bluuebunny/crossref_metadata_2025 huggingface dataset:
Automatic potential energy surface generation directly from ab initio calculations using Shepard interpolation: A test calculation for the H2+H system
Authors: Ishida Toshimasa , Schatz George C.
Published: July 2002

A modified version of the Shepard interpolation scheme proposed recently by Collins et al. is used to determine a potential energy surface for the H2–H reaction. Our modifications are based on preliminary calculations in which the Liu–Siegbahn–Truhlar–Horowitz surface for H3 is used to study convergence of the Shepard procedure. Included in the modifications are changes to the form of the weight functions, the coordinate sets used in the zeroth-order surface, and the transformation of the Cartesian first and second derivatives to internal coordinates. The new ab initio potential energy surface is based on calculations using the basis set of Siegbahn and Liu and second order Mo/ller–Plesset (MP2) perturbation calculations. We have not experienced convergence problems with the self-consistent-field iteration while “growing” the surface. Furthermore, a converged fit can be obtained using only 85 ab initio calculations. This demonstrates that the Shepard interpolation scheme is a powerful candidate for automatic determination of medium quality potential surfaces for dynamical calculations.
Resonant Raman scattering in O2
Authors: Simbotin I. , Marinescu M. , Sadeghpour H. R. , Dalgarno A.
Published: July 2002

Raman scattering of molecular oxygen in the region of the Schumann–Runge absorption band system is calculated from first principles in the second order of perturbation theory in the interaction of photon–electron operator. Quantitative predictions are given for the cross sections for scattering of photons and a detailed comparison is made with available measurements. The behavior of Raman spectra near the predissociating rovibronic level (v′=5,N′) in the B 3Σu− electronic state of O2 is investigated and shown to have significant dependence on the detuning of the incident laser frequency. By casting the Raman transition matrix in a form similar to the Fano formalism, we are able to investigate the nature of the resonant and nonresonant scattering in the language of configuration interaction. By varying the predissociating width of the (v′=5,N′) resonance, we make a comparison with an earlier time-dependent wave-packet analysis. We also calculate the depolarization ratio of the scattered photons parallel and perpendicular to the polarization of the incident laser beam for the different Raman branches for lines terminating in the v′′=1 and v′′=6 levels and comment on the relationship between resonant Raman scattering and fluorescence emission.
Dynamic behavior of the monomer–monomer surface reaction model with adsorbate interactions
Authors: Voigt Christopher A. , Ziff Robert M.
Published: July 2002

The monomer–monomer surface reaction model with an adsorbate interaction term is studied. An epidemic analysis of the poisoning times (tp) for small square lattices as a function of lattice edge length L and interaction strength α at the point of equal adsorption rates yields a dynamic scaling relation which describes the crossover between log-power-law and exponential behavior in L, and is able to fit the entire dependence of tp upon α and L. The phase transition is further explored by varying adsorption rates and is found to follow second-order kinetics. A mean-field approximation is introduced as a comparison for the numerical results.
Adsorption and reactions of NO on NiAl(111) at 75 K
Authors: Schmitz G. , Bartolucci F. , Gassmann P. , Masuch J. , Franchy R.
Published: July 2002

The adsorption and reactions of NO on NiAl(111) at 75 K were studied by high resolution electron energy loss spectroscopy, temperature programmed desorption, Auger electron spectroscopy, and low energy electron diffraction. At low exposure (⩽1 L), NO mainly adsorbs molecularly on top in an upright geometry on Ni atoms. Simultaneously, a small amount of NO dissociates. Higher exposures (⩾2 L up to saturation) lead to the formation of a thin amorphous Al-oxynitride (am-ALON) film. In the presence of am-ALON, a molecular adsorption of NO on am-ALON sites and/or in the neighborhood of ALON islands is observed. Besides the upright geometry, NO molecules are adsorbed in disarranged (bent or tilted) configurations. The growing am-ALON film acts as a catalyst for the reduction of NO to N2O. Substantial amounts of N2O are formed for NO exposures higher than 5 L, and are coadsorbed molecularly. The main thermal desorption products are N2O, N2, and NO. For an exposure of 20 L NO, the ratios of the amounts of desorbing molecules are: N2O:N2:NO=1:0.43:0.36. It could be shown that the N2 signal is due to a recombinative desorption of adsorbed nitrogen atoms.
Brownian dynamics simulations of protein folding
Authors: He Siqian , Scheraga Harold A.
Published: July 2002

The torsional angle space macromolecular conformational dynamics treatment presented in the preceding paper is used to study the mechanism and kinetics of protein folding by using continuum rigid chain molecules. The main purpose is to test the treatment using simple macromolecular systems. It is found that the torsional angle space approach is much faster and more reliable than similar approaches in atomic coordinate space. The simulation results also suggest that the short-ranged Lennard-Jones binary interactions alone are not sufficient to fold the chain molecules, and that hydrophobic collapse is essential for the folding processes. In our simplified protein folding model, the hydrophobic collapse is achieved by introducing global dipole interactions. The collapse of the chain molecule induced by dipole interactions significantly reduces the folding time. The chain collapse processes effectively bring the atoms into the (short) range of Lennard-Jones attractions, which then, in turn, are able to play their role in the folding processes; without such collapse the folding processes are highly frustrated.
Viscosity coefficients of nematic hard particle fluids
Authors: Evans Glenn T.
Published: July 2002

