Dissociation of the CO2 dications in the upper atmospheres of planets - a new route of CO+ and O+ formation

The first observation of the CO22+ 
dication formed by electron impact ionization of carbon dioxide was reported in
1961. Since that experiment, the double ionization of CO2 
has been studied in several laboratories. These kinds of processes are of great interest because of the involvement 
of CO2 in several atmospheric phenomena of the Earth and
 of other planets and in plasma environments. In Mars’ atmosphere, where CO2 is the main component, the importance of
the CO22+ dication and its dissociation has been recently demonstrated. In this contribution we present a double photoionization of CO2 molecules studied in the 34-50 eV photon energy range, by the use of synchrotron radiation and detecting electron-ion and electron-ion-ion coincidences. The experiment has been carried out at the synchrotron light laboratory ELETTRA (Trieste, Italy) by the use of the ARPES end station of the Gasphase Beamline. The CO2 molecular beam and the vuv light beam cross at right angles, with the light polarization vector being parallel to the synchrotron ring plane and perpendicular to the time-of-flight direction of detected ions. 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. In the photon energy range investigated, the double dissociative photoionization reaction of CO2 leads to an isotropic distribution of CO+ and O+ product ions with respect to the polarization vector
of the light. When the photon energy increases, the distribution of products becomes anisotropic, with the two ions preferentially emitted along the direction of the light polarization vector. This implies that the molecule photoionizes when oriented parallel to that direction and also that the CO22+ dication just formed dissociates in a time shorter than its typical rotational period. At low photon energy, the CO+ and O+ product ions separate predominantly with a total kinetic energy 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 kinetic energy between 5 and 6 eV is dominant.