bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational mode mixing in terminal acetylenes: High-resolution infrared laser study of isolated J states
Authors: McIlroy Andrew , Nesbitt David J.
Published: July 2002

Mode–mode vibrational coupling in the acetylinic CH stretch at 3330 cm−1 of 1-butyne and 1-pentyne is studied via high-resolution, direct absorption infrared spectroscopy. As in our previous study of propyne, mixing of the CH stretch vibration carrying oscillator strength (the bright state) with the bath of multiquantum combination states (the dark, or background, states) manifests itself in the spectrum via fragmentation of the isolated bright state transitions into clusters of closely spaced spectral lines in a ∼0.01 cm−1 window about the zeroth order acetylinic CH stretch position. In the 1-butyne spectrum, we find an experimental density of mixed states of 114±30 states/cm−1 compared to a direct state count prediction of 90 total states/cm−1, and thus quantitatively determine that all possible states appear in the spectrum. The 1-butyne line spacing distribution suggests the Wigner distribution expected for a quantum mechanically ergodic system. Analysis of the mode mixing as a function of J′ shows that anharmonic terms in the potential, rather than Coriolis effects, contribute most strongly to the coupling. The acetylinic CH stretch spectrum of 1-pentyne (2400 states/cm−1) reveals only broad rovibrational transitions with ∼0.01 cm−1 Lorentzian width, even at our 10−4 cm−1 resolution. J′ independent, anharmonic coupling with a minimum of 1/3 of all states must be invoked to reproduce the observed broadening. In contrast, the 1-pentype methyl CH stretch spectrum shows broadening greater than five times larger than that observed at the acetylinic end. Via Fourier transform methods, the spectra for both 1-butyne and 1-pentyne indicate vibrational energy localization in the CH stretch for ∼500 ps. However, for the methyl CH stretch, energy redistribution takes place in <40 ps, clearly indicating the presence of mode specific, nonRRKM vibrational relaxation pathways.
Thermal kinetics in small systems. II. Generalized Arrhenius plots with applications to the dissociation of benzene and substituted-benzene cations
Authors: Klots Cornelius E.
Published: July 2002

Rate constants for the dissociation of isolated benzene and substituted-benzene cations are analyzed. In the first section Arrhenius activation energies are deduced. A primitive version of the adiabatic channel model is then used to reduce these activation energies to their values at zero Kelvin. A value for the heat of formation at that temperature of the C6H+5 ion equal to 1130 kJ/mol is reported. Attention is drawn to a remarkable constancy (and also two anomalies) in the frequency factors for these reactions. The predictive capability of the adiabatic channel model is then examined. It is found to yield kinetic isotope effects in good agreement with experiment.
A jump motion of small molecules in glassy polymers: A molecular dynamics simulation
Authors: Takeuchi Hisao
Published: July 2002

Motions of small molecules in an amorphous polymer below its glass transition temperature Tg have been studied using molecular dynamics simulation. An infinite length chain model having 600 interacting sites, each representing a CH2 unit, is used. The system, which is composed of the infinite length chain and ten spherical molecules representing oxygen, is examined at 30 deg below Tg. Although most of the molecules are trapped by cages created by the chain, a molecule reveals clear jump motions. During the jump, no significant change is found in the local free volume as well as in the nonbonded energy acting on the molecule. Potential energy surfaces for the molecules clearly show the cooperative process of jump and chain motions: a generation of a vacant cage and a channel which connects two cages. It is found that the molecule passes through the channel without significant energy barrier and concentration of free volume.
Quasirelativistic calculation of the vibronic spectra of NiH and NiD
Authors: Marian C. M.
Published: July 2002

Large ab initio calculations on the low-lying 2Δ, 2Π, and 2Σ+ electronic states of NiH have been performed employing a relativistically corrected Hamiltonian. The relative ordering of the unperturbed electronic states is found to be 2Δ<2Σ+<2Π. Diagonal and off-diagonal spin–orbit matrix elements have been evaluated within the Breit–Pauli approximation and were used to couple the individual vibronic functions. With the exception of 〈2Σ+1/2‖HSO‖2Π1/2〉, the spin–orbit matrix elements are found to be nearly independent of the internuclear distance. Their magnitude is determined by coupling matrix elements of the components of a pure atomic d9 configuration. The deflection of the 〈2Σ+1/2‖HSO‖2Π1/2〉 matrix element from the d9 limit at shorter bond distances can be attributed to increased admixture of d10 character in the 2Σ+ wave function. For NiH the v=0,  J=2.5 level of the 2Δ3/2 state is calculated at 1014 cm−1, in excellent agreement with experiments (1012 cm−1). The lower of the heavily mixed 2Σ+1/2 and 2Π1/2 combinations with total angular momentum  J=2.5 is located at 2279 cm−1, approximately 150 cm−1 above the corresponding experimental value. Shifting the 2Σ+ potential curve by −250 cm−1 brings the calculated levels of the Ω=1/2 system into almost perfect agreement with observed levels averaged over e and  f  components. The upper, experimentally not yet determined component is predicted at approximately 3655 cm−1, close to the v=2 level of the 2Δ5/2 state. The 2Π3/2 v=0,  J=2.5 and the 2Δ3/2 v=1,  J=2.5 levels, located at 2631 and 3091 cm−1, are considerably mixed. Excitation energies to several higher-lying vibronic states of NiH and a corresponding analysis of the vibronic spectrum of the NiD isotope are also reported.
Catastrophe map classification of the generalized normal–local transition in Fermi resonance spectra
Authors: Xiao Lin , Kellman Michael E.
Published: July 2002

