bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of interaction between a solid inclusion in a liquid with nonuniform viscosity with an advancing solidification front
Authors: Raitchenko A. I. , Mukai K.
Published: July 2002

A theory of behavior of foreign solid inclusions in a liquid being crystallized is developed. As the temperature of the liquid is nonuniform near the solidification front and the viscosity of the liquid depends on temperature, a dependence of viscosity on the distance from the solid-crystal surface is taken into account in the proposed theory. The dependence of the inclusion/liquid interface specific surface free energy at the same distance from the crystal surface has major importance in the present theory. A model is used in which the weight of the particle, its buoyancy, a force due to the gradient of interfacial tension, and that of viscous drag are important. Attention is paid to the case when the velocities of the inclusion and of the solidification front move in the same direction. A peculiarity in the action of viscous drag is ascertained; namely, the drag force diminishes with the movement of the inclusion towards the region of decreasing viscosity in a liquid with nonuniform viscosity. A correction factor for the Stokes formula is derived which comprises the influences of the presence of the solidification front and of the nonuniformity of viscosity. The analysis of inclusion behavior is performed in terms of three dimensionless variables; one of these variables is the modified Weber number; the other two are dimensionless complexes having the following meanings: (1) the relation of the drag force in a homogeneous viscous medium to the buoyancy force, and (2) the relation of the reduction in the viscous drag force to the buoyancy force (the former decreases with distance as a result). The known experimental data are explained in terms of the relationships between inclusion dimensions, the so-called “critical velocity” for engulfment by the solidification front, and the existence of the variability of viscosity with distance ahead of the solidification front.
Resistance degradation in barium strontium titanate thin films
Authors: Zafar Sufi , Hradsky Bruce , Gentile D. , Chu P. , Jones R. E. , Gillespie S.
Published: July 2002

Experimental and modeling results for resistance degradation in thin Ba0.5Sr0.5TiO3 (BST) film capacitors with platinum (Pt) electrodes are reported. The main experimental results are as follows. Under a constant applied voltage, the current density is observed to increase with time until it reaches a maximum value. Once the maximum value is reached, the current density becomes constant with time. The barrier height at the BST/Pt (cathode) interface is observed to decrease after prolonged electrical stressing. The resistance degradation effect is observed to be reversible, particularly at elevated temperatures. Based on the experimental results, a quantitative model for resistance degradation is proposed. In this model, the increase in the current density is attributed to a decrease in the barrier height at the cathode and this decrease is assumed to have a stretched exponential dependence on time. Using experimentally determined parameters, the model calculates the current density as a function of time at various temperatures. The calculated results are verified and the model is shown to be self-consistent. Hence the model provides an accelerated method for determining the lifetime of thin BST films at the operating conditions for advanced memory applications.
Roughness evolution of Cu6Sn5 intermetallic during soldering
Authors: Zuruzi A. S. , Chiu C.-h. , Lahiri S. K. , Tu K. N.
Published: July 2002

The roughness evolution of the Cu6Sn5 intermetallic compound during the soldering reaction of 40/60% (by wt %) PbSn solder on Cu and its effects on wettability were studied due to their importance in the reworking of microelectronic components. The roughness parameters investigated were Rrms and the ratio, Rrms/λa, where Rrms is the root mean square roughness and λa is the average distance between asperities. It was found that Rrms increased with soldering time for the range of soldering times studied. The evolution of Rrms was found to follow a parabolic relation with soldering time—which is similar to that observed for the intermetallic thickness. However, the ratio Rrms/λa was found to increase very gradually for the range of soldering times investigated. From the ratio Rrms/λa obtained, the average inclination of the intermetallic morphology was calculated. The average inclination of the Cu6Sn5 intermetallic morphology was found to be greater than the apparent contact angle observed during the wetting of the 40/60% PbSn solder on Cu substrates. This inconsistency was attributed to the difference in the roughness of the Cu6Sn5 intermetallic in the inner regions and at the reaction band at the edge of the solder cap.
Evidence for the alteration of an organic/metal interface resulting from the formation of a broad interfacial layer
Authors: Bonello B. , Armand F. , Pradeau J-P. , Perez H. , Perrin B. , Louis G.
Published: July 2002

The alteration of the organic/electrode interface is partially responsible for the deterioration of the electrical efficiency of some molecular electronic devices. We used the picosecond ultrasonics to investigate the changes that occur at the interface copper-phthalocyanine/M (M=Al or Au) after the electrode deposition: the absorption of an ultrashort laser pulse sets the heterostructure into vibration and the nature of the interface is deduced from the photoelastic response of the samples. We show that a broad interfacial layer is formed, the thickness of which is estimated. We show also that the slow migration of metallic particles within the organic layer leads, after a few weeks, to a broadening of the interfacial region.
Does normal boiling exist due to laser-pulse or ion bombardment?
Authors: Kelly Roger , Miotello Antonio
Published: July 2002

