bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics simulations of matrix deposition. I. Site structure analysis for porphyrin in argon and xenon
Authors: Kyrychenko Alexander , Waluk Jacek
Published: October 2003

Classical molecular dynamics methodology was applied for the simulation of rare gas matrix deposition and for the structural analysis of the environment around porphyrin embedded in argon or xenon. The results confirm the experimental observations of several trapping sites in argon, as opposed to one dominant location in xenon. The main site in xenon has been assigned to a symmetrical hexagonal cavity, obtained after removing seven host atoms from the {111} crystallographic plane. Differences between argon and xenon matrices are due to the fact that, for the former, a larger number of host atoms have to be removed from the crystal in order to accommodate the porphyrin molecule. This results in less stable and more perturbed site structure in argon with respect to xenon.
General Born–Oppenheimer–Huang approach to systems of electrons and nuclei
Authors: Baer Roi , Kouri Donald J. , Baer Michael , Hoffman David K.
Published: October 2003

We reconsider the Born–Oppenheimer–Huang treatment of systems of electrons and nuclei for the case of their interaction with time-dependent fields. Initially, we present a framework in which all expressions derived are formally exact since no truncations are introduced. The objective is to explore the general structure of the equations under the most unrestricted conditions, including the possibility that the electronic basis is dependent both on the nuclear coordinates and on time. We then derive an application of the theory applicable to cases of interaction with strong time-dependent fields. The method truncates the electronic basis only after the time-dependent interaction is taken into account in the electronic wave functions. This leads to theory which is similar to a Born–Oppenheimer-type truncation within the interaction picture.
Correlations among colloidal particles confined to a spherical monolayer
Authors: Chávez-Páez M. , González-Mozuelos P. , Medina-Noyola M. , Méndez-Alcaraz J. M.
Published: October 2003

The internal structure of spherical colloidal monolayers of charged particles is studied here, both by means of Monte Carlo computer simulations and of an integral equation approach based on the application of the Ornstein–Zernike equation for spherical surfaces. The latter is complemented with a relatively fast and accurate numerical method for its solution, obtained by expanding the corresponding correlation functions in series of Legendre polynomials. It is found that the density correlations among the particles within the monolayer have some special features that differentiate them from the corresponding bulk corrections in open spaces. In particular, for a sufficiently small radius of the spherical monolayer, the distribution of particles around a particle fixed at one of the poles exhibits a peak at the opposite pole which is noticeably larger than the peaks immediately before it. It is also shown here that the introduction of a simple functional form with one adjustable parameter for the bridge function greatly enhances the fit between the theoretical approach and the simulation data.
Electronic transport of benzothiophene-based chiral molecular solenoids studied by theoretical simulations
Authors: Tagami Katsunori , Tsukada Masaru , Wada Yasuo , Iwasaki Tomokazu , Nishide Hiroyuki
Published: October 2003

By the density-functional-derived tight-binding method, the electronic transport properties of two types of benzothiophene-based molecular wires, i.e., the linear and helical molecular wires have been investigated. In the molecular bridge system where these molecules are connected to the gold electrodes by S–Au bonds, the transmission peaks are found to lie at the energies somewhat lower than 0.5 eV below the Fermi level for both cases. Thus the conductances of both types of wires for the bias voltage less than 1.0 V are not so large without doping. Upon iodine doping, however, the new transmission peaks are found to appear around the Fermi level, particularly in the case of helical wires. It means that the conductances of the helical wires are expected to be improved dramatically by the chemical doping. Therefore, the doped helical molecular wires are predicted to work as molecular solenoids even under lower bias voltages. Next, the applicability of the current-induced magnetic field generated in such a molecular solenoid is considered. As an example, we propose a novel helical molecule where the hydrogen atoms connected to the inner C–C bonds of the helix are substituted by some kind of radicals. In this case the current-induced field can control the alignment of the radical spin orientations.
Exit channel dynamics in the ultraviolet photodissociation of the NO dimer: (NO)2→NO(A 2Σ+)+NO(X 2Π)
Authors: Potter A. B. , Dribinski V. , Demyanenko A. V. , Reisler H.
Published: October 2003

The correlated angular and product rotational state distributions obtained in the 221.67 nm photodissociation of (NO)2 yielding NO(A 2Σ+)+NO(X 2Π) have been examined in the molecular beam using the velocity map ion imaging technique. The translational energy and angular distributions of selected rotational states of NO(A 2Σ+) products in N=0, 5, 6 for which the maximum energies available to the NO(X 2Π) products are 202.5, 142.5, and 118.5 cm−1, respectively, have been measured. The recoil anisotropy parameter of the photofragments, βeff, is 1.2±0.1, less than that previously measured at 213 nm (1.36±0.05). The correlated product state distributions near dissociation threshold agree with the predictions of phase space theory. These experimental results, as well as those obtained previously at 213 nm, are compared to statistical calculations, including v⋅J correlations. Application of the β-ET correlation model to the 213 nm results indicates that [NO(A,N),NO(X,J)] pairs with high NO(X,J) rotational levels are produced preferentially via planar dissociation, in contrast to the statistical expectation of the v⋅J correlation, which reveals no preference for planar dissociation. A mechanism involving vibrational predissociation with restricted intramolecular vibrational energy redistribution can explain both the observed scalar and vector properties. Specifically, the low frequency torsional (out-of-plane) mode does not couple efficiently to the other modes, especially at higher excess energies when the dissociation is rapid. On the other hand, the long-range attraction between NO(A) and NO(X), which is revealed both in the photodissociation dynamics of the dimer and in the quenching of NO(A) by NO(X), encourages long-range mode couplings and can explain the largely statistical rotational state distributions observed near threshold. From images obtained near threshold, the bond energy of the NO dimer in the ground state is determined to be 710±10 cm−1, in good agreement with previous results.
Rotational line strengths of the v2-active two-photon transitions of the methyl radical
Authors: Chen Kuo-mei
Published: October 2003

