Zero-field splitting of the self-trapped exciton in alkali fluorides and alkaline-earth fluorides

The observed zero-field splitting parameter D of the triplet self-trapped excitons in alkali fluorides is negative, while in alkaline-earth fluorides of the fluorite structure it is of comparable magnitude but positive. D in these materials is almost completely due to spin-spin interaction. Analysis of D allows an estimate of the separation between the hole and electron charges. These are found to be in satisfactory agreement with the results of recent calculation which predicted the off-centre character of the self-trapped exciton in alkali halide crystals. As for the alkaline-earth fluorides with the fluorite structure, the analysis explains the positive sign of D in this structure and also identifies which of the proposed nearest-neighbour F-H pair configurations corresponds to the observed self-trapped exciton.