bluuebunny/crossref_metadata_2025 huggingface dataset:
Electric birefringence of flexible polymers in high fields: Brownian dynamics simulation
Authors: Goff R. Blake , Karel Steven F. , Prud’homme Robert K.
Published: July 2002

The dynamics of polyelectrolytes and other orientable polymers can be followed experimentally by electric birefringence. In this work, we examine the steady state and transient behavior of polymer chains in electric fields with a Brownian dynamics simulation. The polymers are modeled as bead-spring chains with finitely extensible nonlinear elastic springs. Two models of the interaction between the electric field and the polymer chains are examined. The two models are a local mechanism which acts on individual chain segments, and a global mechanism which depends on the conformation of the entire chain. Steady-state birefringence vs field strength results indicate that both mechanisms have an apparent coil-stretch transition at some critical field strength. Observation of the steady-state conformation of these chains reveals that the local mechanism does not truly uncoil the chain but rather stretches individual links, while the global mechanism results in the entire coil unraveling. The transient behavior is likewise affected by the orientation mechanism. Relaxation spectra for chains oriented by the different mechanisms reveal that the longest relaxation time is seen only for the unraveled chains that result from the global mechanism.
Comment on ‘‘14N spin relaxation of N2 in buffer gases. Cross sections for molecular reorientation and rotational energy transfer’’ [J. Chem. Phys. 95, 5799 (1991)]
Authors: Temkin S. I. , Burshtein A. I.
Published: July 2002

An inappropriate formula was used in Jameson, Jameson, and ter Horst, J. Chem. Phys. 95, 5799 (1991) to find the orientational relaxation time τθ,2 from the longitudinal relaxation time T1 of 14N2 measured in gas phase by NMR. As a result, while experimental data (Jameson, Jameson, and ter Horst) confirm our earlier measurements [Chem. Phys. Lett. 91, 139 (1982)], the wrong interpretation led authors (Jameson, Jameson, and ter Horst) to a contradiction with the fundamental results of J-diffusion theory.
Universal amplitude ratios of surface tensions near a critical point in a liquid binary system: Water-2,5 lutidine
Authors: Karad S. , Amara M. , Laouenan A. , Tronel-Peyroz E. , Bennes R. , Privat M.
Published: July 2002

Experimental data of surface tensions along the critical isochore (1) and the two branches of the coexistence curve (2,3) for the water-2,5 lutidine system have been used in order to check signs and values of the universal amplitude ratios of the critical laws in the forms σα,j=σc+A(T−Tc)+Bj(T−Tc)μ, j=1,2,3, with imposed μ=1.26. A and the Bj’s being highly correlated, several statistical tests using the values of the ratios have been used. Experimental values are compatible with P+=B/Bi=−0.59 (B: isochore; Bi: interfacial tension in the two phases system), maybe with Q=B/B″=−0.83 (B″ one branch of the coexistence curve) and certainly with its sign. The other ratios cannot be verified at all by the experimental data.
Classical dynamics of adsorbate–surface systems: Application to nonthermal desorption
Authors: Dzegilenko Fedor , Herbst Eric
Published: July 2002

The classical trajectory approach has been used to study the nonthermal desorption of CO from a variety of model surfaces to which it is weakly adsorbed. In addition to three degrees of freedom for the CO adsorbate (bond stretching, physisorption, libration), a significant number of lattice degrees of freedom have been included using the generalized Langevin approximation. Nonthermal amounts of energy have been put into both the CO stretching and librational modes at t=0. We find that for initial values of the stretching quantum number vstr=0–10, desorption does not take place at all within 12.5 ps unless there is also significant librational excitation. The detailed mechanism by which librational energy causes desorption is discussed. The role of the surface is also explored; we find that the probability of desorption is a nonmonotonic function of the Debye frequency of the solid in the range 28–915 cm−1, and is larger for lattices with either ‘‘high’’ or ‘‘low’’ Debye frequencies than for lattices with ‘‘intermediate’’ Debye frequencies. This result is partially explained in terms of resonances between low frequency libration and physisorption modes and high frequency phonon modes.
Monte Carlo simulation of the structural properties of concentrated aqueous alkali halide solutions at 25 °C using a simple civilized model
Authors: Llano-Restrepo Mario , Chapman Walter G.
Published: July 2002

In this paper we present the results of a Monte Carlo (MC) simulation study of the structural properties of concentrated aqueous solutions of various alkali halides at 25 °C using a simple civilized model (SCM). A simplified version of the rigid nonpolarizable SPC model of liquid water, in which the Lennard-Jones interaction between intermolecular oxygen sites is changed into a hard-core repulsion, is combined in our SCM with a treatment of the ions as charged hard spheres. Changes in the structure of the solvent, and the behavior of ionic solvation and ion pairing upon varying the concentration and size of the ions, are determined by computing the corresponding ten radial distribution functions from sufficiently long MC simulation runs for various aqueous alkali halide solutions at concentrations above 1 M. Hydration numbers are reported for the first time for NaBr and KBr, and the first simulation-based estimates for LiBr, NaI, and KI are also obtained. Whenever possible, results for the hydration numbers are compared with available experimental data and also with other simulation studies. The excellent predictive capability and simplicity of the SCM proposed here, should lead to the development of tractable theoretical approaches to aqueous 1:1 electrolyte solutions in the near future.
Characterization of particles by modulated dynamic light scattering. I. Theory
Authors: Dahneke Barton E. , Hutchins Darrell K.
Published: July 2002

