bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics simulation study on the transient response of solvation structure during the translational diffusion of solute
Authors: Yamaguchi T. , Matsuoka T. , Koda S.
Published: January 2005

The transient response function of the density profile of the solvent around a solute during the translational diffusion of the solute is formulated based on the generalized Langevin formalism. The resultant theory is applied to both neat Lennard-Jones fluids and cations in liquid water, and the response functions are obtained from the analysis of the molecular dynamics simulations. In the case of the self-diffusion of Lennard-Jones fluids, the responses of the solvation structures are in harmony with conventional pictures based on the mode-coupling theory, that is, the binary collision in the low-density fluids, the backflow effect from medium to high density fluids, and the backscatter effect in the liquids near the triple point. In the case of cations in water, the qualitative behavior is strongly dependent on the size of cations. The pictures similar to simple dense liquids are obtained for the large ion and the neutral molecule, while the solvent waters within the first solvation shell of small ions show an oscillatory response in the short-time region. In particular, the oscillation is remarkably underdumped for lithium ion. The origin of the oscillation is discussed in relation to the theoretical treatment of the translational diffusion of ions in water.
Transport coefficients of the Lennard-Jones model fluid. III. Bulk viscosity
Authors: Meier Karsten , Laesecke Arno , Kabelac Stephan
Published: January 2005

In an extensive computer simulation study, the transport coefficients of the Lennard-Jones model fluid were determined with high accuracy from equilibrium molecular-dynamics simulations. In the frame of time-correlation function theory, the generalized Einstein relations were employed to evaluate the transport coefficients. This third of a series of four papers presents the results for the bulk viscosity. With comprehensive simulation data at over 350 state points, the temperature and density dependences of the bulk viscosity are characterized in this work over a wide range of fluid states. The bulk viscosity exhibits a large critical enhancement similar to that known for the thermal conductivity, but it extends much farther into the supercritical region and can be observed even at 4.5 times the critical temperature. An investigation of the pressure-fluctuation autocorrelation functions shows that the enhancement is caused by extremely slowly decaying pressure fluctuations.
Addition of water to Al5O4− determined by anion photoelectron spectroscopy and quantum chemical calculations
Authors: Das Ujjal , Raghavachari Krishnan , Jarrold Caroline Chick
Published: January 2005

The anion photoelectron spectra of Al5O4− and Al5O5H2− are presented and interpreted within the context of quantum chemical calculations on these species. Experimentally, the electron affinities of these two molecules are determined to be 3.50(5) eV and 3.10(10) eV for the bare and hydrated cluster, respectively. The spectra show at least three electronic transitions crowded into a 1 eV energy window. Calculations on Al5O4− predict a highly symmetric near-planar structure with a singlet ground state. The neutral structure calculated to be most structurally similar to the ground state structure of the anion is predicted to lie 0.15 eV above the ground state structure of the neutral. The lowest energy neutral isomer does not have significant Franck–Condon overlap with the ground state of the anion. Dissociative addition of water to Al5O4− is energetically favored over physisorption. The ground state structure for the Al5O4−+H2O product forms when water adds to the central Al atom in Al5O4− with–H migration to one of the neighboring O atoms. Again, the ground state structures for the anion and neutral are very different, and the PE spectrum represents transitions to a higher-lying neutral structure from the ground state anion structure.
Individual fundamental mode dependence of H2O vibrational excitation in the 6–8 eV resonance region by electron impact
Authors: Makochekanwa C. , Kajita R. , Kato H. , Kitajima M. , Cho H. , Kimura M. , Tanaka H.
Published: January 2005

Resonance phenomena in the bending (010), symmetric (100), and asymmetric (001) stretching vibrational modes in water (H2O) molecules have been investigated by measuring differential cross sections (DCSs) at fixed energy losses of 0.43, 0.46, 0.49, and 0.51 eV while sweeping the impact energies from 1.6 to 10 eV, at angles 60° and 90°, using a crossed-beam method. No resonancelike enhanced features have been observed in the study of the (010) mode. In the DCSs for the combined (100+001) mode, a single broad hump was observed at each angle, whose position shifts towards the higher energy side with increasing scattering angle, a feature typical of the interference profiles previously observed in a study of the (301) vibrational mode. DCSs for the (001) mode exhibited a rather isotropic angular distribution with no evidence of resonances. However, enhancement of DCSs were indeed observed for the (100) mode, at the energy loss of 0.46 eV, which is attributable to the A1 symmetry resonance that was studied by Seng and Linder [J. Phys. B 9, 2539 (1976)], with a typical s-wave characteristic angular distribution. Theoretical analysis based on the continuum multiple-scattering approach has been carried out to provide rationale to the experimental results.
Leakage current of Pt∕(Ba0.7Sr0.3)TiO3 interface with dead layer
Authors: Chen B. , Yang H. , Miao J. , Zhao L. , Cao L. X. , Xu B. , Qiu X. G. , Zhao B. R.
Published: January 2005

