Rotational Quenching of an Interstellar Gas Thermometer: CH₃CN⋯He Collisions
Among all the molecular species found in the interstellar medium, molecules with threefold symmetry axes play a special role, as their rotational spectroscopy allows them to act as practical gas thermometers. Methyl-cyanide (CH3CN) is the second most abundant of those (after ammonia). We compute in this paper the collisional dynamics of methyl-cyanide in collision with helium, for both the A-and the E-symmetries of CH3CN. The potential energy surface is determined using the CCSD(T)-F12b formalism and fit with convenient analytic functions. We compute the rotationally inelastic cross sections for all levels up to 510 cm-1 of collision energy, employing at low energy exact Coupled Channels methods, and at higher energies, approximate Coupled States methods. For temperatures from 7 K up to 300 K, rates of quenching are computed and most are found to differ from those reported earlier (up to a factor of a thousand), calling for a possible reexamination of the temperatures assigned to low density gasses.
M. Ben Khalifa et al., "Rotational Quenching of an Interstellar Gas Thermometer: CH₃CN⋯He Collisions," Physical Chemistry Chemical Physics, vol. 22, no. 31, pp. 17494-17502, Royal Society of Chemistry, Jul 2020.
The definitive version is available at https://doi.org/10.1039/d0cp02985h
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23 Jul 2020