bluuebunny/crossref_metadata_2025 huggingface dataset:
A simplified stochastic description for the A+B2 surface reaction including A diffusion
Authors: Mai J. , Kuzovkov V. N. , von Niessen W.
Published: July 2002

We introduce a simplified stochastic model for the A+B2 surface reaction with diffusion of the monomer particle. We use a stochastic description which consists only of an autonomous differential equation system with four variables (the coverage of A and B and the corresponding pairs). The solution of this system is a straightforward process. We compare the values of the phase transition points with a much more complex stochastic ansatz which has been introduced earlier [Mai, Kuzovkov, and von Niessen, J. Chem. Phys. 100, 6073 (1994)]. We find good agreement in the prediction of the first-order phase transition with the complete treatment. The ansatz also gives the second-order transition, but the results are not quantitative in the neighborhood of the second-order phase transition.
Angle-velocity contour maps for the H+D2→HD+D reaction from quasiclassical trajectory calculations
Authors: Aoiz F. J. , Herrero V. J. , Puentedura O. , Rábanos V. Sáez
Published: July 2002

Quasiclassical trajectory (QCT) calculations have been performed for the H+D2 (v=0,j=0,1,2)→HD+D reaction. With the results of these calculations angle-velocity contour maps are constructed and directly compared to the recent product imaging experiment on the same reaction. While a good agreement is obtained in the shape of the angular distributions, noteworthy discrepancies are found between the measured and calculated recoil velocities. The maxima of the calculated velocity distribution correspond to larger velocities than the experimentally reported, thus predicting a sensibly lower internal excitation.
Monte Carlo study of pancake to brush transition
Authors: Chakrabarti Amitabha
Published: July 2002

We present results from a Monte Carlo study of functional polymers at a solid–liquid interface, when both the ‘‘anchors’’ and the ‘‘backbones’’ of the polymer chains adsorb to the interface, but with unequal strengths. We have studied the configuration of the polymer chains as the surface density of the anchors is increased. For small values of the anchor density, both the anchors and the backbones reside close to the interface, and thus the adsorbed polymer layer takes a so-called ‘‘pancake’’ shape. As the surface density of the anchors is increased, the anchors replace more and more backbone monomers on the interface and as a result the layer becomes more extended, like a grafted polymer brush. Data for density profiles and layer thickness support this picture. We find that the pancake to brush behavior becomes sharper as the chain length is increased.
Far-infrared spectra of HCl in dense Ar and time-dependent anisotropic potential autocorrelation functions. A molecular dynamics study
Authors: Medina A. , Hernández A. Calvo , Velasco S. , Guàrdia E.
Published: July 2002

In many spectral theories, the knowledge of the time autocorrelation functions associated with the solute–solvent intermolecular anisotropic potential is a necessary step to obtain the far-infrared (FIR) spectra of small polar diatomic molecules dissolved in a nonpolar solvent. Within the pairwise additive approximation, we have obtained these functions from a molecular dynamics simulation for HCl in fluid Ar (between 100 and 480 amagat) at T=162.5 K. The simulated correlation functions are tested in the calculation of the pure far-infrared spectrum, appropriately described by a theory incorporating memory effects, line mixing effects, and the influence of any anisotropy order on the diatom–atom intermolecular potential. The comparison between the theoretical and experimental spectra allow us to obtain some conclusions about the relative importance of the first and second order anisotropic interaction correlations on the FIR spectra of HCl in dense Ar.
Numerical test of the generalized Flory and generalized Flory dimer theories
Authors: Mooij G. C. A. M. , Frenkel D.
Published: July 2002

We report results of a numerical test of the generalized Flory and generalized Flory dimer theories. Using continuous space Rosenbluth sampling, we have tested the basic assumptions of these theories. We find that the discrepancy between the predictions of the generalized Flory theory and simulation data is caused by the use of the Carnahan–Starling equation of state to estimate the incremental chemical potential of a chain molecule. Although the assumptions made in deriving the generalized Flory dimer theory are similarly flawed, the predictions of the latter theory are in excellent agreement with simulation results. This is explained by the occurrence of a cancellation of errors. Therefore, it is not to be expected a priori that increasingly higher order extensions of these theories will be improvements. In particular, the disappointing performance of the recently developed generalized Flory trimer theory may be due to the fact that this theory lacks the cancellation of errors that is present in the generalized Flory dimer theory.
Solvent mean force perturbations of diatomic dissociation reactions. Comparison of perturbed hard fluid and computer simulation results
Authors: de Souza Luís E. S. , Ben-Amotz Dor
Published: July 2002

The perturbed hard fluid model, which separates solute–solvent interactions into repulsive hard sphere and mean field attractive contributions, is applied to predict solvent effects on the thermodynamics of diatomic dissociation reactions. Theoretically predicted changes in excess Gibbs free energy (ΔG), entropy (ΔS), enthalpy (ΔH), and volume (ΔV) for the dissociation of a homonuclear diatomic dissolved in a monatomic solvent, with Lennard-Jones solute atom–solvent atom and solvent–solvent interaction potentials, are compared with computer simulation results. The perturbed hard fluid model requires only one adjustable parameter, which is determined using simulation results at a single temperature and density. This parameter is used in the prediction of reaction thermodynamics over the entire vapor, liquid, and supercritical fluid regime. Furthermore, the thermodynamics of other reactions, in which the solute atom–solvent atom attractive well depth changes upon dissociation, can be predicted by including one additional parameter, determined using only simulation results for a system with no well depth change.
Dynamical studies of UV-laser-induced NO-desorption from the polar NiO(111) versus the nonpolar NiO(100) surfaces
Authors: Menges M. , Baumeister B. , Al-Shamery K. , Freund H.-J. , Fischer C. , Andresen P.
Published: July 2002

