bluuebunny/crossref_metadata_2025 huggingface dataset:
The overtone dynamics of acetylene above 10 000 cm−1
Authors: Smith Brian C. , Winn John S.
Published: July 2002

The 10 000–13 000 cm−1 C–H stretching spectra of normal acetylene have been measured with a high resolution Fourier transform spectrometer and a long path gas cell. Over 400 rovibrational lines were assigned and analyzed yielding band origins and rotational constants. Comparison of calculated and observed rotational constants and intensities confirmed the vibrational assignments made in this region. A vibrational analysis of all observed C2H2 transitions above 10 000 cm−1 was made based on the normal mode constants derived earlier. Some reassignments were made, many unassigned bands were assigned, and several new Fermi resonances were analyzed. When combined with our previous results below 10 000 cm−1, the standard deviation of our analysis for 64 unconstrained states up to 24 000 cm−1 is 9.35 cm−1. Local and global rotational perturbations were noted in our spectra, and the possible identity of the interacting states is discussed. The implication of our results for theories of vibrational energy level structure, intramolecular vibrational relaxation, the C2H2 potential surface, the structure of its phase space, and stimulated emission pumping results on acetylene are discussed.
Concerning zero-point vibrational energy corrections to electronic energies
Authors: Grev Roger S. , Janssen Curtis L. , Schaefer Henry F.
Published: July 2002

For comparison with experimentally obtained thermochemical data, zero-point vibrational energies (ZPVEs) are required to convert total electronic energies obtained from ab initio quantum mechanical studies into 0 K enthalpies. The currently accepted practice is to employ self-consistent-field (SCF) harmonic frequencies that have been scaled to reproduce experimentally observed fundamental frequencies. This procedure introduces systematic errors that result from a recognizable flaw in the method, namely that the correct ZPVE, G(0), is not one half the sum of the fundamental vibrational frequencies. Until better methods for accurately determining ZPVEs are presented, we recommend using different scaling factors for the determination of ZPVEs than those used to compare theoretically determined harmonic frequencies to observed fundamentals.
Direct determination of the adiabatic ionization energy of NO2 as measured by guided ion-beam mass spectrometry
Authors: Clemmer D. E. , Armentrout P. B.
Published: July 2002

The adiabatic ionization energy (IE) of NO2 is measured to be 9.60±0.03 eV by studying the charge-transfer reactions of Zn+, NO+, and CH3I+ with NO2 and those of NO+2 with α,α,α-trifluorotoluene and CH3I using guided ion-beam mass spectrometry. This value confirms the accuracy of a very precise spectroscopic value measured by Haber et al. [J. Chem. Phys. 144, 58 (1988)] and Tanaka and Jursa [J. Chem. Phys. 36, 2493 (1962)], IE(NO2)=9.586±0.002 eV, but is much lower than many other measurements that are limited by very unfavorable Franck–Condon factors. The mechanism that allows the charge-transfer reactions to occur at the thermodynamic limit is discussed by examining qualitative potential-energy surfaces for the charge-transfer processes.
Derivation of coupled cluster equations from general many-body relations
Authors: Schork Tom , Fulde Peter
Published: July 2002

Recently, general equations for the ground state energy were derived, which are based on the use of cumulants and which are suitable for various approximation schemes, e.g., projection techniques. Here we want to show that one can derive the well-known coupled cluster equations from them. This provides a link between two different methods of treating many-body systems.
Observation and simulation of electrohydrodynamic instabilities in aqueous colloidal suspensions
Authors: Hu Yue , Glass J. L. , Griffith A. E. , Fraden Seth
Published: July 2002

When an alternating linear electric field is applied to an aqueous suspension of micron-size polystyrene particles, hydrodynamic instabilities are seen at a frequency range between 2 and 200 kHz. The formation of elongated bands of particles oriented at certain angles relative to the direction of the applied electric field and circulation of particles within the bands are observed. We examine the phase lag between the induced dipole moments of the particles and the effects of this on the interactions between neighboring particles. Because the phase lag causes neighboring particles to exert torques upon one another, thereby causing spinning of double layers, we propose a phenomenological model in which there is a symmetry breaking between the direction of the polarization of the particles and the direction of the applied electric field. Based on this modified dipole–dipole interaction, and together with hydrodynamic interactions, we are able to simulate the formation of the tilted bands of particles. The sense of circulation of particles within a band also agrees with our experimental observations.
Quantum scattering studies of electronically inelastic collisions of N+2(X 2Σ+g, A 2Πu) with He
Authors: Berning Andreas , Werner Hans-Joachim
Published: July 2002

