bluuebunny/crossref_metadata_2025 huggingface dataset:
Melt pockets in phenocrysts and decompression rates of silicic magmas before fragmentation
Authors: Liu Yang , Anderson Alfred T. , Wilson Colin J. N.
Published: June 2007

A new method is presented to estimate the decompression rate of silicic magma prior to fragmentation and is applicable to rates of 1–100 kPa/s. This method uses the remaining concentrations of H2O and CO2 in bubble‐free cylindrical melt (now glass) pockets in phenocrysts that are connected to the surrounding bubbly melt. These melt pockets are common in volcanic rocks. During magma ascent, bubbles grow in the surrounding melt, and the concentrations of dissolved H2O and CO2 decrease, establishing a boundary condition of low volatile concentration at the outlet of the melt pocket. In turn, volatiles diffuse from the melt pocket into the surrounding bubbly magma and form a gradient that is time dependent. The volatile concentration at the inner limit of the melt pocket likewise diminishes with time. Knowing the diffusivity and solubility of H2O and CO2 permits the remaining H2O and CO2 contents to be estimated for different decompression rates using diffusion equations. We apply this approach to the measured H2O and CO2 contents in quartz‐hosted melt pockets in pumice from the phreatomagmatic 26.5 ka Oruanui eruption, Taupo, New Zealand. A decompression rate of 1–7 kPa/s (∼5–35 cm/s) was obtained for Oruanui magma.
Electric fields, cloud microphysics, and reflectivity in anvils of Florida thunderstorms
Authors: Dye J. E. , Bateman M. G. , Christian H. J. , Defer E. , Grainger C. A. , Hall W. D. , Krider E. P. , Lewis S. A. , Mach D. M. , Merceret F. J. , Willett J. C. , Willis P. T.
Published: June 2007

A coordinated aircraft–radar project that investigated the electric fields, cloud microphysics, and radar reflectivity of thunderstorm anvils near Kennedy Space Center is described. Measurements from two cases illustrate the extensive nature of the microphysics and electric field observations. As the aircraft flew from the edges of anvils into the interior, electric fields very frequently increased abruptly from ∼1 to >10 kV m1 even though the particle concentrations and radar reflectivity increased smoothly. The abrupt increase in field usually occurred when the aircraft entered regions with a reflectivity of 10–15 dBZ. We suggest that the abrupt increase in electric field was because the charge advection from the convective core did not occur across the entire breadth of the anvil and because the advection of charge was not constant in time. Also, some long‐lived anvils showed enhancement of electric field and reflectivity far downwind of the convective core. Screening layers were not detected near the edges of the anvils. Comparisons of electric field magnitude with particle concentration or reflectivity for a combined data set that included all anvil measurements showed a threshold behavior. When the average reflectivity, such as in a 3‐km cube, was less than approximately 5 dBZ, the electric field magnitude was <3 kV m1. Based on these findings, the Volume Averaged Height Integrated Radar Reflectivity (VAHIRR) is now being used by the NASA, the Air Force, the and Federal Aviation Administration in new Lightning Launch Commit Criteria as a diagnostic for high electric fields in anvils.
On the energetics of mean‐flow interactions with thermally dissipating gravity waves
Authors: Akmaev R. A.
Published: June 2007

Previous studies have demonstrated the importance of downgradient transport by dissipating waves and particularly of downward heat fluxes by gravity waves undergoing thermal dissipation. With a few exceptions, however, this effect has not been represented in gravity‐wave parameterizations commonly employed in global numerical models. A general expression relating the heat flux to the wave energy deposition rate caused by thermal dissipation is obtained within the standard linear‐theory approach. Although the flux is directed down the gradient of potential temperature, it is not proportional to its magnitude, i.e., is not formally diffusive as commonly represented. With necessary assumptions regarding the partitioning of the total wave energy deposition rate between the thermal and frictional channels, the heat flux may be calculated within any suitable parameterization of gravity‐wave drag. The general relation may also be used to estimate net heating rates from observations of wave heat transport. In a more general thermodynamical context, it is noted that gravity‐wave dissipation results in atmospheric entropy production as expected for a dissipative process. Without friction, entropy is produced under the conservation of the column potential enthalpy. Thermally dissipating waves thus represent an example of an entropy‐generating process hypothesized in the literature but not identified before. Although the downward heat transport results in a local cooling of upper levels, the integrated net effect of the wave energy deposition and heat transport combined is always heating of the whole atmospheric layer in which the dissipation occurs.
Retrievals of carbon monoxide profiles from MOPITT observations using lognormal a priori statistics
Authors: Deeter M. N. , Edwards D. P. , Gille J. C.
Published: June 2007

Optimal estimation methods, such as the “maximum a posteriori” solution, are commonly employed for retrieving profiles of atmospheric trace gases from satellite observations. To complement the information actually contained in the measured radiances, such methods exploit a priori information describing the gases’ variability characteristics. We show that in situ surface‐based data sets for carbon monoxide (CO) volume mixing ratio (VMR) indicate that the variability of CO is more accurately modeled in terms of a “lognormal” probability distribution function (PDF) than a “VMR‐normal” PDF. The VMR‐normal PDF is particularly poor at describing CO variability in unpolluted conditions. We also compare retrievals of carbon monoxide (CO) vertical profiles based on Measurements of Pollution in the Troposphere (MOPITT) observations for 1 day using both VMR‐normal and lognormal statistical models. Use of the lognormal model improves retrieval convergence and yields fewer profiles with unphysically small VMR values. Generally, these results highlight the importance of properly representing the variability of trace gas concentrations in optimal estimation‐based retrieval algorithms.
Trend of estimated actual evapotranspiration over China during 1960–2002
Authors: Gao Ge , Chen Deliang , Xu Chong‐yu , Simelton Elisabeth
Published: June 2007

