232 research outputs found
Attosecond Control of Ionization Dynamics
Attosecond pulses can be used to initiate and control electron dynamics on a
sub-femtosecond time scale. The first step in this process occurs when an atom
absorbs an ultraviolet photon leading to the formation of an attosecond
electron wave packet (EWP). Until now, attosecond pulses have been used to
create free EWPs in the continuum, where they quickly disperse. In this paper
we use a train of attosecond pulses, synchronized to an infrared (IR) laser
field, to create a series of EWPs that are below the ionization threshold in
helium. We show that the ionization probability then becomes a function of the
delay between the IR and attosecond fields. Calculations that reproduce the
experimental results demonstrate that this ionization control results from
interference between transiently bound EWPs created by different pulses in the
train. In this way, we are able to observe, for the first time, wave packet
interference in a strongly driven atomic system.Comment: 8 pages, 4 figure
Macroscopic studies of short-pulse high-order harmonic generation using the time-dependent Schrödinger equation
We consider high harmonic generation by ultrashort (27–108 fs) laser pulses and calculate the macroscopic response of a collection of atoms to such a short pulse. We show how the harmonic spectrum after propagation through the medium is significantly different from the single-atom spectrum. We use single-atom data calculated by integration of the time-dependent Schrödinger equation and propose a method, based on an adiabatic approximation, to extract the data necessary to perform a propagation calculation. © 1998 The American Physical Society
0200: Predictors of angiographically visible distal embolization in ST elevation myocardial infarction patients treated by primary percutaneous coronary intervention and thrombectomy
Aspiration thrombectomy during primary percutaneous coronary intervention (p-PCI) in ST elevation myocardial infarction (STEMI) has been proposed to restore myocardial reperfusion. But important controversies remain concerning the usefulness of thrombectomy to improve this perfusion. Our objective was to evaluate the impact of manual thrombectomy on the occur-rence of angiographically visible distal embolization (AVDE) during p-PCI in STEMI. 346 consecutive patients admitted for STEMI who underwent p-PCI and thrombectomy were included. Clinical, angiographic and therapeutics characteristics were assessed. AVDE was defined as an abrupt vessel closure occurring at any point during the PCI procedure and that was not present at baseline. Patients were divided into 2 groups: with AVDE (59 patients) and without AVDE 287 patients.Comparing the 2 groups, patients with AVDE were more likely to be older than 60 year-old (67 vs. 50%; p=0.014), with a higher proportion of women (34 vs. 20%; p=0,027), of right coronary artery lesion (57 vs. 34.5%; p=0.001).Conversely, smoking and left anterior descending coronary artery lesion were more frequent in patients without AVDE (respectively 63 vs. 46%; p=0.014 and 50 vs. 30%; p=0.005).Univariate analysis identify age>60 (OR[95% CI]:2.09(1.15-3.78), p=0.015), female gender (OR[95% CI]:2.02(1.09-3.73), p=0,024), culprit coronary diameter>3mm (OR[95% CI] as predictors of AVDE during p-PCI for STEMI.By multivariate analysis, culprit coronary diameter>3mm (OR[95% CI]: 1.90 (1.01-3.56); p=0.048) and and the right coronary artery culprit lesion site (OR[95% CI]:2.48(1.36-4.52); p=0.003) were independent factors associated with AVDE. AVDE complicating p-PCI during STEMI is frequent (17%). Strikingly we found that successful thrombectomy does not prevent from AVDE. Conversely, we highlight that patients with culprit coronary artery diameter>3mm and STEMI due to right coronary artery occlusion are more likely to develop AVDE
Differences in measures of the fiscal multiplier and the reduced-form vector autoregression
The literature has recently asked whether the effects of fiscal policy vary with the state of the economy (Christiano, Eichenbaum, and Rebelo 2011; Rendahl 2014; Auerbach and Gorodnichenko 2012). We study this question in the context of vector autoregression (VAR) estimation. We show formally that, if (asymptotically) the parameters of the reduced-form VAR differ, then the dynamic effects of fiscal policy differ as well, generically and for any set of identification assumptions. Thus, in theory, the econometrician can detect these differences (either across time or space) generically just by relying on reduced-form VAR estimation
Coherent Electron Scattering Captured by an Attosecond Quantum Stroboscope
The basic properties of atoms, molecules and solids are governed by electron
dynamics which take place on extremely short time scales. To measure and
control these dynamics therefore requires ultrafast sources of radiation
combined with efficient detection techniques. The generation of extreme
ultraviolet (XUV) attosecond (1 as = 10-18 s) pulses has, for the first time,
made direct measurements of electron dynamics possible. Nevertheless, while
various applications of attosecond pulses have been demonstrated
experimentally, no one has yet captured or controlled the full three
dimensional motion of an electron on an attosecond time scale. Here we
demonstrate an attosecond quantum stroboscope capable of guiding and imaging
electron motion on a sub-femtosecond (1 fs = 10-15 s) time scale. It is based
on a sequence of identical attosecond pulses which are synchronized with a
guiding laser field. The pulse to pulse separation in the train is tailored to
exactly match an optical cycle of the laser field and the electron momentum
distributions are detected with a velocity map imaging spectrometer (VMIS).
