bluuebunny/crossref_metadata_2025 huggingface dataset:
New mechanisms for irreversibility in high-T
c superconductors (invited)
Authors: Krusin-Elbaum L. , Malozemoff A. P. , Cronemeyer D. C. , Holtzberg F. , Clem John R. , Hao Zhidong
Published: July 2002

We discuss the zero-field cooled (ZFC), field-cooled (FC), and remanent (rem) magnetization of YBaCuO crystals in the context of an extended Bean critical-state model. We introduce a novel way to include the reversible magnetization in this theory and show how this can explain the concave-downwards shape of the ZFC magnetization near Tc. We explain the persistence of remanence in a temperature range where FC and ZFC magnetizations merge in terms of a residual irreversibility arising from a step in the reversible B(H) curve. We also introduce the concept of a flux-trapping depth which helps explain the field dependence of the Meissner effect.
Analysis of surface oxides of gas-evaporated Si small particles with infrared spectroscopy, high-resolution electron microscopy, and x-ray photoemission spectroscopy
Authors: Hayashi Shinji , Tanimoto Shinichi , Yamamoto Keiichi
Published: July 2002

Oxide layers grown on surfaces of gas-evaporated Si particles as small as 100 Å have been investigated. Results of high-resolution electron microscopy and x-ray photoemission spectroscopy show that the native oxide layer (SiOx) is about 20 Å thick and its chemical composition x is around 1.2. When the particles are annealed in air at 400 °C, the thickness of the oxide layers remains almost the same while the composition x increases up to 2 as the annealing time increases. The evolution of the oxide takes place during the first 15 min of annealing. Due to the increase in x, the infrared absorption band in the region of Si-O-Si stretching vibration shifts to higher frequencies and increases in intensity. A comparison of experimental infrared spectra with those calculated by an effective medium theory suggests that the observed large width of the absorption band is caused by the particle aggregation.
Refractive indices of AlSb and GaSb-lattice-matched AlxGa1−xAsySb1−y in the transparent wavelength region
Authors: Alibert C. , Skouri M. , Joullie A. , Benouna M. , Sadiq S.
Published: July 2002

The refractive indices of AlSb grown by the solute diffusion method and GaSb-lattice-matched AlxGa1−xAsySb1−y alloy grown by liquid-phase epitaxy have been determined at room temperature from accurate measurements of the reflectance of p-polarized light as a function of the angle of incidence. The refractive index variations versus the photon energy were obtained in the spectral range 0.5–1.5 eV. Experimental data in the transparent wavelength region could be matched by calculated curves on the basis of a single-oscillator model.
Optical spectra of undoped and rare-earth-(=Pr, Nd, Eu, and Er) doped transparent ceramic Y3Al5O12
Authors: Sekita Masami , Haneda Hajime , Shirasaki Shinichi , Yanagitani Takagimi
Published: July 2002

Undoped and rare-earth- (Pr, Nd, Eu, and Er) doped transparent Y3Al5O12 ceramics have been prepared, and their optical spectra have been measured. It is found that absorption coefficient of the undoped ceramic Y3Al5O12 is almost independent of wavelength with 0.258 cm−1, which gives transmittance of the undoped ceramic Y3Al5O12 to be, for example, 95% for a 2-mm height. Peaks in the absorption and emission spectra of Pr:Y3Al5O12 are assigned to the transitions in 4f2 configuration. The transitions from the multiplets with total angular momentum J=0 to multiplets with even J momentum are strong as predicted by Judd–Ofelt theory. The overall and detail structure of the absorption and emission spectra of Nd:Y3Al5O12 is the same as those in a previous publication. However, the absorption coefficient at nonabsorbing wavelengths by Nd3+ is reduced from more than 1.7 to 0.25 cm−1. A simple estimation of the population inversion threshold of the Nd:Y3Al5O12 ceramic reveals that the threshold is still about 25 times larger than that of single-crystal Nd:Y3Al5O12. Peaks in the absorption and emission spectra of Er:Y3Al5O12 ceramic are assigned to the transitions in 4f11 configuration. The transition energies agree well with those reported for single-crystal Er:Y3Al5O12 and the other hosts within 30 cm−1. A Stark splitting scheme for some multiplets has been constructed. Peaks in the emission, absorption, and excitation spectra of Eu:Y3Al5O12 ceramic are assigned to the transitions in 4f6 configuration. The transition energies are in very good agreement with those of single-crystal Eu:Y3Al5O12 within 8 cm−1 for the emission peaks and 3 cm−1 for the absorption and excitation peaks. Spectral characteristics and derivation of a broad peak in the absorption spectrum at around 280 nm are discussed in some detail.
The effect of Ag doping on superconducting properties of the Bi-Pb-Sr-Ca-Cu-O tape fabricated by an Ag sheath
Authors: Oota A. , Horio T. , Ohba K. , Iwasaki K.
Published: July 2002

