The relation between ultraviolet-excited fluorescence spectroscopy and aromatic species formed in rich laminar ethylene flames

The exploitation of fluorescence techniques for the characterization of aromatic pollutants formed in combustion processes needs a reliable interpretation for the assignment of fluorescence emission to specific products. To this aim, ultraviolet-excited LIF (laser-induced fluorescence) spectra were measured in premixed rich ethylene/oxygen flames having a different PAH mass loading as verified by sampling and chomatographic analysis of the condensed species produced along the flames. Fluorescence emission in the ultraviolet was mainly found in the flames where PAH formation is relatively low indicating that ultraviolet-fluorescence emission is not related to PAH species. On the opposite, broad visible emission features became prevalent in PAH-rich flames suggesting that the fluorescence of PAH species could be shifted toward the visible for effect of the high-temperature flame environment. In alternative to this hypothesis the visible fluorescence could be due to the heavier unidentified part of the condensed species whose fluorescence emission is shifted toward the visible.