Orbit-based theoretical approaches to modelling strong eld phenomena allow physical intuition to be extracted from complex multi-dimensional quantum processes. Highorder harmonic generation (HHG) has been interpreted relatively successfully for almost two decades as a three step process in which an ionized electron is accelerated by the eld and recombines with its parent ion, resulting in high-order multiples of the laser frequency. This process is often modelled within the strong-eld approximation (SFA), where the eect of the Coulomb potential on the electron is neglected while the electron is accelerated by the eld, and the single-active electron (SAE) approximation. The SFA provides an appealing interpretation of HHG in terms of interfering electron trajectories. Although successful in reproducing experimental observables in atomic systems, in recent years the importance of multi-electron eects, molecular orbital symmetry and the Coulomb potential in atoms and diatomic molecules have been seen experimentally and theoretically. These eects, neglected by the original SFA formulation, mean that either modications to the original SFA, or new trajectory based theories, are essential for a more complete physical understanding of the HHG phenomenon. This thesis investigates these eects in HHG from homonuclear and heteronuclear diatomic molecules in strong elds. We model and assess the importance of multiple molecular orbital contributions, molecular orbital geometry and two-centre interference on the HHG spectrum. These problems are approached within a semi-analytical, SFA, framework and with a static core. It is found that these eects can be seen in the HHG spectrum. By predicting novel features in the spectrum arising from such eects we obtain not only a better understanding and interpretation of current experimental results, but also new insight and applicability to molecular imaging. In addition to these modications, a new theoretical approach, the coupled coherent state (CCS) method is used to model Hydrogen in an intense eld, although it can be extended to multi-electron systems and diatomic molecules. In the CCS method, the Coulomb potential is fully included at all stages in the HHG process, and most notably, during the electron propagation, where it is neglected by the SFA. The CCS method has favourable scaling with dimensionality, compared to other numerical approaches, as well as being fully quantum. It is trajectory based, facilitating comparison with the three step model and the strong eld approximation. Therefore we benet from the physical intuition of semi-classical approaches but within a fully quantum framework and without the approximations of semi-analytical methods.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:625854 |
| Date | January 2012 |
| Creators | Augstein, B. B. |
| 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/1349184/ |
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