bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhanced imaging of biomolecules with electron beam deposited tips for scanning force microscopy
Authors: Zenhausern F. , Adrian M. , ten Heggeler-Bordier B. , Ardizzoni F. , Descouts P.
Published: July 2002

Tip/sample interaction on the scanning force microscope (SFM) is a particularly difficult problem with biological materials. One major factor affecting image quality is the tip shape. Improved electron beam induced deposition technique with a scanning electron microscope (SEM) was used for the reproducible fabrication of carbon sharp tips on the end of commercially available silicon nitride cantilevers for scanning force microscopy. By aligning a fine focused beam of 20 nm diameter directly down the axis of the pyramidal tip at electron energy of 20 kV, carbon deposits grow with full cone angle of about 25°, cone length of 2 μm, and radii of curvature down to 10 nm, making these e-beam tips suitable for biomolecules imaging. The tip dimensions also were controlled by adjusting the beam parameters. Three different types of SFM tips were used to image tobacco mosaic virus (TMV). Conventional pyramidal tips appeared generally worse for imaging helical particles of TMV than SEM-deposited tips which were found more robust than commercially available conical tips. The use of sharper tips for SFM imaging of protein DNA revealed a 25% improvement in lateral resolution.
Computational study of heat transfer and gas dynamics in the pulsed laser evaporation of metals
Authors: Ho J. R. , Grigoropoulos C. P. , Humphrey J. A. C.
Published: July 2002

Pulsed laser irradiation of nanosecond duration is used in a variety of applications, including laser deposition of thin films and micromachining. Of fundamental interest is the prediction of the evaporative material removal rates, as well as the velocity, density, and temperature distributions of the ejected particles as functions of the laser-beam pulse energy, temporal distribution, and irradiance density on the target material surface. In order to address these issues, the present work establishes a new computational approach for the thorough treatment of the heat transfer and fluid flow phenomena in pulsed laser processing of metals. The heat conduction in the solid substrate and the liquid melt is solved by a one-dimensional transient heat transfer model. The ejected high-pressure vapor generates shock waves against the ambient background pressure. The compressible gas dynamics is computed numerically by solving the system of Euler equations for mass, momentum, and energy, supplemented by an isentropic gas equation of state. The aluminum, copper, and gold targets considered were subjected to pulsed ultraviolet excimer laser irradiation of nanosecond duration. Results are given for the temperature distribution, evaporation rate, and melting depth in the target, as well as the pressure, velocity, and temperature distributions in the vapor phase.
Electron-lattice coupling in “colossal” magnetoresistance rare earth manganites
Authors: Millis A. J.
Published: July 2002

The results reported in this conference have been published elsewhere. This article presents a brief precis of the published work, along with a few additional remarks.
The selective etching with H+ ions and its effect on the oriented growth of diamond films
Authors: Zhang W. J. , Jiang X. , Xia Y. B.
Published: July 2002

A novel etching effect of hydrogen ions on the growth of diamond films was observed. The H+ ion bombardment was performed by applying a negative substrate bias during a microwave plasma chemical vapor deposition process, using only hydrogen as a reactant gas. The effect of this bombardment was investigated by means of scanning electron microscopy. It was found that the etching efficiency of H+ ions on non-[001]-oriented grains is more significant than that on grains with their (001) faces parallel to the substrate. A lateral growth of the (001) faces can occur during the bombardment process. As a result, the size of (001) faces increases after H+ etching while grains with other directions are etched off. This effect provides a way to improve the orientation degree of [001] oriented diamond films and might be helpful for obtaining [001] oriented diamond films with small thickness.
Influence of the host lattice on the photoluminescence of Ni2+ ions in Rb1−xCsxCaF3 and RbCa1−xCdxF3 crystals
Authors: Lahoz F. , Dı́az M. , Villacampa B. , Cases R. , Alcalá R.
Published: July 2002

The optical properties of Ni2+ doped Rb1−xCsxCaF3 and RbCa1−xCdxF3 solid solutions were studied. The T2g1(D1)→A2g3(F3) and T2g1(D1)→T2g3(F3) emission bands and the lifetime of the T2g1 level were measured as a function of the crystal composition. Both the band positions and the lifetime show a monotonic change with x between the values corresponding to the pure crystals. In Rb1−xCsxCaF3 crystals the contributions of different Ni2+ centers in the emission bands were resolved. The shift of emissions is due to the Rb+–Cs+ substitution as next nearest neighbors of Ni2+ ions as well as to a change of the lattice parameter with the composition of solid solutions. Both contributions were separated for some of the Ni2+ centers.
Defects and interfaces in epitaxial ZnO/α-Al2O3 and AlN/ZnO/α-Al2O3 heterostructures
Authors: Narayan J. , Dovidenko K. , Sharma A. K. , Oktyabrsky S.
Published: July 2002

