Physics and chemistry of small molecules on palladium(111) studied by scanning tunneling microscopy

Local probe studies of the physical and chemical properties and interactions of molecules adsorbed on metal surfaces generally reveal details inaccessible to macroscopic surface measurements. To explore the behavior of adsorbate systems at the atomic scale, a unique scanning tunneling microscope (STM) system capable of operating from 25 K to 300 K in ultrahigh vacuum (UHV) was designed, constructed, and characterized. Experimental studies of CO, O 2, and O adsorbed on the (111) surface of Pd single crystals were then performed with this instrument.;Initial characterization of the Pd(111) samples indicated that low concentrations of three distinct impurity species occur beneath the Pd(111) surface, despite intensive cleaning procedures. These subsurface impurities are found to strongly interact with adsorbed atoms and molecules, and to play an important role in each of the adsorbate systems examined in this work. These impurities are particularly difficult to detect by traditional surface analysis instruments, raising important questions about their potential influence on previous studies of adsorbate systems on Pd(111). In addition to their interactions with adsorbates, two of the three impurity types exhibit a novel mode of surface-assisted diffusion between the outermost two layers of the crystal.;Multiple ordered phases of CO on Pd(111) were studied by STM. Below 1/3 monolayer coverage, subsurface impurities are found to dominate the nucleation, growth, and ultimate domain structure of the CO adlayer. These impurities also appear to break the energetic equivalence of the two hollow adsorption sites observed for isolated molecules. With increasing coverage, small quantities of coadsorbed hydrogen are found to induce compression of the ( 3 × 3 ) R30° CO layer to form two distinct structures of the c(4 × 2)-2CO phase. A progression of higher coverage structures are examined, including previously unreported low symmetry domains of the 2 × 2-3CO phase.;The adsorption of O2 on Pd(111) was also investigated. At temperatures below 60 K, STM resolves the orientation of individual molecules and distinguishes two binding geometries. By comparison with theoretical calculations, the adsorption state of the molecules is identified with respect to prior vibrational spectroscopy experiments. The initial stages of thermal dissociation occur near 120 K, primarily at the periphery of p(2 × 2) islands and at active sites associated with subsurface impurities. Density fluctuations within the islands are observed, and may be associated with an additional precursor state of the molecule. At temperatures above 180 K, oxygen atoms resulting from O2 dissociation diffuse and form ordered p(2 × 2) islands.;The STM tip was used to manipulate and dissociate O2. Both thermal and tip induced dissociation lead to closely spaced pairs of oxygen atoms. The atoms bind to fcc hollow sites, with metastable occupation of hcp hollow sites at low temperature. The STM tip is also able to displace oxygen atoms via a short-range repulsive interaction.