bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical study of kinetic isotope effects on rate constants for the H2+C2H→H+C2H2 reaction and its isotopic variants
Authors: Kurosaki Yuzuru , Takayanagi Toshiyuki
Published: July 2002

Thermal rate constants have been calculated for the H2+C2H→H+C2H2 reaction (1) and its isotopic variants: HD+C2H→H+C2HD (2); DH+C2H→D+C2H2 (3); D2+C2H→D+C2HD (4); H2+C2D→H+C2HD (5) using variational transition state theory with the multidimensional semiclassical tunneling correction. The geometries were optimized at the MP2(full)/cc-pVTZ level and the potential energy curves for these reactions were calculated at the PMP4(SDTQ,full)/cc-pVTZ and QCISD(T,full)/cc-pVTZ levels. It was thus revealed that these reactions have “early” potential barriers. The calculated rate constants for reactions (1) and (5) were found to be comparable and the largest among these reactions. The calculated rate constants for reactions (1) and (4) showed good agreement with experiment at relatively low temperatures. The reaction-path-curvature effects and secondary kinetic isotope effects [the effects of change in zero-point energies (ZPEs) along the reaction path relative to the reactant ZPEs] were predicted not to be so large in these reactions. Rather, the primary isotope effects (the effects of change in the effective mass for the reaction coordinate) were found to be the main contributions to tunneling. This is because these reactions have “early” characters.
Molecular dynamics simulations of Si etching with energetic F+: Sensitivity of results to the interatomic potential
Authors: Abrams Cameron F. , Graves David B.
Published: July 2002

Comparative analyses of molecular dynamics (MD) simulation studies of reactive ion etching of Si are presented. A recently developed empirical potential is used to model the Si–F system, and applied to the simulation of Si etching with energetic F+ at 10, 25 and 50 eV. These results are compared to those of a similar study using the Stillinger-Weber Si–F potential. This analysis leads to the expected result that different potentials lead to quantitatively different results with regard to Si etch yield, surface structure and composition, etching mechanisms, and product distributions. More importantly, however, it attests to the robustness of the qualitative nature of these results. The degree of qualitative agreement between systems studied with the two potentials is high enough for us to conclude that MD simulations have revealed valuable qualitative insights into the complicated system of reactive ion etching of Si.
Copper gettering at half the projected ion range induced by low-energy channeling He implantation into silicon
Authors: Fichtner P. F. P. , Behar M. , Kaschny J. R. , Peeva A. , Koegler R. , Skorupa W.
Published: July 2002

He + ions were implanted at 40 keV into Si 〈100〉 channel direction at room temperature (RT) and at 350 °C. The Si samples were subsequently doped with Cu in order to study the gettering of Cu atoms at the defective layer. A subsequent annealing at 800 °C was performed in order to anneal the implantation damage and redistribute the Cu into the wafer. The samples were analyzed by Rutherford backscattering channeling and transmission electron microscopy techniques. The Cu distribution was measured by secondary ion mass spectrometry (SIMS). The SIMS experiments show that, while the 350 °C implant induces gettering at the He projected range (Rp) region, the same implant performed at RT has given as a result, gettering at both the Rp and Rp/2 depths. Hence, this work demonstrates that the Rp/2 effect can be induced by a light ion implanted at low energy into channeling direction.
Second-harmonic generation in highly textured LiNbO3 films prepared by pulsed laser deposition
Authors: Chaos J. A. , Pruneri V. , Gonzalo J. , Afonso C. N.
Published: July 2002

LiNbO 3 films have been deposited on (001) sapphire substrates by pulsed laser deposition, in an oxygen environment. Films grown at substrate temperatures of 650 °C are crystalline, strongly textured, and show a degree of twining that is lower the higher the oxygen pressure is in the range of 0.5–1 Torr. Values of the nonlinear optical coefficients d33 and d31 of the films, measured via second-harmonic generation, are close to those for the bulk LiNbO3 single crystal. The dependence of both the degree of twining and the nonlinear optical response on the film thickness suggests that the films become closer to single domain for larger thickness.
Thermoluminescence of nickel-doped synthetic diamond crystals
Authors: Benabdesselam M. , Iacconi P. , Briand D. , Berkane-Krachaı̈ A. , Gheeraert E. , Kanda H.
Published: July 2002

An investigation of the thermoluninescent response of nickel-doped synthetic diamond samples is reported. The glow curve obtained after x-ray irradiation or ultraviolet light illumination exhibits a main peak around 500 K, and three weaker ones around 380, 645, and 715 K. Among these peaks, two are attributed to nickel-related centers at 1.4±0.1 and 2.4±0.2 eV, respectively. Spectral analysis of the main thermoluminescence peak indicated a broad band around 2.3 eV, similar to the photoluminescence spectrum. Linearity of the response versus x-ray doses, thermal, and optical fading properties are reported. The results indicated that nickel-doped synthetic diamond overcomes most of the disadvantages of other synthetic diamond crystals for ionizing radiation dosimetry applications.
Flexible cell designs for simultaneous electrochemical electron spin resonance measurements with a coaxial microwave cavity
Authors: Zhuang Lin , Lu Juntao
Published: July 2002

