bluuebunny/crossref_metadata_2025 huggingface dataset:
Local order and transition dipole coupling in liquid methanol and acetone as the origin of the Raman noncoincidence effect
Authors: Torii Hajime , Tasumi Mitsuo
Published: July 2002

Model calculations are performed on the Raman noncoincidence effect (frequency difference between the isotropic and anisotropic components) observed for the C–O stretching band of liquid methanol and the C=O stretching band of liquid acetone. Microscopic liquid structures are obtained by Monte Carlo simulations, and coupling between molecular vibrations is introduced by the transition dipole coupling mechanism. Ab initio molecular orbital calculations are also performed to check the validity of the assumed direction of the transition dipole for the C–O stretching mode of methanol. The different signs of the Raman noncoincidence between the C–O stretching band of liquid methanol and the C=O stretching band of liquid acetone can be explained by the transition dipole coupling mechanism. The calculated magnitudes of the frequency separations between the isotropic and anisotropic components are in good agreement with the experimental results. Pressure dependence of the Raman noncoincidence is also calculated and compared with the experimental results. In the case of the C–O stretching band of liquid methanol, local anisotropy in the pressure-induced changes of the liquid structure is shown to be important for the pressure dependence of the Raman noncoincidence.
The interatomic structure of argon in water
Authors: Broadbent R. D. , Neilson G. W.
Published: July 2002

The first order difference method of neutron diffraction and isotopic substitution has been applied to argon dissolved under pressure in heavy water. The results show that argon atoms possess a well-defined nearest neighbor hydration shell composed of 16(2) water molecules in the range 2.8≤r (Å)≤5.4. Also evident is a weaker second shell which extends to ∼8 Å. The results are in broad agreement with those obtained from Monte Carlo and M.D. computer simulation of models in which an argon atom interacts with a number of water molecules via pairwise additive forces. Significant differences are discernible when attempts are made to match quantitatively the theoretical results over the entire range of the experimental information.
Vibrational predissociation dynamics of the He79Br2 van der Waals molecule near the B state dissociation limit: Binding energies, lifetimes, and implications for the rare gas–halogen potential
Authors: Jahn Douglas G. , Clement Simon G. , Janda Kenneth C.
Published: July 2002

Excitation spectra of the He79Br2 complex have been obtained for the B state vibrational levels v′=34–48 using the pump–probe spectroscopic technique. The Δv=−1 channel for vibrational predissociation is closed for levels v′=45 and higher. The B and X state van der Waals binding energies are determined to be 13.5±1.0 and 17.0±1.5 cm−1, respectively. Lifetimes for vibrational predissociation were found to range from 5.3 ps for v′=34 to 1.8 ps for v′=44. For v′=45 the lifetime increases to 3.8 ps due to the closing of the Δv=−1 dissociation channel. The lifetimes are consistent with the momentum gap law using a Morse parameter equal to 1.9±0.1 Å−1. The trend of increasing blue shifts as v′ increases reverses at v′=40. This appears to indicate that the He–Br interaction changes dramatically when the Br–Br separation becomes larger than 4.0 Å. Based on the results presented here, we have reevaluated previous data for the HeI2 molecule, and suggest that the ground-state binding energy be revised to 17.6±1.0 cm−1 from the currently accepted value of 18.8 cm−1.
Influence of orientation on coadsorption dynamics: CO displacement from a c(2×2) precovered Ni(100) surface by free oriented NO
Authors: Müller H. , Dierks B. , Fecher G. H. , Böwering N. , Heinzmann U.
Published: July 2002

Collision experiments between two different molecules both oriented with their axis are presented: The kinetics of the CO desorption and NO sticking probability by oriented NO has been investigated using supersonic molecular beam and work function techniques. The work function measurements exhibit mainly molecular adsorption of NO on a CO precovered Ni(100) surface at T=300 K. The desorption and the sticking depend strongly on the initial orientation of the NO molecules in the gas phase. The sticking probability is higher for preferential N-end collisions and the sticking asymmetry is constant, whereas the desorption asymmetry changes its sign from initially negative to positive values, depending on the exposure time or NO coverage. We explain the asymmetries by two different mechanisms, namely direct and indirect molecular exchange. N-end collisions lead preferentially to NO sticking and NO induced CO desorption, whereas O-end collisions enhance the direct CO displacement.
General, energy-separable Faber polynomial representation of operator functions: Theory and application in quantum scattering
Authors: Huang Youhong , Kouri Donald J. , Hoffman David K.
Published: July 2002

