bluuebunny/crossref_metadata_2025 huggingface dataset:
Soft magnetic properties of Fe based amorphous thick sheets with large glass forming ability
Authors: Inoue A. , Makino A. , Mizushima T.
Published: July 2002

The thermal stability of the supercooled liquid, glass forming ability (GFA) and magnetic properties were examined for amorphous Fe72−xAl5Ga2P11C6B4Six, Fe72Al5Ga2P11−xC6B4Six, and Fe72Al5Ga2P11C6−xB4Six alloys. The increases in the thermal stability and GFA and the improvement of the soft magnetic properties were recognized in the replacements of P by 1 to 2 at. % Si and of C by 1 at. % Si. The supercooled liquid region ΔTx defined by the difference between Tx and Tg increases from 53 K for Fe72Al5Ga2P11C6B4 to 58 K for Fe72Al5Ga2P11C5B4Si1. The maximum thickness for glass formation (tmax) by copper mold casting also increases from 1 mm for the Fe–Al–Ga–P–C–B alloy to 3 mm for the Fe72Al5Ga2P10C6B4Si1 alloy. The increases in ΔTx and tmax are presumably because of the increase in the degree of the satisfaction of the three empirical rules for glass formation. The soft magnetic properties are also improved by the replacement of 1% Si through the increase in the squareness ratio of B–H loop (Br/Bs) and the decrease in coercive force (Hc). The best soft magnetic properties for the bulk amorphous alloys are 1.14 T for Bs, 1.5 A/m for Hc, 0.45 for Br/Bs and 594 K for Tc for the Fe72Al5Ga2P10C6B4Si1 alloy. The success of forming the Fe based ferromagnetic bulk amorphous alloys of 3 mm in thickness is promising for future development as a new type of magnetic material.
Effect of magnetic annealing on the behavior of FeTaN films
Authors: Viala B. , Inturi V. R. , Barnard J. A.
Published: July 2002

A study of the effect of field annealing on soft magnetism in FeTaN films is presented. Changes with temperature (Ta) for longitudinal field annealing (LFA) and with time (ta) for transverse field annealing (TFA) are described. For LFA of 60 min, film properties are essentially unchanged up to 300 °C. Higher Ta leads to grain growth and TaN precipitation, along with substantial reduction of the magnetization and poorer soft magnetism. Changes in stress and magnetostriction are also observed. LFA also leads to an increase of the structure constant S. Consequently, lower μi and higher Hc are reported; Hk is gradually reduced. TFA has been performed at 150 °C, as a function of ta. A drastic change in Hk is observed, decreasing with increasing ta, switching from positive to negative values after 60 min and finally stabilizing after 120 min. The sign change of Hk indicates a complete rotation (90°) of the easy axis (EA). μi is highly sensitive to the change in Hk and can be significantly increased by lowering Hk. It is concluded that TFA can be instrumental in improving the softness of FeTaN films, but such treatment can also lead to a rotation of the EA, raising some concerns about the thermal stability of such materials.
Switching field interval of the sensitive magnetic layer in exchange-biased spin valves
Authors: Rijks Th. G. S. M. , Reneerkens R. F. O. , Coehoorn R. , Kools J. C. S. , Gillies M. F. , Chapman J. N. , de Jonge W. J. M.
Published: July 2002

The switching field interval, ΔHs, of Ni–Fe–Co-based thin films and spin-valve layered structures, sputter-deposited on a Ta-buffer layer, was studied. The switching field interval is the field range in which the magnetization reversal of a ferromagnetic layer takes place. In thin films, ΔHs is determined by the uniaxial anisotropy, induced by growth in a magnetic field. This anisotropy increases with the ferromagnetic layer thickness and saturates at a thickness of 10–25 nm. It also depends on the alloy composition as well as on the choice of the adjacent layers. In exchange-biased spin valves, an additional contribution to ΔHs was observed, which increases monotonically with increasing interlayer coupling. We explain this in terms of the effect on the magnetization reversal of the sensitive layer due to a simultaneous small, but temporary, magnetization rotation in the exchange-biased layer and lateral variations of the interlayer coupling. In addition, the effect of biquadratic coupling on ΔHs is discussed. Finally, the thermal stability of ΔHs is investigated.
Failure of planar fiber networks
Authors: Räisänen V. I. , Alava M. J. , Nieminen R. M.
Published: July 2002

We study the failure of planar random fiber networks with computer simulations. The networks are grown by adding flexible fibers one by one on a growing deposit [K. J. Niskanen and M. J. Alava, Phys. Rev. Lett. 73, 3475 (1994)], a process yielding realistic three dimensional network structures. The network thus obtained is mapped to an electrical analogue of the elastic problem, namely to a random fuse network with separate bond elements for the fiber-to-fiber contacts. The conductivity of the contacts (corresponding to the efficiency of stress transfer between fibers) is adjustable. We construct a simple effective medium theory for the current distribution and conductivity of the networks as a function of intra-fiber current transfer efficiency. This analysis compares favorably with the computed conductivity and with the fracture properties of fiber networks with varying fiber flexibility and network thickness. The failure characteristics are shown to obey scaling behavior, as expected of a disordered brittle material, which is explained by the high current end of the current distribution saturating in thick enough networks. For bond breaking, fracture load and strain can be estimated with the effective medium theory. For fiber breaking, we find the counter-intuitive result that failure is more likely to nucleate far from surfaces, as the stress is transmitted more effectively to the fibers in the interior.
Excitonic quantum confinement effects and exciton electroabsorption in semiconductor thin quantum boxes
Authors: Gotoh Hideki , Ando Hiroaki
Published: July 2002

