Visualizing Photon Induced Dynamics in Polyatomic Molecules using Femtosecond Pump-Probe Laser Pulses

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General audience abstract: This project will investigate how light affects and transforms molecules encountered in nature and in living systems. For example, after shining light on molecules they may transform through a phenomenon called isomerization. The first step in human vision is such a transformation. Another light-triggered phenomenon is the ability for hydrogen atoms, in some molecules, to move from one end of a molecule to another. This transformation is crucial for biological photo-protection mechanisms in DNA and melanin. Students will use the ultrafast and ultra-intense table-top lasers in the PI’s lab to conduct experiments on such light-induced changes in molecules. They will also study the light-triggered opening of some ringed shaped molecules, as for example occurs in vitamin D synthesis in the skin. These types of molecules are being explored for optoelectronic applications in optical switching and nano-mechanical motors. Furthermore, the investigation of the breakup of molecules, with many hydrogen atoms, after they absorb light, is of extreme importance in environmental protection. This research program provides a comprehensive hands-on education and training opportunity for undergraduates, graduate students, and postdocs, who are the next generation workforce in STEM fields in academia and are essential in various industries. The team will collaborate with many theorists, allowing the development of computational methods for understanding the phenomena being explored in the lab. The PI is very active in outreach activities to the wider community. She will continue to invite local teachers with their middle school and high school students to day camps, giving many students from groups currently underrepresented in science an early exposure to STEM fields.Technical audience abstract:The goal of the research program is to study, time-resolved, table-top laser-induced femtosecond dynamics in polyatomic molecules in order to better understand non-adiabatic molecular dynamics. Specifically, the PI and her students wish to elucidate how electronic rearrangement in molecules, subsequent to optical field absorption, at different laser pulse energies and femtosecond pulse duration induces and affects nuclear motion. The latter leads to atomic rearrangement, bond elongation, isomerization, ring opening, bond breaking, bond making, and molecular fragmentation. Time-resolved non-adiabatic dynamics have been difficult to observe directly because techniques are often not sufficiently differential to provide detailed information. Building on their current work, the team aims to measure infrared- or ultraviolet-induced multi-hydrogen migration, an isomerization process involving migration of two or more hydrogen atoms in molecules. The team will also investigate ring opening reaction experiments in molecules, for which the deactivation mechanism of the photo-excited molecules along the reaction path crosses conical intersections, which they plan to characterize. The experiments will be carried out using laser pump-probe technique,s and the resulting charged fragments will be detected using a coincidence experimental system, the Cold-Target Recoil Ion Momentum Spectroscopy (COLTRIMS). The underlying dynamics will be revealed by coincident ion-momentum imaging. This work is in the regime where the Born-Oppenheimer approximation breaks down, which needs thorough investigation and understanding.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.