Intrinsic electronic excitations and impurity luminescent centres in NaMgF₃ and MgF₂ doped with Yb²⁺

The relaxation pathways of states in the energy region of the fundamental absorption of NaMgF₃:Yb²⁺ and MgF₂:Yb²⁺ are investigated using time-resolved vacuum ultraviolet spectroscopy. For NaMgF₃:Yb²⁺, excitation into intrinsic free exciton states or above the band gap results in emission features associated with self-trapped excitons and impurity-trapped excitons. Excitation into host-related states of MgF₂:Yb²⁺ similarly exhibits self-trapped exciton emission as well as emission from Yb²⁺ 4f¹³5d states. The excitation features related to Yb²⁺ 4f¹⁴→4f¹³5d transitions are interpreted using semi-empirical crystal field models. For both materials, the interplay between intrinsic distortions and Yb impurity centres, as the excitation wavelength and sample temperature are varied, is presented and analysed.