bluuebunny/crossref_metadata_2025 huggingface dataset:
High-frequency modulation of an electron beam produced by a plasma cathode
Authors: Dunaevsky A. , Krasik Ya. E. , Felsteiner J. , Rosenberg A.
Published: July 2002

We present a detailed study of the recently discovered modulation at frequencies of hundreds of MHz of an electron beam produced by a plasma cathode. The plasma cathode consists of a ferroelectric disk sample [BaTiO3 or Pb(Zr, Ti)O3] that is placed in a cylindrical metal box having an output window covered by a grid. The plasma is prepared by an incomplete discharge that was ignited by a few kV driving pulse applied across the ferroelectric disk. It was shown that the current modulation appears only when the driving pulse is applied to the front gridded electrode of the ferroelectric sample and the solid rear electrode is grounded. The modulation spectrum is quite narrow (⩽3%), with harmonics at frequencies up to a few GHz. The basic frequency of modulation was found to be dependent on the length of the feed cable to the sample. Investigation of the plasma and the beam modulation properties and their dependence on the experimental parameters is presented. The results indicate the formation of a plasma at the vicinity of the front electrode of the ferroelectric sample during the modulation. The results are discussed within the frame of electrons oscillating in the potential well created by the positively charged plasma having feedback coupling with the input cable.
Synchrotron x-ray topographic and high-resolution diffraction analysis of mask-induced strain in epitaxial laterally overgrown GaAs layers
Authors: Rantamäki R. , Tuomi T. , Zytkiewicz Z. R. , Domagala J. , McNally P. J. , Danilewsky A. N.
Published: July 2002

Synchrotron x-ray back reflection section topographs of epitaxial lateral overgrown (ELO) GaAs samples grown on (001) GaAs substrates show images of the GaAs layers bent due to the interaction between the layer and the SiO2 mask. The topographs are simulated under the assumption of orientational contrast. Using the same data the measured x-ray diffraction curve is simulated. The calculations, which are in good agreement with the measurements, are used to gain information on the tilted (001) lattice planes in each ELO layer. We show that the bending of ELO lattice planes reaches a maximum at the center of the ELO stripes, where misorientation is at a minimum, and decreases towards the edges of the stripes, where misorientation reaches a maximum.
Indium-modified growth kinetics of cubic and hexagonal GaN in molecular beam epitaxy
Authors: Adelmann C. , Langer R. , Martinez-Guerrero E. , Mariette H. , Feuillet G. , Daudin B.
Published: July 2002

The kinetics changes in the growth of GaN induced by the presence of In segregated on the surface have been investigated in situ by using reflection high energy electron diffraction intensity oscillations. Two types of surfaces have been studied, namely, Ga-polar (0001) wurtzite, and (001) zinc blende. It has been found that In influenced both Ga and N surface kinetics, leading to a change in the growth rate associated with a change in the amount of Ga and N effectively available for the GaN growth.
Scanning near-field cathodoluminescence microscopy for semiconductor investigations: A theoretical study
Authors: Pastré D. , Troyon M.
Published: July 2002

The spatial resolution and the near-field signal detection efficiency obtained in scanning near-field cathodoluminescence microscopy (SNCLM) are theoretically evaluated in the case of semiconductor investigation. The effect of the electron-beam interaction volume, the surface recombination velocity, and the energy diffusion is taken into account. The lateral resolution of the SNCLM, because of the near-field collection mode, is not energy-transfer dependent. A good lateral resolution can be obtained even when the surface recombination velocity is low and the minority-carrier diffusion length is large [this is the advantage of the near-field collection mode with respect to the tip illumination/far-field collection mode generally used in photoluminescence (PL) imaging]. Further, it is shown that the radiative recombination centers situated far from the tip can perturb the near-field detection efficiency, particularly for large nonradiative surface recombination velocities and low diffusion lengths. Nevertheless, the short range of the electron energy loss limits the contribution of these luminescent centers. Consequently, the near-field/far-field contribution ratio in cathodoluminescence experiments is better than that of PL experiments done in the far-field illumination/tip collection mode for which the far-field signal is more important because of the large laser-beam excitation area.
Effect of the layered structure on the electronic properties of amorphous carbon films on n-Si
Authors: Konofaos N. , Evangelou E. , Logothetidis S.
Published: July 2002

Amorphous carbon (a-C) films rich in sp3 bonds were grown onto n-type (100) silicon substrates by rf magnetron sputtering. The electrical defects created in the bulk of the carbon films as well as those at the a-C/Si interface during the deposition of the films were characterized electrically. Devices having the metal–insulator–semiconductor structure were fabricated and investigated by means of current–voltage (I–V) and capacitance–voltage (C–V) profiling and admittance spectroscopy. The construction of the films and in particular the presence of layered and bilayer structures was shown to affect the device characteristics. This behavior was attributed to Ar+ ions trapped inside the carbon bulk as well as to charge deficiencies due to the specific growth conditions of the amorphous carbon layers. The density of the interface states was found to depend considerably on the film construction, ranging from 1011 to 1013 eV−1 cm−2. The dynamic behavior of the states was expressed by the relevant time constant, which was calculated to vary from 10−3 to 10−6 s.
Theoretical study of valence-band offsets of strained Si1−x−yGexCy/Si(001) heterostructures
Authors: Wu Liqing , Huang Meichun , Li Shuping , Zhu Zizhong , Zhuang Baohuang
Published: July 2002

