Angle- and velocity-resolved inelastic He scattering has been used to study how the surface dynamics of thin rare-gas films evolves on a layer-by-layer basis. Surface-phonon dispersion relations for ordered 1-, 2-, 3-, and 25-layer Kr films physisorbed on Ag(111) are presented along $\overline{\Gamma }\overline{M}$ across the entire Brillouin zone. The monolayer data are dispersionless, indicative of an Einstein oscillator mode. In comparison, the 25-layer film has a well-developed Rayleigh wave, typical of a thick crystal surface. Excitation linewidths for monolayer Kr, which vary across the zone, are also briefly discussed.

• Received 21 March 1985

DOI:https://doi.org/10.1103/PhysRevLett.55.1514