bluuebunny/crossref_metadata_2025 huggingface dataset:
The potential distribution-based closures to the integral equations for liquid structure: The Lennard-Jones fluid
Authors: Lee Lloyd L.
Published: July 2002

The potential distribution theorems for the test particles provide a connection to the chemical potentials and the cavity distribution functions y(r) much used in molecular theory. These relations can be capitalized for establishing the closure relations for the Ornstein–Zernike equation. In this study, we formulate a class of closures with built-in flexibilities in order to satisfy the potential distribution theorems (or the related zero separation theorems) and thermodynamic consistency. The theory is self-contained within the integral equation framework. We test it on the Lennard-Jones fluid over ranges of temperatures (down to T*=0.81) and densities (up to ρ*=0.9). To achieve self-sufficiency, we exploit the connections offered by writing down n members of the mixture Ornstein–Zernike equations for the coincident oligomers up to n-mers. Then the potential distribution theorems generate new conditions for use in determining the bridge function parameters. Five consistency conditions have been identified (three thermodynamic and two based on zero-separation values). This self-consistency allows for bootstrapping and generation of highly accurate structural and thermodynamic information. The same procedure can potentially be extended to soft-sphere potentials other than the Lennard-Jones type.
Aggregate model of liquids
Authors: Kilian H. G. , Zink B. , Metzler R.
Published: July 2002

Atomic-force pictures reveal a heterogeneous microstructure at the surface of glassy layers which should be similar to one of the many equivalent microstructures a liquid is running through. These microstructures are described with the aid of a kinetic model of reversible aggregation which goes back to formulations as used in the description of living polymerization or aggregation of polymers in solution. Aggregates are considered as dynamic subsystems wherein collective modes of motions are excited. Fluctuations of the aggregates, densely packed in a disordered pattern, leads to a broad size distribution which happens to be controlled by Boltzmann’s factor. The disordered structure within the aggregates themselves is optimized, reduced aggregate energy and entropy should be equal. Symmetries are deduced which elucidate many universal properties of the dynamic microstructure of liquids. Thermodynamic properties like the specific heats of aggregation in liquids or the dependence of the glass transition of homologues of linear atactic polystyrene are consistently described.
Structure of liquid carbon tetrachloride up to the subcritical point measured by wide Q-range neutron diffraction
Authors: Akatsuka H. , Misawa M. , Fukunaga T. , Mizutani U. , Furusaka M.
Published: July 2002

Structure of liquid and gaseous CCl4 at elevated temperatures from 195 °C to the subcritical point 275 °C (Tc=285.15 °C) along the liquid–vapor coexistence curve have been studied by means of pulsed neutron diffraction. Structure factor in a wide range of momentum transfer Q from 0.03 to 20 Å−1, covering from the small angle scattering region to atomic-scale structure region, has been obtained. Long-range density fluctuation and atomic scale structure have been analyzed: (1) molecular orientational correlation still exists in liquidCCl4 at 275 °C while it disappears in its saturated vapor; (2) correlation lengths of density fluctuation are 10–13 Å for the liquid at 275 °C and 13–17 Å for its saturated vapor; and (3) molecular-density fluctuation (〈Nm2〉−〈Nm〉2)/〈Nm〉 is estimated to be 2.4 for the liquid at 275 °C and 9.3 for its saturated vapor.
Distribution and Xe129 NMR chemical shifts of Xen clusters in the alpha cages of zeolite AgA
Authors: Jameson Cynthia J. , Lim Hyung-Mi
Published: July 2002

The distributions and 129Xe NMR chemical shifts of xenon in zeolite AgA have been measured in a series of experiments by Moudrakovski, Ratcliffe, and Ripmeester [Proc. Internat. Zeolite Conference, Quebec, 1995; unpublished]. We carry out grand canonical Monte Carlo (GCMC) simulations of xenon in a rigid zeolite AgA lattice to provide the average Xen cluster shifts, and the distributions Pn for comparison with their experiments. The GCMC results for the distributions, the fraction Pn of the alpha cages containing n Xe atoms, are compared with the experimental distributions in 12 samples and the agreement is excellent. The distributions in NaA and in AgA are very similar, as can be established from the comparison of the dispersion of the distributions, {〈n2〉−〈n〉2}, and both are different from the idealized hypergeometric distribution, in which the component atoms occupy eight lattice sites per cage under mutual exclusion. The calculated chemical shift increments [σ(Xen)−σ(Xen−1)]AgA are in good agreement with experiment. The differences between these and the increments in zeolite NaA,{[σ(Xen)−σ(Xen−1)]AgA−[σ(Xen)−σ(Xen−1)]NaA}, are fairly small and are in good agreement with experiment. The absolute 129Xe chemical shifts of Xen in the alpha cages of AgA are nearly uniformly shifted by about 40 ppm compared to the Xen clusters in NaA. This is attributed to the Fermi contact shifts arising from the Ag0 metal atoms that form the linear Ag32+complexes that are found within the beta cages of AgA.
The isotropic–nematic phase transition in a fluid of dipolar hard spherocylinders
Authors: Williamson Dave C. , del Rio Fernando
Published: July 2002

