bluuebunny/crossref_metadata_2025 huggingface dataset:
Combined neutron diffraction and computer simulation study of liquid dimethyl sulphoxide
Authors: Luzar Alenka , Soper A. K. , Chandler David
Published: July 2002

The structure of liquid dimethylsulphoxide (DMSO) at 25 °C is explored using a combination of neutron diffraction with isotope substitution and computer simulation techniques. The potentials used in the computer simulation consist of Coulomb and 6-12 Lennard-Jones interactions for each of the carbon, oxygen, and sulfur sites on the molecule. To interpret the neutron diffraction data most effectively, it is necessary to refine both the molecular internal structure and the intermolecular contributions to the measured structure factors at the same time in order to separate the intermolecular terms correctly, because there is a large degree of overlap between intramolecular and intermolecular distances. This renders the data far more sensitive to the intermolecular forces than if this analysis were not performed. Direct comparison of neutron diffraction data and computer simulation results indicates that existing models of the molecular force field give a sensible description of the liquid structure, although there are some discrepancies which are not fully understood at this time. The question of whether this material can be regarded as an associated liquid, as it is frequently referred to, is discussed. All tests for association that have so far been applied to both the diffraction data and the computer simulation results do not indicate a highly ordered molecular association in the liquid.
Off-lattice Monte Carlo simulation of dilute and concentrated polymer solutions under theta conditions
Authors: Milchev Andrey , Paul Wolfgang , Binder Kurt
Published: July 2002

A recently introduced bead-spring type model of polymer chains with purely repulsive interactions between the beads is modified to allow for attractive forces at intermediate distances. This new model is suitable for the study of thermal properties of three-dimensional polymer solutions and melts and can be efficiently simulated with Link–Cell Monte Carlo methods. As a first step, the single chain lengths up to N=128 beads are studied and the theta temperature θ is located. It is shown that the data are compatible with the theoretically predicted crossover scaling behavior, and that the properties of collapsed chains can be studied for reasonably low temperatures T<θ. In addition to static properties (chain radii, polymer density inside the coil, internal energy, specific heat) also dynamic properties are obtained, namely various mean square displacements and relaxation times and the self diffusion constant. While for T=θ (and long enough chains), there is reasonable agreement with the Rouse model, a different behavior occurs for collapsed chains. As a second step, the behavior of the polymer solution at T=θ is studied as function of concentration. There is a clear increase of the chain gyration radii with concentration φ, although the values of 〈Rg2〉T=Θ in the highly concentrated regime (melts) are still smaller than their counterparts for the corresponding ‘‘athermal’’ model (with purely repulsive interactions) at melt densities. Thus one cannot identify the chain linear dimensions at the θ point with the chain dimensions in melts quantitatively. For the chain lengths studied (N≤64) the dynamic properties are compatible with Rouse behavior at all concentrations.
General strategy for the ab initio calculation of exchange coupling in polynuclear complexes
Authors: Handrick K. , Malrieu J. P. , Castell O.
Published: July 2002

A general strategy for the calculation of energy differences between states of different total spin is proposed. The procedure goes through the definition of a minimal model space and a set of energy-difference contributing determinants is established through the second-order development of the corresponding Hamiltonian in the framework of the quasidegenerate perturbation theory. The forementioned determinants are treated variationally in a so-called differential configuration interaction (CI). Several test calculations for simple model Li clusters have been carried out, and the results of the differential CI compare favorably with those of the exact solution in the full CI procedure.
Rotational motion of charged molecules in aqueous solutions. A change in the diffusion mechanism
Authors: Mikosch W. , Dorfmüller Th. , Eimer W.
Published: July 2002

The rotational motion of the anions polyphenyl 2 (PP2) and pyrene tetrasulfonate (PTS) was studied as a function of the solvent viscosity by time-resolved fluorescence depolarization spectroscopy using the single photon counting technique. The viscosity was varied by changing the temperature and the composition of the water–glycerol mixtures, respectively. At low viscosity the reorientational behavior of PP2 and PTS is well described by the Stokes–Einstein–Debye (SED) equation under stick boundary conditions. With increasing viscosity (η≥25 cP) however the rotational motion did no longer follow the SED predictions. Instead, we observed a much faster relaxation time, approaching an asymptotic value at very high viscosities. It seems that in the high viscosity regime microscopic collisional effects rather than the macroscopic hydrodynamic drag of the solvent determines the reorientational dynamics of the solute molecules.
Thermodynamics of strongly-coupled Yukawa systems near the one-component-plasma limit. I. Derivation of the excess energy
Authors: Hamaguchi S. , Farouki R. T.
Published: July 2002

The excess energy for a system of charged mesoscopic particles or ‘‘particulates’’ immersed in a neutralizing background medium is derived analytically, and is shown to approach that of the classical one-component plasma in the limit of high background temperatures. Examples of such systems, which are known as Yukawa systems due to the form of the interparticle pair potential, include dusty plasmas and colloidal suspensions. The expression for the excess energy allows thermodynamic properties of Yukawa systems to be determined from Monte Carlo or molecular-dynamics simulations.
Breaking in polymer chains. II. The Lennard-Jones chain
Authors: Oliveira F. A. , Taylor P. L.
Published: July 2002

