Radiative decay of polaron-excitons in poly(phenylene vinylene)

Starting from a nondegenerate-ground-state Su-Schrieffer-Heeger-like model Hamiltonian for poly(phenylene vinylene) we study the effect of electron-lattice coupling on the radiative recombination of polaron-excitons, using a lattice relaxation theory which explicitly takes into account multimode effects. We provide a detailed microscopic description of the decay process with emphasis on the fact that the phonon modes of the excited state---the polaron-exciton---are different from the ground-state phonon modes. We find that at zero temperature all accepting modes are within an almost dispersionless ground-state phonon branch. Therefore, as long as the polaron-exciton is in its vibrational ground state a simple, empirical, single-mode description of the recombination (photoluminescence) is feasible. However, the complicated phonon structure of the polaron-exciton---including localized as well as extended modes---renders an Einstein desciption of promoting modes meaningless. We thus expect that in processes where promoting modes are involved a single-mode model breaks down.