To extract information on the rotational population distributions of the methyl radical from photodissociation by the 2+1 resonance-enhanced multiphoton ionization technique, its rotational line strength formulas of the two-photon transitions have been reexamined. Symmetry-adapted rovibronic-nuclear spin wave functions of CH3 and CD3 in the |X̃ 2A2″〉 and |np 2A2″〉 electronic states were utilized in the derivation. Transformation properties of the rovibronic and nuclear spin basis functions under the permutation-inversion group D3h(M) have been employed to construct the total wave functions which follow the appropriate statistics of CH3 and CD3, respectively. Explicit expressions of the two-photon rotational line strengths of the v2-active vibronic bands of the methyl radical were reported.
The dynamics of the Cl+C2H6→HCl(v′,j′)+C2H5 reaction at 0.24 eV: Is ethyl a spectator?
Authors: Bass M. J. , Brouard M. , Vallance C. , Kitsopoulos T. N. , Samartzis P. C. , Toomes R. L.
Published: October 2003

The hydrogen atom abstraction reaction between Cl(2P3/2) and ethane has been studied at a mean collision energy of 0.24 eV. The experiments were performed in a coexpansion of molecular chlorine and ethane, with the atomic Cl reactants generated by laser photodissociation of Cl2 at 355 nm. HCl(v′,j′) products were detected quantum state selectively using (2+1) resonantly enhanced multiphoton ionization, coupled with velocity-map ion imaging. The ion images were used to determine center-of-mass angular and kinetic energy release distributions. Several analysis methods were employed and have been carefully assessed. It is shown that the single beam experiments can be used with confidence to determine both center-of-mass angular and energy release distributions. For the title reaction the angular distribution is found to be forward peaking, with on average 22% of the available energy channeled into internal excitation of the ethyl coproducts. Possible sources of this internal excitation are discussed.
Photodissociation dynamics of dicyclopropyl ketone at 193 nm: Isomerization of the cyclopropyl ligand
Authors: Clegg Samuel M. , Parsons Bradley F. , Klippenstein Stephen J. , Osborn David L.
Published: October 2003

The photodissociation dynamics of dicyclopropyl ketone are investigated using time-resolved Fourier transform infrared spectroscopy and photofragment ion imaging spectroscopy. The photodissociation products are C3H5+CO+C3H5, and the isomerization dynamics of C3H5 are the focus of this paper. Electronic structure calculations are used to define the potential energy surface, while a two-step phase space theory model predicts excitation in the CO product. The vibrational energy distribution of the CO product is not described by this statistical distribution, and is more excited than that observed in the analogous dissociation of acetone. The translational energy distribution of CO indicates an exit barrier on the potential energy surface. Contrary to expectations based on the photodissociation of other aliphatic ketones, the hydrocarbon products are not cyclopropyl radicals. Instead, the excited dicyclopropyl ketone undergoes a ring-opening isomerization to form diallyl ketone, followed by dissociation producing allyl radicals and carbon monoxide. Some of the allyl radicals have sufficient internal energy to decompose to allene+H.
Magnetoresistance oscillations induced by intersubband scattering of two-dimensional electron gas in Al0.22Ga0.78N/GaN heterostructures
Authors: Tang N. , Shen B. , Zheng Z. W. , Liu J. , Chen D. J. , Lu J. , Zhang R. , Shi Y. , Zheng Y. D. , Gui Y. S. , Jiang C. P. , Qiu Z. J. , Guo S. L. , Chu J. H. , Hoshino K. , Someya T. , Arakawa Y.
Published: October 2003

Magnetointersubband scattering (MIS) oscillations of two-dimensional electron gas (2DEG) in Al0.22Ga0.78N/GaN heterostructures have been investigated by means of magnetotransport measurements at low temperatures and high magnetic fields. Double periodic Shubnikov–de Haas oscillations modulated by MIS oscillations have been observed due to the intersubband scattering of the 2DEG at the two lowest subbands in the triangular quantum well at the heterointerface. By using the fast Fourier transform analysis, it is found that the MIS oscillations become slightly weaker with an increase in temperature. From the MIS frequency, the energy separation between the first and the second subbands is determined to be 80 meV. The observation of the MIS effect indicates that the effective masses of the electrons in the first and second subbands are the same in Al0.22Ga0.78N/GaN heterostructures.
Dielectric relaxation in laser ablated polycrystalline ZrTiO4 thin films
Authors: Victor P. , Bhattacharyya S. , Krupanidhi S. B.
Published: October 2003

