bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational energies for NH3 based on high level ab initio potential energy surfaces
Authors: Lin Hai , Thiel Walter , Yurchenko Sergei N. , Carvajal Miguel , Jensen Per
Published: December 2002

Ab initio coupled cluster calculations with single and double substitutions and a perturbative treatment of connected triple substitutions [CCSD(T)] have been carried out to generate six-dimensional (6D) potential energy surfaces (PES) and dipole moment surfaces (DMS) for the electronic ground state of ammonia. Full 6D-PES and 6D-DMS (14400 points) were computed with the augmented correlation-consistent triple-zeta basis (aug-cc-pVTZ). For a selected number of points (420 in C3v symmetry and 1260 in lower symmetry), more accurate energies (CBS+) were obtained by extrapolating the CCSD(T) results for the aug-cc-pVXZ (X=T,Q,5) basis sets to the complete basis set limit and adding corrections for core-valence correlation and relativistic effects. Two procedures were investigated to enhance the quality of the 6D-PES from CCSD(T)/aug-cc-pVTZ by including the CBS+ data points. The resulting 6D-PES were represented by analytical functions involving Morse variables for the stretches, symmetry-adapted bending coordinates, and a specially designed inversion coordinate (up to 76 fitted parameters, rms deviations of about 5 cm−1 for 14 400 ab initio data points). For these analytical surfaces, vibrational energies were calculated with a newly developed computer program using a variational model that employs an Eckart-frame kinetic energy operator. Results are presented and compared to experiment for the vibrational band centers of NH3 and its isotopomers up to around 15 000 cm−1. For our best 6D-PES, the term values of the fundamentals are reproduced with rms deviations of 4.4 cm−1 (NH3) and 2.6 cm−1 (all isotopomers), the maximum deviation being 7.9 cm−1.
Ground and excited states of linked and fused zinc porphyrin dimers: Symmetry adapted cluster (SAC)—configuration interaction (CI) study
Authors: Miyahara T. , Nakatsuji H. , Hasegawa J. , Osuka A. , Aratani N. , Tsuda A.
Published: December 2002

The symmetry adapted cluster (SAC)/SAC-configuration interaction method was applied to calculate the ground and excited states of zinc porphyrin monomers (without and with phenyl groups, ZnP and ZnPPh, respectively) and meso–meso linked (Zn2PMM) and doubly fused (Zn2PDF) zinc porphyrin dimers. Various features of the absorption spectra are studied, clarified, and assigned theoretically. The calculated electronic spectrum of ZnPPh, in comparison with that of ZnP, showed that the phenyl groups affect the spectrum in both the peak positions and intensities. In the dimers, Zn2PMM and Zn2PDF, the interactions of the monomer’s four-orbitals result in an eight-orbital model of the dimers, which plays an important role in the interpretation of the excited states observed in the spectra. In Zn2PMM, the interaction is smaller and each peak in the split Soret (BI and BII) bands consists of two peaks, in contrast to the prediction based on Kasha’s exciton rule. In Zn2PDF, the interaction between the two monomer units is so strong that the small highest occupied molecular orbital–lowest unoccupied molecular orbital gap causes the red-shifted Q (1.16 eV) and BI (2.13 eV) bands. In addition, the excitations out of the eight orbitals appear in the low-energy region of the spectrum, which is very different from the case of the monomer. A comprehensive and pictorial analysis is given for the excited states of the dimers in comparison with those of the monomer.
Force correlation functions and the diffusion coefficient of water
Authors: Evans Glenn T.
Published: December 2002

The zero frequency part of the force correlation function and the diffusion coefficient D are calculated for a square well fluid with waterlike attributes. The force correlation function separates into a part due to the hard core, assessed using kinetic theory, and a part due to the anisotropic square well, assessed using the dynamics prescribed by the Smoluchowski equation. Anisotropic attraction dominates the forces. When properly parameterized, the present theory sensibly portrays the temperature dependence of the friction coefficient and the diffusion coefficient over the normal liquid range.
Image charges in spherical geometry: Application to colloidal systems
Authors: Messina René
Published: December 2002

The effects of image charges (i.e., induced surface charges of polarization) in spherical geometry and their implication for charged colloidal systems are investigated. We study analytically and exactly a single microion interacting with a dielectric sphere and discuss the similarities and discrepancies with the case of a planar interface. By means of extensive Monte Carlo simulations, we study within the framework of the primitive model the effects of image charges on the structure of the electrical double layer. Salt-free environment as well as salty solutions are considered. A remarkable finding of this study is that the position of the maximum in the counterion density (appearing at moderately surface charge density) remains quasi-identical, regardless of the counterion valence and the salt content, to that obtained within the single-counterion system.
Thermodynamics and phase behavior of the lamellar Zwanzig model
Authors: Harnau Ludger , Rowan David , Hansen Jean-Pierre
Published: December 2002

Binary mixtures of lamellar colloids represented by hard platelets are studied within a generalization of the Zwanzig model for rods, whereby the square cuboids can take only three orientations along the x, y or z axes. The free energy is calculated within Rosenfeld’s “fundamental measure theory” adapted to the present model. In the one-component limit, the model exhibits the expected isotropic to nematic phase transition, which narrows as the aspect ratio ζ=L/D (D is the width and L the thickness of the platelets) increases. In the binary case the competition between nematic ordering and depletion-induced segregation leads to rich phase behavior.
Quantum-dot temperature profiles during laser irradiation for semiconductor-doped glasses
Authors: Nagpal Swati
Published: December 2002

