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## Inhaltsverzeichnis

### Introduction

The structure of a solid’s surface influences many of its essential properties, among them the chemical, electronic, and vibrational properties. To fully understand surface properties, one must often first determine to an atomistic accuracy the spatial coordinates of atoms in the surface region. Normally, an accuracy of a few percent of the interatomic distances is needed to fix certain electronic and vibrational characteristics. The interlayer spacings, bond lengths, bond angles, and bonding configurations need to be accurately determined and the results understood. Much work in this field has focused on single-crystal surfaces, but techniques have also been introduced which yield structural information at imperfect, amorphous, and other kinds of surfaces or interfaces. The contributions to these proceedings reflect the great diversity and complexity of surface science, covering theory, techniques, and structural results. They have been grouped according to the following topics.

Michel A. Van Hove, S. Y. Tong

### 1. Theory of Surface Reconstruction

A review is presented of theoretical calculations used to determine surface structure. The focus is on applications of the pseudopotential total energy approach.

Marvin L. Cohen

### 2. Electronic and Magnetic Properties of Transition-Metal Surfaces, Interfaces and Overlayers

Results of calculations of the electronic and magnetic properties of transition-metal surfaces, interfaces and overlayers are presented for a variety of systems. They involve Ni, Co, Fe and Cr in a diversity of forms, including alloys, metastable configurations, and overlayers on nonmagnetic metals. The overall behavior of these systems can be interpreted in terms of four qualitative rules which are presented, analyzed, and illustrated.

L. M. Falicov, R. H. Victora, J. Tersoff

### 3. The Binding of Adsorbates to Metal Surfaces

The total energy of an adsorbate as a function of position outside a metal surface determines, within the adiabatic approximation, the equilibrium position, the chemisorption energy, the vibrational spectrum and the activation energies for adsorption, diffusion and further reactions on the surface. Results obtained using the effective medium approach to calculate the total interaction energy are reviewed. Due to the simplicity of the approach the full potential energy surface has been calculated for a number of adsorption systems, and it is possible to relate the properties of the interaction potential to the parameters describing the atom and surface in question. Specific topics to be discussed are hydrogen and oxygen chemisorption on transition metals, quantum diffusion of chemisorbed hydrogen, and the oxygen incorporation and initial oxidation of metals.

S. Holloway, J. K. Nørskov

### 4. Energy Minimization Calculations for Diamond (111) Surface Reconstructions

Total energies are calculated for a variety of structural models for the reconstructed 2 × 1 diamond (111) surface. An ab initio LCAO approach to local density theory, which incorporates a self-consistent treatment of interatomic charge transfer, is used. Among the structural models considered are the Haneman buckled, the Pandey π-bonded chain, the Chadi π-bonded molecule, and the Seiwatz single-chain models. The results strongly favor the π-bonded chain model; the others are shown to be implausible. When fully relaxed, the π-bonded chain model has an energy of ~0.3 eV/surface-atom lower than that of the relaxed 1 × 1 surface. No dimerization of the surface chain is found to occur.

David Vanderbilt, Steven G. Louie

### 5. Total Energies and Atom Locations at Solid Surfaces

With the advent of modern computers it is now possible to compute total energies at surfaces from first principles. However, the kinds of problems that can currently be dealt with are quite restrictive. A brief overview of such calculations, as well as the recently discovered universality in binding energy relations will be given. We show that it is now possible to calculate surface or cleavage energies of transition metals from first principles. This is done via our self-consistent local orbital (SCLO) method. The total energies of metal films of many thicknesses, for example of three, five, seven, and nine layers, are first computed; a bulk and surface energy are then derived from a least-squares linear plot of total energy versus film thickness. In the examples of copper(100) and silver(100), the deviation from the line is less than 0.1 eV. This shows the accuracy of our method since the slope of the line (the bulk energy) is four Orders of magnitude larger than the intercept (twice the surface energy). Good agreement with experiment is obtained for all metals considered.

R. Richter, J. R. Smith, J. G. Gay

### 6. Theory of Hydrogen on Metal Surfaces

The embedded atom method [6.1, 2] is used to study the classical behavior of hydrogen at all coverages on the Ni(110) and Pd(lll) surfaces. For both surfaces, the hydrogen adsorption site is predicted to be the threefold site in agreement with the experimental observations. For Ni(110) the lowest energy ordered structure at ⊝=1 is computed to be the (2 × 1). Molecular dynamics simulations of this system show a critical temperature for the order-disorder transition to be between 150 K and 200 K. For the Pd(lll) surface, two structures $$\left( {\sqrt 3 \times \sqrt 3 } \right)$$ and $$\left( {\sqrt 12 \times \sqrt 12 } \right)$$ R30º are predicted to be very close in energy. Quantum corrections are expected to make the $$\left( {\sqrt 3 \times \sqrt 3 } \right)$$ R30º lower in energy. Monte Carlo simulations at θ=1/3 and 2/3 indicate that the order-disorder transition occurs near 125 K at both of these coverages with lower critical temperatures away from these ideal coverages. These predictions agree well with experimental observations.

M. S. Daw, S. M. Foiles

### 7. The Surface Topography of a Pd(100) Single Crystal and Glassy Pd81Si19 Studied by Scanning Tunneling Microscopy

The methods available to study surfaces [7.1–3] may be divided into techniques which supply spatially resolved information (on the submicron scale) and into the others which yield integral information. Scanning Auger spectroscopy and electron microscopy used as SEM, TEM and STEM are perhaps the most powerful methods among the first group. A principal resolution limit of these techniques is given by the volume of interaction between the electrons of the “light source” and the specimen. This volume will always be larger than the volume of a single atom. Therefore a new microscopic technique which does not require such an interaction zone had to be found in order to reach the ultimate goal of atomic resolution in imaging surfaces. Such a technique is Scanning Tunneling Microscopy (STM) recently developed by Binnig and Rohrer and collaborators [7.4, 5]. We have built a microscope similar to the published design principles.

