Far infra-red spectra of molecular crystals

Abstract

Absorption spectra of crystalline films of chlorine, bromine and iodine at 77°k have been measured in the region 20–400 cm^-1 using a Michelson interferometer. The two low-frequency bands observed for each sample are assigned as translational lattice modes, in agreement with theoretical predictions from the known crystallographic structure, and the relevant intermolecular force constants are derived. In addition, a weak absorption is observed in the region of the stretching frequency of both bromine and iodine and possible reasons for the appearance of this theoretically infra-red inactive mode are discussed.