We investigate the excitonic optical properties in thin quantum boxes in the intermediate regime between the two-dimensional (2D) and zero-dimensional (0D) with a theoretical analysis that rigorously treats excitonic confinement effects. It is found that the exciton binding energy is substantially enhanced and that the oscillator strength concentrates to the lowest excitonic transition, even in a thin box whose lateral width is considerably (about five times) larger than the Bohr radius. Novel optical properties experimentally observed in semiconductor quantum disks, which are the intense photoluminescence spectrum and ultranarrow photoluminescence excitation spectrum, are explained well by the theoretical results. We also calculate exciton absorption in a thin box in which an electric field is applied in the lateral direction. The present theory can simulate how the electroabsorption evolves from the quantum confined Stark effect in the 0D to the quantum confined Franz–Keldysh effect in the 2D with an increase in the lateral size of the box. In the intermediate regime between 2D and 0D, a strong excitonic electric-field effect, distinct from the well-known electroabsorption effects at 0D and 2D, is found. These theoretical results demonstrate that even though the lateral confinement is weak, it considerably enhances the electron–hole Coulomb interaction and alters excitonic optical features markedly in the thin quantum box.
Investigation of silicon oxide films prepared by room-temperature ion plating
Authors: Yeh Ching-Fa , Chen Tai-Ju , Fan Ching-Lin , Kao Jiann-Shiun
Published: July 2002

To develop excellent silicon oxide film using low temperature method, ion plating (IP) oxide is investigated. Physicochemical characterizations of the IP oxide are studied using ellipsometry, Fourier transform infrared spectrometry, and P-etch rate measurement. The IP oxide is a high-density dielectric with strained bonds. Electrical characterizations are also analyzed using capacitance–voltage and current–voltage techniques through metal-oxide-semiconductor capacitors. The IP oxide has a low leakage current, a high breakdown field, and low interface state density. In addition, IP oxide annealed in N2 ambient is also studied. After high-temperature annealing, the characteristics of IP oxide become comparable to those of thermal oxide. The novel oxide film is successfully applied as a gate insulator to low-temperature processed (⩽620 °C) polysilicon thin-film transistors.
Temperature dependence of the fundamental absorption edge in CuIn3Se5
Authors: Marin G. , Wasim S. M. , Rincón C. , Sánchez Pérez G. , Power Ch. , Mora A. E.
Published: July 2002

From the study of the temperature dependence of the optical absorption spectra, the energy gap EG of CuIn3Se5 between 10 and 300 K are calculated using the model proposed by Elliot. This variation is compared to the semiempirical relation suggested by Manoogian–Woolley. The Debye temperature ΘD, the dielectric constant ε0, and the effective masses of free excitons mex, electrons me, and holes mh are estimated from the analysis of the adjustable parameters of these models.
Model for the chemical erosion of graphite due to low-energy H+ and D+ impact
Authors: Mech B. V. , Haasz A. A. , Davis J. W.
Published: July 2002

A methane erosion yield model has been developed using the principal atomistic reactions outlined by Küppers and co-workers [eg., A. Horn, A. Schenk, J. Biener, B. Winter, C. Lutterloh, M. Wittmann, and J. Küppers, Chem. Phys. Lett. 231, 193 (1994)] with additional terms to account for the energy of the incident particles, namely, kinetic ejection and damage deposition. Furthermore, modifications were made to the previous models by using distributed activation energies for methyl and hydrogen release as well as an activated Eley-Rideal abstraction process. Fitting of this model to experimentally measured methane yield data shows excellent agreement, except for low energy (⩽25 eV) impact at temperatures above ∼800 K. We have provided a sound physical basis for the behavior of the free fitting parameters and conclude that most of the processes associated with low-energy impact on pyrolytic graphite leading to methane production have been incorporated. Possible extensions of the model to include heavy hydrocarbons and total chemical erosion yields are also discussed. Due to the lack of comprehensive experimental data on the flux dependence of hydrocarbon erosion, the flux dependence of the fitting parameters could not be explored and so the effect of flux density requires further modeling considerations.
Charged defects and ferroelectricity in (1−x)PbZrO3−x(Na1/2Bi1/2)TiO3 solid solution
Authors: Lee Jung-Kun , Youn Hyuk-Jun , Hong Kug Sun , Park Seung-Eek
Published: July 2002

The crystal structure, dielectric/ferroelectric properties, and domain configuration of (1−x) PbZrO3−x(Na1/2Bi1/2)TiO3 solid solution with 0.14⩽x⩽0.4 were investigated using x-ray powder diffraction, D-E hysteresis, εr(T) measurements, and transmission electron microscopy. A sluggish transition from normal ferroelectric to relaxor state was observed as a function of composition. With an increasing amount of (Na1/2Bi1/2)TiO3, this transition was accompanied by changes in crystal structure (rhombohedral to pseudocubic), domain size and morphology (normal micron-size domain to polar nanodomain), and thermal phase transition behavior (normal ferroelectric transition to diffuse transition behavior with polarization relaxation). For intermediate compositions (i.e., x=0.25) between normal ferroelectric (x=0.16) and relaxor (x=0.4), a spontaneous relaxor-to-normal ferroelectric transition occurred at 80 °C in the εr(T) curve. It is suggested that this transition originates from the introduction of charged defects such as NaPb′ and BiPb•, resulting in the disturbance of long-range interactions among polar ordered BO6-octahedra and short-range structural order of charged defects (i.e., NaPb′−BiPb•).
A comparison of coprecipitation with microemulsion methods in the preparation of magnetite
Authors: Lee Hyo S. , Lee Woo C. , Furubayashi T.
Published: July 2002

