bluuebunny/crossref_metadata_2025 huggingface dataset:
Photodissociation dynamics of 1-propanol and 2-propanol at 193.3 nm
Authors: Zhou Weidong , Yuan Yan , Zhang Jingsong
Published: October 2003

193.3-nm photodissociation dynamics of jet-cooled 1-propanol and 2-propanol and their partially deuterated variants are examined by using the high-n Rydberg-atom time-of-flight technique. Isotope labeling studies show that O–H bond fission is the primary H-atom production channel in the ultraviolet photodissociation of both 1-propanol and 2-propanol. Center-of-mass (c.m.) product translational energy release of the RO–H dissociation channel is large, with 〈fT〉=0.78 for H+1-propoxy (n-propoxy) and 0.79 for H+2-propoxy (isoproxy). Maximum c.m. translational energy release yields an upper limit of the O–H bond dissociation energy: 433±2 kJ/mol in 1-propanol and 435±2 kJ/mol in 2-propanol. H-atom product angular distribution is anisotropic (with β≈−0.79 for 1-propanol and −0.77 for 2-propanol), suggesting an electronic transition moment perpendicular to the H–O–C plane and a short excited-state dissociation lifetime (less than a rotational period). Information about photodissociation dynamics and bond energies of the partially deuterated propanols are also obtained. The 193.3-nm photodissociation dynamics of 1-propanol and 2-propanol are nearly identical to each other and are similar to those of methanol and ethanol. This indicates a common RO–H dissociation mechanism: after the nO→σ*(O–H)/3s excitation localized on the H–O–C moiety, the H atom is ejected promptly in the H–O–C plane in a time scale shorter than a rotational period of the parent molecule, and it dissociates along the O–H coordinate on the repulsive excited-state potential-energy surface with a large translational energy release.
Probing branching mechanism through resonance-type oscillation in the collision-energy dependence of Penning ionization reaction of Ar (3P) with oriented CH3I
Authors: Okada Seiki , Ohoyama Hiroshi , Kasai Toshio
Published: October 2003

The collision energy and orientation-angle-resolved Penning ionization cross sections were determined for the Ar (3P)+CH3I reaction. A remarkable depletion was recognized in the collision- energy dependence of the orientation-angle-resolved Penning ionization cross section. The depletion could be interpreted as the competitive neutral dissociation from the Rydberg states formed by an electron exchange mechanism related to a 3a1 molecular orbital. The stereo selectivity for the Penning ionization and neutral dissociation were discussed. From the series of our studies on Ar (3P)+CH3X (X=Cl,Br,I) reactions, it was substantiated that the orientation angle and collision-energy-resolved Penning ionization cross section give an important information about the branching mechanism to the Rydberg states that are linked to the neutral dissociation channels.
Influence of the expansion dynamics of laser-produced gold plasmas on thin film structure grown in various atmospheres
Authors: Irissou Eric , Le Drogoff Boris , Chaker Mohammed , Guay Daniel
Published: October 2003

The expansion dynamics of plasma produced by excimer laser ablation of a gold target was measured by means of spatially resolved real time emission spectroscopy. The emission line of the Au(I) neutral gold species at 267.65 nm was used to monitor the expansion dynamics of the plasma in several background gases (He, N2, O2, and Ar). The measurements were performed as a function of the gas pressure (from 4×10−5 to 4 Torr) and target-to-substrate distance (from 1.5 to 11 cm). Gold thin films were prepared in the same conditions and their structure was analyzed by x-ray diffraction. All films prepared fall into one of three categories: highly (111) oriented, mixed, or polycrystalline. All the films prepared herein show a transition from highly (111) oriented to mixed and then to polycrystalline as the velocity of the Au(I) neutral gold species decreases. In the case of inert background gases (He, N2, and Ar), the velocity at which the transition between the various types of structure occurs is remarkably constant. Highly (111) oriented films are obtained for Au(I) neutral gold species exceeding v1=2.4 km s−1 (5.8 eV), while a polycrystalline (nanocrystalline) film is formed when the velocity falls below v2=0.8 km s−1 (0.6 eV). The conditions of distance and pressure at which these velocities are attained differ greatly from one atmosphere to the other, reflecting the fact that the interaction between the expanding plasma and the background gases varies with the molar mass of the gas. In the case of O2, the transition velocities between the different structures are higher than those observed in He, N2, and Ar [v1=8.3 km s−1 (70.4 eV) and v2=3.4 km s−1 (11.8 eV)]. This reflects a significant difference in the growth mechanism of these films compared to those prepared in an inert atmosphere.
Local structure of LiB3O5 single crystal from Li7 nuclear magnetic resonance
Authors: Lim Ae Ran , Kim Ji Won , Yoon Choon Sup
Published: October 2003

