Spectroscopy and Dynamics of Reaction Intermediates

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In this project funded by the Chemical Structure, Dynamics and Mechanisms (CSDM-A) and Environmental Chemical Sciences (ECS) Programs of the Chemistry Division, Professor Marsha I. Lester of the University of Pennsylvania is investigating the chemical properties of reactive atmospheric intermediates that release hydroxyl radicals, which are fragments of the water molecule, with one hydrogen atom missing. Hydroxyl radicals are a key oxidant in the atmosphere and often called the atmosphere's detergent. Professor Lester and her group are exploring atmospheric chemical reactions that produce hydroxyl radicals. Their work is leading to a better understanding of the troposphere, the lowest part of the atmosphere, where we live and breathe, and thus has potential benefits to society. The reactive intermediates that give rise to hydroxyl radicals in the atmosphere are produced by reactions of ozone with isoprene, a small molecule that is produced in large quantities by plants, and other alkenes (often called "volatile organic compounds"). When formed, these intermediates have enough internal energy that they spontaneously fall apart into an organic fragment and a hydroxyl radical, in a process known as "unimolecular decay," or react with other molecules. The problem is a difficult one because the intermediates are large and complex, and understanding their chemistry requires the use of state-of-the-art experimental and theoretical methods. Professor Lester and her group produce these reactive intermediates in the laboratory, and study their unimolecular decay and light-induced dissociation processes using action spectroscopy and theoretical modeling. The students working with Professor Lester gain experience in sophisticated experimental and theoretical methods that probe the physical properties of molecules and chemical reactions, and thus a significant broader impact of the project is its contribution to the training of future scientists.