Periodic solutions of perturbed isochronous hamiltonian systems at resonance

We look for periodic solutions of planar systems obtained by adding an asymptotically positively homogeneous nonlinear term to an isochronous hamiltonian system. Precise computations of the topological degree are obtained by elementary phase-plane analysis.     

Synthesis of Indoles Using Visible Light: Photoredox Catalysis for Palladium‐Catalyzed CH Activation

A combined palladium‐ and photoredox‐catalyzed C? H olefination enables the synthesis of indoles. By using visible light, the direct C? H activation of aromatic enamines can be achieved and a variety of indole derivatives can be obtained in good yields under mild reaction conditions.     

The linkage between macroscopic gettering mechanisms and electronic configuration of 3d-elements in p/p+ epitaxial silicon wafers

We have measured the gettering efficiencies for Cr, Mn, Fe, Co, Ni and Cu in p/p+ epitaxial wafers. The gettering test started with a reproducible spin-on contamination on the front side of the wafers in the 10¹²–10¹⁴ atoms/cm² range, followed by thermal treatment to redistribute the metallic impurities in the wafer. The gettering efficiencies were measured by a novel wet chemical stratigraphic etching technique in combination with inductively-coupled plasma mass spectrometry. The residual bulk metal contamination was also measured by this method. This procedure led to global distributions of the 3d elements on the wafer’s front side, in the bulk of the wafer and on the wafer’s back side. Recovery rates were found to be 34%, 2.3%, 100%, 85%, 100% and 100% for Cr, Mn, Fe, Co, Ni and Cu, respectively. An impurity segregation effect in the wafer bulk was measured for Cu (100%) and Cr (34%), while no detectable segregation-induced gettering mechanisms were detected for the other elements in the applied concentration range. The segregation-induced gettering mechanisms were interpreted from the electronic structure of the metallic impurities. For segregation gettering by increased solubility in p+ silicon, the metallic species must form donors. Only Cu⁺ (3d ¹⁰) and Cr⁺ (3d ⁵) can form singly positively charged species that exhibit a spherical electronic distribution. It is well known from spinell structures that 3d ¹⁰ and, to a smaller extent 3d ⁵, are stable configurations in tetrahedral structures like the silicon lattice. Thus, we link the segregation-induced gettering mechanism in p/p+ epitaxial wafer to the electronic configuration of the 3d elements. Received: 19 January 2001 / Accepted: 31 January 2001 / Published online: 20 June 2001     

Ni reactions with surfaces: dependence of gettering efficiencies for Ni on crystal-growth conditions, back-side-gettering techniques, oxygen precipitates and thermal treatments

We have performed measurements on the gettering efficiencies for Ni in different silicon wafers. Gettering efficiencies were measured of wafers grown by different crystal-growth techniques, such as Czochralski-grown (CZ) and floating zone (FZ), as well as wafers containing crystal-originated particles (COPs) of different size and density. Lightly boron doped CZ wafers covered with an epitaxial layer were also evaluated. In another set of experiments, we compared different back-side-gettering techniques, like poly-silicon, stacking faults and He-implanted back sides and the dependence of back-side gettering on cooling rate and contamination level. Internal surfaces of oxygen precipitates were also investigated. The gettering test started with a reproducible spin-on contamination in the range around 10¹² atoms/cm² and was followed by a thermal treatment to redistribute the Ni impurity in the wafer. Subsequently, wafers were analyzed for their surface and bulk contamination by a novel layer-by-layer etching, stratigraphical technique in combination with inductively coupled plasma mass spectrometry. No detectable gettering effect of COPs was found. FZ wafers differed remarkably in their gettering behavior from CZ wafers, obviously due to differences in aggregated self-point defects. Most remarkably, the deposition process of an epitaxial layer changed the gettering behavior of p/p- wafers. Comparing the gettering efficiencies of different back sides, an extraordinarily high gettering efficiency of He-implanted voids can be anticipated, which was higher than the gettering efficiency of poly-silicon and stacking faults. High cooling rates at the end of the drive-in cycle and low contamination levels lowered the gettering efficiencies of back-side-gettering techniques, suggesting a diffusion-limited gettering process. Based on the dependence of the gettering efficiencies on different drive-in cycles, a surface reaction as a mechanistic initiation of the drive-in must be assumed. Oxygen precipitates exhibited a high gettering effect for Ni contamination. All experimental results are interpreted by available active surfaces in the gettering phases. Received: 30 May 2001 / Accepted: 16 June 2001 / Published online: 30 August 2001     

