bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of hydrogen for the magnetic properties of iron and cobalt in PuNi3 type hydrides Y(Fe1−xCox)2.9Hy
Authors: Kanematsu K. , Ohkubo N. , Itoh K. , Ban S.
Published: July 2002

Magnetization and Mössbauer measurements were performed for PuNi3 type alloys Y(Fe1−xCox)2.9 and their hydrides. The composition dependencies of magnetization, Curie temperature, and average hyperfine field of 57Fe in the alloys show a maximum near x=0.5, and those in hydrides decrease with increasing x. This behavior suggests that the magnetic moments of iron and cobalt are nearly the same, except for cobalt rich alloys. Numerical fitting using a Stoner theory with a parabolic band was performed. It was assumed that the alloys and hydrides are ferromagnets of 3d-band-hole with incomplete saturation. It was shown that both the exchange energy kθ and Fermi energy ε0 decrease with substitution of cobalt for iron, and that the difference in the composition dependencies of kθ/ε0 in alloys and hydrides leads to different composition dependencies of the magnetization and Curie temperature. It was shown that ε0 and kθ decrease with hydrogenation, but kθ/ε0 increases to lead magnetization increase only in YFe3.
Structural, electrical and optical properties of aluminum doped zinc oxide films prepared by radio frequency magnetron sputtering
Authors: Kim Kun Ho , Park Ki Cheol , Ma Dae Young
Published: July 2002

Aluminum doped zinc oxide (AZO) films are prepared by rf magnetron sputtering on glass or Si substrates using specifically designed ZnO targets containing different amount of Al2O3 powder as the Al doping source. The structural, electrical, and optical properties of the AZO films are investigated in terms of the preparation conditions, such as the Al2O3 content in the target, rf power, substrate temperature and working pressure. The crystal structure of the AZO films is hexagonal wurtzite. The orientation, regardless of the Al content, is along the c axis perpendicular to the substrate. The doping concentration in the film is 1.9 at. % for 1 wt % Al2O3 target, 4.0 at. % for 3 wt % Al2O3 target, and 6.2 at. % for 5 wt % Al2O3 target. The resistivity of the AZO film prepared with the 3 wt % Al2O3target is ∼4.7×10−4 Ω cm, and depends mainly on the carrier concentration. The optical transmittance of a 1500-Å-thick film at 550 nm is ∼90%. The optical band gap depends on the Al doping level and on the microstructure of the films, and is in the range of 3.46–3.54 eV. The optical band gap widening is proportional to the one-third power of the carrier concentration.
Stress relaxation and misfit dislocation nucleation in the growth of misfitting films: A molecular dynamics simulation study
Authors: Dong Liang , Schnitker Jurgen , Smith Richard W. , Srolovitz David J.
Published: July 2002

The low-temperature growth and relaxation of misfitting films are analyzed on the basis of two-dimensional molecular dynamics simulations using Lennard–Jones potentials. The temporal evolution of the surface morphology and the mechanisms for misfit dislocation nucleation and stress relaxation are monitored. Pseudomorphic film growth is observed up to a critical thickness. In some cases, the formation of voids within the film relaxes some of the stress. At the critical thickness, dislocations nucleate and relax most of the misfit. The critical thickness increases with decreasing lattice mismatch and depends on the sign of the misfit. The critical thickness of compressively strained films is smaller than that of films with the same magnitude of misfit, but in tension. The mechanism of dislocation nucleation is different in tension and compression and, in all cases, is associated with the roughness of the film surface. In the compressive misfit case, dislocations nucleate by squeezing-out an atom at the base of surface depressions. In the tensile misfit case, however, the nucleation of misfit dislocations involves the concerted motion of a relatively large number of atoms, leading to insertion of an extra lattice (plane) row into an already continuous film. These results show that the critical thickness depends intimately on the film morphology which, in turn, is determined as an integral part of the film growth process.
Effect of atomic bond structure on crystallographic orientation dependence of carbon doping in GaAs
Authors: Park Young K. , Son Chang-Sik , Kim Seong-Il , Kim Yong , Kim Eun Kyu , Min Suk-Ki , Choi In-Hoon
Published: July 2002

The effect of atomic bond structure at the deposition surface on the crystallographic orientation dependence of carbon doping in GaAs was studied. Carbon doping into GaAs epilayers was performed by atmospheric pressure metalorganic chemical vapor deposition using extrinsic carbon sources of carbon tetrachloride (CCl4) and carbon tetrabromide (CBr4). Epitaxial growths were done on the exact (100) and four different misoriented GaAs substrates with orientations of (511)A, (311)A, (211)A, and (111)A. The electrical properties were measured by van der Pauw Hall analysis at room temperature. Electrically active concentrations in excess of 1×1019 cm−3 were obtained so that CCl4 and CBr4 were demonstrated as efficient p-type dopant sources for carbon doping into GaAs. The dependence of hole concentration on the offset angle of CCl4-doped and CBr4-doped GaAs shows the same tendency, whereas the trend of carbon doping from intrinsic carbon doping technique is different from our results. In particular, the hole concentration on the (100) surface is higher than that on the (111)A surface in the cases of CCl4 and CBr4. The result is opposite to that of the intrinsic doping case. A model based on the atomic bond structure of an adsorption site of carbon-containing species is proposed to explain the difference.
Direct evaluation of contact injection efficiency into small molecule based transport layers: Influence of extrinsic factors
Authors: Abkowitz M. , Facci J. S. , Rehm J.
Published: July 2002

