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

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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.