A general, uniformly convergent series representation of operator-valued functions in terms of Faber polynomials is presented. The method can be used to evaluate the action of any operator-valued function which is analytic in a simply connected region enclosed by a curve, Lγ. The three most important examples include the time-independent Green’s operator, G+(E)=1/[E−(H−iε)], where H may be Hermitian or may also contain a negative imaginary absorbing potential, the time-dependent Green’s or evolution operator, exp(−iHt/ℏ), and the generalized collision operator from nonequilibrium statistical mechanics, 1/[E−(ℒ−iε)], where ℒ is the Liouvillian operator for the Hamiltonian. The particular uniformly convergent Faber polynomial expansion employed is determined by the conformal mapping between the simply connected region external to the curve Lγ, which encloses the spectrum of H−iε (or ℒ−iε), and the region external to a disk of radius γ. A locally smoothed conformal mapping is introduced containing a finite number of Laurent series terms. This results in an equal number of terms in the recursion of the Faber polynomials and avoids a serious memory problem in a calculation for a large system. In addition, this conformal mapping uniquely determines a scaled Hamiltonian, which when combined with the radius γ, ensures a completely stable recursion relation for calculating the Faber polynomials of the operator of interest (i.e., the Hamiltonian or Liouvillian). We earlier showed that for Lγ chosen to be an ellipse, the Faber polynomial expansion provides the generalization to non-Hermitian H of the Chebychev polynomial expansion of G+(E) [Chem. Phys. Lett. 225, 37 (1994); 206, 96 (1993)]; the present results provide a similar generalization for the Chebychev expansion of e−iHt/ℏ [Tal-Ezer and Kosloff, J. Chem. Phys. 81, 3967 (1984)]. Nonelliptic Lγ lead to other, new polynomial representations having superior convergence properties.
The computation of electron transfer rates: The nonadiabatic instanton solution
Authors: Cao Jianshu , Minichino Camilla , Voth Gregory A.
Published: July 2002

A computational theory for determining electron transfer rate constants is formulated based on an instanton expression for the quantum rate and the self-consistent solution of the imaginary time nonadiabatic steepest descent approximation. The theory obtains the correct asymptotic behavior for the electron transfer rate constant in the nonadiabatic and adiabatic cases, and it smoothly bridges between those two limits for intermediate couplings. Furthermore, no assumptions regarding the form of the diabatic potentials are invoked (e.g., harmonic) and more than two diabatic states can be included in the calculations. The method thereby holds considerable promise for computing electron transfer rate constants in realistic condensed phase systems.
Quantum Monte Carlo calculation of argon–HF clusters: Nonadditive forces, isomerization, and HF frequency shifts
Authors: Lewerenz Marius
Published: July 2002

Zero point energies and structures have been calculated for ArnHF clusters (n=1–4) using the diffusion quantum Monte Carlo method and potential surfaces based on pairwise additive and non pairwise additive models. Energy differences for related potential surfaces are computed with a correlated sampling technique. The two lowest energy isomers of Ar4HF are found to be energetically equivalent if zero point energy effects are taken into account. Their energetic ordering depends sensitively on threebody contributions to the interaction potential. The calculated size dependent frequency shifts for fundamental and overtone transitions of HF caused by the argon micromatrix are in good agreement with experimental data but are sensitive to nonadditive forces. The line shifts cannot be explained quantitatively without accounting for zero point energy effects. The band origin of the yet unobserved C2v isomer of Ar4HF is predicted to be about 2.8 cm−1 to the red of the C3v isomer band.
Structural and dynamical properties of Cu–Au bimetallic clusters
Authors: López M. J. , Marcos P. A. , Alonso J. A.
Published: July 2002

The effect of alloying on the structural and thermal properties of Cun−xAux (n=13,14) clusters is investigated by constant energy Molecular Dynamics simulations. The interactions between the atoms in the clusters are mimicked by a many-body (Gupta-like) potential based on the second moment approximation to the tight-binding model. The minimum energy structures and the lowest-lying isomers of the pure and mixed clusters are obtained by thermal quenching. We find icosahedral-like ground state structures for the 13- and 14-atom clusters and for all the concentrations, the only exception being Au14 which has C6v symmetry. Mixed structures are preferred over the segregated ones. The lowest-lying isomers of the binary clusters are the permutational ones, i.e., isomers having the same underlying geometry as the ground state structure and different relative arrangement of the unlike atoms in the atomic positions of the geometry. However, presence of these low lying permutational isomers does not affect the gross features of the melting-like transition. The 13- and 14-atom (icosahedral-like) binary clusters melt in one and two stages, respectively, as the corresponding pure Cu clusters. In constrast the melting-like transition of Au14 exhibits a single stage. The melting temperature is studied as a function of cluster concentration and size. The main conclusion is that mixed Cu–Au clusters likely behave as pure Cu clusters, both from the structural and the dynamical points of view, for all concentrations.
Quasiclassical dynamics of the I2–Ne2 vibrational predissociation: A comparison with experiment
Authors: García-Vela A. , Rubayo-Soneira J. , Delgado-Barrio G. , Villarreal P.
Published: July 2002

The vibrational predissociation dynamics of the I2(B,v)–Ne2 complex is investigated for several vibrational levels of I2, using a quasiclassical trajectory approach. The time evolution of the population of nascent I2 fragments is calculated. A model is proposed which reproduces the results of the classical trajectories, and allows to obtain the lifetimes associated with the dissociation of the two van der Waals (vdW) bonds. The classical lifetimes are higher in general than the experimental ones of Zewail and co-workers [J. Chem. Phys. 97, 8048 (1992)]. The classical method appears to overestimate mechanisms of energy redistribution between the modes, which slow down the dissociation of the cluster. However, the behavior of the lifetimes with the initial iodine vibrational excitation is in very good agreement with experiment. A sequential path of fragmentation of the two weak bonds via direct predissociation is found to dominate, producing I2(B,v–2)+2Ne fragments. Although with smaller probability, alternative dissociation paths are observed involving statistical mechanisms of internal energy redistribution. In these paths, the energy initially transferred by the iodine heats the vdW modes without breaking the complex. Further energy transfer produces either simultaneous or sequential dissociation of the two weak bonds in a rather evaporative way, populating the v–2 and v–3 exit channels.
Structure and vibrations of catechol and catechol⋅H2O(D2O) in the S and S1 state
Authors: Gerhards M. , Perl W. , Schumm S. , Henrichs U. , Jacoby C. , Kleinermanns K.
Published: July 2002

