bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantum dynamics of overtone relaxation in benzene. I. 5 and 9 mode models for relaxation from CH(v=3)
Authors: Wyatt Robert E. , Iung Christophe , Leforestier Claude
Published: July 2002

This series is concerned with the quantum dynamics of overtone relaxation in planar benzene and in reduced mode planar benzene fragments. In these studies, ultralarge direct product primitive vibrational spaces (of dimension up to 1010) are contracted to active spaces of dimension 5000–10 000. The contractions are carried out via artificial intelligence tree pruning algorithms, or a new iterative wave operator pruning algorithm. The exact dynamics within the active space is then developed via the recursive residue generation method. In part I of this series, emphasis is placed upon v=3 CH overtone dynamics in the 5 and 9 mode benzene fragments C3H and C3H3. Neither system undergoes complete relaxation, but the survival probability in C3H undergoes large amplitude oscillations with a period characteristic of stretch–wag interaction in the CH chromophore. For C3H3, the two initially nonexcited CH stretch modes do not play a significant role in the dynamics for t<1 ps. However, modes in both systems that have a high degree of wag motion for the initially excited chromophore play a significant role at short times. Comparisons with earlier classical trajectory studies show good correspondence between the classical and quantum results only at short times, t<0.1 ps.
A dielectric theory of the optical-like high-frequency mode in liquid water
Authors: Resat Haluk , Raineri Fernando O. , Friedman Harold L.
Published: July 2002

The ‘‘optical’’-like collective mode (OM) in TIP4P model water recently reported by Ricci et al. is investigated here with a simple dynamical theory, the reference memory function approximation (RMFA), applied to calculate the dynamical dielectric properties of rigid and nonpolarizable interaction site models for water. Our results indicate that the OM is responsible for the high-frequency local charge density fluctuations that are germane to the frequency- and wave vector-dependent longitudinal dielectric function of water. The results for the OM in the TIP4P and SPC/E water models studied here under RMFA are in good agreement with each other as well as with molecular dynamics simulation results of Ricci et al. on the TIP4P model. An interesting connection to ion solvation dynamics is established.
Metal–metal bonding on surfaces: Zn–Au on Ru(001)
Authors: Rodriguez José A. , Hrbek Jan
Published: July 2002

The properties of Zn, Au, and Zn–Au films on Ru(001) have been studied using thermal desorption mass spectroscopy (TDS) and core and valence-level photoemission. TDS spectra show desorption of Zn from Ru(001) at 450 (multilayer), 490 (second layer), and 720 K (first layer). The monolayer of Zn desorbs following zero-order kinetics with an activation energy for desorption of 36 kcal/mol. Photoemission studies show that the Zn(3s) and Zn(3d) levels of Zn bonded to Ru are shifted 0.5 eV toward lower binding energy with respect to the corresponding levels of pure metallic Zn. The direction of this binding-energy shift together with the results of CO-TDS indicate that Zn is withdrawing electrons from the Ru(001) surface. Such charge transfer is in part responsible for the large stability of the Ru–Zn bond. Au desorbs from Ru(001) at temperatures of 1175 (multilayer) and 1260 K (first layer). For a monolayer of Au deposited on Ru(001), the positions of the Au(4f ) and Au(5d) levels are very close to those of the surface atoms of Au(111), indicating that charge transfer between Au and Ru is minimal. A comparison of the desorption temperature for a monolayer of Zn or Au from Ru(001) with values previously reported for the desorption of monolayers of alkali (K and Cs), noble (Cu and Ag), and transition (Mn, Ni, and Pd) metals indicates that the strength of a metal–metal bond in a bimetallic surface depends on (1) the bulk cohesive energy of the individual metals (which gives an indication of the tendency of the element to form strong metal–metal bonds), and (2) the charge transfer within the bond. Zn and Au alloy when coadsorbed on Ru(001). Results for submonolayer coverages of Zn and Au show that the Zn–Ru and Zn–Au bonds in the trimetallic system are more stable than the corresponding bonds in Zn/Ru(001) or Zn–Au alloys. This phenomenon is probably caused by synergistic interactions in three center metal–metal bonding. The formation of ZnAu alloys induces a shift of ∼+1.0 eV in the Au(4f ) and Au(5d) levels, and −0.2 eV in the Zn(3s) and Zn(3d) levels. The shift in the core and valence levels of Au is caused by rehybridization of the Au(5d,6s,6p) levels, while the shift in the Zn levels comes from a contraction in volume.
A simplified theoretical analysis of nonequilibrium effects in bimolecular gas phase reactions
Authors: Cukrowski A. S. , Popielawski J. , Qin Lihong , Dahler John S.
Published: July 2002

There can be significant differences between the rates of activated chemical reactions calculated as if the reactants and products were in states of mutual thermal equilibrium and the rates associated with concentration and species temperatures obtained from solutions of Boltzmann equations. The theory presented here is based on the assumption that the particle distribution functions can be represented by Boltzmann functions with time-dependent concentrations and species temperatures. This leads from the Boltzmann equations to a set of coupled nonlinear equations of change for the concentrations and temperatures. Solutions of these equations have been obtained and used to calculate the rates of reaction. Numerical results generated by this procedure are found to agree quite well with the outputs of Nanbu–Babovsky Monte Carlo simulations and with predictions based on quasistationary perturbative solutions of the Boltzmann equations.
On the stability of the plastic crystal phase of hard dumbbell solids
Authors: Vega C. , Paras E. P. A. , Monson P. A.
Published: July 2002

