Experimental and theoretical evaluation of density-sensitive N VI, Ar XIV, and Fe XXII line ratios

The line ratios of the 2p-3d transitions in the boron-like spectra Ar XIV and Fe XXII have been measured using the electron beam ion traps at Lawrence Livermore National Laboratory. Radiative-collisional model calculations show these line ratios to be sensitive to the electron density in the ranges n(e) = 10(10)-10(12) cm(-3) and n(e) = 10(13)-10(15) cm(-3), respectively. In our experiment, the electron beam density of about 10(11) cm(-3) was varied by about a factor of 5. Our data show a density effect for the line doublet in Ar XIV, and good agreement with theory is found. The relative intensity of the Fe XXII doublet shows good agreement with our predicted low-density limit. The N VI K-shell spectrum was used to infer the actual electron density in the overlap region of ion cloud and electron beam, and systematic measurements and calculations of this spectrum are presented as well. The Ar XIV and Fe XXII spectra promise to be reliable density diagnostics for stellar coronae, complementing the K-shell diagnostics of helium-like ions.