Barrier-free intermolecular proton transfer in the uracil-glycine complex induced by excess electron attachment

The photoelectron spectra (PES) of anions of uracil-glycine and uracil-phenylalanine complexes reveal broad features with maxima at 1.8 and 2.0 eV. The results of ab initio density functional B3LYP and second order M?ller-Plesset theory calculations indicate that the excess electron occupies a π^* orbital localized on uracil. The excess electron attachment to the complex can induce a barrier-free proton transfer (BFPT) from the carboxylic group of glycine to the O8 atom of uracil. As a result, the four most stable structures of the anion of uracil-glycine complex can be characterized as the neutral radical of hydrogenated uracil solvated by the anion of deprotonated glycine. The similarity between the PES spectra for the uracil complexes with glycine and phenylalanine suggests that the BFPT is also operative in the case of the latter anionic species. The BFPT to the O8 atom of uracil may be related to the damage of nucleic acid bases by low energy electrons because the O8 atom is involved in a hydrogen bond with adenine in the standard Watson-Crick pairing scheme. Received 6 April 2002 Published online 13 September 2002     

Fragmentation and reliable size distributions of large ammonia and water clusters

The interaction of large ammonia and water clusters in the size range from <n> = 10 to 3 400 with electrons is investigated in a reflectron time-of-flight mass spectrometer. The clusters are generated in adiabatic expansions through conical nozzles and are nearly fragmentation free detected by single photon ionization after they have been doped by one sodium atom. For ammonia also the (1+1) resonance enhanced two photon ionization through the state with v = 6 operates similarly. In this way reliable size distributions of the neutral clusters are obtained which are analyzed in terms of a modified scaling law of the Hagena type [Surf. Sci. 106, 101 (1981)]. In contrast, using electron impact ionization, the clusters are strongly fragmented when varying the electron energy between 150 and 1 500 eV. The number of evaporated molecules depends on the cluster size and the energy dependence follows that of the stopping power of the solid material. Therefore we attribute the operating mechanism to that which is also responsible for the electronic sputtering of solid matter. The yields, however, are orders of magnitude larger for clusters than for the solid. This result is a consequence of the finite dimensions of the clusters which cannot accommodate the released energy. Received 21 November 2001     

Dynamics of ionization mechanisms in relativistic collisions involving heavy and highly-charged ions

The dynamics of mechanisms associated with the ionization of inner-shell electrons in relativistic collisions involving heavy and highly-charged ions is investigated within a nonperturbative approach formulated explicitly in the time domain. The theoretical treatment is based on the exact numerical solution of the time dependent Dirac equation for two Coulomb centers on a lattice in momentum space. We present results for ionization in encounters between 100 MeV/u Au⁷⁹⁺ projectile ions impinging on a hydrogen-like uranium target. By directly visualizing the collision dynamics we identify a new ionization mechanism in which electrons are emitted from the internuclear region preferentially in the transverse direction with respect to the projectile trajectory. A striking characteristic of this ionization mechanism is that the velocity of the electron is higher than the projectile velocity. Received 26 June 2001 and Received in final form 27 November 2001     

Microscopic three-body force for asymmetric nuclear matter

Brueckner calculations including a microscopic three-body force have been extended to isospin-asymmetric nuclear matter. The effects of the three-body force on the equation of state and on the single-particle properties of nuclear matter are discussed with a view to possible applications in nuclear physics and astrophysics. It is shown that, even in the presence of the three-body force, the empirical parabolic law of the energy per nucleon vs. isospin asymmetry β = (N - Z)/A is fulfilled in the whole asymmetry range 0≤β≤1 up to high densities. The three-body force provides a strong enhancement of the symmetry energy which increases with density in good agreement with the predictions of relativistic approaches. The Lane's assumption that proton and neutron mean fields linearly vary vs. the isospin parameter is violated at high density due to the three-body force, while the momentum dependence of the mean fields turns out to be only weakly affected. Consequently, a linear isospin split of the neutron and proton effective masses is found for both cases with and without the three-body force. The isospin effects on multifragmentation events and collective flows in heavy-ion collisions are briefly discussed along with the conditions for direct URCA processes to occur in the neutron star cooling. Received: 18 February 2002 / Accepted: 16 May 2002     

