Development of an ultrafast low-energy electron diffraction setup
by Max Gulde
Date of Examination:2014-10-15
Date of issue:2014-12-11
Advisor:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Tim Salditt
Referee:Dr. Klaus Sokolowski-Tinten
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Description:Dissertation
Abstract
English
Quasi two-dimensional systems such as surfaces and atomically thin films can exhibit drastically different properties relative to the material's bulk, including complex phases and transitions only observable in reduced dimensions. However, while methods for the structural and electronic investigation of bulk media with ultrahigh spatio-temporal resolution have been available for some time, there is a striking lack of methods for resolving structural dynamics at surfaces. Here, the development of an ultrafast low-energy electron diffraction setup is presented, offering a temporal resolution of a few picoseconds in combination with monolayer structural sensitivity. In particular, a detailed account is given on the defining beam properties of the electron source, based on a nonlinearly driven nanometric photocathode. The emitter parameters within an electrostatic lens assembly are studied by means of a finite element approach. In particular, the optimal operation regime as well as achievable temporal resolution are determined. A prototype emitter comparable to the one used in the simulation is designed, characterized and applied within an ultrafast low-energy diffraction experiment. Specifically, the superstructure dynamics of an ultrathin bilayer of polymer crystallites adsorbed on free-standing graphene are investigated upon strong out-of-equilibrium excitation. Different processes in the superstructure relaxation are identified together with their respective timescales between 40 and 300 ps, including the energy transfer from the graphene to the polymer, the loss of crystalline order and the formation of extended amorphous components. The findings are subsequently discussed in view of an ultrafast melting of the superstructure. To conclude, the contribution of the approach to time-resolved surface science is discussed and an outlook is given in terms of future systems to investigate and further developments of the apparatus.
Keywords: Ultrafast Low-Energy Electron Diffraction; Electron Pulse; Surface Science; Structural Analysis; Superstructure Dynamics; Ultrathin Polymer Film; Graphene; PMMA; LEED; ULEED; polymer dynamics