In this study, the water balance methodology introduced by Thornthwaite and Mather (1955) is modified to estimate monthly actual evapotranspiration for 686 stations over China during 1960–2002. The modification is done by replacing the Thornthwaite potential evapotranspiration estimation with the Penman‐Monteith method. Temporal trend and spatial distribution of the estimated annual actual evapotranspiration during the past 43 years are analyzed. The results show that (1) the actual evapotranspiration had a decreasing trend in most areas east of 100°E, and there was an increasing trend in the west and the north parts of northeast China; (2) the spatial distribution of the trend for the actual evapotranspiration is similar to that of the potential evapotranspiration in south China, while the trends are opposite in north China; (3) for most parts of China, the change in precipitation played a key role for the change of estimated actual evapotranspiration, while in southeast China, the change of potential evapotranspiration appeared to be the major factor; and (4) in general, the hydrological cycle was intensified in western China, whereas it was weakened from the Yellow River basin northward.
Statistical maritime radar duct estimation using hybrid genetic algorithm–Markov chain Monte Carlo method
Authors: Yardim Caglar , Gerstoft Peter , Hodgkiss William S.
Published: June 2007

This paper addresses the problem of estimating the lower atmospheric refractivity (M profile) under nonstandard propagation conditions frequently encountered in low‐altitude maritime radar applications. This is done by statistically estimating the duct strength (range‐ and height‐dependent atmospheric index of refraction) from the sea surface reflected radar clutter. These environmental statistics can then be used to predict the radar performance. In previous work, genetic algorithms (GA) and Markov chain Monte Carlo (MCMC) samplers were used to calculate the atmospheric refractivity from returned radar clutter. Although GA is fast and estimates the maximum a posteriori (MAP) solution well, it poorly calculates the multidimensional integrals required to obtain the means, variances, and underlying posterior probability distribution functions of the estimated parameters. More accurate distributions and integral calculations can be obtained using MCMC samplers, such as the Metropolis‐Hastings and Gibbs sampling (GS) algorithms. Their drawback is that they require a large number of samples relative to the global optimization techniques such as GA and become impractical with an increasing number of unknowns. A hybrid GA‐MCMC method based on the nearest neighborhood algorithm is implemented in this paper. It is an improved GA method which improves integral calculation accuracy through hybridization with a MCMC sampler. Since the number of forward models is determined by GA, it requires fewer forward model samples than a MCMC, enabling inversion of atmospheric models with a larger number of unknowns.
Ar‐Ar dating of late Cenozoic basaltic volcanism in northern Syria: Implications for the history of incision by the River Euphrates and uplift of the northern Arabian Platform
Authors: Demir Tuncer , Westaway Rob , Bridgland David , Pringle Malcolm , Yurtmen Sema , Beck Anthony , Rowbotham George
Published: June 2007

Ar‐Ar dating of basalt flows capping terrace deposits of the River Euphrates in northern Syria has provided a new quantitative chronology for the late Cenozoic evolution of this important river system and for the associated history of surface uplift of the northern Arabian Platform through which it flows, a region of relatively strong crust that has experienced only slow deformation. Notably, fluvial deposits ∼65 m above the Euphrates are overlain by basalt dated to 2717 ± 20 ka, those ∼45 m above the river are overlain by basalt dated to 2116 ± 39 ka, and those 8–9 m above the river are overlain by basalt dated to 402 ± 11 ka. These new dates require the previous dating scheme, based on Paleolithic archaeology, to be revised; the Euphrates terrace deposits and the associated incised valley are much older than was previously thought. Rates of incision by the Euphrates, providing a proxy for regional surface uplift that is inferred to be the isostatic response to regional erosion, have varied significantly over the past ∼3 Ma, with indications that between ∼1.2 and ∼0.9 Ma, there was regional subsidence, which gave rise to fluvial aggradation. This unusual pattern, involving reversals in the sense of vertical crustal motions, is interpreted to be a consequence of a relatively cold and thin mobile lower crustal layer, no more than ∼5 km thick, evidently due to the presence of a much thicker underlying layer of mafic underplating at the base of the crust. This study thus indicates previously unsuspected complexity in the isostatic response to regional erosion in an area of high crustal stability.
A mass‐conserving fast algorithm to parameterize gravitational transport and deposition using digital elevation models
Authors: Gruber S.
Published: June 2007

