Molecular Dications in Planetary Atmospheric Escape

Abstract

Fundamental properties of multiply charged molecular ions, such as energetics, structure, stability, lifetime and fragmentation dynamics, are relevant to understand and model the behavior of gaseous plasmas as well as ionosphere and astrophysical environments. Experimental determinations of the Kinetic Energy Released (KER) for ions originating from dissociations reactions, induced by Coulomb explosion of doubly charged molecular ions (molecular dications) produced by double photoionization of CO 2 , N 2 O and C 2 H 2 molecules of interest in planetary atmospheres, are reported. The KER measurement as a function of the ultraviolet (UV) photon energy in the range of 28-65 eV was 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, Venus and Titan. The measured KER in the case of H + , C + , CH + , CH 2 + , N + , O + , CO + , N 2 + and NO + fragment ions range between 1.0 and 5.5 eV, being large enough to allow these ionic species to participate in the atmospheric escape from such planets into space. In the case of Mars, we suggest a possible explanation for the observed behavior of the O + and CO 2 2+ ion density profiles.

The Escape of O^+ and CO^+ Ions from Mars and Titan Atmospheres by Coulomb Explosion of CO22+ Molecular Dications

“…In the case of the double VUV photoionization of CO 2 molecules (studied in the photon energy range of 34-50 eV), the fragment product ions O + and CO + are characterized by a translational energy ranging between 1.0-5.0 and 0.4-3.0 eV, respectively, that is large enough to allow their escape process from the upper atmospheres of Mars (in the case of O + ions) and Titan (for both O + and CO + ions). Moreover, these studies could be helpful in understanding important details about the chemistry of planetary ionospheres [40][41][42], as in the case of Mars where we were able to propose a possible rationalization of the observed behavior of the O + and CO…”

Molecular Fragmentation of Acetylene by VUV Double Photoionization

“…Finally, it has to be noted that the translational energy content of each H + , C + , CH + and CH2 + product ion is quite big, having an average value ranging between 4.0, 2.2, 2.5 and 2.0 eV, respectively. This kinetic energy is sufficient to allow this species participating in the atmospheric escape from Mars and Titan where they are characterized by a typical escape energy of 0.13 and 0.02 eV (in the case of H + ), 1.5 and 0.28 eV (in the case of C + ), 1.6 and 0.30 eV (for CH + ), 1.8 and 0,32 eV (for CH2 + ), respectively [17,21].…”

“…For such a reason molecular dications are considered as exotic species, and when they are formed in planetary ionospheres the possibility to generate dissociative products with a kinetic energy of several eV, allows these ionic fragments to reach sufficient velocity to escape into space. Therefore, double ionization processes can in principle contribute to the continuous erosion of the atmosphere of some planets of the Solar System, like Mars and Titan (the largest satellite of Saturn), as discussed in recent papers [21][22][23]. In the present work is discussed the production of C2H2 2+ molecular dications by Double Photoionization (DPI) of acetylene molecules characterizing its microscopic dynamical evolution towards the two-body fragmentation channels by the determination of: i) the angular distribution of product ions; ii) the kinetic energy with which they are produced by Coulomb explosion.…”

Bond-forming and electron-transfer reactivity between Ar2+ and O2

“…18 Indeed, the influence of the unimolecular chemistry of molecular dications in atmospheric erosion has been recently recognized. [19][20][21][22] This paper presents a detailed investigation of the interactions between Ar 2+ and O 2 , giving information on dicationic energetics, reactivity and the associated reaction mechanisms. This detailed information allows a better understanding of the relevance and influence of Ar 2+ /O 2 collisions in planetary environments.…”