The powder-in-tube method using an Ag sheath is applied to achieve high current capacity in the tapes of Bi-Pb-Sr-Ca-Cu-O (BPSCCO) with the high-Tc phase (Tc=110 K), by a combination process of cold working and sintering. Two kinds of the BPSCCO powder with and without Ag doping are used as the materials for tape fabrication. The morphology, structure, and critical current density Jc are investigated on the tapes by x-ray diffraction, differential thermal analysis, scanning electron microscope, magnetic susceptibility, and I-V curves. The optimum sintering conditions to obtain a maximum in the value of Jc at 77 K and 0 T are presented in the Ag-doped tapes and the nondoped ones. An improvement in the Jc vs B properties up to 1 T at 77 K is presented in the BPSCCO tape by Ag doping. The Ag doping causes neither change in the value of Tc (=110 K) nor decomposition of the high-Tc phase.
A simple way of describing the diffusion phase growth in cylindrical and spherical samples
Authors: Gusak A. M. , Yarmolenko M. V.
Published: July 2002

It is shown that during diffusional growth of the intermediate phase between substances A and B for cylindrical and spherical samples, the total diffusion flux relative to the center, −S(r)D̃∂c/∂r, where S is the area of the interface, varies by a small amount in the radial direction, despite the concentration dependence of the interdiffusion coefficient D̃(c). In this context we propose a constant-flux method which allows the kinetics of the diffusional phase growth in cylindrical and spherical samples to be described in a rather simple way. Deviations from the parabolic law of phase growth in cylindrical and spherical samples are analyzed.
Combined three-axis surface magneto-optical Kerr effects in the study of surface and ultrathin-film magnetism
Authors: Yang Z. J. , Scheinfein M. R.
Published: July 2002

Surface and ultrathin-film magnetocrystalline anisotropy in epitaxial fcc Fe thin films grown on room-temperature Cu(100) single crystals has been investigated, in situ, by the combined surface magneto-optical Kerr effects (SMOKE). In polar, longitudinal, and transverse Kerr effects, the direction of the applied magnetic field must be distinguished from the direction of magnetization during the switching process. For arbitrary orientations of the magnetization and field axis relative to the optical scattering plane, any of the three Kerr effects may contribute to the detected signal. A general expression for the normalized light intensity sensed by a photodiode detector, involving all three combined Kerr effects, is obtained both in the ultrathin-film limit and for bulk, at general oblique incidence angles and with different orientations of the polarizer, modulator, and analyzer. This expression is used to interpret the results of fcc Fe/Cu(100) SMOKE measurements. For films grown at room temperature, polar and longitudinal Kerr-effect magnetization loops show that the easy axis of magnetization rotates from the (canted) out-of-plane direction to the in-plane direction at a thickness of about 4.7 monolayers. Transverse Kerr-effect measurements indicate that the in-plane easy axes are biaxial.
A fractal criterion for ductile and brittle fracture
Authors: Nagahama Hiroyuki
Published: July 2002

A universal relationship between roughness and toughness of fractures is proposed and a new criterion to define brittle and ductile fracture is derived from the Griffith energy balance concept and fractal geometry.
A GaAs/AlAs/AlGaAs and GaAs/AlGaAs stacked quantum well infrared photodetector for 3–5 and 8–14 μm detection
Authors: Wang Y. H. , Chiang Jung-chi , Li Sheng S. , Ho Pin
Published: July 2002

A new two-color n-type GaAs/AlAs/AlGaAs double barrier quantum well (DBQW) and bound-to-miniband GaAs/AlGaAs quantum well infrared photodetector (QWIP) with photovoltaic (PV) and photoconductive (PC) dual-mode operation in the 3–5 and 8–14 μm atmospheric spectral windows has been demonstrated in this work. It consists of a stack of the midwavelength infrared (MWIR) QWIP and the long-wavelength infrared (LWIR) QWIP. The PV detection scheme uses transition from the ground bound state to the first quasi-bound excited state for the MWIR-QWIP. The PC detection scheme has two different transitions, one identical to the PV mode detection scheme while the other uses transition from the ground bound state to the miniband state of the superlattice barrier LWIR-QWIP. The peak responsivity for the PV mode was found to be 17 mA/W at λp=4.1 μm and T=50 K with a bandwidth Δλ/λp=15%. The peak responsivities for the PC mode were found to be 25 mA/W at λp=4.1 μm, Vb=1.0 V, and 0.12 A/W at λp=11.6 μm, Vb=3.2 V, and at T=50 K, with a bandwidth Δλ/λp=18%. The PV responsivity was found to be 68% of the PC responsivity at λp=4.1 μm and T=50 K, demonstrating the ability for efficient PV mode operation at 3–5 μm by using the DBQW structure. This is the highest ratio reported for the spectral region of 3–5 μm wavelength with a DBQW structure.
Photoluminescence properties of ZnGa2O4:Mn powder phosphors
Authors: Tran T. K. , Park W. , Tomm J. W. , Wagner B. K. , Jacobsen S. M. , Summers C. J. , Yocom P. N. , McClelland S. K.
Published: July 2002

The results of a systematic photoluminescence study of ZnGa2O4:Mn powder phosphor are reported. At room temperature this phosphor exhibits bright green luminescence with a spectral peak at 2.46 eV and Commission International de l’Eclairage chromaticity coordinates of x=0.073 and y=0.696. At low temperatures the luminescence was found to consist of three components assigned to the 4T1–6A1 inner transition of the 3d electrons of Mn2+ ions located on three different sites of the host crystal. Selective excitation and lifetime measurements were used to investigate the assignment of these features. The photoluminescence lifetime showed a single exponential decay of about 4 ms and at T=1.6 K an optical-phonon-related fine structure [Ephonon=(8.2±0.2) meV] of the main photoluminescence line was observed.
Effect of Mg doping on dielectric properties of alumina
Authors: Mollá J. , Moreno R. , Ibarra A.
Published: July 2002

