Embedded-cluster study of core-level binding energies of magnesium and alkali impurities at the surface of MgO

We present the results of a theoretical study concerning the shifts in the binding energy of core electrons at the (001) surface of magnesium oxide. The core levels considered are the Is, 2s, 2p, ones of the magnesium ion, the Is of substitutional lithium, and the Is, 2s, 2p, of substitutional sodium. The shifts are evaluated by means of ab initio embedded-cluster unrestricted Hartree-Fock Delta SCF calculations. The host perfect crystal is modeled by a two-dimensional MgO slab; within the slab, a cluster is considered which contains at its center the ion where the core hole is created. This technique allows the effects of the surroundings to be taken into account accurately and self-consistently. The dependence of the binding energy shift on the location of the ion with respect to the surface is analysed and discussed. Surface effects are evident but well localized, since they are negligible below the second layer from the surface. The entity of the shift is dictated by the local electrostatic potential the polarizability of neighboring anions and their number; for the formation of the substitutional impurities, the repulsion with neighboring oxygens also comes into play. A semi-quantitative explanation of the whole of the calculated data has been provided in terms of three parameters. For the case of lithium in the surface layer, the effects of relaxation around the impurity have also been studied. (C) 1997 Elsevier Science B.V.