<figure><img src="image_32.png" /></figure>

Figure 28. DESIREE [194] at Stockholm University consists of two electrostatic ion-storage rings in a common cryogenic vacuum chamber and with a common straight section where beams of opposite charge polarity can be merged. With the low internal energies that can be attained by waiting for the ions to reach thermal equilibrium and the high degree of control of the inter-particle motion obtained from the merged-beams configuration, unprecedented control of all internal and external degrees of freedom can be obtained.

<figure><img src="image_33.png" /></figure>

Figure 29. Measured fractions of the OH⁻ ions not in the rotational ground state as function of time after injection in DESIREE (blue data points and curve). The green data points and curve show the same fraction but now a cw laser beam is merged with the ion beam for the first 260 s and tuned to a wavelength where non-ground state ions only can be photodetached [197].

the two rings and measure products of mutual neutralization and related processes. The long-time storage of both beams will make it possible to study pairs of ions of complex molecules close to thermal equilibrium with the cryogenic surroundings. While the merged-beams geometry provides the opportunity to control the *inter*-molecular degrees of freedom of the collision partners, the *intra*-molecular degrees of freedom are controlled by the low temperature and the long time storage.

For all the cryogenic storage rings, the ability to demonstrate rotational cooling and to which extent thermal

equilibrium may be reached are essential. In 2017 both the CSR and DESIREE teams reported the observation of stored beams of OH⁻ with well over 90% of the population in the lowest ($J=0$) quantum state [197]. Figure 29 displays data from a threshold photodetachment experiment in DESIREE. By comparing signal rates for wavelengths just above and just below the threshold for ground state photodetachment and using prior knowledge of the photodetachment cross sections, the fraction of OH⁻ ions *not* in the $J=0$ ground rotational level was deduced as function of time after ion injection. For the data set shown in green, the population of ions in excited rotational levels was reduced artificially by depletion through selective photodetachment. For the first 260 s after injection, this depletion laser was on resulting in an asymptotic excitation fraction of 3.6% ± 0.3% corresponding to a rotational temperature of 12.3 ± 0.2 K. After the laser light was blocked at $t = 260$ s the excitation fraction increased by absorption of thermal photons from the 13.5 ± 0.5 K temperature surroundings and by collisions with the few residual-gas molecules left. Finally an asymptotic excitation fraction of 18.1% ± 0.9%, corresponding to a rotational temperature of 20.6 ± 0.5 K, was reached. These results and similar findings in CSR holds great promise for future experiments in the here mentioned and any future cryogenic storage ring systems for experiments where cold molecular ions interact with light, electrons, neutral particles or other ions [194–197].

In parallel to the electrostatic ion-storage ring developments, compact electrostatic ion-beam traps have been developed in which fast-moving ions are confined between two opposing focusing electrostatic mirrors. Here we mention the recent studies from the Weizmann institute of ultraslow isomerization in $C_{10}$ molecular ions [198] and the first experiment with a cryogenic electrostatic storage device in which the He⁻ lifetime was accurately measured [199].

*Concluding remarks.* Novel cryogenic electrostatic ion-storage rings have become operational and cooling of the rotational degrees of freedom has been demonstrated. New insights from experiments with rotationally cold ions interacting with photons, electrons, neutral atoms and other cold ions are expected in the very near future.

## Acknowledgments

I want to acknowledge all colleagues at Stockholm University and elsewhere working on electrostatic ion-rings and traps for all their wonderful work, which is far beyond what can be represented here. I thank the Swedish Research council for its support for the DESIREE infrastructure (2017-00621) and my individual research grant (2018-04092).
## 19. Frontiers of extreme electromagnetic fields: atomic physics with HCIs at FAIR (SPARC collaboration)

Thomas Stöhlker

Friedrich-Schiller-University Jena, Germany; Helmholtz Institute Jena, Germany; GSI Darmstadt, Germany

