Fine-structure splitting in triplet states of the self-trapped exciton in alkali halides

Abstract
By exploiting optical selection rules in a microwave-optical double-resonance experiment, it is shown that the fine-structure parameter D for the lowest triplet state of the self-trapped exciton changes sign from (+) to (-) along the sequence of crystals from alkali iodides to fluorides. This results from interplay between a positive spin-orbit-exchange interaction and a negative magnetic dipole-dipole interaction.