bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of positron beam data by the combined use of the shape- and wing-parameters
Authors: Clement M. , de Nijs J. M. M. , Balk P. , Schut H. , van Veen A.
Published: July 2002

An improved approach is presented for the analysis of positron beam Doppler broadening data. Instead of analyzing the energy-dependent shape parameter, the so-called S(E) data, we combined the shape S(E) and wing W(E) data by plotting them as a trajectory in the S–W plane, using the implantation energy as a running parameter. It is shown that this plot is of particular interest for the qualitative interpretation of the data. Furthermore, it allows the independent determination of the characteristic shape and wing parameters of the different positron trapping layers without the use of a numerical simulation and fitting program. The method and its advantages and limitations are illustrated for three cases: a silicon sample implanted with helium, a metal–oxide–silicon system subjected to a bias voltage and a bare oxide layer on silicon.
Effect of copper on the microstructure and electromigration lifetime of Ti–AlCu–Ti fine lines in the presence of tungsten diffusion barriers
Authors: Domenicucci A. G. , Filippi R. G. , Choi K. W. , Hu C.-K. , Rodbell K. P.
Published: July 2002

A systematic study was performed of the microstructural and electromigration characteristics of Ti–Al(Cu)–Ti laminate structures fabricated from two metal wiring levels 1 μm in width. The total Cu content in the Al(Cu) core layers was varied from 0.5 to 2.0 wt %. A high degree of 〈111〉 texture was found for all Cu concentrations except for the 0.5 wt % film. Grain size statistics were found to be independent of the Cu concentration. The Al grains were supersaturated with Cu which led to shifts in resistance during low temperature baking and electromigration testing. The electromigration lifetime of stripes connected to large reservoirs of Cu and Al was found to be linearly dependent on the total Cu content, whereas there was a ‘‘roll off’’ in the lifetime of two-level W stud structures as the Cu content was increased. The activation energy for electromigration induced failure was found to be 0.78–0.93 eV. Resistance shifts during electromigration and temperature only stressing and the microstructural characteristics of failed structures were explained in terms of the distribution of Cu in the Al matrix and the geometry of the structures using a blocking boundary model.
Electrical and electro-optical properties of Ce-doped barium titanate thin films prepared by pulsed laser deposition
Authors: Liu Yanwei , Chen Zhenghao , Li Chunling , Cui Dafu , Zhou Yueliang , Yang Guozhen , Zhu Yong
Published: July 2002

Ce-doped BaTiO3 (BTO) films were grown by pulsed laser deposition. Highly c-axis-oriented BTO:Ce/YBa2Cu3O7−x bilayer structures on a SrTiO3 (100) substrate were fabricated for electrical property characterization. Typical hysteresis and butterfly type C–V plots were observed. The remanent polarization and coercive field are 3 μC/cm2 and 20 kV/cm, respectively. Highly a-axis-oriented Ce-doped and undoped BTO films were also deposited on MgO(100) substrates. Their linear and quadratic electro-optic effects were measured and compared. The Ce-doped films showed a larger linear electro-optic coefficient and obvious hysteresis in the quadratic electro-optic coefficient.
Noncollinear magnetic structure in Ni0.35Fe0.65
Authors: Wang Yang , Stocks G. M. , Nicholson D. M. C. , Shelton W. A. , Antropov V. P. , Harmon B. N.
Published: July 2002

Magnetic structure of NicFe1−c alloys in the INVAR region has long been a matter of great scientific interest and controversy. Using the locally self-consistent multiple scattering method, which has recently been extended to treat noncollinear magnetic systems, we studied the magnetic structure of Ni0.35Fe0.65 alloys. To simulate the alloys, we constructed a large fcc based sample which contains 256 sites occupied randomly by Ni and Fe atoms. The ground state magnetic structure is found to consist of noncollinear configurations associated with Fe-rich regions. In particular, Fe sites surrounded completely by other Fe atoms have antiferromagnetic alignments, while Fe sites having less than three Ni nearest-neighbors have a variety of noncollinear arrangements.
Microstructure and magnetic properties of Fe–Co alloys
Authors: Fingers R. T. , Kozlowski G.
Published: July 2002

Fe–Co soft magnetic alloys exhibit high magnetic saturation, high yield strength, and moderate core loss. Use of such materials in cyclic high temperature high stress environments, such as generators and magnetic bearings, gives impetus to determining material properties. In particular, Hiperco® Alloy 50HS, provided by Carpenter Technology Corporation, has been a subject of our study. In order to fully understand the overall behavior of the alloy, both mechanical and magnetic properties must be investigated. Magnetic performance is a function of grain size, which varies with the annealing process. Fe–Co samples have been treated by various annealing recipes ranging in temperature from 1300 to 1350 °F and magnetic saturation along with hysteresis loop measurements made using a vibrating sample magnetometer. An etching and sample preparation process was developed and microstructural analyses were performed. The correlation between composition, heat treatment, microstructure, and magnetic properties of these samples is discussed.
Magnetic film inductors for radio frequency applications
Authors: Korenivski V. , van Dover R. B.
Published: July 2002

Responding to the increasing demand for miniaturization and circuit integration of rf inductors we analyze various aspects of designing planar inductive circuits based on magnetic films as well as demonstrate substantially enhanced inductance characteristics for some prototype structures.
Stress relaxation in Si-rich silicon nitride thin films
Authors: Habermehl S.
Published: July 2002

