bluuebunny/crossref_metadata_2025 huggingface dataset:
A magnetic resonance study of pore filling processes during spontaneous imbibition in Berea sandstone
Authors: Chen Quan , Gingras Murray K. , Balcom Bruce J.
Published: October 2003

A new magnetic resonance technique, DDIF (the decay of magnetization due to diffusion in the internal field), was combined with mercury porosimetry to investigate pore geometry, including pore- and throat-size distribution, and pore connectivity for porous media. A comparison of DDIF spectra for a fully water saturated Berea sandstone, with the partially saturated sample by centrifugation in air, indicated that DDIF can be used for the measurement of water filled pore size distribution in partially saturated porous media. Dynamic water imbibition into air-filled Berea sandstone was studied using the DDIF technique. Simultaneously, in situ three-dimensional saturation and capillary driven water penetration were monitored using Conical-SPRITE, which is a rapid, centric scanning, spin-density weighted single point three-dimensional magnetic resonance imaging technique. These measurements provide direct evidence for differences in the pore filling mechanisms for co-current imbibition and counter-current imbibition in Berea sandstone. During co-current imbibition, water flows through the pores and connected throats with a piston-type mechanism. Air is displaced from the sample by the leading edge of the waterfront, resulting in a macroscopic piston-like flow through the entire sample. During counter-current imbibition, water flows through the pores and connected throats with a film-like structure along the corners and surfaces of the pore space. Air escapes from the sample by flowing through the center of the pores and pore throats, in the opposite direction. Once the penetrating waterfronts meet, at the sample center, there is a global, uniform increase in water content.
Oxygen ion migration in orthorhombic LaMnO3−δ
Authors: Woodley Scott M. , Gale Julian D. , Battle Peter D. , Catlow C. Richard A.
Published: October 2003

Interatomic potentials that can model ligand field effects were used to investigate the properties of vacancies in orthorhombic LaMnO3. The minimum energy structures of LaMnO3−δ (where δ=1/192) were calculated for an oxygen vacancy on either the O1 or O2 site, respectively. It is predicted that the “degenerate” activation energy (and pathway) for oxygen diffusion in cubic LaMnO3−δ is lifted after a cubic–orthorhombic phase transition. Within the orthorhombic phase, one of the once triply degenerate activation energies is lowered, indicating that there is a preferred migration pathway, while one is increased, indicating an increased activation energy for unrestricted oxygen migration (and a much closer agreement to that observed in strontium doped systems). The lowest energy pathways within the orthorhombic, as opposed to the cubic perovskite structure, are no longer symmetric. The activation energies of migration indicate a preferential vacancy migration between the O2 and O1(s) sites, where the migrating oxygen ion would simultaneously arc around the central manganese ion with a bond length, Mn–O, which varied between 1.72 and 1.77 Å. This type of pathway suggests that vacancies migrate along O2(m)–O1(s)–O2(m)–O1(s) chains.
Nucleation of liquid bridges and bubbles in nanoscale capillaries
Authors: Vishnyakov Aleksey , Neimark Alexander V.
Published: October 2003

Nucleation of liquid bridges and bubbles during condensation and evaporation of Lennard-Jones fluid in cylindrical pores is explored by Monte Carlo simulation. The isotherm of constrained critical nuclei is constructed using the gauge cell method. We confirm the Everett–Haynes scenario of bridging through the formation of a bump/undulation on the adsorption film. The molecular structure of growing bridges and cavitating bubbles is revealed. A new simulation approach is introduced to calculate the nucleation energy barriers. The method is based on the introduction and subsequent removal of a virtual “ghost” potential field with a tunable magnitude. Two computation schemes for determining the free energy of nuclei are elaborated based on the thermodynamic integration along a trajectory of states generated in the tunable ghost field and on the umbrella sampling. The methods developed are applicable to study various nucleation phenomena.
Fisher information in density functional theory
Authors: Nagy Á.
Published: October 2003

The Euler equation of the density functional theory is derived from the principle of minimum Fisher information. Both time-independent and time-dependent cases are analyzed. It is shown that Fisher information is a measure of the quality of the approximate density.
Ab initio study of the O(1D)+CH4(X 1A1)→OH(X 2Π)+CH3(X 2A2″) reaction: Ground and excited potential energy surfaces
Authors: Hernando Jordi , Millán Judith , Sayós R. , González Miguel
Published: October 2003

The two potential energy surfaces (1 1A and 2 1A PESs) adiabatically correlating the reactants and products asymptotes of the title reaction were studied by means of the CASSCF and CASPT2 ab initio methods. The minimum energy path determined for the ground PES evolved through the barrierless insertion of the O(1D) atom into a C–H bond. The OH+CH3 products result from the dissociation of the CH3OH methanol intermediate formed. Reactivity on the excited 2 1A PES was found to proceed via an abstraction pathway. The energy barrier involved is low enough to expect the 2 1A PES to play a non-negligible role in the title reaction, even at the usual conditions attained in the experiments. The crossing between the 1 1A and 3 1A PESs was also investigated, the latter surface correlating with the excited OH(A 2Σ+) product.
Effects of longitudinal quadrupoles on the phase behavior of a Gay–Berne fluid
Authors: Withers Ian M.
Published: October 2003

