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

By using 193 nm laser photolysis and cavity ring-down spectroscopy to produce and monitor the propargyl radical (CH 2 CCH), 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 N 2 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 ∞ (C 3 H 3 +C 3 H 3 ) = (4.3 ± 0.6) × 10 -11 cm 3 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 (C 3 H 3 +H 665 Pa He) = (2.5 ± 1.1) × 10 -10 cm 3 molecule -1 s -1, k (C 3 H 3 +C 3 H 3 Cl 2 ) = (7 ± 4) × 10 -11 cm 3 molecule -1 s -1, and k (C 3 H 3 +C 3 H 3 Br 2 ) = (2.4 ± 2) × 10 -11 cm 3 molecule -1 s -1 . The association reaction C 3 H 3 + O 2 was found to lie in the falloff region between linear and saturated pressure dependence for each buffer gas (Ar, He, and N 2 ) between 300 Pa and 13300 Pa. A fit of these data derived the high-pressure limiting rate coefficient k ∞ (C 3 H 3 +O 2 ) = (2.3 ± 0.5) × 10 -13 cm 3 molecule -1 s -1 . Three measurements of the propargyl radical absorption cross-section obtained σ 332.5 = (413 ± 60) × 10 -20 cm 2 molecule -1 at 332.5 nm. Stated uncertainties are two standard deviations and include the uncertainty of the absorption cross section, where appropriate.