Catastrophe theory is used to classify the dynamics of spectra of resonantly coupled vibrations, based on earlier work on the bifurcation structure of the Darling–Dennison and 2:1 Fermi resonance fitting Hamiltonians. The goal is a generalization of the language of the ‘‘normal–local transition’’ to analyze experimental spectra of general resonant systems. The set of all fixed points of the Hamiltonian on the polyad phase sphere for all possible molecular parameters constitutes the catastrophe manifold. The projection of this manifold onto the subspace of molecular parameters is the catastrophe map. The map is divided into zones; each zone has its own characteristic phase sphere structure. The taxonomy of global phase sphere structures within all zones gives the classification of the semiclassical dynamics. The 1:1 system, with normal–local transition, is characterized by cusp catastrophes, with elementary pitchfork bifurcations. In contrast, the 2:1 system is characterized by fold catastrophes, with elementary transcritical bifurcations. The catastrophe map can be used in a new method to classify experimental spectra on the basis of the system’s underlying semiclassical dynamics. The catastrophe map classification appears to persist for nonintegrable, chaotic Hamiltonians, indicating the utility of catastrophe theory for understanding the morphology of chaotic systems.
Photoionization of rotationally cooled H2O and D2O in the region 650–990 Å
Authors: Dehmer P. M. , Holland D. M. P.
Published: July 2002

Room temperature and rotationally cooled relative photoionization cross sections for H2O and D2O were determined in the wavelength region 650–990 Å with a wavelength resolution of 0.06–0.11 Å (∼7–12 cm−1 ); the spectra were normalized to the absolute photoionization cross sections determined by Katayama et al. [D. H. Katayama, R. E. Huffman, and C. L. O’Bryan, J. Chem. Phys. 59, 4309 (1973)] with lower resolution. A rotational analysis of the Π subbands in the (0,6,0) and (0,8,0) bands of the (Ã 2A1)3db11B1←X̃1A1 Rydberg transition determined the symmetry of the upper state; in addition, the series was extended to higher principal quantum numbers. The analysis of these and other bands was made difficult both by the large natural linewidths of the rotational transitions and by the absence of transitions to rotational levels with N′≳3.
Hardnesses from electrostatic potentials
Authors: Harbola Manoj K. , Parr Robert G. , Lee Chengteh
Published: July 2002

By generalization of a method due to Politzer et al. [J. Chem. Phys. 79, 3859 (1983)], it is demonstrated how the absolute hardness of an electronic system can be determined from the electrostatic potential, as a function of position of the system and its positive and negative ions. It is shown that to good accuracy the hardness is one-half the electrostatic potential at the covalent radius due to the Fukui function.
Surface tension of binary liquid–vapor mixtures: A comparison of mean-field and scaling theories
Authors: Sahimi Muhammad , Taylor Byron N.
Published: July 2002

We use two different methods to estimate surface tension of binary liquid–vapor mixtures of CO2 and a hydrocarbon near a critical point. The first method is based on the gradient theory, which is essentially a mean-field approximation to the problem that reduces the determination of the interface’s structure and the surface tension to a boundary value problem. The theory’s input is an equation of state of homogeneous fluid and the influence parameters of inhomogeneous fluid. The Peng–Robinson equation and a modification of it are used as the equation of state of homogeneous fluid. The second method is based on the concept of two-scale-factor universality which can predict the surface tension from the singularity in the thermodynamic properties of the bulk fluid. The inputs of the method are an equation of state and certain universal amplitude ratios near the critical point. As the equation of state, we use a modification of a model first proposed by Leung and Griffiths, and further developed by Moldover, Rainwater, and co-workers. We use the two models to examine in detail CO2+n -butane and CO2+n -decane mixtures. While both models provide accurate estimates of surface tension of CO2+n -butane mixtures, only the gradient theory can predict accurately surface tension of CO2+n -decane mixtures. Moreover, while the gradient theory and the Peng–Robinson equation of state use very few adjustable parameters (at most three parameters), calculation of surface tension based on two-scale-factor universality and the corresponding equation of state requires many adjustable parameters whose number has to be increased dramatically as the fluid mixture becomes more complex. We then use the gradient theory to predict surface tension of binary liquid–vapor mixtures of CO2 and benzene, cyclohexane, and n-hexadecane. In all cases, the predictions of the gradient theory are in good agreement with the available experimental data.
Vibronic analysis of the Ã 2A1–X̃ 2E laser-induced fluorescence of jet-cooled CH3S
Authors: Chiang Su-Yu , Lee Yuan-Pern
Published: July 2002

