Spin-allowed and spin-forbidden 4f(n)<-> 4f(n-1)5d transitions for heavy lanthanides in fluoride hosts

Luminescence spectra of lanthanides in the vacuum ultraviolet spectral regi on have revealed interesting features for 4f(n)<->4f(n-1)5d (f-d) transitio ns. For Tb3+, Dy3+, Ho3+, Er3+, and Tm3+ in LiYF4, weak f-d bands are prese nt in the excitation spectra at lower energy than the usually observed stro ng f-d bands. The weakness of the bands is explained by the spin-forbidden character of transitions from the 4f(n) ground state to these 4f(n-1)5d sta tes. For Er3+ and Tm3+, upon excitation in the higher energetic 4f(n-1)5d s tates two kinds of f-d emission are observed: fast spin-allowed emission wi th a decay time in the order of ns, and slow spin-forbidden emission with a decay time in the order of mu s. These two types of f-d emission are also found for Er3+ and Tm3+ in other fluoride hosts. [S0163-1829(99)11239-6].