Studies of interface formation on conventional semiconductor materials are typically carried out under relatively pristine conditions. However, for devices based on the use of electronic polymers there is also compelling interest in exploring the variations in contact behavior that might result under realistic manufacturing conditions like multilayer device assembly based on solution coating technology. Small molecule doped polymers (MDPs) developed principally as large area coatings for electrophotographic use are now finding wider device applications. These polymers are insulators capable of transporting excess injected charge with a unipolar drift mobility which can be tuned over a wide range by varying the concentration of transport active species. Most significant in the present context, MDPs can be rendered trap free by molecular design. These unique characteristics of MDPs make it possible to analyze the relative injection efficiencies of their interfaces with various contacts simply by a direct comparison of current–voltage characteristics with time of flight drift mobility measurements carried out on the same film coatings. In this way, and apart from their intrinsic interest and practical value, MDPs and closely related polymeric media provide the ideal venue for the study of contact phenomena on molecular solids. Almost all the present measurements were carried out by measuring dark hole injection from various preformed metal substrates into the MDP film TPD/polycarbonate. Under these circumstances it was found that while injection efficiency nominally scaled with the estimated interfacial energy step there was significant variance that in some cases could be clearly associated with the specific details of interfacial chemistry. For one exceptional case where Au was evaporated on the free surface of an already cast film a time and temperature dependent contact forming process could be delineated in which the interface systematically evolved from emission limited to ohmic.
Effective conductivity, dielectric constant, and diffusion coefficient of digitized composite media via first-passage-time equations
Authors: Torquato Salvatore , Kim In Chan , Cule Dinko
Published: July 2002

We generalize the Brownian motion simulation method of Kim and Torquato [J. Appl. Phys. 68, 3892 (1990)] to compute the effective conductivity, dielectric constant and diffusion coefficient of digitized composite media. This is accomplished by first generalizing the first-passage-time equations to treat first-passage regions of arbitrary shape. We then develop the appropriate first-passage-time equations for digitized media: first-passage squares in two dimensions and first-passage cubes in three dimensions. A severe test case to prove the accuracy of the method is the two-phase periodic checkerboard in which conduction, for sufficiently large phase contrasts, is dominated by corners that join two conducting-phase pixels. Conventional numerical techniques (such as finite differences or elements) do not accurately capture the local fields here for reasonable grid resolution and hence lead to inaccurate estimates of the effective conductivity. By contrast, we show that our algorithm yields accurate estimates of the effective conductivity of the periodic checkerboard for widely different phase conductivities. Finally, we illustrate our method by computing the effective conductivity of the random checkerboard for a wide range of volume fractions and several phase contrast ratios. These results always lie within rigorous four-point bounds on the effective conductivity.
Determination of the complex dielectric function of epitaxial SrTiO3 films using transmission electron energy-loss spectroscopy
Authors: Ryen L. , Wang X. , Helmersson U. , Olsson E.
Published: July 2002

We have demonstrated that electron energy-loss spectroscopy in transmission electron microscopy is a useful technique for analysis of the high-frequency dielectric properties on a microstructural level. Compositional variations and interfacial elastic strain of epitaxial (001) SrTiO3 thin films had a small effect on the imaginary part, ε2, of the complex dielectric function, εr(E). Changes in the low-frequency dielectric function, between Sr deficient and stoichiometric films, were much larger compared to the changes in the high-frequency function. Improved film stoichiometry and absence of strain caused the absolute value of ε2 to approach that of a stoichiometric single crystal bulk sample. Simultaneously, the low-frequency dielectric function of the films improved. The electronic polarizability, αe, was also extracted from the dielectric function. Indications were found that strain and compositional deviations from the bulk value, resulted in decreased ability to withstand dielectric breakdown.
Modeling the optical constants of HgxCd1−xTe alloys in the 1.5–6.0 eV range
Authors: Djurišić Aleksandra B. , Li E. Herbert
Published: July 2002

The optical constants of HgxCd1−xTe as a function of energy and composition x are modeled over a wide spectral range from 1.5 to 6 eV. The model employed represents an extension of Adachi’s model and incorporates the adjustable broadening function rather than the conventional Lorentzian one. In this way, greater flexibility of the model is achieved, enabling us to obtain an excellent agreement with the experimental data. The relative rms errors obtained for all compositions are below 2.5% for the real part and below 6% for the imaginary part of the index of refraction. The lowest rms errors are obtained for x=0 (0.6% for the real part and 0.7% for the imaginary part of the index of refraction), and the highest for the x=0.91 (2.4% for the real part and 5.8% for the imaginary part).
Piezoelectric Franz–Keldysh effect in strained GaInN/GaN heterostructures
Authors: Wetzel C. , Takeuchi T. , Amano H. , Akasaki I.
Published: July 2002

Pseudomorphic Ga1−xInxN/GaN single heterostructures in the composition range 0<x<0.2 have been investigated by photoreflectance and photoluminescence spectroscopy. Strong Franz–Keldysh oscillations near the band gap of the ternary film are observed and attributed to a large constant piezoelectric field of up to 0.63 MV/cm. This allows an accurate determination of the electric field. A significant redshift between the optical band gap from photoreflectance and the luminescence maximum is observed. Luminescence is proposed to originate in the indirect transitions between the electric field tilted band edges in GaInN. The presence of this field is expected to dominate the bandstructure and the recombination and transport processes in strained nitride structures. We find no evidence for large inhomogeneities or phase separation in this material.
Semi-insulating semiconductor heterostructures: Optoelectronic properties and applications
Authors: Nolte David D.
Published: July 2002

