bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermometric study of CO2 laser heating and radiative cooling of n-butylbenzene ions
Authors: Uechi Guy T. , Dunbar Robert C.
Published: July 2002

Working in the Fourier-transform ion cyclotron resonance ion trap, the competitive photodissociation of n-butylbenzene ions at 440 nm has been developed as a thermometric probe to monitor the instantaneous internal energy of the ion population. Using this probe, the heating of initially thermal ions by CO2 laser irradiation was followed as a function of time at several laser intensities. The IR multiphoton dissociation of the population was followed simultaneously. The ions were observed to heat up progressively during the induction period, during which no dissociation was observed. Dissociation commenced as the ions leveled off at a steady-state internal energy ∼0.8 eV, and dissociation then proceeded approximately as a first-order decay at constant average internal energy. After heating of the population, the CO2 laser was turned off and IR-radiative cooling of the ions was observed with a cooling rate constant of 0.97 s−1. The kinetics of heating, cooling, and dissociation followed the qualitative expectations from previous work, but quantitative modeling required some modification of the previously used simple harmonic photophysical picture. It seemed most likely that this ion possesses a modest inverse pumping bottleneck with up pumping slowing by a factor of 3 at high internal energies (above 0.8 eV). Enhanced radiative relaxation at internal energies above 1.0 eV can also bring the kinetic modeling into quantitative agreement with experiment.
A general method for the prediction of the three dimensional structure and folding pathway of globular proteins: Application to designed helical proteins
Authors: Kolinski Andrzej , Godzik Adam , Skolnick Jeffrey
Published: July 2002

Starting from amino acid sequence alone, a general approach for simulating folding into the molten globule or rigid, native state depending on sequence is described. In particular, the 3D folds of two simple designed proteins have been predicted using a Monte Carlo folding algorithm. The model employs a very flexible hybrid lattice representation of the protein conformation, and fast lattice dynamics. A full rotamer library for side group conformations, and potentials of mean force of short and long range interactions have been extracted from the statistics of a high resolution set of nonhomologous, 3D structures of globular proteins. The simulated folding process starts from an arbitrary random conformation and relatively rapidly assembles a well defined four helix bundle. The very cooperative folding of the model systems is facilitated by the proper definition of the model protein hydrogen bond network, and multibody interactions of the side groups. The two sequences studied exhibit very different behavior. The first one, in excellent agreement with experiment, folds to a thermodynamically very stable four helix bundle that has all the properties postulated for the molten globule state. The second protein, having a more heterogeneous sequence, at lower temperature undergoes a transition from the molten globule state to the unique native state exhibiting a fixed pattern of side group packing. This marks the first time that the ability to predict a molten globule or a unique native state from sequence alone has been achieved. The implications for the general solution of the protein folding problem are briefly discussed.
Resonant excitation and decay of core holes in condensed layers of furan and pyrrole
Authors: Mauerer M. , Zebisch P. , Weinelt M. , Steinrück H.-P.
Published: July 2002

Near edge x-ray absorption spectra and decay electron spectra for resonant and nonresonant excitations in the C 1s and N 1s regions have been measured for condensed furan (C4H4O) and pyrrole (C4H4NH) layers. The decay spectra after resonant excitation into the lowest unoccupied orbital (π*4) are pure autoionization spectra. The participant channels show strong resonant enhancements of the photoelectron peaks, in particular after excitation of the C 1s core electrons. Overall however, the spectra are dominated by the spectator channels. The spectator spectra resemble the normal Auger spectra shifted by spectator shifts of ≊4 eV for furan and ≊2.5 eV for pyrrole. For furan the participant spectra for excitation at the two different carbon atoms show significant differences that can be qualitatively understood in terms of a simple one-electron picture by considering the localization of the corresponding orbitals on the core hole. The decay spectra after excitation into the π*5 orbital show negligible enhancement of the participant channel, a strongly reduced spectator shift, and less-pronounced structures than at higher photon energies. The latter is attributed to a competition of spectator decay and normal Auger decay after internal photoionization into Rydberg derived bands in the condensate. For excitations into the σ*-resonances pure Auger spectra are observed, indicating that these resonances fully ionize before core decay takes place.
Expansion and crystallization of a sediment of charged colloidal spheres
Authors: van Duijneveldt Jeroen S. , Dhont Jan K. G. , Lekkerkerker Henk N. W.
Published: July 2002

An amorphous sediment of charged colloidal particles in a nonaqueous solvent was prepared by centrifugation. The evolution of sediment height and volume fraction profile to equilibrium were studied with light scattering. A simple sedimentation–diffusion equation allows the experimental data to be modeled quantitatively. The top of the sediment crystallizes within a week, whereas the lower part of the sediment does not show any sign of crystallization after several months, indicating a strong concentration dependence of crystallization rate.
Optical response of Ag2, Ag3, Au2, and Au3 in argon matrices
Authors: Fedrigo S. , Harbich W. , Buttet J.
Published: July 2002

We report on the optical response of size selected Ag2, Ag3, Au2, and Au3 embedded in argon matrices. Cluster samples were investigated in situ by excitation, fluorescence, and absorption spectroscopy. The spectra for the dimers are in agreement with previous measurements made on conventionally prepared matrices. Our previously reported spectra of trimers in krypton are confirmed by the argon results. The influence of the matrix gas will briefly be discussed.
Higher-order response theory based on the quasienergy derivatives: The derivation of the frequency-dependent polarizabilities and hyperpolarizabilities
Authors: Sasagane Kotoku , Aiga Fumihiko , Itoh Reikichi
Published: July 2002

