Ultrafast X-rays from XFELs are driving a qualitative advance in our understanding of condensed-phase chemical dynamics and catalysis. Ultrafast hard X-ray pulses reveal the atomic scale structural dynamics of excited-state reactions – revealing relaxation pathways, and the coupling of atomic structure, electronic structure, and solvent dynamics. Ultrafast soft X-rays provide element-specific mapping of chemical bonds, charge distributions, oxidation states and frontier orbitals. This talk will highlight recent results from the Linac Coherent Light source (LCLS) using advanced ultrafast X-ray methods to track excited-state charge-transfer and relaxation pathways, and reveal the influence of molecular structural dynamics, and solvent coupling. Notably, multi-modal methods combining time-resolved X-ray scattering and spectroscopy represent a powerful approach for linking X-ray experimental observables with theory to achieve a deeper understanding of excited-state dynamics to advance the development of design principles for creating molecules, complexes, and assemblies with desired functions. In addition, new science opportunities enabled by the nearly-completed upgrade of LCLS (LCLS-II) coupled with advanced instrumentation and methods will be discussed. LCLS-II will provide tunable soft X-ray pulses (0.25 to 5.0 keV) at high repetition rate (up to 1 MHz) and hard X-rays up to 25 keV (at 120 Hz). This unprecedented capability will support powerful new methods such as time-resolved resonant inelastic X-ray scattering (RIXS). The new ChemRIXS instrument is optimized for studying solvated complexes with C, N, O (K-edges) and 3d transition metal elements (L-edges) – where 2D RIXS maps of excited-state dynamics coupled with quantum chemical calculations will reveal the evolution of frontier orbitals. The Tender X-ray Instrument (TXI, 2.1-5.0 keV), now under development for LCLS-II, will support time-resolved tender X-ray spectroscopy (spanning the 4d transition metal L-edges and key functional ligands including P, S, and Cl), coherent scattering, and novel nonlinear X-ray pump / X-ray probe methods - combining X-rays from two independently tunable XFEL sources.

New insight into excited-state chemical dynamics using ultrafast X-rays from LCLS: Recent highlights, future opportunities & development plans

ACS Spring 2022

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