New cell designs of coaxial microwave cavity type are reported for simultaneous electrochemical electron spin resonance measurements. Adjustable metallic shields are introduced so that the working electrode can be much shorter than the full height of the cavity. Besides the traditional helical electrode, the working electrode can be cylinders made from metallic plates, meshes, or even composite materials. The counter electrode can be placed either inside or outside the cavity. These features make the new designs more flexible than those previously reported. The impacts of eddy currents due to the magnetic-field modulation are examined and discussed. With a nonperforated cylindrical electrode, this cell can be used to determine the standard potential and the number of electrons transferred in electrode reactions involving radicals. Other possible applications are also briefed.
Structural and magnetic studies of sputtered Fe1−xCrx thin films
Authors: Duc N. H. , Fnidiki A. , Teillet J. , Youssef J. Ben , Le Gall H.
Published: July 2002

X-ray diffraction, magnetization, and Mössbauer effect investigations have been performed for sputtered Fe1−xCrx (0⩽x⩽0.54) thin films. The body-centered-cubic (bcc) phase appears for x<0.32, while the σ phase is formed for 0.38⩽x⩽0.44. For 0.32⩽x<0.38 and 0.44<x⩽0.54, the samples are composed of both bcc and σ phases. As the Cr concentration increases, the ferromagnetic fraction, magnetization, and magnitude of the hyperfine field decrease, whereas the magnitude of the isomer shift increases. The Fe and Cr magnetic moments, isomer shift, and hyperfine field of the bcc Fe–Cr phase have been deduced and are discussed consistently in terms of charge and spin distributions, as well as magnetic valence.
Green functions for impulsive free-surface flows due to bottom deflections in two-dimensional topographies
Authors: Tyvand Peder A. , Storhaug Aanund R. F.
Published: July 2002

An analytical investigation is performed of the initial free-surface flow subject to an impulsive concentrated unit flux through an arbitrary point at an otherwise impermeable boundary. The flow is assumed incompressible, inviscid, irrotational and two-dimensional. It obeys a zero-potential condition at the horizontal free surface. This problem of impulsive Green functions is solved for various bottom topographies, such as sloping beaches, submerged ridges, and finite basins. The basic result in each case is the normal derivative of the velocity potential along the free surface, which represents the initial surface velocity due to the concentrated impulsive flux. The results have relevance to tsunami modeling.
Fiber optic displacement sensor using a coated lens optic
Authors: Golnabi Hossein
Published: July 2002

Design and operation of an optomechanical displacement sensor employing a reflecting coated lens is reported. The light intensity modulation is based on the relative motion of the optical fibers and the lens. The modulated intensity has been measured by using a pair of 450 μm core fiber, one acting as the source and the other one as receiving fiber. Displacement resolution of the order of 2.5 μm has been measured with a sensitivity of about 1.6 mV/μm for a dynamic range of 700 μm. The measured output is checked against the expected theoretical one and also other experimental results and good agreement are obtained.
Ab initio calculation and spectroscopic analysis of the intramolecular vibrational redistribution in 1,1,1,2-tetrafluoroiodoethane CF3CHFI
Authors: Pochert Jörg , Quack Martin , Stohner Jürgen , Willeke Martin
Published: July 2002

We report a new mechanism for intramolecular vibrational redistribution (IVR) in CF3CHFI which couples the CH chromophore vibrations through a strong Fermi resonance to the formal CF stretching normal mode (a heavy atom frame mode) involving the trans F-atom across the CC bond. The analysis is made possible by comparing spectroscopic results with extensive ab initio calculations of the vibrational fundamental and overtone spectra in the range extending to 12 000 cm−1. Potential energy and electric dipole moment hypersurfaces are calculated ab initio by second order Møller–Plesset perturbation theory (MP2) on a grid involving the CH stretching, two CH bending modes and one high frequency CF stretching normal mode. The potentials are scaled to obtain agreement between the experimental spectrum and the theoretical spectrum calculated by a discrete variable representation technique on this grid. Both spectra are then analyzed in terms of three-dimensional (3D) and four-dimensional (4D) effective vibrational Hamiltonians including Fermi- and Darling–Dennison-type resonances between the CH stretching mode and the CH bending modes and the CF stretching mode. The interaction between the CH modes and the CF mode is clearly visible in the experimental and calculated (4D) spectra. The effective Fermi resonance coupling constants [ksff′≃(40±10) cm−1 and ksaf′≃(55±10) cm−1] coupling the CH and CF mode subspaces are of about the same magnitude as the intra-CH chromophore Fermi resonances (ksaa′≃56 cm−1 and ksbb′≃42 cm−1, coupling CH stretching mode “s” with the two CH bending modes “a” and “b”). The chiral, pseudo-Cs symmetry breaking coupling (ksab′≃11 cm−1) is complemented by an equally strong coupling through the CF mode (ksfb′≃15 cm−1). It is demonstrated that low order perturbation theoretical analysis using potential constants from a polynomial expansion to represent effective coupling constants gives inadequate results with discrepancies ranging about from factors of 2–5. Time dependent population and wave packet analysis shows essentially complete IVR among the CH chromophore modes within about 100 fs, the 3D and 4D evolutions being similar up to about that time. At longer times of about 250 fs, there is substantial excitation of the CF stretching mode (with initial pure CH stretching excitation). The 4D treatment is then essential for a correct description of the dynamics.
Femtoampere current reference stable over atmospheric temperatures
Authors: Harrison R. Giles , Aplin Karen L.
Published: July 2002

Calibration of ultralow current ammeters deployed in atmospheric ion counters requires stable current references operating at high source impedance. Using standard precision components, this current reference generates equal bipolar currents of nominally ±500 fA, cyclically for 32 s each. The currents were found to be stable to 2 fA over the temperature range −20 to 20 °C, typical of atmospheric conditions. The output current is delivered via a capacitor, and by arranging for the capacitor to be guarded at ground potential when the system is not generating a current, the reference can be permanently connected to an electrometer with minimal leakage.
Angle determination based on the deflection of a laser beam
Authors: López-Ramı́rez Juan Manuel , Malacara-Doblado Daniel , Malacara-Hernández Daniel
Published: July 2002

