Further insight has been sought into the mechanism of optical bleaching in the vicinity of room temperature by studying the temperature dependence of the bleaching curves for KCl crystals which had been initially irradiated with hard (filtered) x-rays. The absorptions at the maxima of the $F$, $M$, $R_1$, and $R_2$ bands were observed to change during illumination with $F$-light and to be strongly temperature dependent in the range of -30°C to +10°C, whereas the absorption at the $V_3$ band was essentially unchanged. For comparison an analysis was made of the data of Petroff on the early growth of the $M$ band during bleaching with $F$-light in additively colored KCl crystals. In this case a unique activation energy $\epsilon$, of 0.35±0.05 ev and a number of defect jumps $N_j\approx {10}^{10}$ are indicated by the data. It appears that the bleaching curves of the x-irradiated samples are composed of a temperature independent and a temperature dependent part. The temperature dependent part probably is the same as that responsible for bleaching in additively colored crystals. The above results for $\epsilon$ and $N_j$ suggest that the temperature dependent bleaching process results from the trapping of photoelectrons at vacancy clusters which are formed during bleaching through the migration of mobile defects, possibly vacancy pairs. This conclusion is supported by the observations by others that the $\alpha$ band is not observed during bleaching at room temperature.

• Received 12 February 1960

DOI:https://doi.org/10.1103/PhysRev.119.114