arXiv subjects

S. Fritzsche

Publications and source records attributed to S. Fritzsche.

At least 55 records · Page 3Linked to original sources

arXiv2019

Radiative electron capture as a tunable source of highly linearly polarized x-rays

The radiative electron capture (REC) into the K shell of bare Xe ions colliding with a hydrogen gas target has been investigated. In this study, the degree of linear polarization of the K-REC radiation was measured and compared with rigorous relativistic calculations as well as with the previous results recorded for U$^{92+}$. Owing to the improved detector technology a significant gain in precision of the present polarization measurement is achieved compared to the previously published results. The obtained data confirms that for medium-Z ions such as Xe the REC process is a source of highly polarized x-rays which can easily be tuned with respect to the degree of linear polarization and the photon energy. We argue, in particular, that for relatively low energies the photons emitted under large angles are almost fully linear polarized.

physics.atom-ph↗

arXiv2019

Plasma environment effects on K lines of astrophysical interest. II. Ionization potentials, K thresholds, radiative rates and Auger widths in Ne- through He-like iron ions (Fe xvii - Fe xxv)

Aims. In the context of accretion disks around black holes, we estimate plasma-environment effects on the atomic parameters associated with the decay of K-vacancy states in highly charged iron ions, namely Fe xvii - Fe xxv. Methods. Within the relativistic multiconfiguration Dirac-Fock (MCDF) framework, the electron-nucleus and electron-electron plasma screenings are approximated with a time-averaged Debye-Huckel potential. Results. Modified ionization potentials, K thresholds, wavelengths, radiative emission rates and Auger widths are reported for astrophysical plasmas characterized by electron temperatures and densities respectively in the ranges 1E5 - 1E7 K and 1E18 - 1E22 cm-3 . Conclusions. We conclude that the high-resolution micro-calorimeters onboard future X-ray missions such as XRISM and ATHENA are expected to be sensitive to the lowering of the iron K edge due to the extreme plasma conditions occurring in accretion disks around compact objects.

physics.atom-ph↗

arXiv2019

Ground-state hyperfine splitting of B-like ions in high-Z region

The hyperfine splitting of the ground state of selected B-like ions within the range of nuclear charge numbers $Z=49-83$ is investigated in detail. The rigorous QED approach together with the large-scale configuration-interaction Dirac-Fock-Sturm method are employed for the evaluation of the interelectronic-interaction contributions of first and higher orders in $1/Z$. The screened QED corrections are evaluated to all orders in $αZ$ by using an effective potential. The influence of nuclear magnetization distribution is taken into account within the single-particle nuclear model.

physics.atom-ph↗

arXiv2019

Ab initio QED treatment of the two-photon annihilation of positrons with bound electrons

The process of a positron $\unicode{x2013}$ bound-electron annihilation with simultaneous emission of two photons is investigated theoretically. A fully relativistic formalism based on $\textit{ab initio}$ QED description of the process is worked out. The developed approach is applied to evaluate the annihilation of a positron with $K$-shell electrons of a silver atom, for which a strong contradiction between theory and experiment was previously stated. The results obtained here resolve this long-standing disagreement and, moreover, demonstrate a sizeable difference with approaches so far used for calculations of the positron $\unicode{x2013}$ bound-electron annihilation process, namely, the $\textit{Lee's}$ and $\textit{impulse}$ approximations.

physics.atom-ph↗

arXiv2018

Diagnostics of polarization purity of x-rays by means of Rayleigh scattering

The synchrotron radiation is commonly known to be completely linearly polarized when observed in the orbital plane of the synchrotron motion. Under actual experimental conditions, however, the degree of polarization of the synchrotron radiation may be lower than the ideal 100%. We demonstrate that even tiny impurities of polarization of the incident radiation can drastically affect the polarization of the elastically scattered light. We propose to use this effect as a precision tool for the diagnostics of the polarization purity of the synchrotron radiation. Two variants of the diagnostics method are proposed. The first one is based on the polarization measurements of the scattered radiation and relies on theoretical calculations of the transition amplitudes. The second one involves simultaneous measurements of the polarization and the cross sections of the scattered radiation and is independent of theoretical amplitudes.

physics.atom-ph↗

arXiv2018

The photon-ion merged-beams experiment PIPE at PETRAIII - The first five years

The Photon-Ion Spectrometer at PETRA III - in short, PIPE - is a permanently installed user facility at the "Variable Polarization XUV Beamline" P04 of the synchrotron light source PETRA III operated by DESY in Hamburg, Germany. The careful design of the PIPE ion-optics in combination with the record-high photon flux at P04 has lead to a breakthrough in experimental studies of photon interactions with ionized small quantum systems. This short review provides an overview over the published scientific results from photon-ion merged-beams experiments at PIPE that were obtained since the start of P04 operations in 2013. The topics covered comprise photoionization of ions of astrophysical relevance, quantitative studies of multi-electron processes upon inner-shell photoexcitation and photoionization of negative and positive atomic ions, precision spectroscopy of photoionization resonances, photoionization and photofragmentation of molecular ions and of endohedral fullerene ions.

