bluuebunny/crossref_metadata_2025 huggingface dataset:
Hysteresis model identification for particulate media
Authors: Andrei Petru , Stoleriu Laurentiu , Hauser Hans
Published: July 2002

A very important question associated with the phenomenological modeling of the magnetic hysteresis is the identification problem. In this article only the Preisach and the energetic model for ferromagnetic hysteresis are considered. An original identification method for the energetic model is presented. As it uses common experimental data such as simple parameters on the major loop: coercive field, saturation, remanence, or susceptibilities on different points of the loop it is not too sensitive to experimental errors. The method is verified on various commercial particulate magnetic media with excellent agreement.
Magnetic hardening studies in sintered Sm(Co,Cux,Fe,Zr)z 2:17 high temperature magnets
Authors: Zhang Yong , Corte-Real Michelle , Hadjipanayis George C. , Liu Jinfang , Walmer Marlin S. , Krishnan Kannan M.
Published: July 2002

Lorentz microscopy combined with conventional transmission electron microscopy were used to image the magnetic domains and microstructures of sintered Sm(CobalCuxFe0.06Zr0.03)z (0.088⩽x⩽0.128; 5.8⩽z⩽7.2) permanent magnets which were specifically designed for high temperature applications. The microstructural data were correlated with the magnetic measurements to understand the origin of coercivity. All sintered magnets showed typical cellular and lamellar microstructures. The cell size and coercivity were found to be more sensitive to z than to the Cu content. For a fixed Cu content, by increasing z from 5.8 to 7.2, the cell size was found to vary dramatically from 10 to 80 nm and the coercivity from 5.6 to 40 kOe, respectively. On the other hand, for fixed z, the cell size decreases slightly with increasing Cu content from 0.08 to 0.128 and the corresponding coercivity increases from 23.6 to 40 kOe. Both z and the Cu content show a smaller effect on the cell boundary width and lamella phase density. Domain wall pinning is observed in all magnets studied, irrespective of their cell size. The smaller the cell size, the less wavy the walls are, and the lower the coercivity. The Lorentz microscopy data indicate that the majority of pinning sites are the cell boundaries with occasional pinning at the intersection of cell boundaries with the lamella phase.
Lateral interface anisotropy in nanomagnets
Authors: Cowburn R. P. , Koltsov D. K. , Adeyeye A. O. , Welland M. E.
Published: July 2002

The results are presented of a study into the magnetic properties of deep submicron elliptical nanomagnets. Small arrays of 2:1 aspect ratio ellipses of major axis size 500–100 nm and of thickness 5–10 nm have been fabricated out of supermalloy. A high sensitivity magneto-optical method has then been used to measure the hysteresis loops of each array. From these measurements the effective shape anisotropy in the elongated elements can be accurately determined. The measured shape anisotropy is found to be significantly smaller (approximately 50%) than that which would be expected from standard magnetostatic calculations. It is shown experimentally and theoretically that lateral interface (i.e., edge) roughness of only a few nanometers, which is present in any real nanomagnet, leads to the appearance of a powerful anisotropy of magnetostatic origin which competes strongly with the in-plane shape anisotropy, thus accounting for the anomalously low experimental values. The significance of this for data stability in magnetic random access memory is discussed.
Pressure dependence of Si/SiO2 degradation suppression by helium
Authors: Afanas’ev V. V. , Stesmans A.
Published: July 2002

The suppression of thermally induced generation of hole traps in SiO2 layers of Si/SiO2 and poly-Si/SiO2/Si structures by He ambient is analyzed as a function of the gas pressure p. The pressure dependence of the suppression efficiency is found close to p2/3. This pressure dependence and the observed reduced suppression efficiency of heavier noble gases (Ne, Ar) are interpreted as indications that the gas atom dissolved in SiO2 should approach close to the degradation-prone site in order to prevent the thermally induced bond break. This result suggests that a short-range interaction between noble gas atoms and the Si/SiO2 network atoms is at the basis of the degradation blockage effect.
The role of activator–activator interactions in reducing in low-voltage-cathodoluminescence efficiency in Eu and Tb doped phosphors
Authors: Seager C. H. , Tallant D. R.
Published: July 2002

High resolution measurements of spectrally resolved cathodoluminescence (CL) decay have been made in several commercial and experimental phosphors doped with Eu and Tb at beam energies ranging from 0.8 to 4 keV. CL emission from the lowest two excited states of both rare-earth activators was compared to the decay of photoluminescence (PL) after pulsed laser excitation. We find that, at long times after the cessation of electron excitation, the CL decay rates are comparable to those measured in PL; at short times, the decay process is considerably faster and has a noticeable dependence on the energy of the electron beam. These beam energy effects are largest for the higher excited states and for phosphors with larger activator concentrations. Measurements of the experimental phosphors over a range of activator fractions from 0.1 to 0.002 show that the beam energy dependence of the steady-state CL efficiency is larger for higher excited states and weakens as the activator concentration is reduced; the latter effect is strongest for Y2SiO5:Tb, but also quite evident in Y2O3:Eu. We suggest that the electron beam dependence of both the decay lifetimes and the steady state CL efficiency may be due to interaction of nearby excited states which occurs as a result of the large energy deposition rate for low energy electrons. This picture for nonradiative quenching of rare-earth emission is an excited state analog of the well-known (ground state-excited state) concentration quenching mechanism.
Linear giant magnetoresistance behavior of submicron scale Co/Cu multilayer strips with antiferromagnetic coupling
Authors: Matsuyama K. , Matsuo K. , Nozaki Y.
Published: July 2002

