bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies on interface states at ultrathin SiO2/Si(100) interfaces by means of x-ray photoelectron spectroscopy under biases and their passivation by cyanide treatment
Authors: Kobayashi Hikaru , Asano Akira , Asada Shinya , Kubota Tomohiro , Yamashita Yoshiyuki , Yoneda Kenji , Todokoro Yoshihiro
Published: July 2002

The energy distribution of interface states at ultrathin oxide/Si(100) interfaces is obtained using a new method, i.e., x-ray photoelectron spectroscopy measurements under biases between the metal overlayer and the Si substrate of the metal-oxide-semiconductor (MOS) devices. Ultrathin thermal oxide layers formed at 450 °C in oxygen have an interface state peak near the midgap and it is attributed to isolated Si dangling bonds with which no atoms in the oxide layer interact. On the other hand, thermal oxide layers formed at 650 °C have a two-peaked structure, one peak above and the other below the midgap, and they are attributed to Si dangling bonds with which an oxygen or Si atom in the oxide layer interacts weakly. The density of the interface states, especially that near the midgap, decreases drastically by cyanide treatment, i.e., the immersion of Si in a KCN solution for a few seconds followed by a rinse in boiling water, performed before the oxide formation. It is suggested that cyanide ions penetrate into the Si, forming Si–CN bonds at structurally imperfect places. The cyanide treatment improves the electrical characteristics of the MOS tunneling diodes.
A combined scanning tunneling microscopy and electron energy loss spectroscopy study on the formation of thin, well-ordered β-Ga2O3 films on CoGa(001)
Authors: Schmitz G. , Gassmann P. , Franchy R.
Published: July 2002

The formation of thin, well-ordered β-Ga2O3 films on CoGa(001) was studied by means of high resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy. The crystalline β-Ga2O3 films on CoGa(001) are prepared upon adsorption of O2 at 300 K and subsequent annealing at 700 K or by oxidation of the sample directly at 700 K, respectively. EEL spectra of these films exhibit Fuchs–Kliewer modes at 305, 455, 645, and 785 cm−1 in good agreement with calculated spectra using the IR parameters of Ga2O3. The band gap was determined to be 4.5±0.2 eV. In addition, a gap state at 3.3 eV was found. The observed LEED pattern of β-Ga2O3/CoGa(001) can be explained by a (2×1) structure in two perpendicularly oriented domains. STM images exhibit atomically flat and large oxide terraces (up to 2500×700 Å2) mainly of rectangular shape. STM pictures with atomic resolution confirm the (2×1) structure. In addition, a square substructure can be observed which is related to the closed-packed oxygen lattice of β-Ga2O3.
Manipulation of graphitic sheets using a tunneling microscope
Authors: Roy H.-V. , Kallinger C. , Marsen B. , Sattler K.
Published: July 2002

Folding and unfolding of atomic layers has been performed at step edges of graphite using a scanning tunneling microscope. The technique consists of modulating the distance or bias voltage between the microscope tip and the sample when scanning selected areas. Calculation of the energies involved in the manipulations shows that tip-sample vibration can account for tearing and folding of graphitic layers.
A field induced ferromagnetic-like transition below 2.8 K in Li2CuO2: An experimental and theoretical study
Authors: Ortega R. J. , Jensen P. J. , Rao K. V. , Sapiña F. , Beltrań D. , Iqbal Z. , Cooley J. C. , Smith J. L.
Published: July 2002

The low temperature magnetic properties of the Li2CuO2 compound have been investigated by means of superconducting quantum interference device magnetometry. We find in addition to an antiferromagnetic phase below 9.5 K a ferromagnetic-like steep rise of the magnetization around 2.8 K. The observed low temperature behavior is discussed by considering second and fourth order magnetocrystalline effective anisotropy coefficients, in addition to the exchange couplings reported in the literature.
Effects of traps on charge storage characteristics in metal-oxide-semiconductor memory structures based on silicon nanocrystals
Authors: Shi Yi , Saito Kenichi , Ishikuro Hiroki , Hiramoto Toshiro
Published: July 2002

Charge storage characteristics have been investigated in metal-oxide-semiconductor memory structures based on silicon nanocrystals, where various interface traps and defects were introduced by thermal annealing treatment. The observations demonstrate that traps have strong influence on the charge storage behavior, in which the traps and defects at the internal/surface of silicon nanocrystals and the interface states at the SiO2/Si substrate play different roles, respectively. It is suggested that the injected charges are mainly stored at the deep traps of nanocrystals instead of the conduction band in long-term retention mode. The long-term charge-loss process is dominantly determined by the direct tunneling of the trapped charges to the interface states in the present experiment. An optimum way to improve the retention time would be to introduce a certain number of deep trapping centers in nanocrystals and to decrease the interface states at SiO2/Si substrate.
Fabrication of thin film inductors using FeTaN soft magnetic films
Authors: Lee Chang-Ho , Shin Dong-Hoon , Ahn Dong-Hoon , Nam Seoung-Eui , Kim Hyoung-June
Published: July 2002

