Buried Ni/Cu(001) interface at the atomic scale

We present a quantitative surface x-ray analysis of the buried Ni/Cu(001) interface structure after deposition of 3 and 5 monolayers of Ni at room temperature. Interface mixing is found where 27+/-10% of top layer Cu atoms are exchanged by Ni. Atomic scale simulations reveal a kinetic pathway for the Ni/Cu-exchange process and explain the observed limited degree of intermixing. A disperse distribution of Ni within the Cu surface with a preferential Ni-Ni separation of 3-4 nearest neighbor distances is determined.     

Developing organoboranes as phase transfer catalysts for nucleophilic fluorination using CsF

Despite the general high fluorophilicity of boron, organoboranes such as BEt₃ and 3,5-(CF₃)₂C₆H₃–BPin are shown herein for the first time, to our knowledge, to be effective (solid to solution) phase-transfer catalysts for the fluorination of certain organohalides with CsF. Significant (up to 30% e.e.) chiral induction during nucleophilic fluorination to form β-fluoroamines using oxazaborolidine (pre)catalysts and CsF also can be achieved. Screening different boranes revealed a correlation between calculated fluoride affinity of the borane and nucleophilic fluorination reactivity, with sufficient fluoride affinity required for boranes to react with CsF and form Cs[fluoroborate] salts, but too high a fluoride affinity leading to fluoroborates that are poor at transferring fluoride to an electrophile. Fluoride affinity is only one component controlling reactivity in this context; effective fluorination also is dependent on the ligation of Cs⁺ which effects both the phase transfer of CsF and the magnitude of the [Cs⋯F-BR₃] interaction and thus the B–F bond strength. Effective ligation of Cs⁺ (e.g. by [2.2.2]-cryptand) facilitates phase transfer of CsF by the borane but also weakens the Cs⋯F–B interaction which in turn strengthens the B–F bond – thus disfavouring fluoride transfer to an electrophile. Combined, these findings indicate that optimal borane mediated fluorination occurs using robust (to the fluorination conditions) boranes with fluoride affinity of ca. 105 kJ mol⁻¹ (relative to Me₃Si⁺) under conditions where a signficant Cs⋯F–B interaction persists.

Simple boranes with the optimal fluoride ion affinity are effective as catalysts for phase transfer nucleophilic fluorination with CsF.     

Magnetic properties of TlCaBaCuO4.5 compounds prepared from different starting materials

It has been shown, that different starting materials result in Tl–Ca–Ba–Cu–O superconductors of different type even at the same heat treatment process. This manifests itself in the developing structure, in the number, share-rate and superconducting properties of the occurring phases as well. Microcrystals, phase-compositions, crystallinities, unit cells, onset and critical temperatures, magnetic flux exclusions, critical currents, magnetizations and critical magnetic fields essentially unlike from each other have been observed, depending on the starting materials. The presence of a starting compound having low melting point, as Ba(NO₃)₂ can cause a more compact, better superconducting final product, implying better connection between the grains.     

Surface structure determination by leed rotation diagrams; Application to the surface relaxation of W(001)

We examine the potentialities of LEED rotation diagrams at large angles of incidence for surface structure determination. From a comparison of relativistic model calculations for the (00) beam from unreconstructed W(001) to experimental results we find that the structure of the rotation diagrams is essentially determined by strong intralayer scattering within the first atomic layer, whereas peak intensities in rotation diagrams depend strongly on the distance to the adjacent layers. By means of a simple criterion based on the comparison of theoretical and experimental peak intensity ratios, the top layer contraction is determined to be 5.5% ± 1.5%. The influence of the parameters entering the calculations and of the experimental uncertainties on the validity of this result are discussed.     

Spin-polarized electron emission from magnetic and non-magnetic solids

The use of spin-polarization analysis in electron spectroscopy of magnetic and non-magnetic surfaces is demonstrated with a few examples. The existence and properties of spin-dependent transmission of electrons through the solid-vacuum interface is shown. The influence of surface reconstruction of Pt(110) on spin polarization and energy distribution curves in photoemission with circularly polarized light is studied. The polarization of secondary electrons from Fe(110) is observed to depend on the spin polarization of primary electrons at low energies. The temperature dependence of the exchange splitting in Ni is studied by means of spin-polarized electron energy loss spectroscopy and found to be at variance with the assumptions of the Stoner-Wohlfarth theory.     

