bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetic transitions in diffusion-reaction space. II. Geometrical effects
Authors: Kozak John J.
Published: July 2002

We extend the stochastic master equation approach described earlier [J. J. Kozak and R. Davidson, J. Chem. Phys. 101, 6101 (1994)] to examine the influence on reaction efficiency of multipolar correlations between a fixed target molecule and a diffusing coreactant, the latter constrained to move on the surface of a host medium (e.g., a colloidal catalyst or molecular organizate) modeled as a Cartesian shell [Euler characteristic, χ=2]. Our most comprehensive results are for processes involving ion pairs, and we find that there exists a transition between two qualitatively different types of behavior in diffusion-reaction space, viz., a regime where the coreactant’s motion is totally correlated with respect to the target ion, and a regime where the coreactant’s motion is effectively uncorrelated. This behavior emerges both in the situation where correlations between the ion pair are strictly confined to the surface of the host medium or where correlations can be propagated through the host medium. The effects of system size are also examined and comparisons with diffusion-reaction processes taking place on surfaces characterized by Euler characteristic χ=0 are made. In all cases studied, the most dramatic effects on the reaction efficiency are uncovered in the regime where the Onsager (thermalization) length is comparable to the mean displacement of the coreactant, a conclusion consistent with results reported in earlier work.
Molecular dynamics simulations of collision-induced desorption. I. Low coverage N2 on Ru(001)
Authors: Romm Leonid , Asscher Micha , Zeiri Yehuda
Published: July 2002

Classical molecular dynamics simulations have been performed to study the details of collision-induced desorption (CID) of nitrogen molecules adsorbed at low coverages on Ru(001). Semiempirical potential energy surfaces (PES) were used to describe the movable two layers of 56 ruthenium metal atoms each, the nitrogen adsorbate, the Ar and Kr colliders, and the interactions between them. An experimentally measured threshold energy for the CID process of 0.5 eV and the dependence of the cross section σdes on incidence energy and angle of incidence have been precisely reproduced in the energy range of 0.5–2.5 eV. Strong enhancement of the σdes is predicted as the angle of incidence increases. Kinetic energy and angular distributions of the scattered rare gas and the desorbing nitrogen were determined as a function of the dynamical variables of the collider. It is predicted that half of the collision energy is transferred to the solid and the other half is shared among the two scattered species. While no vibrational excitation is observed, efficient rotational energy excitation is predicted which depends on both incident energy and angle of incidence. Polar and azimuthal angular distributions were found to be strongly dependent on the incidence angle and energy of the colliders. These results suggest a new CID mechanism for the weakly chemisorbed nitrogen molecules on Ru(001), based on extensive analysis of individual trajectories. According to this mechanism, the CID event is driven by an impact excitation of frustrated rotation or tilt motion of the adsorbed molecule as a result of collision with the energetic rare gas atom. In addition, lateral motion along the surface is also excited. Strong coupling of these two modes with the motion in the direction normal and away from the surface eventually leads to desorption and completes the CID process. The efficiency of this coupling is dictated by the details of the corrugation of the Ru–N2 PES. It is concluded that the simple hard cube–hard sphere model, frequently used to analyze CID processes, is insufficient for the description of this system. While reasonably well predicting threshold energy, it cannot explain the full dynamical picture of the CID event.
Third-order transport coefficients for charged particle swarms
Authors: Vrhovac Slobodan B. , Petrović Zoran Lj. , Viehland Larry A. , Santhanam Thalanayar S.
Published: July 2002

Momentum-transfer theory has been used to obtain a relationship between the nth order tensorial transport coefficients in a swarm experiment, the (n−1)th derivative of the mobility, and the nth derivative of the reaction rate coefficient. Elastic, inelastic, and reactive collisions for gas mixtures have been taken into consideration. Numerical comparisons show that the results obtained from this relationship are in good agreement with those obtained by solution of the Boltzmann equation. Finally, we have analyzed the structure of the third-order tensorial transport coefficient by applying momentum-transfer theory and group theory; both approaches show that in general there are three independent components of this rank-three tensor.
A pseudospectral algorithm for the computation of transitional-mode eigenfunctions in loose transition states. II. Optimized primary and grid representations
Authors: Rasmussen Anthony J. , Gates Kevin E. , Smith Sean C.
Published: July 2002

