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

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Correlated optical and EPR studies have shown that after x or $\ensuremath{\gamma}$ irradiation and proper thermal treatment, a Br${\mathrm{Cl}}^{\ensuremath{-}}$ center with a positive-ion vacancy in a nearest-neighbor position to both nuclei of the molecule ion, called $\mathrm{Br}{\mathrm{Cl}}^{\ensuremath{-}}({v}_{+})$, 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 $\mathrm{Br}{\mathrm{Cl}}^{\ensuremath{-}}({v}_{+})$ at 77 K results in the decomposition of this center and the formation of a new center $\mathrm{Cl}_{2}^{}{}_{}{}^{\ensuremath{-}}({v}_{+})$, a ${\mathrm{Cl}}_{2}^{\ensuremath{-}}$ with a positive-ion vacancy in a nearest-neighbor position to both nuclei. The internuclear axis of $\mathrm{Cl}_{2}^{}{}_{}{}^{\ensuremath{-}}({v}_{+})$ lies in a {100} plane and parallel to a 110> direction; however, the molecular bond is bent through an angle of 2.8\ifmmode^\circ\else\textdegree\fi{}. The $\mathrm{Cl}_{2}^{}{}_{}{}^{\ensuremath{-}}({v}_{+})$ center has a strong $\ensuremath{\sigma}$-polarized transition at 362 nm and a less intense weakly $\ensuremath{\pi}$-polarized transition near 750 nm; optical excitation at 77 K can cause preferential orientation of these molecule ions. Thermal reorientation of $\mathrm{Cl}_{2}^{}{}_{}{}^{\ensuremath{-}}({v}_{+})$ occurs in the neighborhood of 111 K; a study of the reorientation kinetics shows that the reorientation takes place by 90\ifmmode^\circ\else\textdegree\fi{} and 60\ifmmode^\circ\else\textdegree\fi{} jumps.