bluuebunny/crossref_metadata_2025 huggingface dataset:
Confined electronic states in CdTe/Cd0.9Mn0.1Te superlattices: A resonance Raman study
Authors: Lockwood D. J. , Syme R. W. G. , Dubowski J. J. , Deleporte E.
Published: July 2002

Inelastic light scattering from first- and second-order longitudinal optic phonons using infrared excitation near the direct gap (1.6 eV) of CdTe quantum wells in CdTe/Cd0.9Mn0.1Te superlattices grown on (001) Cd0.95Zn0.05Te has revealed higher lying confined electronic states. The energies of the observed excitations compare well with predictions of a three-band Kronig–Penney model including the effects of strain and the nonparabolicity of the conduction band.
Comparison of infrared, Raman, photoluminescence, and x-ray photoelectron spectroscopy for characterizing arc-jet-deposited diamond films
Authors: Haque M. S. , Naseem H. A. , Shultz J. L. , Brown W. D. , Lal S. , Gangopadhyay S.
Published: July 2002

Impurities and growth-related defect structures are mainly responsible for low thermal conductivity of chemical vapor deposited diamond films. Different quality arc-jet-deposited, free-standing diamond samples were obtained from industry. Fourier transform infrared (FTIR), Raman, and x-ray photoelectron spectroscopy (XPS) were used to determine the quality of these samples. The nondiamond carbon was estimated from the 1560 cm−1 broad peak intensity, the CHx integrated peak absorbance, and the C1s plasmon loss features for Raman, FTIR, and XPS studies, respectively. The diamond quality was also determined from the Raman diamond peak full width at half maximum (FWHM) and XPS valence band spectra. It was observed that the higher the hydrogen content (determined by FTIR), the darker the color of the film, the larger the nondiamond 1560 cm−1 peak intensity, and the larger the FWHM of the Raman diamond peak at 1332 cm−1. Negligible difference in the C1s diamond bulk plasmon loss peak was observed for films of wide ranging quality. The FTIR CHx band exhibited the highest sensitivity to film quality. Impurity-related peaks were observed in the one phonon region of the FTIR spectra and the photoluminescence spectra. The photoluminescence background peak centered at 2.0 eV was found to be strongly related to nondiamond carbon impurities. It is shown that a combination of different analytical tools is required to determine diamond quality.
Threshold dependence on pixel size and cell thickness in a multidomain twisted nematic liquid crystal display
Authors: Qiu Chen , Zheng Chunyang , Gao Hongjin , Ou-Yang Zhong-can
Published: July 2002

On the assumption of an infinite liquid crystal-to-substrate anchoring anisotropy with zero pretilt angle, the threshold voltage in a multidomain twisted nematic liquid crystal display (MTN-LCD), where the number of domains of each pixel is 4, is investigated analytically as a function of the pixel size and cell thickness. It is found that under some critical ratio of the pixel size to the cell thickness the threshold voltage can be strongly modulated by the ratio, i.e., the MTN-LCD exhibits threshold instability.
Photonic effects in the deactivation of ion implanted arsenic
Authors: Fair R. B. , Li S.
Published: July 2002

Observation has been made of a photonic enhancement effect during optical rapid thermal annealing (RTA) of high dose, As implants in Si. Arsenic implant activation using optical radiation heating from tungsten–halogen lamps, λpeak=0.8 μm, was compared with annealing in a continuously heated rapid thermal vertical furnace, λpeak=2 μm. Energetic photons assist in the rapid deactivation of electrically active As+ to its equilibrium value in less than 5 s at the annealing temperature of 1000 °C. A model is presented for rapid As deactivation which is based upon the lowering of reaction energies through electron/hole recombination events. Transient-enhanced diffusion is observed in the RTA sample but not in the furnace-annealed sample. Rapid deactivation by recombination-enhanced processes causes the generation of excess self-interstitials of sufficient concentrations to contribute to the growth of end-of-range dislocation loops at the original amorphous/crystalline interface. After deactivation ends and the excess self-interstitials dissipate, the loops are able to dissolve and coarsen, thereby emitting self-interstitials which cause As transient-enhanced diffusion (TED) in the lower concentration portions of the RTA-annealed As profile. By contrast, end-of-range loops in samples similarly annealed in the vertical furnace are somewhat smaller but of higher density, and a 1000 °C, 15 s anneal results in As deactivation overshoot where [As+] drops below the electrical solubility limit. No As TED was observed in this sample due to the absorption of excess self-interstitials by the growing loops over the duration of the anneal.
Influence of interfacial hydrogen on Al thin film nucleation on Si
Authors: Adams D. P. , Mayer T. M. , Swartzentruber B. S.
Published: July 2002

We use scanning tunneling microscopy to study the coverage-dependent effects of hydrogen on Al thin film nucleation on Si(100). Using a quench-and-look procedure we find that small amounts of H affect Al island structure. Specifically, Al films deposited onto Si having 0.15 monolayer of adsorbed monohydride show an increased island density and a preponderance of small (⩽4 atom) clusters, compared with growth on clean Si. We interpret this to be a result of reduced Al adatom diffusion lengths due to site blocking or trapping. Also, the effects of a full monolayer of hydrogen on Al thin film morphology are studied. Al deposited onto Si(100) terminated with a layer of monohydride forms three-dimensional islands, unlike growth on clean Si. Although a change in growth morphology is observed in thin Al films, the out-of-plane crystal texture of thick Al films (300–1000 Å) is unaffected by a monolayer of interfacial hydrogen. Hydrogen is not present at a Si–Al interface after thick film growth at 100 °C on fully passivated substrates.
Physical origin and characteristics of gate capacitance in silicon metal-oxide-semiconductor field-effect transistors
Authors: Nakajima Yasuyuki , Horiguchi Seiji , Shoji Masanari , Omura Yasuhisa
Published: July 2002

