bluuebunny/crossref_metadata_2025 huggingface dataset:
Preparation and transmission electron microscope investigation of sintered Nd15.4Fe75.7B6.7Cu1.3Nb0.9 magnets
Authors: Bernardi Johannes , Fidler Josef
Published: July 2002

Sintered magnets were prepared from jet-milled and hydrogen-decrepitated jet-milled Nd-Fe-B:(Cu,Nb) powder. The HD process started after heating the cast material up to 200 °C. A single-step annealing treatment at 525 °C improved the magnetic properties of all samples. Magnets prepared from the jet-milled powder required a short time sintering (<60 min) to obtain optimum coercivity but had poor density. Magnets prepared from the HD powder showed a higher remanence and energy density product but a slightly smaller coercivity than the magnets prepared from the jet-milled powder. The sintered and annealed magnets contain a multiphase microstructure. Nb was found as small precipitates within the hard magnetic grains and as intergranular boride phase. The addition of Cu lowers the melting point of the liquid phase during sintering and leads to the formation of an orthorhombic NdCu phase that is partly replacing the Nd-rich intergranular phases. The intergranular δ-phase Nd6Fe13Cu was found regularly in the annealed samples. Nd2Fe17 was not detected in the investigated samples.
Two-dimensional laser-induced fluorescence imaging of metastable density in low-pressure radio frequency argon plasmas with added O2, Cl2, and CF4
Authors: McMillin Brian K. , Zachariah M. R.
Published: July 2002

The effect of minor additions of O2, Cl2, and CF4 on the argon metastable relative density and spatial distribution in low-pressure, radio-frequency argon plasmas, generated within a parallel-plate Gaseous Electronics Conference reference reactor, has been investigated using planar laser-induced fluorescence imaging. For the conditions examined (33.3 Pa, 75–300 V, <10 W), the addition of only a few percent of these electron attaching gases was found to decrease the metastable density by as much as an order of magnitude, despite the fact that the excited-state argon emission indicated an increase in the metastable production rate. In the dilute O2/Ar discharges examined here, the spatial distribution of metastables was similar to that of the electropositive, pure argon cases, exhibiting a strong axial peak near the interface between the plasma bulk and the sheath at the powered electrode. In contrast, the addition of either Cl2 or CF4 was found to significantly modify the spatial distribution of the emission intensity and metastable density, resulting in a more symmetric and uniform axial metastable distribution. This change in metastable distribution for these mixtures was particularly apparent at lower powers and/or higher Cl2/CF4 concentrations, and suggests a transition from an electropositive to a somewhat electronegative discharge.
Electron beam induced crystallization of a Ge-Au amorphous film
Authors: Ba Long , Qin Yong , Wu Ziqin
Published: July 2002

An amorphous Ge-Au(a-Ge-Au) film prepared by vacuum deposition was studied in situ and irradiated by an electron beam in a transmission electron microscope. The amorphous Ge-Au film can be crystallized rapidly and locally by electron beam irradiation, forming polycrystalline metastable Ge0.4Au0.6 tetragonal phase and then decomposing to polycrystalline Ge and polycrystalline Au at increased irradiation dose. The results suggest that the crystallization of a-GeAu film, the formation of Ge0.4Au0.6 and the decomposition of the metastable Ge0.4Au0.6 are dependent on the temperature rise of the irradiation process.
Magnetic-field-induced strain in Ni2MnGa shape-memory alloy (abstract)
Authors: Ullakko K. , Huang J. K. , Kanter C. , Kokorin V. V. , O’Handley R. C.
Published: July 2002

Rare-earth/transition-metal alloys can exhibit magnetostrictive strains of order 0.17% in modest fields. Larger strains are of interest for many actuator applications. Certain alloys that undergo martensitic transformations exhibit a shape–memory effect that can yield strains up to 20% upon heating the deformed martensitic phase and they can show superelasticity upon application of a small stress. These methods of activation can be a disadvantage for many applications; magnetic activation of a shape–memory effect is desired. Several magnetic shape–memory alloys exist, among them intermetallics based on Ni2MnGa, which are the subject of this work. These materials experience a 6.6% c-axis contraction on cooling through the martensitic transition temperature, which is near 273 K; this strain is accommodated by formation of an ensemble of strained twin crystals separated by twin boundaries. Strains of several percent could be produced magnetically if the twin boundaries could be moved under application of a magnetic field. We report observations of strains of nearly 0.2% induced along [001] in unstressed crystals of Ni2MnGa with magnetic fields of 8 kOe applied at 265 K (Fig. 1). (This strain is an order of magnitude larger than the magnetostrictive strain we measure in the parent Heusler phase at 283 K.) Our data suggest that these giant strains are associated with the superelastic motion of twin boundaries in the martensitic phase, which is stable below about 274 K; the strength of the measured anisotropy energy density, MSHa/2 is comparable to the elastic energy density, eσ/2, needed for superelastic twin boundary motion.1 The strains we observe are equivalent to those achieved in terfenol-D and represent only a small fraction of the strain available if the twin variants of the martensitic phase can be oriented prior to application of a field.
Crack propagation in piezoelectric ceramics: Effects of applied electric fields
Authors: Wang Hongyu , Singh Raj N.
Published: July 2002

