bluuebunny/crossref_metadata_2025 huggingface dataset:
High-Q wet-etched GaAs microdisks containing InAs quantum boxes
Authors: Gayral B. , Gérard J. M. , Lemaı̂tre A. , Dupuis C. , Manin L. , Pelouard J. L.
Published: July 2002

A two-steps wet-etching fabrication process producing high-quality GaAs microdisks is presented. We report an optical characterization of these microdisks, using the photoluminescence of InAs quantum boxes as an internal light source. Thanks to an improved smoothness of the microdisk sidewall, cavity Q’s as high as 12 000 are observed, which opens very challenging novel application prospects for semiconductor microdisks.
Semiclassical law for the apparent viscosity of non-Newtonian fluids: An analogy between thixotropy of fluids and sintering of solids
Authors: Mezzasalma Stefano A.
Published: July 2002

A theory is presented to describe the apparent viscosity of thixotropic fluids as a function of the rate of shear. It represents the extension of a semiclassical approach that was previously formulated to deal with matter densification phenomena in solids starting from the state equation of the medium. In this context, the Debye expression for the Helmholtz free energy has been provided with a density of vibrational modes that accounts for atomic and microstructural changes occurring at the frequency scale of momentum transport (see diffusion). Working out the steady-state condition with respect to time gives an equation relating reduced apparent viscosity (η̃) and shear rate (γ̃) through the temperature value (θ*) that is energetically equivalent to the medium vibrations implied. Viscosity also turns out to depend on the Debye temperature θD (see φ∼θ*/θD) and an equivalent Gruneisen parameter (μ), defined with respect to viscosity variations. Increasing φ in pseudoplastic and dilatant media, respectively, increases and decreases η̃, which always increases with increasing μ. The analogy between dilatancy/sintering and pseudoplasticity/desintering is suggested, and a correspondence between matter and momentum transports is traced on the basis of the phononic spectrum properties. Application to experimental measurements are presented and discussed for aqueous monodispersions of polystyrene (PS) latex particles, aqueous glycerol solutions of partially hydrolyzed polyacrylamide (PHPAA) at different sodium chloride (NaCl) concentrations, polymethylmethacrylate (PMMA) suspensions in dioctylphthalate (DOP), and for a molecularly thin liquid film of octamethylciclotetrasiloxane (OMCTS). Best fit coefficients for φ and μ have been constrained to the Debye temperature and the effective low-shear viscosity (η0) according to their dependences upon the suspended volume fraction (φ), θD=θD(φ), and η0=η0(φ), and the agreement with experimental data is quite satisfactory in all cases here examined. It is then suggested that the viscous character of a liquid can be described in terms of a coupling between Brownian diffusion and phonon wave motion.
High-performance all-polymer integrated circuits
Authors: Gelinck G. H. , Geuns T. C. T. , de Leeuw D. M.
Published: July 2002

In this letter, we demonstrate the integration of all-polymer field-effect transistors in fully functional integrated circuits with operating frequencies of several kHz. One of the key items is an approach to incorporate low-Ohmic vertical interconnects compatible with an all-polymer approach. Inverters, NAND gates, and ring oscillators with transistor channel lengths down to 1 μm have been constructed. Inverters show voltage amplification at moderate biases and pentacene seven-stage ring oscillators show switching frequencies of a few kHz. The potential to realize large integrated circuits is demonstrated by a 15 bit code generator circuit using several hundreds of devices. The proposed concept was evaluated for three solution-processable organic semiconductors.
Generalized variational density functional perturbation theory
Authors: Putrino Anna , Sebastiani Daniel , Parrinello Michele
Published: July 2002

We present an implementation of variational perturbation theory in the framework of density functional theory. We use an ab initio pseudopotential scheme with a plane wave basis set and expand the energy functional up to second order in the perturbation. The approach is fairly general and does not rely on the representativeness of the perturbation through a Hamiltonian operator and does not require the use of canonical orbitals. Instead, a functional formulation is used to characterize the perturbation. Several types of applications are presented which illustrate the variety of linear response phenomena that can be treated with our method (vibrational modes, Raman scattering, and nuclear magnetic resonance chemical shift computations). In combination with advanced gradient correction formulas, an accurate description of second order effects in periodic and isolated systems can be achieved.
A nonperturbative density functional analysis for nonuniform Lennard-Jones fluid
Authors: Zhou Shiqi
Published: July 2002

A nonperturbative density functional analysis for nonuniform Lennard-Jones (LJ) fluid is proposed in the framework of weighted density approximation. The required inputs (the second-order and first-order direct correlation functions for uniform LJ fluid) are obtained by numerically solving the OZ equation and using a equation of state for uniform Lennard-Jones fluid. The density distribution resulting from the present analysis is compared favorably with the computer simulation results for two cases: (1) a Lennard-Jones fluid confined in a spherical cavity with a hard wall, and (2) a Lennard-Jones fluid confined between two hard walls.
Pedestal profiles and fluctuations in C-Mod enhanced D-alpha H-modes
Authors: Hubbard A. E. , Boivin R. L. , Granetz R. S. , Greenwald M. , Hughes J. W. , Hutchinson I. H. , Irby J. , LaBombard B. , Lin Y. , Marmar E. S. , Mazurenko A. , Mossessian D. , Nelson-Melby E. , Porkolab M. , Snipes J. A. , Terry J. , Wolfe S. , Wukitch S. , Carreras B. A. , Klein V. , Pedersen T. Sunn
Published: July 2002

