Maximum coverage problem

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Ligand K-edge XAS is a spectroscopic technique that allows the direct study of metal-ligand bonding.[1] In a XAS experiment, electrons in ligand 1s orbitals are excited to unfilled p (principal quantum number n <= 4) and continuum states. This results in a large spectral feature known as a rising edge. Transitions at energies lower than the edge can occur, provided they are to orbitals with some ligand p character, and are called pre-edges. Such transitions are often observed in the data of complexes with metal-ligand bonding.

Pre-edge intensities (D0) are related to the amount of ligand (L) character in the unfilled orbital:

D0(L1sψ*)=const|L1s|r|ψ*|2=α2const|L1s|r|Lnp|2

where ψ* is the wavefunction of the unfilled orbital, r is the transition dipole operator, and α2 is the "covalency" or ligand character in the orbital. Since ψ*=1α2|Mdα|Lnp, the above expression relating intensity and quantum transition operators can be simplified to use experimental values:

D0=α2h3nIs

where n is the number of absorbing ligand atoms, h is the number of holes, and Is is the transition dipole integral which can be determined experimentally. Therefore, by measuring the intensity of pre-edges, it is possible to experimentally the determine the amount of ligand character in a molecular orbital.

References