PHTO-INDUCED PHASE TRANSITION AND ITS FORERUNNING PHENOMENON ON THE QUASI-ONE-DIMENSIONAL HALOGEN-BRIDGED TRANSITION METAL COMPOUNDS

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The temporal evolution of photo-excited state in quasi-one-dimensional halogen-bridged platinum complexes [Pt(en)_2][Pt(en)_2X_2][(ClO_4)_4 (abbreviated as Pt-X, X=Cl, Br or I), has been comprehensively studied by femtosecond time-resolved luminescence spectroscopy. In Pt-Br, we have observed a wave-packet oscillation of a period of about 300fs in an adiabatic potential well associated with a self-trapped exciton (STE). The STE's have a lifetime of about 5.5 ps. In Pt-Cl, short-lived hot luminescence is found in the low energy side of a STE luminescence band. The overall behavior of luminescence is explained by the vibrational relaxation within 2 ps and the lifetime of the STE with 30 ps. These behaviors of luminescence are reproduced by a model calculation based on wavepacket propagation on an interaction mode composed of frequency-dispersed bulk phonons. In Pt-I, the STE luminescence decays much faster (0.65 ps) than those in Pt-Cl and Pt-Br. From the observed dependence of STE lifetime, it is supposed that the barrier becomes lower with decrease of the CT gap energy, and the charged-soliton pairs are efficiently produced by photoexcitation in narrow CT gap. In Pt-I, especially, the system seems to go toward the soliton pair before the thermalization as STE.New quasi-one-dimensional halogen-bridged platinum binuclear compound, R_4[Pt_2(pop)_4l](R=C_3H_7)_2NH_2) is found to show both the pressure- and photo-induced phase transitions at room temperature.We have developed the optical Kerr gate fluorescence spectroscopy with a subpicosecond time resolution as a new spectroscopic tool to investigate ultrafast relaxation processes. The typical Kerr efficiency and instrumental response function of the system are 〜5% and 〜250fs. This system is applied to measure the decay of electron-hole plasma state in ZnO.