Table of contents

An account is given of the analysis of bonding of small molecules to metal surfaces, exemplified by acetylene. The problem is of how to return from completely delocalized two-dimensionally periodic calculations to the local, chemical actions. Various analytical tools, such as partitions of the density of states, a fragment molecular orbital approach, and a bond index in the solid state-the COOP, or crystal orbital overlap population-allow this. One can construct interaction diagrams for surface bonding, just as for discrete molecules, and so see the similarities (and the differences) between bonding in molecules and on surfaces.

For many decades swift electrons have provided a powerful tool for the study of the dynamical properties of condensed matter. The development of this technique has involved much important physics. Here the author sketches the historical background of the field and some important developments in theory and experiment. Possible directions for future research are indicated.

The scattering of molecular beams of ⁴He atoms from solid surface provides a very sensitive diffraction probe of surface structure and surface dynamics. The experimental method is described and illustrated by some studies of surface structure of metal surface. Because of the high monochromaticity of the helium beams ( Delta nu / nu approximately=1%) surface phonons with energies from 0.5 meV upwards can be resolved and entire surface phonon dispersion curves can be measured. On Cu(111) and Cu(001) an anomalous longitudinal resonance with large intensity is observed. On W(001) and W(110)+H(1*1) sharp anomalies give evidence of electron-phonon coupling which in the former system is related to the reconstruction at 280 K. Finally, two experiments on the dynamics of thin films as a function of layer thickness are described.

A free-parameter LCAO method for chemisorption problems is presented. The method includes many-body effects by means of a local-density potential. The case of Sb on GaAs(110) has been analysed and the results are discussed in the perspective of Schottky barrier formation.

A survey of the current theoretical understanding of the spin relaxation processes of electrons and holes in confined systems of quasi-two dimensions is given. The theoretical framework is used to form a bridge between the electronic properties and the recent time-resolved polarization measurements. Spin relaxation mechanisms surveyed include single-particle spin flips via spin-orbit interaction and electron-hole-pair spin flips via exchange interaction.

Most natural elements contain several isotopes. Single crystals grown from them are disordered systems in which the translational invariance is violated by the isotopic mass disorder. Its effects on the vibrational properties of solids of various dimensions are discussed and illustrated with examples involving germanium and diamond. Such effects are often the stronger the lower the dimensionality. Their vibrational frequencies vary rather accurately as the 'average' isotopic mass M. Hence, the average zero-point (i.e. low-temperature) vibrational amplitude varies as M^-12/. This leads to changes in many physical properties with average isotopic mass, many of which have been calculated and measured. Among them the authors discuss: the lattice constant, the thermal conductivity, the phonon lifetimes and the electronic gap energies and Lorentzian widths. The possibility of doping superlattices and MQW with layers made of different isotopes is also discussed. Experiments involving isotopically enriched surfaces and thin films are suggested.

This paper refers to the opto-electronic properties of hydrogenated amorphous germanium and its alloys (deposited by the RF-sputtering method). It is shown that enormous progress has been made in the optimization of a-Ge:H films, even though they are not yet fully electronic quality materials. The problems of material stability and doping are discussed in the light of recent results. The important issues of hydrogenated Ge-Sn and Ge-N alloys are presented, as well as recent results in the a-Ge:H device area.

This paper briefly reviews the characteristics of several important vacuum instruments; capacitance diaphragm gauges, ionization gauges, spinning rotor gauges, and partial pressure or residual gas analysers.

The authors have investigated the adsorption of Rb and Cs on high-symmetry Si faces ((100)2*1, (111)7*7, (110)16*2 and (211)2*1) using the X-ray standing wave technique. Several substrate Bragg reflections were employed to locate the adsorbate using a triangulation method. Hollow-type sites are preferred at coverages close to saturation, although adsorbate-adsorbate interactions play an important role in some cases, and give rise to a coverage-dependent adsorption site change. In particular, in the cases of Si(111) and Si(211) the adsorption site changes with coverage (threefold hollow at saturation and on top at low coverages).

The author elucidates surface structural information obtained by XSW by comparison with X-ray diffraction.

The authors discuss the results of density-functional theory calculations on the adsorption of Na and K adatoms on Al(111). They describe the nature of the adsorbate-adsorbate and adsorbate-substrate interactions, and the mechanism of the formation of a periodic adlayer. The latter is shown to be actuated by surface vacancies and steps. They then discuss the formation energies of and the diffusion at steps, and they draw conclusions about the expected shape of adatom (and vacancy) islands and about crystal growth.

On the basis of a full numerical calculation of the complex and non-local electron self-energy Sigma _xc(x,x' mod E) in the GW approximation, the authors discuss the main qualitative features of the self-energy in the vicinity of a metal surface modelled by jellium. They find that Re Sigma _xc is negligible for mod x-x' mod greater than a fraction of a Fermi wavelength, and that the general features of Re Sigma _xc are the same when moving from the bulk to the vacuum with most of its variation being in magnitude only. This behaviour is approximately reproduced with a purely local potential namely V_xc(x) of density functional theory, provided long-range correlations are taken into account. The same is not true for Im Sigma _xc; its structure in the vacuum has a dominant non-local ( mod x-x' mod > lambda _F) contribution. Thus, the use of a local function to describe damping at a metal surface is less well founded.

The authors discuss adsorbate-metal electrostatic interaction in the Anderson-Newns model.

The authors study the ground-state configurations of dipolarly interacting particles on a square lattice by applying a method of searching among all periodic arrangements for that of minimum energy. Their results explain the phases observed for alkali adsorption on FCC(100) transition metals and also some intriguing absences.