Crack propagation in a piezoelectric lead–zirconium–titanate (PZT) material under simultaneous mechanical loading and applied electric fields is studied using the Vickers indentation technique. It is demonstrated experimentally that electric fields can inhibit or enhance crack propagation in piezoelectric materials. Cracks introduced by indentation are observed to propagate less under a positive applied electric field (the polarity of the field was the same as that for poling), whereas under a negative applied electric field, crack propagation is enhanced. Such an effect is observed to be more profound with increasing electric-field strength and decreasing mechanical loading. Attempts are made to compare these experimental observations with the results of various theoretical analyses. A mechanism for the change in crack propagation behavior of the piezoelectric PZT material under applied electric fields is presented.
Electrodynamic trapping of charged particles in a monopole field
Authors: Peik E. , Fletcher J.
Published: July 2002

While the widely used electrodynamic quadrupole traps are based on the dynamic stabilization around a field minimum, we demonstrate here the trapping of charged particles close to a maximum of the electric field. In the experiment charged steel spheres are trapped in the monopole potential of a tip electrode using a combination of static and alternating voltages. As a periodically driven nonlinear oscillator this system might show chaotic dynamics. A period doubling bifurcation was observed experimentally.
Magnetic levitation for hard superconductors
Authors: Kordyuk Alexander A.
Published: July 2002

An approach for calculating the interaction between a hard superconductor and a permanent magnet in the field-cooled case is proposed. The exact solutions were obtained for the point magnetic dipole over a flat ideally hard superconductor. We have shown that such an approach is adaptable to a wide practical range of melt-textured high-temperature superconductors’ systems with magnetic levitation. In this case, the energy losses can be calculated from the alternating magnetic field distribution on the superconducting sample surface.
Dispersion effects of Raman lines in carbons
Authors: Wang Zhaoxiang , Huang Xuejie , Xue Rongjian , Chen Liquan
Published: July 2002

Raman spectra of highly oriented pyrolytic graphite (HOPG), natural polycrystalline graphite (PCG), high pressure graphite (HPG), and low-temperature pyrolyzed polyfurfuryl alcohol (PFA) have been measured. Comparing the Raman spectra of each sample illuminated with laser beams of 488.0 nm and 1064.0 nm wavelengths, respectively, unambiguous dispersion effects have been observed for the D′-line in HOPG and in other forms of PCG as well as for their D lines. Comparisons of the intensity ratios of the D′ line to the D line (ID′/ID) of HOPG and polycrystalline indicate that the dispersion effects of the D′ line and the D line may have the same physical origin. In addition, it has also been observed that the G-line position of low-temperature pyrolyzed PFA is sensitive to the excitation wavelength while that of HOPG, PCG, and HPG is not. This phenomenon is attributed to the nanometer size of the microcrystalline graphites by analyzing the factors that may induce such a dispersion effect.
Optical-field calculations for lossy multiple-layer AlxGa1−xN/InxGa1−xN laser diodes
Authors: Bergmann M. J. , Casey H. C.
Published: July 2002

Optical-field profiles in wide-band-gap AlxGa1−xN/InxGa1−xN multiple-quantum well (MQW) separate-confinement heterostructure (SCH) laser diodes (LDs) were calculated using a 2×2 transfer-matrix approach that accommodates complex refractive indices. The refractive indices of AlxGa1−xN and InxGa1−xN were approximated by shifting the refractive index of GaN according to the band-gap energy of the solid solution. Current LDs were analyzed and show reasonable optical confinement. Optimization of the SCH waveguide for a three MQW active region was performed by varying the waveguide and cladding layer thicknesses. For 0.8μm thick Al0.10Ga0.90N cladding layers, waveguides on sapphire and SiC substrates had a maximum confinement factor of ∼3.3%. Layers outside of the waveguide strongly affected the optical field for thin (∼0.4 μm) cladding layer thicknesses and resulted in resonant coupling of the light out of the waveguide. Sapphire substrates were found to enhance light confinement, while SiC substrates were found to reduce optical confinement as the cladding layer thickness is reduced.
Electronic structure classifications using scanning tunneling microscopy conductance imaging
Authors: Horn K. M. , Swartzentruber B. S. , Osbourn G. C. , Bouchard A. , Bartholomew J. W.
Published: July 2002

The electronic structure of atomic surfaces is imaged by applying multivariate image classification techniques to multibias conductance data measured using scanning tunneling microscopy. Image pixels are grouped into classes according to shared conductance characteristics. The image pixels, when color coded by class, produce an image that chemically distinguishes surface electronic features over the entire area of a multibias conductance image. Such “classed” images reveal surface features not always evident in a topograph. This article describes the experimental technique used to record multibias conductance images, how image pixels are grouped in a mathematical, classification space, how a computed grouping algorithm can be employed to group pixels with similar conductance characteristics in any number of dimensions, and finally how the quality of the resulting classed images can be evaluated using a computed, combinatorial analysis of the full dimensional space in which the classification is performed.
Effect of finite magnetic film thickness on Néel coupling in spin valves
Authors: Kools J. C. S. , Kula W. , Mauri Daniele , Lin Tsann
Published: July 2002

