bluuebunny/crossref_metadata_2025 huggingface dataset:
Graphitization of amorphous carbon at high pressures to 15 GPa
Authors: Onodera Akifumi , Irie Yasushi , Higashi Koji , Umemura Junzo , Takenaka Tohru
Published: July 2002

Graphitization of amorphous carbon has been studied at high static pressures up to 15 GPa and high temperatures up to 1300 °C, primarily under diamond-stable conditions. Upon heating at 2 GPa, amorphous carbon turns into graphite at ∼900 °C. The temperature for the graphitization decreases with increasing pressure and approaches 600 °C at 10 GPa or higher. The graphitization process at 15 GPa comprises two stages over a temperature range 600–1300 °C. In the first stage, below 900 °C, graphite layers develop and the interlayer spacing decreases markedly. The second stage, above 1000 °C, corresponds to in-layer arrangement and partial development of three-dimensional ordering. At temperatures approaching diamond formation under this pressure, the graphitization does not acquire completion. Along 1300 °C isotherm, the structure and texture of graphite exhibit appreciable changes across the graphite-diamond equilibrium line and also across 11 GPa.
Dielectric properties and defect structure in lanthanum doped SrTiO3 ceramics
Authors: Chen Ang , Zhi Yu
Published: July 2002

The dielectric properties and positron annihilation lifetime spectra of the ceramics with the nominal composition Sr1−xLaxTiO3 (0≤x≤0.06) were measured. The dielectric properties and relaxation behaviors, which varied obviously with lanthanum content, were explained by a charge compensation model. The defect structure of the materials was further discussed and the above model proved to be reasonable by the positron annihilation study.
The influence of Cu precipitation on electromigration failure in Al-Cu-Si
Authors: Kim Choongun , Morris J. W.
Published: July 2002

This paper reports a study of the effect of Cu precipitation on electromigration failure in Al-2Cu-1Si thin-film conducting lines. The films were 0.5 μm in thickness, and patterned to widths of 1.3 and 4 μm, providing width-to-grain-size ratios (W/G) of approximately 0.5 and 2. The lines were aged for various times at 226 °C, and were then tested to failure at a current density of 2.5×106 A/cm2. Scanning and transmission electron microscopy were used to study the Cu precipitate distribution, its evolution during aging and electromigration, and the microstructural failure mechanism. Aging produces a dense distribution of intragranular θ′ (Al2Cu; coherent), with stable θ (Al2Cu; incoherent) in the grain boundaries. The θ′ is replaced by θ as aging proceeds. In the wide lines (W/G≊2), the mean time to failure (MTF) increases slowly and monotonically with prior aging time. The failure happens through the growth and coalescence of intergranular voids. In the narrow lines (W/G≊0.5), both the MTF and the time to first failure increase by more than an order of magnitude when the line is aged for 24 h prior to testing, then decrease on further aging. The dominant failure mode is the ‘‘slit’’ failure mode previously observed in pure Al. However, the 24 h specimens fail by gradual, uniform thinning. Failure occurs in regions that have been swept free of intragranular precipitates. The failure time appears to be proportional to the intragranular density of θ precipitates, and related to the time required to sweep these from a critical length of line.
Measurements of anisotropic characteristic lengths in YBCO films at microwave frequencies
Authors: Jiang H. , Yuan T. , How H. , Widom A. , Vittoria C. , Drehman A.
Published: July 2002

We used microwave self-resonant and magnetically modulated microwave absorption techniques to measure the London penetration depth, λ, and coherence length, ξ, in the direction parallel and perpendicular to the a–b plane of YBCO films. We found that both λ and ξ were anisotropic; it appeared that the value of λ∥(0) was about 1800 Å and λ⊥(86.5) about 26 000 Å, where λ∥ is the penetration depth for the applied microwave electric field parallel and λ⊥ perpendicular to the film plane (c-axis is perpendicular to the film plane). We deduced ξ∥ to be equal to 129 Å and ξ⊥ 40 Å at 86.5 K. The anisotropy factor γ we determined to be about 3.
Two-dimensional magnetism in the organic-inorganic composite phenyl-MnPO4⋅H2O (abstract)
Authors: Visser D. , Kremer R. K. , Carling S. G. , Day P. , Gabás M.
Published: July 2002

Magnetic susceptibility and magnetization data are reported for the organic-inorganic composite material phenyl-MnPO4. The phenyl groups are situated between MnPO4 layers. The MnO46− octahedra of these layers form a quasi 2D square lattice. Two-dimensional magnetism has been observed above TN=13.8 (1) K, θCW=53 (1) K. Weak ferromagnetism is observed below TN. An ac-magnetic susceptibility study showed two peaks around TN, which indicates that the magnetic ordering process takes place in two steps. The magnetic susceptibility data indicate the presence of a reorientational structural phase transition around Tph=250 K.
Structure of the modulated magnetic phase of Mn3Sn (abstract)
Authors: Cable J. W. , Wakabayashi N. , Radhakrishna P.
Published: July 2002