The angle measurement method described here measures the difference in arrival time to a detector of a light beam reflected by a surface being measured and a reference ray. This measurement is made by alternately detecting both rays with a photodetector at the focal plane of a lens and electronically obtaining their arrival time difference. With this information, and reading the slopes along a straight line, the profile of a reflecting surface can be found.
Lateral epitaxy overgrowth of GaN with NH3 flow rate modulation
Authors: Zhang X. , Dapkus P. D. , Rich D. H.
Published: July 2002

We demonstrate the effects of NH3 flow modulation on the lateral growth rate and morphology of GaN stripes employing lateral epitaxial overgrowth (LEO) by metalorganic chemical vapor deposition. The self-limiting growth mechanism, enhanced Ga diffusion on the (0001) plane, and Ga lateral supply are used to explain our observations. A lateral overgrowth rate to a vertical growth rate ratio of 2.1 and fully coalesced LEO GaN stripes after 1 h growth have been achieved.
Conical potential model for InGaAs/GaAs quantum dots formed in tetrahedral-shaped recesses
Authors: Endoh Akira , Sakuma Yoshiki , Takatsu Motomu , Awano Yuji , Yokoyama Naoki
Published: July 2002

We developed a conical potential model for InGaAs/GaAs quantum dots formed in tetrahedral-shaped recesses (TSRs) and evaluated the diameter and average indium (In) content of TSR quantum dots by combining with magnetophotoluminescence (PL) measurements. The model calculations of the PL peak energy and its diamagnetic shift of TSR quantum dots were performed using the effective mass approximation method. We found that the diameter and the ratio of average In content of a TSR quantum dot to that of the surrounding quantum well are independent of the flow rate of trimethylindium, which is the source gas of In, in the range of our experiments. The negative diamagnetic shift of the PL peak from the first excited state transition, which was observed for the “In-rich” sample, could be explained by the contributions of two different energy levels with a different angular momentum.
Measurements of second-order susceptibility at λ=1.5 μm in CdTe-based ternary alloys for efficient wavelength conversion
Authors: Zappettini Andrea , Pietralunga Silvia M. , Milani Antonella , Martinelli Mario , Mycielski Andrzej
Published: July 2002

We have characterized the second-order optical nonlinear response of II-VI semiconductor ternary compounds Cd0.8Zn0.2Te and Cd0.78Mn0.22Te at λ=1.5 μm. A spectrally resolved phase-mismatch second-harmonic generation (SHG) technique has been used on bulk single crystals, exploiting 10−13 s optical pulses and multichannel detection. The nonlinear d coefficient has been measured and chromatic dispersion parameters have been validated. By normalizing SHG results in view of applications to all-optical wavelength conversion, the tested compounds prove to be interesting alternatives to more renowned AlxGa1−xAs.
Transport and optical modeling of organic light-emitting diodes
Authors: Tessler N.
Published: July 2002

We present detailed theoretical modeling of organic light-emitting diodes. We incorporate both optical and electrical modeling into one and thus achieve a more comprehensive picture of the device operation. The importance of including the exciton generation and emission rate in a transport model is highlighted and we find that in such devices the charge recombination zone is not necessarily identical to the exciton emission zone. Moreover, both the electrical and optical structure affect the exciton density distribution and thus also the externally measured light.
Composition-structure correlations in strained FexMn1−x/Ir superlattices
Authors: Snoeck E. , Ardhuin H. , Casanove M. J. , Fischer H. , Andrieu S. , Piecuch M. , Suenaga K. , Colliex C.
Published: July 2002

The structural and chemical characteristics of FexMn1−x alloys in [FexMn1−x/Ir(001)] superlattices with iron contents of x=0.3, 0.5 and 0.9 are investigated at the nanometer scale by high resolution transmission electron microscopy and electron energy loss spectroscopy techniques. The transmission electron microscopy experiments show that pseudomorphic growth of the alloy on Ir is only achieved for x⩾0.5. Layers with iron content x=0.5 display, however, structural inhomogeneities and a weak Mn segregation effect. Layers with a high iron content, Fe0.9Mn0.1 are seen to present a quasiperiodic lattice modulation. We suggest that this modulation is associated with spinodal decomposition. All these structural investigations indicate that, in epitaxial strained thin layers, the FexMn1−x alloy reproduces almost all of the complex FexMn1−x phase diagram with, however, a marked shift towards the low Fe content regions.
Optical parameters of silicon carbide and silicon nitride ceramics in 0.2–1.3 μm spectral range
Authors: Atanasov P. A. , Imamova S. E. , Hügel H. , Abeln T.
Published: July 2002

The optical properties—reflectance, absorptance, and transmittance—of silicon carbide (SiC) and silicon nitride (Si3N4) ceramics in the 0.2–1.3 μm spectral range were estimated by means of Monte Carlo simulation using single-crystal macroscopic optical constants assumed to remain uniform over a small unit of ceramic volume. The data obtained are in good agreement with the measurements made by other authors.
Transport in n+(p+) Si–AlQ–Al junctions
Authors: Aziz Anver , Narasimhan K. L.
Published: July 2002

