Pure methane and 10 mol % methane in Ar have been studied in the region of methane’s ν1 totally symmetric stretching mode near 2910 cm−1 (ω0, the signal maximum or zeroth spectral moment frequency). Densities from 0.0795 (3 bar) to 15.0 (2100 bar)×1021 mol/c.c. were examined using Ar+ laser illumination at 514.5 nm. Frequency shifts of ν1’s ω0, and the zeroth moment depolarization ratio, ρD, values as a function of pressure were recorded. Previous ω0 shifts vs density were generally confirmed and were extended to higher densities. This allowed a shallow minimum to be observed in this data. It was correlated with the ρD values. These latter values were examined to see whether or not they were as sensitive as the frequency shift results were to the local reorientational friction coefficient tensor fr (potential-energy interactions) in methane and the Ar/CH4 solutions. The initial (3.5–1000 bar data) depolarization results were interpreted with two models of methane in mind. The first, a conventional view that is based upon the concept that higher densities cause distortions in the molecular shape (lower symmetry), and the second, which makes use of the known fact that the symmetry of methane at low pressures is less than Td. Therefore, as the density and fr increase quantum rotation slows and the rotation-vibration interaction effectively ceases and causes the molecules to pack tightly and the well-known Td symmetry is achieved.

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The Raman depolarization ratios of carbon monoxide were measured via the zeroth spectral moment, M,, (integrated intensities), and simple peak-height determinations under isothermal conditions (323 K) over a pressure range of 20-2400 bar in a mixture of CH, and CO. These conditions correspond to densities ranging from 0.75 to 24 mol I-'. The measurements were carried out so that the depolarization ratios being measured in other experiments from the v , totally symmetric stretching mode of supracritical methane could be corrected for instrumental depolarization effects. The values near a pressure of 1 bar have been calculated and measured for CO by several sets of workers. The cell windows were float glass and as a result they are known to contribute much less than quartz to depolarization. Float glass windows cause only a 0.5% error in scrambling uncertainty of the polarized light exiting the high-pressure cell.

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