bluuebunny/crossref_metadata_2025 huggingface dataset:
Ab initio model potential study of environmental effects on the Jahn–Teller parameters of Cu2+ and Ag2+ impurities in MgO, CaO, and SrO hosts
Authors: Pascual José Luis , Seijo Luis , Barandiarán Zoila
Published: July 2002

In this paper, the differential effects brought about by the MgO, CaO, and SrO family of hosts on the topology of the energy surface corresponding to the ground state of the clusters (CuO6)10− and (AgO6)10− along the a1g and eg,θ vibrational modes, are studied by means of the ab initio embedding model potential method (AIEMP) at the restricted open-shell Hartree–Fock level. For all six defect crystals, the equilibrium geometries, Jahn–Teller energies, vibrational frequencies, and relative energies between D4h elongated and compressed structures (energy barriers) have been calculated using (a) two different definitions of the defect cluster, namely (MeO6)10− and (MeO6M6)2+ (Me=Cu, Ag; M=Mg, Ca, Sr), (b) two different cluster LCAO expansions (including or not impurity second neighbor components), and (c) relativistic Cowan–Griffin ab initio (core and environment) model potentials. The results show the importance of using basis set functions located at lattice sites next to the (CuO6)10− and (AgO6)10− clusters in order to mimic the results of the larger clusters (CuO6M6)2+ and (AgO6M6)2+. The net relativistic effects calculated for SrO:Cu2+, Ag2+ and the individual contributions due to the impurity and to the environmental Sr2+ ions have been analyzed, all of them being rather small. The use of a simple point-charge model for the MgO, CaO, and SrO crystalline environments (Madelung potential) is shown to be inadequate and to lead to meaningless results, in agreement to what has been found in previous studies of bulk and surface defects in other ionic crystals.

Given that the Ag2+ impurity in SrO, CaO, and MgO hosts is accepted to be an exceptionally clear example of transition from static to dynamic Jahn–Teller effect at low temperature, the reproduction of this trend can be considered to be a demanding test for an embedding method like the AIEMP: A joint analysis of our calculations and the available EPR experimental results is presented which reveals an agreement in the conclusion that the environmental effects promote stronger Jahn–Teller coupling from MgO to SrO. Also, the inadequacy of the strong linear Jahn–Teller coupling approximation for MgO:Cu2+ is suggested to be responsible for the present incomplete understanding of the features of its EPR spectrum, in contrast with the other systems.
Cluster studies of La2CuO4: A mapping onto the Pariser–Parr–Pople (PPP) model
Authors: Martin Richard L.
Published: July 2002

The techniques of ab initio electronic structure theory are used to study Cu2O7 and Cu2O11 cluster models of La2CuO4. Fair agreement is obtained with the experimentally determined spin exchange constant J (90 meV calculated vs 125 meV measured) at the expense of quite large configuration interactions (CI) expansions. Results for various charge states of the cluster are well described by a ‘‘single-band’’ Pariser–Parr–Pople (PPP) model. As in earlier local-density-functional (LDF) based parameter determinations, the present work suggests these materials fall in the strong coupling regime. However, a significant intersite Coulomb repulsion is found in the present research. It is of sufficient strength V∼U/5 to indicate that charge fluctuations may be more important in these materials than generally believed.
Low-energy excitation and fast motion near T
g in amorphous cis-1,4-polybutadiene
Authors: Kanaya Toshiji , Kawaguchi Tatsuya , Kaji Keisuke
Published: July 2002

Dynamics of amorphous cis-1,4-polybutadiene was studied by incoherent inelastic and quasielastic neutron scattering below 10 meV in a wide temperature range covering the glass transition temperature Tg. The spectra at low temperatures below Tg show a broad excitation peak at about 2–3 meV, which is the so-called low-energy excitation and believed to be an origin of anomalous thermal properties of amorphous materials at low temperatures. It was found that the temperature dependence of the low-energy excitation intensity can be normalized by the Bose factor at temperatures low enough below Tg. With increasing temperature, a very fast motion appears in the energy range below ∼4 meV at around the Vogel–Fulcher temperature T0, which is ∼50 K below Tg. This fast motion is often interpreted as the β process predicted by the mode coupling theory. In this paper, both the low-energy excitation and the fast motion were analyzed in terms of a common microscopic picture, i.e., an asymmetric double well potential. The natures of the two motions are discussed focusing on the relation between the two motions as well as the glass transition dynamics.
Chain length dependence of static longitudinal polarizabilities and hyperpolarizabilities in linear polyynes
Authors: Archibong Edet F. , Thakkar Ajit J.
Published: July 2002

Ab initio calculations of the static longitudinal dipole polarizability αL and second dipole hyperpolarizability γL are reported for the linear C2nH2 polyynes up to C44H2 . Basis set requirements diminish with increasing chain length. The intermediate neglect of differential overlap (INDO) method is able to mimic the ab initio results if the Ohno–Klopman parameterization is used. The values per acetylenic linkage, αL/n and γL/n, converge very slowly with chain length; they are extrapolated to the infinite chain limit.
Vibronic coupling in the ground cationic state of naphthalene: A laser threshold photoelectron [zero kinetic energy (ZEKE)-photoelectron] spectroscopic study
Authors: Cockett Martin C. R. , Ozeki Hiroyuki , Okuyama Katsuhiko , Kimura Katsumi
Published: July 2002