Spin valves are widely studied due to their application as magnetoresistive material in magnetic recording heads and other magnetic field sensors. An important film property is the interlayer coupling field (called offset field Ho or ferromagnetic coupling field Hf). It has been shown that the Néel model for orange-peel coupling can be applied successfully to describe this interlayer coupling. The waviness associated with the developing granular structure is thereby taken as the relevant waviness. The original Néel model describes the ferromagnetic magnetostatic interaction between two ferromagnetic layers, of infinite thickness, separated by a nonmagnetic spacer with a correlated interface waviness. In this article, this physical picture is refined to account for the effect of the finite thickness of the magnetic films in a spin valve. Magnetic poles created at the outer surfaces of the magnetic layers result in an antiferromagnetic interaction with the poles at the inner surface of the opposite layer. A simple model is presented for the different interactions in a top spin valve (columnar structure with cumulative waviness on a flat substrate) and for a bottom spin valve (columnar structure with conformal waviness on a way substrate). Comparison to experimental data, shows that the free and pinned layer thickness dependence can be understood from this refined picture.
An alternative geometry for quantum-dot cellular automata
Authors: Gin Aaron , Tougaw P. Douglas , Williams Sara
Published: July 2002

Previous research on quantum cellular automata devices has restricted the cells to a single plane. In this paper, a noncoplanar arrangement is considered. These noncoplanar devices are found to be capable of implementing the same logical functions as conventional quantum cellular automata cells, and they typically require 50% of the area required by the conventional devices. Devices simulated in this paper include wires, corners, majority logic devices, wire crossings, inverters, and exclusive-OR gates. The proposed cellular arrangement is actually biplanar, which simplifies fabrication significantly.
Magnetization process of noninteracting ferromagnetic cobalt nanoparticles in the superparamagnetic regime: Deviation from Langevin law
Authors: Respaud M.
Published: July 2002

Magnetization measurements were performed and analyzed on two systems of noninteracting superparamagnetic cobalt nanoparticles displaying narrow size distributions. The experiments were carried out above the blocking temperature, i.e., in the superparamagnetic regime. Several deviations from classical Langevin behavior were pointed out, in particular, at high field and near the blocking temperature. These deviations were interpreted in terms of anisotropy effects on the magnetization process and analyzed using theoretical expressions including uniaxial anisotropy energy. The effect of the anisotropy on the theoretical magnetization curves plotted versus applied field divided by the temperature are characterized by: (i) superposition at low fields, (ii) deviations in the approach to saturation area, and (iii) decrease of the magnetization when lowering the temperature. These three characteristics are present in our experimental curves. It allows us to determine the magnetic moment of the particle in the low-field region, and then the effective anisotropy from the approach to the saturation area for each sample, validating therefore, our theoretical expressions. A more detailed analysis of the experimental magnetization curves showed that the magnetization process proceeds in two steps: orientation of the magnetic moment of the particle, and orientation of the canted spins in the particle along the applied field. Finally, the values of the effective anisotropy are compared with those determined by other techniques.
Vapor sensing using the optical properties of porous silicon Bragg mirrors
Authors: Snow P. A. , Squire E. K. , Russell P. St. J. , Canham L. T.
Published: July 2002

Large wavelength shifts have been measured in the reflectivity spectra of Bragg mirrors etched in porous silicon after exposure of the mirrors to vapor from organic solvents. The shift in the Bragg wavelength of the mirror arises from refractive index changes, induced by capillary condensation of the vapor in the mesoporous silicon, in the layers of the mirrors. Modeling of the reflectivity changes shows that the layer liquid volume fraction occurring in the measurements was 0.29 for acetone and 0.33 for chlorobenzene. Time-resolved measurements show that condensation occurs on the time scale of tens of seconds.
Thermally activated injection limited conduction in single layer N,N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′-biphenyl-4,4′-diamine light emitting diodes
Authors: Campbell A. J. , Bradley D. D. C. , Laubender J. , Sokolowski M.
Published: July 2002

Impedance, current–voltage–luminosity and spectral measurements have been carried out on indium-tin-oxide/N,N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′-biphenyl-4,4′-diamine (TPD)/Al light emitting diodes. The devices have a blue/violet emission with a spectrum peaked at 404 nm. Capacitance–voltage measurements show that at zero bias the devices are fully depleted. The impedance measurements show that the devices can be modeled on a single, frequency-independent parallel resistor-capacitor RPCP circuit with a small series resistance RS. RP changes with applied bias and temperature, while CP remains constant. The values of CP give εr=3.0±0.3. Analysis of the current–voltage (J–V) characteristics show that the dominant conduction mechanism cannot be either ohmic, trap free space charge limited, or tunneling injection. The temperature and thickness dependence indicate that it must be either thermionic emission or thermally assisted tunneling, the carrier density varying from about 1010/1011 to 3×1013 cm−3 over the measured bias range. The EL efficiency increases 20 fold upon cooling but shows little variation with bias at all temperatures, indicating the same mechanism is responsible for the injection of both holes and electrons. Modeling the results with thermionic emission suggests that image force lowering is responsible for the variation of the current with applied bias, but the calculated injection barrier height and Richardson constant are much smaller than expected. This cannot be explained by models based on a backflowing surface recombination current due to the high carrier mobility found in TPD.
Observations of reaction zones at chromium/oxide glass interfaces
Authors: Jiang Nan , Silcox John
Published: July 2002