The intermetallic compound Mn3Sn has the hexagonal DO19 crystal structure with six Mn and two Sn atoms per unit cell. The triangular antiferromagnetic order that develops below TN=420 K becomes modulated below 270 K in what is generally described as a helical modulation. However, magnetic excitation spectra show the presence of an energy gap of about 4 meV for both the triangular and the modulated phases, and this can occur for a helical modulation only if the period is commensurate with the lattice. This prompted us to re-examine the structure of the modulated phase by neutron diffraction. We find two distinct moment modulations, both propagating along the c axis and both of which are commensurate below 120 K with periods of 11c0 and 14c0.  Above 120 K, both modulation wave vectors increase with temperature and become incommensurate. The satellite reflections exhibit quite different intensities and temperature dependencies, and this suggests coexistent modulated phases. For the predominant 11c0 (22 layer) phase, we observe a third harmonic with about 20% of the amplitude of the first harmonic and with no fifth harmonic to 1% in amplitude. This shows that the modulation in Mn3Sn is definitely not that of a uniform helix. The observed intensity distributions are compared with configurational models in which multilayer blocks of the triangular order align along the easy sixfold axes in the basal plane and rotate by 60° per block in proceeding along the c axis.
Optical properties of deuterated zinc tris thiourea sulphate
Authors: Ramabadran Uma B. , McPherson Angela L. , Zelmon David E.
Published: July 2002

We report measurements of the optical properties of deuterated zinc tris thiourea sulfate (d-ZTS). The material is being studied for its potential value in optical frequency conversion and electro-optic applications. The electro-optic and piezoelectric measurements along with those for the dielectric constant indicate that the figure of merit for electro-optic modulation is comparable to that of KDP. However, careful refractive index measurements along with an analysis of the nonlinear coefficient tensor show that d-ZTS will not noncritically phase match for second-harmonic generation despite the promise shown by its undeuterated analog. In addition, the acceptance angle even for critical phase matching is very small.
Photoconductive gain and generation-recombination noise in quantum well infrared photodetectors
Authors: Shadrin V. D. , Mitin V. V. , Kochelap V. A. , Choi K. K.
Published: July 2002

Photocurrent and excess current noise in a quantum well infrared photodetector are considered using a drift-diffusion model of charge carrier transport. The effect of quantum well recharge under the influence of the nonuniform generated charge carriers is addressed. The recharging effect drastically changes the dependency of both photoconductive gain and excess current noise gain upon detector parameters. We have found that for uniform generation, both gains coincide. For nonuniform generation, noise gain is essentially different from photoconductive gain. This distinction is of the order of 100% for the real device parameters. The existing discrepancy in formulae for photoconductive gain and excess current noise derived in different models, which implicitly assumed drift transport of electrons, is cleared up.
Second order Raman spectroscopy of the wurtzite form of GaN
Authors: Murugkar S. , Merlin R. , Botchkarev A. , Salvador A. , Morkoç H.
Published: July 2002

We report on Raman scattering by phonon pairs in GaN films grown on sapphire substrates by plasma-enhanced molecular beam epitaxy. The first order data are consistent with results obtained from GaN bulk crystals of the wurtzite structure. The A1 and the much weaker E2 symmetry components of the second order scattering have been identified. Two-phonon spectra are dominated by contributions due to longitudinal optical phonons.
Magnetic structure and perpendicular magnetic anisotropy of rare-earth (Nd,Pr,Gd)/Fe multilayers
Authors: Honda S. , Nawate M. , Sakamoto I.
Published: July 2002

The structural phase and the magnetic anisotropy of Nd/Fe, Pr/Fe, and Gd/Fe multilayers have been examined using magnetization measurements and conversion electron Mössbauer spectrometry. With decreasing Fe layer thickness dFe, the structural phase of the Fe layer changes from bcc crystalline to amorphous at the critical thickness dFec of 17–20 Å via an intermediate phase, which contains a paramagnetic phase in the Nd/Fe and Pr/Fe systems, but not in Gd/Fe. At just above dFec, Nd/Fe and Pr/Fe multilayers exhibit perpendicular anisotropy, which originates from the interface anisotropy due to the one ion anisotropy of Nd and Pr. However, Gd/Fe multilayers show in-plane magnetization because of the S-ionic Gd. Below dFec, the magnetization increases from nearly zero with decreasing dFe because of the exchange interaction between Fe and the rare earth (Nd,Pr,Gd).
Poling and orientational relaxation: Comparison of nonlinear optical main-chain and side-chain polymers
Authors: Hagen R. , Zobel O. , Sahr O. , Biber M. , Eckl M. , Strohriegl P. , Eisenbach C.-D. , Haarer D.
Published: July 2002

We investigated both time and temperature dependence of chromophore movements by means of in situ second-harmonic generation (SHG) measurements. Using real-time detection of the SHG intensities during and after poling of various nonlinear-optical polymer films we observed differences between main-chain and side-chain polymers regarding the nonlinear coefficients d33 and the glass transition dynamics. We present experimental data on new main-chain polymers with polar stilbene chromophores attached in the most common transverse position to the backbones (MC-T) or incorporated in a linear fashion where they are a part of the main-chain (MC-L). The side-chain polymer (SC) presented here has been developed for photorefractive applications and is a copolymer containing Disperse Red 1 and a photoconducting carbazole unit in the side chain. The influence of the macromolecular structure on the chromophore dynamics is striking. This is verified by means of in situ corona poling experiments and thermal experiments based on temperature ramps on poled polymers. MC-T and especially MC-L polymers show a better thermal stability of the chromophore orientation relative to the glass transition temperature than SC polymers; however, chromophores of main-chain polymers are, as can be expected, less mobile during the poling process. We obtained resonance enhanced nonlinear optical coefficients d33 of 100 pm/V in a SC polymer, 90 pm/V in a MC-T polymer, and 30 pm/V in a MC-L polymer.
Micromagnetics of small size patterned exchange biased Permalloy film elements (invited)
Authors: Zhu Jian-Gang , Zheng Youfeng , Lin Xiangdong
Published: July 2002

