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

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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.