Singlet-triplet splittings in free and self-trapped excitons

We discuss the available experimental data for the singlet-triplet splitting of free and self-trapped excitons in alkali halides. These data are analysed quantitatively using the pseudopotential method of Bartram, Stoneham and Gash. The predictions confirm the trend emerging from the observed data, namely that the splittings are systematically lower for the self-trapped systems. This difference comes principally from the spread of the self-trapped hole onto two ions, and would not be expected, for example, if the hole were localised on a single site.