bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational and hyperfine analysis of the near infrared Φ43–X 3Φ4 transitions of CoCl and CoI
Authors: Wong Ally L. , Tam W. S. , Cheung A. S-C.
Published: July 2003

Electronic transitions of cobalt monochloride and cobalt monoiodide have been studied using laser vaporization–reaction free jet expansion and laser induced fluorescence spectroscopy in the near infrared region. The observed transitions have been identified as the [10.3] Φ43–X 3Φ4 transition of CoCl and [11.0] Φ43–X 3Φ4 transition of CoI. The magnetic hyperfine structure arising from the cobalt nucleus with I=7/2 was resolved and analyzed. Accurate rotational and hyperfine parameters for the [10.3] Φ43 and X 3Φ4 states of CoCl and [11.0] Φ43 and X 3Φ4 states of CoI have been obtained. Comparison of Fermi contact parameters, bF, for the upper states indicated that the observed [10.3] Φ43–X 3Φ4 transition of CoCl and [11.0] Φ43–X 3Φ4 transition of CoI have arisen from the promotion of an electron from the bonding σ orbital to a slightly antibonding σ orbital. Observed low-lying Φ3 states of the cobalt monohalides and hydride are also compared and discussed.
Time averaging the semiclassical initial value representation for the calculation of vibrational energy levels. II. Application to H2CO, NH3, CH4, CH2D2
Authors: Kaledin Alexey L. , Miller William H.
Published: July 2003

A method recently developed for calculating vibrational spectral densities of molecules, previously tested successfully on H2O, is applied here to several larger molecules. The method relies on use of a time averaging procedure in the conventional semiclassical (SC) initial value representation (IVR) expression for the spectral density. The convergence of the SC-IVR average over the phase space of initial conditions (of classical trajectories) is greatly enhanced by time averaging and is generally achieved with as few as 1000 trajectories per degree of freedom. Furthermore, meaningful results can be obtained with only a single trajectory propagated for a long time. (For systems with chaotic dynamics, however, the phase space average converges more slowly.) Results for vibrational energy levels of H2CO, NH3, CH4, and CH2D2 are reported and compared with quantum mechanical calculations available in the literature. The accuracy of the time-averaged SC-IVR is very encouraging, with the vibrational energy levels consistently in agreement with the quantum results to a few parts in a thousand.
Extreme ultraviolet laser excitation of isotopic molecular nitrogen: The dipole-allowed spectrum of 15N2 and 14N15N
Authors: Sprengers J. P. , Ubachs W. , Baldwin K. G. H. , Lewis B. R. , Tchang-Brillet W.-Ü L.
Published: July 2003

Extreme ultraviolet+ultraviolet (XUV+UV) two-photon ionization spectra of the b 1Πu(v=0–9), c3 1Πu(v=0,1), o 1Πu(v=0,1), c4′ 1Σu+(v=1) and b′ 1Σu+(v=1,3–6) states of 15N2 were recorded with a resolution of 0.3 cm−1 full-width at half-maximum (FWHM). In addition, the b 1Πu(v=1,5–7) states of 14N15N were investigated with the same laser source. Furthermore, using an ultranarrow bandwidth XUV laser [∼250 MHz (∼0.01 cm−1) FWHM], XUV+UV ionization spectra of the b 1Πu(v=0–1,5–7), c3 1Πu(v=0), o 1Πu(v=0), c4′ 1Σu+(v=0), and b′ 1Σu+(v=1) states of 15N2 were recorded in order to better resolve the band-head regions. For 14N15N, ultrahigh resolution spectra of the b 1Πu(v=0–1,5–6), c3 1Πu(v=0), and b′ 1Σu+(v=1) states were recorded. Rotational analyses were performed for each band, revealing perturbations arising from the effects of Rydberg-valence interactions in the 1Πu and 1Σu+ states, and rotational coupling between the 1Πu and 1Σu+ manifolds. Finally, a comprehensive perturbation model, based on the diabatic-potential representation used previously for 14N2, and involving diagonalization of the full interaction matrix for all Rydberg and valence states of 1Σu+ and 1Πu symmetry in the energy window 100 000–110 000 cm−1, was constructed. Term values for 15N2 and 14N15N computed using this model were found to be in good agreement with experiment.
Comment on “On the consistency of the collisionless sheath model” [Phys. Plasmas 9, 4427 (2002)]
Authors: Tskhakaya D. D. , Shukla P. K.
Published: July 2003

The shortcoming of a recently published paper by Godyak and Sternberg [Phys. Plasmas 9, 4427 (2002)] on the consistency of the collisionless sheath model is pointed out.
New constraints and energy conversion efficiencies for plasma emission
Authors: Mitchell J. J. , Cairns I. H. , Robinson P. A.
Published: July 2003

