bluuebunny/crossref_metadata_2025 huggingface dataset:
Stress-annealing in Fe73.5Cu1Nb3Si13.5B9 amorphous alloy ribbon
Authors: Aranda G. R. , González J. , Kul/akowski K. , Echeberrı́a J.
Published: July 2002

Investigations on measurements of the coercive field and the magnetization work of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloy are reported. Nanocrystalline samples are prepared from an amorphous precursor by Joule heating in the presence of tensile stress of 420 MPa. Values of the parameters of the thermal treatment are chosen as to develop a different degree of the nanocrystalline character. Results on the magnetization work W are interpreted in terms of the random anisotropy model. Fitting of the experimental curves of W against applied stress allows us to obtain the annealing dependence of the magnetostriction λ and the stress-induced anisotropy Kann.
Defect generation in field-effect transistors under channel-hot-electron stress
Authors: DiMaria D. J.
Published: July 2002

Nonuniform point-defect generation in n-channel metal–oxide–semiconductor field-effecttransistors under channel-hot-electron stress conditions is shown to have a similar origin as defects created with uniform stress conditions using Fowler–Nordheim tunneling, direct tunneling, or substrate-hot-electron injection. For all stressing modes, defect generation is related to the electron energy delivered to the appropriate silicon region near its interface with the thin-gate-oxide layer. A few of these hot electrons release a mobile species (believed to be hydrogen related) which can move hundreds of nm away from any positively biased contacts creating defects along its path in both silicon and oxide layers of the device. Channel-hot-electron degradation due to defect buildup along the channel is studied here as a function of stressing conditions, channel length, gate oxide thickness, and oxide processing. Procedures for predicting circuit failure under any operating conditions for logic and memory chips due to the total buildup of these defects are discussed.
Fractal and spectral dimensions of biopolymer chains: Solvent studies of electron spin relaxation rates in myoglobin azide
Authors: Colvin J. T. , Stapleton H. J.
Published: July 2002

Two methods of computing the fractal dimension of biopolymer chains are compared for 50 proteins. The chain fractal dimension d̄c is the scaling exponent of the contour length with respect to the end-to-end length, while the reentrant fractal dimension d̄r scales the total mass with respect to distance. Electron spin relaxation data, which yield the spectral dimension d̃, reveal a strong correlation between d̄c and d̃. A study of the apparent value of d̃ for myoglobin azide under 11 solvent conditions is presented and explained in terms of a variation in the protein-solvent coupling. A sharp transition in the effective spectral dimension at T=6 K is interpreted as reflecting a crossover from vibrational modes of the solvent to those of the protein.
Infrared spectrum and autodetachment dynamics of NH−
Authors: Neumark D. M. , Lykke K. R. , Andersen T. , Lineberger W. C.
Published: July 2002

The infrared vibration-rotation spectrum of NH− has been obtained by autodetachment spectroscopy in a coaxial laser–ion beam spectrometer. Transitions from the v=0 to v=1 vibrational levels were excited with an F-center laser, and subsequent autodetachment from the v=1 levels was observed. The apparatus resolution was better than 20 MHz, allowing the resolution of the fine structure and Λ-doubling transitions. The linewidths of the autodetachment resonances revealed some of the dynamics of the autodetachment process. The autodetachment rates were, in general, much greater for the upper Λ-doublet levels of NH−(v=1) than for the lower levels. In addition, the increase of the autodetachment rate with rotational energy for the upper levels was much faster than would be predicted if vibrational autodetachment were the primary detachment mechanism. It therefore appears that rotational-electronic coupling plays an important role in this system, and the differences in the Λ-doublet autodetachment rates are explained in terms of this mechanism.
Second order perturbation correction to CI energies by use of diagrammatic techniques: An improvement to the CIPSI algorithm
Authors: Cimiraglia Renzo
Published: July 2002

A usual procedure to get a large fraction of the correlation energy consists in the evaluation of the second order perturbation contribution to the electronic energy by utilizing as zeroth order state a moderate size CI wave function (CIPSI algorithm). A scheme of calculation based on a hole-particle formulation of the Hamiltonian, leading to a diagrammatic pattern quite similar to the one used for the one-determinant case, is proposed and discussed.
Consistently averaged hydrodynamic interaction for Rouse dumbbells in steady shear flow
Authors: Öttinger Hans Christian
Published: July 2002

The idea of consistently averaged hydrodynamic interaction is introduced and applied to Rouse dumbbells in steady shear flow. The main consequences are shear rate dependent viscometric functions and a nonvanishing secondary normal-stress coefficient.
Non-Markovian dynamics and barrier crossing rates at high viscosity
Authors: Okuyama Shoichi , Oxtoby David W.
Published: July 2002

Using the formulation developed in the preceding paper we explain the anomalous simulation results of Straub, Borkovec, and Berne for barrier crossing rates in a double well potential. We find that even in the high friction regime the well dynamics may have a significant effect on rate constants. By using model potentials and memory kernels we show that the dynamics can resemble not only spatial diffusion but also energy diffusion. This is due to the viscoelastic nature of a non-Markovian interaction with a bath. In general, the well dynamics is a complicated mixture of energy and spatial diffusion processes. We find analytical expressions for rate constants valid when the well dynamics is near or far from the Markovian limit.
Comparison of the propagation of semiclassical frozen Gaussian wave functions with quantum propagation for a highly excited anharmonic oscillator
Authors: Kluk Edward , Herman Michael F. , Davis Heidi L.
Published: July 2002

