In this talk, the application of a quasi-CW (80 MHz, picosecond pulsed), tunable Ti:sapphire laser to absorption detection of transient species in a flash photolysis kinetics experiment is presented. The spectral range of the laser, when used with a harmonic generator, covers most of the visible wavelengths allowing for the detection of a wide array of organic radical species. The sensitivity of this instrument is due to the low noise of the solid-state laser system and is further enhanced with a Herriott-type multi-pass resonator. Transient absorbances of ~0.0001 can be measured, corresponding to cross section - concentration products of less than 1x10-7 cm-1. The flexibility and high sensitivity of this instrument allows direct and accurate measurement of many important transient intermediates in combustion and atmospheric chemistry.

Using this apparatus, located in the Massachusetts Institute of Technology's (MIT) Combustion Dynamics Laboratory, we have studied the kinetics of the vinyl radical, which plays a significant role in combustion and in low temperature extraterrestrial atmospheres. It is a key intermediate in combustion during decomposition of higher hydrocarbons. Also, under certain conditions, it may play a role in precyclization chemistry, leading to soot formation. Employing pseudo-first order kinetic methods, we have measured rate constants for the reactions of vinyl with several combustion-relevant small molecules, including: molecular oxygen, ethene, propene, isobutene, 1-butene, and 2-butene.

We have also measured the self-reaction rate of vinyl radical at MIT using the quasi-CW Ti:sapphire laser, and compared to measurements taken at Sandia National Laboratories using a similar apparatus with a CW diode laser probe. A complication with a second order chemical system is that it requires an accurate determination of the initial radical concentration. Our approach to obtaining the initial concentration is to use vinyl iodide as a precursor and to probe both vinyl radical and I atoms simultaneously. The time-resolved vinyl radical concentration is fitted to a second order kinetic equation to extract the self-reaction rate constant. The current experiments find the self-reaction rate to be more than a factor of two slower than previous studies.

This work is supported by the Division of Chemical Sciences, Geosciences, and Biosciences, the Office of Basic Energy Sciences, the U. S. Department of Energy. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the National Nuclear Security Administration under contract DE-AC04-94-AL85000.