Reflection Spectra of PbCl2 in the Exciton Region

The reflection spectra of orthorhombic PbCl 2 crystals are studied in the exciton region with linearly polarized light at 2 K. It is found that the exciton band splits into two in the spectra obtained with the polarized lights along b - and c -axes. This splitting is explained as due to a combined effect of the crystal field and the Davydov-type interaction of the excitons. The latter effect arises from the fact that the unit contains four PbCl 2 molecules different in their orientation. When light propagation in crystal does not coincide with any of crystal axes, an additional structure appears on the higher energy side of the main band, which is attributed to a complex exciton polariton mode peculiar to anisotropic crystals.