bluuebunny/crossref_metadata_2025 huggingface dataset:
Finite-size effects on drying and wetting transitions in a molecular dynamics simulation
Authors: Bruin C. , Nijmeijer M. J. P. , Crevecoeur R. M.
Published: July 2002

We report on finite-size effects in molecular dynamics simulations of the drying and wetting transitions in Lennard-Jones systems. These effects could be the reason for the incompatibility of previous, conflicting results on the nature of the drying of a substrate by a fluid. We indeed observe a crossover from a quasi-first-order to a continuous-like transition with increasing substrate area. We point out that the customary analysis based on the contact angle has disadvantages for a detailed study of the transition, which are absent in the study of the total substrate–fluid surface free energy. Furthermore, we introduce histogram techniques to this problem.
Rigidity of colloidal crystals as studied by the diffusion equilibrium method
Authors: Okubo Tsuneo
Published: July 2002

Rigidity of the colloidal crystals of monodispersed silica and polystyrene spheres has been measured by the diffusion equilibrium method, in which the stock suspension of the crystal state (ca. 0.1 in volume fraction of spheres) is introduced carefully in the bottom of the observation cell. The interface between upper water and lower crystals keeps clear the whole period of the measurements; one to two months for silica spheres. The initial ascending velocity of the interface increases significantly as the ionic concentration of suspension decreases. The translational mutual-diffusion coefficients, Dtr of the colloidal spheres are evaluated from the ascending velocities. The Dtr values of the deionized suspensions are surprisingly large, ca. 700-fold compared with that calculated using the Stokes–Einstein equation, and decrease sharply as ionic concentration of suspension increases. The lattice spacings at various heights in the crystal phase are determined in a diffusion equilibrium from the reflection spectroscopy. The rigidity and the fluctuation parameter, g factor, of the colloidal crystals are evaluated and compared with the previous data from the sedimentation equilibrium method, for example.
Depolarized Rayleigh scattering in water up to supercritical conditions
Authors: Fontana A. , Nardone M. , Ricci M. A.
Published: July 2002

Depolarized Rayleigh spectra of water have been measured as a function of density along several isotherms in both liquid and supercritical phases. At all the thermodynamical states the spectral line shape can be fitted by a Lorentzian line superimposed to an exponential background. The behavior of the fitting parameters shows that at temperatures lower than ∼470 K the temperature is the most relevant parameter in determining the dynamics of the system, whereas above this temperature both temperature and density play a relevant role. Both the line shape and intensity of the spectra can be consistently interpreted in the framework of the first-order dipole induced dipole approximation to the collision-induced light scattering. The data show a clear evidence for cancellation effects.
Interplay of wetting and adsorption at mixed self-assembled monolayers
Authors: Olbris Donald J. , Ulman Abraham , Shnidman Yitzhak
Published: July 2002

Mixed monolayers of CH3/OH-terminated alkanethiolates chemisorbed on gold exhibit anomalous variations of wettabilities, possibly because of adsorption of a minority species from the bulk. Starting from a lattice-gas model with proper quenched averaging over the chemically heterogeneous surface, we derive a mean-field theory appropriate for such systems by generalizing Cahn’s wetting model. To study interplay of wetting and adsorption in the presence of impurity adsorbates, we introduce a second, coupled field representing the minority species concentration. Adsorption of the second species is allowed by assuming a step-function profile for its concentration. The interplay of wetting and adsorption is studied as a function of the chemical heterogeneity, the temperature, and the chemical potential of the minority species. We conclude with a brief discussion of the adequacy and limitations of this model and its possible generalizations for other systems exhibiting similar phenomena involving interplay of wetting, adsorption, and ordering at chemically heterogeneous surfaces.
A theoretical study of the NH+NO reaction
Authors: Bradley Kimberly S. , McCabe Patrick , Schatz George C. , Walch Stephen P.
Published: July 2002

We present a quasiclassical trajectory study of the NH+NO reaction using a global potential energy surface that is capable of describing branching to the H+N2O and OH+N2 products after initial formation of a HNNO intermediate complex. The surface is based on a many-body expansion wherein fragment potentials for the species N2H, HNO, and N2O are incorporated, using either previously developed potentials, or in the case of N2O, a newly developed potential. The three-body parts of these fragment potentials are damped in the four-body region to provide a zeroth order four-body surface, and then additional four-body terms and mapping transformations are applied to make the final four-body potential match the results of ab initio calculations for eight important HNNO stationary points (minima and saddle points) and for several reaction paths. In addition to this ‘‘best fit’’ surface (surface I), a second surface (surface II) is developed in which the ordering of the saddle points leading to formation of H+N2O and OH+N2 is reversed, and the energy release during 1,3 hydrogen migration is modified so that the N–N stretch experiences smaller distortions from N2 equilibrium during the reaction leading to OH+N2. Quasiclassical trajectory results on surface I show generally good correspondence with experiment, with a branching fraction of 13±3% for the formation of OH+N2 at 300 K, and relatively low OH and N2 vibration/rotation excitation. The results on surface II are similar with respect to both branching and energy partitioning, indicating relatively weak sensitivity of the results of key features of the surface.
The SiOH+–HSiO+ system: A high level ab initio quantum mechanical study
Authors: Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