High resolution measurements on the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1551 (1994)] of the transport barrier in the “Enhanced Dα” (EDA) regime, which has increased particle transport without large edge localized modes, show steep density and temperature gradients over a region of 2–5 mm, with peak pressure gradients up to 12 MPa/m. Evolution of the pedestal at the L-H transition is consistent with a large, rapid drop in thermal conductivity across the barrier. A quasi-coherent fluctuation in density, potential, and Bpol, with f0∼50–150 kHz and kθ∼4 cm−1, always appears in the barrier during EDA, and drives a large particle flux. Conditions to access the steady-state EDA regime in deuterium include δ>0.35, q95>3.5, and L-mode target density n̄e>1.2×1020 m−3. A reduced q95 limit is found for hydrogen discharges.
Shape imprinting due to variable disulfide bonds in polyacrylamide gels
Authors: Greytak Andrew B. , Grosberg Alexander Y. , Tanaka Toyoichi
Published: July 2002

Through the use of variable disulfide crosslinkers, we have created polyacrylamide gels whose shape can be altered after polymerization. N,N′-bisacryloylcystamine is incorporated as a crosslinker, along with a smaller amount of a permanent crosslinker. After polymerization, the disulfide bonds are cleaved into thiols through reduction. By reoxidizing the thiols with the gel held in a new macroscopic shape, a new set of disulfide bonds is formed, and the gel is forced to adopt the new shape. Retension of the new shape improves with greater distortion from the original shape, as well as with increased concentration of variable disulfide bonds. A simple theoretical model has been developed to explain these data, although the enigmatic kinetics of relaxation remain unexplained.
Quasiparticle band structures of wurtzite and rock-salt ZnO
Authors: Ni H. Q. , Lu Y. F. , Ren Z. M.
Published: July 2002

Band structures of both wurtzite and rock-salt ZnO were investigated using the ab initio pseudopotential method with both local density approximation (LDA) and GW approximation. The error in approximating 3d electrons as core electrons was investigated for both LDA and quasiparticle calculations. The differences between the band structures obtained by the GW approximation and LDA were explained. The quasiparticle band structures were compared with experimental results. The spin–orbit splitting was calculated for both wurtzite and rock-salt ZnO with the LDA. The density of states was investigated with the GW approximation.
Stochastic simulation of heavy-particle trajectories in turbulent flows
Authors: Reynolds A. M. , Cohen J. E.
Published: July 2002

It is shown how fluid velocities along heavy-particle trajectories in a turbulent flow, which are neither purely Lagrangian nor purely Eulerian in character, can be simulated using a fractional Langevin equation; the analogy of the Langevin equation but with fractional Gaussian noise as the source of randomness. A modified Fokker–Planck equation is derived for such stochastic processes and Thomson’s well-mixed condition is generalized accordingly. Heavy-particle trajectories in grid turbulence are then simulated by combining the fractional Langevin equation with the equation of motion for a heavy-particle in a turbulent flow. Model predictions for the mean concentrations and root-mean-square velocities of heavy-particles are shown to be in reasonable agreement with the experimental data of Snyder and Lumley (1971).
Energy level alignment and two-dimensional structure of pentacene on Au(111) surfaces
Authors: Schroeder P. G. , France C. B. , Park J. B. , Parkinson B. A.
Published: July 2002

X-ray photoemission, ultraviolet photoemission spectroscopy (UPS), and scanning tunneling microscopy (STM) have been used to determine the energy level alignment and the molecular ordering of monolayer and submonolayer pentacene films on Au(111) in ultrahigh vacuum. Pentacene evaporated onto the van der Waals surface of SnS2 was used as a noninteracting substrate for comparison. A large interface dipole was measured for pentacene on Au(111) (0.95 eV) whereas pentacene on SnS2 showed a relatively small interface dipole (0.26 eV). The different interface dipoles are related to the different orientations of the pentacene molecules due to different pentacene substrate interaction energies. Differences in the UPS spectra also support changing molecular orientations of the two substrates. STM images of pentacene on Au(111) revealed that the molecules lay flat on the substrate and are oriented parallel to each other, forming striped structures that are commensurate with the Au(111) lattice. The pentacene coverage influences the packing of the striped structures that can form a variety of unit cells. Three related unit cells with pentacene molecules tilted [(2×2√7), (2×√31), and (2×√39)] or perpendicular (2×3√3) to the row direction were identified on Au(111).
Glass transition, crystallization kinetics and pressure effect on crystallization of ZrNbCuNiBe bulk metallic glass
Authors: Xing P. F. , Zhuang Y. X. , Wang W. H. , Gerward L. , Jiang J. Z.
Published: July 2002

