bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of diffusion coefficients in degenerate electron gas using Monte Carlo simulation
Authors: Thobel J. L. , Sleiman A. , Fauquembergue R.
Published: July 2002

We propose a method for determining diffusion coefficients in degenerate semiconductors from an ensemble Monte Carlo simulation. The basic idea is that what is relevant for this problem is not the whole electron distribution function, but its perturbation in response to an addition of “excess carriers.” Starting from the Boltzmann transport equation, we derive the equation of evolution for this “excess electron distribution function.” We propose an interpretation in terms of scattering events suffered by particles, allowing one to solve the problem by Monte Carlo simulation. We simulate two sets of carriers, coupled by an “exchange scattering” mechanism which is properly defined. The first set represents the uniform background density in the semiconductor, whereas the second one represents the excess carriers. Only the latter is used for calculating diffusion coefficients. This method is applied to a highly degenerate two-dimensional electron gas in a doped GaAs quantum well. The diffusivity-field characteristics are calculated and discussed.
Dynamics of overdamped Josephson junctions driven by a square-wave pulse
Authors: Kim Jinhee , Sosso Andrea , Clark Alan F.
Published: July 2002

The periodic solutions of an overdamped Josephson junction driven by a square-wave pulse were found. Unlike those driven by a sinusoidal ac current, the integer step widths of a Josephson junction driven by a square-wave pulse train are reduced with increasing ac current frequency. The maximum value of any non-zero step width approaches 2Ic, where Ic is the critical current, by decreasing either the pulse repetition rate or the pulse duration time. The characteristic features of any integer step are almost identical and we give an explanation of this.
Measurement of the lattice thermal expansion coefficients of thin metal films on substrates
Authors: Kraft O. , Nix W. D.
Published: July 2002

A difference in thermal expansion between a thin film and its substrate causes mechanical stresses in the film. Therefore, knowledge of the thermal expansion coefficients of thin films are important for their technological applications. In this article, we present an analysis which can be used to extract the thermal expansion coefficient of a thin film material using a commonly used x-ray technique. The major advantage of our approach is that it is not necessary to remove the film from the substrate. The knowledge of the elastic constants of the thin film material and their temperature dependence is not required, which is particularly useful when thin film alloys are studied whose thermal and elastic properties are not available. For verification of the method, we investigated thin Al films because the thermal and elastic properties of bulk Al are well known. The comparison of our results with the bulk properties shows a reasonable agreement, indicating the validity of the new method.
Numerical modeling and experimental considerations for a two-coil apparatus to measure the complex conductivity of superconducting films
Authors: Turneaure Stefan J. , Pesetski Aaron A. , Lemberger Thomas R.
Published: July 2002

We consider the accuracy of measurements of the complex conductivity of superconducting films with a two-coil mutual inductance technique. We present a numerical analysis of the procedure by which we deduce the real and imaginary parts of the conductivity, σ=σ1−iσ2, of thin films from the in-phase and out-of-phase components of the mutual inductance of coaxial coils located on opposite sides of the film. The accuracy of the procedure is verified for the full ranges of film radii, thicknesses, and conductivities that are encountered for typical films of a wide variety of cuprate superconductors. We determine both experimentally and theoretically what effect flaws in the film would have on the accuracy of the measurement by examining the effects of holes located at various places in a superconducting film. The effect of capacitive coupling between the coils is measured and shown to be negligible when care is taken in grounding the drive and pickup coil circuits. The mutual inductance of the coils changes with temperature even with no sample present because the resistance of the coils changes and there is some thermal contraction. We describe a procedure for taking these effects into account.
An increase of the spall strength in aluminum, copper, and Metglas at strain rates larger than 107 s−1
Authors: Moshe E. , Eliezer S. , Dekel E. , Ludmirsky A. , Henis Z. , Werdiger M. , Goldberg I. B. , Eliaz N. , Eliezer D.
Published: July 2002

Measurements of the dynamic spall strength in aluminum, copper, and Metglas shocked by a high-power laser to hundreds of kilobars pressure are reported. The strain rates in these experiments are of the order of 107 s−1, which cannot be reached in impact experiments. The free-surface velocity behavior associated with spallation is characterized by oscillations caused by the reverberations of the spall layer. An optically recording velocity interferometer system was developed to measure the free-surface velocity time history. This diagnostic method has the advantages of being a noninterfering system and produces a highly accurate continuous measurement in time. The spall strength was calculated from the free-surface velocity as a function of the strain rate. The results show a rapid increase in the spall strength, suggesting that a critical phenomenon occurs at strain rates ∼107 s−1, expressed by the sudden approach to the theoretical value of the spall strength.
Fabrication of submicrometric magnetic structures by electron-beam lithography
Authors: Martı́n J. I. , Jaccard Y. , Hoffmann A. , Nogués J. , George J. M. , Vicent J. L. , Schuller Ivan K.
Published: July 2002

Submicrometric magnetic structures have been fabricated by electron-beam lithography on Si substrates. High-quality patterns have been obtained with typical length scale of the structures in the range of 100 nm. The designed geometrical configurations are suitable for investigation of their physical properties by transport measurements in a controlled way. In particular, long chains of connected magnetic dots are useful to analyze magnetization reversal processes, whereas ordered arrays of isolated dots can be used to study pinning effects in superconducting films.
Impurity photovoltaic effect in c-Si solar cells. A numerical study
Authors: Schmeits M. , Mani A. A.
Published: July 2002

