bluuebunny/crossref_metadata_2025 huggingface dataset:
Atomic layer wear of single-crystal calcite in aqueous solution using scanning force microscopy
Authors: Park Nam-Seok , Kim Myoung-Won , Langford S. C. , Dickinson J. T.
Published: July 2002

Scanning force microscopy (SFM) is employed to study nm-scale wear of single-crystal calcite in an aqueous solution. When the SFM tip is drawn back and forth in a linear fashion across the edge of a preexisting single atomic layer etch pit, dissolution is strongly enhanced at the point where the tip crosses the step. The wear rate as a function of contact force is consistent with a thermally activated wear process, where the activation energy is locally reduced in the strain field of the SFM tip. The activation volume for the strain dependence is on the order of the average volume per ion in the CaCO3 lattice. This study provides further support for strain enhanced nucleation of double kinks along preexisting steps.
Effect of oxidation treatments on photoluminescence excitation of porous silicon
Authors: Rigakis N. , Hilliard J. , Abu Hassan L. , Hetrick J. M. , Andsager D. , Nayfeh M. H.
Published: July 2002

We have studied the effect of various controlled surface oxidation treatments on photoluminescence excitation spectra of porous silicon for various emission energies. The UV excitation spectra of fresh samples were found to be peaked with a monotonic relation between peak excitation energy and emission energy, and to have an onset near 3.3 eV, the direct gap of Si. Aging (ambient oxidation) was found to unevenly affect the excitation spectra, resulting in an effective red shift of the peak center. On the other hand, anodic oxidation or oxidation by exposure of the sample to copper ions in solution caused a blue shift of the excitation spectra. These results are explained in terms of surface condition effects on nonradiative loss sites or on the interquantum dot barriers.
Reactively sputtered Ti-Si-N films I. Physical properties
Authors: Sun X. , Reid J. S. , Kolawa E. , Nicolet M.-A.
Published: July 2002

Films of Ti-Si-N were synthesized by reactively sputtering TiSi2, Ti5Si3, or Ti3Si targets in an Ar/N2 gas mixture. They were characterized in terms of their composition by MeV 4He backscattering spectrometry, their atomic density by thickness measurements combined with backscattering data, their microstructure by x-ray diffraction and high-resolution transmission electron microscopy, and their electrical resistivity by four-point-probe measurements. All films have a metal–to–silicon ratio close to that of their respective targets. The as-deposited films are either entirely amorphous or contain inclusions of TiN-like nanometer-sized grains when the overall atomic composition of the films approaches the TiN phase in the ternary Ti-Si-N diagram. A correlation between the resistivity of the as-deposited films and their position in the ternary phase diagram is evident, indicating that at the atomic scale, the spatial arrangement of atoms in the amorphous phase and their bonding character can approximate those of the equilibrium phases. A mixture of nanocrystalline TiN and amorphous Si-N is proposed for some titanium- and nitrogen-rich films. The atomic density of some films exceeds 1023 at./cm3. The resistivity of the films increases with the Si and the N content. A thermal treatment in vacuum at 700 °C for 1 h decreases the resistivity of the Ti-rich films deposited from the Ti5Si3 or the Ti3Si target, but increases that of the Si-rich films deposited from the TiSi2 target when the nitrogen content exceeds about 30 at. %. The effectiveness of these films as diffusion barriers between Si and Al or Cu is reported in Part II.
Magnetism in BaCoS2
Authors: Mandrus D. , Sarrao J. L. , Chakoumakos B. C. , Fernandez-Baca J. A. , Nagler S. E. , Sales B. C.
Published: July 2002

BaCoS 2 is a layered Mott–Hubbard insulator that orders antiferromagnetically near 300 K. We report magnetic susceptibility measurements on polycrystalline BaCoS2 from 77 to 800 K, and in- and out-of-plane measurements on single crystals from 2 to 350 K. We also report a powder neutron refinement of the magnetic structure of BaCoS2. The neutron measurements indicate that the moments lie in the plane, and that the magnetic unit cell is the same as the (orthorhombic) chemical cell. The ordered moment obtained from the neutron refinement is close to 3 μB, indicating that the Co ions are in the high-spin (S=3/2) configuration. The effective moment inferred from the high temperature susceptibility, p=5.26μB, is also consistent with high-spin Co+2.
Electron affinity and Schottky barrier height of metal–diamond (100), (111), and (110) interfaces
Authors: Baumann P. K. , Nemanich R. J.
Published: July 2002

The electron emission properties of metal–diamond (100), (111), and (110) interfaces were characterized by means of UV photoemission spectroscopy (UPS) and field-emission measurements. Different surface cleaning procedures including annealing in ultrahigh vacuum (UHV) and rf plasma treatments were used before metal deposition. This resulted in diamond surfaces terminated by oxygen, hydrogen, or free of adsorbates. The electron affinity and Schottky barrier height of Zr or Co thin films were correlated by means of UPS. A negative electron affinity (NEA) was observed for Zr on any diamond surface. Co on diamond resulted in NEA characteristics except for oxygen-terminated surfaces. The lowest Schottky barrier heights were obtained for the clean diamond surfaces. Higher values were measured for H termination, and the highest values were obtained for O on diamond. For Zr, the Schottky barrier height ranged from 0.70 eV for the clean to 0.90 eV for the O-terminated diamond (100) surface. Values for Co ranged from 0.35 to 1.40 eV for clean- and O-covered (100) surfaces, respectively. The metal-induced NEA proved to be stable after exposure to air. For the oxygen-terminated diamond (100) surface a field-emission threshold of 79 V/μm was measured. Zr or Co deposition resulted in lower thresholds. Values as low as 20 V/μm were observed for Zr on the clean diamond (100) surface. Results for Zr or Co on H- or O-terminated surfaces were higher. H or O layers on diamond tend to cause an increase in the Schottky barrier height and the field-emission threshold field of Zr– and Co–diamond interfaces. The value of the electron affinity and Schottky barrier were correlated with work function and different initial surface preparation. The results were largely consistent with a model in which the vacuum level was related to the metal work function and the measured Schottky barrier.
Ostwald ripening of end-of-range defects in silicon
Authors: Bonafos C. , Mathiot D. , Claverie A.
Published: July 2002

