bluuebunny/crossref_metadata_2025 huggingface dataset:
Artificial neural network applied for predicting rainbow trajectories in atomic and molecular classical collisions
Authors: Braga A. P. , Braga J. P. , Belchior J. C.
Published: July 2002

A simple artificial neural network (ANN) is developed and applied to collision processes. A general discussion of how ANNs can be introduced to study general phenomena in scattering problems is presented and neural networks are proposed to predict classical rainbow trajectories in atomic and molecular collisions. As a result of modeling the collision process, based on the neural network approach, analytical equations were obtained to calculate classical atomic and molecular rainbow trajectories. However, these analytical results just translate the behavior of the input/output data and do not contain any general physical meaning. Although a fitting procedure could be easily used in the present case, the cost of function approximation using ANNs increases only linearly with the number of input variables. This contrasts with classical polynomial fitting procedures for which the computational cost increases exponentially with the input space dimension. This makes the ANN approach worth considering when modeling scattering processes, as shown throughout this paper. At last, an articial network strategy is pointed out to study inversion problems in collision processes.
Low-energy electron scattering from the water molecule: Angular distributions and rotational excitation
Authors: Gianturco F. A. , Meloni S. , Paioletti P. , Lucchese R. R. , Sanna N.
Published: July 2002

The scattering of electrons from the H2O molecule in its ground electronic state is analyzed by carrying out quantum calculations of the coupled equations in the body-fixed (BF) frame and using exact static-exchange interaction forces (ESE) within a single-center expansion (SCE) formulation. The effect of correlation-polarization forces is included via a density functional approach and the necessary corrections to the nonconvergent behavior of the angular distributions from fixed nuclei calculations involving polar molecules are carried out. Elastic and rotationally inelastic differential and momentum transfer cross sections are compared with experiments and the effects on the inelastic processes of the long-range dipolar potential are examined in some detail. The electron efficiency in exciting molecular rotations over a broad range of energies is also obtained and discussed.
Rotational analysis and assignment of the 630 nm band system of FeH to the e 6Π–c 6Σ+ transition
Authors: Goodridge Damian M. , Hullah Daniel F. , Brown John M.
Published: July 2002

An investigation of the electronic spectrum of FeH at ∼630 nm using dispersed and undispersed laser induced fluorescence techniques is reported. Eighty lines have been assigned to the (0,0) vibrational band of the e 6Π–c 6Σ+ transition. The transitions can be arranged in three subbands with all six spin components of the c 6Σ+ state accessed. This has allowed the c 6Σ+ state to be fully characterized and provides parity assignments of individual rotational levels for the first time. The energy levels of the c 6Σ+ state conform approximately to a Hund’s case (a′) coupling scheme with significant Ω-type doubling (1–10 cm−1). An attempt has been made to simulate the levels using an effective Hamiltonian. Although the simulation failed to reproduce the results to within experimental uncertainty (∼0.007 cm−1), the residuals are sufficiently small to confirm the correctness of the assignments.
Temperature effect up to 1100 K on the Xe2* excimer luminescence using two-photon excitation
Authors: Jonin C. , Laporte P. , Saoudi R.
Published: July 2002

The temperature effect on the Xe2* excimer emission near 172 nm was studied up to 1100 K. As the temperature is increased, the line shape becomes asymmetric, its width increases and its maximum shifts to the blue. The results were simulated using Franck–Condon calculations. Good agreement between the experimental data and the derived profiles was obtained using a Morse potential for the excited state. The deduced parameters for this potential are De=0.51±0.01 eV, re=3.06±0.05 Å and β=0.85±0.05a0−1. Comparisons were made with available data.
Spectral random walks and line broadening of impurity molecules in an Ising spin glass environment
Authors: Tanimura Yoshitaka , Takano Hiroshi , Klafter Joseph
Published: July 2002

Transition energy fluctuations in impurity molecules, embedded in an inhomogeneous environment, are investigated within an Ising spin model of the environment. The spatially distributed impurity molecules are assumed to interact with the Ising spin glass through dipole–dipole type of interaction. We calculate the fluctuations in the transition energies of impurity molecules, for different temperatures and various Ising parameters, and find that the spectral distribution of the fluctuations follows approximately a 1/f power law. The fluorescence spectra of the impurity molecules yield microscopic information about domain structures in the Ising environment. In the case of large disorder, the distribution of transition energies shows profiles similar to those observed in single molecule spectroscopy.
Theoretical study on potential-energy surface of C2N2
Authors: Ding Y. H. , Huang X. R. , Li Z. S. , Sun C. C.
Published: July 2002

B3LYP and CCSD(T) (single-point) calculations are performed on structures and interconversion transition states for five C2N2 isomers, i.e., the linear structures NCCN 1, CNCN 2, CNNC 3, and CCNN 4, and the NNC three-membered ring structure with exocyclic C–C bonding 5. A schematic potential-energy surface of the five C2N2 isomers is established. It is shown that the elusive and long searched for isomer 3 lies in a deep potential well against isomerization to the well-studied isomers 1 and 2, while the two transition states, which have been predicted to connect the isomers 1 and 3, are actually associated with the interexchange of the two cyano groups in isomer 2. It is also shown that the experimentally unknown but expectedly important isomer 4 may take an isomerization pathway to the isomer 1 via the intermediate 5, and the isomer 4 also lies in a deep potential well. Both the isomers 3 and 4 may be thermally as stable as the isomer 2. The results presented in this paper may provide useful information for the identification of the two isomers 3 and 4.
Nonstatistical unimolecular dissociation over a barrier
Authors: Mordaunt David H. , Osborn David L. , Neumark Daniel M.
Published: July 2002

