Mollwo-Ivey relation for the electron transition energy of the self-trapped excitons in alkali halides

We investigate the dependence on the interionic distance R of the transition energy {Δ}E of the electron from 1s to 2p states of the self-trapped exciton (STE) for alkali halides. It is found that the {Δ}E-vs-R plots, except for NaBr and NaI, fall in two distinctive straight lines, showing that the STE in alkali halides can be divided into two groups. This grouping is found to be in accordance with the grouping in terms of the Stokes shift presented by Kan'no, Tanaka, and Hayashi. The relaxed configuration of excitons in alkali halides has been discussed on the basis of these results.