bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo simulation study of the scaling of electron transport parameters in crossed dc electric and magnetic fields
Authors: Nakamura Satoru , Ventzek Peter L. G. , Kitamori Kazutaka
Published: July 2002

A Monte Carlo computational experiment is used to study the scaling of electron swarm parameters in crossed direct current electric and magnetic fields in argon and methane. The ranges of reduced electric and magnetic fields investigated are 28–2830 Td and 10 G/Torr–10000 G/Torr, respectively. Significant deviation from classical scaling is observed and characterized. For very high magnetic fields, some swarm parameters (mean energy and total collision frequency) become insensitive to the magnetic field variations. In addition, when swarm parameters are characterized in terms of their mean electron energy, for some ranges of mean electron energy, the correlation between a parameter and the reduced magnetic field strength can disappear. The impact of these results is discussed in terms of controlling the plasma chemistry via the magnetic field.
Monitoring fatigue damage in materials using magnetic measurement techniques
Authors: Lo C. C. H. , Tang F. , Shi Y. , Jiles D. C. , Biner S. B.
Published: July 2002

Measurements of hysteresis and Barkhausen effect (BE) have been made on 0.1 wt % C Fe–C alloys subjected to strain-controlled fatigue at various strain amplitudes. A relationship between the fatigue lifetime and strain amplitude was observed. The hysteresis properties of the samples cycled at different strain amplitudes were found to vary systematically with expended fatigue life. These properties showed significant changes in the initial and final stages of fatigue, while between these stages they remained stabilized. In the stable stage the remanence was found to decrease, whereas the coercivity increased with increasing strain amplitude. Variations in BE signal during fatigue were found to be closely related to the microstructural changes observed on the sample surface. These results are interpreted in the context of the changes in microstructure caused by fatigue damage, and the effects of the formation and propagation of fatigue cracks on the field distribution and domain structure in the vicinity of the cracks.
Driving voltage of reflective cholesteric liquid crystal displays
Authors: Yamaguchi Takeshi , Yamaguchi Hajime , Kawata Yasushi
Published: July 2002

The reflective cholesteric texture mode between planar and homeotropic textures, which can achieve a high contrast ratio, needs to be coupled with an active matrix drive scheme such as a method using thin film transistors. In this article, the threshold voltage of the cholesteric–nematic phase transition, an important characteristic related to the requirements of active matrix drive schemes, is discussed. We investigated the role of the cell gap in determining the reflectance, and studied the effects of varying the nematic liquid crystal and chiral component of the cholesteric mixture. The cell gap needs to be optimized for lower threshold voltage and higher reflectance. The chiral component and the nematic liquid crystal both have a strong effect on the threshold voltage. On the basis of these results, novel chiral materials are proposed. This study demonstrates that some of these materials successfully contribute to the reduction of the threshold voltage.
Polarization distribution in ceramic-polymer nanocomposites
Authors: Bloss Peter , DeReggi Aimé S. , Gläsel Hans-Jürgen , Hartmann Eberhard , Hirsch Dietmar , Schäfer Hartmut
Published: July 2002

We report pyroelectric responsivity versus depth profiles in nanocomposites of powdered barium titanate and tripropylene glycol diacrylate obtained by using the thermal pulse (TP) method. The ceramic nanopowder (mean particle size 270 nm) was prepared by combined solid-state polymerization and pyrolysis of a metallo-organic precursor. Films of powder/monomer dispersion (thickness ∼30 μm) were cured by irradiation with an electron beam (energy 140 keV, dose 80 kGy). The observation of a TP response after electron beam curing (EBC) indicates the appearance of a poling electric field as an effect of the EBC. The polarization depth profiles were studied for different filler contents. The composites show potential promise for piezo and pyroelectric sensors applications and for charge storage in microelectronics.
Variable-charge interatomic potentials for molecular-dynamics simulations of TiO2
Authors: Ogata Shuji , Iyetomi Hiroshi , Tsuruta Kenji , Shimojo Fuyuki , Kalia Rajiv K. , Nakano Aiichiro , Vashishta Priya
Published: July 2002

An interatomic potential model has been developed for molecular-dynamics simulations of TiO2 (rutile) based on the formalism of Streitz and Mintmire [J. Adhes. Sci. Technol. 8, 853 (1994)], in which atomic charges vary dynamically according to the generalized electronegativity equalization principle. The present model potential reproduces the vibrational density of states, the pressure-dependent static dielectric constants, the melting temperature, and the surface relaxation of the rutile crystal, as well as the cohesive energy, the lattice constants, and the elastic moduli. We find the physical properties of rutile are significantly affected by dynamic charge transfer between Ti and O atoms. The potential allows us to perform atomistic simulations on nanostructured TiO2 with various kinds of interfaces (surfaces, grain boundaries, dislocations, etc.).
Magnetic neutron scattering study of ordered Mn3Ir
Authors: Tomeno Izumi , Fuke Hiromi N. , Iwasaki Hitoshi , Sahashi Masashi , Tsunoda Yorihiko
Published: July 2002

Neutron-diffraction measurements were made on an ordered Mn3Ir single crystal in a wide temperature range up to 1030 K. The ordered Mn3Ir alloy was found to maintain an antiferromagnetic (AF) triangular spin structure up to the Néel temperature TN=960±10 K. The lattice parameter a shows a continuous change in the temperature range including TN. In contrast to the isostructural ordered Mn3Pt alloy, these observations indicate that an AF–AF phase transition is absent in the ordered Mn3Ir alloy.
Strain relaxation in graded composition InxGa1−xAs/GaAs buffer layers
Authors: Romanato F. , Napolitani E. , Carnera A. , Drigo A. V. , Lazzarini L. , Salviati G. , Ferrari C. , Bosacchi A. , Franchi S.
Published: July 2002

