bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Zr on the microstructure and magnetic properties of Sm(CobalFe0.1Cu0.088Zrx)8.5 magnets
Authors: Tang W. , Zhang Y. , Hadjipanayis G. C.
Published: July 2002

The effect of Zr content on the microstructure and magnetic properties of Sm(CobalFe0.1Cu0.088Zrx)8.5 magnets with x varying from 0 to 0.1 was systematically studied using magnetic, x-ray diffraction, and transmission electron microscopy measurements. The results have revealed that addition of Zr greatly affects the microstructure and magnetic properties. X-ray diffraction, electron diffraction, and thermomagnetic data showed that the normal microstructure consists of 2:17 cells surrounded by 1:5 cell boundaries and intersected by lamella phase perpendicular to the c axis. The lamella phase appears to have the SmCo3 structure and its amount increases with increasing Zr content. When the Zr content is above 0.08, the microstructure becomes inhomogeneous, showing regions with a considerable amount of mixtures of 1:3 and 2:7 phases. The formation of the cellular microstructure mainly depends on the Cu content, and the existence of the lamellar phase can stabilize a uniform cellular microstructure with larger cells and help the redistribution of Cu at the cell boundaries, improving the coercivity greatly. A room temperature intrinsic coercivity of around 40 kOe has been obtained in magnets with a wide range of Zr contents from x=0.02–0.06.
Blocking temperature distribution and long-term stability of spin-valve structures with Mn-based antiferromagnets
Authors: Nozières J. P. , Jaren S. , Zhang Y. B. , Zeltser A. , Pentek K. , Speriosu V. S.
Published: July 2002

We have determined the blocking temperature distribution Tb(T) in spin-valve sheet films with FeMn, IrMn, PtMn, NiMn and CrPdMn antiferromagnetic layers (AFM). We find a clear dependence of Tb(T) on the field applied during the measurement, which we link to the reversal state of the pinned layer through the torque applied on the AFM. Using fields large enough to fully reverse the pinned layer, NiMn and PtMn show little or no components of the blocking temperature below 150 °C, whereas both IrMn and CrPdMn (the latter in a “synthetic” AFM design) exhibit important low-temperature trailing edges of the distribution. Accelerated annealing experiments in a low reversed field equivalent to the self-demagnetizing field in a micron-size head allows us to access the time to failure and the failure activation energy from which the expected lifetime can be assessed. We find a general correlation between the expected lifetime and the fraction of loose (e.g., unblocked) AFM spins at any given temperature. Accordingly, only NiMn and PtMn are found to exhibit a sufficient long-term stability for disk-drive operations.
Remote query fluid-flow velocity measurement using magnetoelastic thick-film sensors (invited)
Authors: Kouzoudis Dimitris , Grimes Craig A.
Published: July 2002

The characteristic resonant frequency of a thick-film ribbon-shaped magnetoelastic sensor is dependent upon its length, density, Poisson’s ratio, and modulus of elasticity [J. M. Barandiaran and J. Guitierrez, Sens. Actuators A 59, 38 (1997)]. In a manner analogous to surface acoustic wave (SAW) sensors, this characteristic resonant frequency also changes with effective mass load or with friction force applied to the surface of the magnetoelastic sensor [L. D. Landau and E. M. Lifshitz, Theory of Elasticity, 3rd ed. (Pergamon, New York, 1986), Chap. II]. The time varying magnetic flux emitted from magnetoelastic sensors can be monitored remotely using pickup coils. Hence, in contrast with SAW devices, no direct physical connections are required to obtain magnetoelastic sensor information, allowing them to be monitored from within sealed, opaque containers. Reported here is application of magnetoelastic sensors to the measurement of fluid flow from within sealed pipes. Within the laminar fluid flow regime greater fluid velocity results in increased frictional drag on the surface of the sensor, which in turn lowers the resonant frequency of the magnetoelastic sensor.
Nanostructure and magnetic properties of composite CoPt:C films for extremely high-density recording
Authors: Yu M. , Liu Y. , Sellmyer D. J.
Published: July 2002

The nanostructure and magnetic properties of composite CoPt:C films at room temperature were investigated as a function of annealing temperature, carbon concentration, and film thickness. CoPt:C films with a variety of carbon concentrations were fabricated by cosputtering Co, Pt, and C onto water-cooled Si(100) substrates followed by annealing. X-ray diffraction and transmission electron microscopy analyses indicate that nanocrystallites of face-centered-tetragonal (fct) CoPt phase, which has a uniaxial magnetocrystalline anisotropy constant of about 5×107 erg/cm3, can be formed in carbon matrix when the annealing temperature is higher than 600 °C. The grain sizes of the fct CoPt crystallites are about 10 nm and the coercivities can be as high as 12 kOe. Higher annealing temperature and lower carbon concentration generally lead to larger grain sizes and perhaps more complete formation of the fct CoPt phase, and therefore higher coercivities. The coercivity is insensitive to the film thickness until the thickness is smaller than the CoPt grain size, when the coercivity starts to decrease with film thickness. The magnetic activation volumes are typically around 1×10−18 cm3. The nanostructure and the associated magnetic properties of these composite CoPt:C films are promising as potential media for extremely high-density recording.
Influence of Zr and Cu content on the microstructure and coercivity in Sm(CobalFe0.1CuyZrx)8.5 magnets
Authors: Tang W. , Zhang Y. , Hadjipanayis G. C. , Kronmüller H.
Published: July 2002

