bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetization-induced second-harmonic generation in electrochemically synthesized magnetic films of ternary metal Prussian blue analogs
Authors: Ikeda Katsuyoshi , Ohkoshi Shin-ichi , Hashimoto Kazuhito
Published: February 2003

We observed magnetic field effects on transmitted second-harmonic generation (SHG) in electrochemically synthesized (FexIICr1−xII)1.5[CrIII(CN)6]⋅7.5H2O magnetic films. These films showed a variety of temperature dependences for SH intensities below magnetic phase transition temperatures (TC). The SH intensity for x=0.25 increased monotonically with decreasing temperature and that for x=0.13 exhibited a minimum value around the magnetic compensation temperature under a zero magnetic field. These temperature dependences resembled those of the absolute value of magnetization, indicating that the magnetic strain of the films is responsible for the increase in SH below TC. In addition, the polarization of SH light was rotated by an applied external magnetic field. The observed SH rotation angle of 1.3° was much larger than the Faraday rotation angle of 0.079° at 388 nm. This SH rotation can be understood by the mechanism of magnetization-induced SHG caused by interaction between the electric polarization along the out-of-plane of film and spontaneous magnetization. The magnetic linear term [χijkLmagn(1)] contributed particularly to the SH rotation. The value of the magnetic linear tensor component relative to the crystallographic tensor component [|χyyyXmagn(1)|/|χzyycr], which induced the SH rotation, was 0.023 at 50 K under 10 kOe.
Fluorescence properties of trivalent europium doped in various niobate codoped glasses
Authors: Wang Jiwei , Song Hongwei , Kong Xianggui , Peng Hongshang , Sun Baojuan , Chen Baojiu , Zhang Jiahua , Xu Wu , Xia Haiping
Published: February 2003

A series of niobate–phosphate and niobate–silicate glasses doped with Eu3+ ions were prepared. The emissions, phonon-side band spectra, and fluorescence lifetimes of these glasses were studied and the intensity parameters of Eu3+ were obtained. The temperature dependence of emission intensity of the Eu3+ ion in these niobate glasses was investigated. The temperature-quenching rates were determined. The results indicate that in niobate glasses, as the concentration of Nb2O5 increases, the covalence becomes stronger, the symmetry becomes lower and the electron-phonon coupling strength becomes stronger. Thus, the lifetimes become shorter, and the nonradiative transition processes and the temperature-quenching effect become stronger.
Near-ground detection of atmospheric γ rays associated with lightning
Authors: Greenfield M. B. , Domondon A. T. , Tsuchiya S. , Kubo K. , Ikeda Y. , Tomiyama M.
Published: February 2003

Data taken at 6–20 m above ground from March through June of 2002 was used to confirm that the background subtracted atmospheric γ radiation (GRR) was directly related to precipitation rates. An anomalous increase in GRR (AGR) shortly after lightning strokes several times that expected from only the observed precipitation was concurrently observed at two sites separated by a distance of 220 m. The increase in activity in the detector environment subsequently decayed with a half-life of about 50 min. A model, which assumes the presence of an ensemble of radioisotopes produced by proton and photon-induced nuclear reactions in the atmosphere, fits the observed 50 min (33 min) decay half-life without (with) rain with correlation coefficients of 0.98. The possibilities that energetic protons and to a lesser extent brehmsstrahlung resulting from ions accelerated in electric fields during positive lightning produce this delayed activity and that AGR may be used as a remote monitor of lightning as well as the relative intensity of integrated lightning activity are discussed herein.
Magnetically aligned single wall carbon nanotube films: Preferred orientation and anisotropic transport properties
Authors: Fischer J. E. , Zhou W. , Vavro J. , Llaguno M. C. , Guthy C. , Haggenmueller R. , Casavant M. J. , Walters D. E. , Smalley R. E.
Published: February 2003

Thick films of single wall carbon nanotubes (SWNT) exhibiting in-plane preferred orientation have been produced by filter deposition from suspension in strong magnetic fields. We characterize the field-induced alignment with x-ray fiber diagrams and polarized Raman scattering, using a model which includes a completely unaligned fraction. We correlate the texture parameters with resistivity and thermal conductivity measured parallel and perpendicular to the alignment direction. Results obtained with 7 and 26 T fields are compared. We find no significant field dependence of the distribution width, while the aligned fraction is slightly greater at the higher field. Anisotropy in both transport properties is modest, with ratios in the range 5–9, consistent with the measured texture parameters assuming a simple model of rigid rod conductors. We suggest that further enhancements in anisotropic properties will require optimizing the filter deposition process rather than larger magnetic fields. We show that both x-ray and Raman data are required for a complete texture analysis of oriented SWNT materials.
Determination of gas temperature in an open-air atmospheric pressure plasma torch from resolved plasma emission
Authors: Williamson James M. , DeJoseph Charles A.
Published: February 2003

