bluuebunny/crossref_metadata_2025 huggingface dataset:
Verification of H− formation in ultraviolet-laser-irradiated hydrogen: Implications for negative ion and neutral beam technologies
Authors: Pinnaduwage L. A. , Christophorou L. G.
Published: July 2002

Photodetachment and ion mobility measurements are reported confirming the efficient H− formation in UV-laser-irradiated H2 reported earlier [L. A. Pinnaduwage and L. G. Christophorou, Phys. Rev. Lett. 70, 754 (1993)]. The implications of the efficienct H− formation in UV-laser-irradiated H2 (and other types of negative ions in UV-laser-irradiated gases) for negative ion and neutral particle beam technologies are discussed. Also, the possible contribution to H− formation in H2 discharge sources from electron attachment to high-lying electronically excited states of H2 is indicated.
Laser-induced crystallization phenomena in GeTe-based alloys. II. Composition dependence of nucleation and growth
Authors: Coombs J. H. , Jongenelis A. P. J. M. , van Es-Spiekman W. , Jacobs B. A. J.
Published: July 2002

The laser-induced crystallization behavior of GeTe-based amorphous alloys has been measured with a novel multipulse laser technique. This enables the composition dependence of the nucleation rate and crystal growth speed to be independently followed. Two types of crystallization are investigated. The first involves single-phase crystallization of quaternary alloys based on Ge39Sb9Te52, in which the composition dependence of nucleation and growth is followed as Se, S, Sn, and Si are included. Both the nucleation rate and crystal-growth speed vary exponentially with the composition, and a correlation is found between crystallization behavior and bond strengths. The second involves multiphase crystallization in the GeSbTe ternary system. It is shown that the observed variations in crystallization behavior primarily arise from the composition dependence of nucleation rather than crystal growth. The implications of this finding for the importance of long range diffusion during crystallization in the GeSbTe system are discussed.
A stiction model for a head-disk interface of a rigid disk drive
Authors: Gui Jing , Marchon Bruno
Published: July 2002

A theoretical stiction model for a head–disk interface has been developed that takes into account local redistribution of lubricant due to the competition between the capillary pressure inside a meniscus and the disjoining pressure outside the meniscus. Equations for calculating meniscus force and resulting stiction have been obtained for various contacting geometries and lubrication states. The model has been applied to study the effects of disk variables on stiction. Two categories of texture are considered: One has a uniform asperity height (the regular texture) and the other has a Gaussian asperity height distribution (the random texture). The relationship between stiction and the real area of contact is found to be nonmonotonic. The minimum stiction is predicted to occur at intermediate values of the real area of contact. The model also predicts a complicated interdependence between roughness parameters and their role on stiction. The role of lubricant and overcoat on stiction is also considered. Not only the lubricant film thickness, but also the physical and chemical properties of both lubricant and overcoat, as well as the interactions between them, are found to be crucial in determining stiction.
Dislocation multiplication behind the shock front
Authors: Zaretsky E.
Published: July 2002

A new model of fast operated dislocation source based on multiplication and motion of partial dislocations bounding the stacking fault is suggested. Stress-activated stretching of lateral branches of the partial dislocation bowed-out segment results in collapse of these branches with subsequent restoration of the ‘‘initial’’ dislocation half-loop and generation of a ‘‘fresh’’ partial dislocation loop, both capable to produce the next multiplication act. The multiplication results in the exponential increase of both dislocations and stacking faults concentration and is accompanied with the plastic deformation having a strain rate dε/dt∼2t/ΔT. The characteristic time of the process, ΔT, ranges from 10−9 to 10−10 s. The model explains the variations of x-ray diffraction pattern for the material undergoing shock compression, the shock-induced formation of twins, and shear bands and pre-fracture voids nucleation in the rarefaction wave.
Effects of grain shape anisotropy, porosity, and microcracks on the elastic and dielectric constants of polycrystalline piezoelectric ceramics
Authors: Dunn Martin L.
Published: July 2002

A theoretical approach is proposed to estimate the effects of grain shape anisotropy on the overall response of polycrystalline piezoelectric ceramics. The self-consistent approach is applicable to a polycrystal with arbitrary grain shapes, orientations, and anisotropy, but emphasis is placed on the effective bulk and shear moduli and the dielectric constant of a polycrystal consisting of randomly oriented piezoelectric grains. Because typical piezoelectric ceramics often contain substantial porosity, the effects of randomly oriented pores of various shapes on the overall electroelastic properties are considered. Analytical predictions are compared to simplified predictions which ignore the effects of piezoelectricity at the grain level and the effects of grain shape. Analytical predictions are in good agreement with measurements for unpoled, porous BaTiO3 polycrystals. A FORTRAN computer program developed to implement the theory is available upon request from the author.
Effects of cooling field strength on exchange anisotropy at permalloy/CoO interfaces
Authors: Moran Timothy J. , Schuller Ivan K.
Published: July 2002

We have studied exchange anisotropy at ferromagnet/antiferromagnet (FM-AFM) interfaces as a function of cooling field in Ni80Fe20 (permalloy) films deposited on bulk single-crystal CoO substrates. Hysteresis loops measured after cooling through the antiferromagnetic ordering temperature in different magnetic fields show that the exchange bias has little dependence on cooling field. Large cooling fields produced hysteresis loops with larger remanent magnetization and larger coercivity. These trends can be explained using a model that assumes that the FM anisotropy axis directions can be influenced by the magnetic history of the AFM. Large cooling fields also led to larger CoO susceptibility, which implies rotation of the CoO spin axes. The simultaneous rotation of the FM anisotropy axes and the AFM spin axes suggests perpendicular coupling at the FM-AFM interface.
Magnetism and microstructure of nanocrystalline nickel
Authors: Kisker H. , Kronmüller H. , Schaefer H.-E. , Suzuki T.
Published: July 2002

