Experimental and theoretical study of line mixing in methane spectra. III. The Q branch of the Raman v₁ band

The shape of the v₁ Raman Q branch of CH₄ perturbed by Ar and He at room temperature has been studied. Stimulated Raman spectroscopy (SRS) experiments have been made in the 2915-2918 cm^-1 spectral region for total pressures from 0.4 to 70 atm and mixtures of ∼5% CH₄ with He and Ar. Analysis of the spectra demonstrates that the shape of the Q branch is significantly influenced by line mixing and much narrower than what is predicted by the addition of individual line profiles. For the first time, a model is proposed for the calculation and analysis of the effects of collisions on the considered spectra. In this approach, the rotational part of the relaxation matrix is constructed, with no adjustable parameter, starting from semiclassical state-to-state rates. Two empirical constants which account for the shift and broadening of the branch due to vibrational effects are introduced and their values are determined from fits of measured spectra. Comparisons between measurements and results computed with and without the inclusion of line mixing are made. Although not perfect, our model satisfactory accounts for most effects of pressure at low densities, where rotational transfers are dominant, as well as at high densities, where the profile is strongly influenced by vibrational contributions. It is shown that collisions with He and Ar lead to different behaviors at elevated pressure. The influence of the perturbation introduced by the Fermi coupling between the v₁ and v₂ + v₄ levels is discussed and the rotational and vibrational contributions to the spectral shape are pointed out.