Laser-induced Transient Bleaching of Trapped Electrons in Alkali Halide Single Crystals

Abstract

F-center electrons in KBr and KI crystals at room temperature are almost totally bleached during the intense light pulse from a Q-switched ruby laser. Recovery of the absorbance is largely complete within microseconds of the end of the pulse such that net loss of trapped electrons, which would result in permanent bleaching, is negligible (Φ < 10−2). Transient bleaching seems to result from both a loss of F-centers to form F′-centers and saturation of optical transitions for both the F and F′ bands. Comparisons of transient bleaching effects at 633, 1152, and 694 nm for KBr and KI strongly imply that the trapped electron absorption bands in crystals arise from single optical transitions which are homogeneously broadened. The rate at which the ground state of the F-center is repopulated following laser photolysis is of the order of 10−9 s and is very much slower than the analogous process in hydrated electrons. Results are also presented for additively colored and γ-induced F-centers and for F- and F′-centers bleached by ruby laser pulses at low temperatures.