Cr is often used in thin metallic film structures on oxide glasses since it exhibits good adhesion. The most likely explanation of the basic adhesion mechanism is the formation of a graded metal oxide layer at the interface. In general, details of the interface properties are needed to get a complete understanding of phenomena such as adhesion. We report here observations of interface structures using spatially resolved electron energy loss spectrometry with a small probe (2 Å) scanning transmission electron microscope. Two interfaces, evaporated Cr/alkaline earth boroaluminosilicate glass and sputtered Cr/barium boroaluminosilicate glass, are examined. As expected from the classical adhesion theory, very thin partially oxidized intermediate layers are seen in both samples. However, a 5 nm Cr diffusion layer is also found in the evaporated Cr/glass system (without heat treatment), but it appears absent in the sputtered Cr glass system (with heat treatment). This difference suggests that the mechanisms of the adhesion of Cr to oxide glass are dependent on interface history.
Photothermal ionization spectroscopy of shallow nitrogen donor states in 4H–SiC
Authors: Chen C. Q. , Zeman J. , Engelbrecht F. , Peppermüller C. , Helbig R. , Chen Z. H. , Martinez G.
Published: July 2002

Photothermal ionization spectroscopy (PTIS) measurements were carried out on a free-standing, high purity and high quality 4H–SiC epitaxial layer at various temperatures. The two step photothermal ionization process is clearly reflected in the temperature dependence of the photoconductivity. The PTI spectrum at a temperature of 25.6 K exhibits one order of magnitude higher energy resolution than the infrared absorption spectra of 4H–SiC bulk material. It reveals five strong, well resolved electronic transition lines associated with the shallow nitrogen donor. The ionization energy of the shallow nitrogen donor is deduced to be 60.2±0.5 meV based on experimental results. Furthermore, PTI magnetospectroscopy measurements were performed to investigate the symmetry properties of these transitions in Faraday configuration. No linear Zeeman splitting is observed, however, these lines show a diamagnetic shift. It indicates that the excited states of the shallow nitrogen donor are nondegenerate at zero magnetic field, which is consistent with the fact that the effective mass tensor of 4H–SiC has three different diagonal components.
Local vibrational modes of impurities in semiconductors
Authors: McCluskey M. D.
Published: July 2002

Omnipresent impurities such as carbon, oxygen, silicon, and hydrogen play important roles, both detrimental and beneficial, in the fabrication of solid-state devices. The electronic and vibrational properties of semiconductors are significantly altered by the presence of impurities. Atoms that are less massive than the host atoms, typically, show local vibrational modes (LVMs). Unlike lattice phonons, LVMs are localized in both the real and frequency domains, giving rise to sharp peaks in infrared-absorption and Raman-scattering spectra. The isotopic composition of the impurity and the surrounding atoms results in well-defined shifts in the vibrational frequencies. In Ge, GaAs, and CdTe, the host–isotope disorder leads to complex vibrational spectra that can be simulated by empirical, quasimolecular models. External parameters such as temperature and pressure (uniaxial and hydrostatic) have been tuned over a wide range to yield information about symmetry and impurity–host interactions. In this article, issues relating to LVMs in semiconductors are reviewed, with an emphasis on infrared and Raman spectroscopy. Experimental and theoretical studies of LVMs in semiconductors are described for several representative examples.
Condensation rate on a black sphere via Fokker-Planck equation
Authors: Kumar Vinod , Menon S. V. G
Published: July 2002

The general solution of the Fokker–Planck equation near an absorbing sphere is expressed as a sum of asymptotic and transient terms employing Burnett function expansion procedure. The particle condensation rate on the surface of the black sphere is then calculated using truncated expansions. Numerical results are presented showing the convergence of the condensation rate as the order of the approximation is increased.
Radiative decay of multiply excited core hole states in H2O
Authors: Rubensson Jan-Erik , Petersson Lennart , Wassdahl Nial , Bäckström Mats , Nordgren Joseph , Kvalheim Olav M. , Manne Rolf
Published: July 2002

A high resolution x-ray emission spectrum of water vapor has been recorded, using a 7 keV electron beam and a 10 m grazing incidence spectrometer. The spectrum is interpreted in terms of dipole transition moments between CI wave functions built from separately optimized molecular orbitals. The principal features of the spectrum are reproduced considering transitions in the singly and doubly ionized species, giving rise to main lines and Wentzel–Druyvesteyn satellites, respectively.
Molecular orbital predictions of the vibrational frequencies of some molecular ions
Authors: DeFrees D. J. , McLean A. D.
Published: July 2002