Enskog kinetic theory is applied to the calculation of the Miesowicz viscosities and the Leslie coefficients of a nematic liquid crystal comprised of hard ellipsoidal particles. The anisotropies in the calculated viscosities are consistent with those derived from computer simulation, affine transformation theory and from experiment. When the Miesowicz viscosities are combined with the diffusion coefficients for a nematic, approximate Stokes–Einstein relations are found.
Intermittency of droplet growth in phase separation of off-critical polymer mixtures
Authors: Takeno Hiroyuki , Hashimoto Takeji
Published: July 2002

Kinetics of the phase separation in binary off-critical mixtures of polybutadiene (PB) and polyisoprene (PI) was investigated as a function of quench depth and composition by using time-resolved light scattering. The kinetics for the off-critical mixtures is much slower than that for the critical mixture. The scattering maximum for the off-critical mixtures is very broad relative to that for the critical mixture. For the off-critical mixtures, the domain growth follows the same power law (qm∼t−α, Im∼tβ: α=0.25–0.33, β=0.75–1) in the long time limit covered in our experiments, independent of quench depth and composition. However, the domain growth became very slow before the long time limit under a certain condition (intermittency of the domain growth). In the regime where the intermittency was observed, the scaling exponents α and β depend on quench depth. The shallower the quench, the smaller the values of α and β and the longer the intermittent regime. These behaviors are largely different from those for the critical mixture for which the scaling exponents show almost the same value in the late stage for different quenches and the Langer–Bar-on–Miller or Chou–Goldburg scaling postulate is fulfilled. Scaled structure factor in the late stage of phase separation process was found to depend on quench depth, becoming broader as quench is shallower.
An infrared spectrum of the molecular dication (doubly positively charged molecule), D35Cl2+
Authors: Abusen Ramadan , Bennett Frederick R. , McNab Iain R. , Sharp David N. , Shiell Ralph C. , Woodward Clare A.
Published: July 2002

We have observed an infrared spectrum within the X 3Σ− state of DCl2+ using a fast-ion-beam/laser-beam spectrometer. A preliminary analysis shows good agreement with the rotational constants and tunneling lifetimes calculated by Bennett and McNab for the ν=1–2 vibrational band, although the calculated band origin appear to be in error by 21.1 cm−1.
Nuclear relaxation contribution to static and dynamic (infinite frequency approximation) nonlinear optical properties by means of electrical property expansions: Application to HF, CH4, CF4, and SF6
Authors: Luis Josep M. , Martı́ Josep , Duran Miquel , Andrés José L. , Kirtman Bernard
Published: July 2002

Electrical property derivative expressions are presented for the nuclear relaxation contribution to static and dynamic (infinite frequency approximation) nonlinear optical properties. For CF4 and SF6, as opposed to HF and CH4, a term that is quadratic in the vibrational anharmonicity (and not previously evaluated for any molecule) makes an important contribution to the static second vibrational hyperpolarizability of CF4 and SF6. A comparison between calculated and experimental values for the difference between the (anisotropic) Kerr effect and electric field induced second-harmonic generation shows that, at the Hartree–Fock level, the nuclear relaxation/infinite frequency approximation gives the correct trend (in the series CH4, CF4, SF6) but is of the order of 50% too small.
Density functional investigation of the geometric and electronic structure of ethylene adsorbed on Si(001)
Authors: Birkenheuer U. , Gutdeutsch U. , Rösch N. , Fink A. , Gokhale S. , Menzel D. , Trischberger P. , Widdra W.
Published: July 2002

A detailed first-principles density functional analysis of the geometric and electronic properties of ethylene adsorbed on the dimer reconstructed Si(001)-(2×1) surface is presented. This theoretical study was carried out in close reference to a recent angle-resolved photoemission spectroscopy investigation of the same adsorption system. Adsorbate weighted Kohn-Sham one-particle spectra are calculated and compared to the band structure derived from the angle-resolved photoemission spectra. In addition, the symmetry character of the concomitant Bloch waves is determined to yield information which can directly be related to the results of a dipole selection rule analysis of the corresponding photoemission signals. Total energy minimization of a model slab reveals a distortion of the adsorption complex at saturation coverage to local C2 symmetry involving an 11° rotation of the ethylene molecule around the surface normal and a 27° twist of the methylene groups around the CC axis. This finding is confirmed by a comparison of the calculated band dispersions with those found in the angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) experiments. The driving forces for the distortion of the adsorption complex can be traced to direct Pauli repulsion between the hydrogen atoms of neighboring ethylene molecules and to a bonding overlap contribution from the ethylene 1b2g-derived orbitals of the adlayer.
Mean field theory for solutions of main-chain liquid crystalline polymers
Authors: Matsuyama Akihiko , Kato Tadaya
Published: July 2002

