EPR and Optical Absorption Study of BrCl−and Associated Centers in Doped KCl Crystals. I

Correlated optical and EPR studies show that after $x$- or $\gamma$-ray irradiation combined with proper thermal treatment, Br${\mathrm{Cl}}^{-}$ molecule ions aligned along the six directions are present in crystals of KCl doped with a small amount of KBr. The Br${\mathrm{Cl}}^{-}$ molecule ions, which have a strong $\sigma$-polarized transition at 382 nm and a much weaker but still somewhat $\sigma$-polarized transition at 760 nm, are thermally more stable than the $V_K$ centers (${\mathrm{Cl}}_2^{-}$) from which they are formed. The increased thermal stability of Br${\mathrm{Cl}}^{-}$ is believed to arise primarily because the electronegativity of chlorine is greater than that of bromine. A study of the reorientation kinetics at 88°K reveals that the reorientation of Br${\mathrm{Cl}}^{-}$ proceeds by 90° jumps as well as 60° jumps, and very probably also by 120° jumps, whereas many homonuclear molecule ions reorient only by 60° jumps.