bluuebunny/crossref_metadata_2025 huggingface dataset:
Holography and SL(2,ℝ) symmetry in 2D Rindler space–time
Authors: Moretti Valter , Pinamonti Nicola
Published: December 2003

It is shown that it is possible to define quantum field theory of a massless scalar free field on the Killing horizon of a 2D Rindler space–time. Free quantum field theory on the horizon enjoys diffeomorphism invariance and turns out to be unitarily and algebraically equivalent to the analogous theory of a scalar field propagating inside Rindler space–time, no matter the value of the mass of the field in the bulk. More precisely, there exists a unitary transformation that realizes the bulk-boundary correspondence upon an appropriate choice for Fock representation spaces. Second, the found correspondence is a subcase of an analogous algebraic correspondence described by injective  * -homomorphisms of the abstract algebras of observables generated by abstract quantum free-field operators. These field operators are smeared with suitable test functions in the bulk and exact one-forms on the horizon. In this sense the correspondence is independent from the chosen vacua. It is proven that, under that correspondence, the “hidden” SL(2,ℝ) quantum symmetry found in a previous work gets a clear geometric meaning, it being associated with a group of diffeomorphisms of the horizon itself.
The dynamics of conformational isomerization in flexible biomolecules. I. Hole-filling spectroscopy of N-acetyl tryptophan methyl amide and N-acetyl tryptophan amide
Authors: Dian Brian C. , Longarte Asier , Winter Paul R. , Zwier Timothy S.
Published: December 2003

The conformational isomerization dynamics of N-acetyl tryptophan methyl amide (NATMA) and N-acetyl tryptophan amide (NATA) have been studied using the methods of IR-UV hole-filling spectroscopy (HFS) and IR-induced population transfer spectroscopy (IR-PTS), which were developed for this purpose. Single conformations of these molecules were selectively excited in well-defined NH stretch fundamentals. This excess energy was used to drive conformational isomerization. By carrying out the infrared excitation early in a supersonic expansion, the excited molecules were recooled into their zero-point levels, partially refilling the hole created in the ground state population of one of the conformers, and creating gains in population in other conformers. These changes in population were detected using laser-induced fluorescence downstream in the expansion. In HFS, the IR wavelength is fixed and the UV laser tuned in order to determine where the population went following selective infrared excitation. In IR-PTS, the UV is fixed to monitor the population of a given conformation, and the IR is tuned to record the IR-induced changes in the population of the monitored conformer. Besides demonstrating the capability of the experiment to change the downstream conformational population distribution, the IR-PTS scans were used to extract two quantitative results: (i) The fractional populations of the conformers in the absence of the infrared, and (ii) the isomerization quantum yields for each of the six unique amide NH stretch fundamentals (three conformers each with two amide groups). The method for obtaining quantum yields is described in detail. In both NATMA and NATA, the quantum yields show modest conformational specificity, but only a hint of vibrational mode specificity. The prospects for the hole-filling technique for providing insight into energy flow in large molecules are discussed, leaving a more detailed theoretical modeling to the adjoining paper [Evans et al. J. Chem. Phys. 120, 148 (2004)].
Plasma gun supersonic vortex magnetohydrodynamic dynamo for magnetized target fusion
Authors: Winterberg F.
Published: December 2003

Supersonic plasma jets injected with a large azimuthal velocity component into a magnetic cusp field entrap the radial field component in between the coalescing jets, with the cushioning effect of the magnetic field diminishing the formation of shocks which otherwise would occur by the collision of the jets. With the bending of the radial magnetic lines of force into an azimuthal direction by the vortex flow of the coalescing jets, a magnetohydrodynamic dynamo with an axial current is established. With the radial velocity component of the jets compressing the magnetic field and the plasma formed by the coalescing jets, a closed field line configuration is established. If dense denterium–tritium (DT) gas is injected into the vortex core, the axial current passes through the DT resulting in a dense pinch discharge stabilized by the rapidly rotating vortex flow.
WKB analysis of the Regge–Wheeler equation down in the frequency plane
Authors: van den Brink Alec Maassen
Published: December 2003

The Regge–Wheeler equation for black-hole gravitational waves is analyzed for large negative imaginary frequencies, leading to a calculation of the cut strength for waves outgoing to infinity. In the—limited—region of overlap, the results agree well with numerical findings [Leung et al., Class. Quantum Grav. 20, L217 (2003)]. Requiring these waves to be outgoing into the horizon as well subsequently yields an analytic formula for the highly damped Schwarzschild quasinormal modes, including the leading correction. Just as in the WKB quantization of, e.g., the harmonic oscillator, solutions in different regions of space have to be joined through a connection formula, valid near the boundary between them where WKB breaks down. For the oscillator, this boundary is given by the classical turning points; fascinatingly, the connection here involves an expansion around the black-hole singularity r=0.
A polymer expansion for the quantum Heisenberg ferromagnet wave function
Authors: Federbush Paul
Published: December 2003

A polymer expansion is given for the quantum Heisenberg ferromagnet wave function. Working on a finite lattice, one is dealing entirely with algebraic identities; there is no question of convergence. The conjecture to be pursued in further work is that effects of large polymers are small. This is relevant to the question of the utility of the expansion and its possible extension to the infinite volume. In themselves the constructions of the present paper are neat and elegant and have surprising simplicity.
Weak coherent state path integrals
Authors: Hartmann Lorenz , Klauder John R.
Published: December 2003

