bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron field emission properties of tetrahedral amorphous carbon films
Authors: Cheah L. K. , Shi X. , Liu E. , Tay B. K.
Published: July 2002

The electron field emission of tetrahedral amorphous carbon (ta-C) films deposited by filtered cathodic vacuum arc is reported. The ta-C films were found to have a threshold field ranging from 18 to 28 V μm−1, depending on the sp3 content. The nitrogenated ta-C (ta-C:N) films show a lower threshold field of 12 V μm−1 as compared to the ta-C films. The threshold field appears to be dependent on the film thickness. There is a minimum threshold field with the film thickness of around 30 nm for the ta-C:N film. Although the ta-C and ta-C:N films have relatively low threshold fields, the density of emission sites is not high for these films. The density of emission sites can be increased when the film surface is treated with H+, O+, or Ar+ ions after deposition. Moreover, the posttreated films show even lower threshold fields compared to the untreated films. The improvement in the emission after the ion beam treatment appears to be independent of the ions used. The surface before and after ion bombardment was analyzed using atomic force microscopy and scanning tunneling microscopy. This analysis shows the evident surface modification and more segregated cluster regions induced by the ion beam treatment. The posttreated films are analyzed using ultraviolet photospectroscopy. The photoelectrons start to emit at energies as low as about 0.6 eV. A mechanism for the electron field emission is proposed.
Giant magnetoimpedance effect in sandwiched films
Authors: Xiao Shu-qin , Liu Yi-hua , Dai You-yong , Zhang Lin , Zhou Shao-xiong , Liu Guo-dong
Published: July 2002

The giant magnetoimpedance (GMI) effect in films with a sandwiched structure has been studied. FeNiCrSiB/Cu/FeNiCrSiB sandwiched films with a magnetic closed-loop structure were deposited onto a glass substrate by rf sputtering and then annealed at an optimum temperature. GMI ratios as large as 63% and 77% were obtained at 13 MHz in longitudinal and transverse fields, respectively. The ratios are almost twice as large as those obtained in FeNiCrSiB single films.
Pseudopotential methods for superlattices: Applications to mid-infrared semiconductor lasers
Authors: Dente Gregory C. , Tilton Michael L.
Published: July 2002

Many mid-infrared semiconductor laser sources are now being developed with superlattice active regions. Calculations of gain, index of refraction, and intervalence subband absorption for these laser materials require accurate subband energies, wave functions, and radiative matrix elements. We have recently begun using a solution method based on the empirical pseudopotential method (EPM). This method shows particular strength in analyzing structures with short periods or thin layers, for which the standard method, based on k⋅p perturbation theory and the envelope function approximation, may be problematical. We will describe the EPM applied to bulk solids and then demonstrate our direct generalization of the method for applications to superlattice structures. Calculations for recently developed mid-infrared semiconductor lasers using type-II superlattice active regions will be used to illustrate the method.
Nonlinear response of two-dimensional electron plasmas in the conduction channels of field effect transistor structures
Authors: Rudin Sergey , Samsonidze Guram , Crowne Frank
Published: July 2002

The response of an electron gas in a quasitwo-dimensional conduction channel depends on a characteristic frequency ω0, defined as the ratio of the plasma wave velocity to the channel length. For a short-gate high electron mobility transistor ω0 can be in the terahertz range. We have used a self-consistent hydrodynamic model of the confined electron plasma to show that significant nonlinear effects are present in its response to harmonic signals at microwave frequencies much lower than ω0. We obtain the oscillatory time dependence of the terminal currents and study the interior dynamics of the electron plasma. We find that in certain device parameter ranges the essential nonlinearity in the microwave response may lead to nonlinear hydrodynamic effects, such as shock wave propagation in the conduction channel. When the boundary conditions at the source and drain terminals are asymmetric, the nonlinear plasma oscillations result in a nonzero dc component of the terminal current, which could be measured by using standard microwave circuits.
Effects of doping on the transport properties of CoSb3
Authors: Anno H. , Matsubara K. , Notohara Y. , Sakakibara T. , Tashiro H.
Published: July 2002

Effects of doping on the transport properties of CoSb3 have been systematically investigated using Ni, Pd, and Pt as donor impurities. It is shown that the Hall mobility, the Seebeck coefficient, and the electrical conductivity depend strongly not only on the carrier concentration but also on these donor impurities. Our theoretical analysis suggests that the electron effective mass and the conduction band deformation potential are significantly affected by both the doping levels and the donor impurities. These doping effects in CoSb3 can be attributed to (1) the changes in the electronic structure with doping and (2) the specific nature of the conduction band structure, in particular, the nonparabolicity of the band which can be explained in terms of a two-band Kane model. The observed changes in the electronic properties with doping are also consistent with the predictions of a recent band structure calculation of CoSb3. On the other hand, the lattice thermal conductivity decreases markedly with increasing carrier concentration, and is almost independent of the donor impurities. Our analysis based on the Debye model indicates that the coupling of the point-defect (alloy) scattering with the electron-phonon scattering plays an important role in reducing the lattice thermal conductivity in heavily doped n-type CoSb3. The effects of doping on the phonon scattering are also discussed on the basis of a model calculation as a function of the electronic properties and the impurity properties (atomic mass and size). As a result, it is found that the strength of the electron-lattice interaction (the electron-phonon coupling), which is closely related to the effective mass and the deformation potential, is an important factor affecting the scattering of phonons as well as charge carriers in heavily doped n-type CoSb3.
Dielectric characterization of polycrystalline Sr2Bi4Ti5O18
Authors: Srinivas K. , James A. R.
Published: July 2002

