Department of Chemistry

Research Statement 

Motion in liquids is an intrinsically complicated business. One might not say the same of gases, which are easily thought of in terms of sharp, well-defined collisions between molecules, or of crystalline solids, where the periodicity of the crystal lattice makes the dynamics almost as simple as that seen in gases. Molecules in liquids, by contrast, are constantly disordered and in a perpetual state of collision, making it difficult to understand processes such as chemical reactions in liquids. What we have been engaged in over the last several years is a theoretical effort to get at the most elementary, short-time, events in liquid dynamics. We have discovered that it is possible to compute what one might call the instantaneous normal modes of a liquid, and that these modes can serve as a powerful entry into the elementary events in liquids. Partitioning these modes into components arising from nearby and distant solvents and from solvent translation and libration, for example, has made it clear that the ultrafast solvation is accomplished most easily in polar solvents by reorienting selected, neighboring, solvent molecules. Moreover, analyzing the important modes for each instantaneous configuration of the liquid has revealed that a sizeable fraction of the prompt dynamics in both nonpolar solvation and vibrational population relaxation arises from a relatively small number of modes -- and that the main function of most of these is to move just a single solvent molecule at a time. It seems, then, that the complexities of liquid dynamics do indeed begin to disappear at short times. Our group is actively engaged in pursuing the spectroscopic consequences of these findings. In parallel with this effort, we have begun to look at a variety of nonlinear ultrafast spectroscopies to see if having an insight into the organization of liquid motion helps us understand the molecular lessons of these new spectroscopies. Optical-Kerr-effect spectroscopy of liquids can, in principle, measure the very intermolecular vibrations that we have been studying, and two-dimensional (fifth-order) Raman spectroscopy even has the potential to look directly at the coherence of these vibrations. But can we tease out of these spectra any of the genuinely microscopic information that we want? We are working with these and other kinds of novel spectroscopies trying to ascertain what each kind of measurement reveals about liquid dynamics.

Education 

• 1979 - Ph.D., Chemistry: University of California at Berkeley 
• 1975 - S.B., Chemistry: Massachusetts Institute of Technology