bluuebunny/crossref_metadata_2025 huggingface dataset:
Diffusion-limited polymerization of rigid rodlike molecules: Semidilute solutions
Authors: Agarwal U. S. , Khakhar D. V.
Published: July 2002

The rotational diffusivity and the translational diffusivity perpendicular to the rod axis of rigid rodlike (RRL) molecules decreases rapidly with increasing molecular length, during polymerization in semidilute solutions. This can result in slowing of step-growth polymerizations of RRL molecules with reactive groups at the rod ends and a near-collinearity requirement for reaction. Here a theoretical analysis of the rate of RRL polymerization in semidilute solutions, based on Smoluchowski’s approach, and incorporating the rotational and anisotropic translational diffusion of the molecules is presented. The work is an extension of our analysis of polymerization in dilute solutions in which the translational diffusion was assumed to be isotropic [J. Chem. Phys. 96, 7125 (1992)]. The effective second order rate constant for the system is obtained for different parameter values using a numerical finite element method. With reduction in rotational diffusivity, for a fixed translational diffusivity, the effective reaction rate constant is found to decrease to a limiting value determined by only the translational flux of the correctly oriented molecules. Similarly, for a given rotational diffusivity, with reduction in translational diffusivity perpendicular to the rod axis, the reaction rate constant is found to decrease to a limiting value determined by the flux only due to translational diffusion parallel to the rod axis, aided by rotational diffusion. An asymptotic analysis for this case is presented. For low rotational diffusivities, reduction in the translational diffusion perpendicular to the rod axis results in a significant decrease in the effective rate constant, even for reactions with relatively slow intrinsic kinetics. A qualitative comparison of the theoretical predictions with experimental results is presented.
Absorbing boundary conditions by the partial integration exterior scaling method
Authors: Rom Naomi , Moiseyev Nimrod
Published: July 2002

The partial integration exterior scaling (PIES) method is developed. This method enables one to insert absorbing boundary conditions in finite basis set calculations in a rigorous way. The results are parameter independent when the basis set approaches completeness. Illustrative numerical examples are given for the calculations of resonance positions and lifetimes and of cumulative reaction probabilities.
Thermodynamical properties of ionized lithium oxide clusters, Li2n+pO+n
Authors: Bréchignac C. , Cahuzac Ph. , Carlier F. , de Frutos M. , Leygnier J. , Roux J. Ph.
Published: July 2002

Lithium oxide clusters Li2n+pOn+ are generated by combining reactive nucleation in a gas aggregation source and photoionization. Unimolecular dissociation of mass selected cluster ions provides evidence that the excess of metal atoms evaporates first leading to the most stable species Li+(Li2O)n, which then evaporate Li2O molecules. The evaporation rate behavior as a function of cluster size demonstrates that Li+(Li2O)n can be prepared with different temperatures. It is discussed how metal evaporation from metal-rich oxide clusters leads to oxygen saturated clusters with a lower temperature. An estimate of the dissociation energies of Li+(Li2O)n are given for small sizes n≤10 from photoevaporation experiment.
Theoretical study of the intramolecular isotope effect in the reaction of O(3P)+HD. The importance of molecular reorientation
Authors: Song Ju-Beom , Gislason Eric A.
Published: July 2002

A quasiclassical trajectory study has been made of the intramolecular isotope effect for the reaction of O(3P)+HD (v=0, J=0) on the potential energy surface of Johnson and Winter for relative energies E between 15 and 150 kcal/mol. In this energy range OD+H is the favored product channel. During each trajectory the OH and OD bond lengths were monitored to determine which end of the HD molecule was ‘‘hit’’ by the O atom. The results show unambiguously that the O atom exerts a torque on the molecule that rotates the H atom away from O, increasing the likelihood that O will hit and react with D. The effect of this torque increases as the HD molecule stretches during the reactive process. The reorientation of HD is important even at collision energies as high as E=100 kcal/mol. Plots of the potential energy surface for this reaction confirm the importance of the intermolecular torque exerted on HD by O.
Importance of polarization and correlation effects in ab initio prediction of the acetaldehyde methyl torsional barrier
Authors: Leszczynski Jerzy , Goodman Lionel
Published: July 2002

Rigorous coupled cluster ST4CCD and Mo/ller–Plesset fourth order MP4 predictions using a 6–311G(3df,2p) basis at MP2 optimized geometries are reported for the acetaldehyde torsional barrier. These calculations predict the barrier within 4 cm−1 of the measured 408 cm−1 barrier. They reveal the importance of extensive basis set polarization functions and accurate correlation corrections.
Orientation dependent interface mobilities in a kinetic mean field theory of freezing and melting
Authors: Williams Alison , Moss Richard , Harrowell Peter
Published: July 2002

The microscopic dynamics of freezing and melting at the 100 and 111 surfaces of a simple cubic crystal are examined. The kinetics are described using coupled Landau–Ginzberg equations based on a mean field approximation of the face centered cubic lattice gas with nearest and next nearest neighbor interactions. Anomalous large structural fluctuations are found at the equilibrium 100 surface, a result of the weak coupling between close packed 100 planes. This feature also gives rise to a large anisotropy in interface mobilities on freezing with the 100 surface mobility vanishing unless fluctuations or longer range interactions are included. The interface mobilities during melting are found to be substantially larger than those found during freezing for both surfaces, a result attributed to the asymmetry between crystal and liquid in the free energy functional.
An ab initio study of the HNCN radical
Authors: Tao F.-M. , Klemperer W. , McCarthy Michael C. , Gottlieb C. A. , Thaddeus P.
Published: July 2002

