CHARGE-TRANSFER AND COLLISION-INDUCED DISSOCIATION REACTIONS OF OCS2+ AND CO2(2+) WITH THE RARE-GASES AT A LABORATORY COLLISION ENERGY OF 49 EV

Product channels for the reactions of OCS2+ and CO22+ with each of the rare gases are determined at a laboratory collision energy of 49 eV. A beam of dications is generated using electron impact ionization and mass selection by a quadrupole mass spectrometer. The dication beam is focused into a collision region and reaction products are monitored using a time-of-flight mass spectrometer. In addition to rare gas ions, we observe S+, CO+, and OCS+ as products from the reaCtionS of OCS2+; O+, CO+, and CO2+ are detected as products from reactions of CO22+. The relative yields of these product ions are measured directly. For both dications, the total reaction cross section increases dramatically as the collision partner is varied from He to Xe. OCS2+ reacts with He and Ne almost exclusively by collision-induced dissociation, while Ar, Kr, and Xe react predominantly by charge transfer. The charge transfer reaction of OCS2+ with Ar populates the stable ground state of the OCS+ ion, while reactions with Kr and Xe populate dissociative electronic states of OCS+ resulting in the formation of S+ ions. CO22+ reacts with He principally by collision-induced dissociation. Charge transfer reactions occur when CO22+ reacts with Ne and Ar, and these reactions populate stable states of CO2+. Kr and Xe react with CO22+ principally by charge transfer, forming unstable states of CO2+ ion which dissociate to give O+ or CO+ ions. The variations in charge transfer reactivity are modeled successfully using Landau-Zener theory.