A model to compute the strain relaxation rate in InxGa1−xAs/GaAs single layers has been tested on several compositionally graded buffer layers. The existence of a critical elastic energy has been assumed as a criterion for the generation of new misfit dislocations. The surface strain accuracy results are within 2.5×10−4. The influence of different grading laws and growth conditions on residual strain, threading dislocation density, misfit dislocation confinement, and surface morphology has been studied. The probability of dislocation interaction and work hardening has been shown to strongly influence the mobility and the generation rate of the dislocations. Optimization of the growth conditions removes residual strain asymmetries and smoothes the surface roughness.
Quantum computing using entanglement states in a photonic band gap
Authors: Qiao Bi , Ruda Harry E.
Published: July 2002

In this article, we consider a quantum model constructed using N two-level atoms within a photonic band-gap material, and interacting with a single photon mode under suitable nonequilibrium boundary conditions. We solved the eigenvalue problem of the Hamiltonian by using a projection approach, and found that the behavior of the total system is like that of a two-level system with the eigenvectors existing in a dual pair of spaces. Furthermore, we constructed the operators for a quantum logic gate using the entangled states, the eigenvectors of the system. Therefore the interaction between the atoms and the single photon mode cannot introduce any decoherence over the constructed quantum logic gates. Since this approach necessitates solving the eigenvalue problem of the Hamiltonian from first principles, it represents a general formulation for obtaining the spectral decomposition of the Hamiltonian as well as the evolution of the density operator for N two-level atoms interacting with multimode photonic fields; this will be the case independent of whether the system is under equilibrium or not.
Vertical and lateral forces between a permanent magnet and a high-temperature superconductor
Authors: Hull John R. , Cansiz Ahmet
Published: July 2002

The vertical and horizontal forces and associated stiffnesses on a permanent magnet (PM) above a high-temperature superconductor (HTS) were measured during vertical and horizontal traverses in zero-field cooling (ZFC) and in field cooling (FC). In ZFC, the vertical stiffness was greater in the first descent than in the first ascent and second descent, and the stiffness in the second descent was between those of the first descent and the first ascent. At the FC position, the vertical stiffness was two times greater than the lateral stiffness at each height, to within 1% of the vertical stiffness value. The cross stiffness of vertical force with respect to lateral position was positive for FC, but negative for ZFC. Free-spin-down experiments of a PM levitated above a HTS were also performed. These results showed that the coefficient of friction is double valued at frequencies just below the rotor resonance, a result attributed to cross stiffness in the PM/HTS interaction. A frozen-image model was used to calculate the vertical and horizontal forces and stiffnesses, and reasonable agreement with the data occurred for vertical or horizontal movements of the PM less than several mm from the FC position.
Characterization of defect-related optical absorption in ZnGeP2
Authors: Setzler S. D. , Schunemann P. G. , Pollak T. M. , Ohmer M. C. , Goldstein J. T. , Hopkins F. K. , Stevens K. T. , Halliburton L. E. , Giles N. C.
Published: July 2002

A broad optical absorption band with a peak near 1 μm is present in most single crystals of ZnGeP2. These same crystals have an electron paramagnetic resonance (EPR) signal which has been assigned to singly ionized zinc vacancies. A direct correlation between the intensity of the optical absorption at 1 μm and the intensity of the EPR signal has been established using a set of ZnGeP2 crystals where this absorption varied widely. These results suggest that the singly ionized zinc vacancy acceptor plays a direct role in the electronic transition(s) responsible for the 1 μm optical absorption. In separate experiments, it was found that illuminating the ZnGeP2 crystals with a He–Ne laser (632.8 nm) while at temperatures near 25 K produces an increase in the absorption at 1 μm and an increase in the zinc vacancy EPR spectrum. These latter results provide further evidence that the absorption at 1 μm is associated with the singly ionized zinc vacancy acceptor.
Characteristic features of the laser radiation–target interactions during reactive pulsed laser ablation of Si targets in ammonia
Authors: Mihailescu I. N. , Gyorgy E. , Teodorescu V. S. , Steinbrecher Gy. , Neamtu J. , Perrone A. , Luches A.
Published: July 2002

We performed electron microscopy studies of targets subjected to the multipulse laser irradiation in the regime characteristic of reactive pulsed laser deposition from bulk Si in low-pressure ammonia. Experimental evidence is provided concerning the expulsion of liquid droplets from the crater that forms on the target surface. In our opinion, the main mechanisms responsible for droplets spraying directly from the crater are in this case liquid phase expulsion, under the action of the recoil pressure of the ablated substance and hydrodynamic instabilities at the target surface. Cracks of various sizes and orientations have been observed beneath the crater surface. These cracks might cause an increase in the ablation and deposition rates with time. A qualitative theoretical model was developed to account for crack formation based on the peculiar behavior of Si, which has a smaller density in bulk than in liquid phase.
Slip-free processing of 300 mm silicon batch wafers
Authors: Fischer A. , Richter H. , Kürner W. , Kücher P.
Published: July 2002

Under gravitational and thermal constraints of integrated-circuit (IC) process technology, 300-mm-diam silicon wafers can deform via slip dislocation generation and propagation, degrading the electrical characteristics of the leading edge device. We present a force balance model to describe the strain relaxation in large wafer diameter, which includes heat transfer effects and the upper yield point of the silicon material. The material attributes, such as oxygen content and the state of oxygen aggregation, are taken into account. The theoretical approach allows the calculation of wafer mechanics and ramp rate profiles for an arbitrary high-temperature process. Plastic deformation of silicon wafers caused by thermal stresses at high temperatures can be controlled by process design. Deformation due to gravitational forces can be prevented through appropriate equipment design. The quantitative theory proposed here provides guidance for computer simulation to configure stable slip-free wafer process flow under mechanical and thermal loads. Applications include high speed simulation of “what if?” experiments, and initial simulations of large scale experimental sequences. The simulator developed can also be used by IC manufacturers to determine optimum wafer throughput and cycle times in front-end device processes.
Formation and characteristics of silicon nanocrystals in plasma-enhanced chemical-vapor-deposited silicon-rich oxide
Authors: Lin Chi-Fa , Tseng Wei-Tsu , Feng Ming Shiann
Published: July 2002

