bluuebunny/crossref_metadata_2025 huggingface dataset:
Energies of C2H5O and C2H5O+ isomers
Authors: Curtiss Larry A. , Lucas David J. , Pople John A.
Published: July 2002

The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular orbital theory, is used to calculate the energies of C2H5O and C2H5O+ isomers. The ethoxy radical cation (CH3CH2O+) is found to have a 3A″ ground state while the singlet state is predicted to be unstable to rearrangement. The G2 adiabatic ionization potential of ethoxy radical is 10.32 eV in good agreement with a new ionization potential reported by Ruscic and Berkowitz of 10.29±0.08 eV from photoionization studies. The 2-hydroxyethyl radical (CH2CH2OH) has three isomers of nearly equal energy (within 1.6 kcal/mol). No stable 2-hydroxyethyl cation was located as it collapses to either O-protonated oxirane or 1-hydroxyethyl cation. The G2 appearance potential of CH3CHOH+ from ethanol of 10.79 eV is in excellent agreement with the recent photoionization value of 10.801±0.005 eV. The α(C–H), β(C–H), and O–H bond dissociation energies of ethanol are 94.9, 101.9, and 104.6 kcal/mol, respectively. The G2 result for the O–H bond energy is in good agreement with experiment while the results for the other two bond energies suggest that the experimental values for these quantities may be low.
Spectral projection approach to the quantum scattering calculations
Authors: Mandelshtam Vladimir A. , Taylor Howard S.
Published: July 2002

A new method of implementing scattering calculations is presented. For the S-matrix computation it produces a complete set of solutions of the wave equation that need be valid only inside the interaction region. For problems with small sizes the method is one of several that are practical in the sense that it involves merely a real symmetric Hamiltonian represented in a minimal ℒ2 basis set. For more challenging larger systems it lends itself to a very efficient time independent iterative procedure that obtains results simultaneously at all energies. A modified Chebyshev polynomial expansion of (E−Ĥ)−1 is used. This acts on a set of energy independent wave packets located on the edge of the interaction region. The procedure requires minimal storage and is shown to converge rapidly in a manner that is uniform in energy.
Inverse temperature expansion of some parameters arising from the solution of the mean spherical approximation integral equation for a Yukawa fluid
Authors: Henderson Douglas , Blum Lesser , Noworyta Jerzy P.
Published: July 2002

Analytic formulas, within the mean spherical approximation, are given through fifth order for the inverse temperature expansion of the energy and the pair correlation function at contact for a fluid whose molecules interact via a Yukawa potential with a hard core. This series converges rapidly if the Yukawa potential has a long range. Reliable results are obtained from this series even when the potential has a short range.
A simple recursion polynomial expansion of the Green’s function with absorbing boundary conditions. Application to the reactive scattering
Authors: Mandelshtam Vladimir A. , Taylor Howard S.
Published: July 2002

The new recently introduced [J. Chem. Phys 102, 7390 (1995)] empirical recursion formula for the scattering solution is here proved to yield an exact polynomial expansion of the operator [E−(Ĥ+Γ̂)]−1, Γ̂ being a simple complex optical potential. The expansion is energy separable and converges uniformly in the real energy domain. The scaling of the Hamiltonian is trivial and does not involve complex analysis. Formal use of the energy-to-time Fourier transform of the ABC (absorbing boundary conditions) Green’s function leads to a recursion polynomial expansion of the ABC time evolution operator that is global in time. Results at any energy and any time can be accumulated simultaneously from a single iterative procedure; no actual Fourier transform is needed since the expansion coefficients are known analytically. The approach can be also used to obtain a perturbation series for the Green’s function. The new iterative methods should be of a great use in the area of the reactive scattering calculations and other related fields.
Direct measurement of the size of the helium dimer
Authors: Luo Fei , Giese Clayton F. , Gentry W. Ronald
Published: July 2002

The relative transmissions of helium dimer and helium atom beams through a set of nanoscale sieves were measured as a function of hole size in the range from 98–410 nm. From the relative transmission coefficients, the mean internuclear distance of helium dimer was determined to be 〈r〉=62±10 Å. This enormous bond length—by far the largest known—confirms recent theoretical estimates of the potential energy well depth ε and the extremely small binding energy Eb of helium dimer. The ranges of these parameters corresponding to the experimental uncertainty in 〈r〉 are ε/k=10.88–10.98 K and Eb/k=0.65–1.30 mK, where k is the Boltzmann constant.
Adiabatic path integral molecular dynamics methods. II. Algorithms
Authors: Cao J. , Martyna G. J.
Published: July 2002

Efficient numerical algorithms are developed for use with two finite temperature semiclassical approximations to quantum dynamics both of which require trajectories generated on potentials of mean force derived from the path integral expression for the density matrix. The numerical algorithms are formed from the combination of a classical adiabatic relation similar to that used in the Car–Parrinello method and an efficient path integral molecular dynamics scheme. Results on model, an anharmonic oscillator and a realistic, fluid para-hydrogen, problem indicate that semiclassical dynamics can be obtained for virtually the same computational cost as structure and thermodynamics.
The dynamics of matrix trapping. II. Simple spherical species trapped in an argon matrix
Authors: Cruz Astrid J. , López Gustavo E.
Published: July 2002

