bluuebunny/crossref_metadata_2025 huggingface dataset:
The energies of point defects near metal/oxide interfaces
Authors: Duffy D. M. , Harding J. H. , Stoneham A. M.
Published: July 2002

We demonstrate a simple but accurate method for the atomistic modeling of metal/oxide interfaces, even when these are complicated by charged defects and space charge in the oxide. Thus we calculate the structure and energies of Ag/MgO interfaces, in the presence of point defects in the oxide, using well-established computer simulation techniques. Our approach, which is complementary to other methods such as local density approximation calculations, requires very modest computer power. The major terms in the interaction between the oxide and the metal can be decomposed into the short-range interaction between the ions and the metal cores, the energy required to embed the ions in the jellium, and the image interactions between the ionic charges and the metal. The short-wavelength fluctuations in the induced charge distribution were eliminated in order to represent the finite Fermi wave vector of a real metal. Our predictions are in good accord with observed wetting angles; agreement with other calculations is as close as should be expected, given the somewhat different working assumptions. The energies of vacancies and interstitials in the oxide, near the interface, have also been calculated. The vacancies in the second ionic plane were found to have the lowest energies, 2 eV lower than the corresponding bulk energies. The energies of defects situated more than seven ionic planes from the interface were found to be close to the values predicted by the continuum electrostatic model. Whilst the concentration of intrinsic defects was estimated to be too small to be significant, defects introduced artificially, for example, by radiation, would increase the binding energy of the interface significantly. We therefore confirm quantitatively the explanation for adhesion in processes such as radiation-enhanced adhesion and anodic bonding, proposed by Stoneham and Tasker [A. M. Stoneham and P. W. Tasker, J. Phys. 18, L543 (1985)].
A comparison of particle in cell and fluid model simulations of low-pressure radio frequency discharges
Authors: Nitschke T. E. , Graves D. B.
Published: July 2002

A comparison is made between particle-in-cell model (PIC) and fluid model simulations of a radio frequency (rf) gaseous discharge containing a helium model gas. The agreement in the discharge properties predicted by the two models is examined over a range of pressures and applied voltages. It is found that there is quantitative agreement in the overall discharge behavior predicted by the PIC and fluid simulations at pressures above 100 mTorr and applied voltages up to 800 V. With the exception of the ion energy at the electrodes, the two models also agree in their prediction of the scaling of discharge properties as a function of the applied voltage. Below 100 mTorr, disagreement in the period-averaged electron heating predicted by each model leads to differences in the predicted discharge properties. The estimate for the electron power input prediction can be improved in the fluid simulation by including an analytic expression for stochastic heating in the electron energy balance equation.
Hole capture by D-center defects in 6H-silicon carbide
Authors: Saddow Stephen E. , Tipton C. Wesley , Mazzola Michael S.
Published: July 2002

Room-temperature yellow luminescence is a distinctive signature of boron-related deep-level defects in 6H-SiC. This yellow luminescence is associated with the boron-related D center, rather than the more shallow boron acceptor. However, the reported activation energy for photoluminescence (0.7–0.73 eV) is in disagreement with the D center’s reported thermal activation energy (0.58–0.63 eV) as determined by deep-level transient spectroscopy (DLTS). We show that this discrepancy can be eliminated by correcting the DLTS results for the temperature dependence of hole capture at the D center. By use of independent capture and emission measurements, and a two-stage deep-level capture model, the D center’s ground state is resolved to be Ev+0.74 eV±0.02 eV, in good agreement with photoluminescence data.
Atomistic calculations of defects in ZnGeP2
Authors: Zapol Peter , Pandey Ravindra , Ohmer Mel , Gale Julian
Published: July 2002

Atomistic calculations are performed to study defect energetics in ZnGeP2 where two- and three-body interatomic potentials are used to simulate the perfect lattice. Formation energies for native ionic defects and binding energies for some of the electronic defect complexes are calculated. The dominance of antisite defect pairs, ZnGe+GeZn, is predicted in the lattice. However, the defects controlling the spectroscopic properties would seem to be associated with vacancies. For the EPR-active acceptor center, the hole is found to be localized near the zinc vacancy rather than near the zinc antisite (ZnGe). The calculated results suggest that the reported Hall effect and the photoluminescence data are compatible with the existence of two acceptors in the lattice (in a three level model) where one is significantly shallower, experimentally by 0.27 eV, in reasonable agreement with the calculated difference of 0.37 eV.
Effect of deposition temperature on the structural and electrical properties of laser-crystallized hydrogenated amorphous silicon films
Authors: Sridhar Nagarajan , Chung D. D. L. , Anderson W. A. , Coleman J.
Published: July 2002

The deposition temperature of hydrogenated amorphous silicon films deposited by dc glow discharge was found to affect the crystallinity, hydrogen content, and silicon–hydrogen bonding after laser crystallization of the film. This in turn affected the electrical properties of the crystallized film. The crystallinity of the film after laser annealing was always higher than that of the corresponding furnace-crystallized films, for the same deposition temperature, and it increased with decreasing deposition temperature, similar to that observed in furnace crystallized films (650 °C, 30 h). However, the dark and photoconductivity, photoresponse (defined as the ratio of photo to dark conductivity), and the carrier diffusion length increased with increasing deposition temperature (150–350 °C). This was due to both an increase in hydrogen content and the SiH and SiH2 bonding, as shown by evolved gas analysis and infrared spectroscopy. Carrier transport measurements indicated that the dominant transport mechanism changed from localized hopping to extended state transport with increasing deposition temperature for the laser-crystallized films.
Volume effects on the magnetic properties of R2Fe17-based quasiternary compounds
Authors: Wang Jian-Li , Yang Fu-Ming , Tang Ning , Han Xiu-Feng , Pan Hong-Ge , Hu Ji-Fan
Published: July 2002

