Pulse length of ultracold electron bunches extracted from a laser cooled gas

We present measurements of the pulse length of ultracold electron bunches generated by near-threshold two-photon photoionization of a laser-cooled gas. The pulse length has been measured using a resonant 3 GHz deflecting cavity in TM110 mode. We have measured the pulse length in three ionization regimes. The first is direct two-photon photoionization using only a 480 nm femtosecond laser pulse, which results in short (~15 ps) but hot (~10⁴K) electron bunches. The second regime is just-above-threshold femtosecond photoionization employing the combination of a continuous-wave 780 nm excitation laser and a tunable 480 nm femtosecond ionization laser which results in both ultracold (~10 K) and ultrafast (~25 ps) electron bunches. These pulses typically contain ~103 electrons and have a root-mean-square normalized transverse beam emittance of 1.560.1 nm rad. The measured pulse lengths are limited by the energy spread associated with the longitudinal size of the ionization volume, as expected. The third regime is just-belowthreshold ionization which produces Rydberg states which slowly ionize on microsecond time scales.