bluuebunny/crossref_metadata_2025 huggingface dataset:
Three water sites in upper mantle olivine and the role of titanium in the water weakening mechanism
Authors: Walker A. M. , Hermann J. , Berry A. J. , O'Neill H. St. C.
Published: May 2007

Infrared spectroscopy on synthetic olivines has established that there are at least four different mechanisms by which hydrogen is incorporated into the crystal structure. Two mechanisms occur in the system MgO‐SiO2‐H2O associated with silicon and magnesium vacancies, respectively. A third mechanism is associated with trivalent cation substitution, commonly Fe3+ in natural olivine, while the fourth mechanism, which is the one most prevalent in natural olivines from the spinel‐peridotite facies of the Earth's upper mantle, is associated with Ti4+ [Berry et al., 2005]. Here first principles calculations based on density functional theory are used to derive the structure and relative energies of the two defects in the pure MgO‐SiO2‐H2O system, and possible hydrogen‐bearing and hydrogen‐free point defects in Ti4+‐doped forsterite. Calculated structures are used to compare the predicted orientation of the O‐H bonds with the experimentally determined polarization. The energies are used to discuss how different regimes of chemical environment, temperature (T), pressure (P), and both water content and water fugacity impact on which of the different hydroxyl substitution mechanisms are thermodynamically stable. We find that given the presence of Ti impurities, the most stable mechanism involves the formation of silicon vacancies containing two protons charge balanced by a Ti4+ cation occupying an adjacent octahedral site. This mechanism leads to the water‐mediated formation of silicon vacancies. As silicon is known to be the most slowly diffusing species in olivine, this provides a credible explanation of the observed water weakening effect in olivine.
Regional mapping of the crustal structure in southern California from receiver functions
Authors: Yan Z. , Clayton R. W.
Published: May 2007

Lateral variations of the crustal structure in southern California are determined from receiver function (RF) studies using data from the Southern California Seismic Network broadband stations and Los Angeles Regional Seismic Experiment surveys. The results include crustal thickness estimates at the stations themselves, and where possible, cross sections are drawn. The large‐scale Moho depth variation pattern generally correlates well with the current status of the Mesozoic batholith: Deep Moho of 35–39 km is observed beneath the western Peninsula Ranges, Sierra Nevada, and San Bernardino Mountains, where the batholith is relatively intact, and shallow Moho of 26–32 km is observed in the Mojave Desert, where the batholith is highly deformed and disrupted. High‐resolution lateral variations of the crustal structure for individual geographic provinces are investigated, and distinctive features are identified. The crustal structure is strongly heterogeneous beneath the central Transverse Ranges, and deep Moho of 36–39 km is locally observed beneath several station groups in the western San Gabriel Mountains. Moho is relatively flat and smooth beneath the western Mojave Desert but gets shallower and complicated to the east. Anomalous RFs are observed at two stations in the eastern Mojave Desert, where a Moho step of ∼8–10 km is found between the NW and SE back‐azimuthal groups of station DAN in the Fenner Valley. Asymmetric extension of the Salton Trough is inferred from the Moho geometry. Depth extension of several major faults, such as the San Andreas Fault and San Gabriel Fault, to the Moho is inferred.
Scaling and parametrization of clear‐sky solar radiation over complex topography
Authors: Essery Richard , Marks Danny
Published: May 2007

Solar radiation at the land surface is influenced by slope, aspect, shadows, and obstruction of the sky, all of which vary over a wide range of length scales in regions of complex topography, with important consequences for the surface energy balance. Atmospheric models, however, generally assume the surface to be flat on subgrid scales. For four areas in North America, ranging in latitude from 39°N to 69°N and in topography from rolling to mountainous, we simulate spatial patterns of clear‐sky incoming solar radiation. It is found that distributions of slope components and variations in shaded area with solar elevation can be approximated by simple functions that scale to each of the areas studied. From these results, parametrizations are developed for averages, standard deviations, and distributions of direct‐beam and diffuse solar radiation. Results from these parametrizations, and from a modified form of a simpler parametrization presented previously, compare well with statistics from the spatial simulations. The only topographic input required by the parametrizations is the standard deviation of slope components; this parameter is again found to have simple scaling relationships with the resolution and extent of the underlying elevation grid and with the standard deviation of elevation.
Hygroscopic behavior of NaCl‐bearing natural aerosol particles using environmental transmission electron microscopy
Authors: Wise Matthew E. , Semeniuk Trudi A. , Bruintjes Roelof , Martin Scot T. , Russell Lynn M. , Buseck Peter R.
Published: May 2007

