bluuebunny/crossref_metadata_2025 huggingface dataset:
Interplay of Jahn–Teller and pseudo-Jahn–Teller vibronic dynamics in the benzene cation
Authors: Köppel H. , Cederbaum L. S. , Domcke W.
Published: July 2002

The static and dynamic aspects of the vibronic interaction of the B̃ 2E2g and C̃ 2A2u electronic states of C6H+6 are analyzed. In the approximation of linear vibrational and vibronic coupling, the model Hamiltonian for this system comprises eight nonseparable vibrational modes, six of which are degenerate (two of A1g symmetry, four of E2g symmetry, and two of E2u symmetry). The coupling constants are estimated from existing ab initio SCF and semiempirical (CNDO/S) calculations. The topology of the adiabatic potential-energy surfaces of this class of model Hamiltonians is investigated. It is shown that the model exhibits a variety of conical intersections which dominate the vibronic dynamics. The dynamical problem is solved with simultanteous inclusion of six vibrational modes, four of which are degenerate (the Jahn–Teller coupling of two of the E2g modes is negligible). Hamiltonian matrices with dimensions up to 6×106 are diagonalized using the Lanczos algorithm. After some adjustments of coupling constants, the calculation reproduces well the complex structure of the overlapping B̃ 2E2g–C̃ 2A2u bands in the photoelectron spectrum of benzene. The vibronic structure of the lower-energy E2g band is dominated by a two-mode Jahn–Teller effect in the B̃ state. At higher energy, the marked diffuseness of the ‘‘C̃ 2A2u band’’ is shown to be a consequence of complete vibronic mixing with the lower-lying B̃ state. Based on the

numerical solution of the full problem, the reliability of approximations (neglect of nonseparability of modes, introduction of a single effective pseudo-Jahn–Teller mode) is assessed. A time-dependent analysis reveals an ultrafast decay of the population of the C̃ state on a time scale of about 20 fs, followed by quasiperiodic recurrences which are damped on a time scale of a few hundred femtoseconds. These findings underline the importance of conical intersections and strong nonadiabatic effects also for larger molecules such as aromatic hydrocarbons. They demonstrate that nowadays a full quantum treatment is feasible also for these larger systems.
Molecular structure and thermal stability of B2H4 and B2H+4 species
Authors: Ruščic B. , Schwarz M. , Berkowitz J.
Published: July 2002

B2H4 has been produced by the reaction of F atoms with B2H6, in successive abstraction reactions. The B2H4 species was detected and analyzed by photoionization mass spectrometry. The adiabatic ionization potential of this species (9.70±0.02 eV) and the vertical value (∼10.4 eV) are obtained from the photoion yield curve. These values, and the shape of this curve, are consistent with a doubly bridged, C2v structure for both the neutral and ionic species. The fragment ion B2H+2 is observed, with an appearance potential of 11.535±0.03 eV. This value, combined with previous results, yields D0(B2H4–H)≊40.1 kcal/mol, whereas D0(B2H5–H)≲102.7 kcal/mol. The B2H+2 fragment may have as its neutral precursor an isomeric B2H4 (D2d), with approximately the same stability as the C2v species. An earlier value for the appearance potential of B2H+4 from B2H6 is shown to be too high, due to a very small formation probability at the thermochemical threshold.
A Monte Carlo simulation of water molecules near a charged wall
Authors: Aloisi Giovanni , Foresti Maria Luisa , Guidelli Rolando , Barnes Paul
Published: July 2002

A Monte Carlo simulation of liquid water near a charged wall has been carried out for different values of the surface charge density. The surface excess of electric potential as a function of charge density has been determined from the polarization profiles. The differential capacity of the simulated system is compared with the differential capacity of the mercury/water interface.
Theory of electrohydrodynamic instabilities in electrolytic cells
Authors: Bruinsma R. , Alexander S.
Published: July 2002

The classical Nernst theory for diffusion-limited electrical current transport in electrolytic cells predicts limiting currents of order microamps for unstirred cells. For unstirred cells, one encounters much higher current densities for large enough voltages. In this paper we develop the theory of the hydrodynamic stability of an electrolytic cell as a function of the imposed electric current. A new electrohydrodynamic instability is encountered when the current is forced to exceed the Nernst limit. The convection is driven by the volume force exerted by the electrical field on space charges in the electrolyte. This intrinsic instability is found to be easily masked by extrinsic convection sources such as gravity or stirring. We perform a linear stability analysis and derive a dimensionless number Le whose value determines the convection pattern. The quiescent cell becomes hydrodynamically unstable if (i) the current density exceeds the Nernst limit and if (ii) Le≫1. We expect full hydrodynamic convection if Le/Pr≫1 with Pr the Prandtl number.
Spectroscopy and rotational dynamics of oxazine 725 in alcohols: A test of dielectric friction theories
Authors: Simon John D. , Thompson Peggy A.
Published: July 2002

