Addition and recombination reactions of unsaturated radicals using a novel laser kinetics spectrometer

Abstract

This thesis describes the construction of a novel, low-noise laser kinetics spectrometer. A quasi-CW (picosecond pulse), tunable Ti:Sapphire laser is used to detect various transient species in laser flash photolysis kinetics experiments via direct absorption. The spectral range of the laser, when used with a harmonic generator, covers most of the visible wavelength region, allowing for the detection of a wide array of organic radical species. Paired with a temperature-and pressure-controlled flow reactor equipped with a Herriott-type optical multiple pass cell, transient absorptions of ~0.0001 can be measured, corresponding to cross section - concentration products of less than 1x10-7 cm". 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 instrument, we report the self-reaction rate coefficient of vinyl and allyl radicals. Vinyl iodide and allyl iodide are used as precursors to generate respective radicals via laser-flash photolysis at 266 nm. Second order chemical reactions require an accurate determination of the initial radical concentration, which we determined using direct laser absorption by I atom at 1315 nm. The current study finds the self-reaction rate constant for vinyl radical to be more than a factor of two slower than previous studies and allyl self reaction rate constant to be 50% faster than values reported in the literature. The absorption cross sections of the vinyl radical at 404, 423.2, & 445 nm and allyl radical at 404 & 408 nm are also determined. This thesis also reports measurements of rate coefficients for the reaction of vinyl radical with various alkenes: ethylene, propene, isobutene, 1-butene, and 2-butene,performed over a temperature range of 300 K to 700 K at 100 Torr. The measured vinyl radical disappearance rates compare well with ab initio quantum calculations. The combined measurements and calculations provide improved estimates for other vinyl + alkene reactions.