We present a theoretical study of the valence-band offsets (VBOs) of the pseudomorphic Si1−x−yGexCy/Si(001) heterojunctions. Our calculation is based on the ab initio pseudopotential method, the virtual-crystal approximation, and the average-bond-energy theory. It is found that the VBO is sensitive to the strain condition. Initial addition of carbon increases the VBO of the relaxed Si1−x−yGexCy/Si(001) heterostructures. For constant Ge fraction, the VBO decreases and increases with increasing the C fraction in the case of the compressive and tensile strains, respectively. The trends of our results are in agreement with those of other theoretical and experimental results.
Experimental evidence of compositional mixture in CdTeO films grown by radio-frequency sputtering
Authors: Iribarren A. , Menéndez-Proupin E. , Castro-Rodríguez R. , Sosa V. , Peña J. L. , Caballero-Briones F.
Published: July 2002

An analysis of the absorption spectra of amorphous CdTeO films grown by radio-frequency sputtering has been carried out. The behavior and the shape of the spectra suggest the presence of CdxTeyOz variable-composition compounds ranging between CdTe and CdTeO with 67 at. % oxygen. We present a model based on bond statistics, which describes the absorption profile behavior of this and other materials with similar optical behavior, and models of the composition of such films.
Ultrahigh-resolution, frequency-resolved resonance fluorescence imaging with a monoisotopic mercury atom cell
Authors: Podshivalov A. A. , Shepard M. R. , Matveev O. I. , Smith B. W. , Winefordner J. D.
Published: July 2002

A novel method of ultrahigh-resolution, frequency-resolved imaging using atomic vapor cells is proposed. The method is based on the accurate measurement of the fluorescence signal intensity distribution along the absorption path length when the signal frequency is tuned to a wing of the atomic absorption line. Two-step resonance fluorescence of Hg202 vapor was used for one-dimensional imaging of the Hg resonance radiation at 253.7 nm. An imaging signal with a frequency difference of 500 MHz could be easily distinguished visually and even a frequency difference of 80 MHz could be detected after appropriate processing of the fluorescence imaging signal. Several other novel methods of one- and two-dimensional multifrequency imaging are discussed.
Anomalous transmission through near unit index contrast dielectric photonic crystals
Authors: Loschialpo P. , Forester D. W. , Schelleng J.
Published: July 2002

We present calculations that reveal a limitation for devices that demand photonic crystals with very narrow band gaps. The opaque regions of the transmission spectra through finite length crystals may be substantially broader than the band gap of the corresponding infinite crystals. This is a consequence of the crystals’ near unit dielectric index contrast. The opaque spectral width decreases gradually with increasing thickness. Many hundreds of layers may be needed to obtain a transmission null which approaches the width of the band gap for the infinite crystal. The analysis is done for two configurations of interest for optical applications due to their relative ease of manufacturer. The first is a two-dimensional triangular array of dielectric rods for which finite difference time domain simulations are done to determine the transmission spectra. The second is a one-dimensional dielectric stack for which transmission is analytically calculated.
Structural and magnetic properties of a novel DyFe9.16V0.50 intermetallic compound with a disordered CaCu5-type structure
Authors: Psycharis V. , Gjoka M. , Kalogirou O. , Sheludko N. , Mikhov N. , Moraitakis E. , Niarchos D.
Published: July 2002

A Dy–Fe–V intermetallic compound has been prepared by arc melting elemental constituents, followed by annealing at 1123 K. This compound with a nominal composition of DyFe9.16V0.50, crystallizes in a disordered CaCu5-type structure (space group P6/mmm). Structure analysis is performed on x-ray powder diffraction data by the Rietveld method, and the lattice parameters are a=4.8692(1) Å, c=4.1750(3) Å. The calculated stoichiometric formula is Dy1.00(1)(Fe, V)9.9(2). Magnetic measurements give a Curie temperature of 473 K and a saturation magnetization Ms of 69.4 and 76.5 A m2/kg at 5 and 300 K, respectively. The fact that the Ms value is higher at higher temperatures is attributed to the ferrimagnetic structure of the compound. X-ray powder diffraction measurements on magnetically aligned powder show that the magnetocrystalline anisotropy is of an easy plane type at room temperature. ac susceptibility, magnetic polarization measurements and the calculation of the magnetocrystalline anisotropy constants confirm this result at room temperature and indicate that the magnetocrystalline anisotropy remains of an easy plane type over the whole temperature range between 5 and 300 K.
Theoretical investigation of shallow acceptors confined in Si/Si1−xGex quantum well structures
Authors: Zhao Q. X. , Willander M.
Published: July 2002

Energy levels of the shallow acceptor states have been calculated for center-doped Si/Si1−xGex/Si quantum wells. The impurity states were calculated using an effective-mass theory that accounts for valence-band mixing as well as the mismatch of band parameters and dielectric constants between well and barrier materials. Acceptor binding energies and splitting between the acceptor 1S3/2(Γ7) and 1S3/2(Γ6) ground states were studied at various Ge concentrations and well widths. The results are discussed in comparison with the recent conclusion from the lateral transport measurements in boron-doped Si/SiGe quantum wells.
Irreversibility of zero-field birefringence in ferrofluids upon temperature reversal
Authors: Bakuzis A. F. , Da Silva M. F. , Morais P. C. , Neto K. Skeff
Published: July 2002

