SITE-SELECTIVE SPECTROSCOPY OF ND3+ CENTERS IN CAF2ND3+ AND SRF2ND3+

Laser-selective-excitation and optical-absorption techniques have been used to establish comprehensive sets of energy levels of the F- and hydrogenic varieties of the tetragonal symmetry (C4v) centers in CaF2 and SrF2, of three multihydrogenic centers observed in both CaF2 and SrF2 and of the M, N, and M' Nd3+ cluster centers present in CaF2. Polarization of the various spectral lines of the C4v symmetry centers was used to determine the symmetry labels of all energy levels of these centers up to the F-4(3/2) multiplet, for which crystal-field fits are reported. A noteworthy feature of the Nd3+ centers is their strong upconversion fluorescence giving a visual perception of ''white'' emission under selective laser excitation to energy levels of the 4G5/2, 2G7/2 multiplets near 17 350 cm-1 in the yellow. Excitation with a single frequency in this region enables observation of fluorescence from many multiplets up to the 4D3/2 multiplet near 28 200 cm-1. Fluorescence lifetimes for emission from the F-4(3/2) multiplet are reported for several centers. The Nd3+ cluster centers show small splittings in their sharper transitions, which is attributed to Nd3+-Nd3+ coupling effects.