bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational coherence in electron transfer: The tetracyanoethylene–pyrene complex
Authors: Wynne Klaas , Reid Gavin D. , Hochstrasser Robin M.
Published: July 2002

Coherent vibrational wave packet motion is created in the excited charge-transfer state of the electron donor–acceptor complex between tetracyanoethylene (TCNE) and pyrene by an ultrashort (40 fs) 810 nm pump pulse. Observations of the dynamics of the TCNE–anion transient absorption and the disappearance of the bleach of the ground state absorption show that the electron-transfer reaction back to the ground state of the complex occurs on a 250 fs–1.5 ps time scale. The bleach recovery signal shows clear oscillations and both impulsive stimulated Raman scattering in the ground state and coherent repopulation of the ground state surface due to a vibrationally coherent electron transfer reaction were considered as the cause. Vibrational coherence has also been monitored by observing quantum beats in the stimulated emission from the charge transfer state back to the ground state in the near-ir. This observation strongly suggests that the electron transfer reaction is indeed vibrationally coherent and that the reaction rate is modulated by this coherence. This interpretation is corroborated by a classical Monte Carlo simulation of vibrationally coherent reactions in the inverted regime.
Nonadiabatic wave packet dynamics: Predissociation of IBr
Authors: Vrakking Marc J. J. , Villeneuve D. M. , Stolow Albert
Published: July 2002

We present results of fs pump–probe experiments on the predissociation dynamics of the IBr B-state, as a model of nonadiabatic wave packet dynamics on coupled potential energy curves. The observations reflect the complicated wave packet motion on the coupled potentials and oscillatory behavior of the decay rates as a function of excitation energy.
Water’s character from dielectric relaxation above its T
g
Authors: Johari G. P.
Published: July 2002

To determine the merits of the conjecture for a distinct phase of liquid water below 160 K, and of its computer simulation [Nature 380, 328 (1996)], the dielectric relaxation times of pure bulk water and pure water sequestered in the pores of various polymer matrices are examined. The new data combined with the literature values show that the molecular kinetics of pure water at 273 K is connected by a continuous path with that of sequestered water at 160 K and below. This finding seems to remove the basis for the conjecture of a distinct phase of water in the 136–160 K range, and for its phase transformation to normal water in the 160–230 K range.
Nonadiabatic transition and energy relaxation dynamics in the photoisomerization of s-trans butadiene
Authors: Ito Masakatsu , Ohmine Iwao
Published: July 2002

Photoisomerization dynamics of s-trans butadiene is investigated by a semiclassical surface hopping trajectory method. The Heisenberg model Hamiltonian is developed to describe two covalent states involved in this process, 11Ag and 21Ag states. This model Hamiltonian well reproduces a global structure of the potential energy surfaces of these states and the nonadiabatic coupling of an ab-initio method. It was found that the nonadiabatic decay from the 21Ag to the 11Ag state takes place at the three partially twisted CC bond conformations. The CCC bending motions largely enhance the nonadiabatic transitions. The internal vibrational relaxation associated with the nonadiabatic transition is also analyzed.
Multistate vibronic coupling effects in the K-shell excitation spectrum of ethylene: Symmetry breaking and core-hole localization
Authors: Köppel H. , Gadea F. X. , Klatt G. , Schirmer J. , Cederbaum L. S.
Published: July 2002

The vibrational fine structure of the prominent C1s-π* absorption band of ethylene and some of its isotopomers has been investigated theoretically with the aid of a specific (linear) vibronic coupling model. The presence of two equivalent C1s levels gives rise to two nearly degenerate electronic states of g and u symmetry, respectively, which can interact vibronically via the (planar) antisymmetric C–H stretching and bending modes (ν11 and ν12). In addition to these states of immediate interest, the present model comprises three more pairs of “effective” states at higher energy allowing one to describe the excitation of additional asymmetric (nonplanar) modes as a result of linear vibronic coupling. To a good approximation the intrapair vibronic coupling can be eliminated by using a representation in terms of localized C1s hole states. As a further result, the in-plane and out-of-plane modes become separable. The required vibronic coupling constants have been determined with the aid of ab initio calculations at various nuclear conformations using a second-order polarization propagator method. After slight readjustment of some of the parameters, the calculated spectral profiles are found to be in excellent agreement with the experimental findings. The theoretical spectra reflect strong excitation of nontotally symmetric modes (and the concomitant symmetry lowering), involving mainly the planar C–H stretching mode ν11 and the out-of-plane C–H bending mode ν8. While the planar distortion is a consequence of the equivalent core levels, the nonplanar symmetry lowering can be rationalized in chemical terms as a rehybridization effect in which the sp2 bonding scheme in the ethylene ground state is changed to sp3 in the excited state.
Dependency of swelling on the length of subchain in poly(N,N-dimethylacrylamide)-based gels
Authors: Bromberg Lev , Grosberg Alexander Yu. , Matsuo Eriko Sato , Suzuki Yasuo , Tanaka Toyoichi
Published: July 2002

