bluuebunny/crossref_metadata_2025 huggingface dataset:
Shock wave initiation of the Ti5Si3 reaction in elemental powders
Authors: Vreeland T. , Montilla K. L. , Mutz A. H.
Published: July 2002

Elemental powder mixes were subjected to plane-wave shock processing which reduced the initial porosity to essentially zero. Two powder mixes in a 5:3 Ti:Si atomic ratio were used: −325 mesh Ti and Si (<45 μm), and −100 mesh Ti and Si (<150 μm) with shock pressures up to 7.3 GPa and shock energies up to 671 J/g. Shock pressures were calculated using hugoniot parameters for porous elemental powder mixtures and shock energies were taken to be the work done by the shock (PΔV/2). Shock energy thresholds for complete reaction of the elemental powders were found which depend upon powder particle size and the initial porosity of the powder. The threshold energy for the larger powder mix was found to be ∼80% larger than that for the smaller powder. A decrease in initial porosity from 0.49 to 0.40 caused an increase in threshold shock energy of about 75% for both powders. At shock energies slightly below the threshold energy, evidence for the reaction of solid Ti and liquid Si was observed in small isolated regions. These regions contained spherical micronodules with the composition of TiSi2 in Si. The results are compared to those of previous studies reported in the literature, and mechanisms for reaction initiation and the observed threshold values are proposed.
Charge transfer from Cu in YBa2Cu3O7−δ and Ba2Cu3O4Cl2 crystals determined by Auger electron spectroscopy
Authors: Ogawa Koji , Noro Sumiko , Maki Kunisuke
Published: July 2002

A formulation is proposed to estimate the charge transfer ΔqCu from Cu to some anions in metal-oxide crystals, using Auger electron spectroscopy. In the formulation, a parameter, the Auger transition probability ω, is introduced, which is defined as the ratio of Auger intensity due to a certain transition to the total Auger intensity due to other transitions when the same core electrons are primarily excited. By applying the formulation to vacuum-cleaved surfaces of YBa2Cu3O7−δ (YBCO) and Ba2Cu3O4Cl2 (BCOC) crystals, we obtained the following results: (1) YBCO cleaves between CuO2 and BaO layers, and the amount of ΔqCu is 2.3 in units of elementary electric charge. The deviation of ΔqCu from 2.0 suggests the existence of Cu3+-like atoms; (2) for BCOC, ΔqCu varies from 3.3 to 1.7 depending on the Ar ion beam sputtering time.
Influence of initial energy modulation on premodulated electron beam propagating through a drift tube
Authors: Uhm Han S.
Published: July 2002

Influence of the initial energy modulation caused by the self-potential depression on the premodulated electron-beam propagation through a drift tube is investigated. The potential depression κ can significantly vary because of the initial current modulation. Thus, beam’s kinetic energy at the injection varies accordingly. A self-consistent nonlinear theory of current modulation of the premodulated electron beam is developed. It is shown that the initial energy modulation caused by the self-potential depression at injection plays a significant role in the current modulation for long range propagation. It is also found from a small signal theory that reduction of the beam’s kinetic energy due to its potential depression accelerates debunching process of the initial current modulation. Although the initial current modulation is debunched quickly for high current beam, amplitude of the current modulation never becomes zero because of the initial energy modulation.
Oxidation of silicon–germanium alloys. I. An experimental study
Authors: Hellberg P.-E. , Zhang S.-L. , d’Heurle F. M. , Petersson C. S.
Published: July 2002

The oxidation of polycrystalline SixGe1−x films with different compositions (i.e., different values of x) is carried out in pyrogenic steam at 800 °C for various lengths of time. It is found that the oxidation is enhanced by the presence of germanium and that the enhancement effect is more pronounced for the films richer in germanium. A mixed oxide in the form of either (Si,Ge)O2 or SiO2–GeO2 is found at the sample surface if the initial SixGe1−x contains more than 50% of germanium. However, a surface silicon cap layer of about 14 nm is found to have a significant impact on the oxidation of the Si0.5Ge0.5 films; it leads to the growth of about 115-nm-thick SiO2 which is about four times that of the SiO2 resulting from the oxidation of the cap layer itself. On the SixGe1−x films with only 30% of germanium, the SiO2 continues to grow after oxidation for 180 min resulting in 233-nm-thick SiO2 which is about 2.4 times greater than the SiO2 grown on 〈100〉 silicon substrates. Rejection of germanium results in piling up of germanium at the interface between the growing SiO2 and the remaining SixGe1−x. Substantial interdiffusion of silicon and germanium takes place in the remaining SixGe1−x. The experimental results are discussed in terms of thermodynamics and kinetics.
Strain dependence of hole mass and optical anisotropy in (110) quantum wells
Authors: Peng L.-H.
Published: July 2002

We have investigated the effects of strain and confinement on the biaxial properties of hole masses and optical transitions in strained (110) quantum wells. Based on a theory of k×p perturbation and deformation potential, we present an analytical method to treat the band mixing effects. We have found it crucial to incorporate the spin-orbital coupling effect in order to quantify the band structure anisotropy. Our calculations indicate the use of strain and confinement can lead to considerable changes in the biaxial properties of a (110) quantum well. In a compressively strained quantum well, the in-plane effective masses of heavy and light holes are shown to have a light component along [1̄10] and [001], respectively. In addition, the optical interband transitions are found to exhibit similar anisotropy as those of the hole masses. In a tensile strained quantum well, however, the in-plane anisotropy of hole masses and optical transitions are found to undergo a character change after passing the anticrossing condition.
Theory of nonlinear optical properties of C60-derived nanotubes
Authors: Xie Rui-Hua , Jiang Jie
Published: July 2002

