Rotational de-excitation of HNCS by He: Potential energy surface, collision cross-sections, and rate coefficients

ABSTRACT In order to determine the abundance of the HNCS molecule in the interstellar medium, theoretical data are required. Using an explicitly correlated ab initio approach, we construct the potential energy surface (PES) of the HNCS-He complex. This PES is implemented in dynamic calculations to determine rate coefficients of the rotational de-excitation of HNCS by collision with He. For transitions between the first 21 rotational levels of HNCS (up to $E_{\text{rot}}= 46\, \mathrm{cm}^{-1}$), the quantum close-coupling method is used for total energy ${<}100\, \mathrm{cm}^{-1}$, while the coupled state technique is used for energies up to $500\, \mathrm{cm}^{-1}$. Rate coefficients are calculated for temperatures between 2 and 30 K by averaging the cross-sections over a Maxwell–Boltzmann distribution. In this low-temperature range, $\Delta J = -1$ transitions are found to be slightly dominant. These temperatures are sufficient for modeling HNCS excitation in TMC-1, as its kinetic temperature does not exceed 20 K. Overall, a propensity is observed for odd transitions over even transitions, like for many other prolate or rod-like molecules. The HNCS rate coefficients are compared with those of its isomer HCNS and the valence-isoelectronic molecule HNCO for collision with He and H$_2$. The rate coefficients for HNCS are in good agreement with those of its linear HCNS isomer, but compared to those of HNCO, they are found to be sufficiently different (up to a factor of 3) to warrant separate computations, if quantitative ISM assessments are necessary.