Cross-sections and rate coefficients calculations for rotational excitation of cyanoethynylide ions ($C_3{N}^{-}$) induced by collision with He atoms at low temperature

The focal points of the present paper are the calculations of cross-sections and rate coefficients for rotational (de-)excitation of C₃N⁻ induced by collision with He and H₂ atoms for energies of up to 2000 cm⁻¹ and temperatures ranging from 1 K to 300 K. For this study, the new two-dimensional potential energy surface (PES) of the C₃N⁻−He complex has been calculated at the ab initio restricted coupled cluster level of theory, including single, double and perturbative triple excitation [RCCSD(T)-F12a] with the cc-pVTZ-F12 basis set augmented by mid-bond functions. Basis set superposition errors were taken into account. To allow for the dynamical calculations, the PES was then fitted to a global analytical form, the main features of which are discussed. Collisional cross-sections among the first 17 rotational levels were calculated and employed to compute downward and upward rate coefficients. A propensity towards $|\Delta j|=2$ was observed. (paper)