Atomically resolved spin-dependent tunneling on the oxygen-terminated Fe3O4(111)

We employ spin-polarized STM to study the spin-dependent tunneling between a magnetite (111) sample and an antiferromagnetic tip through a vacuum barrier at room temperature. Atomic scale STM images show significant magnetic contrast corresponding to variations in the local surface states induced by oxygen vacancies. The estimated variations in tunneling magnetoresistance of 250% suggest that the spin-transport properties are significantly altered locally by the presence of surface defects.     

New experimental technique of photoelectron holography applied to Bi trimers on Si(111)

The branching ratio, the photoemission intensity ratio of two spin-orbit-split components, has been applied for the first time as a means for obtaining a photoelectron holographic image. Angle-resolved photoemission from a monolayer of Bi adsorbed on Si(111) shows fine-structure oscillations in the branching ratio of the Bi 5d core level due to diffraction effects. These oscillations as a function of photon energy are recorded for a number of emission angles. Three-dimensional holographic inversion of the data yields an atomic image which shows that the Bi adatoms are arranged in a trimer structure.     

Atomically resolved images of I(h) ice single crystals in the solid phase

The morphology and crystal structure of nanoparticles of ice were examined by high-resolution transmission electron microscopy. Two different crystal structures were found and unambiguously assigned to hexagonal (I(h)) and cubic (I(c)) ice crystals. Direct observation of oxygen columns clearly revealed the hexagonal packing of water molecules. Electron energy-loss spectroscopy was used to monitor the electronic excitation in ice, suggesting possible dissociation of water molecules. Dynamic process of phase transition between I(h) and I(c) phases of individual ice nanoparticles under electron beam irradiation was also monitored by in situ transmission electron diffractometry.     

Azimuthal dependence of angle resolved photoelectron diffraction from I on Ag(111) -- multiple scattering analysis to determine the adsorption site

The azimuthal anisotropy from the 4d level of (√3 × √3)R30°I on Ag(111) is calculated and compared with the data of Farrell et al. The comparisons indicate the φ-patterns give a clear and unambiguous determination of the adsorption registry of I on Ag(111).     

Angular resolved photoemission from intrinsic surface states on Al (100)

Using photon energies of 11.7, 16.8 and 21.2 eV, we have recorded angular resolved photoemission spectra from clean Al (100). A dominant, surface sensitive peak is interpreted as emission from a two-dimensional band of surface states. The band mass is $\text{m}{}^1\text{=(1.03±0.10)m}$, and the band minimum, (2.8±0.2) eV below E_F for surface momentum k₆=0, is located within the bulk band gap. The observed existence of the peak for large values of k₆, indicates a transition from a true surface state to a surface resonance.     

X-ray photoelectron holography of ultrathin film and single crystal Cu(111): improving the accuracy of bond-length determination

Forward-scattering diffraction patterns from single-crystal Cu(111) substrates and an epitaxial film of FCC Cu/Ir(111) have been analyzed to produce real-space images of the atomic lattice by Fourier-transform holography. The accuracy of atomic positions is found to be better in the single-crystal holograms than in the ultrathin film of copper. The angular dependence of the atomicscattering factor produces a shift in the position of atoms in the holographic image, and causes the atom image to be asymmetric. Model calculations reproduce the atom shift and asymmetry. A modified Fourier-transform algorithm is shown to correct for both the atom-shift and the image asymmetry.     

Final-state effects in photoemission experiments from graphene on Ni(111)

Photon-energy dependent photoemission experiments using high harmonic radiation as a tunable photon source on epitaxially grown graphene on a Ni(111) substrate are presented. A resonance in the photoemission cross section for the σ and π states of graphene was observed and compared to a simple model for the determination of photoemission cross sections of free standing graphene, pointing out the role of the interaction of graphene with the nickel substrate.     

Photoemission from physisorbed Co on clean and Xe-covered Al(111)

Molecular-orbital energy shifts are observed in photoemission from weakly physisorbed CO on clean and Xe-covered Al(111) surfaces. These shifts in ionization potentials are mainly due to final-state relaxation effects, which can be described approximately by a point-charge image-potential model. Differential distance- and orbital-dependent energy shifts suggest that CO molecules lie flat on the substrates. CO is adsorbed on Al(111) with a heat of formation of 0.21 eV/molecule.     