**Status.** The international Facility for Antiproton and Ion Research FAIR [200–202], currently under construction, will allow for a broad variety of experiments over a wide ranges of ion energy and ion intensity. The intensities can be varied from single ions up to highest particle densities at energies ranging from rest in the laboratory up to the highly relativistic collision domain, corresponding to the strongest and shortest electromagnetic field pulses available. Moreover, stable and exotic nuclei up to the heaviest ones are available in any arbitrary charge state, enabling the extension of atomic physics research across virtually the full range of atomic matter. Of particular interest for atomic physics, FAIR is unique in its ability to deliver these highly intense, brilliant beams with excellent momentum definition. For this purpose, a unique portfolio of trapping and storage facilities will be available at FAIR covering ion energies, that span more than 10 orders of magnitude (figure 30). This will enable experiments with cooled ions aiming at exploring the atomic structure and electron dynamics in the realm of the extreme electromagnetic fields close to, and even beyond, the Schwinger limit. In order to cope with these challenges, the SPARC collaboration (Stored Particle Atomic physics Research Collaboration) has been formed. SPARC has over 430 members and is dedicated to scientifically exploit the unique discovery potential of FAIR [201, 202], in close interaction between experiment and theory.

Accelerator-based atomic physics has opened new, widely unexplored fields of research. Heavy-ion storage cooler rings, such as the ESR at GSI, have provided a first access to basic processes associated with strong electromagnetic fields in collisions of heavy, HCIs. Experiments with stored and cooled one- and few-electron high-Z ions interacting with atomic targets, (polarized) electrons, or laser photons uniquely reveal the effects of relativity, electron correlations and QED on the dynamics and structure of elementary atomic systems in the presence of extremely strong fields. These research activities focus on the structure and dynamics of atomic matter subject to extreme electromagnetic fields and ultrafast electromagnetic interactions, in particular those described by non-perturbative QEDs. On the side of fundamental interactions, we conduct highly sensitive tests of atomic structure theory based on various experimental approaches such as precision measurements of the 1s Lamb shift [139], 1s hyperfine structure [143], bound-state g-factor [146] and mass measurements of heavy few-electron ions. In case of electrons bound to the strong Coulomb fields of heavy nuclei, experiments on quantum dynamics of electrons subject to extreme fields has been studied. This was achieved

<figure><img src="image_33.png" /><figcaption>Figure 30. Main FAIR facilities for storage and trapping within the SPARC collaboration, covering more than 10 orders of magnitude in particle kinetic energies [202]. Reprinted from [202], Copyright 2015, with permission from Elsevier.</figcaption></figure>

by precision studies of DR [203], photon-correlation studies for radiative EC (see section 6) [204], state-selective excitation and EC studies [205], as well as of correlated electron dynamics of atoms interacting with the intense [206], ultra-short fields of high-Z relativistic ions. Moreover, atomic physics techniques have been applied to yield the most precise test of time dilatation by using stored and cooled Li + ions at the ESR storage ring [207]. Last but not least, concerning the intersection of atomic and nuclear physics, novel decay modes of nuclei have been discovered, such as the bound-state $\beta$-decay, which may play an important role in nuclear astrophysics and more specifically in nuclear synthesis (see section 12) [208].

**Current and future challenges.** The storage and trapping facilities of FAIR will substantially enlarge the research capabilities for the exploration of atomic matter within the realm of extreme and ultra-short electromagnetic fields, and has key features that offer a range of new and challenging research opportunities. The investigations will focus on atomic structure, e.g. new concepts for QED in extreme fields (see section 13), collision studies at moderate and highly relativistic energies (ionization, capture and pair production) (see section 6), to generate insights into correlated many-body dynamics via ultra-short and super-intense field pulses ($<10^{-18}$ s), and avenues towards the discovery of new electromagnetic interaction processes. Further examples include laser spectroscopy exploiting the large Doppler boost (both in the photon frequency and time domain) associated with relativistic ions, RCE of ions passing through crystal lattices at a relativistic speed (see section 16), as well as precision x-ray spectroscopy [209]. Furthermore, experiments at the border between atomic and nuclear physics will be performed with an emphasis on rare nuclear decay modes only possible for nuclei in high atomic charge states. At the same time, SPARC will apply these accurate atomic-physics techniques as powerful tools for the determination of nuclear parameters such as nuclear radii and moments, and of fundamental constants (see section 12). More specifically,
# 19. Frontiers of extreme electromagnetic fields: atomic physics with HCIs at FAIR (SPARC collaboration)

Thomas Stöhlker

Friedrich-Schiller-University Jena, Germany; Helmholtz Institute Jena, Germany; GSI Darmstadt, Germany