The propagation of an initially highly excited localized wave packet in an anharmonic oscillator potential is studied within the frozen Gaussian approximation. Comparison is made to quantum mechanical basis set calculations. The frozen Gaussian approximation involves the expansion of the initial wave function in terms of an overcomplete Gaussian basis set. The wave function evolution is evaluated by allowing each Gaussian to travel along a classical trajectory with its shape held rigid. A Monte Carlo algorithm is employed in the selection of the initial Gaussian basis functions. The frozen Gaussian results are very good for times on the order of a few vibrational periods of the oscillator and remain qualitatively correct for the entire length of the calculations which is 12 vibrational periods. The dependence of the calculations on the width of the Gaussian basis functions is investigated and the effect of a simplifying approximation for the prefactor of the Gaussians is tested.
Experimental determination of the t
r
a
n
s–g
a
u
c
h
e energy difference of gaseous n-pentane and diethyl ether
Authors: Kanesaka Isao , Snyder Robert G. , Strauss Herbert L.
Published: July 2002

The difference in enthalpy between the all-trans conformer and the conformer containing one gauche bond has been measured by Raman spectroscopy for n-pentane and for diethyl ether. For n-pentane, ΔH=465±30 cal/mol and for diethyl ether, ΔH=1150±110 cal/mol. The pentane value is close to the value for ΔH found in the neat liquid, and the ether value is close to or a bit below that found in the liquid. These numbers suggest that the effect of intermolecular interaction on the trans–gauche equilibrium in the liquid is small.
Microscopic evidence that the nitromethane aci ion is a rate controlling species in the detonation of liquid nitromethane
Authors: Engelke Ray , Earl William L. , Rohlfing Celeste McMichael
Published: July 2002

We present microscopic evidence that the aci ion (H2CNO−2) of nitromethane (H3CNO2) plays an important role in the detonation kinetics of liquid-phase nitromethane. It is known from previous detonation experiments that very minute additions of organic bases (e.g., amines) have a profound effect on the detonation properties of nitromethane; i.e., the explosive is strongly sensitized. Here we show that, under conditions similar to the detonation experiments, the only new chemical species generated in nitromethane by the bases sodium hydroxide (NaOH), diethylenetriamine (NH2CH2CH2NHCH2CH2NH2), and pyridine (C5H5N) is the aci ion, within the sensitivity of the experiments. The primary tool used to demonstrate this is 13C NMR spectroscopy. Ab initio quantum-mechanical calculations of the chemical shifts are used to support the experimental interpretation. Qualitative arguments concerning the increased reactivity of the aci ion, relative to normal nitromethane, are given. We review earlier work and relate it to the current findings.
Isothermal–isobaric computer simulations of melting and crystallization of a Lennard-Jones system
Authors: Nosé Shuichi , Yonezawa Fumiko
Published: July 2002

By means of constant-temperature, constant-pressure molecular dynamics techniques, we simulate the melting and crystallization processes of a model system composed of 864 Lennard-Jones (LJ) particles under periodic boundary conditions. On heating an fcc crystal of LJ particles, it is ascertained that melting takes place. On the other hand, a LJ liquid, when quenched slowly, crystallizes into a stacking of layers with stacking faults where each layer forms a close-packed structure with occasional point defects. The atomic configuration is not always nucleated into a completely ordered structure. A large hysteresis in the volume-temperature curve is observed. The volume contraction at the transition is characterized by two different growth rates, relatively slow at the first stage and relatively fast at the final stage. The critical cooling rate which separates the crystal-forming cooling rates and the glass-forming cooling rates is between 4×1010 and 4×1011 K/s for argon. On taking advantage of computer simulations, we analyze the microscopic atomic structure of our LJ system on the basis of the Voronoi and Delaunay tessellation.
Sinusoidal electric birefringence of dilute rigid-body suspensions at low field strengths
Authors: Wegener William A.
Published: July 2002

Kerr-limit birefringence expressions are presented for a dilute suspension of arbitrarily shaped rigid bodies in the presence of a sinusoidally oscillating electric field. A body is characterized by its diffusion tensors, its optical anisotropy tensor, its permanent and instantaneously induced electric dipoles, and its electric charge; no algebraic relation is assumed to hold between these quantities. We specifically allow for charged macromolecules with screw-like shapes which will rotate as well as translate under the exertion of an electric force. The birefringence exhibits components oscillating at twice the applied frequency as well as nonoscillatory components. The frequency dependence of these components is determined by the j=1 and j=2 eigenvalues of the rotational diffusion operator. Low and high frequency limits are considered, as are special cases which may reduce the number of distinct contributing terms. As an example, we examine the behavior of a long bent rod.
Atomic and molecular correlation energies with explicitly correlated Gaussian geminals. V. Cartesian Gaussian geminals and the neon atom
Authors: Wenzel Klaus B. , Zabolitzky John G. , Szalewicz Krzysztof , Jeziorski Bogumil/ , Monkhorst Hendrik J.
Published: July 2002