A numerical study is made of the impurity photovoltaic (IPV) effect in crystalline Si p-n junction solar cells. Application is made to In impurities whose energy level is at 0.157 eV above the valence band edge. The numerical resolution of Poisson’s equation and the continuity equations for electrons, holes and defect levels yields the current–voltage curves for different impurity concentrations Nt, from which the short-circuit current density Jsc, the open-circuit voltage Voc, and the energy conversion efficiency η is obtained. It is shown that increasing the defect concentration Nt with respect to the background donor concentration leads to an increase of Jsc but decreases Voc and η. A modified defect concentration profile in a p-n-n+ structure is suggested to extract the maximum benefit from the IPV effect.
Texture analysis of damascene-fabricated Cu lines by x-ray diffraction and electron backscatter diffraction and its impact on electromigration performance
Authors: Vanasupa Linda , Joo Young-Chang , Besser Paul R. , Pramanick Shekhar
Published: July 2002

The texture of electroplated Cu lines of 0.375, 0.5 and 1.5 μm widths with Ta and TiN barrier layers was analyzed using x-ray pole figure and electron backscatter diffraction (EBSD) techniques. Both techniques indicate a strong (111) fiber texture relative to the bottom surface of the trench for samples with a Ta barrier layer and a 400 °C, 30 min, postelectroplating anneal. Samples with a TiN barrier and no anneal exhibit a weak (111) texture. For both barrier layers the quality of the texture, as measured by (111) peak intensity, fraction of randomly oriented grains and (111) peak width, degrades with decreasing linewidth. EBSD data also indicate (111) texture relative to the sidewalls of the trench in samples with a Ta barrier and postelectroplating anneal. Electromigration tests at 300 °C of 0.36 μm damascene Cu lines with the same process conditions show that samples with very weak (111) texture have median time to failures that exceed those of the strongly textured Cu lines. These results indicate that diffusion at interfaces, such as the Cu/barrier and Cu/overlayer interfaces, along with diffusion along an electroplating seam play more dominant roles in electromigration failure in damascene-fabricated lines than diffusion along grain boundaries within the interconnect.
Sheath thickness dependence on resonance condition during fast ion extraction using a radio frequency resonance method
Authors: Goto N.
Published: July 2002

Fast ion extraction from plasma as a function of magnetic flux densities using an rf resonance method has been investigated with X-Windows version plasma device planar 1 dimension (XPDP1), a one-dimensional particle simulation code. The resonance condition is dependent on plasma density, sheath thickness, gap distance between electrodes, magnetic field and radio frequency. Due to the decrease in plasma density during ion extraction, an increase in sheath thickness is observed. The relationship between the plasma density and the sheath thickness, derived from a theory of sheath-plasma resonance in a magnetic field, can explain that the increase in sheath thickness is due to the continuance of the rf resonance. The rate of ion extraction is important in the isotope separation technique, and can be predicted from the fact that the sheath thickness is determined by the sheath-plasma resonance theory in the rf resonance method.
Influence of crystal structure on the perpendicular magnetic anisotropy of an epitaxial CoPt alloy
Authors: Huang J. C. A. , Hsu A. C. , Lee Y. H. , Wu T. H. , Lee C. H.
Published: July 2002

By molecular beam epitaxy CoPt1.1 alloys were simultaneously prepared on Mo seeding layers on Al2O3(11–20), (1–102), and (1–100) substrates, respectively. Distinct crystal structures and chemical ordering of the CoPt1.1 alloys were observed for substrate temperatures of 300 and 400 °C. Structural and magnetic observations for CoPt1.1 alloys grown on separate sapphire substrates show that the appearance of the ordered L11(111) phase results in an enhancement of the perpendicular magnetic anisotropy and Kerr rotations in the CoPt alloys.
Microstructure of low temperature grown AlN thin films on Si(111)
Authors: Auner G. W. , Jin F. , Naik V. M. , Naik R.
Published: July 2002

AlN thin films were grown on HF-etched Si(111) substrates at 400–600 °C by plasma source molecular beam epitaxy. Reflection high energy electron diffraction and transmission electron microscopy studies show that AlN films grown at 400 °C form an initial amorphous region at the interface, followed by c-axis oriented columnar grains with slightly different tilts and twists. AlN films grown at 600 °C showed a significantly reduced amorphous region near the interface promoting an epitaxial growth of AlN with AlN[0001]∥Si[111] and AlN[011̄0]∥Si[112̄] orientations. However, all the films show numerous defects such as stacking faults, dislocations, and grain boundaries.
New carbon nitride phase coherently grown on Si(111)
Authors: Barucca G. , Majni G. , Mengucci P. , Leggieri G. , Luches A. , Martino M. , Perrone A.
Published: July 2002

Carbon nitride films, deposited on Si(111) substrates at room temperature by XeCl laser ablation of graphite targets in low pressure (1, 5, 10, and 50 Pa) N2 atmosphere at the fluence of 12 J/cm2 (∼0.4 GW/cm2), have been submitted to accurate x-ray diffraction and transmission electron microscopy investigations in order to study the structure of the films. Results showed that the samples are constituted of a continuous amorphous film inside which microcrystals of a new coherently grown CNx phase are dispersed. This new phase has a triclinic crystallographic cell with lattice parameters a=b=0.384 nm, c=0.438±0.007 nm, α=110±1°, β=105±1°, and γ=120°. It coherently grows on the (111) Si plane with the following orientation relationships: (001)CNx∥(111)Si, [100]CNx∥[1-10)]Si, and [010]CNx∥[01-1)]Si.
Microhardness-bulk modulus scaling and pressure-induced phase transformations in AIBIIIC2VI chalcopyrite compounds
Authors: Rincón C. , Villareal Isavic , Galindo H.
Published: July 2002