We consider here the process of normal boiling as induced by laser-pulse or ion bombardment. Normal boiling (thenceforth “boiling”) refers to the appearance of heterogeneously nucleated bubbles which diffuse towards the outer surface of a liquid and, if the surface is reached, may possibly escape. We will here present evidence that boiling, whether the distance scale is atomically small (5–15 nm, as for laser-pulse or ion impact on a metal in the absence of thermal diffusion) or much larger, has a prohibitive kinetic obstacle because it requires bubble diffusion. That is to say: boiling will never be a significant process for bombardment with laser pulses which are short (<1 μs) or with ions in general. This leaves vaporization and phase explosion as the only possible thermal-spike processes capable of expelling material from a laser-pulse or ion bombarded surface in a significant quantity.
“Dynamic-thermal” reversal in a fine micromagnetic grain: Time dependence of coercivity
Authors: Safonov Vladimir L. , Bertram H. Neal
Published: July 2002

An analytic model for reversal field versus pulse time in a fine magnetic grain has been developed for the case when the external magnetic field H0 is parallel to the axis of uniaxial anisotropy. The expressions include both thermal fluctuations and magnetization dynamics. Therefore application is for fields both less than and greater than the intrinsic uniaxial anisotropy field HK. For the case of −H0/HK>1 the mean first passage time from the initial saturated state to the thermodynamically equilibrium reversed state is calculated. For the case of −H0/HK<1 we calculate the mean first passage time from the thermodynamically equilibrium state of the metastable well to the thermodynamically equilibrium stable reversed state. It is shown that simple Néel–Arrhenius analysis is applicable for times greater than at least 100/αγHK.
Magnetization processes in two different types of anisotropic, fully dense NdFeB hydrogenation, disproportionation, desorption, and recombination magnets
Authors: Gutfleisch O. , Eckert D. , Schäfer R. , Müller K. H. , Panchanathan V.
Published: July 2002

Two types of textured, fully dense NdFeB hydrogenation, disproportionation, desorption, and recombination (HDDR) magnets were produced. The first type was produced by hot pressing isotropic HDDR powder followed by die upsetting; the second, by hot pressing prealigned, anisotropic HDDR powder (MQA-T). Studies of the magnetization processes revealed that for isotropic HDDR powder and its hot pressed and die-upset magnets a much larger initial susceptibility is found after thermal demagnetization than after reverse dc-field demagnetization. Prealigned, hot pressed magnets made from MQA-T material showed a different virgin magnetization curve, indicating a unique coercivity mechanism. Interaction domains larger than the average grain size can be observed in both cases by Kerr microscopy, with the MQA-T type showing significantly broader interaction domains.
Current source imaging for high spatial resolution magnetocardiography in normal and abnormal rat cardiac muscles
Authors: Uchida S. , Iramina K. , Goto K. , Ueno S.
Published: July 2002

The purpose of our study was to identify the current source produced by acute ischemia and infarction. We measured magnetocardiograms (MCG) and electrocardiograms (ECG) of five male rats using a high-resolution dc superconducting quantum interference device gradiometer in a magnetically shielded room after performing coronary artery occlusion. The spatial resolution of the detecting magnetic field of our system is higher than the typical system, thus permitting the measurement of magnetic fields in small animals. Distribution of the magnetic fields B(t) and distribution of |rot B(t)|, which corresponded to the distribution of the current source, were imaged by 12-channel MCGs. As a result, the distribution of current source changes in the affected area of the myocardium during the ST segment, and amplitude of the peak significantly increased after occlusion. Our system can be used to help clarify the mechanism of the ST shift related to severe heart disease.
Programmable logic using giant-magnetoresistance and spin-dependent tunneling devices (invited)
Authors: Black William C. , Das Bodhisattva
Published: July 2002

Programmable logic functions may be realized with giant-magnetoresistance or spin-dependent tunneling devices in conjunction with relatively simple circuitry. These functions may be implemented as conventional programmed logic arrays using the magnetic devices as nonvolatile programming elements or as arrays of universal logic gates whereby the core logic functions are magnetically programmed. Both methodologies are described in this article along with rapidly reconfigurable variations that have applications in high performance and adaptive computing. Various design considerations of these schemes are presented along with simulation results of proposed circuits. Among the potential advantages of these magnetic logic devices are nonvolatility of both programming information and data and the ability to reconfigure an entire logic array into any one of a number of configurations in a single clock cycle.
Connection between hysteresis, Barkhausen noise, and microstructure in magnetic materials
Authors: Durin G. , Beatrice C. , Appino C. , Basso V. , Bertotti G.
Published: July 2002

The interplay between material microstructure and magnetic hysteresis is studied in rapidly quenched Si–Fe alloys. Two ribbons of different average grain dimension 〈s〉 (35 and 160 μm) were prepared by annealing at different temperatures and studied through two independent approaches: Barkhausen noise measurements, and Preisach analysis of static and dynamic hysteresis loops. In order to monitor the effect of demagnetizing fields on the magnetization process, the strips were progressively shortened from 30 to 10 cm. The correlation length of a domain-wall jump was estimated through the analysis of Barkhausen jump distributions versus apparent permeability. The correlation length of the coherent magnetization reversals controlling excess dynamic losses was estimated through the Preisach analysis of dynamic hysteresis loops. In the sample with lower 〈s〉, both the Barkhausen and the dynamic loss correlation lengths are comparable to 〈s〉, showing that a single structural feature governs all aspects of magnetization reversal. Conversely, in the high 〈s〉 sample, the ribbon thickness competes with 〈s〉 in controlling static and dynamic magnetization processes.
Spectroscopic ellipsometry of epitaxial ZnO layer on sapphire substrate
Authors: Postava K. , Sueki H. , Aoyama M. , Yamaguchi T. , Ino Ch. , Igasaki Y. , Horie M.
Published: July 2002