We consider the thermodynamic properties of the solid phase and solid–fluid phase equilibria of hard dumbbells. The focus of the present work is a more detailed study of the stability of the orientationally disordered (plastic crystal) solid phase relative to other solid phases and to the fluid. Our results indicate that the plastic crystal is not stable for values of L* (the separation of the two spheres of the dumbbell reduced by the sphere diameter) greater than about 0.38. For larger anisotropies than this the hard dumbbell fluid freezes into an orientationally ordered structure. The disappearance of the plastic crystal phase is accompanied by the approach to zero of the density difference between the fluid and the plastic crystal at coexistence and sharp increases in the coexistence pressure and densities. At high pressures the plastic crystal phase transforms into an orientationally ordered structure. These results and those of previous studies allow us to present a fairly complete description of the phase diagram of hard dumbbells.
A theory for treating spatially-dependent friction in classical activated rate processes
Authors: Voth Gregory A.
Published: July 2002

A variational approach is developed to generalize Kramers–Grote–Hynes theory into the regime of spatially-dependent friction. The theory is developed by identifying the probability function for a particle to retain its energy as it leaves the barrier region. This function provides the basis for a variational equation to determine the zero-time value of an effective linear friction. The latter quantity, which characterizes the effect of the spatially-dependent friction in the barrier region, leads to the formulation of an effective Grote–Hynes relationship for the classical correction to transition state theory. The predictions of the analytic theory are compared to computer simulation results for a model system and found to be in excellent agreement.
Space-dependent friction in the theory of activated rate processes: The Hamiltonian approach
Authors: Krishnan R. , Singh Surjit , Robinson G. W.
Published: July 2002

The Hamiltonian approach of Pollak–Grabert–Hänggi to the theory of activated rate processes is generalized to allow for different frictions in the well and the barrier regions. Estimates of the escape rate in the case of exponential friction can be determined from the derived equations of motion, which are exact for all values of the coupling in the well and barrier regions. The rate results are plotted in several cases.
Hydrodynamic instability of chemical waves
Authors: Vasquez Desiderio A. , Wilder Joseph W. , Edwards Boyd F.
Published: July 2002

We present a theory for the transition to convection for flat chemical wave fronts propagating upward. The theory is based on the hydrodynamic equations and the one-variable reaction-diffusion equation that describes the chemical front for the iodate–arsenous acid reaction. The reaction term involves the reaction rate constants and the chemical composition of the mixture. This allows the discussion of the effects of the different chemical variables on the transition to convection. We have studied perturbations of different wavelengths on an unbounded flat chemical front and found that for wavelengths larger than a critical wavelength (λ≳λc) the perturbations grow in time, while for smaller wavelengths the perturbations diminish. The critical wavelength depends not only on the density difference between the unreacted and reacted fluids, but also on the speed and thickness of the chemical front.
Experimental determination of the refractivity virial coefficients of atomic gases
Authors: Achtermann H. J. , Hong J. G. , Magnus G. , Aziz R. A. , Slaman M. J.
Published: July 2002

Values of the second refractivity virial coefficients BR of the atomic gases He, Ne, Ar, Kr, and Xe have been measured with a recently improved differential-interferometric technique to a high degree of accuracy. This device basically consists of two coupled grating interferometers. One interferometer, with two similar cells in series, measures differentially the higher-order effects of density while the second interferometer, with two similar cells in parallel, simultaneously measures the absolute value of the refractive index. In the range of pressure investigated (up to 40 MPa), the third refractivity virial coefficient CR becomes significant for the gases Ar, Kr, and Xe and the fourth refractivity virial coefficient DR becomes significant for Xe. The agreement between our experimental BR and the theoretical prediction based on the classical dipole–induced-dipole model is fairly good for Ar, Kr, and Xe but poor for He and Ne. For He and Ne, however, good agreement is obtained when a semiclassical approach is used. Accurate values of the first refractivity virial coefficients AR were determined independently by making absolute measurements of the refractive index as a function of pressure.
Electron-impact dissociation of carbon monoxide
Authors: Cosby P. C.
Published: July 2002

The electron-impact dissociation of CO to form C and O atoms is observed in a crossed beam experiment at electron energies between the dissociation threshold (14 eV) and 198.5 eV. The center-of-mass energy released in the dissociation of individual molecules is explicitly measured using a position and time sensitive detector for the correlated neutral fragments. The observed energy release distribution is found to be highly structured, reflecting electron-impact excitation to Rydberg states converging to CO+(X 2Σ+) which predissociate to ground state atoms. Little or no dissociation is observed from states above the first ionization limit. Total electron impact dissociation cross sections, exclusive of dissociative ionization contributions, and partial cross sections for the dissociative excitation of specific CO electronic states are presented.
Phase transitions in random copolymers
Authors: Nesarikar A. , Olvera de la Cruz M. , Crist B.
Published: July 2002

