Collisional removal rates for electronically excited CH radicals B 2Σ– and C 2Σ+

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.