Optical properties of epitaxial ZnO layers have been studied in the spectral region from 1.5 to 5.4 eV using four-zone null spectroscopic ellipsometry. An existing model dielectric function based on excitonic structure near direct band gap has been improved by including a high-energy absorption term. Surface layer, corresponding to the surface roughness, was found to be essential to fit the spectroellipsometric data obtained. Two kinds of samples have been studied: ZnO layers prepared on (0001) and (112̄0)-oriented sapphire substrates. The surfaces of the first ones were found to be more rough.
Role of surface Al2O3 clusters on the kinetics of Al(111) oxidation
Authors: Popova I. , Zhukov V. , Yates J. T.
Published: July 2002

The effect of aluminum-oxide clusters on the Al(111) surface on subsequent oxygen adsorption at 300 K was investigated for both low and high degrees of oxidation using x-ray photoelectron spectroscopy and high-resolution electron energy-loss spectroscopy. Oxide clusters on the Al(111) surface were produced by annealing chemisorbed O layers of various coverage up to 773 K. Only the chemisorbed oxygen phase was found to form at 243–300 K after additional O2 exposure on surfaces containing both low- and high-oxide-cluster populations. The chemisorbed O adatoms produced were not influenced by the oxidic phase, exhibiting spectroscopic features similar to those observed upon chemisorption on the clean surface without oxide clusters. Oxygen adsorption on the surface with both low- and high-oxide-cluster populations proceeded with the same rate as found on the clean Al(111) surface. The observed effect is postulated to be due to an increase of the number of the vacant Al adsorption sites liberated upon transformation of chemisorbed O adatoms into oxide clusters.
On the dynamic growth of a 180° domain in a ferroelectric material
Authors: Wang Biao , Xiao Zhongmin
Published: July 2002

This article investigates the dynamic movement process of a 180° domain wall in an external electric field. By modeling the domain wall movement as a dynamic process, the induced electric and magnetic fields due to a growing general ellipsoidal 180° domain under the action of a given electric field was derived explicitly. A general form of the energy release rate was derived through a surface integral, which can serve as the driving force for the movement of the surface. Based on the solution, the released electromagnetic energy during the dynamic process was evaluated. This electromagnetic energy is used to compensate the surface energy increase and the energy dissipation due to the finite velocity of the domain wall movement. Following the result, an equation is established to determine the growth speed of the 180° domain. Finally, the cases where the domain shape is cylindrical and spherical, and the domain wall propagates along the radial direction and the applied electric field direction, respectively, were considered in detail as illustrative examples. It is found that the resistance caused by the depolarization field for the wall to grow along the direction of the applied electric field is only one fifth of the value for the wall to grow along the radial direction.
A perturbation theory of classical solids
Authors: Kang Hong Seok , Ree Taikyue , Ree Francis H.
Published: July 2002

We have developed a new perturbation theory that extends our earlier perturbation theory of fluids to solids and that is reliable over a wide solid region. Characteristic features of this new theory are the use of an optimized reference potential whose repulsive range shrinks with density and its ability to deal with both harmonic and anharmonic thermodynamic properties on equal footing. Thermodynamic properties of face-centered-cubic crystals are computed from the new theory for the Lennard-Jones system, the exponential-6 system, and the inverse nth-power (n=12, 9, 6, and 4) systems. Monte Carlo simulations are also performed to supplement available data. A comparison of theory and computer simulation shows excellent agreement, except for the softest repulsive system (n=4). The agreement extends from an anharmonic region near the melting line to a harmonic region, where the hard-sphere reference system achieves close to 92% of the close-packed density. Beyond this region errors in the analytic fits to the hard-sphere radial distribution functions used in this work make an accurate test of the new theory difficult. Since the present formulation is the same for both solid and fluid phases, we used the theory to compute the melting and freezing data of the aforementioned model systems. Agreement with the corresponding Monte Carlo data is satisfactory. Comparison with other theoretical models of solids is also discussed.
Dielectric relaxation and intramolecular electron transfers
Authors: Sumi H. , Marcus R. A.
Published: July 2002

Intramolecular charge transfer are considered for the case that the motion of the system is on a single potential energy surface. The case where this motion occurred on two surfaces was considered elsewhere. The former is shown to be much preferable for studies of solvent dynamics. Several aspects of the relation between ‘‘constant charge’’ dielectric relaxation time of the polar solvent and the experimental decay time of emission from the polar excited state of the solute are discussed for hydrogen-bonded systems.
Vibrational energy levels of hydrogen cyanide
Authors: Dunn Kevin M. , Boggs James E. , Pulay Peter
Published: July 2002

