High-repetition-rate pulsed electron source based on an atomic Rydberg photocathode

A gas of rubidium atoms has been excited by a combination of ultraviolet and far-infrared excitation to a superposition of Stark states lying just above the classical saddle point. Using an atomic streak camera, we have demonstrated that the atom ejects an ultrafast train of electron subpulses nearly equally spaced in time, with a repetition rate of approximately 50 GHz. The frequency characteristics of this pulse train are seen to be extremely sensitive to small changes in the static electric field. These measurements imply that, by variation of the electric field during the electron emission, it is possible to create shaped ultrafast electron pulses analogous to shaped optical pulses.