We describe a new class of methods for measuring the properties of particles suspended in fluids. We have named these methods modulated dynamic light scattering (MDLS) methods. We present a detailed description of the apparatus and the theoretical foundations for a few examples of MDLS methods in which measured signals are compared to predictions calculated using Brownian motion theory to infer one or more of the particle mass, diffusion coefficient, velocity, shape category, and other properties. The direct measurement of particle mass is novel. MDLS methods can thus be used in characterizing particles with respect to both mass and surface area, size and shape, or other combinations of properties. MDLS measurements of many individual particles provide the distributions of particles over one, two, or more property values, singly or jointly. To be widely used in particle measurements, the method must measure several thousand in a few minutes. We therefore investigate precision and measurement speed and discuss size range of particle characterization by MDLS. We derive an expression for the precision of an MDLS measurement. Provision of both a measured mass or size of a single particle and an estimate of the measurement precision is also novel. The capability to measure mass, size, and other properties of individual particles at known precision should make MDLS useful in characterizing particles of unknown shape including thin flake-shaped and elongated rod-shaped particles.
Photoinduced charge transfer in bichromophoric molecules in the gas phase
Authors: van Dantzig Niels A. , Shou Hansen , Alfano Joseph C. , Yang Nien-chu C. , Levy Donald H.
Published: July 2002

We have studied a series of bichromophores of the form A-(CH2)n-D where A is a 9-anthryl group, D is either a N-methylanilino group or a N-methyl-p-methoxyanilino group, and n=1,2,3, or 4. The fluorescence excitation and emission spectra of these molecules have been observed in a supersonic jet and in a variety of solvents. In the gas phase, in the n=1 and n=3 molecules, and in an extended conformer of the n=4 molecules interactions are weak resulting only in a slightly redshifted anthracene spectrum. No redshifted emission was observed in these cases. For the n=2 molecules and a second conformer of the n=4 molecules we observed a redshifted emission in the gas phase. This emission is assigned as coming from a charge transfer (CT) state or exciplex. On the basis of the excitation and emission spectra we observed two different pathways to reach the CT state. For the molecules with donor N-methyl-N-alkylaniline, excitation took place to the locally excited (LE) state followed by relaxation to the CT state. For the molecules with donor N-methyl-N-alkyl-p-methoxyaniline, the CT state was low enough to interact with the ground state forming an intramolecular electron–donor–acceptor (EDA) complex. From this EDA ground state direct excitation to the CT state occurred. In the molecule with n=2 and donor N-methyl-p-methoxyaniline the EDA interaction is very weak allowing excitation to the LE state as well as to the CT state. This set of molecules exhibits the full range of spectroscopic behavior expected in bichromophores.
Effect of solvation on the shapes, sizes, and anisotropies of polyatomic anions via molecular electrostatic potential topography: An ab initio self-consistent reaction field approach
Authors: Luque F. J. , Orozco Modesto , Bhadane P. K. , Gadre S. R.
Published: July 2002

The effect of solvation, as studied by the self-consistent reaction field procedure, on the shapes, sizes, and anisotropies of eight small prototypical anions (OH−, NH−2, CH−3, CN−, SH−, N−3, NO−2, and NO−3) has been studied at the ab initio 6–31++G(d) level. Both the general molecular electrostatic potential distribution and the molecular electrostatic potential topography have been used for examining the effect of solvation. The results demonstrate both the complexity and specificity of the hydration effect on the solute charge distribution. It is observed that in general anions tend to shrink upon solvation. The molecular electrostatic potential, in general, becomes more negative and the electron density is increased upon solvation. The effect of the solvation on the chemical reactivity and anisotropy of the anions in aqueous environment is discussed.
Multiple time scale simulation of a flexible model of CO2
Authors: Tuckerman Mark E. , Langel Walter
Published: July 2002

The reference system propagator algorithm of Tuckerman et al. is applied to a simple model of flexible CO2. The forces are broken up according to three different time scales corresponding to the internal motion, short-range impulsive collisions, and long-range interactions, and three time steps are introduced to integrate the equations of motion. The use of this scheme allows a factor of 8 to 12 to be saved in CPU time over standard integration schemes.
Measurements of the microwave spectrum and structural parameters for benzene chromium tricarbonyl
Authors: Kukolich S. G. , Sickafoose S. M. , Flores L. D. , Breckenridge S. M.
Published: July 2002