Sixty-nine vibrational energy levels have been calculated for HCN, using an ab initio potential energy function adjusted to the fundamentals, and also an empirical potential function by Murrell et al. The energy levels were calculated variationally, using a novel direct CI procedure analogous to methods widely used in electronic structure theory. The present paper includes states with up to four quanta of excitation distributed among the four modes. Agreement with experimental spectra is good.
Phase transitions in Langmuir monolayers of polar molecules
Authors: Andelman David , Broçhard Françoise , Joanny Jean-François
Published: July 2002

Insoluble Langmuir monolayers are investigated in the presence of dipolar forces which can have two origins: permanent dipoles in neutral monolayers and induced dipoles in charged monolayers. The main effect of the additional long-range repulsive interactions is to stabilize undulating phases at thermodynamic equilibrium. Phase diagrams are obtained in two limits: close to the liquid–gas critical point via a Ginzburg–Landau expansion of the free energy (mainly within a mean-field approximation), and at low temperatures by free energy minimization. Possible applications of this theory to experiments at the liquid–gas, liquid expanded–liquid condensed, and solid–liquid transitions are discussed.
Phase space bottlenecks and statistical theories of isomerization reactions
Authors: Gray Stephen K. , Rice Stuart A.
Published: July 2002

We examine the dynamics of isomerization reactions and point out the existence of important phase space bottlenecks. A three-state statistical theory based on the behavior of trajectories and the properties of the phase space bottlenecks is proposed; this theory is better than standard two-state RRKM theory under some circumstances and offers a new way of conceptualizing isomerization. We test our ideas on the simple system, consisting of an equivalent double well coupled to a Morse oscillator, which was studied by DeLeon and Berne. Approximate methods of obtaining the required elementary rate constants in our theory are also discussed.
An EPR study of the luminescent triplet state of VO3−4 in YVO4 and YP0.96V0.04O4 single crystals at 1.2 K
Authors: Barendswaard W. , Weber R. T. , van der Waals J. H.
Published: July 2002

An EPR study with optical detection is reported of the luminescent triplet state of the VO3−4 ion in single crystals of YVO4 and YPO4 doped with VO3−4 . The experiments in an external magnetic field and in zero field can be fitted by a spin Hamiltonian for an orbitally nondegenerate triplet state. Signals originate from four symmetry-related species; the directions of the principal axes of the four ZFS tensors indicate a lowering of symmetry of the VO3−4 ion in accordance with a static Jahn–Teller effect by which the tetrahedron tends to a trigonal pyramid on excitation. For YVO4 the ZFS parameters are: ‖D‖=0.58±0.04, ‖E‖=13.72±0.04 GHz, and for YPO4: ‖D‖=1.12±0.04, ‖E‖=12.98±0.04 GHz (z axis∥V–O bond and y⊥ac or bc crystallographic plane). For the mixed crystal the lifetimes of the individual spin levels were determined: τx=0.35, τy=3.3, τz=1.4 ms.
The mean spherical approximation for an arbitrary mixture of ions in a dipolar solvent: Approximate solution, pair correlation functions, and thermodynamics
Authors: Wei Dongqing , Blum Lesser
Published: July 2002

The previously obtained solution of the mean spherical approximation (MSA) for an arbitrary mixture of ions and dipoles is discussed in the case of the low ionic densities (up to 3 M however). In this case, the solution is obtained by solving a system of only three algebraic equations for three parameters, an ion–ion scaling parameter Γ, an ion–dipole scaling parameter B10, and finally a dipole–dipole parameter b2. The thermodynamic properties, however, are better expressed in terms of a shifted screening parameter Γsj. The pair correlation functions and thermodynamics are discussed. The thermodynamics is known to agree fairly with more accurate theories like the LHNC. An attempt to fit the experimental hydration free energies shows that this simple model overestimates the structure breaking effect of the ions. If we empirically adjust this effect for one ion, then we get a surprisingly good correlation for monovanlent ions. The results for the pair correlation functions for the dipole–dipole and ion–dipole are reasonable. However the ion–ion correlation function exhibits the low density pathology of the MSA.
Polar ordering of quasiliquid crystals—An optical second harmonic generation study
Authors: Hsiung H. , Rasing Th. , Shen Y. R. , Shvartsman F. P. , Cabrera I. R. , Krongauz V. A.
Published: July 2002

Supercooled quasiliquid crystal films are studied by optical second harmonic generation. The experimental results indicate that a polar ordering of molecules exists in the absence of external fields. By applying an electrostatic field, the polar ordering can be modified in a manner predictable by a mean-field theory.
Methyl internal rotation in partially deuterated molecular solids: The NO2CHD2 and NO2CH2D systems
Authors: Cavagnat D. , Brom H. , Nugteren P. R.
Published: July 2002

Combining optical, neutron scattering and specific heat measurements, a precise determination of the parameters for the internal rotation potential energy function of the partially deuterated methyl group CHD2 and CH2D is obtained in crystallized nitromethane. The effects of other internal motions on the internal rotation are analyzed and compared to the corresponding results for similar molecules in condensed and gas phases. The origin of the difference in the zero point energy of the symmetrical and asymmetrical rotamers of the partially deuterated derivatives is discussed.
Torsional vibrational modes of (HF)3: IR–IR double resonance spectroscopy and electrical interaction theory
Authors: Kolenbrander Kirk D. , Dykstra Clifford E. , Lisy James M.
Published: July 2002

