On 12 January 2012 13:02, Weiergräber, Oliver H. <o.h.weiergrae...@fz-juelich.de> wrote: > I think the problem is related to the term "coherence" being used to describe > both the type of *radiation* and the mode of *scattering*. > When talking about (xray) radiation, it denotes the phase relationship > between photons, and therefore even a monochromatic beam can be incoherent > (whereas a polychromatic one is, of course, always incoherent). In terms of > scattering, however, what matters is the self-coherence between different > "partial waves" scatted from different unit cells. Taking things this way, > the classical crystallographic diffraction experiment with a rotating anode > actually makes use of coherent scattering of an incoherent beam!
I think you're right, one can consider 2 types of coherence: temporal or longitudinal coherence which is measured by the average auto-correlation function of the wave with a copy of itself displaced by some time interval, and spatial or lateral coherence which is measured by the average cross-correlation function of one part of the wave-front with another part at the same instant in time. Spatial coherence is obviously relevant to diffraction because different parts of the wave-front get scattered by different parts of the crystal, so if there's disorder it will lead to spatial decoherence (aka diffuse scattering). In scattering we are considering what happens when an X-ray photon interacts with an electron so then temporal decoherence will only occur if in an inelastic collision the photon loses some of its energy to the electron. So one must take care to distinguish "(in)coherent scattering" from "(in)coherent diffraction". Cheers -- Ian