The two-color (1+1′) threshold photoelectron spectra of naphthalene in a supersonic free jet have been recorded via nine vibronic levels of the S1 electronic state up to about 1420 cm−1 above the S1 band origin. The threshold photoelectron spectra recorded via the S1 band origin and via totally symmetric ag vibronic levels show significant intensity in Δν=+1 transitions in nontotally symmetric vibrations having b1g symmetry indicating that the ionization transition gains significant intensity through a vibronic coupling mechanism between the two lowest doublet cationic states. The strongest departure from the expected Δν=0 propensity in the threshold photoelectron spectra occurs through excitation of the totally symmetric 8 mode having ag symmetry indicating that a considerable displacement occurs along the normal coordinate of this 8 mode upon ionization from the S1 state. The superior resolution of the threshold photoelectron technique over conventional photoelectron methods has allowed accurate values for the fundamental vibrational frequencies of naphthalene in its ground cationic state to be determined and it has also allowed a more rigorous investigation of the vibronic coupling mechanism that occurs between the two lowest doublet cationic states. Moreover, an improved value for the adiabatic ionization energy of naphthalene of 65 687±7 cm−1 (8.1442±0.0009 eV) has been determined.
Density profiles of concentrated colloidal suspensions in sedimentation equilibrium
Authors: Biben Thierry , Hansen Jean-Pierre , Barrat Jean-Louis
Published: July 2002

Sedimentation equilibria in concentrated colloidal suspensions are studied within the framework of density functional theory for inhomogeneous fluids. The density profile in the gravitational field is calculated exactly for the one-dimensional model of hard rods. The predictions of the local density approximation agree very well with coarse-grained density profiles derived from extensive Monte Carlo simulations of three-dimensional systems of hard spheres and charge-stabilized colloidal particles. The simulations show that the shape of the density profiles is very sensitive to the nature of the interactions between colloidal particles, and allow a direct test of a simple inversion procedure, which extracts the osmotic equation of state of colloidal suspensions from a measurement of their density profile. The method is sufficiently sensitive to allow the crystal-fluid tie line to be located at high colloid concentrations. The local density approximation is generalized to calculate density profiles of bidisperse hard sphere suspensions; the competition between gravity and entropy leads to nonmonotonic density profiles which are sensitive to the mass ratio, for a given size ratio. The density functional approach and Monte Carlo simulations are finally extended to study the effect of interparticle attractions on the density profiles, which exhibit a region of rapid variation corresponding to the ‘‘liquid-gas’’ interface for sufficiently strong attractive interactions.
Stark experiments on NO2: Determination of electric dipole moments and observation of perturbations by obscure background levels
Authors: Heitz S. , Hese A.
Published: July 2002

Stark experiments in the A 2B2←X 2A1 transition of NO2 were carried out on a supersonic molecular beam by high resolution laser spectroscopy. The experiments were performed on the qR(0)00 and the qP(2)02 transitions in band 99 at 593 nm. An electric dipole moment value of μb=0.303(4)D could be determined for the X 2A1 ground state from the Stark behavior of the N″KaKc = 202 levels. This result could be confirmed on the N″KaKc = 000 levels. The problem of analyzing the Stark behavior in perturbed electronic states could be examined in the case of the excited 101 levels. Moreover, within the qR(0)00 Stark pattern several perturbations by obscure background states could be analyzed.
Molecular packing in water supported monolayers of F(CF2)11COOH and F(CF2)10CH2COOH
Authors: Acero Anibal A. , Li Mengyang , Lin Binhua , Rice Stuart A. , Goldmann Michel , Azouz Issam Ben , Goudot Anne , Rondelez Francis
Published: July 2002

We report the results of a grazing incidence x-ray diffraction study of monolayers of F(CF2)11COOH and F(CF2)10CH2COOH on water at several temperatures. It is found that in both monolayers the amphiphile molecules have a very small collective tilt angle (relative to the normal to the surface). For the temperature range studied (∼4–30 °C) the data for both monolayers indicate the existence of only a first-order transition between an ordered condensed phase and a disordered dilute phase; there is no evidence, in this temperature range, for the existence of a continuous tilt transition such as has been observed in monolayers of arachidic acid. The experimental data suggest that the critical point for the transition observed is at a lower temperature for a monolayer of F(CF2)11COOH than for a monolayer of F(CF2)10CH2COOH.
Response to ‘‘Comment on ‘The weakest bond: Experimental observation of helium dimer’ ’’ [J. Chem. Phys. 100, 4021 (1994)]
Authors: Luo Fei , McBane George C. , Kim Guensik , Giese Clayton F. , Gentry W. Ronald
Published: July 2002

The alternative hypothesis offered by Meyer, Mester, and Silvera in the preceding Comment is not consistent with the data reported in our original paper. In particular, the pressure dependence observed for the helium dimer ion signal cannot be accounted for by assuming that it arises from a neutral trimer whose population is depleted by the formation of larger clusters. As judged from the previous data on which the arguments of Meyer, Mester, and Silvera are based, our experiments were carried out with total cluster populations about three orders of magnitude lower than would be required for such an effect to be significant.
Potential of mean force for the methane–methane pair in water
Authors: Dang Liem X.
Published: July 2002

