bluuebunny/crossref_metadata_2025 huggingface dataset:
Creep and electrical resistivity of metallic glass Ni78B14Si8 under proton irradiation
Authors: Jung P.
Published: July 2002

Plastic deformation and electrical resisitivity of the metallic glass Ni78B14Si8 were measured during 6.3 MeV proton irradiation at ≈420 K under tensile stresses up to 430 MPa. Irradiation creep rate depended linearly on stress σ and particle flux Φ (or displacement rate K), giving ε*/σΦ≈4.5×10−33 m2/Pa (ε*/σK≈5×10−9 Pa−1 dpa−1). After recrystallization this value was reduced by a factor of ≈25, falling into the range of polycrystalline pure nickel and dilute Ni alloys. The electrical resisitivity in the amorphous state was decreasing under irradiation, while the initially lower resistivity of recrystallized material strongly increased.
Structure and interface-controlled growth kinetics of ZnAl2O4 formed at the (112̄0) ZnO/(011̄2) Al2O3 interface
Authors: Gorla C. R. , Mayo W. E. , Liang S. , Lu Y.
Published: July 2002

The solid state reaction between metalorganic chemical vapor deposition grown epitaxial ZnO films and the R-plane sapphire substrate after annealing at 1000 °C for various times in an O2/N2 atmosphere was studied in detail. Multiple epitaxial relationships between the reaction product (ZnAl2O4) and the reactants were observed, as determined by cross-sectional transmission electron microscopy. In the dominant epitaxial relationship (A1), the (22̄0) plane of ZnAl2O4 was parallel to the (1̄101) plane of Al2O3. A twin (A2) of orientation A1, i.e. (22̄0) ZnAl2O4//(101̄1) Al2O3, and a closely related orientation (B) wherein the (22̄0) ZnAl2O4 plane is parallel to the (1̄21̄0) ZnO plane (which is equivalent to a 5° clockwise rotation about the [1̄1̄2] ZnAl2O4 or [0001] ZnO zone axis relative to A2), were also observed. Enhanced growth was observed at grain boundaries. It was necessary to measure the spinel growth rate from grains with the same orientation far away from grain boundaries because the growth rate was observed to be influenced by the orientation of the grains in addition to the enhanced growth at grain boundaries. The growth rate was observed to follow a linear rate law during early stages (for grains with orientation A1), suggesting an interface-controlled reaction. The structures of the ZnO/Al2O3, ZnO/ZnAl2O4 and ZnAl2O4/Al2O3 interfaces were studied for grains with this orientation (A1). The 13.7% lattice mismatch between ZnO and ZnAl2O4 was relieved by a series of misfit dislocations spaced five to six (11̄00) ZnO planes apart. Due to the small lattice misfit (2.1%) at the ZnAl2O4/Al2O3 interface, very few misfit dislocations were present. This interface was faceted and the sapphire surface had a series of single steps. It is expected that the reaction at the ZnAl2O4/ZnO interface is the rate-controlling step due to the necessity for a dislocation climb (of a large number of misfit dislocations) for movement of this interface.
Rapid-annealing effect on the microstructure and magnetic properties of the Fe-rich nanocomposite magnets
Authors: Kojima Akinori , Makino Akihiro , Inoue Akihisa
Published: July 2002

The rapid-annealing effect on the microstructure and magnetic properties of the nanocomposite Fe93-x-yCoxNb2(Nd, Pr)yB5 (x=0–20, y=5–7 at. %) alloys produced by crystallization of an amorphous phase have been investigated. The melt-spun ribbons consist of an amorphous phase in the as-quenched state and the amorphous phase changes to a nanocomposite structure consisting mainly of bcc-(Fe, Co), (Nd, Pr)2(Fe, Co)14B and residual amorphous phases after annealing at temperatures of 973–1023 K. The nanocomposite alloys exhibit improved values of remanence (Jr), coercive force (HcJ), and maximum energy product [(BH)max] upon annealing at a higher heating rate (α) in the temperature range corresponding to the primary crystallization temperature of bcc-Fe phase. As α increases, the grain sizes of each phase decrease, especially for the Nd2Fe14B phase, and the ratio of total surface area of the Nd2Fe14B to bcc-Fe phases (Shard/Ssoft), which are evaluated assuming each grain is a sphere, becomes close to 1 for the Fe88Nb2Nd5B5 alloy. This result indicates that the homogeneity of soft and hard magnetic phases is improved by rapid annealing, and it causes the improvement of hard magnetic properties of the Fe-rich nanocomposite alloys.
Identification of vector Preisach models from arbitrary measured data using neural networks
Authors: Adly A. A. , Abd-El-Hafiz S. K. , Mayergoyz I. D.
Published: July 2002

It has been recently shown that the identification process of scalar, as well as some family of vector, Preisach-type models may be accomplished by the aid of artificial neural networks. Our purpose in this article is to further generalize the approach through which neural networks may be utilized for the identification of vector Preisach models, while using arbitrary measured data for a magnetic recording tape sample. Using the proposed approach, the identification process has been performed and additional simulations have been carried out and compared to corresponding measurements. Comparison results suggest that the proposed technique can lead to good agreement between measured and predicted values.
Heat treatment effects on sensitivity and hysteresis loops of magnetoelastic torque transducers
Authors: Boley Mark S. , Franklin Doug A. , Rigsbee David K.
Published: July 2002