Si-rich silicon nitride thin films have been deposited by low pressure chemical vapor deposition, at 850 °C from mixtures of dichlorosilane and ammonia. The films’ elastic properties have been studied as a function of film composition. Fourier transform infrared spectroscopy and ellipsometric data indicate that the local atomic strain is a strong function of the calculated volume fraction of Si contained in the films. A relationship is observed that shows the strain to be inversely proportional to the cube root of the Si volume fraction. A model that accounts for distortion in Si–SixN4−x tetrahedra (x=0–4), upon substitution of silicon for nitrogen in the film is applied to the data. The model is shown to be consistent with measurements of intrinsic film stress across a compositional range from stoichiometric silicon nitride, Si3N4, to nitrogen-free amorphous silicon, a-Si.
Underlying physics of the thermochemical E model in describing low-field time-dependent dielectric breakdown in SiO2 thin films
Authors: McPherson J. W. , Mogul H. C.
Published: July 2002

The underlying physics behind the success of the thermochemical E model in describing time-dependent dielectric breakdown (TDDB) in SiO2 thin films is presented. Weak bonding states can be broken by thermal means due to the strong dipolar coupling of intrinsic defect states with the local electric field in the dielectric. This dipole-field coupling serves to lower the activation energy required for thermal bond-breakage and accelerates the dielectric degradation process. A temperature-independent field acceleration parameter γ and a field-independent activation energy ΔH can result when different types of disturbed bonding states are mixed during TDDB testing of SiO2 thin films. While γ for each defect type alone has the expected 1/T dependence and ΔH shows a linear decrease with electric field, a nearly temperature-independent γ and a field-independent ΔH can result when two or more types of disturbed bonding states are mixed. The good agreement between long-term TDDB data and the thermochemical model suggest strongly that the oxygen vacancy is an important intrinsic defect for breakdown and that field, not current, is the primary cause of TDDB under low-field conditions.
Giant magnetoimpedance in a cylindrical magnetic conductor
Authors: Ménard D. , Britel M. , Ciureanu P. , Yelon A.
Published: July 2002

A rigorous treatment of the giant magnetoimpedance (GMI) in soft magnetic wires is presented. A small-signal approximation is used for a cylindrical magnetic conductor which is saturated along its axis by a static magnetic field. The general analysis of GMI includes a discussion of the influence of different parameters on the GMI and of how the calculation can be extended to nonsaturating fields. The comparison with high frequency impedance spectra of CoFeSiB wires measured with a network analyzer, including the observation of the ferromagnetic resonance peaks, confirms that the proposed model gives a satisfactory explanation for the linear GMI effect over a broad frequency range and opens the way to more refined calculations.
Optical functions of silicon at high temperatures
Authors: Šik J. , Hora J. , Humlı́ček J.
Published: July 2002

The optical spectra of silicon are studied ellipsometrically at temperatures between 300 and 1200 K in the spectral range from 2 to 4.3 eV. We present a database of optical constants for pure and heavily doped samples, with the highest concentration of free carriers of 4×1020 cm−3. We cover the photon energy range between 0.2 and 2 eV by using an oscillator representation of the visible and ultraviolet ellipsometric data. We also report the results for the temperature dependence of the E1 interband transition. The values of the energy shift and Lorentzian broadening of this spectral feature are obtained from analytical critical-point line shapes by fitting differentiated dielectric functions; alternatively, we apply the numerical convolution with Lorentzian contours to describe quantitatively the increased broadening with increasing temperature.
Oscillator strength of excitons in (In, Ga)As/GaAs quantum wells in the presence of a large electric field
Authors: Monier C. , Freundlich A. , Vilela M. F.
Published: July 2002

The oscillator strength of the fundamental heavy-hole exciton in strained quantum well (QW) InxGa1−xAs/GaAs p–i–n diode heterostructures is calculated by using a variational approach combined with the transfer matrix formalism. Unlike the weak well thickness dependence of the excitonic properties in the absence of electric field, a completely different picture is observed as the strength of the built-in electric field increases. A dramatic reduction of the QW oscillator strength is noticed for thick wells over the entire indium composition range. This is thought to be induced by a very effective quantum confined Stark effect that becomes a limiting factor for maximized absorption properties. In contrast, calculations for highly strained (x>0.20) thin (less than 50 Å) structures evidence oscillator strength values as large as those found with zero electric field. Finally, the results of this study stress adequate well thickness/composition selection for improved performance of optoelectronic multiple QW-based devices such as modulators and photoconverters.
In-plane magnetic anisotropies in Fe films on vicinal Ag(001) and Au(001) surfaces
Authors: Leeb T. , Brockmann M. , Bensch F. , Miethaner S. , Bayreuther G.
Published: July 2002

