Forbidden transitions in one- and two-electron nickel

Abstract

We describe measurements of the lifetimes of metastable states in one- and two-electron nickel ions. The 2 ${}_{}{}^2{}_{}{}^{}$ ${\mathit{S}}_{1/2}$ level in one-electron nickel was found to have a lifetime of 217.1(1.8) ps, in agreement with a theoretical calculation of 215.45 ps. The lifetimes of the 2 ${}_{}{}^1{}_{}{}^{}$ ${\mathit{S}}_0$ and 2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_2$ levels in two-electron nickel were found to be 156.1(1.6) and 70(3) ps, respectively, in agreement with the theoretical results of 154.3(0.5) and 70.6 ps. We also studied the phenomenon of hyperfine quenching of the He-like 2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_0$ level by comparing decay curves for the isotopes ${}_{}{}^{61}{}_{}{}^{}\mathrm{Ni}$ and ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$. The lifetime of the 2 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_0$ level in ${}_{}{}^{61}{}_{}{}^{}\mathrm{Ni}_{}^{26+}{}_{}{}^{}$ was found to be 470(50) ps, in agreement with a theoretical prediction. The measurements were carried out using the beam-foil time-of-flight technique with a fast beam of highly charged nickel ions. We discuss the handling of systematic effects in the measurements, including the study of yrast cascades following the beam-foil interaction.