Line mixing in the ν₆ Q branches of self- and nitrogen-broadened methyl bromide

Line-mixing effects are studied in the ν₆ ^RQ_K and ^PQ_K (K=0-6) branches of methyl bromide (CH₃Br) perturbed by nitrogen (N₂). Laboratory Fourier transform spectra have been obtained at room temperature, and for a large range of pressure values of atmospheric interest. In order to accurately model these spectra, a theoretical approach accounting for line-mixing effects is proposed. This model is based on the use of the state-to-state rotational cross-sections calculated by a statistical modified exponential-gap fitting law depending on a few empirical parameters. These parameters are deduced by adjusting the calculated diagonal elements of the relaxation matrix to the N₂-broadening coefficients, known from accurate previous measurements. Comparisons between experimental and calculated profiles for various Q branches and under various pressure conditions (0.2-1 atm) demonstrate the adequacy and consistency of the proposed model. To allow accurate laboratory measurements, line-mixing effects are also modeled in the case of self-perturbed CH₃Br.