Five layered Sr2Bi4Ti5O18 has been prepared in the ceramic form with a view to studying its electrical properties. X-ray and microstructural studies were employed for physical characterization. Detailed electrical property measurements were undertaken so as to understand the basic transport physics of this compound. The results of dielectric studies, dc conductivity, and ac impedance are discussed, and a defect formula for Sr2Bi4Ti5O18 has been proposed. The investigations on this material at high temperatures and frequencies reveal its stable behavior.
Effect of anisotropy in the elastic scattering cross sections on the ionization source terms in glow discharges in argon
Authors: Belenguer Ph. , Pitchford L. C.
Published: July 2002

The main purpose of this work is to evaluate, using a Monte Carlo simulation, the extent to which anisotropy in the elastic electron-neutral scattering cross sections influences the spatial distribution of the electron-impact ionization profile in glow discharges in argon. We also briefly discuss the effect of anisotropic elastic scattering on the slowing down of an electron beam and on the transmission factor, the fraction of electrons emitted from the cathode that are not scattered back to the cathode. Monte Carlo simulations were performed using different functional forms for the anisotropy. We show that, for a given momentum transfer cross section and the assumption of isotropic scattering, the ionization profiles over a range of conditions in argon glow discharges in parallel plane electrode geometries are not significantly affected by the inclusion of higher order anisotropies in the elastic cross sections. The anisotropy in the elastic scattering cross section influences only slightly the electron transmission factor for low, uniform electric fields.
Electrical conductivity effects in polyethylene terephthalate films
Authors: Neagu E. , Pissis P. , Apekis L.
Published: July 2002

Electrical conductivity effects in biaxially stretched polyethylene terephthalate (PET) films of 6 μm thickness and 68% degree of crystallinity were investigated by means of dielectric relaxation spectroscopy in the frequency range 10−2–106 Hz and at temperatures higher than the glass transition temperature (∼85 °C) up to 190 °C. The formalisms of complex permittivity, electric modulus, and impedance were employed to analyze the experimental data. The results are discussed in terms of dc conductivity, conductivity current relaxation, interfacial Maxwell–Wagner–Sillars polarization, ρ peak, space-charge polarization, and electrode polarization. They are compared with the predictions of models for the electrical and dielectric properties of ion-conducting polymers. The dc conductivity values determined from dc measurement, from ac conductivity plots and from complex impedance plots agree well with each other. Their temperature dependence is described by the Vogel–Tammann–Fulcher equation and classifies PET as a fragile system.
Ultraviolet-induced densification of fused silica
Authors: Piao Fan , Oldham William G. , Haller Eugene E.
Published: July 2002

A number of fused silica samples were evaluated for their resistance to densification by deep UV radiation at 193 nm wavelength. Density changes for all the samples equal the product of a material dependent constant and the absorbed two-photon dose to a sublinear power of about 2/3. This dose dependence is consistent with earlier compaction studies using UV, electron, and gamma radiation. We also studied the isothermal-annealing behavior of UV-induced compaction in fused silica and found a correlation between thermal recovery of compaction and the compaction rates for different fused silica samples. Preheat-treatment at 950 °C for 1 h increased the UV-induced compaction rate of two types of fused silica samples, but did not affect that of the other two types of samples. Based on these experimental observations and the well-accepted network structure model of glasses, we propose a model to explain the sublinear power dependence on absorbed radiation dose for the ionization-induced compaction.
Anisotropic effective conductivity of materials with nonrandomly oriented inclusions of diverse ellipsoidal shapes
Authors: Shafiro Boris , Kachanov Mark
Published: July 2002

Effective, generally anisotropic, conductivity of a material with ellipsoidal inclusions is analyzed. The results are given in closed form. They cover, in a unified way, mixtures of inclusions of diverse eccentricities (including cracks) and arbitrary nonrandom orientational distributions. Proper parameter of inclusions concentration, in whose terms the effective conductivities are to be expressed, is identified. This parameter correctly represents contributions of the individual inclusions to the overall conductivity. It reflects inclusion shapes and is tensorial; generally, it cannot be replaced by the volume fraction parameter.
Structure and magnetic properties of Tb2Co17−xMnx compounds
Authors: Sun Zhi-gang , Zhang Hong-wei , Zhang Shao-ying , Wang Jing-yun , Shen Bao-gen
Published: July 2002

X-ray diffraction (XRD) and magnetization measurements were employed to investigate the effects of Mn substitution for Co on the structure and magnetic properties of Tb2Co17−xMnx compounds with x=0–7. XRD patterns show that all samples are single phase except the samples with x=6 and 7 which contain a small amount of impurity. The substitution of Mn for Co leads to a monotonic increase in unit cell volume. The saturation magnetization at 5 K remains nearly unchanged upon Mn substitution, while the Curie temperature decreases monotonically. XRD measurements on magnetically aligned powders with x=0–7 exhibit an easy-plane type of magnetic anisotropy at room temperature. For x=2, 3, and 4, spin reorientation transitions above room temperature are observed in the M–T curves. The magnetic phase diagram is given.
Gas phase hydrolysis of sulfur tetrafluoride: A comparison of the gaseous and liquid phase rate constants
Authors: Sauers I. , Adcock J. L. , Christophorou L. G. , Ellis H. W.
Published: July 2002