The technique of matrix isolation has extensively been used to study reactive species which are isolated in an inert host matrix, preventing in this way diffusion or reaction of the trapped species. The present study is concerned with the theoretical description of matrix trapping dynamics. The most common experimental technique of matrix deposition, slow spray on, is simulated using classical molecular dynamics. The systems considered are simple spherical species trapped in argon. The effect of the size of the trapped species on the structure of the matrix and the nature of the trapping site is investigated.
Molecular theory of the yield behavior of a polymer gel: Application to gelatin
Authors: Groot Robert D. , Bot Arjen , Agterof Wim G. M.
Published: July 2002

A microscopic model for the endpoint separation dependent scission rate of a polymer connection in a network is developed. The predicted dissociation rate is proportional to the exponential of the bond force, which is in line with experiments. This atomistic description is thereupon incorporated in a mesoscopic theory to describe strain hardening and failure of physical gels. The resulting theory has been analyzed by a new numerical algorithm, which is some 100 to 1000 times faster than the algorithm described in the literature. We applied this theory to gelatin. To arrive at the correct nonlinear rheologic behavior of the gel, the non-Gaussian nature of the polymer endpoint distribution has to be taken into account. There are four important physical quantities that describe the nonlinear rheology of gelatin. For relatively small shear strain (0<γ<1), stress increases nonlinearly with strain when a gel is deformed (strain hardening). The strain at which the gel ruptures (the yield strain γy) increases quite slowly with shear rate: γy∝γ̇0.05. When experiments are carried out at different shear rate, we find a linear correlation between yield stress and yield strain. Finally it is observed that the yield strain decreases with increasing strain hardening. These four observations are all covered by the theory up to quantitative accuracy. The interpretation that comes forward from this work is that the nonlinearity of the stress–strain curve for γ<1 is correlated to the strain at which the gel yields. The reason for this correlation is that both effects are dominated by a non-Hookean force–distance relationship of the polymer connections. On the one hand side, this function directly causes the upturn of the stress–strain curve. On the other hand, the rate by which polymer connections break is proportional to the exponential of this force. Therefore a nonlinear force–distance relationship leads both to a nonlinear stress–strain relation and to an early and sudden yield behavior.
Collisional recombination reaction H+O2+M→HO2+M: Quantum mechanical study using filter diagonalization
Authors: Mandelshtam Vladimir A. , Taylor Howard S. , Miller William H.
Published: July 2002

We report the results of calculations for the collisional recombination rate of the reaction H+O2+M→HO2+M. This study uses the recently developed quantum mechanical theory of recombination (within the strong collision approximation) based on the flux–flux correlation function analysis [J. Phys. Chem. 99, 12387 (1995)]. The quantum dynamics calculations are based on the new very efficient computational procedure of filter diagonalization [J. Chem. Phys. 103, 10074 (1995)]. This procedure allows one to obtain individual complex eigenenergies and eigenfunctions of a large non-Hermitian matrix representation of the Hamiltonian with an absorbing potential. The computed rates are compared to the Lindemann steady-state approximation result. The latter leads to an overestimation of the rate since it does not take into account the resonance interference effects. This becomes more critical at higher temperatures where many broad high energy overlapping resonances contribute to the rate.
Vibrational enhancement of the charge transfer rate constant of N+2(v=0–4) with Kr at thermal energies
Authors: Kato Shuji , de Gouw Joost A. , Lin Chii-Dong , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

The charge transfer reaction of N+2(v=0–4)+Kr→N2+Kr+ is studied at thermal energy as a function of vibrational excitation in the reactant ion. The selected-ion flow tube technique coupled with laser-induced fluorescence detection is used to measure the vibrationally state specific rate constants. A dramatic vibrational enhancement is observed; measured rate constants are 1.0 (±0.6)×10−12, 2.8 (±0.3)×10−12, 2.1 (±0.2)×10−11, 5.1 (±0.2)×10−11, and 8.3 (±0.4)×10−11 cm3 molecule−1 s−1 for v=0, 1, 2, 3 and 4, respectively. Mass spectrometric kinetics experiments are also performed to confirm that vibrational relaxation, N+2(v)+Kr→N+2(v′<v)+Kr, is a negligible process. The charge transfer for v=0 is extremely slow in spite of the large exothermicity (e.g., 0.915 eV for the production of N2(v′=0)+Kr+(2P1/2) states), yet the reaction is enhanced when the apparent energy mismatch is greater for the vibrationally excited reactant. A simple model is proposed to explain the experimental results at thermal energies (≪1 eV). The model assumes that only the most energy-resonant exothermic transitions, N+2(v)+Kr→N2(v+3)+Kr+(2P1/2), occur within the duration of the ion–molecule collision complex and that the charge transfer takes place with probabilities governed by the corresponding Franck–Condon factors. However, the Franck–Condon factors are modified by a trial displacement of 0.02 Å to account for the changes in vibrational wave functions of N+2 and N2 during a close approach of the (N2–Kr)+ pair; this method gives an excellent description of the experimental results.
Nonlinear electronic responses, Fukui functions and hardnesses as functionals of the ground-state electronic density
Authors: Senet P.
Published: July 2002

