Density-sensitive electric quadrupole decays in Ni-like ions observed in laser-produced plasmas

The electric quadrupole transition 3$p^6$3$d^{10}$ ${\mathrm{}}^1$ $S_0$–3$p^5$3$d^{10}$4f (3/2, 7/1) J=2 has been observed in the spectra of Ta x l v i, W x l v i i, Re x l v i i i, Pt l i, Au l i i, and Hg l i i i. The excited-level populations of Ni-like gold are calculated by means of a collisional-radiative model and show that the intensity ratio of the electric dipole 3$p_{3/2}$-4${\mathrm{d}}_{5/2}$ and electric quadrupole 3$p_{3/2}$-4$f_{7/2}$ resonance transitions is sensitive to density in the range ${10}^{19}$–${10}^{21}$ ${\mathrm{cm}}^{\mathrm{-}3}$, in qualitative agreement with the present observations.