The effects of longitudinal quadrupole moments on the formation of liquid crystalline phases are studied by means of constant NPT Monte Carlo simulation methods. The popular Gay–Berne model mesogen is used as the reference fluid, which displays the phase sequences isotropic-smectic A-smectic B and isotropic-smectic B at high (T*=2.0) and low (T*=1.5) temperatures, respectively. With increasing quadrupole magnitude the smectic phases are observed to be stabilized with respect to the isotropic liquid, while the smectic B is destabilized with respect to the smectic A. At the lower temperature, a sufficiently large quadrupole magnitude results in the injection of the smectic A phase into the phase sequence and the replacement of the smectic B phase by the tilted smectic J phase. The nematic phase is also injected into the phase sequence at both temperatures considered, and ultimately for sufficiently large quadrupole magnitudes no coherent layered structures were observed. The stabilization of the smectic A phase supports the commonly held belief that, while the inclusion of polar groups is not a prerequisite for the formation of the smectic A phase, quadrupolar interactions help to increase the temperature and pressure range for which the smectic A phase is observed. The quality of the layered structure is worsened with increasing quadrupole magnitude. This behavior, along with the injection of the nematic phase into the phase sequence, indicate that the general tendency of the quadrupolar interactions is to destabilize the layered structure. A pressure dependence upon the smectic layer spacing is observed. This behavior is in much closer agreement with experimental findings than has been observed previously for nonpolar Gay–Berne and hard spherocylinder models.
Direct configuration interaction and multiconfigurational self-consistent-field method for multiple active spaces with variable occupations. I. Method
Authors: Ivanic Joseph
Published: October 2003

In order to reduce the number of ineffective configurations in a priori generated configuration spaces, a direct configuration interaction method has been developed which limits the electron occupations of orbital groups making up a total active space. A wave function is specified by first partitioning an active space into an unrestricted number of orbital groups and second by providing limiting values, in the form of minima and maxima, for the electron occupancies of each group. The configuration interaction problem corresponding to all possible determinants satisfying these conditions is solved in a fully direct manner by the use of Slater–Condon expressions in conjunction with single and double replacements. This configuration interaction approach, termed occupation restricted multiple active space-configuration interaction, has also been linked with orbital optimization programs to produce the occupation restricted multiple active space-self consistent field method.
A spectroscopy oriented configuration interaction procedure
Authors: Neese Frank
Published: October 2003

A multireference configuration interaction (MR-CI) based method (Spectroscopy ORiented CI, SORCI) is proposed to calculate energy differences between several electronic states of possibly different multiplicity and which can be extended to fairly large molecules. It combines several well-known concepts in order to achieve efficiency, balance, generality, stability and accuracy: (a) the use of any kind of (preferably small) reference space and any kind of initial self-consistent field treatment, (b) the use of average approximate natural orbitals (AANOs) for the states of interest, (c) a variational treatment of the strongly interacting many electron states combined with second order multireference Møller–Plesset perturbation theory for the much larger weakly interacting remainder, (d) the concept of difference-dedicated CI developed and extensively used by Malrieu and co-workers that avoids contributions which are expected to cancel in energy differences, (e) the use of the resolution-of-the identity approximation to avoid exceedingly large integral files. The method is completely devoid of intruder state problems and makes extensive use of truncation techniques in order to achieve computational efficiency. Currently three cutoffs are introduced that control the size of the reference space, the size of the variational treatment and the size of the AANO basis. Test calculations show that the method is stable against variations of these parameters over a wide range. A study of prototypical molecules ranging in size between diatomics and free-base porphin indicates that the method yields accurate results even with moderate basis sets and CI dimensions that typically do not exceed ∼105 configuration state functions. Transition energies are typically predicted within 0.1–0.3 eV from the experimental values.
Theoretical analysis for the determination of surface composition in molten Ga-Bi metal alloys by rare gas scattering
Authors: Dai Jinze , Manson J. R.
Published: October 2003

A theoretical model of classical mechanical atom-surface scattering which includes both single and double collisions from a many-body target is applied to the determination of surface composition of molten metal alloys by rare gas scattering. Explicit calculations are compared with recently measured data for the scattering of monoenergetic beams of Ar atoms from the surface of liquid Ga-Bi alloys. These calculations indicate that rare gas scattering can provide a precise measure of surface segregation in the outermost layer of dilute liquid alloys.
Local clusters and defects in one-dimensional gold wires
Authors: Hui Li , Pederiva F. , Wang G. H. , Wang B. L.
Published: October 2003

Investigations of the structures of gold nanowires are performed by using genetic algorithm simulation based on the molecular dynamics with a glue potential. Results are reported for some local clusters and defect polyhedra in nanowires. The pair correlation functions and pair analysis technique are used to study the cluster in nanowires. Not only are Frank–Kasper polyhedra and Bernal hole polyhedra detected, but also a variety of defects in nanowires with big diameter. Nanowires with small diameter have a splitting of second peak in PCF and characteristic amorphous features are observed in nanowires with small diameter. The angular correlation functions of gold nanowire have also been studied. Both the favorable energy and the small distortion result in bond pairs having the appropriate population. A suitable proportion of the number of bond pairs is an important factor for the nanowire forming ability.
Bonded hard-sphere theory and computer simulation of the equation of state of linear fused–hard-sphere fluids
Authors: Largo J. , Maeso M. J. , Solana J. R. , Vega C. , MacDowell L. G.
Published: October 2003