The exact equations for the variations of the electronic density induced by an adiabatic external potential are derived to an arbitrary perturbation order in the framework of density-functional-theory. The formal solutions of these density perturbation equations are given and the exact relations between the electronic response functions and the Hohenberg–Kohn functional are derived. Using these relations, the static nonlinear electronic response functions are constructed from the linear one. Nonconserving electron number perturbations are also directly included in the formalism to all perturbation orders. In this way the well-known results of the density-functional reactivity theory are generalized beyond the first and the second-order. This makes it possible to derive the exact relations between the Hohenberg–Kohn functional and the linear and nonlinear Fukui responses, the nonlinear Fukui functions and the nonlinear hardnesses. These relations allow us to reformulate all the derivatives of the electronic energy relative to the external potential and to the particle number in terms of the linear response kernel and in terms of the linear Fukui function. The formalism is applied to the Thomas–Fermi–Dirac-λ von Weiszäcker model of the Hohenberg–Kohn functional.
Adsorption isotherm for flexible molecules in random porous media. Can we regard the system as a binary mixture?
Authors: Padilla P. , Vega C.
Published: July 2002

The adsorption isotherm for repulsive n-alkanes in a random porous medium has been determined by computer simulation. The porous medium or matrix is obtained by quenching an equilibrium configuration of repulsive spheres. Keeping fixed this configuration of the matrix particles, Grand-Canonical Monte Carlo simulations were used to determine the adsorption isotherm of repulsive n-butane and n-hexane models in the random porous medium. In addition to that, canonical Monte Carlo simulations were performed for the equilibrium binary mixture in which the spherical and n-alkane particles are allowed to move. Chemical potential of the n-alkane molecule in the binary mixture is computed by using the test particle method. We found that, at the same chemical potential, the structure of the n-alkane in the binary mixture and in the quenched medium differs at high densities. However, at the same chemical potential, densities of the n-alkane in the random porous medium and in the binary mixture are quite similar. A consequence of that is that adsorption isotherms can be computed if the properties of the binary mixture are known. We have recently proposed an equation of state for mixtures of hard n-alkane molecules which contains as a particular case the sphere+n-alkane mixture. The chemical potential of the n-alkane in the mixture can be computed analytically and we used this result for determining adsorption isotherms. Excellent agreement with simulation is found. The results of this work suggest that the knowledge of the properties of the binary mixture can be very useful for obtaining adsorption isotherms in random porous media. Finally, the effect of attractive forces on the properties of alkanes in binary mixtures and confined in a random porous medium was also studied. In general, the presence of attractive forces reduces the similarity between the properties of alkanes in these two systems.
A small-angle neutron scattering study of nonionic surfactant molecules at the water–oil interface: Area per molecule, microemulsion domain size, and rigidity
Authors: Sottmann T. , Strey R. , Chen S.-H.
Published: July 2002

The small-angle neutron scattering (SANS) of bicontinuous microemulsions of 19 different water-n-alkane-CiEj (n-alkylpolyglycolether) systems has been measured. All scattering curves exhibit a broad scattering peak which permits determining the characteristic length scale ξ for bicontinuous structures at symmetric water and oil volume fractions, i.e., φ=0.5. Various random models predict ξ=aδφ(1−φ)/φc. We find that ξ is indeed inversely proportional to the surfactant volume fraction φc. Approximating the effective surfactant chain length δ by δ=νc/ac, where ac and νc are the area and the volume of the surfactant molecule, the numerical value for a is determined to be a=7.16, which is close to, but significantly different from those used in theoretical models. The head group area ac at the water–oil interface is obtained from the large q part of the scattering curves. It is found to be independent of i and k, the carbon numbers of the alkyl chain of the surfactant and of the alkane, respectively. However, it depends strongly, and nearly linearly, on the head group size j of the surfactant. Within experimental error it is described by ac=29.3+6.20j (Å2).
Viscosity calculations of
n
-alkanes by equilibrium molecular dynamics
Authors: Mondello Maurizio , Grest Gary S.
Published: July 2002

We report shear viscosity results obtained using extensive equilibrium molecular dynamics simulations. By direct numerical comparison, we show the equivalence of the Green-Kubo and Einstein approach to the calculation of viscosity in both the atomic and molecular representations. Comparing the results for two models of linear alkanes, we discuss the molecular factors determining their low-temperature liquid-state transport properties. In the mass range considered here (⩽C16), large corrections to Rouse-dynamics scaling are observed, as expected. We indicate, however, how the scaling relation between rotational-diffusion-time and shear-viscosity still provides a semi-quantitative way of estimating the latter, using simulations which are at least one order of magnitude shorter than required for direct determination of viscosity for n-alkanes.
Ab initio calculations on (MgO)n, (CaO)n, and (NaCl)n clusters (n=1–6)
Authors: Malliavin M.-J. , Coudray C.
Published: July 2002