Molecular dynamics and potential of mean force techniques are used to study methane–methane association in water. The five-site, methane–methane pair potential is taken from recent work by Spellmeyer and Kollman, while the extended simple point charge model is used to model the water–water interactions. The calculated potentials of mean force at 300 and 330 K indicate that the contact wells are significantly deeper than the corresponding solvent-separated wells. The effect of the temperature on the calculated potential of mean force is found to be small. The calculated equilibrium constants suggest that the probability of contact and the solvent-separated pairs are comparable. The shape of our potential of mean force result at room temperature using the extended simple point charge model is very similar to the results of simulations reported by Belle and Wodak using a polarizable water model. It appears that further research is necessary to assess the role of nonadditive effects in modeling of hydrophobic interactions.
Integral equation theory of block copolymer liquids. I. General formalism and analytic predictions for symmetric copolymers
Authors: David Edwin F. , Schweizer Kenneth S.
Published: July 2002

The equilibrium properties of block copolymer liquids are studied using liquid state theory. We first present the formal extension of the polymer reference interaction site model theory to treat block copolymers of general architectures, and then apply this to the symmetric block copolymer using the Gaussian thread model. Contact with Leibler mean field theory is made by employing the ‘‘reference molecular mean spherical approximation’’ closure within the thread idealization. A host of density and concentration fluctuation effects are studied using the ‘‘reference molecular Percus–Yevick’’ closure. In particular, the dependence of the effective chi-parameter and peak scattering intensity on density, chain length, temperature, composition, and spatial range of interactions is examined. Within the thread-polymer/effective incompressibility assumption the chain length dependence of the fluctuation stabilization in the vicinity of the mean field spinodal is found to be the same as in the Brazovskii–Fredrickson–Helfand theory. However, a rich dependence on the nonuniversal prefactors, and the enthalpic origin of the feedback mechanism, distinguishes these results from previous field theoretic work.
Transient shear viscosity of weakly aggregating polystyrene latex dispersions
Authors: de Rooij R. , Potanin A. A. , van den Ende D. , Mellema J.
Published: July 2002

The transient behavior of the viscosity (stress growth) of a weakly aggregating polystyrene latex dispersion after a step from a high shear rate to a lower shear rate has been measured and modeled. Single particles cluster together into spherical fractal aggregates. The steady state size of these aggregates is determined by the shear stresses exerted on the latter by the flow field. The restructuring process taking place when going from a starting situation with monodisperse spherical aggregates to larger monodisperse spherical aggregates is described by the capture of primary fractal aggregates by growing aggregates until a new steady state is reached. It is assumed that the aggregation mechanism is diffusion limited. The model is valid if the radii of primary aggregates Rprim are much smaller than the radii of the growing aggregates. Fitting the model to experimental data at two volume fractions and a number of step sizes in shear rate yielded physically reasonable values of Rprim at fractal dimensions 2.1≤df≤2.2. The latter range is in good agreement with the range 2.0≤df≤2.3 obtained from steady shear results. The experimental data have also been fitted to a numerical solution of the diffusion equation for primary aggregates for a cell model with moving boundary, also yielding 2.1≤df≤2.2. The range for df found from both approaches agrees well with the range df≊2.1–2.2 determined from computer simulations on diffusion-limited aggregation including restructuring or thermal breakup after formation of bonds. Thus a simple model has been put forward which may capture the basic features of the aggregating model dispersion on a microstructural level and leads to physically acceptable parameter values.
Multipole polarizabilities of the helium atom and collision-induced polarizabilities of pairs containing He or H atoms
Authors: Fowler P. W. , Hunt K. L. C. , Kelly H. M. , Sadlej A. J.
Published: July 2002

The transient, collision-induced changes in electric properties of ion–atom or atom–atom pairs at long range are determined by the polarizabilities and hyperpolarizabilities of each of the interacting species. Induction effects on moments of the charge distribution or static susceptibilities depend on the response at zero frequency, and dispersion effects depend on the polarizabilities (linear and nonlinear) at imaginary frequencies. In this paper, we give numerical results for four static multipole hyperpolarizabilities of the helium atom obtained from large-basis, ab initio calculations using many-body perturbation theory and coupled-cluster methods. We report and analyze the numerical results for the R−6 and R−8 terms in the dispersion contributions to the He–He, He–H, and H–H pair polarizability functions with a corrected formula for the contributions from the P-hyperpolarizability tensor. For both parallel and perpendicular components of the polarizability, the numerical results at order R−8 have contributions of similar magnitude arising from the terms containing the P and Q hyperpolarizabilities; these terms approximately cancel for Δα⊥, but reinforce for Δα∥. When R is near to or smaller than the van der Waals minimum distance, the dispersion series is damped by overlap, but the ratio of the undamped R−6 and R−8 terms gives a guide to the convergence behavior. For the pairs treated here, the R−6 and R−8 terms in Δα∥ become equally important between 5 and 8 bohr, but the R−6 term dominates the series for Δα⊥ down to separations of 3–4.5 bohr.
Experimental study of the CO:O3 complex in argon matrices: Irradiation at 266 nm
Authors: Raducu V. , Jasmin D. , Dahoo R. , Brosset P. , Gauthier-Roy B. , Abouaf-Marguin L.
Published: July 2002