The higher-order response theory to derive frequency-dependent polarizabilities and hyperpolarizabilities is examined by means of the differentiation of the ‘‘quasienergy’’ with respect to the strengths of the time-dependent external field, which is referred to as the quasienergy derivative (QED) method. This method is the extension of the energy derivative method to obtain static polarizabilities and hyperpolarizabilities to a time-dependent perturbation problem. The form of the quasienergy W = 〈Φ‖Ĥ − i(∂/∂t)‖Φ〉 is determined from the time-dependent Hellmann–Feynman theorem. The QED method is accomplished when the total sum of the signed frequencies of the associated field strengths, with respect to which the quasienergy is differentiated, is equated to 0. The QED method is applied to the single exponential-transformation (SET) ansatz (up to the fifth-order QEDs) and the double exponential-transformation (DET) ansatz (up to the fourth-order QEDs), where the time-dependent variational principle (TDVP) is employed to optimize the time development of the system. The SET ansatz covers the full configuration interaction (CI) response and the Hartree–Fock response (i.e., the TDHF approximation), while the DET ansatz covers the multiconfiguration self-consistent field (MCSCF) response (i.e., the TDMCSCF approximation) and the limited CI response with relaxed orbitals. Since the external field treated in this paper is always ‘‘polychromatic,’’ the response properties explicitly presented for both the SET and DET ansätze are μA, αAB(−ω;ω), βABC(−ωσ;ω1,ω2), and γABCD(−ωσ;ω1,ω2,ω3), in addition δABCDE(−ωσ;ω1,ω2,ω3,ω4) is presented for the SET ansatz. All variational formulas for these response properties derived in this study automatically satisfy the (2n+1) rule with respect to the variational parameters.
Hydrogen bond dynamics in benzoic acid dimers as a function of hydrostatic pressure measured by nuclear magnetic resonance
Authors: Horsewill A. J. , McDonald P. J. , Vijayaraghavan D.
Published: July 2002

The dynamics of hydrogen atoms in the hydrogen bonds of benzoic acid dimers have been studied as a function of hydrostatic pressure to pressures in excess of 4 kbar. This paper is primarily concerned with results up to 3.3 kbar. The temperature dependence of the correlation time for the motion at a series of pressures has been investigated using measurements of the proton spin–lattice relaxation time. Strong non-Arrhenius behavior is exhibited and the data are in good agreement with a model which invokes phonon assisted tunneling at low temperature and thermally activated Arrhenius dynamics at high temperature. The parameters in the model include the asymmetry of the double minimum potential experienced by the hydrogen atoms and dynamical variables relating to the tunneling and hopping processes. The rate of phonon assisted tunneling is observed to increase exponentially with increasing pressure and this is attributed to the increase in the tunneling matrix element which occurs as the distance between the potential wells is decreased and the overlap of the localized eigenstates beneath the barrier increases. We also observe a decrease in the asymmetry of the potential with increasing pressure which is attributable to modifications to the interdimer contributions to the potential. There is evidence in the nuclear magnetic resonance (NMR) data for two phase transitions below 4 kbar and one of these may be correlated with the reduction in asymmetry of the potential.
Ab initio studies of cyclic water clusters (H2O)n, n=1–6. II. Analysis of many-body interactions
Authors: Xantheas Sotiris S.
Published: July 2002

The magnitudes of the two- through six-body energy terms and their contribution to the interaction energy of small ring water clusters (n=2–6) are computed at the Hartree–Fock (HF) and second- through fourth-order many-body perturbation (MP2, MP4) levels of theory. The analysis is performed at the minimum energy geometries reported earlier [J. Chem. Phys. 99, 8774 (1993)]. The correlation correction is found to account for a 10%–20% increase in the individual two-body terms and a much larger increase of 75% for the three-body and 200% for the small four-body terms. The MP4 results are found to differ only slightly (<2%) from the corresponding MP2 results. We have found that three-body terms have a significant contribution as high as 30% to the interaction energy of the larger clusters and that four-body and higher order terms are negligible. The total and incremental association energies for the processes n H2O→(H2O)n and (H2O)n−1+H2O→(H2O)n, n=2–6 are also reported.
Asymptotic decay of correlations in liquids and their mixtures
Authors: Evans R. , Leote de Carvalho R. J. F. , Henderson J. R. , Hoyle D. C.
Published: July 2002