The effect of Cu and Zr on the microstructure and coercivity of Sm(CobalFe0.1CuyZrx)8.5 magnets has been studied using magnetometry and transmission electron microscopy. For Zr-free samples, when y=0.088, a cellular microstructure is not formed. Instead a rod-like 1:5 phase is observed, distributed in the 2:17 matrix phase. For a sample with y=0.168, the cellular microstructure develops after a short aging. However, the cellular microstructure coarsens and becomes highly nonuniform and finally, breaks down with further aging. This leads to a reduction in coercivity from an optimal value of 5.6–2 kOe. After adding Zr(x=0.04), the cellular microstructure along with a lamellar phase can be formed even with a Cu content as low as 0.048 and the coercivity increases dramatically. Coercivities of up to 8.9 and 38.4 kOe are obtained for the y=0.048 and 0.168 samples, respectively. These results show that the formation of cellular structure strongly depends on the Cu content. But the formation of the lamellar phase can stabilize a uniform cellular microstructure over a wide range of cell size and help the redistribution of Cu at the cell boundaries, which is a key to obtain appropriate microstructures with high coercivity.
Uncertainty limits for the macroscopic elastic moduli of random polycrystalline aggregates
Authors: Chinh Pham Duc
Published: July 2002

Practical polycrystalline aggregates are expected to have macroscopic properties that depend upon the properties of constituent crystals and the aggregate geometry. Since that microgeometry is usually random, there will be some uncertainty in the observed macroscopic behavior of the aggregates. The general shape-independent upper and lower estimates for those uncertainty intervals for the elastic moduli of completely random polycrystals are constructed from the minimum energy and complementary energy principles. Applications to aggregates of cubic crystals are presented.
Molecular jet study of the solvation of toluene by methane, ethane, and propane
Authors: Schauer Mark , Law K. S. , Bernstein E. R.
Published: July 2002

Two color time of flight mass spectroscopy studies of toluene solvated by methane, ethane, and propane in a supersonic molecular jet have been carried out. This work is quite similar to the studies in the preceding paper on benzene. The conclusions and finding in the benzene investigation are strengthened and elaborated. The comparison of calculations and experiments has yielded information on binding enegy, geometry, and spectral shift. A strong correlation is found between observed cluster transition intensity and cluster nucleation processes and a tentative nucleation scheme for the molecular jet formation of solute–solvent clusters is presented.
An improved log derivative method for inelastic scattering
Authors: Manolopoulos D. E.
Published: July 2002

A new method for solving the close coupled equations of inelastic scattering is presented. The method is based on Johnson’s log derivative algorithm, and uses the same quadrature for the solution of the corresponding integral equations. However it differs from the original method in the use of a piecewise constant diagonal reference potential. This results in a reduction in matrix operations at subsequent energies, and an improved convergence of the solution with respect to the number of grid points. These advantages are clearly demonstrated when our method is applied to an atom–diatom rotational excitation problem.
Study of crystallization process of polymer from melt by a real-time pulsed NMR measurement
Authors: Tanaka Hajime , Nishi Toshio
Published: July 2002

Crystallization process of polymer [poly(ε-caprolactone)] has been studied by a real-time pulsed NMR measurement. The temporal change of both the spin–spin relaxation time T2 and the spin-lattice relaxation time in the rotating frame T1ρ were obtained. The growth behavior of crystalline lamellae and interface has been detected successfully by T2 measurement. It becomes clear that the real-time pulsed NMR measurement is a very powerful tool for studying crystallization dynamics. The crystallization dynamics of polymers has been discussed based on the results.
Absolute rate constants for the reaction of silylene with hydrogen, silane, and disilane
Authors: Jasinski Joseph M. , Chu Jack O.
Published: July 2002

Absolute rate constants for the reaction of silylene with hydrogen, silane, and disilane have been determined from direct time resolved measurements of silylene removal at room temperature. Silylene was generated and detected using laser resonance absorption flash kinetic spectroscopy. The rate constants are pressure dependent, consistent with expectations for the insertion reactions typical of silylene. The pressure dependence of the overall rate constants has been determined from 1 to 100 Torr for reaction with hydrogen and silane and from 1 to 10 Torr for reaction with disilane. The results for reaction with hydrogen and silane have been successfully modeled using RRKM theory and high pressure bimolecular rate constants have been extracted. The rate constants determined in this work are significantly (10–104 times) faster than those calculated from literature values for the Arrhenius parameters. These findings require a significant upward revision in the heat of formation of silylene, and may require modification of chemical vapor deposition mechanism in which silylene is invoked as a film growth precursor.
Interfacial tension and vapor–liquid equilibria in the critical region of mixtures
Authors: Moldover Michael R. , Rainwater James C.
Published: July 2002

In the critical region, the concept of two-scale-factor universality can be used to accurately predict the surface tension between near-critical vapor and liquid phases from the singularity in the thermodynamic properties of the bulk fluid [M. R. Moldover, Phys. Rev. A 31, 1022 (1985)]. In the present work, this idea is generalized to binary mixtures and is illustrated using the data of Hsu, Nagarajan, and Robinson for CO2+n-butane. We fit the pressure-temperature-composition-density data for coexisting, near-critical phases of the mixtures with a thermodynamic potential comprised of a sum of a singular term and nonsingular terms. The nonuniversal amplitudes characterizing the singular term for the mixtures are obtained from the amplitudes for the pure components by interpolation in a space of thermodynamic ‘‘field’’ variables. The interfacial tensions predicted for the mixtures from the singular term are within 10% of the data on three isotherms in the pressure range (Pc−P)/Pc<0.5. This difference is comparable to the combined experimental and model errors.
Kinetics of discontinuous volume–phase transition of gels
Authors: Sato Matsuo Eriko , Tanaka Toyoichi
Published: July 2002

