Fluorescence spectroscopy of aromatic species produced in rich premixed ethylene flames

The ¯uorescence spectra of the condensed species (CS) collected in the soot inception region of a rich premixed laminar ethylene/oxygen ¯ame have been measured by excitation in the UV at 266 and 355 nm excitation wavelength. The contribution of the most abundant polycyclic aromatic hydrocarbons (PAH) to the CS ¯uorescence has been evaluated in order to attribute the CS ¯uorescence at di€erent emission wavelengths to speci®c aromatic structures. The ¯uorescence peaks detected in the UV region of the CS ¯uorescence spectrum was found to be mainly due to a typical PAH like ¯uorene, that is, the most ¯uorescent one among the PAH analyzed in the CS by chromatographic analysis. The CS exhibited the larger emission in the visible where the PAH contribution has been shown to be negligible and other ¯uorescing aromatic species, not identi®ed by chromatographic analysis of the CS, have to be considered responsible for the visible ¯uorescence. Laser induced ¯uorescence (LIF) ¯ame measurements excited at 266 nm and detected at two selected wavelengths (310 and 410 nm) have been performed along the ¯ame axis and compared with the CS ¯uorescence intensity. The LIF and CS ¯uorescence signals show quite similar axial trends demonstrating that the LIF signals are related to CS ¯uorescence. In particular, the LIF ¯uorescence signals detected in the UV could be attributed to the PAH ¯uorescence whereas the unidenti®ed species contained in the CS can be followed by LIF detection in the visible region. Ó 2001 Elsevier Science Ltd. All rights reserved.