Alumina samples with different magnesia concentrations between 0.02 and 1 wt% have been sintered and characterized by electron microscopy. Their dielectric properties between 1 kHz and 15 GHz are studied. Different dielectric behaviors are found for different magnesia concentrations. A model based mainly on dipolar relaxation processes is proposed to explain the results obtained. The effect of magnesia on dielectric properties of alumina shown in the present work should be considered in the engineering designs for heating systems of future fusion machines and other applications.
Autocorrelation functions of granular media with free arrangement of spheres, spherical shells or ellipsoids
Authors: Mätzler Christian
Published: July 2002

The decomposition of natural media, such as snow and soil, into grains or pores of variable size and shape is an ambiguous task, depending on how the grains and their size are defined. Volume scattering of electromagnetic waves, adsorption of molecules on the grain surface, percolation of a fluid through the pores, or sedimentation of grains in a fluid do not depend on such definitions; they can be characterized by more general structural parameters, examples of which are the specific surface s or the correlation length pc, both being related to the spatial autocorrelation function (SAF). Therefore, physically meaningful structure information can be obtained even without specifying the grains, by adopting pc as an effective size. In order to get an understanding of how pc is related to the geometrical dimensions of simple particles, we compute the SAF and pc for single spheres, spherical shells, and for isotropically oriented ellipsoids; exact and approximate formulas are derived, plotted, and exact expressions of pc are found. A result is that pc is not related to the maximum particle extent, but in all cases studied, pc is close to the minimum, characteristic extent of the grain. The SAFs can be applied to the computation of volume scattering in the weak and strong fluctuation theory, respectively. Furthermore, it can be shown, based on an assumption of the free arrangement of impenetrable granules, that the SAF of an irregular medium is identical to the SAF of its particles. Thus the single-particle correlation is the dominant structure in media consisting of irregular granules. This result is in contrast to systems of spatially correlated particles, such as atoms in crystals or molecules in liquids. Although this work was driven by the need for a quantitative interpretation of remote sensing data of snow, the results may be applied to other disciplines as well.
Evidence of a low compressibility carbon nitride with defect-zincblende structure
Authors: Martin-Gil Jesús , Martin-Gil Francisco J. , Sarikaya Mehmet , Qian Maoxu , José-Yacamán Miguel , Rubio Angel
Published: July 2002

A carbon-nitride compound with defect zincblende structure (P4̄3m) has been discovered in samples prepared by a chemical precursor route. Crystallographical (high-resolution electron microscopy and electron nanodiffraction) and electron energy loss measurements has been performed to identify the material as cubic zincblende with C3N4 composition. Nanoindentation indicates a high elastic recovery and hardness. The results agree with detailed ab initio calculations on metastable structures and compressibility. Our synthesis method is projected to be a process that could produce large quantities of material by controlling the chemical strategy. The new compound has potential applications for high hardness, elasticity and thermal conductivity materials and thin films.
Magnetic and magnetotransport properties of new III-V diluted magnetic semiconductors: GaMnAs
Authors: Hayashi T. , Tanaka M. , Nishinaga T. , Shimada H.
Published: July 2002

We have studied magnetic and magnetotransport properties of novel III-V diluted magnetic semiconductors, (Ga1−xMnx)As. The GaMnAs thin films were grown on GaAs(001) substrates by low temperature molecular beam epitaxy. We present magnetoresistance, extraordinary Hall effect, and M–H characteristics of two (Ga1−xMnx)As samples having different Mn content x.
Carrier lifetime measurements using free carrier absorption transients. I. Principle and injection dependence
Authors: Linnros Jan
Published: July 2002

A contactless, all-optical technique for semiconductor charge carrier lifetime characterization is reviewed. The technique is based upon measurements of free carrier absorption transients by an infrared probe beam following electron-hole pair excitation by a pulsed laser beam. Main features are a direct probing of the excess carrier density coupled with a homogeneous carrier distribution within the sample, enabling precision studies of different recombination mechanisms. We show that the method is capable of measuring the lifetime over a broad range of injections (1013–1018 cm−3) probing both the minority carrier lifetime, the high injection lifetime and Auger recombination, as well as the transition between these ranges. Performance and limitations of the technique, such as lateral resolution, are addressed while application of the technique for lifetime mapping and effects of surface recombination is outlined in a companion article [J. Appl. Phys. 84, 284 (1998), part II]. Results from detailed studies of the injection dependence yield good agreement with the Shockley–Read–Hall theory, whereas the coefficient for Auger recombination shows an apparent shift to a higher value, with respect to the traditionally accepted value, at carrier densities below ∼2–5×1017 cm−3. Data also indicate an increased value of the coefficient for bimolecular recombination (radiative or trap-assisted Auger) from the generally accepted value. Measurements on an electron irradiated wafer and wafers of exceptionally high carrier lifetimes are also discussed within the framework of different recombination mechanisms.
Bias-enhanced nucleation of diamond on molybdenum: A photoelectron spectroscopy study of the initial stages of the growth process
Authors: Reinke P. , Oelhafen P.
Published: July 2002

