arXiv subjects

Stefan Schippers

Publications and source records attributed to Stefan Schippers.

At least 19 recordsLinked to original sources

arXiv2026

Commissioning of a fast fine-step electron-energy-scan system for electron-ion crossed-beams experiments

We report on the commissioning of a fast electron-energy scan system for measurements of electron-impact ionization cross sections. The Giessen crossed-beams experiment employs a high-power electron gun which has been developed over recent years and which permits wide variations of the beam parameters. A multi-electrode design enables the decoupling of the electron energy from the electron density. The newly implemented control system, which governs the various electrode potentials, is described together with the salient technical features.

physics.atom-ph↗

arXiv2025

Upgrades and maintenance of the CRYRING@ESR electron cooler for improved internal electron target operation

The electron cooler of the CRYRING@ESR storage ring at the GSI-FAIR accelerator complex is a unique instrument, which not only provides beam cooling of the stored ions, but also serves as a low-energy electron target for Dielectronic Recombination experiments. The minimisation of vacuum contamination and the response to rapid energy changes are key requirements of the cooler in electron target mode. Therefore, a test bench was prepared to study the outgassing behaviour of the electron gun components and a setup was constructed to evaluate the drift-electrode-modulation of the acceleration voltage of the cooler. The vacuum studies showed that the electron gun cathode was severely malfunctioning and resulted in its replacement. The drift-electrode-modulation of the acceleration voltage showed significant improvements compared to direct modulation of the terminal voltage of the cooler.

physics.acc-ph↗

arXiv2024

Dissociative recombination of rotationally cold ArH$^+$

We have experimentally studied dissociative recombination (DR) of electronically and vibrationally relaxed ArH$^+$ in its lowest rotational levels, using an electron--ion merged-beams setup at the Cryogenic Storage Ring. We report measurements for the merged-beams rate coefficient of ArH$^+$ and compare it to published experimental and theoretical results. In addition, by measuring the kinetic energy released to the DR fragments, we have determined the internal state of the DR products after dissociation. At low collision energies, we find that the atomic products are in their respective ground states, which are only accessible via non-adiabatic couplings to neutral Rydberg states. Published theoretical results for ArH$^+$ have not included this DR pathway. From our measurements, we have also derived a kinetic temperature rate coefficient for use in astrochemical models.

physics.atom-ph↗

arXiv2024

Experimental and theoretical total cross sections for single and double ionization of the open-$4d$-shell ions Xe$^{12+}$, Xe$^{13+}$, and Xe$^{14+}$ by electron impact

We present new experimental and theoretical cross sections for electron-impact single ionization of Xe$^{12+}$ and Xe$^{13+}$ ions, and double ionization of Xe$^{12+}$, Xe$^{13+}$ and Xe$^{14+}$ ions for collision energies from the respective ionization thresholds up to 3500 eV. The calculations use the fully relativistic subconfiguration-averaged distorted-wave (SCADW) approach and, partly, the more detailed level-to-level distorted wave (LLDW) method. We find that, unlike in previous work, our theoretical cross sections agree with our experimental ones within the experimental uncertainties, except for the near-threshold double-ionization cross sections. We attribute this remaining discrepancy to the neglect of direct-double ionization in the present theoretical treatment.

physics.atom-ph↗

arXiv2024

High-Resolution X-Ray Spectroscopy of Interstellar Iron Toward Cygnus X-1 and GX 339-4

We present a high-resolution spectral study of Fe L-shell extinction by the diffuse interstellar medium (ISM) in the direction of the X-ray binaries Cygnus X-1 and GX 339-4, using the XMM-Newton reflection grating spectrometer. The majority of interstellar Fe is suspected to condense into dust grains in the diffuse ISM, but the compounds formed from this process are unknown. Here, we use the laboratory cross sections from Kortright & Kim (2000) and Lee et al. (2009) to model the absorption and scattering profiles of metallic Fe, and the crystalline compounds fayalite (Fe$_2$SiO$_4$), ferrous sulfate (FeSO$_4$), hematite ($α$-Fe$_2$O$_3$), and lepidocrocite ($γ$-FeOOH), which have oxidation states ranging from Fe$^{0}$ to Fe$^{3+}$. We find that the observed Fe L-shell features are systematically offset in energy from the laboratory measurements. An examination of over two dozen published measurements of Fe L-shell absorption finds a 1-2 eV scatter in energy positions of the L-shell features. Motivated by this, we fit for the best energy-scale shift simultaneously with the fine structure of the Fe L-shell extinction cross sections. Hematite and lepidocrocite provide the best fits ($\approx +1.1$ eV shift), followed by fayalite ($\approx +1.8$ eV shift). However, fayalite is disfavored, based on the implied abundances and knowledge of ISM silicates gained by infrared astronomical observations and meteoritic studies. We conclude that iron oxides in the Fe$^{3+}$ oxidation state are good candidates for Fe-bearing dust. To verify this, new absolute photoabsorption measurements are needed on an energy scale accurate to better than 0.2 eV.