Ab initio electronic structure theory has been employed in order to investigate the ground state potential energy hypersurface of the SiOH+–HSiO+ system. Geometries and physical properties including dipole moments, harmonic vibrational frequencies, and infrared intensities of two equilibrium and isomerization (1,2 hydrogen shift) reaction transition state were determined. The self-consistent-field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory were used with five basis sets. At the highest level of theory employed in this study, CCSD(T) using the triple zeta plus double polarization with diffuse and higher angular momentum functions basis set, linear SiOH+ is predicted to be more stable than linear HSiO+ by 66.1 kcal/mol. This energy difference becomes 64.1 kcal/mol with an appropriate zero-point vibrational energy (ZPVE) correction. At the same level of theory, the classical barrier height for the exothermic isomerization (1,2 hydrogen shift) reaction HSiO+→SiOH+ is determined to be 29.3 kcal/mol and the activation energy (with the ZPVE correction) is 27.3 kcal/mol. The geometrical and energetic features are compared with those of the valence isoelectronic HBO–BOH, HCO+–COH+, and AlOH–HAlO systems.
A guided-ion beam study of the hydrogen atom transfer reaction of state-selected N+2 with H2 at collision energies ranging from subthermal to 2 eV (c.m.)
Authors: Knott W. J. , Proch D. , Kompa K. L. , Rose-Petruck Ch.
Published: July 2002

This article presents detailed internal and kinetic energy dependent cross sections and reaction rates for the hydrogen atom transfer processes N+2(X 2Σ+g, v+=0–4, J+=2)+H2→N2H++H, which were obtained under single-collision conditions in a guided-ion beam/scattering gas experiment. Preparation of ions in specific states relied on single-color excitation within a resonantly enhanced (2+1) multiphoton ionization scheme. The translational energy of the ions, Elab, was varied from 0.1 eV to approximately 30 eV. A small activation barrier impedes the reaction. Vibronic state preparation of the nitrogen ion is influential on the nature of the energy surface—N+2+H2 or H+2+N2—along which the H atom transfer proceeds. Calculations of model potential energy surfaces suggest that the reaction pathway must involve several exoergic and endoergic channels which open successively as the collision energy increases. A purely collision determined cross section—as would be evidenced by the E−1/2 dependence formulated in the Langevin–Gioumousis–Stevenson model—is observed only within a narrow window of kinetic energies.
Lattice relaxation in the 1B
u state for the finite polyenes
Authors: Jin Bih-Yaw , Silbey Robert
Published: July 2002

The effect of electron correlation on the lattice relaxation of the lowest optically allowed 1Bu state of finite polyenes is investigated in this paper. We examine the competition between electron–electron interaction and electron-phonon coupling on the formation of localized lattice distortion in the 1Bu state for finite polyene with chain length up to 30 double bonds, using a number of theoretical models for the electron–electron interaction: short range Hubbard, extended Hubbard, and long-range Pariser–Parr–Pople models. The results show that in the intermediate to strong electron–electron interaction limit, the most stable geometry of the lowest optically accessible excitation is a self-trapped exciton (or a bipolaron), rather than separated solitons.
Classical binary nucleation theory applied to the real mixture n-nonane/methane at high pressures
Authors: Looijmans K. N. H. , Luijten C. C. M. , Hofmans G. C. J. , van Dongen M. E. H.
Published: July 2002

A thermodynamic model of the formation free energy of a droplet, based on a real equation of state, has been implemented in the binary classical nucleation theory to analyze homogeneous nucleation of mixtures of n-nonane and methane in the coexistence region at high pressures. The composition of the critical nucleus is computed by solving the Kelvin equations, the chemical potentials, and molar volumes being evaluated from the Redlich–Kwong–Soave equation of state. Real gas behavior appears to have a strong effect on nucleation due to mutual interactions between methane and nonane molecules. Numerical calculations show an increasing concentration of methane in critical clusters with increasing pressure at fixed temperature and supersaturation. As a consequence, the surface tension of the critical droplets, which is evaluated by means of the Parachor method, is lowered, having a strong increasing effect on the nucleation rate; a 10 bar increase of total pressure leads to an increase of the nucleation rate of several orders of magnitude.
The photodissociation of toluene studied by forward photofragment translational spectroscopy
Authors: Fröchtenicht Ralf
Published: July 2002

The translational energies of different fragments produced in laser induced unimolecular reactions are investigated in a molecular beam experiment. The time-of-flight of the photoproducts is measured in the forward direction with a mass spectrometer. By operating the ion source at low electron impact energies it is possible to nearly eliminate fragmentation and observe the nascent products at their parent mass. With this technique the primary and secondary reaction channels can be identified and branching ratios and product translational energy distribution down to small kinetic energies can be measured. The method is illustrated in a study of the photodissociation of toluene (methylbenzene) at three excitation energies (Eex=51 590, 52 240, and 63 240 cm−1). From the measured product time-of-flight spectra the branching ratio as well as the translational energy distributions of the product molecules benzyl+H and phenyl+CH3 in the two reaction channels were determined. The measured product energy distributions are compared with RRK (Rice–Ramsperger–Kassel) and SACM (statistic adiabatic channel model) statistical theories. The experimental results agree very well with the SACM theory for the phenyl, but not for the benzyl products. This is attributed to a barrier in the exit channel for the latter product.
An ab initio derived torsional potential energy surface for (H2O)3. I. Analytical representation and stationary points
Authors: Bürgi Thomas , Graf Stephan , Leutwyler Samuel , Klopper Wim
Published: July 2002

