bluuebunny/crossref_metadata_2025 huggingface dataset:
A model for the energy levels of rare gas–spherical top van der Waals complexes
Authors: Randall Robert W. , Ibbotson Jonathan B. , Howard Brian J.
Published: July 2002

A model for the rovibrational energy levels of a complex formed between a rare gas atom and a spherical top molecule is developed, specifically for a tetrahedral XY4 molecule in its ground and its triply degenerate vibrational states. Under the assumption that the tetrahedral molecule remains undistorted upon complexation, a large amount of the tetrahedral symmetry is retained in the complex. Using symmetry adapted terms to represent the anisotropy of the intermolecular potential, the perturbations to the energy levels of the tetrahedral molecule caused by the intermolecular potential have been considered. Correlation diagrams showing the pattern of these internal rotor states between the free rotor and rigid molecule limits have been constructed.
Measuring the chemical potential of polymer solutions and melts in computer simulations
Authors: Müller M. , Paul W.
Published: July 2002

We are presenting a new method to measure the chemical potential of polymer chains in computer simulations which has virtually no limitations in chain length. The method uses a thermodynamic integration over the excluded volume interaction of a ghost chain to determine its excess chemical potential. It is exemplified with a Monte Carlo simulation of the athermal bondflucutation model for various densities and the chain lengths N=20,50,80. Furthermore, the osmotic equation of state and the entropy density are determined.
Collisional effects in Q branch coherent anti-Stokes Raman spectra of N2 and O2 at high pressure and high temperature
Authors: Dreier T. , Schiff G. , Suvernev A. A.
Published: July 2002

A temperature and pressure dependent study of coherent anti-Stokes Raman scattering (CARS) Q branch spectra of molecular nitrogen and oxygen has been conducted. Spectra at pressures up to 250 MPa and in the temperature range 298 K<T<850 K have been obtained using a scanning CARS apparatus. The full-width at half-maximum (FWHM) as well as peak position of collapsed Q branch profiles were measured. Measurements also have been made in synthetic air and in mixtures with argon. A detailed comparison of Q branch CARS band shapes with theoretical models of quantum mechanical and quasiclassical origin has been performed. On the one hand existing scaling laws like the modified energy gap (MEG), energy corrected sudden (exponential) polynomial energy gap [ECS-(E)P], polynomial energy gap (PEG), and statistical polynomial energy gap (SPEG) laws that give analytical expressions for rotational relaxation rates are used in a CARS code to calculate half-widths of the collapsed Q branch of nitrogen and oxygen. Many of these models show significant deviations from experimental results in the high pressure regime investigated here. For nitrogen the PEG-law, although not very suitable at lower densities, at room temperature reasonably reproduces the half-widths in the high pressure regime. The same is true for the ECS-EP law at low and high temperatures, whereas the SPEG-law only gives reasonable results at high temperature. For oxygen only the MEG and ECS-EP laws (at room temperature) give half-widths that are within the error limits of the measurement. On the other hand, within experimental error frequency shifts and half-widths of N2 and O2 CARS-spectra are well described by the classical approach throughout the density range. It is found that dephasing contributions to the density induced spectral shift cannot be neglected at room temperature but are less important at higher temperatures. In comparison to experimental data the quasiclassical model provides physical interpretation of temperature dependent cross sections for rotational energy relaxation processes in nitrogen and oxygen at high densities.
State-to-state measurements of internal rotational predissociation in OH–Ar (A 2Σ+)
Authors: Giancarlo Leanna C. , Randall Robert W. , Choi Seung E. , Lester Marsha I.
Published: July 2002

Internal rotational predissociative levels of OH–Ar (A 2Σ+) have been identified lying up to 350 cm−1 above the OH A 2Σ+ (v=0, 1)+Ar dissociation limit. The predissociative level energies, lifetimes, and OH A 2Σ+ rotational product distributions have been measured. Complexes prepared in many of these predissociative levels are long lived with lifetimes ≥50 ps. A novel variation of stimulated emission pumping has enabled quantitative OH A 2Σ+ rotational distributions to be obtained following OH–Ar predissociation. The OH product distributions are highly selective. The highest energetically available channel is always populated, yet in many cases, low rotational levels are conspicuously absent. The OH–Ar predissociative levels have been assigned nearly good quantum numbers based on a rotational contour analysis of the predissociative features and/or the OH A 2Σ+ rotational product distributions. A two-step mechanism involving Coriolis coupling and the potential anisotropy has been proposed to describe the predissociation process. A comparison between the experimentally measured and theoretically calculated observables provides a guide for further refinements of the OH A 2Σ++Ar potential energy surface.
Microscopic structure of low temperature liquid ammonia: A neutron diffraction experiment
Authors: Ricci M. A. , Nardone M. , Ricci F. P. , Andreani C. , Soper A. K.
Published: July 2002

Neutron diffraction measurements on liquid ammonia at two temperature states on the liquid-vapor coexistence curve are presented. Three isotropic mixtures of hydrogenated and deuterated ammonia have been studied and the three radial atom–atom distribution functions extracted at 213 K and 273 K. The comparison of the experimental results with the partial radial distribution functions for orientationally uncorrelated molecules, shows some evidence for hydrogen bonding in liquid ammonia but this is much less pronounced than that found in water under ambient conditions.
Overtone spectroscopy of the hydroxyl stretch vibration in hydroxylamine (NH2OH)
Authors: Scott Jacqueline L. , Luckhaus David , Brown Steven S. , Crim F. Fleming
Published: July 2002

