bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of Raman trace scattering intensities in alkanes with the theory of atoms in molecules
Authors: Gough K. M. , Srivastava H. K. , Belohorcová K.
Published: July 2002

In this paper, we report the detailed ab initio analysis of charge flow that occurs within a molecule during a molecular vibration, under conditions corresponding to a Raman experiment. Theoretical values for the molecular polarizability and polarizability derivatives are obtained in two ways for methane, ethane, and propane. Initially they are obtained from ab initio molecular orbital calculations employing coupled perturbed Hartree–Fock theory. Second, wave functions corresponding to the molecule in the presence and in the absence of an electric field, generated by the calculation, are analyzed according to the theory of atoms in molecules (AIM). The molecular polarizabilities are determined from the amount of charge transferred from one atom to another plus a contribution from reorganization of atomic charge within each atom, due to the presence of an electric field. Derivatives are obtained from the change in the molecular polarizability as the atoms are displaced from their equilibrium positions. It is apparent that a molecule behaves like a dielectric material, developing a depolarizing surface charge while atomic dipoles of interior atoms oppose the charge transfer. For nonspherical molecules, the polarizability derivatives for the symmetric CH stretching modes show considerable dependence on the orientation of the bonds with respect to the principal axes of the molecular polarizability, and hence, the carbon chain. The polarizability derivatives are characterized as arising from the atom-to-atom charge transfer or changes in the induced atomic dipoles. Previously observed dependence of the intensity on bond orientation is due, in large part, to charge transfer along a carbon chain. It is greatest for the terminal CH bonds lying in the plane of the carbon chain and increases regularly with chain length. The question of transferability of intensity parameters from one molecule to another, as is assumed in the bond polarizability model, is addressed in the light of this new information.
A molecular dynamics simulation of polyethylene
Authors: Pant P. V. Krishna , Han Jie , Smith Grant D. , Boyd Richard H.
Published: July 2002

There is recent evidence that united atom CH2 Lennard-Jones nonbonded potentials commonly used for molecular dynamics simulations for liquid polymers such as polyethylene are inadequate in some important respects. In the present work a united atom potential incorporating an offset of its center from the carbon atoms (the ‘‘anisotropic united atom’’ potential form suggested by Toxvaerd) is calibrated for polyethylene. Equation of state (P,V,T) and heat of vaporization representation are used as the criteria for accomplishing this. It is found that good agreement between experiment and molecular dynamics simulation for these quantities can be achieved. Some aspects of the atomistic details of packing are discussed and the onset of vitrification in V–T curves is examined.
Density functional approach to the theory of interfacial properties of nematic liquid crystals
Authors: Osipov M. A. , Hess S.
Published: July 2002

General expressions for the free energy of the nematic liquid crystal in the interfacial region are derived using the density functional approach. The anisotropic part of the surface tension and the coefficients of the Landau–de Gennes theory for the nematic free surface are expressed in terms of the direct correlation function and analytical results are obtained for a reference system composed of molecules with ellipsoidal equipotential surfaces. The assumption of perfect local orientational order is used to account for strong short-range orientational correlations in the nematic liquid. It is shown that in such an idealized system the molecules can be aligned parallel or perpendicular to the free surface of the nematic, depending on the balance between attractive and repulsive forces. However, the condition of the stability of the nematic phase requires this alignment to be parallel. The possible role of specific intermolecular interactions, which can be very important in real liquid crystals, is also discussed.
Ultraviolet-visible absorption spectrum of C60 vapor and determination of the C60 vaporization enthalpy
Authors: Dai Sheng , Mac Toth L. , Del Cul G. D. , Metcalf David H.
Published: July 2002

Ultraviolet-visible absorption spectroscopic studies of C60 vapor were carried out in the temperature range 400 °C–720 °C. The enthalpy of sublimation (180±10 kJ/mol) was obtained through the use of the Clausius–Clapeyron equation.
Prewetting at a liquid mixture–solid interface: A comparison of Monte Carlo simulations with mean field density functional theory
Authors: Kierlik E. , Rosinberg M. L. , Fan Y. , Monson P. A.
Published: July 2002

We present a comparison of the predictions of mean field density functional theory with recent Monte Carlo simulation results for adsorption from a binary liquid mixture in contact with a wall. The symmetric liquid mixture under consideration exhibits liquid–liquid immiscibility with an upper critical solution temperature and the simulation results are consistent with a first order wetting transition with an accompanying prewetting line. The density functional theory gives quite good predictions of the wetting phase diagram for the system when the effect of errors in the bulk thermodynamic properties is removed. However, the composition distribution and adsorption isotherms show significant deviations reflecting the inability of the mean field theory to account correctly for the composition correlations in the system.
A holographic grating study on a crystal-to-crystal photodimerization and thermal backreaction
Authors: Köhler W. , Novak K. , Enkelmann V.
Published: July 2002

The thermally reversible single-crystal-to-single-crystal [2+2] photodimerization of a styrylpyrylium salt, (E)-2,6-Di-tert-butyl-4-[2-(4-methoxyphenyl)-ethenyl]pyrylium- trifluormethansulfonate, has been investigated by means of a laser-induced holographic grating technique. By irradiation far into the long wavelength absorption tail, the single crystalline character of the sample can be retained both during the photodimerization and the thermal backreaction. Without irradiation the holographic gratings are stable at room temperature, but decay at elevated temperatures. From the temperature dependence of the holographic growth and decay curves an upper limit for the activation energy for the photodimerization is obtained to 15.2 kJ/mol. For the thermal cyclobutane cleavage an activation energy of 99 kJ/mol and an attempt frequency of 1.2×1012 s−1 are found.
Electron–ion recombination rate in high-mobility liquids
Authors: Mozumder A.
Published: July 2002