The potential energy surfaces (PESs) of the three lowest electronic states of the system N+2+He have been computed using accurate multiconfiguration-reference configuration (MRCI) wave functions and a large basis set. The approach of the He atom leads to nonadiabatic mixing of the A 2Πu(A′) and X 2Σ+g(A′) states of N+2. The three adiabatic interaction potentials have been transformed into a set of four diabatic potentials, one of which describes the collision-induced nonadiabatic coupling between the two A′ states. The computed potentials have been fitted to analytical functions and used in quantum scattering calculations for electronically inelastic transitions between individual rovibrational levels of the A 2Πu and the X 2Σ+g states of N+2. Our results are compared to transitions observed experimentally by Katayama and co-workers between the rotational levels of the A,v=3 and 4 and X,v=6, 7, and 8 vibrational manifolds. In general, good agreement is found for transitions between nearly isoenergetic vibrational states. However, for transitions which traverse large energy gaps, we obtained cross sections which are several orders of magnitude smaller than experimentally observed. Inclusion of the vibrational degree of freedom of the N+2 molecule in the scattering calculations was found to have only an insignificant effect on the transition probabilities.
Multistate quantum Fokker–Planck approach to nonadiabatic wave packet dynamics in pump–probe spectroscopy
Authors: Tanimura Yoshitaka , Mukamel Shaul
Published: July 2002

The quantum Fokker–Planck equation of Caldeira and Leggett is generalized to a multistate system with anharmonic potentials and a coordinate dependent nonadiabatic coupling. A rigorous procedure for calculating the dynamics of nonadiabatic transitions in condensed phases and their monitoring by femtosecond pump–probe spectroscopy is developed using this equation. Model calculations for a harmonic system with various nonadiabatic coupling strengths and damping rates are presented. Nuclear wave packets in phase space related to electronic coherence are shown to provide an insight into the mechanism of nonadiabatic transitions. The Green’s function expression for these wave packets is used to explore possible algorithms for incorporating electronic dephasing in molecular dynamics simulations of curve crossing processes.
Three-body analytical potential for interacting helium atoms
Authors: Parish Carol A. , Dykstra Clifford E.
Published: July 2002

Large basis set ab initio calculations have been carried out for a dense grid of points on the He3 potential energy surface. Three-body contributions were extracted at every point, and a number of concise functional representations for the three-body potential surface were then examined. Three-body multipolar dispersion terms and other radial and angular terms were used in the representations, and an assessment of relative importance of the different terms is presented. Combined with a two-body He–He potential, the results of this work should offer a high quality interaction potential for simulations of aggregated helium.
Quantum molecular dynamics simulations of liquid alkalies
Authors: Lynch D. L. , Troullier N. , Kress J. D. , Collins L. A.
Published: July 2002

We employ a unified molecular dynamics (MD), quantum mechanical approach to simulate the behavior of large collections of atoms at finite temperature. The nuclei are moved according to classical mechanics while the forces are computed via quantum mechanical models. Two approaches have been used: (1) the extended Hückel method, which is an approximate molecular orbital approach and (2) density functional theory based on the local density approximation and plane wave pseudopotential formulation. We compute properties from the MD trajectories of up to 4 ps duration for samples containing up to 250 atoms of lithium, sodium, and potassium. These results are compared to both previous calculations and experimental results.
Laser phase coherence effects in time-delayed multiphoton transitions: A modified collisional time-correlation function method
Authors: Srivastava Deepak
Published: July 2002

A recently proposed collisional time-correlation function method for resonance laser–target interactions in molecular targets is modified and extended to include time-delayed multiphoton processes in the dynamics. The explicitly time-dependent laser and target dynamics in this method is separated into laser electric field and molecular target dipole/susceptibility correlation functions. The first and second order laser field correlation functions, also known as Glauber field correlation functions, are evaluated in a continuum mode description of photon number representation for both resonance and time-delayed multiphoton processes. We show that such a description allows for the incorporation of laser pulse shape and phase coherence in the formalism, while the laser–target interaction dynamics is still described similarly to the resonance single- and multiphoton processes. As an example, the transition rates for a two-photon Gaussian pump–probe dynamics are derived in which we show that the phase coherence between the Gaussian pump and probe pulses can be incorporated and controlled through a single delay-time-dependent parameter.
Infrared multiphoton dissociation of CBrF2CHClF, CBrF2CHBrF, and CBrClFCBrF2 in a molecular beam
Authors: Yokoyama Atsushi , Yokoyama Keiichi , Fujisawa Ginji
Published: July 2002

Dynamics and mechanisms of infrared multiphoton dissociation of CBrF2CHClF, CBrF2CHBrF, and CBrClFCBrF2 have been studied using a photofragmentation translational spectroscopy. All molecules dissociated through C–Br bond rupture reactions. At high laser fluence, the halogenated ethyl radicals produced by the primary dissociation reactions dissociated through carbon–halogen bond ruptures. Center-of-mass product translational energy distributions for the C–Br and C–Cl bond ruptures of all halogenated ethanes and ethyl radicals studied are essentially consistent with those calculated by Rice–Ramsperger–Kassel–Marcus (RRKM) theory. This indicates that there exists essentially no exit channel barrier on the potential energy surface for the C–Br or C–Cl bond rupture of the halogenated ethanes and ethyl radicals.
Mode-selective photoisomerization in 5-hydroxytropolone. II. Theory
Authors: Nash John J. , Zwier Timothy S. , Jordan Kenneth D.
Published: July 2002

