Extended measurements and calculations of CH3C14N-N2 rotational lineshape parameters

Abstract

Spectroscopic parameters of methyl cyanide are of interest for astrophysical applications because of presence of this molecule in planetary atmospheres, comets, and interstellar medium. For radiative-transfer modeling a key role is played by the pressure-induced linewidths which are strongly influenced by the collision partner and the temperature. To complete the extremely scarce experimental data available, rotational lines of room-temperature CH3C14N in collision with molecular nitrogen have been recorded in a very large (180–1400 GHz) frequency range for J = 9 → 10, 12 → 13, 15 → 16, 21 → 22, 27 → 28, 33 → 34, 42 → 43, 48 → 49, 63 → 64, 69 → 70, 75 → 76 and K = 0–15. The high sensitivity of the frequency-modulated spectrometer has enabled observation of clear deviations of the recorded lineshapes from the usual Voigt profile. Their additional analyses performed with advanced non-Voigt models have demonstrated that the observed lineshapes are mainly governed by the speed dependence of relaxation rates and that the optical diffusion related to velocity changing collisions (Dicke effect) has a nearly negligible role. Compared to infrared-region results available in the literature, our Voigt-profile values lie systematically lower by about 0.5 MHz/Torr, which argues in favor of observable vibrational dependence for the CH3CN-N2 system. Line-broadening parameters have been also evaluated by a semi-empirical method based on the Anderson-Tsao-Curnutte theory. The model parameters determined from fits on some selected experimental values confirm the tendences previously stated for the case of vibrotational transitions and allow better theoretical predictions for extended ranges of rotational quantum numbers. Being temperature-independent, these parameters can be directly used in calculations for other temperatures concerned by radiative transfer models.