Probing metastable Sm²⁺ and optically stimulated tunnelling emission in YPO₄: Ce, Sm

When the model dosimetry system YPO₄: Ce³⁺, Sm³⁺ is exposed to X-rays, the charge state of the dopants changes, becoming Ce⁴⁺ and Sm²⁺ via hole and electron trapping, respectively which are metastable; the original charge states can be achieved through electron transfer back from Sm²⁺ to Ce⁴⁺ via optical stimulation. The work presented here adds further details to the energy levels of the metastable Sm²⁺ defect and the electron transfer processes by undertaking measurements of a) Sm²⁺ excitation spectrum through the internal ⁵D₀ → ⁷F₂ emission at 7 K, b) relaxation lifetime of Sm²⁺ (⁵D₀ state) and its temperature dependence to provide insights into thermal quenching, and c) the kinetics of localised recombination from Sm²⁺ to Ce⁴⁺ on nanoseconds to seconds time scales using sub-band-edge excitation.