Studies on Weak Intermolecular Interactions by Low-Frequency Vibrational Coherence

超短光脉冲可以相干地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，因为振荡和关于特定低频振动模式的动力学的特定知识可以获得。相干光谱学是目前研究生物体系中振动弛豫、振动退相和特定振动对化学反应、分子间振动、分子识别的贡献的一种独立的光谱方法。利用飞秒荧光上转换技术研究了几种电子供体-受体络合物的激发态动力学，发现在许多天然分子体系中都存在着具有电荷转移性质的电子态，如铁卟啉和光合作用中心。配合物的自发荧光表现出与超快衰变分量叠加的振荡特征。对于所使用的三种不同受体的络合物，该振荡被归因于受体的平面外振动，这三种受体分别是四氰乙烯、氯苯腈和氟苯胺。峰移关联函数显示了一个超快弛豫，这主要归因于分子内振动能量重新分配过程与振荡分量的叠加，表明在振动过程中跃迁频率的调制。同时观察到了平均跃迁矩的调制，这是非康登跃迁的一个标志。因此，可以观察到两种机制引起这种振荡：振动对跃迁频率的调制和对平均跃迁力矩的调制。

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.