Imaging Photoinduced Dynamics in Gas-Phase Molecules Using Ultrafast Electron and X-Ray Diffraction

Investigating the ultrafast structural changes in photoexcited molecules provides essential information on their governing reaction mechanisms and product formation. Time-resolved electron and X-ray diffraction are crucial techniques for capturing the structural information of photoexcited molecules on femtosecond timescales. This thesis presents the investigations of photochemical reactions in gas-phase molecules using ultrafast electron and X-ray diffraction experiments. Photoexcitation of conjugated diene molecules with ultraviolet (UV) laser pulses can lead to different reaction mechanisms, depending on their double bonds’ positions and molecular rigidity. Chapter 2 of this dissertation discusses the femtosecond time-resolved UV-induced excited-state structural dynamics of cis,cis-1,3-cyclooctadiene, an eight-membered cyclic conjugated diene molecule, probed using mega electron volt ultrafast electron diffraction (MeV-UED) experiment. This experiment was conducted at the MeV-UED facility of SLAC National Accelerator Laboratory. The results of the experimental and simulated diffraction signals are presented. Combining results from the experiment and theoretical simulations, the molecule’s ring motions after excitation to its first excited state until it relaxes back to the ground state through conical intersections are discussed. Identifying the dissociation pathways and tracking the structural changes along the existing pathways of photodissociation reactions is crucial to understanding their photochemistry and is often challenging in many chemical reactions. In Chapter 3, the investigations of UV photodissociation dynamics of diiodomethane gas-phase molecules imaged using an X-ray scattering experiment are discussed. The experiment was performed using the Coherent X-ray Imaging (CXI) instrument at the Linac Coherent Light Source (LCLS). The analysis and results of the experimental diffraction signals are presented. A preliminary comparison of the experimental results shows a good qualitative match with the theoretical simulations. The major dissociation pathway of this UV-induced dissociation reaction and the structural changes taking place along this channel are identified from these results. Gas-phase diffraction studies of low vapor pressure molecules require a specialized sample delivery system. In the last chapter, an overview of our keV-UED experimental setup at UNL is provided. Sample delivery details of low vapor pressure dicyclopentadiene molecules and their diffraction results are presented. The electron pulse duration measurements from a compact streak camera are also included. Advisor: Martin Centurion