bluuebunny/crossref_metadata_2025 huggingface dataset:
Monolayer orientational fluctuations and a new phase transition at the air water interface detected by second harmonic generation
Authors: Zhao Xiaolin , Eisenthal Kenneth B.
Published: July 2002

Fluctuations in the second harmonic intensity were observed in a number of insoluble amphilphiles spread at an air/aqueous interface. The fluctuations, which depend on the polarization of both the incident light and the second harmonic light generated, indicate a new phase transition for which the orientation of the head group is the order parameter. It is found that the phase transition occurs in certain long chain aromatic species but not in others of the same chain length with different head groups. The phase transition is identified as a weak first order transition. Agreement is obtained with a modified Landau–Ginzburg equation in which the chain–chain interactions provide a friction for the motions of the head groups.
Photoelectron spectroscopy of the solvated anion clusters O−(Ar)n=1–26,34: Energetics and structure
Authors: Arnold Susan T. , Hendricks Jay H. , Bowen Kit H.
Published: July 2002

Negative ion photoelectron spectra of the solvated anion clusters O−(Ar)n=1–26,34 have been recorded. Vertical detachment energies obtained from the cluster anion spectra were used to determine total as well as stepwise stabilization energies. An examination of these energetic values as a function of cluster size demonstrates that the first solvation shell closes at n=12. Furthermore, magic numbers in the energetic data and in the mass spectrum suggest O−(Ar)n clusters of sizes n=12–34 are structurally very similar to homogeneous rare gas clusters and follow a polyicosahedral packing pattern, implying O−(Ar)12 has an icosahedral structure and O−(Ar)18 has a double icosahedral structure. The solvated cluster anion photoelectron data were also analyzed using a generalized cluster size equation, which relates the cluster anion data to bulk parameters. The data for O−(Ar)n≥12 is well represented by the theoretical prediction and was therefore used to estimate several bulk parameters, including the photoemission threshold, the photoconductivity threshold, and the bulk solvation energy.
Depolarized Rayleigh scattering from simple molecular liquids: Investigation of transitions in the melt
Authors: Sen Sumit , Galiatsatos Vassilios , Patterson Gary D.
Published: July 2002

Depolarized Rayleigh scattering photometry has been employed to study the temperature dependence of the optical anisotropy for benzene, hexafluorobenzene, and carbon tetrachloride. The depolarized Rayleigh intensity from carbon tetrachloride is entirely due to scattering by pairs of density fluctuations and increases monotonically with temperature. The total depolarized Rayleigh ratio RVH for benzene and hexafluorobenzene decreases smoothly and monotonically with temperature, in contradiction to a recent report [J. Chem. Phys. 95, 1223 (1991)]. The temperature dependence of the total depolarized Rayleigh ratio can be analyzed in terms of an increasing contribution due to scattering by pairs of density fluctuations and a decreasing contribution due to orientational fluctuations of optically anisotropic molecules. No anomalous behavior is observed anywhere in the liquid range for these three fluids.
Comment on symmetry of the interaction between an asymmetric rigid rotor and a linear rigid rotor
Authors: Green Sheldon
Published: July 2002

In fitting an ab initio potential for H2O–H2, Phillips et al. [J. Chem. Phys. 101, 5824 (1994)] excluded certain terms in the angular expansion they believed to vanish because of ‘‘the requirement that the potential is invariant to inversion of all coordinates through the origin.’’ However, there has been some question in the literature as to whether these terms must, in fact, vanish owing to spatial inversion symmetry. By providing counterexamples, it is demonstrated here that this is not required by fundamental spatial symmetry. However, these terms do appear to vanish for realistic molecular interactions and this symmetry may arise from the two-body nature of the electrostatic Hamiltonian.
Time-dependent light scattering from singly and multiply stranded linear polymers undergoing random and endwise scission
Authors: Reed Wayne F.
Published: July 2002

The time-dependent light scattering signatures for singly and multiply stranded polymers of arbitrary shape undergoing random and endwise degradation are developed. For the case of random scission, useful generalizations at low scattering angle are found for linear polymers of arbitrary conformation and any initial polydispersity, which permit deductions to be made concerning the degree of association of the polymer (single, double, triple strand, mixtures, etc.), in addition to determination of absolute polydispersity-independent depolymerization rates. For the particular case of random scission of multiply stranded random coils the form factor is shown to be the Debye function whose usual argument is simply translated by a function dependent on the average number of cuts per original strand and on the number of strands and monomers per strand. The contrasting case of end-wise cleavage (e.g., ‘‘exocydic’’ enzymatic cleavage) is also presented.
Mechanisms for molecular oxygen desorption from the CaO(100) surface
Authors: Snis Anders , Panas Itai
Published: July 2002

Electronic structure calculations have been employed to study the associative desorption of preadsorbed atomic oxygen from a calcium oxide (100) surface. Two reaction mechanisms were considered, corresponding to symmetric and asymmetric reaction paths. The energy barrier of the latter was found to be the lower of the two. The barrier for the asymmetric reaction was found to be sensitive both to the location of the nearest calcium ions and to the distance between the surface oxygen ions, and is argued to be strongly affected by surface temperature. The desorption mechanism is shown to be conceptually similar to the previously addressed N2O decomposition and CO2 formation reactions. Catalytical aspects are stressed as oxygen desorption from CaO(100) is compared to those of nickel and platinum.
Dynamics of SiO desorption in reactive scattering of O2 with a silicon surface
Authors: Nakamura Kazutaka G. , Kamioka Isao , Kitajima Masahiro
Published: July 2002