Ideal random AB copolymers with degree of polymerization N are mixtures of N+1 types of chains with different compositions (fractions fi of A monomers). Conditions for single phase and multiphase equilibria are studied using Flory–Huggins free energy of mixing with χ representing the A–B interaction parameter. The spinodal for one phase instability is given by χs=[2f(1−f)]−1 for all N, where f is the average A fraction in the system. The transition from one to two phases is continuous at χ=χs when f=0.5 and discontinuous at χ≤χs when f≠0.5. Three, four, and more phases become stable at larger values of χ. Our numerical solution suggests that the stability range for multiple phases approaches Δχ≊0.15 at large (but finite) N. Macroscopically and microscopically phase separated states are investigated with the Landau approach of Fredrickson, Milner and Leibler. The Landau method gives reasonable but inexact results for two macroscopic phases when the random copolymer has compositional symmetry (f=0.5). A disordered mesophase is expected to be the most stable state at least over a range Δχ≊0.15 above the critical point when N≫1. The Landau approach with a single order parameter cannot be used for discontinuous transitions in random copolymers (f≠0.5).
Multiproduct coherent control of photodissociation via two-photon versus two-photon interference
Authors: Chen Zhidang , Brumer Paul , Shapiro Moshe
Published: July 2002

We show that the branching ratios into two or three channel photodissociation products can be controlled by a novel resonant two-photon vs two-photon coherent control scenario. The method allows for control even when the initial molecular system is in a mixed state, e.g., in thermal equilibrium, a significant advance for controlling reactions. Essential to the technique is the phase locking between different frequencies which results in cancellation of incoherences due to laser phase jumps. Reduction of uncontrolled background contributions also obtains. The scenario is demonstrated computationally on Na2 photodissociation, where a wide range of control is achieved over branching into two product channels [Na(3s)+Na(4s) vs Na(3s)+Na(3p)] at lower energies and over three product channels at energies above the Na(3s)+Na(3d) threshold.
A new off-lattice Monte Carlo model for polymers: A comparison of static and dynamic properties with the bond-fluctuation model and application to random media
Authors: Gerroff Ivo , Milchev Andrey , Binder Kurt , Paul Wolfgang
Published: July 2002

A model for a multichain polymer system in three-dimensional continuous space is studied by link cell Monte Carlo methods, using systems up to chain length N=64 and up to 16 384 monomers. The chains consist of beads with a hard core connected by rather stiff harmonic bonds, with a repulsive Lennard-Jones-type interaction between beads chosen such that chains cannot cross each other during their random motions. On RISC workstations the model performs only about a factor of 4 slower than the bond fluctuation lattice model, the qualitative behavior of the time-dependent mean-square displacements and relaxation functions being rather similar to the latter. For the model without obstacles, it is shown that the present continuum model can be approximately mapped on the lattice bond fluctuation model by a suitable rescaling of chain length and volume fraction. But the distinctive advantage of the present model is that it can be applied easily to random media (described by randomly placed rigid obstacles), without the severe ergodicity problems (locked-in configurations) arising in lattice models for such systems. It is shown that static properties of the chains stay nearly unaffected by the obstacles, while the chain motions are considerably slowed down in this frozen environment.
Calculations on the competition between association and reaction for C3H++H2
Authors: Maluendes Sergio A. , McLean A. D. , Yamashita Koichi , Herbst Eric
Published: July 2002

A potential energy surface has been calculated for the competing associative and reactive ion–molecule processes involving the reactants C3H++H2. Our ab initio results show that the linear ion C3H+ and H2 can directly access the deep potential well of the propargyl ion H2CCCH+, which is calculated to lie 390 kJ mol−1 below the zero-point energy of the reactants. Isomerization between the propargyl ion and the lower energy, cyclic C3H3+ ion, calculated to lie 501 kJ mol−1 below the zero-point energy of reactants, can subsequently occur via two pathways. One of these pathways involves a transition state lying 22 kJ mol−1 below the energy of the reactants while the other, which occurs at much lower energies, involves two transition states and an intermediate. The dissociation of c-C3H3+ into c-C3H2++H is calculated to occur directly, without any intermediate potential energy maximum, but the energy of the products lies 7.3 kJ mol−1 above the energy of the reactants. Using the minimum energy potential pathway and properties of the stationary point structures determined via ab initio methods, we have calculated both the association rate coefficient to produce C3H3+ as a function of density and the branching ratio between the propargyl and cyclic structures of the ion. Our results are in good agreement with some experimental results and in conflict with others. Specifically, we agree with the 1:1 branching ratio measured for the propargyl and cyclic isomers of C3H3+ at 80 and 300 K and we agree with the rate coefficient for radiative association measured at 80 K. We cannot reproduce reported measurements that the reactive channel (C3H2++H) is the dominant channel at 80 K and at low gas densities, or that the association channel at high densities saturates at an effective rate coefficient well below the Langevin value −2×10−11 cm3 s−1 at 300 K and 1×10−10 cm3 s−1 at 80 K.
Molecular dynamics simulation of a polymer chain in solution
Authors: Dünweg Burkhard , Kremer Kurt
Published: July 2002