Relaxation and conduction mechanisms under small ac fields of laser ablated ZrTiO4 thin films were analyzed in the light of impedance and modulus spectroscopy. The overall dielectric properties were mainly dominated by a Maxwell–Wagner type of relaxation with grains and the grain boundary two distinct parts of the circuit. Each of these parts was found to follow the universal power law of frequency dispersion. The modulus plot confirmed that the capacitive parts were relatively independent of the frequency and temperature, whereas the impedance and ac conduction studies exhibited significant temperature and frequency dependence. The conduction inside the grains was suggestive of a hopping mechanism through various defect sites whereas the interface barrier potential dictated grain boundary conduction.
Enhancement of sp3 hybridized C in amorphous carbon films by Ar ion bombardment and Si incorporation
Authors: Jung Hae-Suk , Park Hyung-Ho , Mendieta I. R. , Smith D. A.
Published: October 2003

We report an effective method of increasing the sp3 hybridization fraction in sputtered amorphous carbon (a-C) film by the combination of Ar ion bombardment and Si incorporation. In the deposition of an a-C film, Ar ion bombardment by controlling the applied bias voltage plays a role in creating high stress in film and causes the local bonding configuration to change to a sp3 hybridized bond. Simultaneously, the incorporated Si in an a-C network breaks the sp2 hybridized bonded ring and promotes the formation of a sp3 hybridized bond. This enhancement of the sp3 hybridized bonding characteristic is maximized for an a-C film with 23 at. % of Si and 100–150 V of applied bias voltage. In this region, the increase of resistivity, optical band gap, and mechanical hardness of a-C is attributed to the reduction of the sp2 hybridized bonded ring and increased fraction of the sp3 hybridized bond. However, at a higher bias voltage above 150 V, the enhancement effect is reduced due to the resputtering and thermally activated reconversion of a sp3 to a sp2 hybridized bond.
Observation of reflection high-energy electron diffraction oscillation during metalorganic-molecular-beam epitaxy of AlAs and control of carbon incorporation
Authors: Ganapathy Sasikala , Kurimoto Makoto , Thilakan Periyasamy , Uesugi Kasturi , Suemune Ikuo , Machida Hideaki , Shimoyama Norio
Published: October 2003

The in situ observation of reflection high-energy electron diffraction (RHEED) oscillations during the metalorganic-molecular-beam epitaxy deposition of AlAs and AlGaAs epitaxial layers is reported. In situ RHEED oscillations as well as atomic force microscopy measurements confirmed the layer-by-layer growth of the AlAs as well as the AlGaAs layers on GaAs substrates. RHEED oscillation was successfully applied to the precise control of the AlAs/GaAs superlattices and of the alloy compositions in the AlGaAs alloys. High-resolution x-ray diffraction and Hall effect measurements revealed the unintentional doping of carbon into the AlGaAs layers, but it was found that the increase in the V/III ratio is able to reduce the carbon incorporation.
Staircase-like metamagnetic transitions in phase-separated manganites: Influence of thermal and mechanical treatments
Authors: Hardy V. , Yaicle C. , Hébert S. , Maignan A. , Martin C. , Hervieu M. , Raveau B.
Published: October 2003

Substitutions in the Mn sublattice of antiferromagnetic, charge, and orbitally ordered manganites was recently found to produce intriguing metamagnetic transitions consisting of a succession of sharp magnetization steps separated by plateaus. The compounds exhibiting such features can be divided into two categories, depending on whether they are sensitive to thermal cycling effects or not. One compound in each category has been considered in the present study. This article reports on the influence of two treatments: high-temperature annealing and grinding. It is shown that both of these treatments can dramatically affect the phenomenon of magnetization steps. The results provide us with information about the origin of these jumps in magnetization.
Influence of ferromagnetic substrate on the magnetoresistance of Cr film across a nonmagnetic insulating layer
Authors: Jin Xuesong , Shvets I. V.
Published: October 2003

(5 nm) Cr/(x nm) MgO/Mn0.52Zn0.48Fe2O4 (MnZn spinel) substrate (1<x<7 nm), as well as (5 nm) Cr/(7 nm) MgO/glass substrate structures have been grown using molecular beam epitaxy. The influence of the MnZn spinel on the in-plane transport and magnetotransport properties of the Cr layer were studied. The existence of pinholes in the MgO layer was explored by evaluating resistance versus temperature, R(T), dependencies. A hump was observed on the R(T) curves for the MgO layer thickness (tMgO) less than 5 nm. This results from the electrical coupling between the Cr film and the MnZn spinel substrate through the pinholes in the MgO layer. A reversal of the magnetoresistance (MR) sign in Cr film was observed when tMgO is less than 7 nm. It is thought that the effect of the electrical coupling through the pinholes and the magnetostatic coupling are not the reasons for the observed negative MR. A model is proposed which suggests that the magnetic structure in the Cr film adjoining a pinhole is distorted because of the exchange coupling. Such areas in the Cr film are thought to contribute to the negative MR.
X-ray diffraction and Mössbauer studies of structural changes and L1 ordering kinetics during annealing of polycrystalline Fe51Pt49 thin films
Authors: Spada F. E. , Parker F. T. , Platt C. L. , Howard J. K.
Published: October 2003

