Generation, Characterization and Applications of Femtosecond Electron Pulses

Ultrafast electron diffraction is a novel pump-probe technique which aims to determine transient structures during photoinduced chemical reactions and other structural transitions. This technique provides structural information at the atomic level of inspection by using an electron pulse as a diffractive probe. The atomic motions of interest happen on the 100 fs = 10^(−13) s time scale. To observe these atomic motions, a probe which matches this time scale is required. In this thesis, I describe the development of an electron diffractometer which is capable of 200 fs temporal resolution while maintaining high signal level per electron pulse. This was made possible by the construction of an ultra-compact photoactivated 60 keV femtosecond electron gun.\n\nTraditional electron pulse characterization methods are unsuitable for high number density femtosecond electron pulses such as the pulses produced by this electron gun. I developed two techniques based on the laser ponderomotive force to reliably determine the duration of femtosecond electron pulses into the sub-100 fs range. These techniques produce a direct cross-correlation trace between the electron pulse and a laser pulse. The results of these measurements confirmed the temporal resolution of the newly developed femtosecond electron diffractometer. This cross-correlation technique was also used to calibrate a method for the determination of the temporal overlap of electron and laser pulses. These techniques provide the pulse diagnostics necessary to utilize the temporal resolution provided by femtosecond electron pulses.\n\nOwing to their high charge-to-mass ratio, electrons are a sensitive probe for electric fields. I used femtosecond electron pulses in an electron deflectometry experiment to directly observe the transient charge distributions produced during femtosecond laser ablation of a silicon (100) surface. We found an electric field strength of 3.5 × 10^6 V/m produced by the emission of 5.3 × 10^11 electrons/cm^2 just 3 ps after an excitation pulse of 5.6 J/cm^2 . This observation allowed us to rule out Coulomb explosion as the mechanism for ablation under the conditions present in this experiment.