We have investigated the nature of epitaxy, defects (dislocations, stacking faults, and inversion domains), and heterointerfaces in zinc oxide films grown on (0001) sapphire and explored the possibility of using it as a buffer layer for growing group III nitrides. High quality epitaxial ZnO films were grown on sapphire using pulsed laser deposition in the temperature range 750–800 °C. The epitaxial relationship of the film with respect to (0001) sapphire was found to be (0001)ZnO∥(0001)sap, with in-plane orientation relationship of [011̄0]ZnO∥[1̄21̄0]sap. This in-plane orientation relationship corresponds to a 30° rotation of ZnO basal planes with respect to the sapphire substrate, which is similar to the epitaxial growth characteristics of AlN and GaN on sapphire. The threading dislocations in ZnO were found to have mostly 1/3〈112̄0〉 Burgers vectors. The planar defects (mostly I1 stacking faults) were found to lie in the basal plane with density of about 105 cm−1. We have grown epitaxial AlN films at temperatures around 770 °C using ZnO/sapphire heterostructure as a substrate and observed the formation of a thin reacted layer at the AlN/ZnO interface. The implications of low defect content in ZnO films compared to III–V nitrides and the role of ZnO films as a buffer layer for III–V nitrides are discussed.
Photoinduced dichroism and optical anisotropy in a liquid-crystalline azobenzene side chain polymer caused by anisotropic angular distribution of trans and cis isomers
Authors: Blinov Lev M. , Kozlovsky Mikhail V. , Ozaki Masanori , Skarp Kent , Yoshino Katsumi
Published: July 2002

Photochromism has been studied for two comb-like liquid-crystalline copolymers (I) and (II) containing azobenzene chromophores in their side chains. In a smectic glass phase of both copolymers, upon short-time irradiation by UV light, long-living cis isomers are observed. Both copolymers manifest the photoinduced anisotropy, the physical mechanisms of which seem to be quite different. In spin-coated films of polymer (II), the origin of the anisotropy is a strong stable dichroism, which is due to an enrichment and depletion of the chosen angular direction, correspondingly, with trans and cis isomers of the azobenzene chromophores. Polymer (I) manifests no dichroism at all, and its induced optical anisotropy may be accounted for by a rather slow chromophore reorientation. In copolymer (II) a considerable reorientation of the mesogenic groups also occurs as a secondary phenomenon at the stage of the cis isomer formation only. This observation shed more light on the general process of the light-induced molecular reorientation in polymers, liquid crystals, and Langmuir–Blodgett films, which is of great importance for holographic information recording.
PtMn single and dual spin valves with synthetic ferrimagnet pinned layers
Authors: Saito Masamichi , Hasegawa Naoya , Koike Fumihito , Seki Hiroshi , Kuriyama Toshihiro
Published: July 2002

Spin valve films with the PtMn/Co/Ru/Co synthetic ferrimagnet pinned layer, in which the magnetization of the two Co layers is strongly coupled in an antiparallel orientation and the magnetization direction of one of the Co layers is pinned by unidirectional exchange coupling with PtMn, were investigated. PtMn synthetic ferrimagnet pinned layers were demonstrated to exhibit strong unidirectional exchange field Hex exceeding 2500 Oe at an optimum PtMn/Co/Ru/Co thickness combination for either PtMn on top or at the bottom. The Hex of PtMn/Co/Ru/Co rapidly increased at the PtMn layer thickness around 10–12 nm and was nearly constant from 12 to 30 nm. It was also found that the saturation field of the Co(3 nm)/Ru(0.8 nm)/Co(2 nm) sandwich, which represents the antiparallel interlayer coupling strength through Ru, remained 2200 Oe even at 300 °C. Thus the combination of the high blocking temperature of 380 °C for PtMn and the large antiparallel exchange interaction of Co/Ru/Co up to 300 °C makes the pinned layer highly stable against various magnetic fields during Joule heating of spin valve sensor stripes. Further, the PtMn synthetic-ferri-pinned-type dual spin valve films showed a large unidirectional exchange field of 2000 Oe and a giant magnetoresistance ratio of 11.0%.
Magnetic structure and site occupancies in YFe11−xCoxTi (x=1,3,7,9)
Authors: Liang J. K. , Huang Q. , Santoro A. , Wang J. L. , Yang F. M.
Published: July 2002

The crystal and magnetic structures of the system YFe11−xCoxTi have been investigated by high-resolution neutron powder diffraction for the compositions x=1, 3, 7, and 9. All compounds have the ThMn12 type structure, with space group symmetry I4/mmm. The distribution of the atoms over the independent crystallographic sites was found to be significantly different from random, with a strong preference for Ti to be in the 8i site and Co in the 8j and 8f sites. This behavior can be explained in terms of the enthalpy of formation of binary alloys. The magnetic moments are parallel to the c axis for the compositions x=1, 3, 9, and lie in the a-b plane for x=7. No evidence of additional nuclear or magnetic phase transitions was observed for sample YFe4Co7Ti down to 20 K.
Defect clusters in Fe1−xO and their ferrimagnetic properties
Authors: Dimitrov D. V. , Unruh K. , Hadjipanayis G. C. , Papaefthymiou V. , Simopoulos A.
Published: July 2002

Ferrimagnetic properties, large saturation magnetization (Ms) and low temperature coercivity (Hc) was observed in nonstoichiometric Fe1−xO films. This unusual behavior was attributed to the existence of spinel type defect clusters coherently embedded in the FeO matrix. A model, which explains the Fe1−xO properties and predicts a linear dependence between Ms and the Fe1−xO lattice parameter a with the Fe3O4 data falling on the same line, is proposed. The experimental Ms vs a curve is in excellent agreement with the theoretical prediction, which indicates the existence of a fundamental relation between FeO, Fe1−xO, and Fe3O4.
Isotropic–nematic transition in micellized solutionsa)
Authors: McMullen William E. , Gelbart William M. , Ben-Shaul Avinoam
Published: July 2002

