Vibrational and Rotational Distributions of the CH(A2Δ) Product of the C2H + O(3P) Reaction Studied by Fourier Transform Visible (FTVIS) Emission Spectroscopy

The C(2)H + O((3)P) --> CH(A) + CO reaction is investigated using Fourier transform visible emission spectroscopy. The O((3)P) and C(2)H radicals are produced by simultaneous 193 nm photolysis of SO(2) and C(2)H(2) precursors, respectively. The nascent vibrational and rotational distributions of the CH(A) product are obtained under time-resolved, but quasi-steady-state, conditions facilitated by the short lifetime of the CH(A) emission. The vibrational temperature of the CH(A) product is found to be appreciably hotter (2800 +/- 100 K) than the rotational distributions in the v' = 0 (1400 +/- 100 K) and v' = 1 (1250 +/- 250 K) levels. The results suggest that the reaction may proceed through an electronically excited HCCO() intermediate; moreover, the vibrational excitation compared to rotational excitation is higher than expected based on a statistical distribution of energy and may be the result of geometrical changes in the transition state. The CH(A) emission is also observed in a C(2)H(2)/O/H reaction mixture using a microwave discharge apparatus to form O atoms, with subsequent H atom production. The nascent rotational and vibrational distributions of the CH(A) determined by the microwave discharge apparatus are very similar to the CH(A) distributions obtained in the photodissociation experiment. The results support the idea that the C(2)H + O((3)P) reaction may play a role in low-pressure C(2)H(2)/O/H flames, as previously concluded.