Methane is an important constituent of the atmosphere of the Earth and outer planets of the solar system [1]. Precise and detailed knowledge of spectroscopic line parameters of methane is crucial to measure its concentration in these atmospheres using remote sensing techniques.

In this work, high-resolution spectra of pure and air-broadened methane have been recorded in the 2600 – 3200 cm^–1 spectral range at a resolution of 0.006 cm^–1 (maximum optical path difference of 150 cm) with a Bruker IFS125 HR Fourier transform spectrometer available in Brussels. All the spectra were obtained using high purity natural samples of CH₄ and mixtures of the same natural CH₄ with dry air. The experiments were performed at an average temperature of 296 K, with an absorption path length of 19.7 cm and pressures ranging from 0.3 to 678 Torr.

We were able to fit the full range of pressures with a Rautian–Sobel’Man line shape model, which takes into account the Dicke narrowing effect, with Rosenkranz line mixing. For the analysis, a multi-spectrum non-linear least squares fitting software was employed in which all the spectra were fitted simultaneously. Spectral line shape parameters (air broadened widths, shifts, Dicke narrowing and first order line mixing coefficients) have been determined for 166 lines belonging to J" manifolds of the P (J" = 2-11), Q (J" = 1-12) and R (J" = 0-11) branches of the n₃ band of ¹²CH₄ near 3.3µm.

Variations of the measured parameters with tetrahedral symmetry species, rotational quantum numbers and ranking indexes of the lower levels of the studied transitions, and comparisons with previous works will be presented and discussed.

Keywords: methane, n₃ band, High Resolution Infrared Spectroscopy, air broadening and shift coefficients, Dicke-narrowing, line-mixing parameters.

References

[1] Mingmin Zou, Xiaozhen Xiong, Naoko Saitoh, Juying Warner, Ying Zhang, Liangfu Chen, Fuzhong Weng and Meng Fan, 2016, Atmospheric Measurement Techniques, 9, 3567–3576.