bluuebunny/crossref_metadata_2025 huggingface dataset:
Probing electrical conduction behavior of BaSnO3
Authors: Upadhyay Shail , Sahu Ashok Kumar , Kumar Devendra , Parkash Om
Published: July 2002

The ac conductivity, σac of BaSnO3 has been measured in the temperature range 310–520 K and frequency range 100 Hz–1 MHz. Below 420 K the conductivity is almost independent of temperature while between 420 and 520 K it is temperature dependent. At 100 kHz the activation energy of these two regions is 0.08 and 0.35 eV, respectively. It has been proposed that in the low temperature region conduction is due to hopping of charge carriers among the Sn+2 and Sn+4 sites. In the high temperature region conduction is due to excitation of charge carriers at the edges of the conduction band and hopping at energies close to it. Further impedance and modulus spectroscopy studies show that the conduction mechanism is the same for grains and grain boundaries.
Mean energy required to produce an electron-hole pair in silicon for photons of energies between 50 and 1500 eV
Authors: Scholze F. , Rabus H. , Ulm G.
Published: July 2002

The photon energy dependence of the mean energy W required to produce an electron-hole pair in silicon for photons with energies between 50 and 1500 eV was determined from the spectral responsivity of selected silicon photodiodes. The spectral responsivity was measured with a relative uncertainty of less than 0.3% using monochromatized synchrotron radiation whose radiant power was measured with a cryogenic electrical substitution radiometer. In order to deduce W from the spectral responsivity of photodiodes with a relative uncertainty of about 1%, a method for the calculation of photon and electron escape losses from silicon photodiodes was developed and the model for the charge carrier recombination losses was improved. In contrast to recent theoretical and experimental results, a constant value W=(3.66±0.03) eV was obtained in the photon energy range from 50 to 1500 eV. The present experimental results are confirmed by calculation of the pair creation energy in silicon from data from the literature for the relevant material properties. The difference from previous theoretical work is due to different assumptions about the influence of the band structure of silicon.
Three-dimensional super-resolution with a 4Pi-confocal microscope using image restoration
Authors: Schrader M. , Hell S. W. , van der Voort H. T. M.
Published: July 2002

The combination of two-photon excitation 4Pi-confocal fluorescence microscopy with image restoration leads to a fundamental improvement of three-dimensional resolution in the imaging of transparent, fluorescent specimens. The improvement is exemplified by randomly dispersed fluorescent beads and with actin filaments in a mouse fibroblast cell. For an illumination wavelength of 810 nm, we obtained lateral and axial full-width at half-maxima of point-like objects of 120–140 nm, and 70–100 nm, respectively. Fluorescent beads that are 150 nm apart are imaged with an intensity dip of ∼25%. This amounts to a ∼sixfold improvement of the axial resolution over standard two-photon confocal microscopy. In the cell, the 3D-images reveal details otherwise not resolvable with focused light.
Quasiadiabatic switching for metal-island quantum-dot cellular automata
Authors: Tóth Géza , Lent Craig S.
Published: July 2002

Recent experiments have demonstrated a working cell suitable for implementing the quantum-dot cellular automata (QCA) paradigm. These experiments have been performed using metal-island clusters. The most promising approach to QCA operation involves quasiadiabatically switching the cells. This has been analyzed extensively in gated semiconductor cells. Here we present a metal-island cell structure that makes quasiadiabatic switching possible. We show how this permits quasiadiabatic clocking, and enables a pipelined architecture.
Generation of optical and acoustic emissions in laser weld plumes
Authors: Farson D. F. , Kim K. R.
Published: July 2002

The goal of this work is the formulation of an analytical model of optical and acoustic emission generation by laser welds. A simplified model of the visible portion of the laser weld plume is used as a basis for the prediction of optical and acoustic signals. The optical emission is calculated from the strongest helium and iron lines in the visible spectrum. The sound pressure is calculated as a function of vapor flow rate by considering the displacement of ambient air by vapor emanating from the keyhole. The model reveals that generation of visible light increases roughly in proportion to the flow rate of vapor from the plume. However, the acoustic signal is predicted to vary as the time derivative of the flow rate. Experimental measurements of visible light are found to be of the same magnitude as predicted by the model and the fluctuation of the optical and acoustic signals are found to be consistent with the hypothesis that they are both fundamentally due to vapor flow rate variations.
Electronic structure of polycrystalline AMoO3 (A=Sr or Ba)
Authors: Mizoguchi H. , Fukumi K. , Kitamura N. , Takeuchi T. , Hayakawa J. , Yamanaka H. , Yanagi H. , Hosono H. , Kawazoe H.
Published: July 2002

Two strong peaks were observed in the electron energy loss spectra of polycrystalline AMoO3 (A=Sr and Ba) perovskites with metallic conductivities. One, observed at 1.7 eV, was attributed to plasmon loss originating from conduction electrons. This explains the reddish color observed. The other, located at ∼5 eV, was attributed to the charge transfer transition from the Mo4d–O2p bonding band to an unoccupied t2g band through photoemission, ultraviolet inverse photoemission, and x-ray absorption near edge structure spectra. No intense peaks were detected in the energy region between the two absorption bands (2.5<hν<3.5 ev), indicating the possibility of application in transparent conductors for the ultraviolet region.
Co ferrite films with excellent perpendicular magnetic anisotropy and high coercivity deposited at low temperature
Authors: Kitamoto Y. , Kantake S. , Shirasaki F. , Abe M. , Naoe M.
Published: July 2002

