bluuebunny/crossref_metadata_2025 huggingface dataset:
Anomalous splittings of torsional sublevels induced by the aldehyde inversion motion in the S1 state of acetaldehyde
Authors: Chou Yung-Ching , Chen I-Chia , Hougen Jon T.
Published: January 2004

The G6 group-theoretical high-barrier formalism developed previously for internally rotating and inverting CH3NHD is used to interpret the abnormal torsional splittings in the S1 state of acetaldehyde for levels 140−150, 140−151, and 140−152, where 140− denotes the upper inversion tunneling component of the aldehyde hydrogen and 15 denotes the methyl torsional vibration. This formalism, derived using an extended permutation–inversion group G6m, treats simultaneously methyl torsional tunneling, aldehyde–hydrogen inversion tunneling and overall rotation. Fits to the rotational states of the four pairs of inversion–torsion vibrational levels (140+150A,E, 140−150A,E), (140+151A,E, 140−151A,E), (140+152A,E, 140−152A,E), and (140+153A,E, 140−153A,E) are performed, giving root-mean-square deviations of 0.003, 0.004, 0.004, and 0.004 cm−1, respectively, which are nearly equal to the experimental uncertainty of 0.003 cm−1. For torsional levels lying near the top of the torsional barrier, this theoretical model, after including higher-order terms, provides satisfactory fits to the experimental data. The partially anomalous K-doublet structure of the S1 state, which deviates from that in a simple torsion–rotation molecule, is fitted using this formalism and is shown to arise from coupling of torsion and rotation motion with the aldehyde–hydrogen inversion.
Transport of a relativistic electron beam in gas and plasma-filled focusing cells for x-ray radiography
Authors: Welch D. R. , Rose D. V. , Oliver B. V. , Schamiloglu E. , Hahn K. , Maenchen J. E.
Published: January 2004

Gas cells have been used in radiographic sources to assist in the focusing of intense electron beams, produced using pulsed-power accelerators, onto a high atomic number target to generate bremsstrahlung radiation. The quality of the resulting source increases linearly with the dose and inversely with the square of the spot size. The electron beams of order 30 kA current and up to 10 MeV energy drive a rapid breakdown of the gas increasing the gas conductivity through direct ionization by the beam and electron avalanche. Nonideal effects associated with the breakdown result in an axial sweep of the beam focus position that smears the radiographic spot. Higher plasma conductivity in the cell could lead to an improved radiation source. Hybrid particle-in-cell simulations show improvement of the beam spot via pre-ionization of the gas cell prior to arrival of the electron beam pulse. In this paper, the propagation of a 30 kA, 3.5 MeV electron beam, focused into gas and plasma-filled cells is modeled. The simulations compare the effectiveness of beam focusing using neutral gas, partially ionized gas, and fully ionized (plasma-filled) cells. The results show that an initial plasma density approaching 1016 cm−3 is sufficient to prevent significant net currents and the subsequent beam sweep that is observed in an optimized gas cell. The net current is calculated to increase as the plasma density falls to that of the beam due mainly to plasma current resistive decay and electron inertial effects at the target.
Change in the chemical state and thermal stability of HfO2 by the incorporation of Al2O3
Authors: Cho M.-H. , Chang H. S. , Cho Y. J. , Moon D. W. , Min K.-H. , Sinclair R. , Kang S. K. , Ko D.-H. , Lee J. H. , Gu J. H. , Lee N. I.
Published: January 2004

Al 2 O 3 incorporated HfO2 films grown by atomic layer deposition were investigated using various measurement tools. The accumulation capacitance of the Al2O3 incorporated into HfO2 film increases as the postannealing temperature increases because of changes in interfacial and upper layer thickness and in interfacial stoichiometry. The core-level energy state of a 15 Å thick film shows a shift to higher binding energy, as the result of silicate formation and Al2O3 incorporation. The incorporation of Al2O3 into the HfO2 film has no effect on silicate formation at the interface between the film and Si, while the ionic bonding characteristics and hybridization effects are enhanced compared to a pure HfO2 film. Any dissociated Al2O3 on the film surface is completely removed by a vacuum annealing treatment over 850 °C, while HfO2 contributes to Hf silicide formation on the surface of the film.
Calibration of magnetic force microscopy tips by using nanoscale current-carrying parallel wires
Authors: Kebe Th. , Carl A.
Published: January 2004

Experimental results on the characterization of commercially available magnetic force microscopy (MFM) thin film tips as a function of an external magnetic field are presented. Magnetic stray fields with a definitive z-component (perpendicular to the substrate) and a magnetic field strength of up to Hz=±45 Oe are produced with current carrying parallel nanowires with a thickness of t=60 nm, which are fabricated by electron-beam lithography. The magnetic fields are generated by electrical dc-currents of up to ±6 mA which are directed antiparallel through the nanowires. The geometry and the dimensions of the nanowires are systematically varied by choosing different wire widths w as well as separations b between the parallel wires for two different sets of samples. On the one hand, the wire width w is varied within 380 nm<w<2460 nm while the separation b≈450 nm between the wires is kept constant. On the other hand the separation b between the parallel wires is varied within 120 nm<b<5100 nm, while the wire width w=960 nm is kept constant. For all the geometrical configurations of parallel wires the resulting magnetic contrast is imaged by MFM at various tip lift-heights. By treating the MFM tip as a point probe, the analysis of the image contrast as a function of both the magnetic field strength and the tip lift height allows one to quantitatively determine the effective magnetic dipole and monopole moments of the tip as well as their imaginary locations within the real physical tip. Our systematic study quantitatively relates the above point-probe parameters to (i) the dimensions of the parallel wires and (ii) to the characteristic decay length of the z-component of the magnetic field of parallel wires. From this the effective tip-volume of the real thin film tip is determined which is relevant in MFM-imaging. Our results confirm the reliability of earlier tip calibration schemes for which nanofabricated current carrying rings were used instead of parallel wires, thereby proving that the tip calibration equations depend on the underlying stray field geometry. Finally, we propose an experimental approach which allows one to measure the magnetization of nanoscale ferromagnetic elements with an in-plane orientation of the magnetization, quantitatively, by using a calibrated MFM-tip.
Interface structure and adhesion of wafer-bonded GaN/GaN and GaN/AlGaN semiconductors
Authors: Shi Frank F. , Chang Kuo-lih , Hsieh K. C. , Guido Louis , Hoke Bill
Published: January 2004