The temperature dependence of the zero-field birefringence was investigated using acid and basic MnFe2O4 ionic magnetic fluids, in the range of 290–350 K. Approaching a characteristic temperature (Tc) from below, which depends upon the sample characteristics, the zero-field birefringence goes critically down to zero. Furthermore, the birefringence shows an irreversible path upon heating and cooling the samples above Tc. The experimental data are successfully explained as long as dimers are included in the model calculation and the thermal disruption of them follows a critical behavior.
Interface reactions between quaternary cobalt alloys and carbon coating in thin film disk media
Authors: Thiele J.-U. , Pocker D. J. , White R. L.
Published: July 2002

A study of interface reactions between the magnetic layer and the protective carbon coating, and the influence of these interactions on magnetic properties of rigid disk media for magnetic data storage is presented. Quaternary CoPtCrB and CoPtCrTa alloy films and protective hydrogenated or nitrogenated carbon coatings were deposited by dc magnetron sputtering onto metal hard disks (AlMg substrate/electroplated NiP layer/Cr underlayer). Core level x-ray photoelectron spectroscopy of nitrogenated carbon films on B-containing alloys showed the formation of boron nitride and small amounts of chromium nitride at the interface, and also indicated the likely presence of chromium carbide. The amount of boron nitride at the interface varied depending on substrate bias voltage and temperature during deposition of the carbon coating. From a quantitative analysis of the x-ray photoelectron spectra it was inferred that boron nitride formation was controlled by the diffusion of boron from deeper regions of the magnetic layer to the interface. Surprisingly, compared to disks using the same magnetic alloy but with a hydrogenated or pure carbon coating, no effects of these interface reactions on the magnetic properties of the disks could be detected. In contrast, for a Ta-containing alloy the same comparison revealed a drop in the coercivity of up to 200 Oe in disks with nitrogenated carbon overcoats. Strong evidence for the formation of tantalum nitride as well as small amounts of chromium nitride was found in the photoelectron spectra. Thus, while the formation of boron nitride at the interface of CoPtCrB media and protective carbon coating does not affect the magnetic properties of the disks in the range of boron and nitrogen concentrations investigated here, small changes in the chemical environment of Ta and/or Cr can lead to significant changes in the magnetic properties of the CoPtCrTa media.
Surface reaction probability during fast deposition of hydrogenated amorphous silicon with a remote silane plasma
Authors: Kessels W. M. M. , van de Sanden M. C. M. , Severens R. J. , Schram D. C.
Published: July 2002

The surface reaction probability β in a remote Ar–H2–SiH4 plasma used for high growth rate deposition of hydrogenated amorphous silicon (a-Si:H) has been investigated by a technique proposed by D. A. Doughty et al. [J. Appl. Phys. 67, 6220 (1990)]. Reactive species from the plasma are trapped in a well, created by two substrates with a small slit in the upper substrate. The distribution of amount of film deposited on both substrates yields information on the compound value of the surface reaction probability, which depends on the species entering the well. The surface reaction probability decreases from a value within the range of 0.45–0.50 in a highly dissociated plasma to 0.33±0.05 in a plasma with ∼12% SiH4 depletion. This corresponds to a shift from a plasma with a significant production of silane radicals with a high (surface) reactivity (SiHx,x<3) to a plasma where SiH3 is dominant. This has also been corroborated by Monte Carlo simulations. The decrease in surface reaction probability is in line with an improving a-Si:H film quality. Furthermore, the influence of the substrate temperature has been investigated.
Effect of electric field and post-treatment on dielectric behavior of SrTiO3 single crystal
Authors: Ang Chen , Guo Ruyan , Bhalla A. S. , Cross L. E.
Published: July 2002

The effect of dc electric field and post-treatment on the dielectric properties of SrTiO3 single crystals are reported. Both the dielectric constant and the loss tan δ decrease after post-treatment in flowing oxygen environment for 26 h at 1100 °C. The possible physical mechanism is briefly discussed. The dielectric behavior under dc electric field is measured. The rounded dielectric constant peak is induced by application of dc bias, and the corresponding dielectric loss is presented, which show more complicated behavior and usually are missing in the literature. In this work, it is found that the temperature (Tm) of the dielectric constant maximum shifts to higher temperatures with increasing dc electric field E, and follows the Tm∼E2/3 relation, indicating an induced ferroelectric-like behavior with the second-order phase transition under dc bias.
Nanosecond resolved techniques for dynamical magnetization reversal measurements
Authors: Bonfim M. , Mackay K. , Pizzini S. , Arnou M-L. , Fontaine A. , Ghiringhelli G. , Pascarelli S. , Neisius T.
Published: July 2002

In this article we present three techniques developed by our group for probing magnetization dynamics in the nanosecond time scale. All these techniques are based on the magneto-optical interaction of materials with polarized light. Magnetic excitation is provided by microcoils able to generate field pulses of some teslas within a few nanoseconds. Standard Kerr/Faraday dynamic measurements and imaging can be performed as well as time-resolved x-ray magnetic circular dichroism where chemical selectivity can be achieved.
Switching fluctuations and density limitations of pseudospin valve memory
Authors: Fang Tzu-Ning , Zhu Jian-Gang
Published: July 2002

