Vibrational Spectroscopy and Reaction Dynamics of Complexes and Reactive Intermediates

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Marsha Lester of the University of Pennsylvania is supported by the Experimental Physical Chemistry Program to perform experiments that examine reaction pathways of fundamental chemical reactions, including the reversible reaction of OH + CO to form HOCO with subsequent decomposition to H + carbon dioxide, and the reaction of OH + HCl to form water + Cl. The potential energy surface along the reaction pathway from the entrance channel to the transition state will be mapped via infrared spectroscopy of intra- and inter-molecular vibrational modes of the reactant complexes. The transition state region(s) will also be characterized by vibrational overtone studies of the HOCO reactive intermediate, which will access theoretically predicted predissociative resonances of HOCO. The complexes and reactive intermediates will be generated photolytically, and their vibrational overtone spectra will be obtained using various laser pump-probe methods. The structure and stability of the reactant complexes are expected to reveal their role in the reaction dynamics, and may explain unusual temperature behavior observed for these reactions. Vibrational activation of the complexes and reactive intermediates will supply sufficient energy to induce inelastic and/or reactive scattering dynamics, providing a complementary way to explore the reactive potential energy surfaces. The lifetime of the vibrationally excited complexes and intermediates will be determined by direct time-domain measurements or by spectral line broadening in the frequency domain. Quantum state distributions of inelastically scattered fragments and reaction products will be examined, as well as the branching between these decay processes. Intra-and/or inter-molecular mode selective excitation and isotopic substitution will be utilized to prepare initial states that preferentially decay by chemical reaction. There have been many advances in experimental and theoretical investigations of chemical reaction dynamics over the past decade. These studies focus on directly evaluating the influence of long-range forces on chemical reactions, exploring processes that are of practical importance in atmospheric and combustion chemistry.