Surface phonon calculations for noble metals: Comparison with he-surface scattering experiments

We evaluate the surface phonon dispersion relations of Ag(111) and Au(111) within a force constants parametrization of the bu?k dynamics. By changing the surface force constants we are able to explain all the salient features appearing in the experimental time of flight spectra of a 7.9 meV He beam. The parallel momentum Q has been considered along the 11?0] and 112?] directions of the surface Brillouin ozne. We show that the peaks of the time of flight spectra are related to the Rayleigh wave, to the pseudo Rayleigh wave and to a new resonance of longitudinal character.