Exotic atoms, in which electrons are replaced by a heavier negatively charged particle, provide a unique and sensitive laboratory for exploring the frontiers of fundamental physics. Because of the presence of heavier bound particles, such as a muon or antiproton, their electromagnetic interactions with the nucleus are significantly enhanced (due to the inverse dependence on the Bohr radius), allowing precision spectroscopy to probe effects beyond the reach of ordinary atoms. A key technological advancement enabling such measurements is the use of the novel cryogenic microcalorimeters. These detectors offer a great combination of energy resolution and detection efficiency, overcoming limitations of conventional semiconductors and crystal detectors and thus opening new paths for precision X-ray spectroscopy measurement.

In this context, I will present two complementary experimental initiatives, \textbf{QUARTET} and \textbf{PAX}. The QUARTET experiment, conducted at PSI, focuses on X-ray spectroscopy of muonic atoms to determine the nuclear charge radii of light elements (Z = 3–10), achieving an order of magnitude improvement in accuracy over previous results. In parallel, the newly launched PAX experiment aims to test strong-field bound-state quantum electrodynamics (QED) by measuring high-circular Rydberg transition energies in antiprotonic atoms (9 < Z < 83). These circular states are minimally affected by nuclear structure, allowing precise measurements that isolate higher-order QED effects.

When atoms and molecules are exposed to an extreme ultraviolet (XUV) pulse and infrared (IR) laser field that overlap in space and time, the so-called laser-assisted photoionization emission (LAPE) processes take place. Depending on whether the XUV pulse duration is shorter, or longer, than one IR optical cycle (T = 2π/ωIR, where ωIR is the frequency of the IR laser), two different scenarios arise: the sideband or the streaking regime respectively. The photoelecton momentum distribution (PMD) can be recorded for different delays between the pulses, so the photoionization dynamics becomes accessible with attosecond resolution.

In recent years, there has been increasing interest in exploring strong laser–matter in-
teractions beyond the widely used electric dipole approximation [1]. However, the streaking scenario remains unexplored. In this work, we introduce a theoretical model to describe the streaking regime within the strong field approximation including first-order nondipole corrections [2, 3]. In order to study a specific case, we have focused on XUV ionization of a 1s-state hydrogen atom assisted by a ellipctically polarized IR laser.

We will systematically explore the PMD for different delays and making use of the semiclassical model (SCM) to gain an overall understanding of PMD structures, we will analyze the PMD as in the attoclock process [4].

[1] J Maurer and U Keller 2021 J. Phys. B 54 094001
[2] C J Joachain, N J Kylstra, and R M Potvliege 2012 Atoms in Intense Laser Fields, Cambridge University Press.
[3] R Della Picca et al. 2023 Phys. Rev. A 107 053104
[4] R Della Picca et al. 2025 Phys. Rev. A 112 023111

The interaction of strong and short laser pulses with atoms and molecules has received renewed attention, mainly because the advances in laser technology made possible new experimental investigations of atomic and molecular processes on an ultrashort time-scale and under ultra-intense laser radiation.

When the atom (or molecule) is exposed to an intense laser field, ionization occurs through the process called above-threshold ionization (ATI) where the atom absorbs more than the energetically required number of photons. Furthermore, when an extreme ultraviolet (XUV) pulse and an infrared (IR) laser field overlap in space and time with matter, the so-called laser-assisted photoionization emission (LAPE) processes take place. Depending on the XUV pulse duration, two different scenarios arise: the sideband and the streaking regime. The photoelecton momentum distribution (PMD) can be recorded for different delays between the pulses, so the photoionization dynamics become accessible with attosecond resolution.

An interesting aspect of LAPE ionization processes is that, with the usual choice of IR and XUV laser parameters, the energy domains of XUV and IR-induced ionization are well separated: while ATI structures due to the IR laser extend from the energy threshold up to 2Up (twice the ponderomotive energy), the XUV term is approximately centered at the XUV frequency. Then, by selecting the XUV frequencies, the two domains do not overlap [1]. However, in the general situation of LAPE process, both contributions could appear superimposed.

In this work we present a theoretical study within the strong field approximation to analyze the IR and XUV-IR interference terms, i.e., the interference between ATi and LAPE structures in the photoelectron spectrum.

[1] R Della Picca et al, Phys Rev A 102, 043106 (2020)

Experimental data on the spectroscopic properties of SHEs and their ions are extremely limited due to their short half-lives and low production rates, which make direct measurements challenging. Consequently, theoretical calculations are currently the primary tool for investigating their electronic structure. Accurate calculations involving SHEs are computationally demanding because they require methods capable of capturing strong electron correlation and relativistic effects in a self-consistent way.