Temperature profiles around laser irradiated CdX (X=S, Se, and Te) quantum dots in borosilicate glasses were theoretically modeled. Initially the quantum dots heat up rapidly, followed by a gradual increase of temperature. Also it is found that larger dots reach higher temperatures for the same pulse characteristics. After the pulse is turned off, the dots initially cool rapidly, followed by a gradual decrease in temperature.
Effect of defects on the line shape of electron paramagnetic resonance signals from the single-molecule magnet Mn12: A theoretical study
Authors: Park Kyungwha , Dalal N. S. , Rikvold P. A.
Published: December 2002

We estimate the effect of lattice defects on the line shape of electron paramagnetic resonance (EPR) signals from a single crystal of the S=10 single-molecule magnet Mn12, measured with the external magnetic field along the crystal c axis. A second-order perturbation treatment of an effective single-spin Hamiltonian indicates that a small, random and static misorientation of the magnetic symmetry axes in a crystalline lattice can lead to asymmetric EPR peaks. Full spectra are simulated by calculating probability-distribution functions for the resonant fields, employing distributions in the tilt angle of the easy axis from the c axis, in the uniaxial anisotropy parameter, and in the g-factor. We discuss conditions under which the asymmetry in the EPR spectra becomes prominent. The direction and magnitude of the asymmetry provide information on the specific energy levels involved with the EPR transition, the EPR frequency, and the distribution in the tilt angle.
A second-order Kubo response theory-centroid approach to vibrational energy relaxation for single-mode excitations
Authors: Poulsen Jens Aage , Nyman Gunnar , Rossky Peter J.
Published: December 2002

We consider an approach for describing vibrational energy relaxation processes in liquids for solutes excited to states which are dominated by single-mode excitations. The method utilizes the fact that adding a suitable linear term to the solute intramolecular potential, creates excitations in the first excited state of a chosen vibrational mode. The fully quantum energy decay rate of the vibrational excitation can then be derived using quadratic response theory, which expresses the decay rate as the decay of a second-order Kubo transformed correlation function. This correlation function can be exactly related to a path integral centroid second-order correlation function, which can be evaluated approximately by centroid molecular dynamics. The abilities and limitations of the approach are discussed. It is shown that the method should work best when only a single vibrational state is occupied prior to excitation. Practical matters require also that the relaxation is in the pico-second regime or shorter. In contrast to the usual golden rule approach, the present method incorporates quantum effects and does not require explicit evaluation of vibrational coupling elements or Fourier transforms. It also incorporates the intramolecular vibrational coupling, whereby intramolecular relaxation can be monitored explicitly. The approach is tested on asymmetric stretch excited OClO(aq), using a classical bath, and gives results which are in good accord with earlier findings. The theory also points in the direction of how to improve the so-called classical approach to vibrational energy relaxation, where energy is put directly into the mode subsequently undergoing relaxation.
Large-eddy simulation of a compressible flow past a deep cavity
Authors: Larchevêque Lionel , Sagaut Pierre , Mary Ivan , Labbé Odile , Comte Pierre
Published: December 2002

Large-eddy simulations of the flow over a deep cavity are performed. The computations reproduce identically all the parameters of the experiment by Forestier and co-workers [J. Fluid Mech. (to be published)], including the high Reynolds number ReL=8.6×105. Spectra show an accurate prediction of the peak levels of the fundamental frequency and its first harmonics. Results are also analyzed both in terms of Reynolds and phase averages, the procedure used to compute phase averages being identical to the one used during the experiment. Agreement with the experimental data is found to be excellent. The expansion rate of the shear layer is accurately described, and the temporal physics of the flow, including the dynamics of the coherent structures, is fully recovered. By comparison with an auxiliary computation wherein the wind-tunnel upper wall is not taken into account, the cavity is found to oscillate in a flow-acoustic resonance mode. New values for the γ constant of Rossiter’s model are then proposed for a deep cavity resulting in an accurate prediction of the pressure peak frequencies.
Abstraction and desorption kinetics in the reaction of H+D/Si(100) and the relation to surface structure
Authors: Kubo Atsushi , Ishii Yasuhiro , Kitajima Masahiro
Published: December 2002

