bluuebunny/crossref_metadata_2025 huggingface dataset:
Uniform semiclassical IVR treatment of the S-matrix
Authors: Elran Y. , Kay K. G.
Published: July 2002

A new, uniform, semiclassical, initial value representation expression is obtained for the S-matrix in the case of collinear collisions. The derivation is based on an asymptotic analysis (for large inter-fragment distances) of a uniform semiclassical integral expression for the time independent scattering wave function. Although this derivation specifically treats the case of the collision of an atom with a harmonic diatom, the final expression is generalized to arbitrary collinear collisions. The various properties of the expression and its relation to existing semiclassical methods are discussed. Numerical tests are performed for the well-known Secrest–Johnson system. Among other important advantages, the present treatment is a well-defined, uniform, semiclassical approximation that is capable of good accuracy and high computational efficiency, requiring a relatively small number of classical trajectories to obtain converged S-matrix elements for a given energy and initial state.
Dynamic quantum-confined stark effect in self-assembled InAs quantum dots
Authors: Gurioli M. , Sanguinetti S. , Henini M.
Published: July 2002

We present a detailed investigation of the carrier dynamics in a set of InAs/GaAs (N11) quantum dots (QD) by means of time-resolved photoluminesce (PL) techniques. A dynamical red shift of the PL bands when increasing the delay time after the pulse excitation is observed. We attribute this intrinsic optical nonlinearity to the photoinduced screening of internal built-in electric field. The value of the redshift of the QD emission band decays with the carrier population demonstrating the intrinsic nature of the built-in field. Its dependence on the substrate orientation and termination agrees with the expected piezoelectric induced quantum confined Stark effects of the QD optical transitions.
Improved electron emission from carbon film using a resistive layer
Authors: Cui J. B. , Robertson J. , Milne W. I.
Published: July 2002

Field emission of electrons from carbon films deposited on n-type silicon substrate with a resistive layer was investigated by monitoring both the emission current–voltage curves and emission images as displayed by a phosphor anode. The resistive layer was found to effectively improve the emission quality by controlling the emission current and increasing the emission site density. Confinement of the emission current finally results in a uniform emission current across a large range of voltages, which is a key requirement for application to displays.
The surface-ordered phase of n-nonadecane: A molecular dynamics simulation
Authors: Zhen Li Hong , Yamamoto Takashi
Published: July 2002

Behavior of realistic n-nonadecane, C19H40, on the free surface of the melt is investigated by the molecular dynamics simulation. A crystalline thin film made of three layers of lamellas is constructed and used as an initial state of the heating simulation, where we devise a special heating scheme in order to accelerate the rate of convergence to the thermal equilibrium. By very slow heating we find a temperature range (385–410 K) where the middle layer prefers to be in the melt state with both surface layers remaining crystalline; this observation is considered to correspond to the surface freezing recently found in n-alkanes. It is found that the molecules in the surface monolayer align their axes nearly perpendicular to the surface and form well-defined hexagonal packing. It is also found that the molecules in the surface monolayer show large center-of-mass fluctuations, translational and transverse, along the surface normal and parallel to the surface, respectively.
Recombination mechanisms in GaInNAs/GaAs multiple quantum wells
Authors: Kaschner A. , Lüttgert T. , Born H. , Hoffmann A. , Egorov A. Yu. , Riechert H.
Published: July 2002

Recombination processes in Ga1−xInxNyAs1−y/GaAs multiple quantum wells (MQWs) were investigated as function of the nitrogen molar fraction. We found a pronounced S-shaped behavior for the temperature-dependent shift of the photoluminescence emission similar to the ternary nitrides InGaN and AlGaN. This is explained by exciton localization at potential fluctuations. Time-resolved measurements at 4 K reveal an increase of the decay time with decreasing emission energy. A model based on lateral transfer processes to lower-energy states is proposed to explain this energy dependence. The formation of tail states in the Ga1−xInxNyAs1−y/GaAs MQWs is attributed to nitrogen fluctuations.
Precise pulsed time-of-flight laser range finder for industrial distance measurements
Authors: Kilpelä Ari , Pennala Riku , Kostamovaara Juha
Published: July 2002

A pulsed time-of-flight laser range finder with a 1 GHz avalanche photo diode (APD) receiver and a laser pulser with ∼35 ps pulse width has been developed and tested. The receiver channel is constructed using a silicon ASIC chip and a commercially available silicon APD placed on a hybrid ceramic susbstrate. The laser pulser utilizes a single heterostructure laser operating in Q-switching mode. It is shown that the single-shot precision of the complete laser range finder is ∼2.1 mm (σ value) at best. The nonaccuracy in the distance range of 0.5–34.5 m was ∼±2 mm excluding errors caused by the statistical variations and long-term instability. The single-shot precision is clearly better than the single-shot precision of the earlier laser range finders with ∼100–200 MHz bandwidths. Also, two types of optics, coaxial and paraxial, were tested. The linearity of the coaxial optics was better, especially with a long (4 m) receiver fiber. Some possible applications of the laser range finder utilizing ps level pulses are, for example, fast three-dimensional vision in industrial environments and structure analysis of materials.
Finite element modeling of magnetoacoustic emission and of stress-induced magnetic effects at seam welds in steel pipes
Authors: Sablik M. J. , Weidner C. , Augustyniak B.
Published: July 2002