To achieve a balance between accuracy and computational efficiency, we employ a hybrid framework combining the linearised coupled cluster method with the configuration interaction method and perturbation theory. This approach is used to calculate the energy levels, ionisation potentials, electron affinities, field isotope shifts, and static dipole polarisabilities of the SHEs livermorium (Lv) and tennessine (Ts). The accuracy of this method is benchmarked against the lighter homologues tellurium, iodine, polonium, and astatine, for which reliable experimental data are available.

Our calculations provide predictions for Lv and Ts, with energy levels, ionisation potentials, and electron affinities estimated to be accurate within a few percent, and polarisabilities accurate to approximately ten percent. Several strong electric-dipole transitions in Lv and Ts are predicted to be in the optical range, suggesting potential experimental accessibility. For the ions of Lv and Ts, our results provide the first theoretical predictions of their electronic structure.

These results help fill critical gaps in the spectroscopic data for SHEs and their ions, and provide reliable theoretical benchmarks for future spectroscopic experiments.

All elements from tantalum to platinum will be produced in Tokamaks through neutron-induced transmutation of the tungsten of which the divertor walls are composed. Therefore, ionic impurities of all possible charge states should appear in the fusion plasma, contributing to power loss, which does not make it easy to achieve self-maintained fusion reactions. However, the radiation that is emitted by these impurities will be useful for plasma diagnosis (impurity influx, temperature, and density). The identification of the spectral lines in experiments and knowledge of the radiative data for these ions are thus of great interest in this field. This work focuses on calculations of atomic structure, electric dipole transition probabilities, and oscillator strengths for isoelectronic elements of Yb I from tantalum to platinum. A new set of electric dipole transitions from Ta IV to Pt IX are determined and listed using two independent methods: the pseudo-relativistic Hartree–Fock method including core-polarization effects (HFR+CPOL), and the fully relativistic Multiconfiguration Dirac–Hartree–Fock (MCDHF) approach. The results from both methods are compared in order to assess the uncertainty and quality of the new data.

The Multi-Configuration Dirac–Fock (MCDF) approach has been used to calculate the radiative data and energy levels for Cs XLV. For highly ionized Na-like Cs⁺⁴⁴ ions (Cs XLV), these statistics include the transition wavelength, transition rates, oscillator strength, line strength, and radiative rates like electric dipole (E1), quadrupole (E2), magnetic dipole (M1), and magnetic quadrupole (M2) transitions. The Flexible Atomic Code (FAC) and the General Purpose Relativistic Atomic Structure Package (GRASP) were the two codes used to compute the outcome. Our results for Cs XLV show strong agreement with the NIST data and other accessible data in terms of excitation energy, wavelength, oscillator strength, and line strength. Additionally evaluated are the plasma properties, including skin depth, electron density, line intensity ratios, and plasma frequency. We have calculated the strength of the emission and absorption oscillators for the first 50 spectral lines of the E1 transitions for Cs XLV. The impact of plasma temperatures on the skin depth, electron densities, line intensity ratio, and plasma frequency has been studied for Hot Dense Plasma (HDP). Lifetimes for the lowest 20 Cs XLV values have also been determined. The identification and assessment of spectral lines from different fusion plasma, solar, plasma modeling, and astrophysical studies could benefit from our presented atomic and radiative data of Na-like Cs.

The expanding ejecta of binary neutron star mergers (NSMs) have been proven to be the ideal site for the production of lanthanides via rapid neutron capture (r-process) nucleosynthesis. However, identifying specific atomic absorption and emission features in kilonova spectra and linking them with individual elements remains significant challenges [1,2].

One of the primary obstacles is the lack of comprehensive collisional atomic data for modelling the late nebular epochs (> 4 days post-merger). While it is a reasonable approximation to assume that the ejecta is in local thermodynamic equilibrium (LTE) and that atomic absorption processes dominate in the early hours (< 1 day after the NSM), it is not possible to assume LTE for nebular epochs. During these late stages, relevant processes include photoionization, ionization, excitation by electron impact, and electronic recombination [3].

In this work, we address this gap by benchmarking electron-impact excitation collision strengths and line emissivities for Au and Pt against the existing literature [4,5], using the Flexible Atomic Code’s distorted-wave (DW) method [6]. Owing to its lower computational cost, the DW approach enables systematic calculations across the lanthanides, for which we report the results. We also present luminosity predictions derived from our atomic data [7]. Together, these calculations refine non-LTE spectral models, improving the interpretation of kilonova spectra and the tracing of heavy-element production.