The formation of nanosized Si crystals in dual-frequency plasma-enhanced chemical-vapor-deposited silicon oxides is identified in this study. As a higher SiH4N2O gas flow rate ratio is employed during the deposition process, the silicon-to-oxygen atomic ratio and the dangling bond density both increase. The resulting oxide films contain more Si–H bonds and less Si–O and Si–O–H bonds, as determined from the Fourier-transform infrared spectra. The main type of charge defects in these oxides change from ⋅Si≡O3 bonds (E′ centers) to ⋅Si≡Si3 bonds, which eventually cluster together and precipitate out from the oxide network to form the Si nanocrystals. The size of these Si nanocrystals falls within the range of 30–50 nm, as observed by high-resolution transmission electron microscopy. The formation of these nanocrystals inside the silicon-rich oxides results in a lower film density, a tensile stress component, and a higher wet etching rate, even under the ion bombardment provided by the rf bias power during deposition. The underlying mechanisms for the formation of these Si nanocrystals from the silicon oxide will be proposed.
Magnetic-field-induced strains and magnetic properties of Heusler alloy Ni52Mn23Ga25
Authors: Yu C. H. , Wang W. H. , Chen J. L. , Wu G. H. , Yang F. M. , Tang N. , Qi S. R. , Zhan W. S. , Wang Z. , Zheng Y. F. , Zhao L. C.
Published: July 2002

Large magnetic-field-induced strain (MFIS) of −2700 ppm has been obtained from the Heusler alloy Ni52Mn23Ga25 at 300 K. The temperature for structure transformation was confirmed by three types of magnetic measurements. A burst of MFIS up to 5400 ppm could be induced by rotating a steady field of 1 T. In the martensitic phase, the material have a high saturated magnetization of 66 Am2/kg and a high anisotropy of 0.8 T. The variant reorientation mainly occurred in the region of 0.2–0.8 T. The large MFIS is sensitive to temperature, suggesting that it can only be induced when martensite and parent phase are coexisted.
High-temperature giant magnetoimpedance in Fe-based nanocrystalline alloy
Authors: Chen G. , Yang X. L. , Zeng L. , Yang J. X. , Gong F. F. , Yang D. P. , Wang Z. C.
Published: July 2002

Giant magnetoimpedance (GMI) was investigated from room temperature up to 823 K in an Fe-based nanocrystalline Fe73.0Cu1.0Nb2.5V1.0Si13.5B9.0 ribbon. With an increment of the measuring temperature (T), GMI shows notable enhancement followed by a declining dependence, yielding a maximum value around 603 K where the relative GMI is nearly four times that at room temperature. The field at the peak of the GMI vs Hdc curve decreases monotonically with T, but around T=603 K there superimposes a trough-shaped variation. The thermal evolution of the soft magnetic property and magnetic anisotropy is suggested to be responsible for the high-temperature GMI features. Discussion on the intergrain exchange magnetic coupling through the amorphous boundaries in the two-phase Fe-based nanocrystalline alloy is also given.
Viscous flow of the Cu47Ti34Zr11Ni8 glass forming alloy
Authors: Glade S. C. , Johnson W. L.
Published: July 2002

The viscosity of the Cu47Ti34Zr11Ni8 glass forming alloy was determined by beam bending experiments and by a noncontact oscillating drop technique. These viscosity data can be described with the Vogel–Fulcher–Tammann relation. Using the strong/fragile classification of glasses, Cu47Ti34Zr11Ni8 is more fragile than the strong Zr–Ti–Cu–Ni–Be metallic glass formers.
Thermoelectric properties of Si1−xGex(x⩽0.10) with alloy and dopant segregations
Authors: Yamashita Osamu , Sadatomi Nobuhiro
Published: July 2002

The thermal conductivity κ and the thermoelectric properties of Si1−xGex(x⩽0.10) samples doped with B or P which were prepared by arc melting were measured as functions of carrier concentration n and temperature T in the range from 298 to 1273 K. The Si1−xGex(0.03⩽x⩽0.10) samples, doped with a small amount of B or P, have a significantly low κ value, less than one tenth that of pure Si, despite the presence of strong alloy and dopant segregations. The Seebeck coefficients S of the Si0.97Ge0.03 samples over the entire n range from 1018 to 1021 cm−3 were higher than the corresponding values of Si0.7Ge0.3 alloys with little segregation, although the difference between the electrical resistivities ρ of our samples and their alloys is very small. The optimum n value giving the largest thermoelectric figures of merit (ZT=S2T/κρ) for n- and p-type Si0.97Ge0.03 samples was about 2×1020 cm−3, which is almost the same as that for Si0.7Ge0.3 alloys. At n≒2×1020 cm−3 the ZT values of n- and p-type Si0.95Ge0.05 samples with segregation increase linearly with temperature, and become 0.90 and 0.57 at 1073 K, respectively, which corresponds to 92% and 81% of the ZT values for Si0.7Ge0.3 alloys with little segregation.
Guiding large-scale spark discharges with ultrashort pulse laser filaments
Authors: La Fontaine Bruno , Comtois Daniel , Chien Ching-Yuan , Desparois Alain , Génin Frédéric , Jarry Geneviève , Johnston Tudor , Kieffer Jean-Claude , Martin François , Mawassi Raafat , Pépin Henri , Rizk Farouk A. M. , Vidal François , Potvin Carl , Couture Pierre , Mercure Hubert P.
Published: July 2002