Recent experiments have indicated that a torque transducer can be constructed based on local bands of naturally stabilized remanent circumferential magnetization within a hollow steel shaft. A strong coercive force along with the crystalline anisotropy combine to stabilize the circumferential magnetization of the sensing area of the shaft. A torqued shaft has its magnetic axis tilted into a helical orientation, which generates an axial field signal linearly proportional to the applied torque that can be sensed externally to the shaft. Our research has shown that the transducer function can be improved by appropriate sequential heat treatments in a helium atmosphere. Subsequent to all heat treatment steps, the test shafts were measured to determine the changes in axial and circumferential magnetic hysteresis properties and to track the development of the magnetic sensing area to applied torque (sensitivity). Although little change was produced in the originally narrow axial hysteresis loops, a 10%–20% decrease was observed in both axial coercive force and remanent magnetization, and correspondingly, a 50% increase was observed in sensitivity, accompanied by a higher linear saturation limit. Additionally, the heat treatment cycle significantly widened the originally broad circumferential hysteresis loops, resulting in a substantial increase in circumferential coercive force, which improves the stability of the sensory region.
Annealing temperature dependence of permeability spectra and asymmetrical giant magnetoimpedance in Co-based amorphous ribbon
Authors: Jang Kil Jae , Kim Cheol Gi , Kim Ho Chul , Yu Seong Cho , Shin Kyung Ho
Published: July 2002

The permeability spectra and giant magnetoimpedance (GMI) profiles have been measured as a function of annealing temperature, Ta in field-annealed Co-based amorphous ribbon. The permeability spectra show the superposition of two dispersions of Debye type for Ta⩽200 °C. As Ta increases over 250 °C, the low frequency dispersion begins to decrease, and eventually disappears for Ta⩾350 °C due to the exchange coupling with the bias field formed in surface crystalline layer. However, the high frequency dispersion increases with Ta up to 350 °C and then becomes nearly constant. The GMI profile measured at 0.1 MHz shows a one peak in the samples of Ta⩽300 °C. As Ta increases over 350 °C, the GMI-valve is revealed due to the influence of bias field on wall motion. As the Ta increases over 450 °C, the asymmetric two peaks of GMI profile are shown instead of the GMI valve due to the damping of wall motion after the crystallization.
Thermodynamic analysis of InxGa1−xN growth conditions in molecular beam epitaxy
Authors: Li Yanqiu , Koukitu Akinori , Seki Hisashi
Published: July 2002

This article presents a thermodynamic analysis of how the growth parameters are linked together to influence the growth of InxGa1−xN. The calculation results reveal that the growth temperature and the ratio of input group V to group III flux (V/III) most strongly influence the InxGa1−xN growth processes. The optimized growth conditions are determined by the dependence of the growth parameters on the equilibrium partial pressures of desorbed species as well as by ternary phase diagrams. The results agree with the published experimental data.
Effect of magnetic exchange interactions in La(Sr)MnO3/LaMO3 (M=Fe,Cr,Co,Ni) artificial superlattices and their magnetic/ electrical properties
Authors: Tanaka Hidekazu , Kawai Tomoji
Published: July 2002

We have fabricated conductive-ferromagnetic [(La0.8Sr0.2)MnO3/antiferromagnetic-[LaFeO3] spin-frustrated superlattices and insulative-ferromagnetic [La1−δMnO3]/magnetic [LaMO3] (M=Ni,Co,Cr,Fe) superlattices through accurate unit cell control (1–10 units) using a laser-molecular beam epitaxy technique. In [(La,Sr)MnO3]m/[LaFeO3]n superlattices, it has been observed that resistivity is increased and TC is reduced from 270 to 130 K when the antiferromagnetic layer is increased. Metal–insulator transition occurred systematically in a series of superlattices (m=2) with n increasing from 1 to 3 unit cells. This can be explained by spin frustration induced by the neighboring antiferromagnetic spin order. This colossal magnetoresistance effect is enhanced by up to 35% in a magnetic field of 1.0 T. In [LaMnO3]1/[LaMO3]1 superlattices, reduction (M=Cr,Fe) and enhancement (M=Co,Ni) of TC have been observed. We propose an expression for TC on the basis of the molecular field image and demonstrate that the ferromagnetism is systematically affected by the neighboring magnetic layers via the interface.
Monte Carlo transition-state study of angular momentum effects on the unimolecular dissociation of CH4 on the Duchovic–Hase–Schlegel ab initio surface
Authors: Viswanathan R. , Raff L. M. , Thompson Donald L.
Published: July 2002

A previously formulated Monte Carlo transition-state theory approach to unimolecular reactions has been extended to the calculation of microcanonical rate coefficients for specific angular momentum states. The method is applied to a study of two-center dissociation of CH4 on the Duchovic–Hase–Schlegel (DHS) ab initio potential-energy surface. The rotationally averaged microcanonical rate coefficients on the DHS surface are found to be consistently less than those previously calculated on a semiempirical surface due principally to the larger C–H bond dissciation energy on the DHS surface. The rate coefficients are found to be relatively insensitive to the other topographical features of the potential-energy surface. Angular momentum is found to reduce k(E;J) by an amount in excess of the centrifugal effect. The magnitude of the angular momentum effect is found to decrease as the ratio of the rotational energy to the energy in excess of threshold decreases. Overall, the MCTST procedure is found to be computationally efficient for such calculations.
High energy vibrational states of cyanoacetylene: Variational and perturbative analysis of an anharmonic normal mode
Authors: Hutchinson John S.
Published: July 2002

