Laser induced fluorescence spectroscopy of aromatic species produced in atmospheric sooting flames using UV and visible excitation wavelengths

In this work, laser induced fluorescence (LIF) has been applied to probe PAHs in two atmospheric sooting flames: a premixed flat flame of methane and a Diesel turbulent spray one. Different laser excitation wavelengths have been used. UV excitations at 266 and 355nm have been operated from the fourth and the third harmonic frequencies of an Nd: YAG laser while visible excitations were emitted by an OPO pumped by the third harmonic of the YAG laser. Because of the different nature of the flames, the recorded fluorescence spectra highlight different spectral properties. The diffusion flame appears to provide a better selectivity to the LIF measurements because of the stratification of the PAHs size classes along the flame height. In the premixed flame, all PAHs size classes spatially coexist making the analysis of LIF measurements more complex. Upon visible excitations, it is highlighted in this paper that PAHs can absorb and fluoresce up to 680nm. Fluorescence emission spectra are shown to present Stokes and anti-Stokes components. Discussion of these non-conventional absorption and fluorescence features are provided on the basis of the knowledge of PAH spectroscopy and flame kinetics. Hence, different families of PAHs are successively envisaged and discussed to elucidate the experimental spectra recorded in both flames. It is shown that only a limited number of PAHs are able to lead to such spectral features. From this discussion, it appears that large pericondensed PAHs are unlikely to give rise to such signals. Some other possibilities are therefore discussed which could potentially correspond to the latest fluorescent gaseous species at the origin of the soot formation.