Anharmonicities and Nonlinearities in the Ultrafast Dynamics of Liquids

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Richard Stratt of Brown University is supported by the Theoretical and Computational Chemistry Program to develop methods employing the instantaneous normal mode (INM) approach to explore intramolecular vibrational energy flow within polyatomic molecules in liquids, and the intermolecular liquid dynamics revealed by fifth order two-dimensional Raman spectroscopy. Recent progress in incorporating dynamical anharmonicities and nonlinear interactions into the INM formalism are expected to help reveal the ways in which molecules choose between competing pathways for intramolecular vibrational energy relaxation, and help to identify the amount of dephasing to be expected from different intermolecular motions. Insights are also expected into the events that lead to homogeneous broadening in a variety of spectroscopic venues.Chemical processes in liquids take place over a tremendously wide range of time scales, but many of the key "triggering" events seem to occur in times well under a trillionth of a second. This dichotomy is especially evident when one examines how energy flows in liquids at a molecular level. It can take anywhere from a trillionth of a second to seconds to release the energy buried within the vibrations of a molecule into the surrounding solution, but such processes are driven by a solvent friction whose lifetime is invariably less than a trillionth of a second. Only relatively recently, experimental techniques have been able to examine vibrations in liquids at these ultrafast time scales and with sufficient sophistication to learn about underlying molecular mechanisms in the dynamics of liquids. This project aims to develop a framework to interpret the results from these challenging new experiments that study vibrational energy relaxation pathways in molecules dissolved in liquids.