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 determin ed the phonon dispersion curves in commensurate (root 3 X root 3)R30 d egrees Xe monolayers adsorbed on Cu(lll). In addition to the well know n transverse acoustic mode the dispersion curve of the longitudinally polarized phonon branch could be measured. Since this is the first cas e where a finite zone-center phonon gap (here 0.4 +/- 0.15 meV) has be en seen for a Q-resolved phonon branch, a normal mode analysis and the oretical calculations on the phonon excitation probabilities have been carried out to confirm the assignment. From the gap magnitude importa nt information on the corrugation of the Xe-atom-substrate potential i s derived.