Referierte Publikationen

2017

S. Schippers, M. Martins, R. Beerwerth, S. Bari, K. Holste, K. Schubert, J. Viefhaus, D. W. Savin, S. Fritzsche, and A. Müller
Near L-edge Single and Multiple Photoionization of Singly Charged Iron Ions
Astrophys. J., 849 :5 (October 2017)
Abstract:
Absolute cross-sections for m -fold photoionization (m=1, ... , 6 ) of Fe+ by a single photon were measured employing the photon–ion merged-beams setup PIPE at the PETRA III synchrotron light source, operated by DESY in Hamburg, Germany. Photon energies were in the range 680–920 eV, which covers the photoionization resonances associated with 2p and 2s excitation to higher atomic shells as well as the thresholds for 2p and 2s ionization. The corresponding resonance positions were measured with an uncertainty of ±0.2 eV. The cross-section for Fe+ photoabsorption is derived as the sum of the individually measured cross-sections for m -fold ionization. Calculations of the Fe+ absorption cross-sections were carried out using two different theoretical approaches, Hartree–Fock including relativistic extensions and fully relativistic multiconfiguration Dirac–Fock. Apart from overall energy shifts of up to about 3 eV, the theoretical cross-sections are in good agreement with each other and with the experimental results. In addition, the complex de-excitation cascades after the creation of inner-shell holes in the Fe+ ion were tracked on the atomic fine-structure level. The corresponding theoretical results for the product charge-state distributions are in much better agreement with the experimental data than previously published configuration-average results. The present experimental and theoretical results are valuable for opacity calculations and are expected to pave the way to a more accurate determination of the iron abundance in the interstellar medium.
M. Gebhardt, C. Gaida, T. Heuermann, F. Stutzki, C. Jauregui, J. Antonio-Lopez, A. Schulzgen, R. Amezcua-Correa, J. Limpert, and A. Tünnermann
Nonlinear pulse compression to 43  W GW-class few-cycle pulses at 2  μm wavelength
Opt. Lett., 42 :4179 (October 2017)
Abstract:
High-average power laser sources delivering intense few-cycle pulses in wavelength regions beyond the near infrared are promising tools for driving the next generation of high-flux strong-field experiments. In this work, we report on nonlinear pulse compression to 34.4 μJ-, 2.1-cycle pulses with 1.4 GW peak power at a central wavelength of 1.82 μm and an average power of 43 W. This performance level was enabled by the combination of a high-repetition-rate ultrafast thulium-doped fiber laser system and a gas-filled antiresonant hollow-core fiber.
A. Volchkova, A. Varentsova, N. Zubova, V. Agababaev, D. Glazov, A. Volotka, V. Shabaev, and G. Plunien
Nuclear magnetic shielding in boronlike ions
Nucl. Instr. Meth. Phys. Res. B, 408 :89 (October 2017)
Abstract:
The relativistic treatment of the nuclear magnetic shielding effect in boronlike ions is presented. The leading-order contribution of the magnetic-dipole hyperfine interaction is calculated. Along with the standard second-order perturbation theory expression, the solutions of the Dirac equation in the presence of magnetic field are employed. All methods are found to be in agreement with each other and with the previous calculations for hydrogenlike and lithiumlike ions. The effective screening potential is used to account approximately for the interelectronic interaction.
I. Maltsev, V. Shabaev, I. Tupitsyn, Y. Kozhedub, G. Plunien, and T. Stöhlker
Pair production in low-energy collisions of uranium nuclei beyond the monopole approximation
Nucl. Instr. Meth. Phys. Res. B, 408 :97 (October 2017)
Abstract:
A method for calculation of electron-positron pair production in low-energy heavy-ion collisions beyond the monopole approximation is presented. The method is based on the numerical solving of the time-dependent Dirac equation with the full two-center potential. The one-electron wave functions are expanded in the finite basis set constructed on the two-dimensional spatial grid. Employing the developed approach the probabilities of bound-free pair production are calculated for collisions of bare uranium nuclei at the energy near the Coulomb barrier. The obtained results are compared with the corresponding values calculated in the monopole approximation.
