bluuebunny/crossref_metadata_2025 huggingface dataset:
Vacuum-ultraviolet photolysis of N2H2: Generation of NH fragments
Authors: Biehl H. , Stuhl F.
Published: July 2002

The photodissociation of the double bond in HN=NH yielding electronically excited NH(A 3Π) and ground state NH(X 3Σ−) radicals has been studied in the vacuum-ultraviolet above 105 nm. Fragment excitation spectra were taken using tunable synchrotron radiation as the photolysis light source. The excited radicals were detected by their triplet emission to the ground state. A very crude estimate results in 10%, 20%, and 70% of the excess energy to be channeled into fragment vibration, rotation, and translation, respectively, at the Kr resonance line at 123.6 nm. This energy distribution supports a repulsive process with almost equal rotation in the two NH fragments and vibration caused by lengthening all bonds during the N=N bond breaking. An upper limit for the energy necessary to break the double bond is measured to be 510.7±1.2 kJ mol−1. This value yields ΔfH00(N2H2)≥204.1±2.2 kJ mol−1.
O2(X,v=8–22) 300 K quenching rate coefficients for O2 and N2, and O2(x) vibrational distribution from 248 nm O3 photodissociation
Authors: Park H. , Slanger T. G.
Published: July 2002

Vibrationally excited oxygen (O‡2) is produced in the atmosphere by ozone photodissociation in the 200–300 nm Hartley band. It has been suggested that photoexcitation of O‡2 in the O2 Schumann–Runge bands will lead to predissociation, and autocatalytic production of O3. The resultant new source of atmospheric O3 could help alleviate current discrepancies between observed and modeled O3 profiles. To evaluate this possibility, we have examined two critical factors—the nascent distribution of O‡2 levels for 248 nm photodissociation, near the peak of the Hartley band, and the rate coefficients for their relaxation by O2 and N2. We find that the distribution extends to v=22, close to the thermodynamic limit, with a peak near v=8. The 300 K quenching rate coefficients have been evaluated using a cascade model, in which it is assumed that relaxation by O2 occurs through single-quantum vibration–vibration (V–V) and vibration–translation (V–T) steps. By modeling the relaxation from the top of the distribution downwards, we simultaneously obtain both the quenching rate coefficients and the nascent vibrational distribution. Agreement with new rate coefficient measurements carried out in a state-specific manner is good, as is also true for the comparison with new V–V and V–T calculations. Data from experiments on O‡2 quenching by N2 show that in the v=16–22 range, potentially important in the atmosphere, quenching proceeds up to five times faster than for the case of O2.

The hypothesized explanation is that two-quantum V–V transfer, peaking at the resonant condition of O2(v=18–19), is the dominant process. As a consequence, atmospheric quenching of O‡2 for levels above v=14 is basically controlled by N2, and at low stratospheric temperatures, the effect of N2 quenching near v=18 is likely to be 2 orders of magnitude greater than quenching by O2. This unexpected effect probably precludes a significant role for O‡2 photodissociation as a new source of stratospheric O3, but the existence of these high-energy entities can have other consequences, among them being enhanced activity with minor species, and the possibility that energy may flow from the relatively stable O2(v=1) and N2(v=1) levels into infrared-active H2O and CO2, respectively. Measurements have also been made for O‡2 quenching by O3, CO2, and He, particularly to establish whether O3 and CO2 can play a competitive quenching role in the atmosphere. Although O3 is a fast quencher, with CO2 being 2 orders of magnitude slower, they are unlikely to compete with O2 and N2. The data on He is particularly interesting, suggesting that considerably more O‡2 is present in the nascent O3 photodissociation products than subsequently appears from O(1D)+O3 interaction. The implications of this finding are discussed.
Use of second-order perturbation theory for the vibrational energy levels and partition functions at a saddle point
Authors: Isaacson Alan D. , Hung Shu-Chin
Published: July 2002

Recent ab initio information of Kraka and Dunning on the saddle point region for the reaction OH+H2→H2O+H is used to construct an anharmonic potential energy surface valid near the saddle point. Anharmonic vibrational energy levels involving the bound degrees of freedom orthogonal to the reaction coordinate at the saddle point are obtained using perturbation theory through second order for cubic terms and first order for quartic terms, with resonance effects removed. These energy levels are compared to those obtained from an accurate self-consistent field configuration-interaction method, and are used to calculate thermal vibrational partition functions over the temperature range from 200 to 2400 K.
A Born–Green–Yvon equation for flexible chain-molecule fluids. I. General formalism and numerical results for short hard-sphere chains
Authors: Taylor Mark P. , Lipson J. E. G.
Published: July 2002

The Born–Green–Yvon approach is used to construct an integral equation for the site–site distribution functions describing the microscopic equilibrium structure of a fluid of flexible chain molecules. The multisite distribution functions appearing in this formally exact equation are related to the site–site functions via a set of site–site superposition approximations. Intramolecular distribution functions are approximated using exact results for an isolated chain molecule. Numerical results are presented for fluids of tangent hard-sphere 3-mers and 4-mers. These results, which include the site–site distribution functions over a range of densities, second virial coefficients, and the virial pressure equations of state, compare well with Monte Carlo data. In particular, the contact value of the average site–site distribution function is given quite accurately.
State-selected vibrational relaxation rates for highly vibrationally excited oxygen molecules
Authors: Hernández Ramón , Toumi Ralf , Clary David C.
Published: July 2002