The equilibrium swelling degree of poly(N,N-dimethylacrylamide) (polyDMAAm)-based gels in water was systematically studied with various initial monomer (cm) and cross-linker (cxl) concentrations, and as a function of initial concentration of fully ionized co-monomer, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS). Proportionality of the length of a subchain (N) in the gels to [cxl(cm+cxl)]−1 was revealed, both theoretically and experimentally. Equilibrium swelling data indicate non-Gaussian elasticity of the gel network.
Photofragmentation study of Cl2 using ion imaging
Authors: Samartzis Peter C. , Sakellariou Ioannis , Gougousi Theodosia , Kitsopoulos Theofanis N.
Published: July 2002

The Cl(2P3/2) and Cl(2P1/2) photofragments produced by the photolysis of Cl2 at ∼3.5 eV, are detected using (3+1) and (2+1) resonance enhanced multiphoton ionization and their velocity distributions are measured using ion imaging. The measured photofragment translational energy distributions yield a bond dissociation energy of D0=2.474±0.020 eV for ground electronic state Cl2(X1Σg). The angular distributions yield anisotropy parameters of β(2P3/2)=−0.87±0.05 and β(2P1/2)= 1.78±0.05 irrespective of the detection process. The branching ratio [Cl(2P1/2)]/[Cl(2P3/2)] is estimated at 8±1%.
Is the direct observation of electronic coherence in electron transfer reactions possible?
Authors: Lucke Andreas , Mak C. H. , Egger Reingold , Ankerhold Joachim , Stockburger Juergen , Grabert Hermann
Published: July 2002

The observability of electronic coherence in electron transfer reactions is discussed. We show that under appropriate circumstances large-amplitude oscillations can be found in the electronic occupation probabilities. The initial preparation of the system is of crucial importance for this effect, and we discuss conditions under which experiments detecting electronic coherence should be feasible. The Feynman–Vernon influence functional formalism is extended to examine more general and experimentally relevant initial preparations. Analytical expressions and path integral quantum dynamics simulations were developed to study the effects of various initial preparations on the observability of electronic coherence.
Molecular dynamics simulations of dielectric properties of dimethyl sulfoxide: Comparison between available potentials
Authors: Skaf Munir S.
Published: July 2002

We present a molecular dynamics simulation study of the orientational correlations of molecular dipoles and dielectric properties of a series of recently proposed models for liquid dimethyl sulfoxide. The dipole–dipole correlations, as measured by the dipolar symmetry projections h110(r) and h112(r), are very similar for the models with four partially charged sites, but differ somewhat for the potential with no charges on the methyl groups. The effects of the charges on the methyl sites upon the local ordering of the molecular dipoles are discussed by comparing the h110(r) projections for two models which differ only in their site-charge assignments. The Kirkwood g factors for the simulated force fields turn out all greater than unity, in disagreement with some experimental estimates. The dielectric constants, however, are in good agreement with reported experimental data. In addition, we discuss the wave vector (k) dependence of the longitudinal static dielectric responses and also the single-particle molecular reorientation dynamics, which is found to differ from model to model. Likely consequences of that to the dielectric relaxation of these models are briefly discussed.
Degenerate four-wave mixing and photofragment yield spectroscopic study of jet-cooled SO2 in the C̃ 1B2 state: Internal conversion followed by dissociation in the X̃ state
Authors: Okazaki Akihiro , Ebata Takayuki , Mikami Naohiko
Published: July 2002

The predissociation mechanism of the C̃ state of SO2 have been investigated by the measurements of degenerate four-wave mixing (DFWM), laser-induced fluorescence (LIF) and photofragment yield spectra. The DFWM spectrum of jet-cooled SO2 was measured under a high power condition where the DFWM signal corresponds well to the absorption spectrum. Relative fluorescence quantum yields of various vibronic bands were obtained by comparing the DFWM spectrum with the LIF spectrum. It was found that the fluorescence quantum yields of the vibronic levels involving anti-symmetric stretching (ν3) vibration are considerably smaller than those of other levels. The result indicates that the nonradiative process of the C̃ state is initiated as the internal conversion to the X̃ state and ν3 acts as the promoting mode. By observing the O atom photofragment yield spectrum we confirmed that the predissociation occurs from the (1,4,2) level. Vibronic dependence of the rise time of the O atom generation was found to be in accordance with those of the fluorescence decay rate. The obtained results represent that the dissociation occurs immediately after the internal conversion to the X̃ state.
Pressure and temperature effects on H2 spin-lattice relaxation times and H1 chemical shifts in tert-butyl alcohol- and urea-D2O solutions
Authors: Yoshida Koji , Ibuki Kazuyasu , Ueno Masakatsu
Published: July 2002