Extensive simulations were performed in order to determine the conditions under which an anharmonic chain will break. The dynamics of a rectilinear chain of 100 monomers interacting via a Lennard-Jones potential were followed by solving a set of simultaneous Langevin equations. There are two principal results from this study. First, in order for irreversible breaking to occur in a stretched chain, a bond must be extended to a length considerably greater than the length at which the restoring force is maximized. Second, the breaking rate of a bond may be expressed in terms of the product of an attempt frequency and an Arrhenius factor. While the Arrhenius factor may be satisfactorily described in terms of the height of an effective energy barrier, the attempt frequency is found to be several orders of magnitude smaller than the dominant phonon frequencies.
Pulsed-discharge-nozzle Fourier-transform microwave spectroscopy of HC3S(2Πr) and HC4S(2Πi)
Authors: Hirahara Yasuhiro , Ohshima Yasuhiro , Endo Yasuki
Published: July 2002

Rotational spectra of sulfur-bearing carbon-chain free radicals HC3S and HC4S have been observed for the first time by using a Fabry–Perot type Fourier-transform microwave spectrometer combined with a pulsed discharge nozzle. Rotationally cooled (∼1–2 K) HC3S(2Π1/2) and HC4S(2Π3/2) in their ground vibronic states have been produced by a discharge in a supersonic free jet of an Ar-diluted mixtures of C2H2 and CS2. Transitions of an isotopic species, HC334S, have also been observed in the same system, and those of DC3S and DC4S have been observed by using C2D2 instead of C2H2. The observed spectral patterns for these species have clearly indicated that HC3S and HC4S are linear free radicals in the ground electronic states with 2Π symmetry. The spectra of HC3S, HC334S, and DC3S have shown both the Λ-type doubling and the hyperfine splittings due to the H or D nucleus, whereas those of HC4S and DC4S have shown only the hyperfine splittings. The observed transition frequencies for the radicals in the 7–25 GHz region have been fitted to the Hamiltonian appropriate to 2Π electronic states, yielding the effective molecular constants including the hyperfine parameters. The total bond length of HC3S has been determined to be 5.107 Å from the substituted-coordinate analysis.
The structure and stability of BH5. Does correlation make it a stable molecule? Qualitative changes at high levels of theory
Authors: Schreiner Peter R. , Schaefer Henry F. , von Ragué Schleyer Paul
Published: July 2002

Six BH5 structures were examined in detail using the self-consistent field (SCF), configuration interaction including single and double excitations (CISD), and coupled cluster including single, double, and perturbatively included connected triple excitations [CCSD(T)] methods in conjunction with a double-ζ plus polarization (DZP), a triple-ζ plus polarization (TZ2P), and an augmented TZ(3d1f1g,2p1d) basis set. The C4v and the D3h isomers are high in energy [23 and 45 kcal mol−1, respectively, relative to the Cs(I) structure at DZP CCSD]. Although structure Cs(I) is the global minimum, both Cs structures, where BH5 is comprised of nearly planar monoborane (BH3) and a hydrogen molecule, are essentially equal in energy and allow virtually free rotation of the hydrogen moiety. The global minimum was characterized by vibrational frequency analyses at the TZ2P CCSD(T) level. Final energies were obtained with the TZ(3d1f1g,2p1d) basis set and the CCSD(T) method. At room temperature, the borane–hydrogen complex BH5 is unstable toward dissociation by 6.8 kcal mol−1. However, at the absolute zero (0 K), the complex forms exothermically (−1.4 kcal mol−1). The theoretical De value (6.3 kcal mol−1) appears to be effectively converged with respect to basis set and theoretical method. The inclusion of perturbative triple excitations accounts for 36% of the total binding energy, a remarkable effect. Hydrogen scrambling via a C2v structure seems unlikely since the activation barrier for this process is at least 2.8 kcal mol−1 higher than the dissociation barrier of BH5 into BH3 and H2.
UV photostimulated desorption of ammonia from Cu(111)
Authors: Hertel T. , Wolf M. , Ertl G.
Published: July 2002

Upon irradiation with 193 and 308 nm laser light photoinduced desorption of ammonia from Cu(111) was studied at three coverages less than one monolayer (ML). The linear power dependence of the desorption yield and angle-resolved translational energy distributions of desorbed molecules indicate that desorption occurs due to an electronic excitation rather than a thermal process. Polarization measurements indicate an excitation process which is mediated by hot substrate electrons. The isotope effect, i.e., the ratio of the cross sections for photostimulated desorption (at 193 nm) of NH3 and ND3, respectively, decreases from 4.1 ± 1.2 to 1.9 ± 0.5 when the coverage—with respect to the substrate atom density—was raised from ≊0.02 to ≊0.14 ML. The magnitude of this isotope effect suggests that the energy which is required to break the molecule–surface bond is acquired in an intramolecular coordinate during a short-lived electronic excitation. We propose that for high vibrational excitation on the ground-state potential energy surface (PES), efficient coupling of the inversion mode with the molecule–surface coordinate leads to desorption. In order to illustrate the suggested desorption mechanism at a semiquantitative level, we performed trajectory calculations on a two-dimensional model potential energy surface. The results predict that desorption occurs rapidly within a few vibrational periods of the umbrella mode (Tvib∼35 fs)—with comparable energy release into the translational and vibrational degrees of freedom. Ammonia is furthermore expected to desorb in an inverted geometry, i.e., with the hydrogen atoms pointing towards the surface as opposed to the adsorption geometry with the nitrogen end bound to the surface. Angular distributions of flux and mean translational energy are strongly peaked around the surface normal. Their width can be attributed to thermal motions parallel to the surface prior to excitation.
Zero kinetic energy photoelectron spectroscopy of Rydberg excited molecular iodine
Authors: Cockett M. C. R. , Goode J. G. , Lawley K. P. , Donovan R. J.
Published: July 2002

