Tunneling-recombination luminescence betweenAg0andAg2+in KCl:AgCl

Appropriate treatment of a KCl:AgCl crystal results in the trapping of electrons as silver atoms, ${\mathrm{Ag}}^0$, and positive holes as Ag$\mathrm{Cl}_4^{}{}_{}{}^{2-}$, ${\mathrm{Ag}}^{2+}$, centers. Optical excitation of ${\mathrm{Ag}}^0$ in such a crystal at $T<\mathrm{}90\mathrm{}$ K produces a luminescence which lasts for many hours after the excitation. Evidence is presented which indicates that this afterglow results from electron-hole tunneling recombination between nearby ${\mathrm{Ag}}^0$ and ${\mathrm{Ag}}^{2+}$ pairs, similar to the ${\mathrm{Ag}}^0$-$\mathrm{Cl}_2^{}{}_{}{}^{-}$ tunneling-recombination studies we previously reported. We have shown that ${\mathrm{Ag}}^{2+}$ centers are involved in the emission process by preferentially orienting the anisotropic ${\mathrm{Ag}}^{2+}$ at 6 K by excitation with polarized light and observing that the afterglow is polarized. Upon warming to 50 K, where the preferentially oriented ${\mathrm{Ag}}^{2+}$ can change orientation, a strong reversal in the degree of polarization occurs which finally decays to zero. The characteristics of this luminescence can be understood if we assume: (i) a tunneling-recombination mechanism in which the orientation of the electric vector of the emitted radiation depends on the position of the ${\mathrm{Ag}}^0$ relative to the ${\mathrm{Ag}}^{2+}$ and (ii) the tunneling is anisotropic and depends on the location of the ${\mathrm{Ag}}^0$ relative to the anisotropic ${\mathrm{Ag}}^{2+}$. The latter assumption is based on the tetragonal ($d$-like) symmetry of the ${\mathrm{Ag}}^{2+}$ complex. Good quantitative agreement between theory and experiment has been obtained on the decay kinetics, the degree of polarization, and the polarization reversal.