NMR investigations of hexamethyldisilane confined in controlled pore glasses: Pore size distribution and molecular dynamics studies

In this work, the melting-point depression and molecular dynamics of hexamethyldisilane confined within five controlled pore glasses, with mean diameters ranging from 7.9 to 23.9 nm, are studied by high-field (9.4 T) nuclear magnetic resonance (NMR), and the results are discussed with reference to the bulk substance. The melting-point depression in pores with radiusR follows the simplified Gibbs-Thompson equation ΔT=k _p/(R−s) with ak _p value of 74 K · nm and ans value of 1 nm. To our knowledge, this is the first time thek _p value of hexamethyldisilane is reported. Proton spin-lattice relaxation times (T ₁), spin-spin relaxation times (T ₂), and diffusivities (D) are reported as a function of temperature. The confinement in the pores gives rise to substantial changes in the molecular dynamics and the phase behavior. The line-shape measurements reveal a two-phase system assigned to a relatively mobile component at the pore walls and a crystalline solid at the center of the pores. However, theT ₂ measurements show that the mobile phase also embraces a minor component attributed to nonfrozen liquid in pockets or micropores. The diffusivity of the major narrow-line component is approximately three orders of magnitude larger than that in the plastic bulk phase, reflecting fast diffusion of mobile molecules. Below the melting region,T ₁ of the narrow line is significantly shorter thanT ₁ of the broad line, suggesting that the molecular reorientation is more hindered close to the surface than at the center of the pore.     

Polynomial Reproduction in Subdivision

We study conditions on the matrix mask of a vector subdivision scheme ensuring that certain polynomial input vectors yield polynomial output again. The conditions are in terms of a recurrence formula for the vectors which determine the structure of polynomial input with this property. From this recurrence, we obtain an algorithm to determine polynomial input of maximal degree. The algorithm can be used in the design of masks to achieve a high order of polynomial reproduction.     

On Non-Continuous Dirichlet Processes

We introduce here some Itô calculus for non-continuous Dirichlet processes. Such calculus extends what was known for continuous Dirichlet processes or for semimartingales. In particular we prove that non-continuous Dirichlet processes are stable under C ¹ transformation.     

Polarized Lidar Reflectance Measurements of Vegetation at Near-Infrared and Green Wavelengths

There is growing interest in the use of lidar for remote sensing of vegetation owing to the emergence of reliable and rugged lasers and highly sensitive detectors. Lidar remote sensing has a distinct advantage over conventional techniques in vegetation remote sensing due to its capability for three-dimensional characterization of vegetative targets. The Multiwavelength Airborne Polarimetric Lidar (MAPL) system was developed primarily for vegetation remote sensing applications from an airborne platform of up to 1,000 -m altitude. The lidar system has full waveform capture and polarimetric measurement capability at two wavelengths in the near-infrared (1064 nm) and the green (532 nm) spectral regions. This study presents preliminary ground-based lidar reflectance measurements on a variety of deciduous and coniferous trees under fully foliated conditions with a view towards tree species discrimination. Variations in the reflectance characteristics of selected deciduous trees under unfoliated and fully foliated conditions were also investigated. Our study reveals distinct differences in the reflectance characteristics of various trees.     

The g-values of defects in hydrogenated microcrystalline silicon

The anisotropic g-values of defects in hydrogenated microcrystalline silicon prepared by hot-wire chemical vapour deposition have been measured as a function of crystalline volume fraction at room temperature. The defect has been identified as a silicon-dangling bond existing on the surface of crystalline grain. Their anisotropic g-values are discussed in the light of theoretical calculations by Ishii et al. and Ishii and Shimizu. The defect density is also discussed as a function of crystalline volume fraction.     

Electrohydrodynamic instability in some nematic cyanobiphenyls in an a.c. electric field

Electrohydrodynamic instability in homeotropically oriented nematic samples of 4'-n-octyl-4-cyano-biphenyl and 4'n-alkyloxy-4-cyanobiphenyl, (n = 8.9) have been studied in an a.c. electric field. The domain patterns during the instability in these compounds in a very low frequency a.c. field are very similar to those in a d.c. field. The domain patterns observed at higher frequencies have been identified as 'maltese crosses' or 'crossed isogyres'. The electro-convective 'isotropic' flows near the electrode play an important role in the observed instability.     

Conservation de la charge dans les codes PIC

This Note introduces a new integral form of the charge and current deposition scheme given in 1992 by Villasenor and Buneman, which permits the conservation of charge in PIC codes. This integral form extends directly to higher order spline shape factors, whose use produces less numerical noise. We thus get a discrete charge conserving method for high order shape factors. This method extends easily to nonuniform Cartesian grids for first order shape factors. To cite this article: R. Barthelmé, C. R. Acad. Sci. Paris, Ser. I 341 (2005).     

Calculation of Normal Vibrations of Chlorin by the Method of Density Functionals

Using the DFT/B3LYP method with a 6-31G(d) basis set, the structure, normal vibration frequencies, and the absolute band intensities in the IR spectra of the chlorin molecule and its four symmetric isotopomers have been calculated. Scaling of the force field by the Pulay method in independent and natural coordinates has been carried out. A method for obtaining effective force fields without using experimental data on the fundamental vibration frequencies is proposed. By comparing the vibration modes and constructing special matrices, complete assignment of the fundamental frequencies of porphin and chlorin has been carried out. It has been shown that the majority of porphin macroring vibrations upon pyrrolenine ring hydrogenation are frequency-characteristic and only 12 vibrations change considerably. A frequency correlation with regard for the mode transition between chlorin and all its isotopomers under consideration has been established. Comparative analysis of the force fields of porphin and chlorin in dependent natural coordinates has revealed the unique nonlocal character of the change in force constants of the macroring upon hydrogenation of one pyrrolenine ring. Modeling of the IR spectra of chlorin and its isotopomers has been performed. Assignment and interpretation of the normal vibrations of the molecules under consideration have been carried out.     

Role of Bulk Vaporization in the Dynamics of Laser-Induced Erosion Plumes of Metals

In different regimes of exposure of metals to pulsed laser radiation, we show the role played by the liquid-drop phase formed from the target material in a laser-induced erosion plume due to bulk vaporization, in the dynamics of this plume. For some metals, the ranges of power densities of the acting laser radiation, at which the condensed-phase particles influence the passage of laser radiation to the target surface, have been determined experimentally. The mechanism of realization of a low-threshold breakdown has been revealed.     

Loading aircraft for military operations

In this paper, we describe an aircraft loading problem submitted by the French military agency (DGA) as part of a more general military airlift planning problem. It can be viewed as a kind of bi-dimensional bin-packing problem, with heterogeneous bins and several additional constraints. We introduce two-phase methods for solving this NP-hard problem. The first phase consists in building good initial solutions, thanks to two fast algorithms: a list-based heuristic and a loading pattern generation method. Both algorithms call a constraint-based subroutine, able to determine quickly if the items already loaded can be reshuffled to accommodate a new object. The second phase improves these preliminary solutions using local search techniques. Results obtained on real data sets are presented.