bluuebunny/crossref_metadata_2025 huggingface dataset:
Stark field induced perturbations in the ν2+3ν3 vibrational overtone band of acetylene
Authors: Barnes J. A. , Gough T. E. , Stoer M.
Published: July 2002

The ν2+3ν3 vibrational overtone of acetylene was recorded using an optothermal molecular beam spectrometer. The sample excitation region was surrounded by a build-up cavity and by electrodes which allowed the observation of Stark splittings of the spectral transitions. An analysis of the J dependence of the splittings determined that the (01300)0 is mixed by the Stark field with the IR (infrared) dark state (11200)0, and that the energy of the ν1+ν2+2ν3 transition is 11 611.585±0.018 cm−1. The spectra also showed transitions to highly excited bending levels: These transitions were observable because they borrowed intensity from the more intense ν2+3ν3 stretching overtone as they were tuned into coincidence. The bending states were found to have extremely high polarizabilities which may account for previously reported collision dynamics.
Structure and phase behavior of square-well dimer fluids
Authors: Taylor Mark P. , Luettmer-Strathmann Jutta , Lipson J. E. G.
Published: July 2002

A Born–Green–Yvon integral equation approach is used to study the structure and phase behavior of a fluid of fused square-well-sphere dimers. We compute site–site distribution functions for dimers with bond lengths of L=0.6σ, 0.8σ, and 1.0σ (where σ is the hard-sphere diameter) and square-well diameters ranging from λ=1.25σ to 2.0σ over a wide range of temperature and density, and make comparisons with both exact and simulation results. For the tangent square-well dimer fluid having λ=1.5 we obtain a binodal and spinodal via the energy route, and compare our results with Monte Carlo data. The computational intensity of this approach has lead us to search for an alternate route to information about phase behavior, and so we also show how our compressibility results may be used in order to construct approximate spinodal curves.
In situ observation of Si(001) surface in He atmosphere at high temperatures near the bulk melting temperature
Authors: Kimura Masao , Ikari Atsushi
Published: July 2002

Surface melting of Si(001) surface has been investigated by x-ray reflectivity up to the bulk melting temperature: Tm;b. An in situ system was designed for measurements of x-ray reflectivity at high temperatures under an atmosphere of He-gas flow. It has been shown that the surface density changes at T=Tm;s*T>Tm;s*, which is larger than that of the bulk solid, is 2.5(1)×103 kg/m3 which is almost the same as that of the bulk liquid, though a macroscopic melting does not occur. This reversible phenomenon indicates the surface melting at T=Tm;s*.
Finite length diocotron modes
Authors: Hilsabeck T. J. , O’Neil T. M.
Published: July 2002

Diocotron modes are discussed for a finite length nonneutral plasma column under the assumption of bounce averaged E×B drift dynamics and small Debye length. In this regime, which is common to experiments, Debye shielding forces the mode potential to be constant along field lines within the plasma (i.e., ∂δφ/∂z=0). One can think of the plasma as a collection of magnetic-field aligned rods that undergo E×B drift across the field and adjust their length so as to maintain the condition ∂δφ/∂z=0 inside the plasma. Using the Green function (for a region bounded by a conducting cylinder) to relate the perturbed charge density and the perturbed potential, imposing the constraint ∂δφ/∂z=0, and discretizing yields a matrix eigenvalue problem. The mode eigenvector δNl,ω(rj)≡∫dz δnl,ω(rj,z) is the lth azimuthal Fourier component of the z-integrated density perturbation, and the frequency ω is the eigenvalue. The solutions include the full continuum and discrete stable and unstable diocotron modes. Finite column length introduces a new set of discrete diocotron-like modes. Also, finite column length makes possible the exponential growth of l=1 diocotron modes, long observed in experiments. The paper focuses on these two problems. To approach quantitative agreement with experiment for the l=1 instabilities, the model is extended to include the dependence of a particle’s bounce averaged rotation frequency on its axial energy. For certain distributions of axial energies, this dependence can substantially affect the instability.
Perturbation-allowed rotational transitions and A1–A2 splitting transitions in the ground, v2=1 and v4=1 vibrational states of SbH3 observed by microwave Fourier transform spectroscopy: Extension of the effective hyperfine Hamiltonian
Authors: Harder H. , Gerke C. , Fusina L.
Published: July 2002

Using the technique of Fourier transform microwave spectroscopy in the range 1–14 GHz Q-branch rotational transitions have been observed for SbH3122 and SbH3123 in the ground, v2=1 and v4=1 vibrational states with an accuracy of 0.1–100 kHz. A1–A2 splitting transitions for k=±3 in the ground state, k=±3, ±9 in v2=1, and for kl=+1, −2, +4 in the v4=1 vibrational state have been observed. We also measured perturbation-allowed transitions with selection rule Δk=±3 in the ground vibrational state for k=±1↔∓2, with selection rules Δ(k–l)=3,6,9 in the v2=1 state for k=±1↔∓2, 0↔±3, ±2↔∓4, ±2↔∓7, and 0↔±9 and in the v4=1 state for kl=+2↔+3, +2↔+6, and +3↔+5. The transitions show hyperfine structures due to the quadrupole and spin-rotation coupling of the nuclear spin ISb and the rotational angular momentum J. Hyperfine structures in the dyad v2=1, v4=1 have been analyzed using an effective Hamiltonian extended to higher order spin-rotation coupling terms and including spin-vibration coupling. A total of 21 hyperfine parameters has been determined for each isotopomer including quadrupole and spin-rotation constants of the (Δl,Δk)=(0,3), (2,2), and (2,−1) interactions. A similar analysis has been performed for the ground vibrational state yielding 7 (6) hyperfine parameters for SbH3121 (123SbH3) including the (0,3) interaction constants. Splittings of transitions between E-states involving basis states with k=±1 have been observed in the ground, v2=1 and v4=1 vibrational states. This splitting has been unequivocally explained as lifting of parity degeneracy by proton hyperfine interactions. From the analysis of the ground state hyperfine doublets, tensorial constants of the H spin-rotation coupling and the Sb–H spin–spin interaction have been accurately determined.
Transport and confinement properties of stellarators/heliotrons with the multiple-toroidicity magnetic fields
Authors: Smirnova M. S.
Published: July 2002

