F′andKCenters in Additively Colored KCl Crystals

Measurements are reported on the $F-F^{\prime }$ process, and the $K$ band in additively colored KCl. The crystals are colored with distilled potassium, and subjected to repeated cycles of optical bleach with $F$ light (5450 A) at 170°K and thermal bleach of the $F^{\prime }$ centers at room temperature. It is found that the optical quantum efficiency of conversion of $F$ to $F^{\prime }$ centers varies in two ways: (a) The quantum efficiency varies significantly for different specimens of colored KCl all cleaved from the same one-inch cube of KCl grown by the Harshaw Company. (b) The optical quantum efficiency drops significantly after each cycle of $F$-center bleach and subsequent thermal bleach of the $F^{\prime }$ centers. Ultimately, the quantum efficiency appears to decrease to a point where the crystal is essentially unaffected by $F$ light at 170°K. $K$ centers are influenced to a lesser degree, if at all, than $F$ centers by optical bleaching in the $F$ band at 170°K. All absorption measurements are made in the visible part of the spectrum.