bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental study of laser-driven shock waves in stainless steels
Authors: Peyre P. , Berthe L. , Scherpereel X. , Fabbro R. , Bartnicki E.
Published: July 2002

Two industrial stainless steels were investigated under laser-shock loading in plasma confined regime with water at nearly 10 GW/cm2 irradiance, driving to stress loadings close to 7 GPa—20 ns at the surface of the targets. A velocimetry interferometer system for any reflector was used to measure shock wave decay and estimate, through the determination of elastic precursors, the dynamic yield strengths at strain rates approaching 106 s−1. Some 50%–100% increases could be found between dynamic yield strengths and static plastic flow limits at 10−3 s−1. Different behaviors were noticed: on AISI 316L Austenitic steel, elastic precursors were shown to have constant values, whatever the propagation depths, whereas on a Martensitic X12CrNiMo12-2-2 steel, precursor attenuations in depth were evidenced. Concerning the application of laser-shock waves to reinforce surfaces, the determination of Hugoniot elastic limits was shown to be useful to predict the plastified depths and the general evolution of plastic deformations in-depth. For instance, comparisons between shock wave profiles and in-depth residual stress fields were shown to give accurate correlations. Lastly, the propagation of laser-shock waves in work-hardened layers was also investigated, which allowed to point out some stress amplitude increases behind 316L foils in the case of a material saturated with defects (shot peening at very high coverage rate).
Ion energy distributions in inductively coupled radio-frequency discharges in argon, nitrogen, oxygen, chlorine, and their mixtures
Authors: Wang Yicheng , Olthoff J. K.
Published: July 2002

We report ion energy distributions, relative ion intensities, and absolute total ion current densities at the grounded electrode of an inductively coupled Gaseous Electronics Conference radio-frequency reference cell for discharges generated in pure argon, nitrogen, oxygen, and chlorine, and in mixtures of argon with N2, O2, and Cl2. Measured current densities are significantly greater for pure argon and for mixtures containing argon than for pure N2, O2, and Cl2. For all three molecular gases, the ratio of molecular ions to the fragment ions decreases when argon is added to the molecular gas discharges. A possible destruction mechanism for the molecular ions involving metastable argon is discussed.
Polymer light-emitting electrochemical cells with frozen junctions
Authors: Gao Jun , Li Yongfang , Yu Gang , Heeger Alan J.
Published: July 2002

We report on polymer light-emitting electrochemical cells (LECs) with frozen p-i-n junctions. The dynamic p-i-n junction in polymer LECs is stabilized by lowering the temperature below the glass transition temperature of the ion-transport polymer. Detailed studies have shown that the frozen p-i-n junction in LECs based on the luminescent polymer poly[5-(2′ethylhexyloxy)-2-methoxy-1,4-phenylene vinylene] and polyethylene oxide containing lithium triflate (PEO:LiCF3SO3) is stable at temperatures up to 200 K. Frozen-junction LECs offer a number of advantages; they exhibit unipolar light emission, balanced injection, fast response, high brightness, low operating voltage, and insensitivity to electrode materials and film thickness.
Relative importance of the Si–Si bond and Si–H bond for the stability of amorphous silicon thin film transistors
Authors: Wehrspohn R. B. , Deane S. C. , French I. D. , Gale I. , Hewett J. , Powell M. J. , Robertson J.
Published: July 2002

We investigate the mechanism for Si dangling bond defect creation in amorphous silicon thin film transistors as a result of bias stress. We show that the rate of defect creation does not depend on the total hydrogen content or the type of hydrogen bonding in the amorphous silicon. However, the rate of defect creation does show a clear correlation with the Urbach energy and the intrinsic stress in the film. These important results support a localized model for defect creation, i.e., where a Si–Si bond breaks and a nearby H atom switches to stabilize the broken bond, as opposed to models involving the long-range diffusion of hydrogen. Our experimental results demonstrate the importance of optimizing the intrinsic stress in the films to obtain maximum stability and mobility. An important implication is that a deposition process where intrinsic stress can be independently controlled, such as an ion-energy controlled deposition should be beneficial, particularly for deposition temperatures below 300 °C.
Optical properties of TiN films deposited by direct current reactive sputtering
Authors: Adachi Sadao , Takahashi Mitsutoshi
Published: July 2002

Optical properties of TiN films have been studied using spectroscopic ellipsometry in the photon-energy range between 1.2 and 5.4 eV at room temperature. The TiN films are deposited on Si(100) substrates by reactive dc magnetron sputtering. The nearly stoichiometric golden-colored (g∼5.3 g/cm3) and brownish TiN films (g∼4.7 g/cm3) are investigated. The measured ε(E) spectra reveal distinct structures near the screened plasma edge and at interband critical points. These spectra are analyzed on the basis of a simplified model of the interband transitions including the Drude–Lorentz term contribution. Results are in satisfactory agreement with the experimental data over the entire range of photon energies. Dielectric-related optical constants, such as the complex refractive index, absorption coefficient, and normal-incidence reflectivity, of the sputter-deposited TiN films are also presented.
Electrical breakdown of the magnetic tunneling junction with an AlOx barrier formed by radical oxidation
Authors: Shimazawa K. , Kasahara N. , Sun J. J. , Araki S. , Morita H. , Matsuzaki M.
Published: July 2002