Using the nonlinear propagation properties of ultrashort laser pulses in air, we were able to produce long ionized filaments that served to guide spark discharges. With a laser pulse energy of 20 mJ, one or two ionized filaments were created and could guide streamer discharges over 2 m air gaps, where the electric field was fairly uniform and had an average value of 0.6 MV/m. Such a guiding effect was observed for times of 1–3 μs after the laser pulse created the ionized filaments. Longer delays (10–15 μs) were recorded at higher laser pulse energy, with a larger number of filaments. Images of the early stages of the discharge of a uniform air gap show that the laser-produced ionized filaments do not initiate the discharge process but act rather as preferred channels where the leader growth is accelerated. In the end, these straight conductive channels carry the arc current as the voltage in the gap breaks down.
Nascent vibrational and rotational distributions from the charge transfer reaction Ar++CO → CO++Ar at near thermal energy
Authors: Lin Guang-Hai , Maier Jürgen , Leone Stephen R.
Published: July 2002

Saturated laser-induced fluorescence detection is used to study the vibrational and rotational distributions produced in the charge transfer reaction Ar+(2P3/2)+CO(X 1Σ+, v=0) → Ar(1S0)+CO+(X 2Σ+, v″=0–7) at 0.2 eV energy. The apparatus combines a flowing afterglow ion source with a sampling orifice to obtain a supersonic expansion of near thermal energy ions for reaction under nearly single collision conditions in the reaction chamber. Although the available energy is sufficient to populate the CO+ vibrational states up to v″=7, the experiment reveals that the main channels are v″=4–6. The nascent rotational state distributions in the v″=5 and v″=6 states are characterized by Boltzmann distributions with Trot=790+120−100 and 780+60−60 K, respectively. The experimental results are better explained by a potential surface crossing at close approach, than either by considerations of strict Franck–Condon overlaps or energy resonance.
Optical spectra, energy levels, and crystal-field analysis of tripositive rare-earth ions in Y2O3. IV. C3i sites
Authors: Gruber John B. , Leavitt Richard P. , Morrison Clyde A. , Chang N. C.
Published: July 2002

We report an analysis of new and previously existing optical absorption and fluorescence data, far-infrared data, and electronic Raman scattering data for Eu3+, Dy3+, and Er3+ in the C3i sites of Y2O3 and R2O3, where R=a rare earth. Our previous analysis of C2-site spectra yields an effective point-charge model for the host lattice that allows initial estimates to be calculated for the C3i-site crystal-field parameters Bkm. Best-fit values of B20, B40, and B43 are obtained for Eu, and best-fit values of all Bkm allowed by symmetry are obtained for Dy and Er. The best-fit Bkm are in relatively poor agreement with the model; in particular, B20 has the opposite sign from and B44 is much smaller than the model predictions. From the best-fit Bkm we obtain phenomenological crystal-field components Akm, from which we predict Bkm and C3i -site energy levels for the ground states of Tb3+, Ho3+, Tm3+, and Yb3+. While the effective point-charge model is apparently too crude to make accurate, quantitative, a priori predictions, the model and the data allow one to predict confidently the behavior of ions doped into C3i sites for which no data exist.
Instabilities and oscillations in ‘‘The Reversible Oregonator Model’’: A thermodynamic approach to calculate the excess entropy production (δm
P) and the corresponding antisymmetric function (δmΠ)
Authors: Dutt A. K.
Published: July 2002

Near equilibrium thermodynamic analysis of the reversible ‘‘Oregonator kinetic model’’ has been done using the entropy production technique of Prigogine in the range of the validity of the Onsager relations. Expressions for the excess entropy production (δmP) and the corresponding antisymmetric function (δmΠ) have been derived in terms of the rate constants of the steps, the concentrations of the reactants and products, steady state concentrations of the intermediates, and the coefficients of the real and imaginary parts of the complex differentials δX, δY, and δZ, which represent, respectively, the fluctuations from the steady state concentrations of the intermediates X, Y, and Z. Factors destabilizing the steady states have been determined and steps other than autocatalytic and cross catalytic are also found to be involved.
Product vibrational state distributions of thermal energy charge transfer reactions determined by laser-induced fluorescence in a flowing afterglow: Ar++CO→CO+(v=0–6)+Ar
Authors: Hamilton Charles E. , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

The Ar++CO→CO+(v=0–6)+Ar charge transfer reaction is studied at thermal energy in a flowing afterglow and the vibrational state distribution is determined by laser-induced fluorescence on the CO+(A 2Π–X2Σ+) bands. The nascent vibrational state distribution is (0.06±0.04)v=0 :(0.07±0.02)v=1 :(0.09±0.02)v=2 :(0.15±0.03)v=3 :(0.21±0.03)v=4: (0.27±0.02)v=5 :(0.15±0.02)v=6. The rate constant for CO+(v=4) deactivation by CO is measured to be 6.0±2.5×10−10 cm3 s−1; the similarity of this rate constant to that for CO+(v=1) deactivation by CO (5.0±2.0×10−10 cm3 s−1) suggests that vibrational deactivation proceeds by a charge transfer mechanism. The Ar++CO reaction is described as proceeding via a bent ArCO+ intermediate that forms in a side-on attack. Vibrational excitation may then result from delocalization of the bonding electron density of CO and the corresponding dynamical changes in the CO bond length in the intermediate.
Si2, SiH3, and HSiO molecules: ESR at 4 K
Authors: Van Zee R. J. , Ferrante R. F. , Weltner W.
Published: July 2002

