bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of the distribution of insectivorous bats in Israel
Authors: YOM‐TOV YORAM , KADMON RONEN
Published: December 2003

Data on localities from which insectivorous bats were collected or sighted in Israel were compiled into a Geographical Information System (GIS) in order to analyse patterns of species distribution. By intersecting precipitation and temperature data with spatially‐referenced data on species observations stored in the GIS we determined the ‘climatic envelope’ of each species and constructed predictive maps which show the potential distribution of each species. Using cluster analysis, the bats were classified into three main biogeographic groups according to their distribution, namely desert, Mediterranean, and widespread in Israel. The potential distribution maps of all the species indicate that there are areas which have suitable bat habitats but from which bats have never been collected or observed. In the Mediterranean region of Israel, this is attributed to a large reduction in population size due to fumigation of caves, cave visitation and secondary poisoning.
Examination of the ‘founder effect’ in biodiversity research: patterns and imbalances in the published literature
Authors: FRANCE ROBERT , RIGG CATHERINE
Published: December 2003

We reviewed 2524 articles published from 1987 to 1995 in five international journals, Conservation Biology, Biological Conservation, Biodiversity and Conservation, Ecological Applications, and the Journal of Applied Ecology, to assess patterns and imbalances in biodiversity research in the following subject areas: locations where research was conducted, types of organisms studied, types of ecosystems studied, types of methodologies used, and types of stresses investigated. Biodiversity research was found to be narrowly focused with little deviation from its initial course set of being a discipline concerned largely with the implications of forest habitat loss on charismatic terrestrial megafauna; i.e. the ‘founder effect’ is very much in evidence. The ‘sleeping dragon’ of ignored marine biodiversity is really symptomatic of a wider problem, and can perhaps be referred to as a ‘hibernating hydra’ of many imbalances due to, for example, under‐representations of research from developing nations, or on amphibians and invertebrates, alpine and arctic systems, sociological components and chemical stresses etc. It is time to take steps to actively awaken the creature so that conservation biology/applied ecology can become more pluralistic in scope.
Formation of platinum-based silicide contacts: Kinetics, stoichiometry, and current drive capabilities
Authors: Larrieu G. , Dubois E. , Wallart X. , Baie X. , Katcki J.
Published: December 2003

A detailed analysis of the formation of Pt2Si and PtSi silicides is proposed, based on x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and electrical characterizations. Published kinetics of the Pt2Si and PtSi transformations under ultrahigh vacuum condition are consolidated on the basis of XPS measurements performed during an in situ annealing at a constant heating rate. At room temperature, an incomplete PtxSi reaction is clearly identified by XPS depth profiling. Using rapid thermal annealing at 300, 400, and 500 °C, the sequential Pt–Pt2Si–PtSi reaction chain is found to be completed within 2 min. Outdiffusion of silicon to the top surface is shown to be responsible for the formation of a thin SiO2 capping layer at 500 °C. Pileup of oxygen occurring at the Pt2Si/Pt reaction front is clearly identified as an inhibiting factor of the silicidation mechanism. Another incomplete reaction scheme limited to the unique formation of Pt2Si is exemplified in the case of ultra thin silicon-on-insulator films. Finally, current drive measurements on PtSi Schottky contacts have allowed us to identify 300 °C as the optimum annealing temperature while TEM cross sections demonstrate the formation of a smooth and continuous PtSi/Si interface at 300 °C.
Wave simulation in partially frozen porous media with fractal freezing conditions
Authors: Carcione José M. , Santos Juan E. , Ravazzoli Claudia L. , Helle Hans B.
Published: December 2003

A recent article [J. M. Carcione and G. Seriani, J. Comput. Phys. 170, 676 (2001)] proposes a modeling algorithm for wave simulation in a three-phase porous medium composed of sand grains, ice, and water. The differential equations hold for uniform water (ice) content. Here, we obtain the variable-porosity differential equations by using the analogy with the two-phase case and the complementary energy theorem. The displacements of the rock and ice frames and the variation of fluid content are the generalized coordinates, and the stress components and fluid pressure are the generalized forces. We simulate wave propagation in a frozen porous medium with fractal variations of porosity and, therefore, realistic freezing conditions.
Defect relaxation at the origin of reduction in mobile proton generation at Si/SiO2/Si interfaces during annealing in H2
Authors: Stesmans A.
Published: December 2003

Previous work [Vanheusden and Devine, Appl. Phys. Lett. 76, 3109 (2000)] has reported that subjecting Si/SiO2/Si capacitors to a 450 °C preanneal in hydrogen impedes formation of protons in the buried oxide during the subsequent generation anneal at 600 °C in H2. In interpreting, a key role in proton generation was assigned to Si dangling bond (Pb-) type defects, yet with unusual assumptions as to their atomic hydrogen production nature. Here, an explanation of the preanneal effect is offered based on annealing induced structural relaxation of (Pb-type) interface defects residing in edge regions of the capacitor, affecting their atomic hydrogen generation efficiency through altering the spreads in the defect activation energies for passivation in H2 and dissociation. Thus the preannealing effect is seen as a direct result of the occurrence of substantial spreads in regions of enhanced strain.
A new methodology and model for characterization of nucleation and growth kinetics in solids
Authors: Safarik D. J. , Mullins C. B.
Published: December 2003