Two color (2+1′) zero kinetic energy pulsed field ionization (ZEKE-PFI) spectra of I2 ionized via a number of gerade Rydberg excited states have been recorded for the first time. Ionization into both X 2Π3/2,g and X 2Π1/2,g spin–orbit ionic substates was achieved by selective excitation of Rydberg states having the appropriate spin–orbit ionic core. Adiabatic ionization energies have been determined as 75 069±2 cm−1 for the X 2Π 3/2,g state and 80 266±2 cm−1 for the X 2Π1/2,g state with ωe values for the two states determined as 240±1 and 229±1 cm−1, respectively. The ZEKE-PFI spectra recorded for the lower spin–orbit state exhibit non-Franck–Condon behavior which has been interpreted in terms of an autoionization mechanism. The ZEKE-PFI technique has also been used to probe the extent of Rydberg and ion-pair character in a strongly mixed Rydberg/ion-pair state.
Unimolecular dynamics of Cl−...CH3Cl intermolecular complexes formed by Cl−+CH3Cl association
Authors: Peslherbe Gilles H. , Wang Haobin , Hase William L.
Published: July 2002

A previous trajectory study of the dissociation of Cl−...CH3Cl complexes formed by Cl−+CH3Cl association is further analyzed to determine (1) the relationship between classical and quantum Rice–Ramsperger–Kassel–Marcus (RRKM) rate constants for Cl−...CH3Cl→Cl−+CH3Cl dissociation; (2) the importance of anharmonicity in calculating the RRKM dissociation rate constant; (3) the role of angular momentum in interpreting the trajectory distribution N(t)/N(0) of Cl−...CH3Cl complexes versus time; and (4) the pressure-dependent collision-averaged rate constant k(ω,E) for Cl−...CH3Cl dissociation. It is found that only the low-frequency intermolecular modes of Cl−...CH3Cl are initially excited by Cl−+CH3Cl association. Classical and quantum RRKM rate constants for dissociation of this intermolecular complex are in excellent agreement. Anharmonicity lowers the rate constant by a factor of 4–8 from its harmonic value. The dissociation rate for the long-time tail of the trajectory N(t)/N(0) distribution is much smaller than predicted by a RRKM model, which accurately treats angular momentum. It is suggested that the long-lived trajectories may arise from motion on vague tori. The trajectory collision-averaged rate constant k(ω,E) is in agreement with an experimental study at 300 K.
Convergence of molecular potential energy surfaces by interpolation: Application to the OH+H2→H2O+H reaction
Authors: Jordan Meredith J. T. , Thompson Keiran C. , Collins Michael A.
Published: July 2002

A recently proposed scheme for interpolating and iteratively improving molecular potential energy surfaces [Ischtwan and Collins, J. Chem. Phys. 100, 8080 (1994)] is evaluated by comparison with an analytic surface for the OH+H2→H2O+H reaction. An improvement in the procedure for constructing the potential surface is suggested and implemented. The most efficient means of converging the surface is determined. It is found that the probability of reaction, for example, may be accurately calculated using of the order of 200–400 data points to define the potential energy surface.
Rings and chains in sulfur cluster anions S− to S9−: Theory (simulated annealing) and experiment (photoelectron detachment)
Authors: Hunsicker S. , Jones R. O. , Ganteför G.
Published: July 2002

Density functional calculations with simulated annealing have been performed for singly charged anions of sulfur clusters with up to nine atoms. The calculations predict the existence of two classes of structure: open (chain-like) and closed (ring-like), for which the vertical detachment energies (VDE) and the adiabatic electron affinities for transitions to states of the neutral clusters show pronounced differences. These calculations are complemented by photoelectron detachment measurements on sulfur cluster ions with up to 11 atoms using a pulsed arc cluster ion source (PACIS). The measurements provide unambiguous evidence for the existence of two types of isomers in both S6− and S7−. Although ring structures are generally energetically more stable than chain structures, the environment used to generate the larger clusters (n≳7) favors the formation of the latter. The measured VDE and vibration frequencies agree well with the calculated values.
Selective production of photofragments by monitoring the shape of asymmetric resonances in OH photodissociation: Dependence on initial vibrational states
Authors: Lee Sungyul
Published: July 2002

