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Title: Dynamics of gas-liquid interfacial reactions of 0(3P) with long-chain hydrocarbons
Author: Allan, Mhairi
Awarding Body: Heriot-Watt University
Current Institution: Heriot-Watt University
Date of Award: 2007
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The dynamics ofthe gas'-liquid interfacial reaction between O(3P) atoms and long-chain liquid hydrocarbons ha~e been investigated using a unique spectroscopic approach. O(3P) atoms were created above the liquid surface by laser-photolysis ofN02 at 355 nm. A total of five different hydrocarbons were studied. These consisted oftwo branched hydrocarbons, squalane, C30H62, and pristane, CI9!40, and three linear hydrocarbons, n-docosane, C22H46, ntetracosane, C24HSO, and n-octacosane, C2SH60. The nascent gas-phase OH reaction products were detected using laser-induced fluorescence. By combing time-of-flight resolution with state-selective spectroscopic detection, information on both the translational and internal energy distributions was obtained. Molecular dynamics simulations ofthe liquid-vacuum interface of each of the hydrocarbons studied were performed to support any mechariistic conclusions. Vibrationally excited OH molecules were detected for each hydrocarbon studied. This suggests that secondary and/or tertiary hydrogen atoms are exposed to the incoming oxygen atoms, since it is known from related gas-phase experiments that primary hydrogen atoms do not produce measurable amounts ofOH (v' = 1). At least two contributions to the product rotational distributions were identified for each hydrocarbon, confirming and extending previous proposals ofthe participation of both direct ., and trapping-desorption mechanisms. In particular, it was found that for OR (v' = 0) the direct component dominates at the rising edge with an increasing contribution from the trapping-desorption component at later times. On the other hand, it was found that for OR (v' = 1) the direct and thermal contributions a,re difficult to distinguish because oftheir lower kinetic energy release than for (v' = 0). The OH (v' = 0) and (v' = I) yields were measured for squalane over a 70 K temperature range. The yield ofOH (v' = 0) was found to be temperature-independent whereas OH (v' = 1) increases with increasing temperature. Furthermore, the peak in the appearance profile varies more with temperature and is shifted to later times for OR (v' = 1) compared to (v' = 0). An increased OR escape probability at higher temperatures resulting from a more open liquid structure is thought to be the most plausible'mechanistic interpretation. OH yields were measured for the other hydrocarbons. The branched hydrocarbons produce more ofboth (v' = 0) and (v' = 1) compared to the linear hydrocarbons. This may simply reflect the increased reactivity of tertiary sites, but may also have a more complex structural explanation. A structural effect may indeed be required to explain the surprising result that the yield ofboth OH v'=O and 1 depends quite sensitively on the chain length ofthe linear hydrocarbons.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available