The physical origin of gate capacitance in both bulk and silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFETs) is studied. The gate capacitance is theoretically derived as the series capacitance comprising the oxide capacitance, number capacitance (CN), level capacitance (Clevel), and field capacitance (Cfield). CN is in proportion to the thermodynamic density of states under the hypothetical condition that the subband energy levels remain constant when the total electron density is differentiated by the Fermi level. Clevel is inversely proportional to the result of differentiating the subband energy level by the total electron density. In the case of bulk MOSFETs or SOI MOSFETs with thick buried oxide, Cfield, which originates from the electric field at the edge of the depletion layer, is negligible. In addition to the fact that our new theoretical model corresponds to the intuitive conventional model expressed in terms of the effective thickness of the inversion layer in bulk MOSFETs, it is demonstrated that the conventional capacitance model is also applicable to SOI MOSFETs at 300 K. We have calculated self-consistently the capacitance of a bulk MOSFET and of SOI MOSFETs with various top-silicon layer thicknesses at 300 K. At the fixed total electron density near 1012 cm−2, the gate capacitance becomes large with decreasing top-silicon layer thickness. This difference in the gate capacitance mainly comes from Clevel, which means that in the case of SOI MOSFETs with a thin top-silicon layer, a small increase in the subband energy level with increasing electron density results in large gate capacitance and high performance of the SOI MOSFETs at 300 K.
Particle acceleration by laser fields in a dielectric disk-loaded waveguide
Authors: Steinhauer L. C. , Romea R. D. , Kimura W. D. , Fontana J. R.
Published: July 2002

A new method is proposed for accelerating relativistic charged particle beams in a vacuum by a laser. The laser propagates in an overmoded waveguide interrupted periodically by thin dielectric disks spaced many wavelengths apart. The particle beam travels along the waveguide axis, passing through irises in the disks. The disks correct for slippage of the particle phase relative to the laser wave. This concept exploits the inverse of familiar radiation processes (transition radiation, diffraction radiation). Several practical accelerator issues are incorporated in a systems analysis. Acceleration gradients in the GeV/m range are projected using lasers with ∼100 GW power. This represents more than an order of magnitude stronger coupling of the laser energy compared to other laser acceleration schemes.
Local order of polymer derived amorphous SixCyNz
Authors: Uhlig H. , Frieß M. , Lamparter P. , Steeb S.
Published: July 2002

We report on the local order of a-Si39C61, a-Si40C24N36, and a-Si26C40N34. The total structure factors and pair correlation functions obtained by x-ray diffraction as well as the coordination numbers of the first coordination sphere are presented. All materials exhibit a distinct short range order. In the case of a-Si39C61, a stoichiometric amorphous SiC phase besides an amorphous carbon phase is formed. Also in the case of a-Si40C24N36 and a-Si26C40N34 phase separation into two amorphous phases is observed.
Lateral periodicity and elastic stress relaxation in GaInAsP quantum wires on InP investigated by x-ray diffractometry
Authors: Jenichen B. , Grahn H. T. , Kojima T. , Arai S.
Published: July 2002

Synchrotron x-ray diffractometry has been used to investigate GaInAsP quantum wire structures on InP with a quantum well layer between the substrate and the wire. The lateral periodicity was determined with high accuracy. An elastic stress relaxation, which occurs near the free surface of the sidewalls, was observed. It results in deformation gradients in the wires, which influence the distribution of the diffracted intensity in reciprocal space.
Current–voltage characteristics of the partially Ga-terminated Si (111) surface studied by scanning tunneling microscopy
Authors: Kusumi Yukihiro , Fujita Ken , Ichikawa Masakazu
Published: July 2002

We investigated the current–voltage (I–V) characteristics of the Si (111) surface partially terminated by Ga atoms by using scanning tunneling microscopy. On the surface, Si (111) 7×7 and Si (111) √3×√3 Ga terraces alternated. The I–V curves of the 7×7 terraces exhibited semiconductive features, not metallic. The √3×√3 Ga terraces on the surface had narrower surface band gaps than usual √3×√3 Ga surfaces. These features could be explained by taking into account the adatom replacement between Ga and Si adatom sites. An amorphous Si layer was deposited on the surface after Sb was selectively adsorbed on the 7×7 terraces. The alternate structure was preserved after recrystallization of the Si layer.
Microstructure and dielectric properties of silicon nitride films deposited by electron cyclotron resonance plasma chemical vapor deposition
Authors: Ye Chao , Ning Zhaoyuan , Shen Mingrong , Cheng Shanhua , Gan Zhaoqiang
Published: July 2002

