Fluorescence line narrowing (FLN) and site-selective fluorescence decay ofNd3+ centers in CaF2

Site-selective and time-resolved fluorescence and excitation spectroscopy i n the absorption bands of the M and N centers in CaF2:Nd3+ (0.6%) were perf ormed at 1.6 and 4.2 K using a pulsed tunable laser. The site-selective exc itation and fluorescence spectra including the decay kinetics were studied for the crystal-field transitions I-4(9/2)(1) --> (4)G(5/2)(1) in absorptio n and F-4(3/2)(1) --> I-4(9/2)(1) in emission for different Nd3+ centers. I t is shown that the M absorption band mainly formed by (Nd3+-F-i)(2) pair M centers contains also spectral lines of other pair modifications. Similarl y, the N band includes not only lines from (Nd3+-F-i), quartet N centers bu t also from higher aggregated Q and R centers as well as from lower aggrega ted S centers. These centers have different excitation and fluorescence spe ctra and different F-4(3/2) fluorescence lifetimes. The weak absorption bet ween the M and N bands is due to numerous so-called D(2a) centers. An attem pt to identify some of the new centers on the basis of their fluorescence l ifetimes and nonradiative quenching rates was made. It was proved that the I-4(9/2)(1) and the (4)G(5/2)(1) Stark levels in these centers split up int o several sublevels. For the pair M and the quartet N centers the total spl itting could be verified to be about 2.5 and 3.5 cm(-1), respectively. Valu es of 5 and 6 cm(-1) for the Q and Q' centers and a value of 5 cm(-1) for t he R center were found. (C) 1999 Published by Elsevier Science B.V. All rig hts reserved.