Search Results - (Author, Cooperation:H. Davis)

0015-1947

Economics

2018-01-09

Wiley-Blackwell

1525-2027

Chemistry and Pharmacology

Geosciences

Physics

2013-11-23

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2012-08-21

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2018-11-21

National Academy of Sciences

0027-8424

1091-6490

Biology

Medicine

Natural Sciences in General

QnAs

2018-04-26

Royal Society

2054-5703

Natural Sciences in General

ecology

2018-04-27

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Geosciences

Computer Science

Medicine

Natural Sciences in General

Physics

Chemistry

1089-7550

AIP Digital Archive

Physics

Amorphous melt-spun Pr2(Fe1−xCox)14B has been prepared to test the concept of magnetic-field-induced crystallite alignment. Crystallization temperatures (Tx) of alloys with x=0.5, 0.6, and 1.0 were determined to be 622, 646, and 507 °C, respectively. These temperatures are all lower than the Curie temperatures (Tc) of the crystallized 2:14:1 phase. The glasses were then heat treated at a temperature Tt(Tx〈Tt〈Tc), either in no field or in a magnetic field of 0.3 T applied parallel to the long edge of the ribbon. The degree of alignment was investigated by means of x-ray diffraction and magnetization measurements. A texture was found where the c axes tend to lie in the ribbon plane in zero field, which is slightly modified when a magnetic field is applied during the heat treatment.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The reaction of sodium atoms (Na) with ozone (O3) has been studied using crossed molecular beams at a collision energy of 18 kcal/mole. The only product observed was NaO. The reaction occurs via an electron-transfer mechanism that yields NaO which is forward scattered with respect to the sodium beam. The translational energy distribution peaks near the forward stripping limit but extends out to the maximum allowed by energy conservation. The fragmentation ratio of NaO upon electron bombardment ionization I(NaO+)/[I(NaO+)+I(Na+)] as a function of product translational energy has been measured for an electron energy of 200 V.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A three-component model of the electrical double layer is studied using a free energy density functional theory, which incorporates in an appropriate way the excluded volume effects and the electrostatic correlations. The new model differs from the primitive model in that the solvent is represented as a dense fluid of neutral hard spheres instead of as merely a dielectric continuum. The interfacial structure shows much richer features in the three-component electrolyte solution than either in the primitive electrolyte or in the pure hard sphere fluid. In particular we observe the coexistence of the particle layerings and the charge inversion. The electrical double layer in the three-component model is of comparatively shorter range and has a lower electrostatic potential difference across the interface than in the primitive model. The inclusion of the solvent also has large effects on the thermodynamic properties of the electrical double layer.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Angular and velocity distributions of the neutral products resulting from the reaction Ba+NO2 were measured using the crossed molecular beams method. Despite a large reaction exoergicity (ΔH=−61 kcal/mol), formation of the dominant ground state BaO(1∑)+NO products results primarily from decay of long-lived Ba+NO−2 collision complexes, even at incident collision energies as high as 59 kcal/mol or with electronic excitation of the Ba atom. A large fraction of the reaction exoergicity is channeled into product translational energy. This rather unusual behavior results from a large exit potential-energy barrier for decay of the initially formed singly ionic Ba+NO−2 intermediate to ground state doubly ionic Ba2+O2−. A secondary source of forward scattered, internally excited BaO results from a direct reaction without the involvement of long-lived intermediates. An additional minor channel, formation of BaNO+O is observed from ground state Ba+NO2 at high collision energies by a direct reaction mechanism. Unlike the dominant BaO+NO channel, which involves harpooning at the first ionic-covalent curve crossing, formation of BaNO from reaction of ground state Ba likely results from the small range of collision geometries which are able to avoid long range electron transfer. The BaNO+O channel was enhanced substantially by electronic excitation of the incident barium atom. However, BaNO from reactions of electronically excited Ba primarily resulted from decay of collision complexes, rather than from a direct mechanism.