Coprecipitation and microemulsion were used to prepare nanosized magnetite. The average particle sizes of samples A and B were calculated by Brunauer–Emmett–Teller analysis and are 8.16 and 2.24 nm, respectively. The saturation magnetization of sample A at 300 K was 57.2 emu/g, but sample B was not saturated even 7.0 T. The blocking temperature for sample B was 17 K and the crystalline anisotropy energy was 1.2×105 J/m3. At 5 K, sample B is ferrimagnetic with σs=7.62 emu/g. This value is much lower than the σs=65.4 emu/g of the crystallized sample A at 50 K. Mössbauer spectra showed that sample A was ferrimagnetic at room temperature, but sample B was superparamagnetic at T>Tb (critical blocking temperature).
Irreversibility of the antiferroelectric to ferroelectric phase transition in (Pb0.90Ba0.10)ZrO3 ceramics
Authors: Pokharel Bhadra P. , Pandey Dhananjai
Published: July 2002

Dielectric and x-ray diffraction evidences are presented to show that the antiferroelectric to ferroelectric phase transition in (Pb0.90Ba0.10)ZrO3 ceramics is not reversible during the cooling cycle. It is shown that the stable antiferroelectric phase recovers from the metastable ferroelectric matrix on aging at room temperature. The kinetics of recovery of the antiferroelectric phase is very sluggish. It is pointed out that this type of irreversibility occurs in field induced antiferroelectric to ferroelectric transitions also. This may have serious implications for the actuator applications of these materials. It is proposed that the large transformation strains associated with the antiferroelectric orthorhombic to ferroelectric rhombohedral phase transition is responsible for this irreversibility.
Measurements of laser driven spallation in tin and zinc using an optical recording velocity interferometer system
Authors: Moshe E. , Eliezer S. , Dekel E. , Henis Z. , Ludmirsky A. , Goldberg I. B. , Eliezer D.
Published: July 2002

Measurements of the dynamic strength, in tin and zinc, shocked by a high power pulsed laser to tens of kilobars pressures are reported. The strain rates in these experiments are of the order of 107 s−1, higher by two-to-three orders of magnitude than those reached with conventional shock generators like plane impacts or explosives. The free surface velocity time history, which is related to the spallation process, was measured with an optical recording velocity interferometer system. This diagnostic technique is noninterfering and provides a highly accurate continuous measurement in time. The spall strength estimated from the free surface velocity profile was compared with the theoretical upper limit for the spall strength, calculated from a wide range equation of state for metals.
Co
2+
:
ZnS
and Co2+:ZnSe saturable absorber Q switches
Authors: Tsai Tzong-Yow , Birnbaum Milton
Published: July 2002

The materials, Co2+:ZnS and Co2+:ZnSe, serving as Q switches with an Er3+: glass laser at 1.54 μm have been demonstrated and analyzed. The output pulses were typically 15–65 ns in duration and of 1–4 mJ depending on the doping concentration and the crystal quality. The ground-state absorption cross section of Co:ZnS has been determined to be 7.7×10−19 cm2 and 7.6×10−19 cm2 for Co:ZnSe, by bleaching experiments.
Effects of pump heating on laser and spectroscopic properties of the Nd:[YAl3(BO3)4] self-frequency-doubling laser
Authors: Jaque D. , Capmany J. , Rams J. , Solé J. Garcı́a
Published: July 2002

In this work, thermal effects on the spectroscopic and laser properties of Nd:YAl3(BO3)4 crystal caused by pump heating are systematically investigated. Changes in the refractive indices, absorption coefficient, and emission band shape have been studied in a temperature range 20–200 °C. Temperatures up to 88 °C have been measured in a thermally isolated crystal when pumping with 800 mW of laser power at 807 nm. A decrease in infrared laser power at 1.06 μm and its second harmonic (0.53 μm) generated by self-frequency doubling is observed during the early seconds of laser operation. These reductions are explained in terms of the absorption efficiency reduction and of thermal phase mismatch (for second harmonic) induced by pump heating. Instabilities observed in the output (infrared and green) laser power during the initial seconds of laser action are satisfactorily explained in terms of Fabry–Pérot-induced interference effects.
Domain wall traps for low-field switching of submicron elements
Authors: McMichael R. D. , Eicke J. , Donahue M. J. , Porter D. G.
Published: July 2002

In magnetic random access memory, power consumption depends on the coercivity of the magnetic elements in the memory cells. In this article a new method is described that uses a “domain wall trap” element shape to reduce both the coercivity and the dependence of coercivity on element size in submicron magnetic elements. Micromagnetic simulations of a shaped permalloy element show coercivity less than one tenth the coercivity calculated for a rectangular permalloy element of the same size. The switching times for the domain wall traps are shown to be comparable to those of rectangular elements.
Linewidth analysis of the photoluminescence from InAs/GaSb/InAs/AlSb type-II superlattices
Authors: Ongstad Andrew P. , Dente Gregory C. , Tilton Michael L. , Gianardi Donald , Turner George
Published: July 2002