A concerted theoretical and experimental effort has been carried out to characterize and assign in-plane and out-of-plane torsional vibrational modes of hydrogen fluoride trimer. These vibrations are large amplitude motions which sample sizable regions of the intermolecular potential energy surface. The cyclic (HF)3 cluster was modeled as a three-dimensional system of fully coupled hindered rotors. Vibrational energy levels, wave functions, and oscillator strengths moments were calculated for each of the two vibrational systems. Potential energy surfaces for the interaction of the coupled rotors were calculated via a multipole moment and multipole polarizability electrostatic analysis. Complete vibrational manifolds for the two systems are presented. An important feature is that two vibrational bands, an in-plane overtone E′(vA′=0,vE′=2) and an out-of-plane combination E′(vA″=1,vE″=1), were calculated to exist within the frequency range of the CO2 laser. Using a two-laser, double-resonance technique, two rovibrational transitions were observed, corresponding to these predicted bands. Theoretical determinations of oscillator strengths and the effects of deuterium isotopic substitution on the vibrational energy levels were experimentally verified to establish the assignment of the observed transitions to their specific vibrational modes. The theoretical analysis was continued to examine the anharmonicity, amplitude of vibrational motion, and oscillator strengths of the vibrational modes of the cluster in detail. The experimentally determined linewidths of the observed transitions were compared to the previously reported linewidth of the (HF)3 predissociative H–F stretching mode.
Vibrational Stark effect of CO on Ni(100), and CO in the aqueous double layer: Experiment, theory, and models
Authors: Lambert David K.
Published: July 2002

The vibrational Stark effect (VSE), the effect of applied electrostatic field on a molecule’s vibrational frequency, is observed for adsorbates in ultrahigh vacuum (UHV) and in the electrochemical double layer. The VSE has been measured and compared with theory. Models of the double layer have also been compared with experiment. The vibrational Stark tuning rate of c(2×2) CO on Ni(100) at 300 K was measured using IR spectroscopy. The observed Stark tuning rate in terms of the applied E field δνE was (5.27±0.27)×10−7 cm−1/(V/cm). Quantum mechanics has been used to express δνE in terms of a molecule’s potential energy function and dipole moment function. The quantum theory confirms the accuracy of δνE predicted by a classical model. The theory, with potential energy and dipole moment functions observed for the experimental system, gives δνE =(4.7±1.1)×10−7 cm−1/(V/cm), in agreement with the present measurement. The fully resolved CO vibrational line shape was asymmetric with a 16 cm−1 full width at half-maximum. The measured IR cross section is consistent with a previous electron energy loss study of c(2×2) CO on Ni(100). Both experiment and theory find that the effect of applied E field on the IR cross section can be neglected in analyzing the electroreflectance spectrum. The Stark tuning rate in terms of the local E field is denoted δνe. The relationship between the applied E field, and the local E field acting on the adsorbed CO, was determined using the IR cross section. For CO on Ni(100), δνe was (1.42±0.31)×10−6 cm−1/(V/cm), in good agreement with a previously reported ab initio calculation for CO on Cu(100). In comparing different systems, theory predicts δνe to be approximately proportional to the dynamic dipole moment, consistent with the observed trend. The VSE has also been observed previously for CO adsorbed on electrodes in electrochemical cells. The present measurement of δνe is compared with the VSE for CO terminally bonded to Pt, Pd, and Au electrodes in aqueous solution. A direct comparison between experiment in UHV and at the double layer

can be made for Au. A model of the aqueous double layer proposed by Bockris, Devanathan, and Müller is consistent with observation. An alternative model, with the double layer potential across a CO monolayer, is also consistent with the data.
Equation of state of polymer melts: Numerical results for athermal freely jointed chain fluids
Authors: Schweizer Kenneth S. , Curro John G.
Published: July 2002

Our microscopic RISM integral equation theory for the virial equation of state of polymer liquids developed in the preceding paper is numerically implemented for athermal melts composed of freely-jointed chains interacting via hard core site–site potentials. A modified ideal description of the single chain intramolecular correlations is employed which rigorously enforces the nonoverlapping core condition and leads to significant local coil expansion. Comparison of the theoretically computed virial pressure for tangent diatomics and short chains with available Monte Carlo simulation results over a wide range of packing fractions suggests the theory is quite accurate. Significant inconsistencies between the pressure computed via the virial and compressibility routes are found and discussed in light of the known limitations of the RISM method and the importance of self-consistency corrections for flexible chain molecule liquids. A detailed numerical study of the density and degree of polymerization dependences of the total virial pressure, and its individual two- and three-body components, is presented, along with the limiting infinite chain behavior. The integral equation results are also compared with the predictions of several simple mean field and/or lattice models for both short chains and high polymers. Significant, and in some cases massive, differences are found between the predictions of the various approaches and the integral equation calculations which are attributed to the neglect of polymeric connectivity, intermolecular correlations, and/or the use of a lattice model inherent to the simple theories. In particular, both the density dependence of the pressure and its sensitivity to degree of polymerization are found to be much stronger than the simple theories predict due to self-screening and correlation hole effects absent in the latter. Finally, model calculations of the intermolecular radial distribution function and static structure factor at fixed pressure are performed for several degrees of polymerization and are found to be very weakly dependent on chain length due to compensating effects associated with a molecular weight dependent packing fraction.
Multiphoton ionization photoelectron spectroscopy study of OCS: Rydberg vibronic structure and ion state selection
Authors: Yang Baorui , Eslami Mohamad H. , Anderson Scott L.
Published: July 2002