We used conventional and environmental transmission electron microscopes to determine morphology, composition, and water uptake of individual natural inorganic aerosol particles collected from industrial pollution plumes and from clean and polluted marine environments. Five particle types are described in detail. They range from relatively insoluble mineral grains to internally mixed particles containing NaCl with other soluble or relatively insoluble material. We studied the hygroscopic behavior of these particles from 0 to 100% relative humidity (RH). Relatively insoluble materials do not take up water over the experimental RH range. Single crystals of NaCl from both natural and laboratory sources have a well‐defined deliquescence point of approximately 76% RH at 291 K. NaCl‐bearing aggregate particles appear to deliquesce between 74 and 76% RH (same RH within error) when NaCl is internally mixed with relatively insoluble phases, but the particles deliquesce at lower RH when aggregated with other soluble phases such as NaNO3. For all NaCl‐bearing particles studied, hygroscopic growth is pronounced above 76% RH, and water uptake by the particles is dominated by the soluble phase. Furthermore, the soluble phase initiating deliquescence controls the locus of further hygroscopic growth of the aggregate particle. Our results demonstrate that composition and mixing state affect water uptake of natural aerosol particles. Furthermore, internally mixed particles are confirmed to deliquesce at lower RH values than predicted from the individual components.
A detailed evaluation of the Eta‐CMAQ forecast model performance for O3, its related precursors, and meteorological parameters during the 2004 ICARTT study
Authors: Yu Shaocai , Mathur Rohit , Schere Kenneth , Kang Daiwen , Pleim Jonathan , Otte Tanya L.
Published: May 2007

The Eta‐Community Multiscale Air Quality (CMAQ) model's forecast performance for ozone (O3), its precursors, and meteorological parameters has been assessed over the eastern United States with the observations obtained by aircraft, ship, ozonesonde, and lidar and two surface networks (AIRNOW and AIRMAP) during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) study. The results at the AIRNOW sites show that the model was able to reproduce the day‐to‐day variations of observed daily maximum 8‐hour O3 and captured the majority (73%) of observed daily maximum 8‐hour O3 within a factor of 1.5 with normalized mean bias of 22%. The model in general reproduced O3 vertical distributions on most of the days at low altitudes, but consistent overestimations above ∼6 km are evident because of a combination of effects related to the specifications of lateral boundary conditions from the Global Forecast System (GFS) as well as the model's coarse vertical resolution in the upper free troposphere. The model captured the vertical variation patterns of the observed values for other parameters (HNO3, SO2, NO2, HCHO, and NOy_sum (NOy_sum = NO + NO2 + HNO3 + PAN)) with some exceptions, depending on the studied areas and air mass characteristics. The consistent underestimation of CO by ∼30% from surface to high altitudes is partly attributed to the inadequate representation of the transport of pollution associated with Alaska forest fires from outside the domain. The model exhibited good performance for marine or continental clear airflows from the east/north/northwest/south and southwest flows influenced only by Boston city plumes but overestimation for southeast flows influenced by the long‐range transport of urban plumes from both New York City and Boston.
Photolysis of pyruvic acid in ice: Possible relevance to CO and CO2 ice core record anomalies
Authors: Guzmán M. I. , Hoffmann M. R. , Colussi A. J.
Published: May 2007

The abnormal spikes detected in some CO and CO2 polar ice core records indicate persistent chemical activity in glacial ice. Since CO and CO2 spikes are correlated, and their amplitudes scale with reported CO/CO2 yields for the photolysis of dissolved natural organic matter, a common photochemical source is implicated. Given that sufficient actinic radiation is constantly generated throughout ice by cosmic muons (Colussi and Hoffmann, 2003), it remains to be shown that the photolyses of typical organic contaminants proceed by similar mechanisms in water and ice. Here we report that the photodecarboxylation of pyruvic acid (PA, an ubiquitous ice contaminant) indeed leads to the same products nearly as efficiently in both media. CO2 is promptly released from frozen PA/H2O films upon illumination and continues to evolve after photolysis. By analogy with our studies in water (Guzmán et al., 2006b), we infer that 3PA* reacts with PA in ice producing CH3C(O)C(O)O· and (CH3 (OH)C(O)OH) radicals. The barrierless decarboxylation, CH3C(O)C(O)O· → CH3C(O)· + CO2, accounts for prompt CO2 emissions down to ∼140 K. Bimolecular radical reactions subsequently ensue in fluid molecular environments, both in water and ice, leading to metastable intermediates that decarboxylate immediately in water, but protractedly in ice. The overall quantum yield of CO2 production in the λ ~313 nm photolysis of PA in ice at 250 K is ∼60% of that in water at 293 K. The in situ photolysis of natural organic matter is, therefore, a plausible explanation of CO and CO2 ice core record anomalies.
Annual accumulation over recent centuries at four sites in central Greenland
Authors: Banta J. Ryan , McConnell Joseph R.
Published: May 2007