Structure and magnetic properties of the R2Fe17-based quasiternary compounds (R=Sm, Er, Gd, Y, Pr, Nd; M=Al, Ga) and their nitrides have been investigated. The saturation magnetization of R2Fe17−xMx compounds decreases with the increasing M content, while the M concentration dependence of the Curie temperature exhibits a maximum at about x=3.0 for the parent compounds. By nitrogenation the lattice parameters, the saturation magnetization, and the Curie temperature all have a corresponding increase compared with the parent alloys. The volume effects on the Curie temperature of these compounds have been studied. The d ln Tc/d ln V values were derived by comparing the change of the Curie temperature and volume before and after nitrogenation and show a linear dependence on the Curie temperatures for these systems investigated. This behavior can be quite well understood in terms of a combined model of localized and itinerant electrons suggesting more than 90% d electrons are localized, The same conclusion can be obtained from a analysis of the exchange field Hex dependence of the iron sublattice magnetization in the R2Fe17 compounds and their nitrides, and the iron proportionality constant VFe before and after nitrogenation.
Micromechanics of randomly oriented ellipsoidal inclusion composites. Part I: Stress, strain and thermal expansion
Authors: Luo Jie , Stevens R.
Published: July 2002

A framework for micromechanics investigation of randomly oriented ellipsoidal inclusion composites has been proposed and established, based on the eigenstrain concept introduced by Eshelby, and the equivalent inclusion method. The average residual stress and strain and the average stress and strain, in both the matrix and the inclusions, have been derived relative to the mismatch of elastic moduli, thermal-expansion coefficients, and the shape of the ellipsoid. The inclusion shape has a significant effect on the stress and strain field, with the smallest effect generated by a spherical inclusion. An expression for the relationship between the average stress and strain and the inclusion shape has been formulated. The multiphase inclusion composite model has also been established. As an example, the thermal-expansion coefficient for randomly oriented ellipsoidal inclusion composites has been derived and related to the inclusion content, shape and elastic moduli. In Part II of this work, the elastic moduli of the inclusion composites are formulated and discussed.
Optical, electrical, and structural properties of amorphous Ag–Ge–S and Ag–Ge–Se films and comparison of photoinduced and thermally induced phenomena of both systems
Authors: Kawaguchi Takeshi , Maruno Shigeo , Elliott Stephen R.
Published: July 2002

To understand the nature of Ag-rich chalcogenide glasses, the optical, electrical, and structural properties of evaporated amorphous (Ge0.3S0.7)100−xAgx and (Ge0.3Se0.7)100−yAgy films have been examined and compared with each other over a wide compositional range in Ag content. The maximum Ag content for the amorphous films was 67 at. % for the S-based system and 40 at. % for the Se-based system. The physical properties of both systems depended significantly on the Ag content but the compositional trends resembled each other. All the photoinduced and thermally induced phenomena observed for the S-based system were also observed for the Se-based system but with the compositional ranges shifted to lower Ag concentration: the photo- and thermal bleachings (0≤y<22 for the Se-based system, 0≤x<40 for the S-based system), the photoinduced surface deposition (PSD) of metallic Ag phenomenon, and the phase separation on annealing (25<y≤40, 50<x≤67). The Se-based system was found to exhibit the PSD phenomenon at low Ag concentrations where the S-based system never exhibits this effect.
Figures of merit for ferroelectric electron emission cathodes
Authors: Shur D. , Rosenman G.
Published: July 2002

Pyroelectrically induced electron emission from different ferroelectric single crystals and ceramics is studied. It is observed that (Pb,La)(Zr,Ti)O3 ceramics generate rather low ferroelectric electron emission (FEE) current compared with single crystals such as triglycine sulphate and lithium niobate. Figures of merit for FEE cathodes in the pyroelectric excitation mode are derived. It is shown that a physical origin and consequently figures of merit are quite different for FEE cathodes and conventional field electron cathodes.
Epitaxial crystallization during 600 °C furnace annealing of amorphous Si layer deposited by low-pressure chemical-vapor-deposition and irradiated with 1-MeV Xe ions
Authors: Nakata Jyoji
Published: July 2002

