Optical Interactions and Excited-State Dynamics in γ-Irradiated Mn-Doped Phosphate Glasses

The influence of gamma (γ)-irradiation (10 Mrad) on some physical and spectroscopic properties of phosphate glasses doped with MnO₂ as 0.5, 2.0, 4.0, 6.0, and 8.0 mol % was assessed, ultimately focusing on the Mn²⁺ excited-state dynamics. A structural assessment by Fourier-transform infrared spectroscopy was overall consistent with the resilience of the glass structures to undergo significant alteration following γ-irradiation. However, the densities of the more heavily doped glasses (2-8 mol % MnO₂) were somewhat higher after γ-irradiation, indicating a tendency toward more compact glasses. The absorption spectra of the pristine glasses exhibited a distinctive increase in the ⁵E_g → ⁵T_2g transitions band characteristic of Mn³⁺ (3d⁴) ions at high manganese content, which was greatly enhanced after γ-irradiation, consistent with Mn²⁺ photo-oxidation. Optical band gap energies were estimated and compared showing that the γ-irradiated glasses had lower values than the pristine as expected due to electron centers being trapped in the glass matrix. Photoluminescence (PL) spectroscopy showed the red emission from Mn²⁺ (3d⁵) ions was prominent in the pristine glasses, however with indications of a quenching effect above 4 mol % MnO₂. The PL was then drastically quenched following γ-irradiation. The Mn²⁺ excited-state lifetimes ascribed to single and interacting ions were found to be consistently shorter for the γ-irradiated glasses despite the lowering in Mn²⁺ concentration expected given its photo-oxidation to Mn³⁺. Consequently, a Mn²⁺ → Mn³⁺ resonant energy transfer was proposed as a significant emission quenching pathway, which became enhanced after γ-irradiation.