490 research outputs found
Probing scattering phase shifts by attosecond streaking
Attosecond streaking is one of the most fundamental processes in attosecond
science allowing for a mapping of temporal (i.e. phase) information on the
energy domain. We show that on the single-particle level attosecond streaking
time shifts contain spectral phase information associated with the
Eisenbud-Wigner-Smith (EWS) time delay, provided the influence of the streaking
infrared field is properly accounted for. While the streaking phase shifts for
short-ranged potentials agree with the associated EWS delays, Coulomb
potentials require special care. We show that the interaction between the
outgoing electron and the combined Coulomb and IR laser fields lead to a
streaking phase shift that can be described classically
Ionization of helium by slow antiproton impact: total and differential cross sections
We theoretically investigate the single and double ionization of the He atom
by antiproton impact for projectile energies ranging from ~keV up to
~keV. We obtain accurate total cross sections by directly solving the
fully correlated two-electron time-dependent Schr\"odinger equation and by
performing classical trajectory Monte-Carlo calculations. The obtained
quantum-mechanical results are in excellent agreement with the available
experimental data. Along with the total cross sections, we also present the
first fully \textit{ab initio} doubly differential data for single ionization
at 10 and 100~keV impact energies. In these differential cross sections we
identify the binary-encounter peak along with the anticusp minimum.
Furthermore, we also point out the importance of the post-collisional
electron-projectile interaction at low antiproton energies which significantly
suppresses electron emission in the forward direction
Attosecond two-photon interferometry for doubly excited states of helium
We show that the correlation dynamics in coherently excited doubly excited
resonances of helium can be followed in real time by two-photon interferometry.
This approach promises to map the evolution of the two-electron wave packet
onto experimentally easily accessible non-coincident single electron spectra.
We analyze the interferometric signal in terms of a semi-analytical model which
is validated by a numerical solution of the time-dependent two-electron
Schr\"odinger equation in its full dimensionality.Comment: 5 pages, 4 figure
Probing Electron Correlation via Attosecond XUV Pulses in the Two-Photon Double Ionization of Helium
Recent experimental developments of high-intensity, short-pulse XUV light
sources are enhancing our ability to study electron-electron correlations. We
perform time-dependent calculations to investigate the so-called "sequential"
regime (photon energy above 54.4 eV) in the two-photon double ionization of
helium. We show that attosecond pulses allow to induce and probe angular and
energy correlations of the emitted electrons. The final momentum distribution
reveals regions dominated by the Wannier ridge break-up scenario and by
post-collision interaction.Comment: 4 pages, 5 figure
Universal features in sequential and nonsequential two-photon double ionization of helium
We analyze two-photon double ionization of helium in both the nonsequential
and sequential regime. We show that the energy spacing between the two emitted
electrons provides the key parameter that controls both the energy and the
angular distribution and reveals the universal features present in both the
nonsequential and sequential regime. This universality, i.e., independence of
photon energy, is a manifestation of the continuity across the threshold for
sequential double ionization. For all photon energies, the energy distribution
can be described by a universal shape function that contains only the spectral
and temporal information entering second-order time-dependent perturbation
theory. Angular correlations and distributions are found to be more sensitive
to the photon energy. In particular, shake-up interferences have a large effect
on the angular distribution. Energy spectra, angular distributions
parameterized by the anisotropy parameters, and total cross sections presented
in this paper are obtained by fully correlated time-dependent ab initio
calculations.Comment: 12 pages, 8 figure
Multi-electron transitions induced by neutron impact on helium
We explore excitation and ionization by neutron impact as a novel tool for
the investigation of electron-electron correlations in helium. We present
single and double ionization spectra calculated in accurate numerical ab-initio
simulations for incoming neutrons with kinetic energies of up to 150 keV. The
resulting electron spectra are found to be fundamentally different from
photoioniza- tion or charged particle impact due to the intrinsic many-body
character of the interaction. In particular, doubly excited resonances that are
strongly suppressed in electron or photon impact become prominent. The ratio of
double to single ionization is found to differ significantly from those of
photon and charged particle impact.Comment: 5 pages, 5 figure
More-Space â A Simulation Tool for University Room Management
As proposed in various studies, educational facilities hold a high potential yield for improvement of room utilization. The goal of the project âMoreSpaceâ at Vienna University of Technology (TU Vienna) was to develop a hybrid modeling approach which helps to increase the efficiency of the universityâs space utilization. Besides coupling of Discrete Event Simulation (DEVS), Agent-based (AB) methods and Cellular Automata (CA), successful deployment of such a model requires a thorough integration within the peripheral system. Which in turn leads to preconditions that have to be met, (e.g. by input - data, visualization of results, dissemination, etc.). This paper covers the methods applied for analyses of the model and the peripheral system, which enable model integration. For this is necessary to also focus on the psycho-social layer of the institution, as it is this layer that often leads to rejection of otherwise âgoodâ solutions by the people within institutions. The paper further describes a deployment matrix which puts the simulations - models mode of operation (i.e. one time utilization for consulting, recurrent and frequent utilization) into context with met preconditions and the required depth of system integration. This allows it to estimate whether a model can be deployed as intended or not; with alternatives being either a transformation of the system, reformulation of the question(s) towards the model or - in the worst case - abortion of the deployment process. In the latter case the value of the deployment matrix lies within an early judgment of the situation saving resources that would have been spend otherwise. In addition it is possible to use these for developing alternative solutions in support of the intentional goals
Nonsequential two-photon double ionization of helium
We present accurate time-dependent ab initio calculations on fully
differential and total integrated (generalized) cross sections for the
nonsequential two-photon double ionization of helium at photon energies from 40
to 54 eV. Our computational method is based on the solution of the
time-dependent Schroedinger equation and subsequent projection of the wave
function onto Coulomb waves. We compare our results with other recent
calculations and discuss the emerging similarities and differences. We
investigate the role of electronic correlation in the representation of the
two-electron continuum states, which are used to extract the ionization yields
from the fully correlated final wave function. In addition, we study the
influence of the pulse length and shape on the cross sections in time-dependent
calculations and address convergence issues.Comment: 14 pages, 10 figures; final version (acknowledgements and reference
added, typos fixed
Limitation of sucrose biosynthesis shapes carbon partitioning during plant cold acclimation
Cold acclimation is a multigenic process by which many plant species increase their freezing tolerance. Stabilization of photosynthesis and carbohydrate metabolism plays a crucial role in cold acclimation. To study regulation of primary and secondary metabolism during cold acclimation of Arabidopsis thaliana, metabolic mutants with deficiencies in either starch or flavonoid metabolism were exposed to 4 degrees C. Photosynthesis was determined together with amounts of carbohydrates, anthocyanins, organic acids and enzyme activities of the central carbohydrate metabolism. Starch deficiency was found to significantly delay soluble sugar accumulation during cold acclimation, while starch overaccumulation did not affect accumulation dynamics but resulted in lower total amounts of \sucrose and glucose. Anthocyanin amounts were lowered in both starch deficient and overaccumulating mutants. Vice versa, flavonoid deficiency did not result in a changed starch amount, which suggested a unidirectional signalling link between starch and flavonoid metabolism. Mathematical modelling of carbon metabolism indicated kinetics of sucrose biosynthesis to be limiting for carbon partitioning in leaf tissue during cold exposure. Together with cold-induced dynamics of citrate, fumarate and malate amounts, this provided evidence for a central role of sucrose phosphate synthase activity in carbon partitioning between biosynthetic and dissimilatory pathways which stabilizes photosynthesis and metabolism at low temperature
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