We have studied the UV-laser-induced desorption of NO adsorbed on an epitaxial film of NiO(111) grown on Ni(111). The desorbing molecules were detected state selectively via a resonance enhanced ionization technique [REMPI(1+1)] using the A 2Σ(v′=0,1,2)←X 2Π(v″=0,1,2) transition as intermediate state. Our results are compared with our experiments on NO desorption from NiO(100). The similarities and differences of the results due to the different surface structure of the polar NiO(111) and the non polar NiO(100) are discussed. For both surfaces we observe bimodal velocity flux distributions independent of the rovibrational state. Due to a rotational temperature of about 400 K and a vibrational temperature of 1800 K thermal processes can be ruled out. The wavelength dependence of the desorption cross section strongly correlates with the electronic structure of the NiO indicating a surface mediated excitation process. The spin orientation in the NO molecules influences the life time of the excited state depending on the magnetic property of the NiO surface.
Coupled-cluster calculations of indirect nuclear coupling constants: The importance of non-Fermi contact contributions
Authors: Perera S. Ajith , Sekino Hideo , Bartlett Rodney J.
Published: July 2002

Electron correlation effects to the four coupling mechanisms which contribute to the isotropic nuclear spin–spin coupling constant, the Fermi contact (FC), paramagnetic spin–orbit (PSO), spin-dipole (SD), and diamagnetic spin–orbit (DSO) are studied using the equation of motion coupled-cluster (EOM-CC) method. The second-order properties are expressed as a sum-over state (SOS) using EOM-CC intermediate state wave functions. This formulation is simple, accurate, computationally convenient, and involves no truncation. Several molecules, HF, CO, N2, H2O, NH3, and HCl which have been previously shown to have large noncontact contributions are investigated using the EOM-CC method and the results are compared with experiment and other theoretical methods, including polarization propagator and finite-field MBPT(2) methods. Using fairly large basis sets, the EOM-CCSD provides results which agree with experimental indirect nuclear spin–spin coupling constants to within an average error of 13%.
Neumann expansion of the interelectronic distance function for integer powers. II. Modified formula and numerical test
Authors: Budziński J. , Prajsnar S.
Published: July 2002

Using the set of polynomials, orthogonal in the interval <1,∞), the modified form of the Neumann expansion of the interelectronic distance function r12k for integer k≥−1 is obtained. In order to verify this expansion and especially its convergence properties in applications the numerical test has been done.
Time- and frequency-resolved spontaneous emission as a probe of coherence effects in ultrafast electron transfer reactions
Authors: Jean John M.
Published: July 2002

We present results from quantum dynamical simulations of electron transfer, obtained using a multilevel version Redfield relaxation theory, which illustrate the complex dynamics that result when electron tunneling and vibrational relaxation processes occur on similar time scales. By treating the system-field interaction quantum mechanically, we examine the extent to which the time- and frequency-resolved emission signal reflects the electron transfer dynamics. The results from these simulations provide new insight into the role of quantum coherences in ultrafast rate processes and time-resolved spectral measurements.
A classical path theory of collisional redistribution in CaHe spin-changing energy transfer collisions
Authors: Ananthamurthy S. , Sando K. M. , Kleiber P. D.
Published: July 2002

We have applied a semiclassical (classical path quantum close-coupling) nonadiabatic theory of collisional redistribution to model Ca–He spin-changing energy transfer collisions. The theory predicts the total far wing absorption and final state resolved action spectra. The semiclassical model predictions are in excellent agreement with full quantum mechanical close-coupling calculations [J. Chem. Phys. 95, 5861 (1991)] and both are in good accord with the experimental observations.
1.7 GHz electron spin echo envelope modulation due to 29Si in gamma-irradiated fused quartz
Authors: Kurshev V. V. , Buckmaster H. A. , Tykarski L.
Published: July 2002

Gamma-irradiated fused quartz has been investigated at 290 K using 1.7 and 9.8 GHz electron spin echo spectroscopy. Three pulse stimulated echo sequences were used in this study because the electron spin echo signal decay was too fast using two-pulse echo sequences. It was found that no electron spin echo envelope modulation could be observed at 9.8 GHz but that a small relative amplitude envelope modulation signal could be observed at 1.7 GHz if the time interval between the first and second pulses was 1.0 or 3.0 μs but not for 2.0 μs. It was concluded that this modulation is due to the magnetic dipole interaction between the E′1 centers and 29Si nuclei. The natural abundance of this isotope of silicon is 4.7%, I=1/2, and μ=−0.5553 μN. Unsuccessful attempts were made to model this modulation signal assuming that the E′1 center was located at oxygen and silicon vacancies in the hexagonal lattice of quartz. It was concluded that the E′1 center behaves as if it does not interact with the nearest- or next-nearest-neighbor 29Si nuclei. Acceptable agreement was obtained if this electron center is located at a silicon vacancy and only interacts with all nuclei located outside a 0.5 nm radius sphere. This implies that the point charge model may not be applicable because the electron appears to be delocalized within this spherical volume.
Multivibron infrared absorption in solid CD4
Authors: Calvani P. , Nucara A. , Salvaggio M. C. , Lupi S.
Published: July 2002