The reactive scattering of O2, from a pulsed molecular beam, with a hot Si(100) (about 1250 K) has been studied with resonance enhanced multiphoton ionization (REMPI) mass spectroscopy in the wavelength region between 282 and 295 nm (F 1Σ+←X 1Σ+ and H 1Σ+←X 1Σ+). The vibrational and rotational state distributions of scattered SiO species are Boltzmann with temperature near that of the Si(100) surface. An isotropic angular distribution of the SiO desorption is observed and the internal state distribution is independent of the desorbing angle. The results suggest that the SiO desorption goes through a state in which the SiO rotates almost freely.
Threshold zero-kinetic-energy photoelectron spectroscopy of the a 3Σ+ state of NO+
Authors: Wang Kwanghsi , McKoy V.
Published: July 2002

Results of calculations of the photoelectron spectra for single-photon pulsed-field ionization of the 1π orbital of the X 2Π (v″=0) ground state NO leading to the a 3Σ+ (v+=0–2) excited ionic state are reported. Agreement between these calculated and recently measured spectra is very encouraging. Comparison of these spectra for ionization of the 1π level of NO (X 2Π) with those for the 2π orbital reveal significantly different spectral profiles and underlying dynamics for these two cases.
Multiple quantum filtered nuclear magnetic resonance spectroscopy of spin 7/2 nuclei in solution
Authors: Tsoref L. , Eliav U. , Navon G.
Published: July 2002

Multiple quantum dynamics of spin I=7/2 are presented considering modulation of quadrupolar interaction as the relaxation mechanism. The equation of motion of the spin system is calculated in Liouville space using irreducible spherical tensor operators as the orthonormal basis. Relaxation matrices are given explicitly for coherences one to seven. Experiments for measuring the creation of multiple rank tensors and multiple quantum relaxation for spin 7/2 are described and analyzed. Results of double quantum and triple quantum filtered spectra of Cs+ bound to the crown ether 18-crown-6, dissolved in glycerol, were analyzed in terms of the microscopic parameters of the system, the reorientation correlation time and the quadrupolar coupling constant. Numerical calculations indicated that the expected MQF signal intensities decrease in the order TQF≳DQF≫5QF≳4QF≫7QF≳6QF.
Effects of adsorbate lateral repulsion on desorption and diffusion kinetics studied by Monte Carlo simulations
Authors: Farbman I. , Asscher M. , Ben-Shaul A.
Published: July 2002

The effects of adsorbate lateral interactions on the kinetics of surface diffusion and desorption are studied by means of kinetic and thermodynamic Monte Carlo simulations. This study is motivated by recent diffusion and desorption experiments on the NH3/Re(001) system, which show that the activation energies of these processes decrease (in different fashions) with increasing surface coverage, the interactions between the adsorbates are thus assumed to be repulsive. A long range dipole–dipole-like potential is used to simulate both the diffusion and desorption processes. Most calculations are carried out with the interaction range extending up to fourth-order neighbors. Longer ranges are found to barely affect the kinetic behavior. On the other hand, shorter ranges of interaction result in qualitatively and quantitatively different structural (thermodynamic phase) behaviors and, consequently, in very different kinetics of diffusion and desorption.

The model used to calculate diffusion kinetics assumes that the activation barrier to particle diffusion depends, simultaneously, on the local environments of both the initial and the final sites involved in the elementary event of particle jumps. The chemical diffusion coefficient is evaluated based on thermodynamic and kinetic Monte Carlo simulations. It is found to increase with surface coverage, reflecting the repulsive nature of the interactions. Yet, unlike the experimental results, the increase is nonmonotonic but rather, somewhat oscillatory—reflecting the structural phase transitions of the adsorbed layer. The activation energy of desorption is found to decrease by about 15 kcal/mole as the coverage increases from 0 to 1, showing steeper slopes around the coverages corresponding to a perfectly ordered adlayer phase. These results are in satisfactory qualitative and quantitative agreement with experiment. Finally, it is shown that the coverage dependence of the activation barrier to diffusion can be reasonably well evaluated from equilibrium thermodynamic desorption data.
Determination of the conduction band energy minimum in fluid argon by means of field ionization
Authors: Al-Omari A. K. , Altmann K. N. , Reininger R.
Published: July 2002

The evolution of the ionization potential of H2S doped in argon for argon densities between the dilute gas and the triple point liquid was obtained by means of field ionization. The field ionization spectra of H2S in argon were simulated by convoluting the spectrum obtained in pure H2S with the calculated polarization energy distribution between the H2S ion and the medium. The density dependence of the conduction band energy minimum V0(ρ) (relative to vacuum) was obtained from the energy difference between the experimental spectra and the simulations. Excellent agreement was found between these values and those obtained using a larger molecule, CH3I. The values of V0(ρ) are compared to recent theoretical calculations.
Molecular dynamics simulation of n-nonadecane in urea inclusion compound. II. Rotational distribution and elastic incoherent structure factor
Authors: Souaille Marc , Guillaume François , Smith Jeremy C.
Published: July 2002