The spectroscopy and rotational dynamics of oxazine 725 in alcohols are analyzed in terms of dielectric friction theories. From the static spectroscopy, the difference between the ground and electronically excited state dipole moment is found to be 4.5 D. MNDO calculations reveal a ground state moment of 1.7 D. Using dielectric friction theory, the dipole moment difference can also be evaluated from the difference between the ground and electronically excited state rotational times. A commonly used continuum theory expression for evaluating the dielectric friction constant is found to be inconsistent with the static spectroscopic data. Calculation of the dielectric friction constant using the diffusive model developed by Hubbard and Wolynes gives better agreement with the experimental data. The model developed by van der Zwan and Hynes — which connects dielectric friction to time dependent solvation dynamics — results in quantitative agreement with the data obtained from static spectroscopy. Using this model, an accurate measure of the contribution of dielectric friction to the rotational dynamics can be calculated.
High-resolution electronic spectroscopy of jet-cooled hexafluorobenzene and 1,3,5-trifluorobenzene cations, C6F+6 and C6F3H+3
Authors: Yu Lian , Foster Stephen C. , Williamson James M. , Miller Terry A.
Published: July 2002

The nearly completely rotationally resolved electronic spectra of two aromatic organic ions have been obtained. Rotationally cold ions, C6F+6 and C6F3H+3, are produced by laser ionization in a supersonic free jet expansion and probed via laser induced fluorescence with a very high resolution pulse-amplified cw ring dye laser. The spectra are analyzed to obtain band origins, rotational constants, Coriolis and Jahn–Teller parameters.
Theory of end-adsorbed polymer brushes in polymeric matrices
Authors: Shull Kenneth R.
Published: July 2002

A detailed self-consistent field theory is used to calculate the properties of end-adsorbed polymer chains, or polymer ‘‘brushes,’’ in equilibrium with a blend of adsorbing B polymer and nonadsorbing A polymer. The brush properties depend on χab, Na/Nb and β, where χab is the Flory interaction parameter characterizing the thermodynamic interaction between the A and B polymers, Na and Nb are the respective degrees of polymerization of the A and B polymers and kBTβ is the net attractive interaction between the adsorbing chain end and the surface. For χab=0, β≫1 and Na/Nb≫1 an asymptotic limit is reached where the brush properties are completely specified by z*/Rbg, where z* is the integrated surface excess of the adsorbing polymer and Rbg is its unperturbed radius of gyration. In this strong adsorption, dry brush regime the volume fraction of adsorbing polymer at the surface increases rapidly from 0.0 to 0.96 as z*/Rbg increases from 0.0 to 1.6. For z*/Rbg>1.6 the surface volume fraction of the adsorbing polymer is close to one and the brush profiles are accurately described by a hyperbolic tangent form where the only adjustable parameter is a width parameter describing the interpenetration of the brush and the matrix polymer. This width decreases from 2Rbg at z*/Rbg=1.6 to 1.6Rbg at z*/Rbg=3.3. For high values of Na/Nb there is an attractive interaction between opposing polymer brushes, or between a single polymer brush and a nonadsorbing surface, which can be attributed to the free energy associated with the distortion of the matrix chains adjacent to the brush. Results are also extended to cases of weak adsorption characterized by low values of β.
Spinodal decomposition in mixtures containing nematogens. II. Kinetics of spinodal decomposition
Authors: Dorgan J. R.
Published: July 2002

The problem of simultaneous phase separation and ordering kinetics in mixtures containing a nematic component is addressed through the introduction of a time-dependent Landau–Ginzburg model. Dynamic equations for the time evolution of orientation and concentration fluctuations are given; the solution to these equations leads to explicit expressions for the structure factors for concentration and orientation. It is shown that the theory shares some salient features with the Cahn theory of spinodal decomposition. A simulation of a quenching experiment is performed and the time evolution of the structure factors during spinodal decomposition is calculated. Relevant small angle light scattering experiments which could be used to test the model are discussed.
Entropy of simple fluids in terms of pair correlations
Authors: Schmidt A. B.
Published: July 2002

An exact expression for entropy of simple fluids in terms of the pair correlation functions is derived assuming the known form of the bridge functional. The Martynov–Sarkisov closure of the Ornstein–Zernike equation is tested for the Lennard-Jones fluid. The results improve the accuracy of the pair correlation component of the Green–Raveche entropy expansion at low and intermediate densities but fail to describe a dense fluid.
Confined thin films of a linear and branched octane. A comparison of the structure and solvation forces using molecular dynamics simulations
Authors: Wang Yantse , Hill Kirk , Harris Jonathan G.
Published: July 2002

Experiments using the surface force apparatus indicate that the nature of the forces acting on two surfaces separated by liquid film less than 5.0 nm thick are extremely sensitive to the chemical structure of the molecules in the liquid. Using molecular dynamics simulations we probe the structure and solvation forces upon two model surfaces separated by liquid hydrocarbons. We examine two cases—films composed of n-octane and films composed of iso-octane (2-methyl heptane). At certain surface separations the liquid films are strongly layered with chains lying parallel to the surface. These films produce a repulsive force equivalent to as much as 100 MPa on the surfaces, while exhibiting liquidlike self-diffusion constants with no evidence of solidification or glassification. A plot of this force, known as the solvation force, as a function of surface separation reveals strong oscillations with a period of about 0.4–0.5 nm for both the linear and branched molecules. Although the branching in the iso-octane significantly reduces the intensity of the layering, its solvation force curve shows oscillations which are just as strong as those seen in the films of the linear octane. This contradicts experimental results showing that the addition of a single methyl side chain significantly reduces the oscillatory nature of the solvation forces.
Calculation of the shear viscosity of decane using a reversible multiple time-step algorithm
Authors: Mundy Christopher J. , Siepmann J. Ilja , Klein Michael L.
Published: July 2002

