2008A&A...478..567L

Over the next few years, the ALMA and Herschel missions will perform high spatial and spectral resolution studies at infrared and sub-millimeter wavelengths. Modeling of molecular emission requires excitation calculations using radiative, as well as collisional rates, with the most abundant species. In the interstellar medium, the dominant collision partner is H₂, but little data is available for collisions with H₂. If data for collisions with He are available, it has often been proposed to use the more available rate coefficients for collision with He, with the appropriate reduced mass correction, as a first estimate of rate coefficients with H₂(j=0). The validity of this approximation is not known. The present paper focuses on the calculation of rate coefficients among the first rotational levels of the SiS molecule in its ground vibrational state in collision with para-H₂ and compares these new data with recently published He ones to investigate the validity of using He rate coefficients to estimate H₂(j=0) rate coefficients. A new potential energy surface for the SiS-para-H₂ system was obtained using highly correlated ab initio calculations. Dynamical calculations of pure rotational (de)excitation of SiS by para-H₂ were performed for the first rotational levels within the coupled-states approximation. Collisional cross sections among the 51 first rotational levels of SiS were calculated for kinetic energies up to 2500cm^–1. State-to-state rate coefficients are calculated for temperatures ranging from 5K up to 300K. A propensity rule that favors even Δj transitions is found and is explained by the near homonuclear symmetry of the SiS-para-H₂ potential energy surface. A detailed comparison with recent SiS-He rate coefficients is also presented. We demonstrate that collision with He is a reasonable model for collisions with para-H₂, although this approximation must be used with caution.