Decay Cascades of Inner-Shell Vacancies in Rare-Gas Clusters

This thesis explores how multi-electron relaxation cascades develop in rare-gas clusters following inner-shell ionization. Using multi-coincidence electron-photon spectroscopy with synchrotron radiation, the complete decay sequences initiated by the creation of 3d and 3p vacancies in krypton clusters are investigated, revealing a complex interplay of local and non-local electronic processes. The results demonstrate that the presence of a weakly interacting environment profoundly alters the relaxation pathways compared to the atomic case, opening additional channels for charge and energy redistribution. Following 3d ionization, the dominant relaxation mechanism is Auger decay, which generates excited dicationic states that undergo further decay often via interatomic processes such as interatomic Coulombic decay (ICD) and electron-transfer-mediated decay (ETMD). The analysis of coincidence events involving multiple electrons and possibly a UV photon enabled the reconstruction of the entire cascade of up to five emitted particles initiated by the absorption of a single incident photon. Ionization of the 3p shell populates tricationic states through Coster-Kronig–Auger cascades. These states are shown to relax almost exclusively by low-energy electron emission, with radiative decay strongly suppressed. The redistribution of energy and charge across neighboring atoms is suggested to efficiently deexcite and partly neutralize the initial trication, and the high amount of potential energy present in these states favors electron over photon emission throughout the cascade of up to six electrons emitted in total. One of the key findings in this thesis is the identification and experimental verification of a collective decay mechanism, termed ETMD(4) in accordance with common naming conventions. In this process, two neighboring atoms simultaneously transfer electrons to a multiply charged ion and a fourth site is ionized by the excess energy, thereby ejecting a slow electron. ETMD(4) thus enables higher-charged ground-state ions to neutralize non-radiatively, even when all conventional non-local decay channels like ETMD(3) are closed. Experimental evidence for ETMD(4) was found in both pure krypton and mixed argon-krypton clusters, supported by theoretical lifetime calculations. The presented results further corroborate rare-gas clusters as ideal prototype systems for studying non-local relaxation and charge migration in dense media. The observed cascades show how local electronic excitations can decay by interatomic processes, and the discovery of ETMD(4) expands the field of known charge-transfer mechanisms, providing new insight into neutralization pathways in condensed matter systems.