A thermodynamic perturbation theory in the high temperature expansion is presented for a system of hard spherocylinders with a permanent longitudinal dipole positioned at the center of the molecule. The free energy of the reference hard spherocylinder is described using an expression derived by Lee [J. Chem. Phys. 80, 7036 (1988)]. The dipole contribution to the free energy is evaluated using a generalization to the perturbation theory for dipolar hard spheres proposed by Larsen, Rasaiah, and Stell [Mol. Phys. 33, 987 (1977)]. The effect of increasing dipole strength on the isotropic-nematic phase transition of a fluid of spherocylinders of aspect ratio, L/D=5, is studied using a generalization to the well known Onsager theory [Ann. N. Y. Acad. Sci. 51, 627 (1949)]. The single particle orientational distribution function is approximated with a gaussian trial function while an ad hoc approximation is used for the pair and triplet correlation functions of the hard spherocylinder reference system. While these approximations seem quite severe they are not expected to affect the general features of the phase diagram. Increasing the dipole moment is found to destabilize the nematic phase with respect to the isotropic phase, shifting the phase transition to higher densities and pressures. This is in general agreement with the most recent simulation studies of the system. All other theories of this system predict a stabilization of nematic phase. It is suggested that the reason for the failure of these theories lies in the two body character of their approach to the orientational order in the liquid crystal phase. To the knowledge of the authors this is the first theory for the isotropic-nematic phase transition of dipolar hard-spherocylinders which explicitly includes three-body interactions in the orientationally ordered phase.
Phase diagrams of polymer dispersed liquid crystals
Authors: Matsuyama Akihiko , Kato Tadaya
Published: July 2002

A simple model is introduced to describe liquid crystal transitions and phase separations in binary mixtures of a flexible polymer and a liquid crystal. By combining the McMillan theory for the smectic A phase of liquid crystals with the Flory–Huggins theory for the isotropic mixing of two components, we examine binodal and spinodal lines on the temperature-concentration plane. We predict the appearance of phase separations such as the smectic A-nematic, smectic A-isotropic, and smectic A-nematic-isotropic phase separations. We also find a tricritical point caused by the interference between a second-order liquid crystal transition and a phase separation.
Transient dynamics of solvatochromic shift in binary solvents
Authors: Petrov Nikolai Kh. , Wiessner Alexander , Staerk Hubert
Published: July 2002

Using the picosecond spectro-streak method we monitored the dynamics of preferential solvation of charge-transfer dipoles in binary mixtures of solvents strongly different in dielectric constants. The observed slow (⩽1 ns) spectral red-shift of the charge-transfer (exciplex) fluorescence is considered to be mainly due to the formation and the dielectric enrichment of the solvation shell around the solute molecules in their dipolar excited charge transfer state by diffusion of polar molecules from the bulk of the solvent mixture. As a fluorescent probe molecule we used 8-N,N-(dimethylamino)-11H-indeno[2,1-a]pyrene. The characteristic time of the solvatochromic shift varies from 400 ps to 140 ps for toluene/DMSO (dimethylsulfoxide) mixtures as the volume fraction of the polar component increases. Typical Stokes shifts are of the order of 1000–3000 cm−1. A basic concept of the classical Onsager model has been modified and adapted for binary solvents considering a spherical cavity surrounded by a solvent layer with the permittivity of the polar component. To describe the transport of polar molecules to the first coordination shell of the excited dipole we applied the Smoluchowski diffusion model which leads to hyperbolic kinetics that compare reasonably well with the experimental Stokes shift data.
Anomalous diffusion in random pores
Authors: Lee J. C.
Published: July 2002

Molecular dynamics simulations are performed for binary liquids confined in the pores of Vycor glass. The individual molecular motions are studied by computing Fs, the self-part of the concentration fluctuation autocorrelation function. The results for Fs(q,t) show that the diffusion is anomalous as its relaxation rate does not follow the usual q2 dependence on the probing wave vector q. This is explained as a confining effect of the glass walls.
Collision-induced activation of the β-hydride elimination reaction of isobutyl iodide dissociatively chemisorbed on Al(111)
Authors: Lohokare Shrikant P. , Crane Elizabeth L. , Dubois Lawrence H. , Nuzzo Ralph G.
Published: July 2002

The collision-induced activation of the endothermic surface reaction of isobutyl iodide chemisorbed on an Al(111) surface is demonstrated using inert-gas, hyperthermal atomic beams. The collision-induced reaction (CIR) is highly selective towards promoting the β-hydride elimination pathway of the chemisorbed isobutyl fragments. The cross section for the collision-induced reaction was measured over a wide range of energies (14–92 kcal/mol) at normal incidence for Ar, Kr, and Xe atom beams. The CIR cross section exhibits scaling as a function of the normal kinetic energy of the incident atoms. The threshold energy for the β-hydride elimination reaction calculated from the experimental results using a classical energy transfer model is ∼1.1 eV (∼25 kcal/mol). This value is in excellent agreement with that obtained from an analysis of the thermally activated kinetics of the reaction. The measured cross section shows a complex dependence on both the incident energy of the colliding atom and the thermal energy provided by the surface where the two energy modes are interchangeable. The dynamics are explained on the basis of an impulsive, bimolecular collision event where the β-hydride elimination proceeds via a possible tunneling mechanism. The threshold energy calculated in this manner is an upper limit given that it is derived from an analysis which ignores excitations of the internal modes of the chemisorbed alkyl groups.
Flexible conducting polymers: An analytic approach
Authors: Hone Daniel W. , Orland Henri
Published: July 2002