The state-selected vibrational relaxation rates in O2+O2 collisions, with one O2 molecule in a highly vibrationally excited state, have been calculated from first principles. The vibrationally close-coupled, rotationally infinite order sudden approximation has been used to treat the collision dynamics and a potential energy surface based on high quality ab initio calculations, which include the variation of the O2 vibrational coordinates, has been developed. The calculated relaxation rates are in good agreement with those obtained from experiment for 8≤v<26 but fail to reproduce the sharp increase observed experimentally for v≥26 indicating the onset of a new vibrational relaxation mechanism.
Argon cluster evaporation dynamics
Authors: Dumont Randall S. , Jain Shashi , Basile Anthony G.
Published: July 2002

Expansion of argon clusters in a vacuum is simulated via molecular dynamics computations. The resulting evaporation dynamics is investigated with observations of temperature and pV energy loss curves. Observed cooling curves (T vs n) and collapse curves (pV/n vs n) are found to depend on final cluster size but not the initial cluster ensemble. The evaporation mechanism consists of an initial rapid cooling-and-collapse stage of a preliquid dense-gas-like cluster, followed by ‘‘equilibrium’’ evaporation, and then another cooling stage of the resulting relatively incompressible liquidlike subcluster. Elements of this dynamics evaporation mechanism are tested by examination of finite n phase diagrams constructed using Metropolis Monte Carlo simulations of the fixed T and p ensemble.
Vibration–rotation–tunneling dynamics calculations for the four-dimensional (HCl)2 system: A test of approximate models
Authors: Elrod M. J. , Saykally R. J.
Published: July 2002

Several commonly used approximate methods for the calculation of vibration–rotation–tunneling spectra for (HCl)2 are described. These range from one-dimensional models to an exact coupled four-dimensional treatment of the intermolecular dynamics. Two different potential surfaces were employed—an ab initio and our ES1 experimental surface (determined by imbedding the four-dimensional calculation outlined here in a least-squares loop to fit the experimental data, which is described in the accompanying paper [J. Chem. Phys. 103, 933 (1995)]. The most important conclusion deduced from this work is that the validity of the various approximate models is extremely system specific. All of the approximate methods addressed in this paper were found to be sensitive to the approximate separability of the radial and angular degrees of freedom, wherein exists the primary difference between the two potentials. Of particular importance, the commonly used reversed adiabatic angular approximation was found to be very sensitive to the choice for fixed R; an improper choice would lead to results very much different from the fully coupled results and perhaps to false conclusions concerning the intermolecular potential energy surface.
Determination of orientational order parameters of uniaxial films with a commercial 90°-angle fluorescence spectrometer
Authors: Damerau T. , Hennecke M.
Published: July 2002

A method is developed to determine orientational order parameters 〈P2〉 and 〈P4〉 of films by steady state fluorescence polarization in a spectrometer with 90° angle between excitation and emission beam. This is the geometry usually found in commercial fluorescence spectrometers. The method includes the determination of the fundamental molecular anisotropy rM of the fluorescent chromophore (doped or intrinsic) in stretched and unstretched films and the determination of instrumental correction coefficients from an isotropic film. As an example, the method is applied to orientation analysis of polyethylene and polystyrene films doped with fluorescent molecules of repetitive styrene units. It is concluded that in uniaxially stretched polyethylene-films orientation of these molecules is dominated by their molecular shape; in polystyrene films orientation is influenced by additional interactions.
Sticking coefficient for dissociative adsorption of N2 on Ru single-crystal surfaces
Authors: Dietrich H. , Geng P. , Jacobi K. , Ertl G.
Published: July 2002

The dissociative chemisorption of N2 on Ru(0001), Ru(101̄0), and Ru(112̄1) surfaces at 300 K was studied by means of high-resolution electron energy loss spectroscopy and thermal desorption spectroscopy. The initial sticking coefficient was determined to s0=(1±0.8)×10−12, within the limits of error independent of surface orientation. On Ru(101̄0) and Ru(112̄1) small amounts of N can be dissolved into the subsurface region.
Quantum-classical reaction path study of the reaction O(3P)+O3(1A1)→2O2(X 3Σ−g)
Authors: Balakrishnan N. , Billing G. D.
Published: July 2002

The atmospheric reaction O(3P)+O3(1A1)→2O2(X 3Σ−g) is studied using the reaction path approach. In addition to total reaction rate constants and cross sections, product vibrational state-resolved cross sections and rate constants are computed. The product vibrational state distribution shows that one of the product O2 molecules is formed in a higher vibrational state than the other with a broad distribution the tail of which extends beyond v=27. However, no bimodal pattern is seen in the vibrational distribution in contrast to that found for O2 molecules resulting from O3 photodissociation in recent experimental studies. The vibrational excitation of the product O2 molecules is found to be mainly due to the large increase in the coupling element which couples the reaction path motion to the perpendicular vibrational motion.
Influence of stiffness on the dynamics of macromolecules in a melt
Authors: Harnau Ludger , Winkler Roland G. , Reineker Peter
Published: July 2002