Leakage current of Pt∕(Ba0.7Sr0.3)TiO3 (BST)∕YBa2Cu3O7−δ capacitors on a (001) SrTiO3 substrate was studied. By modeling a low-dielectric constant layer, a so-called dead layer, between the Pt∕BST interface as a parasitic capacitor in series with the bulk layer capacitor, the leakage current of Pt∕BST interface was well analyzed based on the modified Schottky emission equation. Furthermore, a two-step schematic energy band diagram is proposed to explain the carrier transport through the Pt∕BST interface.
Quantification of reaction violence and combustion enthalpy of plastic bonded explosive 9501 under strong confinement
Authors: Perry W. Lee , Dickson Peter M. , Parker Gary R. , Asay B. W.
Published: January 2005

The confinement experienced by an explosive during thermal self-initiation can substantially affect performance in terms of deflagration–to–detonation characteristics and explosion/detonation violence. To this end, we have developed an experiment to quantitatively observe enthalpy change and reaction violence in thermally initiated plastic bonded explosive (PBX) 9501. Traditionally, researchers attempt to quantify violence using terminal observations of fragment size, fragment velocity, and through subjective observations. In the work presented here, the explosive was loaded into a heated gun assembly where we subjected a 300 mg charge to a cook-off schedule and a range of static and inertial confinements. Static confinement was controlled using rupture disks calibrated at 34.5 and 138 MPa. The use of 3.15 and 6.3 g projectile masses provided a variation in inertial confinement. This was a regime of strong confinement; a significant fraction of the explosive energy was required to rupture the disk, and the projectile mass was large compared to the charge mass. The state variables pressure and volume were measured in the breech. From these data, we quantified both the reaction enthalpy change and energy release rate of the explosive on a microsecond time scale using a thermodynamic analyisis. We used these values to unambiguously quantify explosion violence as a function of confinement at a fixed cook-off schedule of 190 C for 1 h. P2τ, a measure of critical shock energy required for shock ignition of an adjacent explosive was also computed. We found variations in this confinement regime to have a weak effect on enthalpy change, power, violence and shock energy. Violence was approximately 100 times lower than detonating trinitrotoluene, but the measured shock energy approached the critical shock energy for initiating secondary high explosives.
Numerical simulation of de-NOx performance by repetitive pulsed discharge when added with hydrocarbons such as ethylene
Authors: Onda Kazuo , Kusunoki Hironobu , Ito Kohei , Ibaraki Hiroshi , Araki Takuto
Published: January 2005

Emission regulations are gradually being tightened recently to prevent further air pollution. Cost-effective and efficient technologies must be developed to process the NOx generated in the combustion of fossil fuels. One of the candidate technologies to process NOx is the denitrification of flue gas by pulsed corona discharge, which has been demonstrated experimentally to show high de-NOx performance. However, the optimization of operation conditions and the appropriate understanding of the de-NOx process still remain to be clarified. Therefore, following our previous study on ammonia injection, we have simulated in the present study the de-NOx process to which hydrocarbons such as ethylene have been added to provide guidelines on its proper operation conditions and its main reaction paths to remove NOx. The simulated results show that the removal efficiency in a case of ethylene addition becomes lower than for ammonia addition, but the de-NOx energy consumption rate becomes lower than for ammonia injection. However, with ethylene injection the production of the pollutant, formaldehyde, limits the allowable amount of injected ethylene. The de-NOx performance is better with propylene than ethylene injection because propylene reacts with the OH radical more than ethylene to oxide NOx. However, formaldehyde is also produced in the case of propylene injection, limiting the allowable amount of injected propylene. The de-NOx performance is also assessed in a case where HNO2 is considered as NxOy.
Brownian dynamics simulations of a flexible polymer chain which includes continuous resistance and multibody hydrodynamic interactions
Authors: Butler Jason E. , Shaqfeh Eric S. G.
Published: January 2005

Using methods adapted from the simulation of suspension dynamics, we have developed a Brownian dynamics algorithm with multibody hydrodynamic interactions for simulating the dynamics of polymer molecules. The polymer molecule is modeled as a chain composed of a series of inextensible, rigid rods with constraints at each joint to ensure continuity of the chain. The linear and rotational velocities of each segment of the polymer chain are described by the slender-body theory of Batchelor [J. Fluid Mech. 44, 419 (1970)]. To include hydrodynamic interactions between the segments of the chain, the line distribution of forces on each segment is approximated by making a Legendre polynomial expansion of the disturbance velocity on the segment, where the first two terms of the expansion are retained in the calculation. Thus, the resulting linear force distribution is specified by a center of mass force, couple, and stresslet on each segment. This method for calculating the hydrodynamic interactions has been successfully used to simulate the dynamics of noncolloidal suspensions of rigid fibers [O. G. Harlen, R. R. Sundararajakumar, and D. L. Koch, J. Fluid Mech. 388, 355 (1999); J. E. Butler and E. S. G. Shaqfeh, J. Fluid Mech. 468, 204 (2002)]. The longest relaxation time and center of mass diffusivity are among the quantities calculated with the simulation technique. Comparisons are made for different levels of approximation of the hydrodynamic interactions, including multibody interactions, two-body interactions, and the “freely draining” case with no interactions. For the short polymer chains studied in this paper, the results indicate a difference in the apparent scaling of diffusivity with polymer length for the multibody versus two-body level of approximation for the hydrodynamic interactions.
The influence of temperature and density functional models in ab initio molecular dynamics simulation of liquid water
Authors: VandeVondele Joost , Mohamed Fawzi , Krack Matthias , Hutter Jürg , Sprik Michiel , Parrinello Michele
Published: January 2005

