Experimental and theoretical study of line mixing in methane spectra. II. Influence of the collision partner (He and Ar) in the v3 IR band

Abstract

Line mixing effects are studied in the v3 band of CH4 perturbed by Ar and He at room temperature. Experiments have been made in the 2800–3200 cm−1 spectral region using four different setups. They cover a wide range of total densities, including low (0.25–2 atm), medium (25–100 atm), and high (200–1000 atm) pressure conditions. Analysis of the spectra demonstrates that the spectral shapes (of the band, the Q branch, the P and R manifolds,…) are significantly influenced by line mixing. The theoretical approach proposed in the preceding paper is used in order to model and analyze these effects. As done previously, semiclassical state-to-state rates are used together with a few empirical constants. Comparisons between measurements and spectra computed with and without the inclusion of line mixing are made. They prove the quality of the approach which satisfactorily accounts for the effects of pressure and of rotational quantum numbers on the spectral shape. It is shown that collisions with He and Ar lead to different line-coupling schemes (e.g., more coupling within the branches and less between branches) and hence to different shapes. The influence of line coupling between different branches and manifolds is evidenced and studied using high pressure spectra and absorption in the band wings.