Weak coherent states share many properties of the usual coherent states, but do not admit a resolution of unity expressed in terms of a local integral. They arise, e.g., in the case that a group acts on an inadmissible fiducial vector. Motivated by the recent Affine Quantum Gravity Program, the present article studies the path integral representation of the affine weak coherent state matrix elements of the unitary time-evolution operator. Since weak coherent states do not admit a resolution of unity, it is clear that the standard way of constructing a path integral, by time slicing, is predestined to fail. Instead a well-defined path integral with Wiener measure, based on a continuous-time regularization, is used to approach this problem. The dynamics is rigorously established for linear Hamiltonians, and the difficulties presented by more general Hamiltonians are addressed.
Exact effective Hamiltonian theory. II. Polynomial expansion of matrix functions and entangled unitary exponential operators
Authors: Siminovitch David , Untidt Thomas , Nielsen Niels Chr.
Published: December 2003

Our recent exact effective Hamiltonian theory (EEHT) for exact analysis of nuclear magnetic resonance (NMR) experiments relied on a novel entanglement of unitary exponential operators via finite expansion of the logarithmic mapping function. In the present study, we introduce simple alternant quotient expressions for the coefficients of the polynomial matrix expansion of these entangled operators. These expressions facilitate an extension of our previous closed solution to the Baker–Campbell–Hausdorff problem for SU(N) systems from N⩽4 to any N, and thereby the potential application of EEHT to more complex NMR spin systems. Similarity matrix transformations of the EEHT expansion are used to develop alternant quotient expressions, which are fully general and prove useful for evaluation of any smooth matrix function. The general applicability of these expressions is demonstrated by several examples with relevance for NMR spectroscopy. The specific form of the alternant quotients is also used to demonstrate the fundamentally important equivalence of Sylvester’s theorem (also known as the spectral theorem) and the EEHT expansion.
Direct-drive-implosion experiments with enhanced fluence balance on OMEGA
Authors: Marshall F. J. , Delettrez J. A. , Epstein R. , Forties R. , Keck R. L. , Kelly J. H. , McKenty P. W. , Regan S. P. , Waxer L. J.
Published: December 2003

Direct-drive experiments on OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] are now regularly performed with enhanced single-beam uniformity (1-THz-bandwidth smoothing by spectral dispersion, including distributed phase plates and polarization rotators). The on-target-beam-energy balance is determined by a set of beam pickoffs that are cross-calibrated to full beam calorimeters and by independent measurements for each beam of the losses incurred in transporting the beams to the target chamber. Variations of the illumination due to beam-to-beam, on-target energy differences and beam-to-beam shape differences can then introduce variations of the illumination and hence drive symmetry that will affect implosions. A new technique has been implemented that determines variations of the beam peak fluences at target chamber center on a full-power target shot by simultaneously measuring the x-ray flux produced by all 60 beams separated on a 4-mm-diam, Au-coated spherical target. The inferred beam-to-beam, peak-fluence variations are minimized by changing the gain of the last laser amplifier (enhanced fluence balance). Results of implosion experiments with enhanced balance show more spherically shaped stagnation cores, consistent with the reduction of low-spherical-harmonic-mode components (l⩽6) of the illumination nonuniformity.
Degradation of inter-atomic bonds during structural phase change in intermediate Ni-clusters (Ni39–Ni49)
Authors: Andriotis Antonis N. , Menon Madhu
Published: December 2003

Results based on a symmetry- and spin-unrestricted tight-binding molecular-dynamics study are presented for the ground-state geometries of intermediate Nin, n∈[39,49], clusters. A structural phase change is found to take place around n=43 during which a structural transition from fcc/hcp structure to icosahedral one is observed. This is in good agreement with recent experimental findings. This structural transition is found to be associated with a degradation of the inter-atomic bond energy which indicates that the inter-atomic bond does not only depend on the coordination number of each atom but also on its point group symmetry.
Robust feedback systems for resistive wall modes
Authors: Boozer Allen H.
Published: December 2003

The feedback stabilization of magnetohydrodynamic instabilities that grow on the resistive time scale of the chamber walls is an important issue for steady-state tokamaks. Constraints are derived on the inductance coefficients of the feedback system that will ensure that fixed amplification coefficients can be used to provide stability over the entire region in which feedback is in principle possible. Such feedback systems are called robust. Successful feedback is possible for systems that do not satisfy these constraints, but then the signs of the feedback coefficients will depend on the level of plasma instability.
Classification of derivation-simple color algebras related to locally finite derivations
Authors: Su Yucai , Zhao Kaiming , Zhu Linsheng
Published: December 2003

