bluuebunny/crossref_metadata_2025 huggingface dataset:
Laboratory measurements of infrared excitation cross sections of fast O-atom collisions with CO, CO2, and CH4
Authors: Upschulte B. L. , Caledonia G. E.
Published: July 2002

A unique pulsed fast oxygen atom source has been used to investigate the interactions between 8 km/s oxygen atoms and CO, CO2, and CH4. Absolute cross sections for direct and chemiexcitation of the short wave infrared bands of reaction products have been evaluated.
Structures and vibrational spectra of water clusters in the self-consistent-field approximation
Authors: Knochenmuss R. , Leutwyler S.
Published: July 2002

Fully optimized structures were calculated for (H2O)n, n=5 and 8, at the SCF (self-consistent field) level using the 4–31G and, for n=5, also 6–31G* basis sets. The n=5 cluster was found to have a cyclic structure with five H bonded and five free hydrogens. The n=8 minimum energy structure has almost D2d symmetry, with an approximately cubical oxygen framework and four tetrahedrally arranged free hydrogens; four of the water molecules are single- and four are double-hydrogen donors. Harmonic vibrational frequencies, IR and Raman intensities were calculated for n=5 and 8, as well as for the previously optimized n=2–4 clusters. The band positions and intensities in the 3000–3800 cm−1 region correlate well with IR predissociation spectra of (H2O)n clusters. The O–H stretching frequencies of single- and double-hydrogen donor water molecules are relatively well separated from each other, and both from the frequency region of the free O–H stretches, suggesting a new interpretation for some of the data. The low-frequency translational/librational modes of both n=5 and 8 show strong mixing with intramolecular stretching and bending. The stretch–stretch coupling constants for OH oscillators on different molecules kij(OH,OH) show a strong increase, and those for intramolecular coupling kii(OH,OH) a rapid decrease with increasing cluster size. For n≥5, kij(OH,OH)≫kii(OH,OH), implying that the cluster can be viewed as a supermolecule of strongly coupled O–H oscillators. The n=8 spectra show significant similarity to those of ice.
A molecular dynamics study of methanol near the liquid–glass transition
Authors: Sindzingre Philippe , Klein Michael L.
Published: July 2002

Molecular dynamics simulations of supercooled liquid and glassy methanol are presented, based on an intermolecular potential derived from liquid data. In the supercooled liquid, atom–atom distribution functions for the methyl groups reveal a split first peak that might be observed with diffraction experiments. The nature of the hydrogen-bonded network has been characterized. The fraction of nonbonded molecules is shown to obey an Arrhenius law, with an activation energy similar to that derived from the hydrogen bond lifetimes. The temperature dependence of the self-diffusion coefficient has been analyzed in detail, along with the behavior of the van Hove self-correlation function, the incoherent-structure factor, and orientational correlation functions for intramolecular OH and OC bond vectors. A brief discussion of the collective dynamics is given for the supercooled liquid.
Theoretical and experimental study of dissolution of inhomogeneities formed during spinodal decomposition in polymer mixtures
Authors: Akcasu A. Ziya , Bahar I. , Erman B. , Feng Y. , Han C. C.
Published: July 2002

Dissolution (mixing or melting) of inhomogeneities formed during spinodal decomposition in binary polymer mixtures is studied both experimentally and theoretically. The details of the dissolution experiment with time-resolved light scattering on polystyrene/poly(vinylmethylether) are presented. The theoretical approach differs from that of Langer, Bar-on, and Miller in the way the fluctuations are treated in the nonlinear theory, and in the details of the calculations arising from the chain connectivity (polymer effect). The effect of mode coupling arising from nonlinearity on the relaxation rate is discussed. It is found both experimentally and theoretically that the wave number corresponding to peak intensity decreases in time asymptotically following a t−0.5 power law.
Thermodynamic interactions and correlations in mixtures of two homopolymers and a block copolymer by small angle neutron scattering
Authors: Balsara N. P. , Jonnalagadda S. V. , Lin C. C. , Han C. C. , Krishnamoorti R.
Published: July 2002

Thermodynamic interactions in mixtures of two homopolymers and a block copolymer were obtained from small-angle neutron scattering (SANS) measurements. Experimental SANS profiles from homogeneous, ternary mixtures of model polyolefins—poly(ethyl butylene)/poly(methyl butylene)/poly(methyl butylene)-b-poly(ethyl butylene)—were compared with theoretical predictions based on the multicomponent random phase approximation (RPA). The polymers were nearly monodisperse and were synthesized by saturating the double bonds in anionically synthesized polydienes with H2 and D2, thus yielding polyolefins with neutron scattering contrast. The theoretical scattering profiles depend on 16 structural parameters and six Flory–Huggins interaction parameters χ, all of which were obtained independently. The χ parameters were obtained from SANS measurements on binary poly(ethyl butylene)/poly(methyl butylene) blends. The SANS profiles obtained from the ternary blends were in quantitative agreement with theoretical predictions over the observable q range (0.01–0.08 Å−1) and over a wide range of block copolymer concentrations (from 10 to 80 vol. % copolymer). This suggests that the measured χ parameters are consistent with the original ideas of Flory and Huggins that χ is a measure of monomer–monomer interactions, and is thus independent of block copolymer concentration. Partial structure factors, related to correlations in the concentration of individual components, were examined by selective labeling and were also found to be in quantitative agreement with the predictions of the multicomponent RPA.
Equations of state for hard chain molecules
Authors: Phan S. , Kierlik E. , Rosinberg M. L. , Yu H. , Stell G.
Published: July 2002

