State-to-state rotational rate coefficients for the OCS+H2 collision at low temperatures

Abstract

Context. The physicochemical conditions of interstellar regions with low densities, (e.g., typical molecular clouds), should be analyzed using non-LTE models. In such models, the collisional rate coefficients of the observed molecules with H_{2, He, and H are critical inputs. In the case of OCS, the only set of rate coefficients available for the collision with H2 was computed in the seventies, using a potential energy surface (PES) based on an electron gas model for the collision with He. Furthermore, in a recent study on OCS+He, a mass-scaled approximation for the rates was considered, and different propensity rules were found.Aims. The main goal of this study is to compute a new set of rotational de-excitation rate coefficients of OCS in collision with H2 at low temperatures.Methods. An averaged PES over the orientation of H2 is developed from a large grid of ab initio energies computed at the CCSD(T)/aug-cc-pVQZ level of theory. This surface is employed in close-coupling calculations for studying the collision of OCS with para-H2(j = 0). Furthermore, an available 4D PES was also used in close-coupling calculations to confirm the results of our first approximation.Results. The agreement between the cross sections for the OCS+para-H2 computed using the reduced and 4D PES was very good. The state-to-state rotational de-excitation rate coefficients for the lowest 30 rotational states of OCS by para-H2 are computed from these data. However, the rate coefficients show different behavior with published data; particularly, a different propensity rule, Δj = 1, is found. Furthermore, similarities between the rates with para- and ortho-H2 are found. Finally, the astrophysical implications of the new rate coefficients are explored from non-LTE radiative transfer calculations.}