In-plane fourfold and uniaxial magnetic anisotropies were studied in Fe films epitaxially grown on vicinal Ag(001) and Au(001) surfaces, which were prepared by molecular beam epitaxy on miscut GaAs(001) substrates. The effective fourfold and uniaxial anisotropy constants, K1eff and Kueff, which are determined from magnetisation curves measured with the magneto-optic Kerr effect, are linear functions of the inverse Fe layer thickness. The fourfold anisotropy shows a rotation of the easy and hard axes by 45° below a critical thickness of 6–7 ML. The uniaxial term is mainly an interface contribution. We find that the orientation of the uniaxial easy axis depends on the Fe thickness. In thinner films (tFe⩽20 ML) it is oriented perpendicular to the step edges, i.e., parallel to Fe[100]. This excludes shape anisotropy as the main mechanism. Above a critical thickness the uniaxial easy axis is aligned parallel to the step edges, i.e., along Fe[010]. This step-induced uniaxial anisotropy may be due to modified electronic states and to strain from the large vertical misfit at the steps.
Near-noble transition-metal-based ohmic contacts to p-InP: Comparison of Ni and Pd as a base metal
Authors: Yamaguchi Akira , Asamizu Hirokuni , Okada Takeshi , Iguchi Yasuhiro , Saitoh Tadashi , Koide Yasuo , Murakami Masanori
Published: July 2002

The electrical properties were compared for near-noble transition-metal-based ohmic contacts with a M/Zn/M (M=Ni, Pd) sandwiched structure prepared on p-type InP. The minimum contact resistivities of ∼7×10−5 Ω cm2 were obtained in both the NiZn and PdZn contacts, while the process window for the annealing time of the PdZn contacts was much wider than that of the NiZn contacts. From x-ray diffraction and cross-sectional electron microscopic study at the metal/InP interfaces, it was found that ternary compounds such as Ni2.7InP and Pd2InP were formed at the Ni/InP and Pd/InP interfaces, respectively. However, the Ni2.7InP compounds were not stable at annealing temperatures above 300 °C, which is the primary reason of poor thermal stability of the NiZn contacts, while the Pd2InP compound was stable at such high temperatures. Selection of a suitable base metal for InP ohmic contacts was also discussed.
Magnetic phase diagram of Fe and Ni from crystals to amorphous structures
Authors: Al-Attar H. , Kakehashi Y.
Published: July 2002

Magnetic properties of Fe and Ni have been investigated on the basis of the theory of metallic magnetism which interpolates between crystals and amorphous structures using the average coordination number z* and the fluctuation of interatomic distance Δ. Calculated magnetic phase diagram of Fe is shown to consist of three phases on the z*−Δ plane: the ferromagnetism (F) in the region z*≲10.5, the spin glass (SG) in the region z*≳10.5, and the paramagnetism (P) around z*=11.0 and Δ=0.0. In the region F, the magnetization and the Curie temperature are weakened with increasing z*, while they are considerably enhanced with increasing Δ. The phase diagram explains the ferromagnetism of amorphous Fe powder and the SG in amorphous Fe containing early transition metals. In the case of Ni, F and P are found. In the region F the magnetization first decreases and tends to become constant with increasing Δ. The Curie temperature also decreases but shows a minimum as a function of Δ. Both of them are found to be smaller than those of fcc Ni, and to be sensitive to the degree of structural disorder. This explains qualitatively the nonunique magnetism of amorphous Ni in experiment.
Modeling of the sheath and the energy distribution of ions bombarding rf-biased substrates in high density plasma reactors and comparison to experimental measurements
Authors: Edelberg Erik A. , Aydil Eray S.
Published: July 2002

In plasma etching and deposition processes, the energy distribution of ions incident onto the substrate strongly affects the surface reactions and the film deposition and etching rates. The magnitude and frequency of the rf-bias power applied to the substrate electrode determines the spatiotemporal variations of the sheath potentials and hence the energy distribution of the ions impinging upon the substrate. A self-consistent dynamic model of the sheath, capable of predicting ion energy distributions impinging on a rf-biased electrode, was developed. The model consists of equations describing the charge transport in the sheath coupled to an equivalent circuit model of the sheath to predict the spatiotemporal charge and potential distributions near the surface. Experimental measurements of the energy distributions of ions impinging on a rf-biased electrostatic chuck have also been made in a high density transformer coupled plasma reactor through Ar and Ne plasmas. The predicted ion energy distributions and sheath profiles are in very good agreement with the experimental measurements.
Barrier height enhancement of Ag/n-GaAs and Ag/n-InP Schottky diodes prepared by P2S5/(NH4)2Sx and HF treatments
Authors: Jeng Ming-Jer , Wang Hung-Tsung , Chang Liann-Be , Cheng Yi-Chang , Chou Shu-Tsun
Published: July 2002

A method for surface passivation using both the phosphorus sulfide/ammonia sulfide [P2S5/(NH4)2Sx] solution and hydrogen fluoride (HF) solution has shown great effectiveness on the barrier height enhancement of Ag/n-GaAs and Ag/n-InP Schottky diodes. It is found that, even though the Ag/n-GaAs and Ag/n-InP diodes were baked for 18 h at 300 °C, their Schottky barriers could still reach as high as 1.1 and 0.95 eV, respectively. After the bare semiconductor substrates were bathed successively in P2S5/(NH4)2Sx and HF solutions and then shone by an ultraviolet light, the analysis with x-ray photoelectron spectroscopy indicates a possible formation of ultrathin and stable sulfur fluoride or phosphorus fluoride layers on the substrate surfaces. The formation of these stable interface layers has been attributed to the enhancement of Schottky barrier heights.
Electrical and structural characteristics of lanthanum-doped barium titanate ceramics
Authors: Morrison Finlay D. , Sinclair Derek C. , West Anthony R.
Published: July 2002