Microwave spectra for four isotopomers of benzene chromium tricarbonyl were measured in the 4–17 GHz range using a Flygare–Balle type pulsed beam spectrometer. Rotational constants obtained are B(52Cr)=732.8886(6) MHz and B(53Cr)=732.8966(3) MHz. Asymmetric top spectra were observed for a single 13C substitution on the benzene ring giving B(13C–bz)=729.9606(3) and C(13C–bz)=727.9024(2) MHz. For a single 13C substitution on one of the carbonyl carbons B(13CO)=731.9036(8) and C(13CO)=729.1657(8) MHz. Since no effects of possible internal rotation were observed on the 13C asymmetric top spectra, we can place a lower limit on the V6 potential for internal rotation of V6≳4.0 THz (=1.6 kJ/mole). The centrifugal distortion constants are small, DJ=0.05 kHz and DJK=−0.05 kHz, indicating a fairly rigid structure. The 53Cr quadrupole coupling strength is low, eqQ(53Cr)=−12.11(1) MHz, indicating a near octahedral charge distribution around the Cr atom. Structural parameters obtained are the center of the benzene chromium distance r(Cr–bz)=1.67(2) Å, the chromium–carbonyl bond length r(Cr–CO)=1.86(1) Å and the OC–Cr–CO interbond angle α=88(1)°.
Grand canonical Monte Carlo simulations of the distribution and chemical shifts of xenon in the cages of zeolite NaA. I. Distribution and 129Xe chemical shifts
Authors: Jameson Cynthia J. , Jameson A. Keith , Baello Bernoli I. , Lim Hyung-Mi
Published: July 2002

The equilibrium distribution of the Xe atoms among the alpha cages of the zeolite NaA have been measured directly by nuclear magnetic resonance (NMR) in ten samples ranging from very low xenon loading up to saturation. These distributions are simulated by a grand canonical Monte Carlo (GCMC) method which reproduces the experimental data quantitatively for all ten samples at 296 K and also at 360 K. The adsorption isotherm of the high loading samples has been determined directly from the chemical shift of the gas in equilibrium with the adsorbed xenon. The data compare favorably with the adsorption isotherms resulting from the simulations. The previously reported 129Xe chemical shifts of the individual Xen clusters and their temperature dependences in the range 188–420 K are reproduced quantitatively by the GCMC simulation which makes use of pairwise additive ab initio intermolecular shielding functions. These cluster shifts and their temperature dependence encode the distribution of configurations for a given Xen cluster in an alpha cage. Quantitative agreement with the three experimental measures of the distribution of Xe atoms in NaA (partitioning between the adsorbed phase and the gas phase, distribution of the intrazeolitic atoms among the alpha cages, and the distribution of Xe atoms within an alpha cage containing Xen) as a function of temperature has been achieved for the first time.
Adsorption-stretching analogy for a polymer chain on a plane. Symmetry property of the phase diagram
Authors: Skvortsov Aleksandr M. , Gorbunov Aleksey A. , Klushin Leonid I.
Published: July 2002

We consider a Gaussian chain of finite length adsorbing on a planar surface with the stretching force applied to its free end. The model is exactly solvable and appears to have an unexpectedly rich behavior, exhibiting in the thermodynamic limit N→∞ phase transitions of both the first and the second order. A closed analytical form for the partition function at arbitrary N is obtained. The phase diagram is completely symmetrical with respect to stretching force and adsorption strength parameters. Thus, a rigorous analogy is established between the adsorption of a chain onto a plane in the absence of external force and the stretching of a nonadsorbed chain attached to a plane by one end, both being the second-order transitions. Competition of simultaneous stretching and adsorption leads to a first-order transition. Adsorption of ring polymers is discussed as a natural by-product of this scheme. Nonequilibrium free energy is obtained as a function of the order parameter and compared with the Landau theory of phase transitions. Finite-size effects are discussed.
The spatial evolution of particles diffusing in the presence of randomly placed traps
Authors: Dunlap D. H. , LaViolette Randall A. , Parris P. E.
Published: July 2002

The evolution of a particle undergoing a continuous-time random walk in the presence of randomly placed imperfectly absorbing traps is studied. At long times, the spatial probability distribution becomes strongly localized in a sequence of trap-free regions. The subsequent intermittent transfer of the survival probability from small trap-free regions to larger trap-free regions is described as a time-directed variable range hopping among localized eigenstates in the Lifshitz tail. An asymptotic expression for the configurational average of the spatial distribution of surviving particles is obtained based on this description. The distribution is an exponential function of distance which expands superdiffusively, with the mean-square displacement increasing with time as t2/ln(2D+4)/D(t) in D dimensions.
A linear algebraic method for exact computation of the coefficients of the 1/D expansion of the Schrödinger equation
Authors: Dunn Martin , Germann Timothy C. , Goodson David Z. , Traynor Carol A. , Morgan John D. , Watson Deborah K. , Herschbach Dudley R.
Published: July 2002

The 1/D expansion, where D is the dimensionality of space, offers a promising new approach for obtaining highly accurate solutions to the Schrödinger equation for atoms and molecules. The method typically employs an asymptotic expansion calculated to rather large order. Computation of the expansion coefficients has been feasible for very small systems, but extending the existing computational techniques to systems with more than three degrees of freedom has proved difficult. We present a new algorithm that greatly facilitates this computation. It yields exact values for expansion coefficients, with less roundoff error than the best alternative method. Our algorithm is formulated completely in terms of tensor arithmetic, which makes it easier to extend to systems with more than three degrees of freedom and to excited states, simplifies the development of computer codes, simplifies memory management, and makes it well suited for implementation on parallel computer architectures. We formulate the algorithm for the calculation of energy eigenvalues, wave functions, and expectation values for an arbitrary many-body system and give estimates of storage and computational costs.
A new formulation of quantum transition state theory for adiabatic rate constants
Authors: Hansen Nancy Fisher , Andersen Hans C.
Published: July 2002

