Showing papers by "Stephan Fritzsche published in 2015"

Scattering of twisted relativistic electrons by atoms

Abstract: The Mott scattering of high-energetic twisted electrons by atoms is investigated within the framework of the first Born approximation and Dirac's relativistic equation. Special emphasis is placed on the angular distribution and longitudinal polarization of the scattered electrons. In order to evaluate these angular and polarization properties we consider two experimental setups in which the twisted electron beam collides with either a single well-localized atom or macroscopic atomic target. Detailed relativistic calculations have been performed for both setups and for the electrons with kinetic energy from 10 to 1000 keV. The results of these calculations indicate that the emission pattern and polarization of outgoing electrons differ significantly from the scattering of plane-wave electrons and can be very sensitive to the parameters of the incident twisted beam. In particular, it is shown that the angular- and polarization-sensitive Mott measurements may reveal valuable information about both the transverse and longitudinal components of the linear momentum and the projection of the total angular momentum of twisted electron states. Thus, the Mott scattering emerges as a diagnostic tool for the relativistic vortex beams.

Collinear laser spectroscopy of atomic cadmium

Abstract: Hyperfine structure A and B factors of the atomic 5s 5p 3 P2 → 5s 6s 3 S1 transition are deter- mined from collinear laser spectroscopy data of 107�123 Cd and 111m�123m Cd. Nuclear magnetic moments and electric quadrupole moments are extracted using reference dipole moments and calculated electric field gradients, respectively. The hyperfine structure anomaly for isotopes with s1/2 and d5/2 nuclear ground states and isomeric h11/2 states is evaluated and a linear relationship is observed for all nuclear states except s1/2. This corresponds to the Moskowitz-Lombardi rule that was established in the mercury region of the nuclear chart but in the case of cadmium the slope is distinctively smaller than for mercury. In total four atomic and ionic levels were analyzed and all of them exhibit a similar behaviour. The electric field gradient for the atomic 5s 5p 3 P2 level is derived from multi-configuration Dirac-Hartree-Fock calculations in order to evaluate the spectroscopic nuclear quadrupole moments. The results are consistent with those obtained in an ionic transition and based on a similar calculation.

Narrowband inverse Compton scattering x-ray sources at high laser intensities

Abstract: Narrowband x- and $\ensuremath{\gamma}$-ray sources based on the inverse Compton scattering of laser pulses suffer from a limitation of the allowed laser intensity due to the onset of nonlinear effects that increase their bandwidth. It has been suggested that laser pulses with a suitable frequency modulation could compensate this ponderomotive broadening and reduce the bandwidth of the spectral lines, which would allow one to operate narrowband Compton sources in the high-intensity regime. In this paper we therefore present the theory of nonlinear Compton scattering in a frequency-modulated intense laser pulse. We systematically derive the optimal frequency modulation of the laser pulse from the scattering matrix element of nonlinear Compton scattering, taking into account the electron spin and recoil. We show that, for some particular scattering angle, an optimized frequency modulation completely cancels the ponderomotive broadening for all harmonics of the backscattered light. We also explore how sensitively this compensation depends on the electron-beam energy spread and emittance, as well as the laser focusing.

Polarization measurement of dielectronic recombination transitions in highly charged krypton ions

Abstract: The linear polarization of x rays generated during a dielectronic recombination --- an electron recombines with an ion and simultaneously excites a bound electron --- is measured in good agreement with theoretical predictions, leading to a refined technique to diagnose the electron currents and magnetic fields in both high-temperature astrophysical and laboratory plasmas.

Interaction of twisted light with many-electron atoms and ions

Abstract: The excitation of many-electron atoms and ions by twisted light has been studied within the framework of the density-matrix theory and Dirac's relativistic equation. Special attention is paid to the magnetic sublevel population of excited atomic states as described by means of the alignment parameters. General expressions for the alignment of the excited states are obtained under the assumption that the photon beam, prepared as a coherent superposition of two twisted Bessel states, irradiates a macroscopic target. We demonstrate that for this case the population of excited atoms can be sensitive to both the transverse momentum and the (projection of the) total angular momentum of the incident radiation. While the expressions are general and can be employed to describe the photoexcitation of any atom, independent on its shell structure and number of electrons, we performed calculations for the $3s\ensuremath{\rightarrow}3p$ transition in sodium. These calculations indicate that the ``twistedness'' of incoming radiation can lead to a measurable change in the alignment of the excited $^{2}P_{3/2}$ state as well as the angular distribution of the subsequent fluorescence emission.