Codeposition of argon dilute samples of O3 and CO produced new narrow absorption lines which are assigned to the CO:O3 complex. One line appears at 2140.44 cm−1, 2 cm−1 above the CO frequency which shows that it is a weakly bonded complex. Its spectrum exhibits three new lines near the ν3 doublet of O3. The two lines observed at 1042.56 and 1043.37 cm−1 appear clearly at 5 K; the lowest frequency one, at 1041.35 cm−1, is overlapped at 5 K by the high frequency component of the ν3 doublet of isolated O3 and can only be detected when the temperature is raised, as it is almost not temperature dependent up to 20 K. Irradiation of the sample at 5 K with the 266 nm emission of a quadrupled ND-YAG laser leads to an efficient decomposition of free and complexed ozone and an increase of CO2 with two regimes, a fast beginning similar to the O3 and CO:O3 decays followed by a much slower one. After such an irradiation at 5 K, some recombination of O3 and CO:O3 is observed during a warming at 10 K. A careful analysis of the relative intensities variation shows that CO2 is only produced inside the CO:O3 complex, from the reaction of CO with 1D atomic oxygen. As 1D oxygen atoms are very fastly relaxed to 3P ones during their migration through the matrix, isolated CO does not react. We have shown evidence of a very limited migration of atomic oxygen at 5 K in solid argon, and of an efficient one at 10 K.
Multireference self-consistent size-consistent singles and doubles configuration interaction for ground and excited states
Authors: Malrieu Jean-Paul , Daudey Jean-Pierre , Caballol Rosa
Published: July 2002

One proposes a state-specific self-consistent dressing of the configuration interaction (CI) matrix built on a multireference space and all the singly and doubly substituted determinants. The dressing insures size consistency [and separability when localized molecular orbitals (MOs) are used]. In the here-proposed solution, which generalizes a previous single reference method [(SC)2CI, J. Chem. Phys. 99, 1240 (1993)] valid only for the research of the ground state, all the reference determinants play an equal role and the method is applicable to excited states. The implementation will be simpler if the reference space is a complete active space, but this restriction is not compulsory.
A comparative ab initio study of the Si2C4, Si3C3, and Si4C2 clusters
Authors: Froudakis Georgios , Zdetsis Aristides , Mühlhäuser Max , Engels Bernd , Peyerimhoff Sigrid D.
Published: July 2002

Various structural possibilities for the Si2C4 and Si4C2 clusters are investigated by employing a basis set of triple-zeta plus polarization quality; electron correlation is generally accounted for by second-order Mo/ller–Plesset and, in certain instances, by higher-order perturbation (CASPT2) approaches. The building-up principle recently suggested from an analysis of Si3C3 clusters is found to be fully operative for Si2C4 and Si4C2 clusters. A comparison of the structure and stability of various geometrical arrangements in the series C6, Si2C4, Si3C3, Si4C2, and Si6 shows that linear and planar structures become rapidly less stable if carbons are replaced by silicons and that the three-dimensional bipyramidal forms become less favorable as soon as silicons are exchanged by carbons in the parent Si6 structure. The effects can be rationalized in qualitative terms based on differences in silicon and carbon bonding.
Anisotropic Rayleigh scattering investigations in smectic A liquid crystal phases
Authors: Mondain-Monval O. , Coles H. J. , Claverie T. , Lalanne J. R. , Marcerou J. P. , Philip J.
Published: July 2002

We present here a complete study of Rayleigh scattering by the smectic A phases of 8CB and 8OCB (octylcyanobiphenyl and octyloxycyanobiphenyl). Using two original experimental setups we have been able, for the first time, to measure both the scattered intensities due to orientational fluctuations appearing in a plane containing the mean optical axis of the phase and the ones, more original, in the plane of the smectic layers. We show that these ‘‘in plane’’ fluctuations are unexpectedly important and should be taken into account in the future. We also demonstrate—from both static and dynamic data—that these fluctuations are due to orientational fluctuations of the optical axis and calculate the associated Rayleigh intensities. The confrontation between the calculated and the experimental values allows the determination of both the order of magnitude and the thermal behavior of new elastic constants. Moreover, our results determine the limit of the domain of the so-called linear approximation frequently used in the past in the physics of liquid crystals.
Optical and radiationless intramolecular electron transitions in nonpolar fluids: Relative effects of induction and dispersion interactions
Authors: Matyushov Dmitry V. , Schmid Roland
Published: July 2002