We classify the pairs (𝒜,𝒟) consisting of an (ε,Γ)-color-commutative associative algebra 𝒜 with an identity element over an algebraically closed field 𝔽 of characteristic zero and a finite dimensional subspace 𝒟 of (ε,Γ)-color-commutative locally finite color-derivations of 𝒜 such that 𝒜 is Γ-graded 𝒟-simple and the eigenspaces for elements of 𝒟 are Γ-graded. Such pairs are the important ingredients in constructing some simple Lie color algebras which are in general not finitely-graded. As some applications, using such pairs, we construct new explicit simple Lie color algebras of generalized Witt type, Weyl type.
Observation of radial propagation of electrostatic fluctuations in toroidal plasma
Authors: Singh A. K. , Kaur R. , Mattoo S. K. , Hirose A.
Published: December 2003

There has been a great deal of activity in the area of the detection of radial propagation of electrostatic fluctuations in toroidal magnetic fusion devices. The role of radial propagation is considered extremely important in the understanding of cross field anomalous transport. Radial propagation of low frequency electrostatic turbulence in a purely toroidal device, having no rotational transform, is reported in this paper. The propagation characteristics of the turbulence has been studied using Langmuir probes. It is found that the turbulence has a finite propagation in the radial direction and the radial speed is a sizeable fraction of the poloidal speed.
Import of a set of solutions of the homogeneous Bloch equation
Authors: Kobayashi M.
Published: December 2003

We show for a class of time-varying magnetic fields that a generalized solution of precession is decomposed by two ways into two solutions of the homogeneous Bloch equation, thus having four solutions of which three are independent and form the fundamental systems. These solutions are found by solving the Riccati equation to which the homogeneous Bloch equation reduces. A brief discussion of the magnectic field class characteristics and an illustrative exmaple are given.
Unsteady equipartition MHD solutions
Authors: Bogoyavlenskij Oleg I.
Published: December 2003

The unsteady equipartition solutions are introduced that describe a plasma relaxation in regime of a strong interaction between the plasma velocity V and the magnetic field B and depend on all four variables t,x,y,z. The solutions exist when the kinematic viscosity ν is equal to the magnetic diffusivity η and V=±B/ρμ. The exact equipartition solutions to the boundary value problem with the “no-slip” boundary condition are derived. The solutions depend on infinitely many parameters and describe the plasma relaxation in a ball with a constant pressure at the boundary. The translationally invariant exact solutions are obtained for ν≠η for which plasma is confined in cylindrical domains by the magnetic field B only while the plasma pressure P is zero at the boundary and is positive inside the domains.
Effective Lagrangians for scalar fields and finite size effects in field theory
Authors: Caicedo M. I. , Svaiter N. F.
Published: December 2003

We first discuss the approach of effective field theory in a d-dimensional Euclidean space. We consider a model with two interacting scalar fields (Lint(φ1,φ2)=(λ2/2)(φ1φ2)2) with masses m1 and m2. Assuming m2≫m1 we show that there is a decoupling in the effective theory describing the dynamic of the light mass field. Furthermore, we consider the presence of two parallel hyperplanes which break translational symmetry, with a natural cutoff satisfying m2≫Λ>m1. Then imposing Dirichlet and also Neumann boundary conditions, we study the perturbative renormalization of the effective theory (λφ4) in a region bounded by the two parallel hyperplanes in the one-loop approximation.
Precipitation of oppositely charged polyelectrolytes in salt solutions
Authors: Kudlay Alexander , Olvera de la Cruz Monica
Published: December 2003

We study phase separation in symmetric solutions of weakly charged flexible chains of opposite sign. Precipitation is caused by effective attractions due to charge fluctuations and by short-range attractions between monomers. The contribution from charge fluctuations is computed within the random phase approximation (RPA), which takes into account the connectivity of charges in the polyions. The impenetrability of the ions is accounted for by using a modified Coulomb potential in the RPA. In good solvent conditions the precipitate monotonically swells and eventually dissolves upon addition of salt. However, near the θ-solvent condition, but still in the good solvent, the precipitate can be stable at any salt concentration. Moreover, the density of the precipitate after initial decrease can increase with addition of salt. This effect is a result of redistribution of salt between the precipitate and the supernatant, which is due to an interplay of electrostatic and hardcore interactions. For not too weakly charged polyions the precipitate properties become strongly dependant on temperature even in good solvent conditions.
Two-dimensional Raman and infrared vibrational spectroscopy for a harmonic oscillator system nonlinearly coupled with a colored noise bath
Authors: Kato Tsuyoshi , Tanimura Yoshitaka
Published: December 2003

Multidimensional vibrational response functions of a harmonic oscillator are reconsidered by assuming nonlinear system–bath couplings. In addition to a standard linear–linear (LL) system–bath interaction, we consider a square–linear (SL) interaction. The LL interaction causes the vibrational energy relaxation, while the SL interaction is mainly responsible for the vibrational phase relaxation. The dynamics of the relevant system are investigated by the numerical integration of the Gaussian–Markovian Fokker–Planck equation under the condition of strong couplings with a colored noise bath, where the conventional perturbative approach cannot be applied. The response functions for the fifth-order nonresonant Raman and the third-order infrared (or equivalently the second-order infrared and the seventh-order nonresonant Raman) spectra are calculated under the various combinations of the LL and the SL coupling strengths. Calculated two-dimensional response functions demonstrate that those spectroscopic techniques are very sensitive to the mechanism of the system–bath couplings and the correlation time of the bath fluctuation. We discuss the primary optical transition pathways involved to elucidate the corresponding spectroscopic features and to relate them to the microscopic sources of the vibrational nonlinearity induced by the system–bath interactions. Optical pathways for the fifth-order Raman spectroscopies from an “anisotropic” medium were newly found in this study, which were not predicted by the weak system–bath coupling theory or the standard Brownian harmonic oscillator model.
Combined electronic and nuclear dynamics in a simple model system. II. Spectroscopic transitions
Authors: Erdmann M. , Engel V.
Published: December 2003