Room-temperature x-ray diffraction and Mössbauer effect techniques have been used to characterize the structural features and local atomic environments of sputtered Fe51Pt49 thin films following various isothermal treatments. Both techniques show that no significant changes occur in the chemically ordered L10 tetragonal phase after it has formed. In contrast, changes in the disordered face-centered-cubic (fcc) phase are observed prior to the transformation into the ordered tetragonal phase. Mössbauer measurements indicate the development of increasing short-range order in the disordered fcc phase with increasing annealing temperature. Asymmetries in the fcc x-ray diffraction profiles also suggest the presence of lattice distortions caused by atomic size differences commonly found in the quenched disordered fcc phase of materials that form ordered structures. Quasi-real-time kinetic measurements of the disorder→order transformation in sputtered Fe51Pt49 thin films within the temperature range 300 °C⩽T⩽400 °C have also been conducted using high-temperature x-ray diffraction techniques. Significant differences are observed between the kinetic parameters determined in this study and those of previous reports. It is proposed that these differences arise from the lower temperature range investigated in the present work, where the gradual changes occurring in the fcc phase can influence the rate of the ordering transformation. Furthermore, because the initial state of disorder in Fe∼50Pt∼50 films can be influenced by the deposition conditions, variability in the low-temperature ordering kinetics should be expected among Fe∼50Pt∼50 films prepared under different conditions.
Magnetoresistance in step-edge junctions based on La0.7Sr0.3MnO3 films
Authors: Bosak A. A. , Dubourdieu C. , Chaudouët P. , Sénateur J.-P. , Fournier T.
Published: October 2003

The magnetoresistance of step-edge junctions based on La0.7Sr0.3MnO3 films was investigated using a Wheatstone bridge geometry. Two types of step-edge structures were prepared: The step was either fabricated by ion-beam etching of SrTiO3 (001) substrates or by wet-chemical etching of an insulating NdMnO3 layer deposited on SrTiO3 (001). Both the magnetoresistive and insulating layers were deposited by injection-metalorganic chemical vapor deposition. The temperature dependence of the magnetoresistance was explored for both types of junctions in different field orientations. The largest low-field magnetoresistance was obtained when the step was formed directly in the substrate; it was observed up to room temperature (∼1% for an applied field of ∼0.05 T) and at 40 K, it increased ∼12% under an applied field of ∼0.1 T. The hysteretic behavior for the two types of junctions appeared to be very different.
Fluorescence intensity ratio technique for optical fiber point temperature sensing
Authors: Wade S. A. , Collins S. F. , Baxter G. W.
Published: October 2003

The fluorescence intensity ratio technique for optical fiber-based point temperature sensing is reviewed, including the materials suitable for this technique. The temperature dependence of the fluorescence intensity ratio has been studied using thermally coupled energy levels in seven different rare earth ions doped into a variety of glasses and crystals. Sensor prototypes developed using Pr3+:ZBLANP, Nd3+-doped silica fiber and Yb3+-doped silica fiber as the sensing material have been used to measure temperatures covering the range of approximately −50 to 600 °C with a resolution of the order of 1 °C.
Comment on “Theoretical absorption spectrum of the Ar–CO van der Waals complex” [J. Chem. Phys. 118, 9596 (2003)]
Authors: Havenith M.
Published: October 2003

It is shown that reassignment of the data as proposed by Fernandez et al. is in contradiction to previous experimental results.
Quantum wave packet dynamics of the 1 3A″ N(4S)+NO(X̃ 2Π)→N2(X̃ 1Σg+)+O(3P) reaction
Authors: Gamallo Pablo , González Miguel , Sayós R. , Petrongolo Carlo
Published: October 2003

We present the quantum dynamics of the title reaction using the Gray–Balint–Kurti wave-packet (WP) method, several NO vibro-rotational levels, product coordinates, and an asymptotic analysis. We calculate accurate reaction probabilities at J=0, estimate those at J>0 via a capture model, and discuss the reaction mechanism analyzing the WP time evolution. We also obtain cross sections and rate constants. The potential is barrier-less and thus both probabilities and cross sections do not have a collision-energy (Ecol) threshold. The probabilities present many sharp resonances, due to the Ecol redistribution on the NNO-internal and N2-product degrees of freedom. The reaction is stereo-specific and occurs via a bent abstraction mechanism. The cross sections decrease with Ecol, in agreement with the expected behavior for threshold-less reactions. The present values of the rate constant support previous and less accurate calculations, and are in excellent agreement with laboratory experimental data. This confirms the accuracy of the present 1 3A″ PES.
Density fluctuation correlation length in polymer fluids
Authors: Koshy R. , Desai T. , Keblinski P. , Hooper J. , Schweizer K. S.
Published: October 2003

We employ molecular dynamics simulations and integral equation theory to study the real space collective density correlations in chain polymer fluids over a wide range of concentrations. For a degree of polymerization of 80 and low polymer concentrations, simulation and theory both find the typical behavior that the density–density correlation function decays monotonically with an associated correlation length that decreases with polymer concentration. In contrast, at high polymer concentrations the correlation function decays in an oscillatory manner with a correlation length that grows with fluid density. The correlation length is found to be smallest at a reduced polymer density corresponding to the onset of monomer scale oscillations in the density correlation function. Based on the theory, qualitatively identical behavior is found for chains of 2000 monomers. The observed density dependence of the correlation length is analogous to the dependence of the charge screening length on ionic strength in electrolyte fluids.
Cooperative dynamics in semiflexibile unentangled polymer fluids
Authors: Guenza M.
Published: October 2003