A mean field theory is introduced to describe the nematic-isotropic phase transitions (NIT) in solutions of main-chain liquid crystalline polymers (MLCPs) which consist of rigid mesogens and spacers with various degrees of flexibility. The theory takes into account not only the nematic ordering of mesogens but also the partial ordering, or straightening, of spacer segments in the nematic phase. On the basis of the Onsager-type excluded volume interactions and the Maier–Saupe model for orientational dependent–attractive interactions between rigid segments, we derive the free energy for solutions of the MLCP. We find two different nematic phases: One is a weak nematic phase which is almost formed by the ordering of the mesogens. The other is a high nematic phase where the straightened segments on the spacers and the mesogens are highly ordered. The two different nematic phases and phase behaviors are discussed in the phase diagrams on the temperature-concentration plane.
Linear and nonlinear optical response of polyenes: A density matrix renormalization group study
Authors: Shuai Z. , Brédas J. L. , Saxena A. , Bishop A. R.
Published: July 2002

Using a symmetry adapted density matrix renormalization group (DMRG) formulation, we have calculated various optical spectra, including linear absorption, electroabsorption, third-harmonic generation, and two-photon absorption, for a series of linear polyene molecules (up to 40 carbon sites) modeled by an extended Hubbard–Peierls Hamiltonian within the intermediate correlation regime. The theoretical two-photon absorption spectrum for trans-octatetraene is in good agreement with experiment. By comparing the experimental linear absorption spectra and theoretical higher Ag state energies, it is suggested that the so-called cis-band actually corresponds to the threshold of the mAg band.
Collision induced dissociation, proton abstraction, and charge transfer for low energy collisions involving CH4+
Authors: Peko B. L. , Dyakov I. V. , Champion R. L.
Published: July 2002

Measurements of total cross sections for collision induced dissociation, proton abstraction, and charge transfer have been made for collisions of CH4+ with CD4, H2, and Ar. The laboratory collision energies range from a few up to 400 eV, and isotopic substitutions have been made where possible to investigate any possible isotope effects, and for some reactants, to more closely identify the product ions. Cross sections for all reaction channels are observed to be small; ≲5 Å2 for the methane target, ≲0.5 Å2 for the hydrogen target, and ≲2 Å2 for the argon target. For the methane and hydrogen targets, proton abstraction is observed at low energies while charge transfer occurs for energies ≳10 eV. The implications of internal energy present in the CH4+ projectile for the cross section measurements is discussed. In addition, decomposition of CD4H+ is addressed in terms of recent calculations related to the structure of the CH5+ ion.
Measurement of structural distribution functions in disordered systems: A general approach for sensitivity estimation
Authors: Utz Marcel
Published: July 2002

A novel approach is proposed that allows the evaluation of distribution functions of structural parameters, such as dihedral angles and orientations of molecular segments, from suitable NMR spectra in disordered solids. The relationship between the distribution function and the observable spectrum is represented by a linear integral operator 𝒢. The distribution function corresponding to a measured spectrum is expanded into the eigenfunctions of 𝒢, the expansion coefficients being found by projecting the measured data onto the basis spectra associated with the eigenfunctions. The accuracy of the expansion coefficients, thus obtained, is related to the eigenvalues of 𝒢. Unlike other sets of orthogonal functions, the eigenfunctions of 𝒢 have the interesting property that also their associated basis spectra (“eigenspectra”) are orthogonal. Therefore, we refer to this novel scheme as the “conjugate orthogonal functions” (COF) approach. Beyond the utility of the COF approach for data evaluation and sensitivity analysis, it also represents a means for comparing systematically the analytic power of spectroscopic techniques. The nature of the information delivered by a particular technique (e.g., its symmetry), is reflected in the eigenfunctions of 𝒢, whereas the eigenvalues are related to the “amount” of information that can be extracted from spectra with a given signal-to-noise ratio. By way of example, the approach is applied to the determination of the distribution of dihedral angles in doubly 13C-labeled bisphenol-A poly(carbonate) by solid-state NMR spectroscopy, and experimental data from heteronuclear separated-local-field and double quantum two-dimensional (2-D) correlation experiments are compared.
Dynamics in the spin-boson model by maximum entropy moment imaging
Authors: Bailey D. , Hurley M. , McDowell H. K.
Published: July 2002

The spin–spin time correlation function of the spin-boson model is studied using a maximum entropy imaging procedure founded on knowledge of early time derivatives or moments of the corresponding spectral density. The coherent–incoherent boundary is reproduced in agreement with the results of dynamical path-integral Monte Carlo and the procedure is shown to be stable numerically at both long time and low temperature.
Coil–globule transition of poly(methyl methacrylate) by intrinsic viscosity
Authors: Baysal Bahattin M. , Kayaman Nilhan
Published: July 2002

The coil–globule transition for poly(methyl methacrylate) has been studied in the mixed solvent tert-butyl alcohol+water system. Two polymeric samples having molecular weights Mw=2.55×106 and Mw=4.73×106 were used. The contraction and expansion of the molecular chains were determined by viscometric measurements. The results were compared with light scattering data obtained recently for the same system. A smooth and continuous contraction was observed below the θ temperature (41.5 °C). The temperature dependence of [η] can be represented by a master curve in αη3|τ|Mw1/2 vs |τ|Mw1/2 plot, where αη3=[η(T)]/[η(θ)] is a viscosity expansion factor and τ=(T−θ)/T is the reduced temperature. A comparison of the observed globular radius with a theory for contracted coil confirmed the coil–globule transition for this system.
Quantum dynamics at a conical intersection: The role of the variation of oscillator frequencies in the diabatic transition
Authors: Ferretti Alessandro , Lami Alessandro , Villani Giovanni
Published: July 2002