The infrared absorption spectrum of solid CD4 has been studied in detail in the frequency range between 2000 and 7000 cm−1. In the three normal-pressure phases of CD4 between 30 and 2 K, 25 weak overtone and combination bands have been observed. They have been tentatively assigned on the basis of their energies, structures, and intensities. Overtones of order v4 of the bending mode ν4 have been detected for v4=2, 3, 4, 6. As v increases, one observes the transition from a regime where propagating vibrons are created on different molecules, to a regime where multivibron transitions occur on single molecules. Such behavior is triggered by changes in the intramolecular anharmonicity, which plays the role of a vibron–vibron binding energy.
Line coupling in anisotropic Raman branches
Authors: Bonamy L. , Bonamy J. , Robert D. , Temkin S. I. , Millot G. , Lavorel B.
Published: July 2002

Direct connection between collisional Raman cross sections and state-to-state rotational ones permits, within the sudden approximation, to evidence drastic decouplings from isotropic to anisotropic lines. These decouplings are consistent with the results obtained by using the strong collision model. Convenient energy corrections to the sudden approximation are used for N2 to calculate the resulting effects on the isotropic and anisotropic Q-branch profiles in a large density range (several hundred bars). The comparison of the calculations with experimental coherent anti-Stokes Raman scattering (CARS) and stimulated Raman (SRS) spectra of nitrogen exhibits a good agreement. Such effects could have applications in optical diagnostics.
The direct correlation functions and bridge functions for hard spheres near a large hard sphere
Authors: Henderson Douglas , Chan Kwong-yu , Degrève Léo
Published: July 2002

The recent Monte Carlo data of Degrève and Henderson for the density profiles of hard spheres near a large hard sphere are used to obtain direct correlation functions and bridge functions for this system both directly, using the Ornstein–Zernike relation and an approximation, due to Verlet, for the bridge function.
Role of N2(A′ 5Σ+g) in the enhancement of N2B 3Πg(V=10) populations in the afterglow
Authors: Morrill Jeff S. , Benesch William M.
Published: July 2002

Time-resolved spectroscopic observations of the N2 1PG afterglow, B 3Πg→A 3Σ+u, following a pulsed discharge show both an enhancement in the overall intensity and significant changes the shape of bands which arise from the v=10 level of the B 3Πg. Model results indicate that these changes in shape are produced by an enhancement of the population of the low J levels of the Ω=2 component of the v=10 level. In addition, we also observe bands of the Herman Infrared system of N2 (HIR), C″ 5Πu→A′ 5Σ+g, specifically the (3,1) and (2,0) bands. During the afterglow, both the 1PG and HIR are being produced by energy pooling processes. The time-dependent increase of the 1PG v′=10 band intensities show a strong correlation with the variation in the HIR band intensities which predominately populate the lower levels of the A′ 5Σ+g. Recent work has shown the A′ 5Σ+g to have a significantly deeper potential well than previously thought so that it is now thought to cross the B 3Πg very close to v=10 rather than v=12. Consequently, we consider the A′ 5Σ+g to be the most likely candidate as the source of the observed intensity enhancements in the v=10 level of the B 3Πg. Based on our measurements and a simple model of the afterglow we have estimated the apparent rate coefficient for collisional transfer between A′ 5Σ+g and the high v levels of the B 3Πg due to collisions with the N2 ground state. The value for collisional transfer from A′ to B is approximately 1.0×10−11 cc/molecule s. Our observations indicate the A′ 5Σ+g may have an even deeper potential and we estimate an upper bound for v=0 to be ∼3590±32 cm−1 below the dissociation limit which is ∼500±32 cm−1 deeper than the recent theoretical estimate.
Bridging by adsorbed polymers between two surfaces
Authors: Bonet Avalos J. , Johner A. , Joanny J. F.
Published: July 2002

We analyze the adsorption of flexible polymers between two parallel plates separated a distance 2l. We consider a semidilute polymer solution in contact with the two adsorbing walls and describe the statistics of bridges and loops. A detailed mean-field calculation allows us to obtain the size and density of bridges in two regimes: l≳ξ and l<ξ, where ξ is the bulk correlation length, as well as the size of loop and train sequences. These results are compared with a scaling analysis performed in the spirit of the self-similar profile introduced by de Gennes.
Rotational diffusion in polyacrylamide gels as a function of solvent quality
Authors: Mikosch W. , Dorfmüller Th. , Eimer W.
Published: July 2002