The methylthio (CH3S) radical has been formed by KrF laser photolysis of dimethyl disulfide (CH3SSCH3) in a free-jet expansion. The Ã 2A1←X̃ 2E laser-induced fluorescence excitation spectrum was obtained in the wavelength region 350–382 nm. The origin lies at 26 526.3 cm−1 and the spin–orbital splitting for X̃ (2E3/2 and 2E1/2)was determined to be −259.1 cm−1. Several progressions indicated that ν′2=1098±2,ν3=401±2 cm−1, and (with less certainty) ν′6=635±10 cm−1. The predissociation threshold was estimated to be slightly above 28 016 cm−1, ∼1490 cm−1 above the v′=0 level of the Ã state. The dispersed fluorescence was observed in the 363–530 nm region. The main progressions give ν■2=1313±5 cm−1, ν″3=727±3 cm−1, and ν■5=1496±6 cm−1. Weaker lines are also consistent with some possible vibrational frequencies ν″1=2776 cm−1, ν■4=2706 cm−1, and ν″6=586 cm−1.
Model calculations on the amide-I infrared bands of globular proteins
Authors: Torii Hajime , Tasumi Mitsuo
Published: July 2002

Model calculations are performed on the amide-I infrared (ir) bands of globular proteins by assigning one oscillator with a transition dipole to each peptide group. Coupling between these oscillators is introduced through the transition dipole coupling mechanism. As examples of application of the model, the ir spectra in the amide-I region of eight representative proteins, viz., carbonmonoxy myoglobin, ribonuclease A, α-lactalbumin, lysozyme, flavodoxin, carboxypeptidase A, concanavalin A, and β-trypsin are calculated. Good agreement is obtained between the calculated and observed amide-I band envelopes. Some structure-spectrum correlations are discussed on the basis of the model calculations. The presence of bands with significant ir intensities for myoglobin in the region below 1640 cm−1 is consistent with its x-ray structure having no β sheet. Analysis of the contributions of β sheets to the amide-I band envelopes shows that parallel, antiparallel, and mixed parallel/antiparallel β sheets give rise to strong ir bands at a similar position in the wave number region below 1650 cm−1, and that no band in the region above 1650 cm−1 can be regarded as a reliable marker of antiparallel β sheets. The contributions of non-α-non-β parts spread over a wide wave number region. The differences in the amide-I band envelopes between α-lactalbumin and lysozyme originate most probably from the structural differences between the α-helical parts near the N termini of these proteins.
From uncoupled to coupled Hartree–Fock polarizabilities of infinite polymeric chains. Pariser–Parr–Pople applications to the polyacetylene chains
Authors: Champagne Benoît , Fripiat Joseph G. , André Jean-Marie
Published: July 2002

A general method is formulated to compute the asymptotic longitudinal polarizabilities of infinite systems. This procedure is an extension to infinite systems of the molecular random-phase-approximation method which provides coupled Hartree–Fock values and thus takes into account the field-induced electron reorganizational effects. It is shown that the Genkin–Mednis uncoupled method corresponds to the drastic approximation of the coupled one where there is no electron reorganization. By looking at the asymptotic polarizabilities as the convergence values of the oligomeric results, the method is tested for polyacetylene chains in the Pariser–Parr–Pople approximation.
A theoretical study of the positive and dipositive ions of M(NH3)n and M(H2O)n for M=Mg, Ca, or Sr
Authors: Bauschlicher Charles W. , Sodupe Mariona , Partridge Harry
Published: July 2002

The structure and binding energies are determined for many of the M(H2O)+n and M(H2O)2+n species, for n=1–3 and M=Mg, Ca, or Sr. The trends are explained in terms of metal sp or sdσ hybridization and core polarization. The M(NH3)+n systems, with M=Mg or Sr, are also studied. For the positive ions, the low-lying excited states are also studied and compared with experiment. The calculations suggest an alternative interpretation of the SrNH+3 spectrum.
On the dissociation energy of O2 and the energy of the O2+ b 4Σg− state
Authors: Cosby P. C. , Huestis D. L.
Published: July 2002

We note here that a significant discrepancy exists between the recently determined dissociation energy of the O2 B 3Σu− state and the energy of the O(1D2)+O(3P2) dissociation limit determined in a spectroscopic cycle. The major contribution to this discrepancy appears to be the currently accepted value for the energy of the O2+ b 4Σg− state relative to the ground electronic state of molecular oxygen.
Correlated Monte Carlo wave functions for some cations and anions of the first row atoms
Authors: Moskowitz J. W. , Schmidt K. E.
Published: July 2002

We applied the variational Monte Carlo method to the computation of the correlation energy for several first row cations (Li+ through Ne+) and anions (B− through F−). In addition, we obtained estimates of their ionization potentials and electron affinities. The ionization potentials differ from the experimental results on the average by 0.11 eV and the electron affinites differ on the average by 0.07 eV.
Recursion formulas for quantum statistical partition functions
Authors: Borrmann Peter , Franke Gert
Published: July 2002

Recursion formulas are derived for Fermi–Dirac and Bose–Einstein statistical partition functions for systems where the energy can be written as a sum of one particle energies. Thus, their exact partition functions can be calculated for any N.
Electron-momentum-specific valence-shell electronic structures of cis-, trans-, and iso-butene by symmetric noncoplanar (e,2e) spectroscopy
Authors: Ying J. F. , Zhu H. , Mathers C. P. , Gover B. N. , Banjavčić M. P. , Zheng Y. , Brion C. E. , Leung K. T.
Published: July 2002

