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

本课题的目的是阐明一维四硫代丙烯-对氯胺有机电荷转移晶体（TTF-CA）光诱导离子到中性相变（NI相变）的动力学，从微观角度理解这一光诱导过程的机理。我们利用时间分辨反射光谱在亚ps的时域研究了这一过程的时间演化，并利用激发光谱研究了相变的状态敏感效率，使用了266 ~ 2000 nm的可调谐ns激光器。获得的主要结果总结如下。1) TTF-CA光致NI跃迁的整个过程由三个连续的初级步骤组成；第一个是激发后几ps的前体形成过程，第二个是激发后50 ps左右的局部增殖过程，第三个是形成n相序的过程，持续时间超过300 ps。2)前体形成过程是非线性的，因为一个吸收的光子将多达数百个给体-受体分子的电荷状态转换成堆叠链上的单个中性分子簇。在这个过程中的扩散是强烈的激发波长依赖，显示了初始条件敏感性在这个光诱导转化。3)第二和第三过程与光致变化的非线性增殖有关；增殖的大小在很大程度上取决于光诱导前驱体的量。这种非线性弛豫的扩散也依赖于温度，其大小由0.01eV的活化能控制，这大致对应于两相之间的自由能差。这些结果为我们深入了解这种光诱导相变的机理提供了重要的知识。

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.