Correlation energy through third order and various coupled-pair correlation energies have been calculated for the neon atom using a basis set of explicitly correlated Gaussian geminals. Improved versions of functionals introduced in earlier papers of this series have been used. Our results are in very good agreement with accurate results of Lindgren and Salomonson and Jankowski et al. obtained with techniques specific for atoms. Our values are estimated to be accurate to about 1 mhartree. The results of this paper show that the method of Gaussian geminals is very competitive for systems with p electrons and should be able to achieve ‘‘chemical accuracy’’ for small molecules containing first and second row atoms.
Improved adiabatic corrections for the B 1Σ+u, C 1Πu, and D 1Πu states of the hydrogen molecule and vibrational structures for H2, HD, and D2
Authors: Dressler K. , Wolniewicz L.
Published: July 2002

The adiabatic energy corrections of the (2pσ) B 1Σ+u, (2pπ) C 1Πu, and (3pπ) D 1Πu states of H2 are calculated with higher accuracy than accomplished previously for a wide range of internuclear distances. The vibrational structures of these states are calculated for H2, HD, and D2 in the adiabatic approximation. Comparison with experimental term values of H2 and D2 permits one to separate the nonadiabatic energy shifts from the electronic contributions to the remaining energy discrepancies between ab initio theory and experiment. The electronic ab initio errors at and around the equilibrium internuclear distances of the respective states are thus found to be +0.8±0.2 cm−1 in the B state, +0.7±0.2 cm−1 in the C state, and +6.0±0.2 cm−1 in the D state. The adiabatic vibrational structures are also compared with the multichannel quantum defect treatment of nuclear motion in these states by Atabek and Jungen. The present calculation confirms the magnitudes of the so-called specific mass or mass polarization corrections predicted by these authors.
Theory of activated rate processes: A new derivation of Kramers’ expression
Authors: Pollak Eli
Published: July 2002

The generalized Langevin equation of motion for a particle trapped in a one-dimensional well with a barrier height V0 and coupled to a dissipative medium is modeled by a harmonic bath. Using the properties of the bath and a normal mode analysis we prove that the reactive frequency defined by Grote and Hynes for averaged motion across the barrier is actually a renormalized effective barrier frequency. We then show that the Kramers–Grote–Hynes expression for the rate of escape over the barrier is just the continuum limit of the usual gas phase harmonic transition state theory expression.
Late stage spinodal decomposition of a binary polymer mixture. II. Scaling analyses on Q
m(τ) and I
m(τ)
Authors: Hashimoto Takeji , Itakura Masahiko , Shimidzu Nobuyuki
Published: July 2002

The early-to-late stage of the spinodal decomposition (SD) of the critical mixtures of polystyrene and poly(vinyl methyl ether) was investigated by the time-resolved light scattering technique. The early stage SD where the time evolution of the fluctuations can be described, at least with a good approximation, by the linearized theory of Cahn was found to exist in the reduced time scale τ approximately less than 2. The later stage unmixing process (τ>2) was found to give the scaling laws on the reduced wave number of the dominant Fourier component of the fluctuations Qm and on the reduced maximum scattered intensity Ĩm as given by Qm∼τ−α and Ĩm∼τ β over narrow time scales. In the intermediate stage (2<τ<60), the inequality of β>3α was found, indicating that both the wavelength and amplitude of the dominant mode of fluctuations grow with time. In the late stage (60≲τ), the scaling relation of β=3α was found and dynamical scaling relation was found on the scattering function, indicating that there exists the self-similarity between the structures developed at different times, and the unmixing kinetics is scaled with a single length parameter ξ(t). The experimental behavior of Qm(τ) was compared with those predicted by some theoretical models, and some unique features of unmixing kinetics of polymer (‘‘polymer effect’’) were suggested to arise from entanglements of the long chain molecules.
The elasticity of weakly flocculated suspensions
Authors: Goodwin J. W. , Hughes R. W. , Partridge S. J. , Zukoski C. F.
Published: July 2002

Monodisperse polystyrene lattices, at high electrolyte concentrations and with an adsorbed layer of nonionic surface active agent, produced weakly flocculated dispersions which sedimented rapidly to give a final sediment concentration of ∼30% by volume. Three lattices were used with particle sizes in the range 1–2 μm. The surface active agent used was C12E6. Optical microscopy of dilute systems showed that dense flocs were formed which were easily disrupted by small shear stresses. Shear wave propagation experiments were used to give the volume fraction dependence of the wave rigidity modulus, G̃, of these systems at high frequency. The high frequency limit to the shear modulus, G(∞), was calculated from the particle pair potential and the particle pair distribution function determined by using one of the current perturbation theories. The interparticle pair potential was calculated from a combination of van der Waals’ attraction, and both steric and electrostatic repulsion. Good agreement was obtained between the value of G(∞) obtained from this statistical mechanical analysis and the experimental value of G̃ over the full range of volume fraction studied.
Calculated high-pressure properties of solid acetylene and possible polymerization paths
Authors: LeSar R.
Published: July 2002