**Status.** The international Facility for Antiproton and Ion Research FAIR [200–202], currently under construction, will allow for a broad variety of experiments over a wide ranges of ion energy and ion intensity. The intensities can be varied from single ions up to highest particle densities at energies ranging from rest in the laboratory up to the highly relativistic collision domain, corresponding to the strongest and shortest electromagnetic field pulses available. Moreover, stable and exotic nuclei up to the heaviest ones are available in any arbitrary charge state, enabling the extension of atomic physics research across virtually the full range of atomic matter. Of particular interest for atomic physics, FAIR is unique in its ability to deliver these highly intense, brilliant beams with excellent momentum definition. For this purpose, a unique portfolio of trapping and storage facilities will be available at FAIR covering ion energies, that span more than 10 orders of magnitude (figure 30). This will enable experiments with cooled ions aiming at exploring the atomic structure and electron dynamics in the realm of the extreme electromagnetic fields close to, and even beyond, the Schwinger limit. In order to cope with these challenges, the SPARC collaboration (Stored Particle Atomic physics Research Collaboration) has been formed. SPARC has over 430 members and is dedicated to scientifically exploit the unique discovery potential of FAIR [201, 202], in close interaction between experiment and theory.

Accelerator-based atomic physics has opened new, widely unexplored fields of research. Heavy-ion storage cooler rings, such as the ESR at GSI, have provided a first access to basic processes associated with strong electromagnetic fields in collisions of heavy, HCIs. Experiments with stored and cooled one- and few-electron high-Z ions interacting with atomic targets, (polarized) electrons, or laser photons uniquely reveal the effects of relativity, electron correlations and QED on the dynamics and structure of elementary atomic systems in the presence of extremely strong fields. These research activities focus on the structure and dynamics of atomic matter subject to extreme electromagnetic fields and ultrafast electromagnetic interactions, in particular those described by non-perturbative QEDs. On the side of fundamental interactions, we conduct highly sensitive tests of atomic structure theory based on various experimental approaches such as precision measurements of the 1s Lamb shift [139], 1s hyperfine structure [143], bound-state g-factor [146] and mass measurements of heavy few-electron ions. In case of electrons bound to the strong Coulomb fields of heavy nuclei, experiments on quantum dynamics of electrons subject to extreme fields has been studied. This was achieved

<figure><img src="image_34.png" /><figcaption>Figure 30. Main FAIR facilities for storage and trapping within the SPARC collaboration, covering more than 10 orders of magnitude in particle kinetic energies [202]. Reprinted from [202], Copyright 2015, with permission from Elsevier.</figcaption></figure>

by precision studies of DR [203], photon-correlation studies for radiative EC (see section 6) [204], state-selective excitation and EC studies [205], as well as of correlated electron dynamics of atoms interacting with the intense [206], ultra-short fields of high-Z relativistic ions. Moreover, atomic physics techniques have been applied to yield the most precise test of time dilatation by using stored and cooled Li + ions at the ESR storage ring [207]. Last but not least, concerning the intersection of atomic and nuclear physics, novel decay modes of nuclei have been discovered, such as the bound-state $\beta$-decay, which may play an important role in nuclear astrophysics and more specifically in nuclear synthesis (see section 12) [208].

**Current and future challenges.** The storage and trapping facilities of FAIR will substantially enlarge the research capabilities for the exploration of atomic matter within the realm of extreme and ultra-short electromagnetic fields, and has key features that offer a range of new and challenging research opportunities. The investigations will focus on atomic structure, e.g. new concepts for QED in extreme fields (see section 13), collision studies at moderate and highly relativistic energies (ionization, capture and pair production) (see section 6), to generate insights into correlated many-body dynamics via ultra-short and super-intense field pulses ($<10^{-18}$ s), and avenues towards the discovery of new electromagnetic interaction processes. Further examples include laser spectroscopy exploiting the large Doppler boost (both in the photon frequency and time domain) associated with relativistic ions, RCE of ions passing through crystal lattices at a relativistic speed (see section 16), as well as precision x-ray spectroscopy [209]. Furthermore, experiments at the border between atomic and nuclear physics will be performed with an emphasis on rare nuclear decay modes only possible for nuclei in high atomic charge states. At the same time, SPARC will apply these accurate atomic-physics techniques as powerful tools for the determination of nuclear parameters such as nuclear radii and moments, and of fundamental constants (see section 12). More specifically,
<figure><img src="image_34.png" /><figcaption>Figure 31. Overview of FAIR accelerators and storage rings. Current/future SPARC involvement in addition to ESR and HITRAP is indicated by orange dots [202]. Reprinted from [202], Copyright 2015, with permission from Elsevier.</figcaption></figure>

SPARC will exploit the research potential of the following four research facilities at FAIR (see also figure 31).

