bluuebunny/crossref_metadata_2025 huggingface dataset:
Inductively coupled plasma assisted deposition and mechanical properties of metal-free and Ti-containing hydrocarbon coatings
Authors: Meng W. J. , Meletis E. I. , Rehn L. E. , Baldo P. M.
Published: July 2002

Metal-free amorphous hydrocarbon (a-C:H) and Ti-containing hydrocarbon (Ti–C:H) coatings have been synthesized in a hybrid chemical vapor deposition (CVD)/physical vapor deposition (PVD) system which combines inductively coupled plasma (ICP) and sputter deposition. a-C:H coatings have been fabricated by ICP assisted CVD in inert/hydrocarbon gas mixtures while Ti–C:H coatings have been fabricated by ICP assisted magnetron sputtering of Ti in inert/hydrocarbon gas mixtures. We present results of structural characterization and mechanical property measurements on these a-C:H and Ti–C:H coatings. In particular, the influence of hydrogen on the coating mechanical properties is probed experimentally. We show that hydrogen significantly influences the mechanical properties of a-C:H and Ti–C:H coatings and needs to be considered for a full understanding of the mechanical properties of Ti–C:H coatings. Our results demonstrate that combining ICP with sputter deposition makes a versatile CVD/PVD tool capable of depositing metal-free and metal-containing hydrocarbon coatings with widely varying microstructures and mechanical properties.
Flicker noise properties of organic thin-film transistors
Authors: Martin S. , Dodabalapur A. , Bao Z. , Crone B. , Katz H. E. , Li W. , Passner A. , Rogers J. A.
Published: July 2002

The low frequency noise properties of organic thin film transistors are studied here as a function of frequency and bias. Various n-channel and p-channel devices were evaluated and found to exhibit 1/f-type of noise in the 1 Hz–10 kHz range. The drain current noise is found to vary proportionally with drain current. The noise level is comparable to that found in Si metal–oxide–semiconductor field-effect transistors within the operation region of the devices, owing to the smaller drain currents in organic transistors, although the intrinsic noise is considerably higher in the organic transistors. The viability of using the organic materials in low noise circuits is demonstrated by a ring oscillator.
Surface recombination measurements on III–V candidate materials for nanostructure light-emitting diodes
Authors: Boroditsky M. , Gontijo I. , Jackson M. , Vrijen R. , Yablonovitch E. , Krauss T. , Cheng Chuan-Cheng , Scherer A. , Bhat R. , Krames M.
Published: July 2002

Surface recombination is an important characteristic of an optoelectronic material. Although surface recombination is a limiting factor for very small devices it has not been studied intensively. We have investigated surface recombination velocity on the exposed surfaces of the AlGaN, InGaAs, and InGaAlP material systems by using absolute photoluminescence quantum efficiency measurements. Two of these three material systems have low enough surface recombination velocity to be usable in nanoscale photonic crystal light-emitting diodes.
Vibrational overtone predissociation spectroscopy of hydrogen peroxide
Authors: Dübal H.-R. , Crim F. F.
Published: July 2002

Vibrational overtone predissociation spectroscopy, which detects the products of a unimolecular reaction initiated by overtone vibration excitation as a function of wavelength, is an informative means of studying highly vibrationally excited molecules. The spectrum, obtained by varying the excitation laser wavelength while keeping the probe laser tuned to interrogate a single product state, mirrors the overtone vibration absorption spectrum of those molecules that decompose into the detected quantum state. Using this technique to detect individual rotational states of the OH fragments from hydrogen peroxide excited in the regions of the third to the fifth OH stretching overtone (4νOH, 5νOH, 6νOH) reveals coarse and fine vibrational structure in the spectrum. A vibration-torsion model, in which the high frequency OH stretching vibration and the low frequency torsional vibration are separated adiabatically, recovers the observed transition wave numbers and intensities and predicts previously unobserved transitions. Because the measurements probe individual product quantum states, the excitation spectrum also depends on the partitioning of the products among their states. This appears as a variation in the intensity of different transitions with the product state that is probed, and a statistical model of the unimolecular decomposition in combination with the vibration-torsion model of the absorption spectrum explains the dependence of the overtone vibrational predissociation spectra on the probed rotational state of the OH product.
Calculated yields and fluctuations for electron degradation in liquid water and water vapor
Authors: Paretzke H. G. , Turner J. E. , Hamm R. N. , Wright H. A. , Ritchie R. H.
Published: July 2002

Two sets of physical interaction cross sections for detailed electron track-structure calculations in liquid water and water vapor have been used to investigate the possible magnitude of phase-dependent differences in the primary yields of ionizations and their fluctuations produced during complete slowing down of electrons in the energy range from 10 eV to 10 keV. For fast electrons the calculated values of the mean energy absorbed per ion pair are 25.8 eV/ip for the liquid as compared to 30.0 eV/ip for the vapor; both results are consistent with experimental data. A similar phase effect is found in the ionization yields from each molecular subshell, since essentially the same partitioning of the total ionization cross section has been used in the calculations for the liquid and the vapor. The relative fluctuations of the ionization yields as described by the Fano factor are 0.15 for the liquid and 0.25 for the vapor; the Fano factors for each single molecular orbital are typically between 0.7 and 0.9 in both phases. The distributions in the total number of ionizations produced by the complete slowing down of electrons with initial energies above about 100 eV are Gaussian within about two standard deviations around the mean. The significance of the differences in the yields of ionizations and excitations calculated for both phases is discussed. In the absence of adequate experimental data for the basic processes, the differences are interpreted in terms of the respective assumptions made for single scattering processes.
The electronic spectrum of the amino acid tryptophan in the gas phase
Authors: Rizzo Thomas R. , Park Young D. , Peteanu Linda A. , Levy Donald H.
Published: July 2002