Radio emission near the electron plasma frequency fp and 2 fp due to electron beams is important in many laboratory and space applications. Langmuir waves produced as a result of these beams undergo various interactions leading to radio emission. Two categories of such interactions are decay processes involving ion acoustic waves and processes involving scattering off thermal ions (STI). In this paper energy conversion efficiencies for STI emission processes are derived. These are then compared with existing expressions for emission via decay. It is found that decay dominates STI processes when decay is able to proceed. Conditions are derived for emission near fp, and 2 fp by decay processes. These depend on whether significant nonthermal levels of L′ or S waves are produced by the initial nonlinear processes. These conditions are determined by comparing nonlinear growth rates with Landau damping rates. It is found that where the ratio of the beam speed to the electron thermal speed exceeds roughly 3–6, L′-waves persist and emission near 2 fp can proceed. It is also found that when beams are sufficiently dense, cold, and fast, S-waves persist and emission near fp involving decay processes can proceed.
Semiclassical Monte Carlo model for in-plane transport of spin-polarized electrons in III–V heterostructures
Authors: Saikin Semion , Shen Min , Cheng Ming-C. , Privman Vladimir
Published: July 2003

We study the in-plane transport of spin-polarized electrons in III–V semiconductor quantum wells. The spin dynamics is controlled by the spin-orbit interaction, which arises due to the bulk crystalline-structure asymmetry and quantum-well inversion asymmetry. This interaction, owing to its momentum dependence, causes rotation of the spin-polarization vector, and also produces effective spin dephasing. The density matrix approach is used to describe the evolution of the electron spin polarization, while the spatial motion of the electrons is treated semiclassically. Monte Carlo simulations have been carried out for temperatures in the range 77–300 K.
Fiber-optic thermometer using Cr-doped YAlO3 sensor head
Authors: Uchiyama H. , Aizawa H. , Katsumata T. , Komuro S. , Morikawa T. , Toba E.
Published: July 2003

Cr-doped YAlO3 phosphor crystals have been grown and characterized for applications in fiber-optic thermometers based on the temperature dependence of the photoluminescence (PL) lifetime. PL peaking at λ=735 nm is observed from the crystals following an excitation by a light-emitting diode at λ=520 nm. Long PL lifetime (τ=43.14 ms) is observed from Cr-doped YAlO3 at room temperature as compared with those of ruby (τ=4.2 ms) and spinel (τ=8.0 ms). The temperature coefficient (−0.075 ms/K) of Cr-doped YAlO3 is also larger than those of ruby (−0.010 ms/K) or spinel (−0.037 ms/K). YAlO3 is considered to be a sensitive sensor head material for fiber-optics thermometers.
Formation of fine near-field scanning optical microscopy tips. Part I. By static and dynamic chemical etching
Authors: Lazarev Alexander , Fang Nicholas , Luo Qi , Zhang Xiang
Published: July 2003

The probe tip is the key component in scanning probe microscopes and their applications in the nanoscale imaging and nanofabrication. In this work, we have investigated the formation of near-field scanning optical microscopy probe tips from optical fiber by chemical etching. Static and dynamic etchings and their combinations are studied. The etching process is optimized, and tips with short tapers, small apertures (about 50 nm) and large aperture cone angles (40°) are successfully obtained. Multiple-tapered tips are also fabricated by using different dynamic regimes. It is found that the taper profiles are determined by the nonlinear dynamic evolution of the meniscus of the etchant near the fiber.
Thick photorefractive polymer device with coplanar electrodes
Authors: Hayasaki Yoshio , Ishikura Naofumi , Yamamoto Hirotsugu , Nishida Nobuo
Published: July 2003

We propose a structure of a photorefractive polymer device. An electric field is applied to the device in plane so as to coincide with the direction of a grating formed by two writing beams. This structure permits us to fabricate a thick photorefractive polymer device with a high optical gain. The maximum thickness we fabricated was 600 μm. We investigate the device performance using a two-beam coupling experiment and demonstrate high-beam-coupling gain in the thick device. We also describe position dependence of the photorefractive effect for an apical electrode structure.
Energy dispersive x-ray diffraction and differential scanning calorimetry investigation of the melting behavior of poly(ethylene-succinate) crystallized at low undercooling
Authors: Isopo A. , Albertini V. Rossi , Tentolini U. , Caminiti R.
Published: July 2003

The behavior upon heating of poly(ethylene-succinate) samples crystallized at progressively lower undercooling has been studied. The multiple melting phenomena exhibited by this particular polymer subjected to a temperature ramp were investigated using both ordinary differential scanning calorimetry and an alternative technique, energy dispersive x-ray diffraction applied to phase transition. A critical comparison of the two methods adopted in the melting studies is included. Another application of the energy dispersive x-ray diffraction method, which enables one to distinguish the structural rearrangements that take place on different distance scales, is also described.
Kinetic pathways of carbon nanotube nucleation from graphitic nanofragments
Authors: Louchev Oleg A. , Hester James R.
Published: July 2003

A detailed analysis of nanotube (NT) nucleation from graphitic nanofragments by thermal vibration is given, outlining the role of activation energy barriers which may be significantly decreased by the interaction with amorphous carbon, fullerene-like, and carbonized metal catalyst nanoparticles. This analysis predicts the variety of carbon nanotube chiralities observed experimentally. Heat dissipation by the inert gas and the cooling rate of the metal–carbon nanoparticle are suggested to play an important role in kinetic selection between (i) a carbon nanosheet wrapping around the metal nanoparticle, (ii) NT nucleation and growth on the nanoparticle surface by the so called “root mechanism,” and (iii) the metal nanoparticle surface being covered by an amorphous carbon layer.
Analysis of microstructure of magnetic Fe3C nanograins embedded in amorphous carbon films
Authors: Lee Y. H. , Han T. C. , Huang J. C. A. , Lin C. R.
Published: July 2003

