bluuebunny/crossref_metadata_2025 huggingface dataset:
First order magnetization processes in Nd(Fe, M)12Cy compounds (M=V, Ti, Mo)
Authors: Yang Jinbo , Oleinek Ph. , Müller K.-H.
Published: July 2002

Magnetization measurements on Nd(Fe, M)12Cy (M=Ti, V, Mo) with ThMn12-type structure have revealed well developed first order magnetization processes (FOMPs) when an external field is applied perpendicular to the c axis. The FOMPs depend on carbon concentration, temperature, and the type of M element. The FOMPs appear at a certain carbon concentration and at temperatures below 150–100 K. The FOMP fields of NdFe12−xVxCy are independent of the V content in the range x=1.25–2.0, which is due to the small change of the anisotropy field within this range. No FOMP has been found in the nitrides of the same parent alloys. The evaluation of the crystal field (CF) parameters shows that the higher-order CF parameters of the Nd ions play a key role in producing the observed type-II FOMP. The temperature dependence of the FOMPs is due to the thermal evolution of the anisotropy constants Ki (i=1,2,3) of the carbides.
X-ray resonant magnetic scattering from FePd thin films
Authors: Dudzik E. , Dhesi S. S. , Collins S. P. , Dürr H. A. , van der Laan G. , Chesnel K. , Belakhovsky M. , Marty A. , Samson Y. , Goedkoop J. B.
Published: July 2002

Depending on the growth conditions, FePd thin films can display a perpendicular magnetic anisotropy associated with chemical order. In competition with the shape anisotropy, this can lead to striped magnetic domains, with moments perpendicular to the film plane. Under these circumstances, magnetic flux closure should occur. The striped domains were studied with soft x-ray resonant magnetic scattering using circularly polarized light to demonstrate the presence of closure domains. Magnetic depth profiling was performed both at the Fe and Pd L3 edge, by measuring the magnetic diffraction peak intensities versus angle of incidence θ.
Magnetization reversal in polycrystalline NiO–Co exchange anisotropy coupled bilayers
Authors: Yang David X. , Chopra Harsh Deep , Chen P. J. , Brown H. J. , Swartzendruber L. J. , Egelhoff W. F.
Published: July 2002

Real time investigation of magnetization reversal in exchange anisotropy coupled polycrystalline NiO–Co bilayers show that reversal in exchange coupled Co films is highly local and nonuniform in nature. Growth of reversed domains occur primarily by the abrupt nucleation and coalescence of new reversed domains; wall motion, although observable, is not the dominant growth mode for reversed domains. Clear evidence is presented which shows that the strength of the exchange field varies at the microscopic scale across the sample. This manifests itself as different microscopic regions switching abruptly at different fields, and a given area switching at different fields in the positive and negative field directions due to the unidirectional rather than uniaxial nature of exchange anisotropy.
Hot-electron attenuation lengths in ultrathin magnetic films
Authors: Lu R. P. , Morgan B. A. , Kavanagh K. L. , Powell C. J. , Chen P. J. , Serpa F. G. , Egelhoff W. F.
Published: July 2002

Ballistic electron emission microscopy (BEEM) is used to measure hot-electron transport across magnetic metal multilayers. Room temperature measurements in air have been carried out on Au/M/Si(100), Au/M/Au/Si(100), and Au/M/PtSi/Si diodes, that were sputter deposited at 175 or 300 K, where M is Co, Fe, Ni, Cu, or Ni81Fe19. Plots of log BEEM current versus M thickness are linear giving hot-electron (1.5 eV) attenuation lengths (ALs), for Au/M/Si diodes (M=Co, Fe, Ni81Fe19, and Ni) of 0.3, 0.5, 0.8, and 1.3 nm, respectively (with typical standard uncertainties of ±10%). Magnetic metal sandwich diodes, (Au/M/Au/Si) show larger ALs, 0.8 and 2.1 nm, for M=Co and Ni81Fe19, respectively. PtSi interlayers improve the surface roughness but have little effect on the AL while low temperature depositions increase the AL. We presume that the increases in the AL are due to better microstructure, less silicide reaction, or to changes in elastic scattering at interfaces.
Atomic separation of Ni using tunable pulse lasers
Authors: Ishikawa Toshiaki
Published: July 2002

Two tunable pulse lasers are used to selectively photoionize Ni atoms from an atomic beam containing mixtures of Ni and Cu, by a two-step resonant exitation process. Considering the bandwidth of 0.1 cm−1 of the lasers used, it is expected that 100% pure Ni atoms are separated unless accidental resonant excitations and multiphoton ionizations of Cu atoms occur. The photoionized Ni atoms are extracted by means of an electric field and deposited on a Ag substrate as a pure film. Using a Ni(36%)–Cu(64%) alloy as a starting material, a purified film of Ni(93.3%)Cu(6.7%), which was measured by an inductively coupled plasma-mass spectrometry (ICP-MS), was obtained after laser purification, though no Cu impurities were detected when an electron probe microanalysis was used. The Cu impurity level of 6.7% determined from the ICP-MS is too high, contrary to the expectation of high selectivity of this laser method. The drifting vaporized Cu atoms that contribute to the background pressure are considered to be one of the main sources of the contamination in the purified film.
Complex color centers in ultraviolet-light irradiated PbWO4 single crystal
Authors: Li Wensheng , Tang Tong B. , Feng Xiqi
Published: July 2002