A highly optimized pseudospectral algorithm is presented for effecting the exact action of a transitional-mode Hamiltonian on a state vector within the context of iterative quantum dynamical calculations (propagation, diagonalization, etc.). The method is implemented for the benchmark case of singlet dissociation of ketene. Following our earlier work [Chem. Phys. Lett. 243, 359 (1995)] the action of the kinetic energy operator is performed in a basis consisting of a direct product of Wigner functions. We show how one can compute an optimized (k,Ω) resolved spectral basis by diagonalizing a reference Hamiltonian (adapted from the potential surface at the given center-of-mass separation) in a basis of Wigner functions. This optimized spectral basis then forms the working basis for all iterative computations. Two independent transformations from the working basis are implemented: the first to the Wigner representation which facilitates the action of the kinetic energy operator and the second to an angular discrete variable representation (DVR) which facilitates the action of the potential energy operator. The angular DVR is optimized in relation to the reference Hamiltonian by standard procedures. In addition, a scheme which exploits the full sparsity of the kinetic energy operator in the Wigner representation has been devised which avoids having to construct full-length vectors in the Wigner representation. As a demonstration of the power and efficiency of this algorithm, all transitional mode eigenstates lying between the potential minimum and 100 cm−1 above threshold have been computed for a center-of-mass separation of 3 Å in the ketene system. The performance attributes of the earlier primitive algorithm and the new optimized algorithm are compared.
Reaction cross sections for the H+D2(ν=0,1) system for collision energies up to 2.5 eV: A multiconfiguration time-dependent Hartree wave-packet propagation study
Authors: Jäckle A. , Heitz M.-C. , Meyer H.-D.
Published: July 2002

Cumulative initial-state-selected cross sections for the H+D2(ν,j)→HD+D reaction are presented. Initial states with vibrational quantum numbers ν=0,1 and rotational quantum numbers j=0–4, and initial translational energies up to 2.5 eV are considered. These calculations go beyond previous ones in treating higher energies and vibrationally excited D2. The cross sections are computed by propagating wave packets employing the multiconfiguration time-dependent Hartee scheme, and by analyzing the reactive flux into the configuration channel of the products. The initial wave packets are modified to account for the long-range part of the potential, assuming vibrational adiabaticity. All calculations are performed within the coupled states approximation on the Liu–Siegbahn–Truhlar–Horowitz (LSTH) potential-energy surface. The LSTH surface is expanded in products of one-dimensional functions of the Jacobian coordinates. The initial-state-selected cross sections are averaged over a room-temperature Boltzmann distribution of the D2 rotational states to compare the results with experimentally observed values. The relative efficiency of vibrational, rotational, and translational energy for promoting the reaction is analyzed.
Spin-orbit branching ratios for the Cl atom photofragments following the excitation of Cl2 from 310 to 470 nm
Authors: Samartzis Peter C. , Bakker Bernard L. G. , Rakitzis T. Peter , Parker David H. , Kitsopoulos Theofanis N.
Published: July 2002

Velocity distributions for the Cl(2P3/2) and Cl(2P1/2) photofragments produced by photolysis of Cl2 in the region between 310 and 470 nm are measured using photofragment velocity mapping. Our results indicate that at short wavelengths the absorption spectrum is dominated by the 1u(1Πu) excited electronic state which produces two ground state chlorine atoms. The 0u+(B 3Πu) state which produces a spin-orbit excited and a ground state chlorine atom becomes significant at 350 nm and dominates the spectrum beyond 400 nm. Analysis of the photofragment angular distributions indicates that the Cl(2P3/2) photofragments are aligned and the magnitude of the alignment is quantitatively determined. Nonadiabatic curve crossing between the 1u(1Πu) and the 0u+(B 3Πu) electronic states is observed and quantified below 370 nm. The measured nonadiabatic transition probability is modeled using the Landau–Zener formula and the position of the curve crossing is estimated at ∼3 eV above the zero-point of ground electronic state of Cl2.
Polymer solution viscoelasticity from two-parameter temporal scaling
Authors: Phillies George D. J.
Published: July 2002

A novel approach for computing aspects of the loss modulus G″(ω) of a polymer solution is proposed. The path predicts the functional dependence of G″ on ω over a wide range of ω, independent of assumptions as to the nature of the dominant forces acting between polymer chains in solution. Comparison with representative results from the published literature finds excellent agreement with the predicted functional form for G″(ω). The approach is based on extension of the renormalization-group derivation [G. Phillies, Macromolecules 31, 2317 (1998)] of the hydrodynamic scaling model of polymer solution dynamics [G. Phillies, J. Phys. Chem. 93, 5029 (1989)] to the zero-shear viscosity, considering the fixed-point structure of η(c) as inferred from experiment, and analytic extrapolation of the fixed-point structure to a two-variable η(c,ω).
Crossed beams reaction of atomic carbon, C(3Pj), with d6-benzene, C6D6(X 1A1g): Observation of the per-deutero-1,2-didehydro- cycloheptatrienyl radical, C7D5(X 2B2)
Authors: Kaiser R. I. , Hahndorf I. , Huang L. C. L. , Lee Y. T. , Bettinger H. F. , Schleyer P. v. R. , Schaefer H. F. , Schreiner P. R.
Published: July 2002

