bluuebunny/crossref_metadata_2025 huggingface dataset:
CO adsorption on Pt(111) modified with sulfur
Authors: Kiskinova M. , Szabó A. , Yates J. T.
Published: July 2002

CO adsorption on clean and S-covered Pt(111) was studied using temperature programmed desorption (TPD), electron stimulated desorption ion angular distribution (ESDIAD), LEED, and work function measurements. Special attention was paid to comparing the CO adsorption rate, binding energy, and soft bending modes on a clean surface and on p(2×2) S/Pt(111) with S coverage =0.25 S/Pt. It was found that on p(2×2) 0.25 S/Pt(111), the CO adsorption rate is decreased by a factor of 2 and only one CO adsorption state with maximum coverage, θCO ≂0.25 CO/Pt is detected. On the basis of the ESD data the CO adsorption state on p(2×2) 0.25 S/Pt(111) is assigned to terminal-CO residing on the next nearest Pt atom and separated from S by 3.72 Å. When compared with the same CO configuration on clean Pt(111) in the limit of low θCO, the adsorption binding energy of the terminal CO on sulfided Pt(111) is decreased by 8 kcal/mol. For this same overlayer, the cross sections for production of all ESD products (CO+ , O+ , and metastable CO*) is increased by 30%–50%. This result is interpreted considering the possible S-induced perturbations of the CO–5σ/metal and metal/CO–2π* coupling. An important result in the present study is the observation of a substantial decrease of the polar angle of the ESDIAD patterns of all CO ESD products from the sulfided surface which indicates a decrease of the amplitude of the CO bending modes. This appears to be direct experimental evidence for S-induced stiffening of the soft CO-bending vibrations. Approximate estimations (on the basis of the measured polar angles of the ESDIAD patterns) showed that the frequency of the CO frustrated translational modes increases by about a factor of 2—from 48 cm−1 for CO/Pt(111) to ≃100 cm−1 for CO/p(2×2) 0.25 S/Pt(111).
Resonant two-photon ionization spectroscopy of jet-cooled Pt2
Authors: Taylor Scott , Lemire George W. , Hamrick Yoon Mi , Fu Zhenwen , Morse Michael D.
Published: July 2002

The gas phase optical spectrum of jet-cooled Pt2 has been investigated over the range of 11 300 to 26 300 cm−1 using resonant two-photon ionization spectroscopy in combination with time-of-flight mass spectrometry. Numerous vibronic bands are observed. Analysis of the data gives the location of some 26 excited electronic states, which are characterized by the frequencies of their origin bands, vibrational frequencies, and anharmonicities. Variation of the second color in a two-color resonant two-photon ionization scheme has determined the ionization threshold of Pt2 to be 8.68±0.02 eV. The observation of the onset of predissociation, characterized by a sharp drop in excited state lifetime, places the dissociation energy of Pt2 at 3.14±0.02 eV. In combination with the Pt atomic ionization potential of 8.8±0.2 eV, these results give the bond strength of Pt+2 as D0(Pt−Pt+)=3.26±0.24 eV. The strength of the chemical bond in Pt2, as compared to Au2, demonstrates that there are significant 5d contributions to the chemical bonding in Pt2.
Collision-induced dissociation of Fe+n (n=2–10) with Xe: Ionic and neutral iron binding energies
Authors: Loh S. K. , Hales David A. , Lian Li , Armentrout P. B.
Published: July 2002

Cross sections for collision-induced dissociation (CID) of Fe+n with Xe, 2≤n≤10, are presented. Experiments were performed on a newly constructed guided ion beam mass spectrometer, the design and capabilities of which are described in detail. The single mechanism for dissociation of iron cluster ions is sequential loss of iron atoms with increasing collision energies. There is no evidence for fission to molecular neutral products. The cross section threshold energy dependences are analyzed to give the bond dissociation energies (BDEs), D0(Fe+n−1–Fe). Data analysis employs an empirical model that incorporates RRKM theory to account for inefficient dissociation on the time scale of the experiment. Results show that Fe+6 has the strongest BDE, D0(Fe+5–Fe) =3.44±0.18 eV, while Fe+3 is the most weakly bound, D0(Fe+2–Fe) =1.64±0.15 eV. Neutral cluster BDEs are derived from ionic binding energies and known ionization potentials. Branching ratios and other cross section features are also discussed with respect to cluster size.
Predictions of birefringence and dichroism of hard sphere suspensions in combined electric and shear fields
Authors: Adriani Paul M. , Gast Alice P.
Published: July 2002

We present the optical anisotropy of a hard sphere suspension subjected to combined electric and shear fields. We model the particle structure factor as a shear induced perturbation of the anisotropic structure induced by electric field. We renormalize the nonconvergent birefringence integral to allow calculation of birefrigence from the particle structure factor. The principle axes of the birefringence and dichroism tensors vary with the Mason number, a measure of the relative strength of shear forces to electric dipole interaction forces. The principle axes only coincide in the shear dominated and the electric field dominated limits. The magnitudes of birefringence and dichroism are sensitive to the particle size relative to the wave-length of light employed. We compare theoretical predictions in the limit of pure shear to the experiments of Wagner et al., J. Chem. Phys. 89, 1580 (1988), employing a simple affine deformation theory for the structure factor, which yields fair agreement with experiment for the dichroism but poor agreement for the birefringence. The prediction of optical properties provides both a test for theories of microstructure and insight into the effects of shear and electric fields on microstructure.
Spectroscopy and electronic structure of jet-cooled GaAs
Authors: Lemire George W. , Bishea Gregory A. , Heidecke Scott A. , Morse Michael D.
Published: July 2002

