In this article, we present the experimental protocol and data-processing framework for megahertz X-ray Photon Correlation Spectroscopy (MHz-XPCS) experiments on soft matter samples implemented at the Materials Imaging and Dynamics (MID) instrument of the European X-ray Free-Electron Laser (EuXFEL). Due to the introduction of a standard configuration and the implementation of a highly automated data-processing pipeline, MHz-XPCS measurements can now be conducted and analyzed with minimal user intervention. A key challenge lies in managing the extremely large data volumes generated by the Adaptive Gain Integrating Pixel Detector (AGIPD) - often reaching several petabytes within a single experiment. We describe the technical implementation, discuss the hardware requirements related to effective parallel data processing and propose strategies to enhance data quality, in particular related to data reduction strategies and an improvement of the signal-to-noise ratio. Finally, we address strategies for making the processed data FAIR (Findable, Accessible, Interoperable, Reusable), in alignment with the goals of the DAPHNE4NFDI project.

We investigate the ultrafast dynamics of plasma formation by optical breakdown, filamentation, and cavitation in water, using high spatiotemporal resolution offered by x-ray free-electron laser (XFEL) radiation. A femtosecond infrared laser pulse is focused in a water-filled cuvette and probed by a single femtosecond x-ray pulse, with a time delay covering nearly four orders of magnitude. By exploiting the quantitative contrast values obtained by phase retrieval, we can follow the transition from plasma to gas in terms of a continuous decrease of mass density in the cavity. At the same time, we image the emission of a cylindrical shock wave for the scenario of a single elongated breakdown filament with a high degree of symmetry. Contrarily, the regime of multiple breakdown spots deviates from cylindrical symmetry and the idealized picture expected for a Gaussian beam. Here different scenarios of cavitation and (collective) expansion dynamics as well as bubble fusion are observed. Specifically, we quantify the decrease of the expansion velocity with the number of auxiliary cavitation events due to a redistribution of the deposited laser energy. We also report events with (multi)filamentation reflecting instabilities in the initial distribution of the laser intensity upon formation of the plasma. Filaments with submicron diameter and few-micrometer spacing are observed, as well as the phenomena of filament emergence, splitting, and termination. The different regimes of heterogeneous optical breakdown and cavitation can be distinguished depending on the laser pulse energy. Altogether, the experiments demonstrate the potential of single-pulse XFEL imaging for the investigation of optical breakdown and ultrafast hydrodynamics. The future application of the imaging approach to soft matter environments, tissue, glasses, and opaque materials seems straightforward.

Diamond is a highly suited material for radiation-resistant X-ray optics, particularly for 4th-generation synchrotron radiation sources with high brightness and X-ray free-electron laser (XFEL) facilities operating at high pulse energies. For various imaging applications, critical factors such as spatial resolution, bandwidth flexibility, and compact integration must be addressed in the design of focusing optics. However, the manufacturing process by laser ablation of diamond lenses often leads to residual aberrations and limitations in achievable spot sizes, posing challenges for high-resolution imaging applications. This work introduces an innovative concept of aberration-compensated X-ray lens cubes, composed of bi-concave, two-dimensional diamond lens plates with a 25 µm radius of curvature, fabricated by femtosecond laser ablation. A focal spot size of 52 nm × 51 nm was achieved at 14 keV, with wavefront errors strongly reduced across a wide photon energy range of 14 keV to 20 keV using multiple corrective phase plates. These results demonstrate the strong potential of our approach for nanoimaging applications, advancing high-resolution X-ray focusing capabilities for 4th-generation synchrotron radiation facilities and XFELs.

We demonstrate an advanced scattering method for accessing the 3D reciprocal space of crystalline structures forming in a rapidly supercooled noble-gas liquid using a combination of femtosecond X-ray diffraction and X-ray cross-correlation analysis. The preservation of angular information from the scattering signal allows probing the structure factor along selected directions in reciprocal space and identifying signatures undetectable in azimuthally integrated scattering curves. Therefore, more information from serial diffraction experiments on stochastic crystallization processes can be retrieved despite the inherent variation of the crystal orientation and morphology for each single probe. We also demonstrate how different features in the correlation maps can be associated with certain forms of stacking faults, which enables studying such defects in situ and disentangling them from simultaneous changes in crystal size and temperature.

The structural and dynamical properties of soft-matter systems play an important role in crystallization and nucleation theory. Despite their significance, the dynamical properties are still poorly understood because of experimental constraints and the requirement of performing measurements with high spatial and temporal resolution. Here, we demonstrate MHz X-ray photon correlation spectroscopy (XPCS) using a contactless sample holder at the European X-ray Free-Electron Laser. A millimetre-sized liquid sample droplet was levitated in air via acoustic waves with the solvent slowly evaporating. A colloidal suspension of silica nanospheres was used to track the structural evolutions using small-angle X-ray scattering, and the dynamical information was captured by time-resolved MHz XPCS as a function of evaporation time. This study outlines a new path towards the investigation of metastable structure and dynamics using X-ray speckle techniques, for instance, XPCS, X-ray speckle visibility spectroscopy and X-ray cross-correlation analysis.

