2-ELECTRON DEFECT SYSTEMS IN IONIC-CRYSTALS - APPLICATION TO F' CENTERS IN ALKALI-HALIDES

The extended-ion method for the study of excited states in ionic cryst als is modified for such two-electron defect systems as the F' centre and positronium in alkali halides. Exclusive use of Is floating Gaussi an functions as the basis is the main feature of the method. Some degr ee of electron correlation is built-in in this method. Applied to F' c entres, it is found that in NaI the spin-singlet ground state is deepl y bound, and there is also a bound triplet state. Comparison with expe rimental data regarding the optical and thermal dissociation energies in several alkali halides gives reasonable agreement.