Photoemission energy distributions at high photon energies permit one-electron measurements of the electronic energy levels and level widths of chemisorbed gases on clean metal surfaces at fractional monolayer coverages. Chemisorbed CO on Ni exhibits energy levels at 7.5 and 10.7 eV below the Fermi level $E_{\mathrm{F}}$ while chemisorbed O on Ni shows a 2-eV width level at 5.5 eV below $E_{\mathrm{F}}$. Upon increased oxygen exposure, a surface oxide of Ni is formed which exhibits valence bands about 8.5 eV wide.

• Received 27 August 1971

DOI:https://doi.org/10.1103/PhysRevLett.27.1520