Abstract
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 (∼104 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 (∼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 1.5 ± 0.1 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-below-threshold ionization which produces Rydberg states which slowly ionize on microsecond time scales.
Highlights
Ultrafast electron diffraction (UED) has developed into a powerful technique for studying structural dynamics.1–4 Pumping samples with femtosecond laser pulses and probing them with high energy electron bunches can lead to sample damage, which is true for biological molecules.5 This means that diffraction patterns preferably have to be captured with a single electron bunch: single-shot electron diffraction
We present measurements of the pulse length of ultracold electron bunches generated by near-threshold two-photon photoionization of a laser-cooled gas
The measured pulse lengths are limited by the energy spread associated with the longitudinal size of the ionization volume, as expected
Summary
Ultrafast electron diffraction (UED) has developed into a powerful technique for studying structural dynamics. Pumping samples with femtosecond laser pulses and probing them with high energy electron bunches can lead to sample damage, which is true for biological molecules. This means that diffraction patterns preferably have to be captured with a single electron bunch: single-shot electron diffraction. Pumping samples with femtosecond laser pulses and probing them with high energy electron bunches can lead to sample damage, which is true for biological molecules.5 This means that diffraction patterns preferably have to be captured with a single electron bunch: single-shot electron diffraction. Up to $106 electrons can be extracted in a single shot from the ultracold electron source but strong space charge effects come into play. For UED, the electron pulse length has to be shorter than the shortest timescale associated with the process under investigation This means that often the electron bunch length should preferably be much shorter than one picosecond.
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