The performance of density functional theory methods for the modeling of condensed aqueous systems is hard to predict and validation by ab initio molecular simulation of liquid water is absolutely necessary. In order to assess the reliability of these tests, the effect of temperature on the structure and dynamics of liquid water has been characterized with 16 simulations of 20 ps in the temperature range of 280–380 K. We find a pronounced influence of temperature on the pair correlation functions and on the diffusion constant including nonergodic behavior on the time scale of the simulation in the lower temperature range (which includes ambient temperature). These observations were taken into account in a consistent comparison of a series of density functionals (BLYP, PBE, TPSS, OLYP, HCTH120, HCTH407). All simulations were carried out using an ab initio molecular dynamics approach in which wave functions are represented using Gaussians and the density is expanded in an auxiliary basis of plane waves. Whereas the first three functionals show similar behavior, it is found that the latter three functionals yield more diffusive dynamics and less structure.
Reaction of vinyl radical with oxygen: A matrix isolation infrared spectroscopic and theoretical study
Authors: Yang Rongjing , Yu Liang , Jin Xi , Zhou Mingfei , Carpenter Barry K.
Published: January 2005

The reaction of vinyl radical with molecular oxygen in solid argon has been studied using matrix isolation infrared absorption spectroscopy. The vinyl radical was produced through high frequency discharge of ethylene. The vinyl radical reacted with oxygen spontaneously on annealing to form the vinylperoxy radical C2H3OO with the O–O bond in a trans position relative to the C–C bond, which is characterized by O–O stretching and out-of-plane CH2 bending vibrations at 1140.7 and 875.5 cm−1. The vinylperoxy radical underwent visible photon-induced dissociation to the CH2OH(CO) complex or CH2OH+CO, which has never been considered in previous studies. The CH2OH(CO) product was predicted to be more thermodynamically accessible than the previously reported major HCO+H2CO channel, and is most likely produced by hydrogen atom transfer from the first-formed H2CO–HCO pair in solid argon.
Direct measurement of the dispersion of the electrogyration coefficient of photorefractive Bi12GeO20 crystals
Authors: Deliolanis N. C. , Kourmoulis I. M. , Asimellis G. , Apostolidis A. G. , Vanidhis E. D. , Vainos N. A.
Published: January 2005

We report on the direct measurement of the electrogyration coefficient of Bi12GeO20 sillenite crystals over the visible spectral range. The coefficient is directly measured on a (111)-cut crystal for which the electro-optic and electrogyratory effects are decoupled. The precision of the measurement method over various parameters including absorption and misorientation is examined. The value of the electrogyration coefficient is found to vary from 0.37±0.03to0.05±0.01pm∕V for the 460–760-nm spectrum range, which is within the upper boundaries that have been specified in previous experiments. It is also found that the electrogyration coefficient follows the dispersion law of optical activity and refractive index.
Analysis of wave profiles for single-crystal cyclotetramethylene tetranitramine
Authors: Menikoff Ralph , Dick J. J. , Hooks D. E.
Published: January 2005

Wave profiles measured in the β-polymorph of single-crystal cyclotetramethylene tetranitramine display the characteristic response of an elastic-plastic material, an elastic precursor followed by a plastic wave. Moreover, the elastic precursor decays with the length of run. Numerical simulations with a rate-dependent elastic-plastic model are used to account for nonlinear and transient wave behavior. In addition, to account for the measured anisotropy in propagation, parameters of an isotropic model are fit for two propagation directions, normal to the (011) and (010) planes of the P21∕n space group. Equation of state parameters are constrained by data for the longitudinal sound speed and hydrostatic compression. The fits show that the effective yield strength varies with direction from 0.18GPa for the (011) orientation to 0.31GPa in the (010) orientation.
Negative differential conductance and current bistability in undoped GaAs∕(Al,Ga)As quantum-cascade structures
Authors: Lu S. L. , Schrottke L. , Hey R. , Kostial H. , Grahn H. T.
Published: January 2005

We have investigated negative differential conductivity (NDC) and laser level population in undoped GaAs∕(Al,Ga)As quantum-cascade structures using current–voltage characteristics and interband photoluminescence spectroscopy. While for both GaAs∕Al0.33Ga0.67As and GaAs∕Al0.45Ga0.55As structures a strong, bistable NDC is observed, a weaker NDC without bistability appears only in the GaAs∕Al0.45Ga0.55As structure, which is due to the resonant coupling between injector and upper laser level. Although the bistable NDC is connected with a significant population of the laser levels, it cannot be explained by resonant coupling between electronic states in the active region. We believe that the bistable NDC is caused by an interplay of resonant coupling effects within the injector with the carrier redistribution in the vicinity of the optically active region. Furthermore, a still unidentified state, which exhibits a strong photoluminescence signal, may play an important role for the bistability.
Relaxation mechanisms in metal-organic vapor phase epitaxy grown Al-rich (Al,Ga)N∕GaN heterostructures
Authors: Vennéguès P. , Bougrioua Z. , Bethoux J. M. , Azize M. , Tottereau O.
Published: January 2005