Many natural phenomena such as snow avalanches, debris flows, or lahars involve gravitational transport and deposition that is largely governed by topography. This paper describes a fast and mass‐conserving algorithm to parameterize mass transport and deposition (MTD) over a digital elevation model. The algorithm is an extension to existing flow‐routing and terrain parameterization techniques. Its fast execution allows application over large areas or its incorporation into other models, e.g., distributed glacier mass balance in mountain topography. The proposed method does not include effects of kinetic energy and thus neglects potential uphill flow of fast‐moving mass. The application of MTD is described at the example of small and frequent snow avalanches in steep terrain for which the required parameters are approximated from published data. The algorithm MTD has been developed and is described for the gravitational redistribution of snow, but it is also applicable to other types of mass movements.
Plausible effect of atmospheric tides on the equatorial ionosphere observed by the FORMOSAT‐3/COSMIC: Three‐dimensional electron density structures
Authors: Lin C. H. , Wang W. , Hagan M. E. , Hsiao C. C. , Immel T. J. , Hsu M. L. , Liu J. Y. , Paxton L. J. , Fang T. W. , Liu C. H.
Published: June 2007

The plausible effect of atmospheric tides on the longitudinal structure of the equatorial ionosphere is observed by the FORMOSAT‐3/COSMIC (F3/C) constellation during September Equinox, 2006, near solar minimum. The longitudinal structure was first reported in IMAGE satellite airglow observations at the far‐ultraviolet (FUV) 135.6‐nm wavelength during March Equinox, 2002, near solar maximum. The global three‐dimensional ionospheric electron density observed by F3/C shows a prominent four‐peaked wave‐like longitudinal enhancement in the equatorial ionization anomaly (EIA). The vertical electron density structures observed by F3/C reveal that the feature exists mainly above 250 km altitude indicating that the feature is an F‐region phenomenon. The four longitudinal F‐region enhancements of the EIA peaks may result from a stronger equatorial plasma fountain at each longitude region produced by a stronger F‐region eastward electric field transmitted along the magnetic field lines from E‐region where longitudinal variations in atmospheric tides affect the ionospheric dynamo process.
Effect of non‐synchronous rotation on surface stress upon Europa: Constraints on surface rheology
Authors: Harada Yuji , Kurita Kei
Published: June 2007

In this study visco‐elastic tidal deformation on Europa associated with its non‐synchronous rotation is calculated. The induced surface stress is largely dependent on the rotation period and surface viscosity. On the basis of the relationship between the geological setting, rotation period, and surface rheology, the surface viscosity is inferred to be in the range between 1019 and 1020 Pa · s. The approach presented herein raises the possibility of estimating the surface viscosity on the satellite from not only the rheological law of ice but also the tectonic setting.
Reactive chlorine in the marine boundary layer in the outflow of polluted continental air: A model study
Authors: Pechtl S. , von Glasow R.
Published: June 2007

Results from a numerical one‐dimensional model show that continental pollution that is advected over the ocean leads to substantial acid displacement from newly emitted sea salt aerosol. Oxidation of HCl by OH and cycling between the gas and aerosol phase lead to a build‐up of nighttime Cl2 mixing ratios of up to 90 pmol mol−1 in our base scenario and 125 pmol mol−1 in a sensitivity study with high wind speeds. Therefore the recirculation of continental airmasses over the ocean and back to the coast might explain previous measurements of high nighttime Cl2 in onshore winds. Furthermore, the model runs suggest a substantial contribution of daytime Cl‐radical chemistry to the oxidation of volatile organic compounds under these conditions.
Strong rapid dipolarizations in Saturn's magnetotail: In situ evidence of reconnection
Authors: Jackman C. M. , Russell C. T. , Southwood D. J. , Arridge C. S. , Achilleos N. , Dougherty M. K.
Published: June 2007

The oppositely directed magnetic field in the kronian magnetic tail is expected eventually to reconnect across the current sheet, allowing plasma to escape in an anti‐solar direction down the tail. This reconnection process accelerates ions and electrons both toward and away from the planet, allowing the magnetotail to relax to a more dipolar configuration. Previous missions to Saturn shed no light on the possible presence of this critical process in the kronian magnetosphere. Recent Cassini measurements of the magnetic field in the magnetotail, reported herein, reveal strong, rapid dipolarizations between 40 and 50 Saturn radii (RS) downtail, signalling the episodic release of energy to the magnetosphere and ions to the solar wind.
Heat stress intensification in the Mediterranean climate change hotspot
Authors: Diffenbaugh Noah S. , Pal Jeremy S. , Giorgi Filippo , Gao Xuejie
Published: June 2007

We find that elevated greenhouse gas concentrations dramatically increase heat stress risk in the Mediterranean region, with the occurrence of hot extremes increasing by 200 to 500% throughout the region. This heat stress intensification is due to preferential warming of the hot tail of the daily temperature distribution, with 95th percentile maximum and minimum temperature magnitude increasing more than 75th percentile magnitude. This preferential warming of the hot tail is dictated in large part by a surface moisture feedback, with areas of greatest warm‐season drying showing the greatest increases in hot temperature extremes. Fine‐scale topographic and humidity effects help to further dictate the spatial variability of the heat stress response, with increases in dangerous Heat Index magnified in coastal areas. Further, emissions deceleration substantially mitigates heat stress intensification throughout the Mediterranean region, implying that emissions reductions could reduce the risk of increased heat stress in the coming decades.
Magnetospheric configurations from a high‐resolution data‐based magnetic field model
Authors: Tsyganenko N. A. , Sitnov M. I.
Published: June 2007