Linear polarization of x-ray transitions due to dielectronic recombination in highly charged ions

Abstract: The linear polarization of x rays produced by dielectronic recombination into highly charged xenon ions was measured at an electron beam ion trap using the Compton polarimetry technique. This opens numerous possibilities for diagnostics of anisotropies of hot plasmas. Moreover, it was observed that the polarization of x rays, following the dielectronic capture populating the ${[1s2{s}^{2}2{p}_{1/2}]}_{1}$ state, is highly sensitive to the Breit interaction. The experimental results for this transition rule out by $5\ensuremath{\sigma}$ calculations not taking the Breit interaction into account.

Compton scattering of twisted light: Angular distribution and polarization of scattered photons

Abstract: Compton scattering of twisted photons is investigated within a nonrelativistic framework using first-order perturbation theory We formulate the problem in the density-matrix theory, which enables one to gain new insights into scattering processes of twisted particles by exploiting the symmetries of the system In particular, we analyze how the angular distribution and polarization of the scattered photons are affected by the parameters of the initial beam such as the opening angle and the projection of orbital angular momentum We present analytical and numerical results for the angular distribution and the polarization of Compton scattered photons for initially twisted light and compare them with the standard case of plane-wave light

High-resolution spectroscopy on the laser-cooling candidate La^{-}.

Abstract: The bound-bound transition from the 5d^{2}6s^{2} ^{3}F_{2}^{e} ground state to the 5d6s^{2}6p ^{3}D_{1}^{o} excited state in negative lanthanum has been proposed as a candidate for laser cooling, which has not yet been achieved for negative ions Anion laser cooling holds the potential to allow the production of ultracold ensembles of any negatively charged species We have studied the aforementioned transition in a beam of negative La ions by high-resolution laser spectroscopy The center-of-gravity frequency was measured to be 96592 80(10) THz Seven of the nine expected hyperfine structure transitions were resolved The observed peaks were unambiguously assigned to the predicted hyperfine transitions by a fit, confirmed by multiconfigurational self-consistent field calculations From the determined hyperfine structure we conclude that La^{-} is a promising laser cooling candidate Using this transition, only three laser beams would be required to repump all hyperfine levels of the ground state

Scattering of twisted relativistic electrons by atoms

Abstract: The Mott scattering of high-energetic twisted electrons by atoms is investigated within the framework of the first Born approximation and Dirac's relativistic equation. Special emphasis is placed on the angular distribution and longitudinal polarization of the scattered electrons. In order to evaluate these angular and polarization properties we consider two experimental setups in which the twisted electron beam collides with either a single well-localized atom or macroscopic atomic target. Detailed relativistic calculations have been performed for both setups and for the electrons with kinetic energy from 10 keV to 1000 keV. The results of these calculations indicate that the emission pattern and polarization of outgoing electrons differ significantly from the scattering of plane-wave electrons and can be very sensitive to the parameters of the incident twisted beam. In particular, it is shown that the angular- and polarization-sensitive Mott measurements may reveal valuable information about, both the transverse and longitudinal components of the linear momentum and the projection of the total angular momentum of twisted electron states. Thus, the Mott scattering emerges as a diagnostic tool for the relativistic vortex beams.

Caustic structures in the spectrum of x-ray Compton scattering off electrons driven by a short intense laser pulse

Abstract: We study the Compton scattering of x-rays off electrons that are driven by a relativistically intense short optical laser pulse. The frequency spectrum of the laser-assisted Compton radiation shows a broad plateau in the vicinity of the laser-free Compton line due to a nonlinear mixing between x-ray and laser photons. Special emphasis is placed on how the shape of the short assisting laser pulse affects the spectrum of the scattered x-rays. In particular, we observe sharp peak structures in the plateau region, whose number and locations are highly sensitive to the laser pulse shape. These structures are interpreted as spectral caustics by using a semiclassical analysis of the laser-assisted QED matrix element.