The reaction of atomic carbon, C(3Pj) with per-deutero benzene, C6D6 is investigated at an average collision energy of 32.1 kJ mol−1 using the crossed molecular beams technique combined with a universal mass spectrometric detector. Product angular distributions and time-of-flight spectra of C7D5 and C7D6 are recorded. Forward-convolution fitting of our time-of-flight data (TOFs) and laboratory angular distribution (LAB) together with high level electronic structure calculations on the singlet and triplet C7D6 potential energy surfaces are consistent with the formation of the per-deutero-1,2-didehydrocycloheptatrienyl radical, C7D5. No C7D6 adduct is found experimentally. Our investigations indicate that the carbon atom attacks the benzene molecule face without an entrance barrier to form an initial complex. This undergoes a ring opening to give triplet cycloheptatrienylidene as a C7D6 intermediate. The latter fragments without exit barrier via a C–D bond rupture to yield the per-deutero-1,2-didehydrocycloheptatrienyl isomer, C7D5, and a D atom. This barrierless route for the destruction of benzene may be involved in the synthesis of higher cyclic hydrocarbon derivatives in the interstellar medium, in outflows of dying carbon stars, in hydrocarbon-rich planetary atmospheres, as well as in oxygen-poor combustion flames.
Molecular vibrations and the adiabatic wavefunctions
Authors: Vager Z.
Published: July 2002

The well known existence of vibrational spectra in molecules implies simplicity of the corresponding molecular wavefunctions. This simplicity suggests the existence of quasiparticles with an approximate harmonic oscillator Hamiltonian. The conventional adiabatic approximation is known to reproduce the energy differences of the observed vibrational states. In spite of this it is shown that the adiabatic wavefunctions do not have the simplicity of approximate harmonic oscillator states. In particular, in the adiabatic wave functions the relative distances between the nuclear coordinates are selected to be dynamical variables. This article shows that with such a choice the conjugate momenta density functions could be far from the harmonic oscillator predictions. It is thus concluded that the spacing of the observed molecular vibrational states cannot be used to predict the probability distributions of relative nuclear coordinates or momenta.
What can be stated by the Glansdorff–Prigogine criterion concerning the stability of mass-action kinetic systems?
Authors: Wilhelm Thomas , Hänggi Peter
Published: July 2002

We investigate which general results concerning the local stability of steady states of arbitrary chemical reaction networks can be deduced with the Glansdorff–Prigogine stability criterion. Especially, it is proven that the presence of an autocatalytic reaction is not a necessary condition for a violation of the thermodynamic stability condition. It turns out that every reaction with at least one variable reactant at each side of the reaction equation can potentially destabilize the steady states. An explicit example of a simple reaction system without autocatalytic reactions where the stability of the steady state changes via a supercritical Hopf bifurcation is discussed. Furthermore, in expanding the original concept for proving local stability to global stability analyses, a general way for constructing different Lyapunov functions is given.
Accurate 3 dimensional quantum dynamical study of the Ne+H2+→NeH++H reaction
Authors: Gilibert M. , Giménez X. , Huarte-Larrañaga F. , González M. , Aguilar A. , Last I. , Baer M.
Published: July 2002

In this work a comprehensive, fully converged coupled states (CS) quantum mechanical (QM) study of the endothermic Ne+H2+ ion-molecule reaction is presented. The computed dynamical properties are compared with quasi-classical trajectory (QCT) and with the available experimental data. To this end, the analytical potential energy surface of Pendergast, Heck, Hayes, and Jacquet was employed. The two main features of the dynamical behavior for this system are: (1) the rich structure present in the state-selected integral cross section energy-dependent curves, which may be attributed to resonances surviving the partial wave summation; and (2) the large differences between the quantum and the QCT cross sections which are caused by the inability of classical mechanics to conserve the zero point energy. Also noteworthy are the strong enhancement of the reactivity due to higher vibrational states and the effect of the activated complex, formed during the reaction process, on the angular and the rotational distributions.
Study of a bimolecular annihilation process for coarsening reactants
Authors: Lacasta A. M. , Sagués F. , Sokolov I. M. , Sancho J. M.
Published: July 2002

We consider the spatio-temporal behavior of the A+B→0 bimolecular reaction in a system where both reactants tend to segregate into separated phases. Our study is based on the numerical solution of a pair of reaction–diffusion equations appropriate to capture the underlying coarsening dynamics. The interplay between reaction and coarsening leads to a complex pattern of reactants spatial distribution. At short/intermediate times two distinctive dynamical regimes are seen in the decay of overall concentration and droplet number and the behavior of droplet radii.
Computer simulation of the linear and nonlinear optical properties of liquid benzene: Its local fields, refractive index, and second nonlinear susceptibility
Authors: Janssen R. H. C. , Bomont J.-M. , Theodorou D. N. , Raptis S. , Papadopoulos M. G.
Published: July 2002

Molecular dynamics (MD) simulation and subsequent analysis of the macroscopic polarization developed in response to “a posteriori” applied electric fields or of spontaneous fluctuations in the instantaneous polarization under zero applied field is used to assess the nonlinear optical properties of a polarizable liquid. Three strategies are proposed for the electrostatic analysis, all using as input static “gas phase” (hyper)polarizabilities, obtained from ab initio calculations. All three strategies are shown to accurately reproduce the experimentally measured refractive index and second nonlinear susceptibility of liquid benzene. The simulation also predicts the distribution of orientations and magnitudes of the local electric fields experienced by the molecules in the liquid, and the nonlinear contributions to the local fields. This approach gives an 8% higher estimate of the second nonlinear susceptibility of liquid benzene than the Lorentz local field factor approach, in better agreement with experimental values.
Time evolution of the second derivative time-correlation-function for the depolarized light scattering spectrum of CS2
Authors: Stassen H. , Steele W. A.
Published: July 2002

