How to determine the pressure of a methane‐containing gas mixture by means of two weak Raman bands, ν₃ and 2ν₂

Raman spectra of a pure CH 4 sample, two CH 4 C 2 H 6 mixtures and a CH 4 N 2 mixture were obtained as a function of pressure at pressures up to 39.6 MPa A (MPa absolute). These spectra are presented in the region 3120–2980 cm −1 . A clear pressure dependence of the area ratio between two weak methane bands, ν 3 (asymmetric CH stretching) and 2ν 2 (overtone of the asymmetric CH bending), is observed; the methane ν 3 band intensity is lowered relative to the methane 2ν 2 as the pressure is raised. The intensity ratios between these two bands, I (ν 3 )/ I (2ν 2 ), were determined and plotted as a function of pressure. Surprisingly it is observed that the ratio at a fixed pressure is independent of the composition and thereby of the surroundings in which the methane molecule is vibrating. A model function to predict the pressure is given. From a practical point of view, the present results could be useful for determining directly the total pressure in methane mixtures the composition of which is not known.