**HITRAP [210]:** HCIs (up to bare uranium) from the ESR storage ring will be extracted into the HITRAP facility, where they are decelerated and cooled. The cold ions are delivered to a broad range of atomic physics experiments at very low energies or even in ion traps (see section 11). This facility is ready for operation and is waiting for first beams.

**CRYRING [211]:** A low-energy storage ring, dedicated to precision experiments with HCIs that has already proven its potential in a multitude of experiments at Stockholm. CRYRING has been transferred to GSI/FAIR and ions decelerated in the ESR will be provided to CRYRING for experiments. A unique feature of CYRING is its ultra-cold electron cooler. CRYING is currently being commissioned and will be available for experiments in 2019.

**ESR [201, 202]:** The ESR is the most versatile storage ring with respect to beam energies, acceptance, various beam manipulations, and detection instrumentation. It allows for electron, stochastic, and laser cooling of beams of stable and exotic nuclei. Moreover, beams for HITRAP and CRYRING will be prepared and provided (decelerated) by the ESR.

**HESR [201, 202]:** The HESR will give access for the very first time to cooled, HCIs up to a γ-factor of 6 for precision experiments in atomic physics and neighboring fields.

**Advances in science and technology to meet challenges.** Storage and cooling are essential for the discussed studies, and the development and improvement of the corresponding dedicated tools for particle storage, preparation and detection are part of the current technical challenges. Within the SPARC collaboration, there are numerous efforts to improve instrumentation including devices for deceleration and cooling of ions in storage rings and particle traps, non-destructive detection of ions and their reactions, and

spectrometers in the optical and x-ray regimes. Newly developed calorimetric low-temperature detectors for spectroscopy of Lyman x-rays of HCIs which yield a high energy resolution that is comparable to crystal spectrometers and at the same time exhibit a large wavelength acceptance have been implemented and tested in first experiments at the ESR storage ring. Techniques for cooling of ions from keV to μeV energies (i.e. by more than 8 orders of magnitude) and crystallization within seconds have been devised by application in Penning traps [212].

For laser spectroscopy, a power-scalable nonlinear compression scheme which will provide few-cycle pulses at up to 1 kW of average power has been developed [213]. Thus, efficient high harmonic generation up to the water window will be feasible as well as an isolated attosecond pulse generation. A portable XUV source for experiments at heavy-ion storage rings at GSI/FAIR will be setup and installed at CRYRING [211]. At a later stage, such a system will serve as a light source for experiments at the HESR. Single- and multi-species ion crystals as presently produced in Penning traps will be used for sympathetic cooling of externally produced HCIs (HITRAP and EBIT at FAIR) and will allow us to perform high-precision optical spectroscopy of magnetic dipole transitions as probes of QED calculations in the magnetic sector. Future activities related to micro-calorimeter detectors will focus on large pixel arrays, e.g. 1000 pixels, and the optimization of the electronic read-out schemes. Furthermore, besides Metallic Magnetic Calorimeters [214], Superconducting Tunnel Junctions detectors with their high count-rate capability and unique performance at XUV photon energies will be considered.

**Concluding remarks.** The SPARC collaboration is pursuing a challenging atomic physics program in the realm of strong electromagnetic fields, exploiting the present and future trapping and storage facilities of GSI/FAIR. The focus is on collision experiments of HCIs with atoms, electrons, and photons as well as spectroscopic studies in the x-ray, optical and microwave domains. Beside experiments on stable ions, also experiments at the border between atomic, nuclear and astro-physics will be conducted. Since the future FAIR facility will offer beams of cooled HCIs in energy regimes where no such experiments were possible so far, the SPARC research physics programs exhibit a high discovery potential especially considering the advances in experimental equipment.

