Department of Chemical Physics, Lund University, P.O. Box 124, SE-22100 Lund, Sweden.
Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 3, 121 16 Prague, Czech Republic.
J Chem Phys. 2014 Mar 21;140(11):115103. doi: 10.1063/1.4868557.
Coherent two-dimensional (2D) spectroscopy at 80 K was used to study chlorosomes isolated from green sulfur bacterium Chlorobaculum tepidum. Two distinct processes in the evolution of the 2D spectrum are observed. The first being exciton diffusion, seen in the change of the spectral shape occurring on a 100-fs timescale, and the second being vibrational coherences, realized through coherent beatings with frequencies of 91 and 145 cm(-1) that are dephased during the first 1.2 ps. The distribution of the oscillation amplitude in the 2D spectra is independent of the evolution of the 2D spectral shape. This implies that the diffusion energy transfer process does not transfer coherences within the chlorosome. Remarkably, the oscillatory pattern observed in the negative regions of the 2D spectrum (dominated by the excited state absorption) is a mirror image of the oscillations found in the positive part (originating from the stimulated emission and ground state bleach). This observation is surprising since it is expected that coherences in the electronic ground and excited states are generated with the same probability and the latter dephase faster in the presence of fast diffusion. Moreover, the relative amplitude of coherent beatings is rather high compared to non-oscillatory signal despite the reported low values of the Huang-Rhys factors. The origin of these effects is discussed in terms of the vibronic and Herzberg-Teller couplings.
在 80 K 下进行的相干二维（2D）光谱学用于研究从绿硫细菌 Chlorobaculum tepidum 中分离出的叶绿素体。观察到 2D 光谱演化中的两个不同过程。第一个是激子扩散，在 100fs 时间尺度上发生的光谱形状变化中可见，第二个是振动相干，通过相干拍频实现，频率为 91 和 145cm-1，在最初的 1.2ps 期间相消。2D 光谱中振荡幅度的分布与 2D 光谱形状的演化无关。这意味着扩散能量转移过程不会在叶绿素体内部传递相干。值得注意的是，在 2D 光谱的负区域（主要由激发态吸收主导）中观察到的振荡模式是在正部分（源自受激发射和基态漂白）中发现的振荡的镜像。这种观察结果令人惊讶，因为预计电子基态和激发态中的相干以相同的概率产生，并且在后一种情况下在快速扩散存在下更快地相消。此外，尽管报道的黄昌-瑞斯因子值较低，但相干拍频的相对幅度与非振荡信号相当高。这些效应的起源是根据振子和赫茨伯格-泰勒耦合来讨论的。