Molecular dynamics computer simulations have been performed on liquid CS2 at a temperature of 298 K and a density corresponding to vapor–liquid coexistence. Time-correlation-functions (TCFs) related to the second spectral moments of depolarized Rayleigh scattering were evaluated. The induced molecular polarizability was assumed to be due to dipole-induced dipole interaction. It is shown that second derivative time correlation functions for the spectral TCFs give an insight into the relative importance of translation, rotation and translation-rotation coupling in determining the light scattering. The collective spectral time-correlation-functions were broken down into the contributing component one-, two-, three- and four-body contributions to give detailed insights into the dynamical aspects of the cancellation effect in the light scattering.
The importance of self-interaction and nonlocal exchange corrections to the density functional theory of intracavity electrons in Na-doped sodalites
Authors: Blake Nick P. , Metiu Horia
Published: July 2002

Electrons that are confined to zeolite cavities are modeled using a simplified pseudopotential scheme to represent the interaction of the electrons with both the sodalite framework and the Na+ ions. By comparing theory with recent experimental studies of G centers in Na-doped NaBr-SOD it is demonstrated that restricted forms of density functional theory, where two electrons are forced to pair in the same Kohn–Sham orbital, fail to correctly predict the true nature of the singlet, (spin unpolarized), G center. Electron confinement leads to generalized gradient corrections to the exchange of 0.74 eV and self-interaction corrections (SIC) of 0.7 eV over calculations performed in the local spin density approximation (LSDA). Only the self-interaction corrected generalized gradient approximation and the unrestricted Hartree–Fock approximation are in accord with experiment for the relative stability of the triplet (spin polarized) state. The unrestricted Hartree–Fock method is used to show that G-center absorptions will be blueshifted with respect to absorptions due to the isolated F centers. Constructing a Hubbard Hamiltonian we show that the exchange coupling ranges in values from 2.3 meV(UHF) to 3.6 meV(SIC-LSDA) corresponding to Neel temperatures that range from 27 to 41 K in agreement with experiment.
Dynamics of front propagation in the catalytic CO oxidation on Pt(100)
Authors: Chávez Francisco , Vicente Luis , Perera Aurélien , Moreau Michel
Published: July 2002

The dynamics of CO and O chemical fronts propagating through a Pt(100) substrate are simulated using cellular automaton techniques. The cellular automaton accounts for the adsorption of both molecular species CO and O2 on a platinum substrate, as well as the reaction kinetics between these species and the diffusion of CO on the substrate. The influence of inert sites (defects) is also considered. The chemical front is generated in two different situations: In the first one, the sample is initially covered by CO and the front is generated by depositing a layer of O atoms. The system is then subject to more O2 molecules which can adsorb and react with the other CO molecules and the interface moves as the reaction proceeds. It is found that the fluctuation of the interface in a clean sample follows the Kardar–Parisi–Zhang equation and, in this case, a microscopic description in terms of a stochastic process is also proposed. In a second situation, the adsorption and diffusion of CO are allowed in addition to the previous reaction mechanisms. In this case, it is found that the growth obeys the scaling relation, w(L,t)∼Lαf(t/Lz), and our numerical results tend to indicate a continuous dependence of the exponents β and α on the diffusion of CO. The reflecting and diffracting properties of the waves moving in the media with different composition (one part clean and the other containing inert sites) are also investigated herein. The results agree well with the experimental work of Asakura et al. [Surf. Sci. 374, 125 (1997)] and in particular it is found that the chemical concentration waves during heterogeneously catalyzed reactions obey Snell’s law.
Molecular dynamics of strongly coupled multichain Coulomb polymers in pure and salt-added Langevin fluids
Authors: Tanaka Motohiko , Grosberg A. Yu , Tanaka Toyoichi
Published: July 2002

The multichain effect and also the effect of added salt on randomly copolymerized charged polymers (polyampholytes) in a Langevin fluid are studied with the use of molecular dynamics simulations. The monomers of opposite signs tend to form loose complexes, which makes the Coulomb force attractive on average. With multichain polyampholytes, the typical state at high temperature is a container-bound one-phase state of separated chains with a substantial void among them. The association and dissociation processes occur repeatedly, with the former process a few times faster than the latter. A glass transition occurs when temperature is lowered. A compact and glassy globule in a segregated phase, which resembles that of a single-chain polyampholyte, is a typical state at low temperature due to the Coulomb force. The probability of losing this state is as low as Pdis∼exp(−N3/2), with N the number of monomers. The critical temperature defined by overlapping of the chains increases with molecular weight and stiffness of the chains, and is less sensitive to the number of the chains. An alternate charge sequence makes a difference only when its block size is quite small. The addition of salt suppresses the formation of a dense globule by shielding the electric field; however, this is not effective when the salt ions are not allowed to penetrate well into the globule.
Effective interaction between hard sphere colloidal particles in a polymerizing Yukawa solvent
Authors: Gee R. H. , Henderson D. , Kovalenko A.
Published: July 2002

