Rotational excitation of thioformaldehyde (H2CS) in collisions with molecular hydrogen

Abstract

Thioformaldehyde (H2CS) has been observed in the interstellar medium (ISM). Accurate determination of column densities of the two nuclear spin modifications of H2CS in the low-density environment of the ISM ideally necessitates the use of a radiative transfer model. Such a model requires the availability of rate coefficients for inelastic rotational transitions in H2CS induced by collisions with the dominant hydrogen molecule in the ISM. To compute these rate coefficients, a potential energy surface (PES) for the interaction of H2CS with H2 is computed in this study by the explicitly correlated coupled cluster method including single, double, and (perturbatively) triple excitations [CCSD(T)-f12a] and a correlation-consistent aug-cc-pVTZ basis. The geometries of the molecules were fixed. The calculated points on the PES were fit to a functional form suitable for quantum scattering calculations. The well depth De of the H2CS–H2 PES was determined to equal 318.1 cm−1, and the equilibrium intermolecular separation was found to be 6.18a0. Time-independent quantum scattering close coupling calculations were performed to compute state-to-state cross sections and rate coefficients for transitions between the H2CS rotational levels induced by collisions with the hydrogen molecule.