End-of-range (EOR) defects are interstitial type dislocation loops which nucleate just beneath the crystalline/amorphous (c/a) interface formed by ion implantation in Si, after the preamorphization of the substrate, and during the ramping-up of the anneal. They originate from the presence of a high supersaturation of “excess” Si self-interstitial atoms located just beneath the c/a interface. Upon annealing, the mean radius of the defects increases while their density decreases through the exchange of Si self-interstitial atoms between the loops. The number of interstitials stored in the loops stays constant. For sufficiently high thermal budgets, when the nucleation is finished, and when the local equilibrium between extended and point defects is established, the coarsening of the EOR defects can be modeled through the Ostwald ripening theory applied to the dislocation loops geometry. Indeed, and as expected from the theory, the square of the mean radius of the loop population increases with time while the loop density decreases proportional to 1/t. Furthermore, the theoretical function describing the size distributions perfectly matches the time evolution of the experimental stack histograms, for different annealing temperatures. During the asymptotic steady-state coarsening regime, the activation energy for the loop coarsening is 4.4 eV, which is in the range of values given in the literature for self-diffusion in Si. Nevertheless, an activation energy of about 1–2 eV is found during the transient period preceding the local equilibrium, i.e., in the range of the migration energy of self-interstitials. The limiting phenomenon for the loop growth appears to be diffusion, since it is the hypothesis that leads to the best fit between theory and experiment. An estimate of DiCi* has been derived from the growth laws of the EOR defects. A value of about 1.8×107 cm−1 s−1 at 1000 °C is obtained and compares well with the values given in the literature.
The effect of intragranular domain walls in MgMnZn-ferrite
Authors: van der Zaag P. J. , Kolenbrander M. , Rekveldt M. Th.
Published: July 2002

The effect of grain size, D, on the intragranular domain state and initial permeability, μi, of polycrystalline MgMnZn ferrites has been investigated. The intragranular domain state is found to change from mono- to two domain at D=2.9±0.2 μm. Despite this change, μi follows a continuous dependence on D, which can be described by the nonmagnetic grain boundary model. The results show that in spinel ferrites with an anisotropy of 72 J/m3, μi is still due to rotation of magnetization.
Plasma driven superpermeation of hydrogen through group Va metals
Authors: Livshits A. I. , Sube F. , Solovyev M. N. , Notkin M. E. , Bacal M.
Published: July 2002

A barrier for the absorption of hydrogen molecules generated by a nonmetallic film on a metal surface may result in superpermeation of suprathermal hydrogen particles. Virtually all the sticking or implanted particles may permeate through the metallic membrane, with a permeation flux depending neither on the metal temperature nor on its thickness. Niobium and vanadium membranes with oxicarbide monolayer films were immersed in a hydrogen plasma in order to study their interaction with hydrogen ions in the energy range from a few eV to 250 eV, controlled by changing the membrane bias. A stable superpermeation flux was observed up to ion energies of tens of eV without special efforts to maintain the surface film. We found that the nonmetal monolayer film remains absolutely stable under the sputtering, including chemical sputtering, by hydrogen particles of energy less than ≈50 eV. Thus superpermeability appears to be an intrinsic feature of the investigated systems of “nonmetallic film–metal–hydrogen particles of energy lower than 50 eV.”
Formation of an atomic hydrogen beam by a hot capillary
Authors: Tschersich K. G. , von Bonin V.
Published: July 2002

A beam of atomic hydrogen is produced by passing hydrogen gas through a tungsten capillary heated to 2600 K by electron impact. By means of a differentially pumped quadrupole mass analyzer the angular distribution of the atomic hydrogen is measured for different hydrogen feed pressures. The angular distribution is considerably more peaked than a cosine distribution and is narrower the lower the feed pressure is. Based on the theory of beam formation by molecular flow a simple analytical model is developed which assumes a spatially limited transparent flow at the end of the capillary. The measured angular distribution can be described by the model using the length of transparent flow as the only fitting parameter.
Mechanism of Zn and Si diffusion from a highly doped tunnel junction for InGaP/GaAs tandem solar cells
Authors: Takamoto Tatsuya , Yumaguchi Masafumi , Ikeda Eiji , Agui Takaaki , Kurita Hiroshi , Al-Jassim Mowafak
Published: July 2002

