bluuebunny/crossref_metadata_2025 huggingface dataset:
The K-rotational labeling problem for eigenvectors from internal rotor calculations: Application to energy levels of acetaldehyde below the barrier
Authors: Ortigoso Juan , Kleiner Isabelle , Hougen Jon T.
Published: July 2002

The problem of attaching K rotational quantum number labels to computer-generated numerical eigenvectors with extensive basis set mixing is considered for the internal-rotationoverall-rotation problem in molecules with one methyl top. Quantum number labeling problems arise physically because the torsional and the rotational degrees of freedom both pass from one limiting case to another as the torsional energy moves from below the top of the internal rotation barrier to above it, i.e., the torsional degree of freedom changes from a vibration to an internal rotation, while the rotational degree of freedom moves its direction of quantization from a principal axis to an axis depending also on angular momentum generated by the methyl top rotation. Since the choice of axis system, basis set, and computational scheme all influence the eigenfunction labeling procedure, consideration is limited to a commonly used two-step matrix-diagonalization scheme and to acetaldehyde as a numerical example. Torsional labels vt=0, 1, and 2 for eigenfunctions generated in the first diagonalization step are assigned in order of increasing eigenvalues, and rotational energy surfaces are constructed for the A and E symmetry components of these three torsional states. Projections of rotational eigenvectors over angular momentum coherent states are used to show that classical trajectories on the appropriate rotational energy surface give faithful representations of the quantum wave functions. Irregularities in the rotational energy level structure within a given torsional state can then be understood in terms of extra separatrices in the classical phase space and unusual wave function densities in the coherent state projections. Results are used to devise an automatic K labeling scheme for numerically obtained torsion-rotation levels belonging to torsional states below the top of the internal rotation barrier.
Vibrationally resolved photofragment spectroscopy of FeO+
Authors: Husband John , Aguirre Fernando , Ferguson Peter , Metz Ricardo B.
Published: July 2002

We report the first vibrationally resolved spectroscopic study of FeO+. We observe the 0←0 and 1←0 bands of a Σ6←XΣ6 transition at 28 648.7 and 29 311 cm−1. Under slightly modified source conditions the 1←1 transition is observed at 28 473 cm−1. In addition to establishing an upper limit D0o(Fe+–O)⩽342.7 kJ/mol, our results give the first experimental measurements of the vibrational frequencies in both the ground state, ν0″=838±4 cm−1, and the excited electronic state, ν0′=662±2 cm−1. Partially resolved rotational structure underlying the vibrational peaks has been analyzed to measure the predissociation lifetime and estimate the change in molecular constants upon electronic excitation.
A combination of Kohn–Sham density functional theory and multi-reference configuration interaction methods
Authors: Grimme Stefan , Waletzke Mirko
Published: July 2002

An effective Hamiltonian in a basis of spin- and space-symmetry adapted configuration state functions (CSF), which includes information from Kohn–Sham density functional theory (DFT), is used to calculate configuration interaction (CI) wave functions for the electronic states of molecules. The method emphasizes on states of multiconfigurational character which cannot be represented by conventional DFT. The CI matrix elements are constructed empirically by using the exact operator and corrections from DFT. Both the optimized KS orbitals from the parent determinant and the corresponding KS potential from the parent state density are used. Depending on their energy gap the CI off-diagonal elements between CSF are exponentially scaled to zero to avoid double counting of electron correlation. The selection of the most important CSF describing nondynamical correlation effects and the use of an approximate resolution of the identity (RI) for the evaluation of the two-electron integrals allows a very efficient DFT/MRCI treatment of molecules with several hundreds of electrons. As applications, the prediction of excitation energies for singlet and triplet states of organic molecules and transition metal complexes, the calculation of electronic circular dichroism spectra and investigations of the energetics of diradicals are presented. It is found, that the new DFT/MRCI approach gives results of high accuracy (rms errors for relative energies <0.2 eV) comparable to those from sophisticated ab initio treatments.
Low-order scaling local electron correlation methods. I. Linear scaling local MP2
Authors: Schütz Martin , Hetzer Georg , Werner Hans-Joachim
Published: July 2002

A new implementation of local second-order Mo/ller-Plesset perturbation theory (LMP2) is presented for which asymptotically all computational resources (CPU, memory, and disk) scale only linearly with the molecular size. This is achieved by (i) using orbital domains for each electron pair that are independent of molecular size; (ii) classifying the pairs according to a distance criterion and neglecting very distant pairs; (iii) treating distant pairs by a multipole approximation, and (iv) using efficient prescreening algorithms in the integral transformation. The errors caused by the various approximations are negligible. LMP2 calculations on molecules including up to 500 correlated electrons and over 1500 basis functions in C1 symmetry are reported, all carried out on a single low-cost personal computer.
Time-dependent density functional study on the electronic excitation energies of polycyclic aromatic hydrocarbon radical cations of naphthalene, anthracene, pyrene, and perylene
Authors: Hirata So , Lee Timothy J. , Head-Gordon Martin
Published: July 2002

