Thesis defense of Sara Savio

Core-level photoionization is a fundamental process in light–matter interaction consisting of absorbing a photon by an atom or molecule, ejecting an electron from one of its inner shells, and creating a core-shell vacancy. This vacancy is then filled through various relaxation processes, which can result in the emission of secondary electrons or energy redistribution within the system. The results presented in this thesis contain technical and methodological advances in characterizing the decay dynamics of double-core holes (DCH) in gaseous neon atoms, which have a very short lifetime, using intense and ultrashort X-ray pulses on the attosecond (10−18 s) scale at the European XFEL (Eu-XFEL).
Ultrafast electron dynamics are mapped on a single-shot basis using an angle-resolving electron time-of-flight (e-TOF) spectrometer. A spectrometer was built and commissioned as part of this work and is presented in detail, including technical information and experimentally retrieved performance data. Non-invasive systematic pulse characterization using the angular streaking technique provides spectral and temporal information about the ionizing XFEL pulses with attosecond resolution. This approach enables single-shot DCH probing based on the knowledge of spectro-temporal details about the ionizing pulses.
A comprehensive study was conducted to investigate how the contribution of DCH channels varies with X-ray pulse parameters, including pulse duration, pulse energy, and the photon energy centres of the reconstructed spectra. The results show that the yield of the DCH signal increases in such a way that is compatible with the reconstruction of X-ray pulse durations well below the life time of the single-core hole (SCH) Auger decay in neon, which is on the order of 2.4 femtosecond (10−15 s), thus enabling the characterization of such short-lived ionic states in a single shot. Examining the electronic structure of the core-ionized system before relaxation, combined with detailed information about the ionizing pulse, provides the experimental stage for valuable insights into nonlinear X-ray–matter interaction. Thus the ensuing photoabsorption and relaxation channel intensities achievable at high-repetition-rate, attosecond duration XFEL allow to reveal these ultrafast processes on the natural timescale of electron dynamics.