Kinetics of HD and D2 desorption from D/Si(100)-2×1 surfaces induced by H atoms has been investigated at temperature range of 97–685 K. Desorption rates of HD and D2 are measured in real-time by using mass spectrometers during the exposure of D/Si(100) to H atom beam. HD and D2 rates are not fitted by a hot atom kinetics, but are explained well in terms of adjacent double di-hydrides (DDI)-thermal desorption plus abstraction mechanism. For Ts⩾480 K, the pre-adsorbed D atoms are desorbed as D2 via DDI-desorption (∼22%), as HD via DDI-desorption (∼28%), and HD via abstraction (∼50%). For Ts⩽310 K, DDI-desorption does not occur, whereas the abstraction proceeds at a constant rate. The yield of DDI-desorption decreases as the surface structure of H/Si(100) changes from 2×1 to 1×1, suggesting the DDI-desorption is associated with a re-formation reaction of a mono-hydride dimer from adjacent two di-hydrides. The activation energy (Ea) for DDI-desorption is ∼0.2 eV. The rate of abstraction does not show Arrhenius-type temperature dependence. The abstraction probability PHD is estimated at 0.03±0.015 (cross section σHD=0.4±0.2 Å2) for Ts⩽310 K.
Stacking faults in colloidal crystals grown by sedimentation
Authors: Hoogenboom Jacob P. , Derks Didi , Vergeer Peter , van Blaaderen Alfons
Published: December 2002

A real-space study is presented on the occurrence of stacking faults in crystals of silica colloids with diameters of about 1 and 1.4 μm formed through sedimentation. The softness of the interaction potential is varied from slightly repulsive to hard-sphere like, both intrinsically by variation of the diameter, as well as through the addition of salt, which screens the surface charges. Our results indicate that the equilibrium crystal structure for these colloids is an fcc-crystal, with the number of stacking faults determined by the interplay between sedimentation and crystallization kinetics, irrespective of the softness of the interaction potential. For spheres with a certain diameter the number of stacking faults decreases with decreasing initial volume fractions. These results provide a way to grow fcc-crystals of hard-sphere particles by slow sedimentation. The relative number of stacking faults in the first few layers above the bottom wall can be as much as a factor of 10 higher than deeper into the crystal. This effect is due to the crystallization kinetics on a plain wall in a gravitational field. A patterned bottom wall that favors a specific hexagonal orientation was found to drastically reduce the number of stacking faults in the crystal.
Effects of dipole alignment and channel interference on two-photon absorption cross sections of two-dimensional charge-transfer systems
Authors: Cronstrand Peter , Luo Yi , Ågren Hans
Published: December 2002

The relationship between two-photon absorption cross sections and the architecture of donor–acceptor substitutions in two-dimensional charge-transfer cumulene-containing aromatic molecules is analyzed. It is shown that a molecular design strategy proposed for one-dimensional charge-transfer molecules must be considerably altered for two-dimensional systems. It is found that the maximal two-photon cross section of such two-dimensional systems can often be well described by a four state model. The importance of the alignment of the transition dipole moments and of channel interference on the two-photon absorption cross sections is emphasized.
Effect of roughness on the conductivity of semiconducting thin films/quantum wells with double rough boundaries
Authors: Palasantzas G. , De Hosson J. Th. M.
Published: December 2002

We have investigated the influence of surface/interface roughness on the electrical conductivity in semiconducting thin films/quantum wells with double self-affine rough interface boundaries. The self-affine boundary roughness is characterized by the roughness exponent H, the in-plane correlation length ξ, and the rms amplitude Δ. In addition, nonzero cross correlation between the interfaces are taken into account during the conductivity calculations. The latter is shown to affect strongly the electrical conductivity. Nevertheless, the exact effect depends strongly on the values of the interface correlation lengths and roughness exponents. Finally, the ratio between conductivities slightly below and above the critical thickness for which the second miniband is occupied is shown to be strongly sensitive on the form of the correlation function (or the interface roughness exponents), and the presence of cross correlations.
A permeation theory for single-file ion channels: Corresponding occupancy states produce Michaelis–Menten behavior
Authors: Nelson Peter Hugo
Published: December 2002

A kinetic theory is proposed for permeation in open ion channels. Within the model, the selectivity filter of the ion channel contains either n or (n−1) single-file ions. Association and/or dissociation of the nth ion from the selectivity filter is the rate limiting step. Ion translocation occurs via concerted single-file motion of the (n−1) ions and any water molecules contained within the selectivity filter. This concerted motion is assumed to be sufficiently rapid that translocation is not rate limiting. Ion permeation is thus sorption-limited, and the reduced permeation rate j* is a universal function of the reduced concentration c*, taking the form of a Langmuir adsorption isotherm. A corresponding occupancy-states explanation is thus provided for the Michaelis–Menten kinetics observed experimentally for many ion channels. Published experimental data for cation conduction in the open pores of the KcsA and gramicidin A channels are shown to exhibit the universal saturating behavior predicted by the theory. Additional validation of the theory by asymmetric-solution and tracer counterpermeation experiments is also discussed.
Quantum dynamics of the photoinitiated unimolecular dissociation of HOCO
Authors: Yu Hua-Gen , Muckerman James T.
Published: December 2002

The photodissociation dynamics of HOCO in the overtone νOH=3 are studied using a time-dependent wave packet dynamics approach. The dynamics calculations are carried out using a four-dimensional planar model in which the terminal C=O bond (spectator) is fixed at its equilibrium bond distance in the trans-HOCO conformer based on an empirical potential energy surface for HOCO fit to accurate ab initio calculations of the stationary point energies. The branching fraction into the H+CO2 products, and the resonance states associated with νOH=3 are investigated in detail.
Quantum-classical Liouville approach to molecular dynamics: Surface hopping Gaussian phase-space packets
Authors: Horenko Illia , Salzmann Christian , Schmidt Burkhard , Schütte Christof
Published: December 2002