In this study we present an investigation of the bias-enhanced nucleation and early stages of diamond growth on molybdenum. The progression of the different stages of the nucleation and growth was observed using photoelectron spectroscopy with excitation energies in the ultraviolet and x-ray regime. Subsequent to the film deposition the samples were transferred to the analysis chamber without breaking the vacuum. To achieve high nuclei densities methane concentrations in excess of about 20% diluted in hydrogen and a dc bias of −400 V are required. During the bias-enhanced nucleation period a molybdenum carbide interface is formed and the codeposition of a graphitic carbon layer and diamond is observed. The graphitic carbon layer is subsequently removed during the diamond growth step, where deposition conditions conducive of diamond growth are maintained and a continuous diamond film is formed after 40 min. The results are compared with those obtained for the bias-enhanced nucleation on silicon.
Surface interactions of CF2 radicals during deposition of amorphous fluorocarbon films from CHF3 plasmas
Authors: Capps Nathan E. , Mackie Neil M. , Fisher Ellen R.
Published: July 2002

Surface reactivities for CF2 radicals formed in a CHF3 plasma molecular beam are measured during film deposition on a variety of substrates. The imaging of radicals interacting with surfaces (IRIS) technique was used to collect spatially resolved laser-induced fluorescence (LIF) images of CF2 radicals interacting with SiO2, Si3N4, Si, 304 stainless steel, and system 8 photoresist substrates. Films deposited during IRIS experiments were characterized using x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy and were found to be nearly identical in composition on all substrates. Simulation of LIF cross-sectional data shows high scattering coefficients for CF2 radicals on all substrates. These extremely large scattering coefficients (>1.0) indicate that CF2 molecules are generated through plasma interactions with the substrate. Possible CF2 surface generation mechanisms are discussed, with consideration of CF and ion bombardment contributions to the generation of CF2.
Influence of Joule heating on magnetostriction and giant magnetoimpedance effect in a glass covered CoFeSiB microwire
Authors: Kraus L. , Knobel M. , Kane S. N. , Chiriac H.
Published: July 2002

The influence of annealing parameters (time ta, current Ia, and applied stress σa) on magnetic properties of Joule heated amorphous Co68.15Fe4.35Si12.5B15 glass covered microwire (13 μm) was investigated. Annealing under applied stress induces additional anisotropy which is proportional to σa and can be removed by subsequent heating without stress. The magnetoimpedance, measured on the sample with the lowest anisotropy field (HK≈120 Am−1), shows sharp maxima at H=±HK. For driving currents higher than 0.2 mA nonlinear behavior is observed, and the magnitude of giant magnetoimpedance significantly decreases. The maximum relative change of impedance (60%), observed for the highest frequency, 900 kHz, compares well with the values reported on conventional wires.
Modeling the optical constants of AlxGa1−xAs alloys
Authors: Djurišić Aleksandra B. , Rakić Aleksandar D. , Kwok Paul C. K. , Li E. Herbert , Majewski Marian L. , Elazar Jovan M.
Published: July 2002

The extension of Adachi’s model with a Gaussian-like broadening function, in place of Lorentzian, is used to model the optical dielectric function of the alloy AlxGa1−xAs. Gaussian-like broadening is accomplished by replacing the damping constant in the Lorentzian line shape with a frequency dependent expression. In this way, the comparative simplicity of the analytic formulas of the model is preserved, while the accuracy becomes comparable to that of more intricate models, and/or models with significantly more parameters. The employed model accurately describes the optical dielectric function in the spectral range from 1.5 to 6.0 eV within the entire alloy composition range. The relative rms error obtained for the refractive index is below 2.2% for all compositions.
High-resolution transmission electron microscopy of some Tin+1AXn compounds (n=1, 2; A=Al or Si; X=C or N)
Authors: Farber L. , Levin I. , Barsoum M. W. , El-Raghy T. , Tzenov T.
Published: July 2002

The crystal structures of Ti2AlN, Ti3SiC2, and Ti3AlC1.8, studied by high-resolution transmission electron microscopy confirmed the P63/mmc space group and the layered nature of these ternaries. The structure of Ti2AlN agrees with that previously determined from analysis of x-ray and neutron diffraction data. Conversely, the crystal structures of thinned Ti3SiC2 and Ti3AlC1.8 samples differ from the bulk structure as determined from x-ray and neutron diffraction analysis. Since both structures have identical symmetries and lattice parameters, the differences—which involve the shearing of the Si or Al planes in opposite directions—are indistinguishable by conventional transmission electron microscopy. This polymorphic phase transformation in the Tin+1AXn compounds, for n⩾2, is discussed in relation to dimensionally induced hexagonal close-packed to face-centered-cubic phase transformations in Ti-based thin multilayers.
Theory of phase stabilities and bonding mechanisms in stoichiometric and substoichiometric molybdenum carbide
Authors: Hugosson Håkan W. , Eriksson Olle , Nordström Lars , Jansson Ulf , Fast Lars , Delin Anna , Wills John M. , Johansson Börje
Published: July 2002

First principles, total energy methods have been applied to predict the relative stabilities of the four experimentally verified MoC phases: the cubic δ(NaCl) phase and the three hexagonal γ(WC), η and γ′(TiAs) phases. The effect of vacancies on the relative stability of these four phases was investigated using a model structure with ordered vacancies within the carbon sublattice. For stoichiometric MoC, the γ phase was found to be the most stable followed by γ′, δ, and η, but for substoichiometric MoC0.75, the order of relative stability was changed and the substoichiometric δ phase was found to have the lowest energy followed by γ′ and γ. A study of the electronic structure revealed vacancy induced peaks in the density of state and the electron density attached to these peaks was analyzed and found to emanate from unscreened Mo–Mo bonds through the carbon vacancy site. Finally, the oxygen stabilization of the γ′ MoC phase was studied.
Low-dose radiation effects in extrinsic photoconductors
Authors: Patrashin M. , Hiromoto N. , Fouks B. , Maslov I. A. , Ledenev V. M.
Published: July 2002

