Temperature dependences for the reactions of O^- and O ₂^- with O₂(a¹Δ_g) from 200 to 700 K

Rate constants and product ion distributions for the O^- and O₂^- reactions with O₂(a ¹Δ _g) were measured as a function of temperature from 200 to 700 K. The measurements were made in a selected ion flow tube (SIFT) using a newly calibrated O₂(a ¹Δ_g) emission detection scheme with a chemical singlet oxygen generator. The rate constant for the O₂^- reaction is ∼7 × 10^-10 cm ³ s^-1 at all temperatures, approaching the Langevin collision rate constant. Electron detachment was the only product observed with O₂^-. The O^- reaction shows a positive temperature dependence in the rate constant from 200 to 700 K. The product branching ratios show that almost all of the products at 200 K are electron detachment, with an increasing contribution from the slightly endothermic charge-transfer channel up to 700 K, accounting for 75% of the products at that temperature. The increase in the overall rate constant can be attributed to this increase in the contribution the endothermic channel. The charge-transfer product channel rate constant follows the Arrhenius form, and the detachment product channel rate constant is essentially independent of temperature with a value of ∼6.1 × 10^-11 cm³ s^-1.