Time-dependent density functional theory (TDDFT) and its modification, the Tamm–Dancoff approximation to TDDFT, are employed to calculate the electronic excitation energies and oscillator strengths for a series of polycyclic aromatic hydrocarbon radical cations. For the radical cations of naphthalene and anthracene, TDDFT using the Becke–Lee–Yang–Parr functional and the 6-31G** basis set provides the excitation energies that are roughly within 0.3 eV of the experimental data. The assignments of the electron transitions proposed by TDDFT accord with the previous assignments made by accurate ab initio calculations, except that TDDFT indicates the existence of a few additional transitions of π*←σ character among the several low-lying transitions. The calculated energies for these π*←σ transitions are found to be consistent with the onset of a σ electron ionization manifold in the photoelectron spectra. For the pyrene radical cation, TDDFT supports the previous assignments made by semiempirical calculations, whereas for the perylene radical cation, TDDFT suggests the energy ordering of the three lowest-lying excited states be changed from those of the semiempirical results.
Semiclassical dynamics of nonadiabatic transitions in discrete-state systems using spin coherent-state path integrals
Authors: Lucke Andreas , Mak C. H. , Stockburger Jürgen T.
Published: July 2002

We present a semiclassical method for simulating the dynamics of nonadiabatic transitions in a discrete-state quantum system coupled to a bath of explicit continuous coordinates. This method employs a coherent-state formulation of the path integrals for the discrete system whose dynamics is described by spin operators. This spin coherent-state formulation allows the discrete system to be mapped onto a continuous coordinate. Stationary approximations of the resulting coherent-state path integrals of the system plus bath lead to quasiclassical equations of motion which can be solved numerically by direct integration. This algorithm reduces the problem to a number of simple classical trajectory calculations and does not require calculating any fluctuation determinants.
Optimization of the anisotropic united atoms intermolecular potential for n-alkanes
Authors: Ungerer Philippe , Beauvais Christèle , Delhommelle Jérôme , Boutin Anne , Rousseau Bernard , Fuchs Alain H.
Published: July 2002

The parameters of the anisotropic united atoms potential for linear alkanes proposed by Toxvaerd [S. Toxvaerd, J. Chem. Phys. 107, 5197 (1997)] have been optimized on the basis of selected equilibrium properties (vapor pressures, vaporization enthalpies, and liquid densities) of ethane, n-pentane, and n-dodecane. The optimized parameters for the CH2 and CH3 groups form a regular sequence with those of methane and the force centers are found between the carbon and hydrogen atoms, as expected. The resulting potential, called AUA4, has been compared with Toxvaerd’s potential (AUA3) by using several molecular simulation methods (Gibbs ensemble Monte Carlo, thermodynamic integration, and molecular dynamics). An investigation performed at temperatures ranging from 140 to 700 K and with various chain lengths up to 20 carbon atoms has shown AUA4 to provide systematic improvements of vapor pressures, vaporization enthalpies, and liquid densities for pure n-alkanes. Significant improvements have been also noticed on the critical temperatures of n-alkanes, estimated from coexistence density curves, and on the equilibrium properties of CO2–n-alkane binary mixtures. Self-diffusion coefficients of n-hexane, however, are slightly improved by the new potential, but still exceed experimental measurements at low temperature. As we have only optimized the intermolecular potential in the present study, it is suggested that further optimization of the intramolecular potentials of the anisotropic united atoms model could allow simultaneous prediction of thermodynamic properties and of transport coefficients, particularly in very dense liquids.
H atom and heavy atom tunneling processes in tropolone
Authors: Redington Richard L.
Published: July 2002

The minimum energy pathway leading between the tautomers of tropolone was calculated using molecular orbital (MO) methods. This, with various 1D and 2D cuts of the potential energy surface (PES) topography, reveals the {tunneling skeleton}/{tunneling H atom} mechanism for tautomerization. In the zero-point states the H atom is localized to one of the O atoms until the tropolone skeleton becomes sufficiently vibrationally displaced towards C2v configurations that near-equal double-minimum potential energy functions (PEFs) arise for the H atom vibration. The resulting delocalization of the H atom between the two O atom sites allows the skeletal displacement to proceed through the barrier and the tautomerization process to be completed. The v1 (OH stretching) energies in quantum states N1 are strongly dependent on the skeletal geometry and, adiabatically separated from the slow v22 vibration, they contribute to markedly different 1D effective potential energy functions V22eff[N1] for v22. V22eff[N1=0] is a normal equal double minimum PEF while V22eff[N1≠0] have more complex shapes. Expressed as a function of the v22 skeletal displacement ΔS, the v1 states show a nonadiabatic curve crossing E1(1)→E1(2) contributing to the V22eff[N1=1→2] effective PEF for v22 vibration in the lowest excited OH stretching state. This function, rather than V22eff[N1=1], is strongly supported by the IR observations on v1. The computed effective energy barriers on the “model” tunneling path for the zero point states are 4.97 kcal/mol for the skeletal motion, and 3.22 kcal/mol for the H atom vibration at C2v skeletal geometry. Overall, the independent computational model predicts the major spectroscopic features observed for S0 tropolone(OH) and tropolone(OD): (a) similar IR tunneling doublets with ∼10 cm−1 splittings for the v22 skeletal vibration; (b) weak v1 IR absorbance with 20 and 5 cm−1 tunneling doublet separations for the isotopomers; (c) small tunneling splittings of the zero point states; and (d) unresolved vibrational state-specific IR tunneling doublets for all other fundamentals.
A rigorous modified Thomas–Fermi theory for atomic systems
Authors: Goldstein Jerome A. , Rieder Gisèle Ruiz
Published: July 2002

