Light-scattering properties of random-oriented aggregates: Do they represent the properties of an ensemble of aggregates?

Abstract

We question a common assumption that orientation-averaged light-scattering properties of a single aggregate represent the properties of an ensemble of random aggregates which have the same number of the same constituent particles. Using the T-matrix code for calculation of light scattering by aggregates of spheres, we obtain efficiencies and scattering-angle dependences of intensity and polarization for several samples of aggregates. The aggregates are built using ballistic particle–cluster and cluster–cluster aggregation procedures. Their constituent particles have the same size and the same properties but their arrangement (i.e. specific positions) in the aggregates are different. We have found that the random-oriented efficiencies and asymmetry parameter, as well as the angular dependence of the intensity, vary significantly from one aggregate to another. Linear polarization appears to be not very sensitive to the arrangement of particles within the aggregate, whereas circular polarization has been found the most affected characteristics, which can be zero, positive or negative for the aggregates that differ only in the arrangement of their constituents. This effect originates from the violation of mirror symmetry in aggregates, which reveals itself even at random orientation. Thus, the correct description of light scattering by an ensemble of aggregates requires a realistic averaging over a variety of random aggregates. Such an averaging can be provided by the random-matrix approach.