Collisional processes near the CHB2 Sigma-upsilon '=0, 1 predissociation limit in laser-induced fluorescence flame diagnostics

Excitation and dispersed laser-induced fluorescence spectra of CHB(2)Sigma (-)nu' = 0, 1 in methane flames are analyzed using rotational relaxation mo dels to investigate their applicability for flame diagnostics. The existenc e of non-predissociative and highly predissociative rotational levels in th e same vibrational state provides a unique scenario to test the effects of rotational relaxation in laser-induced fluorescence measurements. Using a s tatistical power gap law for rotational relaxation modeling, we find that t he levels with collision-free lifetimes as short as 100ps have apparent flu orescence yields larger than expected because of the extent of rotational r elaxation at atmospheric pressure. Also, vibrational (nu' = 1 to nu' = 0) a nd electronic energy transfer (B(2)Sigma (-)nu' = 1 to A(2)Delta) are compe titive, and together are half the value for the total collisional removal r ate from CHB(2)Sigma (-)nu' = 0. The measured electronic energy transfer br anching ratio into A (nu' = 0 - 3) depends on the initial rotational level pumped, and energy gap considerations can be used to explain these propensi ties. The combination of measurements and model calculations finds the exci tation of the CHB(2)Sigma (-)nu' = 1, N' = 8 level a good candidate for las er-induced fluorescence quantitative measurements in flames at atmospheric pressure.