Rotational excitation of C2H− anion in collision with H2

Abstract

The discovery of anions in the interstellar medium has shown that they are very reactive species. This gave them great importance in the modeling of the chemical and astrophysical evolution of the interstellar medium. The detection of the first anion C₆H⁻ followed by the other anions C₄H⁻, C₈H⁻ and CN⁻ in the interstellar medium has encouraged research on other detectable anions. The C₂H⁻ anion was observed for the first time in the circumstellar envelope of IRC+10216 and in TMC-1. In these cold and low-density regions, precise modeling of the chemical and physical conditions of the observed emission lines requires knowledge of the radiative and collisional excitation rates. We present here the first new two-dimensional Potential Energy Surface (PES) for C₂H–H₂ interaction. Rotational excitation of the anion by collision with para-H₂(j_H2 = 0) is investigated. The PES is obtained in the super-molecular approach based on a single and double excitation coupled cluster method with perturbative contributions from triple excitations (CCSD(T)). In all our calculations, all atoms were described using the augmented correlation-consistent triple zeta (aug-cc-pVTZ) basis sets and bond f unctions. Fully-quantum close-coupling calculations of inelastic integral cross sections are done on a grid of collision energies large enough to ensure converged state-to-state rate coefficients for the 16 first rotational levels of C₂H⁻ and for temperatures ranging from 5 to 120 K. For this collisional system, rate coefficients exhibit a strong propensity in favor of even Δj transitions.