Nonlinear lattice relaxation of photogenerated charge-transfer excitation in halogen-bridged mixed-valence metal complexes. I. Soliton and self-trapped exciton

The ground and excited states of a one-dimensional extended Peierls-Hubbard model with half-filled-band electrons are studied so as to clarify the lattice relaxation paths of photogenerated charge-transfer excitations in halogen-bridged mixed-valence metal complexes. The adiabatic potential-energy surfaces that describe the nonlinear relaxation from the Franck-Condon state to the solitonic states as well as to the self-trapped state of the exciton (STE) are calculated within the mean-field theory for electrons. It is shown that the lowest excited state is a pair of doubly charged solitons, and it gives a new absorption band with an energy of about a half of the gap. It is also shown that the STE is separated from this soliton pair by only a small barrier, in agreement with the experiments on the unusual short decay time of this state.