Results of a molecular dynamics simulation of a single polymer chain in a good solvent are presented. The latter is modeled explicitly as a bath of particles. This system provides a first-principles microscopic test of the hydrodynamic Kirkwood–Zimm theory of the chain’s Brownian motion. A 30 monomer chain is studied in 4066 solvent particles as well as 40/4056 and 60/7940 systems. The density was chosen rather high, in order to come close to the ideal situation of incompressible flow, and to ensure that diffusive momentum transport is much faster than particle motions. In order to cope with the numerical instability of microcanonical algorithms, we generate starting states by a Langevin simulation that includes a coupling to a heat bath, which is switched off for the analysis of the dynamics. The long range of the hydrodynamic interaction induces a large effect of finite box size on the diffusive properties, which is observable for the diffusion constants of both the chain and the solvent particles. The Kirkwood theory of the diffusion constant, as well as the Akcasu et al. theory of the initial decay rate in dynamic light scattering are generalized for the finite box case, replacing the Oseen tensor by the corresponding Ewald sum. In leading order, the finite-size corrections are inversely proportional to the linear box dimensions. With this modification of the theory taken into account, the Kirkwood formula for the diffusion constant is verified. Moreover, the monomer motions exhibit a scaling that is much closer to Zimm than to Rouse exponents (t2/3 law in the mean square displacement; decay rate of the dynamic structure factor ∝k3). However, the prefactors are not consistent with the theory, indicating that (on the involved short length scales) the dynamics is more complex than the simple hydrodynamic description suggests.
Photoelectron spectroscopy of nickel group dimers: Ni−2, Pd−2, and Pt−2
Authors: Ho Joe , Polak Mark L. , Ervin Kent M. , Lineberger W. C.
Published: July 2002

Negative ion photoelectron spectra of Ni−2, Pd−2, and Pt−2 are presented for electron binding energies up to 3.35 eV at an instrumental resolution of 8–10 meV. The metal cluster anions are prepared in a flowing afterglow ion source. Each dimer exhibits multiple low-lying electronic states and a vibrationally resolved ground state transition. Franck–Condon analyses yield the anion and neutral vibrational frequencies and the bond length changes between anion and neutral. The electron affinities are determined to be EA(Ni2)=0.926±0.010 eV, EA(Pd2)=1.685±0.008 eV, and EA(Pt2)=1.898±0.008 eV. The electronic configurations of the ground states are tentatively assigned. Comparison of the nickel group dimers to the coinage metal dimers sheds light on the d orbital contribution to the metal bonding in the nickel group dimers.
A general purpose exchange-correlation energy functional
Authors: Laming Gregory J. , Termath Volker , Handy Nicholas C.
Published: July 2002

We report a new exchange-correlation functional which incorporates the LYP correlation functional of Lee, Yang, and Parr gives improved molecular geometries and dissociation energies over the standard Becke–LYP composite functional for the 33 molecules in a reduced G2 data set.
A crossed beam study of ion–molecule proton transfer dynamics: Vibrational state-resolved products in the O−+HF reaction
Authors: Levandier D. J. , Varley D. F. , Carpenter M. A. , Farrar J. M.
Published: July 2002

We present the angular and kinetic energy distributions for the products of the proton transfer reaction O−+HF→OH+F− at a center of mass collision energy of 40.5 kJ mol−1 (0.42 eV). The angular distribution shows clear forward–backward symmetry for the formation of products, indicating that the reaction proceeds through a transient [OHF]− complex living several rotational periods. This is the first direct experimental evidence for the participation of a complex in this reaction. The product kinetic energy distributions show clear structure that we attribute to the formation of OH in vibrational states with quantum numbers v′=0, 1, and 2. Approximately 40% of the OH reaction products are formed in v′=1. This value is consistent with drift tube studies at lower collision energies in which increasing reagent translational energy transforms preferentially into product vibration. Such energy partitioning is consistent with the argument that mixed energy release in the Heavy+Light–Heavy mass combination is induced by the strong electrostatic attraction of the reactants.
Brownian dynamics simulations of reversible reactions in one dimension
Authors: Edelstein Arieh L. , Agmon Noam
Published: July 2002

A Brownian dynamics algorithm is developed for simulating many-body effects in one dimensional competitive reversible binding of otherwise noninteracting particles. It allows time steps hundreds of times larger than in conventional lattice random walks and enables us to simulate systems which are sufficiently large to approach the thermodynamic limit. The asymptotic long-time behavior is compared with mean-field predictions.
Experimental observation of the energy threshold in the ion–molecule reaction N++D2→ND++D
Authors: Tosi Paolo , Dmitriev Oleg , Bassi Davide , Wick Oliver , Gerlich Dieter
Published: July 2002

Using two different guided ion beam machines the cross section for the slightly endothermic title reaction has been measured in a collision energy range which includes the threshold onset. High-energy resolution has been achieved by combining a slow guided ion beam with a supersonic deuterium beam, in one case by crossing the two beams, in the other case by merging them. In a brief discussion the kinematic conditions are compared for the two geometries with those of a beam scattering cell arrangement. In contrast to previous low-resolution measurements the present data clearly indicate a threshold, but, despite the high experimental resolution, it is not straightforward to extract a precise value of the reaction endothermicity or barrier since both reactants can contribute internal energy. Variation of the D2 rotational temperature proves that kinetic and rotational energy are equivalent in promoting the reaction. In contrast to that, the two beam experiments give some hints that energy from excited N+ fine structure states is inefficient. This is also corroborated by a low-temperature ion trap study. With this assumption new phase space theory calculations have been performed to estimate threshold values. First evaluations indicate 11 meV for the reaction with H2 and 29 meV for the reaction with D2.
Shape asymmetry of random walks and nonreversal random walks
Authors: Zifferer Gerhard , Olaj Oskar Friedrich
Published: July 2002