This review covers a spectrum of optoelectronic properties of and uses for semi-insulating semiconductor heterostructures and thin films, including epilayers and quantum wells. Compensation by doping, implantation, and nonstoichiometric growth are described in terms of the properties of point defects and Fermi level stabilization and pinning. The principal optical and optoelectronic properties of semi-insulating epilayers and heterostructures, such as excitonic electroabsorption of quantum-confined excitons, are described, in addition to optical absorption by metallic or semimetallic precipitates in these layers. Low-temperature grown quantum wells that have an arsenic-rich nonstoichiometry and a supersaturated concentration of grown-in vacancies are discussed. These heterostructures experience transient enhanced diffusion and superlattice disordering. The review discusses the performance of optoelectronic heterostructures and microcavities that contain semi-insulating layers, such as buried heterostructure stripe lasers, vertical cavity surface emitting lasers, and optical electroabsorption modulators. Short time-scale applications arise from the ultrashort carrier lifetimes in semi-insulating materials, such as in photoconductors for terahertz generation, and in saturable absorbers for mode-locking solid state lasers. This review also comprehensively describes the properties and applications of photorefractive heterostructures. The low dark-carrier concentrations of semi-insulating heterostructures make these materials highly sensitive as dynamic holographic thin films that are useful for adaptive optics applications. The high mobilities of free carriers in photorefractive heterostructures produce fast dielectric relaxation rates that allow light-induced space-charge gratings to adapt to rapidly varying optical fringe patterns, canceling out environmental noise during interferometric detection in laser-based ultrasound, and in optical coherence tomography. They are also the functional layers in high-sensitivity dynamic holographic materials that replace static holograms in Fourier imaging systems and in experimental Tbit/s optical systems. Semi-insulating heterostructures and their applications have attained a degree of maturity, but many critical materials science issues remain unexplored.
Infrared properties of AgGaTe2, a nonlinear optical chalcopyrite semiconductor
Authors: Ohmer M. C. , Goldstein J. T. , Zelmon D. E. , Saxler A. W. , Hegde S. M. , Wolf J. D. , Schunemann P. G. , Pollak T. M.
Published: July 2002

The fundamental optical properties of AgGaTe2, a nonlinear optical semiconductor are reported. These properties include birefringence, indices of refraction, infrared transmission, and the temperature dependence of the band gap. The average index for wavelengths greater than several microns was found to be 3.0. The birefringence was found to be rather large and to range from a near band edge value of 0.038 at 1.3 μm to a value of 0.017 at 15 μm. Additionally, native defect related sub-bandgap absorption, photoluminescence, and electrical transport have been studied in these nominally undoped p-type crystals. An activation energy associated with these defects was determined to be 0.3 eV and the corresponding photoluminescence and absorption data showed, respectively, a broad asymmetric emission band centered at 0.8 eV and two bands at 0.95 and 1.01 eV, the absorption band at 0.95 eV being the most intense. The measured properties were utilized to assess the potential of AgGaTe2 for the wavelength conversion processes of second-harmonic generation and degenerate optical parametric oscillation. It was found that AgGaTe2 will not phase match at room temperature. However, it is estimated that mixed crystals of the form AgGa(Se(1−x))Tex)2 for a Te addition of 19% can exceed the conversion efficiency of AgGaSe2 by more than 100%.
Effect of nitrogen addition on the microstructure and mechanical properties of diamond films grown using high-methane concentrations
Authors: Catledge Shane A. , Vohra Yogesh K.
Published: July 2002

We report on the microstructure and mechanical properties of diamond films grown using varying nitrogen additions to a plasma with a high-CH4 fraction of 15% (in hydrogen) and an operating pressure of 125 Torr. Films were grown at N2/CH4 ratios ranging from 0 to 0.30 by fixing the CH4 flow rate and changing only the N2 flow rate. With increasing nitrogen addition, we observe an increase in intensity and a decrease in the full width at half maximum (FWHM) of the Raman band at 1550 cm−1, while the crystalline diamond peak at 1332 cm−1 decreases in intensity and increases in the FWHM. X-ray diffraction confirms that the film crystallinity and diamond grain size decrease rapidly with increasing nitrogen additions up to a N2/CH4 ratio of 0.10, but then do not change significantly above this ratio. A similar trend is observed for film surface roughness. In addition, we find from indentation testing that all films exhibit high hardness values ranging from 70 to 90 GPa and that the toughness of the films improves with increasing nitrogen addition. Optical emission spectroscopy reveals that an increase in CN species relative to C2 in the plasma is responsible for the formation of tetrahedral amorphous carbon (indicated by the Raman band at 1550 cm−1).
Hydrogen dynamics in the hydrides of Pr2Fe17 as revealed by Mössbauer spectroscopy
Authors: Hautot Dimitri , Long Gary J. , Grandjean F. , Isnard O. , Miraglia S.
Published: July 2002