The shear viscosity of a fully-flexible model for n-decane is calculated via equilibrium molecular dynamics simulations at the state point T=480 K and ρ=0.6136 g/cm3. A reversible multiple-time step approach is used in conjunction with Nosé–Hoover chain dynamics to generate data in the canonical (NVT) ensemble. For comparison the shear viscosity is also computed in the standard microcanonical NVE ensemble. A model that accurately reproduces the experimental vapor–liquid coexistence curve is shown to yield excellent results for the shear viscosity at the state point under study.
Cavity ringdown laser absorption spectroscopy and time-of-flight mass spectroscopy of jet-cooled silver silicides
Authors: Scherer J. J. , Paul J. B. , Collier C. P. , Saykally R. J.
Published: July 2002

The cavity ringdown technique has been employed for the first spectroscopic characterization of the AgSi molecule, which is generated in a pulsed laser vaporization plasma reactor. A total of 20 rovibronic bands between 365 and 385 nm have been measured and analyzed to yield molecular properties for the X, B, and C 2Σ states of AgSi. A time-of-flight mass spectrometer simultaneously monitors species produced in the molecular beam and has provided the first direct evidence for the existence of polyatomic silver silicides. Comparison of the AgSi data to our recent results for the CuSi diatom reveals very similar chemical bonding in the two coinage metal silicides, apparently dominated by covalent interactions.
Guided-ion beam measurements of the O+(4S)+Xe charge-transfer reaction
Authors: Bastian Michael J. , Dressler Rainer A. , Murad Edmond
Published: July 2002

Guided-ion beam integral cross section measurements, product ion time-of-flight (TOF) measurements, and doubly differential cross sections are presented for the O+(4S)+Xe→O+Xe+ charge-transfer reaction. The integral cross section is observed to increase monotonically with mean center-of-mass collision energy (ET) from 0.72 Å2 at 0.1 eV to 26.9 Å2 at 35.2 eV. Product ion TOF measurements show that the primary contribution to the charge-transfer cross section arises from a direct electron transfer at long range of the Demkov type. The collision energy dependence of the cross section for center-of-mass (c.m.) forward-scattered Xe+ ions reveals that this minor contribution to the total cross section arises from a hard-sphere-type scattering that primarily involves a Landau–Zener-type diabatic curve crossing at shorter range. Doubly differential cross sections at 4.4 eV indicate that translational to product internal energy transfer is inefficient even at small impact parameters and that the spin-allowed Xe+(2P)+O(3P) product channels are preferentially populated.
Counting levels within vibrational polyads: Generating function approach
Authors: Sadovskií D. A. , Zhilinskií B. I.
Published: July 2002

Simple analytical formulas for the number of energy levels in the vibrational polyads are given. These formulas account for the resonances between the vibrational modes, and for the symmetry of the problem, so that the number of states of a particular symmetry type can be computed. The formulas are used to estimate the differential and integral densities of states from the minimum initial information about the molecule. Examples of the vibrational structure of triatomic molecules A3, tetrahedral molecules AB4, and linear molecules AB2 are considered. The analytical formulas are compared to the ab initio results for H3+ [J. R. Henderson et al., J. Chem. Phys. 98, 7191 (1993)].
Structure features in binary liquid Li–Tl alloys
Authors: Kuiying Chen , Hongbo Liu , Zhuangqi Hu
Published: July 2002

With the help of potential mapping techniques, molecular dynamics simulations have been performed to study the local structure features in binary liquid Li–Tl alloys. The generalized nonlocal model pseudopotentials have been chosen as the bases for our calculations. Both the bond orientational order and pair analysis approach have been adopted as the basic structural functions to examine the structure features in binary liquid Li–Tl alloys. In addition, bonded pairs with various symmetries, icosahedra, defective icosahedra, Frank–Kasper polyhedra, etc., formed in the Li–Tl alloys are examined in detail, and the relationship between the structure features and composition has been established. The results show that the distribution of attractive parts of the potentials play an important role in characterizing the liquid structure. Finally, the results are discussed in detail.
Electrochemical tuning of the lifetime of the CO stretching vibration for CO/Pt(111)
Authors: Schmidt M. E. , Guyot-Sionnest P.
Published: July 2002

We have used the large electric fields at the interface of a Pt(111) electrode and an acetonitrile electrolyte solution to tune the interaction between adsorbed CO and the Pt(111) surface. The electrode potential is varied over a 2.5 Volt range. As the electrode potential is made more positive, the CO vibrational frequency increases and the vibrational lifetime decreases. Over the potential range investigated, the tuning is about 35–40 cm−1 and the lifetime varies from ≊2.1 to ≊1.5 ps. Ab initio calculations performed for CO/Cu(100) predicted the opposite trend for the lifetimes for that system [M. Head-Gordon and J. C. Jully, Chem. Phys. 175, 37 (1993)]. Within an empirical model of nonadiabatic charge transfer [B. N. J. Persson and M. Persson, Solid State Commun. 36, 175 (1980)], our observations can be explained by a decreasing 2π-derived density of states of the Pt/CO complex at the Fermi level.
A novel method for simulating quantum dissipative systems
Authors: Cao Jianshu , Ungar Lowell W. , Voth Gregory A.
Published: July 2002

