LABORATORY STUDIES OF LOW-TEMPERATURE REACTIONS OF C2H WITH C2H2 AND IMPLICATIONS FOR ATMOSPHERIC MODELS OF TITAN

Rate coefficients for the reaction C2H + C2H2 --> C4H2 + H are measured over the temperature range 170-350 K. The reactions are carried out in a temperature variable flow cell. C2H radicals are produced by pulsed laser photolysis of C2H2, and a tunable infrared color-center laser is used to probe the transient removal of C2H in absorption to derive the rate coefficients. The results show that the rate coefficient is independent of temperature over the range 170-350 K and equal to (1.1 +/- 0.2) x 10(-10) exp[(28 +/- 20)/T] cm3 molecule-1 s-1. The reaction studied is of central importance for models of the photochemistry of the atmospheres of the outer planets, in particular for the satellite Titan, and the implications of the present results for these models are discussed.