Experimental and theoretical surface core level shift study of the S-Rh(100) local environment

The local changes of the Rh(100) electronic structure induced by sulfur adsorption at different coverage have been investigated by combining high-energy resolution core level photoemission spectroscopy, low-energy electron diffraction, Monte Carlo simulations, and ab initio calculations. Our results show that upon adsorption the local density of states does not change appreciably beyond the next neighbors, thus supporting the conclusion that the well-known catalyst's sulfur poisoning effect cannot be related to electronic structure long-range modifications. We also find that the sulfur-induced Rh 3d(5/2) component originated by the second layer Rh, atoms below the sulfur adsorbate shifts by as much as -235 meV with respect to deeper layer contributions. This result points out the importance of considering the contribution of subsurface atoms in the overall 3d(5/2) core-level line shape of transition metal surfaces. Ab initio calculations allow a detailed quantitative understanding of the measured core level shifts. Possible mechanisms that explain the observed core level shifts are discussed.