Abstract
Laser-induced fluorescence (LIF) is commonly used to detect the OH radical and other chemical intermediates in flames. Fluorescence from the strongly predissociating v′ = 3 level of the A2Σ+ state of OH can be excited via absorption of KrF laser radiation in the weak 3,0 band at 248 nm. This approach has been developed1 to potentially circumvent effects of collisional quenching that can complicate the interpretation of LIF signals. However, the dependence of the predissocation rate kpre (approximately 1011 s-1) on rotational quantum number N′ is not known. These relative kpre were measured using low pressure flames where the quantum yields are wholly determined by predissociation. Using a frequency doubled tunable dye laser, LIF excitation scans are made for the 0,0 and 3,0 bands, in the burnt gases of CH4/O2 and CH4/air flames at 27 and 30 Torr, respectively. The 0,0 band yields the true rotational populations (described by a temperature near 1700 K). Apparent populations from the 3,0 scan then furnish the relative quantum yield. These can be well described by an apparent temperature near 1000 K, showing that kpre(N′) increases as exp[cN′(N′ + 1)]. Additionally, kpre for the F1 levels appears slightly faster than that for F2 levels for the same N′.
© 1991 Optical Society of America
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