Ultrafast relaxation dynamics is studied in a quasi-one-dimensional halogen-bridged mixed-valence metal complex [Pt(en)2] [Pt(en)2Cl2](CIO4)4 by pump-probe spectroscopy. A photoinduced absorption spectrum is measured in a broadband energy region of 0.6-2.6 eV, applying optical parametric amplification. The measurement is performed at both 293 and 4 K. The photoinduced absorption spectrum consists of the three contributions with different time and temperature dependence. One is the absorption band ascribed to the self-trapped exciton (STE), and the second is that due to the spatially uncorrelated neutral-soliton pair. Just after the photoexcitation, the STE is in a nonequilibrium state, and the time required for the thermalization is evaluated to be about 1 ps. The formation time of the third one is coincident with the thermalization time of the STE, and it is attributed to the spatially confined neutral-soliton pair.
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