bluuebunny/crossref_metadata_2025 huggingface dataset:
Radar investigations of low‐altitude quasi‐periodic echoes in Chung‐Li
Authors: Pan C. J. , Röttger J. , Chen C. L.
Published: June 2005

Quasi‐periodic (QP) radar echoes from the sporadic E‐layers are also found at altitudes below 100 km which we call Low‐altitude Quasi‐Periodic echoes (LQP). We have used the Chung‐Li VHF radar to study correct locations of these echoes creating irregularities, which are confined to the height range between 90 km and 100 km, have a thickness of 2–4 km and are only slowly descending in altitude. They occur with periods of some tens of seconds to maximum three minutes. Since Chung‐Li is close to the equatorial region, we can prove that these LQP occur in this temperate region as well. The particular progress reported here is that interferometry allows us to determine the position of the scattering irregularities in three dimensions, which provides new information about the motion of the scatterers. We concur with the assumption that the LQP echoes are created in a strong wind shear below 100 km altitude.
Long‐range transport to Europe: Seasonal variations and implications for the European ozone budget
Authors: Auvray M. , Bey I.
Published: June 2005

We use a chemical transport model (GEOS‐CHEM) to quantify the contribution of long‐range transported pollution to the European ozone (O3) budget for the year 1997. The model reproduces the main features observed over Europe for O3, carbon monoxide and nitrogen dioxides, as well as two events of enhanced O3 of North American origin over the eastern North Atlantic and over Europe. North American O3 fluxes into Europe experience a maximum in spring and summer, reflecting the seasonal variation in photochemical activity and in export pathways. In summer, North American O3 enters Europe at higher altitudes and lower latitudes because of deep convection, and because the flow over the North Atlantic is mostly zonal in that season. The low‐level inflow is only important in spring, when loss rates in the boundary layer over the North Atlantic are weaker. Asian O3 arrives mainly via the westerlies, and usually at higher altitudes than North American O3 because of stronger deep convection over Asia. In addition, Asian O3 fluxes are at a maximum in summer during the monsoon period because of enhanced convection over Asia, increased nitrogen oxides sources from lightning and direct transport towards Europe via the monsoon easterlies. Over Europe, total background accounts for 30 ppbv at the surface. North American and Asian O3 contribute substantially to the annual O3 budget over Europe, accounting for 10.9% and 7.7%, respectively, while the European contribution only accounts for 9.4%. We find that in summer, at the surface, O3 decreases over Europe from 1980 to 1997, reflecting the reduction of European O3 precursor emissions. In the free troposphere, this decrease is compensated by the increase in O3 due to increasing Asian emissions. This may explain the lack of trends observed over most of the European region, especially at mountain sites.
Sensitivity of Vatnajökull ice cap hydrology and dynamics to climate warming over the next 2 centuries
Authors: Flowers Gwenn E. , Marshall Shawn J. , Björnsson Helgi , Clarke Garry K. C.
Published: June 2005

The sensitivity of Vatnajökull ice cap to future climate change is examined using spatially distributed coupled models of ice dynamics and hydrology. We simulate the evolving ice cap geometry, mass balance, velocity structure, subglacial water pressures and fluxes, and basin runoff in response to perturbations to a 1961–1990 reference climatology. For a prescribed warming rate of 2°C per century, simulated ice cap area and volume are reduced by 12–15% and 18–25% within 100 years, respectively. Individual outlet glaciers experience 3–6 km of retreat in the first 100 years and a total retreat of 10–30 km over 200 years. For the same applied warming our results suggest a maximum increase in glacier‐derived runoff of ∼25% after 130 years. Ice cap thinning and retreat alters Vatnajökull's subglacial hydraulic catchment structure in the simulations, with up to several kilometers of local hydraulic divide migration. This serves to redistribute water among the major outlet rivers and, in extreme cases, to isolate river basins from glacially derived runoff. Glacier discharge from northern and northwestern Vatnajökull (distal from the coast) appears to be the most robust to climate warming, while discharge from Vatnajökull's southern margin (proximal to the coast) is particularly vulnerable. The latter reflects pronounced changes in the geometry of the southern outlet glaciers and has implications for glacier flood routing and frequency.
Odd‐order probe correction technique for spherical near‐field antenna measurements
Authors: Laitinen T. A. , Pivnenko S. , Breinbjerg O.
Published: June 2005

In this paper, an odd‐order probe for spherical near‐field antenna measurements is defined. A probe correction technique for odd‐order probes is then formulated and tested by computer simulations. The probe correction for odd‐order probes is important, since a wide range of realistic antennas belongs to this class. To the authors' knowledge, the proposed technique is the first practical high‐order probe correction technique that has been formulated in detail, has been tested, and has been shown to work.
Denudational and thermal history of the Early Cretaceous Brandberg and Okenyenya igneous complexes on Namibia's Atlantic passive margin
Authors: Raab M. J. , Brown R. W. , Gallagher K. , Weber K. , Gleadow A. J. W.
Published: June 2005

