| Summary: | The technique of laser selective excitation has been used to study the spectra of 23 Ho³⁺centres present in CaF₂ and SrF₂ crystals, in mixed Ca₀.₉₉Sr₀.₀₁F₂ and Ca₀.₉₉Ba₀.₀₁F₂ crystals and in CsCdBr₃ crystals.
The polarisation of the fluorescence from < 100> and < 111> oriented crystals has been used to assign specific Ho³⁺ site symmetries to the various centres and to label the crystal-field energy levels of each centre by their group irreps.
The two principal Ho³⁺ centres (A and B) observed in both CaF2:Ho³⁺and SrF2:Ho³⁺ crystals have been shown to be of C₄ᵥ and C₃ᵥ symmetry respectively. The four new centres identified in Ca₀.₉₉Sr₀.₀₁F₂:Ho3³⁺ and Ca₀.₉₉Ba₀.₀₁F₂:Ho³⁺ crystals, are derived from the parent CaF₂ C₄ᵥ symmetry (A) centre by an on-axis and an off-axis placement of a dopant cation, Sr²⁺+ or Ba²⁺, in the vicinity of the Ho³⁺ - F- pair. The three additional centres found in SrF₂:Ho³⁺ crystals are from similar Ca²⁺and Ba²⁺ substitutions in the SrF₂ lattice.
Seven centres associated with n- charge compensation were observed in deuterated CaF₂:Ho³⁺ and SrF₂:Ho³⁺ crystals, with no C₄ᵥ symmetry centre being apparent.
For the CsCdBr3:Ho³⁺ system, spectra of the principal dimer centre and three other minority centres are reported. Strong upconversion fluorescence was observed for the principal dimer centre, enabling crystal-field energy levels for 18 Ho³⁺ multiplets to be determined. Low symmetry crystal-field splittings and pair splittings were observed on some of the sharper transitions.
Crystal-field analyses performed for all the C₄ᵥ and C₃ᵥ symmetry centres in CaF2:Ho³⁺, SrF2:Ho³⁺ and CsCdBr3:Ho³⁺ crystals are reported and the resulting crystal-field parameters discussed. Fluorescence lifetimes determined for the major Ho³⁺ centres are also presented.
The high resolution and ODNMR results for the C₄ᵥ symmetry A and SB1 centres in SrF2:Ho³⁺ crystals confirm the centre symmetry assignments and the model configurations proposed on the basis of the polarisation ratio measurements. The superhyperfine ODNMR frequencies correlate well with the changes in the crystal-field parameters between the two centres.
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