The pressure and temperature effects of hydrophobic hydration were studied by NMR spectroscopy. The H1 chemical shifts (δ) were measured at 7.7, 29.9, and 48.4 °C under high pressure up to 294 MPa for HDO contained as impurity in neat D2O, 1 mol kg−1 tert-butyl alcohol (TBA)-D2O, and 1 mol kg−1 urea-D2O solutions, for the methyl group of TBA in the TBA-D2O solution, and for the amino group of urea in the urea-D2O solution. The H2 spin-lattice relaxation times (T1) were measured under the same conditions as the chemical shift measurements for D2O in neat D2O, TBA-D2O and urea-D2O solutions with organic contents up to 8 mol%. The following features are observed for the pressure effect on δ (HDO) and H2-T1 in TBA-D2O solutions: (1) The δ (HDO) exhibits a downfield shift relative to that in neat D2O, and the difference of δ (HDO) between TBA solution and neat D2O becomes larger with increasing pressure at lower temperature. (2) The decrement of the rotational correlation time of water in the hydration shell of TBA (τcs) relative to the value at atmospheric pressure is smaller than that in the bulk (τc0). (3) The pressure coefficients of T1 are positive in dilute solutions but are negative in more than 4 to 5 mol% solutions. These results suggest that the hydrophobic hydration shell of TBA is different than the open structure of water present in bulk, and resists pressure more strongly than the open structure of water in the bulk. In solutions of 4 to 5 mol%, the hydration shell collapses. On the other hand, the τcs in the hydration shell of urea is slightly larger than that in bulk water at lower pressure, but is obviously larger at higher pressure. In view of the rotational motion of water molecules, urea seems to strengthen the water structure slightly rather than weaken it, although δ (HDO) approaches that in the bulk with pressure. It is difficult to classify urea into a structure maker or a breaker.
Stretch–bender calculations of the effects of orbital angular momentum and vibrational resonances in the spectrum of singlet methylene
Authors: Duxbury Geoffrey , Alijah Alexander , McDonald Barry D. , Jungen Christian
Published: July 2002

The stretch–bender Hamiltonian for a symmetric triatomic molecule, which we have recently derived for a symmetric triatomic molecule [Duxbury et al., J. Chem. Phys. 108, 2336 (1998), preceding paper], has been used in conjunction with the Barrow, Dixon, and Duxbury [Mol. Phys. 27, 1217 (1974)] and the Jungen and Merer [Mol. Phys. 40, 25 (1980)] methods of solving the Renner–Teller coupling problem in molecules which are executing large amplitude nuclear motion, to calculate the vibronic structure of the ã 1A1 and b̃ 1B1 states of the methylene radical CH2. The results of the compact stretch–bender variational calculations are compared with the more extensive calculations of Green et al. [J. Chem. Phys. 94, 118 (1991)] to show that good agreement between the two methods has been demonstrated. We have also compared the relative and complementary uses of following the variation of the calculated expectation value of the orbital angular momentum, 〈Lz〉, and of the “end over end” rotation constants B and q, as increasingly complex vibronic mixing is caused by the interplay between Renner–Teller and anharmonic coupling. In the present example the anharmonic coupling is principally due to Fermi resonance terms.
An investigation of dynamical density functional theory for solvation in simple mixtures
Authors: Yoshimori Akira , Day Tyler J. F. , Patey G. N.
Published: July 2002

Linear and nonlinear versions of time dependent density functional theory are solved for a single solute particle in a simple binary solvent. All particles interact with Lennard-Jones potentials. The theoretical results are compared with molecular dynamics calculations. It is shown that the nonlinear theory is necessary in order to obtain a good quantitative description of selective solvation dynamics. The linear theory is only of qualitative value. Also, attention is drawn to a previously little appreciated problem which arises when one attempts to compare time dependent density functional theory with computer simulation or experimental results. The difficulty involves matching the theoretical and absolute time scales and is discussed in detail in this paper.
A perturbative density functional approach to the structure of colloidal suspension
Authors: Choudhury Niharendu , Ghosh Swapan K.
Published: July 2002

A simple perturbative density functional approach which incorporates the three-body correlation in an approximate manner is used to study the structure of inhomogeneous as well as homogeneous colloidal suspension. The suspension is modeled as a one component fluid of charged macro particles interacting with the well known Derjaguin-Landau-Verwey-Overbeek (DLVO) potential. We have calculated the density profiles of inhomogeneous colloidal suspension in front of a highly repulsive charged wall as well as in a planar slit consisting of two parallel planar charged walls. The radial distribution function g(r) of the homogeneous suspensions of charged colloidal particles has also been calculated by making use of the well known Percus identity which relates the density distribution of an inhomogeneous fluid to the g(r) of the corresponding homogeneous fluid when the external potential responsible for the inhomogeneity is the interparticle potential itself. The calculated results on the density profiles and g(r) obtained from the present approach are found to be in closer agreement with the simulation results and show a marked improvement over those obtained from perturbative as well as nonperturbative density functional theories presented in earlier works.
Benchmark calculations with correlated molecular wave functions. IX. The weakly bound complexes Ar–H2 and Ar–HCl
Authors: Woon David E. , Peterson Kirk A. , Dunning Thom H.
Published: July 2002