By using field theoretic, functional integral techniques, we can treat analytically, to very late stages of the calculations, the thermodynamic properties of standard models of conducting polymers in solution. Both the primary interest and calculational difficulty of these systems arise from the coupling between fermionic and conformational degrees of freedom. In the functional integral for the partition function we integrate out exactly the fermionic degrees of freedom and treat the remaining configurational integrals within a loop expansion around the saddle point. We show excellent agreement with Monte Carlo calculations available for some specific models and relatively short chains, and we demonstrate how easily these methods can be used also for long chains and at low temperatures, for which numerical calculations are impractical.
Chemical association and electronic structure: A new theoretical approach
Authors: Lomba E. , López-Martı́n J. L. , Holovko M.
Published: July 2002

Chemical association introduces essential modifications in the microscopic structure of disordered materials, as new species are formed and the density of reactants decreases. These radical changes in the microscopic structure cannot be adequately captured by the pair distribution function, and hence descriptions of the electronic structure at the level of the effective medium approximation break down. We here propose a novel approach closely connected with the single superchain/effective medium approximation, in which the effects of chemical association are explicitly built-in. The new approximation has been tested in a simple minded model of alkali metal, namely a hard sphere fluid with a one-level tight-binding Hamiltonian with transfer matrix elements modeled by Yukawa terms. This fluid is thought to undergo a dimerization process to yield hard dumbbells via an intermediate stage in which a mixture is present. The proposed approach reproduces the band structure obtained by direct diagonalization for various stages along the reaction path, while retaining the simplicity of a linear theory.
Preparation and magnetic properties of manganese(II) phthalocyanine thin films
Authors: Yamada Hiroyuki , Shimada Toshihiro , Koma Atsushi
Published: July 2002

We report the molecular beam epitaxy (MBE) growth of the manganese(II) phthalocyanine thin films with unique crystal structure and the measurement of their magnetic properties. The epitaxial films were grown on hydrogen-terminated Si(111) substrates. Reflection high energy electron diffraction observations showed that the crystal structure of the films is different from that of the bulk crystal. A large magnetic anisotropy was observed in the measurements with the SQUID magnetometer. The films did not show canted ferromagnetism which is usually observed in the β-type crystal of manganese phthalocyanine. The magnetic susceptibility measured with magnetic field normal to the substrate obeyed the Curie–Weiss law with a negative Curie–Weiss constant, indicating the existence of the antiferromagnetic interaction. The negative Curie–Weiss constant was also observed in disordered films of manganese phthalocyanine. The variation of magnetic properties between the bulk crystal and the films is explained in terms of the difference in superexchange interaction caused by microscopic modification of the crystal structure, i.e., the stacking arrangement of the molecules.
Aminoacid zwitterions in solution: Geometric, energetic, and vibrational analysis using density functional theory-continuum model calculations
Authors: Tortonda Francisco R. , Pascual-Ahuir Juan-Luis , Silla Estanislao , Tuñón Iñaki , Ramı́rez Francisco J.
Published: July 2002

Glycine and alanine aminoacids chemistry in solution is explored using a hybrid three parameters density functional (B3PW91) together with a continuum model. Geometries, energies, and vibrational spectra of glycine and alanine zwitterions are studied at the B3PW91/6-31+G** level and the results compared with those obtained at the HF and MP2/6-31+G** levels. Solvents effects are incorporated by means of an ellipsoidal cavity model with a multipolar expansion (up to sixth order) of the solute’s electrostatic potential. Our results confirm the validity of the B3PW91 functional for studying aminoacid chemistry in solution. Taking into account the more favorable scaling behavior of density functional techniques with respect to correlated ab initio methods these studies could be extended to larger systems.
Spectroscopy of buffer-gas cooled vanadium monoxide in a magnetic trapping field
Authors: Weinstein Jonathan D. , deCarvalho Robert , Amar Karine , Boca Andreea , Odom Brian C. , Friedrich Bretislav , Doyle John M.
Published: July 2002

Spectroscopy of buffer-gas cooled vanadium monoxide (VO) is performed in the presence of a magnetic trapping field and at low field. VO is created via laser ablation. A helium buffer gas, chilled by a dilution refrigerator, cools 1012 VO molecules to 1.8±0.2 K within 10 ms. The measured rotational temperature is 1.5±0.8 K. Spatially resolved Zeeman spectra allow the magnetic broadening terms of several optical transitions to be determined. The density of VO decays with a characteristic time of 60 ms, thus precluding the observation of trapping.
Extracting accurate bound-state spectra from approximate wave packet propagation using the filter-diagonalization method
Authors: Beck M. H. , Meyer H.-D.
Published: July 2002