The Early Cretaceous Brandberg and Okenyenya igneous complexes provide ideal locations to quantify the postbreakup denudation history of the continental margin of central Namibia. Apatite fission track analysis has been applied to vertical sample profiles extending over 1–2 km of relief to constrain the denudation history of this region. An advantage of vertical profiles is that they allow the inference of the paleogeothermal gradient directly from the data. An independent, data‐consistent thermal gradient estimate is essential to determine the amount of denudation as well as to resolve phases of slow and accelerated denudation. We obtained independent paleogeothermal gradient estimates for the two vertical profiles from 26 ± 32°C to 23 ± 16°C. The uncertainties are large but are considered pessimistic. The best fit values agree well with each other, and the present‐day gradients of 22°C. The total amount of denudation since the Late Cretaceous has been estimated as 5 km in the Brandberg area and 4 km for the Okenyenya area. The data from the two vertical profiles imply a phase of rapid exhumation in the Late Cretaceous between 80 and 60 Ma, at rates between 0.2 and 0.125 km/m.y. These decline rapidly in the Early Tertiary to a maximum denudation rate between 0.023 and 0.015 km/m.y. The denudation chronology inferred from the new apatite fission track data is consistent with the observation of a major influx of clastic sediment to offshore basins about 50 m.y. after continental breakup. The study shows that vertical fission track profiles provide key insights into postbreakup tectonic and geomorphic processes in the Central Damara Zone in Namibia, where no Phanerozoic sedimentary record has been preserved.
Inverse modeling of groundwater flow using model reduction
Authors: Vermeulen P. T. M. , Heemink A. W. , Valstar J. R.
Published: June 2005

Numerical groundwater flow models often have a very high number of model cells (greater than a million). Such models are computationally very demanding, which is disadvantageous for inverse modeling. This paper describes a low‐dimensional formulation for groundwater flow that reduces the computational burden necessary for inverse modeling. The formulation is a projection of the original groundwater flow equation on a set of orthogonal patterns (i.e., a Galerkin projection). The patterns (empirical orthogonal functions) are computed by a decomposition of the covariance matrix over an ensemble of model solutions. Those solutions represent the behavior of the model as a result of model impulses and the influence of a chosen set of parameter values. For an interchangeable set of parameter values the patterns yield a low‐dimensional model, as the number of patterns is often small. An advantage of this model is that the adjoint is easily available and most accurate for inverse modeling. For several synthetical cases the low‐dimensional model was able to find the global minimum efficiently, and the result was comparable to that of the original model. For several cases our model even converged where the original model failed. Our results demonstrate that the proposed procedure results in a 60% time reduction to solve the groundwater flow inverse problem. Greater efficiencies can be expected in practice for large‐scale models with a large number of grid cells that are used to compute transient simulations.
Storage‐dependent drainable porosity for complex hillslopes
Authors: Hilberts A. G. J. , Troch P. A. , Paniconi C.
Published: June 2005

In hydraulic groundwater theory the parameter drainable porosity f (a storage coefficient that accounts for the effect of the unsaturated zone on water table dynamics) is usually treated as a constant. For shallow unconfined aquifers the value of this parameter, however, depends on the depth to the water table and the water retention characteristics of the soil. In this study an analytical expression for f as a function of water table depth is derived under the assumption of quasi‐steady state hydraulic equilibrium, in this way accounting, in part, for the effects of the unsaturated zone on groundwater dynamics. The derived expression is implemented in the nonlinear hillslope‐storage Boussinesq (HSB) model (Troch et al., 2003) to simulate the drainage response of complex hillslopes. The model's behavior is analyzed by comparison to (1) the HSB model with a constant value for f and (2) measurements of water tables and outflow hydrographs on a 6.0 × 2.5 × 0.5 m laboratory hillslope experiment. The comparison is conducted for a pure drainage case on two different hillslope shapes (linearly convergent and divergent) and for three different slope inclinations (5%, 10%, and 15%). Comparison 1 is run in an uncalibrated and a fully calibrated mode, and it enables us to evaluate the effect of a dynamic, state‐dependent value for f on model output. Comparison 2 allows us to test the HSB model on several hillslope configurations and to analyze whether the concept of a storage‐dependent f enhances the model performance. The comparison of the HSB models to the measurements from the laboratory hillslopes shows that it is possible to capture the general features of the outflow hydrograph during a drainage experiment using either one of the HSB models. Overall, the original (constant f) HSB model, with one fitting parameter more than the revised HSB model, shows a slightly better fit on the hydrographs when compared to the revised (variable f) HSB model. However, the peak outflow values (the first few minutes after initiation of the experiments) are better captured by the revised HSB model. The revised HSB model's performance in simulating water table movements is much more accurate than that of the original HSB model. The improved match of the revised HSB model to piezometric measurements is worth stressing because the ability to model water tables is a key attribute of the model, making it possible to investigate phenomena such as saturation excess runoff. Also noteworthy is the good match between the revised HSB model and the outflow measurements, without any calibration, for the divergent slopes. The changing values of the calibrated drainable porosity parameter for the original HSB model as different configurations are simulated (slope angle, plan shape, initial conditions), together with the ability of the revised HSB model to more accurately simulate water table dynamics, clearly demonstrates the importance of regarding drainable porosity as a dynamic, storage‐dependent parameter.
Validation of ACE‐FTS stratospheric ozone profiles against Odin/OSIRIS measurements
Authors: Petelina S. V. , Llewellyn E. J. , Walker K. A. , Degenstein D. A. , Boone C. D. , Bernath P. F. , Haley C. S. , von Savigny C. , Lloyd N. D. , Gattinger R. L.
Published: June 2005