Kinetics of the first order volume–phase transition of submillimeter gels is investigated for various initial and final states of the transition. Characteristic times for swelling and shrinking critically depend on the final state, but are much less influenced by the intial state. The transition becomes infinitely slow when the final temperature is near the transition threshold. The study of the dependence of the transition time on the gel size reveals that the overall volume change is described approximately as a collective diffusion process, but not precisely. The exponent for time–radius relation is smaller than 2. The transitions having large volume change are accompanied by formation and evolution of transient patterns which appear on the surface of a gel. The patterns for swelling and shrinking are quite different, but both play an important role in the kinetic processes.
A full calculation of multiconfiguration interaction effects up to 120 000 cm−1 (15 eV) on the ground configuration state levels of PrCl3. Zeeman effect interpretation
Authors: Garcia D. , Faucher M.
Published: July 2002

A previous paper gave the results of a full calculation involving free ion and crystal–field interaction within the 4 f2 and 4 f15d1 configurations. Presently, the calculation is extended to 4 f16s1 and 4 f16p1 as well. As a result, the usual experimental/calculated discrepancy of the 1D2 multiplet is completely eliminated. A complete Hamiltonian, including the magnetic contribution, is utilized to reproduce the Zeeman effect in PrCl3.
Prediction of the thermodynamic properties of associating Lennard-Jones fluids: Theory and simulation
Authors: Chapman Walter G.
Published: July 2002

Associating Lennard-Jones (LJ) fluids have been studied at various densities, temperatures, and energies of association by simulation and theory. The association potential is modeled by a highly anisotropic square well. Monte Carlo simulations provide energy, pressure/density information and fraction of monomers. A simple theory is shown to be in good agreement with simulation results. Further, a prediction of the properties of diatomic LJ molecules is made and compared with simulation results.
Observation of the 39K2 a 3Σ+u state by perturbation facilitated optical–optical double resonance resolved fluorescence spectroscopy
Authors: Li L. , Lyyra A. M. , Luh W. T. , Stwalley W. C.
Published: July 2002

Rydberg states of the potassium dimer in the 28 430–29 080 cm−1 and 30 030–30 500 cm−1 regions above the ground state X1Σ+g minimum have been studied using the perturbation facilitated optical–optical double resonance technique (PFOODR). Energy levels in these energy regions have been assigned to both triplet and singlet gerade states based on excitation pattern information as well as intensity considerations. Resolved fluorescence from a mixed triplet–singlet 43Πg ∼1Πg upper state to the ground triplet state a 3Σ+u has been used to construct a potential energy curve for the a 3Σ+u state which is in excellent agreement with recent theoretical results. Since this electronic state and the ground singlet state X1Σ+g share the same dissociation limit, we have determined the dissociation energy for the potassium dimer to be De=4450±2 cm−1.
A surface-electrochemical basis for the direct logarithmic growth law for initial stages of extension of anodic oxide films formed at noble metals
Authors: Conway B. E. , Barnett B. , Angerstein-Kozlowska H. , Tilak B. V.
Published: July 2002

Measurements on anodic surface oxidation of noble metals as a function of time and electrode potential show that the initial extension and subsequent thickening of such oxide films is directly logarithmic in time. A striking feature of this behavior is that the direct logarithmic extension law already applies to increase of coverage of Pt or Au electrodes with time well below the limit of formation of one monolayer of OH or O species on the metal surface. A direct logarithmic law of oxide film growth also applies to post-monolayer growth involving early stages of quasi-three-dimensional film formation. Eventually, as the oxide film thickens, the Mott–Cabrera ‘‘high-field’’ growth mechanism can apply. However, below the monolayer level of oxide film formation, electrochemisorption of two-dimensional (2D) structures of OH or O arises so that the Mott–Cabrera mechanism cannot be applicable to that situation. It is shown that the kinetic relation for direct electrodeposition of OH or O species onto available metal surface sites also cannot lead to a log law in time for extension of a 2D film. A new treatment, based on the changing surface-potential component of the electrode-solution potential difference, due to place exchange between metal atoms in the surface and electrosorbed OH or O species on the surface, is presented and shown to give rise to a direct log law for extension of the film in time. The relation derived has features in conformity with the experimentally demonstrated characteristics of submonolayer and early post-monolayer film extension.
Hole transport in 1,1-bis(di-4-tolylaminophenyl)cyclohexane
Authors: Borsenberger P. M. , Pautmeier L. , Richert R. , Bässler H.
Published: July 2002