Equations of state for hard sphere chain molecules are derived within the framework of second order perturbation theory of polymerization (TPT2). The present formulation differs from the original Wertheim’s treatment by dealing with molecules with a fixed number of beads. However, the two versions yield very close numerical results for chains composed of freely jointed spheres, showing the insensitivity of the pressure of these fluids to polydispersity. The case of star-like molecules and chain mixtures is also considered. Theoretical predictions are compared to available simulation data, using recent results for the triplet correlation function for hard spheres in rolling contact. Some predictions for the pressure of freely rotating chains and that of trimers as a function of the bond angle are given.
Fractal properties and swelling behavior of polymer networks
Authors: Sommer J.-U. , Vilgis T. A. , Heinrich G.
Published: July 2002

The swelling pressure of randomly crosslinked polymer networks is related to the structural properties of the crosslink topology. Using the assumption that the network structure exhibits fractal properties within the correlation length ξ, a scaling relation between the swelling pressure and the polymer volume fraction has been derived. The exponent obtained depends on the internal fractal dimension di of the network and is in general different from the corresponding exponent for linear chains. The latter can be obtained as the special case di=1. As a consequence, a significant difference in mixing entropy between the networks and the corresponding uncrosslinked system is predicted. This explains the experimental results obtained by several authors, which are in contradiction to the Flory–Rehner assumption. Computer simulations based on the bond fluctuation model support the scaling predictions presented. The exponents obtained for the density dependence of the osmotic or swelling pressure are somewhat larger than expected from the theoretical work for both the linear and the crosslinked system.
A new equation of state for hard chain molecules
Authors: Ghonasgi Dhananjay , Chapman Walter G.
Published: July 2002

We present a new equation of state for hard chain fluids. This equation of state is developed by applying an extension of Wertheim’s theory for associating fluids to a nonspherical reference fluid. Since the equation of state is developed in a similar manner to the statistical associating fluid theory (SAFT) we call this improved equation of state SAFT-Dimer (SAFT-D). The equation of state requires only the contact values of the hard sphere and hard disphere site–site correlation functions as input. We compare the compressibility factor from SAFT and SAFT-D with molecular simulation data for flexible hard chains with chain lengths of 16, 51, and 201 segments. The second virial coefficient and compressibility factor from SAFT-D are in better agreement with molecular simulation results than the generalized Flory dimer, TPT2, and Percus–Yevick compressibility equations of state.
Hybrid Monte Carlo simulations of dense polymer systems
Authors: Forrest B. M. , Suter U. W.
Published: July 2002

We investigate the efficiency of two recently introduced hybrid Monte Carlo algorithms for the off-lattice simulation of dense polymer melts. Their performance is assessed both with respect to the molecular dynamics method and new single-chain Monte Carlo techniques proposed recently. Our findings show that the long-time behavior of both hybrid algorithms offer the possibility of calibration with respect to real time.
Observation and characterization of direct inelastic and trapping-desorption channels in the scattering of N2 from Cu(110)
Authors: Siders Jennifer L. W. , Sitz Greg O.
Published: July 2002

Resonantly enhanced multiphoton ionization (REMPI) and time resolved molecular beam techniques were used to study the rotational state and velocity distributions of N2 scattered from Cu(110). At a surface temperature of 85 K, two peaks were observed in time-of-flight (TOF) measurements. We interpret these as a direct inelastic channel at early time and a trapping desorption channel at later time. As the surface temperature is raised the direct inelastic scattering becomes stronger as the trapping desorption channel grows weaker. Rotational state distributions show that the early TOF peak has a large number of populated states characteristic of direct inelastic scattering; the later TOF peak has a rotational state distribution well characterized by the surface temperature. The rotational state distributions of the direct-inelastic channel were found to depend linearly on both surface temperature, Ts and incident energy, Ei. The absence of rotational rainbows at low Ei and low Ts is attributed to multiple collisions with the surface.
Dependence of Turing pattern wavelength on diffusion rate
Authors: Ouyang Qi , Li Rusheng , Li Ge , Swinney Harry L.
Published: July 2002

The relation between the diffusion coefficient of reactants and the wavelength of Turing patterns is examined in experiments on the chlorite–iodide–malonic acid (CIMA) reaction in gel media. The diffusion coefficients in polyacrylamide and agarose gels are varied by varying the gel densities. The diffusion coefficient D of NaCl is found to vary from 0.5×10−5 to 1.8×10−5 cm2/s for the gel conditions considered. The CIMA reactants are assumed to have diffusion coefficients that are directly proportional to that of NaCl. The wavelength λ of the observed hexagonal patterns (0.13–0.28 mm) varies in accord with the predicted relation for Turing patterns, λ∼D1/2. Moreover, the predicted relationship to a characteristic period of oscillation τ, λ=(2πτD)1/2, is supported by measurements of τ just beyond a Hopf bifurcation in a stirred flow reactor.
Polymer Born–Green–Yvon equation with proper triplet superposition approximation. Results for hard-sphere chains
Authors: Attard Phil
Published: July 2002