Ab initio MCDHF calculations of electron–nucleus interactions

Abstract: We present recent advances in the development of atomic ab initio multiconfiguration Dirac–Hartree–Fock theory, implemented in the GRASP relativistic atomic structure code. For neutral atoms, the deviations of properties calculated within the Dirac–Hartree–Fock (DHF) method (based on independent particle model of an atomic cloud) are usually dominated by electron correlation effects, i.e. the non-central interactions of individual electrons. We present the recent advances in accurate calculations of electron correlation effects in small, medium, and heavy neutral atoms. We describe methods of systematic development of multiconfiguration expansions leading to systematic, controlled improvement of the accuracy of the ab initio calculations. These methods originate from the concept of the complete active space (CAS) model within the DHF theory, which, at least in principle, permits fully relativistic calculations with full account of electron correlation effects. The calculations within the CAS model on currently available computer systems are feasible only for very light systems. For heavier atoms or ions with more than a few electrons, restrictions have to be imposed on the multiconfiguration expansions. We present methods and tools, which are designed to extend the numerical calculations in a controlled manner, where multiconfiguration expansions account for all leading electron correlation effects. We show examples of applications of the GRASP code to calculations of hyperfine structure constants, but the code may be used for calculations of arbitrary bound-state atomic properties. In recent years it has been applied to calculations of atomic and ionic spectra (transition energies and rates), to determinations of nuclear electromagnetic moments, as well as to calculations related to interactions of bound electrons with nuclear electromagnetic moments leading to violations of discrete symmetries.

Electron-ion collision spectroscopy: Lithium-like xenon ions

Abstract: The resonant process of dielectronic recombination (DR) has been applied as a spectroscopic tool to investigate intra-$L$-shell excitations $2s\ensuremath{-}2{p}_{j}$ in Li-like ${}^{136}{\text{Xe}}^{51}+$ The experiments were carried out at the electron cooler of the Experimental Storage Ring of the GSI-Helmholtzzentrum f\"ur Schwerionenforschung, Darmstadt, Germany The observed center-of-mass energy range (0--505 eV) covers all resonances associated with the $2s+{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{(2{p}_{1/2}n{l}_{j})}_{J}$ and ${(2{p}_{3/2}n{l}_{j})}_{J}$ DR processes Energies and strengths of isolated $2{p}_{1/2}n$ and $2{p}_{3/2}n$ DR-resonance groups were obtained for principal quantum numbers $n$ up to 43 and 36, respectively The $2s\ensuremath{-}2{p}_{1/2}$ and $2s\ensuremath{-}2{p}_{3/2}$ excitation energies were deduced to be 119816(42) eV and 492174(52) eV The excitation energies are compared with previous measurements of other groups and with recent QED calculations In addition, the experimental spectra and extracted resonance strengths are compared with multiconfiguration Dirac-Fock calculations Measurements and theory are found to be in good agreement with each other

Ionization of H 2 + molecular ions by twisted Bessel light

Abstract: The photoionization of ${\mathrm{H}}_{2}^{+}$ molecular ions is investigated for Bessel beams of twisted light. In particular, the angle-differential photoionization cross sections are evaluated for a macroscopic target of randomly distributed but initially aligned ions by using the nonrelativistic first-order perturbation theory. Detailed calculations of these cross sections and angular distributions are performed for different setups of the electron detectors and for selected opening angles of the Bessel beams and are compared with those for incident plane-wave radiation. It is shown that the modification in the angular distributions of the photoelectrons can be understood quite easily from the variations in the intensity pattern of the Bessel beams, relative to the size of the ${\mathrm{H}}_{2}^{+}$ molecular ions.

Spectroscopy of berylliumlike xenon ions using dielectronic recombination

Abstract: Be-like ions have been investigated employing the resonant electron–ion collision process of dielectronic recombination (DR) as a spectroscopic tool. The experiments were performed at the experimental storage ring in Darmstadt, Germany, using its electron cooler as a target for free electrons. DR Rydberg resonance series for the associated intra-L-shell transitions and were observed with high resolution. In addition to these excitations from the ground state we determined resonances associated with excitations of ions initially in the metastable state. The corresponding excitation energies were determined to be and and . These excitation energies are compared with previous measurements and with recent state-of-the-art atomic structure calculations.