We present an extension of the filter-diagonalization (FD) approach which allows the extraction of accurate bound-state spectra from a time-dependent wave function determined by an approximate propagation scheme. To investigate the method, an approximate wave function is generated by a wave packet propagation on a two-dimensional potential energy surface for the LiCN molecule, employing a second-order differencing integrator with large step size. When applied to this inaccurate wave function, our formulation of the FD technique is capable of yielding eigenenergies considerably more precise than those obtained with the original FD method, while the effort is almost the same. The present FD scheme contains some additional refinements that increase the method’s efficiency, accuracy, and practicability. We demonstrate that the accuracy of the FD procedure can be improved by using cosinetype filters. The quality of different variational principles employed to determine the eigenenergies is examined. These variational principles can also be utilized to estimate the error of the energies and intensities inexpensively and reliably.
Critical-point of the Lennard-Jones fluid: A finite-size scaling study
Authors: Caillol J. M.
Published: July 2002

Monte Carlo simulations of the full, i.e. nontruncated, 3d Lennard-Jones fluid in the critical region are reported. The simulations are performed within the grand canonical ensemble in conjunction with hyperspherical boundary conditions in order to take account of the algebraic attractive part of the pair potential. Using mixed-field finite size scaling analysis and with the assumption of Ising criticality the critical temperature is estimated to be T*=1.326±0.002 and the critical density ρ*=0.316±0.002. Precised estimates of the critical energy per unit volume, pressure, chemical potential, and mixed-field parameters are also reported. The values obtained for the renormalization exponents yτ and yh are compatible with the 3d Ising values within the error bars.
Molecular time scale generalized Langevin equation theory and polynomial maximum entropy imaging of spectral densities
Authors: McDowell H. Keith , Clogston A. M.
Published: July 2002

Molecular time scale generalized Langevin equation (MTGLE) theory is discussed as an approach to condensed phase dynamics. A polynomial maximum entropy (MaxEnt) process for imaging required MTGLE spectral densities based on knowledge of the moments of the spectral density is introduced. The process is based on the use of interpolation polynomials which serve both to image the spectral density as well as provide a numerical procedure to compute the inverse Hessian matrix in a Newton-type minimization. A default model is added to allow for the inclusion of additional information in forming the image. The polynomial MaxEnt imaging process is found to be a fast, numerically stable, computational procedure which produces images comparable in quality to images obtained by other imaging processes. The polynomial MaxEnt imaging process is examined in the context of imaging MTGLE bath spectral densities with special emphasis on a coupled linear chain model. Standard harmonic oscillator, Hamiltonian bath models such as Ohmic-exponential and Ohmic-Gaussian are shown to possess regions of parameter space for which the MTGLE adiabatic frequency is imaginary. When the adiabatic frequency is zero, it is shown that imaging of the friction kernel is the best approach.
Evaluation of analytic molecular orbital derivatives and gradients using the effective valence shell Hamiltonian method
Authors: Chaudhuri Rajat K. , Stevens Jonathan E. , Freed Karl F.
Published: July 2002

Expressions for the analytic energy gradients and the nonadiabatic derivative couplings are derived for the effective valence shell Hamiltonian theory (a variant of degenerate/quasidegenerate many-body perturbation theory) using the diagonal and off-diagonal Hellmann–Feynman formulas and a generalized set of coupled perturbed Hartree–Fock equations to evaluate the derivatives of the molecular orbitals. The method is designed for efficiently treating the energy derivatives and nonadiabatic couplings for several states simultaneously. The generalized coupled perturbed Hartree–Fock equations arise because the reference space orbitals are optimized for simultaneously describing the ground and excited states, a feature lost with the traditional partitioning where the virtual orbitals provide a poor choice for representing the low lying states. A simple correspondence emerges between the new generalized coupled perturbed Hartree–Fock and the traditional coupled-perturbed Hartree–Fock methods enabling the use of the former with straightforward modifications. The derivatives of the second and higher order portions of the effective Hamiltonian are readily obtained using a diagrammatic representation that will be described elsewhere.
Rotational spectrum of a salt-containing van der Waals complex: Ar–NaCl
Authors: Mizoguchi Asao , Endo Yasuki , Ohshima Yasuhiro
Published: July 2002

The first spectroscopic identification of a van der Waals complex containing salt, Ar–NaCl, has been reported. A cm-region rotational spectrum of the complex has been observed for the Cl35 and Cl37 isotopomers by using a Fourier-transform microwave spectrometer combined with a laser ablation nozzle source. The vibrationally averaged geometry of the complex is of the linear Ar⋯Na–Cl configuration, with the internuclear distance of R(Ar⋯Na)=2.887 Å. An almost fully resolved hyperfine structure in low-J transitions has yielded precise nuclear quadrupole coupling constants associated with both Na and Cl nuclei, which indicate a substantial charge rearrangement in the NaCl molecule by complexation with Ar.
Gas-phase ion chemistry and ab initio theoretical study of phosphine. II. Reactions of PH+ with PH3
Authors: Antoniotti Paola , Operti Lorenza , Rabezzana Roberto , Tonachini Glauco , Vaglio Gian Angelo
Published: July 2002

