The Short-Time Dynamics of Liquids

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Richard Stratt, Brown University, is supported by a grant from the Theoretical and Computational Chemistry Program to continue his theoretical research in the short time dynamics of liquids. Stratt seeks to interpret recent spectroscopic data obtained from time-dependent-Stokes-shift measurements of solvation dynamics as well as Fourier transform Raman (Optical Kerr effect) spectra of neat liquids in terms of the instantaneous normal mode (INS) theory of the liquid vibrations. Two fundamentally disparate research directions to be undertaken are: 1) to explore the ways in which INS analysis can assist in the interpretation of ultrafast spectroscopic experiments; and 2) to find ways of extending the workable time span into the nonlinear regime as much as possible, and to discover where the true limits of the technique lie. The effects of nonlinearity will be studied by examining the time evolution, both via exact simulation and somewhat more analytically, by considering the nonlinearity formally as a perturbation. A significant fraction of what one thinks about as chemistry takes place in solution, and chemical processes often look far different in a liquid environment than they would if the molecules were completely isolated. Just what the origins of these differences are, and precisely how these distinctions arise from the molecular properties of the solvent are questions that have been pursued repeatedly with each new generation of spectroscopic technique. Recent theoretical progress in elucidating the concept of linear excitations in liquids suggests that it may indeed be useful to think in terms the short time evolution in the liquid-state being governed by a set of liquid vibrations -- the so-called instantaneous normal modes of the liquid. The major purpose of Stratt's research is to see if this theoretical construct can lead to interpretation of recent results obtained from ultrafast spectroscopy, and to provide a molecular level interpretation of the structure and dynamics of the liquid state.