We present photoacoustic spectra of the second (3νOH), third (4νOH), and fourth (5νOH) overtone bands of the hydroxyl stretch vibration in hydroxylamine. Asymmetric rotor simulations of the rovibrational contours provide rotational constants and an estimate of the homogeneous linewidth. The fourth overtone band appears anomalously broad relative to the two lower bands, reflecting a sharp increase in the rate of intramolecular vibrational energy redistribution (IVR). By contrast, the calculated density of states increases smoothly with energy. The homogeneous linewidth of the fourth overtone transition is similar to that measured by Luo et al. [J. Chem. Phys. 93, 9194 (1990)] for the predissociative sixth overtone band, supporting the conclusion that the broadening arises from increased (ro)vibrational coupling at an energy between the third and fourth overtone states.
Transient electric birefringence of flexible polymers: Orientation and relaxation dynamics
Authors: Nath Sunil , Siddiqui Raheel
Published: July 2002

The orientation and relaxation dynamics of flexible polymers in an electric field is analyzed by following the change in electric birefringence with time. The polymer chain is represented by two beads (dipole) connected by a Hookean spring and interacts with the electric field through a local induced dipole which is proportional to the end-to-end distance between the beads. Equations of motion are written for the beads taking into account (a) the hydrodynamic drag force, (b) the Brownian force, (c) the spring force, and (d) the electric force acting on the molecule. The electric field produces a strong anisotropic orientation of the polymer chain. Thus, equations are derived and analytically solved to yield the time dependence of the rise of birefringence in the electric field, the relaxation of the birefringence from a nonequilibrium state, and the change in the mean-squared end-to-end distance of the molecule with time. The dynamics are found to be governed by the dimensionless number μ0E2/H, where μ0 is a constant related to the polarizability of the molecule, E is the electric field strength, and H the Hookean spring constant. A merit of this analysis lies in the fact that expressions for the time course of the birefringence rise in the presence of an orienting field of strength E as well as relaxation in the presence of a reduced electric field, Er (Er<E) can be readily derived; hence cases other than the field-free decay (Er=0) can be interpreted. Moreover, for a biological system, a state of equilibrium with Er=0 may never be reached. Finally, the field dependence of the birefringence and the orientation and relaxation time constants has been obtained using the model and comparison of the steady state and relaxation behavior of Hookean and finitely extensible nonlinear elastic (FENE) polymer chains has been carried out.
Dynamical structure of water in dioxane aqueous solution by low-frequency Raman scattering
Authors: Tominaga Yasunori , Takeuchi Sachiko Miyoshi
Published: July 2002

The low-frequency Raman spectra of dioxane aqueous solutions have been analyzed from a dynamical aspect of water structure. The reduced Raman spectra χ″(ν̄) of the dioxane aqueous solutions are well explained by a superposition of three characteristic modes of water and one Gaussian mode (∼60 cm−1) of dioxane. The concentration dependence of χ″(ν̄) spectra shows that the 190 cm−1 mode of water disappears below about 0.8 molar fraction of water. This molar fraction corresponds to the ratio of four water molecules to one dioxane molecule. This means that the tetrahedral-like structure of water which is formed by five water molecules is almost destroyed below about 0.8 molar fraction. Thus the basic idea of the five water molecules model of liquid water has been confirmed from Raman spectroscopic point of view.
A state-selected study of the ion–molecule reactions O+(2D,2P)+H2O
Authors: Li X. , Huang Y.-L. , Flesch G. D. , Ng C. Y.
Published: July 2002

State-selected absolute cross sections for H2O+ and OH+ formed by the O+(2D,2P)+H2O reactions have been measured in the center-of-mass collision energy (Ec.m.) range of ≈0.10–30 eV. The charge transfer cross sections for O+(2D)+H2O are significantly higher than those for O+(4S)+H2O. This observation is attributed to the increased number of accessible exothermic product channels for O+(2D)+H2O. While the H2O+ cross sections for O+(2P)+H2O are comparable to those from O+(4S)+H2O at Ec.m.⩾1 eV, the H2O+ cross sections for O+(2P)+H2O at Ec.m.<1 eV are substantially lower than those for O+(4S)+H2O. The lower H2O+ cross sections observed for O+(2P)+H2O are rationalized as due to further dissociation of excited charge transfer H2O+ ions and/or the efficient competition of the OH++OH product channel. The cross sections for OH+ from O+(2D,2P)+H2O are significantly greater than those from O+(4S)+H2O. The majority of OH+ ions from O+(2D,2P)+H2O are associated with exothermic channels corresponding to the formation OH+(X 3Σ−,1Δ,A 3Π)+OH. The comparison of the sum (σT) of the cross sections for H2O+ and OH+ from O+(4S)+H2O to those from O+(2D)+H2O and O+(2P)+H2O shows that σT’s for O+(4S)+H2O and O+(2P)+H2O are comparable, whereas the σT values for O+(2D)+H2O are greater than those for O+(4S)+H2O and O+(2P)+H2O. The σT values are found to conform with the 1/Ec.m. dependence at low Ec.m.’s, indicating that the ion–dipole interaction plays an important role in the formation of the long-lived collision complexes. The high cross sections for H2O+ and OH+ from O+(2D,2P)+H2O observed here suggest that these reactions should be included in the simulation of the H2O+ and H3O+ ion density data obtained in space-borne mass spectrometric experiments.
A first-passage algorithm for the hydrodynamic friction and diffusion-limited reaction rate of macromolecules
Authors: Given James A. , Hubbard Joseph B. , Douglas Jack F.
Published: July 2002

