The Auger Parameter and Plutonium

The Auger parameter, α', the sum of the binding energy of the most prominent core level transition and the kinetic energy of the Auger transition, as measured in a single X-ray Photoelectron Spectrum, can afford chemical state / chemical fingerprinting, non / local screening and initial / final state information. The use of the Auger parameter and the related chemical state plot offer advantages over chemical state / environment determination from the shift in binding energy of a single core hole transition as both charge referencing / binding energy scale calibration and work function corrections are not required, such that data from different laboratories can be easily compared. In the area plutonium XPS there are three separate reports on the Auger parameter and the associated chemical state plots [1-3]. The first two reports [1, 2] have both utilised the Pu P₁VV X-ray induced Auger transition combined with the Pu 4f_7/2 peak and are very similar demonstrating reproducibility as the data were acquire at different laboratories using different samples. However, the most recent report uses the Pu N₇O₅V Auger peak with the Pu 4f_7/2 core level, and the results are distinctly different [3]. The first two reports show that the Auger parameter for PuO₂ is approximately 2 eV higher than plutonium metal which is extremely unusual, indicative of enhanced screening / extra atomic relaxation behaviour in the oxide compared to the metal. In comparison, the most recent report [3] indicate that plutonium dioxide has a remarkably similar Auger parameter to plutonium metal, albeit this is based on an extremely limited data set. The object of this presentation is to investigate this discrepancy and if possible, to explain the differences by evaluating the key assumption used in the Auger parameter of equal binding energy shifts for all core levels involved in the definition of the Auger parameter. To enhance understanding the of the Auger parameter involving the Pu N₇O₅V transition the data set used has been expanded which has resulted in a greater understanding to the factors involved in measuring the Auger parameter on plutonium materials.<br/><br/><br/>[1] Detection of plutonium hydride using X-ray photoelectron spectroscopy, D. T. Larson, K. M. Motyl, J. Electron Spectrosc. Relat. Phenom., 50, 67 (1990).<br/>[2] X-ray excited Auger transitions of Pu compounds, A. J. Nelson, W. K. Grant, J. A. Stanford, W. J. Siekhaus, P. G. Allen, W. McLean, J. Vac. Sci. Technol. A 33, 031401 (2015).<br/>[3] XPS characterization of a PuGa-7 at. % alloy, P. Roussel, S. C. Hernandez, J. J. Joyce, K. S. Graham, T. Venhaus, J. Vac. Sci. Technol. A 41, 023204 (2023).<br/><br/>UK Ministry of Defence © Crown Owned Copyright 2024/AWE