Linear magntoresistance (MR) response of Co/Cu multilayer strips with sub-μm width has been performed with a field sensitivity of sub-Oe. The orthogonal spin orientation for neighboring Co layers is initiated with an additional external field applied along the short axis of the strip. The thickness of the Cu layer was optimized to be 2.1 nm, at which the MR ratio of the as grown sample took a maximum value. The MR ratio of the as-grown film of [Co(2 nm)/Cu(2.1 nm)]3/Co(2 nm)/Cu(2 nm) was 12.4%. The multilayer was patterned into strips with the pattern width w down to 0.2 μm by means of electron beam lithography and Ar ion milling. The measured maximum MR ratio of the strips with w=0.4 and 0.2 μm were 9.3% and 8.2%, respectively. The observed parabolic MR profile shows good agreement with the micromagnetic simulation assuming the symmetric scissors motion of the magnetization in the multilayer. A linear MR response was observed in a giant MR strip with 0.4 μm width by applying alternating external fields, ranged from 0.1 to 10 Oe, under a transverse bias field of 300 Oe.
Magneto-optical observation of surface currents in microelectronic circuits
Authors: Egorov A. N. , Lebedev S. V.
Published: July 2002

A magneto-optical method is applied to observe surface electrical currents in microelectronic devices. We describe here the opportunity of direct experimental localization of latchup effect in complementary metal-oxide-semiconductor integrated circuits (ICs) by visualization of latchup current path in surface layer of ICs substrate. To observe currents greater than 10 mA the garnet films with both uniaxial and in-plane anisotropy are utilized. Optimal polarization geometry providing maximal visual contrast is calculated. Topologies of both solitary and two neighboring electrical currents are experimentally investigated. The sensitivity limit (minimal current surface density) of this method of magneto-optic visualization is estimated to be 0.1 mA/μm.
Muon-spin rotation study of the magnetic correlations in La2−xCa1+xCu2O6+d superconductors
Authors: Klamut P. W. , Dabrowski B. , Dybzinski R. , Bukowski Z. , Shengelaya A. , Khasanov R. , Dottinger S. , Keller H.
Published: July 2002

Muon spin rotation experiments performed on a series of La2−xCa1+xCu2O6 (x=0.12, 0.16, 0.22) superconductors indicate the existence of the magnetic correlations in the superconducting phase below T∼6 K. The increase in sample inhomogeneity strengthens these correlations. For the superconducting phase, the extrapolated σ(0) values fulfill the universal Uemura relation between Tc and σ(0).
Magnetic impurity in a superconductor: Local phase transitions and finite size effects
Authors: Bazaliy Ya. B. , Jones B. A.
Published: July 2002

The influence of a single magnetic impurity on an s-wave superconductor is investigated by solving the Bogoliubov-De Genne equations with the impurity modeled by a spin-dependent square potential well of radius a and strength J. Unlike the one with a δ-function potential, our model shows multiple phase transitions with increase of J or a, which manifest themselves through a change of the nature of impurity bound states. By considering a finite size superconducting host (impurity on a superconducting grain) we suggest a way of observing the transitions in experiment.
Application of an inverse scattering technique to diffusive photon density images
Authors: Han Xiaoyan
Published: July 2002

In this article, I describe a method for inverting diffusive photon density images of objects immersed in a turbid medium. A theoretical model for photon diffusion is presented, and an inversion method that uses only a single image, and which is accomplished with only one step, is proposed. The proposed method is applied to images of several objects immersed in milk. The resulting images show greatly increased contrast and resolution over the original uninverted images.
Spreading of liquid drops on thin cylinders: The ‘‘manchon/droplet’’ transition
Authors: Brochard F.
Published: July 2002

We discuss the spreading of liquids on thin cylinders (radius b), including the effects of long range attractive van der Waals (VW) forces. For volatile liquids, the vapor adsorbs on the cylinders; the thickness e of the film is given by a balance between the disjoining VW pressure and Laplace pressure (e3=ba2, where a is a molecular size). For nonvolatile liquids, a liquid drop cannot spread out over the cylinder if the spreading coefficient S is smaller than a critical value Sc. For S=Sc, we find a first order transition between a droplet and a sheath structure (‘‘manchon’’). For S>Sc the droplet spreads indefinitely. The thickness of the manchon is equal to the thickness reached by a drop spreading over a plane surface. We also calculate the liquid profile just below the threshold Sc. When S→Sc, a precursor manchon appears at the two poles of the droplets.
Monte Carlo studies on the long time dynamic properties of dense cubic lattice multichain systems. II. Probe polymer in a matrix of different degrees of polymerization
Authors: Kolinski Andrzej , Skolnick Jeffrey , Yaris Robert
Published: July 2002