In this article, we present a study on the micromagnetics of exchange biased Permalloy films. Specifically, by combining magnetic force microscopy with micromagnetic modeling simulation, the magnetization reversal processes in exchange biased Permalloy films were studied. The bilayer films were lithographically patterned into micrometer scale rectangular elements. It is shown that the micromagnetic simulations accurately predict domain configurations during magnetization reversal of the exchange biased Permalloy film elements and provide detailed magnetization distributions and transient dynamic magnetization configurations that could not yet be obtained experimentally. The study found that, for both NiO/NiFe and FeMn/NiFe systems, the exchange bias field measured on individual patterned micrometer scale bilayer film elements can be significantly larger than that measured on the sheet film sample.
Magnetic field effect on the complex permeability spectra in a Ni–Zn ferrite
Authors: Tsutaoka Takanori , Nakamura Tatsuya , Hatakeyama Kenichi
Published: July 2002

Complex permeability spectra μ*(=μ′−iμ′′) in a Ni–Zn ferrite was studied in the frequency range from 10 kHz to 3 GHz under dc magnetic field up to 1000 Oe. In the absence of a dc magnetic field, the μ′ spectrum has a frequency dispersion above 1 MHz; the μ′′ spectrum has a maximum at about 2 MHz. This feature can be described by the superposition of the two types of magnetic resonance, domain wall motion with a resonance frequency ωdwr=3.5 MHz and spin rotation with a resonance frequency ωspinr=8.0 MHz. Under dc magnetic field, low frequency permeability μ′ decreases with increasing static field bias. On the other hand, the μ′′ spectrum is broadened and two distinct peaks appear in the external field of 606 Oe. Under about 900 Oe external field, this ferrite becomes to have single-domain structure and the dispersion of domain wall motion in the permeability spectra disappears. In the above 700 Oe external field, high frequency dispersion of μ* shows ferromagnetic resonance characteristics.
Upconversion fluorescence spectroscopy of Er3+/Yb3+-doped heavy metal Bi2O3–Na2O–Nb2O5–GeO2 glass
Authors: Oliveira A. S. , de Araujo M. T. , Gouveia-Neto A. S. , Sombra A. S. B. , Medeiros Neto J. A. , Aranha N.
Published: July 2002

Upconversion fluorescence emission of Er3+/Yb3+-doped Bi2O3–Na2O–Nb2O5–GeO2 heavy metal glass samples excited at 1.06 μm is experimentally investigated. The results reveal the existence of intense emission bands centered around 520, 545, and 655 nm. The germano-niobate based host glass presents high transparency in the region of 400–2700 nm, the capability of incorporating high dopant concentrations, high melting temperature, and large resistance to atmospheric moisture. The observed intensity of the green fluorescence emission, suggested that the niobium based host glass material plays an important role in the efficiency of the upconversion process. Emission lines centered at 425, 483, 503, 608, and 628 nm were also observed.
Application of the shock compaction technique for consolidation of hard magnetic powders
Authors: Leonowicz M. , Kaszuwara W. , Jezierska E. , Januszewski D. , Mendoza G. , Davies H. A. , Paszula J.
Published: July 2002

A shock compaction technique has been successfully applied for the consolidation of hard magnetic powders without appreciable changes in the phase structure and grain size. Pr–Fe–B and Sm–Fe–N materials, prepared with the application of melt spinning and mechanical alloying, respectively, were shock compacted with a shock wave speed of 1200 m/s and a pressure of 4.4 GPa. We found that the coercivity of high RE containing material, for which the magnetic hardness is controlled by good magnetic separation of magnetic grains by a RE-rich paramagnetic phase, decreases slightly, by ∼5 %, due to the closer proximity of the crystallites in a dense material. For low RE, exchange coupled materials, the densification does not change the coercivity.
Fine structure of DX(Sn) centers in AlxGa1−xAs
Authors: Zhan H. H. , Kang J. Y. , Wu Z. Y. , Huang Q. S.
Published: July 2002

High resolution Laplace defect spectroscopy was used to study the fine structure of the electron emission process of DX(Sn) centers in AlxGa1−xAs (x=0.26,0.53). Two groups of peaks in the spectra of electron emission rates were found to correspond to two DX-like centers observed by deep level transient spectroscopy. The line splitting in both groups derives from the alloy disorder effect attributed to the different local configurations of Al and Ga atoms around two DX-like centers. Experimental evidence for the microscopic nature of two DX-like centers in Sn-doped AlGaAs is provided.
Macroscopic and microscopic second-harmonic response from subphthalocyanine thin films
Authors: Rojo G. , Agulló-López F. , del Rey B. , Torres T.
Published: July 2002

