119 research outputs found
Electron Momentum Distributions from Strong-Field-Induced Ionization of Atoms and Molecules
High-intensity femtosecond laser pulses in the visible or infrared range can induce electron emission. This single-ionization process may be interpreted as a sequence of (nonadiabatic) tunnel ionization and subsequent acceleration of the electron by the external oscillating field in the presence of the electrostatic force between electron and parent ion. Based on the analysis of photoelectron momentum distributions from the numerical solution of the time-dependent Schrödinger equation, this thesis theoretically studies a variety of phenomena taking place in atoms as well as in molecules in strong fields. The underlying physical mechanisms are revealed by simplified models which take the nonperturbative character of the ionization process into account.
The simulation results for several settings are directly compared to measurements, offering the possibility to benchmark state-of-the-art theory and experiment against each other. One example of this is an investigation of the nonadiabatic strong-field ionization of atomic hydrogen in an attoclock setting. More generally, the deflection of the photoelectrons is analyzed in different attoclock configurations to explore the initial conditions of electrons at the tunnel exit—the position where the electron appears after tunneling. When a molecule is ionized, its orbital structure influences the liberated electron wave packet. The orbital imprint on the momentum-space phase of the wave packet, which encodes spatial information, is demonstrated and an interferometric approach to access these phases is evaluated. A characterization of the freed wave packet is crucial as it influences subsequent processes.
Such secondary processes are induced when the electron is driven back to the parent ion and scatters off. Similar to focusing of light by a lens, the Coulomb attraction forces scattered electron wave packets through focal points, causing a shift of their phase. Due to the interference of outgoing waves, these phases become visible in electron momentum distributions. For a faithful description, these focal-point effects must be included in a prefactor of the exponentiated action in semiclassical models. Furthermore, the control of electron scattering dynamics is demonstrated for low-energy electrons close to the continuum threshold by means of near-single-cycle terahertz pulses. The temporally-localized preparation of the electron wave packet by a femtosecond laser pulse at a well-defined time within the terahertz field enables a switching between different regimes of dynamics, ranging from recollision-free acceleration to extensive scattering phenomena.
In contrast to most studies in the electric dipole approximation that consider only the temporal evolution of the external electric field, various beyond-dipole effects in strong-field ionization are explored in the present work. The microscopic mechanisms of nondipole modifications are thoroughly analyzed. There, the effects of the spatially-varying electric field and of the magnetic field as well as their fingerprints on the geometry of the momentum distributions are identified. Furthermore, the subcycle time resolution of the light-induced momentum transfer in an attoclock-like setup is explored theoretically. Electron recollisions entirely change the observed nondipole effects and render the observations sensitive to the electronic target structure. The high-order above-threshold ionization caused by large-angle scattering is investigated both for exemplary atoms and for diatomic molecules through examination of nondipole shifts of the lateral momentum distribution. The phases of the electron wave packets are also altered by beyond-dipole effects. It is shown that this results in a displacement of ring-link structures known as above-threshold ionization rings that are caused by intercycle interference. In addition, the holographic structures arising from the subcycle interference of scattered and nonscattered electrons are modified
Signatures of electronic structure in bi-circular high-harmonic spectroscopy
High-harmonic spectroscopy driven by circularly-polarized laser pulses and
their counter-rotating second harmonic is a new branch of attosecond science
which currently lacks quantitative interpretations. We extend this technique to
the mid-infrared regime and record detailed high-harmonic spectra of several
rare-gas atoms. These results are compared with the solution of the
Schrodinger equation in three dimensions and calculations based on the
strong-field approximation that incorporate accurate scattering-wave
recombination matrix elements. A quantum-orbit analysis of these results
provides a transparent interpretation of the measured intensity ratios of
symmetry-allowed neighboring harmonics in terms of (i) a set of propensity
rules related to the angular momentum of the atomic orbitals, (ii)
atom-specific matrix elements related to their electronic structure and (iii)
the interference of the emissions associated with electrons in orbitals co- or
counter-rotating with the laser fields. These results provide the foundation
for a quantitative understanding of bi-circular high-harmonic spectroscopy.Comment: Accepted in Physical Review Letter
Attoclock with bicircular laser fields as a probe of velocity-dependent tunnel-exit positions
Strong-field ionization of atoms can be investigated on the attosecond time scale by using the attoclock method, i.e. by observing the peak of the photoelectron momentum distribution (PMD) after applying a laser pulse with a two-dimensional polarization form. Examples for such laser fields are close-to-circular or bicircular fields. Here, we report numerical solutions of the time-dependent Schrödinger equation for bicircular fields and a comparison with a compact classical model to demonstrate that the tunnel-exit position, i.e. the position where the electron emerges after tunnel ionization, is encoded in the PMD. We find that the tunnel-exit position depends on the transverse velocity of the tunneling electron. This gives rise to a momentum-dependent attoclock shift, meaning that the momentum shift due to the Coulomb force on the outgoing electron depends on which slice of the momentum distribution is analysed. Our finding is supported by a momentum-space-based implementation of the classical backpropagation method