We have investigated the rotational motion of the dianion polyphenyl 2 (PP2) in polyacrylamide (PAA) gels by fluorescence anisotropy decay. A comparison of the dynamics of the probe molecule in various solvent mixtures of water–glycerol and water–methanol in solution as well as in the presence of the polymer matrix revealed a significant retardation of the reorientation with increasing PAA concentration. Having excluded specific interactions of PP2 with the gel matrix, we suggest that a modification of the solvent viscosity in the presence of the polymer is responsible for the alteration of the probe dynamics. The effect is strongly dependent on the solvent quality for the polymer component, i.e., more distinct under good solvent conditions (in pure water) than in a poor solvent (increasing glycerol content). Changing the crosslinker concentration from 0% to 4% we obtained fluid-like solutions as well as viscoelastic gels. However, the reorientational motion of PP2 was independent of the macroscopic constitution of the samples which means that on a microscopic scale the local environment for the probe molecule that determines its dynamics appears similar.
Direct Monte Carlo simulation of chemical reaction systems: Dissociation and recombination
Authors: Dunn Shannon M. , Anderson James B.
Published: July 2002

We report direct Monte Carlo simulations of a chemical reaction system with bimolecular and termolecular dissociation and recombination reactions of the type M+AB■M+A+B. The simulations are carried out at the molecular level using a simple flexible reaction model for termolecular reactions satisfying all the requirements of momentum and energy conservation, microscopic reversibility, and equilibrium. Energy transfer among reactants and products is included. The method is especially useful for treating reaction systems with nonequilibrium distributions and coupled gas dynamic-reaction effects. For systems with thermally equilibrated reactants the observed behavior is identical to that predicted by conventional methods.
Sideband optical–optical double resonance Zeeman spectroscopy. I. Theory of saturation and line shape behavior
Authors: McCarthy Michael C. , Kanamori Hideto , Li Mingguang , Field Robert W.
Published: July 2002

Several variants of the one-laser, two-color technique of sideband optical–optical double resonance Zeeman (SOODRZ) spectroscopy, which enables Zeeman and hyperfine splittings to be measured at sub-Doppler resolution, are described; a detailed theory of the line shape and signal-to-noise ratio for each variant is also presented. In the single-frequency variant of SOODRZ spectroscopy, radio frequency sidebands are imposed onto a single-frequency continuous wave laser beam to determine energy splittings between pairs of closely spaced levels. Each pair of closely spaced levels is coupled to a third isolated level by optical transitions and is tuned into resonance by the Zeeman effect. A theoretical treatment of the SOODRZ effect is presented here. It is shown that both the real and imaginary contributions to the complex third-order optical susceptibility tensor χ(3) can be observed and that the SOODRZ signal originates from a cross term between the amplitude of the unperturbed frequency-modulated beam of the laser and the nonlinear signal amplitude Re{EL*⋅ENL}. The SOODRZ signal is directly proportional to ΔNμ4MI20 (where M is the modulation index), rather than (ΔN)2, μ8, and I30 as in degenerate four-wave mixing and other nonlinear spectroscopies, which means the detection sensitivity of SOODRZ spectroscopy is considerably higher for low laser intensities and at lower number densities.

Moreover, the linewidth of the sub-Doppler feature is independent of the both the Doppler width of the optical transition and the natural width of the common linked level and, hence, is immune from the effects such as upper state predissociation. Two different types of modulators, acousto-optic and electro-optic, can be used in the single-frequency variant of SOODRZ spectroscopy. The unique features of SOODRZ schemes incorporating both types of modulators are described. SOODRZ spectroscopy can also be performed with a multimode broadband laser, where the adjacent cavity modes serve the same function as the sidebands in the single-frequency version. Examples of such multimode SOODRZ spectra will be presented as well. The effect of different detection schemes (i.e., nearly crossed polarizers and/or coherent detection), laser frequency detuning relative to the center of the absorption line, and ac magnetic field modulation conditions on the intensity, phase, and functional form of the sub-Doppler double resonance line shape will be discussed here. The simplest possible form of the SOODRZ line shape is a second derivative of a Lorentzian.
Arrested volume-phase transition of gel around a hole and crossover of domain size scaling
Authors: Kuroki Yoshiki , Sekimoto Ken
Published: July 2002

We theoretically study the phase coexistence in gel upon the volume phase transition. We focus on the geometry where the gel with a rod-like shape has a thin coaxial hole. We demonstrate how the concaved surface of gel affects the coexisting domains in equilibrium. The calculation is similar to the fixed-point analysis of renormalization group theories. We also predict the existence of a crossover temperature across which we have different scaling behaviors of the thickness of the deswollen domain around the free surface of the hole: At temperatures between the transition point and the crossover temperature, the thickness of the domain is roughly proportional to the thickness of the hole, while at the temperatures farther than the crossover temperature the thickness of the domain is rather scaled linearly with the radius of the outer fixed boundary of gel.
Elastic and structural properties of zeolites: Sodalite and dehydrated zeolite A
Authors: Kim Soon-Chul , Keskar N. R. , McCormick A. V. , Chelikowsky J. R. , Davis H. T.
Published: July 2002

A pairwise interatomic potential has been used to investigate elastic and structural properties of two cubic zeolites: sodalite and dehydrated zeolite A. Constant volume energy minimization has been used to determine the variation of lattice constants and atomic coordinates with pressure. The calculated structures of sodalite and dehydrated zeolite A obtained at zero pressure are in reasonably good agreement with the available experimental values. We find that the structures at zero pressure are largely determined by the Coulomb potential. The pressure dependence of bond lengths and bond angles show that both sodalite and dehydrated zeolite A are easily deformed by bending the Si–O–Al angles. As expected for a less dense crystal, the dehydrated zeolite A is softer than the sodalite. We have also obtained the equation of state of these materials.
The vibrational structure of H+4 and D+4
Authors: Ramón Álvarez-Collado Jose , Aguado Alfredo , Paniagua Miguel
Published: July 2002