Herein we describe a new experimental protocol and develop a corresponding kinetic model of phase change that together enable the decoupling of simultaneous nucleation and growth processes and quantification of their kinetics. Growth is effectively isolated from nucleation by dividing a phase transition into two isothermal stages: prenucleation, where product crystallites nucleate and grow concurrently, and growth, in which transformation is completed essentially entirely by the expansion of these “seed” grains. Using the model, the temperature dependence of growth may be extracted from converted fraction versus time data for specimens that are appreciably larger than the mean crystal grain size. Similarly, the growth velocity can be ascertained from thin films that exhibit thickness-dependent transition kinetics owing to geometric constraints imposed by the specimen size. The combination of grain growth kinetics and information derived from the transformation of unseeded materials then allows the subsequent calculation of the temperature dependence and absolute rates of nucleation. A preliminary assessment of this method (for the crystallization of amorphous solid water) indicates that the model captures the basic physics of “seeded” transformations and that the technique appears to be a viable method of quantifying simultaneous nucleation and growth in some solids.
Normal ferroelectric to relaxor behavior in laser ablated Ca-doped barium titanate thin films
Authors: Victor P. , Ranjith R. , Krupanidhi S. B.
Published: December 2003

Ba 1−x Ca x TiO 3 thin films (x=0.05 to 0.17) were deposited on Pt-coated Si substrates using a pulsed excimer laser ablation technique. X-ray diffraction and scanning electron microscope studies of the Ba1−xCaxTiO3 targets exhibit a polycrystalline nature and thin films also show the same but with a significant orientation along the (111) direction. Secondary ion mass spectrometer analysis reveals the presence of a sharper interface existing at the thin film substrate. The dielectric phase transition temperature of (Ba1−xCax)TiO3 targets were sharp and the transition temperature was found to decrease from 140 °C to 110 °C with an increase in the values of x (x>0.05 at. %). The laser ablated Ca-doped BaTiO3 thin films deposited at 100 mTorr exhibited a higher dielectric constant, lower dielectric loss, and an anomalous decrease in phase transition was observed. The anomalous phase transition decrease was ascribed to the occupancy of the Ca2+ in the Ti4+ site. There was a cross over from the sharp to diffused phase transition for a higher composition of Ca (>9 at. %) in BaTiO3 thin films. The diffuse transition behavior might be due to the larger number of the Ca2+ ions occupying the Ti4+ site, eventually introducing larger compositional and structural disorder and this occupancy leads to the generation of oxygen vacancies. The activation energy obtained from impedance spectroscopy was 1.05 eV, and was attributed to the oxygen vacancy motion.
Young’s modulus and hysteretic losses of 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3: single versus polycrystalline forms
Authors: Viehland D. , Li Jie-Fang
Published: December 2003

Young’s modulus (Y) of 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 has been investigated for polycrystals and single crystals oriented along 〈001〉, 〈110〉, and 〈111〉. The value of Y〈001〉 and Y〈110〉 for single crystals was dramatically lower than either Y〈111〉 or the polycrystalline averaged value. For ceramics, field dependent measurements revealed a significant softening of Y, which was not observed for oriented crystals. For both forms, the elastic energy densities were similar, however significantly higher hysteretic losses were found for polycrystals.
Mode transition for power dissipation induced by driving frequency in capacitively coupled plasma
Authors: You S. J. , Kim H. C. , Chung C. W. , Chang H. Y. , Lee J. K.
Published: December 2003

We measured electrical characteristics of capacitively coupled plasma at low pressure (2.67 Pa) with different driving frequencies. From these measurements, we observed a significant change in discharge power characteristics during the frequency increase. While increasing the frequency, a square dependence of power characteristics (P∼I2) changes to a linear dependence (P∼I). This observed result reflects that a power dissipation mode transition from an ion-dominated dissipation mode to an electron-dominated dissipation mode takes place during the driving frequency increase. Both the results calculated from a simple sheath model and a particle-in-cell simulation are in a good agreement with the experimental data.
Effect of polymer–polymer interactions on the surface tension of colloid–polymer mixtures
Authors: Moncho-Jordá A. , Rotenberg B. , Louis A. A.
Published: December 2003

The density profile and surface tension for the interface of phase-separated colloid–polymer mixtures have been studied in the framework of the square gradient approximation for both ideal and interacting polymers in good solvent. The calculations show that in the presence of polymer–polymer excluded volume interactions the interfaces have lower widths and surface tensions compared to the case of ideal polymers. These results are a direct consequence of the shorter range and smaller depth of the depletion potential between colloidal particles induced by interacting polymers.
Mechanism of unidirectional motions of chiral molecular motors driven by linearly polarized pulses
Authors: Hoki Kunihito , Yamaki Masahiro , Koseki Shiro , Fujimura Yuichi
Published: December 2003