Results of theoretical calculations at 0 K on the two known structures of solid acetylene using the modified Gordon–Kim plus damped dispersion model are reported. The lattice energy of the low temperature Cmca structure is compared to that of the high temperature Pa3 structure at zero pressure and found to be slightly less stable, though a very small pressure (about 0.25 kbar) stabilizes the Cmca structure relative to the Pa3 structure. Agreement between the experimental structure and energy for the Cmca structure and our present results is good. Structural properties of the Cmca structure were calculated to 100 kbar as well as the pressure dependence of the Ag libron mode. A possible high-pressure path for the polymerization of solid Cmca acetylene that would lead to a well-ordered polymer was proposed, but calculations of C–C distances suggest that a more cross-linked product would be favored.
The three-dimensional kinetic energy density functional compatible with the exact differential equation for its associated tensor
Authors: Baltin R.
Published: July 2002

The kinetic energy density functional t[n] of a system of electrons with density n(x) moving independently in an effective one-body potential V(x) can be written as the trace ∑3α=1tαα of a tensor tαβ(x) field the divergence of which is shown to be related exactly to third derivatives of n and to the gradient of V. This relation is a ‘‘differential’’, i.e., pointwise valid refinement of the usual ‘‘integral’’ virial theorem. When the approximation is made that tαβ is a function of n and ∇n only, and when V is identified with the Kohn-Sham effective potential occurring in the Euler equation of the total energy minimization problem then V can be eliminated from both the tensor theorem and from the Euler equation whereby a set of differential equations for the dependence of tαβ on n and ∇n is obtained. These equations are solved exactly yielding finally the result that t=κn5/3+λw(∇n)2/n with arbitrary constant κ and λw=ℏ2/(8m) is—within the assumption t[n]≈t(n,∇n)—the only possible solution compatible with the tensor theorem. This indicates that corrections to the Thomas–Fermi–Weizsäcker functional should be done to the TF term, but not to the Weizsäcker term.
From quasiperiodicity to chaos in the Belousov–Zhabotinskii reaction. I. Experiment
Authors: Argoul F. , Arneodo A. , Richetti P. , Roux J. C.
Published: July 2002

We report on the experimental observation of both a primary and a secondary Hopf bifurcation leading to quasiperiodicity in the Belousov–Zhabotinskii reaction. When taking the BZ system away from these local conditions, we witness the occurrence of chaos which comes with the breaking of the underlying torus into a fractal object. We emphasize that most of the alternating periodic–chaotic sequences studied in the literature are reminiscent of such two-frequency dynamics. We argue about the controversal existence of deterministic chaos in such sequences. We anticipate the results of a numerical and theoretical investigation of these sequences which discards definitely any stochastic objection to chemical chaos.
Structure of high-density amorphous water. I. X-ray diffraction study
Authors: Bizid A. , Bosio L. , Defrain A. , Oumezzine M.
Published: July 2002

X-ray diffraction measurements up to momentum transfer q=16 Å−1 were performed at atmospheric pressure on both the high-density form of amorphous ice obtained by pressurization of crystalline ice Ih at 77 K up to 2 GPa and the low-density form obtained by further heating. These two amorphous phases exhibit significant differences at the level of the second- and third-nearest neighbors which involve a decrease of the O–O–O angle on compression at high pressure. Moreover, it is found that the low-density form is quite similar to that obtained by vapor deposition on cooled substrates at 77 K.
Isotopic exchange in gamma-irradiated mixtures of C24H5 and C24D5: Evidence of free radical migration in the solid state
Authors: Clough Roger L.
Published: July 2002

Irradiation of a 1:1 co-melted mixture of tetracosane-h50 and tetracosane-d50 in the solid state resulted in isotopic exchange in very high yield [G(H–D)=25]. Exchange was not observed for a similar isotopic mixture irradiated in the liquid phase. A mathematical model, which operates by iterative computer calculations, has been constructed to simulate the complex changes in isotopic peak patterns which arise in the irradiated solid mixture. The simulations show that the observed isotopic exchange is not random, but rather that an individual molecule which has once undergone an isotopic exchange has an enhanced probability of undergoing additional exchanges. The results are very consistent with the occurrence of a localized chain-reaction process specific to the solid phase, and they provide evidence supporting the hypothesis that free radical transport in solid organics occurs extensively by a repetitive ‘‘hydrogen-hopping’’ mechanism involving successive hydrogen abstractions. It can be estimated that the average number of H–D exchanges occurring per radical pair, from formation to termination, is 14.3.
Picosecond spectroscopic measurement of a solvent dependent change of rotational diffusion rotor shape
Authors: Blanchard G. J.
Published: July 2002

The rotational diffusion behavior of cresyl violet is observed to be solvent dependent, producing an induced orientational anisotropy function with single exponential decays in both ethylene glycol at 26 °C and 1-dodecanol at 37 °C, but a two-component decay in 1-dodecanol at 26 °C. It is clear from this data that cresyl violet is experiencing significant changes in its local solvation environment. Without knowledge of the orientation of the transition moment within the molecule, however, the interpretation of these results is ambiguous. A simple calculation is presented which allows for the estimation of the intramolecular orientation of the transition moment, thereby removing ambiguity from the interpretation. The observed behavior is shown to be consistent with cresyl violet reorienting as an oblate rotor in ethylene glycol and 1-dodecanol at 37 °C, and as a prolate rotor in 1-dodecanol at 26 °C.
Infrared diode laser kinetic spectroscopy of the CCH radical ν3 band
Authors: Kanamori Hideto , Seki Kanekazu , Hirota Eizi
Published: July 2002