Planar-type thin film inductors were fabricated using FeTaN soft magnetic films. It was found that excellent soft magnetic properties (coercivity of 0.1–0.2 Oe, effective permeability of 4000, and saturation magnetization of 15 kG) could be obtained in the Fe78.8Ta8.5N12.7 films after anneals at 400–500 °C. The initial permeability of the films was constant up to 20 MHz. Internal-coil inductors, in which Cu coils were sandwiched by SiO2 insulator/FeTaN magnetic layers, were fabricated and their characteristics were compared with air-core inductors. The inductance was increased by a factor of 4 compared to air-core inductors. However, the high frequency characteristic was degraded due to a frequency-limited permeability of FeTaN magnetic layers.
Optoelectronic and structural properties of amorphous silicon–carbon alloys deposited by low-power electron-cyclotron resonance plasma-enhanced chemical-vapor deposition
Authors: Conde J. P. , Chu V. , da Silva M. F. , Kling A. , Dai Z. , Soares J. C. , Arekat S. , Fedorov A. , Berberan-Santos M. N. , Giorgis F. , Pirri C. F.
Published: July 2002

The optoelectronic and structural properties of hydrogenated amorphous silicon-carbon alloys (a-SiC:H) are studied over the entire compositional range of carbon content. The films are prepared using low-power electron-cyclotron resonance (ECR) plasma-enhanced chemical vapor deposition. The carbon content was varied by using different methane (or ethylene-)-to-silane gas phase ratios and by introducing the methane (or ethylene) either remotely into the plasma stream or directly through the ECR source, together with the excitation gas (hydrogen). Regardless of the deposition conditions and source gases used, the optical, structural and transport properties of the a-SiC:H alloys followed simple universal dependencies related to changes in the density of states associated with their structural disorder. The deep defect density from photothermal deflection spectroscopy, the ESR spin density, the steady state and the transient photoluminescence, the dark and photoconductivity, the temperature of the hydrogen evolution peaks and the bonding from infrared spectroscopy are correlated to the Urbach tail energy, the B factor of the Tauc plot and E04 (defined as the energy at which the absorption coefficient is equal to 104 cm−1). Silicon-rich and carbon-rich regions with very different properties, corresponding approximately to carbon fractions below and above 0.5, respectively, can be distinguished. The properties of the ECR a-SiC:H alloys are compared with those of alloys deposited by rf glow discharge.
Comparative study of the intra- and intervalley contributions to the free-carrier induced optical nonlinearity in n-GaAs
Authors: Shkerdin G. , Stiens J. , Vounckx R.
Published: July 2002

We performed a theoretical study about the optical nonlinearity connected with the redistribution of free electrons between the Γ and L valley of highly doped n-GaAs. An extended comparison was made between the intravalley (or nonparabolicity) and the intervalley contribution. In the energy relaxation process, intravalley transitions in the Γ and L valley by means of the emission of optical phonons and equivalent intervalley transitions in the L valley by means of intervalley phonons were considered as the main mechanism of energy transfer to the lattice. It is demonstrated that when the frequency of light matches with the plasma frequency of the n-doped GaAs the nonlinearity is quite big. At a doping concentration of 7×1018 cm−3 we calculated a nonlinear refractive index n2=1.74×10−8 cm2/W at 10.6 μm. The doping and intensity dependent energy relaxation times of electrons are calculated: for Γ-valley electrons τenΓ∼(3–5) ps and for L-valley electrons τenΓ∼(0.3–0.5) ps. It is believed that the response time of the free-carrier induced nonlinearity is determined by the decay time of longitudinal optical phonons (6–7 ps).
Nondestructive assessment of In0.48(Ga1−xAlx)0.52P films grown on GaAs (001) by low pressure metalorganic chemical vapor deposition
Authors: Feng Z. C. , Armour E. , Ferguson I. , Stall R. A. , Holden Todd , Malikova L. , Wan J. Z. , Pollak Fred H. , Pavlosky M.
Published: July 2002

The quaternary alloy In0.48(Ga1−xAlx)0.52P, lattice-matched to GaAs, has a direct band gap transition in the wavelength range of green-red light and is useful in optoelectronic applications such as visible light emitting diodes and laser diodes. We have investigated a set of six nominal In0.48(Ga1−xAlx)0.52P layers that were grown on GaAs (001) by low pressure metalorganic chemical vapor deposition (MOCVD) turbo disk technology. In order to control and optimize the growth conditions to produce high quality InGaAlP epilayers, a variety of nondestructive techniques, including photoluminescence (PL), Raman scattering, photoreflectance (PR), reflectance anisotropy (RA) spectroscopy, atomic force microscopy (AFM), and high resolution x-ray diffraction (HRXRD) have been applied to evaluate the epitaxial films and growth processes. HRXRD confirmed a good lattice match between the epilayers and the substrate material. The PL and PR spectra showed the variations of the InGaAlP PL peak and the energy band with growth pressure and other parameters. Raman spectral line shape analysis leads to information about the sample crystalline quality. Polarization dependent PR and RA spectroscopy were used to detect the in-plane anisotropy of epitaxial materials. AFM was used to study the surface morphology of these quaternary compounds and to nondestructively detect any possible dislocations in these hetero-epitaxial materials. The optimized parameters for the growth of high quality InGaAlP films on GaAs were obtained. The combination of these nondestructive techniques offers a better understanding of MOCVD-grown In0.48(Ga1−xAlx)0.52P/GaAs and a useful way to optimize the growth parameters of high quality quaternary semiconductor materials.
Near infrared excited micro-Raman spectra of 4:1 methanol–ethanol mixture and ruby fluorescence at high pressure
Authors: Wang X. B. , Shen Z. X. , Tang S. H. , Kuok M. H.
Published: July 2002

