Metastable level in Ni-like ions and time-resolved extreme-ultraviolet observation of Xe ions in an electron beam ion trap

Nickel-like ions have a metastable level (3d^{9}4s ^{3}D_{3}) that decays exclusively by a magnetic octupole (M3) transition. We earlier have measured the lifetime of this level in isotopically pure Ni-like Xe ions and have shown how hyperfine interaction modifies the lifetime. We now demonstrate how this long-lived level enables multistep ionization of Ni-like ions at much lower energies than the usual single-step ionization limit, even at the relatively low densities of our electron beam ion trap. Time-resolved and energy-varying observations of Xe^{26+} (Ni-like) and Xe^{27+} (Co-like) in an electron beam ion trap indicate the notable influence of the metastable level on the charge balance in a plasma. The interpretation of our observations is helped by collisional-radiative modeling with hybrid models based on the Flexible Atomic Code, which furthermore enables quantification.