An optical spectrum, obtained by resonant two-photon ionization spectroscopy, is reported for jet-cooled diatomic gallium arsenide. The ground state is identified as X 3Σ−, deriving from a σ2π2 molecular configuration, and is characterized by ω″e=215 cm−1, ω″ex″e=3 cm−1, and r″0=2.53±0.02 Å. The upper state of the observed band system is 3Πr correlating to the Ga 4s24p, 2P0+As 4s24p3, 2D0 excited separated atom limit. A strong predissociation sets in above v′=0 for the Ω′=2,1 and 0− components of the 3Πr excited state, and it is proposed that this is induced by spin–orbit interaction with the σσ*π2, 5Σ− state which correlates to ground state atomic fragments. Constants for the upper 3Π0+ state are ω′e=152.13±0.70 cm−1, ωex′e=2.89±0.08 cm−1, and re=2.662±0.027 Å for the 69Ga75As isotopic modification. The ionization potential of GaAs has been bracketed as IP(GaAs)=7.17±0.75 eV, and a re-evaluation of the third-law measurement of the bond strength provides D0(GaAs)=2.06±0.05 eV. Comparisons to group IV and other group III-V diatomics, and to the bulk solid materials are also presented.
Evidence for an influence of rotational energy on the rate constants for the reaction of Ar+(2P3/2) with N2
Authors: Viggiano A. A. , Van Doren Jane M. , Morris Robert A. , Paulson John F.
Published: July 2002

Rate constants for the charge transfer reaction of Ar+(2P3/2) with N2 were measured as a function of average center-of-mass kinetic energy (〈KEcm〉) at three temperatures. The rate constants increase rapidly with (〈KEcm〉) at all temperatures. The thermal activation energy is derived to be 0.07 eV, which is approximately the value of the endothermicity of the reaction when N+2 (v=1) is produced. The rate constants vary with temperature at a particular 〈KEcm〉, indicating either an effect due to differing energy distributions in the drift tube at different temperatures or that the reactivity depends on the rotational temperature of N2. Comparing the data at constant average total energy indicates that translational and rotational energy have a similar effect on the reactivity.
Wave-packet dynamics within the multiconfiguration Hartree framework: General aspects and application to NOCl
Authors: Manthe U. , Meyer H.-D. , Cederbaum L. S.
Published: July 2002

The multiconfigurational time-dependent Hartree (MCTDH) approximation to the time-dependent Schrödinger equation is tested for a realistic three-dimensional example, the photodissociation of NOCl. The working equations of the MCTDH scheme introduced earlier are discussed in some detail. A computational scheme is presented which allows for efficient numerical MCTDH calculations. This scheme is applied to the photodissociation of NOCl after excitation to the S1 surface. The results are compared to the results of an exact wave-packet dynamics calculation. Fast convergence of the MCTDH results toward the exact one is found as the number of configurations is increased. The computation times of the MCTDH calculations are found to be much shorter than those of the exact calculation. Even MCTDH calculations including sufficiently many configurations for a fully converged (quasiexact) description require over two orders of magnitude less CPU time than an exact calculation. The so-called ‘‘natural populations’’ that are computed along with the MCTDH wave packet serve as a check of the degree of convergence without the need to resort to an exact (or more accurate) calculation.
Fast reaction products from the oxidation of CO on Pt(111): Angular and velocity distributions of the CO2 product molecules
Authors: Allers K.-H. , Pfnür H. , Feulner P. , Menzel D.
Published: July 2002

Angular and velocity distributions of CO2 desorbing as reaction product of CO oxidation on Pt(111) were measured during heating of layers of initially molecular oxygen and CO adsorbed at a surface temperature of 100 K. In the velocity integrated desorption spectra of the reaction product CO2 four different peaks (α, β3, β2, β1) can be discriminated which, for linear heating rates of 5 K/s, appear at 145, 210, 250, and 330 K, respectively. They can be attributed to different reaction mechanisms which depend on the binding conditions of oxygen and the geometric arrangement and coverages of both species. Whereas α-CO2 coincides with the O2 desorption from and the dissociation of pure chemisorbed molecular oxygen, and thus indicates a reaction channel coupled with desorption and dissociation of O2, β1-CO2 corresponds to the reaction path investigated before by many researchers and is most likely due to the reaction at the boundaries of ordered CO and oxygen islands. The structural conditions for β3 and β2 are less clear, but we believe them to stem from reactions in mixed and/or partly mixed layers at high coverages of O and CO. The α-CO2 species is most likely due to reaction of CO with O atoms stemming from O2 dissociation which react before becoming accommodated.