In order to study the influence of the nanocrystalline microstructure on the magnetic properties of pure nanocrystalline metals, in situ magnetic measurements of high-purity nanocrystalline nickel, prepared by a gas-condensation technique with subsequent compaction at various pressures have been performed. The approach to ferromagnetic saturation indicates free volumes in the nanocrystalline system of the size of individual missing crystallites as well as internal stresses. While the saturation polarization of the nanocrystalline Ni samples appears to be unchanged compared to coarse-grained Ni the coercive field and magnetic domains are strongly influenced by the nanocrystalline structure. The shape of the domains with sizes of 10–200 μm much larger than the crystallite size depends on the annealing state of the specimen. The initial coercive field increases with the compaction pressure during preparation. Upon annealing it further increases to a maximum value at Ta=500 °C and decreases at higher annealing temperatures. Additionally, the effect of oxygen located in the grain boundaries has been studied.
Structural, magnetic, and magnetocaloric properties of (Hf0.83Ta0.17)Fe2+x materials
Authors: Herbst J. F. , Fuerst C. D. , McMichael R. D.
Published: July 2002

We have investigated the sensitivity of structural, magnetic, and magnetocaloric properties of (Hf0.83Ta0.17)Fe2+x alloys (x=−0.18, −0.09, −0.02, 0.00, 0.09, 0.26) to Fe content. As-cast samples for all x consist essentially exclusively of hexagonal MgZn2-type material. Via magnetization and differential scanning calorimetry measurements we find that all the alloys except x=0.26 feature both a ferromagnetic→antiferromagnetic (FM→AFM) transition at temperature T0 and a Néel transition. The most abrupt FM→AFM transition occurs in the x=−0.02 material which also exhibits the lowest T0 and largest low-temperature moment. We report a magnetocaloric property, the field-induced entropy change ΔS, for the x=0.00 and x=±0.09 alloys.
Elastic scattering calculations for electrons and positrons in solid targets
Authors: Dapor Maurizio
Published: July 2002

The differential, total, and transport cross sections for electrons and positrons impinging on free atoms and solid targets have been calculated in the energy range of 100–5000 eV. As an application, the mean number of the wide angle collisions suffered by the particle before slowing down to rest and the backscattering coefficient are analytically calculated; The values of backscattering coefficients are found to be in better agreement with experiment than earlier calculations.
A positron annihilation investigation of porous silicon
Authors: Dannefaer S. , Kerr D. , Craigen D. , Bretagnon T. , Taliercio T. , Foucaran A.
Published: July 2002

Positron lifetime experiments have been conducted on 13 different porous silicon layers formed from p+, p, and n+ substrates. Four distinct positron lifetimes could be associated with the porous layers. One lifetime was close to 0.5 ns and is claimed to arise from positrons trapped by small vacancy clusters. Positronium is formed either on the surface with a lifetime of ∼5 ns, or in the pores, from which exceptionally long lifetimes of up to 90 ns originate. Both of these lifetimes arise from the pickoff annihilation of ortho-positronium; the fourth lifetime is the usual 125 ps component associated with para-positronium. The long lifetime component gives rise to 3-γ annihilations which are readily detectable in the energy spectrum of the annihilation quanta. In situ heat treatments of two of the samples provide evidence that gases play a very important role in the passivation of defects, both in the native oxide layer and on the surface, albeit on very different time scales.
Acoustic detection of laser induced melting of metals
Authors: Mesaros M. , Martı́nez O. E. , Bilmes G. M. , Tocho J. O.
Published: July 2002

Real time detection of pulsed laser surface melting was performed by analyzing the photoacoustic signals produced on the samples. Comparison between the amplitudes of the transversal and longitudinal waves allowed us to identify the fluence thresholds for surface melting. The method was tested with AISI 304 stainless steel samples and the results obtained were checked against direct metallographic analysis.
Analytic solutions for strain distributions in quantum-wire structures
Authors: Faux David A. , Downes James R. , O’Reilly Eoin P.
Published: July 2002

Analytic expressions are derived for the strain field due to a lattice-mismatched quantum wire buried in an infinite medium whose cross-section is composed of any combination of line elements and circular arcs. Expressions for the strain field for rectangular, triangular and circular quantum wires are found confirming published results. For the rectangular wire, useful limiting relations are obtained for the stress components close to the edge of the wire. Good agreement is demonstrated with measurements of lattice spacing reported by Chen et al. [Appl. Phys. Lett. 65, 2202 (1994)] for an In0.2Ga0.8As/GaAs rectangular wire if the indium concentration is assumed to be 24%. The strain field of a single AlGaAs/GaAs crescent-shaped wire, with and without lateral wells, is presented. The lateral wells introduce only minor modifications to the strain distribution when compared to a wire of the same thickness but without lateral wells. For a crescent-shaped quantum-wire stack, it is found that the strain field of each wire is almost independent of other wires in the stack when the wire separation is five times the thickness or more.
Ultrahigh frequency versus inductively coupled chlorine plasmas: Comparisons of Cl and Cl2 concentrations and electron temperatures measured by trace rare gases optical emission spectroscopy
Authors: Malyshev M. V. , Donnelly V. M. , Samukawa S.
Published: July 2002

