Inelastic helium-scattering studies of the surface vibrational spectroscopy and dynamics of ordered Ar, Kr, and Xe multilayers physisorbed on Ag(111). Interim technical report

This is the second of three papers discussing the authors studies of overlayers of argon, krypton, and xenon physisorbed on Ag(1I1). All of these rare gases form ordered structures, which are azimuthally aligned but translationally incommensurate with the Ag substrate. This paper discusses experiments wherein they utilized the angle-resolved time of flight of inelastically scattered 18-meV He atoms to examine the surface dynamics of multilayer rare gas films along the r - M direction. This was done on a layer-by-layer basis for 2, 3, and >20 layers for each of the rare gases. Unlike the monolayers, the vibrational modes observed for the multilayers show dispersion across the surface Brillouin zone, the amount of dispersion increasing with the number of adsorbed layers. These results reveal in detail how the surface dynamical properties of a thin film evolve towards those of a thick crystal as a function of increasing dimension. Lattice-dynamics calculations, which utilize realistic gas-phase pair potentials, reproduce the experimentally observed phonon dispersion relations quite well. The authors also examine the inelastic-scattering probabilities and linewidths of the transitions.