The inter- and intramolecular vibrations in the S0 and S1 state of catechol, d2-catechol, catechol(H2O)1, and d2-catechol (D2O)1 have been investigated experimentally by resonant two photon ionization (R2PI), spectral hole burning (SHB), and dispersed fluorescence spectroscopy (DF). The experimental frequencies are compared to the vibrational frequencies obtained from ab initio normal mode calculations using the 6-31G(d,p) basis set. In order to get a complete interpretation of the S0 state spectra of d2-catechol the strong coupling of the two OD torsional motions has been taken into account. A two-dimensional calculation of the torsional eigenvalues based on an ab initio potential [6-31G(d,p) basis] obtained from single point calculations is presented. Due to these calculations all vibrations in the S0 state can be assigned. Furthermore a new assignment of the vibrations in the S1 state of d2-catechol is given. In the case of catechol (H2O)1 [d2-catechol(D2O)1] different structural isomers are discussed. Using HF ab initio calculations (including MP2, BSSE, and ZPE corrections) a trans-linear hydrogen bonding arrangement turns out to be more stable by an amount of 840 cm−1 compared to a cyclic structure which is also a minimum of the PES. Normal mode calculations have been carried out for both structures and anharmonic corrections are calculated for the τ and β2 mode of the trans-linear arrangement. The prediction of the ab initio calculations is supported by the vibrational transitions observed in the spectra of the S0 and S1 state, which can be assigned on the basis of the vibrations calculated for the trans-linear structure. The most important feature of the R2PI spectrum of catechol(H2O)1 [d2-catechol(D2O)1] is the occurrence of intermolecular vibrations of very low frequencies (14, 37 cm−1). These vibrations and the low frequency torsional modes in the spectra of the S1 state of the catechol monomer strongly support the assumption that catechol is nonplanar in the S1 state with respect to the OH groups. Due to this nonplanarity a double minimum potential for the intermolecular ρ1 mode of catechol(H2O)1 is postulated. Using this assumption the low frequency vibrations of the R2PI spectra as well as the vibrations observed in the spectra of the S0 state can be assigned.
A multi-domain weighted residual method for the one-electron Schrödinger equation: Application to H+2
Authors: Feldkamp Joseph R.
Published: July 2002

The Schrödinger equation is solved for a single electron moving in the coulombic field of some arbitrary configuration of nuclei. Space is partitioned by centering a sphere on each of the individual nuclei without any overlap or touching of the spheres, i.e., muffin-tin spheres. All regions are treated by a weighted residual technique, which is a more general approach than the variational method. Outside the spheres, both the wavefunction and its product with the potential energy function are expanded as a linear combination of solutions taken from the modified Helmholtz equation (M.H.E.). A basis set is prepared by solving the M.H.E. repeatedly for a select set of eigenvalues and boundary conditions, using a boundary integral technique. Inside any sphere, the wavefunction is written as a linear combination of terms, each a product of a radial function and a spherical harmonic. The radial factor is written as product of an exponential and a power series. For either region, an alternate basis set is chosen to supply the weight functions required by the weighted residual approach. Weight functions are chosen according to their ability to provide increased efficiency and accuracy. Only simple integrals over the sphere surfaces are involved in calculating matrix coefficients. In order to demonstrate the method, the H+2 molecule is considered as a test case, with the potential energy function treated in full.
Ab initio molecular dynamics simulation of liquid water: Comparison of three gradient-corrected density functionals
Authors: Sprik Michiel , Hutter Jürg , Parrinello Michele
Published: July 2002

Three frequently used gradient-corrected density functionals (B, BP, and BLYP) are applied in an ab initio molecular dynamics simulation of liquid water in order to evaluate their performance for the description of condensed aqueous systems. A comparison of structural characteristics (radial distribution functions) and dynamical properties (vibrational spectra, orientational relaxation, and self-diffusion) leads to the conclusion that hydrogen bonding is too weak in the usual local density approximation corrected for exchange only according to Becke (B), whereas adding the gradient correction for correlation according to Perdew (BP) yields effective hydrogen bonds in the liquid that are too strong. The combination of B with the semilocal correlation functional according to Lee, Yang, and Parr (BLYP) yields the best agreement with experiment. The computational method, which is the basis for the determination of (adiabatic) electronic structure in the ab initio molecular dynamics simulation, has been validated by an extensive series of test calculations for the water dimer, which will also be presented here.
Combined isothermal and thermally stimulated depolarization measurements in polymers interpreted with the modified coupling model of relaxation
Authors: Marchal Eve
Published: July 2002

