Site selective spectroscopy of the C3v symmetry centre in Er3+ doped BaF2

We report on polarised laser selective excitation studies for BaF2 doped with Er3+. Focussing on the dominant centre at low concentrations, we conclude that it has C3v symmetry of a similar nature to the trigonal symmetry J centre reported for SrF2 crystals doped with Er3+. Deviations from the expected polarisation ratios for exact C3v symmetry indicate the presence of static low symmetry distortions of the rare-earth ion environment, a fact which is confirmed by Zeeman infrared absorption measurements. Crystal-field analyses, performed under the assumption of exact C3v symmetry for both the BaF2 : Er3+and SrF2 : Er3+ centres gives a good account of the experimental energy levels and magnetic splitting factors.