The microstructures and dielectric properties of silicon nitride (SiNx) films have been investigated. These films were prepared by permanent magnet electron cyclotron resonance plasma chemical vapor deposition by varying the substrate temperature between 56 and 400 °C and microwave power between 65 and 520 W. It exhibits a fractal structure with fractal dimension Df=1.45 or fractal-like structures for the films deposited without additional heating. When the substrate temperature is raised above 200 °C, the uniform dense structure can be obtained. The frequency dependence of the dielectric constant (ε′) in the frequency range 5–106 Hz for the films deposited below 100 °C follows a three fractional power law of (ε′−ε∞′)∝ωni−l (i=1, 2, and 3). The exponents ni are n1=0.73–0.93, n2=0.95–0.98, and n3=0.87–0.99 in the frequency range of 5–102 Hz, 102–104 Hz, and 104–106 Hz, respectively. The frequency dependence of ε′ also follows a single power law of (ε′−ε∞′)∝ωn−1 with n=0.95–0.82 for the films deposited in the substrate temperature range of 200–400 °C. The dielectric property of the single power law is in agreement with the intrinsic dielectric property of Si3N4 films, which is related to electron hopping conduction. The dielectric property of the fractional power law is considered to relate to the fractal structure of SiNx films. These are partially in agreement with that of the many-cluster anomalous conduction theory of fractal structure.
Influence of electric potentials on atom interferometers: Increased rotation sensitivity
Authors: Özcan Meriç
Published: July 2002

An atomic interferometer for rotation sensing is proposed. The arms of the interferometer are enclosed in a Faraday cage, and when a potential is applied to the cage, the resultant phase difference in the counterpropagating atom de Broglie waves due to rotation can be much larger than the Sagnac phase shift. This effect, being quantum mechanical in nature, could be used to increase the rotation sensitivity of atomic interferometers.
Theoretical modeling of the photoluminescence spectra associated with free-carrier to acceptor-impurity recombination in quantum-size GaAs wire crystals
Authors: Duque C. A. , Porras-Montenegro N. , Latgé A. , Oliveira L. E
Published: July 2002

A theoretical study of the photoluminescence spectra associated with free-carrier to acceptor-impurity recombination in quantum-size GaAs wire crystals is presented. We model the actual GaAs quantum wires via GaAs low-dimensional systems with finite cylindrical shape and hard-wall boundaries. The acceptor states are described within a variational scheme in the effective-mass approximation. The photoluminescence spectra associated with free carriers recombining with acceptor states are calculated for a homogeneous distribution of acceptor impurities within the low-dimensional heterostructure. We found that for a system whose length is comparable to the radius the photoluminescence line shape shows three structures associated with impurities located at special regions in the heterostructure, whereas for a system whose length is much larger than the radius we obtain two well-defined peaks, associated with acceptors either at the center position or the edge position in the low-dimensional system. Both the peak positions and relative intensities are in good agreement with spectral features present in experimental data on GaAs quantum-wire microcrystals.
Alternating current magnetic properties of cores made from pressed acicular steel particles
Authors: Krause R. F. , Bularzik J. H. , Kokal H. R.
Published: July 2002

This article describes results of magnetic testing of cores made from a composite material that yields powder metallurgically pressed compacts of high density suitable for a variety of ac magnetic applications. Small acicular steel particles are annealed, individually insulated, and uniaxially pressed using standard powder metallurgical techniques. The ac losses and permeability of the pressed compacts are strongly dependent upon particle geometry. The variations of the core loss among the different particle geometries was found to be a function of the cross sectional area of the acicular particles as well as the strain imparted to the particles during the pressing operation. The core loss, when measured at 60 Hz, exhibits a minimum at a certain cross sectional area. The shape of the curve is attributed to a decrease in eddy current loss with decreasing particle cross sectional area, while the hysteresis loss increases with decreasing particle cross sectional area. At a test frequency of 400 Hz, the eddy current component of core loss predominates, and the core loss decreases steadily with decreasing particle cross sectional area. Permeability was also found to depend upon particle geometry. The smaller the demagnetizing factor of the individual particles, the higher the permeability, while the increased strain in the smaller particles overwhelms the smaller demagnetizing factor resulting in a decrease in permeability. The net result is a maximum in permeability for the mid-sized particles.
Exact renormalization group equation for systems of arbitrary symmetry free of redundant operators
Authors: Lisyansky A. A. , Nicolaides D.
Published: July 2002

A generalized exact renormalization group (RG) equation for the Ginzburg–Landau–Wilson functional of an arbitrary symmetry, containing invariants of all orders, with nonlocal vertices is derived. Unlike other RG equations which contain an infinite number of redundant operators, the equation derived is free of them.
Evidence for an extended critical region near the metamagnetic transition of UCoAl
Authors: Kolomiets A. V. , Havela L. , Sechovský V. , DeLong L. E. , Watkins D. B. , Andreev A. V.
Published: July 2002

Electrical resistivity and ac magnetic susceptibility studies of the 5f-metamagnet UCoAl reveal the presence of strong spin fluctuation effects for temperatures below 30 K and magnetic fields below the critical metamagnetic field μ0Hc≈1 T, where a non-Fermi liquid term ∼T3/2 is prominent in the electrical resistivity. A step-wise increase of the residual resistivity and an increasingly prominent Fermi liquid T2 term of the resistivity is observed as the field is increased though the metamagnetic transition.
Structural and magnetic properties of FexSey thin films during their selenization process
Authors: Takahashi T. , Kuno S. , Honda N. , Takemura Y. , Kakuno K. , Saito K.
Published: July 2002

Fe x Se y films were prepared on GaAs(001) substrates by a selenization of Fe films using molecular beam epitaxy equipment. Structural and magnetic properties of FexSey thin films during their selenization process were studied. The selenized films obtained consisted of polycrystalline grains of 100–700 nm. A magnetic anisotropy of in-plane/perpendicular to the films was weakened by increasing the selenization ratio of the samples, which was interesting in contrast to the fact that the grain size of the films became larger.
New phase boundary between magnetic and non-Fermi-liquid in Ce(Rh1−xRux)3B2, for 0⩽x⩽0.4
Authors: Bauer E. , Hauser R. , Galatanu A. , Lindbaum A. , Hilscher G. , Sassik H. , Kirchmayr H. , Sereni J. G. , Rogl P.
Published: July 2002