V. Agababaev, A. Volchkova, A. Varentsova, D. Glazov, A. Volotka, V. Shabaev, and G. Plunien
Quadratic Zeeman effect in boronlike argon
Nucl. Instr. Meth. Phys. Res. B, 408 :70 (October 2017)
Abstract:
Abstract A theoretical investigation of the second-order Zeeman effect in boronlike ions is presented. Rigorous calculations of the one-photon-exchange and one-loop QED corrections allow for predictions of the corresponding theoretical values for boronlike argon with an accuracy of about 2%. The obtained results are important in view of the forthcoming measurements of the Zeeman splitting in 40Ar13+ at GSI (ARTEMIS experiment).
J. Hofbrucker, A. Volotka, and S. Fritzsche
Relativistic effects in the non-resonant two-photon K-shell ionization of neutral atoms
Nucl. Instr. Meth. Phys. Res. B, 408 :125 (October 2017)
Abstract:
Relativistic effects in the non-resonant two-photon K-shell ionization of neutral atoms are studied theoretically within the framework of second-order perturbation theory. The non-relativistic results are compared with the relativistic calculations in the dipole and no-pair approximations as well as with the complete relativistic approach. The calculations are performed in both velocity and length gauges. Our results show a significant decrease of the total cross section for heavy atoms as compared to the non-relativistic treatment, which is mainly due to the relativistic wavefunction contraction. The effects of higher multipoles and negative continuum energy states counteract the relativistic contraction contribution, but are generally much weaker. While the effects beyond the dipole approximation are equally important in both gauges, the inclusion of negative continuum energy states visibly contributes to the total cross section only in the velocity gauge.
G. Sarri, J. Warwick, W. Schumaker, K. Poder, J. Cole, D. Doria, T. Dzelzainis, K. Krushelnick, S. Kuschel, S. P. D. Mangles, Z. Najmudin, L. Romagnani, G. M. Samarin, D. Symes, A. G. R. Thomas, M. Yeung, and M. Zepf
Spectral and spatial characterisation of laser-driven positron beams
Plasma Phys. Contr. F., 59 :014015 (October 2017)
Abstract:
The generation of high-quality relativistic positron beams is a central area of research in experimental physics, due to their potential relevance in a wide range of scientific and engineering areas, ranging from fundamental science to practical applications. There is now growing interest in developing hybrid machines that will combine plasma-based acceleration techniques with more conventional radio-frequency accelerators, in order to minimise the size and cost of these machines. Here we report on recent experiments on laser-driven generation of high-quality positron beams using a relatively low energy and potentially table-top laser system. The results obtained indicate that current technology allows to create, in a compact setup, positron beams suitable for injection in radio-frequency accelerators.
M. Kübel, Z. Dube, A. Yu. Naumov, M. Spanner, G.G. Paulus, M. F. Kling, D. M. Villeneuve, P. B. Corkum, and A. Staudte
Streak Camera for Strong-Field Ionization
Phys. Rev. Lett., 119 :183201 (October 2017)
Abstract:
Ionization of an atom or molecule by a strong laser field produces suboptical cycle wave packets whose control has given rise to attosecond science. The final states of the wave packets depend on ionization and deflection by the laser field, which are convoluted in conventional experiments. Here, we demonstrate a technique enabling efficient electron deflection, separate from the field driving strong-field ionization. Using a midinfrared deflection field permits one to distinguish electron wave packets generated at different field maxima of an intense few-cycle visible laser pulse. We utilize this capability to trace the scattering of low-energy electrons driven by the midinfrared field. Our approach represents a general technique for studying and controlling strong-field ionization dynamics on the attosecond time scale.
F. Karbstein
Tadpole diagrams in constant electromagnetic fields
J. High Energ. Phys., 2017 :75 (October 2017)
Abstract:
We show how all possible one-particle reducible tadpole diagrams in constant electromagnetic fields can be constructed from one-particle irreducible constant-field diagrams. The construction procedure is essentially algebraic and involves differentiations of the latter class of diagrams with respect to the field strength tensor and contractions with derivatives of the one-particle irreducible part of the Heisenberg-Euler effective Lagrangian in constant fields. Specific examples include the two-loop addendum to the Heisenberg-Euler effective action as well as a novel one-loop correction to the charged particle propagator in constant electromagnetic fields discovered recently. As an additional example, the approach devised in the present article is adopted to derive the tadpole contribution to the two-loop photon polarization tensor in constant fields for the first time.
