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This paper is a continuation of a recent one in which, apparently for the first time, theexistence of polaritons in ionic crystals was proven in a microscopic electrodynamictheory. This was obtained through an explicit computation of the dispersion curves. Herethe main further contribution consists in studying electric susceptibility, from which thespectrum can be inferred. We show how susceptibility is obtained by the Green-Kubo methodsof Hamiltonian statistical mechanics, and give for it a concrete expression in terms oftime-correlation functions. As in the previous paper, here too we work in a completelyclassical framework, in which the electrodynamic forces acting on the charges are alltaken into account, both the retarded forces and the radiation reaction ones. So, in orderto apply the methods of statistical mechanics, the system has to be previously reduced toa Hamiltonian one. This is made possible in virtue of two global properties of classicalelectrodynamics, namely, the Wheeler-Feynman identity and the Ewald resummationproperties, the proofs of which were already given for ordered system. The secondcontribution consists in formulating the theory in a completely general way, so that inprinciple it applies also to disordered systems such as glasses, or liquids or gases,provided the two general properties mentioned above continue to hold. A first step in thisdirection is made here by providing a completely general proof of the Wheeler-Feynmanidentity, which is shown to be the counterpart of a general causality property ofclassical electrodynamics. Finally it is shown how a line spectrum can appear at all inclassical systems, as a counterpart of suitable stability properties of the motions, witha broadening due to a coexistence of chaoticity. The relevance of some recent results ofthe theory of dynamical systems in this connection is also pointed out.

the european physical journal. d, atomic, molecular and optical physics/the european physical journal. d, atomic, molecular, optical and plasma physics

Classical microscopic theory of dispersion, emission and absorption of light in dielectrics