We study the third-order optical nonlinearities (characterized by the second-order hyperpolarizabilities γ) of C60-derived nanotubes. It is found that the static γ magnitudes of armchair (C60+i×10) and zigzag (C60+i×18) nanotubes obey their own exponent laws given by γ/γ60=(1+i×10/60)3.15 and γ/γ60=(1+i×18/60)2.98, respectively, where γ60 is the static γ magnitude of C60 and i is a positive integer. Also, the dynamical nonlinear optical responses of armchair and zigzag nanotubes are studied in detail. It is found that the symmetry and caps of these nanotubes have a large effect on their third-order optical nonlinearities. By these detailed studies, we find that armchair and zigzag nanotubes can compete with polymeric materials for photonic applications due to their large nonresonant and resonant third-order optical nonlinearities.
Temperature dependence of impact ionization coefficients in p-Si
Authors: Roze K. , Bannov N. A. , Kim K. W. , Holton W. C. , Littlejohn M. A.
Published: July 2002

An efficient full-band Monte Carlo program for high-energy carrier transport is employed to investigate hole impact ionization in p-Si for a range of electric fields up to 800 kV/cm and lattice temperatures between 77 and 450 K. An empirical expression is developed for the temperature dependence of ionization coefficients. The results are compared with those obtained from existing models. The empirical model agrees well with experiments and other numerically intensive models, providing a means to incorporate these effects into other device simulators and reliability models.
Evaluation of adhesion of diamond coating by thermal quench method
Authors: Fan Qi Hua , Fernandes A. , Pereira E. , Grácio J.
Published: July 2002

Adhesion of a diamond coating on metal was quantitatively evaluated by a thermal quench method combined with micro-Raman spectroscopy. It was found that the diamond Raman peak located at wave numbers higher than 1332 cm−1 before the thermal quench, due to the presence of a compressive stress in the diamond film. After a quench from 413 to 273 K (ice water), the Raman peak shifted to ∼1332 cm−1, implying that the diamond film detached from the substrate and became free from biaxial stresses. The coating adhesion is considered to be comparable to the thermal stress induced by the quench. According to a mismatch of the thermal expansion between the film and the substrate, the thermal stress was calculated and the adhesion of a diamond film coated on copper with a titanium interlayer was estimated, being about 2.54 GPa.
Dynamics of electromagnetic field in imploding conducting shell
Authors: Dolinsky Yu. , Elperin T.
Published: July 2002

In this study we derived explicit analytical solutions of two problems. First is a problem of accumulating of magnetic fields in imploding shells with different geometries. The second problem is determining the eigenmodes of a resonator with moving walls. Using the obtained exact solutions we found the regions of validity of an adiabatic approximation when the frequency of the resonator follows the instantaneous values of geometrical parameters, and the amplitude is determined by a conservation of a magnetic flux. The obtained solutions are of interest in design of devices for generation of ultrastrong magnetic fields and powerful electromagnetic radiation.
Localization and thermal escape of excitons in ultrathin ZnSe/ZnS single quantum wells linked to interfacial ZnSe quantum slabs
Authors: Tomasini P. , Arai K. , Lu F. , Zhu Z. Q. , Sekiguchi T. , Yao T. , Shen M. Y. , Goto T.
Published: July 2002

Ultrathin ZnSe/ZnS single quantum well structures have been grown by molecular beam epitaxy, on GaP substrates with high Miller indices. The optical properties of pseudomorphic ZnSe/ZnS single quantum wells have been investigated by conventional optical methods. Power-dependence and temperature-dependence measurements show that structural disorder plays an important role in ZnSe/ZnS ultrathin single quantum wells. The luminescence emission is the collective response of a group of local potentials with quantum dotlike characteristics. Consequently, the luminescence peak is the convolution of individual narrow lines. In addition, the luminescence spectra are dominated by a thermal escape of carriers when investigating the temperature dependence.
High performance Nd–Fe–B sintered magnets made by the wet process
Authors: Takahashi M. , Uchida K. , Taniguchi F. , Mikamoto T.
Published: July 2002

The wet process for the fabrication of Nd–Fe–B sintered magnets has been developed on the mass production scale. In this process, mineral oil is used as a solvent to prevent the milled powder and green bodies from oxidation. It is removed before sintering. As a result, the oxygen uptake during storage is less than 0.01 wt %/day. By reducing the oxygen content and the total rare earth (TRE) content in sintered magnets to less than 0.2 and 29.3 wt %, respectively, magnets with (BH)max/iHc greater than 50 MGOe/10 kOe, 45 MGOe/15 kOe, and 32 MGOe/32 kOe were obtained with high reliability. The magnets produced by this wet process also show better corrosion resistance and 20% higher mechanical strength than those made by conventional methods. These characteristics may be explained by a smaller grain size, smaller TRE content, and higher density produced by the wet process.
Size and dimensionality effect in single-impurity Anderson model
Authors: Chen Feng , Kioussis Nicholas
Published: July 2002

