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PhD Thesis: Mechanisms of Anion Reactions from the lab to ionospheres
  Thesis defense

Monday 05 February 2018
from 14:00 to 17:00
at FA32
Speaker : Fredrik Lindén (Stockholm University, Department of Physics)
Abstract : A multitude of heavy neutral and ionic molecules have been discovered by the Cassini Plasma Spectrometer in the ionosphere of Saturn's largest moon Titan. However, only three cyano anions were explicitly identified there, namely CN-, C3N- and C5N-. The identity of the heavier anions, which show an abundance maximum at m/z 1000, could, however, not be elucidated and there is, so far, no clear explanation how these were generated. We investigated the reaction of the cyanide anion with methyl iodide using a velocity map imaging spectrometer setup and ab initio calculations. The data indicate a dominant direct rebound mechanism and a high internal excitation of the neutral product. According to the ab initio calculation two possible reaction pathways were expected, but in the experiment the two channels turned out to be indistinguishable due to low resolution. We also studied the reaction between C3N- and acetylene using three different experimental setups: a triple quadrupole mass spectrometer, a tandem quadrupole mass spectrometer, and the ''CERISES'' guided ion beam apparatus. The reaction showed three primary reaction pathways leading to C2H-, CN-, and C5N-. The production of C2H- could either happen via proton transfer or via formation of an adduct. The appearance of CN- could be explained by a reaction sequence involving an intermediate adduct but also via collision induced dissociation. Even though ab initio calculations predict two exoergic pathways leading to CN- and C5N-, all products are only accessible via energy barriers above 1 eV. In addition, we investigated the reaction between C5N- and acetylene. Also in this case the experimental and theoretical studies revealed that all reaction pathways proceed via energy barriers well above 1 eV. The sole exoergic pathway leading to C7N- has an energy barrier of 1.91 eV. Since the chemistry in dark interstellar clouds and planetary ionospheres is restricted to exoergic reactions with energy barriers less than 20 meV or proceed in a barrier-less manner (Vuitton et al. Planetary and Space Science 57, 1558-1572 (2009)), none of the observed pathways are feasible growth mechanism in those environments. We also performed investigations of reactions between charged clusters with and without barriers using electrostatic models. This led to the development of both approximate and exact expressions, which describe the sphere-sphere interaction and the electron transfer from a (neutral or charged) dielectric sphere to another charged dielectric sphere. The exact solutions include sums that describe polarization effects to infinite orders. However, we have shown that these infinite sums can be simplified, and that these approximations can be applied to calculate the charge transfer cross-sections and Langevin-type cross-sections. Keywords: ab initio, anion, complex molecules, cross section, electron transfer, heavy ions, ionosphere, nitriles, quantum chemistry, spectroscopy.

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