Stratospheric ozone profiles measured by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE‐FTS) have been extensively compared with the coincident ozone profiles measured by the Optical Spectrograph and InfraRed Imager System (OSIRIS) on Odin. All coincidences took place between January 31 and August 5, 2004, with the majority in the Northern Hemisphere. ACE‐OSIRIS statistical comparisons were performed in the altitude range of 16–32 km where OSIRIS ozone data are in a good agreement, 5–7%, with another satellite and ground‐based instruments. At altitudes 3–5 km above and below the ozone peak, ACE‐FTS and OSIRIS profiles agree to within ∼4–7%, while in the vicinity of the peak the ACE‐FTS profiles are systematically lower than OSIRIS profiles by about 10%. This result is consistent for all latitudes between 30°S and 82°N.
Plume velocity determination for volcanic SO2 flux measurements
Authors: McGonigle A. J. S. , Hilton D. R. , Fischer T. P. , Oppenheimer C.
Published: June 2005

Ground based volcanic SO2 fluxes provide important insights into the behaviour of volcanoes, and their impacts upon the atmosphere. In order to compute a flux, the plume transport speed, and direction, must be known. In practice these are typically assumed to equal, respectively: (A) a ground based anemometer reading, and (B) the bearing of the vector between the volcanic gas source and the position on the downwind plume cross‐section where the gas concentration is highest. However, use of these proxies is open to question, and they can introduce large errors (possibly > 100%), thereby significantly reducing the utility of the derived fluxes. Here we present direct spectroscopic measurements of volcanic plume velocity; the data were obtained using three ultraviolet spectrometers, at Masaya volcano, Nicaragua, during January 2004. We estimate that flux measurements with overall error budgets < 10% are readily achievable with this approach.
Hydrographic and biological changes in the Taiwan Strait during the 1997–1998 El Niño winter
Authors: Shang S. , Zhang C. , Hong H. , Liu Q. , Wong G. T. F. , Hu C. , Huang B.
Published: June 2005

During the 1997–1998 El Niño event, the average sea surface temperature (SST) in the Taiwan Strait (TWS) in the winter was ∼1.4°C higher than that of the winter climatological mean. The areal ratio of the warm water (≥2°C above the regional mean) to the cold water (≥2°C below the regional mean) in the TWS increased by 25% while the area of the eutrophic water (chlorophyll a >1 mg m−3) was halved. Field observations also indicate that the mixed layer in the TWS became more nutrient‐poor during this winter. These observations are consistent with a diminished advection of the cold and eutrophic Zhe‐Min Coastal Water, and, concomitantly, an expansive intrusion of the warm and oligotrophic South China Sea Warm Current/Kuroshio Branch Water to the TWS as the northeast monsoon was weakened. Thus, El Niño events potentially can have significant ecological impacts on the TWS.
Interaction between Astrocytes and Neurons Studied using a Mathematical Model of Compartmentalized Energy Metabolism
Authors: Aubert Agnès , Costalat Robert
Published: June 2005

Understanding cerebral energy metabolism in neurons and astrocytes is necessary for the interpretation of functional brain imaging data. It has been suggested that astrocytes can provide lactate as an energy fuel to neurons, a process referred to as astrocyte—neuron lactate shuttle (ANLS). Some authors challenged this hypothesis, defending the classical view that glucose is the major energy substrate of neurons, at rest as well as in response to a stimulation. To test the ANLS hypothesis from a theoretical point of view, we developed a mathematical model of compartmentalized energy metabolism between neurons and astrocytes, adopting hypotheses highly unfavorable to ANLS. Simulation results can be divided between two groups, depending on the relative neuron versus astrocyte stimulation. If this ratio is low, ANLS is observed during all the stimulus and poststimulus periods (continuous ANLS), but a high ratio induces ANLS only at the beginning of the stimulus and during the poststimulus period (triphasic behavior). Finally, our results show that current experimental data on lactate kinetics are compatible with the ANLS hypothesis, and that it is essential to assess the neuronal and astrocytic NADH/NAD+ ratio changes to test the ANLS hypothesis.
Characterization of the Effects of Adenosine Receptor Agonists on Cerebral Blood Flow in Uninjured and Traumatically Injured Rat Brain using Continuous Arterial Spin-Labeled Magnetic Resonance Imaging
Authors: Kochanek Patrick M , Hendrich Kristy S , Jackson Edwin K , Wisniewski Stephen R , Melick John A , Shore Paul M , Janesko Keri L , Zacharia Lefteris , Ho Chien
Published: June 2005

Hypoperfusion after traumatic brain injury may exacerbate damage. Adenosine, a vasodilator, regulates cerebral blood flow (CBF). Treatment with adenosine receptor agonists has shown benefit in experimental CNS trauma; however, their effects on CBF after injury remain undefined. We used magnetic resonance imaging to assess CBF in uninjured rats both early and at 24 h after intrahippocampal administration of either the nonselective adenosine receptor agonist 2-chloroadenosine (2-CA, 12 nmol) or the A2A-receptor agonist 2- p-(2-carboxyethyl)-phenethylamino-5'- N-ethylcarbox-amidoadenosine (CGS 21680, 6 nmol). We also assessed the effects of these agents on cerebral metabolic rate for glucose (CMRglu). We then assessed the effect of 2-CA on CBF at 3.5 to 5 h after controlled cortical impact (CCI). Injection of 2-CA into uninjured rat brain produced marked increases in CBF in ipsilateral hippocampus and cortex versus vehicle ( P < 0.05); CBF increases persisted even at 24 h. Measurement of hippocampal levels of 2-CA showed persistent increases to 24 h. CGS 21680 produced even more marked global increases in CBF than seen with 2-CA (2–6-fold versus vehicle, P < 0.05 in 10/12 regions of interest (ROIs)). Neither agonist altered CMRglu versus vehicle. After CCI, 2-CA increased CBF in ipsilateral hippocampal and hemispheric ROIs ( P < 0.05 versus vehicle), but the response was attenuated at severe injury levels. We report marked increases in CBF after injection of adenosine receptor agonists into uninjured rat brain despite unaltered CMRglu. 2-Chloroadenosine produced enduring increases in CBF in uninjured brain and attenuated posttraumatic hypoperfusion. Future studies of adenosine-related therapies in CNS injury should address the role of CBF.
Increased Perfusion in Motor Areas after Constraint-Induced Movement Therapy in Chronic Stroke: A Single-Photon Emission Computerized Tomography Study
Authors: Könönen Mervi , Kuikka Jyrki T , Husso-Saastamoinen Minna , Vanninen Esko , Vanninen Ritva , Soimakallio Seppo , Mervaala Esa , Sivenius Juhani , Pitkänen Kauko , Tarkka Ina M
Published: June 2005

