Precision spectroscopy at the K-edge of astrophysically relevant ions

Fundamental photon-ion interaction processes have been studied using the newly constructed endstation PIPE* which has been set up at the Variable Polarization XUV beamline P04 at PETRA III. The PETRA III storage ring at DESY is presently the brightest synchrotron light source world-wide. Absolute cross sections were measured for single and multiple photo-ionization of Cq+ ions with q=1,2,3,4 and of Neq+ ions with q=1,2,3 by single photons from PETRA III employing the merged-beam technique. The photon beam energy resolution reached down to better than 9 meV at 336 eV, that is, a resolving power of more than 37000 could be accomplished. The investigated photon energy ranges comprise the lowest-energy resonances involving K-shell excitation and reach up to double excitations involving both a K- and an L-shell electron, i.e., to energies beyond the K-shell ionization threshold. Emission of up to 3 electrons from ions with a K-vacancy could be observed. Evidence was obtained for triple-Auger decay processes. In the case of C4+ parent ions, spectroscopic measurements of intermediate hollow atoms (hollow ions) with 2 vacancies in the K-shell were performed. * Photon-Ion Spectrometer at PETRA III