The amorphous Si layers deposited by low-pressure chemical vapor deposition on (100)-crystal-Si substrates and subjected to Xe-ion-beam irradiation are crystallized epitaxially in a layer-by-layer fashion to the surface during 600 °C furnace annealing. Layer-by-layer crystallization can be accomplished by irradiating the layers with a 1-MeV Xe-ion-beam for a 2×1015/cm2 dose at 310 °C prior to furnace annealing. In all cases during furnace annealing that amorphous Si layers are polycrystallized or are grown vertically in isolated epitaxial-columnar-structures and then grown laterally into the amorphous region surrounding each column, the ion-beam-induced epitaxial crystallization (IBIEC) method epitaxially crystallizes them in a layer-by-layer fashion. This is because O atoms that were at the initial interface and that prevented layer-by-layer crystallization or columnar-epitaxial-growth diffuse remarkably because of irradiation. This diffusion decreases the peak concentration and facilitates layer-by-layer crystallization. O atoms at the interface are also diffused by irradiation with 80-keV P, 100-keV As, and 150-keV As ions. This diffusion results in the columnar growth during 600–800 °C furnace annealing. Whether layer-by-layer growth or columnar growth occurs during the furnace annealing depends on the peak concentration of oxygen at the interface. Direct evidence is shown that O diffusion is enhanced by the amount of inelastic electronic scattering of incident ion beam under the same elastic nuclear scattering conditions. The rates of IBIEC and of epitaxial crystallization during furnace annealing after 1-MeV Xe-ion-beam irradiation for a 2×1015/cm2 dose are affected by the amount of oxygen in the amorphous layer. The rate of layer-by-layer IBIEC using a 1-MeV Xe-ion-beam is nearly twice as high for a sample heated in the deposition furnace after evacuation as it is for a sample heated before evacuation. This difference is due to the smaller amount of oxygen in the amorphous Si layer of the former sample.
Spatially resolved experimental investigation of a surface wave sustained discharge in nitrogen
Authors: Dias F. M. , Tatarova E. , Ferreira C. M.
Published: July 2002

A spatially resolved experimental investigation of the electron energy distribution function (EEDF) and wave propagation characteristics in a high frequency surface wave (SW) discharge in nitrogen has been performed. The measurements reveal the specific changes of the EEDF and its integrals which occur as a result of a different coupling between the electrons and the inhomogeneous SW electric field as a function of gas pressure. At a pressure of 0.5 Torr it was found that the EEDF and corresponding integrals depend on the spatial position, which means that local plasma response occurs. As a result, a strong radial inhomogeneity of the discharge is observed which relates to the radial variation of the wave field intensity. On the contrary, at 0.05 Torr nonlocal plasma response is observed. In a molecular plasma, electron energy relaxation occurs faster than in inert gases, this being why the transition from the local to the nonlocal regime takes place here at a lower gas pressure. A turning back of the axial wave number due to the simultaneous effects of collisions and the radial inhomogeneity is also observed. A notable feature of the experiments is that the minimum electron density for the wave to propagate may be smaller than the minimum density for propagation in the collisionless approximation. A strong correlation between the behavior of the plasma parameters and the wave electrodynamics is pointed out.
Blue organic light emitting diodes based on bicarbazyle derivates: Device stability and multilayer configuration
Authors: Bacsa W. S. , Schaer M. , Zuppiroli L. , Adès D. , Siove A.
Published: July 2002

The device stability of blue light emitting diodes based on N,N′-diethyl-3,3′-bicarbazyle, (EtCz)2, are substantially improved by coevaporation with N,N′-diethyl-3,3′-bicarbazyle-6,6′ dicarbaboxylic acid, (OcCzCOOH)2. We attribute this effect to the formation of a network of hydrogen bonds in the alloy related to the presence of carboxyl groups. A three layer device with transport layers for both electrons and holes improves the quantum efficiency: With an Al cathode we obtain a luminance of 100 cd/m2 at current density levels of 50 mA/cm2. We find that the current voltage characteristics of (EtCz)2 is well described by our recent microscopic model for space charge limited currents in an assembly of conjugated polymer segments. The incorporation of an electron transport layer splits and shifts the electroluminescent band to lower energies and we observe a significant additional downshift after a period of three weeks which we attributed to molecular diffusion at the interface between the emitting and electron transport layers.
Nonmonotonic concentration dependence of piezoelectric coefficients of 1–3 composites
Authors: Topolov V. Yu. , Turik A. V.
Published: July 2002

Reported are results of studying nonmonotonic concentration dependences of piezoelectric coefficients e3j*, d3j* and their anisotropy ζe and ζd, respectively, which pertain to the 1–3 composite. Changing the piezoelectric properties of the composite is plotted by variations of ratios between elastic and piezoelectric constants of components and for various volume fractions of one of the components. Systems of conditions providing extreme points of the e3j* and d3j* functions and discontinuity points of the ζe and ζd functions are formulated. The role of electromechanical constants of the components and some connections between these constants in reaching the nonmonotonicity are discussed.
Direct current high-pressure glow discharges
Authors: Stark Robert H. , Schoenbach Karl H.
Published: July 2002

Stabilization and control of a high-pressure glow discharge by means of a microhollow cathode discharge has been demonstrated. The microhollow cathode discharge, which is sustained between two closely spaced electrodes with openings of approximately 100 μm diam, serves as plasma cathode for the high-pressure glow. Small variations in the microhollow cathode discharge voltage generate large variations in the microhollow cathode discharge current and consequently in the glow discharge current. In this mode of operation the electrical characteristic of this system of coupled discharges resembles that of a vacuum triode. Using the microhollow cathode discharge as plasma cathode it was possible to generate stable, direct current discharges in argon up to atmospheric pressure, with estimated electron densities in the range from 1011 to 1012 cm−3. The recently demonstrated parallel operation of these discharges indicates the potential of this technique for the generation of large volume plasmas at high gas pressure through superposition of individual glow discharges.
Gas dynamics and electromagnetic processes in high-current arc plasmas. Part I. Model formulation and steady-state solutions
Authors: Schlitz Lei Z. , Garimella Suresh V. , Chan S. H.
Published: July 2002