In this article, we report on transport across n+(p+) c-Si/AlQ/Al junctions where AlQ is tris(8-hydroxy quinolato)aluminum, and n+ and p+ are n and p type degenerately doped crystalline silicon (c-Si). Specifically, we show that n+ c-Si is a very efficient electron injector into AlQ. We have studied the temperature and thickness dependence of AlQ on the current–voltage characteristics to understand transport in AlQ. From these measurements, we show that transport in AlQ is injection limited and the current is limited by tunneling injection from the contact into AlQ. We also report on the influence of the interface and gap states on transport in AlQ.
Effect of pressure and temperature on the competition between nondissociative and dissociative electron attachment to POCl3
Authors: Williamson D. H. , Mayhew C. A. , Knighton W. B. , Grimsrud E. P.
Published: July 2002

Branching between the nondissociative, POCl3−, and dissociative, POCl2−, products of the POCl3 electron attachment reaction has been examined as a function of buffer gas pressure and temperature. A strong positive pressure dependence is observed for the nondissociative channel, where at 303 K, the %POCl3− increases from 31% at 1 Torr to 94% at 675 Torr total pressure. Conversely, the dissociative channel displays a strong positive temperature dependence. The effect of pressure and temperature on the relative amounts of POCl3− and POCl2− observed is discussed in terms of the competition between the collisional stabilization and dissociation rates of the POCl3−* excited intermediate. The decomposition of POCl3−* is modeled with Rice–Ramsperger–Kassel decomposition kinetics weighted by the vibrational energy distribution of POCl3 neutrals. This model provides an excellent simulation of the experimental pressure and temperature dependencies of the electron attachment process.
Fabrication of acoustic superlattice LiNbO3 by pulsed current induction and its application for crossed field ultrasonic excitation
Authors: Wan Zhi-liang , Wang Quan , Xi Yuan-xin , Lu Yan-qing , Zhu Yong-yuan , Ming Nai-ben
Published: July 2002

An acoustic superlattice LiNbO3 crystal with periodic ferroelectric domain structure was fabricated by introducing a periodic electric current through the solid–liquid interface during the crystal growing process. The domain morphology of an as-grown crystal was observed with a scanning electron microscope, and was found to be of good periodicity. A light diffraction experiment indicated that there was a periodic fluctuation of the dielectric constant along the crystal’s growing direction. Using the “crossed field” scheme, a 340 MHz ultrasonic was excited in the crystal, which means that the acoustic superlattice is suitable for constructing high-frequency bulk-wave acoustic devices.
Stress tuning in crystal ion slicing to form single-crystal potassium tantalate films
Authors: Levy M. , Osgood R. M. , Bhalla A. S. , Guo R. , Cross L. E. , Kumar A. , Sankaran S. , Bakhru H.
Published: July 2002

We report on the implementation of crystal ion slicing in potassium tantalate (KTaO3). Deep-ion implantation is used to create a buried sacrificial layer in (001) single-crystal wafers of KTaO3. 10-μm-thick single-crystal films have been fabricated by adjusting the stress level in the implantation damage layer to induce room-temperature etchless exfoliation. Crack propagation is found to depend critically on implantation dose, with a threshold dose for exfoliation near 1×1016 cm−2. A significant implantation-induced etch selectivity between the sacrificial layer and the rest of the sample is also found. Capacitance measurements show that the films exhibit a bulk-like dielectric constant and loss tangents below 0.01 at low temperatures.
A furnace for the in situ study of the formation of inorganic solids at high temperature using time-resolved energy-dispersive x-ray diffraction
Authors: Geselbracht Margret J. , Walton Richard I. , Cowell E. Sarah , Millange Franck , O’Hare Dermot
Published: July 2002

The design, construction, and use of a furnace from which time-resolved x-ray diffraction data may be measured from reacting mixtures of solids or of solids and liquids is described. The furnace is a vertical tube design, constructed from commercially available components, and can operate at temperatures of up to 1000 °C. The apparatus is designed to heat sample tubes of up to 3 cm diameter. The use of high-intensity synchrotron-generated white-beam x rays allows the tube and its contents to be penetrated; thus x-ray diffraction data can be measured from reactions taking place in laboratory-sized reaction vessels. The energy-dispersive diffraction geometry allows rapid data collection (of the order of seconds); hence reactions can be followed continuously in real time. The use of the furnace is demonstrated by results from experiments performed on Station 16.4 of the Daresbury Synchrotron Radiation Source, UK. Two distinct reaction types are studied, both used to prepare the layered perovskite RbCa2Nb3O10: first, a solid state route at 800 °C and second a flux route, performed in molten RbCl, also at 800 °C.
Simplified radio-frequency generator for driving ion guides, traps, and other capacitive loads
Authors: Jones Ronald M. , Anderson Scott L.
Published: July 2002

We report the design and construction details for a very simple radio-frequency (rf) generator, suitable for driving ion guides, traps, or any capacitive load in the few to few hundred picofarad range. The design is a simple high voltage tube oscillator, and the frequency can be step adjusted by changing capacitors. An optional TTL-triggered keying circuit is provided for applications where the rf needs to be turned off and on under external control.
300 MHz continuous wave electron paramagnetic resonance spectrometer for small animal in vivo imaging
Authors: Koscielniak J. , Devasahayam N. , Moni M. S. , Kuppusamy P. , Yamada K. , Mitchell J. B. , Krishna M. C. , Subramanian S.
Published: July 2002