Diffusion of impurities (Zn and Si) from a tunnel junction during epitaxial growth and the effects of impurity diffusion on InGaP/GaAs tandem cell properties have been investigated. Zn diffusion from the tunnel junction has been found to deteriorate the effect of the back-surface field layer on minority carrier reflectance in the InGaP top cell and degrade the quantum efficiency of the top cell. Furthermore, Zn diffusion has been found to be enhanced around the threading dislocations from a GaAs substrate and creates shunt paths only in the top cell region. Si diffusion, which degrades the quantum efficiency of the GaAs bottom cell, has also been observed when a different substrate with high etch pit density was used. Such anomalous diffusion of Zn has been found to be suppressed by using a double-hetero structure InGaP tunnel junction sandwiched by AlInP layers. It has been found that the Zn diffusion occurs as a layer highly doped with Si being formed nearby and Zn diffuses in the opposite direction from the Si-doped layer. The Zn diffusion is thought to be caused by group III self-diffusion which originates in the highly doped n-type layer. The direction of Zn diffusion is thought to be due to Coulombic repulsion between the substitutional Zn on the Ga site and the substitutional Si on the As site. The large energies of the formation and migration of group III vacancies in the AlInP barrier layers and InGaP tunnel junction layers are thought to suppress Zn diffusion from the tunnel junction.
Theory of electronic and optical properties of 3C-SiC
Authors: Theodorou George , Tsegas George , Kaxiras Efthimios
Published: July 2002

We study the electronic and optical properties of cubic (3C) SiC, using a combination of first-principles and tight-binding electronic structure calculations. We employ pseudopotential density functional theory calculations, with appropriate corrections to the energy of conduction bands, to investigate the band structure of this material and obtain band gaps that are in agreement with experimental results. The optical properties are then studied within the framework of the empirical tight-binding model, which is fitted to reproduce the first-principles calculations. This approach allows for a thorough investigation of the dielectric functions, the reflectivity, and the refractive index. Critical points are identified and connected to the appropriate transitions in the band structure. The results are in good agreement with available experimental data. In addition, we investigate spin splitting effects.
Low stiction/low glide height head–disk interface for high-performance disk drives (invited)
Authors: Fu Ta-Chang , Suzuki Shoji
Published: July 2002

A brief historical review of the head–disk interface evolution is presented, and current limitations when facing stringent tribology requirements for high-density recording are addressed. The tribology performance of sliders with contact landing pads on the air bearing surfaces (the “padded slider”) was studied. Lightly mechanically textured disks, and laser zone-textured disks with shallow bumps (the “light LZT”) to reduce glide avalanche that were specially designed for padded sliders were used. When the smoother mechanically textured surface was applied, we observed inferior constant start-stop (CSS) durability due to possible tribological degradation of the padded slider, and significant landing pad wear was observed on the slider after the CSS test. Rougher surfaces including the light LZT surface, however, were not as sensitive to tribological degradation of the padded slider as the smoother surface, and exhibited better CSS durability as well as less landing pad wear after the test. Hence the optimization effort of padded slider head/disk component design may be reduced if a light texture is applied on the CSS zone. The head–disk interface consisting of a padded slider on a light LZT can better meet the stringent tribology requirements for high density recording needs and therefore it is proposed as an alternative to ramp loading technology in the desktop/server-class disk drives. Hence the better-understood CSS technology is expected to be further extended into future high-performance disk drives.
CF
2
production and loss mechanisms in fluorocarbon discharges: Fluorine-poor conditions and polymerization
Authors: Cunge G. , Booth J. P.
Published: July 2002

The study of CF and CF2 radical production and loss mechanisms in capacitively-coupled 13.56 MHz CF4 plasmas has been extended to CF4 plasmas with an Si substrate, and to C2F6 plasmas, conditions where the atomic fluorine concentration is lower and where more polymer deposition occurs on the reactor surfaces. Processes in the gas phase and at the reactor surfaces were investigated by time resolved axial concentration profiles obtained by laser induced fluorescence, combined with absolute calibration techniques. The results for CF were similar to those observed in the fluorine rich case, whereas the results for CF2 were strikingly different and more complex. This paper focuses on the CF2 radical, which, under these conditions is produced at all of the surfaces of the reactor, apparently via a long-lived surface precursor. The results can only be explained if large polymeric ions and/or neutrals are produced by polymerization in the gas phase. The gas-phase CF2 concentration is high, causing the otherwise slow gas-phase concatenation reactions CXFY(CF2)n+CF2→CXFY(CF2)n+1 to occur. These processes produce high-mass neutrals (and ions) which are the real polymer precursors. The CF2 radical therefore circulates in a closed cycle between the surface and the gas phase. The degree of polymerization is controlled by the fluorine atom concentration, which simultaneously controls the concentrations of CF2, of chain initiating species such as CF3 and of dangling bonds on the growing oligomers. This model appears to apply to fluorocarbon discharges in general, and agrees well with other results presented in the literature.
Temperature dependence of exciton peak energies in ZnS, ZnSe, and ZnTe epitaxial films
Authors: Pässler R. , Griebl E. , Riepl H. , Lautner G. , Bauer S. , Preis H. , Gebhardt W. , Buda B. , As D. J. , Schikora D. , Lischka K. , Papagelis K. , Ves S.
Published: July 2002

