Formation and Bleaching Properties of the T Bands in KCl

Abstract

Optical bleaching of additively colored or 60Co irradiated KCl with ``F light'' and/or red light at 100°C produces three new absorption bands at 839 mμ (T1), 894 mμ (T2), and 992 mμ (T3) which have been designated as ``T bands.'' The most prominent T band is T2, with a thermal stability exceeding the R and N bands and less than the M and F bands. Illumination of a crystal containing T bands at −195°C causes a temporary interconversion between the T2 and T3 bands, the direction depending on the spectral intensity of the ambient light. The transformation kinetics were investigated with 20-μsec pulse and steady bleaching techniques and were found to be of first order. The rate constants are linear functions of the light absorption rates in the T2 and T3 bands with significant dark contributions. The analysis indicates that the temporarily bleached T2 and T3 centers maintain a constant concentration ratio. The available information indicates that the T bands are due to distinct F-aggregate color centers, possibly isomers of F5, and that the T2 and T3 centers are closely related in structure.