Photoionization studies of some closed shell atoms and ions (PhD)

Abstract

Study of interaction of electromagnetic radiation with the atom provides information about the atomic system. When a radiation of sufficiently high frequency is absorbed by an atom one or more electrons may be knocked out of the target, leaving the atom in an ionized state. These ejected electrons, known as photoelectrons, carry information about the atomic structure, electron-electron interactions and relativistic interactions present in the system. In this thesis are reported the photoabsorption process of several closed shell elements including isonuclear and isoelectronic sequence using many body approximation methods Relativistic-Random-Phase Approximation(RRPA),Relativistic Multichannel Quantum Defect Theory (RMQDT) and Multi-configuration Tamm-Dancoff (MCTD). Photoionization cross sections and dipole angular distribution asymmetry parameters, β , of5sand 4d shellsof the closed-shellions (La3+,La9+ and La11+)in the La isonuclear sequence have been studied using RRPA. The positions of the 5s Cooper minima in La3+ and La9+ ions are found to be extremely sensitive to the details of electron correlation. The results show that 5s cross sections of La3+ and La9+ do not lie along the same curve near the thresholds; the 4d cross sections, however, do match well in overlap regions so they liealong same curve,over the isonuclear sequence,exceptfora shift in threshold towards higher energies with increasing degree of ionization. The autoionization resonances in the 5s cross section for La3+, La9+ and in the 4d5/2 cross section for La11+ have been studied using RRPA and RMQDT techniques. The obtained resonance transitions are seen to be in the vicinity of extreme ultra violet region. Angular distribution asymmetric parameter (β5s) results for La3+, La9+ ions isdeviating from its non relativistic value 2. The evolution of the 2s to np autoionization resonances for some of the members in the Mg isoelectronic sequence are reported using the RRPA+RMQDT. The asymptotic quantum defects decrease monotonically with Z in the isoelectronic sequence. The angular momentum coupling scheme which is appropriate for the low-Z atom/ions is not the best suited for the high-Zions,changing from Russell-Saunders to the jj scheme. Photoionization studies of 5d and 6s subshells for atomic Hg using Multi- Configuration Tamm Dancoff method are also reported in the present work.