The mechanism of the unidirectional rotational motion of a chiral molecular motor driven by linearly polarized laser pulses was theoretically studied. A simple aldehyde molecule was adopted as a chiral molecular motor, in which a formyl group (–CHO) was the rotating part of the motor. Temporal evolutions of the instantaneous angular momentum averaged over an ensemble of randomly oriented motors were taken as a measure of the unidirectional motion. The contour plots of the averaged instantaneous angular momentum were obtained by using a quantum master equation approach that took into account relaxation effects and a classical trajectory approach. Two regimes are found in the contour plots. One is an intense laser field regime in which the laser–motor interaction energy exceeds the asymmetric potential barrier. In this regime, the motors are unidirectionally driven in the intuitive direction, i.e., the gentle slope of the potential. The other regime is a subthreshold laser intensity regime in which unintuitive rotational motions also occur. This unintuitive rotation is found to be a quantum effect, as indicated by contour plots calculated by taking into account temperature effects.
Proton damage effects on GaAs/GaAlAs vertical cavity surface emitting lasers
Authors: Le Metayer P. , Gilard O. , Germanicus R. , Campillo D. , Ledu F. , Cazes J. , Falo W. , Chatry C.
Published: December 2003

A series of proton irradiations of GaAs/GaAlAs vertical cavity surface emitting lasers (VCSELs) has been carried out for the purpose of assessing the suitability of these devices for space applications. The irradiations were performed on biased and unbiased devices at energies of 30, 40, 50, and 60 MeV. Both current versus voltage (I–V) and optical power versus current (P–I) characteristics were measured before and after each irradiation phase. A simple circuit equivalent model for the VCSEL has been developed to analyze proton damage effects through the extraction of electrical parameters. The current threshold of VCSEL is shown to be the only important parameter modified by a high fluence (up to 1012 protons/cm2) irradiation. Changes in the threshold current show radiation generated recombination centers to be the main cause of degradation. Due to carrier injection annealing related effects, we observed that unbiased devices show the greatest relative threshold increase (between 15% and 20% at 1013 protons/cm2). The threshold current damage factor was also calculated. The analysis of the I–V characteristics shows that in the range of low fluences (1010–1012 protons/cm2) radiation induced ordering effects may compete with the usual radiation degradation that we observed at higher fluences. Consequently, the nonionizing energy loss approach, which is extensively used to predict the degradation of electronic devices under a full spectrum of energetic particles, is deemed to be not yet applicable for prediction of end-of-life performances of VCSELs.
Systematic study of effects of growth conditions on the (nano-, meso-, micro)size and (one-, two-, three-dimensional) shape of GaN single crystals grown by a direct reaction of Ga with ammonia
Authors: ElAhl Aya Moustafa Sayed , He Maoqi , Zhou Peizhen , Harris G. L. , Salamanca-Riba Lourdes , Felt Frederick , Shaw Harry C. , Sharma Ashok , Jah Muzar , Lakins Darryl , Steiner Todd , Mohammad S. Noor
Published: December 2003

A series of experiments have been conducted to systematically study the effects of growth conditions (NH3 flow rate, growth temperature, chamber pressure, and growth location) on the size (nano, meso, or micro) and the shape (one, two, or three dimensional) of GaN single crystal products grown by a direct reaction of Ga with NH3. A growth map with a wider range of experimental parameters was developed; it has three distinct zones. The size and shape of the products in every zone were found to depend on both temperature and NH3 flow rate with other growth conditions fixed. An effective surface diffusion length consisting of the Ga atomic surface diffusion length and the GaN molecular surface diffusion length, and the anisotropy of the Ga surface diffusion length and the GaN growth rate in different growth directions were introduced into the growth model, in such a way that it allowed successful explanation of all observed results. The optimal growth parameters could thus be determined, which conclusively demonstrated that nanowires with uniform diameter, clear crystal structure, length larger than 1 mm, uniform location distribution, and high yield can be obtained. Such a growth map based on in-depth understanding of the growth mechanism provides a clear direction for growing various materials with desired size and shape.
Theoretical studies of intersystem crossing effects in the O(3P, 1D)+H2 reaction
Authors: Maiti Biswajit , Schatz George C.
Published: December 2003

We have studied the influence of intersystem crossing on the reaction dynamics of the O+H2 reaction by performing trajectory surface hopping (TSH) calculations with accurate potential-energy surfaces and global spin–orbit coupling surfaces that we have generated using a four state model proposed by Hoffmann and Schatz. In the TSH calculations, we develop a new mixed representation that treats the reactant and product asymptotes in the adiabatic representation, and the singlet–triplet crossing region in the diabatic representation. This representation thus correctly describes O and OH fine structure-resolved cross sections, and it also treats intersystem crossing effects arising from the singlet–triplet crossing. Our calculations are based on the 1 3A′ and 1 3A″ states of Walch and Kuppermann, and the 1 1A′ state of Dobbyn and Knowles. The globally determined spin–orbit coupling matrix is derived from complete active space self-consistent field calculations using the two-electron Breit–Pauli Hamiltonian. Our dynamics calculations show that the triplet O+H2 cross section is modestly increased (up to 20% at collision energies >10 kcal/mol above the reactive threshold) by intersystem crossing, and product rotational excitation is also increased. In addition, we find that the OH spin–orbit distributions favor the Π3/22 state by a 2:1 ratio over Π1/22. This result is consistent with observations for O atom reactions with alkanes.
Determination of the width of the carrier recombination zone in organic light-emitting diodes
Authors: Kalinowski J. , Palilis L. C. , Kim W. H. , Kafafi Z. H.
Published: December 2003

