Signatures of autoionization in the angular electron distribution in two-photon double ionization of Ar

A kinematically complete experiment on two-photon double ionization of Ar by free-electron laser radiation with a photon energy of 27.93 eV was performed. The electron energy spectra show that double ionization is dominated by the sequential process. Comparison of the electron angular distributions to our data for single ionization and to theory confirms that even in the sequential process the electrons from both ionization steps are correlated with each other through polarization of the intermediate $\mathrm{A}{\mathrm{r}}^{+}$ state. Furthermore, a very important role of autoionization in both ionization steps is found.