The local structure of LiB3O5 single crystal was investigated with Li7 (I=3/2) nuclear magnetic resonance measurements. We observed four different spectra, which could be divided into two groups corresponding to two kinds of lithium atoms, LiA and LiB, lying at crystallographically equivalent sites and magnetically inequivalent sites. From these results, the quadrupole coupling constants and the asymmetry parameters were determined at room temperature and are e2qQ/h=143±1 kHz and η=0.6±0.1 for Li7. The directions of the principal axes of the electric field gradient tensors were also determined. The spectra for the two groups have the same principal values of the electric field gradient tensor, but different orientations, and originate from magnetically inequivalent sites. Also, the Li7 spin-lattice relaxation rate was measured, and the measured relaxation rate was found to be proportional to the temperature. The temperature-dependent single phonon process is considered to be more effective than the Raman process for nuclear quadrupole relaxation. In addition, we discuss the correlation between the asymmetry parameter and the largest nonlinear optical coefficient.
Magnetic resonance imaging of convection in an electrolyte solution and extracellular fluid associated with stationary electric currents
Authors: Sekino Masaki , Yamaguchi Kikuo , Iriguchi Norio , Ueno Shoogo
Published: October 2003

Convection in an electrolyte solution associated with stationary electric currents has been investigated as one of the Rayleigh–Bénard type instability problems in fluid mechanics. However, details of convective flow associated with current have not yet been clarified because visualization of three-dimensional flow is difficult. In the present study, convections in a 0.9% NaCl solution and extracellular fluid were visualized using magnetic resonance imaging. Flow velocities were measured from the increase of the signal intensity based on in-flow effects of unsaturated spins. The maximum velocity with a stationary current of 1.0 mA/cm2 was 0.85 cm/s in the NaCl solution. The onset of the convection was within a few seconds after turning the current on, which depended on the distance from the electrode. The convection was observed with a current frequency lower than 2.0 Hz. In the muscle tissue of a rat, convection in extracellular fluid was observed with an injection of a 10 mA stationary current from a pair of surface electrodes.
Numerical modeling of large-area electron-beam diodes for KrF lasers
Authors: Rose D. V. , Welch D. R. , Hegeler F. , Swanekamp S. B. , Myers M. C. , Sethian J. D.
Published: October 2003

Particle-in-cell simulations are used to model large-area, electron-beam diodes that pump krypton–fluoride (KrF) laser cells. The simulations include models for following the energy loss and scattering of beam electrons in foils, gas, and support structures. Estimates of energy deposition to the various components of the diode system are obtained and compared with available experimental data. The simulations are in very good agreement with the Faraday cup measurements of electron-beam transport in the KrF gas. Additionally, global energy deposited in the KrF gas predicted from the simulations is in good agreement with experimental measurements for several different diode configurations. The results from this work benchmark and establish the computational procedure as an important tool from which future KrF laser systems can be developed.
Brillouin scattering in planar waveguides. I. Numerical model
Authors: Chiasera A. , Montagna M. , Rossi F. , Ferrari M.
Published: October 2003

A model for the calculation of Brillouin spectra in planar waveguides is presented. The electromagnetic field in a mode of a planar waveguide is a current wave along the guide and a standing wave across the guide with an evanescent tail in the substrate. The modal electromagnetic field can be calculated if the refractive index profile across the waveguide is known. The model for the calculation of Brillouin spectra considers the spatial distribution of the exciting field in the mode, a simple spatial dependence of the elasto-optic coefficients, through the value of the refractive index, and neglects the refraction of phonons. A single parameter, i.e., the sound velocity, is necessary to calculate as many spectra as is the number of modes in the waveguide. Examples of step index waveguides of different thicknesses and of graded-index waveguides are calculated and the results are compared with those of a simpler model based on a ray-tracing approach. In multimode waveguides with a step index profile, it turns out that longitudinal phonons produce two main peaks with an energy separation which increases with the mode index m. In graded-index waveguides, m−1 peaks of comparable intensities may appear.
Brillouin scattering in planar waveguides. II. Experiments
Authors: Chiasera A. , Montagna M. , Moser E. , Rossi F. , Tosello C. , Ferrari M. , Zampedri L. , Caponi S. , Gonçalves R. R. , Chaussedent S. , Monteil A. , Fioretto D. , Battaglin G. , Gonella F. , Mazzoldi P. , Righini G. C.
Published: October 2003

Silica–titania planar waveguides of different thicknesses and compositions have been produced by radio-frequency sputtering and dip coating on silica substrates. Waveguides were also produced by silver exchange on a soda-lime silicate glass substrate. Brillouin scattering of the samples has been studied by coupling the exciting laser beam with a prism to different transverse-electric (TE) modes of the waveguides, and collecting the scattered light from the front surface. In multimode waveguides, the spectra depend on the m mode of excitation. For waveguides with a step index profile, two main peaks due to longitudinal phonons are present, apart from the case of the TE0 excitation, where a single peak is observed. The energy separation between the two peaks increases with the mode index. In graded-index waveguides, m−1 peaks of comparable intensities are observed. The spectra are reproduced very well by a model which considers the space distribution of the exciting field in the mode, a simple space dependence of the elasto-optic coefficients, through the value of the refraction index, and neglects the refraction of phonons. A single-fit parameter, i.e., the longitudinal sound velocity, is used to calculate as many spectra as is the number of modes in the waveguide.
Modeling the absorption spectra of Er3+ and Yb3+ in a phosphate glass
Authors: Gruber John B. , Sardar Dhiraj K. , Zandi Bahram , Hutchinson J. Andrew , Trussell C. Ward
Published: October 2003

