Research

Nonlinear interference and electron dynamics: Probing photoelectron momentum distributions in strong-field ionization

Danish Furekh Dar and Stephan Fritzsche

Phys. Rev. A 109 (2024) L041101

The authors unravel the complexities of nonlinear interference phenomena in laser-atom interaction, demonstrating the role of an electron as a carrier of the obscured fundamental frequencies inherent in the laser pulse. Through theoretical analysis, they identify how interactions between electrons and the concealed fundamental frequencies within intense laser pulses craft distinctive interference patterns and confinement effects in the momentum landscape, offering profound insights into the quantum dynamics underpinning ionization processes.

Near L-edge Single and Multiple Photoionization of Triply Charged Iron Ions

R. Beerwerth, T. Buhr, A. Perry-Sassmannshausen, S. O. Stock, S. Bari, K. Holste, A. L. D. Kilcoyne, S. Reinwardt, S. Ricz, D. W. Savin, K. Schubert, M. Martins, A. Müller, S. Fritzsche, and S. Schippers

Astrophys. J. 887 (2019) 189

Relative cross sections for m-fold photoionization (m = 1,...,5) of Fe³⁺ by single-photon absorption were measured employing the photon-ion merged-beams setup PIPE at the PETRA III synchrotron light source operated at DESY in Hamburg, Germany. The photon energies used spanned the range of 680-950 eV, covering both the photoexcitation resonances from the 2p and 2s shells, as well as the direct ionization from both shells. Multiconfiguration Dirac-Hartree-Fock (MCDHF) calculations were performed to simulate the total photoexcitation spectra. Good agreement was found with the experimental results. These computations helped to assign several strong resonance features to specific transitions. We also carried out Hartree-Fock calculations with relativistic extensions taking into account both photoexcitation and photoionization. Furthermore, we performed extensive MCDHF calculations of the Auger cascades that result when an electron is removed from the 2p and 2s shells of Fe³⁺. Our theoretically predicted charge-state fractions are in good agreement with the experimental results, representing a substantial improvement over previous theoretical calculations. The main reason for the disagreement with the previous calculations is their lack of inclusion of slow Auger decays of several configurations that can only proceed when accompanied by de-excitation of two electrons. In such cases, this additional shake-down transition of a (sub)valence electron is required to gain the necessary energy for the release of the Auger electron.

QED radiative corrections to the ²P_1/2 - ²P_3/2 fine structure in fluorinelike ions

A. V. Volotka, M. Bilal, R. Beerwerth, X. Ma, Th. Stöhlker, and S. Fritzsche

Phys. Rev. A 100 (2019) 010502(R)

Ab initio calculations of QED radiative corrections to the ²P_1/2 - ²P_3/2 fine-structure transition energy are performed for selected F-like ions. These calculations are nonperturbative αZ and include all first-order and many-electron second-order effects in α. When compared to approximate QED computations, a notable discrepancy is found especially for F-like uranium for which the predicted self-energy contributions even differ in sign. Moreover, all deviations between theory and experiment for the ²P_1/2 - ²P_3/2 fine-structure energies of F-like ions, reported recently by [Li et al., Phys. Rev. A 98, 020502(R) (2018)], are resolved if their highly accurate, non-QED fine-structure values are combined with the QED corrections ab initially evaluated here.

Maximum Elliptical Dichroism in Atomic Two-Photon Ionization

J. Hofbrucker, A. V. Volotka, and S. Fritzsche

Phys. Rev. Lett. 121 (2018) 053401

Elliptical dichroism is known in atomic photoionization as the difference in the photoelectron angular distributions produced in nonlinear ionization of atoms by left- and right-handed elliptically polarized light. We theoretically demonstrate that the maximum dichroism always appears in two-photon ionization of any atom if the photon energy is tuned in so that the electron emission is dominantly determined by two intermediate resonances. We propose the two-photon ionization of atomic helium in order to demonstrate this remarkable phenomenon. The maximum elliptical dichroism could be used as a sensitive tool for analyzing the polarization state of photon beams produced by free-electron lasers.

