Surface Dynamics of Xe(111): An Ambiguous Nobility

A new analysis of inelastic helium atom scattering (HAS) data for the (111) surface of Xe reveals a surprising similarity between the surface phonon dispersion curves of the solid noble gas xenon and of the noble metal copper, the only difference being an energy scale factor. Even more surprising is that the poorly understood X phonon branch observed in Xe(111) actually corresponds to the subsurface S2 phonon branch of Cu(111). A subsurface phonon in a closed-shell insulator surface should not be detectable by HAS, as is instead the case for free electron surfaces, where the electron-phonon interaction can couple the scattering He atoms to phonons several layers beneath the surface (quantum sonar effect). A density functional perturbation theory analysis shows that the large atomic polarizability of surface Xe atoms actually provides the electron-phonon coupling sufficient to explain the comparatively large HAS intensity from the S2 branch of Xe(111). It is concluded that the possibility of directly measuring the surface electron-phonon coupling constants from inelastic HAS intensities, originally demonstrated for conducting surfaces, can be extended to polarizable insulating surfaces, as well as oxides and heavier chalcogenides.