Laboratoire de Spectrométrie Ionique et Moléculaire

Gabedit is a freeware graphical user interface, offering preprocessing and postprocessing adapted (to date) to nine computational chemistry software packages. It includes tools for editing, displaying, analyzing, converting, and animating molecular systems. A conformational search tool is implemented using a molecular mechanics or a semiempirical potential. Input files can be generated for the computational chemistry software supported by Gabedit. Some molecular properties of interest are processed directly from the output of the computational chemistry programs; others are calculated by Gabedit before display. Molecular orbitals, electron density, electrostatic potential, nuclear magnetic resonance shielding density, and any other volumetric data properties can be displayed. It can display electronic circular dichroism, UV-visible, infrared, and Raman-computed spectra after a convolution. Gabedit can generate a Povray file for geometry, surfaces, contours, and color-coded planes. Output can be exported to a selection of popular image and vector graphics file formats; the program can also generate a series of pictures for animation. Quantum mechanical electrostatic potentials can be calculated using the partial charges on atoms, or by solving the Poisson equation using the multigrid method. The atoms in molecule charges can also be calculated. Gabedit is platform independent. The code is distributed under free open source X11 style license and is available at http://gabedit.sourceforge.net/.

The nucleic acid sequence bank now contains over 600 protein coding genes of which 107 are from prokaryotic organisms. Codon frequencies in each new prokaryotic gene are given. Analysis of genetic code usage in the 83 sequenced genes of the Escherichia coli genome (chromosome, transposons and plasmids) is presented, taking into account new data on gene expressivity and regulation as well as iso-tRNA specificity and cellular concentration. The codon composition of each gene is summarized using two indexes: one is based on the differential usage of iso-tRNA species during gene translation, the other on choice between Cytosine and Uracil for third base. A strong relationship between codon composition and mRNA expressivity is confirmed, even for genes transcribed in the same operon. The influence of codon use of peptide elongation rate and protein yield is discussed. Finally, the evolutionary aspect of codon selection in mRNA sequences is studied.

Modern laser sources nowadays deliver ultrashort light pulses reaching few cycles in duration, high energies beyond the Joule level and peak powers exceeding several terawatt (TW). When such pulses propagate through optically-transparent media, they first self-focus in space and grow in intensity, until they generate a tenuous plasma by photo-ionization. For free electron densities and beam intensities below their breakdown limits, these pulses evolve as self-guided objects, resulting from successive equilibria between the Kerr focusing process, the chromatic dispersion of the medium, and the defocusing action of the electron plasma. Discovered one decade ago, this self-channeling mechanism reveals a new physics, widely extending the frontiers of nonlinear optics. Implications include long-distance propagation of TW beams in the atmosphere, supercontinuum emission, pulse shortening as well as high-order harmonic generation. This review presents the landmarks of the 10-odd-year progress in this field. Particular emphasis is laid to the theoretical modeling of the propagation equations, whose physical ingredients are discussed from numerical simulations. Differences between femtosecond pulses propagating in gaseous or condensed materials are underlined. Attention is also paid to the multifilamentation instability of broad, powerful beams, breaking up the energy distribution into small-scale cells along the optical path. The robustness of the resulting filaments in adverse weathers, their large conical emission exploited for multipollutant remote sensing, nonlinear spectroscopy, and the possibility to guide electric discharges in air are finally addressed on the basis of experimental results.

Most long-path remote spectroscopic studies of the atmosphere rely on ambient light or narrow-band lasers. High-power femtosecond laser pulses have been found to propagate in the atmosphere as dynamically self-guided filaments that emit in a continuum from the ultraviolet to the infrared. This white light exhibits a directional behavior with enhanced backward scattering and was detected from an altitude of more than 20 kilometers. This light source opens the way to white-light and nonlinear light detection and ranging applications for atmospheric trace-gas remote sensing or remote identification of aerosols. Air ionization inside the filaments also opens promising perspectives for laser-induced condensation and lightning control. The mobile femtosecond-terawatt laser system, Teramobile, has been constructed to study these applications.

We present a novel chemical database for gas-phase astrochemistry. Named the KInetic Database for Astrochemistry
\n(KIDA), this database consists of gas-phase reactions with rate coefficients and uncertainties that will be vetted
\nto the greatest extent possible. Submissions of measured and calculated rate coefficients are welcome, and will
\nbe studied by experts before inclusion into the database. Besides providing kinetic information for the interstellar
\nmedium, KIDA is planned to contain such data for planetary atmospheres and for circumstellar envelopes. Each
\nyear, a subset of the reactions in the database (kida.uva) will be provided as a network for the simulation of the
\nchemistry of dense interstellar clouds with temperatures between 10 K and 300 K. We also provide a code, named
\nNahoon, to study the time-dependent gas-phase chemistry of zero-dimensional and one-dimensional interstellar
\nsources.

