Fundamental Absorption Edge in Cadmium Sulfide

The absorption and reflection spectra of CdS have been determined in the temperature range 90°-340°K by photoelectric measurements on single crystals, using polarized light. The temperature and frequency dependence of the absorption coefficient in the edge, over a substantial range of absorption magnitudes, are well described by expressions of the form $\alpha \left(\nu \right)={\alpha }_0\exp [-\frac{\beta (E_{00}-CT-h\nu )}{\mathrm{kT}}]$, where the energy ($E_{00}-CT$) is closely associated with the position of an absorption peak. For light polarized with $E$ vector parallel to the crystalline $c$ axis, there is one such peak, at $\lambda 4844$ A at 90°K; for light polarized with $E$ vector perpendicular to the $c$ axis, the 4844 A absorption is again present in about equal strength and a stronger line appears at $\lambda 4874$ A, shifting the absorption edge toward longer wavelengths by a corresponding amount. These absorption lines, or sharp bands, are observed in the reflectance spectrum, and correspond to absorption lines observed photographically by Gross and others. These results are discussed in the light of the present theoretical picture of absorption in insulating crystals, and it is noted that an exponential absorption edge of this kind, characteristic also of other ionic crystals, cannot be adequately explained in terms of existing models.