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Dr. Marsha I. Lester - Chairperson and Edmund J. and Louise W. Kahn Professor in the Natural Sciences |
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Our research combines new experimental and theoretical approaches to probe intermolecular
potential energy surfaces. Intermolecular potentials are mappings of the forces, such as dispersion
and electrostatic forces, between atoms or molecules as a function of separation distance and angles.
These potentials control the approach and recoil of molecules in both inelastic and reactive encounters.
We have extensively studied intermolecular interactions involving a free radical--in particular, the
hydroxyl radical, which is a key intermediate in combustion and atmospheric chemistry.
We investigate these intermolecular interactions on a fundamental level by aggregating the molecules of interest in a weakly-bound complex and then probing the binary complexes with high resolution laser spectroscopy. We employ techniques such as laser-induced fluorescence (LIF) and stimulated emission pumping (SEP) to characterize the vibrational levels supported by the intermolecular potential. The intermolecular bending and stretching levels yield information concerning the anisotropy and the radial dependence of the potential, respectively. These experimental methods allow us to access the bound states within the long-range attractive well region of the potential, as well as metastable levels which lie above the dissociation limit. The metastable levels are a sensitive gauge of the short-range repulsive part of the potential. We compare the experimental data with theoretical computations of the observables to guide in refining the intermolecular potential. Complexes prepared in metastable levels may predissociate by converting excess internal energy in one of the molecular subunits into translational energy along the dissociation coordinate. We measure the rate of dissociation in time or frequency domain experiments and the quantum state distribution of the product molecules in pump-probe experiments. These dynamical measurements explore the regions of the potential energy surface sampled in inelastic scattering and, in some cases, the transition state region leading to the chemical reaction. In some systems, nonradiative decay or chemical reaction occurs upon laser excitation. To examine such systems we have implemented a promising new direct absorption technique called cavity ring-down spectroscopy. High sensitivity is achieved by creating extremely long path lengths within an optical cavity and by measuring the rate of absorption rather than the magnitude of absorption. We are applying cavity ring-down and new nonlinear spectroscopic techniques to investigate the intermolecular potentials between chemically reactive species.
Several state-of-the-art spectroscopic methods used in this laboratory are illustrated: laser-induced fluorescence (LIF), direct absorption, stimulated emission pumping (SEP), and pump-probe experiments. These techniques examine both the intermolecular potential energy surface and the dissociation dynamics that take place on the potential surface, shown here for an hydroxyl radical with an atomic or molecular partner (M).
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