An analysis of electron–ion recombination rate in high-mobility liquids is presented from the viewpoint of fractal diffusion (mean-free path effect), followed by a repeated encounter formulation of partially diffusion-controlled reaction of the final step. Good agreement is achieved with experiments in liquid methane, liquid argon, and liquid krypton with an encounter reaction probability of 0.567, 0.060, and 0.383, respectively. The corresponding ratio of the fractal scale parameter ‘‘d’’ to the mean-free path is found to be approximately 2.0, 4.0, and 1.0, respectively. For liquid methane the experimental variation of the recombination rate with the mean-free path agrees fairly well with theoretical calculation using the same value of the reaction probability. Other theoretical models and their limitations are briefly discussed.
Classical S-matrix theory for chaotic atom–diatom collisions
Authors: Tiyapan Ampawan , Jaffé Charles
Published: July 2002

The extension of classical S-matrix theory to chaotic scattering systems is considered. It is shown that if the fractal structure of the chattering region is understood then the contribution to the S-matrix elements and the transition probabilities can be expressed as a sum over the infinite number of contributing trajectories and that by using the scaling laws of the fractal that this sum can be evaluated. It is shown that if the transition is classically forbidden then the contribution from the chattering region is significant.
Molecular dynamics simulations of proteins in solution: Artifacts caused by the cutoff approximation
Authors: Saito Minoru
Published: July 2002

Molecular dynamics (MD) simulations of human lysozyme in water were carried out for 400 ps with/without the truncation of long-range Coulomb interactions in order to investigate artificial effects caused by the cutoff method. MD simulations with the long-range Coulomb interactions included were carried out by using a recently developed method [PPPC method; Mol. Simul. 8, 321 (1992)]. On the other hand, MD simulations with the long-range Coulomb interactions truncated were carried out using the 10 Å cutoff method. Comparing both results, the following artifacts were found. The cutoff method caused a 1.7 Å larger deviation of charged residues from the x-ray structure than the PPPC method. Further, the cutoff method caused unacceptably large fluctuation around the equilibrium structure in water. However, the fluctuation found with the PPPC method correlated well with that derived from x-ray B-factors.
The H–NCS bond energy, ΔH○f (HNCS), ΔH○f (NCS), and IP(NCS) from photoionization mass spectrometric studies of HNCS, NCS, and (NCS)2
Authors: Ruscic B. , Berkowitz J.
Published: July 2002

The species HNCS and NCS have been studied by photoionization mass spectroscopy. The adiabatic ionization potential (IP) of HNCS is ≤9.92 eV; the shape of the ion yield curve near threshold implies a large change in geometry, perhaps to linear HNCS+. Various appearance potentials (AP) from HNCS are determined by a more objective procedure, from which ΔH○f(HNCS)≳29.6 kcal/mol (and very likely ≥31.0±0.7 kcal/mol) is deduced. The adiabatic IP of NCS is 10.689±0.005 eV. This quantity, together with AP(NCS+/HNCS), yields D0(H–NCS)≤97.2±0.2 kcal/mol. However, a measurement of the appearance potential of NCS+ from (NCS)2 results in a still lower value, D0(H–NCS)≤93.3±1.1 kcal/mol, and ΔHf0○(NCS) ≤ 72.7 ± 0.8 kcal/mol. Approximate values are obtained for ΔHf0○(HNCS+) and for the proton affinity of NCS.
A theoretical study of the electronic spectrum of bithiophene
Authors: Rubio Mercedes , Merchán Manuela , Ortí Enrique , Roos Björn O.
Published: July 2002

The electronic spectrum of bithiophene in the energy range up to 6.0 eV has been studied using multiconfigurational second order perturbation theory (CASPT2) and a basis set of ANO type, with split valence quality and including polarization functions on all heavy atoms. Calculations were performed at a planar (trans) and twisted geometry. The calculated ordering of the excited singlet states is 1Bu, 1Bu, 1Ag, 1Ag, and 1Bu with 0–0 transition energies: 3.88, 4.15, 4.40, 4.71, and 5.53 eV, respectively. The first Rydberg transition (3s) has been found at 5.27 eV. The results have been used in aiding the interpretation of the experimental spectra, and in cases where a direct comparison is possible there is agreement between theory and experiment.
Some simple bifurcation sets of an extended Van der Pol model and their relation to chemical oscillators
Authors: Koper Marc T. M.
Published: July 2002

Some typical bifurcation sets of a generalized autonomous Van der Pol-type model are discussed as archetypes of phase diagrams occurring in nonlinear dynamical systems. The relevance of the obtained bifurcation sets is exemplified by several experimental and numerical results from the literature of oscillating chemical reactions.
Theoretical and simulation studies of recombinative temperature programmed desorption
Authors: Meng B. , Weinberg W. H.
Published: July 2002

