bluuebunny/crossref_metadata_2025 huggingface dataset:
Complementary digital logic based on the ‘‘Coulomb blockade’’
Authors: Tucker J. R.
Published: July 2002

A finite charging energy, e2/2C′, is required in order to place a single electron onto a small isolated electrode lying between two tunnel junctions and having a total capacitance C′ to its external environment. Under suitable conditions, this elemental charging energy can effectively block all tunnel events near zero bias voltage in series arrays of ultrasmall junctions, an effect that has come to be known as the ‘‘Coulomb blockade.’’ This article outlines a new approach to the design of digital logic circuits utilizing the Coulomb blockade in capacitively biased double-junction series arrays. A simple ‘‘on’’/‘‘off ’’ switch is described and complementary versions of this switch are then employed to design individual logic gates in precise correspondence with standard complementary metal–oxide semiconductor architecture. A planar nanofabrication technique is also described that may eventually allow the integration of Coulomb blockade logic onto conventional semiconductor chips, thereby realizing hybrid integrated circuits having device densities and operating speeds far in excess of present technology.
Study of the effect of ion implantation on the electrical and microstructural properties of tin-doped indium oxide thin films
Authors: Shigesato Yuzo , Paine David C. , Haynes T. E.
Published: July 2002

Ion implantation of H2+ or O+ ions in the range 0–1.7×1015 and 0–1.3×1015/cm2, respectively, was used to investigate the effect of implant-induced damage on the electrical properties of Sn-doped In2O3 (ITO) films deposited by electron-beam evaporation on SiO2-coated soda-lime glass substrates. The films were characterized as a function of implant dose using low-temperature Hall effect, resistivity, optical transmissivity, x-ray diffraction, and transmission electron microscopy (TEM). A systematic decrease in both carrier density (n) and Hall mobility (μ) was observed with increasing dose of either implant species. The electronic results were analyzed using charged and neutral impurity scattering models which suggest that the observed changes are due to the degradation of electrically active donor centers and the generation of the neutral scattering centers. The microstructure of the implanted films, as revealed by TEM and x-ray diffraction, is consistent with the presence of significant dynamic recovery during implantation.
Behavior of an EL5-like defect in metalorganic vapor-phase epitaxial GaAs:Sb
Authors: Yakimova R. , Paskova T. , Hardalov Ch.
Published: July 2002

An EL5-like trap in metalorganic vapor-phase epitaxially grown GaAs:Sb has been detected by deep-level transient spectroscopy. The trap behavior has been investigated by changing the growth conditions—the V/III ratio and the level of isoelectronic doping with Sb. It has been found that the activation energy of the EL5-like trap changes from Ec−0.43 eV in the undoped GaAs to Ec−0.35 eV in highly Sb-doped GaAs. It has been observed that the EL5 trap concentration increases with increasing the [AsH3] partial pressure in the undoped material, whereas it decreases with Sb doping in correlation with EL2 kinetic decrease. The results obtained suggest that the complex defect VGaAsi can be a possible candidate for the origin of the EL5-like trap.
Analysis of differential gain in GaAs/AlGaAs quantum-well lasers
Authors: Chen P. A. , Chang C. Y. , Juang C.
Published: July 2002

The differential gain of a quantum-well laser is studied theoretically with use of both a parabolic band model and a valence-band-mixing model. In the valence-band-mixing model, the gain profile is derived from the multiband effective mass theory (k⋅p method) as well as the density matrix formalism. The peak gain including the band-mixing effect is significantly reduced to 1.5–2 times when compared to the conventional parabolic band model. There is still a larger differential gain using the parabolic band model than using the band-mixing model. The magnitudes of differential gains for these two models give the order of 10−16–10−15 cm2, which is in agreement with the experimental results. Besides, the quantum-well thickness also influences the differential gain, which is enhanced by a thinner quantum-well structure.
Plasma enhanced chemical vapor deposition of titanium nitride thin films using cyclopentadienyl cycloheptatrienyl titanium
Authors: Charatan R. M. , Gross M. E. , Eaglesham D. J.
Published: July 2002

The use of a low oxidation state Ti compound, cyclopentadienyl cycloheptatrienyl titanium, (C5H5) Ti(C7H7) (CPCHT), as a potential source for TiN and Ti in plasma enhanced chemical vapor deposition processes has been investigated. This precursor provides us with a new chemical vapor deposition route to TiN films that offer an interesting contrast to films deposited from Ti(IV) precursors. Film depositions were carried out by introducing CPCHT, with H2 carrier gas, into the downstream region of a NH3, N2, H2, or mixed H2/N2 plasma. Low resistivity (100–250 μΩ cm) nitrogen-rich TiN films with little carbon or oxygen incorporation and good conformality were deposited with activated N2 or NH3 at deposition temperatures of 300–600 °C, inclusive. Mixed H2/N2 plasmas resulted in more stoichiometric TiN films with similar properties. The most striking feature of these films is the absence of columnar grain growth, in contrast to TiN films deposited using TiCl4 or Ti(NR2)4. Although the film texture was influenced by the plasma gas, the average grain size of the films deposited using activated N2 and NH3 was similar. The TiN films that we deposited were effective diffusion barriers between aluminum and silicon up to 575 °C. Depositions using activated H2 resulted in films with significantly less carbon than CPCHT, but still having a minimum of 2.7:1 C:Ti. The lower oxidation state of the precursor did not facilitate the deposition of a Ti-rich film. No depositions were observed with any of the reactant gases in the absence of plasma activation.
Interface analysis of (110)-oriented Co/Pt superlattices with large magnetic anisotropies using x-ray diffraction
Authors: Pogosova I. S. , Harzer J. V. , Hillebrands B. , Güntherodt G. , Guggi D. , Weller D. , Farrow R. F. C. , Lee C. H.
Published: July 2002

