Fine-structure excitation of CCS by He: Potential energy surface and scattering calculations.

The fine structure excitation of the interstellar CCS radical induced by collisions with He is investigated. The first potential energy surface (PES) for the CCS-He van der Waals complex is presented. It was obtained from a highly correlated spin unrestricted coupled cluster approach with single double and perturbative triple excitations. The PES presents two shallow minima of 31.85 and 37.12 cm for the linear (He facing S) and the nearly T-shaped geometries, respectively. The dissociation energy of the complex was calculated and found to be D = 14.183 cm. Inelastic scattering calculations were performed using the close-coupling approach. Cross-sections for transitions between the 61 first fine structure levels of CCS were obtained for energy up to 600 cm and rate coefficients for the 5-50 K temperature range were derived. This set of collisional data can be used to model CCS emission spectra in dark molecular interstellar clouds and circumstellar envelopes and enable an accurate determination of CCS abundance in these astrophysical media.