A random deposition model with a non-linear threshold-governed diffusivity is presented. Although the asymptotic regime of the model is random deposition, for the intermediate regime the model shows a non-trivial scaling behaviour with a more rapid than random growth.

An Anderson-Newns tight binding model obtained from the Hartree-Fock approximation of a bonding-pairs truncated many-body Hamiltonian, is used to describe the adsorption of H on sodium s-like metal. This model does not require adjustable parameters as all matrix elements between orthonormal states are calculated exactly up to second order in the overlaps of the atomic orbitals involved. The authors results reproduce the expected trends for the broadening and shift of the atomic level as the adatom approaches the surface, predicting a stable on-centre site adsorption with a binding energy of 4 eV.

Recent ion-scattering determinations of the structure of some adsorbate-induced reconstructions of Au and Ni surfaces are summarized.

The authors use the sticky site model in a numerical simulation of adsorption onto a crystalline surface.

The authors comment on the nature of the K-Si interface bond.

The authors extend a model for the adsorption energy of a close-packed hexagonal monolayer deposited on a substrate of the same structure but different lattice constant as a function of the epitaxy angle between the principal axes. The surface substrate potential is expressed in the form of a periodic potential with the symmetry and periodicity of the substrate surface. The case of lead underpotentially deposited on Ag(111) is examined.

Missing neighbours at the edges of regularly stepped surfaces cause the connectivity of percolating clusters to be reduced. The effect is understood on the basis of finite-size scaling arguments.

The unoccupied molecular orbitals of the nucleic acid bases uracil, adenine and guanine adsorbed on a Cu(110) surface have been studied by inverse photoemission spectroscopy (IPES). By comparison with MO calculations the assignment of the pi * affinity levels of these organic systems is suggested. Good agreement with optical measurements is found.

The core chemical shifts of the Si 2p electrons emitted from a Si(111) surface after the adsorption of alkali metals are studied. Initial and final state contributions are discussed. It is found that the changes in the electronic potentials are very similar to the corresponding Fermi level shifts, so only a small chemical shift, if any, will be observed.

An ab initio all-electron cluster model calculation of the adsorption of alkali metals on the Si(111) surface has been performed to study the nature of the M-Si bond. It has been found to have a complex bonding nature that cannot be characterized as either ionic or covalent. Bonding energies, equilibrium distances, populations and charge densities are analysed for Li, Na, K and Rb.

The authors calculate the lattice relaxation and variances of atomic positions for a two-dimensional hexagonal lattice with the (110) surface.

The authors use a tight-binding Hamiltonian to calculate the magnetic properties of the Ni (001) surfaces. They conclude that the magnetic properties are sensitive to sp to or from d charge transfer.

Kr adsorption isotherms near liquid N₂ temperature on a Pyrex glass cell and the same cell covered with a thick film of Na are reported. The authors determine an isoteric heat of adsorption of Kr/Pyrex of 1150 K, and they estimate q_st (Kr/Na) to be about 150 K.

Pulse measurements on the silicon-electrolyte interface are used to study surface states at the Si surface. The authors find that for CP-4-etched Si, surface states exist in the upper half of the energy gap, with a total density of 10¹² cm^-2. Addition of a minute amount of hydrofluoric acid to the electrolyte reduces their density by an order of magnitude. It has been reported that the surface recombination velocity on an oxidized surface is lowered by HF treatment, suggesting a corresponding reduction in the density of surface recombination centres. The results are the first direct evidence for such a reduction. It appears that Si-H bonds are formed at the interface, just as has been reported for an HF-treated free Si surface. Very likely, the formation of such bonds leaves, on both types of surface, considerably fewer dangling bonds and hence fewer surface states.

Secondary ion mass spectrometry (SIMS) operating with primary ion current density (J<5*10^-8 A cm^-2) was used to study the desorption process of hydrogen and deuterium from silicon(100). The description of the desorption process of hydrogen and deuterium was obtained monitoring the H⁺ and D⁺ SIMS signals as a function of the annealing temperature and annealing time. Low-energy electron diffraction (LEED) measurements enabled the surface coverage to be identified as monohydride or dihydride phases. The desorption of both elements displayed second-order kinetics. In addition, the process of reaction between hydrogen and deuterium adsorbed at different temperatures was studied.

The authors study dissociative adsorption kinetics of H₂ and D₂ on W(001).

The CO-TiO₂(110) and SO₂-TiO₂(110) systems have been studied by electron-stimulated desorption (ESD) and Auger electron spectroscopy (AES). AES studies following CO and SO₂ adsorption show clear differences. While the surface is unreactive for CO, i.e. the C peak was not observed for exposures up to 10⁴ L(CO), SO₂ was adsorbed at the surface at much lower exposures. ESD experiments have been shown to be very sensitive to both gases. ESD O⁺ ion yield as a function of the electron incident energy and ion kinetic energy distribution for both systems is presented.

Indium growth on Si(100) is investigated through Monte Carlo simulation. The study supports the models proposed by Knall et al (1989).

Surface roughness shifts the thermal desorption of H₂O to higher temperatures. Based on work function changes upon H₂O adsorption the roughness of Ni, Co, Cu and Pd films correlate well with single-crystal data.

The authors use a Monte Carlo method to investigate the influence of ad-ad interactions on Arrhenius parameters for desorption processes.

Angle-resolved electron energy loss spectroscopy (AREELS) has been used to study surface roughness induced at a polycrystalline Si surface by ion bombardment. A surface plasmon peak which is sensitive to atomic disorder is present in the surface-sensitive (SS) AREELS spectrum, but as expected bulk-sensitive (BS) AREELS data are less sensitive to surface disorder.

