Optical Spectra of Ferrous and Ferric Oxides and the Passive Film: A Molecular Orbital Study

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Abstract
Initial state molecular orbital cluster energy levels for hematite provide assignments for Gardner, Sweett, and Tanners' optical absorption peaks at 2.4, 3.2, and 5.8 eV as , , and transitions, respectively. Absorptions seen around 17 eV by Galuza et al. appear to result from and charge transfer excitations. Ellipsometric peaks measured at 2.2 and 3.0 eV by Cahan and Chen correspond to the 2.4 and 3.2 eV optical absorptions. Ellipsometric peaks of Cahan and Chen occurring at 2.2 and 3.6 eV for ferrous ions on Fe passivated at 1.15V vs. NHE and measured at 1.05V have the same origin, with the second peak shifted 0.6 eV as a result of a structure‐caused shift and an oxidation‐state shift. Buchenau and Müller's broad band for magnetite is a consequence of charge transfer excitations from O 2p to FeIII tetrahedrally split 3d levels and Fe2.5 octahedrally split 3d levels which span a range of ∼2.5 eV. We find that numerous optical excitations should occur in magnetite at energies greater than 6 eV, but as yet this range has not been explored.

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