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Zinc sulfide activated with copper or silver and coactivated with gallium or indium shows two emission bands. The shorter wave‐length band does not involve the ground state of the coactivator or donor, whereas the longer wave‐length band does. Both bands involve the ground state of the activator or acceptor. The factors contributing to the emission intensities from the various associated donor‐acceptor pairs are discussed theoretically. The dependences on temperature and on concentrations of activator and coactivator are in accord with the longer wave‐length band involving a highly associated donor‐acceptor pair. In addition, the energy levels of the donors, as obtained from thermoluminescent data, are correlated on the basis of the model with the differences in the transition energies of the two emission bands of these phosphors.
Apple et al. (Thu,) studied this question.