arXiv subjects

S. Schippers

Publications and source records attributed to S. Schippers.

At least 19 recordsLinked to original sources

arXiv2026

Rate coefficients for dielectronic recombination of N-like Ne

Dielectronic recombination (DR) for the process Ne$^{3+}$ + e$^{-}$ $\rightarrow$ Ne$^{2+}$ was investigated in a merged-beams arrangement at the heavy-ion storage ring CRYRING@ESR. The energy-dependent DR rate coefficient, $\alpha(E)$ was measured over the electron-ion collision energy range from 0 to 25 eV. The measurements cover the complete set of DR resonance series associated with $2s\to2p$ core excitations. The primary ion beam is estimated to have consisted of $44\%$ of the ions in the ground state, with the remainder distributed among long-lived metastable levels. In addition to the measurements we carried out quantum mechanical calculations of DR cross sections. The theoretical treatment includes contributions from the ground and excited metastable initial levels, weighted according to the estimated beam composition. From the experimental energy-resolved spectra, we derive a temperature dependent DR plasma recombination rate coefficient $\alpha_\mathrm{exp}(T)$ (PRRC). In the temperature domain where Ne$^{3+}$ is abundant in collisionally ionized plasmas, the present results show a good agreement with the present and with previous theoretical predictions. In the low-temperature regime characteristic for photoionized plasmas, the experimentally derived DR plasma rate coefficient is slightly larger than the published theoretical ones and does not agree within the experimental uncertainties. Parametrized fits of the experimentally derived DR PRRC are presented in order to facilitate an easy inclusion into astrophysical modelling codes.

physics.atom-ph↗

arXiv2025

Double to tenfold M-shell photoionization of singly charged lanthanum ions

Using the photon-ion merged-beams technique at the PETRA\,III synchrotron light source, we have measured cross sections for double and up to tenfold photoionization of La$^{+}$ ions by a single photon in the energy range 820--1400~eV, where resonances and thresholds occur that are associated with the excitation or ionization of one $M$-shell electron. These cross sections represent experimental benchmark data for the further development of quantum theoretical methods, which will have to provide the bulk of the atomic data required for the modeling of nonequilibrium plasmas such as kilonovae. In the present work, we have upgraded the Jena Atomic Calculator (JAC) and pushed the state-of-the-art of quantum calculations for heavy many-electron systems to new limits. In particular, we have performed large-scale calculations of the La$^+$ photoabsorption cross section and of the deexcitation cascades, which set in after the initial creation of a $3d$ hole. Our theoretical results largely agree with our experimental findings. However, our theoretical product-ion charge state distributions are somewhat narrower than the experimental ones, which is most probably due to the simplifications necessary to keep the cascade calculations tractable.

physics.atom-ph↗

arXiv2025

Electron-ion collision spectroscopy at the CRYRING@ESR electron cooler

Electron-ion collision spectroscopy at heavy-ion storage rings aims at precision measurements of resonance features that occur in the cross sections of electron collision processes such as electron-impact ionization of ions or electron-ion recombination. As part of the international FAIR project, the low-energy ion storage ring CRYRING@ESR has been coupled with the heavy-ion accelerators operated by the GSI Helmholtz Center for Heavy-Ion Research in Darmstadt, Germany. This has created a new opportunity for stringent tests of strong field quantum electrodynamics by electron-ion collision spectroscopy of heavy few-electron ions. The present contribution provides details of the electron-ion collision spectroscopy setup at CRYRING@ESR and of the associated data-analysis procedures along with first results for nonresonant and resonant recombination of berylliumlike lead ions. For nonresonant recombination a recombination rate enhancement factor of 3.5 is found at zero electron-ion collision energy. For resonant recombination there is excellent agreement with recent theoretical results when these are shifted by 340~meV in energy.

physics.atom-ph↗

arXiv2025

Testing strong-field QED to second-order in the highly correlated atomic system berylliumlike Pb78+ by electron-ion recombination spectroscopy

