Mid-infrared high-resolution dual-comb spectroscopy: Line shape parameters of CO2-broadened transitions in the ν4 band of methane

Using a high-resolution dual-comb spectrometer based on quantum cascade lasers, we have measured line shape parameters of CH₄ transitions broadened by CO₂ in the ν₄ vibrational band. 8 spectra were recorded at room temperature for total pressures ranging from 25 mbar to 700 mbar. The line shape parameters were determined by fitting 4 different theoretical profiles to the experimental methane line shapes using a multi-spectrum fitting technique. The Voigt, Nelkin-Ghatak, and speed-dependent Voigt profiles were used for isolated lines, and a Voigt within Rosenkranz’s first-order approximation model to consider the line-mixing effects. To our knowledge, broadening speed dependence and 1^st order line-mixing effect were determined for the first time for CO₂-broadened lines located in the ν₄ band of methane.