The dc magnetron co-sputtering technique was used in fabricating amorphous carbon films containing magnetic Fe3C nanograins. A composite target of a 2-in.-diam graphite disk plus several pieces of iron rods, 2 mm in diameter and 4 mm in length, were used. The films containing pure Fe3C grains could be obtained only in proper sputtering conditions. Films containing only grains of Fe3C phase were subjected to postdeposition annealing at temperatures of 250 to 600 °C for 60 and 90 min, respectively. Auger electron spectroscopy was used to determine the atomic concentration and its fluctuations. The microstructure and phases of grains were determined by high-resolution transmission electron microscopy (HRTEM) and selected area diffraction patterns. Very good crystallinity appeared at Ta⩾250 °C. From the HRTEM image, lattice spacings of 6.7 Å—corresponding to Fe3C (001) planes—and 4.3 Å—corresponding to Fe3C (100) planes—were observed. Higher annealing temperature and larger annealing time caused an increase of grain size. The largest grain (18 nm) was obtained at Ta=550 °C.
Mechanisms of positive charge generation in buried oxide of UNIBOND and separation by implanted oxygen silicon-on-insulator structures during high-field electron injection
Authors: Nazarov A. N. , Kilchytska V. I. , Houk Y. , Ballutaud D.
Published: July 2003

Mechanisms of the generated positive charge and the charge trapping during the high-field electron injection at constant voltage regime in buried oxide (BOX) of silicon-on-insulator (SOI) structures fabricated by UNIBOND and single implanted separation by implanted oxygen (SIMOX) technologies are studied by capacitance–voltage characteristics. It is found, that considerable positive charge is accumulated near the buried oxide/substrate interface independently from direction of the injection (from film or from silicon substrate) for both kinds of structures. Comparison of the theory and experimental data allows to suggest that in the case of the UNIBOND buried oxide a positive charge is generated by two mechanisms—anode hole injection and band-to-band impact ionization. In the case of the SIMOX SOI material at the high-field electron injection in the buried oxide the anode hole injection with defect creation at precursor sites in the strained structure of oxide network is supposed to appear. It is shown, that the positive charge in the UNIBOND BOX can be completely annealed at a temperature of 400 °C while in the SIMOX BOX it is much more stable and the annealing temperature increases with the electron injection electric field rising.
Anode double layer in magnetized radio frequency inductively coupled hydrogen plasma
Authors: Tang Deli , Chu Paul K.
Published: July 2003

The formation of the double layer created around the anode in magnetized radio frequency inductively coupled plasma, which is visually apparent because of enhanced light emission from the neutrals excited by energetic electrons, is investigated in detail in this work. The effects of the external magnetic field and anode voltage on the evolution of a cylindrical luminous anode double layer from the anode glow are evaluated in magnetized hydrogen plasmas. The anode glow is initially produced by the additional dc discharge which forms when a cylindrical anode inserted into the plasma diffusion region is positively biased. If the anode voltage is sufficiently high, the anode glow is transformed into an elongated luminous anode double layer in the plasma diffusion region, to which a diverging magnetic field generated by external magnetic coils is coupled. A weakly magnetized plasma is needed for the formation of the anode double layer in our experiments, and there is a magnetic field strength ceiling beyond which the anode double layer disappears. The dependence of the anode double layer structure on the magnetic field strength, anode voltage and the neutral gas pressure is also investigated.
Annealing behavior of Cu and dilute Cu-alloy films: Precipitation, grain growth, and resistivity
Authors: Barmak K. , Gungor A. , Cabral C. , Harper J. M. E.
Published: July 2003

The impact of 11 alloying elements, namely, Mg, Ti, In, Sn, Al, Ag, Co, Nb, and B, at two nominal concentrations of 1 and 3 at. %, and Ir and W, at only a nominal concentration of 3 at. %, on the resistivity and grain structure of copper was investigated. The films were electron beam evaporated onto thermally oxidized Si wafers and had thicknesses in the range of 420–560 nm. Pure evaporated Cu films were used as controls. Isothermal anneals were carried out at 400 °C for 5 h; constant-heating rate treatments, with no hold at the temperature, were done at 3 °C to 650 and 950 °C. In all cases, annealing resulted in the lowering of resistivity compared with the as-deposited state. Furthermore, annealing to a higher temperature resulted in lower, postannealing, room-temperature resistivity, unless the film agglomerated or showed evidence of solute redissolution. Annealing also resulted in significant growth of grains, except for the Nb- and W-containing films. In addition, the grain sizes for the nominally 3 at. %, 400 °C-annealed films were smaller than those for the nominally 1.0 at. % films. The interesting exceptions in this case were the Co-containing films, which had a larger grain size than the pure Cu film, and which, in addition, exhibited a larger grain size for the film with the higher concentration of Co. After the 400 °C anneal, Cu(0.4B) and Cu(1.0Ag) had the lowest resistivities at 2.0 and 2.1 μΩ cm, respectively, and Cu(2.8Co) showed the largest average grain size at 1080 nm. The resistivity and grain size for the pure Cu film after the same anneal were 2.0 μΩ cm and 790 nm, respectively. Precipitation of a second phase was observed in 8 of 20 alloy films annealed at 400 °C. No alloy film simultaneously showed the combination of a low resistivity and a larger grain size than pure Cu.
Analytical model for saturable aging in semiconductor lasers
Authors: Lam S. K. K. , Mallard R. E. , Cassidy Daniel T.
Published: July 2003