Rayleigh x-ray scattering from many-electron atoms and ions

Abstract: A theoretical analysis is presented for the elastic Rayleigh scattering of x-rays by many-electron atoms and ions. Special emphasis is placed on the angular distribution and linear polarization of the scattered photons for the case when the incident light is completely (linearly) polarized. Based on second-order perturbation theory and the independent particle approximation, we found that the Rayleigh angular distribution is strongly affected by the charge state and shell structure of the target ions or atoms. This effect can be observed experimentally at modern synchrotron facilities and might provide further insight into the structure of heavy atomic systems.

Dielectronic recombination rate coefficients of initially rubidium-like tungsten

Abstract: Ab initio calculations of dielectronic recombination (DR) rate coefficients of initially rubidium-like W37+ ions have been performed for the electron temperatures from 1 eV to 5 × 104 eV, by using the Flexible Atomic Code based on the relativistic configuration-interaction method Special attention has been paid to the partial contributions to total DR rate coefficients as associated with the excitation of individual subshells A detailed comparison of the calculations shows that the excitation from 4p subshell dominates total DR rate coefficients followed by the excitations from 4s and 4d subshells, while the contribution of excitations from 3l(l = s, p, d) subshells becomes important only at high temperatures Besides, it is found that the electron excitations associated with △n = 0,1 dominate at low-temperature plasmas, however, the excitations associated with △n ≥ 2 become non-negligible at high-temperature ones

Relativistically prolonged lifetime of the 2s2p 3P0 level of zero nuclear-spin beryllium-like ions

Abstract: The E1M1 transition rate of the line in beryllium-like ions has been calculated within the framework of relativistic second-order perturbation theory. Both multiconfiguration and quantum-electrodynamical computations have been carried out independently to better understand and test for all major electron–electron correlation contributions in the representation of the initial, intermediate and final states. By comparing the results from these methods, which agree well for all ions along the beryllium isoelectronic sequence, the lifetime of the metastable 2s2p 3P0 level is found to be longer by about 2–3 orders of magnitude for all medium and heavy elements than was estimated previously. This makes the 3P0 level of beryllium-like ions to one of the longest living (low-lying) electronic excitations of a tightly bound system with potential applications for atomic clocks and in astro physics and plasma physics.

Stepwise contraction of the nf Rydberg shells in the 3d photoionization of multiply-charged xenon ions

Abstract: Triple photoionization of Xe3+, Xe4+ and Xe5+ ions has been studied in the energy range 670–750 eV, including the 3d ionization threshold. The photon-ion merged-beam technique was used at a synchrotron light source to measure the absolute photoionization cross sections. These cross sections exhibit a progressively larger number of sharp resonances as the ion charge state is increased. This clearly visualizes the re-ordering of the continuum into a regular series of (bound) Rydberg orbitals as the ionic core becomes more attractive. The energies and strengths of the resonances are extracted from the experimental data and are further analysed by relativistic atomic-structure calculations.

Triple ionization of atomic Cd involving 4 p$^{−1}$ and 4 s$^{−1}$ inner-shell holes

Abstract: The triple ionization spectrum of atomic Cd formed upon the removal of a 4p or a 4s inner-shell electron and subsequent Auger decays has been obtained at 200 eV photon energy. By using a versatile multielectron coincidence detection technique based on a magnetic bottle spectrometer in combination with multiconfiguration Dirac-Fock calculations, Auger cascades leading to tricationic final states have been analyzed and final-state configurations have been identified. The most prominent Auger cascades leading to the ground state of Cd3+ have been identified in good agreement with theory.

Compton polarimetry using double-sided segmented x-ray detectors

Abstract: Hard x-ray polarimetry of radiation emitted in collisions of heavy ions, electrons or photons with matter provides detailed information on the collision dynamics as well as of the atomic structure in the presence of extreme field strengths. Moreover, it also opens a route for polarization diagnosis of spin-polarized ion and electron beams which, for example, might be useful in future parity non-conservation studies. Owing to recent progress in the development of highly segmented solid-state detectors, a novel type of polarimeter for the hard x-ray regime has become available. Applied as Compton polarimeters, two-dimensional position-sensitive x-ray detectors now allow for precise and efficient measurements of x-ray linear polarization properties. In this report recent polarimetry studies using such detector systems are reviewed.