Single phase La-doped BaTiO3 with the formula Ba1−xLaxTi1−x/4O3: 0⩽x⩽0.20 was prepared by solid state reaction of oxide mixtures at 1350 °C, 3 days, in O2. The tetragonal distortion in undoped BaTiO3 decreased with x and samples were cubic for x⩾0.05. Both the tetragonal/cubic and orthorhombic/tetragonal transition temperatures decreased with x, but at different rates, and appeared to coalesce at x∼0.08. The value of the permittivity maximum at the tetragonal/cubic phase transition in ceramic samples increased from ∼10 000 for x=0 at 130 °C to ∼25 000 for x=0.06 at ∼−9 °C. At larger x, the permittivity maximum broadened, showed “relaxor”-type frequency dependent permittivity characteristics and continued to move to lower temperatures. Samples fired in O2 were insulating and showed no signs of donor doping whereas air-fired samples were semiconducting, attributable to oxygen loss.
Two dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures
Authors: Ambacher O. , Foutz B. , Smart J. , Shealy J. R. , Weimann N. G. , Chu K. , Murphy M. , Sierakowski A. J. , Schaff W. J. , Eastman L. F. , Dimitrov R. , Mitchell A. , Stutzmann M.
Published: July 2002

Two dimensional electron gases in AlxGa1−xN/GaN based heterostructures, suitable for high electron mobility transistors, are induced by strong polarization effects. The sheet carrier concentration and the confinement of the two dimensional electron gases located close to the AlGaN/GaN interface are sensitive to a large number of different physical properties such as polarity, alloy composition, strain, thickness, and doping of the AlGaN barrier. We have investigated these physical properties for undoped and silicon doped transistor structures by a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance–voltage profiling measurements. The polarization induced sheet charge bound at the AlGaN/GaN interfaces was calculated from different sets of piezoelectric constants available in the literature. The sheet carrier concentration induced by polarization charges was determined self-consistently from a coupled Schrödinger and Poisson equation solver for pseudomorphically and partially relaxed barriers with different alloy compositions. By comparison of theoretical and experimental results, we demonstrate that the formation of two dimensional electron gases in undoped and doped AlGaN/GaN structures rely both on piezoelectric and spontaneous polarization induced effects. In addition, mechanisms reducing the sheet carrier concentrations like nonabrupt interfaces, dislocations, and the possible influence of surface states on the two dimensional electron gases will be discussed briefly.
Effect of Fe and Zr ion implantation and high-current electron irradiation treatment on chemical and mechanical properties of Ti–V–Al Alloy
Authors: Pogrebnjak Alexander D. , Kobzev Alexander P. , Gritsenko Boris P. , Sokolov Sergey , Bazyl Elena , Sviridenko Nikolai V. , Valyaev Alexander N. , Ivanov Yurii F.
Published: July 2002

Using Rutherford backscattering spectroscopy, nuclear elastic resonance analysis, atomic force microscopy, transmission electron microscopy, and wear resistance and microhardness tests, the alloy Ti41–V41–Al18 was investigated after Fe ion (60 kV) and Zr (40 kV) ion implantation and subsequent high-current electron beam (HCEB) irradiation at an energy flow density of 6 J/cm2, called duplex treatment. Profiles show that the maximum concentration of Fe ions was 16.5 at. % at 85 nm from sample surface and that of Zr ions was 0.85 at. % at 56 nm. The maximum of the Fe concentration profile was found to shift to the large sample depth toward after increasing the implantation dose. The surface alloy layer is composed of a number of structures: grains of dislocation substructure (2×1010 cm−2), grains with plates, and grains with packed martensite. The disorientation of regions is observed. After HCEB treatment, the disorientation of microregions increases (Δα=7.5°) and particles of Ti2Fe are formed. After double implantation, the dry friction wear and the friction coefficient decreased. After the duplex treatment, the thickness of the hardened layer and the wear resistance increased.
Single-crystal magnetic studies of cylindrite (FePb3Sn4Sb2S14)
Authors: Salyer Pamela A. , ter Haar Leonard W.
Published: July 2002

The mineral cylindrite (nominal composition FePb3Sn4Sb2S14) is a layered chalcogenide with an unusual cylindrical morphology that occupies an important role in our search for new magnetic materials. This work capitalizes on cylindrite’s Fe content as a unique opportunity to further understand its structure and properties through magnetism. Magnetic measurements have been carried out with individual cylinders oriented both parallel (∥) and perpendicular (⊥) to the applied magnetic field (Happ).
Simulation of one-dimensional ring quantum interference transistors using the time-dependent finite-difference beam propagation method
Authors: Heller E. K. , Jain F. C.
Published: July 2002

