On the line intensity ratios of prominent Si II, Si III, and Si IV multiplets

Line intensities of singly, doubly and triply ionized silicon (Si II, Si III, and Si IV, respectively) belonging to the prominent higher multiplets, are of interest in laboratory and astrophysical plasma diagnostics. We measured these line intensities in the emission spectra of pulsed helium discharge. The Si II line intensity ratios in the 3s3p22D–3s24p2Po, 3s23d2D–3s24f2Fo, and 3s24p2Po–3s24d2D transitions, the Si III line intensity ratios in the 3s3d3D–3s4p3Po, 3s4p3Po–3s4d3D, 3s4p3Po–3s5s3S, 3s4s3S–3s4p3Po, and 3s4f3Fo–3s5g3G transitions, and the Si IV line intensity ratios in the 4p2Po–4d2D and 4p2Po–5s2S transitions were obtained in a helium plasma at an electron temperature of about 17,000 ± 2000 K. Line shapes were recorded using a spectrograph and an ICCD camera as a highly-sensitive detection system. The silicon atoms were evaporated from a Pyrex discharge tube designed for the purpose. They represent impurities in the optically thin helium plasma at the silicon ionic wavelengths investigated. The line intensity ratios obtained were compared with those available in the literature, and with values calculated on the basis of available transition probabilities. The experimental data corresponded well with line intensity ratios calculated using the transition probabilities obtained from a Multi Configuration Hartree–Fock approximation for Si III and Si IV spectra. We recommend corrections of some Si II transition probabilities.