Recently Parr and Ghosh [Proc. Natl. Acad. Sci. USA 83, 3577 (1986)] proposed a variant of the classical Thomas–Fermi theory of electrons in an atom. They produced a continuous electron density by introducing the constraint that the integral ∫R3 e−2k‖x‖ Δρ(x)dx exists, where k is determined by the nuclear cusp condition. Their results give improved calculations of ground state electron densities and energies. The present paper provides a rigorous mathematical foundation for the work of Parr and Ghosh and converts their results into theorems. Some generalizations are also obtained.
Convergence of compact lattice scalar field theory to its continuum limit
Authors: Capitani Stefano
Published: July 2002

It is shown that the compact lattice scalar field theory converges in the sense of the correlation functions to its continuum limit when simultaneously the lattice spacing tends to zero and the amplitude cutoff and the lattice volume go to infinity. This is achieved in any number of dimensions provided that the last two parameters go to infinity not too rapidly. Besides, it is proved the right upper bound to the limit with no condition on the way it is performed.
A classification of pseudospherical surface equations of type u
t=u
xxx+G(u,u
x,u
xx)
Authors: Rabelo Mauro L. , Tenenblat Keti
Published: July 2002

The equations of type ut = uxxx + G(u,ux,uxx) which describe η-pseudospherical surfaces are characterized. A new class of such equations is obtained. This class together with the Korteweg–de Vries (KdV) equation, the modified Korteweg–de Vries (MKdV) equation, and the linear equation completes the classification.
Symmetry algebras of the potential Nizhnik–Novikov–Veselov model
Authors: Lou Sen-yue
Published: July 2002

Two sets of generalized symmetries of the potential Nizhnik–Novikov–Veselov equation are given in a simple formula by developing a generalized formal series method for searching symmetries of one type of generalized partial differential equations. These two sets of symmetries constitute an infinite-dimensional Lie algebra which contains three types of Virasoro algebras and three w∞ type algebras as its special cases.
The world as a hologram
Authors: Susskind Leonard
Published: July 2002

According to ’t Hooft the combination of quantum mechanics and gravity requires the three-dimensional world to be an image of data that can be stored on a two-dimensional projection much like a holographic image. The two-dimensional description only requires one discrete degree of freedom per Planck area and yet it is rich enough to describe all three-dimensional phenomena. After outlining ’t Hooft’s proposal we give a preliminary informal description of how it may be implemented. One finds a basic requirement that particles must grow in size as their momenta are increased far above the Planck scale. The consequences for high-energy particle collisions are described. The phenomenon of particle growth with momentum was previously discussed in the context of string theory and was related to information spreading near black hole horizons. The considerations of this paper indicate that the effect is much more rapid at all but the earliest times. In fact the rate of spreading is found to saturate the bound from causality. Finally we consider string theory as a possible realization of ’t Hooft’s idea. The light front lattice string model of Klebanov and Susskind is reviewed and its similarities with the holographic theory are demonstrated. The agreement between the two requires unproven but plausible assumptions about the nonperturbative behavior of string theory. Very similar ideas to those in this paper have long been held by Charles Thorn.
The influence of Reynolds number upon the apparent permeability of spatially periodic arrays of cylinders
Authors: Edwards D. A. , Shapiro M. , Bar-Yoseph P. , Shapira M.
Published: July 2002

Flow fields within spatially periodic arrays of cylinders arranged in square and hexagonal lattices are calculated, with microscale Reynolds number ranging between zero and 200, employing a finite element numerical scheme. The terminology of an ‘‘apparent permeability’’ is introduced to establish a relationship existing between mean velocity and macroscopic pressure gradient characterized by a finite Reynolds number flow. In contrast with the low Reynolds number ‘‘true ’’ permeability, the apparent permeability is shown here to generally depend upon the direction of the applied pressure gradient, owing to nonlinearities existing within the local fluid motion. The orientation-dependent permeabilities of both square and hexagonal monodisperse arrays are observed to diminish with increasing Reynolds number. Similar behavior is also observed for a bidisperse square array, though the apparent permeability of the latter is shown less sensitive to Darcy velocity orientation at large Reynolds numbers in comparison to the corresponding monodisperse square array, for all cylinder concentrations examined.
Weak turbulence theory of ion temperature gradient modes for inverted density plasmas
Authors: Hahm T. S. , Tang W. M.
Published: July 2002

Typical profiles measured in H-mode (‘‘high confinement’’) discharges from tokamaks such as JET [Proceedings of the 15th European Conference on Controlled Fusion and Plasma Heating (European Physical Society, Geneva, 1988), Vol. 12B, Part 1, p. 2239] and DIII-D [Phys. Fluids 31, 3738 (1988)] suggest that the ion temperature gradient instability threshold parameter ηi (≡d ln Ti/ d ln ni) could be negative in many cases. Previous linear theoretical calculations [Phys. Fluids B 1, 1185 (1989)] have established the onset conditions for these negative ηi modes and the fact that their growth rate is much smaller than their real frequency over a wide range of negative ηi values. This has motivated the present nonlinear weak turbulence analysis to assess the relevance of such instabilities for confinement in H-mode plasmas. The nonlinear eigenmode equation indicates that the three-wave coupling to shorter wavelength modes is the dominant nonlinear saturation mechanism. It is found that both the saturation level for these fluctuations and the magnitude of the associated ion thermal diffusivity are considerably smaller than the strong turbulence mixing-length-type estimates for the more conventional positive ηi instabilities.
On proofs of stability of drift vortices in magnetized plasmas and rotating fluids
Authors: Muzylev S. V. , Reznik G. M.
Published: July 2002

