Disentangling vibrationally-dependent molecular dynamics in mutual neutralisation reactions

Abstract Products and dynamics of mutual neutralisation (MN) reactions involving even simple molecular ions are largely unknown. Combining cryogenically stored-and-merged ion beams with coincident product-imaging addresses this issue.Application to the reaction of O₂⁺ with O⁻, which occurs in atmospheric phenomena such as sprites and in high-pressure air or O₂ discharge, we find that their low-collision-energy MN always leads to dissociation of the O₂, governed by a two-step mechanism involving Rydberg states in O₂ which couple to the final dissociation channels through Rydberg-valence interactions. We measure the partial kinetic-energy releases both in the initial e⁻-transfer and final dissociation steps which allows us to identify the Rydberg states. We find that the observed products depend on the vibrational levels of the O₂⁺ ion. This type of information has not been available before, and we believe that it will significantly advance theory and models of MN processes involving small molecules.