bluuebunny/crossref_metadata_2025 huggingface dataset:
Stress and heat flux for arbitrary multibody potentials: A unified framework
Authors: Admal Nikhil Chandra , Tadmor E. B.
Published: May 2011

A two-step unified framework for the evaluation of continuum field expressions from molecular simulations for arbitrary interatomic potentials is presented. First, pointwise continuum fields are obtained using a generalization of the Irving–Kirkwood procedure to arbitrary multibody potentials. Two ambiguities associated with the original Irving–Kirkwood procedure (which was limited to pair potential interactions) are addressed in its generalization. The first ambiguity is due to the nonuniqueness of the decomposition of the force on an atom as a sum of central forces, which is a result of the nonuniqueness of the potential energy representation in terms of distances between the particles. This is in turn related to the shape space of the system. The second ambiguity is due to the nonuniqueness of the energy decomposition between particles. The latter can be completely avoided through an alternate derivation for the energy balance. It is found that the expressions for the specific internal energy and the heat flux obtained through the alternate derivation are quite different from the original Irving–Kirkwood procedure and appear to be more physically reasonable. Next, in the second step of the unified framework, spatial averaging is applied to the pointwise field to obtain the corresponding macroscopic quantities. These lead to expressions suitable for computation in molecular dynamics simulations. It is shown that the important commonly-used microscopic definitions for the stress tensor and heat flux vector are recovered in this process as special cases (generalized to arbitrary multibody potentials). Several numerical experiments are conducted to compare the new expression for the specific internal energy with the original one.
Tunable work function of a WOx buffer layer for enhanced photocarrier collection of pin-type amorphous silicon solar cells
Authors: Fang Liang , Baik Seung Jae , Kim Jeong Won , Kang Sang Jung , Seo Jung Won , Jeon Jin-Wan , Kim Yoon Hak , Lim Koeng Su
Published: May 2011

An in situ postdeposition ultraviolet treatment was proposed to improve the electrical properties of a tungsten oxide (WOx) buffer layer of pin-type amorphous silicon-based solar cell. Based on the x-ray and ultraviolet photoelectron spectroscopy and the activation energy measurements, it was found that the work function of WOx is tunable by ultraviolet light treatment, and the collection performance of solar cells incorporating WOx with the lower work function is further improved. Moreover, the optimal band alignment scheme for a window layer is discussed in terms of obtaining enhanced carrier collection without open circuit voltage degradation.
Near threshold Cl−·CH3I photodetachment: Apparent 2P1/2 channel suppression and enhanced 2P3/2 channel vibrational excitation
Authors: Van Duzor Matthew , Mbaiwa Foster , Lasinski Joshua , Dao Diep , Holtgrewe Nicholas , Mabbs Richard
Published: May 2011

Cl−·CH3I cluster anion photoelectron images are recorded over a range of detachment wavelengths in the immediate post threshold region. The photoelectron spectral features fall into two categories. A number of weak, photon energy dependent transitions are observed and attributed to atomic anion fragmentation products. Several more intense, higher electron binding energy transitions result from single photon cluster anion detachment. Comparison with I−·CH3I suggests that the detachment process is more complicated for Cl−·CH3I. The single photon transition spacing is consistent with CH3I ν3 mode excitation, but the two distinct vibronic bands of I−·CH3I detachment are not easily distinguished in the Cl−·CH3I spectra. Similarly, while the spectral intensities for both cluster anions show non-Franck Condon behavior, the level of vibrational excitation appears greater for Cl−·CH3I detachment. These observations are discussed in terms of low lying electronic states of CH3I along the C-I coordinate, and the influence of the CH3I moiety on the neutral halogen atom states.
Scaled opposite-spin CC2 for ground and excited states with fourth order scaling computational costs
Authors: Winter Nina O. C. , Hättig Christof
Published: May 2011

An implementation of scaled opposite-spin CC2 (SOS-CC2) for ground and excited state energies is presented that requires only fourth order scaling computational costs. The SOS-CC2 method yields results with an accuracy comparable to the unscaled method. Furthermore the time-determining fifth order scaling steps in the algorithm can be replaced by only fourth order scaling computational costs using a “resolution of the identity” approximation for the electron repulsion integrals and a Laplace transformation of the orbital energy denominators. This leads to a significant reduction of computational costs especially for large systems. Timings for ground and excited state calculations are shown and the error of the Laplace transformation is investigated. An application to a chlorophyll molecule with 134 atoms results in a speed-up by a factor of five and demonstrates how the new implementation extends the applicability of the method. A SOS variant of the algebraic diagrammatic construction through second order ADC(2), which arises from a simplification of the SOS-CC2 model, is also presented. The SOS-ADC(2) model is a cost-efficient alternative in particular for future extensions to spectral intensities and excited state structure optimizations.
O1s photoionization dynamics in oriented NO2
Authors: Stener Mauro , Decleva Piero , Yamazaki Masakazu , Adachi Jun-ichi , Yagishita Akira
Published: May 2011