Recent spectroscopic advances have led to the first determinations of infrared vibration-rotation bands of polyatomic molecular ions. These initial detections were guided by ab initio predictions of the vibrational frequencies. The calculations reported here predict the vibrational frequencies of additional ions which are candidates for laboratory analysis. Vibrational frequencies of neutral molecules computed at three levels of theory, HF/3-21G, HF/6-31G*, and MP2/6-31G*, were compared with experiment and the effect of scaling was investigated to determine how accurately vibrational frequencies could be predicted. For 92% of the frequencies examined, uniformly scaled HF/6-31G* vibrational frequencies were within 100 cm−1 of experiment with a mean absolute error of 49 cm−1. This relatively simple theory thus seems suitable for predicting vibrational frequencies to guide laboratory spectroscopic searches for ions in the infrared. Hence, the frequencies of 30 molecular ions, many with astrochemical significance, were computed. They are CH+2, CH+3, CH+5, NH+2, NH+4, H3O+, H2F+, SiH+2, PH+4, H3S+, H2Cl+, C2H+, classical C2H+3, nonclassical C2H+3, nonclassical C2H+5, HCNH+, H2CNH+2, H3CNH+3, HCO+, HOC+, H2CO+, H2COH+, H3COH+2, H3CFH+, HN+2, HO+2, C3H+, HOCO+, HCS+, and HSiO+.
A study of the structure of water and its dependence on solutes, based on the isotope effects on solvation thermodynamics in water
Authors: Marcus Y. , Ben-Naim A.
Published: July 2002

The solvation of a particle in water, whether a ‘‘foreign’’ particle or a molecule of water, is expressed in terms of a non-hydrogen-bonding contribution to the pair potential and a contribution from hydrogen bonding. Only the latter depends on whether the water is H2O or D2O, since the hydrogen bond energy differs in these kinds of water. The effect of the isotopic constitution of the water on the solvation is examined for the cases of an inert solute, a water molecule, and a hydrogen-bond-forming solute. The average number of hydrogen bonds in which any water molecule in pure water participates is deduced from the Gibbs free energy and the enthalpy of condensation of H2O, D2O, and T2O molecules into the respective liquids. This number decreases from about 0.95 at 0 °C to about 0.35 at 100 °C, being a few percent higher in D2O than in H2O. The Gibbs free energies of transfer of gaseous solutes, nonelectrolytes, and ions from light to heavy water are interpreted in terms of the average number of hydrogen bonds broken or made in the water per particle of solute.
Theory of electronic transport in molecular crystals. II. Zeroth order states incorporating nonlocal linear electron–phonon coupling
Authors: Munn R. W. , Silbey R.
Published: July 2002

A generalized transformation is applied to a model Hamiltonian incorporating both local and nonlocal exciton–phonon coupling. For the application of the usual transport theory, the difference between the transformed coupling and its thermal average over the free phonon ensemble must be small. This is achieved by a temperature-dependent set of transformation coefficients defined by a transcendental equation which is soluble numerically after some approximation. Nonlocal coupling increases both the exciton binding energy and the usual band narrowing, but it also changes the band shape in a way which may introduce new minima and band broadening. The exciton velocities are then no longer related to the exciton hopping and scattering rates in the same way as in local coupling.
The equation of state of dense argon: A comparison of shock and static studies
Authors: Ross M. , Mao H. K. , Bell P. M. , Xu J. A.
Published: July 2002

New static high pressure measurements for room temperature solid argon to 800 kbar are in excellent agreement with one predicted from a theoretical reduction of shock wave data. The results demonstrate the agreement between shock and static techniques even in cases where shock temperatures are extremely high and a large thermal correction is required to reduce the Hugoniot to an isotherm. The results suggest that solid argon may provide a useful static pressure standard up to about 3–4 Mbar.
The ultraviolet absorption spectrum of the Ã 1A″2←X̃ 1A1 transition of jet-cooled ammonia
Authors: Vaida V. , McCarthy M. I. , Engelking P. C. , Rosmus P. , Werner H.-J. , Botschwina P.
Published: July 2002

The Ã←X̃ absorption spectra of NH3 and ND3, recorded in a cold molecular jet, are presented. Vibrational band progressions resolvable up to v′2=14 appear. No other vibrations are present, either alone or in combinations. Relative band intensities for v2 progressions are recorded, and the homogeneous lifetime broadenings of vibrational levels of the Ã state are reported. The FWHM linewidths span 34–293 cm−1 over all bands of NH3 and 30–135 cm−1 over the v′2=2 through 14 bands of ND3. In general, the rate of dissociation increases nonlinearly with vibrational energy. The band intensity alternation, previously observed only in matrix spectra below 15 K, has been observed in these very cold gas phase samples.
Kinetics of the activated dissociative adsorption of methane on the low index planes of nickel single crystal surfaces
Authors: Beebe Thomas P. , Goodman D. Wayne , Kay Bruce D. , Yates John T.
Published: July 2002

We have measured the kinetics of the methane decomposition reaction on Ni(111), Ni(100), and Ni(110) single crystal surfaces under the high incident flux conditions of 1 Torr methane. We find for these processes apparent activation energies of 12.6, 6.4, and 13.3 kcal mol−1, respectively. Initial methane sticking coefficients at 500 K vary with the Ni surface, but are all ∼10−8 to 10−7. The Ni(110) surface is the most active, followed by Ni(100) and Ni(111). A large (∼ factor of 20) kinetic isotope effect is seen for CH4 vs CD4 on the Ni(100) surface, whereas none is seen on the Ni(110) surface. A comparison is made between measured thermal sticking coefficients and those calculated from the results of recent molecular beam experiments of CH4 on Ni(111) and Ni(100) surfaces. Agreement of our results with the Ni(100) beam results is poor, whereas agreement with the Ni(111) beam results is very good. A comparison is also made between our results and rates of the catalytic steam reforming reaction of methane.
Electrolytic conduction past arrays of charged insulating spheres
Authors: Sen Pabitra N.
Published: July 2002

