Mode assignment of sulfur alpha-S-8 by polarized Raman and FTIR studies at low temperatures

We have measured frequencies of all internal and external modes in natural and S-32 isotopically pure oriented orthorhombic single sulfur crystals using high-resolution Raman and FTIR spectroscopy. Polarized spectra at low temperatures deliver mode frequencies and allow mode assignment to vibrational, librational, and translational character. Experimental data, such as temperature dependent frequencies at constant volume, are analyzed in terms of anharmonic processes. In addition, frequency shifts at the lowest temperature due to isotopic impurities are qualitatively explained by a relative mass defect.