Coulomb explosion of molecular dications and escape of fragment ions from planetary atmospheres

The dissociative double photoionization processes induced by VUV and EUV photons leading to the production of fragment ions with a high kinetic energy content could give in general an important contribution to the ion species escape from the atmosphere of some planets of the Solar System, like Venus, Mars and Titan. In fact, 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 of several eV, and therefore much larger than the limiting thermal escape velocity. In the case of the double photoionization of CO2, C2H2 and N2O molecules the fragment product ions O+, CO+, N+, N2+, NO+, CH2+, CH+, C+, H+ are characterized by a translational energy ranging between 1.0 and 5.5 eV (only for H+ the KER maximum value reaches 6.0 eV), that is large enough to allow their escape process from the upper atmospheres of Mars and Titan (see Table 1). It has to be noted that for CO+, N2+ and NO+, the measured KER ranges between 0.5-2.5 eV, 0.5-2.8 eV and 1.0-2.5 eV, respectively, allowing the possible escape only from the Titan atmosphere. Moreover, these studies are helpful in understanding important details about the chemistry of the planet ionospheres, like that of Mars where we were able to suggest a possible explanation for the observed behavior of the O+ and CO22+ ion density profiles.