Solid state spectroscopy: laser selective excitation studies of neodymium
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
When trivalent rare earth (RE³⁺) ions are introduced into the alkaline earth fluoride, they substitute for the divalent cation and some form of charge compensation is required for the crystal to maintain charge neutrality. A wide variety of symmetry configurations can be produced depending on the charge compensator(s) and the rare earth ion(s) in the lattice. The selective laser excitation technique was used to determine the symmetry of the various dopant centres in CaF₂ and Sr F₂ doped with Nd³⁺ ions; both before and after hydrogenation treatment. Previously unassigned energy levels of F¯ charge compensating centres of tetragonal and orthorhombic symmetry in CaF₂ : Nd³⁺ and SrF₂ : Nd³⁺ have been identified by the energy upconversion processes. The upconversion mechanisms for the tetragonal centre are attributed to be dominated by the sequential two photon excitation process (STEP), whereas the two orthorhombic centres are attributed to be dominated by the phonon-assisted energy transfer upconversion process (ETU). The hydrogenic analogues of the well established tetragonal centre in CaF₂ : Nd³⁺ and SrF₂ : N d³⁺, and other hydrogenic charge compensating centres were studied. The fluorescence of some of the multiple charge compensating hydrogenic centres were found to exhibit 'bleaching' behaviours. These effects are attributed to the migration of the hydrogenic ions adjacent to the rare earth ion. For the tetragonal centres, the C₄v irreducible representation designations of the crystal field levels were identified by their intensity variations in the polarisation studies. Energy transfer between the rare earth ions within a cluster centre was established by studying the M' centres in the CaF₂ : Nd³⁺ co-doped with Ce³⁺, Gd³⁺ or Yb³⁺ rare earth ions. These multiple rare earth ion centres also exhibit energy upconversion processes similar to those observed for the orthorhombic centres in CaF₂ : Nd³⁺. Crystal field analyses have been carried out for the tetragonal centres observed in CaF₂ : Nd³⁺ and SrF₂ : Nd³⁺ and a simplified superposition model was used to analyse the distortions in these centres. The crystal field results are in satisfactory agreement with the experimental data and predicted the experimentally unobserved Z₄ energy level for the various C₄v centres.