When used as pump pulses in transient absorption spectroscopy measurements, femtosecond laser pulses can produce oscillatory signals known as quantum beats. The quantum beats arise from coherent superpositions of the states of the sample and are best studied in the Fourier domain using Femtosecond Coherence Spectroscopy (FCS), which consists of one-dimensional amplitude and phase plots of a specified oscillation frequency as a function of the detection frequency. Prior works have shown ubiquitous amplitude nodes and π phase shifts in FCS from excited-state vibrational wavepackets in monomer samples. However, the FCS arising from vibronic-exciton states in molecular aggregates have not been studied theoretically. Here, we use a model of vibronic-exciton states in molecular dimers based on displaced harmonic oscillators to simulate FCS for dimers in two important cases. Simulations reveal distinct spectral signatures of excited-state vibronic-exciton coherences in molecular dimers that may be used to distinguish them from monomer vibrational coherences. A salient result is that, for certain relative orientations of the transition dipoles, the key resonance condition between the electronic coupling and the frequency of the vibrational mode may yield strong enhancement of the quantum-beat amplitude and, perhaps, also cause a significant decrease of the oscillation frequency to a value far lower than the vibrational frequency. Future studies using these results will lead to new insights into the excited-state coherences generated in photosynthetic pigment–protein complexes.

中文翻译：

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分子聚集体中振动激子飞秒相干光谱理论模型

当飞秒激光脉冲用作瞬态吸收光谱测量中的泵浦脉冲时，可以产生称为量子拍频的振荡信号。量子拍产生于样品状态的相干叠加，最好使用飞秒相干光谱 (FCS) 在傅里叶域中进行研究，该光谱由指定振荡频率随检测变化的一维振幅和相位图组成频率。先前的工作已经表明，FCS 中普遍存在来自单体样品中激发态振动波包的振幅节点和 π 相移。然而，分子聚集体中振动激子态产生的FCS尚未在理论上得到研究。在这里，我们使用基于位移谐振子的分子二聚体中的振动激子态模型来模拟两个重要情况下二聚体的 FCS。模拟揭示了分子二聚体中激发态振动激子相干性的独特光谱特征，可用于将它们与单体振动相干性区分开来。一个显着的结果是，对于跃迁偶极子的某些相对方向，电子耦合和振动模式频率之间的关键共振条件可能会产生量子拍幅的强烈增强，并且也许还会导致量子拍频的显着降低。振荡频率远低于振动频率。未来利用这些结果的研究将对光合色素-蛋白质复合物中产生的激发态相干性产生新的见解。