Absorption spectra of Er3+ and Yb3+ ions, codopants in a phosphate glass, are reported at 8 K and at wavelengths between 350 and 1600 nm. Detailed structure appearing in the spectra, associated with individual multiplet states, LJ2S+1, of Er3+(4f11) and Yb3+(4f13) is interpreted using a ligand-field coordination sphere model to characterize the microscopic environment surrounding the rare earth ions in multiple sites. Inhomogeneous broadening of the spectra is likely due to different configurations of PO4 tetrahedra clustered about a caged rare earth ion in the amorphous host. Similarity between the Er3+ spectrum in the glass and in the spectrum of single-crystal LiErP4O12, where Er3+ occupies sites of C2 symmetry, suggests that an averaged site symmetry of C2 is a reasonable approximation for Er3+ and Yb3+ ions in the phosphate glass. Calculated splitting of multiplet states by the ligand-field cluster model are compared with energy levels derived from the observed absorption peaks and well-defined shoulders. Inhomogeneous broadening of the spectra limit the precision in establishing the energy of the multiplet splittings, but the analysis is useful for modeling studies of the Er:Yb:phosphate glass as an eye-safe laser (1.53 μm). The splitting of the Yb3+(4f13)2FJ states is determined using parameters obtained from the Er3+ set by means of the three-parameter theory. No adjustments were made to the Yb3+ parameters that predict multiplet splittings in reasonable agreement with experimental data.
Counterion distribution in urchinlike charged copolymer micelles Monte Carlo simulation and small angle x-ray scattering
Authors: Belloni L. , Delsanti M. , Fontaine P. , Muller F. , Guenoun P. , Mays J. W. , Boesecke P. , Alba M.
Published: October 2003

The distribution of counterions inside urchinlike charged copolymer micelles is investigated using Monte Carlo simulation. Each highly charged arm of the micelle is fixed in a rodlike geometry. Three different categories can be distinguished in the counterion population: ions locally condensed in the vicinity of the stretched branches, ions trapped inside the globular micelle and free ions localized outside the micelle which monitor the osmotic pressure and the long range electrostatic repulsion between micelles. Comparisons with simple Poisson–Boltzmann theories in various geometries and with small-angle x-ray scattering confirm this picture.
Connectivity and topology of a phase-separating bicontinuous structure in a polymer mixture: Direct measurements of coordination number, inter-junction distances and Euler characteristic
Authors: Jinnai Hiroshi , Watashiba Hideyuki , Kajihara Takashi , Takahashi Masaoki
Published: October 2003

A new set of topological parameters describing connectivity of a phase-separated bicontinuous structure of a polymer mixture in the late stage of spinodal decomposition has been directly measured by use of laser scanning confocal microscopy. A 3D thinning algorithm was used to transform one of the phase-separated domains into a skeletal network that locates at about the center of the domain. Number of channels intersecting at a junction, Nj, and distance between the adjacent junctions, Dj, were obtained from the resulting skeletal network. It was found that the phase-separated domain of the polymer mixture possesses, for the most part, 3 branches at each junction. The Euler characteristic, describing topology of the network, was found to be between ∼−2 to −4, topologically equivalent to a sphere with ∼2 to 3 handles. Time-evolution of the histograms of the coordination number, H(Nj), and distance, H(Dj), clearly demonstrates that the bicontinuous structure grows with self-similarity in topological sense.
Virial coefficients and equation of state of hard chain molecules
Authors: Boublı́k Tomáš
Published: October 2003

The second, third, and fourth virial coefficients of hard chain molecules with number of segments up to 10 (up to 7 in the case of the fourth one) and the reduced distances L*=1 and 0.5 were determined numerically. For fused hard sphere (FHS) molecules the enlarged fused hard sphere model is introduced which forms a connection to the hard convex geometry and makes it possible to determine the virial coefficients of FHS molecules from the expressions derived for hard convex bodies. Our numerical values of the virial coefficients together with data from literature are used to test the proposed method and to compare its results with values from Wertheim’s theory, from its modified version and from the generalized Flory dimer approach. It is found that prediction of the second virial coefficient is very accurate (for L=0.5 the maximum deviation amounts 0.2 percent) and that our values of the third virial coefficient for higher members of the FHS family are superior to those from other considered methods. The model is successfully used to predict geometric characteristics of the hard models of n-butane conformers and to estimate their second virial coefficients. The equation of state of hard prolate spherocylinders in which the nonsphericity parameters of the enlarged FHS model are considered is used to predict the P-V-T behavior of hard dumbbells, hard triatomics, mixture of hard dumbells of different site–site distances, and n-butane taken as a mixture of conformers. In all the cases prediction within error bars is obtained.
Fokker–Planck–Kramers equation treatment of dynamics of diffusion-controlled reactions using continuous velocity distribution in three dimensions
Authors: Ibuki Kazuyasu , Ueno Masakatsu
Published: October 2003

A theory has been developed for the short-time dynamics of diffusion-controlled reactions based on the Fokker–Planck–Kramers equation (FPKE) in three dimensions. A continuous velocity distribution function has been proposed to solve the FPKE approximately. The present theory agrees better with the Langevin dynamics results than the earlier theory using a discontinuous velocity distribution. This indicates the validity of the present theory in three dimensions, because the Langevin dynamics results can be assumed to be the exact solutions to the FPKE. The theory is compared with molecular dynamics (MD) simulations in Lennard-Jones fluids to examine the applicability for realistic systems. The present theory predicts a somewhat smaller rate constant than the MD simulation in the time range of a few picoseconds. The discrepancies can be explained qualitatively in terms of the non-Markovian effect on the molecular motions.
Irreversible thermodynamics of reversible polymerization reactions
Authors: Stier Ulli
Published: October 2003

