Adsorbate-substrate vibrational modes of benzene on Ag(110) resolved with scanning tunneling spectroscopy

Using a low temperature scanning tunneling microscope, we have detected low energy adsorbate-substrate (external or frustrated) vibrational modes of benzene molecules adsorbed on a Ag(110) surface. We demonstrate that such vibrations represent a fingerprint of the molecules' chemical state and environment; two different vibrational spectra are measured on molecules populating two different adsorption states. We also find that the distortion of the adsorption geometry of the molecules may give rise to the excitation of additional (initially hidden) modes. Important differences in the spatial distribution of the inelastic signal are also observed for these external modes.     

Adsorption of pyrimidine molecules on Pd(110) observed by scanning tunneling microscopy

Adsorbed states of pyrimidine molecules on Pd(110) have been studied by a scanning tunneling microscope (STM). The pyrimidine molecules are preferentially adsorbed on terraces, not at steps. The isolated pyrimidine molecule shows a 0.6 nm × 0.6 nm rectangular shape with two parts of elongated protrusions. Two adsorption sites are observed: on-top site of the Pd[1 0] row and the midway between two [1 0] rows. Pyrimidine molecules show a strong tendency to form dimers even at a low coverage (0.01 ML), indicating that there is an attractive interaction between two adsorbed molecules.     

Hydrogen-induced missing-row reconstructions of Pd(110) studied by scanning tunneling microscopy

The effect of hydrogen adsorption on the Pd(110) surface structure at room temperature has been studied by scanning tunneling microscopy. Depending on the partial pressure of hydrogen two different reconstructions of Pd(110) have been observed: a (1 × 3) phase at hydrogen pressures in the 10⁻⁹ mbar range and an additional (1 × 2) phase at p_H2 ≥ 5 × 10⁻⁸ mbar. Both reconstructions are found to be of the missing-row type. The evolution of the surface reconstructions has been followed in situ.     

Anisotropic water chain growth on Cu(110) observed with scanning tunneling microscopy

We report a novel structure of water aggregate by means of scanning tunneling microscopy. Water molecules are self-assembled into one-dimensional chains on Cu(110) at 78 K. The chain exhibits a zigzag structure with a period of 7.2 A and grows to a length of approximately 1000 A. We propose that water hexamers are arranged alternately along the chain. Interchain repulsion due to dipole interaction facilitates the 1D chain growth. A two-dimensional overlayer develops only at high coverage.     

Imaging the phenol molecule adsorbed on TiO2(110) by scanning tunneling microscopy

The image of a phenol molecule adsorbed on a TiO₂(110) single crystal was obtained, for the first time, by using the STM. TiO₂(110) has a tunneling active free region from + 1.35 eV below the Fermi level to −0.25 eV above the Fermi level. This region was found after measuring the surface density of states by using tunneling spectroscopy. The phenol molecule was fixed on TiO₂ utilizing the amphoteric nature of the TiO₂ surface. The image of the phenol molecule was measured at a dp bias of + 443 mV, which almost matches the energy of the OH and C-H stretching bond of the phenol molecule, in the free of energy levels region of the TiO₂(110) substrate. The electron density of phenol ring is apparent in the STM picture.     

CO oxidation on Pt(110): scanning tunneling microscopy inside a high-pressure flow reactor

We have used a novel, high-pressure high-temperature scanning tunneling microscope, which is set up as a flow reactor, to determine simultaneously the surface structure and the reactivity of a Pt(110) model catalyst at semirealistic reaction conditions for CO oxidation. By controlled switching from a CO-rich to an O2-rich flow and vice versa, we can reversibly oxidize and reduce the platinum surface. The formation of the surface oxide has a dramatic effect on the CO2 production rate. Our results show that there is a strict one-to-one correspondence between the surface structure and the catalytic activity, and suggest a reaction mechanism which is not observed at low pressures.     