Accurate accumulation records are fundamental to understanding mass balance of the Greenland ice sheet. Ice cores can provide multicentury, high‐resolution ground‐based point estimates of water accumulation. In 2003 and 2004, four new ice cores were collected in the central western (D4 and D5) and Summit (Sandy and Katie) regions of Greenland. Annual layer counting based on multiple glaciochemical parameters (primarily hydrogen peroxide, sea salt, and dust proxies) was used to date the ice cores, resulting in annual dating. The bottom depth ages for D4, D5, Sandy, and Katie were estimated at 1738, 1673, 1844, and 1934 A.D., with mean annual accumulation rates of 41.4, 35.2, 22.4, and 22.4 cmweq yr−1, respectively. These new records were in good agreement with previous accumulation maps and four previously published records from nearby multicentury ice cores. Spatial variability was quantified through an analysis of variance, thereby allowing for more meaningful comparisons to atmospheric processes. More regionally representative accumulation records were constructed for the central western and Summit regions spanning recent centuries using a principle component analysis of both the new and previously reported ice core measurements. These regional records exhibited 6–7% interannual variability (1 standard deviation of the mean) and had lower spatial variability uncertainties than individual accumulation records. Correlations indicate that ∼20% of the variance in the central western region was explained by the North Atlantic Oscillation (NAO), and suggests that an array of more northern ice cores may correlate more closely with NAO.
Retrieving surface parameters for climate models from Moderate Resolution Imaging Spectroradiometer (MODIS)‐Multiangle Imaging Spectroradiometer (MISR) albedo products
Authors: Pinty B. , Lavergne T. , Voßbeck M. , Kaminski T. , Aussedat O. , Giering R. , Gobron N. , Taberner M. , Verstraete M. M. , Widlowski J.‐L.
Published: May 2007

We present a computer‐efficient software package enabling us to assimilate operational remote‐sensing flux products into a state‐of‐the‐art two‐stream radiation transfer scheme suitable for climate models. This package implements the adjoint and Hessian codes, generated using automatic differentiation techniques, of a cost function balancing (1) the deviation from the a priori knowledge on the model parameter values and (2) the misfit between the observed remote‐sensing fluxes and the two‐stream model simulations. The individual weights of these contributions are specified notably via covariance matrices of the uncertainties in the a priori knowledge on the model parameters and the measurements. The proposed procedure delivers a Gaussian approximation of the PDFs of the retrieved model parameter values. The a posteriori covariance matrix is further exploited to evaluate, in turn, the posterior probability density functions of the radiant fluxes simulated by the two‐stream model, including those that are not measured, for example, the fraction of radiation absorbed in the ground. Applications are conducted using Moderate Resolution Imaging Spectroradiometer (MODIS) and Multiangle Imaging Spectroradiometer (MISR) broadband surface albedo products. It turns out that the differences between these two albedo sets may translate into discernible signatures on some retrieved model parameters. Meanwhile, adding the Joint Research Centre (JRC)‐Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) products into the measurements yields a significant reduction of uncertainties. Results from these applications indicate that the products retrieved from the two‐stream inversion procedure (1) exhibit much less variability than those generated by the operational algorithms for the LAI and FAPAR, and (2) are in good agreement with the available ground‐based estimates.
Enhanced new particle formation observed in the northern midlatitude tropopause region
Authors: Young Li‐Hao , Benson David R. , Montanaro William M. , Lee Shan‐Hu , Pan Laura L. , Rogers David C. , Jensen Jorgen , Stith Jeffrey L. , Davis Christopher A. , Campos Teresa L. , Bowman Kenneth P. , Cooper William A. , Lait Leslie R.
Published: May 2007

The free troposphere and lower stratosphere is a source region for new particle formation. In particular, nucleation can be active near the tropopause because of low temperatures. Here we show enhanced new particle formation observed during midlatitude tropopause folds. Number concentrations and size distributions of particles with diameters from 4 to 2000 nm were measured in the midlatitude tropopause region on 1 and 7 December 2005 during the NSF/NCAR GV Progressive Science Missions. High number concentrations of ultrafine particles with diameters from 4 to 9 nm, ranging from ∼700 to 3960 cm−3, were measured during tropopause folds. Our observations show that stratospheric and tropospheric air exchange during tropopause folding events, with a large gradient of temperature and relative humidity, may have enhanced new particle formation. Our results are consistent with other modeling predictions showing that nucleation rates are increased with mixing of two air masses with different temperatures and relative humidities. In addition, new particle formation events were also associated with vertical motion that may also have brought higher concentrations of water vapor and aerosol precursors (that originate at the ground level) from lower altitudes to higher altitudes where temperatures and surface areas are lower. The average ultrafine particle concentrations for the regions that were not affected by tropopause folds were also high (>100 cm−3), indicating that nucleation is active in the tropopause region, in general. Our results suggest that atmospheric dynamics, such as stratosphere and troposphere exchange and vertical motion, affect new particle formation in this region.
Impacts of enhanced biomass burning in the boreal forests in 1998 on tropospheric chemistry and the sensitivity of model results to the injection height of emissions
Authors: Leung Fok‐Yan T. , Logan Jennifer A. , Park Rokjin , Hyer Edward , Kasischke Eric , Streets David , Yurganov Leonid
Published: May 2007

Carbon monoxide reached record high levels in the northern extratropics in the late summer and fall of 1998 as a result of anomalously large boreal fires in eastern Russia and North America. We investigated the effects of these fires on CO and tropospheric oxidants using a global chemical transport model (GEOS‐Chem) and two independently derived inventories for the fire emissions that differ by a factor of two. We find that it is essential to use both surface and column observations of CO to constrain the magnitude of the fire emissions and their injection altitude. Our results show that the larger of the two inventories appears more reliable and that about half of the emissions were injected above the boundary layer. The boreal fire emissions cause a much larger enhancement in ozone when about half the emissions are released above the boundary layer than when they are released exclusively in the boundary layer, as a consequence of the role of PAN as a source of NOx as air descends in regions far from the fires.
Use of slope creation for rehabilitating incised, regulated, gravel bed rivers
Authors: Elkins Eve M. , Pasternack Gregory B. , Merz Joseph E.
Published: May 2007

