Determination of the U.V.-Visible Optical Properties of Submicronic Carbonaceous Particles at High Temperature from Scattering and Extinction Measurements

Abstract

It is well known that both the U.V. extinction and the I.R. emission spectra measured in interstellar media could be partially attributed to carbon compounds (Huffman, 1977; Allamandola et al., 1987). Furthermore it is normally assumed that the carbon is present in the form either of submicronic solid particles or of large aromatic molecules, mainly polycyclic aromatic hydrocarbons (PAH). Therefore the problem of modelling the observed spectra is partly a problem of light scattering by small particles composed by a medium showing a dispersion of the optical properties and partly a problem of emission, absorption and fluorescence spectroscopy of large molecules. A surprising similar problem is present in the optical diagnostics of combustion of hydrocarbons in defect of oxygen; in fact also in this case carbonaceous particles (soot) and PAH are produced into the flame and often survives into the exhausts of engines or furnaces. This group of research has developed in the last few years different optical methods for sizing soot particles in flames (D’Alessio et al., 1975; D’Alessio et al., 1977) and detecting PAH molecules by laser excited fluorescence (D’Alessio, 1981; Beretta et al., 1985). In this field a specific line of research has been devoted to the measurement of the scattering and extinction coefficients inside rich flames in the wavelength range 250 nm to 550 nm (Menna and D’Alessio, 1983; Vaglieco et al., 1986) and the present communication intends to report on the evaluation of the complex refractive index of soot particles from those measurements. This approach presents the advantage of being an “in situ” technique which gives directly the optical properties of the submicronic soot particles whereas the conventional methods of determination of the optical properties from bulk samples suffer from the uncertainties of the procedure of sampling and compacting the original aerosols.