bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristics of TiNx/n-Si Schottky diodes deposited by reactive magnetron sputtering
Authors: Dimitriadis C. A. , Lee J. I. , Patsalas P. , Logothetidis S. , Tassis D. H. , Brini J. , Kamarinos G.
Published: July 2002

The effects of the substrate bias voltage and the deposition temperature on the electrical characteristics and the 1/f noise of TiNx/n-Si Schottky diodes fabricated by reactive magnetron sputtering are investigated. As the substrate bias voltage varies from −40 to −100 V, the ideality factor of the diodes remain almost unchanged whereas the noise intensity as a function of the current shows a shift parallel by about one order of magnitude. At low current levels, the noise intensity is proportional to the current and is attributed to the mobility and diffusivity fluctuation. At higher current levels, the noise intensity is proportional to the square of the current and is attributed to bulk traps mainly near the interface. Analysis of the noise measurements shows that both the Hooge parameter and the bulk trap density near the interface first are increased and then decreased as the negative substrate bias voltage increases from −40 to −100 V. This is in contrast with the effects of the deposition temperature where we found that the Hooge parameter remains almost constant, while both the ideality factor and the interface states density are decreased as the deposition temperature increases from room temperature to 400 °C. The trap properties of the TiNx/n-Si Schottky diodes are correlated with the stoichiometry of the TiNx films investigated by spectroscopic ellipsometry measurements.
Fine structure of boron nitride nanotubes produced from carbon nanotubes by a substitution reaction
Authors: Golberg D. , Han W. , Bando Y. , Bourgeois L. , Kurashima K. , Sato T.
Published: July 2002

Here we report on the precise structural investigation of multiwalled boron nitride (BN) nanotubes by means of high-resolution transmission electron microscopy and electron energy loss spectroscopy. The nanotubes were produced from carbon nanotubes by applying a recently discovered technique: a substitution chemical reaction [W. Han, Y. Bando, K. Kurashima, and T. Sato, Appl. Phys. Lett. 73, 3085 (1998)]. It is found that in contrast to the starting carbon nanotubes, which exhibited large number of shells (typically >10), a significant proportion of buckled and corrugated graphene-like sheets, poor degree of graphitization, and wide distribution of helicities, the resultant BN nanotubes revealed perfectly straight shapes, limited number of shells (typically 2–6), and remarkable ordering of the graphene-like sheets in the so-called nonhelical “zig-zag” fashion with the [101̄0] direction parallel to the tube axis.
Optical band gap of the filled tetrahedral semiconductor LiMgAs
Authors: Kuriyama K. , Kushida K.
Published: July 2002

The optical band gap (Eg) of the filled tetrahedral semiconductor LiMgAs [viewed as a zinc-blende AlAs-like (MgAs)− lattice partially filled with He-like Li+ interstitials] was studied. The band gap of LiMgAs is found to be direct, with a forbidden gap of 2.29 eV at 300 K (2.38 eV at 77 K) as LiZnP predicted theoretically by an “interstitial insertion rule.” As a result of the replacement of Zn by a lighter element, Mg, the band gap of LiMgAs is much larger than that of LiZnAs (Eg=1.51 eV at 300 K), but also is larger than that of AlAs (Eg=2.16 eV). The ordered structure between Li and Mg is not confirmed exactly by x-ray diffraction studies, since raliability index R for the ordered arrangement is close to that of the random one.
Defect identification in GaAs grown at low temperatures by positron annihilation
Authors: Gebauer J. , Börner F. , Krause-Rehberg R. , Staab T. E. M. , Bauer-Kugelmann W. , Kögel G. , Triftshäuser W. , Specht P. , Lutz R. C. , Weber E. R. , Luysberg M.
Published: July 2002

We use positron annihilation to study vacancy defects in GaAs grown at low temperatures (LT–GaAs). The vacancies in as-grown LT–GaAs can be identified to be Ga monovacancies, VGa, according to their positron lifetime and annihilation momentum distribution. The charge state of the vacancies is neutral. This is ascribed to the presence of positively charged AsGa+ antisite defects in vicinity to the vacancies. Theoretical calculations of the annihilation parameters show that this assignment is consistent with the data. The density of VGa is related to the growth stoichiometry in LT–GaAs, i.e., it increases with the As/Ga beam equivalent pressure (BEP) and saturates at 2×1018 cm−3 for a BEP⩾20 and a low growth temperature of 200 °C. Annealing at 600 °C removes VGa. Instead, larger vacancy agglomerates with a size of approximately four vacancies are found. It will be shown that these vacancy clusters are associated with the As precipitates formed during annealing.
Minimum time for laser induced amorphization of Ge2Sb2Te5 films
Authors: Weidenhof V. , Pirch N. , Friedrich I. , Ziegler S. , Wuttig M.
Published: July 2002

The minimum time t required to form an amorphous spot in a crystalline film of Ge2Sb2Te5 with NaCl structure was investigated for various applied laser powers P. An elementary power law of the form P∝t−0.5 is observed for pulse lengths larger than 10 ns which shows that amorphization occurs as soon as the melting temperature is reached. This implies that kinetic superheating does not occur on this time scale. The growth velocity of amorphous marks was inferred from atomic force microscopy (AFM) both parallel and perpendicular to the film plane. The growth in the vertical direction is shown to dominate the change in reflectivity and thus the size of the readout signal of data storage devices. The experimental data are compared with numerical calculation of the temperature field using finite element analysis. These calculations determine the position of the melt temperature isotherm and reproduce the depth and the area of the amorphous regions as inferred from AFM data.
Inner-shell excitations in weak-bond molecules
Authors: Ishii I. , McLaren R. , Hitchcock A. P. , Robin M. B.
Published: July 2002