Random walks (RWs) and nonreversal random walks (NRRWs) embedded in various lattices and freely jointed (off-lattice) chains—consisting of up to N≊1000 segments—have been produced and analyzed with respect to their instantaneous shape. While the results of different RWs (as expected) coincide for all chain-lengths examined, the short-chain behavior of NRRWs is strongly dependent on the lattice type. In the limit of infinitely long chains, however, quantities characteristic of the shape converge to common values for all types of RWs and NRRWs examined.
Orthogonally spin-adapted state-universal coupled-cluster formalism: Implementation of the complete two-reference theory including cubic and quartic coupling terms
Authors: Piecuch Piotr , Paldus Josef
Published: July 2002

The complete orthogonally spin-adapted Hilbert-space (or state-universal) coupled-cluster (CC) theory involving singly and doubly excited clusters for a model space spanned by two closed-shell configurations is considered. Explicit expressions for the previously neglected cubic and quartic coupling terms are derived using diagrammatic methods of many-body perturbation theory and graphical methods of spin algebras. The resulting formalism has been implemented and the general purpose ab initio program has been written using newly developed procedure for improving the convergence of the reduced linear equation method. Results of the pilot calculations for the two lowest singlet states of the minimum basis set and double zeta plus polarization basis H4 models as well as for the CH2 molecule at equilibrium and displaced geometries are presented and compared with the available single-reference CC and configuration interaction data. They indicate negligible role of the cubic and quartic coupling terms, which justifies the validity of approximations considered so far.
Large, mode-selective vibrational effect on the reaction of C2H+2 with methane
Authors: Chiu Yu-hui , Fu Hungshin , Huang Jui-tsen , Anderson Scott L.
Published: July 2002

The reaction of C2H+2 with CH4 is strongly enhanced by excitation of a C2H+2 bending vibration, with little effect observed for the CC stretch. The enhancement results from efficient use of bending motion in surmounting an activation barrier. Energy in the CC stretch has little effect on the threshold.
A topological approach to statistics and dynamics of chain molecules
Authors: Nitta Koh-hei
Published: July 2002

A topological index W (Wiener index), which is the sum of all the edges between all pairs of vertices in a chemical graph, is used for characterizing branching in random-flight chains. The chains are composed of statistical bonds (or edges) of a length b jointing N beads (or vertices). The mean square radius of gyration 〈S2〉 of random-flight chains is shown to be given by 〈S2〉=(b/N)2W. On the other hand, the set of partial differential equations describing the motion of the chains, whether linear or with any mode of branching, can be expressed by a connectivity matrix (K). We demonstrate that a relationship between the matrix (K) and the Wiener index is given by W=N Tr (K)−1. It follows that the whole of linear chain theory can be generalized to include any form of branching by replacing the molecular weight or N with the Wiener index W.
Two-dimensional potential energy surfaces for CH(X 2Π)+N2(X 1Σ+g) →HCN(X 1Σ+)+N(4S)
Authors: Seideman Tamar , Walch Stephen P.
Published: July 2002

Two-dimensional potential energy surfaces for the CH+N2→HCN+N(4S) reaction are obtained which describe (i) the C2v HCN2 region of the doublet potential energy surface; (ii) the region of the quartet potential energy surface including the C2v minimum and the dissociation pathway to HCN+N(4S); and (iii) the region of crossing of the doublet and quartet surfaces. These surfaces are fit using simple, physically motivated functional forms. In the first and third regions, the active coordinates are the C–N2 [center-of-mass (c.m.)] distance and the N–N distance. In the second region, the active coordinates are the C–N′ separation, where N′ denotes the departing N atom, and the angle H–C–N. In the following paper, this potential is used to study the dynamics of the title reaction and to compute Boltzmann rate constants.
Dynamic viscoelasticity and critical exponents in sol-gel transition of an end-linking polymer
Authors: Takahashi Masaoki , Yokoyama Kuniaki , Masuda Toshiro , Takigawa Toshikazu
Published: July 2002

Evolution of viscoelasticity in an end-linking gel system has been investigated. Storage and loss dynamic shear moduli, G′(ω) and G″(ω), are determined for angular frequencies ω over more than 2.5 decades near the sol-gel transition. Gelation conditions are varied widely to obtain critical exponents which are independent of reaction rates and paths. A viscoelastic scaling exponent n at the gel point (G′∼G″∼ωn) has been observed as n=0.67–0.68. Reaction time dependent shift factors for relaxation time and compliance before the gel point are determined in creating modulus-frequency master curves of G′ and G″. Scaling of these shift factors yields critical exponents k and z for zero shear viscosity and steady state compliance as k=1.3 and z=2.7–2.8, respectively. On the other hand, the critical exponent for equilibrium shear modulus after the gel point is found to be 2.0–2.1, which is much smaller than the exponent for steady state compliance. It is suggested that links in the backbone mainly determine the critical exponent for the equilibrium modulus in chemical gelation. Besides the links in the backbone structure, blobs and dangling chains incorporated into the cluster are considered to contribute to the steady state compliance.
Coupled cluster theory that takes care of the correlation cusp by inclusion of linear terms in the interelectronic coordinates
Authors: Noga Jozef , Kutzelnigg Werner
Published: July 2002