A general formulation is presented to model photodissociation processes in which internal conversion is followed by unimolecular dissociation over an exit barrier; this classification of dissociation mechanism results in a nonstatistical product state distribution. The energy available to products is divided into independent statistical and impulsive energy reservoirs. The statistical reservoir considers direct projections of a vibrational microcanonical ensemble at the transition state (TS) onto product quantum states, conserving vibrational adiabaticity and angular momentum. The impulsive reservoir represents the energy released in passing from the TS to products; this reservoir is treated assuming sudden dissociation of the zero-point TS wave function using a combination of Franck-Condon and impulsive models. We derive the statistical adiabatic impulsive model, which convolutes these two energy reservoirs, to predict the product translational energy distribution for nonstatistical dissociation over a barrier. Two test cases are modeled and compared with experimental data: unimolecular dissociation of acetyl radicals and photodissociation of vinoxy radicals via the B̃ 2A″-X̃ 2A″ band.
Limited coordination number and competitive coordination in ammonia–water mixed-ligand complexes of monopositive metal ions as studied by the laser-ablation–molecular beam method: Experiment and simulation
Authors: Sato Hiroyasu , Matsuzaki Akiyoshi , Nishio Satoru , Ito Osamu , Furukawa Koji , Kawasaki Takashi
Published: July 2002

Ammonia–water mixed-ligand complexes of monopositive metal ions M+ (M=Mg, Al, Mn, and Co) were prepared in the gas phase by reactions of metal ions laser-ablated from a metal substrate in vacuum with ammonia–water binary clusters in a molecular beam injected nearby [the laser-ablation–molecular beam (LAMB) method]. Relative abundances of M+(NH3)m(H2O)n are characterized by intensity gaps which indicate limited (typically 2 or 3) coordination (solvation) numbers in the first coordination (solvation) sphere. Three patterns of competitive coordination (solvation), i.e., selective, nonselective, and magic-number-like, are observed. The patterns are metal-specific and relatively independent of stagnation ratios of two component gases. The coordination numbers as judged from the intensity gaps remain the same throughout the stagnation ratios studied. A model simulation of the dynamic processes involved was made under simple-minded assumptions: (1) the ensemble of metal complex ions starting from the reaction region is characterized with a temperature Tstart (its value being taken as an adjustable parameter), (2) only evaporation of component ligands one by one occurs after metal complex ions start from the reaction region into the quadrupole, (3) activation energy of each evaporation step is determined by binding energy of the leaving ligand, and (4) temperature drop rate of complex ions per one microsecond is constant (its value being taken as an adjustable parameter). Such a simulation procedure is found successful in reproducing the positions of intensity gaps, together with the qualitative features of the metal-specific coordination (solvation) patterns observed.
Femtosecond fifth-order nonlinear response of nuclear motion in liquids investigated by incoherent laser light. Part I. Theory
Authors: Pfeiffer M. , Lau A.
Published: July 2002

The fifth-order nonlinear response is analyzed applying incoherent broadband laser light with femtosecond autocorrelation time under nonresonant conditions. Raman-like excitation is performed combining the broadband incoherent laser with a quasi-cw narrow-band laser with a frequency difference of both lasers tuned to match the frequency of a nuclear mode. The pulsed broadband laser is split into three identical beams allowing application of two different time delays between them. Similar to experiments with single fs pulses, Raman double excitation allows to discern homogeneous and inhomogeneous broadening of the nuclear response. The separation of the broadening contributions, however, demands careful analysis of the signal dependence on delay time in the tail of the observed peaks. Particular features of the experiment, which is described in the second part of this paper, as interferences with cascading of third-order processes and a deconvolution procedure to extract the molecular parameter from the experimental data are theoretically discussed.
Identification of the origin of trans-free base isobacteriochlorin’s S2←S
Authors: Huang Wen-Ying , Johnson Lawrence W.
Published: July 2002

We have identified the S2←S0(ΠΠ*) origin in the Q spectral region of trans-free base isobacteriochlorin. The 0-0 appears as a cluster of about ten sharp bands due to resonant vibronic interference with the S1 state. This cluster is followed by its replicas at normal vibronic intervals.
Comment on “The hyperpolarizability of trans-butadiene: A critical test case for quantum chemical models” [J. Chem. Phys. 106, 1827 (1997)]
Authors: Kirtman Bernard , Toto Joseph L. , Breneman Curt , de Melo Celso P. , Bishop David M.
Published: July 2002

It has been suggested that the random phase approximation is superior to many-body/coupled cluster methods for calculating the second hyperpolarizability of trans-butadiene and, by extension, other π-conjugated species. We show that this conclusion is premature because of basis set inadequacies; approximations in the treatment of frequency dispersion; and omission of vibrational effects. We also hypothesize that trans-butadiene may not provide the most critical test of various computational approaches.
Discrete variational quantum reactive scattering method with optimal distorted waves. II. Application to the reaction H+O2→ OH+O
Authors: Groenenboom Gerrit C.
Published: July 2002

