Comparison of femtosecond- and nanosecond-two-photon-absorption laser-induced fluorescence (TALIF) of atomic oxygen in atmospheric-pressure plasmas

Abstract

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Recent advances in ultrafast-laser-based spectroscopy and imaging for reacting plasmas and flames

“…In addition, laser-induced photolytic effects (photodissociation of molecules) may introduce a significant additional uncertainty into the results. The use of ps and fs lasers with low pulse energies and pulse durations comparable to or shorter than the collisional timescales makes it possible to obtain photolytic interference-free TALIF measurements of atomic-species distributions, as well as direct measurements of the fluorescence quenching rates, which will be discussed in detail in section 3 [44,94,95].…”

“…Schmidt et al recently investigated the merits of applying the fs-TALIF technique to the measurement of atomic oxygen in an atmospheric-pressure plasma jet (APPJ) source with a helium-oxygen feed gas for direct comparison with conventional ns-TALIF [44]. For this comparison, high-pressure microplasma sources with small characteristic length scales were considered, because of the presence of strong spatial gradients for atomic-species concentration within an order of 100 μm or less.…”

“…The timescales associated with collisional quenching can vary from a few ps to hundreds of ps, depending on the collisional environment. Thus, accurate calibrations for quantitatively measuring the concentrations of atomic species are limited only to very lowpressure regimes in ns-laser-based TALIF, because of the difficulty in deconvolving the time-resolved fluorescence decay signal from the excitation-pulse envelope (shown in figure 12) [44]. However, in fs-TALIF, the collision is frozen during the excitation enabling direct measurement of the quenching rate following the excitation.…”

“…A detailed comparison and analysis of ns-TALIF versus fs-TALIF of the O atom has been presented for the APPJ discharge configuration in a 4%-O 2 /He mixture [44], which is discussed in detail in sections 3.1.1-3.1.3. The advantages of fs-TALIF specific to O-atom imaging inferred from this study are: (1) fs-TALIF signal is found to be 15 times stronger than ns-TALIF signal when the signals are normalized with respect to the incident laser intensity, (2) fs-TALIF allows measurements over a wider range of pressures than ns-TALIF, (3) experimental uncertainties in O-atom densities are notably smaller using fs-TALIF (±17%) compared to ns-TALIF (∼±33%), and (4) 2D O-atom images can be acquired in minutes with fs-TALIF compared to multiple hours with ns-TALIF.…”

“…(b) Fs-TALIF for similar conditions in (a); the exponential decays demonstrate the strength of fs-duration excitation as collected with highspeed PMT. Reproduced from[44]. © IOP Publishing Ltd. All rights reserved.…”