He-beam diffraction data evaluated for clean Rh (311) yield a best-fit corrugation, which shows a pronounced corrugation amplitude along the (100)-microfacets and a practically vanishing amplitude along the (111)-microfacets in agreement with expectation, but in contrast to Ni (311) where the reverse situation was found. Hydrogen adsorption leads to a sequence of c(1×1), c(1×3), p(1×1), c(1×3), and p(1×1) REC phases. The fact that the c(1×1) phase was observed with He-diffraction, but not in previous LEED studies, again proves the exceptional sensitivity of He scattering for adsorbed hydrogen. Surface charge density calculations were performed to reproduce the shape and amplitude of the best-fit corrugation function of the c(1×1)H-phase. In this way we found that the H atoms occupy places between the topmost bridge sites and the adjacent threefold hollow sites on the (111)-microfacets; the H-Rh bond length amounts to 1.92±0.1 Å.
Helium scattering structure analyses of the chemisorbed hydrogen phases on Co (1010) Hydrogen adsorption on Rh͑311͒ at 110 K leads to the appearance of ͑1ϫ1͒H, ͑1ϫ3͒H, ͑1ϫ2͒H and ͑1ϫ2͒ REC ordered layers. A previous He-atom scattering ͑HAS͒ investigation of the ͑1ϫ1͒H phase has revealed that its coverage is ⌰ϭ1 ML, whereby the H-atoms are adsorbed between the topmost bridge sites and the adjacent threefold hollow sites on the ͑111͒-microfacets. In this paper we present a combined HAS and HREELS structural analysis of the higher coverage phases. The structures derived from the HAS data show that, starting with the ͑1ϫ1͒H phase, the ͑1ϫ3͒H is formed by the adsorption of H-atoms in the wells between every third close-packed row, whereas adsorption on the same sites but between every second row leads to the completion of the ͑1ϫ2͒H phase. Accordingly, the corresponding coverages are 4/3 ML and 3/2 ML. The HREELS results clearly demonstrate the existence of two different H-species in the ͑1ϫ3͒H phase and three in the ͑1ϫ2͒H phase, supporting the HAS results.
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