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 \u201cUED1-beamline\u201d.\n\nThis work introduces our second-generation beamline, \u201cUED2\u201d, where the electron\nacceleration 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\npulses, 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\u2019s 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.\n \nFor the \u201cUED2-beamline\u201d, 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\u2019s chemical bonds, which vary from relatively weak van der Waals to strong covalent interactions.\n\nIn 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.