Adsorbate vibrations and substrate surface phonons ofp(2×2)O/Mo(110)

We investigated the system $p(2\mathrm{}\times 2)\mathrm{O}/\mathrm{M}\mathrm{o}\mathrm{}\left(110\right)\mathrm{}$ by electron energy-loss spectroscopy. The specular spectra of the ordered $p(2\mathrm{}\times 2)$ superstructure reveal an intense loss peak at $519{\mathrm{cm}}^{\mathrm{-}1},$ which corresponds to the perpendicular vibration of the O atoms adsorbed in the long bridge site. Increasing O coverage or hydrogen coadsorption causes a site conversion towards the triply coordinated hollow site. Measurements of the dispersion curves of the molybdenum Rayleigh wave and the longitudinal mode do not show any significant change compared to the clean surface. This fact is surprising because the experimentally observed Fermi surface of the $p(2\mathrm{}\times 2)$ superstructure should favor Fermi-surface nesting and hence an anomalous softening of the frequencies of the surface phonon modes, as previously observed for the hydrogen-saturated Mo(110) surface.