We consider the asymptotic decay of structural correlations in pure fluids, fluid mixtures, and fluids subject to various types of inhomogeneity. For short ranged potentials, both the form and the amplitude of the longest range decay are determined by leading order poles in the complex Fourier transform of the bulk structure factor. Generically, for such potentials, asymptotic decay falls into two classes: (i) controlled by a single simple pole on the imaginary axis (monotonic exponential decay) and (ii) controlled by a conjugate pair of simple poles (exponentially damped oscillatory decay). General expressions are given for the decay length, the amplitude, and [in class (ii)] the wavelength and phase involved. In the case of fluid mixtures, we find that there is only one decay length and (if applicable) one oscillatory wavelength required to specify the asymptotic decay of all the component density profiles and all the partial radial distribution functions gij(r). Moreover, simple amplitude relations link the amplitudes associated with the decay of correlation of individual components. We give explicit results for the case of binary systems, expanding on and partially correcting recent work by Martynov. In addition, numerical results for g(r) for the pure fluid square-well model and for gij(r) for binary hard sphere mixtures are presented in order to illustrate the fact that the asymptotic forms remain remarkably accurate at intermediate range. This is seen to arise because the higher order poles are typically well-separated from the low order ones. We also discuss why the asymptotics of solvation forces for confined fluids and of density profiles of inhomogeneous fluids (embracing wetting phenomena) fall within the same theoretical framework. Finally, we comment on possible modifications to the theory arising from the presence of power-law attractive potentials (dispersion forces).
Electronic properties of a dilute polarizable fluid: A Green’s function approach
Authors: Schvaneveldt Stephen J. , Loring Roger F.
Published: July 2002

We present a new theoretical analysis of a harmonic model of a polarizable fluid. In this Drude oscillator model, particles with classical mechanical translational degrees of freedom contain internal quantum harmonic oscillators, representing electronic degrees of freedom, which are coupled by dipolar interactions. We relate the calculation of optical properties such as the density of states and the absorption spectrum at finite wave vector to the determination of a Green’s function. A graphical analysis of this Green’s function is shown to suggest a hierarchy of approximations. Calculations based on two members of this hierarchy are compared to recently published simulations of optical properties for this model. Because of the long range of dipolar interactions, this model shows interesting optical properties even in the limit of low particle number density. Our method is shown to agree well with simulation data in this limit.
Cooperative effects in hydrogen bonding: Fourth-order many-body perturbation theory studies of water oligomers and of an infinite water chain as a model for ice
Authors: Suhai Sándor
Published: July 2002

As a step toward the first principles quantum mechanical modeling of the structural and electronic properties of ice, hydrogen-bonded periodic infinite chains of water molecules have been investigated by the ab initio crystal orbital method at the Hartree–Fock (HF) level and by including electron correlation up to the complete fourth order of Mo/ller–Plesset perturbation theory (MP4). The Bloch functions of the crystal have been expanded in a series of high quality atomic orbital basis sets complemented by extended sets of polarization functions, up to TZ(3d2f,3p2d). Basis set superposition errors have been (partly) eliminated by the counterpoise method and the infinite lattice sums have been computed using the multipole expansion technique. The systematically increasing size of the basis sets has allowed the extrapolation of structural and electronic indices of this ice model to the limit of an infinite atomic basis at both the HF and various correlated levels, respectively. For each theoretical model, detailed comparisons have been made with the corresponding physical properties of water monomers, dimers, and some larger linear oligomers.

The results convincingly prove that hydrogen bonding in ice is a highly cooperative phenomenon, both from the structural and energetic points of view. The cohesive energy per hydrogen bond of the crystal is −5.30 kcal/mol at the HF level (with RHFO,O=2.88 Å) as compared with the dimer value of −3.60 kcal/mol (at the optimized distance of 3.03 Å). At the MP2 level of theory, the crystalline binding energy decreases to −6.60 kcal/mol and the lattice contracts to RMP2O,O=2.73 Å (compared with −4.50 kcal/mol at 2.88 Å for the dimer). The correlation corrections at third and fourth order slightly expand the crystal lattice (to RMP4O,O=2.75 Å) and reduce the cohesion by 0.15 kcal/mol. A decomposition of the intermolecular interactions according to different terms of MP4 theory suggests that the cohesive energy of ice results from a delicate balance between different repulsive and attractive terms in third and fourth order, which exhibit different long-range behaviors. The detailed study of the role of high-energy virtual energy bands in computing electron correlation effects in ice provides further insight into the important role that basis set flexibility plays in such investigations. The resulting cohesive energy of −6.83 kcal/mol at the MP4 level is in reasonable agreement with the experimental energy per hydrogen bond in ice I, −6.7 kcal/mol.
Analysis of the crystal-field spectra of the actinide tetrafluorides. II. AmF4, CmF4, Cm4+:CeF4, and Bk4+:CeF4
Authors: Liu G. K. , Carnall W. T. , Jursich G. , Williams C. W.
Published: July 2002

We report a systematic analysis of the crystal-field spectra of four fluoride compounds containing tetravalent actinide ions. The first part of this work [J. Chem. Phys. 95, 7194 (1991)] provided interpretation of the absorption spectra of UF4, NpF4, and PuF4. To extend our analysis to heavier elements of the series, low-temperature absorption spectra of AmF4 and CmF4, and site selective laser-induced emission and excitation spectra of Cm4+:CeF4 and Bk4+:CeF4 were obtained. A model energy level calculation was found to be in good agreement with the experimental results. It is shown that the crystal-field interaction in combination with spin–orbital coupling results in significant J mixing in the excited states, but ground state wave functions are still relatively pure in J character for the tetravalent actinide ions Am4+, Cm4+, and Bk4+. Trends in the parameters of the effective operator Hamiltonian are compared with those of a Hartree–Fock free-ion model. Interpretation of the ground-state splitting of the nominal S-state ion Bk4+ in CeF4 and color center formation in AmF4 are also discussed.
Compact valence bond functions with breathing orbitals: Application to the bond dissociation energies of F2 and FH
Authors: Hiberty Philippe C. , Humbel Stéphane , Byrman Carsten P. , van Lenthe Joop H.
Published: July 2002

