Laser-Assisted Photoemission from Solids with Free Electron Lasers

In this thesis, the topic of laser-assisted photoemission (LAPE) on metals is investigated. By an ultrashort extreme ultra-violet (XUV) laser pulse, photoemitted electrons are released from the solid sample material and interact with a second, spatially and temporally superimposed, ultrashort 800nm infrared (IR) laser pulse. This results in the generation of replicas of the original photoemission line, so-called sidebands. The already well-established technique in gas phase physics offers ideal possibilities for the investigation of ultrafast electronic processes but also, due to the direct dependence on the temporal distribution of the involved laser pulses, a precise method for determining the temporal properties of the radiation sources used. The transfer to the solid-state domain is of particular interest, since the higher cross-section can be used to characterize radiation sources with lower photon density, while surface dynamics can also be investigated. Measurements using time-resolved X-ray photoelectron spectroscopy (trPES) were performed at the Free Electron Laser in Hamburg (FLASH) to measure the respective 4f core levels of Pt(111) and W(110) as well as their corresponding sidebands resulting from the LAPE process. It could be shown that the known theoretical model coincides with the results measured on platinum, both in the temporal distribution and in the resulting intensity. This is in contrast to the data obtained on tungsten. Here, a separation of the sideband signal in the temporal domain indicates a dependence on the place of origin of the electron, which is either emitted from the surface or the bulk. In addition, deviations from the expected value in the sideband intensity also indicate a necessary adjustment of the IR field strength in the theoretical model. Furthermore, it could be shown that this technique is also suitable for the characterization of higher harmonic FEL radiation. From the data obtained on platinum, it was possible to draw conclusions about the temporal distribution of the third harmonic and the beam size in front of the monochromator on the PG2 beamline at FLASH.