Relaxation of the edge atoms of a stepped Cu(410) surface

The edge atom position of the stepped Cu(410) surface has been investigated with low energy ion scattering (LEIS) using 10–20 keV H⁺. It is possible to determine the atomic position by utilizing atomic shadowing effects. The scattered particles are analysed with a time-of-flight spectrometer. This means that we can detect scattered hydrogen leaving the copper surface in the neutral state as well as in the charged state, so the influence of neutralization of the scattered particles along different outgoing trajectories is eliminated. We find an inward relaxation of the edge atoms; assuming that this relaxation takes place in the (100) plane it amounts to about 8% of the interlayer spacing.     

Edge atom depression on stepped Cu(410)

The position of the edge atoms of a stepped Cu(410) surface has been measured by Ion Scattering Spectroscopy using 21 keV H⁺. The edge atoms are depressed 5.0±1.5% of the copper lattice spacing, corresponding to 0.18±0.05 Å.     

Atomic structure of the Cu(410)-O surface: STM visualization of oxygen and copper atoms

The structure of the Cu(410)-O surface is studied by a scanning tunneling microscope (STM) with atomic resolution. Tunneling is accomplished for various states of the tungsten STM probe, which allows oxygen and copper atoms to be visualized separately.     

Low energy ion scattering from a Ni(100) surface

We have tudied the scattering of He and Ne ione from a clean Ni(001) surface in the energy range of 0.5 to 5 keV. The spectra are dominated by scattering from atoms in the first layer for energies belowe 2 keV, whereas for higher energies contributions from the second layer can be identified. In the impact collision mode of ion scattering spectroscopy with a scattering angle =164°, the shape of the shadow cone can be determined. These experimental shadow cone radii agree over a wide range of parameters well with calculations using a Thomas-Fermi-Molière potential with a reduced screening length.     

The structure of oxygen on W(001) by low energy ion scattering

The adsorption of oxygen on the W(001) surface has been studied by LEED, AES, and low energy alkali ion scattering in order to obtain a more detailed understanding of the short-range surface structure. Exposure at room temperature results in oxygen adsorption near bridge sites for all coverages below 0.6 ML as determined by the ion scattering incidence angle dependence. Above 0.6 ML, the oxygen adsorbs in four-fold hollow sites. When the oxygen-covered surface is annealed to 950 K, the surface reconstructs, leaving vacancies in the W lattice. This is confirmed by the observation of ions scattered from second layer W, through the vacancies, at relatively low angles of incidence. The oxygen atoms do not adsorb in the position vacated by the missing W atoms but instead adsorb atop the second W layer.     

The spectroscopy of surface vibrations by atom scattering

The recent progress in the production of highly monochromatic atomic beams is opening new perspectives in surface physics, having paved the way for a full determination of the surface vibrational structure. After a discussion on the possible determination of Rayleigh wave dispersion curves from angular distributions exploiting the kinematical focussing effect, a short review is presented on the direct measurement of surface phonon dispersion curves, first achieved by Brusdeylins, Doak and Toennies in alkali halides, from time-of-flight (TOF) spectra of scattered He atoms. A comparison is made with the existing theories of surface phonons in ionic crystals. The state of the art in the theory of inelastic processes is briefly illustrated in order to discuss the theoretical interpretation of TOF spectra. The one-phonon energy loss spectra of He scattering from LiF(001) calculated for a hard corrugated surface model are found to be in general good agreement with the experimental TOF spectra. From such a comparison evidence is obtained that: i) one-phonon processes are predominant, and ii) in addition to Rayleigh waves important contributions to the inelastic scattering come from the surface-projected density of bulk phonons. Important effects due to inelastic resonances with surface bound states are put in evidence and explained by simple kinematical arguments. The possible observation of surface optical modes in NaF(001) is finally discussed.     

Transformations of the stepped Si(001) surface structure induced by heating the specimen by a current

The system of uniformly distributed atomic steps on a vicinal Si(001) surface was observed to transform into a system of large terraces, separated by step bands, when the specimen is heated by a direct electric current. This transformation is reversible and depends on the current direction and the initial step density. The experimental data are discussed in terms of electromigration of adatoms.     

Adsorption and desorption of oxygen and carbon monoxide on the Si(111) surface studied by ion scattering spectroscopy

The adsorption of O₂ and CO on the Si(111) surface was studied by low-energy helium ion scattering. The adsorption consists of a fast adsorption stage followed by a much slower Sorption process. In the final uptake region CO has a faster rate of increase than O. There is no evidence of He⁺ scattering from C atoms. This fact excludes the CO molecule having its axis parallel to the surface. A comparison of the intensities of the substrate (Si) signals, for the same recorded oxygen content on the surface, shows that carbon monoxide shadows the Si atoms more than oxygen does. An increase in the oxygen signal was observed even after exposures in the range of 10¹⁴–10¹⁵ molecules cm^?2. No substantial diffusion of CO into the bulk can be deduced from these results. Desorption of oxygen by He⁺ ions was observed by following the adsorbate and substrate signals as a function of time. The sputtering cross-section has a maximum for an impact angle of 25° relative to the surface.     

