bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-energy electron scattering cross sections of halofluorocarbons
Authors: Underwood-Lemons Theresa , Winkler Dennis C. , Tossell John A. , Moore John H.
Published: July 2002

The interaction of low-energy electrons with halogenated methanes is important in both their atmospheric and plasma-processing chemistry. In this work, the total electron scattering cross sections of mixed fluorohalomethanes (CFnX4−n) were measured for incident electrons in the energy range of 0.3–12 eV using electron transmission spectroscopy. Resonances in the scattering cross sections may be interpreted as the capture of low-energy electrons into unoccupied molecular orbitals. To aid in the assignments of the resulting negative ion states, we performed quantum-mechanical calculations of the electron attachment energies. The effect of halogen substitution on the orbitals participating in electron capture are examined.
The accuracy of the quasiclassical Landau–Lifshitz formula for matrix elements and its application to the analysis of the intensities of vibrational overtone transitions
Authors: Medvedev Emile S.
Published: July 2002

Intensities of vibrational overtone transitions have been analyzed on the basis of the quasiclassical Landau–Lifshitz formula including the pre-exponential factor derived previously. The accuracy of this formula is shown to be very high, on the order of a few percent. An unusual feature of the Landau–Lifshitz formula is the prediction that the overall rapid decrease in intensity with overtone number is highly sensitive to the behavior of the analytical continuation of the molecular potential into regions where the amplitudes of the wave functions are negligible. The dipole moment enters the slowly varying prefactor, which in general superimposes only small modulations on the above steep intensity falloff. However, this prefactor may go through zero, in which case the intensities of one or two transitions nearest the prefactor’s zero are anomalously low. These results imply that knowledge of the analytical continuation of the potential is crucial for extracting meaningful information about the dipole moment from overtone intensities.
Hyperfine splitting constants studied by the symmetry adapted cluster-configuration interaction method
Authors: Nakatsuji Hiroshi , Ehara Masahiro , Momose Takamasa
Published: July 2002

The accuracy of the symmetry adapted cluster-configuration interaction (SAC-CI) method for calculating hyperfine splitting constants (hfscs) is examined. Two kinds of SAC-CI expansion are performed: one is the SAC-CI(SD-R, DT-R) method in which single and double (double and triple for high-spin multiplicity) excitation operators are included in the linked operators and the other is the SAC-CI (general-R) method in which higher excitation operators are further included. The hfscs for the doublet, triplet, and quartet states of small radicals, OH, CH2, BH2, CH3, and H2O+, calculated by the SAC-CI method compare very well with the full CI results. A convenient configuration selection method, in which both energy and hfsc are used as criteria, is shown to be useful. This method, which is also applicable to the ordinary CI method, is effective for accurate calculations of the hfscs especially for large systems and high-spin systems, where a large number of configurations are required. Finally, the basis-set dependence of the hfscs is examined using the configuration selection method developed here. Within the Gaussian set, the improvement in energy does not necessarily result in the improvement of the hfscs. We have to use the basis set which satisfies the cusp condition, as shown previously.
Comment on ‘‘The ν1+ν3 combination mode of C3 in Ar and Kr matrices: Evidence for a bent structure’’ [J. Chem. Phys. 99, 7371 (1993)]
Authors: Almlöf J. , Jensen P. , Northrup F. J. , Rohlfing C. M. , Rohlfing E. A. , Sears T. J.
Published: July 2002

The weight of recent experimental and theoretical work on C3 shows that the isolated molecule is truly linear, i.e., the minimum of the bending potential is at the linear configuration.
Raman Q-branch line shapes as a test of a H2–Ar intermolecular potential
Authors: Green Sheldon , Schwenke David W. , Huo Winifred M.
Published: July 2002

The Le Roy–Hutson intermolecular potential for H2–Ar, which was derived from analysis of spectral data for the van der Waals complex and also shown to predict gas kinetic transport cross sections in good accord with experimental values, was subsequently found to provide rather poor predictions (small by nearly a factor of 2) for vibrational Q-branch Raman line shift cross sections. More recently, an ab initio intermolecular potential for this system has been calculated and scaled to agree with the earlier one in the region of the minimum where the latter is expected to be accurate [D. W. Schwenke et al., J. Chem. Phys. 98, 4738 (1993)]. We show here that this potential predicts line shifts in much better accord with experiment (about 10% large), but its predictions for linewidths are significantly less satisfactory.
Is adiabatic passage population transfer a solution to an optimal control problem?
Authors: Band Y. B. , Magnes O.
Published: July 2002

We demonstrate that stimulated Raman adiabatic passage (STIRAP) of population from a ground state to an excited state of an atom or molecule is not an optimal control solution to the problem of maximizing the population in the excited state.
Relaxation of an AX2 nuclear spin system. Amplitude modulated pulses for excitation of longitudinal multispin ordered states
Authors: Zieger Günter , Sterk Heinz , Bermel W.
Published: July 2002