The present article describes CoxFe3−xO4 films, which were fabricated at 90 °C by the ferrite-plating method and have the excellent perpendicular magnetic anisotropy and the perpendicular coercivity Hc⊥ above 2 kOe. The films were composed of Co ferrite and Fe3O4 grains. The highest Hc⊥ reached 3 kOe at the composition of Co0.43Fe2.57O4 and the saturation magnetization Ms was 500–550 emu/cc. Hc⊥ was over 2 kOe even for a film thickness of 35 nm without any underlayers. ΔM⊥ evaluation that is measured in the perpendicular direction to the film plane showed that the interactions in the films supported the magnetized state, stabilizing the direct current remanent state. The Mössbauer analyses revealed that the Co0.43Fe2.57O4 films have excellent perpendicular magnetic anisotropy and the magnetization direction inclined from the perpendicular axis due to the demagnetization field. This achievement will open the way to develop the high-density perpendicular recording media even when plastic disks and sheets are used as substrates.
Microstructure, magnetic, and magneto-optical properties of chemical synthesized Co–RE (RE=Ho, Er, Tm, Yb, Lu) ferrite nanocrystalline films
Authors: Cheng Fu-Xiang , Jia Jiang-Tao , Xu Zhi-Gang , Zhou Biao , Liao Chun-Sheng , Yan Chun-Hua , Chen Liang-Yao , Zhao Hai-Bin
Published: July 2002

The sol-gel synthesis of Co–RE (RE=Ho, Er, Tm, Yb, Lu) ferrite nanocrystalline films was accomplished on monocrystalline silicon substrates, and the influence of RE ions on structure, magnetic, and magneto-optical (MO) properties of the resultant ferrites was examined. The results obtained reveal that, when x⩽0.2, the CoFe2−xRExO4 ferrite films have pure spinel phase with slightly enlarged cell constants and lattice distortion due to the large radii of RE ions. The RE ions tend to decrease the room-temperature magnetization as well as the Curie point of products, and may have a certain contribution to coercive force except for a nonmagnetic Lu3+ ion. For the present optical structure in which MO layers are substrated on silica, Co–RE ferrite films show the peak Kerr rotation at a shorter wavelength than the CoFe2O4 film, and Er or Tm doped samples exhibit an interesting enhancement of the MO effect.
Kinetics of Ag/Al bilayer self-encapsulation
Authors: Wang Y. , Alford T. L. , Mayer J. W.
Published: July 2002

The self-encapsulation kinetics of Ag/Al bilayers were studied both experimentally and theoretically as part of the effort to introduce Ag as an alternative metallization scheme for future ultra-large-scale-integrated technologies. Theoretical modeling was based on an analytical solution of a modified diffusion equation, which incorporated the diffusion of Al atoms through the Ag layers during the Ag/Al bilayer encapsulation progress. The amount of segregated Al atoms was monitored by both Rutherford backscattering spectrometry and film resistivity measurements, and correlated well with the theoretical predictions. These findings showed that the kinetics of the self-encapsulation could be significantly affected by both (i) the chemical affinity between Al and Ag atoms, and (ii) the interfacial energy between the metal layer (Ag) and the newly formed AlxOyNz diffusion barriers. Higher anneal temperatures were shown to accelerate the encapsulation process, and hence, achieved a lower resistivity in the underneath Ag layer. This model, in addition, confirmed the self-passivating characteristics of AlxOyNz diffusion barriers formed by Ag/Al bilayers annealed between 500 and 725 °C.
Optical properties of SrMoO3 thin film
Authors: Mizoguchi H. , Kitamura N. , Fukumi K. , Mihara T. , Nishii J. , Nakamura M. , Kikuchi N. , Hosono H. , Kawazoe H.
Published: July 2002

We used an rf magnetron reactive sputtering method to prepare SrMoO3 thin film on a silica glass substrate and evaluated its optical properties. The reflectivity of the blue-colored film showed a cutoff due to plasmon at ∼1.7 eV. From a Kramers–Kronig analysis of the reflectivity, the spectral dependence of dielectric constants and optical constants were obtained. They agreed in tendency with the constants determined by spectroscopic ellipsometry. The plasma frequency due to free carriers (4.5 eV) was obtained by application of a Drude model to reflectivity values, which showed that the effective mass of the conduction carrier was m*=2.1m0, where m0 is the free electron mass.
Analysis of chemical dynamics via Λ doubling: Directed lobes in product molecules and transition states
Authors: Andresen Peter , Rothe Erhard W.
Published: July 2002

Many molecular processes, such as chemical reactions, inelastic collisions, photodissociation, and surface scattering, yield selective populations of Λ-doublet states. The interpretation of such experiments has been difficult, so that little quantitative information could be extracted. Based on the results of our experiment on the photodissociation of H2O, and upon calculations presented here, this situation becomes much better. We discuss processes which yield product 2Π diatomics XY in which the Λ doublets are a manifestation of two different orientations of an unpaired pπ electron orbital. Chemical dynamics which produce such selective populations indicate a stereochemical effect: for example, the unpaired pπ orbital in the XY may point in the direction of the previous bond between XY and the transition complex, and, in an energetic breakup, would have a spatial relationship to J, the total angular momentum of XY. We describe here: (a) the directions of the orbitals at high J, (b) the effect of J, and of the particular XY studied, upon the analysis, so that J-dependent chemical dynamics can be separated from those intrinsic to the molecule, (c) the effect of rotation of the reaction complex, and (d) the close relationship to polarization experiments, both LIF and chemiluminescence. We also calculate the J-dependent degree of alignment of electron density, and its dependence upon the electronic parity.
Field-dependent electron attachment in liquid tetramethylsilane
Authors: Bakale G. , Beck G.
Published: July 2002

