bluuebunny/crossref_metadata_2025 huggingface dataset:
Relationship between the thermal properties and optical emission spectra of the KXeN polyatomic exciplexes
Authors: Fujisaki Akihiro
Published: August 2002

The geometrical structures and optical emission spectra of the K(5S)XeN polyatomic exciplexes are calculated as functions of temperature in the range from 1 to 200 K. The relationships between the emission spectra and the thermal properties are investigated by use of the Metropolis Monte Carlo method. The peak energy values and the linewidths of the transition energy of the calculated emission light spectra of individual exciplexes reflect the structure and the thermal properties of each polyatomic exciplex. As the result of a detailed examination of relationships between the geometrical structure and the optical emission spectra of the KXe3, KXe7, KXe8, and KXe9 polyatomic exciplexes, a change in the structure of a given polyatomic exciplex, or the dissociation into a smaller cluster, can be detected by the discontinuities in the peak energy of the emission light and the linewidth as functions of temperature.
A time-of-flight quadrupole mass spectrometric study of Cn+(n=1–24) ions produced by laser ablation of a graphite target
Authors: Bae Chang Hyun , Park Seung Min
Published: August 2002

Mass spectral distributions and time-of-flight spectra of Cn+ (n=1–24) ions escaping from a carbon plasma plume generated by laser (λ=1064 nm) ablation of a graphite target are analyzed to shed light on their formation mechanisms, expansion dynamics, and laser–plume interaction. In particular, by shining the whole or a slice of the plume with a photodissociation laser (λ=266 nm) delayed with respect to the ablation laser, we examine the effects of the laser–plume interaction on the mass distribution as well as photodissociation channels of carbon-cluster cations. Also, we present experimental data that show a clear linear dependence of the velocities of Cn+ ions on 1/M. This result confirms that Cn+ ions are mainly accelerated by a transient local electric field in the plume.
Improved Gaussian self-consistent method—applications to homopolymers with different architectures in dilute solution
Authors: Timoshenko Edward G. , Kuznetsov Yuri A.
Published: August 2002

A version of the Gaussian self-consistent (GSC) method, which avoids the use of the Edwards’ virial expansion, is presented. Instead, the mean energy is evaluated directly via a convolution of the attractive-part of the pair-wise nonbonded potential with the Gaussian trial radial distribution function. The hard-sphere repulsion is taken into account via a suitably generalized Carnahan–Starling term. Comparison of the mean-squared inter-monomer distances and radius of gyration, as well as of the mean energy, between the results from the GSC calculations and Monte Carlo (MC) simulation in continuous space are made across the coil-to-globule transition for isolated ring, open and star homopolymers of varied lengths and flexibility. Importantly, both techniques utilize the same polymer model so that the data points could be directly superimposed. A surprisingly good overall agreement is found between these GSC and MC results. Caveats of the Gaussian technique and ways for going beyond it are also discussed.
Proton transfer reaction of 4-methyl-2,6-dicarbomethoxyphenol in nonpolar and weakly polar solvents
Authors: Mandal A. , Nath D. N. , Mukherjee S. , Mitra S. , Das R.
Published: August 2002

Proton transfer processes in the ground and excited state of 4-methyl-2,6-dicarbomethoxyphenol (CMOH) have been investigated by means of steady-state and nanosecond transient spectroscopy in some nonpolar and weakly polar solvents at room temperature and 77 K. The results obtained for CMOH were compared with that of 4-methyl-2,6-diformylphenol (MFOH) reported previously. Unlike MFOH the fluorescence spectra of CMOH show dual emission. This was explained as due to the presence of both the enol tautomer and intramolecularly hydrogen-bonded closed conformer as the fluorescing species. At 77 K the emission spectra show phosphorescence only in the presence of a base like triethylamine. The fluorescence decay rates of CMOH are relatively slower than that of MFOH and the nonradiative rates are always found higher than the radiative rates. A theoretical calculation at the Austin model 1 level of approximation revealed that the excited-state intramolecular proton transfer barrier in the ground singlet and excited triplet states is rather large compared to the excited singlet state in the respective potential energy surfaces.
Counterion-only electrical double layer: A constrained entropy approach
Authors: Briscoe Wuge H. , Attard Phil
Published: August 2002

A counterion-only double-layer theory is developed taking a constrained entropy maximization approach based on an isothermal thermodynamic system. The double-layer interaction free energy per unit area between two charged planar surfaces is obtained in a consistent manner for the boundary conditions of constant surface charge density and charge regulation. Once applied to the nonpolar medium, it reveals that the electrical double-layer repulsion is weak and long-ranged. Asymptotically, the interaction decays as a power law, in contrast with that in the presence of an ion reservoir which decays exponentially at large surface separation. The applicability of the counterion-only model for describing colloidal stability in nonpolar media is discussed.
Acceleration and heating of charged particles by a dispersive electrostatic pulse
Authors: Akimoto K.
Published: August 2002

