Simple model for relating EELS and XAS spectra of metals to changes in cohesive energy

An explicit connection between the unoccupied single-particle states which are measured with electron energy loss or x-ray-absorption spectroscopy (EELS or XAS) and the occupied states which are involved in bonding is established by applying the force theorem to the tight-binding bond model. The resulting sum rule relates the first moment of the EELS spectrum to the single-particle bond energy. The sum rule connects a local EELS measurement with a local change in cohesion, thus treating the energetics of defects and nonperiodic structures on an equal footing with perfect crystals. As an example, the sum rule is used to estimate the cohesion of a grain boundary in Ni₃Al from experimental EELS spectra. It should also prove useful for qualitative interpretations of EELS spectra in other metal alloys.