Why does fluorine exhibit an 8 eV K-edge onset energy shift without an oxidation state change?

The fluorine K-edge X-ray absorption spectra (XAS) of various inorganic fluorides were investigated. Despite fluorine maintaining a constant oxidation state of -1, the onset energy for the absorption edge varies by approximately 8 eV across inorganic fluorides. This variation correlates with the electron configuration of the counter cation, allowing classification into three groups: (1) those with a valence electron configuration of n s 0 n p 0 , (2) transition-metal fluorides, the valence electron configuration of which is 3 d m , and (3) those with a valence electron configuration of n s 2 n p 0 ( n − 1 ) d 10 . The Group 1 fluorides exhibit a predominantly ionic character, with the fluorine 2 p orbitals being largely occupied. In these compounds, the K-edge energy is primarily governed by the electrostatic potential at the fluorine site, which scales inversely with the nearest-neighbor distance. A shift of 5.4 eV is observed between the K-edge energies for AlF 3 and CsF, corresponding to bond lengths of 1.80 Å and 3.01 Å, respectively. By contrast, the fluorides of the Group 2 and Group 3 metals exhibit pronounced covalent interactions, resulting in partial delocalization of the fluorine 2 p orbitals and a corresponding downward shift of approximately 6 eV in the K-edge energy. These findings demonstrate that the X-ray absorption edge energy depends not only on the oxidation state of the absorbing atom but also on the local electrostatic environment and the covalency within the coordination structure.