Investigation of Ultrafast Molecular Dynamics via Covariance Mapping : A Tool for Intense XUV Light Sources

Abstract: The study of molecular dynamics involves observations of the motion of nuclei and electrons. While nuclear motion is usually on the picosecond or femtosecond timescale, attosecond precision is necessary to directly observe the motion of electrons. Ultrashort laser pulses have become established as a viable tool for probing femtosecond dynamics by irradiating the target for only extremely short times. This allows snapshots of the motion to be obtained. However, extreme ultraviolet (XUV) or soft X-ray wavelengths are required to generate attosecond pulses. Two light sources that produce ultrashort XUV pulses were used during the work presented in this thesis: the free electron laser in Hamburg (FLASH) and the high-order harmonic generation-based light source at the High-Intensity XUV Beamline in Lund.This thesis describes the application of covariance mapping and pump-probe spectroscopy as tools to investigate molecular dynamics at these XUV light sources. A covariance mapping scheme was implemented in conjunction with a double-sided velocity map imaging spectrometer at the High-Intensity XUV Beamline. Its capabilities were demonstrated in a proof-of-principle experiment on molecular nitrogen. The scheme was subsequently applied to more complex molecules, and results from photoion-photoion covariance mapping of adamantane (C10H16) are presented. The photodissociation behavior of halomethane molecules was investigated with infrared - ultraviolet and ultraviolet - soft X-ray pump-probe schemes during several measurement campaigns at FLASH. While these time-resolved experiments probed dynamics mainly on the picosecond timescale, efforts were made at the High-Intensity XUV Beamline towards attosecond precision XUV-XUV pump-probe experiments. After demonstrating the ability to induce two-photon processes with XUV light, the XUV wavefronts were studied over several experimental campaigns. These campaigns led to a significant reduction of aberrations in the XUV wavefronts. The size and quality of the XUV focal spot as well as the XUV generation yield were improved.