In mixed quantum-classical molecular dynamics few but important degrees of freedom of a molecular system are modeled quantum mechanically while the remaining degrees of freedom are treated within the classical approximation. Such models can be systematically derived as a first-order approximation to the partial Wigner transform of the quantum Liouville-von Neumann equation. The resulting adiabatic quantum-classical Liouville equation (QCLE) can be decomposed into three individual propagators by means of a Trotter splitting: (1) phase oscillations of the coherences resulting from the time evolution of the quantum-mechanical subsystem, (2) exchange of densities and coherences reflecting non adiabatic effects in quantum-classical dynamics, and (3) classical Liouvillian transport of densities and coherences along adiabatic potential energy surfaces or arithmetic means thereof. A novel stochastic implementation of the QCLE is proposed in the present work. In order to substantially improve the traditional algorithm based on surface hopping trajectories [J. C. Tully, J. Chem. Phys. 93, 1061 (1990)], we model the evolution of densities and coherences by a set of surface hopping Gaussian phase-space packets (GPPs) with variable width and with adjustable real or complex amplitudes, respectively. The dense sampling of phase space offers two main advantages over other numerical schemes to solve the QCLE. First, it allows us to perform a quantum-classical simulation employing a constant number of particles; i.e., the generation of new trajectories at each surface hop is avoided. Second, the effect of nonlocal operators on the exchange of densities and coherences can be treated beyond the momentum jump approximation. For the example of a single avoided crossing we demonstrate that convergence towards fully quantum-mechanical dynamics is much faster for surface hopping GPPs than for trajectory-based methods. For dual avoided crossings the Gaussian-based dynamics correctly reproduces the quantum-mechanical result even when trajectory-based methods not accounting for the transport of coherences fail qualitatively.
Fate of isolated CH(B2Σ−,v=0,J) states in inelastic collisions with CO
Authors: Kind M. , Meden P. , Stuhl F.
Published: December 2002

The kinetic fate of the single rotational states 0⩽N⩽7 of electronically excited CH(B2Σ−,v=0) radicals has been studied in the gas phase at room temperature in the presence of CO. Rate constants of the state-to-state relaxation are presented. Further, rate constants were determined for the electronic quenching of single-N states and are compared with the corresponding data for CH(A2Δ,v=0). The radiative lifetimes of the rotational levels are given, too. All these processes were found to be more efficient for the B state than those for the A state. Further, collisional mixing of the B with the A state is described quantitatively.
Helium dimer dispersion forces and correlation potentials in density functional theory
Authors: Allen Mark J. , Tozer David J.
Published: December 2002

The dispersion interaction in the helium dimer is considered from the viewpoint of the force on a nucleus. At large internuclear separations, Brueckner coupled cluster BD(T) forces agree well with near-exact dispersion forces. The atomic density distortion associated with the dispersion force is quantified by comparing the BD(T) dimer density with a superposition of atomic densities. For density functional theory calculations in the Hartree–Fock–Kohn–Sham (HFKS) formalism, the accuracy of the dispersion force is governed by the correlation potential. Calculations using the conventional Lee–Yang–Parr [Phys. Rev. B 37, 785 (1988)] potential only generate a small density distortion, giving forces significantly smaller than BD(T). The BD(T) electron densities are therefore used to determine improved correlation potentials using a modified Zhao–Morrison–Parr (ZMP) approach [Phys. Rev. A 50, 2138 (1994)]. HFKS calculations using these ZMP potentials quantitatively reproduce the distortion, giving dispersion forces in good agreement with BD(T). The dimer ZMP correlation potential is partitioned into two parts, one equal to the sum of two unperturbed spherical atomic correlation potentials and the other representing an interaction potential. HFKS calculations using the former do not generate the distortion; forces are close to Hartree–Fock. Calculations using the latter do generate the distortion, giving forces essentially identical to those from the full dimer potential. The origin of the distortion is traced to the asymmetric structure of the interaction correlation potential in the vicinity of each nucleus.
Effect of nonresonant frequencies on the enhancement of quantum beat amplitudes in rovibrational states of Li2: The role of state spacing
Authors: Mirowski Elizabeth , Stauffer Hans U. , Ballard Joshua B. , Zhang Bo , Hetherington Craig L. , Leone Stephen R.
Published: December 2002

Optical phase manipulation of nonresonant frequencies is investigated as a method of achieving optimal population transfer during resonant impulsive stimulated Raman scattering. Wave packets containing quantum beats between an initially prepared rovibrational level in the A(1Σu+) electronic state of Li2 and states populated via a resonance-enhanced rotational Raman process are created using a shaped ultrafast pulse centered near 800 nm. Study of these wave packets allows a quantitative comparison of population transfer as a function of applied phases in the ultrafast pulse. Two cases are explored to determine the ability to enhance population transfer: one with a wide state spacing [A(νA=11, JA=28)-A(11,30) at 50.1 cm−1] and one with a narrow spacing [A(11,8)-A(11,10) at 16.6 cm−1]. In both cases, several different phase masks are applied to the wave packet preparation pulse to enhance the population transferred to the newly formed state of interest. One phase mask involves the application of a −90° phase shift to the nonresonant optical frequencies that lie between the resonant transition frequencies, resulting in an optimal phase relationship between pairs of nonresonant frequencies contributing to the Stokes–Raman excitation. Another extends the phase modification to the nonresonant frequencies lying outside the two resonant transitions to allow constructive enhancement from a larger range of frequencies. Significant population enhancements, up to a factor of ∼12, of the newly formed A(11,30) and A(11,10) states are demonstrated. In addition, the dependence on the state spacing and therefore the extent to which nonresonant frequencies affect the population transferred in the stimulated Raman process are demonstrated.
Measurement of the polarization profile across a surface-stabilized ferroelectric liquid crystal cell using the pyroelectric laser-intensity-modulation method
Authors: Shtykov N. M. , Vij J. K.
Published: December 2002