The resolved emission spectrum of an open-air atmospheric pressure plasma torch was recorded with a 0.5 m spectrometer and CCD camera. The plasma emission under these conditions was found to be dominated by continuum radiation and emission from species, which obscured large portions of the N2 second positive emission spectrum. Despite these difficulties, the gas temperature of the torch could be determined from a fit of partially resolved N2+ first negative vibrational transitions and a blackbody fit to the continuum radiation. The vibrational temperature, determined from a Boltzmann plot, was 4300±900 K while the blackbody radiation temperature was 4400±400 K. To check these gas temperature determinations, measured spectra over selected spectral regions were compared with spectral simulations using N2+ first negative emission, N2 second positive emission, and a blackbody. Best agreement between measured and simulated spectra was with blackbody temperature, rotational temperature, and vibrational temperature set to 4400 K.
Judd–Ofelt analysis of the Er3+(4f11) absorption intensities in phosphate glass: Er3+, Yb3+
Authors: Sardar Dhiraj K. , Gruber John B. , Zandi Bahram , Hutchinson J. Andrew , Trussell C. Ward
Published: February 2003

A spectroscopic Judd–Ofelt investigation has been performed on Er3+ in the doubly doped phosphate glass:Er3+, Yb3+ in order to assess its potential as a laser system. The standard Judd–Ofelt model was applied to the room-temperature absorption intensities of Er3+(4f11) transitions to determine the intensity parameters: Ω2=6.28×10−20 cm2, Ω4=1.03×10−20 cm2, and Ω6=1.39×10−20 cm2 in the phosphate glass host. The intensity parameters are used to determine the radiative decay rates (emission probabilities of transitions) and branching ratios of the Er3+ transitions from the excited-state J manifolds to the lower-lying J′ manifolds. The radiative lifetimes of these excited states are determined from the radiative decay rates. The predicted decay rates and radiative lifetimes are compared to those of Er3+ transitions in other glass hosts. The quantum efficiency of the eye-safe laser transition I13/24→4I15/2 (1.54 μm) of Er3+ is approximately 80% in the phosphate glass host.
Surface and gas-phase observations of Ar-diluted c-C4F8 plasma by using real-time infrared spectroscopy and planar laser-induced fluorescence
Authors: Ishikawa Kenji , Hayashi Shigenori , Sekine Makoto
Published: February 2003

Using infrared spectroscopy with attenuated-total-reflection methods and planar laser-induced fluorescence, we examined the relationship between the thickness of amorphous fluorinated carbon (a-C:F) film on the surface of a substrate and the distribution of CF2 radical density in an inductively coupled plasma (ICP) generated from an argon-diluted octafluoro-cyclo-butane (c-C4F8) gas. The axial profiles of the CF2 radical density were consistently concave, maximally close to the surface, and minimally present around the bulk plasma. The CF2 density initially increased with increasing the ICP power and then decreased as the plasma density exceeded ∼0.5×1011 cm−3, while the thickness of the a-C:F films deposited on a nonbiased surface consistently increased. When bias power was applied, the thickness of the a-C:F film began to decrease constantly due to the sputtering effect while the CF2 density exhibited a stepwise increase, followed by a gradual decrease in response to the reduction in the thickness of the a-C:F film and its vanishing. The amount of the stepwise increase in the CF2 density depended only on the rate of sputtering of the a-C:F film, and it was negligible at high c-C4F8/Ar ratios. The positive and negative correlations revealed here between the thickness of the a-C:F film and the density of CF2 radicals are discussed in comparison with mechanisms proposed in other studies. Electron-impact-induced destruction of CF2 radicals is demonstrated as the dominant factor affecting the distribution of CF2 radicals.
A survey of ab initio conical intersections for the H+H2 system
Authors: Halász Gábor , Vibók Ágnes , Mebel Alexander M. , Baer Michael
Published: February 2003

In this article we present a survey of the various conical intersections which govern potential transitions between the three lower electronic states for the title molecular system. It was revealed that these three states, for a given fixed HH distance, RHH, usually form four conical intersections: two, between the two lower states and two, between the two upper states. One of the four is the well known equilateral D3h ci and the others are, essentially, C2v cis: One of them is located on the symmetry line perpendicular to the HH axis (just like the D3h ci) and the other two are located on both sides of this symmetry line and in this way form the ci-twins. The study was carried out for four RHH-values, namely, RHH=0.74, 0.5417, 0.52, and 0.4777 Å. It was also established that there exists one single RHH-value designated as R̃HH, located in the interval {0.52, 0.53 Å}, for which all four cis coalesce to become one kind of “super” ci which couples the three states. The numerical study was carried out employing the line integral approach for groups of two and three states. As for the two-state calculations we found that all D3h-cis, at close proximity, are circular (ordinary) Jahn-Teller-type cis, whereas all C2v-cis, at close proximity, are elliptic Jahn-Teller cis [Chem. Phys. Lett 354, 243 (2002)]. Particular attention is given to the 3-state quantization of the nonadiabatic coupling matrix. The quantization is found to be fulfilled in all situations as long as the regions in configuration space are not too far from the relevant cis. In the Discussion and Conclusion we discuss, among other subjects, the possibility to diabatize the adiabatic potential matrix.
Air shear driven flow of thin perfluoropolyether polymer films
Authors: Scarpulla Michael A. , Mate C. Mathew , Carter Malika D.
Published: February 2003