The effective interaction between colloidal hard sphere particles in a Yukawa solvent that can polymerize with the formation of chains and rings is studied and compared with the corresponding results for colloidal hard sphere particles in a solvent of polymerizing hard spheres. The attractive nature of the polymerizing Yukawa solvent particles induces significant changes in the effective interactions between the colloid particles as compared to a polymerizing solvent of hard spheres that was investigated in previous studies. The results for the colloid–solvent mixture are obtained using the associative Percus–Yevick approximation for Wertheim’s Ornstein–Zernike integral equation; the colloidal species are taken at a nonvanishing but very small concentration throughout this article. We present the effects of the size ratio of colloid spheres to solvent spheres, the degree of polymerization, and the solvent density on the effective interactions between colloid and solvent particles. The intercolloidal potential of mean force (PMF) is found to be highly dependent on these parameters for Wertheim’s model. It is found that the correlations between colloid particles obtained using the Yukawa solvent model are longer ranged and more attractive than those found using the hard sphere solvent model. A greater depletion of the solvent density around the colloidal particles is also observed for the Yukawa solvent model as compared to the hard sphere model; an increased polymer chain length also enhances the depletion of the solvent density. The PMF is found to be oscillatory in structure. The oscillatory structure also depends upon the average polymer chain length, specifically, the oscillatory structure in the PMF is strongly diminished as the average polymer chain length increases. Additionally, as the average polymer length increases, the attraction at the colloid–colloid contact distance decreases.
Changing configurations with electric fields
Authors: Murgu E. , Ropke F. , Djambova S. M. , Gallagher T. F.
Published: July 2002

By changing an electric field it is possible to effect the same change in configuration as by altering the time delay between two picosecond laser pulses. In particular, we demonstrate that it is possible to change excited Ba atoms from a doubly excited 5d7d state to a singly excited 6snk Rydberg state by changing an electric field by 630 V/cm.
Luminescence properties of Ni2+ in chlorine or bromine containing fluorozirconate glasses
Authors: Buñuel M. A. , Cases R.
Published: July 2002

Steady state excitation and emission, luminescence decay, and time resolved spectroscopy measurements have been performed in order to study the Ni2+ neighborhood and its optical properties in fluorochloro and fluorobromo–zirconate glasses. The results have been interpreted in terms of the progressive substitution of the fluorine ions in the first coordination shell of Ni2+ by chlorine or bromine ions when their concentration increases in the glasses. Up to four Ni2+ environments have been identified and assigned to 6F−, 5F− and 1Cl− or 1Br−, 1F− and 5Cl− or 5Br−, and 6Cl− or 6Br−; according to previous absorption results. The luminescence results indicate that there is no emission of the Ni2+ ions in NiCl2 and NiBr2 crystals inside the glasses. Values for the Stokes shift and the Huang–Rhys factor have been estimated from the absorption and luminescence data for the hexacoordinated vitreous environments of Ni2+. The luminescence decay and time resolved measurements of the T2g1(D) level of Ni2+ have been explained by the presence of two mechanisms, nonradiative transitions and cross relaxation processes.
Body-centered-cubic lattice model with many-body interactions representing the melting transition in Si
Authors: Sahara R. , Mizuseki H. , Ohno K. , Uda S. , Fukuda T. , Kawazoe Y.
Published: July 2002

A body-centered-cubic (BCC) lattice model with realistic many-body interactions is introduced and investigated by means of the Metropolis’ Monte Carlo method to describe both crystalline and molten states of Si. Under the simplest assumption that atoms surrounded by tetrahedral first-neighbors only have an energy lower than the other atoms, a clear first-order phase transition including hysteresis is observed between a solid with diamond structure and a melt. Nucleation and domain growth are dynamically observed in certain range of the supercooling. In order to introduce more realistic and accurate lattice-gas models, the Tersoff potential is renormalized and the interactions are mapped onto a BCC lattice. Then, it is found that the phase transition temperature and other thermodynamic properties are dramatically improved compared with the case using the Tersoff potential directly in the lattice model without renormalization.
Adsorption site and orientation of pyridine on Cu{110} determined by photoelectron diffraction
Authors: Gießel T. , Schaff O. , Lindsay R. , Baumgärtel P. , Polcik M. , Bradshaw A. M. , Koebbel A. , McCabe T. , Bridge M. , Lloyd D. R. , Woodruff D. P.
Published: July 2002