We present an expression for the change in threshold current of a semiconductor laser diode as a function of the aging time under constant thermal and current aging. The expression forms a sigmoidal shape with the aging time, and is accurate in describing the increase of threshold current during aging. In the case where nonlinearities or “knees” occur in the threshold current aging curve, a multicomponent defect model is utilized to explain the observation. This multicomponent defect model allows multiple sigmoidal curves to be incorporated into the description. Each sigmoidal curve is thought to represent the growth of one type of defect complex. The parameters in the model are designed to have physical meaning that corresponds to the properties of the laser material. The model is compared to five popular models used to describe the threshold current aging characteristics.
Diffuse dielectric anomaly in perovskite-type ferroelectric oxides in the temperature range of 400–700 °C
Authors: Kang B. S. , Choi S. K. , Park C. H.
Published: July 2003

The diffuse dielectric anomaly by the dielectric relaxation found at the high-temperature region of 400–700 °C was investigated in perovskite-type ferroelectric oxides such as BaTiO3, (Pb,La)TiO3, and (Pb,La)(Zr,Ti)O3 ceramics. We observed that the diffuse dielectric anomaly in perovskite oxides was strongly affected by oxygen-related processing parameters. We have modified the Debye relaxation equation by introducing the mobile dipole of the thermal motion of oxygen vacancies in order to explain the temperature-dependent behavior of the diffuse dielectric anomaly. A relationship between the dielectric polarization/relaxation and the electrical conduction by the thermal motion of oxygen vacancies was microscopically suggested to explain the origin of the diffuse dielectric anomaly found at 400–700 °C in perovskite-type ferroelectric oxides.
Demonstration of high-density three-dimensional storage in fused silica by femtosecond laser pulses
Authors: Cheng Guanghua , Wang Yishan , White J. D. , Liu Qing , Zhao Wei , Chen Guofu
Published: July 2003

Three-dimensional optical recording of high contrast spherical bits (diameter<300 nm) at a density of 500 G/cm3 in fused silica using a Ti:sapphire femtosecond laser is demonstrated. Bits are optically read out using both a confocal and a phase-contrast scheme. The recording density for different materials and recording mechanisms are discussed.
Detector-induced dephasing in quantum-dot cellular automata qubit
Authors: Jiang Z. T. , You J. Q. , Zheng H. Z.
Published: July 2003

We investigate the quantum dynamics of the quantum-dot cellular automata qubit in the presence of a quantum point contact detector by modified rate equations. It is demonstrated that the qubit information can be resolved by measuring the detector current variation. Furthermore, we show that this oscillating current and the electron occupation probabilities in states |b〉 and |c〉 decay drastically as the dephasing rate increases, clearly revealing the influence of the dephasing induced by the detector. Moreover, it is shown that the operation speed of the quantum-dot cellular automata qubit may be adjusted by varying the interdot coupling strength.
Analysis of x-ray diffraction as a probe of interdiffusion in Si/SiGe heterostructures
Authors: Aubertine D. B. , Ozguven N. , McIntyre P. C. , Brennan S.
Published: July 2003

We investigate numerical simulations that utilize a nonlinear interdiffusion solver and dynamical x-ray diffraction calculations to predict the local composition evolution in low Ge concentration Si/SiGe superlattices and their diffraction patterns during annealing. Superlattice satellite peak decay rates are compared with experimentally measured values and simulated diffraction patterns are matched directly to data with good success. The simulations are used to test the sensitivity of x-ray diffraction to various uncertainties commonly encountered when measuring interdiffusion at Si/SiGe interfaces. It is found that the most serious errors result from variations in the Ge content across the surface of the wafer. For example, the resolution limit of most experimental techniques used to measure Ge concentration in a SiGe film is ±1 at. %, for a film with 11% mean Ge concentration annealed for 5 h at 870 °C, this level of error will cause the observed interdiffusivity values to deviate by −25% or +50%. The simulations are further used to show that for Si/SiGe interdiffusion, superlattice diffraction produces valid measurements when applied to 004 superlattice satellite peaks and square wave composition modulations even though it is only exactly applicable to satellite peaks about 000 reflections and to sinusoidal composition modulations. Finally, we show that proper interpretation of x-ray scattering data to extract Si/SiGe interdiffusivity values must account for the strong dependence of the interdiffusivity on Ge concentration.
Structural transformations in laser shock-loaded quartz
Authors: de Rességuier T. , Berterretche P. , Hallouin M. , Petitet J. P.
Published: July 2003

The kinetics of the phase transitions occurring in solid materials under strong compression are an issue of considerable interest, which can be studied using dynamic loading processes. Here, we investigate the phase transformation behavior of quartz under laser-induced shock compression of a very short duration (nanosecond order). Time-resolved measurements, coupled with simple computer simulations, have been used to characterize the pressure pulse induced in the samples. Recovery shots have been performed for postshock spectroscopy, that has revealed permanent structural changes of various types, depending on the experimental conditions (loading pressure, sample thickness, and initial temperature). The results, which differ notably from observations reported under quasi-static compression or longer pulsed loads (microsecond order), suggest a partial amorphization of the shocked samples, as well as the formation of a structure that can be compared to known orthorhombic high-pressure phases. They provide an insight into the early stages of the shock-induced transformation mechanisms in this widely studied material.
Benchmark theoretical study of the ionization threshold of benzene and oligoacenes
Authors: Deleuze M. S. , Claes L. , Kryachko E. S. , François J.-P.
Published: July 2003

