We obtain useful information from the intensity autocorrelations of light scattered from systems which exhibit strong multiple scattering. A phenomenological model, which exploits the diffusive nature of the transport of light, is shown to be in excellent agreement with experimental data for several different scattering geometries. The dependence on geometry provides an important experimental control over the time scale probed. We call this technique diffusing wave spectroscopy, and illustrate its utility by studying diffusion in a strongly interacting colloidal glass.

We examine the contribution of rotational diffusion to quasielastic light scattering (QELS) from fractal colloid aggregates, both theoretically and experimentally. Rotational diffusion makes a substantial contribution to QELS when the size of the clusters is large compared with the inverse of the scattering wave vector, due to the anisotropy of the clusters at all length scales smaller than the cluster size. We evaluate the rotational contributions to QELS by performing a multipole expansion of the light scattered from computer-simulated clusters. Experimentally, rotational contributions are observed through measurement of the wave-vector dependence of the first cumulant. We find excellent agreement between cumulants calculated through our multipole-expansion technique and those obtained in our experimental measurements.

We examine the optical properties of aggregate clusters and consider the effects of multiple scattering. The long-range fractal correlations can modify the mean index of refraction of the clusters, but multiple scattering has no effect on the wave-vector dependence of the scattering. By contrast, the short-range correlations inherent in a connected cluster lead to high-order multipole interactions which cannot be treated with a mean-field approach. These are shown to determine the wavelength dependence of the absorption and depolarized scattering from metallic clusters in good accord with experiment.