Atomic relaxation upon multiphoton absorption of soft X-rays

This thesis explores the atomic relaxation processes following multiphoton absorption of soft X-ray free-electron laser pulses within the M shell of xenon at photon energies ranging from 700 to 1700 eV, employing ion time-of-flight spectroscopy and within argon’s 2p subshell to form a double-core-hole state, using electron time-of-flight spectroscopy. The investigation sheds light on the multiphoton multiple ionisation processes of isolated xenon atoms interacting with ultraintense soft X-ray free-electron laser pulses, elucidating the dependence on photon energy across three fluence (photons per unit area) regimes. A map of resonant features as a function of photon energy and charge state is rendered, facilitating the identification of transition energy lines linked to specific resonant excitations through comparison with theoretical calculations. Analysing the calculated ionisation pathways reveals the significance of multiple-core- hole states, shifting the energies where a specific photon energy becomes resonant. The findings further indicate that ion yields in resonance spectra remain constant beyond a specific saturation point, making it possible to study resonances in a pure form and setting the stage for future investigations into pulse duration effects. Furthermore, the thesis presents evidence of a resonant 2p double-core-hole state in singly charged argon induced by two-photon absorption, with calculations confirming the transition of a 2p electron to a 3d orbital. The observed relaxation processes align with theoretical predictions, and further analysis demonstrates that the second step of the double-core-hole decay cascade emits electrons within the same energy range as the first step, enhancing consistency between theory and experiment. A single-core-hole feature in doubly charged argon is also identified through theoretical calculations.