1,586 research outputs found
Pyramidal Atoms: Berylliumlike Hollow States
Based on the idea that four excited electrons arrange themselves around the
nucleus in the corners of a pyramid in order to minimize their mutual
repulsion, we present an analytical model of quadruply excited states. The
model shows excellent comparison with ab initio results and provides a clear
physical picture of the intrinsic motion of the four electrons. The model is
used to predict configuration-mixing fractions and spectra of these highly
correlated states.Comment: 4 pages, 2 figure
Intermanifold similarities in partial photoionization cross sections of helium
Using the eigenchannel R-matrix method we calculate partial photoionization
cross sections from the ground state of the helium atom for incident photon
energies up to the N=9 manifold. The wide energy range covered by our
calculations permits a thorough investigation of general patterns in the cross
sections which were first discussed by Menzel and co-workers [Phys. Rev. A {\bf
54}, 2080 (1996)]. The existence of these patterns can easily be understood in
terms of propensity rules for autoionization. As the photon energy is increased
the regular patterns are locally interrupted by perturber states until they
fade out indicating the progressive break-down of the propensity rules and the
underlying approximate quantum numbers. We demonstrate that the destructive
influence of isolated perturbers can be compensated with an energy-dependent
quantum defect.Comment: 10 pages, 10 figures, replacement with some typos correcte
Urinary naphthalene and phenanthrene as biomarkers of occupational exposure to polycyclic aromatic hydrocarbons.
OBJECTIVES: The study investigated the utility of unmetabolised naphthalene (Nap) and phenanthrene (Phe) in urine as surrogates for exposures to mixtures of polycyclic aromatic hydrocarbons (PAHs). METHODS: The report included workers exposed to diesel exhausts (low PAH exposure level, n = 39) as well as those exposed to emissions from asphalt (medium PAH exposure level, n = 26) and coke ovens (high PAH exposure level, n = 28). Levels of Nap and Phe were measured in urine from each subject using head space-solid phase microextraction and gas chromatography-mass spectrometry. Published levels of airborne Nap, Phe and other PAHs in the coke-producing and aluminium industries were also investigated. RESULTS: In post-shift urine, the highest estimated geometric mean concentrations of Nap and Phe were observed in coke-oven workers (Nap: 2490 ng/l; Phe: 975 ng/l), followed by asphalt workers (Nap: 71.5 ng/l; Phe: 54.3 ng/l), and by diesel-exposed workers (Nap: 17.7 ng/l; Phe: 3.60 ng/l). After subtracting logged background levels of Nap and Phe from the logged post-shift levels of these PAHs in urine, the resulting values (referred to as ln(adjNap) and ln(adjPhe), respectively) were significantly correlated in each group of workers (0.71 < or = Pearson r < or = 0.89), suggesting a common exposure source in each case. Surprisingly, multiple linear regression analysis of ln(adjNap) on ln(adjPhe) showed no significant effect of the source of exposure (coke ovens, asphalt and diesel exhaust) and further suggested that the ratio of urinary Nap/Phe (in natural scale) decreased with increasing exposure levels. These results were corroborated with published data for airborne Nap and Phe in the coke-producing and aluminium industries. The published air measurements also indicated that Nap and Phe levels were proportional to the levels of all combined PAHs in those industries. CONCLUSION: Levels of Nap and Phe in urine reflect airborne exposures to these compounds and are promising surrogates for occupational exposures to PAH mixtures
Elementary Student Knowledge Gains In The Digital Portable Planetarium
Immersive environments provide unique and heightened sensory experiences that focus a learnerâs attention, and thus may be useful learning platforms. Â In particular, portable planetariums may be useful in advancing conceptual knowledge about the night sky, because they afford learners with Earth-based views of celestial motions, and give learners a sense of âbeing there.â Â We demonstrate here that students make gains in knowledge about both apparent celestial motion and general astronomy concepts after viewing a 25-minute planetarium presentation. Â However, the planetarium presentation did not appear to increase interest in astronomy and space science. Â Our results suggest that the portable planetarium may be a useful strategy in supporting learners as they struggle with reconciling observed patterns with underlying, non-observable motions of the Earth, and with visualizing concepts such as the speed of planetary orbits relative to their position with respect to the sun