The equilibrium structure for the ground state of the HNCN radical is calculated at the levels of the self-consistent field theory (SCF), the second-order Mo/ller–Plesset perturbation approximation (MP2), and the full single and double excitation coupled cluster theory including all connected triples in a noniterative manner [CCSD(T)], using various extended basis sets starting from 6–311 G(d,p). At the CCSD(T) level, the outer C–N bond is more than 0.1 Å shorter than the inner one and the N–C–N group departs from linearity by 6°. The total N–C–N length is in good agreement with the experimental value [Herzberg and Warsop, Can. J. Phys. 41, 286 (1963)], however, the H–N–C angle is about 6° smaller. The N–H bond is very close to a normal N–H bond but is about 0.2 Å smaller than the experimental estimate. Except for the smaller H–N–C angle, the geometrical parameters for HNCN closely parallel those for the triplet HCCN molecule. The dipole moment, harmonic frequencies, electric quadrupole, and Fermi contact coupling constants of HNCN are also calculated. The calculated harmonic frequencies confirm the preliminary assignments of Wu, Hall, and Sears [J. Chem. Soc. Faraday Trans. 89, 615 (1993)]. The quadrupole coupling constants for the inner and outer N atoms are comparable, implying a complex pattern of hyperfine split components in the lowest rotational transitions. The present calculation may thus serve as a useful guide for the interpretation of the rotational spectrum.
Spin adapted restricted Hartree–Fock reference coupled cluster theory for open shell systems
Authors: Neogrády Pavel , Urban Miroslav , Hubac̆ Ivan
Published: July 2002

The coupled clusters (CC) method for effective calculations of open shell systems with the single restricted Hartree–Fock (ROHF) reference determinants is formulated. We apply the spin adaptation described in our previous work, aimed at removing the spin contamination in both coupled cluster (CC) amplitudes and CC energy, both for the linear and nonlinear versions of the single and double excitation coupled cluster (CCSD) method. We give a comparison of results with adapted and nonadapted methods. Together with the elimination of the spin contamination, our spin adaptation yields CC equations, which reduce the number of CC amplitudes and the number of arithmetical operations, which results in computational time comparable with analogous closed shell calculations. The complication in the full spin adapted CC method is that individual spin states require specific spin adapted excitations, so that for every spin state, a different implementation is needed. It is possible to define a very effective approximation to the full spin adapted model that is common to all half-spin states. The method is less complicated in implementation, while the computational requirements increase in comparison with the full spin adapted method only slightly. We also suggest how to introduce spin adaptation in a very simple way to existing ROHF CCSD programs.
An assignment of the structured features in the Hartley band absorption spectrum of ozone
Authors: Joens Jeffrey A.
Published: July 2002

Using recent experimental data for the Hartley band absorption spectrum obtained at low temperature, a vibrational analysis of the discrete structure in the spectrum is given. It is found that all of the peaks in the range 238–292 cm can be assigned to transitions between the (0,0,0) vibrational state in the ground electronic state and (v1′,v2′) vibrational states in the upper electronic state. The observed structure in the Hartley band is due to a long progression in the symmetric stretching mode, and a shorter progression in the bending mode. Based on the vibrational assignment, the following information is obtained on the vibrational parameters and geometry of ozone in the 1B2 electronic state (all parameters are given for C2v symmetry): ω1′ = 1108 ± 25 cm−1, ω1′x1′ = 9 ± 3 cm−1, ω2′ = 298 ± 12 cm−1, ω3′ = (1650 ± 300)i cm−1, r00=0.140±0.002 nm, φ=105±2°. The analysis that is presented demonstrates that residual vibrational structure can occur in dissociative transitions, confirming previous suggestions.
Excited states of MgO: A cluster model study
Authors: Bagus P. S. , Illas F. , Sousa C.
Published: July 2002

The character of low-lying excited states of MgO has been examined using cluster models of the MgO crystal. Several different clusters were used; in general, the clusters contained a central Mg atom, one or more shells of nearest-neighbor Mg and O atoms, and a large number of point charges to represent the Madelung potential in the space of the atoms explicitly included in the cluster. The excited orbital was occupied in two ways. First, an electron was added to a low lying excited level while the Mg2+ and O2− anion levels were kept fully occupied. In the second way, an electron was excited from the oxygen 2p band into a low lying excited level. The excited orbitals, for both of these cases are very similar. Although the energetics of the excited states depended on the cluster model, in particular whether it was Mg or O terminated, the general character of the excited orbital remained the same. In all clusters, the excited state orbital was diffuse and delocalized. The excited orbital has no simple or direct relationship to the 3s orbital of Mg2+. It is best described as a ‘‘conduction band’’ orbital. It extends up to and beyond the edge of the atoms explicitly included in the cluster. In our work, we did not find any evidence for localized excitonic states.
Identification of combination and overtone bands in the matrix infrared spectrum of NC3NC
Authors: Smith Alice M. , Bondybey Vladimir E. , Horn Matthias , Botschwina Peter
Published: July 2002