The microhardness H of the AIBIIIC2VI chalcopyrites can be estimated from the bulk modulus B0 by using a simple scaling law. Values of B0 for these compounds, obtained from energy dispersive x-ray diffraction and x-ray absorption spectroscopy techniques under high pressure, have been used in the scaling. The pressure-induced phase transformation from the chalcopyrite to denser structures, observed in these compounds, is also discussed. A relationship between H and the critical pressure at which such a transition occurs is also obtained.
Effects of point defect distribution on arsenic precipitation in low-temperature grown III–V arsenides
Authors: Chang M. N. , Hsieh K. C. , Nee T.-E. , Chyi J.-I.
Published: July 2002

The behavior of As precipitation in low-temperature grown III–V arsenides is investigated and correlated with the doping level, crystal bond strength, and dislocation density. Experimental results reveal that the doping level affects the concentration of charged defects, such as vacancy and antisite point defects, and hence leads to the selective precipitation of excess As in homojunctions. For heterostructures, As precipitates tend to condense in materials with a lower bond strength due to differences in point defect concentrations between the materials. In addition, dislocations are found to be a vacancy source that facilitates As precipitation around them. These results indicate that column III vacancies play an important role in As precipitation of low-temperature grown III–V arsenides.
Sensitivity of near-edge x-ray absorption fine structure spectroscopy to ion beam damage in diamond films
Authors: Laikhtman A. , Gouzman I. , Hoffman A. , Comtet G. , Hellner L. , Dujardin G.
Published: July 2002

In the present work, we study the sensitivity of the near-edge x-ray absorption fine structure (NEXAFS) spectroscopy to ion induced defects in polycrystalline diamond films. The ion bombardment of hydrogenated films is performed using 30 keV Xe+ ions at room temperature for doses ranging from 2×1013 ions/cm2, producing local point defects, to 2×1015 ions/cm2, which results in almost complete amorphization of the diamond surface. Auger electron spectroscopy measurements are not sensitive to the modifications induced by the lowest implantation dose. Whereas partial electron yield (PEY) NEXAFS measurements, applied in surface and bulk-sensitive modes, using 35, 15, and 8 eV secondary electrons, respectively, reveals the formation of a defective structure and gradual deterioration of diamond in the near-surface region. From PEY NEXAFS spectra measured using 15 eV secondary electrons, the position of C(1s) binding energy is measured. The x-ray photoelectron spectra of the samples were measured using an incident photon energy of 450 eV. It is found that the C(1s) binding energy in the implanted samples has a positive shift of 0.6–1 eV, which is indicative of transformation of diamond to disordered carbon. The high sensitivity of NEXAFS spectroscopy to point defects induced by the low dose ion implantation was reflected by a sharp reduction in the intensity of the diamond core exciton peak and by the appearance of a new spectral feature in the pre-edge region, below the C(1s)−π* transition. Analysis of the NEXAFS spectra of ion implanted films is performed on the basis of the electronic structure of diamond.
Raman scattering from very thin Si layers of Si/SiO2 superlattices: Experimental evidence of structural modification in the 0.8–3.5 nm thickness region
Authors: Khriachtchev Leonid , Räsänen Markku , Novikov Sergei , Kilpelä Olli , Sinkkonen Juha
Published: July 2002

Raman study of very thin (⩽3.5 nm) Si layers constituting Si/SiO2 superlattices and grown by molecular beam epitaxy is described. The Raman spectra show systematic dependence on thickness of the Si layers, which highlights the variety of disordered microstructures in the Si/SiO2 superlattices. A clear change in the vibrational properties is found to occur in the 0.8–3.5 nm thickness region. In particular, the Raman spectra are typical for amorphous silicon for the thicker layers, and the characteristic phonon band disappears for the thinner layers, presumably representing another form of Si coordination with a small Raman scattering cross section. In addition, absorption of the material changes essentially with the Si-layer thickness. Photoluminescence is detected from the Si/SiO2 superlattices, the superlattices with 1.2 and 1.8 nm Si layers being the most efficient emitters among our samples, and the photoluminescence is blueshifted with the decrease of the Si-layer thickness. The Raman spectra show no sign of nanocrystalline structure at any thickness of the as-deposited Si layers so that the observed photoluminescence cannot be connected with Si nanocrystallities. Annealing strongly changes the Raman and photoluminescence spectra, a well-ordered Si phase appears in the superlattices, but its increase does not correlate with the photoluminescence, which further disregard it as an emitter. Nevertheless, the emitting phase is not identified in the Raman spectra.
Thin film growth and band lineup of In2O3 on the layered semiconductor InSe
Authors: Lang O. , Pettenkofer C. , Sánchez-Royo J. F. , Segura A. , Klein A. , Jaegermann W.
Published: July 2002

Thin films of the transparent conducting oxide In2O3 have been prepared in ultrahigh vacuum by reactive evaporation of indium. X-ray diffraction, optical, and electrical measurements were used to characterize properties of films deposited on transparent insulating mica substrates under variation of the oxygen pressure. Photoelectron spectroscopy was used to investigate in situ the interface formation between In2O3 and the layered semiconductor InSe. For thick In2O3 films a work function of φ=4.3 eV and a surface Fermi level position of EF−EV=3.0 eV is determined, giving an ionization potential IP=7.3 eV and an electron affinity χ=3.7 eV. The interface exhibits a type I band alignment with ΔEV=2.05 eV, ΔEC=0.29 eV, and an interface dipole of δ=−0.55 eV.
Nucleation and annihilation of magnetic vortices in submicron-scale Co dots
Authors: Fernandez A. , Cerjan C. J.
Published: July 2002

