We experimentally demonstrate that pairs of time-delayed ultrabright and ultrashort X-ray pulses of two different colors, delivered by modern X-ray Free Electron Lasers, can provide two time-delayed snapshots of a sample. We introduce Dichography, a method that algorithmically separates the diffraction signals overlapping on the detector and independently retrieves the two images of the specimen. We employ Dichography to reconstruct two views of individual xenon-doped helium nanodroplets with 20 nm spatial resolution. The consistency of structures observed in both images at delays up to 750 fs provides evidence that, under these illumination conditions, significant structural damage only occurs at longer timescales. We further validate the method by imaging pairs of silver nanoparticles intercepted by the same light pulse. Dichography enables a new class of experiments across physics, chemistry, and materials science, making a significant step toward the original promise of X-ray free-electron lasers to capture ultrafast movies of nanomatter.

The Alvra experimental station at the Swiss X-ray free-electron laser, SwissFEL, investigates ultrafast dynamics in chemical and biological systems using X-ray scattering and spectroscopy techniques. A key feature of Alvra is its unique capability to perform time-resolved X-ray emission and resonant inelastic X-ray scattering in the tender X-ray regime, currently not available at other XFEL endstations, combined with simultaneous access to X-ray absorption spectroscopy and X-ray solution scattering. Together with sub-35 fs time resolution enabled by advanced timing diagnostics, this positions Alvra as a versatile instrument for ultrafast chemical dynamics in the liquid phase. Here we cover the various technical aspects of the beamline and experimental station, and present some examples of experimental results measured during the first years of SwissFEL commissioning and user operation.

Abstract Cyanobacteria have produced Earth’s oxygen for 2.4 billion years by adapting to fluctuating irradiance. This adaptation relies on orange carotenoid protein (OCP), which mediates light-intensity– dependent photoprotective energy dissipation using a unique two-photon absorption mechanism. Photon absorption by ground-state OCP (OCP O ) generates a metastable intermediate (OCP 1hν ) that either relaxes thermally or, upon absorption of a second photon within ∼1 s, converts to the active photoprotective state (OCP R ). By integrating static and time-resolved crystallography, cryo-EM, computation, spectroscopy and biochemistry, we assign the structure of OCP 1hν , establish its functional relevance and capture structural snapshots along the OCP O →OCP 1hν and OCP 1hν →OCP R photochemical pathways. We elucidate the molecular mechanism of OCP, which serves as a unique biological circuit breaker protecting the photosynthetic machinery from high light flux.

Coherent nonlinear light-matter interaction with X-rays gives access to a regime in ultrafast spectroscopy in which atomic resolution meets femtosecond and attosecond timescales1,2. Particularly, X-ray four-wave mixing, involving several resonant transitions in a single coherent nonlinear process, has the potential to provide information on the electronic states coupling, coherent electron motion, correlation and dynamics, with state and site selectivity3-5. Here we demonstrate coherent, background-free four-photon interactions with core-shell electrons using single broadband X-ray pulses from a free-electron laser. The all-X-ray four-wave mixing signals, measured in gaseous neon, arise from doubly resonant nonlinear processes involving Raman transitions6, including X-ray coherent anti-Stokes electronic Raman scattering. The 2D spectral maps (photon-in/photon-out) represent a step towards multidimensional correlation spectroscopy at the atomic scale. Using a multicolour time-delayed X-ray pulse scheme, we further demonstrate the feasibility of extending the proposed methodology to the ultrafast time domain. These results reveal potential for studying localized electron dynamics in multiple systems, from biomolecules to correlated quantum materials, with applications in areas such as energy conversion, biomedical imaging and quantum information technologies.

This paper presents the time-resolved X-ray photoelectron spectroscopy (TR-XPS) setup at the Swiss Free-Electron Laser (SwissFEL) Maloja endstation for investigating the ultrafast dynamics of gas-phase molecules with site-specificity. As a...

We demonstrate a versatile platform for high-power attosecond soft X-ray pulse generation with polarization and photon energy control at the SwissFEL free-electron laser. An isolated high-current spike embedded within a long electron-beam pedestal emits soft X-ray pulses with single-spike spectra and multi-electronvolt bandwidths in the tunable magnetic fields of Apple-X undulators. Demonstrated pulse parameters include a photon energy range of 450–1070 eV, circular as well as linear polarization, and pulse energies from tens to above hundred microjoules. By tuning the longitudinal slice-dependent transverse electron beam orbit we can rapidly switch between attosecond and few femtosecond pulse length. By exploiting magnetic chicanes in the undulator line we can produce two-colour pulse pairs with tunable delay or increase the pulse energy beyond 200 µJ through multi-stage amplification schemes. High-resolution longitudinal phase-space measurements and start-to-end simulations in addition to spectral measurements provide consistent evidence for attosecond-scale pulse durations. This unique combination of high pulse energy and polarization control of attosecond-scale soft X-ray pulses enables the element-specific investigations of spin and chiral dynamics on the natural time scale of electron motion.