Ten overtone and combination bands of iso-dicyanoacetylene (NC3NC), produced by in situ pulsed ultraviolet laser photoisomerization of dicyanoacetylene in an argon matrix, have been assigned in our Fourier-transform absorption spectra. Experimental band frequencies, relative band intensities, and derived anharmonicity constants are compared in detail to CEPA-1 theoretical calculations. The agreement here, even for overtone band intensities, is excellent.
Role of adsorbate interactions in surface dynamics and phase transformations: Mean-field and quasichemical approximation approaches
Authors: Datar Asha , Prasad S. D.
Published: July 2002

The significant influence of adsorbate interactions in surface dynamics is quantified using mean-field approximation (MFA) and quasichemical approximation (QCA) approaches and two typical situations (i) T≳Tc (critical temperature for surface phase transformation) and (ii) T<Tc are analyzed. The formulation involves transition state theory (TST) and the key parameters involved are: (1) the sign and magnitude of the pairwise adsorbate interaction energy (w≳0, w<0 meaning repulsive and attractive interactions, respectively) (2) wA♯, the interaction energy between a molecule in the ground state and the activated complex. wA♯A is in turn related to w by a coupling parameter σ. σ=0, σ=1 are shown to result in extreme divergence of the rate behavior for both repulsive and attractive interactions. First T≳Tc is considered. For σ=0, attractive interactions retard and repulsive interactions enhance the surface rates. The rates display nonmonotonic behavior for attractive interactions and steady increase with surface coverage for repulsive interactions. However, when σ=1, the rates monotonically increase for both types of forces. In addition the attractive forces show an instability of the slope due to a cooperative catalytic effect. Both attractive and repulsive forces display maxima when plotted against temperature, the maxima being sharper for the former case.

The case T<Tc is more interesting, as a discontinuous phase separation can occur for attractive interactions. The density and internal energy differences between the coexisting phases are computed proceeding from closed-form expressions of the canonical ensemble partition functions and employing standard methods of statistical mechanics. Since repulsive forces can only show continuous order–disorder transitions, they are not considered for T<Tc. The surface rate expressions (both corrected and uncorrected for ground-state internal energy differences between the phases) display a symmetric rate curve (symmetric about θ=0.5) vs surface coverage with a maximum at θ=0.5. A certain type of hole-particle symmetry is present in the rate expression as the rate expression is invariant with respect to the exchange of an occupied and vacant site. This conclusion is valid for both σ=0, σ=1. The appearance of symmetry in the rate curve is suggestive of the phase separation. The qualitative differences between the rate predictions of MFA and QCA are significant enough to warrant refinement in the analysis of surface dynamics.
Elementary presentation of self-consistent intermediate Hamiltonians and proposal of two totally dressed singles and doubles configuration interaction methods
Authors: Malrieu J. P. , Nebot-Gil I. , Sanchez-Marin J.
Published: July 2002

Intermediate Hamiltonians are effective Hamiltonians which are defined on an N-dimensional model space but which only provide n<N exact eigenvalues and the projections of the corresponding eigenvectors onto the model space. For a single root research, the intermediate Hamiltonian may be obtained from the restriction of the Hamiltonian to the model space by an appropriate, uniquely defined dressing of the diagonal energies or of the first column. Approximate self-consistent dressings may be proposed. The simplest perturbative form gives the same result as the original 2nd order intermediate Hamiltonian or the ‘‘shifted Bk’’ technique but it is of easier implementation. Self-consistent inclusion of higher order exclusion principle violating corrections greatly improves the results, especially for nearly degenerate problems, as shown on several illustrative applications. Possible generalizations to enlarged or reduced model spaces are discussed.
On the radiative lifetime of the (a 4Σ−,v,N,F
i) levels of the CH radical: An ab initio treatment
Authors: Hettema Hinne , Yarkony David R.
Published: July 2002

The spin–forbidden radiative decay process CH(a 4Σ−)→CH(X 2Π) is studied using the full Breit–Pauli spin–orbit Hamiltonian and multireference configuration interaction wave functions comprised of 250 000–350 000 configuration state functions. The radiative lifetime of the (a 4Σ−,v,N,Fi)→X 2Π transitions were determined in a Hund’s case (b) approximation. The pure radiative lifetimes are found to be quite long, on average being 12, 10, and 8 s for v=0,1,2, respectively.
Charge separation and covalent bonding in metal oxide surfaces: A local density functional study on the MgO(001) surface
Authors: Birkenheuer U. , Boettger J. C. , Rösch N.
Published: July 2002

