Structural and thermodynamic properties of Au2�C20 clusters

Isolated neutral gold clusters with 2?C20 atoms are studied theoretically using a parametrized density-functional tight-binding method combined with genetic algorithms. The structural and energetic properties are analyzed by studying the total energy per atom, the relative stability, the overall shape, and through a common-neighbor analysis. In addition, the temperature dependence of the vibrational heat capacities of the optimized gold clusters has been studied for the first time. We find the vibrational heat capacity of the clusters to be strongly size dependent at low temperature. For instance, the cluster with 6 atoms has a high vibrational heat capacity at low temperature, a finding rationalized in terms of structure.     

Surface electronic structure of the organic superconductor κ-(ET)2Cu(NCS)2 studied via photoemission microscopy

The surface electronic structure of cleaved single crystals of the organic superconductor κ-(ET)₂Cu(NCS)₂ has been studied using photoemission microscopy. Two types of cleaved surfaces were observed, displaying different valence band photoemission spectra and different spectral behavior near the Fermi level, E_F. In particular, spectra from one surface type display relatively broad spectral features in the valence band and finite spectral intensity at E_F, while spectra from the other surface type show well-defined valence band emission features and zero photoemission intensity at E_F. We propose that the spectral differences are due to a very short electron mean free path in this material, and our results are used to explain the differences between previously published photoemission spectra from this superconductor. We also report the results of an investigation of the electronic structure of defects in this material.     

Photoemission study of the interaction of Al with a GaAs (110) surface

A room-temperature reaction between Al adsorbate atoms and a GaAs (110) surface is observed by means of soft X-ray photoemission techniques. Evidence of two states of Al which are distinct from the bulk Al metal is seen at submonolayer coverages. The sequential appearance of these states suggest that both Al chemisorbed on the surface and Al replacing Ga in the surface lattice are present. The possible influence of surface lattice reconstruction and Al proximity effects on the replacement reaction is discussed. The replacement reaction is important in the context of metal contacts to GaAs (both ohmic and Schottky barriers) as well as for GaAs—AlGaAs heterojunctions.     

Magnetic behaviour of ultra-thin iron overlayers on palladium (111)

A LEED, Auger photoemission study of ultra-thin overlayers of iron on palladium (111) is reported. At room temperature and below, the overlayer grows epitaxially in a simultaneous multi-layer mode. On annealing a sub-monolayer coverage to create flat two-dimensional platelets it was found that the density of the Fe 3d states at the Fermi level was reduced and the local iron magnetic moment could not be detected.     

Photoemission investigation on the oxidation of Si(111)Ag interfaces and its relation to the interface structure

We give the first photoemission results on the enhancement of Si reactivity to oxygen when a noble metal (Ag) is present. The tunability of synchrotron radiation (SR) has been used to get high surface sensitivity and to take advantage of cross section energy dependence. We show that when one monolayer of Ag is deposited onto Si(111), the exposure to oxygen (30 × 10⁶L) originates the overgrowth of an oxide phase which is basically SiO₂. This indicates that Ag breaks the sp³ configuration of Si atoms with a consequent dramatic increase in the Si reactivity. This behaviour rules out the model of Ag adsorbed on top of Si with an atomically abrupt interface.     

Applications of the ring oven method in enzymatic analysis

The use of the ring oven technique in enzymatic analysis is described. The “segment technique” for catalyzed reactions is applied for the determination of enzymes, inhibitors and substrates. Three different methods have been worked out for this purpose, none of them needs a stable standard scale. These methods are illustrated by various examples: determinations of the enzymes acid and alkaline phosphatase, hyaluronidase, acetylcholinesterase, lipase and alcohol dehydrogenase; indirect determinations of the inhibitors copper, beryllium, molybdenum, the insecticides carbaryl and paraoxon, the herbicides 2, 4-D, mecoprop and bentazcn, and the fungicide phenylmercury acetate; and determinations of the substrates ethanol and glucose.     

An angle-resolved photoemission study of the chemisorption of chalcogens on Cu(100) III. Cu(100) + p(2 x 2)S

Normal emission ARPES data for Cu(100) with a p(2x2)S overlayer were obtained in the photon energy range hv=11 to 34 eV. These spectra have been compared, within a framework proposed previously, with spectra from clean Cu(100). Changes were found in the Cu emission features, which could be explained by the relaxation of momentum conservation perpendicular to the surface in the optical excitation step and by the relaxation of momentum conservation parallel to the surface in the escape step. These changes include a broadening of the d-band peak and the strong attenuation of the sp-band peak. In addition, a prominent feature appears at about ?4.0 eV at normal emission upon sulfur chemisorption. We tentatively attribute its appearance at normal emission to a new surface umklapp process induced by the overlayer. Changes in the spectrum of scattered electrons were related to modifications of evanescent final states. The sulfur 3p_z^? and 3p_x.y-components could be separated at normal emission and are located at ?4.7 and ?5.4 eV, respectively. No dispersion of the sulfur 3p-bands was detected when the $\text{Γ}\text{X}$ and $\text{Γ}\text{M}$ symmetry lines of the surface Brillouin zone were probed. Sulfur-induced emission above the Cu d-bands was observed and attributed to antibonding states. This emission was directed towards the bulk [011] directions.     

Order-disorder effects in GaAs(110)-oxygen interaction: A LEED-ups analysis

Cleaved GaAs(110) surfaces were exposed to oxygen (10⁶ ? 5 × 10¹⁰ L) and analysed by LEED and UPS. Very important changes in the electron diffraction occur at 10⁸ L and the analysis of the LEED pattern shows a bulk-like diffraction superposed on a high background, implying that a thin layer (1–3 Å) disordered by the oxidation covers the GaAs lattice relaxted to its bulk structure.     

pH-induced protonation of lysine in aqueous solution causes chemical shifts in X-ray photoelectron spectroscopy

We demonstrate the applicability of X-ray photoelectron spectroscopy to obtain charge- and site-specific electronic structural information of biomolecules in aqueous solution. Changing the pH of an aqueous solution of lysine from basic to acidic results in nitrogen 1s and carbon 1s chemical shifts to higher binding energies. These shifts are associated with the sequential protonation of the two amino groups, which affects both charge state and hydrogen bonding to the surrounding water molecules. The N1s chemical shift is 2.2 eV, and for carbon atoms directly neighboring a nitrogen the shift for C1s is approximately 0.4 eV. The experimental binding energies agree reasonably with our calculated energies of lysine(aq) for different pH values.     

Phase preparation in steady-state free precession MR elastography

Strain and motion measurements in balanced steady-state free precession (bSSFP) imaging require high magnetic field homogeneity. This requirement is due to the nonlinear signal response to spin phase variations in bSSFP. Here, a technique that utilizes background gradients for preparing strong in-plane spin phase variations is proposed. As a result, periodic patterns of increased motion sensitivity appear, which are interleaved with bands of low phase-to-noise ratio. Spatial filters commonly used in MR elastography (MRE) remove these bands and leave wave images equivalent to a uniform phase response in bSSFP-MRE. Since phase preparation gradients locally enhance motion sensitivity, the technique can be employed for selectively increasing the wave signal amplitude in MRE. The method is applied without the need for previous shimming, which reduces the examination time. In vivo phase prepared bSSFP-MRE is demonstrated in human liver and heart.