Anharmonicities and Nonlinearities in the Ultrafast Dynamics of Liquids

布朗大学的理查德·M·斯特拉特得到了化学系理论和计算化学项目的支持，他继续研究哪些特定的集体或个人运动，以及哪些分子参与了液体和溶剂化过程中的能量转移的基本步骤。大多数液体实验研究的分析基础是线性响应假设：在实验过程中施加的外部扰动的任何细节都不会对答案产生太大影响。这项研究表明，这些例子并没有被视为例外，而是提供了学习非线性行为的机会。特别是，最近的一些实验涉及液体中高转动激发分子的松弛和溶质电子激发后溶剂的非线性光谱的演变，这些实验为非线性响应提供了有趣的案例研究。这些和相关的例子正在进行理论研究，以了解分子尺度摩擦等概念在多大程度上具有自然的非线性概括，并试图了解是什么产生了这些非线性，以及它们如何在皮秒和亚皮秒的时间尺度上表现出来。这项工作正在产生更广泛的影响，既通过它对生物学和其他地方重要的基本物理过程的阐明，也通过PI向普通受众解释这些基本分子过程的努力。

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.