Material integrations of GaN/GaN and Al0.25Ga0.75N/GaN semiconductors through wafer bonding technology were reported in this work. The wafer surface and interface microstructures were characterized by scanning electron microscopy and energy dispersive x-ray spectroscopy. The interface adhesion (bonding strength) was estimated based upon the interface fracture energy γo measured by double-cantilever beam technique. The interface adhesion properties of several different wafer-bonded III-V semiconductors were also compared. By comparing the atomic chemical bond energy Eo with the measured interface fracture energy γo, the bondability of a few major III-V semiconductors was analyzed.
Bulk and metastable defects in CuIn1−xGaxSe2 thin films using drive-level capacitance profiling
Authors: Heath Jennifer T. , Cohen J. David , Shafarman William N.
Published: January 2004

The drive-level capacitance profiling technique has been applied to ZnO/CdS/CuIn1−xGaxSe2/Mo solar cell devices, in order to study properties of defects in the CuIn1−xGaxSe2 film. Properties studied include the spatial uniformity, bulk defect response, carrier density, and light-induced metastable effects. These results indicate that previous estimates of carrier densities, from C–V profiling, may be significantly overestimated. In addition, a defect response previously thought to be located at the interface is observed to exist throughout the bulk material. Finally, an infrared light-soaking treatment is demonstrated to induce metastable changes in the bulk CuIn1−xGaxSe2 film. Hence, the drive-level capacitance profiling technique provides valuable insights into these films. Herein, the technique itself is fully explained, compared to other junction capacitance methods, and its utility is demonstrated using numerical simulation.
Experimental and theoretical evidence for the existence of photonic bandgaps and selective transmissions in serial loop structures
Authors: El Boudouti E. H. , Fettouhi N. , Akjouj A. , Djafari-Rouhani B. , Mir A. , Vasseur J. O. , Dobrzynski L. , Zemmouri J.
Published: January 2004

We have investigated the electromagnetic band structure, transmission, and phase time through a one-dimensional structure made of loops pasted together with segments of finite length. In this serial loop structure, the loops and segments are constituted of dielectric monomode materials. Analytic expressions are reported for the band structure for a large number N of loops and for transmission coefficients and phase times for any value of N. Experimental and numerical results show the existence of large gaps in these structures. These gaps originate both from the periodicity of the system and the loop resonant states that create zeroes of transmission. The gap widths depend on the lengths of the finite segment and the loop diameters. Defect modes may occur in these bandgaps by introducing defective segments in the structure. The localized states appear as very narrow peaks both in the transmission spectrum and in the transmission phase time of finite serial loop structures. The localized state behavior is analyzed as a function of the length and of the position of the defect segment. The transmission phase measurements enable us to derive the group velocity as well as the density of states in these structures. The experimental results are obtained using coaxial cables in the frequency range of few hundreds of MHz.
1.48 and 1.84 μm thulium emissions in monoclinic KGd(WO4)2 single crystals
Authors: Güell F. , Gavaldà Jna. , Solé R. , Aguiló M. , Dı́az F. , Galan M. , Massons J.
Published: January 2004

By exciting at 788 nm, we have characterized the near infrared emissions of trivalent thulium ions in monoclinic KGd(WO4)2 single crystals at 1.48 and 1.84 μm as a function of dopant concentration from 0.1% to 10% and temperature from 10 K to room temperature. We used the reciprocity method to calculate the maximum emission cross-section of 3.0×10−20 cm2 at 1.838 μm for the polarization parallel to the Nm principal optical direction. These results agrees well with the experimental data. Experimental decay times of the H43→3F4 and F43→3H6 transitions have been measured as a function of thulium concentration.
Kinetic theory of dusty plasmas. V. The hydrodynamic equations
Authors: Tsytovich V. N. , de Angelis U.
Published: January 2004

The collisional hydrodynamic equations for electrons, ions, and dust particles in dusty plasmas are derived from the kinetic equations [V. N. Tsytovich and U. de Angelis, Phys. Plasmas 6, 1093 (1999)] which consistently take into account the elastic and inelastic (charging) collisions of plasma particles with dust and the effects of dust charge fluctuations. These equations are valid in the parameter regime where the collisions of plasma particles with dust dominate with respect to the binary plasma collisions. New expressions for the fluid collision frequencies, transport coefficients, viscosity, and ion drag are found and compared with previous results.
Giant magnetoimpedance of amorphous ribbon/Cu/amorphous ribbon trilayer microstructures
Authors: Amalou F. , Gijs M. A. M.
Published: January 2004

