The Absorption Spectrum of Excited Crystals of Cadmium Sulphide

Crystals of cadmium sulphide show new absorption bands when excited by visible light (0.546 μ). The bands extend from the excitation region out to 1.4 μ with a maximum intensity at 0.78 μ. The bands are stable in the dark at low temperatures but are thermally bleached on warming. They are also bleached by radiation of their own wavelength. Correlation is found between thermal bleaching characteristics and electrical conductivity characteristics. A model is proposed in which thermal bleaching and electrical conduction on warming result from the escape of trapped electrons while optical bleaching frees positive holes which migrate to traps and recombine with trapped electrons.