The 31P and 1H nuclear magnetic resonance (NMR) longitudinal relaxation behavior of the hypophosphite ion (H2PO2)Θ is investigated in D2O solution at various temperatures and two magnetic field strengths. Among a number of different well known methods of preparing the magnetization, a new simple procedure is described. This technique allows the excitation of exclusive longitudinal two- or three-spin order by taking advantage of amplitude modulated pulses. By using the semiclassical Redfield approach and transformation into a normal mode basis, the experimental data are interpreted in terms of an axially symmetric phosphorus chemical shift anisotropy (CSA) tensor and anisotropic rotational diffusion.
Pattern formation during the CO oxidation on Pt(110) surfaces under global coupling
Authors: Falcke M. , Engel H.
Published: July 2002

A reaction-diffusion model for CO oxidation on Pt(110) single crystals proposed by Krischer, Eiswirth, and Ertl is supplemented by an equation for the balance of CO partial pressure in the gas phase. This allows us to study the interaction of local and global coupling with the dynamics of the reaction in the oscillatory regime. In absence of global coupling a stability analysis of the homogeneous oscillatory state predicts parameter regions with negative values of the phase diffusion coefficient indicating the possibility of phase turbulence. In the globally coupled system without diffusion we observe the formation of phase-locked clusters of oscillators and irregular behavior. If both surface diffusion and global coupling through the gas phase are taken into account depending on the range of external parameters we get the following types of structures: phase flips, standing waves, spatially irregular coverage pattern, and the uniformly oscillating surface.
Phase separation in binary hard-core mixtures
Authors: Dijkstra Marjolein , Frenkel Daan , Hansen Jean-Pierre
Published: July 2002

We report the observation of a purely entropic demixing transition in a three-dimensional binary hard-core mixture by computer simulations. This transition is observed in a lattice model of a binary hard-core mixture of parallel cubes provided that the size asymmetry of the large and small particles is sufficiently large (≥3, in the present case). In addition, we have performed simulations of a single athermal polymer in a hard-core solvent. As we increase the chemical potential of the solvent, we observe a purely entropy-driven collapse of the polymer: the scaling of the radius of gyration Rg of the polymer with the number of segments N changes from that of a polymer in a good solvent to that of a collapsed polymer. Both for the study of the hard-core demixing and of the polymer collapse, it was essential to use novel collective Monte Carlo moves to speed up equilibration. We show that in the limit σ1/σ2→0, the pair distribution function for an off-lattice binary hard-core mixture of parallel cubes with side lengths σ1 and σ2 diverges at contact for the large particles. For the lattice system, we calculated the pair distribution functions g(r) up to the fourth virial coefficient. The difference in g(r) at contact for a binary system and a pure system at the same packing fraction gives a rough criterion, whether the mixture phase separates.
Electronic state distribution of Xe+* formed by excitation transfer from Ne(3P0,2) to Xe+(2P3/2) at thermal energy
Authors: Tsuji Masaharu , Kaneko Nobuo , Nishimura Yukio
Published: July 2002

The Ne(3P00,2)+Xe+(5p5 2P01/2,3/2) excitation–transfer reaction has been studied by observing ultraviolet and visible emissions from Xe+* in a Ne flowing afterglow. The relative contribution of Xe+(2P03/2) and Xe+(2P01/2) to the excitation of Xe+* was examined by changing the [Xe+(2P01/2)]/[Xe+(2P03/2)] ratio with the addition of CH4 as a remover of the upper spin–orbit component. Since the intensity distribution of Xe+* lines was independent of the [Xe+(2P01/2)]/[Xe+(2P03/2)] ratio, it was concluded that the contribution of the Ne(3P00,2)+Xe+(2P01/2) reaction was insignificant. The Ne(3P00,2)+Xe+(2P03/2) reaction initially populates nine Xe+(6p,6p′,7s,6) levels in the 13.86–16.60 eV range. The favored product levels are 6p′ 2D03/2,5/2 and 7s 4P5/2 in the 16.36–16.43 eV range and 6p 4P05/2 at 13.88 eV, which are 0.2–0.3 and 2.7 eV below the entrance Ne(3P02)+Xe+(2P03/2) channel, respectively. The former near-resonant levels are excited by either direct curve crossings between attractive entrance potentials and flat exit ones or multiple curve crossings through a strongly attractive Ne−+Xe2+ ionic potential, while the latter nonresonant level is probably formed through the Ne−+Xe2+ ionic potential.
Maximum-entropy representation of non-Gaussian polymer distribution functions
Authors: Poland Douglas
Published: July 2002

The maximum-entropy method is used to construct the end-to-end distribution function for lattice polymers when a large number of moments are known exactly. We use two-dimensional lattice polymers with a finite range of intrachain interaction as examples since, for these systems, the end-to-end distribution function and any number of moments can be calculated exactly using Toeplitz matrices. For chains with strong intrachain interactions the distributions are very non-Gaussian requiring up to six moments to reproduce the main features of the functions.
Inherent structures in the potential energy landscape of solid 4He
Authors: Hodgdon Jennifer A. , Stillinger Frank H.
Published: July 2002