Infrared diode laser kinetic spectroscopy was applied to the photodecomposition of acetylene at 193 nm to observe the ν3 (C–C stretching) band of the CCH radical. More than one hundred absorption lines were observed between 1750 and 1900 cm−1, but none of them could have been assigned to the ν3 band. This is probably due to CCH molecules being excited to high-lying vibrational states by the excess energy of the photolysis. In fact, when either hydrogen or deuterium was added to acetylene as a buffer gas, new absorption lines appeared and were assigned to the ν3 fundamental band. The band origin which was derived from the observed spectrum is 1840.5711(4) cm−1, in agreement with the result of a previous matrix study. The spin–rotation interaction constant was found to decrease by as much as 30% upon ν3 excitation, presumably because of mixing with the lowest excited electronic state.
Spin–spin coupling constants of CO and N2
Authors: Geertsen Jan , Oddershede Jens , Scuseria Gustavo E.
Published: July 2002

We have used the second order polarization propagator method to calculate the indirect nuclear spin–spin coupling constants of 13C–17O and 14N–15N. We have calculated all coupling terms and the vibrationally averaged results are for CO: JFC =7.93 Hz, JSD =−3.99 Hz, JPSO =14.95 Hz, JDSO =0.10 Hz, Jtotal(CO) =18.99 Hz and for N2: JFC =0.82 Hz, JSD =−1.57 Hz, JPSO =3.32 Hz, JDSO =0.03 Hz, and Jtotal(N2) =2.60 Hz. Recent measurements of the two coupling constants gave 1J(13C,17O)=16.4±0.1 Hz and 1J(14N,15N)=1.8±0.6 Hz.
Unified approach to the quantum-Kramers reaction rate
Authors: Hänggi Peter , Hontscha Waldemar
Published: July 2002

The quantum analog of Kramers rate theory is derived from a unique many-body rate approach (Miller formula), being valid at all temperatures. In contrast to the imaginary free energy method (‘‘bounce’’ method) for a dissipative system we do not have to invoke a different prescription of the rate formula for temperatures below the crossover temperature T0 to tunneling dominated escape. Miller’s many-body quantum transition state theory is shown to produce the results of the imaginary free energy technique; in particular it also describes correctly the subtle regime near crossover T∼T0.
Orientation-selective 14N electron spin echo envelope modulation (ESEEM): The determination of 14N quadrupole coupling tensor principal axis orientations in orientationally disordered solids
Authors: Flanagan Heather L. , Gerfen Gary J. , Lai Albert , Singel David J.
Published: July 2002

We present an orientation-selective ESEEM (electron spin echo envelope modulation) technique. From a measurement of the variation of the amplitudes of modulation components at 14N pure quadrupole frequencies as a function of irradiation position within an EPR (electron paramagnetic resonance) powder pattern, we determine the orientation of the principal axes of the quadrupole coupling tensor relative to the principal axes of the tensor which governs the dispersion of the EPR spectrum in an orientationally disordered sample. The pure quadrupole frequencies appear when the EPR frequency is selected such that the nuclear Zeeman and the hyperfine interaction are approximately equal in magnitude. We have applied the method to a mercaptoethanol complex of myoglobin in which the pure quadrupole frequencies originate from 14N in the proximal imidazole ring. Our results enable us to validate the assignment of the quadrupole modulations to the metal-coordinated nitrogen of the imidazole ring, to correlate the principal axes with the principal values of the quadrupole coupling tensor, and to determine the orientation of the imidazole ring relative to the principal axes of the (low-spin) Fe(III) g matrix. These findings are discussed and compared with results of previous studies.
Many-body perturbation calculation on Be using a multiconfiguration model space and an intermediate Hamiltonian
Authors: Heully Jean-Louis , Daudey Jean-Pierre
Published: July 2002

We report a perturbation calculation of the ground state energy of the Be atom starting from a multiconfigurational model space and using the new concept of an intermediate Hamiltonian. The traditional MC-MBPT is known to diverge for this problem, whereas our method strongly converges. This convergence is explained by the fact that this method is free from any intruder state problem. The second-order calculation yields 140% of the correlation energy with a single reference model space, 93% with a multireference, and 98% with our method demonstrating the efficiency of the intermediate Hamiltonian.
Semiclassical quantization of the Liouville formulation of classical mechanics
Authors: Jaffé Charles
Published: July 2002

A new method for the quantization of classical Hamiltonian systems is presented. This method is based upon the correspondence between the Liouville formulation of classical mechanics and the Liouville–von Neumann formulation of quantum mechanics. It does not distinguish between integrable and nonintegrable systems, and consequently, is equally applicable to both types of systems. Further, it treats the indistinguishability of identical particles correctly, and thus, the semiclassical eigenstates have the correct symmetry properties. Application of the method is illustrated by a series of examples. The results are in excellent agreement with quantum mechanics and represent an improvement over results obtained using the uniform semiclassical approximation.
Bimolecular reactions on surfaces. Theory for arbitrary mean free path and interaction potential
Authors: Dawes Judith M. , Sceats Mark G.
Published: July 2002

