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Despite more than a century of study, the fundamental mechanisms behind solid melting remain elusive at the nanoscale. Ultrafast phenomena in materials irradiated by intense femtosecond laser pulses have revived the interest in unveiling the puzzling processes of melting transitions. However, direct experimental validation of various microscopic models is limited due to the difficulty of imaging the internal structures of materials undergoing ultrafast and irreversible transitions. Here we overcome this challenge through time-resolved single-shot diffractive imaging using X-ray free electron laser pulses. Images of single Au nanoparticles show heterogeneous melting at the surface followed by density fluctuation deep inside the particle, which is directionally correlated to the polarization of the pumping laser. Observation of this directionality links the non-thermal electronic excitation to the thermal lattice melting, which is further verified by molecular dynamics simulations. This work provides direct evidence to the understanding of irreversible melting with an unprecedented spatiotemporal resolution. Laser-matter interaction has been intensively studied in equilibrium states, but irreversible processes in a highly nonequilibrium state at nanoscales remains elusive due to experimental challenges. Here, Ihm et al. image heterogeneous melting of gold nanoparticles with nanometer and picosecond resolution.

The Coherent X‐ray Scattering beamline at the Pohang Light Source‐II was constructed in 2011 for coherent diffraction imaging and has now been upgraded in its focusing optics, diffractometer, detectors and endstation. The enhanced photon flux density and modified endstation have enabled routine Bragg coherent diffraction imaging and microbeam diffraction, while the newly implemented ptychography setup has enhanced nano‐imaging capability in transmission geometry. Because coherent diffraction imaging and microbeam diffraction share the same upstream optics, switching between techniques requires only minor adjustments to slit settings, mirror pitch and the sample‐to‐detector distance, enabling efficient integration of user programs without compromising instrument performance. This paper details the upgrade and the new capabilities of the beamline. Comprehensive upgrades to optics, detectors and the endstation at the CXS beamline have transformed its coherent diffraction imaging performance, enabling routine Bragg coherent diffraction imaging and microbeam diffraction, and adding the capability for transmission geometry nano‐imaging via ptychography.

In coherent diffraction imaging (CDI), the coherence properties of photons play critical roles in obtaining structural information from specimens without using lenses. While the impact of coherence has been widely studied in CDI, it has not been systematically investigated within the specific framework of X‐ray free‐electron laser (XFEL)‐CDI. Here, we examined the relationship between the transverse and temporal coherence of XFEL pulses and the quality of image reconstruction using XFELs. Specifically, we investigated the properties of self‐amplified spontaneous emission and self‐seeding beams at an X‐ray energy of 5 keV by collecting diffraction patterns from a single gold nanoparticle. Furthermore, the quality of the reconstructed images obtained using the two beam modes was compared. Our results demonstrate that the self‐seeding beam offers more reliable image reconstruction that is attributable to the narrower bandwidth of the incident X‐rays. This study highlights the advantages of utilizing a self‐seeding beam in CDI experiments, particularly for enhancing the reliability and quality of image reconstruction. This study demonstrates that the superior coherence of narrow‐bandwidth self‐seeded X‐ray pulses significantly improves the quality and reliability of image reconstruction in coherent diffraction imaging compared with conventional broad‐bandwidth SASE pulses. These findings highlight the critical role of spectral purity in achieving high‐fidelity lensless imaging with X‐ray free‐electron lasers.

The photolysis of disulfide bonds is implicated in denaturation of proteins exposed to ultraviolet light. Despite this biological relevance in stabilizing the structure of many proteins, the mechanisms of disulfide photolysis are still contested after decades of research. Herein, we report new insight into the photochemistry of L-cystine in aqueous solution by femtosecond X-ray absorption spectroscopy at the sulfur K-edge. We observe homolytic bond cleavage upon ultraviolet irradiation and the formation of thiyl radicals as the single primary photoproduct. Ultrafast thiyl decay due to geminate recombination proceeds at a quantum yield of >80 % within 20 ps. These dynamics coincide with the emergence of a secondary product, attributed to the generation of perthiyl radicals. From these findings, we suggest a mechanism of perthiyl radical generation from a vibrationally excited parent molecule that asymmetrically fragments along a carbon-sulfur bond. Our results point toward a dynamic photostability of the disulfide bridge in condensed-phase. Disulfide bonds play a key role in the stability of proteins. Here, the authors show such bonds are efficiently reformed after UV photolysis in L-cysteine in solution using femtosecond X-ray absorption spectroscopy and theoretical calculations.