The giant magnetoimpedance (GMI) and giant magnetoresistance (GMR) of amorphous ribbon/Cu/amorphous ribbon trilayer microstructures, based on Metglas™ 2714a ribbons and Cu foils, is measured and analyzed. We obtain GMI and GMR ratios of 830% and 2630%, respectively, in the 0.2–20 MHz frequency range. These very large GMI and GMR values are a direct consequence of the large effective relative permeability due to the closed magnetic flux path in the trilayer structure. We study the effect of magnetocrystalline and shape anisotropy, and analyze our experimental results in terms of the model of Makhnovskiy et al. [Sens. Actuators 81, 91 (2000)].
Coarse grained models for flexible liquid crystals: Parameterization of the bond fluctuation model
Authors: Bates Martin A.
Published: January 2004

We extend the bond fluctuation model, originally devised to investigate polymer systems, to contain anisotropic interactions suitable for the simulation of large flexible molecules such as liquid crystalline polymers and dendrimers. This extended model coarse grains the interaction between the flexible chains at a similar level of detail to the mesogenic units. Suitable interaction parameters are obtained by performing trial simulations on a low molar mass liquid crystalline system. The phase diagram of this system is determined as a function of the molecular stiffness. The nematic to isotropic transition temperature is found to increase with increasing stiffness.
Geometric structure and electronic properties of neutral and anionic Fe2C3 and Fe2C4 clusters, as obtained by density-functional calculations
Authors: Noya E. G. , Longo R. C. , Gallego L. J.
Published: January 2004

We calculated the geometrical structures and electronic properties of neutral and anionic Fe2Cn clusters (n=3,4) using a density-functional method that employs linear combinations of atomic orbitals as basis sets, standard nonlocal norm-conserving pseudopotentials, and the generalized gradient approximation to exchange and correlation. We show that the ground-state structures of Fe2C3 and Fe2C4 are essentially the same in the neutral and anionic states, namely, planar rings that feature nonadjacent Fe atoms. For the anionic clusters, these findings contrast with previously published results.
Current-dependent extension of the Perdew–Burke–Ernzerhof exchange-correlation functional
Authors: Maximoff Sergey N. , Ernzerhof Matthias , Scuseria Gustavo E.
Published: January 2004

The probability current density is used in addition to the electron density and its gradient as a variable in the construction of an exchange-correlation functional. Starting from the Perdew–Burke–Ernzerhof generalized gradient approximation, we employ exact conditions to build a nonempirical exchange functional. Matching the correlation functional to that for exchange yields a current-dependent approximation for correlation. The resulting functional is given in a simple closed form. Application of this approximation to open shell atoms eliminates the artificial level splitting of formally degenerate states observed with generalized gradient approximations.
On the ordering of the first two excited electronic states in all-trans linear polyenes
Authors: Catalán J. , de Paz J. L. G.
Published: January 2004

Reported experimental evidence of the relative position of the first two excited electronic states in linear polyenes was carefully examined and compared with that derived from time dependent density functional theory (TDDFT) theoretical calculations performed at the B3LYP level on optimized geometries. The energy values for the first two triplet states Bu3 and Ag3, obtained from TDDFT calculations, were found to be highly strongly correlated with the experimental values. Also, the theoretical calculations for the electronic transition 1 1Ag→1 1Bu were also extremely well correlated with their experimental counterparts; even more important, the three reported experimental data for 1 1Ag→2 1Ag transitions in these systems conformed to the correlation for the TDDFT 1 1Ag→1 1Bu transition. The first excited electronic state in the linear polyenes studied (from ethene to the compound consisting of 40 ethene units, P40) was found to be Bu1. The energy gap between the excited states 2 1Ag and 1 1Bu decreased with increasing length of the polyene chain, but not to the extent required to cause inversion, at least up to P40. In the all-trans linear polyenes studied, the widely analyzed energy gap from the ground electronic state to the first excited singlet state for infinitely long chains may be meaningless as, even in P40, it is uncertain whether the ground electronic state continues to be a singlet.
Energy transfer in Li(4p)+(Ar,H2,CH4) collisions
Authors: Hattaway Brian C. , Bililign Solomon , Uhl Lionel , Ledentu Vincent , Jeung Gwang-Hi
Published: January 2004

The direct collisional energy transfer processes of the excited states of Li(4p) by several gases are investigated under gas cell conditions. The nonreactive absorption profiles of the collision complex are monitored as a function of laser detuning from the Li(2s–4p) resonances. Pronounced structures in the absorption spectra along with high level ab initio calculations of the relevant potential energy surfaces are used to understand the experimental results.
Reaction dynamics of Cl+CH3SH: Rotational and vibrational distributions of HCl probed with time-resolved Fourier-transform spectroscopy
Authors: Cheng Shin-Shin , Wu Yu-Jong , Lee Yuan-Pern
Published: January 2004

