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 ps and fs spectroscopy techniques. This thesis gives a thorough theoretical and experimental treatment of ultrafast gas-phase electron diffraction (UGED). Classical Monte-Carlo simulations of coherent chemical dynamics were used to demonstrate 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. The circular symmetry of GED patterns was predicted to break during ps rotational coherences, revealing additional structural detail such as bond angles. Instrumentation for UGED was almost entirely home-built. Femtosecond laser pulses were generated in a colliding-pulse, mode-locked ring dye laser and amplified with a four-stage dye cell arrangement pumped by a Nd:YAG laser. The 620-nm output (2 to 3 mJ, 300 fs uncompressed; 30 Hz) 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. The electrons were accelerated with an 18-kV electron gun and focused to a 300-μm diameter. Space-charge effects forced a compromise between electron number density and temporal resolution: streaking experiments revealed that a 1-ps pulse contained 1,000 electrons and a 10-ps pulse contained 10,000 electrons. Thirty centimeters downstream from the exit of the gun, the electrons intersected the pump laser at a 90° angle, directly underneath the molecular beam orifice. A theoretical analysis of the crossed-beam geometry showed that velocity mismatch between the pump photons and the probe electrons also affected the temporal resolution of UGED, making a 3-ps contribution. 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 housed in a separate vacuum chamber. Single-electron sensitivity across two-dimensions was necessary because of the extremely low electron flux, and the estimated detective quantum efficiency of the imaging system was better than 0.5. Ground-state GED patterns of CCl4, SF6, CF3I, CH2I2, and C2F4I2 were recorded using ps electron pulses. The diffraction data were processed with a software package developed in the laboratory, and the resulting modified molecular scattering curves agreed well with theory. Radial distribution functions were also calculated. Time zero for the pump-probe experiment was identified to within 1 to 2 ps using photoionization-induced lensing (PIL) of the unscattered electron beam. The excitation laser ionized a small fraction of the molecular beam sample, and a cylindrical coulombic lens developed within ps as the nascent photoelectrons escaped the interaction region. The effects of this lens on the incident 18-ke V electron beam shape was detected by direct-bombardment on a charge-coupled device located inside the scattering chamber; high spatial resolution of ~ 15-μm was necessary to observe PIL-induced changes in the electron beam. Diiodomethane was selected as the prototype molecule for the first ultrafast GED investigation. After establishing time zero with PIL, diffraction patterns were recorded at several time steps around t0. The transients showed that approximately 10% of the CH2I2 dissociated into CH2I and an iodine atom following excitation with the 310-nm pump laser. The estimated temporal resolution was 5 to 10 ps.