The present state of understanding of the luminescent centers in alkali halide and zinc sulfide phosphors is reviewed. These two classes of phosphors complement each other: the first is ionic; the second, primarily covalent. The energy levels of the luminescent centers in alkali halides are approximated by states of the free impurity ion perturbed by crystalline interactions, whereas the levels of luminescent centers in zinc sulfide phosphors are suitably approximated by states of the crystal band structure perturbed by impurities. The theory of the excitation and emission spectra of thallium-activated potassium chloride is reviewed and extended to the problem of oscillator strengths for luminescent transitions. Semiconductor theory is applied to zinc sulfide phosphors, and the acceptor and donor nature of activator and coactivator, respectively, is thereby revealed. In contrast to semiconductors with small band gaps, association of acceptors and donors is shown to be important in transparent semiconductors. The energy level structure and optical properties of various associated acceptor-donor pairs, as well as of isolated impurities, in zinc sulfide are discussed.
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