High-quality ZnS, ZnSe, and ZnTe epitaxial films were grown on (001)-GaAs-substrates by molecular beam epitaxy. The 1s-exciton peak energy positions have been determined by absorption measurements from 2 K up to about room temperature. For ZnS and ZnSe additional high-temperature 1s-exciton energy data were obtained by reflectance measurements performed from 300 up to about 550 K. These complete E1s(T) data sets are fitted using a recently developed analytical model. The high-temperature slopes of the individual E1s(T) curves and the effective phonon temperatures of ZnS, ZnSe, and ZnTe are found to scale almost linearly with the corresponding zero-temperature energy gaps and the Debye temperatures, respectively. Various ad hoc formulas of Varshni type, which have been invoked in recent articles for numerical simulations of restricted E1s(T) data sets for cubic ZnS, are discussed.
Thermal conductivity of thick meso-porous silicon layers by micro-Raman scattering
Authors: Lysenko V. , Perichon S. , Remaki B. , Barbier D. , Champagnon B.
Published: July 2002

We report here a theoretical model describing specific mechanisms of heat transport in as-prepared and oxidized meso-porous silicon layers. The model is in good agreement with experimental measurements performed by micro-Raman scattering on the layers surface. For the first time, thermal conductivity inhomogeneity along the porous layer thickness of 100 μm is studied. Direct correlation between the thermal conductivity and morphology variations along the layer thickness is brought to the fore. A new approach to estimate local porosity of the porous layers based on thermal conductivity and Si nanocrystallite size measurements is also proposed.
Raman spectra and laser properties of Yb-doped yttrium orthovanadate crystals
Authors: Xia H. R. , Li L. X. , Zhang H. J. , Meng X. L. , Zhu L. , Yang Z. H. , Liu X. S. , Wang J. Y.
Published: July 2002

Yb-doped yttrium orthovanadate YbxY1−xVO4 (YYV), with x=0.02, crystallizes with a zircon-type structure in the tetragonal system, conforming to the space group I41/amd. The lattice constants are a=0.7122(5) and c=0.6291(3) nm at room temperature. The primitive cell contains four formula units. The Raman spectra and crystallography show no distortion of the VO4 and Y/YbO8 groups. The crystal field is similar to that of the YVO4 crystal and is suitable for the dopant Yb3+ ions. A strong absorption at 975 nm arises from the F27/2 to F25/2 transition; with a bandwidth of 70 nm it may provide a useful pump band for InGaAs diode lasers. This band is about seven times broader than that of Nd-doped yttrium aluminum garnet and more than three times as broad as that of Nd-doped yttrium orthovanadate. The measured absorption of the YYV is compared to Judd–Ofelt (JO) theory. When applied, the JO theory of parity-forbidden electric-dipole transitions of rare earth ions on noncentrosymmetric sites demonstrates good agreement. The fluorescence spectrum reveals a ground state splitting of ΔE=463 cm−1. The fitted fluorescence lifetime is 1.18 ms at the wavelength of 1.0 μm. The calculated absorption cross section is 1.38×10−20 cm2. The order of magnitude of the emission cross section is estimated as 10−21 cm2. In Yb-doped yttrium aluminum garnet, the fluorescence lifetime is about 1.16–1.30 ms and the absorption cross section is 1.5×10−20 cm2. In Nd-doped yttrium orthovanadate, the fluorescence lifetime is 115 μs and the emission cross section is 10.7×10−19 cm2. A suitable concentration of the Yb3+ ions doped in the YbxY1−xVO4 crystals should modify the optical and the potential laser properties of the crystals.
Time-resolved photocurrent and electric field measurements in high resistivity CdTe
Authors: Ghislotti G. , Pietralunga S. , Ripamonti L. , Sacco R. , Micheletti S. , Bosisio F.
Published: July 2002

We studied current and electric field transients in high resistivity CdTe:In crystals by means of nanosecond time-resolved photocurrent and electro-optic sampling measurements. Electron and hole dynamics have been investigated for different excitation regimes (incident photon fluence and wavelength). A numerical solution for a drift-diffusion model in presence of several defect centers is given which allows to interpret experimental results. Current decay can be described by two time constants of about 10 and 100 ns, the first one being predominant for low incident photon energy (<1 eV). By comparison with the numerical model, the fast and slow time constants are related to hole and electron trapping, respectively. Electric field decays in a millisecond time scale. This is related to charge trapped in a deep recombination center located at 0.75 eV from the conduction band.
Small signal analysis of frequency response of four-wave mixing in semiconductor optical amplifiers
Authors: Wu Chiming , Fan Honglei , Dutta Niloy K.
Published: July 2002

A theory of small signal frequency response of four-wave mixing (FWM) in a traveling wave semiconductor optical amplifier has been developed. The bandwidth of FWM is >300 GHz. For large wavelength separation between the input signals, where the nonlinear gain effects dominate the generation of FWM, the bandwidth of the FWM signal exceeds 1 THz.
The influence of temperature and surface conditions on surface absorptivity in laser surface treatment
Authors: Wang Jin-Tang , Weng Cheng-I , Chang Jee-Gong , Hwang Chi-Chuan
Published: July 2002

A one-dimensional, transient, inverse heat conduction problem is implemented to investigate the influence of temperature and surface conditions on surface absorptivity in the laser surface heating process. Analysis includes the utilization of the conjugate gradient method (CGM), with temperatures measured near the heated surface. To increase efficiency and accuracy of the calculation, the result retrieved from the least-square method is used as an initial guess for the CGM. Results show that absorptivity decreases when the temperature exceeds a certain value. This decrease is related to structural transformation during the temperature rise. As the surface temperature nears the melting point, the decreasing trend inverts and absorptivity increases. This final abrupt rise is caused by the phase transformation from order to disorder. Additionally, absorptivity is related to surface conditions such as absorption-enhancing coatings and surface roughness.
Determination of anisotropy fields in recording media
Authors: Jones G. R. , Prichard L. S. , Hutchings J. A. , Laidler H. , O'Grady K.
Published: July 2002