Ar adsorptions on Al (111) and Ir (111) surfaces: a first-principles study

We investigate the adsorptions of Ar on Al (111) and Ir (111) surfaces at the four high symmetry sites, i.e., top, bridge, fcc- and hcp-hollow sites at the coverage of 0.25 monolayer (ML) using the density functional theory within the generalized gradient approximation of Perdew, Burke and Ernzerhof functions. The geometric structures, the binding energies, the electronic properties of argon atoms adsorbed on Al (111) and Ir (111) surfaces, the difference in electron density between on the Al (111) surface and on the Ir (111) surface and the total density of states are calculated. Our studies indicate that the most stable adsorption site of Ar on the Al (111) surface is found to be the fcc-hollow site for the (2 × 2) structure. The corresponding binding energy of an argon atom at this site is 0.538 eV/Ar atom at a coverage of 0.25 ML. For the Ar adsorption on Ir (111) surface at the same coverage, the most favourable site is the hcp-hollow site, with a corresponding binding energy of 0.493 eV. The total density of states (TDOS) is analysed for Ar adsorption on Al (111) surface and it is concluded that the adsorption behaviour is dominated by the interaction between 3s, 3p orbits of Ar atom and the 3p orbit of the base Al metal and the formation of sp hybrid orbital. For Ar adsorption on Ir (111) surface, the conclusion is that the main interaction in the process of Ar adsorption on Ir (111) surface comes from the 3s and 3p orbits of argon atom and 5d orbit of Ir atom.     

Oxygen molecule dissociation on the Al(111) surface

The dissociative adsorption of the O2 on the Al(111) surface is studied using ab initio calculations based on density-functional theory. In the sticking probability experiments an activation barrier for the O2 dissociation exists, but our calculations predict a barrier only for one trajectory. Also our results do not support the model of charge transfer from the surface to the molecule as a bond breaking mechanism. Instead, the increasing hybridization between O2 orbitals and Al states, when the adsorbate approaches the slab, is significant for the dissociation.     

Adsorption of H2O on Al(111)

The adsorption of H₂O on Al(111) has been studied by ESDIAD (electron stimulated desorption ion angular distributions), LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and thermal desorption in the temperature range 80–700 K. At 80 K, H₂O is adsorbed predominantly in molecular form, and the ESDIAD patterns indicate that bonding occurs through the O atom, with the molecular axis tilted away from the surface normal. Some of the H₂O adsorbed at 80 K on clean Al(111) can be desorbed in molecular form, but a considerable fraction dissociates upon heating into OH_ads and hydrogen, which leaves the surface as H₂. Following adsorption of H₂O onto oxygen-precovered Al(111), additional OH_ads is formed upon heating (perhaps via a hydrogen abstraction reaction), and H₂ desorbs at temperatures considerably higher than that seen for H₂O on clean Al(111). The general behavior of H₂O adsorption on clean and oxygen-precovered Al(111) (θ_O ? monolayer) is rather similar at low temperature, but much higher reactivity for dissociative adsorption of H₂O to form OH _adsis noted on the oxygen-dosed surface around room temperature.     

The chemisorption of CO on Cu(100) studied with angle resolved photoelectron spectroscopy

Using angle resolved UV photoelectron spectroscopy, coupled with the continuum of polarized light available with synchrotron radiation, we make an interpretation of the photoelectron spectrum of CO adsorbed on Cu(100). We point out that the bonding of CO on Cu as observed by photoemission is considerably different from the bonding of CO on Ni. These differences do not seem to be caused by bonding orientation differences, however, as the CO molecular axis is found to be very close to the surface normal, as was the case for CO on Ni(100). No evidence is found for a second phase of CO on Cu(100).     

Simulation of NC-AFM images of xenon(111)

Experimental results recently obtained for Xe(111) are simulated introducing a method which allows the time-effective simulation of complete non-contact atomic force microscopy (NC-AFM) images for non-reactive surfaces. All features of the experimental image are successfully reproduced. Additionally, the comparison between experiment and simulation allows the maxima in the experimental image to be identified as the actual positions of the xenon atoms.     

X-ray photoelectron diffraction study of discommensurate Cu/Ge(111)

We have studied the atomic structure in the interior of discommensurate domains of the Cu/Ge(111) surface by using scanned-angle X-ray photoelectron diffraction (XPED). XPED patterns of Cu 2p_3/2 intensity provided direct information on the local structure in the vicinity of photoelectron emitters. It has been found that a certain number of Cu atoms are embedded within the surface layer, so that the surface has some structural similarity with the discommensurate Cu/Si(111)-‘5×5’.