Measurement of the Kr xviii 3d 2D5/2 lifetime at low energy in a unitary Penning trap

A different technique is used to study the radiative decay of a metastable state in multiply ionized atoms. With use of a unitary Penning trap to selectively capture Kr¹⁷⁺ ions from an ion source at NIST, the decay of the 3d ²D_5/2 metastable state is measured in isolation at low energy, without any active cooling. The highly ionized atoms are trapped in the fine structure of the electronic ground configuration with an energy spread of 4(1) eV, which is narrower than within the ion source by a factor of about 100. By observing the visible 637-nm photon emission of the forbidden transition from the 3d ²D5/2 level to the ground state, we measured its radiative lifetime to be τ =24.48ms±0.28stat_.ms±0.14syst_. ms. Remarkably, various theoretical predictions for this relativistic Rydberg atom are in agreement with our measurement at the 1% level.