We have studied the wind driven movement of thin perfluoropolyether (PFPE) polymer films on silicon wafers and CNx overcoats using the blow-off technique. The ease with which a liquid polymer film moves across a surface when sheared is described by a shear mobility χS, which can be interpreted both in terms of continuum flow and in terms of wind driven diffusion. Generally, we find that the movement of PFPE films can be described as a flow process with an effective viscosity, even when the film thickness is smaller than the polymer’s diameter of gyration. Only in the special case of sparse coverage of a polymer with neutral end groups is the motion better described by a wind driven diffusion process. The addition of alcohol end groups to the PFPE polymer chain results in strong interactions with the substrate, creating a restricted layer having an effective viscosity an order of magnitude larger than the mobile layer that sits on top of the restricted layer.
On the mechanism of transient bleaching of the optical absorption of ferrofluids and dyed liquids
Authors: Hoffmann B. , Köhler W. , Krekhova M.
Published: February 2003

We have investigated the transient optical bleaching of two sterically stabilized kerosene based magnetite ferrofluids and of a dye solution (quinizarin in toluene). Master plots are constructed from model calculations of the initial rise of the transmitted intensity, which clearly prove that the bleaching is caused by a thermophoretic migration of the absorbers out of the laser beam. The quantitative model allows for the determination of the thermal diffusion coefficients of the two ferrofluids investigated to 0.4 and 1.7×10−8 cm2(s K)−1, which agrees well with the literature value for a similar system.
Polymer adsorption on nanoheterogeneous surfaces: Impact of size and density of heterogeneous sites
Authors: Chun Kyoung-Yong , Huang Yu-Wen , Gupta Vinay K.
Published: February 2003

Model chemically heterogeneous surfaces containing different density of heterogeneous sites were used to study the adsorption of poly(acrylic acid) (PAA) from aqueous solutions. The heterogeneous surfaces were prepared by self-assembly of ω-functionalized alkanethiols on gold substrates such that the surfaces consisted of a patchwise distribution of two types of sites, hydrophobic methyl (DT) and hydrophilic carboxylic acid sites (MPA), that interact differently with PAA. Microscopic imaging by scanning tunneling microscopy and macroscopic characterization by cyclic voltammetry were used to show that the domains of DT on the surface were of nanometer dimension and commensurate with the size of the polymer chain. Surface plasmon resonance was used to measure the adsorption kinetics of PAA at the solid–liquid interface. These experiments represent the first realization of systems modeled or simulated in recent theoretical studies and the results corroborate predictions of the theory. Nanometer scale heterogeneity within an uncharged surface had little effect on the kinetics of adsorption of neutral PAA during the early stages of adsorption. The initial rate of adsorption was unchanged from that measured on a homogeneous surface containing only one type of chemical site (DT or MPA). A threshold concentration of heterogeneous sites was necessary before their impact was significantly manifested in the number of polymer chains adsorbed on the surface. The adsorbed amount of PAA on the heterogeneous surface was found to lie in between the adsorbed amounts found on the two compositionally homogeneous surfaces (DT and MPA) and was dependent on the fraction of heterogeneous sites on the surface.
Precursor states of atomic hydrogen on the Si(100)-(2×1) surface
Authors: Tok E. S. , Engstrom J. R. , Kang H. Chuan
Published: February 2003

Using plane wave pseudopotential density functional theory calculations we have identified for the first time precursor states for hydrogen atom chemisorption on the Si(100)-(2×1) surface. These states exist above clean, partially, and fully monohydride-adsorbed surface dimers. In all three cases the dimer bond is broken in the trapped state. A study of the energetics for atomic desorption, abstraction, chemisorption, and migration was carried out. We find that “hot” hydrogen atoms of energies up to approximately 1.3–1.9 eV can be trapped on the surface. These atoms are highly mobile, and we obtained energetics consistent with experimental data from which precursor-mediated adsorption mechanisms have been inferred. The existence of these states provides an understanding of the non-Langmuirian atomic hydrogen adsorption probability, and also underscores the importance of lattice distortions in the interactions of hydrogen with the silicon surface.
Nuclear magnetic resonance dephasing effects in a spherical pore with a magnetic dipolar field
Authors: Valckenborg R. M. E. , Huinink H. P. , Kopinga K.
Published: February 2003

The NMR dephasing behavior of the nuclear spins of a fluid confined in a porous material can be investigated by Hahn spin echoes. Previous experimental results on water in a magnetically doped clay have shown a nonmonoexponentially decaying magnetization, which can be understood neither by the known dephasing rate of freely diffusing spins in a uniform gradient nor by spins diffusing in a restricted geometry. For a better understanding of NMR measurements on these systems, a systematic survey was performed of the various length scales that are involved. The standard length scales for the situation of a uniform gradient are diffusing length, structure length, and dephasing length. We show that for a nonuniform gradient, a new length scale has to be introduced: the magnetic-field curvature length. When a particle diffuses less than this length scale, it experiences a local uniform gradient. In that case the spin-echo decay can be described by the so-called local gradient approximation (LGA). When a particle diffuses over a longer distance than the structure length, the spin-echo decay can be described by the motional averaging regime. For both regimes, scaling laws are derived. In this paper, a random-walk model is used to simulate the dephasing effect of diffusing spins in a spherical pore in the presence of a magnetic dipole field. By varying the dipole magnitude, situations can be created in which the dephasing behavior scales according to the motional averaging regime or according to the LGA regime, for certain ranges of echo times. Two model systems are investigated: a spherical pore in the vicinity of a magnetic point dipole and a spherical pore adjacent to a magnetic dipolar grain of the same size as the pore. The simulated magnetization decay curves of both model systems confirm the scaling laws. The LGA, characterized by a nonmonoexponential magnetization decay, is also investigated by calculating the spatially resolved magnetization in the pore. For this regime, the magnetization is found to be inhomogeneously distributed within the pore, whereas it is homogeneously distributed in the motional averaging regime.
Calculation of the dielectric permittivity profile for a nonuniform system: Application to a lipid bilayer simulation
Authors: Stern Harry A. , Feller Scott E.
Published: February 2003

