Electronic structure of excited states at low-coordinated surface sites ofMgO

We study a dependence of the optical excitation energies of low-coordinated sites in nano-clusters of MgO on the oxygen coordination. The calculations were performed using the ab initio Hartree-Fock method for a quantum clust er of up to 12 ions embedded in an (MgO),,, cubic nano-cluster. The optical absorption energies were calculated using the configuration interaction te chnique for single (CIS) and double electronic excitations (CISD). The low- coordinated sites included the oxygen corner, cluster edge and cluster grai n. The results show a consistent reduction of the exciton excitation energi es as the oxygen coordination decreases. The hole component of the unrelaxe d excited state at the oxygen corner is localised on the corner oxygen ion, whereas the excited electron is delocalised around the hole outside the cl uster. The hole and electron components of excited states for other coordin ations are delocalised around at least several oxygen ions. We also demonst rate the possibility of energy transfer from the sites with higher coordina tion to those with lower coordination and finally to the localisation centr es. (C) 1999 Elsevier Science B.V. All rights reserved.