Using trace rare gases optical emission spectroscopy, Cl and Cl2 number densities (nCl and nCl2) and electron temperatures (Te) were measured for two source configurations of high-density chlorine plasmas. In one configuration, the reactor was outfitted with a spoke antenna, operated at a resonant ultrahigh frequency (UHF) of 500 MHz. Alternatively, the same reactor was configured with a single loop, inductively coupled plasma (ICP) source operated at a radio frequency of 13.56 MHz. Optical emission from trace amounts (1% each) of rare gases added to the main Cl2 feed gas were recorded as a function of power and pressure. Modeling was used to derive Te from these data. Additional emission from Cl2 (at 3050 Å) and Cl (numerous lines between 7000 and 9000 Å), normalized to the appropriate emission from the rare gases (i.e., actinometry) was used to obtain nCl2 and nCl. In the ICP plasma, Te decreased monotonically from 5.5 to 1.2 eV as a function of increasing pressure between 1 and 20 mTorr. Conversely, with the UHF configuration, Te was 3.3 eV, independent of pressure between 1 and ∼7 mTorr, and then decreased to 1.7 eV as pressure was increased to 27 mTorr. At the same input power (1000 W), both sources resulted in electron densities of 1×1011 cm−3 at 3.5 mTorr, yet the UHF plasma was much less dissociated (30%) than the ICP plasma (70%). This can be attributed to differences in the electron energy distribution functions in the UHF and ICP plasmas, especially at low pressure.
Perpendicular quantized magnetic disks with 45 Gbits on a 4×4 cm2 area
Authors: Cui Bo , Wu Wei , Kong Linshu , Sun Xiaoyun , Chou Stephen Y.
Published: July 2002

Quantized magnetic disks consisting of an array of single domain Ni pillars with a density of 18 Gbits/in.2 were fabricated using nanoimprint lithography (NIL) and electroplating. The total disk area, limited by the NIL mold, is 4 cm×4 cm, leading to a total 45 Gbits. Magnetic force microscope (MFM) images show that all pillars 70 nm in diameter and 400 nm in height are single domain. The magnetostatic interaction between adjacent pillars is fairly strong. The pillars have an average switching field of 360 Oe and can be switched by a MFM tip with a large magnetic moment.
Magnetic properties of induced ferromagnet PrPtAl
Authors: Kitazawa H. , Suzuki H. , Abe H. , Tang J. , Kido G. , Dönni A.
Published: July 2002

The magnetic properties of orthorhombic PrPtAl with TiNiSi-type crystal structure and a nonmagnetic crystalline-electric field (CEF) ground-state singlet were investigated by measurements of specific heat, magnetic susceptibility, magnetization, and electrical resistivity. The results illustrate that in PrPtAl magnetic exchange interactions are strong enough to overcome the singlet–singlet CEF splitting of 21 K and to induce long-range magnetic order at TC=5.8 K.
Fractal-based description for the three-dimensional surface of materials
Authors: Li Jianming , Lu Li , Su Ying , Lai Man On
Published: July 2002

An algorithm called variation–correlation analysis, used to estimate fractal dimension with good accuracy, has been developed. Applying this model to images of the atomic force microscope, magnetic force microscope, and scanning electron microscope, it has been demonstrated that there exists a fractal characteristic length εmax. When the scale ε is within εmax, the variation–correlation Vcor(ε) of the dimensionless field-like variable H(x,y), which may denote the height of a surface or the magnetic domain or the angle distribution, obey a power law, while when ε is over εmax, Vcor (ε) becomes constant for a given image. The concept of “fractal measure” MF is given, MF=(1−δ)/(1+δ), where δ is defined as the dispersed degree of points on a log–log plot. MF is a sort of linear measure of point distribution, which can be used to determine the fractal characteristic length. Investigation shows that the fractal dimension in the range ε<εmax is associated with the irregularity of the different processed surfaces. The fractal characteristic length εmax can represent the statistical maximum size of the surface texture.
Integrated giant magnetoresistance bridge sensors with transverse permanent magnet biasing
Authors: Ku Wanjun , Silva F. , Bernardo J. , Freitas P. P.
Published: July 2002

Two types of giant magnetoresistance (GMR) multilayer bridge sensors with integrated permanent magnet biasing are demonstrated. These sensors differ from previous designs where external permanent magnets were used. The bridges consist of four active GMR multilayer elements at the first antiferromagnetic coupling peak of a NiFe/[NiFe/CoFe/Cu]10 structure (GMR=18%). Bridge linearization is obtained by creating opposite biasing fields of equal amplitude (±200 Oe) in contiguous GMR elements of the bridge structure. This is achieved either by using pairs of permanent magnets with the same Mrt value (14 memu/cm2) but different coercivities (type I bridge, Hc1=1400 Oe, Hc2=800 Oe), or by using a single type of permanent magnet and placing the GMR sensor either under the magnet, or on its side (type II bridge). Linear ranges of ±200 Oe with field sensitivities of 0.3 mV/(V×Oe) were obtained in these bridges.
Effect of metastable oxygen molecules in high density power-modulated oxygen discharges
Authors: Panda Siddhartha , Economou Demetre J. , Meyyappan M.
Published: July 2002