Bilayer organic light-emitting diodes based on tris-(8-hydroxyquinolinato) aluminum III have been fabricated where the thickness of the light-emitting layer was varied between 10 and 80 nm while maintaining a constant total thickness of the organic layers. The electroluminescence quantum efficiency of the devices was measured as a function of the emitter thickness, and used to determine the width of the carrier recombination zone at different electric fields. The width of the carrier recombination zone is found to decrease with an increase in electric field [from 70 nm (E=0.75 MV/cm) to 40 nm (E=1.0 MV/cm)]. It is also related to the field-dependent carrier injection efficiency. An estimate of the light output coupling factor (0.4) is also given based on this analysis.
A systematic ab initio study of the structure and vibrational spectroscopy of HgCl2, HgBr2, and HgBrCl
Authors: Balabanov Nikolai B. , Peterson Kirk A.
Published: December 2003

Near-equilibrium potential energy and dipole moment functions have been calculated for the linear mercury halide molecules HgCl2, HgBr2, and HgBrCl using highly correlated coupled cluster wave functions and large correlation consistent basis sets. After extrapolation to the complete basis set limit, additional corrections due to core-valence correlation and relativity were included in the final anharmonic potential energy functions (PESs). The fitted PESs and dipole moment functions were then used in variational calculations of the low-lying rovibrational band origins. Both the asymmetric stretch and symmetric bend fundamental bands are predicted to carry significant oscillator strength for all three species. A wide range of spectroscopic properties are accurately predicted, which should facilitate the observation of these species by high resolution spectroscopy.
A new approach to calculating the memory kernel of the generalized quantum master equation for an arbitrary system–bath coupling
Authors: Shi Qiang , Geva Eitan
Published: December 2003

The Nakajima–Zwanzig generalized quantum master equation provides a general, and formally exact, prescription for simulating the reduced dynamics of a quantum system coupled to a quantum bath. In this equation, the memory kernel accounts for the influence of the bath on the system’s dynamics. The standard approach is based on using a perturbative treatment of the system–bath coupling for calculating this kernel, and is therefore restricted to systems weakly coupled to the bath. In this paper, we propose a new approach for calculating the memory kernel for an arbitrary system–bath coupling. The memory kernel is obtained by solving a set of two coupled integral equations that relate it to a new type of two-time system-dependent bath correlation functions. The feasibility of the method is demonstrated in the case of an asymetrical two-level system linearly coupled to a harmonic bath.
Quantum dynamics scattering study of AB+CDE reactions: A seven-dimensional treatment for the H2+C2H reaction
Authors: Wang Dunyou
Published: December 2003

A time-dependent wave packet approach is presented for the quantum dynamics study of the AB+CDE reaction system for zero total angular momentum. A seven-degree-of-freedom calculation is employed to study the chemical reaction of H2+C2H→H+C2H2 by treating C2H as a linear molecule. Initial state selected reaction probabilities are presented for various initial rovibrational states. This study shows that the vibrational excitation of H2 enhances the reaction probability, whereas the excitation of C2H has only a small effect on the reactivity. An integral cross section is also reported for the initial ground states of H2 and C2H. The theoretical and experimental results agree with each other very well when the calculated seven-dimensional results are adjusted to account for the lower transition state barrier heights found in recent ab initio calculations.
Highly predissociative levels of CH3S (A 2A1) detected with degenerate four-wave mixing
Authors: Liu Chin-Ping , Matsuda Yoshiyuki , Lee Yuan-Pern
Published: December 2003

Following photodissociation of dimethyl disulfide (CH3SSCH3) in a supersonic jet with a KrF excimer laser at 248 nm, degenerate four-wave mixing spectra of the A 2A1←X 2E3/2 transition of CH3S in a spectral region 317–377 nm are recorded using a dye laser. Observed features extend to 5020 cm−1 above the origin, much greater than 1490 cm−1 for fluorescence spectra and 2979 cm−1 for fluorescence depletion spectra. In addition to extension to higher members of the known progressions 3v and 213v associated with C–S stretching (ν3) and CH3 umbrella (ν2) modes, three new progressions 113v, 3v41, and 213v41 (or 11213v) involving the symmetric CH3 stretching (ν1) and asymmetric CH3 (ν4) stretching modes of the A 2A1 state of CH3S are tentatively assigned, yielding values of ν1=2966 cm−1 and ν4=3006 cm−1 that are consistent with theoretical predictions. Vibrational spectral parameters for ν1, ν2, ν3, and ν4 modes of the A 2A1 state of CH3S are derived.
Theoretical calculations of the Xe chemical shifts in cryptophane cages
Authors: Sears Devin N. , Jameson Cynthia J.
Published: December 2003