Rotationally resolved infrared emission spectra of HCl(v=1–3) in the reaction of Cl+CH3SH, initiated with radiation from a laser at 308 nm, are detected with a step-scan Fourier-transform spectrometer. Observed rotational temperature of HCl(v=1–3) decreases with duration of reaction due to collisional quenching; a short extrapolation to time zero based on data in the range 0.25–4.25 μs yields a nascent rotational temperature of 1150±80 K. The rotational energy averaged for HCl(v=1–3) is 8.2±0.9 kJ mol−1, yielding a fraction of available energy going into rotation of HCl, 〈fr〉=0.10±0.01, nearly identical to that of the reaction Cl+H2S. Observed temporal profiles of the vibrational population of HCl(v=1–3) are fitted with a kinetic model of formation and quenching of HCl(v=1–3) to yield a branching ratio (68±5):(25±4):(7±1) for formation of HCl(v=1):(v=2):(v=3) from the title reaction and its thermal rate coefficient k2a=(2.9±0.7)×10−10 cm3 molecule−1 s−1. Considering possible estimates of the vibrational population of HCl(v=0) based on various surprisal analyses, we report an average vibrational energy 36±6 kJ mol−1 for HCl. The fraction of available energy going into vibration of HCl is 〈fv〉=0.45±0.08, significantly greater than a value 〈fv〉=0.33±0.06 determined previously for Cl+H2S. Reaction dynamics of Cl+H2S and Cl+CH3SH are compared; the adduct CH3S(Cl)H is likely more transitory than the adduct H2SCl.
Resonances in three-dimensional H+HLi scattering: A time-dependent wave packet dynamical study
Authors: Padmanaban R. , Mahapatra S.
Published: January 2004

This paper examines the resonances in H+HLi scattering. The signature of these resonances is obtained from the oscillations in its reaction probability versus energy curves. They are identified here from a set of pseudospectra calculated for different initial locations of a stationary Gaussian wave packet on the ab initio potential energy surface (PES) reported by Dunne, Murrel, and Jemmer [Chem. Phys. Lett. 336, 1 (2001)]. The nuclear motion on this PES is monitored with the aid of a time-dependent wave packet method and the pseudospectrum are calculated by Fourier transforming the time autocorrelation function of the initial wave packet. The resonances are further examined and assigned by computing their eigenfunctions through spectral quantization algorithm. Both the linewidth as well as decay lifetimes of the resonances are reported.
Conformation of p-terphenyl under hydrostatic pressure
Authors: Zhuravlev K. K. , McCluskey M. D.
Published: January 2004

The conformation of p-terphenyl (C18H14) and deuterated p-terphenyl (C18D14) has been investigated, using high-pressure infrared spectroscopy at liquid-helium temperatures. First-principles calculations, together with the experimental results, were performed to determine the structure of p-terphenyl in the twisted conformation. At low temperatures and pressures, p-terphenyl belongs to the C2 point group of symmetry. In this configuration, the central ring is twisted with respect to the plane of the outer rings. The symmetry of the molecule is nearly C2h, consistent with previous x-ray diffraction measurements.
Vacuum ultraviolet-infrared photo-induced Rydberg ionization spectroscopy: C–H stretching frequencies for trans-2-butene and trichloroethene cations
Authors: Woo H. K. , Wang P. , Lau K.-C. , Xing X. , Ng C. Y.
Published: January 2004

We have demonstrated the two-color vacuum ultraviolet (VUV)-infrared (IR) photoinduced Rydberg ionization (PIRI) experiment. Trichloroethene (ClCH=CCl2) and trans-2-butene (trans-CH3CH=CHCH3) were prepared in Rydberg states in the range of effective principal quantum number n*≈7–93 by VUV excitation prior to IR-induced autoionization. The observed VUV-IR-PIRI spectra are found to be independent of n*, suggesting that the electron Rydberg orbital is conserved, i.e., the Rydberg electron is behaving as a spectator during the excitation process. The observed IR active C–H stretching vibrational frequencies ν12+=3072±5 cm−1 for ClCH=CCl2+ and ν23+=2908±3 cm−1, ν25+=2990±10 cm−1 and ν30+=3022±10 cm−1 for trans-CH3CH=CHCH3+ are compared with predictions based on ab initio quantum-chemical procedures and density functional calculations.
Double ionization of fluorinated benzenes: Hole localization and delocalization effects
Authors: Villani Cristian , Tarantelli Francesco
Published: January 2004

The dense double ionization spectra of all the twelve fluoro-substituted benzene molecules are investigated in great detail by Green’s function ADC(2) calculations and a two-hole density mapping. Double ionization is shown to provide an extremely sensitive tool of electronic structure analysis. The calculations evidence and measure quantitatively how the charge distribution is dictated by the complex interplay between the resilience of the aromatic ring electronic structure and the disruptive effect of the electronegative halogen substituents. Successive substitutions are found not to have any synergic effect, but affect the spectra in very identifiable ways. The Auger spectra of the fluorobenzenes are interpreted in the light of the charge distribution results, using the foreign-imaging model. The double charge transfer spectra are also analyzed and discussed.
Orientational order in binary mixtures of hard Gaussian overlap molecules
Authors: Zhou Xin , Chen Hu , Iwamoto Mitsumasa
Published: January 2004

Based on a standard constant-pressure Monte Carlo molecular simulation, we have studied liquid crystal phases of binary mixtures of nonspherical molecules. The components of the mixtures are two types of hard Gaussian overlap (HGO) molecules. The first type of molecule has a small molecularelongation parameter (short HGO molecules) and cannot form stable liquid crystal phase in the bulk by themselves. The second type of molecule has a large elongation parameter (long HGO molecules) and can form a liquid crystal phase easily. In the mixtures, the short HGO molecules can form an orientationally ordered phase because the long HGO molecules form confining surfaces to induce the alignment of the short molecules. We also study the isotropic–nematic phase transition in different mixtures composed of short and long HGO molecules with different elongations and concentrations. The obtained result implies that small anisotropic molecules can show liquid crystal behavior.
The van der Waals interaction between protein molecules in an electrolyte solution
Authors: Song Xueyu , Zhao Xuefeng
Published: January 2004