A microscopic theory of intramolecular optical and radiationless electron transitions in nonpolar fluids is developed. The solute is modeled by a polarizable dipolar hard sphere, and the solvent by polarizable hard spheres. The effect of the induction and dispersion interactions to the spectral line shift and width are calculated as a perturbation expansion in the solute-solvent attractions. The relative contributions of both these effects depend significantly on the solute size. Only for large solutes the dispersions are found to dominate the first order energy shift, while inductions become important if the solute size is comparable to that of the solvent molecules. If the solute dipole moment increases with excitation the dispersion and induction components of the first order spectral shift add up leading to a redshift. In the converse case (dipole moment decreasing) the two components have opposite signs, and the shift may switch from red to blue. Furthermore, both components cause the solvent reorganization energy to decrease sharply with the solute size. However, dispersions are of minor importance relative to inductions, for the parameter values used in this study. The linear correlation of the first order line shift with the solvent dielectric function (ε∞−1)/(ε∞+2) of the dielectric constant ε∞ is traced back to a compensating effect of dispersions and inductions. The continuum theory is shown to overestimate the solvent response substantially. Both the solvent reorganization energy and the Stokes shift (the difference between absorption and fluorescence energies) are predicted to vary inversely with temperature. If not masked by intramolecular reorganization, this dependence can cause a maximum in the Arrhenius coordinates for electron transfer rates in the near-to-activationless region.
A time-dependent approach to the kinetics of homogeneous nucleation
Authors: Olson Terry , Hamill Patrick
Published: July 2002

We present a time-dependent model for homogeneous nucleation and derive a time-dependent solution to the coupled growth-rate equations for molecular cluster concentrations. A correction to the monomer concentration growth-rate equation is applied, which accounts for the gain and loss of molecules by all cluster populations. The condensation rate used considers all embryos to be in motion, rather than assuming that only the monomers move. In order to allow for size-dependent variations in a cluster’s surface tension, the evaporation rate incorporates the revised parametrization of Laaksonen et al. [Phys. Rev. E 49, 5517 (1994)] of the homogeneous nucleation theory of Dillmann and Meier [J. Chem. Phys. 94 3872 (1991)]. Eigenvalue analysis illuminates how the steady-state is approached from the transient period and allows us to estimate the accuracy of a steady-state approximation. It is shown that the distributions of clusters predicted by this theory generate nucleation currents that are more accurate than those produced by other steady-state cluster distributions. More importantly, this model can be used to calculate the time-dependent cluster behavior for any nucleation model that has an expression for the change in Gibbs free energy. The theory agrees well with the experimental data of Miller et al. [J. Chem. Phys. 78, 3204 (1983)] and Viisanen et al. [J. Chem. Phys. 99 4680 (1993)], and with the theoretical predictions of Laaksonen et al.
Improved density functional theory results for frequency-dependent polarizabilities, by the use of an exchange-correlation potential with correct asymptotic behavior
Authors: van Gisbergen S. J. A. , Osinga V. P. , Gritsenko O. V. , van Leeuwen R. , Snijders J. G. , Baerends E. J.
Published: July 2002

The exchange-correlation potentials vxc which are currently fashionable in density functional theory (DFT), such as those obtained from the local density approximation (LDA) or generalized gradient approximations (GGAs), all suffer from incorrect asymptotic behavior. In atomic calculations, this leads to substantial overestimations of both the static polarizability and the frequency dependence of this property. In the present paper, it is shown that the errors in atomic static dipole and quadrupole polarizabilities are reduced by almost an order of magnitude, if a recently proposed model potential with correct Coulombic long-range behavior is used. The frequency dependence is improved similarly. The model potential also removes the overestimation in molecular polarizabilities, leading to slight improvements for average molecular polarizabilities and their frequency dependence. For the polarizability anisotropy we find that the model potential results do not improve over the LDA and GGA results. Our method for calculating frequency-dependent molecular response properties within time-dependent DFT, which we described in more detail elsewhere, is summarized.
Statistical thermodynamics of polymer liquid crystals: Competition between energetic and entropic effects
Authors: Brostow Witold , Walasek Janusz
Published: July 2002

A system of linear polymer liquid crystal (PLC) macromolecules is considered in which each macromolecule constitutes an alternating copolymer of flexible and LC sequences. The distribution function of the system is factorized so that the Gibbs distribution is used for anisotropically interacting LC sequences while Dirac delta functions represent flexible polymer sequences modeled by linear chains of freely jointed statistical segments. A general formula for the Helmholtz function is derived for arbitrary types of anisotropic interactions between LC sequences; the formula of Maier and Saupe for monomer LCs is obtainable from it as a special case. The phase diagram of the system is obtained in the limit of the mean-field approach. Types and orders of phase transitions that the system can undergo are defined and discussed in terms of the Landau classification; all transitions are of the first order. Formation of cholesteric phases in addition to isotropic and nematic or smectic is predicted without involving additional assumptions such as the biaxiality of the LC interactions.
Lifetimes of Rydberg states in ZEKE experiments. III. Calculations of the dc electric field dependence of predissociation lifetimes of NO
Authors: Vrakking Marc J. J.
Published: July 2002