We present a generalized Langevin equation for the dynamics of semiflexible polymer chains of finite size in a dynamically heterogeneous fluid. Local and global dynamical properties, calculated in the framework of this approach, display anomalous behavior in agreement with experiments and computer simulations. The presence of heterogeneous dynamics induces non-Fickian center-of-mass diffusion and an anomalous slowdown of intramolecular modes of motion. Intermolecular interactions mostly perturb lowest-index modes, which correspond to polymer global dynamics. Internal polymer stiffness induces anomalies in the relaxation of highest-index modes, which characterize local dynamics and monomer diffusion.
Dynamics of dendrimer-based polymer networks
Authors: Gurtovenko A. A. , Markelov D. A. , Gotlib Yu. Ya. , Blumen A.
Published: October 2003

We present a theoretical study of polymer networks, formed by connecting dendritic building blocks (DBB’s). We concentrate on the Rouse dynamics of such networks and perform our study in two steps, considering first single generalized dendrimers (GD’s) and then networks formed by such DBB’s. In GD’s the functionality f of the inner branching points may differ from the functionality fc of the core. The GD’s cover wide classes of macromolecules, such as the “classical” dendrimers (fc=f ), the dendritic wedges (fc=f−1), and the macromolecular stars (fc>2, f=2). Here we present a systematic, analytic way which allows us to treat the dynamics of individual GD’s. Then, using a general approach based on regular lattices formed by identical cells (meshes) we study the dynamics of GD-based polymer networks. Using analytical and numerical methods we determine the storage and loss moduli, G′(ω) and G″(ω). In this way we find that the intradendrimer relaxation domain of G′(ω) becomes narrower when Mcr, the number of connections between the neighboring DBB’s, increases. This effect may be understood due to the exclusion of the longest DBB relaxation times from the spectrum of the network, given that the additional connections hinder the mobility of the peripheral DBB branches. We expect that such effects may be readily observed through appropriate mechanical experiments.
Chiral discrimination in liquid 1,1,1-trifluoropropan-2-ol: A molecular dynamics study
Authors: Fioroni Marco , Burger Klaus , Roccatano Danilo
Published: October 2003

The structural and thermodynamical properties of the R and S enantiomers of 1,1,1trifluoropropan-2-ol (TFIP) have been investigated by molecular-dynamics simulations. In particular, the chiral discrimination (Ch.D.) between the two enantiomers in a racemic solution has been analyzed in detail. Differences in density and enthalpy of vaporization between the pure enantiomeric liquid and the racemic mixture have been found. The comparison of the radial distribution functions and the distribution of the reciprocal orientations of TFIP molecules have shown the presence of a slightly different packing organization in the aforementioned solutions explaining the difference in density and enthalpy of vaporization. Furthermore, the structural analysis of the racemic mixture has shown a strong dependence of the homo- and heterochiral preference by the nature of the functional groups present in the molecule. At 298 K, in the case of CH3, CF3, and hydroxy groups, the homochiral interaction is followed by a heterochiral one in the order: OH≫CH3≫CF3. The effect of pressure and temperature on the Ch.D. has been also analyzed. As general trend, the Ch.D. is more effective at lower temperatures and decrease as the temperature is raised. At high pressure (1000 bars) the Ch.D. is still present for the OH and CH3 groups while is negligible for the CF3 group. The Ch.D. has been also quantified by calculating the differential free energy of solvation of the R and S enantiomer in a solution of TFIP(R) molecules, at 298 K and 1 bar pressure. A value equal to −1.7±0.7 kJ mol−1 has been found, indicating a slight immiscibility of the two enantiomers. As a general result, the balance between the first neighbor interactions, affecting the geometrical packing and the 2nd–3rd shell organizations, results in different structural and thermodynamical properties (i.e., density and enthalpy of vaporization) between the racemic TFIP mixture and the pure enantiomers.
Diffusive instabilities in heterogeneous systems
Authors: Vanag Vladimir K. , Epstein Irving R.
Published: October 2003

We investigate the behavior of a system composed of two small identical water droplets loaded with an activator-inhibitor system possessing only a single stable steady state and coupled through a third, “signaling” species able to diffuse through the interdroplet medium. Depending on whether the third species is coupled to the activator or the inhibitor, the system can exhibit wave instability, which is equivalent to out-of-phase oscillations, or Turing instability, which is equivalent to the birth of two new stationary steady states. For a large interdroplet gap, equivalent to a large time delay, droplets can undergo synchronous, in-phase oscillations. Our analysis can be extended to a system of many coupled in-line droplets, as we illustrate.
Aspects of correlation function realizability
Authors: Crawford Jenness , Torquato Salvatore , Stillinger Frank H.
Published: October 2003

The pair-correlation function g2(r) describes short-range order in many-particle systems. It must obey two necessary conditions: (i) non-negativity for all distances r, and (ii) non-negativity of its associated structure factor S(k) for all k. For the elementary unit step-function g2 form, previous work [F. H. Stillinger, S. Torquato, J. M. Eroles, and T. M. Truskett, J. Phys. Chem. B 105, 6592 (2001)] indicates that (i) and (ii) could be formally satisfied, but only up to a terminal density at which the covering fraction of particle exclusion diameters equaled 2−d in d dimensions. To test whether the unit step g2 is actually achievable in many-particle systems up to the apparent terminal density, a stochastic optimization procedure has been used to shift particles in large test systems toward this target g2. Numerical calculations for d=1 and 2 confirm that the step function g2 is indeed realizable up to the terminal density, but with substantial deviation from the configurational preferences of equilibrium hard-rod and hard-disk models. We show that lineal statistical measures are particularly sensitive to this difference. Our results also illustrate the characteristics of “closest approach” to the step function g2 above the terminal density.
Emitter injection efficiency and base transport factor in InAs bipolar transistors
Authors: Wu X. , Maimon S. , Averett K. L. , Koch M. W. , Wicks G. W.
Published: October 2003

