Rotational excitation and de-excitation of interstellar chloronium cation in collisions with helium atoms

Abstract

Chloronium, H2Cl+, is detected in astrophysical media. It is key chemical intermediate for understanding of the physical chemistry of chlorine species there. At present, we compute the collision rates for the rotational excitation and de-excitation of ortho- and para-H2Cl+ colliding with He for T ≤ 150 K, relevant for the interstellar medium (ISM) conditions. Prior to that, we generated the 3D interaction potential of the weakly bound H2Cl+-He complex along the Jacobi coordinates. For electronic structure computations, we used a post-Hartree-Fock explicitly correlated method at the CCSD(T)-F12/aug-cc-pVTZ level. The analytical expansion of this potential was incorporated into close coupling computations of the cross sections for the rotational excitation and de-excitation of ortho- and para-H2Cl+ colliding with He and for collision kinetic energies Ek ≤ 1000 cm−1. The rates were obtained after averaging these cross sections over a Maxwell-Boltzmann distribution of kinetic energies. Our data show that the ∆j=∆ka=∆kc=-1 de-excitation transitions exhibit the largest values, in particular those used to identify this cation in the surveys. Besides, our results should help for determining more accurate abundances of H2Cl+ in the ISM and thus better modeling the chlorine chemistry there.