We derive an expression relating the static dielectric permittivity profile for a system nonuniform in one dimension to correlations between the net system dipole moment and the local polarization density. The permittivity profile of a dipalmitoylphosphatidylcholine (DPPC) lipid bilayer in water is calculated from an all-atom 20-ns molecular dynamics simulation. The component of the permittivity parallel to the bilayer shows a nonmonotonic decrease from the value in bulk water to the value in the membrane interior; the interfacial region itself has a very large permittivity, greater than that of bulk water. In high-dielectric regions, obtaining a quantitative estimate of the component normal to the bilayer is not possible because of large numerical uncertainty. However, the calculated correlation function is consistent with a value for the interface at least as large as that of bulk water. In general, the transition to a low-dielectric environment is sharp and is located on the inner border of the region where there is significant probability of finding the polar head groups.
Temperature induced changes on the structure and the dynamics of the “tetrahedral” glasses and melts of ZnCl2 and ZnBr2
Authors: Yannopoulos S. N. , Kalampounias A. G. , Chrissanthopoulos A. , Papatheodorou G. N.
Published: February 2003

Glassy, supercooled, and molten ZnCl2 and ZnBr2 have been studied by Raman spectroscopy over the broad temperature range −196 to 800 °C in an effort to follow in detail the structural changes caused by temperature variation. A systematic study has also been undertaken for the corresponding crystalline polymorphs showing that each material exists in only one crystalline phase if water traces are not present. The reduced isotropic and anisotropic Raman spectra of the ZnCl2 and ZnBr2 glasses and melts are isomorphous. Unusually drastic changes of the relative intensities of particular bands occur with temperature in the reduced isotropic spectra. A comparison between the spectral features of crystals, glasses, and melts has revealed that the network structure of the glasses and melts consists of ZnX4/2 (X=Br,Cl) tetrahedra bound to each other by apex- and edge-bridged halides. The substructure of the glass/melt is formed by mixing a variety of tetrahedra participating in “open,” “cluster,” and “chain” networks which are bound to each other by bridged halides. The boundaries of the substructure involve neutral or charged terminal halide bonds with zinc of an average threefold coordination. Temperature rise breaks up the substructure to smaller fragments, increases the number of terminal bonds, and rearranges the apex- and edge-bridging networks. The good glass-forming ability of the ZnX2 melts is attributed to the existence and mixing of the three topologically different networks within the substructure. Our data of ZnCl2 are in qualitative agreement with molecular dynamics simulations as regards the frequency distribution of vibrational modes. However, simulations are not able to correctly predict polarization properties in the glass and the melt. The low-frequency Raman spectra reveal the presence of the Boson peak in both glasses, which interestingly persists, well resolved, also into the normal liquid state above the melting point. The spectra in the region of the Boson peak are also discussed in the framework of relevant theoretical models and empirical correlations.
Rotational diffusion of two-segmented macromolecules with a ball–socket joint: A kinetic theory approach
Authors: Ådland Hans Magne , Mikkelsen Arne
Published: February 2003

Semiflexible polymers are often modelled as consisting of rigid segments connected by rigid rods, ball–socket joints or springs. In this paper we focus on polymers consisting of two identical segments connected by a frictionless ball–socket joint. This introduces rigid constraints, resulting in the number of degrees of freedom being reduced. Using kinetic theory we derive the effective hydrodynamic mobility tensor of each segment in the chain, expressed in terms of the hydrodynamic mobility of an identical free segment. The result is used to obtain analytical and numerical expressions for the slowest decay mode in a typical transient electrically induced birefringence experiment. We show that under typical experimental conditions only this single mode will be present. The results we obtain are significantly different from results for a similar once-broken rod published by Wegener and co-workers and Garcia de la Torre and co-workers. In the last part of the paper Brownian dynamics simulations of the two-segment model is used to verify our analytically obtained expressions for the mobility tensor and decay modes and to obtain results for situations where analytical results are unavailable. The model assumes no hydrodynamic interaction between the segments.
Effects of implantation temperature and ion flux on damage accumulation in Al-implanted 4H-SiC
Authors: Zhang Y. , Weber W. J. , Jiang W. , Wang C. M. , Hallén A. , Possnert G.
Published: February 2003

