RATE COEFFICIENTS OF THE REACTIONS OF C2H WITH O-2, C2H2, AND H2O BETWEEN 295 AND 450 K

The absolute rate coefficients of three of the most important C2H-removal reactions in hydrocarbon flames, C2H + O-2 --> products (r1), C2H + C2H2 --> products (r2), C2H + H2O --> products (r3), have been measured over the temperature range 295 < T < 450 K by a laser photodissociation/chemiluminescence (LPD/CL) technique. Ethynyl radicals were generated by 193 nm excimer laser photolysis of C2H2, and their real-time pseudo-first-order decay was monitored by the CH(A(2) Delta-->X(2) Pi) chemiluminescence resulting from their reaction with O-2, present in a high and constant concentration. It was ascertained that in the experimental conditions of the decay experiments, at a pressure of 10 Torr He, the C2H radicals were electronically and vibrationally relaxed. The results for the first two reactions can be expressed by the Arrhenius relations k(1) = (1.9 +/- 0.1) x 10(-11) exp[(+160+/-15)/T(K)] and k(2) = (1.3 +/- 0.2) x 10(-10) exp[0+/-10)/(K)] cm(3) molecule(-1) s(-1), in close agreement with the more recent literature data. For reaction r3, which was investigated for the first time, our data can be fitted to the expression k(3) = (1.9 +/- 0.2) x 10(-11) exp[(-200+/-30)/T(K)] cm(3) molecule(-1) s(-1). The reaction between C2H + H2O appears to be quite fast and should therefore be incorporated in hydrocarbon combustion models.