Cold reactions of He with OCS and CO: competitive kinetics and the effects of the molecular multipole moments.

The reactions of He with OCS and CO have been studied at collision energies between ∼ ⋅ 200 mK and ∼ ⋅ 30 K by merging a beam of Rydberg He atoms with rotationally cold (∼3.5 K) seeded supersonic expansions containing either OCS or CO or a mixture of OCS (mole fraction 23.2%) and CO (76.8%). The observed product ions of the He + CO and He + OCS reactions are CO, and CS and CO, respectively. The He + OCS capture rate coefficient increases by ∼75% with decreasing collision energy over the investigated range, whereas that of He + CO decreases by ∼40%. The analysis of the experimental results using an adiabatic-channel capture model indicates that these opposite collision-energy dependences of the rate coefficients arise from the interaction between the charge of the ion and the electric multipole moments of OCS and CO. From the relative product-ion yields observed when using the mixture of OCS and CO, the He + OCS collisions are inferred to be ∼20% more reactive than those between He and CO. The comparison of the calculated thermal rate coefficients with earlier experiments suggests that about half of the He + CO collisions are reactive.