COMPARISON OF SURFACE PHONON-DISPERSION CURVES FOR CLEAN AND HYDROGEN COVERED RH(111) SURFACES

Surface phonon dispersion curves have been measured for the clean and hydrogen covered Rh(111) surfaces along the [$($) over bar$$ 110] and [$$($) over bar 112] high symmetry directions of the surface Brillouin zone using high resolution He-atom scattering. Interatomic force constants and elastic constants, which are not available for this material, were obtained by a lattice-dynamical analysis. The significant changes of the dynamical properties of the surface upon hydrogen adsorption are shown to be related to hydrogen-induced changes in the surface electronic structure and are discussed within the framework of a pseudocharge model which has been developed recently. An analysis of the experimental phonon excitation probabilities for the two surfaces within the framework of the distorted wave Born approximation indicates large changes in the repulsive He-surface potentials upon hydrogen adsorption.