ИСТИНА

Recently, photoionization experiments with both linearly and circularly polarized, intense XUV radiation became feasible at the free-electron laser FERMI. This opens a way to perform a new type of complete photoionization experiment. It is known that photoelectron angular distributions (PADs) from unpolarized atoms are not enough to extract all complex photoionization amplitudes even within the dipole approximation. To overcome this obstacle different methods of producing polarized targets have been applied, usually involving additional field(s). Here we propose the measurement of PADs in sequential two-photon double ionization (2PDI) to perform a complete photoionization experiment, which is new in three aspects: this is a complete experiment with positive ions, it is generalized for accounting the quadrupole amplitude, and, finally, it provides additional data to complete the photoionization experiment with neutral atoms. We imply the two-step mechanism of the 2PDI with creation of an intermediate single-charged ion. The PADs in the second ionization step have been measured many times for noble gas atoms; the non-dipole effects in these PADs have been reported recently in argon. The important feature of the process is that the intermediate ion is either aligned or oriented depending on polarization of the VUV pulse. Therefore, no additional efforts are needed to produce the polarized ionic target. We showed that for noble gases, within the Cooper-Zare model, five independent asymmetry parameters characterize the PAD in the second-step ionization for each field polarization. Two of the parameters are expressed in terms of the dipole amplitudes, while three parameters originate from the interference between electric dipole and quadrupole amplitudes. At the same time the ionization of the ion is characterized by four amplitudes, two dipole and two quadrupole, and the absolute ratio of the dipole amplitudes of the first ionization step. Thus, the PADs in the second ionization step for linearly and circularly polarized field are enough to extract three ratios of the complex second-step amplitudes and the absolute ratio of the first-step dipole amplitudes, i.e. enabling the complete experiment in the second step. The latter ratio, combined with the PADs of the first ionization step, completes also the experiment for the first step.