M. Ringger, H. R. Hidber, R. Schlögl, P. Oelhafen, H. J. Güntherodt, K. Wandelt, G. Ertl

### 8. Theory of the Scanning Tunneling Microscope

The recent development of the “scanning tunneling microscope” (STM) by Binnig et al. [8.1–5] has made possible the direct real-space imaging of surface topography. In this technique, a metal tip is scanned along the surface while ad justing its height to maintain constant vacuum tunneling current. The result is essentially a contour map of the surface. This contribution reviews the the ory [8.6–8] of STM, with illustrative examples. Because the microscopic structure of the tip is unknown, the tip wave functions are modeled as s-wave functions in the present approach [8.6, 7]. This approximation works best for small effective tip size. The tunneling current is found to be proportional to the surface local density of states (at the Fermi level), evaluated at the position of the tip. The effective resolution is roughly [2Å(R+d)]1/2, where R is the effective tip radius and d is the gap distance. When applied to the 2x1 and 3x1 reconstructions of the Au(l10) surface, the theory gives excellent agreement with experiment [8.4] if a 9 Å tip radius is assumed. For dealing with more complex or aperiodic surfaces, a crude but convenient calculational technique based on atom charge superposition is introduced; it reproduces the Au(l10) results reasonably well. This method is used to test the structure-sensitivity of STM. The Au(l10) image is found to be rather insensitive to the position of atoms beyond the first atomic layer.

J. Tersoff

### 9. Reflection Electron Microscopy Studies of Crystal Lattice Termination at Surfaces

Reflection Electron Microscopy (REM) is applied to the imaging of stacking faults immediately beneath the surface of bulk crystals. Strong contrast is obtained on stacking fault ribbons in graphite. The contrast on Pt(lll) surfaces is also attributed to the stacking sequence change of the topmost layer of atoms.

Tung Hsu, J. M. Cowley

### 10. Surface Structure by X-Ray Diffraction

The diffraction of X-rays by crystalline matter has been well understood for 60 years. In that period a vast amount of practical expertise, in the form of X-ray crystallography, has been developed for the determination of crystal structures on the atomic scale. Low-energy electron diffraction (LEED) has extended crystallography to include crystal surfaces, but, as some of the accompanying papers have shown, data analysis is not so straightforward because of multiple scattering and the need for accurate atomic models. Neither of these is a serious problem in X-ray structure analysis.

I. K. Robinson

### 11. Optical Transitions and Surface Structure

To obtain information on the structure of a solid surface probes should be employed having a wavelength as short as the interatomic distance, which is typically of the order of an angstrom. In the case of electromagnetic radiation, this corresponds to X-rays. On the other hand, optical transitions involving electronic surface states are excited by near IR, visible and near UV light, whose wavelength is too large to probe the atomic structure directly. In some cases, however, valuable structural information can be obtained from optical spectroscopy indirectly, that is via the electronic structure. This is accomplished by exploiting symmetry arguments and making use of models. The present article exemplifies the above concepts by showing their application to the cleavage surface of the covalent semiconductors Si and Ge.

P. Chiaradia, A. Cricenti, G. Chiarotti, F. Ciccacci, S. Selci

### 12. High-Resolution Infrared Spectroscopy and Surface Structure

The potential of high-resolution surface infrared spectroscopy as a structural tool is assessed by combining results on the Si(100)–(2 × 1)H system with ab initio cluster calculations. It is shown that a complete determination of the geometry and vibrational parameters can be obtained.

Y. J. Chabal

### 13. Optical Second Harmonic Generation for Surface Studies

The progress of surface science relies heavily on our ability to probe surfaces and interfaces. For this purpose, many techniques have been developed in the past [13.11. Recently, laser methods for material studies have advanced to a highly sophisticated level; one therefore wonders if they can also be applied to surface studies. Indeed, there have been a number of very interesting recent discoveries in this area. It is found that laser-induced fluorescence or resonant ionization can be used to probe angular, velocity, and internal energy distributions of molecules scattered or desorbed from a surface [13.21. Coherent Raman spectroscopy [13.3], laser-induced desorption [13.4], photoacoustic spectroscopy [13, 5], and photothermal deflection spectroscopy [13.6] can be used to study surface states and molecular vibrations of adsorbates. Lately, we have shown that surface second harmonic generation (SHG) is also an effective tool for surface studies [13.7]. We describe here some of our recent work on this topic.

Y. R. Shen

### 14. NMR and Surface Structure

My students and postdoctoral students [14.1] have been using nuclear magnetic resonance (NMR) to investigate a number of aspects of surfaces in collaboration with Dr. John Sinfelt of the Exxon Research and Development Laboratory. Although NMR has played an enormously important role in solid-state physics and in chemistry, it has been little used to study surfaces since the number of nuclei needed to observe a signal is large, but the number of atoms on a surface is typically small. Nevertheless, NMR is such a powerful spectroscopic technique that we have pursued its use for the study of surfaces.

C. P. Slichter

### 15. Determination of Surface Structure by LEED

After a brief review of some noteworthy recent achievements of LEED crystallography we describe two important advances in methodology. One is experimental and concerns the rapid acquisition of LEED intensity data from display-type equipment with a computer-assisted television camera. The other is theoretical and concerns the calculation of LEED intensities with a new cluster approach that offers notable advantages over the schemes presently used, especially for structures involving large numbers of atoms and for high electron energies.

F. Jona, J. A. Strozier, P. M. Marcus

### 16. Structure Determination of Molecular Adsorbates with Dynamical LEED and HREELS

Molecular adsorbate structures can be determined by combining Low-Energy Electron Diffraction (LEED) with High-Resolution Electron Energy Loss Spectroscopy (HREELS). The HREEL vibrational spectroscopy identifies the molecular species, whose bond lengths and angles are obtained by LEED. Appropriate calculational methods are required in LEED to solve complex molecular structures: such methods will be discussed for large molecules, for large unit cells and for disordered adsorbates. Results have been obtained for the following molecular species adsorbed on several low-Mi11er-index metal surfaces: CO, C2H2, → CCH3, C3H4, → CCH2CH3 and C6H6. The presently available methods should be capable of solving a multitude of molecular adsorbate structures, including large molecules, coadsorbates and disordered species.