A time-dependent finite-difference beam propagation method is presented to analyze quantum interference transistor (QUIT) structures, employing the Aharonov–Bohm effect, in both steady state and transient conditions. Current–voltage characteristics of two ring structures having 0.2 and 0.05 μm channel lengths, respectively, are presented. Additionally, the wave functions are calculated, and reflections are observed in both the ON and OFF states of the device. Cutoff frequency fT values of 3 and 8.5 THz, respectively, are calculated from the switching response to a gate pulse of 200 fs, for the 0.2 μm device, and to a pulse of 50 fs, for the 0.05 μm device. Results indicate that reflections at the drain may degrade frequency performance of these devices, which is not evident from earlier analytical studies. These structures are further explored to investigate the effects of imperfections introduced in fabricating the quantum wire channels. We compare two QUITs, one realized by a 1 nm resolution lithography process (representing an advanced fabrication technique) and the other realized by a 10 nm resolution (representing current state-of-the-art lithography). We also present an asymmetric 10 nm resolution structure, to represent the case when errors in fabrication significantly alter the QUIT topology. This simulation shows strong dependence of the electron transmission probability on the channel topology and roughness determined by the lithographic resolution.
Electric and dielectric properties of Bi12TiO20 single crystals
Authors: Lanfredi S. , Carvalho J. F. , Hernandes A. C.
Published: July 2002

The frequency dependence of the electrical properties of the Bi12TiO20 single crystal was investigated by impedance spectroscopy. The data were presented in the Nyquist diagram form, from which electrical resistivity was determined. Values ranged from 1.93×105 and 1.07×103 Ω cm in the temperature range of 400–700 °C. The electrical conductivity followed the Arrhenius law, with an activation energy of 0.99 eV. The dielectric behavior was investigated from room temperature to 700 °C. Permittivity was calculated based on the relaxation frequency, using an alternative approach based on the variation of the imaginary impedance component as a function of reciprocal angular frequency. The permittivity values obtained by means of these two methods showed good agreement up to 600 °C. The frequency dependence of real and imaginary permittivities from room temperature to 700 °C was also investigated.
Modeling of band-to-band tunneling transitions during drift in Monte Carlo transport simulations
Authors: Ruden P. Paul , Bellotti Enrico , Nilsson Hans-Erik , Brennan Kevin F.
Published: July 2002

The conventional method of semiconductor charge carrier transport investigations using full band ensemble Monte Carlo simulations is extended to allow for tunneling between bands during accelerated drift of the carriers. The essentially classical picture of transport, as simulated, is preserved by implementing a stochastic selection of the band index of the initial state of each scattering process associated with phonons, with impurities, or with impact ionization. Relative probabilities for the band assignment are calculated from the overlap integrals of the cell-periodic parts of Bloch wave functions belonging to different bands, for k-vectors along the carrier k-space trajectory between successive scattering events. As an example, the method is applied to Monte Carlo transport simulations for holes in 4H SiC in a homogeneous applied electric field. Tunneling between valence bands during the drift phases is shown to have a significant impact on the carrier energy distributions when large electric fields are applied, and on physical parameters that directly depend on the carrier energy, such as the hole initiated impact ionization coefficient.
A strategy for time dependent quantum mechanical calculations using a Gaussian wave packet representation of the wave function
Authors: Sawada Shin-Ichi , Heather Robert , Jackson Bret , Metiu Horia
Published: July 2002

We develop methodology for performing time dependent quantum mechanical calculations by representing the wave function as a sum of Gaussian wave packets (GWP) each characterized by a set of parameters such as width, position, momentum, and phase. The problem of computing the time evolution of the wave function is thus reduced to that of finding the time evolution of the parameters in the Gaussians. This parameter motion is determined by minimizing the error made by replacing the exact wave function in the time dependent Schrödinger equation with its Gaussian representation approximant. This leads to first order differential equations for the time dependence of the parameters, and those describing the packet position and the momentum of each packet have some resemblance with the classical equations of motion. The paper studies numerically the strategy needed to achieve the best GWP representation of time dependent processes. The issues discussed are the representation of the initial wave function, the numerical stability and the solution of the differential equations giving the evolution of the parameters, and the analysis of the final wave function. Extensive comparisons are made with an approximate method which assumes that the Gaussians are independent and their width is smaller than the length scale over which the potential changes. This approximation greatly simplifies the calculations and has the advantage of a greater resemblance to classical mechanics, thus being more intuitive. We find, however, that its range of applications is limited to problems involving localized degrees of freedom that participate in the dynamic process for a very short time. Finally we give particular attention to the notion that the GWP representation of the wave function reduces the dynamics of one quantum degree of freedom to that of a set of pseudoparticles (each represented by one packet) moving according to a pseudoclassical (i.e., classical-like) mechanics whose ‘‘phase space’’ is described by a position and momentum as well as a complex phase and width.
Molecular dynamics investigation of the crystal–fluid interface. VI. Excess surface free energies of crystal–liquid systems
Authors: Broughton J. Q. , Gilmer G. H.
Published: July 2002

We present the first direct calculation by simulation of the excess surface free energy of a crystal–liquid interface. We perform these calculations on the (111), (100), and (110) interfaces of a truncated Lennard-Jones face-centered-cubic crystal–liquid system at the triple point by the molecular dynamics technique. Bulk crystal and liquid systems are first cleaved and then combined with one another reversibly. The work required to do this is integrated and equals the excess surface Helmholtz (and Gibbs) free energy. The free energies are found to be 0.35±0.02, 0.34±0.02, and 0.36±0.02 (dimensionless units) for the (111), (100), and (110) faces, respectively. The three faces are therefore energetically isotropic within our error bars and the equilibrium form of the crystal is approximately spherical.
Application of an InGaAsP diode laser to probe photodissociation dynamics: I* quantum yields from n- and i-C3F7I and CH3I by laser gain vs absorption spectroscopy
Authors: Hess Wayne P. , Kohler Susan J. , Haugen Harold K. , Leone Stephen R.
Published: July 2002