CC-R12—a combination of coupled cluster theory and the R12 method, is presented in which the correlation cusp is treated via inclusion of terms explicitly dependent on the interelectronic distance rij into the exponential expansion of the wave function. A diagrammatic derivation of the CC-R12 equations within the so-called ‘‘standard approximation B’’ is given at the level of singles, doubles and triples (CCSDT-R12). MBPT(4)-R12 is derived as a byproduct of CCSDT-R12. Fifth order noniterative corrections are also discussed.
A theory for the activated barrier crossing rate constant in systems influenced by space and time dependent friction
Authors: Haynes George R. , Voth Gregory A. , Pollak Eli
Published: July 2002

A general theory is presented for the thermally activated rate constant in systems influenced by spatially dependent and time correlated friction. The theory is valid at all damping strengths and goes uniformly from the energy diffusion limit to the spatial diffusion limit. Results of the theory for a model system with an exponentially time correlated and spatially dependent friction kernel are compared with results from a numerically exact solution of the equivalent generalized Langevin equation. Predictions of the theory are found to be in excellent agreement with the numerical simulation results. The phenomenon of memory suppression of the rate is observed for long time scale frictions and its modification due to the spatial dependence of the friction is discussed. The effects of spatially dependent friction can be understood through a quantity called the ‘‘average spatial modification’’ of the coupling between the reaction coordinate and the environment.
A conformational interpretation for the peak of reduced viscosity for polyelectrolytes at low ionic strength
Authors: Reed Wayne F.
Published: July 2002

Several articles over the past decades have reported a peak in the curve of reduced viscosity ηr vs polyelectrolyte concentration Cp when diluted with pure water or a fixed, low ionic strength solution. The current article shows that the large changes in conformation which accompany changes in ionic strength Cs, for long, linear, semiflexible polyelectrolytes, and which enter in the first, quadratic interaction term of the solution viscosity, are sufficient to account for the existence, relative magnitudes, and positions of peaks in such viscosity profiles. This is illustrated for hyaluronic acid (HA), for which the variations of radius of gyration with ionic strength have been previously determined experimentally. The conformational interpretation predicts that in the coil limit, the position of the viscosity vs Cp peak is independent of mass, but that its height should be roughly proportional to M at fixed Cs, and to roughly CpCs−1.5 for a fixed M. The explanation is meant as a limiting case for long, linear polyelectrolytes and posits that steric, or hard, interactions between separate polymers dominate over electrostatic interactions when the polyelectrolyte dimensions are much larger than κ−1 the electrostatic screening length. The other extreme, short, chain polyelectrolytes and other inextensible objects, such as spheres, are mentioned in light of previously elaborated theories based on interacting electrostatic atmospheres when the dimensions of such objects are smaller than κ−1.
Translational and orientational dynamics of a water cluster (H2O)108 and liquid water: Analysis of neutron scattering and depolarized light scattering
Authors: Saito Shinji , Ohmine Iwao
Published: July 2002

Molecular dynamics simulations were performed to analyze effects of the translational and orientational motions on the coherent and the incoherent neutron scattering and the depolarized light scattering (DLS) spectra of the water cluster (H2O)108 and liquid water. In the neutron scattering of the water cluster and liquid water, there exist two modes, the high and the low frequency modes, in the collective longitudinal current fluctuations for oxygen atoms. The low frequency mode is almost independent on wave number above 0.6 Å−1, while the velocity of the high frequency mode is faster than 3000 m/s, as experimentally observed in liquid water. The nature of this high frequency mode is analyzed by changing the molecular interaction parameters characterizing the hydrogen bond structure of the system. It was found that the high frequency mode is very sensitive to hydrogen bond structure and a three dimensional network structure of the hydrogen bonds is essential for its existence. In order to characterize the water dynamics reflected in the neutron scattering, the density fluctuations of water are classified into two kinds of dynamics; the local oscillatory dynamics around local potential minima (intrabasin dynamics) and the large amplitude dynamics associated with the structure change of hydrogen bond network (inherent structure transitions; interbasin dynamics). We also analyzed the collective and individual longitudinal current fluctuations for hydrogen atoms.

For the depolarized light scattering, the contribution of the cross correlation is examined and compared with those in the collective orientational relaxation in the far infrared spectrum. The interaction-induced component plays a dominate role below 300 cm−1 while the permanent component does above 300 cm−1 in DLS of the cluster and liquid water. The induced component relaxes very slowly in the cluster and yields almost an identical relaxation to that of the dipole–dipole interaction tensors. It was found that the power spectrum of the DLS base line of liquid water obtained from the molecular dynamics calculation is in good agreement with experimentally observed spectrum by Walrafen et al.
Instabilities in propagating reaction-diffusion fronts of the iodate-arsenous acid reaction
Authors: Horváth Dezső , Showalter Kenneth
Published: July 2002

