Introduction

Above-Threshold Ionization (ATI) is a fundamental strong-field process, in which photoelectrons exhibit a characteristic multi-peak energy spectrum. While well-studied for isolated atoms, this process is less understood in a screened plasma environment. This work utilizes simulations to investigate how a quantum plasma affects the ATI spectrum, comparing the results with those of a classical plasma.

Methods

The one-dimensional time-dependent Schrödinger equation (TDSE) is solved numerically for a hydrogen atom interacting with an intense laser field, defined by a Gaussian pulse envelope. The TDSE is propagated using a stable, unitary Crank–Nicolson algorithm. To compare the two regimes, classical screening is modeled with a Debye–Hückel (Yukawa) potential, while the quantum environment is modeled using an Exponential Cosine Screened Coulomb (ECSC) potential to capture its characteristic oscillatory nature.

Results

The simulations show a clear contrast between the classical and quantum models. In the classical model, stronger screening causes a uniform blue shift of the ATI peaks to higher energies, corresponding to a reduced electron binding energy. The quantum model reproduces this shift but also introduces new features. The oscillatory component of the ECSC potential alters the spectrum's structure, modulating the relative peak heights and creating new spectral peaks indicating absorption of more photons, features that are entirely absent in the classical Debye model. For low laser intensities, the peak heights are reduced due to plasma screening; however, at higher intensities, plasma screening is insufficient to reduce peak heights.

Conclusion

The systematic blue-shift of the spectral peaks with decreasing Debye lengths provides a direct measure of the binding energy reduction caused by plasma screening. Furthermore, the complex modulations and additional spectral features introduced by the oscillatory component of the potential may serve as a unique way of controlling ATI spectra, distinguishing it from simple monotonic screening.

Introduction:

Accurate atomic data for Ti II are essential for abundance analyses in astronomical objects. The aim of this work is to provide accurate and extensive isotope shift transition (TIS) results for Ti II, specifically by comparing our theoretical calculations with existing experimental measurements and other theoretical predictions.

Methods:

The calculations were performed using two robust multiconfiguration approaches:

The non-relativistic Multiconfiguration Hartree–Fock (MCHF) combined with the Breit–Pauli approximation (MCHF-BP).

The fully relativistic Multiconfiguration Dirac–Hartree–Fock (MCDHF) combined with the Relativistic Configuration Interaction (RCI) method.

These methods were implemented using the ATSP2K (Atomic Transfer and Structure Package) and GRASP2018 (General-purpose Relativistic Atomic Structure Package) codes, respectively.

Results:

Energy levels and isotope shift transitions (TIS) were calculated for the 3d² 4s ⁴F_J → 3d² 4p ⁴G_J+1 transitions in Ti II. The calculated excitation energies are found to be in good agreement with the experimental data. We present the results for the isotope shifts (ISs), which are compared extensively with previous theoretical calculations and available experimental measurements. The current theoretical TIS values show, overall, a better agreement with the experimental values than other theoretical predictions, demonstrating the high accuracy of the present work.

Conclusion:

We have performed an extensive comparison of our computed isotope shift transitions (TIS) with existing theoretical and experimental data. Notably, the accuracy of the total isotope shift (TIS) is significantly improved compared to previous theoretical efforts. Our calculated TIS values demonstrate excellent overall agreement with the measured results, confirming the reliability and high quality of the advanced theoretical methods used in this work.

Introduction: Attosecond science has opened the way to probe and control electronic dynamics in matter on their natural timescales. Pump–probe techniques combining attosecond extreme-ultraviolet (XUV) pulses with near infrared (NIR) or visible laser fields provide access to phase and timing information encoded in photoelectron wave packets. The reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) has become the cornerstone of attosecond chronoscopy of photoionization processes in atoms [1].

Methods: We present a time-dependent non-perturbative (respective to the NIR laser intensity, up to 1014 W/cm2) theory of RABBIT for photoelectron emission from atoms encompassing multiphoton transitions. The laser pulse involves a fundamental frequency in the NIR and several harmonics in the XUV. Within the strong-field approximation (SFA), we employ a semiclassical model based on the saddle-point-approximation to gain a better understanding of the physics involved [2].

Results: We derive analytical expressions for the transition amplitudes and demonstrate that the photoelectron probability distribution can be factorized into interferences between trajectories born within the same optical cycle and those born in different cycles. We identify the contributions from trajectories born within each cycle, or within each half-cycle (depending on the considered emission angle), as the mechanism governing attosecond phase delays in the RABBIT protocol. Comparisons with numerical calculations of the SFA and the ab initio solution of the time-dependent Schrödinger equation (TDSE) confirm the accuracy of the semiclassical description.

Conclusions: The present theory thus provides a unified framework for describing attosecond chronoscopy in different emission geometries, for laser intensities covering the entire range from perturbative values up to the non-perturbative domain.

References:

[1] Isinger M, et al., Science ;358(6365):893-896 (2017). doi: 10.1126/science.aao7043

[2] S. D. López, M. L. Ocello, and D. G. Arbó, Phys. Rev. A 110, 013104 (2024) DOI: https://doi.org/10.1103/PhysRevA.110.013104

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.

In the late 1980s, experimenters observed a new phenomenon in the interaction between intense laser fields and gases, which is now known as high-harmonic generation (HHG).
The discovery of HHG paved the way for using infrared (IR) laser sources to produce femtosecond pulses in the extreme ultraviolet (EUV) and soft X-ray spectral ranges.
This nonlinear process also enables the study of two-color photoionization, where an atom is ionized by a combination of EUV and IR laser pulses rather than by a single pulse. In such experiments, the photoelectron energy spectrum reveals that the dominant contributions arise from processes in which an emitted electron, after absorbing an EUV photon, exchanges n infrared photons (n ≥ 1) with the dressing IR field via stimulated absorption or emission.
This results in the formation of photoelectron sidebands (SB±n) that are symmetrically distributed around the main harmonic peak, corresponding to the direct photoionization of the atom.

In this work, we perform numerical simulations of the nonresonant two-color photoionization of argon, using the combined action of an EUV pulse corresponding to the 13th harmonic of an infrared laser and the fundamental IR field itself.
The time-dependent Schrödinger equation (TDSE) is solved numerically within the single-active-electron (SAE) approximation, and the photoelectron wave packet is extracted using the window-operator technique.
This approach allows us to analyze both the photoelectron angular distributions (PADs) and the behavior predicted by the generalized Fano’s propensity rule in IR-assisted EUV photoionization of argon atoms.

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.