HELIUM ATOM SCATTERING SPECTROSCOPY OF SURFACE PHONONS - GENESIS AND ACHIEVEMENTS

New experimental techniques for high-resolution inelastic helium scattering have within the last thirteen years made possible measurements of the dispersion curves of surface phonons in insulators, semiconductors and metals. In air three types of materials a number of anomalies have been observed indicating that the interactions between atoms in the surface region differ significantly from those in the bulk. The transition metals invariably show an unexpected and anomalously soft surface phonon branch below the longitudinal acoustic band while in insulators such as LiF, semiconductors (e.g. Si, GaAs and GaSe) and charge density wave systems (e.g. TaSe2 and TaS2) other substantial deviations from the predicted ideal surface behavior are observed. These findings have stimulated new theoretical investigations based on microscopic model calculations where the role of surface electrons is explicitly taken into account.