Construction d'une paramétrixe pour des opérateurs différentiels hypoelliptiques marimaux sur un espace homogéne d'un groupe de Lie nilpotent gradué de rang 2

Soient G une alébre de Lie nilpotente stratifée de rang 2, une sous-algébre de G, _0, la représentation de G dans l'espace L ²( \ G) indiute par le caractére trivial C, P un opérateur homogène appartenant à l'algébre universelle enveloppante (complexifiée) U(G) tel que l'opérateur _0, (P) soit hypoelliptique maximal. Cet opérateur peut s'exprimer par une intégrale dépendant de la restriction du symbole p de P au sousensemble = G · décrit par les orbites des éléments de dans la représentation contragrédiente de G dans G ^*.Une algèbre de symboles définis sur est construite et permet de déterminer une paramétrixe de _0, (P); des résultats de réguralité de cet opérateur dans des espaces de Sobolev adaptés sont ensuite obtenus.     

Optimized absorption imaging of mesoscopic atomic clouds

We report on the optimization of high-intensity absorption imaging for small Bose–Einstein condensates. The imaging calibration exploits the linear scaling of the quantum projection noise with the mean number of atoms for a coherent spin state. After optimization for atomic clouds containing up to 300 atoms, we find an atom number resolution of $\varDelta_{\rm det}= 3.7$ atoms, mainly limited by photon shot noise and radiation pressure.     

Laser-induced incandescence technique to identify soot nucleation and very small particles in low-pressure methane flames

This paper presents the study we carried out on the formation of soot particles in low-pressure premixed CH₄/O₂/N₂ flames by using Laser-Induced Incandescence (LII). Flames were stabilised at 26.6 kPa (200 torr). Four different equivalence ratios were tested (Φ = 1.95, 205, 2.15 and 2.32), Φ = 1.95 corresponding to the equivalence ratio for which LII signals begin to be measurable along the flame. The evolution of the LII signals with laser fluence (fluence curve), time (temporal decay) and emission wavelength is reported at different heights above the burner. We specifically took advantage of the low-pressure conditions to probe with a good spatial resolution the soot inception zone of the flames. Significant different behaviours of the fluence curves are observed according to the probed region of the flames and Φ. In addition, while the surface growth process is accompanied by an increase in the LII decay-times (indicator of the primary particle diameter) at higher Φ, decay-times become increasingly short at lower Φ reaching a constant value along the flame at Φ = 1.95. These behaviours are consistent with the detection of the smallest incandescent particles in the investigated flames, these particles having experienced very weak surface growth. Flame modelling including soot formation has been implemented in flames Φ = 2.05 and 2.32. Experimental quantitative soot volume fraction profiles were satisfactorily reproduced by adjusting the fraction of reactive soot surface available for reactions. The qualitative variation of the computed soot particle diameter and the relative weight of surface growth versus nucleation were consistent with the experimental observations.     

Wide electrical tunability of a GaInAsP/InP microdisk resonator

We report on the characterization of an InP/InGaAsP-material-based microdisk resonator optical filter. The originality here is constituted by the use of a localized control electrode that is used for the tuning of the resonance wavelength of the filter via the injection of a driving current. Tuning of the resonance wavelength close to 8 nm has been experimentally achieved for a drive current of 80 mA.     

Damage induced in garnets by heavy ion irradiations: a study by optical spectroscopies

The damage induced by heavy-ion irradiation has been studied in yttrium iron garnet (Y₃Fe₅O₁₂ or YIG) films, doped with Ca, Tb and Tm, grown by liquid-phase epitaxy on gadolinium gallium garnet (Gd₃Ga₅O₁₂ or GGG) substrates. Irradiations of doped-YIG epitaxial films and GGG substrates with 36-MeV ¹⁸³W and 12-MeV ¹⁹⁷Au ions were applied for fluences between 1 × 10¹³ and 3 × 10¹⁵ cm^–2 near room temperature. The radiation damage was monitored by micro-Raman spectroscopy and UV–visible optical absorption spectroscopy. Raman spectra revealed that amorphisation was achieved in YIG for both ions, whereas a high lattice disorder was induced in GGG without reaching amorphisation for the Au ion irradiation. Raman spectra also showed that a major damage of the tetrahedral sites was induced in GGG, as previously found for YIG. It is concluded that with such ions reaching the stopping power threshold of track formation in YIG and GGG the observed rate of amorphisation may result from a combination of electronic and nuclear energy losses as calculated using the unified thermal spike model.     

An experimental and kinetic modeling study on the oxidation of 1,3-dioxolane

The modern catalytic or enzymatic advances allow the production of novel biofuel. Among them, 1,3-dioxolane can be produced from formaldehyde and ethylene glycol, both can be obtained from biomass. In this study, the oxidation of 1,3-dioxolane is studied at stoichiometric conditions. The ignition delay times of 1,3-dioxolane/O₂/inert mixtures were measured in a shock tube and in a rapid compression machine at pressures of 20 to 40 bar and temperatures ranging from 630 to 1300 K. The pressure profiles recorded in the rapid compression machine show a first stage of ignition enlightening the influence of the low temperature chemistry of combustion. Furthermore, mole fraction profiles of the stable intermediates produced during the oxidation of 1,3-dioxolane were measured in a jet-stirred reactor at 10 bar. Following these observations, a detailed kinetic model was developed with reaction rate coefficients and thermochemical data calculated by theoretical calculations or estimated by analogies to suitable molecules. In order to get an insight into the most important reaction pathways brute force sensitivity analysis and reaction pathway analysis were performed with the proposed model and discussed. It became clear that in the fuel-in-air case for the alkylhydroperoxide of 1,3-dioxolane the ring opening beta-scission pathway is favored against the further alkane-like second addition to molecular oxygen, which leads to a limited negative temperature coefficient.     

Monte carlo estimates of domain-deformation sensitivities

It is shown that, when a Monte Carlo algorithm is used for estimation of any physical quantity A, a simple and fast additional procedure can be implemented that simultaneously estimates the sensitivity of A to any problem parameter. The proposed approach is general and systematic in the sense that: (i) it includes domain-deformation sensitivities, i.e., cases where a change in the parameter modifies the domain over which the sampled random variables are defined and (ii) a simple generic procedure is presented to address all remaining free choices in terms of variance minimization.     

Synthesis of a C(sp2)-bridged Phosphine-Borane by Ionic Coupling

The vinyl carbenoid H₂C=CBr(Li) has been used as key precursor to prepare a geminal C(sp²)-bridged phosphine-borane. Starting from bromoethene, two sequences of lithiation/electrophilic trapping, with ClPiPr₂ and FBMes₂ respectively, affords iPr₂P–C(=CH₂)–BMes₂ 3 [Mes = 2,4,6-(H₃C)₃C₆H₂]. This new phosphine-borane 3 was characterized by multi-nuclear NMR and mass spectroscopy. It adopts a monomeric open structure without P→B interaction. A few crystals of a secondary product 4 were analyzed by X-ray diffraction, revealing an unusual dimeric structure.