Hole mobilities have been measured in vapor deposited films of 1,1-bis(di-4-tolylaminophenyl)cyclohexane over an extended range of fields and temperatures. At 295 K, the mobility is approximately 10−2 cm2/V s, the highest value for a disordered molecular solid reported thus far. Monte Carlo simulations of random walk in a geometrically and energetically disordered medium demonstrate that the time dependence of the photocurrent and the field and temperature dependencies of the mobility can be described quantitatively in terms of the inherent disorder and its effect on charge transport. No polaronic or trapping phenomena need to be invoked to reproduce even subtle features of the experimental results.
Molecular dynamics analysis of transitions between rotational isomers in polymethylene
Authors: Zúñiga Ignacio , Bahar Ivet , Dodge Robert , Mattice Wayne L.
Published: July 2002

Molecular dynamics trajectories have been computed and analyzed for linear chains, with sizes ranging from C10H22 to C100H202, and for cyclic C100H200. All hydrogen atoms are included discretely. All bond lengths, bond angles, and torsion angles are variable. Hazard plots show a tendency, at very short times, for correlations between rotational isomeric transitions at bond i and i±2, in much the same manner as in the Brownian dynamics simulations reported by Helfand and co-workers. This correlation of next nearest neighbor bonds in isolated polyethylene chains is much weaker than the correlation found for next nearest neighbor CH–CH2 bonds in poly(1,4-trans-butadiene) confined to the channel formed by crystalline perhydrotriphenylene [Dodge and Mattice, Macromolecules 24, 2709 (1991)]. Less than half of the rotational isomeric transitions observed in the entire trajectory for C50H102 can be described as strongly coupled next nearest neighbor transitions. If correlated motions are identified with successive transitions, which occur within a time interval of Δt≤1 ps, only 18% of the transitions occur through cooperative motion of bonds i and i±2. An analysis of the entire data set of 2482 rotational isomeric state transitions, observed in a 3.7 ns trajectory for C50H102 at 400 K, was performed using a formalism that treats the transitions at different bonds as being independent. On time scales of 0.1 ns or longer, the analysis based on independent bonds accounts reasonably well for the results from the molecular dynamics simulations. At shorter times the molecular dynamics simulation reveals a higher mobility than implied by the analysis assuming independent bonds, presumably due to the influence of correlations that are important at shorter times.
Picosecond reorientational dynamics of polar dye probes in binary aqueous mixtures
Authors: Dutt G. B. , Doraiswamy S.
Published: July 2002

Picosecond time dependent fluorescence method has been used to measure the rotational reorientation times (τr) of three kinds of dye probes—oxazine 720 (a monocation), nile red (neutral but polar), and resorufin (a monoanion)—in a series of binary mixtures of water–amides, water–dipolar aprotics, and water–alcohols at 298 K. Most of the binary mixtures are characterized by the fact that at a particular composition (between 25% to 40% of the organic solvent in water), the viscosity (η) of the solution reaches a maximum value that is higher than the viscosities of the two solvents. As a consequence, the viscosity profile of the solution as a function of the organic solvent exhibits a bivaluedness, the extent of which is more, if the liquids are nearly isoviscous. The dielectric properties of the solution also change across the composition range. All the dyes show a near linear behavior of τr vs η in formamide–water, N-methylformamide–water, and methanol–water mixtures. A dual-valued profile for τr vs η is obtained for the cation oxazine 720 in the three dipolar aprotic mixtures (N, N-dimethylformamide–water, N, N-dimethylacetamide–water, and dimethylsulphoxide–water), with the rotational reorientation times being higher in the organic solvent-rich zone, compared to the corresponding isoviscous point in the water-rich zone. However, the anion resorufin shows a bivalued profile of τr vs η only in dimethylsulphoxide–water mixtures, while the neutral nile red shows a linear behavior in all the dipolar–aprotic water mixtures. A hook-type profile of τr vs η is seen for the anion resorufin in ethanol–, 1-propanol–, and 2-propanol–water mixtures and for the cation oxazine 720 in 1-propanol– and 2-propanol–water mixtures; but nile red shows no departure from linear behavior even in alcohol–water mixtures.

The rotational dynamics of the cation oxazine 720 in dipolar aprotic–water mixtures is explained in terms of solvation since the dielectric friction is minimal in these solutions. While the concept of dielectric friction model seems to be adequate to understand the nonhydrodynamic behavior of the oxazine 720, the additional contribution due to solvation effects has to be necessarily taken into account to explain the rotational dynamics of the anion resorufin in alcohol–water mixtures.
Trajectory studies of S
N2 nucleophilic substitution. II. Nonstatistical central barrier recrossing in the Cl−+CH3Cl system
Authors: Cho Young June , Vande Linde Scott R. , Zhu Ling , Hase William L.
Published: July 2002

For the Cl−+CH3Cl SN2 nucleophilic substitution reaction transition-state theory predicts that crossing the central barrier region of the potential-energy surface is the rate-controlling step. In this work classical trajectories are initialized at the central barrier. Four different models are considered for the potential-energy surface. A significant amount of central barrier recrossing is observed in the trajectories, which suggests that transition-state theory is an incomplete model for calculating the Cl−+CH3Cl SN2 rate constant. Two types of recrossings are observed in the trajectories: intermediate recrossings in which trajectories linger near the central barrier and complex recrossings in which trajectories trapped in the Cl−⋅⋅⋅CH3Cl complex return to the central barrier region. Intermediate recrossings are important if, in the trajectory initial conditions, zero-point energy is added to the vibrational modes orthogonal to the reaction coordinate. Rice–Ramsperger–Kassel–Marcus (RRKM) theory predicts extensive dissociation of the Cl−⋅⋅⋅CH3Cl complex to Cl−+CH3Cl and negligible complex recrossings in the trajectory calculations. In contrast to this prediction, negligible Cl−+CH3Cl formation is observed and continual complex recrossings occur, on a time scale longer than the complex’s RRKM lifetime. These results indicate the ergodic assumption is invalid for the Cl−⋅⋅⋅CH3Cl complex. Phase-space bottlenecks which give rise to the intermediate and complex recrossings are considered.
Application of systematic sequences of wave functions to the water dimer
Authors: Feller David
Published: July 2002