Gravel‐bedded channels often become incised and degraded below dams. Gravel can be added to the channel to rehabilitate hydrogeomorphic conditions, including those promoting salmon spawning. When implemented without increasing bed slope, gravel addition at downstream riffles back floods upstream riffles. A 2‐year gravel augmentation project was done to test the efficacy of a new method for “slope creation.” Riffle‐to‐riffle slope was raised from 0.002 to 0.008 by adding gravel to the most upstream riffle. When gravel was added to the next downstream riffle a year later, riffle‐to‐riffle slope decreased to the sought after 0.004. After the study, the area of high‐quality Chinook salmon spawning habitat increased 471%. The number of redds observed went from 62 to 161 during the study despite a 50% decline of in‐river spawners. This eliminates variations in migrant population size and hatchery take as alternative explanations. Slope creation can be a useful aid for rehabilitating regulated rivers.
Calibration of hydrological models in the spectral domain: An opportunity for scarcely gauged basins?
Authors: Montanari Alberto , Toth Elena
Published: May 2007

This study considers the use of the maximum likelihood estimator proposed by Whittle for calibrating the parameters of hydrological models. Whittle's likelihood provides asymptotically consistent estimates for Gaussian and non‐Gaussian data, even in the presence of long‐range dependence. This method may represent a valuable opportunity in the context of ungauged or scarcely gauged catchments. In fact, the only information required for model parameterization is essentially the spectral density function of the actual process simulated by the model. When long series of calibration data are not available, the spectral density can be inferred by using old and sparse records, regionalization methods, or information on the correlation properties of the process itself. The proposed procedure is applied to the case studies of two Italian river basins, for which a lumped rainfall‐runoff model has been calibrated by emulating scarcely gauged situations. It is shown that the Whittle estimator can be applied in such context with satisfactory results.
Estimation of water transport based on in situ measurements of relative humidity and temperature in a dry Tanzanian soil
Authors: Goss Kai‐Uwe , Madliger Michael
Published: May 2007

In situ measurements of relative humidity (RH) and temperature have been conducted along the profile of an air‐dry soil over the course of 3 months. Soil water content data were calculated from RH measurements and the experimentally determined soil water retention curves at various temperatures. In addition, soil water potential data were directly derived from RH measurements, and the water vapor diffusion flux was calculated from the combined RH/temperature data set. The results indicate that traditional concepts for water movement and evaporation under unsaturated conditions must be revised for soils that have an air‐dry surface layer. The hydraulic conductivity coefficients determined under these dry conditions was up to five orders of magnitude higher than predicted with pedotransfer functions found in the literature. Thus liquid water flow contributed more to the overall water transport than would have been expected according to traditional concepts. Water vapor transport only dominated in the top surface layer at very dry conditions. After 3 months of constant evaporation, the evaporation front had not moved deeper than about 3 cm into the soil because water was effectively recharged from deeper layers of the soil.
Deep‐sea observatory gets undersea cable
Authors: Zielinski Sarah
Published: May 2007

More than 50 kilometers of cable have been buried about a meter beneath the seafloor of Monterey Bay, Calif., to provide power and carry data for the first U.S. deep‐sea cabled observatory. When finished later this year, the Monterey Accelerated Research System (MARS) observatory will provide scientists with constant access to any instrumentation, such as seismographs or oceanographic monitoring stations, plugged into its science node. Later this fall, a trawl‐resistant frame will be installed at the end of the cable and provide a home for a computer network hub and electrical substation to which instrumentation can be attached. MARS will support oceanographic research within Monterey Bay and provide a testing ground for technology that can be used in future deep‐sea observatories. MARS (http://www.mbari.org/mars/) is a multiinstitutional effort led by the Monterey Bay Aquarium Research Institute and funded by the U.S. National Science Foundation.
Challenges in atmospheric science and climate
Authors: Serafin Robert J. , Dabberdt Walter F. , Leavitt Peter R. , Busalacchi Antonio J. , Elfring Chris , Mengelt Claudia
Published: May 2007

What constitutes an effective vision for a national weather enterprise? Can barriers to improving subseasonal weather and climate predictions be removed? What are ethical considerations surrounding possible geoengineering approaches to mitigating climate change? These are some of the many questions raised during a recent strategic planning retreat of the U.S. National Academies' Board on Atmospheric Sciences and Climate (BASC).
Outstanding student paper awards
Authors: Anonymous
Published: May 2007

The following members received Outstanding Student Paper Awards at the 2006 AGU Fall Meeting, in San Francisco, Calif.
Impact of recent extreme Arizona storms
Authors: Magirl Christopher S. , Webb Robert H. , Griffiths Peter G. , Schaffner Mike , Shoemaker Craig , Pytlak Eric , Yatheendradas Soni , Lyon Steve W. , Troch Peter A. , Desilets Sharon L. E. , Goodrich Dave C. , Unkrich Carl L. , Youberg Ann , Pearthree Phil A.
Published: May 2007