A three-dimensional computational model has been developed to study the effects of self-induced and external magnetic fields, as well as gassing effects, on arcs involved in switching devices. A commercial computational fluid dynamics code has been adapted and modified to model the fully coupled plasma flow, heat transfer, and electromagnetic field. In this paper, a model is developed to analyze a steady-state, two-dimensional axisymmetric air arc column at low current levels under conditions in which the effects of the self-induced magnetic field are negligible. The model is then extended to analyze a three-dimensional arc column at high current levels with the inclusion of self-induced magnetic effects. The effects of cathode size, distance from the electrode, current level, self-induced magnetic field, and natural convection on the arc plasma are investigated. Predictions from these models compare favorably with published analytical and experimental results. The influence of external transverse magnetic fields, as well as the presence of gassing materials, on a three-dimensional arc column in both steady-state and transient situations will be discussed in L. Z. Schlitz et al. [J. Appl. Phys. 85, 2547 (1999)].
Modeling biaxial stress effects on magnetic hysteresis in steel with the field and stress axes noncoaxial
Authors: Sablik M. J. , Augustyniak B. , Chmielewski M.
Published: July 2002

A model based on the domain wall pinning magnetomechanical hysteresis model of Sablik and Jiles [M. J. Sablik and D. C. Jiles, IEEE Trans. Magn. 29, 2113 (1993)] was formulated to compute changes in magnetic hysteresis under biaxial stress conditions with the magnetic field and stress axes noncoaxial. The model included the Villari effect and other asymmetric stress effects. The magnetic field was taken at various angles relative to the stress axis. The Barkhausen noise, hysteresis loss, and maximum flux density were computed for fields varying between ±1 kA/m. The results compared favorably to experimental data on several steels—Polish St3 steel, Polish St41 steel, and US commercial grade steel pipe.
Low-temperature dynamic properties of photorefractive grating in potassium–strontium–sodium–barium–niobate crystal
Authors: Li Yanqiu , Trung Dang Quoc , Zhang Baoping , Wang Wenxin , Segawa Yusaburo , Itoh Tadashi
Published: July 2002

The low-temperature dynamic properties of photorefractive gratings in cobalt-doped potassium–strontium–sodium–barium–niobate crystals are reported. At low temperatures, the photorefractive space-charge field undergoes quasiperiodic oscillations if the crystal is preilluminated by one of the writing beams. The write–erase cycle of the index grating becomes shorter if the crystal is preilluminated by one of the writing beams, which provides a means for crystal sensitization. The magnitude of the index grating decreases significantly at low temperatures. The temperature dependence of the dark decay of the photorefractive grating is measured to complete our study and demonstrate the multiple shallow levels existing in the crystal. A model of double carriers with multiple shallow levels is assumed to explain the experimental results.
Excess silicon at the silicon nitride/thermal oxide interface in oxide–nitride–oxide structures
Authors: Gritsenko V. A. , Wong Hei , Xu J. B. , Kwok R. M. , Petrenko I. P. , Zaitsev B. A. , Morokov Yu. N. , Novikov Yu. N.
Published: July 2002

The chemical composition and structure of Si3N4/thermal (native and wet) SiO2 interface in oxide–nitride–oxide structures are studied by using secondary ion mass spectroscopy, electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES) measurements. EELS and AES experiments show the existence of excess silicon at the Si3N4/thermal SiO2 interface. Excess silicon (Si–Si bonds) at Si3N4/SiO2 interface exists in the form of Si-rich silicon oxynitride. Numerical simulation of the Si–Si bond’s electronic structure by using semiempirical quantum-chemical method (MINDO/3) shows that Si–Si defects act as either electron or hole traps. This result explains the abnormally large electron and hole capturing at this interface reported earlier.
Generation of high pretilt angle and surface anchoring strength in nematic liquid crystal on a rubbed polymer surface
Authors: Seo Dae-Shik
Published: July 2002

The generation of high pretilt angles in nematic liquid crystal (LC), 4-n-pentyl-4-cyanobiphenyl (5CB), on the rubbed surface of polyimide (PI) containing trifluoromethyl moieties, was investigated. High pretilt angles of 5CB are strongly related to the low surface energy caused more by the existence of trifluoromethyl moieties at low rubbing strength (RS). It is considered that the high pretilt angles of 5CB are most likely due to the van der Waals (VDW) dispersion interactions between the LC molecules and polymer surface having low surface energy. The medium and low pretilt angles at high RS may be caused by the combination of the VDW polymer surface with that of the dispersion and the steric interactions between the LC molecules and the asymmetric triangular structure formed by unidirectional rubbing. Finally, it was found that the polar anchoring strength A of 5CB strongly depended on the surface ordering of the rubbed PI surface containing trifluoromethyl moieties; with A decreasing proportionately with increasing RS.
Photoacoustic determination of the recombination velocity at the AlGaAs/GaAs heterostructure interface
Authors: Reich I. , Díaz P. , Prutskij T. , Mendoza J. , Vargas H. , Marín E.
Published: July 2002