The interaction of Ar with H2 and HCl has been studied using Mo/ller–Plesset perturbation theory (MP2, MP3, MP4) and coupled-cluster [CCSD, CCSD(T)] methods with augmented correlation consistent basis sets. Basis sets as large as triply augmented quadruple zeta quality were used to investigate the convergence trends. Interaction energies were determined using the supermolecule approach with the counterpoise correction to account for basis set superposition error. Comparison with the available empirical potentials finds excellent agreement for both binding energies and transition state. For Ar–H2, the estimated complete basis set (CBS) limits for the binding energies of the two equivalent minima and the connecting transition state (TS) are, respectively, 55 and 47 cm−1 at the MP4 level and 54 and 46 cm−1 at the CCSD(T) level, respectively [the XC(fit) empirical potential of Bissonnette et al. [J. Chem. Phys. 105, 2639 (1996)] yields 56.6 and 47.8 cm−1 for H2 (v=0)]. The estimated CBS limits for the binding energies of the two minima and transition state of Ar–HCl are 185, 155, and 109 cm−1 at the MP4 level and 176, 147, and 105 cm−1 at the CCSD(T) level, respectively [the H6(4,3,0) empirical potential of Hutson [J. Phys. Chem. 96, 4237 (1992)] yields 176.0, 148.3, and 103.3 cm−1 for HCl (v=0)]. Basis sets containing diffuse functions of (dfg) symmetries were found to be essential for accurately modeling these two complexes, which are largely bound by dispersion and induction forces. Highly correlated wave functions were also required for accurate results. This was found to be particularly true for ArHCl, where significant differences in calculated binding energies were observed between MP2, MP4, and CCSD(T).
Photochemistry of (OCS)n− cluster ions
Authors: Sanov Andrei , Nandi Sreela , Jordan Kenneth D. , Lineberger W. Carl
Published: July 2002

We report the photochemistry of (OCS)n− cluster ions following 395 nm (n=2–28) and 790 nm (n=2–4) excitation. In marked contrast to (CO2)n−, extensive bond breaking and rearrangement is observed. Three types of ionic products are identified: S2−(OCS)k, S−(OCS)k/OCS2−(OCS)k−1, and (OCS)k−. For n<16, 395 nm dissociation is dominated by S2−-based fragments, supporting the theoretical prediction of a cluster core with a C2v (OCS)2− dimer structure and covalent C–C and S–S bonds. A shift in the branching ratio in favor of S−-based products is observed near n=16, consistent with an opening of the photodissociation pathway of OCS− core-based clusters. These monomer-based cluster ions may coexist with the dimer-based clusters over a range of n, but electron detachment completely dominates photodissociation as long as their vertical electron detachment energy, increasing with addition of each solvent molecule, is less then the photon energy. An (OCS)2− conformer of C2 symmetry with a covalent C–C bond is believed to be responsible for 790 nm dissociation of (OCS)2−, yielding primarily OCS− products. The yield of OCS−, and thus the importance of the C2 form of (OCS)2− cluster core, decreases with increasing n, perhaps due to more favorable solvation of the C2v form of (OCS)2− and/or a solvent-induced increase in the rate of interconversion of conformers. The (OCS)k− products observed in 395 nm photodissociation of the larger (n⩾7) clusters are attributed to photofragment caging. Formation and dissociation mechanisms of clusters with different core types are discussed. The photochemical properties of (OCS)n− are compared to those of the isovalent (CO2)n− and (CS2)n− species.
Self-consistent field theory of diblock copolymer melts at patterned surfaces
Authors: Petera D. , Muthukumar M.
Published: July 2002

Self-consistent field theory is used to investigate the density profile of diblock copolymer melts in a thin film confined between two surfaces, one of which carries a chemically active pattern. The temperature is slowly lowered through the critical point in order to obtain well organized structures. For strong surface interactions, the lamellae for symmetric diblock copolymers comply with the surface pattern. Their orientation depends on the ratio of natural bulk period L0 to surface period L. For L0⩽L, the lamellae tilt at an angle θ=arcsin L0/L with respect to the surface. For L0⩾L, the diblocks are distorted close to the surface and the necessary relaxation off the surface induces parallel oriented lamellae with respect to the surface for films thicker than 2 lamellar periods. Very thin films still support the perpendicular orientation.
Nonadiabatic wave packet dynamics: Experiment and theory in IBr
Authors: Shapiro Moshe , Vrakking Marc J. J. , Stolow Albert
Published: July 2002

We present an experimental and a theoretical study of nonadiabatic wave packet dynamics in the intermediate coupling regime as exhibited by the IBr molecule. Using a femtosecond pump–probe molecular beam technique, we generated a wave packet which evolves on the electronically excited B 3Π0+/Y(0+) coupled states. The wave packet dynamics was detected by a time-delayed probe pulse which induced two photon ionization to the ground state of the IBr+ ion. The study consisted of a systematic variation of the pump laser wavelength from the crossing point of the two coupled states to the dissociation limit of the bound diabatic state. The theoretical study is based on the convolution of the products of the energy resolved X 1Σ+→B 3Π0+/Y(0+) bound–free dipole matrix elements and the free–bound two-photon ionization amplitudes (calculated exactly using the artificial channel method) with the profiles of the pump and probe pulses. The theoretical calculations reproduce the general decay, recurrence, and revivals observed experimentally. The importance of treating nonadiabatic dynamics beyond the Landau–Zener approximation, as well as the utility of femtosecond pump–probe techniques in probing simultaneously short and long lived resonances is demonstrated.
Symmetry extensions of Euler’s polyhedral theorem and the band theory of solids
Authors: Ceulemans A. , Chibotaru L. F. , Fowler P. W. , Szopa M.
Published: July 2002