The electronic spectrum of the amino acid tryptophan has been measured in the environment of a cold, supersonic molecular beam. The resonantly enhanced two-photon ionization spectrum of tryptophan shows some features characteristic of more volatile indole derivitives, however the region of the spectrum near the origin shows distinctive low frequency structure absent from the simpler indole containing molecules. The power dependence of the spectrum reveals features which can be attributed to several conformers of tryptophan in the molecular beam. One of these conformers shows a nearly harmonic 26 cm−1 vibrational progression which does not appear in the spectra of other indole derivitives, and the intensity pattern of this progression indicates that this particular conformer undergoes a significant geometry change upon electronic excitation. The lack of many extensive vibrational progressions in the electronic spectrum indicates that the excited state conformers of tryptophan are similar to those in the ground electronic state. The identification of a small number of stable tryptophan conformers is important for understanding the photophysics of tryptophan in solution.
A non-mass-dependent oxygen isotope effect in the production of ozone from molecular oxygen: The role of molecular symmetry in isotope chemistry
Authors: Heidenreich John E. , Thiemens Mark H.
Published: July 2002

It was previously reported that the reaction products from the synthesis of ozone in an electric discharge through molecular oxygen display a non-mass-dependent (NoMaDic) oxygen isotope effect. In this paper, we report a detailed characterization of the isotope effect, including the effect of molecular oxygen pressure, and the presence of a chemically inert third body (helium). The NoMaDic effect is due to an isotopically selective stabilization of the O3 formation reaction intermediate, possibly resulting from the ability of the different isotopomers to exhibit different molecular symmetries.
Fluctuation-based computer calculation of partial molar properties. II. A numerically accurate method for the determination of partial molar energies and enthalpies
Authors: Debenedetti Pablo G.
Published: July 2002

A new operational definition of the partial molar energy is derived invoking the grand canonical ensemble formalism. This expression, used in conjunction with the fluctuation-explicit form of the Kirkwood–Buff relation for partial molar volumes, allows the accurate computer calculation of all mechanical partial molar properties in a multicomponent system. The method satisfies two independent consistency checks. Property values for all components in a mixture are obtained simultaneously, and the length of a simulation is independent of the number of species in the system.
Intramolecular vibrational excitations accompanying solvent-controlled electron transfer reactions
Authors: Jortner Joshua , Bixon Mordechai
Published: July 2002

The theory of dynamic solvent effects on outer-sphere electron transfer (ET) was extended to incorporate the modification of the high-frequency quantum modes, which is manifested by the reduction of the electronic coupling by nuclear Franck–Condon factors and by the change of the energy gap. Explicit expressions for the ET rates were obtained in terms of a sum over parallel vibronic channels, each involving a distinct intramolecular vibrational excitation of the final state. In the solvent-controlled adiabatic limit, the effects of intramolecular vibrational excitation are exhibited by the modification of the (partial) activation energies, while the frequency factor is dominated by the longitudinal dielectric relaxation rate of the solvent.
Charge density waves, spin density waves, and Peierls distortions in one-dimensional metals. I. Hartree–Fock studies of Cu, Ag, Au, Li, and Na
Authors: McAdon Mark H. , Goddard William A.
Published: July 2002

A b initio calculations indicate that each of the one-dimensional elemental metals composed of Cu, Ag, Au, Li, and Na is stable with respect to the Peierls distortion if spin polarization is allowed [unrestricted Hartree–Fock (UHF)], leading to a spin density wave. Disallowing spin polarization [restricted Hartree–Fock (HF)] leads to a half-filled energy band, Peierls instability, and a charge density wave. For each case, the UHF wave function leads to an antiferromagnetic (nonmetallic) ground state, with a spin density wave resulting from electron correlation effects, consistent with the Mott–Hubbard low-density antiferromagnetic insulator. The UHF antiferromagnetic (nonmetallic) ground states have large cohesive energies resulting from two-center one-electron bonds (similar to the one-electron bonds of the diatomic molecular cations).
Association of nonionic polymers with micelles, bilayers, and microemulsions
Authors: Nagarajan R.
Published: July 2002

