|UHH > MIN > FB Physik > Institute und Zentren > Institut für Experimentalphysik > AG Wurth > FEL_First||Kontakt Hilfe Druck Sitemap Suche|
In N2 we observe with increasing photon pulse energy a significantly reduced photoemission intensity (I) compared to a linear increase governed by the photoionization cross section s=6.6Mb/atom of N at 38eV [1,2]. This is due to the fact that as the gas molecules in the sample volume get ionized the sample volume is depleted of molecules in the ground state, which is express as a function of photons Np in the saturation formula:
at a focal size of A = 66x66 µm2 and N0 = 6.7 105 atoms.
Left panel: He 1s photoelectron spectra at different photon pulse energies at 38eV photon energy. Additional spectral states occur around 25 eV kinetic energy for high photon pulse energy. These features have a quadratic dependence with the VUV photon pulse energy (right panel).
In contrast, in He (Fig. 2), only weak saturation is observed due to the lower cross section of 2.77Mb/atom at 38eV . However, next to the He 1s photoemission line we observe towards high photon pulse energy additional spectral states which appear to increase quadratically as a function of photon flux.
We acknowledge the HASYLAB VUV FEL staff, in particular R. Treusch, S. Düsterer, E. Plönjes, J. Feldhaus, J. Medved, T. Nunez, V. Kocharian, K. Rehlich, B. Fominykh, and V. Rybnikov. Funding was provided by the BMBF Förderschwerpunkt kondensierte Materie und Synchrotronstrahlung.
|Impressum||Letzte Aktualisierung: 5. Feb. 2008 P. Imperia|