The authors study light scattering from a thin film with one-dimensional roughness of the interfaces using the Rayleigh method. The three layers are characterized by their dielectric constants and magnetic permeabilities. A plane wave of either TE or TM polarization is incident at one of the interfaces. The authors present perturbational results for sinusoidal and sawtooth profiles. They compare their calculations with a nonperturbational study and experimental data.

Oxidation of pure titanium was carried out at room temperature under different oxygen pressures (10^-8-10^-6 torr). The kinetics for each pressure was determined. Measured film thicknesses were larger for higher pressures. Deconvolution of XPS spectra showed than TiO was the major oxide present and TiO₂ the minor component. Formation of Ti₂O₃ and Ti₃O₅ was also observed.

The oxidation in GaAs(100) has been studied by using AES and PCA. It was found that at low oxygen exposures (below 10⁴ L), GaAs oxidizes through a one phase process which involves the simultaneous oxidation of Ga and As.

Ti oxidation has been studied by using AES combined with PCA. The authors found three different compounds for an exposure of 60 L identified as Ti, TiO and TiO₂. They also conclude that the formation of TiO₂ starts at the expense of TiO.

While performing Auger analysis of Si(100) surfaces covered with H₂O, the electron beam induces surface oxidation only at the beam incident point, with formation of SiO. A detectable damage requires an electron dose of approximately=2.4 C cm^-2.

The authors describe photoemission data of a gold monolayer on a Pt(001) surface.

Bi induces restructuring of Cu(100)-Bi into Cu(210)-Bi. The authors report core level spectroscopy results of Cu(100)-Bi phases supporting the idea of these bonds being metallic.

The thermally induced segregation of Mg in a 1% Mg/Al alloy in the temperature range 225 to 350 degrees C has been studied by means of Auger electron depth profiling. An activation energy for the process has been determined and a linear relationship was found between the amount of Mg enrichment in the surface region and temperature. The results suggest that the segregation is chemically driven by the greater affinity of Mg for O.

Metallic and alloy valence electronic structure information can be derived from core level Auger electron spectra through analyses of lineshapes, peak energy shifts and Auger parameters.

The authors present high-resolution measurements of the Auger M₅N₆₇N₆₇ transitions of Au, Pt and W. All the lines studied are narrower than 2 eV. They also measured the atomic sensitivities for the above Auger transitions with a resolution of 0.4% of the Auger energy (somewhat better than commonly available). With this resolution it is impossible to resolve fine structure and gross errors are incurred in the measurements of Auger signals. They corrected the measured sensitivities with the method developed previously; the correction factors for the sensitivities are around ten. These large factors represent also the errors incurred in the as-measured sensitivities if no correction is applied.

The authors discuss the Si Auger electron spectra induced by Ar⁺ and Ne⁺ bombardment.

The authors present a theory for the extremely fast instability of the diamond structure of semiconductors in the presence of a dense electron-hole plasma, which can be generated by an intense laser pulse. They find that the transverse acoustic phonons become unstable if more than 8% of the valence electrons are excited into the conduction band. Furthermore, if 15-20% of the valence electrons are excited, the cubic symmetry of the lattice is destroyed within less than 200 fs after the laser pulse. The crystal itself melts a few ps later. The results are in good agreement with experiments performed on Si and GaAs.

Extrinsic and intrinsic optical surface anisotropies have been observed in (001) and (110) GaAs by a modulation spectroscopy technique. A comparison of the anisotropies observed under two different chemi-mechanical treatments is shown, along with the spectrum obtained for an epitaxial layer to which no treatment is given.

The interest of measuring the DC and optical conductivity of metallic films on Si surfaces is discussed. Experiments on the reflectivity of Cu and Ag deposits on Si(100) 2*1 surfaces are summarized.

The authors have grown CdTe quantum dots in glass, studied the relaxation dynamics in the femtosecond time scale, and measured the non-linear refractive index in the infrared. Their observations, of very large non-linear refractive index in the transparency region and femtosecond recovery times of the resonant non-linearity, indicate that these materials are most promising for all-optical photonic switching applications.

Second-harmonic generation (SHG) may be a very sensitive spectroscopic tool for surface phenomena. However, until now it has not been made clear, either experimentally or theoretically, how strong the bulk contribution to the total radiated field in the second harmonic is. In this work, the author presents a model on the basis of which microscopic estimates could be made towards understanding the interplay of the surface and the bulk.

The authors develop a model for the surface contribution to the second harmonic susceptibility of silver. They use the localized nature of the d bands to account for the interband transitions with screened dipoles immersed in a semi-infinite conduction electron gas. They identify several contributions to the non-linear response, originating from the non-linear dipolar and quadrupolar polarizabilities and from the electronic density inhomogeneities at the crystal surface and they obtain terms that couple the localized and the extended contributions to the response.

From an optical point of view, a crystal with cubic symmetry has only a small anisotropy of the order (a/ lambda )² due to spatial dispersion, where a is an atomic distance and lambda is the wavelength. Therefore any other anisotropy in its optical properties, such as its reflectance, must originate from its surface. The authors explore the optical anisotropy of polar crystals in the far-infrared where the optical response of the system is determined by the ionic motions.

The authors show that the Raman signal from molecules over rough metallic surfaces or colloid clusters is enhanced by up to ten orders of magnitude if the molecules are tightly fitted between curved surface features. The effect of size dispersion and laser frequency over the enhancement are discussed.

