Dielectric screening and susceptibility fluctuations in optical scattering

Abstract

An internally consistent theory of optical scattering in molecular fluids is developed in terms of a ‘screened dipole radiator’ and a screened dipole photon propagator, related by a Bohr-Peierls-Placzek-type relation. The screened dipole radiator describes the propagation of scattered radiation in the dielectric medium, the transition at the surface of the scattering sample, and the ultimate propagation in vacuum outside the material system. The scattering is obtained as a series in terms of Ursell functions, explicit to all orders. In an approximation describing light scattering by matter in bulk, the screened dipole radiator is obtained in closed form involving an intrinsic directional average. The directional average has important consequences for depolarization, for critical scattering, and for questions of convergence. A microscopic representation of local susceptibility fluctuations emerges, and it is shown that polarization fluctuations can naturally and rigorously be decomposed into one contribution from local susceptibility fluctuations and one from field fluctuations. An approximation in closed form to the scattering cross section can be interpreted in macroscopic terms on the basis of this decomposition. In this approximation, critical scattering deviates from Ornstein-Zernike behaviour, but depolarized scattering is incompletely included.