[1] Nanae Domoto et al 2022 ApJ 939 8

[2] A Flörs et al 2023 MNRAS, Volume 524, Issue 2

[3] Quentin Pognan et al 2022 MNRAS Volume 513, Issue 4

[4] Michael McCann et al 2022, MRNAS 509(4):4723

[5] S. J. Bromley et al, 2023, ApJ 268 22

[6] M. F. Gu, “The Flexible Atomic Code,” J Phys 86(5):675 (2008)

[7] M McCann et al, 2025, MNRAS, Volume 538, Issue 1

The study of r-process elements, such as barium (Ba) and strontium (Sr), is key to understanding the formation of heavy elements in the Universe. More precisely, the hyperfine structure (HFS) of their atomic levels is commonly used by astrophysicists to determine abundances via spectral line modeling. Using the Multiconfiguration Dirac–Hartree–Fock (MCDHF) method as implemented in the General Relativistic Atomic Structure Package (GRASP) code, the magnetic dipole and electric quadrupole hyperfine structure constants were determined for the first excited states of Ba II isotopes, as well as for Ba I and Sr II isotopes to monitor the robustness of the developed model. New code developments, such as the use of natural orbitals, the addition of polarization effects and the use of Configuration State Function Generators (CSFGs) as implemented in GRASPG, were tested for these heavy elements. The developed strategy allowed us to achieve encouraging results, with little disagreement with experimental data for all studied level except ²D_5/2 in the first Ba II isotope. This disagreement was observed in another Ba II isotope as well as in Sr II. However, it emerged that the adopted strategy could not describe all the physics crucial for the Ba I states studied, with disagreements reaching up to 70%. This limitation necessitates the introduction of more intensive work employing a multireference (MR) approach to describe configuration mixing. Such efforts are expected to yield improved agreement with experimental values and outcomes from other theoretical computations. Nevertheless, the adopted strategy continues to serve as a valuable benchmark for comparing the MCDHF method with other theoretical approaches.

Abstract

The principles of atomic structure and spectroscopy form the cornerstone of modern biochemical research, offering deep insights into the composition, dynamics, and function of biomolecules. Theoretical models describing atomic and electronic structures—such as quantum mechanics and orbital theory—explain how atoms interact within biological macromolecules and influence biochemical reactivity. Experimental spectroscopic techniques derived from these theories, including UV–Visible, infrared (IR), nuclear magnetic resonance (NMR), fluorescence, and atomic absorption spectroscopy, provide powerful tools for structural elucidation, enzyme characterization, and metabolic profiling. This review bridges theoretical understanding with experimental practice, highlighting how atomic and spectral analysis enables precise detection of trace elements, investigation of metal–protein complexes, and monitoring of biochemical transformations. By integrating atomic theory and spectroscopic experimentation, this work underscores their pivotal role in advancing molecular biochemistry and biomedical science.

Accurate modeling of kilonova spectra, particularly during the late nebular phases (> 4 days post-merger) dominated by non-local thermodynamic equilibrium (non-LTE) processes [1, 2], demands detailed collisional atomic data for r process elements like lanthanides. While electron impact excitation (EIE) proceeds via direct scattering, the indirect pathway of resonant excitation (RE) represents another crucial, complex mechanism. RE proceeds via electron capture into intermediate autoionizing states followed by radiative decay to a bound excited level. Calculating RE accurately is computationally intensive [3, 4], and the requisite atomic data have been largely unavailable for complex lanthanide ions, often leading to the omission of its contribution in astrophysical models despite its potential to dominate total EIE rates [5].

Continuing our systematic effort to generate accurate atomic data for lanthanides, we present large-scale calculations focusing specifically on quantifying the impact of RE on EIE collision strengths for multiple singly and doubly ionized species. We employ the fully relativistic distorted wave (DW) method within the Flexible Atomic Code (FAC) [6]. These calculations utilize improved atomic structures derived from optimized potentials [7] and calibrated energy levels [8]. The RE channels were incorporated via the Independent Process, Isolated Resonance DW framework (IPIRDW) [5].

Our results provide extensive new EIE datasets highlighting the RE contribution, demonstrating that this resonant pathway provides a substantial, often dominant, component to the total collision strength for many astrophysically relevant transitions in lanthanides under nebular kilonova conditions [3, 4]. This work supplies vital atomic data focused on the resonant channel, needed to significantly enhance the fidelity of non-LTE radiative transfer simulations and improve interpretations of late time kilonova spectra.

[1] Pognan et al. MNRAS 510, 3806 (2022)
[2] Gillanders et al. MNRAS 529, 2918–2945 (2024)
[3] M McCann et al. MNRAS 538, 537–552 (2025)
[4] Leo P. Mulholland et al. JQSRT 345, 109545 (2025)
[5] Li et al. Chin. Phys. B. 24, 113401 (2015).
[6] Gu et al. Can. J. Phys. 86, 675 (2008)
[7] Ferreira da Silva et al., Phys. Rev. A 112(1), 012802 (2025)
[8] A. Flörs et al., arXiv:2507.07785 (2025)