M. A. Van Hove

### 17. Computer Controlled LEED Intensity and Spot Profile Determination

Surface-structure analysis, investigations of the degree of order and studies of surface phase changes by means of LEED require collecting as well as handling a large amount of data. Rapid data acquisition was achieved through the computer-controlled system DATALEED designed several years ago, which in the meantime has shown its capabilities in various examples. Applications with respect to surface structure transitions [K/Ir(100), Ir(100)l × 1 → 1 × 5] are given.

K. Müller, K. Heinz

### 18. On the Role of Space Inhomogeneity of Electron Damping in LEED

Surface sensitivity of Low-Energy Electron Diffraction (LEED) results from incident electron damping, theoretically described by a complex optical potential. The imaginary part of this effective potential is then responsible for electron losses from coherent channels in LEED; it reflects the consequences of such processes like excitation of plasmons, creation of electron- hole pairs and interactions of an electron with lattice vibrations. In LEED calculations, an approximate form with uniform magnitude of the imaginary component of the crystal potential is used. Though this description appears to suffice for surface crystallography, the nonuniform nature of several processes involved indicates that the inhomogeneity of the damping should be considered [18.1], especially if subtler features in the intensity profiles are analyzed.

I. Bartoš, J. Koukal

### 19. Attenuation of Isotropically Emitted Electron Beams

The transport features of isotropically emitted electrons inside amorphous samples are analyzed using a Monte Carlo method. This method is used in several electron spectroscopies, e.g., Auger, ESCA, CEMS, which differ only in the excitation mechanism. The scattering model includes realistic cross sections and it uses the continuous slowing down approximation corrected for energy straggling. The energy and angular distributions of the electrons emerging from the surface are computed for several sample compositions and electron initial conditions. Simple scaling rules, which may be useful for a quantitative approach, are found.

R. Mayol, F. Salvat, J. Parellada-Sabata

### 20. LEED, XANES and the Structure of Disordered Surfaces

Surface crystallography is now a well-established discipline, but, like its bulk counterpart, high quality surface crystals tend to be required for a complete structural analysis. This has been particularly true where lowenergy electron diffraction has been concerned. However, there are techniques for studying surfaces which are insensitive to long-range order. I want to talk about two of them: X-ray absorption near edge structure (XANES) and a new technique only recently proposed: diffuse low-energy electron diffraction (DLEED).

J. B. Pendry

### 21. The Structure of Organic Adsorbates from Elastic Diffuse LEED

The novel method of interpreting the diffuse intensity distributions of elasticaly scattered electrons generated by disordered adsorbates on surfaces [21.1] is shown to be capable of being applied even to the case of adsorbates consisting of large organic molecules. Use is made of a matrix formulation of renormalized forward scattering (RFS) perturbation theory in the cluster calculations. The prospects for accurately determining the adsorption sites and orientations of isolated or disordered complex molecules by LEED are discussed

D. K. Saldin, D. D. Vvedensky, J. B. Pendry

### 22. Multiple Scattering Effects in Near-Edge X-Ray Absorption Spectra

We investigate the importance of various multiple-scattering corrections to electron propagation in near-edge X-ray absorption spectra using the 0/Ni(100) system as an example. We find that within approximately 10 of the absorption edge multiple scattering contributions cannot be neglected, though for greater energies the convergence of all perturbation schemes is quite rapid. We also outline our recently developed geometric series representation of renormalized forward scattering perturbation theory.

D. D. Vvedensky, D. K. Saldin, J. B. Pendry

### 23. NEXAFS and SEXAFS Studies of Chemisorbed Molecules: Bonding, Structure and Chemical Transformations

We discuss the application of the surface extended X-ray absorption fine structure (SEXAFS) and near edge X-ray absorption fine structure (NEXAFS) techniques to study the intramolecular and chemisorption bonds of carbon-, nitrogen-, oxygen-, and sulfur-containing molecules and their reaction intermediates on metal surfaces. NEXAFS is dominated by intramolecular multiple scattering resonances and is, therefore, preferentially sensitive to the intramolecular bonding and structure. It provides the intramolecular bond length, the hybridization of the molecular bond and the molecular orientation on the surface. SEXAFS mainly arises from single scattering processes off the atomic cores of substrate neighbor atoms and determines the chemisorption site and the chemisorption bond lengths. An example is given of a complete structure determination for formate (HCO2) on Cu(100). The power of NEXAFS is pointed out to monitor surface-induced molecular bond length variations, e.g., C2H2 on Pt(lll), CO/Na/Pt(111) as well as chemical transformations of complex molecules, e.g., C4H4S/Pt(lll).

J. Stöhr

### 24. Current Status and New Applications of SEXAFS: Reactive Chemisorption and Clean Surfaces

The development of surface extended X-ray absorption fine structure (SEXAFS) as a technique for determining local geometry of adsorbates on ordered single crystals has been extended to the study of surfaces lacking either order or adsorbates. Two examples briefly described here are the reaction of Ni on Si(111) to form a disordered silicide overlayer and the surface structure of clean amorphized Si.

P. H. Citrin

### 25. Electron-Phonon Scattering and Structure Analysis

We have calculated the inelastic cross section of the Ni(001) surface phonons S4 and S6 for an extended energy range of 50–250 eV. The calculated results indicate regions of energy where the S6 mode has a cross section comparable to that of S4. The theoretical results are confirmed by measurements which observed the S6 mode at the predicted energies. The simultaneous measurement of S4 and S5 modes allows a discrimination between different structural models for the Ni(001) surface. From the S4/S6 intensity ratio, we determined a 1.7-3.3% contraction in the surface interlayer spacing of Ni(001).