Prior finite element modeling (FEM) has shown how creep damage reduces the voltage signal in a secondary coil wrapped around a C-core electromagnet positioned above a seam weld in a steel pipe. Prior hysteresis modeling has also demonstrated the reduction of magnetoacoustic emission (MAE) due to uniform creep damage in a steel specimen. In this article, it is shown how to evaluate the MAE due to a nonuniform distribution of creep damage at a seam weld using FEM. Also, FEM is used to evaluate the C-core secondary emf due to a nonuniform stress distribution, which is developed at a seam weld due to the internal pressure of hot steam passing through the pipe. In this latter case, use is made of the stress distribution computed at the seam weld by Stevick. It is found that nonuniform creep damage also reduces the MAE signal, as is found experimentally. The stresses due to internal pressure in the pipe are shown by FEM to increase the C-core secondary emf, which is opposite to the effect of creep damage. A comparison is made between stressed and unstressed cases, with and without creep damage.
Stimulated emission and optical gain in ZnO epilayers grown by plasma-assisted molecular-beam epitaxy with buffers
Authors: Chen Yefan , Tuan N. T. , Segawa Yusaburo , Ko Hang-ju , Hong Soon-ku , Yao Takafumi
Published: July 2002

We report the results of an experimental investigation on lasing mechanisms in optically pumped ZnO epilayers at room temperature. High-quality ZnO epilayers grown on sapphire by plasma-assisted molecular-beam epitaxy employing an MgO buffer were used. Free exciton emissions and their phonon replicas dominate the photoluminescence from low excited samples. Inelastic exciton–exciton scattering contributes to the mechanism of stimulated emission mainly at intermediate excitation. By using the variable stripe length method, we measured the near threshold optical gain spectrum of the ZnO epilayers. Different from the interband transition governed mechanisms, exciton–exciton scattering gives rise to a nearly symmetric gain spectrum with the peak at 3.17 eV. The electron-hole plasma emerges to contribute to the optical gain when excitation exceeds 220 kW/cm2.
Pseudomoment fluid modeling: Electron Landau damping of ion-acoustic waves
Authors: Amendt Peter
Published: July 2002

This Brief Communication introduces a hierarchy of electron pseudomoment fluid equations to derive electron Landau damping of ion-acoustic waves. Exact agreement with kinetic predictions is obtained. The pseudomoment fluid formulation allows simple, analytic estimates of Landau damping for general, nonthermal electron distribution functions in Vlasov plasmas. Successful truncation of the pseudomoment hierarchy suggests a novel definition of electron temperature away from thermal equilibrium in the low-frequency regime.
Surface properties of diluted aqueous solutions of solutes containing isopropyl hydrophobic group
Authors: Gliński Jacek , Chavepeyer Guy , Platten Jean-Karl
Published: July 2002

The surface tensions σ of aqueous solutions of isobutanol, isobutylamine, and L-valine were measured in the temperature range between 278 and 308 K using the Wilhelmy plate method. The surface entropies, enthalpies, densities, and compositions were calculated. The results were interpreted in terms of specific interactions of the hydrophobic parts of the molecules adsorbed at the interface. Increasing surface concentration of two former solutes molecules causes, in both cases, a rearrangement at the surface, similar to that observed earlier for water+tert-butanol. The same concerns L-valine, although it was found to be a very weak surfactant, which can be understood if assuming strong interactions of this solute with the bulk water structure and competition between hydrophilic and hydrophobic hydration of its molecules.
Laser induced fluorescence spectroscopy of the Ã 3Πi←X̃ 3Σ− transition of the CCS radical
Authors: Schoeffler A. J. , Kohguchi H. , Hoshina K. , Ohshima Y. , Endo Y.
Published: July 2002

The Ã 3Πi←X̃ 3Σ− electronic transition of a carbon-chain molecule CCS has been observed for the first time by laser induced fluorescence spectroscopy. The species was generated with a pulsed discharge of 0.5% C2H2 and 0.5% CS2 diluted in Ar via a pulsed supersonic jet. A number of bands in the 690–675 nm and 600–615 nm regions (14 500–14 800, 16 200–16 600 cm−1) were observed and assigned to those of CCS using ground state combination differences. Three of the bands in each region were found to belong to the three Ã 3Πi spin-orbit components of the Ã 3Πi←X̃ 3Σ− transition (Tv=14 662.777(4) and 16 423 cm−1), and an effective set of rotational constants for the upper states were obtained. The higher energy band was heavily perturbed, preventing the determination of effective constants for this band. These bands have been tentatively assigned as two successive bands in the Ã 3Πi(n,0,0) progression with a resultant effective value of 1760 cm−1 for the Ã 3Πi ν1 vibrational fundamental. Dispersed fluorescence spectra from the Ã 3Πi←X̃ 3Σ− band of CCS have also been observed, yielding definitive experimental information on the vibrational structure of the X̃ 3Σ− ground state for the first time. The three harmonic frequencies were determined to be 1708.2(40) cm−1, 862.5(19) cm−1, and 269.3(13) cm−1 for ω1 (CC stretch), ω2 (CS stretch), and ω3 (CCS bend), respectively.
Magneto-optical properties of ZnO-based diluted magnetic semiconductors
Authors: Ando K. , Saito H. , Jin Zhengwu , Fukumura T. , Kawasaki M. , Matsumoto Y. , Koinuma H.
Published: July 2002