The local adsorption geometry of pyridine on Cu{110} has been determined quantitatively using photoelectron diffraction in the scanned-energy mode. At high coverages the molecule adsorbs nearly atop a Cu atom in the close-packed rows with a N–Cu bond length of 2.00 Å. Moreover, the Cu–N axis and the molecular (C2) axis are inclined by 8° and 20°, respectively, to the surface normal. The result shows that not only the adsorption site of the emitter (in this case the N atom) but also the position of relatively light scatterers (the C atoms) can be determined by photoelectron diffraction.
Electronic spectra of gaseous nitric oxide in magnetic fields up to 10 T
Authors: Takazawa Ken , Abe Haruo
Published: July 2002

Fluorescence excitation spectra due to the A 2Σ, v′=0←X 2Π, v″=0 transition of gaseous nitric oxide were measured in magnetic fields up to 10 T. Zeeman effects on rotational lines of the P21 and P1 branches were observed. Transition energy and intensity were calculated using Hund’s case (a) basis set to assign observed lines. Calculation results reproduced observed spectra well and all lines were completely assigned.
Two-electron transfer reactions involving three paraboloidal potential surfaces in solvents with multiple solvation time scales
Authors: Bandyopadhyay T. , Okada Akira , Tachiya M.
Published: July 2002

The effect of solvent nuclear relaxation dynamics on the rate of two-electron transfer reaction is investigated. We present here a generalized treatment of the Zusman and Beratan model of two-electron transfer reaction using a theoretical scheme that starts from the Liouville equation of motion for the electronic population wave packets to obtain the transfer rates following projection operator formalism. This generalization enables us to treat the three free energy surfaces (three surfaces for D–A, D+–A−, and D+2–A−2 donor–acceptor pairs) involved in such reactions on an equal footing such that the rates for each one- and two-electron transfer step can be obtained when all three diabatic surfaces are present in the system with nonzero electronic coupling elements between them. The reaction takes place on a two-dimensional potential energy surface with two coordinates representing the solvent polarization. The dynamics are governed by overdamped diffusion along these polarization coordinates with different solvent polarization time scales. The resulting equations, that can interpolate the situation between the nonadiabatic and the diffusion limits of electron transfer, are solved numerically for the choice of parameters that validates the criterion for solvent dynamics-influenced rate limit. The transfer rates, in this limit, are found to depend strongly on the multiplicity of the solvent polarization coordinate used. New dynamical solvent effects on the transfer rates in solvents with one or more characteristic relaxation time scales are identified because of the effective participation of all three electronic states in the transfer process. The theoretical recipe developed here is not limited to two-electron transfer problems and can be applied for multiple electron transfer events in solvents with multiple relaxation time scales.
Living polymerization of α-methylstyrene in tetrahydrofuran followed by dynamic light scattering near its polymerization temperature
Authors: Ruiz-Garcı́a J. , Castillo R.
Published: July 2002

We followed the polymerization reaction of living poly-α-methylstyrene in a solution of tetrahydrofuran as a function of temperature, near its ceiling temperature, using dynamic light scattering. We obtained three different relaxation times revealing three kinds of macromolecular species in the solution. One of the relaxation times corresponded to the formation of living polymer chains. In this particular case, we found that the average relaxation time increased as temperature was lowered due to polymer growth. Nevertheless, it did not follow the same track along a cooling–heating cycle. These results are consistent with an incomplete depolymerization reaction. For the other two species, we propose that they were the result of ionic aggregation.
Energetics of carbon clusters C8 and C10 from all-electron quantum Monte Carlo calculations
Authors: Shlyakhter Yuri , Sokolova Svetlana , Lüchow Arne , Anderson James B.
Published: July 2002

The energetics of carbon clusters C8 and C10 in several configurations have been determined in all-electron fixed-node quantum Monte Carlo calculations. The total electronic energies obtained are 0.4–1.2 hartrees lower than those of the lowest-energy analytic variational and coupled-cluster calculations, and they are only 0.2 to 0.4 hartrees above the experimentally based total energies. The recovery of correlation energy is 89%–94%. For C8, relative values are found to be in agreement with earlier calculations: the cyclic C4h 1Ag structure, the linear Σg−3 (cumulenic), and the linear Σg+1 (cumulenic) structures are found lowest in energy and within 10 kcal/mol of each other. For C10, relative values are found to be in agreement with earlier calculations: the cyclic D5h 1A1′ (distorted cumulenic) and D10h 1A1g (cumulenic) structures are found to be lowest in energy, within 5 kcal/mol of each other, 60 kcal/mol below the linear Σg−3 (cumulenic) structure. An analysis of the scaling of computational effort with the number of electrons Ne for these quantum Monte Carlo calculations indicates a third-power dependence Ne3.
Improvement of integral equation theories for mixtures
Authors: Cann N. M.
Published: July 2002