Specific features of charged particle confinement and ripple transport in the low-collision frequency (1/ν) regime have been investigated in stellarator-type devices with discrete toroidal or modular field coils. For such stellarator configurations, a multiple-helicity and a multiple-toroidicity character of the magnetic field is typical, manifested itself in the presence of the distant harmonics εj,N with high-order N⩾2 toroidal mode numbers in a Fourier-decomposition of the magnetic field. The main purpose of the paper was an analysis of the role of these distant harmonics in controlling both direct and indirect losses of charged particles in stellarators. It has been discovered that in configurations, as considered, the level of the neoclassical transport in the 1/ν regime can be one order reduced by properly chosen coil currents generated in Fourier decomposition, the distant N=2 satellite harmonics with strong amplitudes and suitable ripple phases.
CO
+
B
2
Σ
+
(ν=0)
emission induced by laser excitation of neutral CO at 230 nm
Authors: Di Teodoro F. , Farrow R. L.
Published: July 2002

We observed emission from electronically excited CO+ B 2Σ+(ν=0) after laser excitation of ground-state CO with a picosecond dye laser. The laser frequency was tuned to the CO two-photon transition X 1Σ+(ν″=0)→→B 1Σ+(ν′=0) at 230 nm, resulting in 2+1 resonance-enhanced ionization of CO with production of CO+ X 2Σ+, followed by resonant excitation of the one-photon X 2Σ+(ν+′=1)→B 2Σ+(ν+=0) transition in CO+. We provide direct evidence for this process by detecting CO+ B 2Σ+(ν+=0)→X 2Σ+ emission after the laser excitation. An analysis of the pulse-energy and pressure dependence of the CO+ emission is presented to derive collisional quenching rate coefficients for CO+ B 2Σ+.
Spectral line shapes of single molecules beyond the sudden jump model
Authors: Plakhotnik Taras
Published: July 2002

Detailed analysis is done for the case of a single pseudo local phonon interacting with an electronic molecular transition and a phonon bath. Both the sudden-jump model and the master equation formalism predict a complex optical line shape for a single-molecule embedded in a solid matrix. In addition to the conventional Lorentzian, the line consists of a dispersive component in the core region and a side band. The sudden jump model is applicable only if the vibration-phonon coupling is independent of the electronic wave functions. However if the line wings are neglected, the sudden jump model can fit experimental data even when the condition of its applicability is not fulfilled. An interesting property of the complex line was found at strong excitation. In this case the line behaves in a fashion intermediate to the homogeneously and inhomogeneously broadened spectra.
Correlation between elastic and inelastic atom scattering from single adsorbed molecules
Authors: Choi B. H. , Graham A. P. , Tang K. T. , Toennies J. P.
Published: July 2002

High resolution measurements of the angular distributions for the elastic and inelastic scattering of helium atoms from the translational vibrations (T modes) of single CO molecules on Pt(111) agree quantitatively with a simple transformation which predicts the inelastic cross section from the measured elastic cross section. The new theory predicts not only the inelastic interference structures and their correlations with the elastic channel, but also the individual state-to-state transitions.
Quantum interference and asymptotic interactions in the photodissociation of SH: Total cross section and branching ratios
Authors: Lee Sungyul , Sun Hosung , Kim Bongsoo , Freed Karl F.
Published: July 2002

A theoretical analysis is presented for the photodissociation processes of SH to S(3P) and S(1D). Transition dipole moments from the ground X 2Π state to the A 2Σ+, Σ−2, Δ,2 2 2Π states are computed by the effective valence shell Hamiltonian method. Two frame transformation matrices are constructed and used to describe the correlations between the two sulfur atomic terms [S(3P) and S(1D)] and the adiabatic Born–Oppenheimer molecular states. Very interesting dynamics of quantum interference effects and asymptotic interactions are found. At energies between the thresholds to the S(3P) and S(1D) limits, the resonances are mostly Lorentzian with more or less constant S(3Pj,j=0,1,2) branching ratios. The effects of the intrastate interactions between the repulsive states are predicted to be very strong. At energies above the threshold to S(1D) limit, quantum interferences between the dissociative pathways through the optically bright repulsive states (A 2Σ+, Σ−2, Δ,2 and 2 2Π states) are predicted to give asymmetric resonances of multichannel character. Partial cross sections to the triplet sulfur fine structure states S(3Pj,j=0,1,2) exhibit different degrees of asymmetry and, consequently, the S(3Pj,j=0,1,2) branching ratios display strong variations across the asymmetric resonances, suggesting the possibility of controlling the product distributions by scanning the excitation wavelengths across a single asymmetric resonance in a one-photon excitation process. At higher energies, the interference between the two direct dissociation routes (by A 2Σ+ and Σ−2 states) is shown to produce highly oscillatory variations of the total cross section for dissociation to S(3P) and of the branching ratios of S(3Pj,j=0,1,2).
The temperature dependent dielectric function of liquid benzene: Interpretation of THz spectroscopy data by molecular dynamics simulation
Authors: Nymand Thomas M. , Rønne Cecilie , Keiding Søren R.
Published: July 2002