## Acknowledgments

The support by FAIR, BMBF, Helmholtz Association, Max-Planck-Society, EU, and DFG is acknowledged. Moreover, special thanks to the members of the SPARC collaboration for the excellent cooperation.
<figure><img src="image_35.png" /><figcaption>Figure 31. Overview of FAIR accelerators and storage rings. Current/future SPARC involvement in addition to ESR and HITRAP is indicated by orange dots [202]. Reprinted from [202], Copyright 2015, with permission from Elsevier.</figcaption></figure>

SPARC will exploit the research potential of the following four research facilities at FAIR (see also figure 31).

**HITRAP [210]:** HCIs (up to bare uranium) from the ESR storage ring will be extracted into the HITRAP facility, where they are decelerated and cooled. The cold ions are delivered to a broad range of atomic physics experiments at very low energies or even in ion traps (see section 11). This facility is ready for operation and is waiting for first beams.

**CRYRING [211]:** A low-energy storage ring, dedicated to precision experiments with HCIs that has already proven its potential in a multitude of experiments at Stockholm. CRYRING has been transferred to GSI/FAIR and ions decelerated in the ESR will be provided to CRYRING for experiments. A unique feature of CYRING is its ultra-cold electron cooler. CRYING is currently being commissioned and will be available for experiments in 2019.

**ESR [201, 202]:** The ESR is the most versatile storage ring with respect to beam energies, acceptance, various beam manipulations, and detection instrumentation. It allows for electron, stochastic, and laser cooling of beams of stable and exotic nuclei. Moreover, beams for HITRAP and CRYRING will be prepared and provided (decelerated) by the ESR.

**HESR [201, 202]:** The HESR will give access for the very first time to cooled, HCIs up to a γ-factor of 6 for precision experiments in atomic physics and neighboring fields.

**Advances in science and technology to meet challenges.** Storage and cooling are essential for the discussed studies, and the development and improvement of the corresponding dedicated tools for particle storage, preparation and detection are part of the current technical challenges. Within the SPARC collaboration, there are numerous efforts to improve instrumentation including devices for deceleration and cooling of ions in storage rings and particle traps, non-destructive detection of ions and their reactions, and

spectrometers in the optical and x-ray regimes. Newly developed calorimetric low-temperature detectors for spectroscopy of Lyman x-rays of HCIs which yield a high energy resolution that is comparable to crystal spectrometers and at the same time exhibit a large wavelength acceptance have been implemented and tested in first experiments at the ESR storage ring. Techniques for cooling of ions from keV to μeV energies (i.e. by more than 8 orders of magnitude) and crystallization within seconds have been devised by application in Penning traps [212].

For laser spectroscopy, a power-scalable nonlinear compression scheme which will provide few-cycle pulses at up to 1 kW of average power has been developed [213]. Thus, efficient high harmonic generation up to the water window will be feasible as well as an isolated attosecond pulse generation. A portable XUV source for experiments at heavy-ion storage rings at GSI/FAIR will be setup and installed at CRYRING [211]. At a later stage, such a system will serve as a light source for experiments at the HESR. Single- and multi-species ion crystals as presently produced in Penning traps will be used for sympathetic cooling of externally produced HCIs (HITRAP and EBIT at FAIR) and will allow us to perform high-precision optical spectroscopy of magnetic dipole transitions as probes of QED calculations in the magnetic sector. Future activities related to micro-calorimeter detectors will focus on large pixel arrays, e.g. 1000 pixels, and the optimization of the electronic read-out schemes. Furthermore, besides Metallic Magnetic Calorimeters [214], Superconducting Tunnel Junctions detectors with their high count-rate capability and unique performance at XUV photon energies will be considered.

**Concluding remarks.** The SPARC collaboration is pursuing a challenging atomic physics program in the realm of strong electromagnetic fields, exploiting the present and future trapping and storage facilities of GSI/FAIR. The focus is on collision experiments of HCIs with atoms, electrons, and photons as well as spectroscopic studies in the x-ray, optical and microwave domains. Beside experiments on stable ions, also experiments at the border between atomic, nuclear and astro-physics will be conducted. Since the future FAIR facility will offer beams of cooled HCIs in energy regimes where no such experiments were possible so far, the SPARC research physics programs exhibit a high discovery potential especially considering the advances in experimental equipment.