M. Rocca, H. Ibach, S. Lehwald, M.-L. Xu, B. M. Hall, S. Y. Tong

### 26. Shape Resonances in OH Groups Chemisorbed on the (100)Surface of Ge-Si Alloys

Andersson and Davenport [26.1] have previously used high-resolution electron-energy-loss spectroscopy (HREELS) to study hydroxyl groups on Ni0(lll). They found that the dipole interaction mechanism completely fails to describe this system as the relative loss intensity was a decreasing function of primary energy for both the 0-H stretching fundamental and overtone as opposed to the monotonically increasing function predicted by the dipole interaction mechanism. In addition, they found that the overtone intensity was more than an order of magnitude larger than a reasonable dipole estimate. Therefore, they concluded that a negative ion resonance was responsible for the observed anomalies.

H. H. Farrell, J. A. Schaefer, J. Q. Broughton, J. C. Bean

### 27. Structure and Temperature-Dependent Polaron Shifts on Si(lll) (2 × 1)

Recently Demuth et al. [27.1] studied the temperature dependence of ultraviolet photoemission (UPS) and electron-energy-loss (EELS) spectra of Si(111)(7 × 7). Theyfound that significant temperature-dependent changes occur in occupied surface states and their transitions, thus suggesting important electron-phonon coupling at the surface.

C. D. Chen, A. Selloni, E. Tosatti

### 28. Surface Structure Analysis by Atomic Beam Diffraction

The main advantages of atomic beam diffraction for surface structure analysis are: high surface selectivity (no penetration),no surface damage (even for physisorbed layers),high sensitivity to structural disorder.

J. Lapujoulade, B. Salanon, D. Gorse

### 29. Structure Analysis of a Semiconductor Surface by Impact Collision Ion Scattering Spectroscopy (ICISS): Si(lll)R30ºAg

A structural model for the Si (111) $$\left( {\sqrt 3 \times \sqrt 3 } \right)$$ R30º Ag surface is discussed on the basis of impact collision ion scattering spectroscopy (ICISS) experiments using a beam of Li+ ions. Silver atoms at the surface are situated ~0.5 Å above the first silicon layer; their lateral positions have not yet been analyzed.

M. Aono, R. Souda, C. Oshima, Y. Ishizawa

### 30. Surface Structure Determination with ARPEFS

We describe a method of surface structure determination based on oscillations in core-level photoemission intensity-Angle-Resolved Photoemission Extended Fine Structure-with particular emphasis on the use of Fourier transformation. Qualitative comparisons of Fourier power spectra reveal adsorption sites and shortcomings in theoretical calculations; quantitative backtransformation analysis allows accurate bond lengths and bond angles to be determined. Examples are drawn from these similar atomic adsorption systems: c(2×2)S/Ni(100), p(2×2)S/Cu(100) and c(2×2)S/Ni(110).

J. J. Barton, S. W. Robey, C. C. Bahr, D. A. Shirley

### 31. Angle Resolved XPS of the Epitaxial Growth of Cu on Ni(100)

In angle-resolved X-ray photoelectron spectroscopy (XPS) of single crystals the core level peaks exhibit enhanced intensities along major crystal axes. This phenomenon is often referred to as electron channeling (or Kikuchi beams) due to an apparent analogy with effects found in electron microscopy. The present analysis of this phenomenon for epitaxial Cu on Ni(100) demonstrates that the electron channeling (or Kikuchi beams) approach fails completely to describe the data. The actual physical basis for this phenomenon is forward scattering of photoelectrons by overlying atoms in the lattice.

W. F. Egelhoff

### 32. Evidence for Diffusion at 80 K of Gold Atoms Through Thin, Defective Oxide Layers

During the last few years, many papers on the initial stages of the interaction of oxygen with metal surfaces have been published. In particular, the initial oxidation of aluminum surfaces has been extensively studied with most of the surface science techniques which are able to provide atomic-scale information on the interaction process [32.1]. However, microscopy studies of the three-dimensional (3D) oxidation of well-characterized metal surfaces are relatively scarce. Most of the published work concerns the study of the kinetics of the oxide growth and usually it is very difficult to infer microscopic mechanisms from measured rate laws. In general terms the most used theory to describe microscopically the growth mechanism at low temperatures of a thin oxide layer on its metal support is the Cabrera-Mott theory for oxidation of metals [32.2]. In Fig.32.1 we describe briefly the physical basis of the mechanism. When an oxygen atom approaches the oxide surface, its electron affinity level E changes by an amount W due to the interaction with the surface. If the resulting energy of the affinity level, E +W, is larger than the work function ϕ of the underlying metal, electrons from the Fermi level of the metal will tunnel through the oxide to raise the affinity level to establish thermodynamical equilibrium. Consequently, a layer of negative ions is formed on the oxide surface and another cationic layer appears at the oxide-metal interface. This is described as a plate capacitor with a potential difference V. An electric field F=V/X (X: oxide thickness) is produced that acts as a driving force for ion diffusion through the oxide and is effective only if the potential drop in one jump is comparable to the activation barrier for ion diffusion. A simple electrostatic estimation gives for V a value of a fraction of a volt. It is known that oxides containing defects have lower activation energy barriers than perfect oxides and therefore they should be more suitable to study the Cabrera-Mott mechanism.

S. Ferrer, C. Ocal, N. Garcia

### 33. Surface Characterization by the Inelastic Scattering of Neutrons from Adsorbates

This review illustrates how neutron inelastic scattering can be used to determine the coordination number or the local geometry of those sites at a catalyst surface at which chemisorption occurs. There are neutron diffraction methods for determining the density of different faces at the surface of a polycrystalline powder but these will not be referred to here [33.1].

C. J. Wright

### 34. Infrared and Neutron-Scattering Studies of Ethene Adsorbed onto Partially Exchanged Zinc A Zeolite

Infrared and inelastic neutron-scattering studies of thane adsorbed onto ZnNaA zeolite show that the adsorbed molecule occupies a single adsorption site. The C-H stretching modes are not observed in the infrared data but are seen as a broad band in the neutron spectrum. Some low-frequency adsorbate-adsorbent modes are assigned.

