VIBRATIONAL PROPERTIES OF THE CLEAN W(110) SURFACE AND OF DIFFERENT HYDROGEN ADSORBATE PHASES STUDIED BY HELIUM ATOM SCATTERING

We have investigated the clean and the hydrogen covered W(110) surface with elastic and inelastic helium atom scattering (HAS). At low temperatures two different hydrogen superstructures can be detected in accordance with earlier LEED investigations. Further hydrogen exposure leads to a weakening of all superstructure spots and at saturation the (1 x 1) periodicity of the clean surface is restored. The surface phonon dispersion curves of the clean surface and of all hydrogen adsorbate phases have been measured by inelastic HAS. The clean surface displays "normal" surface lattice dynamics while the surface phonon dispersion curves of the superstructures show backfolded phonon branches due to the reduced Brillouin zones of these phases. This behaviour contrasts with the hydrogen saturated surface where a sharp surface phonon anomaly has been found. The connection of these properties with an earlier report of a top layer shift reconstruction induced by adsorbed hydrogen is still not known.