bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Mo concentration on the phase composition and magnetic properties of Nd8(Fe,Mo)86B6 nanocomposite magnets
Authors: Yin J. H. , Zhou Y. X. , Tang S. L. , Tang W. , Wang D. H. , Zhang J. R. , Du Y. W.
Published: July 2002

The phase composition and magnetic properties of Nd8Fe86−xMoxB6 (x=0, 1, 2, and 3) nanocomposite magnets prepared by melt spinning have been investigated by x-ray powder diffraction, transmission electron microscopy, thermomagnetic analysis, and vibrating sample magnetometer measurements. The optimal ribbons were found to be mainly composed of the α-Fe phase and the Nd2Fe14B phase. The average crystalline size of α-Fe is 14–18 nm. In the optimally quenched samples, the α-Fe phase content decreases with increasing Mo content; at the same time, an increase of coercivity and a decrease of energy products are observed. The hysteresis loops of the optimal ribbons show a single-phase hard magnetic behavior. The reduced remanence ratio is always above 0.66. The remanence enhancement is due to the exchange coupling between the soft and hard magnetic phases in these ribbons. The Curie temperature of the Nd2Fe14B phase in the samples decreases with increasing Mo content. The optimal hard magnetic properties of remanence, coercivity, reduced remanence, and maximum energy product are 120 emu/g, 4.5 kOe, and 0.75 and 135 kJ/m3, respectively, for Nd8Fe86B6.
Jahn–Teller coupling and double exchange in the two-site Van Vleck–Kanamori model
Authors: Meskine H. , Satpathy S.
Published: July 2002

The effect of the dynamical Jahn–Teller coupling on the Anderson–Hasegawa double exchange (DE) in the manganites is studied in a two-site model taking into account the double degeneracy of the eg orbitals and their coupling to the three MnO6 vibrational modes (Q1, Q2, and Q3). Both exact diagonalization and the Lang–Firsov approach are used. We find that coupling to the Q2 and Q3 modes reduces the DE, while the Q1 mode is ineffective. The isotope dependence of the DE interaction is consistent with recent experiments.
The effect of the spontaneous magnetization in the grain boundary region on the magnetic softness of nanocrystalline materials
Authors: Suzuki K. , Cadogan J. M.
Published: July 2002

The relationship between the mean hyperfine field of the residual intergranular amorphous phase 〈Bhf〉 and the coercivity Hc in various nanocrystalline soft magnetic samples has been investigated in order to clarify the effect of the spontaneous magnetization in the grain boundary region on their magnetic softness. Nanocrystalline samples with various 〈Bhf〉 values were prepared by annealing an amorphous Fe91Zr7B2 precursor for periods of 60 s–108 ks, at temperatures of 823–973 K. Hc shows a clear tendency to decrease with increasing 〈Bhf〉 or decreasing volume fraction of the residual amorphous phase. These effects of the residual amorphous phase on Hc are well understood within the framework of our extended two-phase random anisotropy model where both the exchange stiffness constant and volume fraction of the grain boundary phase are relevant to the exchange correlation length. Our results indicate that the exchange stiffness constant of the intergranular region varies as the square of the spontaneous magnetization of the residual amorphous phase. A small exchange stiffness constant on the order of 10−13 J/m is predicted for the residual amorphous phase in the Fe–Zr–B alloy.
Magnetic hard disk overcoats in the 3–5 nm thickness range
Authors: Anoikin E. V. , Yang M. M. , Chao J. L. , Russak M. A.
Published: July 2002

Protective properties of 3–5 nm thick carbon overcoat layers deposited on magnetic hard disks by ion beam deposition (IBD) and plasma-enhanced chemical vapor deposition (PECVD) were investigated. It is found that these overcoats are superior to the sputtered carbon films at thicknesses below 5 nm. Low-stiction performance of 3-nm-thick IBD films without any detectable wear was observed during 50 000 contact start-stop cycles at 55 °C and 10% relative humidity. Surface concentration of cobalt ions is reduced by as much as an order of magnitude as compared to the media with sputtered overcoats. Polarization resistivity values for IBD and PECVD overcoats are by an order of magnitude higher. The results show that thickness of protective carbon overcoats on magnetic hard disks can be reduced to 3–5 nm without compromising media reliability requirements. This reduction is critical for the continuing growth of storage density.
Growth and characterization of GeO2 films obtained by plasma anodization of epitaxial Ge films
Authors: Busani T. , Plantier H. , Devine R. A. B. , Hernandez C. , Campidelli Y.
Published: July 2002

Amorphous GeO2 has been grown by plasma assisted anodic oxidation at temperatures ⩽100 °C. The growth kinetics can be explained in terms of a constant current anodization model. Infrared absorption spectroscopy has been used to characterize the transverse and longitudinal optic vibrational modes of the oxide network. Values are displaced with respect to bulk oxide due to geometrical optic effects and plasma induced network structure variation. In relaxed GeO2 we estimate the mean Ge-O-Ge bridging bond angle to be >123° and the full width at half peak height of the bond angle distribution is ∼14°. Electrical measurements on thin oxide capacitors suggest that in 25 nm oxides there are ⩽2×1011 negative charges cm−2 and negligible interface state densities.
An analysis of heat transfer and fume production in gas metal arc welding. III
Authors: Haidar J.
Published: July 2002