A new formulation of quantum transition state theory for adiabatic reactions is proposed. It is based on the flux–flux correlation function of Miller, Schwartz, and Tromp, and calculation of a rate constant requires only the calculation of this correlation function and its second derivative at zero time. The theory is tested for a one-dimensional Eckart barrier and a parabolic barrier linearly coupled to a harmonic oscillator. This quantum transition state theory is exact for a one-dimensional parabolic barrier, is accurate for the model problems studied even under highly quantum conditions, and in the classical limit (ℏ→0) becomes classical transition state theory. An analytic continuation procedure for calculating corrections to the transition state theory is discussed.
Kinetic studies of ionic excimers
Authors: Schumann M. , Langhoff H.
Published: July 2002

Ionic excimers are potential candidates for storage media in vacuum ultraviolet (VUV) lasers. In order to study their production mechanisms the VUV fluorescence was observed after exciting corresponding gas mixtures by an intense coaxial electron beam. The deposited energy was increased up to 100 mJ/cm3 bar (Ar). Out of three classes of ionic excimers the species Ar2+2 (190 nm), Ne+Kr (166, 184 nm), and Kr+K (135 nm) were studied in detail. In the case of Kr+K buffer gas He with a pressure up to 30 bar was added in order to increase the fluorescence intensity. Measurements were performed at various gas densities. At optimum conditions with respect to the quantum efficiency ηQ, values of ηQ=8.5×10−2, 4×10−2, and 3×10−2, respectively, were determined. At pumping densities in the order of 10 mJ/cm3 the quantum yield already decreases. The maximum achievable fluorescence energy was found to be 40, 30, and 10 μJ/cm3, respectively. By comparing the results with predictions of rate equation models rate constants for the most important reactions were obtained. It was shown that quenching processes of the excimer molecules and precursor ions by neutrals and/or plasma electrons limit the obtainable excimer densities below values necessary to achieve optical gain.
Computational studies of halogen chemistry on rare-gas surfaces. II. Structure of chlorine and bromine submonolayer films on Ar(111) and Xe(111) prepared by molecular beam dosing
Authors: Christoffel Kurt M. , Trayanov A. L. , Prisant M. G.
Published: July 2002

The deposition by molecular beam dosing of halogen molecules on rare-gas surfaces has been studied with molecular dynamics simulation. Specifically we have considered films formed by the sequential adsorption reactions: X2g+X2[Θ]ads−Rg(111)[T]→X2(ads)−X2[Θ]ads −Rg(111)[T], where Θ is the film coverage defined by X2[adsorbed]/Rg[surface], T indicates the substrate temperature, X2 is the halogen adsorbate which is either chlorine or bromine, and Rg indicates the rare-gas substrate which is either argon or xenon. The structure of halogen adlayers was studied as a function of coverage for films grown on rare-gas substrates at different temperatures. Chlorine and bromine films on argon exhibit orientational ordering and islanding with increasing coverage. The tendency of the halogen diatoms to align along the surface normal with increasing coverage is strongly enhanced by higher temperatures in the case of chlorine on xenon, moderately enhanced in the case of chlorine on argon, and unchanged in the case of bromine on argon. Chlorine and bromine films form three-dimensional aggregated structures on argon at 15 and 25 K. Chlorine forms a two-dimensional amorphous layer on xenon at 15 K and a highly ordered layer at 50 K. The dynamics of the molecular adsorption event were studied at three different coverages: Θ=0.05, 0.25, and 0.5. At all these coverages, we observed finite possibility of diffusional motion of adsorbate on the surface immediately after it lands: This was not seen at zero coverage.
A Monte Carlo method for simulating associating fluids
Authors: Busch N. A. , Wertheim M. S. , Chiew Y. C. , Yarmush M. L.
Published: July 2002

We present association biased Monte Carlo (ABMC), a Monte Carlo method, which is ergodic, microscopically reversible, and specifically designed to simulate associating fluids with long-ranged center-to-center interactions. The canonical ensemble (NVT) algorithm biases sampling to regions of configuration space where particle association is likely to occur, and provides efficient simulation of associating fluids over a broad range of densities. The usual canonical ensemble (NVT) thermodynamic variables (ensemble average internal energy and pressure), as well as the pair distribution functions are presented. The distributions of associated clusters are presented at a selection of state points and are compared with predictions of thermodynamic perturbation theory for the model system. We also present the simulation results for a symmetric, binary associating fluid with a single site on each particle.
Coexistence curve of polystyrene in methylcyclohexane. VIII. Crossover from Ising to renormalized Ising behavior in ternary system
Authors: Dobashi Toshiaki , Nakata Mitsuo
Published: July 2002

In order to examine the crossover from the Ising behavior to the renormalized Ising behavior, we have measured the coexistence curves of the ternary system, polystyrene I+ polystyrene II+ methylcyclohexane, by varying the molecular weight ratio r of polystyrene II to polystyrene I and volume fraction ξ2 of polystyrene II in total polystyrene. The observed critical exponent βt for the concentration difference between two coexisting phases varies from 0.33 to 0.38 with the increase of parameter k=ξ2r(r−1)2. This result is consistent with the theory of Broseta and Leibler [J. Chem. Phys. 90, 6652 (1989)]. The exponent for the diameter μt was found in the range from 0.81 to 0.92. The average of μt=0.86 is close to the exponent μ for binary systems.
A study of HOCO resonances in the OH+CO→CO2+H reaction
Authors: Hernández Marta I. , Clary David C.
Published: July 2002