Quantum mechanical analysis is presented on the vibrational state dependence of the total dissociation cross sections and the branching ratios of O(3Pj, j=0,1,2) in the predissociation of OH. Two transformation matrices, each of which describes the relation between an atomic term limit and the correlating molecular states, are constructed and incorporated in the close coupling calculations. The branching ratios of O(3Pj, j=0,1,2) depend very sensitively on the vibrational levels (v=0–4) of the initial X 2Π state. The variations of the spin–orbit distributions as a function of the excitation energy near the asymmetric resonances change markedly depending on the vibrational levels. The variations are either redshifted or blueshifted from the resonance position, depending on the degree of asymmetry of the resonances. The widths of the variations tend to increase with increasing vibrational quantum number of the initial state, suggesting the possibility of choosing the proper linewidths in the experiments to selectively produce the photofragments in one-photon process. Discussion is presented on the applicability of the theoretical scheme to analyze the recent measurements by Neumark and co-workers [J. Chem. Phys. 103, 2495 (1995)] on the product fine structure distributions in the predissociation of O2.
Refractive index change by photothermal effect with a constant density detected as temperature grating in various fluids
Authors: Terazima Masahide
Published: July 2002

Two origins of the refractive index variation after depositing heat energy to a fluid are separately investigated by using the transient grating (TG) method; the variations are caused by a density fluctuation and a temperature fluctuation with a constant density. Although the relative contribution of the temperature rise (constant density) component is small in the total refractive index change, a precise measurement of the TG signal enables us to determine the magnitude and it is found that they are as large as 3%–6% in magnitude of dn/dT except water, in which the relative contribution strongly depends on temperature. The origin of the temperature rise component is interpreted in terms of the interaction induced polarizability change of the fluid molecules. An application of this component to the ultrafast detection of the photothermal techniques is discussed.
On the determination of domain sizes in polymers by spin diffusion
Authors: Wang Jiahu
Published: July 2002

In this article, a mathematical description of spin diffusion in copolymers is presented that avoids some of the approximations employed by previous authors. The analytical relations obtained can readily be used in solid-state NMR experiments to determine the domain size of each component. A numerical approach to spin diffusion has also been examined and its performance has been compared with the analytical method. For systems with interphase structure, the spin diffusion process has been numerically simulated based on a simple model with linear character changes. Application of the present method to systems with or without interphase has been demonstrated.
Molecular dynamics simulation of liquid water along the coexistence curve: Hydrogen bonds and vibrational spectra
Authors: Martí J. , Padro J. A. , Guàrdia E.
Published: July 2002

Structure and dynamic properties of liquid water at temperatures between 298 and 523 K and densities between 0.75 and 1.20 g/cm3 have been investigated by molecular dynamics simulation. A flexible simple point charge potential has been asssumed for interactions. The hydrogen bonding structure in the different simulated states as well as the influence of the hydrogen bonds on the dynamic properties (self-diffusion coefficients, vibrational spectra) is discussed. Special attention is paid to the intermolecular vibrational spectrum (10–400 cm−1). It has been corroborated that the band around 200 cm−1 can be attributed to intermolecular O–O stretching vibrations of pairs of H-bonded bounded molecules. On the contrary, molecular dynamics results indicate that the band close to 50 cm−1 is independent of the existence of hydrogen bonds but depends on the density and temperature of the system. It is suggested that it is simply associated with vibrations of molecules in the cage formed by their neighbors. Shifts of librational and stretching bands as a function of the thermodynamic state are highly correlated with changes in the percentage of hydrogen bonded molecules.
Condensation of a supersaturated vapor. XII. The homogeneous nucleation of the n-alkanes
Authors: Rudek Markus M. , Fisk Jeffery A. , Chakarov Vasil M. , Katz Joseph L.
Published: July 2002

Homogeneous nucleation rates as functions of both supersaturation and temperature were measured in an upward thermal diffusion cloud chamber for four n-alkanes: n-heptane, n-octane, n-nonane, and n-decane. Nucleation rates from about 10−4 to 5×100 drops cm−3 s−1 were obtained in the temperature range, 241 to 330 K. Their dependences on supersaturation and temperature were compared to predictions of several nucleation theories: the internally consistent Classical theory, two versions of the Kalikmanov–van Dongen theory, and the Delale–Meier theory. Each theory predicted the dependence of the nucleation rate on supersaturation reasonably well. However, large temperature dependent correction factors were needed for quantitative agreement between measured and predicted nucleation rates. A plot of the ratio of measured to predicted nucleation rates vs reduced temperature shows that all n-alkanes investigated can be represented to within a factor of ten by a single, best fit line.
Statistical adiabatic channel model for ion–molecule capture processes. II. Analytical treatment of ion–dipole capture
Authors: Troe J.
Published: July 2002

Thermal rate constants for capture of permanent and induced dipoles by ions are calculated by the statistical adiabatic channel model (SACM) using analytical representations of adiabatic channel eigenvalues. Linear, symmetric, and asymmetric top dipoles with closed and open electronic shells are considered. Analytical representations of the capture rate constants over broad temperature ranges are presented for all systems. Numbers of open channels W(E,J) are also investigated. The present analytical SACM treatment provides an economical, transparent, and accurate approach to the considered type of capture processes. At the same time, the implementation to ion fragmentation processes is elucidated.
Secondary time scales of intramolecular vibrational energy redistribution in CF3H studied by vibrational overtone spectroscopy
Authors: Boyarkin O. V. , Rizzo T. R.
Published: July 2002