Analytical proofs of the stability of drift vortices given by Laedke and Spatschek [Phys. Fluids 29, 134 (1986); 31, 1492 (1988)] and Gordin and Petviashvili [Dokl. Akad. Nauk SSSR 285, 857 (1985)] are considered. It is shown that both of them suffer from shortcomings. The difficulty lies in the fact that all of these proofs are correct only if some quadratic functional L[φ] is positive for perturbations φ, satisfying some constraints. A counterexample is constructed. This shows that known stability theorems should be evaluated in a different manner than before. A complete analytical proof of two-dimensional gradient-driven drift vortices in magnetized plasmas has yet to be given.
Dimension calculations for low flow rate steam injection experiments
Authors: Lee C. K. B. , McDonough J. M. , Huang M. T.
Published: July 2002

The Grassberger–Procaccia dimension is determined from pressure and temperature measurements in a pool of water agitated by steam injection. For this complex flow system, three distinct values of dimension ranging from ∼4.2 to ∼8.0 (corresponding to three different measurement locations) are obtained.
Surface density function in premixed turbulent combustion modeling, similarities between probability density function and flame surface approaches
Authors: Vervisch L. , Bidaux E. , Bray K. N. C. , Kollmann W.
Published: July 2002

To characterize the dynamics and the physical properties of isoconcentration surfaces of a random reactive scalar field, an instantaneous isosurface quantity and its transport equation are introduced. For turbulent flows, the mean area of isoconcentration surface per unit volume is studied through its transport equation derived by using the probability density function (pdf) formalism. This approach allows the value of the reactive scalar used to define the level surface to be treated as an independent variable. It also leads to the definition of a surface density function. The developed formalism is applied to premixed turbulent combustion and a bridge is built between modeling approaches based on pdf, and others based on the flame surface concept.
Compressibility effects in supersonic transverse injection flowfields
Authors: Gruber M. R. , Nejad A. S. , Chen T. H. , Dutton J. C.
Published: July 2002

The flowfields created by transverse injection of sonic gaseous jets through a circular nozzle into a supersonic crossflow have been experimentally investigated using planar Rayleigh/Mie scattering from silicon dioxide particles seeded into the crossflow stream. Helium and air were used as injectant gases allowing an examination of the effects of compressibility on the large-scale structural development and near-field mixing characteristics present within the flowfield. Instantaneous images from end and side view image planes show a highly three-dimensional interaction dominated by both large- and small-scale vortices. Analyses of these image ensembles provide jet spreading and penetration characteristics, standard deviation statistics, large-scale mixing information, and two-dimensional spatial correlation fields. Results indicate that injectant molecular weight variations do not strongly affect the jet’s transverse penetration into the crossflow, although they lead to substantially different compressibility levels that dramatically influence the characteristics of the large-scale structures formed in the shear layer and the entrainment and mixing occurring between the injectant and crossflow fluids. The large-scale eddies tend to rapidly break-up in the low compressibility injection case while those in the high compressibility case remain coherent over a longer spatial range. Mixing layer fluctuations present in the low compressibility case intrude deeply into the jet fluid as compared to the high compressibility case, where these fluctuations are confined near the jet edge.
Cavity dynamics in high-speed water entry
Authors: Lee M. , Longoria R. G. , Wilson D. E.
Published: July 2002

A method is presented for modeling the cavity formation and collapse induced by high-speed impact and penetration of a rigid projectile into water. The approach proposes that high-speed water-entry is characterized by a cavity that experiences a deep closure prior to closure at the surface. This sequence in the physical events of the induced cavity dynamics is suggested by the most recent high-speed water-entry experimental data, by results from numerical experiments using a hydrocode, and by an understanding of the fundamental physics of the processes that govern surface closure. The analytical model, which specifies the energy transfer for cavity production as equivalent to the energy dissipated by velocity-dependent drag on the projectile, provides accurate estimates for variables that are important in characterizing the cavity dynamics, and reveals useful knowledge regarding magnitudes and trends. In particular, it is found that the time of deep closure is essentially constant and independent of the impact velocity for a given projectile size, while the location of deep closure has a weak dependence on impact velocity. Comparison of these analytical results with experimental results from the literature and with results from numerical simulations verifies the analytical solutions.
Ion cyclotron range of frequencies heating and current drive in deuterium–tritium plasmas
Authors: Phillips C. K. , Bell M. G. , Bell R. , Bretz N. , Budny R. V. , Darrow D. S. , Grek B. , Hammett G. , Hosea J. C. , Hsuan H. , Ignat D. , Majeski R. , Mazzucato E. , Nazikian R. , Park H. , Rogers J. H. , Schilling G. , Stevens J. E. , Synakowski E. , Taylor G. , Wilson J. R. , Zarnstorff M. C. , Zweben S. J. , Bush C. E. , Goldfinger R. , Jaeger E. F. , Murakami M. , Rasmussen D. , Bettenhausen M. , Lam N. T. , Scharer J. , Sund R. , Sauter O.
Published: July 2002