We present α-Al2O3 XAS, XES and RIXS measurements across the Al L2/L3 edges at about 79 eV excitation energy. In the emission spectra, we identify two fluorescence peaks, corresponding to electronic transitions into the 2p core hole from mixed states of Al 3s and Al 3d character, both mixed with O 2p orbitals. Even if the XAS spectrum shows more than one resonance, surprisingly only one clear RIXS signal with energy loss equal to 10.7 eV is present in the data. Nevertheless, this allows us to tentatively extract from the measured high-resolution data the linewidths for fluorescence and RIXS transitions, with the latter being almost a factor of two smaller than the former.

In recent years, time-resolved serial crystallography has emerged as a transformative technique for unraveling the intricate dynamics of macromolecules at atomic resolution. By leveraging the high-intensity and ultra-short pulses of X-ray free electron lasers (XFELs) alongside the high brilliance of synchrotron light sources, this technique has enabled the observation of transient states in biomolecules as they catalyze chemical reactions.This presentation will highlight the advancements and applications of time-resolved serial crystallography in the study of macromolecular dynamic. We will discuss the light-sensitive membrane protein Nonlabens marinus halorhodopsin (NmHR) as an example of how this method enables us to capture the structural dynamics from femtoseconds to milliseconds after light activation. Through combining time-resolved studies at the X-ray free electron laser and synchrotron with spectroscopy and chemical simulation, we obtained a comprehensive understanding of the molecular mechanism that allows NmHR to catalyze ion transport across biological membranes. In addition to discussing the rich chemical information that can be obtained in time-resolved crystallographic studies, this talk will highlight how steady-state experiments can provide exciting structural insights while requiring only a limited amount of beamtime and a minimal setup.

We report the generation of intense deep ultraviolet pulses at 200 nm with a duration of 48 fs and a pulse energy of 130 μJ,achieved via cascaded sum-frequency generation using 800 nm femtosecond pulses in barium borate crystals.Efficient frequency up-conversion is realized by optimizing phase-matching conditions and implementing dispersion control,while maintaining the ultrashort pulse characteristics.The generated deep ultraviolet pulses are characterized using two-photon absorption frequency-resolved optical gating,providing detailed insight into their temporal profile and phase.This approach addresses key challenges in ultrashort deep ultraviolet pulse generation,delivering a high-energy,ultrashort source suitable for ultrafast spectroscopy,nonlinear optics and strong-field physics.These results represent a significant advancement in the generation of high-energy,ultrashort deep ultraviolet pulses,opening up new possibilities for time-resolved investigations in ultrafast molecular dynamics,as well as emerging applications in semiconductor science,quantum materials and photochemistry.

The photophysics and photochemistry of isolated phenanthridine have been investigated by time-resolved UV pump/X-ray probe spectroscopy at the SwissFEL free-electron laser combined with computations. Phenanthridine serves as the example for a polycyclic aromatic nitrogen-containing hydrocarbon (PANH), a class of molecules of considerable interest in material science and astrochemistry. It was excited at 268 nm into the bright 2ππ* state. The dynamics was subsequently probed by time-resolved X-ray photoelectron (TR-XPS) and X-ray absorption (TR-XAS) spectroscopy at the nitrogen 1s edge. Two time constants of τ 1 ≈ 0.3 ps and τ 2 ≈ 3 ps were determined. The excited-state dynamics was simulated using the trajectory surface hopping method and computed TR-XAS to support the band assignments. The study reveals a sequential decay to the electronic ground state via internal conversion. Spectra recorded over longer delay times indicate a dissociation on a time scale of several hundred picoseconds.

X-ray free-electron lasers can nowadays deliver pairs of ultrabright, ultrashort light pulses with controllable delays at different photon energies. When combined with diffraction experiments, the interaction of these pulses with a sample at different times can produce two snapshots of an evolving system, enabling spatial and temporal resolution of ultrafast dynamics in nanomatter at and beyond the terahertz time scale. However, light detectors are orders of magnitude slower. The diffraction signals from the two pulses overlap in the recorded data and cannot be separated using currently available analysis methods. Here, we address this challenge by introducing Dichography, a diffraction imaging technique that restores the two unique views of the sample from superimposed scattering signals. We apply Dichography to experimental diffraction patterns of isolated xenon-doped superfluid helium nanodroplets, imaged using time-delayed, two-color X-ray pulses at intensities near the detection limit. The reconstructed images provide evidence of the survival of the xenon structures up to 750 fs after the interaction with the first shot. The capabilities of Dichography are further explored by applying the method to data from a second experiment, in which we retrieve the images of two distinct silver nanoparticles intercepted by the same light pulse. Dichography enables a new class of experiments for spatially and temporally resolving ultrafast phenomena, making a significant step toward the original promise of X-ray free-electron lasers to capture ultrafast movies of nanomatter.