It is proposed that Rydberg and valence σ* conjugate orbitals have separate existences and can be seen in the same spectrum if the σ* MO can be disentangled from the Rydberg manifold. Because the energy of the σ* MO is a consequence of the σ–σ* split resulting from bond formation, the spectra of molecules having weak bonds should show low-lying transitions to σ* in addition to the conjugate Rydberg bands. Inelastic electron scattering spectra in the x-ray region (270–730 eV) of molecules having bond strengths in the 20–50 kcal/mol regime clearly show well-isolated transitions to low-lying σ* MOs, and in some cases the simultaneous presence of virtual σ* and Rydberg conjugate orbitals. The general characteristics of excitations from C 1s, O 1s, and F 1s inner orbitals to σ* MOs are listed and illustrated by the x-ray spectra of several compounds in which the weak bond involves the O–O or O–F linkage. Quantitative inner-shell optical oscillator strengths derived from the energy loss spectra are reported for H2O, F2O, CF3OF, CF3O2CF3, (CH3)3COH, and (CH3)3CO2C(CH3)3. The valence-shell spectrum of F2O also has been determined. Low-lying inner-shell excitations to σ* valence MOs are identified by their relatively large term values and oscillator strengths. The term values of transitions to σ* MOs in weak-bond molecules correlate with the relevant bond lengths when considered together with the sum of the atomic numbers of the atoms forming the weak bonds.
Phase response behaviors of different oscillatory states in the Belousov–Zhabotinsky reaction
Authors: Ruoff Peter , Noyes Richard M.
Published: July 2002

We have studied the behavior of phase response curves of different oscillatory states in a Belousov–Zhabotinsky reaction by using the amplified Oregonator model. As perturbants we have used bromide ion and silver ion. In case of oxidation spikes, the effect of bromide ion is a phase delay as reported previously. However, when the oscillations stay a considerable time in the oxidized state, bromide ion can cause significant phase advances, whenever the perturbation exceeds a lower critical bromide ion concentration. We are able to model recent experiments performed by Marek and co-workers, which have been claimed to give poor agreements with Oregonator-type of models. We have also compared the effect of a low (104 M−1 s−1) or high (109 M−1 s−1) silver bromide precipitation rate constant on phase response curves when silver ion perturbs oxidation spikes. We find that best agreements with experiments are obtained when the low silver bromide precipitation rate constant is used. The computations exhibit virtually identical behavior whether the rate constant values due to Field, Körös, and Noyes, or to Field and Försterling are used to parametrize the oxybromine chemistry.
Structural characterization of glass-forming oil/water microemulsions by neutron scattering
Authors: Alba-Simionesco C. , Teixeira J. , Angell C. A.
Published: July 2002

Small angle neutron scattering (SANS) has been used to establish the dispersed droplet character of a class of pseudo-three-component oil/water (o/w) microemulsions which can be cooled continuously into the glassy state without either crystallization or microemulsion destabilization. SANS of toluene microemulsions at low volume fractions (∼0.5%) of toluene indicate a droplet radius of 270±10 Å. At normal concentrations (∼33 vol % oil), the droplet size cannot be determined precisely but the presence of a droplet state of small polydispersity is established for toluene, p-xylene, benzene, CS2, and CCl4 by the presence of a sharp peak in the scattered neutron intensity vs Q plots. Average interdroplet separations range from 200 to 300 Å, implying average radii of ∼125 Å. Such radii are consistent with freeze–fracture electron microscope studies of the same or similar systems. Variable temperature studies on toluene microemulsions during both cooling and warming cycles in the temperature range ambient to 100 K, show only minor (∼10%) changes in the interdroplet separation down to 220 K at which temperature diffusion through the viscous matrix phase becomes too slow to permit further emulsion structure changes on the time scale of the cooling experiment. The origin, and some possible exploitations, of this unusual stability against temperature change are discussed.
Vibronic and electronic states of doubly charged H2S studied by Auger and charge transfer spectroscopy and by a
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Authors: Cesar A. , Ågren H. , Naves de Brito A. , Svensson S. , Karlsson L. , Keane M. P. , Wannberg B. , Baltzer P. , Fournier P. G. , Fournier J.
Published: July 2002

Doubly ionic states of H2S are investigated by means of Auger and double charge transfer spectroscopy. From the kinetic energy distribution of H− ions arising from double charge-transfer of protons impinging on gaseous H2S several singlet state energies of H2S2+ have been resolved in the 30 to 50 eV energy region. The most intense experimental peak is narrow proving that the doubly ionized ground state is stable or quasi-stable. The LII,IIIVV Auger electron spectrum exhibits a number of well-defined structures which exhibit vibrational fine structure in the outermost bands. The assignments of the charge transfer states and of the Auger bands are given by ab initio MCSCF electronic structure calculations. We also present vertical double ionization energies, optimized geometries and normal coordinate analysis for the neutral, single and double ionized states. A vibrational analysis of the resolved Auger bands is carried out by employing a recently derived theory for vibrational decay of short-lived core hole states in polyatomic molecules.
Bond length, dipole moment, and harmonic frequency of CO
Authors: Barnes Leslie A. , Liu Bowen , Lindh Roland
Published: July 2002

