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Fig. 1

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Simplified Grotrian diagram displaying the most important transitions involved in the N iv emission line problem. The horizontal line marks the N v ionization threshold. N ivλ4058 is formed by the transition 3d 1D  →  3p 1P0, while the absorption line at λ6380 originates from the transition 3p 1P0  →  3s 1S. An efficient drain of 3p is provided by the “two-electron” transitions 3p 1P0  →  2p2 1S, 1D. Cascade processes from 3d′ 1F0 and pumping from 2p 1P0 are the major routes to overpopulate the 3d 1D state. See text.

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