Results are presented of calculations on the experimentally observed lifetime enhancement of high principal quantum number nf(N+=2) and np(N+=0) Rydberg states of the NO molecule in small dc electric fields. A Hund’s case (d) molecular Hamiltonian is introduced, which describes intramolecular interactions between low angular momentum Rydberg states as a result of core penetration and l mixing caused by the electric field. Eigenstates of the molecule in the electric field are obtained by diagonalization of the Hamiltonian and an effective decay time is determined which describes the multiexponential decay of the superposition of (Stark) states which is excited experimentally. The calculations reproduce the striking onset of the lifetime enhancement of the np(N+=0) series, which was observed experimentally. Both for the nf(N+=2) and np(N+=0) Rydberg series, core penetration by low-l states leads to extensive interseries coupling for several values of the principal quantum number.
Coupling model analysis of polymer dynamics in solution: Probe diffusion and viscosity
Authors: Ngai K. L. , Phillies George D. J.
Published: July 2002

The coupling model of Ngai et al. [K. L. Ngai, Disorder Effects in Relaxation Processes, edited by R. Richert and A. Blumen (Springer-Verlag, Berlin, 1994)] is applied to treat polymer dynamics in solution. Important dynamic quantities considered here include the zero-shear viscosity η and the light-scattering spectrum (field correlation function) g(1)(t) of optical probes suspended in solution. The coupling model describes systems in terms of two times, a basic relaxation time τ0 and a crossover time tc, and a coupling exponent n. Use of scaling arguments allows us to extract values for n from the concentration dependence of η and, by three separate paths, from the concentration, time, and scattering vector dependencies of g(1)(t). Values of n from these four distinct physical approaches are shown to be mutually consistent, especially in the higher-concentration, large-probe-particle regime in which the scaling arguments are most likely to be valid. The behavior of n above the solutionlike–meltlike viscosity transition, and the effect of probe size on n, are briefly discussed.
Optical characterization and electronic energy-level structure of Er3+-doped CsCdBr3
Authors: Quagliano John R. , Cockroft Nigel J. , Gunde Kevin E. , Richardson Fred S.
Published: July 2002

Information obtained from optical absorption, excitation, and emission experiments on erbium doped crystalline CsCdBr3 is analyzed, using a semiempirical Hamiltonian, to calculate atomic and crystal-field interaction parameters and electronic state wave functions. A majority of the Er3+ ions substitute at a Cd2+ site giving C3v point group symmetry and forming an Er3+ ion dimer center. This dimerization, together with the material’s low phonon energies, and the specific positioning of states in the Er3+ (4f11) configuration, produce the interesting and useful emission properties of the material. Comparisons are made with other erbium halide crystals, and interaction parameter and energy-level results for Nd3+:CsCdBr3 are also presented. The inclusion of second order correlation crystal-field interaction parameters is shown to be essential for accurately characterizing splittings of several J multiplets important in visible emission pathways.
High resolution mid-infrared spectroscopy of ArH2O: The v2 bend region of H2O
Authors: Weida Miles J. , Nesbitt David J.
Published: July 2002

Six internal rotation/vibration bands of ArH2O are observed in a slit supersonic expansion via direct absorption of a tunable diode laser in the v2 bend region of H2O. The spectra obtained for the ortho H2O manifold are well represented by a pseudodiatomic model with nearly free internal rotation of the H2O subunit. By way of contrast, the para bands show significant mixing between the internal rotor and stretch states, indicative of strong angular-radial coupling in the intermolecular potential. The spectra for the para Ar–H2O species can be deperturbed based on a three state Coriolis plus angular-radial coupling model which includes microwave, far-ir and near-ir data. The results indicate a redshift of ≈0.58 cm−1 upon bend excitation of the H2O subunit, and in general rather modest changes in the excited state intermolecular potential from the ground state potential. No indication of predissociation broadening is found, and the instrument-limited linewidths place a lower limit on the vibrational lifetime in the excited state of τ⩾7.2(6) ns.
Eigenstate resolved infrared–infrared double-resonance study of intramolecular vibrational relaxation in benzene: First overtone of the CH stretch
Authors: Callegari A. , Srivastava H. K. , Merker U. , Lehmann K. K. , Scoles G. , Davis M. J.
Published: July 2002

The first eigenstate resolved, near the infrared spectrum of benzene in the region of the first C–H stretch overtone (6000 cm−1) has been obtained with an IR–IR double-resonance molecular beam optothermal spectrometer. Using a hierarchical tree analysis and level spacing statistics, we show that the intramolecular vibrational relaxation occurs nonergodically over at least seven different time scales ranging from 100 fs to 2 ns.
Measurement of the splay-bend elastic constant in lyotropic ferronematic liquid crystals: The influence of the bounding surfaces
Authors: Fontanini S. , Alexe-Ionescu A. L. , Barbero G. , Figueiredo Neto A. M.
Published: July 2002

We report the first experimental investigation devoted to analyze the influence of a ferrofluid on the surface properties of lyotropic liquid crystals doped with magnetic particles. By means of optical methods, the bulk orientation of the director vs the temperature is determined. The theoretical analysis shows that near the nematic–isotropic transition, the thermodynamical model for the temperature surface transition works reasonably well. On the contrary, in the low temperature region near the lamellar–nematic transition, important deviations from the theoretical behavior are observed. In order to interpret our experimental data, we propose a simple phenomenological extension of the thermodynamical model that takes into account the different contributions to the surface energy and of the residual lamellar order in the nematic phase. We show, furthermore, that the effective splay-bend elastic constant depends on the ferrofluid doping concentration, in agreement with a recently proposed theory.
Conformational properties of flexible polymer chains in highly confined environments
Authors: Jorge Sonia , Rey Antonio
Published: July 2002