A spatially averaged (well mixed) reactor model was used to simulate a power-modulated (pulsed) high density oxygen discharge. Chemistry involving the high energy oxygen metastable molecules O2M(A 3Σu++C 3Δu+c 1Σu−) was included in the simulation. This chemistry was necessary to capture the experimentally observed increase in the O− negative ion density in the afterglow of the pulsed discharge. As the electron temperature drops in the afterglow, the rate coefficient of electron attachment with O2M increases several fold. The wall recombination probability of oxygen atoms affected the O− density drastically. For the conditions studied, the maximum O− density in the afterglow increased with pressure, decreased with power, and showed a maximum with pulse period. The time in the afterglow at which the peak O− density occurred decreased with pressure and power, and was independent of the pulse period. Knowing the temporal evolution of O− in the afterglow may be important for applications requiring extraction of negative ions out of the discharge.
Supersonic cluster beams of III–V semiconductors: GaxAsy
Authors: O’Brien S. C. , Liu Y. , Zhang Q. , Heath J. R. , Tittel F. K. , Curl R. F. , Smalley R. E.
Published: July 2002

Supersonic beams of semiconductor clusters with the formula GaxAsy were generated by laser vaporization of a disc of pure GaAs mounted on the side of a pulsed supersonic nozzle. These cluster beams were characterized by laser photoionization with various fixed-frequency lasers followed by time-of-flight mass spectrometry. Mass analysis of the clusters with x+y>10 showed all clusters in the composition range from Gax+y through GaxAsy to Asx+y to be present in roughly the amount expected from a binomial distribution. In the smaller clusters strong variations were observed from this expected binomial distirbution as a result of kinetic effects in the cluster formation process. Photoionization with an ArF excimer laser at very low pulse energy revealed a pronounced even/odd alternation in the photoionization cross section of the GaxAsy clusters, depending only on the total number of atoms in the cluster. Clusters in the 5–21 atom range with an odd number of atoms were one-photon ionized by the 6.4 eV ArF excimer laser photons. This even/odd alternation in ionization properties of the clusters supports the view that the even clusters have fully paired singlet ground states with no dangling bonds. At higher ArF excimer laser fluences, the observed mass spectrum became increasingly affected by fragmentation. As is true with bulk GaAs surfaces, these GaxAsy clusters evaporate largely by the loss of arsenic (probably As2) when heated by the laser, leaving behind clusters which are richer in gallium.
High-resolution photoionization spectrum of water molecules in a supersonic beam
Authors: Page Ralph H. , Larkin Robert J. , Shen Y. R. , Lee Y. T.
Published: July 2002

We have obtained high-resolution (∼1.5 cm−1) photoionization spectra of supersonically cooled (Trot∼50 K) H2O and D2O in the 1000–900 Å range. The light source, which used the technique of frequency tripling in a pulsed free jet of gas, is described briefly. Spectra are rotationally resolved. Vibrationally excited autoionizing Rydberg series converging to the ground electronic [X̃; (1b1)−1] state of the molecular ion are detected. This may well be the first example of a highly resolved Rydberg spectrum of a stable polyatomic molecule. From the convergence limit, the ionization potential H2O is determined to be 101 777±7 cm−1. Intensities of the Rydberg state autoionization signals are smaller than predicted with known Franck–Condon factors, indicating that predissociation is a competitive decay channel. Rydberg state lifetimes are ∼1 ps, deduced from homogeneous linewidths. Autoionizing features from Rydberg states associated with the ion’s quasilinear Ã (3a1)−1 state are observed with linewidths above 10 cm−1, indicating that their lifetimes are less than ∼0.5 ps. Rotational assignments of some of the bands in this linear←bent transition show that the Rydberg and ionic state geometries are nearly identical. A consistent assignment of the controversial bending (v2) quantum number and Rydberg series quantum defect δ=−0.037 have been provided.
Temperature dependence of structure of liquid carbon tetrachloride measured by pulsed neutron total scattering
Authors: Misawa M.
Published: July 2002

The structure of liquid carbon tetrachloride (CCl4) has been measured at −20, 20, 60, 110, and 160 °C along the liquid–vapor coexisting curve by means of pulsed neutron total scattering technique. Characteristic changes of the structure factor Sm(Q) have been observed with increase of temperature. Experimental results show that orientational correlation still exists even at 160 °C, the highest temperature studied in this work. The observed Sm(Q)’s are discussed in terms of a packing or a number density of effectively uncorrelated molecules and preferred orientations introduced between the nearest neighbors. Experimental facts of broadening of the first peak of Sm(Q) and the shift toward the lower Q side of this peak with an increase of temperature are attributed simply to a decrease of molecular number density, while broadening both of a small second peak located around 2.2 A−1 and a valley following this peak is attributed to a decrease in number and broadening in distribution of correlated molecules.
The influence of adsorbate–absorbate hydrogen bonding in molecular chemisorption: NH3, HF, and H2O on Au(111)
Authors: Kay Bruce D. , Lykke Keith R. , Creighton J. Randall , Ward Stephen J.
Published: July 2002

Molecular beam and temperature programmed desorption (TPD) techniques are used to study the low temperature (85 K) adsorption and subsequent thermal desorption of NH3, HF, and H2O from Au(111). At 85 K the molecular sticking coefficients are near unity and are coverage independent. The TPD spectra are qualitatively different for the various species. Simple hydrogen bonding arguments based on absorbate–absorbate interactions are used to explain the differences in the TPD spectra.
Diffusion of xenon in liquid alkanes: Temperature dependence measurements with a new method. Stokes–Einstein and hard sphere theories
Authors: Pollack Gerald L. , Kennan Richard P. , Himm Jeffrey F. , Stump Daniel R.
Published: July 2002

