X-ray photon correlation spectroscopy

X-ray photon correlation spectroscopy (XPCS) in physics and chemistry, is a novel technique that exploits a coherent X-ray synchrotron beam to measure the dynamics of a sample. By recording how a coherent speckle pattern fluctuates in time, one can measure a time correlation function, and thus measure the timescale processes of interest (diffusion, relaxation, reorganization, etc.). XPCS is used to study the slow dynamics of various equilibrium and non-equilibrium processes occurring in condensed matter systems.

Advantages
XPCS experiments have the advantage of providing information of dynamical properties of materials (e.g. vitreous materials), while other experimental techniques can only provide information about the static structure of the material. This technique is based on the generation of a speckle pattern by the scattered coherent light originating from a material where some spatial inhomogeneities are present. A speckle pattern is a diffraction limited structure factor, and is typically observed when laser light is reflected from a rough surface, or from dust particles performing Brownian motion in air. The observation of speckle patterns with hard X-rays has just been demonstrated in the last few years. This observation is only possible now because of the development of new synchrotron radiation X-ray sources that can provide sufficient coherent flux.

aXPCS
A specific subgroup of these techniques is atomic-scale X-ray photon correlation spectroscopy (aXPCS).