Density sensitive lines of highly ionized iron

Spectral lines from ions in the nitrogen isoelectronic sequence due to transitions of the type, $2s2p^4{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2},\frac{5}{2}}^{}{}_{}{}^{}-2\mathrm{}{p}^5{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$, and $2s2p^4{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}-2\mathrm{}{p}^5{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$, are identified in the 100-Å region in laser-produced plasma spectra of titanium (Ti xvi) through cobalt (Co xxi). It is proposed that the intensities of these lines relative to the intensities of lines of the same ions due to transitions of the type $2s^{2}2p^3-2s2\mathrm{}{p}^4$ are sensitive to electron density in the range from ≈ ${10}^{18}$ to ≈ ${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Calculations are performed for Fe xx, and a similar calculation is performed for a density-sensitive line ratio of Fe xix, i.e., $\frac{(2s2\mathrm{}{p}^5{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}-2\mathrm{}{p}^6{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{})}{(2\mathrm{}{s}^{2}2p^4{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-2s2\mathrm{}{p}^5{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{})}$. This line ratio is also sensitive to electron density between about ${10}^{18}$ and ${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The $2s2p^5{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}-2\mathrm{}{p}^6{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ line is newly indentified in Cr xvii, Co xx, and Ni xxi.