Ab initio calculations are used to explore the ground-state potential energy surface for the syn–anti photoisomerization reaction of 5-hydroxytropolone (5-HOTrOH). Two reaction coordinates are identified, involving 2-OH tunneling and 5-OH torsion. Hartree–Fock (HF) and perturbation theory (at the MP2 level) have been used to calculate the stationary points on the two-dimensional surface associated with these coordinates. Similar calculations on the parent molecule tropolone are carried out for comparison. As observed in previous studies, the 2-OH tunneling barrier drops dramatically at the MP2 level which includes electron correlation. Vibrational frequency calculations are carried out for both tropolone and 5-HOTrOH at the HF/6-31G** and MP2/6-31G** levels in order to correlate the modes with those observed experimentally. A method is introduced for evaluating which normal coordinates should be most strongly coupled to a given reaction coordinate. Normalized, mass-weighted intrinsic and direct reaction coordinates similar in form to the normal coordinates are devised by projecting atomic displacements from the reactant structure toward a transition state (intrinsic) or product (direct) structure. These serve as limiting cases for the initial projections of the multidimensional reaction trajectories. The intrinsic and direct reaction coordinates are then expanded in the basis set of normal coordinates to obtain coefficients of expansion of the reaction coordinates in this basis set. This simple scheme highlights the subset of normal coordinates which are important in promoting reaction by H-atom tunneling or O–H torsion. In 5-HOTrOH, an in-plane mode calculated at 348 cm−1 has a large coefficient of expansion along both intrinsic and direct reaction coordinates. This mode is assigned as the ‘‘promoter mode’’ W observed in the experimental study of paper I.
Vibration and rotation of CO in C60 and predicted infrared spectrum
Authors: Olthof E. H. T. , Avoird A. van der , Wormer P. E. S.
Published: July 2002

We present the Hamiltonian for the vibrations and rotations of CO inside a freely rotating or fixed C60 molecule and we calculate its eigenstates from an atom–atom model potential. The ensuing level structure can be understood in terms of three basic characteristics. (i) Simultaneous rotations of CO and its position vector R, which give rise to a rotational structure similar to that of free CO. The effective rotational constants differ considerably, however. (ii) Splittings of the levels by the icosahedral field of C60 which perturb the regular rotational structure, because they are of the same order of magnitude as the rotational spacings. (iii) Large frequencies associated with the (nearly harmonic) vibrations of CO against the hard walls of the C60 cage: 209 cm−1 for the radial excitation and 162 cm−1 for the twofold degenerate libration. These vibrations give a rovibrational level structure similar to that of a linear triatomic molecule, the radial excitation resembles a bond stretch (Σ) state, the libration a Π-bending state. From the eigenstates we calculate the line strengths of the electric dipole transitions allowed by the icosahedral symmetry. Additional (approximate) selection rules are found, and the infrared spectrum of CO&at;C60 is predicted.
Equilibrium swelling properties of polyampholytic hydrogels
Authors: English Anthony E. , Mafé Salvador , Manzanares José A. , Yu Xiahong , Grosberg Alexander Yu. , Tanaka Toyoichi
Published: July 2002

The role of counter ions and ion dissociation in establishing the equilibrium swelling of balanced and unbalanced polyampholytic hydrogels has been investigated experimentally and theoretically. The swelling dependence on both the net charge offset and the external bath salt concentration has been examined using an acrylamide based polyampholytic hydrogels. By careful consideration of the swelling kinetics, we illustrate the effects of ion dissociation equilibria and counter ion shielding in polyampholytic hydrogels near their balance point where both polyelectrolyte and polyampholyte effects are present. The theory considers a Flory type swelling model where the Coulombic interactions between fixed ions in the hydrogel resemble those of an ionic solid with a Debye screening factor. Theoretical predictions from this model are in qualitative agreement with our experimental results.
Deformation of ferrogels induced by nonuniform magnetic fields
Authors: Zrínyi M. , Barsi L. , Büki A.
Published: July 2002

Materials producing strain in magnetic fields are known as magnetoelastic or magnetostrictive materials. A new type has been developed and the elongation of magnetic field sensitive polymer gels, called ferrogels, has been studied. The driving force of the deformation is the magnetic field gradient which was produced by an electromagnet. Unidirectional deformation measurements have been performed and the dependence of elongation on the current intensity has been reported. An equation for the uniaxial magnetoelastic properties has been derived and it was proven that at small strains the elongation is proportional to the square of steady current intensity. The elastic modulus of ferrogels was found not to be influenced by the magnetic field.
Keeping the shape but changing the charges: A simulation study of urea and its iso-steric analogs
Authors: Tsai Jerry , Gerstein Mark , Levitt Michael
Published: July 2002

As a first step in simulating solvent denaturation, we compare two possible potentials for urea; one based directly on a parameterization for proteins and another generated from ab initio, quantum calculations. Our results, which are derived from numerous, 1 ns simulations, indicate that both potentials reproduce essentially the same observed water structure (as evident in radial distribution functions). However, even though the quantum potential better approximates dimer energies, it is unable to simulate the dynamic behavior of water (as evident in measurements of diffusion) as well as the potential based on protein parameters. To understand its behavior in aqueous solution, we compare the urea simulations with those of solute molecules that possess the same planar, Y-shape as urea but are progressively more hydrophobic. We find that adding urea to a solution increases the number of hydrogen bonds, while adding any of the Y-shaped analogs decreases the number of hydrogen bonds. Moreover, in contrast to the Y-shaped analogs, which aggregate more as they become less polar, we find that urea mixes well in solution and has little tendency to aggregate. For our analysis of aggregation, we used a novel approach based on Voronoi polyhedra as well as the traditional method of radial distribution functions. In conclusion, we discuss how urea’s unique behavior in comparison to its Y-shaped analogs has clear implications for models of urea solvation and mechanisms of urea protein denaturation.
A time-dependent discrete variable representation for (multiconfiguration) Hartree methods
Authors: Manthe Uwe
Published: July 2002

