O- and H-induced surface core level shifts on Ru(0001): prevalence of the additivity rule

Abstract

In previous work on adsorbate-induced surface core level shifts (SCLSs), the effectscaused by O atom adsorption on Rh(111) and Ru(0001) were found to be additive:the measured shifts for first-layer Ru atoms depended linearly on the number ofdirectly coordinated O atoms. Density-functional theory calculations quantitativelyreproduced this effect, allowed separation of initial- and final-state contributions, andprovided an explanation in terms of a roughly constant charge transfer per Oatom. We have now conducted similar measurements and calculations for threewell-defined adsorbate and coadsorbate layers containing O and H atoms:(1 × 1)-H,(2 × 2)-(O+H)and (2 × 2)-(O+3H) on Ru(0001). As H is stabilized in fcc sites in the prior two structures and in hcp sites inthe latter, this enables us to not only study coverage and coadsorption effects on theadsorbate-induced SCLSs, but also the sensitivity to similar adsorption sites. Remarkablygood agreement is obtained between experiment and calculations for the energies andgeometries of the layers, as well as for all aspects of the SCLS values. The additivity of thenext-neighbor adsorbate-induced SCLSs is found to prevail even for the coadsorbatestructures. While this confirms the suggested use of SCLSs as fingerprints of the adsorbateconfiguration, their sensitivity is further demonstrated by the slightly differentshifts unambiguously determined for H adsorption in either fcc or hcp hollowsites.