Substrate and hydrogen phonons of the ordered p(2 x 1) and (2 x 2) phase and of the anomalous (1 x 1) phase of hydrogen on W(110).

Abstract

The adsorbate and substrate phonons of the hydrogen- and deuterium-covered W(110) surface were investigated using electron-energy-loss spectroscopy (EELS). For coverages below saturation, the measured substrate phonon dispersion curves show a continuous change of the Rayleigh wave frequency around the S\ifmmode\bar\else\textasciimacron\fi{} and N\ifmmode\bar\else\textasciimacron\fi{} points with increasing coverage. No effect of the suggested reconstruction could be observed. For the saturated (1\ifmmode\times\else\texttimes\fi{}1) phase the frequency of the Rayleigh wave (RW) in the shallow dip of the phonon anomaly depends on coverage and temperature. In addition we found a shallow dip also for the longitudinal tungsten surface phonon at the same incommensurate wave vector along \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}H\ifmmode\bar\else\textasciimacron\fi{}. The frequency always stays above the RW. The deep dip, which has recently been interpreted as due to electron-hole pair excitations, has not been detected up to now with EELS at all accessible scattering conditions. At the wave vector of the anomalies along \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}H\ifmmode\bar\else\textasciimacron\fi{} we observed a small peak in the elastic diffuse intensity. For the ordered p(2\ifmmode\times\else\texttimes\fi{}1) phase three H modes are resolved (550, 770, and 1250 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$); at least two of them are dipole active. The ordered (2\ifmmode\times\else\texttimes\fi{}2) phase shows six sharp H modes. The linewidth of the modes depends on the order of the overlayer, the coverage, and the adsorbate isotope. The (triply coordinated) hollow site could be determined as the adsorption site starting from low coverages up to about 0.75 ML. The H modes of the (1\ifmmode\times\else\texttimes\fi{}1) phase show an anomalous behavior: they create in specular a continuum and a single sharp loss in the EELS spectra. Additional structures in the frequency range of the continuum develop with increasing coverage and decreasing temperature. \textcopyright{} 1996 The American Physical Society.