Comparison of photoelectron energy spectra of Ce⁻ in the ion-trap-off mode (the upper red line) and in the ion-trap-on mode (the lower black line). In the ion-trap-on mode, Ce⁻ ions are trapped for 45 ms with the buffer gas H₂ + He. The ion trap can be turned off to observe excited states with short lifetimes. The sets of sticks below the spectra indicate the energy levels of the final neutral states of photodetachment channels from the same anionic states labeled on the left or right sides. The black sticks are for the anionic ground state, and green for the excited states. The inset shows the photoelectron image obtained under the trap-on mode. The double arrow corresponds to the polarization of the photodetachment laser.