physics.atom-ph↗

arXiv2018

Controlling quantum random walk with a step-dependent coin

We report on the possibility of controlling quantum random walks with a step-dependent coin. The coin is characterized by a (single) rotation angle. Considering different rotation angles, one can find diverse probability distributions for this walk including: complete localization, Gaussian and asymmetric likes. In addition, we explore the entropy of walk in two contexts; for probability density distributions over position space and walker's internal degrees of freedom space (coin space). We show that entropy of position space can decrease for a step-dependent coin with the step-number, quite in contrast to a walk with step-independent coin. For entropy of coin space, a damped oscillation is found for walk with step-independent coin while for a step-dependent coin case, the behavior of entropy depends on rotation angle. In general, we demonstrate that quantum walks with simple initiatives may exhibit a quite complex and varying behavior if step-dependent coins are applied. This provides the possibility of controlling quantum random walk with a step-dependent coin.

quant-ph↗

arXiv2018

Maximum Elliptical Dichroism in Atomic Two-Photon Ionization

Elliptical dichroism is known in atomic photoionization as the difference in the photoelectron angular distributions produced in nonlinear ionization of atoms by left- and right-handed elliptically polarized light. We theoretically demonstrate that the maximum dichroism |Δ_{ED}|=1 always appears in two-photon ionization of any atom if the photon energy is tuned in so that the electron emission is dominantly determined by two intermediate resonances. We propose the two-photon ionization of atomic helium in order to demonstrate this remarkable phenomenon. The maximum elliptical dichroism could be used as a sensitive tool for analyzing the polarization state of photon beams produced by free-electron lasers.

physics.atom-ph↗

arXiv2018

Elastic scattering of twisted electrons by diatomic molecules

The elastic scattering of twisted electrons by diatomic molecules is studied within the framework of the non-relativistic first Born approximation. In this process, the coherent interaction of incident electrons with two molecular centers may cause interference patterns in the angular distributions of outgoing particles. We investigate how this Young-type interference is influenced by the complex internal structure of twisted beams. In particular, we show that the corkscrew-like phase front and the inhomogeneous intensity profile of the incident beam can strongly modify the angular distribution of electrons, scattered off a single well-localized molecule. For the collision with a macroscopic target, composed of randomly distributed but aligned molecules, the angular-differential cross section may reveal valuable information about the transverse and longitudinal momenta of twisted states. In order to illustrate the difference between the scattering of twisted and plane-wave beams for both, single-molecule and macroscopic-target scenarios, detailed calculations have been performed for a H_2 target.

physics.atom-ph↗

arXiv2018

Rayleigh scattering of twisted light by hydrogenlike ions

The elastic Rayleigh scattering of twisted light and, in particular, the polarization (transfer) of the scattered photons have been analyzed within the framework of second-order perturbation theory and Dirac's relativistic equation. Special attention was paid hereby to the scattering on three different atomic targets: single atoms, a mesoscopic (small) target, and a macroscopic (large) target, which are all centered with regard to the beam axis. Detailed calculations of the polarization Stokes parameters were performed for C^{5+} ions and for twisted Bessel beams. It is shown that the polarization of scattered photons is sensitive to the size of an atomic target and to the helicity, the opening angle, and the projection of the total angular momentum of the incident Bessel beam. These computations indicate more that the Stokes parameters of the (Rayleigh) scattered twisted light may significantly differ from their behaviour for an incident plane-wave radiation.

physics.atom-ph↗

arXiv2018

Dispersive refraction of different light-to-heavy materials at MeV $γ$-ray energies

The dispersive behavior of materials with atomic charge numbers varing from $Z = 4$ (beryllium, Be) to $Z = 82$ (lead, Pb) was investigated experimentally and theoretically at $γ$-ray energies up to 2 MeV. The experiment was performed at the double-crystal gamma spectrometer GAMS6 of the ILL in Grenoble. The experimental results were compared with theoretical calculations which account for all major elastic processes involved. Overall, we found a good agreement between theory and experiment. We find that for the development of refractive optics at $γ$-ray energies beyond those currently in use high-Z materials become increasingly attractive compared to the beryllium lens-stacks used at X-ray energies.

physics.atom-ph↗

arXiv2018

Double-slit experiment in momentum space

Young's classic double-slit experiment demonstrates the reality of interference when waves and particles travel simultaneously along two different spatial paths. Here, we propose a double-slit experiment in momentum space, realized in the free-space elastic scattering of vortex electrons. We show that this process proceeds along two paths in momentum space, which are well localized and well separated from each other. For such vortex beams, the (plane-wave) amplitudes along the two paths acquire adjustable phase shifts and produce interference fringes in the final angular distribution. We argue that this experiment can be realized with the present day technology. We show that it gives experimental access to the Coulomb phase, a quantity which plays an important role in all charged particle scattering but which usual scattering experiments are insensitive to.