Rotationally resolved vibrational overtone spectra of several CH stretch bands of CF3H have been obtained using a combination of jet-cooling, infrared-optical double-resonance, and infrared laser assisted photofragment spectroscopy detection. The high selectivity and sensitivity of this combination allows measurement of spectral splittings that determine the secondary time scales of vibrational energy redistribution. The spectroscopic results show that in the energy region of the vCH=3–5 CH stretch levels, the vibrational energy would remain localized in the mixed CH stretch–bend states on time scales longer than 13–130 ps if coherently excited. This is as much as 2500 times longer than the time scale for energy transfer between the CH stretch and the HCF bend vibrations. Moreover, the results confirm that the energy redistribution dynamics are sensitive to the character of the bath states and not simply to the total vibrational energy.
Kinetics of morphological transition in Polystyrene–block- polybutadiene–block-polystyrene triblock copolymer melt
Authors: Sakurai Shinichi , Umeda Hideo , Taie Kazuhiro , Nomura Shunji
Published: July 2002

The kinetics of morphological transition from cylinders to lamellae was studied using a time-resolved small-angle x-ray scattering (SAXS) technique with synchrotron radiations. The sample examined is a polystyrene–block-polybutadiene–block-polystyrene (SBS) triblock copolymer having a number average molecular weight, Mn=6.31×104 and a volume fraction of polystyrene (PS) blocks, φPS=0.53. We previously reported that this sample formed a nonequilibrium morphology of polybutadiene (PB) cylinders when cast from its methyl ethyl ketone (MEK) solution. MEK is a selective solvent, i.e., good for PS chains but poor for PB chains. The transmission electron microscopy revealed that the PB cylinders transformed into wavy lamellae through coalescence when the samples were annealed at temperatures above the glass transition temperature of PS. It was also revealed that well-ordered lamellae were formed in the fully annealed samples. In order to discuss quantitatively the morphological transition and the subsequent lamellar ordering, we conducted the time-resolved SAXS measurements. Diffraction peaks arising from hexagonally packed cylinders were decreasing while those arising from alternating lamellae were increasing with time. It was found that the temporal change of the integrated intensity of the diffraction peak could not be fitted by a single exponential decay, but could be expressed by an exponential decay of time with β power. The values of β were in the range of 1<β<2. From the resultant decay time, apparent activation energies were further evaluated for coalescing cylinders and for ordering lamellae. These values were approximately in accord with each other, and it can be simply concluded that the kinetics of the morphological transition is governed by the polystyrene matrix phase.
Sublimation dynamics of CO2 thin films: A high resolution diode laser study of quantum state resolved sticking coefficients
Authors: Weida Miles J. , Sperhac Jeanette M. , Nesbitt David J.
Published: July 2002

Nascent quantum states of CO2 subliming from CO2 thin films at rates of 1 to 103 monolayers (ML) per second are probed via direct infrared absorption of the ν3 asymmetric stretch with a frequency ramped diode laser. The high spectral resolution (Δν≊15 MHz) of the diode laser and the use of polarization modulation techniques permit individual rotational, vibrational, translational, and even MJ degrees of freedom of the subliming flux to be studied with quantum state resolution. Measured rotational and ν2 bend vibrational distributions indicate that the molecules sublime from the surface in a Boltzmann distribution characterized by the thin film temperature Ts. Similarly, the velocity distributions parallel to the surface are well described by a Maxwell velocity distribution at Ts, as determined by high resolution Doppler analysis of the individual rovibrational line shapes. The MJ distribution of subliming rotational states is probed via polarization modulation methods; no alignment is detected within experimental sensitivity. This places an upper limit on the anisotropy in the rotational distribution of |n⊥/n∥−1|<0.02, where n⊥/n∥ is the ratio of molecules with J perpendicular vs parallel to the surface normal. By virtue of the direct absorption technique, the absolute sublimation rates from the surface can be obtained from the measured column integrated densities. Via detailed balance, these fluxes are compared with equilibrium vapor pressure measurements to retrieve the absolute sticking coefficients S for gas phase CO2 impinging on a solid phase CO2 thin film. For sublimation rates <103 ML/s, the data indicate S=1.0±0.2, irrespective of quantum state, rotational alignment, and tangential velocity component. For sublimation rates ≳103 ML/s, the onset of a mild supersonic expansion is observed, with post-desorption collisions cooling the rotational temperature by as much as 15 K below Ts. Modeling of the gas–surface interaction using realistic CO2–CO2 pair potentials demonstrates that the gas–surface potential is relatively ‘‘soft’’ and highly corrugated, which promotes efficient translational and rotational energy transfer to the surface. The scattering analysis also suggests that nonequilibrium quantum state distributions in the subliming flux are not expected for translational and rotational energies less than or comparable to the binding energy of CO2 to the surface.
Energy migration and rotational motion within bichromophoric molecules. II. A derivation of the fluorescence anisotropy
Authors: Johansson L. B.-Å. , Edman Peter , Westlund P.-O.
Published: July 2002

