Differential photoelectron holography: a new approach for three-dimensional atomic imaging

We propose differential holography as a method to overcome the long-standing forward-scattering problem in photoelectron holography and related techniques for the three-dimensional imaging of atoms. Atomic images reconstructed from experimental and theoretical Cu 3p holograms from Cu(001) demonstrate that this method suppresses strong forward-scattering effects so as to yield more accurate three-dimensional images of side- and backscattering atoms.     

Monte Carlo simulations of segregation in Pt-Re catalyst nanoparticles

We have investigated the segregation of Pt atoms to the surfaces of Pt-Re nanoparticles using the Monte Carlo method and modified embedded-atom method potentials that we have developed for Pt-Re alloys. The Pt(75)Re(25) nanoparticles (containing from 586 to 4,033 atoms) are assumed to have disordered fcc configurations and cubo-octahedral shapes (terminated by [111] and [100] facets), while the Pt(50)Re(50) and Pt(25)Re(75) nanoparticles (containing from 587 to 4,061 atoms) are assumed to have disordered hcp configurations and truncated hexagonal bipyramidal shapes (terminated by [0001] and [1011] facets). We predict that due to the segregation process the equilibrium Pt-Re nanoparticles would achieve a core-shell structure, with a Pt-enriched shell surrounding a Pt-deficient core. For fcc cubo-octahedral Pt(75)Re(25) nanoparticles, the shells consist of almost 100 at. % of Pt atoms. Even in the shells of hcp truncated hexagonal bipyramidal Pt(50)Re(50) nanoparticles, the concentrations of Pt atoms exceed 85 at. % (35 at. % higher than the overall concentration of Pt atoms in these nanoparticles). Most prominently, all Pt atoms will segregate to the surfaces in the hcp truncated hexagonal bipyramidal Pt(25)Re(75) nanoparticles containing less than 1000 atoms. We also find that the Pt atoms segregate preferentially to the vertex sites, less to edge sites, and least to facet sites on the shell of Pt-Re nanoparticles.     

The diffraction pattern of pp elastic scattering and the composite structure of the proton

The consequences of the quark-glue structure of hadrons for pp elastic scattering are examined in a simple model where the glue is taken to be the active constituent in normal (low p_T) hadronic collisions. Earlier work based on the study of pp inelastic diffraction led to the conclusion that the glue-glue elastic amplitude t_g is approximately Gaussian in the glue-glue impact parameter b_g, with maximum opacity at b_g = 0. A slight refinement of this approximation, in which t_g is almost maximally opaque over a small b_g interval and is Gaussian at larger b_g, is now shown to account for the remarkable diffraction structure observed in pp elastic scattering at ISR energies. A very simple form is obtained for the form factor describing the impact parameter distribution of the glue inside a proton relative to the proton itself.     

Layer iteration calculation of angle-resolved ultraviolet photoemission: c(2 × 2) oxygen overlayer on Ni(001)

We have calculated the angle-resolved photoemission profile from s and p levels of oxygen c(2 × 2) ordered overlayer on Ni(001). Depending on the electron energy and exit angle, final state multiple scattering can produce large distortions, giving rise to complex angular profiles. However, at energies and exit angles where the solid is a poor elastic reflector, the direct term from the initial wavefunction provides the dominant contribution. We suggest that angular photoemission data in such a region can be most effectively used to study the directions of the chemisorption bond, bond angles as well as the binding site.     

A LEED crystallography study of the (2×2)−C2H3 structure obtained after ethylene adsorption on Rh(111)

The structure of the Rh(111)-(2 × 2)-C₂H₃ overlayer that was obtained upon the adsorption of ethylene has been determined using a LEED intensity analysis. In agreement with a prior HREELS study, an ethylidyne (CCH₃) species is found to stand perpendicularly above an hcp hollow site with a carbon-carbon distance of 1.45±0.10 Å and a metal-carbon distance of 2.03±0.07 Å. The Zanazzi-Jona and Pendry R-factors for this structure are 0.49 and 0.52, respectively. By comparison with similar organometallic complexes, the relatively short carbon-carbon distance and long metal-carbon distance can be explained by σ?π hyperconjugation of the surface ethylidyne fragment.     

