2005ApJ...631..653M

Branching ratios and absolute cross sections have been measured for the dissociative recombination of HCS⁺ and OCS⁺ at the CRYRING ion storage ring. In the case of OCS⁺, the channel leading to CO+S (83%) dominates, whereas the other exoergic pathways leading to CS+O (14%) and C+SO (3%) are of lesser importance. In the case of HCS⁺, fracture of the CS bond is predominant (81%), with the production of H+CS accounting for the remainder (19%). The cross section of the reaction could be fitted by the expressions σ=1.41x10^–15E(eV)^–1.11 and 4.47x10^–16E(eV)^–1.14 cm² for HCS⁺ and OCS⁺, respectively. The derived energy dependences of the thermal reaction rate coefficients can be fitted by k(T)=9.7x10^–7(T/300)^–0.57 and 3.5x10^–7(T/300)^–0.62 cm³ s^–1 for HCS⁺ and OCS⁺, respectively. We use these data to perform model calculations on the HCS⁺/CS abundance ratio in dark clouds and find that the models using the UMIST and Ohio State University databases have even more difficulty in accounting for the large observed ratio.