Many important properties of a macromolecule can be expressed in terms of averages over the trajectories of diffusing particles that begin in the medium surrounding the molecule and terminate at its surface. These properties include its translational hydrodynamic friction coefficient and the Smoluchowski rate constant for diffusion-limited reactions. In this paper we introduce a first-passage algorithm (FPA) for calculating such quantities. This algorithm uses certain exact Green’s functions, or propagators, for the Laplace equation to eliminate the need to construct explicitly those portions of a diffusing particle’s trajectory that are not near an absorbing object. The algorithm is especially efficient for studying objects that contain large voids or have very irregular surfaces, such as macromolecules. Diffusion algorithms were previously shown to give accurate results for the quantities we study. In this paper, we show that first-passage methods make these algorithms more accurate and efficient. In future work, we expect to present systematic results for the properties of globular proteins.
Linear scaling computation of the Fock matrix
Authors: Challacombe Matt , Schwegler Eric
Published: July 2002

Computation of the Fock matrix is currently the limiting factor in the application of Hartree-Fock and hybrid Hartree-Fock/density functional theories to larger systems. Computation of the Fock matrix is dominated by calculation of the Coulomb and exchange matrices. With conventional Gaussian-based methods, computation of the Fock matrix typically scales as ∼N2.7, where N is the number of basis functions. A hierarchical multipole method is developed for fast computation of the Coulomb matrix. This method, together with a recently described approach to computing the Hartree-Fock exchange matrix of insulators [J. Chem. Phys. 105, 2726 (1900)], leads to a linear scaling algorithm for calculation of the Fock matrix. Linear scaling computation the Fock matrix is demonstrated for a sequence of water clusters at the restricted Hartree-Fock/3-21G level of theory, and corresponding accuracies in converged total energies are shown to be comparable with those obtained from standard quantum chemistry programs. Restricted Hartree-Fock/3-21G calculations on several proteins of current interest are documented, including endothelin, charybdotoxin, and the tetramerization monomer of P53. The P53 calculation, involving 698 atoms and 3836 basis functions, may be the largest Hartree-Fock calculation to date. The electrostatic potentials of charybdotoxin and the tetramerization monomer of P53 are visualized and the results are related to molecular function.
Density functional theory with approximate kinetic energy functionals applied to hydrogen bonds
Authors: Wesolowski Tomasz Adam
Published: July 2002

Applicability of the approximate kinetic energy functionals to study hydrogen-bonded systems by means of the formalism of Kohn–Sham equations with constrained electron density (KSCED) [Cortona, Phys. Rev. B 44, 8454 (1991); Wesołowski and Warshel, J. Phys. Chem. 97, 8050 (1993); Wesołowski and Weber, Chem. Phys. Lett. 248, 71 (1996)] is analyzed. In the KSCED formalism, the ground-state energy of a molecular complex is obtained using a “divide-and-conquer” strategy, which is applied to the Kohn–Sham-like equations to obtain the electron density of a fragment embedded in a larger system. The approximate kinetic energy functional enters into the KSCED formalism in two ways. First, the effective potential in which the electrons of each fragment move contains a component which is expressed by means of a functional derivative of an approximate kinetic energy functional (functional derivative of the non-additive kinetic energy). Second, the KSCED energy functional contains a component (non-additive kinetic energy) which is expressed using the approximate kinetic energy functional. In this work, the KSCED energies and densities of (H2O)2, (HF)2, (HCl)2, and HFṡṡṡNCH are compared to the ones obtained using the standard supermolecule Kohn–Sham approach. The following factors determining the agreement between the KSCED and supermolecule Kohn–Sham results are analyzed: the analytical form of the gradient-dependent terms in the approximate kinetic energy functional and the number of atom-centered orbitals used to expand electron density of fragments. The best agreement between the supermolecule Kohn–Sham and the KSCED results is obtained with the kinetic energy functional derived following the route of Lee, Lee, and Parr [Lee et al., Phys. Rev. A 44, 768 (1991)] from the exchange functional of Perdew and Wang [Perdew and Wang, in Electronic Structure of Solids ’91, edited by P. E. Ziesche and H. Eschrig (Academie Verlag, Berlin, 1991), p. 11]. The difference between the KSCED and the supermolecule Kohn–Sham energies of studied complexes amounts to less than 0.35 kcal/mol at the equilibrium geometry.
Molecular dynamics simulation of the liquid–vapor interface: The Lennard-Jones fluid
Authors: Mecke Matthias , Winkelmann Jochen , Fischer Johann
Published: July 2002

In this work we present new molecular dynamics simulation results for the liquid–vapor interface of the pure Lennard-Jones fluid. Our aims were further investigations on the simulation setup and the simulation parameters to obtain reliable data for the coexisting densities as well as for the surface tension. The influence of the cut-off distance to the interfacial properties is investigated and long-range corrections to both the dynamics and the surface tension are applied. It is found that the saturated liquid densities from the surface simulations agree with those from the NpT+test particle method within 1% for sufficiently large simulation boxes; the saturated vapor densities agree within 4%. In order to obtain reliable values for the surface tension, cut-off radii of at least 5 molecular diameters supplemented by a tail correction are required.
Vibrational and nonlinear optical properties of rylenes calculated by ab initio methods
Authors: Rumi Mariacristina , Zerbi Giuseppe , Müllen Klaus
Published: July 2002