In this report we present a general formulation to calculate the van der Waals interaction between two protein molecules in an electrolyte solution using boundary element method of solving linearized Poisson–Boltzmann equation. Our formulation is based upon an inhomogeneous dielectric model of proteins at the residue level. Our results for bovine pancreatic trypsin inhibitor at various relative orientations indicate that the anisotropy of the interaction can be tens of kBT.
Instabilities of ablation fronts in inertial confinement fusion: A comparison with flames
Authors: Clavin P. , Masse L.
Published: January 2004

A comparison with flames sheds new light on the dynamics of ablation fronts in inertial confinement fusion (ICF). The mathematical formulation of the problem in ICF is the same as for flames propagating upwards. The difference concerns the Froude number Fr, yielding a different order of magnitude for the nondimensional wave number of the marginally stable disturbances. When the thermal conductivity varies strongly, as is the case in ICF, a wide range of characteristic (diffusive) lengths is involved across the wave structure. For disturbances with intermediate wavelengths, a “universal” diffusive relaxation rate of thermal waves is exhibited with no dependence on the heat conductivity. This is a key point for describing the dynamics of strongly accelerated ablation fronts whose marginally stable wavelength is much shorter than the total wave thickness. The coupling of hydrodynamics and heat conduction is analyzed in a way similar to flame theory, through the derivation of a kinematic relation for the ablation front including its thermal relaxation. A transition between the regimes of flames and ablation fronts in ICF is exhibited with decreasing Fr. For a moderate acceleration, Fr≫1, the result for flames is recovered. For a large acceleration, Fr of order unity, the thermal relaxation, when coupled with hydrodynamics, is shown to damp out the Darrieus–Landau instability, yielding the known result in ICF for strongly accelerated ablation fronts. For a wide class of models, including the simple two-length-scale model, the description is shown to be independent of the model. A weakly nonlinear analysis, valid irrespective of the number of unstable modes, is carried out for describing the early development of nonlinear structures of the ablation front in ICF. The role of the Darrieus–Landau instability at the early stage of irradiation is pointed out.
Theoretical study of the second-order nonlinear optical properties of [N]helicenes and [N]phenylenes
Authors: Botek Edith , Champagne Benoı̂t , Turki Mohamed , André Jean-Marie
Published: January 2004

The second-order nonlinear optical properties of helicenes and phenylenes have been theoretically investigated at the time-dependent Hartree–Fock level using the Austin model 1 semiempirical Hamiltonian. Both the antisymmetric isotropic component of the first hyperpolarizability (β̄) and its projection on the dipole moment (β∥) have been determined for increasingly large helical systems as well as for their analogs substituted by donor/acceptor pairs. It is found that (i) in nonsubstituted helicenes and phenylenes, β̄ increases monotonically with the size of the system and slightly depends on the nature of the helix; (ii) the corresponding β∥ is mostly determined by the radial component of the first hyperpolarizability vector; (iii) in helicenes, β∥ is positive and presents quasiperiodic oscillations with the helix; (iv) in phenylenes, β∥ depends upon the size of the helix and it can be either positive or negative as a result of the differences in evolution with N of the radial components of the dipole moment and first hyperpolarizability. Substituting the helicenes and phenylenes by the prototypical NH2/NO2 donor/acceptor pair provides a diversity of effects on β̄ and β∥ that encompasses decrease, increase, and change in sign.
Precipitate pattern formation in fluctuating media
Authors: Izsák Ferenc , Lagzi István
Published: January 2004

Simulation of the Liesegang pattern formation in low concentration gradient is presented using concentration perturbation in a deterministic model. The precipitation process is based on ion-product supersaturation theory (Ostwald’s model). In the classical experiments with high initial concentration gradients, appearance time and locations of the band formation are well reproducible. Decreasing initial concentration gradients results in a more stochastic pattern structure; this means that the reproducibility of the experiments becomes worse. The presented model and the results of the simulations exhibit the same trend, which were demonstrated and investigated experimentally by Kai et al. [S. Kai, S. C. Müller, and J. Ross, J. Phys. Chem. 87, 806 (1983)] and Kai and Müller [S. Kai and S. C. Müller, Sci. Form 1, 9 (1985)].
A self-consistent reaction field model of solvation using distributed multipoles. I. Energy and energy derivatives
Authors: Rinaldi Daniel , Bouchy Alain , Rivail Jean-Louis , Dillet Valerie
Published: January 2004

The self-consistent reaction field model developed previously by the authors in the case of single center multipole expansion of the electronic structure of the solute has been extended to the case of a distributed multipole expansion. Three different expansions have been tested and two of them have proved to be rapidly convergent. The performances of the code are illustrated by the density functional theory treatment of few test systems: guanine, cytosine, and cytosine hydrated with one and three water molecules. A robust fast computer code has been tested to get the electronic structure, the electrostatic contribution to the solute–solvent free energy of interaction, and the optimized molecular geometry in solution.
Structure of ionic liquids of 1-alkyl-3-methylimidazolium cations: A systematic computer simulation study
Authors: Urahata Sérgio M. , Ribeiro Mauro C. C.
Published: January 2004

