Extending high harmonic generation spectroscopy to new molecular species

• Main Author: McGrath, Felicity
• Other Authors: Marangos, Jon ; Tisch, John
• Published: Imperial College London 2015
• Subjects: 535.8
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.684280

HHG spectroscopy is a powerful tool for observing electronic structure along with electronic and nuclear dynamics with state of the art resolution on the attosecond (10⁻¹⁸ s = 1 as) timescale and angstrom (10⁻¹⁰ m = 1 Å). Thus far, HHG spectroscopy has been applied to small hydrocarbons such as ethylene and methane using an 800 nm drive laser field wherein dynamics can only be measured in a 0.9 - 1.6 fs time window. This PhD has two primary aims: 1) to extend the time window over which we can make measurement up to 4 fs using an 2 μm drive laser field and 2) to perform HHG spectroscopy on molecules which are in the liquid phase at room temperature. The molecules at the focus of this thesis are benzene and substituted benzenes. The design and development of an apparatus to generate a stable vapour jet from a thin continuous nozzle is presented. The completed and tested apparatus has demonstrated that stable and reproducible spectra can be acquired with no contamination between different samples as they are switched over. Comparison of methylated benzene to benzene harmonic spectra show good agreement with theoretical results on coupled electron-nuclear dynamics developed by our collaborators. We also compare HHG spectra acquired under similar generating conditions from deuterated and protonated benzene which enables us to track nuclear dynamics and to determine the origin of the ionized electron in benzene. HHG spectra from the halogenated benzenes, in particular fluorobenzene, seem to illustrate the contribution of more tightly bound orbitals to the HHG signal which is indicative of dynamical interferences.