Design and construction of an electron paramagnetic resonance (EPR) spectrometer, operating in the continuous wave mode in the radio frequency (rf) region, and capable of performing spectroscopy and in vivo imaging of paramagnetic spin probes is described. A resonant frequency of 300 MHz was chosen to provide the required sensitivity at nontoxic levels of commonly used spin probes and penetration of the rf in small animals. Three major components, the magnet, the radio frequency signal detection bridge, and the data acquisition module are described in this article. Integration of a rapid scan capability to reduce imaging time is also described. Two- and three-dimensional EPR images of the spin probe distribution in phantom objects as well as from in vivo experiments are reported. From the EPR images, morphology of some internal organs could be recognized. EPR images of the spin probe distribution in mice suggest differences in perfusion of the spin probe between normal and tumor regions. Addition of a spectral dimension to spatial images should enable differentiation of oxygen status in normal and pathological conditions.
The “Compact Toroid Injection Experiment” tomographic imaging system
Authors: Terry S. D. , Hwang D. Q. , McLean H. S. , Horton R. D. , Evans R. W. , Baker K. L.
Published: July 2002

The design, construction and operation of a tomographic imaging system on the Compact Toroid Injection Experiment is described. The system measures the total radiated power over energies from visible light up into the extreme ultraviolet. It then reconstructs two dimensional profiles from the data. The reconstruction routine is based on a method known as second order regularization which finds a compromise between smoothness and fit to the data. This method was found to have the best overall fidelity to test images. The hardware and overall reconstruction were calibrated using two different sources. First results from the system under real experimental conditions are presented.
Accurate calibration technique for a resonant-mass gravitational wave detector
Authors: Tobar Michael E. , Locke Clayton R. , Ivanov Eugene N. , Heng Ik Siong , Blair David G.
Published: July 2002

The methodology for accurately calibrating the Niobe resonant-mass gravitational wave detector is presented. The transducer is based on a low noise resonant microwave cavity transducer that converts the displacement of the resonating mass to microwave energy. The calibration technique consists of a one off measurement of the microwave frequency versus resonant-mass displacement characteristic. To measure this accurately, known static forces were applied to the resonant mass and the change in the transducer microwave frequency was recorded. With the aid of finite element analysis and accurate measurements of the resonant-mass characteristics, the deflection due to the known force was calculated. The calculated deflections were verified coarsely with measurements from a calibrated linear variable differential transformer. Typically, the detector operates with a 1 mK noise temperature. A best noise temperature of 890 μK between 1300 and 2000 Universal Time Coordinate (UTC) for day 60 in 1997 is reported. The transducer has been upgraded with a new microwave amplifier, which has a measured electronic noise floor 40 dB lower than the previous amplifier, which is only 10 dB above the quantum limit.
Investigation of the Hugoniot equation of state in laser-driven shock experiments by a line-imaging optically recording velocity interferometer system
Authors: Kadono T. , Yoshida M. , Kozu N. , Kondo K.
Published: July 2002

As a new technique for the measurements of the Hugoniot equation of state in laser-driven shock experiments, a line-imaging optically recording velocity interferometer system (Line-ORVIS) is developed and applied to investigate the Hugoniot of nitrobenzene. The experimental result agrees with the previous data obtained by another method. Thus, it is suggested that a Line-ORVIS is effective for the measurements of the Hugoniot equation of state in laser-driven shock experiments.
Vibrational wave packets in the B 1Πu and D 1Σu+ states of Cs2: Determination of improved Cs2+(X) and Cs2(B) spectroscopic constants
Authors: Oldenburg A. L. , John P. C. , Eden J. G.
Published: July 2002

Vibrational wave packets in the B 1Πu and D 1Σu+ excited states of Cs2 have been studied on the ∼100 fs time scale by pump–probe laser spectroscopy. The temporal behavior of the wave packets was monitored by photoionizing the electronically excited molecule with a time-delayed probe pulse and recording the time and energy-integrated photoelectron signal as a function of time delay between the pump and probe pulses. For the B 1Σu+ experiments, wave packets were produced by exciting the B 1Σu+←X 1Σg+ transition in the ∼740–790 nm region and subsequently detected by photoionizing the molecule at wavelengths between 565 nm and 600 nm. By simulating the experimentally observed transients with the density matrix formalism (and explicitly accounting for laser chirp and |Δv|>1 coherences), improved values for the equilibrium internuclear separation for the Cs2(B 1Πu) state and Te for the Cs2+(X) state were determined to be Re(B 1Πu)=4.93±0.03 Å and Te[Cs2+(X)]=29 930±100 cm−1, respectively. Similar experiments were conducted for the D 1Σu+ state. Wave packets composed of vibrational levels (v′≈40–50) perturbed by the bound 2 3Πou state were produced on the D 1Σu+ potential surface by driving the D 1Σu+←X 1Σg+ transition in the 575–610 nm spectral interval.
Severely perturbed vibrational structure in the 266–310 nm electronic transition of C3
Authors: Izuha Mitsuaki , Yamanouchi Kaoru
Published: July 2002

