X-ray photon correlation spectroscopy for the characterization of soft and hard condensed matter

Abstract

Definition of the Topic X-ray photon correlation spectroscopy (XPCS) allows to access a wide variety of dynamic phenomena at the nanoscale by studying the temporal correlations among photons that are scattered by a material when it is illuminated using a coherent X-ray beam. Here, we describe how XPCS is used to study the dynamics of soft and hard condensed matter, review the recent literature and briefly discuss the future of the technique, especially in the context of the emerging diffraction-limited storage rings and X-ray free electron laser sources. Overview When a disordered system is illuminated with coherent light, the interference between the scattered waves gives rise to a speckle pattern. The speckle pattern depends on the exact arrangement of the scatterers. Information about the dynamics of the system can be obtained by analysing the temporal correlations of the speckle intensities. This is the basis of the X-ray photon correlation spectroscopy (XPCS) technique, which is the counterpart of photon correlation spectroscopy (PCS) using coherent X-rays instead of laser light. XPCS is one of the main techniques to probe the slow nanoscale fluctuations and dynamics of soft and hard condensed matter systems. If a coherent X-ray beam from a third-generation synchrotron source is used as the illumination probe, then, depending on the sample and the scattering geometry, the dynamics of materials on timescales ranging from microseconds to thousands of seconds and length scales from microns down to nanometres can be accessed. At present, the main limitations of XPCS are the relatively low coherent flux of existing X-ray sources and the limited speed of X-ray area detectors. The upcoming new X-ray sources (diffraction-limited storage rings and X-ray free electron lasers) will enable to measure dynamics at large momentum transfer values with tenths of picoseconds time resolution. The principles, experimental methodology and recent application of XPCS are reviewed here, followed by a brief discussion about the possibilities that the new X-ray sources will create for future XPCS experiments.