Using inverse photoemission and photoemission the authors have found that the bulk band structure becomes discretized in highly perfect thin-film structures containing alternating layers of a ferromagnet and a noble metal. In noble metal layers on ferromagnets they find such discrete quantum well states as the Fermi level, which exhibit traits connecting them with oscillatory magnetic coupling. The magnetic oscillation period is determined by the periodicity of their 'envelope' wavefunction.

Fe layers stabilized on Ag(100) and W(100) present interesting anisotropy changes that can be accurately established by smoke signals. This magneto-optical technique can be used to determine the magnetization and it is UHV compatible.

The magnetic properties of very thin transition metal films are investigated. The different steps of such a study are discussed determination, from the electronic structure of the chemical order in the monolayer range, then computation of the magnetic order at T=0 K. Finally, the temperature effects are considered. This approach is applied to the Co/Cu(001) system, with a good agreement with experimental results.

The specific heat and the phase diagram of the Ising square lattice with nearest- (J₁) and next-nearest-neighbour (J₂) interactions are studied within the cluster variation method. The Ising Hamiltonian is solved in the square (Wannier-Kramer) approximation. The phase diagram is calculated and compared with other previous studies.

The authors describe calculations of the magnetization of very thin Co films on W.

The local-pairing problem of two electrons with parallel and anti-parallel spins is analysed by looking at the pairing energy, the coherence length and the mobility of pairs in a square lattice. These physical quantities are calculated by a novel method based on mapping the Hubbard problem onto a tight-binding one in a higher-dimensional space, which can be solved exactly.

Fluid motion induced by surface tension variation accelerates the aggregation of particles into a preferential square cell geometry which was observed in precipitated iron sulphate films.

Phonon density of states are presented for the (001) ideal surface of CdTe and HgTe semiconductors and their interface. Different polarization modes are analysed.

The authors present a quantitative r-factor analysis of experimental LEED data for CdTe(110) and InSb(110) surfaces in order to obtain optimum values for the Debye temperature and optical potential.

The authors are interested in the optical properties of molecular hydrogen. They have evaluated isentropic-expansion paths on the pressure-temperature phase diagrams. The optimal initial conditions are derived. The prospect of detecting the resulting clusters using optical techniques is discussed, the ultimate goal being that of observing supercooled liquid hydrogen.

The authors used XAFS to determine the local structure around small CuO clusters on alumina, between room temperature and 400 degrees C. They measured the spectra at the copper and sulphur K edges to obtain information on the structure and chemical state of the clusters.

It was found that the topology of the Laplacian of the molecular charge distribution can be used in the evaluation of the reactivity of model active sites such as those involved in the gas phase H abstraction from CH₄ by MgO and LiO molecules. The improvement of the catalytic activity of the surfaces of these solid oxides based in this result is also discussed.

The adsorption and coadsorption of CO and H₂ on polycrystalline Pt have been examined using temperature programmed desorption (TPD) and ion scattering spectroscopy (ISS). The data show that CO adsorbs molecularly through the C with the O pointing away from the surface and that the surface is about 80% covered. TPD data obtained after coadsorption indicate that a CO-H complex forms at this surface.

The aim of this study is to characterize the structure and catalytic properties of Bi₂MoO₆ compounds, for CO oxidation. These catalysts were prepared by solid state methods with an intended atomic ratio Bi/Mo=2. X-ray diffraction shows the presence of a Koechlinite phase in the samples calcined at low temperature, and a gamma phase (Watanabe) in the samples calcined at high temperature. However, all samples appeared to be bismuth deficient (compared to the stoichiometric ratio of two), as indicated by Auger analysis. The catalytic activity shows a CO₂ yield better than 40% for these catalysts.

The microstructure of molybdenum disulphide and tungsten disulphide unsupported catalysts have been characterized by high-resolution electron microscopy. Lattice resolution of edge planes shows the typical layered structure, however in this case, additional phase curvature and dislocations are seen. The material shows a micro-domain arrangement where planes like (101), (103) and (105) are identified based in the interplanar spacings. In other cases the (002) basal plane was found to exhibit plane shifts or kinks.

The interaction of deuterium with polycrystalline molybdenum at 80 K in the exposure range of 20-600 L, was studied by thermal desorption spectroscopy and work function measurements. Evidence of molecular and dissociative adsorption was found. Desorption peak shapes suggest the presence of a distribution of interacting adsorption sites with different adsorption energies.

Tracer diffusion on an energetically heterogeneous surface is studied by Monte Carlo simulation. The surface is simulated in the framework of a self-consistent description where site-saddle-point energy correlations are involved in a general way. The influence of heterogeneity, temperature and correlations on the temporal behaviour and dynamic of migration is analysed.

Catalysts and pure phases were characterized using XRD, ³¹P MAS-NMR and XPS techniques. The catalysts always showed the XRD patterns of vanadyl (V(IV)) pyrophosphate, while the presence of V(v) phases was detected from the ³¹P spectra. The XPS results did not exclude the existence of low concentrations of V(v) surface centres. The best catalysts contained traces of V(v) compounds.

Using percolation theory the authors describe the effect of porous system connectivity on catalytic deactivation.

The author uses XPES-AES to present experimental evidence of the nature of copper sites in Cu-containing catalysts used in the heterogeneous dehydrogenation of methanol and CO hydrogenation.

The semi-empirical atom superposition and electron delocalization molecular orbital (ASED-MO) theory is used to study the adsorption and dissociation of CO on different sites of Ru (001) preadsorbed with C1 or MoO.

The number of surface OH groups on a low surface area alpha -Al₂O₃ and a Pd (0.3%)/ alpha -Al₂O₃ catalyst, calcined in air at 523 K, has been measured by reaction with (C₄H₉)Li. The Li uptake on alpha -Al₂O₃ was equivalent to 9.5*10¹⁴ OH groups cm^-2. On Pd/ alpha -Al₂O₃ the amount of Li fixed was higher due to reaction of (C₄H₉)Li with PdO.