The velocity distributions of α, β2, and β3 are far from thermal equilibrium with the surface as indicated by average kinetic energies between 220 and 360 meV, corresponding to ≂10 (for β3 and β2) and ≂30 kTs (for α), normalized speed ratios between 0.6 and 0.8, and strongly peaked angular distributions (∼cosn ϑ, n=8 for α, n≳10 for β3 and β2). For β1 both the angular and velocity distributions show bimodal behavior with one channel fully accommodated to the surface whereas the other contains again an appreciable amount of reaction energy as kinetic energy (〈E〉≂330 meV) resulting in a strongly peaked angular distribution with n≂9. Some TOF results for steady state reaction at high temperatures (420–800 K) obtained in the same apparatus are given for comparison. The fraction of reaction energy channelled into the translational degree of freedom for the nonequilibrated part of reaction peak β1 is estimated to about 40%. A discussion of the various possible mechanisms is given.
Raman spectra of mass-selected cobalt dimers in argon matrices
Authors: Dong Jian-Guo , Hu Zhendong , Craig Robert , Lombardi John R. , Lindsay D. M.
Published: July 2002

We report absorption and Raman spectra for cobalt dimers in an argon matrix prepared by the mass-selected ion deposition technique. A weak dimer absorption band was found between 425 and 485 nm. Resonance Raman spectra (obtained by exciting between 457.9–488.0 nm) give ωe″= 296.8(54) cm−1 with ωexe″ = 2.2(8) cm−1. The dicobalt data are discussed in terms of results from recent photodetachment experiments which suggest a changeover from d–d bonding to s–s bonding along the series Fe2→Cu2.
Vibrational spectroscopy of polypyrrole, theoretical study
Authors: Kostić R. , Raković D. , Stepanyan S. A. , Davidova I. E. , Gribov L. A.
Published: July 2002

Theoretical vibrational spectra of polypyrrole are presented, based on a single-periodic-chain model and harmonic potential in-plane and out-of-plane parameters transferred from the pyrrole molecule. The infrared-absorption intensities were included in the calculations for the first time in polypyrrole. Experimental spectra are analyzed and compared with theoretical ones. Some modes in the experimental spectra indicate the presence of nonplanar conformation. Also, Raman frequencies were compared with experimental ones. Significant effects of electron–phonon coupling are absent.
Calculations of the tunneling splittings in water dimer and trimer using diffusion Monte Carlo
Authors: Gregory Jonathon K. , Clary David C.
Published: July 2002

The diffusion Monte Carlo (DMC) method is used to calculate rovibrational bound states of the water dimer and trimer. The rigid body form of DMC is employed, together with correlated sampling of energy differences between states of different symmetry. This allows calculation of the tunneling splittings in (H2O)2 and (H2O)3. The results for (H2O)2 are in quite good agreement with those obtained using a basis set method, and also agree well with experiment. In addition, we have made predictions for similar splittings in (D2O)3 and several water dimer isotopomers. In all the calculations, we have used the potential energy surface due to Millot and Stone which is known to give quite good agreement with experiment for the tunneling splittings in (H2O)2.
Tensor propagator for iterative quantum time evolution of reduced density matrices. I. Theory
Authors: Makri Nancy , Makarov Dmitrii E.
Published: July 2002

For common condensed phase problems described by a low-dimensional system coupled to a harmonic bath, Feynman’s path integral formulation of time-dependent quantum mechanics leads to expressions for the reduced density matrix of the system where the effects of the harmonic environment enter through an influence functional that is nonlocal in time. In a recent Letter [Chem. Phys. Lett. 221, 482 (1994)], we demonstrated that the range of the nonlocal interactions is finite even at zero temperature, such that the nonlocal kernel extends over only a few time steps if the path integral is expressed in terms of accurate quasiadiabatic propagators. This feature arises from disruption of phase coherence in macroscopic environments and leads to Markovian dynamics for an augmented reduced density tensor, permitting iterative time evolution schemes. In the present paper we analyze the structure and properties of the relevant tensor propagator. Specifically, we show that the tensor multiplication scheme rigorously conserves the trace of the reduced density matrix, and that in cases of short-range nonlocality it leads to Redfield-type equations which are correct to all orders in perturbation theory and which take into account memory effects. We also argue that a simple eigenvector analysis reveals (without actual iteration) the nature of the dynamics and of the equilibrium state, and directly yields quantum reaction or relaxation rates.
Local treatment of electron correlation in coupled cluster theory
Authors: Hampel Claudia , Werner Hans-Joachim
Published: July 2002

The closed-shell coupled cluster theory restricted to single and double excitation operators (CCSD) is formulated in a basis of nonorthogonal local correlation functions. Excitations are made from localized molecular orbitals into subspaces (domains) of the local basis, which strongly reduces the number of amplitudes to be optimized. Furthermore, the correlation of distant electrons can be treated in a simplified way (e.g., by MP2) or entirely neglected. It is demonstrated for 20 molecules that the local correlation treatment recovers 98%–99% of the correlation energy obtained in the corresponding full CCSD calculation. Singles-doubles configuration interac- tion (CISD), quadratic configuration interaction (QCISD), and Mo/ller–Plesset perturbation theory [MP2, MP3, MP4(SDQ)] are treated as special cases.
Separation of carbon dioxide–methane mixtures by adsorption: Effects of geometry and energetics on selectivity
Authors: Nicholson David , Gubbins Keith E.
Published: July 2002

