Theory of Inducing and Probing Ultrafast Correlated-Electron Dynamics in Molecules with X-Ray Pulses

This project will focus on the interaction between molecules and x-ray light, as the vehicle for initiating (pump) and observing (probe) ultrafast correlated-electron dynamics. It puts an emphasis on core electrons, which are highly correlated and have large x-ray cross sections. The PI will model (i) ultrafast correlated electron dynamics in molecules, including valence and core excitations, and (ii) their interaction with realistic x-ray pulses from free-electron lasers (XFELs) to pump and probe those dynamics. The PI will consider small- to mid-size organic compounds like para-aminophenol with carbon, nitrogen, and oxygen centers that can be selectively probed via their distinctive K edge energies. The PI will also study one- and multi-photon induced wave packet dynamics in both neutral (photoexcitation) and cationic (photoionization) systems, as well as how the ultrafast correlated dynamics can be observed with x-ray transient absorption (XTAS) and photoelectron spectroscopies (XPS). The PI will continue to make their codes publicly available via GitHub or similar repositories. The PI will also lead the method development for XPS and supervise and mentor the students involved in this project. This project aims to further ultrafast AMO science with x-ray light. Recent attosecond XFEL source developments such high repetition rate, (two-color) attosecond-pulse pair generation, and ongoing efforts for pulse shaping at LCLS-II, are promising exciting developments for ultrafast AMO science and motivate the proposed work. By explicitly treating the pump, the PI will aim to achieve systematic and reliable description of the wave packet launched by the absorption of one or several energetic photons, which remains for the most part an unmet need for fundamental and applied ultrafast science. It will allow them to study ultrafast electron dynamics probing with XTAS and XPS in realistic experimental conditions. The results will help guide the planning and interpretation of future ultrafast experimental campaigns at XFEL facilities. A central goal is to build a platform for ab initio direct numerical simulation and analysis of correlated electron dynamics in mid-size organic molecules, including both the pump and probe steps, that would become accessible to other theoretical and experimental groups. 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.