Experimental results and a model describing long-term radiation-induced changes in extrinsic photoconductors are presented. Experiments, carried out at low temperatures and low photon backgrounds, simulated typical operating conditions of extrinsic infrared detectors used in space- based platforms and instruments. The observed changes in the carrier lifetime and current–voltage characteristics at a low electric field, as well as a long-term relaxation of the effects after removal of the irradiation are related to a recharging of the impurities in the bulk during the irradiation. The proposed model agrees well with experimental data and despite its simplicity, provides relevant information on how to deal with these effects in practical applications.
Silicon and germanium clusters. A theoretical study of their electronic structures and properties
Authors: Pacchioni Gianfranco , Koutecký Jaroslav
Published: July 2002

Silicon and germanium clusters containing three to seven atoms have been studied with the pseudopotential MO-LCAO method followed by configuration interaction procedure. Si and Ge clusters have very similar electronic structures and consequently analogous physico-chemical properties but differ substantially from small carbon clusters. Linear structures are clearly less favorable than more compact structures. On the other hand, some planar geometries possess considerable stability. The Si and Ge clusters which are sections of the diamond-type crystal lattice are less stable than clusters which can be considered as segments of closed-packed lattices or as steps in pentagonal crystal growth. The reason is that the majority of atoms in small clusters are surface atoms which cannot assume the tetrahedral coordination characteristic of Si and Ge bulk atoms. The appearance of typical bulk properties is expected only for very large Si and Ge clusters with small surface atoms/bulk atoms ratio.
Hyperfine structure and lifetime of the C 2Σ+, v=0 state of CH
Authors: Ubachs Wim , Meyer Gerard , ter Meulen J. J. , Dymanus A.
Published: July 2002

From a laser induced fluorescence (LIF) experiment on a molecular beam of CH, we have obtained the b and c hyperfine constants, the γ and γD spin-rotation constants as well as accurate values for the rotational constants B, D, and H for the C 2Σ+, v=0 state. From measurements of the linewidths, that are partially caused by predissociation, and by comparing relative line intensities, we determined different lifetimes for upper (F1) and lower (F2) ρ-doublet states of the C 2Σ+ state. For the F1 states we find a constant lifetime of 3.7±1.0 ns, that is independent of N, while for the F2 states we observed an increase in lifetime for higher N up to 8.0±1.5 ns for N=11.
The sensitivity of IVR in benzene to bend–stretch potential energy coupling
Authors: Lu Da-Hong , Hase William L. , Wolf Ralph J.
Published: July 2002

Quasiclassical trajectory calculations have been performed to study intramolecular vibrational energy redistribution (IVR) from CH overtone states in benzene. The rate and extent of this redistribution is sensitive to details of the potential energy surface. A particularly important potential energy surface property is attenuation of the HCC bending frequency upon CH stretch excitation. This property gives rise to non-irreversible IVR when the CH stretch is highly excited. An analog to quantum beats is observed in phase averaged quasiclassical trajectories.
Resonant excitation transfer: A quantum electrodynamical study
Authors: Andrews David L. , Sherborne Brad S.
Published: July 2002

A new quantum electrodynamical (QED) method is presented, based in the Schrödinger representation, for the calculation of the rate of energy transfer between identical molecules. In contrast to existing methods in this representation, the new treatment gives explicitly causal and energy-conserving results. By returning to perturbation theory the correct, complex form for the electric dipole–electric dipole interaction tensor is obtained, without recourse to the physical, ‘‘outgoing wave’’ arguments of quantum scattering theory necessary if the Fermi rule is used. This method also allows a new interpretation for the role of the time-ordered diagrams involved, which may be useful in the rigorous treatment of higher order cooperative processes. The QED treatment uses virtual photon coupling, and incorporates both the Coulombic, R−6 dependence, and the R−2 dependence characteristic of two-step radiative transfer.
Infrared structural holeburning and orientational tunneling of NH3D+ ions in ammonium cobalt sulfate
Authors: Trapani Andrew P. , Strauss Herbert L.
Published: July 2002

An infrared diode laser has been used to irradiate one of the N–D stretching bands of the title compound. The irradiation induces reorientation of the NH3D+ ions and the N–D stretching spectrum changes accordingly. The perturbed orientational distribution of the NH3D+ ions relaxes back to equilibrium by tunneling. The various rates have been measured.
Supercooled liquids, glass transitions, and the Kauzmann paradox
Authors: Stillinger Frank H.
Published: July 2002

