INSITU EVALUATION OF THE SOOT REFRACTIVE-INDEX IN THE UV-VISIBLE FROM THE MEASUREMENT OF THE SCATTERING AND EXTINCTION COEFFICIENTS IN RICH FLAMES

An in situ chemical and physical characterization of soot particles during the separate stages of formation and oxidation appears to be an important step forward in the understanding of the heterogeneous aspects of the kinetics. To this end the knowledge of the optical properties of soot particles inside rich flames is of great relevance and categorizes "soot" among the large family of carbonaceous materials. This article proposes a new method for the evaluation of the dispersion of the optical properties of absorbing submicronic aerosols, starting from the simultaneous measurements of the scattering and extinction coefficients in the near UV and visible. The experiments were on premixed flat flames at atmospheric pressure with such nonaromatic fuels as CH4, C2H4, and C2H2 and different CO ratios and flow rates. A quantitative determination of the spectral behavior of the real and imaginary parts of the complex refractive index of soot was obtained in conditions where the molecular contribution could be neglected and where the particles were not agglomerated and behaved as Rayleigh scatterers. The optical properties of soot were compared with those obtained for graphite (250-550 nm), different amorphous carbons, and coals in the same wavelength range. The real part of the refractive index exhibits a strong dispersion in the visible whereas the imaginary part has a hump near 300 nm indicating that the resonance of π electrons, typical of graphite, is much enlarged and shifted toward the red. © 1990.