Picosecond planar laser-induced fluorescence measurements of OH A(2)Sigma(+) (v'=2) lifetime and energy transfer in atmospheric pressure flames

A picosecond, excimer-Raman laser (268 nm, 400 ps FWHM) was used for laser sheet excitation of OH in the (2, 0) band. The fluorescence was detected with a fast-gated, intensified camera (400-ps gate width). The effective collisional lifetime of the spectrally integrated fluorescence was measured in two dimensions by shifting the intensifier gate across the decay curve. The average lifetime is similar to 2.0 ns for a stoichiometric methane-air flame with spatial variations of +/-10%. Shorter collisional lifetimes were measured for rich flame conditions that are due to a higher number density of the quenchers. Vibrational energy transfer (VET) was observed in premixed methane-air and methane-oxygen flames by putting the fast-gated camera behind a spectrometer. The spectrum of the methane-air flame shows strong VET in contrast with the methane-oxygen flame. This is because N-2 is a weak electronic quencher but a strong VET agent. By fitting the measured time dependence of the different vibrational populations (nu' = 2, 1, 0) to a four-level model, rate constants for quenching and VET were determined. For the lower states (nu' = 0, 1) our results are in good agreement with Literature values. For a prediction of a spectrally integrated, collisional lifetime in a known collisional environment it is important to consider not only the quenching but also the amount of energy transfer in the excited state as well. as the spectral detection sensitivity. (C) 1997 Optical Society of America.