Hydrogen adsorption on Mo_1-xRe_x(110) (x =0-0.25) surfaces

The effects of adsorbed H on the Mo_1-xRe_x(110), x =0, 0.05, 0.15, and 0.25, surfaces have been investigated using low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). For the x =0.15 alloy only, a c(2 x 2) LEED pattern is observed at a coverage H ∼0.25 ML. A (2 x 2) pattern is observed for H coverages around H ∼0.5 ML from surfaces with x =0, 0.05, and 0.15. Both c(2 x 2) and (2 x 2) patterns are attributed to reconstruction of the substrate. At higher coverages, a (1 x 1) pattern is observed. For the alloy surface with x =0.25, only a (1 x 1) pattern is obtained for all H coverages. Two H vibrations are observed in HREELS spectra for all Re concentrations, which shift to higher energies at intermediate coverages. Both peaks exhibit an isotopic shift, confirming their assignment to hydrogen. For Re concentrations of x =0.15 and higher, a third HREELS peak appears at 50 meV as H (D) coverage approaches saturation. This peak does not shift in energy with isotopic substitution, yet cannot be explained by contamination. The intrinsic width of the loss peaks depends on the Re concentration in the surface region and becomes broader with increasing x. This broadening can be attributed to surface inhomogeneity, but may also reflect increased delocalization of the adsorbed hydrogen atom.