In straightforward continuation of Green’s function studies of the ultraviolet photoelectron spectra of polycyclic aromatic compounds [Deleuze et al., J. Chem. Phys. 115, 5859 (2001); M. S. Deleuze, ibid. 116, 7012 (2002)], we present a benchmark theoretical determination of the ionization thresholds of benzene, naphthalene, anthracene, naphthacene (tetracene), pentacene, and hexacene, within chemical accuracy [0.02–0.07 eV]. The vertical ionization potentials of these compounds have been obtained from series of single-point calculations at the Hartree–Fock, second-, third-, and partial fourth-order Møller–Plesset (MP2, MP3, MP4SDQ) levels, and from coupled cluster calculations including single and double excitations (CCSD) as well as a perturbative estimate of connected triple excitations [CCSD(T)], using basis sets of improving quality, introducing up to 510, 790, 1070, 1350, 1630, and 1910 basis functions in the computations, respectively. A focal point analysis of the convergence of the calculated ionization potentials has been performed in order to extrapolate the CCSD(T) results to an asymptotically (cc-pV∞Z) complete basis set. The present results confirm the adequacy of the outer-valence Green’s function scheme for strongly correlated systems. Adiabatic ionization energies have been further determined by incorporating Beck-three-parameter Lee–Yang–Parr functional corrections for zero-point vibrational energies and for geometrical relaxations. Extension of the analysis to the CCSD(T)/cc-pV∞Z level shows that the energy minimum form of the benzene radical cation is an obtuse structure related to the 2B2g state. Isotopic shifts of the adiabatic ionization potentials, due to deuterium substitution of hydrogens, have also been discussed.
Ab initio study of nonhomogeneous broadening of the zero-field splitting of triplet guest molecules in diluted glasses
Authors: Loboda Oleksandr , Minaev Boris , Vahtras Olav , Ruud Kenneth , Ågren Hans
Published: July 2003

Nonhomogeneous broadening of phosphorescence lines and microwave signals in optical detection of magnetic resonance (ODMR) has been calculated using multiconfigurational self-consistent field wave functions and the polarized continuum model. The solvent effects on the zero-field splitting (ZFS) parameters in the low-lying triplet states of aza-aromatic molecules are found to be linearly dependent on the solvent-induced shifts in the phosphorescence frequency in agreement with experimental data. The main contribution to the ZFS originates in the dipolar interaction of the two electron spins, the spin–spin coupling. The spin–orbit coupling (SOC) contribution to the ZFS parameter is much larger for the 3nπ* state of pyrazine compared to the 3ππ* states of quinoline. The second-order SOC contribution to the splitting of the 3nπ* state in the pyrazine molecule does not show any appreciable dependence on the dielectric constant of the solvent. This raises doubts about earlier theories for explaining the inhomogeneous broadening in triplet-state spectra based exclusively on the SOC-induced mixing of the singlet and triplet states. We complete the interpretation of the ODMR spectrum of pyrazine by calculating the hyperfine coupling (HFC) tensors in the lowest triplet state using the UB3LYP hybrid functional. An appreciable solvent-induced rotation of the anisotropic HFC tensor axes has been obtained for the 3nπ* state of pyrazine, in particular for 13C and 14N nuclei. This produces additional nonhomogeneous broadening not only in electron–nuclear double resonance spectra, but also in electron paramagnetic resonance signals because the anisotropic HFC perturbation results in an intensity redistribution among the magnetic transitions between the spin sublevels. A small in-plane rotation of the ZFS tensor axes upon solvation has been predicted for quinoline. Rotation of the magnetic axes induced by the interaction with isotropic solvents can provide a new mechanism for spin-lattice relaxation in the triplet state.
A real-space model of nondynamical correlation
Authors: Becke Axel D.
Published: July 2003

Local density-functional exchange-correlation approximations perform remarkably well in simulating nondynamical or “left-right” correlation in molecular bonds. Yet, they do so in a haphazard and unintentional way. In this work we carefully examine the nature of left-right correlation in multicenter systems and suggest a new nondynamical correlation model of post-Hartree–Fock style. The conventional approach to nondynamical correlation is based on the mixing of nearly degenerate states in electronic configuration space. Our approach, on the other hand, is based entirely in real space and uses a single determinant only.
Accurate analytic He–H2 potential energy surface from a greatly expanded set of ab initio energies
Authors: Boothroyd Arnold I. , Martin Peter G. , Peterson Michael R.
Published: July 2003