The dynamics of a probe chain consisting of nP =100 segments in a matrix of chains of length of nM=50 up to nM=800 at a total volume fraction of polymer φ=0.5 have been simulated by means of cubic lattice Monte Carlo dynamics. The diffusion coefficient of the probe chain over the range of nM under consideration decreases by about 30%, a behavior rather similar to that seen in real melts of very long chains. Furthermore, the analysis of the probe chain motion shows that the mechanism of motion is not reptation-like and that the cage effect of the matrix is negligible. That is, the local fluctuations of the topological constraints imposed by the long matrix chains (even for nM=800) are sufficiently large to provide for essentially isotropic, but somewhat slowed down, motion of the probe, nP =100, chains relative to the homopolymer melt. The results of these MC experiments are discussed in the context of theoretical predictions and experimental findings for related systems.
Quantum reactive scattering in three dimensions using hyperspherical (APH) coordinates. Theory
Authors: Pack Russell T , Parker Gregory A.
Published: July 2002

The theory of reactive (rearrangement) scattering for three atoms in three physical dimensions using adiabatically adjusting, principal axes hyperspherical (APH) coordinates is given. The relationships of the APH coordinates to Delves and Jacobi coordinates are given, and the kinetic energy operator is shown to be relatively simple. Procedures for solving the equations via either an exact coupled channel (CC) method or an optimum centrifugal sudden (CSAPH) approximation are given as well as procedures for applying scattering boundary conditions. Surface functions of two angles are obtained using a finite element method with an optimized, nonuniform mesh, and the CC equations are solved using the efficient VIVAS method. Sample CC results are given for the H3 system. The present approach has the advantages that all arrangements are treated fully equivalently; it is a principal axis system, so that both axes and internal coordinates swing smoothly with the reactions; it is directly applicable to both symmetric and unsymmetric systems and mass combinations and all total angular momenta; it gives convenient mappings for visualization of potential energy surfaces and wave functions; only regular radial solutions are required; all coordinate matching is by simple projection; and the expensive parts of the calculation are energy independent, so that, once they are done, the scattering matrices can be rapidly generated at the large numbers of energies needed to map out reactive thresholds and resonances. Accurate reactive scattering calculations are now possible for many chemically interesting reactions that were previously intractable.
Resonant enhancement in the valence orbital photoionization cross sections of xenon difluoride
Authors: Tse John S.
Published: July 2002

The nature of the resonances observed in the valence molecular orbitals photoionization cross sections of XeF2 has been characterized using continuum multiple-scattering (MS) Xα calculations. Analysis of the theoretical cross sections of XeF2 and a hypothetical F⋅⋅⋅F molecule with the same bond length in XeF2 reveals that there are at least three independent mechanisms governing the resonance features. Rather broad weak features at high kinetic energy (above 20 eV) are present in both the fluorine dominating 10σg and 3πg cross sections in XeF2 and in the corresponding 1πg and 3σg orbitals of F⋅⋅⋅F. Taken together with an EXAFS-like treatment of the F⋅⋅⋅F molecule, there is strong evidence that these features are due to diffraction of the photoelectrons by the F atoms. The strong enhancement in cross section at ∼11 eV in the 10σg and 3πg orbitals are not present in the F⋅⋅⋅F orbitals, and are identified as shape resonances which are due to electronic excitations from these molecular orbitals to quasibound predominantly Xe kf (l=3) (kπu and kδu) continuum channels. The localization of the kf continuum is due to the trapping of the wave function by a barrier created by a centrifugal potential analogus to that in atomic Xe. The low energy resonances (kπg and kδg) around 4.8 eV above the ionization threshold are assigned to excitations into valence–Rydberg (d) mixed continuum channels.
Dynamic simulation of electrorheological suspensions
Authors: Klingenberg D. J. , van Swol Frank , Zukoski C. F.
Published: July 2002

A simulation method is developed to investigate structure formation in electrorheological suspensions. The suspension is treated as polarizable, spherical particles in a nonconducting medium, with the spheres subject to electric polarization forces due to an applied electric field and to hydrodynamic resistance due to their motion through the continuous phase. The fibrous structures obtained from these simulations are independent of electric field strength and continuous phase viscosity in agreement with experimental observation. We have also found that the details of the simulated structures are sensitive to the treatment of the short-range forces preventing particle overlap. When this force is represented by a form that accurately approximates a hard-sphere interaction, the simulated structures agree well with those obtained experimentally, both with respect to their appearance and the time scale for structure formation.
A temperature effect in the luminescence emission from electron-irradiated MgO
Authors: Caulfield Kevin J. , Cooper Ronald , Boas John F.
Published: July 2002

The temperature dependence of luminescence emissions from electron-irradiated CaO and MgO single crystals has been studied by time resolved luminescence spectroscopy after pulsed nanosecond irradiation with 0.20 to 0.60 MeV electrons. Emissions from CaO at 375 nm at both 293 and 83 K, show similar threshold characteristics for atomic displacement. These have been attributed to the displacement of oxygen ions and subsequent electron trapping, resulting in the formation of F+ centers. The threshold energy of 0.32±0.01 MeV corresponds to an oxygen displacement energy of 58±2 eV. A 380 nm emission from MgO, also attributed to oxygen displacement and F+ center formation, shows no temperature effect, with a displacement threshold virtually identical to that for CaO. A 235 nm emission from MgO shows a significant temperature effect. The threshold energy at 293 K is 0.31±0.01 MeV, whilst at 83 K two thresholds are observed, 0.31±0.01 and 0.41±0.01 MeV.
Inelastic incoherent neutron scattering study of molecular motions of n-nonadecane in urea chlatrate
Authors: Guillaume F. , Sourisseau C. , Dianoux A. J.
Published: July 2002