A generalized Förster theory is presented which includes reorientation of the interacting molecules. The stochastic master equation is, for the first time, derived from the stochastic Liouville equation, so that it accounts for the molecular origin to the stochastic transitions rates. A formal solution to the stochastic master equation is given. This equation is compared with its truncated cumulant expansion. The second-order cumulant contains the correlation function 〈κ2(0)κ2(t)〉, where κ denotes the orientational dependence on dipole–dipole coupling. The solution of the master equation is used to formulate the time-dependent fluorescence anisotropy, which is the relevant observable of energy transfer within donor–donor (dd) pairs, or bichromophoric molecules. Depending on symmetry of the local orientational distributions of the donor molecules, and their rates of reorientation, the fluorescence anisotropy decay becomes more or less complicated. Different simplifying conditions are given. The orientational distribution of the bichromophoric molecules is assumed to be isotropic and their rotational motion is taken to be negligible. The effect of using the cumulant expansion on the excitation probability and the fluorescence anisotropy was numerically examined. Brownian dynamics simulations are used to describe isotropic rotation of the d molecules. In the fast case (or dynamic limit), where the rates of transfer are much slower than those of reorientation, the cumulant expansion is always valid. In the intermediate case the approximation becomes questionable, while for the static limit, a numerical evaluation of the formal solution must be performed. The theory presented here is easily modified to account for the influence of reorientation in studies of donor–acceptor transfer.
Hyperfine decoupling in electron spin resonance
Authors: Jeschke Gunnar , Schweiger Arthur
Published: July 2002

A new class of experiments is introduced to electron spin resonance (ESR) spectroscopy that utilizes hyperfine decoupling for resolution enhancement and spectrum simplification, and that provides a basis for correlation techniques. A general framework is provided for the discussion of pulse ESR experiments on systems with arbitrary effective electron spin S and an arbitrary number of coupled nuclear spins and is used to describe spin decoupling in pulse ESR and ENDOR spectroscopy. Analytical expressions are given for the hyperfine-decoupled nuclear frequencies and the residual hyperfine splittings of spin-1/2 nuclei during strong decoupling. Pulse sequences are proposed for hyperfine-decoupled electron spin echo envelope modulation (ESEEM) and electron nuclear double resonance (ENDOR) experiments as well as for the correlation of the hyperfine-decoupled ESEEM spectrum with two-pulse and three-pulse ESEEM spectra and of hyperfine-decoupled ENDOR with the hyperfine splittings. It is shown that hyperfine-decoupled ESEEM and ENDOR spectra can reveal information on the magnetic quantum numbers involved in an ESR observer transition, and that choosing a transition mS↔mS+1 with mS≠−1/2 can improve the resolution of a nuclear frequency spectrum. In addition, such experiments can be used to determine the relative signs of hyperfine couplings. The potential of the two-dimensional DECENT (decoupled ESEEM correlated to nuclear transition frequencies) experiment is demonstrated on weakly coupled N14 nuclei in both an ordered and a disordered system and on the hexaquo manganese (II) complex (S=5/2) in a single crystal. It is also shown that for the ESR observer transition mS=(−3/2↔−5/2) the S=5/2 system yields highly resolved hyperfine-decoupled ENDOR spectra which allow for a complete assignment of the ENDOR lines.
Surprising rate coefficients for four isotopic variants of O+O2+M
Authors: Anderson S. M. , Hülsebusch Detlev , Mauersberger Konrad
Published: July 2002

Mass spectrometric analysis of nonequilibrium oxygen isotopic mixtures undergoing UV photolysis has been employed to study three-body recombination rate coefficients for the O+O2, Q+O2, O+Q2, and Q+Q2 (O=16O, Q=18O) reactions, all with M=80% N2:10% O2:10% Q2 at 200 Torr and 296 K. kO+O2 is in good agreement with the currently recommended value, while kQ+Q2 is only slightly smaller. Surprisingly, kQ+O2 is close to kO+O2, while kO+Q2 is ≈50% larger. As a consequence of this unusual behavior, kO+OQ must be ≈20% larger than kQ+OQ to produce the well-known enrichments that occur in the free atmosphere and in laboratory experiments involving scrambled mixtures. Contrary to what is usually assumed in discussions of the heavy ozone anomaly, these results indicate that isotopic asymmetry does not guarantee a rate coefficient advantage.
A symplectic method for rigid-body molecular simulation
Authors: Kol Ayla , Laird Brian B. , Leimkuhler Benedict J.
Published: July 2002

Rigid-body molecular dynamics simulations typically are performed in a quaternion representation. The nonseparable form of the Hamiltonian in quaternions prevents the use of a standard leapfrog (Verlet) integrator, so nonsymplectic Runge–Kutta, multistep, or extrapolation methods are generally used. This is unfortunate since symplectic methods like Verlet exhibit superior energy conservation in long-time integrations. In this article, we describe an alternative method, which we call RSHAKE (for rotation-SHAKE), in which the entire rotation matrix is evolved (using the scheme of McLachlan and Scovel [J. Nonlin. Sci. 16 233 (1995)]) in tandem with the particle positions. We employ a fast approximate Newton solver to preserve the orthogonality of the rotation matrix. We test our method on a system of soft-sphere dipoles and compare with quaternion evolution using a 4th-order predictor–corrector integrator. Although the short-time error of the quaternion algorithm is smaller for fixed time step than that for RSHAKE, the quaternion scheme exhibits an energy drift which is not observed in simulations with RSHAKE, hence a fixed energy tolerance can be achieved by using a larger time step. The superiority of RSHAKE increases with system size.
Relative predissociation rates of OH (A 2Σ+, v′=3) from combined cavity ring down—Laser-induced fluorescence measurements
Authors: Spaanjaars J. J. L. , ter Meulen J. J. , Meijer G.
Published: July 2002

