bluuebunny/crossref_metadata_2025_split huggingface dataset:
Laser Investigations of the Dynamics of Molecule-Surface Interaction
Authors: Hager J. , Walther H. 
Published: March 2011

ABSTRACTThe internal energy distribution of NO molecules scattered from different solid surfaces (Pt(111), graphite, and Pt(111) covered with various adlayers) was investigated by the laser-induced fluorescence method. In the case of the NO/graphite system, moreover, the velocity distribution of the scattered molecules could be measured in a time-offlight experiment. The rotational energy distribution, which can always be described as a Boltzmann distribution, exhibits only partial accommodation to the surface temperature for all surfaces investigated. The measurements of the velocity of the NO molecules scattered from the graphite surface show only a small influence of the surface temperature on the average velocity and on the velocity distribution. Furthermore, the measured velocity distribution is independent of the final rotational state of the scattered molecules. On the basis of these results, a rather complete description of the behavior of the NO molecules during the scattering process can be presented.
Spectral Widths in the Rate Equations for Photodesorption by Laser Infrared
Authors: Heidberg Joachim 
Published: March 2011

ABSTRACTDesorption and evaporation from solid surfaces are induced with high yield and wavelength selectivity by excitation of internal adsorbate vibrations with resonant laser infrared. Simple rate equations describe essential features of the process and relate the photoreaction rate and yield to the intensity, the duration of laser-solid surface interaction and molecular properties of the adsorbate. The significance in photodesorption of the experimentally determined spectral widths is shown. Considering the spectral widths in the theory brings the order of the calculated photo-desorption rates for CH3 F-NaCl and the measured rates (also yields) in better concert.
Effects of Surfaces on the Melting/Freezing Behavior of Fluids in Derivatized- and Nude-Porous Silica
Authors: Malhotra V. K. , Mu R. , Natarajan A. 
Published: March 2011

AbstractComparative differential scanning calorimetry (DSC) measurements were made at 200 K < T < 310 K on geometrically restricted cyclohexane and n-decane in nude-, trimethyl derivatized-, and hexyl derivatized-porous (Rp = 4 nm) silica with a view to determine how the surface structure of the confining media affects the thermodynamic behavior of the restricted fluid. Our results suggest that, irrespective of the fact that both trimethyl derivatized- and hexyl derivatized-silica have methyl terminal groups, the freezing or melting transition of cyclohexane is much more depressed in trimethyl derivatized-silica than in hexyl derivatized- or nude-silica. This is not the case for n-decane where the depression in the melting transition is consistent with the fact that the effective pore radius of the hexyl derivatized-silica is smaller than the trimethyl derivatized- or nude-silica.
NMR Studies of Fluid Diffusion in Confining Geometries
Authors: Jerosch-Herold Michael , Thomann Hans , Thompson A. H. 
Published: March 2011

AbstractDiffusion dynamics for fluids in sandstone rocks were studied by CPMG and pulsed field gradient (PFG) NMR measurements. Stretched exponential decays of the transverse magnetization were observed while varying the interpulse spacing, τ, in the CPMG experiment which sets the time for diffusion. A crossover from free to restricted diffusion is evident in the dependence of both T2 and the stretch exponent, β2, on the diffusion time τ. The T2(τ) data is fit to a model which interpolates between free and restricted diffusion and has only one free adjustable length scale despite the fact that the sandstone rocks are characterized by a pore size distribution. This NMR derived length scale correlates with a characteristic length derived from Hg injection experiments at the percolation threshold. We compare these results from the CPMG experiment with stimulated echo PFG-NMR diffusion measurements. This provides a complementary method of probing the diffusion dynamics since the longitudinal magnetization is insensitive to the dephasing effects of diffusion in internal field gradients.
Dipolar Energy Transfer and Knudsen Diffusion Inside Disordered Porous Media: Interfacial Geometry and Chord Distributions.
Authors: Levitz P. 
Published: March 2011

