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978-3-8439-1731-5, Reihe Physik
Nils Christian Gerken
XUV Multiphoton Excitation and Charged State Dynamics in Small Quantum Systems
122 Seiten, Dissertation Universität Hamburg (2014), Softcover, A5
The investigation of nonlinear effects occurring at multi-photon ionization of small quantum systems is the subject of this thesis. The 4d electrons of xenon atoms and iodine containing molecules were used as target systems irradiated by focused, high intensity XUV laser pulses with intensities up to 10E14 W/cm2 created by the free-electron laser FLASH at DESY in Hamburg. Detailed pulse energy, photon energy and pulse duration ependent measurements using time-of-flight spectroscopy were used for a description of the multi-photon processes happening at high intensity radiation. The mechanism which leads to the generation of unusually highly charged xenon states up to Xe21+after irradiation of high intensity XUV radiation is still not understood completely. The still open question of origin of these highly charged states is complemented by new ion yield results of the multi-photon ionization of xenon at five different energies within the vicinity of the giant resonance.
Intensity dependent measurements are used to reveal the number of involved photons in the photoionization process. When using different photon energies in the XUV regime enormous differences in the maximum charge level are observed. This cannot be attributed only to changes in the ability to surmount the ionization thresholds by the photons. The medium charged states are generated through several sequential 4d electron excitation and relaxation processes including Auger decays.
Since the Auger decay time is shorter, but still comparable to the photon pulse duration, already diminutive changes in the photon pulse duration lead to perceptible changes in the medium charged xenon ion signal around Xe5+ which has been created after several Auger decay steps. This effect is going to be presented together with a simple rate equation model which underlines the time dependency of several charge states.
The related multi-photon processes in molecules are investigated by applying the covariance mapping technique at time-of-flight spectra of iodine containing molecules. By using a normalization process the degree of correlation can be determined for all possible charge state combinations of the participating iodine ions. The combination of the correlation information with kinetic energy release (KER) investigations and Monte Carlo simulations is used for a time resolved, kinematic description of the dissociation process and the preceding charge migration processes.