Magnetic circular dichroism (MCD) spectra of Zn1−xTMxO (TM=Sc, Ti, V, Cr, Mn, Co, Ni, and Cu) films were measured to clarify their magneto-optical properties. Noticeable magneto-optical effect was not observed for Zn1−xScxO, Zn1−xTixO, Zn1−xVxO, and Zn1−xCrxO. On the contrary, Zn1−xMnxO, Zn1−xCoxO, Zn1−xNixO, and Zn1−xCuxO showed pronounced negative MCD peaks near 3.4 eV which correspond to the band gap of the host ZnO semiconductor. These results show that ZnO alloyed with Mn, Co, Ni, and Cu is a diluted magnetic semiconductor with strong exchange interaction between sp-band carriers and localized d electrons.
A transversing connection between density functionals
Authors: Tsuneda Takao , Kamiya Muneaki , Morinaga Naoki , Hirao Kimihiko
Published: July 2002

We identified a transversing physical connection between kinetic, exchange, and correlation functionals by using parameter-free (Pfree) exchange and one-parameter progressive (OP) correlation. On the basis of this connection, we investigated how the calculated energies and chemical properties depend on the shape of the functional in the Kohn–Sham scheme. We found that the fundamental conditions of the functionals are connected through Pfree and OP functionals with the exception of a rapidly varying density limit. We also found that the calculated properties are highly affected by the exchange functional shape in particular regions of xσ=|∇ρσ|/ρσ4/3.
Epitaxial structure SrTiO3〈011〉 on Si〈001〉
Authors: Migita S. , Sakai S.
Published: July 2002

An epitaxial structure, 〈011〉-oriented SrTiO3 film on Si〈001〉 substrate, is developed by inserting an epitaxial Ce0.12Zr0.88O2 buffer layer. Films are prepared by pulsed-laser deposition and evaluated by x-ray diffraction. Origin of this epitaxial growth is considered as a result of the ionic bonding at the interface of perovskite (SrTiO3) and fluorite (Ce0.12Zr0.88O2) structures. SrTiO3(011) surface of this epitaxial structure leads to a non-c-axis-oriented epitaxial growth of bismuth-layerstructured-ferroelectric Bi4Ti3O12 film on Si〈001〉. Unique surface morphologies and superior electrical properties are presented.
High frequency impedance spectra on the chromium dioxide thin film
Authors: Fu C. M. , Lai C. J. , Wu J. S. , Huang J. C. A. , Wu C.-C. , Shyu S.-G.
Published: July 2002

We report on the study of high frequency magnetotransport properties of the chromium dioxide (CrO2) thin films, grown on Si substrate using chemical vapor deposition. The film exhibits a ferromagnetic transition with a Curie temperature near 390 K. The temperature dependent spontaneous magnetization follows Bloch’s law. The impedance spectra, being analyzed based on the fundamental electrodynamics, are demonstrated to be in a low-loss dielectric limit along with the occurrence of dielectric relaxation and magnetization response. The specific features of impedance spectra, distinct from the usual metallic ferromagnet, are attributed to the half metallic nature of CrO2. The results explore the possibility for high frequency device applications.
Strained layer (111)B GaAs/InGaAs single quantum well lasers and the dependence of their characteristics upon indium composition
Authors: Fleischmann T. , Moran M. , Hopkinson M. , Meidia H. , Rees G. J. , Cullis A. G. , Sánchez-Rojas J. L. , Izpura I.
Published: July 2002

We have studied a series of (111) oriented GaAs–InGaAs single quantum well, broad area lasers with active regions containing a range of indium concentrations, x, in order to understand the advantages and limitations of pseudomorphic strain. For x⩽0.3, both an increased emission wavelength and reduced threshold current were observed with increasing x. The predominant cause of the wavelength increase is the reduction in bulk InGaAs band gap. The reduction in threshold current is attributed mainly to the reduced in-plane density of states caused by the strain induced lifting of the heavy and light hole degeneracy at the valence band edge. For x>0.3, we see a marked deterioration in laser performance. However, we believe that this deterioration is not directly associated with strain relaxation at layer thicknesses beyond the critical value. Rather, imperfections in the AlGaAs cladding layers appear to seed the formation of dislocations within the strained regions. Within the limitation of strain relaxation, we observed monomode continuous wave emission at room temperature at wavelengths up to 1.072 μm and with threshold current densities as low as 74 A/cm2. The differential gain of 1.45×10−15 cm2 is around four times higher than measured on unstrained GaAs/AlGaAs single quantum well lasers. Like the reduction in threshold current density, this relatively high value is attributed to the strain induced reduction in the in-plane, heavy hole effective mass.
Racemic fluids of hard molecules
Authors: Vatamanu J. , Cann N. M.
Published: July 2002

