![]() Typically, one distinguishes between the transverse coherence, that depends on the source size, and the longitudinal or temporal coherence, that reflect the monochromaticity of the beam. The speckle size is connected to the coherence volume of the X-rays. XPCS ConceptsĪn important quantity in experiments is the speckle size that can be obtained from a spatial autocorrelation of intensity. Finally, future opportunities will be addressed with special attention on possibilities at the new facilities. The possibility of reaching faster time scales in a real time experiment enables the study of in-operando dynamics, e.g., ion transport in lithium batteries, and paves the way to sub- μs dynamics of soft matter and biological systems. Furthermore, new routes of understanding stress relaxation in glasses have been revealed recently. Recent highlights that will be addressed in this review include the observation of two types of liquid water as well as probing real-time dynamics in liquid water on femto- to picosecond time scales. The main part of this review discusses dynamics studies from the last 10 years, focussing on the new possibilities thanks to accessing faster timescales at storage rings and the performance of XPCS at FEL sources. Afterwards, setup parameters for XPCS are discussed. In this review we will first introduce the XPCS concepts including higher-order correlations, XSVS and the role of coherence. Besides the FEL application, XPCS will play a major role at the currently designed and already built next generation of storage ring sources, as they promise to extend the time scale of XPCS towards nanoseconds. Three main directions can be identified, that experienced a strong boost in the last 10 years: (1) extension of XPCS to milli- and microseconds thanks to new type of detectors and increased brilliance, (2) studies of molecular and atomic length scales, and (3) XPCS and XSVS studies at FEL sources. While focussing first on slow dynamics on length scales of several 10 nm (i.e., using a small-angle scattering geometry), the application of XPCS has extended recently on both time and length scales. In addition, the starting dates of new beamlines and the ID10 (ESRF) upgrade are marked as well. Since the late 1990s, the number of XPCS publications grew linearly, resulting to more than 40 publications in 2020. Their appearance has been limited to titles, keywords, and abstract, so that the actual number of XPCS and XSVS publications is likely slightly larger. Therefore, a search on SCOPUS and Web of Science has been performed on “X-ray Photon Correlation Spectroscopy”, its alternative name “X-ray Intensity Fluctuation Spectroscopy” (XIFS), and the related technique “X-ray Speckle Visibility Spectroscopy” (XSVS). ![]() The development of XPCS is highlighted in Figure 2 showing the number of XPCS publications per year as well as the associated subject areas. Finally, XPCS is also a driving force for FEL science, such as at the X-ray Correlation Spectroscopy (XCS) instrument at LCLS or the Materials Imaging and Dynamics (MID) instrument at the European XFEL. A second generation of XPCS beamlines was built at PETRA III and NSLS II during the last decade, that provided an increase of coherent flux and more flexibility on scattering geometries. In the following, dedicated beamlines for XPCS have been established at the European Synchrotron Radiation Facility (ESRF) in France and at the Advanced Photon Source (APS) at Argonne National Laboratory (USA). In this article, we review the major achievements using XPCS during the last decade.Īfter the first measurements of X-ray speckles XPCS has been demonstrated in 1994, see Figure 1b. For instance, XPCS particularly profits from the development of two-dimensional detectors, allowing for real-time experiments up to sub-ms time resolution. Beyond the new facilities, many upgrades in beamline and detector technology have improved the spatial and temporal resolution of these techniques. XPCS benefits in a larger degree than any other technique from the increasing brilliance. The rise of hard X-ray free-electron lasers (FEL) and the step to the next-generation storage rings based on the multibend achromat lattice will significantly expand the applications of these techniques. ![]() Thanks to the marked increase of brilliance the coherent flux has risen many orders of magnitude compared to previous sources, paving the way for new coherence-based techniques, such as coherent diffractive imaging (CDI), X-ray ptychography and X-ray photon correlation spectroscopy (XPCS). This is demonstrated in Figure 1a showing Airy fringes from Fraunhofer diffraction measured at the coherence beamline ID10 of ESRF. With the advent of third generation synchrotron radiation sources in the early 1990s, the application of coherent X-ray beams has become possible for the first time.
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