astro-ph.GA↗

arXiv2023

Dissociative Recombination of Rotationally Cold OH$^+$ and Its Implications for the Cosmic Ray Ionization Rate in Diffuse Clouds

Observations of OH$^+$ are used to infer the interstellar cosmic ray ionization rate in diffuse atomic clouds, thereby constraining the propagation of cosmic rays through and the shielding by interstellar clouds, as well as the low energy cosmic ray spectrum. In regions where the H$_2$ to H number density ratio is low, dissociative recombination (DR) is the dominant destruction process for OH$^+$ and the DR rate coefficient is important for predicting the OH$^+$ abundance and inferring the cosmic ray ionization rate. We have experimentally studied DR of electronically and vibrationally relaxed OH$^+$ in its lowest rotational levels, using an electron--ion merged-beams setup at the Cryogenic Storage Ring. From these measurements, we have derived a kinetic temperature rate coefficient applicable to diffuse cloud chemical models, i.e., for OH$^+$ in its electronic, vibrational, and rotational ground level. At typical diffuse cloud temperatures, our kinetic temperature rate coefficient is a factor of $\sim 5$ times larger than the previous experimentally derived value and a factor of $\sim 33$ times larger than the value calculated by theory. Our combined experimental and modelling results point to a significant increase for the cosmic ray ionization rate inferred from observations of OH$^+$ and H$_2$O$^+$, corresponding to a geometric mean of $(6.6 \pm 1.0) \times 10^{-16}\,\mathrm{s}^{-1}$, which is more than a factor of two larger than the previously inferred values of the cosmic ray ionization rate in diffuse atomic clouds. Combined with observations of diffuse and dense molecular clouds, these findings indicate a greater degree of cosmic ray shielding in interstellar clouds than has been previously inferred.

astro-ph.GA↗

arXiv2022

Experimental determination of the dissociative recombination rate coefficient for rotationally-cold CH$^{+}$ and its implications for the diffuse cloud chemistry

Observations of CH$^+$ are used to trace the physical properties of diffuse clouds, but this requires an accurate understanding of the underlying CH$^+$ chemistry. Until this work, the most uncertain reaction in that chemistry was dissociative recombination (DR) of CH$^+$. Using an electron-ion merged-beams experiment at the Cryogenic Storage Ring, we have determined the DR rate coefficient of the CH$^+$ electronic, vibrational, and rotational ground state applicable for different diffuse cloud conditions. Our results reduce the previously unrecognized order-of-magnitude uncertainty in the CH$^+$ DR rate coefficient to $\sim \pm 20\%$ and are applicable at all temperatures relevant to diffuse clouds, ranging from quiescent gas to gas locally heated by processes such as shocks and turbulence. Based on a simple chemical network, we find that DR can be an important destruction mechanism at temperatures relevant to quiescent gas. As the temperature increases locally, DR can continue to be important up to temperatures of $ \sim 600\,\mathrm{K} $ if there is also a corresponding increase in the electron fraction of the gas. Our new CH$^+$ DR rate coefficient data will increase the reliability of future studies of diffuse cloud physical properties via CH$^+$ abundance observations.

astro-ph.GA↗

arXiv2020

Near L-Edge Single and Multiple Photoionization of Doubly Charged Iron Ions

Using the photon-ion merged-beams technique at a synchrotron light source, we have measured relative cross sections for single and up to five-fold photoionization of Fe$^{2+}$ ions in the energy range 690--920 eV. This range contains thresholds and resonances associated with ionization and excitation of $2p$ and $2s$ electrons. Calculations were performed to simulate the total absorption spectra. The theoretical results show very good agreement with the experimental data, if overall energy shifts of up to 2.5 eV are applied to the calculated resonance positions and assumptions are made about the initial experimental population of the various levels of the Fe$^{2+}$([Ar]$3d^6$) ground configuration. Furthermore, we performed extensive calculations of the Auger cascades that result when an electron is removed from the $2p$ subshell of Fe$^{2+}$. These computations lead to a better agreement with the measured product-charge-state distributions as compared to earlier work. We conclude that the $L$-shell absorption features of low-charged iron ions are useful for identifying gas-phase iron in the interstellar medium and for discriminating against the various forms of condensed-phase iron bound to composite interstellar dust grains.

