N2-broadening coefficients of methyl chloride: Measurements at room temperature and calculations at atmospheric temperatures

- We measured N2-broadening coefficients of the ν5 band of methyl chloride.
- We performed semi-classical calculations at various temperatures.
- We compared our calculation at 296 K to measurements obtained previously.
- Temperature dependence exponents have been retrieved.
- J- and K-rotational dependencies have been studied.

Infrared spectroscopic study on methyl chloride is the first step for its accurate detection in the atmosphere. In our previous work [Barbouchi Ramchani et al. J Quant Spectrosc Radiat Transfer 2013;120:1-15], line positions, intensities and self-broadening coefficients of both 12CH335Cl and 12CH337Cl isotopologues have been studied in the 6.9 μm spectral region. The present work is focused on measurements of N2-broadening coefficients for transitions of 12CH335Cl and 12CH337Cl around 6.9 μm. For that, high-resolution Fourier transform spectra of CH3Cl-N2 mixtures have been recorded at room temperature using a rapid scan Bruker IFS 120 HR interferometer at LADIR. The N2-broadening coefficients have been retrieved using a Voigt profile and a multispectrum fitting procedure. The average accuracy of the N2-broadening obtained in this work has been estimated to be between 5% and 10% depending on the transitions. The rotational J- and K-dependences of the N2-broadening coefficients have been clearly observed and modeled using empirical polynomial expansions. The 12CH335Cl-N2 line-widths of the ν5 band have also been computed using a semi-classical approach for the PR, RR and QR sub-branches. A global comparison with the experimental data from this work but also existing in the literature was then performed. Similar J- and K-rotational dependences have been observed while no clear evidence of any vibrational or isotopic dependence has been pointed out. Finally, performing theoretical calculations of the N2-broadening coefficients at various temperatures of atmospheric interest between 200 and 296 K allowed deducing the temperature exponent of the 12CH335Cl-N2 line-widths.