Quasibound states of the HOCO intermediate in the reaction OH+CO→H+CO2 are studied by means of the stabilization method. A reduced-dimensionality model is used. The calculated resonance positions and lifetimes agree well with previous quantum scattering calculations. Most of the quasibound states can be assigned approximately to a vibrational excitation in the higher frequency OH vibration and different progressions in the HO–CO stretching mode. Some interesting mode-specific effects in the dynamics of the unimolecular decay are predicted.
Solution behavior of buckminsterfullerene (C60) in benzene
Authors: Ying Qicong , Marecek James , Chu Benjamin
Published: July 2002

Solution characterization of buckminsterfullerene C60 in benzene was carried out by laser light scattering. Static and dynamic light scattering (SLS and DLS) experiments were accomplished by using an incident laser beam of 790 nm wavelength, and a fiber optic probe which was coupled to a RCA C31034 photomultiplier tube sensitive to the near ir wavelength. The effective hydrodynamic radius of C60 in benzene solution determined by using DLS was 6.4±0.1 Å, in close agreement with the distance measured from the center of the solvated benzene to the center of the benzene-solvated C60 (C60⋅4C6H6):6.7∼6.8 Å as evaluated from the single crystal structure by x-ray diffraction. C60 was found to aggregate slowly even in fairly dilute solution concentrations ranging from 0.78 to 1.39 mg/mL at temperatures 23∼27 °C. Based on the high specific refractive index increment of C60 in benzene, the molar mass of the C60 aggregates could be estimated. Together with the radius of gyration, the hydrodynamic radius, and its size distribution, the kinetics of the aggregation process was investigated. The aggregation of C60 in benzene solution was reversible, i.e., the C60 aggregates could be dispersed simply by hand shaking the solution. The light-scattering intensity results indicated that the structure of C60 aggregates could be described as a fractal with a dimension of 2.10 and the aggregation kinetics exhibited essentially an exponential behavior.
The ν1 band of ketene
Authors: Escribano Rafael , Doménech José L. , Cancio Pablo , Ortigoso Juan , Santos Julio , Bermejo Dionisio
Published: July 2002

Infrared and Raman spectra of the ν1 band of ketene, H2CCO, have been recorded at Doppler resolution. The infrared spectrum has been obtained with a difference frequency infrared spectrometer, and the Raman spectrum of the Q branches of this band has been recorded using a stimulated Raman spectrometer. The vib-rotational analysis of the data is very complicated because of many crossings with other vibrational states, which can interact with v1=1 through Fermi or Coriolis mechanisms. We present a discussion on global and local resonances, and we are able to extract information on the perturbing levels and on the perturbation parameters, even though the perturbing bands are not always detected in the spectrum.
The dependence of DNA tertiary structure on end conditions: Theory and implications for topological transitions
Authors: Tobias Irwin , Coleman Bernard D. , Olson Wilma K.
Published: July 2002

Explicit expressions are derived for the equilibrium configurations of long segments of a DNA double helix subject to boundary conditions of the type imposed by DNA-bending proteins at the ends of otherwise free segments. The expressions, which are exact within the framework of Kirchhoff’s theory of elastic rods, show that, in appropriate ranges of parameters, small changes in end conditions can result in large changes in tertiary structure. A discussion is given of the implications of this observation for understanding the action of bending proteins and of proteins that induce topological transitions that change the linking number of closed loops of DNA.
Dynamic displacement of O2 from Pt(111): A new desorption mechanism
Authors: Rettner C. T. , Lee J.
Published: July 2002

Exposing an O2-covered Pt(111) surface to beams of O, N, and H atoms leads to prompt desorption of the O2, which leaves the surface with a bimodal velocity distribution that appears the same in each case, but which is different from that obtained for thermal desorption. We propose that the O2 molecules are directly displaced by the incident atoms and that part of the adsorption energy is carried away by the desorbing species. Photodesorption at 213 nm gives an identical velocity distribution, indicating that a similar displacement mechanism may operate.
Pressure-induced concentration fluctuations in N-isopropylacrylamide gels
Authors: Kato Eiji
Published: July 2002

The scattered light intensity as a function of solvent pressure and the equilibrium swelling curve under the high pressure (900 atm) are measured for N-isopropylacrylamide gels. The concentration fluctuations in the gel diverges at a certain pressure which depends on temperature of the gel. The spinodal temperature is obtained as a function of solvent pressure. The divergence of concentration fluctuations induced by pressure is explained by the mean-field theory of phase separation in a binary mixture, which is reported by Tanaka et al., by taking account of the pressure dependence of the polymer solvent interaction parameter. The pressure dependence of the parameter is estimated from the pressure dependence of the swelling curve.
Water near a planar interface: Atom-based integral equation theory
Authors: Booth Michael J. , Duh Der-Ming , Haymet A. D. J.
Published: July 2002