An original computational method of ab initio valence bond type is proposed, aiming at yielding accurate dissociation energy curves, while dealing with wave functions being very compact and clearly interpretable in terms of Lewis structures. The basic principle is that the wave function is allowed to have different orbitals for different valence bond structures. Thus, throughout the dissociation process, the so-called ‘‘breathing orbitals’’ follow the instantaneous charge fluctuations of the bond being broken by undergoing changes in size, hybridization, and polarization. The method is applied to the dissociation of F2 and FH. For each molecule, a wave function involving only three valence bond configurations yields equilibrium bond lengths within 0.01 Å, and dissociation energies within about 2 kcal/mol of the results of estimated or true full configuration interaction in the same basis sets. The effect of dynamical electron correlation on calculated dissociation energies is analyzed. It is shown that restricting the correlation to its nondynamical part results in an improper treatment of ionic terms due to a mean-field compromise in the optimization of the orbitals.
Three-dimensional study of predissociation resonances by the complex scaled discrete variable representation method: HCO/DCO
Authors: Ryaboy Victor , Moiseyev Nimrod
Published: July 2002

Predissociation resonances of the radicals HCO and DCO were calculated using a three-dimensional (J=0) complex scaled discrete variable representation (DVR) method that was applied previously to a study of the weakly bound van der Waals complex NeICl [Lipkin, Moiseyev, and Leforestier, J. Chem. Phys. 98, 1888 (1993)]. This study represents a first application of the complex scaling method to a full dimensional chemical reactive system described by a fitted ab initio potential energy surface [Bowman, Bittman, and Harding, J. Chem. Phys. 85, 911 (1986)]. It is shown that the calculation method, being applied to a strongly coupled three-dimensional system, provides a unique criteria that makes it possible to identify all resonances in a given energy range as stationary solutions with respect to a complex variational parameter, independently of the resonance widths and their mutual overlapping. About 50 resonances were found for the radical HCO in the energy range between the ground and the second vibrational state of the product diatomic CO, whereas only half of them were located in recent calculations [Dixon, J. Chem. Soc. Faraday Trans. 88, 2575 (1992); Wang and Bowman, J. Chem. Phys. 100, 1021 (1994)]. It was found that the labeling procedure based on a spectroscopic Hamiltonian fit of the bound states and resonance positions agrees completely with the assignment of HCO resonances given in previous calculations, and provides an assignment for the whole set of calculated resonances. Eighty-three resonances of the radical DCO were found in the same energy range.
Computer simulation of sodium disilicate glass
Authors: Smith W. , Greaves G. N. , Gillan M. J.
Published: July 2002

Extensive molecular dynamics simulations have been performed on sodium disilicate glass Na2Si2O5, using realistic empirical interaction potentials. The structure of the simulated glass agrees reasonably with experimental diffraction data. Analysis of this structure is performed by a number of different methods and the results provide support for the modified random network model. Quantitative values of the sodium diffusion coefficient over the temperature range 1000–2000 K yield a diffusion activation energy which is close to the experimental value.
An accurate and simple equation of state for hard disks
Authors: Santos A. , López de Haro M. , Bravo Yuste S.
Published: July 2002

An equation of state for a fluid of hard disks is proposed: Z=[1−2η+(2η0−1)(η/η0)2]−1. The exact fit of the second virial coefficient and the existence of a single pole singularity at the close-packing fraction η0 are the only requirements imposed on its construction. A comparison of the prediction of virial coefficients and of the values of the compressibility factor Z with those stemming out of other known equations of state is made. The overall performance of this very simple equation of state is quite satisfactory.
Vibrational spectrum and structure of silicon trioxide SiO3: A matrix isolation infrared and density functional theory study
Authors: Tremblay Benoît , Roy Pascale , Manceron Laurent , Alikhani M. Esmaïl , Roy Denis
Published: July 2002

Molecular silicon trioxide has been isolated in argon matrices at low temperature (i) as a secondary reaction product within Si+2(O2) aggregates or (ii) as a primary product in the direct reaction of silicon monoxide with oxygen molecules after UV excitation at 5.17 eV. The silicon trioxide molecule has been characterized through the observation of five fundamental vibrations for at least eight different isotopic species, namely 28Si16O3, 29Si16O3, 30Si16O3, 16OSi18O2, 18OSi16O18O, 16OSi16O18O, 18OSi16O2, and Si18O3. From the experimental data, a structure of the C2v symmetry can be deduced with a Si=O double bond much shorter than two equivalent Si–O single bonds. These findings are fully supported by DFT calculations, which moreover predict a large binding energy of about 60 kcal/mol with respect to the separate fragments (SiO and O2). The possible reaction pathways are also discussed.
Collisional vibrational energy transfer of OH (A 2Σ+, v′=1)
Authors: Williams Leah R. , Crosley David R.
Published: July 2002