A detailed comparison of some properties of CO is given, at the modified coupled-pair functional, single and double excitation coupled-cluster (CCSD), and CCSD(T) levels of theory (including a perturbational estimate for connected triple excitations), using a variety of basis sets. With very large one-particle basis sets, the CCSD(T) method gives excellent results for the bond distance, dipole moment, and harmonic frequency of CO. In a [6s 5p 4d 3f 2g 1h]+(1s 1p 1d) basis set, the bond distance is about 0.005a0 too large, the dipole moment about 0.005 a.u. too small, and the frequency about 6 cm−1 too small, when compared with experimental results.
Optical properties of gold colloids formed in inverse micelles
Authors: Wilcoxon J. P. , Williamson R. L. , Baughman R.
Published: July 2002

We discuss the formation of gold metal colloids in a variety of surfactant/solvent systems. Static and dynamic light scattering, small angle x-ray and neutron scattering, TEM analysis, and UV-visible absorbance are used to characterize the kinetics of formation and final colloid stability. These gold colloids exhibit a dramatic blueshift and broadening of the plasmon resonance with decreasing colloid size. Several types of reduction method are discussed and differences between micelle (water-free) or microemulsions as reaction media are compared. Use of inverse micelles allows smaller clusters to be formed with greater long-term stability.
Many-body theory of exchange effects in intermolecular interactions. Density matrix approach and applications to He–F−, He–HF, H2–HF, and Ar–H2 dimers
Authors: Moszynski Robert , Jeziorski Bogumil , Rybak Stanislaw , Szalewicz Krzysztof , Williams Hayes L.
Published: July 2002

The first-order exchange energy for the interactions of closed-shell many-electron systems is expanded as a perturbation series with respect to the Mo/ller–Plesset correlation potentials of the monomers. Explicit orbital formulas for the leading perturbation corrections are derived applying a suitable density matrix formalism. The numerical results obtained using the Mo/ller–Plesset perturbation expansion, as well as nonperturbative, coupled-cluster type procedure, are presented for the interactions of He–F−, He–HF, H2–HF, and Ar–H2. It is shown that the correlation part of the first-order exchange energy increases the uncorrelated results by 10% to 30% for the investigated range of configurations. The analysis of the total interaction energies for selected geometries of these systems shows that at the present level of theory the symmetry-adapted perturbation approach correctly accounts for major intramonomer correlation effects and is capable to accurately reproduce the empirical potential energy surfaces.
Investigation of the collisional quenching of CH(A 2Δ and B 2Σ−) by Ar, O2, CS2, alcohol, and halomethane molecules
Authors: Chen Congxiang , Sheng Yue , Yu Shuqin , Ma Xingxiao
Published: July 2002

The quenching rate constants of CH(A 2Δ and B 2Σ−) by Ar, O2, CS2, alcohol, and halomethane molecules have been measured at 290 K by using the direct time-resolved technique. The electronically excited CH radicals were produced by laser photolysis of CHBr3 at 266 nm and the time-resolved emissions of CH(A 2Δ and B 2Σ−–X 2Π) were then monitored as a function of time using a fast digital storage oscilloscope coupled with a microcomputer. For a given partner, the collisional removal rate of CH(B 2Σ−) is faster than that of CH(A 2Δ) except for O2.
Water chemistry on surface defect sites: Chemidissociation versus physisorption on MgO(001)
Authors: Scamehorn C. A. , Harrison N. M. , McCarthy M. I.
Published: July 2002

The following paper presents the results of a theoretical study that probed the chemistry of water at structural defects on the MgO (001) surface. The computational technique used was periodic Hartree–Fock (PHF) theory with density functional based correlation corrections. The adsorption energies for water adsorbed on isolated corner, edge, and surface sites on the MgO surface were compared to the hydroxylation energies for the same sites. As stated in a previous paper, the binding of water to the perfect surface is exothermic by 4.1-5.6 kcal/mol whereas hydroxylating the perfect surface was endothermic by 24.5 kcal/mol. At step-edge sites, the process of water adsorption is exothermic and comparable in magnitude to the hydroxylation of these sites. The binding energies associated with water bound to the step-edge are 6.5–10.5 kcal/mol, and hydroxylation of this site is exothermic by 7.3 kcal/mol. At corner sites we find a strong preference for hydroxylation. The binding of water to a corner is exothermic by 20.7 kcal/mol, and hydroxylation is exothermic by 67.3 kcal/mol. Mulliken populations indicate that the formation of a hydroxylated surface is governed by the stability of the hydroxyl bond where a hydrogen is bonded to a surface oxygen ion. As the coordination number of this oxygen binding site decreases, its ionic character also decreases, and it forms a more stable bond with the incoming hydrogen. This trend is confirmed by the densities of states for these sites. Finally, hydroxylation of the perfect (001) surface was examined as a function of lattice dilation. It was determined that, as the lattice constant increases, hydroxylation becomes more energetically favorable. This may be important in interpreting experimental thin-film results where the lattice constant of the substrate upon which the MgO film is deposited is slightly larger than that of bulk MgO.
Non-coincidence splitting of the 1505 cm−1 adenine base vibration is due to coupling to water via hydrogen bonding
Authors: Terpstra P. A. , Otto C. , Greve J.
Published: July 2002