We study the potential energy landscape (many-atom potential energy as a function of atomic positions) of solid hcp 4He in the vicinity of the 0 K crystal structure using an accurate pair potential. At the melting point, the potential energy of the helium lattice is far above the minimum hcp interatomic potential energy. We confirm previous conclusions (based on less accurate potentials) that all of the classical phonon frequencies at the 0 K melting pressure are imaginary, indicating that the melting-point crystal corresponds to a local maximum in the potential landscape; a pressure of about 1300 bar, however, makes it a local minimum. We find that the atomic arrangements that lie at local minima in the potential landscape (‘‘inherent structures’’) are glassy and porous, and have much lower potential energy than the crystalline form at the same density. We have quantitatively characterized the glassy structures by their radial distribution functions and coordination number distributions; they qualitatively resemble inherent structures for classical monatomic liquids, but exhibit differences of detail. A model variational calculation has been carried out for the melting-density ground state. It utilizes separate basis functions for each of the inherent structures, predicts a large Lindemann ratio for the crystal, and indicates that the probability distribution is a maximum at the perfect lattice configuration.
An efficient method of generating dense polymer model melts by computer simulation
Authors: Gao Jianping
Published: July 2002

A new method of generating equilibrium configurations of dense polymer model systems with freely jointed chains is presented. It uses the idea of self and mutually avoiding random walks combined with molecular dynamics simulation to generate freely jointed polymer chains from a densely packed simple liquid. A prior knowledge of polymer persistence length in the densely packed condition is not needed. The efficiency and the resultant equilibrium polymer configurations show that this method works better than the ones used previously in the literature.
Determination of multiple diabatic potentials by the inversion of atom–atom scattering data
Authors: Boyd Robert , Ho Tak-San , Rabitz Herschel
Published: July 2002

An inversion algorithm based on first-order functional sensitivity analysis and Tikhonov regularization is extended for the determination of multiple (diabatic) potentials from elastic and/or inelastic scattering data. Two methods of inversion are presented. In order to illustrate the methods, two-state and three-state models of the He++Ne system are employed in a simulated inversion, the former for the recovery of an entire potential matrix, and the latter to obtain multiple coupling elements, as well as to compare and contrast the two methods against each other. In the case of the two-state model, good agreement between the recovered and the model potentials is achieved if initial guesses for V11(r) and [V22(r)−V22(∞)] are within ±5% of the model, and those for the coupling element V12(r) are within ±10%, thus indicating that the method may be useful for the simultaneous refining of abinitio calculations and the determination of coupling potentials. For the three-state case, initial guesses differing from the model coupling potentials by as much as ±50% yield successful inversions using either method, therefore indicating that the procedure may be even more useful for the recovery of multiple coupling potentials.
Infrared spectroscopy of the CH stretching modes of partially F-substituted benzenes. II. The fundamental spectrum of fluorobenzene
Authors: Scotoni M. , Oss S. , Lubich L. , Furlani S. , Bassi D.
Published: July 2002

This work is part of a series aimed to investigate the fundamental and overtone transitions of the CH stretching modes in partially F-substituted benzenes. In this article we report the study of the fundamental transitions of fluorobenzene (C6H5F) in the wave numbers range from about 3000 to 3150 cm−1. The absorption spectrum has been measured through both a low-temperature opto-acoustic cell and a supersonic-beam opto-thermal spectrometer. The opto-thermal spectrometer enabled us to measure the detailed vibrational structure, providing the basis for an algebraic analysis of the fundamental CH transitions. We show, in particular, how benzene algebraic procedures must be modified for taking into account the effects induced by the substitution of one hydrogen atom with fluorine, i.e., (i) the reduction of symmetry and (ii) the frequency (chemical) shift due to the change of electron distribution. Moreover we include the contribution of anharmonic resonances between unperturbed CH stretching modes and overtone or combination bands of lower energy molecular vibrations.
Binary nucleation kinetics. I. Self-consistent size distribution
Authors: Wilemski Gerald , Wyslouzil Barbara E.
Published: July 2002

Using the principle of detailed balance, we derive a new self-consistency requirement, termed the kinetic product rule, relating the evaporation coefficients and equilibrium cluster distribution for a binary system. We use this result to demonstrate and resolve an inconsistency for an idealized Kelvin model of nucleation in a simple binary mixture. We next examine several common forms for the equilibrium distribution of binary clusters based on the capillarity approximation and ideal vapor behavior. We point out fundamental deficiencies for each expression. We also show that each distribution yields evaporation coefficients that formally satisfy the new kinetic product rule but are physically unsatisfactory because they depend on the monomer vapor concentrations. We then propose a new form of the binary distribution function that is free of the deficiencies of the previous functions except for its reliance on the capillarity approximation. This new self-consistent classical (SCC) size distribution for binary clusters has the following properties: It satisfies the law of mass action; it reduces to an SCC unary distribution for clusters of a single component; and it produces physically acceptable evaporation rate coefficients that also satisfy the new kinetic product rule. Since it is possible to construct other examples of similarly well-behaved distributions, our result is not unique in this respect, but it does give reasonable predictions. As an illustrative example, we calculate binary nucleation rates and vapor activities for the ethanol–hexanol system at 260 K using the new SCC distribution and compare them to experimental results. The theoretical rates are uniformly higher than the experimental values over the entire vapor composition range. Although the predicted activities are lower, we find good agreement between the measured and theoretical slope of the critical vapor activity curve at a constant nucleation rate of 107 cm−3 s−2.
CaOH has a second linear structure HCaO
Authors: Kong Jing , Boyd Russell J.
Published: July 2002