The effects of implantation temperature and ion flux on damage accumulation on both the Si and C sublattices in 4H-SiC have been investigated under 1.1-MeV Al22+ irradiation at temperatures from 150 to 450 K. The rate of damage accumulation decreases dramatically, and the damage profile sharpens due to significant dynamic recovery at temperatures close to the critical temperature for amorphization. At 450 K, the relative disorder and the density of planar defects increase rapidly with the increasing ion flux, exhibiting saturation at high ion fluxes. Planar defects are generated through the agglomeration of excess Si and C interstitials during irradiation and post-irradiation annealing at 450 K. A volume expansion of ∼8% is estimated for the peak damage region.
FeCo–Zr–O nanogranular soft-magnetic thin films with a high magnetic flux density
Authors: Ohnuma S. , Fujimori H. , Masumoto T. , Xiong X. Y. , Ping D. H. , Hono K.
Published: February 2003

Soft-magnetic thin films with high magnetic flux densities of about 23 kG have been fabricated in the (Fe–Co)–Zr–O nanogranular system. The films were prepared by reactive sputtering under an oxygen–argon atmosphere using a target of Fe–Co–Zr alloys. The microstructure was composed of base-centered-cubic Fe–Co nanograins, where nanoparticles of amorphous Zr oxide are dispersed. These Zr-oxide nanoparticles are thought to hinder the growth of Fe–Co grains during the film deposition, causing low coercivity.
In situ observations of phase transitions in Ti–6Al–4V alloy welds using spatially resolved x-ray diffraction
Authors: Elmer J. W. , Palmer T. A. , Wong Joe
Published: February 2003

In situ spatially resolved x-ray diffraction (SRXRD) experiments were used to directly observe the heat-affected zone phases present during gas tungsten arc welding of a Ti–6Al–4V alloy. The experiments were performed at the Stanford Synchrotron Radiation Laboratory using a 250 μm diam x-ray beam to gather real-time experimental information about the α−Ti→β−Ti phase transition during weld heating. Six different welding conditions were investigated using SRXRD to experimentally determine the extent of the single phase β-Ti region surrounding the liquid weld pool. These data were compared to predicted locations of the β-Ti phase boundary determined by calculated weld thermal profiles and equilibrium thermodynamic relationships. The comparison shows differences between the experimentally measured and the calculated locations of the β-Ti boundary. The differences are attributed to kinetics of the α−Ti→β−Ti phase transition, which requires superheating above the β-Ti transus temperature to take place during nonisothermal weld heating. Analysis of the results reveal that the transition to β-Ti requires an additional 3.96 s (±0.29 s) of time and 169 °C (±25.7 °C) of superheat above the β-Ti transus temperature to go to completion under an average weld heating rate of 42.7 °C/s.
Variational calculations of excited states with zero total angular momentum (vibrational spectrum) of H2 without use of the Born–Oppenheimer approximation
Authors: Bubin Sergiy , Adamowicz Ludwik
Published: February 2003

Very accurate, rigorous and fully variational, all-particle, non-Born–Oppenheimer calculations of the vibrational spectrum of the H2 molecule have been performed. Very high accuracy has been achieved by expanding the wave functions in terms of explicitly correlated Gaussian functions with preexponential powers of the internuclear distance. An indicator of the high accuracy of the calculations is the new upper bound for the H2 nonrelativistic nonadiabatic ground state energy equal to −1.164 025 030 0 hartree.
Temperature dependence of luminescent spectra and dynamics in nanocrystalline Y2O3:Eu3+
Authors: Peng Hongshang , Song Hongwei , Chen Baojiu , Wang Jiwei , Lu Shaozhe , Kong Xianggui , Zhang Jiahua
Published: February 2003

A temperature dependence for emission of Eu3+ in cubic nanocrystalline Y2O3:Eu3+ was studied in contrast with the polycrystalline powders. The emission intensity of Eu3+ decreased solely with elevated temperature under the excitation of a 580 nm light, while it had a maximum at a certain temperature under a 488 nm light. The experimental data were well fitted based on a theory considering both the thermal activated distribution of electrons among FJ7 and the thermal quenching effect. The results indicated that the thermal quenching rate in nanocrystals (NCs) was faster than that in the polycrystals. The nonradiative decay rate, wNR, the radiative transition rate, w0R, and the luminescent quantum efficiency (QE) were obtained according to the temperature dependence of fluorescence lifetime. It can be concluded that wNR and w0R both increase in NCs, and that QE decreases.
Photodissociation of highly vibrationally excited NH3 in the 5νN–H region: Initial vibrational state dependence of N–H bond dissociation cross section
Authors: Akagi Hiroshi , Yokoyama Keiichi , Yokoyama Atsushi
Published: February 2003

Ultraviolet photolysis of highly vibrationally excited NH3(X̃ 1A1′) in the 5νN–H band has been studied using a crossed laser and molecular beams method. Relative cross sections of N–H bond dissociation via the Ã←X̃ transition have been determined by the measurement of the action spectrum, utilizing a (2+1) resonance enhanced multiphoton ionization scheme of product H atoms. The obtained cross section for the photolysis of the 4ν1+ν3 state (4 quanta in the symmetric stretching and 1 quantum in the antisymmetric stretching) was 1.23±0.06 times as large as that of the 5ν1 state. To interpret the ratio of the obtained cross sections, we have evaluated the Franck–Condon factors for these vibrational states, by utilizing time-dependent wave-packet calculations on the potential energy surfaces constructed by an ab initio molecular orbital procedure.
The absorption spectroscopy of the lowest pseudorotational states of tetrahydrofuran
Authors: Melnik Dmitry G. , Gopalakrishnan Sandhya , Miller Terry A. , De Lucia Frank C.
Published: February 2003