Ultrafast laser excitation can drive materials into exotic states beyond thermodynamic limits, offering alternative ways to control how matter stores and releases energy. Yet, whether light can actively steer energy-relaxation pathways during structural transitions remains unclear due to the lack of direct experimental evidence. Here we show, using single-pulse time-resolved X-ray imaging of gold nanorods, that photoinduced localized surface plasmons control ultrafast energy relaxation into distinct deformation modes, transverse or longitudinal deformation modes, each accompanied by characteristic plasmon-induced oscillatory distortions depending on the laser fluence. Numerical simulations further confirm that localized surface plasmons dictate ultrafast energy relaxation process from photoexcited hot electrons to anharmonic nanocrystal deformations. Our results provide direct evidence that surface plasmon-mediated interactions enable ultrafast, nanoscale control of materials’ energetics, opening a pathway for tailoring energy-transfer processes with femtosecond laser fields. This approach lays the foundation for customizing nonequilibrium phase dynamics at the nanoscale and provides a route to tailoring energy-transfer processes using femtosecond laser fields. Ultrafast lasers can drive materials into states beyond equilibrium. Here, the authors use single-pulse X-ray imaging to show that surface plasmons channel energy relaxation into distinct pathways, leading to different shape deformations.

A transmissive single-shot spectrometer has been developed to monitor shot-to-shot spectral structures in the hard X-ray beamline of the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL). The established spectrometer comprises 10 µm-thick Si crystals bent to a radius of curvature of 100 mm. Depending on the photon energy range, either the Si (111) or Si (110) crystal can be selected for spectral analysis. Especially in the energy range 4.5–17 keV, the spectrometer is designed to cover a spectral range wider than the full free-electron laser bandwidth and to guarantee a high resolution sufficient for resolving each spectral spike. This paper presents the design specifications, instruments and performance of this spectrometer, which has also been applied to demonstrate the spectral properties of various XFEL sources, such as self-amplified spontaneous emission, monochromatic and seeded beams.

A Nanobeam X‐ray Experiments (NXE) instrument was developed and installed at the hard X‐ray beamline of the Pohang Accelerator Laboratory X‐ray Free Electron Laser. This instrument consists of a diagnostic system, focusing optics, an X‐ray diffraction endstation and a femtosecond laser delivery system. The NXE instrument enables sophisticated X‐ray experiments using nanofocused X‐rays. At a 9.5 keV X‐ray energy, the beam was successfully focused to 390 nm × 230 nm at the focal plane using Kirkpatrick–Baez mirrors. Following the successful commissioning experiments in December 2021 and April 2022, the instrument became available for regular user experiments in January 2023. The first user experiment was conducted in January 2024. This article provides detailed information on the beamline optics, the NXE instrument, and its performance and capabilities. The Nanobeam X‐ray Experiments (NXE) instrument at the Pohang Accelerator Laboratory X‐ray Free Electron Laser (PAL‐XFEL) is introduced. The NXE instrument enables users to conduct X‐ray experiments with nanofocused X‐rays.

Various X-ray techniques are employed to investigate specimens in diverse fields. Generally, scattering and absorption/emission processes occur due to the interaction of X-rays with matter. The output signals from these processes contain structural information and the electronic structure of specimens, respectively. The combination of complementary X-ray techniques improves the understanding of complex systems holistically. In this context, we introduce a multiplex imaging instrument that can collect small-/wide-angle X-ray diffraction and X-ray emission spectra simultaneously to investigate morphological information with nanoscale resolution, crystal arrangement at the atomic scale and the electronic structure of specimens.

Understanding the ultrafast dynamics of molecules is of fundamental importance. Time‐resolved X‐ray absorption spectroscopy (TR‐XAS) is a powerful spectroscopic technique for unveiling the time‐dependent structural and electronic information of molecules that has been widely applied in various fields. Herein, the design and technical achievement of a newly developed experimental apparatus for TR‐XAS measurements in the tender X‐ray range with X‐ray free‐electron lasers (XFELs) at the Pohang Accelerator Laboratory XFEL (PAL‐XFEL) are described. Femtosecond TR‐XAS measurements were conducted at the Ru L3‐edge of well known photosensitizer tris(bipyridine)ruthenium(II) chloride ([Ru(bpy)3]2+) in water. The results indicate ultrafast photoinduced electron transfer from the Ru center to the ligand, which demonstrates that the newly designed setup is applicable for monitoring ultrafast reactions in the femtosecond domain. Tender X‐ray absorption spectroscopy was successfully performed with the newly developed experimental apparatus at PAL‐XFEL.

A Hanbury Brown and Twiss interferometry experiment based on second‐order correlations was performed at the PAL‐XFEL facility. The statistical properties of the X‐ray radiation were studied within this experiment. Measurements were performed at the NCI beamline at 10 keV photon energy under various operation conditions: self‐amplified spontaneous emission (SASE), SASE with a monochromator, and self‐seeding regimes at 120 pC, 180 pC and 200 pC electron bunch charge. Statistical analysis showed short average pulse duration from 6 fs to 9 fs depending on the operational conditions. A high spatial degree of coherence of about 70–80% was determined in the spatial domain for the SASE beams with the monochromator and self‐seeding regime of operation. The obtained values describe the statistical properties of the beams generated at the PAL‐XFEL facility. Statistical properties of the hard X‐ray free‐electron laser PAL‐XFEL were studied by Hanbury Brown and Twiss interferometry. The results demonstrate high spatial coherence and short average pulse duration of this facility at 10 keV photon energy.