The surface recombination velocity at the AlGaAs/GaAs single heterostructure interface is determined using the photoacoustic technique. We analyze different heat generation mechanisms for this type of heterostructure with different Al composition and thickness of the AlGaAs layers. A theoretical model of the photoacoustic signal generation in the heat transmission detection configuration was developed. The values of the surface recombination velocity obtained by comparing the theoretical expressions with the experimental results are in good agreement with the literature reported ones. The results were also correlated with those of photoluminescence measurements in the same samples.
The semiclassical quantization of nonseparable systems using the method of adiabatic switching
Authors: Skodje Rex T. , Borondo F. , Reinhardt William P.
Published: July 2002

A method for the semiclassical quantization of multidimensional bound systems based on the adiabatic hypothesis is examined. The validity criteria for multidimensional adiabaticity is discussed. It is demonstrated that the quantizing orbits for nonseparable systems can often be obtained by propagating a single trajectory from well defined initial conditions with a time-dependent Hamiltonian for ∼100 periods. Numerical examples using systems with up to five degrees of freedom are presented and show generally excellent results. It is shown that this method can be used to quantize some states using chaotic trajectories.
Rotational state distributions in the photolysis of water: Influence of the potential anisotropy
Authors: Schinke R. , Engel V. , Staemmler V.
Published: July 2002

We report a detailed investigation of rotationally inelastic effects in the photodissociation of water in the first absorption band using a recently calculated ab initio potential energy surface of the Ã 1B1 state. Although the excited state potential anisotropy is large it has only very weak influence on the rotational state distributions which thus simply reflect the angular behavior of the ground state wave function of the parent molecule. The reason is that both potential energy surfaces have roughly the same equilibrium angle. Strong inelastic effects are observed, however, for dissociation out of excited bent states because the corresponding ground state wave function extends over a considerably wider angular range and thus the more anisotropic regions of the excited state potential are probed. Calculations are performed on three levels of accuracy for the continuum wave function: close-coupling, coupled-states (CS), and infinite-order-sudden (IOS) approximation. The ground state wave function is treated numerically exactly. The CS approximation is found to be extremely reliable for those cases studied in this work. The accuracy of the IOS approximation depends very much on the region of orientation angle probed by the ground state and therefore a general conclusion is not possible. Finally, OH rotational state distributions obtained from the dissociation of water and from H–OH scattering at equivalent translational energies are compared and found to be extremely different.
Exact evaluation and recursion relations of two-center harmonic oscillator matrix elements
Authors: Drallos P. J. , Wadehra J. M.
Published: July 2002

Using vibrational wave functions of two relatively displaced harmonic oscillators of arbitrary frequencies, Franck–Condon overlap integrals and matrix elements of xl, exp(−2cx), and exp(−cx2) (x is the internuclear separation) are obtained. Useful three-term, four-term, and five-term recursion relations among these matrix elements are derived. It is shown that all of the relevant matrix elements can be obtained from a mere knowledge of the lowest two Franck–Condon overlap integrals. Results are illustrated by computation of Franck–Condon factors for the A 1∑+u–X 1∑+g and the B 1Πu–X 1∑+g systems of 7Li2.
Vibrational energy disposal in the reactions of F atoms with NH3, ND3, N2H4, and CH3ND2
Authors: Wategaonkar S. , Setser D. W.
Published: July 2002

The F atom reactions with NH3, ND3, N2H4, and CH3ND2 were studied by observation of HF and DF infrared chemiluminescence for very short reaction time in a flow reactor at 300 K. Experiments were done for a wide range of reagent and F atom concentrations in order to obtain HF vibrational distributions that were free from contributions of secondary reaction or vibrational relaxation. The HF and DF distributions from NH3, ND3, and CH3ND2 appear to be normal relative to the typical dynamics associated with H abstraction by F atoms and 〈 fV(HF)〉 is 0.40–0.45. The energy disposal to HF by the N2H4 reaction is anomalously low with 〈 fV(HF)〉 being 1/2 of the normal result for H abstraction by F atoms. Such a low 〈 fV〉 has been associated with reactions for which there is considerable radical stabilization energy, and structural calculations in the literature do indicate that there are significant changes in the N2H3 geometry relative to that in N2H4. The HF formation rate constants (300 K) are larger than 1.0×10−10 cm3 molecule−1 s−1. The highest observed HF(v,J) and DF(v,J) states are used to discuss D0(H–CH2ND2) and D0(CH3ND–D).
Collisions of excited alkali atoms with O2. I. Intermultiplet transfer
Authors: Paillard D. , Mestdagh J. M. , Cuvellier J. , de Pujo P. , Berlande J.
Published: July 2002

The cross sections for the Rb(7S→5D), Rb(5D→7S), and Na(4D→5S) collisional transfers induced by O2 have been measured in absolute values as a function of the collision energy using a crossed beam apparatus. The experimental data have been compared with the predictions given by a multiple curve-crossing model where the symmetries of the states formed by the colliding alkali-metal atom–O2 system as well as the molecular orientation are considered. Good agreement is observed between experimental and calculated cross sections. This tends to show that in spite of its approximations, a multiple curve-crossing model would be useful to obtain fairly accurate informations about collisional processes involving alkali atoms with as much as 4 eV electronic excitation.
An analytic treatment of percolation in simple fluids
Authors: Xu Jiasai , Stell George
Published: July 2002

