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Title: Chemical processes studied by time-resolved FTIR
Author: Saunders, Mark
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2008
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Abstract:
This thesis has applied the time-resolved FTIR emission technique to the study of a number of gas phase cheinical processes, including photodissociation, chemical reaction and energy transfer. Two particular types of species are considered: vibrationally excited NO, formed from photolysis, reaction and energy transfer pathways, and organic radical compounds formed from photolysis and involved in secondary reaction processes. Vibrationally excited NO has been observed as one of the reaction products following the photolysis of N02 over a range of wavelengths in the region 193 - 327 nm. Emission from the fundamental and first overtone bands was analysed to obtain the nascent vibrational distribution at each wavelength considered. The nascent vibrational distribution was found to be strongly inverted and wavelength dependent. Photolysis at A > 244 nm proceeds via one reaction pathway; 1 A>244nm )NO+Oep) (1) and the NO(v) distribution is of atmospheric importance in determining the global concentrations of NO measured from satellite emission studies. There is a smooth trend in the inverted distributions with increasing photon energy. Nascent populations were used to produce a new model to predict the wavelength dependence of the nascent populations of NO(v) formed following photolysis of N02 in the range 265 - 370 nm. Such work has been shown to provide useful new information for atmospheric models of global concentrations of NO. Atmospheric models of NO concentrations also require input parameters for the rates of relaxation of NO(v) in collision with quenching molecules, in particular 02. The rate constants for vibrational quenching of NO(v = 1 - 16) in collisions with a number of species have been measured. Vibrationally excited NO was produced by both the 193 nm photolysis of N02 and the reaction of OeD) with N20, initiated by the 193 nm photolysis of N20. Quenching rates were measured for collisions with O2, He, NO and CO2. The vibrational distribution of NO(v) formed following photolysis of N02 at 193 nm was confirmed to be bimodal with a secondary maximum at v = 14 allowing new rates of relaxation of highly vibrationally excited NO to be determined due to the relatively large nascent populations of these levels. At A< 244 nm at second reaction channel for N02 photolysis opens up and in order to assess the contribution of this channel to the overall distribution at such wavelengths a new nascent distribution was determined following photolysis at 226 nm. The distribution was found to be bimodal and could qualitatively be described by the sum of the Oep) and OeD) channels. Vibrationally excited NO was also formed following energy transfer from NO A z:E+(v = 0), which was achieved by 226 nm pumping of ground state NO. Nascent vibrational distributions were determined for a variety of collision partners and NO was shown to be populated up to at least v = 20. The distributions obtained varied with the collision partners: NO, CO, COz and Ar. Distributions of ground state NO(v) were also acquired under conditions such that the internal energy in the products was governed by the emission coefficients. Comparison of distributions obtained by collision and fluorescence and those predicted from the emission coefficients allowed a purely collisional NO(v) distribution to be estimated when NO and CO were the quenching molecules. Emission was also observed from high vibrational modes of CO and COz indicating a direct energy transfer from NO A z:E\v = 0). Argon was found to not significantly affect the distribution at the pressures employed but afforded a dramatic increase in intensity from NO(v) that was shown to be due to hole burning and subsequent collisional hole filling. The second part of this thesis describes the generation and reactions of organic radical species. The photolysis of vinyl bromide and vinyl chloride at 193 urn was considered and nascent vibrational populations of the HBr and HCI products have been measured and found to be indicative of a three-centre formation mechanism. Secondary reaction products of the vinyl radical (CZH3) produced from this photolysis process have also been considered and mechanisms for their formation proposed. The presence of buffer gas has been shown to generate new features which have been assigned to vibronic emission from the secondary reaction product CzH. Products of the reaction of Oz with the vinyl radical, formed from the 193 nm photolysis of vinyl bromide and vinyl chloride, have been identified. In particular the existence two primary reaction channels have been confirmed by the assignment of emission from CO2, HCO and HzCO, and the mechanism of one secondary reaction channel has been proposed by analysing the kinetics of emission from CO(v). In addition a new reaction channel has been proposed by considering emission from CzHz. A qualitative branching ratio between these three reaction channels has been proposed. The reaction of CZH3 with O2 is of significance to combustion processes and a rate constant for the process has been determined to be 9.5 x 1O,tZ cm3 molecule,l S'I by assessing the kinetics of emission from HzCO. As a comparison to the generation of simple unsaturated free radicals, such as the vinyl radical, the photolysis of acetone at 266 nm to generate the simple saturated methyl radical was investigated. Primary reaction products were identified as the CH3 and CH3CO radicals and emission was observed in the C-H stretching region that was assigned on the basis of its kinetics to the methyl radical recombination product CZH6• CO(v = 1 - 6) was also observed as a secondary product of the process and attributed to photolysis of CH3CO on account of its significantly hotter nascent vibrational distribution than that obtained in previous studies at 193nm.
Supervisor: Not available Sponsor: Not available
Qualification Name: University of Oxford, 2008 Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.491975  DOI: Not available
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