The frequency and temperature dependence of the dielectric response in monocrystalline lead tungstate was measured before and after ultraviolet-light irradiation. Irradiated crystals, but not as-grown samples, exhibited dielectric relaxation characteristics, its activation energy being 0.3 eV and frequency factor, around 108 Hz. We trace its origin to the dipoles of complex color centers formed in W–O tetrahedra of the lattice. That radiation affects the W–O bonds were confirmed by infrared spectroscopy. On the other hand, irradiated crystals did not undergo dielectric relaxation, if they had been previously annealed in oxygen at a temperature where oxygen diffuses easily, as indicated by thermogravimetry. We conclude that reducing oxygen vacancy concentration effectively enhances their radiation hardness.
Shallow buried SiNx layers
Authors: Barbadillo L. , Hernández M. J. , Cervera M. , Rodrı́guez P. , Piqueras J.
Published: July 2002

High dose nitrogen implantations have been performed at an energy of 30 keV. After high temperature annealing, 1200 °C, a buried layer composed mostly of silicon nitride is formed leaving an overlayer with a high fraction of crystalline silicon. The lattice constant of the overlayer and the region below the SiNx are reduced in 0.13% and 0.089%, respectively. The substitutional N seems to be responsible for this reduction.
Bending instability of electrically charged liquid jets of polymer solutions in electrospinning
Authors: Reneker Darrell H. , Yarin Alexander L. , Fong Hao , Koombhongse Sureeporn
Published: July 2002

Nanofibers of polymers were electrospun by creating an electrically charged jet of polymer solution at a pendent droplet. After the jet flowed away from the droplet in a nearly straight line, it bent into a complex path and other changes in shape occurred, during which electrical forces stretched and thinned it by very large ratios. After the solvent evaporated, birefringent nanofibers were left. In this article the reasons for the instability are analyzed and explained using a mathematical model. The rheological complexity of the polymer solution is included, which allows consideration of viscoelastic jets. It is shown that the longitudinal stress caused by the external electric field acting on the charge carried by the jet stabilized the straight jet for some distance. Then a lateral perturbation grew in response to the repulsive forces between adjacent elements of charge carried by the jet. The motion of segments of the jet grew rapidly into an electrically driven bending instability. The three-dimensional paths of continuous jets were calculated, both in the nearly straight region where the instability grew slowly and in the region where the bending dominated the path of the jet. The mathematical model provides a reasonable representation of the experimental data, particularly of the jet paths determined from high speed videographic observations.
A nonorthogonal finite-difference time-domain method for computing the band structure of a two-dimensional photonic crystal with dielectric and metallic inclusions
Authors: Qiu Min , He Sailing
Published: July 2002

A finite-difference time-domain scheme in a nonorthogonal coordinate system is presented to calculate the band structure of a two-dimensional photonic crystal consisting of a skew lattice. The method can be used for a photonic crystal of complicated configuration, such as a photonic crystal with both dielectric and metallic inclusions. The method is verified by comparing with the results obtained by other methods for some special cases. The band structure of a photonic crystal with a dielectric layer coated on a metallic cylinder as an inclusion is studied. For such a case, it is noticed that both the dielectric and metallic characteristics of the band structure are inherited.
Ultraviolet fluorescence from high pressure air excited by an intense, short-pulse electron beam
Authors: Eckstrom D. J. , Dickinson J. S. , Hill R. M.
Published: July 2002

The prompt emission in the 225–290 nm region from atmospheric pressure laboratory air excited by a high current pulsed e beam includes a large number of bands from the Gaydon–Herman and fourth positive systems and from several other N2 band systems. The strongest of these midultraviolet features have fluorescence efficiencies of about 5×10−7, which is 1% of the value for the strongest air fluorescence feature at 337.1 nm.
On the thermodynamics of crystalline solids
Authors: Mullins W. W. , Sekerka R. F.
Published: July 2002

A formalism for the analysis of thermodynamic equilibrium in an arbitrarily stressed multicomponent crystal is developed. We assume that the energy per unit cell is a function of the following per-unit-cell quantities: the entropy, the mole number of each chemical species, and six independent dot products of the three vectors that define the cell. Body forces specific to each species are included, but chemical reactions and capillarity are excluded. We use a Gibbsian variational method but refer all quantities to the unvaried state which may be one of large strain. We obtain the familiar equations of thermal and mechanical equilibrium and the uniformity of generalized potentials μi +φi for each species; the φi are associated with the body forces and the μi are identified as chemical potentials since we show them to be equal locally to the chemical potentials of components present in a fluid with which the crystal is in equilibrium. The equations of equilibrium are then rederived under the assumption that certain equations of constraint hold among the variables that specify the mole numbers per unit cell of chemical species. This allows the treatment of a system composed of both mobile and immobile species, considered by Li, Oriani, and Darken, as well as a famous example of Gibbs in which a single component solid may be in equilibrium with three fluids having different chemical potentials of that component. We then introduce an extended set of variables in order to give an explicit description of point defects and to allow comparison of our treatment with those of Herring and of Larché and Cahn. Equations are obtained that allow the determination of equilibrium defect populations; when defects are in equilibrium, our previous equations are recovered.
A search for the prewetting line
Authors: Schmidt James W. , Moldover Michael R.
Published: July 2002

