Combined He-atom scattering and theoretical study of the low-energy vibrations of physisorbed monolayers of Xe on Cu(111) and Cu(001)

The surface-phonon-dispersion curves of commensurate Xe monolayers on Cu(111) and incommensurate monolayers on Cu(001) surfaces have been measured using He-atom scattering (HAS) time-of-flight (TOF) spectroscopy. The TOP spectra are interpreted by combining quantum scattering calculations with the dynamical matrix description of the surface vibrations. Both a vertically polarized Einstein-like mode and another, acousticlike mode of dominantly longitudinal character are identified, The latter mode is characterized by the presence and absence of the zone-center frequency gap in the commensurate and incommensurate adlayers, respectively, The microscopic description of the,TOF spectral intensities is based on the extensive theoretical studies of,the interplay of the phonon dynamics, projectile-surface potentials, multiple quantum interference, and projectile recoil, and. their effect on the HAS spectra. Both single and multiple quantum spectral features observed over a wide range of He-atom incident energies and substrate temperatures are successfully explained by the theory. [S0163-1829(99)10507-1].