The energy surface of CaOH has been studied and an isomer of the form of HCaO with an electronic state of 2Σ+ symmetry has been found. It is linear with H–Ca and Ca–O bond lengths of 2.021 and 2.002 Å, respectively, at the CISD level with a basis set of triple-zeta plus double polarization quality. The overall electronic structure is largely ionic and can be described as H(1s2)Ca(4s0)O(2s22p2x2p2y2p1z) (H−Ca2+O−). The minimum on the energy surface is well defined and the harmonic vibrational frequencies have been calculated. The O–H bond length and stretching frequency and the bending frequency of CaOH have also been calculated and compared with experiment.
Hydration in polymer studied through magic angle spinning nuclear magnetic resonance and heteronuclear 13C{1H} Overhauser enhancement spectroscopy: Cross-relaxation and location of water in poly(acrylamide)
Authors: Ganapathy S. , Ray Siddharth S. , Rajamohanan P. R. , Mashelkar R. A.
Published: July 2002

A combination of magic angle spinning (MAS) and heteronuclear 13C{1H} Overhauser enhancement spectroscopy (HOESY) is shown to be a powerful technique for studying hydration in polymers. This is demonstrated in poly(acrylamide)–water system. The increased spectral resolution due to MAS is shown to resolve polymer–polymer and polymer–water dipolar correlations in the two dimensional HOESY experiment. The 2D experiment is thus shown to lead to an indirect detection of water interacting with the polymer. The one dimensional transient Overhauser experiment involving selective inversion of water allows the study of cross-relaxation between water protons and carbonyl carbon in the polymer side chain. The cross-relaxation rate is rationalized in terms of a direct dipole–dipole interaction between the carbonyl carbon and the hydrated bound water. Based on temperature dependent 17O spin-lattice relaxation time measurements and a two-step motional model for water, we gather that water molecules close to the observed polymer site reorient anisotropically, typically an order of magnitude slower than in pure water. The correlation time for bound water mobility has been estimated to be 0.58×10−10 s at 298 K, and, in turn, has been used to locate hydrated water at a distance of 3.45 Å from amide carbonyl. This is the first time such an estimate has been made for hydrated water in a polymer using HOESY data.
Electronic structure calculations and dynamics of CC coupling on nickel and cobalt
Authors: Burghgraef H. , Jansen A. P. J. , van Santen R. A.
Published: July 2002

The carbon–carbon coupling of C (carbide) and CH (methylidyne), C and CH2 (methylene), and C and CH3 (methyl) on nickel has been studied with density functional theory using 7- and 13-atom cluster models. Formation of CCH3 (ethylidyne) turned out to be the most exothermic reaction on both clusters. Experimentally, CCH3 has been identified unambiguously with a structure perpendicular to the metal surface. Thus for the C/CH3 coupling forming CCH3, we have determined the structure and potential energy surface in the coadsorbed state, transition state, and CC-formed state. The transition state is explicitly determined on the 7-atom cluster and the 13-atom cluster of both nickel and cobalt. We find transition state barriers of 57 kJ/mol for the Ni7 cluster, 55 kJ/mol for the Co7 cluster, 84 kJ/mol for the Ni13 cluster, and 47 kJ/mol for the Co13 cluster. The overall reaction energies are −215, −184, −66, and −89 kJ/mol, respectively. Analysis of the TS shows a dominant contribution of the CC bond to the reaction coordinate. Cluster size effects play a role in all studied CC coupling reactions.
Alternative treatment of separable quantum-mechanical models: The hydrogen molecular ion
Authors: Fernández Francisco M.
Published: July 2002

We present an alternative approach to separable problems in quantum mechanics and apply it to the time-independent Schrödinger equation for the hydrogen molecular ion under the Born–Oppenheimer approximation. The roots of a pair of Hankel determinants, built from the Taylor coefficients of the regularized logarithmic derivative of the eigenfunctions resulting from separation in spheroidal coordinates, yield the separation constant and the internuclear distance accurately for every value of the electronic energy. The method converges rapidly, giving tight upper and lower bounds to the basic parameters.
Determining regular orbits in the presence of irregular trajectories using optimal control theory
Authors: Botina Jair , Rabitz Herschel , Rahman Naseem
Published: July 2002

Two general algorithms are presented to determine regular orbits in the presence of irregular trajectories in a phase space of n degrees of freedom. The first algorithm searches for regular orbits with the energy as a free-floating parameter. The second algorithm seeks regular orbits at constant energy. These two approaches utilize optimal control theory to employ a small external control field that permits a search among the irregular motion for the regular orbits. The optimizing algorithm naturally seeks regular orbits with the control field turned off. Numerical results with a chaotic Hamiltonian show the method to be effective in determining regular trajectories. If the system is completely chaotic in some region, the method determines which initial condition is the best one in order to achieve a nearly regular orbit.
The C1s shakeup spectra of Buckminsterfullerene, acenaphthylene, and naphthalene, studied by high resolution x-ray photoelectron spectroscopy and quantum mechanical calculations
Authors: Enkvist Christer , Lunell Sten , Sjögren Bo , Brühwiler Paul A. , Svensson Svante
Published: July 2002

