First rotational (de-)excitation of HOCS+( X 1 A ′) by collision with He(1S): rate coefficients and potential energy surface

Abstract In this work, we constructed the first potential energy surface (PES) for the HOCS+–He complex using the explicitly correlated ab initio CCSD(T)-F12/aug-cc-pVTZ method. An examination of this PES revealed the existence of three potential wells, the deepest of which is 297.25 cm−1, located in the HOCS+ plane, on the hydrogen atom side. We used this PES in dynamical calculations to determine the rate coefficients at which the rotational energy of HOCS+ is exchanged by collisions with He. Rotational transitions involving the lowest 31 energy levels of HOCS+ (up to Erot < 40.2 cm−1) were studied. The quantum close-coupling method (CC) is applied for calculating cross sections at total energies below 50 cm−1. The coupled-states approach (CS) is employed for calculations covering energies up to 500 cm−1. By averaging these cross sections over a Maxwell-Boltzmann distribution, we calculated the rate coefficients for temperatures ranging from 2 to 100 K. In the studied temperature range, a slight dominance of rotational transitions with ΔJ = ΔKc = −1 and ΔKa = 0 was observed. The rate coefficients calculated for HOCS+ ion are compared with those of its valence-isoelectronic molecule HOCO+, specifically for collisions involving He. A similarity was observed, especially for low ΔJ. We expect that the new rate coefficients will help to analyze present and future observations of this key molecule for interstellar chemistry.