Femtosecond high-energy electron diffraction


Electron and x-ray diffraction reveal the atomic structure of crystalline materials. Pulsed electrons or x-rays provide a snapshot of the crystal structure during the time window the pulses transmit through the sample. We employ femtosecond electron pulses at variable, defined time delay after the sample was excited with an ultrashort light pulse. This technique reveals structural dynamics of a material with femtosecond temporal and sub-atomic spatial resolution.

Schematic drawing of the femtosecond electron diffractometer. An electron pulse containing a few thousand electrons is generated by photoemission from a 20 nm thick gold film back-illuminated with a femtosecond visible laser pulse. The electrons are accelerated by a DC electric field between cathode and anode up to an energy of 100 keV. After exiting the electron gun through a small aperture in the anode, the electrons transmit through an ultrathin sample. The sample is excited with an optical pump pulse at a defined time prior the electrons take a snapshot of the atomic structure of the material. The diffraction image is taken with an electron camera placed behind a magnetic electron lens.

Figure 1: Schematic drawing of the femtosecond electron diffractometer. An electron pulse containing a few thousand electrons is generated by photoemission from a 20 nm thick gold film back-illuminated with a femtosecond visible laser pulse. The electrons are accelerated by a DC electric field between cathode and anode up to an energy of 100 keV. After exiting the electron gun through a small aperture in the anode, the electrons transmit through an ultrathin sample. The sample is excited with an optical pump pulse at a defined time prior the electrons take a snapshot of the atomic structure of the material. The diffraction image is taken with an electron camera placed behind a magnetic electron lens.

Simulated electron pulse durations.

Figure 2: Simulations of the electron pulse duration (full width half maximum) at the sample positioned 2 mm behind the anode in dependence of the duration of the laser pulse used to generate the electron pulses. The red line shows the duration of an electron pulse containing only a single electron. When generated with a 25 fs laser pulse, the single-electron pulses have a duration of 60 fs at the sample position. The blue lines indicate the duration of electron pulses containing 1000, 2000, …, 10000 electrons. These simulations imply sub-100 fs few-thousand electron pulses at the sample position. The simulations have been performed with the simulation package General Particle Tracer (GPT).

We developed a highly compact femtosecond electron diffractometer working at electron energies up to 100 keV, see Figure 1. The electron pulse propagation through the setup was simulated with a multi-body particle tracing code (GPT, Pulsar Physics)  in order to estimate the electron pulse durations at the sample position (Figure 2). Our simulations show that electron bunches containing few thousands of electrons per bunch are only weakly broadened by space-charge effects and their pulse duration is thus close to the one of a single-electron wave packet. With our compact setup, we can create electron bunches containing up to 5000 electrons with a pulse duration below 100 fs on the sample.

Further reading:

  • L. Waldecker, R. Bertoni, and R. Ernstorfer:
    Compact femtosecond electron diffractometer with 100 keV electron bunches approaching the single-electron pulse duration limit.
    J. Appl. Phys. 117, 044903 (2015) [doi:10.1063/1.4906786]
    open access: arXiv:1412.1942
  • R.J.D. Miller, R. Ernstorfer, M. Harb, M. Gao, C.T. Hebeisen, H. Jean-Ruel, C. Lu, G. Moriena, G. Sciaini:
    Making the molecular movie: first frames.
    Acta Cryst. A 66, 137-156 (2010), [doi: 10.1107/S0108767309053926].