The C1s core photoelectron shakeup spectrum of fullerene has been studied experimentally and theoretically, as well as the corresponding spectra of the smaller model compounds acenaphthylene and naphthalene. It is found that many of the shakeup excitations of C60 can be classified in terms of excitations found in the model compounds. A population analysis of the orbitals involved reveals a systematic behavior, enabling a generalization to extended aromatic systems and to an infinite graphite layer.
Electron spin resonance line shapes for one- and two-dimensional random walk processes
Authors: Tang Jau , Dikshit S. N. , Norris James R.
Published: July 2002

One- and two-dimensional random walk processes and their effects on EPR (electron spin paramagnetic resonance) line shapes are examined. Discrepancies between the coarse continuum model and the discrete hopping model are discussed. Closed-form formulas are prescribed for the distribution function, correlation (memory) function and the EPR line shapes, covering the entire range between the slow and the fast hopping limits. Deviation from Gaussian or Lorentzian line shape is shown for intermediate hopping rates. Differences between the cases of a closed loop and an open chain are illustrated. Time-domain analysis of line shape measurements can provide information about the hopping rate constants, site numbers in an open or closed chain, and the activation energy.
Kinetics at the collapse transition of homopolymers and random copolymers
Authors: Kuznetsov Yu. A. , Timoshenko E. G. , Dawson K. A.
Published: July 2002

We describe the results of Monte Carlo simulations for kinetics at the collapse transition of a homopolymer in a lattice model. We find the kinetic laws corresponding to the three kinetic stages of the process: R2g(t)=R2g(0)−At7/11 at the early stage corresponding to formation and growth of locally collapsed clusters, the coarsening stage is characterized by growth of clusters according to the law S∝t1/2, where S is the average number of Kuhn units per cluster, and the final relaxation stage is described by the law R2g(t)=R2g(∞)+A1(1)e−t/τ1(1) with τ1(1)∝N2. We also present preliminary results on the equilibrium properties and ‘‘collapse’’ transition of a random copolymer. The transition curve is determined as a function of hydrophobic bead concentration na. We discuss the different collapsed copolymer states as a function of the composition. At low hydrophilicity we believe the critical value of the interaction parameter is governed by the law χc(na)∝na−2/3. In the kinetics we see unusual phenomena such as the appearance of a metastable long-lived states with few clusters and nontrivial loop structure.
Interstellar silicon–nitrogen chemistry. III. The spectral signatures of the H2SiN+ molecular ion
Authors: Parisel O. , Hanus M. , Ellinger Y.
Published: July 2002

The recent detection of SiN in the outer envelope of the IRC+10216 carbon star has renewed the interest for the gas phase interstellar silicon chemistry. In this contribution, we present a theoretical study of the H2SiN+ molecular ion, the silicon hydrogenated counterpart of the previously studied SiNH+2. On many points, the differences relative to the SiNH+2 isomer have been found to be dramatic. As an example, the dipole moment is computed to be 3.8 D while being only 0.5 D in SiNH+2. The radio, infrared and electronic signatures have been evaluated at a quantitative level. The rotational constants and vibrational frequencies have been determined using Möller–Plesset MPn (n=2,3,4), coupled cluster (CCSDT) and complete active space self-consistent field (CASSCF) methods for H2SiN+ and some of its isotopomers. These quantities have been corrected using a scaling procedure derived from previous studies on the HNSi, HSiN, HSiNH2, H2SiNH, and SiNH+2 species in order to provide quantitative results. The failure of single-reference perturbation theories to predict a relevant infrared spectrum is discussed. Intense bands around 550, 950, and 2300 cm−1 are predicted. The electronic spectrum has been obtained using a coupled multiconfiguration SCF–perturbation treatment (MC/P): It is characterized by a large number of excited states, none of them having a strong transition moment. The lowest excited state is predicted to lie 0.54 eV above the ground state, but the first allowed transition having a nonnegligible oscillator strength has to be searched at 6.44 eV.
Is surface melting a surface phase transition?
Authors: Bastiaansen Paul J. M. , Knops Hubert J. F.
Published: July 2002

Monte Carlo or Molecular Dynamics calculations of surfaces of Lennard-Jones systems often indicate, apart from a gradual disordering of the surface called surface melting, the presence of a phase transition at the surface, but cannot determine the nature of the transition. In the present paper, we provide for a link between the continuous Lennard-Jones system and a lattice model. We apply the method for the (001) surface of a Lennard-Jones fcc structure pertaining to Argon. The corresponding lattice model is a Body Centered Solid on Solid model with an extended range of interaction, showing in principle rough, flat and disordered flat phases. We observe that entropy effects considerably lower the strength of the effective couplings between the atoms. The Argon (001) face is shown to exhibit a phase transition at T=70.5±0.5 K, and we identify this transition as roughening. The roughening temperature is in good correspondence with experimental results for Argon.
Vibrationally resolved photoelectron angular distributions and branching ratios for the carbon dioxide molecule in the wavelength region 685–795 Å
Authors: West J. B. , Hayes M. A. , Siggel M. R. F. , Dehmer J. L. , Dehmer P. M. , Parr A. C. , Hardis J. E.
Published: July 2002