The authors describe the catalytic activity of a Langmuir-Hinshelwood reaction with surface reconstruction.

Properties of CH_n (n=1,2,3) absorbed species on a Ni (100) surface were calculated by using the MINDO/SR method. Adsorption is stronger on border atoms (OC₁), and follows the order: CH₃<CH₂<CH. There is charge transfer from the surface to the adsorbates. A new parameter (diatomic binding energy) to evaluate interatomic interactions is proposed.

The authors describe a simulation study of benzene desorption from Pd(111). They show why LEED patterns corresponding to benzene ordered structures, produced by repulsive lateral interactions, are not observed.

The authors comment on the topology of correlated energies on heterogeneous surfaces. A large effort is being dedicated to the characterization of the adsorptive energy topography of heterogeneous surfaces. The authors discuss here the dual site-bond model.

A commercial hydrotreatment catalyst of CoMo type was considered in different life stages namely precursor, activated and spent. Its surface properties are discussed.

The interaction of ion beams with solids and solid surfaces is described, over a wide range of energies, by classical trajectories. At low energies the elastic interaction between the ions and target atoms is important in determining the end of the trajectory, radiation damage and sputtering. The inelastic interaction, i.e. elementary excitations of the solid and ionization of target and projectile atoms, is relevant for the electronic stopping power, and hence the range of fast particles, and it is the cause of radiation damage in insulators and biological material. Applications of ion beams are surface analysis, sputtering, ion implantation and ion-beam modified thin-film growth.

Using grazing-angle, ion-induced electron emission spectroscopy at clean and oxygen-covered metal surfaces (Al, Cu, Fe, Ni(110) and Pd(110)), the authors have studied the angle-resolved energy distribution (ARED) of emitted electrons which directly reveals details of the initial electron excitation mechanisms. The authors find that secondary electron cascades are strongly suppressed and that the ion-induced electron excitation occurs predominantly at the topmost surface layer. The major excitation mechanisms for electron emission are single electron excitation via decay of surface and bulk plasmons (for 150 keV H⁺ ions), electron-hole pair excitation and Auger electron excitation.

Electron spectra arising from grazing-incidence ion-surface collisions in the keV energy range are discussed. Spectra due to spontaneous emission and spectra due to kinetic emission are distinguished. In the case of spontaneous emission, results of analyses based on a recently developed theoretical model are briefly outlined. In the case of kinetic emission, experimental spectra for He⁺ colliding with Cu surfaces are presented and analysed with a new model.

Energy spectra of electrons ejected during collisions of H-like nitrogen and bare carbon ions of a Ni surface are analysed. It is found that at most 10% of the KLL Auger electrons are emitted from above the surface.

Average single-ion impact on solids has been studied by a 3D Monte Carlo approach. A spatially non-homogeneous relaxation, applied afterwards, leads to the formation of a crater at the surface. The main features of the crater agree well with experimental observations by STM.

The Keldysh-Green function approximate solutions of the correlation effects on the charge-exchange probabilities for a scattered atom from a three-atom linear chain within the Anderson model are compared with exact results for different velocities of the moving atom.

The authors show that there is no discrepancy between experiment and theory in the energy dependence of critical angles for skipping motion by taking account of the real electron distribution (a jellium model) at the surface.

Energy sharing is considered for collision cascades in a polyatomic medium with constituents of very different atomic numbers and masses. Considerable disagreement is found between predictions of Anderson and Sigmund (1974) and a Monte Carlo simulation on HfC, even for equivalent physical input. An improved theory due to Urbassek and Conrad (1992) is shown to remove this discrepancy.

An extension of the Okorokov effect to the case in which the ion moves near an oriented surface and the electron is ejected into the continuum is considered. The effect of the screening of the initial electronic bound state on the final electron energy is analysed.

The energy position and lifetime of atomic levels brought in front of a metal surface are the important quantities for the description of the resonant charge transfer process in atom-metal surface scattering. Recent results obtained in the non-perturbative CAM method on multielectron systems (H^-, He^-, C^-) are discussed. They stress the importance of a proper description of the atomic system. In particular, the interaction with a metal surface is seen to reveal the differences between the different orbitals in a multielectronic system.

The authors discuss the physical mechanisms contributing to the phase effect in electronic stopping.

A Monte Carlo program describing the penetration of H_n⁺ hydrogen clusters in thin foils has been used to build a model of charge exchange for protons resulting from the breakup of these clusters. The total neutral fractions observed at emergence with cluster projectiles H_n⁺, n<23 in the energy range 40-120 keV/u, as well as the number of 2p excited states are well reproduced by these calculations taking into account geometrical structures of the projectiles and vicinity effects.

Using the dielectric formalism and the specular reflection model, the authors evaluate the probability of surface and bulk plasmon excitation by particles reflected in the proximity of a metal surface. The authors obtain a strong oscillatory behaviour as a function of the penetration distance.

A Green function analysis of the charge transfer probability for an atom scattered by a semi-infinite linear chain is performed within the one-particle Anderson-Newns model. The main intervening ingredients are the finite but continuous limit of the substrate bandwidth and the eventual presence of surface states. The dependence on the velocity of the moving atom, as well as the effects of including surface states, are comparatively discussed.

Cooling of radiation damage cascades in the bulk, close to a perfect heat sink or close to a reflecting boundary has been followed using the liquid droplet model. Time evolutions of the molten volume and of ion mixing showed rather small differences in cooling behaviour.

