel to surface

1.45 / 1.48

shortest Csubstrate bond length [Å] 2.11

1.41 / 1.43 / 1.53

2.11

1.41 / 1.46

2.11

comment

method

Refs.

ring buckling (0.08 Å), substrate buckling (<0.05 Å) ring buckling (0.10 Å), substrate buckling (<0.07 Å) ring buckling (0.08 Å), substrate buckling (<0.05 Å) out of plane bending of CH bonds

LEED-IV

01Bra 95Ste

LEED-IV

01Bra

LEED-IV

01Bra

NEXAFS

98Wei1

TPD

89Jak 88Jak

TPD

87Pol

3.8.7 Cyclic hydrocarbons

surface

90Jak2 LEED TPD workfunction

319

Ru(0001)

p(3×3) coads. w. O (C6D6 + 2O) (0.11 ML benz, 0.22 ML O) expt. at 90 K p(3×3) coads w. O < 0.22 ML O ads. at 120 K p(3×3) coads. w. NO (C6D6 + 2NO) (0.11 ML benz, 0.22 ML NO) expt. at 90 K coads. w. O (0.55 ML O)

Ru(0001)

Ru(0001)

W(110)

W(110) W(110)

coads. w. H (1 ML H) coads. with CO (0.7 ML CO)

Desorption temperature

adsorption site / symmetry/ orientation

C-C bond length [Å]

hcp site, σ(v), ring parallel to surface

1.38 / 1.40

shortest Csubstrate bond length [Å] 2.22

mol. des. 320→470 K hydr. des 320→700 K hcp site, ring rotated by 20° from σ(v), parallel to surface mol. des. 145K (multilayer) 165→220 K mol. des. 130→170 K mol. des. 125→150 K

1.32 / 1.44

2.23

comment

method

Refs.

substrate buckling (<0.04 Å) 2 O atoms (on hcp sites) per unit cell

LEED-IV

97Sti

ratio oxygen/benz < 0.74

90Jak1 LEED TPD workfunction

substrate buckling (<0.02) 2 NO (on hcp sites) per unit cell.

LEED-IV

97Sti

TPD

96Nah

TPD

99Whi

TPD

99Whi

3.8.7 Cyclic hydrocarbons

superstructure (coverage, ML) exp. temperature

320

surface

Landolt-Börnstein New Series III/42A4

Landolt-Börnstein New Series III/42A4

Semiconductor / insulator surfaces surface superstructure Desorption (coverage, ML) temperature exp. temperature Al2O3 / Mo(110) C(0001) (graphite)

Ge / Si(100)(2×1)

Si(100) – (2×1)

Si(100) – (2×1)

(√7×√7)R19.1° (0.14 ML) at 135 K adsorption at 90 K adsorption at 90 K

adsorption at 90 K

method

Refs.

3 desorption states

TPD

96Str

LEED

86Bar

3 desorption states

TPD

96Str

C2v symm

di-sigma bonded 1,4-cyclo-hexadiene like molecule

ARUPS TPD

01Fin

C2v symm

di-sigma bonded 1,4-cyclo-hexadiene like molecule (as on Ge(100) )

TPD

01Fin

DFT, STM simulation

01Hof

DFT

00Sil

C-C bond length [Å]

mol. des. 150→190 K

mol. des. 150→210 K mol. des. 160→270 K (chemis.) peaks at 183, 202, 234, 252 K mol. des. 220→460 K (chemis), peaks at 311, 369, 422 K

single dimer (metastable) and tight bridge (stable) tilted bridge

1.47/1.50

1.89/1.98

1.47-1.57, 1.35 1.49-1.59 1.34-1.36

1.87-2.01 tetra-sigma bonded (2 double bonds)

3.8.7 Cyclic hydrocarbons

MgO(100) / Mo(100) Ge(100)(2×1)

adsorption at 90 K

shortest Csubstrate bond length [Å]

comment

adsortption site / symmetry/ orientation

321

Desorption temperature

adsortption site / symmetry/ orientation

C-C bond length [Å]

Si(100) – (2×1)

0.25 ML 30 K, 90 K

mol. des. 150→210 K (physisorbed) 432, 501 K (chemis.)

1.35, 1.51

Si(100) – (2×1)

diff. coverages at 300 K

C2v symm. “butterfly” stucture, flat-lying molecule on top of dimer rows

Si(100) – (2×1)

saturation at 300 K

Si(100) – (2×1)

100 K

Si(100) – (2×1)

Landolt-Börnstein New Series III/42A4

Si(100) – (2×1) Si(100) – (2×1)

on top of dimer rows (metastable) two different bridging sites (stable) 3 ads. sites: -1,4 single dimer (ontop of dimer rows) - tight bridge - twisted bridge

shortest Csubstrate bond length [Å] 1.97

comment

method

Refs.

di-sigma bonded 1,4-cyclo-hexadiene like molecule

ARUPS HREELS LCGTO-DF

00Sta 98Bir 98Gok

bonding to Si-Si dimer STM dangling bonds, saturation at 0.25 ML dimer – bridge conversion STM activated (0.94 eV), energy RAIRS difference > 0.14 eV

1.47-1.49, 1.35 1.47-1.53, 1.35

1.86-1.89

98Lop1 98Lop2 98Bor

STM MO cluster calc.

98Wol

TPD

98Kon

cluster PM3

95Jeo

SLABMINDO

93Cra

1.87-1.89

mol. des. 450→505 K hydr. des. 780 K pedestal site (bridging) pedestal site

98Sel

1.45, 1.51

1.98

1.33, 1.45-1.47

1.83

one molecule interacts with 2 Si dimers tilt by 24º w/r to surface

3.8.7 Cyclic hydrocarbons

superstructure (coverage, ML) exp. temperature

322

surface

Landolt-Börnstein New Series III/42A4

superstructure (coverage, ML) exp. temperature

Si(100) – (2×1)

mol. des. C2v symm. di-sigma bonded, 145, 153 K (physisorbed) “butterfly” 460, 505 K (chemis.) no hydr. des. adsorption at 110 K mol. des. 130→160 K (physisorbed) 342→363 K (chemis.) no hydr. des. adsorption at 90 K mol. des. 140→160 K (physisorbed) ~320K (chemis.) no hydr. des. adsorption to mol. des. saturation at 300 K 330→450 K (peak at 350 K) mol. des. c(2×2) (0.5 ML) 200→260 K c(4×3) (multilayers) ads. at 113 K

Si(111) – (7×7)

Si(111) – (7×7)

Si(111)

ZnO(1010)

Desorption temperature

adsortption site / symmetry/ orientation

C-C bond length [Å]

shortest Csubstrate bond length [Å]

comment

method

Refs.

HREELS TPD

91Tag1

HREELS TPD

99Cao

HREELS TPD

91Tag2

high temperature mol. des. states due to defect sites

TPD

91Mac

oblique position of low symmetry, Edes = 73-56 kJ/mol

LEED ARUPS TPD

81Pos

adsorption at 90 K and 300 K

di-sigma bonded 1,4-cyclo-hexadiene like molecule, desorption energy of physis. layer 92, 96 kJ/mol

3.8.7 Cyclic hydrocarbons

surface

323

324

Table 1b. Electronic Structure of adsorbed Benzene Binding energies given in eV with respect to Fermi level EF unless indicated otherwise. Some of the values have been extracted graphically from diagrams in the relevant publications. Metal surfaces surface

superstructure (coverage, ML) expt. temp. gas phase multilayer, 120 K multilayer, 100 K

(Mo(100)) (Fe/W(110)) Ag(110)

Ag(111) Ag(111)

1e2u (π*)

1e1g (π)

2e2g (σ)

1a2u (π)

2e1u (σ)

1b2u (σ)

1b1u (σ)

2a1g (σ)

1e2g (σ)

1e1u (σ)

1a1g (σ)

(4.2)

9.2

11.5

12.3

13.9

14.8

15.5

16.9

19.2

22.5

25.9

11.8

13.7

13.7

multilayer, 100 K

8.8

11.4

11.4

14.0

14.0

multilayer, 80 K Θr = 0.8, 90 K

8.3

10.5

11.1

12.9

13.5

Landolt-Börnstein New Series III/42A4

2 ± 1 ML 80 K

Au(111) Co/W(110)

120 K saturation at 200 K saturation at 300 K

method

Refs.

with resp. to Evac

87Car 98Wei1

11.8

p(3×3) 0.11 ML at 120 K 0.4 L 140 K

comments

285.2 9.0

Au(110)

Co/W(110)

unocc. states, NEXAFS resonances

15.1

11.2

13.2

13.9

98Gun

maximum ring tilt 10 - 15°

NEXAFS

90Liu

ring parallel to surface

ARUPS with resp. to Evac HREELS

90Dud

IPES

86Fra

NEXAFS A/SRUPS

98Wei1 95Get

A/SRUPS

95Get

16.4 14.2

15.8

285.5 π* e2u 289.1 π* b2g 294.1 σ* (C-C) 301.0 σ* (C-C) 8.8

MIES with resp. to Evac UPS (HeI) with resp. to Evac ARUPS

16.4

14.7

16.0

18.0

21.8

25.1

C3v σ(d) symmetry, ring parallel to surface

4.95 (3E1u ←1A1g) 6.35 (1B1u ←1A1g) 7.0 (1E1u ←1A1g) e2u 4.8, b2g 8.3 above EF 285.1 π* e2u 4.7

6.1

7.8

8.1

8.7

4.5

5.9

7.6

7.9

8.6

ring parallel to surface ring parallel to surface

98Gun 95Get

81Avo1

3.8.7 Cyclic hydrocarbons

(not specified) (Mo(100))

C1s binding energy (XPS)

Landolt-Börnstein New Series III/42A4

surface

Cu(110)

Cu(110) Cu(111)

superstructure (coverage, ML) expt. temp. 3 L at 80 K

C1s binding energy (XPS) 284.9

unocc. states, NEXAFS resonances

1e2u (π*)

1e1g (π)

2e2g (σ)

1a2u (π)

2e1u (σ)

284.9 π* e2u

00.4

4.4

7.1

7.9

9.3

4.6

7.1

8.3 – 10.3

saturation (0.22 ML) at 160 K Θr = 0.5, 95 K Θr = 0.6, 95 K

0.3

Cu(111)

90 K

0 - 0.25

Cu(111)

5.4

Θr = 0.1 - 3, 85 K, 120 K

image states at 3.30 / 3.45eV above EF

Cu(111)

monolayer, 90 K

Cu(111)

multilayer, 90 K

284.8 π* e2u 288.5 π* b2g 293.5 σ* C-C 301.5 σ* C-C 285.0 π* e2u 287.2 σ* C-H 288.8 π* b2g 293.3 σ* C-C 300.3 σ* C-C

7.0

8.4

2a1g (σ)

1e2g (σ)

1e1u (σ)

11.7

13.7

17.5

11.8

9.2

10.1

11.5

13.6

1a1g (σ)

comments

method

Refs.

planar ring parallel top surface, H bent upwards

XES, XPS, NEXAFS DFT

00Nil 98Pet 98Wei3 97Nil

tilt of ring greater than 20°

ARUPS

96Lom

electronic states at −0.23 (bonding) and +1.0 eV (antibonding) electronic states at −0.3 (bonding benzene copper) and +3.8 eV (image potential state) dispersion of the hybridization state, m* = 0.9 me no azimuthal orientation, ring parallel to surface image states at +3.30, +3.45 eV above EF, attributed to 1st and 2nd layer 1st layer parallel to surface

2PPE

00Mun 98Mun

2PPE

99Mun1

ARPES (Laser)

99Mun2

ARUPS

00Kos2

2PPE

98Vel

NEXAFS

94Xi

NEXAFS

94Xi

1st layer parallel, 2nd layer tilted

325

Cu(111)

6.4

1b1u (σ)

3.8.7 Cyclic hydrocarbons

Cu(111)

1b2u (σ)

1e1g (π)

2e2g (σ)

1a2u (π)

2e1u (σ)

1b2u (σ)

1b1u (σ)

2a1g (σ)

1e2g (σ)

1 ML Cu/ Ni(111)

0.13 ML, 80 K

3.9

6.1

6.6

8.5

9.0

9.8

11.3

13.4

1 ML Cu/ 1 ML Ni/ Cu(111) (sublayer)

none (0.10 ML) 80 K

5.4

6.4

7.0

8.4

9.2

10.1

11.5

13.6

1 ML Cu/ Ru(0001)

0.14 ML, 80 K

5.2

6.5

7.9

8.8

9.4

10.3

11.7

13.8

Fe / W(110) Fe / W(110) Ir(111)

saturation at 200 K saturation at 300 K p(3×3) (0.11 ML) 30 L at 300 K

4.8

6.2

7.8

8.1

8.7

9.9

4.6

6.0

7.6

8.0

8.4

5

6-7

Ir(111)

20 - 30 L at 300 K

Mo(110)

chemisorbed layer (multilayer annealed to 200 K)

Cu(111)

C1s binding energy (XPS)

284.1

unocc. states, NEXAFS resonances

Landolt-Börnstein New Series III/42A4

7 π* ←π (weak) 4–5 π* ←d 286.7 π* 293.0 σ* 299.2 σ*

1e2u (π*)

e2u 4.6 above EF 4.0

6.1

6.8

8.5

9.0

9.8

11.3

13.4

7.5 - 9.2

11.3

1e1u (σ)

1a1g (σ)

comments

ring parallel to surface, no azimuthal ordering ring parallel to surface, no azimuthal ordering, small tilt at high coverage no azimuthal orientation, ring parallel to surface, binding energies like Cu(111) ring parallel to surface, σ(d) orientation ring parallel to surface ring parallel to surface C3v σ(d) symmetry, ring parallel to surface 4-5 eV is charge transfer loss, ring parallel to surface ring parallel to surface

method

Refs.

IPES

86Fra

ARUPS

99Kos2

ARUPS

99Kos2

ARUPS

99Kos1

ARUPS

00Kos1 99Kos1

A/SRUPS

95Get

A/SRUPS

95Get

ARUPS

85Mac

ELS

80Net

XPS, NEXAFS

88Liu 90Liu

3.8.7 Cyclic hydrocarbons

1 ML Cu/ Ni(111)

superstructure (coverage, ML) expt. temp. 2 ± 1 ML 110 K 0.08 ML, 80 K

326

surface

Landolt-Börnstein New Series III/42A4

surface

Ni(100) Ni(100) Ni(110)

superstructure (coverage, ML) expt. temp. c(4×4) (0.125 ML) 200 K c(4×4) (0.125 ML) c(4×2) (0.125 ML)

C1s binding energy (XPS) 284.1

unocc. states, NEXAFS resonances

1e2u (π*)

1e1g (π)

2e2g (σ)

1a2u (π)

2e1u (σ)

1b2u (σ)

1b1u (σ)

2a1g (σ)

1e2g (σ)

1e1u (σ)

285.2 π* e2u

1.8

5.1

6.3

7.8

8.7

8.7

8.7

11.2

13.5

17.0

1.7

4.4

6.7

6.9

7.5

7.9

9.3

11.5

15.0

1.2/ 2.5

4.5

4.8 5.1 5.1

7.1

6.6

7.6

8.2

9.3

11.6

15.1

1a1g (σ)

comments

method

Refs.

parallel to surface

XES, XPS, NEXAFS

98Wei3 97Nil

17.6

hollow site

01Mit

17.7

asymm. hollow site π – π * charge transfer satellite at −10 eV C2v symmtry

DFT (GGA) DFT (GGA) ADC(3) ARUPS

91Hub2

ARUPS

91Hub2

XPS ARUPS, NEXAFS

92Hun 91Ram

ARUPS

91Hub2

ARUPS

89Hub

ARUPS

89Hub

DFT (GGA)

01Mit

Ni(110) Ni(110)

Ni(110) Ni(110)

Ni(110)

Ni(111) Ni(111) Ni(111)

4.5 4.7

5.8 6.4

7.3

7.8 8.1

8.7

9.7

10.9

13.2

4.3 4.6

5.5 6.1

7.3

7.5 7.9

8.7

9.8

11.3

13.2

C1 symmtry, 0.9 eV band dispersion of 3a1g

4.3 4.6

5.8 6.4

7.6

8.2 8.4

8.6

9.7

10.6 -11.3

13.0 13.6

coads. w. CO 0.125 ML ben, 0.125 ML CO 95 K disordered (0.06ML) 95K

4.2 4.5

5.4 6.0

7.1

7.8 8.0

8.5

9.5

10.8

13.0

2a1g band dispersion of 0.75 eV, C2v symmetry, ring parallel to surface (±20º) C2v symmetry

4.8

5.8 6.4

8.3

8.5

11.1

13.3

disordered (0.105 ML) 95 K (√7×√7)R19.1° (0.143 ML)

4.9

5.9 6.5

7.8

8.2

8.6

11.1

13.3

2.1

4.5

7.0

7.0

7.6

8.1

9.6

11.8

15.1

17.8

2.3

4.7

4.9 5.4 5.3

σ(v) orientation, ring parallel to surface σ(v) orientation, ring parallel to surface bridge site

7.1

7.3

7.8

8.1

9.7

11.8

15.3

18.0

fcc hollow site

284.5 c(4×2) (0.125 ML) 170-270 K

285.1 π* e2u 287.9 / 289.3 π* b2g

97Ohn

3.8.7 Cyclic hydrocarbons

Ni(110)

0.10 ML (dilute), 95 K c(4×2) (0.14 ML) 95 K

01Mit

327

Ni(111) Ni(111) Ni(111) Ni(111)

Ni(111)

Ni(111)

1 ML Ni/ Cu(111) (adlayer) Os(0001) Landolt-Börnstein New Series III/42A4

(√7×√7)R19° (0.14 ML) 15 L at 200K

C1s binding energy (XPS)

unocc. states, NEXAFS resonances

1e2u (π*)

1e1g (π)

2e2g (σ)

1a2u (π)

2e1u (σ)

1b2u (σ)

1b1u (σ)

2a1g (σ)

1e2g (σ)

5.1

6.2 6.8

8.1

8.3

8.7

10.1

11.3

13.3

285.2 π* e2u 289.5 π* b2g 294 σ*

284.0

10.5

11.7

12.1

14.2

5.0

5.8 6.3

7.6

8.1

4.7

5.7 6.4

8.1

4.7

5.7 6.2

8.1

4.8

5.5 6.1

8.0

5.2

6.2 6.7

4.5 - 5.5

15.0

8.4

6.5 6.8

7.8 - 8.7

8.8

1a1g (σ)

comments

method

Refs.

σ(d) orientation, ring parallel to surface values extracted from diagram

ARUPS

89Hub

NEXAFS

91Ami

17.0

19.0

w/r to Evac

UPS

74Dem

10.9

13.1

σ(v) orientation, ring parallel to surface

ARUPS

91Hub1 91Hub3

11.0

13.2

σ(v) orientation, ring parallel to surface

ARUPS

98Hel 91Hub3 90Ste 89Hub

10.9

13.1

σ(v) orientation, ring parallel to surface

9.5

10.8

13.0

σ(d) orientation, ring parallel to surface

ARUPS

91Hub1 91Hub3 90Ste

10.1

11.4

13.4

ARUPS

99Kos1

11.1

13.1

σ(v) orientation, ring parallel to surface C3v σ(d) symmetry, ring parallel to surface C-H derived band

ARUPS

90Gra1 90Gra2 87Net1

15.4

8.3

8.0

1e1u (σ)

91Hub1 91Hub3

3.8.7 Cyclic hydrocarbons

Ni(111)

superstructure (coverage, ML) expt. temp. (√7×√7)R19.1° (0.143 ML) 95 K condensed layer at 120 K chemisorbed layer coadsorbed w. O, 95 K: (2√3×2√3)R30° (0.08 ML ben, 0.17 ML O) coadsorbed w. CO, 95 K: (2√3×2√3)R30° (0.08 ML ben, 0.17 ML CO) coadsorbed w. NO, 95 K: (2√3×2√3)R30° (0.08 ML ben, 0.17 ML NO) coadsorbed w. NO, 95 K: p(3×3) (0.11 ML ben, 0.22 ML NO) 0.14 ML 80 K

328

surface

Landolt-Börnstein New Series III/42A4

surface

Pd(110)

Pd(110)

superstructure (coverage, ML) expt. temp. c(2×4) 320 K

C1s binding energy (XPS)

unocc. states, NEXAFS resonances

1e2u (π*)

1e1g (π)

2e2g (σ)

1a2u (π)

4.5

saturation at 300 K (3 – 5 L) saturation at 300 K,

5.0

Pt(100)

saturation at 333 K (0.22 ML)

4.2

6.0

8.2

Pt(110)

p(4×2) (0.25ML), anneal to 275 K expt at 95 K

4.3

6.2

8.5

Pt(111)

saturation at 230 K saturation at 300 K, expt. at 95 K

Pd(111)

Pd(111)

Pt(100)

Pt(111)

Pt(111)

saturation at 300 K

5.9

66.5

4.3

285.0 π* e2u 288.8 π* b2g 290.2 σ* C-H 293.4 σ* C-C 299/ 304 shape resonance 284.2 π* e2u 285.1 π* e2u

6.9

8.0

1b2u (σ)

8.8

1b1u (σ)

9.4

8.0 - 8.6

7.5

2a1g (σ)

10.9

11.3

8.4

1e2g (σ)

13.0

13.4

10.7

10.0

10.7

13.2

11.1

13.5

1e1u (σ)

1a1g (σ)

comments

method

antibonding state (1e1g and Pd 4d) near EF

STM, MO 97Yos calculations, MAES, UPS ARUPS 88Net4

molecules tilted by 10 - 20° circular dichroism of all orbitals; highest asymm. in 2a1g and 1e1g C6v, ring parallel to surface,

UPS

91Wes

ARUPS, ELS

83Net1

ring parallel to surface, clean surface reconstruction is lifted no band assignment made in the orig. publication molecules tilted by 27 / 30°

ARUPS

82Ric

UPS

77Fis

ARUPS, NEXAFS

98Zeb

NEXAFS

98Wei1

HREELS

96Dip

negative ion resonance due to b2g (π*) at 2.1 eV 5.0

6.0

13.3

Refs.

3.8.7 Cyclic hydrocarbons

c(2×4) prep. at 300 K, expt. at 80 K saturation at 300 K, multilayer at 140 K

2e1u (σ)

87Som1 87Som2

329

C1s binding energy (XPS) 284.4

Pt(111)

multilayer (“solid”) at 100 K

284.9

Rh(111)

saturation at 230 K

Rh(111)

(√19×√19) R23.4° – 3C6H6 c(2√3×4)rect (0.13 ML) 4-5 L at 300K

Pt(111)

Rh(111)

Rh(111)

Ru(0001) Ru(0001) Ru(0001)

p(3×3) coads. w. CO at 300 K (0.11 ML benz, 0.22 ML CO) p(√7×√7)R19° (0.14 ML), 80 K saturation at 230 K

Landolt-Börnstein New Series III/42A4

saturated chemisorbed layer (0.16 ML), 90 K

unocc. states, NEXAFS resonances

1e2u (π*)

1e1g (π)

2e2g (σ)

1a2u (π)

2e1u (σ)

1b2u (σ)

1b1u (σ)

2a1g (σ)

1e2g (σ)

286.0 π* e2u 293.7 σ* e1u 299.9 σ* e2g+a2g 285.0 π* e2u 288.9 π* b2g 293.3 σ* e1u 300.1 σ* e2g+a2g 284.8 π* e2u 285.8 π* e2u

4.9

5.8 6.3

7-9

11.0

13.1

4.9

5.6 6.1

7-9

11.2

12.9

5.1

6.1

7.6

11.2

13.4

8.5

9.0

9.9

284.5 π* e2u 286.1 π* e2u 4.9

6.2

7.8

8.3

8.8

10.0

11.2

13.4

1e1u (σ)

1a1g (σ)

comments

method

Refs.

ring parallel to surface

NEXAFS

85Hor

NEXAFS

85Hor

NEXAFS

98Wei1

ARUPS

91Neu

ARUPS

87Net2 85Neu

ARUPS

86Ber

ARUPS

00Kos1

NEXAFS

98Wei1

out of plane bending of C-H bonds 2a1g derived band width 0.5eV C6v adsorbate symmetry, σ(d) orientation ring parallel to surface C6v adsorbate symmetry, ring parallel to surface σ(v) orientation, ring parallel to surface out of plane bending of C-H bonds

89Hei

3.8.7 Cyclic hydrocarbons

superstructure (coverage, ML) expt. temp. saturation at 200 K (“monolayer”)

330

surface

Landolt-Börnstein New Series III/42A4

surface

Ru(0001)

Ru(0001)

W(110)

superstructure (coverage, ML) expt. temp. coadsorbed w. CO: (√13×√13)R14° (0.08 ML ben, 0.33 ML CO) coadsorbed w. NO: p(3×3) (0.11 ML ben, 0.22 ML NO)

C1s binding energy (XPS)

284.3

unocc. states, NEXAFS resonances

1e2u (π*)

1e1g (π)

2e2g (σ)

1a2u (π)

2e1u (σ)

1b2u (σ)

1b1u (σ)

2a1g (σ)

1e2g (σ)

4.7

6.0

7.8

7.9

8.8

9.8

11.2

4.7

5.6

7.8

8.0

8.8

10.0

10.9

1e1u (σ)

1a1g (σ)

comments

method

Refs.

