The Part of Free Excitons in the Luminescence of Molecular Crystals

Abstract

In Chapter I we set out in detail the theory of free and localized excitons in molecular crystals.

In the case of free excitons, one transition in the molecule corresponds to two allowed transitions in the crystal, owing to the Davydov splitting. This splitting is caused by resonance interaction between the molecules, and by the fact that the molecules have different orientations in the elementary cell. The two terms formed in the crystal have different polarizations, but both are polarized along definite crystallographic axes. In the monoclinic system, one term is usually polarized so that the vibrations of the electric vector are parallel to the b-axis, and the other term perpendicular to the b-axis.