Evolution across four min-max cycles of the H, He I, and Fe II lines, representing respectively intermediate, high, and low ionization potential energy atoms. The exact photometric state of each spectrum is indicated in the panels of the first column; the age (in days) is on the left. The He I transition is contaminated by Na I D both of interstellar and ejecta origin. The Na I D transition (emission and absorption) originating in the ejecta is particularly evident in the spectra taken between day 82 and 101.