Site-selective laser spectroscopy has been used to investigate the defect structure of a 0.1 mol % ${\mathrm{Pr}}^{3+}$:${\mathrm{CaF}}_2$ crystal. Twenty-three distinct sites have been identified from the fluorescence spectra. Only three of the sites were found to be single-ion sites while the rest were assigned to be clusters of two or more ions. The single-ion sites are a $C_{4v}$ site, which dominates the spectrum at 0.1 mol %, a cubic site, and a site of low symmetry. Of the 20 cluster sites identified, two sites undergo intracluster energy transfer at a rate slow enough to be directly observed.

• Received 4 June 1987

DOI:https://doi.org/10.1103/PhysRevB.36.9781