physics.atom-ph↗

arXiv2020

Photoionization of astrophysically relevant atomic ions at PIPE

We review recent work on the photoionization of atomic ions of astrophysical interest that has been carried out at the photon-ion merged-beams setup PIPE, a permanently installed end station at the XUV beamline P04 of the PETRAIII synchrotron radiation source operated by DESY in Hamburg, Germany. Our results on single and multiple L-shell photoionization of Fe+, Fe2+, and Fe3+ ions and on single and multiple K-shell photoionization of C-, C+, C4+, Ne+, and Si2+ ions are discussed in astrophysical contexts. Moreover, these experimental results bear witness of the fact, that the implementation of the photon-ion merged-beams method at one of the world's brightest synchrotron light sources has led to a breakthrough for the experimental study of atomic inner-shell photoionization processes with ions.

physics.atom-ph↗

arXiv2019

Electron-impact single ionisation of W14+ ions: Subconfiguration-average and level-to-level distorted wave calculations

The cross section for electron-impact single ionisation of W14+ ions has been calculated by using two different approaches, i.e., the subconfiguration averaged distorted-wave (SCADW) method and the more involved level-to-level distorted-wave (LLDW) method. Both methods are found to yield very similar results except for the 4d->5d excitation-autoionisation (EA) channels that straddles the ionisation threshold. Accordingly, a hybrid theoretical cross section where the $4d\to 5d$ EA SCADW cross section is replaced by its LLDW counterpart is in good agreement with the experimental result from an electron-ion crossed-beams experiment. This is in contrast to pure SCADW calculations for W14+ and neighbouring charge states which exhibit significant deviations from the experimental near-threshold cross sections of Schury et al. [J. Phys. B 53 (2020) 015201].

physics.atom-ph↗

arXiv2019

Near L-edge photoionization of triply charged iron ions

Relative cross sections for $m$-fold photoionization ($m=1,\ldots,5$) of Fe$^{3+}$ by single photon absorption were measured employing the photon-ion merged-beams setup PIPE at the PETRA III synchrotron light source operated at DESY in Hamburg, Germany. The photon energies used spanned the range of $680-950\,\mathrm{eV}$, covering both the photoexcitation resonances from the $2p$ and $2s$ shells as well as the direct ionization from both shells. Multiconfiguration Dirac-Hartree-Fock (MCDHF) calculations were performed to simulate the total photoexcitation spectra. Good agreement was found with the experimental results. These computations helped to assign several strong resonance features to specific transitions. We also carried out Hartree-Fock calculations with relativistic extensions taking into account both photoexcitation and photoionization. Furthermore, we performed extensive MCDHF calculations of the Auger cascades that result when an electron is removed from the $2p$ and $2s$ shells of Fe$^{3+}$. Our theoretically predicted charge-state fractions are in good agreement with the experimental results, representing a substantial improvement over previous theoretical calculations. The main reason for the disagreement with the previous calculations is their lack of inclusion of slow Auger decays of several configurations that can only proceed when accompanied by de-excitation of two electrons. In such cases, this additional shake-down transition of a (sub-)valence electron is required to gain the necessary energy for the release of the Auger electron.

physics.atom-ph↗

arXiv2017

Near L-Edge Single and Multiple Photoionization of Singly Charged Iron Ions

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 have been carried out using two different theoretical approaches, Hartree-Fock including relativistic extensions and fully relativistic Multi-Configuration 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 deexcitation cascades after the creation of inner-shell holes in the Fe+ ion have been 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.

astro-ph.IM↗

arXiv2017

Recombination of open-f-shell tungsten ions

We review experimental and theoretical efforts aimed at a detailed understanding of the recombination of electrons with highly-charged tungsten ions characterised by an open 4f sub-shell. Highly-charged tungsten occurs as a plasma contaminant in ITER-like tokamak experiments, where it acts as an unwanted cooling agent. Modelling of the charge state populations in a plasma requires reliable thermal rate coefficients for charge-changing electron collisions. The electron recombination of medium-charged tungsten species with open 4f sub-shells is especially challenging to compute reliably. Storage-ring experiments have been conducted that yielded recombination rate coefficients at high energy resolution and well-understood systematics. Significant deviations compared to simplified, but prevalent, computational models have been found. A new class of ab-initio numerical calculations has been developed that provides reliable predictions of the total plasma recombination rate coefficients for these ions.