Tailored orbital angular momentum in high-order harmonic generation with bicircular Laguerre-Gaussian beams

W. Paufler, B. Böning, and S. Fritzsche

Phys. Rev. A 98 (2018) 011401(R)

We report on a method to generate extreme ultraviolet vortices from high-order harmonic generation with two-color counter-rotating Laguerre-Gaussian (LG) beams that carry a well-defined orbital angular momentum (OAM). Our calculations show that the OAM of each harmonic can be directly controlled by the OAM of the incident LG modes. Furthermore, we show how the incoming LG modes have to be tailored, in order to generate every possible value of OAM in the emitted harmonics. In addition, we analyze the emitted harmonics with respect to their divergence and find that it decreases with the harmonic order and increases with the OAM of the emitted harmonic.

Fields of research

Atomic data for astro and plasma physics
Atomic decay: Fluorescence and Auger processes
Atomic structure codes
Atomic structure theory
Atomic photoionization
Computer-algebraic techniques for many-particle physics
Highly-charged ions: Structure and dynamics

Atomic data for astro and plasma physics

Computation of several intercombination transitions which are purely caused by spin-orbit interactions and relativistic effects. These transitions are important for the diagnostics of laboratory and astro physical plasmas.
Theoretical analysis of lifetimes and intensity ratios for multiple-charged ions of the iron group along the silicon-, phosphorus-, and chlorine-like isoelectronic sequence.
Calculation of selected fluorescence and Coster-Kronig yields after inner-shell ionization for atomic zinc within a relativistic ab-initio model.
Calculation of the complete low-lying emission spectrum of Ni II for the diagnostics of plasmas in fusion research.
X-ray transitions in nickel-like ions; these ions are discussed in the context of developing efficient x-ray laser schemes by using high-power optical pump lasers.
Computations of the term structure and transition probabilities of possible x-ray laser lines for multiple-charged neon- and nickel-like ions in collaboration with Chenzhong Dong (Lanzhou, China).
Computation of the E1 forbidden transitions of singly-ionized argon together with Biswajit Saha (Kyoto, Japan).
Computation of the plasma shifts for the electric-dipole allowed and spin-forbidden transitions of Be-like ions in collaboration with Biswajit Saha and Prasanta Mukherjee (Kolkata, India).

Atomic decay: Fluorescence and Auger processes:

Spin polarization of emitted electrons following the resonant excitation of inner--shell electrons in noble gases; together with Markus Drescher and Ullrich Heinzmann (Bielefeld).
Theoretical and experimental analysis of the fluorescence and Auger spectra from a (so-called) `complete' experiment in order to determine the quantum mechanical amplitudes and phases in collaboration with Alexei Grum-Grzhimailo (Moscow) and Michael Meyer (Paris).
Analysis of correlation effects in the Auger cascades of noble gases in collaboration with the group of Georgio Stefani (Rom) and Nicolai Kabachnik (Bielefeld).
Experimental and theoretical investigations on the angle-resolved Auger emission of resonantly excited noble gases; first computations on the `coherence transfer' in two-step Auger cascades with 'overlapping' intermediate states and comparison with coincidence experiments in the group of Kyioshi Ueda (Sendai) and together with Nicolai Kabachnik (Bielefeld).
Analysis and computation of the (linear) polarization of the fluorescence light following the inner-shell photoionization of sodium-like ions together with Mokthar K. Inal (Tlemcen, Algeria).