13.2

shifts in CO 4σ and 5σ levels

ARUPS

90Ste 89Hei

13.2

shifts in NO 5σ level, NO on bridge sites

ARUPS

90Ste 89Hei

XPS

96Whi

3.8.7 Cyclic hydrocarbons 331

332

Semiconductor / insulator surfaces surface

Al2O3(111) / Mo(110)

superstructur e (coverage, ML) expt. temp. ca 1 monolayer (0.03-0.04 L) at 90 K

ca 1 ML (0.03-0.04 L) at 90 K

Ge(100)(2×1) Ge(100)(2×1)

physisorbed 90 K 0.4 ML chemisorbed, 90 K

Ge / Si(100)(2×1)

1.0 ML chemisorbed, 90 K

Si(100)(2×1)

(2×1) (0.25 ML) 30 K (2×1) (0.25 ML) 30 K

Si(100)(2×1)

unocc. states, NEXAFS resonances

1e2u (π)

1e1g (π)

2e2g (σ)

1a2u (π)

2e1u (σ)

1b2u (σ)

1b1u (σ)

2a1g

1e2g

1e1u

1a1g

3.94 (3B1u ←1A1g) 4.84 (1B2u ←1A1g) 6.19 (1B1u ←1A1g) 6.86 (1E1u ←1A1g) 3.94 (3B1u ←1A1g) 4.79 (1B2u ←1A1g) 6.20 (1B1u ←1A1g) 6.84 (1E1u ←1A1g) 3.9

5.8

6.9

8.1

8.8

9.9

11.3

13.4

17.1

20.3

2.3

4.0 8.4

5.7 6.5

8.9

7.9 8.4

8.9

10.2

11.2

12.9 14.1

17.0 17.1

19.9

2.3

4.0 8.4

5.7 6.5

8.9

7.9 8.4

8.9

10.2

11.2

12.9 14.1

17.0 17.1

19.9

11.1 11.8

11.8

13.5 14.3

16.6

16.1

16.9

18.3 19.7

22.5

26.1

3b1g 5.7 6ag 6.5

5b3u

4b2u 1b2g 8.4

1b1u 3b2u 8.9

4b3u

5ag

2b1g

4ag

10.2

11.2

12.9

14.1

2b1u

1b3g

2.3

4.0

7.9

comments

method

Refs.

vibronic fine structure (110 meV spacing) observed in the multilayer range

HREELS

96Str

vibronic fine structure (110 meV spacing) observed in the multilayer range

HREELS

96Str

ARUPS

01Fin

ARUPS

01Fin

ARUPS

01Fin

ARUPS DFT cluster calc. ARUPS DFT cluster calc.

98Gok 98Car

C2v symmetry, di-sigma bonded 1,4-cyclohexadiene-like adsorption levels identical to Ge(100), C2v symmetry, di-sigma bonded 1,4-cyclohexadiene-like adsorption w/r to Evac C2v symmetry,

98Gok

3.8.7 Cyclic hydrocarbons

Landolt-Börnstein New Series III/42A4

MgO(100) / Mo(100)

C1s binding energy (XPS)

Landolt-Börnstein New Series III/42A4

surface

Si(100) – (2×1) Si(100) – (2×1)

superstructur e (coverage, ML) expt. temp. physisorbed layer (1 L at 100K) chemisorbed layer (1 L at 100K)

C1s binding energy (XPS)

unocc. states, NEXAFS resonances

1e2u (π)

1e1g (π)

2e2g (σ)

1a2u (π)

2e1u (σ)

1b2u (σ)

1b1u (σ)

2a1g

1e2g

Si(111) – (2×1) cleaved Si(111) – (2×1) cleaved TiO2(100)

285.0 / 285.9 π* e2u 287.7 σ* C-H 289.5 σ* C-Si 292.2 σ* C-C 298.6 σ* C=C

two chemisorption structures, tilt 30 / 45°

ZnO(10-10) ZnO(10-10)

8.2

12.2

13.5 14.2

16.4

15.3

16.4

18.3 19.6

11.4

13.8

15.8

8.1

10.7 12.4

13.8

16.6

6.1

8.5

10.8

9.6

11.1 12.2

5 - 100 L at 60 K

6.3

9.0

saturation at 300 K

5.3 6.2 3.5 3.9 2

4.1

5.0

6.4

7.3

8.1

2

4.3

5.0

6.6

7.3

7.9

5 L - 20 L at 120 K c(2×2) (0.5 ML) 150 K c(4×3) (multilayers) 150 K

comments

285.0

9.8

HOMO: -1.2eV LUMO: >3.5eV at low temp physisorbed layer with spectrum identical to gas phase, transition to chemisorbed layer at 75-105 K 22.5 26.0 probably C2v symmetry, 2 surface bonds 19.5 physisorbed and chemisorbed species coexist at low temp. and low coverage ring parallel to surface ring parallel to surface energies extracted from spectrum plot energies extracted from spectrum plot

method

Refs.

NEXAFS

98Kon

NEXAFS

98Kon

STM

98Sel

UPS

00Car

ARUPS (w/r to Evac) UPS/XPS

98Car

UPS/XPS

87Pia

AES, UPS, XPS ARUPS

98 Raz

ARUPS

81 Pos

87Pia

3.8.7 Cyclic hydrocarbons

saturation at 300 K

1a1g

285.0 π* e2u 288.8 π* b2g

Si(100)(2×1) Si(111) – (7×7) Si(111) – (7×7)

1e1u

81Pos

333

334

Table 1c. Vibrational properties of adsorbed benzene Vibrational energies in cm-1, unless indicated otherwise. Main peaks are underlined. Values for deuterated benzene are given in brackets where available. (δ = in plane deformation mode, γ = out of plane deformation mode, ν = stretch mode) Metal surfaces surface

preparation, super-structure (coverage, ML), expt. temp.

low vibrational modes (frustrated rotations, lateral translations)

gas phase

Ag(110)

Ag(111)

Ag(111) Landolt-Börnstein New Series III/42A4

Cu(110)

400 – 1000 γ C-C γ C-H δ C-C ν C-H 404 (351) 607 (579) 675 (497) 707 (599) 846 (663) 969 (789) 991 (830)

1000– 1200 δ C-H ν C-C

1200- 1650 ν C-C

993 (945) 1010 (970) 1035 (812) 1146 (826) 1177 (868)

1309 (1282) 1346 (1055) 1479 (1330) 1595 (1553)

1850

3000 (2300) ν C-H

1st layer (< 1 L), 85 K

(495) (650) (780)

(945)

(495)

(945)

410 675 820

1000 1155

685 - 683

1360 1480 1590

method

Refs.

IR spectr.

88Jak

vibrational modes as finger prints for chemical state C2v symmetry

STS IETS

01Pas

Raman

84Hal

C3v σ(d) symmetry

Raman

84Hal

C3v σ(d) symmetry, ring parallel to surface flat-lying ring

HREELS

3047 - 3062 (2267-2294)

4 meV, 19 meV (chemisorbed) 7 meV, 44 meV (weak ads.) 5 L (sub monolayer) 90 K, only C6D6 5 L (sub monolayer) 90 K, only C6D6 0.4 L 140 K

comments

3030

RAIRS

81Avo1

96Haq

3.8.7 Cyclic hydrocarbons

Ag(110)

C-M

Landolt-Börnstein New Series III/42A4

surface

Cu(110)

preparation, super-structure (coverage, ML), expt. temp. multilayers (2 L), 85K

low vibrational modes (frustrated rotations, lateral translations)

C-M

400 – 1000 γ C-C γ C-H δ C-C ν C-H 677

1000– 1200 δ C-H ν C-C

1200- 1650 ν C-C

1037

1479

0.24 ML, 140 K

685

970

1550

Cu(110)

multilayers 140 K monolayer 1.5 L at 120 K multilayer 25 L at 120 K

680

990 1150

1460 1550

Cu(111) Cu(111) Cu(111) Ni(100)

Ni(110) Ni(110) Ni(111)

Ni(111)

multilayer at 140 K c(4×4) 3L at 300 K

c(2×4) (0.25 ML) 300 K c(2×4) (0.25 ML) 300 K (2√3×2√3)R30° 3 L at 300 K

2.0 L at 280 K

3000 (2300) ν C-H

comments

method

Refs.

3042 3066 3084 3050

2nd layer molecules tilted tilt of ring greater than 20º

RAIRS

96Haq

HREELS

96Lom

HREELS

96Lom

ring parallel

HREELS

94Xi

ring tilted

HREELS

94Xi

HREELS

88Pat

HREELS

79Ber 77Ber

HREELS

92Hun

HREELS

81Ber

HREELS

79Ber 77Ber

HREELS

78Leh

3050

675 (500)

360

410 680 (500) 845 (680) 685 845 750 (540) 845 (645) 845 (820)

1000 (960) 1160 (845) 1000 1165 1120 (820)

3060 (2300) 1480 1595 1330 (1225) 1430 (1370)

3060 3025 (2260)

743 879 700 845

1110

1420

3020

290

745 (540) 845 (645) 845 (820)

1110 (820)

1320 (1225) 1420 (1360)

3020 (2250)

320

720 830

1130

1430

3000

ring parallel to surface, frequencies very similar to Ni(111) ring parallel to surface

ring parallel to surface, frequencies very similar to Ni(100) ring parallel to surface

3.8.7 Cyclic hydrocarbons

Cu(110)

1850

335

Os(0001)

preparation, super-structure (coverage, ML), expt. temp. saturation at 273 K

low vibrational modes (frustrated rotations, lateral translations)

C-M

450 500

400 – 1000 γ C-C γ C-H δ C-C ν C-H 790

1000– 1200 δ C-H ν C-C

1200- 1650 ν C-C

1050

720 (520) 870 (675)

1115 (830)

comments

method

Refs.

1355

2900

HREELS

90Gra

1320 (1220) 1425 (1370)

3010 (2240)

phenyl and benzyne as precursors for dissociation ring parallel to surface, Cs adsorption site ring parallel to surface, weak surface bond ring parallel to surface, Cs adsorption site ring parallel to surface

HREELS

85Wad

HREELS

87Gra

HREELS

85Wad

HREELS

87Sur

RAIRS

96Haq

RAIRS

96Haq

HREELS (specular) HREELS

96Lut2

RAIRS

86Sch

Pd(100)

saturation at 300 K

Pd(111)

saturation at 180 K

Pd(111)

saturation at 300 K

265 (270)

720 (525) 810 (640)

1100 (830)

1410 (1355)

2990 (2230)

Pt(110)

300 K, diff. coverages

340 (340)

1120 (800)

1335 (1220) 1435 (1370)

3025 (2260)

Pt(111)

chemisorbed layer at 300 K

565 830 (665) 910 (800) 814-810 829-827 900-891

Pt(111)

multilayer (2 L) at 90 K

1037

Pt(111)

chemisorbed layer at 125 K multilayer at 125 K multilayer at 100 K

684 820 831 825 685

1030

684

1032

Pt(111) Pt(111)

280 (285) 435 (400)

695 (505)

3050 (2280)

ring parallel to surface, up to three different adsorption sites ring tilted in 2nd layer

1478 1480

2990

1480

3050

ring parallel to surface ring tilted

96Lut2

3.8.7 Cyclic hydrocarbons

Landolt-Börnstein New Series III/42A4

3000 (2300) ν C-H

1850

336

surface

Landolt-Börnstein New Series III/42A4

surface

Pt(111) Pt(111)

Rh(111)

preparation, super-structure (coverage, ML), expt. temp. 0.58/0.30 L at 200 K sub monolayer at 300 K, saturation at 2-3L c(2√3×3)rect saturation at 290 K

Rh(111)

c(2√3×4)rect (0.12 ML) coads w. CO p(3×3) (0.11 ML) coads w. CO c(2√3×4)rect coads w. CO 300 K (2√3×2√3)R30° (< 0.07) 115 K

Rh(111) Rh(111)

Ru(0001)

C-M

365 (345) 360 (350)

400 – 1000 γ C-C γ C-H δ C-C ν C-H 550 840 (600) 920 (715) 570 830 (610) 920 (700)

1000– 1200 δ C-H ν C-C

1200- 1650 ν C-C

1145 (820)

1405

3005 (2250)

1130 (800)

1420 (1350)

3000 (2240)

1850

3000 (2300) ν C-H

comments

ring parallel to surface

13.1 meV (frustrated lateral transl.) 345 (330)

550 (550) 810 (565) 880 (835)

1130 (835)

1320 (1320) 1420 (1365)

2988 (2250)

12.9 meV (frustrated lateral transl.) 12.7 meV (frustrated lateral transl.)

570 750 (540) 860 (830) 860 (685) 960 (910)

1010 (790) 1110 (790) 1110 (830)

1260 (1210) 1320 1410 (1360)

3020 (2160) (2250)

Refs.

HREELS

85Abo

HREELS

78Leh

HAS

93Wit

C3v σ(d) HREELS symmetry, ring parallel to surface, decomposition starts above 400 K. HAS

ring parallel to surface 290 (270)

method

parallel to surface

86Koe 84Koe 83Koe

93Wit

HAS

93Wit

HREELS, LEED, TPD HREELS, TPD

87Van

3.8.7 Cyclic hydrocarbons

Rh(111)

low vibrational modes (frustrated rotations, lateral translations)

88Jak

337

Ru(0001)

preparation, super-structure (coverage, ML), expt. temp. multilayer (α1, α2, α3) 115 K

low vibrational modes (frustrated rotations, lateral translations)

C-M

240 (250)

400 – 1000 γ C-C γ C-H δ C-C ν C-H 430 610 (580) 690 (510) 700 (580) 850 980 (840) 980 (940)

1200- 1650 ν C-C

1040 (820) 1170 (820) 1180 (840)

1345 1480 (1335) 1600 (1550)

1311 1348 1400 1470 1478 1540 1586 1250 1312 1403 1415 1469 1474 1479 1550 1565 1360 1440 (1385)

Landolt-Börnstein New Series III/42A4

Ru(0001)

multilayer (α2: glassy phase) 53 K

687 703 855 974

1010 1036 1147 1175

Ru(0001)

multilayers (α2: crystalline phase) 120 K

689 707 974 978 987

1009 1032 1034 1039 1142 1148

Ru(0001)

p(3×3) coadsorbed w. 0.16 - 0.22 ML O (0.11 ML Benz) at 120 K

650 785 (555) 880 (850) 890 975 (825) 975 (900)

975 (760) 1125 (825)

280 (270)

1850

3000 (2300) ν C-H

comments

method

Refs.

3045- 3055 (2275-2290)

α1-layer essentially parallel, metastable, α2- layer strongly tilted, α3-layer bulk-like α2-α3 phase transition is irreversible, temperature depends on layer thickness α2-α3 phase transition is irreversible, temperature depends on layer thickness

HREELS

89Jak

RAIRS,

96Jak

RAIRS,

96Jak

ν(O-Ru): 510 (505); net charge transfer from oxygen to benzene.

HREELS

90Jak3

1754 1825 1969

3032 3069 3088

1755 1829 1838 1974 1983

3029 3037 3067 3071 3085 3091

3005 (2230)

3.8.7 Cyclic hydrocarbons

1000– 1200 δ C-H ν C-C

338

surface

Landolt-Börnstein New Series III/42A4

surface

Ru(0001)

preparation, super-structure (coverage, ML), expt. temp. (√13×√13)R14° coadsorbed w. 0.30 ML CO (0.08 ML) formed at 250 K, exp at 120 K

low vibrational modes (frustrated rotations, lateral translations)

C-M

280 (270)

400 – 1000 γ C-C γ C-H δ C-C ν C-H 565 (545) 770 (550) 870 880 (840) 940 (920) 1000 (830) 1000 (840)

1200- 1650 ν C-C

1000 (740) 1120 (830) 1270

1360 1430 (1365) 1530 (1480)

1000– 1200 δ C-H ν C-C

1200- 1650 ν C-C

1158

1468 1564

3068

1100 1230

1520

2880 3010

1050 1160 1286 1378 1042 (754) 1152 (853)

1623

2935 3050

1648 (1610)

3000 (2216) 3090 (2304)

1850

3000 (2300) ν C-H

comments

method

Refs.

3030 (2260)

ν(C-O): 1960 (1960) ν(CO-Ru): 450 (465) attractive interaction between benzene and CO due to indirect charge transfer

HREELS

90Jak2

3000 (2300) ν C-H

comments

method

Refs.

HREELS

96Str

DFT, CarParrinello approach HREELS, LCGTODF cluster calculation LCGTODF cluster calculation

00Sil

Semiconductor / insulator surfaces surface

preparation, super-structure (coverage, ML), expt. temp. MgO(100 ) multilayer / Mo(100) 0.3 L at 90 K

low vibrational modes (frustrated rotations, lateral translations)

C-S

400 – 1000 γ C-C γ C-H δ C-C ν C-H 406 679 841 989 900

Si(100) – (2×1)

p(√8×√8) R45º supercell

Si(100) – (2×1)

0.25 ML, 90 K

315 538 604

776 876 949

Si(100) – (2×1)

0.25 ML

528 (493) 593 (552)

774 (650) 873 (885)

1850

adsorption in tight bridge site geometry

3.8.7 Cyclic hydrocarbons

1000– 1200 δ C-H ν C-C

00Sta

98Bir

339

Si(100) – (2×1)

chemisorbed at 300 K

Si(100) – (2×1)

chemisorbed annealed to 350 K physisorbed , 2 L at 90 K

Si(100) – (2×1) Si(100) – (2×1) Si(111)(7×7) Si(111)(7×7) Si(111)(7×7) Landolt-Börnstein New Series III/42A4

Si(111)(7×7)

low vibrational modes (frustrated rotations, lateral translations)

C-S

615 (550)

400 – 1000 γ C-C γ C-H δ C-C ν C-H 790 (660) 910 (860)

1000– 1200 δ C-H ν C-C

1200- 1650 ν C-C

1075 (785) 1170 (860)

1635 (1595)

1850

3000 (2300) ν C-H

2935 (2190) 3065 (2300) 3030 3036 3067 3086 2945 3044 2899 3044

c(2×4) chemisorbed at 300 K, expt. at 90 K physisorbed , 1.0 L at 110 K

615 (550)

chemisorbed 0.5 L ann to 300 K, expt. at 110 K physisorbed , 1.5 L at 90 K

540

chemisorbed at 300 K, expt. at 90 K

540 (510)

425 (365) 695 (525) 860 (685) 790 (550) 910 (660)

1005 (850) 1040 (850) 1180 (850) 1075 (785) 1170 (860) 1170 (1080)

1500 (1340) 1610 (1585)

3075 (2300)

1635 (1595)

2935 (2190) 3065 (2300)

407 678 851 974 785 877 951

1021 1164

1332 1483 1587

3060

1068 1157

1299 1582 1635

2920 3025

1010 (840) 1040 (840)

1355 1490 (1345) 1620 (1580) 1440

3070 (2295)

420 (365) 695 (520) 865 (670) 790 (630) 965 (930)

1075 (805) 1190 (930)

2985 (2195) 3050 (2260)

comments

cyclohexadienelike structure cyclohexadienelike structure

di-σ bonded to two adjacent Si atoms (SB)

di-σ bonded to two adjacent Si atoms

π-bonded, ring parallel to surface

method

Refs.

HREELS

98Bir

RAIRS

98Kon

RAIRS

98Kon

RAIRS

98Kon

HREELS

91Tag1

HREELS

91Tag1

HREELS

99Cao

HREELS

99Cao

HREELS

91Tag1 91Tag2

HREELS

91Tag1 91Tag2

3.8.7 Cyclic hydrocarbons

Si(100) – (2×1) Si(100) – (2×1)

preparation, super-structure (coverage, ML), expt. temp. (2×1) (0.25 ML) physisorbed at 100 K

340

surface

Landolt-Börnstein New Series III/42A4

surface

Si(111) – (2×1) cleaved Si(111) – (2×1) cleaved Si(111) – (2×1) cleaved

preparation, super-structure (coverage, ML), expt. temp. 0.5 – 25 L at 85 K

low vibrational modes (frustrated rotations, lateral translations)

C-S

30 - 75 L at 85 K saturation at 300 K

540 (500)

1000– 1200 δ C-H ν C-C

1200- 1650 ν C-C

1050

700 780 (500)

400 – 1000 γ C-C γ C-H δ C-C ν C-H 700

comments

method

Refs.

1570

3050

C3v symmetry

HREELS

86Pia

1050 1170

1570

3050

Cs symmetry

HREELS

86Pia

1060 1250 (1170)

1610 (1540)

3060 (2190)

ring tilted, breakage of C-H bond and formation of C-Si bond.

HREELS

84Pia

3.8.7 Cyclic hydrocarbons

3000 (2300) ν C-H

1850

341

342

Table 2. Cyclohexane (c-C6H12) Tmax [K]

Heating Soft mode rate [K/s] [cm-1]

Structure

Dehydrogenation upon heating

Ni(111) 2720 2

(√7x√7)R19.1°

2730

No No

CS C1, Slightly inclined

C3v

To benzene above 220 K

RAIRS, LEED RAIRS TPD Cluster Calc. HREELS

Not up to 340 K

TPD

No Not up to 470 K

TPD TPD Cluster Calc. HREELS TPD, ARUPS

(√7x√7)R19.1°

2620

135-140

195+ 206

2690

No

2 Not up to 380 K

Cu(111)

Cu(100)

178 165+

160+ 181-184

Landolt-Börnstein New Series III/42A4

Cu(110) 181 Cu3Pt(111) 199 Mo(110) 195

3

3 3

C6D12: 205 3

2754 2775

2790 2746 2728 2754 2570+2610 C6H12: 2577

(√7x√7)R19.1°

No

No No No C6H12: 10% dehydrogenation, 90% molecular desorption C6H12: not investigated C6D12: no dehydrogenation

TPD TPD, RAIRS HREELS, RAIRS, LEED, TPD NEXAFS HREELS TPD, RAIRS He-scattering TPD, RAIRS TPD, RAIRS RAIRS TPD, HREELS

Comments

References

Electronic structure: No bonding shifts

74Dem,76Dem

Rate maximum: Depends on coverage Electronic structure: No bonding shifts Coadsorption of O and K Coadsorption of O C6HD11

78Dem, 79Leh 90Hub, 91Zeb1

95Coo 95Rav 82Tsa1, 82Tsa2 84Hof 79Leh 82Tsa1, 82Tsa2

Parallel, C3v

Chair, lower than C3v at low coverage

ΘK=0.34 ML, Cluster growth 91Zeb1 82Tsa1, 82Tsa2 84Hof + no TPD spectra shown 89Rav2 Electronic structure 91Hub3 No bonding shifts 82Tsa1, 82Tsa2 98Tep + no TPD spectra shown 89Rav1+, 89Rav2, 86Che, 93Rav Electronic structure 01Woe, 99Wei + no TPD spectra shown 89Rav2 Edes = 46.4 kJ/mol 98Tep, 96Tep Frustrated translation 95Wit 98Tep 98Tep Coverage dependence 96Wel Coadsorption of S and O 98Tep

3.8.7 Cyclic hydrocarbons

HREELS TPD, LEED, ARUPS

Not up to 340 K

Ni(110)

Conformation, orientation, symmetry

UPS >170 180-192

Ni[5(111) ×(110)] Ni[9(111) ×(111)] K/Ni(111) Ni(100)

Methods

Landolt-Börnstein New Series III/42A4

Tmax

Heating Soft mode rate [K] [K/s] [cm-1] C/Mo(110) C6H12: 2685 C6D12: 203 3 Ru(0001) 227 20 200 200

5-9 10

2520-2600

200

10

2516

Pd(111)

Structure

Disordered

Dehydrogenation upon heating

Methods

C6H12: not investigated C6D12: no dehydrogenation

TPD, HREELS

Little To benzene

2635

Pd(110) Ag(111) W(100) Ir(111) Pt(111)

2630

Chair

C3v

HREELS UPS

Yes

HREELS Chair LEED Decomposition at 300 K UPS, ∆Φ LEED, AES, TPD LEED (√21×√21)R10.9° Some dehydrogenation TPD RAIRS C3v C6H12: to intermediate >200K, HREELS C3v, Parallel to benzene at 300K. C6D12: no dehydrogenation Starting at 195 K LEED, TPD, XPS, To benzene >290 K AES, HREELS Reactivity red. by coads. Bi To benzene TPD, XPS, AES, ∆Φ

Comments

References

(4×4) interstitial carbide overlayer Edes=59 kJ/mol

98Tep

Edes=38-45 kJ/mol Coverage dependence of mode softening O Coadsorption

86Pol 83Hof2, 81Hof

Surface diffusion, Ediff=19 kJ/mol

88Mak

Electronic structure: No bonding shifts also C6D12

(9×9)

~2600 ~2600 236

~7

230

2550

228

~230

TPD, AES, LEED, HREELS

3

TPD, LITD+FTMS Cluster Calc.

Edes =58 kJ/mol Bi Coadsorption

84Hof

86Wad 78Rub 89Rav1, 89Rav2 78Fir 80Bha 76Nie 77Fir 82Tsa1 90Che 93Lan, 78Dem 93Dom, 88Cam, 94Cam, 89Rod, 92Bus 91Dav, 91Ern

Cs coadsorption: Stabilization of cyclohexane Reversal of dipole moment O Coadsorption (SnPt alloy) 94Xu1, 93Xu Reduction adsorption energy Sticking coefficient at 150 K = 1 Radiation damage 92Par, 89Lan, 91Pet 85Kan

343

Starting at 180 K To benzene >280 K To benzene

Adsorption at 300 K Adsorption at 303 K

78Mad

3.8.7 Cyclic hydrocarbons

To benzene at 300 K

ESDIAD, TPD, LEED TPD HREELS,TPD, LEED, UPS HREELS, Cluster Calc. LITD

Conformation, orientation, symmetry

[K]

Heating Soft mode rate [K/s] [cm-1]

Structure

Dehydrogenation upon heating

Cont’d.