The role of frequency variation in the transition between two crossing diabatic electronic states originating in a conical intersection is studied. We have considered a minimal model with two vibrational degrees of freedom, a coupling mode, and a tuning mode, and performed computations varying the frequency of both in the initially unoccupied diabatic state. It is shown that neglecting this effect may result in a wrong estimate of the transition probability. Two simple rules for evaluating qualitatively such an effect are proposed.
Relationships between surface tension and bulk properties of liquids: Organic liquids near 298 K
Authors: Freeman Gordon R. , March Norman H.
Published: July 2002

The ratio of surface tension γ to shear viscosity η has units of speed. For many liquids at 298 K the value of γ/η is about 0.3 of the thermal average speed of the molecules in the liquid. However, for a selection of organic liquids the values of (γ/η)(M/kBT)1/2 range over a factor of 9, so it is not a useful general correlation. The product of γ with the isothermal compressibility κT for the same liquids at 298 K has an average value 27±3 pm, independent of the ΔT from the triple point. The distance γκT is interpreted as the average elastic spacing between the stiff cores of the molecules, related to the multibody intermolecular potential function operating in 2π and 4π steradian space.
A theoretical study of outersphere electron transfer reactions in electrolyte solutions
Authors: Chandra Amalendu
Published: July 2002

A microscopic theory of outersphere electron transfer reactions in electrolyte solutions is presented. Both static and dynamic effects of solvent and ion atmosphere on rates of electron transfer are calculated by employing molecular models. The donor–acceptor system is composed of two spheres and the electrolyte solution is composed of dipolar solvent molecules and ions which are treated at the same molecular level. A microscopic expression for the free energy of activation is derived by using density functional theory. The dynamic effects are calculated by using a molecular hydrodynamic theory which properly includes finite wave vector modes of relaxation of solvent and ion atmosphere. Explicit numerical results are presented for the activation free energy and the rate constant of electron transfer in solutions of varying ion concentration. It is found that ion atmosphere can make an important contribution to the activation free energy at finite ion concentration although the net increase in the activation energy is not very significant for the solutions studied in this work. This happens because, with increase of ion concentration, the ion atmosphere contribution to the total activation free energy increases, whereas the solvent contribution shows a decreasing trend. The solvent behaves as an effective less polar medium due to screening by ions and, therefore, its contribution to the activation free energy decreases as ion concentration is increased.
Asymmetric line shapes and surface enhanced infrared absorption of CO adsorbed on thin iron films on MgO(001)
Authors: Krauth O. , Fahsold G. , Pucci A.
Published: July 2002

We have measured infrared transmission spectra of CO adsorbed on in situ grown iron films on MgO(001) under ultrahigh vacuum conditions. Even at normal incidence we observed strong absorption lines in the C–O stretch region with number, intensities, positions, and shapes dependent on CO coverage and Fe-film thickness and morphology. This absorption must be due to vibrational dipoles oblique to the substrate surface, e.g., due to molecules at island walls. The distinct absorption lines can be assigned to distinct adsorption sites on crystalline facets of epitaxial Fe islands on MgO(001). For each of the films the strongest CO line shows an asymmetric shape. Additionally, the observed absorption is enhanced by at least two orders of magnitude with respect to adiabatic intensities of, e.g., CO on NaCl. Line shapes and intensity let us suggest nonadiabatic coupling of the adsorbate vibration to electronic transitions.
Anharmonic force fields from analytic CCSD(T) second derivatives: HOF and F2O
Authors: Breidung Jürgen , Thiel Walter , Gauss Jürgen , Stanton John F.
Published: July 2002

The recent implementation of analytic second derivatives for CCSD(T) (coupled cluster theory with single and double excitations augmented by a perturbational treatment of connected triple excitations) has been combined with a numerical finite difference procedure to calculate cubic and semidiagonal quartic force fields. Computational details of this approach are outlined. Applications are reported for HOF and F2O. The CCSD(T) results are in excellent agreement with experiment and represent a substantial improvement over the results obtained from MP2 (Mo/ller–Plesset second-order perturbation theory).
Hydrophobic hydration: Heat capacity of solvation from computer simulations and from an information theory approximation
Authors: Arthur Jonathan W. , Haymet A. D. J.
Published: July 2002