The high energy vibrational eigenstates of cyanoacetylene are calculated with variational and perturbation theory, using a new empirical potential energy function. A detailed analysis of the eigenstates reveals that the HC bond in HCCCN is not a good local mode, even as a zeroth-order description, due predominantly to the linear geometry of the molecule. A unique anharmonic normal mode description is developed and applied to calculate accurately the eigenenergy spectrum with second-order perturbation theory. An indirect coupling scheme is presented which accounts for mixing of nearly degenerate but uncoupled high energy zeroth-order states. These results are used to account for spectral congestion in the experimental high energy spectrum of cyanoacetylene.
Stiffness and excluded-volume effects in polymer chains
Authors: Yamakawa Hiromi , Shimada Jiro
Published: July 2002

In order to examine the correlation between stiffness and excluded-volume effects in polymer chains, polymethylene-like rotational isomeric state chains with excluded volume are simulated and their mean-square radii of gyration are evaluated by Monte Carlo methods. The present results and also experimental data for ordinary flexible chains may well be explained by the Kratky–Porod worm-like chain, or more generally the helical worm-like chain, with the excluded-volume effects incorporated in the Yamakawa–Stockmayer scheme. For typical stiff chains with excluded volume, however, there is clear disagreement between their theory and experiment, and the problem remains unsolved.
Dynamics of molecular reorientations: Analogies between quasielastic neutron scattering and deuteron NMR spin alignment
Authors: Fujara F. , Wefing S. , Spiess H. W.
Published: July 2002

The analogies between incoherent quasielastic neutron scattering and deuteron spin alignment in studying single particle reorientations are pointed out. Quasielastic neutron scattering measures the full van Hove correlation function for correlation times shorter than 10−8 s. Spin alignment represents an orientational single particle function for correlation times longer than 10−4 s. The analogy is based on the fact that geometrical information is obtained in both methods by manipulation of an appropriate phase factor. In neutron scattering this phase factor is Q⋅a in momentum space, where Q is the momentum transfer and a is a jump length. In spin alignment the phase factor is δQ⋅τ1 in time space where δQ specifies the strength of the quadrupole coupling and τ1 is a pulse spacing defining the evolution period. It is shown that the measurements of the elastic incoherent structure factor as a function of Q⋅R and the spin alignment echo height as a function of δQ⋅τ1, respectively, yields largely equivalent information about the type of reorientational motion—although at different time scales. Therefore, both methods nicely supplement each other. Some examples (two-, four-, and six-fold jumps of dimethylsulfone, hexamethylenetetramine, and hexamethylbenzene as well as rotational diffusion of polystyrene in its viscous state) are discussed explicitly, where the geometry and time scale of slow molecular reorientations are identified directly.
A vibrational state-selected study of the reaction H+2(v′0)+H2(v′′0=0)→H+3+H using the tandem photoionization mass spectrometry and radio frequency ion guide methods
Authors: Shao J. D. , Ng C. Y.
Published: July 2002

A new ion–molecule rection apparatus, which consists of a photoionization source, a tandem mass spectrometer, and a radio frequency octopole reaction cell is described. Using a quadrupole mass filter to reject H+3 background ions formed at the photoionization source, absolute total cross sections for the reaction H+2 (v′0) +H2 (v′0 =0)→H+3 +H, have been measured as a function of the vibrational state of reactant H+2, where v′0 =0–4, over the center-of-mass collision energy (Ec.m.) range of 0.04–15 eV. The experimental results are compared with phenomenological cross sections obtained in previous single gas cell studies, the quasiclassical trajectory calculations of Stine and Muckerman, and the recent similar calculations of Eaker and Schatz. The absolute total cross sections measured for v0 =0 and 3 at Ec.m. =0.5, 1, 3, and 5 eV are found to be in agreement with ‘‘trajectory surface hopping’’ calculations which include nonadiabatic surface hopping throughout the reaction.
The sulfur induced reconstruction of the Pt(S)-[6(111)×(100)] surface
Authors: Lanzillotto A-M. , Bernasek S. L.
Published: July 2002

The sulfur induced reconstruction of the Pt(S)-[6(111)×(100)] surface to other stable structures was investigated using low energy electron diffraction and Auger electron spectroscopy. These reconstructed surfaces involve a doubling of the clean surface terrace width and step height. The conditions of annealing temperature and sulfur coverage leading to the reconstructions were determined from this study. The mechanism for the reconstruction is strongly dependent on the preferential adsorption of sulfur at the step edge, and the reconstruction occurs even for very small sulfur coverages. The observed nucleation of (2×2) and (√3×√3) R30° sulfur overlayers by the step edge at relatively low sulfur coverages further supports this conclusion. Observation of the LEED pattern in real time in the 200–300 °C temperature region indicates that the temperature dependence of the sulfur mobility on the surface defines the reconstruction time. Several possible mechanisms for the reconstruction process are discussed, and the ranges of stability of the various reconstructions are indicated.
On the nature of the two low-lying singlet states of anthracene: Vibronic activity in the two-photon excitation spectrum
Authors: Salvi P. R. , Marconi G.
Published: July 2002