A first principles local density functional investigation on extended, two-dimensional periodic slab models of the MgO(001) surface is performed, using the linear combination of Gaussian-type orbitals (LCGTO) technique as implemented in the FILMS program package. Stimulated by recent theoretical evidence for a reduced charge separation in MgO(001), a detailed analysis of the charge distribution and its influence on the electrical field above the surface is carried out. Two different methods to quantify the charge separation in the ionic substrate are employed, a local one based on the topological atom approach and a global one derived from the Madelung field of the surface near potential adsorbates. Both procedures lead to a charge separation significantly (10%–20%) below the nominal ionic value of ±2 a.u. A variational atomic orbital analysis is utilized to discuss the origin of the Mg 3s and 3p structures discernible in the crystal orbitals of the MgO slab systems. They are identified as covalent magnesium valence orbital admixtures to the oxygen dominated valence bands in consistence with the reduced charge separation. Their influence on the cohesive energy of crystalline MgO, however, is found to be only 5% (∼0.5 eV).
Frequency-modulation enhanced magnetic rotation spectroscopy of PdH, PdD, NiH, and CuH
Authors: McCarthy Michael C. , Field Robert W.
Published: July 2002

The technique of frequency-modulation enhanced magnetic rotation spectroscopy (FM-MRS) is applied here to the study of the electronic spectra of gas-phase transition metal hydrides and deuterides. Experimental FM-MR results for several bands of PdH, PdD, NiH, and CuH are presented. In the case of PdH, an Ω=0.5–XΩ=0.5 band at 4371 Å has been observed and assigned. Prior to this work, although several bands of PdD had been reported previously, no electronic transitions of PdH had been assigned. All of the PdH, PdD, and NiH bands studied in the 4200–4600 Å wavelength region are strongly predissociated, and in the case of PdH and PdD, strongly and multiply perturbed as well.
Numerical methods for locating stable periodic orbits embedded in a largely chaotic system
Authors: Marcinek Roman , Pollak Eli
Published: July 2002

Monte Carlo methods are combined with a Newton method to construct an efficient numerical procedure for locating stable periodic orbits embedded in a largely chaotic system. We find that the Newton method effectively enlarges the basin of attraction of the stable orbit by orders of magnitude relative to the stable region surrounding the orbit. Three variants of the Newton method are tested. We conclude that an all-points finite difference version is the optimal choice. Use of a Monte Carlo search with importance sampling and combined with the Newton method proves to be the most efficient search procedure. Application to the two and three dimensional quartic oscillator leads to previously unknown stable orbits.
Broadband axialization in an ion cyclotron resonance ion trap
Authors: Guan Shenheng , Wahl Markus C. , Marshall Alan G.
Published: July 2002

A novel broadband ion axialization method has been developed for Fourier transform ion cyclotron resonance mass spectrometry based on Penning trapping of ions. Ions of arbitrary mass-to-charge ratio range(s) may be driven to the center of the trap (axialization) by azimuthal quadrupolar irradiation with repeated low-amplitude stored-wave-form inverse Fourier transform excitations in the presence of a buffer gas. We demonstrate highly mass-selective axialization and subsequent high-resolution Fourier transform ion cyclotron resonance detection of ions spanning a mass-to-charge ratio range of 500.
Calculation of the intermolecular bound states for water dimer
Authors: Althorpe S. C. , Clary D. C.
Published: July 2002

The intermolecular bound states of (H2O)2 are calculated using a simple approach previously found successful for (HF)2. The monomer bond lengths and bond angles are held fixed, and the angular part of the Hamiltonian is solved variationally at three values of the intermolecular radial coordinate. The results enable comparisons of the tunneling splittings obtained from three potential energy surfaces to be made with experiment. Estimates of three of the intermolecular vibrational frequencies are also obtained.
Rotational motion and the dissociation of H2 on Cu(111)
Authors: Darling G. R. , Holloway S.
Published: July 2002

The influence of rotational state on the dissociation probability of H2 on Cu(111) has been investigated with 3- and 4-dimensional close-coupling wave packet calculations. Recent experimental results have shown that the energetic threshold for dissociative adsorption increases and then decreases as the J state is continuously increased. This trend can be faithfully reproduced by modeling the H2 as a planar (cartwheel) rotor scattering from a flat surface. The agreement disappears when the model is extended to a 3-dimensional rotor. Further, the degenerate mJ states have a spread of dissociation probabilities which results in a broad smearing of the dissociation threshold. This effect, which is absent from experiment, increases with Ji. These shortcomings can be partially corrected by corrugating the potential in the azimuthal coordinate in accord with recent ab initio results. The dynamical calculations also exhibit strong rotational inelasticity for the scattered fraction, during dissociation. Since this system has a late barrier for dissociation, we show that the rotational inelasticity should be enhanced by initial vibrational state. Our 4-dimensional modeling is unable simultaneously to match the relative positions of dissociation and vibrational excitation thresholds. We speculate that these processes occur on different surface sites.
The reactive flux method in the energy diffusion regime. I. Effect of slow vibrational energy relaxation
Authors: Tucker Susan C.
Published: July 2002

The Mori–Zwanzig memory function formalism is used to determine the behavior of the solute reactive flux in the energy diffusion regime. We show formally that in the case of slow vibrational energy relaxation the reactive flux can exhibit two plateaus corresponding to two phenomenological rate constants. The second of these plateaus corresponds to the overall solute reaction rate, while the first plateau is a ‘‘false’’ plateau. Such a scenario is equated with a Lindemann-type reaction phenomenology corresponding to two different energy relaxation mechanisms. Time scale arguments are used to show that such a scenario exists in an example of a cubic reaction coordinate coupled to a slow Gaussian friction bath. The geminate recombination of I2 in solution provides experimental confirmation that such behavior also occurs in real chemical systems.
Quantum calculations of unusual mode specificity in H+C2H2→H2+C2H
Authors: Wang Desheng , Bowman Joel M.
Published: July 2002