This technique has enabled us to guide ionized electrons back to their parent
ion and image the scattering event. We envision that coherent electron
scattering from atoms, molecules and surfaces captured by the attosecond
quantum stroboscope will complement more traditional scattering techniques
since it provides high temporal as well as spatial resolution.Comment: 6 pages, 4 figure
Non-sequential triple ionization in strong fields
We consider the final stage of triple ionization of atoms in a strong
linearly polarized laser field. We propose that for intensities below the
saturation value for triple ionization the process is dominated by the
simultaneous escape of three electrons from a highly excited intermediate
complex. We identify within a classical model two pathways to triple
ionization, one with a triangular configuration of electrons and one with a
more linear one. Both are saddles in phase space. A stability analysis
indicates that the triangular configuration has the larger cross sections and
should be the dominant one. Trajectory simulations within the dominant symmetry
subspace reproduce the experimentally observed distribution of ion momenta
parallel to the polarization axis.Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev.
Attosecond electron spectroscopy using a novel interferometric pump-probe technique
We present an interferometric pump-probe technique for the characterization
of attosecond electron wave packets (WPs) that uses a free WP as a reference to
measure a bound WP. We demonstrate our method by exciting helium atoms using an
attosecond pulse with a bandwidth centered near the ionization threshold, thus
creating both a bound and a free WP simultaneously. After a variable delay, the
bound WP is ionized by a few-cycle infrared laser precisely synchronized to the
original attosecond pulse. By measuring the delay-dependent photoelectron
spectrum we obtain an interferogram that contains both quantum beats as well as
multi-path interference. Analysis of the interferogram allows us to determine
the bound WP components with a spectral resolution much better than the inverse
of the attosecond pulse duration.Comment: 5 pages, 4 figure
Generation of ultra-short light pulses by a rapidly ionizing thin foil
A thin and dense plasma layer is created when a sufficiently strong laser
pulse impinges on a solid target. The nonlinearity introduced by the
time-dependent electron density leads to the generation of harmonics. The pulse
duration of the harmonic radiation is related to the risetime of the electron
density and thus can be affected by the shape of the incident pulse and its
peak field strength. Results are presented from numerical
particle-in-cell-simulations of an intense laser pulse interacting with a thin
foil target. An analytical model which shows how the harmonics are created is
introduced. The proposed scheme might be a promising way towards the generation
of attosecond pulses.
PACS number(s): 52.40.Nk, 52.50.Jm, 52.65.RrComment: Second Revised Version, 13 pages (REVTeX), 3 figures in ps-format,
submitted for publication to Physical Review E, WWW:
http://www.physik.tu-darmstadt.de/tqe
XUV digital in-line holography using high-order harmonics
A step towards a successful implementation of timeresolved digital in-line
holography with extreme ultraviolet radiation is presented. Ultrashort XUV
pulses are produced as high-order harmonics of a femtosecond laser and a
Schwarzschild objective is used to focus harmonic radiation at 38 nm and to
produce a strongly divergent reference beam for holographic recording.
Experimental holograms of thin wires are recorded and the objects
reconstructed. Descriptions of the simulation and reconstruction theory and
algorithms are also given. Spatial resolution of few hundreds of nm is
potentially achievable, and micrometer resolution range is demonstrated.Comment: 8 pages, 8 figure
Neutrino - nucleon reaction rates in the supernova core in the relativistic random phase approximation
In view of the application to supernova simulations, we calculate neutrino
reaction rates with nucleons via the neutral and charged currents in the
supernova core in the relativistic random phase approximation (RPA) and study
their effects on the opacity of the supernova core. The formulation is based on
the Lagrangian employed in the calculation of nuclear equation of state (EOS)
in the relativistic mean field theory (RMF). The nonlinear meson terms are
treated appropriately so that the consistency of the density correlation
derived in RPA with the thermodynamic derivative obtained from EOS by RMF is
satisfied in the static and long wave length limit. We employ pion and rho
meson exchange interactions together with the phenomenological Landau-Migdal
parameters for the isospin-dependent nuclear interactions. We find that both
the charged and neutral current reaction rates are suppressed from the standard
Bruenn's approximate formula considerably in the high density regime. In the
low density regime, on the other hand, the vector current contribution to the
neutrino-nucleon scattering rate is enhanced in the vicinity of the boundary of
the liquid-gas phase transition, while the other contributions are moderately
suppressed there also. In the high temperature regime or in the regime where
electrons have a large chemical potential, the latter of which is important
only for the electron capture process and its inverse process, the recoil of
nucleons cannot be neglected and further reduces the reaction rates with
respect to the standard approximate formula which discards any energy transfer
in the processes. These issues could have a great impact on the neutrino
heating mechanism of collapse-driven supernovae.Comment: 16pages, 19figures, submitted to PR
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