The interaction potential energy surface (PES) of He–H2 is of great importance for quantum chemistry, as the simplest test case for interactions between a molecule and a closed-shell atom. It is also required for a detailed understanding of certain astrophysical processes—namely, collisional excitation and dissociation of H2 in molecular clouds—at densities too low to be accessible experimentally. A new set of 23 703 ab initio energies was computed for He–H2 geometries where the interaction energy was expected to be non-negligible. These have an estimated rms “random” error of ∼0.2 mhartree and a systematic error of ∼0.6 mhartree (0.4 kcal/mol). A new analytic He–H2 PES, with 112 parameters, was fitted to 20 203 of these new ab initio energies (and to an additional 4862 points generated at large separations). This yielded an improvement by better than an order of magnitude in the fit to the interaction region, relative to the best previous surfaces (which were accurate only for near-equilibrium H2 molecule sizes). This new PES has an rms error of 0.95 mhartree (0.60 kcal/mol) relative to the 14 585 ab initio energies that lie below twice the H2 dissociation energy and 2.97 mhartree (1.87 kcal/mol) relative to the full set of 20 203 ab initio energies (the fitting procedure used a reduced weight for high energies, yielding a weighted rms error of 1.42 mhartree—i.e., 0.89 kcal/mol). These rms errors are comparable to the estimated error in the ab initio energies themselves; the conical intersection between the ground state and the first excited state is the largest source of error in the PES.
Double-valued potential energy surface for H2O derived from accurate ab initio data and including long-range interactions
Authors: Brandão João , Rio Carolina M. A.
Published: July 2003

In a recent work we have been able to model the long-range interactions within the H2O molecule. Using these long-range energy terms, a complete potential energy surface has been obtained by fitting high-quality ab initio energies to a double-valued functional form in order to describe the crossing between the two lowest-potential-energy surfaces. The two diabatic surfaces are represented using the double many-body expansion model, and the crossing term is represented using a three-body energy function. To warrant a coherent and accurate description for all the dissociation channels we have refitted the potential energy functions for the H2(3Σu+), OH(2Π), and OH(2Σ) diatomics. To represent the three-body extended Hartree–Fock nonelectrostatic energy terms, V1, V2, and V12, we have chosen a polynomial on the symmetric coordinates times a range factor in a total of 148 coefficients. Although we have not used spectroscopic data in the fitting procedure, vibrational calculations, performed in this new surface using the DVR3D program suite, show a reasonable agreement with experimental data. We have also done a preliminary quasiclassical trajectory study (300 K). Our rate constant for the reaction O(1D)+H2(1Σg+)→OH(2Π)+H(2S), k(300 K)=(0.999±0.024)×10−10 cm3 molecule−1 s−1, is very close to the most recent recommended value. This kinetic result reinforces the importance of the inclusion of the long-range forces when building potential energy surfaces.
A real-space analysis of colloidal crystallization in a gravitational field at a flat bottom wall
Authors: Hoogenboom Jacob P. , Vergeer Peter , van Blaaderen Alfons
Published: July 2003

We have studied crystallization in a system of sedimenting colloids in real space using fluorescence confocal microscopy. During sedimentation a gradient in particle concentration develops at the bottom of the sample container. The presence of the lower boundary wall induces layering in the liquid and, upon increasing sediment thickness, crystallization occurs in these liquid layers. Crystallization in the first layer was found to proceed via a first-order transition. The formation of small crystal nuclei in this layer induced epitaxial crystal growth in the second layer on top of these two-dimensional crystal planes, which resulted in both layers undergoing the phase transition simultaneously. The critical osmotic pressures for crystallization at a flat wall as well as for epitaxial crystallization on an already crystallized layer were determined. The nucleation-and-growth mechanism led to a polycrystalline end state. Due to this polycrystallinity we could not determine whether the crystallization process involved an intermediate “hexatic-like” phase. Our results regarding the nature of the transition in the first two layers are in agreement with recent computer simulations [Biben, Ohnesorge, and Löwen, Europhys. Lett. 28, 665 (1994)].
Elastic deformation of helical-conical boron nitride nanotubes
Authors: Xu F. F. , Bando Y. , Golberg D. , Ma R. Z. , Li Y. B. , Tang C. C.
Published: July 2003

Boron nitride nanotubes with hollow conical-helix geometry have exhibited striking flexibility and elasticity comparable to metals. During an electron-beam induced deformation at room temperature, the nanotubes can be bent by a maximum angle as high as 180° and then retrieve the starting morphology without any evidence of structural failure. The outstanding low-temperature elasticity in this nano-material is interpreted by a theoretical model, displaying deformation processes dominated by slide of filaments along with changes in apex angles stepwise. The specific tubular geometry is believed to take advantages of both high stiffness and extraordinary flexibility of BN filaments, and easiness of interlayer slide in graphitic structure, hence leading to high resistance to fracture.
Modeling the bacterial photosynthetic reaction center. VII. Full simulation of the intervalence hole–transfer absorption spectrum of the special-pair radical cation
Authors: Reimers Jeffrey R. , Hush Noel S.
Published: July 2003

