This thesis concerns multi-ionization coincidence measurements of atoms and small molecules using a magnetic bottle time-of-flight (TOF) spectrometer designed for multi-electron coincidence studies. Also, a time-of-flight mass spectrometer has been used together with the TOF electron  spectrometer for electron-ion coincidence measurements. The multi-ionization processes have been studied by employing a pulsed discharge lamp in the vacuum ultraviolet spectral region and synchrotron radiation in the soft X-ray region. The designs of the spectrometers are described in some detail, and several timing schemes suitable for the light sources mentioned above are presented. Studies have been performed on krypton, molecular oxygen, carbon disulfide and a series of alcohol molecules. For the latter, double ionization spectra have been recorded and new information has been obtained on the dicationic states. A recently found rule-of-thumb  and quantum chemical calculations have been used to quantify the effective distance of the two vacancies in the dications of these molecules. For Kr, O2, and CS2, single-photon core-valence spectra have been obtained at the synchrotron radiation facility BESSY II in Berlin and interpreted on the basis of quantum chemical calculations. These spectra show a remarkable similarity to conventional valence photoelectron spectra. Spectra of triply charged ions were recorded, also at BESSY II, for Kr and CS2 by measuring, in coincidence, all three electrons ejected. The complex transition channels leading to tricationic states were mapped in substantial detail for Kr. It was found that for 3d-ionized krypton, the tricationic states are dominantly populated by cascade Auger decays via distinct intermediate states whose energies have been determined. The triple ionization spectra of CS2 from the direct double Auger effect via S2p, S2s and C1s hole states contain several resolved features and show selectivity based on the initial charge localisation and on the identity of the initial state.

Free-electron lasers provide high intensity pulses with femtosecond duration and are ideal tools in the investigation of ultrafast processes in materials. Illumination of any material with such pulses creates extreme conditions that drive the sample far from equilibrium and rapidly convert it into high temperature plasma. The dynamics of this transition is not fully understood and the main goal of this thesis is to further our knowledge in this area. We exposed a variety of materials to X-ray pulses of intensities from 1013 to above 1017 W/cm2. We found that the temporal evolution of the resulting plasmas depends strongly on the wavelength and pulse intensity, as well as on material related parameters, such as size, density, and composition. In experiments on atomic and molecular clusters, we find that cluster size and sample composition influence the destruction pathway. In small clusters a rapid Coulomb explosion takes place while larger clusters undergo a hydrodynamic expansion. We have characterized this transition in methane clusters and discovered a strong isotope effect that promotes the acceleration of deuterium ions relative to hydrogen. Our results also show that ions escaping from exploding xenon clusters are accelerated to several keV energies. Virus particles represent a transition between hetero-nuclear clusters and complex biological materials. We injected single mimivirus particles into the pulse train of an X-ray laser, and recorded coherent diffraction images simultaneously with the fragmentation patterns of the individual particles. We used these results to test theoretical damage models. Correlation between the diffraction patterns and sample fragmentation shows how damage develops after the intense pulse has left the sample. Moving from sub-micron objects to bulk materials gave rise to new phenomena. Our experiments with high-intensity X-ray pulses on bulk, metallic samples show the development of a transient X-ray transparency. We also describe the saturation of photoabsorption during ablation of vanadium and niobium samples. Photon science with extremely strong X-ray pulses is in its infancy today and will require much more effort to gain more knowledge. The work described in this thesis represents some of the first results in this area.