We have performed extensive density functional theory (DFT) calculations, partial cross sections, dipole prepared continuum orbitals, dipole amplitudes and phase shifts, asymmetry parameters β, and molecular frame photoelectron angular distributions, to elucidate the O1s photoionization dynamics of NO2 molecule with emphasis on the shape resonances in the O1s ionization continuum. In the shape resonance region, the β parameters and photoelectron angular distributions have been compared with our experimental results. Fairly good agreement between the theory and experiment has confirmed that the DFT level calculations can well describe the photoionization dynamics of the simple molecule such as NO2. Interference due to equivalent atom photoionization is theoretically considered, and the possibility of detection of the effect in the two degenerate channels with different combinations of light polarization and photoemission direction is discussed.
Solvent-mediated charge redistribution in photodissociation of IBr− and IBr−(CO2)
Authors: Sheps Leonid , Miller Elisa M. , Horvath Samantha , Thompson Matthew A. , Parson Robert , McCoy Anne B. , Lineberger W. Carl
Published: May 2011

A combined experimental and theoretical investigation of photodissociation dynamics of IBr− and IBr−(CO2) on the B (${}^2\Sigma _{1/2}^ +$Σ1/2+2) excited electronic state is presented. Time-resolved photoelectron spectroscopy reveals that in bare IBr− prompt dissociation forms exclusively I* + Br−. Compared to earlier dissociation studies of IBr− excited to the A′ (2Π1/2) state, the signal rise is delayed by 200 ± 20 fs. In the case of IBr−(CO2), the product distribution shows the existence of a second major (∼40%) dissociation pathway, Br* + I−. In contrast to the primary product channel, the signal rise associated with this pathway shows only a 50 ± 20 fs delay. The altered product branching ratio indicates that the presence of one solvent-like CO2 molecule dramatically affects the electronic structure of the dissociating IBr−. We explore the origins of this phenomenon with classical trajectories, quantum wave packet studies, and MR-SO-CISD calculations of the six lowest-energy electronic states of IBr− and 36 lowest-energy states of IBr. We find that the CO2 molecule provides sufficient solvation energy to bring the initially excited state close in energy to a lower-lying state. The splitting between these states and the time at which the crossing takes place depend on the location of the solvating CO2 molecule.
Simultaneous evaluation of the linear and quadratic electro-optic coefficients of the nonlinear optical polymer by attenuated-total-reflection technique
Authors: Zhu Xiaohui , Deng Xiaoxu , Li Honggen , Cao Zhuangqi , Shen Qishun , Wei Wei , Liu Feng
Published: May 2011

A method of simultaneously evaluating the linear electro-optic and quadratic electro-optic coefficients of the nonlinear polymer poly (9,9-dioctyl-2,7-fluorene-co-benzo[c][1,2,5]thiadiazole-co-9-hexyl-3,6-carbazole) (PF8-BT-CZ) was proposed based on the attenuated-total-reflection (ATR) technique. The measurement was sensitively carried out without the lock-in amplifier due to the prism-waveguide configuration of the sample and only one single optical path was applied, which simplified the experimental setup.
Time-delay-induced phase-transition to synchrony in coupled bursting neurons
Authors: Adhikari Bhim Mani , Prasad Awadhesh , Dhamala Mukeshwar
Published: May 2011

Signal transmission time delays in a network of nonlinear oscillators are known to be responsible for a variety of interesting dynamic behaviors including phase-flip transitions leading to synchrony or out of synchrony. Here, we uncover that phase-flip transitions are general phenomena and can occur in a network of coupled bursting neurons with a variety of coupling types. The transitions are marked by nonlinear changes in both temporal and phase-space characteristics of the coupled system. We demonstrate these phase-transitions with Hindmarsh-Rose and Leech-Heart interneuron models and discuss the implications of these results in understanding collective dynamics of bursting neurons in the brain.
Electrical properties of rolled-up p-type Si/SiGe heterostructures
Authors: Demarina N. V. , Grützmacher D. A.
Published: May 2011

We report a theoretical study of the hole density and the low-field mobility in modulation p-doped rolled-up Si/SiGe heterostructures. Solving coupled Poisson and Schrödinger equations, we show that the total hole density is strongly affected by charged surface states and can reach value of 1011 cm−2 for available doping level at room and low temperature. The simulation of the hole transport along the structure axis based on a Monte Carlo method reveals that the interface roughness scattering is a main mechanism limiting the mobility magnitude, which reaches the value of 104 cm2/V s.
The influence of impurities and planar defects on the infrared properties of silicon carbide films
Authors: Rajasekhara S. , Neuner B. H. , Zorman C. A. , Jegenyes N. , Ferro G. , Shvets G. , Ferreira P. J. , Kovar D.
Published: May 2011

Two cubic, single crystal silicon carbide (3C-SiC) films with similar thickness are shown to exhibit significantly different optical properties at mid-infrared wavelengths. Depth profiling by time-of-flight secondary ion mass spectroscopy indicates that these two films have substantially different n-type impurity concentrations that are responsible for the observed differences in optical absorption. The influence of impurities manifests as substantially different planar defect morphologies.
Photoelectron spectra of dihalomethyl anions: Testing the limits of normal mode analysis
Authors: Vogelhuber Kristen M. , Wren Scott W. , McCoy Anne B. , Ervin Kent M. , Lineberger W. Carl
Published: May 2011