A unified thermodynamic theory is developed here to explore the nature of association of nonionic polymer molecules with surfactant aggregates such as globular micelles, rod-like micelles, bilayers, oil in water microemulsions, and water in oil microemulsions. The association complex is visualized as consisting of the polymer molecule wrapped around the aggregate at its interface with water. The association with polymer provides enhanced shielding of the hydrophobic aggregate surface from water, increases the steric repulsions at the aggregate surface and gives rise to hydrophobic interactions between the polymer and the aggregate surface. Simple free energy models are formulated to account for these polymer-induced effects. Illustrative calculations have been carried out to examine the behavior of monoalkyl and dialkyl surfactants with nonionic, anionic, and zwitterionic polar head groups. The results show that polymer–aggregate association is almost always preferred in the case of anionic or zwitterionic surfactants. As a result of the association with the polymer, globular micelles transform into smaller globular micelles; large, polydisperse rod-like micelles change into small, monodispersed globular micelles; spherical vesicles and planar bilayers transform into smaller discoids. In all the cases, the polymer-bound aggregates are formed at a critical micelle concentration (CMC) that is significantly lower than the CMC of the polymer-free systems. In marked contrast, aggregates of nonionic surfactants may or may not associate with the polymer molecule. If association occurs, the polymer-induced changes in the aggregate morphologies are similar to those for the anionic surfactants. However, the CMC for the formation of bound aggregates is usually very close to that for the formation of the free aggregates. In many situations, association is not favored and polymer-free aggregates coexist with free polymer molecules in solution. It is found that subtle variations in the hydrophobic characteristics of the polymer molecule can tilt the equilibrium between polymer-bound and polymer-free aggregates. In the case of microemulsions, calculations show that the volume ratio of the dispersed phase to the surfactant is significantly altered by the presence of the polymer. Also,

the condition for phase inversion from oil in water to water in oil type of microemulsion is modified by the presence of the polymer. The theoretical results suggest ways by which one may control the morphologies of surfactant aggregates using polymer additives.
A mass-independent sulfur isotope effect in the nonthermal formation of S2F1
Authors: Bains-Sahota Swroop K. , Thiemens Mark H.
Published: July 2002

A non-mass-dependent sulfur isotope effect is present in the rotationally symmetric S2F10 molecule, produced in an electrical discharge through sulfur tetrafluoride. A similar isotopic fractionation was observed in the product S2F10 from the electrodissociation of SF5Cl, and reaction between fluorine atoms produced by F2 photolysis and SF4, collectively ruling out the SF5 formation process as the source of the mass-independent fractionation. The mass-independent mechanism is shown to occur in the SF5+SF5→S2F10 formation step. The secondary dissociation of S2F10 as a source of the mass-independent fractionation is ruled out by control S2F10 dissociation experiments which are shown to produce small mass-dependent fractionations. Mass-dependent effects such as sulfur isotopic exchange and secondary dissociation reactions are significant processes for the system under study and have been quantitatively accounted for. The role of symmetry in non-mass-dependent isotope effects is strengthened by the present experiments, and the search and characterization of mass-independent effects is extended to sulfur-containing molecules.
Theory of resonance Raman scattering and fluorescence from strongly vibronically coupled excited states of polyatomic molecules
Authors: Stock G. , Domcke W.
Published: July 2002

A theoretical description of secondary emission from complex absorption bands of isolated polyatomic molecules is developed. The strong non-Born–Oppenheimer coupling associated with conical intersections of the multidimensional excited-state potential-energy surfaces is included in a fully microscopic manner by solving the time-dependent Schrödinger equation for appropriate model systems incorporating the most relevant electronic states and vibrational modes. The effect of the large number of remaining vibrational modes and of the weaker coupling with additional electronic states is modeled by phenomenological relaxation terms (lifetime broadening and pure dephasing) in the framework of the density-matrix formalism. Explicit eigenstate-free expressions for absorption, resonance Raman, and fluorescence spectra are derived via density-matrix perturbation theory. The computational feasibility of the resulting mixed microscopic/phenomenological theory is demonstrated for a simple three-mode model of the vibronic coupling of the S1(nπ*) and S2(ππ*) states of pyrazine. The effect of excited-state vibronic coupling and ultrafast S2→S1 internal conversion on resonance Raman and fluorescence spectra is analyzed on the basis of these model calculations.
Free energy functional expansion for inhomogeneous polymer blends
Authors: Tang Hai , Freed Karl F.
Published: July 2002

Density functional theory for inhomogeneous polymer systems is reformulated using the new ideal system of noninteracting Gaussian chains to replace the Flory–Huggins-like formulation of McMullen and Freed in which the polymer chains have unspecified connectivity. The price paid for introducing this more realistic ideal system is the fact that the density-field relation may only be inverted in powers of the density gradients, so the ideal free energy functional is obtained as a density gradient expansion. The relevant expansion parameter involves the radius of gyration of the polymer, as expected. However, the coefficient of the square gradient term (and those of higher gradients) involves the spatially varying density in the interface as postulated by de Gennes and first derived rigorously here. The nonideal free energy functional is treated by expansions about a homogeneous reference system, and the correlation functions are evaluated in the random phase approximation (RPA). Although truncations are made at second order, there are no difficulties in including higher order terms provided the RPA approximation is retained. The theory is formulated in general for compressible polymer systems, and the incompressible case follows as a special limiting situation. We also analyze the contribution from higher order terms in a traditional Landau-type free energy functional expansion for inhomogeneous polymer systems in which coefficients are evaluated in a homogeneous reference system. Despite the difference of the former coefficients from the de Gennes postulate, it is shown that this Landau expansion may be resummed to produce the identical functional that we obtain by rigorous density functional methods.
Quantum-mechanical derivation of the Bloch equations: Beyond the weak-coupling limit
Authors: Laird Brian B. , Budimir Jane , Skinner James L.
Published: July 2002

