Gilles Doumy

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.

Optical absorption features that are often described as metal-to-ligand charge transfer (MLCT) bands underlie the utility of many metal coordination complexes by harnessing the energy of light to drive otherwise inaccessible chemical reactions. Excitation of these bands triggers rapid electronic dynamics that can be challenging to understand, due to complicated potential energy landscapes and highly correlated electronic and nuclear degrees of freedom in metal-containing compounds. The lowest-energy absorption bands in Mn complexes containing alpha-diimine, carbonyl, and halide ligands are particularly interesting, due to the delocalized nature of charge transfer upon excitation. Here, we report experimental ultrafast dynamics measurements for the series of compounds Mn(CO)₃(^Rbpy)Br (^Rbpy = 4,4'-disubstituted-2,2'-bipyridine; R = H, CF₃, or NO₂) using polarization-resolved, femtosecond X-ray absorption spectroscopy (XAS) at both the Mn and Br K edges. The appearance of a new absorption feature in the Br pre-edge spectrum upon optical excitation reveals the instantaneous formation of an electronic hole that is partially localized on the Br atom and has a lifetime that depends on the electron withdrawing character of the ^Rbpy ligand. For two of the complexes (R = H or CF₃), a large expansion of the Mn-Br distance results in a rapid redistribution of the hole and a corresponding decrease in anisotropy of the absorption feature within 50 fs, after which the absorption into the hole disappears on a time scale shorter than 300 fs. We observe a different result for the NO₂ substituted compound, for which the Mn-Br bond contracts and the absorption into the Br-centered hole persists beyond the 2 ps time scale of our measurement. The Mn pre-edge spectrum also reveals structural changes for these three complexes, but the new Mn absorption features become evident only after the nuclei respond to the initial excitation, which allows mixing of Mn 3d and 4p orbitals. The combined use of ultrafast Mn and Br K-edge spectroscopy provides unique insight into the ways in which bipyridine substitution alters the excited-state dynamics, including a very different structural response for the most strongly electron withdrawing substituent.

An efficient implementation for the relativistic exact two-component core–valence-separated equation-of-motion coupled-cluster singles and doubles (X2C-CVS-EOM-CCSD) method is reported. The explicit exclusion of pure valence excitations in the EOM-CCSD excited-state eigenvalue equations significantly improves the efficiency for calculations of core-excited states. Benchmark relativistic CVS-EOM-CC calculations with systematic inclusion of relativistic, correlation, and basis-set effects are shown to provide highly accurate results for core ionized and excited states involving heavy atoms.

We report resonant inelastic x-ray scattering (RIXS) spectra of neon atoms interacting with intense x-ray pulses generated using an x-ray free-electron laser (XFEL). We find that an unexpected peak emerges near the K α line of Ne, which does not coincide with any physical resonances of neon ions. We perform theoretical calculations based on a quantum-state-resolved rate-equation approach with x-ray-induced processes including possible resonant excitations. Our dynamics simulations demonstrate that a sequence of multiple resonant photoabsorption events are involved and the interplay of those multiple resonances in combination with the relatively large spectral bandwidth of XFEL radiation leads to the emergent resonance-like structure at a position where no resonances exist. Our finding offers critical guidance for future applications of high-intensity RIXS at XFEL facilities.

Excitonics is an emerging field focused on exploiting and manipulating excitons generated through light-matter interactions. Advancing the field into X-ray excitonics requires precise energy and time control over core-exciton resonances. Such control enables the use of core excitons to enhance non-linear optical effects, including X-ray second harmonic generation, and has broad applications in material characterization and quantum information processing. To achieve these objectives, it is essential to comprehend the role of many-body effects governing core-exciton dynamics. In this work, we address this challenge by combining experiments with a novel \emph{ab initio} approach specifically developed to interpret pump-probe excitations. Applied to the prototypical wide-bandgap semiconductor ZnO, first-principles calculations reproduce experimental results and unveil how the density and distribution of photoexcited carriers can tune the dynamical Coulomb screening, thereby controlling core-exciton binding energies, while Pauli blocking remains negligible. These insights lead us to propose a method for dynamically controlling core-exciton resonances at absorption edges, achieving either a uniform spectral blue shift, driven by thermalized carrier distributions on picosecond timescales, or individual spectral blue shifts, driven by time-dependent carrier distributions on femtosecond timescales.

Excitonics is an emerging field focused on exploiting and manipulating excitons generated through light-matter interactions. Advancing the field into X-ray excitonics requires precise energy and time control of core-exciton resonances, enabling non-linear X-ray phenomena such as element-specific X-ray transient gratings, and advancing material characterization. To achieve these objectives, it is essential to comprehend the role of many-body effects governing core-exciton dynamics. In this work, we address this challenge by combining experiments with an ab initio approach specifically developed to interpret pump-probe excitations. Applied to the prototypical wide-bandgap semiconductor ZnO, first-principles calculations reproduce experimental results and unveil how the density and distribution of photoexcited carriers dynamically tune Coulomb screening, thereby controlling core-exciton binding energies, while Pauli blocking remains negligible. These insights inform a method for dynamically controlling core-exciton resonances at absorption edges, achieving either a uniform spectral blue shift caused by thermalized carrier distributions on picosecond timescales, or distinct blue shifts for individual resonances, driven by time-dependent carrier distributions on femtosecond timescales.