The dielectric function of liquid benzene at −4 °C, 21 °C, and 65 °C has been calculated from molecular dynamics simulations. The simulated dielectric loss curves reproduce the experimental temperature and density trends. In order to investigate the detailed influence of temperature and density changes as well as the underlying molecular mechanism we have taken advantage of different partitioning and projection schemes for the total dipole moment time correlation function (TCF). The study of the n-body partitioning showed that the temperature dependence of the two- and three-body contributions at t=0 can be explained solely by density change arguments. The molecular projection scheme showed that the dielectric loss is governed by out-of-plane libration at all temperatures. In-plane libration was found to contribute significantly only below 2 THz. Below 1 THz, diffusion, manifested as the negative cross correlation between the out-of-plane and the in-plane TCF’s, plays a role very different from that of the directly observed diffusion in dipolar liquids. It has further been established that it is highly problematic to carry out an analysis of the dielectric loss function in terms of the molecular axis rotational TCF’s which is a common procedure for the absorption spectrum. This problem was, however, solved by employing a molecular projection scheme.
Numerical investigation on plasma and poly-Si etching uniformity control over a large area in a resonant inductively coupled plasma source
Authors: Kim S. S. , Hamaguchi S. , Yoon N. S. , Chang C. S. , Lee Y. D. , Ku S. H.
Published: July 2002

In high-density plasma etching processes for ultra-large-scale integrated (ULSI) circuits, the uniformity of the plasma over a large area is of major concern. Recently a resonant inductively coupled plasma source [S. S. Kim et al., Appl. Phys. Lett. 77, 492 (2000)] has been proposed for large-area plasma processing, which achieves large-area plasma uniformity by properly tuning its antenna with an external variable capacitor. In the present paper, the plasma transport and poly-Si etching characteristics of this plasma source have been numerically investigated using a self-consistent model for electron heating, plasma transport, and microscopic etching profiles. The numerical simulation results indicate that uniform poly-Si etching over 300 mm in diameter can be easily achieved in this plasma source.
A model for evaluating compression effects in the mθ=1 diocotron instability
Authors: Coppa G. G. M. , D’Angola A. , Delzanno G. L. , Lapenta G.
Published: July 2002

Herein is presented a refinement of the theory published by Finn et al. (1999) for the mθ=1 diocotron instability. A rigorous definition of the plasma length is introduced and the expression for the velocity field is improved, inasmuch as the effect of the finite size of the plasma column is included. The effect of the perturbation of the plasma length is considered rigorously by using a Green-function approach. A parametric study of the instability for a hollow profile is shown.
Scaled higher-order correlation energies: In pursuit of the complete basis set full configuration interaction limit
Authors: Császár Attila G. , Leininger Matthew L.
Published: July 2002

A simple multiplicative approach is presented for approximating the full configuration interaction (FCI) limit at the complete basis set limit from the small basis set FCI and coupled cluster [most notably CCSD(T) and CCSDT] calculations. The proposed scaled higher-order correlation (SHOC) correction scheme is demonstrated to extrapolate CCSD(T) and CCSDT correlation energies for BH and AlH to the FCI limit with remarkable accuracy, and to correct the dissociation energies of [CN,C2,N2] computed at the CCSD(T) and CCSDT levels by [+1.4, +2.3, +1.5] kcal/mol, respectively, bringing them in much closer agreement with the best experimental values. The approach is also well suited for the generation of accurate potential energy hypersurfaces.
Self-organized quantum wires formed by elongated dislocation-free islands in (In,Ga)As/GaAs(100)
Authors: Ma Wenquan , Nötzel Richard , Trampert Achim , Ramsteiner Manfred , Zhu Haijun , Schönherr Hans-Peter , Ploog Klaus H.
Published: July 2002

Long and fairly uniform quantum wire arrays have been fabricated by the growth of (In,Ga)As/GaAs multilayer structures. The structural properties of the quantum wires are characterized by atomic force microscopy, x-ray diffractometry, and transmission electron microscopy. The lateral carrier confinement in the quantum wires is confirmed by linear polarization dependent photoluminescence (PL) and magneto-PL measurements.
Shear Alfvén mode resonances in nonaxisymmetric toroidal low-pressure plasmas. I. Mode equations in arbitrary geometry
Authors: Salat A. , Tataronis J. A.
Published: July 2002

Field line coordinates are used to obtain a concise set of three coupled partial differential equations for the modes in ideal magnetohydrodynamic plasmas of arbitrary geometry. Zero plasma pressure is assumed throughout. The shear Alfvén continuum and the pressureless remnant of the ballooning continuum are readily obtained in a unifying manner. The qualitative differences and similarities of both spectra are discussed. The equations are suitable, in particular, for the study of the spatial dependence of continuum modes close to singular magnetic surfaces or field lines.
The medium response to an impulsive redistribution of charge in solid argon: Molecular dynamics simulations and normal mode analysis
Authors: Jimenez S. , Chergui M. , Rojas-Lorenzo G. , Rubayo-Soneira J.
Published: July 2002

