Using Auger transitions as a route to determine the oxidation state of copper in high-pressure electron spectroscopy

Abstract

Accurate discrimination between metallic copper (Cu⁰) and cuprous oxide (Cu₂O, Cu⁺) in electron spectroscopy commonly relies on the Auger electron spectroscopy (AES) Cu L₃M_4,5M_4,5 transitions, as the X-ray photoelectron spectroscopy (XPS) Cu core-levels do not provide large enough binding energy shifts. The kinetic energy of the AES Cu L₃M_4,5M_4,5 electrons is ∼917 eV, which leaves the AES electron susceptible for efficient scattering in the gas phase and attenuation of the signal above near-ambient pressure conditions. To study copper-based materials at higher pressures, e.g., the active state of a catalyst, Auger transitions providing electrons with higher kinetic energies are needed.

This study focuses on AES transitions involving the Cu K-shell (1s electrons) that exhibit discernible kinetic energy shifts between the oxidation states of Cu. It is shown that the AES Cu KL₂M_4,5 transition, with kinetic energy of ∼7936 eV, provides a large enough kinetic energy shift between metallic copper and Cu₂O. AES signal is demonstrated in an ambient of 150 mbar CO₂.