Sequential Oxygen Atom Chemisorption on Surfaces of Small Iron Clusters

We report photoelectron spectra of iron oxide clusters, ${\mathrm{Fe}}_x{\mathrm{O}}_y^{-}$ $(x=1\mathrm{}–4,y=1\mathrm{}–6)$. For a given x, we find that the electron affinity increases with the number of O atoms, consistent with an increasing degree of oxidation. The results are interpreted based on charge transfer interactions between the F${\mathrm{e}}_{\mathit{x}}$ clusters and the O atoms, and provide key information about the oxide cluster structures, in which each O atom is suggested to locate on the surface of the clusters for the $x=3\mathrm{}$ and 4 series. These clusters provide novel model systems to understand the electronic structure of bulk iron oxides.