We report reduced dimensionality coupled channel scattering calculations of rotationally averaged cross sections for the H+C2H2↔H2+C2H reaction. A new ten degree-of-freedom potential is developed for use in these calculations. This potential surface is based on a previous potential for C2H2 and C2H as well as previous ab initio calculations of the saddle point properties. We focus on the effect of exciting the symmetric and antisymmetric CH stretches, and symmetric CC stretch in C2H2 on the reactivity, and also on the vibrational distribution of the H2 and C2H products. A significant and surprising finding is that excitation of the CC stretch in combination with excitation of either the symmetric and/or antisymmetric stretch increases the reactivity significantly. A simple Franck–Condon model is used to rationalize these mode specific effects. The thermal rate constant for the H2+C2H→H+C2H2 reaction is also calculated in the temperature range from 200 to 400 K, and compared to limited experimental data.
A proposal for the thermodynamic and kinetic modeling of chemical reactions in cluster formation: Illustrated by soot formation in low pressure flames
Authors: Vogelsberger Wolfram
Published: July 2002

A phenomenological treatment for the thermodynamic and kinetic description of chemical reactions in the formation of clusters is developed. This theory is valid for isothermal processes in closed systems. It takes into account the consumption of the reactants as the reaction proceeds. The surface of the Helmholtz free energy of the reaction is calculated dependent on cluster size and cluster concentration. This surface has a bottom; the bottom begins in the initial state of the system (only reactants are present). It ends at the stable equilibrium (bulk phase in equilibrium with reactants and products). The bottom is assumed to be the trajectory of the reaction from the initial state to the stable equilibrium of the system. A kinetic rate equation for the reaction is derived. The rate equation is tested by experimental results. These results are obtained for the combustion of ethine in a low pressure flame by a molecular beam mass spectrometrical technique. Good agreement is found between the experimental results and the solution of the rate equation. Various parameters of the reaction can be calculated.
Excess electron states on the microsurfaces of Ne and H2 clusters
Authors: Rosenblit Michael , Jortner Joshua
Published: July 2002

In this paper we report on the ground and excited electronic states of localized excess electron surface states of (Ne)−N (N=1.1×104–6×1023) and (H2)−N (N=4.6×103–6×1023) clusters. We used an electron-cluster model potential, which consists of a short-range repulsive interaction with a strength V̄0 [with a lower limit V̄0 (≳0) corresponding to the energy of the quasifree electron in the macroscopic condensed material], and a long-range attractive polarization potential, to explore cluster size effects on the energetics and on the charge distribution of these excess electron clusters. The onset of the cluster size for excess electron localization in the ground (n=1, l=0) electronic state was inferred from a near-threshold scaling analysis, being characterized by a ‘‘critical’’ cluster radius R(1,0)c≂2(1−Q)a0/Q, where Q=(ε−1)/4(ε+1) is the effective cluster charge (for the cluster dielectric constant ε), R(1,0)c=39 Å for Ne(s), R(1,0)c=46 Å for Ne(l), R(1,0)c=35 Å for H2(s) and R(1,0)c=41 Å for H2(l), where (s) and (l) denote rigid and nonrigid cluster structures, respectively. With a further increase in the cluster radius R≳R(1,0)c, higher nl electronic states become localized.

Moving up in the cluster size above the localization threshold, the energy levels Enl can be expressed (for low values of ε≤1.5) in terms of a ‘‘universal’’ scaling relation Enl/Ef=Φnl(rf/R), where Ef=(e2/2a0)Q2, rf=a0/Q and the scaling function Φnl is independent of ε. This scaling relation allows for the determination of isotope effects and the state of aggregation of the cluster on the energetics of electron localization. In order to make contact with experiment, we have studied electric field-induced ionization and the electronic spectroscopy of these excess electron clusters. The threshold dc electric field F(nl)c for field-induced ionization from the n,l state (over a broad range of R, i.e., R<320 Å for the 1s and 1p states and R<900 Å for the 2p state) is of the form F(nl)c∝‖Enl‖5/4 (ε−1)−1/4R−3. Information on electronic spectroscopy was inferred from the cluster size dependence of the transition energies and oscillator strengths for the 1s(n=1,l=0)→n′p(n′=1,2,...,l=1) transitions. The cluster size dependence of the spectroscopic data for the 1s→1p transition reveals that both the transition energy ΔE(1s→1p) and the oscillator strength f(1s→1p) are proportional to (1/R)2, with ΔE(1s→1p)→0 and f(1s→1p)→0 for R→∞, exhibiting the l degeneracy of the flat surface. On the other hand, for the 1s→2p transition, the energy ΔE(1s→2p) and the oscillator strength f(1s→2p) increase with increasing R, reaching the flat macrosurface value for R→∞.
On the energy dependence of the steric effect in atom–molecule reactive scattering. I. A quasiclassical approach
Authors: Groenenboom Gerrit C. , Meijer Anthony J. H. M.
Published: July 2002