Pt[6(111) ×(111)] Pt(100) -(5×20) Pt(100) -(1×1)

Methods

Conformation, orientation, symmetry

NEXAFS, Xα -calculation Molecular Beam

182

No molecular desorption

1.7

2550

No softmode

To benzene (more than on Pt(111)) Molecular desorption dominates, some dehydrogenation to benzene Complete dehydrogenation: - to C6H9 at 220 K - to benzene at 300 K

Comments

References

344

Tmax

86Hit Sticking coefficient at 100 K = 1

TPD, LEED

92Jia 82Tsa2, 74Bar

TPD, HREELS, RAIRS

C3v, chair

94Lam2, 97Lam, 90Mar

TPD, HREELS,RAIRS

Lower than C3v

94Lam2, 97Lam, 90Mar

3.8.7 Cyclic hydrocarbons

Landolt-Börnstein New Series III/42A4

Landolt-Börnstein New Series III/42A4

Table 3. Saturated hydrocarbons other than cyclohexane: Cyclopropane (c-C3H6), Cyclobutane (c-C4H8), Cyclopentane (c-C5H10), Cyclooctane (c-C8H16) Tmax Heating Soft mode DehydroMethods Comments References rate genation [K] [K/s] [cm-1] upon heating Cyclopropane (C3H6) Ni(100) Cu(111)

3

No

Minor No

125

3

No

Cu(110)

125

3

No

No

HREELS, TPD

Ru(0001)

150

10

No

No

145

-

145

-

HREELS, ARUPS, LEED, TPD LEED, TPD, ESDIAD LITD TPD

145

-

No (<20 cm-1 ) Yes

Ir(110)1×2 ~135

Yes

Minor 10

No (<40 cm-1 )

3

2812

No

Edes≈29 kJ/mol Electronic structure

95Son, 96Son 00Woe

Electron beam induced bond scission Edes≈38 kJ/mol Electron beam induced bond scission Edes≈38 kJ / mol

97Mar2, 97Mar3, 98Mar 94Mar, 97Mar1, 97Mar2, 97Mar3, 98Mar, 98Roc 83Fel 82Hof

90 K: Molecular adsorption One C-C bond parallel to surface plane CS symmetry, interaction via σ(CC) orbital non dissociative, no ordered LEED pattern Edes= 33 kJ/mol Surface diffusion, Ediff = 8 kJ /mol 400-1000 K: Dissociation via C-C cleavage Eact = 39.5 kJ/mol (no isotope effect)

78Mad 88Mak 97Jac

TPD, HREELS Molecular beam Etrans < 21 kJ/mol: molecular adsorption Etrans > 21 kJ/mol: dissociative adsorption TPD, UPS, 100 K: Low coverage: LEED, ∆Φ Dissociation upon adsorption 100 K: High coverage: Additional molecular adsorption TPD, LEED 100 K: molecular adsorption

84Hof 96Kel

TPD, HREELS

84Hof

RAIRS

98Tep

82Wit1, 82Wit2, 83Szu, 84Szu 83Szu, 84Szu

345

Ir(111) Cyclobutane (C4H8) Ru(0001) 175 Cyclopentane (C5H10) Cu(111) 161

10-15

HREELS, TPD HF calculation, NEXAFS HREELS, TPD

3.8.7 Cyclic hydrocarbons

123

Cu(110) Ru(0001)

Ir(111) Pt(111)

[K] 166 180

Heating Soft mode rate [K/s] [cm-1] 3 2732 -

180

-

2610

200

1

2690

Dehydrogenation upon heating No

No Partly to C5H6

Methods RAIRS LITD TPD, HREELS LITD TPD, HREELS TPD, HREELS, ∆Φ

~2700

250

-

157 - 270

10

No (<40 cm-1)

Edes = 43 kJ/mol Surface diffusion, Ediff =13.8 kJ/mol

90 K: molecular adsorption Edes = 60 kJ/mol

References 98Tep 88Mak 83Hof1, 84Hof 90Are 93Ave 85Ave, 93Ave 90Che

LEED, TPD, ESDIAD TPD, HREELS

Coverage dependent desorption temperature 78Mad 84Hof

3.8.7 Cyclic hydrocarbons

Cyclooctane (C8H16) Ru(0001) ~260

Comments

346

Tmax

Landolt-Börnstein New Series III/42A4

Landolt-Börnstein New Series III/42A4

3.8.7 Cyclic hydrocarbons 347

Table 4. Non-saturated hydrocarbons other than benzene – Cyclopentene (c-C5H8), Cyclopentadiene (c-C5H6), Cyclohexene (c-C6H10), Cyclohexadiene (c-C6H8), Cyclooctadiene (c-C8H12), Cyclooctatraene (c-C8H8) Methods Thermal evolution Comments References Cyclopentene (c-C5H8) Mo(110) HREELS, Decomposition upon heating 96Fru NEXAFS, TPD, XPS, AES Rh(111) ARUPS, TPD, Adsorption at 290 K: Decomposition to C5H5 Weakly π-bonded in monolayer at 150 K 88Net3, 88Net2 LEED, ∆Φ Heating after adsorption at 80 K: Decomposition to C5H6 Ag(111) ESDIAD, LEED, No decomposition upon heating 86Alv TPD Ag(211) ESDIAD, LEED, No decomposition upon heating Tdes =175 K, 220 K 86Alv TPD Double bond oriented along steps Ir(111) TPD, HREELS 160 K: Decomposition to c-C5H6 93Ave 400 K :Decomposition to c-C5H3 Pt(111) TPD, HREELS, <250 K: molecular adsorption c-C5H8: di-σ bonded 84Ave1, 84Ave2, ∆Φ ~300 K: formation of c-C5H5, stable up to 480 K c-C5H5 : π-bonded 93Ave TPD Molecular desorption + decomposition Bi coadsorption 88Cam NEXAFS, Electronic structure 86Hit Xα-calculations TPD, AES, XPS Dehydrogenation to adsorbed c-C5H5 or molecular 89Hen desorption TPD, XPS, AES, Cs coadsorption: 91Ern ∆Φ Stabilization at low Cs coverage 91Dav Reversal of dipole moment STM, RAIRS, No Formation of [2+2] cycloaddition 97Hov, 98Liu, Si(100)-(2×1) XPS product (i.e. di-σ Si-C bonds) 98Hov1, 98Hov2, 98Ham, 99Ham, 00Lee UPS, XPS Formation of di-σ Si-C bonds 01Yam1, 01Yam2 TPD Some dehydrogenation to c-C5H6 Molecular desorption: Tdes = 300-650 K 92Mac Si(111)7×7 Formation of [2+2] cycloaddition product Tdes = 360 K and 410 K 99Ham, 00Lee Ge(100)-(2×1) RAIRS, STM, XPS, UPS, TPD

Pt(111)

TPD, HREELS

W(100) Pt(111)

UPS, ∆Φ, TPD TPD LEED TPD TPD TPS, XPS, AES

Pt(100)1×1 Pt(100)5×20 Si(100)-(2×1) Landolt-Börnstein New Series III/42A4

Ge(100)-(2×1)

TPD TPD, HREELS TPD, HREELS UPS, STM

RAIRS, STM, XPS, UPS, TPD

Comments

References

Complete decomposition upon heating Formation of cyclopentadienyl (c-C5H5) Heating to 350 K: Decomposition to c-C5H5 Heating to 450 K: Decomposition to c-C5H3 1,3: Adsorption at 90 K: Disproportionation reaction to c-C5H5 and c-C5H8

Weak π-bonded in monolayer

88Net2, 88Net3

Irreversibile adsorption

93Ave 86Ave, 93Ave

Molecular desorption: Tdes = 300-650 K Dyhydrogenation to benzene Also stepped surface No decomposition upon heating Tdes = 206 K, Edes =52 kJ/mol No molecular desorption from monolayer C-H and C-C bond breaking at 150 K – formation of C2H4 300 K: Decomposition upon adsorption Benzene formation above 300 K Edes = 75 kJ/mol, Bi coadsorption Electron beam induced dissociation Dissociation for small coverages Sticking coefficient: unity Molecular desorption at higher coverages: Tdes = 239 di-σ bonding and 281 K Decomposition upon heating Molecular desorption at Tdes = 255 and 300 K Also observed: benzene desorption: Tdes =350-500 K Dyhydrogenation to benzene also stepped surface Dehydrogenation to benzene upon heating to 290 K Dehydrogenation to benzene upon heating to 290 K Two stable adsorption states, assigned to boat type and twist boat type geometries Formation of di-σ Si-C bonds Formation of [2+2] cycloaddition products; Tdes = 370 and 455 K minor decomposition upon heating

92Mac 82Tsa2 96Tep 87Fly 80Bha 88Cam 93Dom, 94Cam 93Xu 94Xu2 89Rod1 82Tsa2 97Lam 97Lam 01Yos 01Yam2 00Lee

3.8.7 Cyclic hydrocarbons

TPD Si(111)7×7 Cyclohexene (c-C6H10) Ni(111) TPD Cu(100) RAIRS, TPD Ru(0001) TPD

Thermal evolution

348

Methods Cyclopentadiene (c-C5H6) Rh(111) ARUPS, TPD, LEED, ∆Φ Ir(111) TPD, HREELS

Landolt-Börnstein New Series III/42A4

Methods Cyclohexadiene (c-C6H8) Ni(111) TPD W(100) UPS, ∆Φ Pt(111) SFG

TPD, HREELS, TPD

Cyclooctadiene (c-C8H12) Pt(111) TPD, HREELS STM, RAIRS Si(100)1×2 Cyclooctatetraene (c-C8H8) Pt(111) NEXAFS TPD, HREELS

Comments

1,3 and 1,4: Dehydrogenation to benzene also stepped surface 1,3 and 1,4: Decomposition upon adsorption at 300 K 1,4: Flat adsorption on surface Dehydrogenation to benzene at ~ 300 K 1,3: partial rearrangement to 1,4; Dehydrogenation to benzene 1,3: Dehydrogenation to benzene upon heating: Bi coadsorption Eact = 58 kJ/mol 1,4: Dehydrogenation to benzene upon heating Eact = 58 kJ/mol 1,3 and 1,4: Dehydrogenation to benzene also stepped surface 1,3 and 1,4: Dehydrogenation to benzene 1,3: Mixture of [2+2] and [4+2] cycloaddition products 1,4: Formation of di-σ Si-C bonds

1,5: Formation of [2+2] cycloaddition product Electronic structure

References 82Tsa2 80Bha 97Su

92Hug

82Tsa2 97Lam 98Hov2 00Ham, 01Yam1, 01Yam2 94Hos 98Hov1, 98Hov2, 99Ham

3.8.7 Cyclic hydrocarbons

Pt(100)1×1 Si(100)-(2×1)

TPD TPD STM, RAIRS, XPS LEED, UPS

Thermal evolution

86Hit 94Hos

349

350

Table 5. Ethylene oxide (C2H4O) Tmax Fe(100)

Ni(111)

150-250

120-240

2-5

2

Structure

−1.6

−2.8

DecomMethods position upon heating ARUPS, TPD, ∆Φ

No (<0.05 ML)

c(2×2)

No Yes

Cu(110)

−1.1

150-240

c(2×2)

No (<0.03 ML)

Mo(110) 140-190 Rh(111) 120-210 Pd(111) 255

4 8

Yes Yes Yes

Pd(110)

170-220

4

Yes

Ag(111)

158

3

No

Orientation, Symmetry Molecular plane nearly perpendicular

Theory (ASEDMO) ARUPS, TPD, ∆Φ, HREELS Theory (ASEDMO) TPD, ∆Φ, LEED, XPS, ARUPS, NEXAFS XPD, XPS, LEED, UPS Cluster Calculations (DFT) ARUPS, TPD, ∆Φ, LEED TPD, XPS TPD, HREELS TPD, LEED, HREELS, XPS HREELS, TPD DFT calculations

RAIRS, TPD

Molecular plane tilted, C1

2b1, 6a1 Bonding Shift [eV] 0.7 / 0.45

Comments

Reference

Edes = 55 kJ/mol

87Ben1

Electronic structure

89Sel

0.75 / 0.6

K coadsorption Reaction at high K coverages Electronic structure

87Ben1, 87Ben2, 90Nie1, 90Nie2, 92Nie 89Sel

0.9 / 0.5

Radiation damage observed

93Wei1, 93Wei2

Radiation damage observed Orbital assignment

93Gro, 94Ham

Edes = 50 kJ/mol O coadsorption: weakening of bond

86Ben1, 86Ben2, 87Ben1

Molecular plane tilted, CS Molecular plane nearly perpendicular

0.5 / 0.6

Edes ≈63 kJ/mol Molecular plane tilted, C1

Landolt-Börnstein New Series III/42A4

Molecular plane perpendicular

Edes = 42 - 51kJ/ mol Orbital assignment Ring opening above 200 K Edes = 45 kJ/mol O coadsorption: Edes = 51 kJ/mol Also “High” pressure (1 mbar) Edes ≈41 kJ/mol

95Ulb

89Ser 93Bro 94Lam 96She, 97She

01Sta

87Tan

3.8.7 Cyclic hydrocarbons

Ni(110)

[K] 120-250

Heating ∆Φ rate [K/s] [eV] −2.2

Landolt-Börnstein New Series III/42A4

Tmax [K]

Heating ∆Φ rate [K/s] [eV]

Structure

Cont’d.

DecomMethods position upon heating TPD

Ag(110)

Orientation, Symmetry

2b1, 6a1 Bonding Shift [eV]

TPD, LEED, HREELS ~170

4.5

175

11

−1.44 c(2×2)

No No

Pt(111)

160-190

11

Pt(110)

140-230

5

Yes −2.1

streaky (2×2)

No (<0.02 ML)

Molecular plane nearly perpendicular

TPD, LEED, XPS, HREELS Cluster calculation, INDO/S XPS, TPD, AES, HREELS TPD, ∆Φ, LEED, Mol plane tilted, C1 XPS, ARUPS, NEXAFS

No shifts

Reference

Adsorption between 300 and 500 K Edes ≈ 38 kJ/mol O coadsorption: new adsorption states K-coadsorption Reaction at high K coverages Edes = 42 kJ/mol Only molecular desorption Edes = 40 kJ/mol

85Gra

Electronic structure

0.6 / 0.4

83Bac 86Kru, 86Ben2, 87Ben1, 91Nie 86Cam 92Bar 88Rod2

Edes = 41 to > 50 kJ/mol, 86Cam depending on coverage Two coexisting species 93Wei2

3.8.7 Cyclic hydrocarbons

c(2×2)

AES, TPD, AES, LEED, HREELS, ARUPS, ∆Φ XPS, TPD, AES, HREELS

Comments

351

352

Table 6. Pyridine (C5H5N) Tmax [K]

Heating Structure rate [K/s]

Ni(111)

Methods

Orientation / thermal evolution

References

HREELS, XPS, ARUPS

120 K and 170 K:

90Coh

NEXAFS PED HREELS,ARU PS,XPS

Ni(100)

HREELS

Cu(111)

TPD NEXAFS XPS, HREELS

Des. <240K

Cu(110)

Low cov: 360 K

2

(5×3), (4×3)

IPES EELS LEED, TPD, ESDIAD

Landolt-Börnstein New Series III/42A4

ARUPS RAIRS

91Ami 94Fri 91Coh

84Din1, 84Din2, 84Avo 82Wex 86Bad1 95Dav

86Fra 01Zyl 01Lee 86Bad1 87Con, 87Bri 79Ban, 80Nyb 96Haq

3.8.7 Cyclic hydrocarbons

Ni(110)

Low coverage: π-bonded, molecular plane parallel to surface. High coverage: N-bonded , molecular plane tilts towards surface normal. Decomposition at 520 K Electronic structure 300 K: tilt angle of molecular plane to surface normal: 20o N atom close to on top position, tilt angle to surface normal: 18o N-Ni bond length: 1.97 Å 100 K: Low coverage: π-bonded, molecular plane parallel to surface High coverage: Molecular plane tilts towards surfaces normal Decomposition upon heating to 520 K 100 K: Low coverage: π-bonded, molecular plane parallel to surface High coverage: molecular plane tilts towards surfaces normal 300 K: Reaction to α-pyridile, N-bonded, perpendicular, stable up to 490 K 300 K: Formation of α-pyridile Tilted, tilt angle of molecular plane to surface normal: 20o C2 axis parallel to surface, π-bonded, CS-symmetry No decomposition upon heating, complete molecular desorption <240 K O coadsorption: Stabilization of pyridine up to 380 K C2 axis perpendicular to surface Electronic structure Charge transfer excitations Low coverage: Perpendicular, mol plane along [001]; Edes = 94 kJ/mol Saturation: Perpendicular, mol plane twisted by 25°; Edes = 109 kJ/mol No decomposition upon heating Perpendicular, tilt angle of molecular plane to surface normal: 0° 240 K: N-bonded , perpendicular, molecular plane along [001] 300 K: N-bonded, perpendicular, differential shift of lone pair orbital: 0.9 eV 85 K: N-bonded, C2v, perpendicular at low coverage some tilting with increasing coverage

Landolt-Börnstein New Series III/42A4

Tmax [K]

Heating Structure rate [K/s]

Cont’d.

(2√3×√3)rect

Disordered

Pd(110)

c(4×2)

Ag(111)

Ag(100) Ir(111)

Saturation coverage: near on top-site, N-Cu: 2.00 Å tilt angle of molecular plane to surface normal: 20o, twisted by 30o from [001] ARUPS Electronic structure STM Electron induced dissociation HREELS, TPD < 200 K: low coverage: π-bonded, molecular plane parallel to surface high coverage: N-bonded, inclined 300 K: reaction to α-pyridile different dehydrogenation pathways for π-bonded and N-bonded pyridine LEED,HREELS, Adsorption at 80 K: α-pyridile formed upon heating to at 310 K further decomposition upon heating at 400 K TPD, SHG ARUPS, TPD, 300 K: intact pyrdine, change from parallel to inclined species with coverage ∆Φ ∆Φ for saturated layer: −1.65 eV

99Gie, 00Ter

300 K: CS, tilted, π + N-bonding 180 K, all coverages: π-bonded, molecular plane parallel to surface 310 K, all coverages: N-bonded HREELS 150 K, all coverages: π-bonded, molecular plane parallel to surface ARUPS, LEED, π-bonded, molecular plane parallel to surface, also at saturation coverage ∆Φ N-atom along [001] azimuth NEXAFS Tilted, tilt angle to surface normal changes from 45 to 20° with coverage EELS Change from π-bonded to N-bonded with coverage Vibrational overtones

83Net1, 83Net2 87Gra

ARUPS, EELS HREELS

NEELFS LEED, ARUPS UPS, Raman IPES, UPS SHG, ∆Φ

88Dud 00Lau 87Bri, 87Jak, 90Jak

88Mat 90Net

86Wad 88Net1 86Bad1, 86Bad2 80Dem, 81Dem, 82Dem, 81Avo1, 81Avo2, 83Dem 90Tyl 86Dud 80San 85Ott 88Hes 80Kel 90Tyl 85Mac 87Con

353

Change from small to large tilt angle with coverage Perpendicular, N-bonded Electronic structure Electronic structure Change from flat to upright with coverage Sticking coefficient at 110 K close to unity UPS, TPD, AES Electronic structure NEELFS Change from small to large tilt angle with coverage 300 K, saturation coverage: CS, N-bonded, inclined by 20o (2√3×2√3)R30° ARUPS, ESDIAD 300 K, saturation coverage: reaction to α-pyridile (from data in 85Mac) (√3×√3)R30°

References

3.8.7 Cyclic hydrocarbons

Pd(111)

Ag(110)

Orientation / thermal evolution

PED

Cu(100) Ru(0001)

Rh(111)

Methods

[K]

Heating Structure rate [K/s]

Pt(111)

Methods

Orientation / thermal evolution

References

RAIRS

85 K: 140 K, 140 K, 300 K: 300 K:

96Haq

NEXAFS, Xα-calculations TPD, NEXAFS

Pt(100) Au(111) Au(110) Si(111)7×7 Si(111)2×1 ZnO(0001)

ZnO(1010) ZnO(1100)

Diffuse (1×1)

240 K: Tilted geometry: 38° to surface normal 300 K: α-pyrdile: Perpendicular (16±10°) ARUPS 300 K: reaction to α-pyridile Adsorbed from solution HREELS, Raman 300 K: Inclined: 15-19° to surface normal HREELS, TPD 120 K – high cov: N + π-bonded, tilted 300 K: reaction to α-pyridile, perpendicularly oriented LEED, ∆Φ 300 K: ∆Φ = −2.7 eV HREELS Adsorption at 300 K: no dissociation Change from π-bonded to N-bonded with increasing coverage LEED, ∆Φ 300 K: ∆Φ= −2.4 eV DFT calculation Vibrational frequencies IPES Electronic structure STM Adsorption across dimer row ARUPS, Electronic structure HREELS 45 K and 85 K: physisorbed state 300 K: chemisorbed state Cluster Calc. Electronic structure, charge transfer (INDO/S) TPD, XPS, O-terminated surface: Tdes= 225 K, Ead = 57 kJ/mol, tilt angle: 24o to SN NEXAFS Zn-terminated surface: Tdes= 365 K, Ead = 112 kJ/mol, tilt angle: 19o to SN NEXAFS 295 K: upright with ring plane along [0001], N-bonded UPS 300 K: N + π-bonded

85Hor, 83Joh 85Joh 87Bri, 87Con 90Kah 86Gra 73Gla, 82Gar 83Ric, 87Sur 73Gla 01Tad Fra86 00Yag, 00Shi 83Pia, 85Pia, 86Pia, 90Pia 88Rod 00Hov 93Wal 78Lut

3.8.7 Cyclic hydrocarbons

Diffuse (2×2) Pt(110)

Tilted geometry, rotation about C2 axis with coverage low coverage: π-bonded, molecular plane parallel to surface high coverage: tilting towards surface normal α-pyridile, upright, for all coverages Perpendicular

354

Tmax

Landolt-Börnstein New Series III/42A4

References for this document 73Gla 74Bar 74Dem 76Dem 76Nie 77Fir 77Fis 78Dem 78Fir 78Leh 78Lut 78Mad 78Nie 78Rub 79Ban 79Leh 80Bha 80Dem 80Kel 80Net 80Nyb 80San 81Avo1 81Avo2 81Ber 81Dem 81Hof 81Pos 82Dem 82Gar 82Ric 82Tat 82Tsa1 82Tsa2 82Wex 82Wit1 82Wit2 83Bac 83Dem 83Hof1 83Hof2 83Joh 83Lin 83Net1 83Net2 83Pia 83Ric 83Szu 84And 84Ave1 84Ave2 84Avo 84Din1 84Din2

Gland, J., Somorjai, A.: Surf. Sci. 38 (1973) 157. Baron, K., Blakely, D.W., Somorjai, G.A.: Surf. Sci. 41 (1974) 45. Demuth, J.E., Eastman, D.E.: Phys. Rev. Lett. 32 (1974) 1123. Demuth, J.E., Eastman, D.E.: Phys. Rev. B 13 (1976) 1523. Nieuwenhuys, B.E., Hagen, D.I., Rovida, G., Somorjai, G.A.: Surf. Sci. 59 (1976) 155. Firment, L.E., Somorjai, G.A.: J. Chem. Phys. 66 (1977) 2901. Fischer, T.E., Kelemen, S.R., Bonzel, H.P.: J. Vac. Sci. Technol. 14 (1977) 424. Demuth, J.E., Ibach, H., Lehwald, S.: Phys. Rev. Lett. 40 (1978) 1044. Firment, L.E., Somorjai, G.A.: J. Chem. Phys. 69 (1978) 3940. Lehwald, S., Ibach, H., Demuth, J.E.: Surf. Sci. 78 (1978) 577. Lüth, H., Rubloff, G.W., Grobman, W.D.: Surf. Sci. 74 (1978) 365. Madey, T.E., Yates, J.J.: Surf. Sci. 76 (1978) 397. Nieuwenhuys, B.E., Somorjai, G.A.: Surf. Sci. 72 (1978) 8. Rubloff, G.W., Lüth, H., Demuth, J.E., Grobman, W.D.: J. Catal. 53 (1978) 423. Bandy, B., Lloyd, D.R., Richardson, N.V.: Surf. Sci. 89 (1979) 344. Lehwald, S., Ibach, H.: Surf. Sci. 89 (1979) 425. Bhattacharya, A.K.: J. Chem. Soc. Faraday Trans. I 76 (1980) 126. Demuth, J.E., Christmann, K., Sanda, P.N.: Chem. Phys. Lett. 76 (1980) 201. Kelemen, S.R., Kaldor, A.: Chem. Phys. Lett. 73 (1980) 205. Netzer, F.P., Bertel, E., Matthew, J.A.D.: Surf. Sci. 92 (1980) 43. Nyberg, G.: Surf. Sci. 95 (1980) L273. Sanda, P.N., Warlaumont, J., Demuth, J.E., Tsang, J., Christmann, K., Bradley, J.: Phys. Rev. Lett. 45 (1980) 1519. Avouris, P., Demuth, J.E.: J. Chem. Phys. 75 (1981) 4783. Avouris, P., Demuth, J.E.: J. Chem. Phys. 75 (1981) 5953. Bertolini, J.C., Massardier, J.: J. Chim. Phys. Phys. Chim. Biol. 78 (1981) 939. Demuth, J.E., Sanda, P.N.: Phys. Rev. Lett. 47 (1981) 57. Hoffmann, F.M., Felter, T.E., Thiel, P.A., Weinberg, W.H.: J. Vac. Sci. Technol. 18 (1981) 651. Poss, D., Ranke, W., Jacobi, K.: Surf. Sci. 105 (1981) 77. Demuth, J.E., Avouris, P., Sanda, P.N.: J. Vac. Sci. Technol. 20 (1982) 588. Garwood, G.-J., Hubbard, A.: Surf. Sci. 118 (1982) 223. Richardson, N.V., Palmer, N.R.: Surf. Sci. 114 (1982) L1. Tatarenko, T., Ducros, R.: J. Chim. Phys. Phys. Chim. Biol. 79 (1982) 409. Tsai, M.-C., Friend, C.M., Muetterties, E.L.: J. Vac. Sci. Technol. 20 (1982) 533. Tsai, M.-C., Friend, C.M., Muetterties, E.L.: J. Am. Chem. Soc. 104 (1982) 2539. Wexler, R.M., Tsai, M.-C., Friend, C.M., Muetterties, E.L.: J. Am. Chem. Soc. 104 (1982) 2034. Wittrig, T., Szuromi, P.D., Weinberg, W.H.: J. Chem. Phys. 76 (1982) 716. Wittrig, T., Szuromi, P.D., Weinberg, W.H.: Surf. Sci. 116 (1982) 414. Backx, C., DeGroot, C.P.M., Biloen, P., Sachtler, W.M.H.: Surf. Sci. 128 (1983) 81. Demuth, J.E., Avouris, P., Schmeisser, D.: J. Electron Spectrosc. Relat. Phenom. 29 (1983) 163. Hoffmann, F.M., O'Brien, E., Hrbek, J., DePaola, R.: J. Electron Spectrosc. Relat. Phenom. 29 (1983) 301. Hoffmann, F.M., Felter, T.E., Thiel, P.A.: Surf. Sci. 130 (1983) 173. Johnson, A.L., Muetterties, E.L., Stöhr, J.: J. Am. Chem. Soc. 105 (1983) 7183. Lin, R.F., Koestner, R.J., VanHove, M.A., Somorjai, G.A.: Surf. Sci. 134 (1983) 161. Netzer, F.P., Mack, J.-U.: J. Chem. Phys. 79 (1983) 1017. Netzer, F.P., Mack, J.-U.: Chem. Phys. Lett. 95 (1983) 492. Piancastelli, M.N., Cerrina, F., Margaritondo, G., Franciosi, A., Weaver, J.H.: Appl. Phys. Lett. 42 (1983) 990. Richardson, N.: Vacuum 33 (1983) 787. Szuromi, P.D., Weinberg, W.H.: J. Vac. Sci. Technol. A 1 (1983) 1219. Anderson, A.B., McDevitt, M.R., Urbach, F.L.: Surf. Sci. 146 (1984) 80. Avery, N.R.: Surf. Sci. 146 (1984) 363. Avery, N.R.: Surf. Sci. 137 (1984) L109. Avouris, P., DiNardo, J.J., Demuth, J.E.: J. Chem. Phys. 80 (1984) 491. DiNardo, J.J., Avouris, P., Demuth, J.E.: J. Chem. Phys. 81 (1984) 2169. DiNardo, J.J., Avouris, P., Demuth, J.E.: J. Vac. Sci. Technol. A 2 (1984) 1015.