The two-photon excitation spectrum of anthracene in a n-heptane matrix at 10 K has been measured in the energy region 26 000–32 000 cm−1. Experimental evidence of two-photon band assignment to vibronically induced B1u ×b1u and B2u ×b2u transitions is given. In particular, the two-photon spectrum above ≂28 000 cm−1 shows several vibronic origins built on b2u vibrations and progressions of ag modes on them. The lowest ππ* absorption region (<28 000 cm−1) has, on the contrary, negligible intensity and very weak B1u ×b1u bands are observed. These data can be rationalized in terms of vibronic coupling between electronic states induced by normal

coordinates of b1u and b2u symmetry. By means of the Herzberg–Teller theory and displacing the molecule away from equilibrium along the normal coordinate we have calculated the vibronic interaction between electronic states in the orbital following approach. The results based on CNDO/S-CI wave functions show that B2u ×b2u transitions have a larger vibronic activity than the B1u ×b1u transitions. The 1132, 1384, and 1393 cm−1 b2u modes are particularly strong in inducing two-photon intensity through a vibronic coupling mechanism involving essentially the ground and the final 1 1B2u state. The B2u ×b2u two-photon amplitude tensors are not sensitive to the method of calculation. B1u ×b1u tensors have instead a more pronounced dependence on the method used. This is due to the fact that in the two-photon sums most of the intermediate states play an equivalent

role in determining the amplitude tensor, in contrast with the B2u ×b2u case. It is important to use a reasonably correct representation of the excited state wave functions. It is shown that more accurate calculations (INDO/S and CNDO/S with increased CI) lead to better agreement of the total vibronic intensity of the 1 1B1u state with experiment and predict the largest activity for the 648 cm−1 b1u mode, as observed in the spectrum.
Measurements of the diffusion coefficients of atomic chlorine in rare gases
Authors: Hwang C.-J. , Jiang R.-C. , Su T.-M.
Published: July 2002

The technique for measuring the diffusion coefficients of reactive atomic species through their radiative recombination reactions is applied to multicomponent mediums. A series of diffusion coefficients of chlorine atoms in chlorine molecules mixed with rare gases M(He, Ne, Ar, Kr, and Xe) were measured for various Cl2 and M mole fractions. By extrapolating to zero molecular chlorine pressure, the diffusion coefficients of chlorine atoms in rare gases were obtained. The measured values of the diffusion constant are DCl–He =0.75±0.12, DCl–Ne =0.32±0.05, DCl–Ar =0.19±0.03, DCl–Kr =0.14±0.02, and DCl–Xe =0.12±0.02 cm2/s, respectively, at 296 K and 1 atm rare gas pressure. The corresponding hard sphere collision diameter of chlorine atoms in rare gases was calculated and a comparison was made between the diffusion coefficients of Cl–rare gas and Ar–rare gas systems at 296 K.
Comparison of experiment and theory for the resonance Raman spectrum of I2 in solution. I. The Raman excitation profile of I2 in n-hexane
Authors: Sension Roseanne J. , Strauss Herbert L.
Published: July 2002

The absolute Raman excitation profiles (REPs) of the fundamental and first two overtones of I2 dissolved in n-hexane were determined with excitation frequencies from 15 000 to 22 000 cm−1. Calculations for both the absorption spectrum and the REPs were performed and the results compared to the experimental results. Good agreement was found using gas phase potentials for the A, B, and B″ states and a ground state potential modified to reproduce the observed anharmonicity of the I2 vibrations in n-hexane. The homogeneous linewidth in the excited electronic states is Γ=15–20 cm−1 (HWHM), which corresponds to a T2 of ≊0.3 ps. The spectra have an inhomogeneous component of 400 cm−1 (HWHM). Separate calculations demonstrate the effects of the excited electronic states individually and demonstrate the importance of the interference terms in the REP. Calculations also demonstrate the sensitivity of the depolarization ratio to the contributions of the various excited electronic states. The index of refraction corrections required in solution are close to those expected.
The microwave spectrum of the thiomethoxy radical CH3S
Authors: Endo Yasuki , Saito Shuji , Hirota Eizi
Published: July 2002

The pure rotational spectrum of the thiomethoxy radical CH3S was observed for the first time in the millimeter-wave region. The radical was produced in a 0.5 m long free-space cell by the reaction of CH3SH with 2450 MHz microwave discharge products of CF4. The rotational transitions were observed to be split into doublets by spin-orbit interaction, and each component of the doublets further into K structures spread over several hundred MHz by vibronic Coriolis interaction and vibronic (2,2)- and (2,−1)-type interactions which are characteristic of a symmetric-top molecule in an orbitally degenerate 2E electronic state. The magnetic hyperfine structure due to the three protons was clearly resolved and was analyzed by taking into account the pseudodipolar interaction which had not been considered previously. Ground-state molecular parameters were determined by least-squares analyzing the observed spectrum, and the C–S bond length was estimated to be 1.791 Å from the observed B rotational constant while assuming an appropriate structure for the methyl group.
An ESR investigation of the F−2 radical anion in neon matrices at 4 K
Authors: Knight L. B. , Earl Edward , Ligon A. R. , Cobranchi D. P.
Published: July 2002

The anion radical F−2 has been generated and trapped as a free ion in dilute neon matrices near 4 K and investigated by electron spin resonance (ESR). The magnetic parameters obtained in neon are: g∥ =2.0018(3); g⊥ =2.0185(3); A∥ =913.5(4), and A⊥ =−22.5(4) G. The neon ESR measurements are the first gas-like results and these are compared with earlier findings for F−2 in various crystalline environments. Analysis of the combined results reveal that F−2 is the most gas-like or least perturbed in the larger ionic lattices. The information obtained should be useful in estimating the degree of perturbation that typically exists for the many radicals which have been studied in ionic crystalline environments. Based upon these experimental findings for F−2 in neon matrices, an atomic value of Adip for a fluorine atom is proposed for estimating P orbital spin densities in ionic fluorine compounds. This experimental Adip for 19F is compared to commonly used theoretical values.
A simple quantitative model of hydrogen bonding
Authors: Spackman Mark A.
Published: July 2002

