Inelastic helium scattering studies of ordered Ar, Kr, and Xe monolayers physisorbed on Ag(111): Dispersion curves, scattering cross sections, and excitation line shapes

This paper is the first in a series covering our experiments examining the physical properties of ordered overlayers of Ar, Kr, and Xe physisorbed on Ag(111). In this paper, we examine the desorption kinetics of the Xe monolayer/Ag(111) system. Desorption is zeroth order until ∼90% of the monolayer has desorbed, then becomes first order. We also measured the inelastic scattering of an 18 meV He beam from unconstrained monolayers of (111) oriented Ar, Kr, and Xe. The transitions are mapped across the entire surface Brillouin zone from Gamma-bar to M-bar. The data are dispersionless, indicating that for the measured mode the adatoms are behaving as independent Einstein oscillators. Parametrized physisorption potentials for RG--Ag(111) [RG = Ar, Kr, Xe] are constructed using these results. Inelastic scattering probabilities and linewidths are also presented. The inelastic scattering probabilities vary by at most a factor of 3 across the entire surface Brillouin zone, and are reported as a function of incident angle, final wave vector, and surface temperature. Variations in the inelastic scattering probabilities are indicative of dynamical adatom--substrate coupling. Experimental techniques which turn these dynamical couplings on or off for the same phonon energy are discussed. Limited results for clean Ag(111) are also presented. It is hoped that these measurements, on such ideal systems as ordered rare gas monolayers, will provide further impetus for developing improved theoretical treatments of inelastic single phonon scattering