Direct measurements of rate coefficients for the reaction of ethynyl radical (C 2 H) with C 2 H 2 at 90 and 120 K using a pulsed Laval nozzle apparatus

Rate coefficients for the reaction C 2 H + C 2 H 2 → C 4 H 2 + 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 C 2 H radicals are generated by 193 nm laser photolysis of C 2 H 2 , and the reaction product, C 4 H 2 , 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 cm 3 molecule −1 s −1 are obtained at 90 and 120 K, compared to the room temperature value of 1.3×10 −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.