High-repetition-rate imaging of atomic oxygen in flames
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Two-photon laser-induced fluorescence (TPLIF) is a well-recognized approach for detecting atomic oxygen however, measurements to date have been restricted to a single point within a flow, flame, or plasma, due to the inherently high intensity requirements for the excitation laser. Furthermore, the high-intensity ultraviolet (UV) photons used for two-photon excitation process can photochemically produce O atoms by dissociating other oxygen containing species in the medium. In this work, we demonstrate nearly photolytic-interference-free femtosecond (fs) TPLIF line imaging of atomic oxygen in flames. By using amplified Ti:sapphirebased laser systems, the data acquisition rate can be increased to the 1'10 kHz regime, enabling capture of dynamic events in turbulent media. Furthermore, as a result of efficient two-photon excitation and subsequent generation of strong fluorescence signals, the fs-TPLIF scheme is readily extended for 2-D imaging for studying spatiotemporal dynamic in systems such as practical combustion and plasma devices. In O-atom TPLIF, two-photon-allowed absorption of 225.7-nm light, drives the transition from the ground 2p3P electronic state to the excited 3p3P state. From this upper level, de-excitation to the 3s3S state occurs by single-photon emission, allowing fluorescence detection at 844.6 nm. By using high-peak-power but lowenergy fs pulses, the photolytic interferences are virtually eliminated while substantially increasing the two-photon excitation efficiency. Similar observations have previously been made in TPLIF detection of atomic hydrogen. The broad bandwidth of nearly Fourier transform' limited (TL) fs pulses contributes to enhanced excitation through the combination of a large number of photon pairs within the spectral bandwidth of each pulse. In TL pulses, all photons of different colors- corresponding to different frequencies-have the same spectral phase, thus collectively contributing to two-photon excitation. Additionally, fs-duration pulses become favorable for TPLIF because the signal scales as the laser irradiance squared, whereas single-photon-induced photodissociation processes scale linearly. As a result, photolytic interferences become virtually negligible at reasonable TPLIF detection levels of nascent O atoms. The measurement techniques described in this work can provide invaluable experimental data of spatially and temporally resolved number density of atomic species such as O, H, and N to validate complex combustion and plasma flow models.