ESR spectra of 29Si2, 29SiH3, and H29SiO molecules in argon and/or neon matrices at 4 K have yielded the following new data: Si2 has a zero-field splitting in its ground3Σ−g state of ‖D‖=2.53(1) cm−1 in an argon matrix, which extrapolates to a gas-phase parameter of ‖λ‖=1.35(5) cm−1. The hyperfine interaction constant ‖A⊥(29Si)‖=40(2) MHz. SiH3 has g∥=2.0043(5), g⊥=2.0035(5), ‖Aiso(H)‖=25(3), ‖Aiso(29Si)‖=665(4), and ‖Adip(29Si)‖=166(3) MHz. Probable spin densities are 15% sSi+91% pSi−5% sH with a calculated bond angle of 113° in solid argon. HSiO has been observed for the first time. Its g components and spin distribution are very similar to those of the HCO radical. It is bent with a presumed 2A′ ground state, in accord with unpublished calculations of Bruna and Peyerimhoff and the recently published study of Frenking and Schaefer. The more stable isomer HOSi may have also been detected here.
Reaction surface description of intramolecular hydrogen atom transfer in malonaldehyde
Authors: Carrington Tucker , Miller William H.
Published: July 2002

Intramolecular H atom transfer in malonaldehyde is described using a reaction surface Hamiltonian. This model utilizes two large amplitude, reaction-coordinate-like degrees of freedom—i.e., a surface in the 3N–6 dimensional configuration space of an N atom system—with the remaining 3N–8 degrees of freedom being local harmonic motion away from this 2D surface. The two reaction surface degrees of freedom are chosen to be the two OH bond lengths, i.e., the bond being broken and the bond being formed. The present methodology is developed using 3N–6 internal coordinates, in contrast to our earlier reaction surface model [J. Chem. Phys. 81, 3942 (1984)] that utilized the 3N Cartesian coordinates. From our calculations we conclude that the bare barrier (i.e., with no zero point vibrational corrections) for the H atom transfer is between 6 and 7 kcal/mol.
Tracer diffusion in concentrated colloidal dispersions
Authors: van Megen W. , Underwood S. M. , Snook I.
Published: July 2002

A colloidal tracer system suitable for light scattering experiments has been prepared. The nonaqueous dispersion comprises host and tracer particles of 0.66 μm diameter in which the refractive index of the dispersion medium is adjusted to that of the host particles. Both the host polymer particles and the inorganic tracer particles are sterically stabilized by the same polymeric adsorbed layer and, therefore, differ only in their scattering properties. Dynamic light scattering measurements were carried out on these systems at volume fractions in the range 0.36 to 0.48; this includes the coexisting disordered and crystalline phases of the dispersion.
Mixed regular stack charge–transfer crystals: Fundamental microscopic parameters from optical spectra
Authors: Painelli A. , Girlando A.
Published: July 2002

Diagrammatic valence bond calculations are applied to the study of the charge–transfer (CT) excitation spectrum and of the electron–molecular vibration (emv) coupling in mixed regular stack CT crystals. The intersite electrostatic interactions are treated by both mean-field and unscreened point–charge potential approaches, obtaining quite similar results. The evaluation of microscopic parameters through optical spectra is critically discussed, and a new method is proposed to estimate the parameters relevant to the long-range Coulomb interaction from the analysis of the emv perturbation on the Raman spectra.
Dielectric friction and the transition from adiabatic to nonadiabatic electron transfer. I. Solvation dynamics in Liouville space
Authors: Sparpaglione Massimo , Mukamel Shaul
Published: July 2002

A microscopic theory for electron transfer rates in a polar medium, which interpolates continuously from the adiabatic to the nonadiabatic limits, is developed. Both static (polarity) interactions, which affect the reaction energetics and dynamic (friction) effects, are incorporated using a macroscopic solvation coordinate, whose dynamics and statistical properties are related to the entire frequency and wave vector dependent dielectric function of the solvent ε(k,ω). The present approach is based on using an expansion of the density matrix in Liouville space and utilizing the analogy with the calculation of nonlinear optical line shapes. A new criterion for adiabaticity is derived, and the role of the solvent longitudinal dielectric relaxation in inducing the crossover from the nonadiabatic to the adiabatic regimes is clarified. The applicability of a Landau–Zener-type resumation for the rate is critically analyzed. The origin of the fractional power dependence of the rate on the solvent time scale, observed in several electron transfer and isomerization reactions, is discussed.
Quenching and excitation transfer for the c 3Π−u and a 3Σ+g states of H2 in collisions with H2
Authors: Wedding A. B. , Phelps A. V.
Published: July 2002

Collisional destruction rate coefficients for c 3Π−u metastable states by thermal energy hydrogen molecules have been measured for various vibrational and rotational levels (N=1, v=0,1,2,3 and v=1, N=1,2,3). The rate coefficients were found to be independent of vibrational and rotational quantum number and had a mean value of (1.88±0.10)×10−15 m3 s−1 at 300 K. Destruction rate coefficients and radiative lifetimes of the a 3Σ+g state have also been measured. Collisional quenching rate coefficients at 300 K of (11.5±1.0) and (6.3±0.8)×10−16 m3 s−1 were obtained for the v=0 and 1 levels, respectively. Radiative lifetimes of 11.1±0.3 ns for v=0 and 10.4±0.3 ns for a a 3Σ+g (v=1) are in good agreement with recent experiment and theory. Collisional excitation transfer between the N=1 levels of the a 3Σ+g and c 3Π−u states was observed for v=0 and 1. No collisional excitation transfer between other a 3Σ+g and c 3Π−u levels or between two c 3Π−u levels was detected. Measured fractional absorption signals and detailed balancing arguments were used to determine the excitation transfer rate coefficient for the v=1, N=1 level of the a 3Σ+g state to the v=1, N=1 level of the c 3Π−u state as (3±2)×10−16 m3 s−1 and that for the reverse process as (1.8±1)×10−16 m3 s−1 for a calculated gas temperature of 410 K. Excitation transfer to the c 3Πu state and subsequent rapid quenching accounts for a large fraction, if not all, of the collisional quenching of the a 3Σ+g (v=1, N=1) state.
A laser spectroscopic study of the X 1Σ+−C 1Π1 transition of MgAr: Evidence for Λ-type doubling
Authors: Bennett Robert R. , McCaffrey John G. , Wallace Ingvar , Funk David J. , Kowalski Andrzej , Breckenridge W. H.
Published: July 2002