In this work, the dielectric breakdown in magnetic tunnel junctions (MTJs) was studied. The MTJ structure is Ta50/NiFe100/Co20/AlOx/Co30/RuRhMn100/Ta50 with the bottom lead of Ta50/Cu500/Ta50 and the top lead of Cu2000/Ta50 (in Å), where the tunneling barrier was formed by 2–20 min radical oxygen oxidation of a 10 Å-thick Al layer. The junctions with area from 2×2 to 20×20 μm2 were patterned using the photolithography process, leading to tunneling magnetoresistance up to 17.2% and resistance-area product ranging from 350 Ω μm2 to 200 kΩ μm2. The junctions studied show dc breakdown voltage from 0.7 to 1.3 V, depending on the junction area and the oxidation time. Long oxidation time up to 14 min and a small junction area results in a large dc breakdown voltage. The electrostatic discharge (ESD) of MTJs was tested by using a human body model. The ESD breakdown voltage increases with decreasing junction resistance. These results are discussed in terms of the E-model based on the field-induced distortion of atomic bonds in the oxide barrier.
The initiation of fine grain pentaerythritol tetranitrate by laser-driven flyer plates
Authors: Watson S. , Gifford M. J. , Field J. E.
Published: July 2002

Charges of pentaerythritol tetranitrate (PETN) were impacted by laser-driven flyer plates launched from substrate backed aluminum films. The flyers were produced by single pulses from a Q-switched Nd:YAG laser. The aluminum flyers were between 3 and 5 μm thick, had diameters of 1 mm, and achieved velocities up to 8 km s−1. The stresses were intense but of only nanosecond duration. This falls into an interesting regime since the shock thickness of a few microns is of the order of typical hot-spot sizes and less than the grain size of conventional explosive powders. Ultrafine PETN (∼1 μm grain size) was readily initiated, whereas conventional, 135 sieve PETN with a grain size of 106 μm was not, although limited reaction was observed in some cases. Environmental scanning electron microscopy was carried out to help elucidate the differences in behavior.
The EPR spectra of Gd3+ and Eu2+ in glassy systems
Authors: Brodbeck C. M. , Iton L. E.
Published: July 2002

A critical analysis is given of the EPR spectrum exhibited by the rare-earth S-state ions, Gd3+ and Eu2+, in glassy and disordered polycrystalline materials. The analysis of this spectrum and of its previous interpretations is based on (a) a set of criteria derived from a wide range of experimental EPR and optical data, and (b) a first principles computer simulation method which explicitly incorporates broad distributions in the crystal field interaction parameters. It is found that all four previous interpretations of the glassy spectrum are unsatisfactory, each failing to satisfy two or more of the criteria imposed by the full range of data. The correct general solution to the spectrum is unequivocally established and shown to be a convolution of (a) a broad and essentially unimodal distribution of second-order crystal field parameters, b02, with a maximum in the approximate range 0.051≲b02 ≲0.056 cm−1, and (b) a broad distribution of asymmetry parameters, λ′=b22/b02, with appreciable probability over the whole range 0.0≤ λ′≤1.0. The prominent features in the X-band spectrum at g∼6.0 and 2.8 are found to be the result of specific EPR transitions that are stationary with respect to b02, λ′, and the orientation angles of the applied field H over a wide range. The quantitive results indicate that the site symmetries of the RE ions are essentially very low and disordered, and are best characterized by a single low-symmetry ‘‘glassy type’’ site.
Exciton trapping in the one-dimensional antiferromagnet [(CH3)4N]MnCl3
Authors: Knochenmuss R. , Güdel H. U.
Published: July 2002

The dynamics of exciton motion in tetramethylammonium manganese (II) trichloride (TMMC) are investigated in the presence of copper (II) and cobalt (II) traps. Manganese luminescence decay curves are analyzed using several models and at various trap concentrations and temperatures, with the conclusion that trapping does not occur on every encounter. Trapping efficiencies are about 10−4. On-chain transport is found to be incoherent, with hop rates of approximately 1012 s−1 at 273 K and 1011 s−1 at 160 K. The Mn to trap rates and the rate of hopping between trap-limited chain segments are also estimated.
A three-dimensional, quantum mechanical study of exchange and charge transfer processes in the (Ar+H2)+ system
Authors: Baer Michael , Nakamura Hiroki
Published: July 2002

A three-dimensional quantum mechanical study of the (Ar+H2)+ system was carried out within the reactive infinite order sudden approximation. All three arrangement channels for exchange and charge transfer were treated simultaneously. Steric factors, opacity functions, angular distributions, and integral cross sections were calculated. Whenever possible, these were compared with both experimental and trajectory surface hopping (TSH) results. Whereas the fit with the TSH results was reasonable, the fit obtained with the experiment was less satisfactory. The reason for that can be attributed at least partially to the semiempirical DIM potential employed in the calculation.
Laser flash–chemiluminescence study of O2(1Δ) photoeliminated from 1,4-dimethylnaphthalene endoperoxide
Authors: Chou Pi-Tai , Frei Heinz
Published: July 2002

