Ultracold and Ultrafast Electrons Diffracted from Graphene

A recent development in electron microscopy and diffraction is the achievement of ultrafast time resolution using high-repetition rate single electron sources and single-shot many-electron sources. Intrinsic heating mechanisms due to inter-particle Coulombic forces make it difficult to achieve large coherence lengths under these conditions. We have developed a new class of electron source by using near-threshold, pulsed photoionization of laser-cooled, trapped rubidium atoms which allows us to produce intense electron pulses with large intrinsic coherence length. Several properties of electron pulses extracted from such an ultracold source have already been investigated, showing in particular that temperatures of a few Kelvin and emittances in the nanometer-radian range can be achieved with tens of thousands electrons per pulse for nanosecond pulse lengths. Photo-ionization also permits charge shaping in order to reduce emittance growth. We have recently extended this work to produce picosecond electron pulses by using ultrafast lasers for the ionization step. We show that few-Kelvin electron temperatures can still be achieved under these conditions, despite the spread in photon energy. We explain this surprising result using a classical hydrogen model that considers the electron trajectories resulting from photoionization in detail. A more realistic rubidium model using the GPT particle tracking code confirms the results. In addition, we show how polarization effects arise and that these can be used to futher control the electron beam properties. To demonstrate application of the ultracold electron source, we are currently studying diffraction from graphene-like samples at few-kV beam energy. We observe the familiar powder diffraction rings and use these to characterize the transverse coherence properties of the electron beam. This work in progress will be followed by a characterization of the bunch length using a radio-frequency streak cavity