We investigate the correlated electronic and nuclear motion in a model system as proposed by Shin and Metiu [J. Chem. Phys. 102, 9285 (1995)]. The quantum dynamics is studied during laser induced electronic transitions. Here, the influence of nonadiabatic coupling on the absorption spectrum is investigated and the Franck–Condon principle is illustrated in terms of the temporal changes of electronic and nuclear densities. In the case of intense field excitation, multiphoton processes become important, and electronic as well as vibrational wave packets are prepared.
Theoretical analysis of the relation between H2 dissociation and reflection on Pd surfaces
Authors: Dı́az C. , Martı́n F. , Busnengo H. F. , Salin A.
Published: December 2003

We study the scattering of H2 (v=0, J=0) molecules by the Pd(110) surface using classical trajectory methods. We show that the dissociative adsorption probability barely depends on incidence angle (total energy scaling) up to an impact energy of 200 meV. This is the consequence of a “loss of memory” of the initial incidence angle, mostly due to dynamic trapping, which also reflects itself in a cosinelike angular distribution of reflected molecules. Consequently, a cosinelike distribution can be the result of a subpicosecond process that involves neither energy dissipation to the surface nor transient thermal accommodation.
Virasoro algebra with central charge c=1 on the horizon of a two-dimensional-Rindler space–time
Authors: Moretti Valter , Pinamonti Nicola
Published: December 2003

Using the holographic machinery built up in a previous work, we show that the hidden SL(2,R) symmetry of a scalar quantum field propagating in a Rindler space–time admits an enlargement in terms of a unitary positive-energy representation of Virasoro algebra defined in the Fock representation. That representation has central charge c=1. The Virasoro algebra of operators gets a manifest geometrical meaning if referring to the holographically associated quantum field theory on the horizon: It is nothing but a representation of the algebra of vector fields defined on the horizon equipped with a point at infinity. All that happens provided the Virasoro ground energy h≔μ2/2 vanishes and, in that case, the Rindler Hamiltonian is associated with a certain Virasoro generator. If a suitable regularization procedure is employed, for h=1/2, the ground state of that generator seems to correspond to a thermal state when examined in the Rindler wedge, taking the expectation value with respect to Rindler time. Finally, under Wick rotation in Rindler time, the pair of quantum field theories which are built up on the future and past horizon defines a proper two-dimensional conformal quantum field theory on a cylinder.
Imaging the pair-correlated excitation function: The F+CH4→HF(v′)+CH3(ν=0) reaction
Authors: Shiu Weicheng , Lin Jim J. , Liu Kopin , Wu Malcom , Parker David H.
Published: December 2003

The velocity map ion imaging technique was applied to measure the reaction excitation function for the first time. It was found that the “raw” excitation function was significantly distorted by the density-to-flux transformation of the title reaction. Through a systematic investigation, possible reasons for such a dramatic effect are outlined. In addition, the state-resolved, pair-correlated excitation functions and branching ratios are presented. Effects of imperfect time slicing in the time-sliced velocity imaging technique in general are also discussed.
On-the-fly ab initio trajectory calculations of the dynamics of Cl atom reactions with methane, ethane and methanol
Authors: Rudić Svemir , Murray Craig , Harvey Jeremy N. , Orr-Ewing Andrew J.
Published: December 2003

The dynamics of Cl atom reactions with methane, ethane, and methanol have been studied by calculation of quasi-classical trajectories, with computation of potential energies and gradients only at the geometries through which the trajectories pass. Trajectories were started from the transition state, with 2 kcal mol−1 of energy given to the mode with an imaginary frequency (representing the reaction coordinate at the transition state) and inclusion of zero-point energy in some or all of the remaining vibrational modes. The trajectories were propagated as far as separated products, with the majority of potential energy calculations performed at the HF/6-31G level of theory. The rotational quantum state population distributions of the HCl products from the reactions of Cl atoms with methane, ethane and methanol peaked at J′=1, 2, and 6, respectively. The calculations thereby exhibit somewhat greater rotational excitation than is found experimentally, but correctly describe the trend of increasing HCl product rotation for the three respective reactions. In agreement with previous observations, only 4% of the energy available to the products of the reaction of Cl atoms with methane was channeled into CH3 radical internal energy, and 1% into HCl rotation, with 92% ending up as translational energy. For the reaction of Cl atoms with ethane and with methanol, the corresponding values for radical internal energy, HCl rotation and product translation are 21, 3, and 78%, and 46, 13, and 42%, respectively. For the latter two reactions, the radical internal energy is mostly accounted for by rotational motion. The clear increase in rotational excitation of the HCl products from the Cl atom reaction with methanol is explained in terms of a dipole-dipole interaction between the departing polar fragments. A smaller set of more computationally expensive trajectory calculations using potentials and gradients from the MP2/6-311G(d,p) level of theory were performed for reactions of Cl atoms with methanol, and give results in better agreement with experimentally measured HCl rotational excitation, consistent with the model of dipole-induced product rotation because the MP2/6-311G(d,p) calculations give smaller dipole moments for both products than the HF/6-31G calculations. The calculated angles between the rotational angular momentum vectors and recoil velocities of the radical peak sharply at 90° for the reactions of Cl atoms with ethane and methanol, but exhibit a much broader distribution for reaction with methane.
Theoretical study of the He–HF+ complex. I. The two asymptotically degenerate ground state potential energy surfaces
Authors: Lotrich Victor F. , Wormer Paul E. S. , van der Avoird Ad
Published: December 2003