Low energy electron attachment to formic acid

Using a high resolution electron energy monochromator low energy electron attachment to formic acid is studied for the first time by means of mass spectrometric detection of the product anions. The largest dissociative electron attachment (DA) cross-section produces HCOO ^-+H with weaker channels for OH^- and O^- becoming apparent at higher incident energies. Received 23 January 2002 and Received in final form 9 February 2002 Published online 13 September 2002     

Total, partial, and electron-capture cross sections for ionization of water vapor by 20-150 keV protons

We present experimental results for proton ionization of water molecules based on a novel event by event analysis of the different ions produced (and lost). We are able to obtain mass analyzed product ion signals (e.g., H2O+, OH+, O+, O++, H+) in coincidence with the projectile analyzed after the collision, i.e., either being H+, neutral H after single electron capture during the ionization event, or H- after double electron capture. After proper calibration we are thus able to determine a complete set of cross sections for the ionization of a molecular target by protons including the total and the partial cross sections and in addition also the direct ionization and the electron capture cross sections.     

Electronic excitation and singlet-triplet coupling in uracil tautomers and uracil-water complexes

Electronic spectra of uracil in its diketo (lactam) form and five enol (lactim) tautomeric forms have been investigated by means of combined density functional and configuration interaction methods. We have simulated the effects of hydrogen bonding with a protic solvent by recomputing the spectrum of uracil in the presence of two, four, or six water molecules. Geometries of the electronic ground state and several low-lying excited states have been optimized. Spin-orbit coupling has been determined for correlated wavefunctions employing a non-empirical spin-orbit mean-field approach. In accord with experiment, we find the diketo tautomer to be the most stable one. The calculations confirm that the first absorption band arises from the ¹( π↦π^*) S ₀↦S ₂ excitation. The experimentally observed vibrational structure in this band originates from a breathing mode of the six ring. Complexation with water molecules is seen to cause a significant blue shift of n↦π^* excitations while leaving π↦π^* excitations nearly uninfluenced. Computed radiative lifetimes are presented for the experimentally known weak phosphorescence from the π↦π^* excited T₁ state. Among the uracil lactim tautomers, one is particularly interesting from a spectroscopic point of view. In this tautomer, the π↦π^* excitation gives rise to the S₁ state. Received 18 February 2002 / Received in final form 5 June 2002 Published online 13 September 2002     

Electron diffraction and microscopy of single-wall carbon nanotube bundles produced by different methods

The atomic structure of single-wall carbon nanotube bundles produced by three different techniques (laser ablation, electric arc discharge and catalytic chemical vapor deposition (CCVD)) has been characterized by electron diffraction and microscopy. Information on the helicity and the lattice packing has been obtained. Concerning the helicity, small bundles produced by CCVD exhibit only one or two tube helicities within a single bundle. The diffraction patterns of laser-ablation produced bundles also present well-defined but more diversified chiralities within a single bundle. By contrast the data acquired on bundles formed by arc discharge show a more diffuse pattern, characteristic of a random chirality dispersion within a single bundle. Concerning the lattice packing, informations are obtained via a detailed study of the equatorial line of the diffraction pattern for bundles produced by the three techniques. This electron diffraction study is completed by high-resolution electron microscopy. Received 8 August 2001 and Received in final form 14 March 2002     

Ginzburg-Landau theory and effects of pressure on a two-band superconductor: application to MgB <Emphasis Type="Bold">2</Emphasis>

We present a model of pressure effects of a two-band superconductor based on a Ginzburg-Landau free energy with two order parameters. The parameters of the theory are pressure as well as temperature dependent. New pressure effects emerge as a result of the competition between the two bands. The theory then is applied to MgB₂. We identify two possible scenaria regarding the fate of the two σ subbands under pressure, depending on whether or not both subbands are above the Fermi energy at ambient pressure. The splitting of the two subbands is probably caused by the E_2g distortion. If only one subband is above the Fermi energy at ambient pressure (scenario I), application of pressure diminishes the splitting and it is possible that the lower subband participates in the superconductivity. The corresponding crossover pressure and Gr neisen parameter are estimated. In the second scenario both bands start above the Fermi energy and they move below it, either by pressure or via the substitution of Mg by Al. In both scenaria, the possibility of electronical topological transition is emphasized. Experimental signatures of both scenaria are presented and existing experiments are discussed in the light of the different physical pictures. Received 3 September 2002 / Received in final form 16 December 2002 Published online 14 February 2003 RID="a" ID="a"e-mail: joseph.betouras@ua.ac.be RID="b" ID="b"On leave from N.S. Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, 31 Leninskii prospekt, 117915, Moscow, Russia     