Field-dependent attachment of quasifree electrons to carbon tetrachloride, CCl4, and ethyl bromide, EtBr, was observed in liquid tetramethylsilane (TMS) at 21 °C using a picosecond-pulse-conductivity technique. The field dependences of the electron-attachment rate constants, ke’s, of the two solutes, were measured at electric fields between 15 and 200 kV/cm and were found to parallel the energy dependences of the electron-attachment rates of the two scavengers in the gas phase; i.e., electron attachment to CCl4 decreases with increasing field (energy) whereas electron attachment to EtBr increases with increasing field (energy). The observed field dependence of the ke’s is interpreted as being consistent with electron heating by the field, and we estimate that a field of 150 kV/cm increases the energy of an electron in TMS at 21 °C to ∼0.1 eV. The effects of thermalizing third bodies on the field dependences of the ke’s are also discussed.
Analytic Raman intensities from molecular electronic wave functions
Authors: Frisch Michael J. , Yamaguchi Yukio , Gaw Jeffrey F. , Schaefer Henry F. , Binkley J. Stephen
Published: July 2002

An analytic method for the evaluation of Ramanintensities from closed−shell self-consistent-field wave functions is presented. Predictioinsf or ethylenemolecule are also reported. (AIP)
Laser photoelectron spectroscopy of the formyl anion
Authors: Murray Kermit K. , Miller Thomas M. , Leopold Doreen G. , Lineberger W. C.
Published: July 2002

The 488 nm photoelectron spectra of HCO− and DCO− show vibrational structure in the X̃ 2A′ state of neutral formyl radical up to 10 000 cm−1 above the vibrational ground state. Electron affinities are found to be 0.313±0.005 eV for HCO and 0.301±0.005 eV for DCO. The CH bond strength and heat of formation of HCO− and the gas phase acidity of formaldehyde are derived from these data. A Franck–Condon analysis of the photoelectron spectra provides an estimate of the equilibrium geometry of the anion. Transitions to excited vibrational states of HCO enable the determination of a complete set of quadratic anharmonicities.
Constrained molecular dynamics: Simulations of liquid alkanes with a new algorithm
Authors: Edberg Roger , Evans Denis J. , Morriss G. P.
Published: July 2002

We present a new algorithm for molecular dynamics simulation involving holonomic constraints. Constrained equations of motion are derived using Gauss’ principle of least constraint. The algorithm uses a fast, exact solution for constraint forces and a new procedure to correct for accumulating numerical errors. We report several simulations of liquid n-butane and n-decane performed with the new algorithm. We obtain an average trans population of 60.6±1.5% in liquid butane at T=291 K and  ρ=0.583 g/ml. This result essentially agrees with that from an earlier simulation by Ryckaert and Bellemans [Discuss. Faraday Soc. 66, 95 (1978)]. However, our simulations are substantially more precise; our run lengths are typically ∼20 times longer than those of Ryckaert and Bellemans. Our result also agrees with that from a recent simulation by Wielopolski and Smith (following paper). Thermodynamic and structural data from our simulations also agree well with results from the simulations discussed in the above articles.
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o study of the ground state surface of Cu3
Authors: Langhoff Stephen R. , Bauschlicher Charles W. , Walch Stephen P. , Laskowski Bernard C.
Published: July 2002

An ab initio study that includes relativistic effects via first-order perturbation theory and correlation effects using the coupled pair functional formalism is presented for the ground state surface of Cu3. An analogous calculation is presented for the 1Σ+g ground state of Cu2 for calibration. The ground state of Cu3 is found to be a 2B2 state corresponding to a Jahn–Teller distortion (R=4.396 a0, θ=64.5°) of a 2E′ equilateral triangle geometry. This structure is found to lie 59 cm−1 below the 2A1C2v geometry and 280 cm−1 below the D3h equilateral geometry in good agreement with the pseudorotation barrier and Jahn–Teller stabilization energy deduced by Truhlar and Thompson from analysis of the fluorescence spectrum of Rohlfing and Valentini.
Reduction of 1+1 resonance enhanced MPI spectra to populations and alignment factors
Authors: Jacobs D. C. , Zare R. N.
Published: July 2002

A theory is presented to reduce 1+1 resonance enhanced multiphoton ionization (REMPI) spectra to accurate rovibrational state population distributions. Classical and quantum mechanical treatments are developed to model the polarization dependence of the REMPI signal from an initially aligned ground state having cylindrical symmetry. The theory includes the effects of saturation and intermediate state alignment. It is demonstrated that, for favorable cases, 1+1 REMPI allows the determination of the relative population as well as the quadrupole and hexadecapole moments of the alignment for rovibrational levels of a linear molecule. The classical treatment differs from that of the quantum treatment by less than 5% for rotational quantum numbers greater than J=4, suggesting that the classical treatment suffices for 1+1 REMPI in most molecular systems.
Statistical adiabatic channel model for ion–molecule capture processes
Authors: Troe J.
Published: July 2002