The second-harmonic generation (SHG) yield of both evaporated and spin-coated films of some tri-substituted subphthalocyanines has been measured for three linear polarizations of the fundamental beam. Partial molecular ordering in the spin-coated films was achieved by the corona poling technique. For the evaporated films the ordering is obtained through the evaporation process itself. By proper fitting of the SHG yield data and assuming C∞v symmetry the three nonzero independent components, χ31(2), χ15(2) and χ33(2), of the susceptibility tensor have been determined for the two types of films. Kleinman symmetry condition (χ31(2)=χ15(2)) is not obeyed. Values close to 10−9 esu have been obtained for the highest χ31(2) component of the spin-coated films containing trioctylsulfonylsubphthalocyanine molecules. For the evaporated films the susceptibility values are somewhat higher, indicating a significant degree of molecular ordering. For both types of films χ33(2)≪χ31(2), χ15(2), at variance with the case of linear molecules. From a statistical analysis of the macroscopic SHG emission the nonzero components of the microscopic (molecular) hyperpolarizability tensor, β15, β31 and β33, have also been estimated. The results indicate that β33≈0, and that β31 and β15 are dominant in accordance with the pyramid-shaped molecular geometry. The additional inplane components βyxx=−βyyy=βxxy=−β22 appearing in the C3v molecular symmetry cannot be determined from the macroscopic data.
Monte Carlo study of the classical Heisenberg antiferromagnet with easy-plane anisotropy on a triangular lattice
Authors: Capriotti Luca , Cuccoli Alessandro , Tognetti Valerio , Vaia Ruggero
Published: July 2002

In this paper the thermodynamics of the classical Heisenberg antiferromagnet with easy-plane anisotropy on a triangular lattice is investigated by means of Monte Carlo simulations. The topological frustration and the anisotropy of the exchange interaction turn out in a rich phase diagram showing the signatures of two distinct phase transitions, i.e., a Berezinkii–Kosterlitz–Thouless transition, also present in the unfrustrated counterparts of the model and associated with the vortex–antivortex unbinding, and an Ising-like one with a discrete order parameter, the chirality. These effects of the frustration, already observed in the XY model (or planar rotator), were not yet known in the model we studied, which is, being the spins three dimensional vectors, a more realistic description of a spin system. Data are reported in a broad range of temperatures for four values of the easy-plane anisotropy and for lattice sizes ranging from 24×24 to 120×120.
Unusually strong ferromagnetic correlations in La-doped CaMnO3
Authors: Neumeier J. J. , Goodwin D. H.
Published: July 2002

Magnetic susceptibility measurements are presented on specimens with the composition Ca1−xLaxMnO3 (0⩽x⩽0.10). Saturation moments at 5 K and 4000 Oe of under 0.6 μB/Mn ion, where μB is the Bohr magnetron, are observed; all specimens exhibit Curie–Weiss behavior above the Néel temperature TN. CaMnO3 (i.e., x=0) has a Curie–Weiss temperature Θ=565 K. Substitution of La3+ for Ca2+ results in isolated Mn3+ ions which interact with the surrounding Mn4+ ions via double exchange. This interaction leads to a strong reduction of Θ, thereby indicating that a small number of Mn3+ ions is sufficient to produce strong ferromagnetic correlations above TN.
Enhanced carrier collection observed in mechanically structured silicon with small diffusion length
Authors: Hahn G. , Zechner C. , Rinio M. , Fath P. , Willeke G. , Bucher E.
Published: July 2002

The diffusion length of minority charge carriers in the silicon bulk Ldiff is an important characteristic of optoelectronic devices fabricated from low cost silicon wafers. In this study computer simulations have been carried out to calculate the beneficial effects of a macroscopic surface texturization on the charge carrier generation and the collection probability. Textured solar cells should be able to collect charge carriers more effectively resulting in an increased current due to the special emitter geometry resulting from the texture, decreased reflection losses, and the inclined penetration of the light. In order to prove this expected behavior, deeply V-textured solar cells have been processed and characterized on low cost silicon reaching an Ldiff of about 25 μm. Spatially resolved high resolution measurements of the internal quantum efficiency exhibit a strongly increased signal in the texture tips which is the first experimental proof of the increased charge carrier collection probability of deeply textured solar cells. This effect can further be seen in cross sectional electron beam induced current measurements and the mechanical texture results in an overall gain in short circuit current density of about 11% and in efficiency of about 8% relatively.
Shear moduli of polystyrene thin films determined with quartz crystal resonators in the sandwich configuration
Authors: Wolff Oliver , Johannsmann Diethelm
Published: July 2002

We report on the determination of the shear compliance of spin-cast polystyrene films with quartz crystal resonators. The films with a thickness between 10 and 93 nm were sandwiched between the quartz blank and an overlayer of aluminum, which had been evaporated on top of the polymer film in order to enhance the shear stress. The shear compliance was inferred from the frequency shifts on the various overtones. For ease of analysis, the film was placed directly onto the quartz blank with no electrodes underneath, the oscillation being excited externally across an air gap. The derived shear moduli are consistent with the literature values for the bulk material determined from ultrasonic spectroscopy. However, they depend on thickness. For films thicker than 10 nm the apparent compliance decreases with film thickness, which presumably is a consequence of surface roughness. The 10 nm specimen had a much increased compliance, which we attribute to a less dense packing of the polymer chains caused by geometric confinement.
Photoluminescence of erbium-doped silicon: Excitation power and temperature dependence
Authors: Thao D. T. X. , Ammerlaan C. A. J. , Gregorkiewicz T.
Published: July 2002

