Anharmonicities and Nonlinearities in the Ultrafast Dynamics of Liquids

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Richard M. Stratt of Brown University is supported by an award from the Theoretical and Computational Chemistry program within the Division of Chemistry to continue his research examining which specific collective or individual motions, and which molecules, are participating in the fundamental steps of energy transfer in liquids and in solvation. Underlying the analysis of most experimental studies of liquids is the assumption of linear response: that none of the details of the external perturbations applied in the course of an experiment make much difference in the answer. This research is showing that, rather than viewing such examples as exceptions, they provide opportunities for learning about nonlinear behavior. In particular, some recent experiments involving the relaxation of highly rotationally excited molecules in liquids and the evolution of the nonlinear spectroscopy of a solvent following the electronic excitation of a solute are providing intriguing case studies of nonlinear response. These and related examples are being examined theoretically in order to see to what extent concepts such as molecular-scale friction have natural nonlinear generalizations, and to try to learn both what it is that generates these nonlinearities and how they manifest themselves on picosecond and sub-picosecond time scales. This work is having a broader impact both through its illumination of fundamental physical processes important in biology and elsewhere and through the efforts of the PI to explain these fundamental molecular processes to general audiences.