Chemical and surface core-level shifts of barium studied by photoemission

Polycrystalline Ba films are prepared on an Al(111) substrate with thickness ranging from monolayer to about ten layers. The clean and oxidized films are investigated by uv photoelectron spectroscopy using synchrotron radiation in the energy range 20 eV ≤ħω≤120 eV. Besides the conduction-band region of bulk Ba the 5p and 4d levels are studied, both showing surface core-level shifts of 0.48 eV to higher binding energies. At $E_F$ and extending down to 0.7 eV, d states are identified in the bulk conduction band. For the bulk, oxidation introduces negative (towards $E_F$) chemical shifts of the core levels and a very sharp ${\mathrm{O}}^{2\mathrm{-}}$ peak in the valence-band region. It is argued that these oxygen-2p-derived states, and not the Ba 5d states, are responsible for the negative chemical shift. For the Ba monolayer the chemical shift is positive and no ${\mathrm{O}}^{2\mathrm{-}}$ state is found, in agreement with the new interpretation of the chemical shifts for bulk Ba.