AbstractThe interfacial geometry of a disordered porous medium strongly influences Knudsen diffusion of gases and interfacial dipolar energy transfer. Some interesting comparison can be made between these two transport processes, involving statistical properties of chords belonging either to the solid matrix or to the pore network. A quantitative analysis of these two mechanisms is proposed, based on a chord distribution model. In a first part, we discuss how chord distribution functions contribute to the stastistical characterization of a porous medium. We show that a direct connection between imaging techniques and small angle scattering provides, in many cases, a reliable description of theses functions. In a second part, interfacial direct energy transfer is analyzed. This one step excitation transfer strongly relies on the interfacial autocorrelation function φ2s(r). An analytic expression of φ2s(r) is given and theoretical predictions compared with available experiments. In a third part and following the seminal work of Derjaguin on the Knudsen diffusion, we critically examine how the self diffusion coefficient can be related to the two first moments of the pore chord distribution. A direct comparison with experimental results and numerical simulations is presented in the case of a model porous medium: the random packing of hard spheres. Finally, we analyze a trapping reaction where an excited gas molecule, diffusing in the Knudsen regime, relaxes primarily by wall effects. We show how the chord distribution of the pore network permits to compute the survival probability of the tagged molecules. Two situations are more closely analyzed: the strong and the very weak wall quenching efficiency.
Energy Transfer, Nanometer Crystals and Opfical Nano-probes
Authors: Tan Weihong , Kopelnan Kopelman 
Published: March 2011

AbstractNanometer light and exciton sources and probes have been prepared by adding various inorganic and organic crystals and molecularly doped polymers to micropipettes and nanofabricated optical fiber tips. Specifically, a new nanotechnology, near-field photonanofabrication, has been developed, leading to a thousandfold miniaturization of immobilized Fiber Optical Chemical Sensors and to a billionfold decrease in necessary sample volume. The response time has also been shortened by a factor of at least 100. Applications of these subwavelength probes include biological single cell analysis, supertip development, Förster-energy transfer and Kasha quenching phenomena at the interface between the positionally controlled nanocrystal tip and its photoactive environment. Practically, this leads to enhanced sensitivity of optical probes, nano-optical chemical sensors and near-field exciton light sources.
Dynamics of Polystyrene in Confining Geometry: Transition from Dilute to Semidilute
Authors: Teraoka Iwao , Langley Kenneth H. , Karasz Frank E. 
Published: March 2011

AbstractDynamics of polystyrene molecules inside controlled pore glasses, a typical confining geometry, was studied by dynamic light scattering over a wide range of concentrations of polystyrene in solutions in equilibrium with the porous glasses. Index-matching of the solvent to the silica glasses effectively facilitates the acquisition of information on the dynamics of polymer chains inside the pore without compromising that information by multiple light scattering. When the concentration outside the pore is much smaller than the overlap concentration v*, the apparent diffusion coefficient Dporc of polymers within the pore shows little dependence on concentration. As the outside concentration increases and approaches v*, Dporc rapidly increases. This tendency is more pronounced for polystyrene samples that have higher molecular weights and are predicted to have a lower concentration inside the pore. With further increases of concentration beyond v*, Dporc approaches the apparent diffusion coefficient outside the pore. Moreover, Dporc becomes almost the same for the three different molecular weights of polystyrene fractions studied and depends primarily on the weight concentration of the solute outside the pore. These features are typical of a semidilute solution regime for flexible polymers. The rapid increase in Dporc, is ascribed to a drastic increase of the polymer concentration inside the pore, which results from an equilibration of the chemical potential of the polymer molecule between the interior of the pore and the exterior. Thus, a rapid increase in the osmotic pressure outside the pore drives the polymers into pore channels even at the expense of reduced entropy. We present a quantitative analysis of this highly nonlinear partitioning of polymer molecules.
Segregation at a Single Trap in the Presence of Fields
Authors: Taitelbaum Haim , Weiss George H. 
Published: March 2011

AbstractThere have been a number of recent investigations of segregation properties of diffusing particles in the presence of a single static trap in low dimensions. We study these properties when the diffusing particles are subject to different forms of external fields: global constant bias, random bias fields (Sinai model) and random transition rates. We discuss two measures of segregation, the distances from the trap either to the point at which the concentration profile reaches a specified fraction of its bulk value, or to the nearest unreacted particle. For the cases of global bias (both away from, and towards the trap) and random fields, we found that both measures of segregation have the same asymptotic temporal behavior, while for random transition rates they differ. We explain this difference by relating the nearest-neighbor distance measure to properties of hard-core diffusion in these systems. We also found anomalous spatial shapes for the profile in the vicinity of the trap in the random systems, as well as anomalous reaction rates.
Catalysis on a Fractal Lattice: A Model for Poisoning
Authors: Clement Eric , Leroux-Hugon Patrick , Argyrakis Panos 
Published: March 2011