A site–site Born–Green–Yvon (BGY) equation is derived for polymeric fluids. This relates the pair and triplet site distribution functions, and superposition approximations for the latter are analyzed. It is shown that the pair functions to be superposed are uniquely determined by the exact normalization equations and asymptotic conditions. The Kirkwood superposition of pair distribution functions is shown to be valid only for the case of sites on three different polymers; for the cases of two or three sites on the same polymer different pair functions must be superposed. The polymer BGY equation is derived for a soft bonding potential between adjacent sites; the result for infinitely stiff bonds is given as a limiting case. Numerical results are obtained for soft and stiff tangent hard-sphere chains, and comparison is made with simulations for packing fractions up to 0.4 and chains with up to 12 sites.
Vibrational spectra of water complexes with H2, N2, and CO
Authors: Sadlej J. , Rowland B. , Devlin J. P. , Buch V.
Published: July 2002

Ab initio calculations are carried out on the H2O...N2, H2O...H2, and H2O...CO complexes. Infrared spectra of the complexes are investigated, with an emphasis on the effect of weak bonding on the frequencies and the infrared intensities of the monomers. Connections are explored between the computational results and the experimentally measured infrared spectra of ice surfaces covered by H2, N2, and CO adsorbate. Additional issues addressed include the influence of the counterpoise correction on the equilibrium geometry of the complexes, and the analysis of the different contributions (exchange, dispersion, electrostatic) to the weak bonding, and to the frequency shifts.
Chemical potential and equations of state of hard core chain molecules
Authors: Escobedo Fernando A. , de Pablo Juan J.
Published: July 2002

A novel approach is presented for the development of equations of state for chain molecules. The basic assumptions of the approach are supported by results of computer simulations of the chemical potential of athermal chains. Our model establishes a bridge between some elements of Wertheim’s thermodynamic perturbation theory and the generalized Flory theory. New equations of state are presented for freely jointed tangent hard-sphere chains which are shown to be more accurate than other existing equations. Extensions of our approach are also presented for branched polymers and blends of polymers.
Equation of state for fluids containing chainlike molecules
Authors: Hu Ying , Liu Honglai , Prausnitz John M.
Published: July 2002

An equation of state for chain fluids has been derived through the r-particle cavity-correlation function (CCF) for chains obtained from sticky spheres; here r is the chain length. The r-particle CCF is approximated by a product of effective two-particle CCFs, accounting for nearest-neighbor correlations and next-to-nearest-neighbor correlations. For hard-sphere chain fluids (HSCF), the density dependence for nearest-neighbor effective two-particle CCFs is determined by the equation of Tildesley–Streett for hard-sphere dumbbells and that for next-to-nearest-neighbor effective two-particle CCFs by computer-simulation results for hard-sphere trimers. The final equation of state has a simple form which gives compressibility factors and second virial coefficients for homonuclear HSCFs covering a wide range of chain length (up to r=201) in excellent agreement with computer simulations. Satisfactory comparisons are also obtained between predicted and computer-simulation results for homonuclear HSCF mixtures, HSCFs containing side-chains and rings, and heteronuclear HSCFs (copolymers).
The 2Σ+ states of HBeO, HMgO, and HCaO
Authors: Kong Jing , Boyd Russell J.
Published: July 2002

The 2Σ+ states of HBeO, HMgO, and HCaO have been studied at the CASSCF level with a basis set of at least triple-zeta plus double polarization quality. The correlation treatment includes the nine valence electrons and ten orbitals resulting in an active space denoted by (622). The optimized structures of HMO (M=Be, Mg, and Ca) are all linear and the energy of HMO lies around 250 kJ/mol (2.6 eV) above that of MOH. The M–O bond length in HMO is predicted to be longer than that in the corresponding MOH by amounts ranging from 0.01 Å to 0.05 Å. The M–O stretching frequencies are very similar for the two structures. The H–M bond lengths and stretching frequencies in HMO are very similar to the same properties in the corresponding diatomic molecule. Analysis of the electronic structures suggests that HBeO has two polarized covalent bonds formed from the sp hybrids of Be; HMgO has one covalent bond (between H and Mg) and one ionic bond and can be viewed as (HMg)+O−, HCaO has two ionic bonds represented by H−Ca2+O−.
The evaporation rate, free energy, and entropy of amorphous water at 150 K
Authors: Speedy Robin J. , Debenedetti Pablo G. , Smith R. Scott , Huang C. , Kay Bruce D.
Published: July 2002

Measurement of the rates of evaporation of amorphous water (a) and ice (i) near 150 K can be interpreted as giving a measure of their free energy difference, ΔaiG (150 K)=1100±100 J/mol, which, together with the known enthalpy difference and heat capacity data, suggests a residual entropy difference of ΔaiS (0)=−0.7±2.2 J/(K mol) at absolute zero. Previous theoretical estimates of ΔaiS (0), which are much larger, did not allow the amorph to be connected with normal liquid water by a reversible thermodynamic path at atmospheric pressure. The present value allows such a connection.
Vibronic analysis of the B̃ 2A′–X̃ 2A″ laser-induced fluorescence of jet-cooled C2H5S
Authors: Hung Wen-Ching , Shen Min-yi , Yu Chin-hui , Lee Yuan-Pern
Published: July 2002