Infrared and Raman spectra of perylene and terrylene computed by ab initio methods at the 3-21G level are presented and discussed. The pattern of the Raman spectrum is very similar to that observed experimentally (even if unscaled frequencies are too high) and characterized by few strong totally symmetric bands; moreover, the agreement between calculated and measured Raman intensities is quite good. Vibrational contributions to second-order molecular hyperpolarizability, γijklr, for the two compounds studied are calculated starting from ab initio Raman frequencies and intensities. The values obtained for the orientation average 〈γ(r〉×=2.6010−35 for perylene and ×1.9010−34 esu for terrylene) are in extremely good agreement with the experimental determination. In the case of perylene we have also calculated the electronic second-order hyperpolarizability, γijkle, with an ab initio finite field method at the same level of approximation (3-21G) as the vibrational analysis. For completeness the case of naphthalene has also been considered. The comparison between γe and γrγя© leads to the conclusion that in this class of materials the vibrational contribution to is large and very close to the electronic one. The results obtained are interpreted by means of the effective conjugation coordinate (ECC) theory and a description is made of the coordinate of these molecules. 1998 American Institute of Physics.
Spectroscopic study of Cr3+ ions in fluorochloro- and fluorobromozirconate glasses
Authors: Buñuel M. A. , Alcalá R. , Cases R.
Published: July 2002

Chromium doped fluorochloro- and fluorobromozirconate glasses have been prepared following and modifying standard methods. Electron probe microanalysis has been carried out to obtain the actual chlorine and bromine content of the samples. Optical absorption, steady-state excitation and emission, luminescence decay, and time-resolved spectroscopy measurements have been performed in order to study the Cr3+ neighborhood and its optical properties. The results have been interpreted in terms of the progressive substitution of the fluorine ions in the first coordination shell of Cr3+ by chlorine or bromine ions when their concentration increases in the glasses. From the analysis of the Fano antiresonances of the A2g4(F)→Eg2(G), T1g2(G) intraconfigurational transitions, up to five Cr3+ environments have been identified and assigned to: 6 F−, 5 F− and 1 Cl− or 1 Br−, 3 F− and 3 Cl− or 3 Br−, 1 F− and 5 Cl− or 5 Br− and 6 Cl− or 6 Br−. It is deduced that less than 10% of fluorine ions substitution is enough to have all the Cr3+ ions hexacoordinated to chlorine or bromine ions. Values for the crystal field and Racah parameters, the Stokes shift, and the Huang–Rhys factor have been estimated from the absorption and luminescence data. The luminescence decay and time-resolved measurements of the T2g4(F) level of Cr3+ have been explained by the presence of two opposite mechanisms: nonradiative transitions and energy transfer processes which predominate, respectively, in fluorozirconate and in the most substituted glasses.
Construction of simulation wave functions for aqueous species: D3O+
Authors: Gomez M. A. , Pratt L. R.
Published: July 2002

This paper investigates Monte Carlo techniques for construction of compact wave functions for the internal atomic motion of the D3O+ ion. The polarization force field models of Stillinger et al. and of Ojamae et al. were used. Initial pair product wave functions were obtained from the asymptotic high temperature many-body density matrix after contraction to atom pairs using Metropolis Monte Carlo. Subsequent characterization shows these pair product wave functions to be well optimized for atom pair correlations despite that fact that the predicted zero point energies are too high. The pair product wave functions are suitable to use within variational Monte Carlo, including excited states, and density matrix Monte Carlo calculations. Together with the pair product wave functions, the traditional variational theorem permits identification of wave function features with significant potential for further optimization. The most important explicit correlation variable found for the D3O+ ion was the vector triple product rOD1⋅(rOD2×rOD3). Variational Monte Carlo with 9 of such explicitly correlated functions yielded a ground state wave function with an error of 5–6% in the zero point energy.
Infrared and Raman spectra of bicyclic molecules using scaled noncorrelated and correlated ab initio force fields
Authors: Collier William B. , Magdó Ildikó , Klots Timothy D.
Published: July 2002

This paper reports the application of a scaled ab initio calculated harmonic force field to predict the frequencies, infrared intensities, Raman intensities, and depolarization ratios of benzofuran, benzothiophene, indole, benzothiazole, and benzoxazole. The theoretical calculations were made using the Hartree–Fock HF/3-21G* and HF/6-31G* basis sets and density-functional theory (DFT)B3-LYP/6-31G* levels. The equilibrium calculated force constants are scaled according to the method of Pulay and compared with the experimentally determined frequencies, intensities, and depolarization ratios to assess the accuracy and fit of the theoretical calculation. Methods for quantitative comparison of intensities were developed. The double numerical differentiation algorithm of Komornicki and McIver was analyzed and used to calculate the Raman intensities for the (DFT)B3-LYP/6-31G* model. The (DFT)B3-LYP/6-31G* model is approaching the harmonic limit in the planar and nonplanar refinement of these bicyclics with wave number fits of 5 and 4 cm−1, respectively. It reduces the need for scale factors and increases their transfer accuracy, largely because the scale factors values cluster near unity. The Komornicki and McIver algorithm is still a viable method for calculating Raman intensity information for methods that do not have analytic routines programmed. The main shortcoming to this method may lie in the tighter self-consistent field (SCF) convergence criterion possibly needed to calculate Raman intensities for the totally symmetric modes of large molecules. The (DFT)B3-LYP/6-31G* model was superior for calculating the planar intensities, but equal to the HF methods for predicting the nonplanar intensities.
Photomanipulation of external molecular modes: A time-dependent self-consistent-field approach
Authors: Yan Zong-Chao , Seideman Tamar
Published: July 2002

