Electron diffraction from an ultracold source

For decades, physicists have been able to resolve the atomic-scale details of matter using x-ray and electron diffraction. Extending such techniques to ultrafast time scales at which chemical reactions occur is a more recent achievement: Thanks to advances in accelerator technology, x rays from free-electron lasers can probe molecular processes on the femtosecond scale. And for the past few years a handful of groups have been working to extend electron sources to that time regime as well—no accelerator required. The essential ingredients are very cold atoms and an ultrafast laser. One of the groups, led by Jom Luiten of Eindhoven University of Technology in the Netherlands, has now done crystallography with an ultracold electron beam. To prepare the beam, the group excited rubidium atoms held in a magneto-optical trap and subsequently ionized them using a picosecond laser pulse to create a cloud of a few hundred electrons. The researchers then accelerated the electron cloud with a local electric field through monocrystalline graphite about a meter away. The combination of the externally applied field and the wavelength of the ionization laser pulse determines the kinetic energy distribution of the released electrons—and thus their effective temperature. By varying the laser’s wavelength in a series of runs, Luiten and colleagues changed the electrons’ temperature from 300 K to 10 K; the diffraction pattern shown here was taken at 10 K. The concomitant change in the width of the diffraction peaks at different temperatures allowed the researchers to measure the beam’s coherence length—essentially the spatial extent of the electrons’ wavefunction—which for their 100-µm sample size was no smaller than 15 nm. Reassuringly, that coherence length is high enough to use in complex macromolecular diffraction, a common goal among many groups. (M. W. van Mourik et al., Structural Dynamics, in press.)