The glass transition behavior and crystallization kinetics of Zr48Nb8Cu14Ni12Be18 bulk metallic glass have been investigated by differential scanning calorimetry and x-ray powder diffraction (XRD). The activation energies of both glass transition and crystallization events have been obtained using the Kissinger method. Results indicate that this glass crystallizes by a three-stage reaction: (1) phase separation and primary crystallization of glass, (2) formation of intermetallic compounds, and (3) decomposition of intermetallic compounds and crystallization of residual amorphous phase. The pressure effect on crystallization is studied by in situ high-pressure and high-temperature XRD using synchrotron radiation. Two crystallization temperatures, observed by in-situ XRD, behave differently with varying pressure. The onset crystallization temperature increases with pressure with a slope of 9.5 K/GPa in the range of 0 to 4.4 GPa, while the another crystallization temperature keeps almost unchanged in the applied pressure range. The results are attributed to the competing processes between the thermodynamic potential barrier and the diffusion activation energy under pressure.
On the criteria for superconductivity in PrBa2Cu3O6.6
Authors: Boyko F. A. , Bukin G. V. , Voloshin V. A. , Gusev A. A.
Published: July 2002

The structure of RBa2Cu3Ox is analyzed to ascertain the reason for the absence of superconductivity in PrBa2Cu3O7 or for its presence in PrBa2Cu3O6.6. An empirical formula with a single adjustable parameter is proposed which describes the temperature dependence of the resistance at different pressures. The character of the temperature variation of the superconducting transition temperature at pressures higher than those attained experimentally is predicted.
Nuclear magnetic resonance monitoring of capillary imbibition and diffusion of water into hardened white cement paste
Authors: Ceballos-Ruano J. S. , Kupka T. , Nicoll D. W. , Benson J. W. , Ioannidis M. A. , Hansson C. , Pintar M. M.
Published: July 2002

Nuclear magnetic resonance (NMR) experiments monitoring the imbibition (sorption) and diffusion of water into white cement paste are reported. The sample was a 1.3 cm long cylinder (6 mm o.d.) of hardened ordinary white cement paste, with a water/cement ratio of 0.42 containing 0.5% Ca(NO2)2 and 2% NaCl. Water proton magnetization and T2 values were obtained as functions of time. Imbibition of H2O and diffusion of H2O and D2O were monitored with H1 NMR at 26 and 30 MHz. The countercurrent water imbibition experiments revealed a two-stage process. A rapid uptake of water, involving about 85% of the total, took place in about 45 min. Maximum saturation was reached in about 2 days. Both stages of the process were well described by a nonlinear diffusion-like equation. Diffusion of both H2O and D2O was characterized by a single diffusion coefficient. The diffusion coefficient for H2O and D2O, derived by fitting the data to the diffusion equation, is well predicted by D0/(Fφ).
Relationships between Jaynes entropy of the one-particle density matrix and Shannon entropy of the electron densities
Authors: Sagar Robin P. , Ramı́rez Juan Carlos , Esquivel Rodolfo O. , Hô Minhhuy , Smith Vedene H.
Published: July 2002

Relationships between the Jaynes and Shannon information entropies, both of which are related to the correlation energy, are given and the physical implications are explored. Evidence is provided from a study of the Be and Ne isoelectronic sequences that the Shannon entropy is more sensitive to the effects of the nuclear charge than is the Jaynes entropy. It is shown that the Jaynes entropy may be considered as the difference between the mean orbital Shannon entropy per electron and the mean orbital Shannon entropy of one electron. These mean orbital entropies display the same functional behavior as the total Shannon entropy throughout the isoelectronic sequence. We demonstrate that the Jaynes entropy may be considered as the sum of the Kullback–Leibler distance entropies occurring between natural spin orbital densities and a reference point whose occupation number is one. The large Z asymptotic behavior of the Jaynes and Shannon entropies is discussed. From a study of a Ne isonuclear sequence, it is shown that both Shannon and Jaynes entropies increase with the number of electrons, illustrating that these quantities are sensitive to the amount of electronic interactions present in the system. Relationships for the changes in the Shannon and Jaynes entropies with respect to the number of electrons for an isonuclear sequence are obtained, which depend on the electron affinity, first and second ionization potentials, and the hardness and chemical potential of the system.
Recombination properties of oxygen-precipitated silicon
Authors: Hwang J. M. , Schroder D. K.
Published: July 2002

It is well known that recombination lifetimes are significantly degraded in oxygen-precipitated silicon. The possible sources for lifetime degradation are expected to be oxygen precipitates (OP’s), dislocation loops, stacking faults, and point defects associated with self-interstitials generated during the oxygen precipitation process. From the results of an extensive experimental study using IR absorption, TEM, DLTS, and SPV (surface photovoltage), we have found that OP’s are mainly responsible for the observed lifetime degradation and that recombination at OP’s takes place through Si/OP interface states. In addition we have observed that the lifetime degradation is more severe in p-Si than in n-Si even for identical densities and sizes of OP’s. A model for recombination at OP’s is presented in terms of the surface recombination velocity at the Si/OP interface and their average density and size. To explain the lifetime difference between n-Si and p-Si we propose a band bending around OP’s caused by positive fixed charges in the OP’s.
Deformation behavior of undoped and In-doped GaAs in the temperature range 700–1100 °C
Authors: Guruswamy S. , Rai R. S. , Faber K. T. , Hirth J. P.
Published: July 2002