Irreducible symmetry representations characterize topological invariants of point group molecules. In the present paper this result is extended to periodic structures in two and three dimensions, using plane and space symmetry groups. Applications include a symmetry analysis of the band structure for the graphite sheet and related nanotubes of the leapfrog type.
Double flip of orientation for a lamellar diblock copolymer under shear
Authors: Leist Heike , Maring Daniel , Thurn-Albrecht Thomas , Wiesner Ulrich
Published: July 2002

Lamellar diblock copolymers are investigated under large amplitude oscillatory shear (LAOS) at temperatures close to the order–disorder transition temperature (TODT). Increasing strain amplitude leads to a double flip of orientation. The state of orientation depends on an effective shear rate as in similar experiments on lyotropic lamellar phases.
Concerning the ionization of large polyatomic molecules with intense ultrafast lasers
Authors: DeWitt Merrick J. , Levis Robert J.
Published: July 2002

The relative photoionization/dissociation probabilities are presented for the molecules benzene, naphthalene, and anthracene upon interaction with 780 nm laser radiation of duration 170 fs and intensity 3.8×1013 W cm−2. Both the ionization probability and the dissociation yield increase exponentially from benzene to anthracene as measured by time-of-flight mass spectra. A structure-based model is presented for the excitation of large polyatomic molecules by intense laser irradiation with pulse widths on the time scale of molecular vibration (100 fs) and with peak field strengths of 1–2 V Å−1. The model accounts for molecular structure and is able to accurately predict the transition from multiphoton ionization (MPI) to tunnel ionization. It is also demonstrated that this structure-based model can quantitatively predict the experimentally measured ionization probabilities. In comparison, models employing the more conventional zero-range potential do not accurately predict either the transition or the relative ion yield measured experimentally.
Perturbatively selected CI as an optimal source for externally corrected CCSD
Authors: Peris G. , Planelles J. , Malrieu J.-P. , Paldus J.
Published: July 2002

Perturbatively selected configuration interaction (PSCI) is used as a source of higher than pair clusters in the externally corrected coupled cluster method with singles and doubles (ecCCSD). This significantly decreases the dimension of the standard multireference (MR) CISD that is employed by the so-called reduced MR (RMR) CCSD method, thus enabling the use of relatively large active spaces. The performance of the proposed PSCI CCSD method is illustrated by considering the ground state potential energy curves of the HF molecule using DZP and cc-pVTZ basis sets (breaking of a single bond), and of the N2 molecule using a TZ basis set (breaking of a triple bond). It is shown that notwithstanding a large reduction in the dimension of the external source, the accuracy of the resulting ecCCSD energies is almost the same as that obtained when correcting with the full MR CISD wave function.
Molecular polarization in water chains
Authors: Hermansson Kersti , Alfredsson Maria
Published: July 2002

Molecular dipole moments, charges, and difference electron densities have been calculated from Hartree–Fock calculations for each individual molecule along geometry-optimized water chains with n=1,…,12,∞. The complementarity of the three properties is discussed. Although yielding rather different aspects of the polarization they all show that the polarization converges much faster for the molecules at the ends of the chains compared to the middle. Moreover, the middle molecules in the long chains acquire an induced dipole moment about twice as large as those at the ends. An electron charge transfer along the chains leaves only the central water molecules neutral, and creates an additional contribution to the net system dipole, a phenomenon which should arise also in small real finite clusters and chains.
Coupled-cluster methods with internal and semi-internal triply excited clusters: Vibrational spectrum of the HF molecule
Authors: Piecuch Piotr , Kucharski Stanisław A. , Špirko Vladimir
Published: July 2002

The recently proposed, single-reference, coupled-cluster (CC) methods with singly, doubly, and triply excited clusters, in which triexcited clusters T3 are restricted to internal and semi-internal components defined through active orbitals, designated as the CCSD{t′} and CCSDt approaches [P. Piecuch, S. A. Kucharski, and R. J. Bartlett, J. Chem. Phys. 110, 6103 (1999)] have been used to obtain the potential energy function and the vibrational spectrum of the HF molecule, as described by the large, aug-cc-pvtz, basis set. A comparison has been made with the vibrational term values obtained at the very high, full CCSDT (CC singles, doubles, and triples), level and with the experimental (RKR) data. In spite of using the restricted Hartree–Fock reference, the calculated CCSD{t′} and CCSDt vibrational term values have been found to be in much better agreement with the full CCSDT and RKR data than the results of conventional CCSD (CC singles and doubles) calculations, which produce errors of an order of 2000 cm−1 for high-lying states near dissociation. It has been demonstrated that the CCSDt approach is capable of providing the vibrational spectrum of the full CCSDT quality.
A stochastic study of microsolvation. II. Structures of CO in small helium clusters
Authors: Gianturco F. A. , Lewerenz M. , Paesani F. , Toennies J. P.
Published: July 2002