J. Howard, J. M. Nicol, J. Eckert

### 35. Theoretical Study of the Structural Stability of the Reconstructed (110) Surfaces of Ir, Pt and Au

Within an effective two-band model the structural stability of the T =0 ground state of the (110) surfaces of Ir, Pt, and Au is compared for several reconstruction models. Band-structure contributions to the surface cohesive energy are treated in tight binding via the recursion method in the self-consistent Hartree approximation. Intersite repulsive interactions are taken into accout by a Born-Mayer potential fitted to bulk elastic data and modified at the surface. Hybridization of the 5d orbitals with a nearly free electron band of mixed sp character proves to be sufficient to discriminate between different reconstruction models for Ir and Pt. For Au, additional multilayer surface relaxation effects seem to become important in stabilizing the atomic configuration of lowest energy. For all three metals the fully relaxed missing-row structure is found to be the favored one on energetic grounds. For Au at least, this is in agreement with recent experiments.

H.-J. Brocksch, K. H. Bennemann

### 36. The Structure and Surface Energy of Au(110) Studied by Monte Carlo Method

The surface energy and configuration of a Au(110) surface was calculated using a Monte Carlo technique based on a potential energy function containing two-body and three-body interactions. The potential function, with the parameters calculated from experimental quantities, reproduced the average surface energy and the cohesive energy of Au correctly. Simulation calculations were performed considering discrete atoms up to the third layer from the surface. An ideal defect-free (110) surface was first considered and calculations were performed at T =100 K. The relaxed surface preserved its (1×1) structure; however, the interlayer spacing d12 between the first and second layers displayed a contraction (Δd12 =-9.0%), while the spacing d23 between the second and third layers expanded (Δd23 =+4.9%).

T. Halicioğlu, T. Takai, W. A. Tiller

### 37. Long- and Short-Range Order Fluctuations in the H/W(100) System

The surface reconstruction phase transformations of W(100) can be understood in terms of periodic lattice distortion instabilities. A mechanism of non-linearly coupled phonon soft modes produces a system of antiphase domain walls which can be viewed as a “soliton superlattice”. Many aspects of the behavior of W(100) with variation in temperature and hydrogen coverage can be understood in terms of long- and short-range order fluctuations of this lattice dynamical model.

Roy F. Willis

### 38. Synchrotron X-Ray Scattering Study of a Chemisorption System: Oxygen on Cu(110) Surface

X-ray diffraction has always been the primary means of determining the three-dimensional structure of bulk materials, just as LEED has been for the two-dimensional structure of surfaces. Although both techniques have been applied for decades, only a handful of surface-structural problems have been solved using LEED, while bulk structures are routinely solved using X-ray diffraction techniques. The main reason for this difference is that LEED analysis is based on dynamic scattering theory while X-ray scattering is usually inter-pretable with kinematic theory.

K. S. Liang, P. H. Fuoss, G. J. Hughes, P. Eisenberger

### 39. Helium Diffraction from Oxygen-Covered Nickel Surfaces

Helium atom diffraction results are presented for oxygen-covered Ni(001) and Ni(110) surfaces. For 0/Ni(001) the diffraction data has clearly favored a single vertical distance for oxygen chemisorption in both the p(2 × 2) and c(2 × 2) phases. This conclusion now seems to be quite generally accepted. For 0/Ni(110) there are several competing candidates which include among others the missing row and the saw-tooth model. We have carried out simple model theoretical calculations based on the Esbjerg-Nørskov-Lang relation between the surface charge density and helium-surface repulsive potential. We have used atomic Hartree-Fock charge densities to generate surface charge densities. The corrugation coefficients have been calculated for various geometrical configurations of oxygen on Ni. These results are compared with the experimental data in order to deduce information about the chemisorption of oxygen on Ni(110).

Inder P. Batra, T. Engel, K. H. Rieder

### 40. Competing Reconstruction Mechanisms in H/Ni(110)

The mechanisms of the hydrogen-induced reconstructions of Ni(110) have been investigated. Below ~180 K a (2 × 1) lattice gas structure with өH=1.0 transforms into a 2D-(1 × 2) structure during addition of hydrogen up to өH =1.5. The phase transition, which involves a reconstruction of the surface, exhibits first-order behavior with no apparent activation energy. In contrast, at T >180 K and already at low coverages, an activated, local transformation into a more stable ID structure (’streaked structure’) occurs. A lattice distortion to optimize the local metal structure with respect to the metal-ad-sorbate bond and thus increase the binding energy is introduced as a general model for many such adsorbate-induced surface phase transformations.

R. J. Behm, K. Christmann, C. Ertl, V. Penka, R. Schwankner

### 41. The Uses and Limitations of ESDIAD for Determining the Structure of Surface Molecules

The principles and mechanisms of electron-stimulated desorption (ESD) and photon-stimulated desorption (PSD), as well as the utility of the electron-stimulated desorption ion angular distributions (ESDIAD) method as a tool for determining the structure of surface molecules, have been described in a recent book [41.1] and several review articles [41.2-6]. The present short paper is intended to provide a guide to the relevant literature, and to describe briefly some recent work related to the uses and limitations of ESDIAD for determining the structure of surface molecules.

### 42. The Study of Simple Reactions at Surfaces by High-Resolution Electron Energy Loss Spectroscopy

The use of high-resolution electron energy loss spectroscopy (HREELS) in the study of two surface reactions is described. The first reaction is the deuteration of pyridine at 350 K on a Pt(110) surface. At this temperature, pyridine may adopt one of two different orientations on the surface depending on the coverage. Close to saturation coverage, a nitrogen lone-pair bonded species is dominant, characterized by intense ring breathing modes in the 1100-1200 cm-1 region. The molecules bonded in this manner undergo only a limited deuteration as monitored by the growth of a band, due to C-D stretching vibrations, at 2360 cm-1 and the attenuation of the corresponding C-H at 3050 cm-1. In contrast, at lower coverages, a π-bonded pyridine molecule predominates, characterized by an intense loss feature at 820 cm-1. These molecules undergo complete H/D exchange.

