Richard Stratt of Brown University is supported by an award from the Theoretical and Computational Chemistry program for research to develop theoretical and computational methods to simulate and understand ultrafast spectroscopic studies of solvation dynamics. These studies are focused on investigating the response of the solution to electronic excitation and invoke a three-time portrait of the solution dynamics rather than the traditional two-time picture.
With the advent of ultrafast lasers, scientists are now able to study the details of solvent reorganization that occurs whenever a chemical reaction takes place. This work is helping to broaden our understanding of exactly how those processes occur. In addition to the scientific broader impact, the work is having a broader impact societally through Stratt's efforts in communicating the value and excitement of scientific research to the public through a series of general interest presentations.
" When ultrafast lasers were first developed, one of the most interesting applications was to following, in real time, the way in which solvent molecules rearrange around a newly formed molecule in solution. This rearrangement process, called "solvation," is critical in helping decide whether chemical reactions happen, so the newfound ability to track the changes happening on a trillionth of a second scale captured the attention of numerous experimental and theoretical chemists. What these experiments looked at, however, was just the evolution of the aggregate effect of the solvent’s presence, not whatever specific solvent rearrangements were actually occurring. It was as if we had the audio, but not the video. Newer ultrafast methods that have recently appeared have the promise to take the next step in watching solvation. These, so-called, two-dimensional methods promise to look directly at the ebb and flow of the molecular motion in the solvent and let us see how that motion itself changes as the solvent’s memory of the creation of a new chemical species begins to recede into the past. Such methods have enormous potential because the signals that would come out of them would seem to contain a wealth of detailed information about patterns of molecular motion. But what was lacking was a way to extract and understand that information. Meeting that need was the goal of this project. Our group, including both graduate students in chemistry and undergraduate students from a variety of disciplines (including computer science and applied mathematics), made a multi-pronged attack on this problem. One avenue we took was the most obvious, but also the hardest: we needed to figure out how to compute what the outcome of a two-dimensional spectral measurement would be if we already knew what the molecules were doing. Devising a way of carrying out such calculations proved to difficult, but within the last year we published our first successful prediction of such spectra based on a computer simulation of molecular motion in a liquid. These results made it possible to identify, for the first time, which molecular locations the spectra were most sensitive to. Moreover, since the computed results turned out to show the same kinds of solvent-to-solvent variations of sign that the actual experiments did, we were able to work backwards and infer the specific molecular origins of the experimental sign changes. The other major avenue we pursued had to do with understanding the patterns of molecular motion that experiments might eventually be able to see. Previous NSF –supported work by our group had led to the development of a way of thinking about simultaneous many-molecule motions in liquids as a consequence of molecules taking the most efficient routes through the rugged landscape dictated by intermolecular forces. When we applied this technique to a problem involving solvation in solvent mixtures – the same system we would later compute two-dimensional spectra for – we discovered, first that there really were discernable patterns of motion, and second, that the very slowest dynamics corresponding to an each molecule-taking-its-turn kind of motion noticeably different from the everyone-moving-at-once patterns we had expected. Aside from the opportunities revealed for applying these new kinds of spectroscopic methods, the broader impacts of these projects lay largely in the exposure of students from chemistry and non-chemistry backgrounds to the kinds of research being carried out in modern theoretical chemistry. The cross-fertilization between different fields and the specific experiences provided to our students should both yield future dividends.
