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A major challenge in the scientific research for strategies that use low-cost renewable energy is to design and develop heterogeneous /homogeneous catalysis processes that use waste CO2 to produce fuels in a circular economy regime. In this paper a theoretical and experimental study aiming at reusing CO2 and implementing a validated laboratory technology based on a prototype methanation reactor producing carbon neutral methane through the chemical conversion of CO2 waste flue gases using renewable energies, is presented. The first operational line of the work is the theoretical, computational and experimental treatment of elementary reactive and non-reactive molecular processes occurring inside the reactor in order to optimize its operating conditions and to identify possible technological improvements that are more compatible with the environment. Experimental determinations of methane yield by the reactor have been carried out using CO2 either taken from commercial bottles or produced from fermentation of wine and vegetable exhausted materials. To this end we have also undertaken a computational and experimental investigation of a new methanation pathway aimed at avoiding the use of the solid catalyst, by exploring mechanisms involving a plasma generation by electrical discharges or by vacuum ultraviolet (VUV) photons on CO2 + H2 gas mixtures. The measurements performed using a microwave discharge beam source developed in our laboratory gave useful indications on how to proceed to develop alternative solutions to the present Ni catalysed apparatus by resorting to a gas-phase-only process for the reduction of CO2 to CH4. These results demonstrate that the chemical reactivity of plasmas containing CO2 should be strongly increased thanks to the presence of CO+ and O+ ions having a very high kinetic energy. These ionic species are produced via Coulomb explosion of CO22+ molecular dications by the same process responsible for the erosion of the atmosphere of Mars.

Experimental determinations of the Kinetic Energy Released (KER) for ions, originating by Coulomb explosion of CO22+dications produced by double photoionization of CO2, are reported. The KER are measured by using synchrotron radiation in the range of 34–50eV coupled with electron-ion-ion coincidence technique.For O+and CO+, originating by Coulomb explosion of CO22+, KER are ranging between 1.0–5.0 and 0.4–3.0eV, respectively, being large enough to allow these ions in participating in the atmospheric escape from Mars and Titan into space. An explanation for the observed behavior of O+and CO22+ion density profiles on Mars atmosphere is proposed.

A photoelectron-photoion-photoion coincidence technique, using an ion imaging detector and tunable synchrotron radiation in the 18.0 – 37.0 eV photon energy range, inducing the ejection of molecular valence electrons, has been applied to study the double ionization of the propylene oxide, a simple prototype chiral molecule. Energy thresholds for the formation of different ionic products and the related branching ratios as a function of the photon energy has been measured. The main recorded two-body fragmentation channels yield C2H4+ + CH2O+, and C2H3+ + CH3+ product ions (66.70% and 18.70%, respectively). These new experimental data are relevant information for further experimental and theoretical investigations of oriented chiral molecules.A photoelectron-photoion-photoion coincidence technique, using an ion imaging detector and tunable synchrotron radiation in the 18.0 – 37.0 eV photon energy range, inducing the ejection of molecular valence electrons, has been applied to study the double ionization of the propylene oxide, a simple prototype chiral molecule. Energy thresholds for the formation of different ionic products and the related branching ratios as a function of the photon energy has been measured. The main recorded two-body fragmentation channels yield C2H4+ + CH2O+, and C2H3+ + CH3+ product ions (66.70% and 18.70%, respectively). These new experimental data are relevant information for further experimental and theoretical investigations of oriented chiral molecules.

Molecular dications are doubly charged cations of importance in flames, plasma chemistry and physics and in the chemistry of the upper atmosphere of Planets. Furthermore, they are exotic species able to store a considerable amount of energy at a molecular level. This high energy content of several eV can be easily released as translational energy of the two fragment monocations generated by their Coulomb explosion. For such a reason, they were proposed as a new kind of alternative propellant. The present topic review paper reports on an overview of the main contributions made by the authors’ research groups in the generation and characterization of simple molecular dications during the last 40 years of coupling experimental and theoretical efforts.

