Demonstration of time-resolved Fe K-edge XANES with a self-seeded X-ray free-electron laser at PAL-XFEL

Abstract

We demonstrate, for the first time, time-resolved X-ray absorption near-edge structure (XANES) spectroscopy at the Fe K-edge using a self-seeded X-ray free-electron laser (XFEL) beam at the FXL endstation in Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL). Focusing on the application of self-seeded XFEL for time-resolved XANES, we show advantages in photon flux, measurement speed, and signal-to-noise ratio (S/N). Using the high-stability, narrow-bandwidth self-seeded mode, we achieved an incident X-ray bandwidth of approximately 0.71 eV and improved spectral purity compared to conventional self-amplified spontaneous emission (SASE) operation. A 50 mM aqueous solution of iron(II) tris(1,10-phenanthroline) dichloride was photo-excited by a 400 nm femtosecond laser, and ultrafast electronic dynamics were probed by synchronized XFEL pulses. The enhanced spectral purity and energy stability enabled clear detection of transient features obscured by SASE pulse bandwidth and energy jitter. The results highlight the clear benefits of the self-seeded XFEL source, showing a 25% improvement in signal-to-noise ratios compared to the SASE with double crystal monochromator (DCM) mode, particularly for time-resolved XANES experiments. This work lays the foundation for advanced exploration of chemical and biological dynamics with improved spectral accuracy in complex environments.