J. Buldt, M. Müller, R. Klas, T. Eidam, J. Limpert, and A. Tünnermann
Temporal contrast enhancement of energetic laser pulses by filtered self-phase-modulation-broadened spectra
Opt. Lett., 42 :3761 (October 2017)
Abstract:
We present a novel approach for temporal contrast enhancement of energetic laser pulses by filtered self-phase-modulation-broadened spectra. A measured temporal contrast enhancement by at least seven orders of magnitude in a simple setup has been achieved. This technique is applicable to a wide range of laser parameters and poses a highly efficient alternative to existing contrast-enhancement methods.
R. Müller, A. Volotka, S. Fritzsche, and A. Surzhykov
Theoretical analysis of the electron bridge process in 229Th3+
Nucl. Instr. Meth. Phys. Res. B, 408 :84 (October 2017)
Abstract:
We investigate the deexcitation of the 229Th nucleus via the excitation of an electron. Detailed calculations are performed for the enhancement of the nuclear decay width due to the so called electron bridge (EB) compared to the direct photoemission from the nucleus. The results are obtained for triply ionized thorium by using a B-spline pseudo basis approach to solve the Dirac equation for a local xα potential. This approach allows for an approximation of the full electron propagator including the positive and negative continuum. We show that the contribution of continua slightly increases the enhancement compared to a propagator calculated by a direct summation over bound states. Moreover we put special emphasis on the interference between the direct and exchange Feynman diagrams that can have a strong influence on the enhancement.
A. Varentsova, V. Agababaev, A. Volchkova, D. Glazov, A. Volotka, V. Shabaev, and G. Plunien
Third-order Zeeman effect in highly charged ions
Nucl. Instr. Meth. Phys. Res. B, 408 :80 (October 2017)
Abstract:
The contribution of the third order in magnetic field to the Zeeman splitting of the ground state of hydrogenlike, lithiumlike, and boronlike ions in the range Z=6-82 is investigated within the relativistic approach. Both perturbative and non-perturbative methods of calculation are employed and found to be in agreement. For lithiumlike and boronlike ions the interelectronic-interaction effects are taken into account within the approximation of the local screening potential. The contribution of the third-order effect in low- and medium-Z boronlike ions is found to be important for anticipated high-precision measurements.
F. Siek, S. Neb, P. Bartz, M. Hensen, C. Strüber, S. Fiechter, M. Torrent-Sucarrat, V. Silkin, E. Krasovskii, N. Kabachnik, S. Fritzsche, R. Muiño, P. Echenique, A. Kazansky, N. Müller, W. Pfeiffer, and U. Heinzmann
Angular momentum–induced delays in solid-state photoemission enhanced by intra-atomic interactions
Science, 357 :1274 (September 2017)
Abstract:
Attosecond time-resolved photoemission spectroscopy reveals that photoemission from solids is not yet fully understood. The relative emission delays between four photoemission channels measured for the van der Waals crystal tungsten diselenide (WSe2) can only be explained by accounting for both propagation and intra-atomic delays. The intra-atomic delay depends on the angular momentum of the initial localized state and is determined by intra-atomic interactions. For the studied case of WSe2, the photoemission events are time ordered with rising initial-state angular momentum. Including intra-atomic electron-electron interaction and angular momentum of the initial localized state yields excellent agreement between theory and experiment. This has required a revision of existing models for solid-state photoemission, and thus, attosecond time-resolved photoemission from solids provides important benchmarks for improved future photoemission models.