Acceleration and heating mechanisms of charged particles by a generalized electrostatic pulse, i.e., a dispersive and propagating pulse of various forms, are investigated analytically and numerically. Gaussian-shaped pulses of arbitrary group- as well as phase-velocities are considered, but the results should apply qualitatively to solitons and other pulses. Actions of ordinary wave packets, as well as extremely short pulses with less than one cycle (or wavelength) among others, are investigated. For these general pulses, it is found that there may exist four types of acceleration/heating mechanisms. If a particle’s initial velocity is sufficiently away from the central phase-velocity of the pulse and if the pulse is sufficiently short, there occurs a well-known but generalized transit-time acceleration. However, for relatively long pulses with more than a couple of wavelengths or cycles, the transit-time acceleration becomes rapidly ineffective in accelerating particles. In the vicinity of the central phase-velocity of the pulse, a new process called “quasi-trapping” occurs, usually giving rise to the greatest velocity shifts. It can at times also cause a novel particle reflection. Finally, in the vicinity of the group velocity of the pulse, there occurs another new reflection process driven by the ponderomotive force of the pulse. All of the analytic expressions obtained for these processes are generally in good to excellent agreement with numerical results.
Secondary instabilities of large scale flow and magnetic field in the electromagnetic short wavelength drift-Alfvén wave turbulence
Authors: Smolyakov A. , Diamond P. , Kishimoto Y.
Published: August 2002

It is shown that the short wavelength (k⊥2ρi2>1, k⊥ is the characteristic wave vector, and ρi is the ion Larmor radius) electromagnetic drift wave turbulence in typical conditions is unstable with respect to the excitation of large scale perturbations of the plasma flow and magnetic field. It is found that the generation of zonal flow is reduced (compared to the long wavelength ion temperature gradient turbulence) due to the Boltzmann nature of the ion response. Magnetic fluctuations further reduce zonal flow drive due to competition of the Reynolds and Maxwell stresses. It is shown that secondary magnetic field structures may be generated in the electromagnetic drift wave turbulence thus leading to the increased levels of the electron energy transport.
Effective elastic stiffnesses of InAs under uniform strain
Authors: Ellaway S. W. , Faux D. A.
Published: August 2002

The effective elastic stiffnesses C11, C12 and C44 of InAs and derived elastic constants are determined for states of uniform volumetric strain by atomistic simulation. The effective elastic stiffnesses C11 and C12 are found to increase significantly with strain whereas C44 is found to be nearly independent of strain. Results for the bulk modulus are consistent with high-pressure experiments and nonlinear elasticity theories. The effective Young’s modulus is found to be nearly independent of strain whereas the effective Poisson’s ratio increases by about 40% over the range 0%–20% volumetric strain. The use of effective elastic stiffnesses within the isotropic approximation leads to a hydrostatic strain for an InAs quantum dot which is 16% smaller than the uncorrected value. It is concluded that continuum techniques for calculating quantum-dot-induced strain in the InAs/GaAs system should include strain-dependent InAs elastic stiffnesses for maximum accuracy.
Intersubband transitions in InGaNAs/GaAs quantum wells
Authors: Duboz J.-Y. , Gupta J. A. , Byloss M. , Aers G. C. , Liu H. C. , Wasilewski Z. R.
Published: August 2002

Intersubband transitions are observed in InGaNAs/GaAs quantum wells at wavelengths around 10 μm. The transition energies are correlated with interband transition energies measured in the near infrared. Clear selection rules are observed: the transition is TM polarized. The amplitude of the absorption is consistent with an increase of the electron effective mass as the N content increases.
Suppressing arrhythmias in cardiac models using overdrive pacing and calcium channel blockers
Authors: Stamp A. T. , Osipov G. V. , Collins J. J.
Published: August 2002

Recent findings indicate that ventricular fibrillation might arise from spiral wave chaos. Our objective in this computational study was to investigate wave interactions in excitable media and to explore the feasibility of using overdrive pacing to suppress spiral wave chaos. This work is based on the finding that in excitable media, propagating waves with the highest excitation frequency eventually overtake all other waves. We analyzed the effects of low-amplitude, high-frequency pacing in one-dimensional and two-dimensional networks of coupled, excitable cells governed by the Luo–Rudy model. In the one-dimensional cardiac model, we found narrow high-frequency regions of 1:1 synchronization between the input stimulus and the system’s response. The frequencies in this region were higher than the intrinsic spiral wave frequency of cardiac tissue. When we paced the two-dimensional cardiac model with frequencies from this region, we found that spiral wave chaos could, in some cases, be suppressed. When we coupled the overdrive pacing with calcium channel blockers, we found that spiral wave chaos could be suppressed in all cases. These findings suggest that low-amplitude, high-frequency overdrive pacing, in combination with calcium channel inhibitors (e.g., class II or class IV antiarrhythmic drugs), may be useful for eliminating fibrillation.
Enhanced self-termination of re-entrant arrhythmias as a pharmacological strategy for antiarrhythmic action
Authors: Aslanidi O. V. , Bailey A. , Biktashev V. N. , Clayton R. H. , Holden A. V.
Published: August 2002

