Optical excitation processes in the near band-edge region of KAlSi3O8KAlSi3O8 and NaAlSi3O8NaAlSi3O8 feldspar

A complex variety of excitation and relaxation pathways are available for charge in naturally occurring NaAlSi3O8NaAlSi3O8 and KAlSi3O8KAlSi3O8 feldspar crystals, when excited with photons in the trans-band-edge energy range 4–12 eV. These can involve a mixture of electronic transitions associated with defect states, the conduction and valence bands and their associated band-tails. In order to demonstrate that a relationship exists between these processes, combinations of three spectroscopic techniques are deployed in this work: luminescence excitation/emission mapping, energy-resolved photo-stimulated phosphorescence and energy-dependent photo-transferred optically stimulated luminescence. The photo-transferred OSL confirms the bandgap energy of alkali feldspars to be 7.7 eV at 10 K, and highlights the role played by a dominant optically active defect 4.4 eV below the conduction band, associated with blue emission in the materials. Luminescence and phosphorescence excitation spectra provide additional, complementary, information regarding the excited states of the blue-emitting defects, including their transition lifetimes and decay paths. Red emission associated with Fe3+Fe3+ luminescence is mostly isolated from the blue emission/excitation processes. A new set of transitions are identified, not previously observed experimentally, that are attributed (TdTd symmetry approximation) to the ground-to-excited 6A1(6S)→4F(4A2,4T1,4T2)6A1(6S)→4F(4A2,4T1,4T2)Fe3+Fe3+ transitions, with 6A1(6S)→4F(4A2)6A1(6S)→4F(4A2) yielding a characteristically narrow feature at 4.61 eV: significantly, excitation to this level allows photo-transferred OSL to take place, a process that is not obviously identifiable with the other transitions of the set. The conclusion of the work is that synchrotron-based luminescence methods can potentially provide one of the few routes to firmly establishing the full optical characteristics of naturally occurring wide bandgap luminescence systems, essential for exploiting their radiation dosimetry properties.