The author considers the interaction of a neutral atomic projectile colliding with a solid surface in the limit in which large numbers of vibrational quanta are exchanged, while at the same time the projectile itself may be moving with momenta corresponding to quantum mechanical motion.

The authors analyse the electron emission produced by ionization of target electrons in grazing fast ion-surface collisions. In this approach they describe the target within the jellium approximation. The Coulomb field of the impinging ion sets the surface electrons in motion like in a binary collision. This process gives to some electrons the energy necessary to overcome the surface potential barrier and to escape from the solid. The authors analyse the electron energy spectra produced by this mechanism and study the effect of the solid work function.

The authors investigate the contribution of tunneling processes to the neutralization of ion projectiles impinging on metal surfaces.

The authors studied the forward electron emission produced in collisions of 15-100 keV H⁺ and He⁺ with single-crystals at grazing incidence. The dependence of the electron energy distribution with the sample topography and the projectile energy is discussed.

This paper describes a study of Ar ion bombardment damage in metal oxides. In the energy range 1 to 5 keV, preferential oxygen removal and reduction of the oxides was found to depend on ion current density, but to be independent of beam energy.

The influence of foil roughness on energy-loss measurements is evaluated. Correction terms due to the effective foil thickness and roughness are deduced.

He⁰ atoms of He⁺ ions of 1.5 keV energy are scattered under a grazing angle of incidence of 1 degrees from a clean Al(111) surface. The author observes an effect of the image charge potential on the trajectories of the ions and from this interaction he deduces estimates of the Auger transition rates for the neutralization process.

The authors discuss the behaviour of an electron beam running parallel to the flat surface of a metal or dielectric.

Atomic mixing at Cr-SiO₂ interfaces induced by noble gas (Ar, Kr and Xe) ion bombardment was studied by means of Rutherford backscattering spectrometry. The amount of mixing was determined as a function of fluence (1-20*10¹⁵ ions cm^-2), substrate temperature (300-800 K) and deposited energy at the interface. These results indicate the prevalence of the ballistic mixing mechanism.

Problems encountered in the description of ion-induced compositional changes in non-monatomic solids, on the basis of the binary collision approach (BCA), are reported. The ingredients of a collisional model responsible for eventual ambiguity of computer simulation results are pointed out. The conclusion is made that application of the BCA to ion-induced composition build-up problem can require preliminary parametrization of the collisional model used for a given target composition.

Using hot-filament CVD (HF-CVD) the authors succeeded in preparing polycrystalline diamond films on non-scratched crystalline silicon by depositing a very thin microcrystalline silicon carbide buffer layer. Films were characterized by AES, EELS and SEM.

This paper is aimed at resolving controversies in the literature over the reproducibility of surface potential and lateral conductance data on Langmuir monolayers (LM). It is shown that the alleged non-homogeneity (at the macroscopic level) in surface potential measurements on expanded monolayers must be an experimental artifact. In addition, the recently reported enhanced conductivity in a monolayer packed to a given critical density is confirmed for a phospholipid monolayer.

The authors report results of experiments of valence band (VB) photoemission and dark conductivity of amorphous SiN_x:H (0<or=x<1.5). The most important change observed in the electronic structure when x increases is the replacement of Si-Si bonding states at the top of the VB by Si-N bonding states at the centre of the band. A plot of the band edges is presented which shows that the sudden opening of the gap at x approximately=1 is due mainly to the recession of the conduction band minimum.

A macroscopic technique which involves only a few minutes' processing, for the evaluation of the induction time was previously used only in organic materials. This methodology is extended to evaluate the induction time in inorganic materials (muscovite mica).

In the present study, experimental data referring to the structural characteristics of amorphous germanium-nitrogen (a-Ge:N) thin films are presented and discussed. The nitrogen content of the samples was allowed to vary from typical impurity levels up to around 35 at.%. The proportionality constant relating the total nitrogen content (as determined from a deuteron induced nuclear reaction) and the infrared (IR) integrated absorption of the in-plane stretching vibration mode of the Ge-N dipole has been determined to be K_(N)=5*10¹⁸ cm^-2.

A crossover from displacive to order-disorder behaviour is found in a two-dimensional non-linearly polarizable model with a ferroelectric transition. The dynamics of a shell model with double-well core-shell interaction is simulated by means of the molecular dynamics technique. Snapshots of the lattice displacement pattern reveal the presence of precursor order regions in the high-temperature phase, while the dynamical structure factor exhibits a quasielastic component in addition to the ferroelectric soft mode peak. The local particle dynamics is characterized by two times scales corresponding to the coexistence of inter- and intra-well motions.

By means of a Monte Carlo simulation the author has studied the effect of different growth mechanisms on the intensity evolution of reflection high-energy electronic diffraction (RHEED) and thermal energy atom scattering (TEAS). The author found that due to the different sensitivities to the surface defects RHEED and TEAS give different results for the same surface evolution during the process of film growth.

The energy distribution of interface states at the insulator-silicon interface has been investigated as a function of the injected charge for standard SiO₂ and plasma nitrided oxides in MOS structures. For the nitrided samples, a lower rate of surface state generation and the absence of the midgap peak known in standard oxides was observed.

Al₂O₃ layers have been obtained by vapour evaporation and their surface morphology has been investigated by TEM and HEED. The relation with capacitance is discussed.

Electrochromic nickel oxide films are studied by means of cyclic voltammetry and photometric measurement. A moving intercalation front model is shown to be consistent with the features of the reflectance curves.

The authors show that the chemical separation method generates a carbon-rich product that allows them to study the microstructure of PVC irradiated at doses above 15 MGy. They show the existence of new structures not clearly identifiable with known carbon structures. Studies with HRTEM revealed highly ordered structure which may be related to fullerene rather than graphitic materials.