An intermolecular potential energy surface was derived for the hydrogen-bonded water trimer as a function of the three torsional angles ω1, ω2, ω3, for energies up to 1300 cm−1 (3.7 kcal/mol) above the global minimum. The O...O distances and the intramolecular geometry of the H2O molecules are held fixed. This surface is based on the ab initio calculations presented in a companion paper [W. Klopper et al., J. Chem. Phys. 103, 1085 (1995)], which involve very large basis sets and the most extensive treatment of correlation energy for calculations of (H2O)3 so far. The 70 ab initio interaction energies, multiplied by six due to the S6 symmetry of the surface, were fitted using an analytical potential function, with an average error of ≊11 cm−1. This potential provides a rapidly computable analytical expression for use in calculations of torsional eigenfunctions and -values and other properties of this cluster. Also given is a classification of the low-lying torsional wave functions according to nodal properties.
Comment on ‘‘Backward Euler and other methods for simulating molecular fluids’’ [J. Chem. Phys. 103, 3718 (1995)]
Authors: Schlick Tamar , Peskin Charles S.
Published: July 2002

As a balanced response to the Wu and Watts critique of two novel methods for molecular dynamics, the record is set straight with respect to LIs damping, the mixing of two different aspects of LI, the effects of quantum mechanics, and the performance of LIN.
Improved computational strategy for the state-selective coupled-cluster theory with semi-internal triexcited clusters: Potential energy surface of the HF molecule
Authors: Ghose Keya B. , Piecuch Piotr , Adamowicz Ludwik
Published: July 2002

The recently developed state-selective (SS) multi-reference coupled-cluster (CC) method involving all singly and doubly, and semi-internal triply excited clusters from the formal reference configuration [SSCCSD(T) approach] is tested in the calculation of the potential energy surface (PES) of the HF molecule. Both double zeta and double zeta plus polarization basis sets are employed and a few different choices of active space are considered. The SSCCSD(T) method provides an accurate description of the entire PES at low cost even for the bond breaking region, contrary to the results obtained with the perturbative single-reference CCSD(T) method or various limited configuration interaction approaches. This is the first application of the new SSCC code, which uses an improved computational strategy for handling the semi-internal triexcited clusters. Details of this new implementation of the SSCCSD(T) method are discussed.
Finite-size scaling behavior of the free energy barrier between coexisting phases: Determination of the critical temperature and interfacial tension of the Lennard-Jones fluid
Authors: Hunter John E. , Reinhardt William P.
Published: July 2002

General aspects of the finite-size scaling behavior of constrained free energy barriers between coexisting phases are discussed and illustrated with the two-dimensional Ising model. Particular attention is given to a method introduced by Binder [Phys. Rev. A 25, 1699 (1982)], in the context of lattice models, for extracting the interfacial tension based on an extrapolation to the thermodynamic limit of the barrier height divided by a quantity related to the total interfacial area. These ideas are then applied to a 3D Lennard-Jones system. The height of the constrained free energy barrier ΔF(V) separating coexisting gas and liquid phases in a Lennard-Jones fluid is determined for various values of the temperature T and number of particles N, using an isothermal-isobaric Monte Carlo simulation in conjunction with biased sampling and reweighting techniques. The critical temperature Tc=1.32 is readily established even from the results for very small systems by observing the value of T for which ΔF(V) is essentially independent of N. The extrapolation of the effective interfacial tension to the thermodynamic limit using Binder’s method is complicated because the data display a non-monotonic N-dependence, similar to that observed recently in the 3D Ising model [Berg, Hansmann, and Neuhaus, Z. Phys. B 90, 229 (1993)]. This behavior appears to arise at least in part from interactions between the two interfaces in the periodic simulation box. A self-consistent fit of all of the effective interfacial tension data is made based on a finite-size scaling ansatz and assuming the standard critical exponent μ=1.26; this leads to a value of γ0=2.79 in the universal expression for the surface tension γ=γ0(1−T/Tc)μ, which is in reasonable agreement with the expected value. Definitive extrapolations will require substantially larger simulations.
CO interactions with ferromagnetic surfaces
Authors: Getzlaff Mathias , Bansmann Joachim , Schönhense Gerd
Published: July 2002

Electronic and magnetic interactions of carbon monoxide with different ferromagnetic substrates (amorphous materials, crystals, and thin films) are investigated by means of angle and spin resolving photoelectron spectroscopy. On none of these surfaces, an exchange splitting of the CO orbitals is present indicating only a weak magnetic interaction of the substrate d bands with the occupied molecular orbitals. Via the spin analysis, a differentiation between molecular adsorption and dissociation was possible.
Thermodynamic perturbation theory for fused hard-sphere and hard-disk chain fluids
Authors: Zhou Yaoqi , Hall Carol K. , Stell George
Published: July 2002

