Rate Coefficients for the Propargyl Radical Self-Reaction and Oxygen Addition Reaction Measured Using Ultraviolet Cavity Ring-down Spectroscopy

Abstract

By using 193 nm laser photolysis and cavity ring-down spectroscopy to produce and monitor the propargyl radical (CH2CCH), the self-reaction and oxygen termolecular association rate coefficients for the propargyl radical were measured at 295 K between total pressures of 300 Pa and 13300 Pa (2.25 and 100 Torr) in Ar, He, and N2 buffer gases. The rate coefficients obtained by simple second-order fits to the decay data were observed to vary with the photolytic precursors: allene, propargyl chloride, and propargyl bromide. By using a numerical fitting routine and more comprehensive mechanisms, a distinct rate coefficient for the self-reaction was determined, k∞(C3H3+C3H3) = (4.3 ± 0.6) × 10-11 cm3 molecule-1 s-1 at 295 K. This rate coefficient, which is a factor of 2.8 times slower than reported previously, was independent of total pressure and buffer choice over the entire pressure range. Other rate coefficients derived during the modeling included k(C3H3+H 665 Pa He) = (2.5 ± 1.1) × 10-10 cm3 molecule-1 s-1, k(C3H3+C3H3Cl2) = (7 ± 4) × 10-11 cm3 molecule-1 s-1, and k(C3H3+C3H3Br2) = (2.4 ± 2) × 10-11 cm3 molecule-1 s-1. The association reaction C3H3 + O2 was found to lie in the falloff region between linear and saturated pressure dependence for each buffer gas (Ar, He, and N2) between 300 Pa and 13300 Pa. A fit of these data derived the high-pressure limiting rate coefficient k∞(C3H3+O2) = (2.3 ± 0.5) × 10-13 cm3 molecule-1 s-1. Three measurements of the propargyl radical absorption cross-section obtained σ332.5 = (413 ± 60) × 10-20 cm2 molecule-1 at 332.5 nm. Stated uncertainties are two standard deviations and include the uncertainty of the absorption cross section, where appropriate.