Measurements are reported of the diffusion constant D(T) for xenon gas, in the form of the radioisotope 133Xe, through liquid n-octane, n-decane, and n-tetradecane, in the range 10–40 °C. The values range from D (10.0 °C, Xe→n-C14H30)=1.32×10−5 cm2/s to D (40.0 °C, Xe→n-C8H18)=6.02×10−5 cm2/s. A new experimental method is used in which D is obtained by monitoring the decrease in concentration as gas diffuses into the liquid in an effectively one-dimensional geometry. As expected, the results do not agree with the Stokes–Einstein law. They do follow the usual correlation Dηp=AT, with p=0.708 and A=9.80×10−8, where η is the liquid viscosity in centipoises and T is in K. Application to these results of the rough-hard-sphere theory of diffusion is discussed. A quantitative analysis cannot be made until molecular dynamics results for smooth-hard-sphere diffusion are available.
Surface density of soluble surfactants at the air/water interface: Adsorption equilibrium studied by second harmonic generation
Authors: Vogel Viola , Mullin C. S. , Shen Y. R. , Kim M. W.
Published: July 2002

Optical second harmonic generation is employed to investigate the adsorption of soluble naphthalene sulfonates from the bulk solution to the air/water interface in the presence of excess counter ions. Both the surface density of surfactant molecules and the surface pressure were measured as functions of surfactant concentration in solution to yield the adsorption isotherm and the surface pressure/area isotherm. The system investigated shows nonideal gas behavior. The ratio of the activity coefficients at the surface and in the bulk is not unity; however both appear to be constant over the concentration range probed.
Molecular spectroscopy and Bayesian spectral analysis—how many lines are there?
Authors: Sivia D. S. , Carlile C. J.
Published: July 2002

We demonstrate the Bayesian spectral analysis approach for analyzing neutron scattering molecular tunneling data. It is a generalized form of model fitting, which is appropriate when the number of parameters to be optimized is not known. Specifically, it addresses the question of how many excitation lines there is evidence for in the data. We review the theory of Bayesian spectral analysis relevant to our particular application, describe an efficient algorithm for its implementation, and illustrate its use with both simulated and real data. We believe that this powerful method of analysis will be a very useful tool in experimental molecular spectroscopy.
Solvent effects on excited-state torsional motion and electronic relaxation of c
i
s-1,3,5-hexatriene
Authors: Ci Xiaopei , Myers Anne B.
Published: July 2002

Resonance Raman spectra of cis-1,3,5-hexatriene, including absolute cross sections, have been obtained in both vapor and solution phases and compared with previous results for the trans isomer. Quantitative modeling of the resonance Raman intensities provides information on the excited-state geometry and dynamics and solvent effects on the excited-state potential surfaces. Cis-hexatriene is nonplanar in both ground and excited states as evidenced by the appearance of out-of-plane fundamentals in the resonance Raman spectra, but the Franck–Condon activity of these modes is weak. Population decay from the lowest allowed singlet state on time scales of approximately 20 fs (cis-hexatriene) and 40 fs (trans-hexatriene) is the major source of diffuseness in the gas-phase absorption spectra of both molecules. Solvation decreases the resonance Raman activity of out-of-plane vibrations, although the effects are smaller in cis-hexatriene than previously observed in the trans isomer. Simulation of the spectra indicates that the allowed excited-state potential surfaces of both isomers undergo an increase in the local force constants for double-bond torsional motion upon solvation in increasingly polarizable environments.
Correlation analysis of complex kinetic systems: A new scheme for utilizing sensitivity coefficients
Authors: Hessler Jan P. , Ogren Paul J.
Published: July 2002

A procedure for designing and evaluating kinetic experiments on complex chemical systems is presented. This procedure uses the temporal integral of products of sensitivity coefficients (which may be calculated by standard methods) (1) to calculate the relative importance of reactions affecting the kinetic behavior of a specific observable and (2) to evaluate the correlation between any of the above reactions. The advantages of this procedure are that it is directly related to the analysis of experimental data, conceptually simple, easy to implement, and it provides a quantitative means for assessing the relative accuracy of different experiments. In addition, it provides a new, efficient, and systematic method for calculating the correlation between reactions and determines how the uncertainties in the rate coefficients of these reactions propagate in an analysis of experimental errors. We illustrate the essential features of the approach by analyzing recent measurements of the rate coefficients for H2+M↔2H+M and H+O2↔OH+O [J. Chem. Phys. 96, 1077 (1992)].
A variational theory of classical solids
Authors: Kang Hong Seok , Ree Francis H.
Published: July 2002

We present a variational theory of classical solids based on an inverse 12th-power repulsive reference system. The reference system is in turn represented by a hard-sphere system and an analytic term which is similar to the term accounting for the softness of the inverse 12th-power repulsion in Ross’s variational theory of fluids. Thermodynamic properties of the Lennard-Jones, exponential-six, and inverse nth-power repulsive (n=4, 6, and 9) systems are calculated for a face-centered cubic phase. At densities slightly above the melting lines to densities where atomic vibrations are nearly harmonic, calculated results are in close agreement with Monte Carlo data performed in this and previous work. For a hexagonal close-packed phase, lattice dynamics calculations are carried out to show that the present variational theory gives reliable results, just as it does for the fcc phase. Comparisons with results from our recent solid-state perturbation theory are also discussed.
Local density component of the Lee–Yang–Parr correlation energy functional
Authors: Lee Chengteh , Sosa Carlos
Published: July 2002