The pyroelectric technique, called the laser-intensity-modulation method, was used for investigations of the spatial distribution of the pyroelectric coefficient that gives information about the director profile in ferroelectric liquid crystal (FLC) cells. The pyroelectric current from a cell, illuminated by the intensity-modulated laser beam, was measured as a function of the modulation frequency. The Tikhonov regularization procedure was used for the deconvolution of the pyroelectric current spectrum into the spatial profile of the pyroelectric coefficient. It was found that in the surface-stabilized ferroelectric liquid crystal (SSFLC) structure with a splayed director profile, the flexoelectric contribution to the local polarization has comparable magnitude with the spontaneous one. The effective flexoelectric coefficient of the investigated FLC material and the flexoelectric polarization of the SSFLC structure were determined.
Nondestructive identification of impurities in granular medium
Authors: Hong Jongbae , Xu Aiguo
Published: December 2002

We found differing characteristics in the backscattered signal depending on the presence of light and heavy impurities in a granular chain. The difference is due to soliton confinement in the region of the light impurity of the granular medium, while there is no such phenomenon for the case of a heavy impurity. This difference may be used to discriminate nondestructively between light and heavy impurities in the granular medium.
Energy profile of the interconversion path between T-shape and slipped-parallel benzene dimers
Authors: Tsuzuki Seiji , Uchimaru Tadafumi , Sugawara Ko-ichi , Mikami Masuhiro
Published: December 2002

The energy profile of the interconversion path between the T-shape and slipped-parallel dimers has been studied by high level ab initio calculations. The CCSD(T) (coupled cluster calculation with single and double substitutions with noniterative triple excitations) interaction energy at the basis set limit has been estimated from the MP2 (the second-order Møller–Plesset calculation) interaction energy near the basis set limit and the CCSD(T) correction term using the 6-311G* basis set. The calculated CCSD(T) level energy profile has shown that the potential is very flat and the interconversion barrier height is very small (around 0.2 kcal/mol). The MP2 calculations using large basis sets near the basis set limit considerably overestimate the attraction of the slipped-parallel dimer, which indicates the importance of higher level electron correlation correction for studying the potential energy surface of the benzene dimer.
Role of isomerization channel in unimolecular dissociation reaction H2CO→H2+CO: Ab initio global potential energy surface and classical trajectory analysis
Authors: Yonehara Takehiro , Kato Shigeki
Published: December 2002

We constructed a full dimensional potential energy function of H2CO that can describe both the dissociation and isomerization channels by the modified Shepard interpolation method. Ab initio calculations at the MP2/cc-pVTZ level were carried out to obtain the local potential functions at about 4700 points. The interpolant points were sampled by classical trajectory calculations and by the grid searches in the internal coordinate space. Classical trajectory calculations were performed to examine the intramolecular dynamics associated with the dissociation as well as the product state distributions. The time scale of intramolecular vibrational energy randomization was much faster than that of the dissociation reaction. The dissociation rate was obtained from the classical trajectory results and the effect of the isomerization channel on the dissociation was estimated. The calculated rate constants were compared with those by Rice–Ramsperger–Kassel–Marcus theory.
Vibronic spectroscopy of the H-bonded aminophenol–water complex
Authors: Meenakshi P. S. , Biswas N. , Wategaonkar S.
Published: December 2002

In this work, the vibronic spectroscopy of the p-aminophenol–water 1:1 complex is presented. The S1 vibrational energy levels of the complex were characterized by REMPI spectroscopy up to 2500 cm−1 above the band origin. The dispersed fluorescence spectra were recorded for the B.O., 6a01 and I02 excitations to characterize the vibrational levels in the S0 state of the complex. Stimulated ion depletion spectroscopy was carried out to determine the higher vibrational levels of the ground state all the way up to ∼3075 cm−1. The structure and the vibrational levels of the AP–W1 complex were calculated ab initio at the HF level and DFT with B3LYP functional for S0, and CIS level for S1 using 6-31G** basis set. The structure of the AP–W1 complex compared well with the earlier calculations for this case as well as the other ROH–water (R=aromatic group) complexes reported in the literature. However, the redshift in the electronic band origin was almost half of that observed in other cases. A good correlation was shown to exist between the electronic red shifts and the respective pKa* values (the pKa in the excited state).
Optimization of density matrix functionals by the Hartree–Fock–Bogoliubov method
Authors: Staroverov Viktor N. , Scuseria Gustavo E.
Published: December 2002