The adsorption of methane–carbon dioxide mixtures in micropores having either slit or cylindrical geometries, and different adsorption energies, has been studied using molecular simulation. The pore models were constructed from close packed layers of simple atoms whose Lennard-Jones interaction parameters could be varied. For realistic choices of the adsorbent parameters, adsorption selectivity outside the molecular sieving regime was found to vary over a range between about 4 and about 1/4, denoting a preference for either CO2 or for methane, depending on geometry and choice of parameters. The variations in selectivity are interpreted in terms of a balance between energetic factors, including adsorbate–adsorbent interactions and cooperative adsorbate interactions, and entropic factors associated with the constraints on rotational freedom of the CO2 molecule imposed mainly by the geometry of the pore spaces.
NMR field gradient diffusometry of segment displacements in melts of entangled polymers
Authors: Fischer Elmar , Kimmich Rainer , Fatkullin Nail
Published: July 2002

Segment diffusion in a polyethyleneoxide melt (Mw = 5 000 000) was studied with the aid of the supercon fringe field version of field-gradient NMR diffusometry. The evaluation based on the second moment of the probability density function, i.e., the mean squared displacement, shows reasonable agreement with the predictions of the tube/reptation model. However, taking into account the whole probability density function, leads to substantial discrepancies.
Unified time-path approach to the effect of anharmonicity on the molecular vibrational spectroscopy in solution
Authors: Okumura K. , Tanimura Y.
Published: July 2002

We obtain a closed expression for the response function for damped anharmonic vibrational modes using the Feynman rule obtained through the nonequilibrium generating functional derived in our previous paper [Phys. Rev. E 53, 214 (1996)]. The linear absorption spectra are explicitly calculated through the Feynman diagrams for molecules with anharmonic potential in solution. The dependence of the spectra both on the temperature and on the solvent nature can be taken into account in our theory. The result is examined numerically for various dampings, anharmonicities, and temperatures.
Comparison of shear flow of hexadecane in a confined geometry and in bulk
Authors: Stevens Mark J. , Mondello Maurizio , Grest Gary S. , Cui S. T. , Cochran H. D. , Cummings P. T.
Published: July 2002

We examine the shear flow of hexadecane confined between plates with separation of 1–10 nm using molecular dynamics simulations. We also performed non-equilibrium molecular dynamics (NEMD) simulations of bulk hexadecane to compare with the simulations in the confined geometry. The stiffness of hexadecane and its high melting temperature result in a tendency to crystallize at room temperature or large load. We find that when confined between hydrocarbon walls, shearing hexadecane exhibits a velocity profile with substantial slip at the wall and essentially constant velocity over most of the interior space between the walls. As the strength of the wall-fluid interaction increases the amount of slip decreases, but slip always occurs at the boundary for the range of parameters studied. The results are compared with recent surface force apparatus experiments on hexadecane and with similar simulations of model bead-spring fluids.
Smoluchowski-type theory of stochastically gated diffusion-influenced reactions
Authors: Berezhkovskii Alexander M. , Yang Dah-Yen , Lin Sheng Hsien , Makhnovskii Yu. A. , Sheu Sheh-Yi
Published: July 2002

The Smoluchowski–Collins–Kimball theory of irreversible diffusion-influenced reactions with one of the reactants in excess is generalized to the case of stochastic gating when one of the reactants can be in one of M states. Distinct states are characterized by various efficiencies of the reaction of contacting partners. General expressions are derived for the rate constant and for the survival probability of the reactant which is in deficiency. We present these quantities in terms of the solution of the isolated pair problem. The difference between the cases when gating is due to the reactant, which is in excess, and one, which is in deficiency, is explicitly demonstrated. General relationships between the rate constants and the survival probabilities in the two cases are established. We show that in the former case the reaction goes faster compared to the latter one. To make the problem treatable analytically in the case when gating is due to the reactant which is in deficiency, a partial mean-field approximation is introduced. General theory is applied to a particular case of the two-state gating model. Explicit analytical solutions for the time-dependent rate constant and the survival probability are obtained in one dimension. They illustrate the general theory as well as show how the kinetics depends on the jump rate between the two states of the gate in both cases when gating is due to the reactant, which is in excess, and one, which is in deficiency.
Spin diffusion in melts of entangled polymers
Authors: Fischer Elmar , Kimmich Rainer , Fatkullin Nail
Published: July 2002

Based on theoretical considerations [N. F. Fatkullin, Sov. Phys. JETP 72, 563 (1991)], immaterial spin diffusion mediated by flip–flop transitions of dipolar coupled spins on different macromolecules was predicted to influence the diffusion coefficient measured in nuclear magnetic resonance field-gradient experiments. In order to test this hypothesis, we have carried out supercon fringe field proton magnetic resonance diffusometry experiments with polyethylene oxide melts (Mw=438 000) using field gradients of up to 60 T/m. The polymer chains were dispersed in a matrix of deuterated chains of an equivalent molecular mass. The time-dependent segment diffusion coefficients measured in the diluted and undiluted polymer coincided for diffusion times below about 200 ms. However, increasing the diffusion time up to 1 s leads to a reduction of the diffusion coefficient in the deuterated matrix by a factor of about 2 relative to the undeuterated system. The long-time diffusion coefficient measured with long polymer chains, which are subject to interchain spin couplings, is obviously strongly influenced by spin diffusion mediated by flip–flop transitions of dipolar coupled spin pairs. This holds true in spite of the relatively long flip–flop time, which is estimated to be of the order 0.1 s.
Survival of a walker in gated trapping systems
Authors: Sheu Wen-Shyan , Yang Dah-Yen , Sheu She-Yi
Published: July 2002