Instabilities in propagating fronts of the iodate-arsenous acid reaction are predicted by an empirical rate-law model. The planar front loses stability when the ratio of the reactant diffusivity to the autocatalyst diffusivity exceeds a critical value. The relative diffusivities can be altered by introducing a reagent that reacts specifically with the autocatalyst to form an immobile complex. Experiments were carried out using a convection-free gelled medium, loaded with reaction mixture containing complexing agent at different concentrations. Instabilities were found above a critical concentration, with the appearance of cellular fronts much like those predicted by the empirical rate-law model.
Radiationless decay of the 1,2,3 3Πg states of Al2: A fully first principles treatment using adiabatic and rigorous diabatic states
Authors: Han Seungsuk , Hettema Hinne , Yarkony David R.
Published: July 2002

The decay mechanisms of the metastable 2,3 3Πg states of Al2 are investigated. Both nonadiabatic radiationless decay to the dissociative 1 3Πg state and radiative decay to the ground X 3Πu state are considered. The 1,2,3 3Πg states are described using state averaged multiconfiguration self consistent field/configuration interaction wave functions [ψam(r,Q)]. The derivative couplings famn(Q)≡〈ψam(r, Q)‖(d/dQ)ψan(r,Q)〉r are determined and used to construct a rigorous diabatic basis for this strongly interacting three state problem. The 2 3Πg state and somewhat surprisingly the 3 3Πg state are rapidly predissociated by the dissociative 1 3Πg state. The lifetimes for nonradiative decay of the vibrational levels of the 2 3Πg state are on the order of picoseconds while those of the 3 3Πg state are on the order of nanoseconds being reduced from the direct coupling (3 3Πg∼1 3Πg) rate of milliseconds by indirect coupling through the 2 3Πg state, (3 3Πg∼2 3Πg∼1 3Πg). Radiative decay is found to be on the order of 102 and 30 ns for the 2 3Πg and 3 3Πg states, respectively, so that radiationless decay is principal decay mechanism. Significant variation in the lifetimes of the individual vibrational levels of the 2,3 3Πg states is expected. This is attributed to the mechanism of the predissociation which involves nonadiabatic interactions near the ‘‘inner walls’’ of the 1,2 3Πg states. Although avoided crossings strongly affect the properties of the 1,2,3 3Πg states the adiabatic basis is preferred over the diabatic basis both conceptually and computationally.
A generalized Gaussian overlap model for fluids of anisotropic particles
Authors: Ayton Gary , Patey G. N.
Published: July 2002

The Gaussian overlap model for anisotropic particles is generalized to include ellipsoids which are not axially symmetric. Explicit analytical expressions are given for the potential and for the torques necessary in molecular dynamics (MD) simulations. Employing this model, we have obtained MD results for several systems carefully selected to determine the influence of molecular biaxiality upon the formation of uniaxial nematic phases. By comparing systems of biaxial and axially symmetric particles with the same length-to-breadth ratio and at the same reduced density and temperature, it is clearly shown that even relatively little molecular ‘‘flatness’’ can significantly favor the formation of a uniaxial nematic phase. For example, ellipsoids characterized by the aspect ratios 2:3:9 form a stable nematic whereas the axially symmetric counterpart (i.e., 3:3:9) has no stable liquid crystal phase.
Bidimensional tunneling splitting in the Ã 1B2 and X̃ 1A1 states of tropolone
Authors: Paz Juan J. , Moreno Miquel , Lluch José M.
Published: July 2002

The intramolecular proton transfer in tropolone has been theoretically analyzed. Ab initio calculations using a variety of basis sets have been performed for both the singlet ground state (X̃ 1A1) and the first excited singlet state (Ã 1B2). A configuration interaction all single excitation method (CIS) has been used to deal with the excited singlet state. Tunneling splittings in both electronic states have been obtained by fitting a bidimensional surface into the ab initio results. This way, a new strategy designed to avoid calculations of the intrinsic reaction coordinate (IRC), which require a very long computer time, is proposed and shown to give accurate results. Our calculations provide a theoretical interpretation of previous extensive spectroscopical data from which the tunneling splitting for the excited Ã 1B2 state was shown to be clearly higher than for the ground X̃ 1A1 state. Finally, the experimentally observed diminution of the splitting upon deuteration of the transferring hydrogen is also accounted for by our theoretical results.
Normal mode theory of diffusion in liquids for a broad temperature range
Authors: Keyes T.
Published: July 2002

A theory based upon normal mode analysis (NMA) is used to calculate the self-diffusion constant, D, in the unit density Lennard-Jones liquid. The theory, which uses no adjustable parameters, is in good agreement with computer simulation over the reduced temperature range 20≥T≥0.42. Thus NMA appears applicable to systems besides the cold liquids for which it was originally proposed.
Monte Carlo study of the thermodynamic stability of the nematic phase of a semiflexible liquid crystal model
Authors: Levesque D. , Mazars M. , Weis J.-J.
Published: July 2002

This paper reports a Monte Carlo (MC) study of the ordering in a semiflexible liquid crystal model designed to give a crude representation of 4-n-octyl-4′-cyanobiphenyl (PCB). A mechanically stable nematic phase is obtained and the question of its thermodynamic stability is addressed by comparing the free energy to that of the isotropic phase. The free energies are calculated by thermodynamic integration using an efficient configuration biased MC scheme. Pair distribution functions, orientational correlation functions, and conformational properties of the nematic and isotropic phases are contrasted. In the isotropic phase the equation of state is compared with theoretical predictions for systems of convex molecules and chains made up of tangent hard spheres.
Nonequilibrium photoinduced electron transfer
Authors: Cho Minhaeng , Silbey Robert J.
Published: July 2002

