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

Abstract

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) × 10−10 and (2.5±0.4) × 10−10 cm3 molecule−1 s−1 are obtained at 90 and 120 K, compared to the room temperature value of 1.3×10−10 cm3 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.