A first-passage-time-distribution (FPTD) approach is developed to investigate the survival and derived properties of a random walker in discrete lattices with a static trap gauged by a general gating mechanism. This approach is effective since the FPTD is directly related to the survival probability distribution of the walker. The random walk is allowed to be undertaken under any potential fields, such as an electric field. We find the gated FPTD can be exactly expressed in terms of its corresponding ungated FPTD in any dimension. Hence, the survival statistics can be calculated. Two gating mechanisms, Poisson and periodic gating, are explicitly considered to calculate their FPTDs, respectively. From the distributions, their mean first passage times (MFPTs) or mean survival times, and mean numbers of visits (MNVs) needed for the walker to become trapped are calculated. Based on the results of these two gating mechanisms, we conclude that the gated MFPT is equal to the sum of the ungated MFPT to the trap starting from the initial site, and the ungated MFPT to the trap starting from the trap multiplied by MNV-1. We argue that this statement founds the basis of approximations to other more complicated gated systems.
A multidimensional Landau-Zener description of chemical reaction dynamics and vibrational coherence
Authors: Zhu Leyun , Widom Allan , Champion Paul M.
Published: July 2002

We discuss the observation of coherent nuclear oscillations in chemical and biological systems subsequent to excitation with an ultrashort laser pulse. The experimentally observed coherent signals arising from rapid, photon initiated, chemical reactions are considered separately from vibrational coherences directly excited by the (electric) field interventions of the light pulse. In order to treat the chemical reaction, a modified Landau-Zener theory is developed that is quasi-classical along the traditional reaction coordinate but fully quantum mechanical in all other degrees of freedom. In this work, we explore explicitly the effects of electronic state coupling, electron-nuclear coupling, reaction velocity, vibrational damping and temperature on the product electronic state population development and on the coherent oscillations of the nuclear coordinates. Recent experimental studies of the ligand photolysis reaction in heme proteins are presented as examples.
Stochastic gating influence on the kinetics of diffusion-limited reactions
Authors: Makhnovskii Yurii A. , Berezhkovskii Alexander M. , Sheu Sheh-Yi , Yang Dah-Yen , Kuo Jimmy , Lin Sheng Hsien
Published: July 2002

We study how the kinetics of diffusion-influenced reactions is modified when the reactivity of species fluctuates in time (stochastically gated) with emphasis on the many-particle aspect of the problem. Because of the fact that the dynamics of ligand binding to proteins originally motivated the problem, it is considered in that context. Recently, Zhou and Szabo [J. Phys. Chem. 100, 2597 (1996)] have demonstrated many-particle effects in the problem and found that the kinetics of reaction between a gated protein with a large number of ligands significantly differs from that between a protein and gated ligands. With our approach, the difference between the kinetics of ligand-gated and protein-gated reactions appears formally the same as the difference between the target and trapping problems despite the origin of the corresponding effects and their manifestations are distinctly different. A simple approximate method to treat the many-particle effects is proposed. The theory is applied to a particular two-state gating model. Explicit analytical expressions for the protein survival probability are obtained. We show that (1) for ligand-gated reactions, gating is effectively accounted for by the appropriate reduction of the species reactivity and (2) for protein-gated reactions, the survival probability changes its time behavior from exponential (fast gating) to nonexponential (slow gating). The role of intensity and asymmetry of the gate motion is discussed.
Semiclassical approximations for the calculation of thermal rate constants for chemical reactions in complex molecular systems
Authors: Wang Haobin , Sun Xiong , Miller William H.
Published: July 2002

Two different semiclassical approaches are presented for extending flux correlation function methodology for computing thermal reaction rate constants, which has been extremely successful for the “direct” calculation of rate constants in small molecule (∼3–4 atoms) reactions, to complex molecular systems, i.e., those with many degrees of freedom. First is the popular mixed quantum-classical approach that has been widely used by many persons, and second is an approximate version of the semiclassical initial value representation that has recently undergone a rebirth of interest as a way for including quantum effects in molecular dynamics simulations. Both of these are applied to the widely studied system-bath model, a one-dimensional double well potential linearly coupled to an infinite bath of harmonic oscillators. The former approximation is found to be rather poor while the latter is quite good.
Ab initio quantum chemistry using the density matrix renormalization group
Authors: White Steven R. , Martin Richard L.
Published: July 2002

In this paper we describe how the density matrix renormalization group can be used for quantum chemical calculations for molecules, as an alternative to traditional methods, such as configuration interaction or coupled cluster approaches. As a demonstration of the potential of this approach, we present results for the H2O molecule in a standard gaussian basis. Results for the total energy of the system compare favorably with the best traditional quantum chemical methods.
Quantitative characterization of the water trimer torsional manifold by terahertz laser spectroscopy and theoretical analysis. II. (H2O)3
Authors: Brown Mac G. , Viant Mark R. , McLaughlin Ryan P. , Keoshian Christy J. , Michael Ernest , Cruzan Jeff D. , Saykally Richard J. , van der Avoird Ad
Published: July 2002

We report the measurement of two new (H2O)3 bands by terahertz laser vibration–rotation–tunneling (VRT) spectroscopy. Both bands have been assigned to torsional (“pseudorotational”) transitions and are highly perturbed by Coriolis interactions. The 42.9 cm−1 band corresponds to the k=±2←±1 transition while the 65.6 cm−1 band corresponds to the k=±2←0 transition. A model Hamiltonian is derived which allowed a global fit of 361 VRT transitions of these two new bands and the previously reported torsional band at 87.1 cm−1. Each of the bifurcation tunneling components is accurately described. This global fit represents a complete description of the VRT transitions of (H2O)3 up to 150 cm−1, and complements our similar treatment of the (D2O)3 torsional dynamics.
Calculated and experimental rotational constants of (H2O)3: Effects of intermolecular torsional and symmetric stretching excitations
Authors: Sabo Dubravko , Bačić Zlatko , Graf Stephan , Leutwyler Samuel
Published: July 2002