Ventricular tachycardia and fibrillation are potentially lethal cardiac arrhythmias generated by high frequency, irregular spatio-temporal electrical activity. Re-entrant propagation has been demonstrated as a mechanism generating these arrhythmias in computational and in vitro animal models of these arrhythmias. Re-entry can be idealised in homogenous isotropic virtual cardiac tissues as spiral and scroll wave solutions of reaction-diffusion equations. A spiral wave in a bounded medium can be terminated if its core reaches a boundary. Ventricular tachyarrhythmias in patients are sometimes observed to spontaneously self-terminate. One possible mechanism for self-termination of a spiral wave is meander of its core to an inexcitable boundary. We have previously proposed the hypothesis that the spatial extent of meander of a re-entrant wave in the heart can be directly related to its probability of self-termination, and so inversely related to its lethality. Meander in two-dimensional virtual ventricular tissues based on the Oxsoft family of cell models, with membrane excitation parameters simulating the inherited long Q-T syndromes has been shown to be consistent with this hypothesis: the largest meander is seen in the syndrome with the lowest probability of death per arrhythmic episode. Here we extend our previous results to virtual tissues based on the Luo–Rudy family of models. Consistent with our hypothesis, for both families of models, whose different ionic mechanisms produce different patterns of meander, the LQT virtual tissue with the larger meander simulates the syndrome with the lower probability of death per episode. Further, we search the parameter space of the repolarizing currents to find their conductance parameter values that give increased meander of spiral waves. These parameters may provide targets for antiarrhythmic drugs designed to act by increasing the likelihood of self-termination of re-entrant arrhythmias.
Full-dimensional quantum wave packet study of rotationally inelastic transitions in H2+H2 collision
Authors: Ying Lin Shi , Guo Hua
Published: August 2002

We report full-dimensional accurate quantum dynamical calculations of the rotationally inelastic collision: para-H2(ν1=0,j1=0)+para-H2(ν2=0,j2=0)→para-H2(ν1=0,j1′)+para-H2(ν2=0,j2′), using a wave packet approach based on the Chebyshev polynomial expansion of Green’s operator. The six-dimensional Hamiltonian within the coupled-states approximation is discretized in a mixed grid/basis representation and its action is computed in appropriate representations facilitated by a series of one-dimensional pseudo-spectral transformations. Both the parity and diatomic exchange symmetry are adapted. The S-matrix elements for the rotational transitions are obtained at all energies by the Fourier transform of Chebyshev correlation functions and used to compute transition probabilities, differential and integral cross sections, and state-resolved thermal rate constants. Results are compared for two recently proposed ab initio based potential energy surfaces and with previous quantum results.
Intracluster multiple trimeric cyclization of acrylonitrile clusters initiated by electron transfer from a potassium atom: Size-dependent pathways in metastable dissociation of K+(CH2=CHCN)n photoions
Authors: Ohshimo Keijiro , Misaizu Fuminori , Ohno Koichi
Published: August 2002

Size-dependent stabilities and intracluster reactions of potassium atom and acrylonitrile molecules (AN; CH2=CHCN) clusters were investigated. Previously reported magic numbers (intensity anomalies) of n=3k (k=1–4) using photoionization mass spectrum of K(AN)n, and size-specific elimination reactions (HCN elimination from clusters of n⩾3, and H2 elimination from n=3 and 6 clusters) were explained by a cyclohexane derivative formation in an intracluster trimeric cyclization (anionic oligomerization) initiated by electron transfer from a K atom in K(AN)n. To elucidate larger K(AN)n structures, unimolecular metastable dissociations of K+(AN)n photoions were observed using a reflectron time-of-flight mass spectrometer. A metastable dissociation pathway of n→n−1 (AN-loss) was predominantly observed for all parent sizes; furthermore, for parent ions with n=6, 9, and 12, pathway of n→n−3 [(AN)3-loss] was also observed. These size-dependent dissociation pathways of photoions are related to structures of neutral clusters since intramolecular bonds are expected to be formed in the oligomerization reactions in neutrals and to be conserved in the photoionization process. Parent clusters that cause the n→n−1 dissociations have structures in which at least one AN monomer can coordinate without forming any chemical bonds. The observation of n→n−3 pathways corresponds to the existence of isomers of n= 3k (k=2–4) clusters having k cyclohexane derivatives, which are formed by intracluster multiple trimeric cyclization reactions with 3k AN molecules in neutral clusters. The existence of at least two types of structural isomers (including reacted AN or unreacted AN) in these clusters is shown from these experimental results, and is further supported by calculations of the microcanonical dissociation rate constants for each pathway based on the Rice–Ramsperger–Kassel–Marcus theory.
Rubidium–xenon spin exchange and relaxation rates measured at high pressure and high magnetic field
Authors: Rice Charles V. , Raftery Daniel
Published: September 2002

