Franssen J G H, Frankort T L I, Vredenbregt E J D, Luiten O J
Department of Applied Physics, Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
Struct Dyn. 2017 Mar 23;4(4):044010. doi: 10.1063/1.4978996. eCollection 2017 Jul.
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 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 (∼10K) 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 ultracold (∼10 K) ultrafast (∼25 ps) electron bunches. These pulses typically contain ∼10 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.
我们展示了通过激光冷却气体的近阈值双光子光电离产生的超冷电子束团脉冲长度的测量结果。脉冲长度是使用处于TM模式的3 GHz共振偏转腔进行测量的。我们在三种电离模式下测量了脉冲长度。第一种是仅使用480 nm飞秒激光脉冲的直接双光子光电离，这会产生短（约15 ps）但热（约10K）的电子束团。第二种模式是阈值以上飞秒光电离，采用连续波780 nm激发激光和可调谐480 nm飞秒电离激光的组合来产生超冷（约10K）超快（约25 ps）的电子束团。这些脉冲通常包含约10个电子，并且具有均方根归一化横向束发射度为1.5 ± 0.1 nm rad。如预期的那样，测量到的脉冲长度受到与电离体积纵向尺寸相关的能量展宽的限制。第三种模式是阈值以下电离，它会产生里德堡态，这些里德堡态在微秒时间尺度上缓慢电离。