We have observed a noncoincidence splitting of 1–2 cm−1 for the 1505 cm−1 adenine base vibration in poly(rA), poly(dA), 5rAMP, and 5dAMP. In double stranded polynucleotides, the noncoincidence splitting is smaller than 0.3 cm−1. Noncoincidence splitting is usually associated with a transition dipole–transition dipole coupling between identical vibrational modes on different molecules of the same species, but any angular dependent intermolecular coupling mechanism may cause this effect. It is argued that resonant vibrational coupling of 1505 cm−1 vibrations on different adenine bases cannot be the source of the noncoincidence splitting. As the 1505 cm−1 base vibration does not have a large infrared absorption, it cannot be caused by transition dipole–transition dipole coupling. We propose that a coupling via hydrogen bonding of the 1505 cm−1 adenine base vibration to water vibrations yields the angular dependent intermolecular coupling potential.
Nonlinear Poisson–Boltzmann theory of a double layer at a rough metal/electrolyte interface: A new look at the capacitance data on solid electrodes
Authors: Daikhin L. I. , Kornyshev A. A. , Urbakh M.
Published: July 2002

Nonlinear Poisson–Boltzmann theory is developed to extend our previous work [Phys. Rev. E 53, 6192 (1996)] on the case when the potential drop across the double layer is not small compared to the thermal energy. Close to the potentials of zero charge (pzc) the effect of surface roughness on the double-layer capacitance is mainly determined by an interplay between the lateral correlation length of roughness and the Debye length. However, far from the pzc dramatic effects of electrode potential are found which are not reduced to the potential-induced shortening of the diffuse layer thickness.
Rapid vapor deposition of millimeters thick optically transparent parahydrogen solids for matrix isolation spectroscopy
Authors: Fajardo Mario E. , Tam Simon
Published: July 2002

We report the rapid vapor deposition of millimeters thick parahydrogen (pH2) solids of remarkable optical clarity. Characterization of pure pH2 samples by IR and Raman spectra show a very low orthohydrogen and vacancy content, and a mixed hexagonal-closed-packed/face-centered-cubic (hcp/fcc) polycrystalline structure for as-deposited samples, which converts to hcp upon annealing. Efficient isolation of atomic and molecular dopants is achieved by conventional matrix isolation spectroscopy (MIS) techniques. The increased optical path lengths offer significant improvements in spectroscopic data quality, and reveal novel dopant-induced IR absorptions of the pH2 matrix host itself. Thus, while traditional MIS studies in rare gas hosts can only probe the influence of the matrix environment on the spectrum of the dopant “solute,” in pH2 the response of the host “solvent” is directly observable as well. This complementary information may prove to be the key to identifying conclusively the microscopic structures of dopant trapping sites.
Quantum effects in simulated water by the Feynman–Hibbs approach
Authors: Guillot Bertrand , Guissani Yves
Published: July 2002

Quantum effects in water are investigated by implementing the Feynman–Hibbs effective potential in a molecular-dynamics code. The reference potential chosen for water is a new central force model related to the one proposed in the 1970s by Lemberg and Stillinger [J. Chem. Phys. 62, 1677 (1975)]. The evolution of the thermodynamics, the structure, the diffusivity, and the dynamics in light and heavy water is investigated over a large range of temperature and is compared with experimental data and with classical simulations as well. It is found that quantum effects are significant near ambient conditions and vanish with increasing temperature less drastically than generally assumed. The most affected quantity is the self-diffusion coefficient for which is predicted a marked increase of the isotopic ratio (DH2O/DD2O) in going into the supercooled region. The accuracy of the results and the very low cost in computer time make the Feynman–Hibbs approach a valuable procedure to rapidly estimate the order of magnitude of the quantum contributions to intermolecular properties of water.
A challenge for density functionals: Self-interaction error increases for systems with a noninteger number of electrons
Authors: Zhang Yingkai , Yang Weitao
Published: July 2002

The difficulty of widely used density functionals in describing the dissociation behavior of some homonuclear and heteronuclear diatomic radicals is analyzed. It is shown that the self-interaction error of these functionals accounts for the problem—it is much larger for a system with a noninteger number of electrons than a system with an integer number of electrons. We find the condition for the erroneous dissociation behavior described by approximate density functionals: when the ionization energy of one dissociation partner differs from the electron affinity of the other partner by a small amount, the self-interaction error will lead to wrong dissociation limit. Systems with a noninteger number of electrons and hence the large amount of self-interaction error in approximate density functionals arise also in the transition states of some chemical reactions and in some charge-transfer complexes. In the course of analysis, we derive a scaling relation necessary for an exchange-correlation functional to be self-interaction free.
A molecular dynamics study of the thermodynamic properties of liquid Ni using the Voter and Chen version of the embedded atom model
Authors: Alemany M. M. G. , Rey C. , Gallego L. J.
Published: July 2002

Using the Voter and Chen version of the embedded atom model, we performed molecular dynamics simulations to compute the thermodynamic properties of liquid Ni up to 3000 K, i.e., well above the melting temperature. Our results show good general agreement with available experimental data. Comparison between simulated and experimental heat capacities requires subtraction from the latter of the electronic contribution, which for liquid transition metals is usually an order of magnitude greater than for simple metals.
Nonthermal photodesorption of N2 from Ag(111)
Authors: Rao R. M. , Beuhler R. J. , White M. G.
Published: July 2002