A systematic study of the local density component (LDC) of the Lee–Yang–Parr (LYP) correlation energy functional on several chemical systems is presented. A total of 22 equilibrium geometries, 28 reaction energies, and 22 atomization energies were calculated using the local density component of the Lee–Yang–Parr correlation energy functional (LDC-LYP). The LDC-LYP results were compared with the correlation energy functional of Vosko, Wilk, and Nusair (VWN), that was parametrized using the exact results of the uniform electron gas approximation. The calculations were performed with local density approximation (LDA) optimized Gaussian basis sets of the double-zeta-type plus polarization functions (DZVP2) and the A2 auxiliary basis sets for the density fitting. Comparison with experimental results indicates the geometries and energetics predicted with the LDC-LYP component are in reasonable agreement with those predicted with the VWN approximation for the systems considered. Furthermore, the LDC-LYP+BLYP perturbative approximation is in very good agreement, usually within 2 kcal/mol or less, when compared to the B-LYP self-consistent-field (SCF) approach.
High frequency relaxation of o-terphenyl
Authors: Roland C. M. , Ngai K. L. , Lewis L. J.
Published: July 2002

Results of molecular dynamics simulations (MDS) of o-terphenyl, a glass-forming liquid, are analyzed in terms of the coupling model of relaxation. At low temperatures thermally activated relaxation processes are suppressed, whereby the density–density correlation function, C(t), obtained by MDS is determined entirely by vibrational modes. This enables the low temperature data to be used to deduce the vibrational density of states, g(ω). With g(ω) determined, the vibrational contribution, Cpho(t), is calculated at higher temperatures assuming that g(ω) is independent of temperature. At higher temperatures, relaxation makes its appearance and is modeled here by the fast dynamics of the coupling model. Assuming that vibration and relaxation contribute independently, the density–density self-correlation function is given by the product Cpho(t)Crel(t), with the relaxation part obtained from the coupling model. There is good overall agreement between the calculated C(t) and the MDS data. Microscopic parameters, including the energy barrier for reorientation of the o-terphenyl molecule, are extracted from the MDS results.
Theory of polarization measurements of second-order nonlinear light scattering
Authors: Kauranen Martti , Persoons André
Published: July 2002

We present a complete theory of polarization measurements of second-order nonlinear light scattering in isotropic solutions of nonlinear molecules. The nonlinear interaction between the molecules and input beams at frequencies ω1 and ω2 gives rise to incoherently scattered radiation at frequency ω1+ω2. The fundamental quantity measured by such experiments in the laboratory frame of reference is the quadratic orientational average 〈βiklβjmn*〉 of the hyperpolarizability tensor. The number of independent orientational averages that can be measured is shown to be equal to the number of rotational invariants of sixth rank quantities. The absolute maximum number of independent measurements is 15 and occurs for the most general case in which the hyperpolarizability tensor is complex and the dispersion between all three frequencies is important. This number is reduced to eleven for the case of a real hyperpolarizability tensor and to six and five for the case of hyper-Rayleigh scattering and complex and real tensors, respectively. For the case of planar molecules, these numbers are further reduced to ten, seven, five, and four, respectively. We present explicit expressions that relate the rotational invariants to the components of the hyperpolarizability tensor in the molecular frame of reference. We also present practical measurement schemes that can be used to determine all rotational invariants experimentally and discuss the possibilities and limitations of nonlinear light scattering in determining the values of individual components of the molecular hyperpolarizability tensor.
Ab initio calculations of Ru, Pd, and Ag cluster structure with 55, 135, and 140 atoms
Authors: Jennison D. R. , Schultz P. A. , Sears M. P.
Published: July 2002

A massively parallel ab initio computer code, which uses Gaussian bases, pseudopotentials, and the local density approximation, permits the study of transition-metal systems with literally hundreds of atoms. We present total energies and relaxed geometries for Ru, Pd, and Ag clusters with N=55, 135, and 140 atoms. The N=55 and 135 clusters were chosen because of simultaneous cubo-octahedral (fcc) and icosahedral (icos) subshell closings, and we find icos geometries are preferred. Remarkably large compressions of the central atoms are observed for the icos structures (up to 6% compared with bulk interatomic spacings), while small core compressions (∼1%) are found for the fcc geometry. In contrast, large surface compressive relaxations are found for the fcc clusters (∼2%–3% in average nearest neighbor spacing), while the icos surface displays small compressions (∼1%). Energy differences between icos and fcc are smallest for Pd, and for all systems the single-particle densities of states closely resembles bulk results. Calculations with N=134 suggest slow changes in relative energy with N. Noting that the 135-atom fcc has a much more open surface than the icos, we also compare N=140 icos and fcc, the latter forming an octahedron with close packed facets. These icos and fcc clusters have identical average coordinations and the octahedron is found to be preferred for Ru and Pd but not for Ag. Finally, we compare Harris functional and LDA energy differences on the N=140 clusters, and find fair agreement only for Ag.
Rheological, thermodynamic, and structural studies of linear and branched alkanes under shear
Authors: Khare Rajesh , de Pablo Juan , Yethiraj Arun
Published: July 2002