The rhombohedral Pr2Fe17Hx compounds with the Th2Zn17 structure have been prepared for x=0–5. Their lattice parameters and Curie temperatures have been determined from powder x-ray diffraction and thermomagnetic measurements, respectively, and their Mössbauer spectra have been measured between 4.2 and 295 K. The Mössbauer spectra for x=0, 1, and 2, obtained between 4.2 and 295 K, and those of Pr2Fe17H3, obtained above 90 K, have been analyzed with a seven sextet model, indicative of a basal magnetization in these compounds. The Mössbauer spectra of Pr2Fe17H3 below 90 K, of Pr2Fe17H4 between 4.2 and 295 K, and of Pr2Fe17H5 above 155 K, have been analyzed with a four sextet model, indicative of an axial magnetization in these compounds over the indicated temperature ranges. The axial magnetic anisotropy results from a combination of lattice expansion upon hydrogenation and contraction upon cooling, and the relative importance of the praseodymium Stevens coefficients. A magnetic phase diagram for the Pr2Fe17Hx compounds is proposed on the basis of their magnetic Mössbauer spectra. The Mössbauer spectra of Pr2Fe17H5 indicate that, above 155 K, the two hydrogen atoms occupying one third of the tetrahedral 18g sites are rapidly jumping, on the Mössbauer time scale of 100 ns, between the six available 18g positions, a jumping which slows down or ceases below 155 K. The compositional dependence of the hyperfine parameters of the Pr2Fe17Hx compounds indicates an initial filling of the interstitial 9e sites by the first three hydrogen atoms and then a subsequent filling of the interstitial 18g sites by the last two hydrogen atoms.
Plasma deposition of low-dielectric-constant fluorinated amorphous carbon
Authors: Endo Kazuhiko , Shinoda Keisuke , Tatsumi Toru
Published: July 2002

Fluorinated amorphous carbon thin films (a-C:F) for use as low-dielectric-constant interlayer dielectrics are deposited by helicon-wave plasma enhanced chemical vapor deposition. To improve their thermal stability, the feasibility of adjusting the fluorine-to-carbon (F/C) ratio by changing the deposition pressure was investigated. Decreasing the pressure increased the dissociation of a source fluorocarbon material in the plasma and decreased the F/C ratio of the deposited film. Both the thermal stability and the dielectric constant of the a-C:F films were increased as the F/C ratio was decreased. Thus, there is a tradeoff relationship between a low dielectric constant and high thermal stability and the tradeoff could be optimized by the pressure during deposition. The mechanism of the pressure dependency of the dielectric constant of a-C:F films was investigated by quantifying the contribution of each polarization and found that a decrease in the dielectric constant of a-C:F films can be attributed to decreases in the orientational and electronic polarizations.
Lithium ion migration pathways in LiTi2(PO4)3 and related materials
Authors: Nuspl Gerhard , Takeuchi Tomonari , Weiß Armin , Kageyama Hiroyuki , Yoshizawa Kazunari , Yamabe Tokio
Published: July 2002

The diffusion pathways of Li+ ions within the frameworks of the high conductive solid electrolytes LiTi2(PO4)3 and the Al-doped one Li1.3Ti1.7Al0.3P3O12 are determined by molecular mechanics and molecular dynamics methods. From a potential energy profile for the lithium migration in LiTi2(PO4)3, the barrier height of lithium migration is estimated as 28.95 kJ/mol (0.30 eV) which is in excellent agreement with experimental data. The main influence on the activation energy of lithium diffusion is caused by changes in electrostatic interactions. Long molecular dynamics simulations for Li1.3Ti1.7Al0.3P3O12 also confirm a solid state diffusion process via rare and sporadic hopping of Li+ ions from their energy minimum site to a neighboring energy minimum site.
Thermal stability of plasma deposited thin films of hydrogenated carbon–nitrogen alloys
Authors: Anguita J. V. , Silva S. R. P. , Burden A. P. , Sealy B. J. , Haq S. , Hebbron M. , Sturland I. , Pritchard A.
Published: July 2002

The need to grow high quality semiconducting hydrogenated amorphous carbon (a-C:H) thin films to allow n-type electronic doping by nitrogenation has lead us to deposit films with low paramagnetic defect density (1017 cm−3). The films were grown on the earthed electrode of a radio frequency driven plasma enhanced chemical vapor deposition system using methane, helium and a range of nitrogen concentrations as the precursor gases. The deposited films are shown to be polymer like. Changes in the chemical structure and relative bond fractions as a function of the nitrogen flow rate into the plasma chamber and ex situ annealing are reported. Particular attention is paid to changes in the film structure after annealing at 100 °C, since an increase in the E04 optical band gap is observed as a function of nitrogen flow after the anneal. This suggests a decrease in the defect density of the film.
Young’s fringes from vertically integrated slab waveguides: Applications to humidity sensing
Authors: Cross Graham H. , Ren Yitao , Freeman Neville J.
Published: July 2002