We report the 364-nm negative ion photoelectron spectra of CHX2− and CDX2−, where X = Cl, Br, and I. The pyramidal dihalomethyl anions undergo a large geometry change upon electron photodetachment to become nearly planar, resulting in multiple extended vibrational progressions in the photoelectron spectra. The normal mode analysis that successfully models photoelectron spectra when geometry changes are modest is unable to reproduce qualitatively the experimental data using physically reasonable parameters. Specifically, the harmonic normal mode analysis using Cartesian displacement coordinates results in much more C-H stretch excitation than is observed, leading to a simulated photoelectron spectrum that is much broader than that which is seen experimentally. A (2 + 1)-dimensional anharmonic coupled-mode analysis much better reproduces the observed vibrational structure. We obtain an estimate of the adiabatic electron affinity of each dihalomethyl radical studied. The electron affinity of CHCl2 and CDCl2 is 1.3(2) eV, of CHBr2 and CDBr2 is 1.9(2) eV, and of CHI2 and CDI2 is 1.9(2) eV. Analysis of the experimental spectra illustrates the limits of the conventional normal mode approach and shows the type of analysis required for substantial geometry changes when multiple modes are active upon photodetachment.
The lowest singlet states of octatetraene revisited
Authors: Angeli Celestino , Pastore Mariachiara
Published: May 2011

The two lowest excited singlet states of all-trans-1,3,5,7-octatetraene, 2 1A−g and 1 1B+u, are studied by means of high level ab initio methods computing the vertical and adiabatic excitation energies for both states and the vertical emission energy for the \documentclass[12pt]{minimal}\begin{document}${1{\hspace{1.42271pt}}{^1{\rm A}_g^-}}$\end{document}11Ag−←2 1A−g transition. The results confirm the known assignment of two energies, the 2 1A−g adiabatic excitation energy and the 2 1A−g vertical emission energy, for which well defined experimental values are available, with an excellent agreement between theory and experiment. In the experimental absorption spectrum, the maximum of the band describing the 1 1B+u←\documentclass[12pt]{minimal}\begin{document}${1{\hspace{1.42271pt}}{^1{\rm A}_g^-}}$\end{document}11Ag− excitation is the first peak and it has been assigned to the (0-0) vibrational transition, but in literature it is normally compared with the theoretical vertical excitation energy. This comparison has been questioned in the past, but a conclusive demonstration of its lack of foundation has not been given. The analysis reported here, while confirming the assignment of the highest peak in the experimental spectrum to the (0-0) adiabatic transition, indicates that it cannot be used as a reference for the vertical excitation energy. The theoretical vertical excitation energies for the 2 1A−g and 1 1B+u states are found to be almost degenerate, with a value, ≃ 4.8 eV, higher than that normally accepted in the literature, 4.4 eV. The motivations which have induced in the past other authors to consider this a correct value are discussed and the origin of their feebleness are analyzed.
Multi-bump solutions for the nonlinear Schrödinger-Poisson system
Authors: Li Gongbao , Peng Shuangjie , Wang Chunhua
Published: May 2011

In this paper, we study a kind of nonlinear Schrödinger-Poisson system with a parameter ε. For any positive integer m, we prove that there exists ε(m) > 0 such that, for 0 < ε < ε(m), the equation has an m-bump positive solution under some suitable conditions. As a consequence, the equation has more and more multi-bump positive solutions as ε → 0.
Equivalence of domains for hyperbolic Hubbard-Stratonovich transformations
Authors: Müller-Hill J. , Zirnbauer M. R.
Published: May 2011

We settle a long standing issue concerning the traditional derivation of non-compact non-linear sigma models in the theory of disordered electron systems: the hyperbolic Hubbard-Stratonovich (HS) transformation of Pruisken-Schäfer type. Only recently the validity of such transformations was proved in the case of U(p, q) (non-compact unitary) and O(p, q) (non-compact orthogonal) symmetry. In this article we give a proof for general non-compact symmetry groups. Moreover, we show that the Pruisken-Schäfer type transformations are related to other variants of the HS transformation by deformation of the domain of integration. In particular we clarify the origin of surprising sign factors which were recently discovered in the case of orthogonal symmetry.
Note: Equation of state and compressibility of supercooled water: Simulations and experiment
Authors: Abascal J. L. F. , Vega C.
Published: May 2011

The equation of state and the isothermal compressibility of supercooled water for pressures up to 3000 bar obtained from computer simulations of the TIP4P/2005 model are compared to recent experimental results. The agreement between the simulations and experimental results is quite good. This reinforces the idea that the Widom line and the liquid-liquid phase separation found in the simulations should also exist in real water.
Time-resolved photoelectron spectroscopy of coupled electron-nuclear motion
Authors: Falge Mirjam , Engel Volker , Gräfe Stefanie
Published: May 2011

We investigate pump-probe electron detachment spectroscopy in a model system which is ideally suited to study coupled electronic and nuclear wave-packet dynamics. Time-resolved photoelectron spectra are calculated within the adiabatic approximation and a discretization of the detachment continuum. These spectra are compared to those which derive from a non-Born-Oppenheimer description and a numerically exact treatment of the detachment process. In this way it is possible to identify the influence of non-adiabatic effects on the spectra in a systematic way and also to test commonly applied approximations.
Nitrogen self-diffusion in magnetron sputtered Si-C-N films
Authors: Hüger E. , Schmidt H. , Geue T. , Stahn J. , Tietze U. , Lott D. , Markwitz A. , Geckle U. , Bruns M.
Published: May 2011

