Femtosecond Time-Resolved Studies of the First 200 fs Photochemical and Photophysical Relaxation Dynamics in the Gas and Liquid Phases

在化学系化学结构、动力学和机理(CSDM-A)项目的支持下，鲍林格林州立大学的Alexander Tarnovsky教授和Massimo Olivucci教授正在研究各种溶剂对化学反应中最早的事件的影响。这项研究试图更好地理解前200飞秒内的动力学，因为这些早期运动通常决定了光诱导化学反应的结果。由溶剂引起的变化可能会导致成功的反应利用光能和在分子水平上浪费能量的无效反应之间的差异。研究小组使用极短的激光脉冲来实验测量化学反应动力学，并进行计算机模拟，能够追踪目标分子中化学键的断裂和建立。这项研究的一个广泛目标是开发预测模型，以描述最终确定如何将光能最佳地利用于各种材料和技术应用的基本过程。除了科学目标，跨学科研究计划还为研究生和本科生提供专业发展和培训，为他们在学术界、政府实验室和行业的高级职业生涯做好准备。通过与当地K-12社区教师和托莱多想象站科学博物馆的接触，研究团队将他们项目的影响与更广泛的社会问题以及科学、技术、工程和数学(STEM)研究的总体意义联系起来。在这个项目中，由Tarnovsky教授和Olivucci教授领导的研究小组正在研究分子在溶液和气相中电子激发态动力学的最早阶段(200fs)。这一研究提供了一个系统地解开和理解在常温下仍未被很好理解的时间窗口中的激发态动力学的溶剂效应的机会。这种洞察力很重要，因为在这种短时间尺度上发生的电子和核动力学经常影响反应结果，如果不是决定的话。这项研究考察了多卤代甲烷和杂环芳烃的反应，这两种反应对溶剂效应都很敏感，其中溶剂可能在激发态动力学中发挥关键作用。为了获得这一初始时间尺度，研究团队在溶液和气相中使用了可调谐(从深紫外到近红外)20-30飞秒光脉冲和超快瞬时吸收光谱，并辅之以飞秒受激拉曼光谱、飞秒时间分辨X射线吸收光谱和显式包含溶剂分子的从头算量子经典表面跳跃轨迹计算。激发态动力学的这种实验-理论表征承诺了对反应的多维性、非绝热性和非平衡溶剂对分子内核和电子运动的影响的基本见解，为光触发超快化学过程的有效控制方案提供了一个切入点。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, Professors Alexander Tarnovsky and Massimo Olivucci at Bowling Green State University are studying the effects that a variety of solvents have on the very earliest events in chemical reactions. The research seeks a better understanding of the dynamics within the first 200 femtoseconds, less than 200 millionths of a billionth of a second, because these early motions often determine the outcome of a light-induced chemical reaction. Changes caused by the solvent can make the difference between a successful reaction that exploits the energy from light and an ineffective reaction that wastes energy at the molecular level. The research team uses extremely short pulses of laser light to measure the chemical reaction dynamics experimentally, and also performs computer simulations capable of tracing the breaking and making of chemical bonds in the target molecules. A broad objective of the research is the development of predictive models to describe elementary processes that ultimately determine how light-energy can be optimally exploited for various materials and technological applications. In addition to the scientific objectives, the interdisciplinary research program provides professional development and training for graduate and undergraduate students, preparing them for advanced careers in academia, government labs, and industry. Through the outreach to local K-12 community teachers and the Toledo Imagination Station science museum, the research team relates the impact of their program to broader societal issues and the significance of science, technology, engineering, and mathematics (STEM) research in general. In this project, the research team led by Professors Tarnovsky and Olivucci is investigating the earliest (200 fs) stage of electronically excited-state dynamics of molecules in solution and in the gas phase. This research provides an opportunity to disentangle and comprehend, in a systematic way, the solvent effects on excited-state dynamics occurring in a time-window that is still not well understood at ambient temperature. Such insight is important, because the electronic and nuclear dynamics occurring on this short time scale often influence, if not determine, the reaction outcome. The study examines the reactions of polyhalomethanes and heterocyclic aromatics, both of which are sensitive to solvent effects, and where the solvent is likely to play a crucial role in the excited-state dynamics. In order to access this initial timescale, the research team uses tunable (deep-UV to near-IR) 20-30 fs optical pulses and ultrafast transient absorption spectroscopy in both solution and gas phases, complemented by femtosecond stimulated Raman spectroscopy, femtosecond time-resolved X-ray absorption spectroscopy, and ab initio quantum-classical surface-hopping trajectory calculations with explicit inclusion of solvent molecules. Such experimental-theoretical characterization of excited-state dynamics promises fundamental insights into reaction multi-dimensionality, non-adiabaticity, and non-equilibrium solvent effects on intramolecular nuclear and electronic motions, providing an entry point for effective control schemes of light-triggered ultrafast chemical processes.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.