The dimensionality and the size effect on the magnetic properties of the single-impurity symmetric Anderson model in three-, two-, and one-dimensional lattices has been studied using the quantum Monte Carlo method. We have considered clusters with odd number of sites with the impurity placed at the center of the cluster and have employed open boundary conditions. In 3D and 1D lattices, we find that at sufficiently low temperatures, the system can be in either a spin compensated or noncompensated state as the cluster size is reduced. On the other hand, in 2D the system is always in a spin-compensated state independent of the cluster size. These results imply physics beyond that contained in the simplest Kondo model and may explain the discrepancy between two recent experimental results.
A comparative study of the magnetic separation characteristics of magnetotactic and sulphate reducing bacteria
Authors: Bahaj A. S. , James P. A. B. , Moeschler F. D.
Published: July 2002

Many microorganisms have an affinity to accumulate metal ions onto their surfaces which results in metal loading of the biomass. Microbial biomineralization of iron results in a biomass which is often highly magnetic and can be separated from water systems by the application of a magnetic field. This article reports on the magnetic separation of biomass containing microbial iron oxide (Fe3O4, present within magnetotactic bacteria) and iron sulphide (Fe1−XS, precipitated extracellularly by sulphate reducing bacteria) in a single wire cell. Since such bacteria can be separated magnetically, their affinity to heavy metal or organic material accumulation render them useful for the removal of pollutants from waste water. The relative merits of each bacterium to magnetic separation techniques in terms of applied magnetic field and processing conditions are discussed.
Numerical study of the role of a background gas and system geometry in pulsed laser deposition
Authors: Itina T. E. , Katassonov A. A. , Marine W. , Autric M.
Published: July 2002

The transport of laser ablated particles through a Maxwell-distributed ambient gas is simulated by Monte Carlo method. Three system geometry configurations frequently appearing in laser ablation experiments are considered: plume tilting, use of an interacting gas jet, and deposition on a substrate placed perpendicular to the laser-irradiated surface. The influence of the ambient gas on the formation of film thickness profiles and kinetic energy distributions of the deposited particles is studied. The thermalization of the laser plume and the backscattering of the ablated particles due to collisions with the background gas are investigated from two-dimensional film thickness distributions.
Space-charge influenced-injection model for conduction in Pb(ZrxTi1−x)O3 thin films
Authors: Stolichnov I. , Tagantsev A.
Published: July 2002

A comprehensive study of leakage conduction in Pb(ZrxTi1−x)O3films with Pt electrodes is carried out. The conduction properties of films prepared in different ways (sol–gel coating, metalorganic chemical vapor deposition, sputtering) were studied by using different experimental techniques including variation of the applied voltage profile, photoassisted measurements, measurements at elevated temperatures, and variation of the prehistory of the samples. Based on the collective data, it is concluded that two different regimes of carrier injection (with the critical electric field of the crossover between these regimes being independent of the measuring technique) are responsible for true leakage conduction in Pt–Pb(ZrxTi1−x)O3–Pt films. A space-charge influenced-injection model is proposed for the interpretation of the experimental results obtained. This model describes well the main features of the observed current–voltage characteristics and provides a reasonable fit for the current–voltage curves measured at elevated temperatures, the values of the fitting parameters being in good agreement with the results of other studies. The true leakage current in the Pt–Pb(ZrxTi1−x)O3–Pt system is shown to be time dependent because of the influence of the injected charge entrapment during measurement. According to the present results, an activation energy of about 0.9 eV describes the temperature dependence of conduction in the range of 70–200 °C. It is shown that a possible origin of the crossover in the activation energy at the temperature range 120–140 °C widely reported in the literature can be a consequence of the measuring procedure.
Measurement and analysis of time-domain electric field relaxation: The vitreous ionic conductor 0.4 Ca(NO3)2–0.6 KNO3
Authors: Wagner Hermann , Richert Ranko
Published: July 2002

We have measured the electric modulus M(t) of the ionic conductor 0.4 Ca(NO3)2–0.6 KNO3 (CKN) by recording the electric field relaxation E(t) under the constraints of a constant displacement D0. The vitreous material CKN is studied in its glassy state for temperatures 240 K⩽T⩽330 K and for times 7×10−3 s⩽t⩽3×105 s. In this range the dc conductivity varies from 3×10−11 to 5×10−17 S/cm according to Arrhenius behavior. We show that the time dependent resistivity ρ(t), instead of the conductivity σ(t), is an appropriate quantity for assessing the time dependent and steady state effects of ionic diffusivity on the basis of experimental modulus data M(t) recorded in the time domain. Even substantial electrode polarization is not critical with regard to this data analysis.
Helium-implanted silicon: A study of bubble precursors
Authors: Corni F. , Calzolari G. , Frabboni S. , Nobili C. , Ottaviani G. , Tonini R. , Cerofolini G. F. , Leone D. , Servidori M. , Brusa R. S. , Karwasz G. P. , Tiengo N. , Zecca A.
Published: July 2002

