EPR and optical-absorption study of BrCl−and associated centers in doped KCl Crystals. III

Correlated optical and EPR studies have shown that after x or $\gamma$ irradiation and proper thermal treatment, a Br${\mathrm{Cl}}^{-}$ center with a positive-ion vacancy in a nearest-neighbor position to both nuclei of the molecule ion, called $\mathrm{Br}{\mathrm{Cl}}^{-}\left(v_{+}\right)$, is formed in crystals of KCl doped with small amounts of KBr and Pb${\mathrm{Cl}}_2$. Prolonged excitation of the high-energy transition of $\mathrm{Br}{\mathrm{Cl}}^{-}\left(v_{+}\right)$ at 77 K results in the decomposition of this center and the formation of a new center $\mathrm{Cl}_2^{}{}_{}{}^{-}\left(v_{+}\right)$, a ${\mathrm{Cl}}_2^{-}$ with a positive-ion vacancy in a nearest-neighbor position to both nuclei. The internuclear axis of $\mathrm{Cl}_2^{}{}_{}{}^{-}\left(v_{+}\right)$ lies in a {100} plane and parallel to a direction; however, the molecular bond is bent through an angle of 2.8°. The $\mathrm{Cl}_2^{}{}_{}{}^{-}\left(v_{+}\right)$ center has a strong $\sigma$-polarized transition at 362 nm and a less intense weakly $\pi$-polarized transition near 750 nm; optical excitation at 77 K can cause preferential orientation of these molecule ions. Thermal reorientation of $\mathrm{Cl}_2^{}{}_{}{}^{-}\left(v_{+}\right)$ occurs in the neighborhood of 111 K; a study of the reorientation kinetics shows that the reorientation takes place by 90° and 60° jumps.