Many liquids have heat capacities that substantially exceed those of the corresponding crystal, and this discrepancy magnifies in the supercooled regime. Thus, liquid entropy declines more rapidly with temperature than does crystal entropy, and the former paradoxically seems to fall below the latter for temperatures below the Kauzmann point TK. Although laboratory glass transitions inevitably intervene to prevent observation of this entropy crossing, it has often been argued that a second-order ‘‘ideal glass transition’’ in principle should occur at TK. The inherent structure theory of condensed phases has been modified to describe supercooled liquids, and has been applied to this Kauzmann paradox. The conclusion is that an ideal glass transition of the type normally associated with the Kauzmann phenomenon cannot occur for substances of limited molecular weight and with conventional intermolecular interactions. This result also subverts theoretical expressions for shear viscosity (such as the Tamman–Vogel–Fulcher and the mode-coupling formulas) that diverge to infinity at an ideal glass transition temperature.
A semiclassical tunneling model for use in classical trajectory simulations
Authors: Makri Nancy , Miller William H.
Published: July 2002

A semiclassical model for tunneling from one classically allowed region on a potential energy surface to another is described. The principal feature of this model, compared to earlier (more ‘‘rigorous’’) multidimensional semiclassical tunneling theories, is that it can be implemented in a straightforward way within the framework of a standard classical trajectory simulation. Applications to several examples of unimolecular isomerization and unimolecular dissociation show that the model is capable of providing excellent results over a wide range of conditions (i.e., coupling strengths, different symmetries of couplings, etc.)
Carbon clusters: The structure of C1 studied with configuration interaction methods
Authors: Liang Congxin , Schaefer Henry F.
Published: July 2002

The structure of the C10 carbon cluster is studied by ab initio quantum mechanical methods using self-consistent-field and configuration interaction theories. Three monocyclic and two linear structures are considered. It is found that all the monocyclic structures are considerably lower in energy than the linear ones, in agreement with previous theoretical studies. At the highest level of theory, the lowest energy cyclic structure is predicted to lie 67 kcal/mole below the lowest energy linear structure. Correction for entropic contributions to the free energy at 3700 K reduces this ΔH value by about 40 kcal/mole. It is also concluded with confidence that the delocalized ring structure is more stable than the localized one with alternating single–triple bonds. Although the D5h symmetric monocyclic structure with cumulated double bonds is predicted to be the lowest state at the highest theoretical level (CISD with a DZP basis set) employed here, it is cautioned that the fully symmetric D10h monocyclic form might be more stable. Among the two linear structures, the triplet acetylenic form is almost isoenergetic with the triplet cumulenic one at CISD/DZP, and Davidson’s correction places the former about 5 kcal/mole above the latter. The possibility of the bending of linear C10 was also investigated. Qualitative theoretical arguments are employed to rationalize the ab initio results obtained.
Improved algorithms for reaction path following: Higher-order implicit algorithms
Authors: Gonzalez Carlos , Schlegel H. Bernhard
Published: July 2002

Eight new algorithms for reaction path following are presented, ranging from third order to sixth order. Like the second-order algorithm [J. Chem. Phys. 90, 2154 (1989)] these are implicit methods, i.e., they rely on the tangent (and in some cases the curvature) at the endpoint of the step. The tangent (and the curvature, if needed) are obtained by a constrained optimization using only the gradient. At most, only one Hessian calculation is needed per step along the path. The various methods are applied to the Müller–Brown surface and to a new surface whose reaction path is known analytically to test their ability to follow the reaction path and to reproduce the curvature along the path.
Dielectrons in water clusters
Authors: Kaukonen Harri-Pekka , Barnett R. N. , Landman Uzi
Published: July 2002

The energetics, structure, and stability of a dielectron solvated in an internal cavity in water clusters, (H2O)−2n, at 300 K are investigated using coupled quantum-classical molecular-dynamics simulations. In these calculations the ground state of the dielectron is calculated concurrently with the atomic configurations using the local-spin-density functional method, and the nuclear degrees of freedom evolve classically on the Born–Oppenheimer potential-energy surface. For n=64 and 128 the internal single-cavity dielectron state is unstable, while for n=256 (as well as in bulk water) it is energetically stable, fluctuating between a compact spherical configuration (e2c) and an elongated ellipsoidal dumbbell-shaped one (e2d). Transitions between these two states of the dielectron are accompanied by structural and orientational transformations of the surrounding water molecules. The induced molecular orientational order is enhanced and is of longer range in (H2O)−2256 than is the case for a solvated single excess electron. By extrapolating our results to the bulk limit we conclude that a spin-paired dielectron state in bulk water, at 300 K, is a stable species relative to two single separated hydrated electrons.
The harmonic force field of benzene. A local density functional study
Authors: Bérces Attila , Ziegler Tom
Published: July 2002

The harmonic force field of benzene has been calculated by a method based on local density functional theory (LDF). The calculations were carried out employing a triple zeta basis set with triple polarization on hydrogen and double polarization on carbon. The LDF force field was compared to the empirical field due to Ozkabak, Goodman, and Thakur [A. G. Ozkabak, L. Goodman, and S. N. Thakur, J. Phys. Chem. 95, 9044 (1991)], which has served as a benchmark for theoretical calculations as well as the theoretical field based on scaled Hartree–Fock ab initio calculation due to Pulay, Fogarasi, and Boggs [P. Pulay, G. Fogarasi, and J. E. Boggs, J. Chem. Phys. 74, 3999 (1981)]. The calculated LDF force field is in excellent qualitative and very good quantitative agreement with the theoretical field proposed by Pulay, Fogarasi, and Boggs as well as the empirical field due to Ozkabak, Goodman, and Thakur. The LDF field is closest to the values of Pulay and co-workers in those cases where the force constants due to Pulay, Fogarasi, and Boggs and to Ozkabak, Goodman, and Thakur differ in sign or magnitude.

