Ultrafast gas-phase electron diffraction

The temporal resolution of pump-probe, gas-phase electron diffraction (GED) has been extended to the picosecond time scale, a three order-of-magnitude improvement. With such resolution, GED can now be applied to structural studies of fundamental chemical dynamics, providing complementary information to conventional time-resolved spectroscopy techniques. This thesis gives a thorough theoretical and experimental treatment of ultrafast GED.; Simulations of coherent chemical dynamics demonstrated that the evolution of molecular spatial coordinates can be determined with fs GED. Similarly, ps GED can reveal the structure of short-lived intermediates in kinetic processes, and the circular symmetries of GED patterns were predicted to break during ps rotational coherences.; 620-nm output from an amplified femtosecond laser (2.5 mJ; 300 fs) was split into pump and probe arms and frequency-doubled. 95% of the laser intensity was focused onto a molecular beam. The remaining 5% was directed onto a back-illuminated 450-A silver cathode, where ultrafast electron pulses were created via the photoelectric effect and accelerated to 18 keV. Space-charge effects forced a compromise between electron number density and temporal resolution: streaking experiments revealed that the pulse duration increased by 1 ps for every 1,000 electrons.; The electrons intersected the pump laser directly underneath the molecular beam orifice. Approximately 10% of the electrons scattered elastically from sample molecules within the interaction region, and the resulting diffraction pattern was recorded with a scintillator/fused fiber optic/image intensifier/charge-coupled device imaging system. Single-electron sensitivity across two-dimensions was necessary because of the extremely low electron flux, and the measured detective quantum efficiency of the imaging system was better than 0.5. Ground-state GED patterns of several molecules were recorded using ps electron pulses. Time zero for the pump-probe experiment was identified to within 1 ps using photoionization-induced lensing (PIL) of the unscattered electron beam.; The first ultrafast GED investigation studied diiodomethane, and diffraction patterns were recorded at several time steps around time zero. The resulting structural transients showed that 10% of the CH{dollar}sb2{dollar}I{dollar}sb2{dollar} dissociated into CH{dollar}sb2{dollar}I + I following excitation with the 310-nm pump laser. The estimated temporal resolution was 5 to 10 ps.