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Abstract
The recent great advance in the production of highly inonochromatic atcmic beams is opening new pers2ectives in surface physics, having made way to a full determination of the surface vibrational structure. After a short review of the earlier att- empts to detect surface phonons from the angular distributions of scattered atoms, we re2ort 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 theo- ries of surface phonons in ionic crystals. The state of the art in the theory of inelastic scattering processes is briefly illu- strated in order to discuss the theoretical inter2retation of TOF spectra. The one-phonon energy-loss spectra of He scattered from LiF(001) calculated for a hard corrugated surface model are found to be in good agreement with TOF spectra at all the incidence angles. Evidence is given that, in addition tl ayleigh waves, important contributions to the inelastic scatte g come from the surface- projected density of bulk phonons. The possible observation of optical surface modes in KCl(001) is finally discussed.
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