The bimolecular reaction rate of molecules constrained to a surface is obtained in a simple form. The solution applies for any central force potential and for arbitrary mean free path, giving in the limit of high friction the solution to the diffusion equation with its logarithmic dependence on surface density and in the limit of low friction the result of canonical variational transition state theory.
Vibronic bands of the CCH radical observed by infrared diode laser kinetic spectroscopy
Authors: Kanamori Hideto , Hirota Eizi
Published: July 2002

Five infrared bands of the CCH radical were observed by diode laser kinetic spectroscopy. Two Π–Π type bands were assigned to hot bands from the (0110) state in X̃ 2Σ+, namely to ν2+ν3−ν2 and 7ν12−ν12. The latter band was found to share the upper state with a Π–Σ type band, of which the lower state was the ground vibronic state. Two other Π–Σ type bands corresponded to transitions from the ν3 state in X̃ 2Σ+ to two vibronic Π states. Simultaneous analysis of all the observed bands yielded molecular parameters with high precision including the ν2 fundamental frequency, which was determined to be 371.6034 (3) cm−1. The parity of each rovibronic level was determined unambiguously because Π–Σ+ transitions were observed, allowing us to choose the signs of K-type doubling constants p and q uniquely.
Fundamental equation for systems in chemical equilibrium
Authors: Alberty Robert A. , Oppenheim Irwin
Published: July 2002

At constant temperature and pressure, the differential of the Gibbs energy can be written in terms of species, components, extents of reaction, isomer groups, and homologous series groups under specified conditions. The applicability of each of these options in deriving expressions for equilibrium constants and devising other methods for calculating equilibrium compositions is discussed. If one or more reactants are available to a system at a constant chemical potential, their terms can be removed from the summation by use of a Legendre transform to obtain a new thermodynamic potential that is a minimum at equilibrium. This pemits the definition of homologous series and other groups that can be used as single terms in the fundamental equation for the purpose of equilibrium calculations. By use of these techniques, calculations can be made on organic systems with indefinitely large numbers of species.
Deformation-dependent hydrodynamic interaction in flows of dilute polymer solutions
Authors: Magda J. J. , Larson R. G. , Mackay M. E.
Published: July 2002

We have extended the bead–spring model of Zimm to include hydrodynamic interaction that depends upon the conformation of the polymer and hence the deformation rate. Our computational scheme is based on the model of Fixman, and it allows us to investigate numerically chains as large as 500 beads. The major approximation is the replacement of the hydrodynamic interaction tensor with its configurational average. We find that the incorporation of configuration-dependent hydrodynamic interaction qualitatively changes the Zimm predictions for both the shear and the elongational viscosity. In particular, the elongational viscosity becomes a multivalued function of the rate of elongation. This result is consistent with the coil-to-stretch transition predicted by de Gennes and Hinch.
Higher dispersion coefficients: Accurate values for hydrogen atoms and simple estimates for other systems
Authors: Thakkar Ajit J.
Published: July 2002

The dispersion coefficients Cn (n≤30) and Z(l,λ,L) (l+λ+L≤13) which appear in the multipole expansions of the pair and nonadditive three-body interaction energies, respectively, are calculated to an accuracy of no less than 15 decimal digits for interactions among ground state hydrogen atoms. The pseudostate technique used is as simple and accurate as the momentum-space method recently advocated for this problem. The oscillator and hydrogenic models are used to obtain simple formulas for the estimation of higher dispersion coefficients from two or three of the leading coefficients. These formulas should prove useful in models of intermolecular potentials.
Theory of vibrational cooling in molecular crystals: Application to crystalline naphthalene
Authors: Hill Jeffrey R. , Dlott Dana D.
Published: July 2002

The process of vibrational cooling (VC) is theoretically investigated in the molecular crystal naphthalene. Specificially we consider the process where a highly excited vibration cools by emitting lower energy vibrations (vibrational relaxation, or VR) and phonons. We also consider the subsequent cooling of emitted optic phonons by emission of acoustic phonons. Using previously determined vibrational lifetimes [J. R. Hill et al., J. Chem. Phys. 88, 949 (1988)], a consistent transition rate matrix is obtained which describes VR of all vibrations and optic phonons at all temperatures. Then a Master equation is solved numerically to obtain the time dependent vibrational populations of all states following impulse excitation of a high frequency vibration. These results are compared to a previously derived analytic model for VC in molecular crystals [J. R. Hill and D. D. Dlott, J. Chem. Phys. 89, 830 (1988)]. In that theory, which is shown to be in good agreement with the naphthalene calculation, the excess vibrational excitation moves to lower energy states and broadens as time increases. The motion toward lower energy states is described by a temperature independent ‘‘vibrational velocity’’ (emitted energy per unit time). In naphthalene, the vibrational velocity is V0 ≊9 cm−1 /ps. The VC process occurs on a time scale as much as an order of magnitude longer than an individual VR step. Although VR is highly temperature dependent, VC is not. The VC calculations are used to predict the decay from the initial state, the time dependent populations of transient vibrational excitations, and the return to the vibrationless ground state. All these quantities are directly related to experimental observables such as incoherent anti-Stokes Raman scattering and hot luminescence.
Conformational disorder in conjugated polymers
Authors: Rossi G. , Chance R. R. , Silbey R.
Published: July 2002

