Ultrafast electronic and structural dynamics in molecular systems probed by soft X-ray spectroscopy
Abstract
Understanding how electronic and nuclear motions evolve on femtosecond timescales is central to modern chemical physics, particularly in condensed-phase environments where intermolecular interactions strongly influence reaction pathways. In this talk, I will present recent advances in time-resolved soft X-ray transient absorption spectroscopy (TR-XAS) for probing ultrafast molecular dynamics in both gas and liquid phases with element specificity, site selectivity, and femtosecond time resolution.
The talk will cover several studies that highlight the utility of TR-XAS for tracking electronic and nuclear dynamics. First, proton-transfer dynamics in aqueous urea dimers reveal how ionization initiates hydrogenbond-mediated proton motion, while transient X-ray signatures disentangle proton transfer itself from accompanying electronic-structure rearrangements, providing direct insight into coupled electronic and nuclear dynamics in solution. Second, studies of UV-excited pyrazine demonstrate that conical intersections can create coherent electronic dynamics in isolated molecules, but that these oscillatory electronic rearrangements are rapidly dephased in water within < 40 fs, directly exposing the strong influence of solvation on non-adiabatic relaxation pathways. Finally, investigations of pyridine in gas and liquid phases show how intermolecular interactions fundamentally alter photochemical outcomes, stabilizing long-lived cationic and isomerized states in solution while promoting fragmentation in the gas phase.
Together, these results establish table-top soft X-ray spectroscopy in the water window as a uniquely powerful tool for tracking ultrafast charge redistribution, proton transfer, conical-intersection dynamics, and solvent-controlled chemical transformations. They illustrate how transient X-ray methods can disentangle electronic and structural dynamics in complex molecular systems and provide new mechanistic insight into chemical processes under realistic condensed-phase conditions.