The gas-phase ion chemistry of phosphine has been investigated by ab initio theoretical calculations and experimental techniques. Following a previous study of H and H2 loss pathways from the P3–PH3+ adduct (generated by P+3 reacting with PH3), the quantum chemical study of these processes has been extended to the ion–molecule reactions starting from PH+2 reacting with PH3, as observed by ion trapping. In these experiments, PH+ reacts to give P2Hn+ (n=2,3) product ions, with loss of H2 or H in different pathways, and also reacts in charge–exchange processes to form PH and PH3+. Moreover, elimination of two hydrogen molecules has been observed leading to the formation of the P2+ ion species. All these processes take place at similar rates, their constants ranging from 1.2 to 5.5×10−10 cm3 molecule−1 s−1. The geometrical structures and energies of transition structures, reaction intermediates, and final products have been determined by ab initio theoretical methods. The initial step is formation of the HP2–PH3+ adduct. Then, a hydrogen atom can be directly lost either from dicoordinated or tetracoordinated phosphorus, to give P3–PH3+ or HP1=PH2+, respectively. Alternatively, one hydrogen can first undergo a displacement from the latter to the former P atom to give H22P–PH2+. This migration can then be followed by P–H bond dissociation, yielding again HP1=PH2+. Dissociation of H2 can also occur, from either the initial HPPH3+ or rearranged H2P–PH2+ isomeric ions, yielding the HP2=PH+ or H22P=P+ ions, respectively. These last species are related by a H-migration process. A last H2 loss from H2P=P+ produces P2+2. Other pathways were explored, but proved not to be viable. The heats of formation of the P2Hn+ (n=0–4) ionic species have also been computed and reported with the experimental data in the literature.
Molecular dynamics of thiophene homologues investigated by femtosecond optical Kerr effect and low frequency Raman scattering spectroscopies
Authors: Kamada Kenji , Ueda Minoru , Ohta Koji , Wang Yan , Ushida Kiminori , Tominaga Yasunori
Published: July 2002

We present the results of femtosecond optical heterodyn detected optical Kerr effect (OHD-OKE) spectroscopy and low frequency Raman scattering spectroscopy for three thiophene homologues (C4H4X, X=O, S, and Se). The observed OHD-OKE transients were analyzed both in time and frequency domains. Fourier transformed OHD-OKE data were directly compared with the corresponding spontaneous Raman scattering spectra. The reorientation lifetimes for furan (O), thiophene (S) and selenophene (Se) were determined to be 0.9, 2.0 and 3.2 ps, respectively, and were fairly explained in terms of the Stokes-Einstein-Debye relation. Their subpicosecond dynamics was analyzed with an antisymmetric Gaussian function and a skewed function developed by Bucaro and Litoviz, which gave good curve fits to observed data. Librational and translational dynamics of the molecules was discussed in terms of microscopic environments formed by neighboring molecules and the results of intermolecular interaction energy calculations by an ab initio molecular orbital method.
General methods for determining the droplet size distribution in emulsion systems
Authors: Ambrosone L. , Ceglie A. , Colafemmina G. , Palazzo G.
Published: July 2002

We present a general method that allows us to figure out the size distribution of an isolated collection of droplets of dilute emulsion system using nuclear magnetic resonance pulsed gradient spin echo measurements. We show that the expression to obtain the volume fraction distribution function is equivalent to a Fredholm integral equation of the first kind. We prove, using the Dirac notation, that a solution of this equation can be easily found if its kernel has a complete biorthogonal system of eigenvectors. Two numerical procedures are discussed. The first, termed indirect, is based on the expansion of the unknown distribution function in the eigenfunctions of the kernel. The second one, called direct, uses the properties of shifted Legendre polynomials to integrate numerically the integral equation and evaluates the unknown distribution by means of a constrained least square procedure. The computational limits are analyzed. To extract the distribution’s form directly by experimental data we have constructed a generating function using the shifted Jacobi polynomials. The procedures have been tested on simulated and experimental data and appear to be a powerful and flexible method to obtain the size distribution function directly by the experimental data.
Molecular dynamics of pyrazine after excitation to the S2 electronic state using a realistic 24-mode model Hamiltonian
Authors: Raab A. , Worth G. A. , Meyer H.-D. , Cederbaum L. S.
Published: July 2002

The molecular dynamics of pyrazine after excitation to the S2 electronic state is investigated using the S2 absorption spectrum as a benchmark. We first present a realistic model Hamiltonian including all 24 vibrational modes of the pyrazine molecule. Using this model, we determined the potential energy surfaces of the lowest two excited states, S1 and S2, which are strongly coupled to each other. We then treated the nuclear motion of all 24 vibrational modes using the multiconfiguration time-dependent Hartree (MCTDH) wave packet propagation method. This method obtains results of good accuracy with acceptable computational effort for such a large system. The calculated spectrum is in good agreement with the experimental one. Furthermore, our results shed light on the role of the 20 modes which are only weakly coupled to the system, and demonstrate that essential physical features, such as symmetries, have to be considered when one wants to treat the molecular dynamics of pyrazine realistically.
Structural transitions in small molecular clusters
Authors: Proykova Ana , Radev Rossen , Li Feng-Yin , Stephen Berry R.
Published: July 2002

Clusters of octahedral molecules mimicking TeF6 afford a vehicle to investigate the analog in small clusters of structural phase transitions through molecular dynamics simulations. Three phaselike forms occur for the solid clusters, both closed-shell structures of 51, 89, and 137 molecules, and open-shell structures of 50, 81, and 129 molecules. The indications are that the free energy has at least two minima as a function of the order parameter. However, whether these converge to a single minimum as N, the number of molecules in the cluster, grows large, leading to a second-order transition in the bulk, or remain apart, implying at least a weak first-order transition in the bulk limit, cannot be determined from the simulations.
Geometrical changes during internal rotation: Evidence from polytypism
Authors: Weltner William
Published: July 2002