84Hal 84Hof 84Pia 84Szu 85Abo 85Ave 85Gra 85Hor 85Joh 85Kan 85Mac 85Ott 85Pia 85Wad 86Alv 86Ave 86Bad1 86Bad2 86Bar 86Ben1 86Ben2 86Ber 86Cam 86Che 86Dud 86Fra 86Gar 86Gra 86Hit 86Kru 86Pia 86Pol 86Sch 86Van 86Wad 87Ben1 87Ben2 87Bri 87Car 87Con 87Fly 87Gra 87Jak 87Mye 87Net1 87Net2 87Pia 87Pol 87Som1 87Som2 87Sur

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87Tan 87Van 88Cam 88Dud 88Hes 88Jak 88Liu 88Mak 88Mat 88Net1 88Net2 88Net3 88Net4 88Oht1 88Oht2 88Pat 88Rod1 88Rod2 88Som 89Gra 89Hei 89Hen 89Hub 89Jak 89Lan 89Rav1 89Rav2 89Rod1 89Sel 89Ser 89Ste 90Are 90Che 90Coh 90Dud 90Gra1 90Gra2 90Hub 90Jak1 90Jak2 90Jak3 90Kah 90Liu 90Mar 90Net 90Nie1 90Nie2 90Pia 90Tyl 90Zho 91Ami 91Coh 91Dav

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Self, K.W., Pelzel, R.I., Owen, J.H.G., Yan, C., Widdra, W., Weinberg, W.H.: J. Vac. Sci. Technol. A 16 (1998) 1031. Teplyakov, A.V., Bent, B.E., Eng jr., J., Chen, J.G.: Surf. Sci. 399 (1998) L342. Velic, D., Hotzel, A., Wolf, M., Ertl, G.: J. Chem. Phys. 109 (1998) 9155. Weiss, K., Gebert, S., Wuhn, M., Wadepohl, H., Wöll, C.: J. Vac. Sci. Technol. A 16 (1998) 1017. Weiss, P.S., Kamna, M.M., Graham, T.M., Stranick, S.J.: Langmuir 14 (1998) 1284. Weinelt, M., Wassdahl, N., Wiell, T., Karis, O., Hasselström, J., Bennich, P., Nilsson, A., Stör, J., Samant, M.: Phys. Rev. B 58 (1998) 7351. Wolkow, R.A., Lopinski, G.P., Moffatt, D.J.: Surf. Sci. 416 (1998) L1107. Zebisch, P., Stichler, M., Trischberger, P., Weinelt, M., Steinrück, H.-P.: Surf. Sci. 396 (1998) 61. Cao, Y., Wei, X.M., Chin, W.S., Lai, Y.H., Deng, J.F., Bernasek, S.L., Xu, G.Q.: J. Phys. Chem. B 103 (1999) 5698. Giessel, T., Schaff, O., Lindsay, R., Baumgärtel, P., Polcik, M., Bradshaw, A.M., Koebbel, A., McCabe, T., Bridge, M.E., Lloyd, D.R., Woodruff, D.P.: J. Chem. Phys. 110 (1999) 9666. Hamers, R.J., Hovis, J.S., Greenlief, C.M., Padowitz, D.F.: Jpn. J. Appl. Phys. 38 (1999) 3879. Koschel, H., Held, G., Steinrück, H.-P.: Surf. Rev. Lett. 6 (1999) 893. Koschel, H., Held, G., Trischberger, P., Widdra, W., Steinrück, H.-P.: Surf. Sci. 437 (1999) 125. Munakata, T., Shudo, K.: Surf. Sci. 433-435 (1999) 184. Munakata, T.: J. Chem. Phys. 110 (1999) 2736. Weiss, K., Weckesser, J., Wöll, C.: J. Mol. Struct.: THEOCHEM 458 (1999) 143. Whitten, J.E., Gomer, R.: Surf. Sci. 429 (1999) 14. Carbone, M., Piancastelli, M.N., Casaletto, M.P., Zanoni, R., Comtet, G., Dujardin, G., Hellner, L.: Phys. Rev. B 61 (2000) 8531. Favot, F., Dal-Corso, A., Baldereschi, A.: Europhysics. Lett. 52 (2000) 698. Hamaguchi, K., Machida, S., Mukai, K., Yamashita, Y., Yoshinobu, J.: Phys. Rev. B 62 (2000) 7576. Hovel, S., Kolczewski, C., Wuhn, M., Albers, J., Weiss, K., Staemmler, V., Wöll, C.: J. Chem. Phys. 112 (2000) 3909. Kang, J.-H., Toomes, R.L., Robinson, J., Woodruff, D.P., Schaff, O., Terborg, R., Lindsay, R., Baumgärtel, P., Bradshaw, A.M.: Surf. Sci. 448 (2000) 23. Koschel, H., Held, G., Steinrück, H.-P.: Surf. Sci. 454-456 (2000) 83. Koschel, H.: Ph. D. Thesis, Universität Würzburg, 2000. Lauhon, L.J., Ho, W.: Surf. Sci. 451 (2000) 219. Lee, S.W., Hovis, J.S., Coulter, S.K., Hamers, R.J., Greenlief, C.M.: Surf. Sci. 462 (2000) 6. Munakata, T.: Surf. Sci. 454-456 (2000) 118. Shirota, N., Yagi, S., Taniguchi, M., Hashimoto, E.: J. Vac. Sci. Technol. A 18 (2000) 2578. Silvestrelli, P.L., Ancilotto, F., Toigo, F.: Phys. Rev. B 62 (2000) 1596. Staufer, M., Birkenheuer, U., Belling, T., Nörtemann, F., Rösch, N., Widdra, W., Kostov, K., Moritz, T., Menzel, D.: J. Chem. Phys. 112 (2000) 2498. Terborg, R., Polcik, M., Hoeft, J.-T., Kittel, M., Pascal, M., Kang, J.H., Lamont, C.L.A., Bradshaw, A.M., Woodruff, D.P.: Surf. Sci. 457 (2000) 1. Wöll, C., Weiss, K., Bagus, P.S.: Chem. Phys. Lett. 332 (2000) 553. Yagi, S., Shirota, N., Taniguchi, M., Hashimoto, E.: Surf. Sci. 454-456 (2000) 157. Bao, S., Lindsay, R., Polcik, M., Theobald, A., Gießel, T., Schaff, O., Baumgärtel, P., Terborg, R., Bradshaw, A.M., Booth, N.A., Woodruff, D.P.: Surf. Sci. 478 (2001) 35. Braun, W., Held, G., Steinrück, H.-P., Stellwag, C., Menzel, D.: Surf. Sci. 475 (2001) 18. Fink, A., Menzel, D., Widdra, W.: J. Phys. Chem. B 105 (2001) 3828. Held, G., Braun, W., Steinrück, H.-P., Yamagishi, S., Jenkins, S.J., King, D.A.: Phys. Rev. Lett. 87 (2001) 216102. Hofer, W.A., Fisher, A.J., Lopinski, G.P., Wolkow, R.A.: Phys. Rev. B 63 (2001) 085314/1. Lee, J.-G., Ahner, J., Yates jr., J.T.: J. Chem. Phys. 114 (2001) 1414. Mittendorfer, F., Hafner, J.: Surf. Sci. 472 (2001) 133. Pascual, J.I., Jackiw, J.J., Song, Z., Weiss, P.S., Conrad, H., Rust, H.-P.: Phys. Rev. Lett. 86 (2001) 1050. Pussi, K., Lindroos, M., Barnes, C.J.: Chem. Phys. Lett 341 (2001) 7. Stacchiola, D., Wu, G., Kaltchev, M., Tysoe, W.T.: Surf. Sci. 486 (2001) 9. Tadjeddine, M., Flament, B.-J.: Chem. Phys. 265 (2001) 27. Wöll, C.: J. Synchrotron Rad. 8 (2001) 129.

01Yam1 01Yam2 01Yam3 01Yos 01Zyl

Yamashita, Y., Nagao, M., Machida, S., Hamaguchi, K., Yasui, F., Mukai, K., Yoshinobu J.: J. Electron Spectrosc. Relat. Phenom. 114-116 (2001) 389. Yamashita, Y., Hamaguchi, K., Machida, S., Mukai, K., Yoshinobu, J., Tanaka, S., Kamada, M.: Appl. Surf. Sci. 169-170 (2001) 172. Yamagishi, S., Jenkins, S.J., King, D.A.: J. Chem. Phys. 114 (2001) 5765. Yoshinobu, J., Yamashita, Y., Yasui, F., Mukai, K., Akagi, K., Tsuneyuki, S., Hamaguchi, K., Machida, S., Nagao, M., Sato, T., Iwatsuki, M.: J. Electron Spectrosc. Relat. Phenom. 114-116 (2001) 383. Zylka, G., Otto, A.: Surf. Sci. 475 (2001) 118.

3.8.7 Cyclic hydrocarbons

355

3.8.7.10 Figures for 3.8.7

C6D6-desorption rate

C6D6/Ni(111)

C6D6-exposure [L] 1.8 1.2 0.9 0.7 0.6 0.5 0.4 0.3 0.2-0.05 300

400 500 Temperature T [K]

D2-desorption rate

200

600

700

C6D6-exposure [L] 1.8-1.2 0.7 0.5 0.4 0.3 0.2 0.1 0.05 200

300

Landolt-Börnstein New Series III/42A4

400 500 Temperature T [K]

600

700

Fig. 1. Temperature programmed desorption spectra of D2 (lower Fig; m/e = 4) and C6D6 (upper Fig.; m/e = 84) after different exposures of deuterated benzene onto Ni(111) at a surface temperature of 200 K. Heating rate was 5 K/s; [89Ste].

356

3.8.7 Cyclic hydrocarbons

Fig. 2. Schematic drawing of the σ(d) and σ(v) orientations of benzene adsorbed parallel on a close packed hexagonal surface (fcc 111 or hcp 0001) which lead to the highest possible C3v symmetry of the adsorption complex. Note that either one of the σ(d) or σ(v) mirror planes is parallel to the mirror plane of the substrate (along [11-2] / [11-20]). The adsorption site is chosen arbitrarily; [99Kos1]. Fig. 3: see next page

Fig. 4. Stable adsorption geometries for benzene on Si(100)-(1×2): SB = standard butterfly (1,4-cyclohexadiene-like, ontop of dimer row) TB = tilted-bridge butterfly, DB = diagonal-bridge butterfly, T = tilted, P = pedestal, TiB = tight bridge, TwB = twisted bridge. Only four Si atoms of the first layer (dimers) and four of the second layer are shown; [00Sil].

Landolt-Börnstein New Series III/42A4

3.8.7 Cyclic hydrocarbons

p

a

p(2

b

c(2 3

c

p(

p

p

7

3

p

2

3)

357

R30

4)rect

p

7)

R19

Fig. 3. LEED patterns (200 eV) for the three commensurate adsorption structures of benzene adsorbed on Ru(0001): p(2√3×2√3)R30º, c(2√3×4)rect, and p(√7×√7)R19º; [01Bra]. Landolt-Börnstein New Series III/42A4

358

3.8.7 Cyclic hydrocarbons

Benzene, D6h

C6

σd

σv

σh

z

x

C2

y

1e1g

C2

H C

H

C

H C

C

C

C

2e2g

H

H H

H

C

C

H

C

C

H

C

C

C

C

H

C

C

H

H

C H

H

H

H

C

C

H

C

H

H C

H

C

C

C

H

H

H

H 1a2u H

H C

C

H

C

C

H

C

C

H

C 2e1u

H C

H

H

C

C

C

C

H

C

C H

H

H

C H

H

1b2u H C

C

H

H

C

C

H

H

H

C

C

H

C

C

H C

C

H

C

C

H

C

C H

1b1u H

H

C

C

H

H

2a1g

H H

C

C

H

H

C

C

C

C

H

H 1e2g H H

C

C

C

C C H

H

H H

C H

H C

C C C H

H

C C H

Fig. 5. Top: schematic drawing of benzene with its symmetry elements in the gas phase. Only one out of three equivalent C2 rotation axes and σ(d) / σ(v) mirror planes are indicated. Bottom: occupied molecular orbitals after Jørgensen and Salem [73Jor]; [96Ste].

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359

C6D6/Ni(110)dilute E ϑ

ϑ = 0° e–

E II [110] E II [001]

α = 0° D II E

Count rate

0° 20°

E α

ϑ

40°

e–

α = 45° D II E

Fig. 6. Angle-resolved UPS spectra of a dilute layer (∼0.1 ML) of deuterated benzene on Ni(110) at various experimental geometries, using a photon energy of 30 eV. Orbital positions and assignment are indicated in form of a bar diagram. D indicates the plane of electron detection, E the orientation of the electrical field vector. From the pronounced polarization and emission angle dependencies the orientation of the molecule is determined as C2v; [94Ste].

60° 1b2u 1a2u 1e2g 15.0

2a1g 1b1u 2e2g 2e2g 1e1g

7.5 12.5 10.0 Binding energy Eb [eV]

5.0

Fig. 7: see next page

A1 A2 B1 B2

3015 2935

1623

876 949 1050 1160 1286 1378

315

538

Intensity I

604

× 300

778

HREELS

DF-GGA

0

500

Landolt-Börnstein New Series III/42A4

1000 2000 1500 –1 Electronic energy loss Eloss [cm ]

2500

3000

Fig. 8. HREELS spectrum of benzene on (2×1) Si(100) together with the calculated normal frequencies of a C6H6Si13H12 cluster. The calculated frequencies are indicated by vertical bars, separated according to the four irreducible representations of the symmetry group C2v; [00Sta].

360

3.8.7 Cyclic hydrocarbons

22 Benzene multilayer IP

20

18 1

2 3

4

5 Benzene monolayers

PEY [units of absorption step]

16

14

90°

12

30°

10

Au(111)

8

Fig. 7. NEXAFS spectrum of a benzene multilayer at 120 K (top) and polarization dependence of NEXAFS spectra for benzene monolayers adsorbed on Au(111), Rh(111), and Pt(111). The disappearance of the π-resonance at around 285 eV for normal incidence spectra (dotted) indicates flat-lying molecules on all three surfaces (PEY = partial electron yield); [98Wei1].

6 Rh(111)

4

2 Pt(111)

0 280

285

290

295 300 305 Photon energy E [eV]

310

315

320

Fig. 9.: see next page side view

H C

Fig. 10. Models of adsorbed cyclohexene on Si(100)(2×1); [01Yos].

Si twist-boat type

boat type

Landolt-Börnstein New Series III/42A4

3.8.7 Cyclic hydrocarbons

361

m/e = 84 Cu(110)

181

Cu(110)

2728 ∆R/R = 0.05 %

Cu(110) 181

Cu(100)

Cu(100)

C6H12-desorption rate

Relative reflectance ∆R/R

2746

Cu(111)

Cu(100) 178

Cu(111)

2754

Cu(110) Cu3Pt(111) Cu3Pt(111)

2754

199

Cu3Pt(111) 3200

3100 3000 2900 2800 –1 Wavenumber ν [cm ]

2700

2600

160

180

200 210 Temperature T [K]

220

230

Fig. 9. Comparison of RAIRS (left) and TPD (right) studies of submonolayer coverages of cyclohexane on Cu(110), Cu(100), Cu(111) and Cu3Pt(111) surfaces. The desorption temperature of cyclohexane is independent of the surface coverage up to a monolayer saturation on all surfaces. The wave numbers of the softened modes are denoted; [98Tep]. 0

Work function change ∆F [eV]

0.5 Et-O/Pt(110)(1×2) Et-O/Ni(110)

1.0 1.5

Fig. 11. Change in work function as a function of coverage for ethylene oxide (Et-O) on Pt(110) (1×2) and Ni(110); adsorption temperature: 100 K.; [93Wei2].

2.0 2.5 3.0

0

Landolt-Börnstein New Series III/42A4

0.2

0.4 Coverage Q [ML]

0.6

0.8

362

3.8.7 Cyclic hydrocarbons C2

N

N

a

N

N

b

N

N

c

N

N

d

N e

N

Fig. 12. (a) perpendicular adsorption via the nitrogen lone pair electrons, (b) flat adsorption of the aromatic ring via the π-electrons, (c) tilted adsorption via the nitrogen lone pair orbitals and the π-electrons, and (d,e) edge on adsorption through the N and C(2) atoms with the molecular plane more or less perpendicular to the surfaces - i.e. formation of α-pyridile by breaking the C-H bond; [99Gie].

Landolt-Börnstein New Series III/42A4

References for this document 73Jor 89Ste 93Wei2 94Ste 96Ste 98Tep 98Wei1 99Gie 99Kos1 00Sil 00Sta 01Bra 01Yos

Jørgensen, W.L., Salem, L.: The Organic Chemist’s Book of Orbitals, New York: Academic Press, 1973. Steinrück, H.-P., Huber, W., Pache, T., Menzel, D.: Surf. Sci. 218 (1989) 293. Weinelt, M., Zebisch, P., Steinrück, H.-P.: Chem. Phys. 177 (1993) 321. Steinrück, H.-P.: Appl. Phys. A 59 (1994) 517. Steinrück, H.-P.: J. Phys. Condens. Matter 8 (1996) 6465. Teplyakov, A.V., Bent, B.E., Eng jr., J., Chen, J.G.: Surf. Sci. 399 (1998) L342. Weiss, K., Gebert, S., Wuhn, M., Wadepohl, H., Wöll, C.: J. Vac. Sci. Technol. A 16 (1998) 1017. Giessel, T., Schaff, O., Lindsay, R., Baumgärtel, P., Polcik, M., Bradshaw, A.M., Koebbel, A., McCabe, T., Bridge, M.E., Lloyd, D.R., Woodruff, D.P.: J. Chem. Phys. 110 (1999) 9666. Koschel, H., Held, G., Steinrück, H.-P.: Surf. Rev. Lett. 6 (1999) 893. Silvestrelli, P.L., Ancilotto, F., Toigo, F.: Phys. Rev. B 62 (2000) 1596. Staufer, M., Birkenheuer, U., Belling, T., Nörtemann, F., Rösch, N., Widdra, W., Kostov, K., Moritz, T., Menzel, D.: J. Chem. Phys. 112 (2000) 2498. Braun, W., Held, G., Steinrück, H.-P., Stellwag, C., Menzel, D.: Surf. Sci. 475 (2001) 18. Yoshinobu, J., Yamashita, Y., Yasui, F., Mukai, K., Akagi, K., Tsuneyuki, S., Hamaguchi, K., Machida, S., Nagao, M., Sato, T., Iwatsuki, M.: J. Electron Spectrosc. Relat. Phenom. 114-116 (2001) 383.

Ref. p. 380] 3.8.10 Polyatomic chain-like hydrocarbons on metals and semiconductors

371

3.8.10 Chemisorption of polyatomic chain-like hydrocarbons on metals and semiconductors M. GRUNZE, W. ECK 3.8.10.1 Introduction Long chain hydrocarbons of defined length can adsorb on metal or semiconductor surfaces from solution or the gas phase to form regularly arranged, well-ordered films which are called self-assembled monolayers (SAMs). Constituents of molecules forming self-assembled monolayers are the anchor group that chemically attaches the chains to the substrate, the chain or linker group, and the tail group on the terminus which determines wettability and chemical reactivity of the monolayer. C

Qt B

A

Fig. 1. The constituents of a self-assembled monolayer can be split into three components: An anchor group A, a spacer group or chain B and a tail group C. SAMs are ordered assemblies of such molecules on flat surfaces whose components are tilted by an angle θt from the surface normal.

Such highly ordered, stable monolayers may have applications in diverse areas such as wetting control, corrosion inhibition [97Sch], adhesion promotion [97Kim], [99Kid], the preparation of biocompatible surfaces [91Pri] or the modification of electronic surface properties [97Cam]. SAMs can also be used for lateral structuring of surfaces, e.g. by microcontact printing [98Del], [99Kum] or electron beam lithography [01Göl]. Since SAMs can be very easily prepared from solution, they present a straightforward way to modify physical and chemical properties of a surface by proper choice of their chemical constitution. Several reviews and books on the preparation, structure and properties of SAMs have appeared in the literature [91Ulm], [96Ulm], [98Ulm], [00Sch], [01Fen], [01Ulm], [01Zha] and a large number of spectroscopic, scattering and imaging methods have been applied for their structural characterization. 3.8.10.2 Physical and Chemical Properties 3.8.10.2.1 Structural data: Tilt and twist angles, packing and lattice structures The main numerical values to characterize packing and structure of self-assembled monolayers on a given substrate are the tilt and twist angles of the chains which are tabulated in table 1. The tilt of the chains in a self-assembled monolayer originates from the requirement to fill volume and to maximize interchain van der Waals interactions. It decreases with lower packing density, e.g. alkanethiols on gold (111) are more strongly tilted than alkanethiols on silver (111) due to the higher packing density on gold (see table 1). At a given chain length, the thickness of the monolayer can be calculated from the tilt angle.

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3.8.10 Polyatomic chain-like hydrocarbons on metals and semiconductors [Ref. p. 380

Y z

Qt

y

ct x

Fig. 2. Eulerian angles that describe the orientation of an alkane chain in a self-assembled monolayer: θt is the tilt angle of the chain with respect to the surface normal; ψ is the twist angle of the chain which defines the rotation of the alkyl carbon backbone about the chain axis with respect to the plane defined by this axis and the surface normal. χt is the azimuthal rotation around the z-axis with respect to the substrate lattice and has rarely been determined experimentally; [00Sch].

For the determination of tilt and twist angles, spectroscopic techniques such as X-ray Photoelectron Spectroscopy (XPS), Infrared Reflection Absorption Spectroscopy (IRAS) or Near Edge X-ray Absorption Fine Structure (NEXAFS) have been used, and imaging methods such as Scanning Tunneling Microscopy (STM) or Atomic Force Microscopy (AFM) provide a real space representation of the structure and homogeneity of SAMs. Scattering methods such as Low Energy Electron Diffraction (LEED), Grazing Incidence X-ray Diffraction (GIXD) and Low Energy Atom Diffraction (LEAD) provide direct information about the two-dimensional structure of SAMs, i.e., their structure projected onto the surface plane. Both scattering and spectroscopic methods are spatially averaging, however, the averaging is performed over different regions with different weight. The molecular tilt angle determined by IRAS or NEXAFS, e.g., is averaged over all molecular chains, including disordered regions such as grain boundaries or defect sites. Note, that NEXAFS probes the molecular orientation at the SAM surface (due to the limited electron mean free path) [98Tho], whereas IRAS measurements give the average angles over the whole film thickness. In contrast, the determination of the tilt angle by GIXD includes only ordered and crystalline regions. This is important when results from different techniques are compared. Since the values reported in table 1 may differ slightly from method to method for a specific substrate/anchor group/chain combination, the applied technique has been indicated in each case. For a precise determination of tilt and twist angles by IRAS, it has been reported recently that effects such as the grain size of the substances used for bulk reference spectra have to be taken into account [01Arn]. In this compilation of published data on SAMs, aliphatic and aromatic hydrocarbons and their terminally substituted derivatives are tabulated. In table 1, only systems that form well-ordered aggregates are included. For aliphatic chains, only data for chain lengths of 16 or more carbon atoms are listed, since routinely reproducible values of the tilt and twist angles are obtained only for these highly ordered films, whereas smaller chain lengths may lead to disordered or unstable systems. For aromatic systems, data for biphenyl and terphenyl containing SAMs have been tabulated. The twist angles of the chains have only been determined for a few systems and are included in table 1, when available. In most cases, these values have not been determined on single crystals, but on evaporated metal surfaces that contain small crystals with a predominant lattice orientation. In the few cases where no substrate orientation is given in table 1, SAMs on oxidic surfaces without regular orientation are concerned or no lattice orientation has been indicated in the primary literature. The two-dimensional structure of a SAM (i.e., the structure projected onto the surface plane) describes the type of crystalline long-range order, the symmetry, the lattice parameters and the packing in the plane. For some cases (e.g. alkanethiols on Au(111)), the unit cell may Landolt-Börnstein New Series III/42A4

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373

contain several non-equivalent molecules. Superlattice structures are included in table 1, when data are available. Most of the data listed have been determined using SAMs prepared from dilute solution. Upon preparation from the gas phase, the growth kinetics of a SAM are usually faster due to the absence of solvent interactions, but the final, densely packed equilibrium structures are identical. Many spectroscopic investigations have been done in UHV and little or no structural differences are observed when compared to studies conducted under solvent, except for the terminal groups of the SAM. SAMs on nanoparticle surfaces (see table 3) have not been included in table 1, since these layers are usually more disordered than on flat single- or polycrystalline surfaces due to the particle curvature. All data given in table 1 have thus been determined on flat surfaces. 3.8.10.2.2 Heat of formation and thermal stability As opposed to purely physisorbed systems, self-assembled monolayers are chemisorbed on the underlying surface and form stable covalent bonds between the anchor group and the substrate. The heat of chemisorption, which can be usually measured as the heat of desorption, is in many cases (especially for short chain lenghts) equivalent to the enthalpy of formation of the anchor group-substrate bond (ca. 126 kJ/mol for alkanethiols on gold). As is shown in Fig. 3, only for longer chains (n>14) a further contribution to the heat of desorption from van der Waals interactions between the chains is observed. 200

–1

Desorption energy [kJ mol ]

160

120

80

40

0

4

8 12 16 Number of carbons

20

24

Fig. 3. Chemisorption (full symbols) and physisorption enthalpies (open symbols) for various alkanethiols on Au(111) as a function of the number of carbon atoms. For comparison, physisorption values for simple alkanes are indicated as dashed line. Only for chain lengths higher than 14 carbon atoms, the desorption energy is higher than the bond enthalpy of the anchor groupsubstrate bond. Values have been determined by temperature programmed desorption in UHV; [98Lav].