Inelastic incoherent neutron scattering experiments were performed on semi-oriented samples of C19H40 –urea chlatrates over the 1.5–306 K temperature range. Using appropriate sample orientations and different instrumental resolutions, all the motions parallel or perpendicular to the main chain axes were analyzed in the 0–400 cm−1 frequency range. In the quasielastic region and for the Q momentum transfer vector parallel to the chain axes, a side peak attributed to a longitudinal acoustic mode was evidenced at 12 cm−1 at 1.5 K (overdamped at higher temperatures) in addition to large quasielastic broadenings due to restricted translational motions. For Q perpendicular to the chains, a diffusive rotational process was also evidenced in the disordered solid phase. All these results are discussed and compared to literature data.
Comparison of coupled-cluster methods which include the effects of connected triple excitations
Authors: Scuseria Gustavo E. , Lee Timothy J.
Published: July 2002

Electron correlation energies have been determined for 14 different molecules which represent a range of chemical bonding situations. These have been determined with the coupled-cluster single, double, and triple (CCSDT) excitation model as well as with several coupled-cluster methods that include only an approximate treatment of connected triple excitations, viz. CCSDT-1a, CCSDT-1b, CCSDT-2, CCSDT-3, CCSDT-4, and the recently proposed CCSD(T) method. All of the CCSDT-x methods include the effects of connected triple excitations in an iterative manner, whereas in CCSDT(T) these are included perturbationally. For chemical systems which are well represented by a single-determinant reference function, some of the CCSDT-xmethods (CCSDT-1a, CCSDT-1b, and CCSDT-4) perform marginally better than the CCSD(T) approach in reproducing the CCSDT results. However, as nondynamical correlation becomes more important the good agreement from the CCSDT-x methods deteriorates rapidly, while the error in CCSD(T) remains more consistent. For the 14 molecules considered in this work, the average error of the CCSD(T) method relative to CCSDT (667 μhartrees) is considerably below that obtained from any of the CCSDT-x methods. It is concluded that CCSD(T) is to be preferred over any of the other approximate methods, both because it is the least expensive and also because it is generally the most accurate approximation to CCSDT.
Collisional deactivation studies of the Xe(6p) states in He and Ne
Authors: Xu J. , Setser D. W.
Published: July 2002

The decay kinetics of the Xe(6p[1/2]0 or 2p5), Xe(6p[3/2]2 or 2p6) and Xe(6p[5/2]2 or 2p9) states have been studied in He and Ne buffer gas using the two-photon laser-excitation technique. The pressure dependence of the fluorescence decay rates was used to measure the total quenching rate constants at 300 K. The primary product distribution from two-body collisions with He or Ne were estimated from product fluorescence intensities, and state-to-state quenching rate constants were assigned. Limited information about the decay kinetics of the Xe(2p7),Xe(2p8), Xe(2p10) states was inferred from observation of the decay of these product states at various He and Ne pressures. With the exception of Xe(2p5) in He and Ne and Xe(2p7) in He, the decay constants of the Xe(6p) states are less than 1.0×10−11 cm3 s−1, and these rate constants are smaller then the decay constants for heavier rare gases. The collisional depolarization of Xe(2p6) by Ne was studied by monitoring the decay rates of the vertical and horizontal polarization components of the fluorescence intensity.
Photoelectron spectroscopy of free radicals with cm−1 resolution: The benzyl cation
Authors: Eiden Gregory C. , Weinhold Frank , Weisshaar James C.
Published: July 2002

We report four vibrational bands from 0–650 cm−1 for benzyl+−h7, benzyl+−αd2, and benzyl+−d7. The exocyclic C—C bond of benzyl cation has substantial double bond character. Band assignments from ab initio frequencies illuminate the mechanism of vibronic coupling in the 1 2A2–2 2B2 system of neutral benzyl. The adiabatic ionization potential of benzyl−h7 is 7.2484±0.0006 eV.
Phase equilibria and critical behavior of square-well fluids of variable width by Gibbs ensemble Monte Carlo simulation
Authors: Vega Lourdes , de Miguel Enrique , Rull Luis F. , Jackson George , McLure Ian A.
Published: July 2002

The vapor–liquid phase equilibria of square-well systems with hard-sphere diameters σ, well-depths ε, and ranges λ=1.25, 1.375, 1.5, 1.75, and 2 are determined by Monte Carlo simulation. The two bulk phases in coexistence are simulated simultaneously using the Gibbs ensemble technique. Vapor–liquid coexistence curves are obtained for a series of reduced temperatures between about Tr=T/Tc=0.8 and 1, where Tc is the critical temperature. The radial pair distribution functions g(r) of the two phases are calculated during the simulation, and the results extrapolated to give the appropriate contact values g(σ), g(λσ−), and g(λσ+). These are used to calculate the vapor-pressure curves of each system and to test for equality of pressure in the coexisting vapor and liquid phases. The critical points of the square-well fluids are determined by analyzing the density-temperature coexistence data using the first term of a Wegner expansion. The dependence of the reduced critical temperature T*c=kTc/ε, pressure P*c=Pcσ3/ε, number density ρ*c=ρcσ3, and compressibility factor Z=P/(ρkT), on the potential range λ, is established. These results are compared with existing data obtained from perturbation theories. The shapes of the coexistence curves and the approach to criticality are described in terms of an apparent critical exponent β. The curves for the square-well systems with λ=1.25, 1.375, 1.5, and 1.75 are very nearly cubic in shape corresponding to near-universal values of β (β≊0.325). This is not the case for the system with a longer potential range; when λ=2, the coexistence curve is closer to quadratic in shape with a near-classical value of β (β≊0.5). These results seem to confirm the view that the departure of β from a mean-field or classical value for temperatures well below critical is unrelated to long-range, near-critical fluctuations.
The interplay of tunneling, resonance, and dissipation in quantum barrier crossing: A numerical study
Authors: Tanimura Yoshitaka , Wolynes Peter G.
Published: July 2002