An effective and flexible numerical scheme is proposed to simulate the dissipative quantum dynamics of a linearized system–bath Hamiltonian. Based on the observation that the Feynman path integrals for a Gaussian bath have a quadratic functional form, the bath average can be performed by directly sampling paths of the discretized harmonic modes and then propagating the system under the influence of quantum Gaussian force. The algorithm is amenable to all known quantum propagation methods and can thus be flexibly applied to study quantum dissipation in the condensed phase. Nontrivial numerical examples based on the spin-boson and damped quantum oscillator models are presented to demonstrate the application of the new algorithm.
Reactions of benzene with rhodium cluster cations: Competition between chemisorption and physisorption
Authors: Berg Christian , Beyer Martin , Schindler Thomas , Niedner-Schatteburg Gereon , Bondybey Vladimir E.
Published: July 2002

The reactions of Rh+n cations, n=1–40, with C6H6 and C6D6 were investigated under single collision conditions in a Fourier-transform-ion-cyclotron-resonance mass spectrometer. For clusters with up to 18 rhodium atoms, dissociative chemisorption, accompanied by total or partial dehydrogenation in competition with nondissociative adsorption of an intact benzene molecule (physisorption) was observed. For clusters with n≳18 only the nondissociative adsorption was observed in the primary reaction step. Besides some enhancement of the nondissociative adsorption channel for cluster sizes n=9, 11, 12, and 13, deuteration has little effect upon the observed reactions. The atomic rhodium cation reacts after a few seconds induction time to an arene complex, which then forms a bis-arene complex in a secondary reaction step. Rhodium dimer alone exhibits some cleavage of the Rh–Rh bond, resulting also in the Rh–C6H+6 complex.
Note on the evaluation method of the configurational partition function of a polypeptide molecule: Relationship between the matrix method and the generating function method
Authors: Kobayashi Yukio
Published: July 2002

We show the relationship between the two conventional methods for evaluating the configurational partition function of a polypeptide molecule: (a) the matrix method and (b) the generating function method. Following the spectral theory, the generating function method is another representation of the matrix method, and thus these methods are equivalent to each other.
Multiconfiguration self-consistent field procedure employing linear combination of atomic-electron distributions
Authors: Ten-no Seiichiro , Iwata Suehiro
Published: July 2002

We present a multiconfiguration self-consistent field (MCSCF) procedure employing recently developed approximations for electron repulsion integrals. Molecular charge distributions in the two-electron interaction part are expanded into atomic-electron distributions without linear dependencies, and the four-center quantities of electron repulsion integrals are reduced to two- and three-center quantities. The method is plugged into the approximate second-order MCSCF procedure and applied to calculations of the HNO molecule. This procedure enables us to reduce the CPU time of the integral transformation step which is usually the most time consuming. It is also shown that the present approximation is very accurate not only for the ground state but also for the low-lying excited states, even after a substantial reduction of the number of required integrals.
Ab initio model potential calculations on the electronic spectrum of Ni2+-doped MgO including correlation, spin–orbit and embedding effects
Authors: Llusar Rosa , Casarrubios Marcos , Barandiarán Zoila , Seijo Luis
Published: July 2002

An ab initio theoretical study of the optical absorption spectrum of Ni2+-doped MgO has been conducted by means of calculations in a MgO-embedded (NiO6)10−cluster. The calculations include long- and short-range embedding effects of electrostatic and quantum nature brought about by the MgO crystalline lattice, as well as electron correlation and spin–orbit effects within the (NiO6)10− cluster. The spin–orbit calculations have been performed using the spin–orbit-CI WB-AIMP method [Chem. Phys. Lett. 147, 597 (1988); J. Chem. Phys. 102, 8078 (1995)] which has been recently proposed and is applied here for the first time to the field of impurities in crystals. The WB-AIMP method is extended in order to handle correlation effects which, being necessary to produce accurate energy differences between spin–free states, are not needed for the proper calculation of spin–orbit couplings. The extension of the WB-AIMP method, which is also aimed at keeping the size of the spin–orbit-CI within reasonable limits, is based on the use of spin-free-state shifting operators. It is shown that the unreasonable spin–orbit splittings obtained for MgO:Ni2+ in spin–orbit-CI calculations correlating only 8 electrons become correct when the proposed extension is applied, so that the same CI space is used but energy corrections due to correlating up to 26 electrons are included. The results of the ligand field spectrum of MgO:Ni2+ show good overall agreement with the experimental measurements and a reassignment of the observed Eg(b3T1g) excited state is proposed and discussed.
Sub-Doppler Zeeman spectroscopy of pyrazine: S1 1B3u–S 1A
g 0 band
Authors: Yamamoto Norimasa , Ebi Tsuyoshi , Baba Masaaki
Published: July 2002