Ethylthio (C2H5S) radicals were formed on laser photolysis at 248 nm of diethyl disulfide (C2H5SSC2H5) or ethyl mercaptan (C2H5SH) in a free-jet expansion. The fluorescence excitation spectrum was recorded in the spectral region 398–432 nm. The origin lies at 23 519.6 cm−1, approximately 799 cm−1 greater than previously reported. Two main progressions with spacings near 420.5 cm−1 (C–S stretch) and 256.0 cm−1 (CCS bend) are dominant. Additional active fundamental vibrational modes of the B̃ state are at 718.4, 862.8, 1054.6, 1158.9, and 1203.3 cm−1. Observation of hot bands enables accurate determination of four low-lying vibrational modes of the ground state at 271.9, 296.0, 478.3, and 672.4 cm−1. The dispersed fluorescence was recorded in the spectral region 415–525 nm. We identified several additional vibrational modes of the X̃ state at 890, 957, 1075, 1257, 1290, 1470, 2950, and 3050 cm−1. Theoretical calculations at the MP2 level were performed to predict vibrational frequencies of both B̃ and X̃ states, and for the latter state were also with the B3-LYP density functional theory; the results agree satisfactorily with experimental observations.
Distribution of reorientational times of optically anisotropic molecular liquids from depolarized light-scattering studies
Authors: Ngai K. L. , Floudas G. , Rizos A. K.
Published: July 2002

Several optically anisotropic glass-forming liquids have been studied by depolarized Rayleigh scattering in the region of the α relaxation. These liquids have a fourfold difference in Tg’s, a fivefold difference in optical anisotropies γeff2’s, and a sevenfold difference in hydrodynamic volumes Vh’s. The analysis of the distribution of relaxation times has been carried out using both the raw intensity data and the corresponding susceptibility representation of the same data. These results are combined with light scattering and dielectric data obtained at lower frequencies. Notwithstanding the great differences in Tg’s, γeff2’s, and Vh’s, the liquids show that their distributions of relaxation times from light scattering narrow with increasing temperature. In some, though not all, liquids, an unbiased analysis of the data indicates that the distribution tends to narrow towards a single exponential time (Debye) spectrum consistent with the extrapolation of the dielectric relaxation measurements from lower temperatures. The light-scattering results, consistent with dielectric and shear viscosity data, are discussed in their relation to current theoretical interpretations.
Multidimensional generalization of the Pollak–Grabert–Hänggi turnover theory for activated rate processes
Authors: Hershkovitz Eli , Pollak Eli
Published: July 2002

The turnover theory for activated rate processes, is extended to multidimensional systems. The theory derived in this paper accounts for the competition between intramolecular and intermolecular relaxation. The extent of chaotic motion of the system modes directly affects the rate of energy diffusion in the system. The more chaos, the faster the energy diffusion and the larger the rate. The dependence of the rate on the intramolecular coupling strength is well accounted for. The theory is applied to a model two-dimensional system studied previously by Straub and Berne [J. Chem. Phys. 85, 2999 (1986)]. The theory, which is the multidimensional generalization of the one-dimensional Pollak, Grabert, and Hänggi (PGH) turnover theory [J. Chem. Phys. 91, 4073 (1989)] accounts well for the rate even in the case of extreme anisotropic friction. The theory is cast in terms of the collective normal modes of the system and the bath and is thus applicable also to memory friction.
Symmetrized density matrix renormalization group studies of the properties of low-lying states of the poly-para-phenylene system
Authors: Anusooya Y. , Pati Swapan K. , Ramasesha S.
Published: July 2002

We report the symmetrized density matrix renormalization group (DMRG) study of neutral and doped oligomers of poly-para-phenylene (PPP) system within an extended Hubbard model. Model parameters are determined by comparing the existing results for an interacting small system. We compute a number of properties in the ground state as well as in the one-photon, two-photon and triplet states to completely characterize these states. Bond-order studies show that the lowest two-photon state corresponds to a localized excitation while one-photon and triplet excitations are extended in nature. The bipolaronic state shows clear evidence for charge separation and disproportionation into two polarons. We find that the extended nature of one-photon and triplet states of the neutral system are very similar to those of the bipolaronic ground states.
Oscillating phase separation in microemulsions. I. Experimental observation
Authors: Vollmer Doris , Strey Reinhard , Vollmer Jürgen
Published: July 2002

We examine the phase separation of a single phase of water-in-oil microemulsion droplets towards a phase of smaller water droplets coexisting with a water-rich excess phase. This transition is found to oscillate when induced by a continuous temperature increase. A periodic clouding and clearing is observed under the microscope and in the microcalorimeter, allowing to determine the oscillation period from the extinction of transmitted light and from the specific heat. The period depends on the surfactant concentration and increases as a square root of the scan speed. To the authors’ knowledge this is the first time that oscillations in a temperature induced phase separation of a mixture have been observed.
Molecular dynamics of the α-relaxation during crystallization of a low-molecular-weight compound: A real-time dielectric spectroscopy study
Authors: Dobbertin J. , Hannemann J. , Schick C. , Pötter M. , Dehne H.
Published: July 2002