Using Monte Carlo simulations and both quasichemical (for nearest neighbors) and mean field (for next-nearest neighbors) approximations, we explore a kinetic lattice gas model to investigate recombinative thermal desorption. A previously introduced Monte Carlo algorithm, which correctly relates Monte Carlo simulation time and real time, is extended in order to quantify the kinetics and energetics of recombinative thermal desorption spectra. We consider the effects of lateral interactions between adsorbates, lattice geometry, and limited mobility of the adsorbate (nonequilibrium) on the temperature programmed desorption spectra. Furthermore, we analyze the apparent coverage dependence of both the activation energy and the preexponential factor of the desorption rate coefficient for both repulsive and attractive nearest-neighbor interactions on a square lattice. For a repulsive nearest-neighbor interaction, we find that kinetic compensation occurs for a surface coverage less than 0.6. However, for surface coverages greater than 0.6, we find that the activation energy and preexponential factor do not vary sympathetically. For an attractive nearest-neighbor interaction, kinetic compensation is only observed at high coverage. We elucidate the compensation effect quantitatively by considering the configurational distribution of adsorbates.
Tethered chains in good solvent conditions: An experimental study involving Langmuir diblock copolymer monolayers
Authors: Kent M. S. , Lee L. T. , Factor B. J. , Rondelez F. , Smith G. S.
Published: July 2002

We have employed Langmuir monolayers of polydimethylsiloxane-polystyrene diblock copolymers as a model system for examining layers of tethered chains under good solvent conditions. The range of surface density accessible with this system coincides with the ranges reported in the literature for chains tethered onto solid substrates from dilute solution in good solvents. We have varied both the surface density (σ) and the molecular weight (M) of the submerged polystyrene block independently, covering over a decade in each variable. Both the form of the segmental concentration profile and the variation of the layer height with σ and M are in good agreement with numerical self-consistent-field (SCF) calculations. On the other hand, we do not observe precise agreement with the scaling predictions for strongly stretched chains, in contrast to some previous reports. Through measurements of the surface pressure (Π), we present the first direct comparison between anchoring energies and differential chain energies in tethered layers. We find these to be of equal magnitude at the desorption transition. However, the chain energies reach values roughly an order of magnitude larger than can be accounted for by the osmotic interaction of the polystyrene segments in the good solvent. In addition, the dependence of Π on σ is more consistent with a model of soft spheres with hard cores than that of a semidilute polymer mesh. The hard core areas seem to be loosely related to Rg. We attribute these observations to a steric effect which limits lateral interpenetration of the submerged blocks. The sharp rise in Π with σ is of great practical importance as it limits the maximum surface coverage in this system, and may also do so for other systems.
Charge-transfer dynamics in ion–polyatomic molecule collisions: X++H2O (X=N,Kr) luminescence study
Authors: Dressler Rainer A. , Arnold Susan T. , Murad Edmond
Published: July 2002

H2O+ Ã 2A1–X̃ 2B1 luminescence spectra are presented for the X++H2O (X=N,Kr) charge-transfer systems at collision energies ranging between 0.4 and 130 eV (c.m.). The luminescence is attributed to bending vibrational states near-resonant with N+(3P)+H2O and Kr+(2P1/2)+H2O reactants, in agreement with energy resonance and Franck–Condon predictions. H2O+ Ã-state product vibrational distributions are obtained from spectral simulations. The energy dependence of vibrational state-to-state cross sections is determined for the N+(3P)+H2O charge-transfer system, where the integral charge-transfer cross section is known. The cross sections are related to semiclassical charge-transfer models. Exothermic product states are interpreted to be primarily governed by curve crossings along the bending coordinate of H2O, while endothermic levels can be rationalized using a Demkov-type mechanism. The Kr+(2P1/2)+H2O luminescence study, for which state-selected cross sections are not available, provides product state distributions consistent with this interpretation. The higher optical resolution in that experiment, however, enables a K-substate resolved analysis, as well as an approximate assessment of the product rotational temperature. The observed rotational temperatures are near thermal, implying long-range processes governed by large impact parameters.
Direct energy transfer in solutions of ideal polymer chains
Authors: Oshanin G. , Blumen A. , Moreau M. , Burlatsky S. F.
Published: July 2002

We study the direct incoherent energy transfer in systems, in which the molecules involved in the transfer process are attached to ideal (Gaussian) polymers, randomly arranged in a viscous solvent. In what the molecules’ attachment to polymers is concerned we consider two different situations: (a) each polymer chain contains at its opposite ends a donor and an acceptor molecule, (b) the donor molecules are dispersed randomly in the solvent and all acceptors are attached to polymers. In both cases we derive the donors’ decay forms exactly.
Measuring diffusion in supercooled liquids: The effect of kinetic inhomogeneities
Authors: Perera Donna N. , Harrowell Peter
Published: July 2002

There is growing evidence to suggest that cooperativity in glasses is characterized by a spatial distribution of relaxation times. In this paper we examine the consequences of such inhomogeneities in the measurement of translational and rotational diffusion constants. We find that the decay of a concentration profile (the basis of transient grating experiments) is accelerated by the development of additional gradients parallel to the relaxation time gradients. This may account for recently observed differences in the variation of rotational and translational diffusion constants with viscosity below the glass transition temperature. The additional gradients may also provide the first direct means of examining the length scale associated with these inhomogeneities.
‘‘False tunneling’’ and multirelaxation time nonexponential kinetics of electron transfer in polar glasses
Authors: Dakhnovskii Yuri , Lubchenko Vassiliy , Wolynes Peter
Published: July 2002