Conformational disorder plays an important role in determining the electronic properties of conjugated polymers. To obtain a better theoretical description of the role and extent of conformational disorder, we derive the π electron correlation length and the bond correlation length in terms of the effective torsional potential and geometry of the conjugated polymer chains. These quantites are related to the conjugation lengths and persistence lengths of the chain, which are computed for polyacetylene, polydiacetylene, polythiophene, and polypyrrole. In spite of uncertainties in the torsional potential parameters for these materials, good qualitative agreement is found between experiment and theory.
Kinematic averaging effects in thermal and low energy ion–molecule collisions: Influence on product ion kinetic energy distributions
Authors: Gerlich D.
Published: July 2002

The broadening and the shift of the kinetic energy distributions of the product ions from ion–molecule reactions caused by the velocity distributions of the reactants is discussed for different experimental situations. For a completely thermalized system (e.g., ions in an ideal trap) it is shown analytically that the product ion energy distribution is independent of the angular dependence of the differential cross section. In most of the cases of practical interest, the laboratory product velocity distribution for a state to state process can be approximated by a generalized Maxwell–Boltzmann distribution. Provided the exothermicity exceeds a few kT, the mean value of the corresponding energy distribution deviates from the nominal one by 3/2kT, and its half width increases with the square-root of the translational exoergicity ΔET i.e., FWHM=(11.09⋅m′1⋅m2/M2⋅ (ΔET+1.5⋅kT)⋅kT)1/2. If the ionic and neutral reactants are not in thermal equilibrium, the laboratory kinetic energy becomes strongly dependent not only on the energetics but also on the differential cross section. The problem is formulated in a rather general way and then applied to different experimental methods where the product ion velocity is used directly (e.g., in KEICR, guided ion beam, and differential scattering experiments) or indirectly (e.g., in LIF experiments for the density to flux conversion) to extract information on the energetics of a collision process. The results are used to analyze recent measurements on the collision systems N++CO, Ar++CO, Ar+++He, and H++D2 and it will be shown that a good estimate of the total resolution function is needed for a critical analysis of experimental data.
A semiempirical method for correcting configuration interaction potential energy surfaces
Authors: Varandas A. J. C.
Published: July 2002

A method recently proposed by Brown and Truhlar to correct large-scale configuration interaction calculations for incomplete dynamic correlation of electrons is reformulated using the formalism of the double many-body expansion method. Rather than making the assumption that large-basis-set multireference configuration interaction calculations including all single and double excitations from a complete-active space recover an approximately constant fraction of the external correlation, it is suggested that this fraction is approximately constant for the various n-body terms which arise in a cluster expansion of the total molecular energy. A new variant of this method in which the assumption of geometry-independent fraction of the external correlation is relaxed for the diatomics is also examined. Results are reported for the ground electronic states of the H3 and HO2 systems.
A comparison between simulations and various approximations for Hookean dumbbells with hydrodynamic interaction
Authors: Zylka Waldemar , Öttinger Hans Christian
Published: July 2002

The Brownian dynamics simulation technique is used to calculate exact rheological properties of the Hookean-dumbbell model with hydrodynamic interaction. The simulation results for steady shear flow are compared to the corresponding predictions of the preaveraging approximation, the consistent-averaging approximation, and the Gaussian approximation developed in the preceding article. Our calculations of the viscometric functions of the Hookean-dumbbell model are performed for various hydrodynamic-interaction tensors.
High resolution spectroscopic detection of acetylene–vinylidene isomerization by spectral cross correlation
Authors: Chen Yongqin , Jonas David M. , Kinsey J. L. , Field R. W.
Published: July 2002

Information about the unimolecular acetylene (HC 3/4 CH)↔vinylidene (H2C=C:) isomerization on the S0 energy surface has been extracted from vibrationally unassigned high resolution stimulated emission pumping (SEP) spectra of acetylene. The combination of a new pattern recognition scheme, spectral cross correlation (SCC) with complete nuclear permutation-inversion (CNPI) group theory is shown to be a powerful new technique for characterizing bond rearrangement in highly vibrationally excited normally rigid polyatomic molecules. SCC detects isomerization ‘‘resonances’’ which destroy an approximate vibrational symmetry (e.g., the number of cis-bending quanta). The energies (relative to the zero point level of the stablest isomer) and widths of such resonances provide information about the ‘‘energies’’ of isomer rovibrational levels and the isomer-level-specific isomerization rate. Vinylidene isomerization resonances may be distinguished from ordinary acetylene Fermi or Coriolis perturbations by a unique rotational symmetry dependence due to the correlation between acetylene [D∞h(M)] and vinylidene [C2v(M)] levels in the CNPI group G8. An SCC map of the HCCH S0 15 000–15 900 cm−1 energy region above the zero point level was obtained by comparing SEP spectra recorded via S1(Ã 1Au)33 and 2162 Ka=1 intermediate levels. The predicted rotational symmetry dependence of the SCC was found between 15 410–15 640 cm−1, but the vinylidene resonance line shape was obscured by Franck–Condon interference effects from well known perturbations between the 33 and 2162 SEP intermediate levels. It was therefore not yet possible to determine the height of the isomerization barrier (or even whether a barrier exists) but an upper bound for the vinylidene zero point level of 15 525 cm−1 was inferred from the SCC data.
Investigations on the convergence rate of the thermodynamic and structural parameters from Monte Carlo simulations of aqueous solutions of methanol and methylamine
Authors: Nagy Peter I. , Dunn William J. , Nicholas John B.
Published: July 2002

