Collision dynamics as a probe of gas-liquid and related interfaces
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This thesis presents a body of work aimed at increasing the current understanding of the dynamics of collisions at the gas-liquid and related interfaces. In particular, collisions of open-shell radicals (oxygen atoms in the electronic ground state and hydroxyl radicals) with hydrocarbon surfaces have been investigated. Translationally hot radicals were scattered from liquid hydrocarbons and related surfaces. The products of inelastic scattering and hydrogen abstraction reactions (OH radicals) were detected, with translational and internal-state resolution, by laser-induced fluorescence (LIF). The radicals were used as a ‘chemical probe’ of the interface, providing unique information on the structure and reactivity of the surface, and how this relates to the bulk composition. Key results include the establishment of the relative reactivity towards oxygen atoms of liquid hydrocarbons and alkylthiol SAMs and the penetration depth of O atoms into the SAM surfaces. This was achieved through systematic studies involving custom-synthesised site-selectively deuterated SAMs. It was found that hydrogen abstraction can occur deeper within the monolayer than previously believed but still emerge as OH (or OD) without reacting further to form water. The reactivity of O(3P) atoms towards a technologically important family of ionic liquids ([Cnmim][Im]) was measured. It was found that the reactivity increased non-linearly as a function of alkyl chain length, to an extent which far exceeded any anticipated increase in reactivity based on stoichiometry alone. The interface in this case was found to differ greatly from the bulk composition, with preferential occupation by the alkyl units at the surface. The energy transfer and reactive uptake of OH radicals at a variety of hydrocarbon surfaces was investigated. There was significant transfer of the initial translational energy to all the surfaces as well as substantial translational-to-rotational energy conversion. This energy conversion was dependent on the functional groups present in the liquids as was the reactive uptake of OH. Reactive uptake coefficients (γ) were obtained for alkane, alkene and carboxylic acid-containing organic liquids. The results are discussed in terms of the relevance to the aging of organic particles in the troposphere.