A Wagner plot (also called a chemical state plot) is a way of analyzing XPS and Auger electron spectroscopy (AES) data together. It was introduced in 1975 by Charles D. Wagner, one of the pioneers of XPS, as a tool to better identify and distinguish chemical states of elements in complex materials.(1)

In a Wagner plot, the kinetic energy of an Auger transition is plotted against the binding energy of a core-level photoelectron from the same element. Because both of these quantities shift when the chemical environment of the atom changes, the plot provides a two-dimensional fingerprint of the chemical state.

The advantage of this approach is that the combined shifts can separate contributions from:

• Initial-state effects (changes in the atom’s electronic structure, such as oxidation state or bonding environment), and

• Final-state effects (relaxation and screening after electron emission).

This is particularly useful because in conventional XPS, different oxidation states may produce overlapping shifts that are hard to interpret, while in AES, chemical shifts can also be subtle. But when binding energy and Auger kinetic energy are plotted together, compounds of the same element tend to fall along straight “chemical state lines” with slopes close to 1. This makes it much easier to distinguish, for example, Cu⁰, Cu⁺, and Cu²⁺, or different oxides of a transition metal.

Wagner originally developed this method to help in the surface chemical analysis of solids, especially for catalysts, thin films, and corrosion products, where precise determination of oxidation state is crucial. Today, Wagner plots are still used in surface science and materials characterization as a complementary diagnostic tool alongside standard XPS peak fitting.

To construct a Wagner plot, make an X,Y of your data with binding energy along the X-axis, and kinetic energy of the relevant Auger-Meitner line on the Y-axis. If you draw diagonal lines with a gradient of 1 on your plot, you will find that the Auger parameter sits on the right hand vertical axis, as a sum of the X and Y.

Where data points sit on the Wagner plot can reveal information about relative differences between samples. Since this plot is a visual representation of the Auger parameter, it can be a great way of observing initial and final state contributions across a series of data. The general position is a great fingerprint for chemical state or bonding environment.

• Gradient ≈ 1 (Auger parameter line):

  • If points lie along a slope of +1, it means final-state effects dominate.

  • This is because relaxation shifts B.E.​ up and K.E. down by about the same magnitude → their sum (α′) stays nearly constant.

  • So movement along slope = 1 indicates differences mostly due to screening efficiency of the chemical environment.

• Gradient ≈ 3 (chemical state line):

  • If points lie along a slope of +3, it means initial-state effects dominate.

  • Here, both B.E. and K.E.​ shift together because the overall electronic structure changes (like oxidation state).

  • So movement along slope = 3 indicates differences due to true chemical shifts (oxidation, bonding, electronegativity).(2)

Wagner plots are fantastic for looking at series of data, and trying to pick out trends, patterns or differences across a large number of samples (or in a changing sample, for example in an operando style measurement). In general, Wagner plots are an excellent tool for:

1. Distinguishing oxidation states with overlapping binding energies #

• Problem: Cu⁰, Cu⁺, and Cu²⁺ all have Cu 2p₃/₂ binding energies within ~1 eV, making them hard to separate by XPS alone.

• Solution: In a Wagner plot, their Auger kinetic energies shift in opposite correlation, giving three distinct positions. → You can identify which oxidation state you actually have.

2. Separating initial vs. final-state effects #

• Problem: A peak shifts by ~1 eV compared to a reference. Is it due to a real chemical shift (different oxidation/bonding), or just extra screening in a metallic/covalent environment?

• Solution: Plot BE vs. KE. If the shift is along slope ≈ 3 → initial state (oxidation, ionicity). If along slope ≈ 1 → final state (covalency, screening).

3. Probing covalency vs. ionicity in bonding #

• Problem: In a mixed oxide or halide, you want to know whether the bond is more ionic or covalent.

• Solution: Compare where the compound falls relative to known references.

  • More ionic → higher BE, lower KE (slope ~3).

  • More covalent → smaller BE increase, stronger relaxation (slope ~1).