A time-dependent discrete variable representation for evaluation of potential energy matrix elements in Hartree and multiconfiguration Hartree approaches is presented. In contrast to other time-dependent discrete variable representations, the scheme presented treats separable parts of the potential exactly and not by quadrature. Thus, accurate results can be obtained even for very small sizes of the time-dependent grid. Time-dependent Hartree and multiconfiguration Hartree calculations for the photodissociation of NOCl after S0→S1-excitation demonstrate the accuracy of this discrete variable representation.
Molecular dynamics simulation of elevated temperature interfacial behavior between silica glass and a model crystal
Authors: Webb Edmund B. , Garofalini Stephen H.
Published: July 2002

Elevated temperature atomistic behavior was investigated using classical molecular dynamics simulations of solid state interfaces. Initially, observations on a Lennard-Jones (LJ) crystal surface interfaced with an ideal vacuum were made. Assignment of temperatures associated with specific amounts of crystal surface disorder was possible. A temperature was observed at and above which disorder propagated through all planes of mobile atoms, making it possible to establish an approximate transition temperature for surface nucleated melting of the LJ crystal. Similar high temperature simulations were then performed on silica glass/LJ crystal interfaces at two system stress levels. No significant dependence of interface behavior on the stress states which were studied was observed. The presence of the glass surface resulted in a depression of the temperature needed for the surface most planes of crystal atoms to roughen. This allowed LJ atoms to sample and occupy sites in the glass surface. Additional data presented shows this behavior was at least partly a function of the open structure inherent in glassy oxide surfaces.
Product rotational polarization in photon-initiated bimolecular reactions
Authors: Aoiz F. J. , Brouard M. , Enriquez P. A.
Published: July 2002

This paper concerns the semiclassical description, calculation and measurement of angular momentum polarization in the products of elementary gas-phase bimolecular reactions. A unified, semiclassical treatment of the centre-of-mass correlated (k,k′,j′) angular distribution involving the reagent and product relative velocity and the product angular momentum vectors is described, and is related to other methodologies already existing in the literature. Explicit expressions are provided enabling experimentalists to extract rotational polarization information from crossed-molecular beam and photon-initiated reaction studies, under a variety of experimental conditions. Furthermore, the strategy developed is well suited to the theoretical calculation of reaction product polarization, in particular, using classical trajectory methods. An illustrative example of such a calculation is presented, and the centre-of-mass polarization data provided is used to simulate the laboratory frame rotational moments that can be determined experimentally using 1+1 Doppler-resolved polarized Laser product probing techniques.
An analysis of rotational transition probabilities and cross sections using close coupling, hard shape, and classical trajectory methods
Authors: Belchior J. C. , Braga J. P.
Published: July 2002

Classical and quantum transition probabilities were calculated applying the two (2D) and three (3D) dimensional close-coupled and classical trajectory methods. The collision energy was 0.01 eV and, in this case, there are 12 classical accessible states for Xe−CO2 collisions. It was shown that for Δj≥8 the 2D and 3D transition probabilities agree quantitatively. The state-to-state total cross sections were also compared, in the same level of dimension, and the results showed similar trends. In addition, the hard shape quantum cross sections were compared with the 2D results and showed that the classical turning point cannot be used to obtain the semi-axes of the ellipse, namely A and B. Moreover, the intensity of the hard shape cross sections showed to be different either for the 2D or 3D close coupling calculations. The comparison between the transition probabilities, carried out by the 2D and 3D quantum methods, showed the nature of the 2D results with accurate agreement for large Δj transitions.
Convergence and path cancellation in quantum Monte Carlo real time path integration
Authors: Gentile Ann C. , Evensky David A. , Durant Joseph L. , Brown Nancy J. , Koszykowski Michael L.
Published: July 2002

We find that poor resolution of the tails of the distribution of integrands obtained inhibits convergence in Monte Carlo calculation of real time path integrals. We show that many methods previously tried to improve convergence neither resolve nor diminish the tails effectively. We find that large contributions to the integrand come from paths that have a large variance from the zero-path and/or that have few zero crossings. The results of crude dampings based on where such paths are poorly sampled suggest that exploring cancellations of paths characterized by path variance and zero crossings may be effective in improving convergence.
Adsorption of water on Si(100)-(2×1): A study with density functional theory
Authors: Konečný Robert , Doren D. J.
Published: July 2002

