Stereoselectivity in Autoionization Reactions of Hydrogenated Molecules by Metastable Noble Gas Atoms: The Role of Electronic Couplings

Abstract

Focus in the present paper is on the analysis of total and partial ionization cross sections, measured in absolute value as a function of the collision energy, representative of the probability of ionic product formation in selected electronic states in Ne*-H2 O, H2 S, and NH3 collisions. In order to characterize the imaginary part of the optical potential, related to electronic couplings, we generalize a methodology to obtain direct information on the opacity function of these reactions. Such a methodology has been recently exploited to test the real part of the optical potential (S. Falcinelli et al., Chem. Eur. J., 2016, 22, 764-771). Depending on the balance of noncovalent contributions, the real part controls the approach of neutral reactants, the removal of ionic products, and the structure of the transition state. Strength, range, and stereoselectivity of electronic couplings, triggering these and many other reactions, are directly obtained from the present investigation.

Molecular Fragmentation of Acetylene by VUV Double Photoionization

“…The results indicate a threshold for double ionization of acetylene just below 31.9 eV, in good agreement with the previous determnation of 31.7 eV [37]. Above this photon energy value, reactions (1) and (3) occur with a mechanism that appears to involve a few elementary steps through a limited number of electronic states of the intermediate C2H2 2+ dication. In contrast, reaction (2), occurs with a different microscopic mechanism involving several electronic states of the intermediate dication, and the competition of more dissociation channels.…”

“…In gas phase ionization processes can be induced by different collisional processes involving energetic photons, electrons or excited metastable neutral species (in this case we have the so called collisional autoionization, or Penning ionization reactions [1,2]). All ionization reactions play an important role in several phenomena occurring in low energy ionized plasmas [3], in the upper atmosphere of planets, where they govern the chemistry of ionospheres [4,5], and in electric discharges [6,7].…”

The Selective Role of Long‐Range Forces in the Stereodynamics of Ion–Molecule Reactions: The He++Methyl Formate Case From Guided‐Ion‐Beam Experiments

“…To account for extension and symmetry of the electron-density distributionsf or the involved MOs (the 7a' and 8a' orbitals are both extremely narrow and have an ode in the yz plane;s ee Figure5b), the coupling term H i is represented as ac onstant value H 0 modulated by a( cos f) 2 term. [76][77][78] This chosen representation brings the coupling to zero for geometries (f = 908, À908)g iving null overlap between the atomic so rbitalo fH e + and the involved MOs, but maximises the coupling maximum for geometries (f = 08,1 808)g iving the best overlap( corresponding to maximum electron density in the involved MOs). To tal crosss ections calculated by averaging cross sections for an appropriate number of (f, q)c ouples indicated ifferent but equallyp robabled irections of approach for He + on the whole sphere surrounding the MF molecule.…”

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…”