Analytical expressions are derived for the incoherent and coherent dynamic structure factor of macromolecules in a melt. In particular, the influence of chain stiffness on these quantities is investigated. For scattering vectors q<πp/2, where 1/2p is the persistence length of the macromolecules, the well known results for the Rouse model are reproduced. However, in the scattering vector range q>πp/2 the incoherent dynamic structure factor is shown to be a universal function of only the combination q8/3tp1/3, reflecting the strong influence of the local chain stiffness on dynamical properties at small length scales. Moreover, the q dependence of both the full width at half maximum of the Fourier transformed incoherent dynamic structure factor and the initial decay rate of the coherent dynamic structure factor are calculated. The comparison of the theoretical results with quasielastic neutron scattering experiments of different polymers exhibits good agreement.
The vibrationally resolved C 1s core photoelectron spectra of methane and ethane
Authors: Osborne S. J. , Sundin S. , Ausmees A. , Svensson S. , Saethre L. J. , Svaeren O. , Sorensen S. L. , Végh J. , Karvonen J. , Aksela S. , Kikas A.
Published: July 2002

Recent progress in the development of high-resolution electron spectrometers in combination with highly monochromatized undulator radiation has allowed observation of the vibrationally resolved gas-phase C 1s photoelectron spectra of methane and ethane. For both molecules, the C–H stretching modes are well resolved and for ethane the active C–C stretching mode has been observed for the first time. The spectra have been measured at low kinetic energies and detailed fittings using post-collision interaction line profiles have been made both, using a free parameter fit and a fit adhering to a linear coupling model. The free parameter fit allows for any anharmonicity in the vibrational energies. The linear coupling model, on the other hand, assumes that the initial and final state potential curves are harmonic and differ only in the normal coordinates. This simple model is used to reduce the number of free parameters in the fit, which greatly simplifies the analysis. An intensity model based on the linear coupling predicts that the intensities of the C–H stretching modes are directly related to the number of C–H bonds around the core ionized atom. The result is verified for ethane and shows a potential for further reduction of free parameters for large molecules and polymers. Ab initio calculations of molecular geometry and vibrational frequencies have also been carried out using the equivalent core (Z+1) approximation. The values predicted for the decrease in bond length have then been compared to those determined empirically by the linear coupling approach. The calculation of ethane indicates that symmetric C–H and C–C stretching modes are important upon core ionization. The corresponding vibrational frequencies have been calculated and agree well with observed values.
Determination of elastic constants and rotational viscosity of micellar liquid crystals by conductivity measurements
Authors: Bajc J. , Hillig G. , Saupe A.
Published: July 2002

An experimental study of a lyotropic nematic liquid crystal (CsPFO in a water solution) confined to a cylindrical tube is presented. The structure of the sample is controlled by a magnetic field. A small ac voltage is applied along the tube for conductivity measurements. Elastic constants and conductivity constants of the material are determined from static measurements at different magnetic fields. The method is also used to determine the rotational viscosity γ1 from dynamic response under sudden changes of the magnetic field. The dynamic measurements are analyzed in the framework of the Leslie–Ericksen theory. It is shown that although the dynamic behavior can be qualitatively explained with a single effective rotational viscosity γ1, back flow effects have to be taken into account should the analysis yield consistent values for γ1. The results obtained by this method agree well with other experimental studies.
Determination of adsorption equilibria in pores by molecular dynamics in a unit cell with imaginary gas phase
Authors: Miyahara Minoru , Yoshioka Tomohisa , Okazaki Morio
Published: July 2002

We developed a new molecular dynamics (MD) scheme, introducing the concept of the potential buffering field through which an adsorbed phase could interact with an imaginary gas phase. This simulation cell allowed us to conduct a MD simulation that allowed a change in the number of molecules to attain equilibrium with given equilibrium pressure, like a grand canonical Monte Carlo simulation. By taking another choice for the setting of the cell, the number of molecules stayed constant but the equilibrium pressure was able to be obtained easily by a new technique of “particle counting method.” The thus obtained equilibrium vapor-phase pressure agreed with that obtained by Widom’s particle insertion method. Some adsorption simulations within slitlike pores of 2 and 3 nm were carried out. Adsorption phenomena could be observed from monolayer adsorption on a pore wall under a low relative pressure to the capillary condensation under a high relative pressure. Thus the adsorption equilibrium relation could be determined. The critical relative pressure for capillary condensation was smaller than that predicted by the modified Kelvin equation. This MD method shall provide much benefit in studying interfaces, which is important for analyzing condensation in pores.
Hard rods in the presence of a uniform external field
Authors: Ibsen Jorge , Cordero Patricio , Tabensky Romualdo
Published: July 2002

We sum the canonical partition function for a system of hard rods in a box of finite length in the presence of a linear external potential (gravity). From the canonical partition function closed expressions for the pressure at the top and bottom walls, and the chemical potential follow. The canonical number density and higher distribution functions are also determined. In particular it is shown that the number densities at the extremes of the box are proportional to the associated pressures at those points even though this is not generally true in the bulk of the system. It also is shown that the system is naturally divided in two wall zones and, if the density is low enough, a central zone as it is the case for the free field system. An expression for the local pressure is also derived and it is found that, in the thermodynamic limit and in a sufficiently weak external potential, an exact local relation between the number density and the pressure profile (equation of state) exists in the canonical ensemble within the central region. We also compute the grand canonical partition function for the system and generalize some results from other authors.
A theoretical study of the isotropic cut sphere fluids
Authors: Chamoux Antoine , Perera Aurélien
Published: July 2002

