Doubly Charged Species in the Upper Atmosphere of Planets and Space

Doubly positive charged atomic or molecular ions, also named atomic/molecular dications, are formed whenever a chemical system has a high amount of energy available, as for example in flames. We can find these species in low concentrations in many natural plasmas such as planetary ionospheres, interstellar clouds and comet tails [1,2]. Astronomers found ionized gas containing H2O+, CO+, CN+, and CO2+ molecular dication in the tail of Hyakutake comet in 1996. In the early 2000s, N22+ and CO22+ were predicted to exist in the ionosphere of Mars, Venus and Titan (the largest moon of Saturn) where they have been detected with significant densities [3]. The role of molecular dications in the upper atmosphere of planets has been investigated by our and other groups, pointing out the possibility for such ionic species to be involved in the escape processes by Coulomb explosion [4]. Our group since 2001 is able to generate molecular dications in double photoionization experiments by using tunable UV and EUV photon source at the Synchrotron Facility of Elettra, Basovizza (Trieste). Many molecular dications were produced and characterized during the last decade: N2O2+, C6H62+, C2H22+, CO22+. More recently, the double photoionization of propylene oxide (CH₃CHCH₂O), nitrosyl chloride (ClNO) and allene (CH2CCH2) has been investigated. They are molecular species very interesting for atmospheric chemistry and astrochemistry whose characterization was done at the beamlines “GasPhase” and “CircularPolarization” using the ARPES (Angle Resolved PhotoEmission Spectroscopy) end station exploiting the PEPIPICO (Photoelectron-Photoion-Photoion) coincidence technique coupled with the TOF (time-of-flight) mass spectrometry [5]. Results, obtained in the double photoionization of ClNO molecules, recording data from 24.0 up to 70.0 eV of photon energy, will be showed and discussed at the conference, highlighting the fragmentation microscopic dynamics of the intermediate ClNO2+ dication. In this regard, it was possible to extract the energy thresholds for several fragmentation channels produced by double photoionization of ClNO (see Fig. 1). They are the following: ClNO + hv → NO+ + Cl+ + 2e- hv ≥ 30.08 eV (1) → NCl+ + O+ + 2e- hv ≥ 34.5 eV (2) → O+ + N+ + Cl + 2e- hv ≥ 39.00 eV (3) → O+ + Cl++ N + 2e- hv ≥ 39.0 eV (4) → N+ + Cl++ O + 2e- hv ≥ 40.5 eV (5) → NO+ + Cl2++ 3e- hv ≥ 63.5 eV (6) The measured threshold energies reported above were obtained by accumulating 2-4 hours at each investigated photon energy with a 200 meV energy step. References [1] M. Larsson, W.D. Geppert, G. Nyman, Rep. Prog. Phys., 2012, 75, 066901. [2] M. Alagia, N. Balucani, P. Candori, S. Falcinelli, F. Pirani, R. Richter, M. Rosi, S. Stranges, F. Vecchiocattivi, Rend. Lincei Sci. Fis. Nat., 2013, 24, 53-65. [3] O. Witasse, O. Dutuit, J. Lilensten, R. Thissen, J. Zabka, C. Alcaraz, P.L. Blelly, S.W. Bougher, S. Engel, L.H. Andersen, et al., Geophys. Res. Lett., 2002, 29, 104. [4] S. Falcinelli, F. Pirani, M. Alagia, L. Schio, R. Richter, S. Stranges, F. Vecchiocattivi, Chem. Phys. Lett., 2016, 666, 1–6. [5] S. Falcinelli and M. Rosi, Molecules, 2020, 25, 4157.