The second-order gas phase reaction rate constant for SF4 hydrolysis to SOF2 has been determined by mass spectrometry and found to be in the range (0.9–2.6)×10−19 cm3 s−1 at 350 K. The hydrolysis of SOF2 in the gas phase is 3 to 4 orders of magnitude slower than that of SF4. Since only liquid phase rate constant data exist for SF4 and none for SOF2, our gas phase rate constant for SF4 was extrapolated to liquid H2O density for comparison to the literature and was found to be consistent with the rate constant for complete aqueous hydrolysis of SF4, which was indirectly determined by the conductivity change in a pulse radiolysis study of SF6 in aqueous solution. The agreement between these values provides direct support for the conclusion of the pulse radiolysis study, namely, that the rate-determining step involves water attack on SF4. The present data further indicate that SOF2 cannot be an intermediate in the aqueous hydrolysis of SF4. A reaction is proposed for both gaseous and liquid phase hydrolyses which involve a common intermediate.
The generalized Smoluchowski equation and non-Markovian dynamics
Authors: Okuyama Shoichi , Oxtoby David W.
Published: July 2002

Starting from the generalized Langevin equation, we derive a generalized Smoluchowski equation. This equation provides a general framework for problems of non-Markovian barrier crossing dynamics. We consider the dynamics of a Brownian particle in a harmonic well and on an inverted harmonic potential, and rederive the Grote–Hynes formula for barrier crossing rate constants in the high viscosity limit. In so doing, we answer some questions about possible limits to the validity of that result at high viscosities.
Infrared spectra of the cluster ions H7O+3⋅H2 and H9O+4⋅H2
Authors: Okumura M. , Yeh L. I. , Myers J. D. , Lee Y. T.
Published: July 2002

Infrared spectra of hydrated hydronium ions weakly bound to an H2 molecule, specifically H7O+3 ⋅H2 and H9O+4 ⋅H2, have been observed. Mass-selected parent ions, trapped in a radio frequency ion trap, are excited by a tunable infrared laser; following absorption, the complex predissociates with loss of the H2, and the resulting fragment ions are detected. Spectra have been taken from 3000 to 4000 cm−1, with a resolution of 1.2 cm−1. They are compared to recent theoretical and experimental spectra of the hydronium ion hydrates alone. Binding an H2 molecule to these clusters should only weakly perturb their vibrations; if so, our spectra should be similar to spectra of the hydrated hydronium ions H7O+3 and H9O+4.
Vibrational frequency shifts induced by molecular compression of pyridine in solution
Authors: Zakin M. R. , Herschbach D. R.
Published: July 2002

Pressure-induced vibrational frequency shifts are calculated for a diatomic oscillator immersed in a benign solvent, employing a simplified version of the Schweizer–Chandler model for solute–solvent interaction. The repulsive contribution is determined from the pair distribution function for hard-sphere cavities. Interpolative evaluation of the pair distribution function is facilitated by noting that to an excellent approximation the pertinent expansion coefficients are merely linear functions of the reduced density. The treatment is applied to the quasidiatomic ring breathing vibrations of neat liquid pyridine, benzene, and toluene and to solutions of pyridine in several solvents including H2O, D2O, CH3OH, CHCl3, dimethylformamide, and toluene. The predicted pressure dependence of the ring breathing frequency is in the range ∂ν/∂P≈0.3–0.8 cm−1/kbar for all these systems. The corresponding compression of the mean ring radius is in the range 0.9 to 2.0×10−4 Å/kbar. Especially for the associated solvents, the dominant contribution (>90%) to ∂ν/∂P comes from the effective hard-sphere repulsion. Accurate values of the effective diameters thus can be evaluated from the observed pressure derivatives.
Compression of solid nitromethane to 15 GPa at 298 K
Authors: Yarger F. L. , Olinger Bart
Published: July 2002

The measurement of the isothermal compression of solid nitromethane to 15 GPa at 298 K using high pressure x-ray diffraction techniques is described. The compression data are fit to a model from which bulk moduli are calculated. Most interesting are the linear compression data. The a axis direction, along which the C–N bonds are aligned, shows little increase in repulsion with increasing pressure above 5 GPa. This indicates that the nitro and methyl groups of neighboring molecules may be interacting.
A quantitative experimental study of the core excited electronic states of formamide, formic acid, and formyl fluoride
Authors: Ishii I. , Hitchcock A. P.
Published: July 2002

Optical oscillator strength spectra of formamide (HCONH2), formic acid (HCOOH), and formyl fluoride (HCOF) in the region of K-shell excitation have been derived from electron energy loss spectra recorded under electric dipole dominated scattering conditions (>2.7 keV impact energy, small scattering angle). The observed features are assigned to promotions of 1s electrons to π*(C=O), σ*(HCX), σ*(C–X), σ*(C=O), and Rydberg orbitals. Systematic changes in the term values for the 1s→π*(C=O) transitions are related to the π donor strengths of the X substituents of the carbonyl group. Broad weak features, observed only in the carbonyl C1s and O1s spectra around 7 eV above the IP, are assigned to 1s→σ*(C=O) transitions. The positions of these features are in agreement with a previously documented correlation with bond length, as are the positions of features associated with σ*(C–N) in formamide, σ*(C–O) in formic acid, and σ*(C–F) in formyl fluoride. The oscillator strengths of the 1s→π* features in the various K-shell spectra are compared to HAM/3 calculations and are used to estimate the spatial distributions of the π*(C=O) orbital in the (1s−1, π*) core excited states of these three substituted carbonyl species. We discuss the degree to which these derived orbital maps reflect the spatial distributions of π*(C=O) orbitals in the ground state.
A Fokker–Planck equation for canonical non-Markovian systems: A local linearization approach
Authors: Grigolini Paolo
Published: July 2002

