Multi-Photon Fluorescence Imaging of Flame Species Using Femtosecond Excitation

Abstract

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 = →→ = transition via two-photon excitation and subsequent one-dimensional fluorescence signal at 656 nm from = → = 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 CH 4 /O 2 /N 2 flames 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. Nomenclature LIF= laser-induced-fluorescence TPLIF = two-photon-excited laser-induced-fluorescence CO = carbon monoxide VUV = vacuum ultra violet PLIF = planar laser-induced fluorescence OPA = optical parametric amplifier ICCD = intensified charge couple device

A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics

“…Atomic hydrogen is a critical species in the combustion of hydrogen or hydrocarbon fuels [58], which gets involved in flame ignition, propagation, and extinction. The H fs-TPLIF technique was first demonstrated in combustion by Kulatilaka et al [30,46]. In their scheme, H was excited by a 205 nm fs laser, and the subsequent H α fluorescence at 656 nm from the transition n = 3 → n = 2 was detected, as shown by the red line in Figure 2.…”