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The position-orientation distribution of a dipolar hard sphere fluid in contact with a neutral hard wall is studied using a density functional theory. Effects of hard sphere exclusions are incorporated in the excess hard sphere free energy density functional. Orientational correlations between the dipoles are taken into account through approximating the pair correlation function of the inhomogeneous dipolar fluid by that of the homogeneous one of the same chemical potential and dipole moment. The latter is obtained by solving reference quadratic hypernetted chain equation. Theoretical results agree well with those of computer simulations which are available at high density. The theory predicts a qualitatively different interfacial structure at low density and high dipole moments from that at high density; the former is very similar to the liquid/vapor interface of dipolar fluids with regard to both the density distribution and orientational order. The theory also demonstrates that different density distributions correspond closely to different orientational orderings of the dipoles at the interface.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Angular distributions, orbital alignment dependence, and energy dependence of the relative cross sections of various ions produced in crossed beams collisions of electronically excited barium with O3 and NO2 were used to explore the dynamics of these reactions. The Ba+ product from both reactions showed strong dependence on alignment of the Ba(1P) p orbital with respect to the relative velocity vector. The Ba+ was generally forward or forward-sideways scattered, with the latter favored by the dominant perpendicular orbital alignment. Similar results for Ba+ from both O3 and NO2 suggest that electron transfer is favored for large impact parameter collisions in which the Ba p orbital is directed toward the molecule at the critical configuration, regardless of the symmetry of the available orbital in the isolated molecule. BaO+2 from the O3 reaction exhibited angular distributions and energy dependences which were reminiscent of the Ba+, albeit 2 orders of magnitude lower in intensity, indicating a close relationship between these two channels. Important differences in the dynamics for the BaO+ channel were suggested in the angular distributions, which show a substantial backscattered component, as well as the alignment effects and energy dependence. These indicate that the BaO+ probably derives from neutral BaO* which is sufficiently internally excited to undergo vibrational autoionization.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Molecular dynamics and the grand canonical Monte Carlo techniques are employed to simulate the structure and dynamics of a fluid in a slit micropore at equilibrium and under Couette flow. Calculated quantities include the fluid's density profiles, pair correlation functions, diffusion coefficients, normal pressure, stress tensor, and velocity profiles. Simulation results for fluids in equilibrium with the same bulk fluid, but confined by either atomically smooth or structured face centered cubic pore walls are compared. At the conditions considered, fluid in the contact layer next to structured walls exhibits enhanced fluid order which is not altered by flow for pores capable of accommodating two fully developed fluid layers (i.e., for pores wider than 2.5 molecular diameters across). At narrower pore widths, the equilibrium fluid structure is changed by flow and the fluid is more sensitive to shear-induced changes in the diffusivity and normal pressure. The layer average density profiles of the confined fluid are similar in the structured and smooth pores. However, in the fluid layers adjacent to the structured pore walls, the local density distributions are ordered with a periodicity reflecting that of the atomic structure of the pore walls. At the strongest fluid–wall interaction considered, the confined fluid does not solidify although the diffusion coefficient of the fluid confined by the structured walls is reduced by as much as a factor of 87 compared to the bulk phase.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We construct a free-energy density-functional approximation for the primitive model of the electrical double layer. The hard-sphere term of the free-energy functional is based on a nonlocal generic model functional proposed by Percus. This latter model functional, which is a generalization of the exact solution for the nonuniform hard-rod model, requires as input the free energy of a homogeneous hard-sphere mixture. We choose the extension of the Carnahan–Starling equation of state to mixtures. The electrostatic part of the nonuniform fluid ion–ion correlations present in the interface is approximated by that of a homogeneous bulk electrolyte. Using the mean spherical approximation for a neutral electrolyte, we apply the theory to symmetrical 1:1 and 2:2 salts in the restricted primitive model. We present comparisons of density profiles and diffuse layer potentials with Gouy–Chapman theory and Monte Carlo data. We also compare our results with data from other recent theories of the double layer. For highly charged surfaces, the profiles show the layering of counterions and charge inversion effects, in agreement with Monte Carlo data.