Diffraction with electrons, X-rays, and atoms

Surface diffraction methods should fulfill three conditions  

The resolution must lie in the range of some atomic layers, since only over this depth the surface differs from the volume. This criterion together with the angle of incidence sets an upper limit for the wavelength. 

The method must be surface sensitive, i.e. the observation depth must substantially be defined by the first atomic layer. For this reason the penetration depth of the beam should be small. 

The method must be nondestructive, i.e. no irreversible changes in the surface structure should occur. 

Surface diffraction experiments have to be done in UHV. Otherwise the surfaces are covered with a monolayer of adsorbed molecules. At this point the reader might ask why do we not have to use UHV in scanning tunneling or the atomic force microscope In both techniques the tip penetrates through the surface contamination layer. In the scanning tunneling microscope it is often just invisible because contamination layers are usually not good conductors. In 

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