Double Photoionization of Water and Dissociative Electron Attachment to CF4

Abstract

This thesis is an amalgam of two largely different experiments. While the exper- imental apparatus are similar, the incident particles, fundamental interactions, and dissociation dynamics of the two systems are entirely different. They are tied together via momentum spectroscopy, occasionally called the ”momentum microscope”. The two experiments each represent a fundamental pillar of the modern scientific regime. For one, it is an observational first, producing an experiment that no other research group has produced before. For the other, it is a check on reproducibility, an effort by the scientific community to validate its efforts.

0.1 Water Experiment

The first of the two experiments included in this thesis is the double photoion- ization of water by 57 eV linearly polarized photons. A 57 eV photon collides with a water molecule and excites two molecular bonding electrons into the continuum. The remaining dication, H2O2+, is generally considered unstable and has not been observed in the laboratory. The unstable dication breaks apart, according to the multi-dimensional potential energy surface(s) available to it. The momentum for each of the charged particles - two protons and two electrons - are measured in co- incidence. The measurement of the two protons allows for the full orientation of the water molecule prior to dissociation. Prior to the writing of this thesis, this has only been achieved in a ”recoil axis” frame, aligning two fragments of a complex molecule that breaks apart upon one bond, including the simplest case of a diatomic molecule. The full three dimensional momentum resolution of the entire molecule, and its pho- toelectrons, is new terrain in momentum spectroscopy. The energies of the recoil ions and electrons, as well as their angular distributions, are measured and analyzed in an attempt to match the repulsive dication states of the water molecule to the asymptotic fragment configurations in both two- and three-body reaction pathways. The water experiment was conducted at the ALS with other members of the COLTRIMS collaboration from Kansas State University, University of Frankfurt, and Auburn University. The data was given to the author of this thesis as a training exercise in data analysis - training on the actual acquisition of data using COLTRIMS came later, in the second experiment of this thesis. The details of the experimental apparatus, the excitation of the dication, and its subsequent decay are the focus of the first two chapters of this thesis.

0.2 CF4 Experiment

The second experiment is dissociative electron attachment to C F4. Electron at- tachment is an electron scattering interaction wherein a low energy electron, typically from 0-15 eV in energy, collides with a molecule and is trapped in a local potential minimum, a so-called Shape Resonance. Alternately, the electron can collide with the molecule and couple with some internal degree of freedom, forming a negative ion, a so-called Feshbach Resonance. These thesis chapters focus on the situation in which the electron attachment leads to a dissociative negative ion state, with inci- dent electron energies ranging from 5.5 eV to 9 eV. The angular distributions found match, at least qualitatively, those of previous experiments, although the precise partial wave analysis of those distributions disagree slightly with the literature. The most interesting data, those taken as the primary motivation for the experiment, are the KER measured as a function of the incident electron energy for the reaction pathway leading to C F3−. These results disagree with the most recent results in the literature which, on their own, disagreed with all the previous results in measurement of this quantity. The maintenance, repair, and troubleshooting of a COLTRIMS experiment are rigorous endeavors, and the C F4 experiment gave the author of this dissertation the opportunity to learn the minutiae of the complex experimental procedure. This process took place, chronologically speaking, well after the analysis of the water experiment was under way. While this ordering of training may initially appear backwards, it was an invaluable layering of technical training, granting the author many opportunities to revisit the skills required as a physicist, from deep literature searches to vacuum pump maintenance to data analysis.