We introduce a time-dependent self-consistent-field approach for studying the coupled rotational–translational motion of molecules subject to the macroscopic potential induced by a spatially inhomogeneous, polarized laser field. A hybrid classical-quantum-mechanical variation of the scheme is employed to investigate the possibility of simultaneously focussing the center-of-mass motion of molecules and aligning their figure axis in a field-free region of space. A semiclassical-quantum-mechanical variation is used to describe the rotational–translational dynamics of cold molecules confined to an optical trap.
Complete analysis of the surface-enhanced Raman scattering of pyrazine on the silver electrode on the basis of a resonant charge transfer mechanism involving three states
Authors: Arenas Juan F. , Woolley Mark S. , Tocón Isabel López , Otero Juan C. , Marcos Juan I.
Published: July 2002

A new general procedure to interpret surface-enhanced Raman scattering (SERS) spectra has been developed in order to clarify the controversy concerning the relevant enhancement mechanism of this type of spectra. The analysis consists of detecting the presence of the charge transfer (CT) enhancement mechanism by correlating the most enhanced SERS bands with the ab initio calculated geometries (ΔQ) and vibrational frequencies (Δv) of the isolated molecule and its radical anion. This CT mechanism is assumed to be identical to that of resonance Raman between the electronic ground state of the metal–adsorbate complex and charge transfer excited states. We consider that these excited states arise when one electron is transferred from the metal to pyrazine. For this reason, they have been labeled from the point of view of pyrazine on the basis on the symmetry of the doublet states of its radical anion. The SERS spectra of pyrazine recorded on silver surface at several electrode potentials have been analyzed on the basis of the Franck–Condon and Herzberg–Teller contributions related to B3u2–1Ag and Au2–1Ag transitions. A great deal of experimental facts related to the relative enhancement of in-plane Ag, B3g, and B1u modes as well as out-of-plane B3u, B2g, and Au vibrations has been explained. Likewise, it is possible to account for the observation of Raman inactive fundamentals as well as the influence of the forbidden Au2–1Ag transition on the SERS spectra if the symmetry of the metal–adsorbate complex and the nonplanarity of the Au2 state are taken into account. All the results point out that the CT mechanism is mainly responsible for the SERS features of this molecule studied here.
Direct observation of the 2 3Πu state of Rb2 in a pulsed molecular beam: Rotational branch intensity anomalies in the 2 3Πu(1u)–X 1Σg+(0g+) bands
Authors: Lee Yonghoon , Yoon Youngjee , Baek Sun Jong , Joo Duck-Lae , Ryu Jae-seok , Kim Bongsoo
Published: July 2002

Resonance enhanced two photon ionization study in a pulsed molecular beam identified spin-forbidden transitions of the 2 3Πu(1u)–X 1Σg+(0g+) and 2 3Πu(0u+)–X 1Σg+(0g+) of Rb2. Λ-doubling and interference-induced rotational branch intensity anomalies are observed for 2 3Πu(1u)–X 1Σg+(0g+) transitions. The rotational constant for the 2 3Πu state was determined to be Be=0.018 932 5±0.000 007 4 cm−1 for Rb285, giving Re=4.5796±0.0009 Å. Other molecular parameters for Rb285 are determined as Te=19 784.254±0.009 cm−1, ωe=42.200±0.006 cm−1, ωexe=0.171±0.001 cm−1, and ωeye=−0.001 06±0.000 06 cm−1 for the 2 3Πu(0u+) state and Te=19 785.74±0.01 cm−1, ωe=42.297±0.005 cm−1, ωexe=0.1725±0.0009 cm−1, and ωeye=−0.000 78±0.000 04 cm−1 for 2 3Πu(1u) state. These Re, ωe, and Te values agree well with ab initio calculated values. The spin–orbit coupling constant A and Λ-doubling parameters p and q are determined by simultaneous fitting of the rotational structures of both 2 3Πu(1u)−X 1Σg+ and 2 3Πu(0u+)−X 1Σg+ transitions.
Nonlinear evolution equations associated with a Riemann–Hilbert scattering problem
Authors: Degasperis A. , Santini P. M. , Ablowitz M. J.
Published: July 2002

In an earlier paper nonlinear evolution equations associated with a Riemann–Hilbert scattering problem, which reduces, in an appropriate limit, to the Zakharov–Shabat–AKNS scattering problem, were considered. Here we discuss certain necessary constraints associated with the scattering problem and their impact upon the associated evolution equations. Moreover, the direct linearization of the nonlinear evolution equations and an algorithm to construct anN-soliton solution are given.
Physical constraints on the coefficients of Fourier expansions in cylindrical coordinates
Authors: Lewis H. Ralph , Bellan Paul M.
Published: July 2002

