Dissociative electron attachment to CO2 produces molecular oxygen

Until recently, it was widely regarded that only one reaction pathway led to the production of molecular oxygen in Earth's prebiotic primitive atmosphere: a three-body recombination reaction of two oxygen atoms and a third body that removes excess energy. However, an additional pathway has recently been observed that involves the photodissociation of CO2 on exposure to ultraviolet light. Here we demonstrate a further pathway to O2 production, again from CO2, but via dissociative electron attachment (DEA). Using anion-velocity image mapping, we provide experimental evidence for a channel of DEA to CO2 that produces O2(X(3)Σ(-)g) + C(-). This observed channel coexists in the same energy range as the competitive three-body dissociation of CO2 to give O + O + C(-). The abundance of low-energy electrons in interstellar space and the upper atmosphere of Earth suggests that the contributions of these pathways are significant and should be incorporated into atmospheric chemistry models.