Photoionization of the Cerium Isonuclear Sequence and Cerium Endohedral Fullerene

Abstract

This dissertation presents an experimental photoionization study of the cerium isonuclear sequence ions in the energy range of the 4d inner-shell giant resonance. In addition, single and double photoionization and photofragmentation cross sections of the cerium endohedral ion Ce$@$C$_{82}^{+}$ were also measured and studied in the 4d excitation-ionization energy range of cerium.Relative and absolute cross-section measurements were performed at undulator beamline 10.0.1 of the Advanced Light Source (ALS) for nine parent cerium ions: Ce$^{+}$ -- Ce$^{9+}$. Double-to-single ionization cross-section ratios were measured for photoionization of the endohedral Ce$@$C$_{82}^{+}$ and empty fullerene C$_{82}^{+}$ molecular ions. The merged ion and photon beams technique was used to conduct the experiments. Multiconfiguration Hartree-Fock calculations were performed as an aid to interpret the experimental data. Four Rydberg series for 4d $\rightarrow$ nf (n $\geq$ 4) and 4d $\rightarrow$ np (n $\geq$ 6) autoionizing excitations were assigned using the quantum defect theory for the Ce$^{3+}$ photoionization cross section. The experimental data show the collapse of the nf wavefunctions (n $\geq$ 4) with increasing ionization stage as outer-shell electrons are stripped from the parent ion. The nf orbital collapse occurs partially for Ce$^{2+}$ and Ce$^{3+}$ ion and completely for Ce$^{4+}$, where these wavefunctions penetrate the core region of the ion. A strong contribution to the total oscillator strength was observed in the double and triple photoionization channels for low charge states ( Ce$^{+}$, Ce$^{2+}$, and Ce$^{3+}$), whereas most of the 4d excitations of the higher charge states decay by ejection of one electron.