It is demonstrated that (i) the postulate of infinite differentiability in Cartesian coordinates and (ii) the physical assumption of regularity on the axis of a cylindrical coordinate system provide significant simplifying constraints on the coefficients of Fourier expansions in cylindrical coordinates. These constraints are independent of any governing equations. The simplification can provide considerable practical benefit for the analysis (especially numerical) of actual physical problems. Of equal importance, these constraints demonstrate that if A is any arbitrary physical vector, then the only finite Fourier terms of Ar and Aθ are those with m=1 symmetry. In the Appendix, it is further shown that postulate (i) may be inferred from a more primitive assumption, namely, the arbitrariness of the location of the cylindrical axis of the coordinate system.
How to construct finite-dimensional bi-Hamiltonian systems from soliton equations: Jacobi integrable potentials
Authors: Antonowicz Marek , Rauch-Wojciechowski Stefan
Published: July 2002

A systematic method of constructing finite-dimensional integrable systems starting from a bi-Hamiltonian hierarchy of soliton equations is introduced. The existence of two Hamiltonian structures of the hierarchy leads to a bi-Hamiltonian formulation of the resulting finite-dimensional systems. The case of coupled KdV hierarchies is studied in detail. A surprising connection with separable Jacobi potentials is uncovered and described.
On the Gelfand–Dickey algebra GD(SLn) and the W
n-symmetry. I. The bosonic case
Authors: Saidi E. H. , Sedra M. B.
Published: July 2002

The algebra Ξ of nonlinear (local and nonlocal) differential operators, acting on the ring of analytic functions, is studied. It is shown in particular that this space splits into 3×2 special subalgebras ∑jr, j=0,±1, r=±1. Each subalgebra is completely specified by quantum numbers s and (p,q) describing the conformal spin, and the lowest and the highest degrees, respectively. The algebra ∑++ (and its dual ∑−−) of local (pure nonlocal) differential operators is used to calculate the general expression of the Gelfand–Dickey bracket and the Wn-symmetry Poisson, one in terms of a set of spin j canonical fields uj, 2≤j≤n and a nonlinear u-cubic dependent differential operator D(n,i,j;z,u). The explicit form of this operator is worked out. Other remarkable features are also discussed.
Can elementary quantum mechanics explain the Aharonov–Bohm effect?
Authors: Magni C. , Valz-Gris F.
Published: July 2002

It is shown that a satisfactory treatment of the Aharonov–Bohm problem requires no departure from the basic principles of ordinary quantum mechanics, a fact not entirely clear in the literature. The discussion ends up with the same formalism as originally proposed by Aharonov and Bohm, in agreement with experimental facts, although in a way that forces no alteration in the traditional interpretation of vector potentials.
Calabi-Yau manifolds by Borcea-Voisin method
Authors: Abe Mitsuko , Sato Masamichi
Published: July 2002

We construct Calabi-Yau manifolds and their mirrors from K3 surfaces. This method was first developed by Borcea and Voisin. We examined their properties torically and checked mirror symmetry for the Calabi-Yau four-fold case. From Borcea-Voisin three-fold or four-fold examples, it may be possible to probe the S-duality of Seiberg-Witten.
On the integrable perturbations of the Camassa–Holm equation
Authors: Kraenkel R. A. , Senthilvelan M. , Zenchuk A. I.
Published: July 2002

We present an investigation of the nonlinear partial differential equations (PDE) which are asymptotically representable as a linear combination of the equations from the Camassa–Holm hierarchy. For this purpose we use the infinitesimal transformations of dependent and independent variables of the original PDE. This approach is helpful for the analysis of the systems of the PDE which can be asymptotically represented as the evolution equations of polynomial structure.
A condition for an area-preserving mapping to be in the Engel’s form
Authors: Mao Jian-min
Published: July 2002

We establish explicit expressions of restrictions on the coefficients of nonlinear terms in a two-dimensional area-preserving polynomial map imposed by the property of area preserving. We also establish a necessary and sufficient condition for a two-dimensional area-preserving generic polynomial map to be in the Engel’s form. The condition is that coefficients of nonlinear terms in the first mapping equation are proportional to the corresponding ones in the second mapping equation. As an application of the results, we discuss how to regain the area-preserving property lost in truncating a two-dimensional area-preserving map.
Functions of linear operators: Parameter differentiation
Authors: Prato Domingo , Tsallis Constantino
Published: July 2002

We derive a useful expression for the matrix elements [∂f[A(t)]/∂t]ij of the derivative of a function f[A(t)] of a diagonalizable linear operator A(t) with respect to the parameter t at t0. The function f[A(t)] is supposed to be an operator acting on the same space as the operator A(t) which is assumed to have a nondegenerate, pure point spectrum. We use the basis which diagonalizes A(t0), i.e., [A(t0]ij=λiδij, and obtain [∂f[A(t)]/∂t|t=t0]ij=[∂A/∂t|t=t0]ij{[f(λj)−f(λi)]/(λj−λi)}. In addition to this, we show that further elaboration on the (not necessarily simple) integral expressions given by Wilcox (who basically considered f[A(t)] of the exponential type) and generalized by Rajagopal [who extended Wilcox results by considering f[A(t)] of the q-exponential type where expq(x)≡[1+(1−q)x]1/(1−q) with q∈R; hence, exp1(x)=exp(x)] yields these same expressions. Some of the lemmas first established by the above authors are easily recovered.
Conductivity of normal metal with phase-coherent excitations in the presence of NS boundary
Authors: Chiang Yu. N. , Shevchenko O. G.
Published: July 2002

Resistive properties of 3D systems formed by a normal metal and type I superconductor (Cu–Sn) with a large electron mean free path in the normal metal are investigated in the liquid-helium temperature region. A number of new types of temperature behavior of the normal metal conductivity is revealed at temperatures below the superconducting transition temperature of the superconductor in contact with the normal metal for distances commensurate with a typical mesoscopic scale of electron transport trajectories in the normal metal, in which the phase memory of wave functions is retained.
Probability distribution, conditional dissipation, and transport of passive temperature fluctuations in grid-generated turbulence
Authors: Jayesh  , Warhaft Z.
Published: July 2002