Based on the theory of irreversible thermodynamics explicit expressions are derived for the entropy production during reversible polymerization of bifunctional linear polymers whose initial molecular weight distribution can be chosen arbitrarily. The time-dependent course of the entropy production is explicitly calculated for two cases where the reaction starts (a) from monomer and (b) from monodisperse polymer molecules. In both cases we treat the system to be ideal and the time dependant change of the number of molecules is described by a kinetic approach using two kinetic constants for the forward and backward reactions, respectively. During reversible polymerization the entropy production σred is a monotonously decreasing function approaching zero when the system reaches the equilibrium molecular weight distribution with σred being positive in accordance with the second law of thermodynamics. In case of starting reaction from monodisperse polymer molecules under constraint that the number average chain length remains constant during reaction we calculate the entropy of mixing and discuss it with results obtained from statistical considerations.
Size scaling of intramolecular charge transfer driven optical properties of substituted polyenes and polyynes
Authors: Yong Lee Jin , Jin Mhin Byung , Mukamel Shaul , Kim Kwang S.
Published: October 2003

The origin of dramatic variation in optical polarizabilities of push–pull conjugated chains with respect to the conjugation length is discussed. Ab initio calculations of intramolecular charge transfer (ICT)-driven dipole moment (μCT), polarizability (αCT), and first hyperpolarizability (βCT) show that the values of αCT/μCT and βCT/μCT change linearly and quadratically with respect to the conjugation length, respectively. The maximum ICT-driven coherence sizes of αCT and βCT are consistent with the time-dependent densities to the first (ρ(1)) and second (ρ(2)) orders of the electric field obtained from the collective electronic oscillators method.
Dynamic properties of water/alcohol mixtures studied by computer simulation
Authors: Wensink Erik J. W. , Hoffmann Alex C. , van Maaren Paul J. , van der Spoel David
Published: October 2003

We have studied mixtures of alcohol and water in an extensive series of 465 molecular-dynamics simulations with an aggregate length of 713 ns, in order to study excess properties of mixing, in particular the relation between mobility and viscosity. Methanol/water, ethanol/water, and 1-propanol/water mixtures were simulated using an alcohol content of 0–100 mass % in steps of 10%, using the OPLS (optimized potential for liquid simulations) force field for the alcohol molecules and the TIP4P (transferable intermolecular potential with four particles) water model. Computed densities and energies show very good agreement with experimental data for bulk simulations and the mixtures are satisfactory as well. The shear viscosity was computed using nonequilibrium molecular-dynamics simulations. Other properties studied include diffusion constants and rotational correlation times. We find the mobility to correlate well with the viscosity data, i.e., at intermediate alcohol concentrations the viscosity is maximal and the mobility is minimal. Furthermore, we have combined the viscosity and diffusion calculations in order to compute an effective hydrodynamic radius of the particles in the mixtures, using the Stokes–Einstein relation. This analysis indicates that there is no collective diffusion of molecular clusters in these mixtures. For all properties we find that the excess values are underestimated in the simulations, which, given that the pure liquids are described rather well, raises the question whether the potential function is too simplistic to describe mixtures quantitatively. The set of simulations presented here can hence be regarded as a force-field benchmark.
Hydrogen fluoride phase behavior and molecular structure: A QM/MM potential model approach
Authors: Wierzchowski Scott J. , Kofke David A. , Gao Jiali
Published: October 2003

A molecular-orbital derived polarizable potential function is developed to model liquid and supercritical fluid hydrogen fluoride. The model is based on a novel application of a combined quantum-mechanical and molecular-mechanical (QM/MM) approach, which treats molecular polarization by a semiempirical method. Two geometrical models are examined, differing in the intramolecular bond length for hydrogen fluoride to match values commonly seen in other empirical models. One QM/MM parameter is fit for each model to reproduce the experimental density at one liquid-phase state condition. The models are examined at this state and at one supercritical state condition. Results for the density, radial distribution function, and average molecular dipole moment are considered in comparison to experiment. Also vapor-liquid coexistence data are evaluated, including saturation densities, heat of vaporization, and vapor pressure. Both models perform well in describing the densities, but are no better than other molecular models in characterizing the vapor-liquid critical point, the heat of vaporization, and the vapor pressure. The QM/MM models are slightly better than others in describing the radial distribution functions, although it is clear that this QM polarization model can be further improved. The present study further demonstrates that a QM-based polarization model is a viable alternative to model polar fluids with strong intermolecular interactions.
Phase coexistence in polydisperse liquid mixtures: Beyond the van der Waals approximation
Authors: Kalyuzhnyi Yurij V. , Kahl Gerhard
Published: October 2003

The solution of the mean spherical approximation for a polydisperse fluid mixture of particles interacting via a set of (factorizable) hard-sphere Yukawa potentials allows one to represent those thermodynamic quantities that are relevant to determine phase coexistence (i.e., pressure and chemical potential) by a limited number of (generalized) moments. Being thus a member of “truncatable free energy models,” the equilibrium conditions reduce to a set of coupled and highly nonlinear equations; we have solved these relations, we have determined phase diagrams for polydisperse fluid mixtures (i.e., cloud and shadow curves as well as binodals), and have extracted the daughter distribution functions of the coexisting phases.
Translational diffusion of hydrophobic solutes in supercritical water studied by molecular dynamics simulations
Authors: Ohmori Tsutomu , Kimura Yoshifumi
Published: October 2003

