Electromagnetic coupling and determination of the structure factor of fractal aggregates

International audienceThe Rayleigh-Debye-Gans approximation for fractal aggregates (RDG-FA) is commonly used to evaluate the radiative properties of soot aerosols composed of nano-spheres due to its analytical character. Despite neglecting electromagnetic coupling within an aggregate, the RDG-FA provides a simple interpretation of angular light-scattering measurements in terms of a structure factor. This factor, in turn, enables the determination of the aggregate’s fractal dimension and radius of gyration . The structure factor can be expressed as the Fourier transform of a purely morphological autocorrelation function. Here we employ the discrete dipole approximation and phasor analysis of the internal electric field in an aggregate to study the role of coupling for the specific case of soot particle refractive index. An optical autocorrelation function is defined in terms of an aggregate’s phasors rather than simply its physical distribution of material. The new function conveys the effect of the non-uniformity of the internal field distribution, due to coupling, to the angular scattering. A correction term is then introduced explaining why coupling tends to decrease the structure factor inferred in the power-law regime. Such decrease impacts the determination of the fractal dimension from scattering data. Finally, it is shown that the inferred structure factor is mainly affected by a so-called internal trapping effect associated with a large imaginary part of the refractive index