The production of hyperpolarized Xe via spin exchange with optically pumped Rb atoms was studied at 47 000 G under high-pressure conditions. From variable pressure and temperature studies, the spin exchange efficiency was found to be pressure dependent and lower than comparable low-field measurements. Spin exchange due to short binary collisions is expected to be pressure independent, and the cross section was measured to be 〈σν〉SE=6.0 (±0.1)×10−17 cm3 s−1. The pressure dependent component was attributed to the formation of Rb–Xe van der Waals (vdW) complexes. The rate constant for spin exchange in Rb–Xe vdW complexes was measured to be κ=1400 (±100) s−1 in He buffer gas. This value is comparable, though somewhat smaller than vdW rates measured previously for other buffer gases. While it is well known that spin exchange occurs readily in vdW complexes when B0<200 G, vdW spin exchange at fields >10 000 G is not accounted for in current theories. Modulation of the hyperfine interaction during the vdW complex lifetime is proposed to account for these observations. Finally, relaxation by the walls of the pumping cell was found to increase with buffer gas pressure. These factors impact the equilibrium Xe polarization obtainable with high-field optical pumping.
Properties of the neutral energetic atoms emitted from Earth’s ring current region
Authors: Shen C. , Liu Z. X.
Published: August 2002

Simulations of energetic neutral atoms (ENA) during the geomagnetic storm main phase have been carried out to provide reliable theoretical foundations for the development of an ENA detector on board the polar satellite of the Chinese Double Star Program (DSP), and to make preparation for the future ENA observational data analyses. In this research, an approximate analytical model for the ring current particle distributions, including the ion loss due to charge exchange processes, has been developed. The simulations have shown that there are two maximum ENA flux regions: The ring current inner boundary region, and the particle precipitation region at the northern and southern poles. The stronger the storms, the lower the particle injection, and the larger the flux of ENA emitted from the ring current region. The ENA detector at advantageous positions can measure the inner boundary of the injection region or the injection front. The ENA detector is able to measure the inhomogeneity of the ring current ions. The features of the energy fluxes of H, O, and He are different from each other due to the discrepancy of their charge exchange cross sections. The ENA at 10–80 keV are more easily observed owing to the large fluxes at this range of energy, and ENA H and O are easily measured because of their stronger fluxes. In contrast, ENA He is more difficult to detect due to its rather weak flux. These simulations have indicated that the polar satellite of DSP is capable of detecting the ring current ENA. The ENA detector at low latitudes, e.g., on board the equatorial satellite, is also useful for observing the ring current ENA. More importance should be placed on the detection for low- and mid-energy ENA during the development of the ENA instruments for the polar DSP satellite.
Normalized photoacoustic techniques for thermal diffusivity measurements of buried layers in multilayered systems
Authors: Balderas-López J. A. , Mandelis A. , Garcı́a J. A.
Published: August 2002

The one-dimensional heat diffusion problem for a three-layer system is solved assuming the surface absorption model. The analytical solution is shown to be suitable for the implementation of normalized depth-profilometric photoacoustic methodologies involving the open photoacoustic-cell configuration for thermal diffusivity measurements in buried underlayers within a three-layer stack. Our normalization procedures eliminate the frequency-dependent instrumental electronic contribution (transfer function) and some thermophysically nonrelevant proportionality factors in the theoretical equations, thus making the depth-profilometric analysis feasible. The measurement methodology is achieved by normalizing the theoretical photoacoustic signal from the three layers with the corresponding signal from the uppermost two layers, involving linear fits to measure the thermal diffusivity of the third underlayer. Three different multilayered materials were examined using the proposed methodologies. High reproducibility of the thermal diffusivity measurements and good agreement with values reported in literature were found. Besides the foregoing procedures, a lumped photoacoustic model was developed, which yields the effective thermal-diffusivity value of the multilayer stack.
Study of C3H5+ ion deposition on polystyrene and polyethylene surfaces using molecular dynamics simulations
Authors: Jang Inkook , Phillips Roshenda , Sinnott Susan B.
Published: August 2002

Molecular dynamics simulations of ion deposition processes are used to study the deposition of C3H5+ ions on crystalline polystyrene (PS) and polyethylene (PE) surfaces at energies of 50 and 25 eV. For each system, 80 trajectories are carried out on pristine surfaces and the incident angle in every case is normal to the surface. The forces are determined using the reactive empirical bond order method developed by Tersoff and parametrized for hydrocarbons by Brenner, coupled to long-range Lennard–Jones potentials. The simulations predict that the ions deposited at 50 eV either dissociate and stick to the surface or remain on the surface intact in 98% of the trajectories on PS, and in 89% of the trajectories on PE. At 25 eV, the ions are deposited intact in 70% of the trajectories on PS and dissociate in only 3%. No dissociation of the incident ions is predicted to occur on PE at 25 eV. Rather, the ions scatter away in 90% of the trajectories. Consequently, ion deposition on PE at 25 eV is predicted to be very inefficient for thin-film growth. Many more ions or major ion fragments (such as C2Hn and CH2) remain near the surface on PS than PE at 50 eV. Thus, in general, polyatomic ion deposition for thin film growth is more efficient on PS than PE, and deposition at 50 eV is more efficient than deposition at 25 eV.
Kinetics and mechanisms of laser-induced decompositions of hydrogenated amorphous carbon films on magnetic hard disks
Authors: Zhang Li Hong , Gong Hao , Wang Jian Ping
Published: August 2002

