Interpretation of europium(III) spectra

The trivalent europium ion (Eu3+) is well known for its strong luminescence in the red spectral region, but this ion is also interesting from a theoretical point of view. Due to the even number of electrons in the 4f shell (4f6 configuration), the crystal-field perturbation by the crystalline host matrix lifts partly or completely the degeneracies of the 2S+1LJ levels. The Eu3+ ion has the great advantage over other lanthanide ions with an even number of 4f electrons that the starting levels of the transitions in both the absorption and the luminescence spectrum are non-degenerate (J = 0). Moreover, the interpretation of the spectra is facilitated by the small total angular momentum J of the end levels in the transitions. The number of lines observed for the 5D0 → 7FJ transitions in the luminescence spectrum or the 5DJ ← 7F0 transitions in the absorption spectrum allows determining the site symmetry of the Eu3+ ion. This review describes the spectroscopic properties of the trivalent europium ion, with emphasis on the energy level structure, the intensities of the f–f transitions (including the Judd–Ofelt theory), the decay times of the excited states and the use of the Eu3+ ion as a spectroscopic probe for site symmetry determination. It is illustrated how the maximum amount of information can be extracted from optical absorption and luminescence spectra of europium(III) compounds, and how pitfalls in the interpretation of these spectra can be avoided.