Rotational Excitation Cross Sections for Chloronium Based on a New 5D Interaction Potential with Molecular Hydrogen.

Abstract

Chloronium (H₂Cl⁺) is an important intermediate of Cl-chemistry in space. The accurate knowledge of its collisional properties allows a better interpretation of the corresponding observations in interstellar clouds and, therefore, a better estimation of its abundance in these environments. While the ro-vibrational spectroscopy of H₂Cl⁺ is well-known, the studies of its collisional excitation are rather limited and these are available for the interaction with helium atoms only. We provide a new 5-dimensional rigid rotor potential energy surface for the interaction of H₂Cl⁺ with H₂, calculated from explicitly correlated coupled cluster ab initio theory, which was fitted then with a set of analytical functions, allowing to perform scattering calculations using accurate quantum theories. We analyze the collision-energy dependence of the rotational state-to-state cross sections and the temperature-dependence of the corresponding thermal rate coefficients, with particular attention on the collisional propensity rules. When comparing our results for collisions with H₂ with those obtained with He as a colliding partner, we found very significant differences with nonlinear scaling trends, which proves again that He is not a suitable proxy for collisions between hydride molecules and molecular hydrogen, the most abundant gas particle in the interstellar medium.