Classical electron transfer in polar glasses is described by a theory based on a model microscopic Hamiltonian which includes the discreteness and randomness of the glassy polar modes with distinct orientation. When configurational dynamics is fast, the reaction is described by exponential kinetics with a rate constant of non-Arrhenius type. The temperature dependent rate constant resembles the tunneling rate, despite the classical transfer of the electron. This effect is called ‘‘false tunneling.’’ In this limit the possibility of a self-acceleration of the reaction is pointed out. When configurational dynamics is very slow the reaction kinetics are nonexponential with multirelaxation time behavior. The reaction is shown to be almost insensitive to temperature change pointing out on a possible explanation of a broad temperature-independent range in the ‘‘rate constant’’ in an electron transfer in cytochrome c oxidation in chromatium. At short times, the reaction accelerates compared to the exponential behavior, while at long times it becomes slower. For strongly exothermic reactions the kinetics are always slower than an exponential decay.
Nonexponential structural relaxations in proteins
Authors: Hagen Stephen J. , Eaton William A.
Published: July 2002

Conformational changes in proteins have been observed to exhibit a nonexponential time course. In myoglobin the conformational relaxation that follows photodissociation of the heme ligand is a very extended process that stretches from less than 1 picosecond to nearly 1 microsecond. We explain these kinetics with a model in which the initial protein conformational substates are connected to the final substates and to each other via transition states of a single energy.
Gaussian-2 (G2) theory: Reduced basis set requirements
Authors: Curtiss Larry A. , Redfern Paul C. , Smith Brian J. , Radom Leo
Published: July 2002

Two variations of G2(MP2) theory which employ smaller basis sets in evaluating the quadratic configuration interaction [QCISD(T)] component of the energy are presented. The first, G2(MP2,SVP), uses the split-valence plus polarization (SVP) 6-31G(d) basis, while the second, G2(MP2,SV), uses the split-valence (SV) 6-31G basis. The methods are evaluated on the basis of results for the set of 125 systems used for testing G2 theory. The mean absolute deviation of G2(MP2,SVP) results from experimental values is 1.63 kcal mol−1 compared with 1.58 and 1.21 kcal mol−1 for G2(MP2) and G2, respectively. The G2(MP2,SVP) method thus provides results which are generally very similar in quality to those obtained from G2(MP2) but at considerably reduced computational expense. On the other hand, the mean absolute deviation of G2(MP2,SV) results from experiment is substantially larger (2.13 kcal mol−1). The G2(MP2,SV) method exceeds the 2 kcal mol−1 target accuracy of G2 theory for an unacceptably large number of comparisons.
Symmetrized shape oscillation in the structure of 6Li7 clusters observed by molecular beam electron-spin resonance
Authors: Hishinuma Naoshi
Published: July 2002

The electron-spin-resonance spectra were measured for free 6Li7 clusters by using techniques of molecular beam magnetic resonance. The spectra show a large hyperfine constant a1 from two equivalent nuclei and a much smaller hyperfine constant a2 from an additional five equivalent nuclei. The cluster geometry consistent with the spectra is a regular pentagonal bipyramid. It has been found that the septemer has two isomers which give the same value for a1 to a high accuracy and slightly different values a2a and a2b for a2. There is a relationship between a1 and ā2=(a2a+a2b)/2 of the form a1/5ā2=1.03≂1. The a1 and a2 indicate that the septemer is fluctuating in structure between two geometries, the regular pentagonal bipyramid and a deformed one of it. The former corresponds to an oblate ellipsoid of revolution in the ellipsoidal shell model, while the latter means a superposition of five equivalent deformations from the oblate ellipsoid to prolate ellipsoids. The deformation must be symmetrized by the superposition, because the ground electronic state of Li7 has no orbital degeneracy in spite of the fivefold axial symmetry of the structure. The symmetrized shape oscillation keeps the vibrational motions of the septemer constant, because the five equivalent deformations mutually cancel all restoring forces. The two ellipsoidal configurations, oblate and prolate, are brought into resonance by a Jahn–Teller effect which takes place on the prolate side. The presence of the two like isomers provides evidence of the Jahn–Teller effect, because there are two mutually independent modes of the Jahn–Teller distortion which divide the septemers into two symmetry types. The simple relation a1≂5ā2 further confirms that the electronic states in the two ellipsoidal configurations are in resonance.
Theory of frequency-shifted excitation by phase-incremented pulses in nuclear magnetic resonance
Authors: Zhang Shanmin , Gorenstein David G.
Published: July 2002

Frequency-shifted excitation by phase-incremented pulses of arbitrary shape can be solved by introducing a second rotating frame. The results show that a phase-incremented pulse can be decomposed into an infinite number of unsymmetrically amplitude-scaled and phase-shifted effective radio frequency (rf) fields, which are responsible for the unsymmetrical and phase-shifted centerband and sideband excitations. In addition to a universal phase shift associated with each band of excitation, a phase inversion occurs when the scaling factor of the effective rf field becomes negative. Also if the total phase increment of the pulse is not equal to 2kπ (k integer) an additional phase shift equal to the total phase increment will be introduced. By properly choosing the total phase increment the phase shift of the centerband can be compensated. A computer program based on the Bloch equations is developed to calculate directly the excitation profiles of phase-incremented pulses, which agrees well with the theoretical predications.
The product vibrational, rotational, and translational energy distribution for the reaction O(3P
J)+O3→2O2: Breakdown of the spectator bond mechanism
Authors: Mack J. A. , Huang Yuhui , Wodtke A. M. , Schatz G. C.
Published: July 2002