The structure in four racemic fluids is explored using two integral equation theories: the reference interaction site method (RISM) [D. Chandler and H. C. Andersen, J. Chem. Phys. 57, 1930 (1972)] and the diagrammatically correct theory of Chandler, Silbey, and Ladanyi (CSL) [D. Chandler, R. Silbey, and B. M. Ladanyi, Mol. Phys. 46, 1335 (1982)]. Discrimination is measured by comparison of site pair distributions for sites on identical molecules with the corresponding distributions for sites on mirror-image molecules. We find that discrimination is largest for distributions between the smallest sites in the molecules. Between racemates, those consisting of more asymmetrical chiral molecules (i.e., with a bigger range of site sizes and bond lengths) show the largest discrimination. The indirect correlation function is shown to be nondiscriminating in racemates. Further, exact relationships between like–like and like–unlike differences in the other pair functions have been obtained. From these, the importance of the bridge functions in discrimination is evident. The CSL theory always satisfies the exact relationships, even with approximate bridge diagrams. RISM theory cannot satisfy these exact limits regardless of density and closure relation. We have found that RISM theory predicts qualitatively incorrect pair distributions at low densities, but that the difference in the distributions is more accurate. All bridge diagrams which contribute to O(ρo) have been enumerated and evaluated. Inclusion of these diagrams into the CSL theory leads to exact results at low density. However, we find that the inclusion of the ρo diagrams has dramatically improved the quality of the CSL theory at all densities.
The structure and the thermochemical properties of the H3+(H2)n clusters (n=8–12)
Authors: Barbatti M. , Jalbert Ginette , Nascimento M. A. C.
Published: July 2002

Ab initio calculations were performed for the H3+(H2)n clusters (n=8–12), including complete optimization of several isomers of the n=10 cluster. Binding energies, enthalpies, and ionization potentials are calculated. Well defined patterns of chromism are predicted for the H2 collective vibrations and for the H3+ breathing vibrations. The calculations for the n>10 clusters allow us to understand their shell structure in terms of concentric spheres of H2 molecules. The first and second shells have occupation numbers equal to 3 and 6, respectively, while for the third shell, this number is within the range 12–15.
Effect of pressure, membrane thickness, and placement of control volumes on the flux of methane through thin silicalite membranes: A dual control volume grand canonical molecular dynamics study
Authors: Martin Marcus G. , Thompson Aidan P. , Nenoff Tina M.
Published: July 2002

The flux of methane through the straight channels of thin silicalite membranes is studied via dual control volume grand canonical molecular dynamics. The adsorption layers on the surfaces of the thin membranes are found to provide a significant resistance to the flux of methane. This strong surface effect for thin membranes requires that the control volumes (where insertions and deletions are performed) must be placed far enough away from the membrane surface that they do not overlap with the surface adsorption layer. The permeance (flux/pressure drop) of methane through the surface layer is shown to be insensitive to both the average pressure and the pressure drop. In contrast, the permeance through the interior of the membrane increases with decreasing average pressure. These results are explained using a model which treats the transport through the surface barrier as driven by the pressure gradient and transport through the zeolite as driven by the chemical potential gradient. A new force field named DACNIS is presented which accurately describes the adsorption isotherms of methane and ethane in silicalite.
Seed-layer effect on the microstructure and magnetic properties of Co/Pd multilayers
Authors: Roy A. G. , Laughlin D. E. , Klemmer T. J. , Howard K. , Khizroev S. , Litvinov D.
Published: July 2002

We have investigated Co/Pd multilayers deposited on either Ta or indium tin oxide (ITO) seed layers as a potential perpendicular recording media. We have examined the microstructural evolution of the films deposited on the two different types of seed layers and related it to the magnetic properties of the films. Ta underlayer produces a strong 〈111〉 fiber texture in the multilayer while ITO produces randomly oriented grains. Transmission electron microscopy reveals a microstructure of columnar grains separated by less dense material at the boundaries for the multilayers with an ITO underlayer. However, the less dense material is absent when using a Ta underlayer. The films exhibited strong perpendicular anisotropy and a higher coercivity of ∼6800 Oe and squareness of ∼0.99 are obtained for the films deposited on an ITO seed layer. The differences in the value of coercivity and squareness in the films can be correlated with the differences in the evolution of microstructures for different seed layers.
Studies of angular distributions and cross sections for photodetachment from the oxygen molecular anion
Authors: Lin Ping , Lucchese Robert R.
Published: July 2002

We have studied the photodetachment from molecular oxygen anion O2− leading to eleven O2 electronic states. Both single- and multichannel scattering calculations were performed using the Schwinger variational method with the Hartree–Fock or configuration interaction approximations for the target states. Our calculated total cross sections agree well with the existing experimental data. One Feshbach resonance was found at about 5 eV above the photodetachment threshold. This resonant state has the character of the 1πu31πg4 configuration, and decays to several final channels. Our calculated partial cross sections suggest that shape resonances exist in different channels at higher energies, and the structures of these shape resonances differ from what we have seen in the photoionization from neutral molecules. The most significant feature in this photodetachment process is the channel coupling effect due to electron correlation.
Formation of quantum wires and dots on InP(001) by As/P exchange
Authors: Yang Haeyeon , Ballet P. , Salamo G. J.
Published: July 2002

We report on the use of in situ scanning tunneling microscopy to study As/P exchange on InP(001) surfaces by molecular beam epitaxy. Results demonstrate that the exchange process can be controlled to selectively produce either quantum wires or quantum dots. 15 nm wide self-assembled nanowires are observed, and they are elongated along the dimer row direction of the InP(001)-2×4 surface with a length of over 1 μm and flat top 2×4 surfaces. In addition, when the nanowires are annealed with no arsenic overpressure, the surface reconstruction transforms from 2×4 to 4×2 and the nanowires transform into dots with a rectangular base and flat top.
Slow domains in semi-insulating GaAs
Authors: Neumann A.
Published: July 2002