We have measured translational and rotational energy distributions of N2 molecules following desorption from a Ag(111) surface by infrared (1064 nm) radiation. The observed desorption yields were large even at laser fluences far below that required for laser-induced thermal desorption. State-resolved laser techniques using coherent VUV radiation showed that the rotational and translational energy distributions of the desorbing N2 molecules are not consistent with the predictions of the heat diffusion model governing laser-induced surface heating. These results suggest that physisorbed adsorbates can couple directly to the nascent-phonon distribution or the nascent electron–hole pairs in the photoexcited substrate without heating of the surface.
Thermal mobility of atomic hydrogen in solid argon and krypton matrices
Authors: Vaskonen Kari , Eloranta Jussi , Kiljunen Toni , Kunttu Henrik
Published: July 2002

Decay patterns of atomic hydrogen trapped in argon and krypton matrices are followed by electron paramagnetic resonance (EPR). Hydrogen atoms are generated by uv-photolysis of HBr and HCl precursor molecules. The EPR signals due to interstitially trapped hydrogen atoms in octahedral sites disappear near 16 and 24 K in Ar and Kr, respectively. Substitutionally trapped H atoms are thermally stable up to evaporation temperature of the solids. The fate of thermally released H atoms in Ar is exclusively due to geminate recombination of the parent molecule. The observed kinetics is well fitted with double exponential decay. The kinetic behavior reflects short-range dissociation and recombination dynamics in Ar. In the Kr matrix, a change from first-order to second-order kinetics is observed at higher concentrations as formation of molecular hydrogen becomes important. From bimolecular decay kinetics, a diffusion constant of 4×10−15 cm2 s−1 is deduced for H-atom diffusion in Kr at 26.9 K. The obtained activation energies, 6–7 kJ/mol in Ar and 9–14 kJ/mol in Kr, are measures of thermally activated cage dynamics and show only weak dependence on the hydrogen isotope.
Electron binding to valence and multipole states of molecules: Nitrobenzene, para- and meta-dinitrobenzenes
Authors: Desfrançois C. , Périquet V. , Lyapustina S. A. , Lippa T. P. , Robinson D. W. , Bowen K. H. , Nonaka H. , Compton R. N.
Published: July 2002

Nitrobenzene anions (NB−) in both valence and dipole bound states are examined using laser (photodetachment) photoelectron and Rydberg electron transfer (RET) spectroscopies. Photoelectron spectroscopy of the valence NB− anion yields a valence (adiabatic) electron affinity of 1.00±0.01 eV. The reaction rates for charge transfer between atoms of cesium and xenon in high Rydberg states [Cs(ns,nd) and Xe(nf )] and NB exhibit a prominent peak in their n-dependencies consistent with the formation of a dipole bound anion having an electron affinity of 28 meV. Para-dinitrobenzene (pDNB) has a zero dipole moment and a large quadrupole moment. RET studies with pDNB show a complex n-dependence. The rate of formation of pDNB− ions exhibits a broad peak at low n-values and a second very broad feature extending to large n-values. The peak at low n is tentatively attributed to charge exchange into a quadrupole bound state (EAqb=25 meV). The absence of field-detachment for these ions suggests that if these are in a quadrupole bound state they are strongly coupled to the valence state. Meta-dinitrobenzene (mDNB) has a large dipole moment and a small quadrupole moment. The observation of a weak but narrow peak at a low n-value in the RET measurement is consistent with electron attachment into a primarily dipole bound state (EAdb=68 meV) which is also strongly coupled to the valence state.
Etching process of SiO2 by HF molecules
Authors: Hoshino T. , Nishioka Y.
Published: July 2002

Ab initio theoretical computations have been performed to reveal the mechanism of the etching reaction of silicon oxide (SiO2) by HF molecules. The probable reaction paths, in which the etching reaction proceeds through four sequential steps to remove a single fragment of SiO2, are presented with their potential energy curves. In every step, the insertion of an HF molecule into an Si-O bond leads to the dissociation of the Si-O connection. The potential energy barriers evaluated along the reaction paths suggest that the HF molecule has an ability to etch the SiO2 surface. The strong interaction among HF molecules, however, likely causes HF polymer formation, which is expected to reduce the reaction rate of SiO2 etching.
Interfacing relativistic and nonrelativistic methods. III. Atomic 4-spinor expansions and integral approximations
Authors: Dyall Kenneth G. , Enevoldsen Thomas
Published: July 2002

Two approximations to the normalized elimination of the small component are presented which enable the work of a relativistic calculation to be substantially reduced. The first involves fixing the ratio of the large and small components in atomic calculations, which corresponds to a basis set expansion in terms of positive energy atomic 4-spinors. The second involves the definition of a local, i.e., center-dependent, fine structure constant, which has the effect of making atoms with α=0 nonrelativistic. A series of test calculations on a variety of molecules and properties indicates that the errors incurred in the first approximation are negligible. In the second approximation, the errors are dependent on the property, the chemical environment and the atomic number. For the second period elements the errors in the approximation are for chemical purposes negligible. In the third period this is true for many properties, but for some, such as ligand-metal binding energies, there are discrepancies which may be a cause for concern in more accurate calculations. Beyond the third period it is usually necessary to treat atoms relativistically.
Reactions of C+(2Pu) with CO(1Σ+) from thermal energies to 30 eV
Authors: Lu Wenyun , Tosi Paolo , Filippi Mauro , Bassi Davide
Published: July 2002