Excitation of the A(3sσ) Rydberg state of NO leads to an extensive rearrangement of the environment, which we have investigated by classical molecular dynamics simulations and normal mode analysis, using pair potentials from the literature. We find that the medium response is independent of the details at long range of the excited state NO A-Ar potential, stressing the fact that it is mainly driven by the short range repulsive forces between the Rydberg electron and the matrix atoms. We establish the inertial character of the first shell response in the initial 100–150 fs after excitation, as the next shells are silent over this time scale. The expansion of the first shell at early times, induces the propagation of a supersonic wave along the (011) axis of the crystal, which define 12 linear chains of atoms with the impurity. The early time response is followed by vibrational coherences with a complex behavior. The normal modes analysis of the crystal shell by shell shows good agreement with the power spectra of the MD trajectories. It allows us to identify the most significant modes in the medium response. Overall, the dynamics of the system may be regarded as that of a NOAr12 supermolecule, embedded in an Ar lattice and undergoing vibrational energy redistribution.
Monte Carlo simulation for the adsorption of diblock copolymers. I. In nonselective solvent
Authors: Chen Ting , Liu Honglai , Hu Ying
Published: July 2002

Extensive Monte Carlo simulations have been performed to study the adsorption of diblock copolymers from a nonselective solvent on an impenetrable surface. The efforts were concentrated on depicting the microstructure of adsorption layers. In simulations, diblock copolymer molecules are modeled as self-avoiding linear chains composed of rA segments of A and rB segments of B, where the former is attractive to the surface while the latter is nonattractive. The adsorption information including segment density profiles, adsorption amount and isotherms, adsorption layer thickness, bound fraction and surface coverage were obtained by detailed analyses on comprehensive simulation data under various conditions. The microstructure of adsorption layers, primarily the profiles of the adsorbed segments corresponding to tails, loops, and trains, and the size distributions of these adsorption configurations are presented. As a whole, the adsorption layer thickness is mainly determined by the length of the nonattractive block. The effect of the adsorption energy and the chain composition f, the latter is the proportion of attractive segments A in a diblock copolymer chain, on various adsorption properties has been inspected. Comparisons between results of this work and those of previous simulations as well as corresponding experiments were made and many useful conclusions have been drawn. It is shown that the adsorption amount increases monotonically with the increase of f when the adsorption energy is relatively small. However, if the adsorption energy has a larger value, the adsorption amount exhibits a maximum at certain value of f dependent on the length of the block A and the magnitude of the adsorption energy. This trend coincides well with the experimental results of Tiberg et al. [Langmuir 10, 2294 (1994)] and Evers et al.’s SCF calculations [J. Chem. Soc., Faraday Trans. 86, 1333 (1990)]. Why some previous work failed to simulate this phenomenon is also explained.
Effects of magnetic curvature on the lower-hybrid-drift instability
Authors: Ueno Genta
Published: July 2002

A local dispersion relation is derived for the lower-hybrid-drift instability including the effects of magnetic curvature associated with transverse electromagnetic perturbations. To account for the curvature drift, an alternative method proposed by Nakamura [Phys. Plasmas 4, 3765 (1997)] is applied to obtain the perturbed distribution function. It is found that the previous treatment, i.e., simulating the curvature drift by a virtual gravitational drift, is considerably inaccurate. When an ambient magnetic field has a curvature so that the curvature drift is directed opposite the ∇B drift, the maximum growth rate increases as long as the radius of curvature is larger than a certain value, while the rate decreases for a sharper magnetic field curvature. At the same time, the wave number giving the maximum growth decreases monotonically. The growth rate is increased by a curvature drift coincident with the ∇B drift. The effects of the magnetic curvature become larger in high-beta plasmas.
Submillimeter wave vibration–rotation spectroscopy of Ar⋅CO and Ar⋅ND3
Authors: Melnik Dmitry G. , Gopalakrishnan Sandhya , Miller Terry A. , De Lucia Frank C. , Belov Sergey
Published: July 2002

A new absorption spectrometer operating in the submillimeter wave region has been designed. The apparatus combines the previously reported fast scan submillimeter wave spectroscopic technique (FASSST) and a pulsed supersonic jet sample. It is specially designed for the rapid searching for unknown molecular transitions of weakly absorbing and/or low abundance species. Vibration–rotation transitions of the Ar⋅CO and Ar⋅ND3 van der Waals molecules have been observed using this apparatus. Transitions in Ar⋅CO involve the j=1→j=2 hindered rotor transition in the complex. Transitions in Ar⋅ND3 have been assigned as the rotational structure of the fundamental of the lowest frequency bending mode involving the two moieties. Transitions have been observed emanating from the levels correlating to the two lowest energy inversion components of ND3 yielding a determination of the inversion splitting in the complex.
Comparison of plasma sheet dynamics during pseudobreakups and expansive aurorae
Authors: Fillingim M. O. , Parks G. K. , Chen L. J. , McCarthy M. , Spann J. F. , Lin R. P.
Published: July 2002