Combined linear polarization and angular distribution measurements of x-rays for precise determination of multipole-mixing in characteristic transitions of high-Z systems

Abstract: By applying novel-type position sensitive x-ray detectors as Compton polarimeters we recently performed a study of the linear polarization of Lyman- radiation following radiative electron capture into initially bare uranium ions. It was found that a model-independent determination of the ratio of the E1 and M2 transition amplitudes, and consequently of the corresponding transition rates, is feasible by combining the linear polarization data with a measurement of the angular distribution of the emitted radiation. In this work a detailed description of the underlying experimental technique for combined measurements of the linear polarization and the angular distribution of characteristic transitions in high-Z ions is presented. Special emphasis is given to the application of two, two-dimensional position-sensitive x-ray detectors for Compton polarimetry of hard x-rays. Moreover, we demonstrate the polarimeter efficiency of such detector systems can be significantly improved if events, where the charge is spread over neighboring segments, are reconstructed to be used in the polarization analysis.

Ground-state excitation of heavy highly-charged ions

Abstract: We have studied the excitation of H-like and He-like uranium (U91+ and U90+) in relativistic collisions with gaseous targets by observing the subsequent x-ray emission. The experiment was conducted at the ESR storage ring of the GSI accelerator facility in Darmstadt, Germany. The measurements were performed with a newly developed multi-phase target at different collision energies. This enabled us to explore the proton (nucleus) impact excitation as well as the electron impact excitation processes in the relativistic collisions. The large fine-structure splitting in uranium allowed us to unambiguously resolve excitation to different L-shell levels. Moreover, information about the population of different magnetic sublevels has been obtained via an angular differential study of the decay photons associated with the subsequent de-excitation process. The experimental results are compared with calculations performed within the relativistic framework including excitation mechanisms due to both protons (nucleus) and electrons.

Effect of bound-state dressing in laser-assisted radiative recombination

Abstract: We present a theoretical study on the recombination of a free electron into the ground state of a hydrogenlike ion in the presence of an external laser field. Emphasis is placed on the effects caused by the laser dressing of the residual ionic bound state. To investigate how this dressing affects the total and angle-differential cross section of laser-assisted radiative recombination (LARR) we apply first-order perturbation theory and the separable Coulomb-Volkov continuum ansatz. Using this approach, detailed calculations are performed for low-$Z$ hydrogenlike ions and laser intensities in the range from ${I}_{L}={10}^{12}$ to ${10}^{13}\phantom{\rule{4.pt}{0ex}}\text{W}/{\mathrm{cm}}^{2}$. It is seen that the total cross section as a function of the laser intensity is remarkably affected by the bound-state dressing. Moreover, the laser dressing becomes manifest as asymmetries in the angular distribution and the (energy) spectrum of the emitted recombination photons.

Relativistic configuration-interaction calculation of Kα transition energies in beryllium-like argon

Abstract: Relativistic configuration-interaction calculations have been performed for energy levels of the low-lying and core-excited states of beryllium-like argon, Ar14+. These calculations include the one-loop quantum electrodynamics (QED) effects as obtained by two different methods: the screening-potential approach and the model QED operator approach. The calculations are supplemented by a systematic estimation of the uncertainties of the theoretical predictions.

Nuclear magnetic dipole moment effect on the angular distribution of the Kα lines

Abstract: We present a theoretical analysis of the fine-structure transitions for helium-like heavy ions with non-zero nuclear spin. The angular distribution of these transitions is studied for its sensitivity with regard to the nuclear magnetic dipole moment . Detailed calculations, performed for the helium-like , and ions with nuclear spin , indicate that the emission pattern of the fine-structure resolved photons is significantly affected by and that this effect can be addressed experimentally at present storage ring facilities.