Near infrared (NIR) lasers, as a new excitation source for Raman spectroscopy, has shown its unique advantages and is being increasingly used for some special samples, such as those emitting strong fluorescence in the visible region. This article focuses on some issues related to high-pressure micro-Raman spectroscopy using NIR excitation source. The Raman spectra of 4:1 methanol–ethanol mixture (4:1 M–E) show a linear variation in both Raman shifts and linewidths under pressure up to 18 GPa. This result is useful in distinguishing Raman scattering of samples from that of the alcohol mixture, an extensively used pressure-transmitting medium. The R1 fluorescence in the red region induced by two-photon absorption of the NIR laser is strong enough to be used as pressure scale. The frequency and line width of the R1 lines are very sensitive to pressure change and the glass transition of the pressure medium. Our results manifest that it is reliable and convenient to use NIR induced two-photon excited fluorescence of ruby for both pressure calibration and distribution of pressure in the 4:1 M–E pressure transmitting medium.
Modeling of laser pulsed heating and quenching in optical data storage media
Authors: Volkert C. A. , Wuttig M.
Published: July 2002

In order for rewritable optical data storage to compete with other storage technologies, it is crucial to increase the data transfer rate and storage capacity. Near-field recording should enable considerable progress in these areas, however, currently available near-field light sources are limited by their low output powers. This puts severe constraints on the thermal properties of the phase change media. This article addresses how rewritable optical data storage media can meet data transfer rate and storage capacity requirements. In particular, the required laser power for writing and erasing submicron “bits” is calculated. It is shown using modeling how the power and the write and erase times depend on the structure and thermal properties of the media, as well as on the crystallization kinetics of the recording layer. Guidelines for media optimization and areas that presently pose the most serious limitations are presented.
A matrix formalism for the Hall effect in multicarrier semiconductor systems
Authors: Kim J. S.
Published: July 2002

A matrix formalism for the Hall effect is presented for an arbitrary J-fold multicarrier semiconductor system. Explicit formulas are derived for important transport quantities such as the sheet resistance, the Hall coefficient, and the Hall scattering factor. For J⩽3, these formulas reduce to simple closed-form expressions as a function of the applied magnetic field and the carrier densities and mobilities. The closed-form formulas are useful for the correct interpretation of the Hall effect in multicarrier semiconductor systems. The field dependence and asymptotic behavior of these formulas are also discussed.
Organic semiconductor heterointerfaces containing bathocuproine
Authors: Hill I. G. , Kahn A.
Published: July 2002

The four organic–organic heterointerfaces formed by depositing the organic semiconductor bathocuproine on tris(8-hydroxy-quinoline)aluminum (Alq3), N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′ biphenyl-4,4″ diamine (α-NPD), 4,4′-N,N′-dicarbazolyl-biphenyl (CBP), and copper phthalocyanine (CuPc) have been studied using ultraviolet photoelectron spectroscopy. The relative positions of the vacuum levels and highest occupied molecular orbital levels of the organics were measured at each interface. Within our experimental uncertainty of 100 meV, the vacuum levels were found to align at all four interfaces.
Phase transition behavior of BaTiO3 thin films using high-temperature x-ray diffraction
Authors: Li Chunling , Chen Zhenghao , Cui Dafu , Zhou Yueliang , Lu Huibin , Dong Cheng , Wu Fei , Chen Hong
Published: July 2002

Ferroelectric BaTiO3 thin films have been fabricated on SrTiO3(001) substrate using pulsed laser deposition. In order to investigate the phase transition behavior of the thin films, we have conducted high-temperature x-ray diffraction in the process of heating and cooling. Lattice constants, degree of c-axis orientation and crystal quality of the thin films were characterized as a function of temperature. The results show that the temperature of tetragonal-to-cubic phase transition for a BaTiO3 thin film is much higher than that of BaTiO3 bulk single crystal, and the phase transition of BaTiO3 thin film occurs in a wide temperature range (from 500 to 800 °C). The tetragonal-to-cubic phase transition for BaTiO3 thin film is irreversible.
Charge transport in a blue-emitting alternating block copolymer with a small spacer to conjugated segment length ratio
Authors: Ma Dongge , Hümmelgen I. A. , Jing Xiabin , Hong Zhiyong , Wang Lixiang , Zhao Xiaojiang , Wang Fosong , Karasz F. E.
Published: July 2002

We analyze current versus voltage data obtained using single carrier injection in several metal/polymer/metal sandwich structures. The polymer used in each case is a soluble blue-emitting alternating block copolymer. Our experimental results demonstrate that the electron transport is space-charge limited by the high density of traps having an exponential energy distribution (temperature dependent characteristic energy) in the copolymer. The electron mobility of 8×10−10 cm2/V s is directly determined using space-charge-limited current analytical expressions. Hole transport is also space-charge limited, with a mobility of 2×10−6 cm2/V s. A hole trap with energy 0.17 eV is observed. We compare these results with those obtained for related block copolymers with different spacer and conjugated segment lengths and discuss the influence of spacer length and conjugated segment length on the charge transport properties.
Composition dependence of the Hall effect in amorphous TbxCo1−x thin films
Authors: Kim T. W. , Gambino R. J.
Published: July 2002

