EELS study of the clean and hydrogen-covered Mo(110) surface

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Abstract
We have investigated both the substrate phonons and adsorbate vibrations of the clean and hydrogen-covered Mo(110) surface by high-resolution electron-energy-loss spectroscopy. With increasing hydrogen coverage the specular spectra together with elastic intensity measurements at S-bar and N-bar lead us to the conclusion that, in analogy to the W(110) surface, there exist three ordered phases at 110 K: three domains of a ``(2×1)'' at about 0.5 ML, a (2×2) at about 0.75 ML, and a (1×1) 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 specular spectra. The background increases monotonically with coverage. This background is probably partly due to an electron-hole damping mechanism 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 wave but also the longitudinal mode exhibit a lowering of the frequency at a wave vector along Γ-barH-bar and at the S-bar point. Moreover, we have discovered a shallow dip in the dispersion curve already for the (2×2) phase.