The interaction of helium atoms with the radiation damage imparted to (100) silicon single crystal by He+ implantation at 5×1015 cm−2, 20 keV, and liquid–nitrogen temperature is investigated by means of various complementary techniques during and after thermal treatments. Thermal programmed desorption was used to study the dissociation kinetics of helium from the defects and to plan suitable heat treatments for the other techniques. The helium profiles were determined by 8 MeV N2+15 elastic recoil detection, quantitative data on damage were obtained by channeling Rutherford backscattering spectrometry, double crystal x-ray diffraction, and positron annihilation spectroscopy. Isothermal treatments at 250 °C produce first helium redistribution and trapping in vacancy-like defects, rather than helium desorption from traps. The process is thermally activated with an effective activation energy, dispersed in a band from 1.1 to about 1.7 eV. For higher temperature treatments (2 h at 500 °C) the traps are almost emptied and at 700 °C all vacancy-like defects are annealed out. No bubbles or voids are observed by transmission electron microscopy, either in the as-implanted or in annealed samples.
Effect of annealing on Ge-doped SiO2 thin films
Authors: Fujimaki Makoto , Shimoto Shigeyuki , Miyazaki Nahoko , Ohki Yoshimichi , Seol Kwang Soo , Imamura Kazuo
Published: July 2002

Thermal annealing effects on optical and structural properties of Ge-doped SiO2 thin films prepared by the chemical vapor deposition and flame hydrolysis deposition methods were investigated. The thin film prepared by the former method showed inhomogeneous Ge distribution, and Ge oxygen-deficient centers were observed. When it was thermally annealed at temperatures higher than 800 °C, the Ge distribution became uniform. The concentration of oxygen deficient centers was found to decrease with the thermal annealing in an O2 atmosphere, while it increased with the thermal annealing at 1000 °C in N2. This suggests that improvement of the film quality can be achieved by thermal annealing. On the other hand, neither inhomogeneity of Ge distribution nor the appearance of oxygen deficient centers was observed in the film prepared by the latter method, and its film quality was scarcely affected by the thermal annealing.
Thermoluminescence in ZnO doped ZrO2 induced by 2.5–9 keV x-ray synchrotron radiation
Authors: La Lee-Jene , Su Ching-Shen
Published: July 2002

This work investigates the thermoluminescent (TL) glow in 1% ZnO-doped and undoped ZrO2 samples induced by 2.5–9 keV x-ray irradiation. The low-temperature TL peak at 98 °C is the main TL peak, having an intensity approximately 30 times higher than that of the high-temperature TL peak for doped and undoped samples. The high-temperature TL peak appearing at 200 °C for undoped ZrO2 samples shifts to 220 °C for the 1% ZnO-doped ZrO2 samples with higher intensities. In addition, the ratio of low-temperature to high-temperature TL peaks, R, decreases with respect to the energy of irradiated photons for the doped ZrO2 samples. However, R only slightly changes with the energy of irradiated photons for the undoped ZrO2 samples. The TL intensity of ZrO2 effectively responds to the photon energy between 2.5 and 9 keV linearly, and increases with respect to photon energy for ZnO-doped and undoped ZrO2 samples. The experimental results demonstrate that the impurity of 1% ZnO in ZrO2 can increase the intensity of both TL peaks at 98 and 220 °C. Furthermore, a simple phenomenological model is used to elucidate the mechanism of TL in this soft x-ray range.
Cluster glass state and photoinduced effects on the freezing dynamics in KxCo[Fe(Cn)6]y⋅ZH2O (x∼0.16, y∼0.72, z∼4.4)
Authors: Pejakovic D. A. , Manson J. L. , Miller J. S. , Epstein A. J.
Published: July 2002

The magnetic properties and photoinduced magnetization of a Prussian blue analog, KxCo[Fe(Cn)6]y⋅ZH2O (x∼0.16, y∼0.72, z∼4.4) were systematically studied. The frequency dependence of the linear ac susceptibility, the irreversibility in the field-cooled/zero-field-cooled magnetization (MFC/MZFC), and the relaxation of MZFC suggest a cluster glass behavior. Illumination with red light leads to an increase in the magnetic irreversibility. While MFC is significantly increased after illumination, MZFC at low temperatures is decreased. The observed photoinduced magnetic effects are explained within a cluster glass model. The photoinduced increase in the concentration of spins leads to a shift of dynamics toward longer length and time scales, resulting in freezing of spin clusters at a higher temperature.
Negative-effective-mass ballistic field-effect transistor: Theory and modeling
Authors: Gribnikov Z. S. , Vagidov N. Z. , Korshak A. N. , Mitin V. V.
Published: July 2002

We consider p+pp+ diodes, in which the middle p region (base) consists of a p-type quantum well current-conducting channel that is controlled by a gate potential. Hole concentrations in the channel are assumed to be such that a ballistic current flows only in the lowest quantized subband. This subband contains a negative-effective-mass (NEM) section in the dispersion relation. We carry out numerical simulation for realistic designs of this ballistic field-effect transistor (FET) and compare them to simple analytical estimates. We show that three types of self-organized terahertz current oscillations appear in these FETs. Two of these types originate from the NEM instability, while the third arises from the two-stream instability, predicted before for conventional ballistic diodes and FETs. Frequencies of the NEM oscillations are controlled effectively by a gate potential. They are substantially higher than frequencies of two-stream oscillations. The NEM oscillation frequencies exceed 2.5 THz for large enhancing gate potentials.
Influence of the dopant species on radiation-induced defects in Si single crystals
Authors: Khan Aurangzeb , Yamaguchi Masafumi , Kaneiwa Minoru , Saga Tatsue , Abe Takao , Annzawa Osamu , Matsuda Sumio
Published: July 2002

