Exciton-Phonon Interaction and Optical Spectra –Self-trapping, Zero-Phonon Line and Phonon Sidebands–

Optical properties of interacting exciton-phonon system have been theoretically studied in terms of a simple model consisting of Frenkel excitons and Einstein oscillators. The problem is characterized by two non-dimensional parameters B and S representing the exciton band width and the interaction energy, respectively. Approximate solutions have been obtained through the diagonalization of the Hamiltonian in special sub-spaces, and they reasonably describe the behavior of the system for all values of B and S . Absorption and emission spectra and the effective mass of the composite particle vary from free exciton type to self-trapped exciton type as we go from B > S to B < S . The energy-momentum relation has been qualitatively studied below the one-phonon continuum, and the following new features have been found in the strong coupling region; (1) complete split-off of the lowest branch in the whole Brillouin zone, and (2) the split-off of the second discrete branch. Furthermore, a clue to the mechanism of the Urbach-Martienssen rule has also been obtained.