In earlier papers it was shown that a group of phosphors show an approximately linear increase of their efficiency with increasing intensity of excitation in certain ranges of that intensity. Tentatively, these observations and the interdependence of the effects of intensity, temperature, and “poisoning” found in the same phosphors were interpreted by the assumption that a non-radiative process of first-order kinetics competes with an emission process of the second order. This type of hypothesis, however, does not easily account for more than linear dependences of efficiency on intensity. Indications of such dependences had been found in our earlier work. In view of the possible theoretical significance of these results, a number of measures were taken to eliminate possible sources of errors. New measurements were carried out on a suitable series of poisoned zinc-cadmium sulfide phosphors. The existence of regions in which the intensity dependence of efficiency was even stronger than quadratic was ascertained. Furthermore,a remarkably strong dependence of the efficiency on the carefully controlled concentrations of the poison was found. The problems raised by these effects are briefly discussed.
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