Deactivation of O2(1Δ) photoeliminated from electronically excited 1,4-dimethylnaphthalene endoperoxide in acetonitrile and perdeuteroacetonitrile has been monitored by time resolved detection of O21Δg→3Σ−g chemiluminescence at 1.27 μ. Study of the chemiluminescence decay as a function of laser photolysis pulse energy and endoperoxide concentration was undertaken in order to elucidate the processes that cause the previously reported drastic lifetime shortening of O2(1Δ) photoeliminated from this singlet oxygen storing molecule. Analysis of the non-first-order decay revealed that, aside from solvent collisional deactivation, quenching of O2(1Δ) by O−2 plays a dominant role at higher photolysis laser pulse energy or endoperoxide concentration. Kinetic evidence suggests that superoxide is generated upon 266 nm endoperoxide photolysis mainly along a two photon excitation path. The ionic fragmentation channel has been confirmed by direct observation of dimethylnaphthalene cation radicals in transient absorption in the red spectral range. In order to find out about the possible role of O2(1Δ) self-annihilation through pooling of the energy in O2(1Σ+), the rate constant of this process has been determined in a separate series of photosensitization experiments as (2.6±0.8)×107 ℓ mol−1 s−1(C6F6). This value implies that marked contribution of O2(1Δ) pooling to nonexponential singlet O2 decay occurs only at high laser photolysis pulse energies or endoperoxide concentrations.
Sub-Doppler infrared spectrum of the carbon dioxide trimer
Authors: Fraser G. T. , Pine A. S. , Lafferty W. J. , Miller R. E.
Published: July 2002

A spectrum of the carbon dioxide trimer van der Waals species has been recorded near 3614 cm−1 at sub-Doppler resolution using an optothermal (bolometer-detected) molecular-beam color-center laser spectrometer. A planar, cyclic structure with C3h symmetry has been determined for the complex with a carbon–carbon separation of 4.0382(3) Å. The observed perpendicular band, corresponding to an in-plane E′-symmetry vibration of the trimer, has been attributed to a localized excitation of the 2ν02 +ν3 combination mode of a CO2 subunit by virtue of its small blue shift (∼0.98 cm−1) from that of the isolated monomer.
Sequential clustering reactions of Si+ with SiD4: Identification of a bottleneck preventing rapid growth of hydrogenated silicon particles
Authors: Mandich M. L. , Reents W. D. , Jarrold M. F.
Published: July 2002

Sequential clustering of atomic silicon-29 cations with SiD4 is observed at room temperature in the ion cell of a Fourier transform mass spectrometer. The clustering reactions proceed in a highly specific fashion. Si+ grows initially by sequential addition of three –SiD2 units. The measured reaction rates for these three steps are, respectively, 8.1±0.4, 0.36±0.04, and 2.0±0.3×10−10 cm3 molecule−1 s−1. A back reaction which results in loss of the silicon-29 isotopic label also occurs for these three reactions and represents ∼12%–15% of the reaction products, depending on the reaction step. This cluster growth mechanism then encounters a critical bottleneck and ceases. Further aggregation occurs only by slow bimolecular attachment of SiD4 at a rate of 1.0±0.3×10−13 cm3 molecule−1 s−1 at p(SiD4)=2.0×10−7 Torr. The fundamental mechanisms and energetics for the individual reaction pathways have been calculated by Raghavachari using ab initio electronic structure theory and are presented in a companion paper. The clustering mechanism involves insertion of a cluster ion into a SiD bond of SiD4 followed by elimination of D2. Addition of –SiD2 serves to increasingly saturate the bonds of all of the silicon centers which leads to the observed growth limitations at Si4D+6. The accuracy of these calculated potential surfaces is tested using statistical phase space theory. Since both the forward and reverse reaction rates are measured using isotopic labeling, the phase space theory calculations are used to determine both the insertion and the elimination transition state energies for each of the first three clustering steps as well as a lower limit for the well depth of the [Si5D+10] intermediate complex. Overall, good agreement is found between the transition state energies obtained from phase space theory and those determined by Raghavachari. These results indicate that Si+ clustering with SiD4 encounters an early bottleneck which prevents rapid formation of the critical nucleus size required for spontaneous growth of large hydrogenated silicon particulates.
Surface chemistry of metastable oxygen. II. Destruction of O2(a 1Δg)
Authors: Sharpless R. L. , Slanger T. G.
Published: July 2002

The loss rate O2(a 1Δg) on a variety of surfaces has been investigated. At 300 K, the most rapid deactivants are Cu, Ag, and Co, with Fe, Ni, Pt, and Pd being somewhat less active. Numerous metals exhibit very low activity, including Al, Au, and W. Quite different temperature dependences were observed among the metals, with the activity of Ag decreasing markedly with increasing temperature over the range 220–470 K, that of Ni increasing, and that of Pd showing little change. The loss-rate–temperature profiles of Fe, Co, and Ni are similar, paralleling the behavior observed in a separate study on these three metals for higher-lying electronically excited states O2.
A reinvestigation of the NO3 1492 cm−1 band
Authors: Kawaguchi Kentarou , Hirota Eizi , Ishiwata Takashi , Tanaka Ikuzo
Published: July 2002