The relaxation mechanisms in metal-organic vapor phase epitaxy grown (Al,Ga)N∕GaN heterostructures are studied. The first stage of the relaxation process is a two-dimensional–three-dimensional growth transition with the formation of mesalike islands separated by V-shaped trenches. The tensile stress relief is obtained by an elastic relaxation of the islands edges. In the case of AlN∕GaN, the apexes of the V trenches reach the heterointerface and misfit dislocations are nucleated at the islands coalescence region. These dislocations are a type and glide in the basal plane to promote further relaxation. For (Al,Ga)N∕GaN with an Al concentration below 70%, the apexes of the V trenches do not reach the heterointerface, prohibiting the nucleation of misfit dislocations. For thicker layers, the next stage of the relaxation is the cracking of the films.
Current inhomogeneity effect in single-layer ferromagnetic antirectangular structures
Authors: Wang C. C. , Adeyeye A. O. , Wu Y. H. , Jalil M. B. A.
Published: January 2005

The magnetotransport properties of antirectangular arrays (2×10μm2) embedded into 200-Å-thick continuous Ni80Fe20 films with the interhole spacing equal to 2μm have been investigated. We observed that the inhomogeneous current-density distribution, due to the placement of electrical contacts, caused an unusual positive magnetoresistance (MR) behavior for the in-plane longitudinal measurements. Finite element simulations on current-density distribution are in agreement with experimental data on small area antirectangular structures. For the continuous film, however, the current-density distribution does not play a significant role in determining the shape of the MR curve.
Auger and radiative recombination coefficients in 0.55-eV InGaAsSb
Authors: Kumar R. J. , Borrego J. M. , Dutta P. S. , Gutmann R. J. , Wang C. A. , Nichols G.
Published: January 2005

A radio-frequency photoreflectance technique, which senses changes in sample conductivity as carriers recombine following excitation by a laser pulse, has been used to measure the recombination parameters in 0.55-eV InGaAsSb lattice matched to GaSb. Doubly capped lifetime structures with variable active layer thicknesses are used to extract the surface recombination velocity, while an analysis of the samples with different doping concentrations is used to obtain Auger (C) and radiative (B) recombination parameters. Parameter extraction for the samples evaluated gives C=(1±0.4)×10−28cm6∕s and B=(3±1.5)×10−11cm3∕s for 0.55-eV InGaAsSb lattice matched to GaSb. The Auger and radiative recombination coefficients obtained from high-level injection decay times in low doping concentration samples show very good agreement with the values obtained from low-level injection conditions.
On measurement of thermal diffusion coefficients in multicomponent mixtures
Authors: Haugen Kjetil B. , Firoozabadi Abbas
Published: January 2005

We investigate the steady-state separation of the individual components of an incompressible multicomponent liquid mixture in a narrow two-dimensional thermogravitational column. Analytic working equations for measuring thermal diffusion coefficients analogous to the existing equations for a binary mixture are derived. Similar to the binary results, we find that when compositional variation has negligible effect on fluid density and vertical diffusive flux can be ignored, molecular diffusion does not affect steady-state separation. However, when compositional effects on density are taken into account, molecular diffusion does affect the bulk convective flow and the steady-state separation of the components. There may be also two distinct trends in the velocity and separation profiles. With one or more negative thermal diffusion coefficients, there may be more than one convection cell resulting in oscillatory behavior of separation. The working equations presented can be used to measure thermal diffusion coefficients of multicomponent mixtures. Such measurements have not yet been reported in the literature.
An investigation into the excitation migration in polyfluorene solutions via temperature dependent fluorescence anisotropy
Authors: Vaughan Helen L. , Dias Fernando M. B. , Monkman Andrew P.
Published: January 2005

Fluorescence anisotropy of dilute polyfluorene polymer solutions has been used to show that two processes, exciton migration and conformational relaxation (twisting of part of the chain), occur within polyfluorene polymers of 20 or more repeat units. The former process is dominant and temperature independent for high excitation energy but, as the chain length is decreased, exciton migration is eliminated and the conformational relaxation becomes the only mechanism by which excited state energy relaxation to the emission site can occur. When the polymers are excited in the absorption band tails, again no migration is observed but conformational relaxation is still present.
Rotational dynamics of methyl groups in m-xylene
Authors: Kirstein O. , Prager M. , Dimeo R. M. , Desmedt A.
Published: January 2005

Methyl group dynamics of m-xylene was investigated by using incoherent inelastic and quasi-elastic neutron scattering. Inelastic measurements were carried out at the high flux backscattering spectrometer HFBS at the National Institute of Standards, quasi-elastic measurements at the time-of-flight spectrometer NEAT at the Hahn-Meitner-Institute. Rotational potentials are derived which describe the tunnel splittings, first librational, and activation energies of the two inequivalent CH3 groups. Indications for coupling of the methyl rotation to low-energy phonons have been found. The finite width of one tunneling transition at He temperature is described by direct methyl–methyl coupling. The combined results of the experiments and the calculations allow a unique assignment of rotor excitations to crystallographic sites.
Density functional theory study of the nematic–isotropic transition in an hybrid cell
Authors: Rodrı́guez-Ponce I. , Romero-Enrique J. M. , Rull L. F.
Published: January 2005

