QUANTUM PHYSICS

Electron Diffraction

 diffraction diffraction grating Bragg Diffraction patterns electron, x-ray, neutron diffraction compared

What is Diffraction?

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.

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:

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.

'n' values correspond to:

 n=0     no path difference   n=1     one wavelength path difference   n=2     two wavelengths path difference etc.

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

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

Electron Diffraction Interference Patterns

Electron, X-ray and Neutron Diffraction Compared

The most commonly used method is X-ray diffraction.

Summary:

 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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