617 research outputs found
Strong-Field-Driven Electron Dynamics near an Ionization Threshold
Excitation of atoms with light that spectrally spans across the ionization threshold leads to free electrons initially located close to the parent ions. The dynamics of these ionized electrons driven by external light fields are studied by transient-absorption spectroscopy on helium in this thesis. Implementing an ab initio simulation, the time-dependent Schrödinger equation (TDSE) is solved for a model atom, giving direct access to the strong-field-driven evolution of the electron's wave function. In a separate theoretical approach, absorption spectra are calculated based on classical trajectories. Experimentally, an extension of a transient-absorption beamline allowed polarization-dependent measurements. The recorded absorption spectra of helium, depending on the intensity of a superimposed near-infrared femtosecond laser pulse with both linear and circular polarization, are presented and discussed. The observed absorption features can be assigned to two intensity regimes. For moderate intensities, light-induced states adequately explain the observed structures in the absorption spectra in agreement with the TDSE simulations. For higher laser intensities, the photon picture breaks down and the analysis of the dipole moment in the time domain suggests an interpretation in terms of classical electron trajectories. The combined theoretical and experimental investigation provides access to regimes in which the classical description of strong-field-driven electron dynamics emerges
Edge Intersection Graphs of L-Shaped Paths in Grids
In this paper we continue the study of the edge intersection graphs of one
(or zero) bend paths on a rectangular grid. That is, the edge intersection
graphs where each vertex is represented by one of the following shapes:
,, , , and we consider zero bend
paths (i.e., | and ) to be degenerate s. These graphs, called
-EPG graphs, were first introduced by Golumbic et al (2009). We consider
the natural subclasses of -EPG formed by the subsets of the four single
bend shapes (i.e., {}, {,},
{,}, and {,,}) and we
denote the classes by [], [,],
[,], and [,,]
respectively. Note: all other subsets are isomorphic to these up to 90 degree
rotation. We show that testing for membership in each of these classes is
NP-complete and observe the expected strict inclusions and incomparability
(i.e., [] [,],
[,] [,,]
-EPG; also, [,] is incomparable with
[,]). Additionally, we give characterizations and
polytime recognition algorithms for special subclasses of Split
[].Comment: 14 pages, to appear in DAM special issue for LAGOS'1
EPG-representations with small grid-size
In an EPG-representation of a graph each vertex is represented by a path
in the rectangular grid, and is an edge in if and only if the paths
representing an share a grid-edge. Requiring paths representing edges
to be x-monotone or, even stronger, both x- and y-monotone gives rise to three
natural variants of EPG-representations, one where edges have no monotonicity
requirements and two with the aforementioned monotonicity requirements. The
focus of this paper is understanding how small a grid can be achieved for such
EPG-representations with respect to various graph parameters.
We show that there are -edge graphs that require a grid of area
in any variant of EPG-representations. Similarly there are
pathwidth- graphs that require height and area in
any variant of EPG-representations. We prove a matching upper bound of
area for all pathwidth- graphs in the strongest model, the one where edges
are required to be both x- and y-monotone. Thus in this strongest model, the
result implies, for example, , and area bounds
for bounded pathwidth graphs, bounded treewidth graphs and all classes of
graphs that exclude a fixed minor, respectively. For the model with no
restrictions on the monotonicity of the edges, stronger results can be achieved
for some graph classes, for example an area bound for bounded treewidth
graphs and bound for graphs of bounded genus.Comment: Appears in the Proceedings of the 25th International Symposium on
Graph Drawing and Network Visualization (GD 2017
Multiscale Modeling of Influenza A Virus Infection Supports the Development of Direct-Acting Antivirals
Influenza A viruses are respiratory pathogens that cause seasonal epidemics with up to 500,000 deaths each year. Yet there are currently only two classes of antivirals licensed for treatment and drug-resistant strains are on the rise. A major challenge for the discovery of new anti-influenza agents is the identification of drug targets that efficiently interfere with viral replication. To support this step, we developed a multiscale model of influenza A virus infection which comprises both the intracellular level where the virus synthesizes its proteins, replicates its genome, and assembles new virions and the extracellular level where it spreads to new host cells. This integrated modeling approach recapitulates a wide range of experimental data across both scales including the time course of all three viral RNA species inside an infected cell and the infection dynamics in a cell population. It also allowed us to systematically study how interfering with specific steps of the viral life cycle affects virus production. We find that inhibitors of viral transcription, replication, protein synthesis, nuclear export, and assembly/release are most effective in decreasing virus titers whereas targeting virus entry primarily delays infection. In addition, our results suggest that for some antivirals therapy success strongly depends on the lifespan of infected cells and, thus, on the dynamics of virus-induced apoptosis or the host's immune response. Hence, the proposed model provides a systems-level understanding of influenza A virus infection and therapy as well as an ideal platform to include further levels of complexity toward a comprehensive description of infectious diseases
Cliophysics: Socio-political Reliability Theory, Polity Duration and African Political (In)stabilities
Quantification of historical sociological processes have recently gained
attention among theoreticians in the effort of providing a solid theoretical
understanding of the behaviors and regularities present in sociopolitical
dynamics. Here we present a reliability theory of polity processes with
emphases on individual political dynamics of African countries. We found that
the structural properties of polity failure rates successfully capture the risk
of political vulnerability and instabilities in which 87.50%, 75%, 71.43%, and
0% of the countries with monotonically increasing, unimodal, U-shaped and
monotonically decreasing polity failure rates, respectively, have high level of
state fragility indices. The quasi-U-shape relationship between average polity
duration and regime types corroborates historical precedents and explains the
stability of the autocracies and democracies.Comment: 4 pages, 3 figures, 1 tabl
Requirement of a Membrane Potential for the Posttranslational Transfer of Proteins into Mitochondsria
Posttranslational transfer of most precursor proteins into mitochondria is dependent on energization of the mitochondria. Experiments were carried out to determine whether the membrane potential or the intramitochondrial ATP is the immediate energy source. Transfer in vitro of precursors to the ADP/ATP carrier and to ATPase subunit 9 into isolated Neurospora mitochondria was investigated. Under conditions where the level of intramitochondrial ATP was high and the membrane potential was dissipated, import and processing of these precursor proteins did not take place. On the other hand, precursors were taken up and processed when the intramitochondrial ATP level was low, but the membrane potential was not dissipated. We conclude that a membrane potential is involved in the import of those mitochondrial precursor proteins which require energy for intracellular translocatio
Inhomogeneous point-process entropy: an instantaneous measure of complexity in discrete systems
Measures of entropy have been widely used to characterize complexity, particularly in physiological dynamical systems modeled in discrete time. Current approaches associate these measures to finite single values within an observation window, thus not being able to characterize the system evolution at each moment in time. Here, we propose a new definition of approximate and sample entropy based on the inhomogeneous point-process theory. The discrete time series is modeled through probability density functions, which characterize and predict the time until the next event occurs as a function of the past history. Laguerre expansions of the Wiener-Volterra autoregressive terms account for the long-term nonlinear information. As the proposed measures of entropy are instantaneously defined through probability functions, the novel indices are able to provide instantaneous tracking of the system complexity. The new measures are tested on synthetic data, as well as on real data gathered from heartbeat dynamics of healthy subjects and patients with cardiac heart failure and gait recordings from short walks of young and elderly subjects. Results show that instantaneous complexity is able to effectively track the system dynamics and is not affected by statistical noise properties
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