The factors affecting the common emitter current gain (β) in InAs bipolar junction transistors (BJTs) have been studied by estimating the base transport factor (αT) and the emitter injection efficiency (γ). This has been accomplished by employing a specially designed sequence of InAs npn BJTs. αT, γ, and the minority carrier (electrons) diffusion length in the base (LB) are extracted from measured electrical characteristics by using a simple method. When the emitter doping density is large, the current gain is limited by the base transport factor αT. The value of LB in these BJTs is found to be 0.44 μm, which indicates high quality epitaxial material. The analysis developed is a general technique, also applicable to BJTs and heterojunction bipolar transistors in other material systems.
Optical nonlinear properties and optical limiting for [(n-Bu)4N]4[MoS4Cu]4
Authors: Zhu XiaoRong , Sun ZhenRong , Niu RuiMing , Zeng HePing , Wang ZuGeng , Lang JianPing , Xu ZhiZhan , Li RuXin
Published: October 2003

We report the nonlinear optical properties of one metal cluster [(n-Bu)4N]4[MoS4Cu]4 by picosecond and nanosecond laser pulses. The optical limiting response is measured by a transmission technique and the nonlinear absorption by a Z-scan technique. The experimental data are theoretically fitted, the mechanism for the optical nonlinearities and the optical limiting properties is discussed, and the absorption cross sections of the ground states and the excited states are evaluated by a rate equation model.
Simultaneous, multilayer plasma etching and deposition of fluorocarbon layers on silicon
Authors: Abraham-Shrauner Barbara
Published: October 2003

The plasma etching and deposition of a fluorocarbon layer on a silicon substrate are modeled by the simultaneous etching and deposition of a stack of fluorocarbon monolayers. Langmuir kinetics apply in each of the monolayers to a depth where the energy of the bombarding ions exceeds or equals the threshold energy for breaking carbon fluorine bonds. The ion energy controls many of the terms in the etch rate and deposition rate expressions. The model is based on the diffusion of fluorine etchant through the fluorocarbon layer to the silicon substrate. The deposition and etching of the fluorocarbon layer are ion assisted; the etching of the silicon is thermal and is assumed self-similar based on experiments. The model covers three regimes: reactive sputtering, fluorocarbon suppression, and fluorocarbon deposition. Etch and deposition curves model CF+ ion beam deposition and etch on silicon and CHF3 and C2F6 etch and deposition on fluorocarbon covered silicon substrates.
Prebreakdown phenomena and formation processes of low pressure glow discharges in N2, O2, and N2/O2 mixtures
Authors: Hosokawa Tatsuzo , Ohuchi Mikio , Ichinose Kensuke , Kaneda Teruo
Published: October 2003

The prebreakdown phenomena and the resulting transient processes from the prebreakdown phenomena to the glow discharge in low-pressure N2, O2 gases, and N2/O2 mixtures were investigated. When the applied voltage was a lower overvoltage ratio less than about 2%, nonprebreakdown phenomenon was observed, and the overvoltage ratio was independent of the gas materials. This phenomenon, however, was observed at the overvoltage ratio of higher than about 3%. The prebreakdown phenomenon in N2 gas differed from those in O2 gas, and formed pulseless components. On the other hand, those in O2 gas formed pulsed components. The prebreakdown phenomenon in N2 gas changed to a pulsed discharge by mixing it with a small amount of more than 0.3% O2 gas. These transient processes from the prebreakdown phenomena to the glow discharges were also observed by a high-speed video camera. As a result, the formation processes of the glow discharge became clear.
Improved irreversibility behavior and critical current density in MgB2-diamond nanocomposites
Authors: Zhao Y. , Cheng C. H. , Rui X. F. , Zhang H. , Munroe P. , Zeng H. M. , Koshizuka N. , Murakami M.
Published: October 2003

MgB 2 -diamond nanocomposite superconductors have been synthesized by addition of nanodiamond powder. Microstructural analysis shows that the nanocomposite superconductor consists of tightly packed MgB2 nanograins (∼50–100 nm) with highly dispersed and uniformly distributed diamond nanoparticles (∼10–20 nm) inside the grains. The Jc–H and Hiir–T characteristics have been significantly improved in this MgB2-diamond nanocomposite, compared to MgB2 bulk materials prepared by other techniques. Also, the Jc value of 1×104 A/cm2 at 20 K and 4 T and the Hirr value of 6.4 T at 20 K have been achieved.
Interaction between two cylindrical inclusions in a symmetric lipid bilayer
Authors: Bohinc Klemen , Kralj-Iglič Veronika , May Sylvio
Published: October 2003