Photoluminescence measurements have been made on float-zone and Czochralski-grown silicon samples which were doped with erbium by ion implantation. The characteristic luminescence spectra in the wavelength range between 1.5 and 1.6 μm were observed. Differences in the multiple line structure of the spectra indicated that the active luminescent centers have different symmetries and atomic structure. The dependence of the photoluminescence intensity on the laser excitation power and on the temperature was measured. Results are discussed on the basis of a physical model which includes the formation of free excitons, the binding of excitons to erbium ions, the excitation of 4f inner-shell electrons of the erbium ions, and their subsequent decay by light emission. To obtain a quantitative agreement between model analysis and experimental data the consideration of Auger processes by which erbium-bound excitons and erbium ions in excited state can decay by dissipating energy to conduction band electrons appears to be required. From the temperature dependence two activation energies are derived which are associated with the exciton binding energies and with an energy transfer process from excited erbium ions back to erbium-bound excitons, respectively. A good quantitative agreement can be obtained for suitable values of the model parameters. The luminescent properties of the samples of the different types of crystalline silicon are remarkably similar.
Picosecond photofragment spectroscopy. II. The overtone initiated unimolecular reaction H2O2(vOH=5)→2OH
Authors: Scherer Norbert F. , Zewail Ahmed H.
Published: July 2002

This paper, second in the series, reports on the picosecond time-resolved photofragmentation of the overtone (vOH=5) initiated reaction: HOOH+hν→2OH. The hydrogen peroxide is initially excited by way of a picosecond laser pulse to the fourth overtone level of the OH-stretch local mode. The subsequent unimolecular reaction behavior is obtained by monitoring the laser-induced fluorescence, caused by the picosecond probe–pulse electronic excitation of the OH radical photoproduct (in a given rotational state). The two pulses are scanned relative to one another in time thereby mapping out the product yield for the given delay-time interval. The resultant product formation behavior is found to be nonexponential, and may be modeled as a biexponential rise. Furthermore, the quasibiexponential behavior is sensitive to the exact excitation wavelength—slight variations of which result in large changes in the two time constants and the relative amplitudes of the fast and slow components. These experiments give direct evidence for the inhomogeneous nature of the overtone transition on the picosecond time scale, and provide the dissociation rate contribution to the homogeneous width (0.05–0.15 cm−1).

The apparent width for the main band feature is about 200 cm−1. The rate of product formation (magnitude and form) is interpreted in terms of statistical and nonstatistical theories. The limitations of the applicability of each model is discussed. The fluctuations of the fitting parameters as a function of excitation wavelength may be simulated by a statistical model which considers all possible discrete optical transitions within the simulated laser bandwidth and the details of product formation from each state. For a nonstatistical interpretation, the biexponential form reflects a division of the vibrational phase space, and this is discussed in the spirit of a kinetic model. Finally, experimental results are reported for direct UV initiated photofragmentation. The observed dynamics indicate that a very different type of potential surface (repulsive) is involved, in contrast to the overtone initiated dissociation, which takes place on the ground state surface.
Self-diffusion of Brownian particles in concentrated suspensions under shear
Authors: Bossis G. , Brady J. F.
Published: July 2002

The self-diffusivity of Brownian hard spheres in a simple shear flow is studied by numerical simulation. Particle trajectories are calculated by Stokesian dynamics, with an accurate representation of the suspension hydrodynamics that includes both many-body interactions and lubrication. The simulations are of a monolayer of identical spheres as a function of the Péclet number: Pe =γ̇a2/D0, which measures the relative importance of shear and Brownian forces. Here γ̇ is the shear rate, a the particle radius, and D0 the diffusion coefficient of a single sphere at infinite dilution. In the absence of shear, using only hydrodynamic interactions, the simulations reproduce the pair-distribution function of the equivalent hard-disk system. Both short- and long-time self-diffusivities are determined as a function of the Péclet number. The results show a clear transition from a Brownian motion dominated regime (Pe<1) to a hydrodynamically dominated regime (Pe>10) with a dramatic change in the behavior of the long-time self-diffusivity.
The expansion of product probability distributions from trajectory calculations in two-dimensional Fourier series
Authors: Budenholzer F. E. , Hu S. C. , Jeng D. C. , Gislason E. A.
Published: July 2002

The procedure for expanding product probability density functions determined from a classical trajectory study in a two-dimensional Fourier series is developed. This method has the advantage that essentially all of the information obtained from the trajectories is retained in the expansion; consequently, the resolution of the density functions is high. A smoothing technique using a Gaussian filter is also presented. The results are applied to a quasiclassical study of the F+H2 reaction, and new insights are obtained for this reaction.
The 2 1A
g state in the linear polyene 2,4,6,8,10,12,14,16-octadecaoctaene
Authors: Kohler Bryan E. , Spangler Charles , Westerfield Curtis
Published: July 2002