Ishiwata et al. [J. Chem. Phys. 82, 2196 (1985)] have recently observed an infrared diode laser spectrum of NO3 in the 1492 cm−1 region and have assigned it to the ν3 band in the X̃2A′2 state. However, some of the derived constants such as the Coriolis coupling and spin–rotation constants did not conform well with expected values. In the present study, the observation was extended so as to take combination differences, which led us to revise the previous assignment slightly and to remove all the anomalies in the lower (i.e., ground) state. A most important result of the present study is that a spin–orbit interaction constant aeff‖〈Lz〉‖ =0.17 cm−1 is indispensable to explain the spin splitting observed for the upper state. The first-order Coriolis coupling constant of the upper state (ζ=0.19) remains essentially the same as in the previous study and differs considerably from the value calculated for the ν3 state (ζ=0.7). Possible explanations of these data are discussed in some detail to obtain more information on the molecular structure of the NO3 radical.
Structures of water and primary alcohol studied by microwave dielectric analyses
Authors: Mashimo Satoru , Umehara Toshihiro , Redlin Harald
Published: July 2002

By the use of the time domain reflectometry method dielectric measurements were carried out first on methanol mixtures with ethanol and 1-propanol, and second, water mixtures with methanol, ethanol and 1-propanol in the frequency range 10 MHz–20 GHz. The first mixtures show a Debye relaxation and logarithm of the relaxation time is given by a linear function of the mole fraction of methanol. These mixtures have the same chainlike cluster of pure alcohol. The second mixtures show the same trend of the relaxation time in a region 0≤xw<0.83, where xw is the mole fraction of water. In a region 0.830.83 and that the cluster must be cyclic, consisting of six molecules.
A far wing line shape theory and its application to the water continuum absorption in the infrared region. I
Authors: Ma Q. , Tipping R. H.
Published: July 2002

A theory is presented for the calculation of the continuous absorption due to the far wing contributions of allowed lines. The theory is based on the quasistatic approximation for the far wing limit and the binary collision approximation of one absorber molecule and one bath molecule. In line space, the motion of the dipole moment of the absorber molecule, determined by the time displacement Liouville operator related to the total Hamiltonian of the absorber molecule and the bath molecule, is approximately expressed as the ordered product of two time displacement Liouville operators, one related to the intermolecular potential and the other to the unperturbed Hamiltonian. Using a Laplace transformation, the spectral density is expressed in terms of a Cauchy integral whose integrand is a product of two resolvent operators corresponding to the interaction and to the unperturbed Hamiltonian, respectively. After isolating the effects of the bath variables, the spectral density and subsequently the absorption coefficient are expressed in terms of the individual line coupling functions, or the individual line shape functions. By using two average line shape functions instead of the individual ones, the expression of the absorption coefficient closely matches empirical models. The validity of the present theory is discussed, and some numerical results of the water continuum absorption in the infrared region are presented for comparison with experimental data. A good agreement in both the magnitude and the temperature dependence of the absorption coefficients is obtained. Some further extensions and applications of the present theory are also discussed briefly.
Calculation of density fluctuation contributions to thermodynamic properties of simple fluids
Authors: Salvino Liming W. , White John A.
Published: July 2002

An equation of state, expressed in the form of a recursion relation for the Helmholtz free energy density, is presented. This equation of state represents the free energy density as an initial mean field free energy density, plus a sum of density fluctuation correction terms which serve both to dress the initial mean field equation of state into a more accurate analytic equation of state away from the critical region, and to provide the characteristic asymptotic singular behavior in the critical region. Numerical solutions of the recursion relations are described and presented as pressure isotherms and the calculated density coexistence curve. A comparison is made with simple fluid data for temperatures in the range 0.7≤T/Tc≤1.5 and densities in the range 0≤ρ/ρc≤2.5. In particular, good agreement is found near the critical point. Three critical exponents, δ, γ, and β, are calculated and found to be in good agreement with simple fluid data as well as with renormalization group results for the 3D Ising model.
Quantum reactive scattering in three dimensions using hyperspherical (APH) coordinates. VI. Analytic basis method for surface functions
Authors: Parker Gregory A. , Pack Russell T
Published: July 2002

We continue development of the theory of reactive (rearrangement) scattering using adiabatically adjusting principal axes hyperspherical (APH) coordinates. The surface functions, functions of the APH hyperangles covering the surface of the internal coordinate sphere, are expanded in analytic basis functions centered in each of the arrangement channels. The rotational functions are associated Legendre polynomials, and the vibrational functions are harmonic functions of an ‘‘anharmonic’’ variable which covers an infinite range, allows accurate Gauss–Hermite quadrature, and includes effects of anharmonicity. Example calculations show that these functions provide an efficient basis which can markedly decrease the computational effort required to generate accurate surface functions.
Critical evaluation of some computational approaches to the problem of basis set superposition error
Authors: Gutowski Maciej , Chal/asiński Grzegorz
Published: July 2002