Vibrational energy transfer (VET) and quenching of the v′=1 level of A 2Σ+ OH have been studied using laser-induced fluorescence in a discharge flow cell at room temperature. VET cross sections (Å2) are N2, 30.1±2.8; O2, 2.8±0.3; Ar, 0.56±0.05; H2O, 8.6±0.6. The rotational energy distribution in v′=0 following the VET event was determined for nine colliders. It is nonthermal, generally populating high rotational levels. There are three broad categories of colliders that cause varying degrees of vibrational to rotational energy transfer; H2, D2, and CH4 show the least; N2, CO2, CF4, and N2O more; and O2 and Ar the most, with about one-third of the vibrational energy appearing as OH rotation.
Further analysis of solutions to the time-independent wave packet equations of quantum dynamics. II. Scattering as a continuous function of energy using finite, discrete approximate Hamiltonians
Authors: Huang Youhong , Iyengar Srinivasan S. , Kouri Donald J. , Hoffman David K.
Published: July 2002

We consider further how scattering information (the S-matrix) can be obtained, as a continuous function of energy, by studying wave packet dynamics on a finite grid of restricted size. Solutions are expanded using recursively generated basis functions for calculating Green’s functions and the spectral density operator. These basis functions allow one to construct a general solution to both the standard homogeneous Schrödinger’s equation and the time-independent wave packet, inhomogeneous Schrödinger equation, in the non-interacting region (away from the boundaries and the interaction region) from which the scattering solution obeying the desired boundary conditions can be constructed. In addition, we derive new expressions for a ‘‘remainder or error term,’’ which can hopefully be used to optimize the choice of grid points at which the scattering information is evaluated. Problems with reflections at finite boundaries are dealt with using a Hamiltonian which is damped in the boundary region as was done by Mandelshtam and Taylor [J. Chem. Phys. 103, 2903 (1995)]. This enables smaller Hamiltonian matrices to be used. The analysis and numerical methods are illustrated by application to collinear H+H2 reactive scattering.
Vibrational predissociation spectroscopy of Cs+(H2O)1−5
Authors: Weinheimer Corey J. , Lisy James M.
Published: July 2002

Infrared spectra of Cs+(H2O)1−5 were obtained from vibrational predissociation of mass-selected cluster ions in a triple quadrupole mass spectrometer using a pulsed-tunable infrared laser in the 2.6–3.0 μm region. By comparison to size-selective infrared spectra of neutral water clusters, the structure of hydrogen-bonded water clusters complexed to the Cs+ can be observed for cluster ions with three or more water molecules. The onset of hydrogen bonding is also marked by the presence of structural isomers. There is also evidence for an unusual change in the vibrational transition moments for the symmetric and asymmetric O–H stretch, for isolated (non-hydrogen-bonded) water molecules, where the symmetric stretch is substantially enhanced.
Zero kinetic energy pulsed field ionization (ZEKE-PFI) spectroscopy of electronically and vibrationally excited states of I+2: The A 2Π3/2,u state and a new electronic state, the a 4Σ−u state
Authors: Cockett M. C. R. , Donovan R. J. , Lawley K. P.
Published: July 2002

Both one- and two-color zero kinetic energy-pulsed field ionization (ZEKE-PFI) spectra of the first electronically excited state of I+2 (A 2Π3/2,u) as well as a new electronic state, the a 4Σ−u state, have been recorded for the first time. In the one-color (two photon) experiment, transitions to the quartet state are formally spin-forbidden and this is reflected in the weak transition intensity observed compared with the A 2Π3/2,u state. However, in the two-color (1+2′) experiment, which used the valence B 3Π0+u state as an intermediate, transitions into both A 2Π3/2,u and a 4Σ−u states are fully allowed and appear in the spectrum with comparable intensity. The a 4Σ−u state appears in the one-color experiment by virtue of spin–orbit coupling with excited electronic states for which direct ionization from the neutral ground state is fully allowed. Values for ω̄exe of 0.46±0.01 cm−1 for the A 2Π3/2,u state and 0.38±0.02 cm−1 for the a 4Σ−u state were derived, together with lower limits for ω̄e of 138±2 and 128±2 cm−1, respectively. In addition, highly vibrationally excited levels of the X 2ΠΩ,g state have been probed in the one-color experiments, by exploiting a propensity for extensive electronic autoionization involving Rydberg states based on the excited X 2Π1/2,g and A 2ΠΩ,u states as well as the dissociative B 2Σ+g state. In the case of the X 2Π3/2,g ground state, the vibrational progression extends as far as v+=90 while that associated with the X 2Π1/2,g state extends as far as v+=40.
Phase transitions of quadrupolar fluids
Authors: O’Shea Seamus F. , Dubey Girija S. , Rasaiah Jayendran C.
Published: July 2002

Gibbs ensemble simulations are reported for Lennard-Jones particles with embedded quadrupoles of strength Q*=Q/(εσ5)1/2=2.0 where ε and σ are the Lennard-Jones parameters. Calculations revealing the effect of the dispersive forces on the liquid–vapor coexistence were carried out by scaling the attractive r−6 term in the Lennard-Jones pair potential by a factor λ ranging from 0 to 1. Liquid–vapor coexistence is observed for all values of λ including λ=0 for Q*=2.0, unlike the corresponding dipolar fluid studied by van Leeuwen and Smit et al. [Phys. Rev. Lett. 71, 3991 (1993)] which showed no phase transition below λ=0.35 when the reduced dipole moment μ*=2.0. The simulation data are analyzed to estimate the critical properties of the quadrupolar fluid and their dependence on the strength λ of the dispersive force. The critical temperature and pressure show a clear quadratic dependence on λ, while the density is less confidently identified as being linear in λ. The compressibility is roughly linear in λ.
Exchange and correlation energy in density functional theory: Comparison of accurate density functional theory quantities with traditional Hartree–Fock based ones and generalized gradient approximations for the molecules Li2, N2, F2
Authors: Gritsenko O. V. , Schipper P. R. T. , Baerends E. J.
Published: July 2002