High resolution fluorescence and fluorescence excitation spectra have been obtained for the all-trans polyene 2,4,6,8,10,12,14,16-octadecaoctaene in an n-hexadecane matrix at 4.2 K. This is the longest discrete linear polyene for which such data are known. The 1 1Ag to 2 1Ag 0–0 band at 17 871 cm−1 is 4900 cm−1 below 1 1Ag to 1 1Bu 0–0 band at 22 770 cm−1. Both of these excitation energies are approximately what would have been predicted by fitting the 0–0 excitation energies of shorter polyenes with N double bonds by a 1/N curve and extrapolating to N=8. These spectra provide further information on the vibronic levels of the ground (1 1Ag) and first excited (2 1Ag ) singlet states and provide insight into the structure of the conjugated polymer polyacetylene. As has been found in the case of shorter polyenes, the strongest vibronic feature (assigned as a totally symmetric C=C stretch) increases 1554 cm−1 in the 1 1Ag state to 1799 cm−1 in the 2 1Ag state) also for the octaene. Possible reasons for this increase are discussed.
Unlimited full configuration interaction calculations
Authors: Knowles Peter J. , Handy Nicholas C.
Published: July 2002

In very large full configuration interaction (full CI), nearly all of the CI coefficients are very small. Calculations, using a newly developed algorithm which exploits this fact, on NH3 with a DZP basis are reported, involving 2×108 Slater determinants. Such calculations are impossible with other existing full CI codes. The new algorithm opens up the opportunity of full CI calculations which are unlimited in size.
Integral equation theory of the structure and thermodynamics of polymer blends
Authors: Schweizer Kenneth S. , Curro John G.
Published: July 2002

Our recently developed RISM integral equation theory of the structure and thermodynamics of homopolymer melts is generalized to polymer mixtures. The mean spherical approximation (MSA) closure to the generalized Ornstein–Zernike equations is employed, in conjunction with the neglect of explicit chain end effects and the assumption of ideality of intramolecular structure. The theory is developed in detail for binary blends, and the random phase approximation (RPA) form for concentration fluctuation scattering is rigorously obtained by enforcing incompressibility. A microscopic, wave vector-dependent expression for the effective chi parameter measured in small angle neutron scattering (SANS) experiments is derived in terms of the species-dependent direct correlation functions of the blend. The effective chi parameter is found to depend, in

general, on thermodynamic state, intermolecular forces, intramolecular structure, degree of polymerization, and global architecture. The relationship between the mean field Flory–Huggins expression for the free energy of mixing and our RISM-MSA theory is determined, along with general analytical connections between the chi parameter and intermolecular pair correlations in the liquid. Detailed numerical applications to athermal and isotopic chain polymer blend models are presented for both the chi parameter and the structure. For athermal blends a negative, concentration-dependent chi parameter is found which decreases with density, structural asymmetry, and increases with molecular weight. For isotopic blends, the effective (positive) chi parameter is found to be strongly renormalized downward from its mean field enthalpic value by long range fluctuations in monomer concentration induced by polymeric connectivity and excluded volume. Both the renormalization and composition dependence of the chi parameter increase with chain length and proximity to the spinodal instability. The critical temperature is found to be proportional to the square root of the degree of polymerization in stark contrast to the classical mean field prediction of a linear dependence. Comparison of the theoretical predictions with SANS measurements and computer simulations is presented, along

with brief discussions of nonideal effects and lower critical solution temperature phenomena.
A centrifugal-sudden distorted wave study of the Cl+HCl→ClH+Cl reaction using a ‘‘tight-bend’’ potential energy surface
Authors: Schatz George C. , Amaee B. , Connor J. N. L.
Published: July 2002

Quantum calculations of product rotational distributions, cross sections and rate coefficients have been performed for the Cl+HCl→ClH+Cl reaction using the centrifugal-sudden distorted-wave (CSDW) technique. The CSDW method has been shown to be nearly exact at low total energies where the total reaction probability for each partial wave is small. The potential energy surface used is the extended London–Eyring–Polanyi–Sato surface No. 3 of Persky and Kornweitz. This surface has a tighter bending potential near the saddle point than many of the other commonly used Cl+HCl potentials. This difference in bending potential is found to have a profound influence on the product rotational distributions, producing much colder HCl molecules than is the case for the other potentials. In contrast, quantities such as rate coefficients and cumulative reaction probabilities are found to be only weakly sensitive to the strength of the bending potential. Comparison with quasiclassical trajectory results shows reasonable agreement with respect to the shape of the rotational distributions, but the magnitudes of the cross sections and rate coefficients are substantially different.
Ultraviolet-laser induced dissociation and desorption of water adsorbed on Pd(111)
Authors: Wolf M. , Nettesheim S. , White J. M. , Hasselbrink E. , Ertl G.
Published: July 2002

Ultraviolet-laser irradiation (6.4 eV and 5.0 eV) of the first layer of water adsorbed on a Pd(111) surface at 90 K leads to desorption of H2O and to conversion of the adsorbed state as manifested in the thermal desorption spectra. The latter effect is attributed to photodissociation of water on the surface. Time-of-flight measurements show that water molecules desorb with the same translational energy of about 600 K for both photon energies. While desorption is suppressed with adsorbed multilayers, conversion within the first layer still proceeds.
A
b i
n
i
t
i
o multireference configuration interaction study of CH2NNO2. HONO elimination vs NN bond fragmentation
Authors: Mowrey R. C. , Page M. , Adams G. F. , Lengsfield B. H.
Published: July 2002

