The Escape of Ions From Planetary Atmospheres by Coulomb Explosion of Molecular Dications

The characterization of fundamental properties of multiply charged molecular ions, such as energetics, structure, stability, lifetime and fragmentation dynamics, is of great interest to understand and to model the behavior of gaseous plasmas as well as ionosphere and astrophysical environments. The measurements of the Kinetic Energy Released (KER) and Angular Distributions (AD) for ions originating from dissociations reactions, induced by Coulomb explosion of doubly charged molecular ions (molecular dications) produced by double photoionization of CO2, C2H2 and N2O molecules of interest in planetary atmospheres, are reported. The KER and AD measurement as a function of the ultraviolet (UV) photon energy in the range of 30-65 eV are extracted from the electron-ion-ion coincidence spectra obtained by using tunable synchrotron radiation coupled with ion imaging techniques at the ELETTRA Synchrotron Light Laboratory Trieste, Italy. These experiments, coupled with a computational analysis based on a Monte Carlo trajectory simulation, allow assessing the probability of escape for simple ionic species in the upper atmosphere of Mars and Titan. The KER measured for H+, C+, CH+, CH2+, N+, O+, CO+, N2+ and NO+ fragment ions are ranging between 1.0 and 5.5 eV, and these translational energies are large enough to allow these ionic species in participating in the atmospheric escape from Mars and Titan into space. These studies help in understanding important details about the physical chemistry of planetary ionospheres. In the case of Mars we suggest a possible explanation for the observed behavior of the O+ and CO22+ ion density profiles.