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., \({{{\rm{O}}}}_{2}^{+}\) 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 \({{{\rm{O}}}}_{2}^{+}\) with O− mutual neutralisation reaction results predominantly in dissociation of the \({{{\rm{O}}}}_{2}^{+}\) molecule. Three competing reaction pathways yields both O(3P) (84%) and O(1D) (16%) products, but no O(1S) products. Analysis of the momentum-correlated dynamics of the reaction reveals the dominance of two-step mechanisms involving the 3pλu and 3sσg Rydberg states of O2. Furthermore, use of the 16,18 \({{{\rm{O}}}}_{2}^{+}\) isotopologue shows that the reaction products strongly depend on the vibrational levels of the \({{{\rm{O}}}}_{2}^{+}\) ion for the channel leading to two O(1D) products.