Adsorption of water on the Si(100)-(2×1) surface has been investigated using density functional theory and cluster models of the surface. The reaction pathway and geometries of the product, the transition state and a molecular precursor state are described. There is no energy barrier to dissociative chemisorption. Adsorbed H and OH fragments are most stable when bonded to the same surface dimer with the hydroxyl oriented away from the surface dimer bond. The orbital and electrostatic interactions that determine the adsorbate and transition state geometries are analyzed. Surface distortion (dimer buckling) is a recurring theme in this analysis. Interactions of adsorbed molecular fragments with each other and with dangling bonds have significant effects, modifying the adsorbate geometry and leading to adsorbate islanding. Calculated vibrational frequencies of adsorbed H2O on Si(100)-(2×1) are discussed. The theoretical results are consistent with most available experimental results, and provide a microscopic description of the interactions that account for the observations.
Split in subtransition of phospholipid bilayer under high pressure
Authors: Utoh Shigenori
Published: July 2002

Characteristics of the subtransition in distearoyl phosphatidylcholine multilamellar dispersion were investigated by differential thermal analyses and x-ray diffraction under high pressure. Two phase transitions other than the well-known pre- and main-transitions were seen when the phospholipid was pressed above about 250 MPa and then once heated beyond the melting temperature. These phase transitions were accompanied by distinct changes of the chain-packing conformation from an orthorhombic symmetry to a quasihexagonal one. It is hypothesized that the subtransition under atmospheric pressure splits into these two transitions as a result of high-pressure melting.
Coupled cluster response functions revisited
Authors: Pedersen Thomas Bondo , Koch Henrik
Published: July 2002

We introduce an inherently real coupled cluster time-dependent expectation value of a Hermitian operator. Based on the expansion of this expectation value in orders of the generally time-dependent perturbation, we subsequently identify the coupled cluster time-independent expectation value, the linear response function, and the quadratic response function. The response functions and their residues behave physically correctly. Spectroscopic observables are identified as residues, whereas the identification of individual transition matrix elements is prohibited. Thus the unphysical behavior of previously published coupled cluster response functions may be viewed not as a consequence of the projection, but rather that identifications are made on the basis of an unphysical expectation value or quasienergy.
Interaction potential of Al3+ in water from first principles calculations
Authors: Wasserman Evgeny , Rustad James R. , Xantheas Sotiris S.
Published: July 2002

We present a parametrization of the interaction potential for Al3+ in water from first principles calculations. We have performed a critical study of the Al3+–water interaction using sequences of correlation consistent basis sets that approach the complete basis set limit and include core-valence correlation effects. We suggest as minimum theoretical requirements treatment of the electron correlation at the MP2 level of theory using a triple zeta quality basis set that accounts for the effect of core-valence correlation. The latter amounts for an increase of ∼5 kcal/mol (3%) to the stabilization energy, a shortening of 0.015 Å in the Al–O distance, and an increase of 22 cm−1 in the harmonic frequency of the Al–O vibration. This is the first time that core-valence effects were investigated for this system. The stabilization energy of the Al3+(H2O) cluster is 201 kcal/mol and the corresponding Al–O bond length is 1.719 Å at the MP2 level of theory with the cc-pwCVQZ basis set. This minimum is metastable with respect to the Al2++H2O+ asymptote since even the second ionization potential (IP) of Al is larger than the first IP of water. The hexa-aqua cluster Al3+(H2O)6 is, however, stable upon dissociation to Al3+(H2O)5+H2O by 64.8 kcal/mol, demonstrating the capacity of “effective” solvation in stabilizing the charge on the cation. The optimal structures of the n=5 and 6 clusters (having C2v and Th symmetries, respectively) and their harmonic vibrational frequencies are the first ones reported at the MP2 level with basis sets of this size. Core-valence correlation effects for the n=6 cluster are found to be of similar magnitude with those observed for the n=1 cluster. The stabilization energy of the n=6 cluster with respect to its fragments is 723.7 kcal/mol and the corresponding Al–O distance is 1.911 Å. These results were used in order to parametrize a pairwise-additive interaction potential for aluminum–water interaction that was grafted onto the Toukan–Rahman interaction potential for water. The potential model reproduces the ab initio results for Al3+(H2O)6 within 2.0 kcal/mol for the stabilization energy and 0.003 Å for R(Al–O) distance. Using this potential we estimated the enthalpy of solvation of Al3+ to be −1106±6 kcal/mol, therefore favoring the lower value of the experimentally obtained data (−1115 and −1140 kcal/mol, respectively). In addition, we calculate the first peak of the Al–O radial distribution function at 1.885 Å, in excellent agreement with x-ray diffraction studies that suggest a peak at 1.882±0.004 Å. We compute the first peak of the Al–H radial distribution function at 2.473 Å and the average angle between the plane of a water molecule and the Al–O vector at −28.27°.
Predissociation dynamics of the A 2Σ+ state of SH and SD
Authors: Wheeler Martyn D. , Orr-Ewing Andrew J. , Ashfold Michael N. R.
Published: July 2002