Self-diffusion was studied in magnetron sputtered nitrogen-rich amorphous compounds of the system Si-C-N by using nitrogen as a model tracer. As shown by infra-red spectroscopy a transient metastable region exists, where the structure of the material can be visualized as silicon nitride tetrahedra which are connected by carbo-diimide (-N=C=N-) bonds to a three dimensional amorphous network. In this region diffusion studies are carried out by neutron reflectometry and isotope multilayers as a function of annealing time, temperature and chemical composition. Low diffusivities between 10−20 and 10−24 m2/s were found. In the metastable region, diffusion is faster than diffusion in amorphous silicon nitride by 1 to 2 orders of magnitude, while the activation enthalpies of diffusion between 3.1 and 3.4 eV are the same within error limits. This can be explained by the fact that the diffusion mechanism along SiN4 tetrahedra is identical to that in amorphous silicon nitride, however, the carbo-diimide bonds seem to widen the structure, allowing faster diffusion. A correlation between diffusivities and the number of carbo-diimid bonds present in the material is found, where the highest diffusivities are observed for materials with the highest number of carbo-diimid bonds, close to the composition Si2CN4.
Optical bistability in mesoporous silicon microcavity resonators
Authors: Pham Anh , Qiao Hong , Guan Bin , Gal Michael , Gooding J. Justin , Reece Peter J.
Published: May 2011

We report on the observation of significant optical bistability in the transmission and reflection properties of mesoporous silicon microcavities when illuminated with a 150 ns pulsed laser at 532 nm. The observed optical hysteresis is shown to be transient in nature and the properties are strongly dependent on the porosity of the cavity layer. The onset and damage threshold intensity are also shown to be porosity dependent. Our modeling suggests that the observed effects are due to changes in the nonlinear refractive index where the transient lifetime increases with increasing porosity. We investigate the role of surface states on influencing the bistable process by passivating the internal porous surface with hydrosilylation chemistry.
Plasma discharge efficiency increase by using a small bandgap protective layer material- first-principles study for Mg1-xZnxO
Authors: Li Qiaofen , Tu Yan , Tolner Harm , Yang Lanlan
Published: May 2011

In this work, the properties of Mg1-xZnxO thin films are investigated as an example of a protective layer material with a small bandgap in a plasma display panel, to analyze the impact of these kinds of materials on the discharge properties. Using the first principles calculation method, the electronic structure of Mg1-xZnxO crystal is analyzed, and an analytical formula is obtained for the values of the bandgap. A cubic structure is obtained for x between 0 and 0.625. The secondary electron emission coefficients γ of Neon and Xenon with the Mg1-xZnxO films are then evaluated based on Hagstrum’s theory. The γ value for Xe ions is zero, until a concentration of 0.375 is reached, when the bandgap is about 5.1 eV. At x = 0.375 and beyond the condition for Auger emission by xenon ions is fulfilled, and for x > 0.375 the γ value increases continuously until a value of 0.07 is reached for x = 0.625. The γ value for Ne increases from 0.25 to 0.38 when the ZnO proportion is increased from 0 to 0.625. The discharge characteristics of the SM-PDP with Mg1-xZnxO protective layer are then calculated using the fluid model. When increasing the x value, the working voltage is strongly reduced, while the discharge efficiency is enhanced by about 60% at 20% Xe for a change in x from 0 to 0.625. We find that this increase is mainly caused by increased electron excitation efficiency. Therefore mixed-oxide materials with a small bandgap like MgO-ZnO in principle enable the use of high xenon content plasma displays, while strongly increasing the discharge efficiency.
Stacking sequence dependence of graphene layers on SiC (0001−)—Experimental and theoretical investigation
Authors: Borysiuk J. , Sołtys J. , Piechota J.
Published: May 2011

Different stacking sequences of graphene are investigated using a combination of experimental and theoretical methods. High-resolution transmission electron microscopy (HRTEM) of the stacking sequence of several layers of graphene, formed on the C-terminated 4H-SiC (0001−) surface, was used to determine the stacking sequence and the interlayer distances. These data prove that the three metastable multilayer graphene configurations exist: AB, AA, and ABC. In accordance, those three cases were considered theoretically, using density functional theory (DFT) calculations comparing properties of graphene, both free-standing and positioned on the SiC (0001−) substrate. Total energies were calculated, the most stable structure was identified, and the electronic band structure was obtained. These results were compared with results obtained for a graphene single layer, having six or three H atoms attached to the carbon ring. It was found that sixfold symmetry leads to linear dispersion relations and threefold symmetry leads to hyperbolic dispersion relations. Thus the type of dispersion relation, i.e., linear versus hyperbolic, is independent of the number of graphene layers or the interlayer coupling but depends on the graphene symmetry. Similarly, it was shown that a linear dispersion relation is obtained in the presence of the SiC substrate, and also for various distances between adjacent carbon layers for AA stacking.
Edge-crack problem in a long cylindrical superconductor
Authors: Zeng Jun , Yong Hua-Dong , Zhou You-He
Published: May 2011