Diffusion Monte Carlo (DMC) calculations of the intermolecular vibrational ground states of CO(He)n clusters with n=1–12, for CO v=0 are reported. The intermolecular degrees of freedom of the clusters are treated in full dimensionality and a pairwise additive potential surface is used in which the He–CO interaction is described by a recently developed scheme which combines density functional theory (DFT) with the long-range dispersion contributions obtained from a perturbative theory. The calculations yield intermolecular ground-state energies, He density distributions, radial and angular density probability distributions. Optimal structures by SIMPLEX minimization have been calculated to estimate zero-point energy (ZPE) and quantum effects.
Distributed first and second order hyperpolarizabilities: An improved calculation of nonlinear optical susceptibilities of molecular crystals
Authors: Reis H. , Papadopoulos M. G. , Hättig C. , Ángyán J. G. , Munn R. W.
Published: July 2002

The method of calculating distributed polarizabilities is extended to the first and second dipole hyperpolarizabilities, in order to describe more accurately the molecular response to strong and inhomogeneous external time-dependent electric fields. The dipolar response is expressed in terms of both potential related charge-density response functions and electric field related dipole-density response functions. The macroscopic linear, quadratic, and cubic optical dipole susceptibilities of molecular crystals are expressed in terms of the distributed (hyper) polarizabilities. This formulation differs from previous theories using distributed dipoles in that it allows for a rigorous treatment of both local induced dipoles and charge flow between different regions of the molecule. As an example, the distributed polarizabilities and first hyperpolarizabilities of urea at the self-consistent-field level are used to calculate the linear and quadratic susceptibilities of the urea crystal. The linear susceptibility does not differ substantially from that calculated with previous less rigorous models for distributed response, but the quadratic susceptibility is about 50% of that calculated with previous models. This indicates that the present treatment of distributed response should give a quadratic susceptibility in good agreement with experimental data, once the effects of electronic correlation, frequency dispersion, and the permanent crystal field are taken into account.
Systematic molecular dynamics studies of liquid N,N-dimethylformamide using optimized rigid force fields: Investigation of the thermodynamic, structural, transport and dynamic properties
Authors: Chalaris Michalis , Samios Jannis
Published: July 2002

The properties of the pure liquid N,N-dimethylformamide were investigated by means of microcanonical (NVE) and isothermal–isobaric (NPT) molecular dynamics simulation techniques. Previously proposed five- and six-interaction-site optimized potential models (OPLS) were employed to simulate the liquid at different state points and their properties were obtained and discussed. The results obtained have shown that these models, to a higher or lower degree, yield realistic descriptions of the liquid. We have found, however, that a systematic readjustment of the potential parameters is necessary in order to describe the liquid properties more accurately. Thus, a new six-interaction-site OPLS computational model for liquid N,N-dimethylformamide has been derived. It turns out that this model can describe not only the thermodynamic and structural properties but also the dynamic (single and Debye relaxation) and the transport properties (self-diffusion, shear viscosity) of the system with good accuracy in the entire temperature range at normal pressure.
SU(2) and SU(1,1) time-ordering theorems and Bloch-type equations
Authors: Dattoli G. , Torre A.
Published: July 2002

Algebraic time-ordering techniques for SU (2) and SU (1,1) coherence preserving Hamiltonians are reviewed. The link with Bloch-type equations is pointed out and the extension of the method to higher groups is briefly discussed.
Cluster expansion in terms of knots in gauge theories with finite non-Abelian gauge groups
Authors: Szlachànyi K. , Vecsernyès P.
Published: July 2002

The cluster expansion is developed in lattice gauge theories with finite gauge groups in d≥3 dimensions in which the clusters are connected (d−2)-dimensional complexes, i.e., connected (d−2) surfaces that can branch along (d−3) cells. The interaction between them has a knot theoretical interpretation. It is a many-body interaction, depending on the type of knot they form together. For small enough gauge coupling g analyticity of the correlation functions in the variable exp(−1/g2) is proven.
An effective method to compute N-fold Darboux matrix and N-soliton surfaces
Authors: Cieśliński Jan
Published: July 2002

Fairly simple formulas for an N-fold Darboux matrix and an N-soliton addition to an arbitrary soliton surface embedded in E3 are proven. They depend directly on the background wave function and are evidently symmetric with respect to N added solitons. As an example, an explicit expression for the interaction of N solitons on a single vortex filament is presented. Moreover, there is given a simplified version of the N-soliton formulas of Neugebauer and Meinel.
The commutative limit of a matrix geometry
Authors: Madore J.
Published: July 2002

The noncommutative geometry of complex n×n matrices is considered as an approximation to the geometry of the two-sphere S2 and the differential forms defined over the matrices are mapped into the ordinary differential forms on SU2 considered as a fiber bundle over S2. It is found that the extra gauge-invariant vacuum studied in previous publications has as image a constant multiple of the connection on SU2 that defines the fibration, the Dirac monopole with unit magnetic charge.
Completeness and geometry of Schrödinger minimum uncertainty states
Authors: Trifonov D. A.
Published: July 2002

The symmetry, geometry, and completeness properties of Schrödinger minimum uncertainty states (SMUS) are considered. SMUS are equivalent to the coherent states with maximal symmetry related to the semidirect product G=Hw×)SU(1,1), Hw being the Heizenberg–Weyl group. The invariant measures on G and on the homogeneous space G/K are constructed as well as noninvariant resolution unity measures. The fiber bundle and the symplectic Kaehler structures of SMUS are elucidated with the implications to their stable evolution.
Recursion operators: Meaning and existence for completely integrable systems
Authors: Landi G. , Marmo G. , Vilasi G.
Published: July 2002