We describe experiments on arrays of polycrystalline Co structures fabricated by interference lithography. The dots are thin (15–40 nm), submicron in size, and are patterned with a uniaxial, in-plane, shape anisotropy axis. We use magnetic force microscopy (MFM) in the presence of an applied field to directly observe magnetic reversal in the dots. These experiments reveal that reversal occurs predominantly through the nucleation and annihilation of a single magnetic vortex in each dot. Hysteresis loop measurements indicate that the vortices are stable over a wide range of applied fields and that the limits of this range depend on the size and thickness of the dots. Using the MFM data, we determine the statistical distribution of the single-vortex nucleation field for several different arrays. We attribute the observed variance to the random orientation of the polycrystalline grains. Finally, we show that the average vortex nucleation and annihilations fields are linearly correlated to the demagnetization field of a uniformly magnetized structure.
Effect of annealing in reduced oxygen pressure on the electrical transport properties of epitaxial thin film and bulk (La1−xNdx)0.7Sr0.3MnO3
Authors: Wu Wenbin , Wong K. H. , Li X.-G. , Choy C. L. , Zhang Y. H.
Published: July 2002

A comparative study of the effect of annealing in reduced oxygen pressure on the electrical transport properties of (La1−xNdx)0.7Sr0.3MnO3 (x=0, 0.25, 0.5, 0.75, and 1) epitaxial thin films and bulk materials has been carried out. The epitaxial films grown by pulsed laser ablation were in situ annealed in an oxygen atmosphere of 2×10−6–760 Torr at 700 °C for 1 h. It is found that the electrical transport behavior of the epitaxial film is insensitive to the annealing pressure. A similar thermal treatment on the bulk materials at 5 mTorr oxygen ambient, however, caused a dramatic change in their resistivity-temperature dependence. Our results suggest that the annealing has a prominent effect on the properties of grain boundary, which plays an important role in determining the electrical transport behavior of polycrystalline manganites.
Soft-magnetic properties of Fe–Co–B thin films for ultra-high-frequency applications
Authors: Chen L. H. , Klemmer T. J. , Ellis K. A. , van Dover R. B. , Jin S.
Published: July 2002

Fe 67 Co 18 B 15 amorphous thin films were sputter deposited on glass substrates under a uniform applied magnetic field. The films exhibit excellent soft-magnetic properties of 4πMs∼17.5 kG, Hc∼0.3 Oe, and Hk (anisotropy field) ∼22 Oe. The ultra-high-frequency performance on these films was studied and the relative permeability μ′ is shown to be as high as ∼740 and almost constant to at least 1 GHz. The combination of high 4πMs and relatively high Hk in these films is believed to be partly responsible for the excellent ultra-high-frequency behavior. The soft-magnetic properties of these films show a strong dependence on the film deposition bias voltage, which presumably changes the microstructure of the films and the related magnetic anisotropy.
Domain wall pinning in high temperature SM(Co, Fe, Cu, Zr)7-8 magnets
Authors: Streibl B. , Fidler J. , Schrefl T.
Published: July 2002

A finite element method was used to simulate domain wall pinning in SmCo5/Sm2Co17 based permanent magnets. The finite element model was built according to the cellular microstructure obtained from transmission electron microscopy investigations. The numerical results show a strong influence of the dimension of the cell boundary phase on the coercive field, which significantly increases with the extension of the 1:5/7-type cell boundary phase. The calculated values of the coercive field are in the range from 1000 to 2000 kA/m assuming a cell size varying from 80 to 160 nm. The difference of the magnetocrystalline anisotropy between cell boundary and cell interior phases is determined by the Cu content of the magnet.
Anomalous anisotropic magnetoresistance in Pr0.67Sr0.33MnO3 thin films
Authors: Li Qi , Wang H. S. , Hu Y. F. , Wertz E.
Published: July 2002

Anisotropic magnetoresistance (AMR) in strained Pr0.67Sr0.33MnO3 thin films has been studied by measuring the resistance as a function of the angle between the applied magnetic field direction and the film normal with the current always perpendicular to the magnetic field. The results show that both compressive- and tensile-strained ultrathin films (50–150 Å) exhibit unusually large AMR, but with opposite signs. In contrast, the almost strain free films show much smaller AMR over all the temperature and field ranges studied. The AMR decreases rapidly as the film thickness increases due to the gradual release of strain.
Deposition of nanocrystalline cubic silicon carbide films using the hot-filament chemical-vapor-deposition method
Authors: Yu M. B. , Rusli  , Yoon S. F. , Chen Z. M. , Ahn J. , Zhang Q. , Chew K. , Cui J.
Published: July 2002