S. H. Hendi, B. Panah, S. Panahiyan, and M. Momennia
Dilatonic black holes in gravity's rainbow with a nonlinear source: the effects of thermal fluctuations
Eur. Phys. J. C, 77 :647 (September 2017)
Abstract:
This paper is devoted to an investigation of nonlinearly charged dilatonic black holes in the context of gravity's rainbow with two cases: (1) by considering the usual entropy, (2) in the presence of first order logarithmic correction of the entropy. First, exact black hole solutions of dilatonic Born—Infeld gravity with an energy dependent Liouville-type potential are obtained. Then, thermodynamic properties of the mentioned cases are studied, separately. It will be shown that although mass, entropy and the heat capacity are modified due to the presence of a first order correction, the temperature remains independent of it. Furthermore, it will be shown that divergences of the heat capacity, hence phase transition points are also independent of a first order correction, whereas the stability conditions are highly sensitive to variation of the correction parameter. Except for the effects of a first order correction, we will also present a limit on the values of the dilatonic parameter and show that it is possible to recognize AdS and dS thermodynamical behaviors for two specific branches of the dilatonic parameter. In addition, the effects of nonlinear electromagnetic field and energy functions on the thermodynamical behavior of the solutions will be highlighted and dependency of critical behavior, on these generalizations will be investigated.
D. Khaghani, M. Lobet, B. Borm, L. Burr, F. Gärtner, L. Gremillet, L. Movsesyan, O. Rosmej, M. Toimil-Molares, F. Wagner, and P. Neumayer
Enhancing laser-driven proton acceleration by using micro-pillar arrays at high drive energy
Sci. Rep., 7 :11366 (September 2017)
Abstract:
The interaction of micro- and nano-structured target surfaces with high-power laser pulses is being widely investigated for its unprecedented absorption efficiency. We have developed vertically aligned metallic micro-pillar arrays for laser-driven proton acceleration experiments. We demonstrate that such targets help strengthen interaction mechanisms when irradiated with high-energy-class laser pulses of intensities ~10^17–18 W/cm2. In comparison with standard planar targets, we witness strongly enhanced hot-electron production and proton acceleration both in terms of maximum energies and particle numbers. Supporting our experimental results, two-dimensional particle-in-cell simulations show an increase in laser energy conversion into hot electrons, leading to stronger acceleration fields. This opens a window of opportunity for further improvements of laser-driven ion acceleration systems.
J. Deprince, S. Fritzsche, T. Kallman, P. Palmeri, and P. Quinet
Influence of plasma environment on K-line emission in highly ionized iron atoms evaluated using a Debye–Hückel model
Can. J. Phys., 95 :858 (September 2017)
Abstract:
The influence of plasma environment on the atomic parameters associated with the K-vacancy states has been investigated theoretically for several iron ions. To do this, a time-averaged Debye–Hückel potential for both the electron–nucleus and electron–electron interactions has been considered in the framework of relativistic multiconfiguration Dirac–Fock computations. More particularly, the plasma screening effects on ionization potentials, K-thresholds, transition energies, and radiative rates have been estimated in the astrophysical context of accretion disks around black holes. In the present paper, we describe the behaviour of those atomic parameters for Ne-, Na-, Ar-, and K-like iron ions.
M. Bilal, R. Beerwerth, A. V. Volotka, and S. Fritzsche
Ab initio calculations of energy levels, transition rates and lifetimes in Ni xii
MNRAS, 469 :4620 (August 2017)
Abstract:
We report large-scale multi-configuration Dirac–Hartree–Fock calculations and relativistic configuration interaction calculations for allowed E1 and forbidden transitions (M1, E2, M2) among the fine structure levels of the 3s^2 3p^5, 3s 3p^6 and 3s^2 3p^4 3d configurations for Ni xii. In our systematically enlarged wave functions, we incorporated the effects of relativity, all important electron correlations and rearrangement of the bound electron density within two different computational models. We compare our calculated energies for the fine structure levels with previous calculations and experiments. We validate all the tentative experimental lines recently identified by Del Zanna & Badnell with one exception. We discuss the consistency of our transition rates in comparison to semi-empirical predictions. We present ab initio lifetime values by taking into account all allowed E1 and forbidden transitions (M1, E2, M2) rates among lowest 31 levels. Our results for lifetime values are better than previously reported ab initio and semi-empirical values as compared to available experiments, thus, providing reliable predictions in the prospects of future experiments.
