Multi-photon fluorescence imaging of flame species using femtosecond excitation
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We demonstrate-for the first time-interference-free, detection of atomic hydrogen (H) in reacting flows at 1 kHz using femtosecond, two-photon-excited, laser-induced-fluorescence (fs-TPLIF) line imaging. Broadband, nearly-transform-limited, fs pulses at 205 nm efficiently populate n = 1 n = 3 transition via two-photon excitation and subsequent one-dimensional fluorescence signal at 656 nm from n=1n=3 decay is imaged. Because the TPLIF signal scales as the laser irradiance squared, to produce the same fluorescence signal level, significantly lower pulse energy is required from fs-duration pulses in comparison to ns- or ps-duration pulses. The reduced laser energy virtually eliminates the single-photon-induced photodissociation of flame radicals generating additional H-atoms in the medium. In the premixed CH4/O2/N2flames investigated, we observed no evidence of photoionization or stimulated emission; two other processes that can further complicate the quantitative detection of H-atoms via TPLIF. Similar fs-TPLIF approach is also used for imaging O-atom and CO. Interference-free detection of O-atom and CO are also discussed. 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
