Collisional removal rates for electronically excited CH radicals B2Σ–and C2Σ+

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Abstract
The quenching of electronically excited CH in its B2Σ– and C 2Σ+ states by O2, H2, CO2, N2O, NO, CH4, C2H6, C3H8 and C2H4 has been studied using 248 nm laser photolysis of CHBr3 as the source of CH (B and C). For the B state, quenching is largely by chemical reaction, with the possible exception of N2O, with rate constants that exceed those for A-state quenching and are comparable to or greater than those for the ground state, CH (X), reactions. The hydrocarbon molecules and nitric oxide are particularly efficient in quenching the B state and it is thought that in addition to reaction, collisional removal of CH (B) can take place via and E → E energy transfer to the A state with a corresponding excitation of a C—H or N—O stretching vibration. This is a near-resonant process which can occur with high efficiency. The C-state quenching-rate constants are lower or similar to those for A-state quenching for most of the molecules studied, with the exception of O2 and H2. For the quenching of CH (C) by alkanes, the evidence would suggest that quenching is by collisional removal rather than by reaction.