Using a multiple layer optical waveguide system consisting of two vertically slab waveguides, classical Young’s fringes may be obtained in the far-field diffraction plane. In agreement with the simple theory of diffraction interference the spacing of the far-field fringes is easily observed on mm to cm dimensions without further transformation of the output light. The simple methods of fabrication and means of optical coupling should provide a readily adaptable method for examining the principles of interferometry in an integrated optical format. The structure acts to transform polarized incident plane wave input light into separate slab modes of the device which emerge as two closely spaced and coherent sources at the output. The elements required for a classical Young’s fringe demonstration are therefore all embodied in this approach. The basic concept can be applied to an optical method for sensing. In one example of this we demonstrate measurement of the phase difference induced between the upper and lower propagating modes in structures due to water vapor diffusion into the layers which are formed from hydrophilic polymers. The Young’s fringe patterns exhibit a spatial intensity distribution which is sensitive to water vapor introduced over the surface of the structure. Differences in the effective index between the modes of the two waveguides during the diffusion of the vapor causes phase shifts which result in redistribution in the fringe pattern. The anticipated limit of detection of these devices is lower than 1 ppm for water vapor.
Piezoelectric contributions to the electrical behavior of ZnO varistors
Authors: Verghese P. M. , Clarke D. R.
Published: July 2002

Internal stresses exist in ZnO varistors as a result of both crystallographic anisotropy in thermal expansion of the constituents and thermal expansion mismatch between phases. Due to the piezoelectric nature of ZnO, these stresses induce a net electric dipole moment that modifies the grain boundary Schottky barriers and causes an alteration of the varistor current–voltage response in the nonlinear regime. We report Raman piezospectroscopic measurements of residual strains in polycrystalline ZnO and develop a stochastic model for the distribution of potential barrier heights based on the distribution of internal stresses. The model provides a physical basis for barrier height distributions used in electrical network simulations of varistor transport behavior.
Growth kinetics of SiO2 on (001) Si catalyzed by Cu3Si at elevated temperatures
Authors: Huang H. Y. , Chen L. J.
Published: July 2002

The oxidation of Si catalyzed by 170-nm-thick Cu3Si at elevated temperatures has been investigated by transmission electron microscopy, glancing angle x-ray diffraction, and Auger electron spectroscopy. For wet oxidation at 140–180 °C, the thickness of the oxide was found to increase parabolically with time with an activation energy of 0.4±0.2 eV. The activation energy is close to that of diffusivity of Cu in Si. At 180–200 °C, the growth rate became slower with increasing temperature. The growth of oxide tended to be discontinuous at the surface as the oxidation temperature was increased to a temperature at or higher than 300 °C. The anomalously fast growth of oxide at low temperatures is attributed to the presence of filamentary structures of Cu clusters in the oxide to expedite the diffusion of the oxidants through oxide. At 200–250 °C, more Cu atoms diffuse to the Cu3Si/Si interface and less Cu atoms stay in the oxide, which slows down the oxide growth. The lack of filamentary structures of Cu as diffusion paths retards the growth of SiO2. At 300 °C or higher temperatures, the lack of filamentary structures of Cu clusters stopped the growth of continuous oxide layer altogether.
The equilibrium low-temperature structure of ice
Authors: Leadbetter A. J. , Ward R. C. , Clark J. W. , Tucker P. A. , Matsuo T. , Suga H.
Published: July 2002

Following the discovery of a first-order phase transition in annealed KOH-doped ice at 72 K, which was identified as the order–disorder transition associated with the proton positions, a structural study has now been made. Powder neutron diffraction measurements on KOD-doped D2O above and below the phase transition, together with x-ray diffraction measurements, reveal a partial ordering of the hydrogen atoms at low temperatures. The equilibrium structure of ice at low tempertures has orthorhombic symmetry, space group Cmc21, with the same lattice as the high-temperature Ih (P63/mmc) modification. The structure is polar and there is evidence that the ordered domains are less than about 40 Å in dimension.
Water–water and water–ion potential functions including terms for many body effects
Authors: Lybrand Terry P. , Kollman Peter A.
Published: July 2002

We have developed potential functions for water–water and water–ion interactions that include terms to model nonadditive or many body effects. Using these potential functions, we have obtained good agreement with experimental results for gas phase and condensed phase water calculations and excellent results for gas phase ion hydration enthalpies. Some gas phase ion–water cluster display unusual geometries such as the sodium complex with six waters, where four water molecules bind directly to sodium and the two remaining water molecules form hydrogen bonds with the first four water molecules. The chloride complex with four waters is also interesting in that all four waters cluster on the same side of the anion and form weak hydrogen bonds with each other in addition to the linear hydrogen bond each forms with the anion. We have also used the potentials to estimate solvation enthalpies and coordination numbers for several ionic species Na+, K+, Mg++, and Cl−, again obtaining good overall agreement with experimental data. We feel these results demonstrate the superiority of many body potential functions over pair potentials for some applications problems.
Molecular beam infrared spectra of dimers formed from acetylene, methyl acetylene, and ethene as a function of source pressure and concentration
Authors: Fischer Gad , Miller R. E. , Vohralik P. F. , Watts R. O.
Published: July 2002

Infrared spectra have been obtained for dimers formed from acetylene, methyl acetylene, and ethylene using a wide variety of free jet source conditions. An F-center laser was used to excite various C–H stretch fundamentals in the range 3000–3300 cm−1. Rotational structure was resolved in the spectra of both acetylene and methyl acetylene dimers when dilute mixtures of either gas in helium were expanded from high pressures. The spectrum of the ethylene dimer showed no such structure even when formed in very dilute mixtures. These results contradict previous suggestions that the vibrational relaxation lifetime is uniformly short for all polyatomic dimers.
Study of the kinetic energy density functional in the locally linear potential approximation
Authors: Ghosh Swapan K. , Balbás Luis C.
Published: July 2002