Neville V. Richardson, C. Damian Lackey, Mark Surman

### 43. Triangle-Dimer Stacking-Fault Model of the Si(111) 7×7 Surface Bonding Configuration

In the triangle-dimer stacking-fault (TOSF) model of the7 × 7recons~ruction of Si(lll) surfaces, the reconstruction extends to a depth of two double layers and consists essentially of the removal of 1/7 of the atoms belonging to the second and third layers. The resulting dislocations are dissociated and form a triangular network of partial dislocations. The unit mesh has two triangular subunits in which there are (111) stacking faults below the first and third layers, respectively. Each subunit is bordered by 18 dimers due to reconstruction of partial dislocations. The triangle-dimer geometry and the presence of stacking faults were first inferred from low-energy electron diffraction (LEEO) and Rutherford backscattering (RBS) results, respectively. The TOSF model is supported strongly but not unequivocally by other observations. These include scanning tunneling microscopy (STM), Hchemisorption combined with high-resolution infrared spectroscopy (HRIRS), Ar and Xe physisorption combined with electron spectroscopies, and transmission electron diffraction (TED). Specific observations that are not essential consequences of the TOSF bonding configuration include the quasi-2 × 2 array of “hills” ll in STM and relatively very intense (1 3/7) spots in TED. The possibility that these observations can be accommodated by the model is discussed.

E. G. McRae

### 44. Structure of the Si (111) 2 × 1 Surface

A structure analysis of the Si(lll)2 × 1surface is performed using extensive new LEED data (12 beams). A modified Keating-type strain energy analysis is used to expedite search in multiparameter structural space. Although the ir-bonded chain model in its original form grossly disagrees with LEED, a modification of that structure gives a more reasonable agreement. We show variation of the R factor with various important structural parameters. Our optimization so far has given us a minimum R factor of 0.42. This is not a particularly satisfying value by LEED standards, but the agreement is significantly better than for all chain models proposed to date. The major modification with respect to the earlier models is a strong intrachain buckling in the top chain. Our optimized coordinates down to six layers are given which may be useful for future electronic structure calculations.

Inder P. Batra, F. J. Himpsel, P. M. Marcus, R. M. Tromp, M. R. Cook, F. Jona, H. Liu

### 45. Refinement of the Buckled-Dimer Model for Si(001)2×1

A refinement (YJM2) of the four-layer-distorted buckled-dimer model for Si(001)2 × l presented earlier (YJM1) is described. The YJM2 model (mean R factor Rm =0.155 at normal incidence) fits the experimental LEED intensity data better than YJM1 (R™ =0.180), and both do so significantly better than a model recently proposed by other workers (rm =0.262).

Y. S. Shu, W. S. Yang, F. Jona, P. M. Marcus

### 46. Surface Relaxation and Vibrational Excitations on the Si (001) 2 × 1 Surface

We present a realistic theoretical study of surface vibrational excitations for the 2 × 1 reconstructed Si (001) surface. The extensive reconstruction of this surface is found to localize a number of interesting surface modes. These include a band whose eigenvector consists of a rocking motion of surface dimers and an unanticipated optical mode localized in the first two subsurface layers. Using a tight-binding theory for structural energies, the model incorporates the effects of bond rehybridization at the surface.

Douglas C. Allan, E. J. Mele

### 47. The Geometric Structure of the (2 × 2) GaAs(111) Surface

We present results of a reconstruction model proposed for the (2 × 2) GaAs (111) surface. Our model suggests similar reconstruction mechanisms for the (111) and (110) surfaces. In both cases, surface electronic energies are lowered via orbital rehybridization between nearest-neighbor Ga and As atoms with dangling bonds.

G. Xu, S. Y. Tong, W. N. Mei

### 48. A Comparison Between the Electronic Properties of GaAs(111) and GaAs()

Results of an angle-resolved photoemission investigation of GaAs(lll)(2 × 2) and GaAs($$\bar 1\bar 1\bar 1$$)(2 × 2) are presented. Significant differences were found to exist between spectra for the two surfaces. In addition, the surface band dispersions which are derived directly from the data are distinctly different for the two surfaces.

R. D. Bringans, R. Z. Bachrach

### 49. X-Ray Diffraction from the (3 × 3) Reconstructed B Surface of InSb

X-ray diffraction measurements have been perfgrmed with synchrotron radition under UHV conditions on the Sb rich ($$\bar 1\bar 1\bar 1$$) surface of InSb. This InSb. ($$\bar 1\bar 1\bar 1$$) B surface has a (3 × 3) reconstruction [49.1]. The surface was prepared by argon-ion bombardment and annealing at 400°C and was characterized using LEED and high-resolution photoemission. Using X-rays incident at the critical angle for total reflection (0.31°) we have measured the intensities of fractional-order Bragg rods corresponding to the (3 × 3) reconstruction on the B surface.

R. L. Johnson, J. H. Fock, I. K. Robinson, J. Bohr, R. Feidenhans’l, J. Als-Nielsen, M. Nielsen, M. Toney

### 50. Atomic and Electronic Structure of p (1 × 1) Overlayers of Sb on the (110) Surfaces of III-V Semiconductors

The prediction of the atomic geometries of overlayers for compound semiconductors is a topic of considerable current interest, especially with regard to the mechanisms of Schottky barrier formation and the growth (e.g., by molecular-beam epitaxy) of multilayer heterojunction systems [50.1]. Moreover, such geometries are now being determined experimentally, for example by elastic low-energy electron diffraction (ELEED) intensity analyses [50.2, 3]. Thus, an opportunity exists to develop and test predictive models of the geometrical and electronic structure of ordered overlayers on semiconductor surfaces. In this contribution we present a tight-binding calculation of the atomic geometries and surface-state eigenvalue spectra of p(l × 1) overlayers of Sb on the (110) surfaces of III-V semiconductors which predicts accurately the measured structures and surface-state spectra of these surfaces. The particular systems which we examined are ordered (l × l) saturated monolayers of Sb on the (110) surfaces of GaP, GaAs, GaSb, InP, InAs and InSb. A schematic diagram of the experimental surface atomic geometry is given in Fig.50.1.

