CORE-LEVEL BINDING-ENERGY SHIFTS DUE TO IONIC ADSORBATES

The mechanisms responsible for the core-level binding-energy (BE) shifts due to alkali-metal and halogen adsorption on metal surfaces are identified and characterized through theoretical analyses of the surface electronic structure. By means of cluster model calculations of the adsorption of K and F atoms on the Cu(100) surface, we show that ionic adsorbates, both cationic and anionic, lead to small BE shifts, typically <200 meV, of the substrate metal atoms. These small shifts arise from the cancellation of two large initial-state effects, the electric field created by the ions at the surface and the consequent polarization of the metal conduction-band electrons. These two mechanisms induce rather large shifts of opposite sign and similar magnitude in the substrate core-level BE's, with resulting small final shifts. This is true for all electronic states, clusters, adsorption sites, and substrate-adsorbate distances. Thus, substrate BE shifts do not provide information about the bonding nature and the adsorption site. On the other hand, ionic and covalent bonding between the substrate and the adsorbate lead to significantly different shifts in the core-level BE's of the adsorbate. The BE shifts of alkali-metal atoms adsorbed on metal surfaces as functions of the coverage provide an indication of the transition from an ionic bond at low coverage to a covalent bond at high coverage.