EELS STUDY OF THE CLEAN AND HYDROGEN-COVERED MO(110) SURFACE

We have investigated both the substrate phonons and adsorbate vibratio ns of the clean and hydrogen-covered Mo(110) surface by high-resolutio n electron-energy-loss spectroscopy. With increasing hydrogen coverage the specular spectra together with elastic intensity measurements at (S) over bar and (N) over bar lead us to the conclusion that, in analo gy to the W(110) surface, there exist three ordered phases at 110 K: t hree domains of a ''(2 x 1)'' at about 0.5 ML, a (2 x 2) at about 0.75 ML, and a (1 x 1) phase at full monolayer coverage. Starting from the elastic beam and extending up to the parallel hydrogen and deuterium Vibration an enhanced background is observed for all phases in the spe cular spectra. The background increases monotonically with coverage. T his background is probably partly due to an electron-hole damping mech anism and partly due to an enhanced mobility of the hydrogen adatoms. Additionally we present the dispersion of substrate phonons along the symmetry directions of the surface Brillouin zone. Our results confirm the helium atom scattering data and show that not only the Rayleigh w ave but also the longitudinal mode exhibit a lowering of the frequency at a wave Vector along <(Gamma)over bar>(H) over bar and at the (S) o ver bar point. Moreover, we have discovered a shallow dip in the dispe rsion curve already for the (2 x 2) phase.