Experimental studies have shown that the steric effect in chemical reactions can decrease (e.g., for Ba+N2O→BaO*+N2) or increase [e.g., for Ca(1D2)+CH3F→CaF*+CH3] with increasing translational energy. Decreasing (negative) energy dependences have successfully been modeled with the angle dependent line of centers model. We present a classical model in which a positive energy dependence of the steric effect is explained by an isotropic, attractive long range potential. In this ‘‘trapping’’ model we assume the reaction—apart from a cone of nonreaction at one side of the molecule—to be barrierless. This model shows that a positive energy dependence of the steric effect is not indicative of reorientation of the molecule, as has been suggested in the literature. Rather, the positive or negative energy dependence of the steric effect is shown to correlate with the absence or presence of a barrier to reaction and an attractive or repulsive long range potential. For the reorientation effects which occur in the case of anisotropic potentials, we consider the application of the standard quasiclassical trajectory (QCT) method and we introduce a modified QCT method. We argue that the latter is more suitable for the computation of the orientation dependent reactive cross section.
Reversible charge transfer in dielectric restricted and fractal solids: Low concentration of acceptors
Authors: Mikhelashvili M. S. , Krissinel E. B.
Published: July 2002

We study the influence of dielectric properties of medium with dielectric constant ε in the charge transfer processes in the restricted and fractal solid spaces. It is shown that this influence is determined by the space restriction and its regularity. The increasing of ε promotes charge separation in restricted and fractal systems. The fractal properties grow weakly with increasing of ε; its increasing transfers the irregular fractal space into the regular one with respect to the charge transfer at ε≫α. The corresponding dependencies of the charge transfer efficiency on the value of ε are presented. The space restriction and breach of its homogeneous with alteration of ε give qualitatively identical results. The obtained results are good for low concentration of acceptors.
The rate of collisional quenching of N2O+ (B 2Σ), N+2 (B 2Σ), O+2 (b 4Σ), O+ (3d), O (3p), Ar+ (4p′), Ar (4p,4p′) at the temperature ≤200 K
Authors: Belikov A. E. , Kusnetsov O. V. , Sharafutdinov R. G.
Published: July 2002

Rate constants of collisional quenching of the states N2O+ (B 2Σ+u), N+2 (B 2Σ+u), O+2 (b 4Σ−u), O i (3p 5P), O ii (3d 4F), Ar ii (4p′ 2F0) in their parent gases and the states Ar i (4p,4p′[1/2]) in oxygen have been measured by studying Stern–Volmer dependencies in the temperature range from 20 to 200 K. Radiating states were excited by the electron beam. A flow on free jet axis was used as a gas target. Negative dependencies of rate constants on temperature which were obtained are consistent with the available experimental data on rate of internal energy and charge transfer at low energy collisions.
Molecular theory of phase equilibria in model and real associated mixtures. II. Binary aqueous mixtures of inert gases and n-alkanes
Authors: Nezbeda I. , Kolafa J. , Pavlíček J. , Smith W. R.
Published: July 2002

The phase equilibrium properties of a molecular-based model of binary aqueous mixtures are investigated using an extended primitive model (EPW) for water, which incorporates a mean-field attractive term in addition to the interactions given by the primitive model studied in part I of this series of papers. The second component, representing a nonpolar fluid, is modeled by a general hard body with a mean-field attractive term. Analytical expressions for the Helmholtz free energy of this precisely defined molecular model are obtained from statistical mechanical theory, as a function of the molecular size, the mean-field interaction term, and the shape of the second component. The predictions of our model are compared with the behavior of two classes of real aqueous mixtures: Group A={water+inert gases, hydrogen} and Group B={water+n-alkanes}. The phase equilibrium properties are studied as a function of the ratio of the critical temperature τ and critical volume λ with respect to the corresponding quantities for water, and the global phase diagram (i.e., the type of phase behavior and its dependence on the model parameters) is determined. Since τ and λ are obtainable both from our theory and from experiment, our approach thus contains no adjustable parameters. The theory gives qualitatively correct predictions of the phase behavior of these two classes of mixtures, i.e., of the transition between Type IIIc and Type IIId critical line behavior in the Konynenberg and Scott classification scheme, of the presence or absence of pressure minima for Group B mixtures exhibiting Type IIIc behavior, and the dependence of the temperature and pressure of the Type IIIc temperature minimum on the size of the second-component molecule.
Dipolar interactions in magnetically very diluted spin networks
Authors: Lacelle Serge , Tremblay Luc
Published: July 2002

Dipolar interactions among nuclear magnetic moments embedded in rigid solids are examined. Substitutional disorder in lattices is shown to modify the statistical properties of the distributions of the absolute values of dipolar coupling constants. Numerical studies reveal that a crossover in the average of the coupling constants in dilute spin networks occurs as a function of the random fractional occupancy in the lattice. The origin of this crossover is related to an interplay between the statistical weights of the coupling constants and their values which are distributed over many orders of magnitude. Two different regimes of behavior are observed for the average of the coupling constants as a function of dilution; one regime is dominated by the most probable value of the distribution of coupling constants, while in the other regime, the arithmetic mean prevails. For a macroscopic sample of 1022 sites, the crossover between regimes is predicted to occur when 1011 sites are occupied. Averaging over the substitutional disorder is also investigated. It is suggested that the character of the fluctuations in the disorder average could possibly serve to establish when a measurement on a large system is equivalent to an ensemble average over the subsystems of a sample. An analogous dynamic crossover, originating from similar behavior in the statistical properties of distributions of products of dipolar coupling constants, is pointed out for multiple quantum nuclear magnetic resonance (NMR) spectroscopy of solids.
Melting of neon clusters: Path integral Monte Carlo simulations
Authors: Chakravarty Charusita
Published: July 2002