American College of Cardiology/ European Society of Cardiology international study of angiographic data compression phase III Measurement of image quality differences at varying levels of data compression
AbstractOBJECTIVESWe sought to investigate up to which level of Joint Photographic Experts Group (JPEG) data compression the perceived image quality and the detection of diagnostic features remain equivalent to the quality and detectability found in uncompressed coronary angiograms.BACKGROUNDDigital coronary angiograms represent an enormous amount of data and therefore require costly computerized communication and archiving systems. Earlier studies on the viability of medical image compression were not fully conclusive.METHODSTwenty-one raters evaluated sets of 91 cine runs. Uncompressed and compressed versions of the images were presented side by side on one monitor, and image quality differences were assessed on a scale featuring six scores. In addition, the raters had to detect pre-defined clinical features. Compression ratios (CR) were 6:1, 10:1 and 16:1. Statistical evaluation was based on descriptive statistics and on the equivalence t-test.RESULTSAt the lowest CR (CR 6:1), there was already a small (15%) increase in assigning the aesthetic quality score indicating “quality difference is barely discernible—the images are equivalent.” At CR 10:1 and CR 16:1, close to 10% and 55%, respectively, of the compressed images were rated to be “clearly degraded, but still adequate for clinical use” or worse. Concerning diagnostic features, at CR 10:1 and CR 16:1 the error rate was 9.6% and 13.1%, respectively, compared with 9% for the baseline error rate in uncompressed images.CONCLUSIONSCompression at CR 6:1 provides equivalence with the original cine runs. If CR 16:1 were used, one would have to tolerate a significant increase in the diagnostic error rate over the baseline error rate. At CR 10:1, intermediate results were obtained
Kinematically complete experimental study of Compton scattering at helium atoms near the ionization threshold
Compton scattering is one of the fundamental interaction processes of light
with matter. Already upon its discovery [1] it was described as a billiard-type
collision of a photon kicking a quasi-free electron. With decreasing photon
energy, the maximum possible momentum transfer becomes so small that the
corresponding energy falls below the binding energy of the electron. Then
ionization by Compton scattering becomes an intriguing quantum phenomenon. Here
we report a kinematically complete experiment on Compton scattering at helium
atoms below that threshold. We determine the momentum correlations of the
electron, the recoiling ion, and the scattered photon in a coincidence
experiment finding that electrons are not only emitted in the direction of the
momentum transfer, but that there is a second peak of ejection to the backward
direction. This finding links Compton scattering to processes as ionization by
ultrashort optical pulses [2], electron impact ionization [3,4], ion impact
ionization [5,6], and neutron scattering [7] where similar momentum patterns
occur.Comment: 7 pages, 4 figure
Theory of Subcycle Linear Momentum Transfer in Strong-Field Tunneling Ionization
Interaction of a strong laser pulse with matter transfers not only energy but also linear momentum of the photons. Recent experimental advances have made it possible to detect the small amount of linear momentum delivered to the photoelectrons in strong-field ionization of atoms. We present numerical simulations as well as an analytical description of the subcycle phase (or time) resolved momentum transfer to an atom accessible by an attoclock protocol. We show that the light-field-induced momentum transfer is remarkably sensitive to properties of the ultrashort laser pulse such as its carrier-envelope phase and ellipticity. Moreover, we show that the subcycle-resolved linear momentum transfer can provide novel insights into the interplay between nonadiabatic and nondipole effects in strong-field ionization. This work paves the way towards the investigation of the so-far unexplored time-resolved nondipole nonadiabatic tunneling dynamics. © 2020 authors
Angular dependence of the Wigner time delay upon tunnel ionization of
More than 100 years after its discovery and its explanation in the energy
domain, the duration of the photoelectric effect is still heavily studied. The
emission time of a photoelectron can be quantified by the Wigner time delay.
Experiments addressing this time delay for single-photon ionization became
feasible during the last 10 years. A missing piece, which has not been studied,
so far, is the Wigner time delay for strong-field ionization of molecules. Here
we show experimental data on the Wigner time delay for tunnel ionization of
molecules and demonstrate its dependence on the emission direction of
the electron with respect to the molecular axis. We find, that the observed
changes in the Wigner time delay can be quantitatively explained by
elongated/shortened travel paths of the electrons that are due to spatial
shifts of the electron's birth position after tunneling. This introduces an
intuitive perspective towards the Wigner time delay in strong-field ionization.Comment: 17 pages, 6 figure
Generalised quantum weakest preconditions
Generalisation of the quantum weakest precondition result of D'Hondt and
Panangaden is presented. In particular the most general notion of quantum
predicate as positive operator valued measure (POVM) is introduced. The
previously known quantum weakest precondition result has been extended to cover
the case of POVM playing the role of a quantum predicate. Additionally, our
result is valid in infinite dimension case and also holds for a quantum
programs defined as a positive but not necessary completely positive
transformations of a quantum states.Comment: 7 pages, no figures, added references, changed conten
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