Hemiparesis is the most common deficit after cerebral stroke. Constraint-induced movement therapy (CIMT) is a new neurorehabilitation method that emphasizes task-relevant repetitive training for the stroke hand. Twelve chronic stroke patients were studied with single-photon emission computerized tomography at rest before and after the two-week CIMT period. Increased perfusion was found in motor control related areas. The specific areas with an increase in perfusion in the affected hemisphere were in the precentral gyrus, premotor cortex (Brodmann's area 6 (BA6)), frontal cortex, and superior frontal gyrus (BA10). In the nonaffected hemisphere, perfusion was increased in the superior frontal gyrus (BA6) and cingulate gyrus (BA31). In the cerebellum increased perfusion was seen bilaterally. The brain areas with increased perfusion receive and integrate the information from different sensory systems and plan the movement execution. Regional cerebral perfusion decreased in the lingual gyrus (BA18) in the affected hemisphere. In the nonaffected frontal cortex, two areas with decreased perfusion were found in the middle frontal gyrus (BA8/10). Also, the fusiform gyrus (BA20) and inferior temporal gyrus (BA37) in the non-affected hemisphere showed decreased perfusion. Intensive movement therapy appears to change local cerebral perfusion in areas known to participate in movement planning and execution. These changes might be a sign of active reorganization processes after CIMT in the chronic state of stroke.
Spontaneous Slow Hemodynamic Oscillations are Impaired in Cerebral Microangiopathy
Authors: Schroeter Matthias L , Bücheler Markus M , Preul Christoph , Scheid Rainer , Schmiedel Ole , Guthke Thomas , von Cramon D Yves
Published: June 2005

Small-vessel disease or cerebral microangiopathy (CMA) is a common finding in elderly people. It is related to a variety of vascular risk factors and may finally lead to subcortical ischemic vascular dementia. Because vessel stiffness is increased, we hypothesized that slow spontaneous oscillations are reduced in cerebral hemodynamics. Accordingly, we examined spontaneous oscillations in the visual cortex of 13 patients suffering from CMA, and compared them with 14 agematched controls. As an imaging method we applied functional near-infrared spectroscopy, because it is particularly sensitive to the microvasculature. Spontaneous low-frequency oscillations (LFOs) (0.07 to 0.12 Hz) were specifically impaired in CMA in contrast to spontaneous very-low-frequency oscillations (0.01 to 0.05 Hz), which remained unaltered. Vascular reagibility was reduced during visual stimulation. Interestingly, changes were tightly related to neuropsychological deficits, namely executive dysfunction. Vascular alterations had to be attributed mainly to the vascular risk factor arterial hypertension. Further, results suggest that the impairments might be, at least partly, reversed by medical treatment such as angiotensin-converting enzyme inhibitors/angiotensin II receptor blockers. Results indicate that functional near-infrared spectroscopy may detect changes in the microvasculature due to CMA, namely an impairment of spontaneous LFOs, and of vascular reagibility. Hence, CMA accelerates microvascular changes due to aging, leading to impairments of autoregulation.
Crossed Cerebellar Diaschisis in Acute Human Stroke: A PET Study of Serial Changes and Response to Supratentorial Reperfusion
Authors: Sobesky Jan , Thiel Alexander , Ghaemi Mehran , Hilker Rüdiger H , Rudolf Jobst , Jacobs Andreas H , Herholz Karl , Heiss Wolf-Dieter
Published: June 2005

Crossed cerebellar diaschisis (CCD) is well described in the chronic phase of stroke, but few data describe acute CCD and its serial changes after reperfusion. Using positron emission tomography (PET), we studied acute CCD with respect to supratentorial perfusion and outcome measures. In 19 acute stroke patients receiving intravenous thrombolysis (<3 h), 15O-water PET assessed CCD and supratentorial hypoperfusion volume before thrombolysis, 3, 24 h and 14 days later. Infarct volume at day 14 and NIHSS score at 3 months were assessed. Supratentorial hypoperfusion decreased from 25 cm3 (median) before thrombolysis to 0.1 cm3 at day 14. Baseline CCD was 13.4% and decreased continuously to 6.1% after 14 days. The NIHSS score decreased from 11 to 4 pts after 3 months. Infarct volume was 1.1 cm3. Crossed cerebellar diaschisis correlated to the hypoperfusion volume within the first 24 h after stroke, but not later. Hypoperfusion correlated to outcome measures at the early stage only. In contrast, CCD correlated to outcome values at all four measurements. Reperfusion with recovery of CCD was seen in patients with small infarcts and good clinical outcome and vice versa. Our data suggest that (i) CCD occurs as early as 3 h after stroke and might be reversible; (ii) acute CCD is closely related to the volume of supratentorial hypoperfusion. At later time points, however, CCD is disconnected from supratentorial perfusion but strongly associated to outcome measures; (iii) CCD is not susceptible to nonnutritional reperfusion and adds valuable information to interpret supratentorial reperfusion patterns.
Theory of long-lived nuclear spin states in solution nuclear magnetic resonance. I. Singlet states in low magnetic field
Authors: Carravetta Marina , Levitt Malcolm H.
Published: June 2005

