Experimental and theoretical study of line mixing in methane spectra. III.The Q branch of the Raman nu(1) band

The shape of the nu(1) Raman Q branch of CH4 perturbed by Ar and He at room temperature has been studied. Stimulated Raman spectroscopy (SRS) experime nts have been made in the 2915-2918 cm(-1) spectral region for total pressu res from 0.4 to 70 atm and mixtures of approximate to 5% CH4 with He and Ar . Analysis of the spectra demonstrates that the shape of the Q branch is si gnificantly influenced by line mixing and much narrower than what is predic ted by the addition of individual line profiles. For the first time, a mode l is proposed for the calculation and analysis of the effects of collisions on the considered spectra. In this approach, the rotational part of the re laxation 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 i ntroduced and their values are determined from fits of measured spectra. Co mparisons between measurements and results computed with and without the in clusion of line mixing are made. Although not perfect, our model satisfacto ry 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 collisi ons with He and Ar lead to different behaviors at elevated pressure. The in fluence of the perturbation introduced by the Fermi coupling between the nu (1) and nu(2)+nu(4) levels is discussed and the rotational and vibrational contributions to the spectral shape are pointed out. (C) 2000 American Inst itute of Physics. [S0021-9606(00)02103-6].