This paper describes efforts to locate the prewetting line in a binary liquid system (isopropanol–perfluoromethylcyclohexane) at the vapor–liquid interface. We placed tight upper bounds on the temperature separation (0.2 K) between the prewetting line and the line of bulk liquid phase separation. We did not detect the prewetting line in systems at equilibrium. Experimental signatures indicative of the prewetting line occurred only in nonequilibrium situations. Several theories predict that the adsorption of one of the components (the fluorocarbon, in this case) at the liquid–vapor interface should increase abruptly, at a temperature sightly above the temperature at which the mixture separates into two liquid phases. A regular solution calculation indicates that this prewetting line should have been easily detectable with the instruments used in this experiment. Significant features of the experiment are: (1) low-gradient thermostatting, (2) in situ stirring, (3) precision ellipsometry from the vapor–liquid interface, (4) high resolution differential index of refraction measurements using a novel cell design, and (5) computer control.
Triplet Rydberg states in molecular nitrogen
Authors: Sharpe S. W. , Johnson P. M.
Published: July 2002

A pulsed electric discharge in a pulsed supersonic jet of N2 is found to produce high densities of metastable E3Σ+gN2. Subsequent laser irradiation of these metastables results in single-photon excitation to autoionizing states. Singly charged cations are detected by time-of-flight mass spectrometry or alternatively, ejected electrons are detected by TOF electron energy analysis. From both the laser excitation spectra and photoelectron spectra we have identified a number of autoionizing Rydberg states, including a triplet Rydberg series (np=8−26) 3Πu←E 3Σ+g converging to the ground state ion, the first member of an s-type triplet Rydberg series 3Σ+u←E converging to the B state of the ion and the lowest member of a d-type triplet Rydberg series 3Πu and 3Σ+u←E converging to the A state of the ion. Some of these transitions involve core excitations of a 3s Rydberg state configuration, resulting in a doubly excited electronic configuration [He(2sσg)2(2sσu)(2pπu)4 (2pσg)2(3sσg)]. Other transitions confirm that the E state is a mixture of a 3s Rydberg state with an X ionic core and a 3p Rydberg state with an A ionic core.
Investigation of structure and stability of small clusters: Molecular dynamics studies of water pentamers
Authors: Moore Plummer P. L. , Chen T. S.
Published: July 2002

The method of molecular dynamics was used to examine the structure and stability of water pentamers for a variety of initial configurations. The evolution of each configuration was followed over a temperature range of 5 to ∼300 K. It was found that at low temperatures, insufficient energy was present for structure interconversion. Thus a time average was not equivalent to a phase space average and the pentamer cluster system is not ergodic below ∼60 K. In addition to the structure studies, collisions of a water monomer with the pentamer cluster were modeled using different impact parameters. In the collisions, only small energy transfer was required for monomer absorption. Re-evaporation or evaporation events in either the collision simulations or the heating runs were rare.
Error analysis in computer simulations
Authors: Bishop Marvin , Frinks Sharon
Published: July 2002

Error analysis methods for computer simulations are reviewed and applied to a Brownian dynamics simulation of a ring polymer.
Comparison between interpretations of dielectric behavior of poly(vinyl acetate) by the coupling model and the Havriliak–Negami equation
Authors: Shioya Yoshitsugu , Mashimo Satoru
Published: July 2002

Comparison has been made between the Kohlrausch–Williams–Watts (KWW) function and the Havriliak–Negami (HN) equation in order to examine which is more suitable for describing dielectric relaxation behavior reported recently for poly(vinyl acetate). The measured complex permittivity is fitted satisfactorily by the KWW function as well as the HN equation over a frequency range of −2≤log( f/fm)≤2, where fm is the frequency of maximum dielectric loss. Temperature dependence of the Kohlrausch relaxation time τ* obtained experimentally is also explained by the coupling model, which gives τ*=[(1−n)ωncτ0]1/(1−n), over a wide temperature region including the region below the glass transition temperature. The value of ωc is determined as 7.2×109 s−1, which is quite reasonable compared to the predicted value. No anomalous dielectric behavior was found in the glass transition region. This is not true of the results obtained with the HN equation.
The 193 (and 248) nm photolysis of HN3: Formation and internal energy distributions of the NH (a 1Δ, b 1∑+, A 3Π, and c 1Π) states
Authors: Rohrer F. , Stuhl F.
Published: July 2002

The UV photolysis of HN3 at 193 nm was investigated in detail in the bulk phase at 300 K. NH radicals in the X, a, b, A, and c states were found to be formed with quantum yields ≤0.0019, 0.4, 0.017, 0.000 15, and 0.000 61, respectively. Relative rotational and vibrational populations were measured for all states except for NH(X). Average translational energies were estimated for NH(a,v=0 and 1) and NH(b,v=0). The 248 nm photolysis of HN3 was reinvestigated with respect to processes forming NH radicals other than NH(a). The observed energy distributions differ for both laser wavelengths and for high and low lying NH states. The distribution can be better described by a simple impulsive than by a statistical model. Some conclusions are drawn concerning the upper HN3 potential surfaces involved.
Fluorescence from photoexcitation of C2H2 at 50–106 nm
Authors: Han J. C. , Ye Chao , Suto Masako , Lee L. C.
Published: July 2002

