Some misconceptions concerning the electronic spectra of tri-positive europium and cerium

Tripositive europium attracts wide interest in diverse fields such as phosphors, sensors, and time-gated bioimaging agents based upon its optical emission spectra. Some inaccurate descriptions of these spectra are being amplified throughout the literature. In this tutorial review the background of electronic states, energy levels and transition intensities is provided as a pre-requisite for clarifying these misconceptions. The topics discussed encompass the electric dipole nature of intraconfigurational 4f electronic transitions, the use of europium as site symmetry and centrosymmetry probe, as well as an indicator of nephelauxetic effects. The frequently mis-used term Stokes shift is also clarified and alternative terms are given for the situations where it is incorrectly applied.