Molecular dynamics simulations of room temperature molten salts (ionic liquids) containing imidazolium cations have been performed. Ten different systems were simulated at 323 K by using united atom force fields, in which the anion size (F−, Cl−, Br−, and PF6−) and the length of the alkyl chain of 1-alkyl-3-methylimidazolium cations (1-methyl-, 1-ethyl-, 1-butyl-, and 1-octyl-) were systematically varied. It is shown that the resulting equilibrium structures account for the observed features of experimental static structure factors when available. A detailed analysis of the simultaneous effect of changing the anion and the alkyl chain on the preferential location of nearest-neighbor anions around the cations is provided. It is shown that regions above and below the imidazolium ring are the preferential ones in case of large anions. By increasing the length of the alkyl chain, nearest-neighbor anions are pushed away from the volume occupied by the flexible alkyl chain. Partial structure factors of 1-butyl- and 1-octyl- derivatives display a peak at a wave vector smaller than the main peak, indicating the occurrence of an intermediate range order in these ionic liquids due to the presence of long alkyl chains.
Thermodynamics of soft anisotropic interfaces
Authors: Rey Alejandro D.
Published: January 2004

The Gibbs–Duhem equation for interfaces between nematic liquid crystals and isotropic fluids is formulated and shown to be a generic equation for soft anisotropic surfaces. The one-to-one correspondence between the nematic and crystalline surface Gibbs–Duhem equations is established. Consistency between the surface Gibbs–Duhem equation and the classical equations of interfacial nematostatics is shown. Using a phase space that takes into account thermodynamics, liquid crystalline order, and geometric variables, the generalized nematic surface Gibbs–Duhem equation reveals the presence of couplings between shape, adsorption, temperature, and average molecular orientation. Merging the thermodynamic analysis with nematostatics results in a model for morphactancy, that is, adsorption-induced interfacial shape selection. The specific roles of gradient bulk Frank elasticity, interfacial tension, and anchoring energy are elucidated by analyzing particular paths in the thermodynamic-geometric phase space.
Voronoi diagrams generated by regressing edges of precipitation fronts
Authors: de Lacy Costello Benjamin P. J. , Hantz Péter , Ratcliffe Norman M.
Published: January 2004

Reaction-diffusion systems where one of the reagents (outer electrolyte) penetrates into a gel by diffusion and forms a precipitate with the other reagent (inner electrolyte) homogenized in the gel, are able to produce various complex precipitation patterns. The previously studied NaOH+AgNO3 and recently discovered CuCl2+K3[Fe(CN)6] processes, (where the first reagent is the outer electrolyte and the other is the inner electrolyte homogenized in the gel), when reacted using the above mentioned method, are able to generate tessellations of a plane by a mechanism dependant on the dynamics of so-called regressing edges of the reaction fronts. The spontaneous partitioning of the reacted phases results in the construction of a pattern analogous to a Voronoi diagram or one of their generalizations.
Molecular dynamics simulation of water in a contact with an iron pyrite FeS2 surface
Authors: Philpott Michael R. , Goliney Igor Yu , Lin Ting Ting
Published: January 2004

A strong adsorption of the water molecules to the pyrite surface is shown by a molecular dynamic simulation of the water–iron pyrite FeS2 interface. Water molecules closest to the pyrite surface are bound by an electrostatic interaction to the iron atoms in grooves running parallel to one of the crystal axes. The grooves are about two atoms wide and are directed along 〈010〉 for the (001) surface. The position of the water-surface potential minimum and the energy of adsorption were determined by optimization for a single water molecule at the interface. At room temperature and normal density there are altogether three distinguishable layers of water above the surface. One is associated with the groove: one with H bonding to the sulphur atoms comprising the ridges separating the grooves, and the third with the soft wall boundary between the absorbed water layers and bulk region of water. Simulations were also used to explore the effect of a temperature range significant for geophysical studies.
Photodissociation dynamics of the methyl radical at 212.5 nm: Effect of parent internal excitation
Authors: Wu Guorong , Jiang Bo , Ran Qin , Zhang Jinghui , Harich Steve A. , Yang Xueming
Published: January 2004

Photodissociation dynamics of the CH3 radical at 212.5 nm has been investigated using the H atom Rydberg tagging time-of-flight method with a pure CH3 radical source generated by the photolysis of CH3I at 266 nm. Time-of-flight spectra of the H atom products from the photolysis of both cold and hot methyl radicals have been measured at different photolysis polarizations. Experimental results indicate that the photodissociation of the methyl radical in its ground vibrational state at 212.5 nm excitation occurs on a very fast time scale in comparison with its rotational period, indicating the CH3 dissociation at 212.5 nm occurs on the excited 3s Rydberg state surface. Experimental evidence also shows that the photodissociation of the methyl radical in the ν2=1 state of the umbrella mode at 212.5 nm excitation is characteristically different from that in the ground vibrational state.
Laser induced electron acceleration in a tapered magnetic wiggler
Authors: Singh K. P. , Tripathi V. K.
Published: January 2004

