bluuebunny/crossref_metadata_2025 huggingface dataset:
Unoxidized porous Si as an isolation material for mixed-signal integrated circuit applications
Authors: Kim Han-Su , Xie Ya-Hong , DeVincentis Marc , Itoh Tatsuo , Jenkins Keith A.
Published: March 2003

An isolation technology for radio frequency (rf) applications based on unoxidized porous Si (PS) is demonstrated. This study examines all the important issues pertinent to incorporating PS with Si very-large-scale integration (VLSI) technology, where PS is used as a semi-insulating material. Specifically, the issues on rf isolation performance of PS as a function of porosity [from coplanar waveguide (CPW) line measurements] and PS thickness (from on-chip inductors) and the stress generated from incorporating PS regions by anodization are discussed in detail. CPW line measurements show that the relative dielectric constant of PS films decreases from 9 to 3 with increasing porosity from 24% to 78%. PS is a very low loss material with loss tangent <0.001 at 20 GHz when its porosity is above 51%. rf crosstalk through a Si substrate can be reduced to that through air by inserting a PS trench between noise generating circuit and noise sensing circuit. On-chip spiral inductors fabricated on top of PS regions of through-the-wafer thickness have Qmax of about 29 at 7 GHz and resonant frequency higher than 20 GHz. With the additional advantage of planar topography and mechanical integrity, we show that unoxidized PS is an outstanding material for rf isolation in Si VLSI.
Optical properties of SiGe alloys
Authors: Ahuja R. , Persson C. , Ferreira da Silva A. , Souza de Almeida J. , Moyses Araujo C. , Johansson B.
Published: March 2003

The optical properties of Si1−xGex have been investigated theoretically using a full-potential linear muffin-tin-orbital method. We present the density-of-states as well as the real and imaginary parts of the dielectric function. The calculated dielectric function was found to be in good agreement with the spectroscopic ellipsometry measurements by J. Bahng et al., J. Phys.: Condens. Matter 13, 777 (2001), and we obtained a static dielectric constant of ε0=12.19+2.45x in the Si rich regime (x⩽0.5).
Si
1−y
C
y
/
Si
(001)
gas-source molecular beam epitaxy from Si2H6 and CH3SiH3: Surface reaction paths and growth kinetics
Authors: Foo Y. L. , Bratland K. A. , Cho B. , Desjardins P. , Greene J. E.
Published: March 2003

In situ surface probes and postdeposition analyses were used to follow surface reaction paths and growth kinetics of Si1−yCy alloys grown on Si(001) by gas-source molecular-beam epitaxy from Si2H6/CH3SiH3 mixtures as a function of C concentration y (0–2.6 at %) and temperature Ts (500–600 °C). High-resolution x-ray diffraction reciprocal lattice maps show that all layers are in tension and fully coherent with their substrates. Film growth rates R decrease with both y and Ts, and the rate of decrease in R as a function of y increases rapidly with Ts. In situ isotopically tagged D2 temperature-programmed desorption (TPD) measurements reveal that C segregation during steady-state Si1−yCy(001) growth results in charge transfer from Si surface dangling bonds to second-layer C atoms, which have a higher electronegativity than Si. From the TPD results, we obtain the coverage θSi*(y,Ts) of Si* surface sites with C backbonds as well as H2 desorption energies Ed from both Si and Si* surface sites. θSi* increases with increasing y and Ts in the kinetically limited segregation regime while Ed decreases from 2.52 eV for H2 desorption from Si surface sites with Si back bonds to 2.22 eV from Si* surface sites. This leads to an increase in the H2 desorption rate, and hence should yield higher film deposition rates, with increasing y and/or Ts during Si1−yCy(001) growth. The effect, however, is more than offset by the decrease in Si2H6 reactive sticking probabilities at Si* surface sites. Film growth rates R(Ts,JSi2H6,JCH3SiH3) calculated using a simple transition-state kinetic model, together with measured kinetic parameters, were found to be in excellent agreement with the experimental data.
Vibrational energy levels for symmetric and asymmetric isotopomers of ammonia with an exact kinetic energy operator and new potential energy surfaces
Authors: Rajamäki Timo , Miani Andrea , Halonen Lauri
Published: March 2003

A new vibrational Hamiltonian operator for ammonia is presented. The potential energy part is expressed in terms of symmetrized bond-angle valence coordinates and an inversion coordinate, which is a function of the bond angles. In the exact kinetic energy operator, the stretching part is instead given in terms of unsymmetrized bond displacement coordinates. Six-dimensional ammonia potential energy surfaces are calculated using high-level ab initio tools, the CCSD(T) method with aug-cc-pVQZ and aug-cc-pVTZ basis sets. The potential energy functions are constructed in two, two-dimensional steps. The surfaces are expressed as a Taylor-type series with respect to the doubly degenerate asymmetric stretching and bending coordinates. This representation is given along a two-dimensional surface of the totally symmetric stretching and inversion coordinates of ammonia. Vibrational energies are calculated variationally in a finite basis representation. Employing successive basis set contractions, it is possible to optimize some potential energy parameters simultaneously for seven symmetric and asymmetric isotopomers very effectively. The symmetric part of the surface is fitted to experimentally observed vibrational band centers up to 6000 cm−1. This reduces the mean absolute error from 7.84 cm−1 with a pure ab initio potential to 0.44 cm−1 compared to the experimental values for NH314. In addition, vibrational energy levels of NH314 have been calculated up to about 15 000 cm−1 using the pure ab initio surface obtained with the aug-cc-pVTZ basis set. The nuclear motion calculation converges all levels up to about 10 000 cm−1 to within 0.05 cm−1.
Numerical spurious solutions in the effective mass approximation
Authors: Cartoixà X. , Ting D. Z.-Y. , McGill T. C.
Published: March 2003

We have characterized a class of spurious solutions that appears when using the finite difference method to solve the effective mass approximation equations. We find that the behavior of these solutions as predicted by our model shows excellent agreement with numerical results. Using this interpretation we find a set of analytical expressions for conditions that the Luttinger parameters must satisfy to avoid spurious solutions. Finally, we use these conditions to check commonly used sets of parameters for their potential for generating this class of spurious solutions.
Temperature measurement at the end of a cantilever using oxygen paramagnetism in solid air
Authors: Thurber Kent R. , Harrell Lee E. , Smith Doran D.
Published: March 2003

We demonstrate temperature measurement of a sample attached to the end of a cantilever using cantilever magnetometry of solid air “contamination” of the sample surface. In experiments like our magnetic resonance force microscopy (MRFM), the sample is mounted at the end of a thin cantilever with small thermal conductance. Thus, the sample can be at a significantly different temperature than the bulk of the instrument. Using cantilever magnetometry of the oxygen paramagnetism in solid air provides the temperature of the sample, without any modifications to our MRFM apparatus.
Temperature dependence of the properties of vapor-deposited polyimide
Authors: Tsai F. Y. , Blanton T. N. , Harding D. R. , Chen S. H.
Published: March 2003

The Young’s modulus and helium gas permeability of vapor-deposited poly(4,4′-oxydiphenylenepyromellitimide) were measured at cryogenic and elevated temperatures (10–573 K). The Young’s modulus decreased with increasing temperature from 5.5 GPa at 10 K to 1.8 GPa at 573 K. The temperature dependency of the permeability followed the Arrhenius’ relationship, with different activation energy for permeation for samples imidized under different conditions. The effect of the imidization conditions on the permeation properties could be explained in terms of morphology/crystallinity as determined by x-ray diffraction techniques. Imidizing in air instead of nitrogen increased the permeability while lowering the activation energy for permeation and crystallinity. Imidizing at higher heating rates (in nitrogen) resulted in higher permeability, lower activation energy for permeation, and larger and fewer crystallites with better-aligned lattice planes.
Structural and optical properties of molybdenum–tungsten mixed oxide thin films deposited by the sol-gel technique
Authors: Taurino A. , Catalano M. , Rella R. , Siciliano P. , Wlodarski W.
Published: March 2003

Sol-gel derived molybdenum–tungsten (Mo–W) mixed oxides were investigated by transmission electron microscopy in order to study the dependence of their structural and morphological properties on the relative percentage of the constituent compounds. Optical measurements were performed in the 200–800 nm wavelength range to correlate thin film structure and optical properties. Moreover, the values of the band gap energy were estimated as a function of the percentage of the W precursors added in the solution.
Replica-exchange multicanonical and multicanonical replica-exchange Monte Carlo simulations of peptides. I. Formulation and benchmark test
Authors: Mitsutake Ayori , Sugita Yuji , Okamoto Yuko
Published: March 2003

The replica-exchange multicanonical algorithm and the multicanonical replica-exchange method for molecular dynamics simulations have recently been developed. In the former method the multicanonical weight factor is determined from a short replica-exchange simulation with the multiple-histogram reweighting techniques. A long multicanonical production run with high statistics is then performed with this weight factor. In this method, the process of determining the multicanonical weight factor is faster and simpler than that in the usual iterative determination. The multicanonical replica-exchange method is a further extension of the first in which a replica-exchange multicanonical simulation is performed with a small number of replicas. In this paper, we give the formulations of these two methods for Monte Carlo simulations and demonstrate the effectiveness of these algorithms for a penta peptide in the gas phase.
Replica-exchange multicanonical and multicanonical replica-exchange Monte Carlo simulations of peptides. II. Application to a more complex system
Authors: Mitsutake Ayori , Sugita Yuji , Okamoto Yuko
Published: March 2003

In Paper I of this series the formulations of the replica-exchange multicanonical algorithm and the multicanonical replica-exchange method for Monte Carlo versions have been presented. The effectiveness of these algorithms were then tested with the system of a penta peptide, Met-enkephalin, in the gas phase. In this article the detailed comparisons of performances of these algorithms together with the regular replica-exchange method are made, taking a more complex system of a 17-residue helical peptide. It is shown that these two new algorithms are more efficient than the regular replica-exchange method.
Structures, energetics, and spectra of Br−(H2O)n clusters, n=1–6: Ab initio study
Authors: Masamura M.
Published: March 2003

Br − (H 2 O ) n  (n=1–6) clusters have been studied using ab initio calculations. The structures of the various Br−(H2O)n isomers differ considerably from those of the corresponding F−(H2O)n isomers for n=2 and 4–6, and especially so for n=6. The structures of the various Br−(H2O)n isomers are almost similar to those of the corresponding Cl−(H2O)n isomers for n=1–5. For n=6, the structures of the various Br−(II2O)n isomers differ considerably from those of the corresponding Cl−(H2O)n isomers. The relative stability of Br−(H2O)n isomers is significantly different from that of the corresponding F−(H2O)n isomers at 0 K. The relative stability of Br−(H2O)n isomers is close to that of the corresponding Cl−(H2O)n isomers at 0 K. The stable isomers are identified by means of comparing the calculated ionization potentials with experimental values. The charge-transfer-to-solvent energy and the OH stretching vibrational spectra are reported to facilitate future experimental work.
Effect of InAs quantum dots on the Fermi level pinning of undoped-n+ type GaAs surface studied by contactless electroreflectance
Authors: Jin Peng , Meng X. Q. , Zhang Z. Y. , Li C. M. , Xu B. , Liu F. Q. , Wang Z. G. , Li Y. G. , Zhang C. Z. , Pan S. H.
Published: March 2003

Self-assembled InAs quantum dots (QDs) have been fabricated by depositing 1.6, 1.8, 2.0 and 2.5 monolayer (ML) InAs on surfaces of the undoped-n+ (UN+) type GaAs structure. Room temperature contactless electroreflectance (CER) was employed to study the built-in electric field and the surface Fermi level pinning of these QD-covered UN+ GaAs samples. The CER results show that 1.6 ML InAs QDs on GaAs do not modify the Fermi level, whereas for samples with more than 1.6 ML InAs coverage, the surface Fermi level is moved to the valence band maximum of GaAs by about 70 meV (which is independent of the InAs deposition thickness) compared to bare GaAs. It is concluded that the modification of InAs coverage on the Fermi level on the GaAs surface is due to the QDs, rather than to the wetting layer.
Ab initio elasticity of chalcopyrites
Authors: Łażewski J. , Neumann H. , Jochym P. T. , Parlinski K.
Published: March 2003

A critical review of the experimental data of elastic stiffness constants of AgGaX2 and CuInX2 (X=S,Se) chalcopyrites is given. Using the ab initio pseudopotential method, their elastic constants have been calculated from strain–energy and from strain–stress relationships. On the basis of the linear elasticity approximation, axial and total compressibilities as well as elastic anisotropy factors have been derived. The calculated results are compared with available experimental data and previous calculations based on phenomenological models.
Epitaxial growth of high quality ZnO:Al film on silicon with a thin γ-Al2O3 buffer layer
Authors: Kumar Manoj , Mehra R. M. , Wakahara A. , Ishida M. , Yoshida A.
Published: March 2003

ZnO:Al thin films were grown epitaxially on epi-γ-Al2O3/Si (111) substrates by rf sputtering and pulsed laser deposition. The γ-Al2O3 buffer layer was deposited on Si (111) at a low substrate temperature of 500 °C using the metalorganic chemical vapor deposition method. Reflection high energy electron diffraction and x-ray diffraction measurements indicated a near alignment of the ZnO:Al epilayer on γ-Al2O3/Si (111) as compared to those grown directly on Si (111). Atomic force microscopy results of the films ZnO:Al/γ-Al2O3/Si (111) deposited by pulsed laser deposition revealed a smoother surface in comparison with the films deposited by rf sputtering. The M band observed in the photoluminescence spectra of the films deposited by laser ablation suggests that high quality epitaxial ZnO:Al/γ-Al2O3/Si (111) films can be deposited by pulsed laser deposition.
Electroluminescence and photoluminescence studies on carrier radiative and nonradiative recombinations in metal-oxide-silicon tunneling diodes
Authors: Chen Miin-Jang , Chang Jui-Fen , Yen Jia-Liang , Tsai Chen S. , Liang Eih-Zhe , Lin Ching-Fuh , Liu C. W.
Published: March 2003

Electroluminescence (EL) and photoluminescence (PL) measurements at silicon band gap energy from metal-oxide-semiconductor tunneling diodes on silicon have been carried out to study the carrier radiative recombination and nonradiative recombination near the silicon/silicon dioxide (Si/SiO2) interface. The temporal EL response indicates that the radiative recombination coefficient involved is as much as ten times greater than that of the bulk silicon. However, the nonradiative recombination is still the dominant carrier recombination process. Voltage-dependent PL shows that PL intensity increases with the bias voltage. This voltage dependence is attributed to the variations of nonradiative recombination rates due to the change of Fermi level with the bias voltage. The intensity of EL is found to be less temperature dependent than that of PL. The near-band-edge Fermi level for EL leads to the reduced and less thermal-active nonradiative recombination as compared to PL. This study shows that Fermi level near the Si/SiO2 interface strongly influences the nonradiative recombination rates and the resulting EL and PL intensities.
Model for interface formation and the resulting electrical properties for barium–strontium–titanate films on silicon
Authors: Mueller A. H. , Suvorova N. A. , Irene E. A. , Auciello O. , Schultz J. A.
Published: March 2003

The interface formation between sputtered barium strontium titanate (BST) films and both Si and SiO2 substrate surfaces has been followed using real-time spectroscopic ellipsometry and the mass spectrometry of recoiled ions. In both substrates an intermixed interface layer was observed and subcutaneous Si oxidation occurred. A model for the interface formation is proposed in which the interface includes an SiO2 film on Si, and an intermixed film on which is pure BST. During the deposition of BST the interfaces films were observed to change in time. Electrical characterization of the resulting metal–BST interface capacitors indicates that those samples with SiO2 on the Si surface had the best electrical characteristics.
Thermophoretic interaction of heat releasing particles
Authors: Dolinsky Yu. , Elperin T.
Published: March 2003

This study investigates thermophoretic force acting at heat releasing (absorbing) particles near the interface between two media with different thermal conductivities. This force is caused by the induced temperature gradient which is proportional to the rate of heat release (absorption) by the particle. Therefore the magnitude of the thermophoretic force is proportional to the rate of heat release (absorption) by the particle, and its direction depends upon the sign of the parameter κ1–κ2, where κ1 is thermal conductivity of a host medium and κ2 is thermal conductivity of the adjacent medium. The obtained results imply that a heat releasing (absorbing) particle is attracted (repelled) to the interface when thermal conductivity of a host medium is less than thermal conductivity of the adjacent medium. Thus, e.g., growing in air by condensation particle is attracted to a metal surface while an evaporating in air particle is repelled from a metal surface. The change of temperature distribution caused by heat releasing particles results in the additional thermophoretic interaction of these particles. We determined a condition for mutual attraction of two spherical heat releasing particles and derived an expression for the thermophoretic force acting at the particles. The magnitudes of the considered thermophoretic forces are compared with the classic thermophoretic force.
High-accuracy determination of the dependence of the photoluminescence emission energy on alloy composition in AlxGa1−xAs films
Authors: Robins Lawrence H. , Armstrong John T. , Marinenko Ryna B. , Paul Albert J. , Pellegrino Joseph G. , Bertness Kristine A.
Published: March 2003

In an effort to improve the accuracy of photoluminescence (PL) measurements of the Al mole fraction (x) of AlxGa1−xAs alloys, the PL peak emission energy, EPL,peak, was measured at room temperature for molecular-beam epitaxy-grown AlxGa1−xAs films with 0⩽x<0.37, and correlated with independent measurements of x by in situ reflective high-energy electron diffraction (RHEED) and also by ex situ wavelength-dispersive x-ray spectroscopy in an electron microprobe analyzer (WDS/EMPA). The measurement uncertainty of EPL,peak was minimized through the following procedures: Accurate calibration of the photon energy (or wavelength) scale, correction of the measured spectra for the spectrometer response function, fitting the data with a well-chosen line shape function, and compensation for the effect of ambient temperature drift. With these procedures, the 2σ measurement uncertainty of EPL,peak was of the order 5×10−4 eV for most samples. From correlation of the PL and WDS/EMPA composition data, the slope ∂EPL,peak/∂x near room temperature was determined to be ∂EPL,peak/∂x=(1.4017±0.0090 eV)−[(2.71±0.97)×10−4 eV/K](T−298.3 K). Correlation with the RHEED data gave the same result within measurement uncertainty. Previously published measurements of ∂EPL,peak/∂x were reviewed and compared with the present study. The results of T. F. Kuech et al. [Appl. Phys. Lett. 51, 505 (1987)], based on nuclear resonant reaction analysis of the Al mole fraction, were found to be in good agreement with the present study after the addition of a correction term to account for the sample temperature difference (T=2 K for Kuech et al., T=298 K for the present study).
Phonon heat conduction in micro- and nano-core-shell structures with cylindrical and spherical geometries
Authors: Zeng Taofang , Liu Wei
Published: March 2003

This study examines the definition of temperatures at interfaces and within thin films when the phonons are in nonequilibrium, and provides a general solution for the temperature distribution within the micro- and nanocylindrical and spherical shells. By applying the Boltzmann transport equation and the established methods of thermal radiation heat transfer, analytical solutions for the temperature distribution and equivalent thermal conductivity are obtained for micro- and nanocylindrical and spherical shells. The study shows that significant drops in temperature occur at the interfaces of micro- and nanocylindrical and spherical shells. For cylindrical shells, the effective thermal conductivity is determined by both the film thickness and the diameter of the inner cylinder. For spherical shells, the effective conductivity is mainly determined by the size of the inner sphere.
Flat-top acceleration system for the variable-energy multiparticle AVF cyclotron
Authors: Fukuda Mitsuhiro , Kurashima Satoshi , Okumura Susumu , Miyawaki Nobumasa , Agematsu Takashi , Nakamura Yoshiteru , Nara Takayuki , Ishibori Ikuo , Yoshida Kenichi , Yokota Watalu , Arakawa Kazuo , Kumata Yukio , Fukumoto Yasushi , Saito Katsuhiko
Published: March 2003

A flat-top acceleration system appropriate to minimization of energy spread in an ion beam was investigated for the JAERI AVF cyclotron. A combination of the fundamental- and the fifth-harmonic voltages to obtain a homogeneous energy gain distribution of accelerated particles is ideally suited to a variable-energy multiparticle cyclotron using acceleration harmonics of 1, 2, and 3. The flat topping of the energy gain distribution using the fifth harmonics has the advantages of minimizing amplifier power, reducing power dissipation in a resonator, and increasing the energy gain per turn. The flat-top acceleration system was designed to reduce the energy spread to 0.02%, which fulfills a beam focusing condition for production of a microbeam with a beam spot diameter of 1 μm. Tolerable fluctuations of acceleration voltages, required to achieve the energy spread of 0.02%, were 2.0×10−4 for the fundamental voltage and 1.0×10−3 for the fifth-harmonic voltage. Both fundamental- and fifth-harmonic phases were required to be stabilized within 0.2 rf degrees. The tolerance of the magnetic excitation was 1.9×10−5. In order to enhance compactness of the flat-topping cavity and to make a substantial saving of the amplifier power, optimum geometric parameters of the flat-topping cavity were determined by a cold model test and a calculation using the MAFIA code. A full range of the fifth-harmonic frequency, 55–110 MHz, was covered by the flat-top system, which enables us to apply the flat-top acceleration to a wide range of energy.
Combined rheometry and small-angle x-ray scattering
Authors: Panine P. , Gradzielski M. , Narayanan T.
Published: March 2003

We have developed an apparatus to simultaneously monitor rheological parameters with high-resolution small-angle x-ray scattering (SAXS). The device is based on a commercial rheometer coupled to concentric cylindrical shear flow geometry. The shear cell can be operated from 20 °C–180 °C without significant evaporation loss of the sample. The apparatus has wide ranging applications in the field of complex fluids such as colloidal suspensions, surfactant phases, liquid crystals, and polymer solutions. In addition, it allows performing SAXS in combination with a wide variety of rheological tests including simple shear flows, creep, recovery, as well as oscillatory deformation. The performance of the equipment is demonstrated through experiments involving shear-induced ordering of complex fluids.
Real-time wavelet-transform spectrum analyzer for the investigation of 1/fα noise
Authors: Brogioli Doriano , Vailati Alberto
Published: March 2003

A wavelet-transform spectrum analyzer operating in real time within the frequency range 3×10−5–1.3×105 Hz has been implemented on a low-cost digital signal processing (DSP) board operating at 150 MHz. The wavelet decomposition of the signal allows one to efficiently process nonstationary signals dominated by large amplitude events fairly well localized in time, thus providing the natural tool to analyze processes characterized by 1/fα power spectrum. The parallel architecture of the DSP allows the real-time processing of the wavelet transform of the signal sampled at 0.3 MHz. The bandwidth is about 220 dB, almost 10 decades. The power spectrum of the signal is processed in real time from the mean square value of the wavelet coefficients within each frequency band. The performances of the spectrum analyzer have been investigated by performing dynamic light scattering experiments on colloidal suspensions and by comparing the measured spectra with the correlation functions data obtained with a traditional multitau correlator. In order to assess the potentialities of the spectrum analyzer in the investigation of processes involving a wide range of time scales, we have performed measurements on a model system where fluctuations in the scattered intensities are generated by the number fluctuations in a dilute colloidal suspension illuminated by a wide beam. This system is characterized by a power-law spectrum with exponent −3/2 in the scattered intensity fluctuations. The spectrum analyzer allows one to recover the power spectrum with a dynamic range spanning about 8 decades. The advantages of wavelet analysis versus correlation analysis in the investigation of processes characterized by a wide distribution of time scales and nonstationary processes are briefly discussed.
Neutron lens by superposition of glancing reflections
Authors: Stoica A. D. , Wang X.-L.
Published: March 2003

A neutron lens can lead to substantial intensity gains in neutron scattering experiments by imaging a source directly onto the sample. A design is proposed for a compact neutron lens made of a stack of silicon wafers, each coated with a neutron-reflective supermirror on one side and a neutron-absorbing layer on the other. We show that for imaging, the length of each mirror is a function of its distance from the optical axis, y, following a simple 1/y relationship. Uniform bending of the mirror assembly decreases the spatial aberration. The optimal design to minimize the optical spatial aberrations is discussed and Monte Carlo simulation results are presented.
Three different simple detector manipulators for spatial measurements in a plasma discharge device
Authors: Shinohara Shunjiro
Published: March 2003

Simple and convenient novel manipulators with two dimensions and three dimensions were demonstrated for measuring various parameters directly in a cylindrical vacuum chamber. Examples of experimental data are presented to prove the usefulness of the systems in a plasma device. Three different mechanisms with neither internal driving nor differentially pumped systems for achieving varying degrees of accuracy were proposed: In one of the two three-dimensional driving systems, detectors were introduced along the axial direction with an arbitrary rotation with respect to the axis by the use of two small-diameter bellows. The other system allowed motion along the radial direction with the capability of arbitrary rotation by small-diameter bellows. In the two-dimensional system, detectors could be scanned vertically using a sliding flange in addition to a radial motion mechanism.
Pressure cell for time-resolved calorimetric measurements of photoinitiated reactions on the fractional millisecond and longer time scale
Authors: Edens Gregory J. , Liu Yan , Grzymski Joseph , Mauzerall David
Published: March 2003

Photoacoustics is an excellent method to obtain enthalpy and volume changes of photochemical reactions. It is particularly useful for photobiological or other reactions in water where the thermal and inherent volume changes can be separated by measurement at the temperature of maximum density. However, its time range is limited to less than ∼10 μs because the amplitude of the observed pressure wave is proportional to the rate of volume change. We describe a closed pressure cell which perfectly complements the usual photoacoustic methodology. It has a rise time of 20 μs and a thermal decay time of several seconds. Its single pulse sensitivity [signal/noise (S/N)=1] is 10 pL or 15 μJ absorbed energy in water. The assembly and calibration of the instrument is described along with its verification by measurement of the enthalpy and volume change of the photodissociation of CO-myoglobin. The apparatus is eminently suited for the study of proton pumping in bacteriorhodopsin and oxygen formation in photosynthesis.
Standard sample probes for characterizing optical apertures in near-field scanning optical microscopy
Authors: Imhof Joseph M. , Kwak Eun-Soo , Vanden Bout David A.
Published: March 2003

A quick, cost effective, semiquantitative means for gauging the quality of near-field scanning optical microscopy (NSOM) probe apertures has been demonstrated by employing a nanoperforated thin metal film standard sample. Small 182 nm holes were created by evaporating gold over dispersed latex spheres with subsequent removal of the spheres. The size of the NSOM aperture can be determined from a deconvolution of the image size and the known sample size and geometry. Results from the standard correlate well with aperture size measurements made from scanning electron micrographs.
Highly charged ion beams at eV kinetic energies
Authors: Lebius H. , Brenac A. , Huber B. A. , Maunoury L. , Gustavo F. , Cormier D.
Published: March 2003

An experimental setup that furnishes slow highly charged ion beams is described. A 14.5 GHz CAPRICE electron cyclotron resonance ion source provides highly charged ions at a kinetic energy of 10 to 20 keV per charge. The kinetic energy is reduced in a deceleration system, which consists of two stages, each of which is built up by two electrostatic zoom lenses. The ion beam can be decelerated down to a kinetic energy of 1 eV/charge. Typical beam currents, measured at the experiment with an emittance of (20 π mm mrad) are about 100 pA, even for the highest measured charge states (Ar16+).
Improving sensitivity of a small angle x-ray scattering camera with pinhole collimation using separated optical elements
Authors: Zemb Th. , Taché O. , Né F. , Spalla O.
Published: March 2003

We show that a significant improvement in the sensitivity of a Huxley–Holmes design for a small angle x-ray scattering camera is obtained by separating the mirror and the monochromator. The design of the camera involves a long x-ray mirror close to a point x-ray source associated with a curved focusing crystal located close to the sample. The sample area is located at half the distance between the source and detector planes. Diffuse scattering produced by the mirror is not incident on the focusing crystal, thus reducing the background signal. Complete elimination of hard x rays allows precise calibration and hence absolute determination of sample cross section by means of a semitransparent beam stop. In pinhole geometry, the flux corresponds to a ∼107 photons/s through the sample, collimated to 10−2 Å−1 in q range. This allows determination of scattered intensities on the order of 10−3 cm−1, corresponding to the scattering related to isothermal compressibility of less than 0.1 mm of pure water. As a reference sample, the widely used Lupolen™, a semicrystalline polymer, is calibrated. The high-q limit (q≈4.5 nm−1) of a porous calcite sample can be used as a secondary standard for specific area determination of solid/solid or solid–liquid dispersions.
High-pressure spectroscopic measurement on diffusion with a diamond-anvil cell
Authors: Aoki K. , Katoh Eriko , Yamawaki H. , Fujihisa H. , Sakashita M.
Published: March 2003

We report a diamond-anvil-cell (DAC) technique developed for spectroscopic measurement on the diffusion process in molecular solids at high pressure. The diffusion processes of atoms, molecules, or their ionic species are investigated for a bilayer specimen by measuring the variation of infrared vibrational spectra with time. The experimental procedures for the protonic and molecular diffusion measurements on ice at 400 K and 10.2 GPa are presented as an example study. The in situ spectroscopic technique with a DAC significantly extends the pressure range accessible for diffusion measurement. The diffusion process at a rate of 10−16–10−14 m2/s can currently be observed at temperatures of 300–600 K and pressures up to several tens of gigaPascals.
Spatial deconvolution technique to obtain velocity profiles from chord integrated spectra
Authors: Golingo R. P. , Shumlak U.
Published: March 2003

Passive spectroscopy is used to measure the plasma parameters on the ZaP experiment at the University of Washington. Twenty spectral intensities, which are functions of the plasma’s density, velocity, and temperature along the viewing chord, are recorded on a charged coupled device. The instrument function is different for each viewing chord. A deconvolution technique based on a shell model, which includes the effects of the instrument function, is developed to deduce the local plasma parameters. The error analysis for this technique is also developed. The technique is able to model complicated plasma parameter profiles and is able to deduce the local plasma parameters and position of the plasma.
Zetameter for microelectrophoresis studies of the oxide/water interface at temperatures up to 200 °C
Authors: Zhou X. Y. , Wei X. J. , Fedkin M. V. , Strass K. H. , Lvov S. N.
Published: March 2003

The zeta potential (ZP) is an important and measurable parameter related to the electrical double layer structure at a solid-aqueous solution interface. A high temperature zetameter based on the microelectrophoresis technique was developed to determine the zeta potential and the isoelectric point (IEP) of the metal oxide/water interfaces at temperatures up to 200 °C and pressures up to 50 bar. Design of the microelectrophoresis cell, the main unit of the zetameter, utilized a flow-through concept and the cell internals were made from corrosion resistant materials in order to minimize materials degradation and solution contamination. Two sapphire windows were installed to the microelectrophoresis cell to enable observation of the particle movement under an imposed electrical field. A ZrO2 powder was used to test the zetameter. The ZP for the ZrO2/water system was measured over wide ranges of temperature and pH. The IEP of the ZrO2/water system was found equal to 6.05 at room temperature, 5.00 at 120 °C, and 4.67 at 200 °C. Thus, at room temperature, the IEP obtained was within the confidence interval of the averaged literature data. In addition, it was found that the IEP of ZrO2 was sensitive to temperature and the difference between the IEP and 0.5pKw remained constant (1.00±0.2) with temperatures up to 200 °C.
Electronic sample-and-hold circuit to minimize artifacts during high-temperature Auger electron spectroscopy measurements
Authors: Henson R. P. , Viljoen E. C. , Terblans J. J. , du Plessis J.
Published: March 2003

The accurate control of sample temperature during Auger electron spectroscopy measurements was obtained in this laboratory using a pulsed heater current. This setup is favored instead of adjusting the dc level of the heater current not only because of its simplicity—it also provides the opportunity to measure the Auger signal during the “off” cycle, thus avoiding possible magnetic effects. The output of the lock-in-amplifier responds to this pulsed current as a differentiation circuit: It spikes on the positive and negative going edges. To eliminate this problem, a sample-and-hold circuit synchronized to the pulsed heating current frequency and a low-pass filter were inserted after the lock-in amplifier.
An extension of transition-state theory for shock-induced chemical kinetics
Authors: Valone Steven M.
Published: March 2003

A qualitative chemical kinetics model is developed for shock environments based on a straight-forward extension of transition-state theory. The model assumes that the distribution of initial velocities along a reaction coordinate is centered about the projection of the shock velocity along that coordinate. The resulting model possesses several highly desirable qualitative features. The first is an adiabatic quality in which the reaction rate depends explicitly on the projected-shock velocity instead of relying on some effective temperature. The second is saturation of the shock amplification of the reaction rate at a critical projected-shock velocity related to the barrier height of the reaction. Third is that the model can act as an extrapolation guide for extending thermally-measured rate constants to a shock environment. Finally, the explicit dependence of the reaction rate on projected-shock velocity, rather than the total shock speed, imparts a natural sense of anisotropy in the shock-induced kinetics. A 1D numerical simulation supports the presence of these features in shock-induced kinetics.
Complex absorbing potentials in the framework of electron propagator theory. II. Application to temporary anions
Authors: Feuerbacher Sven , Sommerfeld Thomas , Santra Robin , Cederbaum Lorenz S.
Published: March 2003

In continuation of Paper I of this work we describe a practical application of the combination of complex absorbing potentials (CAPs) with Green’s functions. We use a new approach for calculation of energies and lifetimes of temporary anions, which emerge, e.g., from elastic scattering of electrons from closed-shell targets. This new method is able to treat the continuum and correlation effects simultaneously and reduces the problem to the diagonalization of a number of relatively small, complex symmetric matrices. The efficiency of the proposed method is demonstrated and its dependence on basis set and parameters characterizing the CAP is investigated using the Πg2 resonance state of N2− as an example. We also present the first correlated ab initio calculation of energies and lifetimes of resonances in elastic electron scattering from the organic molecule chlorobenzene. Our results for both examples are in good agreement with existing experimental values and other theoretical calculations. Possible future developments are discussed.
Nondissociative low energy electron attachment to C2Cl4:C2Cl4− ion lifetime
Authors: Suess L. , Parthasarathy R. , Dunning F. B.
Published: March 2003

The lifetimes of long-lived parent anions formed by nondissociative electron attachment to C2Cl4 are investigated using Rydberg atom techniques. The ions are created through electron transfer in collisions with K(np) Rydberg atoms and their lifetimes are measured using both time-of-flight techniques and a permanent magnet Penning trap designed specifically for heavy ion storage. The data show that low-energy electron attachment to C2Cl4 leads to the formation of C2Cl4− ions with a broad range of lifetimes extending from ∼3 to ∼130 μs, which is attributed to capture by molecules in different initial vibrational states.
Deficiencies of the bend symmetry coordinates used for methane
Authors: Wang Xiao-Gang , Carrington Tucker
Published: March 2003

We demonstrate that the bend symmetry coordinates commonly used for methane do not determine the shape of the molecule. We suggest a rule that does enable one to associate a unique shape with a set of symmetry coordinates. However, any such rule entails excluding shapes and therefore cannot be used to calculate high-lying levels.
Further studies of 3d transition metal cyanides: The pure rotational spectrum of NiCN (X 2Δi)
Authors: Sheridan P. M. , Ziurys L. M.
Published: March 2003

The pure rotational spectrum of the NiCN radical (X 2Δi) has been recorded using millimeter/sub-mm direct absorption techniques in the range 360–550 GHz. Transitions arising from four nickel isotopomers (58Ni,60Ni,62Ni,64Ni) and Ni58CN13 were observed in the ground vibrational state, as well as lines originating in the v2 bending and v1 stretching modes. In the vibrational ground state, transitions from both spin–orbit components (Ω=52 and 32) were identified; in the Ω=32 ladder, significant lambda-doubling was observed. Multiple vibronic components were found for each bending quantum recorded, a result of Renner–Teller interactions. These components were only observed in the lower spin–orbit ladder (Ω=52), however, suggesting that spin–orbit coupling dominates the vibronic effects. The ground-state data were analyzed with a case (a) Hamiltonian, generating rotational, spin–orbit, and lambda-doubling constants for NiCN58 and NiCN60. The vibrationally excited lines were modeled with effective rotational parameters, except where a case (c) or case (b) coupling scheme could be meaningfully used. From the ground-state rotational parameters, r0, rs, and rm(1) structures were derived as well. NiCN appears to be a covalently bonded molecule with similar properties to NiH.
Langevin dynamics simulations of early stage shish-kebab crystallization of polymers in extensional flow
Authors: Dukovski I. , Muthukumar M.
Published: March 2003

We have investigated the molecular origins of shish-kebab morphology occurring in polymer crystallization under extensional flow. Emergence of shish-kebabs is intimately related to the discontinuous coil-stretch transition of isolated chains. Our computed free energy landscape shows that there are in general two populations of stretched and coiled conformations at a given flow rate, even for monodisperse chains. While the stretched chains crystallize into shish, the coiled chains first form single-chain lamellae and then adsorb to the shish constituting the kebabs. We have followed the molecular details of formation of the shish and kebabs, and their dependence on initial configurations of chains, polymer concentration, and rate of crystallization. The local inhomogeneity in polymer concentration dramatically alters the population of stretched and coiled conformations, thus significantly influencing the onset of shish-kebab morphology. The propensity of kebabs is reduced by lowering the rate of crystallization and/or increasing the flow rate.
The role of local conformations in the stretching of a poly(ethylene oxide) chain in solution
Authors: Bedrov Dmitry , Smith Grant D.
Published: March 2003

We have conducted molecular dynamics simulations of the stretching of a poly(ethylene oxide) (PEO) chain in water and n-tridecane in order to elucidate the mechanisms of elastic response of the amphiphilic PEO chain in hydrophilic and hydrophobic environments. Quantitative agreement was found for the restoring force as a function of the degree of chain extension between simulation and recent atomic force microscopy measurements in the highly and severely stretched regimes. In these regimes the free energy of the PEO chain and the restoring force are determined by changes in local conformational populations and geometries. In the highly stretched regime stretching of the PEO chain results in an increased trans population of C–C and C–O dihedrals as well as the deformation of dihedral geometries. Here the restoring force is greater in water than in n-tridecane due to the greater free energy (mostly enthalpic) penalties for C–C trans dihedrals and the deformation of C–C gauche dihedrals in the polar solvent. In both solvents the conversion of gauche C–C and C–O dihedrals to trans in the highly stretched regime makes a significantly entropic contribution to the chain free energy and restoring force. No tendency toward extended gauche helical conformations of the PEO chain in water, as has been previously speculated, was observed in this regime. An estimation of the PEO free energy as a function of the degree of chain extension based upon conformational populations relative to equilibrium (unstretched) chains was found to lead to an overestimation of the free energy cost of chain extension in the highly stretched regime in n-tridecane due to neglect of the influence of deformation of dihedral geometries on chain dimensions. In water, the influence of the deformation of dihedral geometries was largely offset by an increase in their free energy due to decreased polar interactions of the distorted dihedrals with water. In the severely stretched regime, the PEO chain was found to be in an all trans conformation, and the restoring force, which is independent of solvent in this regime, was found to be due to deformation of backbone valence angles and the trans dihedral geometry.
The effect of ions on solid–liquid phase transition in small water clusters. A molecular dynamics simulation study
Authors: Egorov Andrei V. , Brodskaya Elena N. , Laaksonen Aatto
Published: March 2003

Small water clusters, containing ions, have been studied using molecular dynamics simulations at temperatures ranging from 0 to 250 K. The simulations are carried out systematically by varying the ion size, shape, and charge as well as the cluster size and the initial configuration. Transitions between solid and liquid phases are followed to study the effects of the ions on the cluster melting temperature, compared to pure water clusters of the same size. The effect of the ion on the ice-cluster melting appears to be a complicated process which depends simultaneously on a variety of factors, such as the initial cluster configuration and the ion position inside the cluster as well as the ion mass, size and its charge. In the case of monovalent cations the most important characteristics for the cluster evolution is the ion mass, while for divalent cations the ion charge is the most dominant factor. In the case of negatively charged ions the main factor of the system evolution is the ion size. Two principally different types of cluster structures can be observed from the simulations: The peripheral structure where the ion takes up a position, preferably on the cluster surface and the interior structure where the ion prefers the center of the system. The peripheral structure is typical for clusters containing the small monovalent Li+ cation but also for those containing the large Cl− anion, while divalent cations, large monovalent Na+ cation and small F− anions gave rise to the interior type of structure. Generally, an increase of the ion size changes the cluster structure making the peripheral variant more preferable.
The hyper-Rayleigh light scattering spectrum of gaseous Ne–Ar mixture
Authors: Głaz W. , Bancewicz T.
Published: March 2003

Hyper-Rayleigh binary collisional spectra for a Ne–Ar gaseous system are computed within the frequency range up to 500 cm−1 with four new ab initio hyperpolarizability models used. The properties of the profiles as well as the depolarization ratio frequency dependence are discussed. The contributions to the spectra given by the dipolar b1(r) and the octopolar b3(r) components of the hyperpolarizability tensor and the multipolar model HR spectrum are compared.