We show the results of speckle contrast analysis at the MID instrument of European XFEL in the hard X-ray regime. Speckle patterns measured from static colloidal samples are compared to results previously obtained at the SPB/SFX instrument. A high degree of coherence of 0.79 is obtained by modelling the q -dependence of the speckle contrast, that corresponds to a number of coherent modes of M = 1.7. Furthermore, the variation of contrast over many pulse trains is exceptional low, resulting in a degree of coherence with a relative standard deviation below 0.1. Our results demonstrate the high stability of coherence properties at European XFEL over many X-ray pulses and pulse trains which is a prerequisite for coherence-based techniques such as MHz X-ray photon correlation spectroscopy.

A hard X-ray Split and Delay Line unit is under commissioning for the MID station at European XFEL. In this contribution, we report on the technical commissioning of the device and the first X-ray results.

Due to the high intensity and MHz repetition rate of photon pulses generated by the European X-ray Free-Electron Laser, the heat load on silicon crystal monochromators can become large and prevent ideal transmission in Bragg diffraction geometry due to crystal deformation. Here, we present experimental data illustrating how heat load affects the performance of a cryogenically cooled monochromator under such conditions. The measurements are in good agreement with a depth-uniform model of X-ray dynamical diffraction taking beam absorption and heat deformation of the crystals into account.

Free-electron lasers (FELs) provide unique possibilities in investigating matter down to femtosecond time and nanometer length scales, as well as in the regime of non-linear light-matter interaction. Due to the nature of FEL sources, the produced beam is significantly more unstable than beams produced by 3 rd generation synchrotrons. As a result, pulse-resolved normalization of measurement data becomes essential and can be challenging. The intensity monitors permanently installed at a facility might indeed accurately measure the pulse intensities at a certain point of the beamline, but cannot precisely normalize experimental data. For example the impact of pointing instabilities and hence different clipping of the beam downstream on the way to the actual experiment is not reflected in the intensity measurement. Here, we show how the integral intensity of the FEL beam transmitted through the sample can be measured by photodiodes providing a proper normalization of measurement data.

The Materials Imaging and Dynamics (MID) instrument at European XFEL aims at the investigation of nanosized structures and nanoscale dynamics using coherent X-ray radiation. A hard X-ray Split and Delay Line unit (SDL) is under manufacture for the MID station. The SDL is optimized to operate in a photon energy range from 5 to 10 keV and provides pairs of jitter-free X-ray pulses with a variable time delay ranging from −10 to 800 ps. With the powerful tunable and synchronized optical laser system at the MID station, not only X-ray pump-probe experiments but also X-ray probe—optical pump—X-ray probe (XOX) and optical pump—X-ray probe—X-ray probe (OXX) experiments are enabled by the SDL. This device will allow the study of ultrafast dynamics using experimental techniques, e.g. time-resolved X-ray Photon Correlation Spectroscopy (XPCS), Speckle Visibility Spectroscopy (SVS), ultrafast X-ray tomography and temporally and spatially resolved X-ray holography. We present here the current status of the mechanical manufacture of the device. Throughput simulations using diamond crystals as X-ray optics have been performed for the optical splitting scheme in SASE configuration and the results are presented.

Solutions of wormlike micelles can form cross-linked networks on microscopic length scales. The unique mechanical properties of these complex fluids are driven by the interplay between the network structure and dynamics which are investigated by plate-plate rheometry and X-ray photon correlation spectroscopy~(XPCS) nanorheology. Intensity auto-correlation functions of tracer nanoparticles~(NPs) dispersed in micelle solutions were recorded which captured both the slow structural network relaxation and the short-time dynamics of NPs trapped in the network. The results are indicative of a resonance-like dynamic behavior of the network on the nanoscale that develops as a consequence of the intrinsic short-range rigidity of individual micelle chains.

The Materials Imaging and Dynamics (MID) station is located at the SASE2 undulator beamline of European XFEL and has become operational in 2019. The MID instrument operates in the medium to hard X-ray range (5 - 25 keV) and its scientific focus is on time-resolved coherent X-ray scattering and diffraction studies in materials science, with particular interest in ultrafast pump-probe experiments where the pump can be either X-rays, an optical laser beam or a pulsed magnetic field. The optical setup of the MID instrument includes two vertically offset mirrors equipped with cryogenic cooling. The top mirror will be employed for grazing incidence experiments on liquid surfaces, and the bottom mirror will be used to spatially overlap two split beams generated by a “split and delay” line. The mirrors are 500 mm long and are coated with boron carbide (B4C) and platinum (Pt) in two adjacent stripes. Deterministic mirror polishing is done to compensate the gravitational sag in order to achieve a perfectly flat mirror when it is installed. The requirements were very challenging for the surface shape and the needed ion-beam deterministic polishing, so also the coating process had to be performed and monitored with particular care. We present the metrological characterization of the mirrors, carried out before and after the B4C and Pt coating, and performed with a large aperture Fizeau interferometer. The measurements were made at European XFEL’s metrology lab in grazing incidence setup and with the mirrors facing sideways. Analyzing these data, we can deduce many important parameters, as the peak-to-valley of the central profile, the bending radius, and the gravity compensation profile. We show metrological results before and after coating and give details about the calculations performed to decide whether the mirror shapes are still complying with specifications after all these processes.

