Metastable F Aggregate Color Centers in Irradiated KCl

The absorption spectra and decay lifetimes of the M center triplet state (3M) and R center quartet state (4R) measured in irradiated KCl are similar to previous results in additively colored crystals. The wavelength dependence of the light-induced decay indicates that the 3M center is deactivated by illumination in its absorption bands at 505, 590, and 685 mμ (85°K), and pumped by illumination in the M1 band and also in the F band and K band regions. The initial yield of 3M centers induced by pulse illumination is proportional to the M center density, both for crystals of different initial F center density bleached to maximum M band and for a crystal that was bleached through the growth and decline of the M band. The temperature dependence of the dark decay follows a configuration coordinate model in which a transition of 45-sec half-time competes with a thermally-activated step of 0.064 eV activation energy. The 14-sec dark decay half-time of the 4R center absorption at 710 mμ was found to be independent of temperature from 85° to 200°K. Possible mechanisms of metastable center formation and decay are discussed.