K-edge

The K-edge refers to a sharp discontinuity in the absorption spectrum of an element, specifically in the context of X-ray absorption spectroscopy (XAS). It arises when the energy of the incident X-ray photon is sufficient to eject a core electron from the innermost (K) shell of an atom.

When an X-ray photon with energy below the K-edge energy interacts with an atom, it can only be absorbed by exciting valence electrons or through scattering processes. However, when the X-ray energy reaches the K-edge energy, the photon possesses sufficient energy to overcome the binding energy of a K-shell electron. This electron can then be ejected from the atom via the photoelectric effect, creating a core hole in the K-shell.

The ejection of a K-shell electron significantly increases the probability of X-ray absorption at that specific energy. Consequently, the absorption coefficient exhibits a sudden jump, forming the K-edge. The energy of the K-edge is characteristic of a specific element, as the binding energy of the K-shell electrons varies from element to element.

Beyond the K-edge energy, the absorption coefficient gradually decreases as the X-ray energy increases further. This decrease occurs because the probability of photoionization generally decreases with increasing photon energy.

The location of the K-edge can be used to identify the presence of a particular element in a sample. Furthermore, the shape and fine structure of the absorption spectrum near the K-edge (XANES - X-ray Absorption Near Edge Structure) provide information about the oxidation state, local coordination environment, and electronic structure of the absorbing atom. Extended X-ray Absorption Fine Structure (EXAFS) analysis, which examines the oscillations in the absorption spectrum at energies beyond the edge, can be used to determine interatomic distances and coordination numbers around the absorbing atom.

In summary, the K-edge is a fundamental feature in X-ray absorption spectroscopy that allows for the elemental identification and the investigation of the electronic and structural properties of materials. It arises from the core-level photoionization of K-shell electrons when the incident X-ray energy matches or exceeds their binding energy.