Photon-Driven Cluster Fluxionality: Breaking the Bulk Stoichiometry Ceiling in Subnanometric Molybdenum Oxides.

We investigate the low-temperature (40 K) oxidation behavior of size-selected molybdenum clusters supported on epitaxial graphene. By combining high-resolution X-ray photoelectron spectroscopy with first-principles density functional theory, we demonstrate that subnanometric clusters can break the stoichiometric ceiling of bulk transition-metal oxides. Quantitative agreement between experiment and theory is achieved only when the clusters accommodate an unusually high oxygen content, with metal-to-oxygen ratios reaching up to Mo:O = 1:5, far exceeding the 1:3 limit of bulk oxide MoO3. This superstoichiometric regime is a consequence of coordination plasticity, where the molybdenum framework undergoes pronounced structural expansion to stable tetrahedral MoO4 motifs that are energetically inaccessible in extended solids. We provide compelling evidence that this process is mediated by photon-driven structural fluxionality: with kinetic energies exceeding 10 eV, secondary electrons generated during photoemission represent the only source to provide the vibrational excitations necessary to overcome the thermal constraint for structural transformation, allowing the clusters to disrupt the atomic connectivity and reach low-energy isomer configurations characterized by large structural expansion. Furthermore, atomic valence, rather than formal oxidation state, provides a physically meaningful and transferable descriptor for interpreting the main trend in core electron binding energy shifts in nanoscale disordered systems. These findings reveal a fundamentally non-bulk oxidation mechanism governed by non-thermal energy-activated structural dynamics, outlining a strategy for engineering oxygen-rich reactive sites at the ultimate size limit.