The bonded hard-sphere (BHS) theory is extended to fluids consisting of rigid, linear, homonuclear molecules, each of them formed by n fused hard spheres. The theory shows excellent agreement with the Monte Carlo NpT simulation data which are also reported for reduced bond lengths l*=0.5 and n=2, 3, 4, 6, 8, and 10. The accuracy of the BHS prediction in comparison to simulation is similar to that of generalized Flory-dimer theory and superior to that of thermodynamic perturbation theory.
The free energy landscape and dynamics of met-enkephalin
Authors: Evans David A. , Wales David J.
Published: October 2003

For the small peptide met-enkephalin in implicit aqueous solvent a connected database of potential energy minima and transition states is constructed and refined based on kinetic criteria, using the discrete path sampling method. A comparison of this technique with parallel tempering Monte Carlo simulations shows that it produces a satisfactory sampling of the configuration space occupied at 298 K. The peptide is predicted to show a folding transition, and time scales for this folding are obtained, along with a description of significant folding intermediates.
Equilibrium properties of confined single-chain homopolymers
Authors: Maury-Evertsz Johnny R. , Estévez L. Antonio , López Gustavo E.
Published: October 2003

The equilibrium thermodynamics of confined linear homopolymers between two impenetrable walls was investigated by means of Monte Carlo simulations in the canonical ensemble with the parallel tempering algorithm. The influence of confinement and chain size on the potential energy, heat capacity, radius of gyration, and the end-to-end distance was investigated as a function of temperature. A bead–spring model was used to simulate the chains. Two conformational changes were observed regardless of the differences in confinement of chain size: The coil-to-globule transition that resembles the gas to liquid transition and the liquidlike to solidlike transition. An additional transition between solid states was also observed for the smallest chain size studied (16 beads). Results indicate a shift of the coil-to-globule transition temperature to lower values as the slit width approaches the two-dimensional case (wall separation equal to bead diameter), and to higher temperature regions as the chain length increases. For separations greater than five bead diameters, the thermodynamic behavior was similar to the behavior in unconfined space.
Basic enhancement of the overall optical efficiency of intracavity frequency-doubling devices for the 1 μm continuous-wave Nd:Y3Al5O12 laser emission
Authors: Lupei V. , Pavel N. , Taira T.
Published: October 2003

The possibility of basic enhancement of the overall optical-to-optical efficiency of intracavity frequency-doubling devices for the 1 μm continuous-wave (cw) Nd lasers by direct pumping into the emitting level and by using concentrated laser materials is discussed. This possibility is demonstrated for 1.0 and 2.4 at. % Nd:Y3Al5O12 (Nd:YAG) crystals pumped by a Ti:sapphire laser at 885 nm. A slope efficiency of 0.79 in absorbed pump power was obtained for the 1064 nm emission of a 1.0 at. % Nd:YAG crystal. The effect of enhancement of intracavity emission on the second-harmonic emission is manifested in a drastic reduction of emission threshold and in an increased dependence on the absorbed power. The use of concentrated Nd:YAG crystal enables a better use of the pump power and improves the overall optical efficiency of the intracavity frequency-doubling devices.
Simulation algorithms for multidimensional nonlinear response of classical many-body systems
Authors: Dellago Christoph , Mukamel Shaul
Published: October 2003

The numerical effort and convergence of equilibrium and nonequilibrium (finite field) techniques for simulating the response of classical systems to a sequence of n short pulses are examined. The former is recast in terms of n point correlation functions and nth order stability matrices which contain higher order generalized Lyapunov exponents, whereas the latter involves sums over perturbed trajectories. The two methods are tested for a highly chaotic system: The Lorentz gas, and for the less chaotic quartic oscillator.
Interpolating moving least-squares methods for fitting potential energy surfaces: Detailed analysis of one-dimensional applications
Authors: Maisuradze Gia G. , Thompson Donald L. , Wagner Albert F. , Minkoff Michael
Published: October 2003

We present the basic formal and numerical aspects of higher degree interpolated moving least-squares (IMLS) methods. For simplicity, applications of these methods are restricted to two one-dimensional (1D) test cases: a Morse oscillator and a 1D slice of the HN2→H+N2 potential energy surface. For these two test cases, we systematically examine the effect of parameters in the weight function (intrinsic to IMLS methods), the degree of the IMLS fit, and the number and placement of potential energy points. From this systematic study, we discover compact and accurate representations of potentials and their derivatives for first-degree and higher-degree (up to nine degree) IMLS fits. We show how the number of ab initio points needed to achieve a given accuracy declines with the degree of the IMLS. We outline automatic procedures for ab initio point selection that can optimize this decline.
Transcorrelated method for electronic systems coupled with variational Monte Carlo calculation
Authors: Umezawa Naoto , Tsuneyuki Shinji
Published: October 2003

