Photoionization dynamics of some free and confined atomic systems (PhD)

Abstract

ABSTRACT The present thesis are reported the photoionization of free and confined atomic systems using the Relativistic Random Phase Approximation (RRPA) and Relativistic Multichannel Quantum Defect theory (RMQDT). The present thesis reports studies for outer shell of high-Z atoms like Radon (Z=86) and Radium (Z=88). The combined effects of interchannel coupling, relativistic interactions and confinement on the photoionization parameters of heavy atoms such as Radon (Rn) and Radium (Ra) have been carried out. For this thesis work, a study of the outer 4s subshell of the Ca atom in a spherical attractive potential well (SAW) of variable depth is performed to determine the effect of an external potential on dipole (E1) and quadrupole (E2) photoionization processes. As the depth of the potential well increases, dramatic changes are observed in the 4s cross section, as well as in the photoelectron angular distribution. The existence of Cooper minima in the 4s dipole channels give rise to very significant effects of quadrupole interactions, even at extremely low energies, in the vicinity of the E1 Cooper minimum. It is shown that the entrapment of Ca in a spherical attractive well (Ca@SAW) further enhances the importance of quadrupole interactions determining the photoionization parameters. The complicated behavior of both dipole and quadrupole Cooper minima, as functions of well depth, is delineated, along with the importance of relativistic effects. The present work reports an investigation of photoemission time delay from endohedral anions A@C60 q. Photoionization of inner and outer subshells of the endohedral Ne@C60 -5 near the 1s threshold is chosen as a case study. Significant enhancement of the time delay in the 1s dipole photoionization channels is found, owing to the emergence of the Coulomb Confined Resonances (CRs). Moreover, by way of interchannel coupling, the inner-shell 1s Coulomb CRs induce a revival of these resonances in the 2s and 2p photoionization channels some 800 eV above their thresholds. These induced resonances, termed correlation Coulomb CR's, result in large time delay in the release of the 2s and 2p photoelectrons from Ne at these high energies. Furthermore, Coulomb CRs are found to emerge in the 1s quadrupole ionization channels as well, thereby also causing considerable time delays in 1s quadrupole photoemission. In the present work, the photoionization time delay in photoionization of the 2p subshell in the energy region of the 2s np autoionization resonances in atomic Neon have been analyzed. In this work, the expressions for the Breit-Wigner resonances developed in the context of scattering theory is used. Photoionization and electron-ion scattering have the same final state, but have different initial states. These two resonances have been determined using RMQDT in which the quantum defect parameters were obtained ab-initio using the RRPA. The resonance lifetimes and the photoionization Wigner time delay through the autoionization resonances have been studied in the present thesis work.