International audience

For the linear triatomic X–H··· system, the separability of the X–H stretching vibrations from the hydrogen bond vibrations is analyzed in the spirit of the adiabatic approximation. The adiabatic wavefunctions for X–H stretching vibrations are shown to be suitable functions for the evaluation of the principal factors determining the infrared spectral properties of the actual species of carboxylic acid dimers and imidazole crystal. Theoretical infrared spectra in the X–H stretching region of these systems are then obtained and compared with the experimental ones. The quantitative reconstitution of the experimental spectra and, in particular, the predictions for the effect of isotopic substitution of H by D are confirmed. The principal features of the unusual spectral properties of the X–H stretching vibrations in hydrogenbonded systems seem therefore to result from a somewhat peculiar coupling mechanism suggested in the theory.

A new method is proposed for the fabrication of a well-defined size and shape distribution of silver nanoparticles in solution; the method employs direct laser irradiation of an aqueous solution containing a silver salt and a surfactant in the absence of reducing agents.

The measurement of a quantitative and macroscopic parameter to estimate the global motility of a large population of swimming biological cells is a challenge. Experiments on the rheology of active suspensions have been performed. Effective viscosity of sheared suspensions of live unicellular motile microalgae (Chlamydomonas Reinhardtii) is far greater than for suspensions containing the same volume fraction of dead cells. In addition, suspensions show shear thinning behavior. We relate these macroscopic measurements to the orientation of individual swimming cells under flow and discuss our results in the light of several existing models.

Ultrashort, high-power laser pulses propagating vertically in the atmosphere have been observed over more than 20 km using an imaging 2-m astronomical telescope. This direct observation in several wavelength bands shows indications for filament formation at distances as far as 2 km in the atmosphere. Moreover, the beam divergence at 5 km altitude is smaller than expected, bearing evidence for whole-beam parallelization about the nonlinear focus. We discuss implications for white-light Lidar applications.

The low-energy cluster beam deposition technique (LECBD) is applied to produce cluster assembled films with hitherto unknown nanostructured morphologies and properties. Neutral clusters having the very low energy gained in the supersonic expansion at the exit of the inert gas condensation-type source are deposited without fragmentation upon impact on the substrate. Depending on the deposition conditions (nature, size and flux of incident clusters, nature and temperature of the substrate, vacuum conditions), granular nanostructures resulting from the diffusion and coalescence of supported clusters are obtained with materials of any type (covalent or metallic). A critical size for coalescence limits the supported grain size and, finally, highly porous thick films growing by random stacking of nanoparticles are obtained. A recent model developed by combining several dynamical processes simultaneously occurring on the substrate (deposition - diffusion - aggregation, DDA) is used to simulate the cluster assembled film morphology in good agreement with the experimental observations. Examples of novel materials obtained by LECBD are presented to illustrate the interesting potentialities of the technique. In the case of covalent materials such as carbon and silicon, 'amorphon'-type disordered structures, different from the conventional amorphous structures (a-C and a-Si), are obtained with some unique properties. With transition metal (Fe, Co and Ni) cluster assembled films, a specific magnetic behaviour, resulting from the competition between the intrinsic properties of the grains (magnetocrystalline anisotropy) and the interactions between grains, is observed. Also, films of clusters embedded in various co-deposited matrices are produced in order to control the interactions between grains via the matrix materials (insulating, conducting ...). Interesting optical properties (from metallic clusters in ) or giant magnetoresistance effects (from Co clusters in silver) are reported for such systems, emphasizing the future role of LECBD in various fields of applications such as optical and optoelectronic nanostructures, magnetic and magneto-optic nanostructures and quantum devices.

We report experiments on gold clusters in the size range 2--4 nm, embedded in an alumina matrix. The metallic particles are produced with a laser vaporization source and codeposited with a dielectric vapor as a thin film on a substrate. Our technique allows varying the cluster size at a given metal concentration. These composite materials are studied through optical absorption and ellipsometric measurements, allowing determination of their complex index of refraction. Various complementary techniques provide information about their morphology, their chemical composition, the thickness of the films, and the size distribution of the clusters. The surface plasmon resonance in the absorption spectra is shown to be damped and blueshifted with decreasing cluster size. Theoretical calculations in the framework of the time-dependent local-density approximation allow a clear understanding of these experimental results.

