Tatiana Marchenko

Double-core-hole (DCH) states offer a unique window into electron correlation and multielectron dynamics. Using intense x-ray free-electron laser (XFEL) pulses and high photon-energy resolution, we generate DCH states in neon via sequential absorption of two x-ray photons within the 1 s core-hole lifetime ( ≅ 2.4 fs). These DCH states are populated, depending on the photon energy, via two different pathways, and they are detected via their KLL hypersatellite Auger decay. A high-photon-energy-resolution study of these two pathways, previously identified in water, allows us to reveal further details of the corresponding processes. The sequential double-ionization with two photons also allows us to study postcollision interaction (PCI) in a four-particle system consisting of the remaining ion, as well as two photoelectrons and one Auger electron, each with well-defined velocities. The observed PCI shift is well described by a theoretical approach based on the eikonal approximation and shows that the influence on the Auger electron is dominated by the slow photoelectron, while the fast photoelectron provides only a minor correction. Calculations show that for the utilized two-photon double-ionization scheme to prepare DCH states, the length of the FEL pulse influences the PCI shift.

We report on the shift of energies released during Auger cascades in rare-gas ions, when the ions are not isolated but embedded in a homogeneous cluster. By combining high-resolution electron spectroscopy with multielectron coincidence spectroscopy of small Ar and Xe clusters, we investigate the dependence of Auger electron emission energies on the charge state of the emitting ion in the polarizable cluster medium up to charge states of + 4 . Our results show deviations from the predictions of the established electrostatic Born model by a factor of ≈ 2 for the final step of the cascades.

Using coincidence spectroscopy and molecular mechanics simulations, we characterized incomplete Coulomb explosion that follows deep inner-shell ionization of two medium-sized organic molecules that are of applied interest as radiosensitizers.

Multiply charged molecules fragment under Coulomb repulsion, leaving characteristic fingerprints in ensuing electronic decay. Using K-shell ionization of HCl as a benchmark, we identify metastable intermediates that persist to internuclear separations beyond 100 a.u., where weakened Coulomb interactions enable the emergence of narrow atomic lines, while short-lived states decay at smaller separations, producing broad, red-shifted backgrounds. This work provides a previously missing dynamical description of coupled electronic and nuclear evolution in the Coulomb fragmentation regime.

The MUSTACHE setup (MUlti-STep photofragmentation studies by Auger electron–ion Coincidences using High Energy photons) is a high-resolution electron–multi-ion coincidence system optimized for gas-phase experiments in the tender (∼2–10 keV) and hard (>5 keV) X-ray range. The system integrates a high-resolution hemispherical electron analyzer with a Wiley–McLaren-type ion time-of-flight (TOF) spectrometer, enabling coincidence measurements of Auger electrons and high-energy photoelectrons. Designed to overcome challenges in high-energy electron detection while maintaining excellent energy resolution, the setup covers a broad kinetic energy range up to 5 keV, allowing investigation of hard-X-ray-induced Auger cascades in molecules containing high-Z elements, where initial fluorescence decay is followed by Auger processes within this 5 keV detection window. The ion TOF spectrometer provides high-resolution ion mass and momentum analysis, essential for studying light and fast ions generated by deep-core ionization. System capabilities are demonstrated through test measurements on benchmark atomic and molecular systems, such as argon, nitrogen and carbon disulfide. These measurements demonstrate energy-resolved high-kinetic-energy photoelectron–ion coincidences and momentum-resolved multi-ion coincidences following deep-core ionization and Coulomb explosion. MUSTACHE enables investigations into deep-core ionization, Auger cascade processes and Coulomb explosion dynamics in isolated gas-phase species, offering insights into fundamental ionization and fragmentation processes. These results demonstrate that the MUSTACHE setup is a powerful tool for high-resolution electron–ion coincidence spectroscopy, extending advanced coincidence techniques into the hard X-ray regime and providing unprecedented opportunities for studying high-energy X-ray-induced phenomena.

The MUSTACHE setup (MUlti-STep photofragmentation studies by Auger electron - ion Coincidences using High Energy photons) is a high-resolution electron-multi-ion coincidence system optimized for gas-phase experiments in the tender (~2-10 keV) and hard (>5 keV) X-ray range. The system integrates a high-resolution hemispherical electron analyzer with a Wiley-McLaren-type ion time-of-flight (TOF) spectrometer, enabling coincidence measurements of Auger electrons and high-energy photoelectrons. Designed to overcome challenges in high-energy electron detection while maintaining excellent energy resolution, the setup covers a broad kinetic energy range up to 5 keV. The ion TOF spectrometer provides high-resolution ion mass and momentum analysis, essential for studying light and fast ions generated by deep-core ionization. System capabilities are demonstrated through test measurements on benchmark atomic and molecular systems, such as argon, nitrogen, and carbon disulfide. These measurements demonstrate energy-resolved high-kinetic-energy photoelectron-ion coincidences and momentum-resolved multi-ion coincidences following deep-core ionization and Coulomb explosion. MUSTACHE enables investigations into deep-core ionization, Auger cascade processes and Coulomb explosion dynamics in isolated gas-phase species, offering insights into fundamental ionization and fragmentation processes. These results confirm the MUSTACHE setup as a powerful tool for high-resolution electron-ion coincidence spectroscopy, extending advanced coincidence techniques into the hard X-ray regime and providing unprecedented opportunities for studying high-energy X-ray induced phenomena.

Dissociation upon sulfur K -shell excitation or ionization of SF 6 is studied by sulfur L -shell emission spectroscopy using synchrotron radiation and multiconfiguration Dirac-Hartree-Fock calculations of emission energies and transition rates. The decay path involves in particular Auger emission with the ejection of one or more electrons, leading to singly or multiply charged intermediate states. Nevertheless, the results of the study show that the observed photon emission at 151–152 eV following excitation at 2485–2489 eV originates dominantly from transitions in neutral sulfur. This clearly indicates that the central atom retains its electrons in a dissociation process where all fluorine atoms detach before the S 2 p decay.

In this study, we integrate experimental observations and theoretical models to elucidate the complex phenomena observed in the resonant S K -edge Auger scattering spectra of the SF 6 molecule. A two-dimensional spectral map, constructed of incident photon energy and kinetic energy of the emitted Auger electron, is shown to be a versatile tool for understanding a character of the core-excited potential energy surface and change of the molecular geometry. Our findings reveal how the distinct dispersion behavior of multiple spectral lines enables mapping of ultrafast dynamics within the short-lived core-excited states. Our results confirm the presence of nuclear dynamics in the S 1 s − 1 6 a 1 g 1 and S 1 s − 1 6 t 1 u 1 core-excited states, while dynamics is absent in the S 1 s − 1 7 t 1 u 1 state. Using a combination of analysis, simulations with Coulomb model potentials, and a simple analytical approximation, we qualitatively demonstrate how the varying characteristics of spectral dispersion—classified as Raman, non-Raman, and anti-Raman—mirror the relative gradients of the intermediate and final states in the resonant x-ray scattering process. This insight allows for the effective mapping of molecular potential energy curves, offering a prospective tool on the underlying mechanisms of resonant Auger scattering and its potential for probing molecular dynamics. Published by the American Physical Society 2024