A hard X-ray Split-and-Delay Line (SDL) under construction for the Materials Imaging and Dynamics station at the European X-Ray Free-Electron Laser (XFEL) is presented. This device aims at providing pairs of X-ray pulses with a variable time delay ranging from −10 ps to 800 ps in a photon energy range from 5 to 10 keV for photon correlation and X-ray pump-probe experiments. A custom designed mechanical motion system including active feedback control ensures that the high demands for stability and accuracy can be met and the design goals achieved. Using special radiation configurations of the European XFEL’s SASE-2 undulator (SASE: Self-Amplified Spontaneous Emission), two-color hard x-ray pump-probe schemes with varying photon energy separations have been proposed. Simulations indicate that more than 109 photons on the sample per pulse-pair and up to about 10% photon energy separation can be achieved in the hard X-ray region using the SDL.

We report x-ray photon correlation spectroscopy (XPCS) results on bidimensional (2D) gels formed by a Langmuir monolayer of gold nanoparticles. The system allows an experimental determination of the fourth order time correlation function, which is compared to the usual second order correlation function and to the mechanical response measured on macroscopic scale. The observed dynamics is anisotropic, heterogeneous and superdiffusive on the nanoscale. Different time scales, associated with fast heterogeneous dynamics inside 2D cages and slower motion of larger parts of the film, can be identified from the correlation functions. The XPCS results are discussed in view of other experimental results and models of three-dimensional gel dynamics.

Results of a x-ray photon correlation spectroscopy experiment on the very weakly first order martensitic transformation of a Au50.5Cd49.5 single crystal are presented. Slow non-equilibrium-dynamics are observed in a narrow temperature interval in the direct vicinity of the otherwise athermal phase transformation. These dynamics are associated with the martensite-aging effect. The dynamical aging is accompanied by an avalanchelike behavior which is identified with an incubation-time phenomenon.

We report the results of x-ray photon correlation spectroscopy (XPCS) experiments on multilayers of a photosensitive azo-polymer which can be softened by photoisomerization. Time correlation functions have been measured at different temperatures and momentum transfers (q) and under different illumination conditions (dark, UV or visible). The correlation functions are well described by the Kohlrausch-Williams-Watts (KWW) form with relaxation times that are proportional to q(-1). The characteristic relaxation times follow the same Vogel-Fulcher-Tammann law describing the bulk viscosity of this polymer. The out-of-equilibrium relaxation dynamics following a UV photoperturbation are accelerated, which is in agreement with a fluidification effect previously measured by rheology. The transient dynamics are characterized by two times correlation function, and dynamical heterogeneity is evidenced by calculating the variance χ of the degree of correlation as a function of ageing time. A clear peak in χ appears at a well defined time τ(C) which scales with q(-1) and with the ageing time, in a similar fashion as previously reported in colloidal suspensions [O. Dauchot, Phys. Rev. Lett. 95, 265701 (2005)]. From an accurate analysis of the correlation functions we could demonstrate a temperature and light dependent cross-over from compressed KWW to simple exponential behavior.

We discuss the complex dynamics in condensed matter studied with x-ray photon correlation spectroscopy (XPCS) in which non-exponential correlation functions and dispersion relations deviating from the simple diffusion law are observed. Results are presented for two systems whose dynamics are characterized by compressed, faster-than-exponential correlation functions associated with hyper-diffusive motion. In the first case, the microscopic response of an aerogel following sectioning is investigated. In the second, the out-of-equilibrium dynamics in a dense colloidal gel recovering from shear is analyzed. In both cases, the dynamics, which can be associated with relaxation of internal stress, exhibits ageing. Included in the analyses are calculations of two-time correlation functions and the variance of the instantaneous degree of correlation, yielding the dynamical susceptibility.

X-ray photon correlation spectroscopy (XPCS) has emerged as a unique technique allowing the measurement of dynamics of materials on mesoscopic lengthscales. One of the most common problems associated with the use of bright x-ray beams is beam-induced radiation damage, and this is likely to become an even more limiting factor at future synchrotron and free-electron laser sources. Flowing the sample during data acquisition is one of the simplest methods allowing the radiation damage to be limited. In addition to distributing the dose over many different scatterers, the method also enables new functionalities such as time-resolved studies. Here, we further develop a recently proposed experimental technique that combines XPCS and continuously flowing samples. More specifically, we use a model colloidal suspension to show how the macroscopic advective response to flow and the microscopic dissipative dynamics (diffusion) can be quantified from the x-ray data. Our results show very good quantitative agreement with a Poisseuille-flow hydrodynamical model combined with Brownian mechanics. The method has many potential applications, e.g. in the study of dynamics of glasses and gels under continuous shear/flow, protein aggregation processes and the interplay between dynamics and rheology in complex fluids.