The first experiments utilizing high-power radio waves in the ion cyclotron range of frequencies to heat deuterium–tritium (D–T) plasmas have been completed on the Tokamak Fusion Test Reactor [Fusion Technol. 21, 13 (1992)]. Results from the initial series of experiments have demonstrated efficient core second harmonic tritium (2ΩT) heating in parameter regimes approaching those anticipated for the International Thermonuclear Experimental Reactor [D. E. Post, Plasma Physics and Controlled Nuclear Fusion Research, Proceedings of the 13th International Conference, Washington, DC, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 3, p. 239]. Observations are consistent with modeling predictions for these plasmas. Efficient electron heating via mode conversion of fast waves to ion Bernstein waves has been observed in D–T, deuterium-deuterium (D–D), and deuterium–helium-4 (D–4He) plasmas with high concentrations of minority helium-3 (3He) (n3He/ne≳10%). Mode conversion current drive in D–T plasmas was simulated with experiments conducted in D–3He–4He plasmas. Results show a directed propagation of the mode converted ion Bernstein waves, in correlation with the antenna phasing.
Trapping of dust and dust acoustic waves in laboratory plasmas
Authors: Prabhakara H. R. , Tanna V. L.
Published: July 2002

Trapping of negatively charged dust particles is observed in a hot cathode plasma discharge when a layer of dust is exposed to the plasma. The particles are visible in the scattered He–Ne laser light. The trajectories of individual particles have been photographed. The dust particles are excluded from the sheath region of any object in the plasma. The intensity of scattered light as well as the potential on a floating Langmuir probe show coherent fluctuations in the frequency range 1–15 Hz. After several hours of exposure to the plasma, the dust layer develops striations similar to those on sand dunes. Trapping of dust particles by the plasma and the possible identification of the observed low-frequency fluctuations with dust acoustic waves are discussed.
Measurements of direct drive laser imprint in thin foils by radiography using an x-ray laser backlighter
Authors: Kalantar D. H. , Key M. H. , Da Silva L. B. , Glendinning S. G. , Remington B. A. , Rothenberg J. E. , Weber F. , Weber S. V. , Wolfrum E. , Kim N. S. , Neely D. , Zhang J. , Wark J. S. , Demir A. , Lin J. , Smith R. , Tallents G. J. , Lewis C. L. S. , MacPhee A. , Warwick J. , Knauer J. P.
Published: July 2002

In direct drive inertial confinement fusion, the residual speckle pattern remaining after beam smoothing plays an important role in the seeding of instabilities at the ablation front. An x-ray laser is used as an extreme ultraviolet backlighter to characterize the imprinted modulation in thin foils for smoothing by random phase plate and by spectral dispersion for both 0.35 and 0.53 μm irradiation, and by induced spatial incoherence for 0.53 μm irradiation. Measurements of the imprinted modulation due to a single optical mode generated by two beam interference, and modification of the imprint with a superposed smooth irradiation to study time dependence of the imprinting process are demonstrated.
Electrical study of Schottky barrier heights on atomically clean and air-exposed n-InP(110) surfaces
Authors: Newman N. , Kendelewicz T. , Bowman L. , Spicer W. E.
Published: July 2002

We report here a systematic study of the electronic properties of Schottky barrier diodes fabricated on atomically clean, as well as air-exposed n-InP (110) surfaces. Using the current-voltage (I-V) measuring technique, we found the barrier heights of 0.33 eV for Ni, Al, Sn, Mn, 0.43 eV for Pd, Cu, Au, Cr, and 0.54 eV for Ag. Contrary to earlier reports based on a limited amount of data, the results of this study do not show a simple relationship between the chemical reactivity and the Schottky barrier height. The large differences between the electrical characteristics of diodes prepared on clean surfaces and those prepared on air-exposed surfaces were also not found.
Polarization bistability in semiconductor lasers
Authors: Chen Y. C. , Liu J. M.
Published: July 2002

A new kind of optical bistability, the polarization bistability, is observed in InGaAsP/InP lasers operating near the polarization transition temperature. This bistability is characterized by large hysteresis loops in the polarization-resolved power versus current characteristics. Fast switching between the two stable polarization states by injection of current pulses is also demonstrated.
Wavelength dependence of laser enhanced plasma etching of semiconductors
Authors: Reksten Grace M. , Holber W. , Osgood R. M.
Published: July 2002

Ultraviolet (350 nm) and visible (514 nm and 647 nm) laser light have been employed to enhance silicon etching and to perform in situ patterning in a plasma reactor containing CF4/O2 or NF3 reactants. The etch rate enhancement is dependent on dopant concentration and laser wavelength. This dependence has been related to the number of photogenerated carriers on the semiconductor surface.
Two-layer organic photovoltaic cell
Authors: Tang C. W.
Published: July 2002

A thin-film, two-layer organic photovoltaic cell has been fabricated from copper phthalocyanine and a perylene tetracarboxylic derivative. A power conversion efficiency of about 1% has been achieved under simulated AM2 illumination. A novel feature of the device is that the charge-generation efficiency is relatively independent of the bias voltage, resulting in cells with fill factor values as high as 0.65. The interface between the two organic materials, rather than the electrode/organic contacts, is crucial in determining the photovoltaic properties of the cell.
Nuclear magnetic resonance measurements of lattice distortions in GaAs:In
Authors: Carlos W. E. , Bishop S. G. , Treacy D. J.
Published: July 2002

