Oxygen 2sspectroscopy

Photoemission from O 2s--derived levels appears at binding energies (BE) between {~}14 and {~}33 eV. It has been investigated systematically by photoelectron spectroscopy with synchrotron radiation for a variety of oxygen-containing adsorbates on Pt(111), Ge(001), and Si(001) surfaces. On Pt(111), also the O 1s binding energies were determined by conventional x-ray photoelectron spectroscopy. The BE of O 2s orbitals varies strongly for different systems and is similarly suitable for distinguishing different species in a fingerprint technique as O 1s spectra. A splitting of the O 2s orbital into bonding and antibonding states is observed if overlap with s or p levels within the adsorbate occurs. This is observed in condensed O₂ and NO, but is strongly reduced or not existent in adsorbed O₂^2-, indicating an appreciable bond-length increase in the latter. Orbital overlap effects influencing the BE are also expected for substrates with s or p levels in the BE range of O 2s. The comparatively low O 2s intensity is no serious disadvantage for most substrates which have no levels or only sharp and localized d levels in this range. O 2s spectroscopy is especially useful in studies with synchrotron radiation where the deep-lying O 1s level frequently cannot be excited.