population transfer between two different offcenter exciton states in KC1:I

ABSTRACT Temperature dependence of emission intensities and lifetimes of hetero-nuclearrelaxed excitons [(C1I)+e], in KCL:I has been studied in the range of 30-200K. Frompopulation conversion between the type II and type III configurations observed belowlOOK, the energy difference between the two configurations of 13meV is obtained. Ananomalous behavior observed above lOOK suggests that the singlet state above 74meVfrom the triplet state of type III is thermally populated to serve as an additional decay channel effective at higher temperatures. 1. INTRODUCTION Excitons in alkali halide crystals relax into self-trapped exciton (STE) states with multiple local minima on the adiabatic potential energy surface ES'2 Three different types (I, II and III) of recombination luminescence bands may appear alone or together,depending upon the kinds of crystals. Type I bands with a small Stokes shift have beenattributed to the STEs relaxed into the symmetric on-center configuration, while type IIIbands with a large Stokes shift have been attributed to the STEs accompanied by anadditional relaxation into the off-center configuration. Type II bands with an intermediate