The cut sphere fluid is studied in the isotropic phase by the Percus Yevick (PY) and the Hypernetted Chain (HNC) integral equation techniques, as well as by the theory recently proposed which is based on a geometrical interpretation of the direct correlation function. Fluids of cut spheres with thicknesses L* ranging from 0 to 0.7 have been studied, and detailed results for L*=0.1, 0.2, and 0.3 are reported. The L*=0 case is also examined. A new simplified version of the numerical implementation of the PY and HNC closures is proposed here. The results for pressures and structural properties are compared with the available simulations results and the recent theoretical results from the authors. The important feature of the present work is to show the ability of the HNC theory to predict the cubatic phase observed in the computer simulations for thicknesses around 0.2. The nematic phase is also predicted by the HNC theory for thicknesses smaller than L*=0.12. In agreement with previously obtained results, the detailed analysis of the PY theory results show that this approximation is unable to predict an instability toward any of the orientationally ordered fluid phases. The geometrical approach shows the correct trend for an isotropic to nematic transition, but exhibits an instability toward the cubatic phase only for thicknesses above L*=0.5, thus providing an illustration of the inability of standard density functional type theories to fully describe complex fluids. This study also sheds some light on the major differences between the three approaches in the treatment of many body density correlations.
Subglass chain dynamics and relaxation in polyethylene: A molecular dynamics simulation study
Authors: Jin Yong , Boyd Richard H.
Published: July 2002

Chain dynamics in amorphous polyethylene (PE) below the glass transition temperature is studied via molecular dynamics (MD) simulations. Very long, by current MD standards, trajectories (up to 450 ns) are generated. It is found that the time autocorrelation function (ACF) based on pendant perpendicular dipoles placed at the methylene units agrees promisingly well with the experimental γ subglass dielectric process in dipole decorated PE. The central relaxation time of the dipolar ACF follows the γ process on a map of experimental loss maxima versus inverse temperature. When transformed to frequency domain, the isothermal process represented by the ACF is of the broad proportions characteristic of the experimental one. Analysis via several filtering processes demonstrates clearly that all of the ACF decay is accomplished via conformational transitions. The transitions effective in ACF decay are found to be highly neighbor correlated, much more so than in the melt. Specifically, most of the correlations belong to the group of several ±2 next neighbor types that are common but not dominant in the melt. Effects of nonergodicity in the glass on the torsional angle dynamics were investigated. The torsional angle populations are fairly close to that expected from the explicit torsional potential invoked in the simulation. However, the torsional angle trajectories show that individual bonds can have long residence times at angle values well removed from the minima in the explicit potential. Conformational transitions are found to and from these long-lived states that result from partial freezing-in of the conformational state of the chain.
Highly predissociative levels of CH B 2Σ− state detected with two-color resonant four-wave mixing spectroscopy
Authors: Kumar Awadhesh , Hsiao Chih-Chang , Hung Wen-Ching , Lee Yuan-Pern
Published: July 2002

We demonstrate an application of two-color resonant four-wave mixing spectroscopy to detect highly predissociative levels of CH in the B 2Σ− state in a hostile environment of an oxyacetylene flame. The probe and grating wavelengths are in resonance with the A 2Δ–X 2Π and B 2Σ−–X 2Π systems, respectively. We measured 49 previously unobserved rovibronic lines of the 0–0 band and 38 additional lines of the 1–0 band of the B 2Σ−–X 2Π system to access rotational quantum number N′ up to 21 and 13 of v′=0 and 1 in the B 2Σ− state, respectively. Inclusion of additional line positions measured for the 0–0 and the 1–0 bands yields spectral parameters of the B 2Σ− state significantly improved over those obtained previously with only nonpredissociative lines; one additional centrifugal–distortion parameter Lv is evaluated significantly. Although power saturation is significant even at smallest practical laser energies, we estimated lifetimes through the power dependence of linewidths. The lifetimes of the highest measurable level of B 2Σ− (v′=0 and 1) are 3±1 (N′=20) and 8±2 (N′=11) ps, respectively.
Three-dimensional nonperturbative analytic model of vibrational energy transfer in atom–molecule collisions
Authors: Adamovich Igor V. , Rich J. William
Published: July 2002

A three-dimensional semiclassical analytic model of vibrational energy transfer in collisions between a rotating diatomic molecule and an atom has been developed. The model is based on analysis of classical trajectories of a free-rotating (FR) molecule acted upon by a superposition of repulsive exponential atom-to-atom potentials. The energy transfer probabilities have been evaluated using the nonperturbative forced harmonic oscillator (FHO) model. The model predicts the probabilities for vibrational energy transfer as functions of the total collision energy, orientation of a molecule during a collision, its rotational energy, and impact parameter. The model predictions have been compared with the results of three-dimensional close-coupled semiclassical trajectory calculations using the same potential-energy surface. The comparison demonstrates not only remarkably good agreement between the analytic and numerical probabilities across a wide range of collision energies, but also shows that the analytic FHO-FR model correctly reproduces the probability dependence on other collision parameters such as rotation angle, angular momentum angle, rotational energy, impact parameter, and collision reduced mass. The model equally well predicts the cross sections of single-quantum and multiquantum transitions and is applicable up to very high-collision energies and quantum numbers. Most importantly, the resultant analytic expressions for the probabilities do not contain any arbitrary adjustable parameters commonly referred to as “steric factors.” The model provides new insight into kinetics of vibrational energy transfer and yields accurate expressions for energy-transfer rates that can be used in kinetic modeling calculations.
Negative ion photoelectron spectroscopy of the AsO− anion
Authors: Lippa T. P. , Xu S.-J. , Lyapustina S. A. , Bowen K. H.
Published: July 2002