A potential energy surface (PES) for the H+4 system which is a fit to local, high quality ab initio multiple reference single and double excitations configuration interaction (CI) calculations (142 data points) is reported. The potential energy surface obtained here has been calculated by maintaining H+3 as a core in its equilibrium geometry and moving the remaining hydrogen atom around it (three-dimensional potential energy surface). The new surface supposes an improvement on the preceding potential energy surfaces considering both the basis set size and the ab initio method used here. The energy minimum of the potential presented here has been found to be about 2.5 kcal/mol lower than those obtained in previous studies, indicating that H+4 and D+4 are more stable ions than previously believed. A three-dimensional vibrational kinetic energy operator in internal coordinates without singularities has been derived. Energy and wave functions of the vibrational levels of the dissociating hydrogen in H+4 and D+4 systems have been calculated by using the derived potential and kinetic energy operators and integrating the vibrational Hamiltonian with the normal coordinates finite elements method. The vibrational states with energies below the new dissociation limit are reported and characterized, giving a more complete description of the vibrational structure. The number of bound vibrational levels obtained here is 7 for H+4 and 24 for D+4.
Photochemistry of adsorbed molecules. XIV. Photoejection of CH3X (X=Cl, Br) from CH3Br/LiF(001)
Authors: Heyd D. V. , Jensen E. T. , Polanyi J. C.
Published: July 2002

At ≳1 ML coverage of methyl bromide adsorbed on LiF(100), CH3Br is efficiently ‘‘photoejected’’ by ultraviolet light at 193 nm, with a mean translational energy of ∼0.3 eV; this is termed PEJ(CH3Br). The system CH3Cl/LiF(100) shows no detectable PEJ(CH3Cl) at this wavelength for which CH3Cl does not efficiently absorb UV. Similarly PEJ(CH3Br) ceased to be observable with 248 nm light, at which wavelength CH3Br does not absorb. Both observations indicate that PEJ is adsorbate mediated. If at 193 nm ∼1 ML of CH3Br is adsorbed beneath ∼1 ML of CH3Cl, efficient PEJ(CH3Cl) is observed. With the sequence of layers reversed (i.e., with the actinic layer on top), PEJ(CH3Cl) is largely suppressed. These observations support the model for PEJ proposed previously [I. Harrison et al., J. Chem. Phys. 89, 1498 (1988)] in which photoexcitation of the underlayer leads, by way of electronic-to-translational (E→T) transfer, to ejection of the top layer. The angular distribution of the photoejected species is peaked along the surface normal, consistent with repulsion operating between the under- and overlayer. The mean conversion of E→T implied by this mechanism is ∼5%, extending to a maximum of 12%.
Direct simulation of slow-motion electron spin resonance spectra by solving the stochastic Liouville equation in the time domain with stochastic dynamics in the form of trajectories
Authors: Usova Natalia , Westlund Per-Olof , Fedchenia Igor I.
Published: July 2002

A new method to solve the stochastic Liouville equation in the time domain has been developed. The effective and accurate algorithm designed conserve the norm of the propagator at every time step. The algorithm is not dependent on how the fluctuations are generated. The method is applied to solve the stochastic Liouville equation of an electron spin, S=1/2, coupled to a nuclear spin system with spin quantum number I=1. The calculated slow-motion electron spin resonance (ESR) line shapes presented are for a model where the stochastic time dependent spin-lattice coupling was obtained from a Brownian dynamic simulation of restricted reorientation in a cone potential. These spectra were then compared with the spectra obtained by solving the stochastic Liouville equation (SLE) using the eigenfunction expansion method. All spectra conform exactly and the computer power used by the two methods are similar.
An ab initio investigation of the charge-transfer complexes of alkali atoms with oligo (α,α′) thiophenes and oligoparaphenylenes: A model calculation on polaronic and bipolaronic defect structures
Authors: Irle Stephan , Lischka Hans
Published: July 2002

Ab initio self-consistent field (SCF), second- and third-order Mo/ller–Plesset calculations on the charge-transfer complexes of one and two alkali atoms with oligothiophenes and oligophenylenes are reported. Complexes up to quaterthiophene and quaterphenyl with Li, Na, and Cs have been investigated. The mono-alkali complexes are related to polaron and the di-alkali complexes to bipolaron defects. Extensive basis set investigations have been performed. The quinoid structures which result from the interaction of the alkali atoms with the oligomers are well reproduced by the SCF method, whereas for accurate interaction energies electron correlation effects are very important. Our calculations show that the spatial extension of the polaronic and bipolaronic defects on the chain depends strongly on the fact whether counterions are explicitly taken into account or not. From previous quantum chemical calculations one finds that, e.g., in the case of oligothiophene dications the bipolaron defects extend over 9–11 thiophene units whereas our calculations under inclusion of the electron donating alkali atoms show that in this case the defects are much more localized. A very crude estimate of the energy balance between two polaron defects and one bipolaron gives about 15 kcal/mol in favor of the bipolaron structure. We also find a rather similar behavior along the alkali series as concerns interaction energies, defect structures, and charge distribution patterns.
The density functional calculation of nuclear shielding constants using London atomic orbitals
Authors: Lee Aaron M. , Handy Nicholas C. , Colwell Susan M.
Published: July 2002