The flexing which occurs during internal rotation in ethane is manifested as a variation of the spacing between eclipsed (E) and staggered (S) bonds in the 6H (ESSESS) polytype of solid silicon carbide.
An infrared study of the competition between hydrogen-bond networking and ionic solvation: Halide-dependent distortions of the water trimer in the X−⋅(H2O)3, (X=Cl, Br, I) systems
Authors: Ayotte Patrick , Weddle Gary H. , Johnson Mark A.
Published: July 2002

Vibrational spectra of the water trimers solvating the halide anions (Cl−, Br−, I−) have been acquired in the OH stretching region by predissociation spectroscopy of the X−⋅(H2O)3⋅Ar3 complexes. These “wet” ions display two groups of bands assigned to normal modes of the (C3) pyramidal structure. We interpret the evolution of the spectra down the halogens in the context of the rings closing up toward the structure of the bare (H2O)3 neutral. This trend is discussed in terms of the disruptive effect of the ionic H bonds on the water network.
Time dependent diffusion coefficient and the transient dynamics of diffusion controlled bimolecular reactions in liquids: A mode coupling theory analysis
Authors: Morita Akihiro , Bagchi Biman
Published: July 2002

As the transient dynamics of many bimolecular reactions occur on ultrafast time scales, this dynamics may not be described accurately by using the standard time independent diffusion coefficient. On the other hand, the full phase space based descriptions are often too complex and cumbersome. Since the key step in many bimolecular reactions is still the mutual diffusion of the two reacting species in the real space, it makes sense to look for a formalism which will include the effects of inertial motion within a diffusion equation based approach. Such a description is presented in this article. The time-dependent diffusion coefficient of a solute molecule in Lennard-Jones liquid is calculated and analyzed on the basis of the mode coupling theory. Not only the usual asymptotic diffusion coefficient, but also the transient diffusion dynamics shows a rich variety with varying curvature of the potential, solute size, or solute interaction strength. The effect of the nascent diffusion on diffusion-influenced bimolecular reaction is examined using the generalized Smoluchowski equation. It is found that there are significant differences in the dynamics from the results obtained with time independent diffusion.
Is there a crystallization pendulum?
Authors: Avramov I. , Höche Th. , Rüssel C.
Published: July 2002

A theoretical model is developed according to which crystals formed in a glass-forming melt can exhibit a previously unreported growth behavior. After growing very fast in one or two directions, the growth stops due to changes in concentration and turns into dissolution. Along the other direction(s) the growth rate is initially lower; it also slows down but never becomes negative. This is the result of a competition between several processes that take place simultaneously in a nonequilibrium system. Due to direction-dependent growth rates (the latter can vary over a large range), such behavior may result in the first step in the formation of plate or needlelike crystals. After a drop in the concentration of some key compounds (the crystals are consuming the latter), in the second stage the crystals approach their equilibrium shape since relaxation processes prevail. It is demonstrated with an example (mica plates formed from an extruded glass) that such effects in fact can be found experimentally. Although at this initial stage the model is quite simplified it leads to a surprisingly good agreement with the experimental data. It seems that there are many other examples of processes in nonequilibrium systems in which a given parameter is developing, at early stages, far beyond its equilibrium value, and later (when the resources of the system are becoming exhausted and consequently the growth rates slow down) relaxation kinetics dominates.
Extent of reaction in energetic particulate mixtures: Role of composition and particle size ratio
Authors: Ettelaie Rammile , Buscall Richard , Frith William J. , Sutton David
Published: July 2002

The extent of reaction in reactive solid mixtures, comprising two or more particulate components, can be significantly affected by the presence of unavoidable compositional fluctuations in these systems. This is particularly the case where the size of the reaction zone is of the order of a few particle diameters. Results of detailed computer simulations, aimed at investigating the influence of composition and particle size ratios, on the fluctuations and thus the extent of reaction in reactive solid mixtures, are reported here. These data are complemented by analytical calculations performed for systems in certain limiting cases. In accord with experimental observations on certain classes of such systems, namely pyrotechnics, both the simulation and the analytical results show that the extent of reaction attains its maximum value at compositions, which compared to the stoichiometric ratio, are somewhat richer in the minor component. The deviation of the optimal composition from stoichiometry becomes more pronounced for small reaction zone sizes, where the relative compositional fluctuations are larger. The effect of altering the particle size ratio, for systems where the stoichiometric volume ratio is different to 1:1, is found to be markedly asymmetric. Reducing the size of the particles of the minor component is seen to dramatically improve the extent of reaction in the system. In contrast, any observed improvements, arising from a reduction in the size of the major component, are found to be at best marginal. The analytical calculations allow an insight into these result to be gained.
Methanol-acetonitrile complexes trapped in argon and nitrogen matrices: Infrared induced isomerization and theoretical calculations
Authors: Coussan S. , Bouteiller Y. , Perchard J. P. , Brenner V. , Millié P. , Zheng W. Q. , Talbot F.
Published: July 2002

