Excitation diagram of Fe II in the December 6, 1997 comet in two different RV ranges: 21–33 km/s (top) and 33–42 km/s (bottom). The excitation temperature seems to be slightly higher in the most redshifted part of the comet, due to the increased abundance of some excited levels (e.g. the a⁴D term at ~12 000 K). As for Fig. 10, the a⁶S_5/2 state was not included in the fit, as it is severely depleted compared to the other Fe II excitation levels.