We discuss the interplay of various quantum effects on barrier crossing for a one-dimensional system with dissipation. This is based on a numerical study using a hierarchy of kinetic equations introduced by Tanimura and Kubo. The numerical work uses a grid in phase space for the Wigner distribution and deals with both the classical limit and the tunneling regimes.
Diffusion and reaction in multicomponent systems via cellular-automaton modeling: A+B2
Authors: Mai J. , von Niessen W.
Published: July 2002

A cellular-automaton model for the diffusion on a lattice is introduced. The spatial and temporal correlations that appear in the model are compared with the results of the correlated random walk, which has been obtained by Monte Carlo simulations. Very good agreement was found between these completely different simulations for the spatial correlations. The temporal correlations are quite different because of the nonequivalent temporal evolution of the two models. This new cellular automaton model is used to describe a reaction-diffusion system that represents the catalytic oxidation of CO on a metal surface.
Dynamic particle aggregation in the bimolecular A+B→0 reaction
Authors: Kuzovkov V. , Kotomin E.
Published: July 2002

The effect of an elastic (U=−λ/r3) interaction between similar particles on the kinetics of the bimolecular A+B→0 reaction is studied for the first time in terms of a novel formalism of the many-point particle densities taking into account the spatial correlations of both similar and dissimilar particles. It is shown that an elastic attraction results in the dynamic aggregation of similar particles and therefore in the reduced reaction rate unusual for the standard chemical kinetics. The distinctive time of such aggregate formation depends on the initial particle concentration and on their spatial distribution but is always considerably shorter than that for statistical aggregation of similar noninteracting particles. The latter is a distinctive feature of the self-organization in bimolecular reactions at long times and/or great particle concentrations. For the same value of the interaction parameter λ an attraction of similar particles has a greater impact on the reaction kinetics than that of dissimilar particles.
Resonance hyper-Raman scattering polarization. A measure of methyl iodide B-state subpicosecond lifetimes
Authors: Campbell D. J. , Ziegler L. D.
Published: July 2002

The resonance hyper-Raman (RHR) scattering of CH3I vapor is observed as blue incident radiation is tuned through two-photon resonance with the ν2 vibronic absorption band of the predissociative B̃←X̃ Rydberg transition in the UV. In analogy to linear resonance Raman spectroscopy, the RHR band shapes and polarization are found to be a sensitive function of the two-photon resonant vibronic state lifetime and detuning. The dependence of these scattering characteristics on the two-photon excited-state dephasing constant provides a technique for determining subpicosecond predissociation rates via nonlinear spontaneous Raman scattering. Theoretical fits to the observed resonant rovibrational hyper-Raman depolarization ratio dispersion curves are obtained for a vibronic B-state lifetime of 0.5±0.2 ps. This lifetime value is in agreement with previous results obtained from linear resonance Raman scattering studies. The polarization analysis of RHR scattering, in particular, provides a technique for measuring excited-state lifetimes of molecular transitions in the VUV.
Millimeter and submillimeter wave spectroscopy of HNC and DNC in the vibrationally excited states
Authors: Okabayashi Toshiaki , Tanimoto Mitsutoshi
Published: July 2002

The rotational transitions of hydrogen isocyanide (HNC) and deuterium isocyanide (DNC) in the vibrationally excited states as well as in the ground states were observed in the millimeter and submillimeter wave region. These compounds were generated in a dc glow discharge plasma containing hydrogen (or deuterium), nitrogen, and carbon atoms. The stretching vibrational modes, ν1 and ν3 states, were selectively excited in the discharge plasma; on the other hand, the bending mode ν2 state was thermally populated at the cell temperature. The precise rotational, centrifugal distortion and l-type doubling constants were obtained for all of the first vibrationally excited states as well as the ground states. The experimental equilibrium rotational constants Be are 45 496.7769(45) and 38 207.7217(105) MHz for HNC and DNC, respectively, where uncertainties correspond to one standard deviation. The equilibrium internuclear distances are also determined to be re (H–N)=0.996 0643(29) Å and re (N≡C)=1.168 3506(16) Å.
Stark quantum beats and electric dipole moment in the S1 state of pyrimidine vapor
Authors: Ohta Nobuhiro , Tanaka Takehiko
Published: July 2002