A systematic series of calculations encompassing a wide range of basis sets and correlated methods has been used to estimate the complete basis set, full CI hydrogen bond strength in the water dimer system. The largest basis set included up through h polarization functions on oxygen and g functions on hydrogen. The complete basis set limit for the self-consistent-field (SCF) interaction energy is estimated to be −3.55 kcal/mol with an accompanying correlation contribution of ∼−1.5 kcal/mol. This leads to an interaction energy of −5.1 kcal/mol, exclusive of vibrational zero-point considerations, and is in good agreement with experimental measurements of −5.4±0.7 kcal/mol. Inclusion of an approximate adjustment for the basis set superposition error via the Boys/Bernardi counterpoise correction was found to substantially improve agreement with ΔE∞, our estimate of the complete basis set interaction energy, at the both the SCF and correlated levels for basis sets that were lacking in sufficient near-valence diffuse functions. For diffuse-function-augmented basis sets, application of the CP correction was found to sometimes worsen agreement with ΔE∞.
Multiphoton ionization/dissociation of osmium tetroxide
Authors: Ding D. , Puretzky A. A. , Compton R. N.
Published: July 2002

The mechanisms leading to laser multiphoton ionization and dissociation (MPI/MPD) of osmium tetroxide (OsO4) have been investigated from measurements of the kinetic energies of product ions (Os+, Os2+, OsO+, O2+, O+) and photoelectrons as a function of the laser wavelength. Neutral channels, intermediate to the dominant Os+ ionization channel, such as OsO4→OsO4−n+nO are examined using resonance-enhanced multiphoton ionization (REMPI) of the fast O atoms. Equipartition of the available photon energy among the fragments is observed. The wavelength dependence of the Os+ ion signal suggests that one or more of the steps leading to Os+ ions involve molecular ions and/or excited neutral atoms. The observed preponderance of very slow (<0.2 eV) electrons also supports this interpretation. Os2+ is shown to result primarily from REMPI of Os+.
van der Waals broadening of the X̃→B̃ (3s Rydberg) transition of acetone by He, Ar, and CH4: Inversion of the spectra to the interaction pair potentials
Authors: Halpern Arthur M. , Ramachandran B. R.
Published: July 2002

The X̃→B̃ absorption spectrum of acetone at 1944 Å, corresponding to the transition to the (n0,3s) Rydberg state, has been measured in the presence of moderate pressures of the perturbers Ar, CH4, and He. The line shape changes, which are ascribed to van der Waals perturbations, are analyzed globally using an iterative Fourier transform technique. This method is successful in inverting the distorted spectra into Lennard-Jones (6-12) potentials for the ground and excited acetone–perturber van der Waals complexes (for Ar and CH4), and into a (6-12) potential for the ground state and a repulsive potential for the excited state of the acetone–He complex. For Ar and CH4, optimized ground state (6-12) parameters are consistent with those obtained with simple combining rules; in these cases, the excited state parameters show that the interaction distance is larger than that for the ground state by ∼0.37 and 0.35 Å, respectively, and that the well depth is smaller by ∼42 and 33 cm−1, respectively. The results with He indicate that a repulsive potential is needed to describe the excited state–perturber interaction.
Comparison of constant pressure and constant volume nonequilibrium simulations of sheared model decane
Authors: Daivis Peter J. , Evans Denis J.
Published: July 2002

We present the results of nonequilibrium molecular dynamics simulations of a model decane fluid performed at constant pressure and compare them with results previously obtained from simulations performed at constant volume. The strain rate dependence of the viscosity at constant pressure is found to differ from that obtained previously at constant volume. The shear thickening at high strain rates observed in constant volume simulations vanishes when the simulations are performed at constant pressure. We also investigate the question of how our low strain rate data for decane can be accurately extrapolated to zero strain rate. We find a well defined first Newtonian region in which the viscosity is independent of strain rate to within errors. The value of the viscosity that we obtain in this region agrees well with the zero strain rate viscosity calculated from the Green–Kubo formula at equilibrium.
Rotational temperature dependence of the reactions of N+ and C+ with H2, HD, and D2
Authors: Sunderlin L. S. , Armentrout P. B.
Published: July 2002

The reactions of N+ and C+ with H2, HD, and D2 have been studied as a function of translational energy and hydrogen temperature in a guided ion beam mass spectrometer. For the N+ systems, the cross sections for these slightly endothermic reactions depend significantly upon the temperature of the hydrogen reagent below ≊0.3 eV. The effect increases with increasing endothermicity for these reactions, with the maximum effect being a factor of 5 change in cross section for formation of NH+ in the reaction with HD at a kinetic energy of 0.02 eV. The branching ratio for formation of NH+ and ND+ in the reaction with HD at 0.02 eV is 1:3 at 305 K and 1:13 at 105 K. These effects are consistent with rotational energy driving the reaction. The moderately endothermic reactions of C+ with H2 and D2 have also been studied as a function of translational energy and hydrogen temperature. Cooling the hydrogen results in a sharper threshold for reaction because of reduced Doppler broadening and reduced hydrogen rotational energy. Results for these systems can be modeled by using phase space theory after accounting for Doppler broadening and internal energy effects.
Molecular dynamics simulation of ionic mobility. I. Alkali metal cations in water at 25 °C
Authors: Lee Song Hi , Rasaiah Jayendran C.
Published: July 2002