The technique of cavity ring-down spectroscopy has been used to investigate predissociation in the A 2Σ+ state of the SH and SD radicals. Spectra were recorded of the A–X (1,0) band of SH and the (1,0), (2,0) bands of SD. Linewidth measurements of transitions to individual rovibrational levels of the A state revealed increasing predissociation rates with vibrational and rotational quantum number. These and all other available data have been reproduced, quantitatively, by Fermi Golden Rule calculations employing the best (experimentally determined) analytic potential for the A state and ab initio repulsive potentials and spin–orbit coupling matrix elements.
Brownian dynamics simulation of a polymer molecule in solution under elongational flow
Authors: Agarwal U. S. , Bhargava Rohit , Mashelkar R. A.
Published: July 2002

We use Brownian dynamics simulation to study coil–stretch transition of macromolecules in solution. Into a simple elongational flow field, we introduce freely jointed bead-rod chain model molecules in their coiled and stretched states, and follow the conformational changes. We find good agreement of our simulation results with the available theoretical predictions for low and high strain rates (ε̇). At the intermediate elongation rates (near the onset of coil–stretch transition) of the flow field, we find that the residence time required for stretching of an initially coiled chain can be extremely large as compared to predicted (1+ln(√N))ε̇−1, especially for the non-free-draining case. Hence, the chain conformation is dependent on the initial state of the chain molecule for residence time as long as 100ε̇−1. Thus, hysteresis is predicted when chain residence time in such an elongational flow field is limited, as in practical situations. Further, at such intermediate ε̇, the chain molecule is seen to undergo Brownian fluctuation induced jumps between a randomly coiled state and another partially stretched state. This suggests the existence of more than one equilibrium conformation that is unstable to Brownian fluctuations.
Broadband dielectric study of anomalous diffusion in a poly(propylene glycol) melt confined to nanopores
Authors: Schönhals A. , Stauga R.
Published: July 2002

The molecular dynamics of an oligomeric poly(propylene glycol) (PPG) melt confined to nanopores (pore dimensions 2.5, 5.0, and 7.5 nm) is studied by broadband dielectric spectroscopy. Because poly(propylene glycol) has an in chain dipole moment, the molecular motion of the whole chain (normal mode relaxation) can be measured by dielectric spectroscopy, in addition to the dynamic glass transition (α relaxation). For the α relaxation it was found that near the glass transition temperature its relaxation rate is greater within the pores compared to the bulk. The possible reasons for this speeding up of the α relaxation in the nanopores are discussed. This discussion includes the density effect, the influence of surface–melt interactions, as well as the concept of the hindered glass transition. Besides the α relaxation, a further relaxation process (N1 process) was found for the PPG molecules confined to the nanopores. The relaxation rate of the N1 process is drastically shifted to lower frequencies, compared to both the α relaxation and to the normal mode process, and shows a pronounced dependence on the pore size. This relaxation process was assigned to the dynamics of a whole chain or larger parts of it. The slowing down of its relaxation rate compared to that of the bulk material was mainly controlled by adsorption effects and the dependence on the pore size could be understood in the framework of a simple model.
Quantum transition state theory: Perturbation expansion
Authors: Shao Jiushu , Liao Jie-Lou , Pollak Eli
Published: July 2002

The exact quantum expression for the thermal rate of reaction is the trace of a product of two operators. It may therefore be written exactly as a phase space integral over the Wigner phase space representations of the two operators. The two are a projection operator onto the product’s space, which is difficult to compute, and the symmetrized thermal flux operator, which can be computed using Monte Carlo methods. A quantum transition state theory was presented recently, in which the exact projection operator was replaced by its parabolic barrier limit. Alternatively, the exact projection operator may be replaced by its classical limit. Both approximations give thermodynamic estimates for the quantum rates. In this paper, we derive a perturbation theory expansion for the projection operator about the parabolic barrier limit and the classical limit. The correction terms are then used to evaluate the leading order corrections to the rate estimates based on the parabolic barrier or classical limits of the projection operator. The expansion is applied to a symmetric and an asymmetric Eckart barrier. The first two terms in the expansion give excellent results for temperatures above the crossover between quantum tunneling and thermal activation. For deep tunneling and asymmetric systems, the use of variational transition state theory, the classical limit, and perturbation theory leads to significant improvement in the estimate of the tunneling rate. Multidimensional extensions are presented and discussed.
Atomic and electronic structure of neutral and charged SinOm clusters
Authors: Nayak S. K. , Rao B. K. , Khanna S. N. , Jena P.
Published: July 2002

Using molecular orbital approach and the generalized gradient approximation in the density functional theory, we have calculated the equilibrium geometries, binding energies, ionization potentials, and vertical and adiabatic electron affinities of SinOm clusters (n⩽6,m⩽12). The calculations were carried out using both Gaussian and numerical form for the atomic basis functions. Both procedures yield very similar results. The bonding in SinOm clusters is characterized by a significant charge transfer between the Si and O atoms and is stronger than in conventional semiconductor clusters. The bond distances are much less sensitive to cluster size than seen for metallic clusters. Similarly, calculated energy gaps between the highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) of (SiO2)n clusters increase with size while the reverse is the norm in most clusters. The HOMO-LUMO gap decreases as the oxygen content of a SinOm cluster is lowered eventually approaching the visible range. The photoluminescence and strong size dependence of optical properties of small silica clusters could thus be attributed to oxygen defects.
Local stimulation induces long-range order in spatio-temporal disorder
Authors: Baier Gerold , Sahle Sven , Chen Jyh-Phen , Hoff Axel A.
Published: July 2002

