State-selective double excitation and hyperfine quenching of two-electron systems studied by photoionization of ions
Zusammenfassung der Projektergebnisse
In summary, the present photoionization project has been extremely fruitful with respect to results relevant to our understanding of fundamental many-particle effects on atomic structure and collisions. Textbook quality results have been obtained for several two-electron and quasi-two-electron atomic systems. Unprecedented resolution in these measurements allowed us to measure natural line widths of K-shell excited states and to observe individual state-to-state specific transitions in Be-like ions. State-of-the-art theory for photoionization of ions could be shown to provide excellent results for fewelectron atoms (ions) with minor discrepancies in resonance energies below 0.1 eV. With increasing complexity of the atomic structure, theoretical predictions rapidly become less reliable. Therefore, data like the ones obtained in this work are very important for applications e.g. in astrophysics. They cannot readily be substituted by theoretical data. Our investigations considering both photoionization of an ion Aq+ and its time-inverse reaction, i.e., photorecombination of A(q+1)+ have highlighted the quality of absolute cross section measurements in both reaction channels accessed via completely different experimental approaches. The photoionization experiments on C³+ ions were carried out with a resolution more than a factor 10 better than dielectronic recombination experiments on C4+ ions at a heavy ion storage ring. Thus, even the lifetime of the [1s(2s2p)1P]²P_ state became accessible to the experiment. By studying photoionization of fullerene and endohedral fullerene ions real many-particle systems were addressed. This work has yielded two Physical Review Letters and found great interest in the literature. In particular, we discovered and identified a higher-order plasmon resonance in the single photoionization of C+ 60. This collective oscillation would not be dipole allowed in metal cluster spheres but is possible in the hollow-sphere structure of the C60 fullerene. For the first time, reliable experiments on endohedral fullerenes in the gas phase have have been carried out. The system most studied so far is Ce@C+ 82. It was exposed to synchrotron radiation in the energy range up to about 180 eV. The presence of the encapsulated Ce atom, which is in the Ce3+ ionized stage, leads to cross section features, that can well be isolated from those of the plain fullerene shell. Fragmentation with ionization after photon absorption by the Ce-4d subshell is the predominant reaction channel of the endohedral fullerene molecule. A surprise is the observation of strong photofragmentation channels of the fullerene shell mediated by the encapsulated cerium atom with an enhancement factor of more than 6, for example, for the ejection of 2 electrons and 6 C2 dimers from Ce@C+ 82 versus C+ 82 parent ions absorbing a 126 eV photon.
