Calculations of the ground and excited states of F-type centers in corundum crystals

The semiempirical intermediate neglect of differential overlap method was used for calculating optical properties of F+ and F centers (oxygen vacancy trapped one and two electrons, respectively) embedded into large quantum clusters, Al26O39. The geometry was optimized for both the ground and excited states of defects. Calculated absorption and luminescence energies obtained for F+ and F centers are in good agreement with experimental data. Their energy levels lie in the gap between the upper valence band and conduction band, like for similar centers in MgO and alkali halides. It is shown that the oxygen vacancy in corundum crystals is able also to trap a third electron; experimental evidence for this is discussed.