Ligand K-edge

Ligand K-edge spectroscopy is a spectroscopic technique used to study the electronic structures of metal-ligand complexes. This method measures X-ray absorption caused by the excitation of ligand 1s electrons to unfilled p orbitals (principal quantum number n <= 4) and continuum states, which creates a characteristic absorption feature called the K-edge.

Transitions at energies lower than the edge can occur, provided they lead to orbitals with some ligand p character; these features are called pre-edges. Pre-edge intensities (D₀) are related to the amount of ligand (L) character in the unfilled orbital:

${\displaystyle D_{0}(L\ 1s\rightarrow \psi ^{*})=const\ \vert \langle L\ 1s\vert \mathbf {r} \vert \psi ^{*}\rangle \vert ^{2}=\alpha ^{2}\ const\ \vert \langle L\ 1s\vert \mathbf {r} \vert L\ np\rangle \vert ^{2}}$

...
Wikipedia