A low-energy storage ring with an ultracold electron cooler has been coupled with a heavy-ion accelerator facilitating high-resolution electron-ion collision spectroscopy of the heaviest few-electron ions. In the present work resonant electron-ion recombination of berylliumlike Pb$^{78+}$ ions was measured in the collision-energy range 9.3-16.5eV and a value of 244.937(30) eV is derived for the Pb$^{78+}$($2s^2\;^1S_0 - 2s\,2p\;^3P_1$) excitation energy. This result agrees with the most recent (less accurate) theoretical value of 244.942(52) eV [Malyshev et al., Physical Review A 110, 062824 (2024)], which has been calculated by applying strong-field QED rigorously up to the second order. The present investigation suggests that further technical improvements can potentially increase the experimental accuracy by an order of magnitude.

physics.atom-ph↗

arXiv2025

Double-$K$-hole resonances in single photoionization of He-like B$^{3+}$ ions

Within a joint experimental and theoretical research project, single photoionization of He-like B$^{3+}$ ions was investigated in the energy range from approximately 250 to 1200~eV. With the parent-ion beam in the experiment containing both $1s^2~^1S$ ground-state and $1s2s~^3S$ metastable B$^{3+}$ ions, double-core-hole resonances could be studied. Two series of hollow resonant states were observed, one populated by $K$-shell double excitation $1s^2~^1S \to 2\ell n\ell'~^1P$ ($\ell=s,p$; $\ell'=p,s$; $n=2,3,..,6$) at photon energies up to about 510~eV, the other by $K$-shell single excitation $1s2s~^3S \to 2\ell n\ell'~^3P$ ($\ell=s,p$; $\ell'=p,s$; $n=2,3,..,6$) at energies up to about 310~eV. High resolving powers up to approximately 29000 were achieved. The relativistic many-body perturbation theory was employed to determine level-to-level cross sections for $K$-shell excitation with subsequent autoionization. The resonance energies were calculated with inclusion of electron correlation and radiative contributions. The energy uncertainties of the most prominent resonances are estimated to be below $\pm 1$ meV. Convergent close coupling (CCC) calculations provided single-photoionization cross sections $\sigma_{34}$ for B$^{3+}$ including the resonant and non-resonant channels. Apart from the resonances, $\sigma_{34}$ is dominated by direct ionization in the investigated energy range. The contribution $\sigma_{34}^{\mathrm{dir}}$ of the latter process to $\sigma_{34}$ was separately determined by using the random-phase approximation with exchange and relativistic Hartree-Fock calculations which agree very well with previous calculations. Direct ionization of one electron accompanied by excitation of the remaining electron was treated by the CCC theory and found to be a minor contribution to $\sigma_{34}$.

physics.atom-ph↗

arXiv2023

Vibrationally Resolved Inner-Shell Photoexcitation of the Molecular Anion C$_2^-$

Carbon $1s$ core-hole excitation of the molecular anion C$_2^-$ has been experimentally studied at high resolution by employing the photon-ion merged-beams technique at a synchrotron light source. The experimental cross section for photo--double-detachment shows a pronounced vibrational structure associated with $1σ_u\to3σ_g$ and $1σ_g \to 1π_u$ core excitations of the C$_2^-$ ground level and first excited level, respectively. A detailed Franck-Condon analysis reveals a strong contraction of the C$_2^-$ molecular anion by 0.2~Å upon this core photoexcitation. The associated change of the molecule's moment of inertia leads to a noticeable rotational broadening of the observed vibrational spectral features. This broadening is accounted for in the present analysis which provides the spectroscopic parameters of the C$_2^-$ $1σ_u^{-1}\,3σ_g^2\;{^2}Σ_u^+$ and $1σ_g^{-1}\,3σ_g^2\;{^2}Σ_g^+$ core-excited levels.