Hydrophobic hydration is studied with an information theory approximation, using the first two moments of the number of solvent centers in a cavity in liquid water, calculated from the density and the pair correlation function. The excess chemical potential, entropy, and heat capacity of solvation are determined for three cases: the two-dimensional MB model of water, in both the (i) NPT and (ii) NVT ensembles, and (iii) the central force CF1 model of water in the NPT ensemble. The results are compared with Monte Carlo simulations and experimental measurements from the literature. The information theory approximation, using only the first two moments, accurately determines the excess chemical potential and entropy of solvation but is unable to predict the excess heat capacity of solvation. Little difference is found between the results obtained using the uniform prior and the ideal gas prior. Molecular dynamics simulations are performed to calculate the excess chemical potential of solvation of soft-spheres as a function of solute size. These results are compared with the solvation of a hard sphere using the information theory approximation and previous molecular dynamics simulations of Lennard-Jones spheres in water. The information theory approximation is found to predict the free energy of solvation as a function of size accurately up to a cavity diameter of approximately 3.5 Å.
The effect of Coulomb interactions on transport processes in one-dimensional systems such as discotic liquid crystals
Authors: Siebbeles L. D. A. , Movaghar B.
Published: July 2002

We evaluate the consequences of interactions between charge carriers on the transport dynamics or currents in normally insulating one-dimensional systems where excess charges have been injected electrically, chemically, or optically. The time-dependent currents and space charge distribution are calculated using a many body Monte Carlo method adapted to stochastic transport. The idea is to study collective interaction phenomena and to learn how to distinguish them from disorder effects. The latter can in principle be modeled to explain almost any experimental result, however, this is often done at the expense of having to use unphysical or inconsistent parameters. Interactions can we argue, give for example, rise to low frequency anomalies in ac conductivity, negative differential resistance, low field spin magnetoresistance, and critical nonlinear noise like oscillations in currents as a function of time.
Molecular switching in a two-dimensional constriction
Authors: Nakamura Hiroki
Published: July 2002

The new idea of molecular switching discussed previously for the one-dimensional system [Nanbu, Nakamura, and Goodman, J. Chem. Phys. 107, 5445 (1997)] is extended to a two-dimensional constriction model. Analytical formulation is provided with use of the Mathieu functions and the semiclassical theory of nonadiabatic transition. Numerical demonstrations are also carried out. Both reflection and transmission, and thus the switching cannot be complete like in the one-dimensional case, but the switching is demonstrated to be quite effective. Although this is a purely theoretical model at this stage, some practical implications are discussed.
Theoretical study on electronic states of carbon monofluoride and on the predissociation of the lower lying states
Authors: Petsalakis Ioannis D.
Published: July 2002

Adiabatic potential energy curves of X 2Π, 12 doublet excited electronic states of CF and the ground state of CF+ have been determined by MRDCI (multi reference single and double excitations configuration interaction) calculations. The potentials of the excited electronic states are complicated by interactions with repulsive states as well as by valence–Rydberg interactions. Potential energy curves of higher-lying Rydberg states have been generated with the aid of quantum defect calculations. Radiative lifetimes have been calculated for the A 2Σ+ and the B 2Δ states while predissociation lifetimes have been obtained by a five-state complex-coordinate rotation calculation involving the lowest five doublet states along with their mutual interactions. The results are in agreement with the existing experimental data regarding the predissociation of the v>1 levels of A 2Σ+, while predissociation is predicted for all the vibrational–rotational levels of the 2 2Σ+, for the v>1 levels of the positive parity substate of D 2Π and for the higher vibrational levels (v>2) of B 2Δ. For the latter two states predissociation increases significantly with the rotational level.
Effect of the transient mobility in two-dimensional continuum deposition
Authors: Belardinelli Rolando E. , Linares Daniel H. , Pereyra Victor D.
Published: July 2002

By using Monte Carlo (MC) techniques, we study the effect of the transient mobility on the irreversible deposition of spherical particles on two-dimensional substrates. The mobility is introduced by the capability of the incoming particles to roll on the flat surface a distance R before coming to rest or, in the case of falling on top of an already adsorbed particle, roll over until it contacts the surface. Depending on the deposition mechanism three models of irreversible adsorption with transient mobility are proposed. The kinetics of the process are studied by the time behavior of coverage ρ(t). The asymptotic regime ρ(∞)−ρ(t) appears as specially interesting and behaves as a function of the degrees of freedom in the motion of the depositing particles going from power law to exponential dependence. The jamming state is characterized through the behavior of the jamming coverage ρ(∞) and the correlation function g(r).
The site-specific reactivity of isopropanol in aqueous silicon etching: Controlling morphology with surface chemistry
Authors: Newton Theresa A. , Huang Yi-Chiau , Lepak Lori A. , Hines Melissa A.
Published: July 2002

The site-specific rates of isopropanol reaction on Si(111) were studied by kinetic competition with an etchant of known anisotropy and quantified using concentration-dependent changes in the etched surface morphology using a combination of STM measurements and kinetic Monte Carlo simulations. The isopropoxide ion was identified as the active species.
Multidimensional tunneling dynamics on HSiOH cis-trans isomerization with interpolated potential energy surface
Authors: Taketsugu Tetsuya , Watanabe Naomitsu , Hirao Kimihiko
Published: July 2002