We report preliminary results of ab initio calculations on (NaCl)n, (MgO)n, and (CaO)n clusters for n=1–6. We determine the isomers and the structure of various neutral clusters. Their relative stabilities are studied by analyzing their binding energies and their dissociation energies. As a particular behavior of (MgO)n is observed for n=3 and n=6, the bondings of (MgO)3, (CaO)3, and (NaCl)3 are studied.
Mode-specific energy analysis for rotating-vibrating triatomic molecules in classical trajectory simulation
Authors: Rhee Young Min , Kim Myung Soo
Published: July 2002

A method for the mode-specific energy analysis in a classical trajectory calculation is developed. The pure rotational energy is evaluated by invoking the Eckart condition. To evaluate the vibrational energy in each normal mode, the vibrational velocity is divided into two parts, the angular motion part and the angular motion free part, and the latter is analyzed with the Cartesian and internal coordinate systems. The potential energy of each normal mode is also evaluated in the two coordinate systems. A simple algorithm to include some anharmonicity correction is presented. Sample calculations with nonreacting triatomic molecules, H2O and HCN, show that the internal coordinate system is more adequate than the Cartesian, especially for the linear molecule HCN. An excellent result is obtained for the product (CHO+) of a reaction, suggesting that the present method is adequate for the mode-specific energy analysis of classical trajectory results.
Photoelectron spectroscopy of C4−, C6−, and C8−
Authors: Xu Cangshan , Burton Gordon R. , Taylor Travis R. , Neumark Daniel M.
Published: July 2002

Photoelectron spectra of C4−, C6−, and C8− were obtained at two photodetachment wavelengths, 266 nm (4.657 eV) and 213 nm (5.822 eV). The spectra reveal considerably more electronic and vibrational structure than was seen in previous studies of these species [D. W. Arnold et al., J. Chem. Phys. 95, 8753 (1991)]. Term values for several low-lying excited electronic states of the neutral carbon clusters have been obtained, as well as new vibrational frequencies for the ground and some of the excited electronic states of the neutral clusters. The assignments of excited electronic states were aided by measurements of the photoelectron angular distributions. A new assignment of the vibrational frequencies for C6 is in considerably better agreement with ab initio results than our original assignment.
Excitation transfer from azo dye to nematic host during photoisomerization
Authors: Galstyan T. V. , Saad B. , Denariez-Roberge M. M.
Published: July 2002

Dynamic holography and polarimetric techniques are used to study the photoisomerization kinetics of D2 bisazo dye and the excitation transfer from dye to nematic liquid crystal host. As a result of this transfer, transient modulation of the orientational order parameter, three-dimensional long-scale molecular transient rotation and equilibrated two-dimensional molecular rotations are identified in the host. Amplified rotation of nematic molecules is observed which is much more efficient for the dye photoexcitation with a wavelength in the red rather than in the blue side of its absorption band. The possible role of dye photoisomerization mechanism in these processes is discussed. The dynamic character of the dye induced torque and the presence of long-living intermediate states during photoisomerization and their diffusion are suggested.
Gas phase proton transfer reaction of nitric acid-ammonia and the role of water
Authors: Tao Fu-Ming
Published: July 2002

The gas-phase proton transfer reaction of nitric acid-ammonia and the effect of the first three water molecules are investigated by high level ab initio calculations on the molecular clusters HNO3–NH3–(H2O)n, n=0,1,2,3. The equilibrium structures, binding energies, and harmonic frequencies of the clusters as well as the potential energy surfaces along the proton transfer pathway of nitric acid-ammonia are calculated at the second-order Mo/ller–Plesset perturbation (MP2) level with two extended basis sets 6-31+G(d) and 6-311++G(d,p). It is found that, either without water or with one water molecule, the nitric acid-ammonia system exists as hydrogen bonded, with nitric acid acting as the hydrogen bond donor and ammonia as the acceptor. With two or three water molecules, the system becomes an ion pair resulting from the complete transfer of a proton from the nitric acid to ammonia. The potential energy surfaces along the proton transfer pathway are analyzed to understand the effect of the water molecules. The water molecules stepwise added into the system are found to increase the stability of the ion pair over the hydrogen bonded form. The first water molecule is not enough to stabilize the ion pair, but it results in a flatter potential energy pathway for the proton transfer. The second water molecule produces additional stabilization energy which helps fully stabilize the ion pair. The third water molecule contributes to further stabilize the ion pair. The harmonic frequencies and infrared intensities of the clusters are analyzed, which provide further evidence in agreement with the transition from the hydrogen bond to the ion pair structure as the water molecules are stepwise introduced. On the basis of these results, we conclude that ammonium nitrate might be formed by the gas phase reaction of nitric acid with ammonia in the presence of adequate water vapor.
A locally analytic density functional theory describing adsorption and condensation in microporous materials
Authors: Nilson Robert H. , Griffiths Stewart K.
Published: July 2002

The fluid density distribution within microscopic pores is determined by solving integral equations relating the local chemical potential to the van der Waals attractions and hard sphere repulsions of surrounding material. To avoid resolving the density distribution on submolecular scales, the governing equations are averaged over zones of molecular size using analytic functions to represent local density variations within each zone. These local density profiles range from singularities to uniform distributions depending on the local variation of the potential field. Sample calculations indicate that this integral approach yields results in very good agreement with those based on traditional density functional theory, while reducing computing times by three or four orders of magnitude for one-dimensional geometries.
Dielectric solvation dynamics of molecules of arbitrary shape and charge distribution
Authors: Song Xueyu , Chandler David
Published: July 2002

