Absorption edge
As absorption edge an abrupt transition from low to generally stronger absorption referred to the one at a specific location electromagnetic spectrum occurs.
Optics and Atomic Physics
In optics and atomic physics , the absorption edge describes the frequency or photon energy (with Planck's quantum h ) at which a sudden change in the absorption cross-section occurs. This is the case if the energy corresponds exactly to an excitation or ionization energy (see photoelectric effect ).
X-rays
Absorption edges also occur with X-rays . The characteristic X-ray emission occurs after an electron of the lowest energy level has been raised to a higher level. It is not a matter of resonance emission, but the electron that emits the X-ray photon comes from the second (the L-shell ) or a higher shell.
An atom can therefore only absorb X-rays when the frequency of the X-rays is at least so high that an X-ray photon has the energy required for excitation. The energy given off by K-radiation is not sufficient because the L-shell is normally fully occupied with electrons ( closed shell ) and cannot absorb any more (see Pauli principle ).
The X-ray spectroscopy can therefore absorption edges for the analysis of fine structures using ( X-ray absorption spectroscopy ). Thus, there EXAFS - or SEXAFS spectroscopy information about the bond distances in a sample or on the surface thereof. In the NEXAFS method, unoccupied electron states are examined from the X-ray absorption directly at the edge.
Semiconductors and insulators
In semiconductors or insulators , the appearance of the absorption edge is the result of an energy gap ( band gap ) in the excitation spectrum of the electrons. The width of this band gap can be determined from the position of the absorption edge in the absorption spectrum .