We present first results of the magnetospheric magnetic field modeling, based on large sets of spacecraft data and a high‐resolution expansion for the field of equatorial currents. In this approach, the field is expanded into a sum of orthogonal basis functions of different scales, capable to reproduce arbitrary radial and azimuthal variations of the geomagnetic field, including its noon‐midnight and dawn‐dusk asymmetries. Combined with the existing method to model the global field of Birkeland currents, the new approach offers a natural way to consistently represent the field of both the tail and symmetrical/partial ring currents. The proposed technique is particularly effective in the modeling of the inner magnetosphere, a stumbling block for the first‐principle approaches. The new model has been fitted to various subsets of data from Geotail, Polar, Cluster, IMP‐8, and GOES‐8, GOES‐9, GOES‐10, and GOES‐12 spacecraft, corresponding to different activity levels, solar wind IMF conditions, and storm phases. The obtained maps of the magnetic field reproduce most basic features of the magnetospheric structure, their dependence on the geomagnetic activity and interplanetary conditions, as well as characteristic changes associated with the main and recovery phases of magnetic storms.
Noise, cycles, and noisy cycles in finite populations
Authors: Pineda-Krch Mario
Published: June 2007

Periodic predator-prey dynamics in constant environments are usually taken as indicative of deterministic limit cycles. It is known, however, that demographic stochasticity in finite populations can also give rise to regular population cycles, even when the corresponding deterministic models predict a stable equilibrium. The existence of quasi-cycles substantially expands the scope for natural patterns of periodic population oscillations caused by ecological interactions, thereby complicating the conclusive interpretation of such patterns. It is, however, feasible and straightforward to accurately distinguish between the two types of cycle through the combined analysis of autocorrelations and marginal distributions of population sizes. By confronting these results with real ecological time series even short and imperfect time series allow quasi-cycles and limit cycles to be distinguished reliably.This work has been published in:Pineda-Krch M, Blok HJ, Dieckmann U, Doebeli M. 2007. A tale of two cycles - Distinguishing between true limit cycles and quasi-cycles in finite predator-prey populations. Oikos 116: 53-64.
A Genomic Comparison of in vivo and in vitro Brain Microvascular Endothelial Cells
Authors: Calabria Anthony R , Shusta Eric V
Published: June 2007

The blood—brain barrier (BBB) is composed of uniquely differentiated brain microvascular endothelial cells (BMEC). Often, it is of interest to replicate these attributes in the form of an in vitro model, and such models are widely used in the research community. However, the BMEC used to create in vitro BBB models de-differentiate in culture and lose many specialized characteristics. These changes are poorly understood at a molecular level, and little is known regarding the consequences of removing BMEC from their local in vivo microenvironment. To address these issues, suppression subtractive hybridization (SSH) was used to identify 25 gene transcripts that were differentially expressed between in vivo and in vitro BMEC. Genes affected included those involved in angiogenesis, transport and neurogenesis, and real-time quantitative polymerase chain reaction (qPCR) verified transcripts were primarily and significantly downregulated. Since this quantitative gene panel represented those BMEC characteristics lost upon culture, we used it to assess how culture manipulation, specifically BMEC purification and barrier induction by hydrocortisone, influenced the quality of in vitro models. Puromycin purification of BMEC elicited minimal differences compared with untreated BMEC, as assessed by qPCR. In contrast, qPCR-based gene panel analysis after induction with hydrocortisone indicated a modest shift of 10 of the 23 genes toward a more ‘ in vivo-like’ gene expression profile, which correlated with improved barrier phenotype. Genomic analysis of BMEC de-differentiation in culture has thus yielded a functionally diverse set of genes useful for comparing the in vitro and in vivo BBB.
Accumulation of Macrophage-Like Cells Expressing NG2 Proteoglycan and Iba1 in Ischemic Core of Rat Brain after Transient Middle Cerebral Artery Occlusion
Authors: Matsumoto Hiroaki , Kumon Yoshiaki , Watanabe Hideaki , Ohnishi Takanori , Shudou Masachika , Chuai Miao , Imai Yoshinori , Takahashi Hisaaki , Tanaka Junya
Published: June 2007

Although neurons and glia inevitably undergo degeneration in the core of ischemic lesions, many cells, particularly immune cells, infiltrate the core and survive in it. Such infiltrating cells may play certain roles in the regeneration and repair of damaged brain tissues. In this study, we characterized macrophage-like cells that accumulated in the ischemic core of a rat brain whose right middle cerebral artery was transiently occluded for 90 mins. Many of the accumulated macrophage-like cells expressed Iba1, a marker of macrophages/microglia, as well as NG2 chondroitin sulfate proteoglycan (NG2), which has been recognized as a marker of oligodendrocyte progenitor cells. Such macrophage-like cells were termed BINCs (brain Iba1+/NG2+ cells) to distinguish them from NG2/Iba1+ or NG2+/Iba1− cells that were also present in the perilesion and the contralateral hemisphere. Electron microscopy showed the localization of NG2 along the plasma membrane of cells that had many phagosomes and irregular-shaped or reniform heterochromatin-rich nuclei, which are characteristics of monocytes/macrophages. Brain Iba1+/NG2+ cells were highly proliferative and their number peaked at 7 days post-reperfusion. An immunoblot analysis of NG2 revealed the presence of two NG2s: one expressed by BINCs with a molecular weight of 300 kDa, and the other found in the contralateral hemisphere with a molecular weight of 290 kDa. Taken the various functions of NG2, BINCs may be involved in not only phagocytosis of degenerated cells but also the healing and regeneration of lesion cores.
Transient Global Cerebral Ischemia Induces a Massive Increase in Protein Sumoylation
Authors: Yang Wei , Sheng Huaxin , Warner David S , Paschen Wulf
Published: June 2007