A Jastrow–Slater-type wave function is often used as a trial function for precise calculations of the total energy of electronic systems, where the correlation effect is taken into account by the Jastrow factor that directly depends on the distance between electrons. Since many-body integrals are inevitable there, the calculation totally depends on Monte Carlo sampling, and so, except for very simple cases, it is very difficult to optimize one-body wave functions in the Slater determinant which determine the nodal surfaces of the total wave function. Here we propose and demonstrate that the total wave function is efficiently optimized by coupling an ordinary variational Monte Carlo (VMC) technique with the transcorrelated method, in which the one-body wave functions are definitely obtained by solving Hartree–Fock-type self-consistent-field (SCF) equations derived from the similarity-transformed Hamiltonian. It is shown that the present method reproduces about 90% of the correlation energy for helium-like two-electron systems (H−, He, Li+, and Be2+) and gives much better results than the conventional VMC method using the Hartree–Fock orbitals for a Li atom, a Be atom, and a H2 molecule. It is also shown that the orbital energy appearing in the SCF equations gives a good approximation to the ionization potential.
Solar observation from space
Authors: Golub Leon
Published: October 2003

Solar observations are carried out in space when necessary for technical reasons, such as detection of wavelengths that do not reach the ground, or to provide observing conditions that are difficult or impossible to achieve on the ground. Examples in the first category are observations at far ultraviolet or soft x-ray wavelengths and direct detection of solar wind particles; in the latter are precise measurements of variations in the solar luminosity and stereo observations of coronal mass ejections. This article samples the wide variety of techniques presently being used to study the Sun. We begin with an overview of the observational problem presented by the particular scientific questions involved in improving our understanding of the Sun, and of solar activity in particular. We then discuss the need for and advantages of observations from space, and give examples of scientific objectives which can only be met by such observations. We then describe in detail some representative examples of space missions and specific space-based experiments for solar studies.
Development of an electrospray approach to deposit complex molecules on plasma modified surfaces
Authors: Kitching K. J. , Lee H-N. , Elam W. T. , Johnston E. E. , MacGregor H. , Miller R. J. , Turecek F. , Ratner B. D.
Published: October 2003

Two established techniques have been coupled to allow surfaces to be precision engineered. Electrospray ionization to bring large, complex, intact molecular ions into the gas phase has been interfaced with a radio frequency (rf) plasma reactor to treat surfaces making them receptive to the deposition of active biomolecules. The new instrument has been designed and used successfully to deposit a number of high molecular weight molecules including the polysaccharide, sodium hyaluronan (HA), that has an important role in a number of physiological functions. Substrate material is treated using a rf glow discharge plasma chamber, to clean and activate the surface in a controlled manner, then exposed to a beam of multiply charged ions in the gas phase that have been generated using electrospray techniques. The ions are deposited gently onto the substrate and become covalently bound. The molecular integrity and stability of HA surfaces prepared in this way was established using x-ray photoelectron spectroscopy, changes in the observed contact angle, time-of-flight secondary ion mass spectrometry, scanning electron microscopy, and a biological assay–platelet adhesion to the surface.
Nuclear magnetic resonance study of the vapor phase contribution to diffusion in nanoporous glasses partially filled with water and cyclohexane
Authors: Ardelean Ioan , Mattea Carlos , Farrher German , Wonorahardjo Surjani , Kimmich Rainer
Published: October 2003

The contribution of the vapor phase to the effective diffusion coefficient of solvents in a partially filled nanoporous silica glass (Vycor) was investigated with the aid of nuclear magnetic resonance diffusometry. Water and cyclohexane as polar and nonpolar adsorbate species show opposite dependencies on the filling factor. The effective diffusion coefficient of water increases with the filling factor, whereas that of cyclohexane decreases. A “two-phase fast-exchange model theory” is presented accounting for these phenomena. In the vapor phase, both Knudsen and ordinary Einstein diffusion were considered. The different dependencies on the filling factor for polar and nonpolar adsorbate species are attributed to different effective tortuosities represented by different exponents in Archie’s law anticipated in the model theory. The fast exchange feature stipulates that the pore size is small enough.
Dynamical signature of two “ideal glass transitions” in nematic liquid crystals
Authors: Cang Hu , Li Jie , Novikov V. N. , Fayer M. D.
Published: October 2003