Comments are made on the meaning and existence of recursion operators for completely integrable systems. It is shown that any nonresonant integrable system admits infinitely many Hamiltonian descriptions and recursion operators.
Linking topological quantum field theory and nonperturbative quantum gravity
Authors: Smolin Lee
Published: July 2002

Quantum gravity is studied nonperturbatively in the case in which space has a boundary with finite area. A natural set of boundary conditions is studied in the Euclidean signature theory in which the pullback of the curvature to the boundary is self-dual (with a cosmological constant). A Hilbert space which describes all the information accessible by measuring the metric and connection induced in the boundary is constructed and is found to be the direct sum of the state spaces of all SU(2) Chern–Simon theories defined by all choices of punctures and representations on the spatial boundary 𝒮. The integer level k of Chern–Simons theory is found to be given by k=6π/G2Λ+α, where Λ is the cosmological constant and α is a CP breaking phase. Using these results, expectation values of observables which are functions of fields on the boundary may be evaluated in closed form. Given these results, it is natural to make the conjecture that the quantum states of the system are completely determined by measurements made on the boundary. One consequence of this is the Bekenstein bound, which says that once the two metric of the boundary has been measured, the subspace of the physical state space that describes the further information that may be obtained about the interior has finite dimension equal to the exponent of the area of the boundary, in Planck units, times a fixed constant. Finally, these results confirm both the categorical-theoretic ‘‘ladder of dimensions’’ picture of Crane and the holographic hypothesis of Susskind and ’t Hooft.
Nöther formalism for conserved quantities in classical gauge field theories. II. The arbitrary Bosonic matter case
Authors: Fatibene L. , Ferraris M. , Francaviglia M.
Published: July 2002

A formalism suited to deal with Bosonic matter represented by tensor densities coupled with a gauge field and gravitational field is developed. Nöther conserved currents and superpotentials are found and the relation between the canonical and the Hilbert stress tensors are investigated in the general case. The particular cases of gauge theories with scalar matter fields and of purely geometrical theories with arbitrary matter are recovered from this general formalism; as an example of application we consider scalar densities and the vector matter fields.
Linear convective patterns in cylindrical geometry for unipolar injection
Authors: Agrait N. , Castellanos A.
Published: July 2002

Linear stability theory is used to discriminate between the possible convective structures (axisymmetric rolls, axially independent columns, and general three-dimensional patterns) in the case of unipolar injection between coaxial cylinders of arbitrary radii. The cases of weak, strong, and arbitrary finite injection, either from the inner or outer cylinder, are considered. In all cases three-dimensional modes are found to be the least stable. Physical mechanisms involved are discussed thoroughly.
Dispersion of particles injected nonuniformly in a mixing layer
Authors: Wang Lian-Ping
Published: July 2002

The dispersion of fluid elements distributed nonuniformly at the interface of a two-dimensional, temporally evolving mixing layer was studied. The average transverse dispersion of fluid elements was found to be either enhanced or reduced markedly by simply varying the initial number density distribution along the interface. The increase in dispersion is due to the nonuniform stretching of the interface during the growth of the vortical structure. This naturally leads to a conclusion that the dispersion rate of particles in a spatially evolving mixing layer can be controlled by injecting particles nonuniformly in time.
Instabilities in two-dimensional spatially periodic flows. Part II: Square eddy lattice
Authors: Thess André
Published: July 2002

A numerical study is performed on the linear stability of square arrays of alternating vortices with emphasis on confinement effects and linear friction. The appearance of a global rotation as a result of instability in a strongly confined system consisting of four vortices is explained in terms of the first unstable mode and the calculated critical parameter is found to be in agreement with magnetohydrodynamic experiments. The existence of bicritical points in systems with more than four vortices is demonstrated. Stability of the unbounded system is treated in the framework of Floquet theory. It is found that, in the presence of linear friction, the basic flow is unstable with respect to quasiperiodic perturbations with generally incommensurable spatial frequencies. This lays the foundation for a hypothesis about transition to turbulence via spatial quasiperiodicity.
The x-ray-driven heating wave
Authors: Kaiser N. , Meyer-ter-Vehn J. , Sigel R.
Published: July 2002

The heating of matter by a source of intense x rays is studied under conditions where the energy transport in the hot material is carried by the flux of incident photons. This leads to the formation of a self-similar heating wave. Particular emphasis is given to the ablative heating wave that exists for a time interval in between supersonic heating initially and diffusive heat transport at later times. An approximate, analytic solution of the similarity equations describing the ablative heating wave is given. A power law dependence of the opacity on temperature and density is assumed, which is strictly valid for a fully ionized plasma and monochromatic incident radiation, but possibly also useful to model more complicated situations in terms of effective parameters.
Modeling the pressure gradient–velocity correlation of turbulence
Authors: Speziale Charles G.
Published: July 2002

