CAREER: Photo-induced Ultrafast Electron-nuclear Dynamics in Molecules

The University Of Central Florida Board Of Trustees, Orlando FL

Investigators

Abstract

General audience abstract: When a molecule is subject to a sudden disturbance, such as the removal of one or more electrons, the remaining electrons and atomic nuclei move. In many cases, one can assume that the light electrons adapt themselves instantly to the positions of the much heavier atomic nuclei as the molecule changes shape (the “Born-Oppenheimer approximation”), but this is only approximately true. The electrons, which exhibit both wave and particle aspects, can interfere with each other. The resulting electron wavepackets can oscillate rapidly compared to the motion of the atomic nuclei. Such “electron coherences” are thought to be important in chemical and biological processes such as vision but have been difficult to study due to the extremely short timescales on which they evolve (a few attoseconds, where one attosecond = 0.000000000000000001 seconds). This extremely fast electron motion will be affected by the comparatively slower motion of the atomic nuclei. The PI and her research team will use state-of-the-art lasers and photon sources on the campus of the University of Central Florida to study the evolution of these electron coherences and their coupling to the underlying nuclear motions. Ultimately such studies may advance our ability to engineer reaction products and optimize energy harvesting. This CAREER award supports undergraduate and graduate students and postdoctoral researchers who will be trained in the use of state-of-the-art laser systems and associated advanced spectroscopic techniques. In addition, the award supports an educational and outreach project which includes a summer program focusing on instrumentation training for students, an introductory video sequence on ultrafast science addressing a broader audience, and events for high-school teachers and students, called “Go Ultrafast!” Technical audience abstract: When coherent light interacts with a molecule, multiple electronic states can be populated with specific relative phases, resulting in electronic coherences. In this project, the evolution of these electronic coherences will be time-resolved in a pump-probe scheme with temporal resolutions at the natural time scales of the electron and ion motion. The effect of electron-nuclear coupling on the electronic-coherence properties, such as their lengths, strengths, and revivals, will be investigated. Femtosecond infrared/near-infrared lasers and attosecond XUV/x-ray table-top light sources will be used as the pump or the probe. Electronic coherences will be monitored through electron kinetic energy and abundance variations as a function of the pump-probe delay, using electron-ion spectroscopy of 3-dimensional momentum imaging and transient absorption spectroscopy. Simultaneous measurement of electrons and ions in the momentum space will be used to show the correlation between the evolution of electron dynamics and ion motion. This project will advance our understanding of charge dynamics and photoenergy transformation and transfer mechanisms in molecules and pave a path to photocontrol schemes for directing energy flows. The results will provide reliable references for validation of theoretical methods in the non-Born-Oppenheimer regime. 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.