Dissociation of the CO2 and C2H2 molecular dications: Their role in the upper atmospheres of planets

The double dissociative photoionization of CO2 and C2H2 molecules has been studied by using synchrotron radiation in the 34-50 eV photon energy range and detecting electron-ion and electron-ion-ion coincidences. These processes are of interest because of the involvement of CO2 and C2H2 in several atmospheric phenomena of the Earth and of other planets and in plasma environments. For example, in Mars' atmosphere, where CO2 is the main component, the importance of the CO22+ dication and its dissociation has been recently demonstrated. The experiments have been carried out at the synchrotron light laboratory ELETTRA (Trieste, Italy). In the case of CO2, three processes have been observed: (i) the formation of the CO22+ molecular dication, (ii) the production of a metastable (CO22+)* that dissociates, with an apparent lifetime of 3.1 µs, giving rise to CO+ and O+ ions, and (iii) the dissociation leading to the same products, but occurring with a lifetime shorter than 0.05 µs. At low photon energy, the CO+ and O+ product ions separate predominantly with a total kinetic energy release (KER) between 3 and 4 eV. This mechanism becomes gradually less important when the photon energy increases and, at 49 eV, a process where the two products separate with a KER between 5 and 6 eV is dominant. In the case of acetylene, the dissociation leading to C2H++H+ products occurs through a metastable dication with a lifetime of 108±22 ns, and a KER of about 4.3 eV. The reaction leading to CH2++C+ occurs in a time shorter than the typical rotational period of the acetylene molecules (of the order of 10−12 s) with a KER of ∼4.5 eV. The symmetric dissociation, leading to CH++CH+, exhibits a KER distribution with a maximum at ∼5.2 eV.