Toward an understanding of the factors that affect the chemical shift in the Xe nuclear magnetic resonance spectrum of Xe atoms trapped in cages which may have applications as biosensors, we carry out calculations of Xe nuclear magnetic shielding using Hartree–Fock and density functional methods. The resulting values for various Xe positions within the cage can be described by an analytical function of Xe and cage atom coordinates. This shielding function is used in Monte Carlo canonical averaging of a Xe atom within cryptophane cages to investigate the dependence of the Xe chemical shifts on cage size (cryptophane-A versus cryptophane-E), isotopic substitution, and temperature. We compare our theoretical average Xe chemical shifts with the experimental values in four types of cryptophane cages, and with the temperature and isotopic dependence of Xe chemical shifts in cryptophane-A, and achieve a quantitative understanding of the factors that influence the Xe chemical shifts in these cages. The predicted effects on the Xe chemical shifts of mechanical distortion of the cryptophane-A cage provide some insight into the applications of Xe in cages as biosensors.
Self-consistent density matrix algorithm for electronic structure and excitations of molecules and aggregates
Authors: Mukamel Shaul , Berman Oleg
Published: December 2003

An ab initio density matrix algorithm for electronic structure computations of many-electron systems is proposed. The reduced single-electron density matrices are derived by mapping the density functional theory nonlinear optical response functions onto an effective multilevel system. These density matrices are then used as a zeroth order iteration into self-consistent equations whose solution should yield the exact energies and the complete set of (transition and diagonal) single-electron density matrices. Higher order (n electron) density matrices are not computed explicitly. The linear and nonlinear optical response functions may be obtained at a low computational cost. Application is made to constructing an exciton Hamiltonian for molecular aggregates using density matrices of isolated molecules, avoiding electronic structure calculations of the entire aggregate.
Structure of polyelectrolytes in 3:1 salt solutions
Authors: Sarraguça J. M. G. , Skepö M. , Pais A. A. C. C. , Linse P.
Published: December 2003

Polyion conformation and the distribution of small ions near the polyion have been investigated using Monte Carlo simulations. The systems of interest contained one polyion and its monovalent counterions, and variable amount of a 3:1 salt. With monovalent counterions only, the polyion is strongly extended. As salt is added, the polyion folds, and the most compact and spherical-like structure appears at a three-fold excess of the trivalent counterions. The polyion exerts a strong influence on the nearest-neighbor distance among the trivalent ions, an effect being relevant for energy transfer reactions.
Current conduction and plasma distribution on dielectric (velvet) explosive emission cathodes
Authors: Saveliev Yuri M. , Sibbett Wilson , Parkes David M.
Published: December 2003

Distributions of emission centers (ECs) on planar explosive emission dielectric (velvet) cathodes at moderate electric fields of 30–70 kV/cm and pulse durations of ∼2 μs with the use of a fast framing camera have been investigated. The experimental results suggest a link between the EC distribution and current conduction paths through which the electron current is supplied to the cathode plasma. On bare velvet, the surrounding metal electrode was shown not to be of primary importance for the current conduction, instead, the current is supplied to explosive emission plasma mainly through the base of the velvet fabric. Development of a circle of brighter and larger ECs along the perimeter of the cathode was normally observed shortly after the beginning of the high voltage pulse. These ECs were found to be a major factor in the diode perveance growth and instability. Domination of the ECs on the cathode periphery has been suppressed by adding a pattern of well defined current conduction points, e.g., perforations of the velvet fabric, to the cathode design.
Laser propagation and energy absorption by an argon spark
Authors: Bindhu C. V. , Harilal S. S. , Tillack M. S. , Najmabadi F. , Gaeris A. C.
Published: December 2003

The laser propagation and energy absorption of an argon spark induced by a laser at different pressures is investigated. 8 ns pulses from a frequency-doubled Q-switched Nd:YAG laser are used to create the spark. The pressure of the argon is varied from 1 atm to 10 Torr. Significant energy absorption by the plasma is observed at high pressures (>100 Torr) while there is negligible absorption when the pressure is lower than 50 Torr. The plasma kernel showed distinct behavior with respect to laser energy. At a laser energy well above the breakdown threshold, the spark moved only in the backward direction and the forward component was absent indicating the strong absorption of the laser by the spark front. A spiky behavior is observed in the transmitted temporal profiles of the laser at higher energies and at high pressures and can be due to the formation of a self-regulating regime.
Dynamics of pulse phenomena in helium dielectric-barrier atmospheric-pressure glow discharges
Authors: Shin Jichul , Raja Laxminarayan L.
Published: December 2003

A study of pulse phenomena in conventional parallel-plate dielectric-barrier controlled atmospheric-pressure glow (DB-APG) discharges in helium is reported. Stable DB-APG discharges are found to occur at arbitrarily low frequencies as long as the gas voltage exceeds the Paschen breakdown voltage, i.e., no lower limit of ∼1 kHz exists for DB-APG operation. The interpulse preionization phenomenon is found to be an artifact of typical ∼10 kHz operation of DB-APG discharges and does not play a role in the formation of a stable pulse. Multiple pulses result from repeated temporally separated breakdown events in the discharge. A relatively simple zero-dimensional model that treats only the Paschen breakdown mechanism in the discharge and charge trapping phenomena at the dielectric surfaces can be used to simulate all important qualitative features of DB-APG phenomena. Finally, we show that control of the pulse intensity, number of pulses in a pulse train, and the time interval between pulse trains can be achieved using “ramp-plus-plateau” voltage input wave forms rather than typical sinusoidal wave forms.
Focusing of surface-acoustic-wave fields on (100) GaAs surfaces
Authors: de Lima M. M. , Alsina F. , Seidel W. , Santos P. V.
Published: December 2003

