SUBSTRATE AND HYDROGEN PHONONS OF THE ORDERED P(2X1) AND (2X2) PHASE AND OF THE ANOMALOUS (1X1) PHASE OF HYDROGEN ON W(110)

The adsorbate and substrate phonons of the hydrogen- and deuterium-cov ered W(110) surface were investigated using electron-energy-loss spect roscopy (EELS). For coverages below saturation, the measured substrate phonon dispersion curves show a continuous change of the Rayleigh wav e frequency around the (S) over bar and (N) over bar points with incre asing coverage. No effect of the suggested reconstruction could be obs erved. For the saturated (1 x 1) phase the frequency of the Rayleigh w ave (RW) in the shallow dip of the phonon anomaly depends on coverage and temperature. In addition we found a shallow dip also for the longi tudinal tungsten surface phonon at the same incommensurate wave vector along <(Gamma)over bar>(H) over bar. The frequency always stays above the RW. The deep dip, which has recently been interpreted as due to e lectron-hole pair excitations, has not been detected up to now with EE LS at all accessible scattering conditions. At the wave vector of the anomalies along <(Gamma)over bar>(H) over bar we observed a small peak in the elastic diffuse intensity. For the ordered p(2 x 1) phase thre e H modes are resolved (550, 770, and 1250 cm(-1)); at least two of th em are dipole active. The ordered (2 x 2) phase shows six sharp H mode s. The linewidth of the modes depends on the order of the overlayer, t he coverage, and the adsorbate isotope. The (triply coordinated) hollo w site could be determined as the adsorption site starting from low co verages up to about 0.75 ML. The H modes of the (1 x 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 decreasin g temperature.