SURFACE PHONONS ON CLEAN AND ADSORBATE COVERED METAL-SURFACES

Recent progress in surface phonon spectroscopy using inelastic electron scattering is reviewed. For a number of adsorbate systems the dispersion curves of the adsorbate modes have been measured. It is demonstrated that important structure parameters can be deduced from the local symmetry of the adsorbate revealed by the absence of a dispersion branch under particular scattering conditions. Among the adsorbates hydrogen is a particularly interesting case, since there the classical picture of vibrational motion must be replaced by a quantum mechanical description. For some adsorbate covered or metastable clean surfaces the dispersion of the Rayleigh wave indicates a compressive or tensile surface stress. Finally the significance of an experimental determination of the dispersion of surface phonons with odd symmetry will be pointed out.