Structural interpretations of static light scattering patterns of fractal aggregates - I. Introduction of a mean optical index: Numerical simulations

A method based on static light scattering by fractal aggregates is introduc ed to extract structural information. in this study, we determine the scatt ered intensity by a fractal aggregate calculating the Structure and the For m factors noted, respectively, S(q) and F(q). We use the approximation of t he mean field Mie scattering by fractal aggregates (R, Botet, P. Rannou, an d M. Cabane, appl. opt. 36, 8791, 1997). This approximation is validated by a comparison of the scattering and extinction cross sections values calcul ated using, on the one hand, Mie theory with a mean optical index (ii) and, on the other hand, the mean field approximation, Scattering and extinction cross sections values differ by about 5%. We show that the mean environmen t of primary scatterers characterized by the optical index n(s) must be tak en into account to interpret accurately the scattering pattern from fractal aggregates. Numerical simulations were done to evaluate the influence of t he fractal dimension values (D-f > 2) and of the radius of gyration or the number of primary particles within the aggregates (N = 50 to 250) on the sc atterers' mean optical contrast (n(s)/(n) over bar). This last parameter pl ays a major role in determining the Form factor F(q) which corresponds to t he primary particles' scattering, In associating the mean optical index ((n ) over bar) to structural characteristics, this work provides a theoretical framework to be used to provide additional structural information from the scattering pattern of a fractal aggregate (Cf. Part II). (C) 2000 Academic Press.