ELECTROSTATIC AND CHEMICAL BONDING CONTRIBUTIONS TO THE CATION CORE-LEVEL BINDING-ENERGY SHIFTS IN MGO, CAO, SRO, BAO - A CLUSTER MODEL STUDY

We have determined cluster model wavefunctions to identify the origin of the trend for the chemical shifts of the core level binding energies of alkaline-earth atoms in ionic oxides with respect to bulk metals. We have identified two initial state effects, both electrostatic in nature, which fully determine the trend. The first one is the increase in the core level binding energies of the dication compared to the neutral metal atom; the second is the decrease of the dication binding energies induced by the Madelung potential of the ionic crystal. The sum of these two cancelling contributions reproduces the observed trend in chemical shifts from MgO to BaO. Chemical bonding effects, in particular the covalent interaction between the unoccupied d-levels of the dications and the oxygen anions, do not contribute in a significant way to the trend.