We present photoluminescent (PL) linewidth measurements on InAs:GaSb, type-II superlattices as a function of temperature and power. The observed PL linewidth for the samples, studied at 80 K, was 40–60 meV, which is significantly larger than a thermally broadened line of width 2 kT. The larger linewidth is well explained by a combination of homogeneous and inhomogeneous broadening. The data suggest that the inhomogeneous broadening is dominated by interface roughness and that the roughness amplitude at the InAs–GaSb interface is on the order of 1 ML. A significant fraction of the broadening can be accounted for by the presence of interfacial regions which show a 1 ML decrease in the GaSb layer thickness. To account for homogeneous broadening, a Lorentzian function of width δ (full width at half maximum) is employed to smooth and broaden the synthetic spectrum which is calculated from a simple model of the spontaneous emission rate. A δ=10 meV was found to give the best fit of the synthetic spectra to the observed spectra. The spectral distribution was also found to be dependent on excitation power, and the data indicate that the photogenerated carriers preferentially occupy and fill the low energy regions of the heterointerface.
Effect of Si doping on strain, cracking, and microstructure in GaN thin films grown by metalorganic chemical vapor deposition
Authors: Romano L. T. , Van de Walle C. G. , Ager J. W. , Götz W. , Kern R. S.
Published: July 2002

The effect of Si doping on the strain and microstructure in GaN films grown on sapphire by metalorganic chemical vapor deposition was investigated. Strain was measured quantitatively by x-ray diffraction, Raman spectroscopy, and wafer curvature techniques. It was found that for a Si concentration of 2×1019 cm−3, the threshold for crack formation during film growth was 2.0 μm. Transmission electron microscopy and micro-Raman observations showed that cracking proceeds without plastic deformation (i.e., dislocation motion), and occurs catastrophically along the low energy {11_00} cleavage plane of GaN. First-principles calculations were used to show that the substitution of Si for Ga in the lattice causes only negligible changes in the lattice constant. The cracking is attributed to tensile stress in the film present at the growth temperature. The increase in tensile stress caused by Si doping is discussed in terms of a crystallite coalescence model.
The lower electronic states of ClOO: A computational investigation
Authors: Jafri Jawed A. , Lengsfield Byron H. , Bauschlicher Charles W. , Phillips Donald H.
Published: July 2002

Eight doublet and eight quartet states of ClOO were investigated by ab initio CI techniques. The potential energy surfaces of the four lowest energy doublet states of both A″ and A′ symmetry indicate that only the 1 2A″ state is bound. In contrast to the model provided by the HO2 radical, all of the excited doublet states investigated were repulsive with respect to dissociation to Cl+O2 and metastable or bound with respect to dissociation to ClO+O. The transitions to the excited states investigated span the visible and near UV spectral regions, but the transition moments indicate that they are very weak. Since the photolysis products are the same as those of the rapid thermal dissociation, photolysis is not expected to be an important atmospheric process. The soft bending potential for the 1 2A′ state and the shape of the 1 4A″ state in the entrance channel of the ClO+O → Cl+O2 reaction provide a qualitative explanation for the underprediction of the low temperature reaction rate by previous trajectory calculations.
Kinetic oscillations in the catalytic CO oxidation on Pt(100): Theory
Authors: Imbihl R. , Cox M. P. , Ertl G. , Müller H. , Brenig W.
Published: July 2002

Experimental investigations on the catalytic oxidation of CO on well-defined Pt(100) surfaces at low pressures (∼10−4 Torr) revealed under certain conditions the occurrence of kinetic oscillations which were associated with periodic transformations of the surface structure propagating wave-like across the surface area. Based on experimental information about the various elementary steps involved (adsorption, desorption, surface reaction, adsorbate-induced surface structural transformation) a set of coupled differential equations describing the kinetics of the various processes was established. A simplified version of these equations allows analytical treatment and provides qualitative insight into the occurrence of oscillations through linear stability analysis. Numerical solution of the full set of reaction-diffusion equations reproduces the essential spatio-temporal features of the experimental observations, although full quantitative agreement would still require better adjustment of the parameters involved as well as further refinement of the model.
Near-resonant electronic-to-rotational energy transfer in Rb(7S→5D)–H2, D2 collisions at thermal energy
Authors: Cuvellier J. , Petitjean L. , Mestdagh J. M. , Paillard D. , de Pujo P. , Berlande J.
Published: July 2002

The intermultiplet transfer Rb (7S→5D) induced by H2, D2, and He has been studied as a function of collision velocity using a crossed beam apparatus for which the initial rotational distribution of the molecular perturber was determined. A calibration using the blackbody induced redistribution of the excited states populations in the Rb beam allowed the absolute value of the cross section to be obtained. A situation where a near resonance exists (perturber H2) is compared with situations where near resonances do not occur (perturbers D2 and He). For low collision velocities (VR≤4000 m/s), the near-resonant process Rb(7S)+H2( j=1)→Rb(5D)+H2( j=3) has a much larger cross section than competing nonresonant processes (rotationally elastic or inelastic). Its cross section decreases with the relative velocity as ∼1/V2R. This behavior is compared with the predictions of two theoretical models (the impulse approximation model, which leads to a quantitative comparison with the measured velocity dependence, and the long range forces model). Above 4000 m/s the behavior of H2 closely resembles that of D2 and He where no near resonances exist. This suggests that for high velocities, the intermultiplet transfer is due to nonresonant processes only.
Quantum mechanical calculation of the longitudinal elastic modulus and of deviations from Hooke’s law in polyethylene
Authors: Suhai S.
Published: July 2002