The negative ion photoelectron spectrum of AsO− has been measured, assigned, and analyzed. The adiabatic electron affinity, EAa, was determined directly from the photoelectron spectrum. The dissociation energy of AsO−, D0(AsO−), was computed via an energetic cycle using our measured value of EAa and existing literature values for other necessary quantities. Franck–Condon analysis provided values for the bond length of the AsO− anion, re(AsO−), its vibrational frequency, ωe(AsO−), and its anharmonicity constant, ωeχe(AsO−). The values of the molecular constants which were determined in this work are: EAa(AsO)=1.286±0.008 eV, D0(AsO−)=4.74±0.08eV, re(AsO−)=1.696±0.010 Å, ωe(AsO−)=827±40 cm−1, and ωeχe(AsO−)=5.54 cm−1. In addition, we determined the X 3Σ−−a1Δ, ground-to-first excited state splitting in AsO− to be ∼0.54 eV.
Temperature dependence of the collisional removal of O2(b 1Σg+, v=1 and 2) at 110–260 K, and atmospheric applications
Authors: Hwang Eunsook S. , Bergman Aaron , Copeland Richard A. , Slanger Tom G.
Published: July 2002

The temperature dependence of the collisional removal of O2 in the v=1 and 2 vibrational levels of the b 1Σg+ state by O2, N2, and CO2 has been investigated over the 110–260 K temperature range. For the v=1 level, O2 removes energy more effectively than N2 or CO2 at all measured temperatures. For v=2, the difference in effectiveness between colliders is not as great, and the removal by O2 exhibits a substantial positive activation energy; at 125 K O2 in the v=2 level is removed about 40 times more slowly than v=1. Even with this large difference, in the terrestrial atmosphere removal via collisions with O2 controls the lifetime of the vibrationally excited species. Recent atmospheric nightglow measurements performed at the W. M. Keck telescope on Mauna Kea, Hawaii, confirm the presence of O2 in these and higher levels through observation of the naturally occurring b 1Σg+–X 3Σg− emission.
Imaging the atomically resolved dissociation of D2S on Si(100) from 80 to 300 K
Authors: Rezaei M. A. , Stipe B. C. , Ho W.
Published: July 2002

Using a variable-temperature, ultrahigh vacuum scanning tunneling microscope (STM), we have induced and imaged and dissociation of D2S on Si(100). D2S dissociates into DS and D below 200 K. Individual DS fragments can be dissociated with the STM at low temperatures. The deuterium atom attaches to a neighboring silicon dimer. At 200 K or above, D2S dissociates into S and two Ds. D2S adsorption affects the surface reconstruction on Si(100), from the buckled dimer configuration to the dynamically flipping configuration and vice versa. We discuss our results in the context of other experiments on the same and similar systems.
Tethered chains in poor solvent conditions: An experimental study involving Langmuir diblock copolymer monolayers
Authors: Kent M. S. , Majewski J. , Smith G. S. , Lee L. T. , Satija S.
Published: July 2002

We have employed Langmuir monolayers of highly asymmetric polydimethylsiloxane-polystyrene (PDMS-PS) diblock copolymers on dioctyl phthalate (DOP) at temperatures ranging from 22 to −35 °C as a model system for tethered chains in poor solvent conditions. The thicknesses of the tethered PS layers extending into the DOP subphase, measured by neutron reflection, decrease with decreasing temperature (T) over this entire range. However, the variation with T becomes weak below −20 °C. At the lowest T, the layer thicknesses are 55%–75% of the values at the theta condition (Tθ=22 °C). The contraction of the layer with decreasing T is determined as a function of surface density and molecular weight, and these data are compared to universal scaling forms. The PS segments are depleted from the near surface region over the entire T range, with the thickness of the depletion layer increasing slightly with decreasing T. The free energy of the surface layer is probed by surface tension measurements. With decreasing T, negative surface pressures are observed at low coverages for both PDMS-PS and PDMS monolayers, indicating metastability toward lateral phase separation. Evidence for a transition from a dispersed phase to a condensed phase with decreasing T was observed in the reflectivity for very low PDMS-PS coverage. At high coverage where the submerged blocks are strongly interacting at 22 °C, only a modest decrease in surface pressure is observed over the experimental range of T despite the strong contraction. This latter result is discussed in terms of the relative contributions of enthalpic and entropic effects to the surface pressure.
Distribution of vibrational potential energy in molecular systems
Authors: Pritchard Huw O. , Raj Vatsya S. , Shen DeLin
Published: July 2002

It is shown that for a collection of n classical harmonic oscillators, the long-time distribution of potential energies P is approximated by sinm(πP) for n⩾4, where m=(8n/π2−1/√2) and P is scaled to lie between 0 and 1. As n→∞, the distribution tends to a δ-function centered about P=0.5. When coupling is present between the oscillators, the effective value of m is reduced, so that the breadth of the potential energy distribution reflects the degree of randomization in the system.
Liesegang patterns: Studies on the width law
Authors: Droz M. , Magnin J. , Zrinyi M.
Published: July 2002

