Excitons and the Absorption Edge of Cadmium Sulfide

Measurements of the absorption coefficient between 10 and 300 ${\mathrm{cm}}^{-1\mathrm{}}$ have been made with crystals of CdS in polarized light between 20 and 300°K, at wavelengths near 5000 A. Analysis of the results at various temperatures near 70°K shows that the absorption is in agreement with that calculated for a process involving the simultaneous creation of an exciton and the absorption of a phonon, both particles having a small wave vector. This agreement is strong evidence that the conduction band minimum and the valence band maximum in CdS both occur at the center of the Brillouin zone.