Hydrogen on Ru(0 0 1) revisited: Vibrational structure, adsorption states, and lateral coupling

A detailed investigation of the vibrational properties of H and D layers on Ru(001) with high-resolution electron energy loss spectroscopy (HREELS) at a resolution of about 2 meV has been carried out. Coverage and angular dependences, and momentum-dependent data are reported. From 0.05 to 0.5 ML and at full coverage (1 ML) two narrow peaks exist (at 1137 and 820 cm ^-1 for the saturated H-layer, 833 and 602 cm ^-1 for D) which are assigned to the perpendicular and the doubly degenerate parallel vibrations. The latter degeneracy is lifted at higher parallel momentum transfer, as demonstrated with the full dispersion behavior in the ΓK direction, which corroborates this assignment. Losses at low frequencies are due to Ru surface excitations. The two-phonon region is analyzed. Some discrepancies with earlier HREELS work are discussed. All results can be explained by a single adsorbate species at all coverages whose properties change with coverage, consistent with existing geometric, kinetic, and spectroscopic data. Subsurface H can be excluded. No evidence for quantum-mechanical delocalization has been found.