Studies of Single and Multiple Ionization Processes in Rare Gases and some Small Molecules

Abstract: In this thesis various aspects of photoionization are investigated with respect to both single and multiple electron emission from atoms and molecules. The studies include both valence and core levels and involve transitions which leave the atoms or molecules in various charge states.S2p electrons in the CS2 molecule were excited into Rydberg orbitals close to the ionization threshold. Subsequent autoionization leads to the emission of single electrons which were detected by a conventional electron spectrometer, bringing the molecule into various cationic states characterized by two valence holes and a Rydberg spectator electron. Vibrational progressions have been assigned as excitations of the totally symmetric v1 and the asymmetric stretching v3 modes in the cationic states.Double ionization spectra of the CS2 molecule were recorded in the S2p and C1s innershell ionization regions using a magnetic bottle many-electron coincidence spectrometer, revealing dicationic states formed out of one inner-shell vacancy and one vacancy in the valence region. The spectrum connected to the S2p vacancy is richly structured in contrast to the spectrum connected to the C1s vacancy, which shows essentially one distinct band.The development of a new variant of the magnetic bottle coincidence technique tailored for valence triple photoionization studies of rare gas atoms at synchrotron radiation sources is presented, overcoming the problem of high repetition rate in single-bunch operation of the storage ring. The studies of the rare gas atoms confirm that a correction of the lowest triple-ionization energy of Kr, currently listed in standard tables, is needed.Also, single-site N1s and O1s double core ionization of the NO and N2O molecules and single-site O1s, C1s and S2p double core ionization of the OCS molecule has been studied with the magnetic bottle technique. Double core holes are of particular interest due to putatively larger chemical shifts compared to single core holes. The observed ratio between the double and single ionization energies are in all cases close or equal to 2.20.