Direct measurements of rate coefficients for the reaction of ethynyl radical (C2H) with C2H2 at 90 and 120 K using a pulsed Laval nozzle apparatus

Rate coefficients for the reaction C2H + C2H2 --> C4H2 + H are measured at 90 and 120 K by using a new pulsed Laval nozzle apparatus equipped with laser ionization, time-of-flight mass spectrometric detection. The C2H radicals are generated by 193 nm laser photolysis of C2H2, and the reaction product, C4H2, is directly detected by single-photon ionization at 118 nm (10.5 eV). Rate coefficients of (2.7 +/- 0.5) x 10(-10) and (2.5 +/- 0.4) x 10(-10) cm(3) molecule(-1) s(-1) are obtained at 90 and 120 K, compared to the room temperature value of 1.3 x 10(-10) cm(3) molecule(-1) s(-1). The results show a significant negative temperature dependence of the reaction rate coefficient below room temperature. The implications for models of planetary atmospheres, such as for Titan, are discussed.