The so-called width law for Liesegang patterns, which states that the positions xn and widths wn of bands verify the relation xn∼wnα for some α>0, is investigated both experimentally and theoretically. We provide experimental data exhibiting good evidence for values of α close to 1. The value α=1 is supported by theoretical arguments based on a generic model of reaction diffusion.
The equation of state of supercritical HF, HCl, and reactive supercritical mixtures containing the elements H, C, F, and Cl
Authors: Fried Laurence E. , Michael Howard W.
Published: July 2002

We develop a model of chemical equilibrium mixtures containing the elements H, C, F, and Cl. The model is based on a recently developed equation of state for the exponential-6 fluid, combined with a simple equation of state for condensed carbon. We show that the shock response of a wide variety of molecular and polymeric fluorocarbons and chlorocarbons can be modeled as a chemical equilibrium mixture of a small number of dissociation product molecules. In particular, we predict that shocked polyvinylidine fluoride (PVF2) decomposes into a fluid phase composed mostly of HF, and a phase containing condensed carbon. HF is known to strongly associate in the supercritical fluid phase. We predict that such an association also occurs under shock conditions.
Clustering behavior of water (D2O) on Pt(111)
Authors: Ogasawara Hirohito , Yoshinobu Jun , Kawai Maki
Published: July 2002

The structure and adsorption environment of water (D2O) on Pt(111) are investigated using infrared reflection absorption spectroscopy. The sample was prepared by the heat and quench technique at a heating temperature between 25 and 165 K. At 25 K, adsorbed water molecules exist as monomers and dimers, where the latter is evidenced for the first time. Upon annealing to 40 K, dimers dissociate and monomers cluster into bilayer ice at the terrace. At 105 K, a liquidlike phase is formed at the step. The liquidlike phase frozen in by quenching the surface to 25 K reveals itself measured in the measurements as amorphous ice. Until 125 K, bilayer ice at the terrace and the liquidlike phase at the step coexist. At 155 K, bilayer ice melts and the remaining molecules are converted to the liquidlike phase at the step. These phenomena are evidenced by the behavior of coadsorbed CO as a spectator molecule.
Relativistic MCSCF by means of quasidegenerate direct perturbation theory. I. Theory
Authors: Kutzelnigg Werner , Liu Wenjian
Published: July 2002

Relativistic MC-SCF (multiconfiguration self-consistent field) in terms of quasidegenerate direct perturbation theory (DPT) of relativistic effects is formulated based on a recently presented theory of effective Hamiltonians for electrons in a model space. The appropriately defined diagonal and nondiagonal parts of operators play a key role in this context. Their definition is based on stationary conditions for the MC-SCF wave function. The formalism starts from nonrelativistic MC-SCF theory. The leading relativistic correction appears as an expectation value in terms of the nonrelativistic MC-SCF function, while the higher-order relativistic corrections require a coupled-MC-SCF type approach.
The adsorption of water on clean and oxygen predosed Rh(111): Surface templating via (1×1)-O/Rh(111) induces formation of a novel high-density interfacial ice structure
Authors: Gibson K. D. , Viste M. , Sibener S. J.
Published: July 2002

Water adsorbed on clean Rh(111) forms an ordered structure with a (∛×∛)R30° diffraction pattern. This is facilitated by the close match of surface lattice constants for Rh(111) and the (0001) face of hexagonal ice, Ih. The preadsorption of small quantities of disordered oxygen improves the long-range ordering of the water overlayer. When a well-ordered half-monolayer of oxygen is grown on the Rh(111) prior to H2O exposure, there is no evidence of any long-range ordering of the water. However, when H2O is adsorbed on a (1×1)-O/Rh(111) surface, where there is a well-ordered monolayer of adsorbed oxygen, the adsorbed H2O forms a new high-density structure exhibiting a (1×1) diffraction pattern. The adsorbed H2O structure is epitaxial with respect to the underlying oxygen and rhodium. This structure persists for many layers of adsorbed water. On the clean Rh(111) surface, water molecules are adsorbed through the oxygen lone pair orbital. When the surface is fully covered with oxygen, the first layer of water can hydrogen bond to the surface, i.e., they likely adsorb with one or both of the hydrogen atoms pointing toward the surface. This creates a template for a novel structure that forms at low pressure, producing a high-density crystalline form of interfacial ice. This discovery suggests that other molecules, especially those that hydrogen bond, may form new structures on metals covered with a high-density oxygen overlayer, with associated consequences for interfacial chemistry.
Vector coherent state representation theory
Authors: Rowe D. J. , Rosensteel G. , Gilmore R.
Published: July 2002

A vector coherent state theory is formulated as a natural extension of standard coherent state theory. It is shown that the Godement representations and the coherent state representations of the Sp(N,R) groups of Rowe and of Deenen and Quesne are special cases of this more general theory.
Boson representations of the real symplectic group and their applications to the nuclear collective model
Authors: Deenen J. , Quesne C.
Published: July 2002