The accuracy of the LDF force field was investigated by evaluating a number of eigenvalue and eigenfunction dependent quantities from the the LDF force constants. The quantities under investigation include vibrational frequencies of seven isotopomers, isotopic shifts, as well as absorption intensities. The calculations were performed at both theoretical optimized and approximate equilibrium reference geometries. The predicted frequencies are usually within 1%–2% compared to the empirical harmonic frequencies. The least accurate frequency deviates by 5% from the experimental value. The average deviations from the empirical harmonic frequencies of C6H6 and C6D6 are 16.7 cm−1 (1.5%) and 15.2 cm−1 (1.7%), respectively, not including CH stretching frequencies, in the case where a theoretical reference geometry was used. The accuracy of the out-of-plane force field is especially remarkable; the average deviations for the C6H6 and C6D6 frequencies, based on the LDF force field, are 9.4 cm−1 (1.2%) and 7.3 cm−1 (1.2%), respectively. The absorption intensities were not predicted as accurately as it was expected based on the size of the basis set applied. An analysis is provided to ensure that the force constants are not significantly affected by numerical errors due to the numerical integration scheme employed.
Bifurcation analysis of chemical reaction mechanisms. II. Hopf bifurcation analysis
Authors: Olsen Robert J. , Epstein Irving R.
Published: July 2002

One- and two-parameter Hopf bifurcation behavior is analyzed for several variants of the Citri–Epstein mechanism of the chlorite–iodide reaction. The coefficients of an equation for the amplitude of oscillations (the universal unfolding of the Hopf bifurcation) are evaluated numerically. Local bifurcation diagrams and bifurcation sets are derived from the amplitude equation. Sub- and supercritical Hopf bifurcations are identified for the general case of a nondegenerate (codimension one) bifurcation. At degenerate (codimension two) points, the necessary higher-order terms are included in the unfolding, and features such as locally isolated branches of periodic orbits and bistability of a periodic orbit and a steady state are found. Inferences about the global periodic orbit structure and the existence of nearby codimension three Hopf bifurcation points are drawn by piecing together the local information contained in the unfoldings. Hypotheses regarding the global periodic orbit structure are tested using continuation methods to compute entire branches of orbits. A thorough discussion of the application of these methods is presented for one version of the mechanism, followed by a comparison of the complete two-parameter steady state bifurcation structure of three versions of the mechanism. In all cases, the potential for experimental verification of the predicted dynamics is examined.
Electric conductivity in microemulsions
Authors: Kahlweit M. , Busse G. , Winkler J.
Published: July 2002

Microemulsions exhibit a rather sudden change of electric conductivity upon varying either composition at fixed temperature, or temperature at fixed mean composition. In literature this has been interpreted as a percolationlike process. In this paper we question this interpretation, and suggest instead that the change of conductivity is caused by a dispersion inversion that takes place in a plane through the symmetry axes of the three-phase body, irrespective of the volume fraction of the conducting phase.
Accurate quantum calculations for H2+OH→H2O+H: Reaction probabilities, cross sections, and rate constants
Authors: Zhang Dong H. , Zhang John Z. H.
Published: July 2002

Following a previous Communication [J. Chem. Phys. 99, 5615 (1993)], which reported several initial state-selected total reaction probabilities for the title reaction for J=0, we present in this paper the methodologies of the previous calculation and show results of new calculations. In particular, the present calculations are extended to all angular momentum J≳0 and obtained reaction cross sections for a range of energies using the centrifugal sudden (CS) approximation. The computed cross sections are used to obtain the state-specific thermal rate constants for both the ground and the excited vibrations of H2. The dynamics calculation, in which the nonreactive OH bond is frozen, includes explicitly five degrees of freedom in the time-dependent quantum dynamics treatment. The comparison of the present accurate cross sections with other approximate theoretical calculations shows discrepancies. The computed rate constants (from the ground rotation state) are larger than experimental measurements at low temperatures, the v=0 rate is larger than the corresponding experimental rate by a factor of 1.8, and the ratio of v=1 to v=0 rate is a factor of 4.8 greater than the experimental ratio at 300 K. The calculated reaction rates are also compared to those of other theoretical calculations and the differences are discussed in the text.
Photoelectron spectroscopy of Cl−, Br−, and I− solvated in water clusters
Authors: Markovich Gil , Pollack Stuart , Giniger Rina , Cheshnovsky Ori
Published: July 2002

We present the photoelectron-spectra of Cl−, Br−, and I−, solvated in water clusters-(H2O)n, where n is 1–7, 1–16, and 1–60, respectively, taken with 7.1 eV photon energy. The vertical binding energies of the solvated anions are used to extract the solvent electrostatic stabilization energies of the anion. The photoelectron spectra of the solvated I− indicate the formation of the first solvation layer with a coordination number of six. Ab initio calculations support solvation shell closure at n=6. This conclusion is not born-out by current molecular dynamics calculations. These calculations favor structures with a surface solvated anion (coordination number of 3–4) and reproduce (within 0.2 eV) our vertical binding energies. The fitting of the experimental binding energies of large I−(H2O)n to the models of classical electrostatic solvation is consistent with surface solvation. In the size range n=34–40 we have detected special cluster structures, with very low electrostatic stabilization.
Turbulence and standing waves in oscillatory chemical reactions with global coupling
Authors: Mertens Florian , Imbihl Ronald , Mikhailov Alexander
Published: July 2002