Nuclear magnetic resonance (NMR) has been used to probe the atomic displacements and bond bending around an isovalent impurity (In) in the GaAs lattice. It is concluded that the In atoms substitute on the Ga sublattice, that they do not cluster, and that the strain field induced by these isolated atoms is sufficient to affect the NMR of arsenic atoms as far away as 30 Å. Estimates of bond angle distortions in the nearest atomic shells around the In atom are discussed.
Femtosecond optical measurement of hot-carrier relaxation in GaAs, AlGaAs, and GaAs/AlGaAs multiple quantum well structures
Authors: Rosker M. J. , Wise F. W. , Tang C. L.
Published: July 2002

The ultrafast relaxation of photoexcited GaAs, AlGaAs, and GaAs/AlGaAs quantum well structures has been studied using the equal-pulse correlation technique. Improvements in experimental method now allow extraction of decay times directly from the measured transmission through optically thin samples. Two or three distinct exponential decays are resolved for each material, with the fastest time constant measured to be ∼40 fs in each case. A tentative explanation of the data in terms of known scattering processes is given.
Extraction of Schottky diode parameters from forward current-voltage characteristics
Authors: Cheung S. K. , Cheung N. W.
Published: July 2002

It is shown that by using the forward current density-voltage (J-V) characteristics of a Schottky diode, a plot of d(V)/d(ln J) vs J and a plot of the function H(J) vs J, where H(J)≡V−n(kT/q)ln(J/A**T2), will each give a straight line. The ideality factor n, the barrier height φB, and the series resistance R of the Schottky diode can be determined with one single I-V measurement. This procedure has been used successfully to study thermal annealing effects of W/GaAs Schottky contacts.
Absorption at 248 nm by Kr2F*
Authors: McCown A. W.
Published: July 2002

The cross section for absorption of 248 nm radiation by the lowest bound electronic excited (42Γ) of the rare gas halide trimer Kr2F* has been measured to be 7.2×10−19 cm2. This is an order of magnitude smaller than the value which is typically used in KrF kinetics codes [cf. T. H. Johnson and A. M. Hunter, J. Appl. Phys. 51, 2406 (1980)]. The lower absorption cross section leaves unaccounted the observed nonsaturable absorption in long pulse KrF lasers.
Co/Si(111) interface: Formation of an initial CoSi2 phase at room temperature
Authors: Veuillen J. Y. , Derrien J. , Badoz P. A. , Rosencher E. , d’Anterroches C.
Published: July 2002

Ultrathin films (≲50 monolayers) of Co have been deposited on atomically clean 7×7 Si(111) surfaces at room temperature and characterized by in situ surface techniques such as Auger electron spectroscopy and low-energy electron diffraction. Formation of a boundary CoSi2-like phase is surprisingly found at a very low coverage range (≲4 monolayers) as evidenced by low-temperature transport measurements (resistivity and Hall effect) and also by cross-sectional high-resolution transmission electron microscopy.
Role of experimental resolution in measurements of critical layer thickness for strained-layer epitaxy
Authors: Fritz I. J.
Published: July 2002

Experimental measurements of critical layer thicknesses (CLT’s) in strained-layer epitaxy are considered. Finite experimental resolution can have a major effect on measured CLT’s and can easily lead to spurious results. The theoretical approach to critical layer thicknesses of J. W. Matthews [J. Vac. Sci. Technol. 12, 126 (1975)] has been modified in a straightforward way to predict the apparent critical thickness for an experiment with finite resolution in lattice parameter. The theory has also been modified to account for the general empirical result that fewer misfit dislocations are generated than predicted by equilibrium calculation. The resulting expression is fit to recent x-ray diffraction data on InGaAs/GaAs and SiGe/Si. The results suggest that CLT’s in these systems may not be significantly larger than predicted by equilibrium theory, in agreement with high-resolution measurements.
InGaAsP/InP high-power semi-insulating blocked planar buried-heterostructure lasers grown entirely by atmospheric organometallic vapor phase epitaxy
Authors: Miller B. I. , Koren U. , Capik R. J. , Su Y. K.
Published: July 2002

High-power semi-insulating blocked planar buried-heterostructure (SIPBH) lasers were grown entirely by atmospheric organometallic vapor phase epitaxy (OMVPE) by using a novel dilution scheme for the trimethylgallium and AsH3. Current thresholds as low as 20 mA and differential quantum efficiencies ≥20% per facet at 1.3 and 1.5 μm were obtained with power outputs of about 25 mW/facet. These results are similar to SIPBH lasers where liquid phase epitaxy was used to grow the active layer, but because of the better planarity and uniformity of OMVPE-grown material, it appears possible to grow material that can give a high yield of distributed feedback lasers.
Measurements of room-temperature band-gap-resonant optical nonlinearities of GaAs/AlGaAs multiple quantum wells and bulk GaAs
Authors: Park S. H. , Morhange J. F. , Jeffery A. D. , Morgan R. A. , Chavez-Pirson A. , Gibbs H. M. , Koch S. W. , Peyghambarian N. , Derstine M. , Gossard A. C. , English J. H. , Weigmann W.
Published: July 2002

We present a systematic study of the dependence of the optical nonlinearities on quantum well thickness for GaAs/AlGaAs multiple quantum wells (MQW’s) at room temperature and compare them with bulk GaAs. The maximum change in the refractive index is greatest for the MQW’s with the smallest well size and decreases with increasing well size, reaching a minimum for bulk GaAs. The maximum index change per photoexcited carrier increases by a factor of 3 as the well size decreases from bulk to 76 Å MQW.
Photoluminescence from AlInAs/InP quantum wells grown by organometallic vapor phase epitaxy
Authors: Aina Leye , Mattingly Mike , Stecker Lisa
Published: July 2002