physics.atom-ph↗

arXiv2022

Multiple Photodetachment of Oxygen Anions via K-Shell Excitation and Ionization: Direct Double-Detachment Processes and Subsequent Deexcitation Cascades

Experimental cross sections for m-fold photodetachment (m=2-5) of oxygen anions via K-shell excitation and ionization were measured in the photon-energy range of 525-1500 eV using the photon-ion merged-beams technique at a synchrotron light source. The measured cross sections exhibit clear signatures of direct double detachment, including double K-hole creation. The shapes of the double-detachment cross sections as a function of photon energy are in accord with Pattard's [J. Phys. B 35, L207 (2002)] empirical scaling law. We have also followed the complex deexcitation cascades that evolve subsequently to the initial double-detachment events by systematic large-scale cascade calculations. The resulting theoretical product charge-state distributions are in good agreement with the experimental findings.

physics.atom-ph↗

arXiv2022

Precise determination of the 2s22p5-2s2p6 transition energy in fluorine-like nickel utilizing a low-lying dielectronic resonance

High precision spectroscopy of the low-lying dielectronic resonances in fluorine-like nickel ions were determined by employing the merged electron-ion beam at the heavy-ion storage ring CSRm. The measured dielectronic resonances are identified by comparing with the most recent relativistic calculation utilizing the FAC code. The first resonance at about 86 meV due to the dielectronic recombination via (2s2p6[2S1/2]6s)J=1 intermediate state was recognized. The experimental determination of the resonance position at 86 meV reaches an uncertainty of 4 meV, which allows precise determination of the 2s22p5[2P3/2] - 2s2p6[2S1/2] transition energy. The Rydberg binding energy of the 6s electron in the (2s2p6[2S1/2]6s)J=1 state is calculated by the multi-configurational Dirac-HartreeFock and stabilization methods. The determined transition energies are 149.056(4)exp(10)theo and 149.032(4)exp(6)theo, respectively. Moreover, the transition energy has also been calculated by fully relativistic and ab initio approaches. Individual theoretical contributions are evaluated by employing the core-Hartree and Kohn-Sham screening potentials, respectively. High-order QED and correlation effects contribute prominently to the total transition energy. The present DR precision spectroscopy study at the CSRm paves the way for future precision measurements of atomic energy levels with heavier highly charged ions.

physics.atom-ph↗

arXiv2022

Near K-Edge Photoionization and Photoabsorption of Singly, Doubly, and Triply Charged Silicon Ions

Experimental and theoretical results are presented for double, triple, and quadruple photoionization of Si$^+$ and Si$^{2+}$ ions and for double photoionization of Si$^{3+}$ ions by a single photon. The experiments employed the photon-ion merged-beams technique at a synchrotron light source. The experimental photon-energy range 1835--1900 eV comprises resonances associated with the excitation of a $1s$ electron to higher subshells and subsequent autoionization. Energies, widths, and strengths of these resonances are extracted from high-resolution photoionization measurements, and the core-hole lifetime of K-shell ionized neutral silicon is inferred. In addition, theoretical cross sections for photoabsorption and multiple photoionization were obtained from large-scale Multi-Configuration Dirac-Hartree-Fock (MCDHF) calculations. The present calculations agree with the experiment much better than previously published theoretical results. The importance of an accurate energy calibration of laboratory data is pointed out. The present benchmark results are particularly useful for discriminating between silicon absorption in the gaseous and in the solid component (dust grains) of the interstellar medium.

physics.atom-ph↗

arXiv2022

X-ray emission associated with radiative recombination for Pb$^{82+}$ ions at threshold energies

