SCATTERING OF LIGHT BY BISPHERES WITH TOUCHING AND SEPARATED COMPONENTS

We use the T-matrix method as described by Mishchenko and Mackowski [O pt. Lett. 19, 1604 (1994)] to compute light scattering by bispheres in fixed and random orientations extensively. For all our computations t he index of refraction is fixed at a value 1.5 + 0.005i, which is clos e to the refractive index of mineral tropospheric aerosols and was use d in previous extensive studies of light scattering by spheroids and C hebyshev particles. For monodisperse bispheres with touching component s in a fixed orientation, electromagnetic interactions between the con stituent spheres result in a considerably more complicated interferenc e structure in the scattering patterns than that for single monodisper se spheres. However, this increased structure is largely washed out by orientational averaging and results in scattering patterns for random ly oriented bispheres that are close to those for single spheres with size equal to the size of the bisphere components. Unlike other nonsph erical particles such as cubes and spheroids, randomly oriented bisphe res do not exhibit pronounced enhancement of side scattering and reduc tion of backscattering and positive polarization at side-scattering an gles. Thus the dominant feature of light scattering by randomly orient ed bispheres is the single scattering from the component spheres, wher eas the effects of cooperative scattering and concavity of the bispher e shape play a minor role. The only distinct manifestations of nonsphe ricity and cooperative scattering effects for randomly oriented bisphe res are the departure of the ratio F-22/F-11 of the elements of the sc attering matrix from unity, the inequality of the ratios F-33/F-11 and F-44/F-11, and nonzero linear and circular backscattering depolarizat ion ratios. Our computations for randomly oriented bispheres with sepa rated wavelength-sized components show that the component spheres beco me essentially independent scatterers at as small a distance between t heir centers as 4 times their radii.