Substrate core chemical shifts upon chemisorption of oxygen on aluminum are observed using variable-excitation-energy photoelectron spectroscopy. At low exposures a unique intermediate oxidation state evolves where the $\mathrm{Al} 2p$ level is shifted by only 1.3 eV toward higher binding energy as compared to the 2.6-eV shift observed for heavily oxidized aluminum (${\mathrm{A}1\mathrm{}}_2$ ${\mathrm{O}}_3$). Simultaneous observation of the $\mathrm{O} 2p$ resonance shows evidence for a sharp phase change at exposures close to monolayer formation.

• Received 7 September 1976

DOI:https://doi.org/10.1103/PhysRevLett.37.1282