Since SAMs are covalently anchored on the substrate, they show higher resistance to desorption than physisorbed systems. Thermal desorption of alkanethiols on gold (111) occurs only at temperatures of about 500 K and this temperature is largely independent of the chain length [98Lav]. Trichlorosilane- or trialkoxysilane-based SAMs on oxide surfaces are thermally extremely stable due to the polymeric siloxane network formed on the surface. SAMs formed from octadecyltrichlorosilane on silicon dioxide have been found to be stable in UHV up to temperatures of 740 K, at which temperature the C-C bonds in the molecular backbone start to decompose [97Klu]. The siloxane network of the anchor groups remains on the surface after decomposition of the monolayers up to about 1100 K. Monolayers formed by reaction of 1-alkenes with hydrogen-terminated silicon surfaces have been reported to be stable up to 615 K [97Sun].

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3.8.10 Polyatomic chain-like hydrocarbons on metals and semiconductors [Ref. p. 380

3.8.10.2.3 Wettability The wettability of self-assembled monolayers is mainly determined by the chemical nature of their terminal group exposed to the surface. In table 2, a survey of advancing contact angle values of water and hexadecane is given for different end groups of SAMs. Generally, for densely packed and well-ordered monolayers and for a given backbone, the wettability is only weakly dependent on the substrate and the packing density. In table 2, only values for SAMs on gold have been listed. The contact angle increases with higher roughness of the substrate; table 2 lists only the limiting values for minimally rough evaporated surfaces. Aromatic SAMs show a smaller variation of the contact angle for different terminal groups since the aromatic backbone delocalizes induced charges, in contrast to aliphatic chains which present more localized terminal dipoles. 3.8.10.2.4 Anchor groups for SAMs on inorganic substrates A broad variety of anchor groups have been used for the covalent attachment of hydrocarbon chains to inorganic surfaces and have been listed in table 3. On noble metals, sulfur containing anchor groups such as thiolates or disulfides which form covalent bonds to the surface are frequently utilized. On metal oxide surfaces, coordinative bonding to the metal ion component of the surface oxide via functional groups such as phosphonic or carboxylic acids often prevails. For most types of metal and semiconductor oxide surfaces, trialkoxysilanes or trichlorosilanes are versatile anchor groups that form two-dimensional crosslinked siloxane networks on the surface, but the long-range order of these SAMs is generally lower than that of e.g. alkanethiols on noble metals. As opposed to most of the other anchor groups listed in table 3, silanes require a minimum of water content in the solvent used for SAM formation in order to polymerize on the surface. For particle (colloid) surfaces, the same anchor groups as for planar surfaces can in general be used. In table 3, data are included for anchor groups on nanoparticles in a size range from some nanometers to several hundred nanometers. Anchor groups for SAMs can also be used for the chemical attachment of thicker, more disordered films on a surface such as oligomeric or polymeric systems and are therefore of technological importance e.g. for adhesion improvement. 3.8.10.3 List of abbreviations AES AFM ATR-IR GIXD IRAS LEAD LEED NEXAFS SAM SPS SPR STM UHV XANES XPS XR

Auger Electron Spectroscopy Atomic Force Microscopy Attenuated Total Reflection Infrared Grazing Incidence X-ray Diffractometry Infrared Reflection Absorption Spectroscopy Low Energy Atom Diffraction Low Energy Electron Diffraction Near Edge X-ray Absorption Fine Structure Self-assembled Monolayer Surface Plasmon Spectroscopy Surface Plasmon Resonance Scanning Tunneling Microscopy Ultrahigh Vacuum X-ray Absorption Near Edge Structure X-ray Photoelectron Spectroscopy X-ray Reflectivity

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375

3.8.10.4 Tables Table 1. Tilt and twist angles, areas per molecule, structures and superlattices of self-assembled monolayers on various substrates. System

Tilt angle

Alkanethiol / Au(111)

32-34° (IRAS) 55° (IRAS) [90Nuz1], [90Nuz2] [90Nuz1] 35° (NEXAFS) [92Häh]

Alkanethiol / oxidized Au(111)

18° (deposition from solution) 28° (vapour deposition) (NEXAFS) [99Yan] 11° (IRAS) [98Tho] 45° (IRAS) 26° (NEXAFS) [98Tho] [98Tho]

Alkanethiol / Au(111) exposed to Hg vapor Dialkylsulfide / Au(111) Alkaneselenol / Au(111) Alkanethiol / Au(001)

Twist angle

HS(CH2)15COOH / Au(111) HS(CH2)15CONH2 / Au(111) HS(CH2)16CN / Au(111) F(CF2)10(CH2)n-SH (n = 2,6,11) / Au(111)

Superlattice

(√3×√3)R30° [88Str], [90Chi]

c(4×2) LEAD [93Cam] GIXD [93Fen] STM [94Poi], [94Del], [94Buc] 6×√3 (STM, after 6 months storage) [02Noh]

15° (GIXD) [92Sam] 22.2 Å2 (LEAD, GIXD) [95Li]

33.5° (LEAD, GIXD) [95Li]

as deposited: c(2×2) (LEED) [93Dub] annealed: c(2×8) (LEAD) [95Li] c(2×2) (LEAD) [93Cam]

23.5 Å2 (LEAD) [93Cam] 28° (IRAS) [90Nuz2] 50° [90Nuz2] 39.6° (NEXAFS, XPS) [97Dan] 32° [90Nuz2] 55° [90Nuz2] 31° [90Nuz2]

42.5° (NEXAFS) [03Fre] 0-16° (fluorinated segment, SPR, AFM) [01Tam] F(CF2)10(CH2)n-SH 32-38° (alkyl (n = 2,11,17) / Au(111) segment, NEXAFS) 12.5-24° (fluorinated segment, XPS, IRAS, NEXAFS) [00Zha], [00Fre] HS(CH2)11(OCH2CH2)3 ∼30° (alkyl segment) OMe /Au (111) ∼0° (oligoether segment, IRAS) [98Har] HS(OCH2CH2)6C10H21/ 32° (alkyl segment) Au(111) ∼0° (oligoether segment, IRAS) [98Van] Landolt-Börnstein New Series III/42A4

Structure

(√3×√3)R30° [99Sch]

Alkanethiol / Au(110)

OH-terminated Alkanethiol / Au(111)

Area per molecule 21.6 Å2 [00Sch]

(√3×√3)R30° [90Nuz2]

55° [90Nuz2]

p(2×2) or c(7×7) [01Tam] 54-58° (alkyl segment) [00Zha]

−30° [98Van]

376

3.8.10 Polyatomic chain-like hydrocarbons on metals and semiconductors [Ref. p. 380

System

Tilt angle

1,1'-Biphenyl-4-thiol / Au(111) 1,1':4',1''-Terphenyl-4thiol / Au(111) CH3(C6H4)2SH / Au(111) CH3(C6H4)2(CH2)nSH (n = 1-6) / Au(111)

32° [01Fre] 23° ± 5 (NEXAFS) [01Fre] 20° ± 5 (NEXAFS) 32° [01Fre] [01Fre] 19° (GIXD) [00Leu]

Alkanethiol / Ag(111)

10° (NEXAFS) [98Him] 0-18° (SPS) [97Ehl] 12-13° (IRAS) [91Lai], [91Wal] 0° ± 5 (GIXD) [96Sam] 29.5° (NEXAFS) [03Fre] 32° [01Fre] 18° ± 5 (NEXAFS) [01Fre] 16° ± 5 [01Fre]

HS(CH2)16CN / Ag(111) 1,1'-Biphenyl-4-thiol / Ag(111) 1,1':4',1''-Terphenyl-4thiol / Ag(111) CH3(C6H4)2(CH2)nSH (n = 1-6) / Ag(111)

45 ± 10 (n = even) 23 ± 7 (n = odd) NEXAFS, IRAS [01Ron], [00Zha]

24° ± 6 (n = even) 42° ± 9 (n = odd) NEXAFS, IRAS [01Ron] HS(CH2)11(OCH2CH2)3 ∼10° (alkyl segment) OMe / Ag(111) ∼0° (oligoether segments, IRAS) [98Har] F(CF2)10(CH2)n-SH 10-12° (alkyl seg(n = 2,11,17)/ Ag (111) ment, NEXAFS) 12.5-24° (fluorinated segment, XPS, IRAS, NEXAFS) [00Zha], [00Fre] Alkanethiol / Hg 0° [96Mag], [98Ulm] Alkanethiol / Cu(111) 12° (IRAS) [91Lai] 12° (NEXAFS) [98Ima], [97Rie] Alkanethiol / Fe 0° (XPS, AES) [90Str] Alkanethiol / Pt <15° (IRAS) [03Li] Alkanethiol / Pd(111) 14-18° (IRAS) [03Lov] Alkanethiol / InP(110) 34° (XANES) [99Zer] Alkanethiol / InP(100) 51° (angle resolved XPS) [02Yam] Alkanethiol / 57° (IR) [92She] GaAs(100) 1,1'-Biphenyl-4-thiol / 31.5° (NEXAFS) GaAs(100) [03Sha] C18H37MgX / 25° (XPS) [98He]

Twist angle

Area per molecule

Structure

Superlattice

(√3×√3)R30° [00Leu] (√3×√3)R30° (n = odd) [01Ron]

61 ± 10 (NEXAFS, IRAS) [01Ron], [00Zha] 18.5 Å2 [00Sch]

(√3×√3)R10.9° [00Sch]

54° ± 10 (NEXAFS, IRAS) [01Ron]

47-48° (alkyl segment) [00Zha]

17.0 Å2 [91Lai]

45° (IRAS) [03Lov]

45° (IR) [92She]

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Ref. p. 380] 3.8.10 Polyatomic chain-like hydrocarbons on metals and semiconductors System Ge-Cl(111) Alkanethiol / Ge-H(111) Biphenyl-4-ol / H-Si(111) p-Terphenyl-4-ol (TPOH) / H-Si(111) (Me)2(C18H37)2N+ / muscovite mica C18H37SiCl3 / Si(001)

Cl3Si-(CH2)16-CN and Cl3Si-(CH2)16-Br / Si/SiO2 C18H37Si(OMe)3 / Cr/CrO2

Tilt angle

Twist angle

Area per molecule

Structure

377

Superlattice

20° (XPS) [01Han1] 28.7° (NEXAFS) [03Zha] 33° (NEXAFS) [03Zha] 38° (NEXAFS) [99Bro] 20° ± 4 (XR, ellipsometry) [90Tid] 21° (GIXD) [91Tid] 10° (IR) [95All] 7° (ATR-IR) [99Val] <10° (NEXAFS, XPS) [95Bie] 21° (ATR-IR) [99Val]

20.2 Å2 [91Tid]

9° (NEXAFS) [98Hil]

Table 2. Advancing contact angles θa of water and hexadecane on differently substituted alkanethiol SAMs on gold (111). Thiol Ref. θa (H20) [°] θa (C16H34) [°] HS(CH2)2(CF2)5CF3 118 71 89Bai HS(CH2)17CH3 112 47 89Bai HS(CH2)17CH=CH2 107 39 89Bai HS(CH2)19Br 97 <5 89Bai HS(CH2)11OCOCF3 96 62 89Bai HS(CH2)19F 95 <5 89Bai HS(CH2)19Cl 83 <5 89Bai HS(CH2)16OCH3 75 41 89Bai HS(CH2)10CO2CH3 67 28 89Bai HS(CH2)11CN 63 <5 89Bai HS(CH2)10CONH2 13 <5 89Bai HS(CH2)15CO2H <10 <5 89Bai HS(CH2)11OH <10 <5 89Bai Thiophenol 80 93Sab 4-Biphenylthiol 85 93Sab 4-Terphenylthiol 80 93Sab HS-(C6H4)2-CH3 85 01Kan HS-(C6H4)2-CF3 85 01Kan HS-(C6H4)2-OH 30 01Kan HS-(C6H4)2-F 84 01Kan HS-(C6H4)2-Cl 90 01Kan HS-(C6H4)2-Br 81 01Kan HS-(C6H4)2-I 79 01Kan Landolt-Börnstein New Series III/42A4

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3.8.10 Polyatomic chain-like hydrocarbons on metals and semiconductors [Ref. p. 380

Thiol HS-(C6H4)2-CO2Et HS-(C6H4)2-CHOHCH3 HS-(C6H4)2-OEtOMe HS-(C6H4)2-NO2 HS-(C6H4)2-N(CH3)2 HS-(C6H4)2-SH HS-(C6H4)2-SCH3 HS-(C6H4)2-COCH3 HS-(C6H4)-pyridine

θa (H20) [°]

θa (C16H34) [°]

65 60 61 64 66 67 70 55 28

Ref. 01Kan 01Kan 01Kan 01Kan 01Kan 01Kan 01Kan 01Kan 01Kan

Table 3. Anchor groups for self-assembled monolayers on flat and particle substrates. Anchor group RSH

Substrate Au

RSH

Ag

RSH RSH

Cu Pd

Flat surfaces [87Por] [88Bai1] [88Bai2] [89Bai] [91Lai] [96Poi] [96Ulm] [98Lai] [00Sch] [00Ulm] [01Ulm] [02Eve] [91Fen] [91Lai] [91Wal] [92Lai] [96Sam] [98Hut] [99Fel] [99Scho] [00Fre] [01Kan] [01Ron] [91Lai] [93Yam] [98Jen] [98Ron1] [00Sun] [02Lov]

RSH

Pt

[91Gui] [95Hin]

RSH RSH

Hg Fe

[93Dem] [96Mus] [99Mus] [01Slo] [89Vol] [92Vol] [95Che] [95Rei] [97Noz]

RSH RSH RSH RSH RSH RSH RSH RSH RSSR'

Ni Ir Ge/GeH GaAs InP Indium tin oxide (ITO) Oxidised Au χ-Fe2O3 Au

[97Mek] [98Kan1] [00Kan]

RSR'

Au

RSR' RSO2H RSO2H RSO3H RSO3H RSO3H RSeH RSeSeR' RSeSeR' R3P

Ag Au Ag Au Ag FeOx Au, Ag Au Ag Au

Particle surfaces [95Bru] [96Bad] [97Bad1] [97Bad2] [97Sar] [98Kan2]

[02Tzh] [99Yee3] [00Che] [00Cli] [01Zam] [02Qui] [98Das] [99Yee2] [01Pet] [02Zha] [94Roz] [96Kat] [97Kat] [98Kat] [99Yee3]

[01Han1] [92Bai] [92She] [95Gu] [99Yam] [99Zer] [00Yan] [02Bre] [94Ron] [98Ron2] [98Ron3] [99Yan] [00Woo] [83Nuz] [92Hic] [93Bie] [94Bie] [94Off] [96Beu] [96Cas] [98Che] [98Lee1] [98Nel] [99Hei] [00Gro] [88Tro] [89Til] [93Hag] [94Zho] [98Tre] [99Zho] [00Tak] [95Hei] [93Cha] [94Gar] [95Gar] [98Lee2] [98Cao] [99Cao] [01Cao] [95Gar] [98Lee2] [93Tar] [98Hut] [01Cao]

[96Liu] [97Kat] [98Pro] [98Por] [01Sho]

[02She]

[99Yee1] [92Sam] [97Dis] [98Hua] [01Han2] [98Ban] [98Hua] [99Ban] [01Han2] [99Ven] [95Uvd] [98Kar] Landolt-Börnstein New Series III/42A4

Ref. p. 380] 3.8.10 Polyatomic chain-like hydrocarbons on metals and semiconductors Anchor group R3P R3P R3P R3P=O RNC

Substrate Ag Cu Rh Graphite Au

Flat surfaces [98Kar] [98Kar] [98Uvd] [01Sad] [00Jia] [92Hic] [96Hen] [99Lin] [00Hen]

RNC RNC RNC RCOOH

Ag Pt Cr Metal oxides (AgO, CuO, Al2O3 etc.) Indium tin oxide (ITO) Metal oxides (AgO, CuO, Al2O3 etc.) SiO2, glass, Fe2O3, Al2O3, Mica, ZnSe, GeO2, AuO, Si3N4 Indium tin oxide (ITO) ZrO2, In2O3/SnO2, CuO, AgO, Al2O3, Fe2O3 Sapphire Ti/TiO2 Ta2O5 Indium tin oxide (ITO) ZrO2 Si/SiH Si/SiH Si/SiCl Ge/GeCl Si/SiH Si/SiH Si/SiH Mica

[99Han] [89Hic] [92Hic] [01Hor] [99Clo] [82Gol] [84All] [85All1 [85All2] [93Sam] [93Tao] [96Ulm] [00Yan] [95Fol]

RCOOH RCONHOH RSiCl3, RSi(OR)3, RSiMe2Cl RSiCl3 RPO3H2 RPO3H2 RPO3H2 RPO3H2 RPO3H2 RPO3H2 (RCOO)2 RLi RLi, RMgX RMgX ROH RCHO RCH=CH2 R4N+ X−

[80Sag] [88Til] [89Was] [90Ulm] [95Moa] [96Ulm] [99Ste] [00Sch] [03Lus] [95Fol] [96Gao] [96Woo]

379

Particle surfaces

[95Shi] [98Ont1] [98Ont2] [99Hor] [95Liu] [99Kat]

[96Van]

[99Yee1] [02Paw]

[01Mes] [01Gaw] [00Tex] [02Bre] [02Yim] [93Lin] [99Kim] [94Che] [01Ban] [98He] [95Cle] [00Bou] [01Bar][03Zha] [98Eff] [00Bou] [95Lin] [98Eff] [99Bou] [01Bar] [97Woo] [98Hay]

Acknowledgement: The author thanks Prof. Michael Grunze for his continuous support and for proofreading the manuscript.

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3.8.10.5 References for 3.8.10 80Sag 82Gol 83Nuz 84All 85All1 85All2 87Por 88Bai1 88Bai2 88Str 88Til 88Tro

Sagiv, J.: J. Am. Chem. Soc. 102 (1980) 92. Golden, W.G., Snyder, C.D., Smith, B.: J. Phys. Chem. 86 (1982) 4675. Nuzzo, R.G., Allara, D.L.: J. Am. Chem. Soc. 105 (1983) 4481. Allara, D.L., Nuzzo, R.G.: Polym. Prepr. 25(2) (1984) 185. Allara, D.L., Nuzzo, R.G.: Langmuir 1 (1985) 45. Allara, D.L., Nuzzo, R.G.: Langmuir 1 (1985) 52. Porter, M.D., Bright, T.B., Allara, D.L., Chidsey, C.E.D.: J. Am. Chem. Soc. 109 (1987) 3559. Bain, C.D., Whitesides, G.M.: J. Am. Chem. Soc. 110 (1988) 3665. Bain, C.D., Whitesides, G.M.: Science (Washington) 240 (1988) 62. Strong, L., Whitesides, G.M.: Langmuir 4 (1988) 546. Tillman, N., Ulman, A., Schildkraut, J.S., Penner, T.L.: J. Am.Chem. Soc. 110 (1988) 6136. Troughton, E.B., Bain, C.D., Whitesides, G.M., Nuzzo, R.G., Allara, D.L., Porter, M.D.: Langmuir 4 (1988) 365. 89Bai Bain, C.D., Troughton, E.B., Tao, Y.T., Evall, J., Whitesides, G.M., Nuzzo, R.G.: J. Am. Chem. Soc. 111 (1989) 321. 89Bai Bain, C.D., Whitesides, G.M.: Angew. Chem., Int. Ed. Engl. 28 (1989) 506. 89Hic Hickman, J.J., Zou, C., Ofer, D., Harvey, P.D., Wrighton, M.S., Laibinis, P.E., Bain, C.D., Whitesides, G.M.: J. Am. Chem. Soc. 111 (1989) 7271. 89Til Tillman, N., Ulman, A., Elman, J.F.: Langmuir 5 (1989) 1020. 89Vol Volmer, M., Stratmann, M., Viefhaus, H.: Fresenius' Z. Anal. Chem. 333 (1989) 545. 89Was Wasserman, S.R., Tao, Y.-T., Whitesides, G.M.: Langmuir 5 (1989) 1074. 90Chi Chidsey, C.E.D., Loiacono, D.N.: Langmuir 6 (1990) 682. 90Nuz1 Nuzzo, R.G., Korenic, E.M., Dubois, L.H.: J. Chem. Phys. 93 (1990) 767. 90Nuz2 Nuzzo, R.G., Dubois, L.H., Allara, D.L.: J. Am. Chem. Soc. 112 (1990) 558. 90Str Stratmann, M.: Adv. Mater. 2 (1990) 191. 90Tid Tidswell, I.M., Ocko, B.M., Pershan, P.S., Wasserman, S.R., Whitesides, G.M., Axe, J.D.: Phys. Rev. B 41 (1990) 1111. 90Ulm Ulman, A.: Adv. Mater. 2 (1990) 573. 91Fen Fenter, P., Eisenberger, P., Li, J., Camillone, N., III, Bernasek, S., Scoles, G., Ramanarayanan, T.A., Liang, K.S.: Langmuir 7 (1991) 2013. 91Gui Gui, J.Y., Stern, D.A., Frank, D.G., Lu, F., Zapien, D.C., Hubbard, A.T.: Langmuir 7 (1991) 955. 91Lai Laibinis, P.E., Whitesides, G.M., Allara, D.L., Tao, Y.-T., Parikh, A.N., Nuzzo, R.G.: J. Am. Chem. Soc. 113 (1991) 7152. 91Pri Prime K. L.; Whitesides G. M.: Science (Washington) 252 (1991) 1164. 91Tid Tidswell, I.M., Rabedeau, T.A., Pershan, P.S., Kosowsky, S.D., Folkers, J.P., Whitesides G.M.: J. Chem. Phys. 95 (1991) 2854. 91Ulm Ulman, A.: An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to SelfAssembly, Academic Press, Boston, 1991. 91Wal Walczak, M.M., Chung, C.S., Stole, M., Widrig, C.A., Porter, M.D.: J. Am. Chem. Soc. 113 (1991) 2370 92Bai Bain, C.D.: Adv. Mater. 4 (1992) 591. 92Häh Hähner, G., Kinzler, M., Thümmler, C., Wöll, Ch., Grunze, M.: J. Vac. Sci. Technol. A 10 (1992) 2758. 92Hic Hickman, J.J., Laibinis, P.E., Auerbach, D.I., Zou, C., Gardner, T.J., Whitesides, G.M., Wrighton, M.S.: Langmuir 8 (1992) 357. 92Lai Laibinis, P.E., Lewis, N.S.: Chemtracts: Inorg. Chem. 4 (1992) 49. 92Sam Samant, M.G., Brown, C.A, Gordon II., J.G.: Langmuir 8 (1992) 1615. 92She Sheen, C.W., Shi, J.X., Maartensson, J., Parikh, A.N., Allara, D.L.: J. Am. Chem. Soc. 114 (1992) 1514. 92Vol Volmer-Uebing, M., Stratmann, M.: Appl. Surf. Sci. 55 (1992) 19. 93Bie Biebuyck, H.A., Whitesides, G.M.: Langmuir 9 (1993) 1766. Landolt-Börnstein New Series III/42A4

3.8.10 Polyatomic chain-like hydrocarbons on metals and semiconductors 93Cam 93Cha 93Dem 93Dub 93Fen 93Hag 93Lin 93Sab 93Sam 93Str 93Tao 93Tar 93Yam 93Zha 94Bie 94Buc 94Che 94Del 94Gar 94Off 94Poi 94Ron 94Roz 94Zho 95All 95Bie 95Bru 95Che 95Cle 95Fol 95Gar 95Gu 95Hei 95Hin 95Li 95Lin 95Liu 95Moa 95Opi 95Rei 95Shi 95Uvd 96Bad

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4.3 Adsorbate induced surface core level shifts of metals REINHARD DENECKE, NILS MÅRTENSSON

4.3.1 Introduction Core level photoelectron spectroscopy is an element specific, surface sensitive and quantitative probe, which can be used to identify the elemental composition of surfaces. The primary information acquired is the electron binding energy. This binding energy depends strongly on the coordination of the respective atom and/or its neigboring atoms. These reflect the chemistry (alloys, oxides or adsorbates) around the atom from which the photoelectron is being emitted. Any differences in these parameters will lead to differences in the electron binding energy, and this change is called chemical shift. The chemical shifts make it possible to distinguish between atoms of the same element in different chemical environments. Of particular interest for the investigation of surface systems is the surface core level shift (SCLS). When it became possible to measure core level photoelectron spectra with sufficiently high resolution and surface sensitivity it was realized that the different surroundings of the atoms at the surface and in the bulk of a sample lead to a chemical shift, the surface core level shift. The surface core level shifts may be further modified by the presence of adsorbed atoms and molecules. From these shifts most important information about the adsorbate-substrate bonding can be obtained. There exist some review articles dealing with parts of the material presented here [86Ege, 95Mar, 01And1, 03Bar2]. Nevertheless, they mainly cover only small fractions of the whole subject, either by concentrating on a specific substrate material, or by considering only data from a particular working group. In the present contribution we review the existing measurements of adsorbate induced core level shifts. Surface core level shifts are present for all types of systems. However, we restrict ourselves to metallic substrates. We exclude semiconductor surfaces due to the fact that these often have very complex surface structures (reconstructions) and complex adsorbate arrangements. These situations would have to be described in much more detail than what is suited for this type of tabulation. For the same reason we exclude compound surfaces. We furthermore restrict ourselves to the adsorption of small (gas) molecules where each adsorbed atom or molecule bonds to a small and well-defined number of surface atoms. Large adsorbate molecules lead to a more distributed influence on the surface atoms and again each situation would have to be discussed in some detail. Furthermore, we only treat adsorbate overlayers. We do not cover for instance the growth of oxide films in general, but we will give some examples where we think it is appropriate. We also exclude metallic adsorbates except for alkali systems. The adsorption of metallic atoms often leads to complex situations, with a strong and often continuous dependence on coverage, and one would have to describe detailed phase diagrams for each system. The shifts for these situations are, however, well understood and for a discussion of these we refer to, e.g., [88Nil, 89Ste1, 89Ste2, 88Mar]. The case of alkali and earth-alkali metals is somehow special since in most cases they do not display typical metallic behavior. We have therefore included some examples. In the tables of the data section we are exclusively dealing with experimental data. There are, however, a large number of theoretical studies on surface and adsorbate-induced core-level shifts as well. In the next section we will briefly outline some theoretical concepts and refer to some others. In the subsections of the data section we will give citations of the suitable theoretical articles and discuss them briefly, where appropriate. For more specific values the reader is also referred to cited publications in the respective experimental articles.