Electron-momentum-specific ionization energy (IE) spectra of cis-, trans-, and iso-butene were obtained from 8 to 44 eV ionization energy using noncoplanar symmetric (e,2e) spectroscopy and compared with the corresponding photoelectron spectra obtained using monochromated synchrotron radiation at 100 eV photon energy. In the independent particle picture, the intensity variation of a particular ionic state as a function of the electron momentum in an (e,2e) IE spectrum is characteristic of electron momentum density of the ionized orbital. Under favorable conditions, the (e,2e) IE spectra measured at two relative azimuthal angles (0° and 8°) can be used to provide a unique characterization of the dominant orbital composition of a particular ionic state. Together with ab initio calculations, these spectra have been used to clarify the nature of some of the ionic states found in the photoelectron spectroscopic studies. In particular, the presence of prominent many-body structures in the inner-valence region (≥20 eV) for all three butene isomers indicates the general breakdown of Koopmans’ ionization picture in this region. The nature of these many-body states is inferred from their electron momentum dependence in the (e,2e) IE spectra and from recently published configuration-interaction calculations.
Preferential adsorption of ortho-H2 with respect to para-H2 on the amorphous ice surface
Authors: Buch V. , Devlin J. P.
Published: July 2002

This study demonstrates preferential adsorption of ortho-H2 with respect to para-H2 on the surface of amorphous ice exposed to normal H2 at 12 K. The experimental part of the study focuses on the time dependence of Fourier-transform infrared bands of ortho-H2 and para-H2 adsorbate and of dangling-OD bonds. In the spectrum of the newly formed adsorbate overlayer (before the ortho-to-para relaxation takes place) only the ortho-H2 IR band is visible, identifying the initial para-H2 content as less than 10%. In the computational part, H2 adsorption is modeled on an amorphous cluster (H2O)450. The computational study consists of the diffusion Monte Carlo investigation of isolated H2 states on the surface, and of path integral Monte Carlo simulation of the adsorbate overlayer. Nuclear symmetry effects are incorporated approximately in the path integral Monte Carlo simulation, which is carried out in the framework of the grand canonical ensemble. The calculated percentage of para-H2 in the adsorbate layer is 7%, corresponding to a factor ∼4 depletion with respect to the normal-H2 gas phase. The para depletion on the surface is due to excess binding energy of ortho-H2 with respect to para-H2 that originates from the anisotropy of the gas–surface potential. Para-H2 is bound to the surface via the spherically symmetric part of the potential, while ortho-H2 has extra binding energy from the anisotropic part.
Correlation corrected periodic Hartree–Fock study of the interactions between water and the (001) magnesium oxide surface
Authors: Scamehorn C. A. , Hess A. C. , McCarthy M. I.
Published: July 2002

A theoretical study of water adsorption on the surface of a three-layer (001) magnesium oxide film has been performed using periodic Hartree–Fock (PHF) theory with density-functional-based correlation corrections. The calculations treated two water molecules per MgO unit cell (one on each side of the film), and for most of the calculations, the size of the unit cell was chosen such that the ratio of water molecules to surface magnesium ions was 1:4. In these configurations the water dipoles were aligned parallel and the water–water spacing was 5.95 Å between molecules in neighboring cells. Nine geometries were examined, three of which were found to be strongly bound to the surface. The binding energies for the three bound configurations range from 4.1 to 8.9 kcal/mol at the PHF level of theory and 6.3 to 12.5 kcal/mol when correlation effects were included. For the two cases where the geometry of the bound water molecule was allowed to relax at the equilibrium water–film distance, the H–O–H angle increased 1–3° from the 6-31G* free molecule value of 105.6° and the O–H bond distance did not change. The six remaining geometries did not show significant binding to the surface. Additional calculations were performed in which the dipoles of the water molecules were aligned antiparallel. These calculations indicate that as the coverage increases the water molecules will tend to form islands on the magnesium oxide surface rather than wet the surface. The formation of a fully hydroxylated surface (one hydroxyl group added to every surface magnesium ion and one hydrogen atom to every surface oxygen ion) was also examined, but was found to be energetically unfavorable. The energetic bias against dissociative chemistry on the clean MgO (001) surface, consisting of fully five coordinated ions, is in agreement with previously published ultraviolet photoemission spectroscopy, x-ray photoemission spectroscopy, and IR studies.
Dielectric relaxation study of hexamethylphosphoramide-water mixtures using time domain reflectometry
Authors: Kumbharkhane A. C. , Helambe S. N. , Doraiswamy S. , Mehrotra S. C.
Published: July 2002

The dielectric relaxation studies of hexamethylphosphoramide-water mixtures have been carried out at different temperatures using time domain reflectometry technique in the frequency range of 10 MHz to 10 GHz. A calibration method based on the least squares fit has been suggested to determine the complex permittivity at the high frequency region for the aqueous solutions. The excess dielectric, Kirkwood correlation factor and thermodynamic properties in the mixture have been determined. These properties suggest a Cage-type structure of hexamethylphosphoramide molecules around water molecules. The structure has a tendency to break down in the presence of water.
Size-consistent self-consistent truncated or selected configuration interaction
Authors: Daudey Jean-Pierre , Heully Jean-Louis , Malrieu Jean-Paul
Published: July 2002