This work describes an experimental study which reviews the various techniques that can be used for determination of anisotropy fields in recording media. At the present time this topic is of considerable importance due to the need to understand the detailed mechanisms of magnetization reversal and the factors that control the switching field distribution in recording media. We have used a metal-particle-based tape material and have employed various magnetic techniques for the determination of the apparent anisotropy field, HK. The techniques included hysteresis loops measured in a direction transverse to the texture direction of the tapes, single-point detection in the successive derivatives of the magnetization with respect to the field, transverse susceptibility measurement, and the variation of the bulk coercivity with temperature. We find the temperature variation of coercivity produces values of HK far smaller than values determined by other methods due to orientation effects. General agreement is seen for the other techniques mentioned and results for samples with differing degrees of texture show that the apparent anisotropy field of the grains as measured by these techniques is influenced greatly by orientation effects.
Crystal-field interaction in the pyrochlore magnet Ho2Ti2O7
Authors: Rosenkranz S. , Ramirez A. P. , Hayashi A. , Cava R. J. , Siddharthan R. , Shastry B. S.
Published: July 2002

Neutron time-of-flight spectroscopy has been employed to study the crystal-field interaction in the pyrochlore titanate Ho2Ti2O7. The crystal-field parameters and corresponding energy-level scheme have been determined from a profile fit to the observed neutron spectra. The ground state is a well separated Eg doublet with a strong Ising-like anisotropy, which can give rise to frustration in the pyrochlore lattice. Using the crystal-field parameters determined for the Ho compound as an estimate of the crystal-field potential in other pyrochlore magnets, we also find the Ising type behavior for Dy. In contrast, the almost planar anisotropy found for Er and Yb prevents frustration, because of the continuous range of possible spin orientations in this case.
Broad-band photoluminescence from ZnGeP2
Authors: Moldovan M. , Giles N. C.
Published: July 2002

The near-infrared photoluminescence from single crystals of bulk ZnGeP2 was studied as a function of excitation power, excitation wavelength, sample temperature, and polarization. The nature of this broad-band luminescence from large single crystals grown for nonlinear optical applications is established. Two distinct bands with quite different polarization, power, and temperature behaviors were resolved. At 5 K, these broad bands have peaks in intensity near 1.58 and 1.36 eV. The 1.58 eV band is partially polarized perpendicular to the crystal’s c axis, has a relatively small thermal quenching activation energy (45 meV), and excitation spectra show a resonance in intensity associated with a shallow level approximately 90 meV below the minimum conduction band. The 1.36 eV band is partially polarized parallel to c, has a much larger quenching activation energy (220 meV), and its excitation spectrum includes two weak resonances corresponding to the A′ and B′ n=1 excitons. The high-energy band was enhanced in crystals containing relatively larger concentrations of phosphorus vacancies (measured by electron paramagnetic resonance).
Crystal growth and structural, electrical, and optical characterization of CuIn3Te5 and CuGa3Te5 ordered vacancy compounds
Authors: Marı́n G. , Delgado J. M. , Wasim S. M. , Rincón C. , Pérez G. Sánchez , Mora A. E. , Bocaranda P. , Henao J. A.
Published: July 2002

X-ray powder diffraction studies of ordered vacancy compounds CuIn3Te5 and CuGa3Te5, prepared by the vertical Bridgman–Stockbarger technique, show that these materials exhibit a tetragonal chalcopyrite-related structure. The unit cell parameters a and c are, respectively, 6.1639(3) and 12.346(2) Å for CuIn3Te5, and 5.9321(8) and 11.825(4) Å for CuGa3Te5. From electrical resistivity characterization as a function of temperature a shallow acceptor level, with an activation energy lower than 30 meV, is found in both these compounds. Their direct energy gaps at room temperature are 1.013 and 1.092 eV for CuIn3Te5 and CuGa3Te5, respectively.
Remanence breakdown in granular alloys at magnetic percolation
Authors: Batlle X. , Franco V. , Labarta A. , O'Grady K.
Published: July 2002

Microstructural effects on the magnetic behavior of rf-sputtered CoFe–AgCu granular alloys are examined through the study of the in-plane remanence-to-saturation magnetization ratio, Mr/Ms, as a function of temperature, ferromagnetic volume content, xv, and annealing temperature. At low ferromagnetic contents (xv⩽0.25), the MrMs ratio in as-deposited samples tends towards 0.5 at low temperature, as expected from the uniaxial perpendicular anisotropy displayed by all samples, which is magnetoelastic in nature and arises from the axial distortion of the CoFe face-centered-cubic cells. In as-deposited samples with xv>0.25 (well below the volume percolation threshold, xp∼0.5–0.55), a collective magnetic behavior develops due to magnetic correlations among particles. Consequently, a domain structure perpendicular to the film plane appears, which results in a remanence breakdown: Mr/Ms is about 0.2. Besides, magnetic correlations prevents the thermal decay of Mr/Ms, which is almost constant between 5 and 300 K, even for ferromagnetic particles no more than 3 nm in size. The axial distortion disappears with annealing, the cubic symmetry is recovered, the out-of-plane magnetic structure is lost and, therefore, the Mr/Ms ratio at low temperature tends towards 0.8 for highly annealed samples. Consequently, the magnetic properties of granular alloys depend on the interplay between anisotropy, exchange, and dipolar interactions, which in turn depend crucially on the microstructure.
Diode laser spectroscopy of the CO2+ ν3 band using magnetic field modulation of the discharge plasma
Authors: Kawaguchi Kentarou , Yamada Chikashi , Hirota Eizi
Published: July 2002

