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

The electric quadrupole transition 3p⁶3d¹⁰ ¹{S}₀$--3${p}⁵$3${d}¹⁰$4f (3/2, 7/1) J=2 has been observed in the spectra of Ta xlvi, W xlvii, Re xlviii, Pt li, Au lii, and Hg liii. 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 3p3/2-4d5/2 and electric quadrupole 3p3/2-4f7/2 resonance transitions is sensitive to density in the range 10¹⁹--10²¹ cm^-3, in qualitative agreement with the present observations.