We find that first-order thermodynamic perturbation theory (TPT1) which incorporates the reference monomer fluid used in the generalized Flory–AB (GF–AB) theory yields an equation of state for fused hard-sphere (FHS) chain fluids that has accuracy comparable to the GF–AB and GF–dimer–AC theories. The new TPT1 equation of state is significantly more accurate than other extensions of the TPT1 theory to FHS chain fluids. The TPT1 is also extended to two-dimensional fused hard-disk chain fluids. For the fused hard-disk dimer fluid, the extended TPT1 equation of state is found to be more accurate than the Boublík hard-disk dimer equation of state.
Bridge-assisted electron transfer driven by dichotomically fluctuating tunneling coupling
Authors: Goychuk I. A. , Petrov E. G. , May V.
Published: July 2002

The influence of dichotomically fluctuating tunneling coupling on long-range electron transfer is studied theoretically. Within an approach similar to the noninteracting blip approximation known from the spin-boson model a set of coupled integrodifferential kinetic equations is derived. These equations describe the time development of the electronic populations difference between the donor and acceptor states averaged with respect to the stochastic process and the quantum fluctuations of the bath. Furthermore, they contain the correlator between the level population difference and the fluctuating tunneling matrix element. A detailed analysis is carried out for the case of a strong coupling of the transferred electron to a single soft reaction coordinate. Within a Markovian approximation and an adiabatic removing of the correlator, the balance type kinetic equations can be derived which contain effective transfer rates. These rates depend strongly on the correlation time of fluctuations and can exhibit a resonancelike behavior.
Theory of nonlinear optical experiments with harmonic oscillators
Authors: Fourkas John T. , Kawashima Hitoshi , Nelson Keith A.
Published: July 2002

We develop a theory of photon-echo phenomena in harmonic vibrational modes. Although classical harmonic oscillators cannot produce any nonlinear optical signal in the linear response limit, we demonstrate that quantum harmonic oscillators that are coupled to any physically reasonable bath can give rise to novel nonlinear optical behavior, even in the perturbative limit. We show that photon echoes in high-frequency vibrational modes are strongly affected by both population relaxation and pure dephasing. The time dependence of the echo signal is shown to be highly sensitive to the amount of inhomogeneous broadening in the vibrational line. As an example, we develop the simple model of population relaxation resulting from linear coupling to the bath and pure dephasing resulting from quadratic coupling to the bath. Counter to the classical picture, echo signal is present when the only coupling to the bath is linear, but absent when the only coupling is quadratic.
Reaction of atomic oxygen with adsorbed carbon monoxide on a platinum surface
Authors: Ree J. , Kim Y. H. , Shin H. K.
Published: July 2002

The reaction of gas-phase oxygen atoms with carbon monoxide molecules adsorbed on a platinum surface is studied by the use of the classical trajectory approach. Collisions taking place at gas temperature 300 K are considered as a function of the incident angle. Gas atoms approaching CO in the angle range of 0°–50° are very efficient at producing vibrationally excited CO2 molecules in the gas phase. Beyond 50°, the extent of desorbing CO2 formation decreases rapidly and becomes negligible as the incident angle approaches 90°. Most of the exothermicity of the reaction O+CO→CO2 minus the CO–surface-binding energy appears to be transferred to the asymmetric stretching vibration of the desorbing CO2. The fraction of reactive collisions producing molecules having vibrational energies corresponding to levels v3=9 to 13 is found to be very high and exhibits a vibrational population inversion. Molecular time scale trajectory calculations show that relatively few atoms making up the solid are needed to obtain reliable data on energy transfer to the solid. The behavior of ensembles at various reaction times is discussed in detail. The surface is considered to be at 0 K.
The role of localization in glasses and supercooled liquids
Authors: Bembenek Scott D. , Laird Brian B.
Published: July 2002

Localized excitations (tunneling modes, soft harmonic vibrations) are believed to play a dominant role in the thermodynamics and transport properties of glasses at low temperature. Using instantaneous normal-mode (INM) analysis, we explore the role that such localization plays in determining the behavior of such systems in the vicinity of the glass transition. Building on our previous study [Phys. Rev. Lett. 74, 936 (1995)] we present evidence that the glass transition in two simple model systems is associated with a transition temperature below which all un- stable INM’s become localized. This localization transition is a possible mechanism for the change in diffusion mechanism from continuous flow to localized hopping that is believed to occur in fragile glass formers at a temperature just above Tg.
State-selective multireference coupled-cluster theory: In pursuit of property calculation
Authors: Ghose Keya B. , Piecuch Piotr , Pal Sourav , Adamowicz Ludwik
Published: July 2002

In this work, we examine the efficiency of the recently developed [P. Piecuch et al., J. Chem. Phys. 99, 6732 (1993)] state-selective (SS) multi-reference (MR) coupled-cluster (CC) method for calculation of molecular properties. In our earlier papers, we demonstrated that the SSMRCC method with inclusion of single, double, and internal and semi-internal triple excitations [SSCCSD(T) approach] is capable of providing an accurate description of the ground-state potential energy surfaces. In this paper, we present the dipole moment and polarizability values of the HF molecule at equilibrium and stretched geometries calculated using finite field technique and SSCCSD(T) ansatz. The calculations use double zeta quality basis sets with and without polarization functions. Molecular orbital basis sets include both relaxed and nonrelaxed orbitals.
Production of the photoacoustic effect and transient gratings by molecular volume changes
Authors: Chen Huxiong , Diebold Gerald J.
Published: July 2002