For bare lead ions, decelerated to the low beam energy of 10 MeV/u, the x-ray emission associated with radiative recombination (RR) at "cold collision" conditions has been studied at the electron cooler of CRYRING@ESR at GSI-Darmstadt. Utilizing dedicated x-ray detection chambers installed at 0° and 180° observation geometry, we observed for the very first time for stored ions the full x-ray emission spectrum associated with RR under electron cooling conditions. Most remarkably, no line distortion effects due to delayed emission are present in the well resolved spectra, spanning over a wide range of x-ray energies (from about 5 to 100 keV) which enable to identify fine-structure resolved Lyman, Balmer as well as Paschen x-ray lines along with the RR transitions into the K-, L and M-shell of the ions. To compare with theory, an elaborate theoretical model has been applied. By considering the relativistic atomic structure of Pb$^{81+}$, this model is based on a sophisticated computation of the initial population distribution via RR for all atomic levels up to Rydberg states with principal quantum number $n=$ 165 in combination with cascade calculations based on time-dependent rate equations. Within the statistical accuracy, the experimental x-ray line emission is in very good agreement with the results of the theoretical model applied. Most notably, this comparison sheds light on the contribution of prompt and delayed X-ray emission (up to 70 ns) to the observed X-ray spectra, originating in particular from Yrast transitions into inner shells.

physics.atom-ph↗

arXiv2021

Multiple photodetachment of silicon anions via K-shell excitation and ionization

Experimental cross sections for $m$-fold photodetachment ($m=3-6$) of silicon anions via $K$-shell excitation and ionization were measured in the photon-energy range of 1830-1900 eV using the photon-ion merged-beams technique at a synchrotron light source. All cross sections exhibit a threshold behavior that is masked by pre-threshold resonances associated with the excitation of a $1s$ electron to higher, either partly occupied or unoccupied atomic subshells. Results from multi-configuration Dirac-Fock (MCDF) calculations agree with the experimentally derived cross sections for photo-absorption if small energy shifts are applied to the calculated resonance positions and detachment thresholds. Moreover, a systematic approach is applied for modeling the deexcitation cascades that set in after the initial creation of a $K$-shell hole. The resulting product charge-state distributions compare well with the measured ones for direct $K$-shell detachment but less well for resonant $K$-shell excitation. The present results are potentially useful for identifying silicon anions in cold plasmas such as interstellar gas clouds.

physics.atom-ph↗

arXiv2020

Multiple photodetachment of atomic anions via single and double core-hole creation

We review the recent experimental and theoretical progress in $K$-shell detachment studies of atomic anions. On the experimental side, this field has largely benefitted from technical advances at 3rd generation synchrotron radiation sources. For multiple detachment of C$^-$, O$^-$, and F$^-$ ions, recent results were obtained at the photon-ion merged-beams setup PIPE which is a permanent end station at beamline P04 of the PETRA III synchrotron light source in Hamburg. In addition to a much increased photon flux as compared to what was available previously, the PIPE setup has an extraordinary detection sensitivity for heavy charged reaction products that allows one to study detachment processes with extremely low cross sections in the kilobarn range, e.g., for processes involving the simultaneous creation of two core-holes by a single photon. The experimental findings pose new challenges for state-of-the-art atomic theory and require calculations combining photoexcitation (ionization) with decay cascade processes that follow after initial core-hole production.

physics.atom-ph↗

arXiv2020

Hybrid subconfiguration-average and level-to-level distorted-wave treatment of electron-impact single ionisation of W$^{15+}$ and W$^{16+}$

Recently, we have demonstrated (Jin et al. 2020, J. Phys. B: At. Mol. Opt. Phys. 53, 075201) that a hybrid subconfiguration-average and level-to-level distorted wave treatment of electron-impact single ionisation (EISI) of W$^{14+}$ ions represents an accurate and manageable approach for the calculation of EISI cross sections of a complex ion. Here we demonstrate the more general validity of this approach by comparing hybrid cross sections for EISI of W$^{15+}$ and W$^{16+}$ with the recent experimental results of Schury et al. 2020, J. Phys. B: At. Mol. Opt. Phys. 53, 015201). Our calculations also account for the resonant-excitation double autoionisation (REDA) process which is important in the electron energy range 370-600 eV and for the possible presence of initially metastable ions in the experiment.

