The rotational excitation of SiS by para- and ortho-H2

Abstract

Over the next few years, the Atacama Large Millimeter Array (ALMA) and Herschel missions will perform high spatial and spectral resolution studies at infrared and submillimetre wavelengths. Modelling of molecular emission requires excitation calculations using radiative as well as collisional rates with the most abundant species. The present paper focuses on the calculation of rate coefficients among the first 41 rotational levels of the silicon monosulfide (SiS) molecule in its ground vibrational state in collision with para- and ortho-H2. Dynamical calculations of pure rotational (de)excitation of SiS by para- and ortho-H2 were performed within the coupled-states approximation using the recently published highly correlated ab initio potential energy surface. Collisional cross-sections among the first 41 rotational levels of SiS are calculated for kinetic energies up to 2500 cm−1. State to state rate coefficients are calculated for temperatures ranging from 5 up to 300 K. SiS rate coefficients are dependent on the initial rotational level of the H2 molecule. The rate coefficients for collisions with H2(j= 0) are significantly lower than the rate coefficients for collisions with H2(j= 1) and H2(j= 2). The strongest collision-induced rotational SiS transitions are the transitions with Δj= 2 for collisions with H2(j= 0) and the transitions with Δj= 1 for collisions with H2(j= 1) and H2(j= 2).