The rotational distribution of n-nonadecane molecules around the channel axis in urea inclusion compound at 180 K is examined using molecular dynamics simulation and compared with data derived from incoherent neutron scattering experiment. The potential surface for rotation and translation of a single guest n-nonadecane molecule in a urea channel contains helical valleys. However, it is found that the incommensurate packing of the n-alkane molecules in the inclusion compound prevents their executing helical motion. The calculated rotational potential of mean force exhibits a barrier of ∼0.3 kcal/mol for transitions of the molecules between equivalent sixfold minima. The motion of the urea host molecules is coupled to that of the guest molecules which undergo jump rotational diffusion when the urea substructure is fixed in the simulation and continuous diffusion when the urea molecules are allowed to move. The elastic incoherent structure factor (EISF), which is determined by the rotational distribution of the alkane chains, is computed from the simulations and compared with experiment. When derived using the same approximations, the experimental and simulation-derived EISFs are in agreement. However, the EISF has not converged to its long-time limit in the experiment. A converged EISF is obtained from the simulation by applying appropriate averaging. The origins of the differences between the converged and experimental EISFs are discussed.
Dynamics of interacting polymer chains and solvents
Authors: Pandey R. B. , Bernardes A. T. , Foo Grace M. , Stauffer D.
Published: July 2002

Conformation and dynamics of charged polymer chains in counter-ion solvent is studied by a Monte Carlo simulation in two and three dimensions. The nodes of the chains are positively charged while the solvent particles possess negative charge. The concentration of the polymer (p), and the empty sites (pe) is varied along with the temperature. Reptation, kink-jump, and a buckling governed by the interaction energy are considered in moving the chain segments. The variation of the root mean square (rms) displacement of the center of mass, and the relaxation of the radius of gyration of the chains with time are studied as a function of polymer concentration and temperature. The rms displacement of the polymer chains exhibits various power laws in different time regimes which depend on the concentration of the chains. Relaxation of the radius of gyration is enhanced on increasing the temperature; the relaxation time seems to decrease on reducing the chain length and increase on increasing the concentration. The motion of solvent particles show subdiffusive behavior which is sensitive to polymer concentration.
The influence of spacer chain conformations on the nematic ordering of main chain polymers
Authors: Serpi H. S. , Photinos D. J.
Published: July 2002

We present a new molecular field theory of the nematic ordering of main chain polymers that allows molecular structure and conformations to be taken explicitly into account. The theory is based on the self-consistency conditions which follow from the invariance of the segmental order parameters with respect to translations along the contour of the polymer. The orientational order parameters of the segments of a polyethylene chain in a nematic medium are calculated and tested with Monte Carlo simulation. The calculations are extended to thermotropic main chain liquid crystalline polymers (LCPs) consisting of rodlike mesogenic units jointed by n-alkane spacers. The temperatures of the nematic–isotropic phase transitions of these LCPs are evaluated for the first time in terms of molecular structure parameters and are contrasted with the temperatures calculated for the respective monomer, dimer and trimer compounds.
Molecular dynamics simulation of n-nonadecane in urea inclusion compound. I. Comparison with quasielastic neutron scattering experiment
Authors: Souaille Marc , Guillaume François , Smith Jeremy C.
Published: July 2002

The diffusive dynamics of n-nonadecane molecules in urea inclusion compound at 180 K are examined using molecular dynamics simulations. Incoherent quasielastic neutron scattering profiles are calculated from the simulation trajectories and compared with experiment. The effect on the calculated scattering functions of varying the model system in the simulation is examined. Interactions between alkane chains within the channels are found to strongly influence their rotational and translational dynamics and associated scattering spectra. The diffusive alkane dynamics is well described in terms of the molecules moving as rigid bodies. Analytical models of restricted translational and rotational rigid-molecule diffusion of the alkanes are fitted to the simulation-derived scattering functions in the time and energy domains. The effects of the experimental instrumental energy resolution function on the scattering functions and on the dynamical parameters obtained by fitting to the analytical models are examined. The calculations indicate that whereas the translational dynamics are converged on the time scales accessible to the experiment and the simulation, rotational motions exist that are too slow to be detected experimentally.
The direct Monte Carlo method applied to the homogeneous nucleation problem
Authors: Hettema Hinne , McFeaters John S.
Published: July 2002

We discuss the application of the direct Monte Carlo method to the theory of cluster formation. Fractal relationships for the kernels appearing in the Smoluchowski equation are implemented in this method and the scaling behavior of the kernels is investigated using computer simulation. We study the effects of cluster disintegrations and also investigate the effects of ‘‘magic’’ numbers in cluster formation.
Photodissociation of methylazide: Observation of triplet methylnitrene radical
Authors: Ying Liming , Xia Yu , Shang Hairong , Zhao Xinsheng , Tang Youqi
Published: July 2002

