Topological models on the lattice and a remark on string theory cloning

The addition of a topological model to the matter content of a conventional closed-string theory leads to the appearance of many, perturbatively-decoupled spacetime worlds. We illustrate this by classifying topological vertex models on a triangulated surface. We comment on how such worlds could have been coupled in the Planck era.Laboratoire Propre du Centre National de la Recherche Scientifique UPR A.0014Laboratoire de la Direction des Sciences de la Matière du Commissariat à l'Energie Atomique     

Matrix integration and combinatorics of modular groups

Integration over Gaussian matrix ensembles was used to obtain the virtual Euler characteristics of mapping class groups. We present some simplifications in the combinatorial part of the calculation.Laboratoire de l'Institut de Recherche Fondamentale du Commissariat à l'Energie Atomique     

Les proprietés electrochimiques de l'hydrure de lithium solide

The electrical conductivity of solid lithium hydride was measured. The electromotive forces of cells (Li)/LiH/H_2p were determined at pressurep equal tol or to the equilibrium pressure of lithium hydride at the resp. temperatures. From the results it seems that lithium hydride, under the conditions where its conductivity is essentially ionic, may be used as a solid electrolyte for measurements of thermodynamic data of other less stable hydrides.

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Ce travail a été effectué au Centre d'Etudes Nucléaires de Grenoble par l'un des auteurs (A.B.), grâce à une bourse accordée par le C.E.A.Nous tenons à remercier MM. P. L. Blum, J. Besson et J. Chevallier pour l'intérêt qu'ils ont manifesté pour ce travail. Nous remercions également M. P. Holba du I.P.H.S. (Prague) pour les discussions prolongées des résultats.     

Fusion and singular vectors inA 1 (1) highest weight cyclic modules

We show how the interplay between the fusion formalism of conformal field theory and the Knizhnik-Zamolodchikov equation leads to explicit formulae for the singular vectors in the highest weight representations ofA ₁ ⁽¹⁾ .Laboratoire de la Direction des Sciences de la Matière du Commissariat à l'Energie Atomique     

Electron dynamics in free electron laser resonator modes

We study the electron phase-space evolution and gain in free electron lasers whose short-wavelength radiation has Gaussian spherial wavefronts. Several free electron laser designs are considered: the undulator, the tapered wavelength undulator, and the optical klystron. We find that the gain spectrum is no longer proportional to the slope of the forward spontaneous emission spectrum, and we determine the design of the Gaussian mode which maximizes the energy extraction from the electron beam.Supported by the Air Force Office of Scientific Research 81-0061, the Office of Naval Research N00014-81-K-0809, NASA NAG-2-48, and NATO Collaborative Grant No. 1876Supported partly by the Centre d'Etudes Nucléaires de Saclay, DPC/SPP/SP and DRET, Contract 81-131     

ClassicalW-algebras

We reconsider the relation between classicalW-algebras and deformations of differential operators, emphasizing the consistency with diffeomorphisms. Generators of theW-algebra that arek-differentials are constructed by a systematic procedure. The method extends, following Drinfeld and Sokolov, toW-algebras based on arbitrary simple Lie algebras.Laboratoire de la Direction des Sciences de la Matière du Commissariat à l'Energie Atomique     

Laser resonant ionization of plutonium

Resonance ionization mass spectrometry (RIMS) has been tested for the isotope-selective determination of trace amounts of plutonium. An atomic beam is formed by evaporating plutonium atoms from a rhenium-filament heated to 1800 °C. The radiation of a pulsed dye laser excites the atoms in a two-photon process (=595.2 nm) followed by photoionization of the excited atoms. Mass selectivity is obtained by use of a time-of-flight spectrometer. A resonant signal of²³⁹Pu was measured with 10¹³ atoms deposited on the filament.     

Canonical ensemble and nonequilibrium states by molecular dynamics

We present a new technique to simulate the contact of a molecular dynamics system with a thermal wall. A canonical ensemble is obtained, and its statistical and thermodynamic fluctuations are studied. The values of the specific heat found by simulation agree with the experimental data. By means of thermal walls at different temperatures, thermal gradients are obtained. The values of the thermal conductivity are consistent with the experimental data.This work was supported in part by the Commissariat à l'Énergie Atomique (France).     

Causality in local quantum field theory: Some general results

General asymptotic causality properties of chronologicalN-point functions and, in massive theories, ofN-particle collision amplitudes, are derived from locality and the spectral condition. Results include specified rates of exponential fall-off, with simple and direct physical content, for large non-causal separations of points or particles in Minkowski space-time depending on values of the energy-momenta and on the mass spectrum. Relevant mathematical results on rates of exponential fall-off of generalized Fourier transforms outside their microsupports are given.Laboratoire de la Direction des Sciences de la Matière du Commissariat à l'Energie Atomique     

Shift Anisotropy Tensors in Amorphous Natural-Abundance Solids: High-Resolution <Superscript>29</Superscript>Si Chemical Shift Anisotropy Distributions under Very Slow Sample Rotation

