Characterization of an Unexpected μ₃ Adsorption of Molecular Oxygen on Ag(100) with Low-Temperature STM

Precise description of the interaction between molecular oxygen and metal surfaces is one of the most challenging topics in quantum chemistry. In this work, we use low-temperature scanning tunneling microscopy (STM) to identify and characterize an adsorption state of molecular oxygen that coordinates to three Ag atoms (μ₃) on Ag(100). Surprisingly, μ₃-O₂ cannot be identified as a stable configuration with generalized gradient approximation (GGA)-level density functional theory (DFT) calculations. Through inelastic electron tunneling spectroscopy (IETS), we identify three vibrational modes of individual μ₃-O₂ and assign them to out-of-plane hindered rotation (HR) at 38.0 meV, in-plane HR at 32.4 meV, and in-plane hindered translation (HT) at 22.0 meV. We determine the barrier for rotational isomerization of μ₃-O₂ to be 69.3 meV from tunneling electrons-induced rotations. The inability of theory to predict the experiment stems most likely from self-interaction errors inherent to GGA-DFT, which leads to an inaccurate description of localized charges. We speculate that the μ₃-O₂ configuration represents a formal molecular oxygen anion and assign the ±11 meV excitation in the IETS to a transition between spin-orbit states of the surface-bound anion.