High-resolution fluorescence excitation spectrum of the S1 1B3u−S0 1Ag 000 band of pyrazine in a collimated molecular beam and the change with the external magnetic field were measured. Transitions to each molecular eigenstate were resolved. For the J′=0 level, the magnetic field effect was not very large. By taking account of the mixing of fine-structure levels, we analyzed and simulated the spectra of the P(1) line. The fairly good agreements with the observed results are obtained. The spectral lines of the J′≠0 levels were observed to split remarkably and the intensity decreased drastically with the magnetic field even at smaller than 100 Gauss. The spectra at zero field and in the magnetic field have been analyzed by taking account of the spin-orbit interaction between the singlet and triplet vibrational levels and Zeeman interaction in the triplet manifold. The intersystem crossing and the magnetic field effect in the S1 origin level of pyrazine are studied as a function of the number of coupled triplet levels.
Dynamic properties of aqueous dispersions of nanometric particles near the fluid–solid transition
Authors: Delsanti M. , Chang J. , Lesieur P. , Cabane B.
Published: July 2002

Here we investigate a colloidal suspension of practical interest, silica particles, which provides a mesoscopic system for the study of a dynamic near fluid–solid transition. Dynamic properties of concentrated solutions of nearly monodisperse charged colloids (standard deviation/mean radius≊0.11) at low ionic strength (≊6×10−4 M) have been studied by quasielastic light scattering. Below a volume fraction of the order of 15%, the solutions are viscous fluids and the refractive index fluctuations relax in two steps. Above a volume fraction of 20%, and over a macroscopic time window extending up to few seconds or more, the solutions are elastic and fluctuations in the refractive index become frozen. This is typical of systems which pass from a fluid to a solid state. The results are critically analyzed within the same framework used for similar macroparticle systems.
Energetically driven liquid–solid transitions in molecularly thin n-octane films
Authors: Ballamudi R. K. , Bitsanis I. A.
Published: July 2002

In this paper we present findings from molecular dynamics simulations that investigated the changes induced in molecularly thin n-octane films, as a result of increasing solid-methylene unit energetic affinity. The solid surfaces were deprived of any topographical features and were modeled as atomically smooth 10-4 Lennard–Jones planes. We observed an abrupt transition in the structural features of the film at a critical value of the characteristic energy that quantified the affinity between solid surfaces and methylene units. The transition was signaled by a discontinuous increase in the degree of intermolecular order and facilitated by a precipitous extension of the octane molecules, which adopted almost fully extended configurations. Furthermore, the transition resulted in the freezing of molecular migration and rotation. The characteristics of the transition showed that it is a mild first order phase transition between a highly ordered liquid and a poorly organized solid. The solid constitutes a phase with order intermediate to that of hydrocarbon ‘‘rotator’’ phases and two-dimensional smectics. These findings demonstrate that solidification of nanoscopically thin films of linear alkanes is a general, energetically driven phenomenon, which does not require the aid of commensurate surface topography. Our simulations provide a natural explanation for the solidlike features exhibited by alkane films studied experimentally.
Numerical simulation of thermally induced phase separation in polymer-dispersed liquid crystals
Authors: Teixeira P. I. C. , Mulder B. M.
Published: July 2002

We have developed a model of polymer-dispersed liquid crystal (PDLC) formation by thermally induced phase separation. Spinodal decomposition in the thermoplastic-LC mixture is modelled by the cell dynamical systems method of Oono and Puri, suitably modified to describe a continuous temperature quench. Numerical calculations performed on a two-dimensional system for a composition of 30% LC +70% thermoplastic reveal that the final morphology depends strongly on the quench rate: If cooling is much faster than phase separation, then complete decomposition is precluded, whereas for slower quenches we recover the usual LC-rich droplet pattern of constant-temperature studies. The droplet size distributions are quite broad, and the mean droplet size at a given temperature decreases as a power of the quench rate, consistently with what is observed in real PDLCs.
Pure rotational spectrum, quadrupole coupling constants and structure of the dimer of pyrrole
Authors: Columberg G. , Bauder A.
Published: July 2002

Rotational transitions of a pyrrole dimer have been identified and measured over the 8–18 GHz range using a pulsed-nozzle Fourier-transform microwave spectrometer. In addition to the parent species, nine N15 and D isotopomers have been analyzed. Apart from the N14 and D nuclear quadrupole splittings, the rotational transitions did not show any additional splittings arising from large amplitude motions. Rotational constants, centrifugal distortion constants and, for two isotopomers, N14 quadrupole coupling constants have been fitted to the measured frequencies of rotational transitions. The observed rotational constants are consistent with essentially a T-shaped structure for the dimer. From the rotational constants of all isotopomers, a partial structure of the dimer has been determined. Three structural parameters have been fitted to the differences in the planar moments of inertia between the isotopically substituted species and the parent species. The planes of the two pyrrole monomers form an angle of 55.4(4)° with the nitrogen side of one ring directed to the π electron system of the other ring establishing a weak hydrogen bond. The centrifugal distortion constants of the dimer of pyrrole have been used to estimate the frequency of the van der Waals stretching mode and the dissociation energy of the complex in a pseudodiatomic approximation.
Hysteresis loss in liquids
Authors: Vázquez Vázquez Alexei , Sotolongo Costa Oscar
Published: July 2002

The hysteresis loss in liquids is an absorption process that cannot be explained by the classical theory of mechanical relaxation. Here a model involving the relaxation of atomic clusters is developed. It is shown that the hysteresis loss is a consequence of the size distribution of large clusters.
The weakly bound dinitrogen tetroxide molecule: High level single reference wavefunctions are good enough
Authors: Wesolowski Steve S. , Fermann Justin T. , Crawford T. Daniel , Schaefer III Henry F.
Published: July 2002

