Dynamics and kinetics in soft matter physics, biology, and nanoscience frequently occur on fast (sub)microsecond but not ultrafast timescales which are difficult to probe experimentally. The European X-ray Free-Electron Laser (European XFEL), a megahertz hard X-ray Free-Electron Laser source, enables such experiments via taking series of diffraction patterns at repetition rates of up to 4.5 MHz. Here, we demonstrate X-ray photon correlation spectroscopy (XPCS) with submicrosecond time resolution of soft matter samples at the European XFEL. We show that the XFEL driven by a superconducting accelerator provides unprecedented beam stability within a pulse train. We performed microsecond sequential XPCS experiments probing equilibrium and nonequilibrium diffusion dynamics in water. We find nonlinear heating on microsecond timescales with dynamics beyond hot Brownian motion and superheated water states persisting up to 100 μs at high fluences. At short times up to 20 μs we observe that the dynamics do not obey the Stokes–Einstein predictions.

Many soft-matter systems are composed of macromolecules or nanoparticles suspended in water. The characteristic times at intrinsic length scales of a few nanometres fall therefore in the microsecond and sub-microsecond time regimes. With the development of free-electron lasers (FELs) and fourth-generation synchrotron light-sources, time-resolved experiments in such time and length ranges will become routinely accessible in the near future. In the present work we report our findings on prototypical soft-matter systems, composed of charge-stabilized silica nanoparticles dispersed in water, with radii between 12 and 15 nm and volume fractions between 0.005 and 0.2. The sample dynamics were probed by means of X-ray photon correlation spectroscopy, employing the megahertz pulse repetition rate of the European XFEL and the Adaptive Gain Integrating Pixel Detector. We show that it is possible to correctly identify the dynamical properties that determine the diffusion constant, both for stationary samples and for systems driven by XFEL pulses. Remarkably, despite the high photon density the only observable induced effect is the heating of the scattering volume, meaning that all other X-ray induced effects do not influence the structure and the dynamics on the probed timescales. This work also illustrates the potential to control such induced heating and it can be predicted with thermodynamic models.

The gelation of PEGylated gold nanoparticles dispersed in a glycerol-water mixture is probed in-situ by X-ray photon correlation spectroscopy. Following the evolution of structure and dynamics over 104 s, a three-step gelation process is found. First, a simultaneous increase of the Ornstein-Zernike length ξ and slowdown of dynamics is characterized by an anomalous q-dependence of the relaxation times of τ∼q-6 and strongly stretched intermediate scattering functions. After the structure of the gel network has been established, evidenced by a constant ξ, the dynamics show aging during second gelation step accompanied by a change towards ballistic dynamics with τ∼q-1 and compressed correlation functions. In the third step, aging continues after the arrest of particle motion. Our observations suggest that gelation is driven by percolation and stress release as evidenced by anisotropic dynamics once gelation sets in.

X-ray photon correlation spectroscopy (XPCS) is a routine technique to study slow dynamics in complex systems at storage-ring sources. Achieving nanosecond time resolution with the conventional XPCS technique is, however, still an experimentally challenging task requiring fast detectors and sufficient photon flux. Here, the result of a nanosecond XPCS study of fast colloidal dynamics is shown by employing an adaptive gain integrating pixel detector (AGIPD) operated at frame rates of the intrinsic pulse structure of the storage ring. Correlation functions from single-pulse speckle patterns with the shortest correlation time of 192 ns have been calculated. These studies provide an important step towards routine fast XPCS studies at storage rings.

Supercrystals composed of plasmonic nanoparticles constitute a promising material platform for tailored light‐matter interactions. The optimization of their optical properties requires precise syntheses and a detailed structural characterization that addresses not only the basic geometrical parameters but also the degree of order. Herein, plasmonic supercrystals with a well‐defined layered structure are studied by a combined transmission electron microscopy, small‐angle X‐ray scattering and X‐ray cross‐correlation analysis (TEM‐SAXS‐XCCA) approach. It is demonstrated that scanning small‐angle x‐ray scattering (SAXS) data can unambiguously be assigned to the number of crystalline layers by comparison with complementary transmission electron microscopy (TEM) experiments on the same regions of interest. A small but significant increase of the lattice constant with increasing number of layers and a high degree of orientational order irrespective of the number of layers is found. This points to specifics of the supercrystal formation mechanism that could be utilized to improve the control of self‐assembly for supercrystal geometries with subnanometer precision.