Quantitative, spatially-resolved measurements of intermediates in flame synthesis reactors are required for the validation of precursor decomposition and oxidation mechanisms, preceding the nanoparticle formation. In this work we demonstrate how the laser-induced fluorescence (LIF) can be used in a self-calibrating fashion to image absolute concentrations of the strong absorber, such as atomic iron generated during the thermal decomposition of iron pentacarbonyl precursor in the preheat zone of synthesis flame. Flame symmetry facilitates deduction of the absolute concentrations. A comparison of LIF fluorescence patterns on both sides of axisymmetric flow configuration cancels out symmetric factors such as fluorescence quantum yield, fluorescence trapping and optical aberrations. This approach, utilizing one laser beam and one spectral transition provides a refinement of previous methods that have used either two spectral transitions or two collinear laser beams in counter-propagating geometry. Its spectral resolution and the detection sensitivity of que are not compromised when the spectral width of the laser exceeds that of the absorber. The measured Fe-atom concentration field is qualitatively consistent with the predictions of nucleation theory approach and suggest that flame synthesis model should be expanded beyond the formation of small incipient iron clusters to several nm-sized iron particles.
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