Bleaching of F-type centres in alkali halides—II: Laser-induced F centre bleaching in KC1

Abstract

Bleaching of the F band in KCl occurs on pulsing with 532 nm laser light. The bleaching is accompanied by the appearance of an absorption on the red side of the F band. Both bleaching and absorption are transient phenomena, and no permanent bleaching or absorption is observed. The same exponential lifetime of about 5μ S was obtained for both bleaching and absorption, and all the features were found to be independent of both temperature (between 200 and 300 K) and the intensity of the analysing light. This eliminutes the possibility of a trapped species, such as F -, being responsible. The phenomena are qualitatively different to the bleaching observed at low light intensity, where bleaching of the F band is accompanied by formation of F -, which is thermally stable below 250K. The most likely explanation of the results is that the high concentration of conduction band electrons following the laser pulse allows formation of free or quasi-free electron pairs, e = 2, which decay back into F centres on a time scale which is long compared with that for individual electrons.