The discrete Kohn variational reactive scattering method presented in the preceding paper is applied to the reaction H+O2→ OH+O. The essential features of the method are the use of an interaction region grid, fully coupled open and closed distorted waves (DWs) and the use of a singular value decomposition technique to construct the optimal boundary conditions for the open DWs. A convergence test is presented at a total energy of 0.817 eV above the bottom of the H+O2 well. It is shown that very well converged results may be obtained in a calculation with a relatively small interaction region grid, when at least a few asymptotically closed DWs are included in the trial wave function. Furthermore, the number of open distorted waves (DWs) may be considerably smaller than the number of open channels. Six additional points are computed in an energy range of 1.2 meV, scanning through a narrow resonance around 0.817 eV. The results are in very good agreement with the hyperspherical coordinate propagation calculations by R. T Pack, E. A. Butcher, and G. A. Parker [J. Chem. Phys. 102, 5998 (1995)].
Regarding the correlation of nuclear spin relaxation and electrical conductivity relaxation in ionic glasses
Authors: Sidebottom D. L. , Green P. F. , Brow R. K.
Published: July 2002

Much attention has been focused recently on the apparent differences between ion dynamics in ion-containing glasses as probed by electrical conductivity relaxation (ECR) and by nuclear spin relaxation (NSR) techniques. In both relaxation processes, a power law frequency dependence is observed. Based upon fluctuation–dissipation arguments, the power law exponents should be equivalent. However, experimentally, it appears that the conductivity exponent is generally smaller than the NSR exponent. While an explanation for this discrepancy based upon fundamental differences in the correlation functions probed by the two techniques has been proffered, we show how this discrepancy may simply arise from differing analyses of the ac conductivity. We review several cases taken from the literature in which the conductivity exponent was obtained from analysis of the electrical modulus. We demonstrate how this analysis approach generally underestimates the conductivity exponent. When we instead determine the exponent directly from the ac conductivity, we find near equivalence between the NSR and ECR exponents.
Location of fluid-β and β-δ coexistence lines of nitrogen by computer simulation
Authors: Meijer Evert Jan
Published: July 2002

The fluid-β and β-δ coexistence lines for a state of the art intermolecular nitrogen potential [R. D. Etters et al., Phys. Rev. B 33, 8615 (1986)] are located by absolute free energy calculations using Monte Carlo simulations. We have found that the Etters potential provides an accurate description of the fluid-β coexistence line, but significantly overestimates the pressures of the β-δ coexistence line. This suggest that for a more accurate description of the δ  phase the Etters potential needs to be improved. The present results constitute, to our knowledge, the first numerical calculation of full coexistence lines of a realistic molecular solid.
Resonant and nonresonant x-ray scattering spectra of some poly(phenylenevinylene)s
Authors: Guo J.-H. , Magnuson M. , Såthe C. , Nordgren J. , Yang L. , Luo Y. , Ågren H. , Xing K. Z. , Johansson N. , Salaneck W. R. , Daik R. , Feast W. J.
Published: July 2002

The electronic structure of some poly(phenylenevinylene)s have been investigated by resonant and nonresonant x-ray inelastic scattering spectroscopies. The nonresonant as well as all resonant spectra for each polymer demonstrate benzene-like features, indicating a local character of the x-ray emission in which the phenyl ring acts as a building block. Theoretical simulations of x-ray energies and intensities taking the repeat unit as a model molecule of the polymer agree with the experimental spectra fairly well. The edges of the occupied bands have been identified in the nonresonant spectra of each polymer. By subtracting the emission energy of the highest occupied molecular orbital in the nonresonant spectrum from the core excitation energy in the resonant spectrum an alternative way to determine the optical band gap is obtained. As for free benzene the outer π band in the polymer spectra show a depletion of the emission going from the nonresonant to the resonant x-ray emission spectra. It is demonstrated that this transition, which is strictly symmetry forbidden for free benzene, becomes effectively forbidden in the polymer case as a result of strong interference effects, and it is argued that this is the general case for resonant x-ray emission of conjugated polymers as far as the frozen orbital approximation holds.
Change in chain stiffness in viscometric and ultracentrifugal fields: Cellulose diacetate in N, N-dimethylacetamide dilute solution
Authors: Kawanishi Hiroyuki , Tsunashima Yoshisuke , Okada Shinichi , Horii Fumitaka
Published: July 2002

The molecular characteristics and the chain stiffness were investigated for fractionated samples of cellulose diacetate (CDA, degree of substitution DS = 2.40) in N, N-dimethylacetamide (DMAc) through the partial specific volume, the viscometric, the sedimentation velocity, and the sedimentation equilibrium measurements at 30 °C. It was found that CDA dispersed molecularly in DMAc under the external field such as in the viscometric and the sedimentation experiments. The molecular weight dependence of the intrinsic viscosity [η] and the sedimentation coefficient at infinite dilution s0 of the single CDA molecule were expressed by the relation [η] = 1.10×10−2Mw0.85 (cm3 g−1) and s0=2.25×10−14Mw0.20 (s), which exhibit the stiff, or semiflexible chain nature. The semiflexible chain parameters were evaluated by the Yamakawa–Fujii (YF) theory of the unperturbed wormlike cylinder model, first via a combination of the viscosity and the partial specific volume data (method A) and second via a combination of the sedimentation and the partial specific volume data (method B). Method A gave the chain parameters that q = 8.0 nm, ML = 523 nm−1, and d = 0.89 nm, whereas method B gave q = 48 nm, ML = 560 nm−1, and d = 0.93 nm. Here q is the persistence length, ML is the molecular weight per unit contour length, and d is the chain diameter of the wormlike cylinder model. Methods A and B deduce, independent of the method, the definite ML and d values, which are very consistent with the ordinary reported values. However, q estimated by the two methods differs by about six times the other. This fact suggests that the CDA molecule in the ultracentrifugal field has a conformation different from that in the viscometric shear field: The CDA chain may be highly stiff in the ultracentrifuge because of the situation that the adjacent glucose residues are stuck in a rigid conformation by the double stapled hydrogen bonds between the intramolecular hydroxyls and oxygens.
Ab initio study on the mechanism of C2H2++NH3 reaction: Efficient charge transfer and proton transfer processes competing with stable complex formation
Authors: Cui Qiang , Morokuma Keiji
Published: July 2002