A kinetic energy density functional based on a locally linearized approximation for the potential has been investigated through calculation using accurate Hartree–Fock densities for several atoms. The integrated and the local values of this kinetic energy density as well as the local behavior of its functional derivative are compared with corresponding quantities for Hartree–Fock and other existing kinetic energy functionals including a newly suggested one. With an N-dependent correction factor, this functional shows better agreement.
Dissociative and double photoionization cross sections of NO from threshold to 120 Å
Authors: Samson James A. R. , Masuoka T. , Pareek P. N.
Published: July 2002

The partial photoionization cross sections for producing the NO+ parent ion and the O+, N+, and NO2+ fragment ions from neutral NO are presented from 120 to 614 Å. The results indicate predissociation of the c 3Π (21.72 eV) and B′ 1Σ+ (22.73 eV) electronic states of NO+. The photoionization threshold for double ionization was found to be 39.4±0.12 eV.
The rotational spectrum, barrier to internal rotation, and structure of NH3–N2O
Authors: Fraser G. T. , Nelson D. D. , Gerfen G. J. , Klemperer W.
Published: July 2002

The rotational spectrum of NH3–N2O has been observed by the molecular beam electric resonance method. The spectrum is characteristic of a T-shaped complex in which the nitrogen of the NH3 subunit is directed toward the N2O subunit. The NH3 unit exhibits nearly free internal rotation about its C3 axis. The A internal rotor state transitions were fit to Watson’s asymmetrical top Hamiltonian and the following spectroscopic constants were determined: A(MHz)=12 722.5(5), δK (MHz)=0.22(2), B(MHz)=4 083.5(2), δJ(MHz) =0.007(1), C(MHz)=3 070.8(2), ΔK(MHz) =−0.3(2), ΔJ(MHz) =0.016(9), ΔJK(MHz) =0.32(3), μa(D) =1.514(9), μb(D) =0.09(9). The E internal rotor state transitions were fit to the Hamiltonian of Kilb, Lin, and Wilson. For the E internal rotor state the rotational constants A [12 743(64) MHz], B[4077.1(12) MHz], and C[3069.9(9) MHz] were determined, as well as the height of the barrier to internal rotation, V3 [12.5(25) cm−1]. The distance between the nitrogen of the NH3 to the center of mass of N2O is 3.088 Å. The angle between the C3 axis of NH3 and the line joining the centers of mass of the NH3 and N2O subunits is 13°. The C3 axis of NH3 is pointed towards the nitrogen end of the N2O subunit.
Hypernetted chain approximation for the distribution of ions around a cylindrical electrode. II. Numerical solution for a model cylindrical polyelectrolyte
Authors: Gonzales-Tovar Enrique , Lozada-Cassou Marcelo , Henderson Douglas
Published: July 2002

The structure of the electrical double layer associated with a cylindrical polyelectrolyte is studied through a simple charged hard sphere/charged hard cylinder model in which the diameter of the ions in the solution is considered. The hypernetted chain (HNC) equation (HNC/MSA version) is established and solved numerically giving the ionic distribution around the polyelectrolyte. Calculations are made for 1-1 and 2-2 electrolytes for various values of the concentration, ionic diameter, and polyelectrolyte radius and electrical charge. Using these ionic distributions, excess charge adsorption isotherms, zeta potentials, and mobilities are calculated. These quantities are compared with results of the nonlinear Poisson–Boltzmann (PB) equation and, in the case of the mobility, with electrophoresis measurements for double-stranded DNA. Important quantitative and qualitative differences between the PB and HNC results are found. The HNC zeta potential is found to be nonmontonic function of various parameters, for example, the charge density. The HNC mobilities are in better agreement with experiment than are the PB results. For the concentrations considered here, no significance is found for any value of the reduced linear charge density parameter.
A quantum reactive scattering study of Mu+H2→MuH+H
Authors: Schatz George C.
Published: July 2002

The results of quantum coupled states (CS) reactive scattering calculations on the reaction Mu+H2→MuH+H are presented and used to determine reactive cross sections and rate constants. The potential surface used is the chemically accurate Liu–Siegbahn–Truhlar–Horowitz surface, and the calculations are believed to be accurate to about 30%. The reaction probabilities show threshold shifts relative to collinear exact probabilities which are consistent with bending zero point energy effects. Likewise, the cross section thresholds are shifted relative to H+H2 cross sections by amounts which can be correlated with adiabatic barrier zero point energies. The resulting thermal rate constants are in excellent agreement with experiment (within the experimental error bars at all but one temperature between 608 and 845 K) and the corresponding activation energies agree to within 0.8 kcal/mol. The CS rate constants are also found to agree very well at high temperatures with variational transition state theory rate constants (ICVT/LAG based on WKB adiabatic potentials), with differences of 0% at 875 K and 31% at 608 K. However, the ICVT/LAG rate constants are too low at lower temperatures (by a factor of 3.8 at 300 K). Quasiclassical trajectory rate constants are found to be in error by orders of magnitude if applied in the standard (forward) direction, but are off by less than a factor of 2 when applied in the reverse direction. The overall good agreement of CS with experiment indicates the adequacy of the LSTH potential surface for a reaction where the bottleneck is well into the product channel region and at relatively high energy.
A close-coupling infinite order sudden approximation (IOSA) to study vibrational predissociation of the HeI2 van der Waals molecule
Authors: Delgado-Barrio G. , Mareca P. , Villarreal P. , Cortina A. M. , Miret-Artés S.
Published: July 2002

