U.V. Laser Induced Fluorescence for the detection of nanosized organic carbon (NOC) and soot particles in laminar premixed flames

Rich premixed flames produced carbon nanoparticles of different sizes and chemical nature. For C/O ratio just above the stoechiometric one, nanosized organic carbon (NOC) particles with typical size around 2-3 nm are produced just at the end of the reaction zone. For C/O ratio above the "so called soot threshold" black nanoparticles with typical size around 20 nm represent in addition with the NOC ones. The aim of this work is to develop and optical diagnostic method based on the interaction of a pulsed U.V. laser source with the particles, which is able to characterize both classes of carbon nanoparticles. At this purpose we used the fifth harmonics of a Nd:YAG laser, ?=213 nm, into a premixed ethylene/air laminar flame and followed the spectra as function of time and wavelengths. We observed an early phase of the signals that is attributed to fluorescence of NOC, while the later part of the signal is attributed to C2 emissions. Indeed, for low values of C/O ratio (below C/O=0.71) the emission is mainly due to fluorescence in the u.v.-visible spectral region between 260 and 450 nm. Moreover, a linear correlation between these spectra and the absorption coefficient is found. For richer flames broadband emission spectra were observed in the region ranged between 350 and 650 nm with longer typical life time around 150 ns. These spectra are a combination of fluorescence; C2 emissions and incandescence attributed to other particles, namely soot particles.