The endoergic reactions of C+(2Pu) with CO(1Σ+) producing CO+ +C, C2++O, and O++C2 have been studied in a guided-ion beam apparatus. For each reaction channel, we have measured the kinetic energy dependence of the integral cross section and the reaction threshold. Analysis of the reaction cross sections yield the heats of formation ΔfH0(C2+)=19.79±0.16 and ΔfH0(C2)=8.37±0.16 eV. In addition the dissociation energy of C2 is estimated to be D0(C–C)=6.37±0.16 eV. A simple state correlation diagram is used to rationalize experimental observations and to discuss reaction dynamics.
A multiple spawning approach to tunneling dynamics
Authors: Ben-Nun M. , Martı́nez Todd J.
Published: July 2002

Quantum mechanical tunneling effects are investigated using an extension of the full multiple spawning (FMS) method. The FMS method uses a multiconfigurational frozen Gaussian ansatz for the wave function and it allows for dynamical expansion of the basis set during the simulation. Basis set growth is controlled by allowing this expansion only when the dynamics signals impending failure of classical mechanics, e.g., nonadiabatic and/or tunneling effects. Previous applications of the FMS method have emphasized the modeling of nonadiabatic effects. Here, a new computational algorithm that accounts for tunneling effects is introduced and tested against exact solution of the Schrödinger equation for two multi-dimensional model problems. The algorithm first identifies the tunneling events and then determines the initial conditions for the newly spawned basis functions. Quantitative agreement in expectation values, tunneling doublets and tunneling splitting is demonstrated for a wide range of conditions.
Electron distribution in water
Authors: Badyal Y. S. , Saboungi M.-L. , Price D. L. , Shastri S. D. , Haeffner D. R. , Soper A. K.
Published: July 2002

The x-ray structure factor of water measured under ambient conditions with synchrotron radiation is compared with those predicted on the basis of partial structure factors describing the nuclear positions obtained by neutron diffraction and of different assumptions for the electron distribution. The comparison indicates that a charge of approximately 0.5 e is transferred from each hydrogen atom to the oxygen on the same molecule, implying an effective dipole moment of 2.9 D, in good agreement with theoretical estimates.
Existence theorems for 90° vortex–vortex scattering
Authors: Abdelwahid F. , Burzlaff J.
Published: July 2002

A Cauchy problem is formulated with data that describe the scattering of two vortices in a superconductor, and the following is proven: First, a unique global finite-energy solution exists. Second, the symmetry of the solution rules out all cases other than 0°, 90°, or 180° scattering. Third, a local analytic solution exists near the origin. The leading terms of this solution show that, of the three cases, 90° scattering is realized.
The Painlevé formulations and exact solutions of the nonlinear evolution equations for modulated gravity wave trains
Authors: Shivamoggi Bhimsen K. , Rollins David K.
Published: July 2002

In this article the integrability aspects of the nonlinear evolution equations for modulated gravity wave trains are investigated by demanding the Painlevé property of the solutions. The Painlevé formulations also lead to kink-shaped exact localized solutions of these equations.
Timelike geodesic motions within the general relativistic gravitational field of the rigidly rotating disk of dust
Authors: Ansorg M.
Published: July 2002

The general relativistic motion of a test particle near a rigidly rotating disk of dust is investigated. Circular orbits within the plane of the disk (centered on the rotation axis) are special cases of the geodesic motion. One finds that there is always a (stable or unstable) circular orbit for positive angular momentum and a given radius. However, for sufficiently relativistic disks there are regions within the plane of the disk in which a particle with negative angular momentum cannot follow a circular path. If the disk is still more strongly relativistic, then one finds circular orbits with negative energies of arbitrary magnitude. Within the theoretical construction of the Penrose effect, this property can be used to produce arbitrarily high amounts of energy. The study of Hamiltonian mechanics forms another topic of this article. It turns out that the stochastic behavior of the geodesics is related to the position of the region containing all the crossing points of the particle through the plane of the disk. If this region contains points lying inside the disk as well as points outside, the geodesic motion shows highly stochastic behavior. However, if the crossing region is completely inside or outside the disk, the motion proves to be nearly integrable. In these cases the corresponding Hamiltonian system is close to an integrable system of the so-called Liouville class.
Semiclassical limits of extended Racah coefficients
Authors: Davids Stefan
Published: July 2002

We explore the geometry and asymptotics of extended Racah coefficients. The extension is shown to have a simple relationship to the Racah coefficients for the positive discrete unitary representation series of SU(1,1) which is explicitly defined. Moreover, it is found that this extension may be geometrically identified with two types of Lorentzian tetrahedra for which all the faces are timelike. The asymptotic formulas derived for the extension are found to have a similar form to the standard Ponzano–Regge asymptotic formulas for the SU(2) 6j symbol and so should be viable for use in a state sum for three dimensional Lorentzian quantum gravity.
An experimental investigation of the transition of a planar jet: Subharmonic suppression and upstream feedback
Authors: Thomas F. O. , Chu H. C.
Published: July 2002

