arXiv subjects

Kristian König

Publications and source records attributed to Kristian König.

At least 19 recordsLinked to original sources

arXiv2026

Raman Transitions in Collinear Laser Spectroscopy for High-Precision Frequency Measurements

We demonstrate stimulated Raman spectroscopy on dipole-forbidden fine-structure transitions in a fast ion beam. It is applied to the $\mathrm{S}_{1/2}\rightarrow\mathrm{D}_{5/2}$ clock transition in $^{88}\mathrm{Sr}^{+}$ and combined with a subsequent E1 transition to allow for its detection by standard fluorescence detection. Building on this, the $\mathrm{D}_{3/2}\rightarrow\mathrm{D}_{5/2}$ fine-structure splitting is measured directly using a Doppler-free scheme employing two subsequent Raman transitions. These measurements demonstrate a path toward precision collinear laser spectroscopy of narrow transitions on short-lived isotopes beyond the linewidth limit of conventional electric-dipole spectroscopy, which can be used to probe fundamental aspects of atomic and nuclear physics.

physics.atom-ph↗

arXiv2026

Collinear Laser Spectroscopy of the $1s2s\,^3\!S_1\rightarrow 1s2p\,^3\!P_{0,2}$ Transitions in Helium-like $^{11}$B$^{3+}$

We report on hyperfine structure measurements of the $1s2s\,{}^{3\!}S_1 \rightarrow 1s2p\,{}^{3\!}P_{0,2}$ fine-structure transitions in the spectrum of $^{11}$B$^{3+}$ ions. The helium-like ions were created in an electron beam ion trap (EBIT) and ejected after a short production period of 15 ms. A challenge of the experiment was the treatment of the complex energy-time-profile of the created ion bunches and the sensitivity of the collinear laser spectroscopy to the starting potential inside the EBIT. Despite these complications and the low statistics of the experiment, we were able to confirm and improve the results of a previous measurement of these transitions. While we find moderate tension in the results for the hyperfine parameters of the $^{3\!}P_{0,2}$ levels, their fine-structure level energies are in reasonable agreement between experiments and with theory.

physics.atom-ph↗

arXiv2026

Accurate Charge Radius Measurement of $^{14}$C Confronts Ab Initio Theory

Located at the neutron shell closure $N = 8$, the long-lived radioactive isotope $^{14}$C plays a critical role in geochronology and nuclear structure studies. Despite its widespread use, the nuclear charge radius of $^{14}$C has remained less precisely known compared to its stable counterpart $^{12}$C. Here, we report a high-precision determination of the $^{14}$C charge radius using collinear laser spectroscopy at the COALA setup at TU Darmstadt, improving upon the precision of previous muonic measurements by a factor $5$ and revealing a $1.9σ$ discrepancy of combined uncertainty, indicating a likely underestimated uncertainty in the muonic determination. This measurement challenges state-of-the-art ab initio nuclear theory calculations, including auxiliary field diffusion Monte Carlo, the valence-space in-medium similarity renormalization group, and the no-core shell model, augmented by neural-network techniques. With $^{12}$C and $^{14}$C now forming one of the most precisely characterized even-even isotope pairs, these results also enable improved QED tests.

nucl-ex↗

arXiv2026

Magnetic Dipole Hyperfine Constants of 15 Energy Levels in $^{169}$Tm II Determined by Collinear Laser Fluorescence Spectroscopy

We present measurements of the hyperfine structure, transition frequencies and relative intensities in 14 lines of 169Tm+ ions starting from the $4f^{13}({}^2\!F^o_{7/2})6s_{1/2} (7/2, 1/2)_{3,4}^o$ levels. The magnetic dipole hyperfine ($A$) constant of all 15 involved ionic levels are determined, 14 of which have sub-MHz accuracy. For five levels no previous measurements were reported, while all others are in excellent agreement with the literature and improve on the previous accuracy by typically 1 - 2 orders of magnitude. We compare transition strengths and include improved transition frequencies. These results are useful for the quantum computing community, modelling of Tm$^+$ abundances in stellar spectra, as well as for an ongoing measurement program on short-lived Tm isotopes at ISOLDE/CERN.

