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.

Figure 1

Two-dimensional energy spectrum of two electrons measured in coincidence in TPDI of Ar.

Figure 2

Angular distribution of the fast electron ejected in the first step of TPDI (left panel) and of the electron ejected in single ionization (center panel). The right panel shows the ratio between both angular distributions. Black dashed curves, fit of Eq. (2) to the data; blue dashed curves, calculations not accounting for autoionization; red solid curves, calculations accounting for autoionization.

Figure 3

Angular distribution of the slow electrons ejected in the second step of sequential two-photon double ionization averaged over all energies smaller than 1 eV (left panel). The center and right panels show the same angular distributions averaged over the energy intervals 0.2 to 0.4 eV and 0.4 to 0.6 eV, respectively. Curves: same as in Fig. 2.

Figure 4

Energy spectrum of the slow electron ejected in the second step of sequential TPDI. The black dashed curve shows the calculated energy spectrum not accounting for the FEL spectral profile or the electron energy resolution. The red solid curve represents the same calculation multiplied by the spectral profile and convoluted with the electron energy resolution.

Figure 5

Electron energy dependence of the ${}^{\left(2\right)}\beta _2$ parameter for the electron ejected in the second step of sequential two-photon double ionization calculated with autoionization included for three different photon energies within the FEL bandwidth.