Electron-lattice energy exchanges are investigated in gold and silver nanoparticles with sizes ranging from 30 to 2.2 nm embedded in different environments. Femtosecond pump-probe experiments performed in the low-perturbation regime demonstrate a strong increase of the intrinsic electron-phonon interaction for nanoparticles smaller than 10 nm due to a confinement effect.

In rare earth (R)–transition metal (M) compounds, large R-M magnetic interactions can occur, which give rise to higher values of the ordering temperature (TC,TN) for compounds with magnetic R elements than for compounds with R nonmagnetic, i.e., La, Lu, Y. Due to the localized character of the 4f shell, these R-M interactions are indirect, mediated by the 5d, 6s conduction electrons. The highest value of the ordering temperature is obtained for Gd compounds, and as a first approximation it is reasonable to write the interaction energy as ER-M=−nRMMSRMSM, where MSR and MSM are the rare-earth and transition metal spin moments, respectively. The molecular field coefficient nRM is generally assumed to be a constant throughout a given series, owing to the similarities of band structure for all R elements. In this paper, the molecular field coefficient nRM has been obtained for a number of series of rare earth-transition metal compounds. The analysis reveals that nRM is not a constant going across a given series but decreases by a factor of 2. The observed variation in nRM is shown to be related to the variation of the exchange interactions between 4f and 5d electrons which are larger for light rare-earth elements since the difference between the spatial extent of the 4f and 5d electrons is reduced. As a general rule the observed enhancement of magnetic interactions for lightest rare earths in all rare-earth metallic systems can be attributed to the same phenomenon.

The absorption of a single isolated metal cluster is directly measured using a novel far-field optical technique based on modulation of its position. Single gold nanoparticles with average diameters down to 5 nm, dispersed on a transparent substrate, are optically detected and their absolute absorption cross section determined.

We demonstrate remote elemental analysis at distances up to 90m, using a laser-induced breakdown spectroscopy scheme based on filamentation induced by the nonlinear propagation of unfocused ultrashort laser pulses. A detailed signal analysis suggests that this technique, remote filament-induced breakdown spectroscopy, can be extended up to the kilometer range.

The relaxation time of the magnetization of a big magnetic molecule is evaluated. Isotropic exchange is assumed to be dominant, resulting in a fixed spin modulus s (equal to 20 in the case of interest). A perturbation of the form ASz2 results from spin-orbit coupling and produces an energy barrier As2 which separates positive and negative values of Sz. Spin-phonon interactions are treated by second-order perturbation theory. An upper limit of the pre-exponential factor in the Arrhenius law is obtained, which is consistent with the very large experimental value.

En admettant que l'équation de diffusion est valable pour décrire le mouvement aléatoire de translation des noyaux porteurs de spin en interaction dipolaire dans les liquides, il faut tenir compte de la distance minimum d'approche des molécules. Dans un modèle de sphère dure, le calcul habituel des densités spectrales J(ω) résultant de ce problème, à partir de la solution classique de l'équation de diffusion, ne tient pas compte correctement des conditions aux limites. On développe un traitement plus rigoureux conduisant à des résultats aussi simples mais assez différents, notamment dans le cas où ωτ ⪢ 1 (τ temps de corrélation) avec un comportement de J(ω) en ω -2 au lieu de ω-3/2. Les résultats obtenus sont appliqués au calcul de la variation en fonction de ωτ du temps de relaxation des protons du benzène dans des solutions benzéniques diluées de diphényl-picryl hydrazyl et comparés à l'expérience. La généralité de la loi en ω -2 est discutée en détail.

We report the optical second harmonic generation from individual 150 nm diameter gold nanoparticles dispersed in gelatin. The quadratic hyperpolarizability of the particles is determined and the input polarization dependence of the second harmonic intensity obtained. These results are found in excellent agreement with ensemble measurements and finite element simulations. These results open up new perspectives for the investigation of the nonlinear optical properties of noble metal nanoparticles.

We present a joint theoretical and experimental investigation of the absorption spectra of silver clusters Ag(n) (4<or=n<or=22). The experimental spectra of clusters isolated in an Ar matrix are compared with the calculated ones in the framework of the time-dependent density functional theory. The analysis of the molecular transitions indicates that the s-electrons are responsible for the optical response of small clusters (n<or=8) while the d-electrons play a crucial role in the optical excitations for larger n values.