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., $O_2^{+}$ 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 $O_2^{+}$ with O^- mutual neutralisation reaction results predominantly in dissociation of the $O_2^{+}$ 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 3pλ_u and 3sσ_g Rydberg states of O₂. Furthermore, use of the ^16,18 $O_2^{+}$ isotopologue shows that the reaction products strongly depend on the vibrational levels of the $O_2^{+}$ ion for the channel leading to two O(¹D) products.