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Lattice dynamics of rare-gas overlayers on smooth surfaces.

We discuss the lattice dynamics of rare-gas overlayers on metal surfaces, with attention to monolayers, bilayers, and trilayers on the Ag(111) surface. For this system, the adsorbate overlayers are incommensurate with the metal substrate. Our interest is in the role of the interaction between the motions of the atoms in the adlayers and those of substrate atoms. In certain regions of the two-dimensional Brillouin zone, the adlayer phonon dispersion curves (calculated with the substrate atoms held in fixed position) overlap the bulk phonon bands of the semi-infinite substrate. A consequence is that when excited, the adlayer phonon may decay by emitting phonons into the substrate. They thus become leaky modes. Also, the adlayer modes may hybridize with the Rayleigh surface phonon on the substrate, to produce anomalous dispersion near those wave vectors where crossover occurs. For Ar, Kr, and Xe overlayers on models of Ag(111), we explore these phenomena through calculations of the phonon spectral density functions probed in inelastic He-surface scattering experiments.