M. Kozák, P. Beck, H. Deng, J. McNeur, N. Schönenberger, C. Gaida, F. Stutzki, M. Gebhardt, J. Limpert, A. Ruehl, I. Hartl, O. Solgaard, J. S. Harris, R. L. Byer, and P. Hommelhoff
Acceleration of sub-relativistic electrons with an evanescent optical wave at a planar interface
Opt. Express, 25 :19195 (August 2017)
Abstract:
We report on a theoretical and experimental study of the energy transfer between an optical evanescent wave, propagating in vacuum along the planar boundary of a dielectric material, and a beam of sub-relativistic electrons. The evanescent wave is excited via total internal reflection in the dielectric by an infrared (λ = 2 μm) femtosecond laser pulse. By matching the electron propagation velocity to the phase velocity of the evanescent wave, energy modulation of the electron beam is achieved. A maximum energy gain of 800 eV is observed, corresponding to the absorption of more than 1000 photons by one electron. The maximum observed acceleration gradient is 19 ± 2 MeV/m. The striking advantage of this scheme is that a structuring of the acceleration element’s surface is not required, enabling the use of materials with high laser damage thresholds that are difficult to nano-structure, such as SiC, Al2O3 or CaF2.
J. Xu, A. Buck, S.-W. Chou, K. Schmid, B. Shen, T. Tajima, M.C. Kaluza, and L. Veisz
Dynamics of electron injection in a laser-wakefield accelerator
Phys. Plasmas, 24 :083106 (August 2017)
Abstract:
The detailed temporal evolution of the laser-wakefield acceleration process with controlled injection, producing reproducible high-quality electron bunches, has been investigated. The localized injection of electrons into the wakefield has been realized in a simple way - called shock-front injection - utilizing a sharp drop in plasma density. Both experimental and numerical results reveal the electron injection and acceleration process as well as the electron bunch's temporal properties. The possibility to visualize the plasma wave gives invaluable spatially resolved information about the local background electron density, which in turn allows for an efficient suppression of electron self-injection before the controlled process of injection at the sharp density jump. Upper limits for the electron bunch duration of 6.6 fs FWHM, or 2.8 fs (r.m.s.) were found. These results indicate that shock-front injection not only provides stable and tunable, but also few-femtosecond short electron pulses for applications such as ultrashort radiation sources, time-resolved electron diffraction or for the seeding of further acceleration stages.
A. V. Malyshev, D. A. Glazov, A. V. Volotka, I. I. Tupitsyn, V. M. Shabaev, G. Plunien, and T. Stöhlker
Ground-state ionization energies of boronlike ions
Phys. Rev. A, 96 :022512 (August 2017)
Abstract:
High-precision QED calculations of ground-state ionization energies are performed for all boronlike ions with nuclear charge numbers in the range 16≤Z≤96. Rigorous QED calculations are performed within the extended Furry picture and include all many-electron QED effects up to the second order of the perturbation theory. The contributions of third- and higher-order electron-correlation effects are accounted for within the Breit approximation. Nuclear recoil and nuclear polarization effects are taken into account as well. In comparison with previous evaluations of the ground-state ionization energies of boronlike ions the accuracy of the theoretical predictions is improved significantly.
D. Wu, and J. W. Wang
Magetostatic amplifier with tunable maximum by twisted-light plasma interactions
Plasma Phys. Contr. F., 59 :095010 (August 2017)
Abstract:
Laser beams with Laguerre–Gaussian (LG) mode carry orbital angular momentum (OAM); however, when interacting with plasmas, the net angular momentum acquired by plasmas is basically zero after interaction. Here, we find when there exists a small magetostatic seed along the laser propagation direction, the barrier would be broken, giving rise to dramatic angular momentum transfer from LG-lasers to plasmas. Hence, the net OAM remaining in the plasmas system would continuously enhance the magetostatic field, until the corresponding Larmor frequency of electrons is comparable to the laser frequency in vacuum. Three-dimensional particle-in-cell simulations are performed to confirm our theory, producing spatial-uniform, temporal-stable and extremely-intense magetostatic fields.