A two–dimensional dynamic theory for predictions of arc and electrode properties in arc welding has been used to investigate heat transfer phenomena in the welding wire in gas metal arc welding (GMAW). The theory is a unified treatment of the welding wire, the plasma and the workpiece and includes a free surface treatment for the welding drops, accounting for the effects of inertia, gravity, surface tension, arc pressure, magnetic forces, and viscous drag by the gas flow around the drop. Also, the theory accounts for the variation of the surface tension coefficient with temperature and includes thermal and dynamic phenomena within the solid and liquid phases of the wire, together with a detailed treatment for the electrode sheath regions. Calculations are made for arcs in argon with wires of mild steel at currents between 150 and 325 A. Results of calculations for heat fluxes within the wire suggest that evaporation from the surface of the droplet during droplet growth has an important influence on the heat balance of the wire. The calculated evaporation rates from the droplet surface during droplet growth at the tip of the wire are found to be much higher than measured rates of fume formation in GMAW, suggesting an important recondensation process at the surface of the workpiece.
Different recrystallization patterns of Si+ implanted GaAs
Authors: Desnica-Franković I. D.
Published: July 2002

Raman spectroscopy was used in a study of the lattice restoration in Si+ implanted GaAs. Investigated samples differed in the type of primary damage induced by selected implantation parameters. Annealing was carried out by thermal treatment in 40° steps starting at 80 °C. Several well defined stages in recrystallization could be followed; de-amorphization being mostly completed after annealing temperature, Ta, as low as 200 °C, whereas for Ta from 200 to 600 °C various processes of ordering in the crystalline phase were dominant. In this temperature range thermal restoration of the crystal lattice follows quite different patterns depending on the types of primary disorder. Monocrystalline character of the recrystallized layer was not entirely restored, even at Ta=800 °C, but still consisted of nanometer-sized crystallites, ∼25 nm, predominantly oriented to follow the underlying substrate orientation. Various types of defects, breaking translational crystal symmetry, such as linear defects, stacking faults, dislocations, dislocation loops, etc. were still present as seen from the finite size of the monocrystalline regions.
High-resolution x-ray and transmission electron microscopic analysis of a GaInAsSb/AlGaAsSb multiple quantum well laser structure
Authors: Gray A. L. , Newell T. C. , Lester L. F. , Lee H.
Published: July 2002

High-resolution x-ray diffractometry (HRXRD) and transmission electron microscopy (TEM) are employed to characterize a quaternary IIIxIII1−xVyV1−y AlGaAsSb/GaInAsSb multiple quantum well (MQW) heterostructure. A method for uniquely determining the chemical composition of the strained quaternary quantum well, information previously thought to be unattainable using HRXRD, is thoroughly described. The misconception that HRXRD can separately find the well and barrier thickness of a MQW from the pendellosung fringe spacing is corrected and, thus, the need for TEM is motivated. Computer simulations show that the key in finding the well composition is the intensity of the higher order satellite peaks in the diffraction pattern. For the AlGaAsSb/InGaAsSb MQWs analyzed in this work, the variation in the intensity of the third-order satellite peak is identified as a sensitive measure of the quantum well composition. Using HRXRD on an MQW semiconductor laser device layer, the ability to resolve this higher order peak and interpret the information contained in it is demonstrated for the first time.
Equivalent-circuit modeling of ferroelectric switching devices
Authors: Rep D. B. A. , Prins M. W. J.
Published: July 2002

A compact equivalent-circuit model for ferroelectric switching devices is derived from a general model for local charge displacements. The general model consists of a matrix of repeat units describing local dissipationless charge displacements (electrostatic channel), as well as dissipative charge displacements (electrochemical channel), the channels being coupled due to the electrical charge of the moving species. The derived model for ferroelectric charge displacements is used to simulate both hysteresis and transient characteristics, and applied to two devices: (i) a ferroelectric capacitor and (ii) a ferroelectric memory field-effect transistor. The circuits are programmed in SPICE-derived analysis software. We find that experimental hysteresis data obtained on Pb(Zr,Ti)O3 ceramic capacitors and on thin-film transistors with a SnO2:Sb semiconductor and a Pb(Zr,Ti)O3 ferroelectric insulator can be reproduced and interpreted with the equivalent-circuit models.
Phonon damping in thin films of Fe
Authors: Röhlsberger R. , Sturhahn W. , Toellner T. S. , Quast K. W. , Hession P. , Hu M. , Sutter J. , Alp E. E.
Published: July 2002

The phonon density of states (DOS) in thin films of polycrystalline α-Fe was measured by inelastic nuclear resonant scattering of synchrotron radiation. The thin-film DOS exhibits significant deviations from the DOS of bulk Fe, which we attribute to phonon lifetime broadening in the confined geometry. The measured DOS can be described with a damped harmonic oscillator model for the phonons with different quality factors Q=25(2) and Q=13(1) for layer thicknesses of 28 and 13 nm, respectively.
Dielectric and ferroelectric response as a function of annealing temperature and film thickness of sol-gel deposited Pb(Zr0.52Ti0.48)O3 thin film
Authors: Cho Chae-Ryong , Lee Won-Jae , Yu Byoung-Gon , Kim Bo-Woo
Published: July 2002