We apply in this paper a model related to the ‘‘infinite order sudden approximation’’ to treat the vibrational predissociation of the HeI2 van der Waals molecule. For each configuration, the stretching motions within the complex are exactly solved in the close-coupling formalism. The bending motion is then considered in an approximate way and averaged predissociation rates are obtained. Our results are compared with the experiment and also with previous models.
The O(4) wave functions in the vibron model for diatomic molecules
Authors: Frank A. , Lemus R.
Published: July 2002

We present explicit expressions for the wave functions in the O(4) dynamical symmetry of the vibron model for diatomic molecules, both in terms of bosonic operators and coordinates. All matrix elements of operators in the model can be evaluated in closed form using simple recurrence relations. We discuss the relevance of these results for the description of molecular rotation vibration spectra.
Maximum entropy analysis of quasielastic light scattering from colloidal dispersions
Authors: Livesey A. K. , Licinio P. , Delaye M.
Published: July 2002

The maximum entropy method (MEM) is used to analyze quasielastic light scattering correlation functions. The central problem, in such data analysis, is the recovery of the distribution of exponentials describing the decay of the correlation function (i.e., inverting the Laplace transform). MEM is shown to give a high-quality reconstruction of these distributions. The success of the method is illustrated with results from computer-generated ‘‘noisy’’ data and with experimental data from colloidal dispersions. Both the theoretical properties, and practical advantages of MEM are presented and compared with current data analysis techniques.
Inverted perturbation approach (IPA) potentials and adiabatic corrections of the X 1Σ+ state of the lithium hydrides near the dissociation limits
Authors: Chan Y. C. , Harding D. R. , Stwalley W. C. , Vidal C. R.
Published: July 2002

Laser-induced fluorescence (LIF) spectra of the A–X system of the LiH isotopes, excited by the UV lines of an argon and a krypton ion laser, were observed and analyzed. Three long LIF series of different LiH isotopes were taken with a 3.4 m high resolution spectrograph. The results were combined with previous experimental data to study the breakdown of the Born–Oppenheimer approximation in the X 1Σ+ electronic ground state of the isotopic LiH molecules. Using a quantum mechanical variational method, potential energy curves of the four LiH isotopes were extended almost up to the dissociation limits.
Percolation behavior of the square-well fluid
Authors: Netemeyer S. C. , Glandt E. D.
Published: July 2002

A numerical procedure is used to solve the connectedness Ornstein–Zernike equation in the Percus–Yevick approximation for the square-well fluid system. The pair-connectedness function, direct connectedness function, and average cluster size are calculated in order to locate the percolation transition from scaling law behavior.
Production and characterization of some mixed Group V A dimers in solid neon
Authors: Rasanen M. , Heimbrook L. A. , Schwartz G. P. , Bondybey V. E.
Published: July 2002

Vaporization of ternary III–V mixtures is shown to provide a convenient source of the Group V heteronuclear diatomic molecules. Several such mixtures were vaporized and the vapors condensed in neon matrix and examined by time resolved, laser induced fluorescence spectroscopy. The excitation spectra of AsP and SbP dimers were examined and several new excited electronic transitions were observed and analyzed. The assignment of the new states is discussed and the electronic structure of these molecules reviewed and summarized.
Measurements of the absolute Raman cross sections of benzene
Authors: Schomacker K. T. , Delaney J. K. , Champion P. M.
Published: July 2002

The absolute Raman cross sections of the 992 cm−1 (ring breathing) and 3060 cm−1 (C–H stretching) modes of benzene are presented at excitation wavelengths of 514.5, 488.0, 441.6, and 325.0 nm. The technique employs a straightforward determination of the total (unrestricted energy bandwidth) scattering cross section along with a simple relative measurement. The effect of the dispersion of the local field correction (via the frequency dependence of the index of refraction) is found to significantly affect the preresonance benzene cross sections and can distort the normally employed ‘‘A-term’’ analysis. When the local field effect is incorporated into the data analysis, the results involving neat benzene and previous studies involving 1% benzene in acetonitrile are brought into reasonable agreement. The ‘‘n2’’ solid angle correction is also verified experimentally using air, water, and benzene as test substances. The preresonance enhancement of the 992 cm−1 mode is found to be consistent with coupling to electronic excitations centered at ∼80 kK which presumably involve the π-electron system. In contrast, the 3060 cm−1 mode is coupled to electronic states centered in the deep ultraviolet, at ∼150 kK, which presumably involve the more energetic σ-electron excitations.
Energy dependence, kinetic isotope effects, and thermochemistry of the nearly thermoneutral reactions N+(3P)+H2(HD,D2)→NH+(ND+)+H(D)
Authors: Ervin Kent M. , Armentrout P. B.
Published: July 2002

The reactions of N+(3P) ions with H2, HD, D2 are examined using guided ion beam tandem mass spectroscopy. Absolute reaction cross sections are measured from near thermal energies to 30 eV relative energy. The low energy cross section behavior is analyzed using empirical threshold models and phase space theory. The results are compared to other recent studies of the N++H2 system. The reaction endothermicity for N+(3P)+H2→NH++H, ΔH○0 =0.033±0.024 eV (0.76±0.55 kcal/mol), and the bond energy of NH+, D○0(N–H+) =3.51±0.03 eV (80.9±0.6 kcal/mol), are derived from the results.
Monte Carlo trajectory and master equation simulation of the nonequilibrium dissociation rate coefficient for Ar+H2→Ar+2H at 4500 K
Authors: Haug Kenneth , Truhlar Donald G. , Blais Normand C.
Published: July 2002

