Femtosecond pulsed excitation of light-harvesting complexes creates oscillatory features in their response. This phenomenon has inspired a large body of work aimed at uncovering the origin of the coherent beatings and possible implications for function. Here we exploit site-directed mutagenesis to change the excitonic level structure in Fenna–Matthews–Olson (FMO) complexes and compare the coherences using broadband pump–probe spectroscopy. Our experiments detect two oscillation frequencies with dephasing on a picosecond timescale—both at 77 K and at room temperature. By studying these coherences with selective excitation pump–probe experiments, where pump excitation is in resonance only with the lowest excitonic state, we show that the key contributions to these oscillations stem from ground-state vibrational wavepackets. These experiments explicitly show that the coherences—although in the ground electronic state—can be probed at the absorption resonances of other bacteriochlorophyll molecules because of delocalization of the electronic excitation over several chromophores.

飞秒脉冲激发光捕获复合物在其响应中产生振荡特征。这种现象激发了大量工作，旨在揭示连贯殴打的起源以及对功能的潜在影响。在这里，我们利用定点诱变来改变Fenna–Matthews–Olson（FMO）复合物中的激子能级结构，并使用宽带泵浦-探针光谱法比较相干性。我们的实验检测到了两个皮秒级时相移的振荡频率-均为77 K和室温。通过使用选择性激励泵浦-探针实验研究这些相干性，其中泵浦激励仅在最低激子态下发生共振，我们表明，对这些振荡的关键贡献来自基态振动波包。