In Ngai’s coupling model, the relaxation rate of bulk amorphous polymers has a power law time dependence which can be measured by isothermal depolarization. We propose a method by which the time needed for one experiment is considerably shortened without loss of information. At temperatures below and above the liquid glass transition we have previously modified Ngai’s model by introducing a parameter t′ which replaces the physical time, while the other parameters of the model are constant. The above experimental method can be used to probe the hypothesis. This is illustrated for an amorphous poly(ethyleneterephtalate) sample studied below Tg. Above Tg the thermally stimulated depolarization current was analyzed for polystyrene. At the α−β bifurcation temperature it appears that t′=t.
The effects of pressure and temperature on molecular dynamics during linear-chain polymerization by dielectric measurements
Authors: Johari G. P. , McAnanama J. G. , Wasylyshyn D. A.
Published: July 2002

Molecular relaxation of the various states formed during the course of growth of a linear-chain polymer by addition reactions of the amine group of cyclohexylamine with the epoxy groups of a diepoxide has been studied at isobaric conditions of hydrostatic pressures up to 206 bar and at several temperatures from 300 K to 314 K, by using dielectric measurements for a fixed frequency of 1 kHz, which are adequate for obtaining information on the relaxation time during the course of polymerization. The reaction occurs faster at high pressures and the curves of permittivity and loss against the polymerization time, which resemble the corresponding spectra, bodily shift to a shorter time. At 206 bar and 307.5 K, the ε″ plot shows contributions from a second, high frequency relaxation. The increase in relaxation time, when the reaction occurred at high pressures, has been discussed in terms of both (a) an increase due to the increase in the rate of chemical reaction and (b) the usual physical effect of pressure on molecular kinetics, and an attempt made to resolve the two effects. The effect of hydrostatic pressure predominates the molecular relaxation dynamics through an increase in the polymerization rate. Formalisms relating the chemical and physical processes are given, but not examined by experiments. The decrease in the configurational entropy is formulated in terms of the polymerization rate and pressure. The increase of the static permittivity of the mixture on compression is marginal. It decreases more rapidly with the progress of polymerization at high pressures. Two issues on obtaining information on molecular dynamics of a time-variant system from single-frequency measurements, raised by others since our earliest studies, have been elaborated and analytically clarified. By using simulated dielectric data it has been shown that the dc conductivity and interfacial polarization alter the shape of the dielectric permittivity and loss plots to make misleadingly alternative parameter fits possible.
Semiflexible chains under tension
Authors: Ha B.-Y. , Thirumalai D.
Published: July 2002

A functional integral formalism is used to derive the extension of a stiff chain subject to an external force. The force versus extension curves are calculated using a mean-field approach in which the hard constraint u2(s)=1 is replaced by a global constraint 〈u2(s)〉=1 where u(s) is the tangent vector describing the chain and s is the arclength. The theory quantitatively reproduces the experimental results for DNA that is subject to a constant force. We also treat the problems of semiflexible chain in a nematic field. In the limit of weak nematic field strength our treatment reproduces the exact results for chain expansion parallel to the director. When the strength of the nematic field is large, a situation in which there are two equivalent minima in the free energy, the intrinsically mean-field approach yields incorrect results for the dependence of the persistence length on the nematic field.
A three-dimensional wave-packet method for the CH overtone spectroscopy and intramolecular vibrational relaxation dynamics of the fluoroform molecule
Authors: Solá I. R. , Muñoz-Crego C. , Dı́az A. , Muñoz-Sanz R. , Santamarı́a J.
Published: July 2002

The overtone spectroscopy and intramolecular vibrational relaxation dynamics of CH chromophore in the fluoroform molecule is studied by a three-dimensional (3D) time-dependent wave-packet method, and the results are compared with the experiment and with those of a 2D (stretch–bend) wave-packet method. A third mode (CF symmetrical stretch) is included in order to explain the long time dynamics and the combination bands between the CF stretch fundamental and the Fermi polyad frequencies. The comparison with the 2D study is carried out by the use of a full set of dynamical and spectroscopic variables, based on the autocorrelation function of the bright states of each polyad. The spectroscopic variables then follow by Fourier transforming the autocorrelation function, while the dynamical ones emerge via survival probability in the frame of the dynamical statistical ensemble. These include several relaxation times and the number of cells and rates of phase–space exploration. The specific effect of the third mode is monitored by following the reduced dynamics of the system irrespective of the polyad stretch–bend dynamics, through population evolution. Dynamical results clearly reveal the third mode effects at very short and long times. In the last regime, we can observe a great span of different behaviors, depending on how the third mode excited states are involved. This richer variety of dynamical patterns cannot be observed in a two-mode model and justifies the present work. The spectroscopic results of both models are in good agreement with the experimental results.
Surface phase behavior in binary polymer mixtures. III. Temperature dependence of surface enrichment and of wetting
Authors: Budkowski A. , Scheffold F. , Klein J. , Fetters L. J.
Published: July 2002