A simple model for the computation of intermolecular interactions is described. It consists of atom–atom potentials for the representation of repulsion and dispersion energies, and an evaluation of the electrostatic energy in terms of partitioned multipole moments of the monomer electron distributions. Applications are given in detail for hydrogen-bonded dimers of the molecules HF, HCl, CO, N2, Cl2, HCN, CO2, N2O, OCS, HCCH, NCCN, and HCCCN, and the results compared with ab initio and experimental results. Hydrogen bond energies are obtained to better than 4 kJ mol−1, intermolecular separations to typically better than 0.15 Å, and intermolecular angles within 5°, all compared with experiment. Force constants and vibrational frequencies are also well reproduced.
STF HF wave functions from Sc to Zn and STF HF wave function for Cu2
Authors: Tatewaki Hiroshi , Sekiya Masahiro
Published: July 2002

The SCF wave functions are calculated with Slater-type functions (STFs) from Sc to Zn. The obtained total energies agree with those of numerical Hartree–Fock (NHF) given by Fischer, while they are somewhat different (≤0.0005 a.u.) from those of NHF given by Hay. The resulting atomic HF wave function for Cu has been applied to the Cu2 molecule as the basis set.
Millimeter-wave optical double resonance spectra of NO2: How good a quantum number is N?
Authors: Coy Stephen L. , Lehmann Kevin K. , De Lucia Frank C.
Published: July 2002

Optical transitions to the 2B2 electronic state region of NO2 have been observed in double resonance with three millimeter-wave ground state rotational transitions. The probed ground state transitions were the N=2 (F1), N=4 (F1), and N=10 (F2), K=0→1 Q branch transitions near 250 GHz. The optical range scanned was 16 800 to 16 950 cm−1. The 100,10–101,9 F2 spectrum was combined with previous double resonance results with the ground state 91,9–100,10 F1 transition. This comparison shows 70 out of 160 upper states with J=19/2 to have indefinite or mixed N values. It establishes violations of the ΔN=ΔJ selection rule in this spectral region which contains bands with a wide range of spin-rotation constants. Lack of intensity correlation between the two spectra suggests two classes of upper states are present: a group of strongly N-mixed upper states, and a group essentially unmixed in N. Heller’s F parameter, which measures the fraction of all possible symmetry-restricted basis states contributing to a typical eigenstate, is similar for the double-resonance spectra of the three N values, but is greater for K=1 than for K=0. By finding upper states in common, R lines from N=2 and P lines from N=4 have been identified.
Brillouin scattering on noncrystalline ZnCl2
Authors: Soltwisch M. , Sukmanowski J. , Quitmann D.
Published: July 2002

Measurements of Brillouin scattering spectra from liquid and glassy (300–600 °C, 25–130 °C) ZnCl2 are reported. High resolution Fabry–Perot spectroscopy has been used, and temperature as well as momentum transfer have been varied so as to go completely across the condition (Brillouin frequency)⋅(characteristic relaxation time)=1, where very non-Lorentzian spectral shapes are observed. They are fully described by viscoelastic theory, i.e., a Mori-form including fourth moment, two sound velocities, and relaxation times. Connection with current ideas on glass-forming liquids is made.
A CASSCF-CCI study of the valence and lower excited states of the benzene molecule
Authors: Matos J. Mauricio O. , Roos Björn O. , Malmqvist Per-Åke
Published: July 2002

A b initio complete active space (CAS) SCF and contracted CI calculations have been carried out for all valence and the lower Rydberg states of the benzene molecule. The CASSCF active space comprised 12 π-type molecular orbitals and the basis set included both polarization functions and diffuse functions in order to describe properly both valence and Rydberg type orbitals. Resulting excitation energies for the Rydberg states are in close agreement with experiment. CASSCF results for the valence states give errors ranging from 0.0 for the covalent states up to more than 1.0 eV for the most ionic states. Inclusion of σ–π correlation effects reduces the errors in the ionic states to less than 0.6 eV. The 1E1u state is computed to lie 7.4 eV above the ground state with a transition moment of 1.70 a.u., experimental values are 7.0 eV and 1.61 a.u., respectively.
A computer simulation study of the effect of molecular mass distribution on mass and momentum transport in molecular liquids
Authors: Brown D. , Clarke J. H. R.
Published: July 2002

Molecular dynamics computer simulation has been used to determine diffusion coefficients and viscosities for four thermodynamical ly identical one component model liquids composed of rigid triatomic molecules of the type XY2 at the same temperature and density. Exactly the same intermolecular potential was used throughout. The molecules comprising these liquids all had the same total mass but the molecular mass distributions were different for each liquid. Differences in the viscosities and the diffusion coefficients clearly show that the molecular mass distribution is a factor which in itself influences transport properties, a point seldom considered in the past. The viscosities were determined by nonequilibrium molecular dynamics and a new method is described for implementing isokinetic shear flow in molecular liquid simulations which employ quaternion algorithms. To obtain satisfactory diffusion coefficients for the equilibrium liquids it was necessary to use simulations extending up to 300 ps. The results are particularly relevant to the phenomenon of rotation–translation coupling and, together with the linear and angular velocity autocorrelation functions, are discussed in this context.
The three-state lattice gas as model for binary gas–liquid systems
Authors: Meijer Paul H. E. , Napiórkowski Marek
Published: July 2002