Angle resolved the photoemission spectroscopy excited with synchrotron radiation was employed to investigate the electronic properties of the alpha -Sn/CdTe(111) heterostructure during the initial stages of epitaxial growth. After deposition of 10 AA Sn an abrupt shift ( approximately 1.1 eV) of the valence band edge of the alpha -Sn/CdTe interface towards the Fermi level (E_F) was observed, the emission at E_F being considerably stronger than that of the metallic beta -Sn film.

The low temperature photoluminescence emission of amorphous silicon-carbon alloys is presented. A linear correlation between the photoluminescence width and the absorption band tails was found, suggesting that disorder is the main source of the radiative broadening.

The valence band states of a-Si_1-xC_x:H have been studied by combining photoemission and Auger spectroscopies. It was possible to establish the relative positions of the onsets and of the maxima of the local electronic-state density at C and Si atoms. It was also found that heteronuclear Si-C bonds are strongly favoured.

Commercially available samples of SnO₂ were treated using a KrF excimer laser (248 nm). These treatments were characterized using normal optical absorption and transmittance in the VIS-NR region. With this treatment, normalized integrated transmittance in the visible region was improved by 15%.

Experimental results are presented demonstrating that hydrogen diffusion is enhanced under illumination and that the enhancement is suppressed when the photo-generated carriers are extracted from the diffusion region by an electric field. The results suggest a close connection between hydrogen diffusion and metastability in a-Si:H.

The authors report on the Raman scattering spectra and fundamental absorption edge of dense semiconductor-insulator nanocomposites. These were synthesized by injection of the semiconducting melt into the channels of dielectric matrices. The Raman spectra exhibit shifts, broadenings, and asymmetries consistent with phonon confinement in nanometre-size shifts, broadenings, and asymmetries consistent with phonon confinement in nanometre-size semiconductor crystallites. Nanoscale networks of small effective mass semiconductors show evidence for electron confinement as manifested by a blue shift of the fundamental absorption edge.

Measurements of conductivity in samples of a-Si:H alloys show dependence on film thickness that can be explained in terms of band bending near the free surface interface.

Among the mechanisms suggested to account for degradation of varistor materials, ion migration has strong support. Mobile, positively charged ions in the depletion layer would be the key element in degradation. In this context, the authors discuss the consequences of an electrical stressing on the current-temperature characteristic of a comment varistor.

Flash-evaporated GaSb films are analysed using a combination of optical, surface and X-ray diffraction techniques. The effects of thermal annealings on nearly stoichiometric GaSb films are studied.

The authors study the evolution of a-CdTe:O samples with thermal annealing using XRD, RS, AES and ESCA techniques. The final compound mixture is quantified. All results obtained agree with the Cd-Te-O ternary phase diagram.

The current-voltage characteristics of Pd/a-Ge:H Schottky barriers are presented. The devices have been deposited by the RF sputtering method. The I-V curve follows an exponential law and the diode ideality factor is eta =2 indicating the existence of recombination currents in the space charge region.

CuInSe₂ thin films have been electroplated on Ti substrates from aqueous solutions containing Cu, In and Se precursors. Structural and morphological properties of these films are reported.

Large changes of conductivity occur in annealed pyrolytic SnO₂ films when irradiated with ultraviolet radiation under ambient conditions. The authors have found that this effect is due to chemisorption-photodesorption of oxygen following an Elovich-type kinetics and that chemisorption proceeds by the transfer of electrons through a potential barrier.

The effects of the incorporation of Al into high-quality RF sputtered a-Ge:H are reported. Al-to-Ge atomic concentration ratios in the range 2.6*10^-5-1.0*10^-1 were studied. Samples with Al concentration below 1.0*10^-2 present an increase of the activation energy decrease of both dark and photoconductivity and constant H content. This is interpreted as due to active p-type doping of the intrinsically n-type material. Al concentration above 1.0*10^-2 are detrimental to the electronic quality of the samples, probably due to a high concentration of threefold coordinated Al.

The authors describe a study of the electric field dependence of electronic states in an In_xGa_1-xAs/GaAs superlattice.

An optical characterization was carried out on GaAs/Si/GaAs heterostructures grown by MBE. From differential photoreflectance experiments it was possible to determine the contribution to the signal of each interface and determine the magnitude of the electric fields. Raman scattering results indicated good confinement of the Si monolayers.

The authors discuss the self-consistent bound states of a strip of donor impurities in GaAs in the Hartree approximation. For electrons to be bound to the strip at least four energy levels must be occupied regardless of the strip width.

A self-consistent and microscopic theory of Coulomb blockade is developed for a 1D mesoscopic heterostructure. It is shown that these systems develop multistable properties as a function of applied voltage.

Working in the effective-mass approximation the authors study the problem of excitons in a superlattice in parallel electric and magnetic fields. Using the translation symmetry of the excitonic effective Hamiltonian and the electric field induced localization, they find an expression for the envelope excitonic function in terms of packet-like functions constructed of non-interacting electron-hole pairs in a one-dimensional superlattice under an electric field.

New selection rules are experimentally evidenced for coherent tunnelling in double-barrier structures under a tilted magnetic field. A simple theoretical model accounts for these results.

Using a variational procedure within the effective-mass approximation, the authors found that spatially dependent screening increases the binding energies of shallow hydrogenic impurity states in GaAs-(Ga,Al)As quantum-well wires. These energies are higher than those found when constant screening is used.