Laser-induced structural decompositions of hydrogenated amorphous carbon films deposited by dc-magnetron sputtering on magnetic hard disks were studied using a microbeam Ar+ laser (514.5 nm). The laser had varying powers between 3.0 and 0.3 mW at a focused circular spot of 2.5 μm diameter, corresponding to approximate fluence rates between 6×104 to 6×103 J cm−2 s−1. The effect of laser power on decomposition rate could be expressed by an exponential equation. Based on the relationship between laser power and temperature, relative apparent activation energies for film decompositions were obtained and found to decrease with the increase of film hydrogen content and the elapse time of laser irradiation. All the films irradiated with laser powers between 0.75 and 0.3 mW followed the apparent first-order reaction. The reaction order tended to increase at higher laser powers and hydrogen content in the films.
A new general method for solving the resistive inner layer problem
Authors: Galkin S. A. , Turnbull A. D. , Greene J. M. , Brennan D. P.
Published: August 2002

Classical resistive tearing mode stability in toroidal plasma configurations can be determined by matching the solutions between the ideal and resistive layers. A technique developed for the outer ideal region [S. A. Galkin et al., Phys. Plasmas 7, 4070 (2000)] is generalized and successfully applied to the inner resistive layer problem with irregular singularities. The new method is shown to agree with previous published results and can be used over an extended parameter range. A generalization of the traditional resistive layer equations for nonuniform inertia is also modeled using a variable density distribution and solved using the new technique. The influence of the total mass and density profiles on the matching conditions is studied and discussed.
Molecular dynamics simulations on devitrification: The PbF2 case
Authors: Silva Maurício A. P. , Monteil André , Messaddeq Younès , Ribeiro Sidney J. L.
Published: August 2002

In this work molecular dynamics simulations were performed to reproduce the kinetic and thermodynamic transformations occurring during melt crystallization, vitrification, and glass crystallization (devitrification) of PbF2. Two potential parameters were analyzed in order to access the possibility of modeling these properties. These interionic potentials are models developed to describe specific characteristic of PbF2, and thermodynamic properties were well reproduced by one of them, while the other proved well adapted to simulate the crystalline structure of this fluoride. By a modeled nonisothermal heat treatment of the glass, it was shown that the devitrification of a cubic structure in which the Pb–Pb distances are in good agreement with theory and experiment.
The total molecular dipole moment for liquid water
Authors: Gubskaya Anna V. , Kusalik Peter G.
Published: August 2002

For the water molecule, the dipole is the first nonzero multipole moment; it represents the polarity of the molecule and has been widely used in describing solvation behavior. A rather wide range of theoretically determined values for the total molecular dipole moment of water in condensed phases has been reported in the literature. This paper describes a means by which the average total dipole moment for the water molecule in the liquid state can be linked to experimental refractive index data. Three components comprise the mean-field approach that is employed. A formal framework is developed that relates the temperature dependence of the effective molecular polarizability to the average local electric field experienced by a liquid water molecule over a chosen temperature range. A characterization of the distributions of local fields and field gradients is also necessary, and this has been determined from the computer simulations of liquid water samples at several different temperatures for two standard water potentials. The final component, the electric response properties of the water molecule (including nonlinear contributions up to fourth order), were determined from ab initio calculations for gas- and liquid-phase molecules, and are reported elsewhere [A. V. Gubskaya and P. G. Kusalik, Mol. Phys. 99, 1107 (2001)]. By combining these three components, the temperature dependence of the average local electric field, and consequently the average total dipole moment, are extracted from data for the refractive index of liquid water. An almost 10% variation in the dipole moment with temperature is observed over the range 273 to 373 K. The value obtained for the molecular dipole moment at 300 K, 2.95±0.2 D, is in excellent agreement with a recently reported result extracted from x-ray scattering data, as well as with some recent theoretical predictions.
Tracer-diffusion in binary colloidal hard-sphere suspensions
Authors: Zhang Haiyan , Nägele Gerhard
Published: September 2002

We calculate the short-time translational and rotational self-diffusion coefficients of mixtures of colloidal hard spheres. The influence of hydrodynamic interactions is accounted for by a series expansion of the two-body mobility tensors, and by considering the leading hydrodynamic three-body term. Explicit results are presented for the rotational–translational diffusion coefficients, Ds,Tr and Ds,Tt, of a dilute tracer component in a concentrated host dispersion of hard spheres. These coefficients are shown to depend strongly on the host volume fraction, φ, and on the size ratio, λ, of tracer and host particles. For large λ, the tracer diffusion coefficients are related to the viscosity of the host dispersion by effective Stokes–Einstein relations.
Simulation of nanocolloid chemical potentials in a hard-sphere polymer solution: Expanded ensemble Monte Carlo
Authors: Tej Marla Krishna , Meredith J. Carson
Published: August 2002