The dc electrical conductivity σ for a three-dimensional (3D) periodic array of charged spheres immersed in an electolyte is computed by augmenting a method due to Lord Rayleigh to include diffusion currents and by using Fixman’s method for the double layer. The key dimensionless parameter that represents the surface effects emerges from Fixman’s boundary conditions, and is given by ξ=Ω+r+/(N0a), where Ω+ is the surface ion number density, r+ is the ratio of the average diffusion coefficient in the double layer to that outside, N0 is the bulk ion density far from the double layer, and a is the particle radius. This calculation extends our previous calculation for 2D and supports our previous assertion that the 3D calculation differs only in details. We find that σ is a nonlinear function of ξ and hence of the electrolyte conductivity σw. σ vs σw shows the experimentally observed bend at low salinity. At high salinity, σ depends linearly on σw, resembling the empirical relations that are commonly used in exploration geophysics. In addition, our theory explains the observed cation dependencies, the particle size dependence of the surface effect, and the geometry dependence of the parameters in the empirical laws. For a large range of salinity, the theory is in good agreement with the data on latex suspensions both at high and low particle concentrations. Data on ion exchange resins are best explained by assuming that the particles are conductive. The nonspherical shape of clay particles rules out quantitative application of the theory; nevertheless, qualitative agreement is obtained. In particular, it is shown that for clay suspensions the isoconductance point, σ=σw, is weakly dependent on concentration, but depends on cation type and size of particle.
Circular dichroism of oriented α helices. I. Proof of the exciton theory
Authors: Olah Glenn A. , Huang Huey W.
Published: July 2002

Moffitt’s exciton theory for α helices, an important cornerstone of the circular dichroism (CD) theory for biopolymers, was recently cast in doubt by a linear dichroism measurement of electric field oriented polypeptides [Yamaoka et al., J. Am Chem. Soc. 108, 4619 (1986)]. In particular the prediction that the polarization of an exciton split component at 208 nm should be parallel to the α-helical axis was not borne out. This revealed the inadequacy of previous experiments which used long polypeptides to prove the theory. We performed two experiments to measure the effect of orientation of α helices on CD, one with a short membrane peptide, alamethicin, oriented in defect-free multibilayers and another with long polypeptides oriented with electric field. We found the result of the experiment with alamethicin to be consistent with the exciton theory. An elaborate procedure was established to measure the CD of protein molecules embedded in lipid multibilayers with light incident on bilayers at various angles (the helical sections of alamethicin are perpendicularly embedded in bilayers). Thus we are able to measure the polarization of the 208 nm band and prove that it is indeed parallel to the α-helical axis. The problem of electric field oriented polypeptides is discussed in paper II.
Circular dichroism of oriented α-helices. II. Electric field oriented polypeptides
Authors: Olah Glenn A. , Huang Huey W.
Published: July 2002

A transition moment polarized parallel to the helical axis, as predicted to occur in an exciton split component (near 208 nm) of the peptide ππ* transition, can be exploited to study the structural orientation of the helical sections in proteins. In order to test this important feature of the exciton theory for helical polymers, many investigators have measured the polarization of electric field oriented polypeptides. The qualitative support for the theory provided by early experiments was recently overshadowed by a more extensive measurement [Yamaoka et al., J. Am. Chem. Soc. 108, 4619 (1986)] which showed the polarization of the 208 nm band not to be parallel to the orientation axis of the polypeptides. This finding is confirmed in this paper by our circular dichroism measurement of electric field oriented poly-γ-benzyl-glutamate in ethylene dichloride and in dioxane. Thus the exciton theory appears to be contradicted, as similarly concluded by Yamaoka et al., if the polypeptides are rod-like and rigid. However, our theoretical study of the bending effect on long polypeptides shows that our experiment is in fact consistent with the prediction of the exciton theory, which was unambiguously proven recently in paper I by using short membrane peptides [Olah and Huang, J. Chem. Phys. 89, 2531 (1988)].
Vibrational dephasing in liquid nitrogen at high densities studied with time-resolved stimulated Raman gain spectroscopy
Authors: Kroon Ron , Baggen Michael , Lagendijk Ad
Published: July 2002

In highly compressed liquid nitrogen at 295 K we observe a sharp increase in the vibrational dephasing rate T−12 with increasing density, indicating that at high fluid densities (2.3 to 2.9×1022 cm−3) the modulation of the vibrational frequency shows a stronger density dependence than the bath correlation time due to the action of short range repulsive forces. The observed behavior shows good agreement with results of previously published molecular dynamics simulations and a fit to a binary interaction model. When the phase transition from the liquid to the solid state takes place we observe a sudden drop in T–12 by a factor of 4. We also present the variation of the Raman line shift of liquid and solid nitrogen with pressure over a region of 0.5 to 4.6 GPa at 295 K.
Coherent radiative control of IBr photodissociation via simultaneous (ω1,ω3) excitation
Authors: Chan C. K. , Brumer P. , Shapiro M.
Published: July 2002

