What does the K edge (pre-edge) refer to in spectroscopy?

The K edge, often observed in X-ray absorption spectroscopy, refers to the energy at which core electrons, specifically those from the K shell (the closest shell to the nucleus), are ionized. This energy corresponds to the threshold energy required to eject a core electron and is significant for determining the electronic structure of atoms.

In this context, the K edge represents the energy required for the transition of these core electrons to unoccupied higher energy states (i.e., empty orbitals) rather than being associated with the lowest energy of electronic transitions. The process reveals insights about atomic interactions and the electronic environment surrounding the atom in various chemical compounds.

Because of this, the description of the K edge as indicating the promotion of core electrons to empty orbitals accurately captures the phenomenon; it highlights both the ionization process and the resulting electronic transition, emphasizing the significance of the pre-edge features in spectroscopic analyses.