physics.atom-ph↗

arXiv2026

Nuclear charge radii of aluminium isotopes at the proton drip line

Understanding the evolution of nuclear size away from stability remains a central challenge in nuclear physics. In neutron-deficient systems, charge radii can be highly sensitive to the interplay between strong and electromagnetic interactions, and the effects of weak binding, giving rise to exotic nuclear phenomena. However, experimental data on these systems has been limited by short lifetimes and low production rates. Here we report the first laser-spectroscopy measurements of nuclear charge radii along the neutron-deficient aluminium isotopic chain, from $^{25}$Al to the proton-drip-line nucleus $^{22}$Al, using the {Resonance Ionization Spectroscopy Experiment} (RISE) at the {Facility for Rare Isotope Beams} (FRIB). Our measurements reveal a step-like increase in charge radius toward the drip line, with similar radii for $^{22,\,23}$Al. A comparison of our results with those of their mirror partners reveals an almost identical correlation with the calculated proton skins and is consistent with the systematic trend of well-bound nuclei. These results offer insight for understanding the evolution of nuclear size at the proton dripline and place important constraints on modern nuclear theory. They also demonstrate the unique combined capabilities of RISE and FRIB to probe the structures of previously inaccessible nuclei at the limits of existence.

nucl-ex↗

arXiv2025

Demonstration of a Raman Velocity Filter in Collinear Laser Spectroscopy: Towards Applications for sub-ppm High-Voltage Measurements

Raman transitions have a wide range of applications in atomic physics and have recently been proposed as a means for improving high-precision high-voltage measurements. Here, we present a theoretical analysis and a first experimental demonstration of $5s\,^2\mathrm{S}_{1/2} \rightarrow 4d\,^2\mathrm{D}_{3/2,5/2}$ Raman transitions in $^{88}$Sr$^+$ ions in collinear laser spectroscopy. For the theoretical description the three-level system is reduced to an effective two-level system, in order to estimate the experimental parameters, while the role of the spatial laser intensity distribution in combination with the radial extension of the ion beam are elucidated by performing simulations of the full four-level system. Experimentally, we realized the first velocity-selective Raman transition in collinear laser spectroscopy. Using a $^{88}$Sr$^+$ ion beam, we demonstrate a reduction in the energy width to less than $200\,$meV, which is about an order of magnitude reduction compared to the usage of an optical dipole transition as in previous works. We also investigate two-photon Rabi oscillations and show that their observed collapse is consistent with the simulations.

physics.atom-ph↗

arXiv2025

Fingerprints of triaxiality in the charge radii of neutron-rich Ruthenium

We present the first measurements with a new collinear laser spectroscopy setup at the Argonne Tandem Linac Accelerator System utilizing its unique capability to deliver neutron-rich refractory metal isotopes produced by the spontaneous fission of 252Cf. We measured isotope shifts from optical spectra for nine radioactive ruthenium isotopes 106-114Ru, reaching deep into the mid-shell region. The extracted charge radii are in excellent agreement with predictions from the Brussels-Skyrme-on-a-Grid models that account for the triaxial deformation of nuclear ground states in this region. We show that triaxial deformation impacts charge radii in models that feature shell effects, in contrast to what could be concluded from a liquid drop analysis. This indicates that this exotic type of deformation should not be neglected in regions where it is known to occur, even if its presence cannot be unambiguously inferred through laser spectroscopy.

nucl-ex↗

arXiv2025

Splitting Isotope Shift in the $1s2p\,^3\!P_{0,1,2}$ Fine-Structure Triplet in $^{12,13,14}$C$^{4+}$: Experiment and Theory

We report measurements and theoretical calculations of the fine-structure splittings in all three $1s2s\,^3\!S_1\rightarrow\,1s2p\,^3\!P_{0,1,2}$ transitions in the heliumlike systems of the isotopes $^{12,13,14}$C. The metastable triplet state was efficiently populated in an electron beam ion source and the C$^{4+}$ ions were electrostatically accelerated to 50\,keV to perform collinear laser spectroscopy. From the determined transition frequencies, the splitting isotope shift (SIS), i.e., the difference in fine-structure splittings between different isotopes of the same element, was extracted. In the SIS, theoretical uncertainties due to higher-order quantum electrodynamic corrections are strongly suppressed since they are independent of both nuclear mass and the fine-structure quantum number $J$ in lowest order. Comparison with theory provides an important test of experimental accuracy, particularly in the $^{13}$C$^{4+}$ case, for which the nuclear spin leads to hyperfine-induced fine-structure mixing. At the same time, the even-even isotopes $^{12,14}$C$^{4+}$ without nuclear spin can be used to confirm theory. Theoretical values of the SIS are given for all the heliumlike ions with $2\le Z\le 10$.