Multiphoton ionization (MPI) in conjunction with photoelectron spectroscopy (PES) has been used to examine Rydberg states of OCS in the energy range from 70 547 to 75 472 cm−1. The combination of techniques allows us to correct previous assignments for several vibronic bands, and to probe vibronic mixing in the Rydberg levels. In the course of this study we have found routes for preparing OCS+ with control over both vibrational mode and fine structure state.
Two-dimensional spin–echo nuclear magnetic resonance in rotating solids
Authors: Kolbert A. C. , Raleigh D. P. , Levitt M. H. , Griffin R. G.
Published: July 2002

Two-dimensional spin–echo nuclear magnetic resonance (NMR) experiments on rotating powders are presented. A single π pulse in the center of the evolution period of a two-dimensional experiment produces an effective reduction of the spinning speed in an orthogonal frequency dimension. Sets of two-dimensional rotational sidebands are produced from which shielding tensors may be derived by comparison with numerical simulations. A skew projection of the 2D spectrum corresponds rigorously to the normal magic angle spinning NMR spectrum obtained at one-half the spinning speed. For small shielding tensors this corresponds to a considerable enhancement in sideband intensity, assisting the estimation of shift anisotropies without the loss in resolution associated with low actual spinning speeds. Symmetries in the intensities and phases of the 2D sidebands are shown to result from a time-domain symmetry in the powder-averaged signal. Cases where the evolution period includes two or more π pulses, and where a heteronuclear dipolar interaction is introduced, are also examined.
Resonant photodissociation of CoAr+ and CoKr+: Analysis of vibrational structure
Authors: Lessen Dan , Brucat P. J.
Published: July 2002

The transition-metal rare-gas diatomic ions, CoAr+ and CoKr+, generated and cooled in a supersonic expansion, are studied by visible resonant photodissociation for the first time. Photofragmentation excitation spectra exhibit sharp vibronic features which are members of several excited electronic state vibrational progressions in each molecular ion. Analysis of over 200 vibronic transitions in these spectra reveals details of the potential-energy surfaces characterizing the bonding in these excited states. The adiabatic ground-state dissociation energies of CoAr+ and CoKr+, determined as 4100 cm−1 and 5400 cm−1, respectively, are ca. 37% larger than the diabatic dissociation energy of an excited state which dissociates into 3d8 3P2 Co+1S Ar(Kr) excited atoms and 95% larger than a state dissociating into 3d74s 3F2Co+1S Ar(Kr) atoms. Vibrational frequencies, anharmonicities, electronic origins, and dissociation limits of three electronic states in each molecule have been determined. A simple electrostatic binding model for these transition-metal rare-gas species is discussed.
Characterization of transition metal–rare-gas cations: VAr+ and VKr+
Authors: Lessen Dan , Brucat P. J.
Published: July 2002

Resonant photodissociation of supersonically cooled and isolated VKr+ reveals a vibronic progression of a single electronic transition in the visible spectrum. Vibrational analysis of these data indicates an upper state vibrational frequency of 99 cm−1 and a diabatic upper state binding energy of 0.26 eV. Assignment of the dissociation limit of this upper state at 17 419 cm−1 to V+(3d84s 5P2)+Kr(1S0) places the adiabatic binding energy of the ground state of VKr+ at 0.49 eV. The spectrum of VAr+ is analogous to that of VKr+ but shows a somewhat reduced ground state adiabatic binding energy for this molecule, 0.38 eV. A simple inductive binding model is proposed to predict the geometries of these species and parametrize the metal–rare-gas interatomic potential. This potential is used to gain insight into the factors contributing to the enhanced stability of the ‘‘coordinatively saturated’’ complexes, VAr+4 and CoAr+6.
Fluids of dipolar hard ellipsoids: Structural properties and isotropic–nematic phase transitions
Authors: Perera A. , Patey G. N.
Published: July 2002

The hypernetted-chain (HNC) approximation is solved for fluids of dipolar hard ellipsoids and the structural, thermodynamic, and dielectric properties of the isotropic phase are discussed in some detail. Both prolate and oblate particles are considered and the isotropic–nematic transition is investigated using density functional theory (DFT). For fluids of prolate particles dipolar forces are found to have a significant effect upon the isotropic–nematic transition which occurs at lower densities as the dipole moment is increased. For the oblate case the dipolar interactions have only a very small, if any, influence upon the isotropic–nematic transition density. For both prolate and oblate particles the present HNC/DFT calculations do not predict ferroelectric nematic phases.
Photochemistry of oriented molecules coadsorbed on solid surfaces: The formation of CO2+O from photodissociation of O2 coadsorbed with CO on Pt(111)
Authors: Mieher W. D. , Ho W.
Published: July 2002