The photoabsorption and fluorescence cross sections of C2H2 were measured in the 50–106 nm region using synchrotron radiation as a light source. Fluorescence observed at several excitation wavelengths was dispersed to identify the fluorescing species that are excited C2H*, C*2 , CH*, H*, and possibly C2H+*2 . The photodissociation process of C2 H2 leading to the formation of fluorescing species is discussed. The C2 (C–A) emission observed at 92.3 and 95.5 nm is produced by the molecular elimination process associated with superexcited state(s). Fluorescence spectra from the two-photon excitation of C2 H2 at 157.5 and 193 nm were also observed and compared with those of single-photon excitation at the equivalent excitation energies.
Structures and vibrational frequencies of small water complexes from electrical molecular mechanics
Authors: Dykstra Clifford E.
Published: July 2002

A model of intermolecular interaction potentials based on simple electrical energetics has been applied to complexes of water with up to ten interacting monomers. Equilibrium structures, harmonic frequencies, and transition dipoles are reported for the smallest clusters, with the results for the dimer helping to assess the reliability of the model predictions. Trends in monomer separation distances and incremental stability are seen from the results on the larger clusters. The model appears capable of yielding satisfactory potentials for water, potentials that incorporate cooperative or nonpairwise effects.
Fluctuation effects in a symmetric diblock copolymer near the order–disorder transition
Authors: Bates Frank S. , Rosedale Jeffrey H. , Fredrickson Glenn H.
Published: July 2002

The thermodynamic and dynamic properties of a partially deuterated poly(ethylene-propylene)–poly(ethylethylene)(PEP–PEE) diblock copolymer containing 55% by volume PEP were characterized above and below the order–disorder transition (ODT) by small-angle neutron scattering (SANS) and rheological measurements, respectively. Both experimental techniques produced unambiguous evidence of composition fluctuations well above TODT(T−TODT≲50 °C) in the disordered state, which increase in magnitude as the weak first-order transition is approached. Based on the SANS results, which are nearly predicted by a recent fluctuation theory, we conclude that the (equilibrium) instantaneous morphology in the disordered state closely resembles a spinodally decomposed binary mixture. Below TODT, long-range order can be obtained by the application of a shear field as evidenced by the resulting highly anisotropic (one-dimensional) SANS pattern. As the ODT is approached in the shear-oriented ordered state, an isotropic scattering component develops (evident at wavevectors transverse to the principle line of reflections) in qualitative agreement with the fluctuation theory. We speculate that the (equilibrium) ordered morphology can be represented by the superposition of a static lamellar structure with a fluctuating spinodal pattern. Temperature quenching into the ordered region from the disordered state in the absence of an external field produces a macroscopically isotropic (nonequilibrium) material with a complex, poorly defined morphology. Associated with each of these different morphologies and phase states are gross differences in the low-frequency rheological properties that we have interpreted based on the SANS results and fluctuation theory.
An all-electron numerical method for solving the local density functional for polyatomic molecules
Authors: Delley B.
Published: July 2002

A method for accurate and efficient local density functional calculations (LDF) on molecules is described and presented with results. The method, Dmol for short, uses fast convergent three-dimensional numerical integrations to calculate the matrix elements occurring in the Ritz variation method. The flexibility of the integration technique opens the way to use the most efficient variational basis sets. A practical choice of numerical basis sets is shown with a built-in capability to reach the LDF dissociation limit exactly. Dmol includes also an efficient, exact approach for calculating the electrostatic potential. Results on small molecules illustrate present accuracy and error properties of the method. Computational effort for this method grows to leading order with the cube of the molecule size. Except for the solution of an algebraic eigenvalue problem the method can be refined to quadratic growth for large molecules.
Molecular-dynamics simulation of aqueous mixtures: Methanol, acetone, and ammonia
Authors: Ferrario Mauro , Haughney Michael , McDonald Ian R. , Klein Michael L.
Published: July 2002

Effective pair-potential models, parametrized to the properties of the pure liquids, have been used in molecular-dynamics simulations of aqueous (binary) mixtures containing methanol, ammonia, or acetone. Results are reported for thermodynamic and structural properties, self-diffusion coefficients, and reorientational correlation times. There is fair agreement with a wide variety of experimental data. The pattern of hydrogen bonding and the distribution of hydrogen-bond lifetimes in the simulated mixtures have been investigated. The observed anomalous behavior of methanol and acetone solutions appears to be related to specific features of the hydrogen bonding—namely, the ability of these molecules to exhibit enhanced acceptor character. As a consequence of the assumed intermolecular potentials, the balance between the competing effects of hydrophobic hydration of methyl groups and hydrogen bonding to oxygen atoms is tipped towards the latter. A number of interesting structural effects have been noted. In particular, the distribution function g(R) for water oxygen atoms sharpens appreciably on addition of the second component. The structure of the water–ammonia system is discussed in detail. There is good accord with available x-ray data at low concentration, but there are no measurements available to test the predictions at high concentration.
Molecular dynamics with electronic transitions
Authors: Tully John C.
Published: July 2002