An investigation of the photodissociation of methylazide at wavelengths from 292 nm to 325 nm is presented. Emission spectra and lifetime analysis show the existence of the triplet CH3N radical. A simple kinetic model is proposed to explain the observed fluorescence real time profile, which reflects the vibrational relaxation of hot radical at high pressures. The photodissociation dynamics of methylazide seems complex. The predominant channel is likely to produce CH2NH via concerted 1,2-hydrogen shift and N2 extrusion process. The triplet CH3N comes from a minor spin-forbidden channel which involves possibly strong interaction between the low-lying excited singlet and triplet states of methylazide.
A comparative study of first hyperpolarizabilities of the acidic and basic forms of weak organic acids in water
Authors: Ray Paresh Chandra , Das Puspendu Kumar , Ramasesha S.
Published: July 2002

Dynamic and static first hyperpolarizabilities (β) of a series of benzoic and cinnamic acids in water have been investigated. These acids exist in a mixture of acidic and basic forms in solution. The hyperpolarizability of the basic form is higher than that of the corresponding acid form which is attributed to the red-shift of the absorption maximum of the basic form relative to that of the acid form. A ZINDO-correction vector (CV) method including solvent effects through a self-consistent reaction field (SCRF) for computing β reproduce measured values within experimental errors. Static ab initio hyperpolarizabilities within SCRF are slightly lower than the ZINDO/CV/SCRF β’s. Semiemperical as well as ab initio gas phase calculations show the correct experimental trend in β but their magnitudes are much smaller than the measured values.
Shear viscosity of polar fluids: Molecular dynamics calculations of water
Authors: Balasubramanian S. , Mundy Christopher J. , Klein Michael L.
Published: July 2002

The shear viscosity of water at 303.15 K is calculated for the extended simple point charge (SPC/E) model of Berendsen et al. [J. Phys. Chem. 91, 6269 (1987)] using both equilibrium and nonequilibrium molecular dynamics (NEMD) methods. Reciprocal space sums to handle long-range electrostatic forces in a noncubic simulation box under shear are used in conjunction with box dynamics to evaluate the Coulombic interactions. At the state point studied, the shear viscosity of SPC/E water is found to be 6.6±0.8×10−4 Pa s, which is about 18% less than the experimental value.
An H-theorem for the Enskog equation of a binary mixture of dissimilar hard spheres
Authors: Goldstein Patricia , Garcı́a-Colı́n L. S.
Published: July 2002

Within the context of a standardlike Enskog theory, we prove the existence of both a global and a local H-theorem for a system consisting of a binary mixture of dissimilar hard spheres. The semipositive character of the entropy production, in addition to previous results on the proof of Onsager’s theorem, exhibits the complete compatibility of our theory with linear irreversible thermodynamics.
Calculation of two-center integrals involving a rapidly oscillating free electron wave function
Authors: Op de Beek S. S. , Driessen J. P. J. , Beijerinck H. C. W. , Verhaar B. J.
Published: July 2002

Optical potentials are used in a quantum mechanical treatment of loss processes, e.g., ionization, where the loss of flux is described by the imaginary part. We present a numerical method for calculating two-center two-electron integrals necessary to construct the imaginary part of the optical potential. By introducing Slater-type orbitals with complex-valued exponents (CSTOs), we are able to represent the free electron wave with a limited number of CSTOs. For the representation of free electron wave functions with many oscillations, i.e., in a large r range or for a high kinetic energy, these new CSTOs form a more natural set of basis functions. The introduction of CSTOs is inevitable for the calculation of integrals concerning collisions in the mK energy range, where the interaction acts over large internuclear distances. Extensive numerical checks show that the final imaginary part of the optical potentials can be calculated with an accuracy better than 2%.
Comparative study of partially coherent anti-Stokes Raman scattering (PCARS) and Rayleigh scattering: Concentration and temperature dependence of scattered intensities
Authors: Ishibashi Taka-aki , Hamaguchi Hiro-o
Published: July 2002

The concentration dependence of PCARS intensity has been measured for the 992 cm−1 band of benzene in five different solvents and compared with that of Rayleigh scattering intensity. The temperature dependence of PCARS and Rayleigh scattering has also been studied for neat benzene. The concentration dependence of PCARS and Rayleigh scattering shows a similar correlation to the refractive index difference between benzene and the solvents. It is shown that PCARS cannot be considered as the Rayleigh scattering of the (phase mismatched) CARS radiation, by comparing the concentration and temperature dependence of their intensities. The fluctuation of the third-order nonlinear susceptibility itself plays an important role in PCARS. This means that PCARS may bear unique information on the microscopic properties of liquids that Rayleigh scattering can not provide.
Vibrational energy relaxation of diatomic molecules in rare gas crystals
Authors: Egorov S. A. , Skinner J. L.
Published: July 2002

We develop a theory, based on Fermi’s golden rule, for state-to-state vibrational multiphonon relaxation rates for a diatomic molecule in a rare gas crystal. The interaction between the diatomic and each of the host atoms is modeled with a Lennard–Jones potential. As such, the coupling between the anharmonic vibration of the diatomic and the acoustic phonons of the crystal is nonlinear (and nonexponential) in the atomic displacements from equilibrium. We compare our theory for the relaxation rates with experimental results from a sequence of vibrational transitions of O2 in Ar crystal.
Vibrational excitation of methane by 15 and 30 eV intermediate-energy electron impact
Authors: Motte-Tollet F. , Heinesch J. , Gingell J. M. , Mason N. J.
Published: July 2002