We have employed the density functional theory formalism to investigate the nematic–isotropic capillary transitions of a nematogen confined by walls that favor antagonist orientations to the liquid crystal molecules (hybrid cell). We analyze the behavior of the capillary transition as a function of the fluid–substrate interactions and the pore width. In addition to the usual capillary transition between isotropiclike to nematiclike states, we find that this transition can be suppressed when one substrate is wet by the isotropic phase and the other by the nematic phase. Under this condition the system presents interfacelike states which allow us to continuously transform the nematiclike phase to the isotropiclike phase without undergoing a sharp phase transition. Two different mechanisms for the disappearance of the capillary transition are identified. When the director of the nematiclike state is homogeneously planar-anchored with respect to the substrates, the capillary transition ends up in a critical point. This scenario is analogous to the observed in Ising models when confined in slit pores with opposing surface fields which have critical wetting transitions. When the nematiclike state has a linearly distorted director field, the capillary transition continuously transforms in a transition between two nematiclike states.
Critical examination of the supermolecule density functional theory calculations of intermolecular interactions
Authors: Cybulski Sławomir M. , Seversen Christopher E.
Published: January 2005

The results of calculations employing twelve different combinations of exchange and correlation functionals are compared with results of ab initio calculations for two different configurations of the water dimer and three different configurations of the thymine-adenine complex. None of the density functional theory (DFT) treatments could properly reproduce the results of coupled-cluster calculations for all configurations examined. The DFT approaches perform well when the interaction energy is dominated by the electrostatic component and the dispersion energy is less important. Two mechanisms that compensate for the missing dispersion component were identified. The first one is the decrease of the magnitude of the intermolecular exchange-repulsion and the second one is the increase of the magnitude of the attractive deformation energy. For some functionals both effects are observed together, but for some other ones only the second effect occurs. The three correlation functionals that were examined were found to make only very small contributions to the deformation energy. The examination of angular and distance dependence of the interactions shows that the currently available DFT approaches are not suitable for developing intermolecular potential energy surfaces. They could however be used to find global minima on potential energy surfaces governed by intermolecular electrostatic interactions. Additional single point ab initio calculations are recommended as the means of validating optimized structures.
Structure, bonding, and magnetism in manganese clusters
Authors: Bobadova-Parvanova P. , Jackson K. A. , Srinivas S. , Horoi M.
Published: January 2005

We investigate the structures and magnetic properties of small Mnn clusters in the size range of 2–13 atoms using first-principles density functional theory. We arrive at the lowest energy structures for clusters in this size range by simultaneously optimizing the cluster geometries, total spins, and relative orientations of individual atomic moments. The results for the net magnetic moments for the optimal clusters are in good agreement with experiment. The magnetic behavior of Mnn clusters in the size range studied in this work ranges from ferromagnetic ordering (large net cluster moment) for the smallest (n=2, 3, and 4) clusters to a near degeneracy between ferromagnetic and antiferromagnetic solutions in the vicinity of n=5 and 6 to a clear preference for antiferromagnetic (small net cluster moment) ordering at n=7 and beyond. We study the details of this evolution and present a picture in which bonding in these clusters predominantly occurs due to a transfer of electrons from antibonding 4s levels to minority 3d levels.
Norm-conserving Hartree–Fock pseudopotentials and their asymptotic behavior
Authors: Trail J. R. , Needs R. J.
Published: January 2005

We investigate the properties of norm-conserving pseudopotentials (effective core potentials) generated by inversion of the Hartree–Fock equations. In particular, we investigate the asymptotic behavior as r→∞ and find that such pseudopotentials are nonlocal over all space, apart from a few special cases such as H and He. Such extreme nonlocality leads to a lack of transferability and, within periodic boundary conditions, an undefined total energy. The extreme nonlocality must therefore be removed, and we argue that the best way to accomplish this is a minor relaxation of the norm-conservation condition. This is implemented, and pseudopotentials for the atoms H–Ar are constructed and tested.
Variational optimization of effective atom centered potentials for molecular properties
Authors: von Lilienfeld O. Anatole , Tavernelli Ivano , Rothlisberger Ursula , Sebastiani Daniel
Published: January 2005

In plane wave based electronic structure calculations the interaction of core and valence electrons is usually represented by atomic effective core potentials. They are constructed in such a way that the shape of the atomic valence orbitals outside a certain core radius is reproduced correctly with respect to the corresponding all-electron calculations. Here we present a method which, in conjunction with density functional perturbation theory, allows to optimize effective core potentials in order to reproduce ground-state molecular properties from arbitrarily accurate reference calculations within standard density functional calculations. We demonstrate the wide range of possible applications in theoretical chemistry of such optimized effective core potentials (OECPs) by means of two examples. We first use OECPs to tackle the link atom problem in quantum mechanics/molecular mechanics (QM/MM) schemes proposing a fully automatized procedure for the design of link OECPs, which are designed in such a way that they minimally perturb the electronic structure in the QM region. In the second application, we use OECPs in two sample molecules (water and acetic acid) such as to reproduce electronic densities and derived molecular properties of hybrid (B3LYP) quality within general gradient approximated (BLYP) density functional calculations.
Ab initio study on the spectroscopy of CuCl2. II. Benchmark calculations on the X 2Πg-C 2Δg and X 2Πg-D 2Δg transitions
Authors: Ramı́rez-Solı́s A. , Daudey J. P.
Published: January 2005