Low-molecular-weight compounds often crystallizes to systems with 100% crystallinity. There are only a few examples where a small amorphous fraction, characterized by a glass transition, remains after long time crystallization from the melt. The crystallization of such a glass-forming low-molecular-weight compound was investigated in order to monitor the change of the molecular dynamics with increasing crystallinity by dielectric spectroscopy and differential scanning calorimetry (DSC). The measurement of the dielectric α-relaxation was performed in real time during isothermal crystallization above the glass transition. At high crystallinities (above 90%) a shift of the peak position and a broadening of the dielectric spectrum was observed. The calorimetric glass transition temperature shifts in the same region for about 15 K to higher temperatures. No direct information about the morphology of the samples is available at the moment but indirect measurements indicate a layerlike crystalline structure. Then the remaining amorphous fraction can be considered between the crystal layers and the observed changes in the relaxation behavior may be caused by spatial confinement in the order of nanometer.
Dynamics and kinetics of oxygen dissociative adsorption on Pt{110}(1×2)
Authors: Walker A. V. , Klötzer B. , King D. A.
Published: July 2002

The interaction of oxygen with Pt{110}(1×2) has been investigated using supersonic molecular beams and low-energy electron diffraction (LEED) at incident kinetic energies of 60–900 meV and surface temperatures from 170 to 1100 K. LEED studies indicate that at low exposures and low beam energies (<190 meV) oxygen does not induce a reconstruction to (1×1). However at high exposures and low beam energies, a variable unit mesh dimension Frenkel–Kontorova domain structure is observed. Sticking probability measurements on the clean surface indicate that oxygen dissociative adsorption on Pt{110}(1×2) is a predominantly precursor mediated process. At low surface temperatures (<200 K) and low beam energies, the precursor has a strong influence on the adsorption kinetics, and mixed islands of dioxygen and adatoms are formed. Adsorption at beam temperatures below 880 K produces two states in the the thermal desorption spectroscopy spectra, but at higher incident gas temperatures a new high temperature desorption state (Tp=1020 K) can be populated, which inhibits the formation of the usual dissociatively chemisorbed states at Tp=720 and 820 K.
Vibrational levels of the transition state and rate of dissociation of triplet acetaldehyde
Authors: Leu Gen-Hou , Huang Cheng-Liang , Lee Shih-Huang , Lee Yu-Chang , Chen I-Chia
Published: July 2002

Fluorescence decay of the S1 state of d4-acetaldehyde is measured. Below the dissociation threshold for formation of fragments of formyl and methyl radicals, single exponential decays of fluorescence are observed. Biexponential decay is observed when the excitation energies are near and above the dissociation threshold. Hence, in this region the mechanism for intramolecular relaxation of energy reaches the “intermediate case.” Strong coupling between S1 and T1 states or reversible reaction for S1↔T1 results in this biexponential behavior in the fluorescence decay of S1. Rates of appearance of DCO from dissociation of acetaldehyde are measured. The stepwise increases in plots of both rate of appearance of formyl radicals and rate of decay of excited acetaldehyde versus excitation energy for both isotopic variants of acetaldehyde are observed. According to fits to Rice–Ramsperger–Kassel–Marcus (RRKM) theory, these sharp increases correspond to vibrational levels of transition state in the triplet surface. Frequencies of torsional and C–C–O bending modes are determined to be (73±10) and (200±10) cm−1 for the transition state of CD3CDO and the torsional frequency (80±10) cm−1 for CH3CHO. That these fitted frequencies are less than values calculated with methods B3LYP and MP2 indicates a looser transition state than calculated for dissociation. From these fits the dissociation threshold for formation of radical fragments is determined to be 31 845 for CD3CDO and 31 650 cm−1 for CH3CHO.
Relationship between the primary and secondary dielectric relaxation processes in propylene glycol and its oligomers
Authors: León C. , Ngai K. L. , Roland C. M.
Published: July 2002

Dielectric relaxation measurements were performed on propylene glycol (PG) and oligomers having different number of repeat units (N=2, 3, and 69). The primary α-relaxation had the Kohlrausch–Williams–Watts (KWW) form, with a stretch exponent (1−n) which decreased with increasing N. The temperature dependence of the α-relaxation time, as reflected in the fragility index, increased with N. A broad, rather symmetric secondary β-relaxation was observed at higher frequencies in the dielectric loss spectrum for all samples with N>1. This is the first observation of the β-relaxation peak in dipropylene glycol (N=2) and tripropylene glycol (N=3). The separation between the α- and β-relaxations increased with increasing N. This trend indicates that the separation is minimal in PG, which makes it difficult to resolve the β-relaxation from the more intense α-relaxation. This, together with the fact that the strength of the β-relaxation decreases with the molecular weight of PPG, as found by Johari and coworkers, explains the absence of an observable β- peak or shoulder in isothermal or isochronal dielectric measurement on PG. It is proposed that the deviation of the dielectric loss peak for PG from either the KWW or the Cole–Davidson functions at higher frequencies is due to a β-relaxation masked by the α-process. The same conclusion was reached by Johari and co-workers, based on evidence from their “difference isochrone method.” Finally, we show that the data from oligomers of propylene glycol support the recently proposed correlation of τβ(Tg) with (1−n) and the rough agreement between τβ(Tg) and the primitive relaxation time of the coupling model at temperatures above Tg.
Infrared spectroscopy of negatively charged water clusters: Evidence for a linear network
Authors: Ayotte Patrick , Weddle Gary H. , Bailey Christopher G. , Johnson Mark A. , Vila Fernando , Jordan Kenneth D.
Published: July 2002