Monte Carlo simulations using statistical perturbation theory (SPT) were applied to calculate the free energy, solvation enthalpy, and partial molar volume changes for transforming the solute methylamine to methanol in an aqueous solution of 264 water molecules. Using preferential, as well as random, sampling the calculated thermodynamic parameters were compared in the averaging phase after considering 1500 and 3000 K configurations. The relative free energy shows little variance after taking 1500 K configurations. The solvation enthalpy and the partial molar volume agree well with the experimental values after taking 3000 K configurations for methanol and 4500 K configurations for methylamine. There are 3.0 to 3.3 water molecules in the first hydration shells of the polar groups of the solutes with ∼2 of the waters involved in hydrogen bonds with methylamine and ∼2.4 hydrogen bonded to methanol. The calculated numbers of hydrogen bonds do not change after taking 1500 K configurations, and coordination numbers of the polar sites with water were stable after 3000 K. The similarity of the solution structures with the two different solutes suggests little difference in the solvation entropy in accordance with experiment.
Self-assembly of surfactant liquid crystalline phases by Monte Carlo simulation
Authors: Larson R. G.
Published: July 2002

Three-dimensional microstructures of surfactant–water–oil systems self-assemble in a Monte Carlo lattice model, as shown here. The microstructures that form depend on the volume ratios of oil, water, and surfactant, and on the length of the surfactant, and on the ratio R of the length of the oil-loving to the water-loving portion. For R=1 we find lamellar phases when the surfactant is mixed with equal amounts of oil and water. The lamellar spacing increases as the surfactant concentration is lowered. In the presence of water only, as the surfactant concentration is lowered the microstructure evolves from lamellar to broken lamellar to cylinders to spheroids. This progression is found to be independent of lattice size for lattices as large as 40×40×40. For R=3, the progression seems to be replaced by a progression from lamellae to regular bicontinuous structures to cylinders, although we are not yet confident that this latter progression is independent of lattice size effects. The Monte Carlo technique can be used to study a wide variety of interesting phenomena, from micelle size and shape transitions to packing transitions and phase behavior to interfacial properties in the presence of surfactant.
Topological and bonding defects in vitreous silica surfaces
Authors: Feuston B. P. , Garofalini S. H.
Published: July 2002

A model structure for an annealed silica surface was obtained through the molecular dynamics simulation technique employing three-body interaction potentials. Nonbridging oxygen and edge-sharing tetrahedra were found to form on the oxygen-terminated surface with three-coordinated silicon, three-coordinated oxygen, and three-membered rings just below the outermost atoms. Four-membered rings were also created in relatively large concentrations during the surface relaxation. When considering the effect of removing periodic boundary conditions along the z direction, the concentration of larger rings, relative to the bulk, remained unchanged throughout the ∼11 Å simulated surface region. A strong physical association between three-membered rings and three-coordinated oxygen was found which may account for the D2 defect peak observed in Raman scattering.
Phase equilibria in polydisperse fluids
Authors: Stapleton M. R. , Tildesley D. J. , Quirke N.
Published: July 2002

The Gibbs simulation technique has been extended to calculate the phase equilibria of fluids of particles exhibiting variable polydispersity, such as micellar solutions. The density-temperature sections of the phase diagrams and the distribution of particle sizes in the coexisting phases have been calculated for two very simple model fluids. In the first model of size polydispersity, spherical particles of different additive diameters interact through a Lennard–Jones potential and the underlying, or low-density, distribution of particle sizes is a Gaussian characterized by a standard deviation, s. In the second model the well depth associated with each particle is additionally correlated with the diameter. In the case of size polydispersity, with s=0.03 and 0.05 (typical of the degree of polydispersity in spherical micelles close to the critical micelle concentration), we observe no change in the phase envelope from that of the monodisperse fluid. At s=0.2 there is a significant shift in the coexisting ‘‘liquid’’ density to smaller values. On average the particles in the liquid phase are larger than those in the ‘‘vapor’’ phase. The balance of attractive and repulsive forces causes particles in the coexisting liquid phase to increase their average size, and then decrease, in moving from the critical point to the triple point. For size polydispersity the packing fraction of the coexisting phases is independent of the degree of polydispersity. Inclusion of the energy correlation in the model magnifies the effects observed with pure size polydispersity. Moreover, the packing fraction at coexistence is now a strong function of polydispersity. These results constitute the first simulations of phase equilibria in polydisperse fluids.