A study of the temperature-dependent magnetic susceptibility and electrical resistivity ρ(T) (0.5–300 K) on single-phase alloys (CaCu5 type), prepared by argon arc melting, reveals a magnetic phase transition with a nonmonotonous decrease of the ordering temperature from Tc=115 K for x=0 to Tc≈0 for x=0.40. A kink in the susceptibility at about 70 K indicates that ferromagnetism (at x<0.1) transforms to a complex magnetic order for 0.125⩽x⩽0.35. Above that concentration ρ(T)=ρ0+ATn changes from n≈2 to n≈1.5, a characteristic for non-Fermi-liquid behavior. Under pressure Tc stays almost constant for x=0.125 but dTc/dp grows with increasing Ru content. On the contrary, applied fields up to 12 T do not affect Tc. Low-temperature specific heat and ac susceptibility for x=0.40 confirm the absence of long-range magnetic order down to 0.5 K. For x=0.35 Cp/T=62 mJ/mol K at 1.5 K, a value which is three times larger than that of CeRh3B2.
Kerr effect enhancement by photon tunneling and possible application to a new scanning probe magnetic microscope
Authors: Kikitsu A. , Falco C. M. , Mansuripur M.
Published: July 2002

Magneto-optical effects are calculated for the film stack consisting of hemisphere glass/magnetic film (10 nm)/air gap (d nm)/glass plate. Polarized light (wave length=800 nm) is irradiated through the hemisphere glass in the total internal reflection configuration. A typical amorphous rare earth-transition metal alloy is used for the magnetic layer. We find a large monotonic change in the figure of merit (product of the reflected amplitude of light and the Kerr rotation angle) as a function of the air gap, ranging from 1 to 800 nm. Similar results are obtained for a magnetic film with a 10 nm SiO2 protective layer and for a 1-nm-thin magnetic film. This phenomenon is mostly caused by a change in the reflectivity at magnetic film/air interface due to photon tunneling. The difference in the figure of merit between perpendicular and longitudinal magnetization is about 0.6°. These results imply that it might be possible to obtain an image of perpendicular magnetic moment with photon scanning tunnel microscopy (STM). This method can be combined simultaneously with a conventional atomic force microscope or STM.
The role of the tunneling component in the current–voltage characteristics of metal-GaN Schottky diodes
Authors: Yu L. S. , Liu Q. Z. , Xing Q. J. , Qiao D. J. , Lau S. S. , Redwing J.
Published: July 2002

The temperature dependence of the current–voltage characteristics of Ni–GaN Schottky barriers have been measured and analyzed. It was found that the enhanced tunneling component in the transport current of metal-GaN Schottky barrier contacts is a likely explanation for the large scatter in the measured Richardson constant.
Formation of induced anisotropy in amorphous Sm–Fe based thin films by field sputtering
Authors: Lim S. H. , Han S. H. , Kim H. J. , Song S. H. , Lee D.
Published: July 2002

Induced anisotropy with a large energy of 6×104 J/m3 is formed in an amorphous Sm–Fe based thin film by sputtering under an applied magnetic field of 500–600 Oe. The induced anisotropy results in a large anisotropy in magnetostriction, a strain anisotropy ratio reaching as high as 35, although intrinsic magnetostriction is affected only slightly. The large strain anisotropy allows one to realize a large strain in a particular direction and, hence, it is of significant practical importance. Induced anisotropy is also found to be formed by postannealing under applied magnetic field, but the magnitude of anisotropy energy formed is very small.
Single-chip computers with microelectromechanical systems-based magnetic memory (invited)
Authors: Carley L. Richard , Bain James A. , Fedder Gary K. , Greve David W. , Guillou David F. , Lu Michael S. C. , Mukherjee Tamal , Santhanam Suresh , Abelmann Leon , Min Seungook
Published: July 2002

This article describes an approach for implementing a complete computer system (CPU, RAM, I/O, and nonvolatile mass memory) on a single integrated-circuit substrate (a chip)—hence, the name “single-chip computer.” The approach presented combines advances in the field of microelectromechanical systems (MEMS) and micromagnetics with traditional low-cost very-large-scale integrated circuit style parallel lithographic manufacturing. The primary barrier to the creation of a computer on a chip is the incorporation of a high-capacity [many gigabytes (GB)] re-writable nonvolatile memory (in today’s terminology, a disk drive) into an integrated circuit (IC) manufacturing process. This article presents the following design example: a MEMS-based magnetic memory that can store over 2 GB of data in 2 cm2 of die area and whose fabrication is compatible with a standard IC manufacturing process.
Magnetic and magneto-optical properties in TbFeCo/(Pt, Pd) multilayers
Authors: Itoh Y. , Suzuki T.
Published: July 2002

The magnetic and magneto-optical properties of TbFeCo/(Pt, Pd) multilayer films prepared by sputter deposition are discussed in conjunction with their multilayer structure. The effect of polarized Pt on the polar Kerr spectra is significant at ultraviolet photon energies (∼5 eV). Based on the magnetic and magneto-optical properties, it is quite reasonable to surmise that an alloy is formed at the interface in real multilayers. It is the most obvious evidence that the saturation magnetization drastically increases with Pt and Pd thickness up to 12 Å, which indicates that the interfacial regions are essentially due to Fe–Pt and Fe–Pd alloys. This possibility was supported by the simulation of Kerr spectrum for the proposed model based on the layered-structural analyses.
A multipole interaction theory of electric polarization of atomic and molecular assemblies
Authors: Applequist Jon
Published: July 2002