This paper deals with the three-state lattice model as applied to binary liquid–gas systems. Schouten, ten Seldam, and Trappeniers [Physica 73, 556 (1974)] used this model to describe the transition between the liquid phases as well as the gas–gas separation when varying the interaction parameters of the model. Our analysis is aimed at the behavior of the system in the vicinity of its tricritical point in the generalized field space. Different shapes of the critical lines are calculated by using the molecular field method as well as by using the lowest approximation of the cluster variation method. The equivalence between the two approaches is demonstrated. In the immediate neighborhood of the tricritical point the shapes of the critical lines are also determined analytically.
Aggregation and percolation in a system of adhesive spheres
Authors: Seaton N. A. , Glandt E. D.
Published: July 2002

We have studied the aggregation and percolation behavior of a fluid of adhesive spheres by means of Monte Carlo simulations. Cluster statistics have been obtained as a function of temperature and density and compared with the virial results of Post and Glandt. The determination of the percolation threshold from computer simulations is discussed. The percolation threshold of the adhesive sphere system has been obtained as a function of temperature and compared with the Percus–Yevick solution of Chiew and Glandt. Agreement is good except where the Percus–Yevick solution would be expected to fail on physical grounds.
Fragmentation of acetone following excitation in the region of the oxygen K edge
Authors: Nelson M. C. , Murakami J. , Anderson S. L. , Hanson D. M.
Published: July 2002

The fragmentation of acetone following excitation in the region of the oxygen K edge was studied by time-of-flight mass spectroscopy. By comparing mass spectra obtained using various photon energies ranging from below to well above the threshold for oxygen 1s excitation, it is found that methyl ion production is strongly associated with excitation of an oxygen 1s electron. None of the excitations generated significant yields of atomic oxygen or carbon ions.
Time-dependent analysis of the Hartley absorption band and resonance Raman spectra in ozone
Authors: Johnson Bruce R. , Kinsey James L.
Published: July 2002

A detailed investigation is made of the short-time dynamics involved in the Hartley absorption band in ozone and the related resonance Raman spectra for low vibrational overtones. A quadratic expansion of the X 1A1 and dissociative 1B2 potential energy surfaces involved leads to an analytic calculation of three-mode vibrational correlation functions, which are then Fourier-transformed to obtain the absorption and Raman spectra. A type of time-dependent perturbation theory is used to approximate the effects of including anharmonic terms in the expansion of the upper state. It is concluded that an improvement in the description of the Raman spectra will require at the least a major modification of the parabolic barrier behavior along the antisymmetric stretch coordinate and, better, a large-amplitude-motion treatment.
Adiabatic compressibility and structure of aqueous solutions of methyl-alcohol
Authors: Onori G.
Published: July 2002

The adiabatic compressibility of methanol–water mixtures has been measured as a function of temperature and of alcohol mole fraction; the measurements were carried out at 10, 15, and 20 °C over the entire range of cosolvent concentration. A simple model, involving the solute–solvent interactions and the molecular aggregation of the solute, is used to explain the observed behavior.
Structure and phase equilibria of microemulsions
Authors: Andelman David , Cates M. E. , Roux D. , Safran S. A.
Published: July 2002

We present a simple phenomenological model to describe the phase equilibria and structural properties of microemulsions. Space is divided into cells of side ξ; each cell is filled with either pure water or oil. Surfactant molecules are presumed to form an incompressible fluid monolayer at the oil–water interface. The monolayer is characterized by a size-dependent bending constant K(ξ), which is small for ξ≥ξK, the de Gennes–Taupin persistence length. The model predicts a middle-phase microemulsion of structural length scale ξ≊ξK which coexists with dilute phases of surfactant in oil and surfactant in water. (These phases have ξ≊a, a being a molecular length.) On the same ternary phase diagram, we find also two regions of two-phase equilibrium involving upper- and lower-phase microemulsions that coexist with either almost pure water or oil. At low temperatures and/or high values of the bare bending constant, K0≡K(a), the middle-phase microemulsion may be entirely precluded by separation to a lamellar phase, whereas at high temperature and/or low values of K0, there is a first-order transition between a disordered microemulsion and a lamellar phase. In the absence of spontaneous curvature the phase diagram is oil–water symmetric. It may be asymmetrized by: (i) spontaneous curvature in the middle phase or (ii) a difference between the free energy of the two dilute phases. If the asymmetry is sufficiently large, the three-phase region disappears.
Rotational energy transfer within the B 3Πg v=3 manifold of molecular nitrogen
Authors: Ali Ashraf , Dagdigian Paul J.
Published: July 2002