ENDOR data suggests that the special-pair radical cation P+ from Rhodobacter sphaeroides is 68% localized on PL while simple interpretations of FTIR difference spectra based primarily on intensity information, but to some extent also bandwidths, suggest near-complete charge localization. We provide a complete a priori spectral simulation of the spectrum of P+ in the range 0–5000 cm−1, including explicit treatment of the high-resolution vibrational transitions, the low-resolution hole–transfer absorption centered at 2700 cm−1, and the resonance with the SHOMO to HOMO transition at 2200 cm−1 that resolve the issues concerning the nature of P+. The description of the vibrational aspects of the problem were taken from results of previous density-functional calculations, and a qualitatively realistic large number of vibrational modes (50 antisymmetric and 18–20 symmetric) were included. To facilitate the calculations, a new representation of the vibronic-coupling Hamiltonian for intervalence hole–transfer or electron–transfer problems is introduced, allowing the spectrum to be simulated efficiently using only up to 4×109 vibronic basis functions and leading also to new general analytical relationships. Observed spectra are fitted using seven adjustable chemical parameters describing the interactions between the four electronic states involved. The resulting fits provide unique descriptions of the parameters that are insensitive to the source of the observed spectrum or the nature of the symmetric modes used in the model, and all fitted parameters are found to be close in value to those from independent estimates. We determine the electronic coupling, antisymmetric-mode reorganization energy, and redox asymmetry to be J=0.126±0.002 eV, λ=0.139±0.003 eV, and E0=0.069±0.002 eV, respectively. Our description forms the basis of understanding for a wide range of other properties observed for Rhodobacter sphaeroides mutants, as well as the properties of the reaction centers from photosystems I, II, etc., facilitating a deeper understanding of the role of the special pair in initiating primary charge separation during photosynthesis.
Anion clusters of anthracene, Ann− (n=1–16)
Authors: Song Jae Kyu , Lee Nam Ki , Kim Jeong Hyun , Han Sang Yun , Kim Seong Keun
Published: July 2003

We studied the anion clusters of anthracene, Ann− (n=1–16), by mass spectrometry, photoelectron spectroscopy, and theoretical calculations. The magic numbers observed at n=5 and 13 indicated formation of the half-filled and completely-filled first solvation shell, respectively. We found that autodetachment could occur via a short-lived excited state of the anion, producing autodetached electrons at a nearly constant kinetic energy, irrespective of the photon energy. Three distinct forms of anion core previously proposed were confirmed that are monomeric, dimeric, and trimeric in nature. As the clusters grow in size from the monomer, the character of the anion core undergoes multiple switching until the first solvation shell is half-filled. Between the half-filled and completelyfilled first solvation shell, the coexistence of the monomeric and dimeric anion cores was observed at certain cluster sizes, most notably at n=8, 10, and 11. Only the monomeric form of anion core was observed once the first solvation shell is completely filled.
Fast time-resolved spectroscopy in shock compressed matter
Authors: Holmes N. C. , Chau R.
Published: July 2003

A new analytical method for optical measurements used to determine time- and spatially-resolved effects in shocked materials is described. This method is quite general and may be applied to a variety of spectroscopic techniques. We apply this method to the interpretation of time-resolved Mie scattering spectroscopy in shocked benzene, showing the effects of nucleation of nanometer-size diamond particles as a separate condensed phase in the shock-reacted material.
Computing the electrostatic free-energy of complex molecules: The variational Coulomb field approximation
Authors: Borgis Daniel , Lévy Nicolas , Marchi Massimo
Published: July 2003

We introduce a novel approximate electrostatic method yielding the electrostatic fields around a molecule of complex shape embedded in a continuum dielectric solvent and the electrostatic solvation free-energies. This method extends the widely used Coulomb field approximation by supposing that the dielectric displacement can be written as the Coulomb field created by a set of fictitious “image” charges placed on the solute atomic sites. The electrostatic problem is solved by minimizing a polarization density functional with respect to the image charges. The method presents computational advantages which are reminiscent to those of the Coulomb field approximation; in particular, the solvation free-energy can be cast into a form which requires only the evaluation of space integrals limited to the interior of the solute. Its accuracy is demonstrated for simple solutes in water, ion pairs, the Tanford–Kirkwood globular protein model, and small polypeptides. It is shown also that our approach provides a systematic correction beyond the Coulomb field approximation which is able to improve the estimation of the atomic self-energies and associated Born radii in the generalized Born method.
Critical-point and coexistence curve properties of a symmetric mixture of nonadditive hard spheres: A finite size scaling study
Authors: Góźdź W. T.
Published: July 2003

The behavior of binary mixtures of nonadditive hard spheres with positive nonadditivity parameter is investigated. The coexistence curves are calculated with a high accuracy in Monte Carlo simulations. Finite size scaling analysis is used to determine precisely the location of the critical point. The calculations strongly support the hypothesis that mixtures of nonadditive hard spheres belong to the Ising universality class. The dependence of the critical packing fraction on the nonadditivity parameter is investigated.
Modeling induced polarization with classical Drude oscillators: Theory and molecular dynamics simulation algorithm
Authors: Lamoureux Guillaume , Roux Benoı̂t
Published: July 2003

A simple treatment for incorporating induced polarization in computer simulations is formulated on the basis of the classical Drude oscillator model. In this model, electronic induction is represented by the displacement of a charge-carrying massless particle attached to a polarizable atom under the influence of the local electric field. The traditional self-consistent field (SCF) regime of induced polarization is reproduced if these auxiliary particles are allowed to relax instantaneously to their local energy minima for any given fixed configuration of the atoms in the system. In practice, such treatment is computationally prohibitive for generating molecular dynamics trajectories because the electric field must be recalculated several times iteratively to satisfy the SCF condition, and it is important to seek a more efficient way to simulate the classical Drude oscillator model. It is demonstrated that a close approximation to the SCF regime can be simulated efficiently by considering the dynamics of an extended Lagrangian in which a small mass is attributed to the auxiliary particles, and the amplitude of their oscillations away from the local energy minimum is controlled with a low-temperature thermostat. A simulation algorithm in this modified two-temperature isobaric–isothermal ensemble is developed. The algorithm is tested and illustrated using a rigid three-site water model with one additional Drude particle attached to the oxygen which is closely related to the polarizable SPC model of Ahlström et al. [Mol. Phys. 68, 563 (1989)]. The tests with the extended Lagrangian show that stable and accurate molecular dynamics trajectories for large integration time steps (1 or 2 fs) can be generated and that liquid properties equivalent to SCF molecular dynamics can be reproduced at a fraction of the computational cost.
Quantum screening effects on the electron-ion occurrence scattering time advance in strongly coupled semiclassical plasmas
Authors: Song Mi-Young , Jung Young-Dae
Published: July 2003