Ab initio studies of dinitrogen tetroxide (N2O4) have been performed to predict the equilibrium geometry, harmonic vibrational frequencies, and fragmentation energy (N2O4→2 NO2). The structure was optimized at the self-consistent field, configuration interaction, and coupled-cluster levels of theory with large basis sets. At the highest level of theory, the N–N bond distance was 1.752 Å, in excellent agreement with the experimental value of 1.756±0.01 Å. In addition, the harmonic vibrational frequencies were predicted with an average absolute error of 51 cm−1 relative to experimental fundamental values with differences largely attributed to anharmonic effects. The fragmentation energy corrected for zero point vibrational energy and basis set superposition error was 7.2 kcal/mol, in fair agreement with the experimental value of 12.7 kcal/mol. Despite the suggestion that a multireference wavefunction may be necessary to accurately describe the biradical nature of N2O4, single reference treatments with large basis sets and high levels of electron correlation yield molecular parameters remarkably close to experimental values.
Atomic orbitals in molecules
Authors: Cioslowski Jerzy , Liashenko Andrew
Published: July 2002

A rigorous definition of atomic orbitals that invokes concepts of the quantum-mechanical theory of atoms in molecules is presented. Being free of any references to nuclei-centered basis functions, this new definition is universally applicable to wave functions furnished by all types of electronic structure calculations. The atomic orbitals in molecules (AOIMs) it produces are continuous and orthonormal. In most cases, linear combinations of only a few such AOIMs accurately represent individual molecular orbitals (MOs) and strongly occupied natural orbitals (NOs). The coefficients of these linear combinations are remarkably insensitive to the quality of basis sets used in the generation of the MOs and NOs. These properties make the AOIMs particularly suitable for compact description of bonding in diverse chemical systems.
Relativistic and correlation effects on molecular properties: The interhalogens ClF, BrF, BrCl, IF, ICl, and IBr
Authors: de Jong W. A. , Styszynski J. , Visscher L. , Nieuwpoort W. C.
Published: July 2002

The effect of relativity on the properties of the interhalogens ClF, BrF, BrCl, IF, IBr, and IBr is studied by comparing relativistic and nonrelativistic calculations. Bond lengths, harmonic frequencies, and dissociation energies show that the bond is weakened in the relativistic formalism. Relativity increases the electric dipole moment whereas the electric quadrupole moment and dipole polarizability display an irregular behavior. The relativistic contributions to the electric dipole and quadrupole moment of the iodine containing molecules are 10%–20% of the total value, whereas the contributions in the other molecules cannot be neglected. The value of the electric quadrupole moment is dominated by the relativistic contributions.
Adsorption of CO on Rh(100) studied by ab initio local-density functional calculations
Authors: Eichler A. , Hafner J.
Published: July 2002

Ab initio local-density functional studies of the adsorption of CO on the (100) surface of Rh have been performed. We show that although adsorption in the bridge site is always energetically more favorable than adsorption in either the on-top or the hollow sites, two different mechanisms can lead to a relatively high occupation of the on-top sites: (i) At higher coverage the interactions between the adsorbates stabilize a pseudohexagonal coincidence lattice with the experimentally observed p(4√2×√2) structure with a bridge/on-top ratio of 2:1 (all adsorbates being slightly shifted from their high-symmetry positions). (ii) At lower coverages there seems to be a contradiction between the energetic preference for bridge-site adsorption and the mixed top/bridge adsorption reported in the experiments. This could simply be dismissed as a failure of density-functional theory. However, we speculate about a possible way to reconcile the calculated potential energy surface and the experimental observations: At distances larger than 1.75 Å from the surface (this is smaller than the equilibrium height for on-top adsorption), the interaction-energy with the on-top site is always larger by ∼0.5 eV than with the bridge site. The on-top adsorbed molecule is metastable, because there is a barrier of 0.06 eV for migration to the bridge site and the reaction channel is very narrow. This leads to the observed mixed occupation of top and bridge sites at lower coverage (Θ⩽0.5).
Accurate variational calculations and analysis of the HOCl vibrational energy spectrum
Authors: Skokov Sergei , Qi Jianxin , Bowman Joel M. , Yang Chao-Yie , Gray Stephen K. , Peterson Kirk A. , Mandelshtam Vladimir A.
Published: July 2002

Large scale variational calculations for the vibrational states of HOCl are performed using a recently developed, accurate ab initio potential energy surface. Three different approaches for obtaining vibrational states are employed and contrasted; a truncation/recoupling scheme with direct diagonalization, the Lanczos method, and Chebyshev iteration with filter diagonalization. The complete spectrum of bound states for nonrotating HOCl is computed and analyzed within a random matrix theory framework. This analysis indicates almost entirely regular dynamics with only a small degree of chaos. The nearly regular spectral structure allows us to make assignments for the most significant part of the spectrum, based on analysis of coordinate expectation values and eigenfunctions. Ground state dipole moments and dipole transition probabilities are also calculated using accurate ab initio data. Computed values are in good agreement with available experimental data. Some exact rovibrational calculations for J=1, including Coriolis coupling, are performed. The exact results are nearly identical with those obtained from the adiabatic rotation approximation and very close to those from the centrifugal sudden approximation, thus indicating a very small degree of asymmetry and Coriolis coupling for the HOCl molecule.
Spin–orbit effects in photodissociation of sodium iodide
Authors: Alekseyev A. B. , Liebermann H.-P. , Buenker R. J. , Balakrishnan N. , Sadeghpour H. R. , Cornett S. T. , Cavagnero M. J.
Published: July 2002