Observations on deep levels introduced in silicon by 1 MeV electron irradiation are reported using boron- or gallium-doped Czochralski (CZ) grown Si space solar cells with different doping concentrations, deep level transient spectroscopy analysis has been carried out to detect the radiation-induced deep levels. Present results provide evidence for new defect states in addition to those previously reported in gallium- and boron-doped Si. The combined boron and gallium data provide enough information to gain valuable insight into the role of the dopants on radiation induced defects in Si. The dominant donor-like electron level at EC−0.18 eV in boron-doped Si has not been observed in gallium-doped CZ-grown Si. A noticeable suppressing generation of the radiation-induced defects in gallium-doped Si is also observed, especially hole level EV+0.36 eV, which is thought to acts as a recombination center.
The cell model for polyelectrolytes with added salt
Authors: Ramanathan G. V. , Woodbury Charles P.
Published: July 2002

Using a comparison theorem from the theory of elliptic equations, we obtain rigorous bounds for the exact solution to the Poisson–Boltzmann equation for the cylindrical cell model, and error bounds for approximations to the exact solution. We present a method for obtaining perturbative solutions, formally to any order, from the exact solution to the salt-free case, and we obtain an explicit solution for the potential correct to the first order. We derive formulas for thermodynamic quantities and compare them with experimental values. We find that the predicted counterion activity coefficients agree well with experimental results; however, agreement of the predicted coion activity coefficient with experiment is only qualitative. We rigorously show that as the concentration of excess salt tends to zero, and as the polyion’s radius becomes very small in comparison to the cell’s radius, the coion activity coefficient given by the Poisson–Boltzmann equation approaches 16/13 for polyion charge densities above a critical value; for subcritical charge densities the limiting value depends on the charge density and lies between 1 and 16/13.
Semiclassical theory of Fermi resonance between stretching and bending modes in polyatomic molecules
Authors: Voth Gregory A. , Marcus R. A.
Published: July 2002

Approximate semiclassical solutions are developed for a system of a Morse oscillator coupled to a harmonic oscillator via a nonlinear perturbation. This system serves as a model for the interaction of an excited stretching mode with a bending mode in a polyatomic molecule. Three semiclassical methods are used to treat this model. In particular, a matrix diagonalization, a two-state model, and a uniform semiclassical approximation (USC) based on Mathieu functions are each used to determine the splittings and state mixing involved in these stretch–bend Fermi resonances. For small perturbations, approximate analytic semiclassical expressions are obtained for the system treated. These analytic expressions are given for the splittings using a two-state or USC method and for the overlaps of the zeroth order states with the eigenstates of the molecule using a USC method.
Multiple-quantum excitation and spin topology filtration in high-resolution NMR
Authors: Levitt M. H. , Ernst R. R.
Published: July 2002

The possibility of designing pulse sequences to select NMR responses from spin systems of a given coupling topology is examined. The selection can be achieved by topology-selective multiple-quantum excitation sequences. The performance of some pulse sequences for exciting high-order multiple-quantum coherences is analyzed and verified experimentally. Treatment is given of the measurement of the intensity of multiple-quantum coherence, the sensitivity and specificity of topology-selective multiple-quantum experiments, and of the relationship between these pulse sequences and ‘‘spin editing’’ methods in heteronuclear NMR. A scheme for general compensation of pulse errors is also described. Practical application of spin topology filtration to two-dimensional NMR of a small protein is demonstrated.
Disorder in crystals of the perylene–pyromellitic dianhydride complex
Authors: Boeyens Jan C. A. , Levendis Demetrius C.
Published: July 2002

Computer simulation of concerted molecular reorientation in terms of lattice potential energy, has been used to rationalize disorder in the room-temperature structure of the perylene–pyromellitic dianhydride molecular complex. A trial structure, based on the simulated disorder of rigid moieties, is not inconsistent with single-crystal x-ray diffractometer data, whereas unconstrained refinement allowing for anisotropic thermal motion converges to a chemically dubious configuration. The disordered trial structure was obtained by simulating gear-like, in-plane concerted rotation of planar molecules as a function of van der Waals lattice energy. Subminima in the potential energy surface were interpreted as possible contributing forms in the disordered arrangement. The total lattice-energy manifold was not explored and positional disorder was not taken into account. The trial structure therefore reflects the main features of the disorder (R=0.14), but is still inadequate to account for all disorder effects measured crystallographically. Vibration analysis as a function of temperature is shown to be an unreliable probe to distinguish between dynamic and static disorder effects.
The second hyperpolarizability of HCl and the effect of basis set variation on this property in hydrogen fluoride by fully coupled Hartree–Fock perturbation theory with a method for circumventing the transformation of two-electron integrals. An a
b i
n
i
t
i
o study
Authors: Waite J. , Papadopoulos M. G.
Published: July 2002

The effect of basis set variation on the second hyperpolarizability γ of HF is discussed. A representative set of basis functions, resulting from the above analysis, has been used to compute the hyperpolarizability of HCl, for which results in good agreement with the experimental value have been attained. Fully coupled SCF perturbation theory, was used for the computation of the hyperpolarizability components. This formalism is derived on the assumption that the basis is orthogonal, thus a procedure is presented which allows the efficient determination of γ, starting with a nonorthogonal set.
On the Yvon–Born–Green approach to the density distribution of inhomogeneous fluid
Authors: Davis H. Ted
Published: July 2002

