Characterization of optical properties of Porous Silicon using Photoacoustic Technique

Photoacoustic absorption spectra of the porous silicon samples (P-Si) of different thickness and porosity percentage were measured at different modulation frequency. The absorption edge of the P-Si layer for all samples shows a blue shift from that of crystalline silicon (C-Si) at 1.1 eV. At low modulation frequency the estimated energy gap (1.88 eV) is almost the same for all samples and the PA signal increases as the porosity percentage increases. At the higher modulation frequency, the spectra show an increase in the energy gap indicating the effect of quantum confinement as the porosity increasing. The Raman shifts of the P-Si samples are correlated with the particle size leading to an estimated average particle size. The quantum confinement interpretation of the PA results is in agreement with the Raman measurements that indicate the presence of such nanostructure in the P-Si layer.     

Bacterial iron transport: coordination properties of azotobactin, the highly fluorescent siderophore of Azotobacter vinelandii

Azotobacter vinelandii, a nitrogen-fixing soil bacterium, secretes in iron deficiency azotobactin delta, a highly fluorescent pyoverdin-like chromopeptidic hexadentate siderophore. The chromophore, derived from 2,3-diamino-6,7 dihydroxyquinoline, is bound to a peptide chain of 10 amino acids: (L)-Asp-(D)-Ser-(L)-Hse-Gly-(D)-beta-threo-HOAsp-(L)-Ser-(D)-Cit-(L)-Hse-(L)-Hse lactone-(D)-N(delta)-Acetyl, N(delta)-HOOrn. Azotobactin delta has three different iron(III) binding sites which are one hydroxamate group at the C-terminal end of the peptidic chain (N(delta)-Acetyl, N(delta)-HOOrn), one alpha-hydroxycarboxylic function in the middle of the chain (beta-threo-hydroxyaspartic acid), and one catechol group on the chromophore. The coordination properties of its iron(III) and iron(II) complexes were measured by spectrophotometry, potentiometry, and voltammetry after the determination of the acid-base functions of the uncomplexed free siderophore. Strongly negatively charged ferric species were observed at neutral p[H]'s corresponding to a predominant absolute configuration Lambda of the ferric complex in solution as deduced from CD measurements. The presence of an alpha-hydroxycarboxylic chelating group does not decrease the stability of the iron(III) complex when compared to the main trishydroxamate siderophores or to pyoverdins. The value of the redox potential of ferric azotobactin is highly consistent with a reductive step by physiological reductants for the iron release. Formation and dissociation kinetics of the azotobactin delta ferric complex point out that both ends of this long siderophore chain get coordinated to Fe(III) before the middle. The most striking result provided by fluorescence measurements is the lasting quenching of the fluorophore in the course of the protonation of the ferric azotobactin delta complex. Despite the release of the hydroxyacid and of the catechol, the fluorescence remains indeed quenched, when iron(III) is bound only to the hydroxamic acid, suggesting a folded conformation at this stage, around the metal ion, in contrast to the unfolded species observed for other siderophores such as ferrioxamine or pyoverdin PaA.     

Gas-liquid and gas-liquid-solid catalysis in a mesh microreactor

A microstructured mesh contactor that can offer residence time of more than minutes is used for gas-liquid-solid hydrogenations and gas-liquid asymmetric hydrogenations. Applications for catalyst/chiral inductor screening and for kinetic data acquisition are demonstrated.     

Structure refinements of cadmium substituted lead fluoroapatites by powder X-ray pattern fitting

Solid solutions of cadmium and lead fluoroapatite [Pb_(10−x)Cd_x(PO₄)₆F₂ (0?x?5)] were synthesized by a wet process in a basic medium. Replacement of lead by cadmium induces a linear variation of the crystallographic parameters “a” and “c” according to Végard's law. The cadmium content, as obtained from the refinement, is in agreement with the chemical analysis. The distribution of the lead and cadmium ions between two non-equivalent crystallographic sites, M(1) and M(2), was determined by the Rietveld method. The site-occupancy factors of atoms clearly indicate a preference of cadmium for site M(1) in the apatite structure in agreement with its smaller ionic radii. A progressive shift of the F⁻ ion toward the center of the triangles formed by the site M(2) metals has been observed with increasing cadmium content.     

Strain analysis of a bonded, dissimilar, composite material T-joint using moiré interferometry

High-sensitivity moiré interferometry and finite-element analysis are used to analyze the state of deformation and stress in the region of contact between a plane orthotropic rectangular punch bonded to a foundation with dissimilar elastic properties which models a highly loaded region of a composite material rocket motor casing. Stress distributions are presented for the contact region and an estimate of the maximum shear stress in the foundation is given. The displacement components show good qualitative agreement between analysis and experiment. The lack of quantitative agreement between the experimental and the finite-element analysis is attributed to uncertainty of the material properties.     

Integral Geometry on Discrete Grassmannians in Z n

We study the Radon transform R on the discrete Grassmannian of rank-d affine sublattices of Z ⁿ for 0 <  d <  n, extending and building on previous work of the first- and third-named authors in codimension 1. By analogy with the integral geometry on Grassmannians in R ⁿ , various natural questions are treated, such as definition and properties of R and its dual transform R ^*, function space setting, support theorems and inversion formulas.     

Projectivity of Countable Direct Products

We show that if the direct product of countably many copies of a noetherian ring R is pure in a direct sum of finitely generated modules, then R satisfies the descending chain condition on two-sided ideals. This extends a recent result of Buchweitz and Flenner.     

Palladium–dppb–borate‐catalyzed regioselective synthesis of cinnamate esters by alkoxycarbonylation of phenylacetylene

The regioselective alkoxycarbonylation of phenylacetylene into various cinnamate esters was achieved with a catalyst system formed from palladium (II), 1,4‐bis(diphenylphosphino) butane (dppb) and salicylborate complex in acetonitrile as a solvent. The influence of various parameters on the overall conversion of phenylacetylene and the selectivity of the reaction were studied systematically by varying the type of palladium complex, acids promoter, CO pressure, temperature and the reaction time. This investigation allowed us to obtain the predominant formation of cinnamate esters with excellent selectivity (90–96%). Copyright © 2008 John Wiley & Sons, Ltd.