Broad band absorption and luminescence of defects in ionic crystals

Abstract

Studies of the optical absorption and emission band shapes of simple defects in ionic crystals have been carried out within the framework of a single configuration coordinate model and its elementary extentions. The semi-classical and linear coupling approximations have been tested against the fully quantum-mechanical treatment with differing ground and excited state force constants. As might be expected, for strong electron-lattice coupling the semi-classical approximation appears to be more useful than the linear coupling model. Using previously computed configuration coordinate curves and the measured effective frequencies, the absorption and emission band shapes of the F center in KCl, KBr, and KI have been computed in various approximations. The Condon approximation has been tested and found to be fairly adequate. The frequency factor multiplying the sums over states in the usual expression for the absorption coefficient is found to be of minor importance. The calculations show that the temperature shift of the maxima of the absorption bands comes about in part because of the temperature dependence of the lattice constant.