Two nondegenerate quantum levels coupled off-diagonally and linearly to a bath of quantum-mechanical harmonic oscillators are considered. In the weak-coupling limit one finds that the equations of motion for the reduced density-matrix elements separate naturally into two uncoupled pairs of linear equations for the diagonal and off-diagonal elements, which are known as the Bloch equations. The equations for the populations form the simplest two-component master equation, and the rate constant for the relaxation of nonequilibrium population distributions is 1/T1, defined as the sum of the ‘‘up’’ and ‘‘down’’ rate constants in the master equation. Detailed balance is satisfied for this master equation in that the ratio of these rate constants is equal to the ratio of the equilibrium populations. The relaxation rate constant for the off-diagonal density-matrix elements is known as 1/T2. One finds that this satisfies the well-known relation 1/T2=1/2T1. In this paper the weak-coupling limit is transcended by deriving the Bloch equations to fourth order in the coupling. The equations have the same form as in the weak-coupling limit, but the rate constants are calculated to fourth order. For the population-relaxation rate constants this results in an extension to fourth order of Fermi’s golden rule. We find that these higher-order rate constants do indeed satisfy detailed balance. Comparing the dephasing and population-relaxation rate constants, we find that in fourth order 1/T2≠1/2T1.
Optical properties of disordered molecular aggregates: A numerical study
Authors: Fidder Henk , Knoester Jasper , Wiersma Douwe A.
Published: July 2002

We present results of numerical simulations on optical properties of linear molecular aggregates with diagonal and off-diagonal disorder. In contrast to previous studies, we introduce off-diagonal disorder indirectly through Gaussian randomness in the molecular positions; this results in a strongly asymmetric distribution for the interactions. Moreover, we do not restrict to nearest-neighbor interactions. We simultaneously focus on several optical observables (absorption linewidth and line shift and superradiant behavior) and on the density and the localization behavior of the eigenstates (Frenkel excitons). The dependence of these optical properties on the disorder is investigated and expressed in terms of simple power laws. For off-diagonal disorder, such a study has not been performed before. In the case of diagonal disorder, we show that, in particular, the superradiant decay rate of the aggregates may be strongly affected by the inclusion of non-nearest-neighbor interactions. Recent results of absorption line shape, superradiant emission, and resonance light-scattering measurements on pseudoisocyanine aggregates can be understood on the basis of these calculations.
Living networks: The role of cross-links in entangled surfactant solutions
Authors: Drye T. J. , Cates M. E.
Published: July 2002

We study theoretically the effects of cross-link formation in the entangled micellar phase of viscoelastic surfactant solutions. We consider equilibrium cross-links, of energy E≫kBT, that can arise through local fusion of the micelles. For simplicity, we assume that the end-cap energy is much larger than E, so that the system forms a network in which free ends can be neglected. We consider cases when the material between cross-links is (i) a rigid rod, (ii) a Gaussian chain, and (iii) an excluded-volume chain. In each case, we define a ‘‘saturated’’ network as one for which L≂Lb, where L is the arc length between cross-links and Lb that of a geometrical subunit or ‘‘blob’’ of the network. (An ‘‘unsaturated’’ network is, in contrast, one for which L≫Lb. ) In the rigid-rod case we predict an unsaturated network at all concentrations. For flexible chains we predict that the network is unsaturated at high concentrations, but as the concentration is decreased, these systems should reach saturation, followed at lower φ by a phase separation between a dilute saturated network and excess solvent. Surprisingly, this phase separation should occur even for the case of a very good solvent: there is an entropy gain associated with creating threefold junction points which increases with dilution, and asymptotically overcomes any finite cross-link energy E. In practice, however, this may often be unobservable due to the very small concentrations required, and/or the effects of free ends. We briefly discuss the possible implications of our results for the viscoelastic properties of surfactant solutions.
Dissociative excitation of CF4, CCl4, and chlorofluoromethanes by collisions with argon and helium active species
Authors: Tsuji Masaharu , Furusawa Makoto , Mizuguchi Toshinori , Muraoka Toshihiko , Nishimura Yukio
Published: July 2002

Dissociative excitation of CFnCl4–n (n=0–4) by collisions with Ar and He active species has been studied by observing uv and visible emissions in the Ar and He afterglows. Emissions from such excited fragments as Cl*, Cl*2, CF*, CCl*, CCl+*, ArCl*, CF*2, CCl*2, CFCl*, and CF*3 were observed. New continuous bands which may be related to Cl*2, CFCl*, CCl*n(n=2,3), and CCl+*n(n=2,3) were found in the reactions of CFCl3 and CCl4. The effects of ion trapping and SF6 addition into the discharge flow indicated that the excited species were produced from primary reactions by the metastable Ar(3P2) and He(2 3S) atoms and (Ar+)* and He+ ions and/or secondary reactions via electron–ion recombination processes. The emission rate constants have been determined for the primary reactions of Ar(3P2), He(2 3S), and He+ with CFnCl4–n (n=0–3). A comparison between the observed emission rate constants and known total quenching rate constants indicated that dissociative excitation processes are minor exit channels (≤4.6%).
A semiphenomenological droplet model of homogeneous nucleation from the vapor phase
Authors: Delale Can F. , Meier Gerd E. A.
Published: July 2002