The X 2Πg-C 2Δg and X 2Πg-D 2Δg transitions on CuCl2 have been studied using the most sophisticated nondynamic and dynamic electronic correlation treatments. We report here ab initio benchmark calculations using especially developed basis sets to study, at the complete active space self-consistent field plus second-order Möller-Plesset algorithm (CASSCF+CASPT2) and CASSCF+ACPF levels, the transition energies as well as the corresponding equilibrium geometries (ACPF—averaged coupled pair functional). The spin-orbit (SO) effects of both atoms were included in a second step through the effective Hamiltonian formalism, using the calibrated SO effective potentials developed by the Stuttgart group. Without SO at the CASSCF+ACPF level, the vertical excitation energy for the Δg2 state is 6711 cm−1 and the symmetric stretching equilibrium Cu–Cl distance is 4.04 a.u. The inclusion of the SO effects leads to a pure Δg2 Ω=5/2C state and a Ω=3/2 (0.7% 2Πg,99.3% 2Δg)D state. The calculated transition energies for the C and D states are 6340 and 8020 cm−1, in good agreement with the spin-orbit splitting recent values from gas-phase and rare-gas matrix isolation laser induced fluorescence experiments. The present benchmark results show, as was recently done for the X 2Πg-2Σg transition, the rather poor performance of all the density functional theory–based descriptions for the Δg2 state, which largely overestimate its Te, systematically placing it around 19 000 cm−1. The CASSCF+CASPT2 method also overestimates, by around 50%, the X 2Πg-2Δg transition energy, showing that only large variational calculations can produce reliable spectroscopic results for this kind of complex systems where delicate electronic correlation effects have to be carefully dealt with.
Restricted open-shell Kohn–Sham theory for π–π* transitions. III. Dynamics of aggregates
Authors: Nonnenberg Christel , Bräuchle Christoph , Frank Irmgard
Published: January 2005

We present molecular-dynamics simulations for 1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and cyclopentadiene at finite temperature using periodic boundary conditions. These systems form weakly bound aggregates in the ground state and exhibit bond formation in the excited state. Monomeric excitation of an ensemble of butadiene molecules leads to a transfer of the excitation between two molecules in the excited state with an intermediate delocalization of the wave function over both moieties.
Electron–molecule scattering calculations in a 3D finite element R-matrix approach
Authors: Tonzani Stefano , Greene Chris H.
Published: January 2005

We have implemented a three-dimensional finite element approach, based on tricubic polynomials in spherical coordinates, which solves the Schrödinger equation for scattering of a low energy electron from a molecule, approximating the electron exchange as a local potential. The potential is treated as a sum of three terms: electrostatic, exchange, and polarization. The electrostatic term can be extracted directly from ab initio codes (GAUSSIAN 98 in the work described here), while the exchange term is approximated using different local density functionals. A local polarization potential approximately describes the long range attraction to the molecular target induced by the scattering electron.
Nonadiabatic effects on peptide vibrational dynamics induced by conformational changes
Authors: Antony Jens , Schmidt Burkhard , Schütte Christof
Published: January 2005

Quantum dynamical simulations of vibrational spectroscopy have been carried out for glycine dipeptide (CH3–CO–NH–CH2–CO–NH–CH3). Conformational structure and dynamics are modeled in terms of the two Ramachandran dihedral angles of the molecular backbone. Potential energy surfaces and harmonic frequencies are obtained from electronic structure calculations at the density functional theory (DFT) [B3LYP/6-31+G(d)] level. The ordering of the energetically most stable isomers (C7 and C5) is reversed upon inclusion of the quantum mechanical zero point vibrational energy. Vibrational spectra of various isomers show distinct differences, mainly in the region of the amide modes, thereby relating conformational structures and vibrational spectra. Conformational dynamics is modeled by propagation of quantum mechanical wave packets. Assuming a directed energy transfer to the torsional degrees of freedom, transitions between the C7 and C5 minimum energy structures occur on a sub-picosecond time scale (700⋯800 fs). Vibrationally nonadiabatic effects are investigated for the case of the coupled, fundamentally excited amide I states. Using a two state-two mode model, the resulting wave packet dynamics is found to be strongly nonadiabatic due to the presence of a seam of the two potential energy surfaces. Initially prepared adiabatic vibrational states decay upon conformational change on a time scale of 200⋯500 fs with population transfer of more than 50% between the coupled amide I states. Also the vibrational energy transport between localized (excitonic) amide I vibrational states is strongly influenced by torsional dynamics of the molecular backbone where both enhanced and reduced decay rates are found. All these observations should allow the detection of conformational changes by means of time-dependent vibrational spectroscopy.
Enhanced emission from liquid-crystal lasers
Authors: Morris S. M. , Ford A. D. , Pivnenko M. N. , Coles H. J.
Published: January 2005