Oligo and polyuronic acids interactions with hypervalent chromium

Selective oxidation of galacturonic residues of oligo and polyuronic acids by Cr^VI affords CO₂/HCO₂H, oxidized uronic acid, and Cr^III as final redox products. Kinetic studies show that the redox reaction proceeds through a mechanism combining Cr^VI → Cr^IV → Cr^II and Cr^VI → Cr^IV → Cr^III pathways. The mechanism is supported by the observation of free radicals, CrO₂²⁺ and Cr^V as reaction intermediates. The EPR spectra show that five- and six coordinated oxo-Cr^V intermediates are formed. Penta-coordinated oxo-Cr^V species are present at any [H⁺], whereas hexa-coordinated ones are only observed at pH <1. At low pH Cr^V predominating species are coordinated by carboxylate groups and O^ring (g_iso = 1.9783/5). At pH 7.5, the predominating ones are those coordinated by alcoholate groups of the ligand (g_iso = 1.9800). Polygal can reduce Cr^VI and efficiently trap Cr^III. This behaviour represents an interesting model for the study of biomaterials, which possess a high proportion of polygal, in order to remove chromium from polluted waters.     

Photoproduction of η -mesons off protons

Photoproduction of η -mesons was studied with the Crystal-Barrel detector at ELSA for photon energies in the range from 0.75 to 3GeV. Total and differential cross-sections are presented. The η mesons are detected in two decay modes, in η → 2γ and in η → 3π⁰ → 6γ . The cross-sections vary slowly as functions of energy and η production angle suggesting that only few resonances make significant contributions to the cross-section. For photon energies above 1.5GeV, the differential cross-sections show a strong forward peak due to meson exchanges in the t channel. A comparison of the η -photoproduction cross-sections with SAID and MAID as well as with a partial-wave analysis performed on our data in combination with other data sets shows good agreement. The Bonn-Gatchina partial-wave analysis suggests that the largest contribution to η photoproduction for energies below 3GeV proceeds via three resonances, the well-known N(1535)S ₁₁ , via N(1720)P ₁₃ , and a newly suggested N(2070)D ₁₅ .     

The Structure of the Aluminum Fumarate Metal–Organic Framework A520

The synthesis of the commercially available aluminum fumarate sample A520 has been optimized and its structure analyzed through a combination of powder diffraction, solid‐state NMR spectroscopy, molecular simulation, IR spectroscopy, and thermal analysis. A520 is an analogue of the MIL‐53(Al)‐BDC solid, but with a more rigid behavior. The differences between the commercial and the optimized samples in terms of defects have been investigated by in situ IR spectroscopy and correlated to their catalytic activity for ethanol dehydration.     

Universal Approach Towards r‐Hirudin Derivatives with High Anti‐Thrombin Activity Based on Chemical Differentiation of Primary Amino Groups

Chemical modification of recombinant hirudin (r‐hirudin) is necessary whenever surface‐confinement to a biomaterial or biotinylation for subsequent conjugation with carriers is intended. Here, we report a modification strategy that permits chemical discrimination between r‐hirudin's amino groups and preserves its thrombin inhibitor activity. By reaction with Msc‐ONSu, protective groups were successively introduced in r‐hirudin yielding four derivatives (Msc)_x‐hirudin (1 ≤ x ≤ 4) and pure fractions were isolated by ion exchange chromatography. Structure–function relationships were studied for all derivatives and revealed a decrease in activity of more than 90% as compared to unprotected r‐hirudin. MALDI‐TOF MS was used to determine the locations of the Msc groups. Furthermore, evidence was provided that r‐hirudin's N‐terminal amino group is highly important for its anti‐thrombin activity. Selective modification of the lysine residues which maintained the free N‐terminal amino group preserved the anti‐thrombin activity of r‐hirudin even after biotinylation and subsequent linkage to streptavidin or confinement to a polymer surface.     

Routine analysis of ultra pure water by ICP-MS in the low- and sub-ng/L level

The chemical analysis with inductively coupled plasma-mass spectrometry (ICP-MS) can help to examine the purity of ultra pure water (UPW) down to 10 part per trillion (ng/L) and lower. For a proper determination of a high number of samples per week the analysis must be divided into two parts: the routine analysis and the reference water analysis. The routine analysis is done by direct measurement of the ultra pure water samples. Applying a standard addition method under particular clean conditions, the reference water analysis leads to the definition of the accurate zero. A quick evaluation scheme is also presented for the reference water analysis. The method is tested for its fitness for application by examining LOD (for relevant element < 2 ng/L), reproducibility and linearity of calibration. The ICP-MS was optimized according to the methodology of G. Taguchi to improve reproducibility and LOD. Received: 16 June 1999 / Revised: 2 August 1999 / Accepted: 6 August 1999