Cold reactions of He+ with OCS and CO2: competitive kinetics and the effects of the molecular multipole moments

Abstract

The reactions of He⁺ with OCS and CO₂ have been studied at collision energies between ∼k_B ⋅ 200 mK and ∼k_B ⋅ 30 K by merging a beam of Rydberg He atoms with rotationally cold (∼3.5 K) seeded supersonic expansions containing either OCS or ¹³CO₂ or a mixture of OCS (mole fraction 23.2%) and ¹³CO₂ (76.8%). The observed product ions of the He⁺ + ¹³CO₂ and He⁺ + OCS reactions are ¹³CO⁺, 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⁺ + ¹³CO₂ 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 ¹³CO₂. From the relative product-ion yields observed when using the mixture of OCS and ¹³CO₂, the He⁺ + OCS collisions are inferred to be ∼20% more reactive than those between He⁺ and ¹³CO₂. The comparison of the calculated thermal rate coefficients with earlier experiments suggests that about half of the He⁺ + ¹³CO₂ collisions are reactive.