Two-photon photemission (2PPE) spectroscopy was employed to investigate the unoccupied electronic states at surfaces of Cu(111) dosed with 0<sub>2</sub> at 400 K and then exposed to styrene at 90 K. Without styrene, the spectrum after 100 L 0<sub>2</sub> 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<sub>z2</sub> 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 0<sub>2</sub> 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 Cu<sub>2</sub>O. As styrene is added, no new features appear in 2PPE and there is no evidence for chemical reaction in thermal desorption.`
The adsorption and ultraviolet photolysis at 300 K of trimethylacetate, (CH<sub>3</sub>)<sub>3</sub>CCOO-, on TiO<sub>2</sub>(110) has been explored by combining scanning tunneling microscopy, isothermal reaction mass spectrometry and electron energy loss spectroscopy. The photolysis, initiated by forming excited electrons and holes in the oxide, is dominated by ejection of CO<sub>2</sub> and C<sub>4</sub>H<sub>8</sub> with no evidence for retention of carbon-containing species. The chemistry is the result of holes extracting electron density from the pi-orbital of a carboxyl group leading to decarboxylation. The accompanying electron is trapped as Ti<sup>3+</sup> at the surface. These trapped species compete for holes and inhibit the hole-mediated decarboxylation. The trapped electrons can be removed by exposure to O<sub>2</sub> at 300 K leading to a transient acceleration of the reaction rate after resuming photolysis. Under aerobic conditions, arriving O<sub>2</sub> scavenges trapped electrons as they are formed reducing the degree of quenching and increasing the rate of trimethylacetate consumption without changing the products.
The chemistry, driven by 6.4 eV photons, of ammonia, phosphine, and arsine, adsorbed on GaAs(100), is compared. When molecularly adsorbed XH<SUB>3</SUB> (XequalsN, P, and As) at low temperatures, ca. 100 K, is irradiated, molecular desorption and hydride bond cleavage occurs. The branching between these two channels varies strongly with X; desorption is favored for NH<SUB>3</SUB> and As-H Bond cleavage favored for AsH<SUB>3</SUB>. The H atoms within the photochemically formed XH<SUB>2</SUB> groups can also be removed with photons, but only by extensive irradiation.