Three different stable oxygen forms (O, O2, and O3) play important roles in atmosphere chemistry and human activities on Earth. It is known well that molecular oxygen (O2) is produced in the photosynthesis of green plants. However, it is widely believed that before the rise of oxygenic photosynthesis in Earth’s prebiotic primitive atmosphere,O2 molecules were produced only in an abiotic pathway, namely the three-body recombination reaction O + O + M → O2 + M (where M is a third body to carry off the excess energy in the reaction). Recently, a research group of University of Science and Technology of China (USTC) at Hefei found a new production mechanism of O2, and their study was published on Nature Chemistry (
http://dx.doi.org/10.1038/nchem.2427).
Fragmentation processes of carbon dioxide anion formed by electron attachment. /Coutesy by TIAN's group
The researchers at USTC studied the dissociative electron attachment (DEA) to carbon dioxide molecule (CO2), using the electron-molecule crossed beam and anion velocity image mapping techniques. They identified two reaction pathways e¯ + CO2 → C¯ + O2 and C¯ + O + O, where the molecular O2 at the stable ground state can be produced by the electron attachment with the kinetic energies less than 20 eV. They further elucidated the dependence of O2 production efficiency on the electron attachment energy and the important roles in the atmospheric reactions of the free O atoms and molecular O2 as the products of DEA process.
The researchers also stress the essentials of DEA process in the atmosphere chemistry because there are a lot of low-energy free electrons and CO2 molecules in the upper spaces of planets (for examples: Mars, Venus, Earth, and so on). Prof. Shan Xi Tian, the supervisor of this study, said ‘the DEA process may contribute much more to the O2 production than the three-body recombination and the direct photodissociation mechanisms (the latter was reported by a USA research group in 2014). Our study may refresh many views on interstellar chemistry’.
(HFNL)