We treat the isotropic-to-nematic transition in polydisperse suspensions of micellar rods. By generalizing an early theory of Onsager, we obtain a first-order transition in which the ordered rods are significantly longer than those in the (coexisting) isotropic phase. We show that this ‘‘growth’’ is driven by the free energy term arising from loss of orientational entropy upon alignment. For large aggregates the product ρ̄L̄2D (where L̄ is a number-averaged length, D the diameter of the rods, and ρ̄ the number density of aggregates) is roughly constant for each phase at coexistence. The transition value of the (nematic) order parameter (≂〈P2(cos θ)〉) first increases linearly with aggregation number s (for small rods) and approaches unity asymptotically (for large s). We compare our results to those of an earlier, monodisperse treatment of this system and interpret the nematic-induced growth in terms of coupling between rod size and alignment in micellar solutions.
Dependence of the optical spectrum of MnF4−6 on the Mn2+–F− distance
Authors: Rodríguez Fernando , Moreno Miguel
Published: July 2002

We have measured by means of photoluminescence the energy of crystal-field peaks for RbCdF3: Mn2+ and KZnF3: Mn2+ where the value of the Mn2+–F− distance, R, derived by EXAFS is R=2.13±0.01 Å and R=2.08±0.01 Å, respectively. From these data and those for RbMnF3 and KMnF3 we have studied the dependence on R of the B, C, and 10 Dq parameters for the MnF4−6 complex. This analysis reveals that within the experimental errors, B and C are constant in the range 2.07<R<2.14 Å, in agreement with recent self-consistent calculations for MnF4−6, which also predict that 10 Dq=KR−n, where K and n are constant. The present study confirms this dependence, n being 4.4 which is also in accord to the theoretical predictions. The best values of R derived from optical spectra are found to be R=2.141±0.004 Å (for RbCdF3: Mn2+) and R=2.075±0.004 Å (for KZnF3: Mn2+). The present analysis also points out that by measuring the changes induced on the optical spectrum of MnF4−6 in a given lattice we can detect changes in the Mn2+–F− distance down to 10−3 Å. In this way we have derived the difference, ΔR, between R at room temperature and at 77 K for KZnF3: Mn2+. The obtained value ΔR=(9±1)10−3 Å is in agreement with the one ΔR=(10±3.5)10−3 Å derived previously from the variations undergone by the isotropic superhyperfine constant As. Finally the present results are compared to those for some complexes of Eu2+, Co2+, Ni2+, and Cr3+.
Thermal diffusion measurements near a critical end point in a microemulsion system
Authors: Dorion P. , Lalanne J. R. , Pouligny B. , Imaizumi S. , Garland C. W.
Published: July 2002

A new optical technique has been developed for the study of thermal conductivity Λth and thermal diffusivity Dth in critical fluids. The new method combines the advantages of two traditional methods (induced thermal lens and thermal dynamic grating) while eliminating difficulties associated with them. This method has been applied to the investigation of dynamic behavior in a microemulsion composed of n-dodecane, water, n-pentanol, and sodium dodecyl sulfate. This system exhibits critical phase separation (into two microemulsions with different dispersed phases concentrations) that can be considered as analogous to binary liquid phase separation near a consolute point. Over a range of microemulsion compositions there exists a line of such critical points terminating at a critical end point. Near this critical end point both Λth and Dth exhibit an anomalous increase as the temperature approaches the critical temperature. The ratio Λth/Dth is found to be essentially constant near Tc, in agreement with the behavior of independently measured heat capacity values. The results are discussed in the context of current theoretical predictions for the critical transport properties of binary liquids.
Reaction rates of H(H2), D(H2), and H(D2) van der Waals molecules and the threshold behavior of the bimolecular gas-phase rate coefficient
Authors: Hancock Gene C. , Mead C. Alden , Truhlar Donald G. , Varandas Antonio J. C.
Published: July 2002

We study low-energy quantal phenomena in the rearrangement of three-atom systems composed of H and D. All calculations are carried out on the double many-body expansion potential energy surface for the hydrogen trimer. The unimolecular rearrangements of van der Waals molecules, such as D⋅⋅⋅H2→HD⋅⋅⋅H, are studied as a model for the exchange transfer reaction in condensed phases, and the gas-phase bimolecular reactions, such as D+H2→HD+H, are studied to probe the limiting low-temperature threshold behavior, which is compared to that predicted by quantum mechanical threshold laws. The reaction rates are studied down to temperatures of 10−3 K. We also predict the spectroscopic tunneling shift on the lowest energy levels of the H⋅⋅⋅H2 complex.
State-selected photodissociation dynamics of HONO(Ã 1A″): Characterization of the NO fragment
Authors: Dixon R. N. , Rieley H.
Published: July 2002