An experimental and computational investigation characterizing the processes following the double photoionization of the methyloxirane and N-methylformamide molecules has been reported. The double photoionization experiments have been performed at the Elettra Synchrotron Facility of Trieste (Italy). Preliminary data show: (i) in the case of methyloxirane, six different two-body fragmentation processes leading to CH+2/C2H4O+,CH+3/C2H3O+,O+/C3H+6, OH+/C3H+5,C2H+3/CH3O+,C2H+4/CH2O+ pairs of final ions; (ii) in the case of N-methylformamide, two main two-body fragmentation processes, leading to CH+3+CH2NO+ and H++C2H4NO+ . The threshold's energy for each dissociation channel is determined with the relative cross sections as a function of the investigated photon energy range. A careful analysis of recorded electron-ion-ion coincidence spectra mainly based on a Monte Carlo trajectory simulation is able to provide also the kinetic energy released (KER) distribution for the final ions of the investigated fragmentation reactions. These important experimental data are mandatory information to unravel the physical chemistry of the elementary processes induced by the interaction of photons, with simple relevant organic molecules: (i) the methyloxirane of astrochemical interest, being the first chiral molecule recently discovered in interstellar cloud Sagittarius B2; (ii) the N-methylformamide, being an important simple molecule containing the peptide bond, recently detected in the interstellar medium, in order to investigate its selective cleavage induced by UV photons. In the latter case, this can improve a deeper definition of formation/destruction routes in astrochemical environments of the more abundant formamide molecule.

An experimental and computational investigation characterizing the processes following the double photoionization of the methyloxirane and N-methylformamide molecules has been reported. The double photoionization experiments have been performed at the Elettra Synchrotron Facility of Trieste (Italy). Preliminary data show: (i) in the case of methyloxirane, six different two-body fragmentation processes leading to \( {\text{CH}}_{2}^{ + } /{\text{C}}_{2} {\text{H}}_{4} {\text{O}}^{ + } ,{\text{CH}}_{3}^{ + } /{\text{C}}_{ 2} {\text{H}}_{ 3} {\text{O}}^{ + } ,{\text{O}}^{ + } /{\text{C}}_{ 3} {\text{H}}_{6}^{ + } , {\text{ OH}}^{ + } / {\text{C}}_{ 3} {\text{H}}_{5}^{ + } ,{\text{C}}_{ 2} {\text{H}}_{3}^{ + } /{\text{CH}}_{ 3} {\text{O}}^{ + } ,{\text{C}}_{ 2} {\text{H}}_{4}^{ + } /{\text{CH}}_{ 2} {\text{O}}^{ + } \) pairs of final ions; (ii) in the case of N-methylformamide, two main two-body fragmentation processes, leading to \( {\text{CH}}_{3}^{ + } + {\text{CH}}_{2} {\text{NO}}^{ + } \) and \( {\text{H}}^{ + } + {\text{C}}_{2} {\text{H}}_{4} {\text{NO}}^{ + } \). The threshold’s energy for each dissociation channel is determined with the relative cross sections as a function of the investigated photon energy range. A careful analysis of recorded electron-ion-ion coincidence spectra mainly based on a Monte Carlo trajectory simulation is able to provide also the kinetic energy released (KER) distribution for the final ions of the investigated fragmentation reactions. These important experimental data are mandatory information to unravel the physical chemistry of the elementary processes induced by the interaction of photons, with simple relevant organic molecules: (i) the methyloxirane of astrochemical interest, being the first chiral molecule recently discovered in interstellar cloud Sagittarius B2; (ii) the N-methylformamide, being an important simple molecule containing the peptide bond, recently detected in the interstellar medium, in order to investigate its selective cleavage induced by UV photons. In the latter case, this can improve a deeper definition of formation/destruction routes in astrochemical environments of the more abundant formamide molecule.

