ASSIGNMENT OF TRANSVERSE OPTICAL LONGITUDINAL OPTICAL MODES IN THE VIBRATIONAL-SPECTRUM OF SOLID SULFUR-DIOXIDE

A recently proposed radically different set of assignments for the infrared and Raman spectra for solid sulphur dioxide have been assessed and rejected because of the misinterpretation of the origin of the lifting of the degeneracy for TO-LO mode pairs in non-centric crystals. The assessment was based on Raman depolarization measurements for oriented single crystals, well established isotope shift values, natural isotope abundance data and Raman measurements with S-34-enriched sulphur dioxide. The reassigned values are consistent with those previously accepted. The assignment of weak peaks due to small concentrations of isotopically different molecules (self-impurities) has been shown to be unexpectedly complicated due to intermolecular coupling between vibrational modes of isotopically different molecules. In the nu-2 region of the spectrum the expected band due to the (SO2)-S-34 impurity remained invisible even in S-34-enriched samples. In the nu-1 and nu-3 regions the relative intensity of the band due to (SO2)-S-34 was anomalously large due to intermolecular coupling.