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

The reactions of He+ with OCS and CO2 have been studied at collision energies between similar to k(B) & sdot; 200 mK and similar to k(B) & sdot; 30 K by merging a beam of Rydberg He atoms with rotationally cold (similar to 3.5 K) seeded supersonic expansions containing either OCS or (CO2)-C-13 or a mixture of OCS (mole fraction 23.2%) and (CO2)-C-13 (76.8%). The observed product ions of the He+ + (CO2)-C-13 and He+ + OCS reactions are (CO+)-C-13, and CS+ and CO+, respectively. The He+ + OCS capture rate coefficient increases by similar to 75% with decreasing collision energy over the investigated range, whereas that of He+ + (CO2)-C-13 decreases by similar to 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 (CO2)-C-13. From the relative product-ion yields observed when using the mixture of OCS and (CO2)-C-13, the He+ + OCS collisions are inferred to be similar to 20% more reactive than those between He+ and (CO2)-C-13. The comparison of the calculated thermal rate coefficients with earlier experiments suggests that about half of the He+ + (CO2)-C-13 collisions are reactive.