In this work, the edge-crack problem is investigated for a long cylindrical high-temperature superconductor (HTS) under electromagnetic force. The distributions of the current density and the magnetic flux density in the cylinder are obtained analytically in the Kim critical state model. And then, based on the finite element method, the stress intensity factors at the crack tip for different magnetic fields and different lengths of the crack are obtained. The results obtained show that the applied field and the length of the edge crack have significant effects on the safety and validity of an HTS.
Electric field effects on chignolin conformation
Authors: Astrakas Loukas , Gousias Christos , Tzaphlidou Margaret
Published: May 2011

The effect of homogeneous and constant external electric fields on the structural stability of chignolin, a typical β-hairpin peptide, has been studied for 10ns using molecular dynamics simulations. The peptide aligns quickly its total dipole moment with the external electric field and then a constant stress is applied on its charged residues. For weaker fields this has mixed effects on the creation, destruction or strength of hydrogen bonds, but ultimately for strong fields chignolin unfolds starting by the separation of the terminal residues. Since the function of proteins is critically linked to their three-dimensional structures, these findings indicate that exposure to external electric fields may induce changes in conformation that can potentially initiate severe cellular dysfunction.
Tunable asymmetric reflectance in silver films near the percolation threshold
Authors: Chen Aiqing , Deutsch Miriam
Published: May 2011

We report on the optical characterization of semicontinuous nanostructured silver films exhibiting tunable optical reflectance asymmetries. The films are obtained using a multi-step process, where a nanocrystalline silver film is first chemically deposited on a glass substrate and then subsequently coated with additional silver via thermal vacuum-deposition. The resulting films exhibit reflectance asymmetries whose dispersions may be tuned both in sign and in magnitude, as well as a universal, tunable spectral crossover point. We obtain a correlation between the optical response and charge transport in these films, with the spectral crossover point indicating the onset of charge percolation. Such broadband, dispersion-tunable asymmetric reflectors may find uses in future light-harvesting systems.
Experimental study on substrate with hierarchical nested channels for thermal interface resistance control
Authors: Luo Xiaobing , Zheng Huai , Liu Minglu , Liu Sheng
Published: May 2011

Thermal interface resistance is an important parameter for the thermal management of electronics packaging. In order to reduce thermal interface resistance, hierarchical nested channels were designed and fabricated on the surfaces of aluminum and copper sheets. Experiments were conducted to measure the thermal interface resistance of the sheets. A sequence of experimental results revealed that thermal interface resistance of the sheets with hierarchical nested channels is 2−3 times larger than that of the original flat sheets without the channels, which is contrary to the experimental objective. A comprehensive experimental analysis was presented to explain the phenomenon. It is found that the roughness of contact interface plays a key role. In the reported references, the thermal interface material composed of metal particles is easy to form particle stacking in the smooth contacting surface, therefore, the hierarchical nested channels can effectively reduce the particle stacking and the corresponding thermal interface resistance. However, in the present experiment samples, the surface roughness is larger than 10 μm, the phenomenon of particle stacking is difficult to occur, the hierarchical nested channels cannot perform well for decreasing particle stacking and thermal resistance. Otherwise, the hierarchical nested channels increases the bulk thermal resistance because of the replacement of the high thermal conductivity metal material with thermal greases that have relatively low thermal conductivity.
Performance of the Argonne National Laboratory electron cyclotron resonance charge breeder
Authors: Vondrasek R. , Kolomiets A. , Levand A. , Pardo R. , Savard G. , Scott R.
Published: May 2011

An electron cyclotron resonance charge breeder for the Californium rare ion breeder upgrade (CARIBU), a new radioactive beam facility for the Argonne Tandem Linac Accelerator System (ATLAS), has been constructed and commissioned. Charge breeding efficiencies up to 15.6% have been realized for stable beams with a typical breeding time of 10 ms/charge state. The CARIBU system has been undergoing commissioning tests utilizing a 100 mCi 252Cf fission source. A charge breeding efficiency of 14.8 ± 5% has been achieved for the first radioactive beam of 143Cs27+.
Magnetophoresis, sedimentation, and diffusion of particles in concentrated magnetic fluids
Authors: Pshenichnikov Alexander F. , Elfimova Ekaterina A. , Ivanov Alexey O.
Published: May 2011

A dynamic mass transfer equation for describing magnetophoresis, sedimentation, and gradient diffusion of colloidal particles in concentrated magnetic fluids has been derived. This equation takes into account steric, magnetodipole, and hydrodynamic interparticle interactions. Steric interactions have been investigated using the Carnahan-Starling approximation for a hard-sphere system. In order to study the effective interparticle attraction, the free energy of the dipolar hard-sphere system is represented as a virial expansion with accuracy to the terms quadratic in particle concentration. The virial expansion gives an interpolation formula that fits well the results of computer simulation in a wide range of particle concentrations and interparticle interaction energies. The diffusion coefficient of colloidal particles is written with regard to steric, magnetodipole and hydrodynamic interactions. We thereby laid the foundation for the formulation of boundary-value problems and for calculation of concentration and magnetic fields in the devices (for example, magnetic fluid seals and acceleration sensors), which use a concentrated magnetic fluid as a working fluid. The Monte-Carlo methods and the analytical approach are employed to study the magnetic fluid stratification generated by the gravitational field in a cylinder of finite height. The coefficient of concentration stratification of the magnetic fluid is calculated in relation to the average concentration of particles and the dipolar coupling constant. It is shown that the effective particle attraction causes a many-fold increase in the concentration inhomogeneity of the fluid if the average volume fraction of particles does not exceed 30%. At high volume concentrations steric interactions play a crucial role.
Osmotic ensemble methods for predicting adsorption-induced structural transitions in nanoporous materials using molecular simulations
Authors: Zang Ji , Nair Sankar , Sholl David S.
Published: May 2011

