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(11 1) and incommensurate monolayers on Cu(001) surfaces have been measured usi ng He-atom scattering (HAS) time-of-flight (TOF) spectroscopy. The TOP spec tra are interpreted by combining quantum scattering calculations with the d ynamical matrix description of the surface vibrations. Both a vertically po larized Einstein-like mode and another, acousticlike mode of dominantly lon gitudinal 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 t he,TOF spectral intensities is based on the extensive theoretical studies o f,the interplay of the phonon dynamics, projectile-surface potentials, mult iple quantum interference, and projectile recoil, and. their effect on the HAS spectra. Both single and multiple quantum spectral features observed ov er a wide range of He-atom incident energies and substrate temperatures are successfully explained by the theory. [S0163-1829(99)10507-1].