An optical–optical double resonance experiment has been carried out to study rotationally inelastic collisions of N2 B 3Πg v=3 by argon with initial and final state resolution. Nitrogen molecules in the metastable A state are generated by collisional excitation transfer from metastable argon atoms in a flow system. Specific B state rotational levels are prepared by pulsed pump laser excitation of isolated rotational lines in the B 3Πg–A 3Σ+u (3,0) band near 688 nm. After a short delay, a probe laser interrogates the rotational populations in the B state by fluorescence excitation in the C 3Πu–B 3Πg (0,3) band near 406 nm. Collisional transfer from incident levels in all three spin–orbit manifolds of the B state was investigated. For molecules initially in the F1 (Ω=0) manifold, a preference for conservation of fine-structure label with even ΔJ changes was observed. This propensity is very pronounced for the J=0 level but is considerably relaxed for the higher levels investigated. By contrast, inelastic collisions involving the F2 (Ω=1) and F3 (Ω=2) manifolds do not exhibit a significant propensity to conserve fine-structure label. A slight residual preference for even ΔJ changes is observed in collisional transitions within the F2 manifold. These experimental results are compared to the propensity rules expected for homonuclear 3Π rotationally inelastic collisions, both in the case (a) and (b) limits. The reduction of the predicted propensities by the transition to intermediate case coupling and ‘‘orbital-correlated’’ scattering is discussed. The latter term refers to the difference potential for the N2–Ar interaction when the N2 unfilled π orbital is in or perpendicular to the triatomic plane. An unsuccessful attempt to detect collisional interelectronic transfer from the B state to the W 3Δu state by G–W laser fluorescence excitation is also reported.
An electron diffraction study of alkali bromide vapors
Authors: Hartley J. G. , Fink M.
Published: July 2002

The structures of NaBr, KBr, RbBr, and CsBr in the vapor phase were determined via standard electron diffraction counting techniques with nozzle temperatures ranging from 820–920 K. The data analysis yielded the structure parameters and the ratio of the monomeric to dimeric compounds. The mean amplitudes of vibration for the monomeric molecules were held at values based on microwave frequencies and the harmonic potential approximations. The corrected monomer distances re agree well with microwave–spectroscopy results. Rhombic geometries with bonded dimer distances ranging from 2.74 to 3.37 Å and Br–M–Br angles that ranged from 85° to 101.6° were observed. The percentage of dimer in the vapor was found to vary from 7% to 18%. The most surprising result was the large value obtained for the KBr dimer bonded distance. It is substantially larger than predicted by current theories and is nearly as large as the interionic distance in the crystal. Thermodynamic calculations were performed to allow comparisons of the dimer mole fractions observed with values derived from existing structure models.
The B 1Σ+u, B′ 1Σ+u, C 1Πu, and D 1Πu states of the H2 molecule. Matrix elements of angular and radial nonadiabatic coupling and improved a
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o potential energy curves
Authors: Wolniewicz L. , Dressler K.
Published: July 2002

The B, B′, C, and D states of H2 are represented over wide ranges of the internuclear distance R by considerably more flexible electronic wave functions than the ones previously employed by Kolos, Rychlewski, and one of us (L.W.). These are used to compute the nonadiabatic coupling matrix elements for the homogeneous B′–B and D–C and the heterogeneous C–B, C–B′, D–B, and D–B′ interactions over a wide range of R values. The adiabatic potential curves obtained from the new electronic wave functions yield vibrational term values which, for the v=0 levels, lie within 0.6 to 2.2 cm−1 of the experimental ones in these four electronic states of H2. After subtracting the nuclear-mass-dependent contributions to these ab initio errors, which can be estimated by comparing H2 with D2, the remaining electronic errors near the equilibrium internuclear distances of these electronic states are approximately 0.3 cm−1 (C and D state) and 1 cm−1 (B and B′ state). In the C, D, and B′ states this represents improvements by 0.7, 6, and 8 cm−1, respectively, over the previous ab initio values.
Microscopic observation of ordered colloids in sedimentation equilibrium and important role of Debye-screening length. VI. Organic solvent aqueous mixtures for heavy and monodisperse spheres (specific gravity=1.50)
Authors: Okubo Tsuneo
Published: July 2002

A metallurgical microscope is used to directly observe the ordered structures of heavy and monodisperse polystyrene-type spheres (specific gravity=1.50) in sedimentation equilibrium and in organic solvent aqueous mixtures. Eight kinds of water miscible organic solvents, i.e., methanol, ethanol, propanol, 1,4-dioxane, ethylene glycol, propylene glycol, N-methylformamide (NMF), and N-methylacetamide (NMA) are used. The center-to-center interparticle distances (D) in the hexagonal ordering increase by the addition of organic solvents of dielectric constants lower than that of water, whereas D decreases in the presence of NMF or NMA, which has a dielectric constant higher than that of water. Young’s modulus for the ordered lattices is estimated to be from 0.44 to 1.5 Pa in methanol (0 to 30 vol %) aqueous mixtures. These results show that electrostatic interparticle repulsion and the elongated Debye-screening length around the spheres are essential for the ordering.
Pressure broadening and cross relaxation of ammonia perturbed by hydrogen and helium: Implications on intermolecular potentials and discussion of rotational effects
Authors: Broquier M. , Picard-Bersellini A. , Aroui H. , Billing G. D.
Published: July 2002

The study of the absorption line profiles in the infrared region (6–7 μm), by means of a diode laser spectrometer and a FT spectrometer in ammonia diluted in He and H2 at room temperature, allowed us to determine pressure broadening cross section and the intradoublet inelastic rate (due to molecular inversion).
Diode laser probes of vinyl radical kinetics: The reaction of C2H3 with HCl and DCl
Authors: Krueger Herman , Weitz Eric
Published: July 2002

