New atomic data for Si/6+/, S/8+/ and Ar/10+/

The paper gives new atomic data, populations of excited levels, and line intensity ratios for the ions Si VII, S IX, and Ar XI of the O I isoelectronic sequence. Ten levels are included in the calculations, i.e., the levels of the 2s/2/2p/4/ and 2s2p/5/ and 2p/6/ configurations. It is noted that the calculations are done for applications to solar plasmas. The line ratios (2s/2/2p/4/3P1 - 2s2p/5/3P0) / (2s/2/2p/4/3P1 - 2s2p/5/3P1) and (2s/2/2p/4/1D2 - 2s2p/5/1P/1/) / (2s/2/2p/4/3P/1/ - 2s1p/5/3P/1/) are two of the ratios useful for electron density determination. Finally, density sensitive line ratios of Ca XIII and Fe XIX are also discussed.