We report the photogeneration of solitons and polarons in the quasi-one-dimensional (1-D) halogen (X)-bridged metal (M) compounds (simply abbreviated as the MX chain compounds). The most significant feature of this system is the remarkable tunability of the charge density wave (CDW) ground states. By substituting the metals (M = Pt, Pd, and Ni), the bridging halogens (X = Cl, Br, and I), the ligand molecules and the counter anions surrounding the 1-D chains, the amplitude of CDW, the optical gap energy, and the degeneracy of CDW can be widely controlled. On the basis of these controls, we have investigated the nature of photoexcited states. By comparing the results of photoinduced absorption (PA), ESR and photoinduced ESR measurements in the degenerate CDW states with those in the non-degenerate CDW states, we clearly characterized the photoinduced gap states as solitons and polarons. In the compounds having relatively large optical gap energies (ECT). spin-solitons and polarons are photogenerated. From a comparison of the excitation profiles of the PA signals with those of the luminescence of the self-trapped exciton (STE), it was demonstrated that the luminescence process strongly competes with the dissociation to spin-soliton pairs. An analysis of the temperature dependence of the luminescence decay time revealed that the conversion from an STE to a solitonic state occurs through a finite potential barrier, the magnitude of which depends on degeneracy of CDW. With decrease of ECT, the nature of the photoexcited states changes considerably; photogenerations of charged-solitons are observed instead of spin-solitons and the STE luminescence is remarkably quenched. While referring to the theoretical expectations based upon the Peierls-Hubbard model, we will discuss the overall view of the relaxation process of the photoexcited states related to solitons, polarons, and excitons in the MX compounds.
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