A comprehensive study of integral equation theories for binary mixtures is presented. The mixture components differ primarily in size (the diameters differ by 10%, 20%, and 30%) and interact either via hard potentials or via Lennard-Jones potentials. For the latter, variation with energy parameter (ε) is considered. This article focuses on improving the accuracy of the theories by systematic inclusion of bridge diagrams into the closure relationships. Specifically, the first two orders of bridge diagrams are exactly evaluated for these mixtures. A general Monte Carlo integration scheme for diagram evaluation is discussed and applied. Comparisons with diagrams obtained from a Legendre expansion technique are made in order to assess whether this approach is practical. The approximation of higher order diagrams has been considered. Specifically, techniques for approximation of all higher order diagrams, which were successful for single component fluids, were found to be problematic for mixtures. However, a simple algorithm for approximate third order diagrams is presented and found to lead to improvements. A detailed analysis of the bridge diagram variation with the nature of the mixture is presented and may be useful in extending the present results to related mixtures. The spatial dependence of the diagrams has also been examined and found to be extremely well reproduced by simple polynomial expansions. In addition, physical arguments have been applied to extract large separation limits of the diagrams. The accuracy of the integral equation theories with order of bridge diagrams is assessed by comparing pressure estimates from the virial expansion and from the integration of compressibilities. With this measure, the quality of the integral equation theories for each mixture is assessed at 18 state points. In all cases, the thermodynamic consistency improves smoothly and rapidly with the order of bridge diagram included in the theory. This result, together with the general Monte Carlo algorithm and the detailed structural and spatial analysis, shows that direct bridge diagram evaluation is practical and consistently improves the quality of the theory for these mixtures.
Quantum mechanical cross sections for the isotopic reactions H+X2O, X=H,D: A comparison with experiment and with other calculations
Authors: Szichman Henrik , Baer Michael , Volpp H. R. , Wolfrum J.
Published: July 2002

In this work are presented 3-mathematical-dimensional quantum mechanical energy-dependent cross sections for the two isotopic reactions H+X2O→Products; X=H,D. The results are compared with experiment and with other calculations. The comparison between theory and experiment leads to two contradictory conclusions regarding the application of the Walch–Dunning–Schatz–Elgersma (WDSE) potential energy surface: (a) The WDSE surface seems to adequately describe the abstraction process; (b) however, the WDSE barrier for the exchange process seems to be too high and therefore yields small cross sections in contrast to those found in the experiment.
Wavelength-resolved stimulated photon echoes: Direct observation of ultrafast intramolecular vibrational contributions to electronic dephasing
Authors: Book Lewis D. , Scherer Norbert F.
Published: July 2002

Novel wavelength-resolved stimulated photon echo measurements on a dye molecule in solution are presented. Data are simulated within the multimode Brownian oscillator model using the spectral density of de Boeij et al. [J. Phys. Chem. 100, 11806 (1996)] for the same solute–solvent system. For photon echo population times <50 fs there are considerable differences between the measured and calculated data. Aided by further simulations, we conclude that these discrepancies result from dephasing dynamics of high frequency intramolecular vibrational modes not included in the previously derived spectral density.
Low-temperature thermoluminescence in solid argon: Short-range mobility of atoms
Authors: Khriachtchev Leonid , Pettersson Mika , Pehkonen Susanna , Isoniemi Esa , Räsänen Markku
Published: July 2002

The 193-nm photolysis of S2 (produced from H2S2) and SO2 in solid argon and the changes introduced by annealing are monitored by using Fourier-transform infrared spectroscopy and laser-induced fluorescence. The results highlight short-range atomic mobility distinguished from global long-range mobility. It is shown that under the 193-nm irradiation a small part of S2 and SO2 molecules undergo photodecomposition yielding quasi-isolated pairs where the fragments are in close vicinity to each other. In annealing, the short-range mobility results in recombination of these pairs at rather low temperatures (<15 K), and this known low-temperature thermoluminescence does not require long-range atomic mobility. Also, recombination of two sulfur atoms can be induced through phonon sideband absorption of the geminate photofragments, which introduces light-induced short-range atomic mobility. The simulations on two sulfur atoms in a static Ar lattice support this scenario, demonstrating that the photofragments can be gently separated by a low potential barrier which favors their recombination in annealing. In particular, this quasistable configuration occurs when the two atoms occupy substitutional sites separated by the lattice parameter of 5.31 Å.
Quantum consideration of electron transfer solvent control
Authors: Frantsuzov P. A.
Published: July 2002

The elementary act of the electron transfer process is considered within the quantum spin-boson model. The expression for the coordinate dependent reaction flux correct to the second order in the intercenter coupling is obtained. It is shown that in the classical bath limit the flux is localized in a narrow nonadiabatic region in the vicinity of the crossing point of the potential surfaces. Thus, the well-known point-transition model describing solvent dynamical effect in the electron transfer kinetics is justified in this limit. The analysis of the Markovian kinetic equations for a quantum two-stage electron system coupled to a classical bath used by a number of authors for the derivation of the point-transition model is performed. It is demonstrated that this approach is often inappropriate for the description of the electron transfer process. Here we show that these equations may lead to unphysical predictions such as negative reaction rate and flux.
On the nature of electronic excitations in poly(paraphenylenevinylene): A quantum-chemical investigation
Authors: Beljonne D. , Shuai Z. , Cornil J. , dos Santos D. A. , Brédas J. L.
Published: July 2002