In this article, a systematic micromagnetic analysis of the pseudospin valve magnetic random access memory device is presented. The article focuses on the correlation between switching field variation and geometric structure of the memory elements. For an elongated memory element with flat ends, magnetization reversal always initiates from the ends, yielding nonrepeatable switching processes and fluctuating switching fields. Tapering the element ends forces the reversal to start from the center region of the element and triggers robust repeatable magnetization rotation. However, sharp tips of the tapered ends are required, which may present limitations on fabrication in practice.
Spin-dependent intergranular transport in magnetite films deposited by ferrite plating
Authors: Kitamoto Y. , Nakayama Y. , Abe M.
Published: July 2002

The present article describes that Fe3−xO4 films prepared by the ferrite plating exhibited negative magnetoresistance associated with spin-dependent intergranular transport. The Fe3−xO4 plated films were composed of Fe3O4 and γ-Fe2O3; the relationship between the degree of oxidation, magnetic and electrical properties by changing the concentration of NaNO2 in the oxidizing solution CNaNO2 was discussed. Current-in-plane (CIP) electrical properties were investigated with a four-probe method at room temperature. Resistivity was about 2–200 Ω cm, which is higher than that of bulk magnetite (10−2 Ω cm). Negative magnetoresistance with peaks at the coercive field was observed and the MR ratios taken in the maximum field of 2.5 kOe increased up to 5.5% with increasing CNaNO2. Mössbauer spectroscopy and the electrical evaluation showed that an increase in the Fe3+ ion content, in other words, an increase in insulating region raised the resistivity of the films with increasing CNaNO2. The dependence of the MR ratio on CNaNO2 suggested that such insulating region existed in the grain boundaries and the MR effect was caused by the spin-dependent intergranular transport of electrons through the insulating regions. The results suggest that appropriate control of the gain boundaries through changing the oxidizing conditions enables us to improve the MR effect.
Transverse spin freezing in the vicinity of amorphous Fe
Authors: Uchida T. , Al-Attar H. , Kakehashi Y.
Published: July 2002

Temperature dependence of magnetism in the vicinity of the ferromagnetic instability point of amorphous Fe has been investigated on the basis of the finite temperature theory of amorphous metallic magnetism which takes into account the transverse spin degrees of freedom. The theory is based on the functional integral technique to the degenerate-band Hubbard Hamiltonian and the distribution function method for local magnetic moments with structural disorder. Numerical calculations have been performed with decreasing temperature near the boundary between the ferromagnetic and the spin glass phases. It is shown that the amorphous metals around Fe exhibit the transition from the collinear to the noncollinear ferromagnetic state at a temperature lower than the Curie temperature, due to the freezing of transverse spin components. The present results are consistent with those of recent Mössbauer measurements on Fe-rich amorphous transition metal alloys.
Photon-noise-limited direct detector based on disorder-controlled electron heating
Authors: Karasik Boris S. , McGrath William R. , Gershenson Michael E. , Sergeev Andrew V.
Published: July 2002

We present a concept for a hot-electron direct detector capable of counting single millimeter-wave photons. The detector is based on a microbridge (1 μm size) transition edge sensor made from a disordered superconducting film. The electron–phonon coupling strength at temperatures of 100–300 mK is proportional to the elastic electron mean free path l and can be reduced by over an order of magnitude by decreasing l. The microbridge contacts are made from a different superconductor with higher critical temperature Nb, which blocks the thermal diffusion of hot carriers into the contacts. The low electron–phonon heat conductance and the high thermal resistance of the contacts determine the noise equivalent power of ∼10−20–10−21 W/√Hz at 100 mK, which is 102–103 times better than that of state-of-the-art bolometers. Due to the effect of disorder, the electron cooling time is ∼10−1–10−2 s at 0.1 K. By exploiting negative electrothermal feedback, the detector time constant can be made as short as 10−3–10−4 s without sacrificing sensitivity.
Spin dependent tunneling at finite bias
Authors: Davis A. H. , MacLaren J. M.
Published: July 2002

Results of theoretical studies of spin dependent tunneling in magnetic tunnel junctions at finite bias are presented. A simple model which extends Slonczewski’s ideas is developed. For each spin it assumes tunneling from a single free electron band through a simple barrier. The model predicts a decrease in conductance ratio with bias in good agreement with experimental observations. We find that the decrease of the magnetoconductance ratio, universally seen in experiment, has an intrinsic component resulting from the underlying electronic structure.
Effect of lead additive on the ferroelectric properties and microstructure of SrxPbyBi2zTa2O9 thin films
Authors: Chen San-Yuan , Lee Ving-Ching
Published: July 2002

Ferroelectric thin films of bismuth-containing layered perovskite SrxPbyBi2zTa2O9 have been prepared using the metalorganic decomposition method. The effect of both Sr and Pb content on the crystal structure, microstructure, and ferroelectric properties of SrxPbyBi2.3Ta2O9 films was investigated. A maximum remanent polarization of 2Pr=19.2 μC/cm2 was obtained for the SrxPb0.2Bi2.3Ta2O9 film with 20 mol % Sr-deficient composition as prepared at 800 °C, which could be the compromising effects of Sr content on both grain growth and second phase formation of BiTaO4. The substitution of Pb for Bi is accompanied by the occurrence of oxygen vacancies to compensate the charge balance, which is responsible for grain growth mechanism in Sr0.8Pb0.2Bi2.3Ta2O9 films. Fatigue endurance of Sr0.8Bi2.3Ta2O9 films becomes problematic after 109 cycles with a decrease in remanent polarization to 85% of the original value. This phenomenon was related to electron injection and creation of electron traps due to the occupation of Sr vacancies by Bi cations. It is demonstrated that the fatigue endurance of Sr0.8Bi2.3Ta2O9 film can be improved by doping with 20 mol % PbO.
Comparison of Peltier and anti-Stokes optical coolings
Authors: Frey R. , Micheron F. , Pocholle J. P.
Published: July 2002