The ν3 fundamental band of CO2+ has been observed with infrared diode laser spectroscopy, while modulating a hollow cathode discharge in CO2 by a magnetic field. The band origin has been determined to be 1423.0821(8) cm−1. This anomalously low value has been explained in terms of a vibronic interaction between X̃ 2Πg and Ã 2Πu through the ν3 vibration, as in the case of BO2.
Shocked states from initially liquid oxygen–nitrogen systems
Authors: Schott Garry L. , Shaw M. S. , Johnson J. D.
Published: July 2002

Sets of pressures and their corresponding specific volumes and internal energies are derived from measurements on steadily propagating, planar shock waves propelled by explosively driven metal assemblies into a 1:1 atomic mixture of the elements nitrogen and oxygen in each of two liquid initial states. One of these is the equimolar solution of O2 and N2, at T≂85 K, v0≂1.06 cm3/g; the other is the pure explosive compound NO, at T≂122 K, v0≂0.79 cm3/g. Results for this system are calculated with effective spherical potentials and presented graphically for comparison with the measurements. Single- and reflected-shock states are reported, as are incidental new results on pure liquid N2 at 85 K. The method of measurement is described, with reference to its previous applications to liquid O2 and Ar. First-shock pressures from both initial forms lie between 10 and 30 GPa, and the Hugoniots intersect at a common state, near 21 GPa, where a single reflected-shock Hugoniot is centered. Concordant measured state variables at this intersection provide novel confirmation of the expectation, inherently incorporated into theory, that unique equilibrium states are reached. Accounting for densities of these states by theory indicates a significant amount of oxidized nitrogen, in reversible equilibrium with major, but not exclusive, N2 and O2 components. This is treated as residual NO only, although the uncertainty in the potentials for other oxides does not assure their absence.
The vibrational predissociation spectroscopy of hydrogen cluster ions
Authors: Okumura M. , Yeh L. I. , Lee Y. T.
Published: July 2002

The first infrared spectra of protonated hydrogen clusters in the gas phase have been observed. Predissociation spectra were taken with a tandem mass spectrometer: mass selected hydrogen cluster ions were irradiated inside a rf ion trap by a tunable infrared laser, and the fragment ions created by photodissociation of the clusters were mass selected and detected. Spectra for each product channel were measured by counting fragment ions as a function of laser frequency. Low resolution spectra (Δν=10 cm−1) in the region from 3800 to 4200 cm−1 were observed for the ions H+5, H+7, and H+9 at 3910, 3980, and 4020 cm−1, respectively. A band was also observed for H+5 at 3532 cm−1. No rotational structure was resolved. The frequencies of the band maxima agree well with the frequencies predicted by previous ab initio calculations for the highest modes.
Polymer collapse in dilute solution: Equilibrium and dynamical aspects
Authors: Allegra Giuseppe , Ganazzoli Fabio
Published: July 2002

The pairwise, long-range attractions experienced by a polymer chain in poor solvent, are resisted by the screened (two-body) and by the three-body interactions. While the former are due to the nonzero effective chain thickness and are especially important for relatively short chains, the latter arise from interdependence among the two-body attractions, and prevail for very long chains. We adopt the Gaussian approximation for the interatomic long-range distances with a lower cutoff for the chain contacts, and a Fourier description of the chain configurational modes. If the average chain diameter is small enough compared with Kuhn’s segment length, a polymer with a sufficiently large molecular mass undergoes a coil→globule first-order transition upon cooling below the Θ temperature; otherwise, the transition is of the second order. For the case of atactic polystyrene, we predict a first-order transition. Within the collapsed globule, polymer portions are essentially unperturbed if their root-mean-square length does not exceed the globule’s diameter, otherwise their end-to-end distance becomes independent of the contour length, thus leading to a complete randomness among the long-range interatomic contacts; however, the end segments of the chain are predicted to get closer to each other than the average intersegment distance. The resulting picture indicates that the collective Fourier-configuration modes undergo a much stronger shrinkage than the localized ones, upon collapse, which implies in turn that the relaxation times of the collective modes are drastically shortened in proportion with the average. These conclusions appear to be in essential agreement with the dynamical, light-scattering results obtained by Nishio, Swislow, Sun, and Tanaka on very dilute cyclohexane solutions of high-molecular weight polystyrene.
Potential energy curves using unrestricted Mo/ller–Plesset perturbation theory with spin annihilation
Authors: Schlegel H. Bernhard
Published: July 2002