It is demonstrated that the “corrected Hartree–Fock” (CHF) density matrix functional proposed by Csányi and Arias is identical with the Hartree–Fock–Bogoliubov (HFB) functional of the generalized density matrix up to the sign of the pairing energy term. Using this analogy, variational CHF calculations can be performed much more efficiently by solving the HFB equations for the generalized density matrix than by optimizing separately the natural orbitals and their occupations numbers. A family of CHF-type functionals with a scaled pairing energy is introduced and compared to the closely related antisymmetrized geminal power method.
Adsorption of H2, H22, O2, and CO on ZrB2(0001)
Authors: Aizawa Takashi , Hayami Wataru , Otani Shigeki
Published: December 2002

Adsorption of H2, H22, O2, and CO on ZrB2(0001) was investigated by using high-resolution electron energy loss spectroscopy. In hydrogen and oxygen adsorption, a strong loss peak appears in the specular spectra, showing the dissociative adsorption on the threefold hollow site. The vibrational frequency shifts toward higher energy with increase in oxygen coverage, while little changes in vibrational frequencies were observed in the hydrogen or deuterium adsorption. This means that the interaction between the adsorbates is stronger in oxygen adsorption than in hydrogen adsorption. The initial stage of CO adsorption shows two vibrational peaks, one of which corresponds to the atomic oxygen described above, the other is therefore the atomic carbon, showing dissociative adsorption.
The effect of two-exciton states on the linear absorption of the third molecular level in linear molecular aggregates
Authors: Chen Xiaoshuang , Kobayashi Takayoshi
Published: December 2002

The effect of the excited two-exciton state on the transition from the ground state to the third molecular state is studied for a three-level molecular aggregate. Based on a Green function technique, the analytical expression is given for the line shape of pump–probe differential spectrum. A redshift peak of the transition from the ground state to the third state has been found because of introducing the coupling of the excited two-exciton states to the third state. Further, the dependence of the spectra on the aggregate length shows that the delocalization length of the exciton is decreased with an increase in the coupling strength. This result indicates that the coupling induces the exciton localization, leading to the reduction of the effective molecular number in the molecular aggregates.
Pressure-induced structural transformations in a medium-sized silicon nanocrystal by tight-binding molecular dynamics
Authors: Martoňák R. , Colombo L. , Molteni C. , Parrinello M.
Published: December 2002

We use a recently developed constant-pressure molecular dynamics method for nonperiodic systems to study pressure-induced structural transformations in medium-sized silicon nanocrystals, where the kinetics is experimentally known to be bulk rather than surface dominated, choosing Si705 as a representative example. Pressure is applied and tuned through a liquid described by a classical potential, while the nanocrystal is treated within a tight-binding scheme. Upon pressurization the nanocrystal undergoes a structural transformation which starts at the surface and gradually propagates into the bulk core. The high-pressure structure is disordered and metallic, with an x-ray diffraction pattern compatible with both the ideal β-tin and simple hexagonal structures. Strong similarities with a recently calculated high-pressure phase of bulk amorphous silicon are evident. Upon pressure release, the original diamond structure is not recovered and a high degree of disorder persists.
Magneto-chiral anisotropy in charge transport through single-walled carbon nanotubes
Authors: Krstić V. , Roth S. , Burghard M. , Kern K. , Rikken G. L. J. A.
Published: December 2002

Carbon nanotubes are chiral molecular objects and therefore exist in two forms that are each other’s mirror image. Many aspects of these fascinating new materials have recently been explored but their chirality has hardly been addressed. We have investigated the charge transport through individual single-walled carbon nanotubes in the presence of a magnetic field parallel to the tube axis. For most of the investigated tubes, a dependence of the resistance that is odd in both the magnetic field and in the current through the tube is observed. This effect is ascribed to the chirality of the nanotube and is called electrical magneto-chiral anisotropy.
Measuring the porosity of porous materials having a rigid frame via reflected waves: A time domain analysis with fractional derivatives
Authors: Fellah Z. E. A. , Berger S. , Lauriks W. , Depollier C. , Fellah M.
Published: December 2002

An ultrasonic reflectivity method is proposed for measuring porosity of porous materials having a rigid frame. Porosity is the relative fraction by volume of the air contained within a material. It is important as one of the several parameters required by acoustical theory to characterize porous materials like plastic foams and fibrous or granular materials. The proposed method is based on a temporal model of the direct and inverse scattering problem for the propagation of transient ultrasonic waves in a homogeneous isotropic slab of porous material having a rigid frame. This time domain model of wave propagation was initially introduced by the authors [Z. E. A. Fellah and C. Depollier, J. Acoust. Soc. Am. 107, 683 (2000)]. The viscous and thermal losses of the medium are described by the model devised by Johnson et al. [D. L. Johnson, J. Koplik, and R. Dashen, J. Fluid. Mech, 176, 379 (1987)] and Allard [J. F. Allard, Chapman and Hall, London, (1993)] modified by a fractional calculus-based method applied in the time domain. Reflection and transmission scattering operators for a slab of porous material are derived from the responses of the medium to an incident acoustic pulse. The porosity is determined from the expressions of these operators. Experimental and numerical validation results of this method are presented. This method has the advantage of being simple, rapid, and efficient.
Effect of horizontal divergence on the geostrophic turbulence on a beta-plane: Suppression of the Rhines effect
Authors: Okuno A. , Masuda A.
Published: December 2002

