Anomalous Saturation of CO Adsorption at 26% on Cu(111) Governed by Nanometer-Scale Substrate-Mediated Interactions

CO adsorption on metal surfaces is a process of fundamental importance in surface science and heterogeneous catalysis. Despite its apparent simplicity, its understanding has often posed a challenge to conventional models. Using scanning probe microscopy (SPM), we have observed that CO adsorption on Cu(111) saturates at ∼26%, significantly below the anticipated 33% for the canonical $\sqrt{3}\times \sqrt{3}$/R30° structure. This anomalous saturation persists across a wide range of dosing amounts and deposition temperatures, indicating an intrinsic thermodynamic constraint rather than kinetic trapping. Statistical analysis of the SPM images reveals a long-range adsorbate interaction radius of ∼5.3 nm that governs the two-dimensional distribution of CO molecules. This nanometer-scale, substrate-mediated indirect interaction induces spatial correlations that suppress higher coverage. First-principles calculations show that increased coverage leads to confinement of Cu(111) surface-state electrons, with overlapping elastic strain fields, both of which reduce CO binding energy. These long-range interactions collectively enforce a self-limiting adsorbate density. Our findings establish that substrate-mediated forces can govern adsorbate arrangements well beyond nearest-neighbor scales. CO/Cu(111) provides a model system in which such effects can be observed directly, serving as a testbed for established theories of adsorption and substrate-mediated interactions.