Picosecond planar laser-induced fluorescence measurements of OH A 2 + ( ' = 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 ~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 N2 is a weak electronic quencher but a strong VET agent. By fitting the measured time dependence of the different vibrational populations ( ' = 2, 1, 0) to a four-level model, rate constants for quenching and VET were determined. For the lower states ( ' = 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.