The basis set extension (BSE) effects such as primary and secondary basis set superposition errors (BSSE) are discussed on the formal and numerical ground. The symmetry-adapted perturbation theory of intermolecular forces offers an independent reference point to determine efficacy of some computational approaches aiming at elimination of BSSE. The formal and numerical results support the credibility of the function counterpoise method which dictates that the dimer energy calculated within a supermolecular approach decomposes into monomer energies reproduced with the dimer centered basis set and the interaction energy term which also takes advantage of the full dimer basis. Another consistent approach was found to be Cullen’s ‘‘strictly monomer molecular orbital’’ SCF method [J. M. Cullen, Int. J. Quantum Chem. Symp. 25, 193 (1991)] in which all BSE effects are a priori eliminated. This approach misses, however, the charge transfer component of the interaction energy. The SCF and MP2 results obtained within the ‘‘chemical Hamiltonian approach’’ [J. Noga and A. Vibók, Chem. Phys. Lett. 180, 114 (1991)] were found to be inconsistent with the interaction energies resulting from the symmetry-adapted perturbation theory. The constraint equations of Sadlej’s ‘‘constrained dimer function’’ approach [A. J. Sadlej, J. Chem. Phys. 95, 6707 (1991)] were shown to degrade the quality of dimer orbitals in comparison with the quality of monomer orbitals obtained with their monomer centered basis sets.
Reevaluation of the transition-moment function and Einstein coefficients for the N2(A 3Σ+u–X 1Σ+g) transition
Authors: Piper Lawrence G.
Published: July 2002

We have measured the relative intensities of the nitrogen Vegard–Kaplan bands N2(A 3Σu+–X 1Σg+) for transitions covering a range in r centroids between 1.22 and 1.48 Å. With this data we constructed a relative electronic transition moment function that diverges significantly from previously reported functions. We place our data on an absolute basis by normalizing our relative function by the experimentally determined Einstein coefficient for the v′=0 to v″=6 transition. Combining our normalized data from 1.22 to 1.48 Å with absolute transition moment data measured by Shemansky between 1.08 and 1.14 Å results in a function covering the range between 1.08 and 1.48 Å. The radiative lifetimes calculated from this function are longer than those currently accepted by amounts varying between 25% for v′=0%–50% for v′=4–6.
H++H, He, and H2 scattering using a new time-dependent method for electron nuclear dynamics
Authors: Longo Ricardo , Deumens Erik , Öhrn Yngve
Published: July 2002

In this paper we apply the recently proposed and implemented electron nuclear dynamics (END) theory [J. Chem. Phys. 96, 6820 (1992)] to the study of prototypical ion–atom and ion–molecule collisions. The END theory obtains the equations of motion from the time-dependent variational principle (TDVP) employing a group theoretical coherent state (CS) parametrization of the wave function. The approach leads to a fully dynamical treatment of electrons and nuclei without invoking potential energy surfaces. The present implementation of the END theory constitutes the simplest ab initio model with the electrons described by a single determinantal wave function and the nuclei treated classically (or equivalently, with frozen Gaussian wave packets in the limit of a narrow widths). The method is applied to the H++H, He, and H2 collision processes in the energy range of 200–5000 eV. Results for the elastic and charge transfer differential cross sections, the differential probabilities, and the rainbow angles are presented and compared with experimental data. Also, the dynamical trajectories, deflection functions, and differential vibrational excitation for the H2 target are calculated and discussed. Effects of initial state molecular orientations, in the case of the H2 target, are considered. In general, the results provided by this model implementation of the END theory are in good agreement with experimental data.
The frustrated motion of benzene on the surface of Si(111)
Authors: Wolkow Robert A. , Moffatt Douglas J.
Published: July 2002

Benzene adsorption on Si(111)-7×7 is studied with scanning tunneling microscopy. Benzene diffusion is found to be inhibited. Ordinarily surface diffusion is controlled by a substantially lower energy of activation than is desorption. In this case diffusion is frustrated by a barrier to diffusion that is comparable to that for desorption. Both desorption and diffusion are monitored. On average, for every two adsorbate disappearances, one readsorption is observed and one molecule desorbs. Site-specific activation barriers of 0.94±0.01 eV and 0.95±0.01 eV for center faulted and corner faulted adatom sites, respectively, are extracted. Residence times increase as coverage decreases, implying adsorbate crowding causes the strength of the surface–adsorbate bond to weaken. Diffusion is generally found to involve jumps to sites beyond nearest neighbors. It emerges that the adsorbate largely breaks its existing bond to a surface site before forming a substantial bonding interaction with a new site. We surmise that this ‘‘break before make’’ scheme leaves the adsorbate in an intermediate, essentially physisorbed state, where it is sufficiently mobile to make longer than nearest neighbor jumps, or from which it desorbs.
Wetting of crystalline polymer surfaces: A molecular dynamics simulation
Authors: Fan Cun Feng , Caǧin Tahir
Published: July 2002

Molecular dynamics has been used to study the wetting of model polymer surfaces, the crystal surfaces of polyethylene (PE), poly(tetrafluoroethylene) (PTFE), and poly(ethylene terephthalate) (PET) by water and methylene iodide. In the simulation a liquid droplet is placed on a model surface and constant temperature, rigid body molecular dynamics is carried out while the model surface is kept fixed. A generally defined microscopic contact angle between a liquid droplet and a solid surface is quantitatively calculated from the volume of the droplet and the interfacial area between the droplet and the surface. The simulation results agree with the trend in experimental data for both water and methylene iodide. The shape of the droplets on the surface is analyzed and no obvious anisotropy of the droplets is seen in the surface plane, even though the crystal surfaces are highly oriented. The surface free energies of the model polymer surfaces are estimated from their contact angles with the two different liquid droplets.
High-resolution infrared diode laser spectroscopy of (CO2)3: Vibrationally averaged structures, resonant dipole vibrational shifts, and tests of CO2–CO2 pair potentials
Authors: Weida Miles J. , Sperhac Jeanette M. , Nesbitt David J.
Published: July 2002

