Studies on Weak Intermolecular Interactions by Low-Frequency Vibrational Coherence

超短光脉冲可以相干激发频率小于脉冲带宽的振动。这种相干性可以通过光谱方法实时观察到振荡，并且可以获得关于特定低频振动模式动力学的具体知识。相干光谱学是目前研究振动弛豫、振动减相以及特定振动对化学反应、分子间振动、生物系统分子识别的贡献的一种独立的光谱学方法。具有电荷转移特征的电子态存在于许多天然分子体系中，如铁卟啉和光合反应中心。利用飞秒荧光上转换技术研究了几种电子给体-受体配合物的激发态动力学。配合物的自发荧光表现出振荡特征，叠加有超快衰变成分。振荡被分配到受体的面外振动，对于使用三种不同受体的配合物，四氰乙烯、氯胺和氟胺。峰移相关函数显示出一种超快弛豫，这主要归因于分子内振动能量再分配过程与振荡分量叠加，表明振动过程中跃迁频率的调制。同时观察到平均跃迁矩的调制，这是非康顿跃迁的一个指示。因此，观察到两种机制是负责振荡：调制的过渡频率和调制的平均过渡矩的振动运动。

Ultrashort light pulses can coherently excite vibrations with the frequency smaller than the pulse bandwidth. This coherence can be observed by spectroscopic methods in real time as oscillations and specific knowledge can be gained about dynamics of the particular low frequency vibrational mode. Coherent spectroscopy is currently a separate spectroscopic method to study vibrational relaxation, vibrational dephasing and contribution of particular vibrations to chemical reactions, intermolecular vibrations, molecular recognition in biological systems. Electronic states having a charge-transfer character can be found in many natural molecular systems, for example, in iron porphyrin and in photosynthetic reaction center.The excited-state dynamics of several electron donor-acceptor complexes was studied by a femtosecond fluorescence up-conversion technique. The spontaneous fluorescence of the complexes exhibits an oscillatory feature superimposed with an ultrafast decay component. The oscillation is assigned to the out-of-plane vibration of the acceptors, for complexes with three different acceptors used, tetracyanoethylene, chloranil, and fluoranil. The peak-shift correlation function shows an ultrafast relaxation attributed mainly to the intramolecular vibrational energy redistribution process superimposed with the oscillatory component that demonstrates the modulation of the transition frequency during the vibration. At the same time the modulation of the mean transition moment is observed, which is an indication of the non-Condon transition. Thus, two mechanisms are observed to be responsible for the oscillations : the modulation of the transition frequency and the modulation of the mean transition moment by the vibrational motion.