Focused surface-acoustic waves (SAWs) provide a way to reach intense acoustic fields for electro- and optoacoustic applications on semiconductors. We have investigated the focusing of SAWs by interdigital transducers (IDTs) deposited on (100)-oriented GaAs substrates. The focusing IDTs have curved fingers designed to account for the acoustic anisotropy of the substrate. Different factors that affect focusing, such as the aperture angle and the configuration of the IDT fingers, were systematically addressed. We show that the focusing performance can be considerably improved by appropriate choice of the IDT metal pads, which, under appropriate conditions, create an acoustic waveguide within the IDT. We demonstrate the generation of narrow (full width at half maximum of approx 15 μm), high-frequency (0.5 GHz), continuous SAW beams with vertical displacement as high as 4 nm collimated over distances that exceed 100 μm.
Application of database methods to the prediction of B3LYP-optimized polyhedral water cluster geometries and electronic energies
Authors: Anick David J.
Published: December 2003

A method is described for a rapid prediction of B3LYP-optimized geometries for polyhedral water clusters (PWCs). Starting with a database of 121 B3LYP-optimized PWCs containing 2277 H-bonds, linear regressions yield formulas correlating O–O distances, O–O–O angles, and H–O–H orientation parameters, with local and global cluster descriptors. The formulas predict O–O distances with a rms error of 0.85 pm to 1.29 pm and predict O–O–O angles with a rms error of 0.6° to 2.2°. An algorithm is given which uses the O–O and O–O–O formulas to determine coordinates for the oxygen nuclei of a PWC. The H–O–H formulas then determine positions for two H’s at each O. For 15 test clusters, the gap between the electronic energy of the predicted geometry and the true B3LYP optimum ranges from 0.11 to 0.54 kcal/mol or 4 to 18 cal/mol per H-bond. Linear regression also identifies 14 parameters that strongly correlate with PWC electronic energy. These descriptors include the number of H-bonds in which both oxygens carry a non-H-bonding H, the number of quadrilateral faces, the number of symmetric angles in 5- and in 6-sided faces, and the square of the cluster’s estimated dipole moment.
Cross-relaxation between macromolecular and solvent spins: The role of long-range dipole couplings
Authors: Halle Bertil
Published: December 2003

Nuclear spin relaxation by intermolecular dipole–dipole interactions between macromolecular and solvent nuclear moments forms the basis of a widely used method for investigating macromolecular solvation. In particular, intermolecular cross-relaxation [or nuclear Overhauser effect (NOE)] between protein and water protons has been used to probe the mobility of water molecules interacting with the protein surface. The method rests on the assumption that the intermolecular NOE is of short (4–5 Å) range and thus provides information about the mobility of individual water molecules in hydration sites near the monitored protein protons. Here, we present a theoretical analysis of the spectral density function (SDF) that governs the cross-relaxation rates in the laboratory-fixed and rotating frames. In contrast to the r−6 dependence of the intramolecular NOEs used for structure determination, the intermolecular NOE is shown to be long-ranged with important contributions from thousands of water molecules. For a consistent interpretation of such NOEs, it is necessary to use a model that explictly incorporates motionally retarded hydration water molecules as well as unperturbed bulk water molecules. We formulate a diffusion model with a nonuniform solvent mobility and solve it to obtain an analytical expression for the SDF. Calculations with this nonuniform diffusion model demonstrate that intermolecular NOEs with surface protons are dominated by long-range dipole couplings to bulk water and therefore provide little or no information about hydration dynamics. The physical basis of this unexpected phenomenon is that the characteristic time scale for relaxation-inducing fluctuations is longer for the more numerous remote water molecules, despite their higher mobility. The analytical results presented here are generally applicable to intermolecular dipolar relaxation of like or unlike (nuclear or electron) spins in a variety of experimental situations.
Time dependent density functional theory study of charge-transfer and intramolecular electronic excitations in acetone–water systems
Authors: Bernasconi Leonardo , Sprik Michiel , Hutter Jürg
Published: December 2003

A recently introduced formulation of time dependent linear response density functional theory within the plane-wave pseudopotential framework [J. Hutter, J. Chem. Phys. 118, 3928 (2003)] is applied to the study of solvent shift and intensity enhancement effects of the A21 n→π* electronic transition in acetone, treating solute and solvent at the same level of theory. We propose a suitable formalism for computing transition intensities based on the modern theory of polarization, which is applicable to condensed-phase and finite systems alike. The gain in intensity brought about by thermal fluctuations is studied in molecular acetone at room temperature, and in gas-phase (CH3)2CO⋅(H2O)2 at 25 K. The latter system is characterized by the appearance of relatively intense features in the low-energy region of the spectrum, attributable to spurious solvent→solute charge-transfer excitations created by deficiencies in the DFT methodology. The n→π* transition can be partially isolated from the charge-transfer bands, yielding a blueshift of 0.17 eV with respect to gas-phase acetone. This analysis is then carried over to a solution of acetone in water, where further complications are encountered in the from of a solute→solvent charge transfer excitations overlapping with the n→π* band. The optically active occupied states are found to be largely localized on either solute or solvent, and using this feature we were again able to isolate the physical n→π* band and compute the solvatochromic shift. The result of 0.19 eV is in good agreement with experiment, as is the general increase in the mean oscillator strength of the transition. The unphysical charge transfers are interpreted in terms of degeneracies in the spectrum of orbital energies of the aqueous acetone solution.
Autoionization in I and I2 observed by multiphoton ionization and photoelectron spectroscopy: Two atomic iodine Rydberg series built on the …5s25p4 3P1 ion core and revised value for the I+(3P1) limit
Authors: Gu Y.-Y. , Chojnacki A. M. , Zietkiewicz C. J. , Senin A. A. , Eden J. G.
Published: December 2003