T r a n s-HONO has been photodissociated through the 220 band of the Ã1A″–X̃ 1A′ system at 355 nm. The energy disposal in the NO fragment, and vector correlations between its motions, have been characterized using polarized one-photon excited laser induced fluorescence. The rotational distribution, rotational alignment, degree of electron alignment, spin–orbit branching ratio, approximate vibrational distribution, and energy partitioning are determined. The rotational distributions in each of v(NO)=3,2 and 1 are highly inverted and approximately Gaussian in quantum number. The peak J decreases with increasing v to give a fairly narrow overall internal energy distribution. The disposal of the available energy into all NO and OH motions is 60% and 40%, respectively, and is also 40% into internal motions, mainly of NO, and 60% into translational recoil. The NO rotation shows a positive alignment. All the vector correlations are consistent with planar fragmentation dynamics, but with a finite dissociation lifetime of about 100 fs. The Λ-doublet populations of NO indicate the preference of the unpaired π electron to occupy an orbital lying perpendicular to the plane of rotation, which is opposite to that found earlier for the OH fragment. The population of the NO spin–orbit states is inverted. These results are discussed in terms of dynamical models and the features of a recently published ab initio surface for the HONO Ã state.
Comment on linewidths and shifts in the Stokes–Raman Q branch of D2 in He
Authors: Green Sheldon , Blackmore Robert , Monchick Louis
Published: July 2002

Collision induced widths and shifts for Raman Q-branch transitions of D2 in He were calculated from S matrices obtained with converged close coupling scattering calculations on an accurate theoretical interaction potential. Results agree well with experimental values. Discrepancies between experimental line shifts and theoretical values from an earlier study are traced to a computational error in that work. The effects of vibrational inelasticity and of centrifugal distortion on energy levels and on potential matrix elements, all of which were ignored in the earlier study, are explicitly considered here and found to be small.
Exotic exciton–phonon states and bottleneck for self-trapping
Authors: Köngeter Andreas , Wagner Max
Published: July 2002

In a previous paper the conventional barrier model of excitonic self-trapping has been shown to display divergencies. In the present work we report on novel ‘‘exotic’’ exciton–phonon states, which have not been noted so far. These states are of non-self-trapping nature, and by way of their small extension in Q space they may acquire a ‘‘bottleneck’’ role in the process of energy dissipation. This seems to allow for a new explanation of the fast luminescence found in rare gas and other crystals.
Effect of spinning on chemical shifts in magic-angle-spinning nuclear magnetic resonance
Authors: Hayashi Shigenobu , Hayamizu Kikuko
Published: July 2002

We have found that the isotropic peak position changes depending on the spinning rate of the sample in 63Cu, 65Cu, 79Br, and 127I magic-angle-spinning nuclear magnetic resonance (NMR) of cuprous halides, CuCl, CuBr, and CuI. With the increase in the spinning rate, the 63Cu and 65Cu signals shift towards lower frequencies, while the 79Br and 127I signals shift towards higher frequencies. The magnitudes of the frequency shift are in the following order: CuCl<CuBr<CuI, and the shifts in the halogen signals are of comparable magnitudes to those in the copper signals except for the shift direction. The magnitude of the spinning-induced shift is found to be proportional to the square of the spinning rate and to the strength of the magnetic field. We have proposed that the spinning-rate-dependent shift has its origin in the Lorentz force working on Cu+ ions being forced to move fast in a strong magnetic field, and have derived formulas to explain the above phenomena. We have also checked other possibilities, and have concluded that no other mechanisms can explain the above phenomena.
Recoil effects in surface dissociation
Authors: Hand M. , Harris J.
Published: July 2002

Using real-time wave packet propagation we consider the effects of lattice recoil, inelasticity and surface temperature in strongly activated dissociation reactions of diatomic molecules at surfaces. The energy diagram governing the dissociation, modeled as suggested by electronic structure calculations for H2 dissociation at Cu surfaces, consisted of an entrance channel barrier separated from the chemisorption region by a ridge, where dissociation takes place. Lattice recoil is simulated by coupling this ‘‘stiff-barrier’’ PES to a harmonic oscillator. Calculations were carried out for masses and potential parameters appropriate to H2/D2 dissociation on Cu and N2 dissociation on Fe. Barrier recoil was found to suppress the dissociation probability as compared with its stiff-barrier value. The effect, marginal for H2 and D2 but pronounced in the case of N2, can be understood in terms of dynamical increases in the barrier width and height. Simulations where the N2–Fe barrier was excited in the initial state showed that the influence of surface temperature on the dissociation can be quite dramatic and led to a strong enhancement in the tunneling region.
Geometry of the steady-state approximation: Perturbation and accelerated convergence methods
Authors: Roussel Marc R. , Fraser Simon J.
Published: July 2002

The time evolution of two model enzyme reactions is represented in phase space Γ. The phase flow is attracted to a unique trajectory, the slow manifold ℳ, before it reaches the point equilibrium of the system. Locating ℳ describes the slow time evolution precisely, and allows all rate constants to be obtained from steady-state data. The line set ℳ is found by solution of a functional equation derived from the flow differential equations. For planar systems, the steady-state (SSA) and equilibrium (EA) approximations bound a trapping region containing ℳ, and direct iteration and perturbation theory are formally equivalent solutions of the functional equation. The iteration’s convergence is examined by eigenvalue methods. In many dimensions, the nullcline surfaces of the flow in Γ form a prism-shaped region containing ℳ, but this prism is not a simple trap for the flow. Two of its edges are EA and SSA. Perturbation expansion and direct iteration are now no longer equivalent procedures; they are compared in a three-dimensional example. Convergence of the iterative scheme can be accelerated by a generalization of Aitken’s δ2 extrapolation, greatly reducing the global error. These operations can be carried out using an algebraic manipulative language. Formally, all these techniques can be carried out in many dimensions.
Brillouin scattering on the glass former GeSBr2
Authors: Loheider S. , Vögler G. , Petscherizin I. , Soltwisch M. , Quitmann D.
Published: July 2002