The percolation behavior of simple fluids, modeled by a pair potential with a hard core and Yukawa tail, is studied by solving the connectedness Ornstein–Zernike equation analytically in the mean-spherical approximation for two different connectedness models. The simplest connectedness model is defined by a ‘‘connectedness shell’’ concentric to the sphere that defines the hard core. The analytical solution applies to the case of the shell width less than the hard-core diameter, which is the case for most applications of physical interest. The percolation loci on the phase diagram have been determined along with the average coordination number, which is found to be essentially constant along each locus, but different for different loci. A second model is introduced in which direct connectedness-in-probability is defined and applied to the gelation problem. This model can be thought of as a continuum generalization of the lattice model of gelation introduced by Coniglio, Stanley, and Klein. In the mean-spherical approximation, the models have percolation exponents (β=1/2, δ=5, γ=2, η=0, fractal dimensionality=2.5) that for the most part are quite close to the best available estimates of these exponents.
Efficient molecular simulation of chemical potentials
Authors: Deitrick G. L. , Scriven L. E. , Davis H. T.
Published: July 2002

This paper evaluates methods of estimating chemical potential—actually fugacity coefficient—from molecular simulations. These methods are based on formulas given by Widom, by Bennett, and by Shing and Gubbins. They are tested with molecular dynamics simulations of Lennard-Jones liquids along the isotherm kT/ε=1.2 for densities from 0.65σ−3 to 0.90σ−3. A new test molecule sampling method, excluded volume map sampling, is found to be as much as 2 orders of magnitude more efficient than uniform test molecule sampling; it is more efficient and reliable than restricted umbrella sampling proposed by Shing and Gubbins. Several difficulties of estimating the variance of the fugacity coefficient estimates are surmounted by novel application of standard statistical methods. The statistical analysis shows that Bennett’s method yields estimates with the least variance and Widom’s method yields estimates with the greatest variance although all methods are consistent to within statistical error. Pressure and fugacity coefficient estimates obey the Gibbs–Duhem equation along the chosen isotherm up to densities of 0.75σ−3 for 108-molecule, 0.8σ−3 for 200-molecule, and 0.9σ−3 for 500-molecule simulations.
Kinetics of metastable states in block copolymer melts
Authors: Fredrickson Glenn H. , Binder Kurt
Published: July 2002

A theory is presented to describe the nucleation and subsequent growth of weakly inhomogeneous lamellar phases from a supercooled disordered phase. Detailed calculations are performed for the case of near-symmetric diblock copolymer melts, although the results have relevance for other physical systems in the Brazovskii ‘‘universality class.’’ Nucleation phenomena in such systems are particularly interesting because the bulk phase transitions have a very weak, fluctuation-induced first order character. We find unusually small nucleation barriers and critically slowed growth kinetics. For a diblock copolymer melt with degree of polymerization N and undercooling δ∝(Tt−T)/Tt≪1, the barrier is of order ΔF*/kBT∼N−1/3δ−2. Our estimate for the completion time of the nucleation and growth processes is θc ∼N1/12 δ−3/4 τd exp(ΔF*/4kBT), where τd is the copolymer terminal relaxation time.
The divide-and-conquer density-functional approach: Molecular internal rotation and density of states
Authors: Lee Chengteh , Yang Weitao
Published: July 2002

The divide-and-conquer density-functional method recently developed by Yang is applied to the calculations of internal rotation energies and density of electronic states of a tetrapeptide. The method, on comparison with the conventional Kohn–Sham method, is found to be capable of accurately describing the density of states and the small electronic energy changes in the internal rotation. The calculations further demonstrate the promise of the method for calculations of large systems beyond the reach of conventional methods.
Comment on broadening of water microwave lines by collisions with helium atoms
Authors: Maluendes Sergio , McLean A. D. , Green Sheldon
Published: July 2002

Earlier theoretical descriptions of interaction forces and collision dynamics in the H2O–He system [A. Palma et al., J. Chem. Phys. 89, 1401 (1988); S. Green et al., ibid. 94, 1346 (1991)] are improved by calculating the interaction at additional orientations and by including diffuse functions in the molecular orbital expansion basis set. The improvements make substantial changes to the potential energy surface, but make only small changes in predicted microwave pressure broadening coefficients. The qualitative temperature dependence and relative widths of different lines appear to be converged with respect to successive improvements to the theoretical description of this system. Remaining discrepancies with available data must reflect problems with the experimental values.
Correlations in a rigid–flexible diblock copolymer system
Authors: Hol/yst Robert , Schick M.
Published: July 2002

We study density–density, nematic–nematic, and density–nematic order-parameter correlation functions in an incompressible system of rigid–flexible diblock copolymers. Rigid parts are modeled as rods of zero thickness, and flexible parts as chains with freely rotating bonds. The Hamiltonian contains interactions of the isotropic Flory–Huggins and anisotropic Maier–Saupe forms. Correlation functions are calculated in the Gaussian approximation, and are found to oscillate with a characteristic period equal to the length of the rigid part. They also show that lamellar domain boundaries of systems with longer rods are sharper than those with shorter ones, and that the linear density measured along any direction is anticorrelated with the order parameter measuring nematic order in that direction. This latter effect is due to the isotropy of the system, and is relatively independent of the Maier–Saupe and Flory–Huggins parameters. We also study the stability of the system against nematic and lamellar perturbations, and show how the general features of the phase diagram change with the Maier–Saupe parameter.
On the stability of type I gas hydrates in the presence of methanol
Authors: Wallqvist Anders
Published: July 2002