State-resolved and thermally averaged rate coefficients for ion–molecule capture processes are determined by detailed statistical adiabatic channel calculations. The equivalence of this treatment with alternative recent approaches is demonstrated. The advantage of the derived results is their general analytical form which facilitates an application over a wide range of conditions. Particular emphasis is given to ion + nonlinear dipole collisions at low temperatures.
Infrared radiative cooling of isolated polyatomic molecules
Authors: Dunbar Robert C.
Published: July 2002

Within a normal-mode picture of the molecular internal degrees of freedom, and accepting the ergodic assumption, methods of calculating the rate of infrared radiative cooling of hot polyatomic molecules are considered. The fundamental validity of the thermodynamic approach through definition of an internal temperature is discussed and supported. The Boltzmann equation is shown to be a remarkably accurate approximation to the thermodynamic equations; a thermodynamic equation derived from steepest-descents integration is also presented which is more accurate for very small molecules. Both of these equations are compared with exact statistical counts for small-molecule model systems, and give excellent results. The thermodynamic approach is virtually equal in accuracy to direct statistical counting, and offers advantages in many situations. Detailed consideration shows no general theoretical basis guaranteeing that the energy of the cooling molecule declines exponentially in time, but several special cases are identified in which such behavior is approached. An illustrative calculation of the cooling of benzene from 2700 to 400 K is described. The cooling rate constant declines gradually from 2.2 to 0.45 s−1, but is nearly constant over the cooling interval from 1000 to 400 K.
Vibrational relaxation of OH(X 2Πi, v=1–3) by O2
Authors: Dodd James A. , Lipson Steven J. , Blumberg William A. M.
Published: July 2002

Rate constants for OH(X 2Πi, v=1–3) vibrational relaxation induced by nonreactive collision with O2 have been measured. OH(v) is created by the H+O3 →OH(v≤9)+O2 reaction in an electron-irradiated O3, H2, Ar mixture. OH(v) fundamental and first overtone IR emission is observed using time-resolved Fourier spectroscopy. Spectral fitting followed by kinetic fitting of the resultant populations using a single-quantum relaxation model yields rate constants of kv=1 =(1.3±0.4)×10−13, kv=2=(2.1±0.3)×10−13, kv=3=(2.9±0.8) ×10−13 (all units are in cm3 /s). Our measurements are consistent with and extend published results on the same system, as well as predictions made by Schwartz–Slawsky–Herzfeld theory.
Rotational spectrum, structure, and chlorine nuclear quadrupole tensor of the vinyl fluoride–HCl dimer
Authors: Kisiel Z. , Fowler P. W. , Legon A. C.
Published: July 2002

The ground-state rotational spectrum of the dimer formed between vinyl fluoride and hydrogen chloride has been detected by using a pulsed-nozzle Fourier-transform microwave spectrometer. The rotational constants A0, B0, and C0; the centrifugal distortion constants ΔJ, ΔJK, δJ and δJK; and the components χaa, χbb−χcc, and χab of the Cl nuclear quadrupole coupling tensor have been determined for each of the three isotopomers CH2CHF⋅⋅⋅H 35Cl, CH2CHF⋅⋅⋅H 37Cl, and CH2CHF⋅⋅⋅D 35Cl. An accidental near degeneracy of the 211 and 303 levels allows an accurate evaluation of the off-diagonal component χab. Interpretation of the rotational constants establishes a near-planar geometry for CH2CHF⋅⋅⋅HCl, with HCl forming a hydrogen bond to F and RF⋅⋅⋅Cl=3.382(3) Å. The angle CF⋅⋅⋅H is 116.1(1)° and there is some evidence that the HCl subunit lies slightly out of the plane (φ=8.0±1.5°). Diagonalization of the Cl nuclear quadrupole coupling tensor to give the principal components χxx, χyy, and χzz leads to an angle of 30.8° between the a axis of the dimer and the H–Cl(z) axis. Comparison with the corresponding angle available from the geometry suggests that the z axis may be bent away by ≊10° from the F⋅⋅⋅H line in the F⋅⋅⋅HCl system. The preference for hydrogen-bond formation to F rather than to the π bond in vinyl fluoride is consistent with the Legon–Millen rules for angular geometries and can also be rationalized by the numerical Buckingham–Fowler model.
Rotational reorientation dynamics of polar dye molecular probes by picosecond laser spectroscopic technique
Authors: Dutt G. B. , Doraiswamy S. , Periasamy N. , Venkataraman B.
Published: July 2002