Nanocrystalline cubic silicon carbide (3C–SiC) films embedded in an amorphous SiC matrix were fabricated by the hot-filament chemical-vapor-deposition technique using methane and silane as reactance gases. High-resolution transmission electron micrographs clearly showed that these films contain naoncrystallites, with an average dimension of about 7 nm, embedded within an amorphous matrix. X-ray photoelectron spectroscopy, x-ray diffraction, infrared absorption, and Raman scattering studies revealed the nanocrystallites as having the structure of that of 3C–SiC. In contrast to 3C–SiC, where no photoluminescence could be observed at room temperature, strong visible emission with a peak energy of 2.2 eV could be seen from the nanocrystalline films at room temperature. The presence of nanocrystalline cubic SiC in these films is believed to result in a change in their energy-band structure, compared to that of 3C–SiC, which promotes radiative recombination of electron–hole pairs.
Micro-Raman scattering and x-ray diffraction studies of (Ta2O5)1−x(TiO2)x ceramics
Authors: Dobal P. S. , Katiyar R. S. , Jiang Y. , Guo R. , Bhalla A. S.
Published: July 2002

High dielectric constant (Ta2O5)1−x(TiO2)x ceramics for x=0.00, 0.05, 0.08, and 0.11 were prepared and studied utilizing micro-Raman scattering and x-ray diffraction techniques. The modification in the lowest frequency phonon mode of pure Ta2O5 with increasing TiO2 content in these ceramics was assigned as Ta–Ti vibrations originating from the interaction between TiO6 octahedra and TaOn5-2n or Ta6O12+6 clusters. Raman spectra, in the range −200–900 °C, reveal a softening of this mode with increasing temperature followed by a structural transformation. A triclinic to monoclinic phase (Hmon′) transition was observed at about 300, 360, 450, and 540 °C for x=0.00, 0.05, 0.08, and 0.11, respectively. It was also found that reannealing at 1150 °C for 12 h transforms these ceramics from triclinic to a metastable monoclinic phase Hmon, which was found to be an irreversible process.
Trapping and detrapping of electrons photoinjected from silicon to ultrathin SiO2 overlayers. I. In vacuum and in the presence of ambient oxygen
Authors: Shamir N. , Mihaychuk J. G. , van Driel H. M.
Published: July 2002

Transient trapping/detrapping of electrons at the Si(100)/SiO2 outer surface is studied studied in vacuum or with an O2 ambient (between 10−3 and 30 Torr) following internal electron photoemission from Si. Photoemission-current (produced by a 150 fs, 800 nm laser source) and contact-potential-difference techniques were used to investigate a wide variety of n- and p-doped samples at 300 K with thermally grown, steam grown, and dry oxides with thickness ⩽5 nm as well as samples with the oxide layers removed. Characteristics of the steam grown oxide were also studied at 400 and 200 K. For samples in vacuum charging is attributed to direct filling of at least two families of traps, one related to the oxide and the other the Si/SiO2 interface. For samples in O2, details of oxygen-assisted surface charging as reported previously [Phys. Rev. Lett. 77, 920 (1996)] are given. A fast, Coulomb-repulsion driven spillover of surface charge from the irradiated spot to the rest of the surface was detected. Oxygen aids trap filling of the in-vacuum filled and gas-sensitive traps and also detrapping (the efficacy of which increases strongly from 400 to 200 K) when the optical excitation source is removed. Surface transient charging and charge trapping efficacy for the oxidized samples are not very sensitive to sample preparation. A mobility of the trapped charges, probably hopping between traps and also Coulomb-repulsion driven, was measured.
Theory of the interaction of light with large inhomogeneous molecular aggregates. I. Absorption
Authors: Keller David , Bustamante Carlos
Published: July 2002

A general method to describe the spectroscopy of large, internally inhomogeneous particles is presented. The theory utilizes an approach similar to the one used by DeVoe in the treatment of the optical properties of polymers. The particle is divided into groups and the internal field is calculated by solving a self-consistent set of linear equations in the field amplitude at each group in the particle. It is found that if the particle is dense the intermediate and radiation coupling mechanisms must be included in addition to the dipole–dipole coupling. Through these coupling mechanisms it is found that the excitation generated at each group in the chromophore can delocalize over regions comparable to the size of the wavelength of light. The response of the particle to the light can then be described in terms of ‘‘collective modes’’ of excitation of the entire particle, the amplitude of each mode being controlled by the geometrical relation between the groups and the efficiency in energy transfer between any two groups in the aggregate. The spatially averaged equations of the absorbance for a collection of large inhomogeneous arbitrarily shaped aggregates are derived.
Effect of high pressure on the Raman spectra of ice VIII and evidence for ice X
Authors: Hirsch K. R. , Holzapfel W. B.
Published: July 2002

Raman spectra of H2O and D2O ice VIII have been measured up to 50 GPa at 100 K. The changes in line positions and intensities of ice VIII and the appearance of a new band above 40 GPa point to the formation of symmetric hydrogen bonds in ice X. A lattice dynamical analysis of the frequency shifts up to 36 GPa suggest a second order phase transition to symmetric hydrogen bonds near 47.5 GPa. However, the observed changes in the lattice modes indicate that this phase transition is already formed by a first order transition at 42 GPa.
Ferromagnetic intermolecular interactions in a series of organic mixed crystals of galvinoxyl radical and its precursory closed shell compound
Authors: Awaga Kunio , Sugano Tadashi , Kinoshita Minoru
Published: July 2002

