Generation of powerful filaments at a long distance using adaptive optics

Because of its extraordinary properties (intensity clamping, white light source, km long plasma channels), filamentation represents an ideal candidate for remote sensing. However, the promising generation of filaments at long distances for such purposes remains an issue.

We propose a specially designed focusing telescope to properly deliver the laser pulses at long distance and generate powerful filaments. The telescope includes a deformable mirror (DM) that corrects the wavefront’s aberrations working in a closed loop system with a wavefront sensor (WFS). Using this setup we are able to generate extraordinarily strong nitrogen signals at a distance as far as 90 m using 40 mJ laser pulses. Compared to the chirped based filament control technique, we believe that an appropriate control of the reservoir like what we have achieved will contribute to reduce the energy of the required laser pulses and at the same time, the cost of the required laser system.     

Interplay of anions and ligands on the nature and reducibility of NiOx/Al2O3 catalysts prepared by impregnation

When NiOx/Al2O3 catalysts (Ni wt% = 1.5) are prepared by impregnation using [NiL2(H2O)2]X2 as precursors (L = diamine, X = Cl- or NO3-), a supported oxidic or metallic phase can be selectively obtained after thermal treatment in N2 depending on the nature of the ligand and counter anion; the oxidic phase can be reduced at a lower temperature than the classical nickel aluminate phase obtained from [Ni-(H2O)6](NO3)2.     

Sorption of Cu(II) onto vineyard soils: macroscopic and spectroscopic investigations

The sorption of Cu on five vineyard soils was examined via macroscopic and spectroscopic investigations. The composition of the soils was previously determined using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). X-ray absorption spectroscopy (XAS) was employed to determine the metal environment with regard to the identity and interaction of the nearest atomic neighbors, the bond distances, and the coordination numbers. The five soils present similar sorption properties and there is no XAS evidence that the nature of the soil samples affects the local chemical environment of Cu(II). The kinetics of the Cu sorption reactions is rapid, with the equilibrium loading of Cu on the surface achieving approximately 200 mumol g(-1), i.e., 12.7 mg g(-1). The XAS data indicate that Cu is adsorbed in the form of inner-sphere complexes with first shell CuO parameters of four equatorial CuO bonds equal to 1.93 A and two axial CuO bonds at 2.43 A. This is in accordance with a Jahn-Teller distorted octahedron environment around copper. Our results provide evidence of the complexation of Cu(II) onto soil organic matter coated with an inorganic surface (quartz, clay, and goethite).     

Transformations of gamma-alumina in aqueous suspensions 1. Alumina chemical weathering studied as a function of pH

Hydration of gamma-Al2O3 is often reported to occur via the superficial transformation of the alumina surface into aluminum hydroxide-like layers. However, very little evidence has been given so far to support this hypothesis. It is demonstrated here by X-ray diffraction, TEM, electron diffraction, and solubility studies that a second process of hydration takes place that involves the dissolution of alumina and subsequent precipitation of well-shaped Al(OH)3 particles from supersaturated alumina aqueous solution. This process can be observed on a macroscopic scale (XRD, TEM) for any pH5, provided that the contact time between alumina and water exceeds 10 h. The least thermodynamically stable phase of aluminum hydroxide, bayerite, becomes favored compared with gibbsite when the pH of the solution is increased. It is assumed that the rate of formation of bayerite germs is greater than that of gibbsite due to variations in aluminum speciation in solution as a function of pH.     

Hydrodeoxygenation of Furfural over Unsupported,SiO2-supported or Metal-promoted Mo Carbides: Tunning the Selectivity between 2-Methylfuran and C10 Furoins Diesel Precursors

The hydrodeoxygenation of furfural (FF) over Mo carbides in liquid phase at 200 °C, 30 bar of H₂ for 4 h in 2-butanol was investigated. Unsupported and SiO₂-supported Mo carbide with different crystallographic phases (β-Mo₂C/SiO₂ and α-MoC/SiO₂), and in the presence of Cu and Ni as promoters were studied. The reactivation treatment under H₂ atmosphere of the passivated Mo carbides was investigated by XAS. The results show that Mo is present in different states of oxidation in the passivated catalysts, with more severe oxidation in the bimetallic systems, in which the original carbides are not restored after reactivation. Finally, the product distribution in the HDO of furfural is modified as a function of catalyst oxidation degree. Using the less oxidized Mo carbide (β-Mo₂C), a higher yield to 2-methylfuran is obtained, while C₁₀ condensation products are formed for the more oxidized catalysts.     

Conséquences de l'agrégation et de la percolation de charges sur le comportement viscoélastique de nanocomposites

The aim of the present Note is to differentiate the relative contributions of the aggregation and the percolation phenomena to the viscoelastic behaviour of composites. This separation is performed through the development of a micromechanical model, by taking into account the percolation concept. To cite this article: P. Mélé et al., C. R. Mecanique 333 (2005).     

Femtosecond filament induced birefringence in argon and in air: Ultrafast refractive index change

The birefringence of a linearly polarized femtosecond laser filament in gases has been previously established. In this work, we report the time-dependent refractive index measurements of the filament based upon the spectral modulation of a weak probe pulse in air and in argon gas at 1 atmospheric pressure. The polarization dependence of the refractive index modulation induced by the delayed molecular alignment and by the electronic Kerr effect is highlighted. A numerical simulation of the refractive indices, which takes into account the molecular alignment, the electronic Kerr non-linearity and the plasma, is in good agreement with the measurements.     

New developments in frequency domain optical tomography. Part II: Application with a L-BFGS associated to an inexact line search

Gradient-based algorithms in optical tomography have shown efficiency and robustness due to their self-regularization effect, as they track the largest errors first, which tends to stabilize the reconstruction scheme. This second part shows the application of the presented developments in the first part for the reconstruction of optical properties in complex geometry while taking into account the collimated source direction with such an algorithm. The reconstruction scheme is based on the limited memory BFGS type associated to an inexact line search in order to avoid numerous evaluations of the objective function. Normalization of the objective function with measurements and independent scaling of its gradient are used to improve the quality of the reconstruction. The results show that the algorithm is efficient compared to other solvers with a better recovering of both the absorption and scattering coefficients.