Unrestricted Hartree–Fock and unrestricted Mo/ller–Plesset perturbation theory are convenient methods to compute potential energy curves for bond dissociation, since these methods approach the correct dissociation limit. Unfortunately, a spin unrestricted wave function can contain large contributions from unwanted spin states that can distort the potential energy surface significantly. The spin contamination can be removed by projection or annihilation operators. As is well known, the spin project unrestricted Hartree–Fock bond dissociation curves have a large kink at the onset of the UHF/RHF instability, and a spurious minimum just beyond. However, the spurious minimum disappears and the kink is very much less pronounced at the unrestricted Mo/ller–Plesset level with spin projection. Bond dissociation potentials for LiH and CH4 were computed at the fourth order Mo/ller–Plesset level plus spin projection,4 and good agreement was found with full CI and MR-CISD calculations.
Rotational inelastic cross sections for OCS–Ar, OCS–He, OCS–H2 collisions: A comparison between theory and experiment
Authors: Broquier M. , Picard-Bersellini A. , Whitaker B. J. , Green S.
Published: July 2002

Pressure broadening parameters resulting from collisional perturbations by Ar, He, and H2 for a number of rotational transitions in the ν3 band of OCS have been measured using a diode laser spectrometer. The data are compared with theoretical values obtained using intermolecular potentials previously suggested for these systems and infinite order sudden approximation molecular scattering calculations.
Dielectric relaxation in water. Computer simulations with the TIP4P potential
Authors: Neumann Martin
Published: July 2002

In this second paper in a series of systematic investigations seeking to relate the dielectric properties of water to the features of the intermolecular potential, extensive molecular dynamics simulations with the empirical TIP4P effective pair potential are compared with the experimental data for water as well as with the results reported previously for the MCY ab initio model. The frequency dependence of the dielectric constant obtained for the two models is contrasted with the predictions of a Mori three-variable theory and analyzed in detail using a phenomenological description of dielectric relaxation. It is shown that both models are capable of reproducing all reorientational processes observed in the experimental spectrum. However, qualitative agreement with the experimental results for the Debye relaxation time and the static Kirkwood g-factor gK is only obtained with the TIP4P model, although the values are still too low and ∂gK/∂T has the wrong sign. The differences between the models are interpreted as being due to the different position chosen for the center of negative charge, and it is argued that moving the latter further towards the oxygen would, for simple rigid point charge models, yield considerably improved agreement with experiment.
Light scattering by adsorbates at Ag particles: Quantum-mechanical approach for energy transfer induced interfacial optical processes involving surface plasmons, multipoles, and electron-hole pairs
Authors: Pettinger B.
Published: July 2002

A quantum-mechanical approach for surface-enhanced optical processes is given, which may solve an old controversy regarding the surface enhancement mechanisms: The present formalism contains three terms: The first is equivalent to the classically derived electromagnetic enhancement; the second represents a loss term due to a competitive dipole–multipole and dipole–electron-hole pair coupling; and the third reveals an additional energy transfer induced gain for excitable molecules. Thus, the total pure surface enhancement can be much greater for ‘‘colored’’ species than for transparent ones.
On double vacancies in the core
Authors: Cederbaum L. S. , Tarantelli F. , Sgamellotti A. , Schirmer J.
Published: July 2002

The energies needed to create different types of double core vacancies as well as the resulting redistribution of the valence electrons are analyzed in comparison with single core vacancies. Numerical results are presented for CH4 and in particular for the molecules C2H2, C2H4, and C2H6. A detailed perturbation theory analysis of the relaxation energies in terms of localized and delocalized molecular orbital pictures is presented. It is shown that the binding energies associated with double core vacancies where each of the two core holes is at a different atomic site sensitively probe the chemical environment of the atoms.
Bulk viscosity of model fluids. A comparison of equilibrium and nonequilibrium molecular dynamics results
Authors: Hoheisel Claus
Published: July 2002

We compare accurate equilibrium and nonequilibrium molecular dynamics calculations for the bulk viscosity of model fluids. Results for the ‘‘soft-sphere fluid’’ as well as for the ‘‘Lennard-Jones fluid’’ are considered. Particle number dependence and errors due to the cutoff of the potential are investigated. The accuracy of the correlation functions is discussed. Main results are the following: (i) For the soft-sphere fluid, good agreement between the two MD methods exists. (ii) For the Lennard-Jones fluid, similar good agreement can be achieved when particle numbers larger than 108 are used. The reason for this is a significant cutoff radius dependence of the bulk viscosity results, when the potential is cut at a separation smaller than 2.5 σ. Such a cutoff radius dependence does not occur for the shear viscosity. (iii) The nonequilibrium MD data given by Heyes for the soft-sphere fluid are incorrect. (iv) The Green–Kubo correlation function for the soft-sphere fluid oscillates at very high density. Such a behavior has not yet been observed for Lennard-Jones systems.
Structure and order in partially oriented solids: Characterization by 2D-magic-angle-spinning NMR
Authors: Harbison Gerard S. , Vogt Veit-Dieter , Spiess Hans W.
Published: July 2002

Theory and applications of a new 2D-NMR technique for characterizing the molecular structure and order of partially oriented solids are described. After deriving a general expression for the signal intensities in two dimensions in cases of perfect order, the influence of disorder on the spectra is discussed. It is demonstrated that an expansion of the orientational distribution function in terms of Wigner rotation matrices allows spectra from residues with an arbitrary degree of order and arbitrary chemical shielding tensor orientation to be fully characterized. The technique is illustrated by obtaining the orientational distribution function for the crystalline and amorphous regions in semicrystalline polyethylene terephthalate, showing that the molecular orientation is quite different in the two phases.
A determination of the stabilities of H+3(H2)n with n=1–9 from measurements of the gas-phase ion equilibria H+3(H2)n−1 +H2=H+3(H2)n
Authors: Hiraoka Kenzo
Published: July 2002