AbstractWe present theoretical calculations and Monte-Carlo simulations of a surface reaction model proposed by Fichthorn, Gulari and Ziff (1), on a geometrically disordered lattice. Here we look at this model on a percolation cluster in euclidean dimensions d=2 and d=3. We address the problem of poisoning by a foreign inert species exactly at the percolation concentration.
Consolute Critical Phenomena in Dilute Silica Gel
Authors: Frisken B. J. , Ferri Fabio , Cannell David S. 
Published: March 2011

AbstractThe effect of even dilute silica networks on the critical phenomena of binary liquid mixtures is profound. The network preferentially adsorbs one component, preventing a portion of the mixture from participating in critical fluctuations. Fluctuations in the remaining mixture are found to decay with a non-exponential correlation function near the consolute point. A correlation function consisting of the sum of an exponential decay and a non-exponential term of either an activated or stretched exponential form fits the data well. In the presence of the silica network, the mixtures are observed to phase separate near the critical temperature of the pure system, but while still in the one-phase region of the pure system.
High Pressure NMR Studies of Confined Liquids
Authors: Jonas J. , Zhang Jing , Xu Shu 
Published: March 2011

AbstractThe main goal of our novel NMR experiments on confined liquids was to determine the effects of pressure on the dynamics of liquids in the surface layer by using the twostate, fast exchange model and compare them to the pressure effects observed for bulk liquids. With this goal in mind, the deuteron NMR spin-lattice relaxation times, T1, in liquid pyridine-d5, nitrobenzene-d5, and methylcyclohexape-d confined to sol-gel porous silica glasses with pore radii in the range from 18Å to 49˚A were measured as a function of pressure up to 5 kbar at 300 K. In another set of high resolution natural abundance 13C NMR experiments, the 13C relaxation behavior of each carbon in 2- ethylhexyl benzoate model lubricant was measured as a function of pressure in porous silica glasses. In fact, the described experimental approach which allows investigation of the effects of pressure on the dynamic behavior of surface-layer liquids, may provide a new tool in studies of model liquid lubricants at extreme conditions of pressure and temperature. In addition selected results of a recent study of acetonitrile-d3 are reviewed. The 2H and 14N spin-lattice relaxation times of acetonitrile-d3 in porous silica glasses were measured in order to study the confinement effects on the anisotropic reorientation characterized by rotational diffusion constants, D⊥ and D‖, of this symmetric-top molecule.
Simulation Study of Strain-Related Morphology in YBa2Cu3−x(Al,Fe)xO7
Authors: Cai Zhi-Xiong , Zhu Yimei , Welch David O. 
Published: March 2011

ABSTRACTA new simulation method which combines the merits of Monte Carlo simulation of a lattice gas model and the continuum elasticity theory is described. This method treats the elastic strain energy due to concentration fluctuation of interstitial as a perturbation of a lattice gas model Hamiltonian. We illustrate this method by calculating the diffuse scattering intensity of YBa2Cu3O7 systems doped with trivalent impurity atoms M such as Fe or Al. The oxygen concentration wave amplitudes cq were obtained from Monte Carlo simulations of an anisotropic lattice gas model which represents well the interaction between oxygen atoms in this system. These results are in turn used to calculate the diffuse X-ray scattering intensity caused by the displacement field using concentration wave/displacement wave approach. The results suggest that the small orthorhombic domains associated with the oxygen “cross-link” around impurity atoms M causes the diffuse scattering intensity to fall off with oxygen concentration wave vector q as 1/q2for small qand as 1/q4 for larger q.We also show that the size of such domain can be obtained from diffuse scattering data.
An Exact Approach to the Diluted Hubbard Model
Authors: Wang Chumin , Navarro O. , Oviedo-Roa R. 
Published: March 2011

