Site-selective fluorescence and laser excitation spectroscopic studies of various types of optical center in Nd3+-doped (0.3 to 1.0%) and in La3+-doped (1.0%) plus Nd3+-doped (0.2%) samples were carried out at 4.2K. The crystal-field levels of the 4I9/2, 4F3/2 and 4G5/2 multiplets for various Nd3+ optical centers were deduced for Nd3+-doped (0.3%) material. An attempt was made to identify some new optical centers from their fluorescence lifetimes and crystal-field splittings. A 2- to 6-times reduction in the 4F3/2 multiplet lifetime was observed in the pair M-center in comparison with the single optical centers. This was attributed to energy cross-relaxation in the pair. A fine splitting of the levels at the 4I9/2(1) → 4G5/2(1) crystal-field transition (1.5/cm), similar to that in CaF2 (2.4/cm), was found for the Nd–Nd pair in SrF2. The variation of the splitting, from CaF2 to SrF2, demonstrated the R−5 dependence of the coherent quadrupole–quadrupole interaction.

Non-Radiative Relaxation and Inhomogeneous Splitting of Aggregated Optical Centers in Nd3+-Doped CaF2 and SrF2 Crystals. Y.V.Orlovskii, V.V.Fedorov, T.T.Basiev, M.Altwein, B.Leu, J.Heber, S.Mirov: Journal of Luminescence, 1999, 83-84, 361-6