Measurement of a photoinduced reaction involving two types of molecular species coadsorbed with well-defined configurations on a solid surface is reported. The photoinduced reaction, occurring on Pt(111) at 100 K, is O2+CO+hν→O+{O→CO}→O+CO2. A mechanism involving photochemically produced hot O atoms (with high translational energy, and possibly electronically excited) is proposed, in which the initial step involves selective photodissociation of O2 coadsorbed with CO. The O atom collides with a neighboring CO and forms CO2 which desorbs immediately from the surface. The nature of the adsorbed species was probed before and after irradiation by thermal desorption spectroscopy (TDS) and high resolution electron energy loss spectroscopy (EELS). It was found that the wavelength dependence of the CO2 production followed that for O2 photodissociation. At 338 nm the cross section for CO2 production is 3.3±0.5×10−20 cm2 and decreases to 2×10−21 cm2 at 443 nm. CO2 was not observed in EEL spectra following quenching of thermoinduced or photoinduced reaction of either molecular or atomic oxygen with CO on Pt(111). CO2 was observed with EELS following direct CO2 adsorption on Pt(111) at 85 K. The results from these studies are believed to be generally relevant to understanding the dynamics of elementary chemical reactions.
The excitation of N2(B 3Πg, v=1–12) in the reaction between N2(A 3Σ+u) and N2(X, v≥5)
Authors: Piper Lawrence G.
Published: July 2002

We have studied the excitation of N2(B 3Πg, v=1–12) in the interaction between N2(A 3Σ+u) and N2(X 1Σ+g, v≥5). The N2(A) and N2(B) are observed spectroscopically between 220 to 400 nm and 560 to 900 nm, respectively, while the N2(X,v) number densities are determined by metastable-helium Penning ionization. The experiments are performed in a discharge-flow reactor with separate discharge sources of N2(A), N2(X,v) and He*(23S). The excitation rate coefficient is (3±1.5)×10−11 cm3 molecule−1 s−1. Observations of N2(A) decay indicate that the N2(A) is removed by N2(X,v) with an apparent rate coefficient of about 3.5×10−12 cm3 molecule−1 s−1. The discrepancy between the excitation and removal rate coefficients probably results from N2(A) regeneration via cascade from the excited N2(B). The appearance of vibrationally excited N2(A) when N2(X,v) is added to a flow of N2(A, v=0) demonstrates this regeneration process. The reaction appears to be a transfer of electronic energy from the N2(A) to the N2(X,v) rather than an excitation of the N2(A) to N2(B) resulting from the input of energy from the N2(X,v).
Chemical reaction driven phase transitions and critical points
Authors: Corrales L. René , Wheeler John C.
Published: July 2002

Two simple examples of model (mean-field) equations of state for phase equilibrium in chemically reactive systems are examined for ‘‘unexpected’’ phase equilibria. They are, essentially, exactly soluble and give classical critical behavior. One of these leads to a lower critical solution temperature that results in closed-loop coexistence curves similar to those seen in hydrogen-bonding mixtures. The second leads to less familiar, but interesting phase diagrams that exhibit a phenomenon analogous to critical azeotropy. The same phenomena occur in two examples of lattice gas models the partition functions of which can be mapped exactly to that of the Ising model thus resulting in nonclassical critical behavior. These models demonstrate how a chemical reaction can provide a mechanism leading to interesting phase equilibria and critical phenomena.
Structural relaxation and dynamical correlations in a molten state near the liquid–glass transition: A molecular dynamics study
Authors: Signorini Giorgio F. , Barrat Jean-Louis , Klein Michael L.
Published: July 2002

Molecular dynamics calculations have been used to study structural relaxation and dynamical correlations near the glass transition in the system [Ca(NO3)2]0.4[KNO3]0.6. As in a typical molten salt, the overall structure is determined by charge ordering. However, the radial distribution function for Ca2+ ions is unusual in that even at high temperatures it shows a split first peak due to specific spatial correlations of the cations with the nitrate anions. Structural relaxation that accompanies cooling of the system has been characterized with the aid of the van Hove real-space correlation functions Gs (r,t) for the constituent atoms (Ca, K, N, O). The relaxation of the incoherent structure factor Fs (k,t), with a wave vector k near the peak of the static structure factor, has been investigated as a function of temperature. The results clearly reveal both the α and β relaxation processes; the former can be well represented by a master curve with a stretched exponential shape. An analysis of the susceptibility, which agrees qualitatively with neutron spin-echo data, suggests that the glass transition for the model occurs around 400 K. The relatively small discrepancy with the experimental transition temperature derived from neutron scattering data (366 K) is likely related to inadequacies in the model employed for the interionic interactions. The functions C1(t) and C2(t), which describe the reorientational relaxation of the threefold symmetry axes of the nitrate ions, are shown to exhibit a scaling behavior analogous to that of the structure factor. In the region of the glass transition, where translational diffusion has essentially stopped, the nitrate ions continue to flip predominantly about their twofold axes.
Equation of state of chain molecules
Authors: Boublík T. , Vega C. , Diaz-Peña M.
Published: July 2002

