LAUSR

Abstract The kinetics of the reactions of the first excited state of methylene, 1CH2, with C2H2, C2H4, and C2H6, has been measured over the temperature range 43–298 K by pulsed laser photolysis, monitoring 1CH2 removal by laser induced fluorescence. Low temperatures were obtained using a pulsed Laval expansion (43–134 K), while a slow flow reaction cell was used for temperatures of 160 K and above. The rate coefficients for the reactions with C2H2, C2H4, and C2H6, all showed a strong negative temperature dependence. In combination with other literature data, the coefficients can be parameterized as: k C 2 H 2 ( 43 ≤ T / K ≤ 298 ) = ( 3.22 ± 0.15 ) × 10 − 10 × ( T / 298 ) ( − 0.394 ± 0.066 ) k C 2 H 4 ( 43 ≤ T / K ≤ 298 ) = ( 2.16 ± 0.14 ) × 10 − 10 × ( T / 298 ) ( − 0.612 ± 0.089 ) k C 2 H 6 ( 43 ≤ T / K ≤ 298 ) = ( 1.78 ± 0.10 ) × 10 − 10 × ( T / 298 ) ( − 0.545 ± 0.078 ) Branching ratios for reactive removal of 1CH2 vs quenching to ground state were also determined for all three colliders and for H2 and CH4, at temperatures between 100 and 298 K. The values measured show that the dominant removal process of 1CH2 by H2, C2H2, and C2H4, changes from reactive removal to quenching to ground state 3CH2 as the temperature decreases from 298 K to 100 K, while for CH4 and C2H6, reactive removal drops from around 85% to around 55%. The impacts of the new measurements for Titan's atmosphere are examined using a 1D chemistry and transport model. A significant increase (∼25%) in the mixing ratio of benzene between 500 and 1550 km is calculated, due to the increased production of C3H3 from the reaction of 1CH2 with C2H2.