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 produ ce 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 N-2 buffer gases. The rate coefficients obtained b y 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 d istinct rate coefficient for the self-reaction was determined, k(infinity)( C3H3+C3H3) = (4.3 +/- 0.6) x 10(-11) cm(3) molecule(-1) s(-1) at 295 K. Thi s rate coefficient which is a factor of 2.8 times slower than reported prev iously, was independent of total pressure and buffer choice over the entire pressure range. Other rate coefficients derived during the modeling includ ed k(C3H3+H 665 Pa He) = (2.5 +/- 1.1) x 10(-10) cm(3) molecule(-1) s(-1), k(C3H3+C3H3Cl2) = (7 +/- 4) x 10(-11) cm(3) molecule(-1) s(-1), and k(C3H3C3H3Br2) = (2.4 +/- 2) x 10(-11) cm(3) molecule(-1) s(-1). The association reaction C3H3+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) betwee n 300 Pa and 13300 Pa. A fit of these data derived the high-pressure limiti ng rate coefficient k(infinity)(C3H3+O-2) = (2.3 +/- 0.5) x 10(-13) cm(3) m olecule(-1) s(-1). Three measurements of the propargyl radical-absorption c ross-section obtained sigma(332.5) = (413 +/- 60) x 10(-20) cm(2) molecule( -1) at 332.5 nm. Stated uncertainties are two standard deviations and inclu de the uncertainty of the absorption cross section, where appropriate.