A new perspective of dielectric continuum theory is discussed. From this perspective a dynamical generalization of a boundary element algorithm is derived. This generalization is applied to compute the solvation dynamics relaxation function for chromophores in various solvents. Employing quantum chemical estimates of the chromophore’s charge distribution, the Richards–Lee estimate of its van der Waals surface, and the measured frequency dependent dielectric constant of the pure solvent, the calculated relaxation functions agree closely with those determined by experiments.
Electron affinities of CO2, OCS, and CS2
Authors: Gutsev Gennady L. , Bartlett Rodney J. , Compton Robert N.
Published: July 2002

The structure of the CO2−, OCS−, and CS2− anions as well as the adiabatic electron affinities of the corresponding CO2, OCS, and CS2 neutral parents are computed using the infinite-order coupled-cluster method with all singles and doubles and non-iterative inclusion of triple excitations (CCSD(T)) and Hartree-Fock-Density-Functional-Theory (HFDFT) levels of theory. The potential energy curves of the CO2 – CO2− and CS2 – CS2− pairs are calculated as a function of the bending angle. All three anions are found to have bent equilibrium configurations. The adiabatic electron affinities of CO2 and OCS are calculated to be negative, whereas the CS2− anion is stable in the linear and relaxed geometries. The existence of CS2− at linear geometries can be related to experimental observations of an electric field-induced detachment of an extra electron from the anion in fields of only a few kilovolts per centimeter.
Density functional theory study of the proton-bound rare-gas dimers Rg2H+ and (RgHRg′)+ (Rg=Ar, Kr, Xe): Interpretation of experimental matrix isolation infrared data
Authors: Fridgen Travis D. , Parnis J. Mark
Published: July 2002

Density functional theory calculations have been performed on the various proton-bound rare-gas dimers Rg2H+ and (RgHRg′)+ (Rg=Ar, Kr, or Xe, and Rg≠Rg′) employing the BP86 method coupled with either a Gaussian split valence basis set (DZVP) or a numerical split valence basis set (DN**). The calculations with the DN** basis represent the first calculations in which correct qualitative agreement is obtained with respect to the trend in experimental data for the antisymmetric stretching wavenumbers of the three Rg2H+ cations. Good qualitative agreement is also obtained for the antisymmetric stretching wavenumber of the mixed-rare-gas species (ArHKr)+. For the xenon-containing mixed-rare-gas cations, the agreement with experimental wavenumbers is not good as is the case for the DZVP basis set with any of the aforementioned cations. This is believed to be due to the inability of these basis sets to predict some physical and chemical properties for these species. Quantitative agreement between theory and experiment with respect to the antisymmetric stretch of the Rg2H+ cations is improved when four radial argon atoms are placed at a fixed distance from the central H, intended to mimic the matrix environment. Based on these calculations, an inverse hydrogen-isotope dependence for the dissociation energy of these species is predicted. No center atom isotope dependence is predicted for the symmetric stretching vibration. Employing a polyatomic model, we have reanalyzed previously published combination band data for Xe2H+ and Xe2D+, and concluded that there is no evidence for an inverse isotope dependence for the symmetric stretching vibrations of these species.
Conductance spectra of molecular wires
Authors: Tian Weidong , Datta Supriyo , Hong Seunghun , Reifenberger R. , Henderson Jason I. , Kubiak Clifford P.
Published: July 2002

A relatively simple and straightforward procedure for characterizing molecular wires is to measure the conductance spectrum by forming a self-assembled ordered monolayer (SAM) on a metallic surface and using a high scanning-tunneling microscope resolution (STM) tip as the other contact. We find that the conductance spectrum (dI/dV vs. V) can be understood fairly well in terms of a relatively simple model, provided the spatial profile of the electrostatic potential under bias is properly accounted for. The effect of the potential profile is particularly striking and can convert a symmetric conductor into a rectifier and vice versa. The purpose of this paper is to (1) describe the theoretical model in detail, (2) identify the important parameters that influence the spectra and show how these parameters can be deduced directly from the conductance spectrum, and (3) compare the theoretical prediction with experimentally measured conductance spectra for xylyl dithiol and phenyl dithiol.
Preliminary study on the charge transfer and bond ionicity of binary crystals
Authors: Wu Z. J. , Meng Q. B. , Zhang S. Y.
Published: July 2002

Charge transfer and bond ionicity of some monovalent, divalent, and trivalent binary crystals of ANB8−N type have been investigated using the self-consistent method. The method divides the binary crystal systems into two subsystems which contain only one kind of element each in physical space. The charge transfer values are obtained by adjusting the charge in a self-consistent way. Based on the obtained charge transfer values, an empirical formula for bond ionicity has been proposed. It has been shown that the present results for bond ionicity are in good agreement with the previous theoretical study delivered by Levine and Pauling. The results also indicate that a large magnitude of charge transfer (or less excess charge in the bonding region) gives rise to high bond ionicity (or low bond covalency); this agrees well with the viewpoint that the excess charge in the bonding region is the origin of the formation of bond covalency.
Configurational temperature: Verification of Monte Carlo simulations
Authors: Butler B. D. , Ayton Gary , Jepps Owen G. , Evans Denis J.
Published: July 2002