The density functional definition of exchange and correlation differs from the traditional one. In order to calculate the density functional theory (DFT), quantities accurately, molecular Kohn–Sham (KS) solutions have been obtained from ab initio wave functions for the homonuclear diatomic molecules Li2, N2, F2. These afford the construction of the KS determinant Ψs and the calculation of its total electronic energy EKS and the kinetic, nuclear-attraction and Coulomb repulsion components Ts, V, WH as well as the (DFT) exchange energy Ex and correlation energy Ec. Comparison of these DFT quantities has been made on one hand with the corresponding Hartree–Fock (HF) quantities and on the other hand with local density approximation (LDA) and generalized gradient approximation (GGA). Comparison with HF shows that the correlation errors in the components T, V, and WH of the total energy are much larger for HF than KS determinantal wave functions. However, the total energies EKS and EHF appear to be close to each other, as well as the exchange energies Ex and ExHF and correlation energies Ec and EcHF. The KS determinantal wave function and the KS orbitals therefore correspond to much improved kinetic and Coulombic energies, while having only a slightly larger total correlation energy. It is stressed that these properties of the Kohn–Sham orbitals make them very suitable for use in the molecular orbital theories of chemistry. Comparison of the accurate Kohn–Sham exchange and correlation energies with LDA and GGA shows that the GGA exchange energies are consistently too negative, while the GGA correlation energies are not negative enough. It is argued that the GGA exchange functionals represent effectively not only exchange, but also the molecular non-dynamical correlation, while the GGA correlation functionals represent dynamical correlation only.
Direct evidence of the failure of electric-dipole approximation in second-harmonic generation from a chiral polymer film
Authors: Van Elshocht Sven , Verbiest Thierry , Kauranen Martti , Persoons André , Langeveld-Voss B. M. W. , Meijer E. W.
Published: July 2002

Second-harmonic generation from Langmuir–Blodgett films of a polythiophene is strongly influenced by the chirality of the polymer. The polarization dependence of the process cannot be explained in the electric-dipole approximation. Evidence of contributions beyond electric dipoles is obtained directly from individual second-harmonic signals.
The simulation of outgoing-wave boundary conditions via a symmetrically damped, Hermitian Hamiltonian operator
Authors: Yu Hua-Gen , Smith Sean C.
Published: July 2002

A new method for simulating the effect of outgoing-wave boundary conditions in the calculation of quantum resonances is presented. The Hermitian Hamiltonian operator H is multiplied on each side by a damping operator D, consisting of a real function d(R), which is unity in the resonance region and falls gradually to zero in the asymptotic region. The spectrum of the symmetrically damped Hamiltonian operator, DHD is shown to provide an excellent approximation to the resonance energies of the Hamiltonian with outgoing-wave boundary conditions. Applications to the calculation of resonance energies for collinear H+H2 scattering and for HO2 dissociation are presented. In addition, we explore the feasibility of extracting resonance widths by using the DHD operator within a filter diagonalization (FD) scheme. Application of the FD scheme to HO2 yields encouraging results.
The use of cross-correlation functions in the analysis of circular dichroism spectra
Authors: Abbate Sergio , Longhi Giovanna , Kwon Kyonghae , Moscowitz Albert
Published: July 2002

Following Gordon, we show that the circular dichroism (CD) spectrum is proportional to the imaginary part of the Fourier transform of the cross-correlation function of the electric dipole moment and of the magnetic dipole moment. Such a result is useful in the study of CD band shapes, and also in obtaining a spectrum from a classical trajectory calculation, as was done for absorption by Noid, Koszykowski, and Marcus. For ergodic and multiperiodic regimes, the formula reduces to the Fourier cross spectrum of the two moment functions. As an application, we have run classical trajectories calculations for two coupled CH stretchings belonging to a chiral HCCH fragment, in the fixed partial charge approximation. Two sets of results are presented, relative to low and high total energy: Comments are made about the spectroscopic manifestation of normal, local, and chaotic modes.
The anharmonic features of the short-time dynamics of fluids: The time evolution and mixing of instantaneous normal modes
Authors: David Edwin F. , Stratt Richard M.
Published: July 2002