We report the thickness and annealing temperature dependence of the structural, morphological, compositional, and electrical properties including ferroelectric characteristics of Pb(Zr0.52Ti0.48)O3 thin films deposited by a sol-gel method. The thickness and annealing temperature of the films were in the range of 0.12–0.36 μm and 520–670 °C, respectively. The crystalline structure and growth behavior of the films have been studied by x-ray diffraction, scanning electron microscopy, and atomic force microscopy. It is demonstrated that the weak ac electric field dependence of the permittivity of the films in terms of the Rayleigh law can be explained. The coercive field obtained from a sweep up and down schedule of the capacitance-voltage curves is asymmetrical and shows different behavior according to the annealing temperature and thickness. We also show that the activation energy coefficient, γ, obtained from frequency dependent polarization-electric field hysteresis loops is related to the annealing temperature and thickness of the films. A low leakage current density (∼100 nA/cm2 at 200 kV/cm) and low annealing temperature (∼520 °C) demonstrate the potential of the deposited films for integrated ferroelectric random access memory and piezoelectric sensor applications.
Alignment of cholesteric liquid crystals using periodic anchoring
Authors: Komitov L. , Bryan-Brown G. P. , Wood E. L. , Smout A. B. J.
Published: July 2002

Previous work has shown that when short-pitch cholesteric liquid crystals are aligned in cells with the helical axis in the plane of the substrate in a well-defined direction, then application of an electric field across the cells may result in microsecond in-plane switching due to the flexoelectric effect. Moreover, the response to the electric field is linear and temperature independent. Hence this device geometry has great potential since fast, temperature-independent switching with analogue gray scale may be realized. Up to now, however, the alignment of the helix in the plane of the substrate achieved, for example, by rubbed polyimide layers has been rather unstable, since it relaxes to a Grandjean texture in a time scale determined by the cell thickness. This time is generally of the order of minutes. Here we report a method for the attainment of stable anchoring of the unidirectional lying helix texture using periodic anchoring conditions.
Dielectric properties of pulsed laser deposited films of PbMg1/3Nb2/3–PbTiO3 and PbSc1/2Nb1/2O3–PbTiO3 relaxor ferroelectrics
Authors: Tyunina M. , Levoska J. , Sternberg A. , Leppävuori S.
Published: July 2002

Highly oriented perovskite films of PbMg1/3Nb2/3–PbTiO3 and PbSc1/2Nb1/2O3–PbTiO3 with compositions near the morphotropic phase boundary were formed by pulsed laser deposition on La0.5Sr0.5CoO3/MgO (100). The dielectric properties of the films were studied over the frequency range of 100 Hz–1 MHz in the temperature range 20–350 °C. The room temperature polarization and dielectric permittivity of 250-nm-thick films were close to those in bulk ceramics. The films exhibited relaxor-type behavior with thermal hysteresis and with the temperatures of the dielectric peaks corresponding to those in the bulk. The width of the transition in the films was larger than in bulk ceramics. The properties of the films were found to be influenced by the small grain size rather than by the interfaces of the films.
Design considerations for x-ray free electron lasers
Authors: Dattoli G. , Ottaviani P. L.
Published: July 2002

We develop design considerations on the possibility of generating free electron laser radiation in the x-ray region of the spectrum (50–60 Å), by exploiting a frequency multiplier scheme. We propose a free electron device consisting of a relatively low energy linac (750 MeV) and three sections: an oscillator at 150 nm, an amplifier tuned at the fifth harmonic of the first and a second amplifier operating at a subharmonic of the second amplifier. The seedless amplification is ensured by the e-beam bunching, induced in the oscillator and in the second section, which plays the role of amplifier and modulator. We also explore the possibility of overcoming the problems associated with the bunching efficiency dilution, due to intracavity saturation of the first section, by discussing the regeneration of the bunching by the use of a cavity dumping, realized with a suitable e-beam energy or cavity detuning modulation.
Si induced size effects in ferroelectric PbTiO3
Authors: Palkar V. R. , Purandare S. C. , Ayyub P. , Pinto R.
Published: July 2002

Physical properties of PbTiO3 in the presence of Si are investigated in detail. Polycrystalline bulk samples with nominal composition of (Pb1−xSix) (Ti1−xSix)O3, where x varies from 0.01 to 0.25, are synthesized using the coprecipitation route. The presence of Si in the matrix leads to a reduction in coherently diffracting domain size (dXRD). The observed reduction in the ferroelectric distortion in the lattice c/a, ferroelectric transition temperature Tc and shift in Raman lines of PbTiO3 with increase in Si content are attributed to the Si induced finite size effect. However, at higher calcination temperatures, the material exhibits properties similar to pure, undoped, bulk PbTiO3. There is no evidence of change in crystal structure and ferroelectric nature of PbTiO3 due to the presence of Si. Our data suggests that Si, which is diffused out of PbTiO3 perovskite lattice, is likely to reside in the grain boundary region in an as yet unidentified chemical form.
Raman characterization of structural disorder and residual strains in micromachined GaAs
Authors: Pizani P. S. , Lanciotti F. , Jasinevicius R. G. , Duduch J. G. , Porto A. J. V.
Published: July 2002