Using computer simulations we have studied the stability of type I gas hydrates for different mixtures of methane/methanol at 270 K and atmospheric pressure. In the nanosecond time regime the empty and the methane-filled hydrate are stable with only small perturbations to the water cavity structure. Inclusion of methane stabilizes the lattice structure over the empty hydrate. A hydrate filled with methanol melts in roughly 1 ns with a concomitant destruction of the cavity structure. The resulting mixture is liquidlike in contrast to the icelike hydrate structure. Small amounts of methanol can be incorporated into the hydrate structure; thus a 4 wt % methanol solution is stable, whereas a 7 wt % solution melts. In the stable methanol hydrate cavity structure is maintained, but the perturbations relative to the empty hydrate emphasize the inherent flexibility of these structures. More alcohol can be incorporated into the hydrate if methane is allowed to stabilize the hydrate.
Femtosecond real-time probing of reactions. IX. Hydrogen-atom transfer
Authors: Herek J. L. , Pedersen S. , Bañares L. , Zewail A. H.
Published: July 2002

The real-time dynamics of hydrogen-atom-transfer processes under collisionless conditions are studied using femtosecond depletion techniques. The experiments focus on the methyl salicylate system, which exhibits ultrafast hydrogen motion between two oxygen atoms due to molecular tautomerization, loosely referred to as intramolecular ‘‘proton’’ transfer. To test for tunneling and mass effects on the excited potential surface, we also studied deuterium and methyl-group substitutions. We observe that the motion of the hydrogen, under collisionless conditions, takes place within 60 fs. At longer times, on the picosecond time scale, the hydrogen-transferred form decays with a threshold of 15.5 kJ/mol; this decay behavior was observed up to a total vibrational energy of ∼7200 cm−1. The observed dynamics provide the global nature of the motion, which takes into account bonding before and after the motion, and the evolution of the wave packet from the initial nonequilibrium state to the transferred form along the O–H—O reaction coordinate. The vibrational periods (2π/ω) of the relevant modes range from 13 fs (the OH stretch) to 190 fs (the low-frequency distortion) and the motion involves (in part) these coordinates. The intramolecular vibrational-energy redistribution dynamics at longer times are important to the hydrogen-bond dissociation and to the nonradiative decay of the hydrogen-transferred form.
Geometries and bond energies of PHnand PHn+ (n=1–3)
Authors: Balasubramanian K. , Chung Young Sir , Glaunsinger William S.
Published: July 2002

All-electron complete active space self-consistent field (CASSCF) followed by full second-order configuration interaction (SOCI) calculations which included up to 1.7 million configurations have been made on several electronic states of PHn and PHn+ (n=1–3). A comparison is made of the results of several basis sets up to the largest set, namely, the (13s10p3d2 f1g/7s6p3d2 f1g) basis set for P and a (10s5p1d/8s4p1d) basis set for the hydrogen atom. Relativistic effective core potentials/CASSCF/multireference configuration interaction computations have also been carried out. The properties of three new excited states for PH3 have been computed as well as the experimentally observed X 1A1, 3A′2, and 1A′2 states. The calculated inversion barrier of PH3 is 34.6 kcal/mol whereas the corresponding inversion barrier of PH3+ is only 2.5 kcal/mol. The bond energies of PHn and PHn+ as well as adiabatic ionization energies of PHn are computed and compared with experimental results. The core-valence and core-external correlation effects are also computed for the diatomic PH molecule.
Dynamics of light gases in rigid matrices of dense polymers
Authors: Gusev Andrei A. , Arizzi Simone , Suter Ulrich W. , Moll David J.
Published: July 2002

Transition-state theory was employed to study the dynamics of light gases dissolved in rigid microstructures of glassy polycarbonate and rubbery polyisobutylene modeled in atomistic detail. The gaseous molecules migrated through the polymer structures in a sequence of ‘‘hops’’ between local minima of the potential energy. The solute dynamics was characterized by three time domains: at short times, <10−12 s, the mobility was very high and the molecules traveled on a scale of 5 Å; this was followed by a domain of anomalous diffusion; at long times the tiniest molecules (He and H2) followed the Einstein diffusion law. Larger molecules (Ar, O2, and N2) did not reach the diffusive regime at the time scale of simulation (up to ca. 10−3 s) but were trapped instead in the vicinity of their initial sites without any progress in translational motion. It was concluded that the rigid-matrix approach is inadequate for studying the dynamics of light gases in dense polymers, except for He.
Density functional study of nitrogen oxides
Authors: Stirling András , Pápai Imre , Mink János , Salahub Dennis R.
Published: July 2002

Equilibrium geometries, bond dissociation energies, dipole moments, harmonic vibrational frequencies, and infrared intensities were calculated for a set of ten neutral nitrogen oxides (NO, NO2, NO3, N2O, sym N2O2, asym N2O3, sym N2O3, sym N2O4, asym N2O4, and N2O5) by applying one local and two gradient-corrected nonlocal functionals in a Gaussian-type-orbital density functional method. Comparison with available experimental data shows that, except for the bond dissociation energies, the local functional gives very accurate molecular properties. Nonlocal functionals considerably improve the bond dissociation energies, but the results still overestimate the experimental values by about 10 kcal/mol on average. For the other properties, the results obtained with nonlocal functionals are not necessarily superior to those calculated with the local functional. The properties of two molecules (sym N2O3 and asym N2O4) are predicted for the first time and several reassignments are proposed in the vibrational spectra of di-nitrogen oxides.
The collapse of chains with different architectures
Authors: Raos Guido , Allegra Giuseppe , Ganazzoli Fabio
Published: July 2002