The results of Brillouin scattering experiments from magnetic excitations in Fe/Mo and Fe/Cr superlattices are presented. Due to the large number of unknown parameters which enter into a complete theoretical description, it has not been possible to quantitatively interpret the Brillouin results. Trends in the magnetic field dependence of the magnon frequencies are however consistent with the interlayer coupling strength obtained from magnetization data.

The authors have grown, characterized and measured the thermal expansion of (FeF2)_n(CoF₂)_n antiferromagnetic/antiferromagnetic superlattices, from which Neel antiferromagnetic transition temperatures T_N(n) were obtained. These results are compared with similar measurements on FeF₂/ZnF₂ antiferromagnetic/non-magnetic superlattices. The authors conclude that for large n, the CoF₂ near the interface depresses the transition temperature of the FeF₂ significantly more than would be expected inFeF₂/ZnF₂ superlattices with similar FeF₂ thicknesses.

Electron spin resonance of Mn²⁺ in AlCl₃ single-crystal and AlCl₃-GICS reveals a reduction of about 30% in crystal field parameter (D) upon intercalation of AlCl₃ in graphite.

A first-principles prediction of the q dependence of the electronic susceptibility of YBa₂Cu₃O₇ and the relation to experimentally observed phonon anomalies is presented.

Heteroepitaxial thin films of superconducting YBa₂Cu₃O_7-x, HoBa₂Cu₃O_7-x and insulating SrTiO₃ have been deposited on (001) SrTiO₃ substrates by DC/RF sputtering at high oxygen pressures. The deposition conditions for the high-T_c films and SrTiO₃ films were completely compatible. The structures of the layers were investigated by X-ray diffraction and transmission electron microscopy. Trilayer planar-type junctions with SIS structure showed a superconducting transition above 90 K. The heterostructures were characterized by susceptibility, resistivity and I-V measurements.

The authors have studied the structure of high-T_c superlattices and films through refinement of the X-ray spectra. The results indicate that interdiffusion, layer thickness fluctuations and strains are present and cannot be neglected.

The authors report on magnetoresistivity rho _ab( theta ,H,T) measurements below T_c in YBa₂Cu₃O_xn films with differing oxygen content x. The observed variations in the angular and field dependence of rho _ab with reduced x indicate a gradual dimensional crossover from an anisotropic 3D to a quasi-2D behaviour. The increasing anisotropy is related to a decrease of the coupling between the Cu2O₂ bilayers when oxygen is removed from the Cu1O_x chain-planes.

The electronic structure of C₆₀, Si₆₀ and Ge₆₀ is calculated by real-space methods, based on a Hubbard-type Hamiltonian which includes s- and p-type electrons. The equations of motion for the Green function are solved by recursion methods. Differences in the electronic structure of the three elements are discussed.

The authors generalize the hydrodynamic model to degenerate electron gases of inhomogeneous conductors in three, two and one dimensions. The limit of sharp interfaces yields expressions for the additional boundary conditions (ABCS) and settles long standing controversy.

The present work has been undertaken in order to find a simple way of evaluating semiquantitatively the thickness of metal films, using a non-destructive procedure. Thin metal films have been deposited on glass and stainless steel, to be used as standards. The thickness of the deposits on glass was measured by interferometry. The intensity of X-ray radiation from the film was measured with an electron probe microanalyser and related to the intensity obtained for the same element in a bulk sample. The ratio of intensities was plotted against thickness and the possibility of estimating the thickness by assuming a linear behaviour of the microanalyser signal was investigated.

The variation of the electronic structure of V(100) slabs in sandwich-type M1-V-M2 systems is studied by using the Green function matching method and empirical tight binding with s, p and d bands. Combinations of Nb, Cr, Mo and Ta are used for M1 and M2. The author chooses to study mainly the local density of states for each V interface as a function of V slab size. For the slab sizes studied he finds that the properties for the sandwich system are reduced to those of uncoupled interfaces of two semi-infinite media in contact.

A model for the decagonal phase of the Al₁₃Fe₄ system is proposed. The electronic structure is calculated within the tight-binding method. Results for the local and the integrated electronic density of states at particular sites are presented.

Correlation between SEM-EBIC images and I-V curves is obtained for dislocations in GaAs and grain boundaries in polycrystalline Si. This method allows a quantitative description of the electrical behaviour of these defects.

Electromigration and thermomigration of metallic islands formed by vapour deposition on the Si(100) 2*1 surface have been investigated by ultra-high-vacuum scanning electron microscopy at temperatures higher than the melting points of the islands. The direction of the electromigration due to passing a direct current through the Si substrate depends on the type of metal, whereas the direction of the thermomigration due to a temperature gradient is always from low to high temperature. The speeds are 0.1 mu m s^-1 to 2 mu m s^-1 and approximately proportional to the island radius for both cases. The activation energies for electromigration and thermomigration are approximately the same and are equal to 0.6 eV. The driving forces of island migrations are discussed for both cases.

From a self-energy approach, a general expression for the energy loss probability in scanning transmission electron microscopy is derived. As an application, the coupling between a small particle and the supporting surface is studied.

The authors present a brief review of their work by generating structures with sizes down to 10 nm by breaking organometallic molecules with the electrons from a scanning tunnelling microscope.

The authors describe the possibility of using STM as an atomic manipulating tool.

Studies of evaporated platinum particles onto substrates of highly oriented pyrolitic graphite (HOPG) and nuclear grade high purity graphite (NGHPG) were performed by scanning tunnelling microscopy (STM) and high resolution transmission electron microscopy (HRTEM). Distortion of the electronic density of states of the graphite surface near a particle was observed with STM, as well as epitaxial growth of the metal particles as demonstrated by the HRTEM images.