Two novel methods in strong-field and attosecond physics are studied in this thesis: Bohmian trajectories and initial-value representations (IVRs). Both methods describe the system dynamics in terms of trajectories of electrons, and include the binding potential and the laser field. The full solution of the time-dependent Schrodinger equation (TDSE) is used as a benchmark. First, Bohmian trajectories are applied to high-order harmonic generation (HHG). We find that the trajectory located at the innermost region of the atomic core is the most important for HHG and yields spectra with a well defined plateau and cutoff. Using time-frequency analysis, we associate this trajectory to an ensemble of unbound classical trajectories leaving and returning to the core, and observe arch-like patterns that build up in the phase of the wave function. Furthermore, we relate the Bohmian trajectories to the short and long trajectories encountered in the strong-field approximation (SFA) and show that the central Bohmian trajectory overestimates the contribution of the long SFA trajectory. Second, IVRs, namely the Herman-Kluk (HK) propagator and the coupled coherent states (CCS) method, are employed in the study of strong-field ionization dynamics. This study is performed in phase space, where signatures of tunneling and over-the-barrier ionization are identified for an initially bound wave packet. Our results indicate that the HK propagator does not fully account for tunneling and over-the-barrier refections, but partly reproduces features associated with the wave packet crossing classically forbidden regions. In contrast, the CCS method accurately reproduces the results from the time-dependent Schrodinger equation (TDSE). The original CCS, compared with the TDSE, works well in short time propagation. If reprojection of the wave function to the initial grid is employed quantitative agreement between these two methods, and the first ever CCS computation of HHG spectra, with a plateau and sharp cutoff, is obtained in long time propagation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:632113 |
| Date | January 2014 |
| Creators | Wu, J. |
| Publisher | University College London (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://discovery.ucl.ac.uk/1455981/ |
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