Note on the behaviour of zinc sulphide phosphors under conditions of periodic excitation

The behaviour of luminescent solids under conditions of periodic excitation has been discussed theoretically and predictions made on the basis of simple models for the electronic processes accompanying the emission of luminescent radiation. The significant features of the luminescence intensity as a function of time are the shift in phase with respect to the exciting radiation and the ratio of maximum to minimum emitted intensities. The variation of these parameters with intensity and period of excitation can be used to distinguish between various mechanisms for the luminescence process. Experiments are described, using a cathode-ray-oscillograph technique, illustrating the application of the method to zinc sulphide and zinc cadmium sulphide phosphors. The general conclusions are that these phosphors do not possess any of the characteristics associated with the monomolecular mechanism, but show a semi-quantitative agreement with the requirements of a simple ionization-recombination mechanism. Deviations from theory are ascribed to the complexity of these phosphors, which are activated by more than one type of activator atom.