C. B. Duke, C. Mailhiot, A. Paton, K. Li, C. Bonapace, A. Kahn

### 51. Models for Si (111) Surface upon Ge Adsorption

In view of the chemical similarity between Ge and Si one would expect the surface structures observed following deposition of Ge onto the Si(111) substrate to be of complexity comparable to that of the clean Si(111)7 × 7 surface. It is therefore remarkable that a simpler reconstruction has been observed by Chen et al. [51.1] for room temperature deposition of Ge onto the Si(111)2 × l surface. They observed a $$\sqrt 3 \times \sqrt 3$$ pattern corresponding to 0.2–0.4 monolayers of Ge. From the symmetry, coverage, and the fact that Ge-Si intermixing does not occur at this temperature, Chen et al. proposed an adatom model for the structure. Motivated by these experiments, we have carried out total energy calculations for two types of $$\sqrt 3 \times \sqrt 3$$ adatom models. The results of these calculations are discussed in Sect.51.2. Ultimately, we hope to provide a definitive characterization of the Si (111)$$\sqrt 3 \times \sqrt 3$$3-Ge surface, since this will provide insight into the more complicated 5 × 5 and 7 × 7 reconstructions which have been observed on Si and SiGe alloys [51.2].

S. B. Zhan, John E. Northrup, Marvin L. Cohen

### 52. The Graphite (0001)-(2 × 2)K Surface Intercalated Structure

Further absorption of potassium (K) onto the AKAKABA…stacked surface of graphite results in the formation of a (2 × 2) superstructure. A test of seven different surface-structure models with dynamical LEED calculations indicated that an A-(2 × 2)Kα-A-(2 × 2)Kβ-AB…stacked model results in intensities in good agreement with the measured diffracted intensities. Such a model is consistent with structures observed in bulk graphite for intercalated potassium.

N. J. Wu, A. Ignatiev

### 53. Energetics of the Incommensurate Phase of Krypton on Graphite: A Computer Simulation Study

The energetics of the weakly incommensurate phase of a quasi two-dimensional system of 20736 krypton atoms on a graphite surface are investigated. We have chosen a krypton-graphite potential which gives the commensurate phase at the lowest-energy state. The energy increase of the incommensurate phase can be described in terms of an hexagonal domain wall network confirming the phenomenological description byBak et al. [53.9]. The wall energy per unit length £ and the energy associated with each wall intersection A are calculated. Since A is significantly negative our results provide strong evidence that the commensurate-incommensurate transition might be first order.

M. Schöbinger, F. F. Abraham

### 54. Theory of Commensurate-Incommensurate Phase Transitions in W(001)

A theoretical model is proposed to explain the commensurate-incommensurate phase transitions on the W(001) surface. The incommensurate phase on W(001) arises from the coupling of the in-plane distortions with the component of displacement normal to the surface. Near the commensurate-incommensurate transition, the surface is composed of domains with in-plane displacements forming a c(2 × 2) structure and walls separating the domains in which the surface atom displacements have mixed in-plane and out-of-plane components. The H adsorption can increase the coupling of the two distortion modes, so enhancing the incommensurate phase. This can account for the observed LEED data on H/W(001). Finally, finite temperature effects are also studied, enabling a theoretical prediction for the entire phase diagram. The general model used in the present approach can also be applied to the study of commensurate-in-commensurate phase transitions on other surfaces and chemisorption systems.

S. C. Ying, G. Y. Hu

### 55. Molecular Dynamics Investigation of Dislocation-Depinning Transitions in Mismatched Overlayers

In recent years great strides have been made towards understanding the structural properties of incommensurate systems [55.1]. This is largely due to the advent of novel experimental methods and the discovery of systems amenable to these methods [55.1, 2]. In an effort to explain the experimental results, theoretical investigations have concentrated mainly on determining the phase diagrams and the order of the structural phase transitions involved [55.1,3–5]. Although understanding the dynamical behavior of these systems is an essential ingredient for interpreting the experimental results, we find very little work done towards that end [55.6,7]. This stems from the inherent complexities involved: nonlinearity, discreteness, and incommensurability.

K. M. Martini, S. Burdick, M. El-Batanouny, G. Kirczenow

### 56. Quantitative Analysis of LEED Spot Profiles

Since the discovery of electron diffraction [56.1] it has been known that surfaces even of single-crystal substrates frequently are not as perfect as expected from a simple periodic structure. In most cases, therefore, the surface has been treated again and again, until a “good” pattern with spots as sharp as possible was obtained, and defects neglected. There was, however, a growing awareness, that less than ideal diffraction patterns contain valuable information on nonperiodic features [56.2, 3]. The evaluation of these diffraction features opens a new field of study of associated phenomena, which include nonperiodic phenomena such as epitaxial growth, surface reactions, random adsorption, phase transitions or thermal disorder. Since several recent reviews are available [56.4–8], the procedures, results and problems are discussed here only briefly. The general features apply to all diffraction geometries (such as LEED, RHEED, atom diffraction or TEM) if the different ranges of reciprocal space are taken into account with the help of the appropriate Ewald sphere.

M. Henzler

### 57. Measurement of the Specific Heat Critical Exponent Using LEED

Near a second-order phase boundary, integrated intensities of “extra” LEED beams exhibit a |T -Tc|1-α singularity, where α is the specific heat critical exponent. We discuss the origin of this effect, apply it to real and Monte-Carlo-generated data, and comment on generalizations.