Brillouin scattering measurements were performed on the partly interrupted network glass former GeSBr2. Spectra were taken in symmetrical transmission arrangement, and in backscattering, thus varying momentum transfer between 2×104/cm and 44×104/cm. The temperature range was −100 °C<T<300 °C; the glass transition occurs at Tg∼−30 °C. The Brillouin spectra were carefully compared with viscoelastic theory. Strong relaxation accompanied by a dispersion of sound velocity occurs between 100 and 200 °C. A comparison of the spectra taken at different scattering angles and temperatures constitutes convincing evidence for a distribution of relaxation times (a Cole–Davidson distribution is used). The temperature dependence of the maximum relaxation time τ0 can well be described as Arrhenius-like, τ0 varying from 10−11 to>5×10−9 s. The temperature dependences of the high frequency and the adiabatic sound velocities are derived. From the q-dependence, a fast relaxation channel can be identified. We discuss the assumption that it is directly connected with the break up of the glass above Tg. Including this term an agreement is reached between experiment and viscoelastic theory on the level of about 1% for the Brillouin line position and of about 10% for the linewidths, for the glassy, highly viscous, and (relatively) low viscosity regime.
Variational calculation of the rovibrational spectrum of H2O using the coupled-rotor basis
Authors: Leitner David M. , Hunter John E. , Natanson Gregory A. , Berry R. Stephen , Villarreal Pablo , Delgado-Barrio Gerardo
Published: July 2002

We present rovibrational energies and transition intensities for H2O calculated at several levels of approximation, beginning with the adiabatic approximation (separating slow bending and rotation from fast stretching modes), then coupling the slow modes with the symmetric stretch, and culminating with the inclusion of all the couplings in the Hamiltonian. Evaluation of each approximation is made by comparing the results of the ab initio spectra to experiment. A space-fixed rovibrational basis set is used for the variational calculations.
A wave packet Golden Rule treatment of vibrational predissociation
Authors: Villarreal P. , Miret-Artés S. , Roncero O. , Delgado-Barrio G. , Beswick J. A. , Halberstadt N. , Coalson R. D.
Published: July 2002

The time-dependent wave packet technique is applied to the Golden Rule treatment of vibrational predissociation. The wave packet at time zero is taken as the product of the quasibound wave function and the coupling inducing predissociation. The rate for vibrational predissociation can then be obtained by Fourier transform into the energy domain of the time-dependent wave packet autocorrelation function. The method has been applied to a model triatomic van der Waals molecule. It is shown that when the bound-state components of the wave packet are projected out, the time-dependent version of the Golden Rule approximation provides an alternative efficient technique to treat intramolecular decay.
Monte Carlo calculation of chemical potential for the Stockmayer fluid
Authors: Han K.-K. , Diestler D. J. , Cushman J. H.
Published: July 2002

The chemical potential of the Stockmayer fluid is computed by the canonical ensemble Monte Carlo methods of Widom, Bennett, and Han. Both spherical cutoff and metallic Ewald sum techniques are employed. For the states studied it is found that the Ewald sum provides little improvement over the simpler spherical cutoff, that Bennett’s and Han’s methods produce essentially the same results, and that for certain states Widom’s method is less accurate than Bennett’s or Han’s. The accuracy of the chemical potential calculated via the three methods was verified with Adams’ version of the grand canonical ensemble Monte Carlo method.
Use of molecular dynamics methods in conformational analysis. Glycol. A model study
Authors: Hooft Rob W. W. , van Eijck Bouke P. , Kroon Jan
Published: July 2002

Different methods to calculate the potential of mean force using molecular dynamics have been used to estimate the difference in free energy between the gauche and trans conformations of the 1,2-ethanediol (glycol) molecule dissolved in water and in carbon tetrachloride. Results indicate that even in this simple model study simulations of many picoseconds are needed to obtain acceptably low estimated standard deviations. The calculated trans/gauche ratio in aqueous solution is larger than the observed one. Assuming that the solvent effects are more reliable than the intramolecular force field we used the observed ratio to predict an abundance of about 3% trans to be present in CCl4 solution. Details of the results for this solvent agree well with infrared spectra.
Structure of acidic haloaluminate melts: Neutron diffraction and quantum chemical calculations
Authors: Blander M. , Bierwagen E. , Calkins K. G. , Curtiss L. A. , Price D. L. , Saboungi M.-L.
Published: July 2002

