Far infra-red spectra of molecular crystals

Abstract
Absorption spectra of crystalline films of ammonia, 3-deutero ammonia, hydrogen sulphide and 2-deuterium sulphide have been measured at liquid nitrogen temperature using a Michelson interferometer. Changes in frequency on deuteration enable the modes to be classified as either translational or librational. In the case of ammonia, the spectrum is consistent with the stable cubic crystal form belonging to space group T 4 but the situation is complicated by the possible presence of metastable phases. The hydrogen sulphide spectrum indicates a site symmetry of C s, and favours an orthorhombic over a tetragonal structure.