A spatially extended model of diffusively coupled (bio)chemical oscillators creates spatio-temporal disorder (hyperchaos) characterized by complex and weakly correlated concentration patterns in space and time. Applying different types of local stimulation we show that the system forms stable patterns with well-defined global order for certain windows of parameters. Spatio-temporal hyperchaos thus supports efficient pattern control with minimum effort. A possible relevance of this mechanism for experimentally observed calcium wave patterns is pointed out.
Comparison of Monte Carlo data and Percus–Yevick calculations of molecular pair correlation functions of a solute molecule in a simple liquid
Authors: Franosch T. , Singh A. P.
Published: July 2002

The solutions of the Percus–Yevick integral equation are compared with Monte Carlo results for different heteronuclear hard-core dumbbells immersed in a one- or two-component hard-sphere liquid for packing fractions up to 0.45. It is shown that the Percus–Yevick theory provides an accurate description even for the angular correlations induced by the anisotropic solute–solvent interaction.
Geometry of dimer reconstruction on the C(100), Si(100), and Ge(100) surfaces
Authors: Yang C. , Kang H. Chuan
Published: July 2002

We performed density-functional calculations using cluster models of the C(100), Si(100), and Ge(100) surfaces in order to address two issues. First, we resolve the differences in the results from slab calculations and from cluster calculations. Second, we want to contribute to an understanding of the nature and energetics of dimer buckling on these surfaces. We performed calculations using a number of different geometry constraints and three different cluster sizes. The results show that for, at least for density-functional cluster calculations, the geometry, and the buckling energetics are both significantly dependent upon both the choice of geometry constraints and the size of the cluster. Our calculations show that the ground state has a symmetric dimer geometry for the carbon surface and an asymmetric dimer geometry for the silicon and germanium surfaces. This is in agreement with the latest first-principles slab calculations and also consistent with experimental results. Some previous cluster calculations favor a symmetric dimer on the silicon surface. Our density-functional results suggest that the use of either inadequate cluster sizes or inappropriate geometry constraints or a combination of both could have affected these previous calculations. The change in energy of the cluster as a function of the dimer buckling angle is also investigated for all three surfaces. We observed that dimer-buckling is driven by a lowering of the kinetic energy of the electrons. We also find that the nature of the dimer bond is qualitatively different between the carbon surface on the one hand and the silicon and germanium surfaces on the other. We rationalize this in terms of the small core size of the carbon atom and relate it to the different ground-state dimer symmetry found for the C(100) surface as opposed to Si(100) and Ge(100) surfaces.
The calculation of molecular parameters for a molecule with an internal rotor
Authors: Duan Yun-Bo , Wang Li , Wu Xudong T. , Mukhopadhyay Indranath , Takagi Kojiro
Published: July 2002

A derivation for the formulas to calculate centrifugal distortion constants, based on a recent formulation [Duan and Takagi, Phys. Lett. A (1995)] of centrifugal distortion effects for a molecule containing a threefold symmetric internal rotor, is presented. Some constants which are independent of the barrier derivatives, especially the constants representing interactions between torsion and rotation, are given in terms of molecular structural parameters and force constants. These calculated constants are helpful in the reduction of the Hamiltonian and in the analysis of observed transitions. The derived formulas are applied to numerical calculations of the centrifugal distortion constants of methanol. It is shown that most of the calculated constants are in good agreement with those obtained from the fitting to experimental data.
Photodissociation of hydrogen sulfide at 157.6 nm: Observation of SH bimodal rotational distribution
Authors: Liu X. , Hwang D. W. , Yang X. F. , Harich S. , Lin J. J. , Yang X.
Published: July 2002

Photodissociation of the H2S molecule at 157.6 nm was studied experimentally using the Rydberg tagging technique. Translational energy distributions of the H-atom product from the H2S photodissociation were measured, and the SH(X 2Π)+H(2S) channel was found to be the dominant dissociation process. Spin-orbit and rovibrational state distributions were also obtained for the SH product, which was found to be both vibrationally and rotationally excited. An intriguing bimodal rotational distribution in the lowest two vibrational states, v=0 and 1, has been clearly observed for the SH product, indicating that there are two distinctive dissociation mechanisms involved in the photodissociation of H2S at 157 nm excitation.
Monte Carlo simulations of hydrophobic polyelectrolytes: Evidence of complex configurational transitions
Authors: Chodanowski Pierre , Stoll Serge
Published: July 2002