The equilibrium properties and the Rouse–Zimm dynamics of polymer molecules with any architecture at temperatures T≤Θ, are treated using a bead-and-spring coarse-grained description. The collapsed globule model is adopted, whereby essentially all atoms are at the same mean-square distance 〈S2〉 from the center of mass; accordingly, at a given temperature the interatomic free energy is a single-valued function of 〈S2〉 and the self-consistent free-energy minimization is greatly simplified, in the Gaussian approximation. We prove that the connectivity matrix B and the bond-vector product matrix M=[〈li⋅lj〉] possess the same eigenvectors; these are the normal modes of the chain conformation. Furthermore, we show that 〈S2〉=N−1at∑kl2α2k/λk, where Nat is the total number of atoms, λk is the general nonzero eigenvalue of B, and l2α2k is the corresponding eigenvalue of M—the expansion ratio of the normal mode. Finally, we prove that in the free-draining limit the normal mode relaxation times are proportional to l2α2k/λk. Defining αS=√〈S2〉/〈S2〉ph as the overall strain ratio with respect to the phantom state, the plots of αS vs the reduced temperature τ=(T−Θ)/T≤0 indicate that polymers with more compact architectures display a prompter contraction for small ‖τ‖’s, although tending to larger αS’s at strong undercoolings, where the average density (∝Nat⋅〈S2〉−3/2) is about the same for all architectures. Concerning the dynamical behavior, at sufficiently large ‖τ‖’s the longest relaxation times reach a typical plateau, as already found for the linear chain.
Stability of polar guest-encaging clathrate hydrates
Authors: Koga Kenichiro , Tanaka Hideki , Nakanishi Koichiro
Published: July 2002

In order to account for the experimental observation that some amines form clathrate hydrates but that alcohols inhibit hydrate formation, we investigate the stability of clathrate hydrates which encage highly polar guest molecules by examining potential energy local minimum structures and also thermally excited structures. First, we examine the local minimum structure at which the total potential energy has a minimum value for amine and alcohol hydrates and inquire whether, in the absence of thermal fluctuations, the conditions for true-clathrate hydrates are satisfied. The local minimum structures of alcohol hydrates are distinguished from those of stable clathrate hydrates of structures I and II, while amine hydrates hold the host lattice of clathrate hydrates. We argue that the difference between the magnitude of the partial charge on the hydrogen atom of the hydroxyl and amino groups plays a much more significant role in the stability of both kinds of clathrate hydrates than the difference in molecular geometry does. Second, we examine kinetic stability by molecular dynamics simulation. Near room temperature the host lattice structure encaging amine remains intact. This is in contrast to the alcohol hydrate, which begins to melt easily with thermal excitation. Interesting is the finding that very long-time-scale (∼100 ps) fluctuations of the host potential energy is observed, as well as fast oscillations caused by temporary partial defects in the host network. Clathrate hydrates of polar guest molecules may experience an unstable state in which the host hydrogen-bonded network is partially broken and guest–host hydrogen-bond generation occurs but the stable state of their host lattices is again restored.
Density functional theory calculations of the reaction pathway for methane activation on a gallium site in metal exchanged ZSM-5
Authors: Broclawik Ewa , Himei Hiroaki , Yamadaya Michiyuki , Kubo Momoji , Miyamoto Akira , Vetrivel Rajappan
Published: July 2002

Density functional theory is used to describe the reaction profile for methane dissociation on Ga-exchanged ZSM-5. Stable structures on the reaction pathway are characterized as weakly adsorbed methane molecule and the C–H dissociation product. The transition state is also explicitly defined and optimized. The nonlocal density functional approximation is invoked to calculate the energy parameters of the reaction. The activation barrier is estimated at about 120 kJ/mol, in excellent agreement with other similar reactions. From vibrational analysis the reaction coordinate is deduced and transformation of a methane molecule on adsorption is discussed.
Entropy-enthalpy compensation: Perturbation and relaxation in thermodynamic systems
Authors: Qian Hong , Hopfield J. J.
Published: July 2002

The response of an equilibrium molecular system to perturbations depends on its environmental constraints. For example, the response of an equilibrium P, V, T system to a small temperature perturbation (specific heat) depends on whether the environmental constraint on the system is constant pressure or constant volume. In general, there are two classes of thermodynamic quantities of a system; one is completely determined by its equilibrium distribution, and the other also depends on how the distribution responds to macroscopic changes. The former class is independent of the environment of the thermodynamic system, while the latter class is a function of environmental constraints. In response to a small perturbation, the free energy change of an equilibrium system belongs to the first class but the entropy and enthalpy changes belong to the second. Therefore the thermodynamics of perturbation exhibit compensation between entropy and enthalpy of systems with different environments. The thermodynamic analysis presented here provides a framework for the interpretation of experimental observations of this phenomenon, and is illustrated by a real experimental example.