An experimental investigation into the transition of an initially laminar planar jet at moderate Reynolds number is presented. Very low level acoustic excitation is applied upstream of the nozzle exit in order to organize the initial instability wave and thereby facilitate investigation of its downstream evolution. Particular attention is focused upon quantifying the quadratic nonlinear wave interactions that characterize the near-field transition as evidenced by measurements of phase locking between resonant wave triads. The spectral evolution of fluctuations in the planar jet shear layer indicate that the subharmonic is significantly suppressed. This subharmonic suppression was found to result from an interesting combination of the dynamics of subharmonic resonance and the upstream feedback of perturbations from the interaction region beyond the jet potential core to the disturbance sensitive region near the nozzle lip. The upstream feedback was found to be associated with the large-scale restructuring of the flow that characterizes the interaction region of the planar jet and results in the loss of symmetry of the coherent vorticity field with respect to the centerline. The imposed near-exit perturbations interact nonlinearly with the shear layer instability wave to produce the observed suppression. The interplay between the initial and interaction regions of the flow provides a unique glimpse of the rich behavior that even seemingly simple flows can exhibit during transition to turbulence.
Description of the pressure effects in the Reynolds stress transport equations
Authors: Groth Johan
Published: July 2002

Three different proposals for the separation into a deviatoric and a nondeviatoric term of the pressure–velocity interactions in the Reynolds stress transport equations are discussed. It is shown from general considerations and two specific examples that the customary separation is likely to be the correct choice.
The linear threshold of the ion-temperature-gradient-driven mode
Authors: Guo S. C. , Romanelli F.
Published: July 2002

A comprehensive stability analysis of the ion-temperature-gradient-driven mode in various parameter domains is presented. The effect of parallel ion dynamics on the threshold value in toroidal geometry is investigated for the short-wavelength and finite-shear limits. A general explicit expression for the threshold is given, which is in agreement with the results obtained by solving the full integrodifferential equation. The destabilizing effect of trapped electrons below the threshold is qualitatively similar to the results of local theory, but electron collisionality has to be sufficiently weak. In the long-wavelength limit, two branches exist, respectively, toroidal and slablike in character. The toroidal branch has been investigated elsewhere [Romanelli et al., Phys. Fluids B 3, 2496 (1991)]. In this paper, the stability of the slablike branch is studied both analytically and numerically. The trapped-ion effects on the threshold are discussed in detail.
Ion-acoustic solitary waves in relativistic plasmas
Authors: Das G. C. , Paul S. N.
Published: July 2002

This is a sequel to our earlier study on ion-acoustic waves studied through the augmentation to a modified Korteweg–deVries (K–dV) equation. We have derived a K–dV equation in a plasma, taking account of weakly relativistic effects, and the result shows that the solitary wave does exhibit the relativistic effect in the presence of ion streaming.
Plasma transport coefficients for nonsymmetric toroidal confinement systems
Authors: Hirshman S. P. , Shaing K. C. , van Rij W. I. , Beasley C. O. , Crume E. C.
Published: July 2002

A variational principle is developed for computing accurate values of local plasma transport coefficients in nonsymmetric toroidal confinement configurations. Numerical solutions of the linearized drift Fokker–Planck equation are used to obtain the thermodynamic fluxes as functions of collision frequency and the radial electric field. Effects resulting from the variation of the longitudinal adiabatic invariant J along an orbit (resulting from particle transitions from helically trapped to toroidally trapped orbits) are treated. The velocity-space distribution resulting from trapped, circulating, and transition particle orbits is well represented by a Legendre polynomial expansion in the pitch angle coordinate. The computational effort is significantly reduced from that required with Monte Carlo methods through use of an efficient treatment of the disparity between the time scales of collisionless and collisional particle dynamics. Numerical computations for a stellarator configuration are presented.
A closure model for conditioned stress equations and its application to turbulent shear flows
Authors: Byggstoyl S. , Kollmann W.
Published: July 2002

A second-order closure model based on intermittency factor and conditioned moments is developed. The transport equations for the nonturbulent zone stresses are included in the model. The resulting model is then compared with measurements in several shear flows and satisfactory agreement between calculation and experiment is obtained.
Bifurcations and bursting in two-dimensional Poiseuille flow
Authors: Jiménez Javier
Published: July 2002

The bifurcations to two-dimensional unsteady behavior of a large amplitude equilibrium shear wave train in two-dimensional Poiseuille flow are studied by direct simulation of the time evolution of the full Navier–Stokes equations. It is found that the wave train becomes unstable at Re∼5600, and sheds a limit cycle which, at higher Re, seems to undergo further transitions to more complex behaviors. It is shown that the site of the original bifurcation is in the neighborhood of the walls and that it shows some characteristics suggestive of the burst generation mechanism in the boundary layer.
Small amplitude theory of Richtmyer–Meshkov instability
Authors: Yang Yumin , Zhang Qiang , Sharp David H.
Published: July 2002

