Electron-impact-ionization measurements using hyperfine-assisted state preparation of ground-state berylliumlike sulfur

We have measured electron impact ionization (EII) of the beryllium-like ion S¹²⁺. The use of an isotopically pure A=33 beam of S¹²⁺ ions is established as a method to eliminate all metastable levels, including the extremely long-lived metastable level 2s2p³ P₀, which, for this isotope, decays by hyperfine-induced radiative transitions. The energy dependence and absolute size of the EII cross section were measured from the resulting pure ground-state population at energies from below the threshold at ≈ 652.2 eV up to 3000 eV. These data provide an experimental benchmark for theory