Electronic and Vibrational Energy Transfer Dynamics Studied Through van der Waals Complexes

美国宾夕法尼亚大学的戴海龙在实验物理化学项目的支持下继续他对范德华（vdW）配合物的研究。诱导电子和振动能量转移的分子间相互作用将通过与相遇中涉及的两个分子形成的vdW配合物进行研究。vdW力将两个分子放置在一个独特的方向上，并在复杂结构定义的固定距离上。通过监测配合物光激发后的荧光衰减和发射光谱，可以通过取向和能量规格来检测能量传递动力学。利用同分异构体的配合物结构来研究取向效应。具体来说，(a)碰撞引起的电子松弛，如系统间交叉，将通过监测激光诱导荧光（LIF）衰减在vdW配合物研究。分子电子弛豫中的分子轨道依赖关系以及不同碰撞伙伴的影响将被表征。(b)将发展简并四波混频（DFWM），以研究具有快速非辐射衰变和低荧光产率的配合物。(c)结构和vdW模式将由LIF、DFWM和受激发射光谱确定。计算分子间电位的模型将根据这些实验测量进行测试，并用于寻找配合物的同分异构体结构。这项研究处理的问题是能量如何在振动和电子激发的分子内部和之间流动。所研究的分子被纳入分子簇中，这些分子簇由弱分子间的vdW力结合在一起。组分分子的振动或电子激发通常会使复合物解离。这种解离过程模拟了发生在碰撞分子之间的能量转移。在vdW复合物中嵌入分子的实验提供了一种控制反应过程的措施，这是分子随机取向的分子气体所无法实现的。该实验将为理解不同分子轨道的反应性和确定表征反应的势能面提供有用的信息。有了这些信息，就有可能设计出新的光化学反应途径。

Hai-Lung Dai of the University of Pennsylvania is supported by the Experimental Physical Chemistry Program to continue his studies on van der Waals (vdW) complexes. Intermolecular interactions that induce electronic and vibrational energy transfer will be studied through vdW complexes formed with the two molecules involved in the encounter. The vdW forces place the two molecules in a unique orientation and at a fixed distance defined by the complex structure. By monitoring the fluorescence decay and emission spectra following optical excitation of the complex, energy transfer dynamics can be examined with orientation and energy specification. Isomeric complex structures will be used to enable the orientation effect to be studied. Specifically, (a) collision-induced electronic relaxation such as intersystem crossing will be studied through monitoring laser-induced fluorescence (LIF) decay in vdW complexes. Molecular orbital dependence in electronic relaxation of molecules will be characterized together with the effect of different collision partners. (b) Degenerate four wave mixing (DFWM) will be developed to study complexes with rapid nonradiative decay and low fluorescence yield. (c) Structures and vdW modes will be determined from LIF, DFWM and stimulated emission spectroscopy. Models for calculating intermolecular potentials will be tested against these experimental measurements and used for searching isomeric structures of the complexes. This research deals with the problem of how energy flows within and among vibrationally and electronically excited molecules. The molecules of study are incorporated in molecular clusters which are held together by weak intermolecular vdW forces. Vibrational or electronic excitation of the component molecules will generally cause the complex to dissociate. This dissociation process mimics the energy transfer that occurs between colliding molecules. The experiments with molecules embedded in a vdW complex provide a measure of control over the reaction process that is not available with molecular gases in which the molecules are randomly oriented. The experiment will provide information useful for the understanding of the reactivity of different molecular orbitals, and for the determination of the potential energy surface which characterizes the reaction. With this information it will be possible to devise novel photochemical reaction pathways.