The structure of molten mixtures of (KX)y(AlX3)1−y was investigated by neutron diffraction for X=Cl and Br and y=0.25 and 0.33. These melts, known as acidic haloaluminates, contain different ionic species such as AlX−4,Al2X−7, Al3X−10. The diffraction patterns were similar, although not identical, for the same halide pairs and showed features indicative of short-range and intermediate-range order. Quantum chemical calculations were carried out on AlX−4, Al2X−7, and Al3X−10 to provide information needed to interpret the structural results within the framework of the random packing of structural units model. It is shown that the bent Al–X–Al bridge for Al2X−7, obtained from the quantum chemical calculations, is necessary to represent well the structure, and that the values of the bond angles at the central halide atoms are around 110°. Aluminum atoms are found to be tetrahedrally coordinated by the halide atoms, and information on the orientational correlations of neighboring Al2X−7 is derived.
Laser photolysis of benzene at 158 nm
Authors: Shimada Tetsuya , Nakashima Nobuaki , Izawa Yasukazu , Yamanaka Chiyoe
Published: July 2002

Gaseous benzene and benzene-d6 have been photolyzed with a 158 nm laser. Transient species have been found to have decay rate constants (2.2±0.2)×107 s−1 and (0.7±0.2)×107 s−1, respectively. These rates are explained in terms of the C–H/C–D dissociation by the hot molecule mechanism.
Properties of liquid acetone in silica pores: Molecular dynamics simulation
Authors: Bródka A. , Zerda T. W.
Published: July 2002

Molecular dynamics simulations are reported for molecular acetone confined in a model cylindrical pore within a structure representing amorphous silica. Calculations are performed for pores of average diameters 1.5, 2.0, 2.5, and 3.0 nm. To evaluate the role of electrostatic interactions on the structure and dynamics of the confined liquid different potentials are considered; a guest molecule with and without a dipole moment, and the host material with and without electrostatic charges. A layered structure of molecules inside the pores is observed. The monolayer is always well defined but when polar forces are included the mobility of the molecules is highly restricted and localized adsorption centers can be identified. The most probable orientation of molecules on the pore walls is discussed. Translational and rotational motions within the monolayer are highly hindered and they are slower for stronger attractive potentials. Diffusion of polar molecules in the pore center is faster than for nonpolar molecules. This effect is explained in terms of increased molecular packing near the pore walls caused by strong adsorbate–adsorbent interactions.
High-nuclearity mixed-valence magnetic clusters: A general solution of the double exchange problem
Authors: Borrás-Almenar J. J. , Clemente J. M. , Coronado E. , Palii A. V. , Tsukerblat B. S. , Georges R.
Published: July 2002

We report here a general solution of the double-exchange problem in the high-nuclearity mixed valence systems containing arbitrary number P of the electrons delocalized over the network of N (P<N) localized spins. The developed approach is based on the successive (chainlike) spin-coupling scheme and takes full advantage from the quantum angular momentum theory. In the framework of this approach the closed-form analytical expressions are deduced for the matrix elements of the double exchange interaction, two-electron transfer, and three-center interaction that can be referred to as the potential exchange transfer. For the arbitrary nuclearity mixed-valence systems the matrix elements of all named interactions are expressed in terms of all relevant spin quantum numbers and 6j symbols and do not contain higher order recoupling coefficients. We describe also the combined approach taking into account both angular momentum consideration and advantages of point symmetry adapted basis set.
Correlation of Kauzmann temperature with odd–even effect in n-alkanes
Authors: Kishore K. , Bharat S. , Kannan S.
Published: July 2002

A unique correlation has been established between Kauzmann temperature (Tk1) and the odd–even effect in n-alkanes. The derived new parameter, i.e., Tm/Tk1 obtained from entropy conservation at Tk1, when plotted against chain length, provides a much sharper odd–even contrast than entropy of fusion plot reported earlier.
The bond energy of Rh2
Authors: Langenberg Jon D. , Morse Michael D.
Published: July 2002

In a spectroscopic investigation of jet-cooled Rh2 by the resonant two-photon ionization method, an abrupt predissociation threshold is observed in a dense set of vibronic levels at 19 405±4 cm−1. Based on the high density of states expected in the rhodium dimer, the sharp definition of the predissociation threshold that is observed, and the validation of a similar conclusion in the case of V2, it is argued that predissociation occurs as soon as the energy of the separated ground state atoms is exceeded. On this basis the bond energy of Rh2 is assigned as D0(Rh2)=19 405±4 cm−1=2.4059±0.0005 eV. This value is compared to the results of other experiments and to theoretical calculations. The bond energy of disilver, D0(Ag2)=1.65±0.03 eV, provides a particularly useful standard of comparison, and suggests that 4d contributions to the bond energy of Rh2 amount to at least 0.76 eV.
A time-independent theory of photodissociation based on polynomial propagation
Authors: Guo Hua
Published: July 2002

The time-dependent quantum theory of molecular photodissociation of Heller is reformulated in the framework of polynomial propagation. The new formulation retains the essential features of the time-dependent approach, but requires neither propagation in time nor interpolation of the evolution operator. In this new approach, the propagation is carried out by recursion of the corresponding orthogonal polynomial, which requires minimal storage. The wave packet can be restricted to real space, further reducing cpu and memory requirements. If the wave packet is propagated by the Chebyshev operator, the total cross section can be obtained via the cosine Fourier transform from the order-dependent autocorrelation function. Like the time-dependent approach, the internal state distributions of the fragment can be projected out from the asymptotic wave packet. The nonadiabatic photodissociation of methyl iodide with two active dimensions is employed to illustrate the applicability of the new approach.
A neutron diffraction and computer modeling study of the interatomic structure of phosphoric acid
Authors: Tromp R. Hans , Spieser Stephane H. , Neilson George W.
Published: July 2002

