Spectroscopy and vibronic transitions of divalent europium in LiBaF 3

Abstract

In LiBaF 3: Eu 2+ ion is in a site with inversion symmetry and has the 4f 65d excited state at higher energy than the lowest excited states ( 6P J ) within the 4f 7 configuration. This makes LiBaF 3:Eu 2+ an ideal system for the study of the magnetic dipole (MD) transitions and vibrationally induced forced electric dipole (ED) transitions within the 4f 7 configuration of Eu 2+. Transition probabilities for the MD transitions ( 8S 7/2 → 6P 5/2 and 8S 7/2) and vibronic transitions are reported and compared with values for Gd 3+. It is shown that the MD transition matrix elements ( L + 2 S) that have been calculated for Gd 3+ can be used to predict MD transition probabilities for Eu 2+. The vibronic transition probabilities are two orders of magnitude higher for Eu 2+ than for Gd 3+ in comparable systems due to the much lower position of the opposite parity 4f 65d state for Eu 2+. The vibronics are not only intense but also very narrow (Δ 1 2 ≈ cm −1) for several vibrational modes. This allows for an accurate determination of the ratios of Stokes and anti-Stokes vibronic lines as a function of temperature. The measured temperature dependence follows Bose-Einstein statistics. Also vibronic transition probabilities for the strong and narrow vibronics could be determined accurately in both excitation and emission. Comparison shows that the vibronic transition probabilities are the same in excitation and emission.