Relative predissociation rates of OH (A 2Σ+, v′=3) have been determined for N′ up to 17 by simultaneously measuring laser-induced fluorescence excitation and cavity ring down absorption in the same experimental setup. The results are in overall agreement with calculated predissociation rates by Yarkony [J. Chem. Phys. 97, 1838 (1992)], but show a stronger increase of the predissociation rate with increasing N. The obtained values for the quantum yield can be used in the application of the laser-induced predissociative fluorescence detection technique in combustion diagnostics.
The effects of pressure on structural and dynamical properties of associated liquids: Molecular dynamics calculations for the extended simple point charge model of water
Authors: Bagchi Ken , Balasubramanian Sundaram , Klein Michael L.
Published: July 2002

Constant temperature and constant pressure molecular dynamics calculations have been performed on the extended simple point charge (SPC/E) model water to understand the effects of pressure on its structural and dynamical properties. The effect of pressure is to increase the presence of interstitial water molecules, which is shown to be responsible for the experimentally observed increase in the first x-ray diffraction peak and a broadening of the OOO∧ bond angle distribution. These structural changes lead to weaker hydrogen bonding and the observed minimum in the rotational correlation time as a function of pressure. The weak maximum in the measured translational diffusion coefficient at around 1.5 kbar was not reproduced. The temperature of the density maximum was estimated to occur at 245±4 K, which suggests that structural changes with increasing pressure and decreasing temperature are not quantitatively reproduced by the SPC/E model.
Attenuation of polarization echoes in nuclear magnetic resonance: A study of the emergence of dynamical irreversibility in many-body quantum systems
Authors: Levstein Patricia R. , Usaj Gonzalo , Pastawski Horacio M.
Published: July 2002

The reversal of the time evolution of the local polarization in an interacting spin system involves a sign change of the effective dipolar Hamiltonian which refocuses the “spin diffusion” process generating a polarization echo. Here, the attenuation of these echo amplitudes as a function of evolution time is presented for cymantrene and ferrocene polycrystalline samples, involving one and two five spin rings per molecule, respectively. We calculate the fraction of polarization which is not refocused because only the secular part of the dipolar Hamiltonian is inverted. The results indicate that, as long as the spin dynamics is restricted to a single ring, the non-inverted part of the Hamiltonian is not able by itself to explain the whole decay of the polarization echoes. A crossover from exponential (cymantrene) to Gaussian (ferrocene) attenuation is experimentally observed. This is attributed to an increase of the relative importance of the spin dynamics, as compared with irreversible interactions, which favors dynamical irreversibility.
Can a single function for χ account for block copolymer and homopolymer blend phase behavior?
Authors: Maurer Wayne W. , Bates Frank S. , Lodge Timothy P. , Almdal Kristoffer , Mortensen Kell , Fredrickson Glenn H.
Published: July 2002

Most theoretical treatments of polymer–polymer phase behavior assume that homopolymer mixtures and block copolymer melts are controlled by a common segment–segment interaction parameter knows as χ. This publication describes the results of small-angle neutron scattering (SANS) experiments conducted as a function of temperature and composition from homogeneous mixtures of poly(ethylene) (PE) and poly(ethylenepropylene) (PEP) of equal molecular weight. Analysis of these SANS measurements based on the random phase approximation indicates that χPE/PEP is independent of composition and linear in T−1. The associated symmetric phase diagram calculated with Flory–Huggins theory contains a stability curve that is consistent with the divergence in single phase susceptibility obtained by SANS. This function χPE/PEP(T) is compared with functions for χPE–PEP(T) associated with the homologous PE–PEP diblock copolymers, extracted both from the temperature dependence of the disordered state scattering structure factor, and from the N dependence of the order–disorder transition (ODT) temperature TODT, where N is the degree-of-polymerization. Analysis of these data using mean-field and fluctuation theories leads to four distinct expressions for χPE–PEP(T), none of which is consistent with χPE/PEP(T). We attribute this disparity to the effects of polarization and stretching of diblock molecules in the vicinity of the ODT. A simple model calculation that corrects the prediction for (χN)ODT based on the experimentally determined shift in the SANS peak location corroborates this deduction. We conclude that current theories cannot account for block copolymer and homopolymer phase behavior based on a single function for χ, and suggest that this deficiency may be rectified with a more sophisticated block copolymer theory that properly accounts for chain stretching.
Abstraction of D adsorbed on Pt(111) surfaces with gaseous H atoms
Authors: Wehner S. , Küppers J.
Published: July 2002

The impact of gaseous H atoms at D covered Pt(111) surfaces at 85 K leads to the formation of gaseous HD and D2 products. The kinetics of formation of these products was measured simultaneously with H exposure for different initial D coverages. The HD and D2 rates as a function of H fluence from the reaction start exhibit common characteristics; a rate step, a rate maximum, and a subsequent exponential rate decay. The HD and D2 rate steps were observed not to scale linear with the D coverage and to increase if on D covered surfaces H was coadsorbed prior to reaction. Of the observed phenomena, only the exponential decay of the HD rate is in line with expectations if an Eley–Rideal mechanism acts in the present reaction. D2 formation, the HD rate step as a function of D coverage, and the presence of a H coadsorbate contradict the Eley–Rideal picture. The results suggest that the reactions towards HD and D2 proceed via hot atom type mechanisms.
Silicide formation by high-temperature reaction of Rh with model SiO2 films
Authors: Labich S. , Kohl A. , Taglauer E. , Knözinger H.
Published: July 2002