The X 1Σ+0 and the C 1Π1 states of MgAr have been characterized spectroscopically. The MgAr van der Waals molecules, created in a new laser-vaporization, pulsed supersonic jet apparatus, were studied using laser-induced fluorescence. High-resolution spectra revealed lambda-type doubling consistent with the presence of the nearby higher-lying repulsive MgAr (D 1Σ+0) electronic state.
Diode laser spectroscopy of C3: The ν2+ν3−ν2, 2ν2+ν3− 2ν2, and 2ν2+ν3 bands
Authors: Kawaguchi Kentarou , Matsumura Keiji , Kanamori Hideto , Hirota Eizi
Published: July 2002

The ν3 hot bands (011)–(010), (021)–(020), and the combination band (021)–(000) of C3 were observed by a diode laser spectroscopic method. The C3 molecule was produced by the photolysis of allene or by the discharge in cyclopropane or furan. Analysis gave molecular constants in each vibrational state and also the energy difference between (020) and (000). The derived vibrational frequencies were compared with the result of a Morse oscillator-rigid bender internal dynamics (MORBID) calculation by Jensen.
The far infrared spectrum of H2O2. First observation of the staggering of the levels and determination of the c
i
s barrier
Authors: Flaud J.-M. , Camy-Peyret C. , Johns J. W. C. , Carli B.
Published: July 2002

High resolution spectra of H2O2, recorded by means of Fourier transform spectroscopy between 30 and 460 cm−1, have been analyzed leading to the determination of the rotational levels of the torsional states (n,τ) for n=0,1,2,3. In order to reproduce these energy levels, Watson type Hamiltonians have been used and it has been possible to observe a staggering of the levels with n=2 and 3 caused by the cis barrier. The torsional band centers have then been fitted using a torsional Hamiltonian of the form {Bγγ,J2γ} +V(γ) with the potential function V(γ) written as V(γ)=V1 cos 2γ+V2 cos 4γ+V3 cos 6γ+V4 cos 8γ where the torsional coordinate 2γ is the dihedral angle defining the relative position of the two O–H bonds. The potential constants in cm−1 are V1=1036.97±23.1 cm−1, V2=657.53±5.2 cm−1, V3=50.89±3.3 cm−1, V4=2.524±0.83 cm−1 which correspond to barrier heights Vtrans =387.07±0.20 cm−1, Vcis =2562.8±60 cm−1, and to a potential minimum located at 2γ=111.9°±0.4° from the cis configuration. It is also shown that the rotational constants derived from the fit to the experimental rotational levels cannot be reproduced using a model which does not take into account vibrational corrections.
Theory of transient hole-burning spectrum for molecules in solution
Authors: Kinoshita Shuichi
Published: July 2002

Theory based on a configuration coordinate model is presented to clarify solute–solvent relaxation effect on the line shape of a time-resolved hole-burning spectrum. The time-resolved hole-burning spectrum is affected by both a hole created in a ground-state population and an excess excited-state population. The solute–solvent relaxation causes thermalization in both populations and then the shift and broadening of the spectrum is expected observable. The line shapes of three typical cases are mainly considered: (1) The excited state is assumed to have an infinitely long lifetime; (2) the excited state has a finite lifetime, the population being assumed to relax toward an electronic state other than the ground state, and (3) the population created in the excited state relaxes to the ground state, where a hole-filling effect is taken into account. Results obtained by numerical calculations show that a time evolution of the ground-state hole is clearly observed when the low-energy tail of the absorption band is excited under the condition that the population decay to the other electronic state is very fast. Further, it is predicted that the hole-filling effect causes a considerable deformation of the spectral shape when the solvent relaxation rate is comparable to the excited-state population decay rate.
Interatomic potentials of the C1 and D+ states of XeNe, XeAr, and XeKr as studied by tunable vacuum ultraviolet laser spectroscopy
Authors: Tsuchizawa Tai , Yamanouchi Kaoru , Tsuchiya Soji
Published: July 2002

Fluorescence excitation and resonantly enhanced photoionization spectra of the C–X and D–X bands of XeNe, XeAr, and XeKr, formed in a supersonic free jet are measured by use of a tunable vacuum ultraviolet light generated by four-wave difference frequency mixing of two lasers in Kr. The potential parameters of the heteronuclear rare gas dimers for the two excited state potentials, C1 and D0+, correlated with the Xe6s′(1/2)1 state are derived by an analysis of rovibronic structures of the C–X and D–X bands. The dissociation energies De for the C and D states are, respectively, determined to be 244 and 112 cm−1 for XeAr and are 1445 and 54 cm−1 for XeKr.
A method for calculating vibrational bound states: Iterative solution of the collocation equations constructed from localized basis sets
Authors: Yang Weitao , Peet Andrew C.
Published: July 2002

We propose a simple and efficient method for calculating vibrational bound states of molecular systems. The technique is based upon iterative solution of the collocation equations. A localized basis set is used which is very efficient for strongly coupled modes and also leads to a diagonally dominant set of collocation equations. The iterative scheme developed is based upon Davidson’s method and takes advantage of this diagonal dominance. The approach is capable of exploiting the efficiency with which the matrix elements are calculated in the collocation method by evaluating the matrix elements as they are required. This combination of techniques should allow the method to be used for systems which have more degrees of freedom than have been treated by conventional methods.
Selection rules for the photoionization of diatomic molecules
Authors: Xie Jinchun , Zare Richard N.
Published: July 2002

