Surface Vibrational Excitations in Noble Metals

Abstract

The increasing sophistication of the experimental techniques recently reached in surface physics has put in evidence new elementary excitations. One of the more exciting developments is related to the mesure of surface vibrational properties [1]. Inelastic atomic scattering [2], electron energy-loss spectrocscopy [3], Brillouin scattering [4] have been used successful I y to study surface phonons of aIkaIi halides, noble metals, semiconductors and chemisorbed systems. In particular, the high resolution (≃0.3 meV) obtained with inelastic atomic scattering has shown the existence of new excitations. These localised and resonant vibrational modes are not only related to the lack of translational invariance normal to the surface, as one would expect, but a I so to the variation of the interatomic forces occurring in the surface region. The difference between the surface interactions and the bulk ones, particularly important in noble metals, is responsible for the occurence of new phonon branches in the continuous part of the spectrum. These lectures are devoted to the interpretation of the experimental inelastic atom-surface reflection coefficients of noble metals through realistic calculations of the surface phonon spectra. At present, a microscopic theory which fully accounts for the termination of the electron gas exists only for simple metals [5,6] but is not applicable to noble and transition metals. Therefore for such materials we will use a phenomenoIogicaI approach based on the force constant parametrisation of the bulk dynamics.