Absorption


Absorption, and why it matters
If crystals were perfect spheres...

If crystals were perfect spheres...

Absorption is the fact that the intensities of the measured reflections are reduced because the diffracted beams are absorbed when travelling through the crystal.

If our crystal was a perfect sphere, then this wouldn’t matter: All diffracted beams will have to travel through the same length through the crystal (assuming that diffraction ‘happens’ at the centre of the crystal).

... but unfortunately, they are not!

... but unfortunately, they are not!

But real crystals aren’t perfect spheres, and the more anisotropic they are, the bigger the problem will be: in certain directions, the diffracted beams will have to travel through a lot more crystal than in other directions.

This is how a typical FOBS plot looks like when there is insufficient absorption correction

This is how a typical FOBS plot looks like when there is insufficient absorption correction

Why does this matter? Remember: the position of the diffracted beams is defined only by the unit cell dimensions (a, b, c, α, β γ). The intensities of the diffracted beams are defined by the contents of the unit cell, i.e. the bit we are interested in: the structure!