A method is proposed for carrying out molecular dynamics simulations of processes that involve electronic transitions. The time dependent electronic Schrödinger equation is solved self-consistently with the classical mechanical equations of motion of the atoms. At each integration time step a decision is made whether to switch electronic states, according to probabilistic ‘‘fewest switches’’ algorithm. If a switch occurs, the component of velocity in the direction of the nonadiabatic coupling vector is adjusted to conserve energy. The procedure allows electronic transitions to occur anywhere among any number of coupled states, governed by the quantum mechanical probabilities. The method is tested against accurate quantal calculations for three one-dimensional, two-state models, two of which have been specifically designed to challenge any such mixed classical–quantal dynamical theory. Although there are some discrepancies, initial indications are encouraging. The model should be applicable to a wide variety of gas-phase and condensed-phase phenomena occurring even down to thermal energies.
The a
b i
n
i
t
i
o model potential method. Second series transition metal elements
Authors: Barandiarán Zoila , Seijo Luis , Huzinaga Sigeru
Published: July 2002

The ab initio core method potential model (AIMP) has already been presented in its nonrelativistic version and applied to the main group and first series transition metal elements [J. Chem. Phys. 86, 2132 (1987); 91, 7011 (1989)]. In this paper we extend the AIMP method to include relativistic effects within the Cowan–Griffin approximation and we present relativistic Zn-like core model potentials and valence basis sets, as well as their nonrelativistic Zn-like core and Kr-like core counterparts. The pilot molecular calculations on YO, TcO, AgO, and AgH reveal that the 4p orbital is indeed a core orbital only at the end part of the series, whereas the 4s orbital can be safely frozen from Y to Cd. The all-electron and model potential results agree in 0.01–0.02 Å in Re and 25–50 cm−1 in ν̄e if the same type of valence part of the basis set is used. The comparison of the relativistic results on AgH with those of the all-electron Dirac–Fock calculations by Lee and McLean is satisfactory: the absolute value of Re is reproduced within the 0.01 Å margin and the relativistic contraction of 0.077 Å is also very well reproduced (0.075 Å). Finally, the relative magnitude of the effects of the core orbital change, mass–velocity potential, and Darwin potential on the net relativistic effects are analyzed in the four molecules studied.
Hydrodynamic stress on fractal aggregates of spheres
Authors: Bossis G. , Meunier A. , Brady J. F.
Published: July 2002

We calculate the average hydrodynamic stress on fractal aggregates of spheres using Stokesian dynamics. We find that for fractal aggregates of force-free particles, the stress does not grow as the cube of the radius of gyration, but rather as the number of particles in the aggregate. This behavior is only found for random aggregates of force-free particles held together by hydrodynamic lubrication forces. The stress on aggregates of particles rigidly connected by interparticle forces grows as the radius of gyration cubed. We explain this behavior by examining the transmission of the tension along connecting lines in an aggregate and use the concept of a persistance length in order to characterize this stress transmission within an aggregate.
Many-body symmetry-adapted perturbation theory of intermolecular interactions. H2O and HF dimers
Authors: Rybak Stanisl/aw , Jeziorski Bogumil/ , Szalewicz Krzysztof
Published: July 2002

A many-body version of the symmetry-adapted perturbation theory is developed for a direct calculation of intermolecular potentials as a sum of the electrostatic, exchange, induction, and dispersion contributions. Since no multipole expansion is used, the obtained interaction energy components are properly dampened at short distance by the charge-overlap (penetration) effects. The influence of the intramonomer correlation is accounted for by the perturbation expansion in terms of the Mo/ller–Plesset type fluctuation potentials WA and WB for the individual molecules. For the electrostatic and for the dispersion energy, the terms of the zeroth, first, and second order in WA+WB are considered. In this way, the leading three-particle correlation contribution to the dispersion energy is taken into account. As a test of our method, we have performed calculations of the interaction energy for the water and hydrogen fluoride dimers. Both the geometry and the basis set dependence of the interaction energy components have been investigated. For a comparison, we have also computed the supermolecular interaction energies through the full fourth order of the many-body perturbation theory. On the basis of our results, we predict the association energy for (H2O)2 equal to −4.7±0.2 kcal/mol in relatively poor agreement with the experimental value of −5.4±0.7 kcal/mol, but still within the experimental error bars. For (HF)2, the predicted association energy is −4.2±0.2 kcal/mol, while the experimental value (corrected by a theoretical zero-point energy) is −4.9±0.1 kcal/mol.
The high energy asymptotic behavior of line shape cross sections and detailed balance
Authors: Monchick Louis
Published: July 2002

