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Licentiate thesis: Anion processes from the lab to ionospheres
  Thesis defense

Wednesday 19 October 2016
from 13:00 to 15:00
at FA:31
Speaker : Fredrik Lindén (Stockholm university, Department of Physics)
Abstract : Anion processes have been in the focus of interest during the last decades, especially due to their importance in planetary ionospheres. Previous observations done by the Cassini Plasma Spectrometer, detected a multitude of heavy neutral and ionic molecules in the ionosphere of Saturn's moon Titan. Unfortunately, only the lighter anions could be identified as CN-, C3N- and C5N-. It has been believed that the ion-neutral reactions involving these cyano anions could explain the reaction mechanism leading to heavier negatively charged anions. In this thesis we present experimental investigations of the reaction of C3N - with C2 H2, using three different guided ion beam mass spectrometry setups. Complementary, we performed ab inito calculations at CCSD(T)/6-311++G** level of theory. These calculations provide a detailed picture of the different reaction pathways. Unfortunately our computations yielded that all these pathways possess high reaction barriers and the above-mentioned process therefore an unlikely reaction to occur in Titans atmosphere. We also performed an investigation using velocity map imaging spectrometry to study the attack mechanism of CN- on halogenated hydrocarbons. In this experiment we studied the nucleophilic substitution 𝐶𝑁− + 𝐶𝐻3𝐼 = { 𝐶𝐻3𝑁𝐶 + 𝐼− 𝐶𝐻3𝐶𝑁 + 𝐼− Akademisk avhandling för avläggande av licentiatexamen vid Stockholms universitet, Fysikum Licentiatseminariet äger rum 19 oktober kl 13:00 i sal FA:31, Fysikum, Albanova universitetscentrum, Roslagstullsbacken 21, Stockholm. The energy distribution at low collision energies reveals isotropic scattering of I- which suggests that the process proceeds via a long-lived transition state complex, whereas at higher collision energies the backward scattering mechanism becomes more dominant. Additionally ab inito calculations were carried out, and even though the reaction energy difference between the two attack mechanisms is quite considerable (approximately 1 eV), we could not identify the ratio between the number of reactions involving an attack from the nitrogen atom leading to methylisocyanide (CH3NC) versus the one proceeding via an attack from carbon atom forming acetonitrile (CH3CN). Furthermore, I present a general semi-classical model that describes the forces and electron-transfer processes between two dielectric spheres in vacuum. The model spheres may be charged, may have different radii and different dielectric constants. Here, the model is used to calculate the so called Langevin cross sections for fusion of two spheres and charge exchange cross sections between two charged spheres. The model may be used to predict properties of reactions relevant in the chemistry of astronomical objects like dark clouds, circumstellar envelopes, protoplanetary disks and star-forming regions.

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