The microscopic longitudinal elastic modulus, deviations from Hooke’s law, the maximum of the retractive force and tensile strength are calculated for single infinite polyethylene chains using a first principle crystal orbital method and including electron correlation effects by perturbation theory. The best theoretical value of the modulus is 334 GPa at the single-particle level and 276 GPa including correlation. Significant deviations from Hooke’s law start at small deformations and amount to 30%–40% at ε=0.10–0.15. The maximum of the retractive force is f=0.814 mdyn at ε=0.38 and f=0.631 mdyn at ε=0.31, using restricted or unrestricted Hartree–Fock states as zeroth-order wave functions for perturbation theory, respectively. The corresponding maxima of the microscopic tensile strength are at 44.6 and 34.2 GPa, respectively.
Structural study of small catalytic particles using differential anomalous x-ray scattering
Authors: Liang K. S. , Laderman S. S. , Sinfelt J. H.
Published: July 2002

Using the differential anomalous x-ray scattering (DAXS) technique, the diffuse diffraction pattern associated with small Pt particles has been successfully separated from the strong scattering of crystalline η-Al2O3 as well as amorphous SiO2 supports. For samples with ≳2.5 wt. % Pt loading, the differential diffraction patterns unambiguously reveal that the Pt clusters are fcc microcrystals. The results demonstrate the feasibility of DAXS technique in structural study of catalytic particles and its advantage over EXAFS in revealing long-range structure.
The ionic correlation contribution to the free energy of spherical double layers
Authors: Woodward Clifford E. , Jönsson Bo , Åkesson Torbjörn
Published: July 2002

An estimate of the ion fluctuation contribution to the free energy for a dispersion of spherical macroions, without additional salt, is made using the cell model. The Poisson–Boltzmann approximation is used to estimate the linear response in a given cell, which together with second order perturbation theory allow us to calculate the free energy of interaction between a pair of cells. As well, an infinite order theory is developed. We find excellent agreement with simulations for monovalent counterions, the comparison becoming poorer for more strongly coupled systems. A cubic lattice of cells is studied, modeling a macroionic dispersion. The fluctuation contribution is shown to be several orders of magnitude larger than the usual quantum dispersion forces. However, it plays a minor role in the thermodynamics of the lattice.
The effects of cross-linking on the equation of state of a polymer solution
Authors: Horkay F. , Hecht A. M. , Geissler E.
Published: July 2002

Measurements of the swelling pressure ω̄ and shear modulus Gs in a set of poly(vinyl acetate) networks swollen to different degrees in toluene and in acetone are reported, using solutions of the uncross-linked polymer to obtain deswelling under known conditions of osmotic pressure. The swelling pressure can be completely described by the difference between two terms, each of which is a simple power law in the polymer volume fraction φ. Identification of the subtractive term with that related to the elastic free energy of the network gives the volume elastic modulus Gv. The shear modulus Gs, obtained from mechanical measurements at constant volume, and Gv are found to coincide for these samples, and neither deviates measurably from a one-third power law dependence on φ, up to values of φ in excess of 0.4. The remaining term in the swelling pressure of the networks behaves like the mixing term in a polymer solution, obeying good solvent scaling predictions as a function of concentration in both diluents. Its magnitude, however, is substantially smaller than the osmotic pressure of an equivalent uncross-linked solution of infinite molecular weight. The difference between the cross-linked and uncross-linked states conflicts with the assumption that the two mixing free energies are identical.
Solving the discretized time-independent Schrödinger equation with the Lanczos procedure
Authors: Groenenboom Gerrit C. , Buck Henk M.
Published: July 2002

A new method is presented to find bound state solutions of the one-, two-, or three-dimensional Schrödinger equation. The equation is turned into a sparse matrix eigenvalue problem by representing the potential energy surface and the wave function on a grid. The Laplacian is represented by a high (10th) order finite difference formula. Eigenvalues are found by the Lanczos procedure [J. Cullum and R. A. Willoughby, J. Comp. Phys. 44, 329 (1981)] and transition probabilities (Franck–Condon factors) are found by the recursive residue generation method [A. Nauts and R. E. Wyatt, Phys. Rev. Lett. 51, 2238 (1983)]. Examples are given for the 1D Morse oscillator and the 2D Hénon-Heiles potential. Numerical convergence can be checked easily and highly accurate results can be obtained. The algorithm is fast, easy to implement, and vectorizable.
Equation of state for fluids of spherical particles in d dimensions
Authors: Song Yuhua , Mason E. A.
Published: July 2002

An analytical equation of state for a d-dimensional fluid is presented, obtained as a generalization of a successful result for d=3. Agreement with available computer simulations and virial coefficients for d=1,2,4, and 5 indicates that the generalization is accurate.
Calculation of relative free energy via indirect pathways
Authors: Mark Alan E. , van Gunsteren Wilfred F. , Berendsen Herman J. C.
Published: July 2002