Compressive deformation of undoped and In-doped GaAs single crystals has been carried out in [001] and [123] orientations in the temperature range 700–1100 °C. Indium additions, at levels of 1–2×1020 atoms cm−3, result in critical resolved shear stress (CRSS) values that are about twice as large as the undoped crystals in the temperature range of 700–1100 °C. The CRSS was weakly dependent on temperature in the temperature range investigated as expected for a model of athermal solid solution hardening. The CRSS value of 3.3 MPa for the In-doped crystal is sufficient to eliminate profuse dislocation formation in a 75-mm-diam crystal on the basis of current theories for the magnitude of the thermal stress experienced during growth. The results also suggest that the process of dislocation climb is slowed appreciably by In doping.
Hysteresis phenomena and Barkhausen-like instabilities in the Sherrington–Kirkpatrick spin-glass model
Authors: Bertotti G. , Pasquale M.
Published: July 2002

We investigate by computer simulations the hysteresis and instability phenomena taking place in the Sherrington–Kirkpatrick (SK) spin-glass model at zero temperature in the presence of field H varying in time at a given rate ‖Ḣ‖. At low ‖Ḣ‖, the system evolves by well-separated Barkhausen-like spin-flip clusters, which generate through a branching mechanism a sequence of metastable spin configurations associated with different values of H. On this basis, a new method to search for the lowest-energy states by applying a proper field history is proposed. The spin behavior versus ‖Ḣ‖ is studied by evaluating the power spectrum of the magnetization rate associated with spin flipping. Close similarities with the properties of the Barkhausen effect in ferromagnetic materials are found. This result suggests that the SK model can to some extent describe the intricate, long-range magnetostatic interactions governing Barkhausen effect phenomenology.
Efficient blue-green and white light-emitting electrochemical cells based on poly[9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl]
Authors: Yang Yang , Pei Qibing
Published: July 2002

Efficient blue-green polymer light-emitting electrochemical cells, based on poly[9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl] (BDOH-PF), have been demonstrated. BDOH-PF has a high photoluminescent quantum efficiency of 73% photon/photon in solid films. The polyether-type side groups of this polymer are built in to promote the ionic conductivity necessary for the operation of light-emitting electrochemical cells (LECs). Blue-green LECs using a blend of BDOH-PF and lithium triflate as the emissive layer, ITO and Al as the electrodes, achieve an external quantum efficiency of 4% photons/electron and a luminous efficiency of 12 lm/W at 3.1 V. The brightness reaches ∼200 cd/m2 at 3.1 V, and exceeds 1000 cd/m2 at 3.5 V. When additional poly(ethylene oxide) (PEO) is blended into the emissive layer, phase separation between PEO and BDOH-PF occurs, and the emission becomes white. These white emitting LECs reach an external quantum efficiency of 2.4% photons/electron and 400 cd/m2 brightness at a bias of 4 V.
Mechanism of incongruent ablation of SrTiO3
Authors: Dam B. , Rector J. H. , Johansson J. , Huijbregtse J. , De Groot D. G.
Published: July 2002

At low fluences, the [Sr]/[Ti] ratio of laser deposited SrTiO3 films appears to be a function of the laser fluence. The deviation from stoichiometry is remarkably constant in time. From an analysis of both the composition of the film and the irradiated target, we deduce a volume-diffusion-assisted preferential ablation process. At high fluences (above 1.3 J/cm2), stoichiometric SrTiO3 films are obtained. This is not due to a change in ablation mechanism, but follows from the fact that at 1.3 J/cm2 the calculated diffusion length of Sr within the irradiated target, becomes of the order of the ablation rate per shot.
Lattice thermal conductivity of wires
Authors: Walkauskas S. G. , Broido D. A. , Kempa K. , Reinecke T. L.
Published: July 2002

The lattice thermal conductivity of free standing wires is calculated within a Boltzmann equation approach. Diffusive and specular phonon scattering at the wire surfaces are included by appropriate boundary conditions on the phonon distribution. The wire thermal conductivity is found to decrease markedly below the bulk value for narrow wires. We show that this decrease in wires is larger than that which occurs in free standing wells of comparable size.
Strain sensitivity of highly magnetostrictive amorphous films for use in microstrain sensors
Authors: Shin Kwang-Ho , Inoue Mitsuteru , Arai Ken-Ichi
Published: July 2002

Strain sensitivity of amorphous (Fe90Co10)78Si12B10 sputtered film was studied in detail, so as to examine the potential of film for highly sensitive microstrain gauges. The film exhibited the figure of merit F=(Δμ/μ)/ε (change in film permeability μ per unit strain ε) of 1.2×105, the value of which is comparable to that of amorphous ribbons. Reflecting the large value of F, the high-frequency impedance of the film was also subjected to change due to strain: the change in impedance of 46% was observed at 100 MHz applying a strain of 300×10−6. It is also demonstrated that utilizing the strain-dependent impedance, a small strain of 0.2×10−6 can be detected with the film.
Nitrogen uptake and rate-limiting step in low-temperature nitriding of iron
Authors: Inia D. K. , Vredenberg A. M. , Habraken F. H. P. M. , Boerma D. O.
Published: July 2002

Recently, a method to nitride iron in NH3 ambients at low temperature (225–350 °C) has been developed. In this method, the Fe is covered with a thin (∼40 nm) Ni layer, which acts as a catalyst for the nitriding process. From experiments, in which the amount of nitrogen uptake is measured as a function of nitriding time, it is concluded that the decomposition of NH3 at the Ni surface contains the rate-limiting step in this low-temperature nitriding process. From a model calculation, it is further concluded that the reaction step NH3→NH2+H at the Ni surface is the rate-limiting step with an activation energy of ∼1.5 eV.
Dependence of Ni/AlGaN Schottky barrier height on Al mole fraction
Authors: Qiao D. , Yu L. S. , Lau S. S. , Redwing J. M. , Lin J. Y. , Jiang H. X.
Published: July 2002