Structural disorder and strain effects in ductile-regime single-point-diamond-turned gallium arsenide monocrystalline samples were probed by Raman scattering. The positive frequency shift of the longitudinal and transverse optical phonons observed in the machined samples indicate a residual compressive stress of about 1.5 GPa. This residual strain was attributed to the hysteresis of phase transformation generated by the high pressure imposed by the cutting tool tip during the machining process. The broadening of the Raman peaks indicate a high degree of structural disorder in the GaAs lattice. Moreover, the Raman spectrum of annealed samples, after machining, shows a less disordered but still misoriented matrix. In addition, it was found that crystalline arsenic formed into the surface vicinity.
Simultaneous measurement of Young’s modulus, Poisson ratio, and coefficient of thermal expansion of thin films on substrates
Authors: Zhao Jie-Hua , Du Yong , Morgen Michael , Ho Paul S.
Published: July 2002

A method of measuring the Yong’s modulus, Poisson ratio, and coefficient of thermal expansion (CTE) is presented. The method uses a wafer curvature technique to measure thermal stresses of thin films of the same material deposited on two different substrates, one isotropic and the other thermomechanically anisotropic. By analyzing the thermal stress data as a function of temperature, the Young’s modulus, Poisson ratio and CTE can be simultaneously determined. The method is demonstrated for Al (0.5 wt % Cu) and Cu thin films by performing measurements on (100) Si wafers and Y-cut single-crystal quartz wafers. The CTE, Young’s modulus, and Poisson ratio are found to be 24.3 ppm/°C, 58.9 GPa, and 0.342, respectively, for Al (Cu) thin film, and 17.7 ppm/°C, 104.2 GPa, and 0.352, respectively, for Cu thin film. They are in good agreement with those measured by other methods. This method is generally applicable to other on-wafer films with in-plane isotropy.
1.54 μm infrared photoluminescence and electroluminescence from an erbium organic compound
Authors: Sun R. G. , Wang Y. Z. , Zheng Q. B. , Zhang H. J. , Epstein A. J.
Published: July 2002

Infrared emission at 1.54 μm excited optically and electrically from an erbium organic compound tris(acetylacetonato)(1,10-phenanthroline) erbium [Er(acac)3(phen)] is observed. The rare-earth complex is dispersed into a polymer matrix of poly(N-vinylcarbazole) (PVK) to fabricate an electroluminescent (EL) device with an ITO/PVK:Er(acac)3(phen)/Al:Li/Ag structure, where ITO represents indium–tin–oxide-coated glass. The device shows infrared EL emission at 1.54 μm, which suggests a simple and cheap method to obtain a light source for 1.54-μm-wavelength devices in optical communications.
Measurement dependence of the exchange bias
Authors: Xi Haiwen , White Robert M. , Rezende Sergio M.
Published: July 2002

Recently, in addition to the conventional hysteresis loop measurement, several different measurement techniques, such as ac susceptibility, ferromagnetic resonance (FMR), and Brillouin light scattering, have been used to study the unidirectional exchange coupling induced in a ferromagnetic film in proximity to an antiferromagnet. In this article, we theoretically investigate how interface exchange coupling manifests itself in different measurements. The magnetic degrees of freedom of the antiferromagnet are incorporated in this analysis. Anisotropies from hysteresis loop measurements, ac susceptibility measurements, and FMR measurements are obtained. These results show that the measured exchange anisotropies differ with measurement techniques.
Anisotropy-dependent macroscopic domain structure in wedged-permalloy/uniform-FeMn bilayers
Authors: Liu Kai , Zhou S. M. , Chien C. L. , Nikitenko V. I. , Gornakov V. S. , Shapiro A. J. , Shull R. D.
Published: July 2002

Macroscopic domain structures have been realized in wedged-permalloy (Py)/uniform-FeMn bilayers during magnetization reversal. When the exchange anisotropy is established perpendicular or parallel to the wedge direction, two macroscopic domains are observed. Separating these domains are a 180° wall in the perpendicular geometry and an intermediate band containing large density of stripe-type microdomains in the parallel geometry. While the exchange field remains practically the same in both geometries, the coercivity and squareness of the loop are much less in the parallel geometry.
Transition metal oxides: CrO, MoO, NiO, PdO, AgO
Authors: Bauschlicher Charles W. , Nelin Constance J. , Bagus Paul S.
Published: July 2002

The transition metal oxides are quite ionic, Mulliken population analyses for several oxides give a negative charge of ∼0.7 electrons for oxygen. When the transition metal d shell is only partially filled, the d orbitals are involved in covalent bonds with O; both two electron (bonding2–antibonding0) and one electron (bonding2–antibonding1) bonds are formed. These covalent bonds occur in addition to the ionic bonding. There is dσ-O2pσ repulsion and this repulsion is reduced when the dσ electron is promoted into an orbital which has dominantly 4spσ character and is polarized away from O. This effect is quite large, greater than half and electron, for some states of CrO and MoO.
Luminescence of some metalloporphins including the complexes of the IIIb metal group
Authors: Ohno Osamu , Kaizu Youkoh , Kobayashi Hiroshi
Published: July 2002

Luminescence yields and decay lifetimes were measured on some metalloporphyrins including the complexes of the IIIb metal group. The S2 emission was observed for AlClTPP (TPP: tetraphenylporphin), GaClTPP, and InClTPP as well as some other TPP metal complexes, ZnP (P:porphin) and H4P2+ (diacid), while it was not detected for H2TPP, H2OEP (OEP: octaethylporphin), AlClOEP as well as other OEP metal complexes. The rate of S2→S1 internal conversion is dependent upon the peripheral substituents rather than the central metal ion and is enhanced in the porphyrins which do not emit the S2 fluorescence. On the other hand, the intersystem crossing plays a more important part in the relaxation process of the S1 state. With increase in the atomic number of the central metal ion as well as its axial ligand, the yield and decay lifetime of the S1 emission are reduced. In the relaxation of the T1 state, the radiative decay is also accelerated for the central metal ion with higher atomic number.
EBK quantization of nonseparable systems: A Fourier transform method
Authors: Martens Craig C. , Ezra Gregory S.
Published: July 2002