We describe a series of molecular dynamics simulations performed on model cation-water systems at 25 °C representing the behavior of Li+, Na+, K+, Rb+, and Cs+ in an electric field of 1.0 V/nm and in its absence. The TIP4P model was used for water and TIPS potentials were adapted for the ion-water interactions. The structure of the surrounding water molecules around the cations was found to be independent of the applied electric field. Some of the dynamic properties, such as the velocity and force autocorrelation functions of the cations, are also field independent. However, the mean-square displacements of the cations, their average drift velocities, and the distances traveled by them are field dependent. The mobilities of the cations calculated directly from the drift velocity or the distance traveled by the ion are in good agreement with each other and they are in satisfactory agreement with the mobilities determined from the mean-square displacement and the velocity autocorrelation function in the absence of the field. They also show the same trends with ionic radii that are observed experimentally; the magnitudes are, however, smaller than the experimental values in real water by almost a factor of 2. It is found that the water molecules in the first solvation shell around the small Li+ ion are stuck to the ion and move with it as an entity for about 190 ps, while the water molecules around the Na+ ion remain for 35 ps, and those around the large cations stay for 8–11 ps before significant exchange with the surroundings occurs. The picture emerging from this analysis is that of a solvated cation whose mobility is determined by its size as well as the static and dynamic properties of its solvation sheath and the surrounding water. The classical solventberg model describes the mobility of Li+ ions in water adequately but not those of the other ions.
An optimized united atom model for simulations of polymethylene melts
Authors: Paul Wolfgang , Yoon Do Y. , Smith Grant D.
Published: July 2002

We present here an optimized united atom model that is able to reproduce properties of melts of n-alkane chains of varying molecular weights. This model differs from previous models in that the Lennard-Jones well depth for the terminal methyl group (0.2264 kcal/mol) differs from that of the methylene units (0.093 kcal/mol). The position of the minimum is at 4.5 Å for both units. Properties of n-C44H90 melts from this model are compared with experiments and those from an explicit atom model. Good agreement with experiment is obtained for static properties of the melt, specifically P–V–T behavior, chain conformations, and x-ray scattering profiles. The large-scale dynamics, as measured by self-diffusion, are found to agree reasonably well with experimental results, being about 30% faster with our best united atom force field. Analysis of the end-to-end vector orientation autocorrelation function in terms of the Rouse model yields a monomer friction coefficient somewhat greater than that determined from the rate of self-diffusion, reflecting the fact that the n-C44H90 chains are not sufficiently long to behave as Gaussian coils. Detailed local chain dynamics for n-C44H90 melts, as measured by the P1(t) and P2(t) orientation autocorrelation functions for C–H vectors, are found to agree reasonably well with results from simulations using an explicit atom model, and yield spin-lattice relaxation times T1 and nuclear Overhauser enhancement values in reasonable agreement with experimental 13C NMR measurements. As with large scale dynamics, local dynamics are faster in general (about 20%) than experimental results.
The electron affinities of the silicon fluorides SiFn (n=1–5)
Authors: King Rollin A. , Mastryukov Vladimir S. , Schaefer Henry F.
Published: July 2002

Several independent density functional methods have been employed to determine the molecular structures and total energies of SiFn and SiF−n (n=1–5). Three significant measures of neutral–anion separation are reported: the adiabatic electron affinity, the vertical electron affinity, and the vertical detachment energy of the anion. The first Si–F ligand dissociation energies D(Fn−1Si–F), D(Fn−1Si−–F), and D(Fn−1Si–F−) as well as the harmonic vibrational frequencies of SiFn and SiF−n are also reported. Trends in the predictions of the different DFT methods are discussed. Self-consistent Kohn–Sham orbitals were obtained using various exchange correlation functionals and a double-ζ plus polarization basis set augmented with diffuse s-type and p-type functions. The method (BHLYP) based upon the Becke half-and-half exchange functional and the Lee–Yang–Parr correlation functional predicts molecular geometries in best agreement with experiment, while the other methods tend to produce bond lengths that are slightly longer. The BHLYP vibrational frequencies are also superior to those obtained via the other three DFT methods utilized. In previous studies of AFn molecules, the density functional methods have predicted electron affinities a few tenths of an electron volt above experiment with the BHLYP value being the best. Although experimental information concerning the electron affinities of the SiFn series is scarce, the BHLYP method does continue to predict electron affinities that are lower than the other methods. The BHLYP adiabatic electron affinities are 0.83 eV (SiF), 0.42 eV (SiF2), 2.50 eV (SiF3), and −0.22 eV (SiF4). The (unknown) Si–F bond distances for the anions are 0.076–0.088 Å longer than their respective neutral counterparts. The SiF−3 anion is more strongly pyramidal than SiF3. The SiF−4 species is predicted to lie energetically above SiF4, despite its experimental identification. No significantly bound minimum was found for the SiF5 molecule, although SiF−5 is predicted to exist and M+SiF−5 has been experimentally observed. The predicted vertical detachment energy of SiF−5 is 8.54 eV with the BHLYP method.
First-, third-, and fifth-order resonant spectroscopy of an anharmonic displaced oscillators system in the condensed phase
Authors: Tanimura Yoshitaka , Okumura Ko
Published: July 2002