Two well-developed Rydberg series of atomic iodine, built on the …5s25p4 3P1 ion core and assigned to nd[2]5/2 and ns[1]3/2 states with principal quantum numbers as high as n=47, have been observed by two photon (566⩽λ≲600 nm) ionization of I(6s 2P3/2), and time-of-flight photoelectron spectroscopy and mass spectrometry. The data reported here extend the nd[2]5/2 and ns[1]3/2 series by 27 and 23 terms, respectively. The limit for both series, 91 387.9±1.3 cm−1, revises upwards by 5.8 cm−1 the value for the I+(3P1) state energy suggested by Minnhagen in 1962 [Ark. Fys. 21, 415 (1962)]. The quantum defects for the nd[2]5/2 and ns[1]3/2 series were determined to be 2.44±0.03 and 4.03±0.02, respectively. Several terms of a third, weaker series were also detected and tentatively assigned to nd[2]3/2 states, also having the P13 ion core. Photoelectron peaks associated with direct, five photon ionization of I2 have been observed and are attributed to electronic autoionization of A 2Π(3/2)u ion core Rydberg states into the X 2Π(1/2)g,(3/2)g ionization continua.
Collision-induced absorption in the rototranslational band of dense hydrogen gas
Authors: Gustafsson Magnus , Frommhold Lothar , Bailly Denise , Bouanich Jean-Pierre , Brodbeck Claude
Published: December 2003

Measurements of the H2–H2 collision-induced absorption spectra at temperatures of 297.5 and 77.5 K are reported in the frequency range from 1900 to 2260 cm−1 at gas densities ranging from 51 to 610 amagat. Ab initio calculations of the absorption are carried out for comparison with the measurements. In these calculations, for the lower temperature close-coupled equations describe the H2–H2 scattering in the presence of a weak electromagnetic radiation field; the anisotropy of the H2–H2 interaction is accounted for. For the room temperature calculations, the isotropic potential approximation is employed. Agreement of measured and calculated spectral shapes is observed. However, in the far wing, at large frequencies (≳2000 cm−1), discrepancies of measured and calculated spectral intensities are observed which are somewhat larger than the combined, estimated uncertainties of theory and measurement. These differences remain unexplained at this stage.
Density-functional theory for polar fluids at functionalized surfaces. I. Fluid-wall association
Authors: Tripathi Sandeep , Chapman Walter G.
Published: December 2003

We present a novel perturbation density-functional theory (DFT) to describe adsorption of associating fluids on surfaces activated with polar sites to which fluid molecules can bond or associate, such as water adsorbing on activated carbon, silica, clay minerals, etc. Association is modeled within the framework of first order thermodynamic perturbation theory (TPT1). In this first of two papers, we explore in detail the changes brought about in a system due to fluid–wall (FW) association. Hence fluid–fluid association is not considered here. However, the theory can be coupled with existing DF theories of associating fluids to study the interplay between the wall–fluid and fluid–fluid association as is shown in a future paper by S. Tripathi. The proposed theory, in excellent agreement with simulations, shows that FW association significantly changes the fluid structure and adsorption behavior. The theory accurately predicts the distribution of bonded and nonbonded species away from the surface, adsorption characteristics and surface coverage over a range of conditions commonly found in several real systems. The most striking feature of the theory is that in addition to properties away from the wall, it also estimates the distribution of the fluid along the surface, or the three-dimensional (3D) structure, despite being one-dimensional (1D) in form.
Core level spectroscopy and reactivity of coadsorbed K+O layers on reconstructed Rh(110) surfaces
Authors: Günther S. , Marbach H. , Imbihl R. , Baraldi A. , Lizzit S. , Kiskinova M.
Published: December 2003

The bonding character of oxygen and potassium and the interactions in K+O coadsorbed layers on a Rh(110) surface have been studied by means of high resolution x-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). The Rh 3d5/2, K 2p, and O 1s spectra and LEED patterns were used as fingerprints for the interfacial reactions and the structural changes. Dramatic changes in the chemical state of the substrate occur in the presence of dense K+O adlayers, when the oxygen coverage exceeds one monolayer. The effect of coadsorbed potassium on the “reactivity” of oxygen was probed by following the evolution of the O 1s spectra during titration with H2. The enhanced surface capacity for oxygen adsorption and the reduced rate of H2O formation with increasing K coverage were discussed considering the influence of K on the dissociative adsorption of O2 and H2 and on the bonding of the coadsorbed species.
Lattice model of equilibrium polymerization. IV. Influence of activation, chemical initiation, chain scission and fusion, and chain stiffness on polymerization and phase separation
Authors: Dudowicz Jacek , Freed Karl F. , Douglas Jack F.
Published: December 2003