High level ab initio calculations have been performed to investigate the mechanism of the ion–molecule reaction NH3+C2H2+. Three channels, covalent complex formation (CC), proton transfer (PT), and charge transfer (CT) have been studied. Among the two pathways found for the PT channel, one leads the reactants NH3+C2H2+ to NH4++C2H(2Π) through a moderately bound complex without any barrier, and the other leads NH3++C2H2 to the H-atom transferred products NH4++C2H(2Σ+) with a modest barrier. These findings support the fast “stripping” mechanism proposed by Anderson et al. As to the CC channel, several isomers of C2H5N+ and the isomerization transition states have been located. No significant barrier relative to the reactants has been found on either the ground or the 2Av″ excited state. To rationalize the experimental fact that no CC channel products have been observed, it is argued that the reactants NH3+C2H2+ correlate adiabatically to excited states of covalent C2H5N+ species, whose formation requires significant alternation of the C2H2+ geometry and electronic structure. Therefore, the system is most likely to follow the PT or the CT channel instead of visiting the CC channel. For the CT channel, limited potential energy surface scans of the three electronic states (1,2 2A′+2A″) indicate that CT at different approach angles or between electronic states of different symmetries (A′→A′,A″→A′) may produce final products of different characteristics, and might account for the two pathways proposed by Anderson et al.
Three-dimensional nuclear magnetic resonance exchange spectroscopy with rotary resonance in rotating solids: Application to tropolone dynamics
Authors: Gan Zhehong , Ernst R. R.
Published: July 2002

A three-dimensional nuclear magnetic resonance (NMR) experiment is described for the investigation of molecular rearrangements in the course of chemical exchange processes. The experiment relies on the two-dimensional correlation of the chemical shift tensors before and after exchange. The chemical shift tensors are retrieved under fast magic angle spinning by rotary resonance induced by a radio-frequency field whose magnitude matches the sample spinning frequency. The third dimension serves for the separation of the two-dimensional chemical shift patterns by the isotropic chemical shifts. The three-dimensional rotary-resonance C13-exchange experiment is demonstrated with an investigation of hydrogen-transfer and molecular diffusion processes in a sample of solid tropolone.
Energetics and kinetics of ethylbenzene adsorption on epitaxial FeO(111) and Fe3O4(111) films studied by thermal desorption and photoelectron spectroscopy
Authors: Zscherpel D. , Ranke W. , Weiss W. , Schlögl R.
Published: July 2002

The adsorption of ethylbenzene (EB) has been studied on thin films of FeO(111) and Fe3O4(111) grown epitaxially on Pt(111) using thermal desorption spectroscopy (TDS), ultraviolet photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED). Applying a threshold analysis of the TDS data, desorption energies Edes and the corresponding frequency factors are deduced. The UPS measurements are performed under adsorption–desorption equilibrium conditions: The spectra are taken at varying sample temperature at constant EB gas phase pressures. From the spectra, the EB-coverages ΘEB are deduced. From the adsorption isobars obtained in this way, isosteric heats of adsorption qst(ΘEB) are obtained which are compared to the desorption energies Edes deduced from TDS. On the oxygen-terminated FeO(111) surface, two adsorption states are observed, a physisorbed first layer (β-EB) followed by condensation (α-EB). Their UP spectra are almost identical and very similar to the spectrum of gas phase EB. On Fe3O4(111), a more strongly chemisorbed species (γ1-EB) is adsorbed first, followed by physisorbed β- and condensed α-EB. The chemisorbed phase exhibits a strong shift and split of the highest occupied π orbitals of the phenyl group. This indicates a strong interaction between the substrate and the adsorbed molecules that are adsorbed with the phenyl ring lying flat on the surface. The desorption energies Edes and the isosteric heats of adsorption qst, respectively, are 91 (85) kJ/mol for γ1-, 55 (58) kJ/mol for β- and 50 (52) kJ/mol for α-EB and agree generally well. The differences are discussed in terms of different coverage ranges accessible for both methods, the nonequilibrium character of the TDS method and to the threshold analysis which yields only data for the most loosely bound molecules desorbing first in each desorption track.
New vibrational analysis of the 1B2(1Δu) state of CS2
Authors: Brasen G. , Leidecker M. , Demtröder W. , Shimàmoto T. , Katô H.
Published: July 2002