Thermally averaged rate coefficients for state-to-state rovibrational transitions and dissociation from individual states in Ar+para-H2 collisions at 4500 K are derived from Monte Carlo quasiclassical trajectory calculations. The problem of multidimensional interpolation of state-to-state rate constants is discussed, and the rate matrix is completed by nonlinear least-squares fitting. The relaxation time, the induction time, and the steady dissociation rate are calculated by a matrix eigenvalue solution of the master equation simulating a shock wave experiment on para-H2 dilute in Ar. Rotational–vibrational nonequilibrium effects are fully included. We have found that multiquantum transitions play a very significant role in determining the observable dissociation rate and that the master equation eigenmodes representing internal energy redistribution are little affected by the reaction process. The final steady dissociation rate is 2.8 times less than at local equilibrium. Various lumping schemes are tested, and we show that the original 162-state system can be well approximated by a 10-state model which predicts a factor of 2.4 for the nonequilibrium effect. However, we are unable to find equally successful models involving smaller numbers of states, and the popular vibrational ladder model and the corresponding rotational ladder model do not reproduce the full results even qualitatively.
The nature of the bonding of CN to metals and organic molecules
Authors: Nelin C. J. , Bagus P. S. , Philpott M. R.
Published: July 2002

The nature of the bonding of the cyanide ligand to a metal atom and an organic group was examined through a study of the model systems CuCN, CuNC, CH3CN, and CH3NC. These particular molecules encompass extremes of bonding. Three properties of the wave functions are used to characterize the bond: (1) projection of the CN orbitals from the wave function of the complete system; (2) the dipole moment as a function of the distance between CH3 or Cu and CN; and (3) the energetic preference for the CN orientation. These measures show that the CuCN and CuNC bonding is dominantly ionic while the bonding for CH3CN and CH3NC is dominantly covalent. For the CH3 containing molecules, there is participation of the cyanide 2π* orbital in the bond; however, it is not involved in the ionic bond with Cu. In all cases considered, there is a large binding energy between CN and CH3 or Cu; CN can form strong bonds that are of either covalent or ionic character.
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o calculation of adiabatic potential surfaces for Ne*(2p53s,1,3P)+H2(1Σ+g)
Authors: Stern B. , Robert J. , Reinhardt J. , Bocvarski V. , Baudon J.
Published: July 2002

Adiabatic potential surfaces have been computed ab initio for molecular states dissociating into Ne* (2p 53s,1,3P)+H2(1Σ+g). The distance R between Ne* and H2 ranges from 5 up to 50 a0. The distance r between H atoms is kept constant, equal to its equilibrium value (1.4 a0). The interaction with the electronic continuum is ignored. Potentials, including spin–orbit interaction, are fitted to analytic forms. Specific molecular properties of the system are discussed.
Photofragmentation dynamics of hydrogen peroxide: Analysis of two simultaneously excited states
Authors: Grunewald A. U. , Gericke K.-H. , Comes F. J.
Published: July 2002

The dynamics of the photodissociation of hydrogen peroxide has been analyzed by a complete characterization of the scalar and vectorial properties of the OH fragment using Doppler and polarization spectroscopy. When hydrogen peroxide is optically excited at 193 nm the hydroxyl radicals are formed exclusively in the X 2Π3/2,1/2 ground state with 84% of the available energy (Eav=417 kJ/mol) being released as OH recoil translation. The remaining energy is transferred in product rotation showing a strongly inverted rotational state distribution peaking at N″=12. Vector correlations between the transition dipole moment of the parent H2O2 and the OH product rotational and translational motions were observed by Doppler broadened spectral lines and evaluated in terms of four bipolar moments. The quantitative contribution of two different electronic excited states in the dissociation process could be determined by analyzing the vector properties of the fragment. 62% of the OH products evolve from the Ã 1A electronic excited state while 38% of the fragments are formed via the B̃ 1B state when hydrogen peroxide is excited at 193 nm. The OH rotational state distributions when produced from the Ã 1A and the B̃ 1B state show no remarkable difference. The vector correlation of the recoil velocity vOH and the rotation JOH is strongly positive and increases with increasing JOH indicating a strong preference towards vOH and JOH being parallel to one another. The major part of product rotation is caused by a strong dependence on the torsion angle of the two upper potential surfaces.
Alignment of CN from 248 nm photolysis of ICN: A new model of the Ã continuum dissociation dynamics
Authors: O’Halloran Maureen A. , Joswig Hellmut , Zare Richard N.
Published: July 2002

A KrF excimer laser (248 nm) is used to dissociate a low pressure (5–10 mTorr) sample of cyanogen iodide (ICN) and the resulting CN X 2Σ+ fragments are probed by laser-induced fluorescence (LIF) via various vibrational sequences of the B 2Σ+–X 2Σ+ transition. In addition to measuring relative vibrational and rotational populations in the CN X 2Σ+ photofragment, the alignment of rotational angular momentum in this fragment is determined from the variation in LIF intensity as the direction of linear polarization of the probe laser is rotated with respect to that of the photolysis laser. A unifying model is proposed for the Ã continuum photodissociation dynamics which is able to account for present and previous experimental measurements characterizing both the I and CN photofragments.