Global auroral images and plasma sheet ion distributions and magnetic field data are examined for two intervals when the Ultraviolet Imager (UVI) onboard the Polar spacecraft was imaging the entire northern auroral oval and, at the same time, the Wind spacecraft was passing through the near-Earth plasma sheet. On 26 July 1997, UVI recorded a series of brief, localized auroral brightenings known as pseudobreakups. On 27 March 1996, UVI observed several global expansions of auroral activity. Large variations in the magnetic field were observed by the Wind magnetometer, large velocity moments were derived from Wind ion measurements, and ions were accelerated to mega-electron-volt energies during both types of activity. The plasma sheet dynamics appear very similar during these two different types of auroral activities. Closer inspection of the ion distribution functions and energy spectra indicate that the plasma sheet dynamics need to be characterized kinetically.
The spinning terrella plasma experiment: Initial results
Authors: Sheldon Robert B. , Spurrier Scott
Published: July 2002

The observation of 40 keV field-aligned plasma flows (Sheldon et al., 1998) has been conjectured as the result of a space-charge driven instability generating kV parallel potentials (Sheldon, 1999), which occur whenever hot plasma drifts in an inhomogeneous magnetic field. Such conditions occur in almost any magnetic field that interacts with an energetic plasma: at the Earth, the Sun, Jupiter, and perhaps even astrophysical magnetospheres of stars, black holes and active galactic nuclei, all of which possess collimated flows. From this ubiquity, field-aligned flows were looked for in a table top plasma experiment involving a permanent magnet and a direct current (dc) discharge source of energetic plasma. It is emphasized that a one-fluid plasma theory such as magnetohydrodynamics is incapable of describing a parallel potential drop, and that most second-order corrections to the theory predict weak parallel voltages proportional to the thermal energy. In contrast, it is shown photographs of the plasma and peculiar high-voltage (500 eV≫kT) discharges suggestive of a large field-aligned potential drop. It is proposed that this may be a manifestation of the quasi-neutral catastrophe heretofore neglected by space and laboratory plasma physicists, which may unify many disparate observations.
Ferromagnetic interactions in doped semiconductors and their nanostructures (invited)
Authors: Dietl Tomasz
Published: July 2002

The current status and prospects of research on ferromagnetism in tetrahedrally coordinated magnetic semiconductors and their nanostructures are reviewed. Results of a quantitative comparison between experimental and theoretical results, notably for Mn-based III–V and II–VI thin films and heterostructures, are presented. This comparison demonstrates that the current theory of the hole-mediated exchange interactions correctly describes the values of Curie temperatures TC, magnetic anisotropy, and magnetic circular dichroism. On this basis, chemical trends are examined and shown to lead to the prediction of semiconductor systems with TC that may exceed room temperature. Hopes associated with materials containing magnetic ions other than Mn are discussed.
Dispersion of the first hyperpolarizability of a strongly charge-transfer chromophore investigated by tunable wavelength hyper-Rayleigh scattering
Authors: Hsu Chia-Chen , Liu Sean , Wang Chen Chi , Wang C. H.
Published: July 2002

The first hyperpolarizability (β) of a strongly charge-transfer (azulenic-barbituric) chromophore is measured by hyper-Rayleigh scattering (HRS) experiment using 11 excitation wavelengths, ranging from 900 nm to 1907 nm. The measured wavelength-dependence of the β value is used to investigate the dispersion relation of the first hyperpolarizability of the charge-transfer chromophore. When excited by the 1064 nm wavelength laser radiation, the experimental HRS spectrum shows a two-photon resonance (TPR) peak, which does not coincide with the maximum of the linear absorption spectrum. The wavelength dependence of the first hyperpolarizability cannot be explained by an undamped or damped two level model. However, incorporating vibrational motion in the damped two-level model results in a good fit between theoretical and experimentally measured β, especially in the vicinity of TPR region.
The role of interfaces in the magnetoresistance of Au/Fe/Au/Fe/GaAs(001)
Authors: Enders A. , Monchesky T. L. , Myrtle K. , Urban R. , Heinrich B. , Kirschner J. , Zhang X.-G. , Butler W. H.
Published: July 2002

The electron transport and magnetoresistance (MR) were investigated in high quality crystalline epitaxial Fe(001) and Au(001) films and exchange coupled Au/Fe/Au/Fe/GaAs(001) trilayer structures. Fits to the experimental data were based on the semiclassical Boltzmann equation, which incorporates the electronic properties obtained from first-principles local density functional calculations. The fits require a surprisingly high asymmetry for the spin dependent electron lifetimes in Fe, τ↓/τ↑=10 at room temperature. Despite the large atomic terraces at the Au/vacuum and Fe/GaAs interfaces the scattering at the outer interfaces was found to be diffuse. The origin of MR in Au/Fe/Au/Fe/GaAs(001) structures is due to electron channeling in the Au spacer layer. The measured MR is consistent with the diffusivity parameters s↑=0.55, s↓=0.77 at the metal–metal interfaces.
Leveraging deep photonic band gaps in photonic crystal impurity bands
Authors: Lan Sheng , Nishikawa Satoshi , Wada Osamu
Published: July 2002

Photonic band gaps can be generated in the impurity bands of photonic crystals formed by periodically placed defects. Even slight periodic modulation of the properties of these defects can open up very deep band gaps in the impurity bands. This phenomenon originates from the concentration of electromagnetic field at the defect regions, making electromagnetic wave extremely sensitive to the small changes of the defects. A dynamical photonic band gap in the impurity band created by a control light, provides a mechanism for constructing high-efficiency optical switches.
The role of exchange and correlation in time-dependent density-functional theory for photoionization
Authors: Stener M. , Decleva P. , Görling A.
Published: July 2002

