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Electron Diffraction



diffraction grating

Bragg Diffraction


electron, x-ray, neutron diffraction compared




What is Diffraction?



diffraction through a single slit        plane waves diffracted at an edge



Classical physics gives us a definition of diffraction in terms of waves:


Diffraction is the phenomenon whereby light or sound waves bend around small


obstacles or spread out through small openings.



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The Diffraction Grating


A diffraction grating is an array of identical, equidistant, parallel lines on a surface.


Gratings are used to produce optical spectra from a single source, parallel beam of light.

There are two types of grating: reflection & transmission



The Transmission Grating:



transmission diffraction grating



Light rays are diffracted at the edge of each gap in the grating.


The waves of light add when the path difference is a whole number of wavelengths.


So discrete images at different angles are produced.


diffraction grating equation


'n' values correspond to:


n=0     no path difference


n=1     one wavelength path difference


n=2     two wavelengths path difference etc.




order of spectra from a diffraction grating



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Bragg Diffraction (an example of reflection diffraction)


Bragg diffraction occurs when particles or waves with wavelength 'λ' comparable with atomic spacings 'd' , interact with atoms in crystals.


At a particular angle θ* waves are diffracted by atoms in adjacent planes and interfere constructively.


As with transmission diffraction gratings, a bright image is only produced when the path difference is a whole number of wavelengths.


* the angle the incident wave makes with a plane of atoms



Bragg diffraction diagram



the Bragg equation


Practically, the incident waves can be provided by X-rays, electrons or neutrons.



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Electron Diffraction Interference Patterns



electron diffraction photos



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Electron, X-ray and Neutron Diffraction Compared


The most commonly used method is X-ray diffraction.




  electron x-ray neutron
scattering by electrostatic repulsion of nucleus by electron cloud around nucleus by interaction with the nucleus
resolution moderate moderate high
penetrating power poor (requires thin specimens) good good
matter interaction high (unreliable results) none moderate
magnetic effects no no yes (neutrons have their own magnetic field)
good for light elements no no yes
particular uses crystals crystals fuel rods, archaeological artefacts




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