Measurements of vibrational branching ratios and photoelectron angular distributions have been made in the regions of the Tanaka-Ogawa, Lindholm, and Henning series for the CO2 molecule. The behavior of these parameters was found to be sensitive to which particular resonance is excited, with considerable intensity going into vibrational modes other than the symmetric stretch. An initial analysis of some of the data taken is presented.
High pressure investigation of absorption spectra of J-aggregates
Authors: Lindrum M. , Chan I. Y.
Published: July 2002

The spectral shift of J-bands under high pressure up to 60 kbar has been investigated for J-aggregates of different cyanine dyes. Under conditions where no J-band can be observed at normal pressure, J-aggregates are formed at higher pressure. A red shift of absorption upon increasing pressure was found with a linear dependence of line position on pressure. The results can be explained by a change of dipolar coupling of monomer molecules in the aggregate due to decreasing center-to-center distances. The different pressure slopes of various aggregates are interpreted from their different aggregate structures. The monomer absorption is also red-shifted under pressure, but the pressure dependence is quite different and can be described by a solvent shift. From the linear dependence, the dipole–dipole coupling energy J can be determined with a simple theory. The general applicability and limitations of this method are discussed.
Application of graph theory to the statistical thermodynamics of lattice polymers. I. Elements of theory and test for dimers
Authors: Brazhnik Olga D. , Freed Karl F.
Published: July 2002

The lattice cluster theory (LCT) is extended to enable inclusion of longer range correlation contributions to the partition function of lattice model polymers in the athermal limit. A diagrammatic technique represents the expansion of the partition function in powers of the inverse lattice coordination number. Graph theory is applied to sort, classify, and evaluate the numerous diagrams appearing in higher orders. New general theorems are proven that provide a significant reduction in the computational labor required to evaluate the contributions from higher order correlations. The new algorithm efficiently generates the correction to the Flory mean field approximation from as many as eight sterically interacting bonds. While the new results contain the essential ingredients for treating a system of flexible chains with arbitrary lengths and concentrations, the complexity of our new algorithm motivates us to test the theory here for the simplest case of a system of lattice dimers by comparison to the dimer packing entropies from the work of Gaunt. This comparison demonstrates that the eight bond LCT is exact through order φ5 for dimers in one through three dimensions, where φ is the volume fraction of dimers. A subsequent work will use the contracted diagrams, derived and tested here, to treat the packing entropy for a system of flexible N-mers at a volume fraction of φ on hypercubic lattices.
The adsorption and desorption of water on single crystal MgO(100): The role of surface defects
Authors: Stirniman M. J. , Huang C. , Scott Smith R. , Joyce S. A. , Kay Bruce D.
Published: July 2002

Adsorption and desorption of water on well-ordered and sputter-damaged single crystal MgO(100) surfaces were studied by a combination of molecular beam reflection and temperature programmed desorption techniques. Adsorption exhibits precursor-mediated kinetics and desorption exhibits a strong dependence on substrate treatment, demonstrating the importance of surface defects.
Laser induced fluorescence measurements of microwave stimulated OH molecules from H2O photodissociation
Authors: Wurps H. , Spiecker H. , ter Meulen J. J. , Andresen P.
Published: July 2002

Laser induced fluorescence of microwave stimulated OH molecules from H2O photodissociation is investigated as a first step in a series of laboratory experiments to understand features of astronomical OH masers. The inversion between the Π3/2, J=7/2 Λ-doublet states of OH generated via photodissociation of cold H2O at 157 nm is shown to be 1.8:1. Within a microwave, Fabry–Perot cavity tuned to the resonance of one of the main hyperfine transitions in this Λ doublet it is possible to stimulate all inverted OH molecules. The linewidth of the two main microwave transitions is measured as a function of the microwave power and interaction time. The results show that Doppler broadening is negligible at short interaction times of the molecules with the microwave field.
Slow passage through a supercritical Hopf bifurcation: Time-delayed response in the Belousov–Zhabotinsky reaction in a batch reactor
Authors: Strizhak Peter , Menzinger Michael
Published: July 2002

When the control parameter of a dynamical system varies continuously, bifurcation is delayed due to the inertia in the system’s response. We study experimentally the time delay of the supercritical Hopf bifurcation that arises from the parametric drift of the oscillating Belousov–Zhabotinsky reaction under batch conditions. The time-dependent oscillation amplitude and period are analyzed using the normal form of the supercritical Hopf bifurcation with a time-dependent control parameter. We show that this approach describes the time evolution of the entire oscillatory domain from high amplitude to vanishing oscillations.
Competition effects in steady-state diffusion-limited reactions: Renormalization group approach
Authors: Traytak S. D.
Published: July 2002