The performance of a photonic band-edge laser fabricated from a low molar mass dye-doped chiral nematic liquid crystal is found to have a strong thermal dependence. At each temperature the performance of the laser has been characterized by the slope efficiency which was calculated from a plot of the emission energy as a function of excitation energy. This slope efficiency was found to increase by 36% when the dye-doped chiral nematic liquid crystal was cooled from 53to43°C. The increase in slope efficiency is considered to be due to a change in the lasing conditions, in particular, changes in the emission efficiency of the dye and possibly the quality factor of the liquid-crystal resonator, which is dependent upon the linewidth of the resonant mode. The wavelength dependency of the spontaneous emission intensity and the quantum efficiency of the dye were not found to influence the lasing conditions in this case. The order parameters relating to the dye-doped chiral nematic liquid crystal were considered to be the principle factors responsible for the increase in performance at the lower operating temperature of 43°C.
Optical properties of InAs quantum dots with InAlAs∕InGaAs composite matrix
Authors: Liu Wei-Sheng , Chyi Jen-Inn
Published: January 2005

InAs quantum dots (QDs) in various matrices, such as InAlAs, InGaAs, InAlAs∕InGaAs, and InGaAs∕InAlAs, are prepared to clarify how the overgrown layers affect their optical properties. It is shown that strain reduction mechanism dominates the emission wavelength extension when the thickness of the overgrown layer is thin. The relaxation of compressive strain at the QD bottom by InAlAs overgrown layer cannot only extend the QD emission wavelength but also narrow the spectral linewidth and increase the state separation between the ground state and the first excited state.
Resonator-based all-optical Kerr-nonlinear phase shifting: Design and limitations
Authors: Priem Gino , Notebaert Inge , Bienstman Peter , Morthier Geert , Baets Roel
Published: January 2005

The design of all-optical phase shifting by means of Kerr-nonlinear resonators is investigated using a one-dimensional analytical model. Dependence on different device parameters and design optimization are discussed. In particular, it is shown that a tradeoff in terms of optical input power and device length is required, which is limited by the signal bandwidth.
Advanced nanosphere lithography for the areal-density variation of periodic arrays of vertically aligned carbon nanofibers
Authors: Park Kyung Ho , Lee Soonil , Koh Ken Ha , Lacerda Rodrigo , Teo K. B. K. , Milne W. I.
Published: January 2005

Periodic arrays of vertically aligned isolated carbon nanofibers (CNFs) have been fabricated using self-assembled polystyrene spheres as shadow masks for catalyst-pattern formation. Proper use of monolayer and bilayer masks, and judicial combination of angle-deposition technique with monolayer masks have allowed us to control the dot size and spacing of catalyst patterns. As long as the catalyst-dot size is not too large, isolated single CNF has grown from each catalyst dot. Combining nanosphere lithography with conventional photolithography, we have been able to realize patterned growth of CNF arrays on selected areas.
Formation of inorganic electride thin films via site-selective extrusion by energetic inert gas ions
Authors: Miyakawa Masashi , Toda Yoshitake , Hayashi Katsuro , Hirano Masahiro , Kamiya Toshio , Matsunami Noriaki , Hosono Hideo
Published: January 2005

Inert gas ion implantation (acceleration voltage 300kV) into polycrystalline 12CaO·7Al2O3 (C12A7) films was investigated with fluences from 1×1016 to 1×1017cm−2 at elevated temperatures. Upon hot implantation at 600°C with fluences greater than 1×1017cm−2, the obtained films were colored and exhibited high electrical conductivity in the as-implanted state. The extrusion of O2− ions encaged in the crystallographic cages of C12A7 crystal, which leaves electrons in the cages at concentrations up to ∼1.4×1021cm−3, may cause the high electrical conductivity. On the other hand, when the fluence is less than 1×1017cm−2, the as-implanted films are optically transparent and electrically insulating. The conductivity is enhanced and the films become colored by irradiating with ultraviolet light due to the formation of F+-like centers. The electrons forming the F+-like centers are photo released from the encaged H− ions, which are presumably derived from the preexisting OH− groups. The induced electron concentration is proportional to the calculated displacements per atom, which suggests that nuclear collision effects of the implanted ions play a dominant role in forming the electron and H− ion in the films. The hot ion implantation technique provides a nonchemical process for preparing electronic conductive C12A7 films.
Frequency dependent heat capacity within a kinetic model of glassy dynamics
Authors: Chakrabarti Dwaipayan , Bagchi Biman
Published: January 2005

