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X-ray absorption and X-ray circular dichroism

enlarged view

X-ray absorption at the 2p level of a 3d transition metal atom (schematic)

X-ray absorption (XAS) spectroscopy
is widely used in the investigation of the electronic structure in physics, chemistry, and materials science. Because XAS involves the promotion of a core electron to valence states, it is local (because the core hole is localized) and element specific (because core electrons have a characteristic binding energy) probe. The core excited final state which is created in XAS can decay via Auger electron emission or x-ray fluorescence. Therefore, electron, ion, or fluorescence yield is often used to monitor x-ray absorption instead of X-ray transmission.

X-ray magnetic circular dichroism (XMCD) spectroscopy is a variant of XAS that makes use of circularly polarized x-ray radiation to explore spin and orbital magnetization via XMCD sum rules: