bluuebunny/crossref_metadata_2025 huggingface dataset:
Proton-induced disorder in InP-based resonant tunneling diodes
Authors: Jackson E. M. , Weaver B. D. , Seabaugh A. C. , Van der Wagt J. P. A. , Beam E. A.
Published: July 2002

We have fabricated arrays of resonant tunneling diodes based on InP substrates for exposure at room temperature with fluences of 3 MeV protons up to 7×1014 H+/cm2. Proton fluences below about 1×1013 cm−2 have little effect on the resonant tunneling diode but higher fluences decrease the peak current and increase the valley current. We find that proton-induced changes in the operating parameters are qualitatively similar to the effects of doping the wells.
Morphology of InAs self-organized islands on AlAs surfaces
Authors: Ballet P. , Smathers J. B. , Salamo G. J.
Published: July 2002

We report an in situ molecular-beam epitaxy-scanning tunneling microscopy study of three-dimensional (3D) self-organized InAs islands on AlAs surfaces. The evolution of the density and morphology of these islands is investigated as a function of the InAs coverage and substrate temperature. It is shown that the 2D island density is already high just prior to 3D island formation and remains constant for 3D structures as the InAs coverage is increased. This observation contrasts with the InAs/GaAs system and makes possible the growth of very high densities of small quantum dots.
Optically pumped type-II interband terahertz lasers
Authors: Vurgaftman I. , Meyer J. R.
Published: July 2002

Optically pumped terahertz lasers based on interband transitions in type-II antimonide heterostructures are proposed and modeled in detail. At cryogenic temperatures, the activated nature of the Auger and phonon-assisted mechanisms should provide substantially longer nonradiative lifetimes and higher gains than are attainable with intersubband devices. For emission at λ=27 μm, pulsed operation is projected up to 60 K, and >25 mW of cw output power is calculated for T=30 K. At T=4 K, lasing is expected out to wavelengths as long as 100 μm.
Inverter made of complementary p and n channel transistors using a single directly deposited microcrystalline silicon film
Authors: Chen Yu , Wagner Sigurd
Published: July 2002

We report a p channel thin-film transistor (TFT) made of directly deposited microcrystalline silicon (μc-Si). By integrating this p TFT with its n channel counterpart on a single μc-Si film, we fabricated a complementary metal-silicon oxide-silicon (CMOS) inverter of deposited μc-Si. The μc-Si channel material was grown at 320 °C by plasma-enhanced chemical vapor deposition in a process similar to the deposition of hydrogenated amorphous silicon. The highest postdeposition TFT process temperature was 280 °C. The low-temperature p channel μc-Si TFT and the integrated CMOS inverter represent building blocks of a digital circuit technology based on ultralow-temperature silicon.
Scanning near-field optical spectroscopy and imaging using nanofabricated probes
Authors: Zhou H. , Midha A. , Mills G. , Donaldson L. , Weaver J. M. R.
Published: July 2002

We have designed and constructed a scanning near-field optical microscopy (SNOM) system which is based on batch-fabricated aperture probes fabricated using micromachining and direct-write electron-beam lithography. The aperture is situated at the hollow tip apex of a modified silicon nitride atomic force microscope (AFM) cantilever. The SNOM/AFM probes are used both for excitation and collection of luminescence due to their excellent optical efficiency. The integration of a cantilever with the aperture allows better control of the aperture–sample distance and also results in considerable simplification of the SNOM system. Using this system, we have obtained near-field luminescence spectra and imaging of GaAs/AlGaAs quantum wires and wells, and have demonstrated spectrally resolved luminescence imaging with a spatial resolution of ∼50 nm.
Molecular-beam epitaxy growth and nitrogen doping of ZnSe1−xTex alloys grown on InP substrates
Authors: Lin W. , Yang B. X. , Guo S. P. , Elmoumni A. , Fernandez F. , Tamargo M. C.
Published: July 2002

High-quality ZnSe1−xTex alloys spanning the entire compositional range have been grown by molecular-beam epitaxy. Good compositional control can be obtained by adjusting the beam equivalent pressure ratio of Se to Zn during growth. Double-crystal x-ray rocking curves with full widths at half maximum as narrow as 70 arcsec were obtained suggesting excellent crystalline quality. The p-type doping with nitrogen of ZnSe1−xTex alloys having Te contents from 0% to 100% has been systematically studied. The free hole concentration increases from 1017 to 1019 cm−3 as the Te content increases from 12% to 40%. The N-doped ZnSeTe lattice matched to InP has a free hole concentration of 2×1019 cm−3. This highly doped material was used as the p-type ohmic contact layer in light-emitting diodes made from ZnCdMgSe/ZnCdSe quantum-well structures grown on InP substrates that emit in the red, green and blue regions.
The nature of the electron distribution functions in quantum cascade lasers
Authors: Harrison P.
Published: July 2002

There has been much speculation about the nature of the electron distribution functions in quantum cascade lasers, in particular: Are they non equilibrium and if so, to what extent? In order to address this, detailed calculations of the intersubband and intrasubband electron-longitudinal phonon and electron–electron scattering rates in a real GaAs/AlxGa1−xAs device design [Sirtori et al., Appl. Phys. Lett. 73, 3486 (1998)] are presented. The results suggest that two important conclusions relating to the electron distribution functions in the quantum well subbands can be drawn: the electron distributions in both the active region and the injector subbands are thermalized, i.e., they can be described by Fermi–Dirac distribution functions and furthermore all the electron distributions have the same electron temperature.
Narrow-linewidth terahertz intersubband emission from three-level systems
Authors: Williams Benjamin S. , Xu Bin , Hu Qing , Melloch Michael R.
Published: July 2002

Narrow-linewidth terahertz spontaneous emission resulting from interwell (or diagonal) intersubband transition from an electrically pumped multiple-quantum-well structure was observed. The center frequency of the emission peak is at 2.57 THz, and its full width at half maximum is 0.47 THz. The emission frequency is in good agreement with the calculated intersubband transition energy of 11.3 meV (corresponding to 2.7 THz) in a three-level system, which was designed to achieve population inversion between two radiative levels.
Imaging of microwave permittivity, tunability, and damage recovery in (Ba, Sr)TiO3 thin films
Authors: Steinhauer D. E. , Vlahacos C. P. , Wellstood F. C. , Anlage Steven M. , Canedy C. , Ramesh R. , Stanishevsky A. , Melngailis J.
Published: July 2002

We describe the use of a near-field scanning microwave microscope to quantitatively image the dielectric permittivity and tunability of thin-film dielectric samples on a length scale of 1 μm. We demonstrate this technique with permittivity images and local hysteresis loops of a 370-nm-thick Ba0.6Sr0.4TiO3 thin film at 7.2 GHz. We also observe the role of annealing in the recovery of dielectric tunability in a damaged region of the thin film. We can measure changes in relative permittivity εr as small as 2 at εr=500, and changes in dielectric tunability dεr/dV as small as 0.03 V−1.
Type-II InAs/InAsSb strained-layer-superlattice negative luminescence devices
Authors: Pullin M. J. , Hardaway H. R. , Heber J. D. , Phillips C. C.
Published: July 2002

Negative luminescence operation is reported for p–n diode devices with type-II InAs/InAsSb strained-layer-superlattice active regions which have a spectral peak at 4.2 μm and a negative luminescence efficiency of up to 20%.
Impurity gettering by high-energy ion implantation in silicon beyond the projected range
Authors: Gueorguiev Y. M. , Kögler R. , Peeva A. , Panknin D. , Mücklich A. , Yankov R. A. , Skorupa W.
Published: July 2002

Deep gettering layers have been formed in Si wafers by MeV implantation of Si+ and P+ ions, followed by annealing. Samples have been subsequently contaminated with Cu. Secondary ion mass spectrometry analysis reveals for P implants gettering of Cu atoms in regions significantly deeper than the projected ion range RP and formation of a separate Cu gettering band there. We call this phenomenon the “trans-RP effect.” The results obtained indicate the presence of an appreciable amount of defects in the region beyond RP. Their gettering ability is much higher than that of the implanted gettering layer at RP. The size of these deep defects is below the resolution limit of transmission electron microscopy. We suggest that they are interstitials and/or small interstitial clusters. An explanation of the mechanism responsible for their migration from RP into the trans-RP region and their clustering is proposed.
Instabilities in low-pressure inductive discharges with attaching gases
Authors: Lieberman M. A. , Lichtenberg A. J. , Marakhtanov A. M.
Published: July 2002

Plasma instabilities at frequencies 1 Hz–900 kHz have been observed in low-pressure inductive processing discharges with attaching gases. Instability windows in pressure and driving power are found. A volume-averaged (global) model of the instability is developed, considering idealized inductive and capacitive energy deposition. As pressure or power are varied to cross a threshold, the instability is born at a Hopf bifurcation, with relaxation oscillations between inductive and capacitive modes causing modulations of charged particle densities, electron temperature, and plasma potential. The oscillations can be so strong that the potential collapses and negative ions flow to the walls.
Photoluminescence and cathodoluminescence of GaN doped with Tb
Authors: Lozykowski H. J. , Jadwisienczak W. M. , Brown I.
Published: July 2002

We report the observation of the visible cathodoluminescence and photoluminescence of Tb3+ ions implanted in GaN. The sharp characteristic emission lines corresponding to Tb3+ intra-4f8-shell transitions are resolved in the spectral range from 350 to 900 nm, and observed over the temperature range of 7–330 K. The luminescence shows transitions which originate in the D35 and D45 levels and terminated in the F7 manifolds. The cathodoluminescence emission is only weakly temperature dependent. The results indicate that Tb-doped GaN epilayers may be suitable as a material for visible optoelectronic devices.
Spin-polarized tunneling in La0.67Sr0.33MnO3
Authors: Worledge D. C. , Geballe T. H.
Published: July 2002

We have developed the ability to grow reproducible La0.67Sr0.33MnO3/SrTiO3/Al tunnel junctions with high quality gap characteristics for spin-polarization measurements. A YBa2Cu3O7 counterelectrode was required to prevent current crowding. In order to fit our data, we have numerically solved Maki’s equations which include the effects of orbital depairing, the Zeeman splitting of the spin states, and spin-orbit scattering. We consistently measure P=+72%, for a variety of deposition conditions.
Piezoelectric field and confinement effects on the dielectric function spectrum of InGaN/GaN quantum wells
Authors: Ramakrishnan A. , Wagner J. , Kunzer M. , Obloh H. , Köhler K. , Johs B.
Published: July 2002

In 0.13 Ga 0.87 N/GaN double heterostructures and quantum wells (QWs) have been studied by room-temperature photoluminescence (PL) and spectroscopic ellipsometry (SE). PL revealed the presence of strong piezoelectric fields, which strongly influence the luminescence properties for InGaN QW widths in the 3–12 nm range. The pseudodielectric function spectrum derived from the SE measurements were analyzed using a multilayer approach, describing the dielectric function of the individual layers by a parametric oscillator model. The fundamental band-gap resonance in the InGaN dielectric-function spectrum was found to broaden for an InGaN layer width of 12 nm, as compared to bulk-like InGaN layers, due to piezoelectric field effects. For a much narrower QW width of 1.7 nm, however, quantum confinement was found to dominate over piezoelectric-field effects, resulting in a much sharper band-gap resonance shifted to higher energies and an increased oscillator strength.
Thickness-independent electron field emission from tetrahedral amorphous carbon films
Authors: Zhao J. P. , Chen Z. Y. , Wang X. , Shi T. S. , Yano T.
Published: July 2002

Electron field emission properties of tetrahedral amorphous carbon films of different thicknesses have been studied. The experimental results indicate that there exists no close relationship between threshold electric field and film thickness. Different field emission models are used to examine the experimental results in order to explain the thickness-independent electron field emission properties.
(GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser with ultrabroad temperature operation range
Authors: Wagner A. , Ellmers C. , Höhnsdorf F. , Koch J. , Agert C. , Leu S. , Hofmann M. , Stolz W. , Rühle W. W.
Published: July 2002

The temperature dependence of the emission of a (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser is investigated. We find laser emission over an extremely broad temperature range from 30 K up to 388 K. The laser threshold varies from 5 kW/cm2 at 373 K down to a minimum of 1 kW/cm2 at 180 K and increases again to 4 kW/cm2 at 30 K. Picosecond emission dynamics after femtosecond optical excitation is obtained with peak delays below 33 ps and pulse widths below 20 ps over the entire operation range.
Analysis of submicron carbon nanotube field-effect transistors
Authors: Yamada Toshishige
Published: July 2002

A theoretical analysis of carbon nanotube based field-effect transistors fabricated by two different groups [Tans et al., Nature (London) 393, 49 (1998); Martel et al., Appl. Phys. Lett. 73, 2447 (1998)] is presented. The metal (electrode)-semiconductor (nanotube) contact influences subthreshold channel conductance versus gate voltage VG, such that the occurrence of a kink depends on the transport mechanism across this contact. Saturation in the turn-on drain current ID vs VG seen in experiments reflects the nanotube state density. Saturationless ID versus drain voltage VD indicates transport in the weak-localization regime in the absence of carrier–carrier scattering so that pinch-off cannot occur. To compensate for saturationless ID(VD) in digital applications, nanotube transistors need to be designed to maximize their transconductance.
Comparison of memory effect between YMnO3 and SrBi2Ta2O9 ferroelectric thin films deposited on Si substrates
Authors: Lee Ho Nyung , Kim Yong Tae , Choh Sung Ho
Published: July 2002

For the ferroelectric gate-type capacitors, we have fabricated Pt/YMnO3(YMO)/Si and Pt/SrBi2Ta2O9(SBT)/Si structures. We have used the highly c-axis oriented hexagonal YMO thin films (εr≈19) and the polycrystalline SBT thin films (εr≈150) with a dominant (115) orientation, respectively. The memory effect resulting from the ferroelectric switching properties is investigated as a function of the dielectric constant of ferroelectric thin films with 150 nm in thickness. About 3 times wider memory window is obtained by using a relatively low dielectric constant of YMO than that using a relatively high dielectric constant of SBT. Typical memory windows of the Pt/YMO/Si and the Pt/SBT/Si capacitors are 1.24 and 0.34 V, respectively, at a gate voltage of 5 V.
Polarization of Xe129 with high power external-cavity laser diode arrays
Authors: Zerger J. N. , Lim M. J. , Coulter K. P. , Chupp T. E.
Published: July 2002

We demonstrate narrowing of a 2 W, broad area laser diode array and present calculations of the resulting improvement of Xe129 polarization by spin exchange with laser optically pumped Rb vapor. This improvement significantly impacts both medical imaging with laser polarized noble gas and spin-exchange pumped noble gas maser research.
Photoelectron spectroscopic study of amorphous GaAsN films
Authors: Zanatta A. R. , Hammer P. , Alvarez F.
Published: July 2002

Amorphous gallium–arsenic–nitrogen (a-GaAsN) thin films were deposited by sputtering a crystalline GaAs target with different mixtures of argon and nitrogen. X-ray photoelectron spectroscopy (XPS) and x-ray excited Auger electron spectroscopy (XAES) were employed to study the Ga and As core levels and the corresponding LMM Auger transitions of films with different N concentrations. Chemical information of these samples was obtained through the analysis of the Auger parameter, which is exempt from problems inherent in the interpretation of XPS and XAES shifts, revealing aspects associated with the composition of the a-GaAsN films. In particular, these experimental results show the preferential bonding of N to Ga atoms in the formation of N-rich amorphous GaAsN films.
Quantitative determination of the magnetization and stray field of a single domain Co/Pt dot with magnetic force microscopy
Authors: Lohau J. , Kirsch S. , Carl A. , Wassermann E. F.
Published: July 2002

The z-component of both the magnetization and the stray field of a nanometer sized single domain magnetic Co/Pt multilayer dot with perpendicular magnetic anisotropy is determined quantitatively within the point probe approximation by magnetic force microscopy (MFM). The MFM tip used is calibrated by probing omega-shaped nanosized current rings fabricated by electron-beam lithography. Since the stray field geometry of the dot and the current rings are similar, the calibrated tip can be used to determine quantitatively the magnetization and the stray field of the dot with perpendicular magnetic anisotropy.
An electron microscopy study of the growth of Ge nanoparticles in SiO2
Authors: Bonafos C. , Garrido B. , Lopez M. , Perez-Rodriguez A. , Morante J. R. , Kihn Y. , Ben Assayag G. , Claverie A.
Published: July 2002

Ion implantation followed by high temperature annealing can be used to synthesize group IV semiconducting nanoparticles in SiO2. The density and the size distribution of these nanocrystals obviously depend on the implantation and annealing conditions. While their size can be measured by “classical” transmission electron microscopy techniques, their density cannot because no diffraction occurs in the amorphous matrix. In this letter, we use electron energy loss spectroscopy to overcome this problem. We have measured the evolution of the size distribution, the density, and the atomic fraction occupied by the Ge precipitates during annealing. We show that the nanocrystals grow in size and reduce their density, while the overall number of atoms they contain remains constant. This observation proves that the nanoparticles undergo a conservative ripening during annealing.
Near-field scanning optical microscopy cross-sectional measurements of crystalline GaAs solar cells
Authors: Herndon M. K. , Bradford W. C. , Collins R. T. , Hawkins B. E. , Kuech T. F. , Friedman D. J. , Kurtz S. R.
Published: July 2002

Near-field scanning optical microscopy (NSOM) was used to study cleaved edges of GaAs solar cell devices. Using visible light for excitation, the NSOM acquired spatially resolved traces of the photocurrent response across the various layers in the device. For excitation energies well above the band gap, carrier recombination at the cleaved surface had a strong influence on the photocurrent signal. Decreasing the excitation energy, which increased the optical penetration depth, allowed the effects of surface recombination to be separated from collection by the pn junction. Using this approach, the NSOM measurements directly observed the effects of a buried minority carrier reflector/passivation layer.
Phase space observables and isotypic spaces
Authors: Cassinelli Gianni , De Vito Ernesto , Lahti Pekka , Levrero Alberto
Published: July 2002

We give necessary and sufficient conditions for the set of Neumark projections of a countable set of phase space observables to constitute a resolution of the identity, and we give a criteria for a phase space observable to be informationally complete. The results will be applied to the phase space observables arising from an irreducible representation of the Heisenberg group.
Probe measurements of electron temperature and density in strongly magnetized plasma
Authors: Ratynskaia S. V. , Demidov V. I. , Rypdal K.
Published: July 2002

It was demonstrated that with increasing probe length the electron saturation current becomes more independent of the probe potential. This provides higher accuracy of measurements of the electron density and plasma potential in a strongly magnetized plasma (B>0.1 T). Long probes oriented parallel to the magnetic field yield better spatial resolution in the perpendicular direction. Parallel probes, however, are very sensitive to alignment errors. To avoid this problem, a cylindrical probe with a side surface coated by a thin insulating layer is proposed.
The structure of liquid and supercritical benzene as studied by neutron diffraction and molecular dynamics
Authors: Tassaing T. , Cabaço M. I. , Danten Y. , Besnard M.
Published: July 2002

The local ordering in benzene has been investigated under isothermal and isobaric conditions from the liquid to the supercritical domain using neutron diffraction. The experimental results have been analyzed at the light of molecular dynamics simulation. A progressive evolution of local ordering is observed when the density decreases when going from liquid to gaslike values in the supercritical domain. The translational ordering, which is characterized by three well-defined shells of neighboring molecules at ambient conditions, is gradually weakened, and extends only to the first shell as the density decreases in the supercritical domain (under isothermal compression). In the same way, the orientational ordering of benzene, which exists under ordinary conditions where parallel and perpendicular configurations of neighboring molecules are slightly favored, is lost in the supercritical state and is found to be almost completely isotropic.
Far field behavior of noncompact static spherically symmetric solutions of Einstein SU(2) Yang Mills equations
Authors: Linden Alexander N.
Published: July 2002

There exist static spherically symmetric solutions of the Einstein equations with cosmological constant Λ coupled to the SU(2) Yang Mills equations that are smooth at the origin r=0 and with a horizon which can be transformed away with a change of coordinates in which the radius increases across the singularity. We establish the global behavior of these solutions.
Optimization and enhancement of H− ions in a magnetized sheet plasma
Authors: Abate Yohannes , Ramos Henry J.
Published: July 2002

Several schemes to improve the efficiency of extraction of H− ions from a magnetized sheet plasma source are reported. Parameters that affect the optimization of the extraction process such as plasma electrode position relative to the core plasma, plasma electrode bias, extraction electrode bias, neutral gas pressure, and discharge current were investigated. The negative hydrogen ion current density extracted from pure hydrogen plasma under optimum conditions was 0.15 A/m2. Enhancement of the H− current density has been observed by as much as 73.3% when argon was mixed with hydrogen at a 10%/90% ratio. The addition of argon raised the electron density by ten times and increased the electron temperature by 40% at the center of the sheet plasma. These plasma parameters were of comparable value with and without argon at the periphery of the sheet. The existence of high-energy electrons at the core and the presence of cold electrons at the periphery of the sheet plasma conform to ideal conditions of H−production of a volume source. The increase in H− current density with the addition of argon correlates well with the product of electron density and the square root of electron temperature at the center of the sheet plasma.
Transient electronic absorption of vibrationally excited CH2I2: Watching energy flow in solution
Authors: Bingemann Dieter , King Andrew M. , Crim F. Fleming
Published: July 2002

Transient electronic absorption of methylene iodide (CH2I2) in CCl4, CDCl3, and C6D6 after excitation of two quanta of C–H stretching vibration with a 100 fs laser pulse allows direct observation of the times for intramolecular vibrational relaxation and energy transfer to the solvent. Intramolecular energy redistribution populates vibrational states with larger Franck–Condon factors for the electronic transition, leading to an increased absorption of probe pulses in the wavelength range of 380–440 nm. A model based on the temperature dependence of the electronic absorption coefficient describes the transient absorption well for all wavelengths. In the model, the temperature rises and decays exponentially with time, reflecting the initial redistribution of energy within the excited molecule and the subsequent transfer of energy from the vibrationally excited molecule into the solvent. The intramolecular vibrational relaxation time for CH2I2 is essentially the same in the solvents CCl4 (10.8±1.5 ps) and CDCl3 (11.2±2.0 ps) and is only slightly shorter in C6D6 (8.0±1.5 ps). Energy transfer to the solvent takes longer, occurring with a time constant of 68±10 ps for CCl4, 51±10 ps for CDCl3, and 23±2 ps for C6D6.
Raman spectroscopy of sol–gel ultrathin films enhanced by surface plasmon polaritons
Authors: Brioude A. , Lequevre F. , Mugnier J. , Dumas J. , Guiraud G. , Plenet J. C.
Published: July 2002

We present experimental results of a Kretschmann-prism coupler method used in Raman scattering experiments with ultrathin (few nanometers) TiO2 sol–gel films deposited on a metallic silver layer characterized by surface plasmon resonance (SPR) experiments. In this article, we describe a new optical device as an application of SPR properties and coupled with a micro-Raman scattering setup. The enhancement factor (150) of the Raman intensity in comparison to the classical way allows us to determine the critical thin films structure. Optical results are then discussed and compared to transmission electron microscopy.
A Brillouin study of the angular dispersion of generalized surface and pseudosurface acoustic waves on the (001) surface of InSb
Authors: Kuok M. H. , Ng S. C. , Zhang V. L.
Published: July 2002

The velocity angular dispersion of the generalized surface (GSW) and pseudosurface (PSW) acoustic waves on the (001) plane of InSb has been measured by Brillouin scattering. The complete range of angular dispersion of these waves on the (001) plane of an anisotropic cubic crystal has been experimentally determined. The p–p and p–s polarization results accord well with theory. In particular, the measured angular dispersion of these surface modes traces out two separate and distinct branches, in contrast to one single curve arising from the branching from GSW to PSW, normally observed for anisotropic cubic crystals.
Vibrational spectra and intramolecular vibrational redistribution in highly excited deuterobromochlorofluoromethane CDBrClF: Experiment and theory
Authors: Beil Andreas , Hollenstein Hans , Monti Oliver L. A. , Quack Martin , Stohner Jürgen
Published: July 2002

The rovibrational spectra of deuterobromochlorofluoromethane (CDBrClF) were measured at intermediate (0.1 cm−1) and high resolution (0.0024 cm−1 full bandwidth, half-maximum) by interferometric Fourier transform infrared spectroscopy in the range from the far infrared at 200 cm−1 to the near infrared (12 000 cm−1) covering all the fundamentals and CD stretching overtones up to polyad N=5. The spectra are completely analyzed in terms of their vibrational assignments to fundamentals, combinations and overtones. At high excitation the analysis reveals the dominant anharmonic coupling between four high frequency vibrational modes; the CD stretching (ν1), two CD bending (ν2,ν3), and the CF stretching mode (ν4). The analysis is carried out using effective model Hamiltonians including three and four vibrational degrees of freedom. We also present vibrational variational calculations on a grid in a four-dimensional normal coordinate subspace. The potential energy and the dipole moment function are calculated ab initio on this grid using self-consistent field second order Møller–Plesset perturbation theory (MP2). Experimental and theoretical results for band positions and integrated intensities as well as effective spectroscopic parameters are found to be in good agreement. The important anharmonic coupling between the CD chromophore and the CF stretching vibration can be described by an effective cubic Fermi resonance coupling constant ksff′≈(50±10) cm−1, which leads to intramolecular vibrational redistribution between the CD and CF chromophores on the femtosecond time scale. Time dependent intramolecular vibrational redistribution processes in CDBrClF are derived in various representations, including time dependent probability densities (“wave packets”) in coordinate space and finally time dependent entropy.
Quantum Monte Carlo determination of the atomization energy and heat of formation of propargyl radical
Authors: Harkless J. A. W. , Lester W. A.
Published: July 2002

The quantum Monte Carlo (QMC) method is used to compute the atomization energy and the heat of formation of the propargyl radical, C3H3. The effective core potential and fixed-node approximations are used in the diffusion Monte Carlo (DMC) variant of QMC. Two generalized gradient approximation density functionals, B3LYP and B3PW91, are also applied for comparison. The atomization energy determined by these methods is 606.12 kcal/mol (B3LYP), 610.24 kcal/mol (B3PW91), and 607.6(0.6) (DMC). The latter compares favorably with separate measurements of 608.0(3.0) and 608.5(1.2) kcal/mol. The ΔHf298 determined by these methods is 84.03 kcal/mol (B3LYP), 79.91 kcal/mol (B3PW91), 82.5(0.6) (DMC), and two independent measurements yield values of 82.5(3.0) and 81.5(1.2) kcal/mol.
Reactions in microemulsions: Effect of thermal fluctuations on reaction kinetics
Authors: Ganesan Venkat , Fredrickson Glenn H.
Published: July 2002

In this paper we address the generic effects arising from the interplay of thermal fluctuations and reactions. This is accomplished by considering specifically the kinetics of reactions effected in microemulsion media. In the first part of this paper we consider the kinetics of the reaction A+B→O/ in bicontinuous microemulsion media, wherein the solutes A and B are assumed to be preferentially attracted to water and oil, respectively, and O/ constitutes an inert product. We formulate the diffusion and reaction of these solutes in a field-theoretical framework within which the fluctuations of the background microemulsion are embedded. We then employ mean-field arguments and a perturbative Wilson-type renormalization group (RG) approach to discern the relevance, at long length scales, of the background fluctuations. Our analysis indicates that the dynamic fluctuations of the microemulsion prove irrelevant in impacting the asymptotic kinetics of the reaction. In view of the fact that our field-theoretic approach enables us to probe only the long time characteristics, moreover, only in the weak-coupling limit, in the second part of this paper we analyze similar issues in the context of the droplet phase of microemulsions. This enables us to surmount some of the restrictions placed upon the results of the first part of this paper. In the second part, our analysis focuses upon a simpler reaction, viz., A→O/, wherein the solute A which is present only in the water phase is anhiliated upon contact with the fluctuating interfaces of the droplets. We employ a standard diffusion equation framework to formulate the transport and reaction of A. The fluctuations of the microemulsion are manifest in the boundary condition positing the vanishing concentration of A. We then employ a perturbation scheme to the solution of the diffusion equation, and thereby discern the explicit effects of the fluctuations of the sinks. Our formulation enables, in a sequentially improvable asymptotic manner, the explicit computation of the time-dependent and the steady state fluctuation contributions to the reaction rate.
Quantum scattering calculations of the O(1D)+HCl reaction using a new ab initio potential and extensions of J-shifting
Authors: Bittererová Martina , Bowman Joel M. , Peterson Kirk
Published: July 2002

We report results of quantum wave packet calculations of the O(1D)+HCl(v=0,j)→ClO+H, OH+Cl, reactions for zero and nonzero total angular momentum, J, (using the centrifugal sudden approximation), and using a new fit to extensive ab initio calculations of a global potential [K. A. Peterson, S. Skokov, and J. M. Bowman, J. Chem. Phys. 111, 2445 (1999)]. Initial state-selected and cumulative reaction probabilities to form each set of products for J=0 are calculated by a direct summation of the initial state-selected reaction probabilities. We propose and test a simple energy-shifting approximation that relates the initial state-selected reaction probability for arbitrary j to the one for j=0. Extensions of standard J- and K-shifting methods are suggested and applied to both reaction channels. In doing this extension the adiabatic rotation approximation is used to determine the rotational barriers in the entrance and exit channels. The energy dependence of the reaction cross sections to form the two products is calculated for O(1D)+HCl(v=0,j=0) using J- and K-shifting and compared at two translational energies to results of quasiclassical trajectory calculations. The thermal rate constants for the two reaction channels are calculated from 200 to 400 K and compared to experiment.
Restricted primitive model of dianions and counterions within the mean spherical approximation: Integral equation and thermodynamic perturbation theory
Authors: von Solms N. , Chiew Y. C.
Published: July 2002

We present an analytical integral equation theory for dimers modeled as hard-sphere tangentially connected anions and cationic hard-sphere monomeric counterions embedded in a continuum dielectric medium. Each hard-sphere segment on the dimer and hard-sphere counterion is univalent with unit diameters. The model was formulated in the context of the two-density Ornstein–Zernike integral equation theory within the mean spherical approximation. Analytical algebraic solutions for the model were obtained except for one parameter which requires simple numerical computation. The contact values of the radial distribution functions, internal energy, Helmholtz energy, and osmotic pressure of the system were derived analytically as a function of density and Bjerrum length via the energy route. Radial distribution functions beyond contact predicted by the theory were calculated numerically using the Perram algorithm. Thermodynamic perturbation theory was used to predict the osmotic pressure of longer chains using the dimer thermodynamic and structural properties as a reference system. Predictions from the theory compared well with computer simulation data reported in the literature although no significant improvement over the monomer reference system was found.
A modified Seya–Namioka monochromator with improved light intensity transfer
Authors: Maletta Anthony M. , Johnson Philip M.
Published: July 2002

A monochromator with improved focusing properties has been designed and constructed using a novel optical geometry based on the Seya–Namioka concept. The only optical elements used are a spherical grating and a cylindrical mirror mounted in the exit arm (within the Rowland circle), retaining the Seya–Namioka scanning simplicity while minimizing reflections. This optical system compensates for the astigmatism associated with spherical optics at large incidence angles (as in the Seya–Namioka geometry) shortening the effective vertical focal length and providing a single, tight focus. A method of constructing a similar apparatus for any given spherical grating is presented. Experiments involving coherent vacuum ultraviolet generation from harmonic mixing demonstrate the utility of the new design.
Induced-dipole contributions to the conductivity and dielectric response of molten ZnCl2
Authors: Gray-Weale Angus , Madden Paul A. , Wilson Mark
Published: July 2002

A molecular dynamics simulation of molten ZnCl2 with a realistic interionic potential is used to evaluate the contribution of interaction-induced dipoles to the dielectric response, or equivalently, to the conductivity. The induced dipoles are included self-consistently in the interionic potential. The contribution is found to be significant across the accessible range of frequencies, modifying various features of the spectrum ordinarily attributed to elementary translations of the ionic charges, and markedly improving agreement with experimental spectra.
Dynamical Monte Carlo study of equilibrium polymers. II. The role of rings
Authors: Wittmer J. P. , van der Schoot P. , Milchev A. , Barrat J. L.
Published: July 2002

We investigate by means of a number of different dynamical Monte Carlo simulation methods the self-assembly of equilibrium polymers in dilute, semidilute and concentrated solutions under good-solvent conditions. In our simulations, both linear chains and closed loops compete for the monomers, expanding on earlier work in which loop formation was disallowed. Our findings show that the conformational properties of the linear chains, as well as the shape of their size distribution function, are not altered by the formation of rings. Rings only seem to deplete material from the solution available to the linear chains. In agreement with scaling theory, the rings obey an algebraic size distribution, whereas the linear chains conform to a Schultz–Zimm type of distribution in dilute solution, and to an exponential distribution in semidilute and concentrated solution. A diagram presenting different states of aggregation, including monomer-, ring-, and chain-dominated regimes, is given. The relevance of our work in the context of experiment is discussed.
Optical bandgap formation in AlInGaN alloys
Authors: Tamulaitis G. , Kazlauskas K. , Juršėnas S. , Žukauskas A. , Khan M. A. , Yang J. W. , Zhang J. , Simin G. , Shur M. S. , Gaska R.
Published: July 2002

We report on the spectral dynamics of the reflectivity, site-selectively excited photoluminescence, photoluminescence excitation, and time-resolved luminescence in quaternary AlInGaN epitaxial layers grown on GaN templates. The incorporation of a few percents of In into AlGaN causes significant smoothening of the band-bottom potential profile in AlInGaN layers owing to improved crystal quality. An abrupt optical bandgap indicates that a nearly lattice-matched AlInGaN/GaN heterostructure with large energy band offsets can be grown for high-efficiency light-emitting devices.
Photoluminescence from polymer-like hydrogenated and nitrogenated amorphous carbon films
Authors: Anguita J. V. , Silva S. R. P. , Young W.
Published: July 2002

The mechanism of photoluminescence (PL) in hydrogenated amorphous carbon (a-C:H), and nitrogenated and hydrogenated amorphous carbon (a-C:H:N) thin films grown by radio frequency driven plasma enhanced chemical vapor deposition is still a subject of much debate. In this work, we investigate the PL signal obtained from a-C:H and a-C:H:N films, paying particular attention to the effect of nitrogen flow rate during growth, and postgrowth, ex situ annealing on the PL properties of the films. We also correlate the PL spectra to the electronic structure of the films. The films had a low paramagnetic defect density (1017 cm−3). The PL spectra were obtained using the 488 nm (2.54 eV) line of an argon ion laser, as the excitation source. It was observed that the nitrogenation of the films leads to the creation of new bands in the PL signal, which were correlated to the bond fraction of CN triple bonds, as measured by infrared spectroscopy.
Field-enhanced trapping in deep levels by multiple phonon emission in semi-insulating GaAs
Authors: Rubinger R. M. , de Oliveira A. G. , Ribeiro G. M. , Bezerra J. C. , Moreira M. V. B. , Chacham H.
Published: July 2002

We have carried out the time, temperature, and illumination dependencies of the current density in a semi-insulating GaAs sample grown at 300 °C under strong electric field. Standard ohmic behavior was observed at room temperature. A negative differential behavior as a function of the applied electric field was observed by lowering the temperature and increasing the photon flux, and this phenomenon was associated to the field-enhanced trapping effect. We have fit our data with a model for enhanced capture by a multiple-phonon emission capture process assisted by the applied electrical field.
Nonlinear ion-drift waves in a nonuniform plasma with nonzero ion-temperature-gradient effects
Authors: Onishchenko Oleg G. , Pokhotelov Oleg A. , Pavlenko Vladimir P. , Shukla Padma K. , Farid Tahir , Stenflo Lennart , Kamenets F. F. , Bogdanov A. V.
Published: July 2002

It is shown that Grad hydrodynamics can be used for the description of nonlinear ion-drift waves in a low β plasma. The set of nonlinear equations which allows us to describe both the ion polarization drift and the nonzero ion Larmor radius effects is derived. The presence of a nonzero ion temperature gradient induces a corresponding perpendicular thermal flux which in turn substantially modifies the transverse stress tensor. Thus, the description of effects associated with the perpendicular ion polarization drift demands corrections in the magnetic viscosity due to the nonzero thermal flux.
Verification of hot hole scattering rates in silicon by quantum-yield experiment
Authors: Kamakura Yoshinari , Kawashima Ichiro , Deguchi Kazuaki , Taniguchi Kenji
Published: July 2002

High-energy hole scattering rates for a full-band Monte Carlo simulation in Si are verified using a quantum-yield experiment. We compare two models that yield the correct drift velocity and ionization coefficient but quite different energy distributions. It is demonstrated that the quantum-yield experiment provides a means for monitoring hole scattering rates in Si; the model based on the ab initio impact ionization rate shows good agreement with the experiments, while the random-k approximation proposed by Kane overestimates the ionization rate of holes near the threshold energy.
Hydrodynamic coupling of a speckled laser beam to an axially flowing plasma
Authors: Bezzerides B. , Vu H. X. , Kopp R. A.
Published: July 2002

A plasma with a background flow along the direction of propagation of a laser beam is examined. The laser beam is subject to the beam-smoothing properties of a random phase plate. A simple model for the speckled properties of the laser beam is employed to show that the combined effect of the ponderomotive pressure of both the incident beam and stimulated Brillouin scattering can significantly perturb the hydrodynamics of the plasma.
The local structure of various hydrogen bonded liquids: Voronoi polyhedra analysis of water, methanol, and HF
Authors: Jedlovszky Pál
Published: July 2002

The local structure of three hydrogen bonded liquids comprising clusters of markedly different topology: water, methanol, and HF are investigated by analyzing the properties of the Voronoi polyhedra (VP) of the molecules in configurations obtained from Monte Carlo computer simulations. For comparisons, the properties of argon as a nonassociating reference liquid are also calculated. It is found that in liquid HF the fluctuation of the local density is much larger than in the other three liquids, and hence, besides the dense regions, this liquid contains numerous large voids as well. The size of the largest of these voids is found to be commensurate with a cluster of 11 HF molecules, whereas the number of voids as large as one molecule is in the order of the number of molecules. It is found that the number of the hydrogen-bonded neighbors of the HF molecules located at the boundary of such voids does not differ considerably from that of the other molecules, and thus they do not have any specific topological position (e.g., chain terminal) in the hydrogen bonded chains of the molecules. The shape of the VP is found to reflect the arrangement of the nearest neighbors of the molecules. Thus, the VP are considerably less spherical and their topological properties are distributed in a wider range in the associated liquids than in argon. Similarly, in the hydrogen bonded liquids considerably more geometric neighbors are found than the number of molecules belonging to the first coordination shell. On the other hand, in liquid argon, which is a closely packed system, the two values are found to be close to each other. The inhomogeneities of the local density of liquid HF are also reflected in the shape of the VP. The molecules located at the boundary of the large voids are found to have more spherical VP with more and larger faces than the other ones.
Characteristics of C3 radicals in high-density C4F8 plasmas studied by laser-induced fluorescence spectroscopy
Authors: Takizawa K. , Sasaki K. , Kadota K.
Published: July 2002

Spatial and temporal variations of C3 density in high-density octafluorocyclobutane (c-C4F8) plasmas were examined using laser-induced fluorescence spectroscopy. The C3 density varied slowly for a long time after the initiation of discharge, suggesting the importance of surface chemistry for the formation of C3. Hollow-shaped spatial distributions (the C3 density adjacent to the chamber wall was higher than that in the plasma column) were observed in the C3 density. This result indicates that C3 radicals are produced from fluorocarbon film on the chamber wall and are lost in the plasma column due to electron impact processes. The surface production of C3 was also observed in the afterglow for 1 ms after the termination of rf power. The decay time constant of the C3 density in the late (>1 ms) afterglow, where the surface production of C3 stopped, was almost independent of discharge parameters, suggesting that the loss of C3 due to gas-phase reactions is negligible.
Identification of the Γ5 and Γ6 free excitons in GaN
Authors: Reynolds D. C. , Look D. C. , Jogai B. , Saxler A. W. , Park S. S. , Hahn J. Y.
Published: July 2002

The Γ5 and Γ6 free excitons have been identified in GaN from emission measurements. Another emission peak is also observed which we believe to be the longitudinal free exciton. These measurements along with electrical measurements, which show the sample to have very high peak mobility, attest to the high quality of the sample.
Estimate of temperature distributions in steady-plane-wave fronts using a Hugoniot function
Authors: Sano Yukio , Abe Akihisa
Published: July 2002

The purpose of this study was to establish a method for estimating temperature distributions in steady-plane-wave fronts in a thermoviscous material using a Hugoniot function. To this end, under the fundamental assumption that the material in the wave front is approximately in an equilibrium state, two irreversible thermodynamic equations for temperature in the wave front were derived. In the first equation, heat transport was neglected, and in the second equation, the work done by thermal stress was offset by heat transport. The temperature distributions were evaluated qualitatively under the assumption of heat transport. This evaluation indicated that the second equation was effective if the effects of viscosity were large. These two equations were applied to the shock compressions up to 140 GPa of yttria-doped tetragonal zirconia. The second equation sufficiently predicted temperature behind the shock and also fairly accurately predicted temperatures in the shock front. The influence of heat transport on both temperatures was also examined.