Two three-dimensional potential energy surfaces (PESs) are reported for the cationic complex He–HF+; they are degenerate for linear geometries of the complex and correlate with the doubly degenerate X 2Π ground state of the HF+ monomer. The PESs are computed from the interaction energies of the neutral dimer and the ionization potentials of the He–HF complex and the HF molecule. Ionization potentials are obtained from the outer valence Green’s function (OVGF) method, while the energies of the neutral species are computed by means of the single and double coupled-cluster method with perturbative triples [CCSD(T)]. For comparison, interaction energies of the ionic complex were computed also by the use of the partially spin-restricted variant of the CCSD(T) method. After asymptotic scaling of the OVGF results, good agreement is found between the two methods. A single global minimum is found in the PES, for the linear He–HF+ geometry. The well depth and equilibrium separation are 2.240 Å and 1631.3 cm−1, respectively, at an HF+ bond length r=1.0012 Å, in rather good agreement with results of Schmelz and Rosmus [Chem. Phys. Lett. 220, 117 (1994)]. The well depth depends much more strongly on the internuclear H–F separation than in the neutral He–HF complex and the global minimum in a full three-dimensional PES occurs at r=1.0273 Å.
Surface forces in polymer fluids: A comparison between simulations and density functional theory
Authors: Forsman Jan , Broukhno Andrei , Jönsson Bo , Åkesson Torbjörn
Published: December 2003

A polymer density functional theory is evaluated in terms of its ability to predict interactions between large surfaces in a polymer fluid. Comparisons are made with results from simulations in an expanded isotension ensemble. The variation of the net surface–surface interaction with adsorption strength is examined. Cases where the monomers interact via a pure hard sphere potential are investigated, but we have also studied the effect of attractions between the monomers. In all cases, we obtain an almost quantitative agreement between the simulated results and the predictions from the polymer density functional theory.
Dynamical multiple-time stepping methods for overcoming resonance instabilities
Authors: Chin Siu A.
Published: December 2003

Current molecular dynamics simulations of biomolecules using multiple time steps to update the slowly changing force are hampered by instabilities beginning at time steps near the half period of the fastest vibrating mode. These “resonance” instabilities have became a critical barrier preventing the long time simulation of biomolecular dynamics. Attempts to tame these instabilities by altering the slowly changing force and efforts to damp them out by Langevin dynamics do not address the fundamental cause of these instabilities. In this work, we trace the instability to the nonanalytic character of the underlying spectrum and show that a correct splitting of the Hamiltonian, which renders the spectrum analytic, restores stability. The resulting Hamiltonian dictates that in addition to updating the momentum due to the slowly changing force, one must also update the position with a modified mass. Thus multiple-time stepping must be done dynamically.
Microscopic theory of orientational order, structure and thermodynamics in strained polymer liquids and networks
Authors: Oyerokun Folusho T. , Schweizer Kenneth S.
Published: December 2003

A microscopic integral equation theory of the segmental orientational order parameter, structural correlations and thermodynamics of strained polymer solutions, melts and networks has been developed. The nonclassical problem of the consequences of intermolecular excluded volume repulsions and chain connectivity is addressed. The theory makes several novel predictions, including effective power law dependences of the orientational order parameter on monomer concentration and chain degree of polymerization, and strain hardening of the bulk modulus. The predictions of a nearly classical strain dependence, and supralinear scaling with segment concentration, of the strain-induced nematic order parameter is in agreement with nuclear magnetic resonance experiments. The absolute magnitudes of the a priori calculated orientational order parameter agree with simulations and experiments to within a factor of 2. The possible complicating influence of “trapped entanglements” in crosslinked networks is discussed. Extensions of the theory are possible to treat the mechanical response of flexible polymer liquids and rubbers, and the structure, thermodynamics, and mechanical properties of strained liquid crystal forming polymers.
Self-organization towards helical states in the Toroidal Pinch Experiment reversed-field pinch
Authors: Piovesan P. , Spizzo G. , Yagi Y. , Koguchi H. , Shimada T. , Hirano Y. , Martin P.
Published: December 2003

