Unoccupied electronic states at the diamond (111) surface are studied by measuring both bulk- and surface-sensitive C 1s partial-yield soft-x-ray absorption spectra. Several absorption features are observed in the bulk band gap below the 289.19-eV bulk-C 1s absorption edge. They are associated with transitions from the C 1s surface core level to unoccupied surface states by their sensitivity to chemisorbed species and changes in their intensity as the electron escape depth is varied. These states have been detected previously with electron-energy-loss spectroscopy but no structure was resolved. The close proximity of the observed surface absorption (onset at hν=284 eV) with the hν=285.35 eV, C 1s→${\pi }^{\mathrm{*}}$ transition in graphite indicates the existence of unoccupied ${\pi }^{\mathrm{*}}$ bands at the diamond (111) surface. An interpretation of these results in terms of the π-bonded chain model for the diamond (111) 2×1 reconstructed surface is given.

• Received 16 July 1985

DOI:https://doi.org/10.1103/PhysRevB.33.1346