The vibrational Raman spectra of the V₁ symmetric mode of liquids methane and carbon tetrafluoride

Abstract

The polarized or isotropic contributions to the totally symmetric V₁ vibrational Raman spectra of methane and carbon tetrafluoride were measured in the liquid phase along the liquid vapour co-existence line from the triple point to the critical point in each case. Attention was focused on three principal characteristics of single component spectra; (1) the line width i.e. the full width at half maximum, (2) the shift in peak frequency relative to that at the triple point and (3) the degree of asymmetry observed in the line shape. Light from Ar⁺ ion laser incident on the sample cell and scattered at 90° was analysed by a Fabry-Perot interferometer and recorded by photon counting systems. In case of methane, the line width remains practically constant from the triple point to the boiling point and increases from the boiling point onwards to the critical point, while in case of carbon tetrafluoride the line width is maximum at triple point and decreases up to the boiling point and afterwards remains constant up to the critical point. The observed variation in line width can be explained on the basis of a hydrodynamic theory of vibrational dephasing due to Hills and Madden. The relative Raman frequency shift has been found to vary linearly with density as previously observed in similar gas-phase experiments. In both the liquids the asymmetry in line shape is observed to be in opposite sense at the triple point to that at the critical point.