Thermal bleaching of V- and F-centres in X-irradiated potassium chloride crystals

Abstract

The kinetics of thermal bleaching at 25 deg C of V- and F-centers were studied in four different single potassium chloride crystals, of varying divalent cation impurity content, that were colored by x irradiation. The V-band comprises both V/sub 2/-centers, which undergo bleaching and V/sub 3-/ centers that are stable at 25 deg C. The maximum number of V/sub 2/ centers is rapidly attained under irradiation and is dependent on the cationfc impurity content; Va- centers are created at a slower rate, and the number formed is related to the dislocation density of the crystal. The bleaching of V/sub 2/-centers results from the thermal excitation of F-centers and the capture of F-electrons by the V/ sub 2/-center, the kinetics being accurately represented over the whole range of measurements by a bimolecular process. There remains, however, apparently a residual concentration of V/sub 2/-centers after the virtual completion of the bleaching process; some possible explanations of this anomaly are suggested. (auth)