Hydrogen cycling of niobium and vanadium catalyzed nanostructured magnesium

The reaction of hydrogen gas with magnesium metal, which is important for hydrogen storage purposes, is enhanced significantly by the addition of catalysts such as Nb and V and by using nanostructured powders. In situ neutron diffraction on MgNb(0.05) and MgV(0.05) powders give a detailed insight on the magnesium and catalyst phases that exist during the various stages of hydrogen cycling. During the early stage of hydriding (and deuteriding), a MgH(1< x < 2) phase is observed, which does not occur in bulk MgH(2) and, thus, appears characteristic for the small particles. The abundant H vacancies will cause this phase to have a much larger hydrogen diffusion coefficient, partly explaining the enhanced kinetics of nanostructured magnesium. It is shown that under relevant experimental conditions, the niobium catalyst is present as NbH(1). Second, a hitherto unknown Mg-Nb perovskite phase could be identified that has to result from mechanical alloying of Nb and the MgO layer of the particles. Vanadium is not visible in the diffraction patterns, but electron micrographs show that the V particle size becomes very small, 2-20 nm. Nanostructuring and catalyzing the Mg enhance the adsorption speed that much that now temperature variations effectively limit the absorption speed and not, as for bulk, the slow kinetics through bulk MgH(2) layers.     

The syntheses of 16a'-homo-leurosidine and 16a'-homo-vinblastine. Generation of atropisomers

The synthesis of 16a'-homo-leurosidine was achieved through enantioselective generation of a ring D'-seco-precursor 33 (without requirement of a chiral auxiliary). Its cyclization provided the N(b')-quaternary salt 35 with a configuration corresponding to the atropisomeric form 8a rather than 8b of the target product. On debenzylation, the amine 8a was obtained and found not to isomerize thermally to the anticipated atropisomer 8b (in contrast to its lower homologue, with its formation of natural leurosidine). However, on protonation, a 1:1 mixture of atropisomers of 16a'-homo-leurosidine was obtained. A synthesis of 16a'-homo-vinblastine provided two atropisomers 5a and 5b for the free base at equilibrium (1:2.3 at room temperature in CDCl(3)), with a shift to the major conformer 5b with increasing solvent acidity or decreasing temperature. The synthesis was achieved through a stereoselective inversion of the tertiary hydroxyl function in the enantioselectively generated C-20' progenitor 39.     

100-kHz diffraction-limited femtosecond laser micromachining

We demonstrate active and adaptive wavefront correction of a 100-kHz amplified femtosecond laser chain, and investigate the subsequent improvement for micromachining applications. Thanks to a non-pixelated liquid-crystal modulator, phase-front distortions are decreased down to /15 peak–valley and /100 rms. Clean 1.7-m diffraction-limited microholes are obtained, both on low-threshold metallic and high-threshold dielectric materials. This high beam quality femtosecond source enables diffraction-limited high-speed micromachining. PACS 87.80.Mj; 42.60.Jf; 42.65.Re     

Spatial mode control of a diode-pumped Nd:YAG laser by use of an intracavity holographic phase plate

We present a new method of realizing phase plates by phase-volume holography on a photopolymer film. We implement such a component in a diode-pumped Nd:YAG oscillator to control the output spatial beam profile. Flattop super-Gaussian and square-shaped beams are obtained.     

Laser mode manipulation by intracavity dynamic holography: Application to mode selection

Intra-cavity dynamic holography is proposed as a self-adaptive, easy to set up technique for efficient laser mode manipulation. A first implementation is presented here as a longitudinal mode selector providing a spectacular spectral narrowing in linear laser cavities operating both in the pulsed and the cw regimes. Received: 12 April 1999 / Published online: 24 June 1999     

Validity of the three-charge-state model in photorefractive BaTiO3:Rh at 1.06 μm in the cw regime

3 :Rh are the only ones involved in the photorefractive effect at 1.06 μm. In this paper, we apply this model to the prediction of the steady-state photorefractive space-charge field. We present experimental measurements of induced absorption and beam-coupling gain in BaTiO₃:Rh at 1.06 μm, which are consistent with the model. From these data, we determine a set of parameters (with error bars) such as the excitation cross sections and density of rhodium. Received: 2 January 1997 / Revised version: 21 March 1997