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

TL;DR

This study measures self and N2 collisional broadening coefficients of methane's far-infrared lines at low temperatures, improving spectral models for Titan's atmosphere and validating them against Cassini data.

Contribution

It provides new measurements of methane broadening coefficients at low temperatures relevant to Titan, enhancing spectral modeling accuracy.

Findings

Good agreement between modeled and observed Titan spectra.

Measured broadening coefficients vary with temperature and pressure.

Validated spectral models for Titan's methane profile.

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)%.