Complete photoionization experiment and autoionizing states in Ne II

The interaction of intense extreme ultraviolet (XUV) pulses with an atom can lead to multi-photon absorption and multiple ionization of the target. By using intense, linearly and circularly polarized XUV pulses of the Free Electron Laser (FEL) FERMI [1], we realized the first experimental demonstration of a quantum mechanically complete experiment (CE) in an ionic system. The quest for a CE, as the strongest test of theory and as minimal source of information from which any observable can be predicted, was first formulated at the end of the 1960s, [2], for electron-atom scattering, and then extended to atomic and molecular photoionization (PI). The complete information cannot be obtained by measuring only the photo-electron angular distribution (PAD) and the cross sections, but additional variables are required. Thus, CEs in atomic PI were realized by measuring the angle-resolved PAD, combined with the observation of other variables or, in case of atoms with open shells, by controlling the initial polarization of the target. All the CEs on PI so far have been performed with neutral targets. As atomic PI leads to polarized residual ions, CE in positive ions can be realized within the two photon double sequential ionization (TPDI) process, now available thanks to the intense XUV pulses of FELs.