Extracting Chemical Information from XPS Spectra: A Perspective

Abstract

Important mechanisms that lead to features, often complex, in X-ray photoelectron spectroscopy (XPS) spectra are defined and described. It is shown that there is much information in an XPS spectrum that can be obtained by examining these features rather than examining only the shifts of main peaks between different materials. These mechanisms are presented with a focus on describing the underlying chemical and physical phenomena responsible for features of the XPS and on showing how these XPS features can be related to the properties and electronic structure of the material studied. While it is necessary to consider certain quantum mechanical rules, the mathematical formalism is not discussed. However, a general awareness of multiplet splittings, which are a result of angular momentum coupling combined with ligand field and spin–orbit splittings, and of covalent mixings in the metal–ligand bond of oxides is essential to properly interpret the significance of XPS features. A conceptual framework of shake excitation from bonding to anti-bonding orbitals is introduced to provide an understanding of the significance of XPS satellites. While the coupling of theory and measurement is required to extract quantitative information from XPS, it may be possible to obtain useful qualitative information directly from features of the XPS spectra provided that one takes into account more than only shifts of the XPS binding energies.

Graphical Abstract

A correct analysis of XPS features may require a careful treatment of many-body effects that distribute intensity over many individual, unresolved final states.

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Notes

  1. 1.

    CLIPS is a program system to compute ab initio SCF and correlated wavefunctions for polyatomic systems. It has been developed based on the publicly available programs in the ALCHEMY package from the IBM San Jose Research Laboratory by P. S. Bagus, B. Liu, A. D. McLean, and M.Yoshimine.

  2. 2.

    DIRAC, a relativistic ab initio electronic structure program, Release DIRAC08 (2008), written by L. Visscher, H. J. Aa. Jensen, and T. Saue, with new contributions from R. Bast, S. Dubillard, K. G. Dyall, U. Ekström, E. Eliav, T. Fleig, A. S. P. Gomes, T. U. Helgaker, J. Henriksson, M. Iliaš, Ch. R. Jacob, S. Knecht, P. Norman, J. Olsen, M. Pernpointner, K. Ruud, P. Sałek, and J. Sikkema (see the URL at http://dirac.chem.sdu.dk).

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Acknowledgements

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences (CSGB) Division through the Geosciences program at Pacific Northwest National Laboratory.

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Correspondence to Paul S. Bagus.

Appendix: List of Acronyms and Abbreviations

Appendix: List of Acronyms and Abbreviations

  1. 1.

    BE binding energy. The binding energy of an electron associated with a photoelectron peak.

  2. 2.

    CI configuration interaction wavefunctions. This is a wavefunction that mixes determinants for different orbital occupations. In the limit of a large expansion, it provides exact properties and energies.

  3. 3.

    CT charge transfer. In the present context it is used to refer to promotion of an electron from a dominantly ligand orbital into a dominantly metal, or cation, orbital.

  4. 4.

    ΔSCF delta self-consistent field. Usually used to contrast a BE obtained by taking the difference of two variational calculations from those obtained using an initial state, or Koopmans’ Theorem approximation.

  5. 5.

    DHF Dirac Hartree-Fock, see HF below, for relativistic Dirac Hartree-Fock wavefunctions and properties.

  6. 6.

    ECA equivalent core approximation. Developed by Jolly to model the valence orbital relation by replacing a core ionized atom by the cation with the nuclear charge increased by 1.

  7. 7.

    FO frozen orbital. Normally FO describes a wavefunction where the orbitals are fixed or “frozen” as they were for a different set of orbital occupations. Especially for XPS, the orbitals are not permitted to relax or screen a core–hole created on an atom.

  8. 8.

    FWHM full width at half-maximum.

  9. 9.

    HF Hartree-Fock. This describes the wavefunctions and other properties obtained from solution of the Hartree-Fock variational equations. This is normally used to describe non-relativistivic wavefunctions as contrasted with DHF, see above.

  10. 10.

    MAD maximum average deviation

  11. 11.

    MAE mean absolute error

  12. 12.

    SA sudden approximation. An approximation for the relative intensity of XPS peaks. The approximation is exact in the limit of high photon energy.

  13. 13.

    SCF self consistent field. SCF can describe a wavefunction obtained from an SCF numerical method of the methodology itself.

  14. 14.

    SCLS surface core level shifts. The difference of the core-level BE of an atom at the surface of a crystal from that of an atom in the bulk. The sign is generally taken so that SCLS > 0 indicates that the BE of an atom at the surface is larger than the BE of an atom in the bulk of a crystal.

  15. 15.

    WF wavefunction

  16. 16.

    XPS X-ray photoelectron spectroscopy. We use this acronym to describe the physical process as well as the spectroscopy rather than creating another acronym as is sometimes done, using XP to distinguish the process from the spectroscopy. Thus “XPS spectra” is used in the text to describe spectra.

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Bagus, P.S., Ilton, E. & Nelin, C.J. Extracting Chemical Information from XPS Spectra: A Perspective. Catal Lett 148, 1785–1802 (2018). https://doi.org/10.1007/s10562-018-2417-1

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Keywords

  • XPS
  • Electronic structure
  • Multiplets
  • Ligand field splittings