A room temperature heterostructure InGaAsP laser diode operating at 1315 nm is employed for the first time for detection of I*(2P1/2) and I(2P3/2) atoms. The cw diode probe laser is used to study I* yields in the photodissociation of n- and i-C3F7I and CH3I by the new technique of time-resolved laser gain vs absorption spectroscopy. Preliminary quantum yields determined at 266 nm for n-C3F7I, i-C3F7I, and CH3I are 102±4%, 102±7%, and 73±4%, respectively. With further refinements to the diode laser set-up, highly accurate quantum yields will be possible.
Dependence of ion pairing and solvation on solution temperature: Evidence from DMSO solution and matrix-solvation spectra
Authors: Wooldridge Robert , Fisher Mark , Ritzhaupt Gary , Devlin J. Paul
Published: July 2002

Infrared and Raman Spectra for alkali-metal nitrate solutions in dimethylsulfoxide (DMSO) have been measured as a function of concentration (0.5 to 2.0 M) and temperature (298 to 400 K) to determine the influence of temperature on structural changes that favor the incorporation of the nitrate anion into the solvation shell of the M+ cations. It is apparent from the spectra observed at elevated temperatures that this contact pairing of the ions in liquid DMSO occurs endothermically. It is also shown, by invoking the known spectra of the solvates (DMSO)3⋅LiNO3 and (DMSO)4⋅LiNO3 that are available from low temperature matrix-solvation measurements, that partial desolvation of the contact-ion-pair solvates occurs endothermically over this temperature range. These observations are interpreted as evidence that the energetics for the structural changes that lead to anion incorporation into the cation inner coordination sphere are dominated by the endothermic partial desolvation of the ions (in particular the cation). The DMSO solvates of the contact ion pair dominate the Raman spectra at 400 K for all of the alkali metal nitrates provided the concentration exceeds 1.0 M. In the case of LiNO3, polarization Raman data indicate that the cation-anion binding for (DMSO)3⋅LiNO3 is bidentate while that for (DMSO)4⋅LiNO3 is monodentate. This switch in cation–anion binding has been anticipated from considerations of steric repulsive forces in the inner coordination shell of the Li+ cation but has proved difficult to document.
Benzonitrile and its van der Waals complexes studied in a free jet. I. The LIF spectra and the structure
Authors: Kobayashi Tohru , Honma Kenji , Kajimoto Okitsugu , Tsuchiya Soji
Published: July 2002

The van der Waals (vdW) complexes consisting of benzonitrile and various partner species were formed in a free jet and their laser-induced fluorescence (LIF) spectra were recorded. For all the species chosen as partners (Ar, Kr, N2O, CF3H, and H2O), the LIF spectra showed a red shift relative to that of benzonitrile monomer. The spectral shift increased with increasing dipole moment of the partner species owing to the large dipole–dipole interaction between the partner species and benzonitrile whose dipole moment amounts to 4.14 D. With the aid of computer simulation, the rotational contours of the LIF spectra of the benzonitrile dimer and benzonitrile–Ar complex were analyzed. The dimer was found to be in planar form with the two CN groups facing each other in an antiparallel geometry, whereas in the Ar complex the Ar atom lies over the benzene ring slightly leaning toward the CN group.
Cluster perturbation theory for interaction site fluids
Authors: Lupkowski M. , Monson P. A.
Published: July 2002

We describe a method for determining the contributions to the structure and thermodynamics of interaction site fluids arising from long-ranged perturbations to the site–site potential. An extension of cluster perturbation theories developed for atomic fluids (optimized cluster theory, Γ ordering) to the interaction site cluster expansion is the basis of the new theory. Given the pair distribution function and the Helmholtz free energy of the reference system, the theory predicts the contribution to the structure and Helmholtz free energy arising from a perturbation to the potential, and contains no nonphysical contributions such as those arising in previous theories for these systems. Various levels of approximation are possible within the theory. We discuss the lowest level in detail. This is the extension of the optimized random phase approximation to the interaction site formalism (ISF-ORPA). The relationship with integral equation theories for these systems is established.
The density of supercooled water. II. Bulk samples cooled to the homogeneous nucleation limit
Authors: Hare D. E. , Sorensen C. M.
Published: July 2002

We have measured the density of supercooled water (H2O) in the range −33.41≤T≤−5.23 °C. Our samples were held in glass capillary tubing with an approximate inside diameter of 0.3 mm=300 μ. These samples were prepared by Mossop’s method and could be cooled to their homogeneous nucleation limit before freezing. We compare our density data to other measurements using capillaries and demonstrate what appears to be an excess density in smaller capillaries which is inversely proportional to the capillary inside diameter. The origins of this excess density are unknown, but we show its effect is insignificant on our measurement. The thermal expansivities derived from our data are fit to a power law in temperature relative to a singular temperature. These results are inconclusive due to a poor knowledge of the background expansivity.
Pressure broadening of rotational bands. II. Water vapor from 300 to 1100 cm−1
Authors: Rosenkranz P. W.
Published: July 2002