Anti-Stokes optical cooling is compared theoretically to Peltier cooling. The influence of reabsorption of fluorescence photons, parasitic absorption, and interaction length is discussed by using a simple modeling for the propagation of laser and fluorescence waves. An equivalence between anti-Stokes and Peltier coolings is presented, which shows that Peltier effect potentially exhibits a higher cooling coefficient but much larger thermal losses than anti-Stokes fluorescence. As a consequence, Peltier coolers are shown to be more effective for heat pump and high temperature operation (T>190 K), while anti-Stokes coolers could be used down to low cooling temperatures (around 70 K) especially if the cooling material could be operated at long pump wavelength. Moreover, if photons emitted at the anti-Stokes frequency are down converted to the laser frequency and recycled into the cooling medium, anti-Stokes cooling could reach very high efficiencies even in the low temperature regime, making this technique highly attractive for cooling down of small devices.
Growth mode and surface morphology of a GaN film deposited along the N-face polar direction on c-plane sapphire substrate
Authors: Sumiya M. , Yoshimura K. , Ito T. , Ohtsuka K. , Fuke S. , Mizuno K. , Yoshimoto M. , Koinuma H. , Ohtomo A. , Kawasaki M.
Published: July 2002

The dependence of polar direction of GaN film on growth conditions has been investigated by changing either the group-V/group-III ratio (V/III ratio) in supplying the source gas or the deposition rate. GaN films were deposited on a nitrided sapphire by two-step metalorganic chemical vapor deposition. The surface morphology changed from flat hexagonal to pyramidal hexagonal facet with the increase of V/III ratio. However, the polar direction of GaN on an optimized buffer layer of 20 nm thickness was N-face (−c) polarity, independent of both the V/III ratio and the deposition rate. The polarity of the GaN epitaxtial layer can be determined by that of an interface (nitrided sapphire, annealed buffer layer or GaN substrate) at the deposition of GaN epitaxial layer. The higher V/III ratio enhanced the nucleation density, and reduced the size of hexagonal facets. The nuclei, forming the favorable hexagonal facets of wurtzite GaN, should grow laterally along the {101̄0} directions to cover a room among the facets until coalescence. After coalescence, −c GaN growth on a flat hexagonal facet results in a pyramidal hexagonal facet. The growth mode for −c GaN has been discussed with respect to surface structure and migration length of adsorbing precursors, in comparison with Ga-face (+c) GaN.
Paramagnetic resonance evidence for fluctuations in the crystal near the solid to liquid crystal transition in p-(methoxybenzlidene) amino phenyl acetate
Authors: Owens F. J.
Published: July 2002

A pronounced broadening of the lines and a decrease in the hyperfine tensor components of the EPR spectrum of vanadyl acetylacetonate (VAA) doped into the crystalline phase of the nematic liquid crystal p-methoxybenzlidene amino phenyl acetate is observed as the temperature of the solid approaches the transition temperature to the liquid crystal phase from below. The effects are indicative of the onset of a thermally activated fluctuation of the VAA complex resulting from pretransitional fluctuations of the liquid crystal molecules in the solid prior to melting.
Molecular-beam studies of the dynamics of organic electron transfer reactions
Authors: Russell J. A. , Hershberger J. F. , McAndrew J. J. , Cross R. J. , Saunders M.
Published: July 2002

Using crossed molecular beams we have studied the dynamics of several electron transfer reactions, A+B→A++B−, where A is an organic base and B is SnCl4, SbF5, or TiCl4. We propose a simple, modified stripping model whereby the electron jumps at the point where the ionic and covalent surfaces cross to form a pair of ions produced by a vertical, Franck–Condon transition. All initial energy in excess of this vertical threshold appears in the translational energy of the products. This model is verified in one case where the vertical ionization potential and electron affinity are known and is then used to obtain a rough vertical electron affinity of SbF5. Except at the lowest energies all the reactions follow this modified stripping mechanism.
A new thermodynamic model for the hard-core fluid with a Yukawa tail
Authors: Mansoori G. A. , Kioussis N.
Published: July 2002

A new and analytical thermodynamic model for simple fluids with an intermolecular pair potential consisting of a hard-sphere core and an attractive Yukawa tail is presented. The model yields simple analytic expressions for all thermodynamic properties of the fluid system. The self-consistent numerical results of the present model for the compressibility factor, internal energy, and Helmholtz free energy are in excellent agreement with the Monte Carlo ‘‘experiments’’ for the whole range of temperatures and densities in which experimental data are available.
Limits on the lifetimes of intramolecular rovibrational relaxation
Authors: Kulp Thomas , Ruoff Rodney S. , McDonald J. D.
Published: July 2002

