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Abstract:
In previous work on adsorbate-induced surface core level shifts (SCLSs), the e ffects caused by O atom adsorption on Rh(111) and Ru(0001) were found to be additive: the measured shifts for fi rst-layer Ru atoms depended linearly on the number of directly coordinated O atoms. Density-functional theory calculations quantitatively reproduced this e ffect, allowed separation of initial- and fi nal-state contributions, and provided an explanation in terms of a roughly constant charge transfer per O atom. We have now conducted similar measurements and calculations for three well-defi ned adsorbate and coadsorbate layers containing O and H atoms: (1 x 1)-H, (2 x 2)-(O + H) and (2 x 2)-(O + 3H) on Ru(0001). As H is stabilized in fcc sites in the prior two structures and in hcp sites in the latter, this enables us to not only study coverage and coadsorption effects on the adsorbate-induced SCLSs, but also the sensitivity to similar adsorption sites. Remarkably good agreement is obtained between experiment and calculations for the energies and geometries of the layers, as well as for all aspects of the SCLS values. The additivity of the next-neighbor adsorbate-induced SCLSs is found to prevail even for the coadsorbate structures. While this confi rms the suggested use of SCLSs as fi ngerprints of the adsorbate con figuration, their sensitivity is further demonstrated by the slightly diff erent shifts unambiguously determined for H adsorption in either fcc or hcp hollow sites.