Semi-insulating GaAs shows current oscillations if a high dc voltage is applied to a sample. These oscillations are caused by traveling high-electric-field domains that are formed as a result of electric-field-enhanced electron trapping. This article describes the various types of experiments that have been carried out with this system, including recent ones that use the electro-optic Pockels effect in order to measure the local electric fields in the sample in a highly accurate manner. An historical overview of the theoretical developments is given and shows that no satisfying theory is currently available. A list of all the required ingredients for a successful theory is provided and the experimental data are explained in a qualitative manner. Furthermore, the main electron trap in semi-insulating GaAs is the native defect EL2, the main properties of which are described.
Near-critical two-dimensional smectic-C to solid-like phase transition in azobenzene-derivative Langmuir monolayers
Authors: Tabe Yuka , Yokoyama Hiroshi
Published: July 2002

We describe the results of a detailed study of two-dimensional (2D) smectic-C to solid-like phase transition in azobenzene-derivative Langmuir monolayers by the surface pressure vs area-per-molecule isotherm measurement, polarizing optical microscopy, and the grazing incidence synchrotron x-ray diffraction. The phase transition was observed to be weakly first order up to slightly above room temperature with a transition entropy less than 1.0 kB per molecule, accompanied by proper characteristics of a nearly ideal horizontal coexistence line on the isotherm, an abrupt change in optical texture, and the emergence of a conspicuous x-ray diffraction peak in the solid-like phase, which indicates a weak positional order with a correlation length of 10–20 nm. Analysis of the x-ray diffraction data within the framework of distorted hexagonal lattice suggests that the solid-like phase may be regarded as a 2D analog of smectic-L phase that is a hexatic smectic phase with the molecules tilted toward a direction between the nearest and the next-nearest bonds. At higher temperatures, the transition became less discontinuous and entirely disappeared above 40 °C in all these experimental aspects. We argue that the transition is viewed as a 2D induced hexatic–hexatic transition (analog of smectic-C to smectic-L transition), which has an isolated critical point where the first-order transition comes to an end.
Size-dependency of polarizabilities of fractal- and nonfractal-structured oligomers modeled after dendron parts in Cayley-tree-type dendrimers
Authors: Nakano Masayoshi , Fujita Harunori , Takahata Masahiro , Yamaguchi Kizashi
Published: July 2002

We investigate the size-dependency of static polarizabilities (α) (in the chain-length direction) of three types of model oligomers (made of phenylene vinylenes), i.e., para-oligomers and meta-oligomers with fractal and nonfractal structures by the finite-field method using molecular orbital calculations. The fractal-structured meta-oligomers are considered to be models of dendron parts involved in Cayley-tree-type dendrimers. The chain-length dependence of α of para-oligomers is found to be much larger than that of nonfractal-structured meta-oligomers, while the fractal-structured meta-oligomers exhibit an attractive chain-length dependence of α; the α values of meta-structured oligomers are near to those of nonfractal-structured meta-oligomers in small chain-length region, while they are close to those of para-oligomers in sufficiently large chain-length region. Using the polarizability density analysis, the spatial contributions of π-electrons to α of nonfractal-structured meta-oligomers are found to be smaller than those of para-oligomers and to be well decoupled at the meta-substituted benzene rings. In contrast, it is found that the spatial contributions of π-electrons to α in linear-leg regions for fractal-structured meta-oligomers are similar to those for the para-oligomers though the α density distributions for fractal-structured meta-oligomers are also well decoupled at the meta-substituted benzene rings. Such features, which reflect the fractal structure, is predicted to cause the attractive size-dependency of α for fractal-structured meta-oligomers.
Single-electron tunneling in a single PbS nanocrystal nucleated on 11-mercaptoundecanoic acid self-assembled monolayer at room temperature
Authors: Jiang Peng , Liu Zhong-Fan , Cai Sheng-Min
Published: July 2002

Nanometer-sized PbS particles in the size of about 3 nm produced by exposing a self-assembled monolayer of 11-mercaptoundecanoic acid salts on gold (111) substrate in a H2S atmosphere were studied by scanning tunneling microscopy (STM) and high-resolution transmission electron microscopy at room temperature in air. The stability of thus-obtained PbS nanoparticles makes STM imaging possible and repeatable scans of the semiconductor nanoparticles were performed. The current-voltage (I–V) characteristics of a single PbS nanoparticle exhibited clearly Coulomb blockade and Coulomb staircase. Furthermore, by varying the gap between a STM tip and the PbS nanoparticle, we also verified the dependence of staircase width on the change of the gap in the local I–V characteristics on the same PbS nanoparticle. The phenomena can be well described by a semi-classical double-barrier tunneling model.
Moving adaptive grid methods for numerical solution of the time-dependent molecular Schrödinger equation in laser fields
Authors: Lu HuiZhong , Bandrauk André D.
Published: July 2002

We present a moving adaptive grid method for solving the time-dependent Schrödinger equation, TDSE, for molecules in intense laser fields, applicable in the nonperturbative nonlinear regime where dissociation ionization occurs. The method is based on a Lagrangian, moving coordinate system. In this representation, the reference system is moving with the laser pulse so that the classical movement of free particles in the field, i.e., in the asymptotic region where electron–molecule potentials are negligible but the laser field is still present, is exactly described. As a consequence, the asymptotic quantum wave functions are exact in presence of a laser pulse. We have tested several discrete propagator methods for the TDSE in different gauges in a Born–Oppenheimer simulation of H2+ in a short, intense laser pulse. Our comparison of convergence between the same discretization methods for different gauges have demonstrated the superiority of the present Lagrangian adaptive grid method to treat the response of molecules to intense time-dependent electromagnetic fields.
Reaction induced by a scanning tunneling microscope: Theory and application
Authors: Alavi Saman , Seideman Tamar
Published: July 2002