The acceleration of electrons by a laser pulse, in the presence of a magnetic wiggler, in vacuum and plasma is studied. The vector potentials of the laser pulse and magnetic wiggler are taken as AL=−x̂A0 sin(ωt−kz)exp⌊−(t−(z−zL)/vg)2/τL2⌋ and Aw=x̂A0w sin[kwz/(1+αz)], respectively, where α is the tapering parameter. For a specific value of k/kw, the inverse free-electron laser resonance condition is satisfied and energy gained by the electron increases. The resonance condition is sensitive to the electron energy and the electron density of the medium. It can be maintained for longer duration for a suitably tapered wiggler period and the electron can gain much higher energy. The wiggler period increases with initial electron energy and with the decrease in plasma density. Energy gained by the electron decreases with plasma density.
Time-resolved photoluminescence on samarium, nickel, and cesium ion doped scintillator CdWO4 (010) single crystal surfaces
Authors: Tanaka Katsumi , Suzuki Satoshi , Choo Cheow-Keong
Published: January 2004

Photoluminescence (PL), cathode luminescence (CL), and x-ray excited luminescence (XL) are studied on Sm-, Ni-, and Cs-doped CdWO4 (010) single crystal surfaces. Luminescence spectra are deconvoluted to three constituent species at 2.77, 2.48, and 2.18 eV. Their intensities of the ion-doped CdWO4 crystals are compared with the undoped crystals. CL and XL intensities on Sm- and Ni-doped single crystals are suppressed to be about 20–40% of the undoped crystal. However, the luminescence component at 2.18 eV is dominant on the Sm-ion doped CdWO4 crystal. The x-ray diffraction peak at (020) face shifts to lower 2θ value on the Sm-ion doped CdWO4 single crystal, indicating that the crystal structure of CdWO4 is disturbed by Sm ions and the distance between (020) faces is enlarged. The distortion of the crystal structure will be induced by the exchange of W6+ ions into Sm3+ ions with larger ion diameter. Time-resolved PL decay analyses are studied on the doped and undoped crystals under excitation below the band gap. While only the slow decay component is detected with a lifetime of 14 μs on the undoped CdWO4 crystal, the fast and the slow PL decay components are detected with a lifetime of 0.7 and 10 μs on the three emission species in the doped CdWO4 crystals, respectively. The Cs-doped CdWO4 crystal has almost the same intensities for PL, CL, and XL as the undoped crystal.
Time-resolved photoluminescence on ion doped CdWO4 thin films prepared with pulsed laser deposition
Authors: Tanaka Katsumi , Suzuki Satoshi , Choo Cheow-Keong
Published: January 2004

Samarium-, nickel-, cesium-ion-doped and -undoped CdWO4 thin films are prepared by pulsed laser deposition (PLD) method. Photoluminescence (PL), cathode luminescence (CL), and x-ray excited luminescence (XL) are studied on these thin films. Luminescence spectra are deconvoluted to three luminescence species typically at 2.77, 2.48, and 2.18 eV. Luminescence intensities and the relative contribution of three deconvoluted components are compared with those on the undoped CdWO4 (010) single crystal surface. PL intensity on the undoped thin film can be compared to that on the (010) single crystal surface. Not only CL and XL intensities but also PL intensity drops drastically on ion-doped CdWO4 thin films. However, the emission component at about 2.2 eV is dominant and the relative contribution of 2.77 eV is diminished on the ion-doped PLD thin films. The x-ray diffraction data indicates that the (002) plane is most intense on these films and the peak shifts to lower 2θ value on the ion-doped CdWO4 thin films. These results imply that these films are polycrystalline and the interplane distance is enlarged on the ion-doped CdWO4 thin films. The stress of the crystal structure is associated with the origin of the PL component at about 2.2 eV which has been assigned to oxygen vacancies such as [WO5]4− moieties in the crystal. Time-resolved PL decay analyses are studied on the PLD thin films at three wavelengths, 440 nm (2.82 eV), 550 nm (2.25 eV), and 650 nm (1.91 eV). The slow and fast decay component can be detected on the PLD thin films with a decay lifetime of 8–10 and 0.6–0.8 μs, respectively. Comparing to the corresponding CdWO4 (010) single crystal surfaces, the decay time of the slow PL component is decreased on PLD thin films. The PL component with fast decay time will be associated with the strain of CdWO4 crystal structure.
Phenomenological theory of tunnel emitter transit time oscillators for the terahertz range
Authors: Gribnikov Z. S. , Vagidov N. Z. , Haddad G. I.
Published: January 2004

We develop an analytic theory based on an earlier model of the admittance of a ballistic transit time diode terahertz oscillator with tunnel emission of electrons into a transit space. The focus of this work is on the actual case when electrons are injected with high enough energy to move from the start with maximal (saturated) ballistic velocity (∼1×108 to 2×108 cm/s). On the one hand, such diodes have maximal oscillation frequencies and, on the other hand, a simple analytic theory describes them and allows us to avoid a cumbersome numerical procedure, which characterizes the general case. Such a description is analogous to the description of oscillatory diodes with diffusive transport and saturated drift velocity. We have also considered a special case when a small part of the ballistic electrons crossing the transit space scatter into a diffusive subsystem with a small drift velocity. The appearance of such slow-drifting electrons substantially increases space charge in the transit space and influences the static JV-characteristic but the high-frequency admittance is almost invariable.
Stress hysteresis and mechanical properties of plasma-enhanced chemical vapor deposited dielectric films
Authors: Thurn Jeremy , Cook Robert F. , Kamarajugadda Mallika , Bozeman Steven P. , Stearns Laura C.
Published: January 2004