Propagation of intense X-ray pulses through dense media has led to the observation of phenomena such as atomic X-ray lasing^1,2, self-induced transparency³ and stimulated X-ray Raman scattering (SXRS)⁴. SXRS has been long predicted as a means to launch and probe valence-electron wavepackets and as a building block for nonlinear X-ray spectroscopies^5,6. However, experimental observations of SXRS to date^4,7,8 have not provided spectroscopic information, and theoretical modelling has largely implemented hard-to-realize phase-coherent attosecond pulses. Here we demonstrate SXRS with spectroscopic precision, that is, detection of valence-excited states in neon with a near Fourier-limited joint energy–time resolution of 0.1 eV–40 fs. We used a new covariance analysis between statistically spiky broadband incident X-ray and scattered X-ray Raman pulses. Using 18,000 single shots, we beat not only the incident (about 8 eV) bandwidth but also the approximately 0.2 eV instrumental energy resolution, thus creating super-resolution conditions, in analogy to super-resolved fluorescence microscopy⁹. Our experimental results, supported by ab initio propagation simulations, reveal the competition between lasing in the ion and stimulated Raman scattering in the neutral. We demonstrate enhanced signal collection efficiency and a broad excitation window, surpassing spontaneous Raman efficiencies by orders of magnitude. This stochastic SXRS approach represents a first step towards tracking elementary events that determine chemical outcomes¹⁰.

A narrow bandwidth x-ray beamline and a multi-crystal von Hamos spectrometer were used to record x-ray absorption and x-ray emission across the uranium L₃ edge of UO₂, UO₃, Cs₂UO₂Cl₄, and Cs₂UCl₆. Measurements were made over 17150-17250 eV with an instrumental resolution of ∽2 eV. This resolution allowed the use of Lorentzian peak fits to the L₃N₄ and L₃N₅ characteristic x-ray fluorescence lines that have ∽12-13 eV lifetime widths. The fluorescence yields of the four compounds display strong white lines near threshold and multiple scattering features at higher energies. The x-ray emission spectra were fit with three Lorentzians, two for the L₃N₄ and L₃N₅ lines and one for inelastic scattering by 2p-6d excitations and 4d-6d final states. The intensities of the white lines were largely due to the inelastic scattering component. To support the measurements, relativistic equation-of-motion coupled-cluster (EOM-CC) and restricted active space configuration interaction (RASCI) calculations were performed for uranium core-excitation energies in Cs₂UO₂Cl₄ and Cs₂UCl₆. These calculations with rigorous treatments of relativistic effects are shown to provide accurate uranium L₃-edge binding energies, L₃N₅ emission energies, as well as the energy losses in the inelastic scattering process. The measured and calculated results show variations between compounds with U(IV) oxidation states that contain 6d⁰5f² electrons in their ground configurations (UO₂ and Cs₂UCl₆) compared with U(VI) compounds (UO₃ and Cs₂UO₂Cl₄) with empty 5f and 6d configurations.

The nonlinear interaction of x rays with matter is at the heart of understanding and controlling ultrafast molecular dynamics from an atom-specific viewpoint, providing new scientific and analytical opportunities to explore the structure and dynamics of small quantum systems. At increasingly high x-ray intensity, the sensitivity of ultrashort x-ray pulses to specific electronic states and emerging short-lived transient intermediates is of particular relevance for our understanding of fundamental multiphoton absorption processes. In this work, intense x-ray free-electron laser (XFEL) pulses at the European XFEL are combined with a gas cell and grating spectrometer for a high-intensity transmission spectroscopy study of multiphoton-induced ultrafast molecular fragmentation dynamics in CF 4 . This approach unlocks the direct intrapulse observation of transient fragments, including neutral atoms, by their characteristic absorption lines in the transmitted broadband x-ray spectrum. The dynamics with and without initially producing fluorine K -shell holes are studied by tuning the central photon energy. The absorption spectra are measured at different FEL intensities to observe nonlinear effects. Transient isolated fluorine atoms and ions are spectroscopically recorded within the ultrashort pulse duration of a few tens of femtoseconds. An isosbestic point that signifies the correlated transition between intact neutral CF 4 molecules and charged atomic fragments is observed near the fluorine K edge. The dissociation dynamics and the multiphoton absorption-induced dynamics encoded in the spectra are theoretically interpreted. Overall, this study demonstrates the potential of high-intensity x-ray transmission spectroscopy to study ultrafast molecular dynamics with sensitivity to specific intermediate species and their electronic structure.