The ab initio potential energy surface (PES) for the HSiOH cis-trans isomerization was generated by the modified Shepard interpolation method at the computational level of the complete active space self-consistent field (CASSCF) method. This isomerization has three reaction paths, i.e., one in-plane and two out-of-plane paths, so the reaction-path PES (RP-PES) has first been generated by setting reference points on these paths for the interpolation. In this RP-PES, there is an artifact of potential ridges between the in-plane and out-of-plane paths. By adding molecular configurations around potential ridges as reference points, the global PES has been much improved (the potential ridges have disappeared). Following trajectory simulations on this full-dimensional PES, the energy level shift of vibrational ground states due to tunneling was estimated by the semiclassical method. It is verified that there is a relatively large tunneling effect in this reaction, which is consistent with the experimental observations.
Thermal diffusivity of fluid oxygen to 12 GPa and 300 °C
Authors: Abramson E. H. , Slutsky L. J. , Brown J. M.
Published: July 2002

The thermal diffusivity of fluid oxygen in the diamond-anvil cell has been measured from 1 to 12.6 GPa and 25 to 300 °C. These constitute the first experimental measurements of thermal transport properties of simple fluids above 1 GPa. Diffusivities are found to rise sharply from a minimum at intermediate pressures and then to level off at ∼6 GPa. Thermal conductivities derived from these measurements do not vary as √T, rather the excess conductivities are approximately independent of temperature. The diffusivities of nitrogen, previously measured to 1 GPa, closely match those of oxygen when scaled as suggested by a simple, corresponding states theory.
Persistent spectral hole-burning and hole-filling in CuBr semiconductor nanocrystals
Authors: Valenta J. , Dian J. , Hála J. , Gilliot P. , Lévy R.
Published: July 2002

Persistent spectral holes (p-SHs) are induced in the Z1,2 excitonic absorption band of CuBr nanocrystals (NCs) in glass by selective excitation with nanosecond dye-laser pulses at low temperatures. The effect can be observed only in samples containing NCs with mean radius smaller than about 5 nm. The kinetics of p-SHs growth and the long-time relaxation (spontaneous hole-filling) after burning are studied. The burning process may be described as the first order dispersive reaction, which proceeds through a phonon-assisted tunneling between different excited states of the NC/matrix system. The same model apply in the backward reaction (hole-filling) but the tunneling takes place between different ground states of the NC/matrix system and the reaction rate is much lower. The photoproduct of persistent spectral hole-burning reaction is a charged NC which energy states are modified by the Stark effect. The quantum efficiency of burning reaction is quite high 4×10−3 (mean rate constant is 4×107 s−1) while the rate of spontaneous HF reaction is very low (0.5 s−1).
An accurate description of the ground and excited states of CH
Authors: Kalemos Apostolos , Mavridis Aristides , Metropoulos Aristophanes
Published: July 2002

With the high accuracy afforded by the sextuple correlation consistent basis set of Dunning, we have calculated energy levels, dissociation energies, equilibrium distances, and other spectroscopic constants for eleven valence and four Rydberg states of the CH radical. Comparisons with experimental and previous theoretical results are made for each state that has been treated. An understanding of their binding is attempted by means of simple valence bond–Lewis diagrams.
The binding of CO to nickel clusters. II. Structural implications and comparisons with electron counting rules
Authors: Kerns K. P. , Parks E. K. , Riley S. J.
Published: July 2002

The saturation levels of small nickel clusters Nin (n=2–20) with CO are determined. Modeling of the CO-covered clusters shows that in almost all cases saturation is governed by the geometrical sizes of the nickel clusters and the van der Waals size of the CO molecules. While electron counting rules, along with an assumed cluster structure, can predict saturation levels, the predictions generally overestimate the levels based on geometry. In the case of the smallest clusters (n⩽13), CO adsorption appears to cause changes in structure to more open ones that result in increased adsorption, giving coverages close to the electron counting rule predictions for the bare cluster geometries. Larger clusters, with internal metal atoms, seem to be more resistant to structural changes. Comparisons with earlier studies of nickel cluster positive and negative ions are made and the significance of a short reaction time scale on the nature of the reaction products is discussed.
Molecular dynamics simulation of the homogeneous nucleation of UF6 molecules: Configurations and infrared spectra of the excited hot clusters
Authors: Tanimura Shinobu , Yasuoka Kenji , Ebisuzaki Toshikazu
Published: July 2002

The temperature, potential energy, and configurations of the clusters produced in the homogeneous nucleation of UF6 molecules from the supercooled (supersaturated) vapor phase were determined by classical molecular dynamics (MD) simulations. We observed two phenomena which demonstrate that the nucleation process occurs in the state far from thermal equilibrium. First, the excited hot clusters, the temperature of which is much higher than that of the monomer, were produced and continued to exist during the nucleation process. Second, the relationship between the potential energy and temperature of the clusters depends on the monomer temperature, that is, the potential energy at a temperature decreases with the increase in monomer temperature. In the simulations, various types of cluster configurations were observed: prolate, oblate, spherelike, and confeitolike. The confeitolike cluster is composed of one core and a few horns, and it was found predominantly in the hotter clusters. The infrared spectra of the UF6 clusters were calculated using a dipole–dipole and dipole-induced dipole interaction model and compared with those measured in a supercooled state by Tanimura et al. [J. Chem. Phys. 107, 7096 (1997)]. As a result, we found that the spectra measured in the supercooled state can be attributed to the excited hot clusters, the configuration of which are confeitolike.
Static inhomogeneities and dynamics of swollen and reactor-batch polymer gels
Authors: Shibayama Mitsuhiro , Shirotani Yasuhiro , Shiwa Yasuhiro
Published: July 2002

