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 ${\mathrm{S}}^{12+}$. The use of an isotopically pure $A=33$ beam of ${\mathrm{S}}^{12+}$ ions is established as a method to eliminate all metastable levels, including the extremely long-lived metastable level $2s2p{}^3{P}_0$, 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 $\approx$ 652.2 eV up to 3000 eV. These data provide an experimental benchmark for theory.

Figure 1

Diagram of the TSR heavy-ion storage ring. See text for details.

Figure 2

EII cross section for ${\mathrm{S}}^{12+}$ forming S${}^{13+}$. Circles indicate experimental values and smaller error bars show the $1\sigma$ statistical uncertainty. Dotted lines illustrate the $1\sigma$ systematic uncertainty limits. Experimental results are compared to the theoretical cross section of Younger [29] and to the distorted-wave calculation of Dere [1].