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 k_pre (approximately 10¹¹ s^-1) on rotational quantum number N′ is not known. These relative k_pre 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 k_pre(N′) increases as exp[cN′(N′ + 1)]. Additionally, k_pre for the F₁ levels appears slightly faster than that for F₂ levels for the same N′.

© 1991 Optical Society of America