We report on studies of the profiles of the strain field components in epitaxially grown (110)-oriented Co/Pt superlattices with fixed Pt layer thickness (nominally 18 Å) and varying Co layer thickness. The strain profiles in the growth direction as well as fluctuations in lattice parameters at the interfaces are obtained using a parameter refinement method for the quantitative analysis of high angle Θ–2Θ x-ray diffraction scans. We find that all strain components originate only from the Co-Pt interfaces. Off-axial x-ray scattering measurements provide for the in-plane strain components, which are found to increase with increasing Co layer thickness. From the obtained strain fields the in-plane uniaxial magnetic anisotropy contributions caused by magnetoelastic interactions are calculated, which agree well with experimentally observed values.
Measurement of rheological properties of ultrathin lubricant films at very high shear rates and near-ambient pressure
Authors: Jonsson Ulf , Bhushan Bharat
Published: July 2002

In liquid-lubricated bearings used in experimental near-contact magnetic recording devices, rheological properties of lubricant films at very high shear rates (up to about 109 s−1) and at near-ambient pressure are required. In this extremely high shear rate regime, shear thinning (non-Newtonian behavior) becomes extremely important. A rheometer is designed and constructed that makes it possible to study the rheology of lubricants at extremely high shear rates. Since traditional rheometers are limited to moderate shear rates due to viscous heating, the design used in this study is based on a magnetic disk drive configuration in which an ultrathin film of lubricant is sheared between a magnetic disk and a slider at relatively low sliding speeds. By monitoring the film thickness between the slider and the disk using a capacitive method, it is possible to accurately measure the shear rate. Two different types of perfluoropolyether lubricants have been studied up to shear rates of 107 s−1, and these data have been compared to previously published data.
Structure of the DX state formed by donors in (Al,Ga)As and Ga(As,P)
Authors: Dobaczewski L. , Kaczor P. , Missous M. , Peaker A. R. , Zytkiewicz Z. R.
Published: July 2002

High-resolution Laplace-transform deep level transient spectroscopy has been used to study the influence of the defect local environment on electron emission from the DX centers related to group-IV (silicon) donors in AlxGa1−xAs (0.20<x<0.76) and δ-doped GaAs and group-VI (tellurium) donor elements in AlxGa1−xAs (0.25<x<0.73) and GaAs0.35P0.65. The experimental evidence that substitutional–interstitial atom motion is responsible for DX behavior and for the associated metastability effects is presented. The atom which is subjected to this transition is for DX(Si) silicon itself, as in the spectra only one group of peaks in AlxGa1−xAs is observed, while for DX(Te) it can be either gallium or aluminum, producing two groups of peaks in AlxGa1−x As and three or four broad emission bands in GaAs0.35P0.65. The present results rule out a possibility that the DX-type defect states are formed by a donor atom in a stable substitutional position with a small lattice relaxation or with a fully symmetric large lattice relaxation effect.
Modeling of permanent magnets: Interpretation of parameters obtained from the Jiles–Atherton hysteresis model
Authors: Lewis L. H. , Gao J. , Jiles D. C. , Welch D. O.
Published: July 2002

The Jiles–Atherton theory is based on considerations of the dependence of energy dissipation within a magnetic material resulting from changes in its magnetization. The algorithm based on the theory yields five computed model parameters, MS, a, α, k, and c, which represent the saturation magnetization, the effective domain density, the mean exchange coupling between the effective domains, the flexibility of domain walls and energy-dissipative features in the microstructure, respectively. Model parameters were calculated from the algorithm and linked with the physical attributes of a set of three related melt-quenched permanent magnets based on the Nd2Fe14B composition. Measured magnetic parameters were used as inputs into the model to reproduce the experimental hysteresis curves. The results show that two of the calculated parameters, the saturation magnetization MS and the effective coercivity k, agree well with their directly determined analogs. The calculated a and α parameters provide support for the concept of increased intergranular exchange coupling upon die upsetting, and decreased intergranular exchange coupling with the addition of gallium.
Carrier transport in porous silicon light-emitting devices
Authors: Peng C. , Hirschman K. D. , Fauchet P. M.
Published: July 2002

This work presents a comprehensive investigation of carrier transport properties in light-emitting porous silicon (LEPSi) devices. Models that explain the electrical characteristics and the electroluminescence properties of the LEPSi devices are developed. In metal/LEPSi devices, the forward current density–voltage (J–V) behavior follows a power law relationship (J∼Vm), which indicates a space charge current attributed to the carriers drifting through the high resistivity LEPSi layer. In LEPSi pn junction devices, the forward J–V behavior follows an exponential relationship (J∼eeV/nkT), which indicates that the diffusion of carriers makes a major contribution to the total current. The temperature dependence of the J–V characteristics, the frequency dependence of the capacitance–voltage characteristics, and the frequency dependence of the electroluminescence intensity support the models. Analysis of devices fabricated with a LEPSi layer of 80% porosity results in a relative permittivity of ∼3.3, a carrier mobility of ∼10−4 cm2/V s, and a free carrier concentration of ∼1013 cm−3.
Oxygen-related 1-platinum defects in silicon: An electron paramagnetic resonance study
Authors: Juda U. , Scheerer O. , Höhne M. , Riemann H. , Schilling H.-J. , Donecker J. , Gerhardt A.
Published: July 2002