Quantum effects on melting of magic number neon clusters Ne13 and Ne19 are studied using Fourier path integral Monte Carlo simulations. The cluster phase transition temperatures are determined from the behavior of the specific heat and the Lindemann index with temperature. Quantum effects are shown to lower transition temperatures by approximately 10%. Quantum and classical potential energies and structural distribution functions are examined in order to understand the quasiclassical behavior of neon clusters.
Monte Carlo study of melting and disordering in monolayer films formed on the (100) face of face centered cubic crystals
Authors: Patrykiejew A. , Sokol/owski S. , Zientarski T. , Binder K.
Published: July 2002

Monte Carlo simulation studies have been carried out for monolayer films formed on the (100) plane of model fcc crystals with different corrugation of the gas-solid potential. It has been demonstrated that the low temperature structure of monolayer films depends strongly on the gas-surface potential corrugation as well as on the size of adsorbed atoms. The effects of the gas-surface potential corrugation on the melting transition have been determined for a series of systems, exhibiting different structure of the solid phase.
Gas–liquid nucleation of nonideal binary mixtures. I. A density functional study
Authors: Laaksonen Ari , Oxtoby David W.
Published: July 2002

The binary gas–liquid nucleation of model Lennard-Jones mixtures is studied using density functional methods. It is shown that the ordinary ‘‘Ar’’–‘‘Kr’’ mixture shows stronger nonclassical nucleation behavior than previously thought. A more nonideal mixture is defined by modifying the usual mixing rules of the Lennard-Jones parameters. This fluid exhibits significantly stronger surface activity than the ordinary mixture. As with water/alcohol systems, classical nucleation theory produces nonphysical predictions for the nucleation of the surface active mixture. On the other hand, the predictions of the density functional theory are thermodynamically consistent.
Numerical multiconfigurational Hartree–Fock calculations of spin and charge densities using the Hiller–Sucher–Feinberg operator identity
Authors: Sundholm Dage
Published: July 2002

The Hiller–Sucher–Feinberg operator has been implemented into the atomic finite-element multiconfiguration spin-restricted Hartree–Fock program lucas. The spin-dependent and spin-independent Hiller–Sucher–Feinberg operators have been tested on Li(2Se), Be(3Po), B(2Po), C(3Pe), N(4So), O(3Pe), and F(2Po) using numerical and Gaussian sp basis sets. In the basis-set calculations, the electron-charge densities at the nucleus obtained using the Hiller–Sucher–Feinberg operator are about 50 times more accurate than the values obtained using the δ-function operator. In the numerical correlation calculations, the spin densities at the nucleus obtained for Li, C, N, O, and F using the Hiller–Sucher–Feinberg operator converge faster with the size of the orbital space than those obtained using the δ-function operator, and for Be and B, the two spin-dependent operators perform about equally well.
Periodic orbit analysis of molecular vibrational spectra: 1:1 resonant coupled modes
Authors: Rouben Daniel C. , Ezra Gregory S.
Published: July 2002

In this paper we analyze the quantum density of states for a model molecular vibrational Hamiltonian describing two coupled anharmonic (Morse) oscillators. Periods of classical periodic orbits as a function of energy and coupling parameter are extracted directly from the quantum spectrum using the Gabor transform. We are able to identify the quantum manifestation of the local-to-normal transition, and of resonant bifurcations of periodic orbits.
Vibrational spectrum and structure of the K2O2 complex in solid argon: A far infrared and density functional theory study
Authors: Tremblay Benoît , Roy Pascale , Manceron Laurent , Pullumbi Pluton , Bouteiller Yves , Roy Denis
Published: July 2002

New fundamental K–O stretching frequencies of the isolated potassium peroxide K2O2 molecule are reported for different isotopic species. The B2u vibration can be measured for 39K216O2, 39K216O18O, K217O18O, 39K218O2, 39K41K16O2 and 39K41K18O2. The B3u vibration can be located for K216O2, K216O18O and K218O2 products, and the formerly B3g symmetry motion, IR-activated by symmetry breaking in 39K216O18O, 39K41K16O18O, 39K41K16O2, and 39K41K18O2, is equally observed. The self-consistency of the attributions is based on semiempirical quadratic force field calculations, and, more importantly, upon theoretical ab initio and Density Functional Theory calculations. The ground state geometry calculated using the all electron density functional theory method corresponds to a slightly out of plane distorted rhombic structure (C2v symmetry), in contrast to the D2h symmetry structure calculated for Li2O2 or at the SCF level for K2O2. The energy difference between rhombic D2h and out-of-plane C2v structure is nevertheless too small (0.08 kcal/mol) to exclude a calculation artefact. The agreement between the positions of observed vibrations and the density functional theory calculation predictions is however remarkable.
Mixing properties of model polymer/solvent systems
Authors: Sheng Y.-J. , Panagiotopoulos A. Z. , Kumar S. K.
Published: July 2002

