DIRECT IDENTIFICATION OF THE C2H(X(2)SIGMA(-3)-]CH(A(2)DELTA)+CO REACTION AS THE SOURCE OF THE CH(A(2)DELTA-]X(2)PI) CHEMILUMINESCENCE IN C2H2())+O(P)O/H ATOMIC FLAMES/

In this work, the intensity of the CH(A(2) Delta) chemiluminescence, I (CH), as well as the concentrations of ground state C2H radicals and O atoms were measured as a function of the reaction time in a variety of helium-diluted C2H2/O/H mixtures in an isothermal flow reactor at t emperatures of 290, 410, 520, 590, 675, and 925 K and at a total press ure of 2 Torr. The species concentrations [O] and [C2H] were measured using molecular beam sampling-threshold ionization mass spectrometry ( MB-TIMS). At each temperature, the intensity I(CH) was found to be di rectly proportional to the [C2H][O] concentration product, over a rang e of two decades, irrespective of the initial mixture composition or t he reaction time. Using the NO + O --> NO2 chemiluminescence as a cal ibration standard, CH formation rates were derived from the measured 1(CH), and the values of the rate coefficient k(2a) of the CH*-formin g reaction channel C2H + O --> CH(A(2) Delta) + CO (r2a) were thus der ived from the slopes of the I(CH) versus [C2H][O] plots. The results, for 290 K < T < 925 K, can be represented by the Arrhenius expression k(2a) = 2.4 x 10(-11) exp[-230/T(K)] cm(3) molecule(-1) s(-1); the po ssible systematic error is a factor of 2, due to the uncertainty of th e C2H calibration factor. The value at 290 K, 1.1 x 10(-11), is in fai r agreement with our recent result obtained in an independent pulse la ser photolysis/chemiluminescence experiment. The addition of methane w as found to suppress I(CH) in quantitative agreement with the C2H for mation mechanism in C2H2/O/H systems elucidated earlier by us. It is a rgued that the fast reaction r2a is a major if not the dominant CH so urce also in hot hydrocarbon flames.