Experimental and theoretical study of line mixing in methane spectra. IV. Influence of the temperature and of the band

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Abstract
Line-mixing effects are studied in infrared bands of CH 4 perturbed by N 2 at various pressures. The effects of temperature are investigated in the ν 3 region whereas spectral shapes of the ν 2 , ν 4 , and ν 3 bands are compared at room temperature. The theoretical approach proposed in preceding papers is used in order to model and analyze the influence of collisions on the spectral shape. All model parameters are now fixed to values determined in the previous studies. Comparisons between measurements and spectra computed with and without the inclusion of line mixing are made. They show that our approach satisfactorily accounts for the effects of temperature, pressure, and of rotational quantum numbers on the absorption by the ν 3 band. Furthermore, the effects of collisions on spectra in the ν 4 region at room temperature are also correctly calculated. On the other hand, the proposed approach fails in modeling the evolution with increasing pressure of absorption in the spectral range containing the ν 2 band. This result is attributed to the Coriolis coupling between the ν 2 and ν 4 vibrational states and to a contribution whose physical origin remains unclear. The latter, which is negligible when CH 4 –He mixtures are considered, behaves as collision-induced absorption.

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