Absolute measurements of differential electron-collision cross sections have been performed for the vibrational excitation of methane in its electronic ground state at impact energies of 15 eV and, for the first time, at 30 eV, in the 8°–95° angular range. The normalization to the absolute scale has been carried out using the inelastic/elastic ratios and the absolute elastic differential cross section obtained from a relative flow technique recently developed in our laboratory, in which the elastic differential cross section of methane was compared to that of nitrogen at each impact energy and at each scattering angle. Special attention has been given to the small scattering angular range (⩽40°) where minima have been observed in the vibrational cross sections and where simultaneous independent measurements have been made on two different electron spectrometers.
Calculated thermodynamics of reactions involving NO2+⋅X complexes (where X=H2O, N2, and CO2)
Authors: Bush Andrew M. , Dyke John M. , Wright Timothy G.
Published: July 2002

Ab initio molecular orbital calculations have been performed on the NO2+ṡX complexes (X=H2O, N2, and CO2). Optimized geometries and vibrational frequencies, as well as computed total energies have been used to calculate standard enthalpies, entropies, and free energies for the complexing reactions, and ligand-switching reactions between the three molecular complexes. The results obtained have been compared with experimental values where available. The following thermodynamic values are recommended for the association reaction NO2++X→NO2+ṡX: ΔH298=−16.8, −4.7, and −7.9 kcal mol−1 and ΔS298=−23.1, −20.2, and −20.8 cal mol−1 K−1, for X=H2O, N2, CO2, respectively. Corresponding thermodynamic values for the ligand-switching reactions have also been calculated. Minimum energy geometries calculated for NO2+ṡH2O and NO2+ṡN2 are found to be in agreement with those obtained in previous theoretical studies. The NO2+ṡCO2 complex has been studied for the first time, and the minimum energy geometry has been shown to be planar and of C2v geometry.
The dissociative recombination of hydrocarbon ions. I. Light alkanes
Authors: Lehfaoui L. , Rebrion-Rowe C. , Laubé S. , Mitchell J. B. A. , Rowe B. R.
Published: July 2002

Rate coefficients for the dissociative recombination of a series of hydrocarbon ions derived from normal alkanes have been measured using the FALP-MS technique. Despite the increasing complexity of the ions along the series, the measured rate coefficients are found to be remarkably similar having values between 5.5 and 8.3×10−7 cm3 s−1.
The reactive flux method applied to complex isomerization reactions: Using the unstable normal mode as a reaction coordinate
Authors: den Otter W. K. , Briels W. J.
Published: July 2002

A basic problem when calculating reaction rates using the reactive flux method is the introduction of a reaction coordinate. In this paper we show that it is advantageous to define a reaction coordinate by means of the unstable normal mode of the saddle point of the potential energy surface. This particular choice is made since it yields a high transmission function. Moreover, the reaction coordinate is calculated via a rapidly converging algorithm, and its derivative, which is needed in constrained runs, is calculated analytically. Calculations on the transmission coefficient of the isomerization of n-butane are in good agreement with results published by others. Runs with an isomerizing calix[4]arene in vacuo produce a very high transmission coefficient, as is the purpose of the reaction coordinate. The same molecule is also studied in chloroform.
Diffusive motions in liquid 18-crown-6: A molecular dynamics study
Authors: Briels W. J. , Leuwerink F. T. H.
Published: July 2002

Transport properties of 18-crown-6 in the liquid phase are investigated by means of molecular dynamics simulations. Three different force fields are used. It is attempted to separate molecular motions into independent contributions from translations, rotations, and deformations. Translational diffusion coefficients are calculated and they are found to depend very much on the molecular flexibility, i.e., on the potential model. With two potential models, diffusion coefficients are obtained which are in good agreement with experimental data. With one of these force fields the possibility is investigated to define molecule-fixed frames which allow a separation of rotations and deformations. Two different definitions are suggested for this purpose. Combining contributions to the hydrogen displacements from translational, rotational, and intramolecular motions, and comparing them to the actual displacements, it is found that one of the definitions fails, and the other performs reasonable well. It is found that the hydrogen displacements may very well be modeled by assuming independent translational and rotational motions. Attempts to obtain rotational diffusion coefficients from fitting the data using a symmetric diffusor model were unsuccessful. This was imputed to the large difference between the time scales for the different orientational motions and illustrates that experimental results should be met with reservation.
On the ab initio evaluation of Hubbard parameters. I. The analytical approach in the absence of orbital relaxation
Authors: Fortunelli Alessandro , Painelli Anna
Published: July 2002

A theoretical approach is considered for the ab initio derivation of the parameters contained in the Hubbard Model, under the fundamental assumption that the orbital relaxation represents a negligible effect for the system of interest. The approach is based on a one-to-one mapping between the states of a general empirical two-electron Hamiltonian and the results of ab initio calculations on a dimeric unit. The localization of the ab initio orbitals and its consequences are discussed in the Appendix. The analysis is carried out by distinguishing two main cases: (a) the symmetric case, in which it is possible to derive the values of the empirical parameters from a limited set of self-consistent calculations, and (b) the asymmetric case, in which several parameters have to be estimated by resorting to frozen-orbital calculations. The dependence of the effective value of the on-site Hubbard repulsion Ua on the chemical environment is discussed. Finally, making contact with an alternative approach, based on the expansion of the Hamiltonian matrix in series of overlaps between non-orthogonal functions centered on neighboring sites, the reliability of the standard Hubbard Model is investigated. It is found that off-diagonal electron-electron interaction terms such as X and W can become non-negligible, even in the absence of orbital relaxation, due to special geometrical arrangements. A discussion of the capabilities and limitations of the approach, as compared with more rigorous methods, is also included.
An accurate single-center three-dimensional numerical integration and its application to atomic structure calculations
Authors: Yamamoto Kazuo , Ishikawa Hideaki , Fujima Kazumi , Iwasawa Misako
Published: July 2002