A variety of phenomena, of apparently different natures, are described within a unifying picture, by properly isolating the role of charge/electron transfer as an interaction component triggering chemical reactivity. This basic quantity is isolated by analyzing, with advanced theoretical methods developed by our group, experimental findings characterized with different techniques, such as double photo-ionization spectra, scattering cross sections and auto-ionization reaction probabilities. Suitable rationalization of such phenomena appears to be crucial for modeling the selectivity of basic elementary processes occurring in systems at increasing complexity of fundamental/applied interest, such as plasmas, flames, interstellar media, planetary atmospheres and biological environments.

The measurements of the Kinetic Energy Release (KER) for various ionic species originating from two-body dissociations reactions, induced by double photoionization of CO 2 , C 2 H 2 and N 2 O neutral molecular precursors of interest in planetary atmospheres, are reported. The KER distributions as a function of the ultraviolet (UV) photon energy in the range of 30–65 eV (Vacuum and Extreme UV photons) are extracted from the electron–ion–ion coincidence spectra obtained by using tunable synchrotron radiation coupled with ion imaging techniques. This experimental method allows assessing the probability of escape for some simple ionic species in the upper atmosphere of Mars and Titan. In fact, the KER measured for H + , C + , CH + , CH 2 + , N + , O + , CO + , N 2 + and NO + fragment ions are ranging between 1.0 and 5.5 eV (only for H + the maximum value reaches 6.0 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 (for CO + , N 2 + and NO + , the measured KER of 0.5–2.5 eV, 0.5–2.8 eV and 1.0–2.5 eV, respectively, allows the possible escape only from the Titan atmosphere).

Fundamental properties of multiply charged molecular ions, such as energetics, structure, stability, lifetime and fragmentation dynamics, are relevant to understand and to 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 CO2, N2O and C2H2 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 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 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 + , CH2 + , N + , O + , CO + , N2 + and NO + fragment ions are ranging between 1.0 and 5.5 eV, being large enough to allow these ionic species in participating in the atmospheric escape from such planets into space.

The fragmentation of the CH 4 2+ dication has been studied in an electron impact experiment by Auger electron-ion and Auger electron-ion–ion coincidence measurements. Different states of the dications have been selected by changing the kinetic energy of the Auger electrons. Particular attention has been paid to the production of the CH 2 + and CH 3 + ions whose Kinetic Energy Release (KER) distributions and dissociation channels have been determined. Moreover, for the first time in an electron impact experiment, the C 2+ ion fragment has been identified.

The double Qphotoionization of N2O and C2H2 molecules by linearly polarized light in the 30-50 eV energy range has been studied by coupling ion imaging and electron-ion-ion coincidence techniques. In the case of N2O, for the two possible dissociative processes leading to N++NO+ and O++N-2+, anisotropic angular distributions of ionic fragments have been measured, indicating that N2O ionizes when its axis is parallel to the light polarization vector and the fragments are separating in a time shorter than the dication rotational period. In the case of C2H2, the two-body dissociation reactions producing C2H+H+ and CH2++C+ have shown almost isotropic angular distributions, indicating the double photoionization occurs when acetylene is mainly oriented perpendicularly to the light polarization vector. The analysis of results based on a Monte Carlo trajectory simulation provides: i) the metastable N2O2+ and C2H22+ dication lifetimes, ii) the kinetic energy release (KER) distribution for the final ions resulting from the Coulomb explosion, and iii) the anisotropy beta parameter values as a function of the investigated photon energy extracted from the measured ionic angular distributions.

The Kinetic Energy Release (KER) for various ionic species coming from two-body dissociations reactions, induced by double photoionization of CO2, C2H2 and N2O neutral precursors of interest in planetary atmospheres, are reported. The KER distributions as a function of the VUV photon energy in the range of 30-65 eV are extracted from the coincidences spectra obtained by using tunable synchrotron radiation and electron-ion-ion coincidences coupled with ion imaging techniques. This experimental method, coupled with a computational analysis of the data based on a Monte Carlo trajectoriy simulation, allows to discuss about the probability for some ionic species to escape the upper atmosphere of Mars and Titan. In fact, the KER measured for H + , C + , CH + , CH\(_{\rm 2}^{\rm +}\), N + , O + and CO + fragment ions are ranging between 1.0 and 6.0 eV, and these translational energy contents are large enough to allow these ionic species in participating in the atmospheric escape from Mars and Titan into space.