Imaging surface electronic structure of NiAl(110) using low-temperature scanning tunneling microscopy

The surface electronic structure of NiAl(110) is examined by means of scanning tunneling microscopy and spectroscopy at a temperature of 4 K. Topography and conductance images for a wide range of bias voltages reveal wavelike patterns around steps and defects. Fourier transforms of conductance images are used to map the surface electronic structure of NiAl(110). We interpret the patterns in the Fourier transforms in terms of surface resonances, and analyze the details of its dispersion relation E(k_). A comparison with density-functional-based calculations and photoemission experiments is presented, and alternative explanations for the appearance of structures in Fourier transforms of conductance images are discussed.     

Novel water overlayer growth on Pd(111) characterized with scanning tunneling microscopy and density functional theory

Scanning tunneling microscopy (STM) images of water submonolayers on Pd(111) reveal quasiperiodic and isolated adclusters with internal structure that would ordinarily be ascribed to icelike puckered hexagonal units. However, density functional theory and STM simulations contradict this conventional picture, showing instead that the water adlayers are composed mainly of flat-lying molecules arranged in planar water hexagons. A new rule for two dimensional (2D) water growth is offered that generates the structures observed experimentally from planar hexamer units.     

The decomposition of ammonia on an oxygen-precovered Ni(110) surface studied by scanning tunneling microscopy

The decomposition of ammonia on a Ni(110) surface with preadsorbed oxygen has been investigated in ultra-high vacuum at room temperature using scanning tunneling microscopy (STM). We propose a reaction model in which the high reactivity observed at low O coverage is ascribed to a direct interaction between the NH₃ molecules and the terminating atoms of the short, mobile -Ni-O- added rows which are observed on the surface under these conditions. This model is consistent with the observation that the surface becomes inert at high O coverage. We believe that the present reaction model can also explain results from some other experiments in which preadsorbed oxygen has been found to act as a promoter for dissociation of H-containing species, such as for NH₃ on Cu(110) and H₂O on Ni(110).     

Observation of charge-density wave domains on the Cr(110) surface by low-temperature scanning tunneling microscopy

We have studied the Cr(110) surface with low-temperature scanning tunneling microscopy at 6 to 145 K. For tunneling voltages below +/-150 mV we observe a surface charge-density wave (CDW) with a wavelength of 42 A and wave fronts aligned with the [001] in-plane direction. The observed wave pattern is identified as the surface projection of the bulk CDW's arising from the spin-density wave ground state of Cr with the Q vector parallel to [100] and [010], respectively. The bulk CDW with Q parallel to the [001] in-plane direction appears, however, to be strongly suppressed at the Cr(110) surface.     

The vibrational modes of hydrogen adsorbed on Ni(110)

By studying the vibrational modes of H on Ni(110) as a function of coverage and temperature, a local picture of H site occupation is obtained in the lattice gas regime and on the (1 × 2) reconstructed surface at low temperature and for the irreversibly disordered surface formed by thermal conversion. Threefold sites are deduced from our data in both the lattice gas and the (1 × 2) reconstructed low temperature phases, with an additional loss in the latter phase ascribed to a second layer site. After thermal conversion, the threefold sites are depleted with a ? 0.5 monolayer (ML) transfer of H to second layer sites which appear to stabilize the surface and with ~ 0.5 ML H desorbing. Readsorption on the disordered surface indicates that a small amount of empty threefold Ni sites are still present after conversion. Various other models and site assignments are also discussed for comparison to the results of this study.     

Adsorbed state and vibrational excitation of N2 on Pd(110)

High-resolution vibrational electron energy loss spectroscopy, low-energy electron diffraction and Auger electron spectroscopy have been used to study the interactions of nitrogen with the Pd(110) surface. At 120 K, N₂ is chemisorbed molecularly on the Pd(110) surface, and the (2 × 1)-N₂ structure is formed. Most probably, the N₂ molecules are chemisorbed in the on-top sites of the bulk-like Pd(110) surface in the upright-linear structure. The Pd---N₂ bond energy is estimated to be ˜ 6 kcal/mol. The Pd---N₂ and N---N stretching vibrations of N₂ admolecules on Pd(110) are observed at 30 and 278 meV, respectively. The primary-energy dependence and angle dependence of their excitation cross sections agree reasonably well with the prediction of the dipole theory. The electron beam-induced effects are briefly discussed.