Infrared induced isomerization of methanol-acetonitrile mixed aggregates trapped in argon and nitrogen matrices has been studied using an optical parametric oscillator for monochromatic irradiations at the OH and CH stretching frequencies of complexed methanol. Several forms of the 1:1 complex have been characterized by most of their vibrational modes; two (Ar) or three (N2) hydrogen-bonded forms are observed after matrix deposition; upon irradiation they are converted into another H-bonded and one non-H-bonded species. This unbonded species is highly unstable, with a lifetime of the order of 24 min in argon at 7.2 K. In the presence of an excess of acetonitrile, larger aggregates are identified, with OH stretching frequencies weakly redshifted with respect to those of the 1:1 complex. They are also infrared photosensitive, giving rise to non-hydrogen- bonded structures. The possible structures of the heterodimer have been calculated theoretically. The deepest minimum of the potential energy surface is the quasilinear H-bonded form, with vibrational properties which agree with the matrix data obtained after sample deposition. As for the photolysis product, its vibrational spectrum is compatible with a secondary minimum with a pseudocyclic structure in which the OH group is perturbed on the oxygen atom.
Formation of ordered structure in Langmuir monolayers of semifluorinated hydrocarbons: Molecular dynamics simulations
Authors: Kim Namshin , Shin Seokmin
Published: July 2002

We report the results of molecular dynamics simulations of Langmuir monolayers of a semifluorinated hydrocarbon molecule F(CF2)12(CH2)18H. Our simulations show that highly ordered structures are formed at low temperature after quenching from a random structure at high temperature. The structural formation process of the monolayer is characterized by a decrease in the gauche defects of the hydrocarbon block and an increase in the global bond-orientational order throughout the chain molecules. It is also found that the monolayer structure consists of mixed orientations for the head-group-free F(CF2)12(CH2)18H molecule, with a slightly larger fraction for a hydrocarbon-down, fluorocarbon-up configuration.
Surface-induced layer formation in polyelectrolytes
Authors: Solis F. J. , de la Cruz M. Olvera
Published: July 2002

We analyze, by means of a random phase approximation (RPA) calculation, the conditions under which a mixture of oppositely weakly charged polyelectrolytes can microsegregate in the neighborhood of a charged surface creating a layered structure. A number of stable layers can be formed if the surface is sufficiently strongly charged even at temperatures at which the bulk of the mixture is homogeneous. The wave-length of the induced concentration fluctuation is proportional to 1/f1/2 near the microphase transition, where f is the charge density of the polyelectrolytes.
Doppler line shapes in the photolysis of laser excited, aligned molecules: Application to the vibrationally mediated photodissociation of HN3
Authors: Barnes Rhett James , Sinha Amitabha , Dagdigian Paul J. , Lambert H. Mark
Published: July 2002

A general expression for the Doppler profile for fragments produced in the photodissociation of laser excited, aligned molecules, as in vibrationally mediated photodissociation (VMP), is presented. In contrast to one-photon dissociation, for which the Doppler profile depends only on the second moment of the fragment molecular-frame angular distribution, the profile in the case of VMP is sensitive to several moments of the angular distribution, up to k=4. In addition, the profile for a near-prolate molecule depends on the angle θa between the electronic transition moment μ and the a inertial axis. This theory is applied to the analysis and interpretation of Doppler profiles in the laser fluorescence detection of NH(a 1Δ) fragments, of rotational angular momenta N=7 and 10, from the 532 nm VMP of HN3 excited to the second N–H stretch overtone level (3ν1). For both Λ-doublets of these rotational levels, the second moment of the molecular-frame angular distribution β2,0=〈P2(cos θm)〉 was found to be positive, in agreement with previous results for high-J fragments from one-photon photolysis of HN3. The profiles are consistent with a value of ∼0° for the angle θa. These values for θa and β2,0 are inconsistent with simple expectations based on planar, prompt dissociation upon excitation to the lowest singlet excited state (1A″) and suggest the importance of nonplanar geometries in the dissociation dynamics.
Adsorption of fluoroform CHF3 on ice Ih(0001): Structure and vibrations
Authors: Graham A. P. , Menzel A. , Toennies J. P.
Published: July 2002

The adsorption of fluoroform CHF3 on an ice Ih(0001) surface grown on Pt(111) under UHV conditions was studied using high resolution elastic and inelastic helium atom scattering. The results indicate that the CHF3 molecules are aligned with their H-atoms pointing downwards and form an ordered p(1×1) overlayer at temperatures less than Ts=75 K. The Debye temperature decreases from ΘiceD=132 K for the clean ice surface to ΘCHF3D=53 K on adsorption of one monolayer. From time-of-flight measurements the reduction in ΘD appears to be due to a substantial increase in multiphonon processes and the creation of a new, broad, dispersionless, low frequency vibrational mode at ℏω=2.1 meV.
Thermal and mass diffusion in a semidilute good solvent-polymer solution
Authors: Zhang K. J. , Briggs M. E. , Gammon R. W. , Sengers J. V. , Douglas J. F.
Published: July 2002

