Multipole-Bound Molecular Negative Ions

 Within the framework of a simple electrostatic model, as compared to recent experimental results, we here discuss the stability of very weakly bound molecular negative ions. In contrast with the case of conventional valence anions, the excess electron is then located in a very diffuse orbital and is mainly bound by electrostatic dipolar, quadrupolar, and polarization forces, at large distances from the neutral molecular core. By fitting a single repulsion parameter of the model to the available experimental data, it is possible to make quantitative predictions of the excess-electron binding energies in these species. Critical values of the dipole moment, quadrupole moment or polarizability required for the observation of stable multipole-bound negative ions are predicted and compared to available experimental data and ab initio calculations. Received October 24, 2001; accepted for publication November 16, 2001     

Suppression of parasitic lasing in high energy, high repetition rate Ti:sapphire laser amplifiers

Transverse parasitic lasing is well known for limiting the signal gain and the pulse energy that can be extracted from Ti:sapphire petawatt amplifiers. We have developed a technique for suppressing these parasitic lasing modes based on perfect refractive index-matching liquid doped with a broad-bandwidth absorber to suppress the transverse lasing while ensuring proper heat removal from the Ti:sapphire crystal. The 800 nm laser output with a bandwidth of 41 nm (FWHM) and peak energy of 22.7 J at a repetition rate of 1 Hz is demonstrated.     

DESIRS: a state‐of‐the‐art VUV beamline featuring high resolution and variable polarization for spectroscopy and dichroism at SOLEIL

DESIRS is a new undulator‐based VUV beamline on the 2.75 GeV storage ring SOLEIL (France) optimized for gas‐phase studies of molecular and electronic structures, reactivity and polarization‐dependent photodynamics on model or actual systems encountered in the universe, atmosphere and biosphere. It is equipped with two dedicated endstations: a VUV Fourier‐transform spectrometer (FTS) for ultra‐high‐resolution absorption spectroscopy (resolving power up to 10⁶) and an electron/ion imaging coincidence spectrometer. The photon characteristics necessary to fulfill its scientific mission are: high flux in the 5–40 eV range, high spectral purity, high resolution, and variable and well calibrated polarizations. The photon source is a 10 m‐long pure electromagnetic variable‐polarization undulator producing light from the very near UV up to 40 eV on the fundamental emission with tailored elliptical polarization allowing fully calibrated quasi‐perfect horizontal, vertical and circular polarizations, as measured with an in situ VUV polarimeter with absolute polarization rates close to unity, to be obtained at the sample location. The optical design includes a beam waist allowing the implementation of a gas filter to suppress the undulator high harmonics. This harmonic‐free radiation can be steered toward the FTS for absorption experiments, or go through a highly efficient pre‐focusing optical system, based on a toroidal mirror and a reflective corrector plate similar to a Schmidt plate. The synchrotron radiation then enters a 6.65 m Eagle off‐plane normal‐incidence monochromator equipped with four gratings with different groove densities, from 200 to 4300 lines mm^?1, allowing the flux‐to‐resolution trade‐off to be smoothly adjusted. The measured ultimate instrumental resolving powers are 124000 (174 µeV) around 21 eV and 250000 (54 µeV) around 13 eV, while the typical measured flux is in the 10¹⁰–10¹¹ photons s^?1 range in a 1/50000 bandwidth, and 10¹²–10¹³ photons s^?1 in a 1/1000 bandwidth, which is very satisfactory although slightly below optical simulations. All of these features make DESIRS a state‐of‐the‐art VUV beamline for spectroscopy and dichroism open to a broad scientific community.     

Tracking elementary particles near their primary vertex: A combinatorial approach

Colliding beams experiments in High Energy Physics rely on solid state detectors to track the flight paths of charged elementary particles near their primary point of interaction. Reconstructing tracks in this region requires, per collision, a partitioning of up to 10³ highly correlated observations into an unknown number of tracks. We report on the successful implementation of a combinatorial track finding algorithm to solve this pattern recognition problem in the context of the ALEPH experiment at CERN. Central to the implementation is a 5-dimensional axial assignment model (AP5) encompassing noise and inefficiencies of the detector, whose weights of assignments are obtained by means of an extended Kalman filter. A preprocessing step, involving the clustering and geometric partitioning of the observations, ensures reasonable bounds on the size of the problems, which are solved using a branch & bound algorithm with LP relaxation. Convergence is reached within one second of CPU time on a RISC workstation in average.     

Cassie's Law and Concavity of Wall Tension with Respect to Disorder

For the semiinfinite Ising model with quenched boundary disorder, we prove concavity inequalities for the difference of wall tensions associated with the minus and plus phases. These inequalities generalize phenomenological equalitiesknown as Cassie's law.     

On the Distribution of Free Path Lengths¶for the Periodic Lorentz Gas

Consider the domain

Formation of ordered micro-porous membranes

Regular micro-porous polymeric membranes have recently been discovered by rapidly evaporating a solution of CS₂ containing poly(p-phenylene)-block-polystyrene [#!ref1!#]. 1,2-dichloroethane (a chlorated solvent in which polystyrene gel phase has never been observed) is also found to produce ordered structures, which definitively excludes eventual effect of the gelation process during the membrane formation. The observation of the solution surface during the solvent evaporation reveals the growing of micron-sized water droplets trapped at the surface and forming compact aggregates. The study of the solution/water interface shows that the water droplets profile is in agreement with the pore shape observed in the membranes. Moreover, the copolymer was found to precipitate at the interface, forming a layer encapsulating the droplets and preventing their coalescence. In that way, the final structure results from the droplets stacking under the action of large surface currents. Finally, we argue that the decisive element in the formation of ordered structures is the ability of the polymer to precipitate at the solution/water interface, which seems to be related the star-polymer microstructure. Received: 18 August 1998     

Dynamical Localization II with an Application to the Almost Mathieu Operator

Several recent works have established dynamical localization for Schrödinger operators, starting from control on the localization length of their eigenfunctions, in terms of their centers of localization. We provide an alternative way to obtain dynamical localization, without resorting to such a strong condition on the exponential decay of the eigenfunctions. Furthermore, we illustrate our purpose with the almost Mathieu operator, H _{, ,} =–+ cos(2(+x)), 15 and with good Diophantine properties. More precisely, for almost all , for all q>0, and for all functions ²( ) of compact support, we show that The proof applies equally well to discrete and continuous random Hamiltonians. In all cases, it uses as input a repulsion principle of singular boxes, supplied in the random case by the multi-scale analysis.     

The Mean-Field Limit for a Regularized Vlasov-Maxwell Dynamics

The present work establishes the mean-field limit of a N-particle system towards a regularized variant of the relativistic Vlasov-Maxwell system, following the work of Braun-Hepp [Commun Math Phys 56:101–113, 1977] and Dobrushin [Func Anal Appl 13:115–123, 1979] for the Vlasov-Poisson system. The main ingredients in the analysis of this system are (a) a kinetic formulation of the Maxwell equations in terms of a distribution of electromagnetic potential in the momentum variable, (b) a regularization procedure for which an analogue of the total energy—i.e. the kinetic energy of the particles plus the energy of the electromagnetic field—is conserved and (c) an analogue of Dobrushin’s stability estimate for the Monge-Kantorovich-Rubinstein distance between two solutions of the regularized Vlasov-Poisson dynamics adapted to retarded potentials.