Energy terms linear in wave vector are allowed in the second valence band of CdS. Perturbation theory shows that excition states formed with holes from this band will also have a linear splitting. The effect of such terms on the optical properties are discussed using a spatial dispersion approach. Previous reflectivity experiments have some anomalous structure which is shown to be caused by the linear term. Comparing theory with these experiments provides an estimate of these linear terms, which have not been previously measured.

• Received 30 January 1964

DOI:https://doi.org/10.1103/PhysRev.135.A428