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The scattering vector is perpendicular to the planes of interest.
This is the standard symmetric reflection geometry
Now the planes, and hence the scattering vector are offset
.
To measure the diffraction peak the sample is rotated by the offset. Now ω is not equal to θ. This is called an asymmetric reflection
How do we collect reflections from these planes?
This transmission mode is only possible when the X-rays can
penetrate the sample, i.e. a low enough absorption coefficient. Typically
this is possible in polymers.