The theory for the calculation of nuclear shielding constants with London atomic orbitals using density functional theory is presented. The theory includes the use of a local exchange–correlation functional which depends on both the electron density ρ(r) and the paramagnetic current density jp(r). The resulting coupled-perturbed Kohn–Sham equations are presented, together with the working expression for the nuclear shielding tensor. The entire theory has been programmed and exhaustively checked, using standard Gaussian basis sets. A variety of ρ(r) dependent exchange–correlation functionals have been used, together with a current dependence suggested by Vignale, Rasolt, and Geldart. Certain numerical difficulties arose with this form which necessitated a cutoff in its evaluation for low densities. Calculations have been performed on HF, N2, CO, F2, H2O, and CH4. Including the current dependence is seen here to have a slightly deshielding effect. The major deficiency in the reported calculations appears to arise from the use of local functionals which give poor values for the Kohn–Sham eigenvalues.
Hydrophobic hydration in methanol aqueous solutions
Authors: Hernández-Cobos J. , Ortega-Blake I.
Published: July 2002

In this work we present a Monte Carlo study of a methanol–water mixture. The model potentials used includes polarization and nonadditive effects in the intermolecular interactions. The parameters were fitted to two- and three-body energy surfaces computed ab initio with a basis set of 2ζ plus polarization quality. Correlation was included at the MP2 level, and the basis set superposition error was corrected with the counterpoise method. Very long runs, 20×106 configurations, were used to assure that equilibrium was attained. Good agreement with experiment was found for the structural data; the carbon to water oxygen and the hydrogen to hydrogen radial distribution functions. We studied the hydrophobic hydration of the methyl group and the effect of the solute on the water structure. We found that there is hydration of the hydroxyl group and a caging of the methyl group; that the solute does indeed affect the water structure at close distance to the solute but that this effect is masked by the average over the whole system. The water structure is disrupted by a slight broadening of the first peak in the O–O rdf and a shift of the second peak toward larger distances, we have also found that there is some methanol–methanol association.
Localized relaxations in the glassy states of several molecular materials before and after their polymerization
Authors: Parthun M. G. , Johari G. P.
Published: July 2002

Dipolar diffusion in the glassy and supercooled liquid states of 12 molecular substances and of their linear chain or network polymerized states formed by condensation polymerization at different temperatures and times have been studied by measuring the dielectric absorption for a fixed ac frequency of 1 kHz. The study showed that as the extent of polymerization increased with increasing isothermal temperature of polymerization, the sub-Tg relaxation peak due to localized molecular motions in the molecular state became gradually extinct, and a corresponding peak at a higher temperature evolved and reached its maximum height. The temperature of the sub-Tg relaxation peak in the polymerized state differed from that of the α-relaxation peak of the supercooled molecular liquid by as much as 70 K, but, in several cases, the two temperatures were similar. Reasons for the latter occurrence are given in phenomenological terms. It is concluded that the localized relaxation modes of the polar segments of the macromolecule are not related to the modes of molecular diffusion in the monomeric liquid state above its Tg. The localized relaxation characteristic of the glassy molecular state persists in the incompletely polymerized state, where it is seen as a γ-relaxation.
The microwave spectrum of the CD2 radical
Authors: Ozeki Hiroyuki , Saito Shuji
Published: July 2002

The pure rotational spectrum of the CD2 radical in the X̃ 3B1 ground state was observed in the frequency region of 257 GHz–526 GHz by using a frequency modulated submillimeter-wave spectrometer. The CD2 radical was generated by the dc glow discharge of deuterated ketene, CD2CO. Twenty one fine and hyperfine component lines of four rotational transitions were measured and were analyzed by a least-square method. Rotational and fine-structure constants were revised in higher accuracy than those of the previous laser magnetic resonance (LMR) study. The hyperfine coupling constants for the deuterium nuclei have been determined for the first time.
Photoelectron spectroscopy of iron–sulfur cluster anions
Authors: Zhang Nan , Hayase Takasuke , Kawamata Hiroshi , Nakao Kojiro , Nakajima Atsushi , Kaya Koji
Published: July 2002

Iron–sulfur cluster anions (FenS−m, n=1–6, m=1–6) have been studied using photoelectron spectroscopy (PES) with a magnetic-bottle-type time-of-flight electron spectrometer. The FenS−m cluster anions were formed in a laser vaporization cluster source. It was found that the stable cluster ions are the ones with compositions of n=m and n=m±1. The electron affinities were measured from the onsets of the PES spectra. Three low-lying electronic states were observed for FeS. The PES spectra of FenS− (n=1–4) series show a unique similarity, indicating that the Fe atom addition to FeS− has little effect on the electronic property of FeS. The PES spectra of FenS−2 series show a similarity among the cluster anions with n=2–4, showing that Fe2S−2 is the structural framework of these clusters. For FenS−3 series, Fe3S−3 is proposed to be the structural framework. The electronic properties and geometrical structures of the clusters are discussed.
The dynamics of matrix trapping. I. Deposition of a pure argon matrix
Authors: Cruz Astrid J. , López Gustavo E.
Published: July 2002

