Atom-surface interaction in inelastic scattering: He/Ag(111)

The amplitude of inelastic He scattering from Rayleigh waves on a Ag(111) surface is calculated in the framework of the distorted wave Born approximation (DWBA) for a two-body exponential potential. Using the potential parameters recently obtained by Bortolani et al. from a quantum mechanical calculation, agreement is obtained with the experimental data. Simple explicit formulas are presented both in the DWBA and in the eikonal approximation. The essential factor in both cases describes an approximate exponential decay of the intensity for increasing parallel momentum transfer.     

Fast atom diffraction during grazing scattering from a MgO(001) surface

Fast neutral atoms and molecules with energies from 0.4 up to 3 keV are scattered under a grazing angle of incidence from a clean and flat MgO(001) surface. For “axial surface channeling” conditions, we observe defined diffraction patterns in the angular intensity distributions for scattered ³He and ⁴He atoms as well as H₂ molecules. The diffraction patterns are analyzed in terms of semiclassical trajectory calculations making use of projectile surface interaction potentials derived from density functional theory and from pair potentials calculated from Hartree–Fock wave functions. From comparison of measured and calculated diffraction patterns we deduced the rumpling of the topmost surface layer of MgO(001), i.e. an inward shift of Mg²⁺ ions with respect to O^2? ions, of (0.03±0.03) Å.     

Symmetry changes in He scattering from a Si(111) surface exposed to H atoms

Changes which occur on, and just below, the surface of a Si(111)-(7×7) crystal when it is dosed with H atoms are complex and still not well described. Measurements of He atom diffraction (HAS) from this surface in the [ 2 ] or [ 11] directions give scans which are asymmetric about the specular angle. The scans become symmetric when the surface is exposed to doses of H atoms sufficient to give coverages of 0.1 to 14 ML, if the atoms all stick. This means that the 3-fold rotational symmetry of the clean surface becomes 6-fold after the addition of the H atoms and shows that the faulted and unfaulted regions of the DAS model become equivalent as they are averaged by the He beam, although the large 49-atom unit cell remains. Thus this surface is not Si(111)-H(1×1), the equilibrated product of H-atom addition which is about 0.3 eV lower in energy, but a metastable intermediate state(s).

Comparison of results from HAS with those from other techniques, such as scanning tunneling microscopy, help to characterize this surface.     

An experimental investigation of the elastic scattering of He and H2 from Ag(111)

Experimental diffraction probabilities for 63 meV He and 66 meV H₂ scattering from Ag(111) along the 〈112̄〉 direction are reported. Debye-Waller experiments for the He/Ag(111) system yield a mean well depth of 9.3 meV and an effective surface Debye temperature of 253 K.     

Diffraction of fast atomic projectiles during grazing scattering from a LiF(001) surface

Light atoms and molecules with energies from 300 eV to 25 keV are scattered under a grazing angle of incidence from a LiF(001) surface. For impact of neutral projectiles along low index directions for strings of atoms in the surface plane we observe a defined pattern of intensity spots in the angular distribution of reflected particles which is consistently described using concepts of diffraction theory and specific features of grazing scattering of atoms from insulator surfaces. Experimental results for scattering of H, D, 3He, and 4He atoms as well as H2 and D2 molecules can be unequivocally referred to atom diffraction with de Broglie wavelengths as low as about 0.001 Angstroms.     

Theory of one-phonon scattering of atoms from metal surfaces: Application to He/Ag(111)

We present explicit expressions for one-phonon scattering of atoms from metal surfaces, within the distorted wave Born approximation by including Van der Waals forces. A comparison is made with the recent high-resolution time of flight spectra for He/Ag(1 1 1). We explain in quantitative terms the position and the intensity of the peaks due to scattering from the Rayleigh wave of silver. Also the bulk phonons contribute to the spectra, but the calculated intensities underestimate the data.     