We develop a theoretical framework for study of chemical dynamics induced by a scanning tunneling microscope. An analytically solvable limit of the expression derived for the reaction rate reveals the information content of the voltage dependence of the observable. The theory is applied to the problem of H-atom desorption from a silicon surface in the 4–10 V range, where desorption is triggered by a single electronic transition into a short-lived excited state localized on the H–Si bond. The resonance lifetime is extracted by fitting the numerical results to an observed desorption yield versus voltage curve [Foley et al., Phys. Rev. Lett. 80, 1336 (1998)].
A reduced-dimensionality quantum model which incorporates the full-dimensional energy of the system: Application to the vibrational predissociation of Cl2–Ne2
Authors: Ceotto M. , Garcı́a-Vela A.
Published: July 2002

A reduced-dimensionality quantum model is proposed which incorporates the zero-point energy of the neglected modes in a systematic, natural way. In this model the reduced-dimensionality Hamiltonian is obtained by averaging the exact Hamiltonian over the dependence of the full-dimensional initial state of the neglected modes. The reduced Hamiltonian conserves all the terms of the full Hamiltonian, providing a more flexible description of the couplings between the modes considered explicitly in the model. The model is applied to simulate the vibrational predissociation dynamics of Cl2–Ne2, considering the three stretching modes of the complex. The results are compared to experimental data and to previous calculations using a reduced-dimensionality quantum model and a full-dimensional quantum–classical approach. The Cl2–Ne2 resonance lifetimes obtained agree only qualitatively with the experimental and previously calculated ones. By contrast, the present model predicts more correctly than previous calculations the behavior of the Cl2 fragment vibrational distributions observed experimentally. The applicability of the model is discussed and further refinements are suggested.
Zero electron kinetic energy photoelectron spectroscopy of weakly bound In–NH2CH3, In–NH(CH3)2, and In–N(CH3)3 complexes
Authors: Rothschopf Gretchen K. , Li Shenggang , Shannon Perkins Jimmye , Yang Dong-Sheng
Published: July 2002

Single-photon pulsed field ionization-zero electron kinetic energy photoelectron spectroscopy has been used to study the indium–methylamine complexes. The photoelectron spectra have been assigned using density functional theory and Franck–Condon calculations. The spectral assignments identify the symmetric In+–N stretch mode for In+–NH2CH3 (259 cm−1), In+–NH(CH3)2 (200 cm−1), and In+–N(CH3)3 (158 cm−1); In–N stretch for In–N(CH3)3 (110 cm−1); In+–N–C bend for In+–NH2CH3 (110 cm−1) and In+–NH(CH3)2 (120 cm−1); In–N–C bend for In–NH2CH3 (91 cm−1) and In–NH(CH3)2 (106 cm−1); and In+–N–H bend for In+–NH(CH3)2 (324 cm−1). Methyl substitutions for hydrogen atoms greatly influence the indium–nitrogen stretch forces and ionization potentials of the complexes. The indium–amine binding in these complexes includes orbital interaction and electrostatic forces.
Calibration procedure and data processing for a TV Thomson scattering system
Authors: Barth C. J. , Chu C. C. , Beurskens M. N. A. , Meiden H. J. v. d.
Published: July 2002

Calibration procedures for Thomson scattering systems based on television-like cameras, so-called TVTS systems, are described. The TVTS systems of the Rijnhuizen Tokamak Project (RTP), the TJ-II stellarator, and the Torus Experiment for Technology Oriented Research (TEXTOR) tokamak combine a 10–15 J ruby laser as a source with an intensified charge coupled device camera as a detector. A tungsten strip lamp in combination with an integrating sphere is used to calibrate all ∼105 pixels of the camera relatively to each other. Rayleigh scattering on hydrogen or nitrogen is used to perform an absolute calibration of the complete detection system. Great emphasis is placed on possible systematic errors on the determination of the electron temperature Te and density ne due to the calibration, such as tungsten lamp temperature, detoriation of the detection window, long term stability, laser beam alignment, and detector linearity. The long term stability of the system was tested by comparing different sets of calibration factors. Over a period of 1/2 yr the values of the calibration factors varied by less than 5%. Using the same Thomson scattering data but different sets of calibration factors the Te values varied even less than 1%. A two camera technique was used to search for possible unknown systematic errors in the determination of Te profiles. It appears that these systematic errors are about half of the observational error on Te. Density fluctuations can be determined with an accuracy equal to the statistical error of ∼3%, while the systematic error on ne appeared to be ∼10%, which can be corrected for using interferometer data. As a result, these TVTS systems can measure Te and ne profiles with ∼100 (RTP) or 120 (TEXTOR) spatial elements along the full plasma diameter with observational errors on Te of ∼6% in the range of 25 eV–6 keV, at ne=2×1019 m−3 and 10 J laser energy.
Isotopic probing of weak intermolecular forces: Infrared spectrum and energy levels of the C13O16 dimer
Authors: McKellar A. R. W.
Published: July 2002