physics.atom-ph↗

arXiv2025

The nuclear charge radius of $^{13}\mathrm{C}$

The size is a key property of a nucleus. Accurate nuclear radii are extracted from elastic electron scattering, laser spectroscopy, and muonic atom spectroscopy. The results are not always compatible, as the proton-radius puzzle has shown most dramatically. Beyond helium, precision data from muonic and electronic sources are scarce in the light-mass region. The stable isotopes of carbon are an exception. We present a laser spectroscopic measurement of the root-mean-square (rms) charge radius of $^{13}\mathrm{C}$ and compare this with ab initio nuclear structure calculations. Measuring all hyperfine components of the $2\,^3\mathrm{S} \rightarrow 2\,^3\mathrm{P}$ fine-structure triplet in $^{13}\mathrm{C}^{4+}$ ions referenced to a frequency comb allows us to determine its center-of-gravity with accuracy better than $2\,\mathrm{MHz}$ although second-order hyperfine-structure effects shift individual lines by several $\mathrm{GHz}$. We improved the uncertainty of $R_\mathrm{c}(^{13}\mathrm{C})$ determined with electrons by a factor of $6$ and found a $3σ$ discrepancy with the muonic atom result of similar accuracy.

physics.atom-ph↗

arXiv2024

Ultra-stable 3D-printed precision voltage divider for calibrations and experiments

This paper presents the concept of an ultra-stable, thermally independent precision voltage divider tailored for direct current (DC) voltages up to 60 kV. Key features of this voltage divider include minimal voltage dependence, excellent stability, and resistance to external temperature variations. The innovative approach involves its fabrication using 3D printing technology, allowing easy replication by project partners. This precision voltage divider leverages commercially available precision resistors, drawing upon successful outcomes from the FutureEnergy 19ENG02 and HVDC ENG07 Projects. In these experiments, which involve ion acceleration and laser probing of electronic transitions, voltage dividers are integrated into setups such as COALA (TU Darmstadt), BECOLA (Michigan State University), COLLAPS (CERN/ISOLDE), and ATLANTIS (Argonne National Laboratory). Monitoring the applied acceleration potential, these dividers allow one to consider and counteract long-term drifts and thereby improving measurement accuracy.

physics.ins-det↗

arXiv2024

High Voltage Determination and Stabilization for Collinear Laser Spectroscopy Applications

Fast beam collinear laser spectroscopy is the established method to investigate nuclear ground state properties such as the spin, the electromagnetic moments, and the charge radius of exotic nuclei. These are extracted with high precision from atomic observables, i.e., the hyperfine splitting and its the isotope shift, which becomes possible due to a large reduction of the Doppler broadening by compressing the velocity width of the ion beam through electrostatic acceleration. With the advancement of the experimental methods and applied devices, e.g., to measure and stabilize the laser frequency, the acceleration potential became the dominant systematic uncertainty contribution. To overcome this, we present a custom-built high-voltage divider, which was developed and tested at the German metrology institute (PTB), and a feedback loop that enabled collinear laser spectroscopy to be performed at a 100-kHz level. Furthermore, we describe the impact of field penetration into the laser-ion-interaction region. This strongly affects the determined isotope shifts and hyperfine splittings, if Doppler tuning is applied, i.e., the ion beam energy is altered instead of scanning the laser frequency. Using different laser frequencies that were referenced to a frequency comb, the field penetration was extracted laser spectroscopically. This allowed us to define an effective scanning potential to still apply the faster and easier Doppler tuning without introducing systematic deviations.

physics.ins-det↗

arXiv2024

Transition frequencies, isotope shifts, and hyperfine structure in $4s \rightarrow 4p$ transitions of Ti$^+$ ions

We have measured transition frequencies, isotope shifts and hyperfine structure splittings in the $3 d^{2}\left({ }^{3\!}F\right) 4 s\,{ }^{4} F_J\rightarrow 3 d^{2}\left({ }^{3\!} F\right) 4 p \,^{4} G_{J+1}$ transitions in Ti$^+$ ions for $J=\frac{3}{2},\, \frac{5}{2},\, \frac{7}{2}$ using collinear laser spectroscopy. Ions were generated by laser ablation in a buffer gas atmosphere and extracted into vacuum through a nozzle and a pair of radiofrequency (RF) funnels. The obtained results are of interest as reference values for on-line measurements of short-lived titanium isotopes and for astrophysical searches for temporal or spatial variations of the fine structure constant $α$ using quasar absorption spectra.

physics.atom-ph↗

arXiv2023

Collinear laser spectroscopy of highly charged ions produced with an electron beam ion source

