Low-Temperature Coloration in KCl and KBr near the Fundamental Edge

Abstract

Crystals of KCl and KBr have been cooled with liquid helium, x rayed, and investigated for coloration near their fundamental absorption edge. Three bands are seen in both materials with the largest band closest to the fundamental edge. In KBr the first band is broad and peaks at 2300 Å, the second is the $\alpha$ band at 2020 Å, and the third, called the $\gamma$ band, is at 1895 Å. The areas of the three bands with respect to the $F$ band are, respectively, 1: 8: 20. In KCl similar bands are found at 1940 Å, 1790 Å, and 1640 Å. The areas are, respectively, 1: 6: 16 as large as the $F$ band. The $\alpha$ and $\gamma$ bands are also found in KBr: H on bleaching in the $U$ band, and the $\alpha$ and $\gamma$ bands have similar bleaching rates on warming. It is concluded that the $\gamma$ band arises when an exciton is created at a slightly larger distance from the negative-ion vacancy than is the case for the $\alpha$ band. It is proposed that the centers responsible for the first band, the $\alpha$ and $\gamma$ bands, and the $F$ and $H$ bands all are formed from the creation of Frenkel defects but differ in the distance of separation of the interstitial and vacancy and in the resulting ability to trap electrons and holes. With this assumption the x-ray energy dissipated in producing a Frenkel defect is 240 eV for KBr and 450 eV for KCl using 50-kV x rays.