The infrared spectrum of the fully substituted 13-C carbon monoxide dimer, (13C16O)2, has been studied in the region of the C–O stretching vibration, around 2096 cm−1. Over 120 transitions have been assigned to transitions involving 49 rotational levels in the excited state and 23 levels in the ground state. Relative energies for all these levels have been accurately (≈0.0003 cm−1) determined without model dependence, except for the interval between the lowest levels of A and B symmetry. The observed stacks of levels were similar to those determined previously for the normal isotope, (12C16O)2, and could therefore be identified with the same labeling scheme, but the differences between them constitute a subtle probe of the intermolecular forces in this system. In both isotopes, the stacks fall into two groups, characterized by larger (4.4 Å) or smaller (4.0 Å) intermolecular separations. The energy splitting between the ground states of these two isomers is 0.877 cm−1 in (12C16O)2, and 1.285 cm−1 in (13C16O)2. The observed isotope shifts may be explained in terms of a simple anharmonic interaction model.
The influence of the detachment of electrons on the properties and the nature of interactions in X−H2O (X=Cl, Br) complexes
Authors: Roszak Szczepan , Kowal Marek , Gora Robert W. , Leszczynski Jerzy
Published: July 2002

The theoretical study of anionic and neutral halogen–water complexes is presented. The detachment of an electron from an anion leads to drastic changes in the structure and thermodynamic properties. Two possible neutral isomers separated by transition state were located. It is suggested that different neutral species are observed in photoelectron and mass spectrometry experiments.
Dynamics of probe particles during sol–gel transition of PBLG–DMF solution and the resulting gel structure
Authors: Oikawa Hidetoshi , Nakanishi Hachiro
Published: July 2002

The dynamics of polystyrene microgel (PSMG), i.e., a probe particle of highly cross-linked polystyrene latex spheres with radii 20.7 and 284 nm, during the sol–gel transition of poly(γ-benzyl-L-glutamate) (PBLG) dissolved in N,N-dimethylformamide (DMF) solution has been investigated by measurements with the dynamic light scattering technique. All the intensity time correlation functions (ICF) measured from PSMG dispersed in PBLG matrix approximately decayed single exponentially. First, it was confirmed that the diffusion coefficients of PSMG at the sol state of 298.8 K obeyed the stretched exponential relation as a function of concentration of PBLG–DMF solutions. Second, the initial amplitude and the normalized variance of ICF, and the diffusion coefficient of PSMG were altered considerably before and after sol–gel transition with decreasing temperature. The initial amplitude and the normalized variance had a minimum and a maximum at the gelation threshold temperature, respectively. It was also found that PSMG almost free-diffused even at the gel state, as well as the sol state, regardless of low temperature. These facts may strongly support that the resulting PBLG gels induced through microphase separation evidently had a porous and bicontinuous structure at the “swollen gel state,” which corresponds to the results observed by a transmission electron microscope.
The convergence of spin–orbit configuration interaction calculations for TlH and (113)H
Authors: Jeong Choi Yoon , Han Young-Ku , Lee Yoon Sup
Published: July 2002

To test the convergence of spin–orbit effects for molecules, the ground states of TlH and (113)H are calculated by configuration interaction(CI) calculations using relativistic effective core potentials with one-electron spin–orbit operators. The employed CI methods are the Kramers’ restricted CI (KRCI) and the spin–orbit CI (SOCI) methods. The KRCI method includes the spin–orbit interactions in the generation of one-electron basis space through the use of the two-component molecular spinors obtained by the Kramers’ restricted Hartree–Fock (KRHF) method, whereas the SOCI adds the spin–orbit term only at the CI level. For systems with heavy atoms, orbital relaxations due to the spin–orbit interaction could become sizable, resulting in slow convergences for the SOCI method. Spin–orbit effects on bond lengths and energies using single- and multireference CI calculations at the SOCI level of theory are evaluated and compared with KRCI results for TlH and (113)H. The spin–orbit effects on energies converge easily for TlH but slowly for (113)H. Especially, bond lengths do not converge for the seventh-row (113)H in our calculations. The present results imply that large-scale multireference SOCI calculations are necessary for some molecules to recover orbital relaxation effects due to large spin–orbit interactions in the SOCI scheme. In those cases, the KRCI scheme based upon two-component spinors will have advantages over SOCI and other one-component orbital based methods.
Coherent control of quantum chaotic diffusion: Diatomic molecules in a pulsed microwave field
Authors: Gong Jiangbin , Brumer Paul
Published: July 2002

Extensive phase control of quantum chaotic diffusion is demonstrated for diatomic molecules periodically kicked with microwave pulses. In particular, both complete suppression of chaotic diffusion as well as its enhancement can be achieved by varying the phase of the initial superposition state. The origin of this control in deviations from random matrix theory is also discussed. The results should motivate experiments that are relevant to both coherent control and to quantum chaos.
Investigation of structural changes in Ni19 and Ni23 induced by adsorption of hydrogen/deuterium and ammonia
Authors: Parks E. K. , Nieman G. C. , Riley S. J.
Published: July 2002

Two-reagent reactions of Ni19 and Ni23 with hydrogen/deuterium and ammonia are studied to probe adsorbate-induced cluster structural changes. H/D and NH3 bind to the clusters noncompetitively, i.e., to different sites, so simultaneous saturation by the two reagents can be achieved. It is found that H/D adsorption causes a change in Ni19 from the double icosahedral structure to a face-centered cubic (fcc) or hexagonal close-packed (hcp) one, but that subsequent saturation with NH3 converts the cluster back to the double icosahedron. No such structural changes are seen for the triple icosahedral Ni23. The results for Ni19 are interpreted in terms of the electron withdrawing or donating character of the ligands and the consequent effect on the extent of d-orbital bonding in the cluster. Possible configurations of the H/D atom binding sites on the icosahedral Ni19 and Ni23 clusters and on the fcc/hcp Ni19 cluster are presented.
High-efficiency red electroluminescence from a narrow recombination zone confined by an organic double heterostructure
Authors: Xie Z. Y. , Hung L. S. , Lee S. T.
Published: July 2002

