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 TM$_{110}$ 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 ($\sim 15$ ps) but hot ($\sim 10^{4}$ K) 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 ($\sim 10$ K) and ultrafast ($\sim 25$ ps) electron bunches. These pulses typically contain $\sim 10^{3}$ electrons and have an rms normalized transverse beam emittance of $1.5\pm0.1$ nm$\cdot$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-below-threshold ionization which produces Rydberg states which slowly ionize on microsecond time scales.