The evolution of the scalar probability density function (pdf), the conditional scalar dissipation rate, and other statistics including transport properties are studied for passive temperature fluctuations in decaying grid-generated turbulence. The effect of filtering and differentiating the time series is also investigated. For a nonzero mean temperature gradient it is shown that the pdf of the temperature fluctuations has pronounced exponential tails for turbulence Reynolds number (Rel) greater than 70 but below this value the pdf is close to Gaussian. The scalar dissipation rate, conditioned on the fluctuations, shows that there is a high expectation of dissipation in the presence of the large, rare fluctuations that produce the exponential tails. Significant positive correlation between the mean square scalar fluctuations and the instantaneous scalar dissipation rate is found when exponential tails occur. The case of temperature fluctuations in the absence of a mean gradient is also studied. Here, the results are less definite because the generation of the fluctuations (by means of fine heated wires) causes an asymmetry in the pdf. The results show, however, that the pdf is close to Gaussian and that the correlation between the mean square temperature fluctuations and the instantaneous scalar dissipation rate is very weak. For the linear profile case, measurements over the range 60≤Rel≤1100 show that the dimensionless heat flux Nu is proportional to Rel0.88 and that the transition from a Gaussian pdf to one with exponential tails occurs at Nu∼31, a value close to transitions observed in other recent mixing experiments conducted in entirely different turbulent flows.
An experimental investigation of the characteristics of free-surface turbulence in channel flow
Authors: Kumar S. , Gupta R. , Banerjee S.
Published: July 2002

The structural features of turbulence at the free surface of a channel flow have been experimentally investigated. The experiments were conducted in a horizontal channel of large aspect ratio in the (depth based) Reynolds number range of 2800–8800. The results indicate that the persistent structures on the free surface can be classified as upwellings, downdrafts, and spiral eddies. Upwellings are shown to be related to the bursts originating in the sheared region at the channel bottom and the eddies are seen to be generated at the edges of the upwellings. The eddies often merge if rotating in the same direction, and form “pairs” if rotating in opposite directions—though there are occasional mergers of such counter-rotating ones. The spiral eddies decay slowly and are sometimes annihilated by fresh upwellings. The population densities and the persistence times of the various structures were measured for different flow conditions. The resulting data show that the physical parameters characterizing the structures at the interface, scale with a mix of inner (wall shear stress and viscosity) and outer variables. Measurement of the streamwise and spanwise velocities at the free-surface were made by particle imaging velocimetry (PIV) and the surface normal velocity near the free-surface estimated by continuity. The results indicate that the upwellings and spiral eddy regions would be expected to dominate scalar transport rates at high Prandtl/Schmidt numbers. The one-dimensional energy spectra of the flow field at the free-surface compare well with direct numerical simulations and show a region with −5/3 slope at low wave numbers. This experimentally confirms a previous result regarding the two-dimensionality of turbulence near the free surface, based on numerical simulations by Pan and Banerjee [Phys. Fluids 7, 1649 (1995)].
Three-dimensional simulation of flows through a rectangular sudden expansion
Authors: Sau Amalendu
Published: July 2002

Detailed three-dimensional numerical predictions of the effect of vortex generators on vorticity dynamics and the phenomenon of axis switching are presented for the laminar incompressible flows through a rectangular sudden expansion. As a special case, the axis switching mechanism for flows through a square sudden expansion has also been investigated. The physical domains consist of 2:1 and 1:1 aspect-ratio channels which undergo a sudden expansion such that the backward facing step height is uniform, and equal to 0.75 times the minor width (or width, for the 1:1 aspect-ratio channel) of the inlet channel. The rectangular-shaped protrusions into the flow (“rectangular tabs”) are used as vortex generators, placed on the plane of the expansion. Passive forcing was provided by the system geometry and particularly due to the presence of the tabs. Suitable placement of the tabs could either stop or augment the axis switching mechanism. The rectangular vortex rings formed in the shear layer of the expanding jet undergo nonuniform self-induction and a complex three-dimensional deformation of the jet takes place in its downstream location. Such a deformation of the jet leads to the generation of streamwise vortices. Two mechanisms, the ωθ-dynamics, originated from the difference in induced velocities for different segments of a rolled-up azimuthal vortical structure; and the ωz-dynamics, caused due to the induced velocities of streamwise vortex pairs in the flow, co-exist and are identified to be active in flows through a rectangular sudden expansion. The tendency of axis switching initiated by the ωθ-dynamics are either resisted or enhanced by the ωz-dynamics. While ωθ-dynamics is known to be responsible for axis switching in periodically forced asymmetric jets, the ωz-dynamics mostly govern the effect of tabs. The results of the present simulation reveal the flow features closely consistent with the axis switching mechanism suggested by the laboratory flow visualization.
Interaction of low-energy implanted atomic H with slow and fast diffusing metallic impurities in Si
Authors: Singh Ranbir , Fonash S. J. , Rohatgi A.
Published: July 2002

