Ab initio study of the optical transitions of F centers at low-coordinated sites of the MgO surface

We report ab initio calculations on the optical transitions of oxygen vacancies (F centers) located at low-coordinated sites of the MgO (100) surface. F and F+ centers at terrace, step and corner sites were investigated by means of cluster models embedded in point charges. The transition energies to the lowest excited states were determined by performing extensive configuration interaction calculations where correlation effects are explicitly taken into account. The transitions due to F centers at the surface of MgO are considerably red-shifted, by similar to 2.5 eV, compared to the bulk excitations. We also found a decrease of similar to 0.5-1.2 eV of the transition energies of oxygen vacancies at step and corner sites compared to those at the (100) terrace sites. This trend is explained in terms of reduced Pauli repulsion in the excited state. According to these results, F centers at terrace, step and corner sites should give rise to two distinct optical bands. We tentatively suggest that the features at 2.05 and 2.3 eV observed experimentally are due to the low-coordinated step and corner F centers and not to centers at the MgO (100) terraces. (C) 1999 Elsevier Science B.V. All rights reserved.