The magnetic properties of the 4:1, 6:1, 9:1, and 19:1 mixed crystals of galvinoxyl (4-[[3,5-bis(1,1-dimethylethyl)-4-oxo-2,5-cyclohexadien-1-ylidene]methyl]-2,6 -bis(1,1-dimethylethyl) phenoxy) radical and its precursory closed shell compound, hydrogalvinoxyl, have been studied. From the measurements of the temperature dependence of the magnetic susceptibility, it is found that the ferromagnetic intermolecular interactions, which are lost below about 85 K in pure galvinoxyl because of the phase transition, are maintained down to 2 K in these mixed crystals, and that the number of galvinoxyl radicals keeping the ferromagnetically coupled structure at low temperature increases as the concentration of hydrogalvinoxyl increases. The magnetic behavior of the mixed crystals at low temperature depends on the thermal history of the sample and is well interpreted by assuming the presence of a glassy state into which the high-temperature, ferromagnetically coupled phase is quenched. The magnetization curves show the spin multiplicity to be almost in proportion to the radical concentration in the mixed crystal and may be qualitatively understood by assuming that the mixed crystal is an assembly of one-dimensional chain segments partitioned by hydrogalvinoxyl within which the galvinoxyl radicals are coupled ferromagnetically.
Sub-Doppler Zeeman spectroscopy of the CeO molecule
Authors: Schall H. , Gray J. A. , Dulick M. , Field R. W.
Published: July 2002

Sub-Doppler Zeeman spectra of several low-J rotational lines in 0–0 bands of CeO[16.5]2–X12, [16.5]4–X23, [18.4]4–X34, and [19.3]3–X43 systems have been recorded by intermodulated fluorescence spectroscopy. Electronic g values for the four lower states [the Ω=Ja states of the Ce+2(4 f 6s)O−2] superconfiguration obtained from Zeeman splittings of low-J lines at magnetic fields up to 1100 G, are found to agree with g values calculated from the eigenvectors of a ligand field effective Hamiltonian. The present results demonstrate the capability of the ligand field model to predict electronic properties in addition to energies and illustrate the value of the Zeeman effect for uncovering the atomic–ion quantum numbers which are not normally specified for molecules in the strong spin-orbit [Hund’s case (c)] limit.
Helical bilayer structures due to spontaneous torsion of the edges
Authors: Helfrich W.
Published: July 2002

A theory is given to explain the helical structures, in particular the strips wound around as cylinders, which were found with solid bilayers made of chiral molecules. It employs a spontaneous torsion of the bilayer edges and the bending stiffness of the bilayer. The gradient angle of the helix is found to be 45°, in good agreement with experiment.
An effective core potential investigation of Ni, Pd, and Pt and their monohydrides
Authors: Rohlfing Celeste McMichael , Hay P. Jeffrey , Martin Richard L.
Published: July 2002

The nickel, palladium, and platinum atoms and their monohydrides are investigated using effective core potentials (ECP’s) recently introduced by Hay and Wadt [J. Chem. Phys. 82, 270, 299 (1985)]. The palladium and platinum ECP’s include relativistic effects. Two types of ECP’s, which differ in their definition of the core region, are used in conjunction with large valence basis sets including  f  functions. Electron correlation is incorporated by Mo/ller–Plesset perturbation theory through fourth order. The results demonstrate the success of the ECP approximation: they are similar in quality to those of all-electron studies and at the same time are achieved at a reduced computational cost. Total correlation energies obtained with both types of ECP’s are compared, and are found to be very sensitive to details of the basis set. Correlation effects are treated more consistently by the ECP’s which include the outermost core orbitals in the valence region. The relative ordering of molecular states is calculated to be 2Δ<2Π≈2Σ+ for NiH, 2Σ+<2Δ<2Π for PdH, and 2Δ≈2Σ+<2Π for PtH. Trends in bond lengths and bond strengths in the nickel, palladium, and platinum series are discussed.
From quasiperiodicity to chaos in the Belousov–Zhabotinskii reaction. II. Modeling and theory
Authors: Richetti P. , Roux J. C. , Argoul F. , Arneodo A.
Published: July 2002

We discuss the quasiperiodic behavior experimentally observed in the Belousov–Zhabotinskii reaction as the result of the interaction of two elementary instabilities, namely the Hopf bifurcation at the origin of the oscillating nature of this reaction and the hysteresis bifurcation which accounts for the phenomenom of bistability. We use a normal form approach to understand the evolution of the dynamics when the BZ system is moved away from the local situation where both these instabilities are competing. We first discuss the transition to chaos which comes with the breaking up of the underlying torus into a fractal object. Then we emphasize that nonlocally such a two-frequency dynamics manifests through alternating periodic–chaotic sequences which look very much like the sequences observed in bench experiments. We propose a seven-variable Oregonator type model which not only accounts for these sequences but also for those which involve only one fundamental frequency as observed in the 1980 Texas experiment. We refer to the dynamical system theory to definitively establish the existence of deterministic chaos in both of these sequences. We conclude with the very promising perspective of using a normal form approach to reduce the evolution equations to their simplest form by selecting the relevant instabilities which control the dynamics of the BZ reaction in the regions of parameter space explored so far.
The electronic state-selective photodissociation of CH2BrI at 248, 210, and 193 nm
Authors: Butler L. J. , Hintsa E. J. , Shane S. F. , Lee Y. T.
Published: July 2002