High-resolution infrared spectra of (CO2)3 formed in a slit jet supersonic expansion are obtained via direct absorption of a tunable diode laser in the ν3 asymmetric stretch region of CO2. Over 100 distinct transitions are recorded in the trimer spectrum, which can be modeled as a perpendicular band of a planar symmetric top with C3h symmetry and no observable tunneling splittings. Results from the spectroscopic fit indicate that the complex is vibrationally averaged planar, with a carbon–carbon atom separation of RCC=4.0376(2) Å. An analysis of the vibrational blue shift for (CO2)3 of 2.5755(2) cm−1 via a resonant dipole–dipole interaction model yields an angular orientation for each CO2 axis of β=33.8(5)° away from a line tangent to the vertex and parallel to the opposite side of the equilateral triangle connecting the centers of mass of each CO2 monomer. Several model CO2–CO2 interaction potentials are tested against the vibrationally averaged structural parameters for (CO2)3. In particular, the potential of Murthy et al. [Mol. Phys. 50, 531 (1983)] reproduces RCC for the complex, but similar to all potentials tested, does not accurately predict the angular orientation β of the monomers within the trimer. Lastly, spectral evidence and model predictions suggest that there is an asymmetric top isomer of the trimer that is energetically comparable to the observed cyclic isomer.
Precipitation of highly charged polyelectrolyte solutions in the presence of multivalent salts
Authors: de la Cruz M. Olvera , Belloni L. , Delsanti M. , Dalbiez J. P. , Spalla O. , Drifford M.
Published: July 2002

Solutions of highly charged polyelectrolyte chains are described by a model that introduces ion condensation as a random charge along the polymer. The degree of condensation is obtained by solving the Poisson–Boltzmann equation with cylindrical geometry. Short range electrostatic attractions between the monomers via the condensed counterions of high enough valency lead to reversible chain precipitation. The range of polymer concentration over which salt-free solutions are unstable is determined, as well as the miscibility of the chains when salt is added. Redissolution at high salt concentration is due to a screening of the short range electrostatic attractions. Precipitation of chains in mixtures of movalent and multivalent salts is also studied. We find the range of salt concentration where chains precipitate. The model explains the experimental results on the precipitation of sodium and lanthanum polystyrene sulfonate solutions in presence of multivalent salts [LaCl3 and Th(NO3)4].
Statics and dynamics of homopolymer adsorption and desorption: A Monte Carlo study
Authors: Zajac Richard , Chakrabarti Amitabha
Published: July 2002

We present results from an extensive lattice Monte Carlo study of the adsorption and desorption of homopolymer chains from semidilute solution onto a solid/liquid interface. We have studied in detail the growth, equilibrium structure and dynamics, and rinsing of layers of homopolymers adsorbed from solution. For solution concentrations at or near the overlap threshold we directly verify the self-similar structure of the adsorbed layer first predicted by de Gennes. This scaling result is confirmed without our having to make any assumptions or inferences as required in experiments. By following the flight of the individual chains, we obtain a detailed description of their structure and dynamics, including direct proof that the adsorbed chains form two broad categories: a tightly bound category with a long characteristic time of exchange with the solution, and a loosely bound category with a very short characteristic time of exchange. We study the differing conformations of these two subpopulations, as well as the kinetics of chain exchange between the adsorbed phases and the bulk solution. Finally, we examine the active displacement of the self-similar adsorbed layer by a solution of different chains. We describe different regimes of desorption, characterized by exponential and faster than exponential decays of the adsorbed layer.
Molecular dynamics simulation of a chloride ion in water under the influence of an external electric field
Authors: Kiselev M. , Heinzinger K.
Published: July 2002

Molecular dynamics simulations were performed for one chloride ion in 213 water molecules under various applied external electrical fields. The field strength varied from 0.5–2⋅1010 V/m. The structure of the solution is described by various radial distribution functions, hydrogen bond statistics, and the deviation from tetrahedrality. The selfdiffusion coefficients and the spectral densities of the hindered translational motions of the water molecules have been calculated from the velocity autocorrelation functions. The dependence of these properties on the strength of the external electrical field is discussed.
Quantum treatment of the effects of dipole–dipole interactions in liquid nuclear magnetic resonance
Authors: Lee S. , Richter W. , Vathyam S. , Warren W. S.
Published: July 2002

Experimental observation of anomalous intermolecular cross-peaks in two-dimensional solution NMR spectra have attracted significant recent attention. Extremely simple pulse sequences on extremely simple samples with large equilibrium magnetization give resonances in the indirectly detected dimension which are simply impossible in the conventional density matrix framework of NMR. Here we extend a recently proposed density matrix treatment [Science 262, 2005 (1993)] to calculate the exact time evolution for a variety of pulse sequences. This density matrix treatment explicitly removes two fundamental assumptions of the standard theory—it includes the dipolar interaction between spins in solution (which is only partially averaged away by diffusion) and completely removes the high temperature approximation to the equilibrium density matrix [exp(−βℋ)≊1−βℋ]. We compare this quantum mechanical treatment to a corrected classical model, which modifies the dipolar demagnetizing field formulation to account for the effects of residual magnetization, and show that the quantum picture can be reduced to this corrected classical model when certain assumptions about the retained dipolar couplings are valid. The combination of quantum and classical pictures provides enormously better predictive power and computational convenience than either technique alone.
Ab initio calculations and dynamical tests of a potential energy surface for the Na+FH reaction
Authors: Laganà Antonio , Alvariño José M. , Hernandez M. Luz , Palmieri Paolo , Garcia Ernesto , Martinez Teresa
Published: July 2002

