Nonlinear interaction of excitons and photoinduced phase transition in quasi-one dimensional organic charge-transfer crystals

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The purpose of this project is to clarify the dynamics of photoinduced ionic-to-neutral phase transition (NI transition) in one-dimensional organic charge-transfer crystal of tetrathiafulvalen-p-chloranil (TTF-CA) for understanding the mechanism of this photoinduced process from microscopic point of view. We have studied the temporal evolution of the process by means of time-resolved reflectance spectroscopy in temporal domains of sub-ps, and the state-sensitive efficiency of the phase transition by means of excitation spectroscopy, using a tunable ns laser from 266 to 2000 nm. Main results obtained are summarized below.1) The whole process of the photoinduced NI transition in TTF-CA consists of the three sequential primary steps ; the first is the precursor-forming process at a few ps after excitation, the second is the process of local proliferation around 50 ps after excitation, and the third is the process of forming N-phase orders, lasting over 300 ps.2) The process of precursor formation is nonlinear in the sense that one-absorbed photon converts the charge state of as many as a few hundreds of donor-acceptor molecules to form individual neutral-molecular clusters on stack chains. The proliferation in this process is strongly excitation-wavelength dependent, showing the initial-condition sensitivity in this photoinduced transformation.3) The second and third processes are associated with nonlinear proliferation of photoinduced changes ; the magnitudes of proliferation depend strongly on the amount of the precursor photoinduced. This proliferation of nonlinear relaxation also dependents on temperature, and the magnitude is governed by an activation energy of 0.01eV, which roughly corresponds to the free-energy difference between the two phases.These results have provided us crucial knowledge for establishing thorough understanding of the mechanism of this photoinduced phase transition.