We present an accurate and efficient method for EBK quantization of bound states in multidimensional systems, based upon a Fourier representation of invariant tori in the quasiperiodic regime. The key feature of the method is the direct determination of good action variables from Fourier transforms of trajectories, using a formula due to Percival. It is possible to calculate actions using this formula provided frequencies and Fourier components can be determined to high accuracy. We describe a special windowing technique for this purpose, which overcomes the stability problems encountered in the recent related work of Eaker, Schatz, De Leon and Heller. Quantized energy levels can be found either by direct search for quantizing trajectories or by linear extrapolation from nearby arbitrary tori. Results are presented for two-, three-, and four-dimensional vibrational Hamiltonians.
Normalization constraint for variational bounds on fluid permeability
Authors: Berryman James G. , Milton Graeme W.
Published: July 2002

A careful reexamination of the formulation of Prager’s original variational principle for viscous flow through porous media has uncovered a subtle error in the normalization constraint on the trial functions. Although a certain surface integral of the true pressure field over the internal surface area always vanishes for isotropic materials, the corresponding surface integral for a given trial pressure field does not necessarily vanish but has nevertheless been previously neglected in the normalization. When this error is corrected, the form of the variational estimate is actually simpler than before and furthermore the resulting bounds have been shown to improve when the constant trial functions are used in either the two-point or three-point bounds.
Translational energy dependence of Ar++XY→ArX++Y (XY=H2,D2,HD) from thermal to 30 eV c.m.
Authors: Ervin Kent M. , Armentrout P. B.
Published: July 2002

Cross sections for the reactions of Ar+ with H2, D2, and HD to form ArH+ and ArD+ are measured using a new guided ion beam tandem mass spectrometer which affords an experimental energy range from 0.05 to 500 eV laboratory. The apparatus and experimental techniques are described in detail. Cross sections for H2 and D2 are found to be nearly identical over this entire energy range when compared at the same barycentric energy. The total HD cross section is the same as H2 and D2 at low energies, but differs significantly above 4 eV c.m., where product dissociation becomes important. The intramolecular isotope effect for reaction with HD exhibits a reversal at low energy, favoring the deuteride product below ∼0.14 eV c.m., and surprising nonmonotonic behavior at energies above 5 eV c.m. In all these systems, a new feature at higher energies is observed. This is interpreted as the onset of a product channel having an energy barrier of 8±1 eV. The room temperature rate constant derived from the data for the reaction with H2 is (9.5±2)×10−10 cm3 s−1, in good agreement with the literature. Analysis of the data indicates an activation energy of between 2 and 15 meV at room temperature. The results are compared to previous experimental determinations and to theoretical reaction models.
New Born–Oppenheimer potential energy curve and vibrational energies for the electronic ground state of the hydrogen molecule
Authors: Kol/os Wl/odzimierz , Szalewicz Krzysztof , Monkhorst Hendrik J.
Published: July 2002

The Born–Oppenheimer potential energy curve for the electronic ground state of the hydrogen molecule has been computed for internuclear distances 0.2≤R≤12.0 bohr. At all R the energies are lower than any previously reported values. At the equilibrium distance the improvement is negligible; its largest value, 0.47 cm−1, has been obtained for R=2.4 bohr. Vibrational energies have been computed for all isotopic species using the new potential energy curve. For H2, HD, and D2 the recent nonadiabatic corrections of Wolniewicz have been included. The resulting dissociation energies of H2 and HD agree with the experimental values within the experimental error limits; for D2 the discrepancy amounts to 0.6±0.3 cm−1.
Bound state semiclassical wave functions
Authors: Knudson S. K. , Delos J. B. , Noid D. W.
Published: July 2002

The semiclassical theory developed by Maslov and Fedoriuk is used to calculate the wave function for a two-dimensional bound state system. We investigate in detail an eigenstate of a coupled anharmonic oscillator system. The primitive semiclassical wave function is obtained from the characteristic function S and the density function J. Each of these functions consists of four branches corresponding to the four possible directions of motion of the classical trajectory through any point. The interference from the four branches determines the basic structure of the wave function. A uniform approximation gives a wave function which is well behaved along each caustic and which is in good agreement with the fully quantal wave function.
Molecular dynamic calculation of elastic constants of silicon
Authors: Kluge Mark D. , Ray John R. , Rahman Aneesur
Published: July 2002

Stillinger and Weber have introduced a model potential to study the solid and liquid forms of silicon. This potential has been used in a number of computer simulation studies of silicon. We have calculated the elastic constants of silicon using the Stillinger–Weber potential at three different temperatures T=888, 1164, and 1477 K. The adiabatic elastic constants are calculated using fluctuation formula appropriate for the microcanonical ensemble. We find that the calculated shear modulus C44 is smaller than the experimental values by ∼30% at T=888 K, ∼40% at the two higher temperatures, and the modulus C12 is larger than the experimental value by 25%–30%. Simulations with N=216 and 1728 particles were carried out to check on the number dependence of the results. These two particle numbers give equivalent results for the elastic constants. The calculated elastic constants show the same softening with rising temperature as shown by the experimental values.
Vibrational state resolved measurements of differential cross sections for H++O2 charge transfer collisions
Authors: Noll M. , Toennies J. Peter
Published: July 2002

