Cluster model wavefunctions for the alkaline-earth oxides are analyzed to determine the origin of the trend of the chemical shifts of the binding energies, BEs, of the metal core levels. Two cancelling initial states mechanisms in the oxides are responsible for the trend. One is the increase of the BEs for the dication over the neutral atom. The second is the decrease of the free dication BEs due to the Madelung potential of the ionic oxides. The trend given by the sum of these two contributions to the chemical shifts is in full agreement with experiment. These mechanisms are electrostatic and do not involve chemical bonding effects. &
The metal atom core-level binding energies, BEs, in alkaline-earth metal oxides are usually large than the BEs in the bulk metals [ 11. This is expected since it is much harder to ionize a core electron from a M*+ cation than from a neutral atom. However, the observed differences [ 1 ] between the core-level BEs of alkaline-earth oxides, MO, and the bulk metals, M, are considerably smaller than the shifts between the BEs of the free metal dication, M*+, and the free neutral atom, Mat. The chemical shift of the corelevel BEs of the oxide from the bulk metal , ABE (MO-M), is small, usually x l-2 eV. There is a consistent trend of the ABE(MO-M) to smaller values for heavier alkaline earths with the shift for Ba being negative [ 1,2 1.
The negative chemical shift for Ba, which means