A new diagnostic that is useful for checking the algorithmic correctness of Monte Carlo computer programs is presented. The check is made by comparing the Boltzmann temperature, which is input to the program and used to accept or reject moves, with a configurational temperature kBTconfig=|∇qΦ|2/∇q2Φ. Here, Φ is the potential energy of the system and ∇q represents the dimensionless gradient operator with respect to the particle positions q. We show, using a simulation of Lennard-Jones particles, that the configurational temperature rapidly and accurately tracks changes made to the input temperature even when the system is not in global thermodynamic equilibrium. Coding and/or algorithm errors can be detected by checking that the input temperature and Tconfig agree. The effects of system size and continuity of Φ and its first derivative on Tconfig are also discussed.
X-ray excited optical luminescence (XEOL) detection of x-ray absorption fine structure (XAFS)
Authors: Soderholm L. , Liu G. K. , Antonio Mark R. , Lytle F. W.
Published: July 2002

The x-ray excited optical luminescence (XEOL) from a variety of rare-earth ions was used as a detection mode for the collection of L-edge x-ray absorption fine-structure (XAFS) data. In order to understand the source of the observed optical signal, advantage is taken of the known luminescent response of f ions in a variety of transparent host materials. Whereas some samples exhibit an optical response that is indistinguishable from the transmission XAFS data, other samples show marked differences between the data obtained with the two different detection schemes. The unexpected optical luminescence of a Gd2O3 sample is traced to a Eu impurity. An optical spectrum of 0.4% Tb in Gd2O2S, excited by x-ray photons at the Gd edge, is used to demonstrate that the optical signal may arise from an ion different from the absorbing ion. The implications of this energy transfer are discussed in terms of the suitability of XEOL as a detection scheme for XAFS spectroscopy.
The merging of the dielectric α- and β-relaxations in poly-(methyl methacrylate)
Authors: Bergman R. , Alvarez F. , Alegrı́a A. , Colmenero J.
Published: July 2002

Using broad-band dielectric spectroscopy (10−2–109 Hz) the merging of the α- and β-relaxations has been investigated for a polymer, syndiotactic poly(methyl methacrylate) (PMMA), in which the dielectric losses are dominated by a strong β-relaxation. The asymmetrically shaped β-relaxation cannot be described by a Cole–Cole function, not even at low temperatures where the α- and β-relaxations are well separated in frequency. At higher temperatures close to Tg (=404 K), the weak α-relaxation enters our dynamic window and rapidly merges with the β-relaxation. To investigate this merging process we first used a simple addition of two Havriliak–Negami (HN) equations to fit the spectra. The obtained relaxation time for the β-relaxation then displays a kink in its temperature dependence close to Tg. To gain further understanding of the merging, the data were analyzed by means of a regularization method in order to calculate the corresponding distributions of relaxation times directly from the actual measurements without the need of invoking any parametrical fitting functions. From these distributions we obtained the relaxation functions in time domain which were analyzed by adopting an ansatz proposed by Williams which implies that the α- and β-relaxations are statistically independent processes. The dynamics in the merging region of PMMA can then be described using extrapolations of the β-relaxation from low temperatures and the kink in the temperature dependence of the relaxation time of the β-relaxation obtained in the simple addition approach, is explained in terms of overlapping effects between the α- and β-processes, i.e., without invoking any change of relaxation mechanism.
Efficient approach to the numerical calculation of optical line shapes for molecular aggregates
Authors: Makhov D. V. , Egorov V. V. , Bagatur’yants A. A. , Alfimov M. V.
Published: July 2002

A new approach is proposed for numerical calculations of optical line shapes for disordered molecular aggregates using the excitonic Hamiltonian. The method proposed previously for Gaussian disorder is generalized. A fluctuating origin of the energy scale is introduced as a new independent variable. The integration over this variable eliminates the energy delta function from the integrand, which drastically simplifies the Monte Carlo procedure. The new approach is suitable for calculations using both the diagonalization of the Hamiltonian and the resolvent of the Hamiltonian. As an example, optical line shapes have been calculated for aggregates with bi-Gaussian disorder.
Infrared spectra of the CO2− and C2O4− anions isolated in solid argon
Authors: Zhou Mingfei , Andrews Lester
Published: July 2002