We report autodetachment spectra of the mass-selected, anionic water clusters, (H2O)n−, n=2, 3, 5–9, 11 in the OH stretching region (3000–4000 cm−1), and interpret the spectra with the aid of ab initio calculations. For n⩾5, the spectra are structured and are generally dominated by an intense doublet, split by about 100 cm−1, which gradually shifts toward lower energy with increasing cluster size. This behavior indicates that the n=5–11 clusters share a common structural motif. The strong bands appear in the frequency region usually associated with single-donor vibrations of water molecules embedded in extended networks, and theoretical calculations indicate that the observed spectra are consistent with linear “chainlike” (H2O)n− species. We test this assignment by recording the spectral pattern of the cooled (argon solvated) HDO⋅(D2O)5− isotopomer over the entire OH stretching frequency range.
Fully coupled six-dimensional calculations of the water dimer vibration-rotation-tunneling states with split Wigner pseudospectral approach. II. Improvements and tests of additional potentials
Authors: Fellers R. S. , Braly L. B. , Saykally R. J. , Leforestier C.
Published: July 2002

The SWPS method is improved by the addition of H.E.G. contractions for generating a more compact basis. An error in the definition of the internal fragment axis system used in our previous calculation is described and corrected. Fully coupled 6D (rigid monomers) VRT states are computed for several new water dimer potential surfaces and compared with experiment and our earlier SWPS results. This work sets the stage for refinement of such potential surfaces via regression analysis of VRT spectroscopic data.
Creep of selenium near the glass temperature
Authors: Roland C. M. , Santangelo P. G. , Plazek D. J. , Bernatz K. M.
Published: July 2002

Creep experiments were carried out on amorphous selenium (Se) at temperatures in the vicinity of the glass temperature. The recoverable compliance lacks a plateau, indicating Se chains are too short to form an entanglement network. The measured compliance function was thermorheological complex, even after subtraction of the glassy level and normalizing by the steady state compliance. The temperature dependence determined from the viscosity was in accord with previous viscosity data, although weaker than the near-Arrhenius dependence deduced from the stress relaxation of Se. Based on a comparison to other, small-molecule glass-formers, the dynamic fragility calculated from the viscosity was larger than expected from Se’s thermodynamic fragility (i.e., steepness of the normalized Kauzmann curve). In contrast, although polypropylene (PP) is substantially more dynamically fragile than Se, PP is less thermodynamic fragile. Thus, when compared to either small-molecule liquids or polymers, Se exhibits a disconnect between dynamic and thermodynamic measures of fragility.
Electronic structure and dynamics of O(3P)+CO(1Σ+) collisions
Authors: Braunstein M. , Duff J. W.
Published: July 2002

The potential energy surfaces of the three lowest electronic triplet states of CO2 which lead to O(3P)+CO(1Σ+), 3A′, 1 3A″, and 2 3A″, have been computed at the completeactive-space-self-consistent-field plus second-order perturbation theory (CASSCF-MP2) level with a modest 631+G(d) basis. Potential energy surfaces are fit with a global functional form. The 3A′ state has a well 0.9 eV deep and the 1 3A″ state has a 0.2 eV well with respect to the O(3P)+CO(1Σ+) dissociation threshold. The 3A′ and 1 3A″ states are both bent at their minima and have a barrier at 0.2 eV and 0.3 eV above threshold, respectively. The 2 3A″ state is mostly repulsive, and has a saddle at C2v geometries. We have run classical trajectory calculations for O(3P)+CO(1Σ+) collisions using these surfaces. Results agree well with available vibrational relaxation and oxygen atom exchange measurements except at low temperature. Comparisons are also made with measured vibrational excitation cross sections and infrared emission spectra of the nascent CO products at 3.4 eV collision energy. These results show a high degree of vibrational and rotational excitation with a nearly statistical population which is evident in a distinct spectral “bandhead” signature. Analysis of the trajectories show that almost all collisions which lead to oxygen atom exchange and/or vibrational energy transfer occur when the O(3P) approaches the CO at OCO angles between 80° and 140°, passes over the barrier and through the wells of the 3A′ and 1 3A″ states, and interacts with the repulsive wall of the carbon end of the CO nearly perpendicular to the CO bond.
X-ray Raman scattering under pulsed excitation
Authors: Gel’mukhanov Faris , Sałek Paweł , Shalagin Anatoly , Ågren Hans
Published: July 2002

Accounting for actual and anticipated developments in synchrotron radiation- and in x-ray and free electron laser techniques, we present a theory of resonant scattering of short-pulsed x rays. Both the instantaneous and integrated-over-observation-time x-ray scattering spectra are studied. Contrary to the commonly accepted notion of an instantaneous probability, we find that the correct quantity describing the resonant x-ray scattering (RXS) experiment is the probability integrated over the time of measurement. It is shown that the integral RXS probability is given by the convolution of the RXS probability for monochromatic excitation with the Wiener–Khintchine spectrum of the nonstationary incident radiation. We have found that short pulses yield a new spectral band following a non-Raman dispersion law. The finite duration of pulses results in a broadening of the Raman resonance. Some aspects of the theoretical analysis of the RXS spectra as functions of the pulse duration time are analyzed by numerical calculations.
The metaplectic group within the Heisenberg–Weyl ring
Authors: García-Bullé Mauricio , Lassner Wolfgang , Wolf Kurt Bernardo
Published: July 2002