The Hall effect in amorphous TbxCo1−x thin films has been studied quantitatively with respect to Tb at. % in films. Amorphous TbxCo1−x thin films exhibit unusually large spontaneous Hall coefficients (Rs≈2×10−9 Ω cm/G, whose sign is strongly dependent on the film composition as governed by the relation ρH=Rs4πMS. At Tb compositions below the compensation composition xcomp, the Hall resistivity ρH is positive and the sign is reversed for Tb compositions above xcomp. Even though the saturation magnetization at x>xcomp is lower than at x<xcomp, the absolute Hall resistivity |ρH| and Hall coefficient |Rs| are higher. As the Tb content increases in the films, the resistivity increases. In amorphous TbxCo1−x thin films, the Hall angle |ρH/ρ| is sensitive to the resistivity of the films. On the basis of electrical resistivities, Hall resistivities, and Hall angles in amorphous TbxCo1−x films, the side jump Δy of 0.16 to 0.26 Å was calculated.
Ion implanted dopants in GaN and AlN: Lattice sites, annealing behavior, and defect recovery
Authors: Ronning C. , Dalmer M. , Uhrmacher M. , Restle M. , Vetter U. , Ziegeler L. , Hofsäss H. , Gehrke T. , Järrendahl K. , Davis R. F. ,   ISOLDE Collaboration
Published: July 2002

The recovery of structural defects in gallium nitride (GaN) and aluminum nitride (AlN) after implantation of In+111 and Sr+89 in the dose range (0.1–3) 1013 cm−2 and ion energies of 60–400 keV has been investigated as a function of annealing temperature with emission channeling (EC) and perturbed γγ angular correlation spectroscopy. The implanted In and Sr atoms occupied substitutional sites in heavily perturbed surroundings of point defects after room temperature implantation. No amorphization of the lattice structure was observed. The point defects could be partly removed after annealing to 1473 K for 10–30 min. Lattice site occupation of implanted light alkalis, Na+24 in GaN and AlN as well as Li+8 in AlN, were also determined by EC as a function of implantation and annealing temperature. These atoms occupied mainly interstitial sites at room temperature. Lithium diffusion and the occupation of substitutional sites was observed in GaN and AlN at implantation temperatures above 700 K. A lattice site change was also observed for sodium in AlN, but not in GaN after annealing to 1073 K for 10 min.
Optical and electrical properties of nitrogen incorporated amorphous carbon films
Authors: Yu Y. H. , Chen Z. Y. , Luo E. Z. , Cheung W. Y. , Zhao J. P. , Wang X. , Xu J. B. , Wong S. P. , Wilson I. H.
Published: July 2002

Nitrogen incorporated amorphous carbon (a-C:N) films on silicon (111) wafer, quartz, and Ti/C substrates with nitrogen concentration up to 20 at. % are prepared by filtered arc deposition. The nitrogen concentration and area density of the films were measured by Rutherford backscattering. The electrical properties of the films were investigated by Hall electrical measurements. The optical properties of the films were characterized by ultraviolet–visible and infrared reflection spectrometry. Results indicate that the optical band gap and area density of a-C:N films decrease with increasing nitrogen pressure, accompanied with an increase of nitrogen concentration and reflectivity of the films. Furthermore, the influence of nitrogen concentration on the optical band gap of the films is discussed. The dielectric constant, refractive index and absorption coefficient of a-C:N films in infrared region were investigated. The results indicate that the optical constants of a-C:N show considerable variation with wave number and nitrogen content. The variation of optical properties and optical constants of a-C:N films may be due to the development of graphite-like structure with the increasing of nitrogen content in these films.
Surface superstructure on fullerenes annealed at elevated temperatures
Authors: An B. , Fukuyama S. , Yokogawa K. , Yoshimura M. , Korai Y. , Mochida I.
Published: July 2002

Scanning tunneling microscopy images of fullerenes annealed at elevated temperatures are investigated. Three types of ringlike superstructures of (∛×∛)R30° of graphite, the circular type, the U type, and the semicircular type, are observed on the curved surface of the fullerenes annealed in the temperature range from 2073 to 2673 K. A new modified interference model, in which both the α and β site atoms contribute to the tunneling current caused by the curved surface, is proposed. The model well simulates the three types of the ringlike superstructure of graphite and the continuous change from the triangular structure of graphite to the ringlike superstructure of graphite.
On the determination of the crystallization activation energy of metallic glasses
Authors: Khonik V. A. , Kitagawa K. , Morii H.
Published: July 2002

The dependence of the crystallization temperature on the heating rate was measured for five conventional melt spun metallic glasses. The results obtained, together with similar data taken from the literature, were analyzed using the Kissinger method. It is shown that application of this method to glass transition and crystallization of metallic glasses leads to unreasonably high apparent attempt frequencies, by many orders of magnitude above the Debye frequency. It is concluded, in accordance with some remarks available in the literature, that this method gives obscure values of the activation parameters of glass transition and crystallization of metallic glasses. A simple equation for approximate estimate of the crystallization onset activation energy of metallic glasses is proposed.
Aging of electrical conductivity in conducting polymer films based on polyaniline
Authors: Jousseaume V. , Morsli M. , Bonnet A.
Published: July 2002