Surface segregation in thin films of binary liquid mixtures consisting of random olefine copolymers was studied by nuclear reaction analysis over a wide temperature and composition range. A divergence of the surface excess Γ was indicated as the binodal of each mixture was approached from the one-phase region, even at temperatures 100 °C below the critical point Tc, and interpreted as the advent of complete wetting behavior. A consistent description of the adsorption isotherms in terms of a mean field approach assuming a short-ranged surface potential fs is feasible, but requires an unexpected temperature dependence of fs. This dependence causes the wetting transition temperature to be located lower than expected on the basis of present models.
Local volume fraction fluctuations in random media
Authors: Quintanilla J. , Torquato S.
Published: July 2002

Although the volume fraction is a constant for a statistically homogeneous random medium, on a spatially local level it fluctuates. We study the full distribution of volume fraction within an observation window of finite size for models of random media. A formula due to Lu and Torquato for the standard deviation or “coarseness’’ associated with the local volume fraction ξ is extended for the nth moment of ξ for any n. The distribution function FL of the local volume fraction of five different model microstructures is evaluated using analytical and computer-simulation methods for a wide range of window sizes and overall volume fractions. On the line, we examine a system of fully penetrable rods and a system of totally impenetrable rods formed by random sequential addition (RSA). In the plane, we study RSA totally impenetrable disks and fully penetrable aligned squares. In three dimensions, we study fully penetrable aligned cubes. In the case of fully penetrable rods, we will also simplify and numerically invert a prior analytical result for the Laplace transform of FL. In all of these models, we show that, for sufficiently large window sizes, FL can be reasonably approximated by the normal distribution.
Reaction on the O−+CH4 potential energy surface: Dependence on translational and internal energy and on isotopic composition, 93–1313 K
Authors: Viggiano A. A. , Morris Robert A. , Miller Thomas M. , Friedman Jeffrey F. , Menedez-Barreto Melani , Paulson John F. , Michels H. H. , Hobbs R. H. , Montgomery, Jr. J. A.
Published: July 2002

Rate constants have been measured for the reactions of O− with CH4, CH2D2, and CD4 as a function of ion-neutral average center-of-mass kinetic energy, 〈KEcm〉, at several temperatures over the range 93 K–565 K using a selected ion flow drift tube apparatus. For the CH4 reaction we also report measurements made using a high-temperature flowing afterglow (HTFA) instrument over the temperature range 300 K–1313 K. The rate constants are found to have a very large isotope effect, with the CH4 rate constant a factor of 15 higher than the CD4 rate constant at 93 K. The rate constants generally have a minimum with respect to temperature and 〈KEcm〉, except for the higher-temperature data for CD4 where the rate constants show only an increase with increasing kinetic energy. The data indicate that increasing rotational temperature decreases the rate constants and that rotational energy behaves similarly to translational energy. Single excitations of bending and twisting vibrations have a negligible effect on the rate constant. Either the stretching vibrations or overtones of the bending vibrations increase the rate constants. If the stretches are responsible for the increase in the rate constants, the derived rate constant for a single quantum of stretch excitation (v=1) is 5×10−10 cm3 s−1, a factor of 6 larger than the rate constant for v=0. The CH2D2 rate constants are approximately equal to the averages of the rate constants for the pure isotopes. The product branching ratio (OH−/OD−) shows no dependence on CH2D2 rotational temperature or low-frequency CH2D2 vibrations. A theoretical study of the minimum energy reaction path was performed to help elucidate the reaction dynamics. The minimum energy reaction surface was characteristic of the standard double minimum pathway for ion molecule reactions. The height of the central barrier was found to be close to the energy of the reactants and varied with isotopic substitution. Conformationally different transition states are found for these isotopic reactions. Theoretical studies at the QCISD(T) level of theory find distinct transition states corresponding to O−+CH4, O−+H-CHD2, O−+D-CH2D, O−+CD4. The transition state barriers increase in the order O−+CH4, O−+H-CHD2, O−+D-CH2D, and O−+CD4, in agreement with experimental reaction rates. The main features of the reactivity are explained by the characteristics of the reaction surface.
Shear behavior of squalane and tetracosane under extreme confinement. III. Effect of confinement on viscosity
Authors: Gupta S. A. , Cochran H. D. , Cummings P. T.
Published: July 2002

This study uses nonequilibrium molecular dynamics simulation to explore the rheology of confined liquid alkanes. Two alkanes that differ in molecular structural complexity are examined: tetracosane (C24H50), which is a linear alkane, and squalane (C30H62), which has six symmetrically placed methyl branches along a 24 carbon backbone. These model lubricants are confined between model walls that have short chains tethered to them, thus screening the wall details. This paper, the third of a three part series, compares the viscosities of the confined fluids to those of the bulk fluids. The alkanes are described by a well-documented potential model that has been shown to reproduce bulk experimental viscosity and phase equilibria measurements. Details of the simulation method, and structural information can be found in the preceding two papers of this series. The measured strain rates in these simulations range between 108 and 1011 s−1, which is typical of a number of practical applications. The confined fluids undergo extensive shear thinning, showing a power-law behavior. Comparison of results for the confined fluid to those for the bulk fluid reveal that, for the conditions examined, there is no difference between the bulk and confined viscosities for these alkanes. This observation is in contrast to experimental results at much lower strain rates (10–105 s−1), which indicate the viscosities of the confined fluid to be much larger than the bulk viscosities. In making the comparison, we have carefully accounted for slip at the wall and have performed simulations of the bulk fluid at the same conditions of strain rate, temperature, and pressure as for the corresponding confined fluid. The viscosity is found to be independent of the wall spacing. The calculated power-law exponents are similar to experimentally observed values. We also note that the exponent increases with increasing density of the fluid.
Surface-hopping modeling of photoinduced relaxation dynamics on coupled potential-energy surfaces
Authors: Müller Uwe , Stock Gerhard
Published: July 2002