Neutralization of charged fullerenes during grazing scattering from a metal surface

The neutralization of C(60)(+) and C(60)(2+) fullerenes with keV energies is studied for grazing scattering from a clean and flat Al(001) surface. From the measured shifts between the angular distributions for scattered projectiles of different incident charge, we derive image-charge interaction energies, which relate to the distances of electron transfer for C(60)(+) and C(60)(2+). These neutralization distances are in accord with a classical over-the-barrier model taking into account the image-charge effects of the Al target and the polarization of the fullerene.     

Inelastic scattering of He atoms from a Cu(110) surface

A He beam has been used to measure inelastic scattering from a Cu(110) surface. The scattering was a result of predominantly single phonon events and both energy gain and loss processes were observed. This was in contrast to Cu(001) where only energy loss was observed. For Cu(110), aligned in the [001] azimuth, partial dispersion relations have been measured which meant that phonons with ΔQ values up to the zone boundary value have not been detected under the present experimental conditions. The elastic incoherent component varied with azimuthal angle for Cu(110).     

Vacancy-defect scattering in Ag(111)-supported silicene interface

The role of vacancy-defect scattering in electron mobility near the two-dimensional (2D) Ag(111)-supported silicene sheet interface was investigated in this paper. The interface structures of Ag(111)-supported silicene and silicene-sheet charge density were considered. Mobility limited by the vacancy-defect scattering was calculated in the order of 10²–10⁵ cm²/V?s, which was comparable with first-principle calculations and experimental results for free-standing and substrate-supported silicene. Results further showed the decay relationship between the electron mobility and vacancy density of silicene sheet. Furthermore, the influence of the Ag(111)-supported silicene sheet structure on electron mobility was confirmed through calculations. This study can contribute to the prediction of the electron mobility of other 2D graphene-like honeycomb-structure systems.     

The Tamm surface state on Cu(111) and Ag(111)

It is shown, that the well-known d-electron Tamm surface-state emission observed in photoelectron spectra from Cu(111) at the M̄ point in the surface Brillouin zone, is indeed due to such a surface state and not a bulk band transition as recently suggested L. Wallden, Solid State Commun. 59, 205 (1986). A similar surface state on Ag(111) is reported.     

The scattering of He from adsorbed layers of gases on Ag(110)

The adsorption of gases on Ag(110) has been studied using inelastic He atom scattering. Vibrational spectra have been obtained for Kr, Xe, C₂H₆, C₂H₄, CH₄, CF₄, CHF₃, CO₂ and H₂O. Spectra have also been obtained for multilayers of Xe (2 layers) and C₂H₆ (3 and 4 layers) where the energy changes move to lower values. The scattering from Kr and Xe can be shown to be dispersionless as has been previously found for these adsorbates on Cu(100) and Cu(110). The energy changes for Kr and Xe are smaller than on Cu surfaces and attempts were made to account for this based on an Einstein model of the adsorbed atoms in the surface holding potential.     

On the relative strength of He and H2 diffraction from Ag(111)

Under equivalent incident conditions, H₂ diffraction beams on Ag(111) have recently been observed to be about one order of magnitude stronger than He beams. We show that this effect can be attributed to details of the interaction, in particular, the exponential increase of the corrugation amplitude towards the surface. By extending a previously developed theory for the interaction between He and a metal surface, we show that the H₂Ag(111) repulsion is roughly 1.5 times larger than for He. However, the Van der Waals attraction is about three times stronger for H₂, so that the classical turning points of low-energy H₂ particles lie closer to the surface. Because of the stronger corrugation at short distances, H₂ diffraction intensities can be up to an order of magnitude larger than for He.     

Diffuse scattering from Ge(111) surface at high temperature

On heating a clean Ge(111) surface above 240°C, Ge(111) 2 × 8 surface structure changes to 1 × 1 one. We have first observed twofold splitting diffuse scattering in RHEED patterns from the Ge(111) 1 × 1 surface at high temperatures. A modulated 2 × 2 structure is proposed as a structural model for the diffuse scattering. The Fermi-surface instability of dangling-bond electrons at the surface is studied as an origin of the formation of the modulated structure.