The Heisenberg–Weyl ring contains the metaplectic group of canonical transforms acting unitarily on ℒ 2(ℛ). These ring elements are characterized through (i) the integral transform kernels, (ii) coset distributions, and (iii) classical functions under any quantization scheme. The isomorphism under group composition leads to several new relations involving twisted products and quantization of Gaussian classical functions. The Wigner inversion operator is a special central group element. It is shown that the only quantization scheme invariant under metaplectic transformations is the Weyl scheme. The structure studied here appears to be relevant to the study of wave optics with aberration.
Scalar potentials for vector fields in quantum electrodynamics
Authors: Müller Eberhard E.
Published: July 2002

For an arbitrary vector field F: x∈R3→F(x)∈R3, the representation F=∇Φ+LΨ+∇∧Lχ, where Φ, Ψ, χ are scalar potentials, is used to disentangle the longitudinal and transversal degrees of freedom of the electromagnetic field. As a result the potentials for the electromagnetic field can be quantized restriction-free. Replacing the conventional vector potential in the Dirac equation by these Debye potentials provides a manifestly covariant and local description for the interaction of the quantized electromagnetic field with a quantized Dirac field.
The unitary irreducible representations of SU(2,1)
Authors: Bars Itzhak , Teng Zhong-jian
Published: July 2002

This paper analyzes the irreducible unitary representations of SU(2,1) in a basis labeled as ‖p, q; jmy〉, where p,q correspond to quantum numbers associated with the quadratic and cubic Casimir operators, j,m label states of the SU(2) subgroup, and y labels the quantum number with respect to the U(1) subgroup. All the irreducible representations are found and the allowed range of these quantum numbers for each representation are given. The results are expressed in the form of diagrams that show the allowed values in a (j,y) plot for fixed values of p,q. A (p,q) plot is also provided that indicates the allowed values of these quantum numbers.
Polyhedral representations of discrete differential manifolds
Authors: Zapatrin Roman R.
Published: July 2002

Any discrete differential manifold M (a finite set endowed with an algebraic differential calculus) can be represented by appropriate polyhedron P(M). The representation is proved to be exact for a special class of manifolds called basic. The links with the finitary substitutes of continuous spaces introduced by R. Sorkin are established.
Exterior differentials of higher order and their covariant generalization
Authors: Abramov V. , Kerner R.
Published: July 2002

We investigate a particular realization of generalized q-differential calculus of exterior forms on a smooth manifold based on the assumption that dN=0 while dk≠0 for k<N. It implies the existence of cyclic commutation relations for the differentials of first order and their generalization for the differentials of higher order. Special attention is paid to the cases N=3 and N=4. A covariant basis of the algebra of such q-grade forms is introduced, and the analogs of torsion and curvature of higher order are considered. We also study a ZN-graded exterior calculus on a generalized Clifford algebra.
Non-linear effects in hopping conduction of single-crystal La2CuO4+δ
Authors: Belevtsev B. I. , Dalakova N. V. , Panfilov A. S.
Published: July 2002

The unusual non-linear effects in hopping conduction of single-crystal La2CuO4+δ with excess oxygen has been observed. The resistance is measured as a function of the applied voltage U (voltage controlled regime) in the temperature range 5 K⩽T⩽300 K and voltage range 10−3−25 V. At relatively high voltage (approximately at U>0.1 V) the conduction of sample investigated corresponds well to variable-range hopping (VRH). That is, in the range 0.1 V<U⩽1 V the conductivity does not depend on U (Ohmic behavior) and the temperature dependence of resistance R(T) follows closely Mott’s law of VRH [R∝exp(T0/T)1/4]. In the range of highest applied voltage the conduction has been non-Ohmic: the resistance decreases with increasing U. This non-linear effect is quite expected in the frame of VRH mechanism, since the applied electric field increases the hopping probability. A completely different and unusual conduction behavior is found, however, in the low voltage range (approximately below 0.1 V), where the influence of electric field and (or) electron heating effect on VRH ought to be neglected. Here we have observed strong increase in resistance at increasing U at T⩽20 K, whereas at T>20 K the resistance decreases with increasing U. The magnetoresistance of the sample below 20 K has been positive at low voltage and negative at high voltage. The observed unusual non-Ohmic behavior at low voltage range is attributable to inhomogeneity of the sample, namely, to the enrichment of sample surface with oxygen during the course of the heat treatment of the sample in helium and air atmosphere before measurements. At low enough temperature (below ≈20 K) the surface layer with increased oxygen concentration is presumed to consist of disconnected superconducting regions in a poorly conducting (dielectric) matrix. This allows us to explain the observed unusual non-linear effects in the conduction of sample studied. The results obtained demonstrate that in some cases the measured transport properties of cuprate oxides cannot be attributed to the intrinsic bulk properties.
The dynamics of helical decaying turbulence
Authors: Polifke Wolfgang , Shtilman Leonid
Published: July 2002