The rotational structure of the pseudorotational (PR) band n=0→n=2 has been observed in jet-cooled tetrahydrofuran in the 170–360 GHz frequency range. The observed transitions were analyzed together with the previously obtained microwave data of Meyer and co-workers [R. Meyer, J. C. Lopes, J. L. Alonso, S. Melandri, P. G. Favero, and W. Caminati, J. Chem. Phys. 111, 7871 (1999)]. The experimentally observed transitions provide direct spectroscopic evidence of the symmetry ordering of the lowest four observed PR states. Based on the symmetry properties of the pseudorotational states involved in this study, an analytical model of the potential energy surface (PES) along the pseudorotational path has been proposed that provides a consistent explanation of all the observed transition frequencies, including those from the early IR work. In addition, an analysis of the variation of the rotational constants of the molecule in different PR states has been performed using the proposed model. This analytical PES and the derived rotational constants are compared to the results of it ab initio calculations. A discussion of the results obtained by different methods is given.
Characterization of neutral, positive, and negative species in a chlorine high-density surface-wave plasma
Authors: Stafford L. , Margot J. , Chaker M. , Pauna O.
Published: February 2003

This article reports an experimental study of the influence of the plasma parameters on the concentration of neutral and ion species in a chlorine high-density plasma sustained by surface waves. The investigation focuses on the dependence of the concentrations of Cl, Cl2, Cl+, Cl2+, Cl−, and electrons on the gas pressure in the 0.1 to 10 mTorr range, and on the intensity of a confinement magnetic field. The results show that a high dissociation degree (up to 90%) can be achieved even with a very modest power level (250 W, power density of about 2 mW/cm3), provided the pressure is low enough (i.e., less than 1 mTorr). It was also found that Cl+ is the main positive ion and that electrons are the main negative charge carrier at lower pressure. When the gas pressure is higher than a few mTorr, Cl2+ becomes dominant with Cl− as the negative charge carrier. The behavior of the positive ion and neutral species concentrations is compared to the results of a simple model based on creation–losses rate equations for the various species. It is shown that for a given magnetic field intensity, there is a critical pressure above which diffusion can be neglected in comparison with ion–ion recombination and charge transfer.
Quantum-statistical equilibrium and the “law” of constant Fermi potential
Authors: Le Coz Yannick L.
Published: February 2003

We apply the general quantum-statistical density-matrix formalism to an independent-electron gas within a space-dependent external electric potential, under equilibrium conditions. This problem is analogous to an ideal semiconductor homojunction diode. We solve the resulting equilibrium density-matrix equation using a perturbation theory. Next, we derive a first-order quantum correction to the classical Maxwell-Boltzmann density-potential formula. The correction appears as an added curvature term in external potential. It represents expected quantum-mechanical scattering against a spatially varying potential. Our results indicate that the commonly encountered thermodynamic or statistical-mechanical “law” of constant, equilibrium Fermi potential—with Fermi potential a parameter in the Maxwell-Boltzmann density-potential formula—is not fundamentally exact. In a general space-dependent potential, this “law,” we prove, is simply a classical approximation.
Parallax measurements of integrated circuit interconnects using a scanning transmission electron microscope
Authors: Levine Zachary H. , Gao James J. , Neogi Suneeta , Levin T. M. , Scott John Henry , Grantham Steven
Published: February 2003

A 300 keV scanning transmission electron microscope was used to obtain tilt-series images of two two-level copper integrated circuit samples. The center-to-center layer spacing obtained from the tilt series showed internal consistency at the level of 15%, and external validity at the level of 20%.
Specific heat of magnetic fluids under a modulated magnetic field
Authors: Chiu Y. P. , Chen Y. F. , Yang S. Y. , Chen J. C. , Horng H. E. , Yang H. C. , Tse W. S. , Hong Chin-Yih
Published: February 2003

Due to the increasing importance of thermal devices in engineering applications, the investigation of the thermal properties of magnetic fluids has become necessary. In this work, we report both the temperature and field dependence of the specific heat of a water-based magnetic fluid. The results show that the specific heat of the magnetic fluid is almost independent of temperature under lower-field strengths (<50 Oe), but decreases significantly upon increasing temperature under higher fields. To clarify this effect of the magnetic field on the specific heat capacity of the magnetic fluid, the evolution of the structure of the magnetic fluid film is examined and compared with its field-modulated heat capacity of the fluid.
Mobile ionic impurities in organic semiconductors
Authors: Rep D.B.A. , Morpurgo A. F. , Sloof W. G. , Klapwijk T. M.
Published: February 2003