Based on the principle of intermediate effective Hamiltonians, a simple procedure is proposed in order to eliminate the unlinked contributions of any truncated or selected configuration interaction (CI). The corrections are diagonal energy shifts, easily calculated. A self-consistent version is proposed, which insures separability if localized molecular orbitals (MO) are used. In the special case of double CI, the present method is an improved version of the coupled electron pair approximation (CEPA), but it may be applied to any selected model space, involving configurations of various degrees of excitation. The efficiency of the proposed algorithms is illustrated on a series of test calculations performed on Be2, F2, N2, and NH3.
Fluctuations in electrochemical systems. I. General theory on diffusion limited electrochemical reactions
Authors: Gabrielli C. , Huet F. , Keddam M.
Published: July 2002

A model of the stochastic behavior of the electrochemical interface is presented when electrochemical reactions limited by diffusion of the reacting species occur on the electrode surface. The random fluctuations of the state variables (concentrations and voltage) are supposed to come from Poisson elementary noise sources which are directly acting on the elementary fluxes, either reactive or diffusive, of the particles. The evolution of the state variables is governed by Langevin equations obtained from the linearization of the nonlinear electrochemical equations deduced from the heterogeneous electrochemical kinetics. The Langevin noise sources are derived from the Poisson elementary noise sources. Their first and second order moments are calculated and appear to be coherent with the master equation approach. Finally, the control and observation variables are defined for the electrochemical interface and the power spectral density of the observable quantity (current or potential) are derived from the Poisson elementary noise sources.
Melting of two-dimensional colloidal crystals: A simulation study of the Yukawa system
Authors: Naidoo Kevin J. , Schnitker Jurgen
Published: July 2002

The two-dimensional melting transition of charged polystyrene spheres in aqueous colloidal suspension has been studied by molecular dynamics simulation of a screened Coulomb system. Some central predictions of the Kosterlitz–Thouless–Halperin–Nelson–Young theory of defect-mediated melting are confirmed, such as an apparent divergence of the correlation lengths for translational and bond-orientational order at different thermodynamic state points, but there are also predictions of the theory that are violated. The defect topology is very complex, with oscillation periods of the defect density of many million time steps duration. The need for extensive sampling and, to a lesser degree, the choice of potential function continue to be the crucial issues for any attempt to generate a hexatic structure by means of computer simulation.
Comparison of exact and approximate density functionals for an exactly soluble model
Authors: Filippi C. , Umrigar C. J. , Taut M.
Published: July 2002

We consider a model, given by two interacting electrons in an external harmonic potential, that can be solved analytically for a discrete and infinite set of values of the spring constant. The knowledge of the exact electronic density allows us to construct the exact exchange–correlation potential and exchange–correlation energy by inverting the Kohn–Sham equation. The exact exchange–correlation potential and energy are compared with the corresponding quantities, obtained for the same densities, using approximate density functionals, namely the local density approximation and several generalized gradient approximations. We consider two values of the spring constant in order to study the system in the low correlation case (high value of the spring constant) and in the high correlation case (low value of the spring constant). In both cases, the exchange–correlation potentials corresponding to approximate density functionals differ from the exact one over the entire spatial range. The approximate correlation potentials bear no resemblance to the exact ones. The exchange energy for generalized gradient approximation functionals is much improved compared to the result obtained within the local density approximation but the correlation energy is only a little improved.
Integrals over B functions basis sets. I. Three-center molecular integrals, a numerical study
Authors: Bouferguene A. , Fares M. , Rinaldi D.
Published: July 2002

The evaluation of multicenter integrals for SCF computation with a Slater-type orbitals (STO’s) basis set can be performed by expanding the atomic orbitals over a B function basis set. This is commonly carried out by means of the so-called Fourier integral transform. However, in spite of the extreme simplicity in developing the formulas, several numerical difficulties have not been fully solved. In fact, the reliability of computational algorithms is highly dependent on the choice of numerical integration techniques that are used to evaluate the multiple integral occurring in the analytical expression of any given multicenter integral. Special attention is paid to this aspect of computation and efficient numerical strategies are proposed.
The effect of apolar solutes on water structure: Alcohols and tetraalkylammonium ions
Authors: Turner J. , Soper A. K.
Published: July 2002

Neutron diffraction with hydrogen/deuterium isotope substitution on the solvent water hydrogens is used to investigate changes in water structure around the apolar groups on alcohols and tetraalkylammonium ions at room temperature. The HH and HX correlations (where X is a nonsubstituted atom) are calculated for the following solutes in water: ethanol (5 mol%), tertiary butanol (3 mol%), tetrapropylammonium bromide (2.4 mol%), and tetrabutylammonium bromide (1.7 mol%). The correlation functions suggest that the water structure in all four systems is very similar to bulk water. There is no evidence of structural enhancement in going from pure water to water in the alcohol–water mixtures. There is a small but probably significant increase in structure in the tetraalkylammonium bromide solutions compared to both pure water and the alcohol–water mixtures. There is very little measurable difference between the two alcohol–water mixtures or the two tetraalkylammonium bromide solutions in terms of water structure. The results are consistent with the view that apolar solutes are located in cavities in the hydrogen-bonded water network, but they suggest that the increase in order associated with apolar hydration is a small effect in terms of structure.
Application of density functional methods for the study of hydrogen-bonded systems: The hydrogen fluoride dimer
Authors: Latajka Zdzislaw , Bouteiller Yves
Published: July 2002