The dynamics of helical decaying homogeneous turbulence is investigated in direct numerical simulations at moderate Reynolds numbers. A new initialization procedure is presented that allows one to control both the energy and the helicity spectral density of the initial flow field. It is observed that large initial helicity impedes the transfer of energy toward smaller scales, inhibits the buildup of enstrophy, and reduces dissipation for several turnover times. Also, the skewness and flatness of the velocity derivatives reach values typical of turbulence much later than in comparable flows without helicity. However, these effects are significant only if the helicity of the flow is quite high. In simulations with small or vanishing initial helicity it is found that the fluctuations of the average helicity and the helicity spectral density lie within the range suggested by a quasi-Gaussian approximation. This suggests that at moderate Reynolds number spontaneous fluctuations of helicity are not large enough to directly influence the energy transfer.
Unsteady free-surface flow due to a line source
Authors: Tyvand Peder A.
Published: July 2002

The initial development of the free-surface flow due to a submerged two-dimensional source (or sink) is studied analytically. The source is turned on at time zero. The fluid depth is infinite. The nonlinear initial/boundary-value problem is solved to second order in a Taylor expansion in time. A critical sink strength is found, for the formation of a surface dip. Its Froude number is equal to 1/3. The leading gravity wave is considered in the context of classical Cauchy–Poisson problems.
Reconnection in orthogonally interacting vortex tubes: Direct numerical simulations and quantifications
Authors: Boratav O. N. , Pelz R. B. , Zabusky N. J.
Published: July 2002

The three-dimensional time evolution of two orthogonally offset cylindrical vortices of equal strength is simulated by solving the hyperviscosity-regularized incompressible Navier–Stokes equations. A Fourier pseudospectral method with a time-split integration scheme is used for the solution. Four runs with different Reynolds numbers ranging between 690–2100 are performed, each with a resolution of 963 collocation points. The sequence of important physical processes and the evolution of local and global quantities such as vorticity, velocity, and mean-square strain rate are presented. It is found that the growth rate of the maximum vorticity is at most exponential. The Reynolds number dependence of the time scale of reconnection, the vorticity growth rate, and the time at which the maximum vorticity is attained are examined and differences between the present results and Saffman’s essentially two-dimensional model predictions are encountered and elucidated. The distributions of the eigenvalues α, β, γ and the corresponding eigenvectors sα, sβ, sγ of the rate of strain tensor Sij are calculated at different times. It is found that as the mean-square strain rate ε increases during the evolution, sβ and the vorticity vector ω are perfectly aligned and β>0 in high ε regions. Strong temporal, spatial, and Reynolds number dependence of the strain fields is also seen. Evidence is presented that, during reconnection, the vorticity growth in newly forming bridges takes place in the vicinity of the upper stagnation line segment of the vortex dipole due to the nature of the vortex stretching term. Also examined is the initial finger formation and it is found that the initial nonuniform axial stretching and the displacement of the vortex cores due to a lift force play an important role in this process.
Statistical analysis of the rate of strain tensor in compressible homogeneous turbulence
Authors: Erlebacher G. , Sarkar S.
Published: July 2002

Recent analysis of direct numerical simulations of compressible homogeneous shear flow turbulence has unraveled some of the energy transfer mechanisms responsible for the decrease of kinetic energy growth when the flow becomes more compressible. In this complementary study, attention is focused on the rate of strain tensor. A Helmholtz decomposition of the velocity field leads to a consideration of a solenoidal and an irrotational rate of strain tensor. Their eigenvalue distributions, eigenvector orientations, and the relative alignment between the eigenvectors and the vorticity and pressure gradient vectors are examined with the use of probability density functions. The irrotational rate of strain tensor is found to have a preferred structure in regions of strong dilatation. This structure depends on the mean shear, and is quite different from that of the solenoidal rate of strain tensor. Compressibility strongly affects the orientation properties of the pressure gradient vector.
Stability of field-reversed configurations in the large s experiment (LSX)
Authors: Slough J. T. , Hoffman A. L.
Published: July 2002

Data from several diagnostics employed on the Large s Experiment (LSX) field-reversed theta pinch were analyzed to seek correlation between plasma distortions and the confinement properties of the field-reversed configurations (FRC’s) formed. In particular, an array of Bθ probes was used to determine separatrix movement, which might indicate the existence of low-order modes, such as a tilt instability. No correlation between the quality of confinement and signal was observed. The parameter s, equal to the average number of ion gyroradii inside the separatrix, has been postulated as a measure of FRC stability with values above 2, leading to instability and loss of confinement. However, the confinement observed in experiments conducted over a large range of s (1<s<8) appeared to correlate more with the shape of the equilibrium radial density profile produced during formation rather than s. Flatter profiles correlated with poorer confinement.
Velocity field measurements of a laminar starting plume
Authors: Tanny(Tanai) Jossef , Shlien D. J.
Published: July 2002

The region of buoyant fluid resulting from the initiation of heating of an infinite fluid is called the starting plume. Here, velocity field measurements of this flow pattern are presented for the first time. The measurements were carried out by processing digitized tracer particle path photographs. Similarity of the velocity field of the starting plume as it rises was found to exist in the limited range of heat injection rates investigated. The vorticity computed from the velocity field was diffuse, there being no evidence of a distinctive core. Additional flow visualization experiments show the spiral structure in the starting plume cap.