The influence of thermal activation, chemical initiation, chain fragmentation, and chain stiffness on basic thermodynamic properties of equilibrium polymerization solutions is systematically investigated using a Flory–Huggins type lattice model. The properties treated include the average chain length L, extent of polymerization Φ, Helmholtz free energy F, configurational entropy S, specific heat CV, polymerization transition temperature Tp, osmotic pressure Π, and the second and third virial coefficients, A2 and A3. The dependence of the critical temperature Tc and critical composition φc (volume fraction of associating species) on the enthalpy Δhp and entropy Δsp of polymerization and on the strength εFH of the FH effective monomer–solvent van der Waals interaction (χ=εFH/T) is also analyzed as an illustration of the strong coupling between phase separation and polymerization. For a given polymerization model, both Tc and φc, normalized by their values in the absence of polymerization, are functions of the dimensionless “sticking energy” hε≡(|Δhp|/R)/(2εFH) (where R is the gas constant) and Δsp.
Molecular-dynamics simulations of the ethanol liquid–vapor interface
Authors: Taylor Ramona S. , Shields Roseanne L.
Published: December 2003

Molecular-dynamics computer simulations have been utilized to study the thermodynamic, structural, and dynamical properties of the liquid–vapor interface of ethanol as a function of temperature. Both the ethanol united-atom and the ethanol all-atom optimized potential models for liquid simulations (OPLS) have been investigated. For both potentials, the calculated ethanol surface tensions as a function of temperature and the orientation of ethanol molecules at the liquid–vapor interface are in agreement with the corresponding experimental data. However, given the degree of agreement between the experimental and simulation data, the OPLS all-atom model better represents the types of interactions present at the liquid–vapor interface of ethanol. In addition, for both potentials the self-diffusion constant of ethanol is found to be larger at the surface than in the bulk of the liquid.
Stationary points and dynamics in high-dimensional systems
Authors: Wales David J. , Doye Jonathan P. K.
Published: December 2003

We present some new theoretical and computational results for the stationary points of bulk systems. First we demonstrate how the potential energy surface can be partitioned into catchment basins associated with every stationary point using a combination of Newton–Raphson and eigenvector-following techniques. Numerical results are presented for a 256-atom supercell representation of a binary Lennard-Jones system. We then derive analytical formulae for the number of stationary points as a function of both system size and the Hessian index, using a framework based upon weakly interacting subsystems. This analysis reveals a simple relation between the total number of stationary points, the number of local minima, and the number of transition states connected on average to each minimum. Finally we calculate two measures of localization for the displacements corresponding to Hessian eigenvectors in samples of stationary points obtained from the Newton–Raphson-based geometry optimization scheme. Systematic differences are found between the properties of eigenvectors corresponding to positive and negative Hessian eigenvalues, and localized character is most pronounced for stationary points with low values of the Hessian index.
Monolithic coupling of a SU8 waveguide to a silicon photodiode
Authors: Nathan M. , Levy O. , Goldfarb I. , Ruzin A.
Published: December 2003

We present quantitative results of light coupling from SU8 waveguides into silicon p-n photodiodes in monolithically integrated structures. Multimode, 12 μm thick, and 20 μm wide SU8 waveguides were fabricated to overlap 40×180 μm2 photodiodes, with three different waveguide-photodiode overlap lengths. The attenuation due to leaky-mode coupling in the overlap area was then calculated from photocurrent measurements. The overlap attenuation ranged from a minimum of 2.2 dB per mm overlap length to a maximum of about 3 dB/mm, comparing favorably with reported nonpolymeric waveguide-Si photodiode attenuations.
Formation of single-phase oxide nanoclusters: Cu2O on SrTiO3(100)
Authors: Lyubinetsky I. , Thevuthasan S. , McCready D. E. , Baer D. R.
Published: December 2003

Selective formation of the single-phase nanoclusters of Cu2O on SrTiO3(100) substrates in the size range of 10–50 nm is found to occur only in a very narrow oxygen plasma-assisted molecular-beam epitaxy growth parameter window, in comparison with the bulk phase diagram (for oxygen pressure versus temperature). There are distinctive parameter regions, where multiple phaselike forms coexist (CuO/Cu2O and Cu2O/Cu), in agreement with theoretical prediction for small systems, and as opposite to the sharp phase boundaries for the bulk. Observed changes in the nanocluster composition are found to correlate with differences in cluster morphologies.
Effects of boundary roughness on a Q factor of whispering-gallery-mode lasing microdisk cavities
Authors: Rahachou A. I. , Zozoulenko I. V.
Published: December 2003

We perform numerical studies of the effect of sidewall imperfections on the resonant state broadening of the optical microdisk cavities for lasing applications. We demonstrate that even small edge roughness (≲λ/30) causes a drastic degradation of high-Q whispering gallery (WG)-mode resonances reducing their Q values by many orders of magnitude. At the same time, low-Q WG resonances are rather insensitive to the surface roughness. The results of numerical simulation obtained using the scattering matrix technique, are analyzed and explained in terms of wave reflection at a curved dielectric interface combined with the examination of Poincaré surface of sections in the classical ray picture.