A monoclinic 1-platinum defect recently detected was investigated more thoroughly by electron paramagnetic resonance (EPR). The defect is one of the dominating defects in platinum doped silicon. With a perfect reproducibility it is observed in samples prepared from n-type silicon as well as from p-type silicon, in float zone (FZ) silicon as well as in Czochralski (Cz) silicon. Its concentration varies with the conditions of preparation and nearly reaches that of isolated substitutional platinum in Cz silicon annealed for 2 h at 540 °C after quenching from the temperature of platinum diffusion. Because of its concentration which in Cz-Si exceeds that in FZ-Si the defect is assumed to be oxygen-related though a hyperfine structure with 17O could not be resolved. The defect causes a level close to the valence band. This is concluded from variations of the Fermi level and from a discussion of the spin Hamiltonian parameters. In photo-EPR experiments the defect is coupled to recently detected acceptorlike self-interstitial related defects (SIRDs); their level position turns out to be near-midgap. These defects belong to the lifetime limiting defects in Pt-doped Si.
Structural characterization of nanocrystalline copper by means of x-ray diffraction
Authors: Zhang K. , Alexandrov I. V. , Valiev R. Z. , Lu K.
Published: July 2002

Quantitative x-ray-diffraction measurements were performed on a nanocrystalline Cu sample made by severe plastic deformation. The shape of Bragg reflections was found to be represented primarily by a Lorentzian function. A difference of as much as 6%±3% was revealed between the integrated intensities from the nanocrystalline and a reference coarse-grained Cu samples. The broadening of Bragg reflections from the nanocrystalline Cu sample was mainly induced by small crystallite sizes and microstrains inside the grains and/or the deformed layers near the grain boundaries. It was found that the grain sizes of nanocrystalline Cu in different crystallographic orientations are essentially the same, while the microstrains exhibit a significant anisotropy. The Debye–Waller parameter B of the nanocrystalline Cu sample was 0.97±0.06 Å2, which suggests that the atomic displacement from their ideal lattice positions equals on average 0.111±0.004 Å or 4.3% of the nearest-neighbor spacing.
Magnetization and magnetostriction processes in Tb(0.27−0.30)Dy(0.73−0.70)Fe(1.9−2.0)
Authors: Armstrong William D.
Published: July 2002

The present analysis provides Tb(0.27−0.30)Dy(0.73−0.70)Fe(1.9−2.0) magnetization and magnetostriction states during applied magnetic field and stress processes by minimizing the total magnetic free energy in the presence of a parameter selected inverse exponential magnetic free energy distribution. A comparison between experimental and model results shows that the model provides a qualitatively accurate description of the magnetostriction and magnetization in Tb(0.27−0.30)Dy(0.73−0.70)Fe(1.9−2.0) during applied magnetic field processes for a range of constant uniaxial applied stresses. Therefore, the model appears to be a useful tool for the analysis of Tb(0.27−0.30)Dy(0.73−0.70)Fe(1.9−2.0) applications with constant stress or strain dependent stress states. Furthermore, the good qualitative agreement between model and experiment supports the contention that the magnetostriction and magnetization response of Tb(0.27−0.30)Dy(0.73−0.70)Fe(1.9−2.0) is influenced by a defect induced defocusing of local magnetization direction about the average magnetization direction.
Refractive indices of InSb, InAs, GaSb, InAsxSb1−x, and In1−xGaxSb: Effects of free carriers
Authors: Paskov P. P.
Published: July 2002

A detailed study of the refractive indices of the narrow-band-gap III–V semiconductors is presented. The calculation is based on a numerical Kramers–Kronig analysis of the carrier-related imaginary part of the dielectric function. The near-band-gap refractive index spectra of InSb, InAs, and GaSb for different free-carrier densities are obtained. The density dependence of the refractive index is analyzed and a comparison with the results from the Drude theory is performed. The refractive indices of In1−xGaxSb and InAsxSb1−x ternary alloys are also investigated. The obtained results are expected to be useful in the design optimization of the phase modulators and the laser diodes for midinfrared applications.
Evolution from point to extended defects in ion implanted silicon
Authors: Benton J. L. , Libertino S. , Kringho/j P. , Eaglesham D. J. , Poate J. M. , Coffa S.
Published: July 2002

