The extended-ion method for the study of excited states in ionic crystals is modified for such two-electron defect systems as the F' centre and positronium in alkali halides. Exclusive use of 1s floating Gaussian functions as the basis is the main feature of the method. Some degree of electron correlation is built-in in this method. Applied to F' centres, it is found that in NaI the spin-singlet ground state is deeply bound, and there is also a bound triplet state. Comparison with experimental data regarding the optical and thermal dissociation energies in several alkali halides gives reasonable agreement.
A general and analytical method of evaluating the zero-field splitting parameter D originating from the spin-spin interaction is presented. Application of this method to the self-trapped excitons (STE) in alkaline earth fluorides identifies the geometry of the STE in an unambiguous way and gives values of the zero-field splitting parameter D in good agreement with experiment. A recently proposed model of the STE in alkali halide crystals involving rotations is briefly discussed on the basis of the zero-field splitting.
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