Systematic studies of the ability of a broad family of density functional methods applied to hydrogen-bonded complexes have been carried out on the hydrogen fluoride dimer. Specifically, calculations have been performed using basis sets of triple-zeta quality with diffuse functions and multiple sets of polarization functions. Various local and nonlocal exchange-correlation functionals have been applied in order to study the structure, energetics, and vibrational properties of the hydrogen fluoride dimer. The comparison with the experimental data, and also with results coming from ab initio methods (Hartree–Fock, Mo/ller–Plesset second order, and quadratic configuration interaction with the single and double excitations) shows good performance of nonlocal density functional methods for the description of hydrogen-bonded systems. The calculated binding energy, with nonlocal Becke exchange and Lee–Yang–Parr correlation functionals and a 6-311++G(3df,3pd) basis set, is 4.48 kcal/mol and is in good agreement with experimental value and prior calculations.
Dipole mechanism of line broadening in amorphous solids
Authors: Ovchinnikov Mikhail A. , Wight Charles A.
Published: July 2002

Dipole–dipole interaction between molecular vibrations is the most important source of inhomogeneous line broadening in the strongest infrared absorption bands of amorphous films of N2O and O3 prepared by vapor deposition at cryogenic temperatures. This type of broadening occurs in all strongly allowed absorption bands of amorphous solids. Comparison of spectra obtained for amorphous and polycrystalline ozone show that the crystal structure is anisotropic, and most likely uniaxial. The absorption line shape of amorphous films is satisfactorily predicted by a computational model in which orientationally disordered molecules on a regular lattice interact through long-range dipole interactions. It is shown that the normal vibrations are delocalized throughout the solid, and the absorption line shape depends on the macroscopic shape and orientation of the sample.
Infrared photofragmentation spectra of size-selected SF6⋅Ar+n cluster ions
Authors: Winkel J. F. , Woodward C. A. , Jones A. B. , Stace A. J.
Published: July 2002

Results are presented of a detailed experimental study of the infrared photofragmentation patterns of size-selected SF6⋅Ar+n cluster ions for n in the range 3 to 70. Line-tuneable CO2 and N2O lasers have been used to excited the ν3 vibrational mode of the SF6 molecule which is followed by the loss of one and two argon atoms as the principal fragmentation routes. Which of the two processes is dominant depends quite strongly on the size of the cluster ion concerned, with very pronounced fluctuations in the relative intensities of photofragments being observed for cluster ions in the range SF6⋅Ar+3 to SF6⋅Ar+25. Only for SF6⋅Ar+3 is the fragmentation pattern markedly different from that found for the other ions; an observation that supports an earlier conclusion regarding the relative ionisation energies of the two constituents [Stace et al. J. Phys. Chem. 97, 11363 (1993)]. A summation of fragment ion intensities as a function of laser wavelength is used to determine infrared absorption profiles and these have been recorded for individual clusters containing up to 70 argon atoms. Clusters containing fewer than 40 argon atoms appear to form single structures, with both the absorption profile shapes and selected hole-burning experiments suggesting that the number of isomers is small.

The presence of isomers only appears to become significant when the clusters contain more than 40 argon atoms. The observation of site splittings for the triply degenerate ν3 vibrational mode of SF6, together with the comparatively narrow linewidths seen for clusters containing between 15 and 40 rare gas atoms, indicates the presence of ordered structures. Such a conclusion implies that the clusters are solidlike rather than liquidlike. Overall, the results demonstrate that there is a clear correlation between those criteria previously used to identify the presence of stable cluster ion structures, i.e., mass spectra and unimolecular fragmentation patterns, and the corresponding infrared fragmentation patterns and absorption profiles. Of the ions studied, SF6⋅Ar+21 stands out as being particularly stable and worthy of future theoretical attention.
Periodic orbit analysis of molecular vibrational spectra: Spectral patterns and dynamical bifurcations in Fermi resonant systems
Authors: Ezra Gregory S.
Published: July 2002

Semiclassical periodic orbit theory is used to analyze the quantum density of states for three model molecular vibrational Hamiltonians describing stretch/bend modes with and without 2:1 (Fermi) resonant coupling. Periods of classical periodic orbits as a function of energy are extracted directly from the quantum spectrum using a Gaussian windowed (Gabor) Fourier transform. The quantum (E,τ) plots so obtained provide an informative representation of the level structure. Qualitative similarities and differences between spectra (i.e., resonant vs nonresonant) are immediately apparent; in this sense, the quantum (E,τ) plot is an efficient device for analysis of spectral patterns. At a more detailed level of analysis, we show that, for sufficiently small effective values of ℏ, the quantum (E,τ) plots reflect in full detail the intricate periodic orbit bifurcation structure for Fermi resonant Hamiltonians previously described by Li, Xiao, and Kellman [J. Chem. Phys. 92, 2251 (1990)].
Simulation of chemical reaction initiation through high velocity collisions of NO clusters with a surface
Authors: Gupta M. , Walters E. A. , Blais N. C.
Published: July 2002