We have recently demonstrated the existence of exceptionally long-lived nuclear spin states in solution-state nuclear magnetic resonance. The lifetime of nuclear spin singlet states in systems containing coupled pairs of spins-1∕2 may exceed the conventional relaxation time constant T1 by more than an order of magnitude. These long lifetimes may be observed if the long-lived singlet states are prevented from mixing with rapidly relaxing triplet states. In this paper we provide the detailed theory of an experiment which uses magnetic field cycling to observe slow singlet relaxation. An approximate expression is given for the magnetic field dependence of the singlet relaxation rate constant, using a model of intramolecular dipole-dipole couplings and fluctuating external random fields.
Variation of geometries and electron properties along proton transfer in strong hydrogen-bond complexes
Authors: Pacios L. F. , Gálvez O. , Gómez P. C.
Published: June 2005

Proton transfer in hydrogen-bond systems formed by 4-methylimidazole in both neutral and protonated cationic forms and by acetate anion are studied by means of MP2∕6-311++G(d,p) ab initio calculations. These two complexes model the histidine (neutral and protonated)-aspartate diad present in the active sites of enzymes the catalytic mechanism of which involves the formation of strong hydrogen bonds. We investigate the evolution of geometries, natural bond orbital populations of bonds and electron lone pairs, topological descriptors of the electron density, and spatial distributions of the electron localization function along the process N-H⋯O→N⋯H⋯O→N⋯H-O, which represents the stages of the H-transfer. Except for a sudden change in the population of electron lone pairs in N and O at the middle N···H···O stage, all the properties analyzed show a smooth continuous behavior along the covalent → hydrogen bond transit inherent to the transfer, without any discontinuity that could identify a formation or breaking of the hydrogen bond. This way, the distinction between covalent or hydrogen-bonding features is associated to subtle electron rearrangement at the intermolecular space.
Six-dimensional quantum dynamics of (v=,j=)D2 and of (v=1,j=)H2 scattering from Cu(111)
Authors: Nave Sven , Lemoine Didier , Somers Mark F. , Kingma Sikke M. , Kroes Geert-Jan
Published: June 2005

We report six-dimensional quantum dynamics calculations of the dissociative scattering of molecular hydrogen from the copper(111) surface. Two potential energy surfaces are investigated and the results are compared with experiment. Our study completes the preliminary work of Somers et al. [Chem. Phys. Lett. 360, 390 (2002)] and focuses on the role of initial vibrational excitation and on isotopic effects. None of the two investigated potential energy surfaces is found satisfactory: the use of neither potential yields reaction and vibrational excitation probabilities and vibrational efficacies that are in close agreement with experiment. In addition to showing the shortcomings of existing potential energy surfaces we point out an inconsistency in the experimental fits for D2.
Morphological structures formed by grafted polymers in poor solvents
Authors: Pattanayek Sudip K. , Pham T. T. , Pereira G. G.
Published: May 2005

We study a system of grafted polymers in a poor solvent by self-consistent-field methods as well as Monte-Carlo simulation methods. We observe a number of different morphological structures including an inverted solvent micelle or hole in the polymer layer, a lamella-like micelle structure, and fused, spherical micelle structures. These structures can be obtained by either varying the grafting density or chain length. We also develop a scaling theory for the existence of these structures and find reasonable agreement between this theory and our numerical calculations.
Effect of platinum on the photophysical properties of a series of phenyl-ethynyl oligomers
Authors: Rogers Joy E. , Hall Benjamin C. , Hufnagle David C. , Slagle Jonathan E. , Ault Andrew P. , McLean Daniel G. , Fleitz Paul A. , Cooper Thomas M.
Published: June 2005

In this work we detail the photophysical properties of a series of butadiynes having the formula H–(C6H4–C≡C)n–(C≡C–C6H4)n–H, n=1–3 and ligands H–(C6H4–C≡C)n–H, n=1–3 and compare these to previous work done on a complimentary series of platinum-containing complexes having the formula trans-Pt[(PC4H9)3]2[(C≡C–C6H4)n–H]2, n=1–3. We are interested in understanding the role of the platinum in the photophysical properties. We found that there is conjugation through the platinum in the singlet states, but the triplet states show more complex behavior. The T1 exciton, having metal-to-ligand charge-transfer character, is most likely confined to one ligand but the Tn exciton appears to have ligand-to-metal charge-transfer character. The platinum effect was largest when n=1. When n=2–3, the S0-S1,S1-S0,T1-S0, and T1-Tn spectral properties of the platinum complex are less influenced by the metal, becoming equivalent to those of the corresponding butadiynes. When n=1, platinum decreases the triplet state lifetime, but its effect diminishes as n increases to 2.
On equilibrium structures of the water molecule
Authors: Császár Attila G. , Czakó Gábor , Furtenbacher Tibor , Tennyson Jonathan , Szalay Viktor , Shirin Sergei V. , Zobov Nikolai F. , Polyansky Oleg L.
Published: June 2005

