The Auger parameter and plutonium

Experimental methods for elemental and chemical signatures of nuclear materials are a key requirement in the field of nuclear forensic science. X-ray photoelectron spectroscopy (XPS) has received little attention in this area, especially for materials containing transuranium elements. The XPS technique offers a nondestructive elemental quantification and chemical state identification method for both amorphous and crystalline materials. A method of XPS data analysis called the Auger parameter affords a chemical state fingerprint of materials. Furthermore, the Auger parameter can be used to separate initial and final state screening effects and provide surface electronic structure information. Here, the x-ray excited Auger peaks acquired in plutonium XPS are assessed for use in the Auger parameter. The N₆O_4,5O_4,5 Auger transition affords suitable data for the Auger parameter and shows that plutonium metal, alloys, and oxides all have an identical final state contribution. Therefore, the different positions in the chemical state plot and the Pu 4f spectra occur from the initial state effects of effective charge and Madelung potential.

X-ray photoelectron spectroscopy (XPS) of actinide materials affords both quantitative surface chemical analysis and fundamental electronic structure information. A direct application of the former is in nuclear forensics signatures, whereas the latter can give an insight to the role of the 5f electrons in these materials. This is frequently achieved from the interpretation of the actinide 4f core-level peak shape and position. The Auger parameter, the sum of the sharpest Auger peak kinetic energy and the binding energy of the most intense core level, can be used as a chemical state fingerprint and the characterization of the initial and final state contributions. Application of the Auger parameter method will help inform which description should be used for the interpretation of plutonium core-level photoemission data. Here, for the first time, from a meticulous study of the Auger parameter it is demonstrated unequivocally that measurements on plutonium metal, alloys, and oxides yield consistent final state contributions. These findings reveal that the peak shape of the Pu 4f core level primarily stems from the net atomic charge or formal oxidation state of plutonium, with small shifts in binding energy arising from the local ligand environment. This provides a clear framework for evaluating the Pu 4f peak shape and position in all XPS data reported on this element from the past 45 years.