Berlin 2015 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 70: Structural Dynamics in Nanoscale Materials Probed by Ultrashort Electron Pulses

O 70.6: Poster

Mittwoch, 18. März 2015, 18:15–21:00, Poster A

Sub 400 fs temporal resolution in reflection high energy electrondiffraction at surfaces — •Tim Frigge, Bernd Hafke, Boris Krenzer, Carla Streubühr, Ping Zhou, Manuel Ligges, Dietrich von der Linde, Uwe Bovensiepen, and Michael Horn-von Hoegen — Universität Duisburg-Essen, Lotharstr. 1, 47057 Duisburg

In time resolved reflection high energy electron diffraction at surfaces an ultra short electron pulse probes the surface at grazing incidence. At energies of 30 keV the electrons travel at 1/3 of the speed of light and require 20 ps to traverse a typical sample width of 2 mm. For pump-pulses at normal incidence the sample surface is excited simultaneously. This resulting geometric velocity mismatch limits the overall experimental temporal resolution. Employing a tilted pulse front scheme for the laser excitation we are able to overcome this limitation and obtain a temporal resolution of 600 fs when probing the entire sample width. Reduction of the electron number per pulse and probing only a narrow part of the sample further improves the temporal resolution to less than 400 fs without electron pulse compression. Temporal broadening due to vaccum dispersion on the 8 cm distance between photocathode and sample is minimized by the narrow initial electron energy spread of only 100 meV of the back illuminated Au photocathode. Additionally, a new electron gun design ensures a vertical coherence length of more than 30 nm. An integration time of 10 s is sufficient for good statistics for one diffraction image, i.e. a diffraction movie with 100 frames at 50 fs delay steps is recorded in less than 30 min.