Pressure Dependence of the CO and CO2 Quantum Yields in the Photolysis of simple Ketones in the Wavelength interval 240-308 nm

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In this work, the quantum yields of the two stable end products CO and CO2 in the UV-photolysis of acetone, butanone and 3-pentanone have been measured while variating the photolysis wavelength, the bath gas composition and the bath gas pressure. The measurements were performed using a pulsed stationary UV laser photolysis and a high resolution diode laser infrared spectroscopy. In the area of the used photolysis wavelength between 240-308 nm all three ketones have a similar behaviour which is known in literature. They dissociate into an acyl and an alkyl radical. The quantum yield of the dissociation is one in the wavelength area around 250 nm, decreasing towards higher wavelengths. The respective acyl radical (acetyl or propinyl) can – depending on the remaining excess energy – either dissociate further into CO and another alkyl radical or be thermalised by the bath gas. In the secondary chemistry of the thermalised acyl radical CO2 is formed. The kinetic modelling of the secondary chemistry shows that based on the known reactions of all products under the conditions of the experiment any other significant channels leading to CO can be excluded. The formation of CO from acyl peroxy which is postulated by literature can not totally be excluded but is not detectable with this method. The formation of CO via OH + acetone is not significant either. The formation of CO2 goes exclusively via thermalised acyl radicals. So it can be concluded that by measuring the stable end products CO and CO2 the yields of acyl formation and the ketone dissociation can be calculated. The CO yield in the photolysis of acetone shows a steady decrease with increasing wavelength induced by the decreasing amount of excess energy in the acetyl radical after photolysis. With increasing bath gas pressure the CO yield also decreases due to a more effective collisional deactivation of the radical. One of the main products of thermalised acetyl is CO2. Due to the competition of dissociation and deactivation the CO2 yields show an inverse behaviour in their pressure and wavelength dependency compared with CO. In the photolysis of acetone with uv of ~245 nm the sum of the yields of CO and CO2 is unity with the CO yields decreasing with increasing pressure while the CO2 yield increases in the same amount. In agreement with literature this leads to a primary quantum yield of the acetone dissociation of unity. Photolysing with higher wavelengths of 280 and 308 nm a different situation is visible. The CO yield is not significant any more, CO2 is the dominant product but showing a different dependence on pressure namely it decreases with increasing pressure. This effect can be explained by the competition of acetone dissociation and thermalisation of the activated acetone. According to literture the life time of an electronically activated acetone is highly depending on the energy absorbed. Is it high when photolysing with wavelengths bigger than 280 nm, the thermalisation by the bath gas can compete with the dissociation. With high wavelengths the primary yield of acetone dissociation is obtained by extrapolating the CO2 yield against zero pressure. At 280 nm and 308 nm it is 76% and 45% respectively. To verify these findings butanone and 3-pentanone were photolysed with UV of identical wavelangths. A similar behaviour can be found. Even though the dissociation of butanone will lead to acetyl as a main product and propionyl as a minor product and the dissociation of 3-pentanone leads to propionyl only, the pressure and wavelength dependencies of the CO and CO2 yields are very similar. From this fact it can be concluded, that acetyl and propionyl have a similar dissociation behaviour and the energy partition in the measured ketones is also similar. In a quantum dynamic modelling of the dissociation of the hot acetyl radical performed in the group of Prof. Zellner by Mr. Holger Somnitz the pressure dependency of the CO yield at 248 nm could be reproduced in good agreement with the experiment. Within this work it was possible to deepen the understanding of the mechanism of the dissociation of the acetyl radical formed from photolytically energized ketones. Using the sensitive method of high resolution diode laser ir spectroscopy it is possible to come closer to atmospheric reality when performing the photolysis experiment. Even though the secondary chemistry in the laboratory took place without NOx being present, a transfer of the yields measured in this work into atmosperic models should be easy since the NOx chemistry is well known.
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Wissenschaftliche Abschlussarbeiten » Dissertation
Fakultät / Institut:
Fakultät für Chemie » Physikalische Chemie
Dewey Dezimal-Klassifikation:
500 Naturwissenschaften und Mathematik » 540 Chemie » 541 Physikalische Chemie
Druckabhängigkeit CO CO2 Quantenausbeuten Photolyse Aceton MEK DEK Ketone UV TDLAS Pressure Dependence Quantum Yields Photolysis Ketones acetone butanone 3-pentanone butanon 3-pentanon
Prof. Dr. rer. nat. Zellner, Reinhard [Thesis advisor]
Prof. Dr. rer. nat. Zellner, Reinhard [Reviewer]
Prof. Dr. Hasselbrink, Eckart [Reviewer]
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siehe Literaturanhang
September 2003 - November 2007