Two-photon photoemission spectroscopy: atomic oxygen on Cu(111) and styrene on oxygen covered Cu(111)

Abstract

Two-photon photemission (2PPE) spectroscopy was employed to investigate the unoccupied electronic states at surfaces of Cu(111) dosed with 0₂ at 400 K and then exposed to styrene at 90 K. Without styrene, the spectrum after 100 L 0₂ exhibits an occupied Cu-derived surface state and an unoccupied state at 2.8 eV above the Fermi level. Consistent with polarization results, we attribute the latter to strong hybridization and covalent bonding between the 2p states of oxygen atoms located in three-fold hollow sites and the d_z2 states of the Cu atoms in the second layer. As styrene is added, the O-induced unoccupied state disappears and a new unoccupied state appears 3.3 eV above the Fermi level. For styrene on clean Cu(111), a different state appears at 3.5 eV above the Fermi level and is attributed to antibonding orbitals formed by hybridization of copper and styrene orbitals. Thermal desorption provides evidence that the chemisorbed oxygen and styrene react. For a 1000 L 0₂ exposure, the occupied Cu-based surface state vanishes, and there is a broad unoccupied state located at 2.8 eV above the Fermi level. These results are consistent with a surface structure that is a precursor to Cu₂O. As styrene is added, no new features appear in 2PPE and there is no evidence for chemical reaction in thermal desorption.`