Fischer’s approximation (ref.1,2) in applying the Yvon−Born−Green equation to the theory of inhomogeneous fluids is used to calculate the density distribution of one dimensional hard−rods in an external field.(AIP)
A Monte Carlo simulation study of electrostatic forces between hexagonally packed DNA double helices
Authors: Guldbrand Lars , Nilsson Lars G. , Nordenskiöld Lars
Published: July 2002

The electrostatic contribution to the force between hexagonally packed B-DNA double helices has been studied using different statistical mechanical descriptions and the Monte Carlo simulation method. The effects of small ion correlations and sizes are considered, and comparison with experimental results is made. It is found that for monovalent counterions, the mean field Poisson–Boltzmann theory can give a reasonable reproduction of the experimental data, even though the simulations show that its description of the electrostatic interaction can be qualitatively wrong. The chemical equilibrium of the ordered DNA phase with the surrounding bulk salt solution is found to be an important feature. Furthermore, the simulations show that the correlation between the ion clouds of different DNA polyions gives rise to a significant attractive contribution to the interaction when divalent cations are present. It is suggested that this electrostatic attraction is an important driving force for the experimentally observed condensation and collapse of DNA solutions to ordered systems, which can be induced by multivalent ions.
Generalized Zimm model for dilute polymer solutions under theta conditions
Authors: Öttinger Hans Christian
Published: July 2002

The consistent averaging approximation for the hydrodynamic interaction is applied to linear chains with Gaussian chain statistics in order to improve the well-known Zimm model, which is based on the preaveraged hydrodynamic interaction. For the resulting generalized Zimm model a rheological equation of state is derived which is then used as a starting point for the derivation of a codeformational memory integral expansion and a retarded motion expansion as well as for numerical investigations. The material functions predicted by the generalized Zimm model for steady shear flow and for small amplitude oscillatory shear flow are discussed in great detail. Finally, the limit of infinitely long chains is thoroughly studied by analytical and numerical techniques.
Oscillatory femtosecond relaxation of photoexcited organic molecules
Authors: Wise F. W. , Rosker M. J. , Tang C. L.
Published: July 2002

The ultrafast relaxation of large organic molecules in solution has been investigated with the transmission correlation technique. The existence of damped sinusoidal decays after photoexcitation has been verified for several organic dyes. This behavior (the first such decay observed on a femtosecond time scale for any material) is explained as quantum beats. For all molecules examined, we have also observed fast initial exponential decays (<100 fs) in addition to previously known picosecond processes.
The configurational entropy of partially ordered ice
Authors: Howe Rachel , Whitworth R. W.
Published: July 2002

A calculation is presented for the configurational entropy arising from partial disorder of the protons in ice. The result is given in terms of a single order parameter, and differs from the usually accepted formula of Nagle. The new result is justified by a simple argument for the case of almost complete ordering.
Density functional theory for the freezing of 1:1 hard sphere mixtures
Authors: Smithline S. J. , Haymet A. D. J.
Published: July 2002

The freezing of 1:1 mixtures of hard spheres is studied using density functional theory. Three different stable solid phases are found: a disordered (phase separated) fcc structure and ordered CsCl and NaCl structures. The relative stability of these phases is predicted as a function of the ratio of the diameters of the spheres.
High-density amorphous ice. IV. Raman spectrum of the uncoupled O–H and O–D oscillators
Authors: Klug D. D. , Mishima Osamu , Whalley Edward
Published: July 2002

The Raman spectrum of the uncoupled O–H and O–D stretching vibrations of HDO dissolved in H2O high-density amorphous ice, that was made from ice Ih, has been measured at 12 K for the sample as recovered at 77 K and after heating it at 5 K min−1 from 12 K to, successively, 80, 90, 100, ..., 170 K and annealing there for 5 min. The distribution of hydrogen-bonded O–H--O and O–D--O distances was estimated by using an experimental curve of the O–H stretching frequency as a function of the O--O distance in ice VII, as suitably adjusted using the frequency in ice I. The most probable bond length, as estimated from the peak frequency, is 2.83 Å, which is 0.08 Å longer than the length in ice I, and the distribution of bond lengths has a width at half-height of 0.19 Å. When the sample was heated to 80, 90, 100, 110, and 120 K in succession, the peak frequency and the width at half-height, as measured at 12 K, decreased by amounts that corresponded to a decrease of the most probable distance from 2.83 to 2.76 Å and a decrease of the width at half-height from 0.19 to 0.09 Å. The phase produced at 120 K is a low-density amorph, and it changes little after heating to 130 and 140 K. After heating to 150 K it changes to ice Ic, whose peak frequency implies an O--O distance of 2.75 Å.
Scavenging of one-dimensional diffusion with random traps
Authors: Glasser M. L. , Agmon Noam
Published: July 2002

For the process of one-dimensional diffusion in a medium with random traps and homogeneous scavenging, we derive exact expressions for the ultimate trapping and scavenging probabilities and for the lifetimes associated with the two processes. This is done for free diffusion, where results agree with those previously found for discrete random walk, as well as for diffusion in a linear potential. The asymptotic expansions, for small and large ratio of trap concentration to scavenging rate, are given and compared numerically to the exact integrals.
Electronic states and spectra of 1,4-naphthoquinone: Evidence for the dual phosphorescence from T 31 (n,π*) and T 32 (n,π*) states
Authors: Itoh Takao
Published: July 2002