The dynamic light scattering study has been carried out on swollen and reactor-batch gels of poly(N-isopropylacrylamide) (NIPA) in water as a function of polymer volume fraction, φ. The scattered intensity from these gels showed strong intensity fluctuations dependent on the sample position. The ensemble (i.e., position) average scattered intensity, 〈I〉E, increased by swelling for all gels prepared at different initial monomer concentrations, indicating an increase of the static inhomogeneities by swelling. On the other hand, the dynamic component, e.g., the intensity component of the thermal fluctuations, IF, decreased by swelling. The collective diffusion coefficient, D, increased with increasing φ and by swelling. The physical meaning of the static inhomogeneities and the dynamics of gels are discussed. Furthermore, the concentration dependence of the diffusion coefficient is discussed as a function of φ.
Boundary conditions, solitons, and spin configuration in interchain coupled organic ferromagnetic polymers
Authors: Wang W. Z. , Yao K. L. , Lin H. Q.
Published: July 2002

With an extended Su–Schrieffer–Heeger Hamiltonian including Hubbard energy and intersite Coulomb repulsion, we study the effects of two kinds of boundary conditions on the spin configuration and lattice distortion in organic ferromagnetic polymers with interchain coupling. It is shown that if the length L of the lattice is even, there is dimerization along the main chain and uniform spin density along side radicals. If L is odd, there exist domain wall solitons describing the displacement of the lattice along the main chain, and spin soliton with envelope shape describing the localization of spin at side radicals. Under the condition of strong interchain coupling, a great fluctuation of spin and distortion appear along the main chain and side radicals, and the system transfers to two-dimensional one. The e–e correlation reduces the amplitude of spin soliton and domain wall while electron–phonon coupling has the reverse effect.
Surface-aligned photochemistry: Aiming reactive oxygen atoms along a single crystal surface
Authors: Tripa C. Emil , Yates John T.
Published: July 2002

Surface-aligned photo-oxidation of CO with O2 was demonstrated on two stepped surfaces, Pt(335) and Pt(779). UV light photolysis of O2, localized exclusively on step sites, generates reactive oxygen atoms which oxidize preferentially the step-site CO. This effect was observed by monitoring labeled-CO consumption from step and terrace sites by reflection–absorption infrared spectroscopy, and also by monitoring labeled-CO2 desorbed in the gas phase from step and terrace sites by mass spectrometry. Photo-oxidation of step-CO was found to be about 2 times more efficient than photo-oxidation of terrace-CO, irrespective of the surface step density. This effect was attributed to the initial alignment of the O2 intramolecular axis on stepped Pt surfaces, parallel to the step edge.
Shear-induced changes of the potential energy landscape underlying
n
-butane liquids and glasses
Authors: Frey Mignon M. , Lacks Daniel J.
Published: July 2002

The effects of shear strain on n-butane liquids and glasses are examined with molecular simulations. The simulations focus on the nonlinear mechanical properties and the transport properties of these systems. These properties are analyzed in terms of the changes in the potential energy landscape with strain, which is quantified by the curvature of the energy minima and the height, position, and curvature of barriers between minima. Shear strain is shown to cause local minima to disappear and barrier heights to decrease to zero, which renders the system mechanically unstable and forces the system toward alternate local minima. These disappearances of energy minima and barriers lead to plastic deformation in mechanically stressed n-butane glasses, and enhanced diffusion and molecular alignment in flowing n-butane liquids.
Quantum dynamics of electrons in a molecular segment with phonon interaction
Authors: Gayen Taposh , McDowell Keith , Burns Allassia
Published: July 2002

A Hamiltonian model for a molecular segment or molecular chain with phonon or vibrational coupling is introduced which admits analytic solutions. A time correlation function Q(t) for the average position of an electron inserted at the end of a chain with a thermal average of the phonons is defined. A prominent feature of the dynamics is that the phonons drive the electron density to decay to a steady-state distribution along the chain. We demonstrate that two imaging methods based on the time derivatives of Q(t) at zero time are capable of producing the average velocity of the electron along the chain using a reasonable number of the time derivatives. We further show that this average velocity increases as the coupling to the phonons is increased and as the temperature is increased; that is, the decay to a steady state is enhanced in both cases.
Anion–anion pairing in concentrated aqueous lithium chloride solution
Authors: Ansell S. , Neilson G. W.
Published: July 2002

The second difference method of neutron diffraction and isotopic substitution was used to determine the pair radial distribution function gClCl(r) in an 8.6 molal solution of lithium chloride in heavy water. The number of direct contacts between the Cl− ions was found to be 1.2±0.5. This observation is in marked contrast to that found in an equivalently concentrated nickel (II) chloride solution (8.6 molal Cl−), where no direct Cl−⋯Cl− contacts are found.
Anisotropic Coulomb explosion of C60 irradiated with a high-intensity femtosecond laser pulse
Authors: Kou Junkei , Zhakhovskii Vasilii , Sakabe Shuji , Nishihara Katsunobu , Shimizu Seiji , Kawato Sakae , Hashida Masaki , Shimizu Keisuke , Bulanov Sergei , Izawa Yasukazu , Kato Yoshiaki , Nakashima Nobuaki
Published: July 2002