An investigation is made on the effect of strong stratification, or horizontal divergence, on almost freely decaying geostrophic turbulence on a β-plane. In a model ocean, the surface layer is assumed to be active above the quiet deep layer, so that barotropification is prohibited a priori to purify the effect of horizontal divergence. Spectral evolution is accelerated numerically by an adjustment to keep kinetic energy constant against the retarding effect of horizontal divergence. First, numerical experiment is carried out for small or moderate horizontal divergence as control runs for comparison; UF/β<O(1), where U is the characteristic velocity of turbulence and F the squared inverse of the radius of deformation. As has been reported repeatedly, the β-effect induces a highly anisotropic field characterized by a band of zonal currents. It is confirmed also that kinetic energy has a one-dimensional spectrum approximately proportional to k−5 at high wave numbers k. Then, horizontal divergence is enlarged enough so that UF/β≫1, for which geostrophic turbulence turns out to behave just as on an f plane: (1) the field becomes isotropic with no significant zonal currents; (2) the inverse cascade of energy is not hindered by the β effect though it takes a longer time for turbulence to transfer energy to longer scales; and (3) the spectrum of kinetic energy (not total energy) is proportional to k−3 at high wave numbers. An argument based on the physics of long baroclinic Rossby waves is presented to explain why strong horizontal divergence suppresses the β effect. Furthermore a transform of variables leads to a modified governing equation, which clearly shows that the β effect should disappear for large horizontal divergence.
Apparent diffusion due to topographic microstructure in shallow waters
Authors: Nachbin André , Sølna Knut
Published: December 2002

Wave propagation in disordered (random) media is the underlying theme. We study the effective behavior of long coastal waves that travel over rough topographies. The topographies analyzed contain a smooth slowly varying profile together with disordered small-scale features. The mathematical model is a conservation law with random coefficients. The main asymptotic (stochastic theory) result is that the medium fluctuations cause the propagating pulse to broaden as it travels. The so called apparent diffusion (or pulse shaping) depends only on the traveling distance and the statistics of the random medium fluctuations. Thus, the broadening can be described in a deterministic way independently of the particular medium realization. This is confirmed numerically. Numerical experiments also show that the theory describing pulse shaping is very robust. Nonlinear shallow water simulations show that small amplitude pulse shaping is not affected by higher order terms. The robustness of the theory is observed numerically for a wide parameter regime. We vary both the microscale fluctuation level as well as the horizontal length scales of the topography. The numerical experiments produce very good results regarding the prediction for the wavefront attenuation.
Modeling velocity autocorrelation functions of confined fluids: A memory function approach
Authors: Krishnan S. H. , Ayappa K. G.
Published: December 2002

Velocity autocorrelation functions (VACF) of a fluid confined in a slit pore have been modeled using the memory equation. Models for the VACF are based on both the truncation and analytic closure approximations of the Mori’s continued fraction representation. The performance of the models is evaluated for gas to liquid-like pore densities and pore widths which accommodate one to four atomic layers. In all cases we compare the predictions from the models with the VACF obtained from molecular dynamics simulations. The truncation models predict an oscillatory behavior for the in-plane VACF with better agreement at lower densities. Among the analytical closure models we observe that the sech model applied at the first level of closure is not only able to capture the short-time dynamics but is also seen to give the best predictions to the in-plane diffusivities at liquid-like pore densities. Although the minima in the VACFs are captured accurately by the sech model, the subsequent plateau regions in the VACF typically observed in confined systems are not predicted. This aspect is due to the slower relaxation of the actual memory kernel, which is not captured by the model. Predictions of the in-plane diffusivities using different levels of analytic closure have been compared with diffusivities obtained from the simulations.
Note on chaotic advection in an oscillating drop
Authors: Angilella J. R. , Brancher J. P.
Published: December 2002

Mixing by chaotic advection within an oscillating drop is investigated by means of an elementary model (Hill’s spherical vortex). The drop remains spherical and the velocity field within the drop is assumed to rotate around a radial axis, in a periodical manner. Poincaré sections in the vicinity of fixed points are obtained asymptotically in the limit of small amplitude oscillations, and show that the trajectory of elliptic points undergoes a resonant mechanism. The flow inside the drop can display Lagrangian chaos when the drop oscillates. In particular, tracers located near the boundary of the drop are observed to contaminate the whole drop within a few periods.
Electronic properties of GaN induced by a subsurface stressor
Authors: Waltereit P. , Romanov A. E. , Speck J. S.
Published: December 2002

The influence of the strain field originating from a subsurface point source of dilatation (point stressor) on the electronic properties of nitride semiconductors is investigated. In the far field, real quantum dots can be viewed as such point stressors. The material surrounding the stressor was considered either to be a uniform GaN matrix or GaN matrix with an (In,Ga)N quantum well which is grown pseudomorphically between the stressor and the free surface. Isotropic elasticity was used to find the strain field around the stressor. A k⋅p perturbation theory approach was then applied to examine the shifts of the conduction and valence band edges caused by the stressor. We find lateral confinement for electrons and holes, which can be utilized to realize a strain-induced quantum dot in the quantum well.
Derivative moments in turbulent shear flows
Authors: Schumacher J. , Sreenivasan K. R. , Yeung P. K.
Published: December 2002