Temperature dependence of the emission spectrum of 1,4-naphthoquinone has been investigated in the vapor phase as well as in degassed carbon tetrachloride. It is shown that the emission consists of the dual phosphorescence from T1(n,π*) and T2(n,π*), in addition to the weak delayed fluorescence from S1(n,π*). The occurrence of the T2 phosphorescence is caused by the thermal population from T1. In the vapor phase the T1 and T2 phosphorescence origins are determined to be, respectively, 20 295 and 20 475 cm−1. These observations lead to the conclusion that the weak absorption band at near 490 nm is attributed to the S0→T2 transition. It is indicated that the T2 state has a symmetry of A2.
Two photon resonance enhanced multiphoton ionization spectroscopy of gas phase O2 a 1Δg between 305–350 nm
Authors: Johnson Russell D. , Long George R. , Hudgens Jeffrey W.
Published: July 2002

The resonance enhanced multiphoton ionization (REMPI) spectrum of O2 a 1Δg between 305 and 350 nm is reported. The spectrum is compared to the REMPI spectrum of ground state molecular oxygen in the same electronic energy region, and a number of differences in the respective spectra are observed. Detection limits for O2 a 1Δg by REMPI is calculated to be 5×109 molecule cm−3 in the ion source of the mass spectrometer.
Multichannel interactions in the resonant photoionization of HCl
Authors: White M. G. , Leroi G. E. , Ho M.-H. , Poliakoff E. D.
Published: July 2002

Vibrational state distributions of the A 2Σ+ excited state of HCl+ were measured by dispersed fluorescence following resonant photoionization. Autoionization of levels excited at the NeI resonance line strongly influence the vibrational branching ratios of the A 2Σ+ state although not in accord with the propensity rule expected for vibrational autoionization. Other measurements utilizing total fluorescence yields and synchrotron radiation confirm the presence of competing dissociation channels for autoionizing Rydberg states converging to the A 2Σ+ limit. These results are discussed in terms of the multichannel interactions responsible for determining the observed ion and fragment product distributions.
Molecular properties by a
b i
n
i
t
i
o quasidegenerate many-body perturbation theory effective Hamiltonian method: Dipole and transition moments of CH and CH+
Authors: Sun Hosung , Freed Karl F.
Published: July 2002

The ab initio effective valence shell Hamiltonian method, based on quasidegenerate many-body perturbation theory, is generalized to calculate molecular properties as well as the valence state energies which have previously been determined for atoms and small molecules. Our approach is applicable to both expectation values and transition moments of any molecular property within and between the valence states, respectively. The procedure requires the evaluation of effective operators for each molecular property. Effective operators are perturbatively expanded in powers of correlation and contain contributions from excitations outside of the large multireference valence space. Expectation values and transition moments are the diagonal and off-diagonal matrix elements, respectively, of the effective property operators between the eigenfunctions of the correlated effective Hamiltonian. Calculations for dipole moments of and transition moments between several low lying states of CH and CH+ to first order in the correlation corrections are compared with large configuration interaction calculations to show that our methods provide a useful ab initio formalism for dipole moments.
A relaxation time study of SF5 reorientation in solid tetrakis (pentafluorosulfanyl) hydrazine, (SF5)2NN(SF5)2
Authors: Choudhury M. R. , Harrell J. W. , Nielsen J. B. , Thrasher J. S.
Published: July 2002

An experimental and theoretical study of the 19F relaxation in solid tetrakis (pentafluorosulfanyl) hydrazine due to the reorientation of the SF5 groups has been undertaken. T1 and T1ρ have been measured from 138 to 292 K. T1 is very long, reaching a maximum of 1140 s at the lowest temperature for one sample, and is determined primarily by paramagnetic impurities. T1ρ appears to be influenced by paramagnetic impurities below 227 K and determined by the reorientation of the SF5 groups above 227 K. Expressions for T1 and T1ρ have been derived for two different reorientational models. In the first model the SF5 group is assumed to undergo 90° jumps about the C4 axis; whereas, in the second model the SF5 group is assumed to undergo rotational diffusion about the C4 axis. The T1ρ measurements above 227 K have been fit to the derived expressions to obtain the correlation time for the reorientation.
On the critical behavior of circular intensity difference for a nonpolar fluid composed of chiral molecules
Authors: Lee Dong J. , Kim Shoon K.
Published: July 2002

The effect of double scattering on circular intensity difference (CID) in a nonpolar fluid composed of chiral molecules is explicitly obtained in the critical region, where the Ornstein–Zernike theory holds, using the stationary phase approximation. The double scattering in the direction of orthogonal to the direction of propagation of incident light makes a minor contribution to CID in the critical region, compared with the single scattering. In the direction of propagation of incident light the double scattering plays the most dominant role in the critical region very close to the critical point and the magnitude of CID is same as that in the direction orthogonal to the propagation of incident light, while its effect on CID may be neglected in the critical region far from the critical point.
Dynamic computer simulation of concentrated dispersions
Authors: van Megen W. , Snook I.
Published: July 2002