Correctly renormalized drift coefficients can be straightforwardly derived using the linear version of the generalized Langevin equation and linear reaction potentials (parabolas or inverted parabolas). Recent investigation via computer simulation of molecular dynamics and numerical solution of stochastic differential equations, shows that interesting cases exist where the nonlinear nature of the interaction between reacting system and ‘‘bath’’ and that of the reaction potential must be taken into account. Then it is shown that a Smoluchowski diffusion equation with correctly renormalized drift coefficients can be obtained by adopting a local linearization assumption, which, nevertheless allows the reaction coordinate to ‘‘feel’’ the influence of different transport properties in different regions of the reaction potential. Under the special condition where the system–bath interaction is assumed to be linear, this Smoluchowski equation is shown to coincide with that recently proposed by Okuyama and Oxtoby [J. Chem. Phys. 84, 5830 (1986)]. In the case where the renormalization corrections are neglected, this equation coincides with that proposed by our group [Fonseca, P. Grigolini, and D. Pareo, J. Chem. Phys. 83, 1039 (1985)].
Effect of collision energy and incidence angle on the precursor-mediated dissociative chemisorption of N2 on W(100)
Authors: Rettner C. T. , Stein H. , Schweizer E. K.
Published: July 2002

The dissociative chemisorption of N2 on W(100) is found to fall rapidly with increasing kinetic energy Ei in the range 26 to 450 meV. For a surface temperature Ts of 300 K, the initial dissociative chemisorption probability S0 falls from ∼0.8 at Ei=26 meV to 0.15 at 450 meV. Over this range of energies the dissociation probability is also found to fall rapidly with Ts, and to be relatively insensitive to surface coverage at low Ts, strongly suggesting that dissociation occurs in this system via a precursor under these conditions. This picture is supported by angular distribution measurements of the scattered molecules which are consistent with an appreciable cosine component, which also becomes smaller as Ei increases. Results are found to be surprisingly insensitive to the incidence angle over the range 0° to 70°, indicating that the trapping process scales quite closely with the total incidence energy.
The calculation of excited state properties with quantum Monte Carlo
Authors: Ceperley D. M. , Bernu B.
Published: July 2002

A new Monte Carlo method for computing excited state properties of quantum systems is introduced. It is a generalization of the transient estimate method used for fermion Green’s function Monte Carlo and of subspace projection methods used for computing eigenstates of matrices. The time dependent autocorrelation function of a vector of trial functions is calculated for a random walk generated by the imaginary-time Schrödinger equation and estimates of energy levels are determined by the eigenvalues of the matrix of correlation functions. This method is especially useful for treating states with the same symmetry as it automatically keeps higher states orthogonal to lower states. The estimated energy converges to the exact eigenvalue with a rate which decreases with increasing excitation energy, thus limiting the method to relatively low-lying states. The method is zero variance in the sense that as better trial functions are introduced, the statistical error decreases to zero. The method has a nontrivial bias which is analyzed. As an illustration, the eigenvalues of a double well are computed.
Dynamics of wetting with nonideal surfaces. The single defect problem
Authors: Raphaël E. , de Gennes P. G.
Published: July 2002

Under static conditions, the macroscopic contact angle θ between a (partially wetting) liquid, a solid, and air, lies between two limiting values θr (receding) and θa (advancing). If we go beyond these limits (e.g., θ=θa+ε, ε>0) the contact line moves with a certain macroscopic velocity U(ε). In the present paper, we discuss U(ε) (at small ε) for a very special situation where the contact line interacts only with one defect at a time. (This could be achieved inside a very thin capillary, of radius smaller than the average distance between defects.) Using earlier results on the elasticity and dynamics of the contact line in ideal conditions, we can describe the motions around ‘‘smooth’’ defects (where the local wettability does not change abruptly from point to point). For the single defect problem in a capillary, two nonequivalent experiments can be performed: (a) the force F is imposed (e.g., by the weight of the liquid column in the capillary). Here we define ε=(F−Fm)/Fm, where Fm is the maximum pinning force which one defect can provide. We are led to a time averaged velocity Ū∼ε1/2. (b) The velocity U is imposed (e.g., by moving a horizontal column with a piston). Here the threshold force is not at F=Fm, but at a lower value F=FU —obtained when the contact line, after moving through the defect, leaves it abruptly. Defining ε̄=(F̄−FU)/FU, where F̄ is the time average of the force, we find here U∼ε̄3/2. These conclusions are strictly restricted to the single defect problem (and to smooth defects). In practical situations, the contact line couples simultaneously to many defects: the resulting averages probably suppress the distinction between fixed force and fixed velocity.
Charge distributions and valency in copper oxide crystals related to superconductivity
Authors: Goodman G. L. , Ellis D. E. , Alp E. E. , Soderholm L.
Published: July 2002