Double logarithmic asymptotics and regge singularities of quark amplitudes with flavour exchange

We derive simple equations in terms of definite signature partial waves for the quark scattering and annihilation amplitudes in the double logarithmic approximation of QCD. We apply the method of isolating the softest virtual particle in the graphs which is based on gauge invariance and unitarity. Besides the particle Regge trajectories and corresponding cuts, there are further singularities in the angular momentum plane generated by perturbative double logarithmic contributions and which may be of phenomenological relevance. We also consider amplitudes with external currents and discuss how to include ultraviolet single logarithms and non-perturbative contributions.     

Growth, morphology, and crystalline structure of ultrathin Fe films on Cu3Au(100)

The growth mode, morphology, and crystalline structure of Fe films on Cu₃Au(100) are studied for different growth temperatures (300 and 160 K), using in situ scanning tunneling microscopy and low energy electron diffraction. Multilayer growth is found to be predominant for both growth temperatures. Only in films of 3–4 monolayers (ML) grown at 300 K is a mixed mode of layer-by-layer growth and island growth observed. An fcc-to-bcc structural transformation, accompanied by a distinct change in the surface topography, starts at about 3.5 ML and 5.5 ML for the growth temperatures of 300 and 160 K, respectively. For both growth temperatures bcc-like Fe in Fe/Cu₃Au(100) assumes, most likely through a Bain path, a surface plane with the (100) rather than the (110) orientation found in the Fe/Cu(100) system. Both the surface morphology and the onset thickness of the fcc–bcc structural transformation are shown to be strongly affected by the growth temperature.     

Spin- and angle-resolved photoelectron spectroscopy from solid surfaces with circularly polarized light

Spin-polarized photoemission with circularly polarized light represents a relatively new technique in surface science. It became feasible with the increased availability of circularly polarized light in the vacuum ultraviolet and soft X-ray regime, generated in dedicated synchrotron radiation facilities. Another important ingredient was the development of efficient electron spin polarization detectors. Supported by group theoretical considerations, this technique was first employed to study the process of optical spin orientation in single crystalline non-magnetic materials. In these samples, spin-dependent effects arise solely from spin-orbit coupling. The experiments revealed the strong influence of spin-orbit coupling on the details of the electronic band structure, such as the symmetry character of the electronic wave functions, hybridization phenomena, and the behavior of degeneracies. The results opened the way to a detailed understanding of the electronic structure and permit a rigorous test of both state-of-the-art bulk band theories and fully relativistic photoemission calculations. Since 1990, spin-polarized photoemission with circularly polarized light is also used to investigate ferromagnetic systems. These experiments led to the discovery of the counterpart of optical spin orientation in ferromagnets, namely, the magnetic dichroisms. Magnetic dichroism means that the spin-dependent photoexcitation from a ferromagnetically ordered system manifests itself already in the photoelectron intensity distributions. Because of this particular property, techniques based on magnetic dichroisms are currently receiving wide interest in the spectroscopic investigation of magnetic systems and phenomena.

The following article reviews the current status of the field of spin- and angle-resolved photoemission from solids using circularly polarized radiation. We survey the development of this technique over the last 10 years, covering its applications to both non-magnetic and ferromagnetic systems.     

Carbon dioxide versus xenon as a mobile phase for flow cell packed column SFC/FT-IR

Xenon is compared to carbon dioxide as a mobile phase for super critical fluid chromatography/Fourier transform infrared spectrometry. The study showed xenon to be comparable to carbon dioxide in terms of resulting chromatography for non-polar analytes. Xenon was confirmed to be a very poor mobile phase, however, for polar analytes. It was determined that small wavenumber shifts in the infrared spectra of probe analytes occurred as either the density or temperature of the mobile phase was increased. The degree of these shifts was often similar for xenon and carbon dioxide. Analyte spectra for five different compounds were produced in both super critical xenon and carbon dioxide and compared to condensed phase and vapor phase library spectra. In all cases, carbon dioxide spectra were readily matched to their corresponding vapor phase spectra, despite having blanked portions of the spectrum due to carbon dioxide infrared absorption. Xenon produced technically superior spectra without such blanked regions, but at a much higher economical cost than carbon dioxide and with no real improvement in terms of library matching.