A new group of proteins, small ubiquitin-like modifier (SUMO) proteins, has recently been identified and protein sumoylation has been shown to play a major role in various signal transduction pathways. Here, we report that transient global cerebral ischemia induces a marked increase in protein sumoylation. Mice were subjected to 10 mins severe forebrain ischemia followed by 3 or 6 h of reperfusion. Transient cerebral ischemia induced a massive increase in protein sumoylation by SUMO2/3 both in the hippocampus and cerebral cortex. SUMO2/3 conjugation was associated with a decrease in levels of free SUMO2/3. After ischemia, protein levels of the SUMO-conjugating enzyme Ubc9 were transiently decreased in the cortex but not in the hippocampus. We also exposed HT22 cells to arsenite, a respiratory poison that impairs cytoplasmic function and induces oxidative stress. Arsenite exposure induced a marked rise in protein sumoylation, implying that impairment of cytoplasmic function and oxidative stress may be involved in the massive post-ischemic activation of SUMO conjugation described here. Sumoylation of transcription factors has been shown to block their activation, with some exceptions such as the heat-shock factor and the hypoxia-responsive factor, where sumoylation blocks their degradation, and the nuclear factor-κB (NF-κB) essential modulator where sumoylation leads to an activation of NF-κB. Because protein sumoylation is known to be involved in the regulation of various biologic processes, the massive post-ischemic increase in protein sumoylation may play a critical role in defining the final outcome of neurons exposed to transient ischemia.
Note on the emergence of large scale coherent structure under small scale random bombardments: The discrete case
Authors: Majda Andrew , Wang Xiaoming
Published: June 2007

We continue our study on mathematical justification of the emergence of large scale coherent structure in a two dimensional fluid system under small scale random bombardments. We treat the case of small scale random bombardments at discrete times which is different from our earlier work [Commun. Pure Appl. Math. 59, 467 (2006)], where we approximated the small scale random kicks by a continuous in time random process. In the absence of geophysical effects, the large scale structure emerging out of the small scale random forcing is the same as the case of continuous in time forcing that we studied before.
Statistical analysis of small bubble dynamics in isotropic turbulence
Authors: Snyder Murray R. , Knio Omar M. , Katz Joseph , Le Maître Olivier P.
Published: June 2007

The dynamics and dispersion of small air bubbles in isotropic turbulence are analyzed computationally. The flow field is simulated using a pseudospectral code, while the bubble dynamics are analyzed by integration of a Lagrangian equation of motion that accounts for buoyancy, added mass, pressure, drag, and lift forces. Probability density functions (pdfs) of bubble velocities, lift and drag forces, and of field velocities and vorticities along bubble trajectories are used to analyze bubble dynamics. Lagrangian bubble trajectories are also employed to determine dispersion characteristics, following the theoretical development of Cushman and Moroni [Phys. Fluids 13, 75 (2001)]. Consistent with available experimental data, bubble rise velocities are increasingly suppressed with increasing turbulence intensity. The analysis also reveals that the vertical bubble velocities are characterized by asymmetric pdfs that are positive or negative-skewed dependent upon the nondimensional turbulence intensity and the Taylor length scale. The role of the lift force in moving the bubbles to the down-flow side of turbulent eddies, and consequently retarding their rise, is consistently observed in all analyses. The dispersion of 40μm bubbles and transition to Fickian behavior is shown to be weakly affected by the turbulence level. Larger, 400μm bubbles are shown to be more sensitive to turbulence level with transition to Fickian behavior delayed in low turbulence fields.
Rotationally resolved electronic spectra of 9,10-dihydrophenanthrene. A “floppy” molecule in the gas phase
Authors: Alvarez-Valtierra Leonardo , Pratt David W.
Published: June 2007

Rotationally resolved fluorescence excitation spectra of several bands in the S1←S0 electronic spectrum of 9,10-dihydrophenanthrene (DHPH) have been observed and assigned. Each band was fit using rigid rotor Hamiltonians in both electronic states. Analyses of these data reveal that DHPH has a nonplanar configuration in its S0 state with a dihedral angle between the aromatic rings (φ) of ∼21.5°. The data also show that excitation of DHPH with UV light results in a more planar structure of the molecule in the electronically excited state, with φ∼8.5°. Three prominent Franck-Condon progressions appear in the low resolution spectrum, all with fundamental frequencies lying below 300cm−1. Estimates of the potential energy surfaces along each of these coordinates have been obtained from analyses of the high resolution spectra. The remaining barrier to planarity in the S1 state is estimated to be ∼2650cm−1 along the bridge deformation mode and is substantially reduced by excitation of the molecule along the (orthogonal) ring twisting coordinate.
Interaction of ionic liquid with water with variation of water content in 1-butyl-3-methyl-imidazolium hexafluorophosphate ([bmim][PF6])/TX-100/water ternary microemulsions monitored by solvent and rotational relaxation of coumarin 153 and coumarin 490
Authors: Seth Debabrata , Chakraborty Anjan , Setua Palash , Sarkar Nilmoni
Published: June 2007