Changes in molecular volume arising from photochemical reactions can produce acoustic waves in a manner analogous to heat release. Here a derivation of two coupled differential equations for the state variables is given for such molecular volume changes based on the linearized hydrodynamic equations for fluids. The results of the calculation are given in terms of a chemical expansion coefficient and a chemical potential, the former describing the change in fluid density and the latter the change in the energy per mass of the fluid on conversion of one chemical species to another. The coupled equations show that chemical changes act analogously to temperature changes in the launching of acoustic waves. Wave equations are derived from the coupled equations. Application of the theory to the problems of generation of sound wave generation by photodissociation of a diatomic molecule in the gas phase, production of transient gratings in solution through exponential change in the concentration of a chemical species, and production of the photoacoustic effect with a Gaussian laser beam is given.
Motion of star-branched vs linear polymer: A Monte Carlo study
Authors: Sikorski Andrzej , Romiszowski Piotr
Published: July 2002

Monte Carlo simulations of lattice model of branched polymer chains were performed. The object of this simulation was a uniform star-branched polymer of equal length which was confined to a simple cubic lattice and consisted of f=3 arms of equal length. An efficient dynamic Monte Carlo algorithm was used to study the motion of a single athermal chain. Simulations were performed for star-branched chains with and without the excluded volume and for linear polymers in the same range of molecular weights. The maximum total chain length was 800 beads in both cases. The detailed comparison of static and dynamic properties of star-branched and linear polymers was made via the analysis of some new correlation functions as well as widely used parameters. The differences in the motion of star-branched and linear chains were described and discussed.
Octatetraene m 1A
g states: Two-photon fluorescence excitation spectrum from 28 000 to 50 000 cm−1
Authors: Kohler Bryan E. , Terpougov Victor
Published: July 2002

The 2-photon fluorescence excitation spectra of all-trans octatetraene in n-hexane and n-octane host crystals at 77 and 1.8 K have been recorded for the region 28 000 to 50 000 cm−1. Measurements of the dependence of emission signal on excitation intensity and fluorescence lifetimes contributed to the assignment of the observed features. On the basis of their frequencies, intensities, and solvent shift behaviors all of the bands are seen in the region 28 000 to 44 000 cm−1 are assigned to vibronic bands of the 1 1Ag–2 1Ag transition. The unusual temperature dependence of one excitation feature is consistent with a near degeneracy of singlet and triplet levels. Above 44 000 cm−1 there are four broadbands, two of which must be assigned to 0–0 transitions from the 1 1Ag ground state to higher lying m 1Ag states at 45 030 and 46 710 cm−1, respectively. The other two bands at 48 000 and 49 860 cm−1 could be either vibronic additions to the 0–0 band at 46 710 cm−1 or 0–0 bands for new m 1Ag states.
Small-angle neutron scattering of D2O–C12E5 mixtures and microemulsions with n-octane: Direct analysis by Fourier transformation
Authors: Strey R. , Glatter O. , Schubert K.-V. , Kaler E. W.
Published: July 2002

Spherical, cylindrical, and lamellar microstructures were examined by (SANS) in the binary system water (D2O)/pentaethylene glycol mono-perdeutero-n-dodecyl ether (C12D25E5) and in the ternary system (D2O)/pentaethylene glycol mono-n-dodecyl ether (C12E5)/perdeuterated n-octane (C8D18). A model-independent picture of the structures emerges from Fourier transformation of the measured SANS spectra. The data analysis makes no a priori assumptions about the type of structure. Rather, the pair distribution functions obtained are so unique that an unambiguous assignment of the local geometry is possible. The different structures for a given mixture are obtained by changing temperature. For water-rich samples the sequence of spherical, cylindrical, and planar structures is obtained with increasing temperature. For oil-rich samples the same sequence occurs as a function of decreasing temperature. As an important additional result, the scattering length density profile normal to the interface is obtained. The interfacial profile is found to be rather diffuse, apparently because of solvent penetration.
The photodissociation threshold of NO2: Precise determination of its energy and density of states
Authors: Jost Rémy , Nygård Jesper , Pasinski Adam , Delon Antoine
Published: July 2002

The NO2 photodissociation threshold has been observed by laser induced fluorescence in a jet at 25 128.57±0.05 cm−1. The unexpected high vibronic level density of 2.7 levels/cm−1 is tentatively attributed to long range interactions between the NO (2Π1/2) and O(3P2) products. The energy spacing and intensity distributions indicate rovibronic chaos.
Swelling/shrinking kinetics of chemically cross-linked poly(vinyl alcohol) gels in the presence of borate ions
Authors: Shibayama Mitsuhiro , Uesaka Masao , Shiwa Yasuhiro
Published: July 2002