The structure and properties of an atom-based model of water next to a planar interface are solved using an approximate integral equation theory. The input to the calculations is the structure of the bulk water in the form of the direct correlation functions. Predictions from the theory include the oxygen and hydrogen density profiles perpendicular to the interface, the mean electrostatic potential, the potential of zero charge, and the differential capacitance. The predicted structure is relatively insensitive to both the surface potential and the details of the short-range wall–water potentials, and exhibits a layered structure which extends approximately 15 Å into the liquid. For our initial choice of the short-range wall–water potential, we predict a value of −32 mV for the potential of zero charge and a differential capacitance of 4.89 μF cm−2 for pure water at a planar interface. The capacitance is apparently independent of the surface charge density and the surface potential.
On the generation of preferential Λ-doublet populations in the collisional relaxation of highly rotationally excited CH(X 2Π)
Authors: Alexander Millard H. , Dagdigian Paul J.
Published: July 2002

By means of full quantum close-coupling and coupled states calculations based on an ab initio potential energy surface for the Ar–CH system, we confirm a propensity seen experimentally by Hancock, Stuhl, and their co-workers. During the rotational relaxation of high rotational levels of the CH(X 2Π) radical, produced by photolysis of a suitable precursor, there appears a clear population imbalance in favor of the Λ-doublet levels of Π(A″) symmetry. A full kinetic simulation, based on the calculated cross sections, reproduces nearly quantitatively the experimental observations of both the temporal evolution and the pressure dependence of this Λ-doublet asymmetry. This asymmetry is a consequence of both an enhanced depletion of high N Π(A′) levels and the enhanced formation of Π(A″) levels in the next lower (N−1) manifolds. The physical origin of this propensity involves a crossing between two adiabatic bender potentials which follow, respectively, the A′ and A″ potential energy surface (PES). This crossing occurs only for the ‘‘helicopter-like’’ approach of the CH molecule, in which its rotational angular momentum is aligned along the initial relative velocity vector. Thus, a strong v, N correlation in the reactant channel results in a strong Λ, N correlation in the product channel.
Kinetics at the collapse transition Gaussian self-consistent approach
Authors: Timoshenko E. G. , Kuznetsov Yu. A. , Dawson K. A.
Published: July 2002

We introduce an approximation to the Langevin equation that can be used to study the nonequilibrium dynamics and kinetics of polymer conformational transitions in dilute solution. The approach we describe involves the introduction of a time-dependent effective potential, ΔVq(t), and effective friction ζq. The potential is used to generate a time-dependent Gaussian ensemble and we derive time-dependent self-consistent equations that can be analyzed numerically, or by asymptotic methods. We work out various examples of the homopolymer kinetics, including relaxation of a Flory coil and the collapse transition. For the latter we argue that there are various characteristic regimes after a fast quench that carry us from the Flory to collapsed state. We have explicitly worked out the early stage kinetics where we find a process rather like spinodal decomposition, but where the degrees of freedom are confined to the internal metric of the polymer chain. The chain evolves to produce a chain with a near-periodic arrangement of locally collapsed blobs, which is then believed to coarsen, and we have discussed this phenomenon elsewhere.
Spectroscopic study of rare-gas excimer formation in a direct-current discharge with supersonic expansion
Authors: Kiik M. J. , Dubé P. , Stoicheff B. P.
Published: July 2002

Emission spectra of the rare-gas excimers Ar2*, Kr2*, and Xe2* were excited in a dc jet discharge with supersonic expansion. Absorption and fluorescence emission measurements provided atomic population densities for levels of the 1s manifold. Changes in intensities of the atomic resonance lines and the VUV bands were examined as the plasma was irradiated with laser radiation tuned to specific atomic transitions between levels of the 1s and 2p manifolds. This technique of optical pumping has established that excimers in the A 3Σ+u state are the main contributors to the observed bands from this source. Rate equations were developed to explain the observed intensity changes. Comparisons of the calculated and observed changes indicated that population mixing amongst levels of the 1s manifold caused by electron collisions is an important process in regulating the population in the 1s5 atomic level that leads to the formation of rare-gas excimers by collisions with ground level atoms.
Chemical potentials of hard molecular solutes in hard sphere fluids. Monte Carlo stimulations and analytical approximations
Authors: Stamatopoulou Argyroula , de Souza Luís E. S. , Ben-Amotz Dor , Talbot Julian
Published: July 2002

Monte Carlo measurements of the chemical potential of hard diatomics and polyatomics dissolved in hard sphere fluids are reported. These are performed as a function of density, solute size, and diatomic bond length. Bond length derivatives are used to determine the mean force along the diatomic bond axis. The results are compared with analytical expressions derived from the hard fluid (HF) model, a model proposed by Boublik, and a spherical approximation to diatomic and polyatomic chemical potentials.
Phase separation dynamics in the presence of surfactant molecules with an asymmetric shape
Authors: Kawakatsu Toshihiro , Kawasaki Kyozi , Furusaka Michihiro , Okabayashi Hirofumi , Kanaya Toshiji
Published: July 2002