The formerly proposed equation of state of fused hard-sphere molecules is applied to evaluate the compressibility factor of systems of flexible chains. A fair agreement with the available Monte Carlo data is obtained. Next, the equation of state extended to mixtures is used to predict the P–V–T behavior of binary systems composed of simple hard-body chains differing in the number of atoms in molecules. Good accordance of the calculated and pseudoexperimental values—within the experimental errors of the data—is obtained. To get further experimental data for pure chain molecules and their mixtures Monte Carlo simulations were performed for the system of linear tetraatomic molecules with the site–site length l*=1 and their equimolar mixtures with hard dumbbells at several densities. Comparison of the results for the linear tetraatomics with data on the corresponding flexible chain molecule system reveals a good agreement of the data. The proposed equation of state describes adequately the behavior of both the pure fluid and mixtures.
Theoretical potential energy function and rovibronic spectrum of CO+2(X 2Πg)
Authors: Brommer Matthias , Chambaud Gilberte , Reinsch Ernst-Albrecht , Rosmus Pavel , Spielfiedel Annie , Feautrier Nicole , Werner Hans-Joachim
Published: July 2002

For the electronic ground state of CO+2 the three-dimensional potential energy, electric dipole, and transition moment functions have been calculated from highly correlated multireference configuration interaction electronic wave functions. Along the antisymmetric stretching displacements the shape of the potential energy functions is found to be very sensitive to the electron correlation effect. Using a modified theoretical potential energy function rovibronic energy levels have been calculated variationally by the method of Carter and Handy. In this approach, anharmonicity, rotation–vibration, electronic angular momenta, and electron spin coupling effects have been accounted for. The vibronic band origins agree to within about 10 to 20 cm−1 with the available experimental data, and the rotational levels agree to within 0.01 cm−1 for low J values. Additional vibrational band origins have been predicted for energies up to 3200 cm−1. The anomalously low frequency of the antisymmetric stretching mode and its inverse anharmonicity in the X 2Πg state of CO+2 have been reproduced with the potential energy functions for the adiabatic states. Previously, it has been assumed that this effect is due to the vibronic coupling. The molecular parameters of one-dimensional effective Hamiltonians obtained from fits of the spectral data are compared with those derived from the theoretical potential.
Analysis of microviscosity and reaction coordinate concepts in isomerization dynamics described by Kramers’ theory
Authors: Åkesson E. , Hakkarainen A. , Laitinen E. , Helenius V. , Gillbro T. , Korppi-Tommola J. , Sundström V.
Published: July 2002

In this work we have studied the isomerization dynamics of a cyanine dye molecule in solution. The viscosity and temperature dependencies of the isomerization rate have been measured in the series of n-alcohols for three different sizes of the isomerizing group. From these measurements we conclude that the shear viscosity of the solvent is not a good measure of the microscopic friction experienced by the isomerizing groups. The friction is varying in a nonhydrodynamic manner with viscosity, which shows that the relative volume of the isomerizing group and solvent molecules (Vp/Vs) is a critical parameter determining the microscopic friction. When the microscopic friction is calculated using a model for molecular rotational relaxation proposed by Dote, Kievelson, and Schwartz [J. Phys. Chem. 85, 2169 (1981)], good fits to Kramers’ equation is obtained. Similar models for microscopic rotational and translational friction combined with Kramers’ equation also yield an apparent improvement over the hydrodynamic Kramers description. The measurements also show that the non-Kramers behavior of the reaction rates have a more complex origin than the (Vp/Vs) dependence of the microscopic friction, that possibly can be traced back to a more general failure of the hydrodynamic description of friction (frequency dependent friction), or to a temperature and solvent dependence of the potential surface parameters. The results also suggest that the detailed nature of the reaction coordinate plays an important role in determining the detailed viscosity dependence of the isomerization. Thus a reactive motion mainly experiencing rotational friction is much more sensitive to the molecular size and free-volume effects, than is the isomerization controlled by translational friction.
Theoretical study of lithium intercalated graphite
Authors: Boehm Randall C. , Banerjee Ajit
Published: July 2002

We have performed a series of calculations on small models (number of atoms ranging from 10 to 34) of graphite and lithium intercalated graphite (LIG) at the UHF level with a minimal basis set for the valence electrons and an effective core potential for the core electrons (CEP-4G), where the basis and the CEP is optimal for free atoms. We have shown that small model hosts, such as C10 (Bernal i.e., AB) and C12 (primitive hexagonal, i.e., AA), enable us to make several predictions regarding LIG. Firstly, lithium looses its valence electron upon entering either type of host lattice and eventually falls into a body-centered position in an AA host lattice. Secondly, lithium strongly destabilizes the AB lattice, while it strongly stabilizes the AA lattice. Thirdly, the barrier for site hopping in the limit of infinite dilution (Ea) can be estimated along with a related quantity which we call the hilltop energy (see text). Further, we have shown that by building up to host models no larger than C32 (AA) we can make a better estimate of Ea (0.72 eV), determine that the dynamics of Li (within any two-dimensional solvated sheet) is largely determined by ionic interactions with screening from adjacent carbon layers effecting an approximately 20% reduction of naked two-dimensional Li Coulombic forces, and give a reasonable indication of how much energy is liberated as Li is moved from infinity to a vacant site in unsaturated LIG (1.1±0.7 eV).