Laser induced fluorescence (LIF) detection of highly vibrationally excited O2 resulting from visible photolysis of pure O3 is attributed to the title reaction. The vibrational and rotational energy distributions as well as Doppler profiles of selected product states of the nascent O2 were obtained. Predictions of quasiclassical trajectory calculations on the ‘‘Varandas-Pais’’ potential energy surface (l) are inconsistent with observation. This points out the need for a more accurate ab initio study of this important reaction. The implications for nonlocal thermodynamic equilibrium chemistry in the stratosphere are discussed.
Ab initio study of the He(1S)+CH(X 2Π) interaction
Authors: Cybulski Sl/awomir M. , Chal/asiński Grzegorz , Szczȩśniak Mal/gorzata M.
Published: July 2002

Potential energy surfaces for the 2A′ and 2A″ states of the He(1S)–CH(X 2Π) complex were calculated using supermolecular unrestricted Mo/ller–Plesset perturbation theory and analyzed via the relevant perturbation theory of intermolecular forces. It has been found that the two states are distinctly different. The potential energy surface (PES) of the A″ state has only a single and relatively deep minimum of De≊335 μEh for the T-shaped geometry, at R=5.0 a0 and Θ=100°. The position of this minimum is determined by the exchange repulsion which is substantially reduced at this geometry. The minimum is unusually deep for a complex of He, and it can be viewed as an example of an incipient chemical bond. In contrast, the A′ state’s PES represents a typical van der Waals interaction which is characterized by two similarly deep minima. The shape and location of these minima are determined primarily by the anisotropy of the dispersion component. The first minimum occurs for the collinear He–C–H arrangement, at R≊7.5 a0, and Θ=0°, and is 55 μEh deep. The second minimum has a troughlike form which joins the region between R=7.5 a0, Θ=140° and R=8.0 a0, Θ=180°. The lowest point is approximately 54 μEh deep and occurs at R=7.5 a0 and Θ=140°.
Approximate factorization of molecular potential surfaces. I. Basic approach
Authors: Madsen D. , Pearman R. , Gruebele M.
Published: July 2002

The intermediate vibrational energy regime—well above the zero point, yet below the first dissociation limit—plays an important role in many molecular processes, such as radiationless transitions or intramolecular vibrational energy redistribution (IVR). For molecules with 𝒩>6 vibrational degrees of freedom, the calculation of energy levels and spectra of the full anharmonic vibrational Hamiltonian at high energies presents a formidable problem. Here we derive an asymptotic factorization of the potential energy surface (PES) based on the global topography of molecular potentials. The resulting approximate models require only a few input parameters such as vibrational frequencies and cubic anharmonicities. They are compared to benchmark PES derived from curvilinear model potentials and Monte Carlo sampled ab initio calculations for several small molecules. Higher order couplings are found to make a significant contribution to IVR decays at long times.
Reactivity induced by complex formation: The reaction of O(3P) with HCl dimers
Authors: Hurwitz Y. , Stern P. S. , Naaman R. , McCoy A. B.
Published: July 2002

The reaction of O(3P) with HCl⋅M (M=HCl, Ar) complexes has been studied. While the monomer HCl, in its ground vibrational state, reacts extremely slow with O(3P), it is shown here that the van der Waals complexes react with an efficiency of about 3 orders of magnitude larger than that of the monomer. The reactivity of DCl, on the other hand, is not enhanced by the complex formation. Molecular dynamics simulation indicates that the collision complex lifetime increases by several orders of magnitude due to the existence of the “third body” in the cluster. A model for explaining the complex induced enhancement of reactivity is presented and is supported by ab initio calculations.
Adsorption of rigid rodlike molecules
Authors: Matsuyama Akihiko , Kishimoto Reiko , Kato Tadaya
Published: July 2002

A theory is introduced to describe the adsorption of rigid rodlike molecules to a planar surface from solution. The configurations of the adsorbed rods are modeled by restricting the possible orientations to three mutually orthogonal directions: two parallel and one normal to the surface. We examine the adsorption isotherm and the most stable configurational state of the adsorbed rods as a function of rod axial ratio, bulk concentration, and adsorption energy. In the case of a strongly attractive wall, the rods are adsorbed parallel to the surface. As the bulk concentration is increased, however, the adsorption isotherm has a kink at a characteristic bulk concentration where the adsorbed rods become preferentially aligned parallel to the surface. For a weaker surface attraction, the adsorbed rods are aligned along a direction normal to the surface. The dependence of the configuration of the adsorbed rods on the temperature is also discussed.
The electronic structure of metal-molten salt solutions: A tight-binding approach
Authors: Koslowski Thorsten
Published: July 2002