Osmotic framework adsorbed solution theory is a useful molecular simulation method to predict the evolution of structural transitions upon adsorption of guest molecules in flexible nanoporous solids. One challenge with previous uses of this approach has been the estimation of free energy differences between the solid phases of interest in the absence of adsorbed molecules. Here we demonstrate that these free energy differences can be calculated without reference to experimental data via the vibrational density of states of each phase, a quantity that can be obtained from molecular dynamics simulations. We show the applicability of this method through case studies of the swelling behaviors of two representative systems in which swelling upon adsorption of water is of importance: single-walled aluminosilicate nanotube bundles and cesium montmorillonite. The resulting predictions show that the aluminosilicate nanotube bundles swell significantly with increasing interstitial adsorption and that the layer spacing of cesium montmorillonite expands up to about 12.5 Å, giving good agreement with experiments. The method is applicable to a wide range of flexible nanoporous materials, such as zeolites, metal-organic frameworks, and layered oxide materials, when candidate structures can be defined and a force field to describe the material is available.
Liquid-solid transition in fully ionized hydrogen at ultra-high pressures
Authors: Liberatore Elisa , Pierleoni Carlo , Ceperley D. M.
Published: May 2011

We study the phase diagram of an effective ion model of fully ionized hydrogen at ultra-high pressure. We assume that the protons interact with a screened Coulomb potential derived from a static linear response theory. This model accurately reproduces the physical properties of hydrogen for densities greater than \documentclass[12pt]{minimal}\begin{document}$\rho _{m}=10\,\text{g/cm}^3$\end{document}ρm=10g/cm3 corresponding to the range of the coupling parameter rs ≲ 0.6. The pressure range, \documentclass[12pt]{minimal}\begin{document}$P \gtrsim 20 T\text{Pa}$\end{document}P≳20TPa, is well beyond present experimental limitations. Assuming classical protons, we find that the zero temperature enthalpy of the perfect bcc crystal is slightly lower than for other structures at \documentclass[12pt]{minimal}\begin{document}$\rho _{m}= 12.47 \text{\,g/cm}^{3}$\end{document}ρm=12.47g/cm3 while the fcc structure gains stability at higher density. Using Monte Carlo calculations, we compute the free energy of various phases and locate the melting transition versus density. We find that on melting, bcc is energetically favored with respect to fcc over the entire range investigated. In the solid phase the system undergoes a structural transition from bcc at higher temperature to fcc at lower temperature. The free energy difference between these two structures is very small so that obtaining a quantitative estimate of this second transition line requires accuracy beyond that provided by our method. We estimate the effect of proton zero point motion on the bcc melting line for hydrogen, deuterium, and tritium by a path integral Monte Carlo method. Although zero point effects on hydrogen are large, since the two competing phases (bcc and liquid) have locally similar environments, the effect on the melting line is small; the melting temperature for hydrogen is lowered by about 10% with respect to the classical value.
Time-resolved photoelectron imaging of S2 → S1 internal conversion in benzene and toluene
Authors: Suzuki Yoshi-Ichi , Horio Takuya , Fuji Takao , Suzuki Toshinori
Published: May 2011

Ultrafast internal conversion of benzene and toluene from the S2 states was studied by time-resolved photoelectron imaging with a time resolution of 22 fs. Time−energy maps of the photoelectron intensity and the angular anisotropy were generated from a series of photoelectron images. The photoelectron kinetic energy distribution exhibits a rapid energy shift and intensity revival, which indicates nuclear motion on the S2 adiabatic surface, while the ultrafast evolution of the angular anisotropy revealed a change in the electronic character of the S2 adiabatic surface. From their decay profiles of the total photoelectron intensity, the time constants of 48 ± 4 and 62 ± 4 fs were determined for the population decay from the S2 states in benzene and toluene, respectively.
Experimental characterization of the weakly anisotropic CN X 2Σ+ + Ne potential from IR-UV double resonance studies of the CN-Ne complex
Authors: Beames Joseph M. , O'Donnell Bridget A. , Ting Melodie , Lester Marsha I. , Stephenson Thomas A.
Published: May 2011