quant-ph↗

arXiv2017

Ground-state hyperfine splitting for Rb, Cs, Fr, Ba^+, and Ra^+

We have systematically investigated the ground-state hyperfine structure for alkali-metal atoms ^{87}Rb, ^{133}Cs, ^{211}Fr and alkali-metal-like ions ^{135}Ba^+, ^{225}Ra^+, which are of particular interest for parity violation studies. The quantum electrodynamic one-loop radiative corrections have been rigorously evaluated within an extended Furry picture employing core-Hartree and Kohn-Sham atomic potentials. Moreover, the effect of the nuclear magnetization distribution on the hyperfine structure intervals has been studied in detail and its uncertainty has been estimated. Finally, the theoretical description of the hyperfine structure has been completed with full many-body calculations performed in the all-orders correlation potential method.

physics.atom-ph↗

arXiv2017

Angle-resolved x-ray spectroscopic scheme to determine overlapping hyperfine splittings in highly charged helium-like ions

An angle-resolved x-ray spectroscopic scheme is presented for determining the hyperfine splitting of highly charged ions. For helium-like ions, in particular, we propose to measure either the angular distribution or polarization of the $1s2p~^{3}P_{1}, F \rightarrow 1s^{2}~^{1}S_{0}, F_{f}$ emission following the stimulated decay of the initial $1s2s~^{1}S_{0}, F_{i}$ level. It is found that both the angular and polarization characteristics of the emitted x-ray photons strongly depends on the (relative) \textit{splitting} of the partially overlapping hyperfine $1s2p~^{3}P_{1}, F$ 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 helium-like ions with a relative accuracy of about $10^{-4}$.

physics.atom-ph↗

arXiv2017

Photoelectron distribution of non-resonant two-photon ionization of neutral atoms

Photoelectron angular distributions following the non-resonant two-photon K-shell ionization of neutral atoms are studied theoretically. Using the independent particle approximation and relativistic second-order perturbation theory, the contributions of screening and relativistic effects to the photoelectron angular distribution are evaluated. A simple nonrelativistic expression is presented for the angle-differential cross section in dipole approximation for two-photon ionization by elliptically polarized photons, and its limitations are analyzed numerically. Moreover, we show that screening effects of the inactive electrons can significantly affect the photoelectron distributions and can also lead to a strong elliptical dichroism. Numerical results are resented for the case of two-photon K-shell ionization of neutral Ne, Ge, Xe, and U atoms.

physics.atom-ph↗

arXiv2017

Theoretical analysis of the electron bridge process in $^{229}$Th$^{3+}$

We investigate the deexcitation of the $^{229}$Th nucleus via the excitation of an electron. Detailed calculations are performed for the enhancement of the nuclear decay width due to this so called electron bridge (EB) compared to the direct photoemission from the nucleus. The results are obtianed for triply ionized thorium by using a B-spline pseudo basis approach to solve the Dirac equation for a local $x_α$ 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.

physics.atom-ph↗

arXiv2017

Particle-in-cell simulations of laser-plasma interactions at solid densities and relativistic intensities: the role of atomic processes

Direct studies of intense laser-solid interactions is still of great challenges, because of the many coupled physical mechanisms, such as direct laser heating, ionization dynamics, collision among charged particles, and electrostatic or electromagnetic instabilities, to name just a few. Here, we present a full particle-in-cell simulation (PIC) framework, which enables us to calculate laser-solid interactions in a "first principle" way, covering almost "all" the coupled physical mechanisms. Apart from the mechanisms above, the numerical self-heating of PIC simulations, which usually appears in solid-density plasmas, is also well controlled by the proposed "layered-density" method. This method can be easily implemented into the state-of-the-art PIC codes. Especially, the electron heating/acceleration at relativistically intense laser-solid interactions in the presence of large scale pre-formed plasmas is re-investigated by this PIC code. Results indicate that collisional damping (even though it is very week) can significantly influence the electron heating/acceleration in front of the target. Furthermore the Bremsstrahlung radiation will be enhanced by $2\sim3$ times when the solid is dramatically heated and ionized. For the considered case, where laser is of intensity $10^{20}\ \text{W}/\text{cm}^2$ and pre-plasma in front of the solid target is of scale-length $10\ μ\text{m}$, collision damping coupled with ionization dynamics and Bremsstrahlung radiations is shown to lower the "cut-off" electron energy by $25\%$. In addition, the resistive electromagnetic fields due to Ohmic-heating also play a non-ignorable role and must be included in real laser-solid interactions.

physics.plasm-ph↗

arXiv2016

Nuclear excitation by two-photon electron transition

A new mechanism of nuclear excitation via two-photon electron transitions (NETP) is proposed and studied theoretically. As a generic example, detailed calculations are performed for the $E1E1$ $1s2s\,^1S_0 \rightarrow 1s^2\,^1S_0$ two-photon decay of He-like $^{225}$Ac$^{87+}$ ion with the resonant excitation of the $3/2+$ nuclear state with the energy 40.09(5) keV. The probability for such a two-photon decay via the nuclear excitation is found to be $P_{\rm NETP} = 3.5 \times 10^{-9}$ and, thus, is comparable with other mechanisms, such as nuclear excitation by electron transition and by electron capture. The possibility for the experimental observation of the proposed mechanism is thoroughly discussed.

physics.atom-ph↗