Study of exciton dynamics in a simple liquid

A quantum-mechanical formulation of the theory of Frenkel excitons in a simple liquid is given with attention focused on the interaction of the exciton with the collective translational excitations of the liquid. Using a Born-Oppenheimer approximation to define a basis, the nonadiabatic term which describes the coupling of the exciton to the momentum density of the liquid is treated as a perturbation. It is shown, by use of Green's-function methods, that this coupling to the collective modes of the liquid is more important than is elastic scattering arising from the disorder in the system. The energy shift, damping, and the line shape of the Frenkel exciton are expressed in terms of the resonance interaction, the structure function, the translational kinetic energy of the liquid, and the lifetime of translational collective excitations.