Preparation and Characteristics of Zinc Sulfide Phosphors Sensitive to Infra-Red*

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Abstract
In the presence of a relatively high percentage of lead or certain other agents and after the usual excitation, zinc sulfide phosphors become capable of storing luminescence energy at such a high potential that, unlike phosphorescence, it can be released only by radiation with the near infra-red. There are two stimulating bands—a strong one at 1.25–1.60μ and a weaker one adjoining the visible. The resulting luminescence is increased by a small content of copper. Its decay, under continued stimulation, is monomolecular in contrast to the decay of phosphorescence, which is bimolecular. In the case of lead, the color of the stimulated luminescence is green, that of the shorter of its two fluorescence bands. These fluorescence bands, in the green and yellow, are excited by different portions of the spectrum, and their phosphorescence decay is at different rates. After excitation at 77°K, there is also a bright emission observed on radiating with infra-red of 1–3μ, but this is due to the release of stored luminescence energy which is normally latent at 77°K, appearing at higher temperatures as phosphorescence. It does not affect the magnitude of the luminescence which is subsequently obtainable, after warming to 25°C, by stimulation with infra-red of the same range.

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