Calculations of the torsional state dependence of the A=B and C rotational constants of (H2O)3 are reported, for torsional excitations up to 144 cm−1 above the ground state, extending our work on the rotational constants of all isotopomers of water trimer using an ab initio four-dimensional torsional-stretching intermolecular potential energy surface [D. Sabo et al., J. Chem. Phys. 110, 5745 (1999)]. Direct comparison was made to the rotational constants of (H2O)3 recently measured and analyzed by Brown et al. [J. Chem. Phys. 111, in press (1999)], for the lowest five torsional levels n=0–5, with torsional energies up to 80 cm−1. The results of the (3+1)-dimensional torsional-stretching calculations are in excellent agreement with experiment. The rotational constants of the next higher torsional level, n=6, were calculated by the (3+1)-dimensional method. A comparison is made to the rotational constants calculated by a three-dimensional purely torsional model, which yields inferior results. In order to consistently reproduce the pronounced variations of the A=B and C rotational constants from one torsional eigenstate to the other, theory must take into account both the changes of rms torsional angles of the “free” O–H bonds and the changes in the oxygen–oxygen separation which accompany torsional excitation.
Dissociative recombination of D3O+ and H3O+: Absolute cross sections and branching ratios
Authors: Neau A. , Al Khalili A. , Rosén S. , Le Padellec A. , Derkatch A. M. , Shi W. , Vikor L. , Larsson M. , Semaniak J. , Thomas R. , Någård M. B. , Andersson K. , Danared H. , af Ugglas M.
Published: July 2002

Dissociative recombination of the polyatomic ions D3O+ and H3O+ with electrons have been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). Absolute cross sections have been determined from 0.001 eV to 0.25 eV center-of-mass energy for D3O+ and from 0.001 eV to 28 eV for H3O+. The cross sections are large (7.3×10−13 cm2 for D3O+ and 3.3×10−12 cm2 for H3O+ at 0.001 eV). At low energies, the cross sections for D3O+ are E−1 energy dependent whereas it is slightly steeper for H3O+. A similar E−1 energy dependence was also observed by Mul et al. [J. Phys. B 16, 3099 (1983)] with a merged electron-ion beam technique for both H3O+ and D3O+ and by Vejby-Christensen et al. [Astrophys. J. 483, 531 (1997)] with the ASTRID storage ring in Denmark, who presented relative cross sections for H3O+. A resonance has been observed around 11 eV for H3O+. It reflects an electron capture to Rydberg states converging to an excited ionic core. A similar structure was reported by Vejby-Christensen et al. Our absolute measurements are in fairly good agreement with those from Mul et al., which were first divided by 2 (Mitchell, 1999, private communication) and from Heppner et al. [Phys. Rev. A 13, 1000 (1976)] for H3O+. Thermal rates were deduced from the measured cross sections for electron temperatures ranging from 50 K to 30 000 K. At 300 K, the thermal rate is equal to 7.6×10−7 cm3 s−1 for H3O+ and to 3.5×10−7 cm3 s−1 for D3O+. Complete branching ratios for all the possible product channels have been determined from 0 eV to 0.005 eV center-of-mass energy for D3O+ and at 0 eV for H3O+, using a well-characterized transmission grid in front of an energy-sensitive surface-barrier detector. No isotope effect was observed within the experimental uncertainties. The three-body break-up channel OX+X+X (where X stands for H or D) is found to occur for 67%–70% of the dissociations. Water or heavy water is produced with an 18%–17% probability and the production of oxygen atoms is negligible. These results support the three-body break-up dominance already found by Vejby-Christensen et al. for the DR of H3O+ in a similar heavy-ion storage ring experiment. However, even if the general trend is the same for both storage rings, significant differences have been observed and will be discussed.
The influence of particle size on the magnetorheological properties of an inverse ferrofluid
Authors: de Gans B. J. , Duin N. J. , van den Ende D. , Mellema J.
Published: July 2002

The dependence of magnetorheological properties on particle size is studied, using a model magnetorheological fluid consisting of nonmagnetic silica particles suspended in a ferrofluid. For small particles a strong increase of magnetorheological properties with particle size was found. At a certain particle size and field strength a crossover occurs to a regime where there is only a very limited size dependence. This can be understood in terms of the length of the chains in relation to the gap size of the geometry. Crossover occurs when the average chain length becomes comparable to the gap size. The remaining size dependence may be explained by polydispersity, variations in the morphology, variations in the sterically stabilizing layer of the particles and surface roughness of the geometry.
The secondary instability in Goertler flow
Authors: Yu Xiuyang , Liu J. T. C.
Published: July 2002

The ‘‘high-frequency’’ secondary instability of nonlinearity developed and strongly modified Goertler vortices is numerically studied by considering two kinds of secondary instability modes, i.e., sinuous modes and varicose modes. A spectral collocation method with Chebyshev polynomials is used for calculations. It is found that secondary instabilities are more correlated with the vertical vorticity and the sinuous type disturbance would prevail over the varicose one for the same frequency and streamwise wave number found in experiments.
Multifrequency excited jets
Authors: Mankbadi Reda R.
Published: July 2002