Heavy rainfall on 27–31 July 2006 led to record flooding and triggered an historically unprecedented number of debris flows in the Santa Catalina Mountains north of Tucson, Ariz. The U.S. Geological Survey (USGS) documented record floods along four watercourses in the Tucson basin, and at least 250 hillslope failures spawned damaging debris flows in an area where less than 10 small debris flows had been documented in the past 25 years. At least 18 debris flows destroyed infrastructure in the heavily used Sabino Canyon Recreation Area (http://wwwpaztcn.wr.usgs.gov/rsch_highlight/articles/20061 l.html). In four adjacent canyons, debris flows reached the heads of alluvial fans at the boundary of the Tucson metropolitan area. While landuse planners in southeastern Arizona evaluate the potential threat of this previously little recognized hazard to residents along the mountain front, an interdisciplinary group of scientists has collaborated to better understand this extreme event.
Satellite repositioned to monitor South American region
Authors: Zielinski Sarah
Published: May 2007

The NOAA geostationary operational environmental satellite GOES‐10 has been repositioned to provide coverage of atmospheric conditions in South America, the agency announced on 10 April. In its new position, the satellite will provide coverage of the region around South America nearly to the South Pole, imaging it once every 15 minutes. Further, coverage will no longer be interrupted during hurricanes and other severe weather events in the United States. Continual satellite coverage of the region should provide better advanced warning of natural disasters, such as storms, cyclones, mudslides, and flooding. Shifting the satellite was part of the U.S. contribution to the Global Earth Observation System of Systems (GEOSS),a global monitoring network.
Russian volcano warnings reinstated
Authors: Zielinski Sarah
Published: May 2007

The Kamchatka Volcanic Eruption Response Team (KVERT) is again issuing warnings for aviation during periods of activity by Kamchatkan volcanoes. KVERT had stopped issuing warnings on 1 March due to a loss of funding by the Federal Unitary Enterprise State Air Traffic Management Corporation of Russia (see Eos 88(12), 2007). The funding for this work has now resumed. KVERT is a collaborative project of scientists from the Russian Institute of Volcanology and Seismology, the Kamchatka Experimental and Methodical Seismological Department, and the Alaska Volcano Observatory.
Mechanical interaction between active volcanoes in Iceland
Authors: Gudmundsson Agust , Andrew Ruth E. B.
Published: May 2007

We test the possibility of mechanical interaction between eight central volcanoes in the central part of the active Iceland rift zone. The average distance between the volcanoes is 30 km; all are thought to have shallow magma chambers, and many contain collapse calderas. We analyzed many finite‐element models with the volcanoes subject to a tensile stress of 5 MPa (equal to the maximum in situ tensile strength of the crust) in a direction parallel to the spreading vector, N105°E. The results show zones between many nearby volcanoes where the tensile stresses exceed the in situ tensile strength of the crust. The results indicate that mechanical interaction between volcanoes in a pair, such as simultaneous dike emplacement, seismogenic faulting, and deformation, may be common in this part of Iceland, in agreement with observations.
The effects of subtherapeutic antibiotic use in farm animals on the proliferation and persistence of antibiotic resistance among soil bacteria
Authors: Ghosh Sudeshna , LaPara Timothy M
Published: May 2007

The use of antibiotics at subtherapeutic concentrations for agricultural applications is believed to be an important factor in the proliferation of antibiotic-resistant bacteria. The goal of this study was to determine if the application of manure onto agricultural land would result in the proliferation of antibiotic resistance among soil bacteria. Chlortetracycline-resistant bacteria were enumerated and characterized from soils exposed to the manure of animals fed subtherapeutic concentrations of antibiotics and compared to the chlortetracycline-resistant bacteria from soils at farms with restricted antibiotic use (dairy farms) and from non-agricultural soils. No significant differences were observed at nine different study sites with respect to the numbers and types of cultivated chlortetracycline-resistant bacteria. Genes encoding for tetracycline resistance were rarely detected in the resistant bacteria from these sites. In contrast, soils collected from a tenth farm, which allowed manure to indiscriminately accumulate outside the animal pen, had significantly higher chlortetracycline-resistance levels. These resistant bacteria frequently harbored one of 14 different genes encoding for tetracycline resistance, many of which (especially tet(A) and tet(L)) were detected in numerous different bacterial species. Subsequent bacterial enumerations at this site, following the cessation of farming activity, suggested that this farm remained a hotspot for antibiotic resistance. In conclusion, we speculate that excessive application of animal manure leads to the spread of resistance to soil bacteria (potentially by lateral gene transfer), which then serve as persistent reservoir of antibiotic resistance.
Relativistic Raman backscattering theory with application in free-electron lasers with helical wiggler and axial magnetic field
Authors: Kordbacheh A. , Maraghechi B. , Aghahosseini H.
Published: May 2007

