Marco Cammarata

The recent availability of X-ray pixel-array detectors with nanosecond time resolution now allows X-ray photon correlation spectroscopy (XPCS) studies of nanosecond dynamics in materials. However, a storage-ring source of synchrotron radiation has temporal structure in the nanosecond domain which can contribute strongly to observed time correlation functions, especially for non-uniform fill patterns. When detectors can resolve them, such source effects extend out to tens of microseconds in typical XPCS analysis. Here it is shown how to characterize and remove the contribution from the source. Because the sample dynamics are uncorrelated with the source, the source time correlation function contributes simply as a multiplicative factor in the observed correlation function. Our results demonstrate that it is possible to measure the effect of source time structure from XPCS datasets and to remove it accurately, to reveal the dynamics of the sample into the nanosecond regime. Since non-uniform fill patterns are often used at storage-ring X-ray sources, this greatly extends the availability of beam time suitable for high-speed XPCS.

Advanced XPCS reveals microsecond diffusion of PEGylated gold nanoparticles in water, quantitatively matching Stokes-Einstein predictions, and accessing intrinsic Brownian timescales even in supercooled water.

We present a multi-tau two-time correlation (MT-2TC) analysis for X-ray photon correlation spectroscopy that enables efficient analysis of large-scale, high-frame-rate datasets collected at modern synchrotron beamlines. The method combines the advantages of the multi-tau autocorrelation and two-time correlation approaches, maintaining sensitivity to non-stationary dynamics while reducing computational complexity and memory requirements from quadratic to linear scaling with the number of frames. This formulation not only improves processing efficiency but also extends the accessible correlation time range, allowing analysis over much broader time scales than conventional techniques. The algorithm is implemented in both dense and sparse data representations, allowing direct processing of sparsified photon-counting data from ultra-fast detectors. We demonstrate the performance of the MT-2TC scheme using experiments on a bulk GeO ₂ glass and on a v-Ta ₂ O ₅ thin film conducted at the ESRF ID10 beamline with a 1 kHz Eiger 4M detector. The results show perfect agreement with standard correlation analyses while extending the temporal correlation range by nearly two decades and improving signal-to-noise ratios at long lag times. The MT-2TC approach thus provides a scalable and flexible tool for XPCS at fourth-generation synchrotrons, where enhanced coherent flux and detector speed demand correlation strategies capable of spanning microsecond-to-day timescales.

Ultrafast control of functional materials by light is key for developing optoelectronic devices. Still, the mechanisms by which microscopic photoinduced excitations evolve into a macroscopic phase transformation remain poorly understood, partly due to the experimental challenges of isolating multiscale electronic and structural dynamics. Here we use femtosecond X-ray techniques to track the non-equilibrium dynamics leading to macroscopic transformation in a ferroelastic charge-transfer Prussian blue analogue. The experimental data evidence a sequence of phenomena. The initial electronic excitation leads to reverse Jahn-Teller distortion within 50 fs, causing intermetallic charge-transfer polarons within 200 fs. Polarons generate considerable lattice strain and trigger phase nucleation within 60 ps. This elastically driven cooperativity, launched by the photoinduced polarons, offers an efficient route to the generation and stabilization of photoinduced phases in many volume-changing materials.

In this contribution, we present a compact differential pumping unit with apertures ≥500 µm. It allows windowless operation for in-air sample environments as well as to connect low-quality in-vacuum sample environments to the beamline UHV (≤10 ⁻⁸ mbar) section. The unit also protects the UHV section of the beamline from accidental venting due to operator errors. To simplify the design, an adjustment-free series of pinholes is used. The positioning of the apertures relies on tight machining tolerances. The assembly consists of just eight parts: one main aluminium body, four threaded cylinders with apertures and three covers with links to pumping units assembled with Viton seals. The overall footprint is restricted to 368 mm on the beam axis.

The fastest pixel array X-ray detectors can record images with nanosecond resolution. This is accomplished by storing only a few images in in-pixel memory cells. In this study, we demonstrate nanosecond resolution over a large number of images by operating a prototype detector in an event driven mode. The performance of this mode is tested by measuring the Brownian dynamics of colloidal nanoparticles. We can achieve sub-100 ns time resolution and overcome the pixel dead time by applying a cross-correlation analysis of the neighboring pixels. The approach used in this work can be extended to study time-resolved fast processes with diffraction, scattering or imaging techniques.

Understanding photoinduced phase transitions is key for developing optoelectronic devices based on the ultrafast control of multifunctional materials. Still, how macroscopic transformations emerge from local excitations remains poorly understood, as it is experimentally challenging to isolate the multiscale electronic and structural dynamics. Here, we use femtosecond X-ray techniques to track the non-equilibrium dynamics leading to macroscopic transformation in a Prussian Blue analogue. The experimental data evidence a sequence of phenomena with different equilibration time scales. The initial electronic excitation leads to reverse Jahn-Teller distortion within 50 femtoseconds, which generates intermetallic charge-transfer polarons within 200 femtoseconds. Our study reveals how these photoinduced polarons generate significant lattice strain and trigger the phase nucleation within 60 picoseconds, where the lattice expansion leads to a self-amplifying macroscopic phase transition.

Many rubidium manganese hexacyanoferrate materials, with the general formula Rb_xMn[Fe(CN)₆]_(x+2)/3·zH₂O, exhibit diverse charge-transfer-based functionalities due to the bistability between a high temperature Mn^II(S = 5/2)Fe^III(S = 1/2) cubic phase and a low-temperature Mn^III(S = 2)Fe^II(S = 0) tetragonal phase. The collective Jahn–Teller distortion on the Mn sites is responsible for the cubic-to-tetragonal ferroelastic phase transition, which is associated with the appearance of ferroelastic domains. In this study, we use X-ray diffraction to reveal the coexistence of 3 types of ferroelastic tetragonal domains and estimate the spatial extension of the strain around the domain walls, which represents about 30% of the volume of the crystal.

The Rb_0.94Mn_0.94Co_0.06[Fe(CN)₆]_0.98 material was designed to exhibit a 75 K wide thermal hysteresis around room temperature, associated with a bistability between the low temperature Mn^IIIFe^II tetragonal phase and the high temperature Mn^IIFe^III cubic phase. We have recently shown that a single laser pulse can be used to drive, at room temperature, the photoinduced phase transition from the Mn^IIIFe^II tetragonal phase to the Mn^IIFe^III cubic phase. By using a specific powder sample streaming technique, we could monitor by time‐resolved X‐ray diffraction how the ultrafast out‐of‐equilibrium dynamics, imposed by the structural relaxation around the local photoinduced charge transfer, changes with the fluence of the laser pulse. In this paper we present a detailed structural analysis of the crystalline reorganizations associated with the charge‐transfer in different regimes of photoexcitation. In the low excitation polaronic regime, the photoinduced charge transfer is local and accompanied by an anisotropic distortion of the Mn coordination sphere and of the tetragonal lattice. In the high excitation regime, a macroscopic and cooperative charge‐transfer phase transition occurs towards the cubic lattice, where the coordination around the Mn reaches octahedral symmetry.