The Soret coefficient ST and collective (mass) diffusion coefficient Dc of polystyrene dissolved in the good-solvent toluene has been measured over a range of concentrations and molecular masses with an optical beam-deflection method. Our measurements indicate that ST scales inversely with the polymer translational diffusion coefficient in dilute solutions, exhibits a power-law scaling with polymer concentration, and an independence of polymer molecular mass in semidilute solutions. These findings are consistent with the known scaling of 1/Dc in dilute and semidilute polymer solutions, the relative insensitivity of the thermal-diffusion coefficient Dth of polystyrene in toluene to polymer concentration, and the relation ST=Dth/Dc from irreversible thermodynamics. We are able to represent our ST and Dc data by theoretically motivated reduced-concentration master curves, but the concentration-molecular mass scaling variables are found to be different for each transport property, a result contrary to theoretical expectations. However, the asymptotic concentration scaling exponents deduced from these data fits are compatible with de Gennes’ scaling arguments for Dc and with modern estimates of the chain-size exponent ν for swollen polymers in good solvents.
Surface-induced reactions of acetone cluster cations
Authors: Mair C. , Fiegele T. , Biasioli F. , Herman Z. , Märk T. D.
Published: July 2002

The occurrence of two different chemical reactions initiated by the surface impact of acetone dimer, trimer, and tetramer cations (energy 20–70 eV) on a stainless-steel surface (covered with hydrocarbons) was observed. The reaction product is the protonated acetone ion, formed in (i) an intracluster ion–molecule reaction, and in (ii) a hydrogen pickup reaction of the cluster ion with the surface material. Only the monomer product ions (and small amounts of their dissociation products) could be observed; the spectra did not show any presence of clustered product ions. A simple model based on the Brauman double-well potential is suggested to explain the formation of the two product ions. In accordance with predictions from molecular dynamics simulations, this appears to be the first observation of competitive chemical reactions of a cluster ion driven by energy transfer in a surface collision.
Ultrafast spectroscopy of dark states in solid state sexithiophene
Authors: Lanzani G. , Cerullo G. , Stagira S. , De Silvestri S. , Garnier F.
Published: July 2002

Femtosecond time resolved photoinduced transmission studies are carried out in nanocrystalline dihexylsexithiophene films with long range structural order. The results are compared with those obtained for sexithiophene in several states of aggregation. We explain the lack of radiative recombination in the solid phase with the formation of two nonradiative excitations, dark excitons and charge transfer states. The dark exciton dynamics is studied in detail in dihexylsexithiophene films; thermalization occurs within 200 fs, then decay takes place by mutual annihilation in the first ps, and by monomolecular recombination at longer times. Optical dynamics indicates that within the pump pulse a second excitation is formed, and we assign it to charge transfer states. The latter decay monomolecularly in the hundred ps time scale.
Split operator method in hyperspherical coordinates: Application to CH2I2 and OClO
Authors: Barinovs Ǧirts , Marković Nikola , Nyman Gunnar
Published: July 2002

3D wave packet calculations on the CH2I2 and OClO molecules have been performed for a total angular momentum equal to zero. The initial wave function is found by solving the time independent Schrödinger equation in internal bond coordinates. The split operator method and the fast Fourier transform in hyperspherical coordinates are used in order to follow the quantum dynamics. An absorption spectrum of CH2I2 is obtained and compared with a previous 2D calculation. A Raman spectrum for the CH2I2 molecule at 355 nm is calculated and compared with experimental results. The absorption spectrum for the X2B1→A2 A2 transition of the OClO molecule is calculated using the same method as for CH2I2. Good agreement with experiment is obtained.
A new time-dependent wave operator approach to the internal eigenstate problems for large matrices
Authors: Jolicard Georges , Killingbeck John P. , Grosjean Alain , Zucconi Jean-Marc
Published: July 2002

A new integration procedure is proposed to solve the nonlinear time-dependent wave operator equation. This procedure can easily be adapted to describe the adiabatic evolution of a system on a long-time scale and also to solve the internal eigenstate problem for large matrices.
The microwave spectrum of the cesium monoxide CsO radical
Authors: Yamada Chikashi , Hirota Eizi
Published: July 2002

The microwave spectrum of CsO has been observed and analyzed, not only in the ground vibrational state, but also in the v=1–3 excited vibrational states. The CsO radical was generated by the reaction of N2O with Cs vapor, which was produced by the reaction of Li metal with CsCl at 500–530 °C. The observed spectra were found to conform to those expected for a Σ2 diatomic molecule, thereby establishing the ground electronic state of CsO to be of Σ2. The observed rotational and centrifugal distortion constants yielded the equilibrium bond length and the harmonic vibrational frequency to be 2.300 745 (16) Å and 356.78 (11) cm−1, respectively, based on the Born–Oppenheimer approximation. A careful examination of the observed spectral pattern definitely concluded that the spin-rotation interaction constant was positive, at variance with the expectation from a simple Σ2/2Π two-states interaction. This observation was interpreted by assuming positive contributions from higher excited electronic states which superseded a negative contribution from the Π2 lowest excited state; the latter state was responsible for the large dependence of the spin-rotation interaction constant on the vibrational quantum number and was estimated from this vibrational dependence to be located at 1225 cm−1 above the ground electronic state. In reverse to the spin-rotation splitting, the hyperfine splitting was found to increase with the vibrational excitation; in the v=3 state the hyperfine structure was found completely resolved. However, the hyperfine coupling constants did not vary much with the vibrational quantum number, namely the vibrational dependence of the hyperfine splitting was caused primarily by that of the spin-rotation splitting. The observed hyperfine interaction constants indicated that CsO was an ionic molecule.