Ultrafast Dynamics of Highly Excited Molecules in the Condensed Phase

University Of Kansas Center For Research Inc, Lawrence KS

Investigators

Abstract

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.