Sub-Doppler excitation spectra of CS2 in the spectral range of 29 600–32 800 cm−1 (V-system) are measured with a frequency doubled continuous wave (cw) single-mode dye laser. The ultraviolet (UV) radiation is generated in an external enhancement cavity. The average output power is about 5 mW, which is sufficient to resolve weak rotational lines in the excitation spectra of hot bands from the (0,22,0) level of the ground state. This increased the number of measured transitions with K>0 considerably. The low temperature achieved by adiabatic cooling of CS2 seeded in a supersonic argon beam simplifies the spectrum and allows an exact assignment of the rotational lines. Based on new ab initio calculations, a revised electronic state and vibrational assignment of the bands were necessary and will be presented. From this vibrational analysis the geometry, the position of the barrier to linearity, and the rotational constants of the CS2 molecule in the V 1B2(1Δu) state have been determined, revising former results. A statistical analysis of the positions of all measured bands shows a pure Poissonian nearest level distribution indicating no correlation between neighboring vibrational levels nor long-range interactions.
A CCSD(T) study of the He⋅NO molecular complex
Authors: Lee Edmond P. F. , Wright Timothy G.
Published: July 2002

Calculations at the CCSD(T) level of theory employing the cc-pVTZ, aug-cc-pVTZ and aug-cc-pVQZ basis sets are reported. Both the A'2 and the A"2 states are considered, as well as the two linear structures, He⋅NO and He⋅ON. The highest level of calculation, CCSD(T)/aug-cc-pVQZ//CCSD(T)/aug-cc-pVTZ, indicates that the global minimum is a skewed T-shaped structure, in agreement with recent CEPA calculations, but in disagreement with MP4 calculations, which concluded that the linear He⋅ON isomer was the lowest energy geometry. Although the highest level of theory used here indicates that the A'2 surface is the lower in energy, the ordering of the A'2 and the A"2 surfaces has not yet been firmly established. The interaction energy is calculated to be ∼27 cm−1, and estimated as being ∼30 cm−1 at the basis set limit.
The SiH4+H→SiH3+H2 reaction: Potential energy surface, rate constants, and kinetic isotope effects
Authors: Espinosa-Garcı́a J. , Sansón J. , Corchado J. C.
Published: July 2002

The potential energy surface for the gas-phase SiH4+H→SiH3+H2 reaction and its deuterated analogs was constructed with suitable functional forms to represent the stretching and bending modes, and using as calibration criterion the reactant and product experimental properties and the ab initio saddle point properties. Using this surface, the rate constants were calculated with variational transition-state theory over the temperature range 200–1000 K, finding good agreement with experiments. We also provide a detailed analysis of the kinetic isotope effects and a comparison with the scarce experimental results.
Electron and anion mobility in low density hydrogen cyanide gas. II. Evidence for the formation of anionic clusters
Authors: Klahn Th. , Krebs P.
Published: July 2002

We have measured the drift mobility of anions in low-density hydrogen cyanide (HCN) gas. They are formed by attachment of photoinjected excess electrons to HCN molecules. Mobility isotherms for the temperatures 294, 333, and 373 K were determined in the HCN number density range 9.36×1017⩽n⩽1.21×1020 cm−3.The density-normalized mobility (μanionn) of the anions shows a rather strong density dependence. At least in the limit of very low density the value (μanionn)0 of some postulated anions can be tested by the statistical adiabatic channel model (SACM) for ion-dipole molecule capture processes published recently by Troe [J. Chem. Phys. 105, 6249 (1996)]. The observed density variation can be explained by cluster anion formation. But additionally one has to consider the influence of HCN dimer formation in the drift gas on the anion mobility. It is further shown that the experimental mobilities recently obtained for cluster ions like NO+(CH3CN)2 and NO+(CH3CN)3 in acetonitrile are in total contradiction not only to our experimental results but also to theory.
The influence of polarizability in metal cluster reactions as seen in the reactions of gas-phase W clusters with cyclopropane
Authors: Pedersen David B. , Parnis J. Mark , Rayner David M.
Published: July 2002

Absolute second-order rate coefficients for the reaction of Wn with cyclopropane have been measured using a large-bore flow tube reactor coupled with a laser ablation metal cluster source and a laser ionization/time-of-flight mass spectrometric detector. Rate coefficients for the depletion of W clusters by cyclopropane in He carrier gas show no helium bath gas pressure dependence between 0.5 and 2.0 Torr but decrease with increasing temperature between 277 and 351 K. Reaction rates are significantly higher than those predicted by a simple hard-sphere collision model. A polarizability-based model has been successfully employed to explain the reactivity observed. The model implicates van der Waals complexes as precursors to chemisorption of cyclopropane by Wn.
Quantum mechanical study of intramolecular vibrational energy redistribution in the second CH stretch overtone state in benzene
Authors: Rashev S. , Stamova M. , Kancheva L.
Published: July 2002

The vibrational level mixing at the second CH stretch overtone state CH(v=3) in benzene has been studied quantum mechanically using a completely symmetrized vibrational basis set in terms of a combined local mode/normal mode description. The employed symmetrized approach has helped to reduce the dimensionality of coupling Hamiltonian matrices and thus allowed for the inclusion of all 30 vibrational modes in the calculations. The absorption spectrum and dynamical intramolecular vibrational redistribution characteristics for initial excitation of a symmetrized local mode “bright” state in the CH(v=3) overtone manifold have been calculated and analyzed in connection with the degree of localization of the CH stretch overtone vibrational system in benzene.
Renormalized Ising behavior of critical nonionic surfactant solution
Authors: Kubota Kenji , Kita Rio , Dobashi Toshiaki
Published: July 2002