Nonlinear wave–wave interactions in turbulent jets were investigated based on the integrated energy of each scale of motion in a cross section of the jet. The analysis indicates that two frequency components in the axisymmetric mode can interact with other background frequencies in the axisymmetric mode, thereby amplifying an enormous number of other frequencies. Two frequency components in a single helical mode cannot, by themselves, amplify other frequency components. But combinations of frequency components of helical and axisymmetric modes can amplify other frequencies in other helical modes. The present computations produce several features consistent with experimental observations on two-frequency excitation such as (1) dependence of the interactions on the initial phase differences between the waves, (2) enhancement of the momentum thickness under multifrequency forcing, and (3) the increase in background turbulence under forcing. In a multifrequency-excited turbulent jet the mechanisms contributing to the development of the jet are identified. Mixing enhancement was found to result from the turbulence enhancement rather than simply from the amplification of forced wave components. The excitation waves pump energy from the mean flow to the turbulence, thus enhancing the latter. The high-frequency waves enhance the turbulence close to the jet exit, while the low-frequency waves are most effective farther downstream.
Numerical evidence of smooth self-similar dynamics and possibility of subsequent collapse for three-dimensional ideal flows
Authors: Brachet M. E. , Meneguzzi M. , Vincent A. , Politano H. , Sulem P. L.
Published: July 2002

Direct numerical simulations of the three-dimensional Euler equations at resolutions up to 2563 for general periodic flows and 8643 for the symmetric Taylor–Green vortex are presented. The spontaneous emergence of flat pancakelike structures that shrink exponentially in time is observed. A simple self-similar model that fits these observations is discussed. Focusing instabilities similar to those leading to streamwise vortices in the context of free shear layers [J. Fluid Mech. 143, 253 (1984)], are expected to subsequently concentrate the vorticity and produce isolated vortex filaments. A finite time singularity for the Euler equation is not excluded as the result of interactions among these filaments.
Saturation of a single mode driven by an energetic injected beam. III. Alfvén wave problem
Authors: Berk H. L. , Breizman B. N.
Published: July 2002

The saturation amplitude of Alfvén waves, excited by alpha particles produced in an ignited tokamak, is estimated. The formalism that has been developed to describe the saturation of a single mode is generalized to toroidal geometry. The saturation level is estimated for the toroidal Alfvén gap mode. The alpha particle radial flux resulting from the finite wave amplitude is found to produce relatively weak energy losses compared to the usual energy drag losses.
Experimental demonstration of compact torus compression and acceleration
Authors: Hammer James H. , Eddleman James L. , Hartman Charles W. , McLean Harry S. , Molvik Arthur W.
Published: July 2002

Tests of compact torus (CT) compression on the RACE device [Phys. Rev. Lett. 61, 2843 (1988)] have successfully demonstrated stable compression by a factor of 2 in radius, field amplification by factors of 2–3 to 20 kG, and compressed densities exceeding 1016 cm−3. The results are in good agreement with two-dimensional magnetohydrodynamic simulations of the CT dynamics. The CT is formed between a pair of coaxial conical conductors that serve as both a flux conserver for stable, symmetric formation and as electrodes for the compression and acceleration phases. The CT is compressed by J×B forces (poloidal current, toroidal field) when a 120 kV, 260 kJ capacitor bank is discharged across the electrodes. The CT reaches two-fold compression to a radius of 8 cm and a length of 20–30 cm near the time of peak current, 10 μsec (many Alfvén times) after the accelerator fire time, and is subsequently accelerated in a 150 cm straight coaxial section to velocities in the range 1.5–6.5×107 cm/sec. A new set of acceleration/focusing electrodes 740 cm in length are projected to give an additional factor of 3 in radial compression with final velocities of 1–3×108 cm/sec (similar to previously achieved on RACE) and incident power densities of a few ×1011 W/cm2. Compact torus accelerators scaled to multimegajoules have the potential to achieve unprecedented plasma velocities and power densities with many applications in high-energy-density physics.
Two-dimensional dynamics of the plasma wakefield accelerator
Authors: Keinigs Rhon , Jones Michael E.
Published: July 2002

A general analysis of the electromagnetic wakefields for an axisymmetric charge distribution moving through a cold uniform plasma is presented. Particular attention is given to the electromagnetic and relativistic effects influencing the waves. It is shown that the plasma provides shielding of the transverse wakefields on a scale length of a few electromagnetic skin depths (c/ωp). The implications for plasma wakefield accelerators are that driving beams with radii of a few skin depths will experience severe pinching, whereas larger beams will be subject to filamentation instabilities. These conclusions are supported by self-consistent particle-in-cell simulations. The deleterious effects of transverse wakefields will severely limit the accelerating gradients that can be obtained even if a magnetic field is used to guide the driving beam.
The effect of hydrodynamic interactions on the orientation of axisymmetric particles flowing through a fixed bed of spheres or fibers
Authors: Shaqfeh E. S. G. , Koch Donald L.
Published: July 2002