The primary photodissociation channels of CH2BrI following excitation at 193.3, 210, and 248.5 nm have been studied with the crossed laser-molecular beam technique. Product translational energy distributions and polarization dependences were derived for the primary dissociation processes observed. The data demonstrate bond selective photochemistry as well as some selective formation of electronically excited photofragments in bond fission and concerted dissociation. Excitation at 248.5 nm, which is assigned to excitation of primarily a n(I)→σ*(C–I) transition with some contribution from an overlapping n(Br)→σ*(C–Br) transition, results in both C–I and C–Br bond fission. C–I bond fission is the dominant channel, producing I atoms in both the 2P3/2 and spin-orbit excited 2P1/2 states in a ratio of 1.0:0.75. Excitation at 193.3 nm, assigned to a transition to primarily predissociated Rydberg levels on the I atom, leads to C–Br bond fission, some C–I bond fission, and significant concerted elimination of IBr. Analysis of the product translational energy distributions for the dissociation products indicates that the IBr is formed electronically excited and that the halogen atom products are spin-orbit excited. Excitation at 210 nm, of the transition assigned as n(Br)→σ*(C–Br) based on comparison with CH3Br, results in selective breaking of the stronger C–X bond in the molecule, the C–Br bond, and no fission of the C–I bond. Some concerted elimination of IBr also occurs; the IBr velocity distribution indicates it is probably formed electronically excited as in photolysis at 193.3 nm. The selective breaking of the C–Br bond over the weaker C–I bond is discussed in contrast to previous photolysis studies of polyhalomethanes.
Mobile Bjerrum defects: A criterion for ice-like crystal growth
Authors: Wooldridge Paul J. , Richardson Hugh H. , Devlin J. Paul
Published: July 2002

There has recently been a substantial increase in information on low-temperature phase transformations of ice and ice-like substances along with a rapid increse in molecular level information on the defect structure and activity of this subset of hydrogen-bonded solids. These data, some quantitative and some qualitative in nature, are examined from the viewpoint that the phase transformation mechanism depends on the availability of mobile orientational defects (Bjerrum L defects) within the new phase. Some of the data that seem particularly apt for establishing any dependence of the phase transformation on the presence of mobile L defects within the new phase, reflect a strong correlation between transformation rates/temperature and the availability of mobile L defects. One possible inference is that the integrity of a growing phase can be maintained only if defects responsible for orientational mobility at the interface can ultimately achieve equilibrium through recombination. The implications of such a dependence on mobile L defects are examined for a variety of systems. For example, one implication is that, at low temperature (<160 K), the structure I clathrate hydrate of trimethylene oxide (TMO) should grow more rapidly than the structure II hydrate, since structure I hydrates are known to be relatively rich in mobile defects. New data for the clathrate hydrates of TMO are presented that confirm the preferential growth of the structure I hydrate from amorphous deposits containing water and TMO in ratios ranging from 7:1 to >17:1.
Effect of rotational coupling on emission probabilities of Lyman and Werner band systems of the vacuum ultraviolet spectrum of H2
Authors: Abgrall Hervé , Launay Françoise , Roueff Evelyne , Roncin Jean-Yves
Published: July 2002

Ratios of emission probabilities of rotational lines of the Lyman and Werner band systems, B 1∑+u →X 1∑+g and C 1∏u →X1∑+g, respectively, are calculated by different methods. These theoretical ratios are compared to the experimental intensity ratios of about a hundred lines selected from a vacuum ultraviolet emission spectrum whose intensity is investigated for the first time at high resolution. The computation permits to compare two ways of solving a system of two coupled equations with discrete eigenvalues. In most cases the effect of the mutual rotational perturbation between B and C states is found significant on the ratio of P and R line intensities. Ambiguity in labeling a few B and C levels is discussed.
Superposition-model analysis of intensity parameters for Eu3+ luminescence
Authors: Reid Michael F.
Published: July 2002

Intensity parameters for the luminescence transitions between crystal-field levels of the 4f6 configuration of Eu3+ in Y2O2S, LaOCl, LiYF4, Na5Eu(MoO4)4, and Na5Eu(WO4)4 are discussed. The superposition model is used to relate the parameters to parameters determined for other ions in both similar and different host crystals. The results confirm our earlier observations that mechanisms outside the point-charge crystal-field model are required in order to rationalize the signs of the intensity parameters for lanthanide ions in solids.
The kinetics of adsorption and thermal desorption of NO on stepped Pt single crystal surfaces
Authors: Kruse N. , Abend G. , Block J. H.
Published: July 2002

The kinetics of adsorption and thermal desorption of nitric oxide on Pt field emitter surfaces has been studied by means of pulsed field desorption mass spectrometry (PFDMS). Adsorption on the high index planes with terraces of (111) orientation occurs molecularly with high initial sticking probabilities S=0.65,...,0.9. By varying the repetition frequency of the desorbing field pulses, i.e., by varying the field free reaction time tR (100 μs to 1 s), the mean lifetimes before thermal desorption of the NOad have been measured. The temperature dependence of the mean lifetimes has been evaluated. An activation energy of 139 kJ/mol and a preexponential of 3.8×10−15 s are found. These rate parameters are dominated by adsorption at step sites as verified on the basis of a model comprising multiple state adsorption and desorption. Dissociation of NOad has not been observed on stepped Pt(111), but has been on stepped Pt(100).
Calculation of magic-angle spinning nuclear magnetic resonance spectra of paramagnetic solids
Authors: Nayeem Akbar , Yesinowski James P.
Published: July 2002