Although line shape relaxation cross sections in the ‘‘impact approximation’’ do not obey detailed balance except in the very high temperature limit, they can be shown, in the Born approximation, to satisfy a somewhat similar asymptotic relation. In this empiric relation, first observed by Boissoles et al. [J. Chem. Phys. 90, 5392 (1989)], the complex conjugate of the cross section for the radiative transition if→i′f′ is asymptotically equal to the cross section for the inverse transition i′f′→if. It is then shown that although cross sections calculated with the presumably more correct Fano collision operator also obey this relation, due to time reversal symmetry they also obey detailed balance. These two observations taken together imply that the imaginary part of line shape relaxation cross sections in the more exact theory decay much more rapidly with energy than the real parts and suggest a quick fix for relaxation cross sections calculated in the ‘‘impact approximation,’’ or as it is sometimes called, the Shafer–Gordon formalism. Numerical calculations of inelastic off-the-energy shell scattering of simple rigid spherelike models support these results and suggest that off-the-energy shell calculations with realistic models are not only desirable but also necessary for computing line shapes of partially overlapping lines.
A neutron scattering study of the structure of a bimodal colloidal crystal
Authors: Bartlett P. , Ottewill R. H.
Published: July 2002

We have studied the freezing of a binary mixture of colloidal poly(methyl methacrylate) spheres of size ratio 0.31 and composition AB4 (here A refers to the larger spheres). When suspended in a suitable liquid these particles interact via a steeply repulsive (approximately hard sphere) potential. The structure of the colloidal crystals formed in this binary system has been established from a combination of small-angle neutron and light scattering measurements. We find that there is an almost complete size separation on freezing. The crystalline phase contains almost exclusively large spheres while the smaller spheres are excluded from the crystal into a coexisting binary fluid. This observation is in agreement with recent density functional calculations for the freezing of hard sphere mixtures.
Dielectric constant measurements near the critical point of cyclohexane-aniline
Authors: Early M. D.
Published: July 2002

Measurements of the static dielectric constant of the cyclohexane+aniline system have been made in the neighborhood of the critical point. Within the experimental resolution of one in 105, no critical behavior was found. This is shown to be consistent with the prediction of the droplet model of this effect, where the amplitude of the expected critical term is calculated by subtraction of two expressions. In the particular case of cyclohexane + aniline these terms are approximately equal. A consequence of this model is that the change in critical temperature due to an applied electric field should be positive for most mixtures (and very small for this particular system) and that previous measurements of negative values are therefore likely to be due to spurious heating effects.
Interoscillator coupling effects on the OH stretching band of liquid water
Authors: Hare D. E. , Sorensen C. M.
Published: July 2002

We have studied the OH stretch spectrum in water to explore the effects of interoscillator coupling. Both H2O, which may have significant intra and intermolecular coupling, and dilute HOD in D2O, which is isotopically decoupled, were studied over a broad range of temperature from −33 °C to 160 °C. By adding a small amount of a calibration dopant, we obtained quantitative spectra. We found interoscillator coupling plays a large role at all temperatures. At high temperature, intramolecular coupling contributes to a downshift in the peak position for H2O as compared to HOD. Intermolecular coupling, however, still has some influence at high temperature. At low temperature, the large excess intensity below 3200 cm−1 in H2O compared to HOD we find is due to an enhanced Raman cross section due to intermolecularly coupled in-phase OH stretch oscillations. We define a degree of delocalization for coupling as the idealized number of perfectly in-phase oscillators that could cause the enhanced scattering and find N≂2 for liquid H2O at −33 °C. An exact isosbestic crossing is not found and this can be understood given the changing line shape that the intermolecular coupling can induce. All the properties of the spectrum approach those found in amorphous solid water or ice near the supercooled water anomalous temperature, Ts≂−45 °C.
Conformational studies of xanthene by laser-induced fluorescence in a supersonic free jet: Inversion potentials in ground and excited singlet states
Authors: Chakraborty Tapas , Lim Edward C.
Published: July 2002

The analyses of fluorescence excitation and dispersed fluorescence spectra of jet-cooled xanthene indicate that the equilibrium conformation of the molecule is nonplanar in the ground state and planar in the lowest excited singlet state. The estimated ground-state inversion barrier is 50 cm−1.
Particle mesh Ewald: An N⋅log(N) method for Ewald sums in large systems
Authors: Darden Tom , York Darrin , Pedersen Lee
Published: July 2002

An N⋅log(N) method for evaluating electrostatic energies and forces of large periodic systems is presented. The method is based on interpolation of the reciprocal space Ewald sums and evaluation of the resulting convolutions using fast Fourier transforms. Timings and accuracies are presented for three large crystalline ionic systems.
Instability in a system of two interacting liquid films: Formation of liquid bridges between solid surfaces
Authors: Forcada Mikel L.
Published: July 2002

A theoretical study of systems composed of two solid-supported liquid films that are subject to a mutual attractive interaction reveals the existence of a mechanical instability: for distances closer than a certain threshold value, the system composed by two separate liquid films has no stable equilibrium configurations, and the system collapses to form a single liquid body. The sudden condensation of a connecting liquid bridge when two solid surfaces are brought to close proximity inside an undersaturated medium has been observed experimentally using the surface-force apparatus [see, e.g., Christenson et al., Phys. Rev. B 39, 11750 (1989)]. In this paper, these results are explained as follows: first, liquid films condense on the surfaces; then, if the distance is short enough, the films jump to contact, because of a mechanical instability due to attractive interactions.
A study of the adsorption of alkanes on a thin-metal film
Authors: Bradberry G. W. , Vukusic P. S. , Sambles J. R.
Published: July 2002

