Ultrafast single-electron diffraction at 100 keV and investigation of carbon-nanotube dynamics

Time-resolved electron diffraction is a powerful tool to observe
ultrafast structural dynamics in materials and molecules with
atomic spatial as well as temporal resolution. Due to Coulomb
repulsion, however, the use of only single-electrons or
few-electrons per pulse is inevitable to reach the shortest pulse
durations. Electrons have rather high scattering cross sections and
thus experiments in transmission require ultrathin samples in the
nanometer-range, making sample preparation very challenging. Up to
now, ultrafast single-electron diffraction was only demonstrated at
an electron energy of 30 keV; these measurements were performed in
our group at the “UED1-beamline”. This work introduces our
second-generation beamline, “UED2”, where the electron acceleration
voltage is upgraded from 30 to 100 kV, which allows the
investigation of significantly thicker samples. This is decisively
widening the range of complex materials that can be studied. In the
experiment, electron pulses are generated by a two-photon
photoemission process and the long-term stability of the source is
shown. The samples can be placed in transmission as well as
grazing-incidence geometry. To achieve phase-matching between the
optical and electron pulses, tilted optical pulses can be applied.
We figured out that to avoid temporal distortions in tilted pulses,
a geometry must be chosen in which the propagation direction of the
tilted pulses is perpendicular to the grating’s surface.
Furthermore, temporal distortions for ultrashort electron pulses
caused by misaligned magnetic lenses are examined. It is found that
a displacement or tilt of the lens causes significant temporal
aberrations on a femtosecond time scale and pulse-lengthening is
only minimized if the beam travels precisely on the symmetry axis.
An experimental procedure detailed here for aligning lens-position
and -tilt reduces the aberrations to less than one femtosecond. For
the “UED2-beamline”, a new laboratory was established and a first
time-resolved electron diffraction experiment at this beamline
performed. Anisotropic ultrafast atomic motion in carbon-nanotubes
was observed, revealing the nature of the system’s chemical bonds,
which vary from relatively weak van der Waals to strong covalent
interactions. In summary, it is thus shown that ultrafast electron
diffraction at 100 keV with single/few electrons per pulse is an
excellent method to study ultrafast atomic-scale dynamics even in
complex solid samples with the highest possible resolution in space
and time.