M. Balden et al., SUBSTRATE AND HYDROGEN PHONONS OF THE ORDERED P(2X1) AND (2X2) PHASE AND OF THE ANOMALOUS (1X1) PHASE OF HYDROGEN ON W(110), Physical review. B, Condensed matter, 53(11), 1996, pp. 7479-7491
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.