Dichroism ofVBands in Potassium and Rubidium Halides

Ultraviolet absorption bands which are homologous to the $V_2$, $V_3$, and $V_4$ bands in KBr have been studied in the potassium and rubidium chlorides, bromides, and iodides. They are generated by x irradiation above liquid-nitrogen temperature, and they are apparently intrinsic in the sense that no tendency to saturate has been observed as a function of radiation dose. The predominant $V$ band which accompanies the $F$ band depends on the conditions of irradiation. The effect of illumination in the bands with polarized uv light shows that the $V_4$ centers can be optically reoriented by $〈100〉$-polarized light absorbed in the band at 100 K. The dichroism is washed out by warming the crystal from 100 to 200 K. The $V_2$ band can be preferentially bleached by $〈100〉$ light absorbed in the band at 100 K. Again the dichroism is washed out upon warming to 200 K. The $V_3$ band is not affected at all by light absorbed at 100 K. Above room temperature, isotropic bleaching occurs. In addition, in KCl an irreversible thermal transformation of some of the $V_4$ centers into $V_2$ and also of some $V_2$ into $V_3$ occurs when the crystal is warmed above the temperature of irradiation. The peak wavelengths of these $V$ bands lie between 200 and 360 nm, and are strongly dependent on the halide, but nearly independent of the alkali species. The close grouping of their peak wavelength positions, and the dependence on halogen species, indicate that they may all be ${X_3}^{-}$ centers, in accordance with a suggestion of Hersh. The $V_4$ and $V_2$ centers show a $〈100〉$ symmetry axis. The irreversible thermal conversions observed in KCl suggest that the three centers differ by ionic constituents and not merely by their state of ionization.