Search

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.

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).

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.

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.

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.