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 (oxyge n vacancy trapped one and two electrons, respectively) embedded into l arge quantum clusters, Al26O39. The geometry was optimized for both th e ground and excited states of defects. Calculated absorption and lumi nescence 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 M gO 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.