Gas-surface interactions with sp 2 carbon in extreme environments
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Date
2018
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Montana State University - Bozeman, College of Letters & Science
Abstract
Molecular beam scattering experiments can determine the relative importance of reactive and non-reactive processes that occur when a surface is bombarded with high energy atoms and molecules. The mechanisms by which these processes proceed are inferred by analyzing the angle-resolved flux and energy distributions of the scattered products. The studies presented in this thesis have been conducted with a crossed molecular beams machine reconfigured for surface scattering. Two molecular beam sources were used. One uses a laser detonation process to produce high translational energy O atoms in the ground electronic state, and the other uses a supersonic expansion to produce continuous beams of N 2, nitromethane, or methyl formate. In the first two studies presented in this thesis, the oxidation of dynamics vitreous carbon and highly oriented pyrolytic graphite (HOPG) held at surface temperatures in the range of 800 - 2300 K by O atoms with a translational energy of ~ 500 kJ mol -1 are presented. These two studies revealed that the reactivity is suppressed at high temperature because O atoms desorb from the surface before they react to form CO and CO 2. Even though the translational energy of the O atoms was high, the surface reactions proceeded primarily through reactions that occurred in thermal equilibrium with the surface. The third study focuses on the scattering dynamics of O, O 2, and Ar with the surfaces of a gold thin-film, SiO 2, and HOPG. The results of the experiments were used to evaluate the efficacy of a proposed gas concentrator. The strong forward scattering on the HOPG surface made it the most suitable surface for the gas concentrator. The fourth study examines the non-reactive scattering dynamics of N 2 with HOPG. At high surface temperature, the residence time of N 2 is too short for the molecule to fully accommodate to the surface. Thus, even if the molecule suffers multiple collisions with the surface, it will scatter into the vacuum before it can come into thermal equilibrium with the surface. The results have been used in conjunction with theoretical calculations by a collaborator to investigate the relationship between the potential energy surface and the scattering dynamics. In order determine the usefulness of an HOPG concentrator with complex molecules, the scattering dynamics of methyl formate and nitromethane on HOPG were studied. These molecules do not shatter upon impact with the surface and they both scatter strongly in the forward direction through direct and indirect mechanisms, suggesting that the proposed HOPG concentrator should perform as desired. In all studies described in this thesis, the fundamental gas-surface scattering dynamics were elucidated from molecular beam experiments, and these fundamental results have direct links to modeling the performance of hypersonic vehicles and designing a gas concentrator for mass spectrometry in tenuous atmospheres.