Double Ionization to CO₂ Produces Molecular Oxygen: A Roaming Mechanism

Abiotic oxygen formation predates photosynthesis, sustaining early chemical evolution, yet its elementary mechanisms remain contested. Here, we show the production pathways for molecular oxygen from doubly ionized carbon dioxide upon electron-impact. Through fragment ions and electron coincidence momentum imaging, we unambiguously determine the ionization mechanism by measuring the projectile energy loss in association with the C⁺ + O₂⁺ channel. Further potential energy and trajectory calculations enable us to elucidate the dynamical details of this fragmentation process, in which a bond rearrangement pathway is found to proceed via the structural deformation to a triangular intermediate. Moreover, we demonstrate a further roaming pathway for the formation of O₂⁺ from CO₂²⁺, in which a frustrated C-O bond cleavage leaves the O atom without sufficient energy to escape. The O atom then wanders around varied configuration spaces of the flat potential energy regions and forms a C-O-O₂⁺ intermediate prior to the final products C⁺ + O₂⁺. Considering the large quantities of free electrons in interstellar space, the processes revealed here are expected to be significant and should be incorporated into atmospheric evolution models.