Stark quantum beat spectroscopy is applied to pyrimidine vapor in a supersonic jet with excitation at the R(0) and R(1) rotational lines of the 0–0 band belonging to the S0→S1 transition. The dependence of the amplitude and phase of the Stark quantum beat on the polarization of both incident light and emission as well as on the geometry with respect to excitation and detection is theoretically predicted, and the results are useful for identification of the observed Stark quantum beats. The electric dipole moment in the S1 excited state of pyrimidine vapor is evaluated to be 0.58 D, consistent results being obtained from experiments with the R(0) and R(1) excitations. A marked decrease of the dipole moment in going from S0 to S1, i.e., from 2.334 to 0.58 D, is consistent with the n→π* transition.
Dissociative photoionization of H2 from 18 to 124 eV
Authors: Chung Y. M. , Lee E.-M. , Masuoka T. , Samson James A. R.
Published: July 2002

Absolute cross sections for the production of H+ and H2+ have been measured from the dissociative ionization threshold at 18.076–124 eV. A broad resonance occurs in the H+ cross section caused primarily by autoionization of the Q1 Rydberg states. The absolute cross section for producing neutral atoms only is obtained by the use of published photoionization yield data. It is shown that the sum of the neutral and ionized H fragments produce a major resonance representing transitions into the Q1 and Q2 Rydberg states.
Diffusion in multicomponent liquids: A new set of collective velocity correlation functions and diffusion coefficients
Authors: Trullàs J. , Padró J. A.
Published: July 2002

A new set of time correlation functions for the description of the microscopic collective dynamic behavior in multicomponent liquids is introduced. These functions, the relative mean molecular velocity correlations, are independent of the considered reference frame and the related diffusion coefficients may be defined. The advantages of this kind of description are discussed and relations between the proposed diffusion coefficients and both phenomenological and empirical transport coefficients are derived. Special attention is devoted to the binary liquids.
Energy-structure relationships for microscopic solvation of anions in water clusters
Authors: Combariza Jaime E. , Kestner Neil R. , Jortner Joshua
Published: July 2002

In this paper we present a quantum-mechanical study of anions in water clusters, X−(H2O)n (X=Cl, Br, I, and n=1–6). Molecular orbital calculations at the self-consistent field (SCF) level and at the second-order Mo/ller–Plesset (MP2) level were performed using extended basis sets. Full structural optimization was conducted at the MP2 level for n=1 and at the SCF level for n=2–6. The energies and charge distribution of X−(H2O) were calculated at the MP2 level, while the energies of the X−(H2O)n (n=2–6) clusters were calculated at the MP2 level using the SCF optimized geometry. Calculations of total and sequential enthalpies of hydration and for the vertical ionization potentials were conducted for X−(H2O), the hydrogen bonded and linear isomers of X−(H2O)2, the pyramidal structure of X−(H2O)3, and the interior and surface isomers of X−(H2O)n, n=4–6. The calculated hydration enthalpies account well for their experimental size dependence for n=1–6. However, the isomer specificity of the hydration enthalpies is reflected by a small energy difference (δ=1–5 kcal mol−1) between the surface and interior isomers at a fixed n, precluding the assignment of structural isomers on the basis of ground-state energetics. The cluster size dependence and isomer specificity of the calculated vertical ionization potentials in conjunction with experimental data provide a diagnostic tool for the structural assignment of isomers and for the distinction between surface and interior structures. The central prediction emerging from the structure-energetic relations based on cluster size dependence and isomer specificity of vertical ionization potentials, is the prevalence of surface structures for Cl−(H2O)n (n=2–6), Br−(H2O)n (n=2–6), and I−(H2O)n (n=2–5), while a ‘‘transition’’ from surface to interior structure may be exhibited for I−(H2O)6.
On some structural properties of various electronic states of the hydroxonium ion
Authors: Di Giacomo F. , Gianturco F. A. , Raganelli F. , Schneider F.
Published: July 2002

Calculations have been carried out for several excited electronic states of the protonated water molecule the hydroxonium ion H3O+ at the level of highly correlated, multireference double-excitation configuration-interaction (MRD-CI) computations for several potential energy profiles of the system. It was found that different structural geometries exist for the ground and for the excited electronic states and that such geometries are very helpful for a quantitative interpretation of previous mass spectrometric fragmentation patterns. The general features of the umbrella motion in several electronic states are described for the title system and their importance for the analysis of the above spectra is shown for the first time. The possible extension of the present study to analyze charge-exchange and direct inelastic collision experiments in molecular beams is briefly outlined.
Comment on ‘‘Adsorption kinetics on chemically modified or bimetallic surfaces’’ [J. Chem. Phys. 100, 664 (1994)]
Authors: Cassuto A.
Published: July 2002

Using relations existing at equilibrium between adsorption, desorption, and migration rates in adsorption models with precursors, it is shown that when the condensation coefficients are equal, an implicit assumption in Zhu and Koel’s paper [J. Chem. Phys. 100, 664 (1994)], the coverage and temperature dependence of the sticking coefficient on two component surfaces simplify. New equations are derived for nondissociative and dissociative adsorption.
A nonlinear integral equation theory for the optical dielectric properties of a polarizable fluid
Authors: Winn M. D. , Kahl G.
Published: July 2002

In a previous paper, we described a fast and reliable numerical method for obtaining the optical absorption spectrum of a fluid of nonpolar linearly polarizable molecules. The fluid is modeled by a generalization of the microscopic classical Yvon–Kirkwood equations, which yields the same dynamic response as the much-studied quantum Drude oscillator model. Numerical results were presented based on a linear closure relation to the central Ornstein–Zernike analog equation. In the present paper, we consider a nonlinear closure which includes but goes beyond the previously studied linear closure. We display the absorption spectrum, as implied by the renormalized polarizability and the dynamic dielectric constant, for both hard sphere and Lennard-Jones fluids. Comparison with available simulation results shows that the nonlinear closure performs well over a wide density range, and in particular corrects the poor low-density behavior of the linear theory.
Steady state thermodynamics for homogeneous chemical systems
Authors: Koutselos Andreas D.
Published: July 2002