The equilibria of clustering reaction H+3(H2)n−1 +H2=H+3(H2)n were studied over the temperature range 330–25 K. The experiments were done with a pulsed electron–beam mass spectrometer. The ion source was thermostatted using a temperature-regulated cryocooler. The thermochemical stabilities of H+3(H2)n with n=1–9 were determined. The ions H+5, H+9, H+15, and H+19 are relatively more stable toward dissociation. The enthalpy changes for the clustering reaction yielding H+5 and H+7 were found to be in good agreement with the recent experimental study of Beuhler, Ehrenson, and Friedman, but in somewhat poor agreement with the earlier examination by Hiraoka and Kebarle.
Pulsed beam Fourier transform microwave measurements on OCS and rare gas complexes of OCS with Ne, Ar, and Kr
Authors: Lovas F. J. , Suenram R. D.
Published: July 2002

A pulsed molecular beam Fourier transform microwave spectrometer, which has been recently constructed at NBS, was employed for measurements on several monomer and van der Waals species of OCS. The absorption–emission cell consists of a Fabry–Perot resonant cavity inside a high vacuum chamber. A pulsed nozzle is used to generate a supersonic molecular beam of a seeded inert gas. New spectra of Ne–OCS, Ar–OC34S, and for four Kr isotopes of Kr–OCS are reported, as well as structure analyses of each species. The nuclear electric quadrupole hyperfine structure of 83Kr–OCS has been resolved and yields eQqaa=1.601(7) MHz and eQqbb=−1.857(3) MHz.
Two-dimensional imaging of state-selected photodissociation products detected by multiphoton ionization
Authors: Chandler David W. , Houston Paul L.
Published: July 2002

A new technique is presented that makes it possible, with a single laser pulse, to determine the three-dimensional spatial distribution of state-selected photoproducts. Initially, absorption of a photon from a laser beam causes fragmentation of a molecule. Multiphoton ionization is used to select the internal state of a desired fragment without perturbing its velocity. Following a short delay, the three-dimensional spatial distribution caused by the fragment velocities is projected onto two dimensions by accelerating the state-selected fragment ions into the surface of a channel plate particle multiplier. Electrons emerging from the multiplier are imaged onto a phosphorescent screen for analysis by a digital-image processing device such as a two-dimensional optical multichannel analyzer. The three-dimensional spatial distribution is reconstructed by taking the Hankel transform of the Fourier transform of the projection. The technique is illustrated by recording the spatial distribution of methyl fragments produced in their vibrational ground state by the 266 nm photodissociation of CH3I. From this study it is determined that the fraction of CH3(v=0) formed in coincidence with I(2P1/2) is greater than 0.95, the rest being formed in coincidence with I(2P3/2) ground state.
Bulk viscosity of the Lennard-Jones fluid for a wide range of states computed by equilibrium molecular dynamics
Authors: Hoheisel C. , Vogelsang R. , Schoen M.
Published: July 2002

Accurate data for the bulk viscosity ηv have been obtained by molecular dynamics calculations. Many thermodynamic states of the Lennard-Jones fluid were considered. The Green–Kubo integrand of ηv is analyzed in terms of partial correlation functions constituting the total one. These partial functions behave rather differently from those found for the shear viscosity or the thermal conductivity. Generally the total autocorrelation function of ηv shows a steeper initial decay and a more pronounced long time form than those of the shear viscosity or the thermal conductivity. For states near transition to solid phases, like the pseudotriple point of argon, the Green-Kubo integrand of ηv has a significantly longer ranged time behavior than that of the shear viscosity. Hence, for the latter states, a systematic error is expected for ηv using equilibrium molecular dynamics for its computation.
Spectroscopic studies of phthalocyanines and their clusters with small molecules
Authors: Menapace J. A. , Bernstein E. R.
Published: July 2002

van der Waals clusters of free base phthalocyanine (H2Pc) and magnesium phthalocyanine (MgPc) with small hydrocarbons [CnH2n+2(n=1,2,3)], hydrogen bonding solvents (H2O, MeOH, EtOH), and CO2 are studied in the gas phase using supersonic molecular jet spectroscopy. Fluorescence excitation spectra of the cluster systems are characterized in the 000 regions of the cluster S1←S0 transitions. Forbidden low frequency cluster chromophore out-of-plane vibronic transitions are induced by clustering in the majority of the cluster systems studied. This low frequency motion is characterized using an out-of-plane normal coordinate analysis on the H2Pc moiety. Calculations of the binding energy and ground state geometry for the clusters are carried out employing Lennard-Jones (6-12-1) and hydrogen bonding (10-12-1) potentials. Comparison between the calculations and experiments allow for the identification of specific configurations responsible for the cluster vibronic transitions observed. The cluster vibronic spectra and theoretical calculations suggest that stable H2Pc and MgPc solvation sites are located over the phthalocyano core and not over peripheral ring centers. The H2Pc/hydrocarbon cluster experimental and calculational results parallel those obtained for benzene and N-heterocycle/hydrocarbon clusters studied previously. The H2Pc and MgPc/alcohol cluster spectra and calculated geometries suggest that the solvent OH group is intimately involved in the intermolecular interactions and contributes significantly to the observed spectroscopic shifts.