We calculate the membrane-mediated interaction between two cylindrical inclusions in a symmetric lipid bilayer. Our theory takes two contributions to the free energy into account, the elastic behavior of the membrane and the conformational restrictions that the flexible hydrocarbon chains of the lipids experience in the vicinity of a rigid inclusion. The description of the elastic behavior is based on two order parameters, the hydrophobic thickness of the membrane and a director field that characterizes the average tilt of the lipid chains. Conformational restrictions of the lipid chains are taken into account by a simple director model. We show that the short-range interaction potential between two inclusions sensitively depends on the degree of hydrophobic mismatch and on the spontaneous curvature of the lipid layers. In particular, we find pronounced attraction if the hydrophobic mismatch is positive. For negative mismatch the attraction is much less pronounced and, additionally, an energetic barrier appears. The inclusions prefer a small but notable negative hydrophobic mismatch. Positive spontaneous curvature amplifies these behaviors.
Reaction pathway and free energy barrier for defect-induced water dissociation on the (101) surface of TiO2-anatase
Authors: Tilocca Antonio , Selloni Annabella
Published: October 2003

The adsorption of a water molecule on a partially reduced TiO2 anatase (101) surface has been studied by first-principles molecular-dynamics simulations. At variance with the stoichiometric surface, dissociation of water close to the oxygen vacancy is energetically favored compared to molecular adsorption. However, no spontaneous dissociation was observed in a simulation of several picoseconds, indicating the presence of an energy barrier between the molecular and dissociated states. The free energy profile along a possible dissociation path has been determined through constrained molecular dynamics runs, from which a free energy barrier for dissociation of ∼0.1 eV is estimated. On the basis of these results, a mechanism for the dissociation of water at low coverage is proposed.
Solid–liquid interfacial free energy of small colloidal hard-sphere crystals
Authors: Cacciuto A. , Auer S. , Frenkel D.
Published: October 2003

Using free-energy calculations on small crystalline clusters, we estimate the free-energy density γSL for the solid–liquid equimolar interface of a system of hard-sphere colloids. By studying the behavior of a crystallite at coexistence, we determine the dependence of γSL on the radius of curvature of the interface. An extrapolation to infinite radius of curvature (flat interface), yields γSL(R→∞)=0.616(3), in good agreement with recent numerical estimates. Subsequently, we consider the dependence of the interfacial free-energy density on the degree of supersaturation. Our simulations suggest that the γSL associated with the equimolar surface is fairly insensitive to changes in supersaturation.
Selectivity of explicit internal signal stochastic resonance in a chemical model
Authors: Li Qian Shu , Lei Aizhong
Published: October 2003

The explicit internal signal stochastic resonance (EISSR) that happened in the Willamowski–Rössler model was studied. EISSR is a cooperative effect of the intrinsic signal and external noise or internal fluctuation. Our studies have shown that EISSR can exist in the stable periodic-2 oscillation state. The output signal to noise ratio (SNR) of the strong signal (SS) versus noise intensity exhibits explicitly nonmonotonic character. The output SNR of the weak signal decreases with the increment of noise intensity. It is clear that EISSR can hold selectivity to the preponderant intrinsic signal when there are two intrinsic signals in the system. At the same time, the fundamental frequency of the SS almost does not alter. These properties imply that the system can resist to the effect of noise and sustain preponderant intrinsic oscillation by itself. Our results will be helpful to better understand circadian rhythm and signal detecting in creatures.
Understanding the structure of metal encapsulated Si cages and nanotubes: Role of symmetry and d-band filling
Authors: Mpourmpakis Giannis , Froudakis George E. , Andriotis Antonis N. , Menon Madhu
Published: October 2003

Using ab initio calculations we study the stability of Si-based cages and nanotubes stabilized by encapsulated transition metal atoms (TMAs). It is demonstrated that the stabilization of these cages and nanotubes as well as their magnetic properties are strongly guided by a delicate interplay between the attainable symmetry of the system and the d-band filling of the encapsulated TMA. As a result, encapsulated TMAs of the early 3-d series lead to tubular stuctures of C6 symmetry and anti-ferromagnetic alignment between the magnetic moment of the TMA and that of the Si atoms. On the other hand, the encapsulated late 3-d elements lead to tubules of the C5 symmetry and to a ferromagnetic alignment of the metal and Si magnetic moments. Encapsulated Fe atoms (being near the middle of the 3-d series) lead to tubular structures of either C6 or C5 symmetry.
Vacuum synthesis and determination of the ionization energies of different molecular orbitals for BrOBr and HOBr
Authors: Qiao Zhimin , Sun Shutao , Sun Qiao , Zhao Jincai , Wang Dianxun
Published: October 2003

Pure BrOBr and HOBr were synthesized in vacuum by heterogeneous reactions of the dried bromine vapor and Br2/H2O mixture vapors (5:1) with HgO, respectively, and then characterized by He I photoelectron spectroscopy (PES) and augmented by ab initio GAUSSIAN 2 and the outer valence Green’s functional calculations. The first PE band at 10.26 eV with vibrational spacing 550±60 cm−1 and the second PE band at 11.23 eV with vibrational spacing 240±60 cm−1 are, respectively, assigned as ionizations of the electrons of the highest occupied molecular orbital (HOMO)(6b1(39)) and the SHOMO(13b2(38)) orbitals of BrOBr. The first PE band at 10.73 eV with vibrational spacing 750±60 cm−1 and the second PE band at 11.56 eV with vibrational spacing 650±60 cm−1 are, respectively, assigned as ionizations of the electrons of the HOMO(6a″(22)) and the SHOMO(16a′(21)) orbitals of HOBr. The study does not only provide vacuum synthesis conditions for preparing pure BrOBr and HOBr, but also provide experimental PES results along with theoretical ionization energies of different molecular orbitals for BrOBr and HOBr.
Effects of the Duschinsky mode-mixing mechanism on temperature dependence of electron transfer processes
Authors: Tang Jau , Lee Ming Tsung , Lin Sheng Hsien
Published: October 2003