The critical behavior of a nonionic surfactant, heptaethyleneglycol mono-n-tetradecyl ether (C14E7), in an aqueous tetramethylammonium chloride (TMAC) solution was investigated by the combined use of turbidity and light scattering measurements concerning the critical exponents of the isothermal osmotic compressibility and the long-range correlation length. Both the critical exponents and the critical points depend on the TMAC concentration. Critical exponents increase with the increase of salt concentration showing a deviation from the three-dimensional Ising model. The critical behaviors are well described by a continuous change from the three-dimensional Ising model to Fisher’s renormalized Ising model.
Critical strain and catalytic growth of single-walled carbon nanotubes
Authors: Cornwell C. F. , Wille L. T.
Published: July 2002

The elastic response and critical strain of single-walled carbon nanotubes (SWT’s) are investigated by means of quenched molecular dynamics simulations using the Tersoff-Brenner potential. The results of this discrete model are found to be in agreement with calculations based on continuum elasticity theory for tube radii greater than 10.0 Å. However, for tube radii less than 10.0 Å the two models diverge. It is shown that there is a correlation between the critical strain and the experimentally measured abundance of SWT’s with various radii. Based on this information, we suggest a model to explain the role of structural stability in determining the radial distribution of SWT’s.
Conformation and dynamics of polymer chains on dirty surfaces: A discrete-to-continuum approach
Authors: Foo Grace M. , Pandey R. B.
Published: July 2002

A discrete-to-continuum (DC) simulation approach is introduced to study the statics and dynamics of polymer chains in two dimensions with quenched barriers, a dirty surface. In our DC hybrid approach, the large-scale relaxation of polymer chains on a discrete disordered lattice is followed by off-lattice simulation using a bead-spring chain model with a finitely extensible nonlinear elastic (FENE) potential for covalent bonds and Lennard-Jones (LJ) potential for nonbonded interactions. Segregation/folding of chains, which occurs at low temperatures (T=0.2, 1.0) with LJ interaction, becomes more difficult as the concentration of barriers increases, due to a screening effect of the barriers. In contrast to the chains’ contraction at high temperature (i.e., T=5) and their collapse in athermal systems, chains are elongated on increasing the barrier concentration—a barrier-induced stretching. Variations of the root-mean-square (rms) displacements of the center of mass (Rcm) of the chains and their center node (Rcn) with time (t) show power-law behaviors (Rcm∼tν1, Rcn∼tν2) with nonuniversal exponents in the range ν1≃0.40–0.05 and ν2≃0.30–0.05, respectively, depending on temperature and barrier concentration. The radius of gyration (Rg) and the average bond length (〈l〉) expand on increasing the barrier concentration at low temperature and contract at high temperature, which is consistent with the variation of the width of the radial distribution function.
A strictly variational procedure for cluster embedding based on the extended subspace approach
Authors: Gutdeutsch Ulrich , Birkenheuer Uwe , Rösch Notker
Published: July 2002

Even if an isolated defect results only in a local perturbation of the electron density, the wave function and the first-order reduced density matrix may still exhibit a long-range response to the defect. We present an axiomatic approach to the construction of a general-purpose embedding scheme which is able to cope with this problem. We start from a list of requirements, which we consider pertinent to an accurate embedding technique, and we proceed to demonstrate that the extended subspace approach recently proposed by Head and Silva [J. Chem. Phys. 104, 3244 (1996)] is the minimal realization of such an embedding scheme. The variational principle, strict fulfillment of the Pauli exclusion principle, a finite dimensional parameter space, and the possibility to perform the minimization by a standard SCF (self-consistent field) procedure are the key requirements which lead to a constrained SCF procedure. Self-embedding consistency and local completeness of the Hilbert space can then be realized by a mathematically very simple construction principle for the active subspace which can be formulated independent of any basis set. We analyze the spatial structure of the resulting minimal orbital space by means of tight-binding model Hamiltonians. For metal systems, we find active and frozen constrained SCF spaces to necessarily interlock in a strong and complicated fashion.
Free convection and surface kinetics in crystal growth from solution
Authors: Baird James K. , Guo Lihong
Published: July 2002

As a crystal grows from solution, there is ordinarily a boundary layer depleted in solute, which forms at the crystal–solution interface. When the normal to the growing crystal surface is oriented in any direction other than parallel to gravity, the boundary layer is set into motion by the force of buoyancy. Using a similarity transformation and a boundary layer approximation, we have solved the Navier–Stokes equation and the equation for convective diffusion for a crystal in the form of a flat plate growing with normal perpendicular to gravity. Parameters in the theory include solute concentration, c0, and diffusion coefficient, D; solution shear viscosity, μ, mass density, ρ, and logarithmic density derivative with respect to concentration, α; crystal solubility, cs, height, h, and linear growth rate, kG; the specific rate, k (sticking coefficient), of the reaction which transfers molecules from the solution to the crystal and the kinetic order, n, of this reaction; and the acceleration due to gravity, g. We find these parameters to be related by the equation log[1−Sh/a (Sc) 1/4(Gr) 1/4φ s1/4]=(1/n) log[a(5/4) n(D/hkc 0n−1)(Sc) 1/4(Gr) 1/4]+[(5/4−n)/n]log φs, where a=0.9, Sh=kGh/D, Sc=μ/ρD, Gr=gαh3ρ2/4μ2, and φs=(c0−cs)/c0. Given a knowledge of the solution physical properties, if Sh is measured as a function of φs and the results plotted in accord with the above equation, both n and k can be determined.
Relativistic corrections to the energies of the EF, GK, and HH̄ 1Σg states of the hydrogen molecule
Authors: Wolniewicz L.
Published: July 2002