We investigate the dynamics of phase separation processes in the presence of added surfactant molecules with an asymmetric shape, e.g., surfactant molecules with asymmetric hydrophilic/hydrophobic sizes, or block copolymers with asymmetric compositions of subchains. Upon adsorption onto interfaces, such asymmetric surfactant molecules produce a spontaneous curvature of interfaces. The analysis is based on the hybrid model, with which one can investigate the macroscopic phase separation phenomena while keeping some essential molecular features of the surfactant. Computer simulations are performed using the hybrid model and we found a spontaneous morphological change from a bicontinuous structure to a globular structure during the phase separation process. Such a morphological change is caused by the spontaneous curvature of the interfaces where the surfactant molecules are adsorbed. A simple argument to explain such a phenomenon is also presented.
Conformation and thermodynamic properties of repeated-block copolymers
Authors: Gan Hin Hark , Eu Byung Chan
Published: July 2002

Conformational properties of isolated linear copolymers are studied by means of the integral equation theory of polymers. We examine two-letter copolymers that have repeated-block symmetry; the potential between like monomers is repulsive and that between unlike ones is a Lennard-Jones potential. This class of copolymer sequences satisfies an approximate translational invariance symmetry for the correlation functions. Conformational behavior of any given copolymer is analyzed by computing its configurational and thermodynamic properties from the information of its sequence and potentials of interaction. All properties calculated show that they are independent of sequence heterogeneity at high temperatures. The influence of sequence heterogeneity becomes significant below the theta point. Sequences that are compact and have low excess entropy generally exhibit two key features: (a) their monomer composition is symmetric and (b) the unlike monomer species tend to alternate in the sequence. These conditions are consistent with the minimization of unfavorable interactions. In addition, the thermodynamics functions of copolymers are discussed in the context of the Flory-type theory.
Spectral comparison of the gerade Rydberg states of Br2 with different angular momenta
Authors: Wang Pei-nan , Xu Lei , Wang Ya-fei , Li Fu-ming
Published: July 2002

Rotational structure of [2Π3/2]6s:2g Rydberg state of bromine has been studied by high resolution resonance enhanced (2+1) multiphoton ionization spectroscopy and computational simulation. Excellent agreement between theoretical analysis and experimental results was achieved. Spectral difference for two-photon transitions to Rydberg states of [2Π3/2]6s:2g and [2Π3/2]4d:1g with different angular momenta Ω is discussed.
Two-dimensional imaging of metastable CO molecules
Authors: Jongma Rienk T. , Rasing Theo , Meijer Gerard
Published: July 2002

Direct time and spatially resolved detection of metastable CO molecules, prepared in selected quantum states via pulsed laser excitation, is experimentally demonstrated in a molecular beam machine. Characterization of the molecular beam in terms of parallel and perpendicular velocity distributions and rotational temperatures is performed. A direct two-dimensional (2D) demonstration of the mass-focusing effect in binary gas mixtures is given. Two-dimensional imaging of the spatial distribution of the metastable a 3Π CO molecules in the beam after passage through a hexapole field is used to study hexapole focusing performance. Structured 2D images demonstrate the dependence of the focusing characteristics on the magnitude of the Λ-doubling and on the angular dependence of the focusing force in a hexapole consisting of cylindrical rods.
A vibrationally adiabatic theory of molecular Penning ionization
Authors: Bevsek H. M. , Siska P. E.
Published: July 2002

Based on available theoretical and experimental information on the Penning ionization of molecules by metastable helium and neon atoms at thermal energies, an approximate theoretical approach for incorporating the vibrational degrees of freedom is developed. The electronically excited, metastable atoms have diffuse outer orbitals, giving rise to relatively soft intermolecular repulsion in nonbonded excited state potentials. A low-energy ionizing collision is then near-adiabatic in respect to its effect on the vibrations of the molecule under attack. In addition for the great majority of experimentally studied molecules, nearly vertical vibrational populations in the Penning molecular ion are observed in Penning ionization electron spectroscopy (PIES). In the simplest limit of vibrational adiabaticity, the bond oscillator remains unperturbed by the collision, and small deviations from verticality may then be interpreted as a reflection of the bond-length dependence of the discrete-continuum coupling that gives rise to ionization. The theory presented may be cast into an approximate but simple form that allows the ready extraction of such information from a complete set of vibrational populations. Recent experimental studies of the He*+H2 system provide both justification and an example of the application of the theory to reduction of population data.
Vibrational dynamics of a stepped metallic surface: Step-edge phonons and terrace softening on Ni(977)
Authors: Niu L. , Koleske D. D. , Gaspar D. J. , Sibener S. J.
Published: July 2002

Inelastic helium atom scattering has been used to measure the surface and step localized phonons on a stepped metallic surface, Ni(977). These time-of-flight measurements were carried out both perpendicular and parallel to the step direction. Surface phonon dispersion data collected across the steps show backfolding of the surface Rayleigh mode, and, most importantly, dramatic softening as compared to the forces present at the smooth Ni(111) surface. This softening suggests significant relaxation perpendicular to the step edge. Single-phonon scattering data collected along the step direction reveals the presence of two new step-edge localized modes, as well as the Rayleigh mode for this direction of the crystal. The Rayleigh mode here does not exhibit the notable softening that was found for the other direction. Novel in- and out-of-phase scattering measurements, with respect to the terraces, lead us to assign the new step induced modes as the two transversely polarized vibrations which propagate along the direction of the step edge. An analytic one-dimensional lattice model is proposed which well represents the dispersion data for these two step modes; its use allows us to determine the effective local force field in the two transverse directions with respect to the step edge. The findings reported herein shed new light on such topics as interface stability, crystal growth, and charge redistribution in the vicinity of well-characterized extended surface defects.
Quantum dynamics of a planar model for the complex forming OH+CO→H+CO2 reaction
Authors: Goldfield Evelyn M. , Gray Stephen K. , Schatz George C.
Published: July 2002