The interaction of water with room temperature ionic liquid (RTIL) [bmim][PF6] has been studied in [bmim][PF6]/TX-100/water ternary microemulsions by solvent and rotational relaxation of coumarin 153 (C-153) and coumarin 490 (C-490). The rotational relaxation and average solvation time of C-153 and C-490 gradually decrease with increase in water content of the microemulsions. The gradual increase in the size of the microemulsion with increase in w0 (w0=[water]∕[surfactant]) is evident from dynamic light scattering measurements. Consequently the mobility of the water molecules also increases. In comparison to pure water the retardation of solvation time in the RTIL containing ternary microemulsions is very less. The authors have also reported the solvation time of C-490 in neat [bmim][PF6]. The solvation time of C-490 in neat [bmim][PF6] is bimodal with time constants of 400ps and 1.10ns.
Equilibrium beta limits in a dipole configuration
Authors: Guazzotto L. , Freidberg J. P. , Kesner J.
Published: June 2007

The levitated dipole configuration is an innovative concept for fusion research. One of the main advantages of the dipole configuration is the possibility of stably confining high plasma pressure compared to the magnetic pressure, that is, the possibility of achieving high β (where β is the ratio between plasma pressure and magnetic pressure). The present work investigates the limit on equilibrium β existing in the dipole system. It is found that a limit exists, which is considerably modified by the presence of plasma rotation in the toroidal direction (the long way around the torus). Plasma anisotropy instead does not modify the limit in any appreciable way for the moderate anisotropies considered in this work.
Microwave enhanced ion-cut silicon layer transfer
Authors: Thompson D. C. , Alford T. L. , Mayer J. W. , Höchbauer T. , Lee J. K. , Nastasi M. , Lau S. S. , Theodore N. David , Chu Paul K.
Published: June 2007

Microwave heating has been used to decrease the time required for exfoliation of thin single-crystalline silicon layers onto insulator substrates using ion-cut processing. Samples exfoliated in a 2.45GHz, 1300W cavity applicator microwave system saw a decrease in incubation times as compared to conventional anneal processes. Rutherford backscattering spectrometry, cross sectional scanning electron microscopy, cross sectional transmission electron microscopy, and selective aperture electron diffraction were used to determine the transferred layer thickness and crystalline quality. The surface quality was determined by atomic force microscopy. Hall measurements were used to determine electrical properties as a function of radiation repair anneal times. Results of physical and electrical characterizations demonstrate that the end products of microwave enhanced ion-cut processing do not appreciably differ from those using more traditional means of exfoliation.
Comparative study of charge division in substituted benzene cations
Authors: Lee Kang Taek , Sung Jiha , Lee Kwang Jun , Kim Hyung Min , Han Kyu Young , Park Young Dong , Kim Seong Keun
Published: June 2007

A recently proposed phenomenon of charge division in a molecular cation [K. T. Lee et al., J. Am. Chem. Soc. 129, 2588 (2007)] was examined in a number of molecules by experiment and theory. We investigated the spatial distribution of electrostatic charge in the cation of the following benzene derivatives: n-propylbenzene (PB), 3-phenylpropionic acid (PPA), 2-phenylethyl alcohol (PEAL), and 2-phenylethylamine (PEA). A density functional theory calculation indicated that the positive charge was divided into two cationic charge cores in both conformers of PEA+, while it is localized mainly on the phenyl group in PB+, PPA+, and PEAL+. This finding was experimentally verified by the characteristic range of electronic transition of these species reflected in the fragmentation pattern of the mass spectra. The degree of charge division in PEA+ was slightly less than in the cationic conformers of L-phenylalanine in its subgroup II. The charge distribution in a phenyl-containing cation is suggested to depend on whether there exists a functional group that can act as a competing charge core against the phenyl ring.
Stokes-Einstein relation for pure simple fluids
Authors: Cappelezzo M. , Capellari C. A. , Pezzin S. H. , Coelho L. A. F.
Published: June 2007

The authors employed the equilibrium molecular dynamics technique to calculate the self-diffusion coefficient and the shear viscosity for simple fluids that obey the Lennard-Jones 6-12 potential in order to investigate the validity of the Stokes-Einstein (SE) relation for pure simple fluids. They performed calculations in a broad range of density and temperature in order to test the SE relation. The main goal of this work is to exactly calculate the constant, here denominated by α, present in the SE relation. Also, a modified SE relation where a fluid density is raised to a power in the usual expression is compared to the classical expression. According to the authors’ simulations slip boundary conditions (α=4) can be satisfied in some state points. An intermediate value of α=5 was found in some regions of the phase diagram confirming the mode coupling theory. In addition depending on the phase diagram point and the definition of hydrodynamics radius, stick boundary condition (α=6) can be reproduced. The authors investigated the role of the hydrodynamic radius in the SE relation using three different definitions. The authors also present calculations for α in a hard-sphere system showing that the slip boundary conditions hold at very high density. They discuss possible explanations for their results and the role of the hydrodynamic radius for different definitions in the SE relation.
Moderate-temperature thermoelectric properties of TiCoSb-based half-Heusler compounds Ti1−xTaxCoSb
Authors: Zhou Min , Chen Lidong , Feng Chude , Wang Dongli , Li Jing-Feng
Published: June 2007