This paper presents a new analysis of small amplitude Richtmyer–Meshkov instability. The linear theory for the case of reflected rarefaction waves, a problem not treated in previous work, is formulated and numerically solved. This paper also carries out a systematic comparison of Richtmyer’s impulsive model to the small amplitude theory, which has identified domains of agreement as well as disagreement between the two. This comparison includes both the reflected shock and reflected rarefaction cases. Additional key results include the formulation of criteria determining the reflected wave type in terms of preshocked quantities, identification of parameter regimes corresponding to total transmission of the incident wave, discussion of an instability associated with a rarefaction wave, investigation of phase inversions and the related phenomenon of freeze-out, and study of the sensitivity of the numerical solutions to initial conditions.
Dipole formation in the transient planar wall jet
Authors: Conlon Brian P. , Lichter Seth
Published: July 2002

An initially quiescent quarter-plane of fluid is set into motion by the action of a wall jet, i.e., a jet parallel and adjacent to a wall. The circumstances for which the jet separates from the wall and forms a dipole at the upstream head of the jet are studied numerically. The streamfunction-vorticity formulation is used to track the time evolution of vorticity over a range of Reynolds numbers, 50≤Re≤1000, based on jet velocity and width. By implementing both a no-slip and a slip boundary condition and comparing the results for the time evolution of vorticity, it is found that the boundary condition has a negligible effect on dipole development. By contrast, the relative magnitude of positive ω+ and negative ω− vorticity present in the inlet jet flow controls whether dipole formation occurs. In particular, dipole formation requires a sufficiently large magnitude of negative vorticity ω−/ω+≳0.65. The results refine a conjecture by Yushina (‘‘Evolution of the near-wall jet,’’ in General Circulations of the Oceans, Technical Report No. WHOI of the 1989 Summer Study Program in Geophysical Fluid Dynamics, Woods Hole, Oceanographic Institute, Woods Hole, MA, 1989, pp. 470–512) that negative vorticity is necessary for dipole creation. The results further indicate that the criterion for the existence of a moving separation point concurrently serves as a criterion for dipole formation. This suggests that, in some situations in an unsteady boundary layer, dipole generation will be associated with unsteady separation.
A note on the vorticity spectrum
Authors: Antonia R. A. , Shafi H. S. , Zhu Y.
Published: July 2002

The vorticity spectrum has been inferred, using local isotropy, from measurements of the lateral vorticity components in a turbulent wake over a small Reynolds number range. The high wavenumber part of the spectrum supports Kolmogorov’s [C. R. Akad. Sci. USSR 30, 301 (1941)] similarity theory. Among the different published analytical expressions for the three-dimensional energy spectrum, the model of Kida and Murakami [Phys. Fluids 30, 2030 (1987)] is closest to the present data.
The circular hydraulic jump
Authors: Higuera F. J.
Published: July 2002

An asymptotic order-of-magnitude description is given of the structure of a circular laminar hydraulic jump for large values of the Reynolds and Froude numbers of the flow entering the jump. The results are compared with numerical solutions of the boundary layer equations for the flow in a liquid layer on a horizontal disk and with experimental results existing in the literature.
Stability of a growing end rim in a liquid sheet of uniform thickness
Authors: Fullana José Maria , Zaleski Stéphane
Published: July 2002

We study the stability of a viscous liquid layer of uniform thickness subject only to viscous stresses and surface tension. We show that the growing cylindrical end rim does not typically break into droplets for moderate wavelengths. Thus, other mechanisms are needed to cause the instabilities, which, for instance, lead to the famous milk crown. The problem remains open for very large wavelengths.
Reflection of ion acoustic solitons in a plasma having negative ions
Authors: Chauhan Sarvesh S. , Malik H. K. , Dahiya R. P.
Published: July 2002

Reflection of compressive and rarefactive ion acoustic solitons propagating in an inhomogeneous plasma in the presence of negative ions is investigated. Modified Korteweg–de Vries equations for incident and reflected solitons are derived and solved. The amplitude of incident and reflected solitons increases with negative to positive ion density ratio. With increasing density ratio, reflection of rarefactive solitons is reinforced whereas that of compressive solitons weakened. The rarefactive solitons are found to undergo stronger reflection than the compressive ones.
Structure of the wake field in plasma channels
Authors: Andreev N. E. , Gorbunov L. M. , Kirsanov V. I. , Nakajima K. , Ogata A.
Published: July 2002

An equation is derived that describes the linear response of an underdense inhomogeneous plasma [ω0≫ωp(r), where ω0 and ωp(r) are the laser-carrier and plasma frequencies, respectively] during the propagation of a laser pulse along the axis of a plasma channel with a characteristic width Rch. For a wide channel, i.e., when Rch/λp0>1 (where λp0=2πc/ωp0 is the wavelength of the excited plasma wave and ωp0 is the plasma frequency at the channel axis), the structure of the wake field is studied analytically. It is shown that this structure changes with the distance from the trailing edge of the pulse. As a result, at a certain distance behind the pulse, the fraction of the plasma wave period in which the simultaneous focusing and acceleration of electrons are possible increases by a factor of 2. For a narrow channel (Rch/λp0<1), the structure of the wake field is studied numerically and it is shown that, in this case, the doubling of the phase interval of the wave where the simultaneous focusing and acceleration of electrons are possible also occurs; but, in contrast to a wide channel, a rapid reconstruction of the wake occurs, so that the amplitude of the axial (accelerating) field in the wake decreases while the radial (focusing) field increases with the distance from the pulse trailing edge. The numerical modeling of the laser pulse (90 fs, 2 TW) guiding and the excitation of plasma waves in a narrow plasma channel is carried out and the possibility of reaching GeV energies of accelerated electrons in an experiment is discussed.