COLLISIONAL REMOVAL RATES FOR ELECTRONICALLY EXCITED CH RADICALS B2-SIGMA- AND C2-SIGMA+

The quenching of electronically excited CH in its B2-SIGMA and C2-SIGMA+ states by O2, H-2, 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 an 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 H-2. For the quenching of CH (C) by alkanes, the evidence would suggest that quenching is by collisional removal rather than by reaction.