Ab initio configuration interaction calculations of the electronic binding energies, spin–orbit coupling matrix elements and transition dipole moments of NaI are presented. The results are used to construct adiabatic and diabatic representations of the 0+ molecular states relevant to predissociation. The dynamics of photopredissociation is elucidated by multichannel time-dependent wave packet propagation in the diabatic representation. Specific features associated with the spatial and temporal evolution of the wave packet are ascribed to those observed in femtosecond pump–probe experiments. In particular, the rate of decay of the electronically excited NaI* complex is found to be in close agreement with time-resolved experimental studies. Partial photoabsorption cross sections for the production of iodine atoms in the ground (2P3/2) and excited (2P1/2) spin–orbit states are calculated and found to peak at excitation wavelengths of 322 and 263 nm, respectively, in accord with experimental data.
Anomaly coefficients: Their calculation and congruences
Authors: Braden H. W.
Published: July 2002

A new method for the calculation of anomaly coefficients is presented. For su(n) some explicit and general expressions are given for these. In particular, certain congruences are discovered and investigated among the leading anomaly coefficients. As an application of these congruences, the absence of global six-dimensional gauge anomalies is shown.
Superposition formulas for nonlinear superequations
Authors: Beckers J. , Gagnon L. , Hussin V. , Winternitz P.
Published: July 2002

Nonlinear superequations, for which the general solution can be expressed algebraically in terms of a finite number of particular solutions, are obtained. They are based on the orthosymplectic supergroup OSP(m,2n) and its action on a homogeneous superspace. Superposition formulas are discussed for the cases m=1, n arbitrary, and m=2, n=1. For OSP(2,2) the number of particular solutions needed to reconstruct the general solution depends on the dimension of the underlying Grassmann algebra, whereas for OSP(1,2n) it does not.
Simple spinors and real structures
Authors: Kopczyński Wojciech , Trautman Andrzej
Published: July 2002

A concept of a real index associated with any maximal totally null subspace in a complexified vector space endowed with a scalar product, and also with any complex simple (pure) spinor, is introduced and elaborated. It is shown that the real Pin group acts transitively on the projective space of simple spinors with any given real index. The connection between the real index and multivectors associated with a simple spinor and its charge conjugate is established. The simple spinors with extreme values of the real index deserve a special attention: The generic simple spinors for which the real index is minimal and simple-r spinors for which it is maximal.
Asymptotics of the multipositon–soliton τ function of the Korteweg–de Vries equation and the supertransparency
Authors: Matveev Vladimir B.
Published: July 2002

The asymptotics of the τ function generating the N-positon–M-soliton solution of the Korteweg–de Vries equation is calculated. This allows to prove that solitons do not experience any phase shift in a collision with positons. The positons themselves survive mutual collisions unchanged. This phenomenon is called the supertransparency or super-reflectionless property of the multipositon solutions. The linear aspects of this phenomenon are also discussed. It is demonstrated that positons acquire two additional but always finite phase shifts in collision with solitons. This result admits a natural extension to any number of solitons and positons in interaction.
Braid group related algebras, their representations and generalized hydrogenlike spectra
Authors: Arik M. , Aydin F. , Hizel E. , Kornfilt J. , Yildiz A.
Published: July 2002

Representations of the n-braid group where generators are given by matrices whose elements belong to a noncommutative algebra are discussed. Representation of this algebra in a Hilbert space generalizes the Burau representation. Two algebras which are closely related and are physically indistinguishable provided that a particular eigenvalue is fine tuned are discussed. It is shown that the generalized oscillator system given by one of the algebras generates a hydrogenlike spectrum.
Eigenfunctions of the curl operator in spherical coordinates
Authors: Torres del Castillo G. F.
Published: July 2002

The eigenfunctions of the curl operator are obtained by separation of variables in spherical coordinates, making use of the spin-weighted spherical harmonics. It is shown that the eigenfunctions of the curl operator with vanishing divergence can be written in terms of a single scalar potential that satisfies the Helmholtz equation. It is also shown that these eigenfunctions give a complete basis for the divergenceless vector fields.
Topology change and the propagation of massless fields
Authors: Gratus Jonathan , Tucker Robin W.
Published: July 2002

The massless wave equation on a class of two-dimensional manifolds consisting of an arbitrary number of topological cylinders connected to one or more topological spheres are analyzed herein. Such manifolds are endowed with a degenerate (nonglobally hyperbolic) metric. Attention is drawn to the topological constraints on solutions describing monochromatic modes on both compact and noncompact manifolds. Energy and momentum currents are constructed and a new global sum rule discussed. The results offer a rigorous background for the formulation of a field theory of topologically induced particle production.
Time-dependent dissipation in nonlinear Schrödinger systems
Authors: Lange H. , Toomire B. , Zweifel P. F.
Published: July 2002