In this work we describe a detailed study of the conformational properties of flexible polymer chains confined into rigid wall spherical cavities of different sizes, with dimensions comparable to those of the chains. An off-lattice Monte Carlo algorithm has been used to simulate the behavior of excluded-volume, freely jointed chains in a wide range of concentrations. The considered properties include the size and shape of the chains, different densities of the system, and the orientation of certain characteristic vectors. All these properties are analyzed as a function of the position in the spherical cavity, trying to determine how each property is affected by the presence of the spherical rigid wall. A comparison with the results found in the literature for polymer systems confined between infinite parallel plates is done whenever possible, with some significant differences.
Structure and stability of the nitric acid–ammonia complex in the gas phase and in water
Authors: Nguyen Minh-Tuan , Jamka Alan J. , Cazar Robert A. , Tao Fu-Ming
Published: July 2002

The nitric acid–ammonia system is studied by high level ab initio calculations. The equilibrium structure, vibrational frequencies, and binding energy of the system in the gas phase are calculated at the second-order Mo/ller–Plesset perturbation level with the extended basis set 6-311++G(d,p). The potential energy surface along the proton transfer pathway is investigated by calculations at the same level of theory, and the effect of water as a solvent on the structure and stability of the system is investigated using self-consistent reaction field theory. It is found that the equilibrium structure contains a strong hydrogen bond with nitric acid acting as the hydrogen bond donor and ammonia as the acceptor. The binding energy is calculated to be D0=12.25 kcal/mol (De=14.26 kcal/mol), which is about three times greater than the binding energy for the water dimer. The OH stretching frequency of nitric acid in the hydrogen-bonded complex is found to be red shifted by over 800 cm−1, with an enhancement of over an order of magnitude in the infrared intensity from the isolated nitric acid molecule. The structure of ammonium nitrate corresponding to the product of a proton transfer reaction is found to be highly unstable on the potential energy surface. The most energetically favorable gas phase reaction of nitric acid and ammonia in the absence of a solvent results in proton exchange, not proton transfer from acid to base. The structure and stability of the system change drastically in the water solvent medium. In the water solvent, the hydrogen-bonded structure is no longer stable and the system exists as an ammonium nitrate ion pair resulting from the completed transfer of a proton from nitric acid to ammonia. On the basis of these results, we conclude that it is unlikely to form gaseous ammonium nitrate from nitric acid and ammonia, and that the formation of particulate ammonium nitrate most likely involves a heterogeneous mechanism.
A phase-space study of Bloch–Redfield theory
Authors: Cao Jianshu
Published: July 2002

A phase-space representation of Bloch–Redfield theory is used to describe the dynamical evolution of quantum dissipative systems. The resulting Liouville operator equations are capable of incorporating both the master equation in eigenstate space and the stochastic equation in classical phase space, and thus provide a useful framework for mixing classical, semiclassical, and quantum dynamics for simulating complicated dissipative systems. In addition, the proper limit of quantum dissipation, the approximate nature of the second-order cumulant truncation, the detailed balance of quantum correlation functions, and the reduction of dissipation by a transformation of the bath Hamiltonian are investigated within the framework of phase-space Bloch–Redfield theory.
A stringent test of the cavity concept in continuum dielectrics
Authors: Papazyan Arno , Warshel Arieh
Published: July 2002

Continuum dielectric representation of solvation requires that a cavity of a certain shape and size is defined for the solute. It is generally assumed that the cavity size is a quantity given by the structure of the solvent around the solute, and that, however complicated, there must be a relationship between the actual solute–solvent structure and the effective cavity size. We show that, when the solvent is not a true continuum (i.e., composed of discrete particles), there is no consistent cavity size even when the geometry of the system is invariant. This requires the ability to separate the structural factors from other system attributes that are mainly expressed in system polarity. Separating the system polarity and structure is virtually impossible in real solvents and “realistic” solvent models, while such a separation is inherent in dipole-lattice models. Thus, for representing the discreteness of the solvent we use a dipole-lattice model which has the unique advantage of eliminating electrostriction or other complicating factors associated with structural changes in more detailed solute–solvent descriptions. The optimum cavity radius that gives agreement with the solvation (the so-called “Born radius”) in a discrete solvent is different for a charge and a dipole, suggesting that a simple cavity-in-a-continuum description of solvation cannot self-consistently capture the correct solvent response to significant changes in the solute charge distribution. Perhaps more interestingly, for a given solute, there is significant variation in the effective cavity size as a function of solvent polarity, even though the structure of the system is constant. Similar trends are obtained with Langevin dipole lattices of simple cubic and face-centered cubic structure, which are known to have rather dissimilar microscopic polarization behavior. Also, the trends obtained with Langevin dipole lattices are very similar to those observed in the more realistic Brownian dipole model of the solvent with explicit thermal fluctuations, whose dielectric constant is known to be a different function of microscopic parameters than a Langevin dipole lattice. Since this diverse set of systems gives similar results, our conclusions are likely to be qualitatively applicable to solvation in real solvents or “realistic” models for which the present computational experiment cannot be directly applied.
Exploring the space of protein folding Hamiltonians: The balance of forces in a minimalist β-barrel model
Authors: Shea Joan-Emma , Nochomovitz Yigal D. , Guo Zhuyan , Brooks Charles L.
Published: July 2002