A statistical theory, with a simple model for intermolecular potentials, is used to calculate the absorption of infrared waves by the far wings of water vapor rotational transitions. The model is similar to ones that have been used to explain the second virial coefficient. The effectiveness of self-broadening relative to broadening by nitrogen is found to increase with frequency displacement from the center of the rotational band and to decrease with temperature, in agreement with the observed characteristics of water vapor.
Fermi resonances and local modes in water, hydrogen sulfide, and hydrogen selenide
Authors: Halonen Lauri , Carrington Tucker
Published: July 2002

A simple vibrational curvilinear internal coordinate Hamiltonian for bent H2X molecules is constracted by expanding the g matrix elements and the potential energy function in terms of the Morse variable y=1−exp(−ar) and retaining important local mode and Fermi resonance terms. The eigenvalues of this Hamiltonian are calculated variationally using Morse oscillator basis functions for the stretches and harmonic oscillator basis functions for the bend. The nonlinear least-squares method is used to optimize the potential energy parameters. The model is applied to water, hydrogen sulfide, and hydrogen selenide. Experimental vibrational levels up to 18 500 cm−1 for five symmetrical isotopic species of water are reproduced with a standard deviation of about 4 cm−1. For both hydrogen sulfide and hydrogen selenide two symmetrical isotopic species were included in the optimization procedure and standard deviations of 1.0 and 0.66 cm−1 were obtained. The potential energy parameters obtained agree well with previous anharmonic force field calculations.
Linear response calculations for large scale multiconfiguration self-consistent field wave functions
Authors: Jo/rgensen Poul , Jensen Hans Jo/rgen Aagaard , Olsen Jeppe
Published: July 2002

It is shown that large scale MCSCF linear response (MCLR) calculations can be carried out efficiently using an iterative algorithm where the linear transformations are carried out directly, i.e., without explicitly constructing the MCLR matrices. Calculations are presented on H2O of frequency dependent polarizabilities with configuration spaces containing up to 128 283 determinants.
Application of unimolecular reaction rate theory for highly flexible transition states to the dissociation of NCNO into NC and NO
Authors: Klippenstein S. J. , Khundkar L. R. , Zewail A. H. , Marcus R. A.
Published: July 2002

A recently described method for implementing RRKM theory for unimolecular reactions with highly flexible transition states is applied to the calculation of energy and angular momentum resolved rate constants and rotational–vibrational energy distributions for the reaction NCNO→hν NCNO*→NCNO(vib. hot)→NC+NO. The dissociation rate results are compared to the recent experimental results of Khundkar et al., and the vibrational and rotational distribution results are compared to the experimental values of Nadler et al. Comparison is also made with phase space theory calculations. The calculated rotational distributions at energies below the vibrational threshold of the products are the same as those of PST. At energies (2348, 2875 cm−1) above this threshold energy the rovibrational distribution is in better agreement with the data than is that of PST. The need for obtaining more accurate ab initio potential energy surfaces is noted, particularly for treating reactions at still higher energies.
Absorption spectroscopy of mass-selected ions in neon matrices
Authors: Forney Daniel , Jakobi Michael , Maier John P.
Published: July 2002

A new approach for the spectroscopic characterization of mass-selected ions is demonstrated. In this a stream of ions coming from a quadrupole mass selector are codeposited with neon to form a 5 K matrix and the absorption spectrum is measured. The potential of this technique is illustrated by the observation of the electronic absorption spectra of C+2, C2N+2, and C2Cl+2, in turn, after mass selection.
Molecular integrals with Slater basis. I. General approach
Authors: Fernández Rico J. , López R. , Ramírez G.
Published: July 2002

A method for the calculation of many-center integrals involving Slater-type orbitals (STOs) is reported. The method is based on the separation of the short- and long-range terms of the potentials generated by charge distributions. The calculation of three-center nuclear attraction integrals and many-center electron repulsion integrals, when the short-range contributions are negligible, is formulated in terms of multipolar moments of charge distributions. Recurrence relations for obtaining the multipolar moments, which enable us to reduce their calculations to the evaluation of some basic integrals, are reported.
Molecular dynamics simulation of real-fluid mutual diffusion coefficients with the Lennard-Jones potential model
Authors: Stoker J. M. , Rowley R. L.
Published: July 2002

Mutual diffusion coefficients for selected alkanes in carbon tetrachloride were calculated using molecular dynamics and Lennard-Jones (LJ) potentials. Use of effective spherical LJ parameters is desirable when possible for two reasons: (i) computer time is saved due to the simplicity of the model and (ii) the number of parameters in the model is kept to a minimum. Results of this study indicate that mutual diffusivity is particularly sensitive to the molecular size cross parameter, σ12, and that the commonly used Lorentz–Berthelot rules are inadequate for mixtures in which the component structures differ significantly. Good agreement between simulated and experimental mutual diffusivities is obtained with a combining rule for σ12 which better represents these asymmetric mixtures using pure component LJ parameters obtained from self-diffusion coefficient data. The effect of alkane chain length on the mutual diffusion coefficient is correctly predicted. While the effects of alkane branching upon the diffusion coefficient are comparable in size to the uncertainty of these calculations, the qualitative trend due to branching is also correctly predicted by the MD results.
A three-dimensional quantum mechanical study of the H2+H+2 system: Calculation of reactive and charge transfer cross sections
Authors: Baer Michael , Ng Cheuk Y.
Published: July 2002