Despite the obvious role of sharply varying repulsive forces in determining the structure of most liquids, for short periods of time, motion in liquids looks remarkably harmonic. That is, there seem to be well-defined collective, but independent, harmonic modes governing the ultrafast dynamics launched from any given liquid configuration. Because liquids are not truly harmonic, however, these modes cannot last forever. In particular, “instantaneous” modes of this sort eventually have to give way to new instantaneous modes—ones more appropriate to whatever new configuration the liquid has evolved into. In this paper we investigate just this process of mode evolution. By concentrating on solely the highest frequency modes, it is possible to formulate analytical models for both the modes and the anharmonic interactions that affect them. We can therefore begin to understand the mechanisms by which modes change in time and the kinds of time scales on which the specific anharmonic processes occur in liquids. What we find is that there are several rather distinct signatures of anharmonicity: we see first that the anharmonicity within a mode itself continually causes the mode frequency to fluctuate. More sporadically, we find that two different but nearly resonant modes will sometimes interact strongly enough with one another to cause a temporary—though not a permanent—mixing between the modes. Of course, both of these processes are, in some sense, breakdowns of instantaneous-normal-mode theory, but neither of them affects the basic identity and existence of instantaneous modes. The eventual destruction of the modes turns out to be an even less frequent event precipitated by an even stronger mixing between a mode and the motion of surrounding atoms. It is precisely this longer time scale that may mark the first point at which diffusive motion plays an essential role in liquid dynamics.
A pseudobond approach to combining quantum mechanical and molecular mechanical methods
Authors: Zhang Yingkai , Lee Tai-Sung , Yang Weitao
Published: July 2002

A major challenge for combined quantum mechanical and molecular mechanical methods (QM/MM) to study large molecules is how to treat the QM/MM boundary that bisects some covalent bonds. Here a pseudobond approach has been developed to solve this problem for ab initio QM/MM calculations: a one-free-valence atom with an effective core potential is constructed to replace the boundary atom of the environment part and to form a pseudobond with the boundary atom of the active part. This pseudobond, which is described only by the QM method, is designed to mimic the original bond with similar bond length and strength, and similar effects on the rest of the active part. With this pseudobond approach, some well-known deficiencies of the link atom approach have been circumvented and a well-defined potential energy surface of the whole QM/MM system has been provided. The construction of the effective core potential for the pseudobond is independent of the molecular mechanical force field and the same effective core potential is applicable to both Hartree–Fock and density functional methods. Tests on a series of molecules yield very good structural, electronic, and energetic results in comparison with the corresponding full ab initio quantum mechanical calculations.
Size selectivity by cation–π interactions: Solvation of K+ and Na+ by benzene and water
Authors: Cabarcos Orlando M. , Weinheimer Corey J. , Lisy James M.
Published: July 2002

Size-specific interaction of alkali metal ions with aromatic side chains has been proposed as a mechanism for selectivity in some K+ channel proteins. Experiments on gas-phase cluster ions of the form M+(C6H6)n(H2O)m, with M=Na or K, have demonstrated that the interaction between benzene and K+ is sufficiently strong to result in partial dehydration of the ion, i.e., benzene will displace some water molecules from direct contact with the ion. In sharp contrast, there is no evidence that benzene can displace water from the first hydration shell of Na+. The resistance of Na+(H2O)4 towards dehydration in an aromatic environment suggests a molecular-level mechanism for the low permeability of Na+ through the pore region of K+ channel proteins: the hydrated Na+ ion is too large to pass, while K+ can shed enough of its hydration shell to fit through the pore. These results also suggest that it may be possible to design a new class of ionophores that take advantage of the cation–π interaction to confer ion selectivity. This is the first experimental evidence that K+ selectively interacts with an aromatic complex in an aqueous environment, while Na+ does not. A remarkable sidelight from this study was the discovery of a self-assembled cluster ion, Na+(C6H6)8(H2O)4, with a single structure: an inner shell of four water molecules and an outer layer of eight benzene molecules, each of the latter fixed by a π–hydrogen bond to one of the eight interior O–H groups.
Properties of a liquid–gas interface at high-rate evaporation
Authors: Anisimov S. I. , Dunikov D. O. , Zhakhovskii V. V. , Malyshenko S. P.
Published: July 2002

The effects of temperature and pressure nonuniformities at evaporation on the properties of liquid–gas interface are studied by molecular dynamics (MD) simulation and thermodynamic perturbation method on the basis of the van der Waals theory of capillarity. The structure and properties of the interfacial layer of equilibrium and nonequilibrium Lennard-Jones (12-6) systems are investigated. The surface tension, the two-particle distribution functions, the density fluctuation correlation lengths, and the evaporation coefficients are calculated using MD simulation. It is shown that the presence of the temperature gradient at the interface due to evaporation leads to reduction of the surface tension. The results of MD simulations are in agreement with the results of thermodynamic approach.
Nonclassical polarization effects in fluorescence emission spectra from microdroplets
Authors: Arnold S. , Goddard N. L. , Hill S. C.
Published: July 2002

We report a pronounced nonclassical polarization effect on the shape of fluorescence emission spectra from isolated microdroplets containing a dilute solution of soluble fluors or a dilute layer of surfactant fluors. We see different spectral shapes for 90° scattering when comparing between IVV, IVH, IHH, IHV. However, we measure the largest difference in spectral shape in the surfactant case, with the incident polarization directed toward the detector (IHV vs IHH). Imaging reveals that the emission in this case principally arises from two distinct regions near the surface of the droplet, which are diametrically opposed and along the axis of the incident laser beam. The effect appears to be the direct result of coupling between molecular emission moments and electromagnetic modes of the droplet. It is not the molecule which radiates but the molecule microvessel. Directional emission is sensitive to the polarization of the electromagnetic mode which is stimulated by the coupling.
Nonexponential “blinking” kinetics of single CdSe quantum dots: A universal power law behavior
Authors: Kuno M. , Fromm D. P. , Hamann H. F. , Gallagher A. , Nesbitt D. J.
Published: July 2002