The swelling/shrinking kinetics of chemically cross-linked poly(vinyl alcohol) (PVA) gels with borate ions has been studied by measuring the gel size as a function of time. The experimental process consisted of two steps, i.e., (I) immersion of a gel in a mixture of NaOH and boric acid aqueous solutions, and (II) subsequent immersion in deionized water or in a NaOH solution without borate ions. In step I, the gel swelled or shrank depending on the borate ion concentration and reached individual equilibrium values within 5000 min. After equilibration, the gel was transferred to the second bath and step II was initiated. When a gel was immersed in deionized water, the gel swelled considerably and then deswelled gradually to the original size. On the other hand, when a gel was dipped in a bath containing NaOH, a reverse process of step I took place and the gel size gradually reduced to the original size. The diffusion coefficients in these steps are discussed in conjunction with those obtained by dynamic light scattering.
Expanded grand canonical and Gibbs ensemble Monte Carlo simulation of polymers
Authors: Escobedo Fernando A. , de Pablo Juan J.
Published: July 2002

A novel formalism is presented for simulation of polymers in expanded grand canonical and expanded Gibbs ensembles. Molecular creation and destruction attempts are replaced by transition attempts between states of a tagged chain of variable length. Results are presented for expanded grand canonical simulations of hard-core chain fluids in the bulk and in a slit pore and for expanded Gibbs ensemble simulations of vapor–liquid equilibria for square-well chains.
Structural relaxation in nitromethanelike molecules
Authors: So/rensen Georg Ole
Published: July 2002

In a molecule with a large-amplitude coordinate like the internal rotation angle of the methyl hydrogens relative to the CNO2 fragment in nitromethane the other structural parameters are not strictly constant, but vary slightly with the angle. This behavior is usually described by saying that relaxations of the vibrational coordinates take place, and it is observed by the presence of special terms in the Hamiltonian needed in fitting the spectra. It is shown how relaxations can be treated theoretically by a perturbation method. Subsequent quantitative analysis of the effects in nitromethane indicates that the dominant relaxations involve the rocking and wagging coordinates of the nitro group. The maximum rocking relaxation in the staggered configuration is found to be θr≊4°, whereas the maximum wagging relaxation in the eclipsed configuration is smaller, θw≊0.7°.
A systematic strategy for design of optimum coherent experiments applied to efficient interconversion of double- and single-quantum coherences in nuclear magnetic resonance
Authors: Nielsen N. C. , Tho/gersen H. , So/rensen O. W.
Published: July 2002

A systematic strategy for design of optimum experiments in multidimensional coherent spectroscopy is described and applied to nuclear magnetic resonance (NMR). First an appropriate bound on the quantum dynamics is employed to determine a theoretical upper limit on coherence transfer efficiency which translates into the sensitivity of the experiment. Next it is investigated whether the theoretical limit can be achieved by a series of selective rotations. Then the final step serves to convert the selective rotations found into a nonselective pulse experiment which has a more general applicability. The example chosen for illustration is double- to single-quantum coherence transfer in INADEQUATE which is one of the most popular NMR experiments. Using the design strategy presented we show how the sensitivity of INADEQUATE can be doubled.
Magnetic field effects on the single-rovibronic-level fluorescence of S1(1B1) pyrimidine: Study of the singlet–triplet coupling by level anticrossings and quantum beats
Authors: Wang Pei-Nan , Liu Haisheng , Lim E. C.
Published: July 2002

Level anticrossings and quantum beats have been observed for the fluorescence from single rotational levels of the vibrationless S1 pyrimidine in a supersonic free jet. The density of triplet levels (ρ) and the coupling matrix elements (v) deduced from the data obtained for R(0) transition confirm that the singlet–triplet coupling belongs to the weak coupling limit (vρ≪1) of interstate interaction, in which electronic relaxation from the prepared singlet state to the triplet manifold does not occur in the absence of collisions.
Quantum statistical mechanical simulation of the ion–water cluster I−(H2O)n: The importance of nuclear quantum effects and anharmonicity
Authors: Gai Huadong , Schenter Gregory K. , Dang Liem X. , Garrett Bruce C.
Published: July 2002

Monte Carlo simulations of quantum statistical mechanical properties using the Feynman path integral method were carried out at temperatures of 70, 100, 200, and 300 K to study the structure and energetics of the ion–water cluster I−(H2O)n, with n=1–6. Simulation results at low temperatures (i.e., 70 K) can be directly compared with data from photoelectron spectroscopy (PES). The temperature dependence of binding enthalpies and stabilization energies indicate that at low temperatures the classical description of nuclear motion is qualitatively incorrect. The binding enthalpies are also computed within the harmonic approximation using the energy and frequencies at the minimum energy geometry, a method widely used in conjunction with electronic structure calculations to extend zero temperature information to finite temperatures. It is found that the harmonic approximation works well for the cluster with a single water molecule, but not for clusters with more than one water molecule, where the temperature dependence of binding enthalpies is in qualitative disagreement with the exact enthalpy obtained from simulation.
Interplay of multiple vibrational spectral densities in femtosecond nonlinear spectroscopy of liquids
Authors: Khidekel Vadim , Chernyak Vladimir , Mukamel Shaul
Published: July 2002

