Primitive causality and optically active molecules

Abstract

The primitive causality condition-namely that an effect cannot precede its cause-yields analytic properties and dispersion relations for the polarizability when the cause is an external electric field acting at the centre of an isolated molecule and the effect is the induced electric dipole moment. For a chiral molecule the lowest approximation that allows for the optical activity involves also the spatial derivatives of the electric field at the molecular centre, and a more stringent condition must be imposed in order to show the analyticity of the optical rotatory parameter as well as that of the polarizability. This condition is illustrated by the classical model of an optically active molecule consisting of two dissymmetrically coupled damped linear oscillators. The various approaches that can be used to derive dispersion relations for optically active isotropic media are briefly reviewed.