Attosecond Ionization Dynamics and Non‑Perturbative X‑ray Strong‑Field Physics at XFELs

Recent advances in X‑ray free‑electron laser (XFEL) technology have opened new opportunities to probe photoionization dynamics on attosecond timescales and to access strong‑field regimes at X‑ray wavelengths that were previously experimentally inaccessible. In this talk, I will discuss two complementary classes of XFEL experiments that together illuminate the timing and mechanisms of electron emission in atoms and molecules.  First, I will address attosecond photoemission delays measured using circular streaking techniques, with a focus on extending ionization‑delay chronoscopy from linearly to circularly polarized probes.  I will also address the challenges of interpreting streaking delays at low photoelectron energies.  Second, I will present recent theoretical work exploring a new non‑perturbative strong‑field X‑ray regime, where multiphoton above‑threshold ionization (ATI) is driven by attosecond XFEL pulses. In helium, this regime produces distinct angular and spectral signatures that are clearly separated from perturbative harmonic contributions. Our theoretical framework captures these nonlinear dynamics and identifies observables that enable ultrafast probing and control of X‑ray–driven strong‑field processes.  Together, these studies demonstrate how XFELs enable both quantitative attosecond chronoscopy and fundamental tests of strong‑field ionization physics at short wavelengths, establishing a foundation for future experiments in atoms and molecules.