Shock tube studies of thermal decomposition reactions using ultraviolet absorption spectroscopy

Several elementary thermal decomposition reactions of importance in combustion systems have been investigated using ultraviolet laser absorption spectroscopy and high-temperature shock tube methods. These studies are broken into three categories: (1) the decomposition of alkanes, (2) the incubation and decomposition of CO2, and (3) the decomposition of the benzyl radical and toluene.; The decomposition of ethane, propane, iso-butane, and n-butane were studied over the temperature range of 1297--2034 K and pressure range of 0.13--8.8 atm. The progress of reaction after shock-heating was monitored by measuring methyl radical concentration using laser absorption at 216.62 nm. The rate coefficient determinations were fit using RRKM/master equation calculations using a restricted Gorin model for the transition states.; The thermal decomposition of CO2 was investigated behind shock waves at temperatures of 3200--4600 K and pressures of 0.44--0.98 atm. Ultraviolet laser absorption was used to monitor the CO2 concentration with microsecond time resolution, allowing the observation of a pronounced incubation period prior to steady CO2 dissociation for the first time, confirming the expected bottleneck in collisional activation of this triatomic molecule. Master-equation calculations, with a simple model for collisional energy transfer, were carried out to describe the measured incubation times and decomposition rate coefficient. The master equation calculations suggest that the energy transferred per collision must have a greater than linear dependence on energy.; The decomposition of benzyl and toluene was investigated behind shock waves at temperatures of 1398--1782 K and pressures around 1.5 atm. Benzyl radicals were detected using laser absorption at 266 nm to monitor the progress of reaction. The fast decomposition of benzyl iodide dilute in argon behind shock waves provided an instantaneous benzyl source enabling the measurement of the rate of benzyl decomposition by monitoring the pseudo-first-order decay in benzyl absorption at 266 nm. Rate coefficients for the two-channel toluene decomposition were determined by monitoring benzyl absorption at 266 nm during the shock heating of toluene dilute in argon. RRKM/master equation calculations were carried out for the two-channel toluene decomposition using a restricted Gorin model for the two transition states providing extrapolation of the measured rate coefficients to the high-pressure-limit.