Some computational results have been obtained for a system of diatomic molecules clustered together and driven to impact on a surface at sufficient energy to induce an observable quantity of chemical reactions. The diatomic molecules were modeled to be energetically similar to nitric oxide, NO, which is a detonable material when in the condensed phase. The system was intended to simulate an experiment devised to examine the initiation phase of a detonation of liquid NO stimulated by impact with a high-speed flyer plate. Classical trajectories were computed for six different cluster sizes, from 4 molecules to 50, and the clusters were directed into a wall at five different impact speeds ranging from 3.0 to 11.8 km s−1. The interatomic forces used for the computations were based on a modification of an empirical potential suggested by Tersoff. The characteristics of the products (O2, N2, NO, and N and O atoms) are examined, as well as the dynamic features of the collisions of the clusters with the wall. The conditions of the cluster impacts produced atom densities that were nearly triple the initial density of the clusters. The reactions in the n=50 cluster are complete in less than 300 fs. These conditions are unusual for studies of chemical reactions so that the many-body effects are expected to be important. They are conditions experienced in the initiation of explosive detonations.
Semiclassical approximations to quantum dynamical time correlation functions
Authors: Cao Jianshu , Voth Gregory A.
Published: July 2002

Semiclassical approximations for quantum time correlation functions are presented for both electronically adiabatic and nonadiabatic dynamics along with discussions of the operator ordering and the classical limit. With the combined use of the initial-value representation of the semiclassical propagator, a discrete algorithm to evaluate the Jacobi matrices, semiclassical operator ordering rules, and the stationary-phase filter technique, a practical algorithm is developed to calculate quantum time correlation functions. This approach holds considerable promise for simulating the quantum dynamics of realistic many-body systems. Some simple illustrative examples are used to demonstrate the feasibility and accuracy of the algorithm.
Rheology, self-diffusion, and microstructure of charged colloids under simple shear by massively parallel nonequilibrium Brownian dynamics
Authors: Rastogi Sanjeev R. , Wagner Norman J. , Lustig Steven R.
Published: July 2002

The simple shearing of a suspension of charge-stabilized, colloidal particles close to the melting line is investigated by massively parallel, nonequilibrium Brownian dynamics (NEBD) simulation. The suspension undergoes a discontinuous transition from a distorted fluid structure to an ordered ‘‘string’’ phase. Comparisons between simulations of 43 000, 4725 particles, and previous NEBD work on ≤500 particles proves that shear-induced ordering is not an artifact of the small system sizes. We also show that the shear-rate dependence of the rheological properties obtained from NEBD is different than those obtained from nonequilibrium molecular dynamics (NEMD), a consequence of the solvent damping not being present in NEMD. The validity of the Ree–Eyring model for viscosity and the stress-optic law for colloids are tested. Further, a type of generalized Stokes–Einstein relationship is discovered for systems under shear.
Concentration fluctuation induced dynamic heterogeneities in polymer blends
Authors: Kumar Sanat K. , Colby Ralph H. , Anastasiadis Spiros H. , Fytas George
Published: July 2002

The presence of two distinctly different local segmental mobilities found in the case of several phase mixed polymer blends by two-dimensional 2H-NMR, dielectric spectroscopy and depolarized dynamic light scattering is rationalized through a simple concentration fluctuation model. Our primary hypothesis is that, although the probability of the occurrence of concentration fluctuations is symmetric about the mean value in a given volume, the ‘‘cooperative volume’’ over which a fluctuation must occur for it to be detected by a dynamic probe is not a constant, but rather depends on the composition of the cooperative volume. Consequently, we suggest that the cooperative volume associated with a concentration fluctuation be determined by the local composition in a self-consistent manner. In the case of systems with weak interactions and large Tg contrast, these ideas are shown to create a bimodal probability density function for dynamic concentration fluctuations, which has a local maximum corresponding to small cooperative volumes rich in the more mobile component, in addition to the bulk composition itself. We also predict the broadening of both the segmental relaxation time spectrum and the single calorimetric glass transition of the blends with increasing concentration of the slow component. Comparisons of the predictions are made for two experimental systems with different thermodynamic and kinetic effects to illustrate the present approach. Further, we predict the effects of concentration fluctuations in the case of three other blends whose segmental dynamics have not been examined experimentally to date. We predict that the dynamics of the two components approximately follow time–temperature superposition either in the case of strongly interacting blends or for systems that have closely matched Tg values and Williams–Landel–Ferry (WLF) coefficients.
Pressure broadening and line shift measurements of carbon monoxide in collision with helium from 1 to 600 K
Authors: Beaky Matthew M. , Goyette Thomas M. , De Lucia Frank C.
Published: July 2002

Pressure broadening and line shift parameters for the 1←0 and 2←1 rotational transitions of carbon monoxide in collision with helium have been measured between 1 and 600 K. Measurements below 30 K were made using the collisional cooling technique, while measurements at higher temperatures were made in an equilibrium cell. The experimental apparatus and techniques employed in the study are described. Pressure broadening and line shift cross sections are compared with theoretical predictions based on the best available potential energy surface, and differences between theoretical predictions and the experimental results are discussed. Potential sources of systematic experimental error are examined as a possible origin of these differences.
Autodetachment spectroscopy and dynamics of dipole bound states of negative ions: 2A1–2B1 transitions of H2CCC−
Authors: Yokoyama K. , Leach Gary W. , Kim Joseph B. , Lineberger W. C.
Published: July 2002