Monte Carlo calculations have been performed on two nominally athermal polymer/solvent mixtures to test molecular theories of mixing properties for these systems. We first used the incremental chemical potential concept to derive an equation of state in the spirit of the generalized Flory dimer theory, without resorting to the concept of excluded volume. The resulting generalized Flory dimer-like theory and a related model, statistical associating fluid theory, were tested against simulation results for the excess volume, excess Gibbs free energy and component activity coefficients. Good agreement was obtained between the statistical associating fluid theory and computer simulations for all properties studied. The generalized Flory dimer theory, when applied self-consistently, was also able to provide quantitative predictions for the thermodynamic properties of these mixtures. An important result that emerges from our calculations is that these polymer solutions behave ideally when examined on the basis of a ‘‘Flory-like’’ reference state augmented by density effects. This asserts that the effects of molecular size disparity on system thermodynamics are properly captured by this approach. By contrast, the incompressible Flory approach fails to capture the dependence of activity coefficients on composition.
A molecular beam optical/Stark study of calcium monoacetylide
Authors: Marr A. J. , Perry J. , Steimle T. C.
Published: July 2002

A supersonic molecular beam optical Stark measurement of the Ã 2∏r–X̃ 2∑+ band system of calcium monoacetylide CaCCH has been performed. The determined values of ‖μ‖ are 3.01±0.03 D and 2.41±0.02 D for the X̃ 2∑+ and Ã 2∏1/2 states, respectively. A simple electrostatic model quantitatively predicts the ground and excited state dipole moments.
Quasielastic light scattering in the presence of a sinusoidal electric field: A comparison of theories to experimental data on poly(lysine)
Authors: Schmitz Kenneth S. , Pontalion Timothy J.
Published: July 2002

The spectral density profile of quasielastic light scattering in the presence of a sinusoidal electric field (QELS-SEF) is theoretically represented by multiple Lorentzians with peaks located at the driving frequency and harmonics as well as the unshifted central peak. QELS-SEF data on poly(lysine) are characterized by a multiple Lorentzian fit in which each peak location, height, and half-width were treated as independent variables. These analyses are compared with two theories that differ only in the functional dependence of the peak amplitude on the electrophoretic mobility, viz., Poisson or Bessel function distributions, that results from basic assumptions regarding the translation of the distribution function in time. An unexpected result of the poly(lysine) system is that the linewidths were not constant for low applied electric fields, which was correlated with the so-called ‘‘ordinary–extraordinary’’ transition reported for polylysine. Thus neither theory was appropriate under these conditions.
Viscosity coefficients for anisotropic, nematic fluids based on structural theories of suspensions
Authors: Kröger M. , Sellers H. S.
Published: July 2002

The relation between the governing equations for low Reynolds number flow of suspensions of rigid, axisymmetric particles and director theories of anisotropic fluids is examined in detail. The role of directors for modeling suspensions is illustrated and it is shown in what sense and how the equations for three common models of suspensions may be related to a director theory with a variable degree of alignment. The viscosity coefficients and molecular fields of the director theory are then expressed in terms of the parameters characterizing the suspension. Some implications of these results are discussed for tumbling/nontumbling of the director in shear flow, for Parodi’s relations, and for thermodynamic restrictions of the viscosity coefficients. Additionally, several previous calculations of the Leslie viscosities are compared to the results obtained.
Excited ν3 vibrational state of the Ar–HCN and Kr–HCN dimers
Authors: Arunan E. , Emilsson T. , Gutowsky H. S.
Published: July 2002

Rotational spectra have been observed for an excited vibrational state of the Ar/Kr–HCN dimers in a Balle–Flygare Fourier transform microwave spectrometer with a high temperature nozzle that can be heated to ∼1300 K. The B, DJ, and HJ rotational constants found for the excited parent Ar–HCN isotopic species are 1602.475(3) MHz, 164.2(6) kHz, and 309(5) Hz, and for Kr–HCN they are 1176.4493(3) MHz, 41.7(1) kHz, and 56(1) Hz. The 14N quadrupole interaction χa(J=1) observed in the excited state is −2.824 (−3.239) MHz for Ar(Kr)–HCN, slightly smaller than the −2.844 (−3.268) MHz in the ground state. A substitution analysis based on the C and N isotopic species reveals that the C–N distance rs (CN) increases by ∼0.013 (0.020 Å) on excitation of Ar(Kr)–HCN. A similar analysis for free DC15N gives an increase in rs(CN) upon ν3 excitation of 0.0022 Å. Assignment of the excited state in the dimer as the ν3C–N stretch of the HCN is proposed and discussed.
Study of the importance of relativistic, correlation, and relaxation effects on ionization energy of atoms by a relativistic and correlated local density method
Authors: Vijayakumar M. , Gopinathan M. S.
Published: July 2002

The effect of relativistic corrections, correlation, and relaxation on the ionization energy of inner and outer orbitals of light and heavy atoms has been studied using the ‘‘fully’’ correlated relativistic local density method—called the RCΞ method. The validity of Koopmans’ theorem is also discussed in this paper.