Ta-doped Ti1−xTaxCoSb (0⩽x⩽0.08) half-Heusler compounds were synthesized by melting and annealing process. Their thermoelectric properties were studied in the temperature range of 300–900K. The Ti1−xTaxCoSb compounds exhibit negative Seebeck coefficients with considerably large absolute values. With increasing Ta substitution, the electrical conductivity was greatly increased, but the thermal conductivity was reduced. Because of the combined effects of increased electrical conductivity and reduced thermal conductivity, the thermoelectric performance of Ti1−xTaxCoSb alloys was apparently improved by doping Ta. The dimensionless figure of merit of 0.3 was obtained for Ti0.92Ta0.08CoSb compound at 900K. This value is about ten times larger than that of the undoped TiCoSb compound.
Particle image velocimetry study of turbulent flow over transverse square ribs in an asymmetric diffuser
Authors: Tachie Mark F.
Published: June 2007

The objective of this paper is to study the combined effects of rib roughness and adverse pressure gradient produced in an asymmetric diffuser on turbulent flows. The two-dimensional asymmetric diffuser was comprised of a straight flat floor and a curved roof. The diffuser section was preceded and followed by straight parallel walls. The complete test conditions were comprised of a reference smooth floor and repeated arrays of transverse square ribs glued onto the floor to produce three pitch-to-height ratios, p∕k=3, 6, and 8. The curved roof was kept smooth in all the experiments. For each of the four test conditions, a particle image velocimetry was used to conduct detailed velocity measurements within the diverging section and also at locations upstream and downstream of the diverging section. From these measurements, the mean streamlines, mean velocities, turbulent intensities, Reynolds shear stress, and production terms in the transport equations for the turbulent kinetic energy and Reynolds stresses were obtained. The results obtained in the diverging section showed that the boundary layers that developed on the ribs thickened considerably at the expense of those adjacent to the roof opposite to the ribs. The streamlines and mean velocity profiles over the ribs showed that adverse pressure gradient increased the roughness sublayer substantially. Adverse pressure gradient and rib roughness also increased the drag and levels of the turbulent intensities, Reynolds shear stress and production terms compared to smooth-wall zero pressure gradient turbulent boundary layer. It appears, however, that adverse pressure gradient enhanced turbulence more effectively than it increased drag.
Quasiballistic transport in nanometer Si metal-oxide-semiconductor field-effect transistors: Experimental and Monte Carlo analysis
Authors: Łusakowski J. , Martínez M. J. Martín , Rengel R. , González T. , Tauk R. , Meziani Y. M. , Knap W. , Boeuf F. , Skotnicki T.
Published: June 2007

Room temperature electron mobility (μ) in nanometer Si metal-oxide-semiconductor field-effect transistors (MOSFETs) with gate length (LG) down to 30 nm was determined by the magnetoresistance method. A decrease of μ with the decrease of LG was observed. Monte Carlo simulations of electron transport in nanometer MOSFETs were carried out for realistic devices as a function of LG. The dependence with LG and electron concentration of simulated mobility and transmission coefficient agree with experimental data. An analysis of scattering events and time of flight gives evidence of the presence of ballistic motion in the investigated structures and proves its influence on mobility degradation in short transistors. The results give arguments that interpretation of the magnetoresistance coefficient as the square of the mobility is valid also in the case of quasiballistic electron transport.
On the performance of local, semilocal, and nonlocal exchange-correlation functionals on transition metal molecules
Authors: Ramírez-Solís A.
Published: June 2007

The lowest singlet-triplet transition (XΣ+1-Σ+3) of AgI has been used to study systematically the performance of local [local density approximation (LDA)], semilocal [generalized gradient approximation (GGA)], and nonlocal (semiempiric hybrid and meta)-type exchange-correlation functionals on a transition metal molecule where dynamic electronic correlation effects are essential. Previous benchmark ab initio calculations showed that the triplet ground state possesses a shallow well in the Franck-Condon region before becoming repulsive at longer internuclear distance [A. Ramírez-Solís, J. Chem. Phys. 118, 104 (2003)]. Several density functional theory (DFT) descriptions are compared with the benchmark complete active space self-consistent-field+averaged coupled pair functional results, using the same relativistic effective core potentials and optimized Gaussian basis sets. A rather unreliable performance of exchange-correlation functionals was found when ascending the various rungs in DFT Jacob’s ladder for this complex molecule. While some of the simpler (LDA and GGA) functionals correctly predict the presence of a short-distance maximum for the Σ+3 state, more sophisticated hybrid and meta-functionals lead to totally repulsive or oscillating curves for the ground triplet state. A thorough discussion addressing the local versus nonlocal character of the exchange and correlation effects on the triplet potential curve is presented. The author concludes that any new efforts directed at producing more accurate exchange-correlation functionals must take into account the more complex electronic structure arising in transition metal molecules, whether these efforts follow the dominant pragmatic semiempiric trend or the more philosophically correct nonempiric pathway to develop better exchange-correlation functionals; only then will the Kohn-Sham version of DFT make the necessary improvements to correctly describe the electronic structure of complex transition metal systems.
Comparative study between continuum and atomistic approaches of liquid flow through a finite length cylindrical nanopore
Authors: Huang Cunkui , Choi Phillip Y. K. , Nandakumar K. , Kostiuk Larry W.
Published: June 2007

