SPECTROSCOPY AND VIBRONIC TRANSITIONS OF DIVALENT EUROPIUM IN LIBAF3

In LiBaF3:Eu2+ the Eu2+ ion is in a site with inversion symmetry and has the 4f(6)5d excited state at higher energy than the lowest excited states (P-6J) within the 4f7 configuration. This makes LiBaF3:Eu2+ an ideal system for the study of the magnetic dipole (MD) transitions and vibrationally induced forced electric dipole (ED) transitions within the 4f7 configuration of Eu2+. Transition probabilities for the MD transitions (S-8(7/2) --> P-6(5/2) and S-8(7/2) --> P-6(7/2)) and vibronic transitions are reported and compared with values for Gd3+. It is shown that the MD transition matrix elements (L + 2S) that have been calculated for Gd3+ can be used to predict MD transition probabilities for Eu2+. The vibronic transition probabilities are two orders of magnitude higher for Eu2+ than for Gd3+ in comparable systems due to the much lower position of the opposite parity 4f(6)5d state for Eu2+. The vibronics are not only intense but also very narrow (DELTAnu1/2 almost-equal-to 3.5 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.