Abstract
Accurate modeling of anionic abundances in the interstellar and circumstellar media requires calculations of collisional data with the most abundant species that are usually He atoms and H 2 molecules. In this paper, we focus on smaller cyclic molecular anion, c-C 3 H − , an astrophysical candidate, following the detection of larger C n H − carbon chains. From a new three-dimensional potential energy surface, the rotational (de-)excitation of the c-C 3 H − (X 1 A 1 ) anion by collision with He is investigated. The surface is obtained in the supermolecular approach at the CCSD(T)-F12/aug-cc-pVTZ level of theory. Fully quantum close-coupling calculations of inelastic integral cross sections are performed on a grid of collisional energies large enough to ensure the convergence of the state-to-state rate coefficients for the 34 first rotational levels up to [Formula: see text] = 7 7,0 of c-C 3 H − and temperatures ranging from 5 to 100 K. For this collisional system, rate coefficients exhibit a strong dominance in favor of 2 1,2 → l 1,1 downward transition. This transition was previously used for the detection of the cyclic parent c-C 3 H. The c-C 3 H − –He rate coefficients (∼10 −11 cm 3 s −1 ) are of the same order of magnitude as those of the detected anions C n H − (as C 2 H − , C 4 H − , and C 6 H − ) in collision with He and one order of magnitude smaller than those with H 2 . The critical densities of H 2 were also estimated, and a discussion on the validity of the local thermodynamic equilibrium conditions is carried out. This work represents the contribution to understanding and modeling abundances and chemistry of hydrocarbon radicals, C n H, in astrophysical media.