PRODUCT DISTRIBUTIONS OF THE C2H2+O AND HCCO+H REACTIONS - RATE-CONSTANT OF CH2(X)OVER-TILDE 3B1) + H

The branching ratios of the two dominant methylene sources in C2H2/O/H systems C2H2 + O --> (CH2(B1))-B-3 + CO (r2a) or HCCO + H (r2b) and HCCO + H --> CH2(1A1) + CO (r3a) or (CH2(B1))-B-3 + CO (r3b) have been determined, at 285 K, using discharge-flow/molecular beam mass spectrometry techniques (D-F/MBMS). The ratios were derived from the observed decrease of MBMS CH2 signals Upon substituting 25% of the helium bath ps by 0.5 Torr of methane, which selectively scavenges CH2(1A1); in the absence of CH4 the singlet CH2 is mainly collisionally converted to (CH2(B1))-B-3. In this way, the ratios of the total formation rates of singlet and triplet CH2 are obtained. As the mte of reaction r3 is linked to the rate of reaction r2b by the known fraction of HCCO reacting with H, in competition with 0, both the ratios k2a/k2b and k3a/k3b can be extracted from the data. Thus, including also probably systematic errors, the HCCO yield of C2H2 + 0 is found to be k2b/k2 = 85(-9)+4% and the CH2(1A1) yield of HCCO + H, k3a/k3 = 92 +/- 15% (95% confidence intervals). In addition, the rate constant of (CH2(B1))-B-3 + H --> CH(2PI) + H-2 (rl) was derived relative to the known (k(CH2+O))-C-3 from CH2 signals at different [H]/[O] ratios:k1 = (1.6 +/- 0.6) X 10(14) cm3 Mol-1 S-1.