We present a series of novel X-ray imaging systems designed specifically for the soft X-ray energy range, optimized for operation in ultra-high-vacuum environments and compactness. These systems achieve micrometre-level spatial resolution with high collection efficiency of visible light by using high numerical aperture optics. Comprehensive characterization of the systems' response was performed, including linearity assessments and X-ray sensitivity measurements, across X-ray photon densities ranging from 1 nJ m−2 to 10−4 nJ m−2. The imaging system was employed for caustic measurements to characterize the X-ray focal spot and to demonstrate its capabilities. Finally, grating interferometry was used to measure the wavefront distortion, yielding a pitch resolution as fine as 3.1 µm. These results underscore the system's potential for high-resolution soft X-ray imaging and wavefront characterization applications.

Coherent Diffraction Imaging (CDI) is an experimental technique to image isolated structures by recording the scattered light. The sample density can be recovered from the scattered field through a Fourier Transform operation. However, the phase of the field is lost during the measurement and has to be algorithmically retrieved. Here we present SPRING, an analysis framework tailored to X-ray Free Electron Laser (XFEL) single-shot single-particle diffraction data that implements the Memetic Phase Retrieval method to mitigate the shortcomings of conventional algorithms. We benchmark the approach on data acquired in two experimental campaigns at SwissFEL and European XFEL. Results reveal unprecedented stability and resilience of the algorithm’s behavior on the input parameters, and the capability of identifying the solution in conditions hardly treatable with conventional methods. A user-friendly implementation of SPRING is released as open-source software, aiming at being a reference tool for the CDI community at XFEL and synchrotron facilities.

We present absorption spectra of thin, free-flowing liquid sheets in the vacuum ultraviolet energy range using a gas-squeezed liquid jet. Compared to liquid flow cells, operation without transmission windows eliminates restrictions on the energy range. The temperature of the water sheet is estimated at 0 ± 3 °C, at the verge of the supercooled regime. By adjusting flow conditions in situ, we recorded absorption spectra at water sheet thicknesses ranging from 20 to 50 nm. We show that the absorption spectra of thin jets contain significant contributions from interference effects that need to be deconvoluted from spectral contributions due to the electronic structure. We employ a Fresnel propagation model to model the spectral changes and understand the impact of thickness variations and thin film interference. This opens the door for the investigation of solvation, interface, and similar effects by recording valence band spectra.

We present an extreme ultraviolet (EUV) transient grating (TG) experiment of the spinel Co3O4 compound using tuneable incident energies across the Co M-2,M-3-edge and a 395 nm probe pulse, detecting both the first and the second diffraction orders (SDOs). While the first diffraction order shows a monotonous behavior as a function of time, with a sharp response at t = 0, followed by a weak sub-picosecond component and a nearly constant signal thereafter, the time dependence of SDO varies dramatically with the incident energy as it is tuned across the Co M-edge, with the appearance of a component at t > 1 ps that grows with increasing energy. The results are rationalized in terms of the deviations of the initial grating from sinusoidal to non-sinusoidal, namely a flattening of the grating pattern, that introduces new Fourier components. These deviations are due to higher order, three-body terms in the population relaxation kinetics. The present results highlight the use of the SDO response in EUV TG as a tool to identify higher order terms in the population kinetics.

The ability to freely control the polarization of X-rays enables measurement techniques relying on circular or linear dichroism, which have become indispensable tools for characterizing the properties of chiral molecules or magnetic structures. Therefore, the demand for polarization control in X-ray free-electron lasers is increasing to enable polarization-sensitive dynamical studies on ultrafast time scales. The soft X-ray branch Athos of SwissFEL was designed with the aim of providing freely adjustable and arbitrary polarization by building its undulator solely from modules of the novel Apple X type. In this paper, the magnetic model of the linear inclined and circular Apple X polarization schemes are studied. The polarization is characterized by measuring the angular electron emission distributions of helium for various polarizations using cold target recoil ion momentum spectroscopy. The generation of fully linear polarized light of arbitrary angle, as well as elliptical polarizations of varying degree, are demonstrated.