A temperature scaling analysis using the same mode coupling theory (MCT) scaling relationships employed for supercooled liquids is applied to optical heterodyne detected optical Kerr effect data for four liquid crystals. The data cover a range of times from ∼1 ps to 100 ns and a range of temperatures from ∼50 K above the isotropic to nematic phase transition temperature TNI down to ∼TNI. The slowest exponential component of the data obeys the Landau–de Gennes (LdG) theory for the isotropic phase of liquid crystals. However, it is also found that the liquid crystal data obey MCT scaling relationships, but, instead of a single scaling temperature TC as found for supercooled liquids, in the liquid crystals there are two scaling temperatures TCL (L for low temperature) and TCH (H for high temperature). TCH is very close to T*, which results from LdG scaling, just below the isotropic to nematic phase transition temperature, TNI, but is 30–50 K higher than TCL. The liquid crystal time dependent data have the identical functional form as supercooled liquid data, that is, a fast power law decay with temperature independent exponent, followed by a slower power law decay with temperature independent exponent, and on the longest time scales, an exponential decay with highly temperature dependent decay constant. For each liquid crystal, the amplitudes of the two power laws scale with expressions that involve TCL, but the exponential decay time constant (long time dynamics) scales with an expression that involves TCH. The existence of two scaling temperatures can be interpreted as a signature of two “glass transitions” in liquid crystals. In ideal MCT developed for spheres, TC is the “ideal glass transition temperature,” although it is found experimentally to be ∼20%–30% above the experimental glass transition temperature, Tg. The transition in nematic liquid crystals at TCL corresponds to the conventional ideal MCT glass transition, while the transition at TCH can occur for nonspherical molecules, and may correspond to the freezing in of local nematic order.
Experimental evidence for the 7,7,8,8-tetracyano-p-quinodimethane dianion in vacuo
Authors: Nielsen Steen Brøndsted , Nielsen Mogens Brøndsted
Published: October 2003

The existence of the unsolvated 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) dianion is proved experimentally. The dianion was formed in 50-keV collisions between TCNQ monoanions and sodium vapor. In the collision process, electron capture to the monoanion occurs with a cross section of about 1 Å2. The lifetime of the dianion is estimated to be in the order of microseconds or longer as the flight time from the collision cell to the detector is 5 μs. The stability of the dianion was elucidated by density functional theory (DFT) and ab initio MP2 calculations. A selection of dissociation reactions have been studied theoretically and compared with the experimental results.
A continuum approach to electron solvation by a layer of polar adsorbates
Authors: Sebastian K. L. , Chakraborty Aniruddha , Tachiya M.
Published: October 2003

We give a continuum description of the solvation of an electron in the image state of a metal surface by a layer of polar adsorbates. In the model, we account for the interaction of the dipole moment of the adsorbate with the electric field exerted by the electron, which is perpendicular to the surface. We also include the dipolar interactions between the adsorbates. With this simple model, it is easy to make an analysis of the self-trapping of electron. Depending upon the values of the parameters, the self-trapped state can have any arbitrary size. Also, there are regimes in which (1) there is no localized state, (2) a localized and delocalized state coexist, with the delocalized state being a saddle point on the potential energy surface, and the localized state a minimum and (3) both the states exist as stable minima, and there is a barrier between the two. In the second case, self-trapping would be a barrierless process while for the third, it would be an activated process. We find that our model can explain the salient features of the experimental results of Harris et al. [Science 297, 1163 (2002)]. At the parameter value required to fit the experimental data, self-trapping is barrierless.
Mode-dependent vibrational autoionization of NO2
Authors: Bell Patrice , Aguirre F. , Grant E. R. , Pratt S. T.
Published: October 2003

Triple-resonance excitation and high-resolution photoelectron spectroscopy were combined to study the mode dependence of vibrational autoionization in Rydberg states of NO2. Photoselection isolates vibrational autoionization via the symmetric stretching vibration, ν1, and the bending vibration, ν2. The previously characterized Fermi resonance between one quantum of ν1 and two quanta of ν2 [H. Matsui et al., J. Mol. Spectrosc. 175, 203 (1996)] allows the comparison of the vibrational autoionization matrix elements for these two modes. The squared matrix element for vibrational autoionization via the symmetric stretch is found to be approximately 35 times greater than that for the bend, which is also consistent with previous results. The results are discussed in terms of existing theoretical models for the autoionization process.
Transition state spectroscopy of the excited electronic states of Li–HF
Authors: Aguado Alfredo , Paniagua Miguel , Sanz Cristina , Roncero Octavio
Published: October 2003

In this work the LiHF(A,B,B′←X) electronic spectrum is simulated and compared with the experimental one obtained by Hudson et al. [J. Chem. Phys. 113, 9897 (2000)]. High level ab initio calculations of three A′2 and one A″2 electronic states have been performed using a new atomic basis set and for a large number of nuclear configurations (about 6000). Four analytic global potential energy surfaces have been fitted. The spectrum involved very excited rovibrational states, close to the first dissociation limit, at high total angular momentum. Two different methods have been used, one based on bound state and the second one on wave packet calculations. Different alternatives have been used to simulate the relatively high temperatures involved. The agreement obtained with the experimental spectrum is very good allowing a very simple assignment of the peaks. They are due to bending progressions on the three excited electronic states. A simple model is used in which only rotational degrees of freedom are included, which simulates the spectrum in excellent agreement with the experimental one, providing a nice physical interpretation. Moreover, the remaining theoretical/experimental discrepancies have been attributed to nonadiabatic effects through the extension of this model to a diabatic representation of excited coupled electronic states.
A simulation study of the quantum hard-sphere Yukawa fluid
Authors: Sesé Luis M. , Bailey Lorna E.
Published: October 2003

