XPCS principle
The method relies on the fact that a particular arrangement of atoms (or scatters) in a sample produces a characteristic "speckle" pattern when it scatters a coherent beam of X-rays. If the arrangement of atoms changes, the speckle pattern changes, and by studying these changes as a function of time, one can obtain information at the atomic dynamics at various wavevector transfer (i.e. at different length scales).
For example, let's consider the 2D Brownian motion of a bunch of dilute spherical particles. The simulated scattering pattern is calculated through the 2D fast Fourier transform of the instantaneous positions and shapes of the particles.
The intensity of a region of interest (ROI) in the reciprocal space fluctuates as the absolute time (frame number for this example) elapses.
If an intensity-intensity auto-correlation is performed as a function of time delay (frame delay here), the characteristic of the dynamics describing the diffusion coefficient of the Brownian motion can be revealed.
As an extension of DLS, XPCS has an ability to study the dynamics on a much smaller length scale than can be achieved by traditional DLS and on a slower time scale than the neutron spin-echo (NSE) technique can usually reach. It allows a study of samples that are opaque to visible light, and the broadening problem of the wave vector due to multiple scattering in DLS can be solved. The following figure shows the frequency and wave vector ranges accessible to XPCS compared with other frequently used techniques for the study of dynamics.
No comments:
Post a Comment