A tunable diode laser probe is used to measure the rate of reaction of vinyl radicals with HCl and DCl at room temperature. Rate constants for reaction with HCl and DCl are (1.3±0.3)×10−12 and (7.8±1.6)×10−13 cc/molecule s, respectively. These rate constants are about five orders of magnitude faster than the estimated rate constant for reaction of vinyl radicals with H2, and potential reasons for these differences are considered within the context of the BSBL (bond strength–bond length) model of atom abstraction reactions. The rate constant for reaction of C2H3 with O2 is determined to be (1.0±0.4)×10−11 cc/molecule s, in excellent agreement with a previous determination. Finally, the temperature dependence of the vinyl radical + HCl rate constant is reported. The rate constant is found to decrease by approximately 33% over the temperature range of 278–370 K. This change, however, is within the experimental error of the measured rate constants. From these measurements, an activation energy of −840±1400 (2σ) cal/mol is obtained.
Dissociative excitation of water by metastable rare gas atoms: Rg(3P0,2)+H2O→Rg+OH(A 2Σ+)+H (Rg=Ar,Kr)
Authors: Tabayashi Kiyohiko , Shobatake Kosuke
Published: July 2002

The title reactions were studied over the relative collision energy range 0.3–1.8 eV in crossed molecular beams. Vibrational and rotational state distributions of the nascent OH(A 2Σ+) product were determined by analysis of fluorescence from the OH(A 2Σ+–X 2Π) bands. The rotational distributions could be represented by simple Boltzmann distributions. With Ar*(3P0,2) excitation, both vibrational and rotational distributions were found to have no significant dependence on the collision energy and compare well with results previously obtained at near-thermal energies. With Kr(3P0,2) excitation, however, the state distributions were found to be strongly collision-energy dependent, the rotational temperature Tr (v=0) increasing from 850 to 1750 K and the vibrational population ratio Nv=1/Nv=0 from ≤0.09 to 0.14 as the collision energy was increased from 0.35 to 0.65 eV. Time-of-flight (TOF) energy selection was used to measure the integral cross sections for the formation of the OH(A). The collision energy dependence of the cross section for the reaction Ar(3P0,2) +H2O→Ar+OH(A 2Σ+)+H was found to be negative, whereas that for the reaction Kr(3P0,2)+H2O→Kr +OH(A 2Σ+)+H exhibited a positive dependence. To understand the above experimental findings, possible mechanisms for the formation of OH(A 2Σ+) from Rg(3P0,2)+H2O are considered that are consistent with the hypothesis that the reaction is governed by spin conservation. It is found that the present results clearly display the characteristic features of reaction dynamics that involve triplet excited potential surfaces. The formation of OH(A) is well interpreted as dissociation of an excited intermediate H2O*(d̃ 3A1) state produced competitively in the decay of triplet Rydberg states.
Phenomenological theory of the dynamics of polymer melts. I. Analytic treatment of self-diffusion
Authors: Skolnick Jeffrey , Yaris Robert , Kolinski Andrzej
Published: July 2002

In the context of dynamic Monte Carlo (MC) simulations on dense collections of polymer chains confined to a cubic lattice, the nature of the dynamic entanglements giving rise to the degree of polymerization n, dependence of the self-diffusion constant D∼n−2 is examined. Consistent with our previous simulation results, which failed to find evidence for reptation as the dominant mechanism of polymer melt motion [J. Chem. Phys. 86, 1567, 7164, 7174 (1987)], long-lived dynamic entanglement contacts between pairs of segments belonging to different chains are extremely rare and are mobile with respect to the laboratory fixed frame. It is suggested that dynamic entanglements involve the dragging of one chain by another through the melt for times on the order of the terminal relaxation time of the end-to-end vector. Employing the physical description provided by the MC simulation, the general expression of Hess [Macromolecules 19, 1395 (1986)] for the friction constant increment experienced by a polymer due to the other polymers forms the basis of a phenomenological derivation of D∼n−2 for monodisperse melts that does not require the existence of reptation. Rather, such behavior is dependent on the relatively benign assumptions that the long distance global motions of the chains are uncorrelated, that the dynamic contacts can be truncated at the pair level, and that the propagator describing the evolution between dynamic contacts contains a free Rouse chain component. The mean distance between dynamic entanglements is predicted to depend inversely on concentration, in agreement with experiment. Moreover, as the free Rouse component is frozen out, for chains greater than an entanglement length ne, a molecular weight independent glass transition is predicted. Extension to bidisperse melts predicts that the probe diffusion coefficient Dp depends on the matrix degree of polymerization, nm, as n−1m. Finally, comparison is made between the theoretical expressions and MC results for mono- and bidisperse melts.
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o evaluation of the Born correction, Born couplings, and higher derivative matrix elements with Gaussian-lobe orbitals
Authors: Zhang Yongfeng , Sukumar Nagamani , Whitten Jerry L. , Porter Richard N.
Published: July 2002

Formulation of the expectations of eight operators required for the evaluation of the Born corrections, the Born couplings, and higher derivatives of the Born–Oppenheimer wave functions are derived for the case of a Gaussian-lobe orbital (GLO) basis. The relative simplicity of these analytical formulas is a special advantage of GLO and reduces the computer time in these calculations. Some operators treated here require a modification of the Slater rules. As examples of applications of these techniques, ab initio calculations of the orbital stresses and the Born corrections for the ground state X 1Σ+g and excited states 1Σ+g(II), C, C′ 3Πu, and 3Πu(II) of N2 are reported. The obtained results show that the Born correction near the avoided crossing region strongly depends upon the nuclear separation; in this region configuration interaction makes an important contribution.