In this paper results of an experimental study of magnetic fluctuations in the Toroidal Pinch Experiment (TPE–RX) [Y. Yagi, S. Sekine, H. Sakakita et al., Fus. Eng. Design 45, 409 (1999)] are reported. This study has permitted the identification of the experimental conditions under which the usually turbulent spectrum of magnetic fluctuations in TPE–RX turns into a single tearing mode, in the so-called quasi-single helicity (QSH) state. The large data set has allowed us to provide a complete two dimensional map of QSH probability in the current F plane, being F=Bφ(a)/〈Bφ〉. Two main basins of high QSH probability have been found. The first one, at high current and low |F|, confirms previous results on QSH in other machines; the second one is completely new, and is found at low current and high |F|. These studies show that QSH is a robust, device-independent phenomenon, and a significant step towards a laminar type of dynamo in the reversed-field pinch configuration.
Confluence expansions of the generalized hypergeometric function
Authors: Nagel Bengt
Published: December 2003

By confluencing a subset of upper and lower parameters in the generalized hypergeometric function FQP(a1,,…,aP,c1,…,cQ;z) with the variable z one obtains a lower-order hypergeometric function in the limit when the confluence parameters go to infinity. It is shown that this function is the first term in a convergent expansion in terms of functions of the same type with parameters increasing stepwise by integers ν and coefficients which are polynomials in the reciprocals of the confluence parameters. These polynomials have nonvanishing lowest degree terms whose power increases with ν. The expansion can hence be used to derive asymptotic expansions for large but finite absolute values of the confluence parameters.
Theta functions on noncommutative T4
Authors: Kim Hoil , Lee Chang-Yeong
Published: December 2003

We construct the so-called theta vectors on noncommutative T4, which correspond to the theta functions on commutative tori with complex structures. Following the method of Dieng and Schwarz, we first construct holomorphic connections and then find the functions satisfying the holomorphic conditions, the theta vectors. The holomorphic structure in the noncommutative T4 case is given by a 2×2 complex matrix, and the consistency requires its off-diagonal elements to be the same. We also construct the tensor product of these functions satisfying the consistency requirement.
Phenylene ring dynamics in bisphenol-A-polysulfone by neutron scattering
Authors: Arrese-Igor S. , Arbe A. , Alegrı́a A. , Colmenero J. , Frick B.
Published: December 2003

We have investigated the dynamics of phenylene rings in a glassy polysulfone (bisphenol-A-polysulfone) by means of quasielastic neutron scattering. Nowadays it is well known that these molecular motions are directly connected with the mechanical properties of engineering thermoplastics in general. The particular system investigated by us has the advantage that by selective deuteration of the methyl groups, the neutron scattering measured is dominated by the incoherent contribution from the protons in the phenylene rings. In this way, the dynamics of such molecular groups can be experimentally isolated. Two different types of neutron spectrometers: time of flight and backscattering, were used in order to cover a wide dynamic range, which extends from microscopic (10−13 s) to mesoscopic (10−9 s) times. Moreover, neutron diffraction experiments with polarization analysis were also carried out in order to characterize the structural features of the sample investigated. Fast oscillations of increasing amplitude with temperature and π-flips are identified for phenylene rings motions. Due to the structural disorder characteristic of the amorphous state, both molecular motions display a broad distribution of relaxation times, which spreads over several orders of magnitude. Based on the results obtained, we propose a model for phenylene rings dynamics, which combines the two kinds of molecular motions identified. This model nicely describes the neutron scattering results in the whole dynamic range investigated.
Studies of translational diffusion in the smectic A phase of a Gay–Berne mesogen using molecular dynamics computer simulation
Authors: Bates Martin A. , Luckhurst Geoffrey R.
Published: December 2003

Molecular dynamics computer simulations are used to determine the self-diffusion coefficients for a Gay–Berne model mesogen GB (4.4,20,1,1) in the isotropic, nematic and smectic A phases along two isobars. The values of the parallel and perpendicular diffusion coefficients, D∥ and D⊥, are calculated and compared in the different phases. For the phase sequence isotropic-smectic A, D⊥*⩾D∥* over the whole smectic A range with the ratio D∥*/D⊥* decreasing with decreasing temperature. At a higher pressure, a nematic phase is observed between these two phases and we find that D∥*>D⊥* throughout the nematic region and the inequality D∥*>D⊥* remains on entering the smectic A phase. However, the ratio D∥*/D⊥* decreases with decreasing temperature within the smectic A range and eventually this ratio inverts such that D⊥*>D∥* at low temperatures. The temperature dependence of the parallel diffusion coefficient in the smectic A phase for this model mesogen is compared to that predicted by a theoretical model for diffusion subject to a cosine potential.
Accurate ab initio pair potentials between helium and the heavier group 2 elements
Authors: Lovallo Christopher C. , Klobukowski Mariusz
Published: December 2003

Interactions between the heavier Group 2 metals (Ca, Sr, and Ba) and helium were studied using the well-tempered model core potential method. Accurate pair potentials, calculated at the coupled-cluster level of theory with very large basis sets, were used in bound state calculations. Three bound rovibrational states were found for each complex. The pair-potential parameters were used to predict how each of the metal atoms would be solvated by a helium nanodroplet. The Ca atom is not fully solvated by the droplet and the interaction between the helium and the metal decreases from Ca to Ba. This agrees with the experimental observation that the spectra of these atoms in a nanodroplet are intermediate between the spectra of the free atoms and the spectra in liquid helium.
The well-tempered auxiliary-field Monte Carlo
Authors: Jacobi Shlomit , Baer Roi
Published: December 2003

