Vibrational Spectrum of Crystalline HF and DF

Abstract

Infrared spectra of pure crystalline HF and DF have been obtained over the region 30–4000 cm−1, and the spectra of the dilute isotopic mixed crystals have been studied in the intramolecular region 2000–4000 cm−1. Unlike previous workers, we find four bands in the stretching region of pure DF crystals, as well as the previously-reported four in the corresponding region of the HF spectrum. Direct observation of a lattice band near 200 cm−1 in both HF and DF, together with the line spacing in the stretching region permits an unambiguous assignment of the extra pair of lines in the intramolecular region as combination bands. The Raman spectra of crystalline HF and DF reveal four lines in the stretching region and one lattice transition. With the aid of simple intensity rules and correlations with the other hydrogen halides, a complete assignment of the vibrational bands is made except for the librational region, where the multiplicity of lines and the peculiar behavior of one of them under different deposition conditions make a firm interpretation impossible. The assignment is discussed with reference to results obtained by inelastic neutron scattering. Within experimental error each Raman line is coincident with an infrared line, and the structural implications of this fact are discussed.