Adsorption, reaction and interfacial electronic structures of aromatic molecules on single crystal surfaces
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Electron transfer at organic/metal interfaces is fundamental to a large number of problems in surface science. Electronic interactions at such an interface are responsible for charge injection from an electrode to the molecular film. The efficiency or rate of charge injection is determined by the energetic alignment of molecular orbitals to the metal Fermi level and the electronic coupling strength (wavefunction mixing) between molecular orbitals and metal bands. Two experimental investigations were performed with two-photon photoemission spectroscopy (2PPE). First, the energetic alignments of naphthalene/Cu(111) were probed. Three transitions involving unoccupied orbitals were found and identified as having π* molecular orbital character—the first lying 0.4 eV above the vacuum level vii (π* b1u), the second 0.3 eV below the vacuum level (π* b3g), and the third 1.1 eV below the vacuum level (π* b2g). In the second experiment, the interfacial electronic structures of chemisorbed styrene on Cu(111) were successfully investigated. We observed unoccupied states 3.5 eV above the Fermi level and occupied states 2.0 eV below the Fermi level. Polarization results reveal that the occupied and unoccupied states arise from bonding and antibonding orbitals formed by hybridization of copper (surface state and d-band orbitals) and styrene (π1* and π2* orbitals). For the first time, two-photon photoemission spectroscopy was employed to explore a surface chemical reaction: epoxidation of styrene on Cu(111). With 100 L oxygen on a Cu(111) surface, the atomic oxygen occupies three-fold hollow HCP sites rather than FCC sites. Its 2p states hybridize strongly with the dz2 states of the Cu atoms in the second layer. After styrene is adsorbed on Cu metal sites of this oxygen-covered surface, it undergoes efficient epoxidation to styrene oxide. The 2PPE results show that the change in the electronic structures of the adsorbed reactant is consistent with the surface reaction: the oxygen-induced feature from the Cu-O bonding disappears and a new state appears. However, 1000 L oxygen-covered Cu(111) is catalytically inert for styrene epoxidation: as styrene is added, no new features appear in 2PPE, and there is no evidence for chemical reaction in thermal desorption. This study could open up a new area of solid state and surface catalytic chemistry.