Long 3d-level lifetimes in highly charged ions of the iron-group elements: a nuisance or a feature?

Atomic lifetime measurements (for example, by beam–foil spectroscopic techniques) of prominent lines and levels in highly charged ions of iron-group elements began decades ago with 3s-3p resonance and intercombination transitions and a few 3p-3d transitions, finding lifetimes in the order of 0.1 ns to dozens of nanoseconds for levels with electric dipole (E1) decay channels. The proximity of the 3p- and 3d-level lifetimes causes well-known problems in multi-exponential decay curve analysis. When, later on, the much longer lifetimes (in the millisecond range) of levels with only E1-forbidden decay channels were addressed in ion traps, the cascade problem initially seemed to be absent, and these long-lived levels were considered to be unique. However, proceeding from Na- and Mg-like ions to Al-like ions and "heavier" isoelectronic sequences, slow cascades appeared that could be traced to specific long-lived 3d levels that also have only E1-forbidden decay channels and thus feature level lifetimes in the same millisecond range, causing the cascade problem to re-appear and to worsen with the increasing number of electrons in the n=3 valence shell. A severe complication arises from the fact that these (often multiple) 3d decays have not been observed directly, so no lifetime measurement has been available to test the computational predictions cleanly. There is at least an astrophysical identification of such a transition in (Al-like) Fe XIV, but this is without time resolution.