Systematic field-theory for the hard-core one-component plasma

A continuous liquid flow in a vacuum (a liquid beam) of an aqueous solution of adenine salt containing hydrochloric acid or sodium hydroxide was irradiated with an intense pulsed IR laser at 3 μm, which is resonant to a vibrational mode related to the OH stretch vibration of H₂O. Neutral species isolated into the vacuum were ionized by a pulsed UV laser at 270 nm, and the product ions were mass-analyzed by a time-of-flight mass spectrometer. It is found that AH ₂ ^{2 + .} 2Cl^- and [A-iH] ^{i - .} iNa⁺ (i = 1-3) are isolated in the vacuum from the aqueous acidic and alkaline solutions, respectively, under irradiation of the IR laser, and undergo four-photon ionization involving decomposition and proton transfer of the intermediate species under irradiation of the UV laser. Received 1st May 2002 Published online 13 September 2002     

Production of medium-weight isotopes by fragmentation in 750 A MeV 238U on 208Pb collisions

Projectile fragmentation of ²³⁸U in a lead target was investigated at a bombarding energy of 750 A MeV. Isotopic production cross sections of about 250 different projectile fragments in the element range Z= 30–53 were measured with the FRagment Separator (FRS). The magnetic selection and the kinematical analysis of the measured isotopes allowed to disentangle fission and fragmentation residues. The mass loss of these residues indicates a violent collision where a large amount of energy is dissipated. The position of the fragmentation corridor defined by the measured residues was used to determine an effective proton-evaporation barrier. Received: 3 October 1997 / Revised version: 27 February 1998     

Antiferromagnetism of almost localized fermions: Evolution from Slater-type to Mott-Hubbard gap

We supplement (and critically overview) the existing extensive analysis of antiferromagnetic solution for the Hubbard model with a detailed discussion of two specific features, namely (i) the evolution of the magnetic (Slater) gap (here renormalized by the electronic correlations) into the Mott-Hubbard or atomic gap, and (ii) a rather weak renormalization of the effective mass by the correlations in the half-filled-band case, which contrasts with that for the paramagnetic case. The mass remains strongly enhanced in the non-half-filled-band case. We also stress the difference between magnetic and non-magnetic contributions to the gap. These results are discussed within the slave boson approach in the saddle-point approximation, in which there appears a non-linear staggered molecular field due to the electronic correlations that leads to the appearance of the magnetic gap. They reproduce correctly the ground-state energy in the limit of strong correlations. A brief comparison with the solution in the limit of infinite dimensions and the corresponding situation in the doubly-degenerate-band case with one electron per atom is also made. Received 16 December 2002 / Received in final form 5 February 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: ufspalek@if.uj.edu.pl     

Balance-equation approach to impact ionization induced by an intense terahertz radiation: Application to InAs/AlSb heterojunctions

We have extended the balance equations to account for conduction-valence interband impact ionization (II) process induced by an intense terahertz (THz) electromagnetic irradiation in semiconductors, and applied them to study the II effect on electron transport and electron-hole pair generation-recombination rate in THz-driven InAs/AlSb heterojunctions (HJ). As many as needed multiphoton channels are self-consistently taken into account for yielding a given accuracy. The time evolution of transport state including THz-radiation-induced II process are monitored in details by an extensive time-dependent analysis. Two different physical stages, the quasi-steady state and the complete steady-state, are clearly identified from the present calculations. Intersubband electron transfer rate and net electron-hole generation rate are derived as functions of the THz radiation strength E _ac for various radiation frequencies from f _ac = 0.42 to 6 THz at lattice temperatures T = 6 K. It's indicated that the THz radiation with a larger E _ac or a lower f _ac, has a stronger effect on electron transport and II process. Qualitative agreement is obtained between the calculated electron-hole generation rate and the available experimental data for InAs/AlSb HJ's at T = 6 K. Received 24 May 2002 / Received in final form 26 August 2002 Published online 31 October 2002 RID="a" ID="a"e-mail: jccao8@hotmail.com     