Brownian dynamics computer simulation has been employed in the diffusive limit to calculate the space-time correlation functions and intermediate scattering functions of concentrated dispersions. At particle concentrations close to crystallization the results indicate that the magnitude of the departure of the single particle probability density from Gaussian behavior is comparable with that calculated and observed for atomic liquids. Also, the inclusion of tensorial hydrodynamic interactions between the particles has only a marginal effect on the magnitude of the non-Gaussian terms.
Theoretical characterization of the minimum energy path for the reaction H+O2→HO2*→HO+O
Authors: Walch Stephen P. , Rohlfing Celeste McMichael , Melius Carl F. , Bauschlicher Charles W.
Published: July 2002

The potential energy surface for the reaction H+O2→HO*2 →HO+O has been characterized in the vicinity of the minimum energy path using CASSCF/contracted CI calculations with a basis set which is triple zeta valence quality plus three sets of polarization functions. CASSCF/CI calculations were carried out along the CCI minimum energy path. The latter calculation shows essentially no barrier for addition of an H atom to O2, in agreement with predictions made in earlier studies. The potential surface for recombination of OH and O is complicated by a crossing, at rOO ≈5.5a0, between the surface for electrostatic (OH dipole–O quadrupole) interaction and that for the formation of an O–O chemical bond. This surface crossing results in a small (≈0.5 kcal/mol) barrier.
Anab initiostudy of the chlorine x-ray emission spectra of the CH3Cl molecule
Authors: Larkins F. P. , Phillips R. A.
Published: July 2002

The x-ray emission processes associated with K, L1, and L2,3 chlorine core shell ionization in the CH3Cl molecule have been studied using ab initio molecular orbital methods. The energy and absolute transition rates of XES processes have been calculated. Comparison with atomic calculations is used to identify weak molecular effects. For the Kβ spectrum relaxation terms must be included to obtain good agreement between the calculated and experimental spectra. It is concluded that three transitions are major contributors to the Kβ spectrum. However, a one-center model is adequate to account for the Kα spectrum. Uncertainty regarding the extent of satellite contribution to the experimental L2,3 spectrum and the breakdown of the single particle model has made direct comparison between theory and experiment difficult. New measurements and a more refined theoretical approach beyond the relaxed Hartree–Fock model are desirable. The L2,3 and L1 or Kβ emission spectra provide complementary information for probing the valence electronic structure of the CH3Cl molecule.
Monte Carlo calculations of cluster statistics in continuum models of composite morphology
Authors: Sevick E. M. , Monson P. A. , Ottino J. M.
Published: July 2002

We describe a simple and efficient algorithm for sampling physical cluster statistics in Monte Carlo simulations of continuum morphology models. The algorithm produces a variety of information including the pair connectedness function, cluster size distribution, and mean cluster size. The approach can be applied to any system, given a definition of a physical cluster for that system. Results are presented for two types of models commonly used in studies of percolation phenomena; randomly centered spheres and the concentric shell (extended sphere) model. The simulation results are used to assess the accuracy of the predictions of the Percus–Yevick closure of the Ornstein–Zernike equation for the pair connectedness function.
On Enskog’s dense gas theory. I. The method of moments for monatomic gases
Authors: Kremer G. M. , Rosa E.
Published: July 2002

Based on Enskog’s dense gas theory and on Grad’s method of moments, the 13-field and the five-field theories are developed for monatomic dense gases of hard spherical particles. The transport coefficients for the five-field theory are obtained from an iteration method akin to the Maxwellian procedure. The problem concerning the propagation of plane harmonic waves of small amplitude is analyzed for both theories.
Dynamics and spectra of a solvated electron in water clusters
Authors: Barnett R. N. , Landman Uzi , Nitzan Abraham
Published: July 2002

The dynamics and spectra of negatively charged water clusters, containing a single excess electron, are investigated. In our calculations the atomic water constituents of the clusters are treated classically while the excess electron is described quantum mechanically using the fast Fourier transform algorithm to solve the Schrödinger equation. Information about ground and excited electronic states corresponding to the equilibrium, finite temperature, ground-state ensemble configurations can be obtained by solving for these states for given nuclear configurations generated via quantum mechanical path-integral molecular dynamics simulations. As an alternative, more efficient way, we introduce the adiabatic simulation method which consists of propagating the nuclei in real time while concurrently annealing the electronic wave functions to their correct values corresponding to the instantaneous, dynamically generated nuclear configurations. The resulting trajectories can be used for analyzing nuclear motion in the ground electronic state as well as for calculating energy distributions for the ground and excited electronic states and the (vertical) excitation line shape. We study the cluster size effect on these quantities, and in particular, by comparing results for(H2O)−64 and (H2O)−128, we conclude that the vertical ionization potential increases while the vertical excitation energy to the bound excited state decreases for larger cluster sizes. For the smallest negatively charged water cluster (H2O)−2, where adiabatic separation of electronic and nuclear motion does not hold, we simulate the time evolution in the TDSCF approximation. The dynamics reveals the close correlation between the electronic binding energy and the cluster dipole, and provides information on intramolecular and intermolecular vibrational motion. Comparison of vibrational density of states evaluated from the nuclear trajectories of the negatively charged and the neutral dimer shows that most of the modes associated with intermolecular motions shift to the red upon electron attachment (a few modes, possibly those associated directly with the magnitude of the total molecular dipole, shift to the blue).