In this talk I will give an overview of ultrafast electron diffraction (UED) using radio-frequency (RF) compressed electron pulses in the 100keV energy range. The concepts involved in recompressing femtosecond laser produced electron bunches with RF cavities will be discussed along with their practical implementation at McGill University. Novel methods to characterize the temporal impulse response function in pump-probe geometry using laser fields and streak cameras will also be described. At pC bunch charges time resolution in this instrument is ~350 fs FWHM, currently limited by RF/laser synchronization jitter not the recompressed electron pulse duration which we estimate to be 100 +/- 50 fs FWHM . To demonstrate the performance of this instrument, experiments on photo-induced structural dynamics in vanadium dioxide and graphite will be described. Using a pair distribution function analysis of the UED data, we have mapped changes in the structure of the vanadium dioxide unit cell through the optically-induced ultrafast insulator-to-metal transition. This analysis has, for example, allowed us to identify the timescales associated with both the formation of the vanadium-vanadium bonding and the oxygen coordination consistent with the high temperature tetragonal phase. In addition, we have studied the complex structural dynamics in graphite following femtosecond laser excitation over a wide-range of fluences in both HOPG and single crystal graphite. I will describe the coherent and incoherent structural dynamics, including changes to both a and c lattice constants and the stacking arrangement of the planes, that follow femtosecond laser excitation as suggested by these observations.
SLAC National Accelerator Laboratory, Menlo Park, CA
Operated by Stanford University for the U.S. Dept. of Energy