We study the stability in time of the current–voltage characteristics of organic thin-film devices on glass substrates. We find for poly(3-hexylthiophene) and for pentacene that the resistance of the devices gradually changes under the application of an electrical bias depending on the sodium content of the glass substrates used in the experiment. For devices on a very common type of glass (with a Na2O content of about 6%) and on sodalime glass (14% Na2O) substrates, the prolonged application of a voltage bias results in a substantial decrease (up to two orders of magnitude) of the bulk and contact resistances, whereas for sodium-free glass substrates the gradual changes in current–voltage characteristics are much smaller. A systematic study of the electrical behavior complemented by chemical analysis shows that the instabilities observed are due to Na+ ions diffusing from the substrate into the organic film, and moving inside the organic material as a result of the applied electric field. Our results show in detail how ion motion in organic materials results in substantial hysteresis and device instabilities.
Model study of soft x-ray spectroscopy techniques for observing magnetic circular dichroism in buried SmCo magnetic films
Authors: Itza-Ortiz S. , Ederer D. L. , Schuler T. M. , Ruzycki N. , Jiang J. Samuel , Bader S. D.
Published: February 2003

X-ray emission and absorption spectroscopy (XES and XAS, respectively) are important and powerful techniques for determining the electronic properties of materials. Both are used to study magnetic circular dichroism (MCD) which is especially useful for analyzing the magnetic properties of materials. We present XAS and XES measurements and a MCD model study of two thin film layered samples containing SmCo layers in order to report on the applicability of soft x-ray spectroscopic techniques to determine the composition, layer thickness, and electronic structure of such materials. Using a transmission by fluorescence attenuation (TFA) technique we determined the composition and thickness of the SmCo layer to be consistent with the intended composition and thickness. We also confirmed the thickness of the other layers by comparing the XES from the thin film with that of a bulk sample. We showed by a model study that TFA could be used to obtain MCD, and thus the anisotropy of the sample, for film thicknesses between about 400 and 800 Å.
Study of photoisomerization of azo dyes in liquid crystals
Authors: Statman D. , Jánossy I.
Published: February 2003

The trans-cis isomerization of azo dyes in liquid crystalline hosts is studied. It is shown that the full set of parameters governing the isomerization process can be deduced from polarized pump–probe transmission measurements. The results indicate that the dye order parameter for the trans isomer is relatively high and is strongly related to the liquid-crystalline order. The cis isomer exhibits a much lower dye order parameter and is not strongly dependent on the liquid-crystalline phase. The barrier to the thermal cis-trans relaxation is found to be relatively unaffected by the liquid-crystalline phase, while changes are observed in the pre-exponential factor.
Experimental and theoretical aspects of proton transfer in 3-methyl-6-hydroxy-m-phthalic acid
Authors: Mandal A. , Mitra S. , Banerjee D. , Bhattacharyya S. P. , Mukherjee S.
Published: February 2003

The ground- and excited-state proton transfer reactions of 3-methyl-6-hydroxy-m-phthalic acid (HmPA) have been studied in different protic and aprotic solvents at room temperature and 77 K both in the presence and absence of a base. In the ground state, intramolecularly hydrogen-bonded closed conformer has been found to be the only component in nonpolar and weakly polar aprotic solvents, whereas anion is detected in highly polar aprotic solvents. The excited-state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted emission in nonpolar and weakly polar solvents due to the formation of enol tautomer of HmPA. Solute–solvent interaction appears to play a major role in determining the nature of the absorbing and fluorescing species. At 77 K, HmPA shows phosphorescence both in the presence and absence of a base. The observed fluorescence decay rates are relatively slow in polar solvents compared to those in nonpolar and weakly polar solvents. Semiempirical calculations with limited configuration interaction demonstrate that ESIPT is favored only in the S1 electronic state both kinetically and thermodynamically, while the ground singlet and the excited triplet are predicted to have considerably higher barriers on the respective proton transfer paths.
Phase behavior and concentration fluctuations in suspensions of hard spheres and nearly ideal polymers
Authors: Shah S. A. , Chen Y. L. , Schweizer K. S. , Zukoski C. F.
Published: February 2003

The phase behavior and concentration fluctuations in suspensions of hard sphere colloids and nonadsorbing polymers under nearly ideal solvent conditions is studied experimentally. A remarkably different qualitative behavior compared to the athermal solvent case is observed for the dependence on polymer/particle size asymmetry of both the gelation and fluid–fluid phase separation boundaries. Near the theta state the effect of increasing the range of depletion attractions leads to a weak monotonic destabilization of the homogeneous phase at high particle volume fractions, with a reversal of the trend at lower volume fractions. In stark contrast to athermal solvent behavior, this nonmonotonic behavior results in multiple “curve crossings” of gel and phase separation boundaries as the polymer/particle size ratio is varied. Quantitative comparisons with no adjustable parameter PRISM integral equation theory for the fluid–fluid spinodals and osmotic compressibilities show good qualitative or semiquantitative agreement with all the experimental trends. The differences between good and ideal solvent conditions are largely attributed to changes in the polymer–polymer pair correlation functions due to the enhanced ability of coils to interpenetrate and cluster in theta solvents. Even for ideal solvent conditions the simplifying polymer model and statistical mechanical assumptions adopted by prior classic free volume and related approaches appear to miss fundamental aspects of the experimental behavior, especially for large size asymmetry ratios and/or moderate-to-high colloid volume fractions. The primary error can be identified with the approximation of a polymer chain by a phantom sphere with no conformational degrees of freedom.
Modeling chemical reactions for conformationally mobile systems with force field methods
Authors: Olsen Per T. , Jensen Frank
Published: February 2003