We contribute to the development of steady state thermodynamics for isothermal and homogeneous chemical systems, through a generalized Einstein fluctuation relation, by utilizing a local steady state interpretation of static concentration fluctuations of reaction intermediates. For systems of constrained total chemical affinity the ‘‘next-particle ratio’’ of the probability density, q=P(N+1)/P(N), derived from a usual chemical master equation is employed for the construction of chemical potential steady state laws. This analysis is based on the identification of the exponent of the distribution, as a generalized availability of local fluctuations. Further, pressure steady state laws are derived through a generalized Gibbs–Duhem equation, restricted to constant ‘‘disequilibrium variables.’’ Such variables are introduced as state variables, in addition to the classic ones, for the characterization of steady states. They relate to externally controlled generalized forces or affinities, which induce the flows of mass through the system. Within the local steady state approach, the state laws enable the construction of the generalized availability, as state function for quasisteady state processes beginning from a reference state. This quantity is found to provide a Liapounov function for the deterministic evolution of the system towards stationary states in analogy to a previously developed local equilibrium theory. The analysis is applied to two-variable chemical systems of high stoichiometry change, but should be capable of extension to general hydrodynamic systems.
Physical interaction between tip and molecules in scanning force microscopy imaging of adsorbed C60 and fullerene tubules
Authors: Dereux Alain , Girard Christian , Martin Olivier J. F. , Lambin Philippe , Richter Henning
Published: July 2002

After the discovery of C60, a large family of fullerene molecules was also identified. Among them, elongated fullerenes are formed by the tubular assembly of carbon atoms. The van der Waals bonds between fullerene molecules are due to the correlations between fluctuating charge densities inside the molecules. The interaction is then dominated by collective excitations which are sensitive to the shape of the molecules. Therefore, van der Waals attraction is expected to be modified when considering successively spherical C60, C70 and more elongated fullerenes (tubules). This paper presents self-consistent computations of the van der Waals interaction between a (111) diamond probe tip and various fullerene molecules adsorbed on a gold surface. Relative to spherical C60, the dependence law of the force experienced by the probe tip as a function of the tip-sample distance decreased when approaching fullerene tubules. Simulations of scanning force microscope scans of carbon tubules next to C60 molecules show that the shape of the molecules affects the interpretation of scanning force microscopy imaging. Particularly, information about the height of the various molecules deposited on the surface must be considered with some care since carbon tubules with the same radius as C60 interact more strongly with the probe tip.
Interaction of platinum films with the (0001̄) and (0001) surfaces of ZnO
Authors: Petrie W. T. , Vohs J. M.
Published: July 2002

In this investigation, the growth, structure, and electronic properties of Pt films on the polar surfaces of ZnO were examined using high-resolution electron energy-loss spectroscopy (HREELS) and low-energy, electron diffraction (LEED). The growth mode of vapor-deposited Pt films on ZnO(0001̄) and ZnO(0001) at 300 K was found to be nearly layer-by-layer. The surfaces of Pt films produced in this manner exhibited hexagonal symmetry and were stable up to 600 K. At higher temperatures, the Pt agglomerated into particles which remained oriented with respect to the ZnO substrate. HREELS results indicate that there are only weak interactions at the Pt/ZnO(0001̄) interface, while charge transfer and Schottky barrier formation occurs at the Pt/ZnO(0001) interface.
Unrestricted compact model potentials for ab initio embedded cluster calculations: Magnetic interactions in KNiF3
Authors: Mejías J. A. , Sanz Javier Fernández
Published: July 2002

Compact model potentials to introduce the effect of spin-dependent environments in ab initio embedded cluster calculations are reported. The groups forming the environment are described by unrestricted Hartree–Fock wave functions. The method is tested for the magnetic description of KNiF3 by using different model clusters. The cluster calculations are done at the unrestricted Hartree–Fock and unrestricted second-order perturbation levels. The obtained values are in excellent agreement with other more sophisticated ab initio calculations if some Ni–F delocalization is allowed. How the superexchange interaction is accounted for in our method is also discussed.
The effect of the range of the potential on the structures of clusters
Authors: Doye Jonathan P. K. , Wales David J. , Berry R. Stephen
Published: July 2002

We investigate the structures of clusters bound by the Morse potential by mapping the structure of the global minimum as a function of both cluster size and the range of the pair potential. We consider values of the range parameter appropriate to a loosely bound diatomic molecule (longest), two C60 molecules (shortest), and at regular intervals between these two limits. We have studied all cluster sizes with 25 atoms or less and a selection of sizes containing between 35 and 80 atoms. The effect of decreasing the range of the potential is to destabilize strained structures. For the larger clusters the structure of the global minimum changes from icosahedral to decahedral to face-centered cubic as the range is decreased. We have also investigated the effects of temperature on the equilibrium structure by performing a model calculation for a 75-atom cluster.
Approximations for the direct correlation function in multicomponent molecular fluids
Authors: Chamoux A. , Perera A.
Published: July 2002