Fluorescence lifetimes and rotational reorientation times for four structurally similar dye molecules—three monocations: cresyl violet, nile blue, and oxazine 720 and one neutral but polar: nile red—have been measured by picosecond time-resolved fluorescence depolarization spectroscopy using the single-photon counting technique, in a number of solvents, which included a wide range of alcohols, other hydrogen-bonding liquids, and a few aprotic liquids. The rotational reorientation of the dye probes (assumed to be oblate ellipsoids) are sought to be explained in terms of the Stokes–Einstein–Debye theory and dielectric friction. The individual contributions to the rotational friction due to the above two factors were calculated using reasonable values for the molecular volume and dipole moment of the solute. The rotational behavior of all the four dyes in amides and aprotic solvents is reasonably well explained in terms of the simple stick hydrodynamic model with the ‘‘molecular volume’’ obtained by using the measured rotational reorientation time in water. On the other hand, in order to describe the rotational reorientation dynamics of all the dye molecules in n-alcohols, it is necessary to include the friction contribution due to the dielectric properties of the solvent. It appears that a change in boundary condition, something intermediate between stick and slip or close to slip, is required to satisfactorily explain the rotational reorientation times of the dye molecules in polyalcohols like ethylene glycol and glycerol. Investigation of the rotational behavior of all the four dyes as a function of viscosity by varying the temperature has been carried out in three solvents: 1-heptanol, 1-undecanol, and ethylene glycol. While the rotational reorientation times had a good linear η/T dependence, it was found that at a particular macroscopic viscosity value the rotational reorientation times obtained by the solvent variation and temperature variation are different. From the temperature variation study it was found that there is a satisfactory agreement between the solvent viscosity activation energy and the activation energy obtained for the reorientation rate of the dye probe molecules.
Self-entanglement in ring polymers
Authors: Koniaris Kleanthes , Muthukumar M.
Published: July 2002

We represent ring polymers with the off-lattice rod-bead model. We produce perfect unbiased model instances that have between 32 and 2048 beads. These rings are produced at several different values of bead radius, to represent polymers in solvents of different quality. These instances are subject to a novel smoothing operation that facilitates the determination of their topological state, which is represented with the Alexander polynomial. We observe that the probability of observing a trivial knot (P) has a decreasing exponential dependence on the contour length (N) of the polymer, or that P=exp(−N/N0). The characteristic length (N0) varies by many orders of magnitude depending on chain flexibility and solvent quality. For Gaussian rings in a theta solvent, the characteristic length (N0) is 2.6×102. For a good solvent, N0∼8×105. We also explore the expectation value of the minimum number of crossings and suggest that it probably cannot be represented as a power law or exponential function of N. We also suggest that sufficiently large rings will always be composite, and not prime.
Nuclear magnetic resonance studies of xenon clusters in zeolite NaA
Authors: Jameson Cynthia J. , Jameson A. Keith , Gerald Rex , de Dios Angel C.
Published: July 2002

We have observed the equilibrium distribution of Xe atoms trapped in the alpha cages of zeolite NaA at 300 and at 360 K for low to high xenon loadings. The experimental distributions obtained by nuclear magnetic resonance (NMR) spectroscopy differ from two previously proposed statistical distributions. The experimental deviations from these statistical models can be explained by the attractive Xe–Xe interactions which favor clustering at low to medium loading, and the higher energies associated with the overcrowded cage disfavoring clusters of eight Xe atoms at high loadings. The temperature dependence of the 129Xe NMR chemical shift of each cluster has been measured in the range 188–421 K, except that for Xe8, which was determined only up to 300 K. The observed shifts and their temperature dependence are interpreted by using the results of ab initio calculations of the intermolecular shielding function in the 39Ar system as a model for the 129Xe system.
Harmonically coupled, anharmonic oscillator model for the bending modes of acetylene
Authors: Lehmann Kevin K.
Published: July 2002

The harmonically coupled anharmonic oscillator model developed by Child and co-workers is extended to treat the coupling of two doubly degenerate ‘‘local benders’’ as may be expected in acetylene. This effective Hamiltonian is transformed to the normal mode representation and a set of ‘‘x-K’’ relations derived that relate the anharmonic and Darling–Dennison-type resonance terms to the diagonal local bending anharmonic constant. It is found that these relations agree very poorly with the experimental constants determined by Pliva in a fit to the acetylene spectrum. This failure is attributed to a remarkable cancellation of cubic and quartic contributions to the constants.
The short-time dynamics of molecular liquids. Instantaneous-normal-mode theory
Authors: Buchner Michael , Ladanyi Branka M. , Stratt Richard M.
Published: July 2002

Since the sharply varying forces that control the arrangement of molecules in liquids are themselves intrinsically anharmonic, the natural assumption would be that any picture that regarded molecular motion as harmonic would be at best a rough phenomenological guide. This expectation is, in fact, not a correct one. While the packing forces that determine liquid structure are indeed strongly anharmonic, the short-time displacements and librations that molecules execute are actually quite harmonic. It is possible to show rigorously that, for short enough (subpicosecond) time intervals, the dynamics of liquids is governed by a set of independent, collective, harmonic modes—the instantaneous normal modes of the liquid. In this paper we illustrate this fact by predicting the translational and rotational dynamics of a model diatomic liquid using the instantaneous normal modes computed by simulation. When compared to the exact molecular-dynamics results for the same autocorrelation functions, we find that perfect agreement is maintained only for very short times, but that if one removes the artificial runaway dynamics caused by the imaginary-frequency modes, reasonable levels of agreement are maintained for much longer time intervals. We also investigate the nature of the coupled translational–rotational motion by looking at the relevant translational and rotational projections of the modes. We find that the negative (backscattering) regions of both the translational- and rotational-velocity autocorrelation functions can be understood in terms of these same instantaneous harmonic modes.
An exact separation of the spin-free and spin-dependent terms of the Dirac–Coulomb–Breit Hamiltonian
Authors: Dyall Kenneth G.
Published: July 2002