We consider photoinduced electron transfer, which is intrinsically a three-state system consisting of electronic ground, electronic excited (electron donor), and electron acceptor states. It is assumed that the bath consists of a collection of harmonic oscillators. Using an elementary time-dependent perturbation theory, it is found that the nonequilibrium Golden rule formula proposed by Coalson et al. [J. Chem. Phys. 101, 436 (1994)] can be rigorously obtained in a certain limit of our results. Invoking a stationary phase approximation, a simple result analogous to the Marcus expression is obtained, except for the presence of time-dependent reorganization energy. The multidimensional nature of the solvation coordinate system is discussed further. Finally a few numerical calculations are presented.
Ostwald ripening: A stochastic approach
Authors: Bhakta Ashok , Ruckenstein Eli
Published: July 2002

The phenomenon of Ostwald ripening is examined theoretically for precipitates in solution, crystallites on a substrate, and islands of insoluble surfactant molecules at the liquid–air interface. The classical theory of Ostwald ripening, developed by Lifshitz and Slyozhov and independently by Wagner, employs a continuum approach in which the size distribution is a continuous function of particle size and the conservation equation is formulated as in hydrodynamics with the particle growth rate serving as a velocity in size space. It was derived for the case in which the diffusion fields of the individual particles do not overlap and provides asymptotic expressions at large times for the two extreme cases of diffusion controlled and kinetic controlled growth. In this paper, the validity of this continuum (hydrodynamic) approach is examined using a more rigorous stochastic microscopic approach (MCE). A detailed comparison is made between the numerical and asymptotic results obtained using the Lifshitz, Slyozhov, Wagner theory and the stochastic microscopic continuity equation (MCE). It is shown that with the MCE, the limiting case of ‘‘diffusion controlled Ostwald ripening’’ ceases to have significance, since the kinetics at the interface play a role even when the transport of molecules from the bulk to the particle is diffusion controlled. For the case of interface kinetic controlled growth in precipitates, the asymptotic trends, which are independent of the initial conditions in both cases, are seen much earlier with the MCE. Besides, for this case (precipitates), the MCE predicts a mean radius which is about 25% larger at any given time. While the scaled (with respect to the maximum) asymptotic distributions obtained from the two approaches are similar, the absolute (not scaled) particle size distributions obtained using the MCE are much broader and the maxima are almost a third of those obtained using the Lifshitz, Slyozhov, Wagner approach. Similar differences in the results are obtained for the two dimensional Ostwald ripening of crystallites on a substrate and islands of insoluble surfactant.
Multispeckle autocorrelation spectroscopy and its application to the investigation of ultraslow dynamical processes
Authors: Kirsch S. , Frenz V. , Schärtl W. , Bartsch E. , Sillescu H.
Published: July 2002

We describe a modification of conventional dynamic light scattering (DLS) using a CCD camera as optical area detector. Scattered intensity autocorrelation functions are determined using both ensemble- and time-averaging. Therefore, the new setup allows for much shorter measurement times compared to conventional DLS. Our apparatus has been checked by investigating a dilute dispersion of polystyrene (PS)–microgel lattices in glycerol. The results agree well with DLS measurements using the standard setup. Further, we present data for ultraslow dynamical processes in highly concentrated nonergodic suspensions of PS–microgel lattices, where the new technique, due to its shorter measurement time and much better statistical accuracy as compared to conventional DLS, is most useful.
The method of symmetrized bosons with applications to vibrations of octahedral molecules
Authors: Chen Jin-Quan , Iachello Francesco , Ping Jia-Lun
Published: July 2002

A new technique for constructing representations of point groups, called symmetrized boson representation is introduced. This method is particularly useful when describing vibrations of large molecules and for high overtones. The method is illustrated by applying it to stretching overtones of octahedral molecules, SF6, WF6, and UF6 with group Oh.
Modulation of resonant multiphoton ionization of CH3I by laser phase variation
Authors: Xing Guoqiang , Wang Xuebin , Huang Xin , Bersohn Richard , Katz Benjamin
Published: July 2002

The phase control of molecular absorption, first proposed by Shapiro, Hepburn, and Brumer was accomplished with CH3I using the technique of Chen, Yin, and Elliott. Red light (λ near 603 nm) was focused in a cell containing CH3I gas at a pressure around 1 Torr. The emerging light, a coherent mixture of the fundamental and the third harmonic (λ near 201 nm), was refocused on a molecular beam of CH3I and multiphoton ionization was detected. Ionization was by two simultaneous processes: a uv photon absorption followed by the absorption of two red photons (1+2 process) and an absorption of three red photons followed by the absorption of two more (3+2 process). Because of the sharp resonant Rydberg transition at 201 nm, the one and three photon matrix elements dominate the transition. Interference between the amplitudes of these two paths was demonstrated by varying the pressure and, hence, the index of refraction of the Ar gas in a tuning cell where the two light beams were refocused by two spherical mirrors. The modulation depth varied with wavelength but had a maximum of 75%, the largest modulation so far observed in this kind of experiment.