Electron transfer processes involving a multimode mixing mechanism (the Duschinsky rotation) are systematically examined. Such processes can be analyzed with a very general “spin–boson” model with N displaced and quadratically coupled harmonic potentials. The very general Franck–Condon factor obtained here is applicable to the studies of electron transfer as well as energy transfer processes, where frequency shifts and the Duschinsky rotation are involved. Although there are several numerical studies of such a mechanism, the derivation of an analytical expression for the rate constant is presented here and the temperature dependence is examined. As, in general, at very low temperatures where the thermal energy is smaller, the electron transfer rate becomes temperature independent due to nuclear quantum tunneling. However, in the presence of Duschinsky rotation, the pre-exponential factor in the rate constant can deviate from the characteristic 1/√T dependence of the Marcus theory. For processes with no or a small activation energy, the rate can be dominated by the pre-exponential factor and becomes 1/T dependence in the high-temperature regime.
Trapping site structures of HArF and HKrF in solid rare gases
Authors: Jolkkonen Santtu , Pettersson Mika , Lundell Jan
Published: October 2003

The trapping sites of HArF and HKrF in crystalline Ar and Kr are investigated computationally. Ab initio calculations are used to evaluate interactions between the rare gas containing molecule and a single rare gas atom. Molecular mechanics and molecular dynamics are used to study the properties of HArF and HKrF in rare gas crystals. Three different trapping configurations have been found for both molecules. The lowest-energy site is a double-substitutional (DS) configuration and the second lowest energy site is a single-substitutional (SS) one. The DS site can be interpreted to involve a 1:1 Rg⋯HRgF complex (Rg=Ar, Kr). The energy difference between these sites is 10.4 and 9.8 kJ/mol for HArF and HKrF, respectively. All the computational evidence shows that the experimentally observed stable site of HArF [J. Am. Chem. Soc. 123, 8610 (2001)] corresponds to a DS site and the unstable site corresponds to a SS site. Relaxation of the SS site to the DS site involves the motion of a vacancy in the lattice and this suggests that HArF and HKrF can be used to study the dynamics of vacancy motion in rare gas solids.
Potential energy, Λ doubling and Born–Oppenheimer breakdown functions for the B 1Πu “barrier” state of Li2
Authors: Huang Yiye , Le Roy Robert J.
Published: October 2003

The potential energy curve for the B 1Πu state of Li2 has a rotationless barrier which protrudes above its energy asymptote. A direct fit to spectroscopic data for all three isotopomers of this species, including Λ-doubling splittings and tunneling predissociation line widths, is used to determine an accurate analytic potential energy function plus Born–Oppenheimer breakdown and Λ-doubling perturbation radial strength functions for this system. This analysis introduces an analytic model for representing a potential function with a rotationless barrier, and shows that a radial perturbation function treatment can determine the symmetry of the perturbing state giving rise to Λ-doubling splittings.
Evolution of classical and quantum phase-space distributions: A new trajectory approach for phase space hydrodynamics
Authors: Trahan Corey J. , Wyatt Robert E.
Published: October 2003

Recently, Donoso and Martens described a method for evolving both classical and quantum phase-space distribution functions, W(q,p,t), that involves the propagation of an ensemble of correlated trajectories. The trajectories are linked into a unified whole by spatial and momentum derivatives of density dependent terms in the equations of motion. On each time step, these nonlocal terms were evaluated by fitting the density around each trajectory to an assumed functional form. In the present study, we develop a different trajectory method for propagating phase-space distribution functions. A hierarchy of coupled analytic equations of motion are derived for the q and p derivatives of the density and a truncated set of these are integrated along each trajectory concurrently with the equation of motion for the density. The advantage of this approach is that individual trajectories can be propagated, one at a time, and function fitting is not required to evaluate the nonlocal terms. Regional nonlocality can be incorporated at various levels of approximation to “dress” what would otherwise be “thin” locally propagating trajectories. This derivative propagation method is used to obtain trajectory solutions for the Klein–Kramers equation, the Husimi equation, and for a smoothed version of the Caldeira–Leggett equation derived by the Diosi. Trajectory solutions are obtained for the relaxation of an oscillator in contact with a thermal bath and for the decay of a metastable state.
Visible and near-infrared photoabsorption spectrum of Li3O: Resonance enhanced two-photon ionization spectroscopy and ab initio calculations
Authors: Neukermans Sven , Janssens Ewald , Tanaka Hiromasa , Silverans Roger E. , Lievens Peter , Yokoyama Keiichi , Kudo Hiroshi
Published: October 2003

We report the measurement of the photoabsorption spectrum of Li3O using resonance-enhanced two-photon ionization spectroscopy in the energy range between 0.7 and 2.75 eV. Ab initio geometry optimization calculations at the CCSD(T)/6-311+G(d) level of theory are carried out, resulting in a stable D3h ground state symmetry for Li3O. Vertical excitation energies are computed from the CCSD(T) potential, and the flatness of the potential energy surface is analyzed. A comparison of the recorded absorption spectrum with the theoretical predictions allows an assignment of all the observed bands and excited states in terms of a D3h ground state structure. It is argued that the width of the bands is governed by the flat-bottomed shape of the potential energy surface.