A previously proposed method for modeling transition structures as minima on the seam of two force field energy functions, denoted SEAM, is analyzed with respect to improvements in the functional form of key energetic terms and parameters. It is shown that the SEAM method is capable of accurately reproducing transition structures from electronic structure methods, at a fraction of the cost. The SEAM method can be used to automatically sample different conformational transition structures for a given reaction. Depending on the underlying force field and the reaction, the relative energies of the transition structures may or may not be sufficiently accurate for selecting the important low-energy structures. A combination of using the SEAM method for determining geometries and a low-cost electronic structure method for single point energy calculations appears to be an attractive combination for identifying low-energy transition structures for systems having many degrees of freedom.
Rotation-vibration states of H3+ at dissociation
Authors: Kostin Maxim A. , Polyansky Oleg L. , Tennyson Jonathan , Mussa Hamse Y.
Published: February 2003

Calculations are presented which estimate energies for all the bound rotation-vibration energy levels of H3+ with rotation angular momentum J=0, 2 and 8. The calculations, which use Radau coordinates with z-axis of the molecule embedded perpendicular to the molecular plane, are performed on 128 nodes of a massively parallel computer. It is found that convergence with respect to basis set size of the higher J states is fairly slow and that further improvements are beyond the capabilities of the current computational set-up.
Infrared spectroscopy of (CO2)N nanoparticles (30<N<14500) flowing in a uniform supersonic expansion
Authors: Bonnamy A. , Georges R. , Benidar A. , Boissoles J. , Canosa A. , Rowe B. R.
Published: February 2003

The infrared signature of carbon dioxide clusters of nanometric size is discussed both in the bending (ν2 mode at 15 μm) and in the asymmetric stretching (ν3 mode at 4.2 μm) spectral region of the monomer. The carbon dioxide nanoparticles were formed using a capillary tube injection inserted upstream of a uniform supersonic flow of argon generated by a Laval nozzle. The size of the formed clusters was varied by changing the stagnation pressure P0 of the capillary. The empirical power law connecting P0 to the number N of monomers per cluster: N∝P02.2 was verified in this work. The cluster mean size was estimated using a Rayleigh scattering experiment showing the formation of nanometric clusters whose radii are in the range 0.7 nm<r<5.3 nm, corresponding to 30<N<14 500. The thermodynamic and kinetic parameters of the flow were determined from the rovibrational absorption lines of the monomer and from a time-of-flight experiment. The measured flow velocity and flow temperature show that CO2 condensation is responsible for both a strong flow warming and a non-negligible flow acceleration. The translational and rotational temperatures of the monomers were found to be identical, highlighting a thermal equilibrium between these two motions. The cluster temperature Tc ranging from 93 to 135 K was estimated assuming a thermal equilibrium between the clusters and the monomer bath, induced by a high flow density of about 1016 molecules cm−3. The double peak feature at 657 and 667 cm−1 reflects the crystalline nature of the clusters. A single Lorentzian peak is observable at 2360 cm−1 whose position however appears to be weakly size dependent. The pronounced narrowing of the peak with increasing N surprisingly stopped evolving for N=820 and Tc=108 K.
Molecular vibrations: Iterative solution with energy selected bases
Authors: Lee Hee-Seung , Light John C.
Published: February 2003

An efficient and accurate quantum method for the calculations of many large amplitude vibrational states of polyatomic molecules is proposed and tested on three triatomic molecules; H2O, SO2, and HCN. In this approach we define zero-order reduced dimensional Hamiltonians ĥk using minimum energy reduced dimensional potentials. The eigenfunctions and eigenvalues of ĥk, φn(k), and εn(k), are used to form an energy selected basis (ESB) for the full system including all the product functions Πkφn(k) for which ∑ε(k)⩽Ecut. We show that ESB can be used efficiently in an iterative solution of the Schrödinger equation by the transformation between the ESB and the direct product quadrature grid. Application of the ESB of one-dimensional basis functions is shown to be very efficient for vibrational states of H2O and SO2 up to 30 000 and 23 000 cm−1, respectively. A combined two-dimensional/one-dimensional basis is used very effectively for HCN above the isomerization energy to HNC. The present approach is shown to be substantially more efficient than either the direct product discrete variable representation (DVR) bases or compact bases from the DVR with the sequential diagonalization/truncation method.
Effect of a nearby charge-ordered phase on correlation functions in ionic systems
Authors: Ciach A. , Góźdź W. T. , Evans R.
Published: February 2003

The charge–charge and number–number correlation functions are investigated using a mean-field density functional for the restricted primitive model (hard-sphere and Coulomb interactions) supplemented with short-ranged attractive forces. The system exhibits phase separation into ion-dilute and ion-dense phases, and the latter phase becomes unstable with respect to charge-ordering along the λ-line. In mean field approximation both the range and the amplitude of the charge-charge correlation function increase as (S−Sλ)−1/2 on the approach to the λ-line, where S=T*/ρ0* and Sλ is the value of S at the λ-line (T* is the reduced temperature and ρ0* is the dimensionless density). The line dividing the phase diagram into regions where the range of charge–charge correlations is longer (shorter) than the range of the number–number correlations is also determined. We argue that the large range and large amplitude of the charge–charge correlation function is consistent with the formation of aggregates (living polymers) observed in simulations of ionic systems.