The adsorption of hydrogen and deuterium has been studied with high resolution electron energy loss spectroscopy (HREELS). At maximum coverage at 105 K the H adlayer exhibits two dominant loss features at 102 (ngr;1) and 141 (ngr;2) meV which show pronounced intensity variations as a function of the primary electron energy (maximum between 5 and 6 eV). The hydrogen is assigned to the threefold coordinated site of the Ruthinsp;(001) surface leading to aC3vsymmetry of the adsorption complex. The angular dependence of both loss peaks shows only a smooth intensity decrease in offhyphen;specular directions thus preventing the separation of dipole and impact scattering contributions. Three additional loss peaks with considerably lower intensities are identified with overtone and combination excitations. The anharmonicity coefficients derived from the respective energies are used to fit a twohyphen;dimensional potential function for the hydrogen atom located in a plane perpendicular to the surface with first order perturbation theory. The resulting differences in the potential gradient allows the assignment of the ngr;1and ngr;2fundamentals to the vibrational modes parallel and perpendicular to the surface, respectively. As a function of increasing coverage both ngr;1and ngr;2shift to higher frequencies with a concomitant decrease in the peak width, the shift of the parallel mode being distinctively larger. At low coverages a third loss peak is observed which is ascribed to a different adsorption state of hydrogen. For deuterium the two fundamentals and the lowest overtone are observed and exhibit reasonably accurately the expected isotopic shift. The adsorption properties are found to be similar.
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