Self and N2 collisional broadening of far-infrared methane lines at low-temperature with application to Titan

Abstract

We report the measurement of broadening coefficients of pure rotational lines of methane at different pressure and temperature conditions. A total of 27 far-infrared spectra were recorded at the AILES beamline of the SOLEIL synchrotron at room-temperature, 200 K and 120 K, in a range of 10 to 800 mbar. Self and N 2 broadening coefficients and temperature dependence exponents of methane pure rotational lines have been measured in the 73-136 cm --1 spectral range using multi-spectrum non-linear least squares fitting of Voigt profiles. These coefficients were used to model spectra of Titan that were compared to a selection of equatorial Cassini/CIRS spectra, showing a good agreement for a stratospheric methane mole fraction of (1.17 $\pm$ 0.08)%.