Temperature dependence of CH3CNN2 line broadening predicted from extended rotational-band measurements

Abstract

As methyl cyanide is one of the complex organic molecules present in planetary atmospheres, comets and interstellar medium but extremely scarce data are available for its radiative transfer modeling, we focus on providing exhaustive sets of theoretical nitrogen-broadening coefficients of CH${}_3$CN lines for a large set of quantum numbers and a wide temperature range of astrophysical interest. Taking advantage of recent extensive measurements of rotational transitions with Voigt- and Speed-Dependent-Voigt profiles at room temperature and the corresponding update of the temperature-independent parameters of the semi-empirical approach suitable for CH${}_3$X-type molecules with large dipole moments, we employ these parameters in calculations for lower and higher temperatures with a further determination of temperature-dependence exponents. CH${}_3$CNN${}_2$ line-broadening parameters are calculated for both line-profile models in the temperature range 100–2000 K for the rotational numbers $J=0$–80, $K=0$–20 typically requested by spectroscopic databases. The temperature-dependence exponents for the reference temperature $T_{\mathrm{ref}}$ = 296 K are extracted for the Earth’s-atmosphere related interval 200–400 K, the interval of theoretical validity 300–2000 K, and the full interval of interest 100–2000 K, showing an excellent applicability of the traditional one-power temperature law. Although the semi-classical treatment is not rigorously justified at low temperatures 100–300 K but seems to be the only feasible one, separate datasets are provided for this interval as well. As no clear experimental evidence exists neither for branch- nor for vibrational dependency of N${}_2$-broadening of CH${}_3$CN lines, the data computed for the rotational band can be used for other branches in other bands of vibrotational transitions.