We present a computer simulation study of simple models of alkali metal–alkali halide melts Mx[MX]1−x. The geometries are modeled using the primitive model of hard spheres, the electronic structure is described by a tight-binding Hamiltonian including the crucial component of Madelung potential fluctuations. Electronic defects like F centers are treated by a novel variational ansatz. We focus on the computation of experimentally measured quantities like the optical absorption or the electronic conductivity and the paramagnetic susceptibility as a function of x. Despite the simplicity of the underlying model, simulation and experiment are in good agreement in the entire region from nonmetallic to metallic states.
Numerical application of the coupled cluster theory with localized orbitals to polymers. IV. Band structure corrections in model systems and polyacetylene
Authors: Förner Wolfgang , Knab Reinhard , Čı́žek Jiřı́ , Ladik Janos
Published: July 2002

We present the formalism for the correction of the band structure for correlation effects of polymers in the framework of a localized orbital approximation, using the quasiparticle model. For this purpose we use in an ab initio framework Mo/ller–Plesset perturbation theory in second order, the coupled cluster doubles method, and its linear approximation. The formalism is applied to a water stack and two different forms of a water chain as model systems to test the reliability of the approximations involved. From our previous work we know that, e.g., in polyacetylene difficulties due to the localizability of the canonical crystal orbitals do not arise from the π or π* bands, but from bands of σ symmetry. Thus we concentrate in this work again on polyacetylene as an example of a realistic polymer. We find that the localized orbital approximation is quite useful also in the case of band structure corrections due to correlation effects. However, the coupled cluster calculations, in particular, turn out to be computationally very costly for infinite systems. But it seems to us that localized orbital approximations are at the moment the only way to make coupled cluster calculations on realistic polymers with covalent bonds between the unit cells possible at all.
The calculation of time-resolved negative-ion-to-neutral-to-positive-ion-spectra with an application to iron-carbonyl
Authors: Rubner O. , Meier C. , Engel V.
Published: July 2002

We investigate “negative-ion-to-neutral-to-positive-ion” (NENEPO) pump-probe spectroscopy as proposed by Wöste and co-workers [Wolf et al., Phys. Rev. Lett. 74, 4177 (1995)] where the dynamics of wave packets in a neutral molecule produced by photodetachment from the negative ion is probed by time-delayed photoionization and detection of positive molecular ions or photoelectrons. Approximations are introduced for an effective numerical treatment of the double-ionization process. The organometallic compound FeCO is used as an example.
Relativistic ab-initio calculations on PdH and PdD: The rovibronic spectra and rotational splittings
Authors: Fleig Timo , Marian Christel M.
Published: July 2002

An ab initio theoretical analysis of the rovibronic spectra of the PdH molecule and its heavier isotope PdD is presented. Potential curves of the lowest ΛS state multiplet 2Σ+, 2Δ, and 2Π are generated in consideration of kinematic relativistic effects in a one-component formalism. We determine the effects of the rotational motion of the nuclei on the vibronic spectra by evaluating matrix elements of a rotational Hamiltonian in the basis of uncoupled vibronic Born–Oppenheimer functions, including the effects of spin-orbit coupling and compare our results to experiment. Moreover, we present predictions for experimentally yet unknown transitions by calculating energy levels, Ω-, and vibrational splittings of several excited states.
Competitive diffusion into two sinks with a finite surface reaction coefficient
Authors: Zoia Giorgio , Strieder William
Published: July 2002

The asymptotic solution for the reaction rate of two identical spherical sinks of radius ρ with a surface reaction rate coefficient k, placed in an infinite medium of diffusivity D, is presented for mid to high surface reaction rates, i.e., 1<λ−1=kρ/D<∞. These results are then discussed in the context of the complete kinetic problem from chemical, λ−1≈0.1, to diffusion control, λ−1→∞.
Substrate-induced order in confined nematic liquid-crystal films
Authors: Gruhn Thomas , Schoen Martin
Published: July 2002

In orientationally biased grand canonical ensemble Monte Carlo (GCEMC) simulations we investigated the microscopic structure of liquid-crystalline films confined between two plane parallel solid surfaces (i.e., walls) consisting of Ns discrete, rigidly fixed atoms. These wall atoms are distributed across the plane of a wall according to the (100) structure of the face-centered cubic lattice. Parameters of the film–wall interaction potential are chosen such that a homeotropic alignment of film molecules is favored. In the simulations the thermodynamic state of the film is determined by the temperature T, the chemical potential μ, the distance between the walls sz, and the film–wall interfacial area A. Thermodynamic states of the film are chosen such that a corresponding bulk liquid crystal is nematic. To simulate nematic phases in the GCEMC we modified the classic Gay–Berne potential for the interaction between a pair of film molecules so that the isotropic–nematic phase transition in the bulk occurs at sufficiently low densities. Reliability of the GCEMC method under these conditions is illustrated by a self-consistent comparison between Monte Carlo simulations in the canonical and grand canonical ensembles. In the bulk the nematic nature of the modified Gay–Berne fluid is established by computing the Mayer–Saupe order parameter S and suitably defined pair correlation functions which show that the bulk phase is not smectic even though S is fairly large. For a single temperature we investigate the isotropic–nematic phase transition in the modified Gay–Berne fluid which turns out to be a first-order phase transition. In the corresponding confined film variations of the microscopic structure with increasing sz are correlated with the normal component of the stress tensor Tzz(sz). Our results show that molecules in inner portions of the film undergo a reorganization from an originally planar orientation of their symmetry axes to a perpendicular one with respect to the plane of a wall. This orientational change is manifested as a periodic sequence of shoulders in Tzz(sz) where the periodicity length Δsz is close to the larger diameter of the ellipsoidal film molecules.
Deuterium to carbon cross-polarization in liquid crystals
Authors: Hodgkinson Paul , Auger Céline , Emsley Lyndon
Published: July 2002