The dependence of the Schottky barrier height of Ni/AlxGa1−xN contact on the Al mole fraction up to x=0.23 was studied. The barrier heights were measured by I–V, capacitance–voltage, and the internal photoemission method. The Al mole fractions were estimated from the AlGaN band gap energies measured by photoluminescence. In the range of x<0.2 a linear relationship between the barrier height and Al mole fraction was obtained. This was consistent with the slope predicted by the Schottky rule. For x=0.23, the measured barrier height was lower than predicted. We believed this was due to crystalline defects at the Ni/AlGaN interface.
Waveform control algorithm for rotational single sheet testers using system identification techniques
Authors: Makaveev D. , Maes J. , Melkebeek J.
Published: July 2002

In rotational single sheet testers (RSST), the magnetic properties of electrical steel sheet are measured for well-defined one- or two-dimensional induction waveforms over a large frequency range (1–200 Hz). For this task, a new iterative waveform control algorithm is presented. It is based on the recursive least squares system identification technique. The modeling of the setup as well as the principles and implementation details of the algorithm are discussed. Simulation and measurement results for alternating sinusoidal magnetization of the sample are presented. The main advantage of the proposed method is that it does not require preliminary information concerning the properties of the tested material or the measurement setup. The method is therefore ideally suited for use in a fully automated measurement system. Furthermore, especially for low frequencies (1–10 Hz) the proposed algorithm exhibits faster convergence compared to previously described methods.
Magnetoresistance of a (γ-Fe2O3)80Ag20 nanocomposite prepared in reverse micelles
Authors: Wiemann Joan A. , Carpenter Everett E. , Wiggins Jason , Zhou Weilie , Tang Jinke , Li Sichu , John Vijay T. , Long Gary J. , Mohan Amitabh
Published: July 2002

The magnetic and transport properties of a (γ-Fe2O3)80Ag20 nanocomposite, prepared by a reverse micelle technique, have been studied. γ-Fe2O3 nanoparticles and Ag particles were individually synthesized in reverse micelles. The nanocomposite material was then prepared by mixing the two different particles in a γ-Fe2O3/Ag molar ratio 80/20. The morphology of the nanoparticles was examined with transmission electron microscopy. Mössbauer spectra revealed no obvious presence of any divalent iron. Zero field cooled and field cooled magnetic susceptibilities indicated a blocking temperature of about 40 K. Negative magnetoresistance was observed resembling that in ball milled γ-Fe2O3/Ag nanocomposites. However, the magnitude of the negative magnetoresistance is smaller and is ∼2.2% at 220 K and 9 T. Two possible mechanisms, spin-dependent hopping and tunneling across magnetic barriers, are discussed.
The vibrational spectrum and the isomerization potential of HCN/HNC
Authors: Szalay Viktor
Published: July 2002

A newly developed theory of molecules with large-amplitude internal motions, the nonrigid rotation–large-amplitude internal motion Hamiltonian (NRLH) method [V. Szalay, J. Mol. Spectrosc. 128, 24 (1988)], and the global potential energy surface of the HCN/HNC system proposed by Murrell, Carter, and Halonen [J. Mol. Spectrosc. 93, 307 (1982)] have been used to calculate the vibrational energy levels of HCN and HNC. When applied to HCN/HNC the NRLH method provides an approximate, effective isomerization Hamiltonian. The Schrödinger equation of this effective isomerization Hamiltonian has been solved by combining the variational method and the discrete variable representation. The comparison of the results to those obtained by different fully variational methods (i.e., by methods which treat all of the internal motions variationally ) using the same potential energy surface shows excellent agreement for the stretching vibrational frequencies, and satisfactory agreement between the bending frequencies. The vibrational energy levels of a number of other triatomic molecules, C3, CH2, and H2O have also been calculated from their ground electronic state potential energy surfaces. The results are discussed and compared to those of fully variational and nonrigid bender calculations.
The C 3Π(v′=0–6)–X 3Π(v″=0) transition in SiC
Authors: Ebben Maarten , Drabbels Marcel , ter Meulen J. J.
Published: July 2002

We report the measurement of seven rovibronic bands belonging to the C3Π(v′=0–6)-X3Π(v″=0) transition in SiC. The molecules are produced by laser vaporization in combination with supersonic cooling and the transitions are detected using pulsed laser induced fluorescence spectroscopy. The vibrational and rotational structure of the C3Π state are determined. Radiative lifetimes vary from 2886 ns for v′=0 to 499 ns for v′=6. Dispersed fluorescence spectra from the excited vibrational levels to the ground state have been measured and the vibrational constants of the electronic ground state were obtained. Both the v′ dependence of the radiative lifetime and observed anomalous intensity distributions in the fluorescence spectra are explained by an electronic transition moment function that varies with the internuclear distance. The results are compared with ab initio calculations.
Off-resonant transient birefringence in liquids
Authors: Cho Minhaeng , Du Mei , Scherer Norbert F. , Fleming Graham R. , Mukamel Shaul
Published: July 2002