ABSTRACTA new method to solve the extended Hubbard Hamiltonian for systems with few electrons is reported. This method is based on mapping the original many-body problem onto a tight-binding one in a higher dimensional space, which can be solved exactly. For one-and two-dimensional periodic lattices, the real-space pairing problem of two electrons with parallel and anti-parallel spins is analyzed by looking at the binding energy, the coherence length and the mobility of electron pairs. Likewise, some results of the three-body correlation are also reported.
Pseudopotential Portability in the Qmc Framework
Authors: Menchi M. , Bosin A. , Meloni F. , Bachelet G. B. 
Published: March 2011

ABSTRACTWe have calculated the binding energies of several two-electron pseudoions using the Diffusion Quantum Monte Carlo method. The comparison between our results and the experiment suggests that HSC pseudopotentials are portable from the original single particle theory (DFT-LDA) to the “exact” many body one. Moreover we are able to evaluate the degree of portability of each effective field.
Ionic Overlayers on Corrugated Surfaces III : Dynamics in the Fluid Phase
Authors: Seong Hyangsuk , Mahanti S.D. 
Published: March 2011

ABSTRACTUsing molecular dynamics simulation, we have studied the dynamics in the fluid phase of ionic overlayers on corrugated substrates. This is a good model to probe the dynamics of intercalants inside the galleries of intercalated systems. We have calculated the diffusion constant from the incoherent dynamic structure factor, de Genne oscillations in the q-dependence of the Rayleigh peak width and dispersion of Brillouin peaks have been obtained from the coherent dynamic structure factor. These results are compared with neutron scattering measurements in stage-2 RbC24 and earlier simulation results.
Constant-Temperature Molecular-Dynamics Simulation of the 2D Melting Transition of Rb and K
Authors: Fan J. D. , Cai Zhi-Xiong 
Published: March 2011

ABSTRACTThe energy histogram method, introduced by Ferrenberg and Swendsen [Phys. Rev. Lett., 61, 2635, (1988) and 63, 1195, (1989)], was applied for the first time to the constant temperature molecular dynamics (MD) simulation of a two-dimensional (2D) system with incommensurate structures. We performed MD simulations for the stage-2 graphite intercalation compounds (GIC's) with Rb or K being the intercalants (Rb-GIC's and K- GIC's). The temperature dependence of the specific heat, Cv, is calculated for various sizes up to 864 atoms. The melting temperature was found to be 158 K for Rb-GIC's and 119 K for K-GIC's, respectively, which are in agreement with the experimental observations.
Methods for Determining Vacancy Formation Thermodynamic
Authors: Zhao L. , Najafabadl R. , Srolovitz D. J. 
Published: March 2011

ABSTRACTThe vacancy formation thermodynamics in six fcc metals Ag, Au, Cu, Ni, Pd and Pt are determined from atomistic simulations as a function of temperature. This investigation is performed using the Embedded Atom Method interatomic potentials and the finite temperature properties are determined within the local harmonic and the quasiharmonic frameworks. We find that the temperature dependence of the vacancy formation energy can make a significant contribution to the vacancy concentration at high temperatures. An additional goal of the present study is to evaluate the accuracy of the local harmonic method under circumstances in which the excess entropy associated with the formation of a defect is very small. Our data demonstrate that while the errors associated with determining the vacancy formation entropy in the local harmonic model are large, a simple extension to the local harmonic method yields thermodynamic properties comparable to that obtained in the quasiharmonic model, but with much higher computational efficiency.
Equilibrium Unstable Point Defects in Solids
Authors: Koshkin V.M. 
Published: March 2011

ABSTRACTIt is shown that in addition to Frenkel’s and Schottky’s defects there is another type of equilibrium defects, such as an unstable pairs (UP) : vacancy and interstitial atom. UP has the life time of the order of 10-1010-12s, independent of temperature. The review of the unstable pairs physics is presented. Numerous experimental evidences of equilibrium UP in semiconductors, insulators, especially in superionic crystals, and metals are discussed.
A Comparative Atomistic Study of the Structure of Grain Boundaries in Tungsten
Authors: Marinopoulos A. , Sob M. , Vitek V. , Carlsson A. E. 
Published: March 2011

