Molecular dications in planetary ionospheres

In the chemistry of planetary ionospheres molecules can interact with electromagnetic waves: γ, X rays and ultraviolet (UV) light. The dissociative double photoionization processes induced by VUV and EUV photons leading to fragment ions formation with a high kinetic energy content could give an important contribution to the ions escape from the atmosphere of some planets of the Solar System, like Venus, Mars and Titan. These processes occur via formation of intermediate molecular dications that can dissociate by Coulomb explosion towards the formations of two ion fragments having a kinetic energy released (KER) of several eV, and therefore much larger than the limiting thermal escape velocity. When CO2, C2H2 and N2O molecules are ionized using photons with an energy in the range of 30 to 65 eV, molecular fragmentation can be induced. These ionic species (CO+, O+, N2+, N+, NO+, H+, C+, CH+, CH2+ and C2H+) are characterized by a translational energy ranging between 1.0 and 5.5 eV, which is large enough to allow some of them to escape from the atmospheres of Mars and Titan. In the case of O+, we can conclude that the dissociative double photoionization of CO2 in the ionosphere of Mars can contribute to explain the observed behavior of the O+ ion density profile measured by the Viking 1 lander. Finally, the angular distributions of product ions with respect to the light polarization vector direction are discussed.