INFLUENCE OF MODE POLARIZATIONS ON THE INELASTIC HE-SCATTERING SPECTRUM - HIGH-ORDER COMMENSURATE XE MONOLAYER ADSORBED ON CU(110)

The phonon dispersion curves for monolayer Xe adsorbed on Cu(110) were measured along the <(Gamma X)over bar> and <(Gamma Y)over bar> direct ions using inelastic He scattering. The dispersion along these two ort hogonal high-symmetry directions is fundamentally different as a conse quence of the substrate anisotropy. Coupled dynamics calculations usin g a slab model are performed on the square low-order commensurate c(2 x 2) structure and on the (26 x 2) high-order commensurate phase of th e Xe monolayer, the latter being stable at low temperature. The calcul ations allow a quantitative interpretation of the hybridization and av oided crossings between the longitudinal and perpendicular monolayer b ranches and the Rayleigh mode along <(Gamma X)over bar>. They can also account for the unexpected behavior of the monolayer longitudinal mod e along <(Gamma Y)over bar>. revealing a different type of dynamical c oupling between this monolayer branch and the longitudinally polarized Rayleigh mode of the substrate. This coupling is responsible for a po larization exchange by which both modes become susceptible to He scatt ering detection.