The laser induced fluorescence (LIF) spectra of C3 are measured under jet-cooled conditions in the wide UV region of 266–310 nm, and 173 vibronic bands are identified. Among them, 77 and 68 vibronic bands were found to exhibit Σ–Σ type and Π–Σ type rotational structures, respectively. From the rotational analysis, the band-origin wave numbers and the rotational constants of the upper Σ and Π vibronic levels and K-type doubling parameters for the Π vibronic levels are determined. The rotational constants for the Σ and Π vibronic levels exhibit, respectively, similar distributions with almost the same mean values, B̄′=0.395(14) and 0.398(17) cm−1, indicating that the Σ and Π vibronic levels have the same electronic origin and that the average C–C bond distance of the upper electronic state in the observed energy range is about 1.331(25) Å, which is longer than the electronic ground X̃ 1Σg+ state by 0.054(25) Å. The convolution and Fourier-transform analyses are performed separately for the spectra composed only of Σ–Σ type and Π–Σ type vibronic bands to derive estimates of the vibrational fundamentals in the upper electronic state. The low ν1 fundamental of 940(60) cm−1 derived from these analyses and the seemingly regular ν1 progression in the convoluted spectra indicate that the bright character is carried primarily by the transitions to the vibrational levels in the Δu1 electronically excited state and is distributed into a large number of the originally dark transitions to the vibrational levels in the Πg1 electronically excited state, resulting in the exceedingly complex vibronic band system. © 2000 American Institute of Physics.
Combined Hartree–Fock quantum mechanical and molecular mechanical molecular dynamics simulations of water at ambient and supercritical conditions
Authors: Tu Yaoquan , Laaksonen Aatto
Published: July 2002

Combined ab initio Hartree–Fock quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations have been carried out for water at ambient and supercritical conditions. The QM/MM coupling potential is optimized for ambient water by scaling down the original MM charges in the MM potential model. Simulation results show that the modified QM/MM coupling potential behaves well in the studied water system. The evolution of the water structure from ambient to the supercritical conditions is studied and compared to the available experimental results. Our simulation results suggest that, at supercritical conditions, the data refined from the experiment by neutron diffraction with the isotopic substitution technique probably overestimate the hydrogen bonding interactions.
Ab initio/spectroscopic interaction potential for He+Ne+
Authors: Falcetta M. F. , Dorko M. J. , Siska P. E.
Published: July 2002

High-level ab initio calculations have been carried out on the lowest Σ and Π states of HeNe+. These have been used to construct a new interaction potential in a Hund’s case (e) representation, by fitting spectroscopic vibrational spacings ΔGv+1/2 and rotational constants Bv using a close-coupling method and a potential function whose form is established by the ab initio data and a long-range analysis. The characteristics of the resulting Born–Oppenheimer potential curves, particularly for the X state, where only the higher vibrational levels were observed, differ considerably from those derived by extrapolation of the experimental spectroscopic constants. A new set of constants is proposed, and functions for the X-state G(v) and Bv are given that are well-behaved from the bottom of the well to the dissociation limit. The asymptotic formula for Bv of Le Roy is extended to improve its accuracy. The X state of HeNe+4 is predicted to support 15 bound vibrational states, the A2 state 7. Good agreement with experiment in magnitude and trends is found for spin-splitting and Ω-doubling constants. Comparison is made between energy levels calculated in the close-coupling formalism and in the customary case (c) Born–Oppenheimer approximation; the largest differences, which are readily resolved spectroscopically, are reflected in Bv for high vibrational levels of the X state, including those observed.
Eight-potential-well order-disorder ferroelectric model and effects of random fields
Authors: Liu Zhi-Rong , Gu Bing-Lin , Zhang Xiao-Wen
Published: July 2002

An eight-potential-well order-disorder ferroelectric model was presented and the phase transition was studied under the mean-field approximation. It was shown that the two-body interactions are able to account for the first-order and the second-order phase transitions. With increasing the random fields in the system, a first-order phase transition is transformed into a second-order phase transition, and furthermore, a second-order phase transition is inhibited. However, proper random fields can promote the spontaneous appearance of a first-order phase transition by increasing the overcooled temperature. The connections of the model with relaxors were discussed.
Control of polarity of ZnO films grown by plasma-assisted molecular-beam epitaxy: Zn- and O-polar ZnO films on Ga-polar GaN templates
Authors: Hong Soon-Ku , Hanada Takashi , Ko Hang-Ju , Chen Yefan , Yao Takafumi , Imai Daisuke , Araki Kiyoaki , Shinohara Makoto
Published: July 2002

We report on the growth of polarity-controlled ZnO films by plasma-assisted molecular-beam epitaxy. Different polar (Zn- and O-polar) ZnO films on unipolar (Ga-polar) GaN epilayers are selectively grown. Polarity of ZnO films is evaluated by coaxial impact collision ion scattering spectroscopy. Zn preexposure prior to ZnO growth results in Zn-polar ZnO films (Zn face), while O-plasma preexposure leads to the growth of O-polar ZnO films (O face). High-resolution transmission electron microscopy reveals the formation of an interface layer between ZnO and GaN epilayers in O-plasma preexposed samples, while no interface layer is observed in Zn preexposed samples. The interface layer is identified as single crystalline, monoclinic Ga2O3. We propose models for interface configurations at ZnO/GaN heterointerfaces, which can successfully explain the different polarities of the ZnO films.
Characteristics of free-standing hydride-vapor-phase-epitaxy-grown GaN with very low defect concentration
Authors: Visconti P. , Jones K. M. , Reshchikov M. A. , Yun F. , Cingolani R. , Morkoç H. , Park S. S. , Lee K. Y.
Published: July 2002

