The spectroscopy of surface vibrations in ionic crystals by inelastic scattering of atoms

Abstract

The recent progress in the production of highly monochromatic atomic beams is paving the way for a full determination of the surface vibrational structure. Account is given of the recent determination of Rayleigh wave dispersion curves from angular distributions exploiting the kinematical focussing effect. A short review is then 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 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.