Off-lattice Monte Carlo simulations, including electrostatic and Lennard-Jones potentials, are performed to investigate at the limit of counterion condensation the dilute conformations of quenched polymer chains in poor solvent conditions. The Monte Carlo (MC) search procedure is improved to achieve dense conformations in the limit of both strong attractive and repulsive interaction potentials. Configurational properties such as the radius of gyration, and single chain structure factors are calculated as a function of attractive monomer–monomer interactions, ionic concentration, and monomer number. It is observed that hydrophobic polyelectrolytes exhibit a large variety of conformations compared to flexible or semiflexible polyelectrolytes without hydrophobic groups. MC results demonstrate that there is a range of electrolyte concentration and hydrophobicity for which polymers exhibit exotic but stable conformations, namely the pearl necklace and the cigar-shape conformation. It is shown that by gradually increasing the monomer hydrophobicity of a strong polyelectrolyte, it undergoes a cascade of transitions from an extended structure to a pearl necklace, a pearl necklace to a cylinder, and a cylinder to a coil, successively. Smooth transitions are observed when competition between hydrophillic and hydrophobic interactions is important, whereas sharp transitions are obtained in the other cases. A good agreement is observed with the theoretical description of weakly charged polyelectrolytes in poor solvent conditions. To have an insight of the spatial organization of the chain monomers, scattering functions are calculated for each characteristic conformation. It is shown that large changes in the scattering curve profiles can be expected when the hydrophobicity of the backbone has a definite influence on the chain dimensions.
Vector signatures of adiabatic and diabatic dynamics in the photodissociation of ICN
Authors: Costen Matthew L. , North Simon W. , Hall Gregory E.
Published: July 2002

Nascent Doppler profiles of CN (X 2∑+) fragments from the Ã band photodissociation of room temperature ICN have been measured using high-resolution transient frequency modulated absorption spectroscopy. Results for dissociation at 222 nm, 248 nm, 266 nm, and 308 nm are presented. From the Doppler profiles of multiple CN states, we determine branching ratios of the coincident atomic iodine states, and bipolar moments characterizing the CN velocity and angular momentum anisotropy. The measurements provide sensitive tests of the strengths of optical coupling to the excited states contributing to the Ã band continuum, and the adiabatic and diabatic dynamics leading to the observed product states. Precise velocity measurements resolve differences in the average energy of the ICN molecules leading to selected fragment channels. We find a bond energy for ICN of 26 980±100 cm−1, somewhat higher than previous literature values.
Structure and vibrations of dihydroxybenzene cations and ionization potentials of dihydroxybenzenes studied by mass analyzed threshold ionization and infrared photoinduced Rydberg ionization spectroscopy as well as ab initio theory
Authors: Gerhards M. , Unterberg C. , Schumm S.
Published: July 2002

In this paper structures of resorcinol (1,3-dihydroxybenzene) and hydroquinone (1,4-dihydroxybenzene) cations are presented. The adiabatic ionization potentials of the dihydroxybenzenes and the vibrations of the ionic ground state of the dihydroxybenzene cations are experimentally obtained by mass analyzed threshold ionization (MATI) and infrared photoinduced Rydberg ionization (IR-PIRI) spectroscopy. In contrast to the catechol (1,2-dihydroxybenzene) cation, two isomers of different symmetry can be obtained for resorcinol and hydroquinone cations. The MATI and IR-PIRI spectra of the isomers of resorcinol will be interpreted by comparing the experimental results with the results of density functional theory (DFT) and ab initio calculations performed at the complete active space self-consistent field (CASSCF) level of theory. Furthermore, the adiabatic ionization potentials as well as the excitation energies (for the S1←S0 transition of neutral dihydroxybenzenes) are derived from CASSCF calculations for the S0, S1, and D0 states including second order Mo/ller–Plesset (MP2) and zero point energy corrections. The experimentally observed ionization potentials of all isomers of all dihydroxybenzenes can be predicted from the calculated ionization potentials with excellent accuracy. The relative shifts of ionization potentials of different isotopomers can also be predicted. In this paper partially deuterated d1-resorcinol up to d5-resorcinol is investigated. By comparing the experimentally observed excitation energies and ionization potentials (between deuterated and undeuterated resorcinol) with the calculated values, an assignment of the electronic origins of the resonant two-photon ionization (S1←S0 excitation) and MATI spectra to different isotopomers can be given.
Local density augmentation in attractive supercritical solutions: Inhomogeneous fluid approach
Authors: Egorov S. A.
Published: July 2002

We study the local solute–solvent structure in supercritical solutions, using as a model system a dilute Lennard-Jones solute in a supercritical Lennard-Jones fluid. The study is focused on attractive mixtures, which are characterized by enhanced local solvent density around the solute molecule. We employ the integral equation theory for inhomogeneous fluids to calculate the solute–solvent radial distribution function and the local coordination number of the solute. We show that this theory is in excellent agreement with Monte Carlo simulations and provides a substantial improvement over the integral equation theory formulated for homogeneous fluids. Using the inhomogeneous fluid theory, we perform a detailed study of the local density enhancement in attractive supercritical mixtures, and analyze its dependence on the model potential parameters and on the solvent thermodynamic conditions.
Highest weight representations for gl(m/n) and gl(m+n)
Authors: Le Blanc R. , Rowe D. J.
Published: July 2002

It is shown how vector coherent state (VCS) theory enlightens the simultaneous discussion of representation theory for classical Lie algebras and superalgebras and provides an optimal framework for the explanation of the noted similarities and dissimilarities of their representations. Reducibility, atypicality, and the positive-definitiveness of inner products in VCS representation spaces are discussed. The discussion is exemplified through the parallel and explicit construction of highest weight ladder representations of the Lie algebra gl(m+n) and superalgebra gl(m/n) in gl(m)⊕gl(n) bases.