The multimode Brownian oscillator model of nonlinear response functions is generalized to include a multielectronic level system interacting with several spectral densities representing solvent and vibrational coupling to electronic energies, transition dipoles, and permanent dipoles. Applications to resonant and off-resonant transient grating as well as to infrared and fifth-order Raman photon echoes illustrate how the various spectral densities may be probed separately.
Born–Oppenheimer type separation in the study of the dynamics: Application to photodetachment of ClHCl− and ClDCl−
Authors: Rougeau N. , Marcotte S. , Kubach C.
Published: July 2002

The Born–Oppenheimer type separation between light and heavy nuclear motions is applied to the study of the dynamics of the ClHCl and ClDCl systems. Approximate and elaborate approaches are used to calculate the photodetachment spectra of the ClHCl− and ClDCl− anions. The approximate approach is based on a Born–Oppenheimer approximation where the nuclear wave function of the ClHCl or ClDCl systems is obtained as a single product of two wave functions which describe the motion of the light H(D) nucleus for clamped chlorine nuclei and the motion of the heavy chlorine nuclei, respectively. The elaborate approach is a multistate description which goes beyond the Born–Oppenheimer approximation. The relevant close coupling equations are solved numerically subject to proper photodissociation asymptotic conditions. The results obtained in this approach confirm the validity of the approximate one and provide additional information on the ClH+Cl and ClD+Cl decay channels. Resonances are obtained at energies of 0.640 eV and 0.899 eV for the ClHCl system, and at 0.538 eV and 0.715 eV for the ClDCl system. They are identified as shape or Feshbach resonances and analyzed in terms of the normal modes of a linear symmetrical triatomic molecule.
Analytical derivatives of atomic zero-flux surfaces and properties of atoms in molecules with respect to external perturbations
Authors: Cioslowski Jerzy , Stefanov Boris B.
Published: July 2002

A complete theory of the response of the atomic zero-flux surfaces to external perturbations is presented. The resulting expressions for the analytical derivatives of the properties of atoms in molecules (AIMs) are implemented in an efficient computer code. Several test calculations demonstrate the clear advantages of the new approach over methods based upon numerical differentiation. The present development opens an avenue to routine calculations of the second-order response properties of AIMs.
Convergence of symmetry-adapted perturbation theory expansions for pairwise nonadditive interatomic interactions
Authors: Korona Tatiana , Moszynski Robert , Jeziorski Bogumil
Published: July 2002

Convergence properties of symmetry-adapted perturbation expansions for nonadditive interactions are tested by performing high-order calculations for three spin-aligned hydrogen atoms. It is shown that the strong symmetry forcing characteristic of the Hirschfelder–Silbey theory leads to a rapidly convergent perturbation series. The symmetrized Rayleigh–Schrödinger perturbation theory employing a weak symmetry forcing is shown to provide in low orders accurate approximations to the nonadditive part of the interaction energy. In very high orders the convergence of this perturbation expansion becomes very slow, and the series converges to an unphysical limit, very close to the exact interaction energy. The nonadditive part of the interaction energy for the lowest quartet state of H3 is interpreted in terms of the first-order exchange, induction, exchange-induction, exchange-dispersion, induction-dispersion, and dispersion contributions. It is shown that even for such a simple trimer the correct description of these components is necessary to obtain quantitative agreement with variational full configuration interaction results.
Hybrid DFT-MD simulations of geometry and hyperfine structure of the CCH radical in argon matrices at low temperatures
Authors: Eriksson Leif A. , Laaksonen Aatto
Published: July 2002

The thermal motion of the CCH radical embedded in a matrix of solid argon is simulated at 4 and 40 K, using a hybrid density functional theory—molecular dynamics (DFT-MD) approach. The DFT calculations are performed at the B3LYP/6-311G(d,p) level. It is concluded that the CCH molecule when embedded in the Ar matrix favors an oscillating, slightly bent geometric struct- ure, whereas in vacuum the molecule is linear. In the matrix at 4 K, the oscillations lie centered at a CCH bond angle of 170±5°. At 40 K far larger oscillations are noted (up to ±19° bending motion, centered at a 154° CCH angle), due to the increased thermal energy. As a consequence of the vibrational motion, the radical hyperfine structure becomes significantly modified, and agree far better with experimental data than do the linear optimized vacuum geometry results. The B3LYP/6-311G(d,p) computed vibrationally averaged isotropic couplings in an ordered Ar matrix at 4 K are 935, 173 and 42 MHz for C–C–H, respectively, to be compared with the experimental values (Ar matrix, 4 K) 902, 156 and 44 MHz, and the data for the B3LYP/6-311G(d,p) optimized linear structure in vacuum: 1043, 224 and 54 MHz. The present hybrid DFT-MD results also agree well with previous vibronically corrected MRDCI data.
Thermodynamic properties of critical clusters from measurements of vapour–liquid homogeneous nucleation rates
Authors: Ford I. J.
Published: July 2002