The Dirac Hamiltonian is transformed by extracting the operator (σ⋅p)/2mc from the small component of the wave function and applying it to the operators of the original Hamiltonian. The resultant operators contain products of Pauli matrices that can be rearranged to give spin-free and spin-dependent operators. These operators are the ones encountered in the Breit–Pauli Hamiltonian, as well as some of higher order in α2. However, since the transformation of the original Dirac Hamiltonian is exact, the new Hamiltonian can be used in variational calculations, with or without the spin-dependent terms. The new small component functions have the same symmetry properties as the large component. Use of only the spin-free terms of the new Hamiltonian permits the same factorization over spin variables as in nonrelativistic theory, and therefore all the post-self-consistent field (SCF) machinery of nonrelativistic calculations can be applied. However, the single-particle functions are two-component orbitals having a large and small component, and the SCF methods must be modified accordingly. Numerical examples are presented, and comparisons are made with the spin-free second-order Douglas–Kroll transformed Hamiltonian of Hess.
Calculation of nuclear magnetic shieldings. IX. Electron correlation effects
Authors: Fukui H. , Baba T. , Matsuda H. , Miura K.
Published: July 2002

Nuclear magnetic shieldings were calculated with electron correlations through third order. The calculation was performed on the use of London’s gauge invariant atomic orbitals (GIAO) and the finite field many-body perturbation theory (FF-MBPT), in which the two kinds of perturbation fields, i.e., the external magnetic field and the field due to the nuclear magnetic moment, were introduced to get the perturbed one-electron states. The Hartree–Fock (HF) values and the second- and third-order Mo/ller–Plesset (MP) correlation corrections of the nuclear magnetic shieldings were calculated in (i) four first-row hydrides HF, H2O, NH3, and CH4; and (ii) three linear molecules with a multiple bond N2, CO, and HCN. The calculations showed that the post-HF correlations are important and much improve the calculated shielding values, and that the second-order corrections are positive, but the third-order ones are negative. Furthermore, the rovibrational corrections to the shielding constants in the HF molecule at 300 K were estimated to be −9.9 ppm for F and −0.66 ppm for H.
The structure of a rotational isomeric state alkane melt near a hard wall: Comparison of density functional theory with related theories
Authors: Sen Sudeepto , McCoy John D. , Nath Shyamal K. , Donley James P. , Curro John G.
Published: July 2002

Density functional theory is applied to inhomogeneous, rotational isomeric state polymer melts. In particular, a melt of tridecane near a hard wall is investigated, and the variation of polymer–wall correlation functions as a function of packing fraction is of primary interest. In addition to the evaluation of the wall–polymer density profile and the fractional distribution of sites, we use the relation between pressure and contact density to calculate the equation of state of the bulk. Agreement with the generalized Flory dimer equation of state is excellent, and this, in conjunction with our earlier comparison [Sen et al. J. Chem. Phys. 101, 9010 (1994)] with full, multichain simulation, indicates that the density functional theory gives an accurate description of inhomogeneous polymer melts.
Theory for long time polymer and protein dynamics: Basis functions and time correlation functions
Authors: Tang Wilfred H. , Chang Xiao-yan , Freed Karl F.
Published: July 2002

We develop methods for alleviating the major impediment in the extension to larger and more complex systems of our matrix method theory for describing the long time dynamics of flexible polymers and proteins in solution. This impediment is associated with the enormous growth in size of the required basis set with the addition of higher order mode coupling basis functions, which are needed to describe the influence on the dynamics of the ‘‘internal friction,’’ or equivalently of the memory function matrices. We use the first order eigenfunctions (the generalized Rouse modes) to construct an approximate mode coupling basis. Specific applications are made to united atom models of alkanes with a white noise structureless solvent, where the theory is compared with Brownian dynamics simulations to provide a no-parameter stringent test of the theory. Good convergence is found to the full second order treatment with the new basis set whose size scales more nearly with the size of the system rather than the cube of the system with the previous full basis. These technical improvements enable us to test the need for third order contributions to the dynamics of the longer alkanes and to compute the orientational time correlation functions probed by fluorescence depolarization and NMR experiments. Additional symmetry considerations provide further reductions in the required basis set sizes.
Extended molecular dynamics and optimized Rouse–Zimm model studies of a short peptide: Various friction approximations
Authors: Hu Yi , Kostov Konstantin , Perico Angelo , Smithline Shepard , Freed Karl F.
Published: July 2002

Developing a theory for the long time dynamics of polypeptides requires not only a proper choice of the relevant dynamic variables, but also a meaningful definition of friction coefficients for the individual atoms or groups of atoms in the reduced system. We test various aspects of the optimized Rouse–Zimm model for describing the long time rotational dynamics of a peptide fragment. The necessary equilibrium input information is constructed from a 1 ns molecular dynamics simulation for the solvated peptide by using a parallel Cray version of CHARMm, whose new features are described here. The simulations also provide time autocorrelation functions for comparisons with both theoretical predictions and with experiment. Two atomic friction models (van der Waals radii and accessible surface area) are chosen, and tests are made of the applicability of two combining rules for calculating the group friction coefficients. While the rotational dynamics of the peptide is insensitive to the friction models used, the combining rules are found to impact profoundly upon the theoretical descriptions for the behavior of the peptide dynamics for the reduced descriptions with fewer variables. The calculations study the role of the memory functions, neglected in this dynamical theory, and the interatomic hydrodynamic interactions in constructing the group friction coefficients. While the 1 ns trajectory is longer than customarily used for very complex systems, there are nontrivial influences of the duration of the molecular dynamics trajectory on the description of the dynamics.
Inelastic neutron scattering study of low energy excitations in glassy 1-butene
Authors: Yamamuro Osamu , Matsuo Takasuke , Takeda Kiyoshi , Kanaya Toshiji , Kawaguchi Tatsuya , Kaji Keisuke
Published: July 2002