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The surface core level shift Chemical shifts for metallic systems are well understood theoretically. The final state system is identical to the initial state with the exception that one core electron is missing for one of the atoms in the system. This core-ionized atom is a different chemical species with different properties. Furthermore it has been shown that for metallic systems there is a complete screening of the core ionized state (e.g. [80Joh]). This implies that the valence electron distribution can be treated as fully adjusted to the new situation. When calculating core ionization energies one contribution corresponds to the energy of a (chemical) replacement reaction where the original atom is replaced by a core ionized one. In this section we derive an expression for the surface chemical shift, which can be used to make some general statements about the shifts. For a more detailed derivation we refer to, e.g., [80Joh, 83Joh, 88Nil, 95Mar]. The binding energy for a core level denoted c in a metal M is defined as the difference in total energy between the system with a core electron missing in shell c and the total energy of the initial state system EB(M, c) = ETot(M, c) − ETot(M)

(1)

When discussing chemical shifts in metallic systems, one can make the following partitioning of the expression for the binding energy: EB(M, c) = E0(M, c) + EBond(M) − EBond(Mc, M)

(2)

EBond(M) is the energy by which an atom in the metal M bonds to the lattice. We use the sign convention that a stable metal corresponds to a positive value of EBond(M). EBond(Mc, M) is the corresponding energy for a core ionized atom Mc in the M metal host. E0(M, c) contains all other contributions to the core ionization energy. It can be shown that E0(M, c) is independent of the detailed coordination of the core ionized atom and that this quantity will cancel when chemical shifts are considered. For the bulk and the surface of the metal M, Eq. (2) can be written as EB(M, c, bulk) = E0(M, c) + EBond, bulk(M) − EBond, bulk(Mc,M)

(3)

EB(M, c, surf) = E0(M, c) + EBond, surf(M) − EBond, surf(Mc, M)

(4)

The surface core level shift is defined as

∆s = EB(M, c, surf) − EB(M, c, bulk)

(5)

Before inserting (3) and (4) in (5) we make the following definitions EBond, surf(M) = β EBond, bulk(M)

(6)

EBond, surf(Mc, M) = β´ EBond, bulk(Mc, M)

(7)

β and β´ are parameters that describe the effects of the reduced atomic coordination at the surface. Due to

the reduced number of neighbors the bonding for a surface atom to the lattice will be smaller than for the bulk and hence β and β´ are parameters with values <1. β and β´ are of the order of 0.8 for a close-packed surface and smaller for more open surfaces. For the present qualitative discussion we assume that β = β´ and furthermore we rewrite this factor as 1 − α. Hence α ≈ 0.2. With these definitions and assumptions we arrive at the following expression for the surface core level shift

∆s = α (EBond, bulk(Mc, M) − EBond, bulk(M))

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(8)

390

4.3 Adsorbate induced surface core level shifts of metals

[Ref. p. 418

In order to evaluate this expression we first of all note that EBond, bulk(M) is normally identical to the cohesive energy of the metal M. There are some exceptions where there are significant configuration changes between the free atom and the metal [80Joh]. This is for instance the case for several lanthanides. For the term EBond, bulk(Mc, M) it has been shown that a first and fairly accurate approximation is obtained by considering the corresponding energy in a lattice where all M atoms are replaced by core ionized atoms, i.e. EBond(Mc). Furthermore, a good estimate of this energy is obtained by noticing that the chemical properties of a core ionized atom are very similar to those of the next element in the Periodic Table. This is due to the fact that the valence electron distribution reacts in very much the same way upon the removal of a core electron as for the addition of a unit charge to the nucleus of the atom. EBond, bulk(Mc, M) can thus be approximated by the cohesive energy of the next element in the Periodic Table [80Joh]. Surface core level shifts are typically of the order of a few tenths of an eV up to about one eV. In some systems the surface components shift to lower binding energies and in some systems they shift to higher binding energies. This can be understood from Eq. (8). For an early transition metal (to the left in the Periodic Table) the fully screened final state atom bonds stronger to the lattice than the initial state atom (Ecoh(MZ+1) > Ecoh(MZ)). This is due to the fact that one more valence electron is occupying a bonding orbital. This yields a positive surface core level shift (towards higher binding energies). For a late transition metal (to the right in the Periodic Table) the bonding of the final state atom is instead reduced due to an additional occupation of an unoccupied orbital. This leads to a negative shift. In order to illustrate this trend we show in Fig. 1 a compilation of measured and calculated SCLS for various 5d transition metals [89Mar]. Only values for close-packed surfaces are included. For a refined analysis of the surface core-level shifts one has to consider further details, such as the surface structure, and more specific details of the electronic structure [80Joh, 80Ros, 83Cit, 83Joh, 88Nil, 94Ald, 95Mar]. 0.75

0.50

Rosengren Tomanek Johansson & Mårtensson experiments

SCLS [eV]

0.25

0

– 0.25

– 0.50

– 0.75 Yb Lu Hf Ta W Re Os Ir Pt Au fcc hcp hcp bcc bcc hcp hcp fcc fcc fcc (111) (0001) (0001) (111) (110) (0001)(0001) (111) (111) (111)

Fig. 1. Comparison of experimentally and theoretically determined SCLS’s of the 5d transition metals. Only close-packed surfaces are included. Reproduced from [89Mar].

In this section we have presented an approximate evaluation of the shifts in order to give a qualitative account of trends in the surface core level shifts. However, the expressions in Eqs. (3) and (4) are exact and they serve as a powerful basis for detailed ab initio shift calculations using DFT (Density Functional Theory). In these calculations factors like the detailed surface structures can be included. Examples of this approach can be found in [93Bag, 94And2, 96Ham, 98Bih, 99Bag, 01Cho, 01Gan, 01Liz, 03Bir] and further references cited therein. Landolt-Börnstein New Series III/42A4

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An important aspect of the chemical shifts in metallic systems is the fact that they are usually dominated by short-range effects. In most cases the main influence on the chemical shifts comes from the nearest neighbors. This makes the shifts very useful for many purposes, such as for obtaining information on the structural arrangements of the adsorbates. From the treatment above it is also clear how the influence of an adsorbate could be included in the shift calculations. In the presence of an adsorbate the terms EBond, surf(M) and EBond, surf(Mc, M) in Eq. (4) will contain contributions corresponding to the bonding of the adsorbate to a surface atom before and after the ionization, respectively. In this way we obtain the following modified expression for the core ionization energy for the adsorbate covered surface: EB(M, c, surf, ads) = EB(M, c, surf) + Eads(M) − Eads(Mc, M)

(9)

We use the sign convention that Eads is positive for a stable adsorbate. In this case it is not always straightforward to make simple quantitative statements about the shifts. Eads(M) is normally known. In the final state one of the M metal atoms has changed to an Mc atom and independent information about this situation is usually not available. In the case of on-top adsorption where the adsorbate bonds only to one substrate atom and assuming only nearest neighbor interaction, the last term can be written as Eads(Mc). If the atom or molecule bonds at this site also for the Z+1 metal independent information can be obtained also for this term. However, in the general case total energy calculations are required in order to calculate the shifts. Based on the theoretical understanding of the core level shifts it has also been realized that the shifts are directly related to other most relevant properties of the systems. The surface core level shift has thus direct links to surface segregation energies [80Joh, 83Ege]. This implies that there is a similar connection between adsorbate induced surface core level shifts and adsorbate effects on the surface segregation energies [88Mar]. Binding energy scales For the utilization of chemical shifts it is very important to define what reference energy is used for the shift scale. In this work we give all shifts relative to the bulk binding energies. The shift between the surface and the bulk components is usually seen directly in the spectrum. This way of using relative binding energies from the same measurement avoids all problems related to properly calibrating absolute energy scales. For each surface we present in the tables also the determined surface core level shift for the clean surfaces. In this way one can in each case derive by how much the surface components are shifted due to the presence of the adsorbates. The utilization of the bulk component as the energy zero for the shift scale usually gives well-defined shift values. This choice is based on the assumption that the bulk binding energy can be accurately determined. This is usually a relevant assumption. In some cases there may be significant shifts also of one or more layers below the first surface layer. These are often unresolved and lead to a broadening of the bulk peak. If the measurements are performed under similar conditions in different investigations this way of establishing the reference energy does not introduce any additional errors since the same ratio of peaks from the different layers will be determined. However, if the measurements are different in terms of, e.g., surface sensitivity, the measurements will locate the bulk component at slightly different energies. Whenever a second layer shift has been identified also this value is included in the tables. Beryllium provides a striking example of the existence of shifts also for deeper layers. For the Be(0001) surface distinct and well resolved core level shifts are observed for the four outermost surface layers [01And2]. A spectrum with the deconvoluted components (“B” for the bulk and “S1”, “S2”, “S3”, “S4” for the outermost surface layers, counted from the surface) is shown in Fig. 2. This is, however, a rather unique case due to the semimetallic character of Be. Be is not included in the present review since no adsorbate measurements have been performed.

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4.3 Adsorbate induced surface core level shifts of metals

[Ref. p. 418

Be 1 s hν = 132 eV

S2

Intensity I

S1

B

S3 S4

112.5

112.0

111.5

111.0

110.5

Fig. 2. Be 1s core level recorded at a photon energy of 132 eV measured with a resolution of 20 meV (upper curve) and 70 meV (lower curve). Included are the results of a deconvolution into the bulk component “B” and four outermost surface layers “S1”, “S2”, “S3”, and “S4”. The fine structure in the upper curve is due to vibrational excitation. Reproduced from [01And2].

Binding energy Eb [eV]

Determination of surface core level shifts One of the important issues is that of correctly determining the core-level line positions from the measured spectra. There are many contributions to the line profiles and furthermore these are not completely known. The shifts are also usually rather small compared to the line widths leading to overlapping spectral features. Basically one needs to fit a model line profile to each core electron line in the system. If all contributions are well described by these model line profiles the determined core level positions are also well defined. Since there is no strictly unique way to determine core level binding energies, it is important that one reports how the binding energies have been determined. When determining surface core level shifts many line-shape contributions are similar for the different core level peaks. Several of the broadening contributions are similar for a surface, with or without an adsorbate, and for the bulk of the metal. This makes the shift determinations less model dependent than the determination of the absolute binding energies. This also implies that the shifts are transferable from one investigation to another. What is furthermore improving the situation very much is, that one is considering shifts between atoms in very similar environments. The core electron line profiles can be described as a convolution of a number of broadening functions. Several of these are due to effects which are intrinsic to the core ionization process. First of all a core excitation has a finite lifetime. This usually leads to a Lorentzian contribution to the line profile. This is the case for all core levels that are used for detailed determinations of chemical shifts. The Lorentzian broadening is totally symmetric. Usually the Lorentzian broadening can be considered to be identical for a core level independent of the chemical environment. The Lorentzian function can be described as follows: L(E, E0, λ) ∝ 2λ/[λ2−(E−E0)2]

(10)

E0 denotes the peak position, 2λ is the full width at half maximum (FWHM). The creation of a core-hole at the surface or in the bulk of a metal leads to a disturbance of the lattice. This will generate replicas of the core electron line, shifted by the energy required to excite one or more phonons in the system. Usually this leads to unresolved spectral features and the net result is a broadening of the core electron lines. In the case of Be, however, distinct phonon side bands have been resolved (see Fig. 2). In most cases many phonons are excited and the net broadening is to a good approximation a symmetric Gaussian: Landolt-Börnstein New Series III/42A4

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4.3 Adsorbate induced surface core level shifts of metals

G(E, E0, σ) ∝ 1/[√(2π)σ] exp [−(E−E0)2/(2σ2)]

393 (11)

Here, 2σ is the FWHM, E0 is again the peak position. The core ionization process also leads to the excitation of valence electrons. These processes are denoted shake-up and shake-off. For a metallic system there is a continuum of excitation energies all the way down to zero. This leads to asymmetric line-shapes with tails towards the high binding energy side of the core level spectra. The resulting line-shapes depend on the details of the electronic structure of the system. However, usually the line profiles can be well described by model line profiles with one or possibly two parameters. One such description was introduced by Doniach and Šunjić [70Don] (D-S): DS(E, γ, α) ∝ Γ(1−α)cos[πα/2+(1−α)arctan(E/γ)]/[(E2+γ2)(1−α)/2]

(12)

with 2γ being the Lorentzian FWHM, and α describing the asymmetry parameter. This time, E0 = 0 is assumed. There is also a contribution in the spectra due to photoelectrons which have undergone inelastic scattering processes after the core ionization. Also this contribution leads to an asymmetric broadening towards the low kinetic energy or high binding energy side. These two contributions usually cannot be separated in a straightforward manner. In some cases there may also be discrete shake-up states in the vicinity of the main lines. There may also be other intrinsic line-shape contributions. In some systems there are effects due to the coupling between the open core shell created by the removal of a core electron and open valence shells. This may lead to complex final state spectra. The problem of unresolved shifts for different atomic layers was also discussed above. A most important contribution is that caused by the finite resolution of the experimental setup. This is due to the energy profile of the exciting photon spectrum as well as the finite resolution of the electron analyzer. This may lead to different types of line-shapes which also depend on the experimental setup. However, in most cases the instrumental broadening is reasonably well described by a Gaussian line profile (see Eq. 11). However, if shift measurements are performed at very different energy resolution any inaccuracies in the used model line profiles will lead to differences in the determined shifts. For the final fitting procedure convolutions of the various broadening effects have to be calculated. For a symmetric line this results in the Voigt profile which is a convolution between a Gaussian and a Lorentzian line profile, whereas for asymmetric line shapes a D-S function is convoluted with a Gaussian function. As a word of caution we want to emphasize again that fitting procedures not necessarily produce unique results. In fact, changing the number of components or changing some constraints (like going from variable peak positions to fixed ones) can easily change binding energy values by significant amounts. Therefore, the values quoted in the tables below should be used with this in mind. For further specific fitting procedures the reader is referred to the original articles. We have chosen not to describe, discuss and evaluate these in connection with the tabulations. Systematics of surface core level shifts For the tabulation of adsorbate-induced surface core level shifts it is important to describe the adsorbate structures correctly. We refer in each case to the original reference for details of the structures. Whereas for the adsorbate itself a classification according to the binding site seems to be the most relevant one, this does usually not provide sufficient information to describe the situation for the substrate atoms involved. When comparing different adsorbate-induced surface core-level shifts and identifying trends in these it may be helpful to describe and quantify the surface atom coordination to the adsorbed atoms or molecules. We have included this type of analysis in those cases where it is published. As a parameter we have used the coordination number nads of the metal surface atoms to the adsorbates. It is based on the assumption that an adsorbate of a particular kind and for a particular type of surface has a well-defined total influence on the substrate atoms. This influence, in turn, is very much localized to the nearestLandolt-Börnstein New Series III/42A4

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4.3 Adsorbate induced surface core level shifts of metals

[Ref. p. 418

neighbor atoms [88Nil, 91And, 00Sur, 01Gan]. An on-top adsorbate gives nads = 1 for the atom to which it is attached and nads = 0 for all other atoms. For a bridge bonded species the influence is shared between the two substrate atoms and both of these get a contribution of nads = 1/2. Furthermore, the influence is considered to be additive. This implies, for instance, that a surface atom that has two bridge bonded adsorbates attached to it gets nads = 1. There is no strict physical support for this model. In fact, it does sometimes not even work for very similar systems. However, there are several systems for which it provides a fairly reasonable way of systemizing and rationalizing the observed shifts. In brief, one could state that it seems to be applicable for atomic oxygen and CO adsorption. In the case of oxygen adsorption on Rh(111) a combined experimental and theoretical investigation using this concept has been performed [01Gan]. Oxygen adsorbs in threefold hollow sites on this surface. The different adsorbate phases correspond to situations with Rh surface atoms bonding to one, two or three oxygen atoms. These Rh sites correspond to nads = 1/3, nads = 2/3 and nads = 1, respectively. The p(2×2)-O phase has an adsorbate coverage of 0.25 ML and consists of 25 % surface atoms with nads = 0 and 75 % of atoms with nads = 1/3. In the p(2×1)-O phase (0.5 ML) there are 50 % surface atoms with nads = 1/3 and 50 % with nads = 2/3. It was found that the peaks in the spectra can be identified according to the parameter nads. The fact that surface atoms in different situations but with the same nads undergo approximately the same shift gives strong evidence that the shifts are dominated by nearest neighbor interaction. Furthermore, it was found that each bond to oxygen shifts the Rh 3d core level of a substrate atom by about 0.3 eV. A theoretical analysis of this correlation was also made [01Gan]. In an analysis of CO on Pd(111) a similar simple relationship was found [00Sur]. Typical spectra are shown in Fig. 3. In this case the core level data could be used to distinguish between different proposed models for the adsorbate structures. In this system different adsorption sites are populated. In the (√3×√3)R30° phase (0.33 ML adsorbate coverage) each surface atom is ideally attached to one CO bonded in a threefold hollow site (“H1”), yielding nads = 1/3. Also for the c(4×2)-2CO phase (0.5 ML) there are only hollow site adsorbates. There are 50 % Pd surface atoms with nads = 1/3 (“H1”) and 50 % with nads = 2/3 (“H2”). In the (2×2)-3CO phase (0.75 ML) there is instead a mixture of hollow and on-top sites (“T”). In this case 75 % of the Pd surface atoms bond to two hollow site CO molecules corresponding to nads = 2/3. The remaining 25 % of the surface atoms bond to one on-top adsorbate giving nads = 1. For intermediate coverages CO is also found on bridge sites (“Br”), as seen in the top panel of Fig. 3; here, nads = 1/2. Also in this case the shifts correlate fairly well with nads. This is the case in spite of the fact that different adsorption sites are involved [00Sur]. However, the clean surface with nads = 0 does not fit to the proposed linear dependence between core-level shift and nads, in contrast to the case of O/Rh(111) (see discussion above). Experimental requirements In order to observe the adsorbate-induced surface core-level shifts of interest here, the experimental conditions have to be such, that the photoemission signal is tuned to be quite surface sensitive. Since the critical parameter is the kinetic energy of the released photoelectron [79Sea], this sensitivity varies with the binding energy of the core level under study and with the excitation energy used. Thus, for a special choice of levels laboratory photon sources with fixed energy sometimes yield very good results [86Duc]. In most cases, however, synchrotron radiation is the photon source of choice for these high-resolution experiments. Synchrotron radiation is tunable in energy and with state-of-the-art monochromators high energy resolution (up to E/∆E=10000) can be achieved (see for example [99Den]). Another limiting factor is the width of the core-level lines. Depending on the natural lifetime and its related peak width it may be difficult to resolve surface related contributions, even with high resolution of the exciting radiation. Typical examples of these inherently wide lines are p levels, as e.g., encountered in Ni or Cu. This is reflected in the lack of experimental data for these systems. In contrary, the d and especially the f levels are quite narrow and thus yield well separated core-level components.

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395

θco = 0.72 ML

Pd 3d5/2 H2 B

T Br

θco = 0.50 ML

Intensity I [arb.units]

H1

B

H2

θco = 0.28 ML H1 B

H2 S

338

337

336

335

334

333

Fig. 3. Pd 3d5/2 core level spectra recorded with a photon energy of 400 eV for varying CO coverages at a sample temperature of 120 K. Components described in the text are also inlcuded. “B” marks the bulk emission, “S” the clean surface component. Peaks “H1” and “H2” are related to Pd atoms bonded to one or two CO molecules in threefold hollow sites, component “Br” is due to Pd atoms in contact with CO in a bridging position, and peak “T” represents Pd atoms with CO in on-top position. Reproduced from [00Sur].

Binding energy Eb [eV]

The apparent intensity of the different contributions can also influence the possibility to clearly identify adsorbate-induced features. In this respect, energy-dependent photoelectron diffraction plays an important role (see, e.g., [02Woo] and references therein). Again, the tunability of the photon energy allows to use kinetic energies which emphasize the contributions of interest. The detection angle of the photoelectrons (usually measured relative to the surface normal) also has a drastic effect on the surface sensitivity. The more grazing the escape angle, the more surface sentitivity is obtained. Additionally, again photoelectron diffraction effects, but now in the angular distribution, can further change relative intensities of different contributions [97Fad]. Being able to change this detection angle is an elegant way to identify surface related features, as is done in most of the studies using laboratory sources. Another important aspect in the studies summarized in this contribution is the cleanliness of the sample. Possible undetected surface contaminations not only strongly suppress the surface component, but can also lead to new features which are mistakenly interpreted as clean surface components. On the other hand, unwanted coadsorbates could also alter the binding energy of a certain adsorbate under study, thus leading to wrong values. These difficulties are usually more pronounced on more reactive metals, such as Aluminum. Therefore, discrepancies in the published data can also be caused by varying contamination levels.

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4.3 Adsorbate induced surface core level shifts of metals

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4.3.2 Data section In this chapter we present an overview of the reported values for adsorbate-induced surface core-level shifts. As mentioned in the introduction, we are giving the shifts relative to the bulk peak, which is the natural reference level. Since the shifts are closely related to the clean surface core-level shifts, we are including values for this as well. However, since measuring absolute binding energies in a reliable and comparable way is rather difficult, we will only quote difference values with respect to the individually reported bulk binding energies. If, for orientation only, absolute binding energy values of the bulk components are included, they are referenced to the Fermi energy. We use the convention that increasing binding energies (“stronger binding”) are denoted by larger positive numbers. Please note that some authors are using a scale going to more negative values. If that is the case we have “converted” the values accordingly. We will only quote numerical values given in the publications, and have not derived values from figures. Accuracies (quoted number of digits) are taken from the literature, but no error margins are given in our tables. As will be noticed, there are remarkable differences in some reported values. Sometimes they are due to improved experimental resolution with easier identification of components in the line shapes. Sometimes the origin of these differences is not clear. While some publications try to explain these disagreements with varying sample cleanliness or different sample preparation procedures, we are not commenting on differing results. Interested readers are referred to the original articles. As suggested guideline, more recent publications are more likely to quote more reliable numbers, but not necessarily. For each surface plane we will summarize the binding energy shifts in a separate table, ordered by adsorbate. For each adsorbate we will use the reported adsorbate binding configurations, if possible. In order to improve the readability we use the following abbreviations: H for hollow sites, T for on-top sites, B for bridge sites. This assignment of the adsorption site is rather difficult since we are dealing with the substrate atoms just influenced by the adsorbates. In some cases we will see that a correlation to adsorbate binding site is not possible. In those cases we give the different substrate atom types as quoted by the respective reference. For more general remarks, see the Introduction. We will discuss further details in the respective subsections. Data for the 3d transition metals are rather scarce. The 3d metals are more difficult to measure at high resolution than the narrowest levels for the 4d and 5d metals. This is due to the fact that the narrowest core-electron lines, the 2p lines, have rather high binding energies. This implies that higher photon energies are needed which limits the resolution. The 2p electron lines for the 3d transition metals are also intrinsically broader than the narrowest lines for the 4d and 5d transition metals as well as for the narrowest lines of all simple metals [81Nyh]. The intrinsic lifetime broadening is slightly larger. However, the additional broadening has mainly other reasons. One reason is the narrow band character of the 3d band. The 2p-3d coupling in the spectroscopic final states leads to a more complex line shape of the core-levels in the 3d metals. These effects do not lead to resolved effects in the spectra but are only seen as additional broadenings of the core electron lines. These make any surface core level shift harder to resolve. Below we show an example of a table given for the various substrates. In the beginning we display the quoted binding energies of the respective substrate bulk core level, with respect to the Fermi level. If there are different published values, we will give all of them. In the next row the clean surface core-level shift is given. As mentioned in the introduction, the shifts are all given relative to the respective bulk binding energies. Here, the 1st column gives details about the position of the substrate atoms considered. “1st layer” denotes the obvious surface layer, while “2nd layer” refers to the first underlayer, if it is distinguishable from the bulk value. Values for different adsorbates are given in the following rows, grouped in sections. The columns are organized as follows. The first column gives a short description of the adsorbate layer structure and the adsorption site or bonding configuration considered. We reproduce the classification used in the cited publication, even if it has been proposed to be wrong by more recent publications. If applicable, the next column gives the (fractional) coordination number of the substrate surface atom considered. The 3rd column gives the coverage for the adsorbate structure under consideration. This is usually the ideal (nominal) value, not the value obtained in the actual experiment. Here, 1 ML is defined as one adsorbate Landolt-Börnstein New Series III/42A4

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397

molecule/atom per substrate surface atom. The 4th column displays the values of the core-level shifts in units of eV. If different values exist in the literature, we give all of them without commenting on possible reasons for the discrepancies. The number of digits quoted is from the original reference, but no error bar values are reported. The last column gives the cited references and the line shapes used in the data analysis; if no fitting procedure has been used, we will also notice. The used abbreviations are as follows: “D-S”: Doniach-Sunjic line shape; “+G”: additional Gaussian function; “V”: Voigt profile; “Sci”: fitting routine supplied by Scienta, using an asymmetric line profile with a combined FWHM and a ratio of Lorentzian and Gaussian contributions; “no fit”: no fitting routine has been used by the authors. “special” denotes line shapes used by only one working group; for details see the original paper. If the line shape is not specified in the original paper (“n.s.”), the data are deconvoluted, but but no details are given. Example: 21.65 eV

Ref. 1, 2 (D-S+G)

+0.74 eV +0.14 eV

Ref. 1, 2 (D-S+G)

low coverage

+0.99 eV

Ref. 1 (D-S+G)

O oxide-like

higher coverage

+1.29 eV

Ref. 1 (D-S+G)

H adsorption H

low coverage

+0.93 eV

Ref. 1 (D-S+G)

Ta 4f7/2 level, Eb(bulk) Clean surface 1st layer 2nd layer atomic O adsorption O chemisorbed

The different substrates are listed in the following sections in the order of their appearance in the Periodic Table. Within each substrate system, (100), (110) and (111) surface orientations are shown in this order. If values for stepped surfaces exist, they are displayed next. The different adsorbates considered for the various substrates are not given in a special order. Usually the adsorbed species is given, not the gas phase molecule used in the experiment. With a few exceptions, only substrates are included for which adsorbate data have been published. There are some additional substrate surfaces, for which either only clean surface measurements or only theoretical investigations exist; they are not listed here. 4.3.2.1 Al(001) In a theoretical study using the linearized augmented-plane-wave (LAPW) method a negligible clean surface core-level shift was found, whereas for a p(1×1)-O layer a shift of −1.5 eV was predicted [81Kra]. From a slightly more sophisticated full-potential self-consistent LAPW (FLAPW) calculation including crystal field splitting effects, a continuous shift for the first four surface layers was predicted, with a value of −0.12 eV for the surface layer [81Wim]. Al 2p3/2 level, Eb(bulk)

73 eV 72.26 eV

78Bac (no fit) 91Bag (n.s.)