The rheological, thermodynamic, and structural behavior of linear and branched alkanes in simple shear is investigated using nonequilibrium molecular-dynamics simulation of united atom model fluids. Our results for the zero-shear viscosity of pure linear alkanes as well as mixtures of alkanes are in reasonable agreement with experiment. Simulation results for intermediate molecular weight linear alkanes indicate that the simple models employed here are capable of describing the pressure and temperature dependence of the viscosity. More importantly, our calculations indicate that addition of short, flexible branches to alkanes leads to a viscosity enhancement of a factor of 2 or more, thereby offering interesting possibilities for formulation of lubricants with specific properties.
The ultrafast ground and excited state dynamics of
cis
-hexatriene in cyclohexane
Authors: Pullen Stuart H. , Anderson Neil A. , Walker Larry A. , Sension Roseanne J.
Published: July 2002

One- and two-color kinetics have been combined with broadband ultraviolet transient absorption spectroscopy in the 265–300 nm region to elucidate the photophysics of cis-hexatriene in cyclohexane solvent. The lowest singlet excited state, the 2 1A1 state, is observed to have a lifetime of 200±50 fs. The ground-state hexatriene is produced vibrationally hot. The excess vibrational energy permits ultrafast isomerization around the C–C single bonds in hexatriene. This results in a dynamic equilibrium of the three cis-hexatriene rotamers, which then relaxes multiexponentially to the room-temperature distribution in which the di-s-trans-Z-hexatriene form predominates. The peak of the mono-s-trans (cZt-HT) population is estimated to be ∼50%. Vibrational cooling results in trapping of a small amount, ∼8%, of cZt-HT that relaxes on a much longer time scale as the barrier to isomerization becomes important. An estimate of the absorption spectrum of cZt-HT is deduced from analysis of the spectral data at 50 ps.
Calculating the Keldysh adiabaticity parameter for atomic, diatomic, and polyatomic molecules
Authors: DeWitt Merrick J. , Levis Robert J.
Published: July 2002

A numerical model is presented to determine the Keldysh adiabaticity parameter for the interaction of an intense laser with a polyatomic molecule. The adiabaticity parameter is a guide to determining whether the ionization process is in the field or multiphoton ionization regime. The adiabaticity parameters are compared for potentials including the simple zero-range potential, the Coulomb potential, an atomic potential (Xe), a diatomic (N2) molecular potential, and a polyatomic (C6H6) molecular potential. It is demonstrated that the Coulomb potential is approximately equal to the atomic and diatomic potentials and differs from the zero-range potential employed in the Keldysh model in a way which is predominantly dependent upon the ionization potential. Both simple models substantially overestimate the adiabaticity parameter for C6H6 at all field strengths and at fields above 1.25 V/Å both simple models become completely unphysical. This is because barrier suppression ionization is predicted to occur for benzene above 1.25 V/Å using the ab initio potential. This field for barrier suppression ionization is significantly lower than the 1.5 V/Å field predicted using the Coulomb model.
Water confined in Vycor glass. II. Excluded volume effects on the radial distribution functions
Authors: Soper Alan K. , Bruni Fabio , Ricci Maria Antonietta
Published: July 2002

The results of a recent neutron diffraction experiment on water confined in Vycor glass, reported in the preceding paper [F. Bruni, M. A. Ricci, and A. K. Soper, J. Chem. Phys. 109, 1478 (1998)], are analyzed by attempting to correct for the distortions brought into the site–site radial distribution functions by the presence of regions of the sample where water molecules are not allowed. These so-called excluded volume effects are evaluated through calculation of the radial distribution function of a uniform fluid in the same confinement conditions as our water samples, and are shown to have a strong effect on the relative intensity of the peaks of the site–site distribution functions. The corrected data are compared with corresponding data on bulk water where appropriate, suggesting that confinement in porous Vycor glass strongly affects the orientational arrangements of water molecules, even at room temperature. The general consequences of this analysis for diffraction studies of other confined liquids, as well as water, are discussed.
Reaction of S2 and SO2 with Pd/Rh(111) surfaces: Effects of metal–metal bonding on sulfur poisoning
Authors: Rodriguez José A. , Jirsak Tomas , Chaturvedi Sanjay
Published: July 2002

The surface chemistry of S2 and SO2 on Rh(111), Pd/Rh(111) and polycrystalline Pd has been investigated using synchrotron-based high-resolution photoemission and ab initio self-consistent-field calculations. Pd adatoms lead to an increase in the rate of adsorption of S2 on Rh(111), but they are less reactive than atoms of pure metallic palladium: Rh(111)<Pd/Rh(111)<Pd. The adsorption of sulfur induces a large reduction in the density of states (DOS) near the Fermi level of Pd/Rh(111) surfaces. The decrease in the DOS is smaller than in S/Pd(111) but bigger than in S/Rh(111). The chemistry of SO2 on Rh(111), Pd/Rh(111), and Pd is rich. At 100 K, SO2 adsorbs molecularly on these systems. Above 200 K, the adsorbed SO2 decomposes (SO2,a→Sa+2Oa) or transforms into SO3/SO4 species. The molecular SOx species disappear upon annealing to 450 K and only atomic S and O remain on the surfaces. A Pd monolayer supported on Rh(111) is not very active for the dissociation of SO2. In this respect, the Pd1.0/Rh(111) system is less chemically active than pure Pd or Rh(111). The electronic perturbations associated with the Pd–Rh bonds reduce the electron donor ability of Pd, weakening the interactions between the Pd 4d orbitals and the lowest unoccupied molecular orbitals of S2 and SO2. The behavior of the S2/Pd/Rh(111) and SO2/Pd/Rh(111) systems shows that bimetallic bonding can reduce the reactivity of Pd towards sulfur-containing molecules. A very large drop in reactivity can be expected when Pd is bonded to s,p or early transition metals.
Ab initio study of He2H+ and Ne2H+: Accurate structure and energetics
Authors: Kim Soon Tai , Lee Jae Shin
Published: July 2002

