Redox equilibrium and spectral hole burning in Sm²⁺-doped Al₂O₃–SiO₂ glasses

Abstract

Sm²⁺ ion-doped Al₂O₃–SiO₂ glasses were prepared using sol-gel and melt-quenching methods; the redox equilibrium and spectral hole burning were investigated. The Sm³⁺ ions were reduced into Sm²⁺ by heating in H₂ gas or x-ray irradiation. The redox between the Sm³⁺ and Sm²⁺ obeyed first-order kinetics, the rate of which was larger for the sol-gel glasses. The Sm³⁺ ions were also reduced by x-ray irradiation and the activation energy for redox equilibrium was half of that for the glasses treated in H₂ gas. Two different mechanisms were proposed for the redox reaction of the samarium ions. In the x-ray irradiated glasses, the Sm³⁺ ions were reduced into Sm²⁺ by electron transfer from the oxygen defect center, whereas the H₂-gas reaction removed the oxygen ions to reduce the Sm³⁺ ions. The spectral hole burning of the x-ray-irradiated glasses could be burned by the reverse reaction of the reduction of the Sm³⁺ ions; that is, the electron transfer from the excited Sm²⁺ into the surrounding oxygen. A short distance between the Sm²⁺ and oxygen defect centers allowed fast hole burning. On the other hand, the hole burning in the H₂-treated glasses was performed by electron transfer between Sm²⁺ and another trapping center such as Sm³⁺.