Electron angular distributions of noble gases in sequential two-photon double ionization

We present an angle resolved study of photoelectrons emitted from ions of the noble gases neon, argon and krypton by means of time-of-flight spectroscopy. The ionic targets are generated in a sequential two-photon process induced by the free-electron laser FLASH. Values of the anisotropy parameters β2 and β4 are derived from electron angular distribution measurements in the photon energy range from 38 to 91 eV and compared with recent theoretical calculations.