We have obtained Nth-order response functions for a two-level system described by displaced anharmonic potential surfaces coupled to a heat bath. The anharmonicity of the potentials has been taken into account as a perturbation of harmonic potentials. The heat-bath was assumed to be an ensemble of harmonic oscillators. Coupling between the two-level system and the bath was assumed to be bilinear. The calculations were done analytically using the Liouville-space generating functional, which had been obtained by way of the path-integral approach. The response functions have been defined in terms of line-shape functions with these line-shape functions being expressed in terms of the bath spectral density and the temperature. We have carried out model calculations of the first-, third-, and fifth-order optical processes for various parameters of anharmonicity to show that anharmonicity plays a minor role in linear absorption, impulsive pump–probe, and photon echo experiments, but plays a major role, in some cases, in fifth-order two-dimensional resonant spectroscopy which is proposed in this paper.
The dynamic mean-field density functional method and its application to the mesoscopic dynamics of quenched block copolymer melts
Authors: Fraaije J. G. E. M. , van Vlimmeren B. A. C. , Maurits N. M. , Postma M. , Evers O. A. , Hoffmann C. , Altevogt P. , Goldbeck-Wood G.
Published: July 2002

In this paper we discuss a new generalized time-dependent Ginzburg-Landau theory for the numerical calculation of polymer phase separation kinetics in 3D. The thermodynamic forces are obtained by a mean-field density functional method, using a Gaussian chain as a molecular model. The method is especially aimed at describing the formation kinetics of the irregular morphologies which are typical for many industrial systems. As proof of concept we present the formation of irregular morphologies in quenched symmetric and asymmetric block copolymer melts.
Detecting wave packet motion in pump–probe experiments: Theoretical analysis
Authors: Cao Jianshu , Wilson Kent R.
Published: July 2002

The Zewail–Bersohn model [Ber. Bunsenges. Phys. Chem. 92, 373 (1988)] of pump–probe experiments is generalized to nonstationary wave packets and more realistic forms of probe pulses. The analysis illustrates the important role of probe linear chirp rate, as pointed out by Sterling, Zadoyan, and Apkarian [J. Chem. Phys. 104, 6497 (1996)], in detecting the motion of wave packets and the physical reason for the existence of optimal probe pulses to yield the best probe signal. Since the pump–probe process can be viewed as delayed two-photon resonant absorption, the probe signal can be readily optimized within the framework of quantum control theory, as discussed by Yan [J. Chem. Phys. 100, 1094 (1994)]. Numerical calculations based on quantum control theory are used to confirm our theoretical predictions. We point out that the same analysis can be extended to other impulsive nonlinear optical processes, such as multiphoton pump–probe absorption and stimulated Raman scattering.
Gaussian–Markovian quantum Fokker–Planck approach to nonlinear spectroscopy of a displaced Morse potentials system: Dissociation, predissociation, and optical Stark effects
Authors: Tanimura Yoshitaka , Maruyama Yutaka
Published: July 2002

Quantum coherence and its dephasing by coupling to a dissipative environment play an important role in time-resolved nonlinear optical response as well as nonadiabatic transitions in the condensed phase. We have discussed nonlinear optical processes on a multi-state one-dimensional system with Morse potential surfaces in a dissipative environment. This was based on a numerical study using the multi-state quantum Fokker–Planck equation for a colored Gaussian–Markovian noise bath, which was expressed as a hierarchy of kinetic equations. This equation can treat strong system-bath interactions at a low temperature heat bath, where quantum effects play a major role. The approach applies to linear absorption measurements as well as four-wave mixing including pump-probe spectroscopy. Laser induced photodissociation and predissociation have been studied for the potential surfaces of Cs2. We have calculated nuclear wave packets in Wigner representation and their monitoring by femtosecond pump-probe spectroscopy for various displacements of potentials and heat-bath parameters. Numerical calculations of probe absorption spectra for strong pump pulse are also presented and discussed. The results show dynamical Stark splitting, but, in contrast to the Bloch equations which contain an infinite-temperature dephasing, we find that at finite temperature their peaks have different heights even when the pump pulse is on resonance.
Can imaginary instantaneous normal mode frequencies predict barriers to self-diffusion?
Authors: Gezelter J. Daniel , Rabani Eran , Berne B. J.
Published: July 2002

We discuss whether or not local information on the potential energy surface embodied by the distribution of unstable instantaneous normal modes can be used to predict the hopping rates and barrier heights for Zwanzig’s model of self-diffusion [R. Zwanzig, J. Chem. Phys. 79, 4507 (1983)] in simple liquids. Results from a set of simulations of Lennard-Jones particles done at multiple temperatures and densities are presented. These simulations show that the theories which predict diffusive barrier heights from the distribution of imaginary frequencies are questionable. This discrepancy is due to the presence of imaginary frequency instantaneous normal modes which persist into the solid phase. Model systems are used to show that imaginary frequency instantaneous normal modes (and even those at the top of the barrier along that mode) are not necessarily indicators of diffusive barrier crossing as used in Zwanzig’s model. These false barriers are shown to be the cause of all of the imaginary frequency zero-force modes in the solid as well as many of the imaginary frequency modes in the high-density super-cooled liquid. We therefore dispute their utility as predictors of barrier heights or hopping rates in related liquid systems. We also show that attempts to separate the modes that are truly diffusive from those with false barriers using a frequency cutoff or local information on the potential energy surface are not successful at removing all of the non-barrier modes.
Absorption spectroscopy of SiH2 near 640 nm
Authors: Escribano Rafael , Campargue Alain
Published: July 2002