Collinear laser spectroscopy has been performed on He-like C$^{4+}$ ions extracted from an electron beam ion source (EBIS). In order to determine the transition frequency with the highest-possible accuracy, the lineshape of the fluorescence response function was studied for pulsed and continuous ion extraction modes of the EBIS in order to optimize its symmetry and linewidth. We found that the best signal-to-noise ratio is obtained using the continuous beam mode for ion extraction. Applying frequency-comb-referenced collinear and anticollinear laser spectroscopy, we achieved a measurement accuracy of better than 2\,MHz including statistical and systematic uncertainties. The origin and size of systematic uncertainties, as well as further applications for other isotopes and elements are discussed.

physics.atom-ph↗

arXiv2023

Collinear Laser Spectroscopy of $2\,{}^3\!S_1 \rightarrow 2\,{}^3\!P_{\!J}$ transitions in helium-like $^{12}\mathrm{C}^{4+}$

Transition frequencies and fine-structure splittings of the $2\,{}^3\!S_1 \rightarrow 2\,{}^3\!P_{\!J}$ transitions in helium-like $^{12}\mathrm{C}^{4+}$ were measured by collinear laser spectroscopy on a 1-ppb level. Accuracy is increased by more than three orders of magnitude with respect to previous measurements, enabling tests of recent non-relativistic QED calculations including terms up to $mα^7$. Deviations between the theoretical and experimental values are within theoretical uncertainties and are ascribed to $mα^8$ and higher-order contributions in the series expansion of the NR-QED calculations. Finally, prospects for an all-optical charge radius determination of light isotopes are evaluated.

physics.atom-ph↗

arXiv2023

Nuclear charge radii of silicon isotopes

The nuclear charge radius of $^{32}$Si was determined using collinear laser spectroscopy. The experimental result was confronted with ab initio nuclear lattice effective field theory, valence-space in-medium similarity renormalization group, and mean field calculations, highlighting important achievements and challenges of modern many-body methods. The charge radius of $^{32}$Si completes the radii of the mirror pair $^{32}$Ar - $^{32}$Si, whose difference was correlated to the slope $L$ of the symmetry energy in the nuclear equation of state. Our result suggests $L \leq 60$\,MeV, which agrees with complementary observables.

nucl-ex↗

arXiv2023

Surprising charge-radius kink in the Sc isotopes at N=20

Charge radii of neutron deficient 40Sc and 41Sc nuclei were determined using collinear laser spectroscopy. With the new data, the chain of Sc charge radii extends below the neutron magic number N=20 and shows a pronounced kink, generally taken as a signature of a shell closure, but one notably absent in the neighboring Ca, K and Ar isotopic chains. Theoretical models that explain the trend at N=20 for the Ca isotopes cannot reproduce this puzzling behavior.

nucl-ex↗

arXiv2022

Ground State Magnetic Dipole Moment of $^{40}$Sc

The hyperfine coupling constants of the proton dripline odd-odd $^{40}$Sc nucleus were deduced from the hyperfine spectrum of the $3d4s\,^3\text{D}_2$\,$\leftrightarrow$\,$3d4p\,^3\text{F}^\circ_3$ transition in Sc II, measured by the bunched beam collinear laser spectroscopy technique. The ground state magnetic dipole and electric quadrupole moments were determined for the first time as $μ$\,=\,$+5.57(4)(2)\,μ_N$ and $Q$\,=\,$+42(38)(28)\,e^2\;{\rm fm^2}$, respectively. The magnetic moment is well reproduced by the additivity rule with magnetic moments of neighboring odd-even nuclei in the vicinity of the doubly-magic $^{40}$Ca nucleus. An ab-initio multishell valence-space Hamiltonian was also employed to calculate the magnetic moment of $^{40}$Sc, which spans across the $sd$ and $fp$ nuclear shells, where we obtained good agreements.

nucl-ex↗

arXiv2021

Charge Radius of Neutron-deficient $^{54}$Ni and Symmetry Energy Constraints Using the Difference in Mirror Pair Charge Radii

The nuclear root-mean-square charge radius of $^{54}$Ni was determined with collinear laser spectroscopy to be $R(^{54}$Ni) = 3.737\,(3)~fm. In conjunction with the known radius of the mirror nucleus $^{54}$Fe, the difference of the charge radii was extracted as $ΔR_{\rm ch}$ = 0.049\,(4)~fm. Based on the correlation between $ΔR_{\rm ch}$ and the slope of the symmetry energy at nuclear saturation density ($L$), we deduced $20 \le L \le 70$\,MeV. The present result is consistent with the $L$ from the binary neutron star merger GW170817, favoring a soft neutron matter EOS, and barely consistent with the PREX-2 result within 1$σ$ error bands. Our result indicates the neutron-skin thickness of $^{48}$Ca as 0.15\,-\,0.19\,fm.

nucl-ex↗