We present a quantitative study of the evolution of point defects into clusters and extended defects in ion-implanted Si. Deep level transient spectroscopy (DLTS) measurements are used to identify and count the electrically active defects in the damaged region produced by Si ion implantation at energies of 145 keV–2 MeV, and fluences from 1×108 to 5×1013 Si/cm2. Analyses of silicon annealed in the temperature range 100–680 °C allow us to monitor the transition from simple point defects to defect clusters and extended defects that occur upon increasing the ion fluence and the annealing temperature. At low doses, <1010 Si/cm2, only about 2% of the Frenkel pairs generated by the ion beam escape recombination and are stored into an equal number of interstitial and vacancy-type point defects. Thermal treatments produce a concomitant annealing of interstitial and vacancy-type defects until, at temperatures above 350 °C, only two to three interstitial-type defects per ion are left, and the DLTS spectra contain signatures of second-order point defects. Interstitial clusters at Ev+0.29 and Ev+0.48 eV are found to dominate the residual damage of silicon implanted at higher fluences, 1×1012–7×1013 Si/cm2, and at annealing temperatures, T⩾600 °C. These interstitial clusters have point defect capture kinetics and are not observable in transmission electron microscopy (TEM), suggesting that they are smaller than ≈50 Å. Finally, for silicon implanted at higher Si doses, ⩾5×1013 Si/cm2, thermal treatments at 680 °C result in a strong decrease in the concentration of the interstitial cluster signatures and in the introduction of a different DLTS signal, Ev+0.50 eV, which exhibits logarithmic rather than exponential carrier capture kinetics, a feature typical of an extended defect. Comparison of the formation and dissolution of this extended defect signature with TEM analyses indicates that this level is a signature of the rodlike {311} defects that are known to store the interstitials responsible for transient enhanced diffusion. These results suggest that the small interstitial clusters are either the precursors of the {311} defects or that they compete with {311} defects as sinks for self-interstitials.
Approximations to the Whipple solution for grain boundary diffusion and an algorithm for their avoidance
Authors: Evans J. W.
Published: July 2002

This paper concerns the Whipple model for grain boundary diffusion. In particular, it examines the various approximations that have been used to calculate solute concentration from the equation provided by Whipple. Included are approximations intended to permit the determination of grain boundary diffusivities from experimental data. An algorithm is presented which avoids these approximations. It is demonstrated that the algorithm is robust in the face of moderate experimental error, and, unlike an alternative, can be used to minimize the impact of that error.
Cascaded TeO2 acousto-optic devices for high efficiency multifrequency modulation
Authors: Kastelik J. C. , Gazalet M. G. , Boudy P.
Published: July 2002

An original combination of two cascaded multifrequency acousto-optic modulators, based on the tangential phase matching anisotropic interaction, is presented. This configuration allows to greatly reduce the phase grating intermodulation products and to increase the diffraction efficiency. For experimental validation, multifrequency paratellurite acousto-optic devices have been realized and tested. The whole modulation line operates with eight diffracted and independently modulated beams. A high diffraction efficiency (80%) is obtained.
Theoretical investigation of the apparent spectral radiance from the metal/window interface in shock temperature experiments
Authors: Tang Wenhui , Zhang Ruoqi , Jing Fuqian , Hu Jinbiao
Published: July 2002

When a shock wave passes through an ideal metal/window interface, the interface temperature is time independent; accordingly, the apparent spectral radiance at the interface is also regarded as time independent. However, each material has a definite transparency, so that the apparent spectral radiance at the interface must be the total radiance of a thin layer rather than the surface. In this article, the relationship among the optical absorption coefficient of a metal and its pressure and temperature is studied based on classical electromagnetics. It is obtained that the absorption coefficient decreases and the optical depth increases as shock pressure increases. By using the radiative transport equation, the apparent spectral radiance at the ideal iron/sapphire interface following shock compression is calculated, which shows that the apparent spectral radiance is time dependent, although the interface temperature is time independent. As an example, the apparent spectral radiance history at the iron/sapphire interface under shock pressure of about 230 GPa is presented.
Structural characterization of BaTiO3 thin films grown by molecular beam epitaxy
Authors: Yoneda Yasuhiro , Okabe Tooru , Sakaue Kiyoshi , Terauchi Hikaru , Kasatani Hirofumi , Deguchi Kiyoshi
Published: July 2002

Epitaxial ultrathin films of BaTiO3 were prepared using molecular beam epitaxy. For the substrate, (001)-oriented SrTiO3 single crystals were used. Controlling the growth conditions of these films as well as the semiconductor thin films, led to the successful growth of the BaTiO3 films as single crystals, characterized by x-ray diffraction even in the ultrathin range. The ultrathin BaTiO3 films are highly c-axis-oriented tetragonal phaselike bulk BaTiO3 crystals. The tetragonality of the thin film crystals is much larger than bulk crystal’s. We also measured the saturated polarization (Ps) of the BaTiO3 films at temperatures ranging from room temperature to 600 °C. The results confirmed again that the films are ferroelectric tetragonal phase crystals. Moreover, they showed that the transition temperature for the ferroelectric-paraelectric phase transition of the films is higher than bulk crystal’s.
Indium phosphide vapor phase epitaxy at high growth rates, growth kinetics, and characterization
Authors: Mimila-Arroyo J. , Dı́az-Reyes J. , Lusson A.
Published: July 2002

InP epitaxial layers have been grown at high growth rates by vapor phase reactive transport. A systematic study of the influence of growth conditions on the growth rate, surface morphology, and photoluminescence properties of the grown materials has been performed. The results show that the growth rate can be controlled at least between 0.3 and 75 μm per hour, the latter being over 75 times of the growth rate obtained with the usual techniques. The changes of the surface morphology of the layers with the growth conditions have been understood and layers having mirror-like surfaces can be grown for any growth rate. The mass transport mechanism occurs through three chemical reactions whose activation energies have been determined and a reliable model for the growth rate as a function of the growth conditions is now available. Double diffraction x-ray measurements demonstrate that the layers are characterized by a peak whose width at half maximum is 18 arcsec, even for the highest growth rates. Low temperature luminescence exhibits transitions involving free and bound excitons, and shallow donors and acceptors. C and Zn are identified as the only residual acceptors and no radiative deep levels are detected. Depending on the growth conditions, C or Zn are incorporated preferentially as the dominant acceptor. The intensity ratio of the excitonic transition to the transition involving Zn as acceptor varies exponentially with the growth temperature.
Elastic moduli, strength, and fracture initiation at sharp notches in etched single crystal silicon microstructures
Authors: Suwito Wan , Dunn Martin L. , Cunningham Shawn J. , Read David T.
Published: July 2002

