The scattering of light. II. The complex refractive index of a molecular fluid

We treat the complex refractive index of a finite molecular fluid on the basis of a classical many-body theory; the surface problem is handled through surfacedependent propagators. We develop a density expansion generalizing the Lorentz-Lorenz relation and sum all two-body terms to a closed form with intermolecular correlations determined by a Lennard-Jones pair potential; the dependence on density, temperature and frequency is discussed and the case when the frequency is near a molecular resonance is considered. The refractive index and the extinction coefficient are compared with experiments for gases. We also derive a generalization of the macroscopic relation of Onsager and Bottcher from the many-body theory, essentially as an expansion in an effective polarizability of a molecule in the many-body system. Exact microscopic expressions for an effective polarizability and for a reaction field are identified, and it is shown that they are related like the Bottcher polarizability and the Onsager reaction field in a well-defined decorrelation approximation. The relation with, and validity of, the macroscopic formulae of Lorentz and Bottcher are analysed in depth.