The quantum dynamics of three and four degrees-of-freedom planar models of the OH+CO→H+CO2 reaction are discussed. These computationally intensive calculations, which are carried out on a scalable parallel computer, illustrate the role of HOCO reaction intermediates or scattering resonances. The results are contrasted with previous two and three degrees-of-freedom quantum results, as well as with two, three, four, and six degrees-of-freedom quasiclassical trajectory calculations. While our quantum calculations are restricted to total angular momentum J=0, it is possible to estimate the thermal rate constant using a J-shifting approximation, and to make comparison with experiment and previous full-dimensional classical trajectory results.
Global dipole moment function for the X 1Σ+ ground state of CO
Authors: Langhoff Stephen R. , Bauschlicher Charles W.
Published: July 2002

We have studied the dipole moment function (DMF) for the X 1Σ+ state of CO as a function of the completeness of the one- and n-particle treatments. Our best DMF is obtained using an augmented correlation-consistent quadruple-zeta basis set with external correlation included using the averaged-coupled-pair functional approach from a complete-active-space self-consistent-field zeroth-order reference. The DMF evaluated using the finite-field approach is in far better agreement with the experimentally deduced DMF than all previous theoretical determinations, but systematic differences still remain in the DMF at larger internuclear distances that give rise to significant discrepancies between the theoretical and experimental Einstein coefficients for transitions involving vibrational quantum numbers above about v=15.
Ab initio finite oligomer method for nonlinear optical properties of conjugated polymers. Hartree–Fock static longitudinal hyperpolarizability of polyacetylene
Authors: Kirtman Bernard , Toto Joseph L. , Robins Kathleen A. , Hasan Muhammad
Published: July 2002

Ab initio restricted Hartree–Fock 6-31G calculations are reported for the static longitudinal hyperpolarizability of the linear polyenes C4H6 through C44H46. Using a new extrapolation technique the infinite chain value in polyacetylene is determined with an accuracy similar to that achieved for small molecules. This is the first in a series of articles leading to a comprehensive ab initio treatment for the nonlinear optical properties of conjugated polymers.
Si adatom binding and diffusion on the Si(100) surface: Comparison of ab initio, semiempirical and empirical potential results
Authors: Smith A. P. , Wiggs J. K. , Jónsson H. , Yan H. , Corrales L. R. , Nachtigall P. , Jordan K. D.
Published: July 2002

The binding energies and configurations for single Si adatoms on the Si(100) surface are investigated theoretically. Detailed comparisons between previously published and new calculations using classical potentials, semiempirical formulations, and density functional theory (DFT) are made. The DFT calculations used both the plane-wave-pseudopotential approach in a periodic slab geometry and the Gaussian-orbital based all-electron approach employing cluster geometries. In the local-density approximation excellent agreement between the cluster and slab results was obtained. Inclusion of gradient corrections to the exchange-correlation energy significantly improves absolute binding energies and changes relative energies by as much as 0.3–0.5 eV depending on the particular exchange-correlation functional used. Binding energies and relative energies obtained using the classical potentials disagree with the gradient corrected DFT energies at about the 0.6–0.9 eV level, and most find qualitatively different local minima from those found in the DFT calculations. The semiempirical approaches give results intermediate in quality between those of the classical potentials and the ab initio calculations. Analysis of the energies and binding site geometries provides insight into the shortcomings of some of the classical potentials.
Isotopic dependence of predissociation linewidths in the Schumann-Runge bands of oxygen
Authors: Cheung A. S-C. , Mok D. K-W. , Yoshino K. , Parkinson W. H. , Jamieson M. J. , Dalgarno A. , Child M. S.
Published: July 2002

It is demonstrated that, according to semi-classical theory, the isotopic dependence of the predissociation linewidths in the Schumann-Runge bands of oxygen cannot be removed by simple scaling of the reduced mass. This is in contrast to the isotopic dependence of the predissociated vibrational energy levels.
Rotational state selected vibrational overtone spectroscopy of jet-cooled molecules
Authors: Boyarkin O. V. , Rizzo T. R.
Published: July 2002

The combination of infrared-optical double resonance excitation with infrared laser assisted photofragment spectroscopy (IRLAPS) detection permits the measurement of rotationally resolved vibrational overtone spectra of jet-cooled CF3H. The IRLAPS detection technique is sufficiently selective to be able to discriminate against the dissociation of molecules in the ground state and the vCH=1 level used as an intermediate in the double resonance scheme. The combination of this selective detection technique with both supersonic cooling and double resonance produces J state resolved overtone spectra of the 42 vibrational overtone band. These spectra put a limit on the homogeneous linewidths of the rovibrational transitions of ∼0.5 cm−1.