We demonstrate that the cooperative charge transfer among constructive molecules in organic crystals of tetrathiafulvalene-chloranil, i.e., the bidirectional transition between the neutral and ionic phases, can certainly be induced by irradiation of a laser pulse with an 80-fs width. We also report the dynamics of the photoinduced neutral-ionic phase transition by means of the time-resolved spectroscopic technique. Studies using various excitation intensities have shown that the photoconverted fraction (Φ) depends nonlinearly on the excitation intensity. In addition, it has been found that the growth dynamics of the macroscopic domain of the metastable phase strongly depends on the excitation intensity as well. The role of the cooperative charge-transfer interaction in the driving process of this unusual photoeffect is discussed from both the experimental and the theoretical viewpoints.
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