Rotational Excitation of Vinyl Cyanide by Collisions with Helium Atoms at Low Temperature

Abstract

Among the numerous molecular systems found in the interstellar medium (ISM), vinyl cyanide is the first identified olephinic nitrile. While it has been observed in various sources, its detection in Sgr B2 is notable as the 2${}_{11}$-2${}_{12}$ rotational transition exhibits maser features. This indicates that local thermodynamic equilibrium conditions are not fulfilled, and an accurate estimation of the molecular abundance in such conditions involves solving the statistical equilibrium equations taking into account the competition between the radiative and collisional processes. This in turn requires the knowledge of rotational excitation data for collisions with the most abundant species - He or H${}_2$. In this paper the first three-dimensional CH${}_2$CHCN - He potential energy surface is computed using explicitly correlated coupled-cluster theory [(CCSD(T)-F12] with a combination of two basis sets. Scattering calculations of the rotational (de-)excitation of CH${}_2$CHCN induced by He atoms are performed with the quantum-mechanical close-coupling method in the low-energy regime. Rotational state-to-state cross sections derived from these calculations are used to compute the corresponding rate coefficients. The interaction potential exhibits a high anisotropy, with a global minimum of $-53.5$ cm${}^{-1}$ and multiple local minima. Collisional cross sections are calculated for total energies up to 100 cm${}^{-1}$. By thermally averaging the cross-sections, collisional rate coefficients are determined for temperatures up to 20 K. A propensity favouring the transitions with $\Delta k_a=0$ is observed.

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