Atomic photoionization cross sections are calculated by time-dependent density-functional (TDDF) methods using different exchange-correlation potentials including the exact one. The exchange-correlation kernel is treated in the adiabatic local density approximation (ALDA). Results for the exact full and the exact exchange-only Kohn–Sham (KS) potential are very similar, the calculated photo cross section agree very well with experimental data. Thus the exact correlation potential seems to have no influence on photoionization and the ALDA for the exchange-correlation kernel seems to be sufficient for most features of the cross sections. The TDDF method employing the exact exchange-only KS potential in combination with the ALDA exchange-correlation kernel therefore is a promising approach to describe photoionization. Deviations from experiment are observed for the widths and shape of the autoionization resonances and have to be attributed to deficiencies of the ALDA exchange-correlation kernel. The calculation of widths and shapes of autoionization resonances therefore may serve as a severe test for new approximate exchange-correlation density-functionals. The asymptotically exact exchange-correlation potential of van Leeuwen and Baerends also leads to quite good photo cross section, which, however, shows deficiencies close to the ionization threshold and in the energetic position of the autoionization resonances. Supplementation of the exact exchange potential with the LDA correlation potential leads to a worsening of the photo cross section because the LDA correlation potential is too attractive.
Cylinder
↔
sphere
epitaxial transitions in block copolymer melts
Authors: Matsen M. W.
Published: July 2002

We examine the Landau free energy surface of a diblock copolymer melt using self-consistent field theory. The topography of its surface is found to exhibit a low-energy pathway connecting the local minima associated with the cylindrical (C) and spherical (S) morphologies. The pathway corresponds to an epitaxial transition where the periodicity of the two phases is matched with the cylinder axis oriented in one of the [111] directions of the bcc spheres. The energy barriers and stability limits are evaluated, and from them we conclude that the C↔S transition should normally occur by a nucleation and growth mechanism. Based on the generally small energy barriers, we suggest that fluctuation effects are much stronger than previously anticipated. Furthermore, we examine the way the morphology evolves during a transition and discuss its signature in a small-angle scattering experiment.
What determines the emission peak energy of the blue luminescence in highly Mg-doped p-GaN?
Authors: Colton J. S. , Yu P. Y.
Published: July 2002

We report a study of the 2.8 eV blue luminescence (BL) in heavily Mg-doped p-GaN via resonant excitation with a tunable blue dye laser. The dependence of the BL on the excitation photon energy (Eex) is unlike that of the yellow luminescence found in n-type GaN. An Urbach-type band tail, with Urbach parameter of 33 meV is observed in the vicinity of the BL energy. We propose that the peak energy of the BL marks the transition from localized states to delocalized states within this band tail.
Constructing global functional maps between molecular potentials and quantum observables
Authors: Geremia J. M. , Rabitz Herschel , Rosenthal Carey
Published: July 2002

The relationships that connect potential energy surfaces to quantum observables can be complex and nonlinear. In this paper, an approach toward globally representing and exploring potential-observable relationships using a functional mapping procedure is developed. Based on selected solutions of the Schrödinger equation, it is demonstrated that an observable’s behavior can be learned as a function of the potential and any other variables needed to specify the quantum system. Once such a map for the observable is in hand, it is available for use in a host of future applications without further need for solving the Schrödinger equation. As formulated here, maps provide explicit information about the global response of the observable to the potential. In this paper, we develop the mapping concept, estimate its scaling behavior (measured as the number of times the Schrödinger equation must be solved during the learning process), and numerically illustrate the technique’s globality and nonlinearity using well-understood systems that demonstrate its capabilities. For atom–atom scattering, we construct a single map capable of learning elastic cross sections (i.e., differential cross sections at 2° intervals over angle, as well as integral, diffusion, and viscosity cross sections for scattering energies between 50 meV and 2 eV) involving collisions between any pair of atoms from the Periodic Table. The map for each class of cross sections over the Periodic Table is quantitative with prediction errors shown to be ≪1%. We also consider a 3Σu+ Na2 and create a rovibrational spectral map that encompasses all of the currently proposed potentials for that system. The Na2 map is highly accurate with the ability to predict rovibrational spectra with errors less than 1×10−3 cm−1 over variations in the potential that exceed 130 cm−1.
Reaction of acetaldehyde cations with water: The effects of CH3CHO+ vibrational mode and impact parameter on reactivity and product branching
Authors: Kim Ho-Tae , Liu Jianbo , Anderson Scott L.
Published: July 2002

Scattering of mode-selectively excited acetaldehyde cations from D2O was studied in a guided ion beam instrument. The effects of reactant vibrational state and collision energy on reactivity, product branching, and product ion recoil velocity distributions were measured. Ab initio calculations were performed to help understand the reaction coordinate. The dominant reaction is H/D exchange, which occurs in about 40% of low energy collisions, dropping to just a few percent at high energies. H/D exchange is also inhibited by CH3CHO+ vibration, but with a smaller effect than the equivalent amount of collision energy. H/D exchange is mediated by a long-lived complex, and several candidates are identified. The other low energy channel corresponds to methyl elimination from the collision complex. This channel is the most energetically favorable, but is only a few percent efficient, even at low energies, and is negligible at high energies. Methyl elimination is strongly suppressed by both collision energy and vibration, and the vibrational effects are nonmode specific. The most interesting channel is proton transfer (PT), which occurs by a direct mechanism at all collision energies. At low energies, PT occurs only in small impact parameter collisions, while at high energies, PT occurs primarily for large impact parameters, and is suppressed for small impact parameters. PT also shows strongly mode-specific dependence on CH3CHO+ vibrational state.
Collisional decomposition of the sulfur hexaflouride anion (SF6−)
Authors: Champion R. L. , Dyakov I. V. , Peko B. L. , Wang Yicheng
Published: July 2002