A free-standing 300-μm-thick GaN template grown by hydride vapor phase epitaxy has been characterized for its structural and optical properties using x-ray diffraction, defect delineation etch followed by imaging with atomic force microscopy, and variable temperature photoluminescence. The Ga face and the N face of the c-plane GaN exhibited a wide variation in terms of the defect density. The defect concentrations on Ga and N faces were about 5×105 cm−2 for the former and about 1×107 cm−2 for the latter. The full width at half maximum of the symmetric (0002) x-ray diffraction peak was 69 and 160 arc sec for the Ga and N faces, respectively. That for the asymmetric (10–14) peak was 103 and 140 arc sec for Ga and N faces, respectively. The donor bound exciton linewidth as measured on the Ga and N faces (after a chemical etching to remove the damage) is about 1 meV each at 10 K. Instead of the commonly observed yellow band, this sample displayed a green band, which is centered at about 2.44 eV.
Intersubband electroluminescence from long-side-cleaved quantum-cascade lasers above threshold: Investigation of phonon bottleneck effects
Authors: Colombelli Raffaele , Capasso Federico , Gmachl Claire , Tredicucci Alessandro , Sergent A. Michael , Hutchinson Albert L. , Sivco Deborah L. , Cho Alfred Y.
Published: July 2002

A technique is reported which allows the observation of intersubband spontaneous emission in unipolar quantum-cascade lasers above threshold. The technique consists of cleaving the laser stripe in the direction perpendicular to its facets. This does not negatively affect the operation of the lasers thanks to their unipolar nature. To show the potential of the method, we apply it to superlattice quantum-cascade (QC) lasers with various active region designs. We directly observe the saturation of the luminescence intensity at the laser transition, and a bottleneck effect for transitions separated from the lasing one by less than one optical phonon. This technique should help in the optimization of QC lasers.
Formation of Y2O3 interface layer in a YMnO3/Si ferroelectric gate structure
Authors: Choi Jae Hyoung , Lee Jeong Yong , Kim Yong Tae
Published: July 2002

During the crystallization of amorphous YMnO3 thin film on Si (100) at 870 °C in a dry O2 ambient, a nanoprecipitate layer was found between the YMnO3 and the Si substrate. Lattice image processing as well as high-resolution transmission electron microscopy showed that the nanoprecipitate layer was a cubic Y2O3 phase. Also, it showed that a native oxide was consumed by the reaction with the Y atoms. This [111] Y2O3 layer exhibited a local epitaxial relationship to the c-axis oriented (0001) YMnO3. The formation of Y2O3 phase and the consumption of native oxide at the YMnO3/Si interface are due to the Y atom which is better than Mn in its ability to oxidize during heat treatment in O2 ambient.
Velocity angular distribution of ground level atomic silicon in the plume of laser ablated silica
Authors: Gal G. , Sgulim S. , Livneh T.
Published: July 2002

Polished silica (synthetic SiO2 glass) samples were ablated to vacuum by 266 nm laser pulses at a flounce of 110 J/cm2. The concentration of ground level atomic silicon in the plume was measured as a function of time, by laser-induced fluorescence (LIF), at distances of 5–7 mm from the ablated point. The angular velocity distribution was found to be highly asymmetric with respect to surface normal, as the LIF signal dropped abruptly at angles approaching the laser direction. The symmetric morphology of the craters that were formed by ablation indicates that the above asymmetry is not likely to be attributed to the interaction of the laser or the ablated plume with the surface. Therefore, it is suggested that the silicon atoms, ejected in the direction of the incident laser beam, were preferentially ionized via a multiphoton off-resonance absorption process. The velocity of the silicon atoms was detected in the 1–13 km/s range. Since the velocity distribution for a thermal ablation process is expected to extend down to zero, the photochemical channel is suggested to be dominant in this case. Doppler broadening, calculated from the lateral velocity distribution, was found to be consistent with the broadening of the silicon absorption spectrum.
Differences between interfacial and surface molybdenum in the formation of TiSi2
Authors: Zhang S.-L. , Zhang Z.-B. , Zhu D.-Z. , Xu H.-J.
Published: July 2002

Backscattering and diffraction results are presented for the effects of an interfacial or a surface Mo layer on the formation of Ti-silicides during solid-state interaction between Ti films and Si substrates. It is shown that the interfacial and surface Mo are fundamentally different in their involvement in the Ti-silicide formation. The interfacial Mo induces the formation of C40 (Mo,Ti)Si2 at the interface adjacent to the Si substrate already after annealing at 550 °C, in agreement with our previous results. Hence, the desired C54 TiSi2 can grow directly on top of the C40 (Mo,Ti)Si2 at relatively low temperatures as a result of the template effect. The surface Mo is, however, found in a metal-rich silicide presumably (Mo,Ti)5Si3 at 550–600 °C, which eventually converts to (Mo,Ti)Si2 upon annealing at higher temperatures. Underneath this metal-rich silicide lies a fully developed C49 TiSi2 layer. Consequently, the formation of C54 TiSi2 in the presence of surface Mo follows the usual path of the C49–C54 phase transition. This important difference in the participation of Mo in the silicide formation spreads doubts about the validity of using interfacial Mo versus surface Mo to study the dominant mechanism(s) responsible for the enhanced formation of C54 TiSi2.
Irradiation effect of low energy nitrogen-ion beam during pulsed laser deposition process on the structural and bonding properties of carbon–nitride thin films
Authors: Zhao J. P. , Chen Z. Y. , Yano T. , Ooie T. , Yoneda M.
Published: July 2002

