Photoionization of positive ions: The nitrogen isoelectronic sequence

This dissertation presents an experimental photoionization study of the first three ionic members of the nitrogen isoelectronic sequence performed by merging an ion beam with a photon beam produced by an electron synchrotron. The ion-photon-beam apparatus and the electron-cyclotron-resonance ion source designed and constructed for this research are described, along with the principles of synchrotron radiation. Absolute cross sections for photoionization of O+, F2+ and Ne3+ ions are reported in the photon energy range from the ionization thresholds of their metastable (2Po, 2Do) and ground states (4 So) to the series limits for the dominant Rydberg series of resonances. The photon energy resolution of these measurements exceeds that of work performed elsewhere on photon-ion interactions. Most of the resonant structure observed in the recorded photoion-yield spectra has been identified and assigned to eleven different Rydberg series by means of quantum defect theory. The behavior of the quantum defect parameters is analyzed for these three ions, providing information about the electronic screening of the nucleus as a function of ion charge along the isoelectronic sequence. The ground-state and metastable-state fractions present in the primary ion beams were determined for O+ and estimated for F2+ and Ne3+ . The absolute cross-section measurements for the state admixtures are compared with the R-matrix calculations reported in the atomic database (TOPBase) that is widely used in astrophysics.