The aim of this work is to present an electrical study of the aging of conducting polyaniline–polystyrene (PANl–PSt) blends. Results concerning time dependence of the electrical conductivity of some samples artificially aged at different temperatures are presented. It is shown that the electrical conductivity decreases with time with a rate depending principally on the nature of the dopant. When aged at the same temperature, similar degradation kinetics are observed as well for doped polyaniline as for PANl–PSt blend. A study of the electrical conductivity variations versus temperature is presented for samples which have been aged at the same temperature but for different durations. Experimental results are discussed in the frame of the heterogeneous picture already proposed for nonaged samples.
Emission channeling studies of Pr in GaN
Authors: Wahl U. , Vantomme A. , Langouche G. , Araújo J.P. , Peralta L. , Correia J. G. ,   ISOLDE Collaboration
Published: July 2002

We report on the lattice location of Pr in thin film, single-crystalline hexagonal GaN using the emission channeling technique. The angular distribution of β− particles emitted by the radioactive isotope Pr143 was monitored by a position-sensitive electron detector following 60 keV room temperature implantation of the precursor isotope Cs143 at a dose of 1×1013 cm−2 and annealing up to 900 °C. Our experiments provide direct evidence that Pr is thermally stable at substitutional Ga sites.
Molecular beam studies of the F+D2 and F+HD reactions
Authors: Neumark D. M. , Wodtke A. M. , Robinson G. N. , Hayden C. C. , Shobatake K. , Sparks R. K. , Schafer T. P. , Lee Y. T.
Published: July 2002

The F+D2 and F+HD reactions were investigated in a high resolution crossed molecular beams experiment at several collision energies. The DF product from both reactions was predominantly backward scattered although some forward scattered DF(v=4) was observed at the highest energy studied. The HF angular distributions from F+HD were quite different, showing considerable forward scattered (v=3) and no other identifiable structure. These results disagree with classical trajectory studies, which predict only small variations in the product angular distributions among F+H2 and its isotopic variants. They agree, however, with the predicted dependence of dynamical resonance effects on isotopic substitution. The results therefore support the conclusions drawn in the previous paper regarding the role of dynamical resonances in the F+H2 reaction.
MC test of perturbation theories based on site–site potentials
Authors: Abascal J. L. F. , Martín C. , Lombardero M. , Vazquez J. , Bañon A. , Santamaria J.
Published: July 2002

The extension of the Barker–Henderson (BH) theory to diatomic potentials is examined by comparison of the predicted thermodynamic properties with ‘‘exact’’ MC results. Two choices of the potential separation have been considered; corresponding to the BH scheme and that of Weeks–Chandler–Andersen (WCA). Attention has been paid to the effect of the usual approximations to the exact properties of the hard reference system (equation of state and pair correlation functions) and to the convergence rate of the expansion. Conclusions can also be drawn for the extension of the WCA-blip function theory to these systems. It is shown that the behavior of the perturbation expansions exhibits the same features observed in atomic fluids. Nevertheless, the presence of the molecular anisotropy leads to a rather different assessment of the relative usefulness of each theory. Incidentally, small inaccuracies of the well-known Boublik and Nezbeda equation for the compressibility factor of dumbbells have been discovered at very high densities (outside the range previously reported).
The shape of ring polymers
Authors: Bishop Marvin , Michels J. P. J.
Published: July 2002

Ring and linear polymers, with an without excluded volume, have been studied by Brownian dynamics. The shape of the polymers was investigated by calculating the principal orthogonal components of the radius of gyration. Ring polymers are found to be more spherical than linear ones. (AIP)
Calculated proton affinities for some molecules containing group VIA atoms
Authors: Jasien Paul G. , Stevens Walter J.
Published: July 2002

The proton affinities and structures of a series of small molecules containing group VIA atoms are calculated via ab initio electronic structure techniques. The series under study included CX, OCX, XCX, and H2CX, where X=O, S, Se, and Te. In those cases where multiple protonation sites are available, a definitive assignment of the most stable site is reported. Excellent agreement with the experimentally known proton affinities is found in almost all cases. The results indicate that the general trend which one would expect upon moving down a column of the periodic table is born out, with a particularly large change on going from the first to the second row. Calculations were performed at both the SCF and correlated levels with compact effective potentials used to replace the core electrons. Complete structural optimizations via analytic gradients were performed utilizing basis sets of at least double zeta plus polarization quality.
Corresponding states for thermal conductivities via nonequilibrium molecular dynamics
Authors: Grover Richard , Hoover William G. , Moran Bill
Published: July 2002

We follow Rosenfeld in comparing fluid-phase thermal conductivities for several simple pair potentials. Within about ten percent these (nonelectronic) conductivities satisfy a corresponding states relation involving the equilibrium entropy. This corresponding states relation, deduced directly from the results of computer simulations, is also suggested by hard-sphere perturbation theory and by the quasiharmonic cell-model approach. The conductivity-entropy relation should be useful for estimating transport coefficients from the equation of state of monatomic fluids with arbitrary pair potentials.
Effect of exchange reactions on transport processes: A comparison of thermodynamic treatment with random walk on lattice points
Authors: Ruff Imre , Botár László
Published: July 2002

The effect of an AX+A ⇄ A+AX exchange reaction on the transport of X is discussed for a general transport process driven by the gradient in the chemical potential of AX, as well as its special cases of isothermal diffusion and electric conductivity. This treatment based on macrodifferentials is compared with a stochastic treatment of random walk on regular lattice points in one-, two- , and three-dimensional cases. The basic equivalence is proved for the two methods which ensures the application of the latter, more simple, procedure with general validity. It is also shown that the stochastic treatment reveals some aspects that remain hidden in the thermodynamic approach.
Bottlenecks to intramolecular energy transfer and the calculation of relaxation rates
Authors: Davis Michael J.
Published: July 2002

