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Recent interest in lead sulphide, selenide and telluride as photoconductors makes information on their absorption spectra valuable. It is found that the absorption spectrum of each material is characterized by strong absorption in the visible and ultraviolet region, with a further band in the infra-red. The effects of temperature and oxidation are also studied. In PbS the forbidden energy region between the filled and empty bands is found to be about 1.3 ev. in width, not 0.4 ev. as previously suggested. Hence photoconductivity in PbS cannot be associated with the main lattice. The corresponding energy gaps in PbSe and PbTe are found to be about 1.05 ev. and 0.9 ev. respectively.
A F Gibson (1950) studied this question.