Insulating gas mixtures containing SF6 have been promoted to serve as replacements for pure SF6 in order to reduce SF6 atmospheric emission. It has been argued that some synergism may be achieved by choosing proper buffer gases in mixtures with SF6 such that the buffer gases efficiently slow down electrons into an energy range where the electron attachment cross section for SF6 is large. A complete understanding of the dielectric properties of SF6 mixtures obviously requires information about electron detachment from SF6− as collisional electron detachment may be the principal source of discharge initiation in SF6 mixtures. In this paper, we report total cross-section measurements for electron detachment and collision induced dissociation for collisions of SF6− with N2 for collision energies ranging up to a few hundred eV. The experimental results are analyzed using a two-step collision model where the unimolecular decomposition of collisionally excited SF6− ions is described in a statistical framework.
Two-color amorphous silicon image sensor
Authors: Mulato M. , Lemmi F. , Ho J. , Lau R. , Lu J. P. , Street R. A.
Published: July 2002

A large-area two-color image sensor array made with amorphous silicon (a-Si:H) technology is described. Mesa-isolated double-junction p-i-n-i-p a-Si:H sensors discriminate the two spectral bands—blue/green and red—according to the polarity of the applied voltage bias. The 512×512 element active-matrix array with 75 μm pixel pitch is addressed using a-Si:H thin-film switching transistors. Under steady state illumination, the array exhibits a linear response, good color separation, and good spatial response as measured by the line-spread function, so that images obtained with the array are clear and sharp. The response to transient illumination exhibits image lag and a strong dependence of the signal on integration time, both of which depend on the bias polarity. Switching the bias voltage also induces strong transient properties. These effects are attributed to the back-to-back diodes, which act as capacitative dividers in the generation and readout of the signal. The transient effects compromise the practical application of the color sensors for video color imaging.
Variational transition state theory and quasiclassical trajectory studies of the H2+OH→H+H2O reaction and some isotopic variants
Authors: Troya Diego , Lakin Matthew J. , Schatz George C. , González Miguel
Published: July 2002

Variational transition state theory (VTST) methods and quasiclassical trajectories (QCT) have been used to study the dynamics of the OH+H2 reaction, along with the isotopic counterparts OD+H2, OH+HD, OD+H2, OD+D2, and the reverse H+H2O→H2+OH reaction. Two new global analytical potential energy surfaces (PES) for H3O are employed, Wu, Schatz, Lendvay, Fang, Harding (WSLFH) and Ochoa, Clary (OC), both of which are based on high quality electronic structure calculations. Extensive comparisons with earlier results based on the Walch, Dunning, Schatz, Elgersma (WDSE) PES are also presented. The WSLFH PES surface, in combination with our best VTST estimate (ICVT/μOMT), yields rate constants for OH+H2 in quantitative agreement with experiment, while the OC PES yields somewhat less accurate results. The agreement with the OH+D2 experimental rate constants is less quantitative, but the WSLFH PES rate constant agrees with experiment to within a factor of 2 at all temperatures for which there are measurements. The OH+HD, OD+H2, and OD+D2 WSLFH PES rate constants calculated at the ICVT/μOMT level are in very good agreement with the less detailed experimental information that is available for these isotopes. The two surfaces give comparable predictions for the reverse H+H2O reaction at high temperatures, with deviations of less than 30%. This good agreement is maintained by the WSLFH PES at room temperature, while the OC PES predicts rate constants one order of magnitude larger than experiment. The QCT excitation functions for OH+H2, OH+D2, and OH+HD are well below experiment for both potentials, as was the case for earlier accurate quantum mechanical calculations that employed the WDSE PES. The WSLFH PES improves the agreement with the experimental vibrational state selected rate constants for the OH+H2 reaction compared to the WDSE PES. OC is also less accurate and presents antithreshold behavior for H2(v=1)+OH. H2 and OH rotational excitation have opposing effects: while rotation in H2 promotes reactivity, OH rotation impedes it. This impeding effect applies likewise to HD for high rotational excitation, explaining the selectivity toward HOH+D products in the OH+HD reaction.
Control of thermal photoinduced electron transfer reactions in the activated and activationless regimes
Authors: Pollak Eli , Plimak Lev
Published: July 2002

Photoinduced electron transfer rates depend on the internal energy distribution of the locally excited donor state. This energy distribution may be hot or cold relative to the temperature of the donor in the ground electronic state and is dependent on the photoexcitation frequency. In the activated regime, the electron transfer rate depends exponentially on the temperature of the locally excited donor state. Therefore, the electron transfer rate is sensitive to the photoexcitation frequency. In the activationless regime, even if the vibrational frequencies of the locally excited donor state and the acceptor state differ, the electron transfer rate is rather insensitive to the internal energy distribution of the locally excited donor state. Therefore, changing the photoexcitation frequency does not lead to a significant change in the transfer rate. Model computations are presented to demonstrate this qualitative difference between the two regimes, as well as to confirm that the photoinduced electron transfer rate is well-approximated as a thermal electron transfer rate, but at an effective temperature of the locally excited donor state that depends on the photoexcitation frequency.
Modification of magnetic properties of epitaxial Co/Cu multilayers by 1 MeV C+ irradiation
Authors: Sung M. C. , You D. G. , Park H. S. , Lee J. C. , Ie S. Y. , Kim I. S. , Lee J. , Whang C. N. , Im S. , Jeong K. , Kim T. G. , Yang J. S. , Song J. H.
Published: July 2002