Carbon–nitride thin films were deposited by pulsed laser ablation of graphite with assistance of low energy nitrogen-ion-beam irradiation. The nitrogen to carbon (N/C) atomic ratio, bonding state, microstructure, surface morphology, and electrical property of the deposited carbon–nitride films were characterized by x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, micro-Raman spectroscopy, x-ray diffraction (XRD), atomic force microscopy, and four-probe resistance. The irradiation effect of low energy nitrogen-ion beam on the synthesis of carbon–nitride films was investigated. The N/C atomic ratio of the carbon–nitride films reached the maximum at the ion energy of ∼200 eV. The energy of ∼200 eV was proposed to promote the desired sp3-hybridized carbon and the C3N4 phase. Electrical resistivity of the deposited films was also influenced by the low energy nitrogen-ion-beam irradiation. However, the low energy irradiation had little effect on the surface morphology of the films. XRD results revealed the coexistence of the α- and β-C3N4 phases in the deposited thin films.
Effect of emitter disorder on the recombination zone and the quantum yield of organic electroluminescent diodes
Authors: Kalinowski J. , Picciolo L. C. , Murata H. , Kafafi Z. H.
Published: July 2002

The electroluminescence (EL) quantum yield (QY) of double- and triple-layer organic electroluminescent diodes based on a N,N′-diphenyl-N,N′bis(3-methylphenyl)-1,1′-biphenyl4,4′-diamine /tris (8-hydroxyquinolinato) aluminum III (Alq3) junction has been measured as a function of the electric field and the emitting guest (6,13-diphenylpentacene) concentration in the host Alq3. The well-resolved maxima of the QY plots versus applied field for neat and low dopant concentration emitter layers (EMLs) shift strongly toward high fields and disappear at high dopant concentrations. Based on the EL QY data and the measured absolute photoluminescence quantum efficiency of the emitter, the recombination zone width is determined and shown to be a decreasing function of electric field for all of the diodes. The dopant reduces the width of the recombination zone at low dopant concentrations and increases at high dopant concentrations (>0.5 mol %). The results are discussed in terms of a two-step recombination mechanism, assuming disorder-controlled charge carrier mobilities. The dopant concentration effect on the recombination zone width and EL QY can be explained using the disorder formalism that predicts low dopant concentrations create a high degree of positional (off-diagonal) disorder whereas energetic (diagonal) disorder dominates at higher doping levels in the EMLs. The latter makes the recombination zone width as well as EL QY practically field independent.
Nitrogen delta doping in 6H silicon carbide layers
Authors: Yokogawa Toshiya , Takahashi Kunimasa , Uenoyama Takeshi , Kusumoto Osamu , Uchida Masao , Kitabatake Makoto
Published: July 2002

Nitrogen delta-doped silicon carbide (SiC) layers were grown by a pulse doping method in a chemical vapor deposition. Doping distribution with high peak concentration (1×1018 cm−3) and narrow distribution width (12 nm) was fabricated in the nitrogen delta-doped structure of SiC. Mobility enhancement due to spatial separation of electrons and their ionized parent donors was observed for the delta-doped structure. Metal-semiconductor field-effect transistors with a nitrogen delta-doped channel and a recess gate structure were fabricated. The devices had large source-drain (200 V) and source-gate (140 V) breakdown voltages, high drain current capability and easy control of the threshold voltage with a good pinch-off characteristics.
Electroreflectance of CuInS2 thin film solar cells and dependence on process parameters
Authors: Henninger R. , Klaer J. , Siemer K. , Bruns J. , Bräunig D.
Published: July 2002

The electroreflectance (ER) of CuInS2 thin film solar cells prepared by sequential processing is investigated as a function of the process parameters Cu/In ratio and sulfurization temperature in order to get a better understanding of the growth mechanisms. A detailed interpretation of the ER spectra is presented. The disappearance of the ER signal under indium-rich conditions is explained by a superficial CuIn5S8 secondary phase. An additional structure in ER spectra below the band gap energy is attributed to electroabsorption and is explained by the transition between copper vacancies and sulfur vacancies in CuInS2 as identified by photoluminescence. The strong dependence of this electroabsorption structure on the process parameters and the occurrence of the superficial CuIn5S8 secondary phase is explained by phase transitions in the equilibrium phase diagrams of the metal sulfides formed during sulfurization.
First experimental observation on different ionic states of the CH3SS radical: A HeI photoelectron spectrum
Authors: Maofa Ge , Jing Wang , Zheng Sun , Xinjiang Zhu , Dianxun Wang
Published: July 2002

A continuous flowing CH3SS radical beam is produced in situ by pyrolysis of CH3SSCH3 at 215(±0.5) °C. An obvious and complete photoelectron (PE) spectrum of the CH3SS radical is recorded in situ for the first time. Five sharp peaks at 8.63, 9.36, 9.94, 10.29, 10.72 eV and one broader band at 11.82 eV are observed in the PE spectrum below 12 eV. The first peak with the lowest ionization energy, 8.63 eV, is attributed to removal of an electron from the highest occupied molecular orbital (HOMO) 5a″ of the CH3SS radical, corresponding to the CH3SS+(X 1A′)←CH3SS(X 2A″) ionization. The vibrational spacing 600±60 cm−1 on the first peak corresponds to the S–S stretch mode excited in the CH3SS+ cation upon photoionization process. The sharp peaks at 9.36 and 9.94 eV come from removal of the electron from the 16a′ orbital, leading to 3A″ and 1A″ ionic states of the CH3SS radical, and the sharp peaks at 10.29 and 10.72 eV should be the result of removal of the electron from the 15a′ orbital of the CH3SS radical. To assign the photoelectron spectroscopy (PES) bands of the CH3SS radical, both GAUSSIAN2 (G2) and improved density functional theory (DFT) calculations on different ionic states of the CH3SS radical are also performed. The PES experimental ionization energies of the CH3SS radical agree with those calculated with G2 and DFT.