Rotationally resolved infrared laser induced fluorescence spectra are presented for dimethyl ether and methyl formate. Q branch spectra are presented for 1,4-dioxane, methyl formate, and dimethyl carbonate. It has been demonstrated earlier that all of these states intramolecularly vibrationally relax. Lower limits on the lifetime of this process are inferred from the present data. For methyl formate and dimethyl ether the J=1 and 2 states have linewidths <0.31 cm−1. This sets a lower limits on the lifetime of 15 ps, although for the limited number of coupled states an exponential decay is not expected. This is also the lower limit for relaxation in the higher J=20 states of 1,4-dioxane and methyl formate, for which exponential decays probably do occur. Dimethyl carbonate relaxes in greater than 10 ps. All measurements were performed in a pulsed supersonic beam with a infrared optical parametric oscillator.
Quantum statistical mechanics via the recursive residue generation method
Authors: Friesner Richard A. , Wyatt Robert E.
Published: July 2002

Using the recursive residue generation method (RRGM), it is shown how Green’s function matrix elements calculated in large basis sets (N≫103) can be utilized to obtain controlled approximations to average values, correlation functions, and power spectra in multimode anharmonic systems. Convergence of the method is demonstrated through studies on five and seven mode model systems with basis sets containing up to about 8×103 states.
Bounds on fluid permeability for viscous flow through porous media
Authors: Berryman James G.
Published: July 2002

General properties of variational bounds on Darcy’s constant for slow viscous flow through porous media are studied. The bounds are also evaluated numerically for the penetrable sphere model. The bound of Doi depending on two-point correlations and the analytical bound of Weissberg and Prager give comparable results in the low density limit but the analytical bound is superior for higher densities. Prager’s bound depending on three-point correlation functions is worse than the analytical bound at low densities but better (although comparable to it) at high densities. A procedure for methodically improving Prager’s three point bound is presented. By introducing a Gaussian trial function, the three-point bound is improved by an order of magnitude for moderate values of porosity. The new bounds are comparable in magnitude to the Kozeny–Carman empirical relation for porous materials.
Use of carbon–carbon nuclear spin diffusion for the study of the miscibility of polymer blends
Authors: Linder Max , Henrichs P. Mark , Hewitt J. Michael , Massa Dennis J.
Published: July 2002

This high-resolution solid-state NMR work explores the usefulness of carbon–carbon nuclear spin diffusion to probe the miscibility of polymer blends. Experiments involving one- and two-dimensional NMR spectra, applied to blends of poly(ethylene terephthalate) (PET) and bisphenol-A polycarbonate (BPAPC), show that spin exchange occurs between carbon-13 nuclei located in monomer units that belong to the two different homopolymers. Carbon spin diffusion, which strongly depends on the internuclear distance, is explored as a possible probe for specific interactions between polar groups of the two polymers. Our results clearly show that the PET/BPAPC blends studied are homogeneously mixed at distances of 4.5 to 6 Å.
Higher excited states of benzene: Symmetry assignments of six gerade Rydberg series by four-photon absorption spectroscopy
Authors: Grubb S. G. , Otis C. E. , Whetten R. L. , Grant E. R. , Albrecht A. C.
Published: July 2002

We report the observation of six gerade Rydberg series in benzene converging to the first ionization potential, seen in the region from 14 500–18 500 cm−1 as four-photon resonances in the five-photon ionization spectrum of the jet-cooled molecule. Polarization ratios measured for each transition in two- as well as four-photon resonance aid in determining excited state symmetries. We assign a Rydberg series with a quantum defect of 0.76 to higher members of the 3s Rydberg state first seen by Johnson in the near UV two-photon resonant three-photon ionization spectrum of benzene. The observed quantum defect, polarization ratios, and correlation with synchrotron absorption data all strongly support this assignment. Another series, with a quantum defect of 0.24, exhibits a polarization ratio of 4.5±0.5 signifying pure rank four activity. This is the first report of a purely rank four transition in a molecule. We assign this as an excitation to a d2 orbital which when converging to the first ionization potential (the 2E1g cation) gives rise to B1g and B2g Rydberg transitions. Within the D6h point group, these symmetries are allowed only in a four-photon transition. The number of gerade Rydberg series observed as four-photon resonances exceeds those available from the s-(1) and d-(3) bound members of a hydrogenic basis set. The participation of g(l=4) orbitals is indicated as the lowest members of two of the Rydberg series are at the n=5 level.
Spectroscopic studies of AlO in argon matrices
Authors: Bares S. J. , Haak M. , Nibler J. W.
Published: July 2002

The diatomic species AlO has been isolated in argon at 18 K as an Al+O2 reaction product and shown to produce intense laser induced fluorescence in the visible. An infrared-optical double resonance experiment has established the X state vibrational fundamental at 975 cm−1, one of four possibilities previously proposed from infrared studies. 18O experiments further establish the assignment. The apparent low infrared intensity of the AlO stretch is consistent with a recent ab initio calculation of an oscillator strength of 4×10−7. This anomalously low value is confirmed by comparison of infrared and visible absorption intensities for similar samples. A v′=1 lifetime in excess of 6 ms was directly measured for AlO by the double resonance method. This slow rate of radiative relaxation makes AlO attractive as a probe molecule for vibrational relaxation studies.
Conical intersections in a system of four identical nuclei
Authors: Keating Sean P. , Mead C. Alden
Published: July 2002

