Relaxation processes yielding nonlinear excitations in the extended Peierls-Hubbard model for photoexcited MX chains

The relaxations occurring after photoexcitation in the MX chains are investigated, using a one-dimensional extended Peierls-Hubbard model. The special emphasis is placed on the processes in which the nonlinear excitations such as solitons and polarons are created in the background of the charge-density wave (CDW). The way of relaxations changes its nature, depending mainly on the relative strengths of the on-site and nearest-neighbor Coulombic repulsions U and V, respectively, to the on-site electron-lattice interaction S. When U or both L and V dominate over S, soliton-pair states are unstable compared with the self-trapped exciton (STE) in the localized Limit of the model. However, the neutral-soliton pair can be the lowest structural excitation in intermediate-transfer cases. stabilized by the effect of itineracy. As for the adiabatic potential surfaces, the uniform CDW is connected to the neutral-soliton pair, while the STE to the charged-soliton pair. It is expected that the STE will make a transition to the neutral-soliton pair nonadiabatically. When S dominates over U and V, on the other hand! the charged-soliton pair is the lowest structural excitation even in the localized limit. In those cases, the uniform CDW is connected to the charged-soliton pair, while the STE to the neutral-soliton pair on the same potential surfaces. Following these considerations, the calculation of the photoinduced absorption is also performed assuming that the former case is realistic. The obtained spectrum in the presence of a neutral soliton is compared with the experimental result.