We show that simultaneous one- and three-photon photodissociation of IBr can be used to control the relative product yield of ground and excited state Br atoms. The control attained is substantial insofar as it is possible to vary the Br* yield over the range of 25%–95%, even for a high J case which involves extensive averaging over MJ states.
Heteronuclear cross polarization in liquid-state nuclear magnetic resonance: Mismatch compensation and relaxation behavior
Authors: Levitt Malcolm H.
Published: July 2002

Nuclear spin dynamics in liquid-state heteronuclear cross-polarization experiments were explored by experiments on di-15N -labeled histidine in DMSO-d6. Compensation for Hartmann–Hahn mismatch was achieved by simultaneously shifting the phases of both spin-locking fields by π at intervals short compared to the spin–spin coupling. With rf compensation, slightly more 15N polarization was achieved than with the conventional selective INEPT scheme. This was attributed to relaxation by slowly fluctuating random fields associated with proton exchange. Theory for the rf mismatch compensation and relaxation behavior of the Hartmann–Hahn method is presented for the case of a heteronuclear 2-spin system.
Structure of the Si45 cluster
Authors: Jelski Daniel A. , Swift Brian L. , Rantala Tapio T. , Xia Xinfu , George Thomas F.
Published: July 2002

Six structures for the Si45 cluster are compared using a tight-binding model. Two new structures are proposed which appear to be the low-energy isomers and to explain much of the existing experimental data. Cluster reactivity is distinguished from cluster stability, and several reasons are discussed which may lead to a reactive or unreactive species. These criteria are applied to the Si45 isomers, and the results are also correlated with experimental data.
Mobilities of silicon cluster ions: The reactivity of silicon sausages and spheres
Authors: Jarrold Martin F. , Bower J. Eric
Published: July 2002

The mobilities of size selected silicon cluster ions, Si+n (n=10–60), have been measured using injected ion drift tube techniques. Two families of isomers have been resolved by their different mobilities. From comparison of the measured mobilities with the predictions of a simple model, it appears that clusters larger than Si+10 follow a prolate growth sequence to give sausage-shaped geometries. A more spherical isomer appears for clusters with n>23, and this isomer completely dominates for unannealed clusters with n>35. Annealing converts the sausage-shaped isomer into the more spherical form for n>30. Activation energies for this ‘‘sausage-to-sphere’’ structural transition have been estimated for several cluster sizes and are ∼1.2–1.5 eV. We have examined the chemical reactivity of the sausages and spheres towards both C2H4 and O2. With C2H4 large differences in reactivity of the isomers were found, with the spherical isomer often being more reactive than the sausage form by more than an order of magnitude. With O2 the variations in reactivity were smaller. Despite the substantial differences in reactivity observed for the two isomers in the cluster size regime where both forms coexist, examination of a broader range of cluster sizes shows that there is not a systematic change in reactivity associated with the geometry change.
Comparison of coupled-cluster results with a hybrid of Hartree–Fock and density functional theory
Authors: Scuseria Gustavo E.
Published: July 2002

The performance of a hybrid of Hartree–Fock and density functional theory is tested on a set of ‘‘pathological’’ quantum chemistry problems. The predictions of this hybrid model are in qualitative agreement with coupled-cluster results and with experiment. Given the modest computational cost of the procedure, this is an extremely encouraging development.
Barrier crossing with concentration boundary conditions in biological channels and chemical reactions
Authors: Barcilon Victor , Chen Duanpin , Eisenberg Robert S. , Ratner Mark A.
Published: July 2002

Ions move into biological cells through pores in proteins called ionic channels, driven by gradients of potential and concentration imposed across the channel, impeded by potential barriers and friction within the pore. It is tempting to apply to channels the chemical theory of barrier crossing, but important issues must first be solved: Concentration boundary conditions must be used and flux must be predicted for applied potentials of all sizes and for barriers of all shapes, in particular, for low barriers. We use a macroscopic analysis to describe the flux as a convolution integral of a mathematically defined adjoint function, a Green’s function. It so happens that the adjoint function also describes the first-passage time of a single particle moving between boundary conditions independent of concentration. The (experimentally observable) flux is computed from analytical formulas, from simulations of discrete random walks, and from simulations of the Langevin or reduced Langevin equations, with indistinguishable results. If the potential barrier has a single, large, parabolic peak, away from either boundary, an approximate expression reminiscent of Kramers’ formula can be used to determine the flux. The fluxes predicted can be compared with measurements of current through single channels under a wide range of experimental conditions.
Ab initio studies of cyclic water clusters (H2O)n, n=1–6. I. Optimal structures and vibrational spectra
Authors: Xantheas Sotiris S. , Dunning Thom H.
Published: July 2002

