Vibrations of the commensurate monolayer solid Xe/Pt(111)

High resolution helium atom scattering has been used to study the structure and phonons of the (root 3 x root 3)R30 degrees commensurate lattice of a monolayer of xenon on Pt(111). For in-plane vibrations, the data indicate a Brillouin zone-centre gap of about 1.3 meV and a dispersion of less than 0.5 meV from the Brillouin zone centre to the zone edge along the <(Gamma)over bar>(K) over bar azimuth of the Xe solid, the Pt(111)[11 (2) over bar] direction. The data are consistent with a model of top-site adsorption of the Xe and with lateral Xe-Xe interactions being the same as in the gas phase.