We present an overview of the discrete variational method for calculating orbital states of a molecular cluster in the local density approximation. Then we introduce a new method of embedding a finite molecular cluster in a crystal lattice. This new technique of cluster weighting is based on the bulk stoichiometry of the crystal and the Mulliken populations for chemically complete atoms—those atoms that have all their covalently bonded nearest neighboring atoms also in the cluster. We have applied our new approach to calculate linear-combination-of-atomic-orbitals ground states for seven copper oxide crystals and Cu metal. The average net Mulliken charges calculated for Cu atoms in this way are shown to correlate well with the observed energies of the Cu K edge feature in x-ray absorption spectra of these materials. Several of these compounds are important to understanding the high Tc superconductivity of copper oxides.
Studies of the temperature-dependent phase behavior of long chain n-alkyl thiol monolayers on gold
Authors: Nuzzo Ralph G. , Korenic Eileen M. , Dubois Lawrence H.
Published: July 2002

The influences of temperature on the infrared spectra of a docosylthiol (C22) monolayer on a gold substrate are reported. The data suggest that this adsorbate exhibits a complex phase state at 300 K in which gauche conformations concentrated at the chain termina figure prominently. The detailed temperature dependence of the spectral band intensities is found to be very similar in many regards to that of bulk hydrocarbon crystals. At low temperature, band splittings are observed which suggest the lowest energy structure for this adsorbate is one containing two chains per unit cell. A simple lattice argument, consistent with steric models and low-energy electron diffraction data, is presented to explain these observations.
Line-mixing effects in Ar-broadened doublets of a hot band of OCS
Authors: Blanquet G. , Walrand J. , Bouanich J. -P. , Boulet C.
Published: July 2002

The spectra of several Ar-broadened symmetric doublets in the hot band ν1+ν2−ν2 (at 850 cm−1) of OC 32S have been recorded with a tunable diode-laser spectrometer. The pressure-broadening coefficients measured for sufficiently separated lines belonging to 7 doublets are in agreement with the results previously obtained in the ν1 band. From these data, we have determined, for different pressures of argon, line-coupling coefficients for 15 doublets in the P and R branches of the hot band. These coefficients appear to be small and negative, at the limit of our estimated accuracy, which implies very weak line-mixing effects in the doublets, in agreement with the results of a semiclassical calculation as well as with results of coupling cross sections estimated at large-j values from the infinite order sudden (IOS) approximation.
Hydrogen bonding between the water molecule and the hydroxyl radical (H2O⋅OH): The 2A″ and 2A′ minima
Authors: Kim Kwang S. , Kim Hyun Sik , Jang Jee Hwan , Kim Ho Soon , Mhin Byung-Jin , Xie Yaoming , Schaefer Henry F.
Published: July 2002

The two degenerate components of the OH radical 2Π ground electronic state give rise to independent minima (of 2A″ and 2A′ symmetries) upon interaction with the water molecule. These two minima have been investigated here for the first time using ab initio quantum mechanical methods. Minimum, double zeta, double zeta plus polarization, and triple zeta plus double polarization basis sets have been employed in conjunction with self-consistent-field, second-order perturbation, and configuration interaction methods. At all levels of theory, the 2A″ state is predicted to be the global minimum, with a dissociation energy De of about 3.5 kcal/mol. The 2A′ state is predicted to lie about 1 kcal higher in energy. Both minima occur for structures with OH⋅⋅⋅O linkages close to linear and are reminiscent of the water dimer. However, the H⋅⋅⋅O distances (∼2.1 Å for 2A″, ∼2.2 Å for 2A′) are significantly longer than observed for the water dimer. Preliminary estimates of the H2O⋅OH vibrational frequencies are made.
Structure and dynamics of liquid water between plates
Authors: Zhu S.-B. , Robinson G. W.
Published: July 2002

Using the recently developed SPC-FP water model (simple point charge model with flexible bonds and polarization) and the molecular dynamics method, we investigate the structure and properties of liquid water between two rigid plates. In one case the plates are neutral and in the other the plates are electrically charged. In both cases substantial differences from bulk state water are found, structurally and dynamically. We observe some anomalies compared with normal liquids and attribute these to the breakage of hydrogen bonds under the influence of the solid–liquid interface. Adding an external torque enhances such breakage through the attempted alignment of the water molecules. A combination of these two contributions determines the resultant dynamical behavior of water between charged plates. The information obtained from this work should be helpful in the understanding of ‘‘hydrophobic effects’’ in aqueous solutions. The behavior of water near large polar or nonpolar molecular solutes is also revealed by these studies.
Reactive scattering using efficient time-dependent quantum mechanical wave packet methods on an L-shaped grid
Authors: Mowrey R. C.
Published: July 2002

Most quantum mechanical time-dependent wave packet methods represent the wave packet on Cartesian grids. For reactive systems these grids can contain many points at which the wave function amplitude is negligible throughout the calculation. A significant fraction of the computational resources in a calculation may be used in calculating and storing values of the wave function at these grid points. This work describes a method which uses the Fourier transform method to calculate the kinetic energy of a wave function on a grid which covers only reactant and product regions of the potential. For a two-dimensional system this corresponds to the use of an L-shaped grid. The time propagation is performed using the Chebyshev propagation method. The method is demonstrated for a model problem treating dissociative adsorption and associative desorption of H2 from a flat surface. The amount of computational resources required for these calculations is much less than would be needed if the full rectangular grid was used.
Theoretical x-ray absorption spectra of SF6 and H2S
Authors: Nakamatsu Hirohide , Mukoyama Takeshi , Adachi Hirohiko
Published: July 2002