Features of the potential energy surface pertinent to the thermal decomposition mechanism of methylenenitramine (CH2NNO2) are investigated. At issue are the activation energies both for concerted molecular dissociation leading to HCN and HONO and for the simple NN bond cleavage leading to CH2N and NO2. We use multiconfiguration (MC)SCF wave functions with double-zeta and double-zeta plus polarization quality basis sets to locate minimum energy and transition state structures as well as determine harmonic vibrational frequencies and resulting zero-point vibrational energies. The energetics are determined using configuration interaction wave functions which include single and double excitations from a selected set of reference configurations constructed with the MCSCF molecular orbitals. Our best calculations, without adjustment, predict the molecular dissociation to have an activation energy slightly lower than that for simple bond rupture. Calculations at the level of those reported here are known, however, to underestimate bond dissociation energies by several kcal/mol. Based on trends from past calculations, and new calculations on a related compound, we estimate the activation energy for the HONO elimination path to be 31±4 kcal/mol and the NN bond dissociation energy to be 35±4 kcal/mol. This reverses the energetic ordering of previous theoretical predictions and is consistent with recent molecular beam experiments.
Potential energy hypersurfaces for the interaction of NO with the Ag(111) surface
Authors: DePristo Andrew E. , Alexander Millard H.
Published: July 2002

We present a new potential energy surface for the interaction of the NO molecule with a close-packed Ag(111) surface obtained using the recently developed corrected effective medium (CEM) method. The dual NO–Ag(111) interaction potentials, which reflect the electronic orbital degeneracy of the NO molecule in its ground(X 2Π) electronic state, were determined at a large variety of distances and orientations of the NO molecule above the surface, and for approach of the molecule over three different sites on the surface unit cell, described by a cluster of greater than 10 Ag atoms. The more than 600 calculated interaction energies were then fit to a compact analytic form, which allows the determination of the interaction potential at any position above the surface. The strongest binding corresponds to a nearly flat orientation of the NO molecule, with approach of the N end slightly preferred, and with the single antibonding π orbital of NO pointing toward the surface. In this geometry, the binding energy above, respectively, the atop, two-fold bridge, and three-fold center sites site is 2200, 1773, and 1574 cm−1. By contrast when the antibonding π electron is oriented in a plane parallel to the surface, the binding energy diminishes dramatically, and the dependence of the interaction energy on the orientation of the NO molecule is also diminished. Here, the three-fold center site is preferred, but with a well depth of only 885 cm−1 (0.110 eV).
Monte Carlo simulations and integral equation theory for microscopic correlations in polymeric fluids
Authors: Yethiraj Arun , Hall Carol K.
Published: July 2002

Monte Carlo simulations are performed for polymer chains modeled as pearl necklaces of freely jointed tangent hard spheres; chains composed of 20, 50, and 100 beads are studied at volume fractions ranging from 0.1 to 0.35. The mean-square end-to-end distance, the radius of gyration, and the intramolecular and intermolecular site–site distribution functions are monitored in the simulations. Various approximations for the intramolecular structure factor, ŵ(k), are tested. It is found that the ŵ(k) from the semi-flexible chain model is the most accurate. The polymer ‘‘reference interaction site model’’ (PRISM) theory of Curro and Schweizer is tested using both approximate and exact expressions for ŵ(k). It is found that, at the densities examined here, the theory is accurate for the local structure except near contact where it tends to overestimate the value of the intermolecular site–site distribution function, g(r). The polymer-RISM theory is also solved with the generalized mean spherical approximation (GMSA), which uses a Yukawa closure for the direct correlation function. The contact value of g(r), required in the GMSA, is obtained approximately, but accurately, via a perturbation expansion for a hypothetical fluid and the generalized Flory dimer equation of state. The GMSA theory results in improved predictions for g(r) when compared with the original polymer-RISM theory, but there are still some differences between theoretical predictions and simulation results near contact.
Infrared spectroscopy of carbo-ions. VI. C–H stretching vibration of the acetylene ion C2H2+ and isotopic species
Authors: Jagod Mary-Frances , Rösslein Matthias , Gabrys Charles M. , Rehfuss Brent D. , Scappini Flavio , Crofton Mark W. , Oka Takeshi
Published: July 2002

The infrared spectra of the band of the 2Π–2Π asymmetric hydrogen stretching vibration in the three isotopic acetylene ions C2H2+ (ν3), 13C2H2+ (ν3), and DCCH+ (ν1) have been observed and analyzed. The high resolution infrared spectra were recorded using a difference-frequency laser spectrometer as the tunable coherent infrared source probing an ac glow discharge. Velocity modulation, noise subtraction, and unidirectional multipassing of the infrared beam through the discharge cell provided high sensitivity. C2H2+ was produced in a gas mixture of H2, He, and either CH4 or C2H2, with a total pressure of ≊7 Torr in multiple-inlet–outlet air-, water-, and liquid-nitrogen-cooled discharge tubes; C2H2 freezing precluded its use in liquid-N2-cooled discharges. Complicated by a strong perturbation whose maximum occurred at N′=15 for F1 and N′=14 for F2, the assignment of the spectrum of normal C2H2+ was made possible by (1) fortuitous discharge conditions which provided unambiguous discrimination of C2H2+ lines from among concurrent CH3+ and C2H3+ lines, and (2) fitting the ground state combination differences. Sufficiently high N transitions were observed where Λ doubling was evident. The average bond lengths rz(CH)=1.077 (5) Å and rz(CC)=1.257 (8) Å were calculated from the spectroscopic constants determined from nonlinear least-squares fitting. Vibration–rotation interactions, the Renner–Teller interaction of perturbing states, plasma chemistry, and the relevance of the work in astrophysics are discussed.
The cumulative reaction probability as eigenvalue problem
Authors: Manthe Uwe , Miller William H.
Published: July 2002

