Tunneling-recombination luminescence between Ag0 and Ag2+ in KCl:AgCl

Appropriate treatment of a KCl:AgCl crystal results in the trapping of electrons as silver atoms, Ag0, and positive holes as AgCl42-, Ag2+, centers. Optical excitation of Ag0 in such a crystal at T < 90 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 Ag0 and Ag2+ pairs, similar to the Ag0-Cl2- tunneling-recombination studies we previously reported. We have shown that Ag2+ centers are involved in the emission process by preferentially orienting the anisotropic Ag2+ at 6 K by excitation with polarized light and observing that the afterglow is polarized. Upon warming to 50 K, where the preferentially oriented Ag2+ 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 Ag0 relative to the Ag2+ and (ii) the tunneling is anisotropic and depends on the location of the Ag0 relative to the anisotropic Ag2+. The latter assumption is based on the tetragonal (d-like) symmetry of the Ag2+ complex. Good quantitative agreement between theory and experiment has been obtained on the decay kinetics, the degree of polarization, and the polarization reversal