Imaging Multi-Particle Atomic and Molecular Dynamics in Dissociative Electron Attachment to CF4 and Double Photoionization of NH3 Molecules
Type of DegreePhD Dissertation
Restriction TypeAuburn University Users
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The experimental process and scientific study of two molecular interactions is presented. The first experiment consist of the phenomenon of dissociative electron attachment, (DEA), to Carbon Tetrafluoride, CF4, with the second experiment consisting of single Photon Double Ionization, (PDI), of Ammonia, NH3. These two experiments, along with their respective experimental setups, calibration, analysis and results are detailed within this work. The initial experiment, the DEA to Carbon Tetrafluoride, CF4, is a scattering process where a free electron of low energy, typically under 20eV, collides with a molecule and is either coupled to the molecule and forms a negative ion, or is trapped in a potential minimum. These two processes are known as either Shape, or Feshbach resonances, and will be discussed in depth within this work. This experiment also works to explain the dissociation process and gives evidence of a sequential breaking of the C-F bonds through an intermediate CF3-* electronically excited anion. This intermediate step forms near-zero energy F- anions as confirmed by isotropic angular distributions as well the presence of a heavy CF3-anion imprinted onto the F- fragment distribution. Following the success of the DEA experiment introduced above, a new energy source involving linearly polarized photons was used to ionize atomic molecules. In this project, we measure the fragmentation channels following direct single Photon Double Ionization of NH3 where two photoelectrons and two protons are measured in coincidence using Three-Dimensional imaging.The breakup process following photoionization seeks to uncover the dication electronic states that correspond to either a sequential or concerted process by calling upon theoretical reports utilizing calculations on multi-reference configuration interactions of the dication potential energy surfaces. Evidence is given that the neutral NH fragment is ro-vibrationally excited, yielding much higher internal energy. Additionally, the energies of both the cations and electrons are measured, along with angular distributions in an effort to explain the energy sharing, bond angle breaks and symmetric behavior of the Ammonia molecule. The common denominator with these two experiments is the capture technique known as COLTRIMS, which stands for COLd Target Recoil Ion Momentum Spectroscopy. This approach provides a deep insight into the physics and chemistry of the fundamental interactions that drive the chemical process in molecular systems. The emphasis of this scientific branch is centered around low-energy dissociative electron attachment and multiple-ionization dynamics of small molecular targets. The dissociative electron attachment experiment took place at Auburn University whereas the Double Photoionization experiment was performed at the Advanced Light Source synchrotron in Berkeley, California and analyzed at Auburn University. The main goal for these experiments is to pursue the understanding of a new molecular species, building on prior work with other molecules utilizing similar techniques. This, in part with previous theoretical findings, seeks to pursue the research in this field and usher in new understanding for the scientific community.