Vibrationally-dependent molecular dynamics in mutual neutralisation reactions of molecular oxygen ions

Product distributions and dynamics of low-collision-energy mutual neutralisation reactions involving even simple molecular ions are largely unknown. Reactions which involve oxygen ions, e.g., O₂⁺ O 2 + with O − , are expected to be important in atmospheric phenomena such as sprites and in high-pressure air or oxygen discharges. Here we show, by combining cryogenically stored-and-merged ion beams with coincident product-imaging techniques, that the O₂⁺ O 2 + with O − mutual neutralisation reaction results predominantly in dissociation of the O₂⁺ O 2 + molecule. Three competing reaction pathways yields both O( 3 P) (84%) and O( 1 D) (16%) products, but no O( 1 S) products. Analysis of the momentum-correlated dynamics of the reaction reveals the dominance of two-step mechanisms involving the 3p λ u and 3s σ g Rydberg states of O 2 . Furthermore, use of the 16,18 O₂⁺ O 2 + isotopologue shows that the reaction products strongly depend on the vibrational levels of the O₂⁺ O 2 + ion for the channel leading to two O( 1 D) products.