Red light-emitting diodes (LEDs) with both a conventional bilayer structure and a double heterostructure (DH) have been investigated. In these LEDs, N,N′-bis-(1-naphthl)-diphenyl-1, 1′-biphenyl-4,4′-diamine (NPB), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and tris(8-quinolinolato) aluminum (Alq3) were used as hole-transporting, hole-blocking, and electron-transporting layers, respectively. The bilayer and DH LEDs had a configuration of ITO/NPB/Alq3:red dopant/Alq3/MgAg and ITO/NPB/Alq3:red dopant/BCP/Alq3/MgAg, respectively. Three kinds of red fluorescent dyes—nile red, DCJTB, and DCM—were used as dopants. Compared with the bilayer structures, the luminance efficiencies of the DH LEDs were found to increase as much as 100%. We attribute the efficiency enhancement to the formation of a narrow recombination zone, in which both charge carriers and excitons were confined. High charge concentrations in the emissive layer resulted in efficient collision capture in the electron–hole recombination process. Exciton confinement led to improved energy transfer. The two factors were simultaneously operating and consequently benefitted from efficiency enhancement.
The solid–liquid interfacial free energy of close-packed metals: Hard-spheres and the Turnbull coefficient
Authors: Laird Brian B.
Published: July 2002

Largely due to its role in nucleation and crystal-growth, the free energy of the crystal-melt interfacial free energy is an object of considerable interest across a number of scientific disciplines, especially in the materials-, colloid-, and atmospheric sciences. Over 50 years ago, Turnbull observed that the interfacial free energies (scaled by the mean interfacial area per particle) of a variety of metallic elements exhibit a linear correlation with the enthalpy of fusion. This correlation provides an important empirical “rule-of-thumb” for estimating interfacial free energies, but lacks a compelling physical explanation. In this work we show that the interfacial free energies for close-packed metals are linearly correlated with the melting temperature and are therefore primarily entropic in origin. We also show that the slope of this linear relationship can be determined with quantitative accuracy using a hard-sphere model, and that the correlation with the enthalpy of fusion reported by Turnbull follows as a consequence of the fact that the entropy of fusion for close-packed metals is relatively constant.
Electron-beam enhancement of ion charge state fractions in the metal-vapor vacuum-arc ion source
Authors: Bugaev Alexey , Gushenets Vasily , Yushkov George , Oks Efim , Kulevoy Timur , Hershcovitch Ady , Johnson Brant M.
Published: July 2002

We report demonstrations of ion charge-state enhancement for an electron-beam metal-vapor vacuum-arc (E-MEVVA) ion source. Results with a lead cathode yielded a maximum ion charge state of Pb7+, which implies an ionization potential of at least 130 eV. Electron current densities j=70 A/cm2 and ionization times τ≅100 μs produced jτ=9.2×10−3 C/cm2 (5.8×1016 electrons/cm2). Standard analysis for these conditions indicates—somewhat surprisingly—that successive single (stepwise) ionization accounts for the present observations, even though the charge states are substantially higher than most previous results with MEVVA-based ion sources.
Path integral molecular dynamics method based on a pair density matrix approximation: An algorithm for distinguishable and identical particle systems
Authors: Miura Shinichi , Okazaki Susumu
Published: July 2002

In this paper, the path integral molecular dynamics (PIMD) method has been extended to employ an efficient approximation of the path action referred to as the pair density matrix approximation. Configurations of the isomorphic classical systems were dynamically sampled by introducing fictitious momenta as in the PIMD based on the standard primitive approximation. The indistinguishability of the particles was handled by a pseudopotential of particle permutation that is an extension of our previous one [J. Chem. Phys. 112, 10 116 (2000)]. As a test of our methodology for Boltzmann statistics, calculations have been performed for liquid helium-4 at 4 K. We found that the PIMD with the pair density matrix approximation dramatically reduced the computational cost to obtain the structural as well as dynamical (using the centroid molecular dynamics approximation) properties at the same level of accuracy as that with the primitive approximation. With respect to the identical particles, we performed the calculation of a bosonic triatomic cluster. Unlike the primitive approximation, the pseudopotential scheme based on the pair density matrix approximation described well the bosonic correlation among the interacting atoms. Convergence with a small number of discretization of the path achieved by this approximation enables us to construct a method of avoiding the problem of the vanishing pseudopotential encountered in the calculations by the primitive approximation.
Nanoscopic inhomogeneities induced by spinodal decomposition and irreversible chemical growth reactions
Authors: Schulz Michael , Reineker Peter
Published: July 2002

We present a hydrodynamic limit description of the freezing of spinodal decomposition in multifunctional monomer mixtures, which allow for simultaneous noninterfering irreversible chemical reactions. From our results we conclude that the chemical reaction can lead both to an onset of spinodal decomposition also in the case of an initially completely miscible system and to an extreme retardation of the dynamics of the spinodal decomposition with the probability of a general freezing of these processes. A typical process in mind is the formation of interpenetrating polymer networks which allow the experimental observation of the freezing of spinodal decomposition at mesoscopic or nanoscopic scales.