The diffusion process of hydrophobic solutes (oxygen and methane) in water at various temperatures and densities has been studied by molecular dynamics simulation. We found anomalous temperature dependence of the self-diffusion constants of hydrophobic solutes in water in the medium-density region, i.e., the diffusion constants are almost independent of the temperatures. In the case of oxygen, even the inversion of the temperature dependence is observed. To investigate the reason of this anomaly, we have analyzed the velocity auto correlation function (VACF) and memory function of the friction on the diffusion based on the generalized Langevin theory. The VACFs of hydrophobic solutes decay almost exponentially, which suggests that the Enskog theory holds. According to the analysis of the memory functions, it has been revealed that the binary contribution of the friction decreases with decreasing temperature from 973 to 647 K in the density region below 663 kg m−3, which is the main reason for the anomalous temperature dependence of the diffusion constant of the hydrophobic solutes. The radial distribution function of water around the hydrophobic solutes shows the water deficient structure. This deficiency is enhanced with decreasing the temperature which causes the decrease of the binary friction.
A coarse grain model for n-alkanes parameterized from surface tension data
Authors: Nielsen Steve O. , Lopez Carlos F. , Srinivas Goundla , Klein Michael L.
Published: October 2003

Molecular dynamics simulations are carried out in a systematic manner to develop a coarse grain model for multiple-of-three carbon n-alkanes. The procedure involves optimizing harmonic bond and bend parameters, and Lennard-Jones nonbonded parameters, to match observables taken from fully atomistic simulations and from experiment. The experimental values used consist of surface tension and bulk density data. Scaling relations are introduced to allow for the representation of the remaining n-alkanes. As n increases these relations converge to the multiple-of-three carbon values. The model is assessed by comparing it to both fully atomistic simulation and experimental data which was not used in the fitting procedure.
A density-functional theory for bulk and inhomogeneous Lennard-Jones fluids from the energy route
Authors: Tang Yiping , Wu Jianzhong
Published: October 2003

A new density-functional theory is developed for representing the structural and thermodynamic properties of Lennard-Jones fluids by unifying the modified fundamental measure theory for the short-range repulsion and the first-order mean-spherical approximation (FMSA) via the energy route for the attractive part of the intermolecular potential. This theory significantly improves the conventional mean-field approximation for the attractive forces and is applicable to both bulk and inhomogeneous systems using a single set of molecular parameters. The new theory is computationally very affordable and self-consistent with FMSA for bulk systems. It provides accurate radial distribution function, phase diagram, saturation pressure, and chemical potential of bulk Lennard-Jones fluids except very close to the critical point. In addition, it agrees well with simulation results for inhomogeneous systems including the adsorption isotherms and the density profiles of Lennard-Jones molecules near hard walls as well as in attractive slit pores.
Nitrogen-induced magnetic transition in small chromium clusters
Authors: Wang Q. , Sun Q. , Rao B. K. , Jena P. , Kawazoe Y.
Published: October 2003

Using density functional theory with generalized gradient approximation for exchange and correlation, we show that otherwise antiferromagnetically coupled chromium atoms in very small chromium clusters couple ferromagnetically when doped with a nitrogen atom, thus leading to giant magnetic moments. For example, the magnetic moment of Cr2N is found to be 9μB while that of Cr2 is 0μB. Strong bonding between Cr and N atoms brings about this magnetic transition. The Cr atoms nearest neighbor to N couple ferromagnetically with each other and antiferromagnetically with nitrogen. The significance of these results in understanding the ferromagnetic order in Cr-doped GaN is discussed.
Geometry and torsional energies of a C–C-protonated n-alkane
Authors: Li Qingbin , Hunter Ken C. , Seitz Christa , East Allan L. L.
Published: October 2003

The geometry and relative energies of torsional conformers of centrally protonated C4H11+ were studied with ab initio methods, to (a) obtain the most accurate geometry of the three-center–two-electron CHC bond to date, (b) evaluate the performance of lower levels of approximation upon this challenging structure, and (c) gain an understanding of the torsional dynamics of C4H11+. Twenty-nine combined levels of theory were used to optimize the geometry of the C2-symmetry minimum for trans-C4H11+, and the most accurate one [CCSD(T)/cc-pVTZ] gave the following CHC bond geometry: θCHC=122.4°, RCC=2.177 Å, RCH=1.2424 Å. Molecular-orbital-based methods generally perform better than density functional methods for describing the three-center–two-electron bond. A smaller subset of levels of theory was used to optimize other torsional conformers of centrally protonated C4H11+, varying the CCCC dihedral (trans, gauche, cis) and the dihedral for the bridging proton (various eclipsed and staggered positions). The results show that all conformers lie within a 4 kJ mol−1 range, with the lowest-energy conformer being either trans or gauche with a staggered dihedral for the bridging proton. The effect of core-valence correlation was also investigated. Finally, the potential energy surface as a function of the CCCC and bridging-proton dihedral angles was qualitatively estimated and drawn, based on our computed data, to aid in understanding the fluxional character of C4H11+.
Self-assembly of well-aligned gallium-doped zinc oxide nanorods
Authors: Yan M. , Zhang H. T. , Widjaja E. J. , Chang R. P. H.
Published: October 2003