A general method is presented to reduce the simulation time required to compute the relative free energy between two states X and Y of a molecular system by computer simulation. Although the free energy difference ΔAx→y is, in principle, independent of the pathway chosen to change X into Y, in practice its choice strongly affects the accuracy of the obtained ΔA value. The optimum path is the one for which the relaxation time of the system τsystem attains a minimum, allowing the system to remain as close as possible near equilibrium during a simulation. Downscaling the relevant parts of the potential energy function before the change from X to Y is made, and upscaling afterwards is a rather general way to shorten τsystem and thus save computing time. For a model system of butane like molecules the proposed procedure is more than 1 order of magnitude more efficient than the conventional technique of direct interconversion from state X to state Y.
Reactions of boron cluster ions (B+n, n=2–24) with N2O: NO versus NN bond activation as a function of size
Authors: Hintz Paul A. , Sowa Marianne B. , Ruatta Stephen A. , Anderson Scott L.
Published: July 2002

Cross sections for reaction of mass-selected boron cluster ions (B+n, n=2–24) with N2O are reported for collision energies from 0.1 to 10 eV. The major product channels are addition of a single nitrogen or oxygen atom to the intact cluster ion. For small clusters, there are no activation barriers and cross sections are large, however, as cluster size increases, bottlenecks and activation barriers reduce reactivity substantially. Significant size effects are observed in the product branching distributions. The dominant reaction mechanism at low collision energies is proposed to be complex formation, where only one bond in N2O is broken, followed by desorption of the stable leaving group (N2 or NO). Reactions with boron cluster ions larger than 16 atoms in size have been studied for the first time, and in addition, supporting evidence is given for our previous suggestion that B3 has an anomalously high ionization potential. Comparisons are made with other oxidation reactions of boron cluster ions, and with aluminum and silicon cluster ion reactions with N2O.
The overlooked bond-stretching contribution in free energy perturbation calculations
Authors: Pearlman David A. , Kollman Peter A.
Published: July 2002

We examine the validity of an assumption commonly made when carrying out free energy perturbation (FEP) calculations: The contributions to the free energy difference from intraperturbed group interactions are negligible. We show: (A) that these contributions are not, in fact, necessarily negligible; and (B) that, in particular, the bond-stretching contribution resulting from changes in equilibrium bond lengths is important, but can be extremely difficult to calculate using standard procedures. We show how this contribution can be determined using an alternative ‘‘pseudopotential of mean force’’ correction. We also examine the behavior of the calculated free energy with the length of the simulation, and conclude that simulation times significantly longer than those typically used at present (e.g., ≥200 ps) may be required to yield reliable and accurate results.
A study of the ferroelectric phase transition of vinylidene fluoride–trifluoroethylene copolymers by ac calorimetry
Authors: Mizuno Hideki , Nagano Yatsuhisa , Tashiro Kohji , Kobayashi Masamichi
Published: July 2002

The ferroelectric phase transition in vinylidene fluoride–trifluoroethylene (VDF–TrFE) random copolymers was studied by ac calorimetry (ACC) and differential scanning calorimetry (DSC). By this combined approach, a higher-order component of the phase transition in the VDF 52% copolymer was separated successfully as a symmetric heat capacity peak with a response faster than 3.6 Hz. The ACC thermogram of the VDF 65% copolymer does not show any heat capacity peak. This disappearance of heat capacity peaks is interpreted in terms of the Martensitic transition. A method of obtaining the kinetic constant of the phase transition from the heating frequency dependence of sample heat capacity is also proposed. On the basis of this method, a value of the kinetic constant for the slower component of the phase transition in the VDF 52% copolymer was determined to be 2 s.
Crystallization properties of a supercooled metallic liquid
Authors: Qi D. W. , Lu J. , Wang S.
Published: July 2002

With the aid of molecular-dynamics technique, a calculation of the thermodynamic and structural properties of the crystallization of a supercooled metallic liquid is presented. The calculated results demonstrate a number of features of this crystallization. It now appears that the transition associated with this crystallization is a first-order-like phase transition in the Ehrenfest scheme.
State-to-state spin–orbit and rotational energy transfer of very highly vibrationally excited nitric oxide
Authors: Yang Xueming , Wodtke Alec M.
Published: July 2002

PUMP–DUMP–PROBE experiments were performed on nitric oxide (NO) in order to study the rotational and spin–orbit energy transfer in a highly vibrationally excited diatomic. Total depopulation as well as state-to-state rotational and spin–orbit energy transfer rate constants were measured for v=8 and 19 of the ground electronic state. By comparing this new set of results with previously measured results for v=2 [Aa. Sudbo and M. M. T. Loy, J. Chem. Phys. 76, 3646 (1982)] the vibrational influence on the rotational and spin–orbit energy transfer dynamics could be evaluated. Total depopulation rate constants for individual rovibrational states were found to be very similar for v=2, 8, and 19. State-to-state rotational energy transfer rate constants could be compared for v=8 and 19, and again only small differences were found. It appears that vibrational energy does not have a significant impact on the rotational and spin–orbit energy transfer dynamics for NO–NO collisions.
Study of the hyperfine coupling constants (14N and 1H) of the NH2 molecules in the X 2B1 ground state and the A 2A1 excited state
Authors: Engels B. , Perić M. , Reuter W. , Peyerimhoff S. D. , Grein F.
Published: July 2002

The hyperfine coupling constants (hfcc) Aiso and Aij are calculated for the atoms of NH2 in its two lowest-lying electronic states at various molecular geometries by means of the ab initio multireference configuration interaction method. The vibronically averaged values of the hfccs for the K=0 and 1 levels in 14N 1H2 in the energy range up to 20 000 cm−1 are computed. Polarization effects which determine Aiso as well as a simple model to describe the dipolar hfccs are discussed. All results are in excellent agreement with experimental data.
Beam-gas study of bismuth–fluorine reactions using laser-induced fluorescence of BiF
Authors: Oberlander M. D. , Parson J. M.
Published: July 2002