We designed and fabricated a series of micromechanical test structures for microtensile testing by anisotropically etching epitaxial silicon. Specimens were fabricated to study Young’s moduli, the uniaxial tensile strength, and the strength of T-structures which are tensile bars with an abrupt reduced cross section that have a 90° corner at the point of reduction. They are a generic mimic of actual transitions that occur in micromechanical structures due to anisotropic etching. The test structures were loaded in uniaxial tension in a piezoactuated microtensile test apparatus. The applied force and crosshead displacement were recorded and displacements in the specimen gage section were directly measured using a speckle interferometry technique. During tensile loading of the T-structures, fracture always initiates at the sharp 90° corners. This results in an interesting apparent strength scaling where the nominal strength of the structures increases as their width decreases. In order to understand the fracture initiation from the sharp 90° corners of the silicon T-structures, we carried out a complete analysis of the elastic fields at the 90° corners by coupling an asymptotic analysis (to compute the asymptotic radial and angular dependence of the elastic fields up to an arbitrary constant for each loading mode, the stress intensity), and full-field finite element calculations (to determine the magnitude of the stress intensities for specific geometries and loadings). Excellent results are obtained by using a single parameter, the critical mode I stress intensity, to correlate fracture initiation from the sharp 90° corners of the T-structures.
X-ray absorption of Si–C–N thin films: A comparison between crystalline and amorphous phases
Authors: Chang Y. K. , Hsieh H. H. , Pong W. F. , Tsai M.-H. , Dann T. E. , Chien F. Z. , Tseng P. K. , Chen L. C. , Wei S. L. , Chen K. H. , Wu J.-J. , Chen Y. F.
Published: July 2002

X-ray absorption near edge structure (XANES) spectra of crystalline (c)- and amorphous (a)-Si–C–N thin films were measured at the C, N, and Si K edge using the fluorescence and sample drain current modes. A sharp peak similar to the C 1s core exciton in chemical vapor deposition diamond is observed, which can be assigned to the transition from the C 1s to sp3 hybridized states in c-Si–C–N. The C K edge XANES spectrum of a-Si–C–N contains a relatively large 1s→π* peak, implying that carbon atoms in the a-Si–C–N film are bonded largely in graphite-like sp2 configurations. A shift of the a-Si–C–N π* peak towards the lower energy by ∼0.3 eV relative to that of c-Si–C–N is observed, which can be attributed to a higher degree of disorder-induced localization of excited electrons. Both a- and c-Si–C–N N K-edge XANES spectra resemble that of α-Si3N4. The Si K-edge absorption spectra of the Si–C–N thin films indicate a proportional combination of local Si–N and Si–C bonds. The increase of the binding energies of excited electrons and the broadening of the spectral features by structural disorder are also observed in the Si K-edge XANES spectrum of the a-Si–C–N film.
Faraday-rotation spectra of the diluted magnetic semiconductor Cd1−xGdxTe
Authors: Chen Chenjia , Wang Xuezhong , Liu Anwen , Wu Hao
Published: July 2002

The Faraday rotation as a function of photon energy for Cd1−xGdxTe with x=0.012 and 0.030 in the temperature range of 85–300 K has been measured. The Verdet constant includes a positive part and negative part due to pure Zeeman and sp-f exchange interaction contributions, respectively. The sp-f exchange interaction parameter has been determined in Cd1−xGdxTe: (q=9.3×10−4 at 85 K and q=5.3×10−4 at 300 K) in terms of a microscopic model, which takes into account the k dependence of the spin-spin exchange interaction. The normalization parameter C has been found to be independent of both the compositions of Gd and temperature. The agreement between the data and the microscopic model is very good in the whole spectra region even far from the band gap.
Direct quantitative detection of the sp3 bonding in diamond-like carbon films using ultraviolet and visible Raman spectroscopy
Authors: Gilkes K. W. R. , Prawer S. , Nugent K. W. , Robertson J. , Sands H. S. , Lifshitz Y. , Shi X.
Published: July 2002

The bonding in a series of unhydrogenated amorphous carbon films has been analyzed quantitatively using Raman spectroscopy excited by laser light in both the visible and ultraviolet regions of the spectrum. The asymmetry of the peak near 1550 cm−1 in the visible Raman spectra is correlated with the percentage of sp3 bonding in the films. The ultraviolet Raman spectra exhibit two broad Raman peaks at 1650 and 1100 cm−1, due to sp2 and sp3 vibrational modes, respectively. The former is a resonance feature associated with a large proportion of paired sp2 sites, while the latter is a weighted phonon density-of-states for the distorted random network of sp3 sites. The position and relative intensity of the two peaks are shown to be strongly correlated with the percentage of sp3 sites in the films, providing a reliable measure of sp3 bonding which is both semiquantitative and nondestructive.
Dynamics of gas–surface interactions: Scattering and desorption of NO from Ag(111) and Pt(111)
Authors: Muhlhausen Carl W. , Williams Leah Ruby , Tully John C.
Published: July 2002

