Ultrafast Optical Pump-Hard X-Ray Probe Spectroscopy of Transition Metal Complexes in Solution at SLAC.

Time-resolved x-ray spectroscopy has become a powerful approach for probing the ultrafast dynamics of photoexcited systems in solution, offering a window into the structural and electronic transformations that govern function for instance in catalytic or biological systems. At SLAC, the colocation of the Stanford Synchrotron Radiation Lightsource (SSRL) and the Linac Coherent Light Source (LCLS) enables a uniquely synergistic platform for ultrafast studies. Together, these facilities span a broad temporal range, from fs to ns, allowing multiscale investigations of excited-state dynamics using complementary techniques such as x-ray absorption, emission, and scattering. This article provides an update on solution-phase optical pump x-ray probe capabilities at the XCS instrument of LCLS and beamline 15-2 at SSRL. Recent studies demonstrate how coordinated use of both sources enables the characterization of transient electronic states, charge-transfer pathways, and photoinduced structural rearrangements.