Resonance structure in the Li^- photodetachment cross section
We report on the first observation of resonance structure in the total cross
section for the photodetachment of Li^-. The structure arises from the
autodetaching decay of doubly excited ^1P states of Li^- that are bound with
respect to the 3p state of the Li atom. Calculations have been performed for
both Li^- and H^- to assist in the identification of these resonances. The
lowest lying resonance is a symmetrically excited intrashell resonance. Higher
lying asymmetrically excited intershell states are observed which converge on
the Li(3p) limit.Comment: 4 pages, 2 figure, 19 references, RevTeX, figures in ep
Exposure to an enriched CO 2 atmosphere alters carbon assimilation and allocation in a pine forest ecosystem
We linked a leaf-level CO 2 assimilation model with a model that accounts for light attenuation in the canopy and measurements of sap-flux-based canopy conductance into a new canopy conductance-constrained carbon assimilation (4C-A) model. We estimated canopy CO 2 uptake ( A nC ) at the Duke Forest free-air CO 2 enrichment (FACE) study. Rates of A nC estimated from the 4C-A model agreed well with leaf gas exchange measurements ( A net ) in both CO 2 treatments. Under ambient conditions, monthly sums of net CO 2 uptake by the canopy ( A nC ) were 13% higher than estimates based on eddy-covariance and chamber measurements. Annual estimates of A nC were only 3% higher than carbon (C) accumulations and losses estimated from ground-based measurements for the entire stand. The C budget for the Pinus taeda component was well constrained (within 1% of ground-based measurements). Although the closure of the C budget for the broadleaf species was poorer (within 20%), these species are a minor component of the forest. Under elevated CO 2 , the C used annually for growth, turnover, and respiration balanced only 80% of the A nC . Of the extra 700âg Câm â2 âa â1 (1999 and 2000 average), 86% is attributable to surface soil CO 2 efflux. This suggests that the production and turnover of fine roots was underestimated or that mycorrhizae and rhizodeposition became an increasingly important component of the C balance. Under elevated CO 2 , net ecosystem production increased by 272âg Câm â2 âa â1 : 44% greater than under ambient CO 2 . The majority (87%) of this C was sequestered in a moderately long-term C pool in wood, with the remainder in the forest floorâsoil subsystem.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73982/1/j.1365-2486.2003.00662.x.pd
Ecological Science Infrastructure for Sustainability Transformations in Rangelands
Sustainability transformationsâdeliberate and radical shifts in values, governance, and management regimes to achieve sustainabilityâare needed in rangelands as in other components of the Earth system. We review four concepts comprising an ecological science infrastructure to support such transformations. The foundation is standard measurement of rangeland conditions in the field, especially vegetation and soil properties that underpin the environmental aspects of sustainability. Big data resources, especially gridded spatial datasets produced by models and remote sensing, can be combined with field data and computational approaches to upscale information about rangeland conditions and produce additional indicators of ecosystem functions and services. State and transition models (STMs) linked to land types provide a means to interpret indicators and link interpretations to sustainable land management practices to manage change. Technologies for climate adaptation in rangelands also need to be linked to STM databases. Web and mobile technologies can put multifaceted science knowledge into the hands of pastoralists worldwide to support transformational changes in how rangelands are managed
NASA in-house Commercially Developed Space Facility (CDSF) study report. Volume 1: Concept configuration definition
The results of a NASA in-house team effort to develop a concept definition for a Commercially Developed Space Facility (CDSF) are presented. Science mission utilization definition scenarios are documented, the conceptual configuration definition system performance parameters qualified, benchmark operational scenarios developed, space shuttle interface descriptions provided, and development schedule activity was assessed with respect to the establishment of a proposed launch date
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