We present a novel application of the expanded ensemble Monte Carlo (EEMC) simulation method to calculation of the chemical potential of nanocolloidal particles in nanocolloid–polymer mixtures. This approach uses an expanded canonical ensemble in which the colloidal particle diameter is an additional ensemble variable, allowed to vary between zero and the maximum colloid size desired. Using a hard-sphere model system, we demonstrate that this approach is superior to the Widom method for calculating chemical potentials in colloid–polymer systems. Specifically the EEMC leads to lower uncertainties and is capable of calculating accurate colloid chemical potentials for particle sizes where Widom insertion fails due to overlap. The EEMC method is applied to calculate the colloid chemical potential for an infinitely dilute colloidal particle (hard-sphere) in a dilute polymer (hard-sphere chain) solution over a wide range of relative sizes, 0.1<Rg/R<12, where Rg is the polymer radius of gyration and R is the colloid radius. The simulation results are compared to the predictions of models developed by others: an integral equation model (FS) [Fuchs and Schweizer, Europhys. Lett. 51, 621 (2000)] and a field theoretic (FT) approach [Eisenriegler et al., Phys. Rev. E 54, 1134 (1996)]. Very good agreement is observed with the FS model over a wide range of Rg/R values, whereas the FT model agrees well only at large Rg/R. An empirical power law function is found to represent the simulation results well, potentially useful for analysis of free energy data for colloid–polymer mixtures.
Exact solution for the geminate ABCD reaction
Authors: Popov Alexander V. , Agmon Noam
Published: September 2002

We obtain exact analytic solution for the Green functions and survival probabilities of the reversible, geminate diffusion-influenced reaction A+B↔C+D in Laplace space, and in the time domain for various spatial dimensionalities. The asymptotic behavior (in one and three dimensions) goes as t−1/2, like in the irreversible case, but approach to this limit may occur from either above or below. The implications of these results are demonstrated and discussed.
Vibrational interactions of acetonitrile: Doubly vibrationally resonant IR–IR–visible four-wave-mixing spectroscopy
Authors: Kwak Kyungwon , Cha Sangyob , Cho Minhaeng , Wright John C.
Published: September 2002

The origin of the vibrational coupling that was observed between the CC and CN stretching modes of acetonitrile by doubly vibrationally enhanced (DOVE) IR–IR–Vis four-wave-mixing (IIV-FWM) spectroscopy is investigated by various ab initio calculations including DFT(B3LYP), HF, and MP2 methods with the same baisis set, 6-311++G**. The linear and nonlinear susceptibilities of the combination bands and cross peaks are numerically calculated and compared with the experimental values, and the agreement between ab initio results and experiments are quantitative. By separately analyzing the contributions from each coherence pathway to the vibrational coupling of the CC and CN stretching modes, a quantitative understanding of the DOVE IIV-FWM signals is possible. Although the direct coupling of the CC and CN stretching modes by mechanical and electric anharmonicity coupling is sizable, the CH bending and CH stretching modes are also involved in the vibrational coupling between CC and CN stretching modes as promoting modes. The numerically simulated two-dimensional (2D) DOVE spectrum for a CH3CN sample is presented and compared with experiment. It is found that the interference among distinctive pathways plays a central role in describing the distorted, asymmetric shape of the 2D DOVE spectrum. In addition, the IIV-FWM cross peak associated with the vibrational coupling between the CH and CN stretching mode is also calculated and its magnitude is compared with that of the CC and CN stretching modes.
Statistical sampling of semiclassical distributions: Calculating quantum mechanical effects using Metropolis Monte Carlo
Authors: Sun Sean X. , Miller William H.
Published: September 2002

A statistical sampling method is proposed for computing oscillatory integrals associated with the semiclassical initial value representation. The semiclassical expression is rewritten as an integral over a phase distribution P(s). The phase distribution is obtained from Metropolis sampling of trajectories according to a properly chosen weight function. The averaging of oscillatory integrals is converted into a Monte Carlo algorithm where one diffuses through trajectory space. A histogram of phases is collect from importance sampling. Techniques of Metropolis Monte Carlo such as umbrella (or biased) sampling are generalized to the present context. From example calculations, phase distributions are seen to be multi-peaked, thus clearly demonstrating the origin of quantum interference. Trajectories that are responsible for the interference patterns can be collected using this method.
Failure of density-functional theory and time-dependent density-functional theory for large extended π systems
Authors: Cai Zheng-Li , Sendt Karina , Reimers Jeffrey R.
Published: September 2002

Density-functional theory (DFT) is widely used for studying large systems such as metals, semiconductors, and large molecules, with time-dependent density-functional theory becoming a very powerful tool for investigating molecular excited states. As part of a systematic study of both the intrinsic weaknesses of DFT and the weaknesses of present implementations, we consider its application to the one and two-dimensional conjugated π systems: polyacetylene fragments and oligoporphyrins, respectively. Very poor results are obtained for the calculated spectra, and polyacetylene is predicted by all functionals considered, including gradient-corrected functionals, to have a triplet ground state. The cause of this is linked to known problems of existing density functionals concerning nonlocality and asymptotic behavior which result in the highest-occupied molecular-orbital being too high in energy so that semiconductors and low-band-gap insulators are predicted to have metal-like properties. The failure of modern density functionals to predict qualitatively realistic molecular hyperpolarizabilities for extended systems is closely related.
Combining ab initio and density functional theories with semiempirical methods
Authors: Cui Qiang , Guo Hong , Karplus Martin
Published: September 2002