Both non-Hermitian Dyson and Hermitian Holstein–Primakoff representations of the Sp(2d,R) algebra are obtained when the latter is restricted to a positive discrete series irreducible representation 〈λd +n/2,...,λ1+n/2〉. For such purposes, some results for boson representations, recently deduced from a study of the Sp(2d,R) partially coherent states, are combined with some standard techniques of boson expansion theories. The introduction of Usui operators enables the establishment of useful relations between the various boson representations. Two Dyson representations of the Sp(2d,R) algebra are obtained in compact form in terms of ν=d(d+1)/2 pairs of boson creation and annihilation operators, and of an extra U(d) spin, characterized by the irreducible representation [λ1⋅⋅⋅λd]. In contrast to what happens when λ1=⋅⋅⋅=λd=λ, it is shown that the Holstein–Primakoff representation of the Sp(2d,R) algebra cannot be written in such a compact form for a generic irreducible representation. Explicit expansions are, however, obtained by extending the Marumori, Yamamura, and Tokunaga method of boson expansion theories. The Holstein–Primakoff representation is then used to prove that, when restricted to the Sp(2d,R) irreducible representation 〈λd +n/2,...,λ1+n/2〉, the dn-dimensional harmonic oscillator Hamiltonian has a U(ν)×SU(d) symmetry group. Finally, the results are applied to the Sp(6,R) nuclear collective model to demonstrate the existence of a hidden U(6)×SU(3) symmetry in this model.
A Hamiltonian approach to normal mode coupling in a Coulomb plasma
Authors: Spencer Richard G.
Published: July 2002

A Hamiltonian approach is used to treat the nonlinear problem of normal mode coupling in an unmagnetized homogeneous Coulomb plasma. It is shown that this method yields results consistent with well-known equations of plasma physics.
Hamilton’s form for the Kadomtsev–Petviashvili equation
Authors: Wickerhauser Mladen Victor
Published: July 2002

The scattering transform for the Kadomtsev–Petviashvili equation (KPII) is a local symplectomorphism. Pulling back the Hamiltonians for the linear evolutions of scattering data gives Hamiltonians for the KPII hierarchy: they are values of the associated scattering data at distinguished points. This method yields simple proofs that KPII has infinitely many commuting flows and simplifies their calculation. It also provides a Plancherel-type theorem.
Degenerate Siegert states
Authors: Huestis David L.
Published: July 2002

In a previous paper [D. L. Huestis, J. Math. Phys. 16, 2148 (1975)] a superposition principle was developed that allows the representation of an arbitrary wave function in an explicit uniformly convergent expansion over the discrete Siergert states for finite-range potentials. Possible difficulties were identified that could arise for special values of the potential strength due to degeneracy of the complex Siegert eigenvalues or vanishing of the norm of the Siegert state. In this paper these difficulties are addressed. By generalizing the Siegert eigenvalue problem, distinct orthogonal eigenfunctions with nonvanishing norm are obtained, recompleting the Siegert basis.
On Hamiltonian systems with first-class constraints
Authors: Goldberg Joshua , Newman Ezra T. , Rovelli Carlo
Published: July 2002

Some new results are presented on the theory of Hamiltonian systems with first-class constraints. In these systems it is possible to separate the physical part from the gauge part by transforming to canonical coordinates in which the constraints are a subset of the new momenta; this construction is accomplished by algebraic methods and the use of a set of Hamilton–Jacobi-like equations. Finally, the problem and meaning of evolution in systems with weakly vanishing Hamiltonian is commented on.
On the global gravitational instanton and soliton that are homotopy spheres
Authors: Baadhio Randy A. , Lee Pascal
Published: July 2002

A generalization of the Dirac string trick for higher-dimensional manifolds is presented. As a rather unexpected result, the procedure is shown to yield homotopy spheres. Because of their relevant interest to physical theories, particular emphasis is given to even-dimensional homotopy spheres. For instance, it is demonstrated that a nontrivial representation of 6–10-dimensional exotic spheres in dimensions 4k+2 can be performed by using the Pontrjagin–Thom construction. The physical content of the results is consistent with Witten’s hypothesis [Commun. Math. Phys., 100, 197–229 (1985)] that n–10-dimensional exotic spheres can be interpreted as gravitational instanton and/or soliton. It is also shown that three mutually diffeomorphic elements can be isolated from the 6–10-dimensional exotic spheres and their spectrum is provided.
Non-Riemannian model of the space-time responsible for quantum effects
Authors: Rylov Yuri A.
Published: July 2002

A class of homogeneous isotropic space-time models including pseudo-Euclidean space as a special case is considered. Such a model is chosen, where the particle motion is described in the most adequate way. It means that the world tubes of all free particles (both classical and quantal) are characteristic geometrical structures of the space-time. The world function σ of this model has the form σ=σE+σ0, σE>σ0 where σE is the world function of the pseudo-Euclidean space, σ0 is a constant responsible for quantum effects. It is proportional to Planck’s constant ℏ.
Solutions of Yang–Baxter equation in the vertex model and the face model for octet representation
Authors: Hou Bo-Yu , Hou Bo-Yuan , Ma Zhong-Qi , Yin Yu-Dong
Published: July 2002

In this paper, the solutions of the Yang–Baxter equation both in the vertex model (R matrix) and in the face model (braiding matrix) are computed explicitly for the octet representation of the quantum Kac–Moody A(1)2 enveloping algebra where there is multiplicity greater than one in the decomposition of the coproduct 8⊗8.
Casimir operators of U
q sl(3)
Authors: Rodríguez-Plaza M. J.
Published: July 2002

