Department of Chemistry, UC Berkeley, Berkeley, California 94720, USA.
J Chem Phys. 2011 Jul 28;135(4):044201. doi: 10.1063/1.3607236.
We experimentally demonstrate a nonlinear spectroscopic method that is sensitive to exciton-exciton interactions in a Frenkel exciton system. Spatial overlap of one-exciton wavefunctions leads to coupling between them, resulting in two-exciton eigenstates that have the character of many single-exciton pairs. The mixed character of the two-exciton wavefunctions gives rise to a four-wave-mixing nonlinear frequency generation signal. When only part of the linear excitation spectrum of the complex is excited with three spectrally tailored pulses with separate spatial directions, a frequency-shifted third-order nonlinear signal emerges in the phase-matched direction. We employ the nonlinear response function formalism to show that the emergence of the signal is mediated by and carries information about the two-exciton eigenstates of the system. We report experimental results for nonlinear frequency generation in the Fenna-Matthews-Olson (FMO) photosynthetic pigment-protein complex. Our theoretical analysis of the signal from FMO confirms that the emergence of the frequency-shifted signal is due to the interaction of spatially overlapped excitons. In this method, the signal intensity is directly measured in the frequency domain and does not require scanning of pulse delays or signal phase retrieval. The wavefunctions of the two-exciton states contain information about the spatial overlap of excitons and can be helpful in identifying coupling strengths and relaxation pathways. We propose this method as a facile experimental means of studying exciton correlations in systems with complicated electronic structures.
我们实验演示了一种对弗伦克尔激子体系中激子-激子相互作用敏感的非线性光谱方法。单激子波函数的空间重叠导致它们之间的耦合，从而产生具有许多单激子对特征的双激子本征态。双激子波函数的混合特征导致四波混频非线性频率产生信号。当仅用三个具有独立空间方向的光谱调谐脉冲激发复合体线性激发光谱的一部分时，在相匹配方向上会出现频移的三阶非线性信号。我们采用非线性响应函数形式理论来表明，信号的出现是由系统的双激子本征态介导并携带其信息的。我们报告了在 Fenna-Matthews-Olson（FMO）光合作用色素蛋白复合体中的非线性频率产生的实验结果。我们对 FMO 信号的理论分析证实，频移信号的出现是由于空间重叠激子的相互作用。在该方法中，信号强度直接在频域中进行测量，不需要扫描脉冲延迟或信号相位检索。双激子态的波函数包含激子空间重叠的信息，可有助于识别耦合强度和弛豫途径。我们提出该方法作为研究具有复杂电子结构的体系中激子相关性的简便实验手段。
