Tunneling recombination of trapped electrons and holes in KCl:AgCl and KCl:TlCl

It is well established that exposure of KCl:AgCl (or KCl:TlCl) crystals at 77 K to ionizing radiation results in the trapping of holes as $\mathrm{Cl}_2^{}{}_{}{}^{-}$, $V_K$ centers, and electrons as ${\mathrm{Ag}}^0$ (${\mathrm{Tl}}^0$). These crystals emit luminescence at 77 K which persists for many hours after the irradiation. Evidence is presented that indicates that this afterglow results from electron-hole recombination between nearby metal-atom-$\mathrm{Cl}_2^{}{}_{}{}^{-}$ pairs, and that the recombination does not occur by a thermally activated process but by a tunneling process. The time dependence of the intensity of the afterglow is qualitatively understood on the basis of a simple model. When the $\mathrm{Cl}_2^{}{}_{}{}^{-}$ are preferentially oriented in the crystal it is observed that the afterglow is partially polarized with the electric vector of the dominant component parallel to the $\mathrm{Cl}_2^{}{}_{}{}^{-}$ molecular axis. If a crystal containing oriented $\mathrm{Cl}_2^{}{}_{}{}^{-}$ is warmed, the degree of polarization remains constant up to the temperature range where the $\mathrm{Cl}_2^{}{}_{}{}^{-}$ disorient at an appreciable rate; in the latter range the degree of polarization goes to zero and then reverses sign before finally going to zero again. This reversal of the polarization which occurs when the $\mathrm{Cl}_2^{}{}_{}{}^{-}$ change orientation is interpreted as indicating that the tunneling probability is anisotropic.