Target effects in negative-continuum-assisted dielectronic recombination

Abstract: The process of recombination of a quasifree electron into a bound state of an initially bare nucleus with the simultaneous creation of a bound-electron--free-positron pair is investigated. This process is called negative-continuum-assisted dielectronic recombination (NCDR). In a typical experimental setup, the initial electron is not free but bound in a light atomic target. In the present work, we study the effects of the atomic target on the single- and double-differential cross sections of positron production in the NCDR process. Calculations are performed within the relativistic framework based on QED theory, accounting for the electron-electron interaction to first order in perturbation theory. We demonstrate how the momentum distribution of the target electrons removes the nonphysical singularity of the differential cross section which occurs for the initially free and monochromatic electrons.

Linear polarization of x-rays emitted in the decay of highly-charged ions via overlapping resonances

Abstract: The linear polarization of x-rays, emitted from highly-charged ions, has been studied within the framework of the density matrix theory and the multiconfiguration Dirac-Fock method. Emphasis was placed especially on two-photon cascades that proceed via intermediate overlapping resonances. For such two-step cascades, we here explore how the level-splitting of the resonances affects the linear polarization of the x-rays, and whether modifications in the degree of polarization may help determine small level-splittings in multiply- and highly-charged ions, if carefully analyzed along isoelectronic sequences. Detailed calculations are carried out for the 1s2p2 Ji = 3/2 → 1s2s2p J = 1/2, 3/2 + γ1 → 1s22s Jf = 1/2 + γ1 + γ2 radiative cascade of lithium-like W71+ ions. For this cascade, a quite remarkable increase of the (degree of) linear polarization is found for the second-step γ2 photons, if the level-splitting becomes smaller than Δω 0.2 a.u. ≈ 5.4 eV. Accurate polarization measurements of x-rays may therefore be also utilized in the future to ascertain small level-splittings in multiply- and highly-charged ions.

Stepwise contraction of the nf Rydberg shells in the 3d photoionization of multiply-charged xenon ions

Abstract: Photoionization of Xe3+, Xe4+ and Xe5+ ions was measured in the energy range 670-750 eV, including the 3d ionization threshold. The cross sections exhibit a progressively larger number of sharp resonances as the ion charge state is increased. This clearly visualizes the re-ordering of the ef continuum into a regular series of (bound) Rydberg orbitals as the ionic core becomes more attractive.

Reply to “Comment on ‘Hyperfine-induced modifications to the angular distribution of the K α 1 x-ray emission’ ”

Abstract: In a recent work [Phys. Rev. A 89, 022513 (2014)], the $K{\ensuremath{\alpha}}_{1}$ ($1s2{p}_{3/2}{}^{1,3}P{}_{1,2}\ensuremath{\rightarrow}1{s}^{2}\phantom{\rule{0.16em}{0ex}}{}^{1}{S}_{0}$) x-ray emission following the radiative electron capture into initially hydrogen-like ions has been explored for ions with nonzero nuclear spin $(I\ensuremath{ e}0)$. A rather strong influence upon the angular distribution of the (hyperfine- and fine-structure averaged) $K{\ensuremath{\alpha}}_{1}$ radiation was found, especially for isotopes with nuclear spin $I=1/2$, while this effect are less important for isotopes with nuclear spin $Ig1/2$. Two comments were made by Inal and Benmouna about this work with regard to (i) the incoherent summation of the individual hyperfine components of the $1s2{p}_{3/2}\phantom{\rule{0.16em}{0ex}}{}^{1}{P}_{1}\ensuremath{\rightarrow}1{s}^{2}\phantom{\rule{0.16em}{0ex}}{}^{1}{S}_{0}$ transition and (ii) the treatment of the hyperfine-induced $E1\text{\ensuremath{-}}M2$ multipole mixing in the $1s2{p}_{3/2}\phantom{\rule{0.16em}{0ex}}{}^{3}{P}_{2}\ensuremath{\rightarrow}1{s}^{2},{}^{1}{S}_{0}$ fine-structure component. While we agree with the first comment and here provide updated anisotropy parameters, the hyperfine-induced modification of the $K{\ensuremath{\alpha}}_{1}$ emission remains valid and may help in the future to determine the nuclear parameters of radioactive isotopes. We also explain that the hyperfine-induced $E1\text{\ensuremath{-}}M2$ mixing has already been fully taken into account in our previous work.