Analytical approximations for the pair direct correlation function (DCF) of molecular fluids and their mixtures are derived within the frame of a new formalism based on weighted density functional methods which represents a generalization of Rosenfeld theory for hard spheres mixtures [J. Chem. Phys. 89, 4271 (1988)]. These approximations rest upon the geometrical properties of individual molecules such as the volume, the surface, and the mean radius. They are Percus–Yevick (PY) like in nature and reduce to the analytical PY solution for DCF in the hard sphere case. By construction the approximations incorporate several interesting features: They yield the Mayer function in the low density limit as expected, and they are anisotropic at zero separation as well as at contact. In addition they predict an orientational instability of the isotropic phase with respect to the nematic phase, a feature that is absent from the Percus–Yevick theory. Comparisons are made with the Percus–Yevick numerical results for the DCF for various convex hard bodies such as hard ellipsoids of revolutions (prolate and oblate), prolate spherocylinders, cutspheres, and generally the agreement is very good for a large range of liquid densities. Analytical expressions for the virial and compressibility routes for the pressures are also given. The results obtained for a large varieties of convex bodies are in very good agreement with corresponding numerical Percus–Yevick results. These approximations can be generalized to inhomogeneous systems in a straightforward manner.
Perturbative treatment of triple excitations in coupled-cluster calculations of nuclear magnetic shielding constants
Authors: Gauss Jürgen , Stanton John F.
Published: July 2002

A theory for the calculation of nuclear magnetic shielding constants at the coupled-cluster singles and doubles level augmented by a perturbative correction for connected triple excitations (CCSD(T)) has been developed and implemented. The approach, which is based on the gauge-including atomic orbital (GIAO) ansatz, is illustrated by several numerical examples. These include a comparison of CCSD(T) and other highly correlated methods with full configuration interaction for the BH molecule, and a systematic comparison with experiment for HF, H2O,NH3, CH4, N2, CO, HCN, and F2. The results demonstrate the importance of triple excitations in establishing quantitative accuracy. Finally, the ability of GIAO-CCSD(T) to make accurate predictions for difficult cases is explored in calculations for formaldehyde (CH2O), diazomethane(CH2NN), and ozone (O3).
Mechanism for the desorption of molecularly and dissociatively adsorbed methane on Pt(111) probed by pulse-laser heating
Authors: Watanabe Kazuo , Lin M. C. , Gruzdkov Yuri A. , Matsumoto Yoshiyasu
Published: July 2002

Dynamics of the thermal desorption of CD4 molecularly and dissociatively adsorbed on Pt(111) has been studied by laser-induced thermal desorption (LITD). CD4 adsorbates were prepared at the surface temperature of 55 K and the coadsorbates of CD3 and D were prepared by UV photolysis of CD4 adsorbates. The thermal desorption was induced by irradiating the surface with pulses of a frequency-doubled Nd:YAG laser and time-of-flight distributions of CD4 in each desorption channel were measured as a functions of the desorption angle (θ). The desorption of molecularly adsorbed CD4 shows a broad angular distribution (cos2.5 θ) and its translational energy distribution can be approximately described by the Maxwell–Boltzmann distribution with the temperature close to the estimated maximum surface temperature. In contrast, associatively formed CD4 desorbs sharply along the surface normal (cos31 θ) and its average translational energy is almost constant at 0.43 eV in the range of θ=0°–25°. The experimental results are compared with the calculated ones obtained from the statistical model developed by Ukraintsev and Harrison [J. Chem Phys. 101, 1564 (1994)]. Although the statistical model apparently succeeds in describing some of adsorption/desorption features by fitting the data of sticking coefficient measurements with molecular beams, it fails to predict the LITD data for the reverse process when their fitted data are applied in total.
Melts of semiflexible diblock copolymer
Authors: Matsen M. W.
Published: July 2002

Melts of semiflexible AB diblock copolymers are studied using the wormlike chain model and self-consistent-field theory. The diblocks considered here are symmetric in composition and in A- and B-block flexibility. Under these conditions, the microstructure formed is lamellar. We find that an increase in molecular rigidity moves the order–disorder transition from χN=10.495 for Gaussian chains to χN=6.135 for rigid rods. In the ordered state, rigidity has little effect on the exponents that describe the scaling of the domain spacing and of the interfacial width with molecular weight. However, the proportionality constants are significantly affected. The spatial and orientational distribution of segments in the ordered microstructure is examined for a selection of conditions.
Efficient and general algorithms for path integral Car–Parrinello molecular dynamics
Authors: Tuckerman Mark E. , Marx Dominik , Klein Michael L. , Parrinello Michele
Published: July 2002

In path integral molecular dynamics, efficient sampling of the phase space is not guaranteed due to the stiff harmonic part of the action arising from the quantum kinetic energy. This problem has been eliminated by incorporating a sufficient number of thermostats into the dynamical scheme and by introducing a transformation of the path ‘‘bead’’ variables. In this paper, an efficient Car–Parrinello path integral molecular dynamics algorithm, sufficiently general to include the use of ultrasoft pseudopotentials is introduced. Difficulties encountered when combining thermostats and transformations of the Cartesian ‘‘bead’’ coordinates with the generalized orthonormality condition are circumvented by employing a constrained nonorthogonal orbital method.