Highly oriented Ga-doped ZnO nanorod arrays have been fabricated on GaN and sapphire substrates by pulsed laser deposition. X-ray diffraction shows that these nanorods are grown epitaxially with the c-axis normal to the substrate. The ZnO nanorod formation proceeds as follows. After a wetting layer of ZnO film grows to approximately 14 nm in thickness, the Stranski–Krastanov instability takes place due to interface strain. As a result, ZnO nanodots are formed uniformly on the ZnO layer. These nanodots then serve as nucleation sites for the subsequent electric-field-assisted growth of nanorods. Ga-doping level plays a key role in the formation of ZnO nanorods, while the oxygen partial pressure and the substrate temperature also affect the morphology of nanorods. These self-assembled and ordered ZnO nanorod arrays may be used in field emission and optoelectronic applications.
Optimized pulsed laser deposited barium ferrite thin films with narrow ferromagnetic resonance linewidths
Authors: Song Young-Yeal , Kalarickal Sangita , Patton Carl E.
Published: October 2003

Hexagonal M-type barium ferrite (BaM) films have been prepared by pulsed laser deposition. Optimal preparation conditions gave pure single phase films with the narrowest possible ferromagnetic resonance (FMR) linewidths. The films were deposited on c-plane sapphire substrates with a KrF excimer laser at wavelength of 248 nm and fluence of 1.7 J/cm2. The minimum linewidth films were obtained for an oxygen partial pressure of 300 mTorr and a substrate temperature of 910 °C, and were 0.85 μm thick. X-ray diffraction indicated pure single crystal BaM phase with local c-axis deviations less than 0.15°. Vibrating sample magnetometry data gave hysteresis loops with minimal hysteresis and small coercive force, saturation induction 4πMs of 4.2 kG and uniaxial anisotropy field HA of 16.4 kOe. The HA matches the anisotropy for bulk and the 4πMs is about 10% below literature values for the bulk. The FMR spectra were measured from 50 to 75 GHz by shorted wave guide techniques with the static field perpendicular to the film. The optimized film uniform mode half power FMR linewidth at 60.3 GHz was 27 Oe. This linewidth was a linear function of the frequency with a response coefficient of 0.5 Oe/GHz and nearly zero frequency intercept. Within experimental error, this linewidth response is the same as for bulk crystals. The FMR spectra show additional standing spin wave (SSW) resonances which could be indexed to give field positions that scale with the square of index n except for low order modes. The scaling gave a standard exchange energy parameter, Aex, of 6.4×10−7 erg/cm. The SSW linewidths scaled with n2. Application of the Kittel thickness variation model to the SSW linewidth data gave an estimated thickness variation of the film of about 7%.
Aluminum oxynitride at pressures up to 180 GPa
Authors: Sekine Toshimori , Li Xijun , Kobayashi Takamichi , Yamashita Yasuyuki , Patel Parimal , McCauley James W.
Published: October 2003

Hugoniot equation-of-state data of shock compressed aluminum oxynitride (AlON), consisting of 64.1 mol% Al2O3⋅35.9 mol% AlN with a density of ∼3.68 g/cm3, have been determined to 180 GPa. The relationship between shock velocity (Us) and particle velocity (Up) is expressed by a straight line: Us(km/s)=8.08+0.761Up(km/s). Although there is no evidence of phase transition in the data, the determined Hugoniot of AlON has been compared with those of oxide spinels such as MgAl2O4 and Fe3O4. We discuss the systematics of high pressure phase transitions of spinels that indicate a phase transition to CaTi2O4-type phases. The phase transition to CaTi2O4-type structures implies that the recently discovered Si3N4 spinel also may be transformed into a CaTi2O4-type phase with increasing pressure.
Fictive temperature dependence of density fluctuation in SiO2 glass
Authors: Watanabe T. , Saito K. , Ikushima A. J.
Published: October 2003

Fictive temperature dependence of density fluctuation in SiO2 glass was investigated by the small-angle x-ray scattering measurement. The density fluctuation in SiO2 glass varies linearly to the fictive temperature. Contribution of structural relaxation to the density fluctuation is larger than that of phonons. Isothermal compressibility, relaxational isothermal compressibility and high-frequency adiabatic compressibility are estimated. If N,  ̄, and Δ represent the number of SiO4/2 unit, average and difference from average, respectively, the density fluctuation in SiO2 glass is less than 3.6% as (ΔN)2¯/N̄.
Microstructure changes in fired ceramics quantified by magnetic resonance relaxation and imaging
Authors: Fantazzini P. , Salem A. , Timellini G. , Tucci A. , Viola R.
Published: October 2003