Proton-neutron structure of collective excitations in 94Mo

Due to the recent development of radioactive beam production, various direct reaction studies in reversed kinematics have been made to investigate the behavior of the N = 20 shell closure in the neutron-rich region. Coulomb excitation, proton inelastic scattering, and fragmentation of unstable nuclei have been studied with γ-ray detection. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: motobaya@rikkyo.ne.jp     

Electron capture in the ion-cluster collision: effect of the electronic interaction

We investigate the electron capture occurring in the collision between an ion A⁺ and a cluster A_n (n = 5). The process has been modelled within the Hubbard Hamiltonian,which takes into account the intrasite U electron correlation. An exact procedure has been numerically applied which involves all the excited states to examine the time evolution of the system during the collision. We have applied the model to the sodium case. We have investigated the time evolution of the electron population during the collision on the projectile versus the kinetic energy of the projectile. It displays some oscillations which means that the electron exchanges between the ion and the cluster occurs alternatively in one direction and the other. We also vary U and examine its influence on the dynamics of the oscillation of the average population. Finally the cross section is derived versus the energy and U. Received 29 November 2000     

Frequency-dependent fluctuational conductivity above in anisotropic superconductors: effects of a short wavelength cutoff

We discuss the excess conductivity at nonzero frequencies in a superconductor above T_c within the Gaussian approximation. We focus the attention on the temperature range not too close to T_c: within a time-dependent Ginzburg-Landau formulation, we phenomenologically introduce a short wavelength cutoff (of the order of the inverse coherence length) in the fluctuational spectrum to suppress high momentum modes. We treat the general cases of thin wires, anisotropic thin films and anisotropic bulk samples. We obtain in all cases explicit expressions for the finite frequency fluctuational conductivity. The dc case directly follows. Close to T_c the cutoff has no effect, and the known results for Gaussian fluctuations are recovered. Above T_c, and already for ε = ln(T/T _c) > 10^-2, we find strong suppression of the paraconductivity as compared to the Gaussian prediction, in particular in the real part of the paraconductivity. At high ε the cutoff effects are dominant. We discuss our results in comparison with data on high-T_c superconductors. Received 19 March 2002 Published online 25 June 2002     

Persistence of opinion in the Sznajd consensus model: computer simulation

The density of never changed opinions during the Sznajd consensus-finding process decays with time t as 1/t ^θ. We find θ ≃ 3/8 for a chain, compatible with the exact Ising result of Derrida et al. In higher dimensions, however, the exponent differs from the Ising θ. With simultaneous updating of sublattices instead of the usual random sequential updating, the number of persistent opinions decays roughly exponentially. Some of the simulations used multi-spin coding. Received 22 August 2002 / Received in final form 12 November 2002 Published online 31 December 2002     

Stern Gerlach interferometry with metastable argon atoms: an immaterial mask modulating the profile of a supersonic beam

A new Stern Gerlach interferometer operating with a nozzle beam of metastable argon atoms Ar^* (3p⁵ 4s, ³ P ₂) is described. The selection of incoming (polarisation) and outgoing (analysis) Zeeman sublevels is achieved by use of laser induced transitions at two wavelengths, 811.5 nm (closed J = 2 → J = 3 transition) and 801.5 nm (open J = 2 → J = 2 transition). Linear superpositions of Zeeman sublevels, just beyond the polariser and just before the analyser, are prepared by means of two zones where Majorana transitions take place. In between, a controlled magnetic field configuration (the phase object) is produced within a triple μ-metal shielding. Standard interference patterns are obtained by scanning the field and detecting the atoms by secondary electron emission from a Faraday cup. When a static radial magnetic gradient is used, the beam profile is modulated by interference. The transverse pattern, which can be translated at will by adding a homogeneous field, is observed for the first time using a multi-channel electron multiplier followed by a phosphor screen and a CCD camera. The results satisfactorily agree with all theoretical predictions. Received 27 June 2002 / Received in final form 20 September 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: perales@lpl.univ-paris13.fr RID="b" ID="b"UMR 7538 du CNRS