The quantum hard-sphere fluid with attractive Yukawa interactions is studied along several isochores, at low and medium densities, by varying the rest of the parameters that define its state points (the temperature, and the well-depth and inverse range of the attractive tail). Path integral Monte Carlo simulations are performed to fix the thermodynamics (energies and pressures) and the pair structures in r-space (instantaneous, linear response, and centroids). The k-space structure factors associated with these radial structures are determined via direct correlation functions defined by suitable Ornstein–Zernike equations. The results obtained allow one to assess quantitatively the reduction in the quantum features of the hard-sphere system due to the inclusion of attractive interactions. Besides, it is observed that under certain conditions the Yukawa interactions can lead to the formation of cavities in the system. For the sake of comparison, some complementary simulations of the bare quantum hard-sphere system have also been carried out, which have helped to resolve certain unclear features near a fluid–solid change of phase of this system. At the onset of increasing fluctuations in the Yukawa system one can observe, from the computation of k-space properties, the negligible role of the repulsive forces in this issue, and also the direct correlation function features associated with the attractions. In addition, the current quantum results give evidence of the decrease in both the critical temperature and the critical density of the Yukawa system, as compared with its classical counterpart.
Osmotic de-swelling of ionic microgel particles
Authors: Fernández-Nieves A. , Fernández-Barbero A. , Vincent B. , de las Nieves F. J.
Published: October 2003

In this work, we study experimentally the effect of an external osmotic pressure πext on the swelling of ionic mesoscopic gels in the weak screening limit, where the Debye screening length is larger than the mesh size of the gel. Variations in the osmotic pressure were induced by adding dextran to the solution. The results show that ionic microgels do not respond to πext below a given value of the normal stress; above this value the system de-swells with increasing osmotic pressure. The start of de-swelling is set by the gel charge density. The Flory thermodynamic theory for ionic gels captures the essential characteristics of the de-swelling behavior; in particular, it predicts with fairly good accuracy the value of πext at which de-swelling begins. Finally, due to the colloidal character of the gels, we observe that the system flocculates by a depletion interaction mechanism at high dextran concentrations.
Fragmentation mechanisms of aqueous clusters charged with ions
Authors: Consta Styliani , Mainer Kirkland R. , Novak William
Published: October 2003

Fragmentation processes of mesoscopic aqueous clusters charged with ions of similar sign are studied by computer simulations. In order to examine differences in the fragmentation that depend on the nature of the ions and the charge distribution, the clusters contain positive (Na+ and Ca2+) or negative (Cl−) ions. Insight into the fragmentation mechanism is obtained by theories of activated processes. Critical to this approach is the use of a new reaction coordinate that captures the shape fluctuations of the droplet that are responsible for the reaction. Reversible work profiles for the reaction are constructed along the reaction coordinate, and dynamics is performed. The dynamics validates the use of the reaction coordinate, and shows diffusive barrier crossing. It is found that clusters with even number of charges fragment unevenly in contrast to analytical theories that predict even fission by considering only the energetic factors that determine the stability of charged droplets.
Short-channel effects in contact-passivated nanotube chemical sensors
Authors: Bradley Keith , Gabriel Jean-Christophe P. , Star Alexander , Grüner George
Published: October 2003

We report a design for carbon nanotube field-effect transistors which tests the nanotube depletion length. In this design, the metal contacts and adjacent nanotubes were coated with impermeable silicon oxide, while the central region of nanotubes was exposed. We tested the devices by measuring sensitivity to NH3 and poly(ethylene imine). NH3 caused similar responses in passivated devices and in normal, nonpassivated devices. Thus, the device design passivates the metal-nanotube contacts while preserving chemical sensor characteristics. Poly(ethylene imine) produced negative threshold shifts of tens of volts, despite being in contact with only the center region of devices. Based on the observed device characteristics, we conclude that the length scale of the covered nanotubes in our structure is comparable to the decay length of the depletion charge in nanotube transistors.
Deconvolution of positron annihilation coincidence Doppler broadening spectra using an iterative projected Newton method with non-negativity constraints
Authors: Ho K. F. , Beling C. D. , Fung S. , Cheng Vincent K. W. , Ng Michael K. , Yip A. M.
Published: October 2003

A generalized least-square method with Tikonov–Miller regularization and non-negativity constraints has been developed for deconvoluting two-dimensional coincidence Doppler broadening spectroscopy (CDBS) spectra. A projected Newton algorithm is employed to solve the generalized least-square problem. The algorithm has been tested on Monte Carlo generated spectra to find the best regularization parameters for different simulated experimental conditions. Good retrieval of the underlying positron–electron momentum distributions in the low momentum region is demonstrated. The algorithm has been successfully used to deconvolute experimental CDBS data from aluminum.
Filament size of floating-emissive probe for low density plasmas with large space potential
Authors: Himura H. , Fukao M. , Wakabayashi H. , Yoshida Z.
Published: October 2003