Wide angle neutron diffraction in combination with H/D substitution was used to determine the inter- and intramolecular structure of 100% phosphoric acid (H3PO4, PA). From radial distribution functions gHH(r), gHX(r), and gXX(r) (where X is either O or P) the hydrogen bonds were found to be characterized by a very short O…H distance (1.54 Å). Within a molecule, the orientation of an OH group was found to be preferably in one of the three O–P–O planes. In the interpretation of the radial distribution functions, use was made of preliminary results of molecular dynamics simulations. Temperature effects on the structure of PA were only found in the hydrogen bond structure, which becomes somewhat less well defined when heating up from room temperature to 60 °C. Polyphosphates could not be detected, probably due to the small degree of polymerization.
A method of determination of quantum yields of S3⇝S2, S3⇝S1, and S3⇝S intramolecular radiationless transitions
Authors: Maciejewski Andrzej , Milewski Marek , Szymański Marian
Published: July 2002

An effective method for determination of the quantum yields of the S3⇝S2, S3⇝S1 and S3⇝S0 intramolecular radiationless transitions, based on steady-state measurements of absorption, S2 fluorescence and T1 phosphorescence or S1 fluorescence under selective excitation to the S2 and S3 states has been described and used for three aromatic thioketones. The results show that in the compounds studied the deactivation of the S3 state is not strictly sequential and the S3⇝S1 as well as, unexpectedly, S3⇝S0 transitions occur with relatively high yields. Measurement of the quantum yields of S3⇝S2, S3⇝S1, and S3⇝S0 intramolecular radiationless transitions is essential for the determination of the intramolecular properties of molecules in the S3 state as well as pathways of their decay.
Condensation of supersaturated water vapor on submicrometer particles of SiO2 and TiO2
Authors: Chen Chin-Cheng , Tao Chun-Ju
Published: July 2002

Condensation of supersaturated water vapor on monodisperse submicrometer SiO2 and TiO2 particles is investigated in a flow cloud chamber (FCC). The dependence of the critical supersaturation Scr on particle size in the range of 15 to 120 nm and on temperature in the range of 10 to 50 °C are determined experimentally. The results show that the experimental Scr decreases with increasing particle size, at a rate in reasonable agreement with that predicted by Fletcher’s version of Volmer’s theory of heterogeneous nucleation, but decreases with increasing temperature at a rate higher than that predicted by Fletcher’s theory. The experimental Scr is smaller than the theoretical prediction even with the line tension and surface diffusion taken into account; the particles induce heterogeneous nucleation better than theory predicts even for perfectly wetted particles. The discrepancy cannot be fully accounted for by the effects of line tension, surface diffusion, or the curvature dependence of the physical properties. The condensation of supersaturated vapor on singly-positively-charged particles with diameters of 30, 60, and 90 nm is also examined. No obvious effect of charge on Scr is observed, in agreement with the theoretical prediction. The law of corresponding states was extended to heterogeneous nucleation, and a simple correlation was obtained.
Chiral symmetry breakdown. III. Delbourgo’s gauge technique
Authors: Atkinson D. , Hulsebos A. , Johnson P. W.
Published: July 2002

The quark propagator in massless quantum chromodynamics (QCD) is analyzed using the gauge technique. In both the Feynman and Landau gauges with a Pauli–Villars cutoff, a chirally symmetric solution is found, while a nonsymmetric solution appears at a critical coupling λc>0. As the cutoff is removed, λc tends to 0 but the nonsymmetric solution vanishes in the continuum limit, so that chiral symmetry is then restored.
Phase shifts in the collision of massive particles
Authors: Sidharth B. G.
Published: July 2002

In this paper phase shifts for inverse power potentials and their superpositions are considered. These potentials govern a large number of collision processes involving massive particles. But they are problematic as they do not conform to the norms obeyed by standard potentials: The usual rules for estimating the error of the phase shifts break down and an abnormally large number of phase shifts have to be computed, even if they are small. A sufficiency condition is deduced for the applicability of the Born approximation which shows that, for a given potential strength and l, surprisingly, the Born approximation is good at low energies and bad at high energies. The magnitude and direction of the error committed are also estimated. These conclusions are then verified in a number of special cases, for instance, the inverse fourth power potential used in the scattering of electrons by atoms and the Lennard–Jones potential used in the scattering of beams of molecular hydrogen with mercury atoms.
On the compatibility of relativistic wave equations in Riemann–Cartan spaces. II
Authors: Buchdahl H. A.
Published: July 2002

In a Riemann–Cartan space-time U4 the minimally coupled massive spin-S Dirac equations are subject to constraints for all S> 1/2 . One may seek to modify these equations by the inclusion of appropriate supplementary terms α̂, β̂ (indices suppressed) in such a way that the modified equations are unconstrained. This is done here explicitly in the case where the field spinors are ξν1⋅⋅⋅νn and ημ̇ν1⋅⋅⋅νn−1, with n=2S. The spin-1 tensor equations, i.e., the minimally coupled Proca equations, are transcribed into spinorial form. The explicit expressions for α̂ and β̂ that so appear are in harmony with those already obtained. The freedom to choose α̂ to be a zero spinor is shown to be circumscribed. Finally, it is pointed out that whether the unconstrained (spin-1) equations are minimally coupled or not depends on whether one chooses to write them as tensor or spinor equations.
Parametrization of SU(n) with n−1 orthonormal vectors
Authors: Bincer Adam M.
Published: July 2002