We present a highly accurate single-center three-dimensional numerical integration technique and apply it to atomic structure calculations based on density functional theory. Our integration scheme employs a Stroud–Lebedev formula for spherical integration. For radial integration a division of a radial interval 0⩽r<∞ into several subregions is adopted and the Gauss–Legendre and the Gauss–Laguerre quadratures are applied to finite and semi-infinite intervals, respectively. The present method can represent the orthonormality of the analytical hydrogen wave functions with 15-figure accuracy at a few hundred integration points per atom. In the atomic structure calculations using numerical basis functions orthonormal integrals and Hamiltonian and dipole matrix elements are calculated with more than 10-figure accuracy. The accuracy of the matrix elements brings the more reliable total energies of atoms.
Geometric and electronic structures of silicon–sodium binary clusters. II. Photoelectron spectroscopy of SinNam− cluster anions
Authors: Kishi Reiko , Kawamata Hiroshi , Negishi Yuichi , Iwata Suehiro , Nakajima Atsushi , Kaya Koji
Published: July 2002

Electronic properties of silicon–sodium cluster anions (SinNam−; 4⩽n⩽11, 1⩽m⩽3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The electron affinities (EAs) of SinNam were determined from the threshold energies in the photoelectron spectra of SinNam−. Besides, the geometries and energies of SinNa− (n=1–7) were investigated with ab initio MO (molecular orbital) calculations; Mo/ller–Plesset perturbation and coupled cluster methods were used to take into account the electron correlation. By an Na adsorption on Sin, the EA decreases for n=4–6, but increases for n⩾7. The calculations for SinNa− show that the Sin framework for n=4–6 accepts at least two excess electrons. For Si7Na−, however, the charge transfer from the Na atom scarcely occurs to avoid distorting the close-packed Si7 framework. Moreover, the ionization energy (Ei) and EA of SinNam show several patterns with the number of the Na atoms, which reflect the charge transfer mechanism in the cluster reflecting the nature of the electron-accepting orbitals of the corresponding Sin and the adsorption types of the Na atoms.
Effects of reactant composition and nonuniformities on temperature fronts
Authors: Annamalai J. , Ballandis C. , Somani M. , Liauw M. A. , Luss D.
Published: July 2002

Stationary and spatially oscillating temperature fronts separating regions with high and low temperatures (amplitudes up to 120 K) formed during the atmospheric oxidation of carbon monoxide on a Pd on alumina ring kept in a mixed reactor. The sharp fronts bounded either one or two low temperature regions. The transition from stationary to spatially oscillating fronts occurred at a feed CO/O2 concentration ratio close to 2.0 and was independent of the reactor temperature. The net rate of local heat generation on the ring was nonuniform leading to angular variation of the temperature of the ignited state. Front motions were affected by both the nonuniformity of the system and the interaction between the catalyst and the ambient gas. The two fronts bounding a low temperature region usually moved at different velocities and their velocities increased with increasing oxygen concentration. In most cases the two fronts did not move in phase, but sometimes they moved in tandem generating a back-and-forth pulse motion. The spatial amplitude of the oscillations decreased as the slope of the net heat generation activity became steeper. The spatial variation of the standard deviation of the temperature was helpful in assessing the dynamics of the oscillating fronts.
Is the Gaussian-2 technique reliable for determining the energetics of small molecular dications?
Authors: Petrie Simon
Published: July 2002

We present a quantum chemical study of 39 molecular dications comprising between two and four atoms. The calculations, which employ the G2 procedure and several variants (G1, G2(MP2), G2Q, and G2Q(QCI)), allow an assessment of the performance of these popular “model-based” techniques when applied to molecular dications. Of the dications included here, all except HeAr2+ and AlCl2+ have been studied previously by different theoretical methods: both HeAr2+ and AlCl2+ are calculated to be metastable against dissociation into atomic ions. Overall, the results of the present study indicate that G2 and its variants are less well suited to the characterization of molecular dications than of neutral or singly charged species, and that such theoretical methods should therefore be used with caution in determining the properties of dications. Several factors (including adequacy of the level of theory implemented in geometry optimizations; adequacy of the basis sets employed for single-point total energy calculations; and additivity of the basis-set extensions implicit in G2 and related methods) are responsible for the lapses in performance of the ab initio methods surveyed in the present study. Particularly severe discrepancies are evident in the calculated enthalpies of formation of SO2+, OOO2+, and AlNC2+, which for the G2 and G2Q methods differ by 30, 98, and 162 kJ mol−1, respectively; other dications which are poorly treated at some or all of the levels of theory employed here include NeBeNe2+, OSO2+, and F22+.
Resolving the structural relaxation of a two-dimensional liquid using apertured cross correlation functions
Authors: Hurley M. M. , Harrowell Peter
Published: July 2002