A recently developed theory of MacKay, Meiss, and Percival [Physica D 13, 55 (1984)] and Bensimon and Kadanoff [Physica D 13, 82 (1984)] is applied to the intramolecular relaxation of highly excited, collinear OCS. This theory, which was originally developed to understand the long time relaxation of mappings, possesses many of the features of statistical theories of reactions. Bottlenecks, dividing surfaces, and transition states are all part of the theory, which we employ here to describe the relaxation dynamics of collinear OCS. In particular, the theory is used to find a bottleneck to intramolecular energy transfer, generate a dividing surface, derive the flux across this dividing surface, and then calculate the rate across it. A simple kinetic model then employs this rate to accurately predict the rate of relaxation for collinear OCS. At present the theory is purely classical mechanical, has only been applied to systems of two degrees of freedom, and has only been used to describe the dynamics of bound systems. We discuss extensions of the theory in these three areas.
The correlation length divergence of Tolman’s length
Authors: Phillips Philip , Mohanty Udayan
Published: July 2002

The curvature dependence of the surface tension of liquid drops is studied. A closed expression is derived for the first curvature correction, δ (Tolman’s length) to the surface tension. From this expression, we show that when phase interchange symmetry is absent, δ(1) vanishes identically if the density profile is sharp (a step function), (2) is nonzero otherwise, and (3) diverges as the correlation length. It is also shown that the curvature dependence of the surface tension for a finite-sized drop in an Ising model cannot be extracted from the flat interface.
Multiple d-type basis functions for molecules containing second row atoms
Authors: Magnusson Eric , Schaefer Henry F.
Published: July 2002

The importance of multiple sets of d functions on the energy of SO2 and other molecules of Si, P, S, and Cl has been tested by comparing total energies from the SWPS set (supplementation by five sets of d functions as recommended by Strömberg, Wahlgren, Pettersson, and Siegbahn) with those obtained with one or two sets of d functions. The energy improvement due to supplementation is very sensitive to the nature of the attached atoms and to valence shell expansion but the optimum d function exponents are not appreciably affected by either of these factors. When used in conjunction with Dunning–Hay basis sets for second row atoms, supplementation by a single set of d functions provides about 80% of the self-consistent field (SCF) energy lowering obtained with five sets; use of a second set generates about 15% more. These results are compared with the results of adding f or replacing nuclear-centered functions with bond functions. Varying d function exponents about the optimum values produces an energy surface with the same general shape for all ten molecules studied. Based on this uniformity within a limited sample of molecules containing second row elements, recommendations are presented for the inclusion of supplementary sets of d functions in basis sets and the level of energy improvement to be achieved by using them is estimated.
Effect of hydrogen bonding on the direction of the dipole-moment derivative of the O–H bond in the water molecule
Authors: Whalley Edward , Klug D. D.
Published: July 2002

The ratio of the integrated absorption intensities of the ν3 and ν1 vibrations of H2O vapor is 20.6 [J. M. Flaud and C. Camy-Peyret, J. Mol. Spectrosc. 55, 278 (1975)] and so the dipole-moment derivatives for the stretching of the two bonds independently are 25.2° outside the bonds themselves. The corresponding ratio for the uncoupled D2O molecule isolated in H2O low-density ice I at∼100 K and zero pressure has been measured as 1.77 and 1.73, and for D2O isolated in the low-density H2O amorph as 1.90, 2.72, and 1.94 from the spectra of Devlin and his colleagues. The derivatives of the water molecule in ice are, therefore, directed along the bonds within the experimental uncertainty. The ratio of the integrated intensities of the ν3 and ν1 vibrations of H2O ice VIII at −10 °C and 32 kbar is 1.95 [D. D. Klug and Edward Whalley (in preparation)], which implies that the dipole-moment derivative is directed ∼2° outside the bond. This spectacular change of direction of the dipole-moment derivative on condensation is a newly recognized effect of hydrogen bonding.
Structures and phase diagrams of N2 and CO to 13 GPa by x-ray diffraction
Authors: Mills R. L. , Olinger Bart , Cromer D. T.
Published: July 2002

The structures and phase transitions of N2 and CO were studied by powder x-ray diffraction from 100 to 300 K and 4 to 13 GPa. Three solid phases, β, δ, and ε, were observed in each material. The known β and δ solids were confirmed to have hexagonal space group P63/mmc and cubic space group Pm3n, respectively. From refinements using photographic x-ray intensities, the new ε-N2 and ε-CO structures were determined to be rhombohedral R3̄c. There are eight ordered molecules in the ε-N2 unit cell with a=5.928 Å and α=85.14° at 110 K and 7.8 GPa, and eight ordered molecules in the ε-CO unit cell with a=6.059 Å and α=85.73° at 100 K and 5.5 GPa. The CO molecules are randomly oriented head to tail. The δ–ε transition takes place through an ordering and small displacement of the N2 and CO molecules, accompanied by a slight extension of the lattice along a cube diagonal. Molar volumes are presented over an expanded P-T region. Recent theoretical calculations using lattice energies, molecular dynamics, and symmetry correlations correctly predict features in the N2 and CO phase diagrams.
Self-consistent integral equations for fluid pair distribution functions: Another attempt
Authors: Zerah Gilles , Hansen Jean-Pierre
Published: July 2002