In the photoionization of the diatomic molecule AB to yield AB++e− the photoelectron may be charatcterized by a partial wave expansion in terms of its orbital angular momentum quantum number l. For a given value of l, conservation of angular momentum implies that transitions can only occur for ΔJ=l+ (3)/(2) , l+ (1)/(2) , ... ,−l− (1)/(2) , −l− (3)/(2) , where ΔJ=J+−J is the change (half-integer) in the total angular momentum (excluding nuclear spin) of the AB+ ion rovibronic level and the AB neutral rovibronic level. Other selection rules are ΔΩ=−λ+ (3)/(2) , −λ+ (1)/(2) , ... , −λ− (3)/(2) , and ΔM=−ml+ (3)/(2) , −ml+ (1)/(2) , ... , −ml− (3)/(2) . In addition, for Hund’s case (a) and case (b) coupling, ΔS=S+−S=± (1)/(2) , ΔΣ=± (1)/(2) , and ΔΛ=−λ, −λ±1. Parity selection rules have been derived for transitions connecting levels described by one of the four coupling schemes, Hund’s case (a), case (b), case (c), and case (d). In particular, for a case (a)–case (a) transition, ΔJ−ΔS+Δp+Δs+l=odd, where the symbols have their traditional spectroscopic meanings. The parity label p=0,1 has been associated with the e, f label, from which it may be shown that (e/f )↔(e/f ) for ΔJ− (1)/(2) +l=odd and (e/f )↔( f/e) for ΔJ− (1)/(2) +l=even. It also follows that ±↔± for l odd and ±↔∓for l even. Moreover, Σ± is connected to Σ± in general, but Σ± is only connected to Σ∓ for l≥2 and λ=±1 (π wave). For homonuclear diatomics, the additional selection rules are (g/u)↔(g/u) for l=odd, (g/u)↔(u/g) for l=even, and (s/a)↔(s/a) but (s/a)↔/(a/s).
A random walk chain reptating in a network of obstacles: Monte Carlo study of diffusion and decay of correlations and a comparison with the Rouse and reptation models
Authors: Reiter Johannes
Published: July 2002

The integrated decay times for correlations of the end-to-end vector of random walk chains of length N are well known to be proportional to N2 and N3 for the Rouse and the reptation models, respectively. For subchains of length n situated in the center and at the end of a chain, respectively, these times are about nN and n2 in the Rouse model and nN2 and n2N in the reptation model if n≪N holds. For a random walk chain in a network of obstacles which moves along its contour by defect diffusion with Monte Carlo simulations, the autocorrelation time for the end-to-end vector and the disentanglement time for the primitive path are found to vary as about N3.5 and N3.7, respectively, for chain lengths varying from 15 to 63. The curvilinear diffusion coefficient varies as about N−1.2 and the center-of-mass diffusion coefficient varies as about N−2.4. The integrated autocorrelation times of small subchains vary with n/N also faster than predicted by the reptation model.
Rotational dependence of the Rydberg–valence interactions in the 1Πg states of molecular oxygen
Authors: Sur Abha , Friedman R. S. , Miller Paul J.
Published: July 2002

The two-photon d 1Πg(v′)← ←X 3Σ−g(v″) transition in O2 is studied using resonance enhanced multiphoton ionization (REMPI) spectroscopy and the lowest four vibrational levels of the d 1Πg Rydberg state are characterized. Anomalies in the rotational constants of the Rydberg vibrational levels and perturbations in the REMPI spectra are attributed to the interaction of the Rydberg state with a bound diabatic 1Πg valence state. The rotational dependence of the Rydberg–valence interaction is explored by solving the coupled eigenvalue equations describing the interaction of the two states. The results from solution of the coupled equations as well as observations from photoelectron spectroscopy experiments are used to understand the additional structure seen in the REMPI spectra in the vicinity of the v′=2 Rydberg vibrational level.
A new triplet band system of C3: The b̃ 3Πg–ã 3Πu transition
Authors: Sasada H. , Amano T. , Jarman C. , Bernath P. F.
Published: July 2002

A triplet band system of C3 has been observed for the first time in absorption with a distributed-feedback (DFB) diode laser spectrometer and in emission with a Fourier transform spectrometer at around 6500 cm−1. The band has been assigned to the b̃ 3Πg–ã 3Πu system, and the spectroscopic constants in both the upper and lower states have been determined. It is demonstrated that near-infrared DFB lasers are promising radiation sources for spectroscopy of unstable molecules.
Absolute state-selected and state-to-state total cross sections for the Ar+(2P3/2,1/2)+CO reactions
Authors: Flesch G. D. , Nourbakhsh S. , Ng C. Y.
Published: July 2002

Absolute spin–orbit state-selected total cross sections for the reactions, Ar+(2P3/2,1/2)+CO→CO++Ar [reaction (1)], C++O+Ar [reaction (2)], O++C+Ar [reaction (3)], and ArC++O [reaction (4)], have been measured in the center-of-mass collision energy (Ec.m.) range of 0.04–123.5 eV. Absolute spin–orbit state transition total cross sections for the Ar+(2P3/2,1/2)+CO reactions at Ec.m. have also been obtained. The appearance energies (AE) for C+(Ec.m.=6.6±0.4 eV) and O+(Ec.m.=8.6±0.4 eV) are in agreement with the thermochemical thresholds for reactions (2) and (3), respectively. The observed AE for reaction (4) yields a lower bound of 0.5 eV for the ArC+ bond dissociation energy. The kinetic energy dependence of the absolute cross sections and the retarding potential analysis of the product ions support that ArC+, C+, and O+ are formed via a charge transfer predissociation mechanism, similar to that proposed to be responsible for the formation of O+ (N+) and ArO+ (ArN+) in the collisions of Ar+(2P3/2,1/2)+O2(N2).
Thermodynamic and kinetic properties of the reaction Cl+O2+M=ClOO+M in the range 160–300 K and 1–1000 bar
Authors: Baer S. , Hippler H. , Rahn R. , Siefke M. , Seitzinger N. , Troe J.
Published: July 2002