We have performed molecular orbital calculations for SF6 and H2S, using the discrete–variational Xα method. Energies and cross sections of virtual states are obtained for theoretical spectra of sulfur K, LII,III and fluorine K x-ray absorptions for the SF6 molecule. They are in good agreement with the experimental results. Through the same calculation procedures, the theoretical spectra of sulfur K and LII,III absorptions for H2S are derived. The obtained molecular orbitals represent the experimental spectra below the ionization energy very well. SF6 has distinct shape resonances above the ionization energy, in contrast to H2S which has pre-edge peaks as a main structure. For the SF6 molecule, shape resonances come from the scattering at the steep change of attractive potential of surrounding fluorine atoms. It is demonstrated that phase shift due to the scattering causes the resonances without a potential barrier. Various spectral differences among the sulfur K, LII,III and the fluorine K absorptions in peak width and in base line are related to the character of wave functions. Other resonances among the fluorine wave functions contribute to the characteristics of the base line which was explained as the continuum state alone previously. For the H2S molecule, the reason for the absence of resonances is attributable to the weak scattering power of hydrogen atoms.
Stochastic linkage: Effect of random fluctuations on a two-state process
Authors: Di Cera Enrico
Published: July 2002

When the two-state process A⇄B is asymmetrically perturbed by random fluctuations, a unidirectional shift of the equilibrium between the two states is observed. Random fluctuations destabilize the state that is populated nondeterministically by the fluctuating rate. This effect of random fluctuations on the preexisting equilibrium between the two states is analogous to the allosteric regulation of conformational transitions in biological macromolecules induced by ligand binding to one of the two states. The unidirectional effect of random fluctuations of the two-state process A⇄B can be interpreted as a stochastic linkage that couples noise to conformational transitions.
Multireference configuration interaction calculations of the low-lying electronic states of ClO2
Authors: Peterson Kirk A. , Werner Hans-Joachim
Published: July 2002

Using extensive internally contracted multireference configuration interaction wave functions and large basis sets, near-equilibrium potential energy functions for the first four doublet electronic states (X 2B1, 2B2, 2A1, and A 2A2 ) of OClO and the X 2A″ electronic state of ClOO have been calculated. Electric dipole moment functions have also been computed for the ground states of both isomers. Spectroscopic constants derived for the X and A states of OClO, as well as for the X state of ClOO, are compared to the available experimental data, and predictions for the other states are made. In agreement with previous assumptions about the photodissociation of OClO, strong interactions between the first three excited electronic states in the Franck–Condon region of the A←X transition are indicated from cuts of the potential energy surfaces. In particular, the experimentally observed predissociation of the A 2A2 state is proposed from this work to proceed initially by an interaction with the close-lying 2A1 state. Additionally, the calculated asymmetric stretch potential for the A 2A2 state of OClO does not show evidence of a double minimum as has been previously proposed.
Vibrational relaxation and Bloch–Redfield theory
Authors: Figueirido Francisco E. , Levy Ronald M.
Published: July 2002

Bloch–Redfield theory helps bridge the gap between classical molecular dynamics (MD) simulations and (still unavailable) full quantum-mechanical simulation of processes in condensed media. It allows one to describe quantum processes in terms of a few parameters which can be computed from MD simulations. In this letter we present results which show that it is practical to use Bloch–Redfield theory to describe the relaxation of high-energy (≥kT) vibrational modes and that it gives answers which differ from the classical MD ones.
Vibrationally induced rotational axis switching: A novel mechanism for vibrational mode coupling
Authors: Li H. , Ezra Gregory S. , Philips Laura A.
Published: July 2002

High resolution IR spectra of small- to medium-sized molecules such as 2-fluoroethanol (2FE) show that the effective density of coupled states is often greater than that obtained by a direct count of vibrational states. A novel mechanism for rotation–vibration interaction, vibrationally induced rotational axis switching (VIRAS), is proposed as a possible explanation for these discrepancies. VIRAS has its origin in centrifugal distortion, and is physically distinct from Coriolis coupling. In the case of 2FE, we explicitly treat the coupling of overall rotation with large-amplitude internal rotation about the C–C bond. Assuming a uniform coupling of all dark vibration–torsion states to the bright state, we predict a density of coupled states in good agreement with that observed in the C–H stretching region at 2980 cm−1.
Spectroscopic characterization of the X 1Σ+ and C 1Π states of the Mg⋅Ne molecule
Authors: Wallace Ingvar , Breckenridge W. H.
Published: July 2002

The X 1Σ+ ground state and C 1Π1 excited state of the Mg⋅Ne molecule have been characterized by laser induced fluorescence in a supersonic jet. The spectroscopic constants are compared to those for analogous electronic states of metal–rare-gas van der Waals complexes. The bonding in such states is discussed in terms of models for metal–rare-gas interactions which we have proposed earlier.
Conjugated-circuit computations on two-dimensional carbon networks
Authors: Zhu Hongyao , Balaban Alexandru T. , Klein Douglas J. , Živković Tomislav P.
Published: July 2002