C 60 vapor was irradiated with an intense femtosecond laser pulse (1×1016 W/cm2 in 120 fs at a wavelength of 800 nm). Multiply charged carbon ions, up to C4+, were detected in the time-of-flight spectra. We have measured both energy and angular distributions with respect to the laser polarization direction for ions with different charge number. The kinetic energy of ions are distributed above 1 keV. The angular distribution measurement shows that the highly charged ions of C3+ and C4+ are mostly distributed in the directions parallel to the laser polarization, and the C+ ions are slightly distributed in the direction perpendicular to it. These observations clearly indicate that an anisotropic explosion takes place. The average energy of ions with the different charge number is found to be proportional to the square of the charge number, while the maximum energy is proportional to the charge number. Classical molecular dynamics simulations have been successfully carried out reproducing not only the energy spectra but also the angular distributions of ions. The C3+ ions are shown to be produced by the cascade hopping of electrons induced by the intense laser light. The simulations suggest that the most crucial process for the anisotropic Coulomb explosion of C60 is not the electron impact ionization, but the cascade hopping of electrons.
Quantum dynamics of photoexcited cyclohexadiene introducing reactive coordinates
Authors: Hofmann Angelika , de Vivie-Riedle Regina
Published: July 2002

The ultrafast photochemical ring opening reaction of 1,3-cyclohexadiene is investigated theoretically. To allow for a quantum mechanical treatment of this high-dimensional problem we introduce reactive coordinates which contain the modes active on the femtosecond time scale. A reduced Hamiltonian is derived in the corresponding subspace and two-dimensional potential energy surfaces are interpolated using ab initio data points. The quantum dynamics in the S1 state toward two conical intersections with the ground state is monitored. We find a bifurcation of the wave packet already in the excited state so that both conical intersections are reached but by different parts of the wave packet. By manipulating the initial conditions the branching ratio can be influenced.
The relationship between nonexponential relaxation and molecular stiffness in aromatic model compounds
Authors: Privalko V. P. , Ezquerra T. A. , Zolotukhin M. , Baltá-Calleja F. J. , Nequlqueo G. , Garcı́a C. , de la Campa J. G. , de Abajo J.
Published: July 2002

The α relaxation of a series of ether–ketone model compounds was studied to quantify the effect of molecular stiffness on the Kohlrausch–Williams–Watts stretching exponential. The obtained values decrease with increasing stiffness and are within the theoretical bounds predicted for systems with spatial dimensionality varying from 2 to 1.
Does variational transition state theory provide an upper bound to the rate in dissipative systems?
Authors: Drozdov Alexander N. , Tucker Susan C.
Published: July 2002

By comparing variational transition state theory (VTST) against exact numerical calculations for the Brownian motion of a reactive particle, we uncover the unexpected result that VTST does not provide a rigorous upper bound to the least nonvanishing eigenvalue of the corresponding Fokker–Planck equation, a quantity which can be shown to correspond to the phenomenological rate constant. This result is tied to deviations between the reactive flux rate and the least nonvanishing eigenvalue which arise for intermediate to low barriers.
Transport mechanisms during spreading of MoO3 on Al2O3 supports investigated by photoelectron spectromicroscopy
Authors: Günther S. , Gregoratti L. , Kiskinova M. , Taglauer E. , Grotz P. , Schubert U. A. , Knözinger H.
Published: July 2002

The initial stage of spreading of micron-sized MoO3 crystals deposited on a flat Al2O3 support have been studied by synchrotron radiation spectromicroscopy. The lateral resolution of 150 nm and the surface sensitivity of the microscope allowed us to detect submonolayer quantities of Mo–oxide, spread on the support in the vicinity of the MoO3 crystals after annealing in oxygen atmosphere. Detailed analyses of the Mo–oxide wetting the support have been performed combining chemical imaging with spectroscopy. The concentration of the spread Mo–oxide was quantified as a function of the distance from the MoO3 crystals. The determined concentration profiles were fitted to the analytical solution of an equivalent diffusion model for the MoO3 mass transport in dry oxygen at 630 K yielding a diffusion constant of D=7.8×10−11 cm2/s. The results are discussed in the framework of the possible mass transport mechanisms causing the spreading. The simple unrolling carpet was ruled out as an active process.
Measurement of 10−1 s state-specific lifetimes in the neutral CO molecule
Authors: Sykora Thomas , Vidal C. R.
Published: July 2002

Recently, a technique has been demonstrated which allows the measurement of metastable lifetimes in neutral molecules of up to 10−3 s [J. Chem. Phys. 110, 6319 (1999)]. The present article extends the method by two orders-of-magnitude to molecular lifetimes as long as 10−1 s. Using the CO metastable a 3Π state, lifetimes of eight rovibrational a 3Π(v=3,Ω,J) levels were measured. Within their experimental error of 30%, these lifetimes agree with previous theoretical predictions [J. Chem. Phys. 110, 6319 (1999)]. The presented technique is suggested for identification of molecular quintet states, in particular the CO a″ 5Π state.