Studying Intermolecular Interactions and Bimolecular Reactions Through van der Waals Complexes

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In this project in the Experimental Physical Chemistry Program of the Chemistry Division, H.L. Dai of the University of Pennsylvania will investigate intermolecular interactions such as electronic energy transfer and bimolecular reactions through van der Waals complexes. The study consists of several parts,namely investigation of collision induced electronic relaxation processes in van der Waals complexes by means of laser induced fluorescence spectroscopy; molecular orbital dependence in the electronic relaxation of molecules like glyoxal and pyrimidene and the effect of different collision partners; and the chemistry of electronically excited molecules, or molecules excited to selected vibrational levels in a van der Waals complex. The structure and frequencies of the van der Waals vibrational modes will be determined by means of fluorescence excitation and stimulated emission spectroscopy. These results will be used to test theoreticel models for calculating intermolecular potentials. %%% 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 van der Waals 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 van der Waals complex provide a measure of control over the reaction process that is not available with molecular gases in which the molecules have random orientation. The experiments 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.