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A free energy density functional theory is applied to predict the electrostatic force between charged surfaces immersed in the restricted primitive electrolyte, in which ions are represented by charged hard spheres and solvent by a uniform dielectric continuum. The particle correlation due to hard-core exclusions is incorporated in the nonlocal density functional of inhomogeneous hard sphere fluid. The ion–ion electrostatic correlation is treated in the mean spherical approximation. The surface force is found to depend strongly on the electrolyte concentration, surface charge density, and valency of the counterions. Attractive forces are often observed, especially in electrolytes containing divalent counterions. The maximum of the attraction can be an order of magnitude stronger than the van der Waals force evaluated at the same surface separation. The prediction of the classical Poisson–Boltzmann theory that the surface force is always repulsive is qualitatively correct only for monovalent electrolytes at low concentrations and surface charges. The results for an asymmetrical electrolyte resemble those of a symmetrical electrolyte with counterions of the same valency. The theoretical predictions agree well with available computer simulation results.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The mean-field approximation, widely used in the nonlocal-density-functional theories of inhomogeneous simple fluids, is reexamined. Comparing its predictions of the density profiles of a Lennard-Jones fluid near a hard wall with those of Monte Carlo simulations reveals that the approximation is qualitatively incorrect at low densities and quantitatively inaccurate at intermediate and high densities. A density-functional perturbation theory is proposed. It combines the nonlocal-density-functional model of an inhomogeneous hard-sphere system with the Barker–Henderson second-order perturbation theory of uniform simple fluids. It also takes into account the softness of the short-range repulsive potential. The new theory is shown to be qualitatively correct and quantitatively more accurate over the whole range of liquid densities. The effects of the pair potential truncation and the self-consistency of the nonlocal-density-functional theories are discussed.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Polymer–metal interfaces are of increasing technological importance in a variety of applications. These interfaces are characterized by specific interactions between functional groups of the organic polymer and the metallic substrate. In order to study the structure of these interfaces at the molecular level, the energetics of the segment–surface interactions must be well characterized. We have used density functional theory to investigate the interactions of poly(methyl methacrylate) (PMMA) oligomers with aluminum surfaces. The aluminum surface is represented by the simple jellium model. The energetics of the interactions between the organic molecules and the aluminum surface are calculated as a function of the orientation with respect to the surface of the organic molecule and various internal degrees of freedom. The computed energy hypersurfaces exhibit a rich structure characterized by several energy minima and barriers. Implications of such an energy hypersurface for the architecture of long chain molecules at the interface are discussed. The energy hypersurfaces also show that the rotational conformational statistics of PMMA interacting with an aluminum surface should be quite different from that of bulk PMMA. The reported energy hypersurfaces have been used to construct an empirical force-field for future molecular simulations of long chains subject to the proper potentials.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Nonlocal density functional free energy theories are used to investigate the nature of the wetting transition of a Lennard-Jones fluid near a Lennard-Jones wall. In particular, the wetting behavior is investigated for argon films on a solid carbon dioxide substrate for which the Lennard-Jones parameters are known. In accordance with previous predictions of local density functional theories, the transition is found to be of first order. The locus of the prewetting line of the Ar–CO2 system is calculated. Density profiles obtained from the Tarazona and generalized hard rod models are both in qualitative agreement with Monte Carlo simulations of Finn and Monson, although Tarazona's model performs better quantitatively.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Flow of fluids confined in molecularly narrow pores is studied by molecular dynamics. Strong density variations across the pore render the usual dependence of the local viscosity on local density inappropriate. At separations greater than four molecular diameters flow can be described by a simple redefinition of local viscosity. In narrower pores a dramatic increase of effective viscosities is observed and is due to the inability of fluid layers to undergo the gliding motion of planar flow. This effect is partially responsible for the strong viscosity increases observed experimentally in thin films that still maintain their fluidity.

Electronic Resource