Glassy 1-butene (CH2=CHCH2CH3) was studied by incoherent inelastic neutron scattering below 10 meV in the temperature range 18≤T≤80 K covering the glass transition temperature (Tg=60 K). A broad excitation peak due to the low-energy excitation was found at 3–4 meV. The spectra corrected for the Bose factor could be scaled to a unique curve for temperatures below Tg. The absolute density of vibrational states was derived from the spectrum at 18 K and the previously measured heat capacity data. The number of vibrational states associated with the low energy excitation was 1.4 per molecule. This is much larger than those for SiO2 and other amorphous materials studied so far. The density of vibrational states was well reproduced by the soft potential model originally conceived for network glasses.
Stretch-collapse transition of polyelectrolyte brushes in a poor solvent
Authors: von Goeler F. , Muthukumar M.
Published: July 2002

This paper describes the behavior of charged, polymer brushes in electrolyte solutions of varying solvent quality. The brush height, d, dependence on the chain length, L (=Nl, where l is the Kuhn length), the grafting density σ, and solvent conditions is determined. We consider a monomer–monomer potential consisting of three components: (1) a long-ranged, screened Coulombic component of strength v̄/l (l is the Kuhn length) and range κ−1; (2) a short-ranged, two-body component of strength w̄l; and (3) a short-ranged, three-body component of strength ūl3. In particular, we examine the transition from a stretched state to a collapsed state in a poor solvent (w̄<0) as the solvent quality is decreased. Using dimensional analysis, Monte Carlo methods, and a variational technique, a first order transition is observed as predicted by the scaling arguments of Ross et al. and Borisov et al. for high charge/grafting densities. Using a variational procedure, we derive an analytical expression for the brush size and determine, quantitatively, the critical conditions for a first order transition in terms of key dimensionless variables, vN5/2, κlN1/2, wN3/2, and uN2 (where v=2πσl2v̄, w=σl2w̄, and u=σ2l4ū).
Describing van der Waals Interaction in diatomic molecules with generalized gradient approximations: The role of the exchange functional
Authors: Zhang Yingkai , Pan Wei , Yang Weitao
Published: July 2002

Generalized gradient approximations have been used to calculate the potential energy curves for six rare gas diatomic molecules. Several generalized gradient approximations are found to provide a good description of binding in these diatomic molecules and show a significant improvement over the local density approximation in the prediction of bond lengths and dissociation energies. It is shown here that the behavior of an exchange functional in the region of small density and large density gradient plays a very important role in the ability of the functional to describe this type of van der Waals attraction.
Noniterative energy corrections through fifth-order to the coupled cluster singles and doubles method
Authors: Kucharski Stanisław A. , Bartlett Rodney J.
Published: July 2002

Perturbation corrections through fifth order in the many-body perturbation theory energy with respect to a coupled cluster singles and doubles reference have been derived and analyzed. The formulas employ the T1 and T2 amplitudes obtained as a solution of the coupled cluster singles and doubles equations. Four different energy functionals have been considered as a starting point in the derivation: the regular coupled cluster energy expression, the coupled cluster functional incorporating Λ amplitudes, the one constructed via an expectation value coupled cluster method, and that obtained on the basis of the extended coupled cluster method. The proposed corrections have been applied to several small molecules to test their performance compared to full configuration interaction. The fourth-order Λ-based formulas improve upon CCSD(T), (coupled cluster singles and doubles with noniterative triples), while the best fifth-order formulas reduce the fourth-order error by about two-thirds. We also introduce a factorized evaluation of connected T4 in fifth order, which reduces its calculation from an n9 algorithm to n7. This permits T4 to be included at approximately the same cost as CCSD(T).
Theory for the nonequilibrium dynamics of flexible chain molecules: Relaxation to equilibrium of pentadecane from an all-trans conformation
Authors: Tang Wilfred H. , Kostov Konstantin S. , Freed Karl F.
Published: July 2002