We propose a generalized perspective on the behavior of high-order derivative moments in turbulent shear flows by taking account of the roles of small-scale intermittency and mean shear, in addition to the Reynolds number. Two asymptotic regimes are discussed with respect to shear effects. By these means, some existing disagreements on the Reynolds number dependence of derivative moments can be explained. That odd-order moments of transverse velocity derivatives tend not to vanish as expected from elementary scaling considerations does not necessarily imply that small-scale anisotropy persists at all Reynolds numbers.
Concentrations and concentration fluctuations in two-dimensional turbulence
Authors: Krane B. , Pécseli H. L. , Trulsen J.
Published: December 2002

The dispersion of a cloud of passive test particles in a model for homogeneous and isotropic two-dimensional turbulence is studied by a direct numerical simulation in a driven-dissipative turbulent flow. The basic statistical properties of the concentrations of the clouds are analyzed with particular attention to their space–time evolutions in the center of mass as well as the rest frames of reference. Approximate analytical expressions for the concentration and the concentration fluctuations are discussed. The cloud concentration is modeled by a multiple contour distribution, where the concentration is constant within boundaries convected by the flow. Two different types of initial releases are considered: compact, core-type distributions, and alternatively “annulus” shaped distributions, with vanishing concentration at the center. It is demonstrated that the time evolution of the mean square radius of a cloud can be obtained solely from the power spectrum of the velocity fluctuations with a given, straightforward, model for the relative velocity correlation function. The analysis also provides results for the elongation of material lines convected by the turbulent flow, the observations confirming the expected exponential elongation for large times.
Homogeneous bubble nucleation in liquids: The classical theory revisited
Authors: Delale Can F. , Hruby Jan , Marsik Frantisek
Published: December 2002

The classical theory of homogeneous bubble nucleation is reconsidered by employing a phenomenological nucleation barrier in the capillarity approximation that utilizes the superheat threshold achieved in experiments. Consequently, an algorithm is constructed for the evaluation of the superheat temperatures in homogeneous boiling (tensile strengths in cavitation), the critical radii and steady-state nucleation rates. The nucleation theorem is written in this framework and is applied to the classical theory of homogeneous bubble nucleation for the phenomenological nucleation barrier employed. The superheat temperatures calculated show excellent agreement over a wide range of liquid pressures for most of the substances investigated. The steady-state nucleation rates are also altered by many orders of magnitude, in agreement with the results of previous investigators using different approaches.
Thermally stable exchange-biased magnetic tunnel junctions over 400 °C
Authors: Matsukawa Nozomu , Odagawa Akihiro , Sugita Yasunari , Kawashima Yoshio , Morinaga Yasunori , Satomi Mitsuo , Hiramoto Masayoshi , Kuwata Jun
Published: December 2002

Exchange-biased magnetic tunnel junctions (MTJs) with interposed Fe1−xPtx metal alloy layers between the Al oxide barrier and the ferromagnetic electrodes maintain large tunneling magnetoresistance (TMR) after thermal treatment in excess of 400 °C, owing to an improved barrier interface. After 400 °C annealing, TMRs of MTJs with Fe1−xPtx (x=0.1–0.2) exhibit over 40% and retain 30% TMR after 420 °C annealing. The tunnel barrier height derived from the current–voltage curve fitted to the Simmons equation increases with richer Pt content. Secondary ion mass spectroscopy depth profiles and cross-section transmission electron micrographs of MTJs with Fe0.85Pt0.15 show a clear interface around the Al oxide barrier even after annealing at 400 °C.
Backilluminated GaN/AlGaN heterojunction ultraviolet photodetector with high internal gain
Authors: Zhang S. K. , Wang W. B. , Shtau I. , Yun F. , He L. , Morkoç H. , Zhou X. , Tamargo M. , Alfano R. R.
Published: December 2002

We report on a backilluminated GaN/Al0.18Ga0.82N heterojunction ultraviolet (UV) photodetector with high internal gain based on metal-semiconductor-metal structures. A narrow band pass spectral response between 365 and 343 nm was achieved. When operating in dc mode, the responsivity reaches up to the order of 102 A/W under weak UV illumination, which is due to enormous internal gain up to 103. The linear dependence of photocurrent on bias and its square root dependence on optical power are found and explained by a trapping and recombination model. The high photocurrent gain is attributed to trapping and recombination centers with an acceptor character induced by dislocations in GaN.
δf
method to calculate plasma transport and rotation damping
Authors: Williams J. D. , Boozer A. H.
Published: December 2002

In this paper, the case of rotation damping for plasmas that lack perfect symmetry is studied. Results from a δf Monte Carlo code are used to calculate the damping. The δf method has proven to greatly improve the efficiency of computing the distribution function for fusion plasmas compared to standard Monte Carlo techniques. A method is developed that improves upon the efficiency of the δf code by combining the results of the kinetic simulation with the single-fluid force-balance equation.