Vibrational state-to-state charge transfer cross sections have been obtained for H++O2 collisions at Ecm =23.0 eV in a differential scattering experiment by measuring the product H atom energy distribution in the range 0°≤θ≤11°. The results show a strong dependence of the distribution of vibrational transition probabilities P(O2, v=0→O+2, v″) on the scattering angle. At very low angles (θ≲2°) we find a strongly enhanced contribution of the more resonant states v″=3–6 compared to a Franck–Condon distribution, which is peaked at v″=1 and, on the other hand, at moderate angles of about θ=4° comes very close to the measured spectra. At even larger angles increasing deviations from the Franck–Condon distribution towards larger excitation of higher vibrational states are observed. An explanation of these and the related results for the nonexchange collisions leading to neutral O2 vibrational excitation is given in terms of the underlying potential energy surfaces.
The determination of the vector correlation between photofragment rotational and translational motions from the analysis of Doppler-broadened spectral line profiles
Authors: Dixon R. N.
Published: July 2002

A theoretical framework is presented to describe the possible correlation between the translational and rotational motion of a molecular photofragment, and to relate this to experimental observables. The correlation is defined by the values of a number of bipolar moments of the translational and rotational angular distributions which reflect the dynamics of the dissociation process. Detailed equations are presented for the polarization dependence of the profiles of recoil Doppler–broadened spectral lines of the photoproduct for two distinct classes of system: (i) molecular photodissociation, followed by spontaneous fluorescence by a molecular product with dispersion of its emission spectrum, and (ii) LIF excitation of a photoproduct, with or without dispersion of the resulting emission. The application of the theory is illustrated by calculating the line profiles for two model systems, each with several excitation–detection geometries. It is pointed out that there may still be detectable correlation between the product motions even for weakly predissociated parent molecule transitions in which the memory of the initial excitation is lost through extensive rotation before dissociation.
Multiple-quantum NMR spectroscopy of S=3/2 spins in isotropic phase: A new probe for multiexponential relaxation
Authors: Jaccard Guy , Wimperis Stephen , Bodenhausen Geoffrey
Published: July 2002

In nuclear magnetic resonance of quadrupolar spins with S≥3/2, it is shown that excitation and observation of multiple-quantum coherence is possible in the absence of scalar, dipolar, or quadrupolar splittings, in contrast to the widely accepted view that nonvanishing couplings are a prerequisite for the creation of multiple-quantum coherence. In the absence of splittings, multiple-quantum coherence can be excited because the longitudinal (‘‘T1’’) or transverse (‘‘T2’’) relaxation is multiexponential, which occurs if the motional correlation times τc are comparable to or larger than the inverse of the Larmor frequency (violation of the extreme narrowing approximation). Two experiments are described, which combine multiple-quantum filtration with conventional spin-echo and inversion-recovery sequences. For isotropic motion each experiment allows one to determine the motional correlation time without knowledge of the magnitude of the quadrupolar coupling constant.
Multiphoton coherent excitation of molecules
Authors: Diels J.-C. , Besnainou S.
Published: July 2002

Two different approaches to multiphoton excitation of an anharmonic multilevel system are presented. Both methods result in nearly complete population of the upper state. The first approach involving phase shifting portions of the coherent excitation is treated analytically with square pulses. The second approach uses sequences of several pulses with selected relative phase and delays. Some numerical examples are presented for three- and four-photon excitation of the vibrational level structure of CH3F. This particular molecule is again taken as model to demonstrate four-photon excitation with a five pulse sequence, taking into account the rotational line structure. The rotational line distribution in the upper state is seen to correspond to a lower temperature than in the ground state.
Use of molecular symmetry in coupled-cluster theory
Authors: Čársky Petr , Schaad Lawrence J. , Hess B. Andes , Urban Miroslav , Noga Jozef
Published: July 2002

It is shown that intermediate arrays resulting from the quasilinearization of coupled-cluster theory with double excitations, and having the form of pseudointegrals 〈ab‖ij〉n, retain symmetry properties of the ordinary two-electron integrals 〈ab‖ij〉. The 〈ab‖ij〉n elements vanishing on symmetry grounds may be eliminated a priori, as well as contributions to nonvanishing 〈ab‖ij〉n pseudointegrals from vanishing 〈ab‖ij〉n−1 or 〈ab‖ij〉 terms. A program using molecular symmetry in this way has been coded and its speed compared with that of gaussian 82.
Empirically fitting the potential energy curves for the X, B, and B′ double minimum and the D 2Σ+ states in CaH using united knowledge of CaH and CaD
Authors: Martin H.
Published: July 2002

A set of potential energy curves was generated for the three lowest 2Σ+ states in CaH using a semiempirical model. Spectroscopic data from both CaH and CaD was used in a least-squares fit of potential energy parameters. Eigenvalues for vibrational motion in the resulting potentials were calculated as a function of rotation. Special attention was given to the B state double minimum potential. Calculations on the potential barrier penetrations were performed and compared with observations. Franck–Condon factors for transitions between the ground state and the very anharmonic excited states were calculated. An explanation for the predissociations known in the C state was given.
On the ionization potential of small metal and dielectric particles
Authors: Makov Guy , Nitzan Abraham , Brus Louis E.
Published: July 2002