+0.22 eV −0.096 eV

91Bag (n.s.) 91Nyh (D-S+G)

not seen +1.4 eV +2.6 eV +2.7 eV

78Bac (no fit) 78Ebe (no fit) 78Bac (no fit) 78Ebe (no fit)

Clean surface

O adsorption O chemisorbed subsurface oxygen Al2O3 Al2O3 Landolt-Börnstein New Series III/42A4

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4.3 Adsorbate induced surface core level shifts of metals

Alkali adsorption Na c(2×2) 100K, 4-fold hollow Na c(2×2) RT - highly coord. Na c(2×2) RT - less coord.

0.5 ML

−0.150 eV −0.500 eV −0.115 eV

[Ref. p. 418

92And (no fit) 92And (no fit) 92And (no fit)

4.3.2.2 Al(111) Aluminum as a very reactive metal has caused some attraction in the context of the oxide formation. Although we do not generally deal with oxides in this contribution, we have included the bulk oxide values for comparison here. The species denoted “subsurface oxygen” is most likely identical to a surface oxide. The preparation in [93Ber] was basically a room temperature oxidation process. The various peaks could be assigned to some chemisorbed species and some “bulk oxide”, which in fact seems to be a surface oxide. The peak with a shift of –0.14 eV was proposed to be due to Al atoms at the interface between the Al crystal and the surface oxide network. It is not bonded to any oxygen [93Ber]. Al 2p3/2 level, Eb(bulk) Clean surface O adsorption Al at interface, no O bond O chemisorbed, Al-1O O chemisorbed, Al-2O O chemisorbed, Al-3O O chemisorbed (3-fold hollow)

73 eV

78Flo, 78Bac (no fit)

< 0.015 eV

91Nyh (D-S+G)

−0.14 eV +0.37 eV +0.84 eV +1.37 eV +1.3 eV

(1 × 1) overlayer [79Ebe]

+1.4 eV

subsurface oxygen

+2.7 eV

surface oxide

+2.7 eV

“bulk oxide” Al2O3 Al2O3 Alkali adsorption Cs (2×2) Cs (√3×√3) Rb (2×2) Rb (√3×√3) K (√3×√3) Na (3×3) Na (4×4) Na (√3×√3)

+2.63 eV +3.5 eV +3.3 eV

93Ber (D-S+G) 93Ber (D-S+G) 93Ber (D-S+G) 93Ber (D-S+G) 95Ruc (no fit) 78Flo, 78Bac, 79Bia, 79Ebe (no fit) 95Ruc (no fit) 78Flo, 78Bac, 79Bia, 79Ebe (no fit) 93Ber (D-S+G) 95Ruc (no fit) 79Bia (no fit)

−0.130 eV −0.145 eV −0.130 eV −0.140 eV −0.130 eV −0.070 eV −0.080 eV −0.130 eV

93And (no fit) 93And (no fit) 93And (no fit) 93And (no fit) 93And (no fit) 93And (no fit) 93And (no fit) 93And (no fit)

1 ML

0.33 ML

4.3.2.3 Ni(100) The problem of accurately resolved surface core level shifts for the 3d transition metals was discussed in the introduction to this section. The measurements in [93Nil] give shifts for the main line as well as for the so-called “6 eV satellite”. These represent differently screened final states. Landolt-Börnstein New Series III/42A4

Ref. p. 418]

4.3 Adsorbate induced surface core level shifts of metals

Ni 2p3/2 level, Eb(bulk) Clean surface main line satellite not specified CO adsorption c(2×2) on-top - main line satellite c(2×2) - not specified H adsorption c(2×2) O adsorption c(2×2)

0.5 ML 0.5 ML

0.5 ML

399

852.60 eV 852.77 eV

93Nil (no fit) 84Ege (no fit)

−0.3 eV −0.7 eV −0.43 eV

93Nil (no fit) 93Nil (no fit) 84Ege (no fit)

+0.7 eV +1.5 eV +0.24 eV

93Nil (no fit) 93Nil (no fit) 84Ege, 85Ege (no fit)

−0.36 eV

85Ege (no fit)

−0.18 eV

84Ege, 85Ege (no fit)

4.3.2.4 Mo(110) Mo 3d5/2 level, Eb(bulk) Clean surface

227.9 eV

95And (n.s.)

−0.33 eV −0.333 eV

95And (n.s.) 93Lun (D-S+G)

−0.33 eV

95And (n.s.)

Na adsorption saturated ML

4.3.2.5 Ru(0001) Oxygen on Ru(0001) is a prototype system. Oxygen atoms occupy hcp hollow sites only, thus making it necessary to think about the correlation of peak components and bonding situations. This has been done in terms of coordination number, as has been discussed in Sect. 4.3.1 already. “1O”, “2O”, and “3O” denote Ru atoms coordinated to one, two, and three oxygen atoms, respectively. In [01Liz] the spectra for all coverages are fitted including the 1st and 2nd layer clean surface peaks. The binding energy values for these peaks change slightly as well with changing oxygen coverage. However, for clarity we do not include these values here. Just briefly, the 1st layer clean surface component vanishes for coverages higher than 0.25 ML, while the 2nd layer component is still close to its clean surface value up to coverages of 0.75 ML. For the (1×1)-O structure the 2nd layer Ru atoms get coordinated to one oxygen as well, thus shifting to a position identical to the bulk Ru position. The behavior observed experimentally is also described theoretically by DFT calculations, yielding very similar values for the core-level shifts [01Liz]. The case of RuO2 (110) has been included, since it is discussed in close relationship with the oxygen adsorption [01Ove]. In addition, the so-called coordinationally unsaturated (cus) Ru atoms on the surface of this oxide are rather much involved in the oxide formation, as judged by their large binding energy difference with respect to the surface Ru atoms of the clean surface (see table below). Within this study, the adsorbate-induced core-level shifts have also been calculated, taking both initial state and final state contributions into account. Qualitatively the results follow the trend in the experimental values, slightly overestimating the shift [01Ove].

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400

4.3 Adsorbate induced surface core level shifts of metals

Ru 3d5/2 level, Eb(bulk) Clean surface 1st layer 2nd layer 1st layer 2nd layer O adsorption p(2×2) 1O p(2×1) 1O p(2×1) 2O (2×2)-3O 2O (2×2)-3O 3O (1×1)-O 3O 1st layer (1×1)-O 1O 2nd layer (2×1) (not res.) RuO2 (110) - oxide RuO2 (110) - cus CO adsorption not specified H adsorption (1×1) (2×1) - 1st layer 2nd layer NO+O coadsorption NO+2 O NO+O - “1st layer” “2nd layer”

280.1 eV −0.366 eV +0.125 eV −0.363 eV +0.135 eV 1/3 0.25 ML 1/3 0.50 ML 2/3 0.50 ML 0.5 ML 2/3 0.75 ML 3/3 0.75 ML 3/3 1 ML 1 ML 1 ML

[Ref. p. 418 01Liz, 01Ove (D-S+G) 01Liz (D-S+G) 97Sti (Sci)

+0.020 eV −0.050 eV +0.390 eV +0.40 eV +0.387 eV +0.980 eV +0.960 eV +0.93 eV 0 +0.371 eV +0.63 eV +0.35 eV

01Liz (D-S+G) 01Liz (D-S+G) 01Liz (D-S+G) 01Ove (D-S+G) 01Liz (D-S+G) 01Liz (D-S+G) 01Liz (D-S+G) 01Ove (D-S+G) 01Liz (D-S+G) 97Sti (Sci)

−0.353 eV

97Sti (Sci)

−0.254 eV −0.304 eV +0.113 eV

97Sti (Sci)

+0.341 eV +0.530 eV +0.756 eV

97Sti (Sci)

01Ove (D-S+G)

97Sti (Sci)

97Sti (Sci)

4.3.2.6 Ru (10 1 0) In contrast to the papers about Ru(0001), where the binding energy of the bulk component of the Ru 3d5/2 level was given explicitly, the references on Ru (10 1 0) give no absolute binding energy values. From [00Bar2] we have taken from the figure the value of 280.1 eV, which also corresponds to the value reported for the Ru(0001) surface. Oxygen is again adsorbed in hcp hollow sites. “1O” and “2O” denote Ru atoms bonded to one or two O atoms.

Ru 3d5/2 level, Eb(bulk) Clean surface 1st layer 2nd layer O adsorption c(4×2) 1O (2nd layer) (2×1)p2mg 1O (2nd layer) c(4×2) 1O (1st layer) (2×1)p2mg 2O (1st layer)

280.1 eV

00Bar2 (D-S+G)

−0.480 eV −0.240 eV

00Bar1, 00Bar2, 01Bar (D-S+G)

all coverages

+0.465 eV

00Bar2, 01Bar (D-S+G)

<0.5 ML >0.5 ML

−0.085 eV +0.215 eV

00Bar2, 01Bar (D-S+G) 00Bar2, 01Bar (D-S+G) Landolt-Börnstein New Series III/42A4

Ref. p. 418]

4.3 Adsorbate induced surface core level shifts of metals

401

4.3.2.7 Rh(100) CO adsorption gives rise to two clearly resolved states in C 1s core-level emission. However, in the Rh 3d5/2 level only three components due to bulk, clean surface and adsorbate covered Rh atoms are observed [98Str]. From a comparison between Rh 3d5/2 and C 1s spectra the on-top contribution is proposed to overlap with the bulk component. 307.3 eV 307.6 eV

98Str (D-S+G) 94Bor (D-S+G)

−0.65 eV −0.62 eV −0.64 eV −0.655 eV

98Str (D-S+G) 94Bor (D-S+G) 97Pri (D-S+G) 96Zac (D-S+G)

0.5 ML saturation > 0.5 ML

< ±0.05 eV 0 −0.300 eV

98Str (D-S+G) 96Zac (D-S+G) 98Str (D-S+G)

0.5 ML

−0.25 eV

96Zac (D-S+G)

+0.385 eV

96Zac (D-S+G)

−0.47 eV

96Zac (D-S+G)

−0.65 eV

94Pri (D-S+G)

−0.08 eV

94Pri (D-S+G)

−0.35 eV

94Pri (D-S+G)

Rh 3d5/2 level, Eb(bulk) Clean surface

CO adsorption on-top c(2×2) “split (2×1)” bridge O adsorption O hollow (2×2)p4g NO adsorption NO H adsorption H

4.3.2.8 Rh(110) Rh 3d5/2 level, Eb(bulk) Clean surface CO adsorption (2×1)p2mg H adsorption 20 L

4.3.2.9 Rh(111) For oxygen adsorption, three types of Rh surface atoms are considered (see Fig. 4): type “A” for clean Rh, type “B” for Rh coordinated to one O adatom, type “C” for Rh coordinated to two O adatoms [01Gan]. A linear relationship between the core-level shift and the number of O neighbors can be observed; approximately one finds 0.3 eV per bond. This is very well reproduced by the calculations. The theoretical prediction for Rh coordinated to three O adatoms (type "D") in a (1×1) layer with 1 ML coverage is +0.94 eV. In the case of CO adsorption, [98Beu] does not give coordination numbers. If we calculate these, we get 1 for on-top and 2/3 for the hollows in the (2×2) structure. That does not give a linear relationship, not even with respect to the clean surface component.

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402

4.3 Adsorbate induced surface core level shifts of metals

[Ref. p. 418

Fig. 4. Core-level shift relative to bulk signal for O adsorption on Rh(111). Comparison between experiment and theory is shown; reproduced from [01Gan].

For Rh(111) a number of theoretical studies exist as well. While some concentrate on the clean surface core-level shift [82Fei, 94And2], obtaining values close to the experimentally observed –0.5 eV, others also calculate adsorbate induced shifts [03Bir]. For CO/Rh(111), results of 0.24 eV for the on-top species in a (√3×√3)R30° structure and –0.22 eV for threefold hollow sites in the high coverage (2×2)3CO phase [03Bir] agree very well with experimental values listed below. As shown above, also for O adsorption the theoretical description seems to work well [01Gan]. Rh 3d5/2 level, Eb(bulk) Clean surface

1st layer 2nd layer CO adsorption (√3×√3)R30° on top (2×2)-3CO hollow O adsorption p(2×2) B hollow p(2×1) C hollow Alkali adsorption K, Rb, Cs

307.18 eV

94And2, 98Beu (D-S+G)

−0.50 eV

94And2, 97Beu, 98Beu (D-S+G) 01Gan (D-S+G)

−0.485 eV −0.46 eV +0.071 eV

0.33 ML 0.75 ML 1/3 0.25 ML 2/3 0.5 ML

03Bar (D-S+G)

+0.27 eV −0.22 eV

97Beu, 98Beu (D-S+G) 98Beu (D-S+G)

−0.140 eV +0.295 eV

01Gan (D-S+G) 01Gan (D-S+G)

−0.50 eV

95And (n.s.)

Landolt-Börnstein New Series III/42A4

Ref. p. 418]

4.3 Adsorbate induced surface core level shifts of metals

403

4.3.2.10 Stepped Rh surfaces The (111) terraces of the stepped surfaces exhibit a different SCLS as the Rh(111) surface, approaching the flat surface value with increasing terrace width. O adsorption on the steps gives rise to a binding energy shift of the step atoms only [03Gus]. Rh 3d5/2 level, Eb(bulk) Clean surface terrace (553) terrace (151513) step underneath step O adsorption step adsorption (553)

0.06 ML

307.15 eV

03Gus (D-S+G)

−0.43 eV −0.48 eV −0.72 eV −0.14 eV

03Gus (D-S+G) 03Gus (D-S+G) 03Gus (D-S+G) 03Gus (D-S+G)

−0.35 eV

03Gus (D-S+G)

4.3.2.11 Pd(100) The case of CO on Pd(100) has been studied in some detail [91And]. There are two interesting aspects. First of all, there seems to be a linear relationship between the number of adsorbed CO molecules and the binding energy shift, increasing from 0.5 eV for one CO neighbor (called “bridge 1”, coord. 1/2) to about 1.0 eV for two CO neighbors (“bridge 2”, coord. 1). The other interesting aspect is that the binding energies of both Pd species change slightly with increasing coverage, i.e., with next nearest neighbors. A similar effect is also observed for other systems (e.g., CO on Pd(111)). In [94Gur] no spectrum of the clean surface is shown. Therefore it is not possible to check the value given for the core-level shifts. However, we assume that the given SCLS is meant to be towards lower binding energies (−0.41 eV), despite the given value of +0.41 eV. As stated by some of the authors [92Nyh, 94And2], the D-S line shape does not adequately describe the Pd 3d core level, due to the details of the density of states close to the Fermi level. Therefore, some groups use subtraction procedures which are labelled as “no fit”. However, the same authors often use a D-S+G line shape for comparison as well. If discrepancies are small, we give only one value for the binding energy shift. 334.95 eV

Pd 3d5/2 level, Eb(bulk)

334.99 eV 334.96 eV

91And (D-S+G) 02Jaw2 (no fit) 94Gur (Sci) 96Par (no fit)

−0.43 eV −0.44 eV −0.41 eV −0.40 eV

91And, 02Jaw2 92Nyh (no fit) 94Gur (Sci) 02Jaw1 (no fit)

+0.48 eV +0.56 eV +0.97 eV +0.60 eV +1.04 eV +0.47 eV

91And (D-S+G) 91And (D-S+G) 91And (D-S+G) 91And (D-S+G) 91And (D-S+G) 96Par (no fit)

Clean surface

CO adsorption p(2√2×√2)R45° bridge 1 p(3√2×√2)R45° bridge 1 p(3√2×√2)R45° bridge 2 p(4√2×√2)R45° bridge 1 p(4√2×√2)R45° bridge 2 unspecified CO

Landolt-Börnstein New Series III/42A4

1/2 1/2 1 1/2 1

0.50 ML 0.67 ML 0.67 ML 0.75 ML 0.75 ML

404

4.3 Adsorbate induced surface core level shifts of metals

O adsorption c(2×2) 4-fold hollow c(2×2) 4-fold hollow NO adsorption p(4×2) 4-fold hollow p(2√2×√2)R45° bridge saturation unspecified NO H adsorption c(2×2) Alkali adsorption Na

0.5 ML

[Ref. p. 418

0.5 ML

+0.55 eV +0.61 eV +0.55 eV

96Par (no fit) 96Par (Sci) 94Gur (Sci)

0.25 ML 0.5 ML 0.65 ML

+0.3 eV +0.8 eV +1.0 eV +0.5 eV

02Jaw1 (no fit) 02Jaw1 (no fit) 02Jaw1, 02Jaw2 (no fit) 96Par (no fit)

≈0 eV

92Nyh (no fit)

+0.7 eV

95And (n.s.)

saturation

4.3.2.12 Pd(110) CO on Pd(110) is a system that shows a reconstruction of the substrate for adsorption at room temperature. The Pd surface is still unreconstructed for CO coverages up to 0.3 ML. For CO coverages larger than 0.3 ML, a missing-row (1×2) reconstruction is found [97Ram], which coexists with the (1×1) structure up to coverages of 0.75 ML. For 0.75 ML (the saturation coverage at room temperature) the missing-row reconstruction of the substrate is complete. For higher coverages reachable at lower adsorption temperature or higher ambient pressures, the reconstruction is lifted again and CO forms the (2×1) p2mg structure [97Ram]. In [97Ram] only an averaged CO induced core-level position could be determined, leading to a continuous energy shift between 0.3 and 0.75 ML. The high coverage phase had to be prepared separately so that no information about the coverage range between 0.75 ML and 1 ML is given. The authors of [96Bon] use exponentially-modified Gaussian line shapes for thr deconvolution of the different contributions. 335.3 eV 335.2 eV

97Ram (Sci) 91Com (D-S+G) 96Bon (special)

−0.5 eV −0.24 eV −0.55 eV −0.4 eV

97Ram (Sci) 91Com (D-S+G) 94And2 (no fit) 96Bon (special)

<0.3 ML 0.75 ML 1 ML 1 ML 1 ML 1 ML

+0.45 eV +0.63 eV +0.94 eV +0.93 eV +0.98 eV +0.95 eV

97Ram (Sci) 97Ram (Sci) 97Ram (Sci) 94Loc (D-S+G) 91Com (D-S+G) 96Loc (D-S+G)

0.25 ML 22800 L surface oxide

+0.48 eV +0.2 eV +1.1 eV +1.5 eV

91Com (D-S+G) 96Bon (special) 96Bon (special) 97Pil (no fit)

Pd 3d5/2 level, Eb(bulk) Clean surface

CO adsorption randomly, unrec. substrate (4×2) (2×1)p2mg bridge (2×1)p2mg bridge (2×1)p2mg bridge (2×1)p2mg bridge O adsorption c(2×4) bridge O2 at 400 K (1×10−4 mbar) O2 at 400 K (4×10−2 mbar) PdO PdO

Landolt-Börnstein New Series III/42A4

Ref. p. 418]

4.3 Adsorbate induced surface core level shifts of metals

405

4.3.2.13 Pd(111) [00Sur] makes use of the coordination number concept (see section 4.3.1). For the example of the c(4×2) structure, the existence of two adsorbate-induced features in the Pd 3d5/2 core level favors a structural model with fcc and hcp hollow sites over a bridge site model. While the bridge-only model would only yield one adsorbate related feature, the hollow sites model has Pd atoms bonded to one CO molecule only (H1) and Pd atoms bonded to two CO molecules (H2). Therefore, H1 corresponds to 1/3 Pd-CO coordination (CO shared by three Pd atoms), H2 corresponds to 2/3 Pd-CO (two “1/3-CO molecules” bound to Pd) (=associated with occupation of both fcc and hcp sites, different to H1 not because of site but because of coordination), on-top has coordination number 1 (one CO per Pd). Again, as for the Pd(100) surface, a linear relationship between coordination number and shift is observed, from 0.37 eV (1/3) to 0.7 eV (2/3) to 1.05 eV (1). Bridge (1/2) features as expected at 0.5 eV. We want to mention that the binding energies of the different components display some very small shifts (some meV) upon increasing the CO coverage. Shown are only the values for coverages closest to the nominal values of the observed adsorbate structures. 334.9 eV

98San, 00Sur (D-S+G) 00Lei (Sci)

−0.28 eV −0.3 eV

98San, 94And2, 00Sur (D-S+G) 00Lei (Sci)

0.33 ML 0.50 ML 0.50 ML 0.75 ML 0.75 ML 0.75 ML > 0.1 ML > 0.1 ML

+0.34 eV +0.40 eV +0.69 eV +0.75 eV +1.05 eV +0.52 eV +0.32 eV +0.59 eV

00Sur (D-S+G) 00Sur (D-S+G) 00Sur (D-S+G) 00Sur (D-S+G) 00Sur (D-S+G) 00Sur (D-S+G) 00Sur (D-S+G) 00Sur (D-S+G)

0.25 ML

+0.32 eV

00Lei (Sci)

+0.40 eV

00Lei (Sci)

0.33 ML 0.21 ML

+0.58 eV +0.58 eV

98San (D-S+G) 98San (D-S+G)

0.33 ML

+0.72 eV

98San (D-S+G)

> 0 eV

98San (no fit)

Pd 3d5/2 level, Eb(bulk) Clean surface

CO adsorption (√3×√3)R30° fcc hollow H1 c(4×2)-2CO hollow H1 c(4×2)-2CO hollow H2 (2×2)-3CO hollow H2 (2×2)-3CO on-top (2×2)-CO bridge (√3×√3)R30° hollow (300 K) CO bridge (300 K) O adsorption (2×2) hollow (300 K) CO coadsorption on (2×2)-O CO induced core-level shift C2H2 adsorption (√3×√3)R30° hollow (2×2) hollow C2H3 adsorption (√3×√3)R30° H adsorption (1×1)

Landolt-Börnstein New Series III/42A4

1/3 1/3 2/3 2/3 1 1/2

406

4.3 Adsorbate induced surface core level shifts of metals

[Ref. p. 418

4.3.2.14 Ta(100) In a theoretical study, Krakauer has calculated a clean SCLS of +0.96 eV for the first and no shift for the second layer [84Kra]. Since no final state effects are taken into account, the agreement seems reasonable. A better agreement is found in [85Gui] by using a microscopic model, where for the 1st layer a value of +0.9 eV and for the 2nd layer +0.14 eV is found for the unrelaxed surface. If some relaxation is included, both values are reduced by about 0.05 eV. 21.65 eV

84Gui2, 85Gui, 85Spa (D-S+G)

+0.74 eV +0.14 eV

84Gui2, 85Gui, 85Spa (D-S+G)

low coverage higher coverage

+0.99 eV +1.29 eV

84Gui2 (D-S+G) 84Gui2 (D-S+G)

low coverage

+0.93 eV

84Gui2 (D-S+G)

+0.64 eV

85Sou (D-S+G)

Ta 4f7/2 level, Eb(bulk) Clean surface 1st layer 2nd layer O adsorption O chemisorbed O oxide-like H adsorption H Cs adsorption not specified 4.3.2.15 Ta(110)

[95Ruc] uses the Ta 4f5/2 level to derive the shifts, since the components of the 7/2 level overlap with the 5/2 peak. In [94And1] no adsorbate induced component is observed, but the position of the clean surface component is shifting with increasing coverage. This shift is coverage-dependent and non-linear. For saturation coverages the quoted values are obtained. Theoretical calculations of the clean SCLS yield 0.4 eV [85Gui]. 21.65 eV 21.58 eV

84Gui2, 85Gui (D-S+G) 94And1 (no fit)

+0.28 eV +0.3 eV +0.31 eV +0.360 eV +0.065 eV

85Gui (D-S+G) 95Ruc (no fit) 94And1 (no fit)

(both coexist)

+1.1 eV +1.9 eV +4.5 eV

95Ruc (no fit) 95Ruc (no fit) 95Ruc (no fit)

saturation saturation

+0.355 eV +0.330 eV

94And1 (no fit) 94And1 (no fit)

Ta 4f7/2 level, Eb(bulk) Clean surface

1st layer 2nd layer O adsorption O chemisorbed p(2×1) [81Tre] monolayer oxide [81Tre] bulk oxide Ta2O5 Alkali adsorption Na - surface component Rb - surface component

93Rif (D-S+G)

Landolt-Börnstein New Series III/42A4

Ref. p. 418]

4.3 Adsorbate induced surface core level shifts of metals

407

4.3.2.16 Ta(111) Oxygen adsorption on Ta(111) at room temperature leads to monolayer adsorption for exposures up to 1 L and to various oxidation states for higher exposures [82Vee]. State “A” is related to the adsorbate phase, while “C” and “D” are related to different sub-oxides. For hydrogen adsorption, continuous shifts of the clean surface components are observed [82Vee]. Published calculations of the clean surface core-level shift give very large shifts of +0.86 eV for the first and 0.14 eV for the 2nd layer [85Gui]. Ta 4f7/2 level, Eb(bulk) Clean surface 1st layer 2nd layer 1st layer 2nd layer O adsorption OA OC OD H adsorption 1st layer surface component 2nd layer surface component

21.64 eV +0.4 eV +0.19 eV +0.39 eV +0.11 eV

saturation saturation

82Vee (D-S+G) 82Vee (D-S+G) 84Wer (D-S+G)

+1.12 eV +1.3 eV +2.4 eV

82Vee (D-S+G) 82Vee (D-S+G) 82Vee (D-S+G)

+0.65 eV +0.36 eV

82Vee (D-S+G)