A three-dimensional quantum mechanical study of the H2+H+2 system is presented. The numerical study was carried out on the two lowest adiabatic surfaces, to permit charge transfer processes. All seven possible reactive arrangement channels were assumed to take part in the exchange process. The treatment was carried out within the framework of the rotational decoupling infinite order sudden approximation (IOSA). The reactive cross sections were compared both with surface hopping calculations and with experiment. Whereas the fit with experiment was reasonable, a less satisfactory fit was obtained with the semiclassical results. The analysis leads to the conclusion that the main source for the discrepancy are the nonadiabatic coupling terms which are included in the quantum mechanical treatment but ignored in the semiclassical treatment.
A dynamic reaction coordinate approach to a
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o reaction pathways: Application to the 1,5 hexadiene Cope rearrangement
Authors: Maluendes S. A. , Dupuis M.
Published: July 2002

A modified dynamic reaction coordinate algorithm for tracing reaction paths is implemented in the framework of ab initio molecular orbital calculations. This method requires fewer energy and gradient evaluations than the traditional intrinsic reaction coordinate methodology and produces reaction pathways of acceptable accuracy. The approach is applied to the 1,5 hexadiene Cope rearrangement for which we trace the pathways passing through the chair and boat transition states. Analysis of the lowest energy pathway indicates that the rearrangement is concerted and synchronous.
A transition state theory-based statistical distribution of unimolecular decay rates with application to unimolecular decomposition of formaldehyde
Authors: Miller William H. , Hernandez Rigoberto , Moore C. Bradley , Polik William F.
Published: July 2002

A statistical distribution of state-specific unimolecular decay rates is derived (within the framework of random matrix theory) that is determined completely by the transition state properties of the potential energy surface. It includes the standard χ-square distributions as a special case. Model calculations are presented to show the extent to which it can differ from the χ-square distribution, and specific application is made to the state-specific unimolecular decay rate data for D2CO→D2+CO.
The dipole moment of water. I. Dipole moments and hyperfine properties of H2O and HDO in the ground and excited vibrational states
Authors: Shostak Shelley L. , Ebenstein William L. , Muenter J. S.
Published: July 2002

Molecular beam electric resonance spectroscopy has been used to study H2O in its ground state and in each of its singly excited vibrational states. HDO was studied in its ground state and first excited O–H stretching state. Precise dipole moments have been obtained for a total of 14 rotational levels of six vibrational states. Centrifugal distortion corrections to the Stark coefficients were made and rotationless moments were calculated for each vibrational state. The magnitude of the H2O moment can be written μ(v1v2v3) =1.857 04+0.005 08(v1+ (1)/(2) ) −0.031 66(v2+ (1)/(2) ) +0.022 46(v3+ (1)/(2) ). Individual components of the HDO moment were obtained for the ground and excited states. Hyperfine properties were also determined for each of the vibrational states studied. In the following paper, the dipole moments are combined with infrared transition moments to obtain an improved dipole moment function for water.
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o potential energy surfaces and trajectory studies of A-band photodissociation dynamics: CH3I*→CH3+I and CH3+I*
Authors: Amatatsu Yoshiaki , Morokuma Keiji , Yabushita Satoshi
Published: July 2002

A b initio contracted spin–orbit configuration interaction (SOCI) calculations have been carried out to obtain potential energy surfaces of 3Q0 and 1Q1 excited states of methyl iodide as functions of all the geometrical parameters except for the three C–H stretches. The results are fitted to six-dimensional diabatic potential functions and their couplings. Classical trajectory calculations have been performed using these potential functions. The rotation of the CH3 product in the I channel has been calculated to be perpendicular to the top axis and to have a peak at N=5 and extend up to N=8, whereas it is cold in the I* channel, in good agreement with recent experiments. The CH3 rotation is excited by the time trajectories arrive at the conical intersection region; this excitation is retained in the I-channel product because the 1Q1 surface has a small bending force constant outside the conical intersection, whereas it is damped in the I* channel because 3Q0 still has a large bending force constant. The calculated distribution in the ν2 umbrella vibrational mode of the CH3 product is hot and has a peak at v=2 for the I channel, and is cool for the I* channel, in good agreement with recent experiments. This channel selectivity is due to the difference in the preferred structure of the CH3 group outside the conical intersection region; while the 3Q0 surface prefers a bent CH3 until the CH3–I distance becomes very large, 1Q1 wants a planar CH3. The location of conical intersection and the ground-excited energy difference there are in good agreement with those deduced from experiment if a dynamical effect is taken into account.
Identification of defect chain motions in the low temperature orthorhombic phase of binary mixtures of n-alkanes by means of nuclear magnetic resonance spin-lattice relaxation time measurements
Authors: Basson I. , Reynhardt E. C.
Published: July 2002

Powder x-ray diffraction, differential scanning calorimetry, and nuclear magnetic resonance spin-lattice relaxation time measurements on a series of n-alkane binary mixtures are reported. The results have been interpreted in terms of a model in which the longer chains execute thermally activated end-gauche defect motions, while the shorter chains are involved in twofold screw motions. The results are compared with infrared and x-ray diffraction results.