The ionization potential of small metal and dielectric spheres is considered in different frameworks: classical, semiclassical, and quantum mechanical density functional approach. Classical calculations give conflicting results, and the generally accepted result for the ionization potential of a metal sphere of radius R: WI(R)=bulk work function+(3/8)q2/R is shown to be wrong, resulting from the classical image potential too close to the metal surface. Using appropriate cutoff to the image potential, the result WI(R)=bulk work function+(1/2)q2/R (previously obtained from solvation energy considerations) is recovered. Experimental results on relatively large particles are in agreement with the latter result. For very small clusters, deviations of experimental results from this classical behavior are shown by a density functional calculation to arise from quantum mechanical effects. These are first the spilloff of the electronic wave functions beyond the cluster edge and secondly from exchange and correlation contributions.
A surface constrained all-atom solvent model for effective simulations of polar solutions
Authors: King Gregory , Warshel Arieh
Published: July 2002

A consistent simulation of ionic or strongly polar solutes in polar solvents presents a major challenge from both fundamental and practical aspects. The frequently used method of periodic boundary conditions (PBC) does not correctly take into account the symmetry of the solute field. Instead of using PBC, it is natural to model this type of system as a sphere (with the solute at the origin), but the boundary conditions to be used in such a model are not obvious. Early calculations performed with our surface constrained soft sphere dipoles (SCSSD) model indicated that the dipoles near the surface of the sphere will show unusual orientational preferences (they will overpolarize) unless a corrective force is included in the model, and thus we implemented polarization constraints in this spherical model of polar solutions. More recent approaches that treated the surface with stochastic dynamics, but did not take into account the surface polarization effects, were also found to exhibit these nonphysical orientational preferences. The present work develops a surface constrained all-atom solvent (SCAAS) model in order to consistently treat the surface polarization effects in all-atom molecular dynamics simulations. The SCAAS model, which was presented in a preliminary way in previous works, introduces surface constraints as boundary conditions in order to make the necessarily finite system behave as if it was part of an infinite system. The performance of the model with regard to various properties of bulk water is examined by comparing its results to those obtained by PBC simulations. The results obtained from SCAAS models of different sizes are found to be similar to each other and to the corresponding PBC results. The performance of the model in simulations of solvated ions is emphasized and a comparison of the results obtained with spheres of different sizes demonstrates that the model does not possess significant size dependence. This indicates that the model can be used with a relatively small number of solvent molecules for convergent simulation of structure, energetics, and dynamics of polar solutions. The much simpler fixed

center Langevin dipoles (FCLD) model is also examined and found to provide a

powerful tool for estimating solvation free energies. Finally, a preliminary

study of the dielectric properties of the SCAAS model is reported and the potential of this model for exploring the correct implementation of the solvent reaction field is discussed.
Effect of incidence energy and angle on the adsorption probability of Xe on Pt(111): Energy–angle scaling relations
Authors: Rettner C. T. , Bethune D. S. , Auerbach D. J.
Published: July 2002

The sticking probability of Xe on Pt(111) at a surface temperature of 85 K is found to fall rapidly with increasing kinetic energy over the range 63 to 800 meV. Increasing the incidence angle from 30° to 60° generally raises this probability, in a manner consistent with an approximate scaling with Ei cos1.6θi.
Spectroscopy and electronic structure of jet-cooled Al2
Authors: Fu Zhenwen , Lemire George W. , Bishea Gregory A. , Morse Michael D.
Published: July 2002

Resonant two-photon ionization spectroscopy has been used to study the jet-cooled Al2 molecule. The ground state has been conclusively demonstrated to be of 3Πu symmetry, deriving from the σ1gπ1u electronic configuration. High resolution studies have established the bond length of the X3Πu state as re(X3Πu) =2.701±0.002 Å. The third-law estimate of the Al2 bond strength has been reevaluated using the observed and calculated properties of the low-lying electronic states to give D00 (Al2)=1.34±0.06 eV. In addition to the previously reported E 2 3Σ−g←X3Πu and F 33Σ−g←X3Πu band systems, the E′ 33Πg←X 3Πu, F″–X, F′–X, G 3Πg←X 3Πu, H′ 3Σ−g←X 3Πu, and H3Δg←X3Πu band systems have been observed for the first time. Bands of the G–X, H′–X, and H–X systems have been rotationally resolved and analyzed, providing rotational constants and electronic state symmetries for the upper states of these systems. A discussion of all of the experimentally known states of Al2 is presented, along with comparisons to previous experimental and theoretical work.
Broadband multiple-quantum nuclear magnetic resonance with frequency-modulated ‘‘chirp’’ pulses: Applications to pairs of scalar-coupled spin I=1/2 nuclei
Authors: Burghardt Irene , Böhlen Jean-Marc , Bodenhausen Geoffrey
Published: July 2002