The theory describing nuclear cross-polarization from a spin-1 to spin-1/2 is generalized to sweeps of the radio-frequency field amplitude. This allows efficient cross-polarization over the entire width of the deuterium NMR spectrum of partially oriented systems, such as liquid crystals. Such experiments permit the assignment and measurement of the deuterium quadrupolar couplings, and hence the quantification of the order parameters which characterize the dynamics of such systems. The evolution of the spin system can be understood in terms of individual quasiadiabatic population exchanges, allowing the prediction of the effects of the various experimental parameters on the efficiency of cross-polarization.
Ion-pair formation in near-thermal energy collisions of sodium clusters with electron acceptors
Authors: Hampe Oliver , Gerhardt Philip , Gilb Stefan , Kappes Manfred M.
Published: July 2002

Sodium clusters (Nax, x⩽100) were collided with A=TCNQ, C84, C60, Br2, or SF6 at near-thermal energies under crossed beam conditions. Efficient single collisional ionization was observed for all A. Resulting cations and anions were studied by mass spectroscopy. Resonant electron transfer followed by charge separation to yield parent cluster cation (Nax+*) and anion (A−*) pairs is an important channel—given sufficient center of mass kinetic energy. It is the only charging process observed for the fullerenes and TCNQ. For Br2 and SF6, electron transfer can give rise to either charge separation or bond rearrangement/chemi-ionization. We discuss factors influencing the energetics, inelasticity, and probability of electron transfer/ion-pair formation in these systems.
Density functional calculations of molecular polarizabilities and hyperpolarizabilities
Authors: Calaminici P. , Jug K. , Köster A. M.
Published: July 2002

This paper presents dipole moments, static polarizabilities, first hyperpolarizabilities and second hyperpolarizabilities calculated in the framework of density functional theory. All calculations have been performed using a finite field approach implemented in our new density functional theory program ALLCHEM. The calculations were of all-electron type. Both local and gradient-corrected functionals have been used. The influence of first- and second-order field-induced polarization functions, the external field strength, the numerical integration technique and the exchange-correlation functionals on the calculation of polarizabilities and hyperpolarizabilities is discussed in detail. A systematic study including 23 small and medium size molecules demonstrates that the obtained polarizabilities as well as the first and second hyperpolarizabilities are in good qualitative agreement with experimental data. The described density functional method provides polarizabilities and hyperpolarizabilities considerably better than the Hartree–Fock method and almost as accurate as much more expensive correlation treatments. This work demonstrates that reliable predictions of electro-optical properties for molecules with 20 and more atoms are possible using an efficient implementation of density functional theory.
Spectroscopic characterization of the unusually strongly bound, doubly excited van der Waals state, Mg(3pπ3pπ 3PJ)⋅Kr[3Σ−]
Authors: Leung Allen W. K. , McCaffrey John G. , Breckenridge W. H.
Published: July 2002

The unusual doubly excited van der Waal’s state, Mg(3pπ3pπ 3PJ)⋅Kr[3Σ−], has been characterized using a laser-vaporization, supersonic-jet source and R2PI (Resonance Two-Photon Ionization) spectroscopy. This state is very strongly bound (De=3966 cm−1) and has a short bond length (Re=2.45 Å) compared to its singly excited analogue, Mg(3s3pπ 3PJ)⋅Kr[3Π0−], for which De=267 cm−1 and Re=3.48 Å. In fact, this state is even more than twice as strongly bound as the ground-state Mg(3s)+⋅Kr ion, where De=1949 cm−1 and Re≈2.8 Å. Possible reasons for the strong van der Waal’s bonding are discussed, and it is concluded that the lack of σ-σ repulsion because there is no Mg(3sσ) valence electron must be a major factor; the similar ionic van der Waal’s state Mg+(3pπ)⋅Kr[2Π], which would be obtained by removing one of the Mg(3pπ) electrons, is even more strongly bound, with De≈7200 cm−1 [J. S. Pilgrim, C. S. Yeh, K. R. Berry, and M. A. Duncan, J. Chem. Phys. 100, 7945 (1994)].
Isomers of N2O3: Observation of trans-cis N2O3 in solid Ar
Authors: Lee Cheng-I , Lee Yuan-Pern , Wang Xuefeng , Qin Qi-Zong
Published: July 2002

