Enhanced One-Color-Two-Photon Resonant Ionization in Highly Charged Ions by Fine-Structure Effects

TL;DR

The study addresses the challenge of disentangling inner-shell dynamics and ionization pathways in highly charged ions under ultra-intense XFEL irradiation, where relativistic fine-structure and core-hole screening strongly influence transitions. It combines EBIT-based preparation of HCIs with ultrafast, narrow-bandwidth XFEL pulses to realize and resolve doubly-resonant two-photon ionization via sequential $2p_{1/2}$ and $2p_{3/2}$ excitations and subsequent Auger decay. The authors demonstrate that the double-resonance channel enhances two-photon ionization by more than two orders of magnitude and that near-cancellation between screening and fine-structure effects brings relevant resonances into the XFEL bandwidth for Kr$^{26+}$, with data in good agreement with FAC/GRASP-based rate-equation modeling. This mechanism offers a path toward state-selective X-ray multiphoton processes and underpins potential future X-ray metrology, two-color pump-probe experiments, and X-ray optical clock concepts with next-generation XFEL/XFELO sources.

Abstract

Ultraintense pulses from X-ray free-electron lasers can drive, within femtoseconds, multiple processes in the inner shells of atoms and molecules in all phases of matter. The ensuing complex ionization pathways of outer-shell electrons from the neutral to the final highly charged states make a comparison with theory enormously difficult. We resolve these pathways by preparing highly charged ions in an electron beam ion trap before exposing them to the pulsed radiation. This reveals how relativistic fine-structure effects shift electronic energies, largely compensate the core-screening potential, and enable the consecutive, resonant absorption of two quasi-monochromatic X-ray photons that would generally be unfeasible. This doubly-resonant channel enhances the efficiency of two-photon ionization by more than two orders of magnitude, dominating in this regime the nonlinear interaction of light and matter with possible application for future precision X-ray metrology.

Figures (4)

• Figure 1: Resonance-enabled X-ray photoionization: (a) Linear: Resonant excitation ($$) followed by autoionization (AI). (b) Nonlinear REMPI: Resonant excitation followed by direct photoionization. (c, d) Double-resonance ionization: Two sequential resonances populate a doubly excited autoionizing state. (e) Calculated absorption spectra for the isoelectronic sequence of ten-electron (Ne-like) ions from iron to neodymium. Red: $3C$ ($2p_{1/2} \rightarrow 3d_{3/2}$) and $3D$ ($2p_{3/2} \rightarrow 3d_{5/2}$) transitions to singly excited states. Grey/blue: Transitions to doubly excited autoionizing states. The $3C$ position is set to zero as a reference. Blue spectra ($3C'$, $3D'$), with a spectator electron, represent absorption after initial $3C$ excitation. For iron, ($3C$, $3D$) and ($3C'$, $3D'$) are well separated due to screening differences ($\Delta \text{E}_{\text{SC}} \approx 40\,\text{eV}$, $\Delta \text{E}_{\text{FS}} \approx 15\,\text{eV}$); in heavier ions, relativistic effects shift $3D'$ toward $3C$ that leads to a total cancellation i.e. $\Delta \text{E}_{\text{SC}}-\text{E}_{\text{FS}} \approx 0$ for Kr, Rb and Sr. The red band shows the SASE XFEL bandwidth ( 0.5%) near the $3C$ energy, as determined from this experiment—much narrower than the fine-structure splitting in Kr$^{26+}$.
• Figure 2: Top: Scheme of the SASE3 beamline at EuXFEL. XFEL pulses with the depicted pattern from an undulator pass XGMD1 (for intensity measurements), a gas attenuator and a second intensity monitor (XGMD2). Two KB mirrors focus the beam into the experimental chamber (bottom) of SQS-EBIT to irradiate HCIs produced and trapped in it. A silicon drift detector records their fluorescence, while an ion-ToF spectrometer downstream of the EBIT registers their charge-state distribution after each exposure period. The bottom panel depicts the electrostatic potential of EBIT and ion extraction beamline.
• Figure 3: Overview spectrum from 1.6 keV to 3.1 keV with highly charged krypton in EBIT. (Top) Fluorescence yield; (bottom) charge-state distribution (CSD) versus incident photon energy; (left) initial CSD without XFEL irradiation. Note the change in the scaling of the photon-energy axis around 2.1 keV. Predicted energies of major ground-state transitions for each charge state are marked with colors according to the upper state $n$ and symbols for the orbital angular momentum, e.g., blue circles for $2p\rightarrow4d$ transitions. The white dashed line marks the ionization threshold.
• Figure 4: Experimental data (red) and predictions (grey) for the photon energies near the $3C$ and $3D$ resonant transitions for varying pulse energy. (a): Average fluorescence yield. (b): ion-yield spectra for fluorine-like krypton. (c): ion yield spectra for oxygen-like krypton. Five identified peaks are labeled from 1 to 5. (d) Comparison of Kr$^{27+}$ yield evaluated at 1800 eV (peak (1)) and 1850 eV (peak (4)) as a function of the pulse energy.