An ab initio investigation employing highly correlated electronic methods and large basis sets has been carried out to determine the equilibrium geometries and binding energies of He2H+ and Ne2H+. The theoretical methods employed include the Moller–Plesset Perturbation Theory (MP2, MP4), and coupled cluster with single and double substitution with noniterative triple excitation [CCSD(T)] ab initio methods with two different type of basis sets, the segmented contracted 6-311++G basis sets with multiple polarization functions, and the correlation consistent aug-cc-pVxZ (x=D,T,Q,5) basis sets. It is found that both species have in common linear symmetric XHX+ equilibrium geometries with dissociation energies of more than 4000 cm−1 to X+XH+ (X=He, Ne). A convergence study comparing the uncorrected and counterpoise (CP) corrected dissociation energies with respect to the complete basis set (CBS) limiting values shows that the CP method generally yields less accurate dissociation energies than the uncorrected ones in both ionic species; a possible explanation is given in terms of differences in geometries between the dissociated and complex state. The dissociation energies for He2H+ to He+HeH+ at MP4/aug-cc-pV5Z and CCSD(T)/aug-cc-pV5Z levels are, in cm−1, 4622.2(4621.7) and 4631.1(4631.7), respectively, with the values in parentheses representing the dissociation energies at the CBS limit. The corresponding dissociation energies for Ne2H+ at MP4/aug-cc-pVQZ and CCSD(T)/aug-cc-pVQZ levels are 5846.9(5746.3) and 5807.1(5703.9), though the estimated CBS limit in this case is less reliable than in the case of He2H+.
Stress distributions in flowing aggregated colloidal suspensions
Authors: Silbert L. E. , Farr R. S. , Melrose J. R. , Ball R. C.
Published: July 2002

Simulations of the flow of concentrated aggregated colloidal systems, at the particulate level, are used to investigate the distribution of stresses in the shear-thinning regime. It is found that the distribution of shear stress carried by interparticle bonds decays approximately exponentially at large stresses, but with a double-exponential distribution for values of positive stress. The microstructural mechanisms associated with large stresses are manifested in clusters which dominate the positive contribution to the stress in the system. Towards the end of shear thinning the highest forces occur along bonds defining rods of particles aligned approximately along the flow-compression direction. We propose that the rheology of such systems is determined by a rupture–reformation process of these clusters of stress concentration during the flow. The aggregation forces play the role of enhancing such stress concentration by stabilizing clusters against buckling.
Singlet and triplet energies of α-oligothiophenes: A spectroscopic, theoretical, and photoacoustic study: Extrapolation to polythiophene
Authors: de Melo J. Seixas , Silva Luı́s M. , Arnaut Luı́s G. , Becker R. S.
Published: July 2002

The triplet state energies, ET(0–0), for the α-oligothiophenes (αn’s) with n=1–7, were determined and vary from 18 000 cm−1 (α2) to 13 000 cm−1 (α7). The data were obtained from spectroscopic and calorimetric measurements. It was found that the triplet energies decrease with the increase ring number (n), leveling off beginning with the α-oligothiophene with n=5. A linear relation was obtained for the triplet energy as a function of the reciprocal of n. The data has allowed the extrapolation to polythiophene (n=∞) and the evaluation of its triplet energy (≅10 000 cm−1). The energies (0–0) of the first singlet excited state of n=1–7 were also evaluated and again a linear relation was found for the E[S1(0–0)] as a function of 1/n. The S1−T1 energy splitting, ΔE(S1−T1), was determined and also found to decrease with the increase in the number ring. Semiempirical molecular orbital theoretical calculations reproduce this same trend of decreasing ΔE(S1−T1) with n. Extended FEMO theory was applied to the singlet and triplet energies as a function of the number of π electrons of the system. The data obtained are discussed regarding the length of the chain needed for an effective conjugation similar to that of the polymer (n=∞) and the geometries of the singlet and triplet states of the α-oligothiophenes.
Time-dependent quantum dynamics study of the Cl+H2 reaction
Authors: Yang Ben-Hui , Gao Hong-Tao , Han Ke-Li , Zhang John Z. H.
Published: July 2002

We report rigorous quantum dynamics studies of the Cl+H2 reaction. The time-dependent wave packet calculations are performed on the semiempirical G3 and the new BW2 potential energy surfaces. Initial state-specific total reaction probabilities and cross sections are calculated and the thermal rate constant is obtained. The quantum dynamics calculations show a striking difference in the energy dependence of reaction probabilities between the G3 and BW2 surfaces, with the former having a smaller reaction threshold. The calculated rate constant on the G3 surface is in good agreement with previous theoretical calculations at room temperature. The calculated rate constant on the new BW2 surface is smaller than that on the G3 surface, and comparison of them with experimental measurement is shown.