Using the model of the complex Ginzburg–Landau equation with global coupling, the influence of long-range interactions on the turbulent state of oscillatory reaction–diffusion systems is investigated. Experimental realizations of such a system are, e.g., oscillatory reactions on single crystal surfaces where some of the phenomena we simulate have been observed experimentally. We find that strong global coupling suppresses turbulence by transforming it into a pattern of standing waves or into uniform oscillations. Weaker global coupling gives rise to an intermittent turbulent state which retains partial synchrony.
Intracavity laser absorption spectroscopy detection of HCO radicals in atmospheric pressure hydrocarbon flames
Authors: Cheskis Sergey
Published: July 2002

Formyl radical, HCO, was monitored for the first time in an atmospheric pressure premixed hydrocarbon flame. Intracavity laser absorption spectroscopy based on quasi-(cw) argon-ion pumped dye laser was used. The sensitivity of the detection is ∼5×1012 cm−3 and can be improved with better flame and laser stabilization.
The wurtzite to rock salt structural transformation in CdSe nanocrystals under high pressure
Authors: Tolbert Sarah H. , Alivisatos A. P.
Published: July 2002

Structural transformations in CdSe nanocrystals are studied using high pressure x-ray diffraction and high pressure optical absorption at room temperature. The nanocrystals undergo a wurtzite to rock salt transition analogous to that observed in bulk CdSe. Both the thermodynamics and the kinetics of the transformation, however, are significantly different in finite size. The nanocrystal phase transition pressures vary from 3.6 to 4.9 GPa for crystallites ranging from 21 to 10 Å in radius, respectively, in comparison to a value of 2.0 GPa for bulk CdSe. The size dependent data can be modeled using thermodynamics when surface energies are accounted for. Surface energies calculated in this way can be used to understand the dynamic microscopic path followed by atoms during the phase transition. X-ray diffraction data also shows that unlike bulk CdSe, crystalline domain size is conserved upon multiple transition in the nanocrystals, indicating that the transition only nucleates once in each nanocrystal.
Ion-induced nucleation. II. Polarizable multipolar molecules
Authors: Kusaka I. , Wang Z.-G. , Seinfeld J. H.
Published: July 2002

Density functional theory is applied to ion-induced nucleation of polarizable multipolar molecules. The asymmetric nature of the ion–molecule interaction is shown to cause the sign preference in ion-induced nucleation. When the ion–molecule interaction is weak, the observed sign preference is consistent with that of the bare ion–molecule interaction potential and decreases with increasing supersaturation. However, as the ion–molecule interaction becomes stronger, the sign preference in the reversible work exhibits some nontrivial behavior. For molecular parameters applicable for CS2 and CH4, the predicted values of the reversible work of nucleation depend on the sign of the ion charge, yielding a difference in the nucleation rate by factors of 10 to 102 and 10 to 105, respectively.
The lattice dynamics of β-hydroquinone clathrate
Authors: Belosludov R. V. , Belosludov V. R. , Zubkus V. E.
Published: July 2002

Dynamic properties of β-hydroquinone (Qβ) clathrate with various guest molecules have been calculated by the method of lattice dynamics within the atom–atom potential approximation; van der Waals interactions, H-bond interactions, and electrostatic interactions for charges localized on the atoms are included. External vibration spectra have been calculated for the empty framework of Qβ and with different guest molecules (HCN, CO2, SO2, C2N2, C2H6, CH3OH, O2CO). Vibrational frequencies of the guest molecules in cavities are determined. The phonon density of states for these compounds has been calculated. The influence of the encaged guest molecules on the vibrational spectrum of the host lattice has been investigated. The most visible changes are observed upon the insertion of guest molecules with large dipole moment or those positioned asymmetrically inside the Qβ clathrate cages. Peculiarities of different interactions have been revealed. Analysis of the calculated and experimental results have been carried out.
Electronic transition moment and rotational transition probabilities in CH. I. A 2Δ–X 2Π system
Authors: Luque Jorge , Crosley David R.
Published: July 2002

Vibrational band transition probabilities of the A–X electronic system of CH, v′=0, 1, and 2, have been measured in a low pressure flame using laser-induced fluorescence. These results improve earlier experiments that were complicated by energy transfer contributions, and expand measurements to very weak off-diagonal bands such as 0,2 and 1,3. The electronic transition moment derived from the vibrational branching ratio measurements differs slightly from ab initio calculations. Hönl–London factors using a mixed coupling scheme have been tested by experiment. Finally, a combination of these results with previously measured radiative lifetimes yields absolute absorption and emission coefficients for this system. A comparison between computed radiative lifetimes and collision-free experimental lifetimes suggests a predissociation rate in v′=0, N′≳17 of ∼1.5×105 s−1.
Diffusive dynamics of the reaction coordinate for protein folding funnels
Authors: Socci N. D. , Onuchic J. N. , Wolynes P. G.
Published: July 2002

The quantitative description of model protein folding kinetics using a diffusive collective reaction coordinate is examined. Direct folding kinetics, diffusional coefficients and free energy profiles are determined from Monte Carlo simulations of a 27-mer, 3 letter code lattice model, which corresponds roughly to a small helical protein. Analytic folding calculations, using simple diffusive rate theory, agree extremely well with the full simulation results. Folding in this system is best seen as a diffusive, funnel-like process.