N. C. Bartelt, T. L. Einstein, L. D. Roelofs

### 58. Short-Range Correlations in Imperfect Surfaces and Overlayers

Extended defects (such as steps) on imperfect, periodic surfaces and relative positions of surface adsorbates may both exhibit short-range order. One powerful technique for studying this surface ordering is spot profile analysis of diffraction (low-energy and high-energy electron, atom and X-ray) measurements. This analysis requires only kinematic (single-scattering) diffraction calculations and thus is more tractable than conventional multiple-scattering problems. Here we discuss kinematic surface scattering results and their applicability to surface ordering studies.

J. M. Pimbley, T.-M. Lu

### 59. Domain Size Determination in Heteroepitaxial Systems from LEED Angular Profiles

The angular distribution of intensity in the fundamental beams diffracted from a surface with a lattice gas overlayer consists of two components, a broad “diffuse intensity” distribution reflecting the part of the correlation function resulting from the size distribution of ordered domains at the surface, and a delta-function component reflecting the part of the correlation function due to the order of the clean substrate. The magnitude of the diffuse component depends on electron-beam energy, degree of order, and coverage. We present an investigation of the relative weighting of the two components as a function of these parameters. Results of kinematic and multiple-scattering models are compared with each other and with measurements for GaAs(llO) p(l × l)-Sb.

D. Saloner, M. G. Lagally

### 60. Diffusion and Interaction of Adatoms

Compared to the wealth of information now available about atomic arrangements in overlayers and surfaces, rather little is known about the energetics governing the atomic interactions which underlie the structures formed. This imbalance has been redressed somewhat, as recently considerable information has been derived about atomic interactions from Monte Carlo simulations [60.1–3]. In these studies, interactions between atoms in a lattice gas are varied until the simulations match the experimentally observed phase diagram for the adsorbed layer. This macroscopic and rather indirect approach has also been complemented by insights concerning interactions at surfaces derived by direct examination on the atomic level.

G. Ehrlich

### 61. Atom-Probe and Field Ion Microscope Studies of the Atomic Structure and Composition of Overlayers on Metal Surfaces

The field ion microscope can give a two-dimensional view of the atomic structure of a solid surface. The composition of the top and deeper surface layers can be analyzed atomic layer by atomic layer with a time-of-flight atom probe If an adsorption layer structure is commensurate with the substrate, then its structure can be derived reliably from the field ion image. We have observed an adsorption layer superstructure, silicon on W(110), which can be directly correlated to the experimentally measured pair energies of silicon adatom-ad-atom interactions. The true atomic layer by atomic layer compositional analysis of surfaces can be best illustrated by a study of surface segregation of platinum base binary alloys, where the number of surface layers enriched with the segregants is found to vary from one system to another, ranging from 1 to 4 atomic layers. For some alloys, the second atomic layer is depleted with se gregants. This result is consistent with an oscillatory structure in the single-ion potential in the near surface layers.

### 62. LEED Studies of Physisorbed Noble Gases on Metals and Interadatom Interactions

Low-energy electron diffraction studies of noble gas adsorption provide opportunities to study a number of interesting phenomena. There are also a number of experimental problems. Approaches to these opportunities and problems are discussed and illustrated with selected results.

M. B. Webb, E. R. Moog

### 63. Phases and Phase Transitions in Two Dimensional Systems with Competing Interactions

We summarize the results of synchrotron X-ray studies of the phases and phase transitions in two prototypical two-dimensional systems where competing interactions play an essential role. The first, stage–4 bromine intercalated graphite, C28Br2, exhibits a transition from a centered $$\sqrt 3 \times 7$$ commensurate phase to a stripe domain incommensurate phase where technically there are no Bragg peaks but only power law singularities with exponents nn =2n2/49 where n is the harmonic number. The second, monolayer krypton on graphite, is one of the most extensively studied surface overlayer systems. This system exhibits a $$\sqrt 3 \times \sqrt 3$$ commensurate phase, a modulated hexatic reentrant fluid phase and both nonrotated and rotated incommensurate triangular solid phases. These phases and the transitions between them have been studied in detail. Finally, we discuss briefly perspectives for the future.

R. J. Birgeneau, P. M. Horn, D. E. Moncton

### 64. Diffraction from Pinwheel and Herringbone Structures of Nitrogen and Carbon Monoxide on Graphite

At low coverages and temperatures nitrogen monolayers and carbon monoxide monolayers form structures commensurate with the triangular array of carbon hexagons which makes up the basal plane of graphite [64.1–3]. For the 4.26 Å molecule center-to-center spacing of the commensurate monolayer, the molecule-molecule and molecule-substrate interactions for nitrogen are reasonably well represented by the lowest-order crystal field term dependent only on the orientation of a given molecule and an anisotropic interaction term primarily due to the electrostatic quadrupole-quadrupole interaction [64.6]. Under these conditions the mean-field theory of Harris and Berlinsky (HB) predicts four possible molecular axis ordered structures [64.5]: a 4-sublattice “pinwheel” in which one out of every 4 molecules is perpendicular to the substrate, a 2-sublattice herringbone structure in wich the mean molecular-axis orientation is in-plane (“1/2-in”) , a 2-sublattice herringbone in which there is a systematic tilt of the molecules out-of-plane (“1/2-outl”), and a “ferrol” phase in which the molecular axes are parallel to each other, but not quite perpendicular to the substrate.

Samuel C. Fain, Hoydoo You

### 65. X-Ray Scattering Studies: The Structure and Melting of Pb on Cu(110) Surfaces

The melting of two-dimensional structures has been the subject of considerable theoretical [65.1] and experimental [65.2] interest. One such system which several groups have studied is Pb deposited onto a Cu(110) surface, first reported by Herwion andRhead [65.3]. Mavva, Fuoss and Eiseribevgev (MFE) used Grazing Incidence X-ray Scattering (GIS) to confirm that the as-deposited monolayer exhibited a p(5 × l) structure [65.2] and determined that the initial melting temperature of the overlayer was near the bulk melting temperature of 327°C, but that the melted monolayer resolidified in an incommensurate structure which melted at 240°C. Subsequent melting of the incommensurate phase was reversible with an apparent second-order melting transition.

S. Brennan, P. H. Fuoss, P. Eisenberger

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