Photoconductivity ofZBands in KCl and an Associated New Band

Abstract

Photoconductivity measurements have been made on colored KCl crystals, both pure and strontium doped. Assuming the room-temperature quantum efficiency for photoconductivity of the $F$ center to be unity, the room-temperature quantum efficiencies of the $Z_1$ and $Z_2$ bands are found to be 0.3±0.03 and 1.0±0.2, respectively. The comparison of these results to the previously reported observations on optical bleaching is discussed. Estimates of quantum efficiencies for other bands in pure crystals are $R_1$, 0.9; $R_2$, 0.5; $M$, 0.15; $N$, 0.6, with an accuracy of about 25%. Low-temperature absorption measurements (100°K) accompanying this work show a small band at 860 millimicrons in the doped crystals. Its proximity to the $M$ band prevents complete resolution, but its behavior is similar to that of the $Z_1$ band. The possibility of contamination by an unknown impurity exists, but it seems likely that the new band is genuinely due to the strontium additive. It has been named the $Z_4$ band, and probable models for the $Z_4$ center are considered.