Single molecule confocal microscopy is used to study fluorescence intermittency of individual ZnS overcoated CdSe quantum dots (QDs) excited at 488 nm. The confocal apparatus permits the distribution of “on” and “off” times (i.e., periods of sustained fluorescence emission and darkness) to be measured over an unprecedentedly large dynamic range (109) of probability densities, with nonexponential behavior in τoff over a 105 range in time scales. In dramatic contrast, these same τoff distributions in all QDs are described with remarkable simplicity over this 109-fold dynamic range by a simple inverse power law, i.e., P(τoff)∝1/τoff1+α. Such inverse power law behavior is a clear signature of distributed kinetics, such as predicted for (i) an exponential distribution of trap depths or (ii) a distribution of tunneling distances between QD core/interface states. This has important statistical implications for all previous studies of fluorescence intermittency in semiconductor QDs and may have broader implications for other systems such as single polymer molecules.
Experimental and theoretical study of rotationally inelastic collisions of highly rotationally excited CN(A 2Π) with Ar
Authors: Yang Xin , Dagdigian Paul J. , Alexander Millard H.
Published: July 2002

A collaborative experimental and theoretical study of rotationally inelastic collisions of CN(A 2Π,v=3,N=60) fine-structure Λ-doublet levels with argon is presented. Experimental state-to-state rate constants were determined with an optical–optical double resonance technique. The CN radical was prepared by 193 nm photolysis of BrCN diluted in slowly flowing argon at a total pressure of ∼0.9 Torr. Specific levels of CN(A 2Π,v=3,N=60) were prepared by excitation with a pulsed dye laser on various rotational lines in the A 2Π–X 2Σ+(3,0) band, and collisionally populated levels were probed after a short delay by laser fluorescence excitation in the B 2Σ+–A 2Π(3,3) band. Final state distributions (relative state-to-state rate constants) are reported. To calibrate their magnitude, absolute total removal rate constants and the large state-to-state rate constants for ΔN=−1 fine-structure conserving, Λ-doublet symmetry-conserving transitions were determined. The measured rate constants were compared with theoretical rate constants computed in a quantum scattering treatment of the dynamics with ab initio CN(A 2Π)–Ar potential energy surfaces. The agreement of measured and computed rate constants is very good. The rate constants display dramatic Λ-doublet propensities which depend upon the reflection symmetry of the initial level. From examination of coupled-state partial cross sections as a function of the projection quantum number specifying the angle of approach of the Ar atom to the CN plane of rotation, these propensities are shown to arise from a “helicopter” approach orientation which facilitates curve crossings between effective potential energy curves correlating with different rotational asymptotes.
The structure and ordering of ices III and V
Authors: Lobban Colin , Finney John L. , Kuhs Werner F.
Published: July 2002

The structures of ices III and V have been studied under their thermodynamic conditions of stability by neutron diffraction. The results clearly indicate partial ordering of the water molecule orientations for both ice structures. For ice V the ordering is both pressure and temperature dependent, while no significant changes in ordering were noted for ice III within the small region of stability. No reduction in symmetry, necessary for complete orientational ordering, was observed for ice V at low temperatures. The ordering behavior of ice V at low temperatures (<150 K), when considered in conjunction with dielectric measurements at high temperatures, suggests that while relaxation is achieved predominantly through the diffusion of rotational defects at high temperatures, the mechanism at low temperatures appears to be the migration of ionic defects which require only a small activation energy for mobilization.
On the Painlevé property of nonlinear field equations in 2+1 dimensions: The Davey–Stewartson system
Authors: Leo R. A. , Mancarella G. , Soliani G. , Solombrino L.
Published: July 2002

With the purpose of clarifying some aspects of the complete integrability of nonlinear field equations, a singular-point analysis is performed of the Davey–Stewartson system, which can be considered as an extension in 2+1 dimensions of the nonlinear Schrödinger equation. It is found that the system under consideration possesses the Painlevé property and allows a set of Bäcklund transformations obtained by truncating the series expansions of the solutions about the singularity manifold.
Killing vector fields in self-dual, Euclidean Einstein spaces with Λ≠0
Authors: Przanowski M.
Published: July 2002

Killing equations for self-dual, Euclidean Einstein spaces with nonvanishing cosmological term Λ are considered. It is shown that in this case one can reduce ten Killing equations to one master equation. Some important solutions of the master equation are found and one of these solutions is analyzed in detail as it leads to metrics defined by the field equation well known in Euclidean or complex relativity. Ernst potentials for the solutions considered are found.
Riemannian symmetric superspaces and their origin in random-matrix theory
Authors: Zirnbauer Martin R.
Published: July 2002

Gaussian random-matrix ensembles defined over the tangent spaces of the large families of Cartan’s symmetric spaces are considered. Such ensembles play a central role in mesoscopic physics, as they describe the universal ergodic limit of disordered and chaotic single-particle systems. The generating function for the spectral correlations of each ensemble is reduced to an integral over a Riemannian symmetric superspace in the limit of large matrix dimension. Such a space is defined as a pair (G/H,Mr), where G/H is a complex-analytic graded manifold homogeneous with respect to the action of a complex Lie supergroup G, and Mr is a maximal Riemannian submanifold of the support of G/H.