Simultaneous laser-induced emission of soot and polycyclic aromatic hydrocarbons within a gas-jet diffusion flame

Simultaneous images of laser-induced fluorescence (LIF) due to polycyclic aromatic hydrocarbons (PAHs) and laser-induced incandescence (LII) visualization of soot concentrations are presented for laminar gas-jet diffusion flames. Spatially integrated measurements reveal similar spectral emission for LIF and LII, but vastly different time scales associated with radiative decay. Comparison of spatially resolved images using either 266-nm or 1064-nm excitation light reveals distinct regions of molecular fluorescence and soot incandescence. Consideration of photophysical properties of PAHs suggests that the fluorescence wavelength distribution is dependent on the size of the PAH. Using different detection spectral bands, spatially resolved regions attributed to different-sized PAHs are presented. The spatial distribution of PAH size is consistent with the putative growth mechanism of PAHs. In the region between the LIF due to PAHs and LII due to soot, a dark zone is observed that is attributed to the presence of soot precursor particles. Current understanding of soot formation indicates that through both physical and/or chemical condensation, large PAHs react first to form soot precursor particles prior to the formation of soot particles. Transmission electron microscopy analysis of thermophoretically collected material from within this dark region confirmed the presence of soot precursor particles 2 to 5 nm in diameter. (C) 1997 by The Combustion Institute.