Absolute efficiency and dispersion of Raman scattering by phonons in silicon

We have measured the absolute Raman efficiency for first order scattering of the optical zone center phonon with two different incident photon energies (1.16 and 0.94 eV) by comparison with the Raman scattering from diamond. A value significantly lower than the experimental data published previously in this region has been found. For the first order scattering no dispersion of the scattering efficiency has been observed at the indirect gap, in contrast to the second order scattering which shows a pronounced resonance behaviour in this energy range.     

Polarization of hot photoluminescence in GaAs/AlAs superlattices

The polarization characteristics of hot photoluminescence in GaAs/AlAs superlattices are investigated experimentally and theoretically. It is shown that the formation of an electronic miniband in the superlattice substantially changes the polarization characteristics of the photoluminescence. As a result of the quasi-three-dimensional character of the motion of hot electrons in the superlattice, the polarization depends on the ratio of the electron kinetic energies in the plane of the superlattice and along the axis of the superlattice. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 4, 285–289 (25 February 1996)     

X-ray standing wave analysis of the effect of isotopic composition on the lattice constants of Si and Ge

The x-ray standing wave (XSW) technique is used to measure the isotopic mass dependence of the lattice constants of Si and Ge. Backreflection allows substrates of moderate crystallinity to be used while high order reflection yields high accuracy. The XSW, generated by the substrate, serves as a reference for the lattice planes of an epilayer of different isotopic composition. Employing XSW and photoemission, the position of the surface planes is determined from which the lattice constant difference Deltaa is calculated. Scaled to DeltaM = 1 amu we find (Deltaa/a) of -0.36x10(-5) and -0.88x10(-5) for Ge and -1.8x10(-5) and -3.0x10(-5) for Si at 300 and 30 K, respectively.     

Lanthanum-pyBOX complexes as catalysts for the enantioselective conjugate addition of malonate esters to β,γ-unsaturated α-ketimino esters

Abstract

In this paper, we report the application of chiral complexes of La(III) with pyBOX ligands as Lewis acid catalysts in the conjugate addition of malonic esters to N-tosyl imines derived from β,γ-unsaturated α-keto esters to give the corresponding chiral α,β-dehydroamino esters. pyBOX complexes with La(III), Yb(III), Sc(III), and In(III) triflates were assessed in this reaction but only La(III) showed good activity and enantioselectivity, while Yb(III) provided the expected product with low yield and stereoselectivity, and the Sc(III) and In(III) complexes were completely inactive. The complex of La(OTf)₃ with the diphenyl-pyBOX ligand prepared in situ provided the best results and allowed obtaining chiral α,β-dehydroamino esters 3 with excellent yields, E:Z diastereomeric ratios (29:71–99:1) and high enantiomeric excesses (20–95%). The reaction could be applied to imines having a substituted aromatic ring or a heterocycle attached to the double bond, although the presence of electron-withdrawing groups on the aromatic ring was detrimental for stereoselectivity. The reaction products were obtained with the S configuration at the stereogenic center and the Z configuration at the enamine double bond as determined by NOESY experiments and X-ray analysis. Based on the experimental results a stereochemical model involving a nine-coordinate La(III) species has been proposed.     

Finite element modeling of welding processes

This work deals with the simulation of fusion welding by the Finite Element Method. The implemented models include a moving heat source, temperature dependence of thermo-physical properties, elasto-plasticity, non-steady state heat transfer, and mechanical analysis. The thermal problem is assumed to be uncoupled from the mechanical one, so the thermal analysis is performed separately and previously to the mechanical analysis at each time step. The mechanical problem is based on the thermal history. A special treatment is performed on mechanical elements during the liquid/solid and solid/liquid phase changes to account for stress states. The three-dimensional stress state of a butt-welded joint is obtained as an example of an application.     

cis-Glyco-fused benzopyran compounds as new amyloid-β peptide ligands

A small library of glyco-fused benzopyran compounds has been synthesised. Their interaction features with Aβ peptides have been characterised by using STD-NMR and trNOESY experiments. The conformational analysis of the compounds has also been carried out through molecular mechanics (MM) and molecular dynamics (MD) simulations.     

Half-life measurements of 137, 139Cs excited nuclear states

A fast-timing setup has been developed to measure the nuclear-state half-lives of neutron-rich nuclei. The first on-line measurements were performed on ^{137, 139}Cs obtained from the β decay of ^{137, 139}Xe. These neutron-rich Xe nuclei were produced by the ²³⁸U photofission induced by the 50MeV electron beam delivered by the ALTO facility. The half-lives of the first excited state in ¹³⁷Cs and of six excited states in ¹³⁹Cs were measured for the first time. Taking into account new information given by the deduced transition probabilities, we discuss the structure of the first excited states in ^{137, 139}Cs : it is shown that, in both nuclei, the first excited $\tfrac{5} {2}^ +$ level corresponds mainly to the one-proton state issued from the π2d _5/2 orbital and that the most probable spin values for the states located at 289.8, 393.5, and 393.8keV in ¹³⁹Cs are $\tfrac{5} {2}^ +$ , $\tfrac{3} {2}^ +$ , and $\tfrac{1} {2}^ +$ , respectively.     

Synthesis of a New β-Galactosidase Inhibitor Displaying Pharmacological Chaperone Properties for GM1 Gangliosidosis

GM1 gangliosidosis is a rare lysosomal disease caused by the deficiency of the enzyme β-galactosidase (β-Gal; GLB1; E.C. 3.2.1.23), responsible for the hydrolysis of terminal β-galactosyl residues from GM1 ganglioside, glycoproteins, and glycosaminoglycans, such as keratan-sulfate. With the aim of identifying new pharmacological chaperones for GM1 gangliosidosis, the synthesis of five new trihydroxypiperidine iminosugars is reported in this work. The target compounds feature a pentyl alkyl chain in different positions of the piperidine ring and different absolute configurations of the alkyl chain at C-2 and the hydroxy group at C-3. The organometallic addition of a Grignard reagent onto a carbohydrate-derived nitrone in the presence or absence of a suitable Lewis Acid was exploited, providing structural diversity at C-2, followed by the ring-closure reductive amination step. An oxidation-reduction process allowed access to a different configuration at C-3. The N-pentyl trihydroxypiperidine iminosugar was also synthesized for the purpose of comparison. The biological evaluation of the newly synthesized compounds was performed on leucocyte extracts from healthy donors and identified two suitable β-Gal inhibitors, namely compounds 10 and 12. Among these, compound 12 showed chaperoning properties since it enhanced β-Gal activity by 40% when tested on GM1 patients bearing the p.Ile51Asn/p.Arg201His mutations.