SURFACE PHONONS OF THE CLEAN AND H-COVERED W(110) SURFACE MEASURED WITH EELS

The surface phonons of the clean W(110) surface were investigated by EELS. At the GAMMABAR-point a dipole active surface resonance at about 190 cm-1 has been resolved and confirmed by slab calculations using central forces up to third neighbors. The Rayleigh wave (RW) has been observed throughout the whole surface Brillouin zone along the GAMMAHBAR, GAMMANBAR, and GAMMASBAR direction. The frequencies are 128, 116 and 130 cm-1 at HBAR, NBAR, and SBAR, respectively. The dispersion curves in all three directions agree with the data obtained by He-scattering where such data are available. A second mode within the bulk band has been detected by both techniques and is interpreted as the longitudinal polarized surface phonon of the first layer. For the hydrogen covered surface the RW dispersion curves along GAMMAHBAR of the (2 x 1) and (2 x 2) superstructure show a slightly higher frequency than for the clean surface around HBAR. On the nearly H-saturated (1 x 1) surface a dip in the dispersion curve of the RW was found at a wave vector of 0.9 angstrom-1 along GAMMAHBAR: the frequency drops from 110 cm-1 on the clean surface down to 83 cm-1. A second feature detected with He-scattering, showing an even stronger anomaly at the same wave vector, could not be observed in our measurements.