6.2.2.1.1 Surface phonons

This chapter discusses surface phonons in meals and semimetals. Data presentation is limited to the electron energy loss spectroscopy (EELS) cross section of the surface-phonons and to the evaluation of the effective charges in case of dipole active modes. The importance of the analysis of the inelastic cross section to understand the surface dynamics is discussed for the significant case of Ag(001). For this surface a displacement of loss and gain peaks in the EELS spectra was observed as a function of E_i which is caused by the relative contribution of the Rayleigh wave and of unresolved modes to the inelastic signal. The comparison between the measured and the computed shift of the loss peak maximum vs. E_i allowed to establish that L₁ and not S₂ contributes to the inelastic intensity and to discriminate between different lattice dynamical models. The excitation and annihilation continuum of bulk phonons was shown to be responsible for the tail of the specular peak observed in the case of Cu(001) and Ni(001). The effective charge connected with dipole active mode for the different crystals is summarized. In case of Cu(110) the frequency shift vs T due to anharmonicity was studied and resulted 4…5 times larger than in the bulk.