The dynamics of matrix deposition is considered using classical molecular dynamics simulations. The slow spray-on and pulsed experimental deposition techniques are modeled using pairwise Lennard-Jones potentials. The structure of the deposited matrix is characterized after equilibration is allowed. The effect on the matrix structure of such factors as deposition rate, equilibration time, and initial structure of the substrate, are considered. Results show a layer-by-layer formation of the deposited material with no dependence on the deposition rate. Both experimental deposition techniques modeled in this study produce an hexagonal close packing structure.
Diffusional spinning as a probe of DNA fragments conformation
Authors: Collini Maddalena , Chirico Giuseppe , Baldini Giancarlo
Published: July 2002

The dependence of the spinning diffusion coefficient of a wormlike chain upon contour length L, persistence length P, and radius R is shown here to follow a ‘‘Lorentzian’’ law of width Γ vs σβL/R, where σ2β=l0/P is the variance of the bending angles distribution of Monte Carlo simulated chains with bond length l0. This description is equivalent to that of a spinning cylinder of length L and effective radius Reff=R(L,P), with Reff≥R. When considering experimental data it is found that fluorescence polarization anisotropy (FPA), a technique very sensitive to spinning, also yields apparent DNA radii depending upon fragment length. In order to derive DNA parameters which are independent of fragment length, we introduce a procedure for fitting FPA data which takes into account thermal distortions and employs the parametric expressions for rigid body rotations, spinning and tumbling, depending only upon L, P, and the actual DNA radius, R. Then the apparent persistence length P can be estimated once a value of R is assumed together with the value of the dynamic persistence length, the latter affecting the internal bending motions of the fragments. Fitting the FPA data is easily accomplished with the value of R=10 Å as suggested by a number of recent measurements.
Effective bridge spectral density for long-range biological energy and charge transfer
Authors: Kühn Oliver , Rupasov Valery , Mukamel Shaul
Published: July 2002

The role of medium-induced relaxation of intermediate (bridge) sites in energy and charge transfer processes in molecular aggregates of arbitrary size and geometry is explored by means of Green’s function techniques. The coupling of electronic and (solvent and intramolecular) nuclear degrees of freedom is incorporated using the Brownian oscillator model, which allows an exact calculation of the necessary two-point and four-point correlation functions of exciton operators. The signatures of energy transfer and spectral diffusion in time- and frequency-resolved fluorescence spectroscopy are studied. A unified expression for the frequency-dependent transfer rate is derived, which interpolates between the sequential and superexchange limits. Numerical results and a Liouville space pathway analysis for a donor–acceptor system coupled through a single bridge molecule are presented.
Supported nickel and copper clusters on MgO(100): A first-principles calculation on the metal/oxide interface
Authors: Pacchioni Gianfranco , Rösch Notker
Published: July 2002

The interaction of Ni and Cu atoms as well as Ni4 and Cu4 clusters with cationic and anionic sites of the MgO(100) surface has been studied by means of gradient-corrected density functional calculations using cluster models. We found that the cationic surface atoms and the fourfold hollow sites are essentially inert while Ni and Cu atoms as well as their clusters are weakly oxidized by the surface oxygens. The adhesion energy is 0.62 eV/atom for Ni4 and 0.36 eV/atom for Cu4. This reflects the stronger bonding of a surface oxygen with a Ni atom, 1.24 eV, compared to a Cu atom, 0.28 eV. The reason for the stronger bonding of Ni is the presence of the uncomplete 3d shell. In fact, the mixing of the 3d orbitals with the O 2p band leads to the formation of a covalent polar bond of moderate strength. Cu binds mainly via the 4s electrons and the interaction is weaker. An important conclusion is that the metal–metal bonds in the cluster are stronger than the metal–substrate bonds. The adsorbed clusters feature somewhat longer intermetallic distances than in the gas phase, but clusters epitaxially adsorbed on top of the surface oxygens feature significantly longer metal–metal bonds and thus are less stable.
Proton tunneling assisted by the intermolecular vibration excitation. Temperature dependence of the proton spin-lattice relaxation time in benzoic acid powder
Authors: Sakun V. P. , Vener M. V. , Sokolov N. D.
Published: July 2002

Temperature dependence of the proton spin-lattice relaxation time (T1) in powdered benzoic acid dimer and in its deuterated analog is calculated. The model assumes that two protons (deuterons) synchronously move in the double-minimum potential of the dimer. The two-dimensional potential energy surface was constructed previously, which adequately describes the static properties of the hydrogen-bonded complex. The important characteristics of this potential are a very strong mode coupling and a very high proton potential barrier (≳25 kcal/mol), whereas the experimental activation energy for the proton transfer is known to be on the order of 1 kcal/mol only. This apparent discrepancy is removed by our suggestion that the proton transfer is driven by the transitions between OHO fragment vibrational levels under the action of random forces of the surrounding. The excitation of the low-frequency intermolecular vibrations assists such transfer mechanism strongly. Using four fitting parameters to allow for the medium repolarization, the calculated T1 temperature dependence is found to be in good agreement with the experiments in the natural and deuterated benzoic acid dimer. The agreement is best at high temperature where the apparent activation energy for proton transfer was found to be 2.3 kcal/mol.
Nonlinear effects of number density of solvent molecules on solvation dynamics
Authors: Yoshimori Akira
Published: July 2002

