UV AND VISIBLE LASER-EXCITED FLUORESCENCE FROM RICH PREMIXED AND DIFFUSION FLAMES

Fluorescence spectra were obtained in laminar, gaseous, rich premixed and diffusion flames employing different harmonics of a pulsed Nd: YAG laser. Spectra obtained with the fourth harmonic at lambda0 = 266 nm exhibit a peak in the u.v. at lambda = 320 nm and a broader one in the visible with a maximum beyond 400 nm. The relative role of these two features has been followed in rich premixed flames with different C:O ratios and heights above the burner and along the radius of a cylindrical, coflowing diffusion flame. The u.v. fluorescence detected in our flames should be attributed to structures containing no more than two aromatic rings, while the fluorescence in the visible is attributed to larger aromatic structures. Fluorescence and absorption spectra obtained from the different fractions of the material sampled into the flames showed qualitative agreement with the ''in situ'' spectra. Fluorescence spectra in the visible, originating from structures heavier than 300 a.m.u., has been also detected in the sampled material but are not considered the main cause of the visible fluorescence because their low quantum efficiency for u.v. excitation.