The auxiliary-field Monte Carlo (AFMC) is a method for computing ground-state and excited-state energies and other properties of electrons in molecules. For a given basis set, AFMC is an approximation to full-configuration interaction and the accuracy is determined predominantly by an inverse temperature “β” parameter. A considerable amount of the dynamical correlation energy is recovered even at small values of β. Yet, nondynamical correlation energy is inefficiently treated by AFMC. This is because the statistical error grows with β, warranting increasing amount of Monte Carlo sampling. A recently introduced multi-determinant variant of AFMC is studied, and the method can be tuned by balancing the sizes of the determinantal space and the β-parameter with respect to a predefined target accuracy. The well-tempered AFMC is considerably more efficient than a naı̈ve AFMC. As a welcome “byproduct” low lying excitation energies of the molecule are supplied as well. We demonstrate the principles on dissociating hydrogen molecule and torsion of ethylene where we calculate the (unoptimized) torsional barrier and the vertical singlet-triplet splitting.
Density functional theory with alternative spin densities: Application to magnetic systems with localized spins
Authors: Pérez-Jiménez Ángel J. , Pérez-Jordá José M. , Illas Francesc
Published: December 2003

A new method to improve the excess spin density obtained from unrestricted Hartree–Fock wave functions in terms of natural orbitals is proposed. Using this modified excess spin density to evaluate the correlation energy by means of density functionals leads to large improvements in the computed magnetic coupling constants of several materials without need to modify the exchange contribution. This is important because it reconciles the density functional theory description with the one provided by multi-determinant wave functions. Using the present approach, the leading contribution to the magnetic coupling constant arises from electron correlation effects. The performance of the new method is illustrated on various materials including high-critical-temperature superconductors parent compounds.
A diffusion quantum Monte Carlo study of geometries and harmonic frequencies of molecules
Authors: Lu Shih-I
Published: December 2003

This article describes an approach in determination of equilibrium geometries and harmonic frequencies of molecules by the Ornstein–Uhlenbeck diffusion quantum Monte Carlo method based on the floating spherical Gaussians. In conjunction with a projected and renormalized Hellmann–Feynman gradient and an electronic energy at variational Monte Carlo and diffusion quantum Monte Carlo, respectively, the quasi-Newton algorithm implemented with the Broyden–Fletcher–Goldfarb–Shanno updated Hessian was used to find the optimized molecular geometry. We applied this approach to N2 and H2O molecules. The geometry and harmonic frequencies calculated were consistent with some sophisticated ab initio calculated values within reasonable statistical uncertainty.
Fock space multireference coupled cluster calculations based on an underlying bivariational self-consistent field on Auger and shape resonances
Authors: Sajeev Y. , Mishra Manoj K. , Vaval Nayana , Pal Sourav
Published: December 2003

The Fock space multireference coupled cluster based on an underlying bivariational self-consistent field is applied to the problem of computing complex energy associated with Auger and shape resonances in e-atom scattering. It is concluded that the Fock space multireference coupled cluster based on a bivariational self-consistent field provides a useful and practical approach to calculation of resonance parameters. Numerical results are presented for the P2 shape resonance of Mg and Auger 1 s−1 hole of Be.
The size of neutral free clusters as manifested in the relative bulk-to-surface intensity in core level photoelectron spectroscopy
Authors: Tchaplyguine M. , Marinho R. R. , Gisselbrecht M. , Schulz J. , Mårtensson N. , Sorensen S. L. , Naves de Brito A. , Feifel R. , Öhrwall G. , Lundwall M. , Svensson S. , Björneholm O.
Published: December 2003

A new approach for obtaining an estimate of the effective size of the free neutral clusters is proposed. The approach relies on an experimental measure of the surface and interior or “bulk” cluster atoms provided by the x-ray photoelectron spectroscopy and on a model for the attenuation of photoelectrons ejected from the bulk of the cluster as the result of the ionizing irradiation. The experimental part gives the ratio of the electron signal from the bulk cluster atoms to that from the cluster surface atoms for a wide range of cluster sizes and electron kinetic energies. The attenuated response of the bulk atoms is modeled using an exponential law with the cluster size and kinetic-energy-dependent electron escape depth as parameters. For the experimental size range, model-based calculations for Ar, Kr, and Xe clusters are presented. The cluster size estimates obtained from comparison of the model calculations and experimental results agree well with those determined from the parameters of the cluster creation process. The combination of experiment and modeling also makes it possible to estimate the effective escape depth for electron propagation in free clusters. For Ar, Kr, and Xe clusters of varying mean size, absolute determination of the surface and bulk electron binding energies of the core levels used in the experiments has also been made.
Electron attachment to PSCl3
Authors: Knighton W. B. , Miller Thomas M. , Grimsrud E. P. , Viggiano A. A.
Published: December 2003