For the number density of solvent molecules, nonlinear effects on solvation dynamics are studied using the dynamical density functional method. The present method includes nonlinear coupling between the number density and a polarization field only in the free energy functional. By means of the nonlinear free energy functional, nonlinear differential-integral equations are developed for the polarization field and number density. Numerical calculations show that solvent molecules relax more slowly around an ion than around a neutral solute. This result agrees qualitatively with nonlinear effects observed in many molecular dynamics simulations. In addition, the nonlinear dynamics of hydrogen bonds can be understood by considering the slow relaxation of the number density of solvent molecules.
Dielectric and pressure virial coefficients of imperfect gases: CO2–SF6 mixtures
Authors: Hourri A. , St-Arnaud J. M. , Bose T. K.
Published: July 2002

Dielectric and pressure virial coefficients of five mixtures of CO2–SF6 have been determined at 322.85 K. The quadrupole moment of CO2 calculated from the dielectric second virial coefficient of the CO2–SF6 mixture is (4.48±0.30)×10−26 esu cm2. This value is in excellent agreement with the most recent value determined by the direct method of induced birefringence by Battaglia et al. The hexadecapole moment of SF6 calculated from the second dielectric virial coefficient of the pure gas is (10.75±0.75)×10−42 esu cm4.
Laser-induced fluorescence spectroscopy of the C̃ 2Σ+−X̃ 2Π1/2 band system of jet-cooled CCN radical
Authors: Kohguchi Hiroshi , Ohshima Yasuhiro , Endo Yasuki
Published: July 2002

Laser-induced-fluorescence spectra of the C̃ 2Σ+−X̃ 2Πr system of the CCN radical were observed under jet-cooled conditions. The radical was generated in a supersonic expansion by a pulsed electric discharge of methyl cyanide diluted in Ar. Sixteen C̃ 2Σ+(v1,v2l,v3)–X̃ 2Π1/2(0,00,0) vibronic bands with v1=0–3, v2=0–2 (l=0–2), and v3=0–2 were observed, including some very weak vibronically induced parallel-type transitions with v2=1 (l=1). Rotational analyses were performed to give the band origins and the rotational constants for vibrationally excited states of all the three normal modes and their combinations in the C̃ 2Σ+ state. From the determined band origins, the term value and the vibrational parameters in the C̃ 2Σ+ state have been determined: Te=24 627.1(20) cm−1, ω1=1903.1(11) cm−1, ω2=469.88(38) cm−1, ω3=1256.6(40) cm−1, and g22=0.86(11) cm−1. A combination of the present ω2 frequency in the C̃ state with the data on the hot bands reported so far in literature has yielded the vibronic structures in the X̃ 2Πr and Ã 2Δ states, which are subject to a prominent Renner–Teller effect.
Geometry optimization of a water molecule in water. A combined quantum chemical and statistical mechanical treatment
Authors: Moriarty Nigel W. , Karlström Gunnar
Published: July 2002

A Hartree-Fock optimized geometry for a water molecule in a statistical mechanical Monte Carlo bath of 89 water molecules was determined using a polarization model. Both the O–H bond lengths and ∠HOH bond angle were found to increase from the gas to the liquid phase. The bond length increase is in good agreement with recent neutron diffraction results; liquid water is closer in bond length to gas phase water than ice. In combination with the optimized ∠HOH bond angle approaching tetrahedral, the conclusion is that quadrupole moment dominates the water geometry in the liquid phase.
Intrinsic resonance representation of quantum mechanics
Authors: Carioli M. , Heller E. J. , Mo/ller K. B.
Published: July 2002

The choice of basis states in quantum calculations can be influenced by several requirements, and sometimes a very natural basis suggests itself. However often one retreats to a “merely complete” basis, whose coefficients in the eigenstates carry little physical insight. We suggest here an optimal representation, based purely on classical mechanics. “Hidden” constants of the motion and good actions already known to the classical mechanics are thus incorporated into the basis, leaving the quantum effects to be isolated and included by small matrix diagonalizations. This simplifies the hierarchical structure of couplings between “zero-order” states. We present a (non-perturbative) method to obtain such a basis-state as solutions to a certain resonant Hamilton–Jacobi equation.
Response to “Comment on: ‘Successive capture and coagulation of atoms and molecules to small clusters in large liquid helium clusters’ ’’ [J. Chem. Phys. 106, 5785 (1997)]
Authors: Lewerenz M. , Schilling B. , Toennies J. P.
Published: July 2002

The alternative formula for the determination of the capture cross section is found to lead only to small deviations from the original results. A labelling error is acknowledged. The measurement of coagulation cross sections is explained in additional detail.
Dielectric constant of the Drude-Lorentz model of a nonpolar fluid
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

The frequency-dependent dielectric constant of the Drude–Lorentz model of a nonpolar fluid is shown to have universal scaling behavior in the limit of low density. We derive an exact expression for the universal dielectric constant, and calculate it in a self-consistent ring approximation. The absorption line shape is found to be a shifted and asymmetrically distorted Lorentzian, as suggested by computer simulation data for systems of hard spheres and of atoms with Lennard-Jones interactions. A calculation of correction terms accounting for pair correlation effects leads to fairly good agreement with existing computer simulation data.