It is shown by simulation and experiment that frequency-modulated ‘‘chirp’’ pulses are very effective for exciting multiple-quantum coherences over large bandwidths. Chirp pulses can be considered as an alternative to composite pulses to combat the effects of large offsets and tilted effective fields. By way of example, we discuss the excitation of double-quantum coherences for pairs of scalar-coupled spin I=1/2 nuclei over large ranges of chemical shifts. Refocusing of the phase dispersion of double-quantum coherence can be combined with suitable detection sequences to yield pure absorption two-dimensional double-quantum spectra. The idea of symmetrical excitation and detection by time reversal may be applied to obtain t1-modulated longitudinal magnetization, which may then be converted into observable single-quantum coherence by a chirp echo sequence. Similar approaches can be used for many other NMR experiments involving coherence transfer.
Lattice walks by long jumps
Authors: Wrigley John D. , Twigg Mark E. , Ehrlich Gert
Published: July 2002

Diffusion on surfaces has in the past been modeled as a random walk in continuous time between nearest-neighbor sites on a lattice. In order to allow tests for the possible participation of long jumps in actual diffusion processes, we examine the properties of random walks in which transitions are not limited to jumps between nearest-neighbor sites. Two features of such walks are of special interest: (a) the moments of the displacements, which are directly related to the diffusivity and the statistical uncertainties in its determination; (b) the distribution function governing the probability of displacements, which is an important indicator of the contributions from long jumps. The techniques used to develop expressions for these quantities are illustrated for random walks in one dimension, with transitions allowed between neighbors up to three spacings removed. The appropriate probability generating function is derived starting from the Kolmogoroff equation. This is then manipulated to yield both the moments and the distribution of displacements in terms of the jump rates to first-, second-, and higher-nearest neighbors. Inasmuch as this approach to the distance distribution is not universally feasible, the latter is also evaluated directly as a combinatorial problem for diffusion by single and double jumps. Generating function techniques are then used to describe two-dimensional diffusion with contributions from long jumps on the (110) plane of the bcc lattice. Atomic jumps of different lengths are allowed along the close-packed directions, and jumps along the Cartesian coordinates are considered as well. The most challenging problem is to describe diffusion on the (111) plane of the fcc lattice, on which atoms can be bound at two different types of positions, one surrounded by the other, so that double jumps are no longer uncorrelated with single jumps. Lower moments of the displacements are developed from rather complicated generating functions, but combinatorial methods have to be used to derive the complete distribution function governing displacements.
Aluminum hydride desorption from hydrogen covered aluminum single crystal surfaces
Authors: Winkler A. , Resch Ch. , Rendulic K. D.
Published: July 2002

Formation and desorption of aluminum hydride from hydrogen covered Al(110), Al(100), and Al(111) has been investigated, using thermal desorption spectroscopy. Both desorption of aluminum hydride and molecular hydrogen has been detected with branching ratios depending on surface structure. Production of aluminum hydride is negligible on the rough (110) surface, whereas on the flat (111) plane up to 50% of the adsorbed hydrogen is desorbed in form of aluminum hydride. Furthermore, aluminum hydride formation is strongly enhanced with an increase of the heating rate. Desorption of molecular hydrogen follows a close to zero order reaction, with a desorption energy around 17±1 kcal/mol. Aluminum hydride desorption can be described best by a fractional order (≊0.5), with a desorption energy of 27±1 kcal/mol. Angular distribution measurements reveal desorption distribution functions of D(Θ)≊cos Θ−cos3 Θ for aluminum hydride and D(Θ)≊cos2 Θ−cos15 Θ for molecular hydrogen, strongly dependent on surface structure.
Turing patterns visualized by index of refraction variations
Authors: Lee Kyoung J. , McCormick W. D. , Swinney Harry L. , Noszticzius Zoltan
Published: July 2002

Gel pattern is visualized by the refractive index variations.The fossil patterns correspond to a spatial variation in the refractive index. (AIP)
Vacuum ultraviolet absorption spectra and photodissociative excitation of CHBr2Cl and CHBrCl2
Authors: Ibuki Toshio , Hiraya Atsunari , Shobatake Kosuke
Published: July 2002

Photoabsorption and fluorescence excitation spectra of CHBr2Cl and CHBrCl2 trihalomethanes were measured in the 106–200 nm region using synchrotron radiation as a light source. Main photoabsorption bands observed are assigned as the Rydberg transitions of the outer shell orbitals. In the photoexcitation of CHBr2Cl a diffuse emission band of CHCl(Ã 1A″→X̃ 1A′) was observed in the 500–800 nm region. The radiative lifetime of the CHCl(Ã 1A″) state was first determined to be 7.01±0.25 μs. In the photolysis of CHBrCl2 at 121.6 nm, the formation of CHCl(Ã 1A″) and CCl2(Ã 1B1) radicals were observed. The fluorescence thresholds for CHBr2Cl and CHBrCl2 start at 162 and 159 nm with the maximum yields of 5% and 11% at 122 and 119 nm, respectively.
Rotational level dependence of predissociation in the v′=3 level of OH A 2Σ+
Authors: Heard Dwayne E. , Crosley David R. , Jeffries Jay B. , Smith Gregory P. , Hirano Akira
Published: July 2002

Using laser-induced fluorescence in a low-pressure flame, relative predissociation rates have been determined as a function of rotational level in the v′=3 level of the A 2Σ+ state of the OH molecule. A Boltzmann analysis of an excitation scan in the (3,0) band yields an apparent temperature of 970 K in a 1670 K flame, showing that kpre increases rapidly with rotational level. The F1 levels predissociate slightly faster than F2 for the same N′, agreeing with previous multichannel scattering predictions. The results can be placed on an absolute basis using recent linewidth measurements in a flow tube.