Abstract

Optical measurements in combustion environments have been very successful for detection of molecules containing two and three atoms, but no species-specific results have been reported for larger compounds. However, intermediate-size hydrocarbons as a group have been detected by laser-induced broadband fluorescence in the visible region. The observed spectra have been widely attributed to polycyclic aromatic hydrocarbons, which are thought to be important in soot formation chemistry as precursors and perhaps as building blocks. Recently, we extended these fluorescence measurements to the ultraviolet region as part of an investigation of the soot inception zone in laminar diffusion flames. Detailed concentration profiles have been determined in a methane–air flame using a variety of optical and mass spectrometric techniques. Broadband ultraviolet emission in the 300-400-nm region is observed on excitation at 282 nm. The spatial profiles of this fluorescence exhibit two distinct contributions, which show different behavior as a function of height above the burner. One contribution is located in the soot inception zone and overlaps the broadband visible fluorescence profile, whereas the second feature is unconnected with chemical growth processes and arises from a stable combustion by-product. Many candidates exist for the observed ultraviolet fluorescence including polyenes, polyynes, and the polycyclic aromatic hydrocarbons.

© 1985 Optical Society of America