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

本课题的目的是研究一维有机电荷转移晶体四硫富瓦仑-对氯苯醌（TTF-CA）中光诱导离子-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。我们已经研究了时间演化的过程中，通过时间分辨的反射光谱在时间域的亚ps，和状态敏感的效率的相变通过激发光谱，使用可调谐的ns激光从266到2000 nm。主要结果如下：1）TTF-CA光致NI转变的全过程由三个主要步骤组成;第一个是在激发后几ps的超导体形成过程，第二个是在激发后50 ps左右的局部增殖过程，第三个是形成N相有序的过程，持续超过300 ps。2）前体形成的过程是非线性的，在这个意义上，一个吸收的光子转换多达几百个供体-受体分子的电荷状态，以在堆叠链上形成单独的中性分子簇。在这个过程中的增殖是强烈的激发波长依赖性，显示在这个光诱导的转变的初始条件的敏感性。3）第二和第三个过程是与光诱导的变化的非线性增殖;增殖的幅度强烈地依赖于前体光诱导的量。这种非线性弛豫的扩散也依赖于温度，其大小由0.01eV的激活能控制，这大致对应于两相之间的自由能差，这些结果为我们深入理解这种光致相变的机制提供了重要的知识。

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.