The polarization of an assembly of polarizable units in the presence of an arbitrary external electric field and the fields of the permanent and induced electric multipole moments of the units is treated by the method of Cartesian polytensors, which formally include the contributions of multipoles of all orders. The linear polarizabilities of the system of all orders and the electrostatic energy of the system are derived in terms of a set of relay polytensors, which express the multipole moments of all orders induced in each unit by the external electric field gradients of all orders at each other unit. The relay polytensors are calculated by a matrix method from the inherent multipole polarizabilities and positions of the units. The net charges on the units are allowed to be unconstrained, constrained to a fixed net charge of the system, or fixed to permanent values. General-order multipole polarizabilities are derived for two types of bodies which are treated as units in assemblies for numerical calculations: (i) homogeneous dielectric and conducting spheres, which are shown to bear a close analogy to atoms and ions; and (ii) polyatomic molecules in which the atoms are characterized by isotropic point dipole polarizabilities. Numerical examples illustrate the importance of higher order induced multipole effects in ion–atom pair energies, anisotropies of molecular pairs, and optical rotations of simple chiral assemblies.
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o calculations of low-energy electron scattering by HCN molecules: Dependence on internuclear distance in linear geometry
Authors: Jain Ashok , Norcross D. W.
Published: July 2002

Low-energy electron scattering with HCN molecules is studied in the SEP (static-exchange plus parameter-free polarization potential) model as a function of both bond (CH and CN) stretches. A 2Π resonance at the equilibrium geometry behaves very similarly to the CO 2Π resonance as the CN bond is stretched; the corresponding HCN−(2Π) potential surface seems to cross the neutral curve (HCN, 1Σ+) around 2.9 a.u. of CN distance. In 2Σ symmetry, where no shape resonance is present at equilibrium geometry, a broad resonance appears when the CH or CN bonds are stretched well beyond the equilibrium position; the former appears to cross the 1Σ+ curve at about 2.8 a.u., i.e., just below the H+CN− asymptote, the latter to approach the neutral curve much more slowly and tangentially. Structure interpreted as a Π resonance in vibrational excitation, and as necessarily of Σ character in dissociative attachment to the lowest anion–neutral asymptote, can be explained as due to the mixing of the lowest linear 2Π and 2Σ resonances through bending (the Renner–Teller effect) and the peculiarity (cusp behavior) associated with anion–neutral curve crossings in polar molecules.
Translational and rotational disorder in solid n-alkanes: Constant temperature–constant pressure molecular dynamics calculations using infinitely long flexible chains
Authors: Ryckaert Jean-Paul , Klein Michael L.
Published: July 2002

Molecular dynamics calculations are used to study the effect of temperature on the interchain packing in solid n-alkanes. The model used consists of infinite chains initially arranged in a centered orthorhombic structure with the lateral packing found in the low temperature phase of n-alkanes with an odd number of carbon atoms. An atom–atom interchain potential is employed and the chains have flexible backbones. The problem of equilibrating the inter- and intramolecular motions is overcome using the technique of massive stochastic collisions proposed by Andersen. The calculated isobaric lateral thermal expansion of the orthohombic a and b cell parameters is in excellent agreement with experimental data over a wide temperature range. For T> 250 K translational (jump) diffusion is observed along the chain axis (c direction) without any accompanying rigid body chain rotation. At higher temperatures, the diffusion becomes liquid- like and rotational diffusion sets in between four well defined sites. Possible implications of our results for understanding of real alkanes are explored.
Application of a folding transformation to the Magnus solution for the evolution of periodically time dependent systems
Authors: Maricq M. Matti
Published: July 2002

Two difficulties arise in the Magnus expansion for the evolution operator of a periodic time dependent Hamiltonian when the eigenvalues of the Hamiltonian are separated by more than 2π/τ, where τ is the period: (1) The Magnus expansion generally does not converge, and (2) the equilibrium properties of the system cannot be determined by statistical mechanical techniques analogous to those for time independent systems. A transformation is introduced that folds the eigenvalue spectrum into the interval −π/τ<λi<π/τ, yet retains the periodicity of the Hamiltonian, thereby alleviating the above difficulties. The method is compared to the approach taken by Shirley [Phys. Rev. B 138, 979 (1965)] for the Floquet solution to the evolution operator. It is applied to a dipole coupled spin system in an oscillating transverse field and the implications of the folding transformation on the spin thermodynamics of this system and the ‘‘Magnus paradox’’ are discussed.
Theoretical spectroscopic parameters for the low-lying states of the second-row transition metal hydrides
Authors: Langhoff Stephen R. , Pettersson Lars G. M. , Bauschlicher Charles W. , Partridge Harry
Published: July 2002