Averaged equations describing the evolution of the orientation distribution in a dilute suspension of axisymmetric ‘‘tracer’’ particles flowing through a fixed bed of spheres or fibers are derived. The orientations of the freely suspended ‘‘tracer’’ particles change because of the stochastically fluctuating velocity field induced by the randomly distributed fixed-bed particles. In the limit of small fixed-bed volume fraction the orientation distribution satisfies a local orientational advective-diffusion equation with an effective rotary diffusivity and ‘‘drift’’ velocity, which are functions of the orientation of the particle relative to the direction of the average flow. Solutions of this equation show that the tracer particles tend to align with their ‘‘thin side’’ toward the direction of the bulk flow and that the degree of this alignment depends on the aspect ratio of the particle.
A microscopic model of electrorheology
Authors: Adriani Paul M. , Gast Alice P.
Published: July 2002

An electrorheological fluid is modeled as a concentrated suspension of hard spheres with aligned field-induced electric dipole moments. The presence of dipole moments causes clustering of the particles into an anisotropic suspension characterized by a particle probability distribution function. The elastic shear modulus and the dynamic viscosity are calculated from a perturbation to the particle distribution as a result of a small amplitude, high frequency, oscillatory flow. The high frequency elastic shear modulus and the dynamic viscosity are shown as a function of particle concentration and electric dipole strength. Both the modulus and the viscosity increase strongly with particle concentration. Dynamic viscosity is insensitive to dipole strength, but elastic shear modulus increases strongly with dipole strength indicating the relative importance of the particle distribution to elastic properties. The results of these calculations provide insight into the relationship between the macroscopic properties of an electrorheological fluid and microscopic structural changes.
On the mechanism of sinuous and varicose modes in three-dimensional viscous secondary instability of nonlinear Görtler rolls
Authors: Yu Xiuyang , Liu Joseph T. C.
Published: July 2002

The present study begins with the solution of the linearized partial differential equations for viscous, secondary instabilities of nonlinearly developed primary Görtler rolls. A global energy balance verifies the amplification rates obtained from secondary instability analysis and confirms that the sinuous mode would dominate. This therefore explains the strong resemblance between the rms-streamwise u-velocity structure obtained by hot-wire measurements [Swearingen and Blackwelder, J. Fluid Mech. 182, 255 (1987)] in the cross-sectional yz plane and that of the sinuous mode. The u contours of the secondary disturbance have peaks at locations corresponding to the shoulder regions of the mushroom-like primary streamwise U-velocity structure and even more intense regions surrounding the mushroom stem near the wall. This structure is explained by detailed analysis of the energy-balancing mechanisms for u2/2 in yz plane: the Reynolds stress-conversion mechanism predominantly affects the primary spanwise rate of shear strain ∂U/∂z, whereas of less importance is the effect on the normal rate of shear strain ∂U/∂y; these are delicately balanced by the isotropizing mechanism of pressure rate of strain correlation and viscous dissipation.

The appropriate pressure rates of strain correlation then provide the sources for the vertical v2/2 and spanwise w2/2 contributions to the kinetic energy. The sinuous mode can be identified with streamwise-propagating structures having spanwise waviness in flow visualization studies. The spanwise-antisymmetric w, which has intense regions in the upper and lower levels of the boundary layer, tends to wag the primary mushroom structure both at the upper domed region and in the stem region near the wall, not necessarily in phase, in the spanwise direction in the y,z plane, as shown in the flow-visualization work of Peerhossaini and Wesfreid [Intl. J. Heat Fluid Flow 9, 12 (1988)]. On the other hand, the infrequency of the varicose mode can be attributed to its relatively lower amplification rate for its most amplified mode. When observed, it is identifiable with the spanwise-symmetric streamwise propagating, repetitive ‘‘horseshoe’’ structures appearing in flow visualization. Energy balance analysis reveals that this mode is attributable mainly to the Reynolds stress-conversion mechanism associated with rate of strain ∂U/∂y in the high shear layer in the peak region, although the mechanism associated with ∂U/∂z is not dramatically smaller. Viscous dissipation, though not large enough to overcome the production mechanisms, is nevertheless prominent in the global balance as well as in the detailed structural balances in the yz plane. The secondary instability vorticity structure indicates that the sinuous mode is dominated by its vertical component ηs and the varicose mode by its spanwise component ζv.
Reversal in spreading of a tabbed circular jet under controlled excitation
Authors: Zaman K. B. M. Q. , Raman G.
Published: July 2002

Detailed flow field measurements have been carried out for a turbulent circular jet perturbed by tabs and artificial excitation. Two “delta tabs” were placed at the nozzle exit at diametrically opposite locations. The excitation condition involved subharmonic resonance that manifested in a periodic vortex pairing in the near flow field. While the excitation and the tabs independently increased jet spreading, a combination of the two diminished the effect. The jet spreading was most pronounced with the tabs but was reduced when excitation was applied to the tabbed jet. The tabs generated streamwise vortex pairs that caused a lateral spreading of the jet in a direction perpendicular to the plane containing the tabs. The excitation, on the other hand, organized the azimuthal vorticity into coherent ring structures whose evolution and pairing also increased entrainment by the jet. In the tabbed case, the excitation produced coherent azimuthal structures that were distorted and asymmetric in shape. The self-induction of these structures produced an effect that opposed the tendency for the lateral spreading of the streamwise vortex pairs. The passage of the distorted vortices, and their pairing, also had a cancellation effect on the time-averaged streamwise vorticity field. These led to the reduction in jet spreading.