Collision excitation of c-C₃H^-(X¹A₁) by He

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 H₂ molecules. In this paper, we focus on smaller cyclic molecular anion, c-C₃H^-, an astrophysical candidate, following the detection of larger C_nH^- carbon chains. From a new three-dimensional potential energy surface, the rotational (de-)excitation of the c-C₃H^-(X¹A₁) 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 j_K = 7_7,0 of c-C₃H^- 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-C₃H. The c-C₃H^--He rate coefficients (∼10^-11 cm³ s^-1) are of the same order of magnitude as those of the detected anions C_nH^- (as C₂H^-, C₄H^-, and C₆H^-) in collision with He and one order of magnitude smaller than those with H₂. The critical densities of H₂ 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_nH, in astrophysical media.