CONFIGURATION OF THE SELF-TRAPPED EXCITON IN ALKALI-HALIDES

Vinti sum-rule analysis of the optical absorption of self-trapped exci tons (STEs) in alkali halides reveals two STE classes: (1) Large excit ons with electron orbits encompassing many ions. The STE radii correla te roughly with the dielectric constant as in the Wannier model. (2) S mall 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 lumi nescence-based scheme of Kan'no et al. The classes correspond to on-ce nter and off-center STEs, respectively.