Studies of Attosecond Time Delay and Superheavy Elements in Atomic Photoionization

This research consists of two distinct but related elements in the realm of atomic photoionization. The first was to explore the photoionization in superheavy elements. Employing the fully Relativistic Random Phase Approximation (RRPA), we calculated the photoionization cross sections for the Oganesson (Og), the heaviest element, since the influence of relativity becomes more significant as the atomic number increases. The results revealed the importance of relativistic interactions along with the predominant influence of interchannel coupling on the photoionization cross sections. Encouraged by the ability of the RRPA to deal with the combination of relativistic and many-body correlation effects, we proceeded to investigate the angle-dependent time delay in the photoionization process using the RRPA. Specifically, we investigated the time delay in argon. The matrix elements calculated using RRPA were used to calculate the phases of the various photoionization amplitudes, and the energy derivative of these phases provided us with the time delay information. We computed a weighted average of the time delays associated with dipole and quadrupole components, averaged over photoelectron spin and orientation of the residual ion, considering different transitions involved in the photoionization process. Time delay in photoionization exhibits angular dependence, a phenomenon explored in previous calculations considering only dipole transitions. In the nonrelativistic dipole approximation, time delay for atomic ns-states is angle-independent. However, incorporating relativistic effects, such as spin-flip transitions and non-dipole (quadrupole) effects, introduces angular dependence to the time delay. This is especially prominent when dominant dipole photoionization channels (without spin-flip) vanish at specific angles due to angular momentum geometry. In these instances, quadrupole and spin-flip transitions dominate. When the dipole amplitude vanishes, time delay results from a combination of spin-flip dipole and quadrupole time delays. Relativistic expressions have been formulated to delineate the conditions where quadrupole and/or spin-flip channels influence the time delay. We focus on the 3s subshell in argon to highlight the complex effects of spin-flip and quadrupole transitions. Of particular interest is the presence of a significant Cooper minimum in the cross section, leading to notably long time delays. Our goal is to explore these dynamics and understand the intricate processes in photoionization