The interaction of hydrogen, injected into silicon using low-energy ion bombardment, with slow (Ti and V) and fast (Cr and Au) diffusing impurities was investigated. It was found that this H ion bombardment of the Si surface was effective in reducing the electrically active concentration of only the fast diffusing impurities. The results are explained by damage enhanced diffusivity and surface gettering of the fast diffusing impurities.
AlGaAs/GaAs double barrier diodes with high peak-to-valley current ratio
Authors: Huang C. I. , Paulus M. J. , Bozada C. A. , Dudley S. C. , Evans K. R. , Stutz C. E. , Jones R. L. , Cheney M. E.
Published: July 2002

We report the largest peak-to-valley current (PVC) ratios to date from AlGaAs/GaAs double barrier (either alloy barrier or superlattice barrier) diodes. PVC ratios as high as 3.6 and 21.7 were obtained from an AlAs/GaAs superlattice barrier structure at 300 and 77 K, respectively. In an alloy barrier structure with x=0.42 (x=0.3), PVC ratios of 3.9 (2.2) and 14.3 (7.0) were observed at 300 and 77 K, respectively. We attribute these excellent results to a ‘‘two-step’’ spacer layer incorporated in the devices studied which facilitated the growth of high material quality.
Low-threshold (∼600 A/cm2 at room temperature) GaAs/AlGaAs lasers on Si (100)
Authors: Chen H. Z. , Ghaffari A. , Wang H. , Morkoç H. , Yariv A.
Published: July 2002

Low-threshold graded-refractive-index GaAs/AlGaAs laser structures were grown on Si (100) by molecular beam epitaxy and tested at room temperature under a probe station. Broad area devices having widths of 110–120 μm and cavity lengths of ∼500–1210 μm exhibited threshold current densities as low as 600 A/cm2 and total slope efficiencies of as high as 0.75 W/A. The thresholds fell in the range of 600–1000 A/cm2 in three different wafers, and it is assumed that the quality of the facets accounts for most of the spread in results.
HgCdTe photovoltaic detectors on Si substrates
Authors: Kay R. , Bean R. , Zanio K. , Ito C. , McIntyre D.
Published: July 2002

HgCdTe infrared photovoltaic detectors were fabricated on silicon substrates for the first time by using intermediate CdTe and GaAs epitaxial layers. No cracking or degradation was observed after thermal cycling these devices (cutoff wavelength of 5.5 μm and R0A as high as 200 Ω cm2 at 80 K). Secondary ion mass spectrometry and Auger data substantiate that a CdTe buffer layer can prevent Ga diffusion from the intermediate GaAs epitaxial layer from inadvertently converting the p-HgCdTe to n-type at growth temperatures as high as 500 °C.
Inhibition of macrophage migration inhibitory factor (MIF) tautomerase and biological activities by acetaminophen metabolites
Authors: Senter Peter D. , Al-Abed Yousef , Metz Christine N. , Benigni Fabio , Mitchell Robert A. , Chesney J. , Han Jianlin , Gartner Carlos G. , Nelson Sidney D. , Todaro George J. , Bucala Richard
Published: July 2002

The cytokine macrophage migration inhibitory factor (MIF) has emerged to be an important regulator of the inflammatory response and is critically involved in the development of septic shock, arthritis, and glomerulonephritis. Although the biological activities of MIF are presumed to require a receptor-based mechanism of action, the protein is also a tautomerase and has a catalytically active N-terminal proline that is invariant in structurally homologous bacterial isomerases. This observation raises the possibility that MIF may exert its biological action via an enzymatic reaction. Physiologically relevant substrates for MIF have not been identified, nor have site-directed mutagenesis studies consistently supported the requirement for a functional catalytic site. Small molecule inhibitors of MIF's isomerase activity also have been developed, but none have been shown yet to inhibit MIF biological activity. We report herein that the iminoquinone metabolite of acetaminophen,N-acetyl-p-benzoquinone imine (NAPQI), inhibits both the isomerase and the biological activities of MIF. The reaction between NAPQI and MIF is covalent and produces a NAPQI-modified MIF species with diminished cell binding activity and decreased recognition by anti-MIF mAb. These data are consistent with a model by which the NAPQI reacts with the catalytic Pro-1 of MIF to disrupt the integrity of epitope(s) critical to MIF's biological activity and point to the importance of the catalytic domain, but not the catalytic activityper se, in MIF function. These results also point to a powerful approach for the design of small molecule inhibitors of MIF based on interaction with its catalytic site and constitute an example of a pharmacophore capable of irreversibly inhibiting the action of a proinflammatory cytokine.
A method for soluble overexpression of the α7 nicotinic acetylcholine receptor extracellular domain
Authors: Fischer Markus , Corringer Pierre-Jean , Schott Karin , Bacher Adelbert , Changeux Jean-Pierre
Published: July 2002

We describe the construction of a soluble protein carrying the
N-terminal extracellular domain (ECD) of the α7 subunit of the
nicotinic acetylcholine receptor. The approach was to fuse the α7 ECD
at the C and N termini of several monomeric and pentameric soluble
carrier proteins and to investigate the soluble expression of the
product in
Escherichia coli
. An initial screening of six
carrier proteins resulted in the selection of a fusion protein
comprising, from the N to the C terminus, the maltose binding protein,
a 17-aa linker containing an enterokinase binding site, and the α7
ECD. This protein is soluble upon expression in bacteria and is
purified by affinity chromatography. It binds the competitive nicotinic
antagonist α-bungarotoxin with 2.5 μM affinity and displays a CD
spectrum corresponding to a folded protein. The method might be
suitable to produce large quantities of protein for crystallization and
immunochemical experiments.