Structure of the Ag on Si(111) 7 × 7 interface by means of surface exafs

Polarization dependent surface extended X-ray absorption fine structure (SEXAFS) measurements are used to determine the structure of the Ag on Si(111)7 × 7 system at the early stages (< 3 monolayers (ML)) of interface formation. At room temperature (RT) Ag is found to initially (< 0.5 ML) chemisorb in the threefold hollow site, approximately 0.7 Å above the outermost Si layer with an average Ag-Si distance of 2.48±0.05 Å. Above monolayer coverage the SEXAFS spectrum is dominated by the Ag-Ag distance indicating Ag island formation on the surface. Upon heating (200 ?T? 600°C) a (√3 ×√3)R30° LEED pattern is observed. At the lowest coverage ( < 0.7 ML) this pattern is determined to arise from Ag atoms which are embedded in the threefold hollows, ~ 0.7 Å below the first and above the second Si layer, with a Ag-Si distance of 2.48 ± 0.04 Å. At higher coverage ($\text{\$}\text{?}$1 ML) Ag clusters are found to grow on this interface with the same Ag-Ag distance as in Ag metal. Our results are discussed in the context of previous experimental and theoretical results.     

Bestimmung einiger Aminosäuren unter Verwendung der Ringofenmethode

Zusammenfassung Die mit Hilfe der Ringofensegmenttechnik im Nanogrammbereich durchgeführten Bestimmungen der Aminosäuren Tryptophan, Prolin, Hydroxyprolin, Arginin, Histidin, Tyrosin, Cystein und Cystin wurden beschrieben. Die Fehler liegen innerhalb der bei der Ringofenmethode üblichen Grenzen.Die bereits früher entwickelte Segmenttechnik dient zur Bestimmung von Substanzen in all jenen Fällen, bei denen das Reaktionsprodukt einer zeitlichen Veränderung unterworfen ist. Dabei ist keine haltbare Standardskala notwendig.

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Determination of some amino acids by means of the king oven

Summary Determinations of the amino acids tryptophane, proline, hydroxyproline, arginine, histidine, tyrosine, cysteine and cystine in the nanogram range with the aid of the ring oven segment technique are described. The range of error of these determinations is about the same as usual in ring colorimetry.The segment technique already earlier described serves for the determination of substances in cases, where the product of reaction is unstable; this is possible because a stable standard scale is not necessary.

     

Sn-containing liquid crystalline side group homopolymers with two glass transitions

Three homologous tin-containing homopolymers with a terminal CN-dipole in the side group have been synthesized and characterized by dynamical calorimetry, polarization microscopy, X-ray and dielectric methods. AFM was used to evaluate the texture at room temperature. Four different phase transitions were detected by DSC. The high temperature phases were identified by polarization microscopy as SmA and SmC. AFM-measurements show focal-conic domains at room temperature and confirm so the smectic nature of all phases. X-ray measurements on nonoriented samples give hints to a phase segregation on nanometer scale. Dielectric investigation and temperature-modulated DSC (TMDSC) confirm clearly phase separation by appearance of two glass transitions related to the liquid order of the main chains and the liquid crystalline of the side groups.     

Determination of the xenoestrogens 4-nonylphenol and bisphenol A by high-performance liquid chromatography and fluorescence detection after derivatisation with dansyl chloride

An easily performable and highly selective method for the determination of the xenoestrogens bisphenol A (BPA) and technical 4-nonylphenol (mixture of isomers) from environmental samples was developed. The method consists of fluorigenic labelling of the substances by dansylation followed by HPLC separation of the derivatives. Specific wavelengths (lembda(ex)=354 nm, lambda(em)=545 nm) for detection of the dansylated phenols were determined in order to reduce the signals of interfering compounds. The applicablility of the method for environmental samples was demonstrated by using sewage sludge spiked with BPA and 4-n-nonylphenol (as internal standard).     

Photoenhancement mechanism for oxygen chemisorption on GaAs(110) using visible light

Visible light from an argon ion laser (514.5 nm, 3 W/cm²) is seen to increase oxygen chemisorption on cleaved GaAs(110) surfaces up to a final coverage between one and two monolayers. Using photoemission spectroscopy to measure the oxygen coverage after simultaneous exposure of the surface to oxygen and light, we have determined that oxygen uptake for photoenhanced exposures is independent of sample temperature and doping type. In addition, significantly less enhancement is seen for weakly bound oxidizing molecules (N₂O) relative to the effects with molecular oxygen. These results are explained by a photoenhancement mechanism in which energy is released in a surface recombination event, possibly in the form of nonthermal phonons, causing physisorbed gas molecules to dissociate and thereby overcoming a major rate limiting step of the reaction in the dark. This reaction mechanism is supported by calculations of the surface recombination rates and free carrier densities at the surface which show that only the recombination rate is correlated with enhanced oxygen uptake. Other mechanisms and experimental data are also discussed.     

Surface core level shifts on InP(110): Use of Sb overlayers

The surface core level shift of the P2p from a clean InP(110) surface has been studied by use of the Sb overlayer. It is shown that the nonreactive Sb overlayer removes the surface core level shift and does not introduce any other component. The unique possibility to separately characterize the bulk component allows a precise determination of the surface core level shift. We show that the surface P2p core level is shifted to lower binding energy by −0.29 eV, while an opposite and nearly equal in magnitude +0.30 eV shift is established for the In4d.     

Magnetic resonance elastography and diffusion-weighted imaging of the sol/gel phase transition in agarose

The dynamics of the sol/gel phase transition in agarose was analyzed with magnetic resonance elastography (MRE) and diffusion-weighted imaging, providing complementary information on a microstructural as well as on a macroscopic spatial scale. In thermal equilibrium, the diffusion coefficient of agarose is linearly correlated with temperature, independent of the sol/gel phase transition. In larger agarose samples, the transition from the sol to the gel state was characterized by a complex position and temperature dependency of both MRE shear wave patterns and apparent diffusion coefficients (ADC). The position dependency of the temperature was experimentally found to be qualitatively similar to the behavior of the ADC maps. The dynamics of the temperature could be described with a simplified model that described the heat exchange between sol and gel compartments. The experiments supported the approach to derive temperature maps from the ADC maps by a linear relationship. The spatially resolved dynamics of the temperature maps were therefore employed to determine the elasticities. For this reason, experimental MRE data were simulated using a model of coupled harmonic oscillators. The calculated images agreed well with the experimentally observed MRE wave patterns.     

Photoemission studies of the energy dependence of the bulk plasmon loss intensity in Si and Al

Photoemission, using synchrotron radiation in the photon energy range 100 to 600 eV, has been utilized to study the energy dependence of the bulk plasmon loss intensity in Si and Al. The results are compared to the predictions of an extrinsic model for plasmon creation.