The optical band gap model in the interpretation of the UV-visible absorption spectra of rich premixed flames

A spectral analysis of the light extinction of near-sooting premixed benzene/air flames at atmospheric pressure has been performed between 200 and 700 nm, together with UV laser-excited fluorescence and light scattering. The absorption spectra, attributed to high molecular mass structures, are interpreted in the framework of the optical properties of solid state matter, according to the Tauc band-gap model. This is a powerful approach to characterize carbonaceous material; in fact, theoretical computations on carbon structures containing conjugated ? bonds indicate that the extension of aromatic clusters or islands in side the structures can be obtained from the measured optical band-gap energies. The absorption measurements, performed both in the soot-preinception and soot-inception regions, give a clear indication that a spread of structures, with a different degree of aromatization, is present, since at least three optical band gaps are identified. The soot inception itself is characterized by a sharp decrease of the band gap caused by the progressive aromatization of the structures. This process takes place before the particles grow by surface reactions. Finally, the extinction spectra of fully grown soot particles, so far expressed by an empirical dispersion law, are reinterpreted in terms of the optical band gap, thus allowing a more rational correlation between the optical and chemical properties of the particles