Anharmonic damping in rare-gas multilayers

We have calculated the linewidths from anharmonic damping, for the phonon modes of rare-gas bilayers and trilayers adsorbed on the Pt(111) surface. This is done through use of perturbation theory, which in lowest order provides no damping for the monolayer vibration polarized normal to the surface. We also assume that the source of anharmonicity is the lateral interactions between adsorbates. We find linewidths in the range 0.1--0.2 meV at the temperature T=25 K for all cases studied, save for the Ar bilayer, which is found to be more heavily damped. The results, supplemented by inclusion of radiative damping produced by coupling to substrate phonons, are compared with new experimental results for both the linewidth and dispersion of the normal modes of Ar, Kr, and Xe monolayers, bilayers, and trilayers adsorbed on the Pt(111) surface. The theoretical and experimental results are in good accord.