Ab initio calculations of the potential energy surface for the ground state Na+FH reaction were performed. Calculated potential energy values were fitted using a polynomial in bond order coordinates. Quasiclassical trajectories integrated on the fitted surface were used to calculate reactive properties of the system. Calculated quasiclassical properties agree with available experimental information. Quasiclassical trajectories allowed also a rationalization of the reactive dynamics of the system.
Microwave spectroscopic detection of HCCP in the X 3Σ− electronic state: Phospho-carbene, phospho-allene, or phosphorene?
Authors: Ahmad Imtiaz K. , Ozeki Hiroyuki , Saito Shuji
Published: July 2002

Microwave spectrum of the HCCP radical was detected for the first time in the X 3Σ− ground electronic state using a source-modulated microwave spectrometer. In total, 24 rotational transitions of HCCP in the 90–360 GHz region, 9 rotational transitions of DCCP in the 260–360 GHz range, and 24 rotational transitions of H13C13CP between 130–360 GHz were measured. Hyperfine structure pertaining to the phosphorus and hydrogen nuclei was observed for HCCP, and in the case of H13C13CP, only for phosphorus. The corresponding hyperfine coupling constants were ascertained in addition to the rotational, centrifugal distortion, and fine structure constants by a least-squares analysis of the measured frequencies. From the hyperfine coupling constants determined, the spin density of unpaired electrons was estimated to be 76% for the phosphorus atom and 42% for the carbon adjacent to the hydrogen. The r0 structure of HCCP was established from the rotational constants of HCCP and its isotopically substituted species: r0(CP)=1.685 Å, r0(CC)=1.241 Å, and r0(CH)=1.057 Å. These structural features are consistent with a linear phospho-allenic form that has been somewhat modified by a phosphorene.
Kinetics of microemulsion formation in polymer mixtures
Authors: Lee J. H. , Jeon H. S. , Balsara N. P. , Newstein M. C.
Published: July 2002

The kinetics of microemulsion formation in a multicomponent blend of two homopolymers and a block copolymer was studied by neutron scattering. The scattering profiles indicate the presence of large length-scale defects that coexist with the periodic microemulsion. With time, the large-scale structures undergo coarsening, while the periodic structures undergo refinement, i.e., they become more organized. The defect density at a given temperature can be controlled to some extent by thermal history.
Interfacing relativistic and nonrelativistic methods. II. Investigation of a low-order approximation
Authors: Dyall Kenneth G.
Published: July 2002

The formalism presented in a previous paper for the introduction of relativistic effects into nonrelativistic calculations is used to develop an approximation which is correct to order α2 and is similar to the Breit–Pauli approximation. Although it corresponds to a projection of the Dirac Hamiltonian onto the nonrelativistic (Lévy–Leblond) states which in principle should guarantee a lower bound, the bound depends on the form of the potential. The physical eigenstates correspond to a local minimum in the exponential parameter space which vanishes at large Z. In an extended basis set an approximation to the hydrogenic ground state can always be identified, but the relativistic correction to the energy is grossly overestimated. In molecular calculations, the restriction of the variational space involving the high-exponent functions by general contraction did not yield improved numerical stability in a variational scheme. The use of a low-order approximation in a quasivariational method is therefore not expected to yield reliable results.
Femtosecond relaxation of 2-amino-7-nitrofluorene in acetonitrile: Observation of the oscillatory contribution to the solvent response
Authors: Ruthmann J. , Kovalenko S. A. , Ernsting N. P. , Ouw D.
Published: July 2002

Transient absorption measurements of aminonitrofluorene in acetonitrile reveal for the first time an oscillatory behavior in the dynamic Stokes shift of stimulated emission. The measured relaxation curve for the maximum of the stimulated emission band is in excellent agreement with the solvation correlation function C(t) obtained from the simple continuum theory of dipolar solvation.
Analytical treatment of a simple fluid adsorbed in a slit-pore
Authors: Schoen Martin , Diestler Dennis J.
Published: July 2002

A model for a simple fluid confined to a slit-pore (fluid sandwiched between two plane-parallel substrates with infinitesimally smooth surfaces) is presented. The analysis is based on thermodynamic perturbation theory, in which the free energy is split into a zero-order (unperturbed) contribution from a hard-sphere fluid reference system and a correction accounting for both fluid-fluid and fluid-substrate attractions. The correction is evaluated in the mean-field approximation and the (unperturbed) local density is assumed uniform in order to obtain a closed expression for the correction. The resulting equation of state has the same temperature and density dependence as the van der Waals equation of state for the bulk fluid, although it differs from the latter in that the a parameter (ap) is a function of the separation sz of the substrate surfaces. The inequality ap(sz)⩽ab holds, from which it follows that the critical temperature of the pore fluid is lower than that of the bulk fluid. For mesoscopic pores, the lowering is in semi-quantitative agreement with experiment. The excess coverage Γ is calculated for bulk isochoric paths T→T0b, where T0b is the bulk liquid-gas coexistence temperature for a given isochore. If the fluid-substrate attraction is great enough, Γ vs T may exhibit a discontinuity reflecting pore condensation. The model predicts pore condensation over a density range comparable with the experimental one.
Application of free rotational models of molecular reorientation to the explanation of high frequency effects in dielectric relaxation
Authors: Coffey W. T. , Déjardin P. M. , Walsh M. E.
Published: July 2002