Photoluminescence from AlInAs/InP quantum wells and single heterojunctions is reported for the first time. An emission centered around 1.1 eV which is most intense in multiquantum well structures, is shown to originate from confined-particle transitions involving spatially separated electrons and holes in quantum wells in the InP and AlInAs, respectively. The AlInAs/InP heterostructure is shown to have a staggered band lineup with an effective band gap of 1.06 eV.
Receptor sensitivity in bacterial chemotaxis
Authors: Sourjik Victor , Berg Howard C.
Published: July 2002

Chemoreceptors in
Escherichia coli
are
coupled to the flagella by a labile phosphorylated intermediate,
CheY∼P. Its activity can be inferred from the rotational bias of
flagellar motors, but motor response is stochastic and limited to a
narrow physiological range. Here we use fluorescence resonance energy
transfer to monitor interactions of CheY∼P with its phosphatase,
CheZ, that reveal changes in the activity of the receptor kinase, CheA,
resulting from the addition of attractants or repellents. Analyses of
cheR
and/or
cheB
mutants, defective in
receptor methylation/demethylation, show that response sensitivity
depends on the activity of CheB and the level of receptor modification.
In
cheRcheB
mutants, the concentration of attractant
that generates a half-maximal response is equal to the dissociation
constant of the receptor. In wild-type cells, it is 35 times smaller.
This amplification, together with the ultrasensitivity of the flagellar
motor, explains previous observations of high chemotactic gain.
Protein and ligand dynamics in 4-hydroxybenzoate hydroxylase
Authors: Wang Jian , Ortiz-Maldonado Mariliz , Entsch Barrie , Massey Vincent , Ballou David , Gatti Domenico L.
Published: July 2002

para
-Hydroxybenzoate hydroxylase catalyzes a two-step reaction that demands precise control of solvent access to the catalytic site. The first step of the reaction, reduction of flavin by NADPH, requires access to solvent. The second step, oxygenation of reduced flavin to a flavin C4a-hydroperoxide that transfers the hydroxyl group to the substrate, requires that solvent be excluded to prevent breakdown of the hydroperoxide to oxidized flavin and hydrogen peroxide. These conflicting requirements are met by the coordination of multiple movements involving the protein, the two cofactors, and the substrate. Here, using the R220Q mutant form of
para
-hydroxybenzoate hydroxylase, we show that in the absence of substrate, the large βαβ domain (residues 1–180) and the smaller sheet domain (residues 180–270) separate slightly, and the flavin swings out to a more exposed position to open an aqueous channel from the solvent to the protein interior. Substrate entry occurs by first binding at a surface site and then sliding into the protein interior. In our study of this mutant, the structure of the complex with pyridine nucleotide was obtained. This cofactor binds in an extended conformation at the enzyme surface in a groove that crosses the binding site of FAD. We postulate that for stereospecific reduction, the flavin swings to an out position and NADPH assumes a folded conformation that brings its nicotinamide moiety into close contact with the isoalloxazine moiety of the flavin. This work clearly shows how complex dynamics can play a central role in catalysis by enzymes.
C2A activates a cryptic Ca
2+
-triggered membrane penetration activity within the C2B domain of synaptotagmin I
Authors: Bai Jihong , Wang Ping , Chapman Edwin R.
Published: July 2002

Synaptotagmin (syt) I, an integral membrane protein localized to secretory vesicles, is a putative Ca
2+
sensor for exocytosis. Its N terminus spans the membrane once, and its cytoplasmic domain contains two conserved C2 domains, designated C2A and C2B. The isolated C2A domain penetrates membranes in response to Ca
2+
; isolated C2B does not. Here, we have addressed the function of each C2 domain, but in the context of the intact cytoplasmic domain (C2A-C2B), by using fluorescent reporters placed in the Ca
2+
-binding loops of either C2A or C2B. Surprisingly, these reporters revealed that, analogous to C2A, a Ca
2+
-binding loop in C2B directly penetrates into lipid bilayers. Penetration of each C2 domain was very rapid (
k
on
≈10
10
M
−1
⋅s
−1
) and resulted in high affinity C2A-C2B–liposome complexes (
K
d
≈13–14 nM). C2B-bilayer penetration strictly depended on the presence, but not the membrane binding activity, of an adjacent C2A domain, severing C2A from C2B after protein synthesis abolished the ability of C2B to dip into bilayers in response to Ca
2+
. The activation of C2B by C2A was also displayed by the C2 domains of syt III but not the C2 domains of syt IV. A number of proteins contain more than one C2 domain; the findings reported here suggest these domains may harbor cryptic activities that are not detected when they are studied in isolation.
Association of SWAP-70 with the B cell antigen receptor complex
Authors: Masat L. , Caldwell J. , Armstrong R. , Khoshnevisan H. , Jessberger R. , Herndier B. , Wabl M. , Ferrick D.
Published: July 2002