Empirical potential energy surfaces have been constructed to describe the nondissociative interaction of NO with the (111) faces of Ag and Pt. Stochastic trajectory simulations employing these interaction potentials accurately reproduce experimental angular and velocity scattering distributions. Measured rotational energy distributions of scattered molecules, including the observed ‘‘rotational rainbow’’ features, are also reproduced quantitatively. Arrhenius prefactors for desorption are computed to be large (1016 s−1), and the translational and rotational ‘‘temperatures’’ of desorbed molecules are found to be lower than the surface temperature, in agreement with experiment. Sticking probabilities, desorption rates, and the rotational energy of desorbed and scattered molecules are all found to be strongly influenced by the dependence of the attractive region of the gas-surface potential on molecular orientation.
The experimental and theoretical expressions for the molecular electronic oscillator strength in solution
Authors: Abe Takehiro , Iweibo Idowu
Published: July 2002

In this paper we propose the appropriate fundamental definition of the oscillator strength in solution and clearly show that both the experimental and theoretical expressions of the oscillator strength should contain refractive index correction factors of α and β, respectively. Therefore, in the discussion of the solvent effects on the oscillator strength the experimental and theoretical expressions containing the correction factors should be used instead of the usual expressions of α=1 and β=1.
On distributed Gaussian bases for simple model multidimensional vibrational problems
Authors: Hamilton I. P. , Light J. C.
Published: July 2002

Distributed Gaussian bases (DGB) are defined and used to calculate eigenvalues for one and multidimensional potentials. Comparisons are made with calculations using other bases. The DGB is shown to be accurate, flexible, and efficient. In addition, the localized nature of the basis requires only very low order numerical quadrature for the evaluation of potential matrix elements.
Electrostatic interactions in colloidal solutions: Comparison between primitive and one-component models
Authors: Belloni Luc
Published: July 2002

In the framework of the primitive model, colloidal solutions are considered as mixtures of charged hard spheres interacting through unscreened Coulomb potentials. The structure factors are calculated using the mean-spherical approximation (MSA) with a rescaling method (R) which improves the results in the low concentration regime. With a view to study such solutions as one-component systems, different expressions for the polyion–polyion effective pair potential veff(r) are derived which depend on the volume fraction of the colloidal particles and on the finite size of the small ions. With these expressions for veff(r), the conventional RMSA formalism of the one-component model leads to polyion–polyion structure factors which are nearly identical to those obtained in the primitive model. The effective potential presents a larger electrostatic repulsion than the classical DLVO potential. As a consequence, the use of the DLVO expression induces an overestimation of the colloidal charge. Due to its exponential form, the HNC integral equation in the primitive model, compared to the RMSA equation, produces a larger accumulation of counterions on the colloidal surface and, then, a lower effective repulsion between macroions.
Reduced state relationship for limiting electrical conductances of aqueous ions over wide ranges of temperature and pressure
Authors: Marshall William L.
Published: July 2002

A reduced state relationship for limiting electrical conductances of aqueous ions to high temperatures and pressures is presented in which ionic conductance is a simple function of solvent density. Walden’s rule is not observed. Specific parameters for 13 ions are included that can be used to calculate ionic conductances by the relationship. From these values, limiting equivalent conductances of many single and mixed aqueous electrolyte systems may be obtained over temperature–pressure ranges of 0–800 °C and up to 400 MPa (4000 bar), with reasonable estimates to 1000 °C and 1000 MPa.
Intensity distribution in the overtone spectra of methyl halides: A local mode analysis of the spectra of methyl halides and methyl cyanide
Authors: Ahmed M. Khalique , Henry Bryan R.
Published: July 2002

The CH-stretching overtone spectra of CH3Cl, CH3Br, CH3I, and CH3CN are measured at room temperature in the liquid phase in the regions of ΔvCH=2–6. We successfully analyze these spectra with a harmonically coupled local mode model. The local mode parameters ω, ωx, and γ′ω, the coupling parameter between the methyl CH bonds, are determined. Oscillator strengths are obtained from the spectra of the three methyl halides. We demonstrate that these oscillator strengths fit the simple intensity distribution law derived by Medvedev. A parameter determined from this fit is used to discuss the sensitivity of the CH bond potential to the successive replacement of one, two, or three hydrogen atoms of methane by halogens.
Vibrationally resolved inelastic and charge transfer scattering of H+ by H2O
Authors: Friedrich Bretislav , Niedner Gereon , Noll Martin , Toennies J. Peter
Published: July 2002

Inelastic and charge transfer scattering of protons by water molecules at collision energies of 27.0 and 46.0 eV have been investigated in a high-resolution crossed beam experiment up to the rainbow scattering angles. Excitation of the stretching (symmetric or asymmetric) and bending mode vibrations within the electronic ground state, X 1A1, of H2O was observed in the proton energy-loss spectra. In the case of charge transfer, formation of H2O+ in the X̃ 2B1 and Ã 2A1 electronic states was identified in the corresponding H-atom spectra; the vibrational states within the X̃ and Ã bands were for the most part resolved and, at small angles (θ≤2°), they were found to be nearly the same as in photoionization (symmetric stretch and bending mode excitation within the X̃ state and pure bending mode excitation within the Ã state). The vibronic transition probabilities deviate, however, considerably from the corresponding Franck–Condon factors in favor of the enhancement of the quasiresonant states. For both the inelastic and charge transfer scattering, state-selected quantities characteristic of the detailed collision dynamics have been derived. In addition, rotational excitation superimposed on the vibrational transitions could be estimated and for both processes it was found to be of the order of 50–100 meV.
The effects of bead inertia on the Rouse model
Authors: Schieber J. D. , Öttinger Hans Christian
Published: July 2002

