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Diffractive Imaging of Laser Induced Molecular Reactions with Kiloelectron-Volt Ultrafast Electron Diffraction
Capturing the structural changes during a molecular reaction with ultrafast electron diffraction (UED) requires a high spatiotemporal resolution and sufficiently high signal-to-noise to record the signals with high fidelity. In this dissertation, I have focused on the development of a tabletop gas phase keV-UED setup with a femtosecond temporal resolution. A DC electron gun was employed to generate electron pulses with a high repetition rate of 5 kHz. The space charge effect in the electron pulse was ameliorated by compressing the 90 keV electron pulse longitudinally with a time varying electric field in an RF cavity. The velocity mismatch between electron and laser pulses was mitigated using a tilted laser pulse with an incident angle such that longitudinal component of the laser velocity is matched to the speed of electron pulse. The combination of these two techniques enabled the setup to reach a temporal resolution of 240 fs, more recently ~200 fs, and a timing drift of 50 fs rms over several hours. The UED was used to capture the laser induced alignment of linear and nonlinear molecules. The high beam current and femtosecond resolution allowed us to extract the molecular orientation distribution (MOD) of the molecular ensemble with high fidelity as it evolved from the prompt alignment to the past multiple revivals. To retrieve the MOD of nonlinear molecules, I developed a theory that maps the MOD to the atom-pair angular distributions. The retrieval method does not require solving Schrödinger equation and works for any alignment methods. We also investigated ionization, fragmentation and isomerization of toluene generated by an IR strong laser field. Combined with the time-of-flight mass spectrometry, UED can determine the structure and yield of cations. A comparison of measurements to scattering calculations shows that scattering computation with independent atom model is inadequate to describe electron scattering from cations, and ab-initio calculation is required. Finally, the molecular photodissociation experiments with CF3I and iodobenzene induced by a UV pulse were demonstrated with the keV-UED.
Xiong, Yanwei, "Diffractive Imaging of Laser Induced Molecular Reactions with Kiloelectron-Volt Ultrafast Electron Diffraction" (2023). ETD collection for University of Nebraska - Lincoln. AAI30423175.