It is shown that the cumulative reaction probability for a chemical reaction can be expressed (absolutely rigorously) as N(E)=∑kpk(E), where {pk} are the eigenvalues of a certain Hermitian matrix (or operator). The eigenvalues {pk} all lie between 0 and 1 and thus have the interpretation as probabilities, eigenreaction probabilities which may be thought of as the rigorous generalization of the transmission coefficients for the various states of the activated complex in transition state theory. The eigenreaction probabilities {pk} can be determined by diagonalizing a matrix that is directly available from the Hamiltonian matrix itself. It is also shown how a very efficient iterative method can be used to determine the eigenreaction probabilities for problems that are too large for a direct diagonalization to be possible. The number of iterations required is much smaller than that of previous methods, approximately the number of eigenreaction probabilities that are significantly different from zero. All of these new ideas are illustrated by application to three model problems—transmission through a one-dimensional (Eckart potential) barrier, the collinear H+H2→H2+H reaction, and the three-dimensional version of this reaction for total angular momentum J=0.
Quantum correction for electron transfer rates. Comparison of polarizable versus nonpolarizable descriptions of solvent
Authors: Song Xueyu , Marcus R. A.
Published: July 2002

The electron transfer rate constant is treated using the spin-boson Hamiltonian model. The spectral density is related to the experimentally accessible data on the dielectric dispersion of the solvent, using a dielectric continuum approximation. On this basis the quantum correction for the ferrous–ferric electron transfer rate is found to be a factor 9.6. This value is smaller than the corresponding result (36) of Chandler and co-workers in their pioneering quantum simulation using a molecular model of the system [J. S. Bader, R. A. Kuharski, and D. Chandler, J. Chem. Phys. 93, 230 (1990)]. The likely reason for the difference lies in use of a rigid water molecular model in the simulation, since we find that other models for water in the literature which neglect the electronic and vibrational polarizability also give a large quantum effect. Such models are shown to overestimate the dielectric dispersion in one part of the quantum mechanically important region and to underestimate it in another part. It will be useful to explore a polarizable molecular model which reproduces the experimental dielectric response over the relevant part of the frequency spectrum.
Predissociation dynamics on a highly anisotropic potential: OH–Ar (A 2Σ+)
Authors: Lester Marsha I. , Randall Robert W. , Giancarlo Leanna C. , Choi Seung E.
Published: July 2002

Metastable levels of OH–Ar which lie significantly above the OH (A 2Σ+)+Ar dissociation limit are identified. The rates of internal rotational predissociation and the resultant OH (A 2Σ+) product rotational distributions are determined. Comparison with theoretical calculations of the predissociative states provides a sensitive test of the short-range interaction potential.
Electric field modulation studies of optical absorption in CdSe nanocrystals: Dipolar character of the excited state
Authors: Colvin V. L. , Cunningham K. L. , Alivisatos A. P.
Published: July 2002

The Stark effect on the electronic absorption spectrum of CdSe nanocrystals has been studied for nanocrystals ranging in size from 80 to 20 Å in diameter. For all but the smallest clusters, a second derivative line shape is observed, indicative of a dipole moment in the excited state. This result is independent of the surface modification and appears in both CdS and CdSe systems. The Δμ ranges from 15±10 D in the smallest clusters and up to 100±10 D in the largest; however, the increase is not monotonic, and in the very largest clusters studied (d≳70 Å), the dipole moment decreases. The dipolar character is lost in clusters less than 25 Å. These results can be explained by a model in which there is resonance of an interior state with a surface state at a particular size, with the mixing occurring on a preferred axis.
The effect of temperature on the structure of amorphous hydrogenated carbon
Authors: Walters J. K. , Fox D. M. , Burke T. M. , Weedon O. D. , Newport R. J. , Howells W. S.
Published: July 2002

The results of a neutron diffraction study on the structure of amorphous hydrogenated carbon a-C:H are presented up to a maximum temperature of 1000 °C. The data show clearly the effect on atomic correlations of elevated temperatures, with the initial room-temperature amorphous network (a mixture of single bonds and olefinic double bonds) becoming progressively aromatic, then graphitic as hydrogen is evolved. Complementary x-ray diffraction and infrared spectroscopy data are also presented, the infrared data enabling a more detailed discussion of the temperature-dependent hydrogen environment, and the x-ray data are used to highlight the change in the carbon network. Comparisons have been made with previous work on similar systems and a brief summary of these results is given.
Statistics of simple chains in a sea of blockers
Authors: Leung Kevin , Chandler David
Published: July 2002

We carry out a simulation of freely jointed ring polymers in the presence of uncorrelated blockers on a cubic three-dimensional lattice. The size of the polymer is shown to obey Lifshitz scaling at large polymer mass. The simulation results are compared with the predictions of the RISM polaron theory [D. Chandler, Y. Singh, and D. M. Richardson, J. Chem. Phys. 81, 1975 (1984)] derived for the nearly isomorphic problem of thermal excess electrons in simple fluids. Implications of the Gaussian density field treatment of the fluid in that theory are discussed. Further comparisons are drawn between the blocker model and the quantum percolation problem. This correspondence allows us to compare the values of mobility edge (the energy of electronic states at the bottom of the conduction band) in hard-sphere-like fluids with those in lattice models.