Photodissociation Dynamics of Small Molecules: Collisionless and Collisional Processes

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Hanna Reisler of the University of Southern California is supported by the Experimental Physical Chemistry Program to continue her studies of the unimolecular and bimolecular dynamics of open shell molecular species such as free radicals and electronically excited molecules that react via multiple pathways. Photoinitiated unimolecular decomposition of molecules and radicals relevant to processes in the atmosphere will be examined, emphasizing nonadiabatic transitions, competitive pathways, correlated distributions, and the dependence of branching ratios on excitation energy. State-to-state studies will be extended to larger molecules, important in a variety of environments, where pathways are controlled by several multidimensional potential energy surfaces. The photofragment velocity map imaging technique will be exploited and further developed to afford simultaneous product resolution in the mass, frequency, velocity, and angular domains. The systems chosen for photodissociation studies include halomethyl and nitrogen trioxide radicals, and a weakly bound species such as nitric oxide dimers. As well, infrared laser-induced vibrational predissociation of ammonia dimers (such as ammonia-carbene and ammonia-acetylene) will be examined by imaging techniques. Recent advances in laser and detection methodologies are providing the technical capability to achieve understanding of complex chemical reaction systems that have potential environmental and technological importance. The present effort capitalizes on these new technologies, to examine in microscopic detail the chemistry of molecules that can react by competing pathways when energy is added to them by the absorption of light. This research project is additionally expected to provide broad training for graduate students and postdoctoral associates in the experimental and theoretical foundations of reaction dynamics.