Investigations of Ice Photodynamics: Molecular Dynamics Studies and an Apparatus for Solid Sample Velocity Map Imaging

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Crouse, Jeff
Photodissociation , Photodynamics , Molecular Dynamics , Spectroscopy , Time-of-Flight , Ice , Velocity Map Imaging
Water ice is prevalent in many environments, from the surface and atmosphere of Earth to molecular clouds in the interstellar medium. High energy photons initiate photoreactions via excitation of water or contaminant species within these environments. This thesis describes increasingly sophisticated Molecular Dynamics (MD) simulations applied to investigate photodynamics in amorphous solid water (ASW), ice XI, and ice Ih. Additionally, results from a velocity map imaging apparatus, which has been augmented for the study of low temperature solid samples, are presented. Specifically, preliminary results for NO2:Ar photodissociation are shown. In the MD simulations, dynamics of the primary photodissociation products - H and OH - are followed until they desorb, recombine, or are trapped in the ice matrix. Photoexcitation is described using ground and excited ab initio potential energy surfaces for H2O. Additionally, the impact of including flexible water models, zero point energy consistent vibrational motion, and a secondary reaction (formation of H3O) is discussed. Overall, OH radicals are much less mobile than H. For this reason, OH desorption is rare and OH typically traps very close to the photoexcitation location. In contrast, H usually travels past several water molecules, to settle between hexagonal rings of water molecules present in crystalline ice. H atom desorption is frequent and occurs with a kinetic energy distribution reflective of surface H atoms which leave the matrix with little to no collisions. Excitation deeper in the ice leads to a broadened kinetic energy distribution which favors lower energies. Inclusion of H3O formation leads to H exchange reactions and slowed H and OH dynamics. H3O formation is thought to be a source of H2, but the ice structure prevents H approaching H2O along the reaction coordinate. H2 is not formed in the simulations.
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