Relativistic corrections to the electronic energies are computed for the EF, GK, and HH̄ 1Σg states of the hydrogen molecule in a wide range of internuclear distances. Term values for the J=0 levels are given for the EF state and for the outer well of the HH̄ state. A comparison with experiment shows that the adiabatic, relativistic theoretical HH̄ energies are too low by about 1 cm−1.
Vibrational spectroscopic study of 5,5″-bis(dicyanomethylene)-5,5″-dihydro-Δ2,2′:5′,2″-terthiophene bearing a heteroquinonoid structure as a model of doped polythiophene
Authors: Hernandez V. , Hotta S. , Lopez Navarrete J. T.
Published: July 2002

We present the results of a spectroscopic study carried out on a solid sample of 5,5″-bis(dicyanomethylene)-5,5″-dihydro-Δ2,2′:5′,2″-terthiophene. This compound bears a heteroquinonoid structure in its ground state and could be considered as a model of the structural changes induced by the different types of charge carriers in doped polythiophene. We have identified two very strong Raman bands that are indicative of the attainment of a pure heteroquinonoid structure in α-oligothienyl compounds. The large frequency downshifts of these Raman bands with respect to the pure heteroaromatic structure are a measure of the extent of the electronic modification caused by the quinoidization of the ring molecular system.
Direct observation of infrared band intensity transfer between coadsorbates having widely separated oscillator frequencies: Intermixed NO/CO adlayers on ordered iridium electrodes
Authors: Weaver Michael J. , Tang Catherine , Zou Shouzhong , Severson Mark W.
Published: July 2002

The occurrence of substantial (two–threefold) transfer of infrared band intensity between juxtaposed coadsorbates having widely separated (200–250 cm−1) oscillator frequencies is demonstrated directly for intermixed NO/CO adlayers on ordered Ir(111) and (110) electrodes by selectively removing CO or NO by electrochemical oxidation and reduction, respectively. The surprisingly large effect is nevertheless semiquantitatively consistent with the predictions of theoretical dipole-coupling models.
Ab initio all-electron fully relativistic Dirac–Fock–Breit calculations for molecules of the superheavy transactinide elements: Rutherfordium tetrachloride
Authors: Malli Gulzari L. , Styszynski Jacek
Published: July 2002

Ab initio all-electron fully relativistic molecular spinor (RMS) Dirac–Fock (DF) self-consistent field (SCF) and nonrelativistic limit Hartree–Fock (HF) calculations are reported at four Rf–Cl bond distances for the ground state of tetrahedral (Td) rutherfordium tetrachloride (RfCl4) with our universal Gaussian basis set. The optimized Rf–Cl bond distance computed from our relativistic and nonrelativistic SCF wave functions for RfCl4 (Td) is 2.39 and 2.45 Å, respectively. The relativistic correction to the total electronic energy of RfCl4 was calculated as ∼−4355 hartrees (−118 504 eV) at the Dirac–Fock level. The dominant magnetic part of the Breit interaction correction for RfCl4 is estimated by perturbation method as 66.8509 hartrees. Our Hartree–Fock, Dirac–Fock, and Dirac–Fock–Breit calculations predict the tetrahedral RfCl4 to be bound with the calculated dissociation energy of −14.14, −15.56, and −15.53 eV, respectively. Mulliken population analysis of our Dirac–Fock wave function indicates RfCl4 (Td) to be more volatile than that estimated from the corresponding Hartree–Fock wave function.
Nuclear magnetic resonance study of the internal magnetic field distribution in water base ionic and surfacted ferrofluids
Authors: González Cecilia E. , Pusiol Daniel J. , Figueiredo Neto Antonio M. , Ramia Máximo , Bee Agnes.
Published: July 2002

This work reports a nuclear magnetic resonance (NMR) study of both hydrogen and deuterium on two different kinds of ferrofluids. One of them is a colloidal suspension of surfacted magnetic grains dispersed in a mixture of light and heavy water. The second one is an ionic ferrofluid composed by positively charged magnetic grains of Co–Fe2O4 dispersed in the same solution of water. A model of the local magnetic field distribution in the interstitial (bulk) water is worked out in order to account for the broadening of the NMR spectra observed in both samples.
Response to “Comment on a critique of the instantaneous normal mode (INM) approach to diffusion” [J. Chem. Phys. 109, 4693 (1998)]
Authors: Gezelter J. Daniel , Rabani Eran , Berne B. J.
Published: July 2002

In our original critique [J. Chem. Phys. 107, 4618 (1997)] of the instantaneous normal mode (INM) theory for self-diffusion, we concentrated on atomic Lennard-Jones systems, in which we found a large number of “false-barrier” modes. These are modes which quench to the same local minimum from either side of the imaginary frequency region. We now extend our inquiry to a molecular system (CS2), and find very similar results to what we observed in the atomic system, i.e., a large number imaginary frequency instantaneous normal modes that persist into the crystalline solid.
Simulation of polymers with rebound selection
Authors: Alexandrowicz Z.
Published: July 2002