SWAP-70 is a component of an enzyme complex that recombines Ig switch regions
in vitro
. We report here the cloning of the human cDNA and its B lymphocyte-specific expression. Although its sequence contains three nuclear localization signals, in small resting B cells, SWAP-70 is mainly found in the cytoplasm. On stimulation, SWAP-70 translocates to the nucleus. In activated, class-switching B cell cultures, it is associated with membrane IgG, but not IgM. The membrane Ig association requires a functional pleckstrin homology domain and is controlled by the C terminus. We suggest that SWAP-70 is involved not only in nuclear events but also in signaling in B cell activation.
The inhibitory effect of hormones associated with stress on Na appetite of sheep
Authors: Weisinger R. S. , Blair-West J. R. , Burns P. , Denton D. A. , McKinley M. J. , Purcell B. , Vale W. , Rivier J. , Sunagawa K.
Published: July 2002

Stress is a large stimulus of Na appetite in rabbits, rats, and mice. This study investigated the influence of some peptides implicated in stress, i.e., adrenocorticotropin (ACTH), corticotropin-releasing factor (CRF), and the recently discovered member of the CRF family, urocortin, on the ingestive behavior of sheep. Intracerebroventricular infusion of these peptides over 4 days decreased the need-free Na intake of Na-repleted sheep. Intracerebroventricular infusion of urocortin, however, did not alter Na intake of Na-depleted sheep. Systemic infusion of ACTH increased, whereas systemic infusion of either urocortin or CRF decreased, Na intake of Na-repleted sheep. The increase in Na intake caused by the peripheral infusion of ACTH was blocked by concurrent i.v. infusion of urocortin, substantiating the inhibitory role of this peptide on Na appetite. Central administration of all peptides and i.v. administration of urocortin or urocortin and ACTH combined decreased food intake. Water intake was not directly influenced by the peptides. Rather, decreased water intake, when observed, was secondary to decreased food intake, as determined by pair-feeding experiments. Whereas systemic infusion of ACTH mimics the increase in Na intake observed in several different stressful situations, CRF and urocortin actually inhibit Na intake, indicating a direct central action overriding any effect of these peptides on ACTH release. Indeed, the inhibition of Na intake by urocortin occurred despite its stimulation of ACTH release and the subsequent increase in peripheral level of cortisol. Thus it would appear that hormones associated with stress have both excitatory and inhibitory influences on Na intake. Presumably, other physiological processes entrained by stress also will be important in determining the quantitative outcome on Na appetite.
Invariant scaling relationships for interspecific plant biomass production rates and body size
Authors: Niklas Karl J. , Enquist Brian J.
Published: July 2002

The allometric relationships for plant annualized biomass
production (“growth”) rates, different measures of body size (dry
weight and length), and photosynthetic biomass (or pigment
concentration) per plant (or cell) are reported for multicellular and
unicellular plants representing three algal phyla; aquatic ferns;
aquatic and terrestrial herbaceous dicots; and arborescent monocots,
dicots, and conifers. Annualized rates of growth
G
scale
as the 3/4-power of body mass
M
over 20 orders of
magnitude of
M
(i.e.,
G
∝
M
3/4
); plant body length
L
(i.e., cell length or plant height) scales, on average, as the
1/4-power of
M
over 22 orders of magnitude of
M
(i.e.,
L
∝
M
1/4
); and photosynthetic biomass
M
p
scales as the 3/4-power of
nonphotosynthetic biomass
M
n
(i.e.,
M
p
∝
M
n
3/4
). Because these scaling
relationships are indifferent to phylogenetic affiliation and habitat,
they have far-reaching ecological and evolutionary implications (e.g.,
net primary productivity is predicted to be largely insensitive to
community species composition or geological age).
Amyloid fibers are water-filled nanotubes
Authors: Perutz M. F. , Finch J. T. , Berriman J. , Lesk A.
Published: July 2002

A study of papers on amyloid fibers suggested to us that cylindrical β-sheets are the only structures consistent with some of the x-ray and electron microscope data. We then found that our own 7-year-old and hitherto enigmatic x-ray diagram of poly-
l
-glutamine fits a cylindrical sheet of 31 Å diameter made of β-strands with 20 residues per helical turn. Successive turns are linked by hydrogen bonds between both the main chain and side chain amides, and side chains point alternately into and out of the cylinder. Fibers of the exon-1 peptide of huntingtin and of the glutamine- and asparagine-rich region of the yeast prion Sup35 give the same underlying x-ray diagrams, which show that they have the same structure. Electron micrographs show that the 100-Å-thick fibers of the Sup35 peptide are ropes made of three protofibrils a little over 30 Å thick. They have a measured mass of 1,450 Da/Å, compared with 1,426 Da/Å for a calculated mass of three protofibrils each with 20 residues per helical turn wound around each other with a helical pitch of 510 Å. Published x-ray diagrams and electron micrographs show that fibers of synuclein, the protein that forms the aggregates of Parkinson disease, consist of single cylindrical β-sheets. Fibers of Alzheimer Aβ fragments and variants are probably made of either two or three concentric cylindrical β-sheets. Our structure of poly-
l
-glutamine fibers may explain why, in all but one of the neurodegenerative diseases resulting from extension of glutamine repeats, disease occurs when the number of repeats exceeds 37–40. A single helical turn with 20 residues would be unstable, because there is nothing to hold it in place, but two turns with 40 residues are stabilized by the hydrogen bonds between their amides and can act as nuclei for further helical growth. The Aβ peptide of Alzheimer's disease contains 42 residues, the best number for nucleating further growth. All these structures are very stable; the best hope for therapies lies in preventing their growth.