First, a new approach to treat the strongly correlated conjugated-circuit model on two-dimensional networks is made with computational effort comparable to that for corresponding tight-binding models. Toward this end a translationally symmetric arrow assignment is used to construct an antisymmetrically signed ‘‘adjacency’’ matrix for two-dimensional networks. Then symmetry blocking is used to manipulate this ‘‘adjacency’’ matrix, and make the associated conjugated-circuit computations. Second, a series of two-dimensional translationally symmetric structures related to graphite is constructed by means of a kind of local rearrangement on the graphite lattice. A consequent detailed description of π-electron resonance energy via conjugated-circuit computations is presented for these novel two-dimensional nets as well as several other regular and semiregular nets with vertices of degree 3. Approximate energy estimates indicate that resonance stability depends dominantly on the local topology of the networks, and in particular on the fraction of faces which are hexagonal.
Two-photon time-of-flight spectra of Xe2
Authors: Dimov S. S. , Cai J. Y. , Lipson R. H.
Published: July 2002

Resonantly enhanced multiphoton ionization (REMPI) spectra of jet-cooled Xe2 are presented, covering the spectral region between ≊74 627 and 80 849 cm−1. Dimer ions produced by (2+1) REMPI excitation were mass selected in a linear time-of-flight (TOF) mass spectrometer. The vibrational and isotopic structure of several band systems dissociating to Xe* 5p56p and 5p55d asymptotes have been analyzed, many unambiguously for the first time, and molecular constants derived. Equilibrium bond lengths were estimated from Franck–Condon calculations. Insight into excited state predissociation was also obtained by recording atomic TOF excitation spectra.
Laser-induced fluorescence spectroscopy of the Ã 1Πu–X̃ 1Σ+g transition in jet-cooled C3
Authors: Balfour Walter J. , Cao Jianying , Prasad C. V. V. , Qian Charles X. W.
Published: July 2002

C3 radicals have been produced in a plasma of rhenium with methane doped in helium and cooled in a supersonic free jet expansion. More than 50 vibronic bands in the laser induced fluorescence spectrum of the Ã 1Πu–X̃ 1Σ+g electronic transition have been recorded in the region 370–415 nm at 0.4 cm−1 resolution. The observations include bands identified by Gausset et al. (1965) and many bands not previously characterized. Rotational analyses have been made for a majority of the bands and the number of located Renner–Teller levels of the Ã state has been considerably extended. The (002) level is tentatively identified. A comparison of the experimentally determined energy level pattern in the Ã state with theoretical predictions has suggested a number of revisions to previous assignments and evidence is presented to contradict suggestions from earlier work that the (020) level of the Ã state is perturbed.
Barrier crossing and solvation dynamics in polar solvents: Photoisomerization of trans-stilbene and E,E-diphenylbutadiene in compressed alkanols
Authors: Mohrschladt R. , Schroeder J. , Schwarzer D. , Troe J. , Vöhringer P.
Published: July 2002

The viscosity dependence of the photoisomerization of trans-stilbene in compressed liquid ethanol shows deviations from a simple power law description in the viscosity range from 1 to 4 mPa s. Corresponding deviations are observed in the solvents methanol, n-propanol, and n-butanol. This behavior is attributed to a competition between solvent relaxation and barrier crossing in the S1 state of trans-stilbene. The relative time scales of barrier crossing and solvent relaxation change as the pressure increases, because the dielectric relaxation rate of the solvent decreases more rapidly with increasing viscosity than the barrier crossing rate. Consequently, the reaction takes place in an increasingly retarded solvent environment which no longer relaxes completely around the changing charge distribution of the solute along its reaction path, giving rise to ‘‘dielectric friction.’’ In contrast to trans-stilbene, the corresponding reaction of diphenylbutadiene in n-alkanols shows a much weaker sensitivity to solute-solvent interaction and, consequently, a simple inverse viscosity dependence of the photoisomerization rate is observed in all alkanols such as described by the Kramers–Smoluchowski theory. This significant difference is probably caused by smaller sudden polarization effects along the reaction path in diphenylbutadiene. The observed dependence of the trans-stilbene barrier crossing rate on pressure is compared either to a model with density dependent effective barrier height, or to a simple continuum model of the frequency dependence of the dielectric friction in the limit of weak coupling. Neither model works well unless a very strong viscosity dependence of the dielectric relaxation time of the solvent (τD∝η10) is employed to obtain agreement with the observed viscosity dependence of the barrier crossing rate.
The microwave spectrum of CH4– –H2O
Authors: Suenram R. D. , Fraser G. T. , Lovas F. J. , Kawashima Y.
Published: July 2002

