Raman spectra of a pure CH4 sample, two CH4-C2H6 mixtures and a CH4-N2 mixture were obtained as a function of pressure at pressures up to 39.6 MPaA (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, n3 (asymmetric C-H stretching) and 2n2 (overtone of the asymmetric C-H bending), is observed; the methane n3 band intensity is lowered relative to the methane 2n2 as the pressure is raised. The intensity ratios between these two bands, In3/I2n2, 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.
|Journal||Journal of Raman Spectroscopy|
|Publication status||Published - 2002|