The H2CCC− ion is studied by autodetachment spectroscopy in a coaxial laser-ion beam spectrometer. Sharp resonances at photon energies near the photodetachment threshold energy are ascribed to a 2A1←2B1 transition followed by autodetachment of the dipole-bound state (DBS). Some 2500 rotational transitions are assigned and the band origin is determined to be 14 284.420(5) cm−1. The observation of individual rotational lines allowed determination of the rotational spectroscopic constants as A=9.651 53(4) cm−1 and (B+C)/2=0.346 461(3) cm−1 for the DBS as well as the spin-rotation coupling constant εaa=2.17(6)×10−3 cm−1. Based on an electron affinity of 14 469±64 cm−1, the binding energy of the electron in the DBS is 170±50 cm−1. Anomalous rotational line positions are found in the vicinity of Ka=7–10 in the DBS and have been attributed to the centrifugal distortion couplings caused by mixing with the CCC out-of-plane bending mode (ν6) and the CCC in-plane bending mode (ν9). The linewidths provide information about autodetachment rates that make it possible to obtain electron detachment dynamics for individual rotational states.
The role of structural changes in the excitation of chemical waves in the system Rh(110)/NO+H2
Authors: Mertens F. , Schwegmann S. , Imbihl R.
Published: July 2002

Previous investigations have demonstrated that the formation of chemical waves in the NO+H2 reaction on Rh(110) involves a cyclic transformation of the surface structure via various N,O-induced reconstructions, i.e., starting form the c(2×6)-O a cycle is initiated comprising the formation of a (2×3)/(3×1)-N and a mixed c(2×4)-2O,N structure. The stability and reactivity of these structures has been investigated in titration experiments as well as under stationary reaction conditions employing LEED, work function, rate measurements, and thermal desorption spectroscopy. It was shown that the c(2×6)-O and c(2×4)-2O,N structures exhibit a low reactivity whereas the (2×1)/(2×1)-N displays only a small to moderate decrease in catalytic activity (≈20%–30%) compared to the clean surface. On the basis of these results, an excitation mechanism for pulses in the NO+H2 reaction on Rh(110) was constructed consisting of the sequence c(2×6)-O, (2×1)/(3×1)-N c(2×4)-2O,N, c(2×6)-O.
Dewetting of thin polymer films adsorbed on solid substrates: A Monte Carlo simulation of the early stages
Authors: Milchev Andrey , Binder Kurt
Published: July 2002

Using an off-lattice bead–spring model of flexible polymer chains containing N=32 beads under bad solvent conditions, thin films of polymer melts are simulated. The films are confined between two parallel plates, the upper plate being purely repulsive while the lower plate exerts a short range attraction on the polymer layer so that a dense thin film is adsorbed on this plate for large enough attraction strength ε. Then “quenching experiments’’ are simulated by suddenly reducing |ε| at time t=0 and monitoring the time evolution of the polymer film. While for large enough final values of |ε| only the density in the film decreases somewhat, but the film stays laterally homogeneous, for |ε| less than a critical value |εc| it is found that the film breaks up into droplets. The early stages of the time evolution of this process in the framework of a dynamic Monte Carlo simulation are studied both by recording the time dependence of the adsorbed amount, the average thickness of the layer, the distribution function of meansquare displacements, and with the help of snapshot pictures of the system configurations. Also equilibrium properties of the films are investigated including both collective properties such as density profiles and radial distribution functions, and single-chain properties such as parallel and perpendicular parts of meansquare gyration radii, in dependence on the adhesive strength |ε| of the substrate.
Experimental investigation and computational modeling of hot filament diamond chemical vapor deposition
Authors: Zumbach Volker , Schäfer Jörg , Tobai Jens , Ridder Michael , Dreier Thomas , Schaich Thomas , Wolfrum Jürgen , Ruf Bernhard , Behrendt Frank , Deutschman Olaf , Warnatz Jürgen
Published: July 2002

A joint investigation has been undertaken of the gas-phase chemistry taking place in a hot-filament chemical vapor-deposition (HFCVD) process for diamond synthesis on silica surfaces by a detailed comparison of numerical modeling and experimental results. Molecular beam sampling using quadrupole mass spectroscopy and resonance-enhanced multiphoton ionization time of flight mass spectroscopy (REMPI-TOF-MS) has been used to determine absolute concentrations of stable hydrocarbons and radicals. Resulting species of a CH4/H2, a CH4/D2 (both 0.5%/99.5%) and a C2H2/H2 (0.25%/99.75%) feedgas mixture were investigated for varying filament and substrate temperatures. Spatially resolved temperature profiles at various substrate temperatures, obtained from coherent anti-Stokes Raman spectroscopy (CARS) of hydrogen, are used as input parameters for the numerical code to reproduce hydrogen atom, methyl radical, methane, acetylene, and ethylene concentration profiles in the boundary layer of the substrate. In addition, the concentration of vibrationally excited hydrogen is determined by CARS. Results reveal only qualitative agreement between measured data and simulations, concerning concentrations of stable species and radicals probed near the surface, on filament and substrate temperature dependence, respectively. Hydrogen and deuterium experiments show similar behaviour for all species. In the case of CH4 as feedgas the model describes measured concentration profiles of CH3, CH4, and C2H2 qualitatively well. Large differences between model and experiment occur for hydrogen atoms (factor of 2) and C2H4 (factor of 3). For acetylene as feedgas the model is not able to give any predictions because no conversion of C2H2 is seen in the model in contrast to the experiment.