The adsorption of vapor on a thin gold film has been studied for three alkanes: pentane, hexane, and heptane. Adsorbed overlayers were monitored by coupling visible radiation to excite a surface plasmon on the gold film and detecting the minimum of reflectivity which is sensitive to the thickness of any overlayer. The relationship between the relative pressure of the vapor surrounding the gold film and the overlayer thickness has been investigated and found to have a linear form for sub-monolayer thicknesses. For overlayer thicknesses greater than about 4 nm the change in the chemical potential, deduced from the pressure measurements, is found to vary inversely as the square of the thickness. This behavior is compared with other experimental observations and theoretical predictions.
Laser photodissociation of C7H7+ created through multiphoton ionization of para-chlorotoluene
Authors: Lablanquie Pascal , Ohashi Kazuhiko , Nishi Nobuyuki
Published: July 2002

The nature of the C7H7+ ion created through resonant dissociative multiphotoionization of para-chlorotoluene by an ultraviolet (UV) laser was investigated thanks to its interaction with a second laser beam. The dissociation pattern corresponding to one or several photon absorption could be observed. Cross section for the one-photon absorption in the 265/530 nm range revealed the presence of the tropylium and benzyl isomers and suggested they possessed substantial internal energy. This was confirmed by the study of the C7H7++hν■C5H5++C2H2 reaction, and more precisely of its rate and of the kinetic energy released. A ladder switch mechanism for the three-photon dissociative ionization of para-chlorotoluene leading to C7H7+ is shown to agree with our results.
Theoretical study of the monocarbonyls of first-row transition metal atoms
Authors: Fournier René
Published: July 2002

The results of density functional calculations on the most stable high-spin and low-spin states of MCO are given, where M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. The ground states are found to be high spin for M=Sc, Ti, V, Cr, and Cu (2S+1=4, 5, 6, 7, and 2, respectively) and low spin for M=Mn, Fe, Co, and Ni (2S+1=4, 3, 2, and 1, respectively). From Sc to Cu, the M–CO binding energies with respect to ground state products are estimated to be 9, 16, 26, 13, −14, 14, 30, 54, and 19 kcal/mol. Where comparison with experiment is possible, the estimates are apparently too large by about 6 kcal/mol (FeCO), 13 kcal/mol (NiCO), and 12 kcal/mol (CuCO). The high-spin state MCO complexes with metal to the right of vanadium in the Periodic Table all have bent equilibrium geometries; all others are found to be linear. The calculated CO harmonic stretch frequencies generally overestimate the observed values, but follow a similar trend. The CO bond lengths, CO stretch frequencies, and metal–CO bond strengths all correlate well with the extent of π back donation. However, these correlations hold only within either the group of all high-spin states, or the group of all low-spin states. Thus, there are no simple trends in the calculated properties of ground state MCO complexes.
Tricritical theory of dilute polymer solutions with quenched disorder
Authors: Rajasekaran J. J. , Muthukumar M.
Published: July 2002

We have performed a renormalization group analysis of the tricritical behavior of dilute polymer solutions containing quenched impurities in three dimensions. We have treated the quenched disorder using the replica formalism. The logarithmic corrections to the mean field results for the quenched problem are found to be in a different universality class from those for the pure or annealed problem. For example, the leading corrections to the second virial coefficient and the specific heat are proportional to L1/2(ln L/a0)−2/11 and L(ln L/a0)7/11, respectively, where L is the chain length and a0 is the cutoff; the coexistence curve is found to follow the universal equation u∼−𝒞(‖ln 𝒞2a0‖)−9/11 when 𝒞2a0≪1, where 𝒞 is the monomer concentration and u is the two-body excluded volume interaction parameter.
The vibrational spectrum of H2O3: An ab initio investigation
Authors: Jackels Charles F.
Published: July 2002

Theoretically determined frequencies and absorption intensities are reported for the vibrational spectra of the covalent HOOOH and hydrogen bonded HO...HOO intermediates that may form in the reaction of the hydroxyl and hydroperoxyl radicals. Basis sets of double-zeta-polarized quality, augmented by diffuse and second sets of polarization functions have been used with complete-active-space, self-consistent-field wave functions. The calculated harmonic vibrational frequencies of HOOOH have been corrected with empirical factors and presented in the form of a ‘‘stick’’ spectrum. The oxygen backbone vibrations, predicted to occur at 519, 760, and 870 cm−1, are well separated from most interferences, and may be the most useful for the species’ identification. In the case of the hydrogen bonded isomer, emphasis has been placed upon prediction of shifts in the intramolecular vibrational frequencies that take place upon formation of the complex. In particular, the HO stretch and HOO bend of HO2 are predicted to have shifts of −59 and 53 cm−1, respectively, which should facilitate their identification. It is also noted that the antisymmetric stretching frequency of the oxygen backbone in HOOOH exhibits a strong sensitivity to the degree of electron correlation, similar to that which has been previously observed for ozone.
On quantum mechanical phase-space wave functions
Authors: Wl/odarz Joachim J.
Published: July 2002