Equilibrium structures are fundamental entities in molecular sciences. They can be inferred from experimental data by complicated inverse procedures which often rely on several assumptions, including the Born–Oppenheimer approximation. Theory provides a direct route to equilibrium geometries. A recent high-quality ab initio semiglobal adiabatic potential-energy surface (PES) of the electronic ground state of water, reported by Polyansky et al. [Polyansky et al.Science 299, 539 (2003)] and called CVRQD here, is analyzed in this respect. The equilibrium geometries resulting from this direct route are deemed to be of higher accuracy than those that can be determined by analyzing experimental data. Detailed investigation of the effect of the breakdown of the Born–Oppenheimer approximation suggests that the concept of an isotope-independent equilibrium structure holds to about 3×10−5Å and 0.02° for water. The mass-independent [Born–Oppenheimer (BO)] equilibrium bond length and bond angle on the ground electronic state PES of water is reBO=0.95782Å and θeBO=104.485°, respectively. The related mass-dependent (adiabatic) equilibrium bond length and bond angle of H2O16 is read=0.95785Å and θead=104.500°, respectively, while those of D2O16 are read=0.95783Å and θead=104.490°. Pure ab initio prediction of J=1 and 2 rotational levels on the vibrational ground state by the CVRQD PESs is accurate to better than 0.002cm−1 for all isotopologs of water considered. Elaborate adjustment of the CVRQD PESs to reproduce all observed rovibrational transitions to better than 0.05cm−1 (or the lower ones to better than 0.0035cm−1) does not result in noticeable changes in the adiabatic equilibrium structure parameters. The expectation values of the ground vibrational state rotational constants of the water isotopologs, computed in the Eckart frame using the CVRQD PESs and atomic masses, deviate from the experimentally measured ones only marginally, especially for A0 and B0. The small residual deviations in the effective rotational constants are due to centrifugal distortion, electronic, and non-Born–Oppenheimer effects. The spectroscopic (nonadiabatic) equilibrium structural parameters of H2O16, obtained from experimentally determined A0′ and B0′ rotational constants corrected empirically to obtain equilibrium rotational constants, are resp=0.95777Å and θesp=104.48°.
Effect of thermic emission current on charge of grains in plasma
Authors: Wu Hai-Cheng , Xie Bai-Song
Published: June 2005

The effect of thermic emission from the dust surface on the charge of the grains, depending on different grains’ work function and temperature, has been researched. It is found that the thermal emission current plays a significant role in modifying the dust grains’ charge for a low dust surface work function with high dust temperature. Thermic emission also makes the dust grains charge positively, in contrast to those charging negatively in plasma without thermic emission.
Characteristics of a micromachined floating-gate high-electron-mobility transistor at 4.2K
Authors: Teh W. H. , Crook R. , Smith C. G. , Beere H. E. , Ritchie D. A.
Published: June 2005

We use micromachined, free-standing Ni cantilevers to develop a mechanical field-effect transistor based on III-V material systems. The device consists of an electrostatically actuated microcantilever, acting as the floating gate, fabricated over a defined two-dimensional electron gas (2DEG) in a modulation-doped GaAs∕AlGaAs heterostructure. The gating effects on the conductance of the 2DEG channel of the biased floating gate at different operating points are studied at 4.2K. Preliminary resonance measurements based on the 2DEG as a deflection sensor are presented.
Finite Larmor radius assisted velocity shear stabilization of the interchange instability in magnetized plasmas
Authors: Ng Sheung-wah , Hassam A. B.
Published: June 2005

Finite Larmor radius (FLR) effects, originally shown to stabilize magnetized plasma interchange modes at short wavelength, are shown to assist velocity shear stabilization of long wavelength interchanges. It is shown that the FLR effects result in stabilization with roughly the same efficacy as the stabilization from dissipative (resistive and viscous) effects found earlier.
Phosphorescent-sensitized triplet-triplet annihilation in tris(8-hydroxyquinoline) aluminum
Authors: Tanaka Isao , Tokito Shizuo
Published: June 2005

We characterized the photoluminescence properties of an amorphous tris(8-hydroxyquinoline) aluminum (Alq3) thin film heavily doped with fac tris(2-phenylpyridine) iridium [Ir(ppy)3] at 8 K. Not only green fluorescence but also red phosphorescence from Alq3 was clearly observed, where Ir(ppy)3 plays the important role as a phosphorescent sensitizer for Alq3. The triplet energy of Alq3 was estimated to be 2.03 eV from the highest energy peak of the phosphorescence spectrum. The fluorescence intensity was proportional to the excitation power. On the other hand, the deviation from the linearity of the phosphorescence intensity to the excitation power was observed above ∼0.01W∕cm2. This nonlinear phosphorescence behavior is well explained by the simple triplet-triplet annihilation theory. It was demonstrated that the efficient triplet energy transfer from Ir(ppy)3 enables us to observe triplet-triplet annihilation in Alq3.
Alternative electronic parts for multiphonon-broadened photoionization cross sections of deep levels in SiC
Authors: Pässler Roland
Published: June 2005