Steady state pressure driven flow of liquid argon through a finite length cylindrical nanopore was investigated numerically by classical Navier-Stokes (NS) hydrodynamic models and nonequilibrium molecular dynamics (MD) simulations. In both approaches, the nanopore was nominally 2.2nm in diameter and 6nm long. For the MD simulations, the intermolecular properties of the walls were specified independently from the liquid. Comparisons between the approaches were made in terms of the gross feature of total flow rate through the nanopore, as well as the more refined considerations of the spatial distributions of pressure, density, and velocity. The results showed that for the NS equations to predict the same trends in total flow rate with increasing pressure difference as the MD simulation, submodels for variations in density and viscosity with pressure are needed to be included. The classical NS boundary conditions quantitatively agreed with the flow rate predictions from MD simulations only under the condition of having a neutral-like solid-liquid interaction. Under these conditions, the NS and MD models also agreed well in streamwise distributions of pressure, density, and velocity, but not in the radial direction.
Lagrangian coherent structures in n-dimensional systems
Authors: Lekien Francois , Shadden Shawn C. , Marsden Jerrold E.
Published: June 2007

Numerical simulations and experimental observations reveal that unsteady fluid systems can be divided into regions of qualitatively different dynamics. The key to understanding transport and stirring is to identify the dynamic boundaries between these almost-invariant regions. Recently, ridges in finite-time Lyapunov exponent fields have been used to define such hyperbolic, almost material, Lagrangian coherent structures in two-dimensional systems. The objective of this paper is to develop and apply a similar theory in higher dimensional spaces. While the separatrix nature of these structures is their most important property, a necessary condition is their almost material nature. This property is addressed in this paper. These results are applied to a model of Rayleigh-Bénard convection based on a three-dimensional extension of the model of Solomon and Gollub.
Filling and emptying transitions in cylindrical channels: A density functional approach
Authors: Husowitz B. , Talanquer V.
Published: June 2007

The authors use density functional theory in a square gradient approximation to investigate capillary condensation and evaporation in cylindrical channels of finite lengths. The model allows them to systematically explore the effect of the pore’s length, width, and surface fields on the location of the transition between “empty” (vapor-filled) and “full” (liquid-filled) states. In general, their results indicate that decreasing the length of the channel drastically reduces the range of pore widths where a transition between liquidlike and vaporlike configurations may be observed. For the wide pores, the transition occurs at very low pressures where the liquid is no longer stable, while for the narrow pores, the transition is hindered by the solid-fluid interactions that favor the vapor phase in lyophobic pores. For the limited range of sizes where the transition can occur, the authors’ results confirm the existence of two competing minima that may explain the density oscillations observed in a computer simulation of nanochannels. Comparisons between these results with those generated using a phenomenological model based on the capillary approximation indicate that this simplified approach yields fairly good predictions for medium size pores. However, the capillary approach fails to properly describe the properties of the very small and very large pores.
Evolution of a forced stratified mixing layer
Authors: Rotter J. , Fernando H. J. S. , Kit E.
Published: June 2007

Laboratory measurements were carried out in a spatially developing stably stratified shear layer generated downstream of a splitter plate. The instabilities were controlled using a flapper spanning the entire shear layer, with the flapper forced at the fastest growing frequency of the primary [Kelvin-Helmholtz (KH)] instability. The measurements were taken as the KH instabilities roll up, break down, and degenerate into stratified turbulence. Both stratified and homogeneous shear layers were considered, the latter acting as the “baseline” case. The measurements included the streamwise and vertical velocities (made using X-wire hot film probes), which allowed calculation of the mean and rms velocities, turbulent kinetic energy (TKE) dissipation, and TKE production. The density and its gradients were measured using miniature conductivity probes. The measurements and flow visualization elicited interesting features of KH evolution, namely that KH billows may be turbulent from the onset, the TKE dissipation is largest at early stages of evolution, the production of TKE is a maximum at the breakdown of billows, the decay of turbulence to fossilized motions and concomitant formation of fine (layered) structure occur rapidly after the breakdown of billows, and episodic rebirth of (zombie) turbulence develops before a final permanently fossilized state is achieved.
Spin wave interferometer employing a local nonuniformity of the effective magnetic field
Authors: Vasiliev S. V. , Kruglyak V. V. , Sokolovskii M. L. , Kuchko A. N.
Published: June 2007

We have investigated scattering of exchange spin waves by a model nonuniformity of the effective magnetic field. In particular, certain profiles of the nonuniformity are characterized by a total transmission of the spin wave intensity while inducing large shifts to the phase of transmitted spin waves. These properties are discussed in the context of potential application within a spin wave logic device—a spin wave interferometer containing such a nonuniformity in one of its branches. We demonstrate limitations imposed upon the size and the speed of operation of such a device by a requirement that it be controlled by an external uniform magnetic field.