Observation of a Zone-Center Gap in the Longitudinal Mode of an Adsorbate Overlayer: Xenon on Cu(111)

Using inelastic scattering of thermal energy He atoms we have determined the phonon dispersion curves in commensurate $(\sqrt{3}\times \sqrt{3})R30\mathrm{}°$ Xe monolayers adsorbed on Cu(111). In addition to the well known transverse acoustic mode the dispersion curve of the longitudinally polarized phonon branch could be measured. Since this is the first case where a finite zone-center phonon gap (here $0.4\pm 0.15\mathrm{meV}$) has been seen for a $Q$-resolved phonon branch, a normal mode analysis and theoretical calculations on the phonon excitation probabilities have been carried out to confirm the assignment. From the gap magnitude important information on the corrugation of the Xe-atom–substrate potential is derived.