Off-resonant transient birefringence measurements are analyzed using a reduced equation of motion for the ground state density matrix, which is expanded using an effective Hamiltonian. Assuming that the pump field is weak, we express the polarization relevant for the birefringence signal in terms of a convolution of the tensorial polarizability response function with the external fields. The homodyne-detected birefringence signal is directly compared with the coherent Raman signal. The relationship between off-resonant birefringence and spontaneous Raman experiments is discussed. By expanding the polarizability in powers of the nuclear coordinates and applying the Brownian oscillator model to the coordinate response function, we separate the birefringence signal into intra- and intermolecular coordinate response functions. Off-resonant transient birefringences of acetonitrile, chloroform, dimethylsulfoxide, and a series of alcohols were measured. The data are transformed to the frequency domain by using a model independent analysis method. The spectra are discussed in the context of various models for the distribution of intermolecular modes (spectral density) in liquids.
How far is far from critical point in polymer blends? Lattice cluster theory computations for structured monomer, compressible systems
Authors: Dudowicz Jacek , Lifschitz Masha , Freed Karl F. , Douglas Jack F.
Published: July 2002

Although the lattice cluster theory (LCT) incorporates many features which are essential in describing real polymer blends, such as compressibility, monomer structures, local correlations, chain connectivity, and polymer–polymer interactions, it still remains a mean field theory and is therefore not applicable in the vicinity of the critical point where critical fluctuations become large. The LCT, however, permits formulating the Ginzburg criterion, which roughly specifies the temperature range in which mean field applies. The present treatment abandons the conventional assumptions of incompressibility and of composition and the molecular weight independent effective interaction parameter χeff upon which all prior analyses of the Ginzburg criterion are based. Blend compressibility, monomer structure, and local correlations are found to exert profound influences on the blend phase diagram and other critical properties and, thus, exhibit a significant impact on the estimate of the size of the nonclassical region. The LCT is also used to test various methods which employ available experimental data in computations of the Ginzburg number Gi. The reduced temperature τ=‖T−Tc‖/T defining the range of the validity of mean field theory (τ≳τMF) and the onset of the Ising-type scaling regime (τ≳τcrit) are quite different, and renormalization group estimates of τMF and τcrit are presented as a function of Gi to more precisely specify these scaling regimes.
A quantitative approximation for the quantum dynamics of hydrogen transfer: Transition state dynamics and decay in ClHCl−
Authors: McCoy Anne B. , Gerber R. Benny , Ratner Mark A.
Published: July 2002

A general method for studying transition state spectroscopy and dynamics in hydrogen atom transfer reactions is presented. This approach is based on the time-dependent self-consistent field (TDSCF) approximation and is applied to a study of the ClHCl− photodetachment experiments of Metz et al. [Metz et al., J. Chem. Phys. 88, 1463 (1988)]. Comparison of results of exact time-dependent and TDSCF calculations are made for collinear and three-dimensional (J=0) approximations for the quantum dynamics. When ClHCl is constrained to be collinear, the TDSCF calculation overcorrelates the motions in the H atom displacement and ClCl extension coordinates. This results in relatively poor agreement with the exact result for many properties of the wave function. In contrast, when the system is propagated in the three vibrational coordinates of the system, the transition state dynamics are effectively over much more rapidly. Consequently, the TDSCF approximation yields results of very good quantitative accuracy over the time required for most of the wave function to decay off of the transition state. Comparison is also made between the wave function that results from the exact propagation and from TDSCF when the wave function in the ClCl stretch coordinate is approximated by a Gaussian wave packet. Here the magnitude of the overlap between the two TDSCF wave functions in the H atom coordinates, for quantum and semiclassical propagations of the wave function in the ClCl distance coordinate, is greater than 0.98 over the time of the propagations. These TDSCF calculations are repeated for a wave function that is approximated by a product of a two-dimensional wave function in the hydrogen atom coordinates and a one-dimensional wave function in the ClCl extension coordinate and even better quantitative agreement with the exact propagation is achieved. The success of this method for studying ClHCl gives us confidence that TDSCF will provide a general powerful tool for studies of hydrogen and proton transfer reactions in large systems.
Rovibrational energy levels and equilibrium geometry of HCP
Authors: Puzzarini Cristina , Tarroni Riccardo , Palmieri Paolo , Demaison Jean , Senent Maria Luisa
Published: July 2002

The ground state potential energy surface for HCP has been investigated theoretically. A large fraction of electron correlation is included by multireference internally contracted configuration interaction from CASSCF reference wave functions using large orbital expansions. The origin of the potential is then shifted and the force constants scaled to reproduce all spectroscopic data available for the four isotopically substituted species. Variational calculations of vibrational and rotational frequencies for transitions up to J = 7 ← 6 have finally been performed, with accuracy which is typically ± 5 cm−1 for vibrations and ± 10 MHz for most rotations. By comparison with the results of the perturbation treatment the importance of the ν1:2ν3 Fermi interaction for vibrational frequencies and effective rotational constants has been determined. From computed and experimental ground state rotational constants, the molecular equilibrium geometry has also been estimated.
Nonequilibrium fluctuation theory on pitting dissolution. I. Derivation of dissolution current equations
Authors: Asanuma Miki , Aogaki Ryoichi
Published: July 2002