The Rouse model for dilute polymer solutions undergoing homogeneous flows has been generalized to include the inertia of the beads in the equations of motion. To obtain the correct ‘‘diffusion equation’’ for the probability density distribution function in phase space, we generalize the diffusion equation derived by Murphy and Aguirre [J. Chem. Phys. 57, 2098 (1972)] from Hamilton’s equations of motion for an arbitrary number of interacting Brownian particles at equilibrium. Material functions are found, and the noninertial case is seen to be obtained as the zero mass limit in all steps of the development. In particular, the steady-state shear results are unaffected by the inclusion of inertia. It is also shown how two assumptions, ‘‘equilibration in momentum space,’’ and the neglect of acceleration, made independently by Curtiss, Bird, and Hassager in their phase-space kinetic theory, are actually the result of assuming zero mass.
The calculation of dynamic molecular polarizability
Authors: Parkinson William A. , Zerner Michael C.
Published: July 2002

The frequency dependence of molecular polarizability is examined with the Tamm–Dancoff approximation (TDA) and the random phase approximation (RPA) methods. Two computational algorithms for obtaining these values are discussed. The first uses diagonalization of the TDA state energy representation, which permits a rapid evaluation of several frequency points. The second involves matrix inversion, which has the advantage that the actual states need not be constructed, thus avoiding a large diagonalization. It is demonstrated that the inversion method may be performed with the atomic orbital (AO) two-electron integrals. This speeds the computation time for the static or dynamic polarizability in two ways. First there is a reduction in execution time due to circumventing the integral transform. Also when employing semiempirical methods, the I/O savings for retrieving the substantially shorter AO list further enhances performance. Test cases are evaluated using the intermediate neglect of differential overlap spectroscopic (INDO/S) wave function and include CH+, CO, and ferrocene. Results are compared against available ab initio calculations, followed by a discussion of the relative merit of the computational strategies.
Photoionization of hydrogen atoms near the ionization threshold
Authors: Norwood K. , Ng C. Y. , Vary J. P.
Published: July 2002

The photoionization cross sections for the hydrogen atom were measured in the wavelength range from 700−920 Angstroms. It was found that the experimental results agree with the theoretical predictions.(AIP)
Fluorescence upconversion study of cis-stilbene isomerization
Authors: Todd David C. , Jean John M. , Rosenthal Sandra J. , Ruggiero Anthony J. , Yang Ding , Fleming Graham R.
Published: July 2002

The isomerization dynamics of cis-stilbene in the first excited singlet state were studied by the technique of fluorescence upconversion. Lifetime measurements were made with subpicosecond resolution in 2-propanol, decanol, n-hexane, and tetradecane. The cis-stilbene fluorescence decay curves are single exponential in all solvents except for decanol, where they are adequately described by a double exponential. A weak viscosity dependence of the decay times is observed in both alcohols and alkanes. These results are discussed in terms of the Bagchi, Fleming, Oxtoby theory [J. Chem. Phys. 78, 7375 (1983)] for activationless electronic relaxation in solution, and the limitations of hydrodynamic models of microscopic friction. The fluorescence decay times of α,α′ dideutero cis-stilbene (cis-stilbene-D2) in tetradecane are found to be approximately 20% greater than those of cis-stilbene. A picosecond component in the fluorescence anisotropy decay measurements made in 2-propanol suggest that we are directly measuring motion along the reaction coordinate.
Dynamics of an ammonium ion in water: Molecular dynamics simulation
Authors: Karim Omar A. , Haymet A. D. J.
Published: July 2002

The translational and rotational dynamics of an aqueous ammonium ion are examined using an NVE molecular dynamics computer simulation and a rigid model of water. The linear momentum, angular momentum and reorientational autocorrelation functions of the ammonium ion have been measured. From these, the memory functions for the linear and angular momentum correlation functions are calculated. Reorientation of the ammonium ion about its center of mass shows unambiguous departure from Debye rotational behavior. The translational and rotational diffusion constants have been calculated.
Convergence to the configuration-set limit in multireference configuration-interaction calculations on the He dimer
Authors: Vos R. J. , van Lenthe J. H. , van Duijneveldt F. B.
Published: July 2002

The multireference configuration-interaction (MR-CI) method is used to calculate the binding energy of the He dimer. The convergence of the binding energy to the configuration-set limit (full-CI) is followed by progressively extending the multireference configuration set. Two variants of the Pople size-extensivity correction are applied. The distance dependence of the corrections and hence the effect upon the binding energy turns out to be very small. The effect of orbital optimization is studied and it is shown that it is sufficient to optimize the orbitals used for the multireference space in an atomic multiconfiguration self-consistent field (MCSCF) calculation. In a basis of 50 atomic orbitals, the full-CI binding energy of −9.08 K can be reproduced to 0.00 K (0.02 K) in calculations using only 37 (27) reference configurations, built from the atomic 1s, 2s, 2p, and 3s natural orbitals. Using a very large basis, the 37-reference set gives a best binding energy of −10.87 K, in satisfactory agreement with Aziz’s recent semiempirical result of −10.95 K. These findings suggest that the MR-CI method can be developed into an efficient tool for calculating accurate van der Waals interaction energies for larger systems.
Superradiance quenching by confined acoustic phonons in chemically prepared CdS microcrystallites
Authors: Misawa Kazuhiko , Yao Hiroshi , Hayashi Toyoharu , Kobayashi Takayoshi
Published: July 2002