Time-delayed intensity cross correlation functions are used to analyze the structural relaxation of a two-dimensional liquid of soft disks as simulated using a molecular dynamics algorithm. The structural relaxation exhibits an increasing long time tail as the freezing transition is approached. A cross correlation function designed to select out only local hexagonal fluctuations is used to demonstrate explicitly that this slow relaxation arises from crystalline fluctuations. A theoretical treatment of the role of aperture size and cluster distribution in the scattering correlation functions is presented.
The forbidden radiative transitions with ΔS=±1 in small homonuclear diatomic molecules
Authors: Bhattacharyya Shampa , Bhattacharyya Dilip K.
Published: July 2002

A theoretical formalism has been developed for forbidden radiative processes with ΔS=±1 in small homonuclear diatomic molecules. Spin-dependent electric multipole and magnetic multipole operators which are responsible for spin-forbidden radiative transitions are expressed as tensor operators and forbidden transition probabilities are worked out by using Racah Algebra. Spin-dependent electric dipole (SE1) and magnetic quadrupole (SM2) interactions are assumed to connect two states of different spin multiplicities as well as different inversion symmetry whereas spin-dependent electric quadrupole (SE2) and magnetic dipole (SM1) interactions are assumed to connect two states of the same inversion symmetry but of different spin multiplicities. Transition probabilities are presented for a few forbidden radiative transitions with ΔS=±1 between states having different symmetry as well as having the same symmetry.
High-resolution spectroscopy of 4-fluorostyrene-rare gas van der Waals complexes: Results and comparison with theoretical calculations
Authors: Lakin N. M. , Pietraperzia G. , Becucci M. , Castellucci E. , Coreno M. , Giardini-Guidoni A. , van der Avoird A.
Published: July 2002

High-resolution laser excitation spectra of the S1←S0 000 bands of the 1:1 van der Waals complexes of 4-fluorostyrene with atoms of argon and neon are presented. The rotational structure of each is fully assigned using a rigid asymmetric rotor Hamiltonian. The rotational constants for the complexes are used to extract effective coordinates for the rare-gas atoms which contain both dynamic and geometric information. Semiempirical potentials for the clusters in the ground and excited states are determined by fitting to the rotational constants and vibrational frequencies from three-dimensional quantum calculations to the experimental data. The effective coordinates are interpreted by comparison with the results of these quantum calculations on the potential surfaces obtained.
A study of the dissociation of CH3CH2SH+ by collisional activation: Evidence of nonstatistical behavior
Authors: Chen Y.-J. , Stimson S. , Fenn P. T. , Ng C. Y. , Li Wai-Kee , Ma N. L.
Published: July 2002

The absolute total cross sections for CH3CH2+, C2H4+, C2H3+, CH3+, CH2SH+(CH3S+), CH2S+(HCSH+), CHS+(CSH+), and H2S+ produced by the collision-induced dissociation (CID) reaction of CH3CH2SH++Ar have been measured in the center-of-mass collision energy (Ec.m.) range of 1–42 eV. Using the charge transfer probing technique, we found that the mass 47 product ions have overwhelmingly the CH2SH+ structure. The onsets for CH3CH2+, C2H4+, C2H3+, CH2SH+, H2S+, and CH3+ are consistent with their corresponding thermochemical thresholds. The formation of the higher energy channels CH3CH2++SH and CH3+CH2SH+, which involve the C–S and C–C bond scissions, are found to dominate in the entire Ec.m. range. The lower energy channel corresponding to the formation of CH3CHSH++H is not found. The strong preference observed for the formation of the higher energy channels is in accord with the conclusion obtained in the recent CID study of CH3SH+, providing evidence that the CID of CH3CH2SH+ is also nonstatistical. The high yields of CH3CH2++SH and CH2SH++CH3 are attributed to the more efficient translational to vibrational energy transfer for the low frequencies C–S and C–C stretching modes than for the high frequencies C–H and S–H stretching modes, along with the weak couplings between these low and high frequencies vibrational modes of CH3CH2SH+. The relative abundances of product ions formed by the single-photon ionization of CH3CH2SH were also measured for comparison with the CID results. The CH3CHSH++H channel is observed in the photoionization of CH3CH2SH. Similar to the finding in the photoionization of CH3SH, the relative abundances of fragment ions formed in the photoionization of CH3CH2SH are in qualitative accord with statistical predictions. To rationalize the dissociation mechanisms of CH3CH2SH+, we have also performed ab initio calculations to locate the possible transition structures for the observed dissociation channels.
Low-frequency Raman scattering of liquid CCl4, CHCl3, and acetone
Authors: Amo Yuko , Tominaga Yasunori
Published: July 2002

We report herein depolarized low-frequency Raman scattering measurements of liquid CCl4, CHCl3 and acetone. The reduced Raman spectra were analyzed for the first time using a relaxation function based on the multiple random telegraph (MRT) model of dielectric relaxation which takes into account inertia and memory effects. The imaginary part of the dielectric function of the MRT model reproduces the spectral profile of the low-frequency region of the reduced Raman spectra quite well. This indicates that the origin of the complicated central component of Raman spectra of liquids can be explained by intermolecular dynamics based on the MRT model.
Laser fluorescence excitation spectra of the AlNC and AlCN isomers
Authors: Gerasimov Irina , Yang Xin , Dagdigian Paul J.
Published: July 2002

