Ultrafast Dynamics of Highly Excited Molecules in the Condensed Phase

该项目由化学系化学结构、动力学和机制（CSDM-A）项目资助，支持堪萨斯大学克里斯托弗·埃莱斯教授和一群研究生和本科生的研究，以更好地理解甚至控制受光激发的分子的行为。这项研究使用复杂的激光技术，连续两次快速激活分子，以引起原本不会发生的新反应。这些实验揭示了分子在任何类型的光激活反应中的反应方式的新信息。学习如何用一系列激光脉冲来控制化学反应，也可能有助于开发利用单个分子运动的新材料和新技术，包括可以用作分子开关或马达的变色分子。该项目为研究生和本科生提供先进的技术培训，并为有抱负的高中科学教师提供课程发展机会。该项目研究了在第一电子激发态以上被激发的分子动力学。处于这些较高状态的分子通常寿命很短，很难用实验来探测，也很难用理论来计算。然而，通过顺序激发进入更高的状态开辟了新的反应途径，这是不可能通过平衡基态几何的直接激发来实现的。换句话说，允许一个分子在一个势能表面上进化，然后再激发到更高的状态，这为有选择地控制分子的反应路径提供了机会。实验方法结合了超快光谱技术，包括三脉冲瞬态吸收（泵浦-再泵-探针，PReP）测量和激发态共振拉曼散射（或飞秒受激拉曼散射，FSRS）。PReP测量直接监测高激发分子的演化，而FSRS测量基于拉曼散射信号的特定模式共振增强来探测更高势能表面的拓扑结构。这些测量为理论提供了重要的基准，同时也推动了实验超快光谱学的极限。这项工作的广泛影响包括对基本反应动力学的更深入的理解，用于探测化学反应的超快光谱方法的发展，以及一个教育推广计划，使有抱负的科学教师接触到他们将用于课堂活动的前沿研究课题。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This project, funded by the Chemical Structure, Dynamics, and Mechanisms (CSDM-A) program in the Division of Chemistry, supports Professor Christopher Elles and a group of graduate and undergraduate research students at the University of Kansas in their quest to better understand, and even control, the behavior of molecules excited by light. The research uses sophisticated laser techniques to energize molecules twice in rapid succession in order to cause new reactions that would not otherwise occur. These experiments reveal new information about the way in which molecules react in any type of light-activated reaction. Learning how to control chemical reactions with a sequence of laser pulses may also enable the development of new materials and technologies that take advantage of the motions of individual molecules, including color-changing molecules that can be used as molecular switches or motors. The project provides advanced technical training for graduate and undergraduate research students, as well as curriculum development opportunities for aspiring high school science teachers.The project examines the dynamics of molecules excited above the first electronically excited state. Molecules in these higher-lying states typically have very short lifetimes that are difficult to probe experimentally, and challenging to calculate using theory. However, accessing the higher-lying states through sequential excitation opens new reaction pathways that are not possible via direct excitation from the equilibrium ground-state geometry. In other words, allowing a molecule to evolve on one potential energy surface before re-exciting to a higher state provides an opportunity to selectively control the reaction path of the molecule. The experimental approach uses a combination of ultrafast spectroscopy techniques, including three-pulse transient absorption (pump-repump-probe, PReP) measurements and excited-state resonance Raman scattering (or femtosecond stimulated Raman scattering, FSRS). The PReP measurements monitor the evolution of highly excited molecules directly, whereas the FSRS measurements probe the topology of higher-lying potential energy surfaces based on mode-specific resonance enhancements of the Raman scattering signal. These measurements provide important benchmarks for theory, while simultaneously pushing the limits of experimental ultrafast spectroscopy. The broader impacts of the work include a deeper understanding of fundamental reaction dynamics, development of ultrafast spectroscopy methods for probing chemical reactions, and an educational outreach program that exposes aspiring science teachers to cutting-edge research topics that they will use to develop classroom activities.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.