The q-deformed Casimir operators of U sl(3) are constructed using the Hopf algebra structure and the fundamental representation of Uq sl(3).
A hierarchic array of integrable models
Authors: Freund Peter G. O. , Zabrodin Anton V.
Published: July 2002

Motivated by Harish–Chandra theory, starting from a simple CDD-pole S matrix, a hierarchy of new S matrices involving ever ‘‘higher’’ (in the sense of Barnes) gamma functions are constructed. These new S matrices correspond to scattering of excitations in ever more complex integrable models. From each of these models, new ones are obtained either by ‘‘q deformation,’’ or by considering the Selberg-type Euler products of which they represent the ‘‘infinite place.’’ A hierarchic array of integrable models is thus obtained. A remarkable diagonal link in this array is established. Though many entries in this array correspond to familiar integrable models, the array also leads to new models. Setting up this array led to new results on the q-gamma function and on the q-deformed Bloch–Wigner function.
Symmetry breaking by propagation
Authors: Mekideche F. , Giraud B. G.
Published: July 2002

Matrix elements of propagators are calculated variationally. Two analytically soluble models confirm the validity of the time independent mean field variational approximation. The models have many solutions (7 and 85, respectively), the physical relevance of which can be completely analyzed and screened. One of the models shows solutions with a ‘‘spontaneous’’ symmetry breaking. The restoration of the broken symmetry is implemented. It describes correlations occurring during propagation.
A Wigner-function approach to (semi)classical limits: Electrons in a periodic potential
Authors: Markowich P. A. , Mauser N. J. , Poupaud F.
Published: July 2002

A rigorous derivation of the semiclassical Liouville equation for electrons which move in a crystal lattice (without the influence of an external field) is presented herein. The approach is based on carrying out the semiclassical limit in the band-structure Wigner equation. The semiclassical macroscopic densities are also obtained as limits of the corresponding quantum quantities.
Perturbed factorization of the singular anharmonic-oscillator eigenequation
Authors: Bessis N. , Bessis G.
Published: July 2002

The perturbed-ladder-operator method is applied to the solution of the perturbed eigenequation {(d2/dx2)−[m(m+1)/x2]−b2x2+V(x)+Λ}Ψ θix)=0 where V(x)=b1(1/x)2+b2(1/x)4+... is a singular perturbation. This method, which is the extension of the Schrödinger–Infeld–Hull factorization method within the perturbation scheme, provides closed form expressions of the perturbed eigenvalues and ladder functions, by means of algebraic manipulations. As an illustrative application, an analytical solution of the spiked-harmonic-oscillator eigenequation {(d2/dx2)−b2x2−(λ/x4)+E}Ψ(x)=0 is worked out up to the second order of the perturbation, by considering specifically adapted m- and λ-dependent perturbing and unperturbed potentials in order to tentatively avoid the known difficulties of convergence of the perturbation series. Closed form expressions of the λ/x4-anharmonic-oscillator energies are obtained in terms of the coupling constant λ and the quantum number v: results following from these expressions are compared with exact available values.
An algebraic extension of Dirac quantization: Examples
Authors: Ashtekar Abhay , Tate Ranjeet S.
Published: July 2002

An extension of the Dirac procedure for the quantization of constrained systems is necessary to address certain issues that are left open in Dirac’s original proposal. These issues play an important role especially in the context of nonlinear, diffeomorphism invariant theories such as general relativity. Recently, an extension of the required type was proposed using algebraic quantization methods. In this paper, the key conceptual and technical aspects of the algebraic program are illustrated through a number of finite dimensional examples. The choice of examples and some of the analysis is motivated by certain peculiar problems endemic to quantum gravity. However, prior knowledge of general relativity is not assumed in the main discussion. Indeed, the methods introduced and conclusions arrived at are applicable to any system with first class constraints. In particular, they resolve certain technical issues which are present also in the reduced phase space approach to quantization of these systems.
On the Noether identities for a class of systems with singular Lagrangians
Authors: Chaichian Masud , Martinez Domingo Louis
Published: July 2002

A straightforward method for obtaining the generators of Lagrangian gauge transformations within the Lagrangian formalism is presented. This procedure can be carried out completely without the need of developing the Dirac–Hamilton formalism. Singular Lagrangians which may generate Lagrangian constraints are considered. It is assumed that the consistency conditions on the Lagrangian constraints do not generate new independent equations for the accelerations. From the structure of these consistency conditions, the Noether identities are explicitly constructed and the Lagrangian generators of gauge transformations are obtained. It is shown that in the generic case of systems having chains of Lagrangian constraints up to a level k (k≥2), the variations δq under which the action is invariant depend on q, dq/dt, and higher time derivatives up to dkq/dtk. The relation between this Lagrangian approach and the standard Dirac–Hamiltonian approach is discussed.
A geometric setting for higher-order Dirac–Bergmann theory of constraints
Authors: Krupková Olga
Published: July 2002

Hamilton equations for degenerate time-dependent, and generally higher-order mechanical systems are considered and their geometric meaning is analyzed. The local dynamics of constrained systems is clarified and an algorithm for finding it is presented.