Kinetic and Product Study of the Reactions of C(1D) with CH4 and C2H6 at Low Temperature.

The reactions of atomic carbon in its first excited 1D state with both CH4 and C2H6 have been investigated using a continuous supersonic flow reactor over the 50-296 K temperature range. C(1D) atoms were generated in situ by the pulsed laser photolysis of CBr4 at 266 nm. To follow the reaction kinetics, product H atoms were detected by vacuum ultraviolet laser-induced fluorescence at 121.567 nm. Absolute H-atom yields for both reactions were determined by comparison with the H-atom signal generated by the reference C(1D) + H2 reaction. Although the rate constant for the C(1D) + CH4 reaction is in excellent agreement with earlier work at room temperature, this process displays a surprising reactivity increase below 100 K. In contrast, the reactivity of the C(1D) + C2H6 system decreases as the temperature falls, obeying a capture-type rate law. The H-atom product yields of the C(1D) + CH4 reaction agree with the results of earlier crossed-beam experiments at higher collision energy. Although no previous data is available on the product channels of the C(1D) + C2H6 reaction, comparison with earlier work involving the same singlet C3H6 potential energy surface allows us to draw conclusions from the measured H-atom yields.