Ionized Electron Centers in Alkali Halide Crystals

Abstract

New optical-absorption bands in alkali halide crystals are attributed to transitions of ionized $F$-aggregate centers. These bands are produced by cooling crystals containing $F$-aggregate centers to liquid-helium temperature and exposing them first to x rays and then to near-ultraviolet light. The x-ray exposure introduces trapped-hole centers, namely, $V_K$ and $H$. Optical excitation at the spectral region of the overlapping $V_K$ and $H$ bands results in a decrease of both $F$-aggregate center and $V_K-H$ absorption, and the production of the new bands in the near infrared. In KCl and KBr containing $M$ centers, the $M$ band transforms with a fixed ratio to bands at 1.40 and 1.49 μ, respectively. This ratio is also maintained in the restoration of the $M$ band with subsequent $F$-light exposure. When optical dichroism is initially present in the $M$ bands, the same dichroism appears in the new bands following the transformation process. These bands have been indentified as transitions of the ${F_2}^{+}$ center. For crystals containing $R$ centers, a one-to-one correspondence of $R$ bands with a new near-infrared band attributed to the ${F_3}^{+}$ center is similarly observed. In KCl this band strongly overlaps the spectral position of the $N_1$ and at 0.96 μ. In KBr the ${F_3}^{+}$ transition lies at 1.02 μ and is clearly resolved from the $N_1$ band which apparently peaks at 1.07 μ. There is evidence to indicate that ${F_3}^{+}$ centers are also produced at room temperature as a result of either x-ray exposure of a virgin crystal or bleaching with $F$ light of an additively colored crystal. For KCl and KBr crystals containing relatively large $R$ and $N$ bands, x-ray and near-uv exposure also results in the production of additional bands. First, a new absorption band is superimposed on the ${F_2}^{+}$ transition. The origin of this band is not known. Second, a band forms at 1.67 μ in KCl and 1.80 μ in KBr in crystals containing an $N_1$ band enhanced by annealing an additively colored crystal at 85°C. This band is tentatively ascribed to the singly ionized $N_1$ center, ${N_1}^{+}$.