## Acknowledgments

The support by FAIR, BMBF, Helmholtz Association, Max-Planck-Society, EU, and DFG is acknowledged. Moreover, special thanks to the members of the SPARC collaboration for the excellent cooperation.
## ORCID iDs

[1] Friedrich Aumayr https://orcid.org/0000-0002-9788-0934

[2] Emma Sokell https://orcid.org/0000-0002-7063-7205

[3] Stefan Schippers https://orcid.org/0000-0002-6166-7138

[4] Frédéric Merkt https://orcid.org/0000-0002-4897-2234

[5] Paul Scheier https://orcid.org/0000-0002-7480-6205

[6] Olof Echt https://orcid.org/0000-0002-0970-1191

[7] Tom Kirchner https://orcid.org/0000-0002-6644-3379

[8] Stephan Fritzsche https://orcid.org/0000-0003-3101-2824

[9] Xinwen Ma https://orcid.org/0000-0001-9831-0565

[10] Lokesh Tribedi https://orcid.org/0000-0002-9282-9734

[11] José R Crespo López-Urrutia https://orcid.org/0000-0002-2937-8037

[12] Yuri A Litvinov https://orcid.org/0000-0002-7043-4993

[13] Henning Zettergren https://orcid.org/0000-0002-2493-4161

[14] Richard A Wilhelm https://orcid.org/0000-0001-9451-5440

[15] Henning T Schmidt https://orcid.org/0000-0002-8209-5095

## References

[1] Gallagher T F 1994 *Rydberg Atoms* (Cambridge: Cambridge University Press)

[2] Merkt F, Willitsch S and Hollenstein U 2011 High-resolution photoelectron spectroscopy *Handbook of High-Resolution Spectroscopy* vol 3 ed M Quack and F Merkt (Chichester: Wiley) pp 1617–54

[3] Hogan S D, Seiler C and Merkt F 2009 Rydberg-state-enabled deceleration and trapping of cold molecules *Phys. Rev. Lett.* 103 123001

[4] Seiler R, Hollenstein U, Greetham G M and Merkt F 2001 Rydberg-state-resolved zero-kinetic-energy photoelectron spectroscopy *Chem. Phys. Lett.* 118 10024–33

[5] Deiglmayr J, Herburger H, Saßmannshausen H, Jansen P, Schmutz H and Merkt F 2016 Precision measurement of the ionization energy of Cs I *Phys. Rev. A* 93 013424

[6] Seaton M J 1983 Quantum defect theory *Rep. Prog. Phys.* 46 167–257

[7] Jungen C 2011 Elements quantum defect theory *Handbook of High-resolution Spectroscopy* vol 1 ed M Quack and F Merkt (Chichester: Wiley)

[8] Schäfer M, Raunhardt M and Merkt F 2010 Millimeter-wave spectroscopy and multichannel quantum-defect-theory analysis of high Rydberg states of xenon: the hyperfine structure of $^{129}$Xe$^+$ and $^{131}$Xe$^+$ *Phys. Rev. A* 81 032514

[9] Haase C, Beyer M, Jungen C and Merkt F 2015 The fundamental rotational interval of para-H$_2$ $^+$ by MQDT-assisted Rydberg spectroscopy of H$_2$ *J. Chem. Phys.* 142 064310

[10] Cheng C *et al* 2018 Dissociation energy of the hydrogen molecule at 10$^{-9}$ accuracy *Phys. Rev. Lett.* 121 013001

[11] Beyer M, Hölsch N, Agner J A, Deiglmayr J, Schmutz H and Merkt F 2018 Metrology of high-*n* Rydberg states of molecular hydrogen with $\Delta v/v = 2 \times 10^{-10}$ accuracy *Phys. Rev. A* 97 012501

[12] Pratt S T, Dehmer J L, Dehmer P M and Chupka W A 1994 Reactions of Rydberg states of molecular hydrogen *J. Chem. Phys.* 101 882–90

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[16] Sanz-Sanz C, Aguado A, Roncero O and Naumkin F 2013 Non-adiabatic couplings and dynamics in proton transfer reactions of H$^+$ systems: application to H$^+ + H \rightarrow H + H$ $^+$ collisions *J. Chem. Phys.* 143 2–234303

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[27] Nipper J *et al* 2012 Highly resolved measurements of Stark tuned forster resonances between Rydberg atoms *Phys. Rev. Lett.* 108 113001