High-intensity femtosecond pulses from an X-ray free-electron laser enable pump–probe experiments for the investigation of electronic and nuclear changes during light-induced reactions. On timescales ranging from femtoseconds to milliseconds and for a variety of biological systems, time-resolved serial femtosecond crystallography (TR-SFX) has provided detailed structural data for light-induced isomerization, breakage or formation of chemical bonds and electron transfer 1 , 2 . However, all ultrafast TR-SFX studies to date have employed such high pump laser energies that nominally several photons were absorbed per chromophore 3 – 17 . As multiphoton absorption may force the protein response into non-physiological pathways, it is of great concern 18 , 19 whether this experimental approach 20 allows valid conclusions to be drawn vis-à-vis biologically relevant single-photon-induced reactions 18 , 19 . Here we describe ultrafast pump–probe SFX experiments on the photodissociation of carboxymyoglobin, showing that different pump laser fluences yield markedly different results. In particular, the dynamics of structural changes and observed indicators of the mechanistically important coherent oscillations of the Fe–CO bond distance (predicted by recent quantum wavepacket dynamics 21 ) are seen to depend strongly on pump laser energy, in line with quantum chemical analysis. Our results confirm both the feasibility and necessity of performing ultrafast TR-SFX pump–probe experiments in the linear photoexcitation regime. We consider this to be a starting point for reassessing both the design and the interpretation of ultrafast TR-SFX pump–probe experiments 20 such that mechanistically relevant insight emerges.

Time-resolved serial crystallography at X-ray Free Electron Lasers offers the opportunity to observe ultrafast photochemical reactions at the atomic level. The technique has yielded exciting molecular insights into various biological processes including light sensing and photochemical energy conversion. However, to achieve sufficient levels of activation within an optically dense crystal, high laser power densities are often used, which has led to an ongoing debate to which extent photodamage may compromise interpretation of the results. Here we compare time-resolved serial crystallographic data of the bacteriorhodopsin K-intermediate collected at laser power densities ranging from 0.04 to 2493 GW/cm2 and follow energy dissipation of the absorbed photons logarithmically from picoseconds to milliseconds. Although the effects of high laser power densities on the overall structure are small, in the upper excitation range we observe significant changes in retinal conformation and increased heating of the functionally critical counterion cluster. We compare light-activation within crystals to that in solution and discuss the impact of the observed changes on bacteriorhodopsin biology. Time-resolved serial crystallography at XFELs reveals ultrafast photochemical reactions, but high laser densities can cause photodamage to biological samples. Here, the authors study the early K-intermediate in bacteriorhodopsin at high power, showing overall conformation remains robust over a wide range.

Nonlinear wave mixing in the X-ray range can provide valuable insights into the structural and electron dynamics of atomic and molecular systems on ultrafast time scales, with state- and site-selectivity and atomic resolution. This promising experimental toolbox was so far limited by requiring at least one near-visible laser, thus preventing core-shell two-dimensional X-ray spectroscopy. In this work, we demonstrate the generation of background-free all-X-ray four-wave mixing (XFWM) signals from a dilute gaseous sample (Ne). The measured and simulated two-dimensional spectral maps (ω_in,ω_out) show multiple contributions involving the coherent response from core electrons. Notably, two-color resonant XFWM signals, essential for generalized multi-color schemes that allow to locally probe the electronic excitation of matter, are observed in neutral Ne. Moreover, stimulated Ne^+ emission in each of the propagating X-ray pulses leads to an increase of the temporal coherence in a narrow-bandwidth, which results in the coherent mixing of three X-ray lasers. Preliminary X-ray excitation experiments making use of multi-color time-delayed X-ray pulses demonstrate temporal resolution capability and show a time dependency consistent with a signal dominated by resonant XFWM processes. This first all-X-ray four-wave-mixing approach represents a major breakthrough towards multidimensional X-ray correlation spectroscopy and the general application of nonlinear all-X-ray wave-mixing.

To fully exploit ultra-short X-ray pulse durations routinely available at X-ray free-electron lasers to follow out-of-equilibrium dynamics, inherent arrival time fluctuations of the X-ray pulse with an external perturbing laser pulse need to be measured. In this work, two methods of arrival time measurement were compared to measure the arrival time jitter of hard X-ray pulses. The methods were photoelectron streaking by a THz field and a transient refractive index change of a semiconductor. The methods were validated by shot-to-shot correction of a pump–probe transient reflectivity measurement. An ultimate shot-to-shot full width at half-maximum error between the devices of 19.2 ± 0.1 fs was measured.