Spectroscopic parameters (De,re,μe) are determined for the second-row transition metal hydrides using large valence basis sets in conjunction with relativistic effective core potentials (RECPs). All-electron calculations are also performed for YH and AgH to calibrate the RECP results. Electron correlation is incorporated using singles-plus-doubles configuration interaction (SDCI), the coupled pair functional (CPF) method, and a modified version (MCPF) of CPF. Although similarities exist between the bonding in the first- and second-row transition metal hydrides, the greater overlap of the d orbitals in the second row with the hydrogen 1s orbital, tends to lead to larger dissociation energies and some changes in the relative ordering of the states. For example, the ground state of ZrH is predicted to be a 2Δ state whose bonding involves 4d–5s hybrid orbitals, whereas in TiH the ground state is a 4Φ state with primarily 4s–1s bonding. The bonding in the second-row transition metal hydrides involves a mixture of all three atomic asymptotes, 4dn5s2, 4dn+15s1, and 4dn+2, whereas contribution from the 3dn+2 asymptote is unimportant in the first-row TM hydrides. However, the bonding is generally much simpler to describe in the second-row as compared with the first-row TM hydrides, and the spectroscopic parameters are much less sensitive to the level of correlation treatment.
A study of the structure and dynamics of the hydronium ion by high resolution infrared laser spectroscopy. II. The ν4 perpendicular bending mode of H3 16O+
Authors: Gruebele Martin , Polak Mark , Saykally Richard J.
Published: July 2002

Sixty-six lines in the ν4 band of H3O+ have been measured using diode laser velocity modulation spectroscopy. 36 have been assigned to the a–a component of the spectrum and 30 to the s–s component. The observed spectra were fit to a standard symmetric top Hamiltonian including the effects of l-type doubling. The band origin of the s–s transitions is at 1625.947(5) cm−1, whereas that of the a–a component lies at 1638.533(3) cm−1. The effect of the asymmetric bending vibration on the inversion splitting is in qualitative agreement with previous theoretical results, but the experimental band origins deviate by about 60 cm−1 from calculated values. Coriolis perturbations are found to have an important effect on the rotational constants of the ν4 and ν2 levels.
Determination of the structure of OCS CO2
Authors: Novick Stewart E. , Suenram R. D. , Lovas F. J.
Published: July 2002

The rotational spectrum of the weakly bound complex OCS CO2 has been measured using a pulsed beam Fourier transform microwave spectrometer. The rotational constants of the major isotopomer are A=4454.606 MHz, B=1517.778 MHz, and C=1129.666 MHz. The molecule is planar with a ‘‘slipped near parallel’’ structure analogous to the structure of the dimer of carbon dioxide. The sulfur atom occupies the ‘‘inner’’ position; this is the obtuse rather than the acute vertex of the O–O–O–S quadrangle. The distance from the center-of-mass (c.m.) of CO2 to the c.m. of OCS is 3.552 Å.
Radiation from oscillating dipoles embedded in a layered system
Authors: Crawford Oakley H.
Published: July 2002

Maxwell’s equations are solved for the radiation due to a source consisting of an oscillating point dipole located in a layered system. Solutions are developed first for the related problem of the fields generated in the system by a distant dipole source, and the problem of interest is then solved by application of the Lorentz reciprocity theorem. The effects of extremely thin layers are considered in detail. Some of the results are illustrated by calculations of the emission from dipoles located in, or near, a film covering a plane-bounded silver metal substrate. It is found that the surface selection rule for absorption, emission, or Raman scattering is not valid for molecules contained in this film.
Measurement of the 2D5/2–2D3/2 fine structure interval in metastable nitrogen atoms at 1.15 mm by laser magnetic resonance
Authors: Bley Ulrich , Koch Martin , Temps Friedrich , Davies Paul B. , Davis Ian H.
Published: July 2002

Far infrared laser magnetic resonance (LMR) spectra of metastable (2p3)2D nitrogen atoms have been measured using laser lines in carbonyl fluoride (COF2) at 1135 μm and methanol (CH3OH) at 1224 μm. The concentration of N 2D was optimized by electron paramagnetic resonance (EPR) using a hybrid EPR/LMR spectrometer. The far infrared cavity of this spectrometer was designed to oscillate at wavelengths longer than 1 mm. The LMR spectra were analyzed using g factors derived from the EPR spectrum [Radford and Evenson, Phys. Rev. 168, 70 (1968)] and yield a precise value of the 2D5/2–2D3/2 fine structure separation of −261 447.7(2) MHz, −8.720 957(7) cm−1(2σ).
Rigorous formulation of quantum transition state theory and its dynamical corrections
Authors: Voth Gregory A. , Chandler David , Miller William H.
Published: July 2002

We describe a procedure for computing the thermal rate constants for infrequent events that occur in complicated quantum mechanical systems. Following the ideas of Gillan, the procedure focuses on the equilibrium statistics of the centroids for the imaginary time quantum paths. We argue that the imaginary time statistics can be used to efficiently bias Monte Carlo sampling of the real time reaction dynamics. Consideration of imaginary time paths or equilibrium statistics alone leads to a quantum transition state theory. Analytical versions of this transition state theory are developed with the aid of a variational principle. Numerical applications of the quantum transition state theory are given for the one-dimensional Eckart barrier problem and for the nonseparable two-dimensional collinear H2+H reaction. Remarkably accurate results are obtained. The quantum transition state theory we describe provides a rigorous basis and generalizes algorithms recently employed to treat electron transfer and also ionization in polar media.
Molecular dynamics simulation of water beween two ideal classical metal walls
Authors: Hautman J. , Halley J. W. , Rhee Y.-J.
Published: July 2002

We have simulated a slab of water with two-dimensional periodic boundary conditions between two metallic walls. The entire compliment of charges, arising from periodic reproductions and from classical images in the metal, are included explicitly by mapping onto a problem with three-dimensional periodicity which is handled by usual Ewald summation methods. Results are presented for charged and uncharged surfaces, permitting an estimate of the differential capacitance arising from the layer of water near the walls. The estimate is about a factor of 2 smaller than the observed differential capacitance of metal–aqueous electrolyte interfaces.
Hydrogen desorption from the monohydride phase on Si(100)
Authors: Sinniah Kumar , Sherman Michael G. , Lewis Lisa B. , Weinberg W. Henry , Yates John T. , Janda Kenneth C.
Published: July 2002