Space potential φs of non-neutral plasmas with a low density of ne∼1012 m−3 are measured by two floating-emissive probes. Nothing is different between them except the area S of filaments. Despite the fact that the thermionic current is sufficiently emitted, floating potential φf outputted from the smaller filament is much larger than the realistic φs at some measurement points, which is contrary to the widely known relation of φf⩽φs in probe measurements. The result is attributed to the insufficient probe current Ip collected in low-ne plasmas with a large φs. This is because, in such a plasma, Ip does not always satisfy the necessary condition of Ip>φs/RHI, where RHI is a high impedance resistor, although the value of Ip required for the floating emissive method is very small. In order to correctly determine the φs of the plasmas, S must be larger than φs/ene〈ve〉RHI, where e is the electron charge and 〈ve〉 is the mean speed of electrons collected to the probe.
Extending the surface force apparatus capabilities by using white light interferometry in reflection
Authors: Connor Jason N. , Horn Roger G.
Published: October 2003

An important factor in the success of the surface force apparatus (SFA) in measuring interactions between surfaces over nanometer separations has been the optical interference technique used to measure the surface separation. Until recently, this technique has only been used when both of the materials are transparent. As a result, thin sheets of mica have been the material of choice. We describe a simple method to extend the capabilities of the SFA so that a wide variety of material surfaces can be studied while retaining an optical measurement technique. The key to this technique is to modify the optics so that reflected, rather than transmitted, light is used to produce the interference pattern. Now, only one material is required to be thin and transparent while the other can be any material providing it is at least partially reflective. To succeed with this technique, it is necessary to maximize the visibility of the interference fringes. This is achieved by optimizing the thickness of a partially reflective coating (often silver) deposited on the back side of the transparent material.
An in-line micro-Fourier rheometer
Authors: Glasscock J. A. , Smith R. S. , Vanajek J. , Winter J. G.
Published: October 2003

A rheometer has been designed which is capable of being used in industrial process flows. The advantage of this instrument is that the rheological parameters of a process flow can be measured in a very short time across a frequency range of at least two orders of magnitude in situ and thus eliminates the necessity of removing a sample from the flow and making off line measurements. The body of the rheometer is made from stainless steel and is constructed such that no dead spaces exist that could potentially trap process fluid and lead to fouling or contamination of subsequent measurements. The operation of the instrument is fully computerized and the rheological parameters such as viscosity, storage moduli, or loss moduli can be used as process controls in a feedback system and thus reduce wastage of the fluid being produced. The operation of this rheometer is described and results from measurements of flowing sunflower oil are presented. The instrument shows an excellent level of reproducibility and its ability to capture and release discrete samples is demonstrated.
Gain-switched, all-acousto-optic, femtosecond pulse amplifier
Authors: Gruetzmacher Julie A. , Horn Matthew A. , Flanders Bret N. , Shang Xiaoming , Scherer Norbert F.
Published: October 2003

The design and performance of a gain-switched, all acousto-optic (AO) Ti:Sapphire regenerative laser amplifier is presented. An AO Bragg cell is used to send pulses into and out of the amplifier cavity, and an AO modulator serves as an active isolation device. Pumping the high-Q amplifier with a short duration (∼40 ns) 532 nm pulse allows gain-switched operation in which the seed pulse dominates the amplified spontaneous emission; no Q switch is required. The amplified pulse energy is >110 μJ at a 4 kHz repetition rate, and the compressed pulse duration is ⩽50 fs. Detailed measurements are reported demonstrating that this design facilitates low-noise operation.
Comment on “A simple molecular thermodynamic theory of hydrophobic hydration” [J. Chem. Phys. 116, 2907 (2002)]
Authors: Graziano Giuseppe
Published: October 2003

It is shown that, in the statistical mechanical model developed by Ashbaugh, Truskett, and Debenedetti [J. Chem. Phys. 116, 2907 (2002)], the reorganization of H-bonds, as a response to the direct perturbation caused by the excluded volume effect due to the cavity growth in liquid water, plays a pivotal role (a) in determining the occurrence of entropy convergence, and (b) in rendering the partial molar excess heat capacity a large positive and decreasing function over a large temperature range. The analysis emphasizes the need to distinguish the direct perturbation of the system from the response of the system itself as a consequence of the direct perturbation.
Dissociative adsorption of pyrrole on Si(111)-(7×7)
Authors: Yuan Ze Liang , Chen Xian Feng , Wang Zhong Hai , Yong Kian Soon , Cao Yong , Xu Guo Qin
Published: October 2003

Pyrrole adsorption on Si(111)-(7×7) has been investigated using high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy, scanning tunneling microscopy (STM), and theoretical calculations. Compared to physisorbed pyrrole, chemisorption leads to the appearance of N–Si and Si–H vibrational features, together with the absence of N–H stretching mode. This clearly demonstrates the dissociative nature of pyrrole chemically binding on Si(111)-(7×7) through the breakage of N–H bond. Based on STM results, the resulting fragments of pyrrolyl and H atom are proposed to bind with an adatom and an adjacent rest atom, respectively. The STM images further reveal that the adsorption is site selective. The faulted center adatoms are most favored, followed by unfaulted center adatoms, faulted corner adatoms, and unfaulted corner adatoms. In addition, the chainlike pattern of reacted adatoms was observed, implying the possible existence of attractive interaction between adsorbed pyrrolyl and the precursor state. Theoretical calculation confirms that the dissociative adsorption with pyrrolyl bonded to an adatom and H atom to an adjacent rest atom is energetically favored compared to the associative cycloaddition involving the two α-carbon atoms of pyrrole and an adatom–rest atom pair.
Scandium/carbon filters for soft x rays
Authors: Artioukov I. A. , Kasyanov Yu. S. , Kopylets I. A. , Pershin Yu. P. , Romanova S. A.
Published: October 2003