We propose a new mixed integral equation for the pair distribution function of classical fluids, which interpolates continuously between the soft core mean spherical closure at short distances, and the hypernetted chain closure at large distances. Thermodynamic consistency between the virial and compressibility equations of state is achieved by varying a single parameter in a suitably chosen switching function. The new integral equation generalizes a recent suggestion by Rogers and Young to the case of realistic pair potentials containing an attractive part. When compared to available computer simulation data, the new equation is found to yield excellent results for the thermodynamics and pair structure of a wide variety of potential models (including atomic and ionic fluids and mixtures) over an extensive range of temperatures and densities. The equation can also be used to invert structural data to extract effective pair potentials, with reasonable success.
Stable highly excited vibrational eigenvalues without the variational principle
Authors: Chang Johnny , Moiseyev Nimrod , Wyatt Robert E.
Published: July 2002

The energy spectrum in the stochastic region for a model Hamiltonian of two strongly coupled modes is calculated by diagonalizing small matrices which do not provide the Hylleraas–Undheim–MacDonald variational energy upper bounds. In addition, a method for selecting the most important basis functions by artificial intelligence algorithms is utilized. The energy convergence is determined by the Hazi–Taylor stabilization method and by the nearest-neighbor-spacing distribution function which measures the local fluctuations in the spectrum.
Vibrational state distributions of H+2(v″) resulting from the electron transfer reactions H+2(v′=0, 1) +H2(v″=0)→H2(v′)+H+2(v″) in the collisional energy range of 2–16 eV
Authors: Liao C. L. , Ng C. Y.
Published: July 2002

The vibrational state distributions of product H+2(v″) resulting from the symmetric charge transfer reactions H+2(v′0=0 or 1) +H2(v″0=0) →H2(v′) +H+2(v″) in the center-of-mass collisional energy (Ec.m.) range of 2–16 eV have been measured by the charge exchange method. When reactant H+2 ions are prepared in v′0 =0, the majority (>80%) of product H+2 ions are formed in v″=0. The vibrational relaxation channel for forming H+2(v″=0) is found to be much more efficient than the vibrational excitation process for producing H+2(v″=2) in the H+2(v0=1) +H2(v″0=0) charge transfer collisions. The experiment also reveals that inelastic charge transfer channels become more important as Ec.m. is increased. The vibrational state distributions of product H+2(v″) determined at Ec.m. =8 and 16 eV are compared with results of the semiclassical energy conserving trajectory calculations of Lee and DePristo. A better agreement between experimental and theoretical results is observed at Ec.m. =16 eV, a collisional energy at which charge transfer is the overwhelming channel.
Boron quadrupole coupling in o
r
t
h
o-carborane. NMR study of an oriented liquid sample
Authors: Ruessink B. H. , MacLean C.
Published: July 2002

Line splittings observed in the 10B and 11B NMR spectra of ortho-carborane-d2, when a solution of the compound is exposed to a very strong electric field, have been used to derive the components of the boron quadrupole coupling tensors along the molecular dipole moment direction. Furthermore, the magnitudes of the boron quadrupole coupling constants were measured from the spin-lattice relaxation times (T1). The boron linewidths observed indicate the presence of a substantial scalar coupling contribution to T2 for both boron isotopes. The direction of the axis to be associated with the quadrupole coupling constant has been derived for some of the borons in o-carborane by combing the EFNMR results with those of the T1 study.
Statistical thermodynamics of particles adsorbed onto a spherical surface. I. Canonical ensemble
Authors: Post Albert J. , Glandt Eduardo D.
Published: July 2002

A system of equisized hard spheres adsorbed onto the surface of a substrate sphere of a different diameter is treated by application of the concepts of small-system thermodynamics. The various thermodynamic representations (or statistical mechanical ensembles) are not equivalent in such a case, and the Gibbs adsorption isotherm does not apply. For spaces of nonzero curvature, two different forms of the pressure equation are possible, depending on whether the interparticle separations are measured within the non-Euclidian space or in a Euclidian space of one higher dimensionality. The two forms of the third virial coefficient are reported, including finite size and finite volume corrections, for all particle sizes. The available data on closest packing of spheres around a central sphere is used together with the virial expansion to construct two-point approximants to the equation of state. The results are compared to canonical ensemble simulations of adsorbed hard spheres.
The transport properties of rod-like particles via molecular dynamics. I. Bulk fluid
Authors: Magda J. J. , Davis H. T. , Tirrell M.
Published: July 2002