We extend to nonequilibrium processes our recent theory for the long time dynamics of flexible chain molecules. While the previous theory describes the equilibrium motions for any bond or interatomic separation in (bio)polymers by time correlation functions, the present extension of the theory enables the prediction of the nonequilibrium relaxation that occurs in processes, such as T-jump experiments, where there are sudden transitions between, for example, different equilibrium states. As a test of the theory, we consider the “unfolding” of pentadecane when it is transported from a constrained all-trans conformation to a random-coil state at thermal equilibrium. The time evolution of the mean-square end-to-end distance 〈Rend2(t)〉noneq after release of the constraint is computed both from the theory and from Brownian dynamics (BD) simulations. The lack of time translational symmetry for nonequilibrium processes requires that the BD simulations of the relaxation of 〈Rend2(t)〉noneq be computed from an average over a huge number of independent trajectories, rather than over successive configurations from a single trajectory, which may be used to generate equilibrium time correlation functions. Adequate convergence ensues for the nonequilibrium simulations only after averaging 9000 trajectories, each of 0.8 ns duration. In contrast, the theory requires only equilibrium averages for the initial and final states, which may be readily obtained from a few Brownian dynamics trajectories. Therefore, the new method produces enormous savings in computer time. Moreover, since both theory and simulations use identical potentials and solvent models, the theory contains no adjustable parameters. The predictions of the theory for the relaxation of 〈Rend2(t)〉noneq agree very well with the BD simulations. This work is a starting point for the application of the new method to nonequilibrium processes with biological importance such as the helix-coil transition and protein folding.
Computer simulation study of gas–liquid nucleation in a Lennard-Jones system
Authors: ten Wolde Pieter Rein , Frenkel Daan
Published: July 2002

We report a computer-simulation study of homogeneous gas–liquid nucleation in a Lennard-Jones system. Using umbrella sampling, we compute the free energy of a cluster as a function of its size. A thermodynamic integration scheme is employed to determine the height of the nucleation barrier as a function of supersaturation. Our simulations illustrate that the mechanical and the thermodynamical surfaces of tension and surface tension differ significantly. In particular, we show that the mechanical definition of the surface tension cannot be used to compute this barrier height. We find that the relations recently proposed by McGraw and Laaksonen [J. Chem. Phys. 106, 5284 (1997)] for the height of the barrier and for the size of the critical nucleus are obeyed.
Solvation in high-temperature electrolyte solutions. II. Some formal results
Authors: Chialvo A. A. , Cummings P. T. , Simonson J. M. , Mesmer R. E.
Published: July 2002

Our molecular-based formalism for infinitely dilute supercritical nonelectrolyte solutions is extended to electrolyte solutions by establishing rigorous connections between the microscopic behavior of the solvent around individual ionic species and their macroscopic solvation behavior. The formalism relies on the unambiguous splitting of the mixture’s properties into short-ranged (finite) and long-ranged (diverging) contributions, associated with the corresponding solvation and compressibility-driven phenomena, respectively. The salt (solute) and solvent’s residual chemical potentials are linked to the change of the local solvent’s environment around the infinitely dilute anion and cation, and the salt partial molar properties are interpreted in terms of the individual ion partial molar counterparts without introducing any extra-thermodynamic assumption. This is achieved with the use of Kusalik and Patey’s version of the Kirkwood–Buff fluctuation theory of mixtures. Moreover, the salt-and the individual ion-induced effects are connected to the solvent’s electrostriction around the ions, and to the coefficients of the Helmholtz free energy expansion for dilute mixtures. The ion-induced effects are also linked to well-defined excess solvation numbers which do not rely on any choice of solvation shell radius. Finally, some theoretical implications concerning the modeling of high-temperature aqueous-electrolytes solutions are discussed.
Size dependent properties of Au particles: Coherent excitation and dephasing of acoustic vibrational modes
Authors: Hodak Jose H. , Henglein Arnim , Hartland Gregory V.
Published: July 2002

Ultrafast laser spectroscopy has been used to characterize the low frequency acoustic breathing modes of Au particles, with diameters between 8 and 120 nm. It is shown that these modes are impulsively excited by the rapid heating of the particle lattice that occurs after laser excitation. This excitation mechanism is a two step process; the pump laser deposits energy into the electron distribution, and this energy is subsequently transferred to the lattice via electron–phonon coupling. The measured frequencies of the acoustic modes are inversely proportional to the particle radius; a fit to the data for the different sized particles yields v̄R=0.47cl/Rc, where R is the particle radius, cl is the longitudinal speed of sound in Au, and c is the speed of light. This functional relationship exactly matches the prediction of classical mechanics calculations for the lowest frequency radial (breathing) mode of a free, spherical particle. The inverse dependence of the frequency on the radius means that the modulations are damped for polydisperse samples. Analysis of our data shows that this inhomogeneous decay dominates the damping, even for our high quality samples (8%–10% dispersion in the size distribution). The size dependence of the electron–phonon coupling constant was also examined for these particles. The results show that, to within the signal to noise of our measurements, the electron–phonon coupling constant does not vary with size for particles with diameters between 4 and 120 nm. Furthermore, the value obtained is the same as that measured for bulk gold.
Qualitative theory of rubber friction and wear
Authors: Persson B. N. J. , Tosatti E.
Published: July 2002

When rubber is slid on a hard, rough substrate, the surface asperities of the substrate exert oscillating forces on the rubber surface leading to energy “dissipation” via the internal friction of the rubber. We present a qualitative discussion of how the resulting friction force depends on the nature of the surface roughness and comment on the origin of the wear of sliding rubber surfaces.
Symmetries and conditional symmetries of differential-difference equations
Authors: Levi D. , Winternitz P.
Published: July 2002

Two different methods of finding Lie point symmetries of differential-difference equations are presented and applied to the two-dimensional Toda lattice. Continuous symmetries are combined with discrete ones to obtain various reductions to lower dimensional equations, in particular, to differential equations of the delay type. The concept of conditional symmetries is extended from purely differential to differential-difference equations and shown to incorporate Bäcklund transformations.