SIMBAD references

Parity forbidden radiative transitions from metastable levels are observed in spectra of low-density astrophysical plasmas. These lines are used as probes of the physical conditions, made possible due to the long lifetime of their upper level. In a joint effort, the FERRUM project aims to obtain new and accurate atomic data for the iron-group elements, and part of this project concerns forbidden lines. The radiative lifetimes of the metastable energy levels 3 d⁴(a ³ D)4 s c⁴ D_5/2 and 3 d⁴(a ³ D)4 s c⁴ D_7/2 of singly ionized chromium have been measured. The experiment has been performed at the ion storage ring CRYRING. We employed a laser-probing technique developed for measuring long lifetimes. In this article, we present the lifetimes of these levels to be τ_5/2= 1.28(16) s and τ_7/2= 1.37(7) s, respectively. A comparison with previous theoretical work shows good agreement and the result is discussed in a theoretical context.