A relativistic theory for Raman backscattering, in the beam frame of electrons, is presented and is used to find the growth rate of a free-electron laser in Raman regime. The effects of the self-electric and self-magnetic fields, on the equilibrium trajectories of electrons, are taken into account. The wiggler effects, on the linear dispersion relations of the space-charge wave and radiation, are included in the analysis. A numerical computation is conducted to investigate the wiggler and self-field effects on the growth rate. The results of the relativistic and nonrelativistic treatments are compared.
Kinetic reversed-shear Alfvén eigenmodes
Authors: Konovalov S. V. , Mikhailovskii A. B. , Shirokov M. S. , Kovalishen E. A. , Ozeki T.
Published: May 2007

Alfvén eigenmodes in tokamak discharges with reversed shear are theoretically studied taking into account the effects of finite Larmor radius. It is predicted that, similarly to the kinetic toroidal Alfvén eigenmodes in positive-shear discharges, a new branch of eigenmodes in reversed-shear discharges exists, called here the kinetic reversed-shear Alfvén eigenmodes.
Amplification of electrostatic ion-cyclotron waves in a plasma with magnetic-field-aligned ion flow shear and no electron current
Authors: Kim Su-Hyun , Agrimson Erick , Miller Michael J. , D’Angelo Nicola , Merlino Robert L. , Ganguli Gurudas I.
Published: May 2007

Experimental demonstration of the amplification of electrostatic ion-cyclotron waves in a plasma with parallel ion flow shear but no magnetic-field-aligned electron current is presented. Waves with frequencies near the ion gyrofrequency and multiple harmonics launched from an antenna are observed to grow in amplitude in a region of ion flow shear. Amplification of multiple cyclotron harmonics was observed when a broadband (white noise) signal was applied to the launching antenna. Implications of these results for plasma wave excitation in the Earth’s auroral region are discussed.
Three-dimensional electroacoustic solitary waves induced by ion holes in magnetized plasmas
Authors: Mamun A. A. , Eliasson B. , Shukla P. K.
Published: May 2007

Properties of three-dimensional (3D) electrostatic solitary waves, introduced by ion holes in magnetized plasmas have been investigated. A modified Zakharov–Kuznetsov, equation or a 3D modified Korteweg–de Vries equation has been derived analytically and solved numerically. The 3D electroacoustic solitary waves have a stronger nonlinearity due to the trapping of hot ions in the localized electroacoustic wave potential, in contrast to that associated with the traditional harmonic generation. The obtained results exhibit 3D electroacoustic solitary structures with bipolar parallel electric fields similar to the ones observed by the S3-3, Viking, Freja, Fast Auroral Snapshot, and Polar spacecrafts in the auroral acceleration region of the Earth’s magnetosphere.
Bohm criterion for dust-electron plasmas induced by UV irradiation
Authors: Wang Zheng-Xiong , Liu Jin-Yuan , Liu Yue , Wang Xiaogang
Published: May 2007

Bohm criterion for dust-electron plasmas induced by UV irradiation is investigated by Sagdeev potential, taking into account the self-consistent dust charge variation. In the collisionless limit, the Boltzmann electron distribution and dust cold fluid in the sheath are adopted. It is found that the dust Bohm velocity decreases with increasing dust density in the bulk plasma. The results are examined to be reliable by the quantitative analysis of Sagdeev potential.
Experimental setup for low-energy laser-based angle resolved photoemission spectroscopy
Authors: Koralek J. D. , Douglas J. F. , Plumb N. C. , Griffith J. D. , Cundiff S. T. , Kapteyn H. C. , Murnane M. M. , Dessau D. S.
Published: May 2007

A laser-based angle resolved photoemission (ARPES) system utilizing 6 eV photons from the fourth harmonic of a mode-locked Ti:sapphire oscillator is described. This light source greatly increases the momentum resolution and photoelectron count rate, while reducing extrinsic background and surface sensitivity relative to higher energy light sources. In this review, the optical system is described, and special experimental considerations for low-energy ARPES are discussed. The calibration of the hemispherical electron analyzer for good low-energy angle-mode performance is also described. Finally, data from the heavily studied high Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) is compared to the results from higher photon energies.
Formation of FePt nanoparticles by organometallic synthesis
Authors: Bagaria H. G. , Johnson D. T. , Srivastava C. , Thompson G. B. , Shamsuzzoha M. , Nikles D. E.
Published: May 2007

Our interest in determining the mechanism of FePt nanoparticle formation has led to this study of the evolution of particle size and composition during synthesis. FePt nanoparticles were prepared by the simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl. During the course of the reaction, samples were removed and the particle structure, size, and composition were determined using x-ray diffraction, transmission electron microscopy (TEM), and scanning electron microscopy–energy dispersive x-ray spectrometry. Early in the reaction the particles were Pt rich (greater than 95at.% Pt) and as the reaction proceeded the Fe content increased to the target of 50%. The particle diameter increased from 3.1to4.6nm during the reaction. Energy dispersive x-ray spectrometry measurements of individual particle compositions using a high resolution TEM showed a broad distribution of particle compositions with a standard deviation greater than 15% of the average composition.
Two-dimensional infrared surface spectroscopy for CO on Cu(100): Detection of intermolecular coupling of adsorbates
Authors: Nagata Yuki , Tanimura Yoshitaka , Muckamel Shaul
Published: May 2007

