High-resolution inelastic He-atom surface scattering experiments have been carried out on an in situ cleaved NaCl͑001͒ crystal surface for the incident wave-vector range of 4.29-6.07 Å Ϫ1 in the ͗110͘ high-symmetry direction to detect the recently predicted effect, the focused inelastic resonance ͑FIR͒. The scattering angle for the resonance in the ͗110͘ direction is uniquely predicted based on the reciprocal lattice vector exchanged in the process for the bound state ͉⑀ 1 ͉ϭ3.4Ϯ0.1 meV. This value is slightly lower than the 3.7Ϯ0.4 meV value found in the literature. The convincing evidence for the FIR effect comes from a systematic analysis of the angular distributions in combination with an energy analysis based on time-of-flight spectra.
The structure and dynamics of physisorbed carbon dioxide on in situ cleaved single crystal sodium chloride surfaces was studied by means of elastic as well as inelastic helium atom scattering. At Tsurface=80–83.5 K the diffraction patterns indicate a commensurate (2×1) monolayer superstructure on the (001) plane of the substrate, the unit cell containing a glide plane. This is in agreement with results obtained from low energy electron diffraction and infrared spectroscopy. In time-of-flight experiments single phonon low-energy loss and gain features were observed which can be attributed to acoustic and optical modes. Two higher-energy features are probably due to the first combination modes observed by helium atom scattering so far. The growth of solid CO2 adsorbed on NaCl(001) was also studied.
Dispersion curves of surface vibrational modes of monolayer CO2/NaCl(001) have been measured by inelastic He atom scattering along the ΓX azimuth. Eight phonon modes could be followed across almost the entire Brillouin zone. The identification of the modes is discussed based on previously calculated normal mode energies at the zone origin.
Helium atom scattering has been used to investigate the structure of the Si͑111͒ surface in the temperature range from 900 to 1600 K. Even below the well-known ͑7ϫ7͒ to ''͑1ϫ1͒'' transition the adatoms become mobile, and, when the transition is reached near 1140 K, the specular-and integral-order diffraction peaks have sudden intensity changes, some up and others down, while the seventh-order peaks disappear. Above the transition the adatoms remain, moving rapidly on, and supported by, the ordered but relaxed, outer bilayer of the surface. A second transition, first reported by Ishizaka and co-workers occurs near 1470 K. The loss of all diffraction peaks and the attenuation of the specular peak indicate a completely disordered surface as the temperature approaches the melting point.
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