We report the luminescence spectrum and picosecond dynamics of excitons in CdS microcrystallites embedded in an acrylonitrile–styrene copolymer film by a new method. The intensity ratio of band-edge emission due to the direct recombination of excitons to red-shifted emission from the trapped states was much larger than any other samples previously reported. This indicates much higher quality of our microcrystallites. From the comparison of luminescence spectrum of samples prepared by different cadmium salts as raw materials, we propose a ‘‘site–substitution’’ model for the origin of the trapped states. According to the model, the trapped states yielding the red-shifted emission are impurities substituting the sulfur sites. The band-edge emission was found to be emitted mainly from the larger microcrystallites among the size distribution. The luminescence lifetime of the band-edge emission was almost constant at 60 ps below a threshold temperature (45±5 K) and increases nearly proportionally with temperature above this temperature. This temperature dependence of the lifetime suggests the quenching of superradiance of excitons by acoustic phonons confined in the microcrystallites. The threshold temperature corresponds to the frequency of radial compression mode (30.0 cm−1=43.2 K) of the spherical microcrystallites.
The relativistic open shell coupled cluster method: Direct calculation of excitation energies in the Ne atom
Authors: Ilyabaev Ephraim , Kaldor Uzi
Published: July 2002

The relativistic Fock space coupled-cluster method for the direct calculation of excitation energies (including fine structure) is presented and applied to excitations of atomic Ne. The coupled clusters with single and double excitations (CCSD) approximation, which includes single and double virtual excitations in a self-consistent manner, is implemented. The no-pair Dirac–Coulomb Hamiltonian is taken as the starting point. Mean length basis sets of balanced Gaussian spinors are used to span the atomic orbitals. The model (P) space is quasicomplete, consisting of determinants with one valence hole and one valence particle. Fourteen excitation energies of the atom are calculated. They all agree with experiment to 0.08 eV or better, with an average error of 0.02 eV. Fine structure splittings are also in very good agreement with experiment.
A study of FeCO− and the 3Σ− and 5Σ− states of FeCO by negative ion photoelectron spectroscopy
Authors: Villalta Peter W. , Leopold Doreen G.
Published: July 2002

The 488 and 514 nm negative ion photoelectron spectra of FeCO−, obtained at an instrumental resolution of 5 meV (40 cm−1), show vibrationally resolved transitions from the anion ground state to the ground state and a low-lying excited state of the neutral molecule. The ground state of FeCO is assigned as the 3Σ− state and the excited state, lying 1135±25 cm−1 higher in energy, as the 5Σ− state. The fundamental vibrational frequencies are νCO=1950±10, νFeC=530±10, and νbend=330±50 cm−1 in the 3Σ− state, and νCO=1990±15, νFeC=460±15, and νbend=180±60 cm−1 in the 5Σ− state. Principal force constants are estimated from these results. Based on a Franck–Condon analysis of the spectrum and other considerations, the Fe–C bond is determined to be 0.15±0.04 Å shorter, and the C–O bond 0.05±0.02 Å longer, in the 3Σ− state than in the 5Σ− state. These results demonstrate the importance of sdσ hybridization in reducing the σ repulsion between the metal 4s electron and the CO 5σ lone pair, a mechanism that is available only when the electrons in the singly occupied 3dσ and 4s orbitals are singlet coupled as in the 3Σ− state. The FeCO− anion displays a high Fe–C stretching frequency (465±10 cm−1), as well as an asymptotic Fe–CO bond energy, a bending frequency (230±40 cm−1) and equilibrium bond lengths intermediate between those in the 3Σ− and 5Σ− states. Since the FeCO− ground state is assigned as a 4Σ− state in which the extra electron occupies a σ orbital, these results indicate that the increased σ repulsion is partially offset by stronger metal–CO π bonding in the anion. The electron affinity of FeCO is measured to be 1.157±0.005 eV.
Molecular orbital calculations on (MgO)n and (MgO)n+ clusters (n=1–13)
Authors: Recio J. Manuel , Pandey Ravindra , Ayuela Andres , Kunz A. Barry
Published: July 2002

We report ab initio molecular orbital calculations on neutral and single-ionized stoichiometric clusters of MgO containing up to 26 atoms. Geometrical parameters of the neutral clusters are optimized at the Hartree–Fock level, whereas for the ionized clusters we have applied the vertical approximation. Correlation corrections in the clusters with 2–12 atoms are included at the equilibrium geometries by means of second order Moller–Plesset calculations. We have found that the structures based on the (MgO)3 subunit are preferred in comparison to cubelike configurations, although the energy difference decreases with the increase in cluster size. The relative stability of neutral and single-ionized clusters has been studied by means of the fragmentation path involving the loss of a neutral MgO molecule. The calculated ‘‘magic numbers’’ for the charged clusters, (MgO)n+, are in complete agreement with the abundance maxima observed in the mass spectra. Finally, we explore the size dependence of structural, energetic, and electronic properties. These properties show a large variation from the monomer to the (three-dimensional) eight atom cluster, followed by a softer approach towards the corresponding bulk limit.