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Evaluation of the zero-field splitting of the triplet self-trapped exciton in CaF2, SrF2 and BaF2

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.