S. Fuchs, M. Wünsche, J. Nathanael, J. J. Abel, C. Rödel, J. Biedermann, J. Reinhard, U. Hübner, and G.G. Paulus
Optical coherence tomography with nanoscale axial resolution using a laser-driven high-harmonic source
Optica, 4 :903 (August 2017)
Abstract:
Extreme ultraviolet microscopy is technologically demanding and thus largely confined to synchrotron radiation facilities. However, specific benefits like high resolution and exceptional material contrast provide strong motivation for the development of table-top alternatives. We report on the first demonstration of coherence tomography, i.e., noninvasive cross-sectional imaging, with high harmonics. A depth resolution of 24 nm and very good material contrast are achieved. Excessively demanding optics for extreme ultraviolet radiation are avoided and artifacts due to the elementary geometry are suppressed with a novel three-step one-dimensional phase-retrieval algorithm. The images are recorded in reflection geometry, facilitating the analysis of, e.g., operating semiconductor samples.
A. A. Peshkov, D. Seipt, A. Surzhykov, and S. Fritzsche
Photoexcitation of atoms by Laguerre-Gaussian beams
Phys. Rev. A, 96 :023407 (August 2017)
Abstract:
In a recent experiment, Schmiegelow et al. [Nat. Commun. 7, 12998 (2016)] investigated the magnetic sublevel population of Ca^+ ions in a Laguerre-Gaussian light beam if the target atoms were just centered along the beam axis. They demonstrated in this experiment that the sublevel population of the excited atoms is uniquely defined by the projection of the orbital angular momentum of the incident light. However, little attention has been paid so far to the question of how the magnetic sublevels are populated when atoms are displaced from the beam axis by some impact parameter b. Here, we analyze this sublevel population for different atomic impact parameters in first-order perturbation theory and by making use of the density-matrix formalism. Detailed calculations are performed especially for the 4s ^2S_1/2 -> 3d ^2D_5/2 transition in Ca^+ ions and for the vector potential of a Laguerre-Gaussian beam in Coulomb gauge. It is shown that the magnetic sublevel population of the excited ^2D_5/2 level varies significantly with the impact parameter and is sensitive to the polarization, the radial index, as well as the orbital angular momentum of the incident light beam.
C. Scullion, D. Doria, L. Romagnani, A. Sgattoni, K. Naughton, D. R. Symes, P. McKenna, A. Macchi, M. Zepf, S. Kar, and M. Borghesi
Polarization Dependence of Bulk Ion Acceleration from Ultrathin Foils Irradiated by High-Intensity Ultrashort Laser Pulses
Phys. Rev. Lett., 119 :054801 (August 2017)
Abstract:
The acceleration of ions from ultrathin (10–100 nm) carbon foils has been investigated using intense (∼6×10²⁰ W cm⁻²) ultrashort (45 fs) laser pulses, highlighting a strong dependence of the ion beam parameters on the laser polarization, with circularly polarized (CP) pulses producing the highest energies for both protons and carbons (25−30  MeV/nucleon); in particular, carbon ion energies obtained employing CP pulses were significantly higher (∼2.5 times) than for irradiations employing linearly polarized pulses. Particle-in-cell simulations indicate that radiation pressure acceleration becomes the dominant mechanism for the thinnest targets and CP pulses.
Z. W. Wu, A. V. Volotka, A. Surzhykov, and S. Fritzsche
Angle-resolved x-ray spectroscopic scheme to determine overlapping hyperfine splittings in highly charged heliumlike ions
Phys. Rev. A, 96 :012503 (July 2017)
Abstract:
An angle-resolved x-ray spectroscopic scheme is presented for determining the hyperfine splitting of highly charged ions. For heliumlike ions, in particular, we propose to measure either the angular distribution or polarization of the 1s2p ^3P_1,F -> 1s^2 ^1S_0,F_f emission following the stimulated decay of the initial 1s 2s ^1S_0,F_i level. It is found that both the angular and polarization characteristics of the emitted x-ray photons strongly depend on the (relative) splitting of the partially overlapping hyperfine 1s 2p ^3P_1,F resonances and may thus help resolve their hyperfine structure. The proposed scheme is feasible with present-day photon detectors and allows a measurement of the hyperfine splitting of heliumlike ions with a relative accuracy of about 10^-4.