The reaction Cl+O2+M■ClOO+M was studied by laser flash photolysis in the bath gases M=He, Ar, N2, and O2 over the temperature range 160–300 K and the pressure range 1–1000 bar. UV absorptions of ClOO were monitored, a maximum absorption cross section of σ(248 nm)=3.4×10−17 cm2 was determined. An expression for the equilibrium constant Kp =5.3×10−6 exp(+23.4 kJ mol−1/RT) bar−1 was derived between 180 and 300 K, which, by a third law analysis, yields ΔH○0 =−20.2±0,2 kJ mol−1. Limiting low pressure rate coefficients for Cl+O2 recombination of k0=[He]8.8×10−34(T/300 K)−3.0, k0=[O2]1.6×10−33(T/300 K)−2.9, k0=[N2]1.4×10−33(T/300 K)−3.9 cm6 s−1 were obtained over the range 160–260 K, as well as k0 (160 K)=[Ar]2.2×10−32 cm6 s−1. Rate constants for the reactions Cl+ClOO→Cl2+O2 or 2ClO, ClOO+ClOO→products, and ClOO+Cl2→Cl2O+ClO were also derived. The recombination Cl+O2(+M)→ClOO(+M) at pressures above 10 bar shows a transition to a high pressure plateau and, at pressures above 200 bar, to diffusion control. It is suggested that, like O+O2(+M)→O3(+M), the reaction is governed by a radical complex mechanism.
Microwave and tunable far-infrared laser spectroscopy of the ammonia–water dimer
Authors: Stockman Paul A. , Bumgarner Roger E. , Suzuki Sakae , Blake Geoffrey A.
Published: July 2002

Microwave and far-infrared spectra of the H3N–HOH dimer have been recorded from 36 to 86 GHz and 520 to 800 GHz with a planar supersonic jet/tunable laser sideband spectrometer. The a-type pure rotational microwave data extend the previous m=0, K=0 A symmetry manifold measurements of Herbine and Dyke [J. Chem. Phys. 83, 3768 (1980)] to higher frequency and also provide an additional set of microwave transitions in the mK=+1 E symmetry manifold. Two sets of five b-type rotation–tunneling bands, one set shifted from the other by an approximately constant 113 MHz, have been observed in the far infrared. The splitting into two sets arises from water tunneling, while the overall band structure is due to internal rotation of the ammonia top. Nonlinear least-squares fits to an internal rotor Hamiltonian provided rotational constants, and an estimation of V3=10.5±5.0 cm−1 for the barrier height to internal rotation for the NH3 monomer. A nonlinear equilibrium hydrogen bond is most consistent with the vibrationally averaged rotational constants; with the angle cos−1[〈λz〉] determined from 〈λz〉, the projection of the ammonia’s angular momentum onto the framework; and with the nitrogen quadrupole coupling constants of Herbine and Dyke. The water tunneling splitting and observed selection rules place constraints on the barrier height for proton exchange of the water as well as the most feasible water tunneling path along the intermolecular potential energy surface. An estimated barrier of ∼700 cm−1 is derived for the water tunneling motion about its c axis.
Is there a transition state for the unimolecular dissociation of cyclotetraoxygen (O4)?
Authors: Seidl Edward T. , Schaefer Henry F.
Published: July 2002

The cyclo-O4 molecule is isoelectronic with cyclobutane and has been mentioned as a potential new high energy density material (HEDM). The important unresolved question has been whether or not cyclo-O4 is a genuine minimum on the O4 potential energy hypersurface. Here the transition state for cyclo-O4 dissociation to two O2 molecules has been located at a number of levels of theory using a double zeta plus polarization (DZP) basis set. The theoretical methods with which the transition state geometry was optimized include two-configuration self-consistent-field (TCSCF), configuration interaction including all single and double excitations (CISD), coupled cluster including all single and double excitations (CCSD), and the latter with a noniterative correction for connected triple excitations, CCSD(T). The equilibrium geometry of O4 has D2d symmetry, while a stationary point of D2 symmetry is of Hessian index two (i.e., two imaginary vibrational frequencies) at the highest level of theory, DZP CCSD(T). The true transition state lies only slightly lower energetically (classical barrier 7.9 kcal/mole) and is of C2 symmetry with the DZP CCSD(T) method. The activation energy was predicted in a completely consistent manner to be 6.2 kcal/mole. The possibility that this barrier could disappear entirely at higher levels of theory is discussed. It is concluded that the O4 barrier is too low for O4 to be useful as a HEDM.
Theoretical modeling of spatiotemporal self-organization in a surface catalyzed reaction exhibiting bistable kinetics
Authors: Bär M. , Zülicke Ch. , Eiswirth M. , Ertl G.
Published: July 2002

A two-variable Langmuir–Hinshelwood mechanism for isothermal CO oxidation on a catalytically active surface is presented. It shows bistability stemming from 2 cusp bifurcations, which can be obtained analytically for low pressure. Inclusion of CO diffusion on the surface leads to a system of partial differential equations, which exhibits nucleation and front propagation phenomena in the bistable region. While the line of equistability could with good accuracy be solved for analytically, the front velocities and critical radii for nucleation had to be determined numerically (using the method of heteroclinic orbits). Throughout the calculations the kinetics and rate constants for the CO oxidation on Pt(111) are used. Here the model can be reduced by adiabatic elimination of one variable (namely oxygen coverage) allowing a comparison to the exactly solved one-variable Schlögl model. Possible implications for future experimental work are briefly discussed.