New rate coefficients for rotationally inelastic collisions of OCS with para- and ortho-H2

Abstract

Carbonyl sulfide OCS(X¹Σ⁺) is one of the most abundant sulfurated molecules detected in the gas phase toward star-forming regions and plays a major role in interstellar chemistry. Accurate collisional rate coefficients for its rotational excitation with the most abundant colliding species (H₂) in the ISM are crucial for modeling OCS lines. In this work, we calculate a four-dimensional potential energy surface (4D-PES) of OCS interacting with H₂ using the CCSD(T)-F12 method and the aug-cc-pVTZ basis sets. A test on the cross sections showed that considering a single rotational level of H₂ is sufficient. In the case of para-H₂ (j₂ = 0), we treated H₂ as a spherical body and the dynamic calculation is based on an average PES over five orientations of H₂, which greatly reduces the computational cost, whereas for ortho-H₂ (j₂ = 1), a 4D-PES was used. Inelastic cross sections were calculated using the close-coupling theory for collision energies up to 1000 cm⁻¹. From the Maxwell–Boltzmann distribution of kinetic energies, cross sections were averaged to generate the collisional rate coefficients below 200 K and j₁ ≤ 22 (18) for OCS–para-H₂ (-ortho-H₂). This work provides the first calculation of the rate coefficients of OCS–ortho-H₂, as well as the first calculation performed using the ab initio PES and quantum close-coupling approach for OCS–para-H₂. The rotational rate coefficients are compared with previous data obtained for OCS–H₂ by Green and Chapman in 1978 for j₁ ≤ 12 levels with the coupled state approach, using an interaction potential adapted from an electron gas model for OCS–He. The agreement between the two results is discussed. The present new data will help to accurately model the OCS emission lines toward the young stellar objects.