Quantum screening effects on the occurrence scattering time advance for elastic electron-ion collisions in strongly coupled semiclassical plasmas are investigated using the second-order eikonal analysis. The electron-ion interaction in strongly coupled semiclassical plasmas is obtained by the pseudopotential model taking into account the plasma screening and quantum effects. It is found that the quantum-mechanical effects significantly reduce the occurrence scattering time advance. It is also found that the occurrence scattering time advance increases with increasing Debye length. It is quite interesting to note that the domain of the maximum occurrence time advance is localized for the forward scattering case. The region of the scaled thermal de Broglie wave length (λ̄) for the maximum occurrence time advance is found to be 0.4⩽λ̄⩽1.4.
Investigation of a steady-state cylindrical magnetron discharge for plasma immersion treatment
Authors: Levchenko I. , Romanov M. , Keidar M.
Published: July 2003

Ion current distribution in a system with crossed magnetic and electrical fields for plasma immersion ion implantation has been investigated. It is found that the ion current to a target has a nonmonotonic behavior with bias voltage when a magnetic field is applied. For instance, the current density has a maximum of about 150 A/m2 at bias voltage of about 1 kV in the case of a magnetic field parallel to the target of about 0.035 T. These results are explained in terms of ionization by magnetized electrons in the E×B system. Our findings suggest that the system with crossed fields can be used for intense plasma immersed processing.
The n-component KP hierarchy and representation theory
Authors: Kac V. G. , van de Leur J. W.
Published: July 2003

It is the aim of the present article to give all formulations of the n-component KP hierarchy and clarify connections between them. The generalization to the n-component KP hierarchy is important because it contains many of the most popular systems of soliton equations, like the Davey–Stewartson system (for n=2), the two-dimensional Toda lattice (for n=2), the n-wave system (for n⩾3) and the Darboux–Egoroff system. It also allows us to construct natural generalizations to the Davey–Stewartson and Toda lattice systems. Of course, the inclusion of all these systems in the n-component KP hierarchy allows us to construct their solutions by making use of vertex operators.
A note on monopole moduli spaces
Authors: Murray Michael K. , Singer Michael A.
Published: July 2003

We discuss the structure of the framed moduli space of Bogomolny monopoles for arbitrary symmetry breaking and extend the definition of its stratification to the case of arbitrary compact Lie groups. We show that each stratum is a union of submanifolds for which we conjecture that the natural L2 metric is hyper-Kähler. The dimensions of the strata and of these submanifolds are calculated, and it is found that for the latter, the dimension is always a multiple of four.
Diamond deposition on modified silicon substrates: Making diamond atomic force microscopy tips for nanofriction experiments
Authors: Tanasa G. , Kurnosikov O. , Flipse C. F. J. , Buijnsters J. G. , van Enckevort W. J. P.
Published: July 2003

Fine-crystalline diamond particles are grown on standard Si atomic force microscopy tips, using hot filament-assisted chemical vapor deposition. To optimize the conditions for diamond deposition, first a series of experiments is carried out using silicon substrates covered by point-topped pyramids as obtained by wet chemical etching. The apexes and the edges of the silicon pyramids provide favorable sites for diamond nucleation and growth. The investigation of the deposited polycrystallites is done by means of optical microscopy, scanning electron microscopy and micro-Raman spectroscopy. The resulting diamond-terminated tips are tested in ultra high vacuum using contact-mode atomic force microscope on a stepped surface of sapphire showing high stability, sharpness, and hardness.
Stresses arising from a solid state reaction between palladium films and Si(001) investigated by in situ combined x-ray diffraction and curvature measurements
Authors: Gergaud P. , Thomas O. , Chenevier B.
Published: July 2003

By combining simultaneously in situ x-ray diffraction and curvature measurements, the stress evolution during silicide formation in thin Pd films deposited on Si substrates has been reliably investigated. The in-plane strain in the silicide is compressive. The combination method also permits the determination of the elastic constants and thermal expansion coefficient of Pd2Si ([001] fiber texture): Esil=86±8 GPa, νsil=0.34±0.05, and αsil=13.2×10−6 K−1. It is shown that during growth the average compressive stress increases progressively up to a maximum temperature-dependent value (−1.5 GPa at 220 °C) and then vanishes for long annealing. This behavior is in reasonable agreement with the predictions of a model by Zhang and d’Heurle [S. L. Zhang and F. M. d’Heurle, Thin Solid Films 213, 34 (1992)]. A strain gradient in the growing silicide layer, which is predicted by the model, has been shown.