Epitaxial [Co/Cu]10 grown on Si (001) has been irradiated by 1 MeV C+ with a dose of 1×1016/cm2. The intrinsic characteristics of epitaxy, such as crystal structure and fourfold magnetic anisotropy, are conserved after ion irradiation. However, the extrinsic magnetic properties are changed such that the coercivity decreases and the squareness of the hysteresis loop is noticeably improved. The intensity of the Cu (200) peak in the x-ray diffraction pattern increases about three times and its half-width decreases after irradiation. It is concluded that the changes of magnetic properties induced by ion irradiation result from the grain growth and the improved crystalline quality.
Infrared spectrum of the OCS–He complex
Authors: Tang Jian , McKellar A. R. W.
Published: July 2002

The weakly-bound van der Waals complex OCS–He has been observed by infrared spectroscopy in the region of the OCS ν1 vibration (≈2062 cm−1) in a pulsed supersonic jet expansion using a tunable diode laser probe. The results are in good agreement with previous microwave observations of OCS–He, but they extend to the vibrationally excited state, as well as to a considerably larger number of rotational levels with higher J-values. Prominent a-type transitions (ΔK=0) are observed, together with weaker b-type transitions (ΔK=±1). The rotational energy level pattern in the excited vibrational state is quite similar to that in the ground state, and the band origin is slightly blue-shifted (+0.111 cm−1) from that of the free OCS molecule. These results are of special interest due to recent observations in the same spectral region of OCS molecules trapped in ultra cold helium droplets.
Adsorption of HgCl2 molecules on Au(111) surfaces studied by scanning tunneling microscopy
Authors: Laakso A. , Lahtinen J. , Levlin M. , Hautojärvi P.
Published: July 2002

We have studied the adsorption of HgCl2 on Au surfaces by scanning tunneling microscopy (STM). HgCl2 was found to form a stable, ordered overlayer on Au(111) surface in ambient conditions and it was imaged with molecular resolution. We suggest that the revealed structure is (7×7)R19.1°-2HgCl2. This model supposes that the HgCl2 molecule bends in the adsorption to have a 123° angle between Cl–Hg bonds. In addition, the adsorption capacity of Au(111) surface for HgCl2 was determined to be 2:7.
Effects of pressure and temperature on the reactions of niobium cluster cations with hydrogen and deuterium
Authors: Vakhtin Andrei B. , Sugawara Ko-ichi
Published: July 2002

The effects of pressure and temperature on the reactivities of niobium cluster cations Nbn+(n=4–16) toward molecular hydrogen and H2/D2 mixture were studied using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer combined with a laser-ablation/supersonic-jet external ion source. It was shown that the rate constants of the reactions of most Nbn+ clusters with the first H2 molecule exhibit pronounced fall-off behavior in the total pressure range of 10−6–10−4 Torr. For the Nbn++H2/D2 reactions, the H/D scrambling in the reaction products and the apparent kinetic isotope effects were also found to be strongly pressure-dependent. The observed phenomena are consistent with a reaction mechanism that involves a sequence of reversible stages of dissociative chemisorption of H2 (or D2) molecules on Nbn+ clusters. The fall-off effects can explain the apparent negative temperature dependence for the Nbn++H2 reaction rate constants, the difference between the kinetic data obtained in FT-ICR and fast-flow reactor studies, and the observed enhanced kinetic isotope effects. Indirect evidence was obtained showing that the low reactivities of certain Nbn+ clusters (n=8, 10, and 16) toward hydrogen are due to activation barriers rather than low chemisorption energies.
A quasiclassical trajectory study of product energy and angular distributions for the OH+D2 reaction
Authors: Lakin Matthew J. , Troya Diego , Lendvay György , González Miguel , Schatz George C.
Published: July 2002

Product energy disposal, vibrational state populations, and angular distributions for the OH+D2→HOD+D reaction have been determined using the quasiclassical trajectory method with the recently developed WSLFH and OC potential energy surfaces (PES). The calculations agree well with recent crossed molecular beam experiments as well as reduced-dimensionality quantum methods. Angular distributions calculated using the WSLFH PES agree quantitatively with experiments, and the trends with increasing translational energy and vibrationally exciting reagents are presented. The measured average fractions of energy and HOD vibrational distributions are satisfactorily reproduced by both surfaces with slight differences between them. The new surfaces show a fair improvement over earlier potential surfaces, with the product HOD clearly exhibiting vibrational excitation in the nonspectator OD stretch mode. In addition, vibrational excitation of the reagent OH is converted completely to OH stretch in the product HOD. Product vibrational state distributions resulting from increases in reagent translational energy or vibrational excitation are presented as well, with a discussion of the differences. Analysis of such differences, along with the different predictions of earlier surfaces, can hopefully lead to a better understanding of reaction fundamentals.