Necessary contact conditions for charge injection development into KBr single crystals

We have studied the current vs time behaviour of KBr single crystals by applying a constant voltage between a pointed cathode and a flat anode at constant temperature. The obtained curves are characterized by three or four different zones. We report here very detailed results on the first one. We conclude that in this zone the current through the crystal is purely ionic. As a consequence of this ionic transport, K atoms accumulate at the pointed cathode to form a new electrode. This acts as an injecting contact for electronic current. Electron injection currents and, therefore, colour center clouds that developed upon the formation of this new injecting contact, mark the start of zone II of the current vs time curve. Simple equations are proposed to explain the kinetics of zone I. Experiments by other authors are also analyzed. Finally, a discussion on the simpler experimental conditions required to create the new injecting contact is presented. Results on zones II and III will be published in the near future.     

Analytical properties of 2-oximinodimedone dithiosemicarbazone

The synthesis and analytical properties of 2-oximinodimedone dithiosemicarbazone are described.     

Non-aqueous fluorimetric determination of scandium in silicate rocks

A method for the fluorimetric determination of scandium with 1,2,7-trihydroxyanthraquinone in dimethylformamide medium is described. The calibration graphs obtained by the normal, synchronous, and synchronous first and second derivative techniques are linear between 12 and 225 ng/ml, and the detection limit is 2 ng/ml. The method is applied to the determination of Sc(III) in two simulated and two naturally occurring rocks.     

Mise en evidence de la chiralité de l'édifice spirophosphoranique

The chirality of the spirophosphorance skeleton of the recemic mixture 2a , 2ā , is revealed by the NMR chiral shift reagent.     

Improvement of the PLD process assisted by RF plasma for AlN growth

The pulsed-laser-deposition (PLD) method is particularly well suited for the growth of oxide thin films, but in the case of other compounds, such as nitrides, PLD presents some limitations which are mainly due to the low reactivity of nitrogen in comparison with oxygen. A possible way to overcome this problem is to increase the reactivity of the constituent species, via plasma assisted-pulsed-laser deposition. A plasma is coupled to the ablation chamber, in order to increase the density of reactive atomic species, which can be further incorporated in the growing film. This approach is described in this paper as well as the nature, energy, and concentration of the atomic and molecular species in the plasma as determined by various plasma diagnostics. These results are correlated to the growth of thin films in the particular case of the aluminum nitride compound. The composition and structure of the films are studied as a function of the growth conditions, and the positive effects of the additional discharge are evidenced on the film purity and properties. The fundamental problem with the PLD technique, especially with metallic targets, is the production of unwanted droplets that significantly worsen the properties of the films. To eliminate these droplets, a thin film has been grown with an experimental setup using two targets and crossed laser beams which gave positive results. PACS 81.15.Fg; 52.80.Pi; 77.84.Bw     

Nuclear charge radii of 8,9Li determined by laser spectroscopy

The 2s-->3s transition of (6,7,8,9)Li was studied by high-resolution laser spectroscopy using two-photon Doppler-free excitation and resonance-ionization detection. Hyperfine structure splittings and isotope shifts were determined with precision at the 100 kHz level. Combined with recent theoretical work, the changes in the nuclear-charge radii of (8,9)Li were determined. These are now the lightest short-lived isotopes for which the charge radii have been measured. It is found that the charge radii monotonically decrease with increasing neutron number from 6Li to 9Li.     

Synthesis and structure determination of bimetallic Au/Cu nanoparticles

The synthesis of bimetallic nanoparticles has become so important in recent times due to its multi-faceted applications. The structure of the synthesised particles influences directly their properties. In this paper, we report the synthesis of Au/Cu nanoparticles by a simultaneous reduction method, considering three different molar concentrations (AuCu, AuCu₃ and Au₃Cu) of the components. In order to determine the size and structure of the obtained clusters, the particles were examined by optical spectroscopy, transmission electron microscopy and high-resolution transmission electron microscopy. One of the major factors, the minimum bimetal formation energy, was calculated by an embedded-atom method. The relation between the Au/Cu proportions has been demonstrated to affect the size of the particles and the corresponding structures. From the basic structural analysis it is found that the particles were fcc-like, multiple twin or multiple defect particles for the different Au/Cu concentrations. The smallest particles were identified for the Au/Cu sample with 50% of each, and the most crystalline structures were also obtained in the same sample. PACS 61.46.+w; 61.16.-d; 31.15.Qg     

S-alkyl dithioformates as 1,3-dipolarophiles. Generation of C(2)-unsubstituted penems

S-Alkyl dithioformates, generated by a cycloreversion process, react as 1,3-dipolarophiles with beta-lactam-based azomethine ylids to provide, after (net) elimination of MeSH, C(2)-unsubstituted penems. The overall cycloreversion/cycloaddition sequence was accelerated by microwave irradiation. [reaction: see text]     

Fast and heteroclinic solutions for a second order ODE related to Fisher-Kolmogorov’s equation

We study the existence of fast and heteroclinic solutions of an ODE that arises in connection with travelling wave solutions of Fisher-Kolmogorovs equation. They are obtained by solving a minimum or constrained minimum problem. A variational characterization of the minimum speed for which heteroclinic solutions exist is obtained.Received: 14 November 2003, Accepted: 29 January 2004, Published online: 2 April 2004M. Arias: Supported by MCYT, Acción Integrada HP2000-0039, SpainJ. Campos: Supported by MCYT, BFM2002-01308 and Acción Integrada HP2000-0039, SpainA.M. Robles-Pérez: Supported by Acção Integrada E-15/01 and Fundaçâo para a Ciência e a Tecnologia, Portugal     

Coherent beam combining and phase noise measurements of ytterbium fiber amplifiers

Phase noise characterization of a 10-W Yb fiber amplifier is presented, and we demonstrate phase locking of two fiber amplifiers with near-perfect fringe contrast at power levels of as much as 10 W per fiber. Coherent beam combining is maintained during the turn-on transient as well as in thermal steady-state operation.