Microwave spectra of CH4– –H2O, CH4– –H218O, CH4– –H217O, CH4– –D2O, and CH4– –DOH have been measured using a pulsed-nozzle Fourier-transform microwave spectrometer. The spectra were recorded to aid the assignment of the high-resolution far-infrared spectrum of CH4– –H2O reported recently [L. Dore, R. C. Cohen, C. A. Schmuttenmaer, K. L. Busarow, M. J. Elrod, J. G. Loeser, and R. J. Saykally, J. Chem. Phys. 100, 863 (1994)]. Spectral assignments were guided by Stark-effect and nuclear-spin hyperfine measurements. For the primary isotopic species, CH4– –H2O, four K=0 (Σ) and six K=1 (Π) rotational progressions were observed at the ∼1 K rotational temperature of the supersonic expansion. The internal-rotor state of the complex correlating to j=0 H2O+j=0 CH4 is found to have a rotational constant B=4346.7202(7) MHz and centrifugal distortion constant DJ=119.72(9) kHz, where the numbers in parentheses represent one standard deviation of the fit. These constants imply a zero-point center-of-mass separation of 3.7024 Å between the two subunits and a pseudodiatomic weak-bond stretching force constant of 1.53 N/m and stretching frequency of 55 cm−1. Stark-effect measurements reveal that two of the K=1 progressions originate from degenerate states while the other four K=1 transitions arise from two Π states which are K (or l) doubled. The effective electric dipole moments vary from 1.95×10−30 to 2.67×10−30 C m (0.58–0.83 D) for the states studied. The isotopic results are consistent with a CH4– –H2O structure in which one of the hydrogens of H2O proton donates to CH4, analogous to structures previously reported for CH4 with HCN and HCl. A combined analysis of the microwave and far-infrared data allow estimates of the barriers to internal rotation of the H2O and CH4 units. The H2O internal rotation potential is found to be much more anisotropic than that of Ar– –H2O.
Phase transitions and chemical transformations of nitromethane up to 350 °C and 35 GPa
Authors: Courtecuisse Stéphane , Cansell François , Fabre Denise , Petitet Jean-Pierre
Published: July 2002

Nitromethane has been studied as a model of the energetic nitro compounds. The phase diagram has been determined by Raman scattering in the pressure and temperature ranges of 0–35 GPa and 20–350 °C, respectively. Three new solid phases of nitromethane called III, IV, V, and their domain of stability have been located. A first chemical transformation is observed by the disappearance of nitromethane Raman modes and by the irreversible formation of a transparent solid called compound I (CI). A second chemical transformation [compound I–compound II (CII)], at higher temperature than the first one, is observed by the sudden darkening of the sample.
Long-time tails in angular momentum correlations
Authors: Lowe C. P. , Frenkel D. , Masters A. J.
Published: July 2002

We compare computer simulation results for the angular velocity autocorrelation function (AVACF) of a colloidal particle with theoretical predictions. We consider both spherical and nonspherical particles in two and three dimensions. The theoretical prediction for the long-time decay of the AVACF in three dimensions is well known, here we also give the two-dimensional result, along with a sketch of how it was derived. For spherical particles we find excellent agreement between the simulations results and theoretical predictions in both two and three dimensions. We also find that the same expressions apply to the nonspherical particles when the particles have had time to undergo a significant angular displacement. This observation is again in agreement with theory.
Recursion polynomial expansion of the Green’s function with absorbing boundary conditions: Calculations of resonances of HCO by filter diagonalization
Authors: Grozdanov Tasko P. , Mandelshtam Vladimir A. , Taylor Howard S.
Published: July 2002

An iterative method for calculating resonance positions and widths is developed. The system Hamiltonian with an asymptotic complex absorbing potential is represented by a large and sparse matrix. A small set of ‘‘good’’ basis functions suitable for diagonalizing the Hamiltonian matrix in a given energy window is generated by acting with a polynomial expansion of the imaginary part of the system Green’s function onto a generic initial wave packet. As an application to a realistic three-dimensional system, the calculation of 65 resonances of the nonrotating HCO molecule up to the energy 9000 cm−1 is presented. The method is shown to be rapidly convergent and accurate, especially for narrow resonances.
Mass-resolved two-photon spectra of ArXe in the region of Xe*(6p)
Authors: Dimov S. S. , Hu X. K. , Mao D. M. , Cai J. Y. , Lipson R. H.
Published: July 2002

(2+1) resonantly enhanced multiphoton ionization (REMPI) spectra of ArXe have been recorded between ≊78 000 and 80 110.0 cm−1. Single isotopomer data was obtained using a time-of-flight (TOF) mass spectrometer. Vibrational analyses for several transitions involving ArXe excited states that dissociate to Ar(1S0)+Xe*(6p) are presented, in some instances, for the first time. In addition to vibrational numbering and constants, excited state symmetries were deduced from separate REMPI/TOF spectra recorded with linearly and circularly polarized light, while excited state bond lengths were derived from Franck–Condon factor calculations. Some of the excited states were found to have potential humps and/or unusual vibrational band intensity distributions. Where possible, the nature of the perturbations is discussed.
Complex scaling of ab initio molecular potential surfaces
Authors: Mandelshtam Vladimir A. , Moiseyev Nimrod
Published: July 2002

The energies and lifetimes (i.e., inverse decay rates) of resonance (quasibound) states in chemical reactions are associated with the complex eigenvalues of the complex scaled Hamiltonian. The corresponding eigenfunctions are square integrable and are compact, localized functions in the coordinate space. Complex scaling is applicable when the potential, V(x), is dilation analytic. Ab initio potentials, however, are given on a grid, Vn=V(xn). Starting from the theoretical work of Moiseyev and Hirschfelder [J. Chem. Phys. 88, 1063 (1988)], we propose an efficient numerical method of calculating V(xn exp(iθ)) by acting on the unscaled potential with a grid represented scaling operator Ŝ.