A semiphenomenological droplet model, which corrects for the macroscopic surface tension and monomer–monomer interactions from real gas behavior (second virial coefficient) and for the correlation between the mean surface area of a cluster and the number of molecules constituting the cluster over all ranges of temperature below the critical point, is proposed by modifying Fisher’s droplet theory of condensation. A steady-state nucleation rate equation is derived and compared with expansion and diffusion cloud chamber data for a variety of substances. An overall good agreement is achieved for the range of temperatures investigated in contrast to comparison with the classical nucleation rate equation.
Coupled-cluster methods with noniterative triple excitations for restricted open-shell Hartree–Fock and other general single determinant reference functions. Energies and analytical gradients
Authors: Watts John D. , Gauss Jürgen , Bartlett Rodney J.
Published: July 2002

A new, noniterative triples correction to the coupled-cluster singles and doubles (CCSD), method, for general single determinant reference functions is proposed and investigated numerically for various cases, including non-Hartree–Fock (non-HF) reference functions. It is correct through fourth-order of perturbation theory for non-HF references, and unlike other such methods, retains the usual invariance properties common to CC methods, while requiring only a single N7 step. In the canonical Hartree–Fock case, the method is equivalent to the usual CCSD(T) method, but now permits the use of restricted open-shell Hartree-Fock (ROHF) and quasirestricted Hartree–Fock (QRHF) reference determinants, along with many others. Comparisons with full configuration interaction (FCI) results are presented for CH2, CH2+, CH3, NH2, and SiH2. The paper also reports the derivation and initial computational implementation of analytical gradients for the ROHF-CCSD(T) method, which includes unrestricted Hartree–Fock (UHF) CCSD(T) and RHF-CCSD(T) as special cases. Applications of analytical gradients are presented for HOO, the CN radical, which is highly spin contaminated at the UHF level, and HCO, the latter with several large basis sets. With these developments of analytical gradients, these highly accurate generalized CCSD(T) methods can be widely applied.
Simulations of electrorheological and particle mixture suspensions: Agglomerate and layer structures
Authors: Melrose J. R. , Heyes D. M.
Published: July 2002

Brownian dynamics (BD) simulations have been carried out of the rheology of two model weakly aggregated suspensions—model electrorheological (ER) fluids with anisotropic dipole interactions and depletion flocculated (DF) systems with isotropic well potential interactions. At rest, both systems form an infinite gel-like aggregate network. Under shear, they shear thin and undergo shear-induced agglomeration to structures more compact than the gels at rest. The present work reports simulations of much larger model systems (N=500) than considered previously and makes a comparison between the two systems under shear. We investigate a range of volume fractions in the ER case and elucidate further the microscopic mechanisms of flow in the ER and DF cases. In the case of ER fluids at 10% and 30% volume fraction, layer structures form normal to the vorticity axis at low shear. These are not found at 50% volume fraction. The layer transition is associated with some hysteresis in the Stokesian (large particle/high shear rate) regime. An intermediate liquid phase exists between the high shear rate hexagonal string phase and the layered phase. The ER model appears to have a yield stress followed by power law shear thinning. In the case of the isotropic depletion system, an additional structural transition occurs. At low shear rates, globular shaped agglomerates are formed, but on increasing the shear rate, a transition occurs to a new structure layered normal to the vorticity axis. This is associated with a change in the slope of the viscosity with shear rate, also seen in the experimental system.
Vibronically state-selective photoisomerization in 5-hydroxytropolone
Authors: Ensminger Frederick A. , Plassard Jason , Zwier Timothy S. , Hardinger Steven
Published: July 2002

Fluorescence excitation, dispersed fluorescence, and population labeling spectra of 5-hydroxytropolone cooled in a supersonic free jet demonstrate that syn–anti photoisomerization in the molecule can be driven with high vibrational state selectivity in either direction (syn↔anti).
Studies on free energy calculations. I. Thermodynamic integration using a polynomial path
Authors: Resat Haluk , Mezei Mihaly
Published: July 2002

The difference in the free energy of hydration between the C7 and αR conformations of alanine dipeptide at infinite dilution is computed using Monte Carlo method in a canonical ensemble with the united-atom AMBER force field. The recently introduced polynomial path with different exponents for the calculation of liquid state free energies using thermodynamic integration (TI) is tested. This is achieved by separating the interaction energy between the solute and the solvent molecules into the constituent r−12, r−6 and Coulomb terms. The separate contribution of each term is calculated and the comparison shows that the shape of the TI integrand as a function of the coupling parameter is mainly determined by the Coulomb term. Analysis of the convergence characteristics shows that five-point Gaussian quadrature integration would be sufficient to obtain quantitatively reliable results when a polynomial path is employed.
Substrate dependence of electron-stimulated O− yields from dissociative electron attachment to physisorbed O2
Authors: Huels M. A. , Parenteau L. , Sanche L.
Published: July 2002