We consider the steady-state competition between perfectly absorbing sinks distributed uniformly in a spherical region. An equation for the coarse-grained concentration of diffusing particles inside the region is derived following the renormalization group approach. The total flux of diffusing particles, their coarse-grained concentration, and the characteristic penetration length are found. We then compare our results with so-called mean field approximation, and show that the latter approach cannot correctly describe the coarse-grained concentration in the system under consideration.
Transient reactions in a binary suspension of diffusing spheres
Authors: Felderhof B. U. , Jones R. B.
Published: July 2002

Transient diffusion-controlled reactions in a binary suspension of equal-sized spheres are studied on the basis of the time-correlation function of concentration fluctuations in thermal equilibrium. The transient effects are characterized by a memory function, which is calculated in an Enskog type approximation from the nonequilibrium distribution function of a pair of diffusing and converting spheres. In Ising model language, the explicit calculation is performed for hard spheres with a square well Ising interaction and with a magnetic field present. The memory function exhibits appreciable dependence on the parameters of the model.
Electronic structure, ground state, and electron paramagnetic resonance spectroscopy of the matrix-isolated (η6-C6H6)V and (η6-C6D6)V half-sandwich transients
Authors: Mattar Saba M. , Sammynaiken Ramaswami
Published: July 2002

The electron paramagnetic resonance (EPR) spectra of the (η6-C6H6)V, (η6-C6D6)V transients and (η6-C6H6)2V, (η6-C6D6)2V molecules are obtained by codepositing C6H6 and C6D6 with vanadium atoms in an Ar matrix at 15 K. Under optimal experimental conditions, the spectra of these axial species are well resolved and the elusive parallel resonance components are identified for the first time. Mathematical expressions for the resonance field positions and line shapes are derived and are used to simulate the experimental spectra. For (η6-C6H6)V and (η6-C6D6)V the g and vanadium hyperfine tensors are diagonal with gxx=gyy=1.942(3), gzz=2.007(3), Axx(V)=Ayy(V)=−113.2(5)×10−4 cm−1, and Azz(V)=−9.3(5)×10−4 cm−1. The (η6-C6H6)V and (η6-C6D6)V effective spin Hamiltonian tensor components and resonance field positions are also derived in terms of their molecular orbital (MO) coefficients assuming a 2A1 ground state. Consequently, the g, hyperfine, and superhyperfine tensor components, obtained from the simulation of the experimental spectra, are compared with those computed using the local-density-functional method. The very good agreement between the experimental and computed parameters indicate that the (η6-C6H6)V and (η6-C6D6)V complexes have 2A1 ground states. The explicit formulas derived for the carbon and hydrogen hyperfine tensors, using symmetry adapted linear combinations of atomic orbitals, imply that they are not diagonal and do not have coincident principal axes. Although this suggests that, in principle, the carbon and hydrogen centers in (η6-C6H6)V and (η6-C6D6)V must be magnetically inequivalent, the resulting effects are negligible due to the small 2p(C) character of the 10a1 singly occupied molecular orbital. Understanding and simulating the (η6-C6H6)V and (η6-C6D6)V EPR spectra is essential in the analysis of the EPR spectra of more complicated molecules like (η6-C6H3F3)V, (η6-C6F6)V, and (η6-C6F6)2V.
On the coherent vibrational phase in polarization sensitive resonance CARS spectroscopy of copper tetraphenylporphyrin
Authors: Voroshilov Artemy , Otto Cees , Greve Jan
Published: July 2002

Polarization sensitive multiplex spectroscopy of resonance coherent anti-Stokes Raman scattering (CARS) of copper(II)-tetraphenylporphyrin in solution (CH2Cl2) is reported. The measurements were performed in the Q band range of the porphyrin electronic absorption. Four polarized CARS spectra were resolved in 1300–1650 cm−1 Raman range and were simultaneously fitted with a single set of vibrational parameters (band positions, bandwidths, amplitudes, depolarization ratios, and phases). The obtained coherent vibrational phases of A1g, A2g, and B1g skeletal modes of the porphyrin macrocycle appeared to correlate strongly with the mode vibrational symmetry. The origin of such correlation is analyzed within a model of the third-order nonlinear electric susceptibility χ(3). The proposed model is based on multidimensional displaced harmonic oscillator in the Herzberg–Teller expansion of Raman polarizability. The coherent vibrational phases of modes of different symmetry classes are directly affected by the symmetry dependent vibronic couplings between the pairs of Qx, Qy and Bx, By electronic transitions of the porphyrin. The phase contrast between the modes of various symmetries is most pronounced near the Q00 resonance.
Comparison study of pivot methods for global optimization
Authors: Serra Pablo , Stanton Aaron F. , Kais Sabre , Bleil Richard E.
Published: July 2002