Relative rotational and vibrational populations have been extracted by computer simulation of laser excited fluorescence spectra of BiF (X0+) formed in a thermal molecular beam of bismuth reacting with molecular fluorine under single-collision conditions. The observed rotational distribution is colder than the prior distribution calculated assuming three product fragments. The observed vibrational distribution is also cold, indicating that comparatively little energy is available to the ground state molecules probed. Since the dependence of the detected BiF (X0+) product on the beam time of flight points toward Bi2 as the major beam reactant, the product energy distributions presented here suggest a mechanism in which BiF (X0+), Bi (4S), and F (2P) are formed in an end-to-end attachment of Bi2 and F2. There were no indications that BiF (X0+) can be readily formed by reaction of atomic bismuth.
An ab initio investigation of the significance of the HOOF intermediate in coupling reactions involving FOOx and HOx species
Authors: Francisco J. S.
Published: July 2002

Ab initio calculations are used to investigate the stability and role of HOOF in the reaction of FO with HO radicals. The heat of formation for HOOF is estimated as 0.4±2 kcal mol−1 using an isodesmic reaction scheme. Spectroscopic properties of the HOOF intermediate is also provided in order to facilitate its identification. Decomposition pathways of the intermediate are examined. The lowest energy pathway is the formation of F atoms and HO2 radicals and requires 27.2 kcal mol−1 to proceed. Reactions leading to the formation of the HOOF intermediate are examined in regard to their importance in understanding stratospheric chemistry involving the coupling of fluorine and fluorine oxide with HOx species in catalytic cycles.
Observation of the one-exciton to two-exciton transition in a J aggregate
Authors: Fidder Henk , Knoester Jasper , Wiersma Douwe A.
Published: July 2002

We report on the first observation of the one-exciton to two-exciton transition in J aggregates. A theoretical analysis supports our interpretation.
Laser-induced gratings in free jets. I. Spectroscopy of predissociating NO2
Authors: Butenhoff Thomas J. , Rohlfing Eric A.
Published: July 2002

We use two-color laser-induced grating spectroscopy (LIGS) to obtain double-resonance, absorptionlike spectra of jet-cooled NO2 below and above the threshold for predissociation at 398 nm. The grating-laser frequency is tuned through the dissociation threshold while the probe-laser frequency is fixed to a specific rotational line of an isolated, cold vibronic band well below threshold. The signal beam diffracts off the ground state depletion grating formed when the grating laser excites a transition out of the rotational level selected by the probe laser. The above-threshold spectra observed for NO2 indicate that, under collision-free conditions in the free jet, the LIGS spectrum maps the square of the absorption spectrum. We determine predissociation lifetimes from LIGS linewidths and discuss the temporal decay of the induced grating as a function of its orientation with respect to the jet axis. Finally, we consider the effects of phase matching on two-color LIGS in the forward geometry and derive expressions for the variation in signal-beam angle and for the phase matching bandwidths for experiments in which the frequency of either the grating or probe laser is scanned.
Shape-resonance-enhanced continuum–continuum coupling in photoionization of CO2
Authors: Siggel M. R. F. , West J. B. , Hayes M. A. , Parr A. C. , Dehmer J. L. , Iga I.
Published: July 2002

We have measured photoionization branching ratios and photoelectron asymmetry parameters for photoionization of CO2 leading to the first four electronic states of CO2+ over the photon energy range from 20 to 50 eV. The motivation for this work was the prediction by Lucchese [J. Chem. Phys. 92, 4203 (1990)] that the σu shape resonance in the (4σg)−1 C̃ 2Σg+ ionization channel would influence the photoionization dynamics in the other valence-shell continua through continuum–continuum channel interaction, with the main effect occurring in the (3σu)−1 B̃ 2Σu+ channel. Indeed, clear evidence for this phenomenon is observed in this channel, the most prominent indication being a broad, shallow minimum in the asymmetry parameter at ∼40 eV. Comparisons of the present results with theory and other measurements reflect good overall agreement and provide some guidance regarding the effectiveness of alternative approximations used in the theoretical calculations.
The car-parking limit of random sequential adsorption: Expansions in one dimension
Authors: Bartelt M. C. , Evans J. W. , Glasser M. L.
Published: July 2002

We consider the irreversible random sequential adsorption of particles taking k sites at a time, on a one-dimensional lattice. We present an exact expansion for the coverage, θ(t,k)=A0(t)+A1(t)k−1+A2(t)k−2+..., for times, 0≤t≤O(k), and at saturation t=∞. The former is new and the latter extends Mackenzie’s results [J. Chem. Phys. 37, 723 (1962)]. For these expansions, we note that the coefficients Ai≥1(∞) are not obtained as large-t limits of the Ai≥1(t). Finally, we comment on the Laurent expansions for general O(k)<t<∞, which reveal the occurrence of additional kn terms, with n≳0.
Optical dephasing of organic dye molecules doped in cross-linked polyvinyl alcohol derivatives: Incoherent photon echo and hole-burning studies
Authors: Nakanishi Shunsuke , Miyawaki Yuuichi , Nishikawa Mitsuo , Amano Masayuki , Fujiwara Seiji , Jitou Mayumi , Itoh Hiroshi , Kawase Masaya
Published: July 2002