Quadrupole mass spectrometry has been used to detect BaX2+ (X = F, Cl, Br, I), SrCl2+, CaBr2+, and MgBr2+. Dications were produced by electron bombardment of an effusive beam containing the stable alkaline earth monohalide radical precursor. All species but MgBr2+ are predicted to be thermodynamically stable. MgBr2+ is likely to be metastable but with a lifetime sufficiently long to allow its detection in the millisecond timescale. Simple arguments are presented for the expected trends in stability.

Theoretical calculations of the energy and structure of dissociation product ions have been performed to provide detailed information about the dynamics of the charge separation reactions following the photoionization event of benzene molecules. The calculations are in agreement with the experimental results obtained in a study where a time-of-flight mass spectrometer with a position sensitive ion detector has been used in the dissociative double ionization of benzene by UV synchrotron radiation.

International audience; Reactions of the hydrocarbon dication C4H32+ with a series of atomic and molecular targets (Kr, Xe, N2, NO, NH3, C2H2, and CH4) were investigated in crossed-beam scattering and guided-beam (quadrupole–octopole–quadrupole, QOQ) experiments. Non-dissociative charge transfer (NDCT), leading to the product ion C4H3+, and proton transfer (PT), leading to the product ion C4H2+, were observed. In several systems, however, dissociative charge transfer (DCT) contributed significantly to the formation of the product ion C4H2+. Full scattering diagrams of the products C4H3 and C4H2+ were obtained for the reaction with acetylene. For the other systems, energy profiles of the products close to the angular maximum were obtained. The hydrocarbon products C4H2+ and C4H3+ were scattered mostly forward, suggesting an impulsive mechanism of their formation at the collision energies investigated; the only exception was proton transfer with H2, where the energy profile of C4H2+ indicated substantial backward scattering and formation of an intermediate. From relative translational energy distributions of the products, conclusions could be made about electronic states of the hydrocarbon cations formed in these reactions.

An application of previously introduced correlation formulas in terms of polarizabilities of interacting species permits an assessment of the relative role of dispersion and induction contributions to describe bond lengths and energies in some doubly charged ion-neutral systems (the dications of alkaline earth atoms and rare gases). An extension to ion-ion systems is also presented, on the basis of an empirical analysis of available data on alkali halides, in order to establish the role of both dispersion and induction interactions as compared to Coulomb attraction. An improved version of the Slater-Kirkwood formulation with an appropriate choice for the effective electron numbers is suggested to estimate the long range effective dispersion coefficients. The present approach provides a unifying framework for the characterization of bonds for systems which greatly vary in size and strength and recipes are suggested for interatomic potential energy curves. It is concluded that this approach is also relevant as a starting point for extensions to include anisotropies due both to orbital overlap for open shell partners and to mutual orientation when molecules are considered.

The bimolecular reactivity of the CO(2)(2+) dication with neutral CO(2) is investigated using triple quadrupole and ion-ion coincidence mass spectrometry. Crucial for product analysis is the use of appropriate isotope labelling in the quadrupole experiments in order to distinguish the different reactive pathways. The main reaction corresponds to single-electron transfer from the neutral reagent to the dication, i.e. CO(2)(2+) + CO(2) --2CO(2)(+); this process is exothermic by almost 10 eV, if ground state monocations are formed. Interestingly, the results indicate that the CO(2)(+) ion formed when the dication accepts an electron dissociates far more readily than the CO(2)(+) ion formed from the neutral CO(2) molecule. This differentiation of the two CO(2)(+) products is rationalized by showing that the population of the key dissociative states of the CO(2)(+) monocation will be favoured from the CO(2)(2+) dication rather than from neutral CO(2). In addition, two bond-forming reactions are observed as minor channels, one of which leads to CO(+) and O(2)(+) as ionic products and the other affords a long-lived C(2)O(3)(2+) dication.