Configuration of the self-trapped exciton in alkali halides

Vinti sum-rule analysis of the optical absorption of self-trapped excitons (STEs) in alkali halides reveals two STE classes: (1) Large excitons with electron orbits encompassing many ions. The STE radii correlate roughly with the dielectric constant as in the Wannier model. (2) Small excitons with orbits less than the lattice spacing. The radii are of the same size as F centers and show the same linear dependence on lattice spacing. This absorption-based classification mirrors the luminescence-based scheme of Kan'no et al. The classes correspond to on-center and off-center STEs, respectively.