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

宾夕法尼亚大学的戴海龙得到了实验物理化学计划的支持，继续他对范德华(VDW)络合物的研究。诱导电子和振动能量转移的分子间相互作用将通过与参与相遇的两个分子形成的VDW络合物来研究。VDW力将两个分子以独特的方向和由复杂结构定义的固定距离放置。通过监测络合物光激发后的荧光衰减和发射光谱，可以用取向和能量规范来研究能量转移动力学。将使用同分异构体的络合物结构来研究取向效应。具体地说，(A)将通过监测VDW络合物中的激光诱导荧光(LIF)衰减来研究碰撞诱导的电子驰豫，如系统间交叉。分子的电子驰豫与分子轨道的依赖关系将与不同碰撞伙伴的影响一起被表征。(B)将发展简并四波混频(DFWM)来研究快速无辐射衰减和低荧光产额的络合物。(C)结构和VDW模式将由LIF、DFWM和受激发射光谱确定。计算分子间势的模型将根据这些实验测量结果进行测试，并用于搜索络合物的异构体结构。这项研究涉及能量如何在振动和电子激发的分子内部和之间流动的问题。所研究的分子被结合在分子簇中，分子簇通过弱的分子间VDW力结合在一起。组成分子的振动或电子激发通常会导致络合物解离。这种解离过程模拟了碰撞分子之间发生的能量转移。用嵌入VDW复合体中的分子进行的实验提供了一种对反应过程的控制措施，这是分子气体无法实现的，因为分子在分子中是随机定向的。该实验将为理解不同分子轨道的反应性以及确定表征该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.