pH-dependent structure and energetics of H2O/MgO(100)

The structure and energetics of water on MgO(100) surfaces are studied by atomic force microscopy (AFM) and density-functional theory (DFT). Computationally, the adsorption of water monomers, small water clusters and water monolayers on MgO(100) surfaces is considered. The calculations predict the non-dissociative adsorption for water monomers. The potential energy surface for single monomers is characterized by very low diffusion barriers. Increasing water coverage leads to the formation of structures containing alternatively dissociated and molecularly adsorbed water molecules. The magnitude of the calculated adsorption energy per water molecule increases from 0.57 eV for the water monomer to 0.79 eV for the water monolayer. The present experimental and theoretical results show furthermore that the stability of MgO(100) surfaces in the presence of water depends on its pH value. The etching of MgO(100) surfaces in aqueous medium is studied with the AFM in situ with pH value changing from basic to acidic. While the atomically flat MgO(100) surface remains stable in basic and neutral pH ranges, it is easily etched when the pH turns below a value of 6. This agrees qualitatively with the present DFT calculations showing that square pits resulting from the etching reduce the MgO(100) surface energy in acidic environments.     

Slow complexation dynamics between linear short polyphosphates and polyallylamines: analogies with "layer-by-layer" deposits

Polyelectrolyte "complexes" have been studied for almost a century and find more and more applications in cosmetics and DNA transfection. Most of the available studies focused on the thermodynamic aspects of the "complex" formation, mainly to determine phase diagrams and the influence of diverse physicochemical aspects on the formation of "complexes", but conversely less effort has been given to the kinetics of such processes. We describe herein the "complexation" kinetics of a short linear sodium polyphosphate (PSP) with poly(allylamine hydrochloride) (PAH) in the presence of 10 mM, 0.15 M and 1 M NaCl. We find, by using a combination of physicochemical techniques, that mixtures containing a 1 to 1 molar ratio of phosphate and amino groups allow the formation of "complexes" having a few 100 nm in diameter which progressively grow to particles up to 1.5 microns in hydrodynamic diameter, the growth process being accompanied by some progressive sedimentation. During this slow aggregation kinetics, the polyelectrolytes undergo a release of counterions and the zeta potential changes from a positive value to a negative one of -20 mV which is close to the zeta potential of (PSP-PAH)(n) films deposited under identical physicochemical conditions. Even though the complexes have a negative electrophoretic mobility, they contain an equimolar amount of amino and phosphate groups. This allows us to make some assumption about the structure of such "complexes" and to compare them with other published structures. We will also compare them with the aggregates found during the "layer-by-layer" deposition of the same species under the same conditions.     

Self-compression of millijoule pulses to 7.8 fs duration in a white-light filament

We demonstrate a novel technique for pulse compression of few-millijoule pulses with shorter than 10 fs duration. Our technique relies on spectral broadening in a white-light filament generated in a noble gas. In this filament we observe self-compression of 45 fs pulses down to below 8 fs duration without the need for any additional dispersion compensation. Using input pulses of 5 mJ, we generate compressed pulses with up to 3.8 mJ pulse energy. Therefore this method is much more efficient than previously demonstrated compression schemes. The generated peak powers of more than 100 GW at a kilohertz repetition rate open up a perspective for compression of few-cycle pulses with energies well beyond the capacity of hollow-fiber compressors.     

Evaluation of whole-genome amplification of low-copy-number DNA in chimerism analysis after allogeneic stem cell transplantation using STR marker typing

Whole-genome DNA amplification (WGA) is a promising method that generates large amounts of DNA from samples of limited quantity. We investigated the accuracy of a multiplex PCR approach to WGA over STR loci. The amplification bias within a locus and over all analyzed loci was investigated in relation to the amount of template in the WGA reaction, the specific STR locus, and allele length. We observed reproducible error-free STR profiles with 10 ng down to 1 ng of DNA template. The amplification deviation at a locus and between loci was within the intra-method reproducibility. WGA is the method of choice for amplifying nanogram amounts of genomic DNA for different applications. We detected unbalanced STR amplifications at one locus and between loci, allelic drop-outs, and additional alleles after WGA of low-copy-number DNA. We found that the high number of drop-outs and drop-ins could be eradicated using pooled DNA from separate WGA reactions even with as little as 100 pg of starting template. Nevertheless, the quality of the results was still not sufficient for use in routine chimerism analysis of limited specific cell populations after allogeneic stem cell transplantation.     

AB initio calculation of the atomic and electronic structure for Sb adsorbed on GaAs(110)

We report results obtained by a systematic study of Sb adsorption on the relaxed GaAs(110) surface, using density-functional theory within the local-density approximation (LDA) and norm-conserving, fully separable, ab-initio pseudopotentials. The GaAs(110) surface is simulated by a slab geometry wherein the atomic structure of the Sb atoms at the preferred adsorption positions and the uppermost substrate layer is optimized by minimizing the total energy, in contrast to previously reported theoretical approaches obtaining the surface bandstructure for given geometrical equilibrium structures. Sb coverages of Q=1/2 and Θ=1 are considered. We give a detailed analysis of the total-energy surface of the Sb/GaAs(110) system and identify stable and metastable adsorption sites. The resulting equilibrium geometries are discussed: We interpret these results in terms of the Sb-Sb interaction within the chains parallel to the [1¯11] direction and of possible structural instabilities in such chains. The atomic positions are compared with results of LEED analysis, stating an overall agreement except the buckling of the chain atoms. The resulting electronic properties (surface bandstructure, photothreshold, Schottky barrier) are discussed within the context of experimental data available from STM, photoemission spectroscopy, etc.     

Rigorous derivation of nonlinear plate theory and geometric rigidityDérivation rigoureuse de la théorie non linéaire des plaques et rigidité géométrique

We show that nonlinear plate theory arises as a Γ-limit of three-dimensional nonlinear elasticity. A key ingredient in the proof is a sharp rigidity estimate for maps $\text{v:(0,1)}{}^3\text{→}\text{R}{}^3$. We show that the L² distance of ?v from a single rotation is bounded by a multiple of the L² distance from the set SO(3) of all rotations. To cite this article: G. Friesecke et al., C. R. Acad. Sci. Paris, Ser. I 334 (2002) 173–178