Laser ablation of transition metal targets with concurrent 11 to 12 K condensation of CO2–Ar mixtures produces a sharp metal independent infrared absorption at 1657.0 cm−1 due to CO2−, which is formed from the capture of ablated electrons by CO2 molecules during the condensation process. Two additional metal independent absorptions are produced at 1856.7 and 1184.7 cm−1 on matrix annealing to 25 K to allow diffusion and reaction of CO2 and CO2−. Isotopic substitution (13CO2, C18O2, C16,18O2, and mixtures) shows that these two vibrations involve two equivalent CO2 subunits. The excellent agreement with frequencies, intensities, and isotopic frequency ratios from density-functional calculations supports assignment to the symmetrical C2O4− anion with D2d symmetry. Photodissociation (470–580 nm) and failure to observe these absorptions in identical experiments doped with the electron trapping molecule CCl4 further support the molecular anion assignments. Although absorptions were observed for weak (CO2)(CO2−) complexes, no evidence was found for the asymmetric O2C⋅OCO− molecule–anion complex characterized by calculations.
Site specificity in molecular hydrogen elimination from photodissociation of propane at 157 nm
Authors: Wu S. M. , Lin J. J. , Lee Y. T. , Yang X.
Published: July 2002

Site effects on the molecular hydrogen elimination from propane at 157 nm excitation have been studied using the photofragment translational spectroscopic technique. Experimental results indicate that H2 elimination from the internal carbon of propane (2,2-elimination) is predominant while eliminations from the terminal carbon (1,1- and 1,3-elimination) and the vicinal carbons (1,2-elimination) are minor. The translational energy distributions obtained for these processes also show that the dynamics of H2 eliminations from different sites are significantly different. Relative branching ratios of the atomic hydrogen (H) and the molecular hydrogen (H2) elimination processes were also determined.
Lubrication corrections for three-particle contribution to short-time self-diffusion coefficients in colloidal dispersions
Authors: Cichocki B. , Ekiel-Jeżewska M. L. , Wajnryb E.
Published: July 2002

It is shown that the standard treatment of lubrication effects in many-particle hydrodynamic interactions leads to divergent three-particle contributions to the short-time translational self-diffusion coefficient. To resolve the problem the improved method to account for lubrication is proposed. The translational and rotational self-diffusion coefficients of the Brownian semidilute suspension are then evaluated up to terms of the second order in volume fraction.
Photoswitch and nonlinear optical switch: Theoretical studies on 1,2-bis-(3-thienyl)-ethene derivatives
Authors: Majumdar D. , Lee Han Myoung , Kim Jongseob , Kim Kwang S. , Mhin Byung Jin
Published: July 2002

The 1,2-bis-(3-thienyl)-ethene derivatives are known to be good photoswitches. A large number of experiments have been carried out on different classes of these molecules to find out the most effective photoswitch. We have selected several highly efficient representative model photoswitch molecules of this class and studied their structures, photophysics, and different molecular properties at the ground and vertically excited states using density functional technique together with its time-dependent analog. These analyses are motivated toward the understanding of the effective molecular criteria, which are to be satisfied by a molecule in order to be good photoswitch. The theoretical investigations indicate that the capped ethene derivatives of this class of molecules are more effective photoswitches than the uncapped ones. Our contention has been verified by carrying out similar calculations on a well-known thermally irreversible photoswitch molecule of this class. Since the transition of the open to the closed form in photoswitching devices is the key factor for the molecules to exhibit such properties, and, moreover, since the molecules are thermally stable, they could be used for designing nonlinear optical (NLO) switches. One such possibility has been explored theoretically using a model molecular system.
Collision model for activated rate processes: turnover behavior of the rate constant
Authors: Berezhkovskii A. M. , Bicout D. J. , Weiss G. H.
Published: July 2002

A theory of reaction rates is developed on the basis of the Bhatnagar–Gross–Krook model, which assumes instantaneous Maxwellization of the particle velocity at each collision. This model may be regarded as an alternative to the Kramers model for reaction dynamics in the condensed phase. The main results are two expressions for the rate constant for single- and double-well potentials. These cover the entire range of collision frequency. These expressions predict a turnover of the rate constant as a function of the collision frequency, analogous to the Kramers–Mel’nikov–Meshkov solution for the rate constant in the Kramers model. In contrast to the prediction for the Kramers model, the maximal value of the rate constant is noticeably below the TST estimate even for so high a barrier as 30kBT. This is a consequence of two facts: (1) The rate constant grows slowly from zero at small collision frequencies. (2) In addition, the rate of growth increases weakly with the barrier height, ΔU, as ln(ΔU/kBT). Simulated results indicate good agreement with the theory.
Detection of O(1D) produced in the photodissociation of O2. I. Identification of the Σu-3 and Πu3 Rydberg states in 113–130 nm
Authors: Lee P. C. , Nee J. B.
Published: July 2002

The metastable O(1D) atom produced in the photodissociation of O2 in the wavelength region 113–130 nm was detected by measuring the 762 nm fluorescence of the transition O2(b 1Σg+→X 3Σg−). The excited states of O2 were classified as either the Σu-3 state that produces O(1D)+O(3P), or the Πu3 state that produces O(3P)+O(3P). Our quantum yield measurements of O(1D) found bands of both pure and overlapped Σu-3 and Πu3 states. Five bands were found to produce O(1D), including the longest, second, and third bands and two additional bands near 115.3 and 113.8 nm that were assigned as the E 3Σu−(v′=3–4) states. Three bands were assigned to the F 3Πu(v′=0–2) state. We also found a state at 116.2 nm, which did not produce O(1D), but produced weak visible fluorescence. The possibility of attributing the fluorescence to a transition D 3Σu+–C 3Πg was discussed.
Phase behavior of two-dimensional hard rod fluids
Authors: Bates Martin A. , Frenkel Daan
Published: July 2002