This study provides the first quantitative interpretation of the intensity pattern of spinning sidebands observed in the magic-angle spinning (MAS) NMR spectra of paramagnetic solids. The 200 MHz 1H MAS-NMR spectrum of copper chloride dihydrate and its deuterated analog are reported. The inhomogeneous interactions predominantly responsible for the sidebands are the magnetic dipolar couplings between the nucleus and the thermally-averaged magnetic moments due to the unpaired electrons on copper atoms. It is demonstrated that even in the presence of many such couplings to a given nucleus, a g anisotropy of the unpaired electron, and a chemical shift anisotropy of the nucleus, the net inhomogeneous interaction responsible for the sideband intensities is formally equivalent to a chemical shift tensor. However, inhomogeneous dipolar couplings to other nuclei give rise to subspectra corresponding to individual spin states of the other nucleus; the resultant composite spectrum no longer resembles that arising from a chemical shift tensor. The 1H MAS-NMR spectrum calculated using the known structure of copper chloride dihydrate correctly predicts the overall spectral width and sideband intensity pattern experimentally observed for the deuterated compound, and indicates that the unpaired electron density on the copper atom is partially delocalized (∼15%) onto the neighboring chlorine atoms. Two comparable sources of line broadening in deuterated copper chloride dihydrate are demonstrated to be the magnetic susceptibility anisotropy and T2 relaxation. The isotropic proton chemical shift is shown to be influenced by a small pseudocontact shift (∼10 ppm upfield) and a larger Fermi contact shift (∼76 ppm downfield).
Determinant based configuration interaction algorithms for complete and restricted configuration interaction spaces
Authors: Olsen Jeppe , Roos Björn O. , Jo/rgensen Poul , Jensen Hans Jo/rgen Aa.
Published: July 2002

A restricted active space (RAS) wave function is introduced, which encompasses many commonly used restricted CI expansions. A highly vectorized algorithm is developed for full CI and other RAS calculations. The algorithm is based on Slater determinants expressed as products of alphastrings and betastrings and lends itself to a matrix indexing C(Iα, Iβ ) of the CI vector. The major features are: (1) The intermediate summation over determinants is replaced by two intermediate summations over strings, the number of which is only the square root of the number of determinants. (2) Intermediate summations over strings outside the RAS CI space is avoided and RAS calculations are therefore almost as efficient as full CI calculations with the same number of determinants. (3) An additional simplification is devised for MS =0 states, halving the number of operations. For a case with all single and double replacements out from 415 206 Slater determinants yielding 1 136 838 Slater determinants each CI iteration takes 161 s on an IBM 3090/150(VF).
Electron impact excitation of the 3p(5P) state of atomic oxygen
Authors: Germany G. A. , Anderson R. J. , Salamo G. J.
Published: July 2002

The 157.6 nm output from a fluorine excimer laser is focused in a vacuum chamber containing O2 gas at a pressure of 20 mTorr. Laser photodissociation of the O2 target gas produces 2p4(3P) and 2p4(1D) oxygen atoms with unit quantum efficiency. A low-energy electron beam is crossed with the laser beam to produce 3p(5P) excited states of OI via electron impact excitation of the 2p4(3P) ground state. Intensity measurements of the 777.4 nm radiation, corresponding to the 3p(5P)–3s(5S) transition, are used to calculate the excitation cross section of the 3p(5P) state of atomic oxygen. Absolute optical cross sections are reported for a range of incident electron energies less than 18 eV.
Crossed-beam reactive scattering of F2 plus C6H6: Heat of formation of i
p
s
o-fluorocyclohexadienyl radical
Authors: Grover J. R. , Wen Y. , Lee Y. T. , Shobatake K.
Published: July 2002

Angular distributions and relative total cross sections were measured for products of the collisions of F2 with C6 H6 at relative collision energies of 14 to 26 kcal mol−1 . The most conspicuous product is the ipso-fluorocyclohexadienyl radical i-C6 H6 F ̇, which displays a rather narrow peak very near the center-of-mass angle at all collision energies studied. Product C6 H5 F is masked at M/e=96 by dissociative ionization of C6 H6 F ̇ near the center-of-mass angle, but becomes observable at smaller angles to which little or no C6 H6 F ̇ is scattered. This means that at least one reaction C6 H6 +F2 →C6 H5 F+HF occurs, and does so in such a way that the products acquire high translational energies. The dependence on collision energy of the total cross section for the production of C6 H6 F ̇ was probably measured, and displays a well-marked onset at 13.9±0.3 kcal mol−1. The center-of-mass system means final state translational energy as a function of collision energy was obtained from the angular distributions and goes to zero at 13.9±0.5 kcal mol−1 . This probably means that the onset is a true threshold, from which the heat of formation of the ipso-fluorocyclohexadienyl radical is calculated to be 14.8±0.4 kcal mol−1. The mean final state translational energy asymptotically approaches 3.7 kcal mol−1 as the collision energies are increased, which is so large that only a fraction of the vibrational modes of C6 H6 F ̇ can be involved in the energy partitioning. This conclusion is confirmed by the center-of-mass angular distribution, which favors forward scattering of product F atoms, but only by about 10%–30%. This work shows that the crossed-beam method can be used to make precision thermochemical measurements of gaseous polyatomic free radicals. It also reports the investigation of a polyatomic–polyatomic reaction in crossed beams, for which type of study there are still very few examples.
Why does unrestricted Mo/ller–Plesset perturbation theory converge so slowly for spin-contaminated wave functions?
Authors: Gill Peter M. W. , Pople John A. , Radom Leo , Nobes Ross H.
Published: July 2002

The poor convergence of the unrestricted Mo/ller–Plesset (UMP) series for some systems and the presence of significant spin contamination in the underlying unrestricted Hartree–Fock (UHF) wave functions have both been shown to result from the presence of low-lying, doubly excited determinants. The contributions to the UCI energy from any determinants which mix with the UHF wave function through the problematic doubly excited determinants will be described poorly by UMP theory.