The general properties of conical intersections between Born–Oppenheimer potential energy surfaces for the X4 system are considered. This system is one of intermediate complexity, sharing some, but not all, of the simplifying features of the X3 system studied previously. As in X3, one can define an internal coordinate system in terms of the internuclear distances which treats all nuclei equivalently. For X4, however, the degeneracy manifolds are of higher dimension than the symmetry manifolds, which has as a consequence that the degeneracy manifolds are only partially determined by symmetry. We obtain general properties of the locations of the conical intersections, and of the phase factors which compensate the sign change in the electronic wave function resulting from traversing a closed path around the intersection. The phase factor is also chosen in such a way that the resulting electronic wave functions have simple behavior under permutations of the nuclei. This choice of phase factor leads to consistency defined electronic functions which are smooth functions of the nuclear coordinates everywhere. The application of permutation operators to the nuclear functions in the presence of the altered phase factor is also considered. We also consider some simple consequences for scattering in this system. A result of practical interest is that the superposition of direct and exchange scattering for H2+H2 should be superposed with opposite sign from what one would obtain with a conventional calculation.
The critical-point conditions for classical polydisperse fluids
Authors: Johnson K. A. , Jonah D. A. , Kincaid J. M. , Morrison G.
Published: July 2002

The critical-point conditions for a polydisperse mixture are shown to be equivalent to those for the existence of nontrivial solutions to two homogeneous integral equations of the Fredholm type. This mathematically rigorous treatment is not dependent on the form of any particular model free energy and hence shows that there is no formal distinction between the critical-point conditions of a polydisperse fluid and those conditions derived by Gibbs for the critical point of a mixture with a finite number of components. Using the method of Fredholm, we express the critical-point conditions in terms of the zeros of two absolutely convergent expansions, and demonstrate how the expansions may be used to determine the shifts in critical density and temperature caused by changes in the composition of the fluid.
Structural and dynamic properties of lithium sulphate in its solid electrolyte form
Authors: Impey Roger W. , Klein Michael L. , McDonald Ian R.
Published: July 2002

The properties of solid lithium sulphate have been studied by computer simulation. At sufficiently high temperatures, the simulated crystal behaves as a solid electrolyte with lithium ion (jump) diffusion and sulphate group rotation. The atomic radial distribution functions in the rotator phase are discussed in relation to the low temperature, fully ordered, monoclinic structure and the nature of the orientational disorder of the sulphate groups is characterized in terms of tetrahedral rotor functions. The crystal structure factor is found to be sensitive to the model adopted for the charge distribution of the anions; good agreement with experimental neutron diffraction data is obtained when a charge distribution consistent with ab initio quantum mechanical calculations is used. The phase transition whereby the low temperature monoclinic structure transforms to the disordered cubic phase has been investigated by the constant pressure molecular dynamics method. The nature of the lithium ion diffusive motion and its coupling to the anion reorientation, the relaxation of the orientational order and the lattice vibrations are all briefly discussed.
A new method for computing properites of negative ion resonances with application to 2Σ+u states of H−2
Authors: DeRose Eugene , Gislason E. A. , Sabelli N. H.
Published: July 2002

A new method for computing properties of negative ion resonances is reported. The first step is to carry out a CI calculation of the lowest 15 or so states of proper symmetry of the negative ion system. A Feshbach projection-operator technique is then used to project out the various resonances from the CI states. The projection is based on the assumption that resonances have small expectation values for the one-electron operator z2 and continuum states have large values. The energies, energy widths, and lifetimes of the resonances are then straightforward to calculate. The method has been applied to the 2Σ+u states of H−2. Two resonances reported here have been seen by other workers, but a third, which lies 5.8 eV above the v=0 level of H2, has not been identified before. The implication of this resonance for electron-hydrogen scattering experiments is discussed.
Rovibrational effects on nuclear shielding of apex nuclei in bent molecules
Authors: Osten H.-Jörg , Jameson Cynthia J.
Published: July 2002

The isotope shifts of non-end nuclei with lone pairs are generally larger than those of nuclei without lone pairs. Using the bent triatomic molecule as a prototype, we examine the mass and temperature dependence of the mean bond angle deformation and the mean displacement along a bond due to centrifugal stretching and anharmonic vibration. We find that the temperature dependence of 〈Δα〉 determines whether the temperature dependence of the shielding of the apex nucleus will be normal [(dσ0/dT)<0] or abnormal. The dominant contribution of the rotation to the temperature dependence of 〈Δα〉 in the hydrides can lead to opposing temperature effects on shielding while the mass effects lead to normal isotope shifts. We performed similar calculations for the trigonal pyramidal molecules in order to explain the observed abnormal temperature dependence of the 15N and 31P nuclear shielding in NH3 and PH3.
Effect of van der Waals forces on the location of the critical point of a model molten salt
Authors: Arauz-Lara J. L. , Medina-Noyola M.
Published: July 2002

A mixture of charged hard spheres (restricted primitive model) with an attractive Yukawa tail added to represent van der Waals forces between the ions is considered as a model of molten salts. The location of the critical point of the gas–liquid phase transition is followed as a function of the strength A of the attractive term. It is found that the critical density increases with increasing A, but that a quantitative agreement between the theoretical results and the available experimental estimates of the critical constants for alkali halides requires the use of an effective dielectric constant greater than unity.
The microwave spectrum of propyne in the 17–70 GHz region for the V10=2 vibrational state
Authors: Ware J. M. , Roberts J. A.
Published: July 2002

