NONLINEAR LATTICE-RELAXATION PROCESS OF EXCITONS IN QUASI-ONE-DIMENSIONAL HALOGEN-BRIDGED MIXED-VALENCE METAL-COMPLEXES - SELF-TRAPPING, SOLITONS, AND POLARONS

Self-trapped excitons, solitons, and polarons of a one-dimensional extended Peierls-Hubbard model are investigated, in order to clarify the lattice-relaxation paths of photogenerated excitons in halogen-bridged mixed-valence metal complexes. This theory takes into account the lattice distortion of halogen ions in a direction perpendicular to the chain, as well as in a parallel direction. It is mainly based on the adiabatic approximation for phonons and the mean-field theory for interelectronic interactions, but is also reinforced by taking into account the electron-hole correlation to obtain the exciton. Potential surfaces relevant to the relaxation of the exciton are clarified in terms of various nonlinear excitations. This result can explain rather diverse experiments from a unified point of view, such as photoabsorption and luminescence spectra, the Stark effect, and electron-spin-resonance data. The origin of the photoinduced absorption is concluded to be polarons.