Top panel: Fe ii column densities for the ground level (open circle), first fine structure level (filled circle), and first excited level (upper limit) transitions for component I in the spectrum of GRB 090926A. Column density predictions from our time-dependent photo-excitation code are also shown. They refer to the ground level (dotted line), first fine structure level (thick solid line), and first excited level (dashed line) transitions, in the case of an absorber placed at 2.6 kpc from the GRB. The two thin solid lines display the models that enclose the fine structure data at 1σ level. Bottom panel: same as top panel, but for ground and the first fine structure level transitions of Si ii.