The modeling of the pressure gradient–velocity correlation of turbulence is considered. Two distinctly different approaches have been proposed in the turbulence literature: one in which the pressure gradient–velocity correlation is decomposed into a pressure-strain correlation and a pressure-diffusion correlation, and another in which the pressure gradient–velocity correlation is split into its deviatoric and isotropic parts. By examining the limit of two-dimensional turbulence, it is demonstrated that the models obtained from the former approach are inconsistent with the Navier–Stokes equations in a fundamental way, whereas the models obtained from the latter approach are not. Consequently, it appears that the direct modeling of the pressure gradient–velocity correlation in its deviatoric and isotropic parts should be favored. The implications that this result has on turbulence modeling are discussed briefly.
Comparison of the molecular dynamics method and the direct simulation Monte Carlo technique for flows around simple geometries
Authors: Meiburg Eckart
Published: July 2002

The molecular dynamics method (MD) and the direct simulation Monte Carlo technique (DSMC) are compared with respect to their capability of simulating vorticity distributions. The statistical assumptions underlying the evaluation of the collision term by the DSMC are analyzed. They lead to the nonconservation of angular momentum for the interaction of particles. Both methods yield equally good results for the Rayleigh–Stokes flow. Using the present parameters, however, only the MD simulation shows the generation of vortices for the flow past an inclined flat plate. This might indicate an effect on the computation of vortical flows. It is suggested that further systematic studies of the effect of the cell size and particle dimensions be carried out.
Particle size effects on Lagrangian turbulence
Authors: McLaughlin John B.
Published: July 2002

Perturbation methods are used to show that the equations of motion for a small rigid sphere in a steady laminar flow take the form of a dynamical system in which phase volume is not conserved. In the absence of stagnation points, particle inertia and virtual mass effects destroy Lagrangian turbulence and the particles are captured by periodic or quasiperiodic orbits that are associated with the vortices of the flow. When gravitational effects are included, it is found that point particles can sediment chaotically, but that particle inertia and virtual mass effects tend to eliminate the chaotic behavior. An interesting consequence of the latter phenomenon is that finite particles which are denser than the fluid can be permanently suspended in three-dimensional cellular flows. Numerical results are presented for the Arnold–Beltrami–Childress [C. R. Acad. Sci. Paris 261, 17 (1965)] flows.
Fluctuation spectra and variances in convective turbulent boundary layers: A reevaluation of old models
Authors: Yaglom A. M.
Published: July 2002

Most of the existing theoretical models for statistical characteristics of turbulence in convective boundary layers are based on the similarity theory by Monin and Obukhov [Trudy Geofiz. Inst. Akad. Nauk SSSR 24(151), 163 (1954)], and its further refinements. A number of such models was recently reconsidered and partially compared with available data by Kader and Yaglom [J. Fluid Mech. 212, 637 (1990); Turbulence and Coherent Structures (Kluwer, Dordrecht, 1991), p. 387]. However, in these papers the data related to variances 〈u2〉=σ2u and 〈v2〉=σ2v of horizontal velocity components were not considered at all, and the data on horizontal velocity spectra Eu(k) and Ev(k) were used only for a restricted range of not too small wave numbers k. This is connected with findings by Kaimal et al. [Q. J. R. Meteorol. Soc. 98, 563 (1972)] and Panofsky et al. [Boundary-Layer Meteorol. 11, 355 (1977)], who showed that the Monin–Obukhov theory cannot be applied to velocity variance σ2u and σ2v and to spectra Eu(k) and Ev(k) in energy ranges of wave numbers. It is shown in this paper that a simple generalization of the traditional similarity theory, which takes into account the influence of large-scale organized structures, leads to new models of horizontal velocity variances and spectra, which describe the observed deviations of these characteristics from the predictions based on the Monin–Obukhov theory, and agree satisfactorily with the available data. The application of the same approach to the temperature spectrum and variance explains why the observed deviations of temperature spectrum in convective boundary layers from the Monin–Obukhov similarity does not lead to marked violations of the same similarity as applied to temperature variance 〈t2〉=σ2t.
Interaction of wake turbulence with a free surface
Authors: Logory Larry M. , Hirsa Amir , Anthony Douglas G.
Published: July 2002

The turbulent wake of a flat plate aligned with a uniform water flow and extending through the free surface was investigated experimentally. Laser-Doppler velocimetry (LDV) measurements show good agreement with published data for a two-dimensional wake, except in a shallow layer near the free surface. In this surface layer, the wake width is observed to double while the wake centerline velocity remains essentially unchanged from its value at depth, resulting in a wake momentum deficit that is twice that at depth. Instantaneous, full-field measurements of the velocity were made using digital particle image velocimetry (DPIV) to elucidate the role of vortical structures in the development of the surface layer. DPIV measurements reveal that in the deep wake, vortex structures predominantly of opposite sign exist on opposite sides of the wake centerline and contribute to a velocity and vorticity field that is two-dimensional in the mean. However, near the surface, vortex structures tend to become either surface-normal or surface-parallel and contribute to a velocity and vorticity field that is highly three-dimensional in the mean. The resulting surface layer is characterized by surface–parallel structures interacting with their images above the surface to retard and widen the surface flow to a depth comparable to the size of the vortex structures. Histograms taken from many independent DPIV realizations of the flow characterize the distribution of vorticity in the wake and verify a mean flow consistent with the LDV measurements. The existence of these structures in the surface layer is further confirmed by flow visualization using laser-induced fluorescence.