Spectroscopy and Reaction Dynamics of Activated Entrance Channel Complexes

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Marsha Lester of the University of Pennsylvania is supported by the Experimental Physical Chemistry Program in her ongoing studies to characterize the intermolecular potential, inelastic scattering and reaction dynamics of vibrationally activated entrance channel complexes composed of a hydroxyl radical and a reactive molecular partner (carbon monoxide, nitric oxide, water). Infrared excitation will provide sufficient energy to overcome the activation barrier to reaction while intermolecular excitations will orient the reactants such that the structure of the complex approximates that of the transition state. Prof. Lester will use double resonance techniques to record the mid- and near-IR spectra of the OH stretch as well as intermolecular bending levels of the OH complexes. Spectral measurements should yield information on the potential energy surfaces in the entrance channel to reaction for ground state and vibrationally excited reactants. The hydroxyl radical plays a pivotal role in the chemistry of the lower atmosphere. It is known as the `atmosphere's detergent` because its reactions prevent the accumulation of most trace gases in the atmosphere, many of which would act as air pollutants and greenhouse gases. For the majority of hydrogen-containing molecules emitted into the troposphere, reaction with the OH radical is their major, if not sole, chemical loss process. Thus, reaction with OH radicals leads to the removal of these compounds from the atmosphere and limits their atmospheric concentrations. Much experimental work has already been performed to evaluate the OH radical kinetics and reaction mechanisms under the conditions encountered in the atmosphere. In these studies, Prof. Lester will use techniques developed in her laboratory to take a much closer look at the reactions dynamics of OH radicals with molecular partners of atmospheric relevance such as carbon monoxide, nitric oxide and water.