This Note deals with thin-film soft x-ray filters for operation at the wavelengths near carbon K edge (∼4.5 nm). The filters were fabricated by magnetron sputtering deposition of thin layers of scandium (total thickness 0.1–0.2 μm) onto films of polypropylene (thickness 1.5 μm) and polyimide (thickness 0.15–0.3 μm). To protect the scandium layers from oxidation processes in the ambient environment, the filters were coated with 3 nm layers of carbon. The measured transmissions of 0.1 μm Sc/C filters are about 0.6 at the working wavelength and ∼10−4 in the visible. The developed soft x-ray filters can be useful for soft x-ray investigations of carbon-containing materials and biological structures, utilizing radiation of laser produced plasmas and other compact x-ray sources.
An instrument to control parallel plate separation for nanoscale flow control
Authors: White J. , Ma H. , Lang J. , Slocum A.
Published: October 2003

The handling of extremely small samples of gases and liquids has long been a subject of research among biologists, chemists, and engineers. A few scientific instruments, notably the surface force apparatus, have been used extensively to investigate very short-range molecular phenomena. This article describes the design, fabrication, and characterization of an easily manufactured, gas and liquid flow control device called the Nanogate. The Nanogate controls liquid flows under very high planar confinement, wherein the liquid film is, in one dimension, on the scale of nanometers, but is on the scale of hundreds of microns in its other dimensions. The liquid film is confined between a silica (Pyrex) surface with a typical roughness of Ra≈6 nm and a gold-covered silicon surface with a typical roughness of Ra≈2 nm. During the manufacturing process, the Pyrex flows and conforms to the gold-covered silicon surface, improving the mating properties of the two surfaces. The fluid film thickness can be controlled within 2 Å, from sub-10 nm up to 1 μm. Control of helium gas flow rates in the 10−9 atm cm3/s range, and sub-nl/s flow rates of water and methanol have been predicted and experimentally verified.
Atomic-scale depth selectivity of soft x-ray resonant Kerr effect
Authors: Lee Ki-Suk , Kim Sang-Koog , Kortright J. B.
Published: October 2003

By the use of resonant soft x-ray Kerr rotation measurements with its varying incident angle and energy, we observed various shifts of the exchange bias field of a 3.5-nm-thick Co layer in oppositely exchange-biased Ni81Fe19/Fe50Mn50/Co/Pd films. The results in conjunction with their model simulations clearly reveal that the measurements enable one to resolve varying magnetization with depth in the individual magnetic layers of such a multicomponent ultrathin layered structure on the atomic scales. Significant interference effects combined with penetration depth of resonant soft x rays, which are closely associated with their absorptive and refractive contributions, offer remarkably different depth sensitivities into the Kerr effects depending on grazing angle and resonance energy.
Infrared reflectivity of (GaAs)m/(AlAs)n superlattices
Authors: Yu G. , Rowell N. L. , Lockwood D. J. , Wasilewski Z. R.
Published: October 2003

We report an original analysis method to model the p-polarized infrared reflectivity (IR) spectra of (GaAs)m/(AlAs)n superlattices (m=36, 4<n<7) grown on semi-insulating GaAs (100). In our fitting procedure, the first derivative of the IR spectrum was calculated numerically and the respective dielectric functions of the AlAs barriers and GaAs quantum wells were described by two independent factorized models, with the thickness of the AlAs layers treated as a free parameter. The longitudinal and transverse optical phonon frequencies of the GaAs layers were found to be independent of the AlAs layer thickness, while those of the AlAs layer decreased in accordance with theory and their linewidths increased with increasing confinement.
Nearly diffraction-limited focusing of a fiber axicon microlens
Authors: Eah Sang-Kee , Jhe Wonho , Arakawa Yasuhiko
Published: October 2003

We report a simple method of generating nearly diffraction-limited focused spot with a fiber axicon microlens. It is fabricated at the end of a commercially available single-mode optical fiber by the selective chemical etching method that allows fine control of the cone angle of the fiber. The walking distance of the microlens is estimated to be ∼1 μm, which is measured by the photoluminescence from semiconductor single quantum dots.
Spin-valve photodiode
Authors: Appelbaum Ian , Monsma D. J. , Russell K. J. , Narayanamurti V. , Marcus C. M.
Published: October 2003

An optical spin-valve effect is observed using sub-bandgap internal photoemission to generate and collect hot electrons in magnetic multilayers grown on n-Si. Approximately 1.5%–2.5% magnetoresistance is observed in this two-terminal device at low temperature, and this effect is reduced only to 1.1% at room temperature. A simple model is presented to explain the results.