IR-UV double resonance spectroscopy has been used to characterize hindered internal rotor states (nK = 00, 11, and 10) of the CN-Ne complex in its ground electronic state with various degrees of CN stretch (νCN) excitation. Rotationally resolved infrared overtone spectra of the CN-Ne complex exhibit perturbations arising from Coriolis coupling between the closely spaced hindered rotor states (11 and 10) with two quanta of CN stretch (νCN = 2). A deperturbation analysis is used to obtain accurate rotational constants and associated average CN center-of-mass to Ne separation distances as well as the coupling strength. The energetic ordering and spacings of the hindered internal rotor states provide a direct reflection of the weakly anisotropic intermolecular potential between CN X 2Σ+ and Ne, with only an 8 cm−1 barrier to CN internal rotation, from which radially averaged anisotropy parameters (V10 and V20) are extracted that are consistent for νCN = 0-3. Complementary ab initio calculation of the CN X 2Σ+ + Ne potential using MRCI+Q extrapolated to the complete one-electron basis set limit is compared with the experimentally derived anisotropy by optimizing the radial potential at each angle. Experiment and theory are in excellent accord, both indicating a bent minimum energy configuration and nearly free rotor behavior. Analogous experimental and theoretical studies of the CN-Ne complex upon electronic excitation to the CN B 2Σ+ state indicate a slightly more anisotropic potential with a linear CN-Ne minimum energy configuration. The results from these IR-UV double resonance studies are compared with prior electronic spectroscopy and theoretical studies of the CN-Ne system.
Electric quadrupole contribution to the nonresonant background of sum frequency generation at air/liquid interfaces
Authors: Yamaguchi Shoichi , Shiratori Kazuya , Morita Akihiro , Tahara Tahei
Published: May 2011

We study an electric quadrupole contribution to sum frequency generation (SFG) at air/liquid interfaces in an electronically and vibrationally nonresonant condition. Heterodyne-detected electronic sum frequency generation spectroscopy of air/liquid interfaces reveals that nonresonant χ(2) (second-order nonlinear susceptibility) has a negative sign and nearly the same value for all eight liquids studied. This result is rationalized on the basis of the theoretical expressions of χ(2) with an electric quadrupole contribution taken into account. It is concluded that the nonresonant background of SFG is predominantly due to interfacial nonlinear polarization having a quadrupole contribution. Although this nonlinear polarization is localized at the interface, it depends on quadrupolar χ(2) in the bulk as well as that at the interface. It means that the sign of nonresonant χ(2) bears no relation to the “up” versus “down” alignment of interfacial molecules, because nonresonant χ(2) has a quadrupolar origin.
Distinguishing between relaxation pathways by combining dissociative ionization pump probe spectroscopy and ab initio calculations: A case study of cytosine
Authors: Kotur Marija , Weinacht Thomas C. , Zhou Congyi , Kistler Kurt A. , Matsika Spiridoula
Published: May 2011

We present a general method for tracking molecular relaxation along different pathways from an excited state down to the ground state. We follow the excited state dynamics of cytosine pumped near the S0–S1 resonance using ultrafast laser pulses in the deep ultraviolet and probed with strong field near infrared pulses which ionize and dissociate the molecules. The fragment ions are detected via time of flight mass spectroscopy as a function of pump probe delay and probe pulse intensity. Our measurements reveal that different molecular fragments show different timescales, indicating that there are multiple relaxation pathways down to the ground state. We interpret our measurements with the help of ab initio electronic structure calculations of both the neutral molecule and the molecular cation for different conformations en route to relaxation back down to the ground state. Our measurements and calculations show passage through two seams of conical intersections between ground and excited states and demonstrate the ability of dissociative ionization pump probe measurements in conjunction with ab initio electronic structure calculations to track molecular relaxation through multiple pathways.
Robust three-body water simulation model
Authors: Tainter C. J. , Pieniazek P. A. , Lin Y.-S. , Skinner J. L.
Published: May 2011

The most common potentials used in classical simulations of liquid water assume a pairwise additive form. Although these models have been very successful in reproducing many properties of liquid water at ambient conditions, none is able to describe accurately water throughout its complicated phase diagram. The primary reason for this is the neglect of many-body interactions. To this end, a simulation model with explicit three-body interactions was introduced recently [R. Kumar and J. L. Skinner, J. Phys. Chem. B 112, 8311 (2008)10.1021/jp8009468]. This model was parameterized to fit the experimental O–O radial distribution function and diffusion constant. Herein we reparameterize the model, fitting to a wider range of experimental properties (diffusion constant, rotational correlation time, density for the liquid, liquid/vapor surface tension, melting point, and the ice Ih density). The robustness of the model is then verified by comparing simulation to experiment for a number of other quantities (enthalpy of vaporization, dielectric constant, Debye relaxation time, temperature of maximum density, and the temperature-dependent second and third virial coefficients), with good agreement.
Length-scale estimates for the 3D simplified Bardina magnetohydrodynamic model
Authors: Catania Davide
Published: May 2011

We consider the MHD-α model known as double viscous simplified Bardina MHD (SBMHD) on a 3D periodic box (MHD stands for magnetohydrodynamic). This system is a large eddy simulation model useful to approximate the turbulent behavior of an incompressible homogeneous magnetofluid because of the actual impossibility to handle the MHD model neither analytically nor via direct numerical simulation. In a previous paper (joint work with Secchi), the global existence of strong solutions to the SBMHD has been proved as well as the existence of a global attractor of finite fractal dimension. Upper bounds for such dimension are provided both in terms of the modified Grashof number and the dissipation length associated to the mean rate of energy dissipation. In this paper, we commute the same bound in an estimate in terms of the modified Reynolds number R. This result is useful because the classical Kolmogorov theory of turbulence is expressed using the Reynolds number. The global attractor estimate in terms of R is consistent with scaling theories of turbulence and can be interpreted in terms of degrees of freedom of the flow in Landau's sense. However, it does not take into account the effect of strong dissipation-range intermittency, where significant energy lies in wavenumbers larger than the inverse Kolmogorov length. Hence, we prove that there exists a series of time-averaged inverse square length-scales whose members are estimated in terms of R.
On the simulation of photoelectron spectra complicated by conical intersections: Higher-order effects and hot bands in the photoelectron spectrum of triazolide (CH)2N3−
Authors: Dillon Joseph , Yarkony David R. , Schuurman Michael S.
Published: May 2011