Two isomers of N2O3, asym-N2O3 and sym-N2O3, are identified with infared (IR) absorption when mixtures of NO–NO2–Ar or NO–O2–Ar are deposited onto a target at 13 K. Upon irradiation with a XeCl excimer laser at 308 nm, asym-N2O3 is converted to sym-N2O3 and a new form of N2O3, trans-cis N2O3; the latter is readily converted to sym-N2O3 upon further irradiation with red light. Assignments of IR absorption lines to each conformer in its isotopic variants are based on O18-isotopic substitution and photoconversion experiments. For asym-N2O3, we observed O18-isotopic shifts of a few vibrational modes previously unresolved in a N2 matrix, confirming that O-atoms in the NO2 moiety are inequivalent. For sym-N2O3, a more nearly complete set of isotopic shifts for absorption lines at 1688.6, 971.0, and 704.6 cm−1 enables us to provide refined assignments. In addition, lines at 1722.5 and 1721.1 cm−1 are assigned to symmetric stretching modes of two terminal N=O groups of ON18ON16O16 and ON18ON18O16; they gain IR activity because C2V symmetry is broken. Trans-cis N2O3 with an asymmetric ONONO structure is identified with absorption lines at 1704.5, 1665.7, 877.8, and 243.0 cm−1. Isotopic experiments indicate that this species contains two nearly isolated N=O groups. Spectral assignments are supported by theoretical calculations with density-functional theories (BLYP and B3LYP); previous assignments of low-energy vibrational modes of asym-N2O3 and sym-N2O3 are revised based on comparison with calculations. Photoconversion among these isomers is discussed.
Viscoelastic effects in three-dimensional microphase separation of block copolymers: Dynamic mean-field density functional approach
Authors: Maurits N. M. , Zvelindovsky A. V. , Fraaije J. G. E. M.
Published: July 2002

In the present paper, we extend the dynamic mean-field density functional method which describes microphase separation phenomena in polymer liquids, to account for viscoelastic effects. The effect of simple steady shear on polymer orientation and elongation is taken into account by adapting the polymer configurational distribution function. We propose a simplified model for polymer chains in a simple steady shear flow and show numerically that this model correctly reproduces expected conformational changes. The conformational effect is only of importance for high viscosity liquids and/or high shear rates.
Dynamics of surface directed mesophase formation in block copolymer melts
Authors: Sevink G. J. A. , Zvelindovsky A. V. , van Vlimmeren B. A. C. , Maurits N. M. , Fraaije J. G. E. M.
Published: July 2002

The dynamic mean-field density functional method is adapted to describe phase separation in the presence of geometrical constraints. We observe that inclusion of small filler particles (such as rods) already has a dramatic effect on the morphology of polymer melts. The effect is comparable to the effect of applied simple steady shear. Mesostructures in the presence of large filler particles such as plates are totally governed by the geometry of the particle. Effects of polymer–surface interactions on morphology formation are investigated in detail.
Pressure and temperature-dependent gauche-trans isomerization of 1-bromopropane: Raman measurement and statistical thermodynamic analysis
Authors: Hu Min-Hom A. , de Souza Luis E. S. , Lee Meng-Rong , Ben-Amotz Dor
Published: July 2002

Raman measurements of the isomerization equilibrium in liquid 1-bromopropane are compared with perturbed hard-body fluid predictions. The integrated areas of the Raman bands arising from the C–Br stretch of the gauche and trans conformations are used to track the isomerization equilibrium as a function of pressure and temperature. Repulsive solvent–solute interactions are treated using the recently developed excluded-volume-anisotropy model (based on realistic molecular structures for the two isomers and the equation of state of liquid 1-bromopropane), and cohesive interactions are treated using the van der Waals mean field approximation. The results illustrate the delicate balance of attractive and repulsive solute–solvent interactions which underlie the effects of solvation on chemical equilibria. Comparison of the measured and predicted changes in ΔH with pressure, and ΔV with temperature, are used to determine parameters describing the attractive mean field and cavity formation energies of the two isomers, as well as the isomerization enthalpy and entropy of the isolated molecule.
Enrichment of binary van der Waals clusters surviving a surface collision
Authors: Fort E. , De Martino A. , Pradère F. , Châtelet M. , Vach H.
Published: July 2002

We present experimental results on the collision of large binary noble gas clusters with a hot graphite surface at an incident velocity of 430 ms−1. The mixed ArnXm clusters (X=Kr, Xe) with average sizes n+m between 1000 and 13 000 are obtained from pure argon clusters by the pickup technique. The surface scattering dynamics belongs to the thermal evaporation regime and large surviving binary fragments are detected at grazing angles. As expected from simple binding energy considerations, in all cases the surviving clusters are richer in the dopant species X. This enrichment has been measured as a function of incidence angle, incident cluster size, and dopant molar fraction x. For the lowest values of x (about 0.7%), the measured enrichment is the same for Kr and Xe, and decreases slowly with increasing size and incidence angle. On the other hand, when x is raised up to 15%, this enrichment decreases substantially for Xe, and much less for Kr. This unexpected behavior clearly shows that collision induced evaporation cannot be viewed as a mere distillation of an homogeneous binary mixture. The possible role of the incoming cluster structure is discussed in connection with the preparation of the binary clusters by pickup.
A spectral transform minimum residual filter diagonalization method for interior eigenvalues of physical systems
Authors: Yu Hua-Gen , Nyman Gunnar
Published: July 2002

A spectral transform technique is introduced into the minimum residual (MINRES) filter diagonalization (FD) algorithm for the computation of eigenvalues of large Hermitian matrices. It is a low storage method, i.e., only four real vectors are required to calculate all bound states of the system. In the MINRES FD step, the finite Krylov subspace is built up by a Lanczos iteration using a spectral transform operator which is expanded in a series of Chebyshev polynomials. A guided spectral transform method is suggested to achieve high efficiency of this new algorithm. As an example, all even parity bound states of NO2 have been calculated on the adiabatic ground state potential energy surface of NO2 by a single propagation using a hyperbolic tangent function guided filter operator. The results show that the method is accurate and highly efficient. A statistical analysis of the spectrum is also given.