The magic-angle turning technique is applied to amorphous natural-abundance silicate materials and high-resolution silicon-29 correlations between the isotropic and anisotropic chemical shifts are obtained. Very narrow tilted spinning sideband patterns are resolved in the two-dimensional spectra whose line width corresponds to the natural line width of the silicon nuclei. Various numerical approaches are implemented to extract the distribution of the chemical shift anisotropy tensors in these materials and their results are compared. Authors' address: Dimitris Sakellariou, Laboratoire Claude Fréjacques, Service de Chimie Moléculaire, Commissariat à l'Energie Atomique Saclay, 91191 Gif-sur-Yvette cedex, France     

Modular transformations ofSU(N) affine characters and their commutant

We describe the algebra of matrices commuting with the action of the modular group on characters ofSU(N) _k integrable representations. Using methods of finite quantum mechanics we find a canonical basis for this commutant over and prove the existence of an equivalent basis over with integral matrix elements. A final section is devoted to the case ofSU(3).Laboratoire de l'Institut de Recherche Fondamentale du Commissariat à l'Energie Atomique     

Analysis of the magnetic moments of mirror nuclei

The analyses of magnetic moments of mirror nuclei originally carried out by Sugimoto are brought up to date. Numerous new measurements strengthen and extend the earlier conclusions. Comparisons are made with the current large shellmodel calculations in the p and s-d shells.Visitor, Spring 1984, Centre de Recherches Nucléaires, Strasbourg.     

Resonance ionization spectroscopy of thorium

Here we describe experiments aimed at developing an element-selective ion source for thorium (Th). The technique applied is resonance ionization spectroscopy (RIS) with a thermal atom beam. Ionization schemes for isotopically nonselective ionization of Th as well as for isotopically selective ionization of ²³⁰Th are proposed. The RIS-scheme used is two-photon two-colour ionization with excitation in the ultraviolet spectral range between 244 nm and 267 nm or in the visible spectral range between 485 nm and 529 nm. Ionization of the excited atoms is performed either by ultraviolet photons or by visible photons, depending on the energy required for this process.     

Control scheme for two photon ionization of condensate atoms

The efficient photoionization of a Bose-Einstein condensate requires the creation of ions with the smallest possible transfer of atoms from the condensate into the thermal phase. The spontaneous decay from excited states into atomic states with momentum different from the initial one should be reduced. We investigate theoretically two-photon ionization of a rubidium condensate using near resonant excitation to the 6P state and second photon at 421 or 1002 nm into the continuum. Different ionization schemes with coherent control of the first excitation and reduction of the spontaneous decay are presented.     

Photoelectron angular distributions from strong-field coherent electronic excitation

Photoelectron Angular Distributions (PADs) resulting from nonperturbative excitation of potassium atoms using shaped femtosecond laser pulses are presented. We study control exerted by (1) the polarization of an unshaped, i.e., a bandwidth-limited light pulse, (2) shaped linearly polarized light, and (3) a combination of both degrees of freedom, i.e., polarization-shaped laser pulses. A theoretical approach to describe PADs from nonperturbative Resonance Enhanced Multi-Photon Ionization (REMPI) with ultrashort polarization-shaped laser pulses is presented and compared to experimental results. Applications of this technique to the generation and observation of atomic ring currents are discussed.     

Alternate technique for simultaneous measurement of photoionization cross-section of isotopes by TOF mass spectrometer

New measurements of photoionization cross-sections of the lithium isotopes are reported employing a Time of Flight (TOF) mass spectrometer in conjunction with an atomic beam apparatus. Using a two-step selective photoionization and saturation technique, we have simultaneously measured the photoionization cross-section of the 2p excited state of both the isotopes Li⁶ and Li⁷ as 15±2.5 Mb and 18 ±2.5 Mb where as the corresponding number densities have been determined as N₀≈5.3×10¹⁰ atoms/cm³ and N₀≈6.2×10¹¹ atoms/cm³ respectively.     

Three-dimensional tomographic reconstruction of ultrashort free electron wave packets

We present a tomographic technique based on Photoelectron Angular Distributions (PADs) measured by Velocity-Map-Imaging (VMI) to reconstruct the three-dimensional shape of ultrashort free electron wave packets obtained from 1+2 Resonance Enhanced Multi-Photon Ionization (REMPI) of potassium atoms. To this end the laser pulse is rotated about its propagation direction and a set of PADs are recorded at different rotation angles. The tomographic reconstruction technique is described and results for linear and elliptical polarization are presented. Results for linearly polarized light producing cylindrically symmetric electron wave packets confirm the validity of our method whereas elliptically polarized light serves as a prototype for polarization-shaped laser pulses.     

Power series for level spacing functions of random matrix ensembles

A remarkable set of identities among (odd and even) spheroidal functions and their eigenvalues is indicated. This is then used to derive explicitly the general term in the power series expansion ofE (r, s), the probability that a randomly chosen interval of lengths contains exactlyr levels. The parameter takes the values 1, 2 or 4 according as the ensemble considered is orthogonal, unitary or symplectic.Laboratoire de la Direction des Sciences de la Matière du Commissariat à l'Energie Atomique     

How much can one break chiral symmetry in the linear sigma model?

We show that the hedgehog soliton solution describing the nucleon in theSU(3) ×SU(3) linear sigma model breaks down when the pion mass becomes too large.On leave of absence from the Laboratoire de Physique Théorique, Université de Nice, parc Valrose, F-06034 Nice Cedex, France