physics.atom-ph↗

arXiv2016

The Cryogenic Storage Ring CSR

An electrostatic cryogenic storage ring, CSR, for beams of anions and cations with up to 300 keV kinetic energy per unit charge has been designed, constructed and put into operation. With a circumference of 35 m, the ion-beam vacuum chambers and all beam optics are in a cryostat and cooled by a closed-cycle liquid helium system. At temperatures as low as (5.5 $\pm$ 1) K inside the ring, storage time constants of several minutes up to almost an hour were observed for atomic and molecular, anion and cation beams at an energy of 60 keV. The ion-beam intensity, energy-dependent closed-orbit shifts (dispersion) and the focusing properties of the machine were studied by a system of capacitive pickups. The Schottky-noise spectrum of the stored ions revealed a broadening of the momentum distribution on a time scale of 1000 s. Photodetachment of stored anions was used in the beam lifetime measurements. The detachment rate by anion collisions with residual-gas molecules was found to be extremely low. A residual-gas density below 140 cm$^{-3}$ is derived, equivalent to a room-temperature pressure below 10$^{-14}$ mbar. Fast atomic, molecular and cluster ion beams stored for long periods of time in a cryogenic environment will allow experiments on collision- and radiation-induced fragmentation processes of ions in known internal quantum states with merged and crossed photon and particle beams.

physics.atom-ph↗

arXiv2015

An Efficient, Movable Single-Particle Detector for Use in Cryogenic Ultra-High Vacuum Environments

A compact, highly efficient single-particle counting detector for ions of keV/u kinetic energy, movable by a long-stroke mechanical translation stage, has been developed at the Max-Planck-Institut für Kernphysik (Max Planck Institute for Nuclear Physics, MPIK). Both, detector and translation mechanics, can operate at ambient temperatures down to $\sim$ 10 K and consist fully of ultra-high vacuum (UHV) compatible, high-temperature bakeable and non-magnetic materials. The set-up is designed to meet the technical demands of MPIK's Cryogenic Storage Ring (CSR). We present a series of functional tests that demonstrate full suitability for this application and characterise the set-up with regard to its particle detection efficiency.

physics.ins-det↗

arXiv2014

Electron-ion merged-beam experiments at heavy-ion storage rings

In the past two decades, the electron-ion merged-beams technique has extensively been exploited at heavy-ion storage rings equipped with electron coolers for spectroscopic studies of highly charged ions as well as for measuring absolute cross sections and rate coefficients for electron-ion recombination and electron-impact ionization of multiply charged atoms ions. Some recent results are highlighted and future perspectives are pointed out, in particular, in view of novel experimental possibilities at the FAIR facility in Darmstadt and at the Cryogenic Storage Ring at the Max-Planck-Institute for Nuclear Physics in Heidelberg.

physics.atom-ph↗

arXiv2013

The effect of an external magnetic field on the determination of E1M1 two-photon decay rates in Be-like ions

In this work we report on ab initio theoretical results for the magnetic field induced 2s2p ^3P_0 - 2s^2 ^1S_0 E1 transition for ions in the beryllium isoelectronic sequence between Z=5 and 92. It has been proposed that the rate of the E1M1 two-photon transition 2s2p ^3P_0 - 2s^2 ^1S_0 can be extracted from the lifetime of the ^3P_0 state in Be-like ions with zero nuclear spin by employing resonant recombination in a storage-ring. This experimental approach involves a perturbing external magnetic field. The effect of this field needs to be evaluated in order to properly extract the two-photon rate from the measured decay curves. The magnetic field induced transition rates are carefully evaluated and it is shown that, with a typical storage-ring field strength, it is dominant or of the same order as the E1M1 rate for low- and mid-Z ions. Results for several field strengths and ions are presented and we also give a simple Z-dependent formula for the rate. We estimate the uncertainties of our model to be within 5% for low- and mid-Z ions, and slightly larger for more highly charged ions. Furthermore we evaluate the importance of including both perturber states, ^3P_1 and ^1P_1, and it is shown that excluding the influence of the ^1P_1 perturber overestimates the rate by up to 26% for the mid-Z ions.

physics.atom-ph↗

arXiv2012

Storage-Ring Measurements of Hyperfine Induced Transition Rates in Berylliumlike Ions

The status of experimental measurements and theoretical calculations of the hyperfine induced 2s2p 3P0 -> 2s2 1S0 transition rate in Be-like ions is reviewed. Possible reasons, such as external electromagnetic fields and competing E1M1 two-photon transitions, for presently existing significant discrepancies between experiment and theory are discussed. Finally, directions for future research are outlined.

physics.atom-ph↗