Luminescence properties and afterglow in spinel crystals doped with trivalent Tb ions

Luminescence properties of Tb3+-doped spinel crystals have been investigated at various temperatures between 8 and 300 K under excitation in the Tb3+ absorption bands and in the intrinsic absorption region. Two Tb3+ absorption bands are observed at 5.3 and 6.3 eV in the intrinsic absorption edge region. With excitation in these bands, two series of sharp luminescence bands appear in the region between 1.8 and 3.5 eV, which are to be well established as those due to the series transitions from the 5D3 and 5D4 excited states to the 7DJ (J=6, 5, 4, 3,2) ground states, with radiative lifetimes of 1.5 and 2.0 ms, respectively. The same luminescence spectrum is produced with excitation in the intrinsic absorption region above 7 eV. In this case, prominent slow rise and afterglow components of luminescence are observed which last for up to an hour depending on the temperature. The origins of the slow components of luminescence are connected to the presence of oxygen vacancies as electron trapping centers and Tb4+ ions as hole trapping centers. Optically created electrons and holes would be trapped at the above-mentioned centers and released thermally to activate Tb3+ ions giving rise to the slow luminescence components. Discussion will be made on the dynamical processes of electrons and holes in the excited states.