The optimal structures and harmonic vibrational frequencies of cyclic water clusters, (H2O)n, have been determined at the Hartree–Fock (for n=2–6) and second order perturbation theory (for n=2–4) levels of theory with an augmented correlation consistent double zeta basis set. At the MP2 level this basis set yields very accurate results for the structure, dipole moment, and polarizability of the water monomer as well as results of comparable accuracy for the structure, binding energy, and harmonic vibrational frequencies of the water dimer. The optimal structure of (H2O)4 and the harmonic frequencies of (H2O)3,4 are the first ones reported at a correlated level for these species. Analysis of the structural trends reveals that the separation between neighboring oxygen atoms decreases exponentially with increasing cluster size. The predicted R0(O–O) for the ring hexamer is less than 0.02 Å shorter than the interoxygen separation in ice Ih. Furthermore, the trends in the harmonic vibrational frequencies suggest that, for large clusters, the intramolecular bends are blue shifted by ∼70 cm−1 with respect to the monomer frequency. The frequencies corresponding to the ‘‘free’’ OH stretches show little (≤50 cm−1) shift to the blue for n=2–6, whereas the ones corresponding to the ‘‘bridge’’ hydrogens are shifted to the red by ∼500 cm−1 with respect to the average of the harmonic stretching frequencies in water.
Detection of the perpendicular Ã state transitions of CH3I by imaging of photofragment angle-velocity distributions
Authors: Hertz Robert A. , Syage Jack A.
Published: July 2002

Photofragment angle-velocity distributions are measured in a molecular beam using a new two-dimensional imaging technique. We report on the detection of weak perpendicular transitions for CH3I Ã state photodissociation at 266 and 304 nm. The ratio of cross sections and the 3Q0−1Q crossing probability are σ⊥/σ∥=0.00−0.10 and p=0.22−0.27 at 266 nm and σ⊥/σ∥=0.20−0.30 and p=0.43−0.48 at 304 nm.
Absorption of far-infrared radiation by alkali halide aqueous solutions
Authors: Dodo Taro , Sugawa Masao , Nonaka Eiji , Honda Hiroshi , Ikawa Shun-ichi
Published: July 2002

The absorption of radiation in the frequency range 10–80 cm−1 by 2 mol/l electrolyte solutions has been investigated. The absorption at low frequencies below 20 cm−1 is dominantly attributed to the dielectric relaxation of water dipole. At frequencies higher than 40 cm−1, resonance absorption by ion oscillation in a potential well formed by surrounding water molecules comes to contribute to the absorption.
Criticality of charged systems. I. The restricted primitive model
Authors: Zhou Yaoqi , Yeh Showguei , Stell George
Published: July 2002

A new method, which improves upon the mean spherical approximation (MSA), is developed by including the ionic-pairing contribution using a recent theory of association. The association constant of the new approximation is obtained through the second ionic virial coefficient. In the simplest version of our theory, which we call the pairing MSA 1 (PMSA1), we neglect the activity coefficient of the fully associated ionic-pairs, which are regarded as a separate dipolar species, and obtain the critical point (ρc*, Tc*) at (0.025, 0.075). In the second PMSA ( or PMSA2), we include the activity coefficient of these dipolar particles at the MSA level. The new critical point is located at (0.023, 0.073). In the third PMSA ( or PMSA3), we further include the effect of the presence of the dipolar-particle cores. The final critical point is located at (0.0245, 0.0745). These critical points are considerably closer than the MSA result (0.014, 0.079) to the most recent Monte Carlo estimates of ρc* from 0.025 to 0.04 and Tc* from 0.053 to somewhat over 0.057. Both PMSA2 and PMSA3 appear to improve the critical values of pressure and the degree of association significantly over PMSA1. All expressions for the thermodynamic properties in the PMSA1, PMSA2, and PMSA3 are of simple analytic form. The equation of state in the PMSA3 reduces to the very accurate Carnahan-Starling equation of state for hard spheres if the charges are turned off, and it reduces to an accurate equation of state for a mixture of hard spheres and hard dumbbells if the charges of the associated pairs are turned off. A comparison is made between our theory and that of a recent approach of Fisher and Levin, which is in good agreement with the simulation results if the hard-core contribution to the thermodynamics is neglected, but which falls out of agreement when an accurate core contribution is included. A discussion of the importance of an accurate core term in the treatment of the restrictive primitive model is given. Finally, the most likely reasons that the Tc* predicted by the PMSA is somewhat too high are briefly noted.
A semiclassical self-consistent-field approach to dissipative dynamics: The spin–boson problem
Authors: Stock Gerhard
Published: July 2002

A semiclassical time-dependent self-consistent-field approach for the description of dissipative quantum phenomena is proposed. The total density operator is approximated by a semiclassical ansatz, which couples the system degrees of freedom to the bath degrees of freedom in a self-consistent manner, and is thus in the spirit of a classical-path description. The capability of the approach is demonstrated by comparing semiclassical calculations for a spin–boson model with an Ohmic bath to exact path-integral calculations. It is shown that the semiclassical model nicely reproduces the complex dissipative behavior of the spin–boson model for a large range of model parameters. The validity and accuracy of the semiclassical approach is discussed in some detail. It is shown that the method is essentially based on the assumption of complete randomization of nuclear phases. In particular, the assumption of phase randomization allows one to perform the trace over the bath variables through quasiclassical sampling of the nuclear initial conditions without invoking any further approximation.