Absorption Spectra of Lead Sulphide at Different Temperatures

The absorption spectrum of lead sulphide, as measured between 1 and 10 microns, shows a sharp decrease (in some specimens from 100 cm -1 to 5 cm -1 ) in the absorption coefficient at a wavelength corresponding to the limit of photoconductivity. In many galena crystals additional absorption exists that masks completely the sharp absorption edge. The absorption edge and the photoconductive limit change their position similarly with temperature, the rate of shift of the edge being +3.7 × 10 -4 ev/°k between 90°k and 400°k. The shift is in the opposite direction to that observed in the elementary semiconductors silicon and germanium. At long wavelengths the absorption coefficient increases with the square of the wavelength, but there is a large discrepancy between the magnitude of the measured absorption coefficient and that calculated on the free electron theory. The long wavelength absorption increases with the temperature; this may correspond either to an increase in the number of carriers or to a decrease in their mobility.