We present measurements of O− electron stimulated desorption yields obtained under identical experimental conditions from 0.15 monolayers (ML) of O2 deposited onto disordered substrates consisting of 4 ML of either Kr, Xe, C2H6, C2H4, N2O, CH3Cl, or H2O, all condensed on Pt (polycrystalline). The resulting O− yield functions, for incident electron energies below 20 eV, are compared to that obtained from the O2/Kr solid; this allows us to assess the order of magnitude effects of the local substrate environment on dissociative electron attachment (DEA) via the 2Πu and gas phase forbidden 2Σ+g,u resonances of O−2. We note that, in addition to electron energy losses in the substrate prior to DEA to O2 and post-dissociation interactions of the O− with the substrate molecules, charge or energy transfer from the O−2 transient anion to a substrate molecule, and capture of the incident electron into a dissociative anion resonance of the substrate molecule may contribute to a reduced O− yield from the physisorbed O2. In the case of O2 deposited on amorphous ice, we find that the O− signal from DEA to O2 is completely absent for electron energies below 14 eV; we attribute this to a complete quenching of the dissociative O−2(2Πu, 2Σ+) resonances by the adjacent water molecules.
Structures of Cl−(H2O)n and F−(H2O)n (n=2,3,...,15) clusters. Molecular dynamics computer simulations
Authors: Perera Lalith , Berkowitz Max L.
Published: July 2002

We have performed molecular dynamics calculations on Cl−(H2O)n and F−(H2O)n (n=2,3,...,15) clusters. The calculations show that the F− ion is solvated in these clusters, while Cl− remains attached to the water in the clusters. We also obtained the minimum energy structures for the Cl−(H2O)n and F−(H2O)n (n=6,7,8) clusters. From the comparison of these structures with the dynamical structures we conclude that the solvation of the F− ion is due to the entropy effect.
Structural determination of a NiO(111) film on Ni(100) by dynamical low-energy electron-diffraction analysis
Authors: Warren O. L. , Thiel P. A.
Published: July 2002

The geometric structure of a NiO(111) film on Ni(100) has been determined by dynamical low-energy electron-diffraction analysis. The oxide film is sufficiently thick and uniform to carry out the analysis in the bulk crystal limit. Four orientational domains of NiO(111) are present. We consider four possible terminations of the oxide film (oxygen or nickel, fcc or hcp sites). Our results indicate that the oxide film terminates with a topmost layer of oxygen in fcc sites, and exhibits a 14.8% contraction in the first interlayer spacing relative to the ‘‘bulk’’ interlayer spacing of the film. The ‘‘bulk’’ lattice parameters of the oxide film are found to be uniformly compressed by 2.6% relative to bulk NiO(111). Comparison with other work suggests that oxygen termination, a strong contraction in the first interlayer spacing, and retention of cubic symmetry in deeper layers may be general characteristics of unreconstructed rocksalt(111) surfaces of metal oxide films.
Symplectic integrators for large scale molecular dynamics simulations: A comparison of several explicit methods
Authors: Gray Stephen K. , Noid Donald W. , Sumpter Bobby G.
Published: July 2002

We test the suitability of a variety of explicit symplectic integrators for molecular dynamics calculations on Hamiltonian systems. These integrators are extremely simple algorithms with low memory requirements, and appear to be well suited for large scale simulations. We first apply all the methods to a simple test case using the ideas of Berendsen and van Gunsteren. We then use the integrators to generate long time trajectories of a 1000 unit polyethylene chain. Calculations are also performed with two popular but nonsymplectic integrators. The most efficient integrators of the set investigated are deduced. We also discuss certain variations on the basic symplectic integration technique.
Germanium quantum dots: Optical properties and synthesis
Authors: Heath James R. , Shiang J. J. , Alivisatos A. P.
Published: July 2002

Three different size distributions of Ge quantum dots (≳200, 110, and 60 Å) have been synthesized via the ultrasonic mediated reduction of mixtures of chlorogermanes and organochlorogermanes (or organochlorosilanes) by a colloidal sodium/potassium alloy in heptane, followed by annealing in a sealed pressure vessel at 270 °C. The quantum dots are characterized by transmission electron microscopy, x-ray powder diffraction, x-ray photoemission, infrared spectroscopy, and Raman spectroscopy. Colloidal suspensions of these quantum dots were prepared and their extinction spectra are measured with ultraviolet/visible (UV/Vis) and near infrared (IR) spectroscopy, in the regime from 0.6 to 5 eV. The optical spectra are correlated with a Mie theory extinction calculation utilizing bulk optical constants. This leads to an assignment of three optical features to the E(1), E(0′), and E(2) direct band gap transitions. The E(0′) transitions exhibit a strong size dependence. The near IR spectra of the largest dots is dominated by E(0) direct gap absorptions. For the smallest dots the near IR spectrum is dominated by the Γ25→L indirect transitions.
A generalized reaction field method for molecular dynamics simulations
Authors: Tironi Ilario G. , Sperb René , Smith Paul E. , van Gunsteren Wilfred F.
Published: July 2002