ABSTRACTMost atomistic studies of grain boundaries have been carried out using central forces to describe atomic interactions. However, in transition metals with unfilled d-bands the angular dependence of interatomic forces may be important. The purpose of this paper is to investigate the significance of angular forces in the case of Tungsten. The calculations have been performed for the Σ5(210) symmetrical tilt grain boundary using two alternate approaches. First are the central-force many-body potentials of the Finnis-Sinclair type. The second are the angular dependent potentials obtained via a moment analysis of the electronic density of states. The results of these two approaches are compared by analyzing the boundary structures, the relative displacements of the adjoining grains and the expansion. Differences in structural characteristics are discussed in terms of the effect of angular forces.
A New Approach for Calculating Phonon Dispersions and Elastic Constants of Simple Metals
Authors: Li M. , Liu S.J. , Chen N.X. 
Published: March 2011

ABSTRACTA new method to calculate the phonon dispersions for simple metals is first introduced, based on the Mobius transform. The cohesive energy can be obtained from experiment or theory, then the pair and three—body potentials between atoms in metals can be obtained by use of the modified Mobius theorem. Based on only the pair potentials the phonon dispersions for Cu, A1 and Ni have been obtained without any adjustable parameters. The calculation of elastic constants has involved both the 2—and 3—body interactions, and the restriction of the Cauchy relation has been erased. The above result represents a significant improvement over the Carlsson — Gelatt — Ehrenreich method.
Hydrogen Complexes and their Vibrations in Proton and Deuteron Implanted Inp: Theory and Experiment
Authors: Talwar D. N. , Fischer D. W. , Manasreh M. O. , Matous G. 
Published: March 2011

ABSTRACTBy using a Van de Graaff accelerator we have implanted 2 MeV protons and deuterons into InP samples. These implanted samples were annealed in 50 °C increments in the 200 to 600 °C temperature range for 30 minutes each and then measured by the infrared absorption spectroscopy. A group of four different local vibrational mode absorption lines is observed, each of which arises from the hydrogen-phosphorus stretching vibration with different defects or impurities at nearest-neighbor sites. Each mode exhibits an annealing behavior which is different than any of the other absorption lines. The annealing results are shown and discussed in terms of Green's function calculations of the defects responsible for the two absorption lines.
Structure and Growth of Small Palladium Clusters
Authors: Li Y. S. , Cai Y. , Newsam J. M. 
Published: March 2011

ABSTRACTWe study the structure and growth sequence of small palladium clusters Pdn using the Many-Body Alloy (MBA) potential and simulated annealing techniques. Our results show the preference of compact polyhedral structures. These equilibrium structures are compared with the bulk Pd crystal in terms of cohesive energies and nearest neighbor distances. Both the cohesive energy and the nearest neighbor distance show a slow convergence to bulk behaviors. By analyzing the detailed structures and cohesive energies, we find that Pd4, Pd7 and Pd13 are magic number structures, which are the consequence of their high symmetry and large coordination number.
First-Principles Theory of Polarization in Ferroelectrics
Authors: Resta R. , Posternak M. , Baldereschi A. 
Published: March 2011

ABSTRACTWe outline a modern theory of the spontaneous polarization P in pyroelectric and ferroelectric materials. Although P itself isnot an observable, the difference ΔP between two crystal states can indeed be measured and calculated. We define P as the difference between the polar structure and a suitably chosen nonpolar prototype structure. We previously proposed and implemented a supercell scheme in order to evaluate P in pyroelectric BeO; here we adopt an approach recently developed by King-Smith and Vanderbilt, where ΔP is obtained from the computation of Berry's phases, with no use of supercells. We apply this novel approach, which is numerically very convenient, in order to revisit our previous work on BeO. We then perform a first-principles investigation of the spontaneous polarization P of KNbO3 in its tetragonal phase, which is a well studied perovskite ferroelectric. Our calculated P value confirms the most recent experimental data. The polarization is linear in the ferroelectric distortion; the Born effective charges show strong variations from nominal ionic values, and a large inequivalence of the 0 ions. Only the highest nine valence-band states (O 2p) contribute to P, while all the other states behave as rigid core states.