European X-Ray Free-Electron Laser

We demonstrate a grazing-incidence X-ray platform that simultaneously records time-resolved grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence X-ray diffraction (GID) from a femtosecond-laser-irradiated gold film above the melting threshold, with picosecond resolution using an X-ray free-electron laser (XFEL). By tuning the X-ray incidence angle, the probe depth is set to tens of nanometres, enabling depth-selective sensitivity to near-surface dynamics. GISAXS resolves ultrafast changes in surface nanomorphology (correlation length, roughness), while GID quantifies subsurface lattice compression, grain orientation, melting and recrystallization. The approach overcomes photon-flux limitations of synchrotron grazing-incidence geometries and provides stringent, time-resolved benchmarks for complex theoretical models of ultrafast laser-matter interaction and warm dense matter. Looking ahead, the same depth-selective methodology is well suited to inertial confinement fusion (ICF): it can visualize buried-interface perturbations and interfacial thermal resistance on micron to sub-micron scales that affect instability seeding and burn propagation.

Dense plasma environment affects the electronic structure of ions via variations of the microscopic electrical fields, also known as plasma screening. This effect can be either estimated by simplified analytical models, or by computationally expensive and to date unverified numerical calculations. We have experimentally quantified plasma screening from the energy shifts of the bound-bound transitions in matter driven by the x-ray free electron laser (XFEL). This was enabled by identification of detailed electronic configurations of the observed K , K and K lines. This work paves the way for improving plasma screening models including connected effects like ionization potential depression and continuum lowering, which will advance the understanding of atomic physics in the Warm Dense Matter regime.

Roaming-mediated isomerization is a universal reaction mechanism in photochemistry, yet solvent-dependent pathways of roaming intermediates remain poorly understood, particularly for environmentally relevant halogen compounds involved in ozone depletion. Here, using femtosecond time-resolved X-ray solution scattering, we resolve the solvent-dependent roaming dynamics of CHBr3 in methanol and methylcyclohexane. By combining multi-method experimental analysis, machine learning-assisted ab initio molecular dynamics simulations, and density functional theory calculations, we uncover distinct solvent-steered reaction pathways. In methanol, roaming enhances solute-solvent interactions, leading to solvolysis before a stable isomer forms. In methylcyclohexane, roaming facilitates isomerization to a long-lived iso-CHBr2-Br product. Direct dissociation into CHBr2 + Br competes with both pathways in either solvent. By tracking bond-length oscillations and angular dynamics in real time, we visualize how the condensed-phase environment governs the branching ratio between competing pathways. Our findings establish solute-solvent interactions as key factors controlling roaming-mediated reactions in CHBr3, with broad implications for photochemical outcomes in solution.

Abstract We report transient highly strained structural states in individual palladium (Pd) nanocrystals, electronically heated using an optical laser, which precede their uniform thermal expansion. Using an X-ray free-electron laser probe, the evolution of individual 111 Bragg peaks is measured as a function of delay time at various laser fluences. Above a laser fluence threshold at a sufficient pump-probe delay, the Bragg peak splits into multiple peaks, indicating heterogeneous strain, before returning to a single peak, corresponding to even heat distribution throughout the lattice expanded crystal. Our findings are supported by a lattice displacement and strain model of a single nanocrystal at different delay times, which agrees with the experimental data. Our observations have implications for understanding femtosecond laser interactions with metals and the potential photo-catalytic performance of Pd.

Abstract Turkish Accelerator and Radiation Laboratory (TARLA) is proposed as the first accelerator-based user facility within the framework of the Turkish Accelerator Center project initiative. Following the completion of technical design studies and infrastructure development, the installation of accelerator components commenced. TARLA is designed to produce free-electron laser (FEL) and bremsstrahlung radiation utilizing state-of-the-art superconducting accelerator technology. The facility aims to operate FELs in the wavelength range of 5–350 µm by accelerating electrons to energies between 15 and 40 MeV, and to generate bremsstrahlung radiation with electrons accelerated up to 30 MeV. As a major milestone, the first stable electron beam was successfully generated in April 2024, marking the completion of the facility’s first operational phase. This study presents the commissioning efforts for the electron beam, outlines the current operational status of the facility, and discusses future plans and the research potential of TARLA.