There has been renewed interest in the frequency dependent specific heat of supercooled liquids in recent years with computer simulation studies exploring the whole frequency range of relaxation. The simulation studies can thus supplement the existing experimental results to provide an insight into the energy landscape dynamics. We here investigate a kinetic model of cooperative dynamics within the landscape paradigm for the dynamic heat capacity C(ω,T) behavior. In this picture, the β-process is modeled as a thermally activated event in a two-level system and the α-process is described as a β-relaxation mediated cooperative transition in a double well. The model resembles a landscape picture, apparently first conceived by Stillinger [Science 267, 1935 (1995)], where an α-process is assumed to involve a concerted series of β-processes. The model provides a description of the activated hopping in the energy landscape in close relation with the cooperative nature of the hopping event. For suitable choice of parameters, the model predicts a frequency dependent heat capacity that reflects the two-step relaxation behavior. The separation between the two peaks grows as the temperature drops, indicating the stringent constraint on the α-process due to the cooperativity requirement. The temperature dependence of the position of the low-frequency peak, due to the α-relaxation, shows a non-Arrhenius behavior as observed experimentally. The shape of the α-peak is, however, found to be temperature independent. The high-frequency peak appears with considerably larger amplitude than the α-peak. We attempt a plausible reason for this observation that is in contrast with the general feature revealed by the dielectric spectroscopy. The relative amplitudes of the β- and α-peaks in the present framework are found to depend on several characteristic features of the energy landscape, including the extent of cooperativity requirement for the α-relaxation and the asymmetry of the double well.
Chemical reaction dynamics within anisotropic solvents in time-dependent fields
Authors: Hershkovits Eli , Hernandez Rigoberto
Published: January 2005

The dynamics of low-dimensional Brownian particles coupled to time-dependent driven anisotropic heavy particles (mesogens) in a uniform bath (solvent) have been described through the use of a variant of the stochastic Langevin equation. The rotational motion of the mesogens is assumed to follow the motion of an external driving field in the linear response limit. Reaction dynamics have also been probed using a two-state model for the Brownian particles. Analytical expressions for diffusion and reaction rates have been developed and are found to be in good agreement with numerical calculations. When the external field driving the mesogens is held at constant rotational frequency, the model for reaction dynamics predicts that the applied field frequency can be used to control the product composition.
Fluorescence correlation spectroscopy studies of diffusion of a weak polyelectrolyte in aqueous solutions
Authors: Pristinski Denis , Kozlovskaya Veronika , Sukhishvili Svetlana A.
Published: January 2005

We apply fluorescent correlation spectroscopy (FCS) to investigate solution dynamics of a synthetic polyelectrolyte, i.e., a weak polycarboxylic acid in aqueous solutions. The technique brings single molecule sensitivity and molecular specificity to dynamic measurements of polyelectrolyte solutions. Translational diffusion of Alexa-labeled poly(methacrylic acid), PMAA*, chains was studied in very dilute, 10−4 mg/ml, solutions as a function of solution pH and ionic strength. The observed changes in diffusion coefficients were consistent with about twofold expansion of PMAA* coils when pH was changed from 5 to 8, and with chain contraction for alkaline metal ion concentrations from 0.01 to 0.1 M. The dependence of the hydrodynamic size of PMAA* chains on the counterion type followed the sequence: Li+>Na+≈Cs+>K+. The dependence of translational diffusion on polyacid concentration was weak at the low concentration limit, but chain motions were significantly slower at higher polymer concentrations when PMAA chains overlapped. Finally, measurements of dynamics of PMAA* chains in “salt-free” solutions showed that self-diffusion of PMAA* chains significantly slowed down when PMAA concentration was increased, probably reflecting the sensitivity of PMAA* translational motions to the onset of interchain domain formation. These results illustrate the utility of the FCS technique for studying hydrodynamic sizes of polyelectrolyte coils in response to variation in solution pH or concentration of salt and polyelectrolytes. They also suggest that FCS will be a promising technique for selective observation of the dynamics of polyelectrolyte components in complex polymer mixtures.
Biased diffusion of Si magic clusters on Si(111) surface
Authors: Ho Mon-Shu , Hwang Ing-Shouh , Tsong Tien T.
Published: January 2005

The glide and lock-in mechanism of Si magic cluster diffusion on the Si(111)-7×7 surface is investigated using a scanning tunneling microscope. The biased long jumps parallel and normal to the direction of the heating current are determined separately and quantitatively. The tendency of Si magic clusters to detach from the corner step edge of a Si lake on the side of the anode and their subsequent attachment to that at the side of the cathode is studied. The biased detachment of Si magic clusters from Si islands near the cathode side is also observed and studied. The driving force of this anisotropic behavior is significantly stronger than that indicated by current theories.
High-order harmonic generation in a two-electron quantum dot molecule
Authors: Terzis Andreas F. , Paspalakis Emmanuel
Published: January 2005

We present results on the occurrence of high-order harmonic generation in a two-electron quantum dot molecule. The two-electron quantum dot molecule consists of two interacting electrons in a double quantum dot system. We consider coherent interaction of the quantum dot molecule with a low-frequency and high-intensity driving field. We calculate the emission spectrum numerically for material parameters corresponding to GaAs and show that high-order harmonic generation is possible in this system. Our findings are in very good agreement with a model system based on level crossing theory. The model system is used for the derivation of a relation for the cutoff frequency of the harmonics. This cutoff frequency is shown to be crucially dependent on the external field frequency and intensity.
Unbiased estimators for spatial distribution functions of classical fluids
Authors: Adib Artur B. , Jarzynski Christopher
Published: January 2005

We use a statistical-mechanical identity closely related to the familiar virial theorem, to derive unbiased estimators for spatial distribution functions of classical fluids. In particular, we obtain estimators for both the fluid density ρ(r) in the vicinity of a fixed solute and the pair correlation g(r) of a homogeneous classical fluid. We illustrate the utility of our estimators with numerical examples, which reveal advantages over traditional histogram-based methods of computing such distributions.