Electron attachment to PSCl3 was studied in 133-Pa pressure of helium gas at temperatures from 298–550 K. Measurements of rate constants and branching fractions were made in a flowing-afterglow Langmuir-probe (FALP) apparatus. These experiments yielded an electron attachment rate constant of 5.1×10−8 cm3 s−1 that was found not to change significantly in the 298–550 K temperature range. This rate constant represents an attachment efficiency of about 14%. Attachment in 133 Pa of He gas yielded only the dissociative ion products PSCl2− and Cl−. The FALP data suggest that there is an activation energy of about 17 meV for production of PSCl2−. Attachment to PSCl3 was also studied at high pressure (9–93 kPa) of N2 in an ion mobility mass spectrometer, at 298 K. In contrast to the low-pressure data, the parent anion product channel (PSCl3−) was observed (along with the dissociative channels), and increased in importance with N2 pressure. Gaussian-3 (G3) calculations were carried out for PSCl3 and PSCl2 neutrals and anions to aid in interpretation of the experimental results. The calculations indicate that the electron affinity EA(PSCl2) is slightly smaller than EA(Cl), which may account for the observed branching fractions for PSCl2− and Cl− in the low-pressure experiments. A natural population analysis was performed to obtain the charges associated with each atom in the molecules in order to estimate how the attached electron is distributed. Comparison is made between the present study of electron attachment to PSCl3 and our earlier work on attachment to POCl3, and G3 calculations are reported here for neutral and anionic POCl2 and POCl3. In contrast to PSCl2, the calculations imply that EA(POCl2) is slightly greater than EA(Cl). For both PSCl3 and POCl3, the calculations show that the dissociative electron attachment process is close to thermoneutral.
Electronic interactions in a branched chromophore investigated by nonlinear optical and time-resolved spectroscopy
Authors: Lahankar Sridhar Arun , West Richard , Varnavski Oleg , Xie Xingbin , Goodson T. , Sukhomlinova Ludmila , Twieg Robert
Published: December 2003

The third order nonlinear optical properties of a trimer branched chromophore system and its linear molecule analog are investigated. Two-photon absorption and degenerate four wave mixing measurements were carried out on both systems. An enhancement in the nonlinear optical effect is observed for the branched trimer molecule in comparison to the linear chromophore system. Ultrafast time-resolved measurements were carried out to probe the excited state dynamics in the branched structures. The time-resolved measurements suggest that the two important processes affecting the nonlinear optical properties in the trimer system, charge transfer stabilization and initial electronic delocalization, occur on two different time scales.
Proton transfer dynamics of the reaction H3O+(NH3,H2O)NH4+ studied using the crossed molecular beam technique
Authors: Li Yue , Farrar James M.
Published: December 2003

The proton transfer reaction of H3O+ and NH3 was studied using the crossed molecular beam technique at relative energies of 0.41, 0.81, and 1.27 eV. At all three energies, the center-of-mass flux distribution of the product ion NH4+ exhibits sharply asymmetry, and the maximum is close to the velocity and direction of the precursor ammonia beam. The reaction transforms almost all of the 1.69 eV exothermicity into internal excitation of the products at all three collision energies. At the lowest collision energy of 0.41 eV, nearly 77% of the total energy appears in NH4+ internal excitation. However, almost 100% of the incremental translational energy in the two higher-energy experiments appears in the product translational energy. Such an observation provides a classic example of the “induced repulsive energy release” mechanism that is expected to be operative on the highly skewed potential energy surfaces characteristic of the heavy+light–heavy mass combination. These results indicate that the proton transfer proceeds through a direct reaction mechanism; a Rice–Ramsperger–Kassel–Marcus theory calculation shows that the lifetime of the intermediate complex [NH3–H–H2O]+ is about 100 fs. Proton transfer occurs early on the reaction coordinate, when the incipient N–H bond is extended, and results in highly vibrationally excited NH4+ products, with excitation primarily in N–H stretching modes.
Physics of non-thermal Penning-trap electron plasma and application to ion trapping
Authors: Schauer M. M. , Barnes D. C. , Umstadter K. R.
Published: December 2003

A modified Penning–Malmberg trap is used to produce a reflexing beam electron distribution. Positive ions are trapped in the electron space charge within a shaped anode at the center of the beam. Electron reflexing of greater than 106 circuits is observed, with a perveance several times that of earlier single pass devices. Classical slowing of the electron beams is observed. Two-stream instability is avoided because the emission limited space charge is subcritical. Trapped ions are observed in two modes of operation; a quiescent mode in which ions are directly counted after destroying the space charge configuration, and a rf mode in which the ion resonance instability is active. Ion inventory up to quasineutral conditions is implied for the quiescent trapping mode.
Low temperature pressure broadening of NH3 by D2
Authors: Willey Daniel R. , Timlin Robert E. , Ruggiero Charles D. , Sulai Ibrahim A.
Published: December 2003

We report experimentally measured cross sections for pressure broadening of ammonia inversion transitions by J=0, ortho-D2 at temperatures of 18–40 K. These measurements were made in a quasiequilibrium cell using the collisional cooling technique. Cross sections for broadening of the metastable (J,K)=(1, 1), (2, 2) and (3, 3) inversion transitions ranged from 67.5 Å2 for (1, 1) at 20.0 K to 100.1 Å2 for (3, 3) at 25.0 K. The J=0, ortho-D2 cross sections were found to be consistently larger than previously measured cross sections for low temperature broadening of NH3 by both He and H2.