A mixed quantum-classical description of nonadiabatic photoreactions such as internal conversion and electron transfer is outlined. In particular the validity and limitations of Tully’s surface-hopping (SH) model [J. Chem. Phys. 93, 1061 (1990)] is investigated in the case of photoinduced relaxation processes which are triggered by a multidimensional conical intersection (or avoided crossing) of two potential-energy surfaces. Detailed numerical studies are presented, adopting (i) a three-mode model of the S2→S1 internal-conversion process in pyrazine, (ii) a multimode model of ultrafast intramolecular electron-transfer, (iii) a model exhibiting nonadiabatic photoisomerization dynamics, and (iv) various spin-boson-type models with an Ohmic bath for the description of electron-transfer in solution. The SH simulations are compared to exact quantum-mechanical calculations as well as to results obtained by an alternative mixed quantum-classical description, that is, the self-consistent classical-path method. In all cases, the SH data are shown to reproduce the quantum results at least qualitatively; in some cases the SH results are in quantitative agreement with the complex electronic and vibrational relaxation dynamics exhibited by the quantum calculations. Depending on the physical situation under consideration, either the SH or the self-consistent classical-path method was found to be superior. The characteristic features of a mixed quantum-classical description of photoinduced bound-state dynamics (e.g., the start of the trajectories on a diabatic electronic potential-energy surface, high chance of a trajectory undergoing multiple electronic transitions) as well as the specific problems of the SH approach are discussed in some detail. In particular, the focus is on the ability of a method to account for the branching of trajectories, to correctly describe the electronic phase coherence and the vibrational motion on coupled potential-energy surfaces, and to obey the principle of microreversibility. Furthermore, an alternative way to handle classically forbidden electronic transitions is proposed, which is shown to lead to significantly better results than the usual procedure.
Cavity formation energy in hard sphere fluids: An asymptotically correct expression
Authors: Matyushov Dmitry V. , Ladanyi Branka M.
Published: July 2002

Exact geometrical relations valid for hard sphere (HS) fluids are used to derive analytical expressions for the cavity formation energy equal to the free energy cost of insertion of a HS solute into a HS solvent and the contact value of the solute-solvent pair distribution function (PDF) in the limit of the infinite solute dilution. In contrast to existing relations from the Boublik–Mansoori–Carnahan–Starling–Leland (BMCSL) equation of state, the derived expressions are self-consistent and result in correct asymptotics when the solute size goes to infinity. The proposed equations are tested against Monte Carlo simulations at diameter ratios d in the range 1⩽d⩽3.5 and three reduced densities 0.7, 0.8, and 0.9. The BMCSL theory is shown to systematically underestimate contact PDF values as compared to simulations both for finite solute concentrations and in the infinite dilution limit calculated by extrapolation of the results obtained at several concentrations. These infinite-dilution values of the solute-solvent PDF at contact calculated from simulations are in excellent agreement with the analytical expression derived in the paper. An analogy to the BMCSL equation for HS mixtures is used to extend this equation into the range of finite concentrations of the solute. The proposed equation is found to agree well with our simulation results.
Attraction of electrostatic origin between colloids
Authors: Belloni Luc , Spalla Olivier
Published: July 2002

The interaction between two spherical colloids immersed in an electrolyte of screening constant κ is studied using the recent charge regulation primitive model and the hypernetted chain integral equation. The charge of the particles is not fixed a priori but results from the adsorption of positive and/or negative ions onto the colloidal surfaces. In the case of symmetrical adsorption, the model exhibits a long-range attraction between the globally neutral colloids. At large distance, the effective colloid–colloid potential behaves as −exp(−2κr)/r2 for small colloids where r is the center to center interparticles distance and as −exp(−2κh)/h for large colloids where h is the surface to surface distance. For nonsymmetrical cases, such an attraction adds to the usual screened Coulombic repulsion between the globally charged colloids. The numerical results and the physical origin of the attraction are explained in terms of elementary diagrams. The attraction arises from the ion exchange between the adsorbed layers and the bulk and from the non mean-field ion–ion correlations.
Diamond lattice model of semicrystalline polyethylene in the amorphous region
Authors: Duan Zhong-Hui , Howard Louis N.
Published: July 2002

The statistics of polyethylene chains in the amorphous region between two crystallites have been computed, modeling the chains as random walks on a diamond lattice with two absorbing boundaries. The probabilities of trans and gauche states are considered, thereby determining some effects of temperature, e.g. at higher temperature more loops are formed and the average length of loops is shorter, in contrast to less ties formed with a longer average length. It is found that the lower bound on the adjacent reentry is 79% at very high temperature and it increases to 81% at about 400 K.
Angular distributions for photodissociation of O2 in the Herzberg continuum
Authors: Buijsse B. , van der Zande W. J. , Eppink A. T. J. B. , Parker D. H. , Lewis B. R. , Gibson S. T.
Published: July 2002