For large reactive systems, the calculation of energies can be simplified by treating the active part with a high-level quantum mechanical (QM) (ab initio or density functional) approach and the environment with a less sophisticated semiempirical (SE) approach, as an improvement over the widely used hybrid quantum mechanical/molecular mechanical (QM/MM) methods. An example is the interaction between an active region of an enzyme and its immediate environment. One such method is the original “Our-own-N-layer Integrated molecular Orbital+Molecular Mechanics (ONIOM)” approach. In this paper, the interaction between the QM and SE region is described explicitly by two different schemes. In the iterative QM/SE schemes (QM/SE-I), the electrostatic interaction and polarization effects are introduced explicitly for both the QM and SE atoms by a self-consistent procedure based on either polarizable point charges or the electron density. In the noniterative QM/SE scheme, based on the ONIOM model (QM/SE-O), the exchange (Pauli repulsion) and charge transfer effects are taken into account at the SE level, in addition to the explicit electrostatic interaction and polarization between the two regions. Test calculations are made on a number of model systems (including small polar or charged molecules interacting with water and proton transfer reactions in the presence of polar molecules or an extended hydrogen-bond network). The quantitative accuracy of the results depend on several parameters, such as the charge-scaling/normalization factors for the SE charge and the QM/SE van der Waals parameters, which can be chosen to optimize the result. For the QM/SE-O approach, the results are more sensitive to the quality of the SE level (e.g., self-consistent-charge density-functional-tight-binding vs AM1) than the explicit interaction between QM and SE atoms.
Resetting and annihilation of reentrant activity in a model of a one-dimensional loop of ventricular tissue
Authors: Comtois Philippe , Vinet Alain
Published: August 2002

Resetting and annihilation of reentrant activity by a single stimulus pulse (S1) or a pair (S1–S2) of coupled pulses are studied in a model of one-dimensional loop of cardiac tissue using a Beeler–Reuter-type ionic model. Different modes of reentry termination are described. The classical mode of termination by unidirectional block, in which a stimulus produces only a retrograde front that collides with the activation front of the reentry, can be obtained for both S1 and S1–S2 applied over a small vulnerable window. We demonstrate that another scenario of termination—that we term collision block—can also be induced by the S1–S2 protocol. This scenario is obtained over a much wider range of S1–S2 coupling intervals than the one leading to a unidirectional block. In the collision block, S1 produces a retrograde front, colliding with the activation front of the pre-existing reentry, and an antegrade front propagating in the same direction as the initial reentry. Then, S2 also produces an antegrade and a retrograde front. However, the propagation of these fronts in the spatial profile of repolarization left by S1 leads to a termination of the reentrant activity. More complex behaviors also occur in which the antegrade fronts produced by S1 and S2 both persist for several turns, displaying a growing alternation in action potential duration (“alternans amplification”) that may lead to the termination of the reentrant activity. The hypothesis that both collision block and alternans amplification depend on the interaction between the action potential duration restitution curve and the recovery curve of conduction velocity is supported by the fact that the dynamical behaviors were reproduced using an integro-delay equation based on these two properties. We thus describe two new mechanisms (collision block and alternans amplification) whereby electrical stimulation can terminate reentrant activity.
Critical role of inhomogeneities in pacing termination of cardiac reentry
Authors: Sinha Sitabhra , Stein Kenneth M. , Christini David J.
Published: August 2002

Reentry around nonconducting ventricular scar tissue, a cause of lethal arrhythmias, is typically treated by rapid electrical stimulation from an implantable cardioverter defibrillator. However, the dynamical mechanisms of termination (success and failure) are poorly understood. To elucidate such mechanisms, we study the dynamics of pacing in one- and two-dimensional models of anatomical reentry. In a crucial realistic difference from previous studies of such systems, we have placed the pacing site away from the reentry circuit. Our model-independent results suggest that with such off-circuit pacing, the existence of inhomogeneity in the reentry circuit is essential for successful termination of tachycardia under certain conditions. Considering the critical role of such inhomogeneities may lead to more effective pacing algorithms.
Local equilibrium of nonrotating plasmas
Authors: Boozer Allen H.
Published: August 2002

What is the minimal information that must be supplied on a constant pressure surface of a toroidal plasma to completely specify a local magnetohydrodyamic equilibrium? Clearly the shape of the surface must be specified, but other information such as the separation between neighboring surfaces must also be given. This paper explores the conditions required for three-dimensional equilibria and how local equilibrium constraints are related to global constraints. The resulting conditions provide useful checks on the accuracy of equilibrium solvers, particularly for solvers for three-dimensional equilibria, and allow local refinements in the accuracy of equilibria.
Parallel tempering method for reconstructing isotropic and anisotropic porous media
Authors: Makrodimitris K. , Papadopoulos G. K. , Philippopoulos C. , Theodorou D. N.
Published: September 2002

A parallel tempering method was developed for the reconstruction of porous media using information extracted from 2D digitized microscopy images. The method affords significant improvement over simulated annealing, can be easily parallelized, and works generally for isotropic and anisotropic structures. Inclusion of the pore size distribution function in the reconstruction method was decisive for best results. The validity of the results has been checked by comparing cluster size distribution functions, percolation, and transport properties between original and reconstructed images.
Measured energy distributions of ions driven by an asymmetrically pulsed bias during magnetron sputtering
Authors: Barnat E. V. , Lu T. -M.
Published: August 2002