Exact solutions obtained by Gross [J. Chem. Phys. 23, 1415 (1955)] and Sack [Proc. Phys. Soc. London, Sect. B 70, 402 (1957)] for the complex polarizability of assemblies of nonelectrically interacting rotators subjected to a variety of collisions and various approximations to that quantity, specifically the Rocard equation are reappraised in view of recent attempts to use a variety of forms of that equation for the interpretation of far infrared resonance absorption spectra. It is shown that for small values of the inertial parameter (heavy damping) the Rocard equation yields a really good approximation for the complex polarizability only for the small collision model considered by Gross and Sack. In the case of large inertial parameter values it is emphasized by means of plots of the complex polarizability that such an approximation always exhibits behavior characteristic of a sharply resonant system, i.e., a pronounced absorption peak well in excess of the Debye peak and a strongly negative real part, while the exact complex polarizability spectrum for the same parameter values merely displays inertia corrected Debye relaxation. Therefore, an explanation of the resonant term other than that based on a Rocard equation with a large inertial parameter must be sought as that equation strictly applies to inertia corrected Debye (heavily damped) relaxation only. The application of the itinerant oscillator model and the three variable Mori theory to the problem is discussed in view of this conclusion.
Persistence length of flexible polyelectrolyte chains
Authors: Ha B.-Y. , Thirumalai D.
Published: July 2002

We calculate the dependence of the electrostatic persistence length, le, of weakly charged flexible polyelectrolyte chains using a self-consistent variational theory. The variation of le with κ, the inverse Debye screening length, is controlled by the parameter l0lB/A2 where l0 is the bare persistence length, lB is the Bjerrum length, and A is the mean distance between charges along the chain. Several distinct regimes for the dependence of le on κ emerge depending on the value of l0lB/A2. We show that when l0lB/A2≪1 we recover the classical result, le∝κ−2. For intermediate values of l0lB/A2, le∝κ−1. In this regime one can also get le∝κ−y with y<1 depending on the strength of the Coulomb interaction. When l0lB/A2≫1 we find le∝κ−2. Qualitative comparisons between our theory and simulations as well as other theories are presented.
Marangoni flow in liquid crystal interfaces
Authors: Rey Alejandro D.
Published: July 2002

This letter presents the equation of capillary hydrodynamics for interfaces between viscous isotropic fluids and nematic liquid crystals (NLCs) to establish the existence of a new class of Marangoni flow. The interfacial stress balance equation involves the surface divergence of the surface stress tensor. It is shown that the anisotropic elastic contribution to the surface stress tensor gives rise to bending stresses and tangential forces. When the tensor order parameter that describes the NLC structure has surface gradients, a tensor order parameter-driven Marangoni flow is predicted. The strength of the predicted effect is proportional to the nematic–isotropic interaction energy characteristic of the interface, and the direction of flow is from low energy regions towards high energy regions.
Molecular processes of adsorption and desorption of alkanethiol monolayers on Au(111)
Authors: Kondoh H. , Kodama C. , Sumida H. , Nozoye H.
Published: July 2002

The adsorption and desorption of n-alkanethiol monolayers on Au(111) have been studied under ultrahigh-vacuum condition by the use of scanning tunneling microscopy (STM), thermal desorption spectroscopy (TDS), and Auger electron spectroscopy (AES). Molecularly resolved STM observations for the alkanethiol monolayers have revealed that at least four different phases evolve during growth, which results in a multistep growth of the monolayer. The desorption species drastically changes at a critical coverage, which is accompanied by a structure change from a low-density flat-lying phase to a denser standing-up phase: While the latter phase bimolecularly desorbs as disulfides, the former phase unimolecularly desorbs as thiolate radicals. The coverage-dependent change of the desorption mode is explained in terms of the difference in the molecule-substrate bonding.
Approaches to the approximate treatment of complex molecular systems by the multiconfiguration time-dependent Hartree method
Authors: Burghardt I. , Meyer H.-D. , Cederbaum L. S.
Published: July 2002

A consistent treatment of environmental effects is proposed in the framework of the multiconfiguration time-dependent Hartree (MCTDH) method. The method is extended in view of treating complex molecular systems which require an exact quantum dynamics for a certain number of “primary” modes while an approximate dynamics is adequate for a class of “secondary” modes. The latter may correspond to the weakly coupled modes in a polyatomic molecule, or the first solvent shell in a solute-solvent complex. For these modes, a description in terms of parameterized functions is introduced. The MCTDH working equations are generalized to allow for the nonorthogonality of these functions, which may take, e.g., a multidimensional Gaussian form. The formalism is developed on the level of both the wave function description and the density matrix description. Dissipative effects are accounted for in terms of a stochastic Hamiltonian approach versus master equation approach in the respective descriptions.