The observation of laser-induced visible fluorescence in sooting diffusion flames

Abstract

Visible fluorescence is produced using an argon-ion laser for excitation in sooting methane air and methane oxygen diffusion flames. This emission is attributed to small (two to four ring) polycyclic aromatic hydrocarbons, as suggested by previous studies. The key finding in the present investigation is that the fluorescence spectra can be altered dramatically by changes in the flame stabilization conditions. Spectra obtained with a slot burner and a cylindrically symmetric burner are presented, and are usually found to be broad and unstructured. However, under certain experimental conditions the laser-induced fluorescence spectra show considerable structure; four distinct peaks are identified at different excitation wavelengths. Evidence is presented that the structured spectra are caused by recirculation of downstream combustion gases (and particles) into the optically sampled area. Thus, these observations point out the importance of characterizing the flow conditions when optical measurements are used to study the flame chemistry of polycyclic aromatic molecules. Possible temperature effects and the relevance of the results to soot formation processes are also discussed.