A coupled nonlinear Schrödinger–Poisson equation is considered which contains a time-dependent dissipation function as a specific model of dissipation effects in nonlinear quantum transport theory and other areas. The Wigner–Poisson equation associated with this system is derived. Using conservation and quasiconservation laws and certain growth assumptions for the nonlinearities and the dissipation function, global existence of solutions to the Cauchy problem of the time-dependent Schrödinger–Poisson system is shown both for small (attractive case) or arbitrary data (repulsive case).
Antisymmetric tensor fields on spheres: Functional determinants and non-local counterterms
Authors: Elizalde E. , Lygren M. , Vassilevich D. V.
Published: July 2002

The Hodge–de Rham Laplacian on spheres acting on antisymmetric tensor fields is considered. Explicit expressions for the spectrum are derived in a quite direct way, confirming previous results. Associated functional determinants and the heat kernel expansion are evaluated. Using this method, new non-local counterterms in the quantum effective action are obtained, which can be expressed in terms of Betti numbers.
Linear connections on graphs
Authors: Cho Sunggoo , Park Kwang Sung
Published: July 2002

In recent years, discrete spaces such as graphs have attracted much attention as models for physical spacetime or as models for testing the spirit of noncommutative geometry. In this work, we construct the differential algebras for graphs by extending the work of Dimakis et al. and discuss linear connections and curvatures on graphs. Especially, we calculate connections and curvatures explicitly for the general nonzero torsion case. There is a metric, but no metric-compatible connection in general except the complete symmetric graph with two vertices.
Nonextensive Bose–Einstein condensation model
Authors: Michoel T. , Verbeure A.
Published: July 2002

The imperfect Boson gas supplemented with a gentle repulsive interaction is completely solved. In particular, it is proved that it has nonextensive Bose–Einstein condensation, i.e., there is condensation without macroscopic occupation of the ground (k=0) state level.
A reverse log-Sobolev inequality in the Segal-Bargmann space
Authors: Sontz Stephen Bruce
Published: July 2002

This article is based on recent results of the author on the properties of the reproducing kernel of the Segal-Bargmann space. Those results are used here to demonstrate a family of energy-entropy inequalities in the Segal-Bargmann space. In some cases this is a log-Sobolev inequality while in other cases this is actually a reverse log-Sobolev inequality, which means that the energy term is bounded above by the entropy term, plus a norm term. This implies that in the Segal-Bargmann space the entropy is finite if and only if the energy is finite. Applications of this result to the Segal-Bargmann transform are given as well as a discussion of its possible relation with reverse hypercontractivity. It should be noted that all of the results of this article are proved without using hypercontractivity estimates.
Darboux transformation for differential–difference principal chiral equation and its continuous limits
Authors: Gao Jining
Published: July 2002

This paper gives the Darboux transformations for differential–difference integrable systems and their applications to differential–difference principal chiral equation. We also give some exact solutions of principal chiral equation corresponding to the harmonic map R1+1→SU(n) by using two kinds of limits.
Transient double-diffusive convection experiments in a horizontal fluid layer extending over a bed of spheres
Authors: Kazmierczak M. , Poulikakos D.
Published: July 2002

The problem of double-diffusive convection for a thermohaline system consisting of a horizontal fluid layer situated above a saturated porous bed is studied experimentally. The porous bed is constructed from 5 mm spherical glass beads. The system initially contains a linear stabilizing salt distribution and is suddenly heated uniformly from below. The evolving temperature and concentration fields were determined at various times and were used to piece together a reliable picture of the behavior of the composite layer. In the majority of the experiments, convective flow was initiated and sustained in the fluid portion of the system. The flow was characterized by thorough mixing, resulting in uniform temperature and concentration distributions. The presence of the porous layer was responsible for a large delay in the flow initiation near the bottom wall, resulting in a considerable temperature increase. In some cases, the fluid saturating the porous matrix remained stagnant throughout the experiment, despite the fact that a vivid flow region existed directly above the porous region. The effect of the thermal Rayleigh number, the salinity Rayleigh number, and the height of the porous substrate are also investigated in the course of the present study.
The Saffman–Taylor instability: From the linear to the circular geometry
Authors: Thomé H. , Rabaud M. , Hakim V. , Couder Y.
Published: July 2002

The Saffman–Taylor fingers are studied in cells that have the form of sectors of a disk. The less viscous fluid can be injected at the apex (divergent flow) or at the periphery (convergent flow). As in the linear geometry, at large velocities, a unique finger tends to occupy a well determined fraction λ of the cell angular width. This fraction is a function of the angle of the cell, being larger than 0.5 in the divergent case and smaller in the convergent case. In both cases these fractions tend linearly toward λ=0.5 when the angle of the cell tends to zero. In support of recent theories, these results show how the selection is changed when the geometry induces an increase or a decrease of the curvature of the profiles. The formation of fingers in the circular geometry is revisited. In a divergent flow, the circular front appears to break into independent parts so that each finger grows as if it were contained in a sector shaped cell. The rate of occupancy of the cell by one of the fluids as a function of the distance to the center is then discussed. Finally, the existence of the mathematical counterpart to the well-known Saffman–Taylor finger solutions in a nonparallel cell is discussed in the Appendix.