The invention of the laser transformed optics by providing intense, coherent light in the visible region, but extending this concept to X-rays has been hindered by a lack of suitable gain media and mirrors. Current hard X-ray free-electron laser (XFEL) facilities1–5 overcome this by amplifying shot noise from a high-peak-current electron bunch via self-amplified spontaneous emission6 in a single pass through long undulators, delivering very high brightness but with a noisy, multi-spiked temporal and spectral profile. Cavity-based XFELs (CBXFELs)7–9 were proposed to close this gap by recirculating spectrally filtered X-ray pulses in a Bragg-reflecting cavity synchronized to a high-repetition-rate electron beam. Here we show lasing with multi-pass gain at 6.952 keV in a 132.8-m round-trip diamond-based Bragg cavity10 at the European XFEL, matched to the 2.23-MHz bunch spacing of the superconducting accelerator5. Under stringent length and angular stability requirements, a ring-up in the cavity across successive bunches was observed, producing spectrally pure, microjoule-level pulses. This establishes the feasibility of CBXFELs in an accelerator environment and validates diamond Bragg optics for X-ray resonators. The demonstrated spectral purity opens a path to next-generation X-ray science, which demands highly coherent, stable sources.

Attosecond x-ray pulses are a critical tool for tracking ultrafast electron dynamics in condensed matter, molecular systems, and strongly correlated materials. Recent breakthroughs have pushed x-ray free-electron lasers (XFELs) into the attosecond domain, substantially surpassing their previous femtosecond capabilities. Building on these advancements, this work investigates the potential of the Shanghai HIgh repetitioN rate XFEL and Extreme light facility (SHINE), China’s first continuous-wave (CW) XFEL, to generate intense attosecond x-ray pulses, thereby offering transformative capabilities for x-ray science. Through comprehensive start-to-end simulations, we show that SHINE is capable of producing terawatt-scale pulses at both soft and hard x-ray ranges. This is achieved using a self-chirping scheme within the existing machine configuration, requiring no additional hardware. We further find that superradiant behavior in the postsaturation regime plays a central role in attosecond XFEL, enabling higher peak power and shorter attosecond pulses. Our results illustrate the wide applicability of the self-chirping scheme and demonstrate that CW XFELs can generate intense attosecond x-ray pulses at megahertz repetition rates, opening new opportunities for real-time studies of electronic dynamics in complex systems.

Fe-based pnictides have evolved as a practical alternative to Cuprate superconductors. Electrons are transported along As–Fe–As path of inter-connected FeAs4, utilizing Fe 3d-As 4p orbital hybridization. This leads to an obvious question — what are the key tetrahedral parameters that control hybridization? Unfortunately, empirical results from discrete experiments generated conflicting structure-transport correlations. The present work attempts to fill this logistic gap by probing the missing structure-orbital correlation link. To this effect, pressure-dependent XRD and As K-edge XANES were measured on the same sample set of PrOFe0.9Co0.1As. A point of inflection emerges at the compressibility limit of Fe–As bond-length. At this point, the trajectory of As atom and (correspondingly) parameters of FeAs4 undergo parabolic reversal whilepd hybridization undergoes nonreversible maximization. This incongruence generates nonlinear structure-orbital correlation, which is a novel finding. Two novel concepts emanate from this exercise — (a) compressibility limit of Fe–As bond-length emerges as the new critical reference and (b) nonlinearity warrants that pd hybridization is sensitive to the phase of As trajectory (relative to this reference) rather than the absolute values of tetrahedral parameters. These mark major corrections from previous works, which should settle the existing conflicts.