Reactions of small negative ions with O 2(a 1Δ g) and O 2(X 3Σ g−)

Abstract

The rate constants and product ion branching ratios were measured for the reactions of various small negative ions with O 2(X 3Σ g −) and O 2(a 1Δ g) in a selected ion flow tube (SIFT). Only NH 2 − and CH 3O − were found to react with O 2(X) and both reactions were slow. CH 3O − reacted by hydride transfer, both with and without electron detachment. NH 2 − formed both OH −, as observed previously, and O 2 −, the latter via endothermic charge transfer. A temperature study revealed a negative temperature dependence for the former channel and Arrhenius behavior for the endothermic channel, resulting in an overall rate constant with a minimum at 500 K. SF 6 −, SF 4 −, SO 3 − and CO 3 − were found to react with O 2(a 1Δ g) with rate constants less than 10 −11 cm 3 s −1. NH 2 − reacted rapidly with O 2(a 1Δ g) by charge transfer. The reactions of HO 2 − and SO 2 − proceeded moderately with competition between Penning detachment and charge transfer. SO 2 − produced a SO 4 − cluster product in 2% of reactions and HO 2 − produced O 3 − in 13% of the reactions. CH 3O − proceeded essentially at the collision rate by hydride transfer, again both with and without electron detachment. These results show that charge transfer to O 2(a 1Δ g) occurs readily if the there are no restrictions on the ion beyond the reaction thermodynamics. The SO 2 − and HO 2 − reactions with O 2(a) are the only known reactions involving Penning detachment besides the reaction with O 2 − studied previously [R.S. Berry, Phys. Chem. Chem. Phys., 7 (2005) 289–290].