Molecular dynamics studies of pure bulk fluids of hard, infinitely thin rods are reported. In the semidilute fluid above 70 rods/L3, the rotational diffusion coefficient scales with concentration, with an exponent approaching, but not reaching −2, the value predicted by the Doi–Edwards theory. We estimate the semidilute transition concentration to be at least 70 rods/L3, a concentration twice as large as previously reported from simulations of this rod fluid, but more consistent with recently published theoretical estimates. In the Doi–Edwards theory, a given rod executes a series of step rotations through a sequence of ‘‘cages’’ formed by neighboring rods. Using the simulation results, we estimate a value of τt, the average lifetime of a cage. As one would expect from the Doi–Edwards theory, we find that τt decreases sharply when we turn off the hard core repulsive interactions between the rods which comprise the cages. This result is in disagreement with a recently published Brownian dynamics study. In a previous molecular dynamics study of this model rod fluid, the translational self-diffusion coefficient was observed to increase with concentration above 30 rods/L3; an increase attributed in the theory of Frenkel and Maguire to the decreasing amount of rotational motion. Our results confirm the increase above 30 rods/L3. However, at higher concentrations the theory is inadequate; it underestimates the diffusivity and the lifetime of the rod velocity along its axis.
Far infrared laser magnetic resonance detection of NH and ND (a 1Δ)
Authors: Leopold K. R. , Evenson K. M. , Brown J. M.
Published: July 2002

Rotational spectra of the excited a 1Δ state of NH and ND have been observed by far infrared laser magnetic resonance spectroscopy. For ND(a 1Δ) the spectroscopic constants are B0=264 750.263(30) MHz, D0=14.383 83(91) MHz, aN =109.63(22) MHz, aD =11.03(23) MHz, eqQ(N)=−4.0(15) MHz, gr=−0.000 86(10), and gL =1.000 506(17). For NH (a 1Δ), the constants are B0=493 043.182(95) MHz, D0=50.453 MHz (constrained in fit), aN =109.65(85) MHz, aH =70.9(14) MHz, eqQ(N)=−4.0 MHz (constrained in fit), gr =−0.001 58(6), and gL =1.001 03 (constrained in fit). Aspects of the electronic structure of the radical as revealed by the magnetic hyperfine constants are discussed in relation to those of chemically similar systems. The Zeeman parameters are interpreted in terms of mixing of the a 1Δ state with the c 1Π state.
Block copolymers and polymer mixtures in dilute solution: General crossover analysis and comparison with Monte Carlo calculations
Authors: Douglas Jack F. , Freed Karl F.
Published: July 2002

Gell-Mann–Low style renormalization group is applied to the two-parameter type model of block copolymers and mixtures. These systems have multiple excluded volume interaction variables between like and unlike chemical species in addition to variables characterizing the lengths of the substituent blocks. This leads to a five parameter model for monodisperse binary polymer mixtures and diblock copolymers. We derive the full crossover dependence of dilute solution block copolymer and mixture properties on all five parameters by calculating effective exponents in the crossover region to second order in ε=4−d and the prefactors to order ε. The multiparameter renormalization group equation is solved to first order in ε, and a simplifying approximation is introduced to derive closed analytic forms charcterizing average polymer dimensions. Specific radial observables of block copolymers, such as the radius of gyration and the mean square end-to-end vector distance of di- and triblock copolymers as a whole and of the individual blocks, are evaluated in order to compare with Monte Carlo calculations in the avoiding block limit. An important component of this comparison involves the use of a physical reference state to replace the nonexistent theta state for blocks and mixtures. Good agreement is found between the Monte Carlo data and the renormalization group predictions. Moreover, the renormalization group calculations cover a much larger domain of the parameter space of variable excluded volume interaction and composition than explored in previous Monte Carlo studies. Our calculations also provide the basic input parameters for a renormalization group description of semidilute solutions of block copolymers and mixtures.
Full CI benchmark calculations for several states of the same symmetry
Authors: Bauschlicher Charles W. , Taylor Peter R.
Published: July 2002

Full CI (FCI) wave functions are used to compute energies for several electronic states of the same symmetry for SiH2, CH2, and CH+2. It is found that CASSCF/multireference CI (MRCI) wave functions yield results very similar to FCI, irrespective of whether the CASSCF MOs are optimized independently for each state or using an average of the CASSCF energies for all desired states. The ionization potentials (IPs) and excitation energies obtained from the FCI calculations should help calibrate methods such as Green’s function approaches, equations of motion and propagator methods, and cluster expansions, in which energy differences are computed directly.
Rational function representation for accurate exchange energy functionals
Authors: DePristo Andrew E. , Kress Joel D.
Published: July 2002

A representation of the exchange energy functional as the product of a homogeneous gas functional and a rational function approximation in the density gradient is developed and shown to describe accurately both the total exchange energy and the local exchange energy density of atomic systems. For the atoms H through Kr, the error in the total exchange energy is generally <0.02 hartree except for Cr through Zn where it is <0.21 hartree.
Electrolytic conduction in periodic arrays of insulators with charges
Authors: Kan Rui , Sen Pabitra N.
Published: July 2002

We compute dc electrical conductivity σ for a periodic array of charged cylinders immersed in an electrolyte in order to elucidate conduction in brine saturated porous medium containing clay particles. We extend a method due to Lord Rayleigh to charged systems and include diffusion currents. The key dimensionless parameter ξ=Ω+r+/(N0a) that represents the surface effects is the surface ion density Ω+ times the ratio r+ of the average diffusion coefficient in the double layer to that outside divided by the bulk density N0 times the particle radius a. We show that σ is a nonlinear function of ξ and hence of brine conductivity σw and thus provide the first explanation, resting on first principles, of this well known experimental fact. With a given geometry, when Ω+→0, or r+→0 or when N0≫1, the parameter ξ→0 and one obtains a linear dependence of σ on σw. The constant charge system have Ω+ constant, but when the surface potential ζ is constant, Ω+=2δN0eζ/2 varies with N0, δ being the screening distance.