The rotational spectrum of the V10=2 vibrational state of propyne was investigated in the 17–70 GHz region with a total of 18 lines measured. Rotational constants for the V10=2 state have been derived. The rotational constants for the V10=2, V10=1, and ground vibrational states have been compared and the constants of anharmonicity describing the change of B with the level of excitation of the V10 vibrational mode have been derived. The constants derived in this manner are quite different from those calculated by Meyer and Sergent-Rozey based on infrared data. An explanation for this discrepancy is given.
Molecule formation in an optically oriented vapor
Authors: Allegrini Maria , Bicchi Paola , Gozzini Silvia
Published: July 2002

In this work an experimental observation is reported of the predicted shift in chemical equilibrium between an atom and its dimer in the gas phase upon optical orientation of the atoms. The previously derived relationship between the equilibrium constant and the degree of atomic orientation is also verified.
Infrared studies of autoionization of thin films of dinitrogen tetroxide
Authors: Jones L. H. , Swanson B. I. , Agnew S. F.
Published: July 2002

The autoionization of dinitrogen tetroxide to form nitrosonium nitrate in thin films at 150–200 K has been studied using infrared absorption spectroscopy. It is found that at these temperatures and low pressure the process is intramolecular, involving only one of two isomers of the nitrite form (ONONO2). (AIP)
Theoretical investigation of Li and Be atom complexes with H2O
Authors: Curtiss Larry A. , Pople John A.
Published: July 2002

The structures, energetics, and vibrational frequencies of the Li–OH2 and Be–OH2 complexes have been examined by ab initio molecular orbital methods including the effects of electron correlation by Mo/ller–Plesset perturbation theory. Previous studies, at the Hartree–Fock level, indicated the existence of only a very weak van der Waals-type complex between Be and H2O, while the Li–OH2 complex was strongly bound (10–11 kcal/mol) at this level. In contrast, we find that, when correlation effects are included, the Be interaction with H2O is quite similar to that of Li with H2O. The Be–OH2 complex is predicted to be nonplanar and significantly bound with a short Be–O distance of 1.736 Å and a dissociation energy of 5.53 kcal/mol. The Li–OH2 complex at the correlation energy level is nonplanar with a Li–O bond distance of 1.90 Å and a dissociation energy of 13.77 kcal/mol. Calculated shifts in the vibrational frequencies of H2O in Li–OH2 at the correlated level are in good agreement with the experimental values, whereas Hartree–Fock level frequency shifts are in disagreement.
High pressure Raman study of vibrational relaxation of the Fermi coupled modes of ammonia
Authors: Bradley M. , Zerda T. W. , Jonas J.
Published: July 2002

The isotropic line shapes for the N–H vibration of NHD2 and the ν1+2ν4 Fermi resonance coupled peaks of NH3 have been measured as a function of pressure (to 2 kbar) and temperature (from 0 to 100 °C). The coupled oscillator model is used to calculate the uncoupled linewidths. Inhomogeneous broadening is proposed to give the major contribution to the uncoupled linewidths, but Fermi resonance broadening is also shown to be important. This broadening, experimentally determined by partially deuterating ammonia, is explained as being in part due to intramolecular energy relaxation and in part as due to modulation of the Fermi coupling by intermolecular forces.
Molecular orientation in a dielectric liquid–vapor interphase
Authors: Chacón E. , Mederos L. , Navascués G. , Tarazona P.
Published: July 2002

The density functional theory of Chacón et al. is used to study the molecular orientation in an interphase of a weak dipolar fluid. Explicit expressions are obtained using standard perturbation techniques. Molecular orientation, local susceptibility, and the Gibbsean surface susceptibility are evaluated for a Stockmayer model of dipolar fluid. The effect of the surface structure on the bulk ferroelectric transition is discussed in the light of the present theory and the numerical results.
The translational friction coefficient and time dependent cluster size distribution of three dimensional cluster–cluster aggregationa),b)
Authors: Meakin Paul , Chen Zhong-Ying , Deutch J. M.
Published: July 2002

The diffusion coefficient of clusters 𝒟 formed by cluster–cluster aggregation is computed according to the Kirkwood–Riseman Theory. In three dimensions one finds 𝒟 proportional to sγ where s is the number of particles in the cluster and γ=−0.544±0.014. This relationship is employed to simulate the time evolution of the cluster size distribution Ns(t) which is found to exhibit simple scaling behavior Ns(t) ∼s−2g(s/tz) with z∼1.1.
A self-consistent theory of nonequilibrium excitation transport in energetically disordered systems
Authors: Parson Robert P. , Kopelman Raoul
Published: July 2002

The migration of incoherent excitations in energetically disordered systems is studied theoretically using a self-consistent diagrammatic approximation. Spatial diffusion and energy relaxation observables are related to the solutions of a nonlinear integral equation. Extensive numerical illustrations are given for two-component and multicomponent systems. In the latter, spatial transport is found to be highly dispersive (nondiffusive) over an extremely wide range of timescales, in accordance with results from simulations and experiments. The dependence of spatial and spectral transport properties upon the spatial range and the energy dependence of the intermolecular hopping rates is examined. Several measures of energy relaxation, including detailed probability distributions in energy space, relaxation-time spectra, and the nonequilibrium entropy are calculated and compared. The intimate relationship between spatial transport and energy relaxation is discussed in detail.