Nuclear magnetic resonance (NMR) relaxation and imaging are shown to give information on the changes of the pore space structure in ceramics during the sintering process. On a set of ceramic samples held at fixed maximum firing temperature of 1280 °C for different lengths of time, called “soaking times” in the ceramics industry, NMR methods were applied to quantify pore space properties such as connected porosity and pore size distributions. The longer the soaking time the lower the connected porosity, but no corresponding general statement can be made with regard to the pore size distribution. The homogeneous distribution of pores where the porosity is 17% for minimal soaking times becomes heterogeneous and with bigger pores but with porosity only about 5% after 10 min of soaking time. The results obtained are combined with those from traditional techniques such as scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). The measured connected porosities and SEM analysis are in agreement with the NMR results. The “pore size” distributions obtained by magnetic resonance relaxation and MIP differ substantially and give complementary information, because in relaxation measurements “size” tends to correspond to pore dimensions, while in MIP “size” corresponds to the dimensions of the pore “throats,” the channels connecting the pores. The discrepancy between pore and pore-channel size increases with the soaking time and makes it clear that a higher level of sintering allows the formation of larger pores connected by smaller channels. For the longest soaking time sample, the SEM photos and the lack of a NMR signal make it clear that there is substantial isolated porosity and essentially no connected porosity.
Perfect two-dimensional porous alumina photonic crystals with duplex oxide layers
Authors: Choi Jinsub , Luo Yun , Wehrspohn Ralf B. , Hillebrand Reinald , Schilling Jörg , Gösele Ulrich
Published: October 2003

A perfect two-dimensional porous alumina photonic crystal with 500 nm interpore distance was fabricated on an area of 4 cm2 via imprint methods and subsequent electrochemical anodization. By comparing measured reflectivity with theory, the refractive indices in the oxide layers were determined. The results indicate that the porous alumina structure is composed of a duplex oxide layer: an inner oxide layer consisting of pure alumina oxide of 50 nm in thickness, and an outer oxide layer of a nonuniform refractive index. We suggest that the nonuniform refractive index of the outer oxide arises from an inhomogeneous distribution of anion species concentrated in the intermediate part of the outer oxide.
Optical properties of GaSe grown with an excess and a lack of Ga atoms
Authors: Shigetomi S. , Ikari T.
Published: October 2003

Measurements of the photoluminescence (PL) and photocurrent (PC) have been made on GaSe with excess Ga or Se atoms. The 1.77 eV emission band for the sample with excess Ga atoms is attributed to the transition from the donor level at 0.175 eV to the acceptor level at 0.152 eV. This donor level is considered to be related to the defects formed by the interstitial Ga atoms. The 1.92 eV peak in the PC spectrum of the sample with excess Se atoms is thought to be associated with the interstitial Se atoms. Moreover, an emission band at 2.036 eV (PL spectra at 77 K) and a shoulder at 1.99 eV (PC spectra at 295 K) related to the indirect band transition were observed in the samples with excess Ga or Se atoms. It was found that the indirect band transition in GaSe is caused by the excess Ga and Se atoms.
Formation of self-assembled epitaxial nickel nanostructures
Authors: Zhou H. , Kumar D. , Kvit A. , Tiwari A. , Narayan J.
Published: October 2003

Highly orientated nickel magnetic nanoparticles were obtained by pulsed laser deposition technique on silicon (100) substrate using epitaxial titanium nitride film as the template. These nanoparticles have been characterized by conventional and high-resolution transmission electron microscopy, scanning transmission electron microscopy Z-contrast imaging, and x-ray diffraction techniques. The results have shown that the growth of nickel on epitaxially grown titanium nitride follows a three-dimensional island growth mechanism. The predominant orientation of nickel islands observed is Ni(100)∥TiN(100)∥Si(100), the so-called “cube-on-cube” orientation relation. The islands are faceted with a truncated pyramidal shape and bounded by (111) planes at sides and (100) plane at the top. Islands with nontruncated pyramidal shape were also found in some samples, but with rotational orientation relations, where the nickel crystal rotates with an approximate angle of 90° with respect to one of TiN 〈110〉 directions parallel to the interface. The appearance of this rotational epitaxial growth did not show any obvious deposition temperature dependence in the range of 400–650 °C, rather it seemed to be closely related to the crystalline quality of TiN template. The actual size of islands varies from a few nanometers to tens of nanometers, depending on the deposition time and temperature. The three-dimensional growth of nickel islands and the island faceting could be explained by the surface energy anisotropy of both nickel and titanium nitride.
Selectively excited photoluminescence of GaAs1−xNx single quantum wells
Authors: Luo X. D. , Tan P. H. , Xu Z. Y. , Ge W. K.
Published: October 2003

GaAsN bulk and GaAsN/GaAs single quantum wells grown by molecular beam epitaxy are studied by selectively excited photoluminescence (PL) measurements. A significant difference is observed in the PL spectra when the excitation energy is set below or above the band gap of GaAs for the GaAsN/GaAs quantum well samples, while the spectral features of GaAsN bulk are not sensitive to the excitation energy. The observed difference in PL of the GaAsN/GaAs quantum well samples is attributed to the exciton localization effect at the GaAsN/GaAs interfaces, which is directly correlated with the transfer and trap processes of the photogenerated carriers from GaAs into GaAsN through the heterointerfaces. This interface-related exciton localization effect can be greatly reduced by a rapid thermal annealing process, making the PL be dominated by the intrinsic delocalized transition in GaAsN/GaAs.
Optical characterization of AlN/GaN heterostructures
Authors: Ursaki V. V. , Tiginyanu I. M. , Zalamai V. V. , Hubbard S. M. , Pavlidis D.
Published: October 2003

