Collisional processes near the CH B 2Σ- v′=0,1 predissociation limit in laser-induced fluorescence flame diagnostics

Excitation and dispersed laser-induced fluorescence spectra of CH¿B¿2S-v'=0,1 in methane flames are analyzed using rotational relaxation models to investigate their applicability for flame diagnostics. The existence of non-predissociative and highly predissociative rotational levels in the same vibrational state provides a unique scenario to test the effects of rotational relaxation in laser-induced fluorescence measurements. Using a statistical power gap law for rotational relaxation modeling, we find that the levels with collision-free lifetimes as short as 100 ps have apparent fluorescence yields larger than expected because of the extent of rotational relaxation at atmospheric pressure. Also, vibrational (v'=1 to v'=0) and electronic energy transfer (B¿2S-v'=1 to A¿2¿) are competitive, and together are half the value for the total collisional removal rate from CH¿B¿2S-v'=0. The measured electronic energy transfer branching ratio into A¿(v'=0-3) depends on the initial rotational level pumped, and energy gap considerations can be used to explain these propensities. The combination of measurements and model calculations finds the excitation of the CH¿B¿2S-¿v'=1,N'=8 level a good candidate for laser-induced fluorescence quantitative measurements in flames at atmospheric pressure.