A rapid and effective method for obtaining thermodynamic quantities for Hamiltonians whose configurational space has not been examined through a direct simulation has been developed. This approach extends the scope of the weighted histogram analysis method and is applied to the exploration of the balance of forces within the off-lattice Honeycutt–Thirumalai 46-mer beta-barrel model. Specificity is introduced into the long range hydrophobic interactions by scaling back the non-native attractive component of the hydrophobic interactions through a scaling factor λ (0<λ<1). Thermodynamic properties for incremental values of λ are extrapolated from the sampling of the original (λ=1) Hamiltonian. The results were found to be in good agreement with the thermodynamic signatures obtained by direct simulations. Decreasing the strength of the non-native attractive hydrophobic interactions leads to a more cooperative folding with the folding and collapse temperatures nearly coinciding at λ=0.0. The free energy surfaces were also seen to become progressively smoother while retaining a pronounced native well. Thus, this methodology may be used in the development, refinement, and exploration of folding for off-lattice protein models.
Transport coefficients of liquid transition metals: A computer simulation study using the embedded atom model
Authors: Alemany M. M. G. , Rey C. , Gallego L. J.
Published: July 2002

Using the Voter and Chen version of the embedded atom model, we carried out molecular dynamics simulations to compute the diffusion constants and shear viscosities of liquid Pd, Pt, Cu, Ag, and Au as representative of single-particle and collective dynamic properties, respectively. The generally good agreement between the calculated values and available experimental data is evidence that the Voter and Chen embedded atom model allows a reliable description of the dynamic properties of liquid transition metals in spite of being derived from solid-state data and the properties of the diatomic molecule. The discrepancy between the reported experimental diffusion constant of liquid Cu and the values calculated in this work, together with the consistency of these latter, suggests that the reported experimental value may be in error.
Polarization energy distribution for impurity molecules in dense gases
Authors: Al-Omari A. K. , Reininger R. , Huber D. L.
Published: July 2002

The polarization energy distribution of impurity molecules in dense gases is investigated. An approximate expression is derived for a random distribution and compared with results inferred from finite-temperature Monte Carlo studies for NO in argon (ρ⩽2.7×1021 cm−3). It is found that when ρ⩽1.8−2.0×1021 cm−3, the distribution has a similar shape with approximately the same peak position as that obtained from the Monte Carlo calculations. This result indicates that the peak position in this density range is due primarily to the r−4 dependence of the polarization energy and the random positions of the host molecules and does not reflect the hard core of the intermolecular interactions.
Observation of the I′ 1Πg outer well state in H2 and D2
Authors: Reinhold E. , de Lange A. , Hogervorst W. , Ubachs W.
Published: July 2002

We observed bound levels of the I′ state in H2 and D2, confined in the outer well of the lowest Πg1 adiabatic potential close to its (1s+2p) dissociation limit, with an equilibrium internuclear distance of ≈8 a.u. Rovibronic levels (v=0–2, J=1–5 for H2 and v=0–5, J=1–6 for D2) are populated with pulsed lasers in resonance enhanced XUV+IR (extreme ultraviolet+infrared) excitation, and probed by a third laser pulse. Level energies are measured with an accuracy of ≈0.03 cm−1, and are in reasonable agreement with predictions from ab initio calculations in adiabatic approximation; the smallness of Λ-doublet splitting indicating that nonadiabatic interactions with Σg+1 states are generally weak. Additional resonances are observed close to the n=2 dissociation limit, some of which can be assigned as high vibrational levels of the EF 1Σg+ state.
A perturbation theory and simulations of the dipole solvation thermodynamics: Dipolar hard spheres
Authors: Matyushov Dmitry V. , Ladanyi Branka M.
Published: July 2002

Padé truncation of the thermodynamic perturbation theory is used to calculate the solvation chemical potential of a dipolar solute in a model fluid of dipolar hard spheres. Monte Carlo simulations of the solvation thermodynamics are carried out over a wide range of solute and solvent dipoles in order to address the following major issues: (i) testing the performance of the Padé perturbation theory against simulations, (ii) understanding the mechanism of nonlinear solvation, and (iii) elucidating the fundamental limitations of the dielectric continuum picture of dipole solvation. The Padé form of the solvation chemical potential constructed in the paper agrees with the whole body of simulation results within an accuracy of 3%. Internal energy and entropy of solvation are also accurately described by the perturbation treatment. Simulations show a complex nonlinear solvation mechanism in dipolar liquids: At low solvent polarities the solvation nonlinearity is due to orientational saturation that switches to the electrostriction mechanism at higher dipolar strengths of the solvent. We find that the optimum cavity radius of the Onsager reaction-field theory of solvation depends substantially on solvent polarity. A general method of testing the performance of linear solvation theories is proposed. It shows that the fundamental failure of continuum theories consists in their inaccurate description of the internal energy and entropy of solvation.