The Ã 1B1–X̃ 1A1 absorption spectrum of SiH2 has been observed using intracavity laser absorption spectroscopy with an equivalent path length of up to 13.0 km and the Ã 1B1(0,  0,  0)–X̃ 1A1(0,  0,  0) band near 640 nm recorded for the first time. The silylene radical was generated in a continuous discharge in a flowing mixture of silane in argon, giving a concentration of the order of 1010 SiH2/cm3. The spectrum spans the region between 15350 and 16100 cm−1. Rotational transitions have been assigned to levels up to J=16 and Ka=9, with ΔKa up to 5, ΔKc up to 4. Perturbations have been detected in the spectrum, due to Renner–Teller and spin-orbit interactions between both electronic states and the 3B1 state, predicted to be between them. However, the strength of the irregular perturbations affecting the rotational states of Ã 1B1(0,0,0) state is found to be much weaker than that affecting the other (0,  v2′, 0) levels previously studied. The analysis of the spectrum has allowed the determination of the rotational constants of the 1B1 (0,0,0) level, and a new estimation of those of the vibrational and electronic ground state. The geometry of the excited electronic level has also been determined for the first time from accurate experimental data. A change in the structure of this molecule takes place with this transition, the equilibrium angle opening from 92° to 122.4°, while the bond distance is reduced from 1.51 to 1.485 Å.
Critical point and phase behavior of the pure fluid and a Lennard-Jones mixture
Authors: Potoff Jeffrey J. , Panagiotopoulos Athanassios Z.
Published: July 2002

Monte Carlo simulations in the grand canonical ensemble were used to obtain liquid-vapor coexistence curves and critical points of the pure fluid and a binary mixture of Lennard-Jones particles. Critical parameters were obtained from mixed-field finite-size scaling analysis and subcritical coexistence data from histogram reweighting methods. The critical parameters of the untruncated Lennard-Jones potential were obtained as Tc*=1.3120±0.0007, ρc*=0.316±0.001 and pc*=0.1279±0.0006. Our results for the critical temperature and pressure are not in agreement with the recent study of Caillol [J. Chem. Phys. 109, 4885 (1998)] on a four-dimensional hypersphere. Mixture parameters were ε1=2ε2 and σ1=σ2, with Lorentz–Berthelot combining rules for the unlike-pair interactions. We determined the critical point at T*=1.0 and pressure-composition diagrams at three temperatures. Our results have much smaller statistical uncertainties relative to comparable Gibbs ensemble simulations.
On the heats of formation of formyl cyanide and thioformyl cyanide
Authors: Nguyen Thanh Lam , Nguyen Minh Tho
Published: July 2002

Ab initio molecular orbital calculations have been applied to determine the standard enthapies of formation of formyl cyanide and thioformyl cyanide. Using electronic energies at the coupled cluster single double (triple)/ 6-311++G(3df,2p) level in conjunction with different working reactions, the following values have been obtained: ΔHf,2980(HCOCN)=56 kJ/mol and ΔHf,2980(HCSCN)=271 kJ/mol, with a probable error of ±8 kJ/mol. These values differ significantly from the experimental ones of 26±20 and 222±30 kJ/mol, respectively, recently derived from mass spectrometric measurements by Born et al. [J. Phys. Chem. 100, 17662 (1996)].
Polymer translocation through a hole
Authors: Muthukumar M.
Published: July 2002

The average residence time τ of a polymer of length N passing through a narrow hole under a chemical potential gradient is calculated. In the proposed model, details of the hole parametrize the rate constant k0 for transporting a monomer across the hole, independent of the chain length. We show that any asymmetry in the average conformations of the polymer across the hole is sufficient to generate the driving force for the polymer translocation. If chemical potential gradient is absent, τ∼N2, with the proportionality constant depending on the size exponent of the polymer before and after the translocation. For translocation along the chemical potential gradient Δμ, τ is proportional to N(T/k0Δμ) and N2/k0, respectively, for large and small NΔμ/T. For translocation against the chemical potential gradient, τ∼exp(N) for long polymers.
Collapse of flexible polyelectrolytes in multivalent salt solutions
Authors: Solis Francisco J. , de la Cruz Monica Olvera
Published: July 2002

The collapse of flexible polyelectrolytes in a solution of multivalent counterions is studied by means of a two state model. The states correspond to rodlike and spherically collapsed conformations. We focus on the very dilute monomer concentration regime where the collapse transition is found to occur when the charge of the multivalent salt is comparable (but smaller) to that of the monomers. The main contribution to the free energy of the collapsed conformation is linear in the number of monomers N, since the internal state of the collapsed polymer approaches that of an amorphous ionic solid. The free energy of the rodlike state grows as N ln N, due to the electrostatic energy associated with that shape. We show that practically all multivalent counterions added to the system are condensed into the polymer chain, even before the collapse.