A generalization to SU(n) of a well-known relation in SU(2) is proposed. It relies on the observation that an element of SU(n) has associated with it in a natural way n−1 orthonormal vectors in Rn2−1. The meaning of these n−1 vectors is discussed as they relate to the geometry of the adjoint representation of SU(n).
On induced scalar products and unitarization
Authors: Gruber Bruno , Lenczewski Romuald , Lorente Miguel
Published: July 2002

Infinitesimal unitarity of representations of the simple real Lie algebras su(2), su(1,1), and so(4) is discussed with respect to scalar products induced by sesquilinear forms on their universal enveloping algebras. Sesquilinear forms are explicitly calculated. The Verma modules, their irreducible quotients, their irreducible submodules, and their infinitesimal unitarizability are discussed.
The Kadomtsev–Petviashvili equation under rapid forcing
Authors: Moroz Irene M.
Published: July 2002

We consider the initial value problem for the forced Kadomtsev–Petviashvili equation (KP) when the forcing is assumed to be fast compared to the evolution of the unforced equation. This suggests the introduction of two time scales. Solutions to the forced KP are sought by expanding the dependent variable in powers of a small parameter, which is inversely related to the forcing time scale. The unforced system describes weakly nonlinear, weakly dispersive, weakly two-dimensional wave propagation and is studied in two forms, depending upon whether gravity dominates surface tension or vice versa. We focus on the effect that the forcing has on the one-lump solution to the KPI equation (where surface tension dominates) and on the one- and two-line soliton solutions to the KPII equation (when gravity dominates). Solutions to second order in the expansion are computed analytically for some specific choices of the forcing function, which are related to the choice of initial data.
Conservation laws and Calapso–Guichard deformations of equations describing pseudo-spherical surfaces
Authors: Reyes Enrique G.
Published: July 2002

The relation between the Chern and Tenenblat approach to conservation laws of equations describing pseudo-spherical surfaces (conservation laws from pseudo-spherical structure) and the more familiar “Riccati equation” approach (conservation laws from associated linear problem) is investigated. Two examples [cylindrical Korteweg–de Vries (KdV) and Lund–Regge equations] are presented. Chern and Tenenblat’s point of view is then connected with the theory of soliton surfaces. A generalization of the original Chern–Tenenblat construction of conservation laws results, and a reasonable family of large deformations for scalar equations describing pseudo-spherical surfaces, the “equations describing Calapso–Guichard surfaces,” can be introduced. It is shown that these equations are also the integrability condition of linear problems.
Principles of finite-dimensional perturbation theory
Authors: Krasovskiı̆ I. V. , Peresada V. I.
Published: July 2002

The theory of finite-dimensional perturbations of self-adjoint operators, which is aimed at the solution of physical problems, is reviewed. Special attention is paid to the special kind of operators, which permits efficient application of the J-matrix technique. The spectral density of a periodic J-matrix is calculated.
Structure and superfluidity of He4 films on plated graphite
Authors: Nyéki J. , Ray R. , Sheshin G. , Maidanov V. , Mikheev V. , Cowan B. , Saunders J.
Published: July 2002

The results of an experimental study using torsional oscillators of the superfluidity of 4He films adsorbed on hydrogen plated graphite are reported. The evolution of superfluidity with the growth of the film shows considerable structure arising from the atomic layering of the film. There is evidence that the superfluidity of a single fluid layer is strongly suppressed, possibly due to the influence of the periodic potential arising from the underlying solid layer. The behavior of two fluid layers is quite distinct, but shows similarities to that of thicker films on heterogeneous substrates; we suggest that the usual theory of the superfluidity of two-dimensional 4He should be extended to account for superfluid onset temperatures in such a film.
Approach to the steady solution for a plane Chapman–Jouguet detonation
Authors: Fickett Wildon
Published: July 2002

In the context of the author’s mathematical analog for reactive flow, the late-time approach to the steady solution for a plane Chapman–Jouguet (CJ) detonation is studied. The reaction kinetics are those of the ‘‘small resolved heat release’’ model, in which the bulk of the chemical energy is released instantaneously in the shock and the remainder is released at a finite rate in the following reaction zone. Calculated shock histories and particle histories (Lagrangian gauge records) are presented for two driver prescriptions that initiate the detonation close to the steady solution. The implications of these results for the interpretation of CJ pressure measurements is discussed.
Rigorous bounds on the fluid permeability: Effect of polydispersivity in grain size
Authors: Torquato S. , Lu B.
Published: July 2002

Rigorous bounds on the fluid permeability (on resistance) of porous media composed of spherical grains with a continuous size distribution are computed. For any finite degree of polydispersivity, scaling the resistance bound by the square of the specific surface (relative to the monodisperse case) yields effectively universal behavior at a fixed sphere volume fraction. A new proposition regarding an exact relationship between the permeability and another effective parameter, the trapping constant associated with diffusion-controlled reactions among traps, is employed to assess the accuracy of the rigorous bound.