A new molecular electronic band system has been observed by laser fluorescence excitation in a free-jet supersonic expansion of photolyzed trimethylaluminum diluted in seed gases containing molecular nitrogen. The overwhelming majority of observed bands displayed the rotational structure of a Π1←Σ1 transition. The change in the rotational constant B″ of a band at 36 389 cm−1, assigned as the origin band, upon N15 substitution was consistent with the assignment of the molecular carrier as AlNC, and the value of B″ for the Al14NC(X̃ 1Σ+) isotopomer agreed with that previously determined by pure rotational spectroscopy. To carry out a vibrational assignment, resolved emission spectra were recorded for excitation of the ten strongest bands. With one exception, all these bands were found to involve excitation out of the ground vibronic level. Vibrational quantum numbers for the upper levels were assigned for most bands, and the upper state appears to be linear, with Π1 symmetry. A band at 28 754 cm−1, assigned to AlNC by Fukushima [Chem. Phys. Lett. 283, 337 (1998)], was also investigated. The lower-state vibrational spacings in its resolved emission spectrum were different than for the bands discussed above. Based on the rotational constants determined in a rotational analysis of this band, the molecular carrier of this transition is assigned as the isomer AlCN.
Can stacking faults in hard-sphere crystals anneal out spontaneously?
Authors: Pronk Sander , Frenkel Daan
Published: July 2002

We estimate the rate at which randomly stacked hard-sphere crystals transform into the thermodynamically stable face-centered cubic phase. As an input for this estimate we need both the free-energy difference between bulk face-centered cubic (fcc) and hexagonal close packed (hcp) phases, and the hcp–fcc interfacial free energy. The latter quantity was computed using a lattice-switch Monte Carlo (MC) simulation method. We find the interfacial free energy to be nonzero but extremely small: 26±6⋅10−5kT/σ2, where σ is the particle diameter. The free energy difference between the bulk phases was calculated using two different techniques. On the basis of our simulation results we estimate that in hard-sphere colloidal suspensions millimeter-sized randomly stacked crystal will anneal to form essentially pure fcc crystal on a time scale of months to years.
Confinement-induced enhancement or suppression of the resonant dipole–dipole interaction
Authors: Cho Minhaeng
Published: July 2002

By using the quantized Fresnel modes in the planar cavity, the resonant molecular interaction is theoretically described and the general interaction potential tensor is obtained. The resonant dipole-dipole (D–D) interaction or excitation transfer matrix element are found to be dramatically suppressed or enhanced by the surrounding planar boundaries. The analytic expressions of the resonant D–D interaction potential are obtained for a few limiting cases. The results thus obtained are closely related to various phenomena associated with the spatial confinement, such as confinement-induced phase transitions of liquids, resonant excitation transfer between molecules bound in membrane etc. In the case of the high-Q cavity, the reduction of the effective dimensionality is discussed. Secondly, the short-distance limit of the general resonant molecular interaction potential tensor and its frequency dependence are investigated. Numerical calculations of the high-Q cavity limit and the lipid-water system are presented to demonstrate the crucial role of the planar boundaries in the resonant D–D interaction. The relevances of the current investigation with the excitation transfer in the photosynthetic antenna system, coherent energy transfer between neighboring peptide bonds, and static D–D interaction in the molecular dynamics simulation study of the membrane-bound proteins are briefly discussed.
Structures and energetics of the water heptamer: Comparison with the water hexamer and octamer
Authors: Kim Jongseob , Majumdar D. , Lee Han Myoung , Kim Kwang S.
Published: July 2002

In spite of a spate of studies of various water clusters, a few theoretical studies on the water heptamer are available. State-of-the-art ab initio calculations are thus carried out on twelve possible water heptamer structures to explore the conformation as well as spectroscopic properties of this water cluster. Two three-dimensional cagelike structures comprised of seven-membered cyclic rings with three additional hydrogen bondings were found to be the lowest-lying energy heptamer conformers. The global minimum energy structure was found to be 0.5 kcal/mol lower than the other. The zero-point energy uncorrected and corrected binding energies of the global minimum energy structure are 55.2 and 37.9 kcal/mol, respectively. An almost two-dimensional ring conformer, which is only 1 kcal/mol above the global minimum at 0 K, could be more stable above 150 K. The vibrational spectra of different heptamer conformers are discussed and compared with the spectra of the hexamer and octamer water clusters.
Empirical Jahn–Teller potential surfaces for silver doped xenon matrices
Authors: Dietrich P. , Schwentner N.
Published: July 2002

Potential energy surfaces for 5s and 5p states of Ag atoms in the vibrationally distorted Oh symmetry of a Xe matrix are constructed by numerical diagonalization of the Hamiltonian and using coupling parameters derived from experimental absorption and emission spectra. The 5p state exhibits both spin-orbit and Jahn–Teller coupling. The large structural relaxation after excitation indicated by the large Stokes shift of the emission necessitates higher-order terms in both the vibrational potentials and the Jahn–Teller coupling. Only the two Eg modes are considered. With this restriction the experimental and calculated spectra agree very well. We discuss implications for dynamical studies of wave packet motions.