A crossover from dilute to semidilute regimes of lattice chains has been simulated with the help of a novel method. A Monte Carlo step tries to replace an existing chain by a new chain constructed in biased, excluded volume avoiding steps. Reserve bifurcations are allocated at small intervals of steps and after each interval a selective lottery restores an effectively nonbiased probability distribution. If a construction fails the selection at an nth step, it falls back to n−1, n−2, until it “rebounds” from an unutilized bifurcation. Selection on the go combined with a freedom to rebound, make the construction self-corrective; the principle may have a broad applicability.
Biexciton photoseparation and photoconductivity
Authors: Frantsuzov P. A. , Burshtein A. I.
Published: July 2002

The integral kinetic theory of biexciton photoseparation of charges followed by geminate and bimolecular recombination of ions is developed. The kinetics of charge accumulation and dissipation after δ-pulse excitation is studied, as well as the stationary generated concentration of free carriers at arbitrary excitation lifetimes.
Ab initio calculations of dipole polarizabilities of Na and K in their 32D-state and determination of long-range coefficients for S+D molecular states of Na2, K2, and NaK
Authors: Rérat Michel , Mérawa Mohammadou , Honvault-Bussery Béatrice
Published: July 2002

Dynamic polarizabilities of Na and K in their first excited states are calculated with a time-dependent gauge-invariant (TDGI) method, using one-electron wave functions and pseudo-potentials. The values obtained for Na and K in the low-lying S and P states are compared to our previous all-electron results. Then, the dynamic polarizabilities of Na and K in their 3d D2 state are calculated in order to determine the long-range dispersion coefficients for the dissociative S+D states of Na2, K2, and NaK.
A Born–Green–Yvon integral equation theory for self-interacting lattice polymers
Authors: Taylor Mark P. , Lipson J. E. G.
Published: July 2002

A Born–Green–Yvon (BGY) integral equation is constructed for the end-to-end distribution function of an isolated polymer on a lattice. The polymer is modeled as a self-avoiding walk for which nonbonded sites interact via an attractive nearest-neighbor contact potential. The BGY equation is solved analytically using a Markov approximation for the required three-site distribution function and a delta-function pseudopotential to model the lattice contact potential. The resulting recursive algebraic equation is readily evaluated for a polymer on any Bravais lattice with equal length base vectors. Results are presented for the mean-square end-to-end separation as a function of chain length and contact energy for polymers on several two-, three-, and four-dimensional lattices. The variation of the scaling exponent 2ν with contact energy is used to locate the theta energies for these lattices.
Molecular conformation of a polyaramid in nematic solution from small angle neutron scattering and comparison with theory
Authors: Picken S. J. , Noirez L. , Luckhurst G. R.
Published: July 2002

Zero-average contrast small angle neutron scattering measurements have been performed on an 11%(w/w) nematic aramid polymer (PPTA) solution in concentrated sulphuric acid. The radii of gyration parallel and perpendicular to the director were determined to be 250 and 70 Å, respectively, at room temperature. In addition, the temperature dependence of the radii of gyration was investigated. The values for the radii of gyration are compared to a worm-like path model for the polymer chain statistics where the segment orientation is described using a Maier–Saupe-type orientational distribution function. From this analysis, it is found that the persistence length of the aramid molecules is the same in the nematic phase as in a dilute isotropic phase, about 290 Å. We conclude that there is no effect of the nematic environment on the polymer chain statistics in this main chain polymer solution. As a consequence, the nematic phase of aramid solutions is expected to contain polymer chains with a substantial number of hairpin defects.
Time-dependent self-consistent-field dynamics based on a reaction path Hamiltonian. II. Numerical tests
Authors: Fang Jian-Yun , Hammes-Schiffer Sharon
Published: July 2002

Numerical tests are presented for a method that combines the time-dependent self-consistent-field (TDSCF) method with the reaction path Hamiltonian (RPH) derived by Miller, Handy, and Adams [J. Chem. Phys. 72, 99 (1980)]. The theoretical basis for this TDSCF-RPH method was presented in a previous paper. The equations of motion were derived for three different cases: (1) zero coupling matrix (i.e., zero reaction path curvature and zero coupling between the normal modes); (2) zero reaction path curvature and nonzero coupling between the normal modes; and (3) zero coupling between the normal modes and nonzero but small reaction path curvature. For these three cases the dynamics can always be reduced to a one-dimensional numerical time propagation of the reaction coordinate. In this paper the TDSCF-RPH methodology for all three cases is tested by comparing the TDSCF-RPH dynamics to exact quantum dynamics based on the exact Hamiltonian for simple model systems. The remarkable agreement indicates that the TDSCF-RPH method could be useful for the calculation of the real-time quantum dynamics of a wide range of chemical reactions involving polyatomic molecules.
Finite-difference methods for simulation models incorporating nonconservative forces
Authors: Novik Keir E. , Coveney Peter V.
Published: July 2002

We discuss algorithms applicable to the numerical solution of second-order ordinary differential equations by finite differences. We make particular reference to the solution of the dissipative particle dynamics fluid model, and present extensive results comparing one of the algorithms discussed with the standard method of solution. These results show the successful modeling of phase separation and surface tension in a binary immiscible fluid mixture.