Analytical expressions for multiphonon-broadened photoionization cross sections of deep levels are generally represented by convolutions of temperature-independent electronic parts with thermally broadened Franck–Condon (FC) factors. As a simple analytical representation of the FC factors, we use here the semiclassical (Gaussian) approximation. For the electronic part we consider a variety of conventional alternatives such as the familiar Lucovsky model, Ridley’s billiard ball model, and Inkson’s model. From corresponding numerical analyses of experimental photoionization cross section data available for the R center in 6H-SiC and a vanadium-related center in 4H∕6H-SiC we conclude that, among these conventional models, Inkson’s model (for allowed transitions) is the only one that provides satisfactory fits to the experimental data. As a physically plausible alternative to the latter we also consider a Taylor series expansion for the electronic part, which is capable of accounting for competition (superposition) of qualitatively different components due to allowed and forbidden transitions. This alternative model leads, particularly for the vanadium-related center in 4H∕6H-SiC, to a marked improvement of the numerical fit in conjunction with a significant change in the estimated optical ionization energy. We show a simple way of estimating FC shifts and the associated thermal ionization energies on the basis of the fitted semiclassical parameter sets.
Cluster expansion of electronic excitations: Application to fcc Ni–Al alloys
Authors: Geng H. Y. , Sluiter M. H. F. , Chen N. X.
Published: June 2005

The cluster expansion method is applied to electronic excitations and a set of effective cluster densities of states (ECDOS) is defined, analogous to effective cluster interactions (ECIs). The ECDOSs are used to generate alloy thermodynamic properties as well as the equation of state (EOS) of electronic excitations for the fcc Ni–Al systems. When parent clusters have a small size, the convergence of the expansion is not so good but the electronic density of state (DOS) is well reproduced. However, the integrals of the DOS such as the cluster expanded free energy, entropy, and internal energy associated with electronic excitations are well described at the level of the tetrahedron–octahedron cluster approximation, indicating that the ECDOS is applicable to produce electronic ECIs for cluster variation method (CVM) or Monte Carlo calculations. On the other hand, the Grüneisen parameter, calculated with first-principles methods, is no longer a constant and implies that the whole DOS profile should be considered for EOS of electronic excitations, where ECDOS adapts very well for disordered alloys and solid solutions.
Mechanical stability of ultrathin Ge∕Si film on SiO2: The effect of Si∕SiO2 interface
Authors: Huang Minghuang , Nairn John A. , Liu Feng , Lagally M. G.
Published: June 2005

We perform two-dimensional linear elastic finite element analysis to investigate the mechanical stability of ultrathin Ge∕Si film grown on or bonded to SiO2, using imperfect interface elements between Si and SiO2 to model Si∕SiO2 interfacial slippage. We demonstrate that the overall composite film is stable when only the tangential slippage is allowed, however, it becomes unstable when normal slippage is allowed: the coherently strained Ge island induces a large local bending of Si layer, and separates the Si layer from the underlying SiO2 forming a void at the Si∕SiO2 interface.
Proton acceleration mechanisms in high-intensity laser interaction with thin foils
Authors: d’Humières Emmanuel , Lefebvre Erik , Gremillet Laurent , Malka Victor
Published: June 2005

The interaction of short and intense laser pulses with plasmas or solids is a very efficient source of high-energy ions. This paper reports the detailed study, with particle-in-cell simulations, of the interaction of such a laser pulse with thin, dense targets, and the resulting proton acceleration. Depending on the laser intensity and pulse duration, the most energetic protons are found to come from the front, the core, or the back of the target. The main accelerating mechanisms discussed in this paper are plasma expansion acceleration, where proton acceleration is driven by the hot electron population, and shock acceleration, originating from the laser ponderomotive potential imposed at the front target surface. Three main regimes of proton acceleration are defined and the parameters for which each regime is dominant are obtained. For irradiances close to 1020W∕cm2, the highest proton energies are obtained from thin foils efficiently heated by relativistic transparency. At larger intensities, a complex interplay between collisionless shock acceleration and plasma expansion acceleration is evidenced.
Shock propagation in deuterium-tritium-saturated foam
Authors: Collins T. J. B. , Poludnenko A. , Cunningham A. , Frank A.
Published: June 2005

Adaptive-mesh-refinement hydrodynamic simulations have been performed of cross sections of fibrous foams saturated with cryogenic deuterium and tritium (DT). Material tracking indicates that the fibers and DT mix rapidly behind the shock. In addition, fluctuation decay lengths are on the order of a micron even in the absence of radiative and thermal energy transport. Outside the mix region, the Rankine–Hugoniot equations are satisfied to the degree to which the turbulence and transverse motion decay, a few percent or less. Simulations also show that the shock-front perturbations decay rapidly after the shock leaves the foam and enters a layer of DT ice, suggesting that the foam microstructure will not contribute to feedthrough.
Critical size effects on the magnetic resonance in Fe3O4 nanoparticles
Authors: Hsu K. H. , Wu J. H. , Huang Y. Y. , Wang L. Y. , Lee H. Y. , Lin J. G.
Published: June 2005

In this work, Fe3O4 nanoparticles with two distinct diameters of 10 and 2nm are synthesized with different chemical methods in order to investigate the critical size effects on the magnetic resonance of Fe3O4 nanoparticles. Our results clearly demonstrate a difference in the dynamic behaviors of resonance field (Hr) and resonance linewidth between these two nanoparticles. Furthermore, the nonlinear temperature dependency of Hr in 2-nm Fe3O4 suggests that the surface anisotropy becomes dominating on the magnetodynamics of nanoparticles when the diameter of the nanoparticle is smaller than the critical size.
Microcrystalline silicon solar cells deposited at high rates
Authors: Mai Y. , Klein S. , Carius R. , Wolff J. , Lambertz A. , Finger F. , Geng X.
Published: June 2005

