Avoided level crossings in very highly charged ions

Abstract

We report a systematic measurement of the ${(2{p}_{1/2}^{\ensuremath{-}1}3{d}_{3/2})}_{J=1}$ and ${(2{s}_{1/2}^{\ensuremath{-}1}3{p}_{1/2})}_{J=1}$ levels in 14 neonlike ions between ${\mathrm{Ba}}^{46+}$ and ${\mathrm{Pb}}^{72+}$ and document the effects of their avoided crossing near $Z=68$. Strong mixing affects the oscillator strengths over a surprisingly wide range of atomic numbers and leads to the vanishing of one transition two atomic numbers below the crossing. The crossing voids the otherwise correct expectation that the ${(2{p}_{1/2}^{\ensuremath{-}1}3{d}_{3/2})}_{J=1}$ level energy is only weakly affected by quantum electrodynamics (QED). For about 10 atomic numbers surrounding the crossing, its QED contributions are anomalously large, attaining almost equality to those affecting the ${(2{s}_{1/2}^{\ensuremath{-}1}3{p}_{1/2})}_{J=1}$ level. As a result, the accuracy of energy level calculations appears compromised near the crossing.