Comments on the Theory of Complex XPS Spectra: Extracting Chemical Information from the Fe 3p XPS of Fe Oxides

XPS analyses of open shell ionic compounds, especially oxides of the first row transition metals, for information such as oxidation state tend to focus on characteristics of the metal 2p XPS features alone. These analyses could be made considerably more robust with simultaneous characterization of the XPS of the metal 3p features as well as the 2p features. In these comments, we provide a perspective on the conceptual and theoretical framework needed to extract chemical information from the complex multiplet structure of the 3p XPS of Fe oxides as representative of 3d transition metal oxides. We also present information about a novel kind of many-body effects that may contribute to a further redistribution of the Fe 3p XPS intensity. The concern here is not to develop a complicated mathematical formalism but to explain the complexity in terms of fundamental quantum mechanical concepts. This is done on the basis of ab initio Dirac Hartree-Fock wavefunctions where we examine the physical nature of the 3p XPS features of the representative ferrous and ferric oxides of FeO and Fe2O3, respectively. The key objectives of this paper are as follows: (1) to demonstrate the importance of the angular momentum coupling of open shell electrons, which is done more easily with RS multiplets; (2) to show that a single configuration description of the final state multiplets is woefully inadequate; (3) to identify a novel atomic many-body effect that can lead to a rich satellite structure. The considerations discussed here should have implications for making useful interpretations of the XPS of lower BE levels of other ionic, high spin materials. This paper provides a manifestation of a new tradition by which Comments on Inorganic Chemistry starts publishing original research content that, nonetheless, preserves the Journal's identity as a niche for critical discussion of contemporary literature in inorganic chemistry; for previous manifestations, see Comments Inorg. Chem. 2020, 40, 277-303 and references cited in the abstract thereof.