We report simulated photoelectron spectra for 1,2,3-triazolide (CH)2N3−, which reveal the vibronic energy levels of the neutral radical 1,2,3-triazolyl, (CH)2N3. The spectral simulation using a quasidiabatic Hamiltonian Hd comprised of polynomials through 4th order (thereby extending conventional quadratic expansions), is compared to both the experimental spectrum and a standard Franck-Condon (adiabatic) simulation. The quartic Hd is far superior to the quadratic Hd, reproducing the main features of the experimental spectrum and allowing for their subsequent assignment. The contributions from excited anion states successfully reproduce the observed vibronic transitions to the red of the assigned band origin of the neutral species. The algorithmic extensions required for the determination of these hot band contributions to the total spectrum are discussed. Convergence of the spectral envelope with respect to the vibronic basis, including both the principal and hot bands, required more than 109 terms.
Structural and microscopic relaxations in glycerol: An inelastic x-ray scattering study
Authors: Cunsolo A. , Leu B. M. , Said A. H. , Cai Y. Q.
Published: May 2011

The THz dynamics of liquid glycerol has been probed by inelastic x-ray scattering at different pressure spanning the 0.66–3 \documentclass[12pt]{minimal}\begin{document}$\text{Kbar}$\end{document}Kbar range. A comparison with ultrasound absorption results available in literature leads us to identify the presence of two different relaxations, a structural (slow) relaxation and a microscopic (fast) one. Although the former has been already thoroughly studied in glycerol by lower frequency spectroscopic techniques, no hints on the latter are so far available in literature. We observe that the characteristic timescale of this fast relaxation ranges in the sub-picosecond, tends to decrease with increasing the wave-vector and seems rather insensitive to pressure changes. Finally, the timescale and strength of the fast relaxation have a direct link revealing the microscopic, single particle, nature of the involved process.
A novel method for analyzing energy relaxation in condensed phases using nonequilibrium molecular dynamics simulations: Application to the energy relaxation of intermolecular motions in liquid water
Authors: Yagasaki Takuma , Saito Shinji
Published: May 2011

We present a novel method to investigate energy relaxation processes in condensed phases using nonequilibrium molecular dynamics simulations. This method can reveal details of the time evolution of energy relaxation like two-color third-order IR spectroscopy. Nonetheless, the computational cost of this method is significantly lower than that of third-order response functions. We apply this method to the energy relaxation of intermolecular motions in liquid water. We show that the intermolecular energy relaxation in water is characterized by four energy transfer processes. The structural changes of the liquid associated with the energy relaxation are also analyzed by the nonequilibrium molecular dynamics technique.
First-principles study towards the reactivity of the Pd(111) surface with low Zn deposition
Authors: Huang Yucheng , He Xiang , Chen Zhao-Xu
Published: May 2011

Methanol steam reforming (MSR) is an important means to produce hydrogen. While metal Pd shows no selectivity to MSR, PdZn alloy exhibits both high selectivity and activity towards this process. Recently a high temperature desorption peak of formaldehyde is observed when methanol is dosed onto Pd(111) surfaces on which 0.03–0.06 monolayer Zn is deposited. Strikingly such surface which is predominated by Pd atoms was suspected to be active for MSR. To determine the structure on which the high desorption peak is observed and its performance to MSR, we studied adsorption and dehydrogenation of formaldehyde on various models. It is demonstrated that the high desorption peak of CH2O may originate from the supported surface clusters. The calculated energy barriers of CH2O dehydrogenation show that while formaldehyde can decompose easily into formyl on the supported PdZn and Pd2 clusters, this process is kinetically difficult on the surface Zn3 clusters. It is further revealed that formation of dioxymethylene, the proposed precursor for CO2 production, from formaldehyde and oxygen is feasible on the surface Zn cluster. Based on these calculations we predict that compared with 1:1 PdZn alloy, the activity of the Zn clusters to MSR is lower, though its selectivity may be higher.
Possible mechanism in dry micro-electro-discharge machining of carbon-nanotube forests: A study of the effect of oxygen
Authors: Dahmardeh Masoud , Nojeh Alireza , Takahata Kenichi
Published: May 2011

The working principle of dry micro-electro-discharge machining of vertically aligned carbon-nanotube forests is investigated by evaluating the effect of oxygen on the process. The machining experiments with controlled oxygen/nitrogen ratios indicate a correlation between the peak current of discharge pulses and the oxygen concentration, suggesting not only a vital role for oxygen in the process, but also a removal mechanism fundamentally different from that in typical electro-discharge machining based on direct melting and evaporation of the sample material. The highest surface quality and uniformity in the machined forest microstructures as well as smooth machining without short circuiting are achieved at an approximate oxygen concentration of 20% under the discharge condition of 30 V and 10 pF, revealing that air is an optimal medium for the removal process. Elemental and molecular analyses show no evidence of significant crystalline deterioration or contamination in the nanotubes processed with the technique.