Zeitaufgelöste Messungen mit ultrakurzen XUV-Pulsen des Freie-Elektronen-Lasers "FLASH"

The interaction of high-energy photons in the XUV regime with surface systems may lead to new and unexpected results due to the large number of accessible product states. In the past this photon energy regime has mainly been investigated using synchrotron radiation sources. The development of the Free Electron Laser at Hamburg (FLASH) now opens a new area in this spectral range. It provides pulsed radiation in the photon energy range from 20 to 200 eV, with a pulse energy in the tens of microjoule regime, and a pulse duration of 20 to 50 fs [23]. This intensity is sufficient to induce processes at surfaces which can further be analyzed by a second synchronized probe laser pulse. Thereby dynamic properties of the reaction dynamics can be assessed. In preparation experiments a highly-oriented pyrolytic graphite (HOPG) surface covered with D2O is irradiated and desorbing products are analyzed. O2+ ions are formed by the FLASH radiation on the surface and directly emitted from the ice. The O2+ yield shows a highly nonlinear dependence on the FEL intensity [24]. This opens the way to nonlinear correlation measurements with an autocorrelator using two FLASH pulses synchronized on a femtosecond time scale. In first two-pulse correlation measurements the used XUV radiation is characterized by interfering two time-delayed partial beams obtained by wave front beam splitting in an autocorrelator operating at photon energies from hν = 30 to 200 eV. The temporal coherenceproperties of XUV laser pulses at FLASH are measured at λ = 23.9 nm with a maximum visibility of (0.63 ± 0.04). A coherence time of 6 fs is obtained. The visibility function displays a non-monotonic decay, which can be explained by the presence of a multiple pulse structure [129]. The pulse duration of the XUV laser pulses at FLASH is directly determined by time-resolved measurement of doubly charged helium ions at 23.9 nm. As a result a duration of tL = (29 ± 5) fs is obtained [142].In further experiments the pulse duration could be verified by the two photon ionisation (Ne2+ → Ne3+) of neon. As an astrophysically interesting system the desorption of neutral H atoms from graphite with FLASH pulses is reported. The velocity distribution of the atoms peaks at extremely low kinetic energies. A DFT-based electron scattering calculation traces this distribution to desorption out of specific adsorption sites on graphite, and identifies the highest vibrational state in the adsorbate potential as a major source for the slow atoms. It is evident that multiple electron scattering processes are required for this desorption. A direct electronic excitation of a repulsive hydrogen carbon bond seems not to be important [165]. For a better understanding of the formation of interstellar molecules a D2O ice covered HOPG surface was investigated. As already mentioned this analog of interstellar ice covered dust grains was irradiated with XUV radiation (λ = 32 eV) provided by FLASH. To investigate the formation process of the desorbing molecular oxygen ions a non-linear XUV pump - XUV probe measurement with the STUVE was performed. The observed time resolved ion signal distribution points to an electronic process initiating the reaction and a shifted maximum of the O2+- signal suggests a vibration of the ice bulk triggered by proton transfer processes with a life time of tPT P = 3 ps.