We compare two implementations of a new algorithm called the pivot method for the location of the global minimum of a multiple minima problem. The pivot method uses a series of randomly placed probes in phase space, moving the worst probes to be near better probes iteratively until the system converges. The original implementation, called the “lowest energy pivot method,” chooses the pivot probes with a probability based on the energy of the probe. The second approach, called the “nearest neighbor pivot method,” chooses the pivot probes to be the nearest neighbor points in the phase space. We examine the choice of distribution by comparing the efficiency of the methods for Gaussian versus generalized q-distribution, based on the Tsallis entropy in the relocation of the probes. The two implementations of the method are tested with a series of test functions and with several Lennard-Jones clusters of various sizes. It appears that the nearest neighbor pivot method using the generalized q-distribution is superior to previous methods.
A novel approach for determining the droplet size distribution in emulsion systems by generating function
Authors: Ambrosone L. , Ceglie A. , Colafemmina G. , Palazzo G.
Published: July 2002

We present a novel approach that allows us to figure out the size distribution of an isolated collection of droplets of a liquid dispersed in another one (emulsion) using NMR pulsed gradient spin echo (PGSE) measurements. The echo decay coming from the ensemble of droplets with different radii was related to the particle size distribution function and to the echo attenuation of an isolated sphere. This latter term was evaluated according to the second cumulant approximation, while the unknown distribution was approximated using a generating function series. The coefficients of the series are evaluated, using a least squares procedure, fitting the obtained relationship to the experimental data. We find that when the volume fraction distribution is contained in the range of lengths determined by the parameters of the PGSE measurement, the experimental data (echo attenuation) contain enough information to describe the distribution function in all the details. The method has been tested on both experimental and simulated data and appears to be powerful and flexible. It is able to work with highly asymmetric distributions, still giving accurate results.
A femtosecond midinfrared pump–probe study of hydrogen-bonding in ethanol
Authors: Woutersen S. , Emmerichs U. , Bakker H. J.
Published: July 2002

We present a femtosecond midinfrared pump–probe study of hydrogen bonding. It is shown that upon excitation of the OH-stretching vibration of hydrogen-bonded ethanol dissolved in CCl4, the hydrogen bonds are predissociated on a femtosecond time scale. The measured predissociation time constant depends strongly on the excitation frequency, and ranged from ∼250 fs at 3330 cm−1 to ∼900 fs at 3450 cm−1. The time constant of the subsequent reassociation of the hydrogen bonds was found to be 15 ps, in accordance with previous picosecond studies. Furthermore, polarization-resolved measurements show that orientational relaxation takes place on a time scale much shorter than the pulse length of ∼200 fs. This rapid orientational relaxation can be explained from the fast delocalization of the O–H stretching excitation over the hydrogen-bonded ethanol oligomers. The orientational anisotropy R reaches a value of 0.15 instantaneously, and remains constant for all delays.
The reaction volume Hamiltonian model: Further development and application
Authors: Koch A. , Billing G. D.
Published: July 2002

The reaction volume Hamiltonian model is extended and applied to the OH+H2 reaction. A minimum energy path, which is a function of the hyperspherical coordinates ρ, Θ, and Φ, characterizing the reactive centers, has been calculated for the H spectator atom bound to the oxygen atom. Vibrational coupling coefficients, which occur in the Hamiltonian for this reactive system, have been computed.
Constrained free-energy functional of deformed polymer systems
Authors: Sun Tao , Balazs Anna C. , Jasnow David
Published: July 2002

The constrained free-energy functional for deformed polymer systems is derived in a regime of small fluctuations. The free energy contains an elastic contribution due to chain deformation as well as the usual Ginzburg–Landau form. Up to terms quadratic in fluctuations, the linear elastic modulus tensor is obtained. We find that the shear modulus scales with the concentration in the same way as that suggested by scaling arguments in the mean-field regime. The elastic Poisson ratio is 1/2 for the dilute system and decreases as the volume fraction increases. In the melt limit, where the volume fraction is close to unity, the Poisson ratio is down to 0.2.
A thermodynamic basis for charged particles transport in viscoelastic fluids
Authors: Estrada-Flores A. , del Castillo L. F. , Goldstein Patricia
Published: July 2002

A calculation for the generalized longitudinal stress modulus and electric conductivity in an ionic-viscoelastic medium is presented. The system is described within the framework of extended irreversible thermodynamics in terms of the regular conserved variables and of an additional set of independent variables consisting of the stress tensor, its trace, and the electric flux. Using the standard procedure, we derive the time evolution equations for the additional variables. These equations are used to obtain general wave vector and frequency dependent expressions for measurable properties of the system. We obtain for the linear case results that have not been derived previously in the literature.
Vibrational phase and energy relaxation of CN− in water
Authors: Rey Rossend , Hynes James T.
Published: July 2002

Classical molecular dynamics simulations complemented with semiclassical perturbation theory have been applied to the study of the cyanide ion vibrational relaxation in liquid water. The model provides reasonable agreement with known experimental results as well as with ab initio calculations for small clusters. The role of Coulomb and non-Coulomb forces is studied in detail. A dominant role of the former in the vibrational energy (population) relaxation is found, while in contrast, the bandshape—and thus the dephasing—are determined by both forces. Further, and at variance with existing theories, the present model provides the first example in which nonlinear intermolecular terms in the vibration-solvent coupling are critical in the instantaneous frequency shift.