Electron momentum spectroscopy, scanning tunneling microscopy, and photoelectron spectroscopy provide unique information about electronic structure, but their interpretation has been controversial. This essay discusses a framework for interpretation. Although this interpretation is not new, we believe it is important to present this framework in light of recent publications. The key point is that these experiments provide information about how the electron distribution changes upon ionization, not how electrons behave in the pre-ionized state. Therefore, these experiments do not lead to a “selection of the correct orbitals” in chemistry and do not overturn the well-known conclusion that both delocalized molecular orbitals and localized molecular orbitals are useful for interpreting chemical structure and dynamics. The two types of orbitals can produce identical total molecular electron densities and therefore molecular properties. Different types of orbitals are useful for different purposes.
Bibliographical noteFunding Information:
Helpful discussions with J. Vince Ortiz and W. H. Eugen Schwarz are gratefully acknowledged. S.S. acknowledges support of the Israel Science Foundation (ISF grant 520/18). D.G.T. acknowledges the support of the U.S. Department of Energy (award number DE-SC0015997). M.S.G. acknowledges the support of software infrastructure (SI2) grants from the US National Science Foundation (OCI-1047772 and ACI-1450217).
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- Dyson orbital
- Koopmans’ theorem
- electron momentum spectroscopy
- localized orbitals
- photoelectron spectroscopy