1,709 research outputs found
Nonsequential Two-Photon Double Ionization of Atoms: Identifying the Mechanism
We develop an approximate model for the process of direct (nonsequential)
two-photon double ionization of atoms. Employing the model, we calculate
(generalized) total cross sections as well as energy-resolved differential
cross sections of helium for photon energies ranging from 39 to 54 eV. A
comparison with results of \textit{ab initio} calculations reveals that the
agreement is at a quantitative level. We thus demonstrate that this complex
ionization process is fully described by the simple model, providing insight
into the underlying physical mechanism. Finally, we use the model to calculate
generalized cross sections for the two-photon double ionization of neon in the
nonsequential regime.Comment: 4 pages, 4 figure
Coherent adiabatic theory of two-electron quantum dot molecules in external spin baths
We derive an accurate molecular orbital based expression for the coherent
time evolution of a two-electron wave function in a quantum dot molecule where
the electrons interact with each other, with external time dependent
electromagnetic fields and with a surrounding nuclear spin reservoir. The
theory allows for direct numerical modeling of the decoherence in quantum dots
due to hyperfine interactions. Calculations result in good agreement with
recent singlet-triplet dephasing experiments by Laird et. al. [Phys. Rev. Lett.
97, 056801 (2006)], as well as analytical model calculations. Furthermore, it
is shown that using a much faster electric switch than applied in these
experiments will transfer the initial state to excited states where the
hyperfine singlet-triplet mixing is negligible.Comment: 4 pages, 3 figure
Multiphoton ionization and stabilization of helium in superintense xuv fields
Multiphoton ionization of helium is investigated in the superintense field
regime, with particular emphasis on the role of the electron-electron
interaction in the ionization and stabilization dynamics. To accomplish this,
we solve ab initio the time-dependent Schr\"odinger equation with the full
electron-electron interaction included. By comparing the ionization yields
obtained from the full calculations with corresponding results of an
independent-electron model, we come to the somewhat counterintuitive conclusion
that the single-particle picture breaks down at superstrong field strengths. We
explain this finding from the perspective of the so-called Kramers-Henneberger
frame, the reference frame of a free (classical) electron moving in the field.
The breakdown is tied to the fact that shake-up and shake-off processes cannot
be properly accounted for in commonly used independent-electron models. In
addition, we see evidence of a change from the multiphoton to the shake-off
ionization regime in the energy distributions of the electrons. From the
angular distribution it is apparent that correlation is an important factor
even in this regime
Human impacts on soil carbon dynamics of deep-rooted Amazonian forests and effect of land use change on the carbon cycle in Amazon soils
The main objective of these NASA-funded projects is to improve our understanding of land-use impacts on soil carbon dynamics in the Amazon Basin. Soil contains approximately one half of tropical forest carbon stocks, yet the fate of this carbon following forest impoverishment is poorly studied. Our mechanistics approach draws on numerous techniques for measuring soil carbon outputs, inputs, and turnover time in the soils of adjacent forest and pasture ecosystems at our research site in Paragominas, state of Para, Brazil. We are scaling up from this site-specific work by analyzing Basin-wide patterns in rooting depth and rainfall seasonality, the two factors that we believe should explain much of the variation in tropical soil carbons dynamics. In this report, we summarize ongoing measurements at our Paragominas study site, progress in employing new field data to understand soil C dynamics, and some surprising results from our regional, scale-up work
Land-use in Amazonia and the Cerrado of Brazil: State of Knowledge and GIS Database
We have assembled datasets to strengthen the LargeScale Biosphere Atmosphere Experiment in Amazonia (LBA). These datasets can now be accessed through the Woods Hole Research Center homepage (www.whrc.org), and will soon be linked to the Pre-LBA homepages of the Brazilian Space Research Institute's Center for Weather and Climate Prediction (Instituto de Pesquisas Espaciais, Centro de Previsao de Tempo e Estudos Climaticos, INPE/CPTEC) and through the Oak Ridge National Laboratory, Distributed Active Archive Center (ORNL/DMC). Some of the datasets that we are making available involved new field research and/or the digitization of data available in Brazilian government agencies. For example, during the grant period we conducted interviews at 1,100 sawmills across Amazonia to determine their production of sawn timber, and their harvest intensities. These data provide the basis for the first quantitative assessment of the area of forest affected each year by selective logging (Nepstad et al, submitted to Nature). We digitized the locations of all of the rural households in the State of Para that have been mapped by the Brazilian malaria combat agency (SUCAM). We also mapped and digitized areas of deforestation in the state of Tocantins, which is comprised largely of savanna (cerrado), an ecosystem that has been routinely excluded from deforestation mapping exercises
The Care/Security Nexus of the Humanitarian Border: Assisted Return in Norway
While Assisted Return and deportation are frequently viewed as two different return policies, the first represented as humanitarian and the latter as enforcement, this article argues that there is a continuum between these policies and that they form part of humanitarian border enforcement. Drawing on policy document analysis and interviews with NGOs and with irregular migrants, the article provides a two‐level analysis by examining how AR is presented from the Norwegian governmental perspective and how it is experienced from the Afghan migrant perspectives. The article argues that the government bases its AR policy on the need to maintain the credibility and sustainability of the asylum system, as part of fighting crime, while presenting it as a humanitarian solution. For irregular migrants, however, the experienced lack of proper asylum procedures delegitimizes return policies. Overall, the performative aspects of humanitarianism in return policies contribute to depoliticizing return.publishedVersio
09-05 "The Environmental Impacts of Soybean Expansion and Infrastructure Development in Brazil’s Amazon Basin"
For decades, the development of transportation infrastructure in the Brazilian Amazon has been the government’s main social and economic development policy in the region. Reductions in transportation costs have not only opened the agricultural frontier to cattle ranching and logging but have also caused more than two-thirds of Amazonian deforestation. Currently, soybean cultivation is a new economic force demanding improvements to roads in the region. Profitable soybean crops have spread over the Mato Grosso’s cerrados and now head toward the core of the Amazon rain forest. One of the main constraints for soy expansion into the Amazon has been the poor condition of roads. In this study, we analyze the effect Amazon transportation infrastructure programs have on soybean expansion by lowering transport costs. The analysis is based on spatial estimates of transportation costs for the soybean sector, first using current road networks and then projecting changes based on the paving of the Cuiabá-Santarém road. Our results indicate that paving the Cuiabá-Santarém road would reduce transportation costs by an average of 180 million for soybean farmers over a period of twenty years. These benefits, however, ignore the project’s environmental impacts. If the destruction of ecological services and products provided by the existing forests is accounted for, then the Cuiabá-Santarém investment would generate a net loss of between 1.9 billion. This result shows the importance of including the value of the natural capital in feasibility studies of infrastructure projects to reflect their real benefits to society as a whole.
A numerical study of two-photon ionization of helium using the Pyprop framework
Few-photon induced breakup of helium is studied using a newly developed ab
initio numerical framework for solving the six-dimensional time-dependent
Schroedinger equation. We present details of the method and calculate
(generalized) cross sections for the process of two-photon nonsequential
(direct) double ionization at photon energies ranging from 39.4 to 54.4 eV, a
process that has been very much debated in recent years and is not yet fully
understood. In particular, we have studied the convergence property of the
total cross section in the vicinity of the upper threshold (54.4 eV), versus
the pulse duration of the applied laser field. We find that the cross section
exhibits an increasing trend near the threshold, as has also been observed by
others, and show that this rise cannot solely be attributed to an unintended
inclusion of the sequential two-photon double ionization process, caused by the
bandwidth of the applied field.Comment: 7 pages, 3 figure
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