The metal–support interaction between rhodium and silica has been studied by x-ray photoelectron spectroscopy for a Rh/SiO2/Mo model system. This system consists of a thin silicon oxide layer, prepared by chemical vapor deposition on molybdenum with a nominal load of one monolayer rhodium. Heating in ultrahigh vacuum (UHV) results in changes of the cluster size and binding energies of surface species. Thermal treatments above 850 K in UHV results in the formation of a rhodium silicide, Rh3Si, which has not been reported so far. For the formation of this new phase a surface reaction mechanism is proposed.
Effect of convective transport on droplet spinodal decomposition in fluids
Authors: Kumaran V.
Published: July 2002

The effect of convective transport on the late stage growth of droplets in the presence of sedimentation and shear flow is analyzed. The high Peclet number limit (UR/D)≫1 is considered, where U is the characteristic velocity, R is the radius of the droplet, and D is the diffusion coefficient. The growth of the droplet depends on the boundary condition for the fluid velocity at the droplet interface, and two types of boundary conditions are considered. For a rigid interface, which corresponds to the interface between a solid and a fluid, the tangential velocity is zero and the normal velocity is equal to the velocity of the surface. For a mobile interface, which corresponds to an interface between two fluids, the tangential and normal velocities are continuous. These results indicate that the scaling relations for the critical radius are Rc(t)∝t(1/2) for a sedimenting droplet with a rigid interface, Rc(t)∝t(2/3) for a sedimenting droplet with a mobile interface, Rc(t)∝t(3/7) for a droplet with a rigid interface in a simple shear flow, and Rc(t)∝t(1/2) for a droplet with a mobile interface in a simple shear flow. The rate of droplet growth is enhanced by a factor of Pe(1/3) for rigid interfaces and Pe(1/2) for mobile interfaces.
Temperature dependance of properties of star-branched polymers: A computer simulation study
Authors: Romiszowski Piotr , Sikorski Andrzej
Published: July 2002

Dynamic Monte Carlo simulations of single star-branched polymer models were made. Star macromolecules were confined to a simple cubic lattice with the nearest-neighbor attractive interactions. Every star consisted of f=3 arms of equal length. Length of a star varied between 49 and 799 statistical segments. Static and dynamic properties of model stars were calculated in good solvent conditions, Θ-state and in the collapsed state. Change of the chain dimensions, diffusion coefficients, and their scaling exponents with the temperature was shown and discussed. The locations of the Θ temperature and the collapse transition temperature TC were estimated for all chain lengths under consideration. The differences in motion of inner and outer parts of a star-branched polymer in different solvent conditions were described. The existence of a high-density core in the center of star macromolecules was confirmed and the influence of the temperature on its magnitude was studied. An analysis of motion of different parts of star polymers and of the number of inter- and intra-armal contracts was performed.
Ab initio relativistic pseudopotential study of small silver and gold sulfide clusters (M2S)n, n=1 and 2
Authors: Bagatur’yants Alexander A. , Safonov Andrei A. , Stoll Hermann , Werner Hans-Joachim
Published: July 2002

Small semiconductor silver and gold sulfide clusters (Ag2S)n and (Au2S)n, n=1,2, are studied by valence ab initio calculations with the inclusion of electron correlation at the second-order perturbation theory (MP2) and coupled-cluster [CCSD and CCSD(T)] levels. Various relativistic and nonrelativistic pseudopotentials are employed to describe the effects of core electrons. Correlation and relativistic effects are essential in determining the geometry and relative stability of monomer and dimer structures. Relativistic effects result in a notable decrease in the calculated interatomic distances, which is especially significant in the case of gold sulfide structures (up to 10%). Correlation effects markedly increase the stability of compact structures with an increased number of relatively short M…M contacts (M…M distances of about 280–330 pm). Excluding the correlation of lower-lying valence orbitals (sulfur 3s and silver 4d or gold 5d) results in completely opposite predictions. This fact suggests that the effects of d–d and d–outer valence (metal ns and sulfur 3p) electron correlation give rise to attractive short-range interactions of intramolecular van der Waals type, which determine the increased stability of more compact cluster structures. However, large-core pseudopotentials strongly exaggerate this effect in the case of gold and give results rather different from those obtained with more valid and accurate small-core pseudopotentials. It is shown that the reason for this deficiency lies in the nature of pseudopotentials themselves rather than in basis-set shortcomings. The atomization and dissociation energies, equilibrium geometrical parameters, dipole moments, and Mulliken populations are calculated and discussed.
Gas-phase reactions of some positive ions with atomic and molecular nitrogen
Authors: Scott Graham B. I. , Fairley David A. , Freeman Colin G. , McEwan Murray J. , Anicich Vincent G.
Published: July 2002

The reactions of the cations CN+, HCN+, HCNH+, HC3N+, HC3NH+, H3+, H2O+, H3O+, N2+, CO+, HCO+, O2+, CO2+, HCO2+, and C2H2+ with atomic and molecular nitrogen have been characterized using a selected ion flow tube (SIFT) operating at room temperature. Rate coefficient and branching ratio data are reported for all ion–neutral reactions studied. Constraints arising from spin conservation considerations are found to be unimportant in cation-N atom processes.