Photodissociation in the Herzberg continuum of molecular oxygen has been studied at 236, 226 and 204 nm. Using ion-imaging and monitoring of O(3Pj), j=0, 1, and 2 product-atom angular distributions, the amount of parallel character of the transition was measured. In order to interpret these data, analyses of the photoabsorption oscillator strengths and the parallel-perpendicular nature of the Herzberg I, II and III bands, and extrapolation of these properties into the Herzberg-continuum region have been performed. Our measured fine-structure-averaged angular distributions are found to be consistent with this photoabsorption model. In addition, the dynamics of the dissociation process is discussed, based on the O-atom fine-structure distributions.
The spectrum of antimony hydride: An ab initio configuration interaction study employing a relativistic effective core potential
Authors: Alekseyev Aleksey B. , Liebermann Heinz-Peter , Lingott Rainer M. , Bludský Ota , Buenker Robert J.
Published: July 2002

An ab initio configuration interaction (CI) study including the spin–orbit interaction is carried out for numerous valence and Rydberg states of the SbH radical by employing a relativistic effective core potential for the antimony atom. The computed spectroscopic constants are in good agreement with available experimental data, with a tendency toward a slight overestimation of bond lengths (by 0.01–0.03 Å) and Te values (by 370–550 cm−1) for the lowest singlet states. Measured excitation energies and spin–orbit splittings for the A 3Π multiplet are also accurately reproduced in the present calculations and the Ω=0−, 1, and 2 components of this state are shown to be strongly predissociated due to spin–orbit interaction with the corresponding components of the repulsive Σ−5 state. The most stable representative of the A 3Π multiplet, A40+, is found to possess a very unusual potential curve with a double minimum and a fairly low barrier to dissociation. Based on a vibrational analysis of this state it is concluded that the earlier observed B0+ and C0+ electronic states should be attributed to the v=0 and 2 vibrational levels of the A40+ state, while the state experimentally assigned as A 3Π0+ corresponds to the A40+, v=1 level. Dipole moments μ(v=0) for the …σ2π2 X3Σ−, a 1Δ and b 1Σ+ states are computed to have small (e.g., −0.238 D for X1 3Σ0+−) and nearly equal negative values (Sb+H− polarity). The dipole transition moments and the corresponding radiative lifetimes for a number of low-energy electronic transitions have also been computed. Many other bound states and avoided crossings are indicated in the calculations which may be of relevance in future experimental studies of this system.
Finite extensibility effects in nematic elastomers
Authors: Mao Y. , Warner M. , Terentjev E. M. , Ball R. C.
Published: July 2002

We present a model calculation of the elasticity of a nematic elastomer of finite-length network strands under affine deformation. The result is derived in its full tensorial structure, and its predictions are examined. In particular, we look at the non-Gaussian corrections to the effects of soft elasticity and elastic anisotropy. We find that the soft modes can at most be hardened at O(N−2) when compared to the conventional rubber elastic energy, where N is the number of monomer units along a typical strand. The elastic moduli for extensions parallel and perpendicular to the nematic director are found to differ at O(N−1).
The dipole moment and infrared transition strengths of nitric oxide
Authors: Rawlins W. T. , Person J. C. , Fraser M. E. , Miller S. M. , Blumberg W. A. M.
Published: July 2002

The electric dipole moment function for NO(X 2Π) is determined by nonlinear least-squares analysis of experimental data, including previously reported (Δv=2)/(Δv=1) transition branching ratios for v′⩽13. Polynomial fits to the dipole moment function are determined over the range of internuclear separation from 0.97 to 1.50 Å. The fitted dipole moment functions agree well with previous ab initio predictions, but differ somewhat at the larger internuclear separations. The observed dipole moment functions are used to compute rotationless Einstein coefficients for Δv=1–4 vibrational transitions with v′⩽20. The absolute Δv=1 and Δv=2 Einstein coefficients are well determined by this analysis, especially for v′⩽12. The results provide a firm basis for determinations of vibration-rotation emission intensities of NO(X 2Π) in high-temperature or nonequilibrium chemiexcitation systems.
Initial vibrational level distribution of HCN[X̃ 1Σ+(v10v3)] from the CN(X 2Σ+)+H2→HCN+H reaction
Authors: Bethardy G. A. , Northrup F. J. , He G. , Tokue I. , Macdonald R. Glen
Published: July 2002