Molecular dynamics simulations of ionic systems require the inclusion of long-range electrostatic forces. We propose an expression for the long-range electrostatic forces based on an analytical solution of the Poisson–Boltzmann equation outside a spherical cutoff, which can easily be implemented in molecular simulation programs. An analytical solution of the linearized Poisson–Boltzmann (PB) equation valid in a spherical region is obtained. From this general solution special expressions are derived for evaluating the electrostatic potential and its derivative at the origin of the sphere. These expressions have been implemented for molecular dynamics (MD) simulations, such that the surface of the cutoff sphere around a charged particle is identified with the spherical boundary of the Poisson–Boltzmann problem. The analytical solution of the Poisson–Boltzmann equation is valid for the cutoff sphere and can be used for calculating the reaction field forces on the central charge, assuming a uniform continuum of given ionic strength beyond the cutoff. MD simulations are performed for a periodic system consisting of 2127 SPC water molecules with 40 NaCl ions (1 molar). We compare the structural and dynamical results obtained from MD simulations in which the long range electrostatic interactions are treated differently; using a cutoff radius, using a cutoff radius and a Poisson–Boltzmann generalized reaction field force, and using the Ewald summation. Application of the Poisson–Boltzmann generalized reaction field gives a dramatic improvement of the structure of the solution compared to a simple cutoff treatment, at no extra computational cost.
Influence of surface topology and electrostatic potential on water/electrode systems
Authors: Siepmann J. Ilja , Sprik Michiel
Published: July 2002

We have used the classical molecular dynamics technique to simulate the ordering of a water film adsorbed on an atomic model of a tip of a scanning tunneling microscope approaching a planar metal surface. For this purpose, we have developed a classical model for the water–substrate interactions that solely depends on the coordinates of the particles and does not require the definition of geometrically smooth boundary surfaces or image planes. The model includes both an electrostatic induction for the metal atoms (determined by means of an extended Lagrangian technique) and a site-specific treatment of the water–metal chemisorption. As a validation of the model we have investigated the structure of water monolayers on metal substrates of various topology [the (111), (110), and (100) crystallographic faces] and composition (Pt, Ag, Cu, and Ni), and compared the results to experiments. The modeling of the electrostatic induction is compatible with a finite external potential imposed on the metal. This feature is used to investigate the structural rearrangements of the water bilayer between the pair of scanning tunneling microscope electrodes in response to an applied external voltage difference. We find significant asymmetry in the dependence on the sign of the applied voltage. Another result of the calculation is an estimate of the perturbation to the work function caused by the wetting film. For the conditions typical for operation of a scanning tunneling microscope probe, the change in the work function is found to be comparable to the applied voltage (a few hundred millivolts).
Electrochemical growth of superparamagnetic cobalt clusters
Authors: Becker J. A. , Schäfer R. , Festag R. , Ruland W. , Wendorff J. H. , Pebler J. , Quaiser S. A. , Helbig W. , Reetz M. T.
Published: July 2002

The magnetization of stabilized cobalt colloids in tetrahydrofuran has been studied by a superconducting quantum interference device (SQUID) and magnetic balance measurements in dependence of applied magnetic field and temperature. The colloids are generated by a newly developed electrochemical method which allows one to generate clusters containing about 1000 atoms with a narrow size distribution. The final size distribution of the clusters is examined by high resolution transmission electron microscopy and small angle x-ray scattering. The magnetization curves have been determined with special emphasis on changes at the freezing point of the solution. The curves of the liquid phase can be reasonably described by the Langevin function and the magnetic moments of isolated cobalt clusters that have been recently measured by Stern–Gerlach experiments. Deviations that appear at the freezing point can be understood in terms of magnetic anisotropy effects. It is shown that the cluster sizes and the susceptibilities of the dispersions are related. Therefore the growth of the clusters during the electrolysis can be directly observed by measuring the susceptibility in dependence of the charge transport in the cell.
Thermal desorption of hydrogen from atomic nitrogen precovered Ru(001)
Authors: Seets D. C. , Wheeler M. C. , Mullins C. B.
Published: July 2002

We report a previously unobserved, low-temperature desorption peak in the hydrogen temperature programmed desorption spectra from atomic nitrogen precovered Ru(001). This low-temperature feature was observed after preparing the nitrogen atom overlayer by two distinctly different methods.
On the importance of the fragment relaxation energy terms in the estimation of the basis set superposition error correction to the intermolecular interaction energy
Authors: Xantheas Sotiris S.
Published: July 2002

The inclusion of the fragment relaxation energy terms in the estimation of the basis set superposition error (BSSE) correction to the interaction energy is necessary in order to ensure formal convergence to the uncorrected result at the complete basis set (CBS) limit. The problems associated with their omission are demonstrated for F−(H2O), Cl−(H2O), and (H2O)2 especially when very large basis sets are used. The family of correlation consistent basis sets allows for a heuristic extrapolation of both uncorrected and BSSE-corrected electronic energy differences of the three complexes to the MP2 CBS limits of −27.1, −15.1, and −4.9 kcal/mol respectively.
Improved radial grids for quadrature in molecular density-functional calculations
Authors: Mura Michael E. , Knowles Peter J.
Published: July 2002

