Oxygen as a Classifier of Superatomic Electronic States in Au-Based Clusters

The electronic structure of Au-based superatomic clusters governs their stability, spin state, and reactivity, yet facile experimental methods to classify frontier electronic states remain scarce. Here, we demonstrate that oxygen can serve as a direct classifier for distinguishing open-/closed-shell-like electronic manifolds in Au-based superatoms under near-ambient conditions. Using a custom plasma source that delivers both atomic and molecular oxygen, we reveal a strong correlation between oxygen uptake stoichiometry and superatomic shell character: closed-shell-like clusters preferentially form molecular oxygen adducts, whereas open-shell-like clusters favor atomic oxygen incorporation. Collisional cross-sectional area measurements coupled with comprehensive global geometric structure searches and density functional theory analyses rationalize these distinct channels in terms of frontier orbital patterns and binding mechanisms. The approach generalizes to Au-Ag, Au-Cu, and Au-W alloy clusters, where isomer-resolved oxygen branching ratios directly report on coexisting electronic isomers. This oxygen-enabled classification provides a practical and intuitive route to link geometric structure, electronic states, and chemical functionality in superatomic cluster systems, with implications for cluster-based catalysis and materials design.