The kinetics of the thermal recombinative desorption of hydrogen from the monohydride phase on the Si(100) surface has been studied by laser-induced thermal desorption (LITD). A rate law that is first order in the atomic hydrogen coverage with an activation energy of 45 kcal/mol gives an accurate fit to the data over a temperature range of 685–790 K and a coverage range of 0.006 to 1.0 monolayer. A new mechanism is proposed to explain these surprising results, namely, that the rate limiting step of the reaction is the promotion of a hydrogen atom from a localized bonding site to a delocalized band state. The delocalized atom then reacts with a localized atom to produce molecular hydrogen which desorbs. Evidence to support these conclusions comes from isotopic mixing experiments. Studies of recombinative desorption from other surfaces of silicon, which had been assumed to obey second-order kinetics, are discussed in the light of these results.
Kinetic energy, temperature, and derived rotational temperature dependences for the reactions of Kr+(2P3/2) and Ar+ with HCl
Authors: Viggiano A. A. , Morris Robert A. , Dale Fred , Paulson John F. , Giles Kevin , Smith David , Su T.
Published: July 2002

Rate constants for the reactions of Kr+(2P3/2) with HCl and DCl and of Ar+ with HCl have been measured as a function of reactant ion/reactant neutral average center-of-mass kinetic energy (〈KEc.m.〉 ) at several temperatures. The measurements were made using helium as the carrier gas. From these data we have derived the dependences of the rate constants on the rotational temperature of H(D)Cl. Rate constants for the reaction of Kr+(2P1/2) with HCl have also been measured as a function of temperature. The rate constants for all of the reactions were found to decrease with increasing temperature. The rate constants were also found to decrease with increasing 〈KEc.m.〉 at low 〈KEc.m.〉 but then to increase at higher 〈KEc.m.〉 . A significant rotational temperature dependence of the rate constant was derived for the reaction of Kr+(2P3/2) with H(D)Cl. The analogous derivation for Ar+ reacting with HCl showed the rate constant for this reaction to be independent of the rotational temperature of HCl within experimental uncertainty.
Kinetics of binary nucleation: Multiple pathways and the approach to stationarity
Authors: Shi G. , Seinfeld J. H.
Published: July 2002

Explicit analytical expressions are obtained for the rate of nucleation over different paths in a binary system. It is shown that anisotropy in reaction rates and anisotropy in the free energy surface can cause nucleation to occur bypassing the saddle point. Homomolecular nucleation is demonstrated to be the natural limit of binary nucleation as the concentration of one component goes to zero. Explicit expressions are also obtained for the time lag of binary nucleation by using the singular perturbation approach. It is shown that the time lag associated with different paths of nucleation is essential in determining the relative importance of different nucleation pathways.
The low-lying states of the second-row transition metal hydrides (YH–CdH)
Authors: Balasubramanian K.
Published: July 2002

Complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by full second-order configuration interaction calculations which included up to 713 000 configurations are carried out on the low-lying states of all second-row transition metal hydrides (YH–CdH). A large (6s5p5d1f) valence Gaussian basis set together with relativistic effective core potentials which included the outer 4s24p64dn5s2 shells, were employed. The spectroscopic constants and potential energy curves of six electronic states of CdH are also obtained and compared with available experimental spectra. The ground states of YH, ZrH, NbH, MoH, TcH, RuH, RhH, PdH, AgH, and CdH were found to be 1∑+, 2Δ3/2, 3(I), 6∑+, 5∑+, 4∑−1/2, 3Δ, 2∑+, 1∑+, and 2∑+, respectively. For NbH, TcH, and RuH, 5Π, 7∑+, and 4Φ states, respectively, were found to be close to the ground states. The 5Π3 and 5Δ3 states of NbH undergo avoided crossing. The spectroscopic constants (re, μe, De, ωe, Te) of the low-lying states of these hydrides are computed and compared with available data on YH, RhH, PdH, and AgH. The theoretical re and ωe values are within 0.01 Å and 20 cm−1 of the experimental constants for these three hydrides.
Molecular orbital calculations on methyl alkyls and hydrides used in the organometallic vapor phase epitaxy of some III–V semiconductor films. I
Authors: Edwards Arthur H. , Jones K. A.
Published: July 2002

We report ab initio and semiempirical calculations of the equilibrium geometries, bond strengths, and vibrational spectra of NH3, PH3, AsH3, (CH3)3Al, (CH3)3Ga, (CH3)3In, N(CH3)3, P(CH3)3, As(CH3)3, and some of their derived adducts as a function of basis set and of self-consistent field approximation. We compare our results to recent IR and Raman data, and to standard thermochemical data. We obtain very good agreement with experimental geometries, good agreement with experimental bond strengths, and only fair agreement (10%–20%) with experimental vibrational spectra. We argue that this purely theoretical approach is, with qualification, appropriate for predicting thermochemical parameters for gas-phase reactions that occur in organometallic vapor phase epitaxy of III–V semiconductor thin films. We expect to make reasonable predictions for barriers to chemical reactions, while use of our predictions of free energies will probably require care.
Tetrahedral coordination of Zn ions in molten zinc halides
Authors: Allen D. A. , Howe R. A. , Wood N. D. , Howells W. S.
Published: July 2002

