Whether the X-ray source and electron source must be coherent for XRD and electron diffraction? For X-rays emitted from X-ray tubes and electrons from electron guns, the photon and electron hardely can be coherent. Their wave should be radom and why interference pattern can be formed and Bragger diffraction condition can be meet? What is the underlying physics. As I understand, coherent x-ray or electron source is not necessary for coherent scattering, not knowing why.
Hi
It depends on the application of X-rays.
If you want to measure the degree of cristallinity of a material, you don't need coherent X-rays as is the case for X-ray Lasers where the X-ray photon are in phase.
In most cases of routine XRD experiments on macroscopic (or even microscopic) samples, you don't need coherence, you need just to have a relative monochromaticity (delta lamda almost zero) of X-rays and with relative high directionnality (focusing of x-ray to avoid scattering).
If so it is sufficient to determine cristalline properties of most crytals or even non highly cristalline materials.
Because XRD is a long range phenomenon randomized as a mean value over several crystal lattice unit cells.
When you need to look to details at a very small scale, e.g. in some (real) nanostructures (quantum dots, nanotubes, graphene, ...) then you have to care about coherence of your photons.
Otherwise you need sometimes extended X-rays fine structures (EXAFS) experiments for investigating local effects as defects even in macroscopic samples,
Best regards
Professor A. Kadri
DR HOU!! In the case of bimetallic nanoparticles, XRD is important to confirm whether the bimetallic ... X-ray diffraction patterns of PVP-protected metal nanoparticles: (a) ... Atoms scatter X-ray waves, primarily via the atoms' electrons [313]. ... It is essential to account for instrumental sources of peak broadening, and this is commonly ...
https://www.sciencedirect.com/topics/physics-and-astronomy/x-ray-diffraction
Electron microscopes, such as the scanning electron microscope (SEM) and ... the same principle as that of the generation of x-rays used in x-ray diffraction ... type of interfaces between different phases (i.e. coherent, semi-coherent and incoherent) ... The material must be less than 200 nm thick in order to pass the electron
http://www.eng.uc.edu/~beaucag/Classes/XRD/Labs/Keithhtml/Keith.html
Regards--
KIn real world we don't have completely coherent nor completely incoherent sources; one talks about degree of coherence. One can see interference frings with Na lamp because it is not completely incoherent. One narrows down the slit so that only light from a small portion is used. Same is true for x-ray and electron diffraction.
Hi
It depends on the application of X-rays.
If you want to measure the degree of cristallinity of a material, you don't need coherent X-rays as is the case for X-ray Lasers where the X-ray photon are in phase.
In most cases of routine XRD experiments on macroscopic (or even microscopic) samples, you don't need coherence, you need just to have a relative monochromaticity (delta lamda almost zero) of X-rays and with relative high directionnality (focusing of x-ray to avoid scattering).
If so it is sufficient to determine cristalline properties of most crytals or even non highly cristalline materials.
Because XRD is a long range phenomenon randomized as a mean value over several crystal lattice unit cells.
When you need to look to details at a very small scale, e.g. in some (real) nanostructures (quantum dots, nanotubes, graphene, ...) then you have to care about coherence of your photons.
Otherwise you need sometimes extended X-rays fine structures (EXAFS) experiments for investigating local effects as defects even in macroscopic samples,
Best regards
Professor A. Kadri
Dear Huaming Hou ,
in addition to what has been already said above,
you know from visible optics that diffraction occurs for samples having structures in the order of the incoming wave length, although no special measures have been taken to achieve coherence. They are per se locally coherent, even they arise from a complete incoherent source.
That's is the same for x-rays and electrons.
Atomic distances and lattice constants are at the same order of the wave lengths of the x-rays and electrons (via De Broglie wave length).
However for extending the spatial coherence to regimes much larger than the wave length you need slits or a set of slits*) for achieving sufficient degree of coherence as already partly mentioned above.
*) in the case of grating based x-ray differential contrast systems these slit systems are commonly named the G0-grating.
the answer is tricky for this question. why because - to occur diffraction, the wavelength of incident ray should approximately equal to the 'd' spacing (planar distance) in the sample.
Dear Artur Braun ,
to my opinion the term 'to be in phase' (let me add: in phase with themselves) for me is just coherence.
X-rays and electron waves are spatially coherent in their own wavelength regime.
Thus XRD and LEED, both of these techniques are working in the x-ray and electron wavelength regime (~ atomic distances,) are working without further measures for setting up or increase coherence.
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