Dependence of surface properties on adsorbate-substrate distance: Work function changes and binding energy shifts for I/Pt(111)

This paper presents an analysis of the character of the bond of I adsorbed at on-top and 3-fold sites of Pt(111). At both sites, the bonding is dominated by an ionic interaction supplemented with some covalent character due to donation from the adsorbed I anion to the Pt surface. The way in which the I-Pt interaction affects observed properties has been established. In particular, the origins of the anomalous work function changes induced by the adsorption of I and the shifts of I core level binding energies are explained. It is shown that the magnitudes of the changes in these properties can be directly correlated with the distance of the I from the Pt surface, Thus, these shifts can be interpreted to indicate adsorbate height. The fact that the negatively charged I adsorbate leads to a work function decrease, rather than the increase expected due to the charge of the adsorbate, may appear to be an anomaly. However, it is shown that this decrease arises from electronic reorganizations that cancel the dipole due to the charge of the adsorbate. Furthermore, the electronic terms that contribute to a lowering of the work function are larger as the adsorbate moves closer to the surface. (C) 2008 Elsevier B.V. All rights reserved.