Hydrogenated microcrystalline silicon (μc-Si:H) thin-film solar cells were prepared at high rates by very high frequency plasma-enhanced chemical vapor deposition under high working pressure. The influence of deposition parameters on the deposition rate (RD) and the solar cell performance were comprehensively studied in this paper, as well as the structural, optical, and electrical properties of the resulting solar cells. Reactor-geometry adjustment was done to achieve a stable and homogeneous discharge under high pressure. Optimum solar cells are always found close to the transition from microcrystalline to amorphous growth, with a crystallinity of about 60%. At constant silane concentration, an increase in the discharge power did hardly increase the deposition rate, but did increase the crystallinity of the solar cells. This results in a shift of the μc-Si:H∕a-Si:H transition to higher silane concentration, and therefore leads to a higher RD for the optimum cells. On the other hand, an increase in the total flow rate at constant silane concentration did lead to a higher RD, but lower crystallinity. With this shift of the μc-Si:H∕a-Si:H transition at higher flow rates, the RD for the optimum cells decreased. A remarkable structure development along the growth axis was found in the solar cells deposited at high rates by a “depth profile” method, but this does not cause a deterioration of the solar cell performance apart from a poorer blue-light response. As a result, a μc-Si:H single-junction p-i-n solar cell with a high efficiency of 9.8% was deposited at a RD of 1.1 nm∕s.
Dynamics of a Xe cluster plasma produced by an intense ultrashort pulse KrF laser
Authors: Petrov G. M. , Davis J. , Velikovich A. L. , Kepple P. , Dasgupta A. , Clark R. W.
Published: June 2005

The dynamics of Xe clusters with initial radius between 10 and 100 Å irradiated by an IR subpicosecond laser pulse is investigated. The evolution of the cluster is modeled with a relativistic time-dependent three-dimensional particle simulation model. The focus of this investigation is to understand the energy absorption of clusters and how the absorbed energy is distributed among the various degrees of freedom. The consequence of the initial cluster radius on the absorbed energy, average charge per atom, mean electron and ion energies, ionization, removal of electrons from the cluster, and cluster expansion was studied. The absorbed energy per cluster scales as N5∕3, and the mean electron and ion energies scale as N1∕3 and N2∕3, respectively (N is the number of atoms per cluster). A significant fraction of the absorbed energy (∼90%) is converted into kinetic energy with comparable contribution to electrons and ions. The energy balance suggests that smaller clusters are more efficient as radiators, while larger clusters are more conducive to particle acceleration. The radiation yield of clusters with initial radius 20–50 Å irradiated by a laser with peak intensity 1016W∕cm2 is determined to be 1%–2%.
Polarization charge effects on ion-dust grain collisions in presence of ion drift
Authors: Lee Myoung-Jae , Jung Young-Dae
Published: May 2005

The nonuniform polarization charging effects due to the ion flow on the ion-dust elastic collision process is investigated in the presence of ion drift. The Born analysis is considered to obtain the scattering amplitude and cross section as a function of the momentum transfer, Mach number, and Debye length. The result shows that the nonuniform charging effects significantly enhance the differential scattering cross section for backward-scattering directions. It is also found that the nonuniform charging effects are significant for large momentum transfers.
Natural convection in a bottom heated horizontal cylinder
Authors: Hasan Nadeem , Anwer Syed Fahad , Sanghi Sanjeev
Published: June 2005

In this work a numerical investigation of two-dimensional steady and unsteady natural convection in a circular enclosure whose lower half is nonuniformly heated and upper half is maintained at a constant lower temperature has been carried out. An explicit finite difference method on a nonstaggered rectangular grid is used to solve the momentum and energy equations subject to Boussinesq approximation. The study is carried out for a range of Rayleigh number (Ra) varying between 102 and 106 at a fixed Prandtl number (Pr) taken as 0.71. The numerical experiments reveal that for Ra⩽8500, the flow always attains a steady state. In the steady regime, at very low Rayleigh numbers (Ra<300), it is shown that the velocity field is very weak and the heat transfer is predominantly by conduction. A series solution for the temperature field obtained by neglecting the fluid velocities is shown to agree well with the computed data for Ra<300. The convection takes place in the form of two cells with their interface aligned along the vertical diameter. As Ra is increased further, the isotherms distort to form a plume–like structure of hot fluid rising from the hottest point on the lower half of the cylinder wall. Local Nusselt number distribution over the wall shows that only a portion of the nonuniformly heated bottom half of the cylinder wall is responsible for heating the fluid. For Ra⩾8900, the numerical simulations show that the steady flow looses its stability and the flow undergoes bifurcations to periodic and quasiperiodic states. On the basis of the data on the amplitude of the periodic flows obtained for a set of Ra slightly greater than 8900, it is shown that the steady flow undergoes a supercritical Hopf bifurcation at Ra≈8830. An analysis using the proper orthogonal decomposition shows that the instability is in the form of a standing wave. The structure of the unstable mode is examined via empirical eigenfunctions obtained by the method of snapshots. In the unsteady regime (Ra>8.9×103), the cells start to swing their interface in an oscillatory manner with time. As Ra is increased further, the character of flow changes from periodic to quasiperiodic.