Monte Carlo simulations are used to study two-dimensional hard rod fluids consisting of spherocylinders confined to lie in a plane. The phase behavior is mapped out as a function of the aspect ratio (L/D) of the particles, from the hard disc limit at one extreme (L/D=0) to the thin hard needle limit at the other (L/D=∞). For long rods, a 2D nematic phase is observed at high density in which the orientational correlation functions decay algebraically, indicating that the phase does not possess true long range orientational order. The simulation data indicate that the transition from this phase to the low density isotropic phase is continuous, via a Kosterlitz–Thouless disclination unbinding type mechanism, rather than first order. For short rods the nematic phase disappears so that, on expansion, the solid phase undergoes a first order transition directly to an isotropic phase. Although the latter phase is globally isotropic, we find evidence for strong local positional and orientational correlations between the particles. Where possible, the simulation results are compared and contrasted to experimental, simulation and theoretical data for other two-dimensional liquid crystalline systems.
Decomposition formulas of exponential operators and Lie exponentials with some applications to quantum mechanics and statistical physics
Authors: Suzuki Masuo
Published: July 2002

Decomposition formulas of general exponential operators in a Banach algebra and in a Lie algebra are presented that yield a basis of Monte Carlo simulation of quantum systems. They are applied to study the relaxation and fluctuation from the initially unstable point and to confirm algebraically the scaling theory of transient phenomena. A global approximation method of transient phenomena is also formulated on the basis of decomposition formulas. It is applied to the laser model as a simple example.
A method for generating exact Bianchi type II cosmological models
Authors: Hajj-Boutros J.
Published: July 2002

A method for generating exact Bianchi type II cosmological models with a perfect fluid distribution of matter is presented. Two new classes of Bianchi type II solutions have been generated from Lorenz’s solution [D. Lorenz, Phys. Lett. A 79, 19 (1980)]. A detailed study of physical and kinematic properties of one of them has been carried out.
On the integrability of systems of nonlinear ordinary differential equations with superposition principles
Authors: Bountis T. C. , Papageorgiou V. , Winternitz P.
Published: July 2002

A new class of ‘‘solvable’’ nonlinear dynamical systems has been recently identified by the requirement that the ordinary differential equations (ODE’s) describing each member of this class possess nonlinear superposition principles. These systems of ODE’s are generally not derived from a Hamiltonian and are classified by associated pairs of Lie algebras of vector fields. In this paper, all such systems of n≤3 ODE’s are integrated in a unified way by finding explicit integrals for them and relating them all to a ‘‘pivotal’’ member of their class: the projective Riccati equations. Moreover, by perturbing two parametrically driven projective Riccati equations (thus making them nonsolvable in the above sense) evidence is discovered of chaotic behavior on the Poincaré surface of section—in the form of sensitive dependence on initial conditions—near a boundary separating bounded from unbounded motion.
Conformal collineations and anisotropic fluids in general relativity
Authors: Duggal K. L. , Sharma R.
Published: July 2002

Recently, Herrera et al. [L. Herrera, J. Jimenez, L. Leal, J. Ponce de Leon, M. Esculpi, and V. Galino, J. Math. Phys. 25, 3274 (1984)] studied the consequences of the existence of a one-parameter group of conformal motions for anisotropic matter. They concluded that for special conformal motions, the stiff equation of state (p=μ) is singled out in a unique way, provided the generating conformal vector field is orthogonal to the four-velocity. In this paper, the same problem is studied by using conformal collineations (which include conformal motions as subgroups). It is shown that, for a special conformal collineation, the stiff equation of state is not singled out. Non-Einstein Ricci-recurrent spaces are considered as physical models for the fluid matter.
R-separation of variables for the time-dependent Hamilton–Jacobi and Schrödinger equations
Authors: Kalnins E. G. , Miller Willard
Published: July 2002

The theory of R-separation of variables is developed for the time-dependent Hamilton–Jacobi and Schrödinger equations on a Riemannian manifold V n where time-dependent vector and scalar potentials are permitted. As an application it is shown how to obtain all R-separable coordinates for the n-sphere and Euclidean n-space.
Symmetry properties of product states for the system of N n-level atoms
Authors: Alicki Robert , Rudnicki Sl/awomir , Sadowski Sl/awomir
Published: July 2002

Suppose that the state of a system of N n-level atoms is given by a tensor product of N identical density matrices. The exact formulas are presented that describe the probability that such a system may be found in a pure state with a given symmetry with respect to permutations of atoms. The asymptotic form of these probabilities valid for large N is also derived.
Cauchy surfaces in a globally hyperbolic space-time
Authors: Dieckmann Jan
Published: July 2002

With the help of volume functions it is shown that a globally hyperbolic space-time possesses a Cauchy surface which is a three-dimensional, connected, spacelike C∞ hypersurface.
Low-dimensional real Lie algebras
Authors: Turkowski P.
Published: July 2002

All real Lie algebras of dimension up to 8 that admit a nontrivial Levi decomposition are found.