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ORCID iDs

Friedrich Aumayr https://orcid.org/0000-0002-9788-0934

Emma Sokell https://orcid.org/0000-0002-7063-7205

Stefan Schippers https://orcid.org/0000-0002-6166-7138

Frederic Merkt https://orcid.org/0000-0002-4897-2234

Paul Scheier https://orcid.org/0000-0002-7480-6205

Olof Echt https://orcid.org/0000-0002-0970-1191

Tom Kirchner https://orcid.org/0000-0002-6644-3379

Stephan Fritzsche https://orcid.org/0000-0003-3101-2824

Xinwen Ma https://orcid.org/0000-0001-9831-0565

Lokesh Tribedi https://orcid.org/0000-0002-9282-9734

José R Crespo López-Urrutia https://orcid.org/0000-0002-2937-8037

Yuri A Litvinov https://orcid.org/0000-0002-7043-4993

Henning Zettergren https://orcid.org/0000-0002-2493-4161

Richard A Wilhelm https://orcid.org/0000-0001-9451-5440

Henning T Schmidt https://orcid.org/0000-0002-8209-5095

References

[1] Gallagher T F 1994 *Rydberg Atoms* (Cambridge: Cambridge University Press)

[2] Merkt F, Willitsch S and Hollenstein U 2011 High-resolution photoelectron spectroscopy *Handbook of High-Resolution Spectroscopy* vol 3 ed M Quack and F Merkt (Chichester: Wiley) pp 1617–54

[3] Hogan S D, Seiler C and Merkt F 2009 Rydberg-state-enabled deceleration and trapping of cold molecules *Phys. Rev. Lett.* 103 123001

[4] Seiler R, Hollenstein U, Greetham G M and Merkt F 2001 Rydberg-state-resolved zero-kinetic-energy photoelectron spectroscopy *Chem. Phys. Lett.* 118 10024–33

[5] Deiglmayr J, Herburger H, Saßmannshausen H, Jansen P, Schmutz H and Merkt F 2016 Precision measurement of the ionization energy of Cs I *Phys. Rev. A* 93 013424

[6] Seaton M J 1983 Quantum defect theory *Rep. Prog. Phys.* 46 167–257

[7] Jungen C 2011 Elements quantum defect theory *Handbook of High-resolution Spectroscopy* vol 1 ed M Quack and F Merkt (Chichester: Wiley)

[8] Schäfer M, Raunhardt M and Merkt F 2010 Millimeter-wave spectroscopy and multichannel quantum-defect-theory analysis of high Rydberg states of xenon: the hyperfine structure of ¹²⁹Xe⁺ and ¹³¹Xe⁺ *Phys. Rev. A* 81 032514

[9] Haase C, Beyer M, Jungen C and Merkt F 2015 The fundamental rotational interval of para-H₂⁺ by MQDT-assisted Rydberg spectroscopy of H₂ *J. Chem. Phys.* 142 064310

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[203] Brandau C, Kozhuharov C, Lestinsky M, Müller A, Schippers S and Stöhlker T 2015 Storage-ring experiments on dielectronic recombination at the interface of atomic and nuclear physics *Phys. Scr.* 2015 014022

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[205] Gumberidze A et al 2013 Electron- and proton-impact excitation of hydrogenlike uranium in relativistic collisions *Phys. Rev. Lett.* 110 213201

[206] Hagmann S et al 2013 Few-body quantum dynamics of high-Z ions studied at the future relativistic high-energy storage ring *Phys. Scr.* 2013 014086

[207] Botermann B et al 2015 Test of time dilation using stored Li⁺ ions as clocks at relativistic speed *Phys. Rev. Lett.* 113 120405

[208] Bosch F, Litvinov Y A and Stöhlker T 2013 Nuclear physics with unstable ions at storage rings *Prog. Part. Nucl. Phys.* 73 84

[209] Nakano Y et al 2013 Resonant coherent excitation of the lithiumlike uranium ion: a scheme for heavy-ion spectroscopy *Phys. Rev. A* 87 060501

[210] Kluge H J et al 2008 HITRAP: a facility at GSI for highly charged ions *Adv. Quantum Chem.* 53 83

[211] Lestinsky M et al 2016 Physics book: CRYRING@ESR *Eur. Phys. J. Spec. Top.* 225 797–882

[212] Murböck T, Schmidt S, Birkl G, Nörtershäuser W, Thompson R C and Vogel M 2016 Rapid crystallization of externally produced ions in a Penning trap *Phys. Rev. A* 94 043410

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