AlN/GaN/sapphire heterostructures with AlN gate film thickness of 3–35 nm are characterized using photoreflectivity (PR) and photoluminescence (PL) spectroscopy. Under a critical AlN film thickness, the luminescence from the GaN channel layer near the interface proves to be excitonic. No luminescence related to the recombination of the two-dimensional electron gas (2DEG) is observed, in spite of high 2DEG parameters indicated by Hall-effect measurements. The increase of the AlN gate film thickness beyond a critical value leads to a sharp decrease in exciton resonance in PR and PL spectra as well as to the emergence of a PL band in the 3.40–3.45 eV spectral range. These findings are explained taking into account the formation of defects in the GaN channel layer as a result of strain-induced AlN film cracking. A model of electronic transitions responsible for the emission band involved is proposed.
Postgrowth annealing of defects in ZnO studied by positron annihilation, x-ray diffraction, Rutherford backscattering, cathodoluminescence, and Hall measurements
Authors: Chen Z. Q. , Yamamoto S. , Maekawa M. , Kawasuso A. , Yuan X. L. , Sekiguchi T.
Published: October 2003

Defects in hydrothermal grown ZnO single crystals are studied as a function of annealing temperature using positron annihilation, x-ray diffraction, Rutherford backscattering, Hall, and cathodoluminescence measurements. Positron lifetime measurements reveal the existence of Zn vacancy related defects in the as-grown state. The positron lifetime decreases upon annealing above 600 °C, which implies the disappearance of Zn vacancy related defects, and then remains constant up to 900 °C. The Rutherford backscattering and x-ray rocking curve measurements show the improvement of crystal quality due to annealing above 600 °C. Although the crystal quality monitored by x-ray diffraction measurements is further improved after annealing at above 1000 °C, the positron lifetime starts to increase. This is due to either the formation of Zn vacancy related defects, or the change of the Zn vacancy charge state occupancy as a result of the Fermi level movement. The electron concentration increases continuously with increasing annealing temperature up to 1200 °C, indicating the formation of excess donors, such as oxygen vacancies or zinc interstitials. The cathodoluminescence measurements reveal that the ultraviolet emission is greatly enhanced in the same temperature range. The experimental results show that the ZnO crystal quality, electrical and optical characteristics are improved by postgrowth annealing from 600 to 1200 °C. The disappearance of Zn vacancy related defects contributes to the initial stage of improved crystal quality.
Strained silicon on SiGe: Temperature dependence of carrier effective masses
Authors: Richard Soline , Cavassilas Nicolas , Aniel Frédéric , Fishman Guy
Published: October 2003

A strain Bir–Pikus Hamiltonian Hst, based on a 20 band sps* k⋅p Hamiltonian Hkp, is used to describe the valence band and the first two conduction bands over the entire Brillouin zone. This full-band k⋅p computation of the carrier dispersion relation is used to calculate electron and hole effective masses in strained silicon. Hole density of states masses are found to be very temperature dependent whereas electron effective masses can be considered temperature independent to first order.
Stress development in sputtered NiO thin films during heat treatment
Authors: Brückner W. , Kaltofen R. , Thomas J. , Hecker M. , Uhlemann M. , Oswald S. , Elefant D. , Schneider C. M.
Published: October 2003

Nickel oxide thin films with a thickness of 100 nm were deposited on oxidized silicon wafers by rf magnetron sputtering from a NiO target in an Ar (nonreactive case) and an Ar+O2 atmosphere with various oxygen contents (reactive cases). The as-deposited films possess high compressive stresses (up to 3700 MPa) which decrease irreversibly during annealing between 150 and 500 °C. Compositional and microstructural analyses were performed on as-deposited and annealed films by means of electron probe microanalysis, transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, thermal-desorption spectrometry, and magnetization measurements. All as-deposited thin films consist of NiOx with x ranging between about 1.15 and 1.27. These oxygen-excess films are thermally unstable. They decompose during heat treatment into thermally more stable, oxygen-poorer NiO and/or metallic Ni. This decomposition is the reason for the observed irreversible stress changes.
Nonequilibrium molecular dynamics simulation of electro-osmotic flow in a charged nanopore
Authors: Thompson Aidan P.
Published: October 2003

Nonequilibrium molecular dynamics simulations were performed for Poiseuille and electro-osmotic flow in a charged cylindrical nanopore. The goal was to examine any deviations from continuum flow behavior and to compare and contrast the Poiseuille and electro-osmotic flow situations. The fluid was composed of cationic counterions and nonpolar monatomic solvent molecules. The cylindrical surface of the pore wall was represented by a stochastic scattering boundary condition. The lack of any surface roughness and the computational efficiency of the fluid model enabled the velocity profile near the wall to be measured at very high spatial resolution. The simulation results indicate that both Poiseuille and electro-osmotic flow conform to continuum transport theories except in the first monolayer of fluid at the pore wall. The apparent viscosity in this region was highly nonuniform and exhibited singularities. Despite this, the viscosity profiles obtained from Poiseuille and electro-osmotic flow were in good mutual agreement at all locations. The singularities were caused by a local maximum in the solvent and counterion velocity profiles occurring at the edge of the first monolayer of liquid. This apparent channeling of fluid near the pore wall has been observed in previous studies of Poiseuille flow. The exact cause is not clear, but it may be due to cooperative transport of the fluid molecules facilitated by two-dimensional ordering at the wall.