Radio-frequency controlled low-energy electron pulses
by Dennis Epp
Date of Examination:2024-08-06
Date of issue:2024-08-23
Advisor:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Stefan Mathias
Persistent Address:
http://dx.doi.org/10.53846/goediss-10691
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Abstract
English
Electron beams are invaluable for probing material structures and dynamic processes due to their large scattering cross sections and the versatility of tunable electron optics for diffraction and microscopy. Pulsed electron beams generated by photoemission using femtosecond laser pulses provide unique insights into the structural evolution of non-equilibrium states on the intrinsic time scales of light-induced dynamics. Such studies cover a wide range of phenomena, including structural phase transitions, electron correlation, and lattice vibrations. The temporal resolution of ultrafast electron diffraction (UED) instruments depends critically on the duration of the electron pulse at the sample location, which is influenced by factors such as the initial velocity distribution of the photoelectrons and their mutual Coulomb interaction. While femtosecond electron pulses are routinely achieved at high kinetic energies (tens and hundreds of kiloelectronvolts), the challenges become more pronounced at lower electron energies due to electron pulse dispersion effects. These lower energies allow for the probing of single atomic layers, making them highly valuable for studying surface reconstructions, adsorbates, and mono- or bilayers. Recently, Ultrafast Low-Energy Electron Diffraction (ULEED) techniques have emerged, providing insights into adsorbate dynamics, phase-ordering kinetics, lattice thermalization, and coherent control of structural phase transformations. This dissertation contributes technology to enhance ULEED capabilities in terms of temporal resolution by radio frequency (RF) compression of low-energy electron pulses. To this end, the phase-space distribution of electrons is manipulated by an RF compression cavity, allowing for a fourfold reduction in pulse duration and an adjustment of the temporal focus position along the propagation path. Experimental validation of pulse compression is achieved by characterization methods such as lateral streaking, retarding-field analysis, and pulse duration measurements via the transient electric field effect (TEFE). The first demonstrates the synchronization between a cavity field and an electron pulse, the second measures the manipulation of the average kinetic energy and energy width by the compression cavity, and the third provides a precise value for the pulse duration. The results presented here demonstrate the potential of RF compression in achieving sub-picosecond low-energy electron pulses, thereby improving the temporal resolution of the ULEED techniques. These advances pave the way for further exploration of ultrafast structural dynamics in low-energy electron scattering with surfaces and nanostructures, providing new insights into fundamental processes and enabling the development of advanced materials and devices.
Keywords: ultrafast low-energy electron diffraction; ULEED; structural dynamics; surface science; ultrashort electron pulses; nanoscale photoemitter; laser-driven electron source; radio frequency; electron pulse compression; electron pulse streaking; retarding-field analysis