A new radial coordinate transformation and associated integration grid scheme is presented for the problem of integrating functions of atomic electron density. Remarkable accuracy and stability is attained. Together with standard atomic partitioning and angular quadrature schemes, the new radial grid is applied to molecular density-functional theory, and it is shown that acceptable accuracy is attained with significantly fewer grid points than in previously presented schemes.
Kinetic theory and the Flory–Huggins approximation
Authors: Taylor P. L. , Yu Yi-Kuo , Wang X. Y.
Published: July 2002

A kinetic approach is used to examine the structure of the approximations that lead to the Flory–Huggins expression for the free energy of mixing of a polymer solution. The kinetic model that we study can be solved exactly in one dimension, and also on an infinite-dimensional lattice, in which case it reduces to the Flory–Huggins formula. An approximate solution that interpolates between these limits gives results for a three-dimensional lattice that differ rather little from those of the original Flory–Huggins theory.
λ-dynamics: A new approach to free energy calculations
Authors: Kong Xianjun , Brooks Charles L.
Published: July 2002

We present a novel and efficient method for performing free energy calculations. Treating the conventional λ variables associated with the ‘‘progress’’ in the chemical coordinates dynamically, our approach transforms free energy calculations into calculating potentials of mean force in the λ-space with molecular dynamics. This extended Hamiltonian formalism utilizes the full power of umbrella sampling techniques and the weighted histogram analysis method. Applications to free energies of hydration and a model illustrating competitive binding calculations are presented. These calculations demonstrate the robustness and flexibility of the new λ-dynamics approach.
Interactions in binary mixtures: Partial structure factors in mixtures of sodium dodecyl sulfate micelles and colloidal silica
Authors: Kline Steven R. , Kaler Eric W.
Published: July 2002

We have investigated the stability of a mixed colloidal system of sodium dodecyl sulfate (SDS) micelles and colloidal silica (Ludox TM). The colloidal silica is stably dispersed in water by virtue of its surface charge, but at SDS concentrations greater than 0.45 M the silica particles slowly aggregate. This transition is monitored with small angle neutron scattering (SANS) using a solvent contrast variation technique. Partial structure factors for the binary mixture are extracted from the SANS data using an optimization-regularization procedure. Knowledge of the partial structure factors in this asymmetric mixture (the Ludox radius is five times that of the micelles) allows us to determine the three pair interaction potentials. Model calculations for a multicomponent sticky hard sphere fluid are fitted to the experimental partial structure factors, thereby quantifying all three pair interaction potentials (which are potentials of mean force). For a fixed silica vol- ume fraction of 0.063, attractive interactions between the silica particles increase as the volume fraction of micelles is increased from 0.021 to 0.181. The micelles interact with a screened Coulomb repulsion that decreases in strength as the micelle concentration increases. The micelle–Ludox interaction has not been previously measured, and can be modeled as a hard-sphere–charged-sphere interaction for all of the mixture compositions studied.
A constrained maximum entropy method for the interpretation of experimental data: Application to the derivation of single particle orientation-conformation distributions from the partially averaged nuclear spin dipolar couplings of n-alkanes dissolved in a liquid crystalline solvent
Authors: Catalano D. , Emsley J. W. , La Penna G. , Veracini C. A.
Published: July 2002

A new method is described of deriving a distribution, P(x), in a set of variables, x, which is consistent with a set of data, A(x), and which is the least biased modification (maximizes entropy) of a constraining distribution, r(x). The method is demonstrated using nuclear spin dipolar couplings obtained for n-alkanes dissolved in a liquid crystalline solvent. The constraining distributions are obtained from computer simulations of the molecular dynamics.
Site–site pair correlation functions of water from 25 to 400 °C: Revised analysis of new and old diffraction data
Authors: Soper A. K. , Bruni F. , Ricci M. A.
Published: July 2002

Recent controversy in the literature about the structure of water away from ambient conditions is manifested by significant differences between the site–site pair correlation functions of water as derived from neutron diffraction data and the same quantities obtained in computer simulations using an effective pairwise potential for the intermolecular interactions. One possible explanation of the discrepancies between computer models of water and neutron diffraction results near the critical point is that they arise from uncertainties in the inelasticity correction, which is particularly large for light water. To test out this idea, a new method of obtaining the pair correlations is described. This new analysis method is applied both to the earlier neutron data on non-ambient water and on recently reported data for a range of densities at 573 K. For this newer data the incident neutron spectrum is derived from an ambient water moderator instead of the liquid methane (100 K) moderator of previous work, and this has the effect of altering the shape of the inelasticity distortion to the diffraction data. It is found that the resulting site–site correlation functions from different experiments are now closely consistent with one another, indicating that the inelasticity correction has been successfully performed by this new analysis. It is also demonstrated that the new site–site correlation functions can be generated from three-dimensional distributions of water molecules. The previously reported discrepancies between simulation and neutron data are not as severe with this new analysis, but there remain systematic trends with density and temperature which apparently are not reproduced by computer simulation with empirical pair potentials.