| The construction and utilization of the fourth-generation ultrafast electron diffraction apparatus, UED4, is the subject of this thesis. With UED4 and its novel and universal sample delivery method based on laser desorption, we were able to vaporize thermally labile molecular samples and determine their ground-state structures and the structures of their photochemical and photophysical reaction products. Each component part of the new UED4 apparatus is described, and the experimental and computational procedures used to extract structural information from the time-resolved diffraction patterns are presented.;Several molecules were studied in their ground states and photoinduced excited states or product states on the time scale of picoseconds and nanoseconds. With UED3, nitrobenzene was shown to undergo intramolecular rearrangement prior to NO loss in an ultrafast fragmentation reaction. In indole, the chromophore of the amino acid tryptophan, the involvement of a dark structure, formed on the picosecond time scale, was revealed in the nonradiative decay pathway of the initially excited state. By determining the ground state structures of the thermally labile nucleobases uracil and guanine, the first use of surface-assisted laser desorption in a pulsed electron diffraction experiment was reported using the newly developed UED4 apparatus. The determined structures of the photochemically generated species of the photochromic molecule 6-nitro-BIPS further demonstrated the capability of laser desorption electron diffraction to function as a time-resolved experiment. Finally, the fragmentation reaction of the amino acid tryptophan upon UV laser irradiation was studied with UED4. The ability to deliver increasingly large and conformationally heterogeneous molecules into the gas phase now provides new challenges and opportunities of both experimental and theoretical nature for the field of ultrafast electron diffraction. |