Measurements of the scattering of thermal neutrons from natural samples of molten ZnCl2, ZnBr2, and ZnI2 are reported. The experiments were undertaken on the liquids and amorphous materials diffractometer of the ISIS spallation source at the Rutherford–Appleton laboratory. The data reveal some shortcomings in earlier studies of ZnCl2 but confirm a structural model in which the small Zn2+ ions occupy tetrahedrally coordinated sites in a closely packed anion structure, with strong intermediate range ordering. Features arising from changes in the anion species are identified and temperature-dependent effects are resolved in ZnCl2.
Dissociative single, double, and triple photoionization of silicon tetrafluoride in the valence shell and silicon 2p regions (hν=33–133 eV)
Authors: Imamura Takashi , Brion Chris E. , Koyano Inosuke , Ibuki Toshio , Masuoka Toshio
Published: July 2002

The photoionization of SiF4 in the valence shell and Si2p innershell regions has been studied using time-of-flight mass spectrometry and synchrotron radiation over the photon energy range 33–133 eV. Photoionization branching ratios are reported for stable singly and doubly charged ions arising from the various possible molecular and dissociative photoionization processes. The time-of-flight mass spectrometer used in the present work employs very efficient ion extraction fields and this results in very much higher relative abundances of energetic fragmentations than were reported in earlier published work [Lablanquie et al., J. Chem. Phys. 90, 7078 (1989)]. Photoion–photoion coincidence (PIPICO) techniques have been used to investigate the relative yields of Coulomb explosion decomposition products and threshold energies for dissociative double photoionization in the valence shell and silicon 2p regions. The dominant process in the Si2p region is (F++Si+). The PIPICO spectra show additional peaks with thresholds in the Si2p region and these structures arise from the dissociation of triply charged ions into the exit channels (F++SiF2+2), (F++SiF2+) and (F++Si2+). Of these the latter is the most probable process.
Metastable 3Σ−g ground state of F++2 and the bonding in molecular dications
Authors: Senekowitsch Jörg , ONeil Stephen
Published: July 2002

Large multireference configuration interaction (MR-CI) calculations on the F++2 ion predict a 3Σ−g ground state, metastable with respect to tunneling into the F++F+ nuclear continuum. The potential energy curve displays a 0.40 eV barrier at Rb=1.607 Å, between the local potential minimum (Re=1.289 Å) and the 3Pg(F+)+3Pg(F+) asymptote at 7.69 eV lower energy. The potential barrier traps four quasibound vibrational levels, with a tunneling lifetime of 16 ms for v=0. A Dunham analysis at the well minimum gives ωe=919.4 cm−1, ωexe=16.31 cm−1, Be=1.073 cm−1, and αe=0.0316 cm−1. In a departure from an earlier viewpoint, the origin of the barrier in this and other dications is interpreted as a sum of the e2/R Coulomb repulsion and the ordinary chemical bonding of the constituent ions. This model also explains the purely repulsive character found for the valence-excited 1Δg and 1Σ+g states.
Ionization potentials of aluminum–sodium bimetallic clusters (AlnNam)
Authors: Nakajima Atsushi , Hoshino Kuniyoshi , Naganuma Takashi , Sone Yasutomo , Kaya Koji
Published: July 2002

Aluminum–sodium bimetallic clusters (AlnNam, n=2–26) were produced by two independent laser vaporization methods. To investigate the electronic structures, ionization potentials of the AlnNam clusters were measured up to m=3 using a tunable ultraviolet laser combined with time-of-flight (TOF) mass spectrometer. In general, the ionization potentials monotonically decrease with the number of sodium atoms, and the ionization potentials of Aln Na1 generally decrease by 0.2–0.6 eV compared to those of the corresponding Aln cluster. In contrast, the ionization potentials of Al13Na1 and Al23Na1 are higher than (or equal to) those of Al13 and Al23. The anomalous change in the ionization potentials of Al13Na1 and Al23Na1 can be explained by the electronic shell model: By the addition of one sodium atom to the bare aluminum clusters, the total number of their valence electrons strictly satisfies the shell closing of 2p and 3s shells, respectively, and therefore, the clusters can be stabilized electronically.
Vibrational branching ratios from the dissociation of the NeIBr van der Waals molecule
Authors: Walter Sarah A. , Stephenson Thomas A.
Published: July 2002

The degree of vibrational excitation in the IBr fragment from the vibrational predissociation of NeIBr (A 3Π1) has been measured using two-color pump–probe laser-induced fluorescence spectroscopy. We find that for the lowest initial vibrational states examined, Δv=−1 dissociation pathways dominate the dynamics, while this channel is closed for v≥17. From this result, the A state binding energy (D0) of the complex is determined to be 67±4 cm−1, while that in the X electronic state is found to be 73±4 cm−1. The X state binding energy is identical to that for NeI2 and NeBr2, suggesting that the potential energy surface for NeIBr can be constructed from a summation of atom–atom pair potentials; we present such a model potential energy surface. The variations in the vibrational branching ratios, when combined with the trends in the predissociation rates, point to the importance of fragment rotational excitation in the dynamics of the dissociation.
Empirical models for the hydration of protons
Authors: Kozack R. E. , Jordan P. C.
Published: July 2002

Two empirical models are considered for the hydrates H+(H2O)n. In one, a rigid hydronium unit interacts with n−1 water molecules, and in another, a single proton with an effective hard-core radius interacts with n waters. In each case, the polarizable representation of the water molecule developed by Kozack and Jordan is used. The two models are employed in a study of clusters with 1≤n≤7 and it is found that while both yield minimum-energy structures in good agreement with ab initio studies, the experimental hydration energies are much better accounted for within the proton model.