Following the foregoing papers, in this paper, mathematical procedures to describe the time development of the unstable nonequilibrium fluctuations in pitting dissolution are presented: First, general equations depicting the time development of the amplitudes of the fluctuations are deduced, which are the same as those describing nucleation phenomena. Then, with the same definition as nucleation, the average values of the fluctuations are decided, and measurable pitting current is calculated. Actually, some analytical equations are derived for some characteristic parts of the current–time transient. As a result, after applying a constant potential step to the electrode, a slow relaxation current, which follows a double-layer charging current, is decided as the diffusion current of the fluctuations. This current gives the values of passive film coverage and average critical concentration fluctuation on the completely active surface. Moreover, from the minimum current attained by the diffusion current, the ratio of the average critical concentration fluctuation to the autocorrelation distance is determined. Finally, resultant pitting growth current arising from the unstable growth of the fluctuations is deduced as a function of supporting-electrolyte concentration and overpotential, which is attributed to the fact that the instability occurs from the electrostatic interaction of the solution particles in the double layer.
Electronic structure of molecular van der Waals complexes with benzene: Implications for the contrast in scanning tunneling microscopy of molecular adsorbates on graphite
Authors: Lazzaroni R. , Calderone A. , Brédas J. L. , Rabe J. P.
Published: July 2002

We investigate the electronic structure of molecular model systems in order to improve our understanding of the nature of the contrast, which is observed in the scanning tunneling microscopy (STM) imaging of organic adsorbates on graphite. The model systems consist of a benzene molecule, representing the substrate surface, interacting with various molecules representing alkyl chains, oxygen- and sulfur-containing groups, fluorinated species, and aromatic rings. We perform quantum-chemical calculations to determine the geometric structure, stability, and electronic structure of these molecular complexes and analyze the theoretical results in relation with experimental STM data obtained on monolayers physisorbed on graphite. It appears that the STM contrast can be correlated to the energy difference between the electronic levels of the substrate and those of the adsorbate. Finally, we observe that the introduction of a uniform electric field in the quantum-chemical modeling can enhance the electronic interaction between the partners in the complex.
Gas adsorption effects on structural and electrical properties of activated carbon fibers
Authors: Kobayashi Norikazu , Enoki Toshiaki , Ishii Chiaki , Kaneko Katsumi , Endo Morinobu
Published: July 2002

Activated carbon fibers (ACF) are microporous carbon consisting of a three-dimensional network of micrographites having the size of ∼3 nm. We investigate structural and electronic properties of ACFs in relation to the adsorption of helium, nitrogen, and oxygen gases, by means of adsorption isotherm, ESR (electron spin resonance), magnetic susceptibility, and electrical conductivity measurements. The linewidth of ESR associated with dangling bond spins on micrographites decreases with gas uptake at low pressures below 0.1 kPa regardless of gas species. The similar behavior of oxygen to that of nonmagnetic helium and nitrogen demonstrates that adsorbed oxygen molecules having chemisorption feature are stabilized in the singlet ground state in the low-pressure range. Taking into account that the linewidth is governed by dipole–dipole interaction between dangling bond spins, the reduction in the ESR linewidth proves the swelling of micropores induced by gas uptake. The conductivity decreases with gas uptake in the same pressure range regardless of gas species. This behavior is also explained by the modification of microstructure of ACFs. At higher gas pressures 0.1–10 kPa where adsorption is characterized as physisorption for all gaseous species, nitrogen, and helium do not change the ESR linewidth, whereas the linewidth increases with introduced oxygen pressure, due to the dipolar field of paramagnetic oxygen molecules. The mean distance between a dangling bond spin and an oxygen molecule is estimated at ∼0.8 nm.
Raman scattering from a Brownian oscillator with nonohmic Drude dissipation: Applications to continuous wave, impulsive, and noisy excitation
Authors: Ulness Darin J. , Kirkwood Jason C. , Albrecht A. C.
Published: July 2002

An extension of the Brownian oscillator model in nonlinear optical spectroscopy to include frictional memory is treated. Although we have obtained analytic expressions for this model, their unwieldiness makes the understanding of the effect of nonzero frictional memory difficult. However, by focusing on the behavior of the analytically continued oscillator coordinate correlation function in the complex frequency plane, qualitative insight is obtained. Applications to spontaneous Raman scattering and two time-resolved coherent Raman scattering spectroscopies are briefly explored.
VUV photochemistry of CH4 and isotopomers. I. Dynamics and dissociation pathway of the H/D-atom elimination channel
Authors: Wang Jen-Han , Liu Kopin
Published: July 2002

The Doppler-selected time-of-flight technique was used to study the formation of H and D in the photolysis of CH4 and its isotopomers. The combination of measurements for the photofragment kinetic energy release P(ET) and the anisotropy parameter β(ET) distributions allows us to differentiate, for the first time, three distinct pathways which are involved in C–H (C–D) bond fission. In conjunction with a recent ab initio theoretical investigation, the mechanisms for this complicated multichannel dissociation process are proposed. In particular, two distinct dissociation pathways are elucidated for the two-fragments channel CH3(X̃ 2A″)+H. One pathway invokes a perpendicular-type transition in absorption, which subsequently undergoes intersystem crossing to the triplet surface and then dissociates. The fragmentation via this route yields fast CH3+H with a negative β parameter. Alternatively, a parallel-type excitation is involved, followed by internal conversion to the ground-state surface on which dissociation occurs. This pathway results in less kinetic energy release and yields a positive β parameter. An intriguing isotope effect is revealed, which calls for further theoretical investigations.
Static and frequency-dependent polarizabilities of excited singlet states using coupled cluster response theory
Authors: Hättig Christof , Christiansen Ove , Coriani Sonia , Jo/rgensen Poul
Published: July 2002