Surface-specific infrared signals obtained by subjecting the system to two infrared laser pulses are calculated for an admixture of CO and isotopic CO on Cu(100) by using molecular dynamics simulation based on a stability matrix formalism. The two-dimensional profiles of the signals in the frequency domain show both diagonal and cross peaks. The former peaks mainly arise from the overtones of the CO and isotopic CO, while the latter represent the couplings between those. As temperature is increased, the phases of cross peaks in a second-order infrared response function change significantly, while those of diagonal peaks are unchanged. The authors show that the phase shifts are originated from the potential anharmonicities due to the electronic interaction between adsorbates. Using a model with two dipole moments, they find that the frustrated rotational mode activated with temperature has effects on the anharmonicity. These results indicate that two-dimensional infrared surface spectroscopy reveals the anharmonic couplings between adsorbates and surface atoms or between adsorbates which cannot be observed in first-order spectroscopy.
Experimental study and two-dimensional modeling of avalanche breakdown voltage in polycrystalline silicon p-n junctions
Authors: Amrani Mohammed , Benamara Zineb , Chellali Mohammed , Tizi Schahrazade , Mohammed-Brahim Tayeb
Published: May 2007

A two-dimensional (2D) model of the avalanche breakdown mechanism is examined to achieve a lateral polycrystalline silicon (polysilicon) p+-n diode with high forward current and high breakdown voltage (BV). Samples with different film thicknesses (tf) were deposited by low-pressure chemical vapor deposition process. The p+ zone and n zone are doped by ionic implantation with boron and phosphorus, respectively. The measured current-voltage (I-V) characteristics show that BV varies between 6.4, 7.5, and 8.25V when tf varies between 250, 350, and 450nm, respectively. These data also show that when tf decreases, the forward current is high, the leakage current becomes higher under reverse bias, and BV decreases. We reveal that the breakdown phenomenon of our samples is dominated by the impact ionization effect. A 2D simulation of avalanche breakdown voltage versus the critical parameters of polysilicon diodes is implemented. The algorithm is based on the solution of Poisson’s equation and calculating the ionization integral along various electric field lines computed from the potential distribution. By taking into account the localization of trap states in the grain boundaries, the effects on the breakdown voltage of the doping concentration ND, the intergranular trap state density NT, the grain sizes Lg, the disposition of the grain boundaries, and the film thickness tf are investigated. The simulation results show that the impact ionization mechanism is more accelerated in polysilicon than in single-crystalline silicon, and the BV(Lg), BV(ND), BV(NT), and BV(tf) curves are characterized by a succession of descending stair shapes due to the trapping of free carries by trap states contained in grain boundaries that are parallel to the metallurgic junction. By comparing simulation results with experimental data, we select the electron-hole ionization coefficients characterizing our samples: αn∞=1.0×106cm−1, Encrit=5.87×106Vcm−1, αp∞=1.582×106cm−1, and EPcrit=2.036×106Vcm−1. The fit shows that the extracted NT varies from 1.07×1013, 4×1012, and 2×1012cm−2 when the film thickness varies from 250, 350, and 450nm, respectively. These results validate the developed model and confirm that structural and electronic properties of the polycrystalline deposition films are improved during the layer growth.
Evaluation of ultrafast laser-based hard x-ray sources for phase-contrast imaging
Authors: Toth R. , Fourmaux S. , Ozaki T. , Servol M. , Kieffer J. C. , Kincaid R. E. , Krol A.
Published: May 2007

The 2D projection phase-contrast imaging performance of the ultrafast laser-based x-ray (ULX) source has been investigated. The potential of such a novel x-ray source has been assessed by imaging a reference object (Contrast Detail Evaluation phantom) in the in-line holography geometry and by applying a simple 1D numerical model to the data analysis. The results indicate that the ULX is a promising technique for 2D projection phase-contrast imaging and for implementation of phase-contrast micro-Computed Tomography (μ-CT). This is because by using high contrast laser pulse ULX simultaneously provides a very small x-ray source size along with a high average x-ray flux. In addition, due to the ultrashort x-ray burst duration, ULX might allow practical implementation of ultrafast phase-contrast stroboscopy and time-of-flight based electronic scatter rejection. This technique is also of interest for time resolved radiography to follow shock waves and radiative fronts propagating in an opaque matter.
Two-dimensional electromagnetic model of a microwave plasma reactor operated by an axial injection torch
Authors: Álvarez R. , Alves L. L.
Published: May 2007

This paper presents a two-dimensional electromagnetic model for a microwave (2.45 GHz) plasma reactor operated by an axial injection torch. The model solves Maxwell’s equations, adopting a harmonic time description and considering the collision dispersion features of the plasma. Perfect-conductor boundary conditions are satisfied at the reactor walls, and absorbing boundary conditions are used at the open end of the coaxial waveguide powering the system. Simulations yield the distribution of the electromagnetic fields and the average power absorbed by the system for a given spatial profile of the plasma density (tailored from previous experimental measurements), with maximum values in the range 1014−1015 cm−3. Model results reveal that the system exhibits features similar to those of an air-filled, one-end-shorted circular metal waveguide, supporting evanescent or oscillatory solutions for radial dimensions below or above a critical radius, respectively. Results also show that the fractional average power absorbed by the plasma is strongly influenced by the system dimensions, which play a major role in defining the geometry pattern of the electromagnetic field distribution. Simulations are used to provide general guidelines for device optimization.
Defect study on the indium-gallium alloy system of copper chalcopyrites performed on solar cell heterostructures
Authors: Mertens Verena , Parisi Jürgen , Reineke-Koch Rolf
Published: May 2007

