Single site multiphonon and energy transfer relaxation phenomena in BaF2:Er3+

The individual charge compensated sites of Er3+ have been identified in BaF2 crystals containing 0.01–1.0 mole % Er3+ using a narrow bandwidth tunable dye laser and it was found that the sites are not adequately described by the model of single Er3+–F−I pairs. The sites have been classified as single Er3+–F−I pair sites or cluster sites on the basis of the concentration dependence, the number of transitions exhibited, and the existence of efficient nonresonant energy transfer between ions within a site. The energy transfer rates have been found for the cluster sites for two different transfer processes and are compared with previous results reported for similar processes in CaF2:Er3+. The appearance of cluster sites at concentrations as low as 0.01 mole %Er3+ is in agreement with previous work in CaF2 and SrF2 and exemplifies the need for consideration of clustering to describe the defect site equilibria in crystals of this type.