We report an implementation of the linear response function for singlet excited states for the coupled cluster models CCS, CC2 and CCSD. The implementation is based on the derivation of excited state response functions as derivatives of excited state quasienergy Lagrangians. Secular divergencies are explicitly eliminated and response equations and response functions therefore are numerical stable in the static limit. Calculations are performed for the polarizabilities of pyrimidine and s-tetrazine in their lowest singlet excited states. We find that the results for the excited state polarizabilities are sensitive to the accuracy of the excitation energies and that a qualitative correct description of the dispersion is first obtained at a correlated level.
The double-funnel energy landscape of the 38-atom Lennard-Jones cluster
Authors: Doye Jonathan P. K. , Miller Mark A. , Wales David J.
Published: July 2002

The 38-atom Lennard-Jones cluster has a paradigmatic double-funnel energy landscape. One funnel ends in the global minimum, a face-centered-cubic (fcc) truncated octahedron. At the bottom of the other funnel is the second lowest energy minimum which is an incomplete Mackay icosahedron. We characterize the energy landscape in two ways. First, from a large sample of minima and transition states we construct a disconnectivity graph showing which minima are connected below certain energy thresholds. Second, we compute the free energy as a function of a bond-order parameter. The free energy profile has two minima, one which corresponds to the fcc funnel and the other which at low temperature corresponds to the icosahedral funnel and at higher temperatures to the liquidlike state. These two approaches show that the greater width of the icosahedral funnel, and the greater structural similarity between the icosahedral structures and those associated with the liquidlike state, are the cause of the smaller free energy barrier for entering the icosahedral funnel from the liquidlike state and therefore of the cluster’s preferential entry into this funnel on relaxation down the energy landscape. Furthermore, the large free energy barrier between the fcc and icosahedral funnels, which is energetic in origin, causes the cluster to be trapped in one of the funnels at low temperature. These results explain in detail the link between the double-funnel energy landscape and the difficulty of global optimization for this cluster.
Fifth-order nonlinear Raman processes in molecular liquids using quasi-cw noisy light. I. Theory
Authors: Kirkwood Jason C. , Albrecht A. C. , Ulness Darin J.
Published: July 2002

Fifth-order nonlinear Raman processes using broadband, incoherent light are treated for a multiply resonant, multicomponent mixture. In particular, the theoretical development of the direct and the sequential fifth-order analogs of coherent Raman scattering is presented. Of the complete formalism, only the dominant doubly Raman resonant hyperpolarizability contributions to the signal intensity are discussed in this article. Furthermore, application is made to simulate fifth-order signals from a variety of hypothetical molecular liquids. It is seen how the direct and the sequential processes can distinguish themselves in a mixture, in a neat liquid with more than one Raman coherence, and also whenever the Raman active modes are taken to be anharmonic. This theoretical treatment anticipates experimental results presented in the following paper.
Benchmark calculations of chemical reactions in density functional theory: Comparison of the accurate Kohn–Sham solution with generalized gradient approximations for the H2+H and H2+H2 reactions
Authors: Schipper P. R. T. , Gritsenko O. V. , Baerends E. J.
Published: July 2002

The Kohn–Sham (KS) solution is constructed from an accurate CI density and the KS exchange and correlation energies Ex and Ec, as well as the corresponding exchange and exchange-correlation energy densities εx(r) and εxc(r), which are obtained for the hydrogen abstraction reaction H+H2 and the symmetrical four-center exchange reaction H2+H2. The KS quantities are compared with those of the standard GGAs. Comparison shows that the GGA exchange functional represents both exchange and molecular nondynamical left–right correlation, while the GGA correlation functional represents only the dynamical part of the correlation. This role of the GGA exchange functional is especially important for the transition states (TS) of the reactions where the left–right correlation is enhanced. Standard GGAs tend to underestimate the barrier height for the reaction H+H2 and to overestimate it for the reaction H2+H2. For H2+H2 the Kohn–Sham orbital degeneracy in the square TS is represented with an equi-ensemble KS solution for both accurate KS/CI and GGA, while near the TS ensemble solutions with unequal occupations of the degenerate highest occupied orbitals are obtained. For the GGA ensemble solution a special ensemble formula for the GGA exchange functional is proposed. Application of this formula to the H2+H2 reaction reduces appreciably the reaction barriers calculated with GGAs and leads to much better agreement with the accurate value. The too low GGA barriers for the H+H2 reaction are attributed to overestimation of the dynamical correlation in the TS by the GGA correlation functionals. In order to correct this error, it is recommended to modify the dependence of the approximate correlation functionals on the local polarization ζ with the purpose of reducing the approximate correlation energy for intermediate ζ values, which are expected to characterize the TS’s of radical abstraction reactions.