Correlated quantum-chemical calculations are performed on phenylenevinylene oligomers containing up to eleven repeat units, to characterize the nature of the electronic excitations relevant for the photophysical properties of the corresponding polymer. The focus is first on the nonlinear optical response of model conjugated chains and the simulation of their frequency-dependent (third-harmonic generation, electroabsorption, and two-photon absorption) response. From the assignment of the calculated resonance features, the excited states dominating the third-order nonlinear polarizability are identified and their chain-length dependence is investigated. On that basis, we build an essential-state single-chain model (that includes the 1Bu, 2Ag, mAg, and nBu states) and apply it to the interpretation of recent experimental data reported for poly(paraphenylenevinylene) and derivatives. We then examine how the exciton binding energy, here defined as the difference between the energies of the charge-separated nBu and the strongly optically allowed 1Bu excited states, is affected by both intrachain and interchain polarization effects.
Stochastic simulations of micellization kinetics
Authors: Mavelli F. , Maestro M.
Published: July 2002

The Monte Carlo method introduced by Gillespie (cf. D. T. Gillespie, J. Phys. Chem. 81, 2340 1977), was applied to the kinetics of micelle formation according to a mechanism that allows associations and dissociations among ennamers of whatever aggregation number. A careful choice of thermodynamic and dissociation kinetic constants made it possible to reproduce both equilibrium and kinetic properties of hypothetical surfactant solutions. The results obtained by stochastic simulations are shown and compared to the accepted theory and to the experimental evidence available in literature.
Theoretical characterization of the ground- and excited-state structures and properties of indole-(H2O)n (n=1,2) complexes
Authors: Fang Wei-Hai
Published: July 2002

The structures and properties of indole and its complexes in S0 and S1 were characterized with the Hartree–Fock (HF), configuration interaction with single excitation (CIS), and complete active space self-consistent field (CASSCF) methods, using the 3-21G and 6-31G* basis sets. The influence of the intermolecular interaction on the structures and properties of the complexes was discussed in detail. A comparison is made between the results reported here and those from previous theoretical and experimental studies.
Gas–liquid nucleation in partially miscible systems: Free-energy surfaces and structures of nuclei from density functional calculations
Authors: Napari Ismo , Laaksonen Ari
Published: July 2002

Gas–liquid nucleation in partially miscible Lennard-Jones systems is studied using the density functional theory in a perturbation approach. Nucleation properties are examined in a symmetric and an asymmetric mixture. Both systems are shown to form two kinds of mixed critical nuclei in one vapor composition. Spherically symmetric clusters with a phase separated structure are found in the asymmetric mixture; these are shown to correspond to local maxima of the Gibbs free-energy surface. A classical calculation also suggests that the cylindrically symmetric clusters found in earlier studies have higher formation energies than the spherical nuclei in the same vapor.
Integral equations for the density profiles of infinitely polydisperse fluids at a hard wall
Authors: Bryk P. , Patrykiejew A. , Reszko-Zygmunt J. , Sokolowski S. , Henderson D.
Published: July 2002

An extension of the singlet-level equations for the density profile to the case of adsorption of polydisperse fluids on solid surfaces is presented. Explicit calculations and comparisons with canonical ensemble Monte Carlo data have been performed for a polydisperse hard sphere fluid in contact with a hard wall by using the Percus–Yevick, hypernetted chain, and a modified Verlet approximation. A numerical solution of the density profile equations makes use of the orthonormal polynomials with the weight function corresponding to the distribution function of the diameters of fluid particles.
Integral equation theory of flexible polyelectrolytes. II. Primitive model approach
Authors: Dymitrowska Magdalena , Belloni Luc
Published: July 2002

The multicomponent Polymer Reference Interaction Site Model is solved numerically for solutions of linear flexible polyelectrolytes in the Primitive Model approach with an explicit description of the counterions and salt ions. All the ionic species interact via bare Coulomb and hard sphere potentials. The wormlike chains modeled as connected interacting charged sites are characterized by their persistence length. The hypernetted chain integral equation is used as the closure relation. The structural and thermodynamic properties of semidiluted polyelectrolyte solutions are presented. The model gives the pair correlation functions for monomer–monomer, monomer–ion, and ion–ion pairs. From the monomer–counterion correlations information on the ionic condensation around flexible chains is deduced. The ion-averaged monomer–monomer potential obtained from the monomer–monomer correlations presents a classical screened Coulomb behavior at low coupling and reveals an attractive character at high electrostatic coupling.
Moderately dense gas transport coefficients via time correlation functions. II. Shear viscosity and thermal conductivity
Authors: Alavi Saman , Snider R. F.
Published: July 2002

An equation for a moderately dense gas transport coefficient, derived in the preceding article from time correlation function theory, is specialized to the estimation of the coefficients of viscosity and thermal conductivity. These equations include the first order in density correction to the transport coefficient, but are restricted to being valid only for repulsive potentials. It is shown that the resulting expressions are, within a series of analogous approximations, identical to those obtained by solving the generalized Boltzmann equation.