A retarding field analyzer is used to obtain a measure of the energy of the ions extracted from a direct current discharge driven by a low-frequency (100–100 kHz) asymmetrically pulsed bias. Measurements are made to illustrate the effects of the pulse’s shape (frequency and duty) on the distribution in the energies of the ions extracted from the plasma. Ion energy distributions are computed by a quasistatic treatment of the ion’s response to the measured wave form of the pulsed electrode. Comparisons are made between the measured and calculated ion energy distributions to demonstrate the effectiveness of the technique used to calculate the ion energy distribution. Expressions are then produced for the time scales in which the quasistatic treatment of the sheath remains a valid approximation to predict the energy distribution of the ions.
Estimation of dissociation degree of N2 in an inductively coupled plasma by vacuum ultraviolet emission spectroscopy
Authors: Nakano Toshiki , Kumagai Shinya , Samukawa Seiji
Published: August 2002

A simple procedure for estimation of the N2 dissociation degrees in low-pressure, high-density plasmas is proposed. This procedure is applied to the estimation of the N2 dissociation in an N2 inductively coupled plasma and confirmed to give N2 dissociation degrees similar to those reported previously. The rf power and N2 pressure dependence of the N2 dissociation degree is also presented. The contribution of stepwise excitation out of N2 A3Σu+ [N2(A)] metastables to the N2 second positive emission is found to be negligible from the net excitation rate computed using an assumed cross section of the electron-impact excitation from N2(A) to the N2C3Πu state and the N2(A) number density deduced from a simple kinetic model. The recombination of N atoms on the inner surfaces of the plasma reactor used in this study is discussed briefly. Finally, the advantages of the N2 dissociation estimation by vacuum ultraviolet optical emission spectroscopy are mentioned.
Structure and electronic properties of SiO2/Si multilayer superlattices: Si K edge and L3,2 edge x-ray absorption fine structure study
Authors: Sammynaiken R. , Naftel S. J. , Sham T. K. , Cheah K. W. , Averboukh B. , Huber R. , Shen Y. R. , Qin G. G. , Ma Z. C. , Zong W. H.
Published: August 2002

We report an x-ray absorption fine structure study at the Si K and L3,2 edges of a series of Si/SiO2 superlattices (SL). The SL system comprises four periods of elemental silicon with a spacing of 1, 1.4, 2.2, and 2.6 nm sandwiched by a 1.5 nm silicon oxide and capped by a 3 nm silicon oxide layer. These systems exhibit electroluminescence and photoluminescence. X-ray absorption near edge structure (XANES) at both the Si K and L3,2 edge confirms that the Si layers are amorphous. Polarization dependent measurement at the Si K edge reveals that a distinct Si/SiO2 interface exists with strong Si–O bonding oriented preferentially closer to the surface normal. High resolution XANES at the Si L3,2 edge shows a noticeable blueshift of the edge threshold as the lattice spacing decreases, in good accord with quantum confinement. The results and their implications for the origin (quantum confinement and interface/oxide defects) of luminescence in these superlattice systems are discussed.
Numerical modeling of highly doped Si:P emitters based on Fermi–Dirac statistics and self-consistent material parameters
Authors: Altermatt Pietro P. , Schumacher Jürgen O. , Cuevas Andres , Kerr Mark J. , Glunz Stefan W. , King Richard R. , Heiser Gernot , Schenk Andreas
Published: August 2002

We have established a simulation model for phosphorus-doped silicon emitters using Fermi–Dirac statistics. Our model is based on a set of independently measured material parameters and on quantum mechanical calculations. In contrast to commonly applied models, which use Boltzmann statistics and apparent band-gap narrowing data, we use Fermi–Dirac statistics and theoretically derived band shifts, and therefore we account for the degeneracy effects on a physically sounder basis. This leads to unprecedented consistency and precision even at very high dopant densities. We also derive the hole surface recombination velocity parameter Spo by applying our model to a broad range of measurements of the emitter saturation current density. Despite small differences in oxide quality among various laboratories, Spo generally increases for all of them in a very similar manner at high surface doping densities Nsurf. Pyramidal texturing generally increases Spo by a factor of five. The frequently used forming gas anneal lowers Spo mainly in low-doped emitters, while an aluminum anneal (Al deposit followed by a heat cycle) lowers Spo at all Nsurf.
Control of thermal runaway in microwave resonant cavities
Authors: Wu X. , Thomas J. R. , Davis W. A.
Published: August 2002

This article reports direct experimental evidence of the so-called “S curve” of temperature versus electrical field strength when materials with positive temperature dependence of dielectric loss are heated in a microwave resonant cavity applicator. A complete discussion of how the experimental results were achieved is presented. From the experimental results, we believe the S curve theory provides an incomplete explanation of thermal runaway in microwave heating. To understand microwave heating in a resonant cavity, cavity effects must be considered. To explain the experimental results, a theoretical model based on single-mode waveguide theory is developed. Finally, a method to control thermal runaway is described.