Identification of competing ionization processes in the generation of ultrafast electron bunches from cold-atom electron sources

Abstract

We make direct measurements of the duration of ultrafast cold-electron bunches produced by photoionization of laser-cooled atoms. We show that the bunch duration can vary by up to six orders of magnitude for relatively small changes in laser wavelength that enhance or inhibit specific photoexcitation pathways and below-threshold tunneling. By selecting a two-color multiphoton excitation process, bunches with durations as low as the measurement resolution limit of $130\phantom{\rule{0.28em}{0ex}}\mathrm{ps}$ are measured using a streak technique. Verification that ultrafast cold-electron bunches can be generated by photoionization of cold atoms is an important step towards their application in high-brightness ultrafast electron diffraction and injectors for particle accelerators.