We report on defect spectroscopy on Cu(In,Ga)Se2 based solar cells with varying gallium content. Our investigations using deep level transient and admittance spectroscopy do not reveal a pronounced qualitative difference in the defect spectra for the case of absorbers containing indium and gallium. Therefore, we conclude that there exists no detrimental defect in the bulk material that generally prohibits one to achieve an increase in efficiency even for solar cells based on absorbers with a gallium to gallium plus indium ratio (GGI) larger than 0.3. The boundary compositions with CuInSe2 and CuGaSe2 absorbers (i.e., GGI=0 and GGI=1, respectively) show additional trap signals. Additionally, a peak width analysis was performed for two defect signals that occur for all absorber compositions. The relative peak width was found to be independent of GGI whereas the activation energies show some correlation with the gallium content of the absorber layer.
Role of ionized nitrogen species in the optical and structural properties of GaInNAs/GaAs quantum wells grown by plasma-assisted molecular beam epitaxy
Authors: Miguel-Sánchez J. , Guzmán A. , Jahn U. , Trampert A. , Ulloa J. M. , Muñoz E. , Hierro A.
Published: May 2007

We report on the impact of the nitrogen ion density on the structural and optical properties of GaInNAs quantum wells (QWs) grown by molecular beam epitaxy. The optical emission is strongly increased when the nitrogen ion density is reduced, as we found from photoluminescence experiments. Cathodoluminescence mappings of QWs grown under different ion densities are compared, showing a stronger modulation depth, and thus a higher structural disorder when a higher ion density was present during the growth. Atomic force microscopy measurements of equivalent epilayers showed that ions cause an important structural disorder of the layers. A nearly double root-mean-square roughness is observed when the density of ions is not reduced by external magnetic fields. Additionally, results of transmission electron microscopy measurements of buried GaInNAs QWs are presented, showing that lateral compositional fluctuations of In and N are suppressed when the QWs are protected from the ions. Finally, we find that QWs exposed to higher ion densities during the growth show deeper carrier localization levels and higher delocalization temperatures. These results provide clear evidence that the density of nitrogen ions present in the chamber during the epitaxial growth of GaInNAs QWs directly limits both the structural and optical properties.
Effect of water on sulfur dioxide (SO2) and nitrogen oxides (NOx) removal from flue gas in a direct current corona discharge reactor
Authors: Yang Jiaxiang , Chi Xiaochun , Dong Limin
Published: May 2007

A direct current (dc) corona discharge reactor composed of needle-plate electrodes in a glass container filled with flue gas was designed. To clarify the influence of water on discharge characteristics, water was introduced in the plasma reactor as electrode where plate electrode is immersed, under the application of dc voltage. Experiment results show that (1) corona wind forming between high-voltage needle electrode and water by corona discharge enhances the cleaning efficiency of flue gas due to the existence of water and the cleaning efficiency will increase with the increase of applied dc voltage within definite range and (2) both removal efficiencies of NOx and SO2 increased in the presence of water, which reach up to 98% for SO2, and about 85% for NOx under suitable conditions. These results play an important role in flue gas cleanup research.
Anchoring energy and cell gap effects on liquid crystal response time
Authors: Nie Xiangyi , Lu Ruibo , Xianyu Haiqing , Wu Thomas X. , Wu Shin-Tson
Published: May 2007

The anchoring energy and cell gap effects on liquid crystal response time (τ0) is analyzed theoretically and validated experimentally. Analytical expressions are derived using two different approaches: effective cell gap and surface dynamic equation methods. Consistent results are deduced from these two approaches. A simplified equation τ0∼dx also fits the experimental data well, where d is the liquid crystal cell gap and x is the exponent. Under two extreme (strong and weak) anchoring limits, the exponent x approaches 2 and 1, respectively. This information is helpful for optimizing liquid crystal devices for display applications.
Measurement of dynamic tensile strength of nanocrystalline copper by laser irradiation
Authors: Wang Yonggang , He Hongliang , Boustie Michel , Sekine Toshimori
Published: May 2007

An approach is developed to investigate the dynamic tensile fracture of nanocrystalline copper by laser irradiation loading. A push-pull type velocity interferometer system for any reflector is used to measure the rear free surface velocity profiles. The dynamic tensile strength of nanocrystalline copper films is determined from these velocity profiles as a function of the tensile strain rate. Results show that the dynamic tensile strength of nanocrystalline copper film is about 3 GPa, which is much higher than that of polycrystalline bulk copper, but lower than that of single crystal copper. This dynamic tensile strength increase may be attributed to constraints on dislocation motion by more grain boundaries in nanocrystalline materials.