The reaction of the cyano radical (CN) with hydrogen was studied by time-resolved infrared absorption spectroscopy of individual rovibrational states of HCN. The initial vibrational level distribution of HCN(v10v3) was determined by plotting the time dependence of the fractional population of a vibrational level and extrapolating these curves to the origin of time. The experiments were carried out at two temperatures, 293 and 324 K, with similar results. It was estimated that about 50% of the available reaction exothermicity was deposited as vibrational excitation of the HCN product. Surprisingly, the HCN(101) vibrational level received a significant fraction of the observed vibrational population, implying that the CN vibration was not really a spectator bond in the reaction dynamics. Furthermore, the observed HCN(v10v3) vibrations only account for about 27% of the initial HCN population produced in the title reaction. A significant fraction of the product HCN molecules must have been produced with the bending mode excited, likely in combination with the H–C stretch vibrations.
Nonperturbative analytic approach to the interaction of intense ultrashort chirped pulses with molecules in solution: Picture of “moving” potentials
Authors: Fainberg B. D.
Published: July 2002

A nonperturbative analytic approach to the problem of the interaction of high-power chirped ultrashort pulses with a molecular system coupled with a dissipative environment has been developed. We considered the model of the Gaussian–Markovian stochastic modulation of the optical transition of a molecule in solution. The calculation results agree qualitatively with the experimental results by Shank et al. The theory naturally leads to the picture of “moving” potentials which are “photonic replications” of the ground and excited electronic states. An electronic optical transition induced by chirped pulses can be considered as an electron transfer reaction between a “moving photonic replication” and the corresponding term occurring at their instantaneous intersection. This clear picture explains not only the main features of the behavior of the excited state population as a function of the chirp rate, but fine details of it as well.
Accurate determination of molecular structures by x-ray absorption spectroscopy
Authors: Filipponi Adriano , D’ Angelo Paola
Published: July 2002

High-quality x-ray absorption fine structure (XAFS) spectra of the Br2, GeCl4, and BBr3 molecules, collected at a third generation synchrotron radiation source above the Br or Ge K-edges, are presented. Excellent fits are obtained using model spectra calculated in the muffin-tin approximation assuming Gaussian atomic distributions. The extended energy ranges of the spectra (up to 24 Å−1 for Br2) contribute to the reduction of the statistical errors in the structural parameters. We show that the potential accuracy of present XAFS determinations is 0.001 Å in bond lengths and 0.0001 Å2 in vibrational amplitudes. These results demonstrate that XAFS is nowadays competitive with electron diffraction in the determination of simple molecular structures in the presence of heavy atomic species.
A molecular dynamics simulation of the vibrational properties of the Ar1−x(N2)x crystal
Authors: Raugei Simone , Cardini Gianni , Schettino Vincenzo , Jodl Hans J.
Published: July 2002

A classical molecular dynamics calculation of the vibrational properties (and in particular of the infrared and Raman response) of the Ar1−x(N2)x mixed crystal is presented based on an intermolecular interaction potential including electrostatic terms. The lattice and internal vibration region are discussed separately. The vibrational properties are correlated with the structural properties and with the orientational ordering in the crystals.
Effect of electrostatic interactions on the structure and dynamics of a model polyelectrolyte. I. Diffusion
Authors: Liu Hui , Skibinska Lidia , Gapinski Jacek , Patkowski Adam , Fischer Erhard W. , Pecora R.
Published: July 2002

The dynamics of a 20 base pair oligonucleotide is studied by dynamic light scattering-photon correlation spectroscopy and depolarized Fabry–Perot interferometry. The 20 base pair oligonucleotide is a well-defined, albeit short, rigid rod molecule that serves as a model for polyelectrolyte solution dynamics. The effects of added salt on the solution rotational and translational dynamics are examined in detail as functions of the 20-mer concentration. Coupled mode theory together with counterion condensation theory gives good predictions for the effects of salt on the translational diffusion of the 20-mer at the relatively low oligonucleotide concentrations studied. Comparison of the experimental results with these theories shows that the effective charge density of the polyion in solution is approximately equal to the reciprocal of the product of the Bjerrum length and the counterion charge, νeff≅1/NλB. Calculation shows that the numerical solution of the coupled mode theory matrix gives a better fit of our measured polyion diffusion coefficients than the approximate equation derived by Lin, Lee, and Schurr. Simple approximations for the effective rod length, Leff=L+κ−1, and effective rod diameter, deff=d+κ−1, are used to model the thermodynamic-hydrodynamic interactions for charged rodlike molecules and to make predictions for the diffusion second virial coefficient as a function of added salt concentration. This alternative to the coupled mode theory also gives good agreement with experiment. The rotational diffusion constants of the oligonucleotide measured by depolarized Fabry–Perot interferometry show a slowing down of the rotation at low added salt concentrations as the oligonucleotide concentration is increased.
A sequential-analytic method for determining the radial distribution function and predicting the thermodynamic properties of real fluids
Authors: Chen Guang-Jin , Guo Tian-Min
Published: July 2002

A new concept of differential phase has been proposed. The flux of molecules between two phases, rather than the fugacity, is used to formulate the phase equilibrium. A double-resistance physical model is presented for modelling the flow of molecules through the interface of two phases. By equating the equilibrium between differential phase and bulk phase, a generalized sequential-analytic method for determining the radial distribution function (RDF) is developed. The proposed method is capable of giving RDF values close to those obtained by molecular simulation. It has been shown that when coupled with the Lennard-Jones potential model, the pressure-volume-temperature (PVT) relation of pure fluids with molecular size up to n-hexane can be predicted satisfactorily in wide ranges of temperature and pressure.