Atomic structure codes:

Design and development of a `continuum generator' for multiconfiguration Dirac-Fock wave functions as needed for the calculation of atomic ionization and autoionization processes.
Coordination and maintenance of the program package RATIP for the calculation of a large variety of Relativistic Atomic Transition and Ionization Properties; c.f. Programs.
Classification of many-electron states with open d- and f-shells for relativistic structure calculations. Derivation and implementaion of the corresponding `angular coefficients' in collaboration with Gediminas Gaigalas (Vilnius) and Ian P. Grant (Oxford, UK).
Development of the relativistic CI program RELCI for large-scale studies of open-shell (high-Z) atoms and ions; together with Charlotte Froese Fischer.
Implementation of the TOOLBOX for the RATIP package.
Relativistic Wave and Green's functions for hydrogen-like ions and general spherical potentials.

Atomic structure theory:

Calculation of the C_3 Lennard-Jones coefficients for Van-der-Waals interactions among neutral atoms and with (ultra) cold surfaces; third-order MBPT and Coupled-Cluster calculations for the ground states of alkali-metal atoms in collaboration with Walter R. Johnson and Andrei Derivianko (University of Notre Dame, Indiana).
Computations of the low-lying level structure of super-heavy elements; collaboration with Hartmut Backe and Michael Sewtz (Mainz).
Investigations on the photoabsorption of beryllium and neon in the region of the 1s2l double resonances in collaboration with Fumi Koike and others (Japan).

Atomic photoionization:

Investigations on the window resonance structures in the photoionization spectra of alkaline atoms; together with Fumi Koike (Sagami-ono, Japan).
Photoionization and coherence transfer in open-shell atoms in collaboration with the group of Helena und Seppo Aksela (Oulu, Finnland); computation of partial ionization cross sections and angular distributions for different resonant excitations.
Studies on the relation between the vector correlation coefficients and the shape of Fano resonances in photoionization experiments together with Alexei Grum-Grzhimailo (Moscow).
Observation and interpretation of the spin interferences (of the electrons) in the photoemission of magnetized gadolinium; together with Ullrich Heinzmann and Nicolai Kabachnik (Bielefeld).
Interpretation of the photoionization and Auger spectra of the alkaline-earth metals in collaboration with the group of Helena und Seppo Aksela (Oulu, Finnland).
Analysis of the angular distributions of electrons emitted in the ionization of hydrogen-like ions by twisted photons.
Control and analysis of the entanglement between the electrons and photoions in the photoionization of hydrogen-like ions and earth-alkaline atoms.

Computer-algebraic techniques for many-particle physics:

Application of computer-algebra (CA) methods in deriving atomic and molecular perturbation series for closed- and open-shell systems.
Analysis and simplification of characteristic expressions from the theories of angular momentum and spherical tensor operators. The rotational symmetries of a system must be often exploited efficiently before a quantitative treatment of many-particle systems become feasible. During the last decade, we developed the CA package RACAH in the framework of Maple which automatically simplifies such expressions and which is a part of the CPC library.
Derivation and implementation of the jj-LS transformation coefficients for identifying the (leading) LS-components in relativistic structure computations for open-shell atoms; together with Gediminas Gaigalas (Vilnius).
Design of a general, computer-algebraic ansatz for dealing with and the application of point-group symmetries in physics and chemistry.
Simulation of n-qubit quantum gates and circuits and analysis of their separability and entanglement.

Highly-charged ions: Structure and dynamics

Studies on the interference and polarization effects in the radiative recombination of highly-charged ions in collaboration with the group of Thomas Stöhlker (GSI, Darmstadt).
Calculations on the angle-angle correlations in the two-photon decay of hydrogen-like ions.
Theoretical investigations on the polarization and alignment transfer in course of the electron capture into highly-charged ions.
First measurement and interpretation of the (linear) polarization of the REC radiation of high-Z ions; together with Thomas Stöhlker (GSI, Darmstadt).
Accurate atomic and QED computations for several M1 forbidden transitions in highly-charged few-electron ions in collaboration with the group of Joachim Ullrich (MPI Heidelberg).
Analysis of the relativistic and multipole effects on the two-photon ionization of multiple and highly charged ions.