One- and two-photon spectroscopy of f → d and f → f transitions of Eu 2+ ions in M 1- xN xF 2 mixed fluoride crystals (M, N = Ba, Sr, Ca; 0 ⩽ x ⩽ 1)

Abstract

Divalent europium ions exhibit either normal blue or anomalous yellow emission in fluoride crystals, the latter being due to an impurity-trapped exciton resulting from a photoionization process. In (Ca-Ba) and (Sr-Ba) mixed crystals both luminescences are observed. Absorption and emission spectra have been analyzed for several crystals, all doped with 0.02 at% Eu 2+. As the Ba concentration increases, the broadening of the zero-phonon line is due to inhomogeneity and the smoothing of the first absorption band is interpreted as a Fano resonance due to the coupling of the excited “d” electron with the conduction band (i.e. the autoionization of Eu 2+ ion). For the first time, the two-photon transition 8S 7/2 → 6P 7/2 in BaF 2 as well as in M 1- x N x F 2, (M, N = Ba, Sr, Ca; 0 ⩽ x ⩽ 1) mixed crystals is presented. We analyze the evolution of the splitting of the 6P 7/2 energy level in the mixed crystals and show that the lowering of the site symmetry from O h to C 2v induces a fourth component to grow up. Besides, this level is essentially broadened by the inhomogeneity of the crystal but no Fano resonance process with the conduction band occurs. This is in good agreement with the expected fact that the coupling of well shielded “f” electrons with the conduction band is much weaker than this of the outer “d” electrons.