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Title: High-order harmonic generation in laser ablation plumes
Author: Hutchison, Christopher
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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High harmonic generation (HHG) is a powerful and well established technique to study ultra fast processes in atoms and molecules. Originally HHG was restricted to the study of atomic or small molecular gases. In the last few years the use of laser ablation to create plumes that are suitable media for HHG has gained interest due to is ability to allow almost any solid material to be potentially studied using HHG. Most of this thesis focuses on our advancements in this eld of ablation plume HHG spectroscopy with a small section devoted to our continuation of work focused on controlling electron trajectories in HHG through the use of an orthogonally polarised second harmonic eld. We show how through the use of a rotating target system we have been able to stabilise the ablation process to work at 1 kHz repetition rate. The ablation plumes of some transition metals have previously be shown to exhibit enhancement of particular harmonic orders linked to resonances in the parent ion. We performed investigations into tin and manganese plumes and were able to nd good agreement between the experimental data and a theoretical model. This showed that autoionising states in the ions of these materials were the most likely source of the enhancements. We present our attempts to extend the ablation plume HHG technique to work with soft materials. It was found that graphite plumes were able to produce a very strong harmonic response that is comparable with generation from an argon gas jet. It was discovered that nano particles were present in the plumes and these were mostly likely to cause of the stronger e ciency. Finally we present the rst use of laser ablation plume HHG to study molecules of biological interest. We performed HHG studies on uracil and thymine, a signal was obtained from former but not the latter. Were able to determine that during ablation there was a higher degree of molecular fragmentation from thymine molecules compared to uracil.
Supervisor: Frasinski, Leszek ; Marangos, Jon Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available