Vibrations of linear cobalt clusters on a stepped copper surface

The results of calculations of vibrational properties of cobalt dimers and trimers on the Cu(110) surface are reported. The local phonon density of states and their polarizations are analyzed. The calculations are performed using interatomic interaction potentials constructed in a tight-binding approximation. It was shown that the vibrational modes of the free standing dimer and trimer remain in the supported clusters but the revealed anisotropy of the local surface relaxation leads to deformation of the interatomic bonds and, as a result, to splitting and shifting of the phonon frequencies. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 67–73, January, 2009.     

The surface electronic structure of (100) centered tetragonal copper

We present a theoretical investigation of the surface electronic structure for the (100) surface of centered tetragonal copper. The calculated band structure and k_- and layer-resolved densities of states for the surface and bulk are compared with corresponding quantities obtained for the face centered cubic (fcc) structure. We have found a surface state at about 5 eV below the Fermi level which is splitted off the bulk s-d band for both the tetragonal and the fcc structure. Furthermore, we have calculated the normal photoemission spectra within the one-step approach. The direct transition model explains all features of available experimental data. Particularly, we can conclude that the invisibility of the surface state for the fcc structure in most of the photoemission experiments is driven by the final states.     

Structure study of oxygen-adsorbed Ni(111) surface by high energy ion scattering

Surface atomic structures of clean, oxygen-adsorbed, and oxidized (111) nickel have been studied quantitatively by using MeV ion scattering in combination with AES and MEED. We show that; the clean (111) nickel surface has the bulk-like structure with reconstruction or relaxation less than 0.02 Å, the surface thermal vibration amplitude is enhanced by ~20% compared to the bulk value, adsorbed oxygen results in surface lattice expansion by ~0.15 Å which is closely correlated to the p(2 × 2) or (√3 × √3) R30° superstructure, and oxidation at room temperature saturates at a stage which incorporates ~ 3 monolayers of nickel in a stoichiometric amorphous film of NiO whereas at temperatures higher than ~200° C thicker oxide films are produced. Our study indicates that each oxygen atom adsorbed on the Ni(111) surface interacts with and relaxes three nearest neighbor nickel atoms, and after saturation of the relaxation, oxidation of three monolayers takes place abruptly after which the oxide layer on the surface apparently blocks further reaction.     

The surface composition of platinum-palladium alloys determined by auger electron spectroscopy

Pt-Pd alloys were prepared as powders by a chemical reduction method and as films by evaporating the specpure metals onto a quartz substrate. Results were evaluated by means of the Gallon equation; the parameters of this equation — the signal intensity of a monolayer and of the bulk material — were determined experimentally for all relevant Auger peaks. The backscattering factor for the alloy system was evaluated from experiments in which a layer of one metal was evaporated on top of the other. The value determined in this way was compared with results obtained in the literature on similar systems. The theoretical surface compositions are calculated for several models and compared with the experimental results. Experimental results reveal surface enrichment in Pd, in good agreement with the theoretical calculations. Interactions of alloys with carbon monoxide and propane are discussed and a possible explanation of the effects observed is suggested.     

Reconstructed domains on a stepped W(100) surface

A model calculation is carried out for the scattering of low energy electron diffraction from reconstructed (√2 × √2)R45° domains on a stepped W(100) surface. The existence of monotonically increasing steps causes the integral order beams to split and the separation of the splitting oscillates with the incident electron energy. We show that due to the existence of an antiphase relationship among the randomly nucleated reconstructed domains, the half-order beams neither split nor oscillate with the incident electron energy. This nonsplitting of half order beams is in agreement with the observation by Debe and King (DK). We also show that the measured intensity profile of a half order beam is equal to the signal intensity profile from the individual finite size domains convoluted with the instrument response function. This gives a simple way to evaluate the reconstructed domain size quantitatively. From the angular distribution of the half-order beam intensity we deduce that the reconstructed domains are somewhat round in shape, instead of the “long strips” proposed by DK. Also, the long range inhibition (20 Å) of the reconstruction near the step edge suggested by DK does not necessarily follow from our analysis. As a matter of fact, there is evidence showing that the inhibition (if it exists at all) can be short range in nature. Our suggestion is in agreement with the observation of the reconstructed W(100) surface by Melmed, Tung, Graham and Smith using FIM technique.