Optical dephasing of the zero-phonon line (ZPL) of organic dye molecules doped in polyvinyl alcohol (PVA) and several cross-linked PVA derivatives was studied by using the incoherent photon echoes and the photophysical persistent hole-burning. It was found in the incoherent photon echo measurements that optical dephasing time of the ZPL increases with increasing the length of a cross-linker introduced to the PVA backbone. The difference in the temperature dependence of the dephasing time was also observed between a dye doped in PVA with and without the cross-link. When a longer cross-linker was introduced, the phonon sideband component in the incoherent photon echo signal became dominant in contrast to that in PVA with a relatively short cross-linker, while the dephasing time of the ZPL was still longer than that in PVA without the cross-link. Hole-burning studies showed that the phonon sideband spectrum of a dye doped in the cross-linked PVAs is essentially similar to that in PVA. However, the depth of the zero-phonon hole burnt with the same burning power was found to depend on the PVA hosts with the cross-linkers. These our findings suggest that the introduced cross-link does not alter the electron–phonon coupling modes or pseudo-local mode, but it drastically reduces the effect of the two-level tunneling systems in PVA on the optical dephasing of a doped dye. As a result, the dephasing time of the dye in the cross-linked PVAs becomes longer than that in PVA without the cross-link. The decrease of the optical dephasing by introducing the cross-link can be interpreted by assuming a void space in PVA, which is created near to the cross-linker. In addition, absorption and fluorescence spectra in these systems were measured, which imply the occurrence of weak dye–dye interaction when the dye is doped in PVA with a much longer cross-linker.
High pressure range of the addition of HO to HO, NO, NO2, and CO. I. Saturated laser induced fluorescence measurements at 298 K
Authors: Forster R. , Frost M. , Fulle D. , Hamann H. F. , Hippler H. , Schlepegrell A. , Troe J.
Published: July 2002

Saturated laser induced fluorescence is used for the sensitive detection of radicals in high pressure gases. The method and its application to a series of addition reactions of HO radicals in the high pressure regime are described. Experiments between 1 and 150 bar of the bath gas He allow for falloff extrapolations to the high pressure limit of the recombination reactions. Limiting rate constants (in cm3 molecule−1 s−1) of 2.2×10−11 for HO+HO→H2O2, of 3.3×10−11 for HO+NO→HONO, of 7.5×10−11 for HO+NO2→HONO2, and of 9.7×10−13 for HO+CO→HOCO (and H+CO2) are derived at 298 K.
Competition between vitrification and crystallization of methanol at high pressure
Authors: Brugmans Marco J. P. , Vos Willem L.
Published: July 2002

We have studied methanol at high pressure up to 33 GPa at room temperature with x-ray diffraction, optical (polarization) microscopy, Raman spectroscopy, and detection of hydrostaticity. A competition between crystallization and vitrification is observed when methanol is superpressed beyond the freezing pressure of 3.5 GPa: between 5.0 and 10.5 GPa crystals can nucleate, but if this region is surpassed quickly enough (within a few seconds), methanol remains amorphous. For the first time the nucleation rate and the crystal growth velocity have been studied as a function of pressure. These kinetic properties can be described by classical nucleation theory in agreement with, respectively, Turnbull–Fisher and Wilson–Frenkel type behavior using one and the same activated hard-sphere diffusion coefficient. The experimental nucleation rate and the crystal growth velocity are both effectively reduced to zero above 10.5 GPa, because the diffusion is suppressed. At these pressures methanol is compressed into a glass.
The identification of the FeH2 radical in the gas phase by infrared spectroscopy
Authors: Körsgen Helga , Mürtz Petra , Lipus Klaus , Urban Wolfgang , Towle Jonathan P. , Brown John M.
Published: July 2002

The FeH2 radical has been detected in the gas phase through the observation of transitions in the fundamental band of the antisymmetric stretching vibration ν3. The molecule was generated in an electrical discharge through a mixture of hydrogen and iron pentacarbonyl in helium and the signals were recorded by the technique of laser magnetic resonance. It is established that the molecule is linear in its ground state and that this state is 5Δ in character; theoretical calculations suggest that it has gerade symmetry. The vibrational band origin is determined to be 1674.7196(4) cm−1 and the rotational constant for the (0,0,0) level is 3.01328(3) cm−1. This latter value corresponds to an r0 Fe–H bond length of 0.1665 nm.
Theoretical characterization of the low-lying excited states of the CuCl molecule
Authors: Sousa C. , de Jong W. A. , Broer R. , Nieuwpoort W. C.
Published: July 2002

The character of the low-lying excited states of diatomic CuCl is studied primarily by means of the complete active space self-consistent field (CASSCF), method and a second order perturbation approach with the CASSCF wave function as reference state [complete active space perturbation theory to second order (CASPT2)]. For comparison, the lower levels of the spectra of the Cu+ ion are also analyzed. A first order treatment of the scalar relativistic effects, the mass–velocity and Darwin terms, is included in the calculations. The importance of spin–orbit interactions is investigated by comparing our nonrelativistic valence shell CI (VCI) and relativistic results obtained with our four-component program suite MOLFDIR. The six lowest excited states of the CuCl molecule, which are related to the Cu+(3d94s1)Cl−(3s23p6) ionic configuration, are assigned. The assignments agree with earlier theoretical work. Where they can be compared, the calculated spectroscopic constants are in good agreement with the experimental data.