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

TL;DR

This study investigates surface core level shifts (SCLSs) on Ru(0001) caused by O and H adsorbates, confirming the additivity rule and demonstrating the sensitivity of SCLSs to different adsorption sites through combined experimental and theoretical analysis.

Contribution

The paper extends the understanding of SCLSs by analyzing coadsorbate layers of O and H on Ru(0001), confirming the additivity rule and demonstrating site-specific shifts with high accuracy.

Findings

Additivity rule holds for coadsorbate-induced SCLSs.

Experiment and calculations show excellent agreement.

SCLSs are sensitive to adsorption site differences.

Abstract

In previous work on adsorbate-induced surface core level shifts (SCLSs), the effects caused 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 of directly coordinated O atoms. Density-functional theory calculations quantitatively reproduced this effect, allowed separation of initial and final 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-defined 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 confirms the suggested use of SCLSs as fingerprints of the adsorbate configuration, their sensitivity is further demonstrated by the slightly different shifts unambiguously determined for H adsorption in either fcc or hcp hollow sites.