Surface structures of W(110) and W(100) faces by the dynamical LEED approach

By means of an efficient dynamical LEED method, the surface structures of W(110) and W(100) faces are examined. It is found that the W(110) surface maintains the bulk structure, despite the possibility for the top tungsten atoms to settle into sites of higher coordination number. The W(100) face exhibits a possible contraction of the top atomic layer by 0.10 ± 0.10 Å. These results fit into a trend correlating a top-layer contraction with the absence of close-packing in the top layer, i.e. with the roughness of the surface.     

Fast beam collinear laser-rf double resonance

The method and application of fast beam collinear laser-rf double resonance are discussed. Following a short theoretical treatment of the collinear interaction of fast atoms with rf and laser fields, the experimental procedure and some technical details are presented. Selected applications of the method: (i) hyperfine structure and atomicg-factor measurements, and (ii) experimental studies of fundamental aspects of atom-field interactions, are described. prospective applications in nuclear physics experiments are discussed.     

Prospects for resolving chemical structure by atomic force microscopy: a first-principles study

In a recent paper, the chemical structure of a molecule was resolved by means of atomic force microscopy (AFM): using a metal tip terminated in a CO molecule, the authors could image the internal bonding arrangement of a pentacene molecule with remarkable spatial resolution (notably better than with other tip terminations), as verified by their first-principles calculations. Here we further explore with first-principles calculations the mechanisms, applicability, and capabilities of this approach for a wider range of situations, by varying the imaged molecule and the tip beyond the experimental cases. In our simulations, a high atomic resolution is found to be dominated by the electronic structure of the last two atoms on the tip apex which are set perpendicularly to the sample molecule. For example, tips terminated in CH(4) or pentacene itself (both having a C-H apex) yield similar images, while tips terminated in O(2) or CO give quite different images. While using a CO-terminated tip successfully resolves the chemical structure of pentacene and of other extended planar networks based on C(6) rings, this tip fails to resolve the structures of benzene (with its single C(6) ring) or nonplanar C(6) networks, such as C(60) or small-diameter carbon nanotubes. Defects (such as N substitution for a C-H group) were also found to significantly influence the image resolution. Our findings indicate that further application of this approach requires, for each sample, careful selection of a suitable "imaging" molecule as tip termination.     

A concise review of mass spectrometry imaging

Mass spectrometric imaging allows the investigation of the spatial distribution of molecules at complex surfaces. The combination of molecular speciation with local analysis renders a chemical microscope that can be used for the direct biomolecular characterization of histological tissue surfaces. MS based imaging advantageously allows label-free detection and mapping of a wide-range of biological compounds whose presence or absence can be the direct result of disease pathology. Successful detection of the analytes of interest at the desired spatial resolution requires careful attention to several steps in the mass spectrometry imaging protocol. This review will describe and discuss a selected number of crucial developments in ionization, instrumentation, and application of this innovative technology. The focus of this review is on the latest developments in imaging MS. Selected biological applications are employed to illustrate some of the novel features discussed. Two commonly used MS imaging techniques, secondary ion mass spectrometric (SIMS) imaging and matrix-assisted laser desorption ionization (MALDI) mass spectrometric imaging, center this review. New instrumental developments are discussed that extend spatial resolution, mass resolving power, mass accuracy, tandem-MS capabilities, and offer new gas-phase separation capabilities for both imaging techniques. It will be shown how the success of MS imaging is crucially dependent on sample preparation protocols as they dictate the nature and mass range of detected biomolecules that can be imaged. Finally, developments in data analysis strategies for large imaging datasets will be briefly discussed.