Two nucleation theorems are proved using small system thermodynamics. The first is well known and has been used before to determine the number of molecules in the critical nucleus controlling the nucleation of droplets from supersaturated vapours. The second appears to be new, and relates the temperature dependence of the nucleation rate to the excess internal energy of the critical cluster. An analysis of measured homogeneous nucleation rates can therefore provide the size, internal energy, free energy, and entropy of the critical cluster, which should provide important guidance for the construction of theoretical models of the process. This is illustrated using water, n-butanol and n-nonane nucleation data. While there is often a close correspondence between the droplet free energy and the form suggested by classical theory, the excess internal energy seems to show a linear dependence on molecular number, rather than the classical 2/3 power behaviour.
Thermal diffusion factors of polyatomic gas mixtures: He-, Ne-, Ar-N2, 20Ne-CH4, and isotopic sulfur hexafluoride
Authors: Taylor William L.
Published: July 2002

Results of thermal diffusion experiments on mixtures of gases where at least one component is a diatomic or polyatomic molecule are reported herein. The experiments were conducted in a 20-tube trennschaukel, or ‘‘swing separator,’’ over a temperature range of several hundred degrees. Of the two mixture characteristics which influence thermal separation of the components, Δmij/μij and the intermolecular forces between components, the former was varied over a considerable range in a search for uncharacteristic behavior of the thermal diffusion factor. Three mixtures containing noble gas-nitrogen were chosen for study because of recently available theoretical calculations of the thermal diffusion factor, which included inelastic collision effects for these systems. However, anomalous behavior was observed only in the isotopic system. The effect of the molecular forces was investigated by comparing theoretical calculations of the thermal diffusion factor versus the experimental results using the best intermolecular potentials available in the literature.
Dissociative electron attachment and intramolecular electron transfer in linear haloalkenes
Authors: Underwood-Lemons Theresa , Sághi-Szabó Gotthard , Tossell John A. , Moore John H.
Published: July 2002

Intramolecular electron transfer through alkyl chains has been investigated by measuring the cross section for halide detachment following resonant π* electron capture in linear n-halo-1-alkenes, for halo = chloro, bromo and n=2–6. The magnitude of the cross section decreases with increasing chain length for all the haloalkenes, with the exception of the halopropenes, but at a considerably faster rate for the chloro than for the bromo compounds. The decrease in cross section for the chloroalkenes occurs at a rate consistent with the decrease in electron-tunneling rates in hydrocarbons with through-bond interactions. For the bromoalkenes it appears that σ*–π* coupling is quite strong and thus the results are not consistent with an electron transfer interpretation. Excluding the propenes, the energy of the cross section maximum is essentially constant for the chloralkenes while it decreases with chain length for the bromoalkenes. Hartree–Fock calculations have been used to determine the equilibrium geometries of various conformers of the n-halo-1-alkenes for n=2–5. The 4- and 5-bromo-1-alkenes show considerably smaller conformational energy differences than do the chloro compounds. Electron attachment energies have been calculated for the most stable conformers at Hartree–Fock, density functional, and Mo/ller–Plesset second order levels. Trends in calculated attachment energies parallel experimental trends in the energies of dissociative attachment maxima only for the Mo/ller–Plesett second order calculations. At the Hartree–Fock level the singly occupied molecular orbitals of the haloalkene radical anions show a somewhat greater admixture of C–halogen σ* and C=Cπ* character in the bromo compounds than in the chloroalkenes, but the distinct difference in σ*–π* coupling in the bromoalkenes compared to the chloroalkenes is represented accurately only in the calculations that include electron correlation.
Multireference self-consistent size-extensive state-selective configuration interaction
Authors: Adamowicz Ludwik , Malrieu Jean-Paul
Published: July 2002

In this work, we propose a state-specific self-consistent ‘‘dressing’’ of the multireference configuration interaction (MRCI) space to include all single- and double-substituted determinants for the most important reference configurations. The aim of the method is to provide a size-extensive description of the dynamic electron correlation effects for states which mandate a multideterminantal reference wave function. Such states can represent electronic excited states or ground states of the molecular systems which are significantly deformed from their equilibrium structures. The proposed approach follows the concept introduced in our recently proposed quasilinear ansatz for the state-selective multireference coupled-cluster method. The purpose of the dressing procedure is to eliminate the contributions which introduce size-extensivity violating terms in the MRCI approach.
Vibrational relaxation rates of a polar molecule in polar liquids
Authors: Cho Minhaeng
Published: July 2002

Both the vibrational energy relaxation and pure dephasing of a polar solute in polar liquids are theoretically studied and particularly the role of the long-range Coulomb interaction between the solute and the polarization modes of the polar liquid is focused on in this paper. If the linear coupling potential between the vibrational displacement and the solvent polarization mode is taken into account and assuming that the anharmonic contribution to the pure dephasing is the dominant mechanism, the vibrational relaxation rates are shown to be related to the dielectric friction. However the pure dephasing of a perfect harmonic oscillator is an effect arising from the nonlinear (with respect to the vibrational coordinate) vibrational coupling potential, and the relationship between the pure dephasing rate and the frequency-dependent friction is no longer valid. By expanding the bare electric field of the polar solute in terms of the multipoles, the vibrational coupling potential is obtained as a power series summation with respect to the vibrational displacement. Each expansion coefficient is found to be associated with the interaction of the projected multipole fields with the solvent polarization. A few simple cases are discussed in detail with an emphasis on the connection between the vibrational relaxation rates and solvation dynamics of static multipoles in polar liquids.