An approach to quantum mechanics based on the notion of a phase-space wave function is proposed within the Weyl–Wigner–Moyal representation. It is shown that the Schrödinger equation for the phase-space wave function is equivalent to the quantum Liouville equation for the Wigner distribution function. The relationship to the recent results by Torres-Vega and Frederick [J. Chem. Phys. 98, 3103 (1993)] is also discussed.
Periodic, quasiperiodic, and chaotic potentials generated by electrochemical concentration cells: Local and global dynamics
Authors: Zeyer K.-P. , Münster A. F. , Hauser M. J. B. , Schneider F. W.
Published: July 2002

We extend previous work describing the passive electrical coupling of two periodic chemical states to include quasiperiodic and chaotic states. Our setup resembles an electrochemical concentration cell (a battery) whose half cells [continuous-flow stirred tank reactors (CSTRs)] each contain the Belousov–Zhabotinsky (BZ) reaction. For a closed electrical circuit the two half cells are weakly coupled by an external variable resistance and by a constant low mass flow. This battery may produce either periodic, quasiperiodic, or chaotic alternating current depending on the dynamic BZ states chosen in the half cells. A lower fractal dimensionality is calculated from the electrical potential of a single chaotic CSTR than from the difference potential (relative potential) of the two chaotic half cell potentials. A similar situation is observed in model calculations of a chaotic spatiotemporal system (the driven Brusselator in one space dimension) where the dimensionality derived from a local time series is lower than the dimensionality of the global trajectory calculated from the Karhunen–Loeve coefficients.
Algebraic approach to molecular rotation-vibration spectra: Rotation-vibration interactions
Authors: Iachello Francesco , Oss Stefano , Viola Lorenza
Published: July 2002

We study rotation-vibration interactions of triatomic molecules within the framework of the vibron model. We use this method to analyze rotation-vibration interactions in HCN. The method provides an alternative to the usual analysis.
High-resolution Fourier transform emission spectroscopy of YH
Authors: Ram R. S. , Bernath P. F.
Published: July 2002

The electronic emission spectrum of YH has been investigated in the 690 nm to 3 μm spectral region using a Fourier transform spectrometer. The YH bands were excited in an yttrium hollow cathode lamp operated with neon gas and a trace of hydrogen. The observed bands have been classified into three different electronic transitions: C 1Σ+–X 1Σ+, d0+(3Σ−)–X 1Σ+, and e 3Φ–a 3Δ. The rotational analysis of the 0–0, 1–1, 2–2, 3–3, 0–1, 1–2, and 2–3 bands of the C 1Σ+–X 1Σ+ system provided the following set of equilibrium molecular constants for the ground state: ωe=1530.456(15) cm−1, ωexe=19.4369(72) cm−1, ωeye=0.0361(9) cm−1, Be =4.575 667(38) cm−1, αe=0.091 449(23) cm−1, and re=1.922 765(8) Å. Two weaker bands with origins at 14 264.7256(49) and 12 811.5753(49) cm−1 have been assigned as the 0–1 and 0–2 bands of a new d0+(3Σ−)–X 1Σ+ electronic transition with T0=15 756.4251(49) cm−1. Three bands observed in the red region with origins at 11 377.9784(39), 11 499.3408(26), and 11 583.8294(23) cm−1 have been assigned as the 3Φ2–3Δ1, 3Φ3–3Δ2, and 3Φ4–3Δ3 subbands of a new e 3Φ–a 3Δ electronic transition.
Vibrational energy transfers in ozone excited into the (101) state from double-resonance measurements
Authors: Menard-Bourcin F. , Doyennette L. , Menard J.
Published: July 2002

The vibrational energy transfer processes involved in the relaxation of O3 molecules from the excited states of the triad [(200)(101)(002)] have been investigated by double resonance (DR) measurements in O3–O2/N2 gas mixtures. It is shown that as long as the O3 molar fraction is higher than a few percent, the major processes are near-resonant V–V transfers occurring upon O3–O3 collisions. The rate coefficients of these processes have been determined. Also, from measurements performed with small O3 molar fractions, it has been possible to investigate the processes occurring upon O3–O2/N2 collisions. In this way, it is shown that the vibrational energy located in the stretching modes relaxes essentially through transfers to the ν2 mode. Finally, all the experimental DR signals have been simulated using a numerical model based on a kinetic scheme involving all the processes occurring in the relaxation, allowing us to deduce the corresponding rate coefficients.
Orientational correlation function for molecular liquids: The case of liquid water
Authors: Soper A. K.
Published: July 2002

Recently it has been shown how the measured partial structure factors for diatomic molecular liquids might be used to generate a detailed view of the local intermolecular orientational correlation function, via the standard spherical harmonic expansion. The present work generalizes this analysis to the case of molecules of arbitrary shape. An analysis of the measured atom–atom partial structure factors for liquid water is presented, and the corresponding maps of the orientational correlation function between water molecules are derived. It is seen that this method of presenting diffraction data is rich in detail about the nature of the short-range interactions between water molecules in the liquid state. In particular, it is found that while there is a pronounced directionality in the organization of neighboring molecules around any given molecule at the origin, corresponding to the hydrogen bonding in the liquid, there are nonetheless a range of local orientations compatible with the neutron data. This argues against the notion of water forming short lived, ‘‘icelike’’ clusters at ambient temperature and pressure, as has been speculated in the past. The general implications for this kind of analysis for other molecular liquids, solutions, and mixtures is discussed.