physics.atom-ph↗

arXiv2019

Multiple photodetachment of carbon anions via single and double core-hole creation

We report on new measurements of m-fold photodetachment (m=2-5) of carbon anions via K-shell excitation and ionization. The experiments were carried out employing the photon-ion merged-beams technique at a synchrotron light source. While previous measurements were restricted to double detachment (m=2) and to just the lowest-energy K-shell resonance at about 282 eV, our absolute experimental $m$-fold detachment cross sections at photon energies of up to 1000 eV exhibit a wealth of new thresholds and resonances. We tentatively identify these features with the aid of detailed atomic-structure calculations. In particular, we find unambiguous evidence for fivefold detachment via double K-hole production.

physics.atom-ph↗

arXiv2019

Electron-impact single ionisation of W$^{q+}$ ions: Experiment and theory for $\mathbf{11\leq q \leq 18}$

Absolute cross sections for electron-impact single ionisation (EISI) of multiply charged tungsten ions (W$^{q+}$) with charge states in the range $ 11 \leq q \leq 18$ in the electron-ion collision energy ranges from below the respective ionisation thresholds up to 1000~eV were measured employing the electron-ion crossed-beams method. In order to extend the results to higher energies, cross section calculations were performed using the subconfiguration-averaged distorted-wave (SCADW) method for electron-ion collision energies up to 150~keV. From the combined experimental and scaled theoretical cross sections rate coefficients were derived which are compared with the ones contained in the ADAS database and which are based on the configuration-averaged distorted wave (CADW) calculations of Loch et al. [Phys. Rev. A 72, 052716 (2005)]. Significant discrepancies were found at the temperatures where the ions investigated here are expected to form in collisionally ionised plasmas. These discrepancies are attributed to the limitations of the CADW approach and also the more detailed SCADW treatment which do not allow for a sufficiently accurate description of the EISI cross sections particularly at the ionisation thresholds.

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

Near-K-edge single, double, and triple photoionization of C+ ions

Single, double, and triple ionization of the C+ ion by a single photon have been investigated in the energy range 286 to 326 eV around the K-shell single-ionization threshold at an unprecedented level of detail. At energy resolutions as low as 12 meV, corresponding to a resolving power of 24000, natural linewidths of the most prominent resonances could be determined. From the measurement of absolute cross sections, oscillator strengths, Einstein coefficients, multi-electron Auger decay rates and other transition parameters of the main K-shell excitation and decay processes are derived. The cross sections are compared to results of previous theoretical calculations. Mixed levels of agreement are found despite the relatively simple atomic structure of the C+ ion with only 5 electrons. This paper is a follow-up of a previous Letter [Müller et al., Phys. Rev. Lett. 114, 013002 (2015)].

physics.atom-ph↗

arXiv2018

Photoionization of metastable heliumlike C4+(1s 2s 3S1) ions: Precision study of intermediate doubly excited states

In a joint experimental and theoretical endeavour, photoionization of metastable C4+(1s 2s 3S1) ions via intermediate levels with hollow, double-K-vacancy configurations 2s2p, 2s3p, 2p3s, 2p3d, 2s4p, 2p4s and 2p4d has been investigated. High-resolution photon-ion merged-beams measurements were carried out with the resolving power reaching up to 25,000 which is sufficient to separate the leading fine-structure components of the 2s2p 3P term. Many-body perturbation theory was employed to determine level-to-level cross sections for K-shell excitation with subsequent autoionization. The resonance energies were calculated with inclusion of electron correlation and radiative contributions. Their uncertainties are estimated to be below $\pm$1 meV. Detailed balance confirms the present photoionization cross-section results by comparison with previous dielectronic-recombination measurements. The high accuracy of the theoretical transition energies together with the present experimental results qualify photoabsorption resonances in heliumlike ions as new, greatly improved energy-reference standards at synchrotron radiation facilities.

physics.atom-ph↗