A comprehensive survey is described of the responses of three plasma-enhanced chemical vapor deposited dielectric film systems to thermal cycling and indentation contact. All three films—silicon oxide, silicon nitride, and silicon oxy-nitride—exhibited significant nonequilibrium permanent changes in film stress on thermal cycling or annealing. The linear relationship between stress and temperature changed after the films were annealed at 300 °C, representing a structural alteration in the film reflecting a change in coefficient of thermal expansion or biaxial modulus. A double-substrate method was used to deduce both thermoelastic properties before and after the anneal of selected films and the results were compared with the modulus deconvoluted from small-scale depth-sensing indentation experiments (nanoindentation). Rutherford backscattering spectrometry and hydrogen forward scattering were used to deduce the composition of the films and it was found that all the films contained significant amounts of hydrogen.
Vacancy-type defects in electroplated Cu films probed by using a monoenergetic positron beam
Authors: Uedono A. , Suzuki T. , Nakamura T.
Published: January 2004

Positron annihilation was used to probe vacancy-type defects in electroplated Cu films. Doppler broadening spectra of the annihilation radiation for Cu films deposited on samples with a Ta(20 nm)/SiO2(100 nm)/Si structure were measured with a monoenergetic positron beam. For an as-deposited Cu film, the line-shape parameter S measured 20 days after deposition was larger than that measured 1 day after deposition. The observed increase in the value of S was attributed to grain growth at room temperature and the corresponding increase in the fraction of positrons trapped by vacancy clusters in the grains. In isochronal annealing experiments, the value of S for an electroplated Cu film increased for annealing below 200 °C, suggesting agglomeration of vacancy-type defects (vacancy clusters). A decrease in the S value was observed for annealing above 300 °C, and this was attributed mainly to the decrease in the concentration of vacancy clusters. The annealing stages of the defects in electroplated Cu were found to agree with those for irradiated pure Cu.
Shock wave diffraction over wedges, cylinders, and spheres in gases, liquids, and condensed matter
Authors: Gonor A. L. , Gottlieb J. J. , Hooton I.
Published: January 2004

The problem of the initial diffraction of a planar shock wave moving along the front surface of a wedge, cylinder, and sphere is solved analytically for three cases when the fluid around the body is a gas, liquid, and condensed matter, and when the reflection pattern is regular shock reflection. The conservation equations of mass, momentum, and energy are solved across the incident and reflected shocks at the reflection point moving along the body surface, using the equation of state p=ρRT for gases, Tait’s equation of state for liquids, and D=su+a for condensed matter. The monotonic increase in the reflected shock pressure along the front surface of cylinders and spheres during regular shock reflection in liquids and condensed matter is unexpectedly different from well-known results for gases in which the reflected pressure decreases from the stagnation point to a minimum farther along the body.
Effects of partially occupied sub-bands on two-dimensional electron mobility in AlxGa1−xN/GaN heterostructures
Authors: Asgari A. , Kalafi M. , Faraone L.
Published: January 2004

In this article we present a study of the effect that partial occupancy of sub-bands on the two-dimensional electron mobility at different temperatures, using a fully numerical calculation in unintentionally doped AlxGa1−xN/GaN heterostructures with different Al mole fraction in the AlxGa1−xN barrier. The analysis of our results clearly indicates that the effect of partial sub-band occupancy be considerable, especially at higher temperatures and higher Al mole fractions when more than one sub-band is occupied. Comparison of our calculated results with published experimental data shows good agreement.
Electron emission properties of Spindt-type platinum field emission cathodes
Authors: Gotoh Y. , Nagao M. , Nozaki D. , Utsumi K. , Inoue K. , Nakatani T. , Sakashita T. , Betsui K. , Tsuji H. , Ishikawa J.
Published: January 2004

Electron emission properties of Spindt-type platinum field emission cathodes were investigated. The current–voltage characteristics together with the current fluctuation during long term operation were evaluated in ultrahigh vacuum. The changes of the emission properties in hydrogen, oxygen or carbon monoxide gas ambient were also investigated. Significant improvement of the emission properties was found when the cathode was operated in carbon monoxide ambient under a certain condition. It was found that the effect of the improvement lasted at least 200 h, and resistance against the oxygen exposure was also improved. The changes of the electron emission properties during long term operation in ultrahigh vacuum and in gas ambient were analyzed with the Seppen–Katamuki chart, of which ordinate and abscissa are slope and intercept of Fowler–Nordheim plot. The electron emission properties were distributed along a straight line in the Seppen–Katamuki chart, and those of the cathode improved by carbon monoxide exposure were distributed along another line. The deviation of the electron emission property in ultrahigh vacuum was considered to be due to change in the geometry of the emitting surface. The major reason for the modification of electron emission properties in carbon monoxide ambient was considered to be deposition of carbon onto the emitting surface, resulting in reduction of the work function. From the deviation of the Fowler–Nordheim characteristics in the Seppen–Katamuki chart, we presented an empirical equation for field emission.
Ab initio tight-binding approach to photonic-crystal based coupled cavity waveguides
Authors: Leuenberger David , Ferrini Rolando , Houdré Romuald
Published: January 2004

We develop a model to study coupled cavity waveguides (CCWs) in photonic crystals, composed of linear chains of coupled cavities. By extending previous work on single mode cavities, we have developed a model applicable to any finite number of quasi-degenerate cavity modes. Our model, which uses a multimode tight-binding approach, allows us to study the interaction between the cavity modes as well as the coupling between the CCW bands. In contrast to previous work the coupling parameters are calculated ab initio from the single cavity field profile. The calculated dispersion relations are shown to be in good agreement with those obtained from the plane wave expansion method.