The motion of electrons in atoms, molecules, and in condensed matter unfolds on a time scale from the attosecond (1 as = 10-18 s) to several femtoseconds (1 fs= 10-15 s). The advent of attosecond light pulses has opened the way to the time-resolved study of electronic motion and of ionization processes using pump-probe spectroscopy. In this thesis, we examine three aspects of the ionization dynamics in poly-electronic atoms: i) the coherence of the emerging charged fragments, ii) two-photon ionization pathways, and iii) the reconstruction of photoionization amplitudes from experimental observable. We focus on the role of autoionizing states and of inter-channel coupling, which are pervasive features in the spectra of atoms and molecules. Autoionizing states are localized electronic configurations with energy above the ionization threshold. Due to electronic motion correlated character, the excess energy in these configuration can be transferred to a single electron, which is emitted to the continuum. Here, we use state-of-the-art techniques to accurately reproduce these processes. In photoionization, the photoelectron and its parent ion form an entangled pair. Even if the initial state is pure, therefore, either of the two fragments is only partially coherent. We simulate ab initio XUV-pump IR-probe experiment on helium to control this loss of coherence. To a first approximation, the states of the He+ ion with the same principal quantum number n are degenerate. On a femtosecond timescale, the partially coherent population of nl ionic states results in a stationary, delay-dependent dipole, controllable through the intermediate nln'l' autoionizing states of the atom. Fine-structure corrections cause picosecond real-time fluctuations of this dipole, from which we reconstruct the coherence in the residual ion at its inception. In argon, resonant phase structures below the (3s-1) threshold are revealed using the Reconstruction of Attosecond Beating by Interference of Two-Photon Transition (RABBITT) technique. Our calculations, which are in excellent agreement with experimental measurements, mark a significant improvement over previous models
| Identifer | oai:union.ndltd.org:ucf.edu/oai:stars.library.ucf.edu:etd2020-2256 |
| Date | 01 January 2022 |
| Creators | Mehmood, Saad |
| Publisher | STARS |
| Source Sets | University of Central Florida |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Electronic Theses and Dissertations, 2020- |
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