4.3.2.17 Ta (poly) [84Him] reported an oxidation study of polycrystalline Ta. In order to identify oxidation states, different oxidation procedures have been used; average values are presented here. Fairly mild conditions lead to adsorbate phases or surface oxides. Ta 4f7/2 level, Eb(bulk) Clean surface O adsorption oxidation state +1 oxidation state +3 oxidation state +5 oxidation state +5 in bulk oxide

+0.48 eV +1.22 eV +2.05 eV +5.2 eV

84Him (no fit) 84Him (no fit) 84Him (no fit) 84Him (no fit)

4.3.2.18 W(100) The clean surface, which has a (1×1) structure at room temperature, is reconstructed in a c(2×2) structure at low temperatures [81Vee2, 89Jup, 96Jup, 93Mul]. While [81Vee2] attributed two surface related components to unreconstructed (S1) and reconstructed (S2) domains, later publications identified these peaks with 1st and 2nd layer W atoms, the former of which exhibit a small shift upon reconstruction [84Gui1, 89Jup, 93Mul, 96Jup]. For hydrogen adsorption the new position of the original surface components of clean W(100) is noted. No additional H induced features are observed [81Vee2, 82Vee]. In [81Vee2] the S1 component vanishes for H coverages above 0.1 ML, while the S2 component gradually shifts to lower binding energies; however, at saturation H coverages, where the reconstruction is completely lifted, S2 does not reach the binding energy of the clean surface S1 component. In contrast, in the work of Guillot et al. [82Gui], two pairs of peaks are used without allowing for a shift. Landolt-Börnstein New Series III/42A4

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4.3 Adsorbate induced surface core level shifts of metals

[Ref. p. 418

Theoretical calculations of the SCLS are included in [99Kim] for the clean and the Li or K covered surfaces. In [85Gui] values of –0.55 and –0.23 eV are reported for the clean SCLS of the 1st and 2nd layer atoms, respectively. 31.42 eV 31.41 eV

W 4f7/2 level, Eb(bulk)

31.5 eV 31.44 eV

82Vee (V) 81Vee2 (V) 86Jup, 96Jup (D-S+G) 85Spa (D-S+G) 93Mul (D-S+G)

Clean surface

unreconstructed (S1) reconstructed (S2) 1st layer high temp. (1×1) low temp. (2×2) 2nd layer 1st layer high temp (1×1) low temp. (2×2) 2nd layer 1st layer (2×2) (1×1) 2nd layer 1st layer 2nd layer 1st layer unreconstructed 2nd layer O adsorption O chemisorbed p(2×1) O chemisorbed 2D reconstr. oxide WO2 O induced (110) facets (900 K) H adsorption surface component c(2×2) (S2) p(1×1) (S2) c(2×2) 1st layer 2nd layer p(1×1) 1st layer 2nd layer W bound to H (unrecon. dom.) 2nd layer W pinched surf. molecules W2H Cs adsorption p(2×2)

0.5 ML 0.6 ML 1.0 ML >1.25 ML saturation <0.2 ML >0.8 ML 0.5 ML saturation

0.1 ML 0.57 ML

−0.35 eV −0.36 eV −0.4 eV −0.35 eV −0.13 eV −0.37 eV −0.35 eV −0.14 eV −0.40 eV −0.35 eV −0.16 eV −0.35 eV −0.45 eV −0.11 eV −0.39 eV −0.19 eV −0.4 eV −0.16 eV

82Vee (V) 84Wer (D-S+G) 85Spa (D-S+G)

+0.53 eV +0.53 eV +1.3 … +1.4 eV +1.7 ... +1.8 eV +0.7 eV

82Vee (V) 89Aln (special) 89Aln (special) 89Aln (special) 89Aln (special)

−0.255 eV −0.14 eV −0.25 eV −0.25 eV −0.09 eV −0.32 eV −0.15 eV −0.22 eV −0.10 eV +0.06 eV

82Vee (V) 81Vee2 (V) 81Vee2 (V)

−0.35 eV −0.46 eV

85Sou (D-S+G) 85Sou (D-S+G)

81Vee2 (V) 89Jup,96Jup (D-S+G) 84Gui1 (D-S+G)

93Mul (D-S+G) 86Jup (D-S) 82Gui (special)

82Gui (special) 82Gui (special) 96Jup (D-S+G)

Landolt-Börnstein New Series III/42A4

Ref. p. 418]

4.3 Adsorbate induced surface core level shifts of metals

S adsorption p(2×2) (1S/W) c(2×2) (2S/W) N adsorption c(2×2) fourfold hollow – S1 c(2×2) fourfold hollow – S2 c(2×2) fourfold hollow – S3 Li adsorption 1st layer 2nd layer K adsorption 1st layer 2nd layer

409

0.25 ML 0.5 ML

−0.25 eV +0.07 eV

93Mul (D-S+G) 93Mul (D-S+G)

<0.3 ML

−0.09 eV +0.18 eV +0.41 eV

86Jup (D-S) 86Jup (D-S) 86Jup (D-S)

0<θ<0.5 ML >0.5 ML 0<θ<1.0 ML

−0.32…−0.50 eV 99Kim (D-S+G) −0.50 eV −0.15…−0.20 eV 99Kim (D-S+G)

0<θ<1.0 ML 0<θ<1.0 ML

99Kim (D-S+G) −0.32 eV −0.15…−0.11 eV 99Kim (D-S+G)

4.3.2.19 W(110) The system O/W(110) has been studied in some detail. The result is a fairly complicated spectral decomposition [98Rif]. The various components given in the table below can be attributed to W surface atoms bonded to varying numbers of O atoms, which are all located in threefold hollow sites. “O1” describes W atoms bonded to one oxygen atom, “O2” describes W atoms bonded to two O atoms, and “O3” are W atoms with three oxygen neighbors. Even more details can be observed. “O1a” denotes the W atom labelled “A” in Fig. 5, while “O1b” represents atom “B”. The p(2×1) structure with a coverage of 0.5 ML ideally consists of “O1b” and “O2” surface atoms only, while at a coverage of 0.75 ML (with a p(2×2) structure) “O2” and “O3” atoms are observed. For 1 ML a p(1×1) structure with “O3” surface atoms only has been found [98Rif]. Since usually domains of certain surface structures develop consecutively, mixed phases are observed. In [98Rif] binding energy positions of the various components have been allowed to vary within certain boundaries, giving rise to slightly coverage-dependent core-level shifts, even for the clean surface component. The range of values is reflected in the table by values for certain coverages.

B

A

B A

B

B

Fig. 5. Surface model describing special W surface atoms. Reproduced from [98Rif]. Lattice gas, θ < 0.5 ML

In [00Ynz] the two different O1 components have not been resolved separately from the bulk. They are included in a single component named “bulk+O1”.

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4.3 Adsorbate induced surface core level shifts of metals

[Ref. p. 418

For hydrogen adsorption, a H-induced surface reconstruction of the W substrate is found to be one reason for the H-induced core-level shifts. In addition, there is a chemical effect on the binding energies as well [90Rif]. The result is a continuous shift of the H-induced component with increasing coverage. [83Gui] gives an alternative analysis with fixed binding energy positions. They then use three H induced features, namely for W atoms bound to one, two and three H atoms. For completeness we include their attempt with a shifting component summarizing the 1 H and 2 H components as well. In [94And1] for alkali adsorption, no adsorbate induced component is observed, but the position of the clean surface component is shifting with increasing coverage. This shift is coverage-dependent and nonlinear. For saturation coverages the quoted values are obtained. In a theoretical study a value of –0.30 eV was found for the clean surface core-level shift [01Cho], in good agreement with the experimental data. A similar value (−0.28 eV) has already been reported by [85Gui]. In addition, Oguchi calculated a core-level shift of +0.60 eV for the (1×1)-O adsorbate layer [99Ogu]. 31.4 eV

W 4f7/2 level, Eb(bulk)

31.42 eV 31.5 eV

98Rif, 90Rif, 98Tun (D-S+G) 94Ped (V) 94And1 (no fit) 85Spa (D-S+G) 83Gui (special)

Clean surface average low cov. 0.34 ML

−0.3 eV −0.32 eV −0.29 eV −0.32 eV −0.321 eV −0.3 eV −0.30 eV −0.320 eV −0.29 eV

O adsorption O1a O1b O2 O2 O3 p(2×1) O2 p(2×2) O2 p(2×1) O3 p(2×2) O3 (1×1)×12 O3 p(2×1) 3-fold sites (1×1)-like (1×1) isolated O p(2×1) oxide (?)

low cov. 0.34 ML 0.5 ML low cov. 0.5 ML 0.2 ML 0.5 ML 0.5 ML 0.75 ML 0.5 ML 0.75 ML 1 ML 0.5 ML 1 ML 1 ML small 0.5 ML high

−0.16 eV −0.08 eV +0.08 eV +0.059 eV +0.28 eV +0.34 eV +0.26 eV +0.66 eV +0.345 eV +0.365 eV +0.63 eV +0.73 eV +0.73 eV +0.3 eV +0.6 … +0.7 eV +0.73 eV −0.1 eV +0.32 eV +0.6 eV

98Rif (D-S+G) 00Ynz, 94And1 90Rif (D-S+G) 94Ped (V) 83Gui (special) 85Spa (D-S+G) 79Tra (no fit) 98Tun (D-S+G) 89Pur (D-S+G) 98Rif (D-S+G) 98Rif (D-S+G) 98Rif (D-S+G) 00Ynz (Sci) 98Rif (D-S+G) 00Ynz (Sci) 00Ynz (Sci) 00Ynz (Sci) 00Ynz (Sci) 00Ynz (Sci) 94Ped (V) 94Ped] (V) 98Dai1, 98Dai2 81Tre (G) 81Tre (G) 81Tre (G) Landolt-Börnstein New Series III/42A4

Ref. p. 418]

4.3 Adsorbate induced surface core level shifts of metals

H adsorption p(2×1) p(1×1) (reconstr.) isolated H (bound to 2 H) W bound to 1 H W bound to 2 H p(2×1) (bound to 3 H) Alkali adsorption Na – surface component Cs – surface component K – surface component Ba

0 to 0.5 ML 0.5 to 1ML cov. dep.

saturation saturation saturation > 0.7 ML

−0.32 … −0.26 eV −0.26 … −0.07 eV −0.22 … −0.08 eV −0.22 eV −0.08 eV +0.2 eV

83Gui (special)

−0.350 eV −0.340 eV −0.330 eV −0.400 eV

94And1 (no fit) 94And1 (no fit) 94And1 (no fit) 98Tun (D-S+G)

411

90Rif (D-S+G) 90Rif (D-S+G) 83Gui (special) 83Gui (special)

4.3.2.20 W(111) For the clean surface two different fitting approaches were used in [87Pur]: a two-peak model with only one “underlayer” peak with a larger width as the surface peak, and a three-peak model with two “underlayer” peaks (2nd and 3rd layer) with the same width. Since the quality of the fit increases and since a different shift for the 2nd layer seems plausible, the authors prefer the three-peak model. For comparison, we display values for both models. There is a striking difference between H and O adsorption. While for H adsorption the original surface components for 1st and 2nd layer shift by the displayed amounts, a new feature appears for oxygen adsorption. In addition, the original surface component for the 1st layer shifts as well, but its intensity vanishes [82Vee]. The theoretically predicted values for the clean SCLS are –0.43 eV for the 1st layer, −0.20 eV for the nd 2 layer and –0.10 eV for the 3rd layer [85Gui]. W 4f7/2 level, Eb(bulk) Clean surface 1st layer 2nd layer

81Vee1, 82Vee (D-S+G)

−0.43 eV

81Vee1, 82Vee, 84Wer, 87Pur (D-S+G) 81Vee1, 82Vee (D-S+G) 84Wer,87Pur (D-S+G)

−0.10 eV −0.11 eV −0.46 eV −0.36 eV −0.11 eV

1st layer 2nd layer 3rd layer O adsorption O chemisorbed (disordered) H adsorption 1st layer surface component 2nd layer surface component

31.42 eV

saturation saturation

87Pur (D-S+G)

+0.41 eV

81Vee1, 82Vee (D-S+G)

−0.26 eV 0

81Vee1, 82Vee 81Vee1, 82Vee

4.3.2.21 W(320) and other stepped W In [89Pur] three stepped W surfaces have been investigated. The nomenclature for the sites distinguishes between terrace rows, rows on top of the steps (upper row), rows at the bottom of the steps (lower row) and 2nd layer contributions.

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4.3 Adsorbate induced surface core level shifts of metals

[Ref. p. 418

The evaluation in [84Cha] assumed, that the terrace atoms show the same SCLS as on the flat W(110) surface. With this constraint the other positions were fitted. The rows in this publication are counted from the upper row of the steps, yielding three (110) rows, a row before the edge row and the edge row itself (lower step row). A theoretical calculation considering the differently coordinated W atoms on the (320) surface yields results which are giving the same trends in core-level shift, but still with small offset relative to experiment [01Cho]. W 4f7/2 level, Eb(bulk)

31.5 eV 31.39 eV

84Cha (D-S+G) 89Pur (D-S+G)

−0.140 eV −0.270 eV −0.080 eV −0.310 eV −0.200 eV −0.190 eV −0.580 eV −0.300 eV −0.440 eV −0.180 eV −0.41 eV −0.25 eV −0.25 eV −0.10 eV −0.38 eV −0.28 eV −0.38 eV −0.12 eV −0.43 eV −0.29 eV −0.12 eV

94Rif (n.s.) 94Rif (n.s.)

Clean surface average S1 (3 rows/terr) S2 (2 rows/terr) (3 line fit) S1 S2 S3 (4 line fit) steps (row 1, upper row) terrace (row 2,3,4) (110) row 5 (last row before edge edge (row 6, lower row) (320) - steps (upper row) - steps (lower row) - terrace (110) - 2nd layer (610) - steps (upper row) - steps (lower row) - terrace (100) - 2nd layer (310) - steps (upper row) - steps (lower row) - 2nd layer

94Rif (n.s.)

84Cha (D-S+G)

89Pur (D-S+G)

89Pur (D-S+G)

89Pur (D-S+G)

4.3.2.22 W (poly) [84Him] reported an oxidation study of polycrystalline W. In order to identify oxidation states, different oxidation procedures and even bulk oxide samples (WO2 and WO3) have been used. In the table average values for the different samples are presented. Fairly mild conditions lead to adsorbate phases or surface oxides. W 4f7/2 level, Eb(bulk) Clean surface O adsorption oxidation state +2 oxidation state +4 oxidation state +6 oxidation state +6 in bulk oxide

+0.71 eV +1.62 eV +2.7 eV +4.3 eV

84Him (no fit) 84Him (no fit) 84Him (no fit) 84Him (no fit)

Landolt-Börnstein New Series III/42A4

Ref. p. 418]

4.3 Adsorbate induced surface core level shifts of metals

413

4.3.2.23 Os(0001) Os 4f7/2 level, Eb(bulk) Clean surface CO adsorption (√3×√3)R30°

0.33 ML

50.74 eV

89Mar (D-S+G)

−0.41 eV

89Mar (D-S+G)

−0.33 eV

89Mar (D-S+G)

4.3.2.24 Ir(100) The Ir(100) surface can be prepared in a reconstructed and a (1×1) metastable phase. Interestingly, different SCLS are observed for these surfaces. By comparing with the Ir(111) surface (included here), a close relation between the reconstructed Ir(100)-(5×1) and the (111) surface is found [80Vee]. In order to account for instrumental broadening, a triangular function was convoluted with the D-S function. Ir 4f7/2 level, Eb(bulk) Clean surface (5×1) (1×1) metastable (111) surface H adsorption H on (1×1)

60.7 eV

80Vee (D-S)

−0.49 eV −0.68 eV −0.50 eV

80Vee (D-S) 80Vee (D-S) 80Vee (D-S)

0

80Vee (D-S)

60.7 eV

89Duc (D-S+G)

−0.5 eV

89Duc (D-S+G)

−0.35 eV

89Duc (D-S+G)

4.3.2.25 Ir(110) Ir 4f7/2 level, Eb(bulk) Clean surface (1×2) CO adsorption (√3×√3)R30°

0.33 ML

4.3.2.26 Ir(332) The Ir(332) surface consists of six rows of terrace atoms of (111) orientation and single atomic steps with ( 11 1 ) orientation. Therefore, the terrace SCLS is very similar to the value for the flat Ir(111) surface (see also Sect. 4.3.2.24). Upon dosing of molecular hydrogen, only the step sites are covered with hydrogen, causing their binding energy to shift to the value for the clean terrace atoms [81Vee3]. Ir 4f7/2 level, Eb(bulk) Clean surface terrace step (111)

−0.48 eV −0.75 eV −0.53 eV

81Vee3 (D-S+G)

H adsorption step adsorption (H2 exposure)

−0.48 eV

81Vee3 (D-S+G)

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4.3 Adsorbate induced surface core level shifts of metals

[Ref. p. 418

4.3.2.27 Pt(110)

The two publications mainly listed here, foremost differ in the excitation source used. While [89Duc] used a laboratory source, [87Duc] depends on synchrotron data. However, since the laboratory source is a specially made Y Mζ anode with a photon energy of 132.3 eV, a very good surface sensitivity is achieved [86Duc]. The clean Pt(110) surface exhibits a (1×2) reconstruction, which is of the missing-row type. In the (1×1) phase one can distinguish Pt atoms on the rows from Pt atoms in the valleys between. In the case of (1×2), these valleys are twice as deep as for (1×1) and there are row atoms, facet atoms, and valley atoms [87Duc]. For CO adsorption in the p2mg structure the reconstruction is lifted and CO bonds on top of row atoms. The second component observed is related to the valley Pt atoms, which are obviously also affected by the CO adsorption. If CO is adsorbed at low temperatures (≤110 K), the (1×2) reconstruction of the substrate is preserved [87Duc]. 71.1 eV

Pt 4f7/2 level, Eb(bulk)

70.83 eV Clean surface (1×2) (1×2) (1×2) - row atoms - valley atoms CO adsorption p1g1 (p2mg) on (1×1) - row atoms p1g1 (p2mg) on (1×1) - valley atoms on (1×2) - row atoms on (1×2) - facet atoms unspecified H adsorption on (1×2) - row/facet atoms on (1×2) - facet/valley atoms O2 adsorption on (1×2) - row/facet atoms on (1×2) - valley atoms O adsorption on (1×2) Alkali adsorption K

coadsorption K (0.25 ML) + CO (saturation)

87Duc (D-S+G) 82Bae (special) 89Duc (D-S+G)

−0.46 eV −0.44 eV −0.55 eV −0.21 eV

87Duc (D-S+G) 89Duc (D-S+G)

+0.84 eV +0.5 eV +0.8 eV +0.35 eV +0.66 eV

87Duc (D-S+G) 87Duc (D-S+G) 87Duc (D-S+G) 87Duc (D-S+G) 89Duc (D-S+G)

−0.28 eV +0.35 eV

87Duc (D-S+G)

−0.31 eV +0.31 eV

87Duc (D-S+G)

0.9 ML

+0.55 eV

87Duc (D-S+G)

>0.25 ML 0.16 ML 0.05…0.1 ML

−0.12 eV −0.32 eV −0.41 eV

89Duc (D-S+G)

+0.74 eV

89Duc (D-S+G)

1 ML

82Bae (special)

4.3.2.28 Pt(111)

NO/Pt(111) is a case, where the coordination number approach does not explain the observed core-level shifts [03Zhu]. The reason might be the different bonding configuration as compared to CO. H1 marks Pt atoms bonded to one NO molecule in fcc hollow position, while H2 is related to Pt atoms bonded to two NO molecules in hcp and fcc hollow sites. Landolt-Börnstein New Series III/42A4

Ref. p. 418]

4.3 Adsorbate induced surface core level shifts of metals

415

[82Bae] reported an interesting behavior upon oxygen adsorption at high temperatures (900 °C). Although oxygen was detected on the surface, no change in the Pt 4f core level was found. The authors explain this by formation of subsurface oxygen. 70.90 eV 71.1 eV

Pt 4f7/2 level, Eb(bulk)

70.87 eV 71.09 eV 71.0 eV

94Bjo, 95Pug (Sci) 02Rad (D-S+G) 82Bae, 83Apa (special) 86Duc (D-S+G) 88Leg (D-S+G) 02Kin, 03Zhu (D-S+G)

Clean surface −0.40 eV −0.37 eV −0.4 eV −0.42 eV

94Bjo, 95Pug (D-S+G) 82Apa, 83Apa (special) 82Bae (special) 86Duc, 96Jan, 01Jan 02Rad, 03Zhu (D-S+G) 88Leg (D-S+G)

no saturation saturation saturation

+1.01 eV +1.0 eV +0.9 eV +0.33 eV +0.4 eV +0.35 eV +1.0 eV +0.63 eV +0.9 eV

94Bjo (Sci) 02Kin (D-S+G) 02Rad (D-S+G) 94Bjo (Sci) 02Kin (D-S+G) 02Rad (D-S+G) 83Apa (special) 86Duc (D-S+G) 82Bae (special)

0.25 ML 0.50 ML 0.75 ML

+0.2 eV +0.5 eV +0.7 eV

03Zhu (D-S+G) 03Zhu (D-S+G) 03Zhu (D-S+G)

0.25 ML 0.25 ML surface oxide

+0.22 eV +0.2 eV +0.75 eV

94Bjo (Sci) 95Pug (Sci) 88Leg (D-S+G)

0.33 ML 0.23 ML 0.15 ML

−0.4 eV +0.4 eV +0.3 eV

95Pug (Sci) 95Pug (Sci) 95Pug (Sci)

+0.28 eV +0.14 eV

95Her1 (Sci) 95Her1 (Sci)

+0.4 eV +0.36 eV

95Her2 (Sci) 94Bjo (Sci)

+0.26 eV

01Jan (D-S+G)

+0.27 eV

01Jan (D-S+G)

CO adsorption c(4×2) on top

1

0.5 ML

c(4×2) bridge

1/2

0.5 ML

15 L CO unspecified CO unspecified NO adsorption (2×2)-NO fcc hollow H1 (2×2)-2NO on-top T (2×2)-3NO fcc/hcp hollow H2 O adsorption (2×2) hollow (2×2) hollow O2 at 700 K (2×2) O2 adsorption phys. phase (25 K) chem. phase I hollow (90 K) chem. phase II hollow/bridge (138 K) C2H4 adsorption monolayer at 30 K monolayer at 90 K C2H3 adsorption C2H3 „2×2“ 90 K C2H3 hollow (2×2) C3H6 adsorption C3H6 monolayer 100 K C4H6O adsorption C4H6O monolayer Landolt-Börnstein New Series III/42A4

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4.3 Adsorbate induced surface core level shifts of metals

K adsorption unspecified

+0.3 eV

[Ref. p. 418

83Apa (special)

4.3.2.29 Stepped Pt surfaces

In this subsection measurements reported for different stepped surfaces are summarized. These results clearly demonstrate that the very narrow 4f lines can be well resolved. 71.1 eV 71.08 eV

83Apa (special) 88Leg (D-S+G)

−0.30 eV −0.60 eV −0.39 eV −0.30 eV −0.57 eV

83Apa (special) 83Apa (special) 88Leg (D-S+G) 83Apa (special) 83Apa (special)

no saturation

+0.7 eV

83Apa (special)

saturation

+0.15 eV

83Apa (special)

+0.71 eV

88Leg (D-S+G)

+1.2 eV

88Leg (D-S+G)

83.99 eV

81Hei (L)

−0.28 eV −0.38 eV

81Hei (L)

83.83 eV 83.99 eV

89Duc (D-S+G) 81Hei (L)

−0.35 eV −0.35 eV

89Duc (D-S+G) 81Hei (L)

+0.77 eV

89Duc (D-S+G)

0

89Duc (D-S+G)

Pt 4f7/2 level, Eb(bulk) Clean surface Pt(557) terrace Pt(557) step Pt(557) (unres.) Pt(331) terrace Pt(331) step CO adsorption Pt(331) step NH3 adsorption Pt(331) step O adsorption Pt(557) - O2 at 700 K (2×2) “surface oxide” Pt(557) - O2 at 700 K (2×2) “bulk oxide” - PtOx 4.3.2.30 Au(100) Au 4f7/2 level, Eb(bulk) Clean surface (5×20) surface (1×1) surface (metastable) 4.3.2.31 Au(110) Au 4f7/2 level, Eb(bulk) Clean surface

(2×1) surface CO adsorption saturation Alkali adsorption K

0.3 ML

Landolt-Börnstein New Series III/42A4

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417

4.3.2.32 Au(111) Au 4f7/2 level, Eb(bulk) Clean surface

83.99 eV

81Hei (L)

−0.35 eV

81Hei (L)

84.00 eV

78Cit (D-S+G)

−0.399 eV

78Cit (D-S+G)

4.3.2.33 Au (poly) Au 4f7/2 level, Eb(bulk) Clean surface

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4.3 Adsorbate induced surface core level shifts of metals

4.3.3 References 70Don 78Bac 78Cit 78Ebe 78Flo 79Bia 79Ebe 79Sea 79Tra 80Joh 80Ros 80Vee 81Hei 81Kra 81Nyh 81Tre 81Vee1 81Vee2 81Vee3 81Wim 82Apa 82Bae 82Fei 82Gui 82Vee 83Apa 83Cit 83Ege 83Gui 83Joh 84Cha 84Ege 84Gui1 84Gui2 84Him 84Kra 84Wer 85Ege 85Gui 85Sou

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4.3 Adsorbate induced surface core level shifts of metals 85Spa 86Duc 86Ege 86Jup 87Duc 87Pur 88Leg 88Mar 88Nil 89Aln 89Duc 89Jup 89Mar 89Pur 89Ste1 89Ste2 90Rif 91And 91Bag 91Com 91Nyh 92And 92Nyh 93And 93Bag 93Ber 93Lun 93Mul 93Nil 93Rif 94Ald 94And1 94And2 94Bjo 94Bor 94Gur 94Loc 94Ped 94Pri 94Rif

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97Pil 97Pri 97Ram 97Sti 98Beu 98Bih 98Dai1 98Dai2 98Rif 98San 98Str 98Tun 99Bag 99Den 99Kim 99Ogu 00Bar1 00Bar2

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4.3 Adsorbate induced surface core level shifts of metals 00Lei 00Sur 00Ynz 01And1 01And2 01Bar 01Cho 01Gan 01Jan 01Liz 01Ove 02Jaw1 02Jaw2 02Kin 02Rad 02Woo 03Bar1 03Bar2 03Bir 03Gus 03Zhu

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