Characterization\nof an Unexpected μ<sub>3</sub> Adsorption of Molecular Oxygen\non Ag(100) with Low-Temperature STM

Precise description of the interaction between molecular\noxygen\nand metal surfaces is one of the most challenging topics in quantum\nchemistry. In this work, we use low-temperature scanning tunneling\nmicroscopy (STM) to identify and characterize an adsorption state\nof molecular oxygen that coordinates to three Ag atoms (μ<sub>3</sub>) on Ag(100). Surprisingly, μ<sub>3</sub>-O<sub>2</sub> cannot be identified as a stable configuration with generalized\ngradient approximation (GGA)-level density functional theory (DFT)\ncalculations. Through inelastic electron tunneling spectroscopy (IETS),\nwe identify three vibrational modes of individual μ<sub>3</sub>-O<sub>2</sub> and assign them to out-of-plane hindered rotation\n(HR) at 38.0 meV, in-plane HR at 32.4 meV, and in-plane hindered translation\n(HT) at 22.0 meV. We determine the barrier for rotational isomerization\nof μ<sub>3</sub>-O<sub>2</sub> to be 69.3 meV from tunneling\nelectrons-induced rotations. The inability of theory to predict the\nexperiment stems most likely from self-interaction errors inherent\nto GGA-DFT, which leads to an inaccurate description of localized\ncharges. We speculate that the μ<sub>3</sub>-O<sub>2</sub> configuration\nrepresents a formal molecular oxygen anion and assign the ±11\nmeV excitation in the IETS to a transition between spin–orbit\nstates of the surface-bound anion.